Research Highlights

Manifestations of Exotic and New Physics in Nature ()

David Eichler

We examine astrophysical consequences of extreme, exotic, and new physics.

In Gamma Ray Bursts, for example, general relativistic effects , especially the event horizon of the black hole, may play a crucial role. We examine whether observations can reveal these effects and the existence of the event horizon. Preliminary results suggest that general relativity may be crucial to understanding the qualitative nature of gamma ray bursts.

In magnetars, collapsed stars with extraordinarily high magnetic fields, the strength of the magnetic alters the nature of quantum electrodynamics (The magnetic field-particle spin interaction term in the Langrangian exceeds the elecron rest mass). The vacuum develops an index of refraction that affects the propagation of light. Condensed matter, e.g. the crust of the magnetar, is strongly affected by such a magnetic field. We attempt to understand the effects of the ultrastrong magnetic fields on the observed phenomena.

In cosmology, we attempt to understand dark energy, which is matter with negative pressure, as a field that is generated by dark matter. This hypothesis aims to resolve the present mystery as to why dark matter and dark energy both presently exist in the universe in comparable quantities.

Collisionless Shock Waves and the Origin of Cosmic Rays ()

David Eichler

Astronomy in the radio, X-ray, and gamma ray and ultrahigh energy gamma ray frequency bands is made possible largely by relativistic particles, which emit at these frequencies when their paths are bent by magnietic fields. The Crab Nebula is but one famous example of synchrotron radio, X-rays and gamma rays being emitted by electrons moving at 0.999999 of the speed of light. We attempt to understand how, and under what circumstances, violent explosions accelerate a small minority of particles to such enormous energies.

Figure 1: The Crab Nebula in X-ray.
Credits: X-ray: NASA/CXC/ASU/J. Hester et al.; Optical: NASA/HST/ASU/J. Hester et al.
Source: http://chandra.harvard.edu/photo/2002/0052/more.html

Figure 2: Synchrotron emission simulation of the Crab Nebula.
Source: Komissarov S.S. Lyubarsky Y.E. 2003, MNRAS, 344, L93.

Space Weather ()

Michael Gedalin

The Sun governs the life of the Earth. Solar-terrestrial relations occur not only via radiation coming to the Earth but also via time-varying plasma flow (Fig. 1). This solar wind is decelerated and diverted by the bow shock forming at the distance of about 10 Earth radii toward the Sun. The diverted plasma flows around the Earth, shaping a magnetosphere (Fig.2) with a long tail and a current sheet where the so called "magnetic reconnection" (Mov. 1) occurs, causing magnetic substorms (Mov. 2). We study the basic processes in this interaction: mirror waves generated behind the shock because ions are heated more in a direction perpendicular to the magnetic field (1, 2), nonlinear waves propagate in plasma and steepen similar to "shallow water waves" (3), bow shocks are formed around the planets with or without global magnetic field and energize ions and electrons (4, 5, 6), magnetic field lines locally reconnect in the current sheet in an avalanching way (7, 8).

Figure 1: The Solar Wind. Click to enlarge. Figure 2: The Magnetosphere. Click to enlarge.

Movie 1: Magnetic Reconnection (mov, avi)

Movie 2: Magnetic Substorms (mov, avi)

Collisionless shocks ()

Michael Gedalin

Shocks form in a supersonic flow which encounters an obstacle and has to suddenly decelerate to subsonically flow around the body. Alternatively, a gas expanding supersonically into ambient medium produces a shock moving ahead of the gas. Such shocks are very ubuquitous in space where they propagate in collisionless plasmas and generate high energy particles. The latter are responsible for the emission coming from such objects as supernovae remnants (Fig. 1) and gamma-ray bursts (Fig. 2) and their afterglows (Fig. 3). Collisionless shocks are wonderful natural laboratories which allow to learn fundamental processes such as particle dynamics in electromagnetic fields (which may be very peculiar and unexpected), acceleration to relativistic energies in natural accelerators, nonlinear interactions in plasmas and nonlinear structure formation. We study the internal structure of collisionless shocks (1, 2) and the processes of the charged particle energization (3, 4, 5).

Figure 1: Supernovae Remnants. Click to enlarge.Figure 2: Gamma Ray Bursts. Click to enlarge.
Figure 3: Gamma Ray Burst Afterglow. Click to enlarge.

The largest objects: galaxy clusters ()

Uri Keshet

Galaxy clusters, as the largest virialized objects in the present day Universe, provide an important link between cosmology and astrophysics. These relatively young (redshift up to a few), massive (up to a few 1015 solar masses, i.e. ~1045 kg) objects appear optically as concentrations of hundreds or thousands of galaxies, typically found at the nodes of the filamentary cosmic web (see Figure 1). Gravitational lensing maps suggest that the mass is predominately dark (see Figure 2 in blue). X-rays reveal that most of the baryonic mass is in the form of diffuse, hot (10-100 million K) plasma, called the intracluster medium (ICM), extending a few Megaparces (~1023 m) out from the center (see Figure 2 in red). Radio images reveal the presence of relativistic particles (AKA cosmic-rays) and magnetic fields that permeate the ICM.
We address central open questions in the study of galaxy clusters, such as: what stabilizes the dense cluster cores from catastrophic cooling and collapse? what is the nature of the subtle features recently found in the centers of clusters, including spiral features, cold fronts, and X-ray cavities/radio bubbles (see Figure 3)? how do the cosmic-rays and magnetic fields form, evolve, and decay? can we identify the virial shocks that are thought to surround clusters?

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Figure 1: Baryon number density n in a simulated Universe (Keshet et al. 2003). Galaxy clusters typically form in the nodes of the filamentary cosmic web. The image shows n in cm-3 units in a (32Mpc)2 slice of an SPH cosmologicalFigure 2: Multi-messenger image of the bullet cluster (credit). The cluster is shown in optical (galaxies), X-rays (red, showing gas), and gravitational lensing (blue, tracing mass). The image size corresponds to ~(3Mpc)2. Figure 3: The Perseus cluster core in a Megasecond Chandra X-ray exposure, with highlighted features: spiral cold fronts and X-ray cavities (Keshet 2011). The image size is 8' on the side, corresponding to ~250kpc.

Black holes ()

Uri Keshet

Black holes play a double role in physics, both as astronomical objects routinely discovered nowadays in binaries and in the centers of galaxies, and as central targets on the path to understanding quantum gravity. The presence of super-massive black holes in the centers of essentially all galaxies is strikingly illustrated by the observed trajectories of stars in the very center of our Milky Way Galaxy (see Movie 1). Among the many observational implications of such massive black holes, they are thought to induce characteristic steady-state, mass-segregated stellar cusps in the centers of galaxies, as illustrated in Figure 1. Among the tests of strong gravity and clues to quantum gravity obtained from the study of black holes, the quasinormal ringing of perturbed black holes induces characteristic gravitational waves and may shed light on the quantum description of black holes (see Figure 2).

Movie 1: Observed stellar trajectories around Sagittarius A* (see tour), the massive black hole in the center of our Galaxy. Although the central object inferred from these trajectories is ~4 million times more massive than our sun, its radius must be less than the pericenter of the nearest star shown (denoted S2), 17 light hours.

Figure 1: Bahcall & Wolf (1976,1977) have shown that within the radius of influence of a massive black hole,stars of different masses segregate, each species forming a power-law density cusp. The first image illustrates mass segregation of stars (light stars in green, intermediate mass-yellow, and massive-red) around a massive black hole (black disk). We derived an analytic solution for the energy distribution \( f(x,m) \) of stars with an arbitrary mass function \( g(m) \) . At intermediate distances \( f\sim x^p \) , where \( p(m)=m/4M \) and \( M \) is an averaged stellar mass (see details). The figure shows the landscape of the energy power-law indices pH of the most massive stars, for different power-law mass functions \( g(m_{L} < m < m_{H}) \sim m_\alpha \) , as a function of power-law index \( \alpha \) and the mass range \( \zeta=m_H/m_L \) .

Figure 2: The intermediate and asymptotic quasinormal spectrum and greybody factors of a generic (Kerr-Newman, i.e. rotating and charged) black hole in flat space were analytically derived 1, 2 ,3. They reveal a peculiar structure, reminiscent of special black holes where a dual quantum description has been established.|

Quantum Chromo-Dynamics at high energies()

Michael Lublinsky

We have entered the fascinating era of the Large Hadron Collider: the initial proton and heavy
ion collisions are already underway. Heavy ion collisions provide the unique possibility of
creating and studying a new state of matter, known as quark gluon plasma, at energy densities
and temperatures similar to those of the early Universe at 10􀀀5 seconds after the Big Bang.
The microscopic theory describing the structure of protons and nuclei is the theory of strong
interactions, know as Quantum ChromoDynamics (QCD). Even though the fundamental theory
is known, it is extremely dicult to deduce from the QCD results of collision processes. This is
due to the high level of complexity of the theory involving mutual interactions between gluons,
the "photons" of strong interactions. When probed at very high energies, heavy nuclei, and
even protons, appear as very dense clouds of gluons. The main objective of this proposal is to
develop the theory of high energy collisions of dense gluonic objects, using rst principle QCD
calculation and apply it to experimental data on heavy ion collisions at the LHC.

Fluid-Gravity correspondence and its application to Quark Gluon Plasma()

Michael Lublinsky

One of the most intriguing and fundamental questions is a formation of Quark Ggluon Plasma (QGP). Experimentally, QGP is created in the heavy ion collisions. A quest for QGP is the driving force behind two major experimental programs, one at the Relativistic Heavy Ion Collider (RHIC) and another one at the LHC.

Among most striking recent discoveries is the observation made at the RHIC that QGP produced there at temperatures about twice the QCD critical temperature is in fact strongly coupled. The RHIC data indicate that QGP at not too high temperatures behaves like a nearly perfect fluid with relativistic hydrodynamics being an appropriate description of the observed phenomena. Remarkably, gauge theories at strong coupling can be studied using the AdS/CFT duality: from the string theory point of view, QGP is holographically dual to weakly coupled string theory in the 5-dimensional Anti-de-Sitter (AdS) Black Hole background metric. Many interesting phenomena relevant for heavy ion collisions can be learn from the (super) gravity approximation to the string theory. In particular by studying graviton`s absorption into the AdS Black Hole within Einstein`s general relativity, one can learn a great deal about dissipative processes taking place in QGP.

Pulsars ()

Yuri Lyubarsky

Enigmatic, rapidly pulsating radio sources, called pulsars, were discovered more than 40 years ago. They produce beams of radio waves which sweep around the sky like a lighthouse, often hundreds of times a second. Some of them generate also intense beams of gamma-rays. It was found that the beams are radiated out from rapidly rotating, extremely dense neutron stars; a typical pulsar is only around 10 km in diameter (with the mass as large as the solar mass!) and has a magnetic field trillions of times stronger than Earth’s. Such extreme physical conditions are well beyond those attainable in terrestrial laboratories and it is no wonder that, in spite of the great amount of effort that has been expended to understand physics of pulsars, the mechanisms that generate their intense radiation beams remain largely a mystery. We develop theoretical models of the electromagnetic emission from the relativistic plasma within the pulsar magnetosphere. Confronting the calculated emission pattern with observations would tell us a great deal about the fundamental physics behind pulsars.

Figure 1: Pulsar magnetosphere. Figure 2: Pulse shapes of a few pulsars from the radio to gamma-ray bands.

Relativistic jets ()

Yuri Lyubarsky

The primary observational fact that emerged from studies of a large class of astrophysical sources is the occurrence of highly collimated outflows. On a galactic scale, jets are found emerging from radio galaxies and quasars powered by the release of gravitational energy near the supermassive black hole at the galaxy's center. The observed velocities in these jets reach 0.999 of the speed of light. Similar jets, though on a much smaller scale, can develop around the accretion disks of neutron stars and stellar black holes; these systems are often called microquasars. The relativistic jets are also invoked to explain the powerful cosmic gamma-ray
bursts even though they could not be resolved by our telescopes. All cosmic jet sources may be connected by a common basic mechanism. A promising model is magnetohydrodynamic acceleration by rotating, twisted magnetic fields. The basic idea is that a strong magnetic field threading the rotating central object (black
hole and the surrounding accretion disk) serves to convert the rotational energy to the outward directed Poynting flux. We study the basic properties of such electromagnetic jets paying special attention to physical mechanisms responsible for the transformation of the electromagnetic energy into the plasma and radiation energy.

Figure 1: Jet from the radio galaxy Virgo A in different bands. Figure 2: Jet from the quasar 3C175.

Determining the nature of dark energy ()

Ramy Brustein

The accelerated expansion of our universe, and consequently that it apparently contains a substantial amount of dark energy are still unexplained. Interpreting and understanding the accelerating universe is arguably one of the major scientific challenges of our times. The challenge can be approached on several levels, from the most practical level: how to interpret the data, through an intermediate level: phenomenological models that can be embedded into theories of fundamental physics, to the most profound level: the cosmological constant problem.

Particle physics & Cosmology in flux compactifications ()

Ramy Brustein

A novel view of the space of solutions of string theory is starting to emerge, "the stringy landscape" in which there is a very large number of candidate classical solutions - perhaps on the order of 10500. Only a very small fraction of these (less than 10-120) would have a small enough cosmological constant and will live long enough to be acceptable, and when additional conditions such as cosmological viability and particle physics constraints are added, this number will no doubt be substantially reduced. However, it is still likely to be extremely large.

Our main objective is to determine whether any of the solutions in the landscape is an acceptable solution that can be used as a viable model of cosmology and particle physics. We will need to determine a meaningful set of prior assumptions that can define what constitutes an acceptable solution and a viable model and determine their phenomenological consequences.

Vacuum Energy and Child Universes ()

Eduardo Guendelman

In particle physics, spontaneous breaking of scale invariance (SSB of SI) is applied to mechanisms for confinement of quarks and gluons, while in gravity and cosmology it relates to the acceleration of the present universe, quintessence scenarios and inflation of early universe. In gravity and cosmology, SSB of SI is nicely implemented in the context of the Two Measures Theory, which I have developed with Alex Kaganovich. This model addresses all the cosmological questions mentioned before and also provides an interesting solution to the 5th force problem that most quintessence scenarios suffer. I have also been studying the possibility of creating a universe in the laboratory starting from a bubble of false vacuum (see figure).

In particular, with Jacob Portnoy, I have considered the creation of a universe from a stable particle like configuration and with Idan Shilon, I study the gravitational trapping of particles by these stabilized particle like configurations.

Interplay between measurements in particle colliders and theories beyond the Standard Model()

Yevgeny Kats

The most powerful particle physics experiment ever built, the Large Hadron Collider (LHC) at CERN, holds great promise. In 2012, it discovered the Higgs boson, and the significantly higher energy and collision rate that it provides today may allow discovering new types of particles, some of which could explain the various open questions in fundamental physics (the origin of the electroweak scale, dark matter, the matter-antimatter asymmetry in the Universe, and others).

While waiting for discoveries, the results of measurements and new-physics searches from the LHC can be re-used for examining the viability of new physics scenarios other than those for which they were originally designed. Determining the status of those scenarios involves simulating the relevant physical process, the resulting signatures in the detectors, and the analysis done in the experimental studies. As a welcome by-product, we identify general gaps in the experimental coverage of potential new physics signatures.
In one such project, our goal was to clarify the experimental status of supersymmetry as a natural explanation for the electroweak symmetry breaking scale. Besides requiring the existence of a light superpartner (as at least the higgsino is expected to be light), and a gluino within the kinematic range of the 8 TeV LHC, we have kept our analysis quite general, allowing for arbitrary departures from any minimal model of supersymmetry. We were able to argue that gluino decays always give rise to either a significant amount of missing energy and/or frequently produce top quarks and/or large jet multiplicity, to the extent that they are covered by a certain class of LHC searches in each case. We have found that gluinos are almost always excluded up to masses above 1 TeV. We have also identified several classes of scenarios in which the limits were weaker, and proposed strategies for addressing these gaps.

Further reading: Toward Full LHC Coverage of Natural Supersymmetry, J. A. Evans, Y. Kats, D. Shih, M. J. Strassler, JHEP 1407, 101 (2014) [pdf]

In an earlier work, motivated by the lack of any signals in supersymmetry searches based on missing energy, we have addressed the status of more general models of supersymmetry, those that do not assume R-parity. As a compromise between minimizing the fine-tuning of the electroweak symmetry breaking scale, and the apparent absence of significant production of colored superpartners, we considered scenarios in which the only light colored superpartners are the third-generation squarks (and in particular, one of the stops). We constructed a set of simplified models that span the parameter space of the R-parity violating couplings and the mediators through which the stop may decay. We derived limits on these models using a complete set of potentially relevant recent LHC searches. We then looked into the least constrained scenarios in more detail and suggested several ideas for search methods that may allow addressing many of them.

Further reading: LHC Coverage of RPV MSSM with Light Stops, J. A. Evans and Y. Kats, JHEP 1304, 028 (2013) [pdf]

Currently we are analyzing certain novel LHC signatures predicted by the Clockwork Theory. Stay tuned!
CMS search
One of the LHC searches motivated by our models
[CMS collaboration, Phys. Lett. B 739 (2014) 229]

Apart from providing feedback to the experimental community, this type of studies make us prepared to interpret any hints of new particles that the LHC might report in the future. One hint, near the mass of 750 GeV, was reported at the end of 2015. The hint has disappeared since then, by it made us think about various theoretical ideas that may still turn out useful in other contexts.

Further reading:
Interpreting a 750 GeV Diphoton Resonance, R. S. Gupta, S. Jaeger, Y. Kats, G. Perez, E. Stamou, JHEP 1607, 145 (2016) [pdf]
Resonances from QCD bound states and the 750 GeV diphoton excess, Y. Kats and M. J. Strassler, JHEP 1605, 092 (2016) [pdf]
Colorful twisted top partners and partnerium at the LHC, Y. Kats, M. McCullough, G. Perez, Y. Soreq, J. Thaler, JHEP 1706, 126 (2017) [pdf]

Methods to measure quark polarizations at the LHC()

Yevgeny Kats

While it's easy for the LHC detectors to reconstruct the momentum of an energetic quark by measuring the jet it produces, there is no straightforward way to determine the quark's polarization. We have pointed out that it is actually possible. For the bottom and charm quarks, which are heavy relative to the QCD scale, the polarization is expected to be largely preserved when they hadronize into the Λb and Λc baryons, respectively. With collaborators from the CMS experiment, we analyzed how such measurements can be done using various decays of these baryons.

The most interesting application would be characterization of new physics processes producing bottom or charm quarks. While new physics is yet to be discovered, we motivated a set of Standard Model analyses for ATLAS, CMS, LHCb, BaBar and Belle that would help calibrate the polarization measurements.

Further reading:
Heavy baryons as polarimeters at colliders, M. Galanti, A. Giammanco, Y. Grossman, Y. Kats, E. Stamou, J. Zupan, JHEP 1511, 067 (2015) [pdf]
Measuring c-quark polarization in W+c samples at ATLAS and CMS, Y. Kats, JHEP 1611, 011 (2016) [pdf]

An example top-antitop event that can be used for measuring the polarization of c quarks produced in W boson decays.

For the strange quark, the heavy-quark approximation cannot be used. However, it is known from experiments at LEP that Λ baryons in fact preserve much of the strange-quark polarization. We argued that this allows measuring the polarization of strange quarks at the LHC. Furthermore, there are reasons to believe that up and down quarks hadronizing into a Λ may also be transferring some of their polarization. This may open the way for measuring their polarization as well. We motivated studies in top-quark samples in ATLAS and CMS that would provide additional information about the polarization transfer from the strange, up and down quarks to the Λ.

Further reading: Measuring polarization of light quarks at ATLAS and CMS, Y. Kats, Phys. Rev. D 92, 071503(R) (2015) [pdf]

Several additional interesting projects on these topics are underway or planned for the near future.

For more highlights see: http://physics.bgu.ac.il/~katsye

Mechanisms of Species Diversity Change in Stressed Environments()

Ehud Meron

The impacts of environmental changes on species diversity, and thus on ecosystem function and stability, is a central topic of current ecological research. While such changes may independently affect animal and plant species, plants standout in being primary producers; by storing solar energy in chemical compounds they constitute the basal trophic level of the food chain that animal species, including humans, depend on. Plant communities respond to environmental changes at different scales. At the landscape scale, where symmetry breaking vegetation patterns appear (Fig. 1), a transition from one pattern state to another may take place (Animation 1). At smaller, single-patch scales, environmental changes may affect inter- and intra-specific plant interactions. Using a mathematical modeling approach, we developed a theory of plant communities in water limited system, and are currently using it to highlight mechanisms of species diversity change in response to climate changes and disturbances. Special attention is given to mechanisms that involve different levels of organization, e.g. mechanisms by which pattern transitions at the landscape level affect plant interactions and species richness at the single-patch level.

Figure 1: Aerial photograph of vegetation bands on hill slopes in Niger. Reprinted from C. Valentin, J. M. d'Herbes, and J. Poesen, Catena 37, 1 (1999), ©1999.

Animation 1: Model simulation showing a transition from a banded vegetation pattern to a spotted pattern induce by a local clear-cut disturbance. Downhill direction is to the right. Produced by Erez Gilad.
Can't see the movie? here it is in animated GIF.

Further reading:
See also Recent Publications link in http://www.bgu.ac.il/~ehud

Non-equilibrium Localized Structures ()

Ehud Meron

Localized spatial structures, such as defects in periodic patterns, fronts and vortices, are ubiquitous in non-equilibrium systems. Understanding the mechanisms by which they form, their stability and how to manipulate them, is significant for exploiting them in technological applications.

A localized structure may involve a single mode, e.g. a spiral wave in an oscillating medium (showcasing a Hopf mode), but quite often richer structures appear. This is particularly true when a system is driven far from thermal equilibrium and the number of growing modes increases. The system may still be governed by a single dominant mode that damps all other modes by means of nonlinear mode coupling. However, localized structures of the dominant mode, where its amplitude vanishes or becomes sufficiently small, can host the hidden damped mode, giving rise to multimode structures. We studied structures of this kind in systems that go through dual (Hopf-Turing) instabilities. Among our findings are defect lines acting as self-organized wave-guides for traveling pulses (Fig. 1), breathing defects, and multi-state localized structures, that are potentially significant for data storage applications.

Figure 1: Self-organized waveguide. A pulse of a Hopf mode (amplitude B) propagating along a defect line in a Turing pattern (amplitude A). Time proceeds from top to bottom. Produced by Adam Lampert.

Further reading:
  • A. Lampert and E. Meron, "Localized structures as spatial hosts for unstable modes", Europhys. Lett. 78 (2007) 14002.

On asymptotic integrability of perturbed evolution equations ()

Yair Zarmi

Evolution equations yield approximations to solutions of many complex dynamical systems: Propagation of light signals in an optical fiber, and of surface waves in a deep fluid layer (NLS equation); Propagation of disturbances on the surface of a shallow fluid layer, in Plasma ion acoustic waves, and the continuum limit of the Fermi-Pasta-Ulam problem (KdV equation); Propagation of weak shock waves in a fluid (Burgers equation). The evolution equations have wave solutions (e.g., solitons, shock fronts). However, perturbed evolution equations lose all the nice properties of the unperturbed equations.

The research is aimed at finding simple approximations to solutions of perturbed evolution equations that can tell us the extent of validity of the approximation. For instance, over what distance an optical signal in an optical fiber, or a wave on the surface of a fluid layer, will preserve their soliton nature.

Figure 1: Two-Soliton solution of the KdV equation.Figure 2: Wave generated spontaneously by perturbation added to KdV equation, with capacity to destroy simple structure of two-soliton solution.

Further reading:
  • On spatially non-local Burgers-like dynamical systems - A. Veksler and Y. Zarmi, Nonlinearity, 16, 1367-1380 (2003).
  • Freedom in the Normal Form Expansion and Obstacles to Asymptotic Integrability: The Perturbed KdV Equation - A. Veksler & Y. Zarmi, WSEAS Transactions on Mathematics, 3, 560-565 (2004).
  • Perturbative Analysis of Wave Interactions in Nonlinear Systems - A. Veksler & Y. Zarmi, Th. & Math. Phys. 144, 1227-1237 (2005).
  • Wave Interactions and the Analysis of the Perturbed Burgers Equation - A. Veksler & Y. Zarmi, Physica D, 211, 57-73 (2005).
  • Freedom in the Expansion and Obstacles to Integrability in Multiple-Soliton Solutions of the Perturbed KdV Equation - A. Veksler & Y. Zarmi, Physica D 217. 77-87 (2006).
  • Spontaneously Generated Waves in Perturbed Evolution Equations - A, Veksler & Y. Zarmi, Nonlinearity 20 1-14 (2207).

Stochastic analysis of algal-cell motion ()

Yair Zarmi

Single-cell algae are mass-produced as an income source for modern desert settlers. The algae can grow in relatively low-quality water, which is recycled. In large ponds, the optimal biomass production rate cannot be improved. However, in thin bioreactors (containers that can be meters long and high, but a few cm thick), the optimal rate increases by about one order of magnitude. Light hits the transparent wall of the container. As the density of algae is extremely high, only a thin layer near the illuminated wall is exposed to light. Most of the cells are in the dark. They perform a random walk owing to turbulent motion induced in the water by passing air bubbles. The purpose of this research is to analyze the stochastic equations governing the motion of cells in and out of the illuminated layer, and find its effect on biomass productivity.

Further reading:
  • Combined effects of Light Intensity, Light-Path and Culture Density on Output Rate of Spirulina platensis (Cyanobacteria) - Hu Qiang, Y. Zarmi & A. Richmond, Eur. J. Phycol., 33, 165-171 (1998).
  • Biological Principles of Mass Cultivation - A. Richmond, pp. 125-177 in Handbook of Microalgal Culture (ed. A. Richmond), Blackwell, Oxford 2004.

The Laser-Matter Interaction Group ()

Ilana Bar

Our group conduct experimental, theoretical and computational studies of laser-matter interactions.
For further details, please visit our Group homepage

The Atom Chip Group ()

Ron Folman

Quantum theory is one of the scientific revolutions of the 20th century. It is with us for nearly a century and we still don’t fully understand it and its implications. The quantum nature of atoms becomes dominant when their deBroglie wavelength becomes comparable to the size of the potential in which the atoms are held. This occurs at ultra low temperatures ( \( <1\mu K \) ). The wave properties of cold atoms (which are thus named "matter waves") can be exploited for fundamental measurements such as the study of nature's symmetries, search for new forces or analyzing the border between the quantum and classical worlds (with implications even to the question of freedom of thought). The field of quantum optics has made leaps in the past 15 years or so, and in this period 4 Nobel prizes have been awarded (1997, 2001, 2005, 2012).

Our main research tool is the AtomChip (Fig. 1). This device enables the manipulation and detection of isolated cold atoms for quantum operations. Such systems also have technological applications. For example, they have already set the best time standards; they are now being developed via interferometric schemes into acceleration sensors for ultra-accurate navigation systems, as well as for detecting minute changes in the gravitational field; magnetometry can be made so sensitive ( \( 10^{-17} \) Tesla) that it can be used for medical imaging of the brain; more futuristic applications involve secure communication (quantum cryptography) and the super-fast quantum computer.

The "AtomChip" group, and the nano-fabrication facility at Ben-Gurion University, combine in developing AtomChips for new fundamental insights into the laws of nature as well as new technological applications. Our students study for degrees in Theoretical and Experimental Physics (and combinations thereof), and are exposed to a variety of fundamental theory as well as advanced technology, ranging from lasers and optics, to electronics and computer interfaces. Our students have won numerous excellence awards in Israel and abroad (last one in 2014). See our group web site for more information.

Bottom line: We talk to atoms, and you are invited to talk to them too!

Figure 1: An Atom Chip recently fabricated at BGU, with current carrying wires forming magnetic traps and guides for cold atoms.

Figure 2:Figure 2: An interference pattern made by matter-waves in our lab. This interference pattern is the direct result of putting an atom in two places at the same time, as allowed by quantum rules. Reference: S. Machluf, Y. Japha, R. Folman, Nature Communications 4, 2424 (2013).

Attosecond Science and Nanophotonics group ()

Eugene Frumker

In our group, we focus on both experimental and theoretical studies at the interface of ultrafast nonlinear optics, attosecond science and nanoscience.
More specifically, our work involves generation, measurement and control of the interaction of light and matter in atoms,
molecules and nanosystems in space and time at extremely short (attosecond=10(-18)sec) time scales.
Our interests range from fundamental physical phenomena associated with these processes to practical applications.

There are currently positions available for excellent and highly motivated students at MSc/PhD levels.

We also have exciting projects for the undergraduate students.

For further details, please contact [Eugene Frumker:|efrumker@bgu.ac.il]

Figure 1: Attosecond burst interference lies at the heart of
high harmonic generation from oriented molecules.
Further reading: Visit our website

Quantum Interferences and Lasing without inversion ()

Reuben Shuker

Quantum-interference-related phenomena have great interest in many aspects of physics. Quantum interference between two independent quantum channels in three-level systems gives rise to various coherent phenomena, such as electromagnetically induced transparency (EIT), coherent population trapping (CPT), lasing/gain without inversion (LWI/GWI), enhancement of refraction index, sub- and super-luminal light propagation etc. These quantum-interference-related phenomena open a wide-range perspective for new type of phase-sensitive spectroscopy. In particular, the movie in Fig.1 shows the absorption/dispersion properties of the atom controlled by various parameters. Another example is the possibility to get sub-natural line widths (see movie in Fig.2).

Figure 1: Phase-controlled absorption (red) and dispersion (blue).Figure 2: Sub-natural-width peak.

Development of Novel Lasers ()

Salman (Zamik) Rosenwaks

In our group, we focus on experimental and theoretical studies of diode pumped alkali lasers (DPALs)

Figure 1: Low Lying States of Alkali Atoms and Excitation and Lasing Processes in DPALs

Further reading: Visit my website

Constraints on quantum mechanics()

Daniel Rohrlich

The axioms of quantum mechanics are well known, but most of them are abstract and mathematical. They are not clear physical statements like the two axioms of special relativity, defining a maximal signalling speed (the speed of light) and a fundamental symmetry (Lorentz invariance). Can we derive quantum mechanics from clear physical statements? Or, if we are not there yet, can we at least formulate clear physical statements that constrain quantum mechanics and any generalization of quantum mechanics?

Suprisingly - to say the least - the assumption that every physical value exists before we measure it (regardless of what goes on elsewhere) does not constrain quantum mechanics. This "local realism" assumption implies Bell's inequality, and quantum mechanics violates Bell's inequality. In particular, one form of Bell's inequality says that a certain combination \( C \) of measured correlations must not exceed \( 2 \) . But in quantum mechanics, \( C \) can reach \( 2\sqrt{2} \) . This fact is called "Tsirelson's bound", and it is a theorem of quantum mechanics.

Quantum mechanics does obey the no-signalling constraint (the first axiom of special relativity): quantum correlations are useless for sending faster-than-light signals. Does Tsirelson's bound follow from this constraint? No, it does not; as Sandu Popescu and I showed in Ref. \( [1] \) , hypothetical "superquantum" correlations (now also called Popescu-Rohrlich-box ("PR-box") correlations as in Ref. \( [2] \) ) can reach \( C = 4 \) without violating the no-signalling constraint. So where does Tsirelson's bound come from?

Quantum mechanics obeys another constraint, besides no-signalling: it has a classical limit, in which Planck's constant \( h \) vanishes and all physical values are measurable. In this limit, I have found 3 that PR-box correlations do not obey the no-signalling constraint. Generalized to all stronger-than-quantum correlations, this result is a derivation of Tsirelson's bound without assuming quantum mechanics. So we can derive at least a part of quantum mechanics from the two axioms of no-signalling and a classical limit, together with the negation of local realism.

For further details, see Refs. \( [3{-}5] \) . The work of \( [4] \) continues in collaboration with Avishy Carmi and Daniel Moskovich of the Mechanical Engineering Department and the Center for Quantum Information Science and Technology at BGU.

  1. S. Popescu and D. Rohrlich, Quantum nonlocality as an axiom, Found. Phys. 24 (1994) 379.
  2. J. Barrett and S. Pironio, Popescu-Rohrlich Correlations as a Unit of Nonlocality, Phys. Rev. Lett. 95, 140401 (2005).
  3. D. Rohrlich, PR-box correlations have no classical limit, in Quantum Theory: A Two-Time Success Story (Yakir Aharonov Festschrift), eds. D. C. Struppa and J. M. Tollaksen (New York: Springer), 2013, pp. 205-211.
  4. D. Rohrlich, Stronger-than-quantum bipartite correlations violate relativistic causality in the classical limit.
  5. Video of my talk at the Aharonov-80 Conference in 2012 at Chapman University.

Magnetism and ferroelectricity in rare earth oxides ()

Amnon Aharony
Ora Entin

Insulating rare-earth oxides often exhibit rich magnetic phase diagrams, with interesting competing magnetic structures. Often, these phase diagrams are due to structures which are close to cubic or tetragonal. In these highly symmetric structures, the magnetic ground states are highly degenerate, implying magnetic frustration. We have studied several mechanisms which lift the frustration and yield a variety of magnetic ordered states. One mechanism involves quantum fluctuations. The zero point motion associated with these fluctuations creates "order out of disorder", and chooses some ground states over others. The second mechanism involves small magnetic anisotropies, usually due to spin-orbit interactions. Again, these anisotropies lift the degeneracy, and pick specific structures. Examples include several cuprates (the parents of high temperature superconductors) and nickel vanadate, whose structure is close to a planar Kagome lattice. The figure shows the temperature-magnetic field phase diagram of the latter material, which contains various types of (commensurate and incommensurate) magnetic order. Interestingly, this material shows multiferroic behavior: the low temperature incommensurate magnetic phase ("LTI") also exhibits ferroelectric order, which can thus be switched on and off by both a magnetic and an electric field. This has potential applications for spintronics.

Phase measurements in quantum mechanics: The Aharonov-Bohm interferometer and quantum noise ()

Amnon Aharony
Ora Entin

When a wave splits through two slits, one obtains an interference pattern which depends on the phase difference between the two paths. Recently, experiments studied mesoscopic solid state interferometers. A magnetic flux \Phi enclosed between the paths adds a Aharonov-Bohm (AB) phase difference, yielding a current with a term containing \( \cos\left(2\pi e\Phi/hc+\beta\right) \) . The phase shift \( \beta \) was interpreted as the intrinsic phase \( \alpha \) of the quantum transmission via a quantum dot (QD) located on one of the paths. This intrinsic phase is important for studies of the quantum properties of the QD, relevant to many nanoscopic devices. We have shown that in fact, the measured \beta is usually not equal to \( \alpha \) ; except for very specific conditions (not yet discussed in the textbook chapters on the AB interferometer), the measured phase shift is due to losses of current out of the paths. We have then found theoretical conditions for when \( \beta=\alpha \) . We have also given detailed predictions for the conductance of a closed interferometer (with no losses), and shown that the AB oscillations are strongly affected by the finite width of the interferometer's arms (see Fig. 2). Given the complications of the interpretations of such measurements, we have recently proposed an alternative: the quantum noise of the current through a quantum dot was shown to also contain information on the transmission phase.

Figure 1: electrodes on top of a two-dimensional electron gas, determining the potential seen by the electrons. The quantum dot, which can capture or scatter electrons, is in the center.Figure 2: Right: electrodes in the AB interferometer. Left: schematic picture of the interferometer, with several bound states on the quantum dot, which sits on one arm, and with the magnetic field B in the center.

Non-equilibrium steady state of stochastic circuits ()

Doron Cohen

The study of systems with non-equilibrium steady state (NESS) has become of great interest in recent years. The paradigm for NESS is a system which is coupled to two equilibrated reservoirs "A" and "B" that are characterized by different temperatures \( T_A \) and \( T_B \) . The steady state of the system is not canonical. A particular case of special interest is having one reservoirs (call it "A") that is replaced by a stationary driving source, while the relaxation is provided by a bath (call it "B") that has some finite temperature \( T_B \) . This is still the same paradigm because formally the driving source "A" can be regarded as a bath that has an infinite temperature \( T_A=\infty \) .

A prototype model system is illustrated in the above figure: a ring that is made up of N isolated sites with on site energies \( E_n \) . The ring is coupled to a heat reservoir (represented by the blue "environment") and subjected to a noisy driving field (represented by the red circle) that induces a current in the ring. In the figure below the current is plotted as a function of the scaled driving intensity and imaged for various strengths ("sigma") of disorder. The right panel demonstrates the statistics of the current over many realizations of the disorder.

More recently we have we have addressed questions that concern the relaxation of such currents, and how the relaxation depends on percolation and localization properties of the model. Currently we are trying to extend the theory, considering general stochastic active networks.

The ring model belongs to a class of "glassy" systems for which the rate of energy absorption is a semi-linear (rather than linear) functional with respect to the power spectrum of the driving source. This is due to the percolation-like nature of the dynamics. We have demonstrated that a weak coupling to a bath would lead in such case to a novel non-canonical steady state that has glassy characteristics. Furthermore, we have discovered a related quantum saturation effect that sets an upper limit on the NESS temperature, irrespective of the driving intensity.

For references see: http://www.bgu.ac.il/~dcohen/#refsNonEq
For more highlights see: http://physics.bgu.ac.il/~dcohen/HOMEPAGE/Highlights.html

Dynamics of condensed particles in a few site system ()

Doron Cohen

The physics of \( N \) bosons in an \( M \) site system is described by the Bose-Hubbard Hamiltonian. The \( M=2 \) problem is integrable and equivalent to \( j=N/2 \) spin problem, while the \( M=3 \) problem has both non-trivial topology and mixed phase space. Our studies concerns the analysis of fluctuations, occupation statistics and quantum stirring in such systems.

The time dependent dynamics in a Bosonic Josephson junction with no driving can be regarded as a quantum version of a pendulum. Adding driving to this system, we have studied the adiabatic-diabatic-sudden crossover of the many-body Landau-Zener dynamics; the Kapitza pendulum dynamics; and the Quantum Zeno dynamics due to erratic driving. In particular we have studied the stabilization and the suppression of collision-induced dephasing by periodic, erratic, or noisy driving.

The \( M=4 \) problem is the minimal configuration for the study of quantum thermalization and many-body localization. Specifically we consider the equilibration of the occupation in a weakly coupled subsystems.

Our recent studies are focused in Bose-Hubbard superfluid circuits. The expected results are novel due to the quantum chaos perspective. In particular we predict drastic differences between 3 site rings and rings that have more than 3 sites. In the former instability of flow states is due to swap of separatrices, while in the latter it has to do with a web of non-linear resonances. We also argue that it is not likely to observe coherent operation for rings that have a weak link and more than 5 sites.

Fig1: From left to right: a. The Wigner function of a prepared Fock state. The red dashed line is the separatrix. b. The same (viewed from the right) after finite time evolution. c. The time evolution of the Bloch vector for various coherent preparations. d. The quantum Zeno effect for Pi preparation due to the presence of noise.

Circuits with condensed bosons can support superflow. Such circuits, if realized, will be used as QUBITs (for quantum computation) or as SQUIDs (for sensing of acceleration or gravitation). We are studying the feasibility and the design considerations for such devices. The key is to develop a theory for the superfluidity in an atomtronic circuit. Such theory goes beyond the traditional framework of Landau and followers, since is involves ''Quantum chaos'' considerations.

Fig2: Regime diagram for the stability of flow-states in a superfluid circuit. The axes are: the rotation frequency of the device; and the interaction . On the right the stable regions are indicated by blue color. Red indicates instability.

Fig3: Phase space tomography for a kicked top: the participation number for all coherent preparations is imaged for an integrable phase-space (left), for a mixed phase-space (middle) and for a chaotic phase-space (right). In the latter case one observes the effect of scarring.

For references see: http://www.bgu.ac.il/~dcohen/#refsBHH
For more highlights see: http://physics.bgu.ac.il/~dcohen/HOMEPAGE/Highlights.html

Topological Superconductors and Majorana Fermions ()

Eytan Grosfeld

Majorana fermions have been ardently sought after in particle physics since their theoretical prediction in the 1930s. Conclusive experimental evidence for their presence is still lacking. Much of the focus has shifted in recent years to condensed matter physics, where Majorana states are expected to form on the edges of topological superconductors and carried by magnetic vortices through the superconductors. The presence of a Majorana state on the vortex core has far reaching consequences for the quantum statistics of the vortex: it increases the degeneracy of the quantum state in such a way that when two vortices are interchanged, the quantum state gets rotated in a non-trivial way. Detection of Majorana states and their induced vortex statistics requires the design of clever methods of probing the system, but the effort is worthwhile: their discovery can open a pathway to novel quantum computation schemes, largely immune to decoherence.

Figure 1: A vortex interferometer, sensitive to the quantum statistics of solitons trapping Majorana modes
Figure 2: A vortex line in a quasi-2D geometry supporting Majorana modes on the end points of the vortex line
Figure 3: A topological phase transition for a vortex line - Majorana end states appear in a finite parameter regime

For details see homepage: Baruch Horovitz

Research Interests - Condensed Matter Theory ()

* Nanoscopics and noise: Dissipative environmnets, the ESR-STM phenomena, ESR noise.()

* Luttinger liquids: Coupling to condensates, mobile impurity, Coulomb box; phase transitions, dynamics.()

* Surface physics: Noise from metal surfaces and vortices, trapping of cold atoms, plasmons.()

* Superconductivity: Vortex matter, p and d wave effects, disorder.()

* Quantum Hall effects: Disordered superconductors, localization phenomena.()

* Conducting Polymers: Charged defects, spectroscopy, photocurrent.()

Interacting topological phases ()

Dganit Meidan

Topological phases of matter are characterized by a gapped bulk, yet a perfectly conducting surface. The flow on the surface is topologically protected such that it cannot decay even in the presence of defects and imperfections. These insulating phases with perfectly conducting edge states hold promise for a multitude of applications, most prominent of which is the development of new quantum computational schemes that are inherently protected against errors.
When the system is strongly interacting, the emergent surface states may have additional striking properties, such as carry a fraction of the electron charge and obey an exotic exchange statistics. Despite the growing interest in the interplay between interactions and topology, the study of interacting topological phases remains a formidable task. In particular, finding experimental realizations of interacting topological phases, and their signatures in transport measurements, constitutes a vital step towards motivating experimental work.

Fig1:In the thin torus limit, a two dimensional system can be mapped onto a one dimensional pump.
Fig2: Three different classes of pumps for systems with a two-fold degenerate ground state and a time-reversal restriction on the pumping cycle: the trivial class, the integer pump, and the fractional pump which pumps a fractional spin without pumping charge.
Fig3:Interacting Majorana fermions at the end of multiple Kitaev chains form emergent many-body end states identified to be a topological Kondo-like resonance.

Solving the "0.7 Anomaly" ()

Yigal Meir

The simplest nanoelectronics device, and the basic building block for more complicated devices, is a "quantum point contact," a constriction connecting large electron reservoirs (see picture on the right). According to quantum mechanics the conductance through such a device should increase as the gap grew bigger by integer steps of universal value. Surprisingly, an additional first step approximately 0.7 times the expected universal value had also been observed, which became known as "the 0.7 anomaly" thomas5.gif(see picture on the left). Previously we demonstrated that the phenomenology of this anomaly can be explained by the existence of a magnetic impurity, a localized electron in the quantum-point contact PRL 89, 196802 (2002) . Using extensive density-functional calculations we demonstrated impurity.png the emergence of a magnetic impurity at the quantum point contact (see picture on the right) because a lower density of the electrons near the quantum point attracts the other electrons towards it. The wavy nature of such electrons then causes ripples, trapping an electron and causing the 0.7 anomaly.


Intracellular networks in bacteria ()

Yigal Meir

Bacteria are constantly sensing their environments and adjusting their behavior accordingly. Signaling occurs through networks of proteins and nucleic acids, culminating in changes of gene expression and so changes in the proteins of the cell. We are focused on the architecture of these intracellular networks. What is the relation between network architecture and function? For example, can we understand the selection of architectures in terms of general information-processing concepts such as signal to noise, memory, and adaptation? Even in a single bacterium such as E. coli, there are hundreds of coexisting networks. Our belief is that a deep study of a small number of "model" networks will yield general tools to analyze information processing by cell. It is important to choose these model networks carefully. The network components should be well characterized and the physiological function of the network should be known and subject to quantitative measurement. Probes of the internal dynamics of the network such as fluorescence resonance energy transfer (FRET) or direct imaging of dynamic spatial structure, will be critical in developing and testing quantitative models. It will also be important to choose networks which complement each other well, spanning a broad range of architectures and functions. A preliminary list includes (i) chemotaxis, which requires adaptation and rapid response to changing chemical concentrations, (ii) cell-division networks, where accuracy and checkpoints are essential, and (iii) metabolic networks which tie together diverse inputs to maintain homeostasis.


Low temperature universality in disordered solids ()

Moshe Schechter

A large variety of otherwise very different amorphous and disordered solids show, at temperatures lower than 3 Kelvin, remarkable universality in their properties.
Most astounding, the ratio \( \lambda/l \) of the phonon wavelength divided by its mean free path is roughly 1/150, this value being constant in wavelength, temperature, and very similar between otherwise very different materials.
For 4 decades this phenomenon is discussed within the framework of the phenomenological model of tunneling two level systems (TLSs). However, crucial questions such as the nature of the tunneling states, the universality and smallness of the ratio \( \lambda/l \) , and the energy scale dictating the temperature of \( 3K \) below which the phenomenon is observed, remain unanswered.

We have recently suggested a novel model to explain the above questions, where tunneling states are classified to symmetric and asymmetric with respect to local inversion (examples are \( 180^o \) flips and \( 90^o \) rotations of the CN impurity in the picture at the bottom left). The “symmetric” TLSs couple weakly to the phonons, yet gap the asymmetric TLSs at low energies – the DOS of the latter is shown at the bottom right. We are now interested in using the theory to calculate further relevant physical quantities, and in the rigorous generalization of the theory to amorphous solids. For more details see arXiv:0910.1283.

Quantum disordered magnets ()

Moshe Schechter

The interest in magnetism, and specifically quantum magnetism, is twofold. Firstly, magnetic materials are of immense significance for the advance of technology. With reduced size of current and future devices, quantum effects become relevant, and their understanding is crucial to further advance. At the same time, magnetic systems are an ideal tool in the study of various physical phenomena, as they allow the realization of theoretical models with negligible extraneous interactions, and with the ability to tune the relevant parameters.

Perhaps the most studied model for interacting systems is the Ising model. With the addition of a transverse field term and a random field term the model is described by the Hamiltonian
\( H_{Is}=-\sum_{ij}J_{ij}S_i^zS_j^z-\Gamma\sum_iS_i^x-\sum_ih_iS_i^z \)

and allows the study of the interplay of interactions with quantum fluctuations and disorder. This interplay is of much recent interest, as it is essential in phenomena such as the superconducting-insulator transition, the quantum Hall effect, and high superconductivity. Recently we have shown that this model is realized in anisotropic dipolar magnets, allowing its experimental study in new regimes. This both led to an understanding of existing experiments, and motivated new experiments which were recently done and raised new questions regarding the disordering of spin glasses and ferromagnets by random fields. Current questions of interest include the transition between the ferromagnetic and glassy phase as function of disorder, random fields in nano magnetic grains, and entanglement of different degrees of freedom near the quantum phase transition.

Coherent vortex motion in superconductors ()

Jung Grzegorz
The experimental superconductivity and magnetism group

Superconducting quantum phase coherence gives rise to a variety of macroscopic quantum phenomena, among them to the Josephson effects. Due to extremely short coherence length, voltages appearing across high-TC superconducting microbridges are not related to Josephson effects but to the motion of current-created vortices. Josephson-like effects may however appear in relatively large high-TC bridges due to the coherence in current driven vortex flow. To enforce the coherent regime we employ laser written channels of easy vortex motion in c-axis oriented YBCO thin films, and heavy ion irradiated channels in BSCCO single crystals. The project is supported by the Israeli Science Foundation grant.

Figure 1: Schematic of the layout of bridge with laser written channel for easy vortex motion. Figure 2: Magneto-optical image of field penetration into a laser written channel in the bridge constriction. Bright areas mark high magnetic field.

  • Other active research projects, supported by four research grants, are opened for Ph.D. and M.S. students and include:
      • Dynamics of vortices in spatially restricted superconductors (Grzegorz Jung)
      • Anisotropy of flux-flow noise in vicinal YBCO thin films (Grzegorz Jung)
      • Intrinsic tunneling in colossal magnetoresistive materials (CMR) (Grzegorz Jung, Vladimir Markovich)
      • Metastable resistivity in CMR manganites (Grzegorz Jung, Gad Gorodetsky, Vladimir Markovich)
      • Local magnetic properties of CMR manganites. (Grzegorz Jung, Gad Gorodetsky, Vladimir Markovich).
      • Magnetic and transport noise in CMR manganites (Grzegorz Jung, Vladimir Markovich)
      • Current noise in quantum wells. (Grzegorz Jung)
      • Noise and plasmons enhanced charge separation in nanoparticles based devices. (Grzegorz Jung)
      • Ferromagnetic resonance and phase separation in bulk and nano-scale CMR materials (Evgeny Rosenberg, Alexander Shames, Gad Gorodetsky, Grzegorz Jung)
      • Transport, resonance and optical properties of carbon nanotubes filled with charge or magnetically ordered materials. (Evgeny Rosenberg, Grzegorz Jung)
Further details will be available soon at Prof. Jung's web site.

Noise spectroscopy of a single spin ()

Yishay Manassen

We have found that tunneling with a STM (scanning tunneling microscope) to a surface in the neighborhood of a single paramagnetic atom or molecule (individual magnetic dipole) in the presence of an external magnetic field results in an elevated noise at the Larmor frequency. This high frequency noise results is due to the precession of a single magnetic dipole around the field (see Figure1).

The precession frequency depends strongly on the local environment of spin center, and can be used to identify molecules under the tip on the single molecule level. Fundamental information on the single spin physics is studied in this way. Additional potential applications are in single molecule data storage and quantum information.

Steps to improve the system which are implemented in these days, such as cooling to low temperature, working in time domain and scanning the field rather than the frequency are expected to improve the sensitivity of the detection system, to enable getting more information on the single spin physics.

Figure 1 Figures 2,3: The image on the right is an image of a gold sample covered with DPPH molecule. The spectrum on the left is from the lowest molecule in the image (#31). We collaborate on this subject with a group from Italy and also with other groups around the world.

  • Further reading:
      • J. Appl. Phys. 101, 053916 (2007)

Subsurface STM imaging of electrically floating islands ()

Yishay Manassen

We are studying hydrogen absorption and corrosion on a metal surface. The system studied was Gd on W(110). This system was exposed to several Langmuirs of hydrogen. In the image below we see a series of consecutive images taken above the same place. The time between two images is about ½ hour. The phenomenon which is clearly distinguishable is the periodic appearance and disappearance of several islands. Since the islands appear and disappear exactly in the same shape, it clear that the island is not vanishing but somehow becomes transparent to the STM imaging process. As seen below, in some cases, the STM tip is imaging the surface under the island.

The process that is causing this phenomenon is the fact that hydrogen creates an insulating barrier between the Gd island and the W substrate. Thus the island is disconnected electrically from the substrate. As a result, the potential of the island is floating and when it is equal to the potential of the tip, the island is becoming part of the tip and the surface under the island is imaged.

The periodicity of the process is due to a competition between the stress induced by the hydride nuclei seen in the island as white protrusions and the stress in the insulating layer between the tungsten substrate and the gadolinium island.

  • Further reading:
      • Surf. Sci. 600 2795 (2006).

Time-Resolved Cathodoluminescence Studies of Semiconductor Quantum Dots Grown On Silicon Substrates ()

Dan Rich

We study the optical properties of vertically stacked GaN/AlN self-assembled quantum dots (SAQDs) grown on Si substrates. We use temporally and spatially resolved cathodoluminescence (CL). CL is generated by the injection of a tunable and position-controllable high-energy electron beam from a scanning electron microscope (SEM) into a semiconductor sample. The electron beam generates a large density of electron-hole pairs, many of which recombine radiatively across the bandgap of the semiconductor, thereby generating the luminescence which is subsequently detected and analyzed. Potential applications of these quantum dots include light source for UV-blue-green LEDs, lasers, optical information storage, single photon emitters and sources of entangled photons (quantum information technology).

While the presence of thermal stress-induced micro-cracks is clearly undesirable when attempting to obtain high-quality thin film growth of III-Nitride films and SAQDs on Si substrates, the presence of micro-cracking however enables a study of interesting stress-induced energy shifts and lifetime changes in the QD optical transitions that can be measured with a high-resolution spatially resolved probe, such as with CL. Normally, it is difficult to locally perturb the strain tensor of QDs in a reliable and reproducible way with an external stress. We have found such a way using the micro-cracks as stressors in our research. Three-dimensional 6*6 \vec k \cdot \vec p calculations of the QD electron and hole wave functions and eigenstates were performed to examine the influence of biaxial and uniaxial stresses on the optical properties in varying proximity to the micro-cracks.

Figure 1: Growth process of Self Assembled Quantum Dots (SAQD) in the Stranski-Krastanov growth mode. Click to enlarge.Figure 2: Schematic showing the structure of SAQD sample. Click to enlarge.
Figure 3: Schematic explaining the CL experimental method used in studying the effects of the micro-cracks.

Further Details and Recent Experimental Results.

Pressure Induced Structural and Electrical Phase Transitions in disordered materials: Silicon monoxide and the Anderson transition ()

Reuben Shuker

Application of very high pressure significantly alters the nature of intermolecular interaction, chemical bonding, molecular configuration, and crystal structure of solids. Practically we use Diamond Anvil Cell (Fig. 1) to produce pressures in the range from 1 and 40 GPa. Electronic correlations effects induced by high pressure results unusual phase transitions. Our main interest is focused on amorphous systems such as silicon

monoxide, where we observed an insulator to metal transition at pressure of 12 GPa (all in the amorphous phase). The temperature dependence of the conductivity is in very good agreement with the Anderson model for amorphous materials indicating Anderson transition in disordered systems. The material response to high pressure is studied by a variety of experimental methods: x-ray diffraction using synchrotron radiation, IR spectroscopy, Raman scattering and electrical conductivity measurements.

For example, the Raman scattering spectra and the pressure dependence of the resistivity for SiO sample are presented in figures 2 and 3 respectively. IR and Raman results assign bond-bending to Raman active modes since it is hardened under pressure, whereas IR active modes are dominated by bond-stretching modes.

The miniature sample (~100 \( \mu m \) in diameter) we use in high pressure studies makes the electrical resistivity measurements a challenging experimental problem. However, we managed to observe a dramatic drop of the resistivity on one hand (Fig. 3), and significant change in the temperature dependence of the resistance on the other, both happen at pressure 12 GPa. Short-rang order on the molecular scale helps us explain the insulator to metal transition in terms of Anderson's theory of localization, where lone-pair electrons create states in the gap.

Figure 1: Diamond Anvil CellFigure 2: The Raman scattering spectra of SiO sample at different pressures. Click to enlarge.Figure 3: The pressure dependence of the resistivity for SiO sample. Click to enlarge.

Single Cell Dynamics ()

Mario Feingold

We use single cell phase-contrast and fluorescence time-lapse microscopy to monitor the morphological changes during the division of E. coli. To bypass the limitations of optical resolution, we process the images using pixel intensity values for edge detection. We study the dynamics of the constriction width, W, and find that its formation starts shortly after birth much earlier than can be detected by simply viewing phase-contrast images. A simple geometrical model is shown to reproduce the behavior of W(t). Moreover, the time-dependence of the cell length, L(t), consists of three linear regimes. The growth rates in the different regimes are related to each other and to the parameters of our model.

Phase-contrast (right) and fluorescence (left- stained membrane) images of bacteria (E.coli).

This is life: 1.5 min (a), 8.6 min (b) and 19.4 min © from birth. In © the bacterium has just divided.

During the lifetime of a bacterium it elongates linearly in three regimes. The transition between these regimes is rather sharp. Click to enlarge.

Single Molecule Studies of DNA-protein interactions ()

Mario Feingold

We use Optical Tweezers to manipulated single DNA molecules. This method can be used to probe various processes in which the DNA plays a role. In particular, we use this approach to study the interaction between the DNA and sequence specific proteins. Such protein will first search for the appropriate sequence on the DNA and once it has found it will initiate a binding process. These processes will influence the DNA that, in turn, will affect the position of the bead in the optical trap. The displacements that need to be observed in order to monitor such process are in the nanometer range and are the limit of the resolution of such a setup. On the other hand, one could amplify the effect by using a DNA that has several copies of the binding sequence.

Figure: Optical Tweezers trap microbeads that can be attached to single DNA molecules. It is like having a handle at the end of the DNA.

Internal dynamics of biological polymers: DNA molecules, actin filaments ()

Oleg Krichevsky

The problem of polymer dynamics is rather old, going back to the 1930-s.

How the stochastic thermal motion (diffusion) reveals itself in the dynamics of polymer segments which are bound by connectivity along the chain, by polymer stiffness, by topological constrains, by hydrodynamic and other interactions? The question does not have simple solutions either in theory, nor in computer simulations, and neither in experiments. We have developed an original experimental approach to measure the dynamics of biological polymers, such as DNA at the level of single monomer with high temporal and spatial resolution.

Furthermore, one does not have to rely on classical thermal fluctuations to drive the dynamics of the system. We introduce now molecular motors "little nanomachines" which actively push polymers around at the nanoscale.

In collaboration with Dr. Anne Bernheim-Groswasser, Chem. Eng.

Animation 1: The dynamics of a semi-flexible polymer.

Bacterial nucleoid structure and dynamics and its interaction with the membrane ()

Oleg Krichevsky

Bacterial DNA (nucleoid) is highly compacted. Once the nucleoid is gently removed from bacteria, it expands almost 100-fold and occupies the volume many times that of bacteria. What forces keep the nucleoid in its condensed form inside the bacteria? What are the main features of nucleoid structure and dynamics? How nucleoid interacts with the bacterial membrane?

In collaboration with Prof. Itzhak Fishov, Life Sciences.

Figure 1: E. Coli bacteria: the membrane is stained with fluorescent dyeFigure 2: The extracted bacterial nucleoid expands to a volume ~ 30 times that of bacteria.


Optical diagnosis using advanced spectroscopic and computational techniques ()

Shaul Mordechai

The rapid developments in the field of infrared spectroscopy in the past decade have shown a potential for disease diagnosis using non-invasive technologies. Several earlier studies have highlighted the advantage of IR spectroscopy both in the near and mid IR regions for diagnostic purposes at clinical levels . The areas of focus have been the distinction of premalignant and malignant cells 1 and tissues from the normal using specific parameters obtained from FTIR spectra, making it a rapid and reagent free method. While it still requires pilot studies and designed clinical trials 2 to ensure the applicability of such systems for cancer diagnosis, substantial progress has been made in incorporating advances in computational methods into the system to increase the sensitivity of the entire set up making it an objective and sensitive technique suitable for automation to suit the demands of the medical community. The development of fiber-optics systems 3,4 for IR spectroscopy (Fig. 1 and Fig. 2) have further opened up new and modern avenues in medical diagnosis in-vivo at various levels of cells, tissues and organs under laboratory and clinical conditions. Autofluorescence from intracellular chromophores upon illumination of cells by a monochromatic light (Fig. 4) has also been found as a powerful complementary and sensitive technology for the early detection of cancer 5.

Figure 1Figure 2
Figure 3 Figure 4