/Tag: WTe2

Electrically tuneable nonlinear anomalous Hall effect in two-dimensional transition-metal dichalcogenides WTe2 and MoTe2. (arXiv:1804.11069v1 [cond-mat.mtrl-sci])

2018-05-01T19:59:29+00:00 May 1st, 2018|Categories: Publications|Tags: , |

We studied the nonlinear electric response in WTe2 and MoTe2 monolayers. When the inversion symmetry is breaking but the the time-reversal symmetry is preserved, a second-order Hall effect called the nonlinear anomalous Hall effect (NLAHE) emerges owing to the nonzero Berry curvature on the nonequilibrium Fermi surface. We reveal a strong NLAHE with a Hall-voltage that is quadratic with respect to the longitudinal current. The optimal current direction is normal to the mirror plane in these two-dimensional (2D) materials. The NLAHE can be sensitively tuned by an out-of-plane electric field, which induces a transition from a topological insulator to a normal insulator. Crossing the critical transition point, the magnitude of the NLAHE increases, and its sign is reversed. Our work paves the way to discover exotic nonlinear phenomena in inversion-symmetry-breaking 2D materials.

Published in: "arXiv Material Science".

Proton-driven patterning of bulk transition metal dichalcogenides. (arXiv:1803.09825v1 [cond-mat.mtrl-sci])

2018-03-28T19:59:13+00:00 March 28th, 2018|Categories: Publications|Tags: , , , , , |

At the few-atom-thick limit, transition metal dichalcogenides (TMDs) exhibit a host of attractive electronic optical, and structural properties. The possibility to pattern these properties has a great impact on applied and fundamental research. Here, we demonstrate spatial control over the light emission, lattice deformation, and hydrogen storage in bulk TMDs. By low-energy proton irradiation, we create uniquely favorable conditions for the production and accumulation of molecular hydrogen just one or few monolayers beneath the crystal basal plane of bulk WS2, WSe2, WTe2, MoSe2, and MoS2 samples. H2 therein produced coalesces to form bubbles, which lead to the localized swelling of one X-M-X plane prevalently. This results eventually in the creation of atomically thin domes filled with molecular hydrogen at 10 atm. The domes emit light strongly well above room temperature and can store H2 indefinitely. They can be produced with the desired density, well-ordered positions, and size tunable from the nanometer to the micrometer scale, thus providing a template for the manageable and durable mechanical and electronic structuring of two-dimensional materials.

Published in: "arXiv Material Science".

Calculations of point defects in the layered MX2 (M=Mo, W; X=S, Te): Substitution by the groups III, V and VII elements. (arXiv:1803.04334v1 [cond-mat.mtrl-sci])

2018-03-13T20:00:32+00:00 March 13th, 2018|Categories: Publications|Tags: , , , , |

Dopability in semiconductors plays a crucial role in device performance. Using the first-principles density-functional theory calculations, we investigate systematically the doping properties of layered MX2 (M= Mo, W; X=S, Te) by replacing M or X with the groups III, V and VII elements. It is found that the defect BM is hard to form in MX2 due to the large formation energy originating from the crystal distortion, while AlM is easy to realize compared to the former. In MoS2, WS2 and MoTe2, Al is the most desirable p-type dopant under anion-rich conditions among the group III components, since AlM has relatively low transition and formation energies. With respect to the doping of the group V elements, it is found that the substitutions on the cation sites have deeper defect levels than those on the anion sites due to the strong electronegativity. AsTe and SbTe in MoTe2 and WTe2 are trend to form shallow acceptors under cation-rich conditions, indicating high hole-concentrations for p-type doping, whereas SbS in MoS2 and PTe in WTe2 are shown to be good p-type candidates under cation-rich conditions. In despite of that the substitutions of group VII on X site have low formation energies, the transition energies are too high to achieve n-type MoS2 and WS2. Nevertheless, for MoTe2, the substitutions with the group VII elements on the anion sites are suitable for n-type doping on account of the shallow donor levels and low formation energies under Mo-rich condition. As to WTe2, F is the only potential

Published in: "arXiv Material Science".

Room-temperature nanoseconds spin relaxation in WTe2 and MoTe2 thin films. (arXiv:1803.00305v1 [cond-mat.mtrl-sci])

2018-03-02T19:59:20+00:00 March 2nd, 2018|Categories: Publications|Tags: , |

The Weyl semimetal WTe2 and MoTe2 show great potential in generating large spin currents since they possess topologically-protected spin-polarized states and can carry a very large current density. In addition, the intrinsic noncentrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry-controlled spin-orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. Here, we report a room-temperature spin relaxation time of 1.2 ns (0.4 ns) in WTe2 (MoTe2) thin film using the time-resolved Kerr rotation (TRKR). Based on ab initio calculation, we identify a mechanism of long-lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron-hole recombination rate and suppression of backscattering required by time-reversal and lattice symmetry operation. In addition, we find the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. Our work provides an insight into the physical origin of long-lived spin polarization in Weyl semimetals which could be useful to manipulate spins for a long time at room temperature.

Published in: "arXiv Material Science".

Anisotropic Thermal Transport in Phase-Transition Layered 2D Alloys WSe2(1-x)Te2x. (arXiv:1802.10009v1 [cond-mat.mtrl-sci])

2018-02-28T19:59:29+00:00 February 28th, 2018|Categories: Publications|Tags: , |

Transition metal dichalcogenide (TMD) alloys have attracted great interests in recent years due to their tunable electronic properties, especially the semiconductor-metal phase transition, along with their potential applications in solid-state memories and thermoelectrics. However, the thermal conductivity of layered two-dimensional (2D) TMD alloys remains largely unexplored despite that it plays a critical role in the reliability and functionality of TMD-enabled devices. In this work, we study the temperature-dependent anisotropic thermal conductivity of the phase-transition 2D TMD alloys WSe2(1-x)Te2x in both the in-plane direction (parallel to the basal planes) and the cross-plane direction (along the c-axis) using time-domain thermoreflectance measurements. In the WSe2(1-x)Te2x alloys, the cross-plane thermal conductivity is observed to be dependent on the heating frequency (modulation frequency of the pump laser) due to the non-equilibrium transport between different phonon modes. Using a two-channel heat conduction model, we extracted the anisotropic thermal conductivity at the equilibrium limit. A clear discontinuity in both the cross-plane and the in-plane thermal conductivity is observed as x increases from 0.4 to 0.6 due to the phase transition from the 2H to Td phase in the layered 2D alloys. The temperature dependence of thermal conductivity for the TMD alloys was found to become weaker compared with the pristine 2H WSe2 and Td WTe2 due to the atomic disorder. This work serves as an important starting point for exploring phonon transport in layered 2D alloys.

Published in: "arXiv Material Science".

Liquid phase mass production of air-stable black phosphorus/phospholipids nanocomposite with ultralow tunneling barrier. (arXiv:1801.06997v1 [cond-mat.mtrl-sci])

2018-01-23T20:00:54+00:00 January 23rd, 2018|Categories: Publications|Tags: , , , |

Few-layer black phosphorus (FLBP), a recently discovered two-dimensional semiconductor, has attracted substantial attention in the scientific and technical communities due to its great potential in electronic and optoelectronic applications. However, reactivity of FLBP flakes with ambient species limits its direct applications. Among various methods to passivate FLBP in ambient environment, nanocomposites mixing FLBP flakes with stable matrix may be one of the most promising approaches for industry applications. Here, we report a simple one-step procedure to mass produce air-stable FLBP/phospholipids nanocomposite in liquid phase. The resultant nanocomposite is found to have ultralow tunneling barrier for charge carriers which can be described by an Efros-Shklovskii variable range hopping mechanism. Devices made from such mass-produced FLBP/phospholipids nanocomposite show highly stable electrical conductivity and opto-electrical response in ambient conditions, indicating its promising applications in both electronic and optoelectronic applications. This method could also be generalized to the mass production of nanocomposites consisting of other air-sensitive two-dimensional materials, such as FeSe, NbSe2, WTe2, etc.

Published in: "arXiv Material Science".

Planar Hall effect in type-II Weyl semimetal WTe2. (arXiv:1801.05929v1 [cond-mat.mtrl-sci])

2018-01-19T19:58:55+00:00 January 19th, 2018|Categories: Publications|Tags: |

Adler-Bell-Jackiw chiral anomaly is a representative feature arising from the topological nature in topological semimetal. We report the first experimental observation of giant planar Hall effect in type-II Weyl semimetal WTe2. Our comprehensive analyes of the experimental data demonstrate that the detected planar Hall effect is originated from the chiral anomaly of Weyl fermions. Unlike the somewhat elusive negative magnetoresistance, the planar Hall effect is robust and easy to be detected in type-II Weyl semimetal. This work reveals that the planar Hall effect is an effective transport probe to determine the topological nature of topological semimetals, especially in type-II Weyl semimetals.

Published in: "arXiv Material Science".

Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal

2018-01-05T02:27:43+00:00 January 5th, 2018|Categories: Publications|Tags: , |

A variety of monolayer crystals have been proposed to be two-dimensional topological insulators exhibiting the quantum spin Hall effect (QSHE), possibly even at high temperatures. Here we report the observation of the QSHE in monolayer tungsten ditelluride (WTe2) at temperatures up to 100 kelvin. In the short-edge limit, the monolayer exhibits the hallmark transport conductance, ~e2/h per edge, where e is the electron charge and h is Planck’s constant. Moreover, a magnetic field suppresses the conductance, and the observed Zeeman-type gap indicates the existence of a Kramers degenerate point and the importance of time-reversal symmetry for protection from elastic backscattering. Our results establish the QSHE at temperatures much higher than in semiconductor heterostructures and allow for exploring topological phases in atomically thin crystals.

Published in: "Science".

Non-stoichiometry effects on the extreme magnetoresistance in Weyl semimetal WTe2. (arXiv:1712.10200v1 [cond-mat.mtrl-sci])

2018-01-01T19:59:18+00:00 January 1st, 2018|Categories: Publications|Tags: |

The extremely large non-saturated magnetoresistance is an unusual property of many semimetals, some of which have topological quantum states. In the present paper, the non-stoichiometry effect on the magnetoresistance is systematically investigated for the Weyl semimetal WTe2. The as-grown samples have a slight difference in Te vacancies, whose magnetoresistance and Hall resistivity are measured and analyzed with a two-carrier model. The fitting results show that the extreme magnetoresistance is strongly dependent on the residual resistivity ratio, which is interpreted as the degree of non-stoichiometry and the ratio of electron-type and hole-type carriers. Such an electron-hole compensation mechanism is further confirmed by the measurements on the annealed samples with artificial Te vacancies. The non-stoichiometry effect is eventually understood in terms of electron doping which breaks the balance between electron-type and hole-type carriers. These facts demonstrate that the compensation effect is the dominant mechanism of the extreme magnetoresistance in WTe2, in spite of other possible origins.

Published in: "arXiv Material Science".

Electron-hole balanced dynamics in the type-II Weyl semimetal candidate WTe2. (arXiv:1711.05002v1 [cond-mat.mes-hall])

2017-11-15T19:59:01+00:00 November 15th, 2017|Categories: Publications|Tags: |

We present a time- and angular-resolved photoemission (TR-ARPES) study of the transition- metal dichalcogenide WTe2, a candidate type II Weyl semimetal exhibiting extremely large magne- toresistence. Using femtosecond light pulses, we characterize the unoccupied states of the electron pockets above the Fermi level. We track the relaxation dynamics of photoexcited electrons along the unoccupied band structure and into a bulk hole pocket. Following the ultrafast carrier relaxation, we report remarkably similar decay dynamics for electrons and holes. Our results corroborate the hypothesis that carrier compensation is a key factor in the exceptional magnetotransport properties of WTe2.

Published : "arXiv Mesoscale and Nanoscale Physics".

Observation of the Quantum Spin Hall Effect up to 100 Kelvin in a Monolayer Crystal. (arXiv:1711.03584v1 [cond-mat.mes-hall])

2017-11-13T19:59:04+00:00 November 13th, 2017|Categories: Publications|Tags: , , |

The field of topological insulators (TI) was sparked by the prediction of the quantum spin Hall effect (QSHE) in time reversal invariant systems, such as spin-orbit coupled monolayer graphene. Ever since, a variety of monolayer crystals have been proposed as two-dimensional (2D) TIs exhibiting the QSHE, possibly even at high temperatures. However, conclusive evidence for a monolayer QSHE is still lacking, and systems based on semiconductor heterostructures operate at temperatures close to liquid helium. Here we report the observation of the QSHE in monolayer WTe2 at temperatures up to 100 Kelvin. The monolayer exhibits the hallmark quantized transport conductance, ~ e2/h per edge, in the short edge limit. Moreover, a magnetic field suppresses the conductance, and the observed Zeeman-type gap indicates the existence of a Kramers degenerate point, demonstrating the importance of time reversal symmetry for protection from elastic backscattering. Our results establish the high-temperature QSHE and open a new realm for the discovery of topological phases based on 2D crystals.

Published : "arXiv Mesoscale and Nanoscale Physics".

Anomalous magnetotransport properties of high-quality single crystals of Weyl semimetal WTe2: Sign change of Hall resistivity. (arXiv:1710.00570v1 [cond-mat.mtrl-sci])

2017-10-03T19:58:51+00:00 October 3rd, 2017|Categories: Publications|Tags: |

We report on a systematic study of Hall effect using high quality single crystals of type-II Weyl semimetal WTe2 with the applied magnetic field B//c. The residual resistivity ratio of 1330 and the large magnetoresistance of 1.5times10^6 % in 9 T at 2 K, being in the highest class in the literature, attest to their high quality. Based on a simple two-band model, the densities (n_e and n_h) and mobilities (mu_e and mu_h) for electron and hole carriers have been uniquely determined combining both Hall- and electrical-resistivity data. The difference between ne and nh is ~1% at 2 K, indicating that the system is in an almost compensated condition. The negative Hall resistivity growing rapidly below ~20 K is due to a rapidly increasing mu_h/mu_e approaching one. Below 3 K in a low field region, we found the Hall resistivity becomes positive, reflecting that mu_h/mu_e finally exceeds one in this region. These anomalous behaviors of the carrier densities and mobilities might be associated with the existence of a Lifshitz transition and/or the spin texture on the Fermi surface.

Published : "arXiv Mesoscale and Nanoscale Physics".

Cleavage Tendency of Anisotropic Two Dimensional Materials: ReX2 (X=S, Se) and WTe2. (arXiv:1709.05999v1 [cond-mat.mtrl-sci])

2017-09-19T19:59:23+00:00 September 19th, 2017|Categories: Publications|Tags: |

With unique distorted 1T structure and the associated in-plane anisotropic properties, mono- and few-layer ReX2 (X=S, Se) have recently attracted particular interest. Based on experiment and first-principles calculations, we investigate the fracture behavior of ReX2. We find that the cleaved edges of ReX2 flakes usually form an angle of ~120{deg} or ~60{deg}. In order to understand such phenomenon, we perform comprehensive investigations on the uniaxial tensile stress-strain relation of monolayer and multi-layer ReX2 sheets. Our numerical calculation shows that the particular cleaved edges of ReX2 flakes are caused by unique anisotropic ultimate tensile strengths and critical strains. We also calculate the stress-strain relation of WTe2, which explains why their cleaved edges are not corresponding to the principle axes. Our proposed mechanism about the fracture angle has also been supported by the calculated cleavage energies and surface energies for different edge surfaces.

Published in: "arXiv Material Science".

Evidence of Electron-Hole Imbalance in WTe2 from High-Resolution Angle-Resolved Photoemission Spectroscopy. (arXiv:1708.08265v1 [cond-mat.mtrl-sci])

2017-08-29T19:59:27+00:00 August 29th, 2017|Categories: Publications|Tags: , |

WTe2 has attracted a great deal of attention because it exhibits extremely large and nonsaturating magnetoresistance. The underlying origin of such a giant magnetoresistance is still under debate. Utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we reveal the complete electronic structure of WTe2. This makes it possible to determine accurately the electron and hole concentrations and their temperature dependence. We find that, with increasing the temperature, the overall electron concentration increases while the total hole concentration decreases. It indicates that the electron-hole compensation, if it exists, can only occur in a narrow temperature range, and in most of the temperature range there is an electron-hole imbalance. Our results are not consistent with the perfect electron-hole compensation picture that is commonly considered to be the cause of the unusual magnetoresistance in WTe2. We identified a flat band near the Brillouin zone center that is close to the Fermi level and exhibits a pronounced temperature dependence. Such a flat band can play an important role in dictating the transport properties of WTe2. Our results provide new insight on understanding the origin of the unusual magnetoresistance in WTe2.

Published in: "arXiv Material Science".

Pressure-Induced Phase Transition in Weyl Semimetallic WTe2

2017-08-28T10:30:32+00:00 August 28th, 2017|Categories: Publications|Tags: |

Tungsten ditelluride (WTe2) is a semimetal with orthorhombic Td phase that possesses some unique properties such as Weyl semimetal states, pressure-induced superconductivity, and giant magnetoresistance. Here, the high-pressure properties of WTe2 single crystals are investigated by Raman microspectroscopy and ab initio calculations. WTe2 shows strong plane-parallel/plane-vertical vibrational anisotropy, stemming from its intrinsic Raman tensor. Under pressure, the Raman peaks at ≈120 cm−1 exhibit redshift, indicating structural instability of the orthorhombic Td phase. WTe2 undergoes a phase transition to a monoclinic T′ phase at 8 GPa, where the Weyl states vanish in the new T′ phase due to the presence of inversion symmetry. Such Td to T′ phase transition provides a feasible method to achieve Weyl state switching in a single material without doping. The new T′ phase also coincides with the appearance of superconductivity reported in the literature. The plane-parallel/plane-vertical vibrational anisotropies of WTe2 single crystal under high pressure are investigated by Raman spectroscopy and ab initio calculations. WTe2 undergoes a phase transition from orthorhombic Td to monoclinic T′ phase at 8 GPa, where the Weyl states vanish in the new T′ phase due to the presence of inversion symmetry.

Published in: "Small".

Tungsten Dichalcogenides (WS2, WSe2, and WTe2): Materials Chemistry and Applications

2017-07-31T19:17:57+00:00 July 31st, 2017|Categories: Publications|Tags: , , |

J. Mater. Chem. A, 2017, Accepted ManuscriptDOI: 10.1039/C7TA04268J, Review ArticleAli EftekhariTungsten is the heaviest transition metal in the family of common transition metal dichalcogenides (TMDCs). Despite the essential similarities of TMDCs, the considerable differences in the size and charge of…The content of this

Published in: "Journal of Materials Chemistry A".

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