Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S31: Superconductivity and Correlated States in 2D Materials IIFocus
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Sponsoring Units: DMP Chair: Mike Lodge, University of Central Florida Room: 294 |
Thursday, March 16, 2017 11:15AM - 11:51AM |
S31.00001: Local Probe Characterization of Novel Electronic Phases in 2D Transition Metal Dichalcogenides Invited Speaker: Michael Crommie The transition metal dichalcogenides (TMDs) exhibit a rich variety of behavior due to their strong spin-orbit coupling, electron-electron interactions, and electron-phonon coupling. This causes, for example, charge density wave (CDW) ordering, superconductivity, and even Weyl semimetal behavior in different TMD materials. Much recent activity has been focused on understanding how this behavior is modified as materials are thinned from the bulk 3D regime to the single-layer 2D regime. Such dimensional reduction can cause significant changes to band structure, electron screening properties, and structural ground states, leading to new behavior such as the quantum spin Hall effect which was recently predicted for some 2D TMD materials. Here I will discuss our scanning tunneling microscopy measurements of single-layer TMD materials that exhibit CDW behavior, such as NbSe$_{\mathrm{2}}$ and TaSe$_{\mathrm{2}}$. I will describe how the behavior of these materials changes in some ways and remains unchanged in others as they are thinned to the 2D limit. I will also discuss our recent search for the quantum spin Hall effect in the 1T' phase of single-layer TMD materials such as WSe$_{\mathrm{2}}$ and WTe$_{\mathrm{2}}$. This work is a collaboration between UC Berkeley, Stanford, LBNL, and SIMES researchers. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S31.00002: Local electronic structure of charge ordering in monolayer 2H-TaSe$_2$ Yi Chen, Hyejin Ryu, Heejung Kim, Hsin-Zon Tsai, Franklin Liou, Arash A. Omrani, Choonkyu Hwang, B.I. Min, Sung-Kwan Mo, Zahid Hussain, Michael Crommie Metallic transition metal dichalcogenides (TMDs) are ideal platforms for exploring collective electronic phases such as charge density wave (CDW) order and superconductivity. Bulk 2H-TaSe$_2$, a well-studied CDW system, is known to undergo an incommensurate CDW transition at T $\approx$ 122K and a commensurate CDW transition at T $\approx$ 90K. Reducing TaSe$_2$ sample thickness to the single-layer limit is expected to affect this behavior due to the absence of inter-layer coupling and the dimensionality effect [1]. Here we present an electronic structure study of single-layer TaSe$_2$ by means of scanning tunneling microscopy/spectroscopy, angle-resolved photoemission spectroscopy, and first-principle simulations. We observe charge ordering in monolayer TaSe$_2$, thus providing new insight into the interplay between charge order and dimensionality in this model CDW system. [1] Miguel M. Ugeda, et.al. Nature Physics 12, 92-97 (2016). [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S31.00003: Charge density wave metastability in Tantalum based transition metal dichalcogenide monolayers David Miller, Subhendra D. Mahanti, Phil Duxbury Understanding the interplay of charge density wave states, metal-insulator transitions and superconductivity in Tantalum based transition metal dichalcogenides is an important challenge for both theory and experiment. Depending on the choice of chalcogen; and the temperature, pressure and doping, different commensurate structures can occur. Using density functional theory we have studied the energetic stability of six different commensurate charge density wave structures in monolayer undoped TaX$_2$ for three chalcogens (X = S, Se, Te), providing an overview of low lying metastable structures that can occur in this family of materials. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S31.00004: Effect of doping on lattice instabilities of 1H-TaS$_2$ Oliver R. Albertini, Amy Y. Liu, Matteo Calandra Recent ARPES measurements of single-layer 1H-TaS$_2$ grown on Au(111) suggest strong electron doping from the substrate.\footnote{C. E. Sanders, et al. \textbf{Phy. Rev. B} 94, 081404} In addition, STM/STS measurements on this system show the suppression of the charge-density-wave (CDW) and superconducting instabilities occurring in bulk 2H-TaS$_2$.$^2$ We present results from ab initio DFT calculations of free-standing single-layer 1H-TaS$_2$ to explore the effects of doping on the CDW. For the undoped monolayer, we find a lattice instability along the $\Gamma-M$ line, consistent with the bulk $3\times3$ CDW ordering vector. We then study the behaviour of the CDW instability as a function of doping from the substrate. We show that doping progressively removes the CDW instability, in agreement with the experimental findings. We explore the interplay between doping and strain, and we examine the electron-phonon couping and the electronic susceptibility to understand the doping-induced changes in lattice instabilities. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S31.00005: Symmetry-protected superconductivity in ultrathin layered transition metal dichalcogenides Sergio de la Barrera, Michael Sinko, Devashish Gopalan, Benjamin Hunt Recent studies of few-layer NbSe$_2$ and MoS$_2$ have resulted in reports of a robust two-dimensional superconducting states at low temperature. Notably, in both cases the superconductivity persists in the presence of external in-plane magnetic fields significantly larger than the Pauli paramagnetic limit. Following this work, we investigate related phenomena in ultrathin 2H-TaS$_2$ using low-temperature, high-field magnetotransport to probe the associated superconducting critical behavior. We discuss the connection between the observed transport, spin-orbit coupling, and Ising superconductivity resulting from dimensional and symmetry considerations. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S31.00006: Current-induced unprecedented metastable phases in two-dimensional 1T-TaS2 crystals Masaro Yoshida, Takashi Gokuden, Ryuji Suzuki, Masaki Nakano, Yoshihiro Iwasa 1T-TaS2 thin crystal is an attracting correlated 2D material with CDW ordering. The 2D 1T-TaS2 crystal was revealed to have extremely slow ordering kinetics [1], providing a platform to realize novel metastable states. Here we discovered unprecedented metastable electronic phases by current injection [2]. The current-induced metastable phases are thermally inaccessible, absent in the energy landscape established in its bulk crystal. The results indicate the existence of multiple unexplored electronic phases accessible only in 2D materials. In this talk, we will show the unique properties of the current-induced phases unveiled through detailed measurements. [1] M. Yoshida et al. Sci. Rep. 4, 7302 (2014); [2] M. Yoshida et al. Sci. Adv. 1, e1500606 (2015). [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S31.00007: New insight into the Mott-insulating state in 1T-TaS$_{2}$ Liguo Ma, Cun Ye, Yijun Yu, Xiu Fang Lu, Xiaohai Niu, Sejoong Kim, Donglai Feng, David Toma´nek, Young-Woo Son, Xianhui Chen, Yuanbo Zhang In correlated materials, electron-electron and electron-phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. In layered compound 1T-TaS$_{2}$, the intricate interplay between the two generates a Mott-insulating ground state with a peculiar charge-density-wave (CDW) order. We explore a metastable CDW phase induced by voltage pulses, and find that the new phase exhibits electronic structures entirely different from that of the original Mott ground state. The metastable phase consists of nanometer-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layers underneath. We discover that the nature of the new phase is dictated by the stacking order, and few of the corresponding stacking is demonstrated. Our results shed fresh light on the origin of the Mott phase in 1T-TaS$_{2}$. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S31.00008: Low-Frequency Raman Modes of 2H-TaSe$_{\mathrm{2}}$ in the Charge Density Wave Phase Sugata Chowdhury, J. Simpson, T. L. Einstein, A. R. Hight Walker With changes in temperatures, tantalum diselenide (2H-TaSe$_{\mathrm{2}})$, a layered, transition metal chalcogenides (TMD) exhibits unique super-lattice structures. The metallic ground state changes to an incommensurate charge density wave (CDW) state at $\approx $122?K followed by a commensurate CDW state at $\approx $90?K, and eventually a superconducting state $\approx $ 0.14 K. These phase transitions are driven by strong electron-phonon coupling and favored by the particular form of the Fermi surface of these systems. Here we theoretically studied the structural origin of low-frequency Raman modes of bulk 2H-TaSe$_{\mathrm{2\thinspace }}$in the CDW phases. Our calculations reveal that changes observed in the Raman modes are associated with the thermal expansion in the basal plane of 2H-TaSe$_{\mathrm{2}}$. The Gr\"{u}neisen parameters of these two Raman modes increase in the CDW phases. Changes in the lattice parameter ``a'' are large compared to ``c'' which induces strain along the a-axis. We compared our results with experimental data which show low-frequency Raman phonon modes are very sensitive to temperature and are not observed in the metallic room-temperature state. In addition, we found that cation displacement is more than anion in CDW phase. Our results may shed more light on exact nature of the CDW instability and optical properties in this system. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S31.00009: Competing quantum phases in Ising superconducting dome of monolayer WS2 Jianming LU, Oleksandr Zheliuk, Qihong Chen, Inge Leermakers, Nigel Hussey, Uli Zeitler, Jianting Ye A fascinating phenomenon in two dimensional electronic systems (2DES) is the quantum phase transition between an insulating, metallic and superconducting. Recent advances in crystalline 2D materials now make it possible to address quantum phase transitions in a truly 2DES. Here, we present a complete set of competing quantum phases from band insulator, superconductor, to an unexpected re-entrant insulator in a truly 2DES -- a monolayer of WS2 -- where we can control field effect without altering the chemical structure over a large range. In the phase diagram we observe an Ising superconducting dome with dual quantum critical points (QCP). Over the entire dome, the spin of Cooper pairs is pinned by a strong internal spin orbit interaction (\textasciitilde 30 meV). In the strong gating limit, the weak screening in monolayer causes reentrance to a highly insulating state, providing the key insight into many dome-like superconducting phases observed in field induced quasi-2D superconductors. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S31.00010: Superconductivity in atomically thin WS$_2$ from Migdal-Eliashberg study Appalakondaiah Samudrala, E. H. Hwang Recently, the possibility of extrinsic superconducting properties and enhancement of the critical temperature (T$_c$) have been widely studied in two dimensional materials such as graphene, phospherene, MoS$_2$ etc. This process includes several approaches such as applying intercalation/adsorption of metal atoms, carrier doping, strain and electric fields to thin 2D materials. In this work, we consider the effects of carrier doping on stability, electron-phonon, and superconductivity of atomically thin WS$_2$. For this, ab initio calculations were performed using Migdal-Eliashberg theory with the combination of Wannier interpolation. From our results, the pristine single layer WS$_2$ is a direct band semiconductor with similar electronic properties of MoS$_2$. Employing carrier doping makes WS2 have the metallic nature, and doping enhances the electron-phonon coupling strength(from $\alpha^2$F($\omega$)) is from 0.3 to 1.5 as doping levels change from 0.02 to 0.10 per formula unit. This trend gives rise to the enhancement of superconducting T$_c$ from 2K to above 10K by electron doping. Overall, present results indicated that the approximate tuning of electronic band structure results possibility of phonon mediated supercomputing properties in atomically thin single layer WS$_2$. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S31.00011: Ionic liquid gated field effect transistor enhanced superconductivity Jianli Wang, Cong Wang, Chi Zhang Motivated by the high doping ability of ionic liquid gated field effect transistors (FET), substantial attempts had been made on the research of material properties which have a close relationship with carrier concentrations. Among these subjects, superconductor-insulator transition attracts the most attention owing to the rich physical properties. Different from chemical doping induced superconductivity, FET surface doping method injects high density carriers without introducing unintentional disorder and is free from deterioration. Based on the ionic liquid gating method, superconductivity has been reported found in materials known to be non-superconducting. Here, we show a transition-metal dichalcogenide thin film transistor which shows a coexistence of CDW phase and superconductivity based on ionic liquid electrolyte. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S31.00012: High-Tc Odd-parity Superconductivity in Lightly Hole-doped Monolayer Transition Metal Dichalcogenides Through Proximity Yi-Ting Hsu, Kyungmin Lee, Eun-Ah Kim Lightly hole-doped monolayer transition metal dichalcogenides (p-doped TMDs) are a family of multi-valley materials that host opposite species of spinless electrons in each valley due to the intrinsic Ising spin-orbit coupling. Such spin-valley locked low-energy bandstructure together with the moderate electronic correlations expected for d-electrons make p-doped TMDs a promising platform for unconventional superconductivity, as we pointed out in Ref. [1]. Here, we propose an alternative approach to obtain unconventional pairings besides hoping for intrinsic superconductivity, i.e. proximitizing. We investigate the pairing symmetry induced in p-doped TMDs when coupled to a d-wave superconductor, such as cuprates, in proximity. By solving the Bogoliubov–de Gennes (BdG) equation on the interface of the heterostructure self-consistently, we find that nodal odd-parity pairing is induced. The proposed setup offers a platform for high-temperature odd-parity two-dimensional superconductivity. [1]: Y.-T. Hsu, A. Vaezi, M. H. Fischer, E.-A. Kim, Topological superconductivity in monolayer transition metal dichalcogenides, arXiv:1606.00857 (2016) [Preview Abstract] |
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