arXiv subjects

Yiming Chen

Publications and source records attributed to Yiming Chen.

170 records · Page 10Linked to original sources

arXiv2018

Modular Flow as a Disentangler

In holographic duality, the entanglement entropy of a boundary region is proposed to be dual to the area of an extremal codimension-2 surface that is homologous to the boundary region, known as the Hubeny-Rangamani-Takayanagi (HRT) surface. In this paper, we study when the HRT surfaces of two boundary subregions R, A are in the same Cauchy slice. This condition is necessary for the subregion-subregion mapping to be local for both subregions and for states to have a tensor network description. To quantify this, we study the area of a surface that is homologous to A and is extremal except at possible intersections with the HRT surface of R (minimizing over all such possible surfaces), which we call the constrained area. We give a boundary proposal for an upper bound of this quantity, a bound which is saturated when the constrained surface intersects the HRT surface of R at a constant angle. This boundary quantity is the minimum entropy of region A in a modular evolved state -- a state that has been evolved unitarily with the modular Hamiltonian of R. We can prove this formula in two boundary dimensions or when the modular Hamiltonian is local. This modular minimal entropy is a boundary quantity that probes bulk causality and, from this quantity, we can extract whether two HRT surfaces are in the future or past of each other. These entropies satisfy some inequalities reminiscent of strong subadditivity and can be used to remove certain corner divergences.

hep-th↗

arXiv2017

Automated Generation and Ensemble-Learned Matching of X-ray Absorption Spectra

We report the development of XASdb, a large database of computed reference X-ray absorption spectra (XAS), and a novel Ensemble-Learned Spectra IdEntification (ELSIE) algorithm for the matching of spectra. XASdb currently hosts more than 300,000 K-edge X-ray absorption near-edge spectra (XANES) for over 30,000 materials from the open-science Materials Project database. We discuss a high-throughput automation framework for FEFF calculations, built on robust, rigorously benchmarked parameters. We will demonstrate that the ELSIE algorithm, which combines 33 weak "learners" comprising a set of preprocessing steps and a similarity metric, can achieve up to 84.2% accuracy in identifying the correct oxidation state and coordination environment of a test set of 19 K-edge XANES spectra encompassing a diverse range of chemistries and crystal structures. The XASdb with the ELSIE algorithm has been integrated into a web application in the Materials Project, providing an important new public resource for the analysis of XAS to all materials researchers. Finally, the ELSIE algorithm itself has been made available as part of Veidt, an open source machine learning library for materials science.

cond-mat.mtrl-sci↗

arXiv2017

Tunable Quantum Chaos in the Sachdev-Ye-Kitaev Model Coupled to a Thermal Bath

The Sachdev-Ye-Kitaev (SYK) model describes Majorana fermions with random interaction, which displays many interesting properties such as non-Fermi liquid behavior, quantum chaos, emergent conformal symmetry and holographic duality. Here we consider a SYK model or a chain of SYK models with $N$ Majorana fermion modes coupled to another SYK model with $N^2$ Majorana fermion modes, in which the latter has many more degrees of freedom and plays the role as a thermal bath. For a single SYK model coupled to the thermal bath, we show that although the Lyapunov exponent is still proportional to temperature, it monotonically decreases from $2π/β$ ($β=1/(k_BT)$, $T$ is temperature) to zero as the coupling strength to the thermal bath increases. For a chain of SYK models, when they are uniformly coupled to the thermal bath, we show that the butterfly velocity displays a crossover from a $\sqrt{T}$-dependence at relatively high temperature to a linear $T$-dependence at low temperature, with the crossover temperature also controlled by the coupling strength to the thermal bath. If only the end of the SYK chain is coupled to the thermal bath, the model can introduce a spatial dependence of both the Lyapunov exponent and the butterfly velocity. Our models provide canonical examples for the study of thermalization within chaotic models.

hep-th↗

arXiv2017

Competition between Chaotic and Non-Chaotic Phases in a Quadratically Coupled Sachdev-Ye-Kitaev Model

The Sachdev-Ye-Kitaev (SYK) model is a concrete solvable model to study non-Fermi liquid properties, holographic duality and maximally chaotic behavior. In this work, we consider a generalization of the SYK model that contains two SYK models with different number of Majorana modes coupled by quadratic terms. This model is also solvable, and the solution shows a zero-temperature quantum phase transition between two non-Fermi liquid chaotic phases. This phase transition is driven by tuning the ratio of two mode numbers, and a Fermi liquid non-chaotic phase sits at the critical point with equal mode number. At finite temperature, the Fermi liquid phase expands to a finite regime. More intriguingly, a different non-Fermi liquid phase emerges at finite temperature. We characterize the phase diagram in term of the spectral function, the Lyapunov exponent and the entropy. Our results illustrate a concrete example of quantum phase transition and critical regime between two non-Fermi liquid phases.

cond-mat.str-el↗

arXiv2016

Observation of tunneling-assisted highly forbidden single-photon transitions in a Ni$_4$ single-molecule magnet

Forbidden transitions between energy levels typically involve violation of selection rules imposed by symmetry and/or conservation laws. A nanomagnet tunneling between up and down states violates angular momentum conservation because of broken rotational symmetry. Here we report observations of highly forbidden transitions between spin states in a Ni$_4$ single-molecule magnet in which a single photon can induce the spin to change by several times $\hbar$, nearly reversing the direction of the spin. These observations are understood as tunneling-assisted transitions that lift the standard $Δm = \pm 1$ selection rule for single-photon transitions. These transitions are observed at low applied fields, where tunneling is dominated by the molecule's intrinsic anisotropy and the field acts as a perturbation. Such transitions can be exploited to create macroscopic superposition states that are not typically accessible through single-photon $Δm = \pm 1$ transitions.

cond-mat.mes-hall↗

arXiv2015

Instrument for in-situ orientation of superconducting thin-film resonators used for electron-spin resonance experiments

When used in Electron-Spin Resonance (ESR) measurements, superconducting thin-film resonators must be precisely oriented relative to the external magnetic field in order to prevent the trapping of magnetic flux and the associated degradation of resonator performance. We present a compact design solution for this problem that allows in-situ control of the orientation of the resonator at cryogenic temperatures. Tests of the apparatus show that when proper alignment is achieved, there is almost no hysteresis in the field dependence of the resonant frequency.

cond-mat.mtrl-sci↗

arXiv2014

Detection and Decoding for 2D Magnetic Recording Channels with 2D Intersymbol Interference

This paper considers iterative detection and decoding on the concatenated communication channel consisting of a two-dimensional magnetic recording (TDMR) channel modeled by the four-grain rectangular discrete grain model (DGM) proposed by Kavcic et. al., followed by a two-dimensional intersymbol interference (2D-ISI) channel modeled by linear convolution of the DGM model's output with a finite-extent 2D blurring mask followed by addition of white Gaussian noise. An iterative detection and decoding scheme combines TDMR detection, 2D-ISI detection, and soft-in/soft-out (SISO) channel decoding in a structure with two iteration loops. In the first loop, the 2D-ISI channel detector exchanges log-likelihood ratios (LLRs) with the TDMR detector. In the second loop, the TDMR detector exchanges LLRs with a serially concatenated convolutional code (SCCC) decoder. Simulation results for the concatenated TDMR and 2 x 2 averaging mask ISI channel with 10 dB SNR show that densities of 0.48 user bits per grain and above can be achieved, corresponding to an areal density of about 9.6 Terabits per square inch, over the entire range of grain probabilities in the TDMR model.

cs.IT↗

arXiv2013

Iterative Detection and Decoding for the Four-Rectangular-Grain TDMR Model

This paper considers detection and error control coding for the two-dimensional magnetic recording (TDMR) channel modeled by the two-dimensional (2D) four-rectangular-grain model proposed by Kavcic, Huang et. al. in 2010. This simple model captures the effects of different 2D grain sizes and shapes, as well as the TDMR grain overwrite effect: grains large enough to be written by successive bits retain the polarity of only the last bit written. We construct a row-by-row BCJR detection algorithm that considers outputs from two rows at a time over two adjacent columns, thereby enabling consideration of more grain and data states than previously proposed algorithms that scan only one row at a time. The proposed algorithm employs soft-decision feedback of grain states from previous rows to aid the estimation of current data bits and grain states. Simulation results using the same average coded bit density and serially concatenated convolutional code (SCCC) as a previous paper by Pan, Ryan, et. al. show gains in user bits/grain of up to 6.7% over the previous work when no iteration is performed between the TDMR BCJR and the SCCC, and gains of up to 13.4% when the detector and the decoder iteratively exchange soft information.

cs.IT↗