Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L31: Superconductivity and Correlated States in 2D Materials IFocus
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Sponsoring Units: DMP Chair: Masahiro Ishigami, University of Central Florida Room: 294 |
Wednesday, March 15, 2017 11:15AM - 11:51AM |
L31.00001: TBD - 2D Materials: Metals, Superconductors, and Correlated Materials Invited Speaker: Kin Fai Mak |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L31.00002: Disorder-enhanced superconductivity in epitaxial monolayer NbSe$_{2}$ Kun Zhao Noncentrosymmetric monolayer NbSe$_{2}$ film has successfully been synthesized on graphene/SiC(0001) substrate by molecular beam epitaxy. Using the \textit{in situ} ultralow-temperature scanning tunneling microscopy, the superconductivity of epitaxial monolayer NbSe$_{2}$ film has been confirmed. The superconducting gap is 0.12 meV and the transition temperature is 0.90 K. Impurity scattering has successfully been introduced into monolayer NbSe$_{2}$ film by depositing Si atoms on its surface at low temperature. Remarkably, an anomalous enhancement of superconductivity induced by disorder has been discovered and a dome-shaped superconducting phase diagram has been observed. At the optimal-doping coverage, the superconducting gap is 0.44 meV and the transition temperature is 2.37 K. Our discovery not only sheds light to better understanding of superconductivity in layered transition metal dichalcogenides but also paves a new avenue of achieving higher $T_{c}$ in noncentrosymmetric superconductors. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L31.00003: Anistrotropic two-gap superconductivity and electron-phonon coupling in 2H-NbS$_2$ Christoph Heil, Samuel Ponc\'{e}, Henry Lambert, Elena R. Margine, Feliciano Giustino We investigate from first principles the nature of the superconducting state of 2$H$-NbS$_2$ using the fully anisotropic Migdal-Eliashberg theory, including Coulomb interactions. In agreement with experiment we find two superconducting gaps, which we map onto the Fermi surface and analyse in terms of the electronic orbital character. In addition, we investigate the electron-phonon interaction underlying the superconducting pairing and compare it to similar transition-metal dicalchogenides. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L31.00004: Resonance Raman Spectroscopy of Layered, Metallic Niobium Diselenide Heather Hill, Albert Davydov, Angela Hight Walker Transition metal dichalcogenides (TMDCs) are a popular set of materials due to their ability to be isolated into atomically-thin, 2D layers. Most recent research has focused on semiconducting TMDCs, but TMDCs can also be insulating, metallic, and superconducting. Niobium diselenide (NbSe$_{\mathrm{2}})$ is a metallic, superconducting TMDC that exhibits a charge density wave phase below 33.5 K in bulk. In our work, we investigate the laser energy dependence of the Raman modes of NbSe$_{\mathrm{2}}$ using resonance Raman spectroscopy and compare our results with reflectance contrast spectroscopy. We measure the Raman intensity, shift, and FWHM as a function of polarization and excitation energy in the range from 1.6 to 2.7 eV. An increasing intensity of one Raman mode, A$_{\mathrm{1g}}$, with decreasing energy in the visible range is observed, while the opposite behavior is seen for the other dominant Raman mode, E$_{\mathrm{2g}}$. We use the absorption peaks in the reflectance contrast of NbSe$_{\mathrm{2}}$ to relate the resonance Raman spectrum to the band structure. Finally, we compare our results with the existing literature on the resonance Raman spectroscopy on molybdenum disulfide, a semiconducting TMDC with surprising band structure similarities. [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L31.00005: Strain tuning of electronic phase transitions in atomically thin transition metal dichalcogenide metals Shengwei Jiang, Hongchao Xie, Kaifei Kang, Helmuth Berger, László Forró, Jie Shan, Kin Fai Mak Atomically thin transition metal dichalcogenide (TMD) metals have aroused considerable interests recently, owing to their rich collective phenomena, such as the coexistence of charge-density-wave (CDW) order and superconductivity down to the monolayer limit. The ultrathin and flexible nature of the atomically thin TMD metals also opens the possibilities for control of these electronic phase transitions by elastic strain engineering. In this talk, we present our results on continuous and reversible tuning by strain of the superconducting transition in atomically thin NbSe2. Suspended samples of atomically thin NbSe2 were studied by transport measurements whereas a variable tensile stress was applied to the samples through an electrostatic force. The physical mechanism for the observed strain dependence of the superconducting transition temperature will be discussed. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L31.00006: Probing the superconducting gap in two-dimensional NbSe2 Egon Sohn, Xiaoxiang Xi, Zefang Wang, Ju-Hyun Park, Helmuth Berger, László Forró, Jie Shan, Kin Fai Mak Recent experimental advances in atomically thin NbSe2 have unveiled a range of interesting collective phenomena. In particular, in monolayer NbSe2 where inversion symmetry is broken, strong spin-orbit coupling locks the spin to the out-of-plane direction, resulting in unconventional Ising pairing of Cooper pairs. The observation of an ultrahigh in-plane upper critical field from transport measurements has provided an experimental evidence of Ising superconductivity. However, direct measurements of the superconducting order parameter and its field dependence have not been explored. In this work, we develop the point contact Andreev reflection spectroscopy to measure the superconducting gap in atomically thin NbSe2. Differential conductance measurements of few-layer NbSe2 as a function of in-plane magnetic field at temperatures down to 0.3 K will be presented. These results will be discussed in relation with Ising superconductivity and spin paring symmetry in two-dimensional NbSe2. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L31.00007: Superconducting Properties of Atomically-Thin NbSe2 Sheets Julia Sell, Steven Tran, Albert Davydov, James Williams The superconducting members of the transition metal dichalcogenides (TMDCs) family display unique superconducting properties arising from broken in-plane mirror symmetry and multiply occupied bands. Here, we report on device fabrication and low-temperature transport measurements of atomically-thin sheets of superconducting NbSe2. Deviations from conventional I-V curves and anomalous RF responses are observed and discussed in the context of superconducting behavior predicted for this material. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L31.00008: Investigation of short and ballistic coupling in vertical NbSe$_{\mathrm{2}}$ - graphene - NbSe$_{\mathrm{2}}$ Josephson junctions Minsoo Kim, Geon-Hyoung Park, Jongyoon Yi, Jae Hyeong Lee, Jinho Park, Hu-Jong Lee 2H-NbSe$_{\mathrm{2}}$ is a layered two-dimensional superconducting material, which can be constructed into a van der Waals heterostructure with versatile functionality. Here we fabricated a vertically stacked NbSe$_{\mathrm{2}}$ - graphene - NbSe$_{\mathrm{2}}$ heterostructure by the dry transfer technique, where defect-free contact via van der Waals force provides the high interfacial transparency. Insertion of an atomically thin graphene layer between two NbSe$_{\mathrm{2}}$ flakes ensures the formation of highly coherent proximity Josephson coupling. Observed temperature dependence of the junction critical current ($I_{\mathrm{c}})$ and large value of $I_{\mathrm{c}}R_{\mathrm{n}}$ product (as large as 2.3$\Delta _{\mathrm{NbSe2}})$ reveal the short and ballistic Josephson coupling characteristics. Large junction critical current density of \textasciitilde 10$^{\mathrm{4}}$ A/cm$^{\mathrm{2}}$, multiple Andreev reflection in the subgap structure of the differential conductance, and magnetic field modulation of $I_{\mathrm{c}}$ also suggest the strong Josephson coupling via the graphene layer. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L31.00009: Normal-Insulator-Superconductor van der Waals tunnel junctions Tom Dvir, Freek Masse, Lotan Attias, Maxim Khodas, Marco Aprili, Charis Quay Huei Li, Hadar Steinberg The ability to stack multiple layers of van der Waals materials has given rise in the recent years to a large variety of heterostructures. We present a Normal - Insulator - Superconductor tunnel junction fabricated in this method. By using NbSe$_2$ as the target superconductor we observe a hard gap in the density of states, which agrees with the model of a two gap superconductor. Under the application of perpendicular and parallel magnetic fields, we observe the evolution of the density of states, both below and above the critical field for penetration of vortices (Hc$_1$). We propose that van der Waals tunnel junctions can be further implemented to the study of other exotic states of matter. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L31.00010: Nonreciprocal transport in superconducting MoS$_{\mathrm{2}}$ Yu Saito, Ryohei Wakatsuki, Shintaro Hoshino, Toshiya Ideue, Motohiko Ezawa, Naoto Nagaosa, Yoshihiro Iwasa The breaking of the inversion symmetry causes variety of interesting phenomena, such as optical nonlinear response, shift current and multiferroics, in condensed matter physics. Nonreciprocal charge transport, i.e., nonlinear resistance depending on the direction of current, is one of the important phenomena which is expected in such systems. MoS$_{\mathrm{2}}$, which is an archetypal layered semiconductor, can be a candidate material to investigate nonreciprocal phenomena because its monolayer, which possesses three-fold symmetry, shows out-of-plane spin polarization at the K-points due to intrinsic Zeeman-type spin-orbit coupling. Moreover, by ionic-liquid gating, almost all carriers are confined only to topmost layer, realizing 2D superconductivity without in-plane inversion symmetry [1]. In this talk, we report nonreciprocal charge transport in superconducting MoS$_{\mathrm{2}}$. We observed the nonreciprocal signals in superconducting state enhanced compared to the normal state, which is the consistent with the theoretical prediction. [1] Y. Saito et al. Nature Phys. 12, 144 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L31.00011: Nonreciprocal charge transport in noncentrosymmetric superconductors Ryohei Wakatsuki, Yu Saito, Shintaro Hoshino, Toshiya Ideue, Motohiko Ezawa, Yoshihiro Iwasa, Naoto Nagaosa In systems without the inversion symmetry, the I-V characteristics can show the asymmetric behavior between positive and negative directions. It is called nonreciprocal charge transport, and the most famous example is the p-n junction, where the inversion symmetry is broken by its structure. Even in bulk crystals, the nonreciprocal transport occurs, however, the amplitude is usually very small. In this talk, we focus on the resistive regime of noncentrosymmetric superconductors, where the superconducting fluctuation conductivity is dominant. As an example, we study the monolayer transition metal dichalcogenides MoS2 theoretically, and show that the nonreciprocal charge transport is dramatically enhanced compared to the normal regime, which is consistent with the experiment. This dramatic enhancement of the nonreciprocal transport comes from the difference between the Fermi energy and the superconducting gap. Therefore, we expect that this enhancement occurs in any other noncentrosymmetric superconductors. [Preview Abstract] |
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