Schechter's research group

Low temperature universality and the two-TLS model

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 5 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, have remained unanswered.

At the same time, since 2005 it is recognized that these TLSs, which are generic to bulk amorphous solids and to surfaces, are the dominant source of noise, decoherence, and relaxation in many quantum devices, including superconducting qubits and resonators.

Recently, the assertion that the two-TLS model applies also to amorphous solids received strong experimental support in observed excess loss of superconducting resonators in far from equilibrium conditions. Some other phenomena in which the strongly interacting TLSs are dominant, and thus the two-TLS model gives distinct predictions from the STM are the Ramsey and Echo decay rates of TLSs as function of applied strain, and the spectral mixing of TLS under thermal cycling. Effects of the strongly interacting TLSs on the response of qubits and resonators and on the stability of superconducting quantum circuits is a central research direction in our group. Some of our current interests are the frequency shift of superconducting resonators induced by biasing the TLS bath, and the effect of the strongly interacting TLSs on this frequency shift.

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 allows the study of the interplay of interactions, quantum fluctuations, and disorder. This model is experimentally realized in anisotropic dipolar magnets in transverse field. We have thoroughly studied theoretically the phase diagram of such systems, pure and disordered, in their ferromagnetic and glass phases, and in mixed Ising – XY magnets.

Glassy dynamics, including phenomena such as slow relaxation and memory, are common to a wide variety of disordered and frustrated systems. Some of these systems, for example spin glasses, also show a thermodynamic phase transition to a low temperature glass phase, having long range glass order. Other systems, for example structural glasses, show a crossover, within which time scales quickly diverge, yet no thermodynamic transition nor a low temperature glass phase are present.

Anisotropic dipolar glasses at low dipole concentrations have intrigued much attention, as fluctuations beyond mean field become exceedingly important. Using cluster Monte Carlo algorithm we have shown that at all concentrations, however small, a thermodynamic glass phase is present, with the critical temperature being linear in the concentration, in agreement with mean field theory.

More recently we have addressed the question of the existence of a thermodynamic phase in the electron glass system. For systems with short range interactions, adding site disorder destroys the glass phase. For the electron glass, however, onsite disorder drives the glassy phase, introducing a thermodynamic glass phase at low temperature above a threshold disorder. Other questions we recently addressed is the dependence of glass dynamics on interaction and disorder, in dipolar glasses and in the electron glass, near and far from equilibrium.