/Tag: NbSe2

Comprehensive optical characterization of atomically thin ${mathrm{NbSe}}_{2}$

2018-10-05T14:33:34+00:00October 5th, 2018|Categories: Publications|Tags: |

Author(s): Heather M. Hill, Albert F. Rigosi, Sergiy Krylyuk, Jifa Tian, Nhan V. Nguyen, Albert V. Davydov, David B. Newell, and Angela R. Hight WalkerTransition-metal dichalcogenides (TMDCs) have offered experimental access to quantum confinement in one dimension. In recent years, metallic TMDCs like NbSe2 have taken center stage with many of them exhibiting interesting temperature-dependent properties such as charge density waves and superconduc…[Phys. Rev. B 98, 165109] Published Fri Oct 05, 2018

Published in: "Physical Review B".

The effect of Sn intercalation on the superconducting properties of 2H-NbSe2. (arXiv:1808.07219v1 [cond-mat.supr-con])

2018-08-23T02:29:17+00:00August 23rd, 2018|Categories: Publications|Tags: |

2H-NbSe2 is known to be an archetype layered transitional metal dichalcogenide superconductor with a superconducting transition temperature of 7.3 K.In this article, we investigate the influence of Sn intercalation on superconducting properties of 2H-NbSe2. Sn being nonmagnetic and having no outer shell d-electrons unlike transition metals, one naively would presume that its effect on superconducting properties will be very marginal. However, our magnetic and transport studies reveal a significant reduction of both superconducting transition temperature and upper critical field [Tc and BC2 (0)] upon Sn intercalation. With a mere 4 mole% Sn intercalation, it is observed that Tc and BC2 (0) get suppressed by ~ 3.5 K and 3 T, respectively. Werthamer-Helfand-Hohenberg (WHH) analysis of magneto-transport data is performed to estimate BC2 (0). From the low temperature Raman scattering data in the normal phase of intercalated NbSe2, it is inferred that the suppression of superconductivity cannot be ascribed to strengthening of charge density wave (CDW)ordering. The effects such as electron-doping induced Fermi surface change and/or disorder scattering upon intercalation are speculated to be at play for the observed phenomena.

Published in: "arXiv Material Science".

Heat transfer at the van der Waals interface between graphene and NbSe2. (arXiv:1807.07216v1 [cond-mat.mes-hall])

2018-07-20T00:30:21+00:00July 20th, 2018|Categories: Publications|Tags: , |

Graphene has been widely used to construct low-resistance van der Waals (vdW) contacts to other two-dimensional (2D) materials. However, a rise of electron temperature of the graphene under a current flow has not been seriously considered in many applications. Owing to its small electronic heat capacity and electron-phonon coupling, electron temperature of the graphene can be increased easily by the application of current. The heat generated within the graphene is transferred to the contacted 2D materials through the vdW interface and potentially influences their properties. Here, we compare the superconducting critical currents of an NbSe2 flake for two different methods of current application: with a Au/Ti electrode fabricated by thermal evaporation and with a graphene electrode contacted to the NbSe2 flake through a vdW interface. The influence of the heat transfer from the graphene to NbSe2 is detected through the change of the superconductivity of NbSe2. We found that the critical current of NbSe2 significantly reduces when the current is applied with the graphene electrode compared to that from the conventional Au/Ti electrode. Further, since the electron heating in graphene exhibits ambipolar back-gate modulation, we demonstrate the electric field modulation of the critical current in NbSe2 when the current is applied with graphene electrode. These results are attributed to the significant heat transfer from the graphene electrode to NbSe2 through vdW interface.

Published : "arXiv Mesoscale and Nanoscale Physics".

Inter Landau level Andreev reflection at the Dirac point in graphene quantum Hall state coupled to NbSe2 superconductor. (arXiv:1807.07318v1 [cond-mat.mes-hall])

2018-07-20T00:30:14+00:00July 20th, 2018|Categories: Publications|Tags: , |

Superconductivity and quantum Hall effect are distinct states of matter occurring in apparently incompatible physical conditions. Recent theoretical developments suggest that the coupling of quantum Hall effect with a superconductor can provide a fertile ground for realizing exotic topological excitations such as non-abelian Majorana fermions or Fibonacci particles. As a step toward that goal, we report observation of Andreev reflection at the junction of a quantum Hall edge state in a single layer graphene and a quasi-two dimensional niobium diselenide (NbSe2) superconductor. Our principal finding is the observation of an anomalous finite-temperature conductance peak located precisely at the Dirac point providing a definitive evidence for inter-Landau level Andreev reflection in a quantum Hall system. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in Andreev physics. This study paves the way for investigating analogous Andreev reflection in a fractional quantum Hall system coupled to a superconductor to realize exotic quasiparticles.

Published : "arXiv Mesoscale and Nanoscale Physics".

Experimental and Theoretical Electronic Structure and Symmetry Effects in Ultrathin NbSe2 Films. (arXiv:1806.02521v1 [cond-mat.mes-hall])

2018-06-08T02:29:21+00:00June 8th, 2018|Categories: Publications|Tags: |

Layered quasi-two-dimensional transition metal dichalcogenides (TMDCs), which can be readily made in ultrathin films, offer excellent opportunities for studying how dimensionality affects electronic structure and physical properties. Among all TMDCs, NbSe2 is of special interest; bulk NbSe2 hosts a charge-density-wave phase at low temperatures and has the highest known superconducting transition temperature, and these properties can be substantially modified in the ultrathin film limit. Motivated by these effects, we report herein a study of few-layer NbSe2 films, with a well-defined single-domain orientation, epitaxially grown on Gallium Arsenide (GaAs). Angle-resolved photoemission spectroscopy (ARPES) was used to determine the electronic band structure and the Fermi surface as a function of layer thickness; first-principles band structure calculations were performed for comparison. The results show interesting changes as the film thickness increases from a monolayer (ML) to several layers. The most notable changes occur between a ML and a bilayer, where the inversion symmetry in bulk NbSe2 is preserved in the bilayer but not in the ML. The results illustrate some basic dimensional effects and provide a basis for further exploring and understanding the properties of NbSe2.

Published in: "arXiv Material Science".

Anisotropic band splitting in monolayer NbSe<sub>2</sub>: implications for superconductivity and charge density wave

2018-05-04T09:09:06+00:00May 4th, 2018|Categories: Publications|Tags: , |

Anisotropic band splitting in monolayer NbSe2: implications for superconductivity and charge density waveAnisotropic band splitting in monolayer NbSe<sub>2</sub>: implications for superconductivity and charge density wave, Published online: 03 May 2018; doi:10.1038/s41699-018-0057-3The interplay between symmetry breaking, superconductivity and charge density wave in monolayer NbSe2 is unveiled by spectroscopic techniques. A team led by Takafumi Sato at Tohoku University used molecular beam epitaxy to fabricate atomically thin NbSe2 films on bilayer graphene, and investigated their electronic properties by a combination of electrical transport measurements, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy. A charge density wave transition with a strong periodic charge modulation was detected below 1.5 K, accompanied by the emergence of superconductivity. Owing to the inversion symmetry breaking occurring in monolayer NbSe2, a band splitting developed along the Γ-Κ direction of the Brillouin zone. Such band splitting was found to be related to the robustness of the observed superconductivity, as well as the formation of the charge density wave.

Published in: "NPJ 2D Materials and Applications".

An unusual continuous paramagnetic-limited superconducting phase transition in 2D NbSe<sub> 2 </sub>

2018-04-30T16:34:03+00:00April 30th, 2018|Categories: Publications|Tags: |

An unusual continuous paramagnetic-limited superconducting phase transition in 2D NbSe 2 An unusual continuous paramagnetic-limited superconducting phase transition in 2D NbSe<sub> 2 </sub>, Published online: 30 April 2018; doi:10.1038/s41563-018-0061-1Tunnelling spectroscopy reveals a continuous closing of the superconducting gap at low temperature and high in-plane magnetic field in few-layer NbSe2, due to the Ising spin–orbit coupling of these materials.

Published in: "Nature Materials".

Bursting at the seams: Rippled monolayer bismuth on NbSe2

2018-04-13T18:32:58+00:00April 13th, 2018|Categories: Publications|Tags: |

Bismuth, one of the heaviest semimetals in nature, ignited the interest of the materials physics community for its potential impact on topological quantum material systems that use its strong spin-orbit coupling and unique orbital hybridization. In particular, recent theoretical predictions of unique topological and superconducting properties of thin bismuth films

Published in: "Science Advances".

Tuning Ising superconductivity with layer and spin–orbit coupling in two-dimensional transition-metal dichalcogenides

2018-04-12T10:32:03+00:00April 12th, 2018|Categories: Publications|Tags: , , |

Tuning Ising superconductivity with layer and spin–orbit coupling in two-dimensional transition-metal dichalcogenidesTuning Ising superconductivity with layer and spin–orbit coupling in two-dimensional transition-metal dichalcogenides, Published online: 12 April 2018; doi:10.1038/s41467-018-03888-4Monolayer transition-metal dichalcogenide (TMD) is promising to host features of topological superconductivity. Here, de la Barrera et al. study layered compounds, 2H-TaS2 and 2H-NbSe2, in their atomic layer limit and find a largest upper critical field for an intrinsic TMD superconductor.

Published in: "Nature Communications".

Investigation of the two-gap superconductivity in a few-layer ${mathrm{NbSe}}_{2}$-graphene heterojunction

2018-02-16T16:30:45+00:00February 16th, 2018|Categories: Publications|Tags: , |

Author(s): Tianyi Han, Junying Shen, Noah F. Q. Yuan, Jiangxiazi Lin, Zefei Wu, Yingying Wu, Shuigang Xu, Liheng An, Gen Long, Yuanwei Wang, Rolf Lortz, and Ning WangWe investigated the superconductivity in a few-layer NbSe2-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance …[Phys. Rev. B 97, 060505(R)] Published Fri Feb 16, 2018

Published in: "Physical Review B".

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+00:00January 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".

Superconductivity and Magnetotransport of Single-Crystalline NbSe2 Nanoplates Grown by Chemical Vapour Deposition

2017-10-03T14:24:27+00:00October 3rd, 2017|Categories: Publications|Tags: |

Nanoscale, 2017, Accepted ManuscriptDOI: 10.1039/C7NR06617A, CommunicationYi-Chao Zou, Zhi-Gang Chen, Enze Zhang, Faxian Xiu, Syo Matsumura, Lei Yang, Min Hong, Jin ZouNbSe2 is a typical transition metal dichalcogenide with rich variety of electronic ground states existing in its nanostructures, including two-dimensional superconductivity and charge

Published in: "RSC Nanoscale".

High-quality monolayer superconductor NbSe<sub>2</sub> grown by chemical vapour deposition

2017-08-30T10:30:02+00:00August 30th, 2017|Categories: Publications|Tags: |

High-quality monolayer superconductor NbSe2 grown by chemical vapour depositionNature Communications, Published online: 30 August 2017; doi:10.1038/s41467-017-00427-5Two-dimensional superconductors will likely have applications not only in devices, but also in the study of fundamental physics. Here, Wang et al. demonstrate the CVD growth of superconducting NbSe2 on a variety of substrates, making these novel materials increasingly accessible.

Published in: "Nature Communications".

Interlayer bond polarizability model for stacking-dependent low-frequency Raman scattering in layered materials. (arXiv:1708.02614v1 [cond-mat.mes-hall])

2017-08-10T19:58:48+00:00August 10th, 2017|Categories: Publications|Tags: , , , , |

Two-dimensional (2D) layered materials have been extensively studied owing to their fascinating and technologically relevant properties. Their functionalities can be often tailored by the interlayer stacking pattern. Low-frequency (LF) Raman spectroscopy provides a quick, non-destructive and inexpensive optical technique for stacking characterization, since the intensities of LF interlayer vibrational modes are sensitive to the details of the stacking. A simple and generalized interlayer bond polarizability model is proposed here to explain and predict how the LF Raman intensities depend on complex stacking sequences for any thickness in a broad array of 2D materials, including graphene, MoS2, MoSe2, NbSe2, Bi2Se3, GaSe, h-BN, etc. Additionally, a general strategy is proposed to unify the stacking nomenclature for these 2D materials. Our model reveals the fundamental mechanism of LF Raman response to the stacking, and provides general rules for stacking identification.

Published : "arXiv Mesoscale and Nanoscale Physics".

Observing imperfection in atomic interfaces for van der Waals heterostructures. (arXiv:1707.08140v1 [cond-mat.mes-hall])

2017-07-27T19:59:10+00:00July 27th, 2017|Categories: Publications|Tags: , , , , , , |

Vertically stacked van der Waals heterostructures are a lucrative platform for exploring the rich electronic and optoelectronic phenomena in two-dimensional materials. Their performance will be strongly affected by impurities and defects at the interfaces. Here we present the first systematic study of interfaces in van der Waals heterostructure using cross sectional scanning transmission electron microscope (STEM) imaging. By measuring interlayer separations and comparing these to density functional theory (DFT) calculations we find that pristine interfaces exist between hBN and MoS2 or WS2 for stacks prepared by mechanical exfoliation in air. However, for two technologically important transition metal dichalcogenide (TMDC) systems, MoSe2 and WSe2, our measurement of interlayer separations provide the first evidence for impurity species being trapped at buried interfaces with hBN: interfaces which are flat at the nanometer length scale. While decreasing the thickness of encapsulated WSe2 from bulk to monolayer we see a systematic increase in the interlayer separation. We attribute these differences to the thinnest TMDC flakes being flexible and hence able to deform mechanically around a sparse population of protruding interfacial impurities. We show that the air sensitive two dimensional (2D) crystal NbSe2 can be fabricated into heterostructures with pristine interfaces by processing in an inert-gas environment. Finally we find that adopting glove-box transfer significantly improves the quality of interfaces for WSe2 compared to processing in air.

Published : "arXiv Mesoscale and Nanoscale Physics".

Strongly enhanced charge-density-wave order in monolayer NbSe2

2016-10-15T11:35:12+00:00July 20th, 2015|Categories: Publications|Tags: |

By Xiaoxiang Xi Nature Nanotechnology.

Authors: Xiaoxiang Xi, Liang Zhao, Zefang Wang, Helmuth Berger, László Forró, Jie Shan & Kin Fai Mak
Two-dimensional materials possess very different properties from their bulk counterparts. While changes in single-particle electronic properties have been investigated …read more

Published in: Nature Nanotechnology (Advanced Online Publication)

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