arXiv subjects

Binghai Yan

Publications and source records attributed to Binghai Yan.

At least 235 records · Page 13Linked to original sources

arXiv2016

Observation of pseudo two dimensional electron transport in the rock salt type topological semimetal LaBi

Topological insulators are characterized by an inverted band structure in the bulk and metallic surface states on the surface. In LaBi, a semimetal with a band inversion equivalent to a topological insulator, we observe surface state like behavior in the magnetoresistance. The electrons responsible for this pseudo two dimensional transport, however, originate from the bulk states rather topological surface states, which is witnessed by the angle dependent quantum oscillations of the magnetoresistance and ab initio calculations. As a consequence, the magnetoresistance exhibits strong anisotropy with large amplitude (~ 10^5 %).

cond-mat.mtrl-sci↗

arXiv2016

Chiral Quasiparticles at the Fermi Surface of the Weyl Semimetal TaAs

Tantalum arsenide is a member of the non-centrosymmetric monopnictides, which are putative Weyl semimetals. In these materials, three-dimensional chiral massless quasiparticles, the so-called Weyl fermions, are predicted to induce novel quantum mechanical phenomena, such as the chiral anomaly and topological surface states. However, their chirality is only well-defined if the Fermi level is close enough to the Weyl points that separate Fermi surface pockets of opposite chirality exist. In this article, we present the bulk Fermi surface topology of high quality single crystals of TaAs, as determined by angle-dependent Shubnikov-de Haas and de Haas-van Alphen measurements combined with ab-initio band-structure calculations. Quantum oscillations originating from three different types of Fermi surface pocket were found in magnetization, magnetic torque, and mag- netoresistance measurements performed in magnetic fields up to 14 T and temperatures down to 1.8 K. Of these Fermi pockets, two are pairs of topologically non-trivial electron pockets around the Weyl points and one is a trivial hole pocket. Unlike the other members of the non-centrosymmetric monopnictides, TaAs is the first Weyl semimetal candidate with the Fermi energy suffciently close to both types of Weyl points to generate chiral quasiparticles at the Fermi surface.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity in Weyl Semimetal Candidate MoTe2

In recent years, layered transition-metal dichalcogenides (TMDs) have attracted considerable attention because of their rich physics; for example, these materials exhibit superconductivity, charge density waves, and the valley Hall effect. As a result, TMDs have promising potential applications in electronics, catalysis, and spintronics. Despite the fact that the majority of related research focuses on semiconducting TMDs (e.g., MoS2), the characteristics of WTe2 are provoking strong interest in semimetallic TMDs with extremely large magnetoresistance, pressure-driven superconductivity, and the predicted Weyl semimetal (WSM) state. In this work, we investigate the sister compound of WTe2, MoTe2, which is also predicted to be a WSM and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that MoTe2 exhibits superconductivity with a resistive transition temperature Tc of 0.1 K. The application of a small pressure (such as 0.4 GPa) is shown to dramatically enhance the Tc, with a maximum value of 8.2 K being obtained at 11.7 GPa (a more than 80-fold increase in Tc). This yields a dome-shaped superconducting phase diagram. Further explorations into the nature of the superconductivity in this system may provide insights into the interplay between strong correlations and topological physics.

cond-mat.mtrl-sci↗

arXiv2016

Visualizing "Fermi arcs" in the Weyl semimetal TaAs

One of the hallmarks of Weyl semi-metals is the existence of unusual topological surface states known as 'Fermi arcs' [1-3]. The formation of these states is guaranteed by the existence of bulk Weyl points with opposite chirality. Tantalum Arsenide (TaAs) [4-9], a member of the newly discovered family of Weyl semi-metals [4,5], harbors a host of non-topological ('trivial') surface states overlapping in energy with the predicted 12 'Fermi arcs'. This overlap poses a major challenge in identifying the signatures of the arcs [10]. Here we harness the inherently distinct spatial structure of trivial and Fermi arc states to visualize the Fermi arcs for the first time using scanning tunneling microscopy. We do so in four distinct ways, each of which highlights a different aspect of their unusual nature. We reveal their relatively isotropic scattering signature, their energy dispersion and its relation to the bulk Weyl points, their deep bulk penetration relative to that of non-topological surface states and their weak coupling to the atomic structure. The latter is obtained by accounting for the spatial structure of the Bloch wavefunction and its effect on the scattering properties of the electrons off lattice defects in general. It thus provides a novel analysis tool for the spectroscopic characterization of electronic wavefunctions using scanning tunneling microscopy.

cond-mat.mes-hall↗

arXiv2016

Rectangular Tantalum Carbide Halides TaCX (X = Cl, Br, I) monolayer: Novel Large-Gap Quantum Spin Hall Insulator

Quantum spin Hall (QSH) insulators possess edge states that are topologically protected from backscattering. However, known QSH materials (e.g. HgTe/CdTe and InAs/GaSb quantum wells) exhibit very small energy gap and only work at low temperature, hindering their applications for room temperature devices. Based on the first-principles calculations, we predict a novel family of QSH insulators in monolayer tantalum carbide halide TaCX (X = Cl, Br, and I) with unique rectangular lattice and large direct energy gaps larger than 0.2 eV, accurately, 0.23$-$0.36 eV. The mechanism for 2D QSH effect in this system originates from a intrinsic d$-$d band inversion, different from conventional QSH systems with band inversion between s$-$p or p$-$p orbitals. Further, stain and intrinsic electric field can be used to tune the electronic structure and enhance the energy gap. TaCX nanoribbon, which has single-Dirac-cone edge states crossing the bulk band gap, exhibits a linear dispersion with a high Fermi velocity comparable to that of graphene. These 2D materials with considerable nontrivial gaps promise great application potential in the new generation of dissipationless electronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Pressure-driven Superconductivity in Transition-metal Pentatelluride HfTe5

Layered transition-metal tellurides have attracted considerable attention because of their rich physics; for example, tungsten ditelluride WTe2 exhibits extremely large magnetoresistance; the tritelluride ZrTe3 shows a charge density wave transition at low temperature; and the pentatelluride ZrTe5 displays an enigmatic resistivity anomaly and large thermoelectric power. Recently some transition-metal tellurides are predicted to be quantum spin Hall insulators (e.g. ZrTe5 and HfTe5) or Weyl semimetals (e.g. WTe2 and MoTe2) and were subjected to intensive investigations. Here, we report on the discovery of superconductivity in hafnium pentatelluride HfTe5 under high pressure. We observe two structural phase transitions and metallization with superconductivity developing at around 5 GPa. A maximal critical temperature of 4.8 K is attained at a pressure of 20 GPa, and superconductivity persists up to the maximum pressure in our study (42 GPa). Theoretical calculations indicate that the superconductivity develops mainly in the Te atom layers at medium pressure and in the Te atom chains at high pressure.

cond-mat.supr-con↗

arXiv2016

Observation of Unusual Topological Surface States in Half-Heusler Compounds LnPtBi (Ln=Lu, Y)

Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host non-trivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angle-resolved photoemission spectroscopy (ARPES) and ab initio calculations on rare-earth half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observed the unusual topological surface states on these materials, establishing them as first members with non-trivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are non-centrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors.

cond-mat.mtrl-sci↗

arXiv2016

Heart and diamond Fermi arcs in Pd and Pt oxide topological Dirac semimetals

Topological Dirac semimetals (DSMs) exhibit nodal points through which energy bands disperse linearly in three-dimensional (3D) momentum space, a 3D analogue of graphene. The first experimentally confirmed DSMs with a pair of Dirac points (DPs), Na$_{3}$Bi and Cd$_{3}$As$_{2}$, show topological surface Fermi arc states and exotic magneto-transport properties, boosting the interest in the search for stable and nontoxic DSM materials. Here, based on the {\it ab-initio} band structure calculations we predict a family of palladium and platinum oxides as robust 3D DSMs. The novel topological properties of these compounds are distinct from all other previously reported DSMs, they are rendered by the multiple number of DPs in the compounds. We show that along three unit axes of the cubic lattice there exist three pairs of DPs, which are linked by Fermi arcs on the surface. The Fermi arcs display a Lifshitz transition from a heart- to a diamond-shape upon varying the chemical potential. Corresponding oxides are already available as high-quality single crystals, which is an excellent precondition for the verification of our prediction by photoemission and magneto-transport experiments. Our findings not only predict a number of much robust 3D DSM candidates but also extend DSMs to transition metal oxides, a versatile family of materials, which opens the door to investigate the interplay between DSMs and electronic correlations that is not possible in the previously discovered DSM materials.

cond-mat.mtrl-sci↗

arXiv2016

Quantum oscillations and the Fermi-surface topology of the Weyl semimetal NbP

The Weyl semimetal NbP was found to exhibit topological Fermi arcs and exotic magneto-transport properties. Here, we report on magnetic quantum-oscillation measurements on NbP and construct the 3D Fermi surface with the help of band-structure calculations. We reveal a pair of spin-orbit-split electron pockets at the Fermi energy and a similar pair of hole pockets, all of which are strongly anisotropic. The Fermi surface well explains the linear magnetoresistance observed in high magnetic fields by the quantum-limit scenario. The Weyl points that are located in the $k_z \approx π/c$ plane are found to exist 5 meV above the Fermi energy. Therefore, we predict that the chiral anomaly effect can be realized in NbP by electron doping to drive the Fermi energy to the Weyl points.

cond-mat.mtrl-sci↗

arXiv2016

Negative magnetoresistance without well-defined chirality in the Weyl semimetal TaP

Weyl semimetals (WSMs) are topological quantum states wherein the electronic bands linearly disperse around pairs of nodes, the Weyl points, of fixed (left or right) chirality. The recent discovery of WSM materials triggered an experimental search for the exotic quantum phenomenon known as the chiral anomaly. Via the chiral anomaly nonorthogonal electric and magnetic fields induce a chiral density imbalance that results in an unconventional negative longitudinal magnetoresistance, the chiral magnetic effect. Recent theoretical work suggests that this effect does not require well-defined Weyl nodes. Experimentally however, it remains an open question to what extent it survives when chirality is not well-defined, for example when the Fermi energy is far away from the Weyl points. Here, we establish the detailed Fermi surface topology of the recently identified WSM TaP via a combination of angle-resolved quantum oscillation spectra and band structure calculations. The Fermi surface forms spin-polarized banana-shaped electron and hole pockets attached to pairs of Weyl points. Although the chiral anomaly is therefore ill-defined, we observe a large negative magnetoresistance (NMR) appearing for collinear magnetic and electric fields as observed in other WSMs. In addition, we show experimental signatures indicating that such longitudinal magnetoresistance measurements can be affected by an inhomogeneous current distribution inside the sample in a magnetic field. Our results provide a clear framework how to detect the chiral magnetic effect.

cond-mat.mtrl-sci↗

arXiv2016

Facet-dependent giant spin orbit torque in single crystalline antiferromagnetic Ir-Mn / ferromagnetic permalloy bilayers

There has been considerable interest in spin-orbit torques for the purpose of manipulating the magnetization of ferromagnetic (FM) films or nano-elements for spintronic technologies. Spin-orbit torques are derived from spin currents created from charge currents in materials with significant spin-orbit coupling that diffuse into an adjacent FM material. There have been intensive efforts to search for candidate materials that exhibit large spin Hall angles, i.e. efficient charge to spin current conversion. Here we report, using spin torque ferromagnetic resonance, the observation of a giant spin Hall angle of up to ~0.35 in (100) oriented single crystalline antiferromagnetic (AF) IrMn3 thin films, coupled to ferromagnetic permalloy layers, and a spin Hall angle that is about three times smaller in (111) oriented films. For the (100) oriented samples we show that the magnitude of the spin Hall angle can be significantly changed by manipulating the populations of the various AF domains through field annealing. Using ab-initio calculations we show that the triangular AF structure of IrMn3 gives rise to a substantial intrinsic spin Hall conductivity that is three times larger for the (100) than for the (111) orientations, consistent with our experimental findings.

cond-mat.mtrl-sci↗

arXiv2015

New Family of Quantum Spin Hall Insulators in Two-dimensional Transition-Metal Halide with Large Nontrivial Band Gaps

Topological insulators (TIs) are promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, currently realized 2D TIs, quantum spin Hall (QSH) insulators, suffer from ultra-high vacuum and extremely low temperature. Thus, seeking for desirable QSH insulators with high feasibility of experimental preparation and large nontrivial gap is of great importance for wide applications in spintronics. Based on the first-principles calculations, we predict a novel family of two-dimensional (2D) QSH insulators in transition-metal halide MX (M = Zr, Hf; X = Cl, Br, and I) monolayers with large nontrivial gaps of 0.12$-$0.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV) and larger than ZrTe$_5$ (0.1 eV) monolayers and graphene-based sandwiched heterstructures (30$-$70 meV). Their corresponding 3D bulk materials are weak topological insulators from stacking QSH layers, and some of bulk compounds have already been synthesized in experiment. The mechanism for 2D QSH effect in this system originates from a novel d$-$d band inversion, which is different from conventional band inversion between s$-$s orbitals, or p$-$p orbitals. The realization of pure layered MX monolayers may be prepared by exfoliation from their 3D bulk phases, thus holding great promise for nanoscale device applications and stimulating further efforts on transition metal-based QSH materials.

cond-mat.mtrl-sci↗

arXiv2015

Prediction of the Weyl semimetal in the orthorhombic MoTe2

We investigate the orthorhombic phase (Td) of layered transition-metal-dichalcogenide MoTe$_2$ as a Weyl semimetal candidate, which was discovered to be a superconductor in our recent experiment. MoTe$_2$ exhibits four pairs of Weyl points lying slightly ($\sim$ 6 meV) above the Fermi energy in the bulk band structure. Unlike its cousin WTe$_2$, which was predicted to be a type-II Weyl semimetal recently, the spacing between each pair of Weyl points is found to be as large as 4 percent of the reciprocal lattice in MoTe$_2$ (six times larger than that of WTe$_2$). When projected to the surface, Weyl points are connected by Fermi arcs, which can be easily verified by ARPES due to the large Weyl point separation. In addition, we show that the correlation effect or strain can drive MoTe$_2$ from the type-II to type-I Weyl semimetal.

cond-mat.mtrl-sci↗

arXiv2015

Topological surface states and Fermi arcs of the noncentrosymmetric Weyl semimetals TaAs, TaP, NbAs, and NbP

Very recently the topological Weyl semimetal (WSM) state was predicted in the noncentrosymmetric compounds TaAs, TaP, NbAs, and NbP and soon led to photoemission and transport experiments to verify the presumed topological properties such as Fermi arcs (unclosed Fermi surfaces) and the chiral anomaly. In this work, we have performed fully \textit{ab initio} calculations of the surface band structures of these four WSM materials and revealed the Fermi arcs with spin-momentum-locked spin texture. On the (001) polar surface, the shape of the Fermi surface depends sensitively on the surface terminations (cations or anions), although they exhibit the same topology with arcs. The anion (P or As) terminated surfaces are found to fit recent photoemission measurements well. Such surface potential dependence indicates that the shape of the Fermi surface can be manipulated by depositing guest species (such as K atoms), as we demonstrate. On the polar surface of a WSM without inversion symmetry, Rashba-type spin polarization naturally exists in the surface states and leads to strong spin texture. By tracing the spin polarization of the Fermi surface, we can also distinguish Fermi arcs from trivial Fermi circles. The four compounds NbP, NbAs, TaP, and TaAs present an increasing amplitude of spin-orbit coupling (SOC) in the band structure. By comparing their surface states, we reveal the evolution of topological Fermi arcs from the spin-degenerate Fermi circle to spin-split arcs when the SOC increases from zero to a finite value. Our work will help us understand the complicated surface states of WSMs and allow us to manipulate them, especially for future spin-revolved photoemission and transport experiments.

cond-mat.mtrl-sci↗

arXiv2015

Towards rational design of catalysts supported on a topological insulator substrate

Exotic and robust metallic surface states of topological insulators (TIs) have been expected to provide a promising platform for novel surface chemistry and catalysis. However, it is still an unprecedented field how TIs affect the activity of catalysts. In this work, we study the effects of topological surface states (TSSs) on the activity of transition metal clusters (Au, Ag, Cu, Pt, and Pd), which are supported on a TI Bi2Se3 substrate. It was found the adsorption energy of oxygen on the supported catalysts can be always enhanced due to the TSSs. However, it does not necessarily mean an increase of the activity in catalytic oxidation reaction. Rather, the enhanced adsorption behavior in the presence of TSSs exhibits dual effects, determined by the intrinsic reactivity of these catalysts with oxygen. For the Au case, the activity of catalytic oxidation can be improved because the intrinsic binding between Au and O is relatively weak. In contrast, a negative effect is found for the Pt and Pd clusters since the intrinsic binding of Pt and Pd with oxygen is too strong. We also found that the effect of TSSs on the activity of hydrogen evolution reaction (HER) is quite similar, i.e. the metals with original weak reactivity can gain a positive effect from TSSs. The present work can pave a way for more rational design and selection of catalysts when using TIs as substrates.

physics.comp-ph↗

arXiv2015

Orbital ordering of Ir-t2g states in the double perovskite Sr2CeIrO6

The electronic and magnetic properties of monoclinic double perovskite Sr$_2$CeIrO$_6$ were examined based on both experiments and first-principles density functional theory calculations. From the calculations we conclude that low-spin-state Ir$^{4+}$ (5$\textit{d}^5$, S=$\frac{1}{2}$) shows t$_{2g}$ band derived anti-ferro type orbital ordering implying alternating occupations of $\textit{d}_{yz}$ and $\textit{d}_{xz}$ orbitals at the two symmetrically independent Ir sites. The experimentally determined Jahn-Teller type distorted monoclinic structure is consistent with the proposed orbital ordering picture. Surprisingly, the Ir-5$\textit{d}$ orbital magnetic moment was found to be $\approx$ 1.3 times larger than the spin magnetic moment. The experimentally observed AFM-insulating states are consistent with the calculations. Both electron-electron correlation and spin-orbit coupling (SOC) are required to drive the experimentally observed AFM-insulating ground state. This single active site double perovskite provides a rare platform with a prototype geometrically frustrated fcc lattice where among the different degrees of freedom (i.e spin, orbital, and lattice), spin-orbit interaction and Coulomb correlation energy scales compete and interact with each other.

cond-mat.mtrl-sci↗

arXiv2015

Discovery of a Weyl Semimetal in non-Centrosymmetric Compound TaAs

Three-dimensional (3D) topological Weyl semimetals (TWSs) represent a novel state of quantum matter with unusual electronic structures that resemble both a "3D graphene" and a topological insulator by possessing pairs of Weyl points (through which the electronic bands disperse linearly along all three momentum directions) connected by topological surface states, forming the unique "Fermi-arc" type Fermi-surface (FS). Each Weyl point is chiral and contains half of the degrees of freedom of a Dirac point, and can be viewed as a magnetic monopole in the momentum space. Here, by performing angle-resolved photoemission spectroscopy on non-centrosymmetric compound TaAs, we observed its complete band structures including the unique "Fermi-arc" FS and linear bulk band dispersion across the Weyl points, in excellent agreement with the theoretical calculations. This discovery not only confirms TaAs as the first 3D TWS, but also provides an ideal platform for realizing exotic physical phenomena (e.g. negative magnetoresistance, chiral magnetic effects and quantum anomalous Hall effect) which may also lead to novel future applications.

cond-mat.mtrl-sci↗

arXiv2015

Extremely large magnetoresistance and ultrahigh mobility in the topological Weyl semimetal NbP

Recent experiments have revealed spectacular transport properties of conceptually simple semimetals. For example, normal semimetals (e.g. WTe$_2$) have started a new trend to realize a large magnetoresistance, which is the change of electrical resistance by an external magnetic field. Weyl semimetal (WSM) is a topological semimetal with massless relativistic electrons as the three-dimensional analogue of graphene and promises exotic transport properties and surface states, which are different from those of the famous topological insulators (TIs). In this letter, we choose to utilize NbP in magneto-transport experiments because its band structure is on assembly of a WSM and a normal semimetal. Such a combination in NbP indeed leads to the observation of remarkable transport properties, an extremely large magnetoresistance of 850,000 % at 1.85 K (250 % at room temperature) in a magnetic field of 9 T without any signs of saturation, and ultrahigh carrier mobility of 5$\times$10$^6$ cm$^2$ V$^{-1}$ s$^{-1}$ accompanied by strong Shubnikov-de Hass (SdH) oscillations. NbP presents a unique example to consequent design the functionality of materials by combining the topological and conventional phases.

cond-mat.mtrl-sci↗