Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K35: 2D Materials - Superconductivity and Charge Density Waves IFocus
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Sponsoring Units: DMP Chair: Daniel Rhodes, Columbia Univ Room: LACC 409B |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K35.00001: Enhanced coupling and anisotropy in the low carrier density 2D superconductors Yuji Nakagawa, Yu Saito, Tsutomu Nojima, Kei Inumaru, Shoji Yamanaka, Yuichi Kasahara, Yoshihiro Iwasa Electron system with low carrier density and low dimension is a candidate for the exotic superconductor. Li intercalated ZrNCl and HfNCl can offer the model systems for such research of unconventional superconducting phenomena, because they exhibit the relatively high superconducting transition temperatures above 15 K even at the low carrier density in order of 1020 cm-3 and the strong anisotropy. However, the studies on these systems so far have been carried out in polycrystalline samples in many cases. Therefore, it is highly desired to study the intrinsic properties of superconductivity containing the anisotropic parameter, the coupling strength of the Cooper pairs and the relation between them using single crystals, which has been left to be uncovered. Here we report the transport and tunneling spectroscopy studies on Li intercalated ZrNCl and HfNCl single crystals by utilizing the in-situ characterization techniques of intercalation processes established recently[1]. We succeeded in tuning Tc, anisotropy and the superconducting gap, as a function of carrier density, both in Zr and Hf systems. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K35.00002: Supercurrent in Multi-Terminal Ballistic Graphene Josephson Junctions Anne Draelos, Ming-Tso Wei, Andrew Seredinski, Chung-Ting Ke, Hengming Li, Yash Mehta, Ethan Mancil, Tate Fleming, Kenji Watanabe, Takashi Taniguchi, Michihisa Yamamoto, Seigo Tarucha, Ivan Borzenets, Francois Amet, Gleb Finkelstein We investigated electronic properties of encapsulated graphene samples with multiple superconducting terminals. Clean ballistic devices were fabricated from graphene sandwiched by boron nitride flakes and contacted with superconducting molybdenum-rhenium. These multi-terminal devices were studied at zero magnetic field to first investigate the supercurrent among adjacent and non-adjacent Josephson junctions. We then also measured the samples in the quantum Hall (QH) regime and studied the interplay between edge states and the superconducting contacts. We discuss our results in light of various mechanisms of chiral state propagation along the QH and superconducting interfaces. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K35.00003: Gate-induced superconductivity in monolayer WTe2 Ebrahim Sajadi, Tauno Palomaki, Zaiyao Fei, Wenjin Zhao, Philip Bement, Christian Olsen, Silvia Luescher, Xiaodong Xu, Joshua Folk, David Cobden We present the discovery that exfoliated monolayer WTe2, recently reported to be a 2D topological insulator, turns superconducting under electrostatic doping. The observed superconductivity, with a Tc around 700 mK, occurs at much lower levels of electron doping as compared to any other known 2D superconductor. The transition from normal to superconducting is gradual in temperature, and responds very differently to perpendicular and in-plane magnetic fields. The critical in-plane magnetic field exceeds the Pauli limit by over a factor of 2, indicating strong spin-orbit interaction. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K35.00004: Scanning SQUID microscopy of ion-gel-gated MoS2 Alexander Jarjour, Brian Schaefer, George Ferguson, David Low, Rusen Yan, Menyoung Lee, Debdeep Jena, Grace Xing, Katja Nowack Atomically thin exfoliated MoS2 devices have been reported to superconduct at an n-type charge carrier density of ~ 1014 cm-2 [1] with a critical temperature of approximately ~2 K in a monolayer [2]. To achieve the high charge carrier density ionic gating has been employed. Here, we report progress towards implementing ion-gel-gating of MoS2 devices compatible with scanning probe measurements. To achieve this we spin coat an ionic gel to produce thicknesses below 1 μm, making it possible to bring a mesoscopic probe near the sample. This will allow us to perform local DC magnetometry and AC magnetic susceptibility measurements on these devices using scanning superconducting quantum interference device (SQUID) microscopy. By measuring the diamagnetic response of the superconducting state, we can identify spatial inhomogeneity in the transition temperature as well as the local London penetration depth at varying temperature. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K35.00005: Superconductivity and quantum transport in thin crystals of TaSe2 tuned by ionic gating Yueshen Wu, Hailong Lian, Jiaming He, Shun Wang, Hui Xing, Ying Liu, Jinyu Liu, Zhiqiang Mao Superconductivity and quantum transport in two-dimensional (2D) crystals of transition metal dichalcogenides have been a focus of materials and condensed matter physics research in recent years. These 2D crystals have been found to exhibit novel properties not found in the bulk. Bulk 2H-TaSe2 is a metal featuring superconductivity at very low temperatures (Tc = 0.15 K), but few-layer TaSe2 was found to show enhanced Tc. On the other hand, the charge density wave (CDW) did not show similar behavior. We studied superconductivity and quantum transport properties of thin crystal of 2H-TaSe2 obtained by mechanical exfoliation with its electronic properties tuned by ionic gating using a polymer electrolyte. We found that the superconducting transition temperature increases monotonically with decreasing thickness. The transition temperature of CDW was found to decrease only slightly as the crystal gets thinner, consistent with the expectation that superconductivity and CDW compete. We also found that the magnetoresistance (MR) of thin crystals of TaSe2 showed a crossover from positive to negative MR at low magnetic fields as the gate voltage was ramped up. The change in the sign of MR is argued to be a result of competition between weak antilocalization and weak localization. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K35.00006: Absence of Local Fluctuating Dimers in Superconducting Ir1-x(Pt,Rh)xTe2 Emil Bozin, Runze Yu, Soham Banerjee, Hechang Lei, A.M.M. Abeykoon, Cedomir Petrovic, Zurab Guguchia, Ryan Sinclair, Haidong Zhou The local atomic structure of Ir0.96Pt0.04Te2 and Ir0.8Rh0.2Te2 dichalcogenide superconductors at10 K and away from the Ir-Ir dimer/superconductor boundary is examined by x-ray total scattering based pair distribution function (PDF) approach. Hypothesis that dimer fluctuations persist in the superconducting compositional range, which would leave footprints in the local structure, is tested by qualitative PDF analysis and explicit modeling of the experimental data. Trigonal dimer-free local structure model explains the PDF data well, ruling out the possibility of nanoscale dimer fluctuations in this regime. In the vicinity of the dimer/superconductor boundary PDF evidences trigonal/triclinic phase separation, compatible with weakly first order character of the dimer-superconductor transition. Observations are inconsistent with proposed quantum-critical-point-like competition of the dimer state and superconductivity, and preclude scenarios for dimer fluctuations mediated superconducting pairing. Local structure of IrTe2 at 300 K conforms to the same dimer-free model, which is at odds with dimer order-disorder description of the intriguing electronic/structural transition seen in the endmember. |
Wednesday, March 7, 2018 9:12AM - 9:48AM |
K35.00007: Topology, correlations, and superconductivity in 2D Invited Speaker: Pablo Jarillo-Herrero In this talk I will review our recent quantum electronic transport experiments in a variety of 2D materials and van der Waals heterostructures, where we show interplay between topology, strong electron-electron correlations and electrically tunable superconductivity. In some of these materials, different phases can be achieved simply by tuning the electric field applied to the material. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K35.00008: Surface suppression of the superconducting energy gap and critical temperature in atomically thin NbSe2 Ekaterina Khestanova, John Birkbeck, Mengjian Zhu, Yang Cao, Geliang Yu, David Ghazaryan, Jun Yin, Laszlo Forro, Kenji Watanabe, Takashi Taniguchi, Roman Gorbachev, Artem Mishchenko, Andre Geim, Irina Grigorieva The superconducting transition temperature of amorphous thin films often decreases dramatically as their thickness is reduced. This suppression can be attributed to either disorder-induced localization of Cooper pairs or generation of free vortices during Berezinskii-Kosterlitz-Thouless (BKT) transition. In the case of crystalline superconductors such as exfoliated NbSe2, superconductivity is preserved down to monolayer thickness, however, the transition temperature is also found to be suppressed. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K35.00009: Superconductivity in NbSe2 epitaxial thin films grown on sapphire substrates by molecular-beam epitaxy. Hideki Matsuoka, Masaki Nakano, Yue Wang, Yuta Kashiwabara, Satoshi Yoshida, Kyoko Ishizaka, Masashi Kawasaki, Yoshihiro Iwasa Transition metal dichalcogenides (TMDs) provide a rich variety of emerging properties at monolayer limit originating from broken inversion symmetry and large spin-orbit coupling as typified by valley-polarized luminescence and transport in semiconducting TMDs such as MoS2 and WSe2, unconventional superconductivity in NbSe2, enhancement of superconducting critical temperature in TaS2, topological phase transition in WTe2, and so on. The main fabrication methods of monolayer TMDs so far have been mechanical exfoliation and chemical-vapor deposition. The state-of-the-art molecular-beam epitaxy (MBE) should offer an alternative way to fabrication of TMD thin films and heterostructures, while so far conducting graphene substrates have been mainly used for spectroscopic studies. We have recently developed a fundamental route to layer-by-layer epitaxial growth of various TMD thin films on insulating sapphire substrates by MBE for transport studies [1], and demonstrated ambipolar transistor operation in WSe2 epitaxial thin films. In this presentation, we report on superconducting transition with zero-resistance in NbSe2 epitaxial thin films grown on sapphire substrates by MBE. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K35.00010: Scanning tunneling microscopy and spectroscopy study of graphene/NbSe2 heterostructures Zhiming Zhang, Shawulienu Kezilebieke, Kenji Watanabe, Takashi Taniguchi, Peter Liljeroth, Brian LeRoy We investigate vertical van der Waals heterostructures consisting of monolayer graphene and bulk NbSe2. Due to interactions between the layers, coherent states in NbSe2, including charge density wave (CDW) and superconductivity, can be inherited by the graphene. The graphene/NbSe2 heterostructures were characterized with liquid helium temperature scanning tunneling microscopy. The graphene lattice, NbSe2 lattice and a moiré pattern due to their lattice mismatch are observed by scanning tunneling microscopy measurements. A CDW is also observed in the graphene, which is aligned with the NbSe2 lattice and 3 times its length, indicating it originates from the NbSe2. From the scanning tunneling spectroscopy measurements, a 1.1 meV superconducting gap is observed in the graphene. Vortices are also observed in a perpendicular magnetic field of 0.3 T. The superconductivity in the magnetic field is further confirmed by electrical transport measurements, which show a sharp drop in resistance at 6.2 K. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K35.00011: Enhanced Superconductivity and Suppression of Charge-density Wave Order in 2H-TaS2 in the Two-dimensional Limit Yafang Yang, Shiang Fang, Valla Fatemi, Jonathan Ruhman, Efren Navarro-Moratalla, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Pablo Jarillo-Herrero As superconductors are thinned down to the 2D limit, their critical temperature Tc typically decreases. Here we report the opposite behavior, an enhancement of Tc with decreasing thickness, in the 2D crystalline superconductor 2H-TaS2. Remarkably, in the monolayer limit, Tc increases by over a factor of four compared to bulk crystals. Accompanying this trend in superconductivity, we observe progressive weakening and suppression of the charge-density wave (CDW) transition with decreasing thickness. To explain these trends, we perform electronic structure calculations showing that a reduction of the CDW amplitude results in a substantial increase of the density of states at the Fermi energy, which contributes to the enhancement of Tc. |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K35.00012: Disentangling Amplitude and Phase Dynamics in Nonequilibrium Charge Density Wave Formation Alfred Zong, Anshul Kogar, Timm Rohwer, Changmin Lee, Edoardo Baldini, Emre Ergecen, Mehmet Yilmaz, Ya-Qing Bie, Joshua Straquadine, Byron Freelon, Edbert Jarvis Sie, Hengyun Zhou, Boris Fine, Pablo Jarillo-Herrero, Ian Fisher, Nuh Gedik At thermal equilibrium, a broken symmetry phase characterized by a complex order parameter often develops fluctuating order with a finite amplitude above the transition temperature Tc. This is particularly pronounced in low-dimensional systems, where short-range, fluctuating order establishes its long-range phase coherence only below Tc. By using an intense laser pulse that transiently restores the high-symmetry state, we study the nonequilibrium version of such a phase transition on a prototypical charge density wave compound LaTe3. With a combination of time-resolved diffraction, optical, and spectroscopic measurements, we can monitor how the amplitude and phase of the order parameter separately recover with distinct signatures in both coherent and incoherent responses after the photoexcitation. The present work provides a comprehensive picture on how the broken symmetry phase develops at the femto- to picosecond timescale in a low dimensional charge density wave system. |
Wednesday, March 7, 2018 10:48AM - 11:00AM |
K35.00013: Re-entrant Quantum Spin Hall State in Charge-Density Wave Phase of Doped Single-Layer Transition Metal Dichalcogenides Jun-Ho Lee, Young-Woo Son We explore doping induced phase transitions in single-layer 1T'-TMDs, MTe2 (M = Mo and W), based on first-principles calculations. As increasing electron doping, we find that several new collective phases with nontrivial topological properties stabilize. At a low doping region, we obtain a dome-shaped conventional superconducting phase and the transition temperature reaches around 9 K. At an intermediate doping region, we find a 2 x 5 charge-density wave (CDW) phase with nontrivial topological properties. Such a peculiar lattice distortion retains a nonsymmorphic symmetry with/without inversion symmetry that gives rise to symmetry-protected Dirac/Weyl points. At a high doping region, we find a 2 x 2 CDW with the diamond shape chain structure with rather larger energy gap. The 2 x 2 CDW phase also supports the symmetry protected Weyl points. We anticipate that present results will pave the way for the research on the correlated phenomena and nontrivial topological aspect in TMDs by controlled manner. |
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