Session P42: Superconductivity: Fluctuation Phenomena

Anomalous Noise in the Pseudogap Regime of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$

The pseudogap regime of high temperature superconductors is usually associated with electronic correlations (e. g. charge stripes). In symmetry-broken correlated systems, slow switching between different configurations contributes to low-frequency noise. Transport noise measurements performed in underdoped YBCO and Ca-doped YBCO thin films reveals a well-defined temperature of order 250K below which an extra noise component sets in that exhibits sensitivity to magnetic fields, aging effects, and large discrete fluctuators. Such effects are characteristic of noise from collective states in disordered systems. The noise onset temperature varies with measurement frequency but is only weakly doping-dependent. To explain this behavior, we speculate on a model involving thermally-activated motion of charge stripes partially pinned to underlying magnetic order.   

Noise in SQUID inductance as a result of motion of scattering centers

Recent experiments [1] studied 1/f noise in the inductance of superconducting quantum interference devices (SQUIDs) which showed correlation with the usual flux noise. In this work we analyze theoretically the noise in the kinetic inductance of a disordered superconductor produced by motion of scattering centers. The phase coherent diffusion of electrons at the scales shorter than the coherence length of the superconductor is sensitive to changes in spatial configuration of scatterers in the sample and hence to their motion. We use diagrammatic perturbation theory in $k_F\ell \gg 1$ to compute the magnitude of the effect which turns out to be much larger than a naive order of magnitude estimate. Our result sets a lower bound on the magnitude of the inductance noise in realistic superconducting devices such as those studied in [1]. \\[4pt] [1] S. Sendelbach, D. Hover, M. Muck, and R. McDermott, Phys. Rev. Lett. 103, 117001 (2009)   

On-chip photon-assisted shot noise detection to probe the near THz fundamental quantum noise of QPC

The physics of quantum shot noise becomes particularly interesting when the voltage energy is close to the frequency operation. However, high-frequency noise is usually limited to few GHz due to the large mismatch between the high impedance of the quantum conductor and the low impedance of the external circuit. To circumvent this problem, on-chip detection schemes have been recently implemented using Quantum Dots or SIS junctions. The proximity (few tens of microns) between the emitter and the detector prevents capacitive shunting and very high frequency noise (up to THz) can be measured. We propose a new type of universal high frequency noise detector based on Photo-Assisted Shot Noise (PASN) using universal effect of quantum noise from two quantum point contacts (QPCs), one as an emitter of high frequency shot noise and the other as a PASN detector, each of them placed on separate mesa Hall bars of a two-dimensional electron gas. Two extra QPCs are used as tuning impedance for cut-off frequency. At high frequency the detector and the noise source emitter are coupled via an interdigitated capacitor. By separately varying the noise Fano factor with the transmission of the QPCs, it is possible to check the detection is actually based on PASN mechanism and that the emitter actually generates quantum shot noise. We will present detailed studies of this new universal on-chip detection.   

Vortex Measurements in Niobium on a Alumina Substrates with 100nm Triangular Nanopore Lattice

Anodized aluminum films self-assemble into uniformly sized and spaced nanopores dependent upon the current and voltage used during the anodization procedure. This process has been optimized to provide a regular triangular nanopore lattice with a spacing of 100nm. The insulating alumina serves as a rigid substrate/template to investigate the effects of the periodic array on a superconducting thin film of niobium subject to a perpendicular applied field. We observe vortex matching effects in magnetization, current, and resistance measurements due to an overlap of the nanopore and vortex spacing at field multiples of 2.3 kOe. Further, we have investigated flux noise in our samples and find significant voltage noise due to the interaction of our superconducting film with the periodic geometry of our alumina substrate. We show that at fixed DC current values the power spectrum exhibit frequency structure dependent upon the applied field. These flux noise measurements help provide a better understanding of the flux flow in superconductors with non-magnetic periodic pinning.   

Anisotropic Superconductivity and Vortex Dynamics in Magnetically Coupled F/S and F/S/F Hybrids

Magnetically coupled superconductor-ferromagnet hybrids offer advanced routes for nanoscale control of superconductivity. Magnetotransport characteristics and STM images of vortex structures in superconductor-ferromagnet hybrids reveal rich vortex phase diagram. We use a combination of STM and Ginzburg-Landau simulations to reveal that the magnetic stripe domain structure induces periodic local magnetic induction in the superconductor, creating a series of pinning-antipinning channels for externally added superconducting flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. In F/S/F hybrids we demonstrate the evolution of the anisotropic conductivity in the superconductor that is coupled with two adjacent ferromagnetic layers. The anisotropic conductivity of up to three orders of magnitude can be achieved.   

Vortex Matter in nano-structured superconductors

We have fabricated anti-dots array in Nb and NbN, and high-Tc superconductor Bi-2212 with the diameter of r and the spacing of a, and have measured the flow-resistance of vortices perpendicular to the array and magnetic field. Depending also to the ratio of r/a, we can produce a variety of vortex-matching effect; the usual and the fractional matching. In low-Tc superconductors, the usual matching effect shows ``dips'' in the flow resistance at the matching field. However, it shows ``humps'' at higher magnetic fields, which may be related to the formation of giant vortices and interstitial vortices, depending on the ratio r/a. The matching phenomena seem to occur just like the Bloch electrons in two-dimensional electron system with crystalline lattice under magnetic field. The anti-dot array acts as a crystalline lattice to the vortices. In this case, vortex flow-resistance corresponds to Tc in the linearlized GL equation. Generation and annihilation of the fractional matching effect might be well reproduced. In high-Tc superconductor, the matching effect is closely related to the first order vortex lattice melting of the pristine samples in the presence of the anti-dot arrays and with changing the potential energy of the vortex pinning.   

Fractional Flux Vortices in Mesoscopic Two-Component Superconductors

Conventional superconductors have vortices carrying integer multiples of magnetic flux quantum while unconventional ones, with p- or d-wave order parameter, allow half-integer fluxes. In this presentation we show that mesoscopic size effects stabilize fractional flux vortices in the \textit{thermodynamical ground state} of s-wave two-gap superconductors. The value of these fluxes can be an \textit{arbitrary fraction of flux quantum} and can be measured directly from distributions of magnetic fields on the samples.   

Dissipation-driven phase transitions in superconducting wires

Narrow superconducting wires with diameter $d\ll\xi_{0}$ (where $\xi_{0}$ is the bulk superconducting coherence length) are quasi-1D systems in which fluctuations of the order parameter strongly affect low-temperature properties. Indeed, fluctuations cause the magnitude of the order parameter to temporarily vanish at some point along the wire, allowing its phase to slip by $2\pi$, and to produce finite resistivity for all temperatures below $T_c$. In this work, we show that a weak coupling to a diffusive metallic film reinforces superconductivity in the wire through a quench of phase fluctuations. We analyze the effective phase-only action of the system by a perturbative renormalization-group and a self-consistent variational approach to obtain the critical points and phases at $T=0$. We predict a quantum phase transition towards a superconducting phase with long-range order as a function of the wire stiffness and coupling to the metal. Finally we discuss implications for the DC resistivity of the wire.   

Influence of thermal phase fluctuations on the spectral density function in a 2D d-wave superconductor

We present the study of the spectral density for a two-dimensional superconductor in a regime of strong phase fluctuations. The focus of the talk is on $d$-wave superconductors. We obtain analytical expressions for the Green function below the transition temperature and have worked out a way to extrapolate it for a finite temperatures above $T_c$. The quasi-classical approximation is inapplicable in the vicinity of the nodes of the order parameter. We have summed the most singular terms of the perturbation theory in pairing potential. Along with analytical expressions we present the qualitative features of the spectral function.   

A scaling analysis of the superconducting fluctuations in 2D InOx thin films

We apply a broadband microwave Corbino spectrometer covering the range from 10MHz to 20GHz to the study of 2D disordered superconducting InOx thin films. Explicit frequency dependency of the superfluid stiffness and conductivity are obtained down to 300mK. Via vacuum annealing, we investigate a broad range of disorder levels and transition temperatures in a single film. We perform a scaling analysis in which we can extract characteristic relaxation time of superconducting fluctuations. We discuss our results in terms of prevailing scenarios for fluctuation superconductivity and make connection to other experimental results.   

Universal critical behavior in single crystals and films of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$

We studied the normal-to-superconducting phase transition in optimally-doped YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ in zero external magnetic field using a variety of different samples and techniques [1]. Using DC transport measurements, we find that the dynamical critical exponent $z $= 1$.$54 $\pm $ 0$.$14, and the static critical exponent \textit{$\nu $ }= 0$.$66 $\pm $ 0$.$10 for both films (when finite-thickness effects are included in the data analysis) and single crystals (where finite-thickness effects are unimportant). We also measured thin films at different microwave frequencies and powers (as well as DC), which allowed us to systematically probe different length scales to avoid finite-thickness effects. These microwave and DC measurements yielded a value of z consistent with the other results, $z $= 1$.$55 $\pm $ 0$.$15. The neglect of finite-thickness, finite-current, and finite-frequency effects may account for the wide ranges of values for \textit{$\nu $ }and $z $previously reported in the literature. \\[4pt] [1] Hua Xu, \textit{et al}., Phys. Rev. B 80 104518 (2009). (http://link.aps.org/doi/10.1103/PhysRevB.80.104518)   

Shape resonances and proximity to a structural phase transition for high Tc in La2CuO4

The high temperature superconductivity in cuprates and pnictides occurs in multicomponent superconducting materials made of superlattices of superconducting units separated by spacer units. The critical temperature is amplified at the shape resonance or Feshbach resonance by the tuning the chemical potential at an electronic topological transition (ETT) [Antonio Bianconi ``Process of increasing the critical temperature Tc of a bulk superconductor by making metal heterostructures at the atomic limit ''United State Patent No. :US6, 265, 019 B1, July 24, 2001]. An anomalous electron-lattice interaction at the resonance is related with the disorder driving the system to a lattice instability with multiscale structural phase separation. Here we present compelling evidence for multiscale phase separation in the optimum doped phase in a oxygen doped cuprates with mobile dopants that can be controlled by external fields. These results shed light on the critical complexity in the high Tc multilayered materials and open new perspectives for manipulation of the critical temperature via controlling dopant ordering.   

Magnetization currents in thermomagnetic effects

In a finite sample, besides the bulk currents given by the Kubo formula, additional charge and energy are transferred by surface magnetization currents. At the same time, electric and energy magnetization currents are divergence-free and corresponding net currents are always zero. Therefore, instead of adding the surface magnetization currents, one can subtract the bulk magnetization current. We show that for the energy current, the corresponding correction to the Kubo's bulk current is expressed in terms of the magnetization component in the Poynting vector. For the heat current any such corrections are absent. It means that magnetization heat currents are absent, obviously because of the absence of circular heat currents and circular temperature gradients. Thus, the Kubo method gives an exact expression for the heat current, while the electric current should be corrected due to magnetization contribution. This theory satisfies the Onsager relation. It is applied to fluctuation and vortex thermomagnetic effects in superconductors.