Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y31: Magnetism, Superconductivity, and Spin-Orbit Effects in 2D Materials |
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Sponsoring Units: DCMP DMP GMAG Chair: Benjamin Hunt, Carnegie Mellon Room: 294 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y31.00001: Field dependence of magnetic order and excitations in the Kitaev candidate alpha-RuCl3 Arnab Banerjee, Paula Kelley, Barry Winn, Adam Aczel, Mark Lumsden, David Mandrus, Stephen Nagler The search for new quantum states of matter has been one of the forefront endeavors of condensed matter physics. ~The two-dimensional Kitaev quantum spin liquid (QSL) is of special interest as an exactly solvable spin-liquid model exhibiting exotic fractionalized excitations. Recently, alpha-RuCl3 has been identified as a candidate system for exhibiting some aspects of Kitaev QSL physics.~ ~The spins in this material exhibit zig-zag order at low temperatures, and show both low energy spin wave excitation arising from the ordered state as well as a continuum excitation extending to higher energies that has been taken as evidence for QSL relate Majorana fermions.~ ~In this talk, we show that the application of an in-plane magnetic field suppresses the zig-zag order possibly resulting in a state devoid of long-range order. Field-dependent inelastic neutron scattering on single-crystal shows a remarkable effect on the excitation spectrum above the critical field. [Preview Abstract] |
Friday, March 17, 2017 11:27AM - 11:39AM |
Y31.00002: On the Origin of Charge Order in RuCl3 Tom Berlijn RuCl3 has been proposed to be a spin-orbit assisted Mott insulator close to the Kitaev-spin-liquid ground state, an exotic state of matter that could protect information in quantum computers. Recent STM experiments [M. Ziatdinov et al, Nature Communications (in press)] however, show the presence of a puzzling short-range charge order in this quasi two dimensional material. Understanding the nature of this charge order may provide a pathway towards tuning RuCl3 into the Kitaev-spin-liquid ground state. Based on first principles calculations I investigate the possibility that the observed charge order is caused by a combination of short-range magnetic correlations and strong spin-orbit coupling. From a general perspective such a mechanism could offer the exciting possibility of probing local magnetic correlations with standard STM. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y31.00003: Electrical Transport in Encapsulated Few Atomic Layer Bi2Sr2CaCu2O(8+x) Shu Yang Frank Zhao, Margaret G. Panetta, Cyndia Yu, Nicola Poccia, Ruidan Zhong, Genda Gu, Takashi Taniguchi, Kenji Watanabe, Philip Kim We investigate electronic transport in few atomic layer high temperature superconductor $Bi_2Sr_2CaCu_2O_{8+x}$ (BSCCO), obtained via mechanical exfoliation of optimally doped single crystals. In order to avoid sample surface degradation during lithography, we fabricate electrical contacts using a silicon nitride stencil mask, aligned to the sample inside an argon environment with an integrated evaporator. After fabrication, an exfoliated, insulating hexagonal boron nitride crystal is placed on top of the device, sealing it from the environment. In thicker samples containing more than nine copper oxide planes, we observe bulk-like behavior: a linear decrease in normal-state resistance followed by a sharp superconducting transition, with a transition temperature consistent with bulk optimally doped BSCCO. We will also discuss anomalies of resistance versus temperature observed in thinner samples, where a slightly broadened superconducting transition at ~85K is followed by an anomaly in resistance, before falling back to zero resistance. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y31.00004: Thickness Dependence Study of Superconducting Phase Transition in Atomically-Thin FeTe$_{1-x}$Se$_{x}$ Sheets Chunlei Yue, Xue Liu, Jin Hu, Zhiqiang Mao, Jiang Wei, Ana Sanchez Different to other unconventional superconductors, iron chalcogenide FeTe$_{1-x}$Se$_{x}$ (x\textasciitilde 0 to 1) has a unique layered crystal structure, which makes atomically-thin single-crystal sheets achievable by micro-mechanical exfoliation. By using high resolution transmission electron microscopy, we found that exfoliation-prepared FeTe$_{1-x}$Se$_{x}$ sheets with thickness above 10nm exhibit excellent air stability. To obtain FeTe$_{1-x}$Se$_{x}$ sheets below 10nm with uncompromised crystal quality, crystal thinning with electrochemical etching technique that is based on the chemically inert ionic liquid has been experimented. The high crystal quality from obtained thinner sheets is characterized by atomic force microscopy and low-temperature transport measurements. We have observed that the onset transition temperature shifts continuously towards lower temperature with decreasing thickness. With the combination measurements of temperature-dependent resistance (R-T) and magnetoresistance (R-H), it can be clearly identified that four distinct transition stages, including Ginzburg-Landau fluctuation, BKT transition, finite size dominated region and superconducting region, exist in atomically-thin FeTe$_{1-x}$Se$_{x}$ sheets at different temperature range. Moreover, inhomogeneous nanoscale superconducting islands caused by the random lattice distribution of Te and Se atoms may introduce another factor that affects the density of vortices. Finally, a comprehensive phase diagram including sample thickness as the extra dimensionality is presented. [Preview Abstract] |
Friday, March 17, 2017 12:03PM - 12:15PM |
Y31.00005: Electronic transport in thin crystals of sodium iridate David Rosser, Nicholas P. Breznay, Drew Latzke, Christopher Gonzalez, Christopher Kim, Sabrina Kaplan, Joseph Guzman, Irene Lo Vecchio, Alessandra Lanzara, James G. Analytis, Robert Kealhofer, Claudia Ojeda-Aristizabal Sodium iridate (Na$_{2}$IrO$_{3})$, known as a novel relativistic Mott insulator, is a layered material that has shown signature of a metallic surface state [1]. Here we present preliminary transport measurements on thin crystals of Na$_{2}$IrO$_{3}$ as well as analysis of high resolution angle resolved photoemission spectroscopy (ARPES) data at different photon energies and the effect of dopants deposited in-situ. [1] N. Alidoust et al. Phys. Rev. B \textbf{93, }245132 (2016). [Preview Abstract] |
Friday, March 17, 2017 12:15PM - 12:27PM |
Y31.00006: Tunneling Spectroscopy of Graphene Nanodevices Coupled to Type-II Superconductors Joel I-Jan Wang, Landry Bretheau, Daniel Rodan-Legrain, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero By coupling a graphene sheet to type-II superconductors, it was recently shown that the Josephson effect could persist in the quantum Hall regime. Microscopically, the supercurrent arises from the existence in graphene of electron-hole resonance states called Andreev bound states (ABS). However, the way ABS form in graphene subject to high magnetic field remains unclear. For this purpose, we have performed tunneling spectroscopy of graphene proximitized by Nb/NbN electrodes, using graphite probes and hBN tunneling barriers. The geometry of our device allows for spectroscopic and transport measurement in the same graphene flake. In the superconducting regime, Fabry--P\'{e}rot oscillation of the critical Josephson current suggests ballistic transport characteristics in the device. In the presence of magnetic field, graphene density of states (DOS) is modulated by the superconducting phase, as expected for ABS in a normal weak link. Finally, tunneling measurement performed through spurious quantum dots, presumably embedded in the heterostructures, manifests coupling between discrete energy levels and proximitized graphene DOS with evident phase dependence. [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y31.00007: Current-phase relation of encapsulated graphene Josephson junctions Gaurav Nanda, Juan Luis Aguilera, Peter Rakyta, Andor Korm\'{a}nyos, Reinhold Kleiner, Dieter Koelle, Kenji Watanabe, Takashi Taniguchi, Lieven Vandersypen, Srijit Goswami In the past few years there has been remarkable progress in the study of graphene-superconductor hybrids. This surge in interest has primarily been driven by the ability to combine high-quality graphene with superconductors via clean interfaces. We use such ballistic graphene Josephson junctions to create a superconducting quantum interference device (SQUID) which can be tuned continuously from a symmetric to asymmetric configuration. The symmetric SQUID produces typical flux-periodic oscillations in the critical current with a large modulation amplitude. More interestingly, we show that the highly asymmetric configuration allows one to directly obtain the current-phase relation (CPR) of these ballistic graphene JJs. The CPR is found to be skewed, deviating significantly from a sinusoidal form. The skewness can be tuned with the gate voltage and shows correlations with Fabry-Perot oscillations in the ballistic cavity. We compare our experiments with tight-binding calculations which include realistic graphene-superconductor interfaces and find a good qualitative agreement. [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y31.00008: Andreev reflection near the Dirac point at Graphene - NbSe2 junction Manas Sahu, Pratap Raychaudhuri, Anindya Das Despite extensive search for about a decade, specular Andreev reflection (SAR) is only recently realized in bilayer graphene-superconductor interface. The experimental observation of retro to specular Andreev reflection is not only fundamentally important but also has potential application like quantum computing etc. Here, we have carried out the transport measurements at the van der Walls interface of single layer graphene and $NbSe_2$ superconductor. We investigate the Andreev reflection near the Dirac point by measuring the differential conductance as a function of Fermi energy and bias energy. We find that the normalized conductance (G$_{T |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y31.00009: Critical Current Statistics of a Graphene-Based Josephson Junction Infrared Single Photon Detector Evan D Walsh, Gil-Ho Lee, Dmitri K Efetov, Mikkel Heuck, Jesse Crossno, Takashi Taniguchi, Kenji Watanabe, Thomas A Ohki, Philip Kim, Dirk Englund, Kin Chung Fong Graphene is a promising material for single photon detection due to its broadband absorption and exceptionally low specific heat. We present a photon detector using a graphene sheet as the weak link in a Josephson junction (JJ) to form a threshold detector for single infrared photons. Calculations show that such a device could experience temperature changes of a few hundred percent leading to sub-Hz dark count rates and internal efficiencies approaching unity. We have fabricated the graphene-based JJ (gJJ) detector and measure switching events that are consistent with single photon detection under illumination by an attenuated laser. We study the physical mechanism for these events through the critical current behavior of the gJJ as a function of incident photon flux. [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y31.00010: Graphene-Based Superconducting Weak Links in Low Magnetic Field Scott Mills, Piranavan Kumaravadivel, Xu Du The impact of magnetic field on Andreev reflection is studied in graphene-based superconducting weak links. We found, through studying weak links with different adhesion layers and superconducting leads (including Graphene-Ti/Au-Nb, Graphene-Ti/Pd-Nb, Graphene-V-Nb, Graphene-Ti-Nb, Graphene-Ti/Pd-NbN), that in low field ($B \ll B_{c2}$) Andreev reflection is strongly suppressed by the magnetic field. Magnetic field suppression of Andreev reflection is found to be dependent on both the effective gap of the weak link and on Abrikosov vortex dynamics at the graphene-superconductor interface. As the effective gap of the weak link approaches the intrinsic gap of the superconducting leads, a remnant of Andreev reflection can survive into the quantum Hall regime, allowing study of the interplay between the quantum Hall effect and Andreev reflection in high quality suspended graphene-superconductor weak links. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y31.00011: Gate-tunable control from diffusive to ballistic proximity superconducting regime in a long graphene Josephson junction S Dubey, Z Han, B Wen, H Goto, C Dean, V Bouchiat We study a wide and long graphene Josephson junction, where the graphene is 1D-contacted by NbN electrodes and encapsulated between two boron nitride flakes. Additional normal-metal side-electrodes enable a non-invasive measurement of the voltage drop (4W) across the junction. Both below and above transition parameters of the superconducting state (critical current and temperature), a gate-dependent zero-voltage state between these normal contacts is observed. This indicates the dual nature of the zero-voltage as the junction can be either in a normal ballistic regime or in a proximity superconducting state. Measuring the 4W voltage as a function of the current bias, temperature, gate voltage and magnetic field enable to build phase-diagram for both the diffusive/ballistic states and normal/superconducting states, defining a gate-controlled transition from ballistic to diffusive Josephson junction. A diffusive intermediate state is found close to the charge neutrality point, while ballistic regime is found both at high electron and hole doping regimes. The temperature dependence of the critical current provides a second and independent measurement of diffusivity in the proximity Josephson junction. It is found to be strongly affected by the diffusive/ballistic regime. [Preview Abstract] |
Friday, March 17, 2017 1:27PM - 1:39PM |
Y31.00012: Superconducting quantum interference devices with graphene junctions. Michael Thompson, Jonathan Prance, Richard Haley, Yuri Pashkin, Moshe Ben Shalom, Vladimir Fal’ko, Anthony Matthews, Jeremy White, Roman Viznichenko, Ziad Melhem We present measurements of DC superconducting quantum interference devices based on Nb/graphene/Nb Josephson junctions. The superconducting proximity effect in graphene can be used to build Josephson junctions whose critical current can be controlled by field-effect gates. These junctions combine the tunability of semiconductor Josephson junctions with the high critical currents and low contact resistances of metal SNS junctions [1]. By using local gates, the SQUID junction critical currents can be modified individually and this allows the sensitivity and symmetry of the SQUID to be controlled in-situ. We compare the critical current of the SQUID with simulations that include a non-sinusoidal current phase relation in the junctions, as expected for ballistic graphene junctions. We also investigate the transfer function of the device in both symmetric and asymmetric configurations and find a highest transfer function of 300 $\mu $V/$\Phi _{0}$. Graphene Josephson junctions have the potential to add functionality to existing technologies; for example, to make SQUID magnetometers with tunable sensitivity or superconducting qubits with fast electrical control. [1] Ben Shalom et al., Nature Physics 2015, 12, 318--322 [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 1:51PM |
Y31.00013: Spin orbit coupling in graphene through gold intercalation Paromita Mukherjee, Eoin O'Farrell, Jun You Tan, Yuting Yeo, G.K.W. Koon, Barbaros \"Ozyilmaz, K. Watanabe, T. Taniguchi Graphene has a very low value of spin orbit coupling. There have been several efforts to enhance the spin orbit interaction in graphene. Our previous work has provided clear evidence that spin orbit coupling can be induced in graphene through Rashba interaction with intercalated gold. By applying an additional electric field, this splitting can be increased or decreased depending on its relative direction with the internal electric field induced by gold in graphene. A large negative magnetoresistance due to an in-plane magnetic field has been observed which can be attributed to the fact that a magnetic moment is induced in gold due to spin-orbit coupling. Anomalous Hall Effect which decreases with an in-plane magnetic field further suggests the formation of a collective magnetic phase. We would like to further elaborate on the spin-orbit coupling in graphene using non local measurements. Hence, by intercalating graphene with gold, we can have a direct electric manipulation of the spin degrees of freedom and lead to its much awaited applications in spintronics, quantum computing. \\ \\E.C.T O'Farrell, J.Y. Tan, Y.Yeo, G.K.W Koon, B.Özyilmaz, K. Watanabe, T. Taniguchi Phys. Rev. Lett. 117,076603(2016). [Preview Abstract] |
Friday, March 17, 2017 1:51PM - 2:03PM |
Y31.00014: Controlling spin polarization in graphene by cloaking magnetic and spin-orbit scatterers Diego Pereira, Tatiana Rappoport Because of its very particular structure, electrons in the graphene behave like massless relativistic particles and the electronic properties of this material can be described in terms of Dirac fermions, which allow us to use the Dirac equation of relativistic quantum mechanics and all its properties. In this work, we consider scatterers that induce Zeeman spin-orbit and intrinsic spin-orbit coupling on the surface of graphene with a length comparable with the size of the electronic wave. Starting with an analogy with the optics, we consider a cloak around the scatterer and, using partial waves expansions, we show that a combination of resonant scattering and the variation of the cloak parameters can produce an efficient control of the spin-dependent transport, like spin current polarization and spin Hall angle [Preview Abstract] |
Friday, March 17, 2017 2:03PM - 2:15PM |
Y31.00015: Enhanced spin-orbit coupling in single layer CVD WSe2/graphene heterostructures Mark Lohmann, Bowen Yang, David Barroso, Ludwig Bartels, Kenji Watanabe, Takashi Taniguchi, Jing Shi Spin-orbit coupling (SOC) in graphene can be strongly enhanced via proximity effect when graphene is in contact with transition metal dichalcogenides (TMDs) [1]. However, bulk TMDs are hard to be exfoliated to single or few layers while maintaining a practically large size. An alternative approach by using chemical vapor deposition (CVD) grown WSe$_{\mathrm{2}}$ allows us to overcome this obstacle. We have succeeded in picking up a single layer CVD WSe$_{\mathrm{2}}$ flake with hBN and transferring it onto an exfoliated graphene flake. Due to the lower carrier density per unit area of the CVD WSe$_{\mathrm{2}}$, we are able to tune the Fermi level in graphene over a wide range, and observe a clear weak antilocalization at various carrier densities, indicating a strong enhancement of the SOC strength in graphene due to the proximity interaction of the WSe$_{\mathrm{2}}$. Meanwhile, the universal conductance fluctuation is also suppressed owing to the large size of the WSe$_{\mathrm{2}}$. We extract the spin relaxation time which is roughly one fourth that of previously studied WS$_{\mathrm{2}}$/graphene heterostructures [1] and thus a 100{\%} increase in the SOC strength. [1] B. Y. et al., 2D Mater. 3, 031012 (2016). [Preview Abstract] |
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