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Moshe
Schechter
Assistant
Professor, Physics
Department, Ben-Gurion University
Office:
Bulding 54, room 331 |
| 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.
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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
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.
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