Session Q25: Superconductivity: Transport Properties

Networks of ultra-small MoGe nanowires: fabrication and properties

By developing a template-based method we were able to fabricate networks of MoGe nanowires with widths and thicknesses of few nanometers. Resistive measurements reveal magnetoresistance oscillations with a field period up to 2 Tesla. Detailed information on sample fabrication will be presented. Possible mechanisms on the observed magnetoresistance oscillations will be discussed.   

Magnetization-dependent resistance of double ferromagnet-superconductor junctions

Studies of the crossed Andreev reflection (CAR) process in double ferromagnet-superconductor junctions have attracted a lot of attention as way of realizing solid-state entanglement. Here, we perform a theoretical analyses of such a system motivated by our surprising experimental findings that the resistance in the antiparallel alignment of the magnetization of ferromagnets is larger than that in the parallel state. We model the system using an extended Blonder-Tinkham-Klapwijk (BTK) treatment with spin-dependent interfacial barriers associated with the magnetization. We compute scattering amplitudes of CAR and other possible processes as well as the resistance as a function of interfacial parameters. Our results reveal significantly altered physics due to the magnetization-dependent scattering, such as a sign change in the relative resistance between the parallel and antiparallel cases. We can model the positive relative resistance corresponding to our experimental findings as well as the negative results observed in other experiments, both within sufficiently large parameter regions.   

Measurements of the critical current of small Sr$_2$RuO$_4$ crystals

We report critical current measurements of chiral $p$-wave superconductor Sr$_2$RuO$_4$. Because of the strong anisotropy possessed by Sr$_2$RuO$_4$, vortex lines along the in-plane direction are expected to be pinned more strongly than those along the $c$ axis, resulting in anisotropic critical currents. We prepared small single crystals of Sr$_2$RuO$_4$ with a typical size of 50$\mu$m$\times$ 10$\mu$m$\times$1$\mu$m by mechanical exfoliation and characterized them by Raman spectroscopy and high-resolution transmission electron microscopy, showing that they were either pure Sr$_2$RuO$_4$ or eutectic phase containing one or more Ru microdomains. Four-point or Hall probes were prepared on the small crystals by photo lithography. While samples of pure Sr$_2$RuO$_4$ exhibited a typical transition temperature ($T_c$) of 1.2K, slightly lower than the optimal bulk $T_c$, 1.5K, those with Ru microdomains showed multiple resistive transitions with the highest $T_c$ around 2K. The critical current and critical field phase diagrams were determined for these small crystals. Surprisingly, the in-plane critical current density, measured for the first time, was found to be significantly larger than that along $c$-axis of the bulk. The physical implications of these observations will be discussed. Supported by DOE.   

Angle-dependent transport behavior near the magnetic-field tuned superconductor-insulator transition

Thin films of superconducting, amorphous indium oxide were driven insulating by the application of magnetic field and their transport behavior at different magnetic field values are studied. Well below the critical field of the transition, the current-voltage characteristics follow a power law V$\propto$ I$^{p}$, where p depends on the magnetic field. The dependence of the power p on magnetic field and the angle between the sample plane and the magnetic field direction will be presented. In particular, we find two distinct magnetic field values, well above the critical field, where the sample resistance is independent of the angle. Implications of these results in improving our current understanding of the transition will be presented.   

$T_c$ Enhancement in Electron-Doped Cuprate Heterostructures

Multilayer thin films of La$_{2-x}$Ce$_x$CuO$_4$ (LCCO) and Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO) were fabricated as superlattices of different dopings. Pairing over-doped and under-doped (or un-doped) layers is found to increase $T_c$ significantly above that of the single-phase films corresponding to the under- or over-doped layers. We report transport measurements on these mulitlayer films and discuss possible mechanisms for the $T_c$ enhancement. This work was supported by the US-Israel Binational Science Foundation Grant \#2006385 and the Center for Nanophysics and Advanced Materials (CNAM).   

Electronic Transport and Superconductivity in Bi Confined in a 200nm Opal Host

While bulk bismuth at ambient pressure is not a superconductor, changes in morphology are known to induce superconductivity in Bi at low temperatures. We present a study of bismuth nanoparticle arrays fabricated by confining bismuth into a porous opal host consisting of close-packed 200 nm silica spheres. Electrical transport was studied down to temperatures of 0.3K and magnetic fields up to 2T. We find the onset of superconductivity at 4.4K (confirmed by AC magnetic susceptibility) and global superconductivity at a temperature of 1.3K. This two step transition is typical for granular superconductors. Measurements of the critical temperature in magnetic field show that the higher temperature transition consists of at least two transitions. The low temperature upper critical field was calculated to be approximately 0.82T. These results are discussed in terms of the morphologies, namely amorphous and granular structure, that are known to make Bi a superconductor, and the three characteristic sizes for the nanoparticles in that has been used to describe superconductivity in lead imbedded in these host materials   

Electrostatic Modification of Properties of Ultrathin YBCO Films using an Electronic Double Layer Transistor

We have modified the transport properties of ultrathin films of YBCO using an electronic double layer transistor configuration employing the ionic liquid DEME-TFSI [1]. The films were prepared on STO substrates using high pressure oxygen sputtering. The electronic double layer configuration permits extraordinarily large transfers either involving the accumulation or depletion of carriers, employing relatively low gate voltages. Thus far the transition temperature of a 10 unit cell thick film has been shifted by as much as 30K, and the insulating state has been induced in a 7 unit cell thick film. The latest results will be reported on the use of this technique as an alternative to chemical doping. This work was supported by the National Science Foundation under grant NSF/DMR-0709584. \\[4pt] [1] J.T. Ye et al., Nature Materials 9, 125(2010).   

Phase-fluctuating superconductivity in overdoped La$_{2-x}$Sr$_x$CuO$_4$

In underdoped cuprates, an energy gap (pseudogap), appears in the electronic density of states well before superconductivity develops. Similarities between the pseudogap and superconducting gap have led to the idea that the pseudogap is a precursor superconducting state in which the superconducting order parameter is finite but the phase fluctuates. However, this picture of precursor pairing has been challenged by measurements indicating that the pseudogap itself closes at a critical doping concentration just beyond optimal doping. By tracking the restoration of the normal state magnetoresistance in overdoped La$_{2-x}$Sr$_x$CuO$_4$, we show that the phase fluctuation regime remains broad across the entire superconducting composition range, in contrast to the evolution of the pseudogap. The universal low phase stiffness is shown to be correlated with a low superfluid density, a characteristic of both underdoped and overdoped cuprates. The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity.   

Voltage Oscillations in Silver Doped YBa$_{2}$Cu$_{3}$O$_{7-x}$

Nonlinear transport phenomena and time effects were investigated by the time evolution of sample voltage ($V \quad - \quad t$ curves) on long time scales in Ag-doped YBa$_{2}$Cu$_{3}$O$_{7-x}$ sample (YBCO/Ag). We also investigated influence of bidirectional square wave current with various periods ($P)$ and dc currents ($I)$ on the evolution of $V \quad - \quad t$ curves in YBCO/Ag sample material at different temperatures ($T)$ and external magnetic ($H)$ fields. It was observed that a non-linear response seen in $V \quad - \quad t$ curves to bidirectional square wave (BSW) current with sufficiently short periods or sufficiently low amplitude reflects itself as regular sinusoidal- type voltage oscillations. The observed oscillating mode was correlated to the dynamic competition between pinning and depinning. Further, the similarity between the flux dynamics and the charge density waves is considered as a possible explanation of voltage oscillations in YBCO/Ag. Detailed analysis of $V \quad -$ $t$ curves and voltage oscillations reveals that adding of Ag causes degradation in both intergranular and surface pinning of YBCO material.   

Andreev and Single-Particle Tunneling Spectra of Underdoped Cuprate Superconductors

We study tunneling spectroscopy between a normal metal and an underdoped cuprate superconductor modeled by a phenomenological theory in which the pseudogap is a precursor to the undoped Mott insulator. In the low barrier tunneling limit, the spectra are enhanced by Andreev reflection only within a voltage region of the small superconducting energy gap. In the high barrier tunneling limit, the spectra show a large energy pseudogap associated with single particle tunneling. Our theory semi- quantitatively describes the two gap behavior observed in tunneling experiments.   

Doping evolution of nodal quasiparticles in the cuprate superconductor YBCO via low-temperature thermal conductivity

The thermal conductivity of the cuprate superconductor YBa$_2$Cu$_3$O$_y$ was measured at temperatures down to $T \sim 50$~mK in magnetic fields up to $H=15$~T on high-quality single crystals with a hole doping ranging from $p = 0.08$ to $p = 0.18$. The residual linear term at $T \to 0$, a direct measure of the nodal quasiparticle velocities [1], is tracked as a function of doping, and compared to recent, high-resolution ARPES measurements of the Fermi velocity and gap magnitude as a function of doping, in the related cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ [2]. \\[4pt] [1] D.G. Hawthorn {\it et al.}, Phys. Rev. B {\bf 75}, 104518 (2007). \\[0pt] [2] I.M. Vishik {\it et al.}, Phys. Rev. Lett. {\bf 104}, 207002 (2010).   

Correlation Strength, Optical Conductivity and d-d excitons in high $T_c$ cuprates

A single site dynamical mean field analysis is presented of models of high $T_c$ copper-oxide superconductors, including oxygen orbitals and both $x^2-y^2$ and $3z^2-r^2$ $Cu$ d-orbitals. The optical conductivity, doping dependent effective mass and the $e_g$ portion of the d-d exciton spectrum are determined. The details of the oxygen-oxygen hopping are shown to be unimportant. A general connection between $d$ valence and the metal/charge transfer insulator phase boundary is outlined.   

Determining Transport Parameters for Superconductor/Normal Metal Point Contacts in an Applied Magnetic Field from Conductance versus Field Data at Fixed Temperature

Superconductor/normal metal point contact transport data often consists of normalized conductance as a function of reduced temperature, from which the elastic scattering parameter and the inelastic scattering parameter for the contact can be determined, in addition to other features of interest. We show a strategy for determining these parameters from normalized conductance as a function of reduced applied magnetic field, at fixed temperature, even when conductance versus variable temperature data is absent. This analysis strategy will be demonstrated for several point contacts, over a wide range of parameter values.