1,244 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 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
Detecting cells and analyzing their behaviors in microscopy images using deep neural networks
The computer-aided analysis in the medical imaging field has attracted a lot of attention for the past decade. The goal of computer-vision based medical image analysis is to provide automated tools to relieve the burden of human experts such as radiologists and physicians. More specifically, these computer-aided methods are to help identify, classify and quantify patterns in medical images. Recent advances in machine learning, more specifically, in the way of deep learning, have made a big leap to boost the performance of various medical applications. The fundamental core of these advances is exploiting hierarchical feature representations by various deep learning models, instead of handcrafted features based on domain-specific knowledge.
In the work presented in this dissertation, we are particularly interested in exploring the power of deep neural network in the Circulating Tumor Cells detection and mitosis event detection. We will introduce the Convolutional Neural Networks and the designed training methodology for Circulating Tumor Cells detection, a Hierarchical Convolutional Neural Networks model and a Two-Stream Bidirectional Long Short-Term Memory model for mitosis event detection and its stage localization in phase-contrast microscopy images”--Abstract, page iii
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