/Tag: MoTe2

Evidence of Coulomb interaction induced Lifshitz transition and robust hybrid Weyl semimetal in Td MoTe2. (arXiv:1808.08816v1 [cond-mat.str-el])

2018-08-28T02:29:38+00:00August 28th, 2018|Categories: Publications|Tags: , |

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of Td MoTe2. We found that on-site Coulomb interaction leads to a Lifshitz transition, which is essential for a precise description of the electronic structure. A hybrid Weyl semimetal state with a pair of energy bands touching at both type-I and type-II Weyl nodes is indicated by comparing the experimental data with theoretical calculations. Unveiling the importance of Coulomb interaction opens up a new route to comprehend the unique properties of MoTe2, and is significant for understanding the interplay between correlation effects, strong spin-orbit coupling and superconductivity in this van der Waals material.

Published in: "arXiv Material Science".

Barkhausen effect in the first order structural phase transition in type-II Weyl semimetal MoTe2. (arXiv:1808.05735v1 [cond-mat.mtrl-sci])

2018-08-20T02:29:14+00:00August 20th, 2018|Categories: Publications|Tags: |

We report the first observation of the non-magnetic Barkhausen effect in van der Waals layered crystals, specifically, between the Td and 1T’ phases in type-II Weyl semimetal MoTe2. Thinning down the MoTe2 crystal from bulk material to about 25nm results in a drastic strengthening of the hysteresis in the phase transition, with the difference in critical temperature increasing from 40K to more than 300K. The Barkhausen effect appears for thin samples and the temperature range of the Barkhausen zone grows approximately linearly with reducing sample thickness, pointing to a surface origin of the phase pinning defects. The distribution of the Barkhausen jumps shows a power law behavior, with its critical exponent {alpha} = 1.27, in good agreement with existing scaling theory. Temperature-dependent Raman spectroscopy on MoTe2 crystals of various thicknesses shows results consistent with our transport measurements.

Published in: "arXiv Material Science".

Ultrafast Dynamics of Electron-phonon Coupling in Transition-metal Dichalcogenides. (arXiv:1807.10879v1 [cond-mat.mtrl-sci])

2018-07-31T02:29:17+00:00July 31st, 2018|Categories: Publications|Tags: |

Time-domain femtosecond laser spectroscopic measurements of the ultrafast lattice dynamics in 2H-MoTe2 bulk crystals were carried out to understand the carrier-phonon interactions that govern electronic transport properties. An unusually long lifetime coherent A1g phonon mode was observed even in the presence of very large density of photo-excited carriers at room temperature. The decay rate was observed to decrease with increasing excitation laser fluence. Based on the laser fluence dependence including the inducement of significant phonon softening and a peculiar decrease in phonon decay rate, we attribute the long lifetime lattice dynamics to weak anharmonic phonon-phonon coupling and a carrier-density-dependent deformation potential electron-phonon coupling.

Published in: "arXiv Material Science".

Transport properties of semimetallic transition metal dichalcogenides. (arXiv:1807.08227v1 [cond-mat.mtrl-sci])

2018-07-24T02:29:16+00:00July 24th, 2018|Categories: Publications|Tags: |

The Weyl semimetal requires the breaking of either the time-reversal symmetry (TRS) or the lattice inversion symmetry. When the TRS and inversion symmetry coexist, a pair of degenerate Weyl points may exist, leading to the related Dirac semimetal phase. In other words, a Dirac semimetallic state can be regarded as two copies of Weyl semimetal states. In this dissertation, we study tellurium based compounds like the Weyl semimetal candidate MoTe2 and the Dirac semimetal candidate PtTe2 within the transition metal dichalcogenides family.

Published in: "arXiv Material Science".

High performance Tunnel Field Effect Transistors based on in-plane transition metal dichalcogenide heterojunctions. (arXiv:1807.07128v1 [cond-mat.mes-hall])

2018-07-20T00:30:24+00:00July 20th, 2018|Categories: Publications|Tags: , , |

In-plane heterojunction tunnel field effect transistors based on monolayer transition metal dichalcogenides are studied by means of self-consistent non-equilibrium Green’s functions simulations and an atomistic tight-binding Hamiltonian. We start by comparing several heterojunctions before focusing on the most promising ones, i.e WTe2-MoS2 and MoTe2-MoS2. The scalability of those devices as a function of channel length is studied, and the influence of backgate voltages on device performance is analysed. Our results indicate that, by fine-tuning the design parameters, those devices can yield extremely low sub-threshold swings (below 5mV/decade) and Ion/Ioff ratios higher than 1e8 at a supply voltage of 0.3V, making them ideal for ultra-low power consumption.

Published : "arXiv Mesoscale and Nanoscale Physics".

Light induced sub-picosecond topological phase transition in MoTe2. (arXiv:1806.09075v1 [cond-mat.mtrl-sci])

2018-06-26T02:29:21+00:00June 26th, 2018|Categories: Publications|Tags: |

Recent development of ultrashort laser pulses allows for optical control of structural and electronic properties of complex quantum materials. The layered transition metal dichalcogenide MoTe2, which can crystalize into several different structures with distinct topological and electronic properties, provides possibilities to control or switch between different phases. In this study we report a photo-induced sub-picosecond structural transition between the type-II Weyl semimetal phase and normal semimetal phase in bulk crystalline MoTe2 by using ultrafast pump-probe and time-resolved second harmonic generation spectroscopy. The phase transition is most clearly characterized by the dramatic change of the shear oscillation mode and the intensity loss of second harmonic generation. This work opens up new possibilities for ultrafast manipulation of the topological properties of solids, enabling potentially practical applications for topological switch device with ultrafast excitations.

Published in: "arXiv Material Science".

Efficient Topological Materials Discovery Using Symmetry Indicators. (arXiv:1805.07314v1 [cond-mat.mes-hall])

2018-05-21T19:59:42+00:00May 21st, 2018|Categories: Publications|Tags: , |

Although the richness of spatial symmetries has led to a rapidly expanding inventory of possible topological crystalline (TC) phases of electrons, physical realizations have been slow to materialize due to the practical difficulty to ascertaining band topology in realistic calculations. Here, we integrate the recently established theory of symmetry indicators of band topology into first-principle band-structure calculations, and test it on a databases of previously synthesized crystals. The combined algorithm is found to efficiently unearth topological materials and predict topological properties like protected surface states. On applying our algorithm to just 8 out of the 230 space groups, we already discover numerous materials candidates displaying a diversity of topological phenomena, which are simultaneously captured in a single sweep. The list includes recently proposed classes of TC insulators that had no previous materials realization as well as other topological phases, including: (i) a screw-protected 3D TC insulator, b{eta}-MoTe2, with gapped surfaces except for 1D helical “hinge” states; (ii) a rotation-protected TC insulator BiBr with coexisting surface Dirac cones and hinge states; (iii) non-centrosymmetric Z2 topological insulators undetectable using the well-established parity criterion, AgXO (X=Na,K,Rb); (iv) a Dirac semimetal MgBi2O6; (v) a Dirac nodal-line semimetal AgF2; and (vi) a metal with three-fold degenerate band crossing near the Fermi energy, AuLiMgSn. Our work showcases how the recent theoretical insights on the fundamentals of band structures can aid in the practical goal of discovering new topological materials.

Published : "arXiv Mesoscale and Nanoscale Physics".

Exciton states in monolayer MoSe2 and MoTe2 probed by upconversion spectroscopy. (arXiv:1805.04440v1 [cond-mat.mes-hall])

2018-05-14T19:58:49+00:00May 14th, 2018|Categories: Publications|Tags: , , , |

Transitions metal dichalcogenides (TMDs) are direct semiconductors in the atomic monolayer (ML) limit with fascinating optical and spin-valley properties. The strong optical absorption of up to 20 % for a single ML is governed by excitons, electron-hole pairs bound by Coulomb attraction. Excited exciton states in MoSe$_2$ and MoTe$_2$ monolayers have so far been elusive due to their low oscillator strength and strong inhomogeneous broadening. Here we show that encapsulation in hexagonal boron nitride results in emission line width of the A:1$s$ exciton below 1.5 meV and 3 meV in our MoSe$_2$ and MoTe$_2$ monolayer samples, respectively. This allows us to investigate the excited exciton states by photoluminescence upconversion spectroscopy for both monolayer materials. The excitation laser is tuned into resonance with the A:1$s$ transition and we observe emission of excited exciton states up to 200 meV above the laser energy. We demonstrate bias control of the efficiency of this non-linear optical process. At the origin of upconversion our model calculations suggest an exciton-exciton (Auger) scattering mechanism specific to TMD MLs involving an excited conduction band thus generating high energy excitons with small wave-vectors. The optical transitions are further investigated by white light reflectivity, photoluminescence excitation and resonant Raman scattering confirming their origin as excited excitonic states in monolayer thin semiconductors.

Published : "arXiv Mesoscale and Nanoscale Physics".

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+00:00May 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".

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