Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R15: 2D Materials: Superconductivity and Correlations IFocus
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Sponsoring Units: DMP Chair: Jie Shan, Penn State University Room: 314 |
Thursday, March 17, 2016 8:00AM - 8:36AM |
R15.00001: Universal increase in the superconducting critical temperature of two-dimensional semiconductors at low doping by the electron-electron interaction Invited Speaker: Matteo Calandra In two-dimensional multivalley semiconductors, at low doping, even a moderate electron-electron interaction enhances the response to any perturbation inducing a valley polarization. If the valley polarization is due to the electron-phonon coupling, the electron-electron interaction results in an enhancement of the superconducting critical temperature. By performing first-principles calculations beyond density functional theory, we prove that this effect accounts for the unconventional doping dependence of the superconducting transition temperature (Tc) and of the magnetic susceptibility measured in LixZrNCI. Finally, we discuss what are the conditions for a maximal Tc enhancement in weakly doped two-dimensional semiconductors. References: M. Calandra, P. Zoccante and F. Mauri, Pys. Rev. Lett. 114, 077001 (2015) B. Pamuk, J. Baima, R. Dovesi, M. Calandra and F. Mauri, in prepararion. [Preview Abstract] |
Thursday, March 17, 2016 8:36AM - 8:48AM |
R15.00002: Superconducting phases of monolayer transition-metal dichalcogenides Evan Sosenko, Vivek Aji Layered group-VI dichalcogenides, e.g., MoS$_2$, are two dimensional materials that engender novel coupled spin and valley physics. Characterized by strong spin-orbit coupling and inversion symmetry breaking, they give rise to novel phenomena such as the spin Hall and valley Hall effect. In this talk, I focus on the intrinsic and substrate induced superconducting phases expected in this new class of materials. We will discuss the nature of the quasiparticles resulting from valley discriminating, pair breaking processes, and the effect of the BCS phase on the nature of opto-electronic coupling and nontrivial Berry curvature associated with the bands near each valley. [Preview Abstract] |
Thursday, March 17, 2016 8:48AM - 9:00AM |
R15.00003: Two Dimensional Ising Superconductivity in Gated MoS}$_{\mathrm{\mathbf{2}}}$ Noah Yuan, Jianming Lu, Kam Tuen Law, Oleksandr Zheliuk, Inge Leermakers, Ulrich Zeitler, Jianting Ye The Zeeman effect, which is usually considered to be detrimental to superconductivity, can surprisingly protect the superconducting states created by gating a layered transition metal dichalcogenide. This effective Zeeman field, which is originated from intrinsic spin orbit coupling induced by breaking in-plane inversion symmetry, can reach nearly a hundred Tesla in magnitude. It strongly pins the spin orientation of the electrons to the out-of-plane directions and protects the superconductivity from being destroyed by an in-plane external magnetic field. In magnetotransport experiments of ionic-gate MoS$_{\mathrm{2}}$ transistors, where gating prepares individual superconducting state with different carrier doping, we indeed observe a spin-protected superconductivity by measuring an in-plane critical field $B_{\mathrm{c2}}$ far beyond the Pauli paramagnetic limit. The gating-enhanced $B_{\mathrm{c2}}$ is more than an order of magnitude larger compared to the bulk superconducting phases where the effective Zeeman field is weakened by interlayer coupling. Our study gives the first experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field. [Preview Abstract] |
Thursday, March 17, 2016 9:00AM - 9:12AM |
R15.00004: Disorder-enhanced superconductivity in Li intercalated ZrNCl Yuji Nakagawa, Yu Saito, Wu Shi, Yuichi Kasahara, Yoshihiro Iwasa Electrolyte gating, as represented by electric-double-layer transistor (EDLT), possesses various functionalities; forming of $p-n$ junction, electrostatic control of phase transitions, electrochemical etching and intercalation. These different functions, namely, electrostatic function or electrochemical function, strongly depends on the geometry of the device and the temperature in which a gate voltage is applied, and are considerably useful to unveil hidden intrinsic properties of a system. In this talk, we report a study on the transport properties in Li intercalated ZrNCl, which exhibits a maximum transition temperature of 15.2 K in lightly doped regime near the superconductor-insulator transition point. By the application of gate voltages in an EDLT configuration, we succeeded in \textit{in-situ} resistance measurement in the electrochemical intercalation process, and thereby superconductivity in a single crystal. We found that superconductivity in Li intercalated ZrNCl changed from 2D to anisotropic 3D. Furthermore, enhancement of $T_{c}$ in the lightly doped regime is accompanied by the increase of disorder and superconducting fluctuation. These results suggest that superconductivity of ZrNCl in the lightly doped regime is enhanced by disorder. [Preview Abstract] |
Thursday, March 17, 2016 9:12AM - 9:24AM |
R15.00005: Magnetic response and pair-breaking effect in superconducting transition metal dichalcogenides Junhua Zhang, Evan Sosenko, Vivek Aji The low-energy physics of monolayer transition metal group-VI dichalcogenides is significantly affected by the strong spin-orbit interaction in company with inversion symmetry breaking. As a result, the superconducting state in this system exhibits different physical behaviors compared to the conventional superconductors. Motivated by this, we study in detail the effects of the in-plane magnetic field and the non-magnetic disorder on this superconducting state. In particular, we discuss the unusual magnetic response and the pair-breaking effect in this system and their indication to experiments. [Preview Abstract] |
Thursday, March 17, 2016 9:24AM - 9:36AM |
R15.00006: Numerical study of giant nonlocal resistance in 2D spin orbital coupling system Zibo Wang, Hua Jiang, Xincheng Xie Recent experiments find the signal of giant nonlocal resistance $R_{NL}$ in H-shaped graphene sample due to the Spin/Valley Hall Effect. Interestingly, compared with the local resistance $R_L$, $R_{NL}$ decreases much more quickly when the Fermi energy deviates from the Dirac point, which does not satisfy the classical relation: $R_{NL} \propto R_L^3$. In this work, we simulate such transport phenomenon in H-shaped graphene based on the non-equilibrium Green function method. Near the Dirac point, there does exist a large nonlocal resistance signal, which exhibits much sharper than the local one. Moreover, we investigate the relationship between $R_L$ and $R_{NL}$, which can be affected by spin-orbital coupling strength, Fermi energy, sample size, etc. At last, we discuss the possible mechanism that leads to the deviation of $R_{NL}$ from classical $R_{NL} \propto R_L^3$. [Preview Abstract] |
Thursday, March 17, 2016 9:36AM - 9:48AM |
R15.00007: Thermoelectric Powerfactor and Density of States in 2D MoS$_{\mathrm{2}}$ Kedar Hippalgaonkar, Ying Wang, Yu Ye, Hanyu Zhu, Yuan Wang, Joel Moore, Xiang Zhang Efficient thermoelectric devices require high voltage generation from a temperature gradient and a large electrical conductivity, while maintaining a low thermal conductivity. For a given thermal conductivity and temperature, thermoelectric powerfactor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to high powerfactor due to unique density of states (DOS) of confined electrons and holes. Emerging 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only because of their discretized density of states, but also due to their large effective masses and high carrier mobilities. We report a measured powerfactor of MoS2 as large as 8.5 mWm$^{\mathrm{-1}}$K$^{\mathrm{-2}}$ at room temperature, which is amongst the highest among all thermoelectric materials and we show that the powerfactor scales with mobility for 1L and 2L samples. Moreover, measurement of thermoelectric properties of monolayer MoS$_{\mathrm{2}}$ allows us to determine the confined 2D DOS near the conduction band edge and in the insulating state, which cannot be measured by electrical conductivity alone. The demonstrated record high electronically tunable powerfactor in 2D TMDCs holds promise for efficient thermoelectric energy conversion. [Preview Abstract] |
Thursday, March 17, 2016 9:48AM - 10:00AM |
R15.00008: Thermoelectric Transport Measurements of Graphene on hBN Junxi Duan, Xiaoming Wang, Guohong Li, Xinyuan Lai, Mona Zebarjadi, Eva Y. Andrei The unique electronic transport properties of graphene, arising from massless charge carriers whose sign and density can be tuned by gating, have been studied extensively. Much less work was devoted to graphene's thermal properties. Unlike electrical transport which depends on total carrier density, the thermopower is determined by the net charge transferred and not by the carrier density. This leads to profound differences between the two phenomena. For example, when the Fermi level is close to the Dirac point (DP) where electron-hole (e-h) puddles are populated symmetrically, the electron and hole contributions to the thermopower cancel out. In contrast, their contributions to the electrical current add up. We studied the thermoelectric properties of high quality graphene supported on an hBN substrate, where the e-h puddle regime is significantly reduced compared to that on SiO$_{\mathrm{2}}$ substrates, which allows closer access to the DP. At room temperature we find that the maximum Seebeck coefficient close to the DP reaches up to twice the values on SiO$_{\mathrm{2}}$ substrates. Upon cooling down to 77K it decreases in a non-linear fashion with temperature. We will discuss possible origins of this behavior. [Preview Abstract] |
Thursday, March 17, 2016 10:00AM - 10:12AM |
R15.00009: Gate-induced Gap in Bilayer Graphene Suppressed by Coulomb Repulsion Yu-Zhong Zhang, Jin-Rong Xu, Ze-Yi Song, Hai-Qing Lin We investigate the effect of on-site Coulomb repulsion $U$ on the band gap of the electrically gated bilayer graphene by employing coherent potential approximation in the paramagnetic state, based on an ionic two-layer Hubbard model. We find that, while either the on-site Coulomb repulsion $U$ or the external perpendicular electric field $E$ alone will favor a gapped state in the bilayer graphene, competition between them will surprisingly lead to a suppression of the gap amplitude. Our results can be applied to understand the discrepancies of gap size reported from optical and transport measurements, as well as the puzzling features observed in angular resolved photoemission spectroscopic study. [Preview Abstract] |
Thursday, March 17, 2016 10:12AM - 10:24AM |
R15.00010: A comparative study of the tunable spin-orbit coupling in graphene proximity coupled to topological insulators Zhuonan Lin, Wei Qin, Jiang Zeng, Wei Chen, Ping Cui, Jun-Hyung Cho, Zhenyu Zhang We present a comparative study of the electronic properties of the heterostructures consisting of a graphene sheet proximity coupled to the surfaces of three-dimensional topological insulators (TIs). Using density functional theory method, we first calculate the band structures of a single-layer graphene on the Bi$_2$Te$_3$ thin film. Counterintuitively, the spin-orbit coupling (SOC) can be barely induced in the graphene even though the intrinsic SOC strength of Bi$_2$Te$_3$ is stronger than that of Sb$_2$Te$_3$, which can readily introduce a giant SOC interaction into the graphene through proximity effect. In order to understand this exotic phenomenon, we next investigate the differences of the work functions and the charge transfers between the graphene and the TI substrates. It is found that the proximity-induced SOC in the graphene sheet can be enhanced by reducing the work function difference. These findings provide a simple work-function criterion for searching realistic materials that can be utilized as substrates to induce a large SOC gap in the graphene. Our criterion extends the posibities of experimental realization of quantum spin Hall state in graphene. [Preview Abstract] |
Thursday, March 17, 2016 10:24AM - 10:36AM |
R15.00011: Odd frequency pairing of interacting Majorana fermions Zhoushen Huang, Peter Woelfle, Alexandar Balatsky Majorana fermions are rising as a promising key component in quantum computation. While the prevalent approach is to use a quadratic (i.e. non-interacting) Majorana Hamiltonian, when expressed in terms of Dirac fermions, generically the Hamiltonian involves interaction terms. Here we focus on the possible pair correlations in a simple model system. We study a model of Majorana fermions coupled to a boson mode and show that the anomalous correlator between different Majorana fermions, located at opposite ends of a topological wire, exhibits odd frequency behavior. It is stabilized when the coupling strength $g$ is above a critical value $g_c$. We use both, conventional diagrammatic theory and a functional integral approach, to derive the gap equation, the critical temperature, the gap function, the critical coupling, and a Ginzburg-Landau theory allowing to discuss a possible subleading admixture of even-frequency pairing. [Preview Abstract] |
Thursday, March 17, 2016 10:36AM - 10:48AM |
R15.00012: Indirect bonding mechanism for proximity-induced giant spin-orbit coupling in graphene-topological insulator van der Waals heterostructure Shivani Rajput, Yaoyi Li, Michael Weinert, Lian Li We demonstrate proximity-induced spin-orbit coupling in graphene/topological insulator van der Waals (vdW) heterostructures fabricated by transferring chemical vapor deposited graphene onto Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ film grown by molecular beam epitaxy. Using scanning tunneling microscopy/spectroscopy, we observe a spin-orbit splitting of the graphene Dirac states up to 80 meV, with a spatial variation of \textpm 20 meV due to the inherent lack of epitaxial relation in the graphene/Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ vdW junction. Density functional theory calculations further reveal that this giant spin-orbit splitting of the graphene bands is a consequence of the orthogonalization requirement on the overlapping wave functions, rather than simple direct bonding at the interface. This revelation of an indirect bonding mechanism of the proximity effect will facilitate more effective engineering of desired properties in vdW heterostructures. [Preview Abstract] |
Thursday, March 17, 2016 10:48AM - 11:00AM |
R15.00013: Magnetic field induced suppression of the forward bias current in Bi$_2$Se$_3$/Si Schottky barrier diodes Haoming Jin, Arthur Hebard Schottky diodes formed by van der Waals bonding between freshly cleaved flakes of the topological insulator Bi$_2$Se$_3$ and doped silicon substrates show electrical characteristics in good agreement with thermionic emission theory. The motivation is to use magnetic fields to modulate the conductance of the topologically protected conducting surface state. This surface state in close proximity to the semiconductor surface may play an important role in determining the nature of the Schottky barrier. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics were obtained for temperatures in the range 50-300 K and magnetic fields, both perpendicular and parallel to the interface, as high as 7 T. The I-V curve shows more than 6 decades linearity on semi-logarithmic plots, allowing extraction of parameters such as ideality ($\eta$), zero-voltage Schottky barrier height (SBH), and series resistance ($R_s$). In forward bias we observe a field-induced decrease in current which becomes increasingly more pronounced at higher voltages and lower temperature, and is found to be correlated with changes in $R_s$ rather than other barrier parameters. A comparison of changes in $R_s$ in both field direction will be made with magnetoresistance in Bi$_2$Se$_3$ transport measurement. [Preview Abstract] |
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