33 research outputs found
Interferometric probes of many-body localization
We propose a method for detecting many-body localization (MBL) in disordered
spin systems. The method involves pulsed, coherent spin manipulations that
probe the dephasing of a given spin due to its entanglement with a set of
distant spins. It allows one to distinguish the MBL phase from a
non-interacting localized phase and a delocalized phase. In particular, we show
that for a properly chosen pulse sequence the MBL phase exhibits a
characteristic power-law decay reflecting its slow growth of entanglement. We
find that this power-law decay is robust with respect to thermal and disorder
averaging, provide numerical simulations supporting our results, and discuss
possible experimental realizations in solid-state and cold atom systems.Comment: 5 pages, 4 figure
Giant Nernst Effect due to Fluctuating Cooper Pairs in Superconductors
A theory of the fluctuation-induced Nernst effect is developed for arbitrary
magnetic fields and temperatures beyond the upper critical field line in a
two-dimensional superconductor. First, we derive a simple phenomenological
formula for the Nernst coefficient, which naturally explains the giant Nernst
signal due to fluctuating Cooper pairs. The latter is shown to be large even
far from the transition and may exceed by orders of magnitude the Fermi liquid
terms. We also present a complete microscopic calculation (which includes
quantum fluctuations) of the Nernst coefficient and give its asymptotic
dependencies in various regions on the phase diagram. It is argued that the
magnitude and the behavior of the Nernst signal observed experimentally in
disordered superconducting films can be well-understood on the basis of the
superconducting fluctuation theory.Comment: 4 pages, 3 figure
Non-Linear Algebra and Bogolubov's Recursion
Numerous examples are given of application of Bogolubov's forest formula to
iterative solutions of various non-linear equations: one and the same formula
describes everything, from ordinary quadratic equation to renormalization in
quantum field theory.Comment: LaTex, 21 page
Nernst effect as a probe of superconducting fluctuations in disordered thin films
In amorphous superconducting thin films of and ,
a finite Nernst coefficient can be detected in a wide range of temperature and
magnetic field. Due to the negligible contribution of normal quasi-particles,
superconducting fluctuations easily dominate the Nernst response in the entire
range of study. In the vicinity of the critical temperature and in the
zero-field limit, the magnitude of the signal is in quantitative agreement with
what is theoretically expected for the Gaussian fluctuations of the
superconducting order parameter. Even at higher temperatures and finite
magnetic field, the Nernst coefficient is set by the size of superconducting
fluctuations. The Nernst coefficient emerges as a direct probe of the ghost
critical field, the normal-state mirror of the upper critical field. Moreover,
upon leaving the normal state with fluctuating Cooper pairs, we show that the
temperature evolution of the Nernst coefficient is different whether the system
enters a vortex solid, a vortex liquid or a phase-fluctuating superconducting
regime.Comment: Submitted to New. J. Phys. for a focus issue on "Superconductors with
Exotic Symmetries
Fluctuations of the superconducting order parameter as an origin of the Nernst effect
We show that the strong Nernst signal observed recently in amorphous
superconducting films far above the critical temperature is caused by the
fluctuations of the superconducting order parameter. We demonstrate a striking
agreement between our theoretical calculations and the experimental data at
various temperatures and magnetic fields. Besides, the Nernst effect is
interesting not only in the context of superconductivity. We discuss some
subtle issues in the theoretical study of thermal phenomena that we have
encountered while calculating the Nernst coefficient. In particular, we explain
how the Nernst theorem (the third law of thermodynamics) imposes a strict
constraint on the magnitude of the Nernst effect.Comment: 6 pages, 5 figures, extended versio
Controlling quantum many-body dynamics in driven Rydberg atom arrays
The control of nonequilibrium quantum dynamics in many-body systems is challenging because interactions typically lead to thermalization and a chaotic spreading throughout Hilbert space. We investigate nonequilibrium dynamics after rapid quenches in a many-body system composed of 3 to 200 strongly interacting qubits in one and two spatial dimensions. Using a programmable quantum simulator based on Rydberg atom arrays, we show that coherent revivals associated with so-called quantum many-body scars can be stabilized by periodic driving, which generates a robust subharmonic response akin to discrete time-crystalline order. We map Hilbert space dynamics, geometry dependence, phase diagrams, and system-size dependence of this emergent phenomenon, demonstrating new ways to steer complex dynamics in many-body systems and enabling potential applications in quantum information science
Many-body localization in a quantum simulator with programmable random disorder
When a system thermalizes it loses all local memory of its initial
conditions. This is a general feature of open systems and is well described by
equilibrium statistical mechanics. Even within a closed (or reversible) quantum
system, where unitary time evolution retains all information about its initial
state, subsystems can still thermalize using the rest of the system as an
effective heat bath. Exceptions to quantum thermalization have been predicted
and observed, but typically require inherent symmetries or noninteracting
particles in the presence of static disorder. The prediction of many-body
localization (MBL), in which disordered quantum systems can fail to thermalize
in spite of strong interactions and high excitation energy, was therefore
surprising and has attracted considerable theoretical attention. Here we
experimentally generate MBL states by applying an Ising Hamiltonian with
long-range interactions and programmably random disorder to ten spins
initialized far from equilibrium. We observe the essential signatures of MBL:
memory retention of the initial state, a Poissonian distribution of energy
level spacings, and entanglement growth in the system at long times. Our
platform can be scaled to higher numbers of spins, where detailed modeling of
MBL becomes impossible due to the complexity of representing such entangled
quantum states. Moreover, the high degree of control in our experiment may
guide the use of MBL states as potential quantum memories in naturally
disordered quantum systems.Comment: 9 pages, 9 figure
Strain-induced partially flat band, helical snake states, and interface superconductivity in topological crystalline insulators
Topological crystalline insulators in IV-VI compounds host novel topological
surface states consisting of multi-valley massless Dirac fermions at low
energy. Here we show that strain generically acts as an effective gauge field
on these Dirac fermions and creates pseudo-Landau orbitals without breaking
time-reversal symmetry. We predict the realization of this phenomenon in IV-VI
semiconductor heterostructures, due to a naturally occurring misfit dislocation
array at the interface that produces a periodically varying strain field.
Remarkably, the zero-energy Landau orbitals form a flat band in the vicinity of
the Dirac point, and coexist with a network of snake states at higher energy.
We propose that the high density of states of this flat band gives rise to
interface superconductivity observed in IV-VI semiconductor multilayers at
unusually high temperatures, with non-BCS behavior. Our work demonstrates a new
route to altering macroscopic electronic properties to achieve a partially flat
band, and paves the way for realizing novel correlated states of matter.Comment: Accepted by Nature Physic
Discrete Time-Crystalline Order Enabled by Quantum Many-Body Scars: Entanglement Steering via Periodic Driving
The control of many-body quantum dynamics in complex systems is a key
challenge in the quest to reliably produce and manipulate large-scale quantum
entangled states. Recently, quench experiments in Rydberg atom arrays
(Bluvstein et. al., arXiv:2012.12276) demonstrated that coherent revivals
associated with quantum many-body scars can be stabilized by periodic driving,
generating stable subharmonic responses over a wide parameter regime. We
analyze a simple, related model where these phenomena originate from
spatiotemporal ordering in an effective Floquet unitary, corresponding to
discrete time-crystalline (DTC) behavior in a prethermal regime. Unlike
conventional DTC, the subharmonic response exists only for Neel-like initial
states, associated with quantum scars. We predict robustness to perturbations
and identify emergent timescales that could be observed in future experiments.
Our results suggest a route to controlling entanglement in interacting quantum
systems by combining periodic driving with many-body scars