48 research outputs found

### 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

### Quantum Interference of Hydrodynamic Modes in a Dirty Marginal Fermi Liquid

We study the electrical transport of a two-dimensional non-Fermi liquid with
disorder, and we determine the first quantum correction to the semiclassical dc
conductivity due to quantum interference. We consider a system with $N$ flavors
of fermions coupled to SU($N$) critical matrix bosons. Motivated by the SYK
model, we employ the bilocal field formalism and derive a set of
finite-temperature saddle-point equations governing the fermionic and bosonic
self-energies in the large-$N$ limit. Interestingly, disorder smearing induces
a marginal Fermi liquid (MFL) self-energy for the fermions. We next consider
fluctuations around the saddle points and derive a MFL-Finkel'stein nonlinear
sigma model. We find that the Altshuler-Aronov quantum conductance correction
gives linear-$T$ resistivity that can dominate over the Drude result at low
temperature. The strong temperature dependence of the quantum correction arises
due to rapid relaxation of the mediating quantum-critical bosons. We verify
that our calculations explicitly satisfy the Ward identity at the semiclassical
and quantum levels. Our results establish that quantum interference persists in
two-particle hydrodynamic modes, even when quasiparticles are subject to strong
(Planckian) dissipation.Comment: v2: corrected semiclassical conductivity; 39 pages, 19 figures; v3:
published versio