/Tag: MoTe2

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

2018-05-21T19:59:42+00:00 May 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+00:00 May 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+00:00 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".

On Low-Resistance Contacts to 2-D MoTe<sub>2</sub> by Crystalline Phase Junctions

2018-04-10T02:29:36+00:00 April 10th, 2018|Categories: Publications|Tags: |

Low contact resistance to 2-D semiconductor materials plays a critical role on their device applications. It has been experimentally demonstrated recently that the crystalline phase homojunctions between the 1T’ metallic and 2H semiconducting phases of 2-D transition metal dichacolgenide (TMDC) materials can be formed by a variety of fabrication techniques. A multiscale simulation approach that integrates atomistic ab initio simulations with quantum transport calculations based on the nonequilibrium Green’s function formalism is used to examine the contact properties of the crystalline phase junctions of monolayer MoTe2. It is shown that the following mechanisms can contribute to the low contact resistance of crystalline phase metal-semiconductor junctions of 2-D materials. First, the electric field is significantly enhanced at the 2-D phase junction interface due to the extremely thin body, which results in a thin Schottky barrier. Second, the coupling of electron wave functions cross the 1T’-2H junction interface is strong. Third, different from 3-D bulk metal-semiconductor junctions, metal-induced band gap states (MIGS) do not pin the Fermi level in the 2-D material junctions due to low dimensionality of the MIGS charge. The results provide insights into the possibility and limits of achieving low-contact-resistance contacts to 2-D TMDC semiconductors by using crystalline phase metal-semiconductor junctions.

Published in: "IEEE Transactions on Electron Devices".

Vacancy-driven phase transition in MoX2 (X: S, Se and Te) nanoscrolls

2018-03-29T02:24:19+00:00 March 29th, 2018|Categories: Publications|Tags: , , |

Nanoscale, 2018, Accepted ManuscriptDOI: 10.1039/C7NR08634B, CommunicationDa Young Hwang, Kyoung Hwan Choi, Dong Hack SuhAtomically thin MoX2 (MoS2, MoSe2 and MoTe2) exhibits semiconducting, metallic, and semi-metallic properties associated with different polymorphic phases such as 2H, 1T and distorted 1T (1T’), respectively. The phase transitions…The

Published in: "RSC Nanoscale".

Some say, that 2D Research is the best website in the world.