Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T52: Superconductivity at Mesoscopic and Nanometer Scales |
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Sponsoring Units: DCMP Chair: Sergey Bud'ko, Ames Laboratory Room: Mile High Ballroom 1F |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T52.00001: Interaction effects in proximity-coupled spin-orbit quantum wires Ronny Thomale We review recent progress on understanding interaction effects in spin-orbit quantum wires in the presence of a magnetic field and a proximity-coupled superconductor. With the use of adapted density matrix renormalization group techniques, we are able to compute the low-energy tunneling density of states in the presence of interactions. This enables us to make a further step towards a realistic simulation of the experimental scenario. Among other aspects, we analyze how an interaction-driven transition between a topologically trivial superconducting state and a topologically non-trivial state with Majorana edge modes relates to other tuning parameters. In particular, we contemplate on experimentally measurable consequences of interactions such as zero bias peak broadening or gap closing modes at the topological phase transition, and discuss further models accessible through our numerical approaches. [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T52.00002: Anomalous Josephson effect in semiconductor nanowire with strong spin-orbit interaction and Zeeman effect Tomohiro Yokoyama, Mikio Eto, Yuli Nazarov We theoretically investigate the Josephson junction using quasi-one dimensional semiconductor nanowires with strong spin-orbit (SO) interaction, e.g., InSb. First, we examine a simple model using a single scatterer to describe the elastic scattering due to impurities and SO interaction in the normal region.\footnote{T.\ Yokoyama, M.\ Eto, and Yu.\ V.\ Nazarov, J.\ Phys.\ Soc.\ Jpn.\ {\bf 82}, 054703 (2013).} The Zeeman effect is taken into account by the spin-dependent phase shift of electron and hole through the system. The interplay between SO interaction and Zeeman effect results in a finite supercurrent even when the phase difference between two superconductors is zero. Moreover, the critical current depends on its current direction if more than one conduction channel is present in the nanowire. Next, we perform a numerical simulation by the tight-binding model for the nanowire to confirm our simple model. Then, we show that a spin-dependent Fermi velocity due to the SO interaction causes the anomalous Josephson effect. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T52.00003: Fluxon Controlled Resistance Switching in Centimeter-Long Superconducting Galium-Indium Eutectic Nanowires Weiwei Zhao, Jesse Bischof, Xin Liu, Jimmy Hutasoit, Thomas Fitzgibbons, Lin Wang, Zhonghou Cai, Si Chen, John Hayes, Pier Sazio, Chaoxing Liu, Jainendra Jain, John Badding, Moses Chan We observe unexpected hysteretic behavior in centimeter long quasi 1D nanowires of Ga-In eutectic in transport measurements in the presence of a magnetic field. In particular, in some parts of the phase diagram, the system can exist in one of two stable states with different resistances. We propose that the nonzero resistance occurs when a spontaneously nucleated Ga droplet along the length of the nanowire traps a superconducting fluxon and, thereby, triggers phase slips in a nearby Ga droplet. The Ga-In nanowires thus provide a platform wherein the resistance can be switched on and off by the addition of a single fluxon. The presence of pure Ga droplets in the Ga-In nanowire was confirmed by X-ray flourescence studies conducted in Advanced Photon Source. The long length of the nanowire increases the probability of a wire containing two nearby droplets. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T52.00004: Determination of the Transition Temperature of a Superconducting Nanowire through its kinetic inductance by coupling to 3D Microwave Cavity Resonator Jaseung Ku, Alexey Bezryadin A thin superconducting nanowire exhibits a broad resistive transition, due to the thermal fluctuations of the superconductor order parameter, namely Little's phase slips. The transition temperature, Tc, extracted from resistive measurements vary depending on the model used, e.g., the LAMH model versus the Little model. We have demonstrated a new method to determine the transition temperature, utilizing a 3D cavity resonator and the property of the wire kinetic inductance to saturate at Tc. A MoGe nanowire was placed in the microwave cavity and the transmission characteristics were probed as a function of temperature and microwave power. The transition temperatures obtained by this method were compared with DC transport data and confirmed that Little's model provides more accurate predictions for the Tc compared to LAMH. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T52.00005: The electron-hole superfluidity in two coaxial nanotubes Ilya Grigorenko, Oleg Berman, Roman Kezerashvili The superfluid phase and Coulomb drag effect caused by the pairing in the system of spatially separated electrons and holes in two coaxial cylindrical nanotubes are predicted. It is found that the drag resistance as a function of the temperature experiences a jump at the critical temperature and can be used for the detection of the superfluid transition. It is also demonstrated that at sufficiently low temperatures the order parameter exhibits a kink, as the electron-hole asymmetry monotonously increases. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T52.00006: ``Giant'' enhancement of the upper critical field and fluctuations above the bulk Tc in superconducting ultrathin Pb nanowire arrays Mingquan He Highly interesting effects may occur in 1D superconductors with diameter smaller than the superconducting coherence length. Superconductivity in the form of a zero resistance state may be largely suppressed in 1D superconductors. Thermal activated slips in the phase of the order parameter will cause finite resistance unless at T$=$0 K. On the other hand, Van Hove singularities in the density of states of 1D superconductors could cause significant enhancement of the transition temperature and the low dimensionality may strongly increase the upper critical field. In this report, I will present our research on a quasi-1D superconducting system---5 nm Pb nanowire arrays embedded in the pores of mesoporous silica SBA-15. It will be demonstrated that bulk Pb (type I superconductor, Tc $=$ 7.2 K, Hc$=$800 Oe) can be modified by nanostructuring to become a type II superconductor with an upper critical field exceeding 15 T and superconducting fluctuations up to $\sim$ 4 K above the bulk Tc. The material undergoes a crossover from a one-dimensional fluctuating superconductivity at high temperatures to a three-dimensional long-range-ordered superconductivity at lower temperatures [1]. \\[4pt] [1] ACS Nano 7, 4187 (2013). [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T52.00007: Two-dimensional superconductivity with broken inversion symmetry in one-atomic-layer metal films on cleaved GaAs surfaces Takayuki Sekihara, Takahiro Miyake, Hiroki Ichinomiya, Ryuichi Masutomi, Tohru Okamoto We have studied the parallel-magnetic-field dependence of the superconducting transition temperature $T_c$ by magnetotransport measurements on one-atomic-layer Pb and indium films deposited on cleaved GaAs surfaces. Superconductivity was stable even in parallel magnetic field $H_\parallel$ much higher than Pauli paramagnetic limit. Especially the reduction of the transition temperature in the Pb films was found to be rather small even in $H_\parallel$ up to 14 T. Furthermore, the perpendicular magnetic field dependence of the sheet resistance in the Pb films was almost independent of the presence of the parallel field component. For the case of the Pb films, the observed parabolic $H_\parallel$ dependence of $T_c$ is quantitatively explained in terms of an inhomogeneous superconducting state, called a helical state, theoretically proposed for a two-dimensional superconductor with a large Rashba spin splitting $\Delta_R \gg \hbar \tau^{-1}$. For the case of the indium films, we developed the theory for a moderate Rashba spin splitting. The values of $\Delta_R$ are estimated to be 0.04 eV, which is one order of magnitude smaller than that expected for the one-atomic-layer Pb films. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T52.00008: Two dimensionality in electric field induced superconductivity Yu Saito, Jianting Ye, Yijin Zhang, Yuichi Kasahara, Tsutom Nojima, Yoshihiro Iwasa Applying electric field is recognized as a useful tool for search of novel superconductors by using the ionic gating. The method allows us to accumulate carrier density exceeding 1 $\times$ 10$^{14}$ cm$^{-2}$, which is sufficiently large enough for inducing superconductivity. Although such superconductivity has been demonstrated in several systems, physical properties have not been well investigated. In this presentation we will report on the phase diagram and two-dimensional (2D) nature of electric field induced superconductivity. We fabricated electric double layer transistor on superconductivity on ZrNCl [1] and MoS$_2$ [2] using mechanical exfoliation followed by electron beam lithography. First we have established a relation between ${\it T}_c$ and the sheet carrier density on both compounds, and compared with the bulk phase diagram. Second, we measured angle dependence of upper critical field and confirmed the 2D nature of superconductivity in both compounds. We have obtained the thickness of superconductivity as 1-2 nm in both compounds. \\[4pt] [1] J. T. Ye et al., Nat. Mater. 9, 125 (2010).\\[0pt] [2] J. T. Ye et al., Science 338, 1193 (2012). [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T52.00009: Superconductivity in Quasi-2D Electron System with Ultra-high Electron Density Adam Neal, Hong Zhou, Yuchen Du, Peide Ye Superconductivity is widely observed in transition metals, but not easily obtained in conventional semiconductors such as Si or III-V compound semiconductors. Here we report the first electrical experiments performed at room pressure on a set of AlInN/AlN/GaN heterojunction samples with different electron doping concentrations. The state-of-the-art AlInN/AlN/GaN 2DEG grown by MOCVD for GaN HEMT applications has a high electron density of 2 $\times$ 10$^{13}$/cm$^{2}$ and it can be increased to 7 $\times$ 10$^{15}$/cm$^{2}$ by solid metal doping. With such 2-3 orders of magnitude higher electron density, superconducting state has been observed in the conventional III-V semiconductors with T$_{\mathrm{c}}$ $\sim$ 1K and two-step H$_{\mathrm{c}}$ of 0.3T and 1.5T with magnetic field parallel to the sample surface and H$_{\mathrm{c}}$ of 0.1T and 0.8T with magnetic field perpendicular to the sample surface. Angular dependence H$_{\mathrm{c}}$ reveals the quasi-2D nature of the electron system. More details of these new experiments related with superconducting III-V semiconductors will be presented. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T52.00010: Macroscopic Quantum Cotunneling of Phase Slips Andrey Belkin, Maxim Belkin, Victor Vakaryuk, Sergei Khlebnikov, Alexey Bezryadin Quantum phenomena that do not have analogues in the classical world include quantum superposition and tunneling. Despite significant efforts invested into demonstration of quantum effects at the macroscopic level, the main principles that govern the transition from classical to quantum are not well understood. Here we report a study of macroscopic quantum tunneling of phase slips that involve both superconducting and normal degrees of freedom in a superconducting nanowire loop. We discover that in addition to single phase slips that unwind the phase difference along the loop by 2$\pi$, there are transitions that change the phase by 4$\pi$. Experimentally we identify the regime in which, surprisingly, 4$\pi$ phase slips are more likely than 2$\pi$ ones. We interpret our observations in terms of macroscopic cotunneling effect defined as an exact synchronization of two macroscopic phase slip events. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T52.00011: Angular momentum blockade in nanoscale high-Tc superconducting grains Francesco Mancarella, Alexander Balatsky, Mats Wallin, Anders Rosengren We discuss the angular momentum blockade in small $d$-wave SC grains in an external magnetic field. We find abrupt changes in angular momentum state of the condensate ("angular momentum blockade") as a result of the variation of the external field. The effect represents a direct analog of the Coulomb blockade. We use the Ginzburg-Landau theory to illustrate how the field turns a $d$-wave order parameter (OP) into a($d_{x^2-y^2}+id_{xy}$)-OP. We derive the volume magnetic susceptibility as a function of the field, and corresponding small jumps in magnetization at critical values of the field that should be experimentally observable in SC grains. The observation of these jumps requires a small grain, since their extent is inversely proportional to the number of Cooper pairs in the sample. The general source of instability of the pure $d$-wave gap is the presence of gap nodes, completely lifted by the secondary OP component. A $d+id'$-state is chiral and hence has an orbital moment carried by Cooper pairs. We consider fields $H \ll H_{c2}$, making negligible the vortex perturbations of the OP. Boundary effects will be also discussed. Recent experiments suggest that nanoscale $d$-wave SC can be fully gapped and this minimal gap can be modified by an external field. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T52.00012: Enhancing bulk superconductivity by engineering granular materials James Mayoh, Antonio Garc\'Ia Garc\'Ia The quest for higher critical temperatures is one of the main driving forces in the field of superconductivity. Recent theoretical and experimental results indicate that quantum size effects in isolated nano-grains can boost superconductivity with respect to the bulk limit. Here we explore the optimal range of parameters that lead to an enhancement of the critical temperature in a large three dimensional array of these superconducting nano-grains by combining mean-field, semiclassical and percolation techniques. We identify a broad range of parameters for which the array critical temperature, $T_c^{\rm Array}$, can be up to a few times greater than the non-granular bulk limit, $T_{c0}$. This prediction, valid only for conventional superconductors, takes into account an experimentally realistic distribution of grain sizes in the array, charging effects, dissipation by quasiparticles and limitations related to the proliferation of thermal fluctuations for sufficiently small grains. For small resistances we find the transition is percolation driven. Whereas at larger resistances the transition occurs above the percolation threshold due to phase fluctuations. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T52.00013: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T52.00014: Superconductivity at 82K in half-unit-cell thick Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ Da Jiang, Tao Hu, Qiao Li, Lixing You, Ang Li, Haomin Wang, Gang Mu, Zhiying Chen, Haoran Zhang, Guanghui Yu, Xiaoming Xie, Mianheng Jiang, Jie Zhu, Qiujuan Sun, Chengtian Lin, Hong Xiao We report an experimental study of superconductivity in high quality single crystal Bi2212 down to half-unit-cell thick in the form of graphene/Bi2212 heterostructure. Sharp superconducting transitions were always observed above liquid nitrogen temperature (77 K). Thickness dependent T-linear $\rho$ behavior in Bi2212 was found to be related to the superconductor-insulator quantum phase transition (S-I QPT) in 2D superconductor. The S-I QPT was supposed to occur in the disordered boson system as the critical sheet resistance equaled to the quantum resistance for pairs h/4e2 (6.45k$ \Omega$) according to our experiments. Our research revealed that besides protecting the underlying Bi2212, graphene might have helped in damping the 2D fluctuation in Bi2212. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T52.00015: Pair-breaking of the superconducting thin films induced by the intense terahertz pulses Hironaru Murakami, Caihong Zhang, Iwao Kawayama, Biaobing Jin, Jian Chen, Peihen Wu, Masayoshi Tonouchi High-power terahertz time-domain spectroscopy (THz-TDS) was used to examine YBCO and NbN thin films when transmitted by intense single-cycle THz pulses. This allowed for an investigation of the nonlinear, time-resolved behavior of superconducting thin films in the presence of strong THz electric fields with the field strengths of tens of kV cm$^{\mathrm{-1}}$. In the case of low field strengths, the behavior of the thin films agrees with previous examinations by means of conventional, low-power THz-TDS. However, for strong THz electric fields, it was found by analysis with the two-fluid model that the superfluid population decreases dramatically, possibly due to Cooper pair breakup. This was accompanied by a drop in the imaginary part of the conductivity in the THz frequency range. Moreover, a high-intense THz-punp - THz-probe measurement was conducted with the both YBCO and NbN thin films and estimated the recombination time of quasiparticles excited by intense THz electric field in superconductos. As a results, It was found that the recombination time of YBCO was several picosecond and much shorter than that of NbN. [Preview Abstract] |
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