arXiv subjects

Jie Shan

Publications and source records attributed to Jie Shan.

At least 91 records · Page 5Linked to original sources

arXiv2020

Spatially mixed moiré excitons in two-dimensional van der Waals superlattices

Moiré superlattices open an unprecedented opportunity for tailoring interactions between quantum particles and their coupling to electromagnetic fields. Strong superlattice potential generates moiré minibands of excitons -- bound pairs of electrons and holes that reside either in a single layer (intralayer excitons) or two separate layers (interlayer excitons). The twist-angle-controlled interlayer hybridization of carriers can also mix the two types of excitons to combine the strengths of both. Here, we report a direct observation of spatially mixed moiré excitons in angle-aligned WSe2/WS2 and MoSe2/WS2 superlattices by optical reflectance spectroscopy. The strongly interacting interlayer and intralayer moiré excitons in WSe2/WS2 manifest energy level anticrossing and oscillator strength redistribution under a vertical electric field. We also observe doping-dependent exciton miniband renormalization and mixing near half filling of the first electron miniband of WS2. Our findings have significant implications for emerging correlated states in two-dimensional semiconductors, such as exciton condensates and Bose-Hubbard models, and optoelectronic applications of these materials.

cond-mat.mtrl-sci↗

arXiv2020

Gate-tunable spin waves in antiferromagnetic atomic bilayers

The emergence of two-dimensional (2D) layered magnetic materials has opened an exciting playground for both fundamental studies of magnetism in 2D and explorations of spinbased applications. Remarkable properties, including spin filtering in magnetic tunnel junctions and gate control of magnetic states, have recently been demonstrated in 2D magnetic materials. While these studies focus on the static properties, dynamic magnetic properties such as excitation and control of spin waves have remained elusive. Here we excite spin waves and probe their dynamics in antiferromagnetic CrI3 bilayers by employing an ultrafast optical pump/magneto-optical Kerr probe technique. We identify sub-terahertz magnetic resonances under an in-plane magnetic field, from which we determine the anisotropy and interlayer exchange fields and the spin damping rates. We further show tuning of antiferromagnetic resonances by tens of gigahertz through electrostatic gating. Our results shed light on magnetic excitations and spin dynamics in 2D magnetic materials, and demonstrate their unique potential for applications in ultrafast data storage and processing.

cond-mat.mes-hall↗

arXiv2020

Two-dimensional magnetic nanoelectromechanical resonators

Two-dimensional (2D) layered materials possess outstanding mechanical, electronic and optical properties, making them ideal materials for nanoelectromechanical applications. The recent discovery of 2D magnetic materials has promised a new class of magnetically active nanoelectromechanical systems (NEMS). Here we demonstrate resonators made of 2D CrI3, whose mechanical resonances depend on the magnetic state of the material. We quantify the underlining effects of exchange and anisotropy magnetostriction by measuring the field dependence of the resonance frequency under a magnetic field parallel and perpendicular to the easy axis, respectively. Furthermore, we show efficient strain tuning of magnetism in 2D CrI3 as a result of the inverse magnetostrictive effect using the NEMS platform. Our results establish the basis for mechanical detection of magnetism and magnetic phase transitions in 2D layered magnetic materials. The new magnetic NEMS may also find applications in magnetic actuation and sensing.

cond-mat.mes-hall↗

arXiv2019

Imaging and control of critical spin fluctuations in two-dimensional magnets

Strong spin fluctuations are expected near the thermodynamic critical point of a continuous magnetic phase transition. Such critical spin fluctuations are highly correlated and in principle can occur at any time- and length-scales; they govern critical phenomena and potentially can drive new phases. Although theoretical studies have been made for decades, direct observation of critical spin fluctuations remains elusive. The recent discovery of two-dimensional (2D) layered magnets, in which spin fluctuations significantly modify magnetic properties as compared to their bulk counterparts and integration into heterostructures and devices can be easily achieved, provides an ideal platform to investigate and harness critical spin phenomena. Here we develop a fast and sensitive magneto-optical imaging microscope to achieve wide-field, real-time monitoring of critical spin fluctuations in single-layer CrBr3, which is a 2D ferromagnetic insulator. We track the critical phenomena directly from the fluctuation correlations and observe both slowing-down dynamics and enhanced correlation length. Through real-time feedback control of critical spin fluctuations, we further achieve switching of magnetic states solely by electrostatic gating without applying a magnetic field or a Joule current. The ability to directly image and control critical spin fluctuations in 2D magnets opens up exciting opportunities to explore critical phenomena and develop applications in nanoscale engines and information science.

cond-mat.mes-hall↗

arXiv2019

Magneto-memristive switching in a two-dimensional layer antiferromagnet

Memristive devices whose resistance can be hysteretically switched by electric field or current are intensely pursued both for fundamental interest as well as potential applications in neuromorphic computing and phase-change memory. When the underlying material exhibits additional charge or spin order, the resistive states can be directly coupled, further allowing for electrical control of the collective phases. Here, we report the observation of abrupt, memristive switching of tunneling current in nanoscale junctions of ultrathin CrI$_3$, a natural layer antiferromagnet. The coupling to spin order enables both tuning of the resistance hysteresis by magnetic field, and electric-field switching of magnetization even in multilayer samples.

cond-mat.mtrl-sci↗

arXiv2019

WSe2/WS2 moiré superlattices: a new Hubbard model simulator

The Hubbard model, first formulated by physicist John Hubbard in the 1960s, is a simple theoretical model of interacting quantum particles in a lattice. The model is thought to capture the essential physics of high-temperature superconductors, magnetic insulators, and other complex emergent quantum many-body ground states. Although the Hubbard model is greatly simplified as a representation of most real materials, it has nevertheless proved difficult to solve accurately except in the one-dimensional case. Physical realization of the Hubbard model in two or three dimensions, which can act as quantum simulators, therefore have a vital role to play in solving the strong-correlation puzzle. Here we obtain a quantum phase diagram of the two-dimensional triangular lattice Hubbard model by studying angle-aligned WSe2/WS2 bilayers, which form moiré superlattices because of the difference in lattice constant between the two two-dimensional materials. We probe both charge and magnetic properties of the system by measuring the dependence of optical response on out-of-plane magnetic field, and on gate-tuned carrier density. At half filling of the first hole moiré superlattice band, we observe a Mott insulating state with antiferromagnetic Curie-Weiss behavior as expected for a Hubbard model in the strong interaction regime. Past half filling, our experiment suggests an antiferromagnetic to paramagnetic quantum phase transition near 0.6 filling. Our results establish a new solid-state platform based on moiré superlattices which can be used to simulate outstanding problems in strong correlation physics that are manifested by triangular lattice Hubbard models.

cond-mat.mtrl-sci↗

arXiv2019

Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide $β$-MoTe$_2$

Single-crystal materials with sufficiently low crystal symmetry and strong spin-orbit interactions can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets, such as the out-of-plane antidamping torque previously observed in WTe$_2$/ferromagnet heterostructures. Here, we present measurements of spin-orbit torques produced by the low-symmetry material $β$-MoTe$_2$, which unlike WTe$_2$ retains bulk inversion symmetry. We measure spin-orbit torques on $β$-MoTe$_2$/Permalloy heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and MoTe$_2$ thickness down to the monolayer limit. We observe an out-of-plane antidamping torque with a spin torque conductivity as strong as 1/3 of that of WTe$_2$, demonstrating that the breaking of bulk inversion symmetry in the spin-generation material is not a necessary requirement for producing an out-of-plane antidamping torque. We also measure an unexpected dependence on the thickness of the $β$-MoTe$_2$ -- the out-of-plane antidamping torque is present in MoTe$_2$/Permalloy heterostructures when the $β$-MoTe$_2$ is a monolayer or trilayer thick, but goes to zero for devices with bilayer $β$-MoTe$_2$.

cond-mat.mes-hall↗

arXiv2019

Pressure-controlled interlayer magnetism in atomically thin CrI3

Stacking order can significantly influence the physical properties of two-dimensional (2D) van der Waals materials. The recent isolation of atomically thin magnetic materials opens the door for control and design of magnetism via stacking order. Here we apply hydrostatic pressure up to 2 GPa to modify the stacking order in a prototype van der Waals magnetic insulator CrI3. We observe an irreversible interlayer antiferromagnetic (AF) to ferromagnetic (FM) transition in atomically thin CrI3 by magnetic circular dichroism and electron tunneling measurements. The effect is accompanied by a monoclinic to a rhombohedral stacking order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer AF coupling energy can be tuned up by nearly 100% by pressure. Our experiment reveals interlayer FM coupling, which is the established ground state in bulk CrI3, but never observed in native exfoliated thin films. The observed correlation between the magnetic ground state and the stacking order is in good agreement with first principles calculations and suggests a route towards nanoscale magnetic textures by moiré engineering.

cond-mat.mes-hall↗

arXiv2019

Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides

We conduct a comprehensive study of three different magnetic semiconductors, CrI$_3$, CrBr$_3$, and CrCl$_3$, by incorporating both few- and bi-layer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, as well as magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr$_3$, with transition temperature still close to that of the bulk.

cond-mat.mtrl-sci↗

arXiv2019

Material Segmentation of Multi-View Satellite Imagery

Material recognition methods use image context and local cues for pixel-wise classification. In many cases only a single image is available to make a material prediction. Image sequences, routinely acquired in applications such as mutliview stereo, can provide a sampling of the underlying reflectance functions that reveal pixel-level material attributes. We investigate multi-view material segmentation using two datasets generated for building material segmentation and scene material segmentation from the SpaceNet Challenge satellite image dataset. In this paper, we explore the impact of multi-angle reflectance information by introducing the \textit{reflectance residual encoding}, which captures both the multi-angle and multispectral information present in our datasets. The residuals are computed by differencing the sparse-sampled reflectance function with a dictionary of pre-defined dense-sampled reflectance functions. Our proposed reflectance residual features improves material segmentation performance when integrated into pixel-wise and semantic segmentation architectures. At test time, predictions from individual segmentations are combined through softmax fusion and refined by building segment voting. We demonstrate robust and accurate pixelwise segmentation results using the proposed material segmentation pipeline.

cs.CV↗

arXiv2019

Long valley lifetime of dark excitons in single-layer WSe2

Single-layer transition metal dichalcogenides (TMDs) provide a promising material system to explore the electron's valley degree of freedom as a quantum information carrier. The valley degree of freedom in single-layer TMDs can be directly accessed by means of optical excitation. The rapid valley relaxation of optically excited electron-hole pairs (excitons) through the long-range electron-hole exchange interaction, however, has been a major roadblock. Theoretically such a valley relaxation does not occur for the recently discovered dark excitons, suggesting a potential route for long valley lifetimes. Here we investigate the valley dynamics of dark excitons in single-layer WSe2 by time-resolved photoluminescence spectroscopy. We develop a waveguide-based method to enable the detection of the dark exciton emission, which involves spin-forbidden optical transitions with an out-of-plane dipole moment. The valley degree of freedom of dark excitons is accessed through the valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a short valley lifetime for the dark neutral exciton, likely due to the short-range electron-hole exchange, but long valley lifetimes exceeding several nanoseconds for dark charged excitons.

cond-mat.mes-hall↗

arXiv2019

Probing Many-Body Interactions in Monolayer Transition-Metal Dichalcogenides

Many-body interactions in monolayer transition-metal dichalcogenides are strongly affected by their unique band structure. We study these interactions by measuring the energy shift of neutral excitons (bound electron-hole pairs) in gated WSe$_2$ and MoSe$_2$. Surprisingly, while the blueshift of the neutral exciton, $X^0$, in electron-doped samples can be more than 10~meV, the blueshift in hole-doped samples is nearly absent. Taking into account dynamical screening and local-field effects, we present a transparent and analytical model that elucidates the crucial role played by intervalley plasmons in electron-doped conditions. The energy shift of $X^0$ as a function of charge density is computed showing agreement with experiment, where the renormalization of $X^0$ by intervalley plasmons yields a stronger blueshift in MoSe$_2$ than in WSe$_2$ due to differences in their band ordering.

cond-mat.mtrl-sci↗

arXiv2018

An unusual continuous paramagnetic-limited superconducting phase transition in 2D NbSe$_2$

Time reversal and spatial inversion are two key symmetries for conventional Bardeen-Cooper-Schrieffer (BCS) superconductivity. Breaking inversion symmetry can lead to mixed-parity Cooper pairing and unconventional superconducting properties. Two-dimensional (2D) NbSe$_2$ has emerged as a new non-centrosymmetric superconductor with the unique out-of-plane or Ising spin-orbit coupling (SOC). Here, we report the observation of an unusual continuous paramagnetic-limited superconductor-normal metal transition in 2D NbSe$_2$. Using tunneling spectroscopy under high in-plane magnetic fields, we observe a continuous closing of the superconducting gap at the upper critical field at low temperatures, in stark contrast to the abrupt first-order transition observed in BCS thin film superconductors. The paramagnetic-limited continuous transition arises from a large spin susceptibility of the superconducting phase due to the Ising SOC. The result is further supported by self-consistent mean-field calculations based on the ab initio band structure of 2D NbSe$_2$. Our findings establish 2D NbSe$_2$ as a promising platform for exploring novel spin-dependent superconducting phenomena and device concepts, such as equal-spin Andreev reflection and topological superconductivity.

cond-mat.supr-con↗

arXiv2018

Observation of the nonlinear anomalous Hall effect in 2D WTe2

The Hall effect occurs only in systems with broken time-reversal symmetry, such as solids under an external magnetic field in the ordinary Hall effect and magnetic materials in the anomalous Hall effect (AHE). Here we show a new Hall effect in a nonmagnetic material under zero magnetic field, in which the Hall voltage depends quadratically on the longitudinal current. We observe the effect (referred to as nonlinear AHE) in two-dimensional Td-WTe2, a semimetal with broken inversion symmetry and only one mirror line in the crystal plane. Our angle-resolved electrical measurements reveal that the Hall voltage changes sign when the bias current reverses direction; it maximizes (vanishes) when the bias current is perpendicular (parallel) to the mirror line. The observed effect can be understood as an AHE induced by the bias current which generates an out-of-plane magnetization. The temperature dependence of the Hall conductivity further suggests that both intrinsic Berry curvature dipole and extrinsic spin-dependent scatterings contribute to the observed nonlinear AHE. Our results open the possibility of exploring the intrinsic Berry curvature effect in nonlinear electrical transport in solids .

cond-mat.mes-hall↗

arXiv2018

Spin transistor built on 2D van der Waals heterostructures

Spin transistors (whose on-off operation is achieved by electric-field-controlled spin orientation 1), if realized, can revolutionize modern electronics through the implementation of a faster and a more energy-efficient performance as well as non-volatile data storage 2, 3. The original proposal by Datta and Das 1 that relies on electric-field-controlled spin precession in a semiconductor channel faces significant challenges including inefficient spin injection, spin relaxation and spread of the spin precession angle 4, 5. Recent demonstration of electric-field switching of magnetic order 6-8 and spin filtering 9-12 in two-dimensional magnetic insulator CrI3 has inspired a new operational principle for spin transistors. Here we demonstrate spin field-effect transistors based on dual-gated graphene/CrI3 tunnel junctions. These devices show an ambipolar transistor behavior and tunnel magnetoresistance widely tunable by gating when the CrI3 magnetic tunnel barrier undergoes an antiferromagnetic-ferromagnetic spin-flip transition. Under a constant magnetic bias in the vicinity of the spin-flip transition, the gate voltage can repeatedly alter the device between a high and a low conductance state with a large hysteresis. This new spin transistor concept based on the electric-field-controlled spin-flip transition in the magnetic tunnel barrier is immune to interface imperfections and allows spin injection, control and detection in a single device.

cond-mat.mes-hall↗

arXiv2018

Electric-field switching of two-dimensional van der Waals magnets

Controlling magnetism by purely electrical means is a key challenge to better information technology1. A variety of material systems, including ferromagnetic (FM) metals2,3,4, FM semiconductors5, multiferroics6,7,8 and magnetoelectric (ME) materials9,10, have been explored for the electric-field control of magnetism. The recent discovery of two-dimensional (2D) van der Waals magnets11,12 has opened a new door for the electrical control of magnetism at the nanometre scale through a van der Waals heterostructure device platform13. Here we demonstrate the control of magnetism in bilayer CrI3, an antiferromagnetic (AFM) semiconductor in its ground state12, by the application of small gate voltages in field-effect devices and the detection of magnetization using magnetic circular dichroism (MCD) microscopy. The applied electric field creates an interlayer potential difference, which results in a large linear ME effect, whose sign depends on the interlayer AFM order. We also achieve a complete and reversible electrical switching between the interlayer AFM and FM states in the vicinity of the interlayer spin-flip transition. The effect originates from the electric-field dependence of the interlayer exchange bias.

cond-mat.mes-hall↗

arXiv2018

Valley-selective exciton bistability in a suspended monolayer semiconductor

We demonstrate robust power- and wavelength-dependent optical bistability in fully suspended monolayers of WSe2 near the exciton resonance. Bistability has been achieved under continuous-wave optical excitation at an intensity level of 10^3 W/cm^2. The observed bistability is originated from a photo-thermal mechanism, which provides both optical nonlinearity and passive feedback, two essential elements for optical bistability. Under a finite magnetic field, the exciton bistability becomes helicity dependent, which enables repeatable switching of light purely by its polarization.

cond-mat.mes-hall↗

arXiv2018

Controlling magnetism in 2D CrI3 by electrostatic doping

The atomic thickness of two-dimensional (2D) materials provides a unique opportunity to control material properties and engineer new functionalities by electrostatic doping. Electrostatic doping has been demonstrated to tune the electrical and optical properties of 2D materials in a wide range, as well as to drive the electronic phase transitions. The recent discovery of atomically thin magnetic insulators has opened up the prospect of electrical control of magnetism and new devices with unprecedented performance. Here we demonstrate control of the magnetic properties of monolayer and bilayer CrI3 by electrostatic doping using a dual-gate field-effect device structure. In monolayer CrI3, doping significantly modifies the saturation magnetization, coercive force and Curie temperature, showing strengthened (weakened) magnetic order with hole (electron) doping. Remarkably, in bilayer CrI3 doping drastically changes the interlayer magnetic order, causing a transition from an antiferromagnetic ground state in the pristine form to a ferromagnetic ground state above a critical electron density. The result reveals a strongly doping-dependent interlayer exchange coupling, which enables robust switching of magnetization in bilayer CrI3 by small gate voltages.

cond-mat.mtrl-sci↗