Session T54: Superconductivity: Mostly Spectroscopy and Pairing

Probing the Photoresponse of Superconducting Materials and Devices by Laser Scanning Microscopy

We present the results of photoresponse experiments on superconducting rf/microwave devices, including Niobium and Cuprate resonators and metamaterials, by means of laser scanning microscopy. The spatially inhomogeneous photoresponse of these devices reveals the distribution of rf/microwave currents. Moreover, it will be discussed that the dependence of the phase of the photoresponse to the exciting microwave frequency may be used to bifurcate the kinetic and resistive parts of the photoresponse mechanism, which, in turn, could be used to probe the evolution of the local order parameter at different temperatures. Furthermore, we will examine the possibility of imaging the anisotropy of superconducting properties, as in the anisotropic Meissner effect, by investigating the photoresponse of a working superconducting resonator to visible light. While most of the aforementioned devices consist of thin films and cast into resonant structures, we will further discuss the techniques through which these measurements can be extended to bulk materials as well as non-resonant structures. These results are particularly useful for identifying the sources of quench and hot spots in a variety of superconducting devices including superconducting RF cavities.   

Far infrared study of magnetic field induced normal states of La$_{1.94}$Sr$_{0.06}$CuO$_{4}$

We report on the ab-plane optical properties of the magnetic field induced normal state of La$_{1.94}$Sr$_{0.06}$CuO$_4$ ($T_c=5.5 K$), the first such study. We apply strong magnetic fields (4 T and 16 T) along the c-axis . We find that at 4 T fields, which are strong enough to destroy superconductivity, the normal state at 1.4 K is very similar to the normal state at 20 K in zero field. However at higher fields we observed a gap-like depression in the optical conductivity at low frequency along with parallel growth of a broad absorption peak centered at higher frequency. The spectral weight loss in the depression at low frequency is recovered by the spectral weight in the broad peak. We attribute the magnetic field induced gap-like depression and the broad peak to a competing charge order to superconducting order or charge localization in ab-plane of the system.   

Electron energy loss spectroscopy study of superconducting Nb and its native oxides

Niobium has attracted increasing attention in recent years due to its usage in superconducting RF-cavities in next generation particle accelerators. In particular, the possible role of oxidation or the presence of oxygen vacancies on the superconducting properties of niobium metals used in superconducting RF cavities has been the focus on many recent studies. Here, we present a series of electron energy-loss spectroscopy (EELS) studies on niobium (Nb) and its oxides (NbO, NbO$_{2}$, Nb$_{2}$O$_{5})$ to develop a reliable method for quantifying the oxidation state in mixed niobium oxide thin films. Our approach utilizes a combination of transmission electron microscopy and EELS experiments with density functional theory calculations to distinguish between metallic niobium and the different niobium oxides. Based on these observed changes in the core-loss edges, we propose a linear relationship that correlates the peak positions in the Nb M- and O K-edges with the Nb valence state. The methods developed in this paper will then be applied to ultrathin niobium oxide films to examine the effects of low-temperature baking on the films' oxidation states. In addition to oxides, Niobium hydrides are considered as one of the main reasons for Q-decrease under high field. The different phases of Nb hydride can be identified directly using electron diffraction and EELS, which allows for the local hydrogen concentration to be examined at room temperature as well as 95 K.   

Strong-Field Pulsed THz Study of Superconductivity Breakdown in NbN

We report the ultra-fast breakdown of the superconducting state in a NbN thin film (${T}_C\approx\mathrm{14K}$) when exposed to an intense single-cycle THz pulse. The THz pulse's transform-limited spectral content was kept below the NbN pair-breaking energy threshold near 2$\Delta/hc=$ 35 cm$^{-1}$ (i.e., $<$1 THz). Thus, the initial electronic response was dominated by the inductive behavior of the pair condensate. At low THz E-field strength, the NbN film transmitted less for the superconducting state than for the normal state, as expected. As a function of increasing THz E-field strength, the film transmittance remained constant until a threshold range was reached, after which the transmittance changed over to its normal state value. Through this threshold range we also observed a significant non-linear response in the form of THz upconversion to frequencies approaching 3 times the optical gap, corresponding to time scales well below 1 picosecond.   

Spatial Complexity Due to Bulk Electronic Liquid Crystals in Superconducting Dy-Bi2212

Surface probes such as scanning tunneling microscopy (STM) have detected complex electronic patterns at the nanoscale in many high temperature superconductors. In cuprates, the pattern formation is associated with the pseudogap phase, a precursor to the high temperature superconducting state. Rotational symmetry breaking of the host crystal (i.e. from C4 to C2) in the form of electronic nematicity has recently been proposed as a unifying theme of the pseudogap phase [Lawler Nature 2010]. However, the fundamental physics governing the nanoscale pattern formation has not yet been identified. Here we use universal cluster properties extracted from STM studies of cuprate superconductors to identify the funda- mental physics controlling the complex pattern formation. We find that due to a delicate balance between disorder, interactions, and material anisotropy, the rotational symmetry breaking is fractal in nature, and that the electronic liquid crystal extends throughout the bulk of the material.   

Search for order parameter domains in single crystal flakes of chiral $p$-wave superconductor Sr$_2$RuO$_4$ using micrometer size Josephson junctions

Even though the odd-parity, spin-triplet superconductivity in Sr$_2$RuO$_4$ is supported by many experimental studies, the existence of order parameter domains and domain walls remains to be an open question. In addition, how these domains can be controlled experimentally is not understood. We fabricate small Al-Sr$_2$RuO$_4$ Josephson junctions using flakes of singlecrystalline Sr$_2$RuO$_4$ prepared by mechanical exfoliation, using Ti as an interlayer for adhesion purpose. The 3 $\mu$m wide junctions defined by photolithography on a natural $ac$ plane was found to show Josephson coupling. Experiments on the effect of domain walls on Josephson coupling and the control of domain walls are being explored. The work is supported by DOE under Grant No. DE-FG02-04ER46159 and Penn State nanofabrication lab.   

T-linear resistivity and hot Fermi surface from spin-density wave quantum critical fluctuations

The linear dependence in temperature $T$ of the resistivity observed in the normal phase of unconventional superconductors is often attributed to quantum critical behavior. We use the two-particle self-consistent (TPSC) approach to the Hubbard model to study this problem. The quantum critical point is associated with a spin-density wave (SDW) phase transition at zero-temperature on the 2D square lattice at finite doping. Our approach satisfies the Mermin-Wagner theorem, the Pauli principle and conservation laws, and is valid from weak to intermediate coupling. We take into account vertex corrections. For the model with nearest neighbors only and also with $t'$ and $t''$, we compute, as a function of $T$, contributions to the conductivity coming from different directions in the Brillouin zone. Our results show that the low temperature resistivity is linear because the whole Fermi surface is hot down to the lowest temperatures studied. This occurs because the SDW correlation length and the thermal de Broglie wavelength both scale as $1/T$.   

Controlled Intergrowth of 248 and 247 Phases of Y-Ba-Cu-O in Epitaxial Films and Heterostructures

Recent studies have shown that superconductivity in the Y-Ba-Cu-O (YBCO) family of cuprates can also be rooted in the quasi-1D Cu-O chains [1], even when the $\rm{CuO_2}$ planes are not conducting [2]. The critical temperature ($T_c$) depends on the phase of YBCO present, such as $\rm{Y_2Ba_4Cu_8O_{16}}$ (248) and $\rm{Y_2Ba_4Cu_7O_{15}}$ (247). Recent studies have also shown that heterostructuring YBCO with other oxides such as $\rm{La_{2/3}Ca_{1/3}MnO_3}$ (LCMO) can strongly influence the former's $T_c$ [3]. This talk reports on a reexamination of these issues, by probing and controlling the intergrowth of the various YBCO phases in thin films and heterostructures. The samples were grown epitaxially by pulsed laser-ablated deposition, and characterized by electrical transport, XRD and high-resolution TEM with in-situ EELS. We observed the presence of 248 and 247 phases in YBCO/LCMO heterostructures. We also observed conversion between different phases of YBCO depending on the thermodynamics of the growth and annealing conditions. The implication of our results on the $T_c$ variation in YBCO/LCMO heterostructure is discussed. \\[4pt] [1] J. Ngai \emph{et al.}, PRL. 98, 177003 (2007).\\[0pt] [2] E. Berg \emph{et al.}, PRB. 76, 214505 (2007).\\[0pt] [3] Z. Sefrioui \emph{et al.}, PRB. 67, 21451 (2003)   

Quantitative measurements of the magnetic field profile in superconductors

Measurement of the magnetic field profile $B(z)$, $z$ being the distance from the sample surface, in the Meissner state of superconductors is one of the longest standing problems of experimental superconductivity. Importance of $B(z)$ follows, in particular, from the fact, that it provides a direct way to determine the key intrinsic parameters, such as the London penetration depth at zero temperature $\lambda_L(0)$ and the Pippard coherence length $\xi_0$. None of these parameters is known with justified uncertainty for $any$ superconductor. $B(z)$ can be measured using Low-Energy Muon Spin Rotation spectroscopy (LE-$\mu$SR) and Polarized Neutron Reflectometry (PNR). To verify abilities of these techniques for quantitative measurements of $B(z)$ in unconventional superconductors and to examine the nonlocal electrodynamics effect predicted by Pippard in 1953, we performed an extensive series of cross LE-$\mu$SR and PNR measurements of $B(z)$ with two extreme type-I superconductors, In and Sn. Results obtained at the initial stage of this project were reported last year. Now the project is completed. Results unambiguously validate the nonlocal effect. Conditions which have to be met to use LE-$\mu$SR and/or PNR for measurements of $\lambda_L(0)$ and $\xi_0$ will be discussed.   

The critical magnetic field in the intermediate state of a Pippard superconductor

One of the fundamental problems of unconventional superconductivity is the magnetic structure of vortices. In order to contribute to the solution of this problem and to address number of unsolved questions of the intermediate state (IS) we undertook an experimental and theoretical study of IS in an extreme type-I superconductor focusing on the critical field of the IS-N (normal) transition. Results shed new light on the structure and evolution of the magnetic flux density in normal domains and may lead to new insights in the structure of vortices in type-II materials. A 2.5 $\mu$m thick indium film with mean free path 11 $\mu$m was placed in the superconducting vector (3D) magnet. Magneto-optical images were taken simultaneously with measurements of the sample resistance using a small low frequency AC current. The equilibrium domain structure of the IS state was investigated as a function of independently controlled in- and out-of-plane magnetic fields and/or DC transport current applied to the sample. The observed critical field varied in a range from 100\% down to 40\% of the thermodynamic critical field. A theoretical model based on the classical Landau laminar structure quantitatively accounts for the experimental results for both ordered and disordered domain patterns.   

Evidence for s+d wave pairing in copper oxides superconductors from an analysis of NMR and NQR data

Knight shift and spin-lattice relaxation rate data of high temperature copper oxide superconductors are analyzed within a two-band model for superconductivity with coupled s+d wave superconducting gaps. The two-gap approach leads to substantial modifications of the coherence factors, which reflects itself in the Knight shift and the relaxation rate 1/T$_{1}$T. From the analysis it is concluded that the data are consistent with 40{\%} s-wave and 60{\%} d-wave gap admixtures in agreement with earlier penetration depth data.   

Pseudogap studied by optical conductivity spectra of Zn-substituted YBa$_{2}$Cu$_{3}$O$_{y}$

The pseudogap and the superconducting gap cause a similar suppression of the low energy optical conductivity, but the behaviors of the spectral weight transfers are different, which enables us to distinguish these two gaps. In the \textit{c}-axis spectra of YBa$_{2}$Cu$_{3}$O$_{y}$, however, it is difficult to discuss these spectral weight transfers because of the additional structures due to a transverse Josephson plasma mode [1]. To overcome this problem, we substituted Zn for Cu, which is known to suppress those supplementary structures [2]. In this study, we performed temperature dependent reflectivity measurements in Zn-substituted YBa$_{2}$Cu$_{3}$O$_{y} $ system. We have revealed the continuous transfer of the low energy spectral weight to the higher energy region even below Tc, which suggests the coexistence of the pseudogap and the superconducting gap. [1]C. Bernhard et al. Phys. Rev. B, 61 (2000) 618. [2]R. Hauff et al., Phys. Rev. Lett., 77 (1996) 4620.   

Evidence of gate-tunable topological excitations in two-dimensional electron system

We report experimental observation of a new mechanism of charge transport in two-dimensional electron systems (2DES) in the presence of strong Coulomb interaction and disorder. We show that at low enough temperature the conductivity tends to zero at a non-zero carrier density, which represents the point of essential singularity in a Berezinskii-Kosterlitz-Thouless (BKT)-like transition. Our experiments with many 2DESs in GaAs/AlGaAs heterostructures suggest the charge transport at low carrier densities to be due to the melting of an underlying ordered ground state through proliferation of topological defects. Independent measurement of low-frequency conductivity noise supports this scenario. \\[4pt] [1] R. Koushik\textit{ et al}., Phys. Rev. B \textbf{83}, 085302 (2011) \\[0pt] [2] M. Baenninger\textit{ et al}., Phys. Rev. Lett. \textbf{100}, 016805 (2008).