Research Highlights

Navigate by Group

Astrophysics & Cosmology


eichler1

Eichler David: Manifestations of Exotic and New Physics in Nature

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.

BACK TO TOP


eichler2

Eichler David: Collisionless Shock Waves and the Origin of Cosmic Rays

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.

eichler1.jpg
Figure 1: The Crab Nebula in X-ray.
Source
eichler2.jpg
Figure 2: Synchrotron emission simulation of the Crab Nebula.
Source
BACK TO TOP


eichler3

Eichler David: Cellular Automata

Cellular automata can be used to study many, perhaps all, branches of physics - including quantum field theory, morphogenesis of life forms, genetic evolution, and even traffic flow. Easily programmable, these discrete models raise many deep mathematical questions, and may provide a key to answering certain questions that are difficult to address in other approaches (e.g. how can elementary particles be stable? What is the optimal traffic routing system for any given city? etc.)

BACK TO TOP


gedalin1

Gedalin Michael: Space Weather

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).
 solarexp.jpg
Figure 1: The Solar Wind. Click to enlarge.
magnetosphere.gif
Figure 2: The Magnetosphere. Click to enlarge.
Movie 1: Magnetic Reconnection (mov, avi)
Movie 2: Magnetic Substorms (mov, avi)
BACK TO TOP


gedalin2

Gedalin Michael: Collisionless shocks

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).
ssc2004-15a_small.jpg
Figure 1: Supernovae Remnants. Click to enlarge.
grb_animation.gif
Figure 2: Gamma Ray Bursts. Click to enlarge.
grb_bepposax.png
Figure 3: Gamma Ray Burst Afterglow. Click to enlarge.
BACK TO TOP


gedalin3

Gedalin Michael : Relativistic Plasma

Most energetic astrophysical objects and most powerful radiation sources produce relativistic plasma flows which are further responsible for the emission of electromagnetic waves in a wide spectrum - from radio to hard gamma rays. Relativistic plasma powered engines are in the core of the most spectacular discoveries, like pulsars (relativistic plasma outflow empowers the famous Crab nebula (Fig. 1)) and gamma ray bursts (relativistically expanding plasma produces shocks (Fig. 2)which in turn energize electrons which in turn emit from gamma down to radio). Relativistic plasma are expected to be present in the vicinity of black holes. We study the global features of relativistic flows (1, 2), as well as wave propagation in such plasmas (3, 4) and implications for non-thermal radiation processes (5).
crab_xrayradioopt.gif
Figure 1: The Crab Nebula.
Source
meszaros.jpg
Figure 2: Gamma Ray Bursts. Click to enlarge.
BACK TO TOP

High Energy Physics


ramyb1

Brustein Ramy: Determining the nature of dark energy

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.

Further reading:

BACK TO TOP


ramyb2

Brustein Ramy: Particle physics & Cosmology in flux compactifications

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.

Further reading:

BACK TO TOP


ramyb3

Brustein Ramy: Quantum Entanglement, thermodynamics & area

Interpreting the thermodynamic properties of black holes and other space-times with horizons and uncovering their underlying statistical mechanics remains a challenge in spite of the intense efforts and the progress that has been achieved over the last 30 years. What does the black hole entropy measure, the degeneracy of microstates, entanglement entropy between the inside and outside of the horizon, or some intrinsic gravitational entropy? Is the quantum mechanics of space-times with causal boundaries unitary? If so, why do some of them look thermal and non-unitary in some approximation?

Aiming to understand the physics of space-times with causal boundaries, we pursue the entanglement point of view: that their statistical properties arise because classical observers access only a part of the whole quantum state.

Further reading:

BACK TO TOP


guendel1

Guendelman Eduardo : Vacuum Energy and Child Universes

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.

child_universe1

Further reading:

BACK TO TOP

Non-Linear Dynamics


ehud1

Meron Ehud: Mechanisms of Species Diversity Change in Stressed Environments

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.
ehud1.png
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.
clearcut_b1.jpg
Animation 1 (AVI format): 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.

Further reading:

See also Recent Publications link in http://www.bgu.ac.il/~ehud
BACK TO TOP


ehud2

Meron Ehud: Non-equilibrium Localized Structures

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.

ehud2.pngcReB.gif animation
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.
BACK TO TOP


zarmi1

Zarmi Yair: On asymptotic integrability of perturbed evolution equations

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.

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

Further reading:

  • What are solitons + animations (external link)

  • Lecture notes on the Burgers equations

  • Lecture notes on the Fermi-Pasta-Ulam problem

  • 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).
BACK TO TOP


zarmi2

Zarmi Yair: Stochastic analysis of algal-cell motion in bioreactors - effect on bio-mass productivity

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.
BACK TO TOP

The Lasers and Atomic Spectroscopy Groups


ibar1

Bar Ilana: Probing dynamics and structure of molecules using laser spectroscopy

One area of active research is the study of dynamics, structure and bond cleavage in gas phase molecules utilizing pulsed lasers. These characteristics are probed via spectral signatures of the species with various spectroscopies, employing tunable lasers spanning the infrared, visible and ultraviolet regions of the spectrum. In particular, we seek for the possibility of controlling bond fission via energy deposition in different skeletal motions of molecules. The major goals are to find ways for selecting desired reaction pathways, for altering state distributions, for developing an understanding regarding vibrational energy flow within molecules, and for finding non-statistical behaviour so that it can be predicted and may be even controlled by lasers.

Another research project involves the development of a new spectroscopic method for the study of floppy biomolecules. This is due to the crucial role that conformations and shapes of molecular constituents involved in biological processes play in determining their selectivity and functionality. The exploration of these properties is thus a prerequisite for gaining molecular-level understanding of the complex dynamics of these processes. Structures and interactions of biomolecules are of fundamental importance in elucidating the mechanisms of biological processes and might have implications on designing drugs that mimic, activate or suppress the function of bioactive systems.

expermintal_system3.png
Figure 1: The experimental setup. Click to enlarge.
  
photodis2.jpg
Figure 2: Mode-dependent photodissociation and photoionization of methylamine. Click to enlarge.

Further reading:

  • A. Golan, S. Rosenwaks, and I. Bar, J. Chem. Phys. 125, 151103 (2006).

  • A. Portnov, L. Blockstein, and I. Bar, J. Chem. Phys. 124, 164301 (2006).

  • E. Bespechansky, A. Portnov, A. Zwielly, S. Rosenwaks and I. Bar, J. Chem. Phys. 125, 133301 (2006).
BACK TO TOP


ibar2

Bar Ilana: Label-free detection of nano-biochip arrays

we are studying biochip-arrays, which are concerned with the specific and simultaneous profiling of biological markers, while hosting recognition sites for the analysis of multiple parameters. We focus on the possibility of combining the nano fountain pen (NFP) method for printing nanoscale features with controlled surface densities with Raman and surface enhanced Raman spectroscopies for establishing a platform for imaging of protein-protein or antibody-protein interactions. It is recognized that common strategies used for creating biochips and for reading them have extensive limitations, motivating development of miniature biochips and label-free formats for identification of biochemical activities. The fusing of NFP with Raman spectroscopies will allow obtaining the fingerprints of the studied systems and assessing their response, which may lead to exquisite sensitivity and specificity in biodetection, improving medical diagnostics and biomedical research, and enabling monitoring of toxins and microorganisms in the environment (in collaboration with Dr. Gheber Levi A. - Department of Biotechnology Engineering).
raman.png
Figure 1: The Raman spectra of deposited macro-drops of avidin and pre-formed avidin-biotin complex and of small spots of biotin (~ 4 $\mu$m diameter) printed with NFP onto smooth gold surfaces.
BACK TO TOP


folman1

Folman Ron: The Atom Chip Group

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 decade, and in this period 3 Nobel prizes have been awarded (1997, 2001, 2005).

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 T) 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 insight 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. Bottom line: We talk to atoms, and you are invited to talk to them too!

folman1.png
Figure 1: An Atom Chip recently fabricated at BGU, with current carrying wires forming magnetic traps and guides for cold atoms.
 folman2.png
Figure 2: A micro disk high finesse optical resonator for the detection and trapping of "cold" atoms.

Further reading:

BACK TO TOP


shuker1

Shuker Reuben: Quantum Interferences and Lasing without inversion

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).
Phase_controlled_absorption_(red)_and_dispersion_(blue)
Figure 1: Phase-controlled absorption (red) and dispersion (blue).
 Sub_natural_width_peak.gif
Figure 2: Sub-natural-width peak.
BACK TO TOP

Condensed Matter Theory


aaharony1

Aharony Amnon and Entin Ora: Magnetism and ferroelectricity in rare earth oxides

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.

aaharony1.png
BACK TO TOP


aaharony2

Aharony Amnon and Entin Ora: Phase measurements in quantum mechanics: The Aharonov-Bohm interferometer and quantum noise

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.

aaharony2.png
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.
aaharony3.png
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.

Further reading:

Relevant publications are listed in
BACK TO TOP


dcohen1

Cohen Doron: Quantum stirring of particles in closed devices

We study the quantum analog of stirring of water inside a cup using a spoon. This can be regarded as a prototype example for quantum pumping in closed devices. The current in the device is induced by translating a scatterer. Its calculation is done using the Kubo formula approach. The transported charge is expressed as a line integral that encircles chains of Dirac monopoles. For simple systems the results turn out to be counter intuitive: e.g. as we move a small scatterer "forward" the current is induced "backwards". One should realize that the route towards quantum-classical correspondence has to do with "quantum chaos" considerations, and hence assumes greater complexity of the device. We also point out the relation to the familiar S matrix formalism which is used to analyse quantum pumping in open geometries.
 

pmt_fig.gif
       Figure: A scatterer (represented by a black circle) is translated through a systems that has a Fermi occupation of spinless non-interacting electrons. In (a) the system is a simple ring.  In (b) it is a chaotic ring (Sinai billiard). In (c) and in (d) we have network systems that are of the same type of (a) and (b) respectively. In (c)-(e) the scatterer is a delta function (represented by a big circle), while the current is measured at a section (represented by a dotted vertical line). In (e) we have an open geometry with left and right leads that are attached to reservoirs that have the same chemical potential.       
 
Additional highlights of the quantum chaos group can be found here
 
BACK TO TOP


dcohen2

Cohen Doron: Energy absorption by "sparse" systems: beyond linear response theory

In a series of publications (see review), one of them selected as the EPL editorial board highlight of 2006, and another as the Editor's choice of 2007-2008, we develop (in various collaborations involved) a new paradigm for energy absorption of driven chaotic systems, called "semi-linear response theory". The theory uses a resistor network picture of transitions, and gives different results compared with linear response theory (Kubo) whenever the perturbation matrix in the Hamiltonian is "textured" or "sparse" [notably if its elements have log-normal or log-box rather than Gaussian distribution]. The theory has been applied to obtain novel results for the mesoscopic conductance of rings with very strong or very weak disorder, and also in order to resolve long standing questions regarding the applicability of the variable range hopping picture and the absorption of low frequency radiation by small metallic grains.
 
           
 
The figure illustrates model systems of interest: a Billiard with a moving wall, and a Ring with a time dependent magnetic flux. The "sparsity" of the perturbation matrix (right panel) reflects the slight deviation from intelligibility due to a deformation of the boundary, or due to the presence of weak disorder.
 
Additional highlights of the quantum chaos group can be found here
 
BACK TO TOP


hbaruch1

Horovitz Baruch : Interference in presence of dissipative environments

The research studies quantum systems that are affected in a fundamental manner by dissipative environments. The experimentally relevant systems are of two types: (i) Solid state devices probed by electron transport in confined geometries such as rings, quantum dots and wires. These are controlled by gate electrodes that form inevitably a dissipative environment. (ii) Cold atoms and ions as well as condensates trapped by electric or magnetic fields created by surface currents on a chip. These chips necessarily generate noise that limits the visibility of quantum interference of the atoms or ions. E.g., Fig. 1 shows the absorption image of Rb atom cloud with its reflection from a superconducting chip [Nirrengarten et al. PRL 97, 200405 (2006)].

Our theoretical study considers a well known interference phenomena -- the Aharonov-Bohm oscillations of a particle on a ring due to a magnetic flux through the ring. The chip or gate produce noise, inducing dissipation on the particle and affecting the oscillation amplitude; the geometry is shown in Fig. 2. This connection between interference and dissipation is an essential aspect of all quantum information devices. Our system allows one to learn a variety of methods of theoretical physics, such as renormalization group and nonequilbrium Keldysh methods, and apply these to the front edge of research on mesoscopic devices and cold atoms.

hbaruch1.png
Figure 1
hbaruch2.png
Figure 2
BACK TO TOP


ymeir1

Meir Yigal: Solving the "0.7 Anomaly"

pointcontact.gifThe 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.

Further reading:

BACK TO TOP


ymeir2

Meir Yigal: Intracellular networks in bacteria

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.
cell1.JPG

Further reading:

BACK TO TOP


smoshe1
Low temperature universality in disordered solids

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 λ/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 λ/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° flips and 90° 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.


KBr4n.jpgStau

smoshe2 Quantum disordered magnets

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

http://physweb.bgu.ac.il/cgi-bin/mimetex.cgi? 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.

Experimental Condensed Matter


jung1

Grzegorz Jung: Coherent vortex motion in superconductors

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.

 jung1.png
Figure 1: Schematic of the layout of bridge with laser written channel for easy vortex motion.
jung2.png
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.
BACK TO TOP


manassen1

Manassen Yishay: Noise spectroscopy of a single spin

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.

 manassen1.png
Figure 1
manassen3.png manassen2.png
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)
BACK TO TOP


manassen2

Manassen Yishay: Subsurface STM imaging of electrically floating islands

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.

 manassen4.png

Further reading:

  • Surf. Sci. 600 2795 (2006).
BACK TO TOP


danrich1

Rich Dan: Time-Resolved Cathodoluminescence Studies of Semiconductor Quantum Dots Grown On Silicon Substrates

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.

fig1
Figure 1: Growth process of Self Assembled Quantum Dots (SAQD) in the Stranski-Krastanov growth mode. Click to enlarge.
fig2
Figure 2: Schematic showing the structure of SAQD sample. Click to enlarge.
fig3
Figure 3: Schematic explaining the CL experimental method used in studying the effects of the micro-cracks.

Further Details and Recent Experimental Results.

BACK TO TOP


shuker2

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

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.

shuker1.png
Figure 1: Diamond Anvil Cell

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.

shuker2_small.png
Figure 2: The Raman scattering spectra of SiO sample at different pressures. Click to enlarge.
shuker3.png
Figure 3: The pressure dependence of the resistivity for SiO sample. Click to enlarge.
BACK TO TOP

Biophysics


mario1

Feingold Mario: Single Cell Dynamics

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.png fluorescence
Phase-contrast (right) and fluorescence (left- stained membrane) images of bacteria (E.coli).
bacterium_life.png
This is life: 1.5 min (a), 8.6 min (b) and 19.4 min (c) from birth. In (c) the bacterium has just divided.
Tree_elongation.bmp
During the lifetime of a bacterium it elongates linearly in three regimes.
The transition between these regimes is rather sharp.
Click to enlarge.
BACK TO TOP


mario2

Feingold Mario: Single Molecule Studies of DNA-protein interactions

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.
optical_tweezers.png
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.
BACK TO TOP


okrichev1

Krichevsky Oleg: Internal dynamics of biological polymers: DNA molecules, actin filaments

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.

SemiFlexible_v1.gif
Animation 1: The dynamics of a semi-flexible polymer.

Further reading:

BACK TO TOP

okrichev2

Krichevsky Oleg: Bacterial nucleoid structure and dynamics and its interaction with the membrane

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.

okrichev1.jpg
Figure 1: E. Coli bacteria: the membrane is
stained with fluorescent dye
okrichev2.jpg
Figure 2: The extracted bacterial nucleoid
expands to a volume ~ 30 times that of bacteria.

Further reading:

BACK TO TOP

okrichev3

Krichevsky Oleg: The dynamics of environmental stress response in Yeast cells

The living cell is "designed" through evolution to respond to a number of external stimula in a robust manner. We are studying the response of Yeast cells to different types of external stress: so called environemental stress response (ESR) system. ESR contains all of the major control elements normally found in human engineered systems: logical gates, switches, modular architecture. The question is how these elements are organized to work in a robust manner in the presence of thermal noise.

In collaboration with Prof. Dina Raveh, Life Sciences.

Further reading:

BACK TO TOP

shaulm1

Mordechai Shaul: Optical diagnosis using advanced spectroscopic and computational techniques

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].
introd1.jpg
Figure 1
introd3.jpg
Figure 2
introd2.jpg
Figure 3
introd4.jpg
Figure 4
BACK TO TOP


Updated on April 26, 2007 by Assaf Kletter