Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q20: Focus Session: Superconductivity in the 2D Limit II |
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Sponsoring Units: DMP Chair: Hanno Weitering, University of Tennessee, Knoxville Room: Ballroom B |
Wednesday, March 4, 2015 2:30PM - 3:06PM |
Q20.00001: Remarkable effects of disorder on superconductivity of single atomic layers of lead on silicon Invited Speaker: Christophe Brun It is well known that conventional superconductivity is very robust against non-magnetic disorder [1]. Nevertheless for thin and ultrathin films the structural properties play a major role in determining the superconducting properties, through a subtle interplay between disorder and Coulomb interactions [2]. Unexpectedly, in 2010 superconductivity was discovered in single atomic layers of lead and indium grown on silicon substrate using scanning tunneling spectroscopy [3] and confirmed later on by macroscopic transport measurements [4]. Such well-controlled and tunable crystalline monolayers are ideal systems for studying the influence of various kinds of structural defects on the superconducting properties at the atomic and mesoscopic scale. In particular, Pb monolayers offer the opportunity of probing new effects of disorder because not only superconductivity is 2D but also the electronic wave functions are 2D. Our study of two Pb monolayers of different crystal structures by very-low temperature STM (300 mK) under magnetic field reveals unexpected results involving new spatial spectroscopic variations [5]. Our results show that although the sheet resistance of the Pb monolayers is much below the resistance quantum, strong non-BCS corrections appear leading to peak heights fluctuations in the dI/dV tunneling spectra at a spatial scale much smaller than the superconducting coherence length. Furthermore, strong local evidence of the signature of Rashba effect on the superconductivity of the Pb/Si(111) monolayer is revealed through filling of in gap states and local spatial variations of this filling. Finally the nature of vortices in a monolayer is found to be very sensitive to the properties of step edges areas. \\[4pt] [1] P.W. Anderson, J. Phys. Chem. Solids 11, 26 (1959)\\[0pt] [2] M.V. Feigel'man et al. Ann. Phys. 325, 1390 (2010)\\[0pt] [3] T. Zhang et al. Nature Phys. 6, 104 (2010)\\[0pt] [4] Y. Yamada et al. Phys. Rev. Lett. 110, 237001 (2013)\\[0pt] [5] C. Brun et al. Nature Phys. 444, 10 (2014) [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q20.00002: Josephson Vortex in Indium Monatomic Superconductor on Silicon Terraces Takuto Kawakami, Yuki Nagai, Shunusuke Yoshizawa, Howon Kim, Tomonobu Nakayama, Yukio Hasegawa, Takashi Uchihashi, Xiao Hu Superconductivity in Indium monatomic layer on a surface of Silicon substrate is intriguing where the terraces and steps exist. Recently, elliptic vortices trapped at steps have been observed by STM/STS measurement under magnetic field [1]. Motivated by this experiment, we clarify the quasiparticle excitation by using Bogoliubov-de Gennes approach [2]. The current distribution and zero energy density of states at vortex core show elliptic shape with longer axis parallel to the step. Moreover, the order parameter is restored at the vortex core. By comparing theoretical results with experiments, we conclude that the recent STS measurement has directly detected Josephson vortex. [1] S. Yoshizawa, {\it et al}., {\it Phys. Rev. Lett.} (in press, arXiv:1405.5953). [2] T. Kawakami, {\it et al}., {\it J. Phys.: Conf. Ser.} (in press). [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q20.00003: High temperature superconductivity in one unit cell FeSe and superconductivity in two-monolayer Ga Jian Wang By direct transport and magnetic measurements, we provide definitive evidence for high temperature superconductivity in the 1-UC FeSe films on insulating STO substrates with the onset Tc and critical current density much higher than those for bulk FeSe. In addition, by both in situ scanning tunneling microscopy/spectroscopy and ex situ transport and magnetization measurements, we find that the two-atomic-layer Ga film with hexagonal structure on wide band-gap semiconductor GaN is superconducting with Tc up to 5.4 K. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q20.00004: Ballistic superconductivity in high mobility two dimensional electron gas in GaAs heterostructures Zhong Wan, Aleksandr Kazakov, MIchael Manfra, Loren Pfeiffer, Ken West, Leonid Rokhinson Introduction of a Josephson field effect transistor (JoFET) concept sparked active research on proximity effects in semiconductors. Induced superconductivity and electrostatic control of critical current has been demonstrated in two-dimensional gases in InAs, graphene and topological insulators, and in one-dimensional systems including quantum spin Hall edges. Recently, interest in superconductor-semiconductor interfaces was renewed by the search for non-Abelian states when fractional quantum Hall edge states interface with superconductivity. However, the highest mobility 2D gases in GaAs, where variety of strongly correlated states including fractional quantum Hall effect can be observed, are notoriously absent from the list of materials where superconductivity have been induced. We report formation of transparent superconducting contacts to the high mobility 2DEG in GaAs and demonstrate induced superconductivity across several microns. Ballistic supercurrent has been observed across 0.6 $\mu$m of 2DEG, a regime essential to the formation of well separated non-Abelian states. High critical fields ($>16$ Tesla) in NbN contacts enables investigation of a regime of an interplay between superconductivity and strongly correlated states in a 2DEG at high magnetic fields. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 4:18PM |
Q20.00005: Forward scattering from phonons as a pathway to increasing Tc in quasi-two-dimensional unconventional superconductors Invited Speaker: Steven Johnston One of the primary goals of superconductivity research is engineering materials that become superconducting at high temperatures. To this end, I will examine forward scattering from phonons as a general means to increase Tc in unconventional superconductors. First I will introduce the mechanisms for generating electron-lattice interactions in quasi-two-dimensional correlated systems that are peaked in the forward scattering direction. Then I will present case studies comparing theory to angle-resolved photoemission spectroscopy data for the Bi-family of cuprate superconductors [S. Johnston {\it et al}., PRL {\bf 108}, 166404 (2012)] and the recently discovered FeSe monolayer in SrTiO3 substrates [J. J. Lee {\it et al}., Nature {\bf 515}, 245 (2014)]. These studies demonstrate the general principle of using particular electron-phonon interactions for enhancing superconductivity in unconventional pairing channels. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q20.00006: Synthetic Superconductivity in Single-Layer Crystals Leonid Levitov, Dan Borgnia, Patrick Lee Electronic states in atomically thin 2D crystals are fully exposed and can couple to extrinsic degrees of freedom via long-range Coulomb interactions. Novel many-body effects in such systems can be engineered by embedding them in a polar environment. Superconducting pairing interaction induced in this way can enhance the intrinsic electron-phonon pairing mechanism. We take on this notion, which was around since the 60's ("excitonic superconductivity"), and consider synthetic superconductivity (SSC) induced in 2D crystals by a polar environment. One interesting aspect of this scenario is that Coulomb repulsion acts as superconductivity friend rather than a foe. Such repulsion-to-attraction transmutation allows to access strong-coupling superconductivity regime even when intrinsic pairing interaction is weak. We analyze pairing interaction in 2D crystals placed atop a highly polarizable dielectric with dispersive permittivity $\epsilon(\omega)$ and predict that by optimizing system parameters a substantial enhancement can be achieved. We also argue that the SSC mechanism can be responsible, at least in part, for 100 K superconductivity recently observed in FeSe monolayers grown on SrTiO3 substrate, with $T_c$ more than 10 times larger than in bulk 3D FeSe crystals, arxiv:1406.3435. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q20.00007: The Meissner and Mesoscopic Superconducting States in the Ultrathin FeSe-Films L. Z. Deng, B. Lv, Z. Wu, Y. Y. Xue, W. H. Zhang, F. H. Li, L. L. Wang, X. C. Ma, Q. K. Xue, C. W. Chu We carried out a detailed investigation on the superconductivity in eight 1-4 unit-cell FeSe-films on SrTiO3(STO) substrates by measuring their magnetization and resistivity in a field between 5E2 and 7E4 Oe over the last one and half years as a function of temperature and frequency, from 2 to 300 K and 0 to 1.5 kHz, respectively. Systematic aging effect for these samples was also well studied. The results show that samples display a complex superconducting structure, i.e. a Meissner state but populated with weak-links below 20 K, and an unusual superconducting mesostructure up to 45 K. A model is proposed to account for such a superconducting mesoscopic structure, similar to the Andreev reflection between the normal and superconducting carriers. Above 45 K, collective glass-like excitations are evident although their nature is yet to be determined. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q20.00008: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q20.00009: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q20.00010: Superconductivity in a two-dimensional repulsive Rashba gas at low density Luyang Wang, Hong Yao We study the superconducting instability and the resulting superconducting states in a two-dimensional repulsive Fermi gas with Rashba spin-orbit coupling at low electron density. We find that the superconductivity is enhanced as the Fermi energy $E_F$ decreases, due to two reasons: first, the density of states at $E_F$ increases as $1/\sqrt{E_F}$; second, the particle-hole bubble gains a more magnificent structure, resulting in an increasing effective attraction. The superconducting state is always in the total angular momentum $j_z=\pm2$-channel, and breaks time-reversal symmetry. Once a sufficiently large Zeeman coupling is applied to the superconducting state, the spectrum Chern number becomes $\pm1$, depending on the direction of the Zeeman field, and Majorana zero modes appear in the vortex cores. Collective modes in this superconducting state are also studied. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q20.00011: Metallic ground state in an ion-gated two-dimensional superconductor Yoshihiro Iwasa, Yu Saito, Yuichi Kasahara, Jianting Ye, Tsutomu Nojima Ever since a discovery of insulator-superconductor transition in metallic thin films, the ground state of clean two-dimensional (2D) superconductors has been a long standing fundamental question. Recently emerging electric double layer transistors enabled researchers to realize electric-field-induced superconductivity in various substances, which provides us with new opportunities to investigate the ground state of 2D superconductivity. In this presentation, we report that the majority of the field-temperature phase diagram of electric-field-induced superconductivity in ZrNCl including T = 0 K is occupied by a metallic state with finite resistance due to the quantum tunneling of isolated vortices and flux flow caused by extreme two-dimensionality and weak pinning potential. These results imply that electric-field-induced superconductivity can be an ideal platform for accessing quantum vortex states in clean 2D superconductors. [Preview Abstract] |
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