Arnab Das

Fate of Many-Body Localization Under Periodic Driving.

We study many-body localized quantum systems subject to periodic driving. We find that the presence of a mobility edge anywhere in the spectrum is enough to lead to delocalization for any driving strength and frequency. By contrast, for a fully localized many-body system, a delocalization transition occurs at a finite driving frequency. We present numerical studies on a system of interacting one-dimensional bosons and the quantum random energy model, as well as simple physical pictures accounting for those results.

Electrical transport in paratoluene sulfonate doped polypyrrole films at low temperature

Transport data for paratoluene sulfonate dope polypyrrole films in the insulating regime and near the metal-insulator (M-I) boundary are presented and analyzed. Samples in the insulating region show a crossover from Mott to Efros–Shklovskii variable range hopping conduction at T=5u2009K and magnetoconductance of these samples is also explained by variable range hopping theory. The power law dependence of conductivity σ(T)∝Tβ is observed for the sample close to metallic side of the M-I transition with β=0.83 for 20u2009K<T<300u2009K and β=1/2 for 1.8u2009K⩽T⩽20u2009K. The magnetoconductance of this sample is analyzed by three dimensional electron-electron interaction and weak electron localization theory. The inelastic scattering length Lin obeys a power law temperature dependence, Lin∝T−p/2 with p=1.

Dynamical many-body localization and delocalization in periodically driven closed quantum systems

Quantum interference lies at the heart of several surprising equilibrium and non-equilibrium phenomena in many-body Physics. Here we discuss two recently explored non-equilibrium scenarios where external periodic drive applied to closed (i.e., not attached to any external bath) quantum many-body systems have apparently opposite effects in respective cases. In one case it freezes/localizes a disorder free system dynamically, while in the other it delocalizes a disordered many-body localized system, and quantum interference is responsible for both the effects. We review these in the perspective of more general questions of ergodicity, energy absorption, asymptotic behavior, and finally the essential role of quantum mechanics in understanding these issues in periodically driven closed many-body systems. In this article we intend to deliver a non-technical account of some recent developments in this field in a manner accessible to a broad readership.

Quantum annealing: The fastest route to quantum computation?

The Baryon Antibaryon Symmetry Experiment (BASE) aims at performing a stringent test of the combined charge parity and time reversal (CPT) symmetry by comparing the magnetic moments of the proton and the antiproton with high precision. Using single particles in a Penning trap, the proton/antiproton g-factors, i.e. the magnetic moment in units of the nuclear magneton, are determined by measuring the respective ratio of the spin-precession frequency to the cyclotron frequency. The spin precession frequency is measured by nondestructive detection of spin quantum transitions using the continuous Stern-Gerlach effect, and the cyclotron frequency is determined from the particle’s motional eigenfrequencies in the Penning trap using the invariance theorem. By application of the double Penning-trap method we expect that in our measurements a fractional precision of δg/g 10−9 can be achieved. The successful application of this method to the antiproton will represent a factor 1000 improvement in the fractional precision of its magnetic moment. The BASE collaboration has constructed and commissioned a new experiment at the Antiproton Decelerator (AD) of CERN. This article describes and summarizes the physical and technical aspects of this new experiment. ar X iv :1 60 4. 08 82 0v 1 [ ph ys ic s. at om -p h] 2 9 A pr 2 01 6 2 Will be inserted by the editor

Electrical transport properties of thin cadmium films at low temperatures

We report here the results of an extensive study of localization and electron-electron interaction effects in thin cadmium films with thickness ranging from 80 Å to 350 Å. Measurements of the resistance as a function of both temperature and magnetic field have allowed us to separate the contributions of localization and electron-electron interaction. The low resistive films of thicknesses 300 Å and 350 Å do not show any localization. The resistance of these samples decreases logarithmically with decreasing temperature below 10 K, while a ln(T) increases in resistance is observed for the high resistive films of thickness in the range of 80–120 Å. Magnetoresistance of low resistive samples obeys the expression ΔR(H)/R(O)=ATn. But, both the resistance and magnetoconductance of the high resistive films are well explained by weak localization and electron-electron interaction effect. From the magnetoconductance measurement, we have calculated the inelastic scattering time (τi) and the spin-orbit scattering time (τso). The magnitude of spin-orbit scattering time is smaller than the inelastic scattering time. The inelastic scattering has been shown to arise due to the electron-electron scattering and the absolute magnitude of this scattering rate agrees reasonably well with the theory within the temperature range 1.8 K≤T≤5 K. At higher temperature (5 K

Periodic dynamics of fermionic superfluids in the BCS regime.

We study the zero temperature non-equilibrium dynamics of a fermionic superfluid in the BCS limit and in the presence of a drive leading to a time-dependent chemical potential μ(t). We choose a periodic driving protocol characterized by a frequency ω and compute the fermion density, the wavefunction overlap, and the residual energy of the system at the end of N periods of the drive. We demonstrate that the BCS self-consistency condition is crucial in shaping the long time behaviour of the fermions subjected to the drive and provide an analytical understanding of the behaviour of the fermion density nkF (where kF is the Fermi momentum vector) after a drive period and for large ω. We also show that the momentum distribution of the excitations generated due to such a drive bears the signature of the pairing symmetry and can be used, for example, to distinguish between s- and d-wave superfluids. We propose experiments to test our theory.

Multifractality without fine-tuning in a Floquet quasiperiodic chain

Periodically driven, or Floquet, disordered quantum systems have generated many unexpected discoveries of late, such as the anomalous Floquet Anderson insulator and the discrete time crystal. Here, we report the emergence of an entire band of multifractal wavefunctions in a periodically driven chain of non-interacting particles subject to spatially quasiperiodic disorder. Remarkably, this multifractality is robust in that it does not require any fine-tuning of the model parameters, which sets it apart from the known multifractality of

Locating topological phase transitions using nonequilibrium signatures in local bulk observables

critical

Resistivity and Magnetoresistance Studies of Nb3Ir and V3Sb Compounds

wavefunctions. The multifractality arises as the periodic drive hybridises the localised and delocalised sectors of the undriven spectrum. We account for this phenomenon in a simple random matrix based theory. Finally, we discuss dynamical signatures of the multifractal states, which should betray their presence in cold atom experiments. Such a simple yet robust realisation of multifractality could advance this so far elusive phenomenon towards applications, such as the proposed disorder-induced enhancement of a superfluid transition.

Electrical transport in doped polypyrrole films at low temperature

Topological quantum phases cannot be characterized by local order parameters in the bulk. In this work, however, we show that nonanalytic signatures of a topological quantum critical point do remain in local observables in the bulk, and manifest themselves as nonanalyticities in their expectation values taken over a family of nonequilibrium states generated using a quantum quench protocol. The signature can be used for precisely locating the critical points in parameter space. A large class of initial states can be chosen for the quench, including finite temperature states. We demonstrate these results in tractable models of noninteracting fermions exhibiting topological phase transitions in one and two spatial dimensions. We also show that the nonanalyticities can be absent if the gap closing is nontopological, i.e., when it corresponds to no phase transition.