Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P09: Superconductivity in Dichalchogenides and Similar MaterialsFocus
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Sponsoring Units: DCMP Chair: Patrick Vora, George Mason Univ Room: BCEC 151A |
Wednesday, March 6, 2019 2:30PM - 2:42PM |
P09.00001: Infinite-randomness fixed point of the quantum superconductor-metal transitions in amorphous thin films Nicholas Lewellyn, Ilana M Percher, JJ Nelson, Javier Garcia-Barriocanal, Irina Volotsenko, Aviad Frydman, Thomas Vojta, Allen M Goldman The quantum superconductor-metal transition of amorphous indium oxide films has been investigated. As is typical of a field tuned quantum phase transition the crossing of the magnetoresistance isotherms is observed. In contrast with what is found in the case of direct superconductor-insulator transitions in films with lower mobilities, the isotherms do not cross at a single value of magnetic field. Instead the crossing field changes systematically with temperature. Applying a conventional power-law scaling analysis to these films using isotherms near selected crossing fields results in both a temperature and magnetic field dependent exponent product, νz. The temperature dependence of this effective νz value was fit to a theoretically predicted form. Using this fit as a starting point an activated scaling analysis was applied to the data. The critical field was varied until the isotherms collapsed. The ability to fit this transition using activated scaling shows that this transition is well described by an infinite-randomness critical point and quantum Griffiths singularities. |
Wednesday, March 6, 2019 2:42PM - 2:54PM |
P09.00002: ABSTRACT WITHDRAWN
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Wednesday, March 6, 2019 2:54PM - 3:06PM |
P09.00003: Superconductivity in few-layer NbSe2 grown by molecular beam epitaxy Cliff Chen, Peng Wei Superconductors with strong spin-orbit coupling (SOC) are attractive for generating triplet Cooper pairs and topological superconductivity (TSC). In few-layer NbSe2, strong SOC is reported to give rise to TSC. However, as of date, few-layer NbSe2 is produced mainly through mechanical exfoliation. There is a demand to construct large scale NbSe2 thin films to further enable fabrication of scalable heterostructures that can exhibit the unique properties of TSC. Recent reports show that NbSe2 can be grown on graphene by molecular beam epitaxy (MBE). In contrast, in this talk, we report our progress on few-layer NbSe2 grown on insulating substrates by MBE. The layers are characterized by in-situ reflection high energy electron diffraction (RHEED) and ex-situ Raman spectroscopy. A clear superconducting transition has been observed with the transition temperature (Tc) above 2K. We will discuss correlations between layer quality and the Tc. We will also present our studies regarding the critical field in thin film NbSe2. |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P09.00004: Ising superconductivity in NbSe2-xTex with tunable spin-orbit coupling Zhenyu Zhang, Leiqiang Li, Wei Qin, Ping Cui, Changgan Zeng The Ising superconductors of monolayered transition metal dichalcogenides such as MoS2 and NbSe2 have been shown to possess strongly enhanced upper critical fields. In these systems, the spin-orbit coupling (SOC) pins the electron spins to the out-of-plane direction because of in-plane mirror symmetry breaking, thereby becoming insensitive to the lateral external magnetic fields. Here, using first-principles density functional theory calculations, we demonstrate that the Te substitution of Se in NbSe2 will enhance the strength of the SOC, as characterized by the increased spin splitting in the electronic structures of NbSe2-xTex. As a consequence, the upper critical fields of these systems will be further enhanced. Moreover, as x approaches to 2, the enhanced SOC leads to a band inversion around the high symmetry M point in the Brillouin zone, indicating a quantum phase transition towards a topological superconducting state. |
Wednesday, March 6, 2019 3:18PM - 3:30PM |
P09.00005: The enhancement of in-plane Hc2 in monolayer centrosymmetric superconductor 1T'-WTe2 via strong spin-orbital coupling Yingming Xie, Wenyu He, Kam Tuen Law Recently, it was reported that the two-dimensional quantum spin Hall insulator 1T'-WTe2 can be gated into the superconducting phase. The in-plane Hc2 in this centrosymmetric superconductor is enhanced up to 2∼4 times Pauli limit. However, unlike the typical enhancement of Hc2 in a non-centrosymmetric superconductor, the mechanism here is not clear. In this work, we find due to the strong spin-orbital coupling (SOC), the spin susceptibility is reduced in the low gating region. In other words, the magnetic field felt by electrons near the Fermi energy is renormalized, which in turn enhances the Hc2. The amplitude of enhancement is comparable to the experiments. Moreover, in this mechanism, the anisotropic SOC will lead to obvious anisotropy in Hc2, which can be tested in experiments by rotating the magnetic field. |
Wednesday, March 6, 2019 3:30PM - 3:42PM |
P09.00006: Superconductivity in Br-doped misfit compounds (PbSe)1.12(TaSe2) Zhuan Xu We report the discovery of superconductivity with a maximum Tc of 1.28 K in Br-doped (PbSe)1:12(TaSe2), which is a new misfit compound consisting of alternating layers of distorted rocksalt PbSe and dichalcogenide TaSe2. The Br-doping is required for the formation of this misfit compound and superconductivity can be tuned by changing Br content. The large anisotropic parameters in both resistivity and upper critical field Hc2 are found. The estimated c-axis coherence length of 6.3 nm is larger than the c-axis lattice constant, which implies that this compound is an anisotropic three-dimensional superconductor. The Hall coefficient measurements suggest that the charge transport is dominated by the hole-type charge carrier and there is a charge transfer from the PbSe layer to the conducting TaSe2 layer. The small normalized specific heat jump and electron-phonon coupling constant of 0.61 indicate that Br-doped (PbSe)1:12(TaSe2) should be a weak-coupling BCS superconductor. The point-contact Andreev reflection spectroscopy is also studied. |
Wednesday, March 6, 2019 3:42PM - 3:54PM |
P09.00007: Superconductivity and Charge Density Wave in SnSe2-based Heterostructures Yimin Zhang, Jia-Qi Fan, CanLi Song, Xucun Ma, Qikun Xue We report the direct observation of interface superconductivity and interface-induced charge density wave on monolayer SnSe2-based heterostructures. Tunneling spectrums on epitaxial monolayer SnSe2 grown on graphited SiC(0001) substrate showed a fully-gapped superconducting state with rather conventional character. Occurrence of vortices under external magnetic field further confirmed the superconductivity. We attribute the superconductivity to the two-dimensional gas formed at the interface of SnSe2 and graphene. Besides, charge density waves (CDW) with 2 × 2 periodicity were observed in SnSe2 thin films grown on SrTiO3(001) and Si(111), which disappear as the film thickness increases. Our finding opens new perspectives to understand not only interface superconductivity but also possible relationship between supercondcuvity and CDW. |
Wednesday, March 6, 2019 3:54PM - 4:06PM |
P09.00008: Superconductivity in electron-doped layered 1T-SnSe2 Hanlin Wu, Sheng Li, Timothy Haugan, Michael Susner, Bing Lv Layered metal chalcogenide 2D materials have attracted great attention in the recent years due to their emergent new physics phenomena such as surface states, magnetism, giant magnetoresistance, quantum spin Hall effects, and superconductivity. Chemical intercalation, or physical electron-gating, has been found out to be very effective to tune the electronic structures and the associated physical properties. In this presentation, we will focus on our experimental intercalation studies of the CdI2-type 1T-SnSe2 phase. Both electron doping by alkaline metal intercalation and polar organic solvent cointercaltion have been carried out through soft chemical methods. The associated structural characteristics, magnetic, electrical resistivity changes upon doping, and the unconventional superconductivity will be presented. The possible competing orders of charge density wave and superconductivity upon intercalation will be discussed as well. |
Wednesday, March 6, 2019 4:06PM - 4:18PM |
P09.00009: Theory of unconventional superconductivity in gated monolayer WTe2 Yi-Ting Hsu, William Cole, Rui-Xing Zhang, Jay Sau Recent observation of superconductivity in monolayer WTe2 upon gating has attracted much interest since the undoped WTe2 has been experimentally established as a quantum spin Hall insulator. Although the nature of the superconductivity remains elusive, the prevailing expectation is that inducing superconductivity in already topological materials is a promising route to identifying new exotic superconductors. Here we investigate the dominant pairing instabilities under different microscopic interactions by solving the linearized gap equation. We find equal-spin paired state that is consistent with the experimentally found high critical field, and we discuss the possibility of topological paired state. Finally, we consider the effects of a single impurity, as the first step towards determining how disorder impacts the phase diagram. |
Wednesday, March 6, 2019 4:18PM - 4:30PM |
P09.00010: Nodeless superconductivity in the type-II Dirac semimetal PdTe2: London penetration depth and pairing-symmetry analysis Peter Orth, Serafim Teknowijoyo, Na Hyun Jo, Mathias Scheurer, Makariy A Tanatar, Kyuil Cho, Sergey Budko, P.C. Canfield, Ruslan Prozorov London penetration depth and normal-state resistivity were measured in single crystals of type-II Dirac semimetal PdTe2. Superfluid density is exponential at low temperatures and follows single-gap BCS model with weak-coupling Delta(0)/Tc = 1.76 in the whole temperature range. Electrical resistivity is compatible with a classical metal with dominant electron-phonon scattering obtained from the fit to Bloch-Grüneisen formula with a Debye temperature of 207 K from specific heat data. We compare these experimental results with expectations from a detailed theoretical symmetry analysis and reduce the number of possible superconducting pairing states in PdTe2 to only three nodeless candidates: a regular, topologically trivial s-wave pairing, and two distinct odd-parity triplet states that both can be topologically nontrivial depending on the microscopic interactions driving the superconducting instability. Finally, we theoretically discuss the effect of disorder on the different pairing states. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P09.00011: Robust parity-mixed superconductivity in disordered 2D transition metal dichalcogenides David Möckli, Maxim Khodas Monolayer NbSe2 is a nodal topological Ising superconductor at magnetic in-plane fields exceeding the Pauli limit, with nodal points strictly on high symmetry lines in the Brillouin zone. Here, we use a combined numerical and group-theoretical approach in real-space to characterize the unconventional superconducting state in monolayer transition metal dichalcogenides. Even with a conventional pairing interaction, the superconducting state is intrinsically parity-mixed and robust against on-site disorder. The interplay between the Zeeman magnetic field, strong spin-orbit interaction, and electronic orbital content confer the unique superconducting and topological properties. The discussion also extends to strongly hole-doped MoS$_2$ and its relatives. |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P09.00012: Ising superconductors in in-plane magnetic fields Stefan Ilic, Julia Meyer, Manuel Houzet
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Wednesday, March 6, 2019 4:54PM - 5:06PM |
P09.00013: Superconductivity in Re6Se8Cl2, a Superatomic Two Dimensional Semiconductor Evan Telford, Jake Russell, Josh Swann, Kihong Lee, Xiaoyang Zhu, Xavier Roy, Cory R Dean We report on low temperature transport measurements of Re6Se8Cl2 - a synthesized van der Waal cluster solid with a hierarchical structure composed of covalently linked Re6Se8 clusters. Samples are prepared by mechanical exfoliation onto a PDMS stamp, then transferred to metallic leads pre-patterned on a SiO2 substrate. As grown, this material is a weakly-doped semiconductor, with strongly insulating behavior observed at cryogenic temperatures. Using a current annealing technique we demonstrate the ability to induce an insulator to superconductor transition, in situ. This transition is accompanied by an increase in the carrier density by 3-4 orders of magnitude. The chemical (EDX) and structural (Raman) analysis after current annealing show no change in chemical composition or lattice structure. We conjecture that the transition results from a small loss of interplanar chlorine atoms through the application of large currents. Details of the superconducting state and its possible nature will be discussed. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P09.00014: Superconductivity in 1D Zigzag Nanowires Yun-Yi Pai, Megan Briggeman, Hyungwoo Lee, Jung-Woo Lee, Mengchen Huang, Jianan Li, Chang-Beom Eom, Patrick Irvin, Jeremy Levy We investigate the effects of the geometrical shapes of the 1D nanowires created at LaAlO3/SrTiO3 in order to study the origin of superconductivity of SrTiO3, which has puzzled researchers for more than 50 years. It was recently suggested that electron pairing in SrTiO3 may be related to ferroelastic domain walls [1]. By using conductive-AFM lithography [2] to create 1D superconducting nanowires at the LaAlO3/SrTiO3 interface, we can investigate the influence of nanowire shape, and in particular the effect of sharp turns in the nanowire, on superconducting behavior in the nanowires. We find a significant stabilization of the superconducting state in “zigzag” wires compared with straight control wires, both created at the same time and located only a few micrometers apart. We discuss possible origins for this behavior within the context of ferroelastic domain structure that surrounds the nanowires. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P09.00015: Vestigial nematic phase due to superconductivity in doped Bi2Se3, and its influence on the superconducting ground state in an applied magnetic field Matthias Hecker, Erez Berg, Joerg Schmalian Under Cu or Nb doping, the topological insulator Bi2Se3 becomes superconducting with a maximal Tc~3-4 K. The pairing symmetry has been intensely studied and in recent years, experiments have consistently pointed towards an unconventional, odd-parity triplet pairing in the two-dimensional irreducible representation Eu of the crystal point group D3d. Due to a low carrier density and a small ratio of the superconducting coherence length and the Fermi wavelength, fluctuations are increasingly important, even to the extent that they allow for a preformed nematic state with T_n> T_c. Inside this performed phase, we study resistivity, elastic constants, and fluctuating diamagnetism. Furthermore, we investigate how the superconducting ground state in a constant magnetic field is affected by the preformed phase. |
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