Spectroscopic probes of isolated nonequilibrium quantum matter: Quantum quenches, Floquet states, and distribution functions

We investigate radio-frequency (rf) spectroscopy, metal-to-superconductor tunneling, and ARPES as probes of isolated out-of-equilibrium quantum systems, and examine the crucial role played by the nonequilibrium distribution function. As an example, we focus on the induced topological time-periodic (Floquet) phase in a 2D $p+ip$ superfluid, following an instantaneous quench of the coupling strength. The post-quench Cooper pairs occupy a linear combination of "ground" and "excited" Floquet states, with coefficients determined by the distribution function. While the Floquet bandstructure exhibits a single avoided crossing relative to the equilibrium case, the distribution function shows a population inversion of the Floquet bands at low energies. For a realization in ultracold atoms, these two features compensate, producing a bulk average rf signal that is well-captured by a quasi-equilibrium approximation. In particular, the rf spectrum shows a robust gap. The single crossing occurs because the quench-induced Floquet phase belongs to a particular class of soliton dynamics for the BCS equation. The population inversion is a consequence of this, and ensures the conservation of the pseudospin winding number. As a comparison, we compute the rf signal when only the lower Floquet band is occupied; in this case, the gap disappears for strong quenches. The tunneling signal in a solid state realization is ignorant of the distribution function, and can show wildly different behaviors. We also examine rf, tunneling, and ARPES for weak quenches, such that the resulting topological steady-state is characterized by a constant nonequilibrium order parameter. In a system with a boundary, tunneling reveals the Majorana edge states. However, the local rf signal due to the edge states is suppressed by a factor of the inverse system size, and is spatially deconfined throughout the bulk of the sample.Comment: 22 pages, 15 figures. v2: Added calculated ARPES spectr

Response theory of the ergodic many-body delocalized phase: Keldysh Finkel'stein sigma models and the 10-fold way

We derive the finite temperature Keldysh response theory for interacting fermions in the presence of quenched disorder, as applicable to any of the 10 Altland-Zirnbauer classes in an Anderson delocalized phase with at least a U(1) continuous symmetry. In this formulation of the interacting Finkel'stein nonlinear sigma model, the statistics of one-body wave functions are encoded by the constrained matrix field, while physical correlations follow from the hydrodynamic density or spin response field, which decouples the interactions. Integrating out the matrix field first, we obtain weak (anti)localization and Altshuler-Aronov quantum conductance corrections from the hydrodynamic response function. This procedure automatically incorporates the correct infrared physics, and in particular gives the Altshuler-Aronov-Khmelnitsky (AAK) equations for dephasing of weak (anti)localization due to electron-electron collisions. We explicate the method by deriving known quantum corrections in two dimensions for the symplectic metal class AII, as well as the spin-SU(2) invariant superconductor classes C and CI. We show that conductance corrections due to the special modes at zero energy in nonstandard classes are automatically cut off by temperature, as previously expected, while the Wigner-Dyson class Cooperon modes that persist to all energies are cut by dephasing. We also show that for short-ranged interactions, the standard self-consistent solution for the dephasing rate is equivalent to a diagrammatic summation via the self-consistent Born approximation. This should be compared to the AAK solution for long-ranged Coulomb interactions, which exploits the Markovian noise correlations induced by thermal fluctuations of the electromagnetic field. We discuss prospects for exploring the many-body localization transition from the ergodic side as a dephasing catastrophe in short-range interacting models.Comment: 68 pages, 23 figure

Traveling Dark Solitons in Superfluid Fermi Gases

Families of dark solitons exist in superfluid Fermi gases. The energy-velocity dispersion and number of depleted particles completely determines the dynamics of dark solitons on a slowly-varying background density. For the unitary Fermi gas we determine these relations from general scaling arguments and conservation of local particle number. We find solitons to oscillate sinusoidally at the trap frequency reduced by a factor of $1/\sqrt{3}$. Numerical integration of the time-dependent Bogoliubov-de Gennes equation determines spatial profiles and soliton dispersion relations across the BEC-BCS crossover and proves consistent with the scaling relations at unitarity.Comment: Small changes in response to referee's comments; fig 1 revised and refs updated. Cross listed to nucl-th due to interest in the unitary Fermi ga

Anomaly inflow mechanism using Wilson line

It is shown that the anomaly inflow mechanism can be implemented using Wilson line in odd dimensional gauge theories. An action of Wess-Zumino-Witten (WZW) type can be constructed using Wilson line. The action is understood in the odd dimensional bulk space-time rather than in the even dimensional boundary. This action is not gauge invariant. It gives anomalous gauge variations of the consistent form on boundary space-times. So it can be used to cancel the quantum anomalies localized on boundary space-times. This offers a new way to cancel the gauge anomaly and construct anomaly-free gauge theory in odd dimensional space-time.Comment: 4 pages, 1 figure; title changed; text and figure improved; references adde

InGaAs implant-free quantum-well MOSFETs: performance evaluation using 3D Monte Carlo simulation

In this paper we use numerical simulations to evaluate the performance of III-V Implant-Free Quantum-Well (IFQW) MOSFET devices that offer simultaneously high channel mobility, high drive current and excellent electrostatic integrity. Using 3D Monte Carlo simulations we show that to fully understand the performance of this device architecture, Fermi-Dirac statistics and quantum-corrections must be considered to account for the impact of low density-of-states and quantum confinement in the channel layer respectively