arXiv subjects

Xuefeng Wang

Publications and source records attributed to Xuefeng Wang.

At least 73 records · Page 4Linked to original sources

arXiv2017

Nontrivial Berry phase and type II Dirac transport in layered material PdTe2

Here we report the evidence of the type II Dirac Fermion in the layered crystal PdTe2. The de Haas-van Alphen oscillations find a small Fermi pocket with a cross section of 0.077nm-2 with a nontrivial Berry phase. First-principal calculations reveal that it is originated from the hole pocket of a tilted Dirac cone. Angle Resolved Photoemission Spectroscopy demonstrates a type II Dirac cone featured dispersion. We also suggest PdTe2 is an improved platform to host the topological superconductors.

cond-mat.mes-hall↗

arXiv2016

Unsaturated magnetoconductance of epitaxial La0.7Sr0.3MnO3 thin films in pulsed magnetic fields up to 60 T

We report on the temperature and field dependence of resistance of La0.7Sr0.3MnO3 thin films over a wide temperature range and in pulsed magnetic fields up to 60 T. The epitaxial La0.7Sr0.3MnO3 thin films were deposited by laser molecular beam epitaxy. High magnetic field magnetoresistance curves were fitted by the Brillouin function, which indicated the existence of magnetically polarized regions and the underlying hopping mechanism. The unsaturated magnetoconductance was the most striking finding observed in pulsed magnetic fields up to 60 T. These observations can deepen the fundamental understanding of the colossal magnetoresistance in manganites with strong correlation of transport properties and magnetic ordering.

cond-mat.str-el↗

arXiv2016

Evidence of both surface and bulk Dirac bands in ZrSiS and the unconventional magnetoresistance

The unconventional magnetoresistance of ZrSiS single crystals is found unsaturated till the magnetic field of 53 T with the butterfly shaped angular dependence. Intense Shubnikov-de Haas oscillations resolve a bulk Dirac cone with a nontrivial Berry phase, where the Fermi surface area is 1.80*10^-3 Å-2 and reaches the quantum limit before 20 T. Angle resolved photoemission spectroscopy measurement reveals an electronic state with the two-dimensional nature around the X point of Brillouin zone boundary. By integrating the density functional theory calculations, ZrSiS is suggested to be a Dirac material with both surface and bulk Dirac bands.

cond-mat.mtrl-sci↗

arXiv2016

Quantum oscillation and nontrivial transport in the Dirac Semimetal Cd3As2 nanodevice

Here we demonstrate the Shubnikov de Haas oscillation in high-quality Cd3As2 nanowires grown by a chemical vapor deposition approach. The dominant transport of topological Dirac fermions is evident by the nontrivial Berry phase in the Landau Fan diagram. The quantum oscillations rise at a small field of 2 Tesla and preserves till up to 100K, revealing a sizeable Landau level gap and a mobility of over 2000 cm2/V-1s-1. The angle-variable oscillations indicates the isotropy of the bulk Dirac transport. The large estimated mean free path appeals the one-dimensional transport of Dirac semimetals.

cond-mat.mes-hall↗

arXiv2016

Repairing atomic vacancies in single-layer MoSe2 field-effect transistor and its defect dynamics

Here we repair the single-layer MoSe2 field-effect transistors by the EDTA processing, after which the devices' room-temperature carrier mobility increases from 0.1 to over 70cm2/Vs. The atomic dynamics is constructed by the combined study of the first-principle calculation, aberration-corrected transmission electron microscopy and Raman spectroscopy. Single/double Se vacancies are revealed originally, which cause some mid-gap impurity states and localize the device carriers. They are found repaired with the result of improved electronic transport. Such a picture is confirmed by a 1.5cm-1 red shift in the Raman spectra.

cond-mat.mes-hall↗

arXiv2015

Enhancing Magnetic Ordering in Cr-doped Bi2Se3 using High-TC Ferrimagnetic Insulator

We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi2-xCrxSe3, via the proximity effect using a high-TC ferrimagnetic insulator Y3Fe5O12. The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced TC of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 K to 50 K) penetration depth compared to that of diluted ferromagnetic semiconductors (DMSs), which could indicate a novel mechanism for the interaction between FMIs and the nontrivial TIs surface.

cond-mat.mtrl-sci↗

arXiv2015

Lateral Heterojunction Sb2Te3/Bi2Te3 and its topological transport

A lateral heterojunction of topological insulator Sb2Te3/Bi2Te3 was successfully synthesized using a two-step solvothermal method. The two crystalline components were separated well by a sharp lattice-matched interface when the optimized procedure was used. Inspecting the heterojunction using high-resolution transmission electron microscopy showed that epitaxial growth occurred along the horizontal plane. A rectification curve was observed at low temperatures. Quantum correction from the weak antilocalization reveals the transport of the topological surface state. There was, therefore, a staggered-gap lateral heterojunction with a small junction voltage, which is appealing for a platform for spin filters and one-dimensional topological interface states.

cond-mat.mes-hall↗

arXiv2015

Enhanced quantum coherence in graphene caused by Pd cluster deposition

We report on the unexpected increase in the dephasing lengths of a graphene sheet caused by the deposition of Pd nanoclusters, as demonstrated by weak localization measurements. The dephasing lengths reached saturated values at low temperatures. Theoretical calculations indicate the p-type charge transfer from the Pd clusters, which contributes more carriers. The saturated values of dephasing lengths often depend on both the carrier concentration and mean free path. Although some impurities are increased as revealed by decreased mobilities, the intense charge transfer leads to the improved saturated values and subsequent improved dephasing lengths.

cond-mat.mes-hall↗

arXiv2014

Ranks and eigenvalues of states with prescribed reduced states

For a quantum state represented as an $n\times n$ density matrix $σ\in M_n$, let ${\cal S}(σ)$ be the compact convex set of quantum states $ρ= (ρ_{ij}) \in M_{m\cdot n}$ with the first partial trace equal to $σ$, i.e., ${\rm tr}_1(ρ) = ρ_{11} + \cdots + ρ_{mm} = σ$. It is known that if $m \ge n$ then there is a rank one matrix $ρ\in {\cal S}(σ)$ satisfying ${\rm tr}_1(ρ) = σ$. If $m < n$, there may not be rank one matrix in ${\cal S}(σ)$. In this paper, we determine the ranks of the elements and ranks of the extreme points of the set ${\cal S}$ We also determine $ρ^* \in {\cal S}(σ)$ with rank bounded by $k$ such that $\|{\rm tr}_1ρ^*) - σ\|$ is minimum for a given unitary similarity invariant norm $\|\cdot\|$. Furthermore, the relation between the eigenvalues of $σ$ and those of $ρ\in {\cal S}(σ)$ is analyzed. Extension of our results and open problems will be mentioned.

quant-ph↗

arXiv2014

Experimental evidence and control of the bulk-mediated intersurface coupling in topological insulator Bi2Te2Se nanoribbons

Nearly a decade after the discovery of topological insulators (TIs), the important task of identifying and characterizing their topological surface states through electrical transport experiments remains incomplete. The interpretation of these experiments is made difficult by the presence of residual bulk carriers and their coupling to surface states, which is not yet well understood. In this work, we present the first evidence for the existence and control of bulk-surface coupling in Bi2Te2Se nanoribbons, which are promising platforms for future TI-based devices. Our magnetoresistance measurements reveal that the number of coherent channels contributing to quantum interference in the nanoribbons changes abruptly when the film thickness exceeds the bulk phase relaxation length. We interpret this observation as an evidence for bulk-mediated coupling between metallic states located on opposite surfaces. This hypothesis is supported by additional magnetoresistance measurements conducted under a set of gate voltages and in a parallel magnetic field, the latter of which alters the intersurface coupling in a controllable way.

cond-mat.mes-hall↗

arXiv2014

Indications of the topological transport by the universal conductance fluctuations in the Bi2Te2Se microflakes

Universal conductance fluctuations (UCFs) are extracted in the magnetoresistance responses in the bulk-insulating Bi2Te2Se microflakes. Its two-dimensional character is demonstrated by the field-tilting magnetoresistance measurements. Its origin from the surface electrons is determined by the fact that the UCF amplitudes keep unchanged while applying an in-plane field to suppress the coherence of bulk electrons. After considering the ensemble average in a batch of micrometer-sized samples, the intrinsic UCF magnitudes of over 0.37 e2/h is obtained. This agrees with the theoretical prediction on topological surface states. All the evidence point to the successful observation of the UCF of topological surface states.

cond-mat.mes-hall↗

arXiv2014

Aging the Cu-doped Bi2Te3 crystals for the topological transport and its atomic tunneling-clustering dynamics

We report on the observation of the two-dimensional weak antilocalization in (Cu0.1Bi0.9)2Te3.06 crystals relying on measurements of the magnetoresistance in a tilted field. The dephasing analysis and scanning tunneling spectroscopy corroborate the transport of the topological surface states (SS). The SSs contribute 3.3% conductance in 30μm-thick material and become dominant in the 100nm-thick flakes. Such optimized topological SS transport is achieved by an intense aging process, when the bulk conductance is suppressed by four orders of magnitude in the long period. Scanning tunneling microscopy reveals that Cu atoms are initially inside the quintuple layers and migrate to the layer gaps to form Cu clusters during the aging. In combination with first-principles calculations, an atomic tunneling-clustering procedure across a diffusion barrier of 0.57eV is proposed.

cond-mat.mtrl-sci↗

arXiv2014

Perturbing eigenvalues of non-negative matrices

Let $A$ be an irreducible (entrywise) nonnegative $n\times n$ matrix with eigenvalues $$ρ, b+ic,b-ic, λ_4,\cdots,λ_n,$$ where $ρ$ is the Perron eigenvalue. It is shown that for any $t \in [0, \infty)$ there is a nonnegative matrix with eigenvalues $$ρ+ \tilde t,λ_2+t,λ_3+t, λ_4 \cdots,λ_n,$$ whenever $\tilde t \ge γ_n t$ with $γ_3=1, γ_4 = 2, γ_5=\sqrt 5$ and $γ_n = 2.25$ for $n \ge 6$. The result improves that of Guo et al. Our proof depends on an auxiliary result in geometry asserting that the area of an $n$-sided convex polygon is bounded by $γ_n$ times the maximum area of the triangle lying inside the polygon.

math.SP↗

arXiv2013

Shubnikov de Haas quantum oscillation of the surface states in the metallic Bismuth Telluride sheets

Metallic Bi2Te3 crystalline sheets with the room-temperature resistivity of above 10 mΩ cm were prepared and their magnetoresistive transport was measured in a field of up to 9 Tesla. The Shubnikov de Haas oscillations were identified from the secondly-derived magnetoresistance curves. While changing the angle between the field and normal axis of the sheets, we find that the oscillation periods present a cosine dependence on the angle. This indicates a two-dimensional transport due to the surface state. The work reveals a resolvable surface contribution to the overall conduction even in a metallic topological insulator.

cond-mat.mes-hall↗

arXiv2012

Two-dimensional universal conductance fluctuations and the electron-phonon interaction of topological surface states in Bi2Te2Se nanoribbons

The universal conductance fluctuations (UCFs), one of the most important manifestations of mesoscopic electronic interference, have not yet been demonstrated for the two-dimensional surface state of topological insulators (TIs). Even if one delicately suppresses the bulk conductance by improving the quality of TI crystals, the fluctuation of the bulk conductance still keeps competitive and difficult to be separated from the desired UCFs of surface carriers. Here we report on the experimental evidence of the UCFs of the two-dimensional surface state in the bulk insulating Bi2Te2Se nanoribbons. The solely-B\perp-dependent UCF is achieved and its temperature dependence is investigated. The surface transport is further revealed by weak antilocalizations. Such survived UCFs of the topological surface states result from the limited dephasing length of the bulk carriers in ternary crystals. The electron-phonon interaction is addressed as a secondary source of the surface state dephasing based on the temperature-dependent scaling behavior.

cond-mat.mes-hall↗

arXiv2012

Visualizing plasmon coupling in closely-spaced chains of Ag nanoparticles by electron energy loss spectroscopy

Anisotropic plasmon coupling in closely-spaced chains of Ag nanoparticles was visualized using the electron energy loss spectroscopy in a scanning transmission electron microscope. For dimers as the simplest chain, mapping the plasmon excitations with nanometers' spatial resolution and 0.27 eV energy resolution intuitively identified two coupling plasmons. The in-phase mode redshifted from the ultraviolet region as the inter-particle spacing was reduced, reaching the visible range at 2.7 eV. Calculations based on the discrete dipole approximation confirmed its optical activeness, where the longitudinal direction was constructed as the path for light transportation. Two coupling paths were then observed in an inflexed 4-particle chain.

cond-mat.mes-hall↗

arXiv2012

Experimental evidence on the Altshuler-Aronov-Spivak interference of the topological surface states in the exfoliated Bi2Te3 nanoflakes

Here we demonstrate the Altshuler-Aronov-Spivak (AAS) interference of the topological surface states on the exfoliated Bi2Te3 microflakes by a flux period of h/2e in their magnetoresistance oscillations and its weak field character. Both the osillations with the period of h/e and h/2e are observed. The h/2e-period AAS oscillation gradually dominates with increasing the sample widths and the temperatures. This reveals the transition of the Dirac Fermions' transport to the diffusive regime.

cond-mat.mes-hall↗

arXiv2010

Calibrating the atomic balance by carbon nanoclusters

Carbon atoms are counted at near atomic-level precision using a scanning transmission electron microscope calibrated by carbon nanocluster mass standards. A linear calibration curve governs the working zone from a few carbon atoms up to 34,000 atoms. This linearity enables adequate averaging of the scattering cross sections, imparting the experiment with near atomic-level precision despite the use of a coarse mass reference. An example of this approach is provided for thin layers of stacked graphene sheets. Suspended sheets with a thickness below 100 nm are visualized, providing quantitative measurement in a regime inaccessible to optical and scanning probe methods.

cond-mat.mes-hall↗