Session X48: Superconductivity in Low Dimensional Systems III and Mesoscopic and Nanometer Scale Phenomena

Disorder Effects on Superconducting Island Arrays Proximity-Coupled to Graphene

Two-dimensional superconductors have been extensively studied due to the presence of quantum phase transitions (QPT) in these systems at millikelvin temperatures. One of the most investigated QPT is the Superconductor-(Metal)-Insulator Transition (SMIT). However, in the presence of disorder, the nature of transport across the SMIT is not well understood, largely because disorder is difficult to controllably tune in 2D superconducting films. To overcome this difficulty, we have developed arrays of superconducting Sn islands proximity-coupled to graphene with controllable island point disorder and graphene disorder. We studied the SMIT in these devices as a function of magnetic field and analyzed the transport signatures using scaling. Depending on the amount of point disorder, graphene disorder, and device geometry, we observed an evolution in the critical exponents from diffusive to percolative transport. We additionally performed Hall measurements, in which we observed peaks in the transverse resistance corresponding to pinning and depinning of magnetic field-induced vortices. These transport signatures suggest unusual behavior as a function of disorder near the Quantum Critical Point (QCP).
  

The effects of charge carriers, organic solvents and magnetic ions on the superconductivity of doped 1T-SnSe2

We reported bulk superconductivity up to 7.6 K through careful manipulation of the charge carrier density and basal spacing d in the chemically intercalated two dimensional CdI₂-type 1T-SnSe₂ phase. We found out the very unusual scaling behaviour of T_c with the basal spacing distance caused by different organic co-intercalation, meanwhile the T_c appears to be almost independent with the alkali metal concentration. In this presentation, we focus on more chemical doping studies that close to the insulator-superconducting transition regime, more precisely-controlled doping level and various organic molecules have been tested through soft chemical methods. In addition, intercalation of the magnetic molecules are also carried out to study the interplay of superconductivity, magnetism, and possible charge density wave transition in these intercalated system. The details of the results will be presented with their physics origins being further elaborated.   

Uniaxial strain effect on superconductivity in 1D and 2D LaAlO3/SrTiO3 channels

We investigate the effects of uniaxial strain on superconductivity of nanowires in LaAlO₃/SrTiO₃. The interface of LaAlO₃ and SrTiO₃ has a high-mobility 2D electron gas which can be superconducting at sub-Kelvin temperatures. We are able to create 1D conducting channels at the interface using conductive atomic force microscope lithography [1]. Superconductivity in the system is associated with ferroelastic domain boundaries [2] and therefore is in 1D regimes. Application of an external uniaxial stress is expected to displace the ferroelastic domain boundaries. Our experiments indicate that uniaxial stretching of the nanowire in the parallel direction completely suppress the superconducting state, while reversal of the applied strain restores superconductivity. We will discuss implications of possible electron-pairing mechanism in the 1D superconductor.
[1] C. Cen, et al., Nature Materials 7, 298 (2008).
[2] Y.-Y. Pai et al., Phys Rev Lett 120, 147001 (2018).   

Effect of quantum fluctuations on the critical supercurrent through a mesoscopic normal-metal island

We consider the transport properties of a single-electron transistor consisting of a diffusive normal-metal island and superconducting electrodes. We show that the interplay of a dissipative impedance of the gate electrode, a weak magnetic field, and a charging energy of the island causes a suppression of the critical supercurrent of the transistor. When the device is voltage biased, pumping of single electrons can be achieved by a periodic modulation of the gate voltage [1-3]. We derive the analytic expressions for the error rates and show that they are suppressed by the applied magnetic field [4].

[1] J. P. Pekola, J. J. Vartiainen, M. Möttönen, O.-P. Saira, M. Meschke, and D. V. Averin, Nat. Phys. 4, 120 (2007).
[2] D. V. Averin and J. P. Pekola, Phys. Rev. Lett. 101, 066801 (2008).
[3] S. Nakamura, Y. A. Pashkin, M. Taupin, V. F. Maisi, I. M. Khaymovich, A. S. Mel’nikov, J. T. Peltonen, J. P. Pekola, Y. Okazaki, S. Kashiwaya, S. Kawabata, A. S. Vasenko, J.-S. Tsai, and N.-H. Kaneko, Phys. Rev. Appl. 7, 054021 (2017).
[4] V. Bubanja, Phys. Rev. B 97, 224516 (2018).   

Low Temperature Terahertz Nano-Imaging of BSCCO High Temperature Superconductor

Terahertz (THz) near-field imaging and nano-spectroscopy is a capable tool to research nano-scale low energy electromagnetic phenomena. Of particular interest are high-temperature superconductors (HTSC) whose characteristic energy scale exists in the THz regime. We discuss our recent development of a cryogenic, scanning near-field microscope suitable for nano-scale imaging and spectroscopy throughout the THz frequency range. We report THz near-field measurements of optimally doped and over-doped BSCCO-2212 thin film devices above and below their superconducting transition temperatures. Near-field can measure the c-axis Josephson plasma resonance (JPR) spectroscopically. Further, the large anisotropy between the in and out of plane plasma frequency caused by JPR also contributes to a hyperbolic polariton modes in the bulk of a thin film which can be detected by near-field imaging [1][2].
[1] H. T. Stinson et al, PRB 90, 014502 (2014)
[2] Dai, S., et al. Science 343.6175 (2014): 1125-1129.
  

Electric Field Effects in Mesoscopic Metal Superconductors

The superconducting state of metals is typically thought to be negligibly affected by the presence of externally applied electric fields, given sub-nanometer screening lengths. But recent studies of mesoscopic metallic superconducting channels at millikelvin temperatures observe a reduction of the superconducting critical current under applied gate voltage [1]. We extend these measurements of Ti and Al nanowires and find revealing effects, including voltage tuned oscillations in the critical current as a function of magnetic field. We present possible explanations and experiments underway to evince the origin of the effect.

[1] G. D. Simoni, et al., Nat. Nanotech. 13, 802-805 (2018)   

Quasiparticle Poisoning of a Superconducting Island by Nonequilibrium Phonons

Quasiparticle excitations are detrimental to the operation of various superconducting devices. The density of quasiparticles should be exponentially suppressed at low temperatures, but commonly a quasiparticle density many orders of magnitude larger than the thermal expectation is observed. Phonons with energy greater than twice the superconducting gap can break Cooper pairs to form quasiparticles, and conversely two quasiparticles recombining to form a Cooper pair will emit such phonons. In our experiment, we demonstrate how the superconducting island of a single-electron transistor is poisoned with quasiparticles and the parity effect is consequently destroyed, when another heated superconducting island is used as an emitter of nonequilibrium phonons, while a thermal phonon emitter, a similarly heated normal metal island, creates much fewer quasiparticles.   

InAs nanowire Josephson junctions with in-situ grown full shell Al

Superconductor-semiconductor hybrid nanowires are ideal material systems for studying various quantum transport phenomena such as Andreev and Majorana bound states. For quantum information technologies based on these quantum phenomena, it is highly desirable to develop hybrid nanowires with high-quality superconductor-semiconductor interfaces and ballistic transport in the semiconductor. In this study, we use shadow mask structures on the growth substrate to grow full shell Al-InAs-Al Josephson junction nanowires in situ. The in-situ grown junction can be free from surface damage and over-etching associated with the Al etching process. Electrical characterization of the Josephson junctions shows a hard superconducting gap. We observe multiple Andreev reflections in the open-channel regime and sub-gap states in the tunnel coupled regime. Furthermore, as a result of fluxoid quantization in the Al shell, the superconductivity is modulated by the magnetic flux and its effect on the Josephson junction is investigated.

  

Theory of superconductivity in an ultra-dilute fermionic liquid in the proximity of a quantum phase transition.

We study the problem of superconductivity in an extremely weakly doped narrow gap semiconductor, driven by electron-phonon coupling in a non-adiabatic regime. We derive an analytic theory analogue to Eliashberg equations that shall allow us to determine the critical temperature. In the first stage, based on our recent correlated electron-ion dynamics study, we obtain the effective non-adiabatic electron-phonon coupling function. We then add long-range Coulomb type interactions to derive propagators for the emergent collective state. Since the electron-phonon coupling is highly anisotropic, in the second step we test whether coupling to cooperons is able to induce a thermodynamic phase transition. Finally, we discuss transport properties of the emergent superconducting condensate, in particular (magneto-)thermoelectricity driven by vortex states. Our study is applicable in various classes of novel superconducting phases in materials with extremely small carrier concentrations such as doped SrTiO, but also a mysterious case of elementary bismuth.   

Suppression of quantum metallic state in disordered Ta thin films with the addition of artificial periodic pinning centers

We studied the low temperature transport properties of perforated superconducting Ta films with periodic triangular array of holes to investigate the role of pinning effect on superconductor-insulator transition. Magnetic field and temperature dependence of transport characteristics in 7 perforated Ta thin film samples with uniform thickness but different densities of holes are investigated and compared with a reference sample without holes. High density artificial periodic pinning centers increase activation energies for motion of vortices and induce cusp-like magnetic field dependence of resistance due to commensurate matching effect of interstitial vortices between holes as well as trapped vortices insides holes. As hole density increases, matching effects is observed up to higher field. Besides Resistance of field cooling decreases as Arrhenius form of thermal activated model up to higher magnetic field and, zero-temperature limit metallic state is more suppressed.   

Thickness dependent study of superconducting behavior and contact dynamics in atomically thin Fe(Te0.7Se0.3) Flakes

We present evidence of thickness dependent superconducting behavior in strain-free Fe(Te_0.7Se_0.3) flakes. Due to non-uniform Te/Se spatial distribution, we find that R(T) behavior in flakes <10nm can be explained and confirmed using the BKT transition for inhomogeneity model, as well as finite state effects(FSE). For >10nm flakes we find a systematic suppression of the superconductivity and broadening of the superconducting phase transition. Additionally, we present a self-heating model at the metal-superconductor-metal(MSM) interface to explain unusual observed switching behavior in I-V characteristicsfor flakes thinner than 10nm. We propose a 2D network of superconducting paths connecting superconducting islands within Fe(Te_0.7Se_0.3) thin flakes to describe the thickness-dependent behavior of this natural percolating system.   

Gaussian state superposition ansatz for quantum impurity problems: Numerical methods and applications

Recent work of Bravyi and Gosset [1] demonstrated that superposition of Gaussian states can approximate efficiently the ground state of gapped quantum impurity problems. We introduce an efficient numerical method for finding the ground state approximation in this variational manifold. As a benchmark, we first apply our method, based on imaginary time evolution, to the Anderson impurity model. We then study a generalization of this approach to models of superconducting islands in the Coulomb blockade regime.
[1] Bravyi and Gosset, Commun. Math. Phys. 356, 451 (2017)   

Mapping Dynamical Magnetic Responses of Ultra-thin Micron-size Superconducting Films using Nitrogen-vacancy Centers in Diamond

Two-dimensional superconductors have attracted growing interest because of their scientific novelty, structural tunability, and useful properties. Studies of their magnetic responses, however, are often hampered by difficulties to grow large-size samples of high quality and uniformity. We report here an imaging method that employed NV^- centers in diamond as sensor capable of mapping out the microwave magnetic field distribution on an ultrathin superconducting film of micron size. Measurements on a 33nm-thick film and a 125nm-thick bulk-like film of Bi₂Sr₂CaCu₂O_8+δ revealed that the ac Meissner effect set in at 78K and 91K, respectively; the latter was the superconducting transition temperature (T_c) of both films. The unusual ac magnetic response of the thin film presumably was due to thermally excited vortex-antivortex diffusive motion in the film. Spatial resolution of our ac magnetometer was limited by optical diffraction. The technique could be extended with better detection sensitivity to extract local ac conductivity/susceptibility of ultrathin or monolayer superconducting samples as well as ac magnetic responses of other two-dimensional exotic thin films of limited lateral size.