Efrat Shimshoni

Transport through Quantum Melts

We discuss superconductor to insulator and quantum Hall transitions which are first order in the clean limit. Disorder creates a nearly percolating network of the minority phase. Electrical transport is dominated by tunneling or activation through the saddle point junctions, whose typical resistance is calculated as a function of magnetic field. In the Boltzmann regime, this approach yields resistivity laws which agree with recent experiments in both classes of systems. We discuss the origin of dissipation at zero temperature.

A different view of the quantum Hall plateau-to-plateau transitions

We demonstrate experimentally that the transitions between adjacent integer quantum Hall (QH) states are equivalent to a QH-to-insulator transition occurring in the top Landau level, in the presence of an inert background of the other completely filled Landau levels, each contributing a single unit of quantum conductance, e 2 /h, to the total Hall conductance of the system.

Onset of dissipation in Zener dynamics: Relaxation versus dephasing

Abstract We study the onset of dissipation in an externally driven small system, coupled to an environment. More specifically, we focus on the interplay between Zener dynamics (induced by the driving source), and the interaction with the environment, which gives rise to both dephasing and relaxation. We first consider a toy model, consisting of an externally driven two-level system coupled to various tupes of environments. We demonstrate the onset of dephasing, manifested as a decay of interference terms. In addition, we derive an effective equation of motion for the density matrix of the system, in the presence of a dissipative coupling to a thermal bath characterized by a broad spectrum. We extend our analysis of dephasing and relaxation to deal with the dynamics of a multi-level system. We show that dephasing and relaxation have competing effects on the dynamics. Dephasing tends to enhance the absorption of energy into the system, due to destruction of localization in the energy space; relaxation, on the other hand, gives rise to a loss of energy, thus limiting the rate of energy absorption. We calculate the dependence of the conductance on the external bias and find that it is nonmonotonous due to the interplay between diffusion and relaxation in energy space. In the weak dephasing regime we obtain fluctuations in the conductance, due to quantum interference effects. Our results can be applied to one-dimensional conducting rings threaded by time-varying Aharonov-Bohm flux.

Evidence for Charge-Flux Duality near the Quantum Hall Liquid-to-Insulator Transition

A remarkable symmetry has been observed between the diagonal, nonlinear, current-voltage (I-Vxx) characteristics taken in the fractional quantum Hall effect (FQHE) liquid state of the two-dimensional electron system and those taken in the bordering insulating phase. When properly selected, the I-Vxx traces in the FQHE regime are identical, within experimental errors, to Vxx-I traces in the insulator, that is, with the roles of the currents and voltages exchanged. These results can be interpreted as evidence for the existence of charge-flux duality symmetry in the system.

Quantum zigzag transition in ion chains.

A string of trapped ions at zero temperature exhibits a structural phase transition to a zigzag structure, tuned by reducing the transverse trap potential or the interparticle distance. The transition is driven by transverse, short wavelength vibrational modes. We argue that this is a quantum phase transition, which can be experimentally realized and probed. Indeed, by means of a mapping to the Ising model in a transverse field, we estimate the quantum critical point in terms of the system parameters, and find a finite, measurable deviation from the critical point predicted by the classical theory. A measurement procedure is suggested which can probe the effects of quantum fluctuations at criticality. These results can be extended to describe the transverse instability of ultracold polar molecules in a one-dimensional optical lattice.

Large violation of the Wiedemann-Franz law in Luttinger liquids.

We show that in weakly disordered Luttinger liquids close to a commensurate filling the ratio of thermal conductivity kappa and electrical conductivity sigma can deviate strongly from the Wiedemann-Franz law valid for Fermi liquids scattering from impurities. In the regime where the umklapp scattering rate Gamma(U) is much larger than the impurity scattering rate Gamma(imp), the Lorenz number L = kappa/(sigmaT) rapidly changes from very large values L approximately Gamma(U)/Gamma(imp) >> 1 at the commensurate point to very small values L approximately Gamma(imp)/Gamma(U) << 1 for a slightly doped system. This surprising behavior is a consequence of approximate symmetries existing even in the presence of strong umklapp scattering.

Magnetothermal transport in the spin-1 2 chains of copper pyrazine dinitrate

We present experiments on the thermal transport in the spin-1/2 chain compound copper pyrazine dinitrate Cu(C4H4N2)(NO3)2. The heat conductivity shows a surprisingly strong dependence on the applied magnetic field B, characterized at low temperatures by two main features. The first one appearing at low B is a characteristic dip located at muBB approximately kBT, that may arise from umklapp scattering. The second one is a plateaulike feature in the quantum critical regime, muB|B - Bc| < kBT, where Bc is the saturation field at T=0. The latter feature clearly points towards a momentum and field-independent mean free path of the spin excitations, contrary to theoretical expectations.

Duality near quantum Hall transitions

A recent experiment by Shahar et al, on the phase transitions between quantum Hall states and the insulator, found that the current-voltage characteristics in the two phases are related by symmetry. It was suggested in this work that this is evidence for charge-flux duality near quantum Hall transitions. Here we provide details of this analysis. (Appearances notwithstanding, this is a theoretical paper.)

On the nature of the Hall insulator

Abstract We have conducted an experimental study of the linear transport properties of the magnetic-field induced insulating phase which terminates the quantum Hall (QH) series in two dimensional electron systems. We found that a direct and simple relation exists between measurements of the longitudinal resistivity, ϱxx, in this insulating phase and in the neighboring QH phase. In addition, we find that the Hall resistivity, ϱxy, can be quantized in the insulating phase. Our results indicate that a close relation exists between the conduction mechanism in the insulator and in the QH liquid.

Disorder induced superconducting ratchet effect in nanowires

A dc voltage drop develops along amorphous indium oxide nanowires that are exposed to an ac bias source. This voltage is anti-symmetric with magnetic field and is characterized by sample specific quasi-periodic magneto-voltage oscillations. The voltage magnitude increases with decreasing temperature below