arXiv subjects

Shizeng Lin

Publications and source records attributed to Shizeng Lin.

14 recordsLinked to original sources

arXiv2020

Skyrmion lattice creep at ultra-low current densities

Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities $j_c$ to depin from the underlying atomic lattice. Above $j_c$ skyrmions exhibit well-controlled motion, making them prime candidates for race-track memories. In thin films thermally-activated creep motion of isolated skyrmions was observed below $j_c$ as predicted by theory. Uncontrolled skyrmion motion is detrimental for race-track memories and is not fully understood. Notably, the creep of skyrmion lattices in bulk materials remains to be explored. Here we show using resonant ultrasound spectroscopy--a probe highly sensitive to the coupling between skyrmion and atomic lattices--that in the prototypical skyrmion lattice material MnSi depinning occurs at $j_c^*$ that is only 4 percent of $j_c$. Our experiments are in excellent agreement with Anderson-Kim theory for creep and allow us to reveal a new dynamic regime at ultra-low current densities characterized by thermally-activated skyrmion-lattice-creep with important consequences for applications.

cond-mat.str-el↗

arXiv2018

Quantum Algorithm Implementations for Beginners

As quantum computers become available to the general public, the need has arisen to train a cohort of quantum programmers, many of whom have been developing classical computer programs for most of their careers. While currently available quantum computers have less than 100 qubits, quantum computing hardware is widely expected to grow in terms of qubit count, quality, and connectivity. This review aims to explain the principles of quantum programming, which are quite different from classical programming, with straightforward algebra that makes understanding of the underlying fascinating quantum mechanical principles optional. We give an introduction to quantum computing algorithms and their implementation on real quantum hardware. We survey 20 different quantum algorithms, attempting to describe each in a succinct and self-contained fashion. We show how these algorithms can be implemented on IBM's quantum computer, and in each case, we discuss the results of the implementation with respect to differences between the simulator and the actual hardware runs. This article introduces computer scientists, physicists, and engineers to quantum algorithms and provides a blueprint for their implementations.

cs.ET↗

arXiv2017

Elasticity in the skyrmion phase unveils depinning at ultra-low current densities

Controlled movement of nano-scale stable magnetic objects has been proposed as the foundation for a new generation of magnetic storage devices. Magnetic skyrmions, vortex-like spin textures stabilized by their topology are particularly promising candidates for this technology. Their nanometric size and ability to be displaced in response to an electrical current density several orders of magnitude lower than required to induce motion of magnetic domain walls suggest their potential for high-density memory devices that can be operated at low power. However, to achieve this, skyrmion movement needs to be controlled, where a key question concerns the coupling of skyrmions with the underlying atomic lattice and disorder (pinning). Here, we use Resonant Ultrasound Spectroscopy (RUS), a probe highly sensitive to changes in the elastic properties, to shed new light on skyrmion elasticity and depinning in the archetypal skyrmion material MnSi. In MnSi, skyrmions form a lattice that leads to pronounced changes in the elastic properties of the atomic lattice as a result of magneto-crystalline coupling. Without an applied current, the shear and compressional moduli of the underlying crystal lattice exhibit an abrupt change in the field-temperature range where skyrmions form. For current densities exceeding $j_c^*$ the changes of elastic properties vanish, signaling the decoupling of skyrmion and atomic lattices. Interestingly, $j_c^*$, which we identify as the onset of skyrmion depinning, is about 20 times smaller than $j_c$ previously measured via non-linear Hall effect. Our results suggest the presence of a previously-undetected intermediate dynamic regime possibly dominated by skyrmion-creep motion with important consequences for potential applications.

cond-mat.str-el↗

arXiv2009

Stability of the Kink State in a Stack of Intrinsic Josephson Junctions

A new dynamic state characterized by $(2m_l+1)\pi$ static phase kink with integers $\{m_l\}$ is proposed recently in a stack of inductively coupled Josephson junctions. In the present paper, the stability of the phase kink state is investigated against many perturbations and it is shown that the kink state is stable. It is also discussed that the suppression of the amplitude of superconducting order parameter caused by the kink is weak.

cond-mat.supr-con↗

arXiv2009

Kink State in a Stack of Intrinsic Josephson Junctions in Layered High-$T_c$ Superconductors and Terahertz Radiation

A new family of dynamic states are found in a stack of inductively coupled intrinsic Josephson junctions in the absence of an external magnetic field. In this state, $(2m_l+1)\pi$ phase kinks with integers $m_l$'s stack along the c axis and lock neighboring junction together. Large dc power is pumped into plasma oscillation via kinks at the cavity resonance. The plasma oscillation is uniform along the c axis with the frequency satisfying the ac Josephson relation. Thus this state supports strong terahertz radiation and seems to be compatible with the recent experimental observations.

cond-mat.supr-con↗

arXiv2009

Comment on "Mechanism of Terahertz Electromagnetic Wave Emission from Intrinsic Josephson Junctions"

In a recent Letter, Tachiki \emph{et al.} argued that the dynamic state of superconductivity phase in multi junctions characterized by $\pmπ$ kinks is hard to realize from an energetic point of view. Instead, they proposed a so-called "state without kink". In this Comment, we point out that their energetic consideration is inconsistent with the well established treatment for Josephson junctions, and that the results derived for their "state without kink" cannot explain the radiations observed in the recent experiment.

cond-mat.supr-con↗

arXiv2009

Cavity phenomena in mesas of cuprate high-$T_c$ superconductors under voltage bias

Modeling a single crystal of cuprate high-$T_c$ superconductor, such as $\rm{Bi_2Sr_2CaCu_2O_{8+δ}}$, as a stack of intrinsic Josephson junctions, we formulate explicitly the cavity phenomenon of plasma oscillations and electromagnetic (EM) waves in mesas of cylindrical and annular shapes. When the mesa thickness is small compared with the EM wavelength, the boundary condition for the inductively coupled sine-Gordon equations is the Neumann-type one to a good approximation, addressed first theoretically and verified in a recent experiment. This renders the superconductor mesa a cavity. Biasing a dc voltage in the $c$ direction, a state with $\pmπ$ kinks in the superconductivity phase difference piled up alternatively along the c axis is stabilized. The $\pmπ$ phase kinks provide inter-lock between superconductivity phases in adjacent junctions, taking the advantage of huge inductive couplings inherent in the cuprate superconductors, which establishes the coherence across the whole system of more than $\sim 600$ junctions. They also permit a strong coupling between the lateral cavity mode and the transverse Josephson plasma, enhance the plasma oscillation significantly at the cavity modes which radiates EM waves in the terahertz band when the lateral size of mesa is set to tens of micrometers. In order to overcome the heating effect, we propose to use annular geometry. The dependence of frequency on the aspect ratio is analyzed, which reveals that the shape tailor is quite promising for improving the present technique of terahertz excitation. The annular geometry may be developed as a waveguide resonator, mimic the fiber lasers for visible lights.

cond-mat.supr-con↗

arXiv2008

Phase Dynamics in Intrinsic Josephson Junctions and its Electrodynamics

We present a theoretical description of the phase dynamics and its corresponding electrodynamics in a stack of inductively coupled intrinsic Josephson junctions of layered high-$T_c$ superconductors in the absence of an external magnetic field. Depending on the spatial structure of the gauge invariant phase difference, the dynamic state is classified into: state with kink, state without kink, and state with solitons. It is revealed that in the state with phase kink, the plasma is coupled to the cavity and the plasma oscillation is enhanced. In contrast, in the state without kink, the plasma oscillation is weak. It points a way to enhance the radiation of electromagnetic from high-$T_c$ superconductors. We also perform numerical simulations to check the theory and a good agreement is achieved. The radiation pattern of the state with and without kink is calculated, which may serve as a fingerprint of the dynamic state realized by the system. At last, the power radiation of the state with solitons is calculated by simulations. The possible state realized in the recent experiments is discussed in the viewpoint of the theoretical description. The state with kink is important for applications including terahertz generators and amplifiers.

cond-mat.supr-con↗

arXiv2008

Three-Dimensional Phase-Kink State in Thick Stack of Josephson Junctions and Terahertz Radiation

The dynamics of superconductivity phase in thick stack of Josephson junctions with strong inductive coupling, such as the one realized in layered high-$T_c$ cuprates and possibly the recently discovered FeAs-based superconductors, is investigated under a c-axis bias voltage and in the absence of an external magnetic field. The kink state found previously by the present authors is extended to three dimensions for both rectangular and cylindrical geometries. The IV characteristics are calculated and the distributions of electromagnetic field inside the samples are clarified. The solution for a cylindrical mesa exhibits a higher resonating frequency than that of a square mesa with the same linear size by a factor of $\sim 2.4$. More importantly, from the radius dependence of the resonance frequency for the cylinder geometry it is possible to confirm directly the kink state, and thus to reveal the mechanism of the strong radiation discovered in recent experiments.

cond-mat.supr-con↗

arXiv2008

Two Parallel Swendsen-Wang Cluster Algorithms Using Message-Passing Paradigm

In this article, we present two different parallel Swendsen-Wang Cluster(SWC) algorithms using message-passing interface(MPI). One is based on Master-Slave Parallel Model(MSPM) and the other is based on Data-Parallel Model(DPM). A speedup of 24 with 40 processors and 16 with 37 processors is achieved with the DPM and MSPM respectively. The speedup of both algorithms at different temperature and system size is carefully examined both experimentally and theoretically, and a comparison of their efficiency is made. In the last section, based on these two parallel SWC algorithms, two parallel probability changing cluster(PCC) algorithms are proposed.

physics.comp-ph↗

arXiv2008

Computer simulations of two-dimensional melting with dipole-dipole interactions

We perform molecular dynamics and Monte Carlo simulations of two-dimensional melting with dipole-dipole interactions. Both static and dynamic behaviors are examined. In the isotropic liquid phase, the bond orientational correlation length 6 and susceptibility 6 are measured, and the data are fitted to the theoretical ansatz. An algebraic decay is detected for both spatial and temporal bond orientational correlation functions in an intermediate temperature regime, and it provides an explicit evidence for the existence of the hexatic phase. From the finite-size scaling analysis of the global bond orientational order parameter, the disclination unbinding temperature Ti is estimated. In addition, from dynamic Monte Carlo simulations of the positional order parameter, we extract the critical exponents at the dislocation unbinding temperature Tm. All the results are in agreement with those from experiments and support the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory.

cond-mat.soft↗

arXiv2008

Possible Dynamic States in Inductively Coupled Intrinsic Josephson Junctions of Layered High-$T_c$ Superconductors

Based on computer simulations and theoretical analysis, a new dynamic state is found in inductively coupled intrinsic Josephson junctions in the absence of an external magnetic field. In this state, the plasma oscillation is uniform along the c axis and there are $(2m+1)π$ phase kinks, with $m$ being an integer, periodic and thus non-uniform in the $c$ direction. In the IV characteristics, the state manifests itself as current steps occurring at all cavity modes. Inside the current steps, the plasma oscillation becomes strong, which generates several harmonics in frequency spectra at a given voltage. The recent experiments on terahertz radiations from the mesa of a BSCCO single crystal can be explained in terms of this state.

cond-mat.supr-con↗

arXiv2007

Short-time critical dynamics at perfect and non-perfect surface

We report Monte Carlo simulations of critical dynamics far from equilibrium on a perfect and non-perfect surface in the 3d Ising model. For an ordered initial state, the dynamic relaxation of the surface magnetization, the line magnetization of the defect line, and the corresponding susceptibilities and appropriate cumulant is carefully examined at the ordinary, special and surface phase transitions. The universal dynamic scaling behavior including a dynamic crossover scaling form is identified. The exponent $β_1$ of the surface magnetization and $β_2$ of the line magnetization are extracted. The impact of the defect line on the surface universality classes is investigated.

physics.comp-ph↗

arXiv2007

Computer Simulation on Terahertz Emission from Intrinsic Josephson Junctions of High-T_c Superconductors

Solving coupled nonlinear sine-Gordon equations and Maxwell equations numerically, we study the electromagnetic and superconducting properties of the single crystal of high-$T_c$ superconductor $\rm{Bi_2Sr_2CaCu_2O_{8+δ}}$ with a static magnetic field applied parallel to the $ab$-plane and a dc current fed in along the c-axis. Cavity resonances of transverse plasma occur in the intrinsic Josephson junctions with frequencies in terahertz regime. It is revealed that the electromagnetic wave can transmit from the junctions into space. The emitted energy counted by the Poynting vector is about $400\rm{W/cm^2}$. The frequency as well as the energy of emission can be tuned almost continuously by the current and magnetic field.

cond-mat.supr-con↗