/Tag: NbSe2

Scanning Tunneling Microscopy of an Air Sensitive Dichalcogenide Through an Encapsulating Layer. (arXiv:1902.04808v1 [cond-mat.mes-hall])

2019-02-14T02:29:24+00:00February 14th, 2019|Categories: Publications|Tags: , |

Many atomically thin exfoliated 2D materials degrade when exposed to ambient conditions. They can be protected and investigated by means of transport and optical measurements if they are encapsulated between chemically inert single layers in the controlled atmosphere of a glove box. Here, we demonstrate that the same encapsulation procedure is also compatible with scanning tunneling microscopy (STM) and spectroscopy (STS). To this end, we report a systematic STM/STS investigation of a model system consisting of an exfoliated 2H-NbSe2 crystal capped with a protective 2H-MoS2 monolayer. We observe different electronic coupling between MoS2 and NbSe2, from a strong coupling when their lattices are aligned within a few degrees to 2 essentially no coupling for 30{deg} misaligned layers. We show that STM always probes intrinsic NbSe2 properties such as the superconducting gap and charge density wave at low temperature when setting the tunneling bias inside the MoS2 band gap, irrespective of the relative angle between the NbSe2 and MoS2 lattices. This study demonstrates that encapsulation is fully compatible with STM/STS investigations of 2D materials.

Published in: "arXiv Material Science".

Absence of dissipationless transport in clean 2D superconductors. (arXiv:1901.09310v1 [cond-mat.mes-hall])

2019-01-29T02:29:53+00:00January 29th, 2019|Categories: Publications|Tags: |

Dissipationless charge transport is one of the defining properties of superconductors (SC). The interplay between dimensionality and disorder in determining the onset of dissipation in SCs remains an open theoretical and experimental problem. In this work, we present measurements of the dissipation phase diagrams of SCs in the two dimensional (2D) limit, layer by layer, down to a monolayer in the presence of temperature (T), magnetic field (B), and current (I) in 2H-NbSe2. Our results show that the phase-diagram strongly depends on the SC thickness even in the 2D limit. At four layers we can define a finite region in the I-B phase diagram where dissipationless transport exists at T=0. At even smaller thicknesses, this region shrinks in area. In a monolayer, we find that the region of dissipationless transport shrinks towards a single point, defined by T=B=I=0. In applied field, we show that time-dependent-Ginzburg-Landau (TDGL) simulations that describe dissipation by vortex motion, qualitatively reproduce our experimental I-B phase diagram. Last, we show that by using non-local transport and TDGL calculations that we can engineer charge flow and create phase boundaries between dissipative and dissipationless transport regions in a single sample, demonstrating control over non-equilibrium states of matter.

Published in: "arXiv Material Science".

Topological Landscape of Competing Charge Density Waves in 2H-NbSe2. (arXiv:1812.11350v1 [cond-mat.mtrl-sci])

2019-01-01T02:29:30+00:00January 1st, 2019|Categories: Publications|Tags: , |

Despite decades of studies on charge density wave (CDW) of 2H-NbSe2, the origin of its incommensurate CDW ground state has not been understood. We discover that CDW of 2H-NbSe2 is composed of two different, energetically competing, structures. The lateral heterostructures of two CDWs are entangled as topological excitations, which give rise to a CDW phase shift and the incommensuration without a conventional domain wall. A partially melt network of the topological excitations and their vertices explain an unusual landscape of domains. The unconventional topological role of competing phases disclosed here can be widely applied to various incommensuration or phase coexistence phenomena in materials.

Published in: "arXiv Material Science".

Visualizing topological edge states of single and double bilayer Bi supported on multibilayer Bi(111) films. (arXiv:1812.02879v1 [cond-mat.mtrl-sci])

2018-12-10T02:29:28+00:00December 10th, 2018|Categories: Publications|Tags: , |

Freestanding single-bilayer Bi(111) is a two-dimensional topological insulator with edge states propagating along its perimeter. Given the interlayer coupling experimentally, the topological nature of Bi(111) thin films and the impact of the supporting substrate on the topmost Bi bilayer are still under debate. Here, combined with scanning tunneling microscopy and first-principles calculations, we systematically study the electronic properties of Bi(111) thin films grown on a NbSe2 substrate. Two types of non-magnetic edge structures, i.e., a conventional zigzag edge and a 2×1 reconstructed edge, coexist alternately at the boundaries of single bilayer islands, the topological edge states of which exhibit remarkably different energy and spatial distributions. Prominent edge states are persistently visualized at the edges of both single and double bilayer Bi islands, regardless of the underlying thickness of Bi(111) thin films. We provide an explanation for the topological origin of the observed edge states that is verified with first-principles calculations. Our paper clarifies the long-standing controversy regarding the topology of Bi(111) thin films and reveals the tunability of topological edge states via edge modifications.

Published in: "arXiv Material Science".

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".

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