Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A52: Superconductor Insulator Transitions |
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Sponsoring Units: DCMP Chair: Michael Osofsky, Naval Research Laboratory Room: Mile High Ballroom 1F |
Monday, March 3, 2014 8:00AM - 8:12AM |
A52.00001: Dynamical conductivity across the disorder-tuned superconductor-insulator transition Mason Swanson, Yen Lee Loh, Mohit Randeria, Nandini Trivedi We study the superconductor-insulator transition (SIT) in both clean and disordered systems by calculating the dynamical conductivity $\sigma (\omega )$ and the bosonic (pair) spectral function $P(\omega )$ using quantum Monte Carlo simulations. We identify characteristic energy scales in the superconducting and insulating phases that vanish at the transition due to enhanced quantum fluctuations, despite the persistence of a robust fermionic gap across the SIT [1]. While $\sigma (\omega )$ shows a energy scale for absorption associated with a Higgs (amplitude) mode in the clean superconductor, disorder leads to enhanced low frequency absorption in $\sigma (\omega )$ on both the superconducting and insulating side of the transition. Disorder also expands the quantum critical region, due to a change in the universality class, with an underlying $T =$ 0 critical point with a universal low-frequency conductivity $\sigma $* $\cong $ 0.5 (4e$^{2}$/h) [2]. \\[4pt] [1] K. Bouadim, Y.L. Loh, M. Randeria, and N. Trivedi, \textit{Nat. Phys.} 7 884 (2011)\\[0pt] [2] M. Swanson, Y.L. Loh, M. Randeria, and N. Trivedi\textit{, arXiv} 1310.1073 (2013) [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A52.00002: Divergence of Dynamical Conductivity at Certain Percolative Superconductor-Insulator Transitions Rajesh Dhakal, Yen Lee Loh, John Neis, Evan Moen Coarse-grained superconductor-insulator composites can be modeled as random inductor-capacitor (LC) networks, which exhibit percolative superconductor-insulator transitions (SITs). We use a simple and efficient algorithm to compute the dynamical conductivity $\sigma(\omega,p)$ of one type of LC network on large ($6144 \times 6144$) square lattices, where $\delta=p-p_c$ is the tuning parameter for the SIT [1]. We confirm that the conductivity obeys a scaling form near criticality, so that the characteristic frequency scales as $\Omega \propto \left|\delta\right|^{\nu z}$ with $\nu z \approx 1.91$, the superfluid stiffness scales as $\Upsilon \propto \left| \delta \right|^t$ with $t \approx 1.3$, and the electric susceptibility scales as $\chi_E \propto \left| \delta \right|^{-s}$ with $s = 2\nu z - t \approx 2.52$. In the insulating state, the low-frequency dissipative conductivity is exponentially small, whereas in the superconductor, it is linear in frequency. The sign of m $\sigma(\omega)$ at small $\omega$ changes across the SIT. Most importantly, right at the SIT, Re $\sigma(\omega) \propto \omega^{t/\nu z-1} \propto \omega^{-0.32}$, so that the quasi-dc conductivity $\sigma^*$ is infinite, in contrast with most other classical and quantum models of SITs. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A52.00003: Superfluid-insulator transition in a two-dimensional bond-disordered quantum rotor model Min-Chul Cha, Sung-Been Park We study the critical properties of the superfluid-insulator transition in a disordered two-dimensional quantum rotor model with random spatial bonds. Via worm-algorithm Monte Carlo calculations of superfluid density, compressibility, and correlation function, we find the dynamical critical exponent $z\approx 1.17$ and the correlation length critical exponent $1/\nu \approx 1.2$ at commensurate filling, and$z\approx 2.0$ and $1/\nu \approx 1.5$ at incommensurate filling. These exponents are not consistent with the Bose-glass-to-superfluid transition and suggest possibility of new disordered insulating phase. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A52.00004: New method for the controlled creation of sub-15 nm aluminum nanowires to probe the 1D superconductor-insulator transition Tyler Morgan-Wall, Hannah Hughes, Nik Hartman, Tyrell McQueen, Nina Markovic We have developed a new method for the creation of sub-15 nm aluminum nanostructures using a sodium bicarbonate solution. Using PMMA masks patterned with e-beam lithography, we can controllably etch lithographically-produced nanostructures while measuring their resistances in-situ using a 4-probe measurement. This technique allows for precise control over the final resistance and thus can be used to create a wide variety of nanodevices. In particular, this technique allows for the creation of nanowires to probe the superconductor-insulator transition in 1D. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A52.00005: Evolution of the Cooper Pair Insulator Phase in a-Bi Films Grown on Nanohoneycomb Substrates with Varying Surface Topography J.C. Joy, X. Zhang, C. Zhao, S.M. Hollen, J.M. Valles, Jr., G. Fernandes, J.M. Xu The Cooper Pair Insulator (CPI) phase has been observed in a variety of systems close to both the disorder and field tuned Superconductor to Insulator Transition (SIT) in two dimensions. A number of recent experimental and theoretical studies suggest that the CPI phase arises due to inhomogeneities in the superconducting coupling constant on the nanoscale. Anodized Aluminum Oxide (AAO) substrates provide a convenient experimental platform for studying the influence of inhomogeneity on the CPI state, as the substrates exhibit both a nanohoneycomb structure which allows flux periodic behavior to be measured, as well as a controllable morphology which permits control of the level of inhomogeneity present. We will discuss recent experiments and analyses which examine the behavior of the CPI phase as the level of inhomogeneity in the films is reduced. We will also examine the potential implications of this work in understanding the extent of the CPI phase in two-dimensional systems. This work was supported by the NSF through grants No. DMR-1307290 and DMR-0907357 and by the AFRL, the ONR, and the AFOSR. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A52.00006: The Role of Mesoscopic Disorder in Determining the Character of the Field-Induced Insulating regime of Amorphous Ultrathin Films J.J. Nelson, Yen-Hsiang Lin, Allan Goldman A series of quench-condensed amorphous Bismuth films of different thicknesses were shown to exhibit nonmonotonic magnetoresistances and Arrhenius conduction in the magnetic field induced insulating regime. Neither behavior is found in similar measurements carried out on amorphous Bismuth films grown on smooth, thin, amorphous Antimony underlayers. Arrhenius behavior is found for films grown on thicker Antimony underlayers. \textit{ Ex situ }Atomic Force Microscopy measurements of a series of Antimony films of different thicknesses showed that the mesoscopic scale roughness of the surface increased with increasing thickness. This suggests that film roughness plays the role of nucleating superconducting clusters through thickness variations in the subsequently deposited amorphous Bismuth layer. The properties of the insulating regime appear to depend upon the level of mesoscopic scale disorder. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A52.00007: Current Bias Induced Negative Magneto-Resistance in Superconducting Tantalum Thin Films Sun-gyu Park, Eunseong Kim Negative Magneto-Resistance (MR) of 2D superconducting thin films has received attentions because the decreasing resistance with increasing magnetic field cannot be simply understood by conventional superconductivity. This behavior was ascribed to localized bosons, indicating the existence of a Bose insulator (BI) phase[1-3]. We found negative MR within a range of dc current bias in tantalum thin films, whereas no negative MR appears without bias. We measured R$_{\mathrm{xx}}$ and R$_{\mathrm{xy}}$ simultaneously as functions of current bias and magnetic field and construct the phase diagram at T$=$0 limit. We found that the DC biased negative MR in Ta thin film shows substantially different characteristics from those of reported no biased negative MR. We also found that the induced BI can be understood by the vortex instability state [4, 5].\\[4pt] [1] M. A. Paalanen, A. F. Hebard, and R. R. Ruel, PRL 69, 1604 (1992).\\[0pt] [2] G. Sambandamurthy et al., PRL 92, 107005 (2004).\\[0pt] [3] Y. Zou, G. Refael, and J. Yoon, PRB 80, 180503 (2009).\\[0pt] [4] A. I. Larkin, and Y. N. Ovchinnikov, Sov. Phys.-JETP 41, 960 (1975).\\[0pt] [5] D. Y. Vodolazov, and F. M. Peeters, PRB 76, 014521 (2007). [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A52.00008: Probing superfluid rigidity in ultrathin 2D superconductor at microscopic, mesoscopic, and macroscopic length scales H.D. Nam, J.S. Kim, C.D. Zhang, J. Yong, T.R. Lemberger, P.A. Kratz, J.R. Kirtley, K.A. Moler, C.K. Shih Within the conventional picture, two-dimensional (2D) superconductivity is fragile because phase fluctuations disrupt the long-range order of Cooper pairs. Investigations on epitaxially grown conventional superconductors in the ultra-thin regime, however, revealed rather surprisingly robust superconductivity. Since the robust T$_{\mathrm{C}}$ at this extreme limit was observed primarily using scanning tunneling spectroscopy (STS), it has been suggested that while Cooper pairing remains robust, phase fluctuations can still destroy long range coherence, leading to a much more fragile superconductivity. This work is aimed at addressing this issue by probing superfluid rigidity in ultra-thin Pb films at microscopic, mesoscopic, and macroscopic length scales, using STS, scanning SQUID (SSM), and double coil mutual inductance method, respectively. All three methods yield very similar Tc, attesting the robustness of the supercurrent at macroscopic scale. We further discuss the underlying mechanism for the strong phase rigidity for ultra-thin Pb films at microscopic and macroscopic length scales. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A52.00009: Thickness Dependent Superconductor-Insulator Transition in K$_{0.33}$WO$_{3}$ Phillip Wu, Chris Hart, Katherine Luna, Ko Munakata, Theodore H. Geballe, Malcolm R. Beasley We observe a thickness dependent superconductor to insulator transition in K-doped tungsten bronze superconductors. Via a two-step deposition and post-annealing procedure, K-doped WO$_{3}$ films with reproducible transport properties are obtained. Reducing the film thickness by reducing the film deposition time results in a superconductor to insulator transition. Scanning electron microscopy (SEM) images show that KWO$_{\mathrm{3}}$ crystallites become both thinner and less connected as the deposition time is reduced. Suppression of the density of states at the Fermi level observed using point contact tunneling spectroscopy in the superconducting films demonstrates that disorder-induced increased Coulomb interactions are present. Using the theory of Belitz [1] for the reduction of Tc due to disorder, we can infer that the film with highest observed Tc has a relatively large disorder dependent electron-phonon interaction parameter $\tilde{\lambda} \sim$ 1.2. Understanding microscopically why certain films display higher Tc will aid in the search for the trace high Tc superconducting anomalies observed in lightly surface doped bronzes. This work supported by an AFOSR under DoD MURI grant FA9550-09-1-0583. \\[4pt] [1] D. Belitz. Phys. Rev. B 40, 111 (1989). [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A52.00010: Electronic disorder, spin glass and large magnetoresistance in FeSr$_{2}$Y$_{2-y}$Ce$_{y}$Cu$_{2}$O$_{8+x}$ Sebastian Sambale, Grant Williams, Jibu Stephan, Shen Chong We have successfully synthesized FeSr$_{2}$Y$_{2-y}$Ce$_{y}$Cu$_{2}$O$_{8+x}$ (Fe1222) with a wide range of Ce and oxygen concentrations. Fe1222 belongs to an interesting group of compounds that contains a 2D-like CuO$_{2}$ layer and an oxygen deficient FeO$_{x}$ layer. They are structural very similar to the well-studied superconducting and magnetically ordered RuSr$_{2}$R$_{2-x}$Ce$_{x}$Cu$_{2}$O$_{10-x}$ (Ru1222). However, we do not observe superconductivity in Fe1222 and there is a spin-glass transition with antiferromagnetic exchange interactions arising from the disordered FeO$_{x}$ layer at $\sim$25 K, which does not depend on the Ce or oxygen concentration. The electronic transport in the oxygen reduced samples is highly disordered and involves variable range hopping between localized states where there is a large negative magnetoresistance of $\sim$-22\% at 8 T. The oxygen saturated samples are highly conducting at room temperature and display weak localization at low temperatures. The absence of superconductivity may be due to pair-breaking by a small Fe fraction in the CuO$_{2}$ planes. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A52.00011: Textured electronic states of the triangular lattice Hubbard model and Na$_x$CoO$_2$ near $x=1/3$ Kun Jiang, Sen Zhou, Ziqiang Wang The interplay between geometric frustration and strong correlation is studied in the triangular lattice Hubbard model near electron doping $x=1/3$, in connection to the sodium cobaltates Na$_x$CoO$_2$. We found a mechanism of alleviated magnetic frustration via charge and spin inhomogeneity. At $x=1/3$, the uniform paramagnetic ground state for $U < U_{c1}$ transforms into a $\sqrt{3}\times\sqrt{3}$ spin-charge textured insulating state for $U > U_{c2}$ with antiferromagnetic order on the underlying unfrustrated honeycomb lattice. The transition region, $U_{c1} < U < U_{c2}$, shows several textured semi-metallic states with both collinear and noncollinear magnetic order. We obtain the phase diagram and show that the strongly correlated phases near $x=1/3$ corresponds to doping the ``1/3 state'' with excess carriers forming electron or hole Fermi surface pockets, and compare to experimental findings. We thus propose that the cobaltates near $x=1/3$ are in proximity to such ``hidden'' textured phases with spin and charge order and the enhanced electronic fluctuations can mediate the superconducting pairing interaction. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A52.00012: Non-flux avalanches in the tunneling density of states of a superconductor in a high Zeeman field Joseph Prestigiacomo, Philip Adams We report an ongoing experimental study of the effects of disorder and temperature on the glassy dynamics of the Zeeman-limited critical field transition in ultrathin Al films. We have measured the tunneling density of states of the films through the first-order parallel critical field transition. We find that films with sheet resistance of a few hundred ohms exhibit large avalanches on the superheating branch of the critical field hysteresis loop. In contrast, the transition back into the superconducting phase (i.e., along the supercooling branch) is always continuous. Similar avalanche behavior is also observed in transport. We will discuss what our results imply about nature of the superconducting order parameter in the regime where the Zeeman splitting is of the order of the superconducting gap energy. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A52.00013: Disorder-Driven Superconductor-Insulator Transition in $d$-Wave Superconductors Yun Song, Long He We study the superconductor-insulator transition (SIT) in $d$-wave superconductors. By means of the kernel polynomial method, the Bogoliubov-de Gennes equations are solved self-consistently, making it possible to observe fully the nanoscale spatial fluctuations of the superconducting order parameters. It is shown that Anderson localization can not entirely inhibit the occurrence of the local superconductivity in strongly-disordered $d$-wave superconductors. Separated by an insulating ``sea'' completely, a few isolated superconducting ``islands'' with significant enhancement of the local superconducting order parameters can survive across the SIT. The disorder-driven SIT, therefore, is a transition from a $d$-wave superconductor to a boson insulator which consists of localized Cooper pairs. Unlike an $s$-wave superconductor which presents a robust single-particle gap across the SIT, the optical conductivity of a $d$-wave superconductor reveals a gapless insulating phase, where the SIT can be detected by observing the disappearance of the Drude weight with the increasing disorder. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A52.00014: Liquid-gated superconductor-insulator transition in an electron-doped cuprate Shengwei Zeng, Zhen Huang, Nina Bao, Weiming Lv, Zhiqi Liu, T.S. Herng, K. Gopinadhan, Linke Jian, J. Ding, T. Venkatesan, Ariando Ariando Doping charge carriers will causes the change of cuprates from antiferromagnetic Mott insulators to high-$T_{c}$ superconductors. Continuous changing of carrier density is necessary to understand the nature of such phase transition, and thus, further our understanding of cuprate superconductors. Electric field-effect doping, especially with electronic double layer transistors (EDLT) configuration which use ionic liquids (ILs) and polymer electrolyte as the gate dielectrics, is a potential avenue for this investigation and it has been shown its effectiveness in inducing phase transition in strongly correlated electron system. Owing to EDLT, superconductor-to-insulator transition (SIT) has been observed in hole-doped cuprates La$_{2-x}$Sr$_{x}$CuO$_{4}$ and YBa$_{2}$Cu$_{3}$O$_{y}$. Here we use EDLT to tune the carrier density in electron-doped cuprates Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ ultrathin films and cause the sample evolves from a superconducting state to an insulating state. This present results could be helpful to study SIT between electron- and hole-doped cuprates. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A52.00015: Influence of spatial disorder on the superconducting state of a 3D superconductor Carolina Parra, Francis Niestemski, Paula Giraldo-Gallo, Alex W. Contryman, Theodore H. Geballe, Ian R. Fisher, Hari C. Manoharan We present the first measurements of the local tunneling density of states on the three-dimensional superconductor $BaPb_{1-x}Bi_{x}O_{3}$ as a function of Bi doping. Scanning tunneling spectroscopy measurements are performed on a sequence of samples which exhibit a field-tuned superconductor-to-insulator (SIT) transition. Our study shows that gap variations in the superconducting (SC) state (as a sign of SC disorder level) increase when the system moves towards the SIT phase boundary, with spatial inhomogeneity comparable in size to the material's coherence length. We demonstrate that this highly inhomogeneous local gap size is always finite at every location, even for Bi concentration closest to the SIT, where local insulating behavior is expected and globally confirmed in transport experiments. Our results also suggest a method for increasing the critical temperature for this material by reducing its spatial disorder in the appropriate part of the phase diagram. [Preview Abstract] |
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