arXiv subjects

Wei Han

Publications and source records attributed to Wei Han.

At least 181 records · Page 10Linked to original sources

arXiv2019

Observation of Interfacial Antiferromagnetic Coupling between Magnetic Topological Insulator and Antiferromagnetic Insulator

Inducing magnetic orders in a topological insulator (TI) to break its time reversal symmetry has been predicted to reveal many exotic topological quantum phenomena. The manipulation of magnetic orders in a TI layer can play a key role in harnessing these quantum phenomena towards technological applications. Here we fabricated a thin magnetic TI film on an antiferromagnetic (AFM) insulator Cr2O3 layer and found that the magnetic moments of the magnetic TI layer and the surface spins of the Cr2O3 layers favor interfacial AFM coupling. Field cooling studies show a crossover from negative to positive exchange bias clarifying the competition between the interfacial AFM coupling energy and the Zeeman energy in the AFM insulator layer. The interfacial exchange coupling also enhances the Curie temperature of the magnetic TI layer. The unique interfacial AFM alignment in magnetic TI on AFM insulator heterostructures opens a new route toward manipulating the interplay between topological states and magnetic orders in spin-engineered heterostructures, facilitating the exploration of proof-of-concept TI-based spintronic and electronic devices with multi-functionality and low power consumption.

cond-mat.mtrl-sci↗

arXiv2018

Gauge-potential-induced rotation of spin-orbit-coupled Bose-Einstein condensates

We demonstrate that a spin-orbit-coupled Bose-Einstein condensate can be effectively rotated by adding a real magnetic field to inputting gauge angular momentum, which is distinctly different from the traditional ways of rotation by stirring or Raman laser dressing to inputting canonical angular momentum. The gauge angular momentum is accompanied by the spontaneous generation of equal and opposite canonical angular momentum in the ground states, and it leads to the nucleation of quantized vortices. We explain this by indicating that the effective rotation with the vortex nucleation results from the effective magnetic flux induced by the gauge potential, which is essentially different from the previous scheme of creating vortices by synthetic magnetic fields. In the weakly interacting regime, symmetrically placed domains separated by vortex lines as well as half-integer giant vortices are discovered. With relatively strong interatomic interaction, we predict a structure of coaxially arranged annular vortex arrays, which is in stark contrast to the familiar Abrikosov vortex lattice. The developed way of rotation may be extended to a more general gauge system.

cond-mat.quant-gas↗

arXiv2018

Joint Frequency Reuse and Cache Optimization in Backhaul-Limited Small-Cell Wireless Networks

Caching at base stations (BSs) is a promising approach for supporting the tremendous traffic growth of content delivery over future small-cell wireless networks with limited backhaul. This paper considers exploiting spatial caching diversity (i.e., caching different subsets of popular content files at neighboring BSs) that can greatly improve the cache hit probability, thereby leading to a better overall system performance. A key issue in exploiting spatial caching diversity is that the cached content may not be located at the nearest BS, which means that to access such content, a user needs to overcome strong interference from the nearby BSs; this significantly limits the gain of spatial caching diversity. In this paper, we consider a joint design of frequency reuse and caching, such that the benefit of an improved cache hit probability induced by spatial caching diversity and the benefit of interference coordination induced by frequency reuse can be achieved simultaneously. We obtain a closed-form characterization of the approximate successful transmission probability for the proposed scheme and analyze the impact of key operating parameters on the performance. We design a low-complexity algorithm to optimize the frequency reuse factor and the cache storage allocation. Simulations show that the proposed scheme achieves a higher successful transmission probability than existing caching schemes.

eess.SP↗

arXiv2018

Halos in Dark Ages: formation and chemistry

Formation of halos in the Dark Ages from initial spherical perturbations is analyzed in a four component Universe (dark matter, dark energy, baryonic matter and radiation) in the approximation of relativistic hydrodynamics. Evolution of density and velocity perturbations of each component is obtained by integration of a system of nine differential equations from $z=10^8$ up to virialization, which is described phenomenologically. It is shown that the number density of dark matter halos with masses $M\sim10^8-10^9\,\mathrm{M_{\odot}}$ virialized at $z\sim10$ is close to the number density of galaxies in comoving coordinates. The dynamical dark energy of classical scalar field type does not significantly influence the evolution of the other components, but dark energy with a small value of effective sound speed can affect the final halo state. Simultaneously, the formation/dissociation of the first molecules have been analyzed in the halos which are forming. The results show that number densities of molecules $\rm{H_2}$ and $\rm{HD}$ at the moment of halo virialization are $\sim10^3$ and $\sim400$ times larger, respectively, than on a uniformly expanding background. It is caused by increased density and rates of reactions at quasilinear and nonlinear evolution stages of density and velocity of the baryonic component of halos. It is shown also that the temperature history of the halo is important for calculating the concentration of molecular ions with low binding energy. So, in a halo with virial temperature $\sim10^5$ K the number density of the molecular ion HeH$^+$ is approximately 100 times smaller than that on the cosmological background.

astro-ph.CO↗

arXiv2018

A Strategy of MR Brain Tissue Images' Suggestive Annotation Based on Modified U-Net

Accurate segmentation of MR brain tissue is a crucial step for diagnosis,surgical planning, and treatment of brain abnormalities. However,it is a time-consuming task to be performed by medical experts. So, automatic and reliable segmentation methods are required. How to choose appropriate training dataset from limited labeled dataset rather than the whole also has great significance in saving training time. In addition, medical data labeled is too rare and expensive to obtain extensively, so choosing appropriate unlabeled dataset instead of all the datasets to annotate, which can attain at least same performance, is also very meaningful. To solve the problem above, we design an automatic segmentation method based on U-shaped deep convolutional network and obtain excellent result with average DSC metric of 0.8610, 0.9131, 0.9003 for Cerebrospinal Fluid (CSF), Gray Matter (GM) and White Matter (WM) respectively on the well-known IBSR18 dataset. We use bootstrapping algorithm for selecting the most effective training data and get more state-of-the-art segmentation performance by using only 50% of training data. Moreover, we propose a strategy of MR brain tissue images' suggestive annotation for unlabeled medical data based on the modified U-net. The proposed method performs fast and can be used in clinical.

cs.CV↗

arXiv2018

Chiral Supersolid in Spin-Orbit-Coupled Bose Gases with Soft-Core Long-Range Interactions

Chirality represents a kind of symmetry breaking characterized by the noncoincidence of an object with its mirror image and has been attracting intense attention in a broad range of scientific areas. The recent realization of spin-orbit coupling in ultracold atomic gases provides a new perspective to study quantum states with chirality. In this Letter, we demonstrate that the combined effects of spin-orbit coupling and interatomic soft-core long-range interaction can induce an exotic supersolid phase in which the chiral symmetry is broken with spontaneous emergence of circulating particle current. This implies that a finite angular momentum can be generated with neither rotation nor effective magnetic field. The direction of the angular momentum can be altered by adjusting the strength of spin-orbit coupling or interatomic interaction. The predicted chiral supersolid phase can be experimentally observed in Rydberg-dressed Bose-Einstein condensates with spin-orbit coupling.

cond-mat.quant-gas↗

arXiv2018

Probe of Spin Dynamics in Superconducting NbN Thin Films via Spin Pumping

The emerging field of superconductor (SC) spintronics has attracted intensive attentions recently. Many fantastic spin dependent properties in SC have been discovered, including the observation of large magnetoresistance, long spin lifetimes and the giant spin Hall effect in SC, as well as spin supercurrent in Josephson junctions, etc. Regarding the spin dynamic in SC films, few studies has been reported yet. Here, we report the investigation of the spin dynamics in an s-wave superconducting NbN film via spin pumping from an adjacent insulating ferromagnet GdN layer. A profound coherence peak of the Gilbert damping is observed slightly below the superconducting critical temperature of the NbN layer, which is consistent with recent theoretical studies. Our results further indicate that spin pumping could be a powerful tool for investigating the spin dynamics in 2D crystalline superconductors.

cond-mat.mtrl-sci↗

arXiv2018

Dirac-Surface-State Modulated Spin Dynamics in a Ferrimagnetic Insulator at Room Temperature

This work demonstrates dramatically modified spin dynamics of magnetic insulator (MI) by the spin-momentum locked Dirac surface states of the adjacent topological insulator (TI) which can be harnessed for spintronic applications. As the Bi-concentration x is systematically tuned in 5 nm thick (BixSb1-x)2Te3 TI film, the weight of the surface relative to bulk states peaks at x = 0.32 when the chemical potential approaches the Dirac point. At this concentration, the Gilbert damping constant of the precessing magnetization in 10 nm thick Y3Fe5O12 MI film in the MI/TI heterostructures is enhanced by an order of magnitude, the largest among all concentrations. In addition, the MI acquires additional strong magnetic anisotropy that favors the in-plane orientation with similar Bi-concentration dependence. These extraordinary effects of the Dirac surface states distinguish TI from other materials such as heavy metals in modulating spin dynamics of the neighboring magnetic layer.

cond-mat.mtrl-sci↗

arXiv2018

Quantum Materials for Spin and Charge Conversion

Spintronics aims to utilize the spin degree of freedom for information storage and computing applications. One major issue is the generation and detection of spins via spin and charge conversion. Quantum materials have recently exhibited many unique spin-dependent properties, which can be used as promising material candidates for efficient spin and charge conversion. Here, we review recent findings concerning spin and charge conversion in quantum materials, including Rashba interfaces, topological insulators, two-dimensional materials, superconductors, and non-collinear antiferromagnets. Important progress in using quantum materials for spin and charge conversion could pave the way for developing future spintronics devices.

cond-mat.mtrl-sci↗

arXiv2018

Pressure-induced spin reorientation transition in layered ferromagnetic insulator Cr2Ge2Te6

Anisotropic magnetoresistance (AMR) of Cr2Ge2Te6 (CGT), a layered ferromagnetic insulator, is investigated under an applied hydrostatic pressure up to 2 GPa. The easy axis direction of the magnetization is inferred from the AMR saturation feature in the presence and absence of the applied pressure. At zero applied pressure, the easy axis is along the c-direction or perpendicular to the layer. Upon application of a hydrostatic pressure>1 GPa, the uniaxial anisotropy switches to easy-plane anisotropy which drives the equilibrium magnetization from the c-axis to the ab-plane at zero magnetic field, which amounts to a giant magnetic anisotropy energy change (>100%). As the temperature is increased across the Curie temperature, the characteristic AMR effect gradually decreases and disappears. Our first-principles calculations confirm the giant magnetic anisotropy energy change with moderate pressure and assign its origin to the increased off-site spin-orbit interaction of Te atoms due to a shorter Cr-Te distance. Such a pressure-induced spin reorientation transition is very rare in three-dimensional ferromagnets, but it may be common to other layered ferromagnets with similar crystal structures to CGT, and therefore offers a unique way to control magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2018

Image Super-Resolution via Dual-State Recurrent Networks

Advances in image super-resolution (SR) have recently benefited significantly from rapid developments in deep neural networks. Inspired by these recent discoveries, we note that many state-of-the-art deep SR architectures can be reformulated as a single-state recurrent neural network (RNN) with finite unfoldings. In this paper, we explore new structures for SR based on this compact RNN view, leading us to a dual-state design, the Dual-State Recurrent Network (DSRN). Compared to its single state counterparts that operate at a fixed spatial resolution, DSRN exploits both low-resolution (LR) and high-resolution (HR) signals jointly. Recurrent signals are exchanged between these states in both directions (both LR to HR and HR to LR) via delayed feedback. Extensive quantitative and qualitative evaluations on benchmark datasets and on a recent challenge demonstrate that the proposed DSRN performs favorably against state-of-the-art algorithms in terms of both memory consumption and predictive accuracy.

cs.CV↗

arXiv2018

Experimental Signatures of Spin Superfluid Ground State in Canted Antiferromagnet Cr2O3 via Nonlocal Spin Transport

Spin superfluid is a novel emerging quantum matter arising from the Bose-Einstein condensate (BEC) of spin-1 bosons. We demonstrate the spin superfluid ground state in canted antiferromagnetic Cr2O3 thin film at low temperatures via nonlocal spin transport. A large enhancement of the nonlocal spin signal is observed below ~ 20 K, and it saturates from ~ 5 K down to 2 K. We show that the spins can propagate over very long distances (~ 20 micro meters) in such spin superfluid ground state and the nonlocal spin signal decreases very slowly as the spacing increases with an inverse relationship, which is consistent with theoretical prediction. Furthermore, spin superfluidity has been investigated in the canted antiferromagnetic phase of the (11-20)-oriented Cr2O3 film, where the magnetic field dependence of the associated critical temperature follows a two-thirds power law near the critical point. The experimental demonstration of the spin superfluid ground state in canted antiferromagnet will be extremely important for the fundamental physics on the BEC of spin-1 bosons and paves the way for future spin supercurrent devices, such as spin-Josephson junctions.

cond-mat.mtrl-sci↗

arXiv2018

Role of La doping for Topological Hall Effect in Epitaxial EuO Films

We report the critical role of La doping in the topological Hall effect observed in LaxEu1-xO thin films (~ 50 nm) grown by molecular beam epitaxy. When the La doping exceeds 0.036, topological Hall effect emerges, which we attribute to the formation of magnetic skyrmions. Besides, the La doping is found to play a critical role in determining the phases, densities, and sizes of the skyrmions in the LaxEu1-xO thin films. The maximum region of the skyrmion phase diagram is observed on the La0.1Eu0.9O thin film. As the La doping increases, the skyrmion density increases while the skyrmion size decreases. Our findings demonstrate the important role of La doping for the skyrmions in EuO films, which could be important for future studies of magnetic skyrmions in Heisenberg ferromagnets.

cond-mat.mtrl-sci↗

arXiv2018

Hierarchical environment-assisted dynamical speedup control

We investigate the qubit in the hierarchical environment where the first level is just one lossy cavity while the second level is the N-coupled lossy cavities. In the weak coupling regime between the qubit and the first level environment, the dynamics crossovers from the original Markovian to the new non-Markovian and from no-speedup to speedup can be realized by controlling the hierarchical environment, i.e., manipulating the number of cavities or the coupling strength between two nearest-neighbor cavities in the second level environment. And we find that the coupling strength between two nearest-neighbor cavities and the number of cavities in the second level environment have the opposite effect on the non-Markovian dynamics and speedup evolution of the qubit. In addition, in the case of strong coupling between the qubit and the first level environment, we can be surprised to find that, compared with the original non-Markovian dynamics, the added second level environment cannot play a beneficial role on the speedup of the dynamics of the system.

quant-ph↗

arXiv2018

Learning $3$D-FilterMap for Deep Convolutional Neural Networks

We present a novel and compact architecture for deep Convolutional Neural Networks (CNNs) in this paper, termed $3$D-FilterMap Convolutional Neural Networks ($3$D-FM-CNNs). The convolution layer of $3$D-FM-CNN learns a compact representation of the filters, named $3$D-FilterMap, instead of a set of independent filters in the conventional convolution layer. The filters are extracted from the $3$D-FilterMap as overlapping $3$D submatrics with weight sharing among nearby filters, and these filters are convolved with the input to generate the output of the convolution layer for $3$D-FM-CNN. Due to the weight sharing scheme, the parameter size of the $3$D-FilterMap is much smaller than that of the filters to be learned in the conventional convolution layer when $3$D-FilterMap generates the same number of filters. Our work is fundamentally different from the network compression literature that reduces the size of a learned large network in the sense that a small network is directly learned from scratch. Experimental results demonstrate that $3$D-FM-CNN enjoys a small parameter space by learning compact $3$D-FilterMaps, while achieving performance compared to that of the baseline CNNs which learn the same number of filters as that generated by the corresponding $3$D-FilterMap.

cs.LG↗

arXiv2017

The Role of Oxygen in Ionic Liquid Gating on 2D Cr2Ge2Te6: a Non-Oxide Material

Ionic liquid gating can markedly modulate the materials' carrier density so as to induce metallization, superconductivity, and quantum phase transitions. One of the main issues is whether the mechanism of ionic liquid gating is an electrostatic field effect or an electrochemical effect, especially for oxide materials. Recent observation of the suppression of the ionic liquid gate-induced metallization in the presence of oxygen for oxide materials suggests the electrochemical effect. However, in more general scenarios, the role of oxygen in ionic liquid gating effect is still unclear. Here, we perform the ionic liquid gating experiments on a non-oxide material: two-dimensional ferromagnetic Cr2Ge2Te6. Our results demonstrate that despite the large increase of the gate leakage current in the presence of oxygen, the oxygen does not affect the ionic liquid gating effect (< 5 % difference), which suggests the electrostatic field effect as the mechanism on non-oxide materials. Moreover, our results show that the ionic liquid gating is more effective on the modulation of the channel resistances compared to the back gating across the 300 nm thick SiO2.

cond-mat.mtrl-sci↗

arXiv2017

Observation of long phase-coherence length in epitaxial La-doped CdO thin films

The search for long electron phase coherence length, which is the length that an electron can keep its quantum wave-like properties, has attracted considerable interest in the last several decades. Here, we report the long phase coherence length of ~ 3.7 micro meters in La-doped CdO thin films at 2 K. Systematical investigations of the La doping and the temperature dependences of the electron mobility and the electron phase coherence length reveal contrasting scattering mechanisms for these two physical properties. Furthermore, these results show that the oxygen vacancies could be the dominant scatters in CdO thin films that break the electron phase coherence, which would shed light on further investigation of phase coherence properties in oxide materials.

cond-mat.mtrl-sci↗

arXiv2017

Dilated Recurrent Neural Networks

Learning with recurrent neural networks (RNNs) on long sequences is a notoriously difficult task. There are three major challenges: 1) complex dependencies, 2) vanishing and exploding gradients, and 3) efficient parallelization. In this paper, we introduce a simple yet effective RNN connection structure, the DilatedRNN, which simultaneously tackles all of these challenges. The proposed architecture is characterized by multi-resolution dilated recurrent skip connections and can be combined flexibly with diverse RNN cells. Moreover, the DilatedRNN reduces the number of parameters needed and enhances training efficiency significantly, while matching state-of-the-art performance (even with standard RNN cells) in tasks involving very long-term dependencies. To provide a theory-based quantification of the architecture's advantages, we introduce a memory capacity measure, the mean recurrent length, which is more suitable for RNNs with long skip connections than existing measures. We rigorously prove the advantages of the DilatedRNN over other recurrent neural architectures. The code for our method is publicly available at https://github.com/code-terminator/DilatedRNN

cs.AI↗