arXiv subjects

Xuefeng Wang

Publications and source records attributed to Xuefeng Wang.

At least 55 records · Page 3Linked to original sources

arXiv2020

A Gd@C82-based single molecular electret device with switchable electrical polarization

Single molecular electrets exhibiting single molecule electric polarization switching have been long desired as a platform for extremely small non-volatile storage devices, although it is controversial because of the poor stability of single molecular electric dipoles. Here we study the single molecular device of GdC82, where the encapsulated Gd atom forms a charge center, and we have observed a gate controlled switching behavior between two sets of single electron transport stability diagrams. The switching is operated in a hysteresis loop with a coercive gate field of around 0.5Vnm. Theoretical calculations have assigned the two conductance diagrams to corresponding energy levels of two states that the Gd atom is trapped at two different sites of the C82 cage, which possess two different permanent electrical dipole orientations. The two dipole states are stabilized by the anisotropic energy and separated by a transition energy barrier of 70 meV. Such switching is then accessed to the electric field driven reorientation of individual dipole while overcoming the barriers by the coercive gate field, and demonstrates the creation of a single molecular electret.

cond-mat.mes-hall↗

arXiv2020

Investigation of plasmonic evolution of atomically size-selected Au clusters by electron energy loss spectrum--from solid state to molecular scale

Versatile quantum modes emerge for plasmon describing the collective oscillations of free electrons in metallic nanoparticles when the particle sizes are greatly reduced. Rather than traditional nanoscale study, the understanding of quantum plasmon desires extremal atomic control of the nanoparticles, calling for size dependent plasmon measurement over a series of nanoparticles with atomically adjustable atom number over several orders of magnitude. Here we report the N dependent plasmonic evolution of atomically size selected gold particles with N= 100 70000 using electron energy loss (EEL) spectroscopy in a scanning transmission electron microscope. The EEL mapping assigns a feature at 2.7 eV as the bulk plasmon and another at 2.4 eV as surface plasmon, which evolution reveals three regimes. When N decreases from 70000 to 887, the bulk plasmon stays unchanged while the surface plasmon exhibits a slight red shift from 2.4 to 2.3 eV. It can be understood by the dominance of classical plasmon physics and electron boundary scattering induced retardation. When N further decreases from 887 to 300, the bulk plasmon disappears totally and the surface plasmon shows a steady blueshift, which indicates that the quantum confinement emerges and modifies the intraband transition. When N 100 300, the plasmon is split to three fine features, which is attributed to superimposed single electron transitions between the quantized molecular like energy level by the time dependent density functional theory calculations. The surface plasmon's excitation ratio has a scaling law with an exponential dependence on N ( N^0.669), essentially the square of the radius. A unified evolution picture from the classical to quantum, molecular plasmon is thus demonstrated.

physics.atm-clus↗

arXiv2019

Observation of Shubnikov-de Haas Oscillations in Large-Scale Weyl Semimetal WTe2 Films

Topological Weyl semimetal WTe2 with large-scale film form has a promising prospect for new-generation spintronic devices. However, it remains a hard task to suppress the defect states in large-scale WTe2 films due to the chemical nature. Here, we significantly improve the crystalline quality and remove the Te vacancies in WTe2 films by post annealing. We observe the distinct Shubnikov-de Haas quantum oscillations in WTe2 films. The nontrivial Berry phase can be revealed by Landau fan diagram analysis. The Hall mobility of WTe2 films can reach 1245 cm2V-1s-1 and 1423 cm2V-1s-1 for holes and electrons with the carrier density of 5 * 10^19 cm^-3 and 2 * 10^19 cm^-3, respectively. Our work provides a feasible route to obtain high-quality Weyl semimetal films for the future topological quantum device applications.

cond-mat.mtrl-sci↗

arXiv2019

Stack Pressure Considerations for Room Temperature All-Solid-State Lithium Metal Batteries

All-solid-state batteries are expected to enable batteries with high energy density with the use of lithium metal anodes. Although solid electrolytes are believed to be mechanically strong enough to prevent lithium dendrites from propagating, various reports today still show cell failure due to lithium dendritic growth at room temperature. While cell parameters such as current density, electrolyte porosity and interfacial properties have been investigated, mechanical properties of lithium metal and the role of applied stack pressure on the shorting behavior is still poorly understood. Here, we investigated failure mechanisms of lithium metal in all-solid-state batteries as a function of stack pressure, and conducted in situ characterization of the interfacial and morphological properties of the buried lithium in solid electrolytes. We found that a low stack pressure of 5 MPa allows reliable plating and stripping in a lithium symmetric cell for more than 1000 hours, and a Li | Li6PS5Cl | LiNi0.80Co0.15Al0.05O2 full cell, plating more than 4 um of lithium per charge, is able to cycle over 200 cycles at room temperature. These results suggest the possibility of enabling the lithium metal anode in all-solid-state batteries at reasonable stack pressures.

physics.app-ph↗

arXiv2019

Intrinsic magnetic topological insulator phases in the Sb doped MnBi2Te4 bulks and thin flakes

Magnetic topological insulators (MTIs) offer a combination of topologically nontrivial characteristics and magnetic order and show promise in terms of potentially interesting physical phenomena such as the quantum anomalous Hall (QAH) effect and topological axion insulating states. However, the understanding of their properties and potential applications have been limited due to a lack of suitable candidates for MTIs. Here, we grow two-dimensional single crystals of Mn(SbxBi(1-x))2Te4 bulk and exfoliate them into thin flakes in order to search for intrinsic MTIs. We perform angle-resolved photoemission spectroscopy, low-temperature transport measurements, and first-principles calculations to investigate the band structure, transport properties, and magnetism of this family of materials, as well as the evolution of their topological properties. We find that there exists an optimized MTI zone in the Mn(SbxBi(1-x))2Te4 phase diagram, which could possibly host a high-temperature QAH phase, offering a promising avenue for new device applications.

cond-mat.mtrl-sci↗

arXiv2019

Comparison of deuterium retention in tungsten exposed to deuterium plasma and gas

Deuterium(D) retention behavior in tungsten(W) exposed to deuterium plasma and gas was studied by means of thermal desorption spectroscopy (TDS): deuterium plasma exposure in which W was exposed to D plamsa with 35 eV/D at 393 K to the fluence of 3.8E24 D/m2; D2 gas charging in which W was exposed to D2 gas of 500 kPa at 773 K for 4 hours. TDS shows that the total D retention in plasma exposure W is 1.00E22 D/m2, one order of magnitude higher than that of gas charging W; however, the D2 desorption peak of gas charging W is 952 K, much higher than 691 K of plasma exposure W. The detrapping energies of deuterium were determined experimentally from the measured peak temperatures at different heating rates and were found to be 2.17 eV for gas charging W and 1.04 eV for plasma exposure W, respectively.

cond-mat.mtrl-sci↗

arXiv2019

Ultrasensitive charge detection utilizing coupled nonlinear micromechanical resonators

Since the discovery of electrons, an accurate detection of electrical charges has been the dream of scientific community. Due to some remarkable advantages, micro/nano-electromechanical systems (M/NEMS) based resonators have been used to design electrometers with exquisite sensitivity and resolution. Inevitably, some limits including requisite ultra-low environmental temperature, complicated resonator structure and measurement circuit, required linear dynamic response, will cause a gap with respect to practical application. Here, we demonstrate a novel ultra-sensitive charge detection based on the linear dependence of peak frequency drift on the coupling voltage variation of two coupled nonlinear micromechanical resonators. We achieved ultra-high resolution of 2.051E-3 fC (about 13 electrons) of charge detection at the room temperature. We also show an extra application of this device for weak and ultra-low-frequency (ULF) signal detection. Our findings provide a simple strategy for measuring electron charges in an extreme accuracy and developing electrometers in smaller sizes.

cond-mat.mes-hall↗

arXiv2019

Quantifying Inactive Lithium in Lithium Metal Batteries

Inactive lithium (Li) formation is the immediate cause of capacity loss and catastrophic failure of Li metal batteries. However, the chemical component and the atomic level structure of inactive Li have rarely been studied due to the lack of effective diagnosis tools to accurately differentiate and quantify Li+ in solid electrolyte interphase (SEI) components and the electrically isolated unreacted metallic Li0, which together comprise the inactive Li. Here, by introducing a new analytical method, Titration Gas Chromatography (TGC), we can accurately quantify the contribution from metallic Li0 to the total amount of inactive Li. We uncover that the Li0, rather than the electrochemically formed SEI, dominates the inactive Li and capacity loss. Using cryogenic electron microscopies to further study the microstructure and nanostructure of inactive Li, we find that the Li0 is surrounded by insulating SEI, losing the electronic conductive pathway to the bulk electrode. Coupling the measurements of the Li0 global content to observations of its local atomic structure, we reveal the formation mechanism of inactive Li in different types of electrolytes, and identify the true underlying cause of low Coulombic efficiency in Li metal deposition and stripping. We ultimately propose strategies to enable the highly efficient Li deposition and stripping to enable Li metal anode for next generation high energy batteries.

cond-mat.mtrl-sci↗

arXiv2019

Experimental observation of the gate-controlled reversal of the anomalous Hall effect in the intrinsic magnetic topological insulator MnBi2Te4 device

Here we report the reserved anomalous Hall effect (AHE) in the 5-septuple-layer van der Waals device of the intrinsic magnetic topological insulator MnBi2Te4. By employing the top/bottom gate, a negative AHE loop gradually decreases to zero and changes to a reversed sign. The reversed AHE exhibits distinct coercive fields and temperature dependence from the previous AHE. It reaches the maximum inside the gap of the Dirac cone. The newly-seen reversed AHE is attributed to the competition of the intrinsic Berry curvature and the Dirac-gap enhanced extrinsic skew scattering. Its gate-controlled switching contributes a scheme for the topological spin field-effect transistors.

cond-mat.mes-hall↗

arXiv2018

Phase transition and anomalous scaling in the quantum Hall transport of topological insulator Sn-Bi1.1Sb0.9Te2S devices

The scaling physics of quantum Hall transport in optimized topological insulators with a plateau precision of ~1/1000 e2/h is considered. Two exponential scaling regimes are observed in temperature-dependent transport dissipation, one of which accords with thermal activation behavior with a gap of 2.8 meV (> 20 K), the other being attributed to variable range hopping (1-20 K). Magnetic field-driven plateau-to-plateau transition gives scaling relations of (dR$_{xy}$/dB)$^{max}$ \propto T$^{-κ}$ and \DeltaB$^{-1}$ \propto T$^{-κ}$ with a consistent exponent of κ~ 0.2, which is half the universal value for a conventional two-dimensional electron gas. This is evidence of percolation assisted by quantum tunneling, and reveals the dominance of electron-electron interaction of the topological surface states.

cond-mat.mes-hall↗

arXiv2018

Direct demonstration of the emergent magnetism resulting from the multivalence Mn in a LaMnO3 epitaxial thin film system

Atomically engineered oxide heterostructures provide a fertile ground for creating novel states. For example, a two-dimensional electron gas at the interface between two oxide insulators, giant thermoelectric Seebeck coefficient, emergent ferromagnetism from otherwise nonmagnetic components, and colossal ionic conductivity. Extensive research efforts reveal that oxygen deficiency or lattice strain play an important role in determining these unexpected properties. Herein, by studying the abrupt presence of robust ferromagnetism (up to 1.5 uB/Mn) in LaMnO3-based heterostructures, we find the multivalence states of Mn that play a decisive role in the emergence of ferromagnetism in the otherwise antiferromagnetic LaMnO3 thin films. Combining spatially resolved electron energy-loss spectroscopy, X-ray absorption spectroscopy and X-ray magnetic circular dichroism techniques, we determine unambiguously that the ferromagnetism results from a conventional Mn3+-O-Mn4+ double-exchange mechanism rather than an interfacial effect. In contrast, the magnetic dead layer of 5 unit cell in proximity to the interface is found to be accompanied with the accumulation of Mn2+ induced by electronic reconstruction. These findings provide a hitherto-unexplored multivalence state of Mn on the emergent magnetism in undoped manganite epitaxial thin films, such as LaMnO3 and BiMnO3, and shed new light on all-oxide spintronic devices.

cond-mat.mtrl-sci↗

arXiv2018

On existence of out-of-plane equilibrium points in restricted three-body problem with oblateness

We analyze in this paper the existence of the "out-of-plane" equilibrium points in the restricted three-body problem with oblateness. From the series expansion of the potential function of an oblate asteroid, we show analytically all equilibrium points locate on the orbital plane of primaries and how artificial equilibrium points may arise due to an inappropriate application of the potential function. Using the closed form of the potential of a triaxial ellipsoid, we analytically demonstrate that the gravitational acceleration in $z$-direction is always pointing toward the equatorial plane, thus it could not be balanced out at any value of $z\neq 0$ and the out-of-plane equilibrium points cannot exist. The out-of-plane equilibrium points appear only when additional acceleration other than the gravitation from primaries is taken into account. We suggest that special attention must be paid to the application of the spherical harmonics expansion of potential to find the equilibrium points, especially when these points may be very close to the celestial body.

astro-ph.EP↗

arXiv2018

Topological Phase Transition-Induced Tri-Axial Vector Magnetoresistance in (Bi1-xInx)2Se3 Nanodevices

We report the study of a tri-axial vector magnetoresistance (MR) in nonmagnetic (Bi1-xInx)2Se3 nanodevices at the composition of x = 0.08. We show a dumbbell-shaped in-plane negative MR up to room temperature as well as a large out-of-plane positive MR. MR at three directions is about in a -3%: -1%: 225% ratio at 2 K. Through both the thickness and composition-dependent magnetotransport measurements, we show that the in-plane negative MR is due to the topological phase transition enhanced intersurface coupling near the topological critical point. Our devices suggest the great potential for room-temperature spintronic applications, for example, vector magnetic sensors.

physics.app-ph↗

arXiv2018

Quantum oscillations and nontrivial transport in (Bi0.92In0.08)2Se3

Quantum phase transition in topological insulators has drawn heightened attention in condensed matter physics and future device applications. Here we report the magnetotransport properties of single crystalline (Bi0.92In0.08)2Se3. The average mobility of about 1000 cm2/Vs is obtained from the Lorentz law at the low field up to 50 K. The quantum oscillations rise at a field of about 5 T, revealing a high mobility of 1.4*10^4 cm2/Vs at 2 K. The topological Dirac fermions are evident by the nontrivial Berry phase in the Landau Fan diagram. The properties make the (Bi0.92In0.08)2Se3 a promising platform for the investigation of quantum phase transition in topological insulators.

cond-mat.mtrl-sci↗

arXiv2017

Diagnosing the magnetic field-tuned symmetry nature of the topological electrons by conductance fluctuations in bulk-insulating BiSbTeSe2 devices

We extract the quantum conductance fluctuations and study its magnetic field dependence in the gate-dependent transport of the topological electrons in bulk-insulating BiSbTeSe2 devices. While increasing the magnetic field from 0 to 12 Tesla, the fluctuation magnitudes are found reduced by a ratio of sqrt(2) and form a quantized step. The step is observed both in n-type and p-type transport. It is also confirmed in the nonlocal measurements. This essentially demonstrates the breaking of the time reversal symmetry of the three-dimensional Z2 topological insulators.

cond-mat.mes-hall↗

arXiv2017

Three-Dimensional Anisotropic Magnetoresistance in the Dirac Node-Line Material ZrSiSe

The family of materials defined as ZrSiX (X = S, Se, Te) has been established as Dirac node-line semimetals, and subsequent study is urgent to exploit the promising application of unusual magnetoresistance property. In this work, we systematically investigated the anisotropic magnetoresistance in the newly-discovered Dirac node-line material ZrSiSe. By applying a magnetic field of 3 T by a vector field, the three-dimensional (3D) magnetoresistance (MR) shows strong anisotropy. The MR ratio of maximum and minimum directions can reach 7 at 3 T and keeps increasing at the higher magnetic field. The anisotropic MR forms a butterfly-shaped curve, which indicates the quasi-2D electronic structures. This is further confirmed by the angular-dependent Shubnikov-de Haas (SdH) oscillations. The first-principles calculations establish the quasi-2D tubular-shaped Fermi surface near the X point in the Brillouin zone. Our findings shed light on the 3D mapping of MR and the potential applications in magnetic sensors based on ZrSiSe Dirac materials.

cond-mat.mes-hall↗

arXiv2017

The study on quantum material WTe2

WTe2 and its sister alloys have attracted tremendous attentions recent years due to the large non-saturating magnetoresistance and topological non-trivial properties. Herein, we briefly review the electrical property studies on this new quantum material.

cond-mat.mtrl-sci↗

arXiv2017

Stepwise quantized surface states and delayed Landau level hybridization in Co cluster-decorated BiSbTeSe2 topological insulator devices

In three-dimensional topological insulators (TIs), the nontrivial topology in their electronic bands casts a gapless state on their solid surfaces, using which dissipationless TI edge devices based on the quantum anomalous Hall (QAH) effect and quantum Hall (QH) effect have been demonstrated. Practical TI devices present a pair of parallel-transport topological surface states (TSSs) on their top and bottom surfaces. However, due to the no-go theorem, the two TSSs always appear as a pair and are expected to quantize synchronously. Quantized transport of a separate Dirac channel is still desirable, but has never been observed in graphene even after intense investigation over a period of 13 years, with the potential aim of half-QHE. By depositing Co atomic clusters, we achieved stepwise quantization of the top and bottom surfaces in BiSbTeSe2 (BSTS) TI devices. Renormalization group flow diagrams13, 22 (RGFDs) reveal two sets of converging points (CVPs) in the (Gxy, Gxx) space, where the top surface travels along an anomalous quantization trajectory while the bottom surface retains 1/2 e2/h. This results from delayed Landau-level (LL) hybridization (DLLH) due to coupling between Co clusters and TSS Fermions.

cond-mat.mes-hall↗