Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V25: Superconductivity: Superconductor-Insulator Transitions |
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Sponsoring Units: DCMP Chair: Eleanor Clements, University of South Florida Room: 324 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V25.00001: Negative magneto resistance and anisotropic transport in DC biased superconducting Ta films Junghyun Shin, Sungyu Park, Eunseong Kim We investigated the field-tuned superconductor-insulator transition in DC biased Ta thin films. Differential resistance in direction parallel (x axis) and perpendicular (y axis) to DC bias (x axis) was measured simultaneously as functions of DC bias and magnetic field. The DC biased magneto-resistance, dV/dI, showed anisotropy; differential magneto-resistance exhibited peaks at high DC bias in the parallel measurements while monotonic increase in dV/dI was obtained in the perpendicular measurements. Besides, the critical fields determined by magneto-resistance isotherms cross-over reveal substantially different values depending on the measurement directions. Furthermore, the E-field scaling exponents show different values of 1.33 in perpendicular direction and of 0.82 in parallel direction which cannot be simply understood by the temperature scaling with electron heating. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V25.00002: Signature of Cooper pairs in the Metallic and Insulating Phases of Homogeneously Disordered Superconducting Ta Films Yize Stephanie Li With the increase of magnetic field or the decrease of sample thickness, homogeneously disordered superconducting Ta films undergo a superconductor-metal-insulator phase transition [1][2]. Each phase displays remarkably different nonlinear current-voltage (I-V) characteristics. The evolution of the nonlinear transport in the insulating phase exhibits a non-monotonic behavior as the magnetic field is increased, which could be evidence of the presence of localized Cooper pairs in the insulating phase [3]. As the metallic phase intervenes the superconducting and insulating states in Ta films, we further suggest that Cooper pairs also exist in the metallic ground state. References: [1] Y. Qin et al., Phys. Rev. B 73, 100505(R) (2006). [2] Y. Li et al., Phys. Rev. B 81, 020505 (R) (2010). [3] Y. S. Li, Supercond. Sci. Technol. 28, 025002 (2015). [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V25.00003: Superconductor-Insulator Transition and Fermi-Bose Crossovers Nandini Trivedi, Yen Lee Loh, Mohit Randeria, Chia-Chen Chang, Richard Scalettar The direct transition from an insulator to a superconductor (SC) in Fermi systems is a problem of long-standing interest, which necessarily goes beyond the standard BCS paradigm of superconductivity as a Fermi surface instability. We introduce here a simple, translationally-invariant lattice fermion model that undergoes a SC-insulator transition (SIT) and elucidate its properties using analytical methods and quantum Monte Carlo simulations. We show that there is a fermionic band insulator to bosonic insulator crossover in the insulating phase and a BCS-to-BEC crossover in the SC. The SIT is always found to be from a bosonic insulator to a BEC-like SC, with an energy gap for fermions that remains finite across the SIT. The energy scales that go critical at the SIT are the gap to pair excitations in the insulator and the superfluid stiffness in the SC. In addition to giving insights into important questions about the SIT in solid state systems, our model should be experimentally realizable using ultracold fermions in optical lattices. Ref: arXiv:1507.05641 [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V25.00004: Critical Exponents of Dynamical Conductivity in 2D Percolative Superconductor-Insulator Transitions: Three Universality Classes Pragalv Karki, Yen Lee Loh We simulate three types of random inductor-capacitor (LC) networks on 4000x4000 lattices. We calculate the dynamical conductivity using an equation-of-motion method in which timestep error is eliminated and windowing error is minimized [1]. We extract the critical exponent $a$ such that $\sigma(\omega) \propto \omega^{-a}$ at low frequencies. The results suggest that there are three different universality classes. The $L_{ij} C_i$ model, with capacitances from each site to ground, has $a=0.32$. The $L_{ij} C_{ij}$ model, with capacitances along bonds, has $a=0$. The $L_{ij} C_i C_{ij}$ model, with both types of capacitances, has $a=0.30$. This implies that classical percolative 2D superconductor-insulator transitions (SITs) generically have $\sigma(\omega) \rightarrow \infty$ as $\omega \rightarrow 0$. Therefore, experiments that give a constant conductivity as $\omega \rightarrow 0$ must be explained in terms of quantum effects. $~~~~~~~$ [1. Yen Lee Loh, Rajesh Dhakal, John F. Neis and Evan M. Moen, ``Divergence of dynamical conductivity at certain percolative superconductor-insulator transitions'', Journal of Physics: Condensed Matter 26, 50 (2014)] [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V25.00005: The phase diagram and Bose metal in superconducting nanowires Tyler Morgan-Wall, Hannah Hughes, Nikolaus Hartman, Nina Markovic We experimentally investigated the transport properties of thin, narrow superconducting aluminum nanowires as a function of magnetic field and temperature. We characterized the full superconducting phase diagram with respect to magnetic field and temperature, and show the onset of a flux-flow phase for certain values of temperature and magnetic field. The flux-flow resistance follows the Bardeen-Stephen model and it is shown that the resistance increases linearly with respect to magnetic field in this region of the phase diagram. In addition, we show the saturation to a non-zero finite resistance state below the normal state resistance for certain magnetic fields as the temperature decreases to zero. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V25.00006: Shrinking of the Cooper Pair Insulator Phase in Thin Films with Ultrasmall Superconducting Islands J.C. Joy, X. Zhang, C. Zhao, J.M. Valles, Jr., G. Fernandes, J.M. Xu The ubiquity of the bosonic Cooper Pair Insulator (CPI) phase near the two-dimensional superconductor to insulator transition (SIT) is a long standing question. While a number of two dimensional materials exhibit bosonic insulating phases similar to the Mott Insulator in arrays of ultrasmall, Josephson coupled superconducting islands, others show behaviors consistent with a fermionic insulating phase. Utilizing specially prepared anodized aluminum oxide substrates, we are able to fabricate films reminiscent of arrays of superconducting islands whose properties are tunable by varying the substrate morphology. Our recent work has focused on arrays of islands which possess an energy level spacing comparable to the mean field superconducting gap, where one expects pair breaking followed by fermionic Anderson Localization as the dominant mechanism by which superconductivity is destroyed. Early results show that the paradigmatic bosonic insulator exists only very near the disorder tuned SIT, while films only marginally deeper in the insulating phase exhibit transport distinct from the CPI’s reentrant, activated transport. We are grateful for the support of NSF Grant No. DMR-1307290, the AFOSR, and the AOARD. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V25.00007: Disorder induced superconductor-insulator transition in epitaxial La$_{1.85}$Sr$_{0.15}$CuO$_{4}$ thin films Han-Byul Jang, Chan-Ho Yang La$_{2-x}$Sr$_{x}$CuO$_{4\, }$is a well-known superconducting system showing various electronic properties as a function of Sr content. Especially, epitaxial thin layers of the compound show enormous increase of superconducting critical temperature ($T_{c})$ by a compressive strain. It has been reported that $T_{c}$ can be controlled by misfit strain, thickness, and oxygen annealing. In this study, we report structural and transport properties of high quality epitaxial La$_{1.85}$Sr$_{0.15}$CuO$_{4\, }$thin films. According to x-ray diffraction study, $c$-axis lattice parameter shows no significant change for various film thicknesses and the in-plane lattice parameters of the films are coherently matched with that of substrate. Electronic transport measurements show a clear superconductor-to-insulator transition (SIT), accompanying variation of $T_{c}$ depending on film thickness. These results are analyzed by using the McMillan equation to find the relation between the $T_{c}$ and a disorder correlating with film thickness. We have found the disorder exhibits an explicit power-law behavior with respect to film thickness in our La$_{1.85}$Sr$_{0.15}$CuO$_{4}$ thin films. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V25.00008: Origin and consequences of the disorder-induced inhomogeneities in cuprate superconductors Debmalya Chakraborty, Rajdeep Sensarma, Amit Ghosal The effect of potential impurities on cuprate superconductors are investigated within a formalism suitable for addressing the complex interplay of the bare repulsive electronic correlations and disorder, both being strong. We show that the mechanism governing the demise of superconductivity is rather subtle and differs from the conventional weak-coupling descriptions. While the superconductivity remains surprisingly robust for up to moderate disorder, it crashes down sharply at stronger disorders. The initial robustness is attributed to the strong repulsive correlations that smear out charge inhomogeneities by reorganizing the hopping on the bonds prohibiting formation of superconducting ``islands". However, with increasing strength of disorder, the potential difference across some bonds reach the scale of the bandwidth and the overall energy of the system is reduced by prohibiting hopping on such links. Integrating this concept within our formalism, we show that the correlations fail to homogenize the system across these ``cut-bonds". This produces Mott-insulating, Anderson-insulating, as well as locally superconducting regions interspersed among each other at strong disorder, eventually destroying the global superconductivity. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V25.00009: The half-filling paradox in Pr$_{2}$CuO$_{4+\delta}$ Yoshiharu Krockenberger, Ai Ikeda, Hiroshi Irie, Hideki Yamamoto In a type-II superconductor, the onset of the superconducting state as a function of decreasing magnetic field $H$ occurs at the upper critical magnetic field $H_{c_{2}}$, dictated by the pairing gap $\Delta$ through the coherence length $\xi_{0}\sim\nu_{F}/\Delta$, via $H_{c_{2}}=\Phi_{0}/2\pi\xi_{0}^{2}$. We show that the mean-free length $\ell$ in superconducting Pr$_{2}$CuO$_{4}$ can be as large as 85nm, or 200 unit cell lengths. Such large $\ell$ values are directly related to residual resistivity ratios. While Pr$_{2}$CuO$_{4+\delta}$ is commonly known as an antiferromagnetic insulator (AFI), we show that synthesis and annealing conditions govern $\delta$. For AFI Pr$_{2}$CuO$_{4}$, i.e. $\ell\ll 2$nm, $\delta$ is larger than 0.10 whereas $\delta\ll 0.04$ in the superconducting state. In fact, this de-intercalation of apical oxygen is mandatory for the induction of superconductivity and is counter to early conclusions that all cuprate superconductors are AFI in their undoped state. It is not surprising that the AFI state found in cuprates with 5- and 6-fold coordinated copper is not established in cuprates with 4-fold coordinated copper, i.e. $\delta<0.04$. Such coordination-driven phase transition is at the core of the long-term assumed half-filling paradox. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V25.00010: Investigating Links Between Enhanced Inhomogeneity and Robustness of the Superconducting State in Severely Underdoped, Ultrathin CaYBCO Stanley Steers, Thomas Lemberger, Brian Baker Multiple experiments in recent years have shown evidence for intrinsic inhomogeneity in the cuprate superconductors upon approach to the superconductor to insulator transition (SIT). We present experimental evidence of anomalous suppression of both the diamagnetic response and the resistivity in ultrathin Ca-doped YBCO over tens of Kelvin. We then present further experiments to investigate to what extent this suppression is related to increased inhomogeneity upon approach to the SIT. Finally, we compare these results in CaYBCO to experiments in other cuprate compounds from the literature. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V25.00011: Modulation of Superconductor-Insulator Transition in NdBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7-x\thinspace }}$through Oxygen Migration by Electrolyte Gating Lingchao Zhang, S.W. Zeng, D.Y. Wan, K. Han, L.K. Jian, A. Ariando, T. Venkatesan The technique of electric double layer transistor (EDLT) has been applied to several HTS, such as LSCO and YBCO. The interpretation of SIT in all these studies are attributed to electrostatically induced carriers. However, in several electrolyte gating experiments recently, the effect is mainly attributed to oxygen vacancy formation, with migration of oxygen from the film into ionic liquid. In this study, the modulation of SIT is performed in a 7uc NBCO EDLT. By applying positive V$_{\mathrm{g}}$, the SC NBCO gradually transits to insulating. When V$_{\mathrm{g}}$ changes back to 0V, it remains insulating. If the mechanism is electrostatically induced carriers, it should recover SC. However, it is only when applying a reverse negative V$_{\mathrm{g}}$ that it can gradually recover SC. Meanwhile, after SIT and V$_{\mathrm{g}}$ back to 0V, another sample is taken out from PPMS. After careful remove of ionic liquid, it remains insulating. After annealing at O$_{\mathrm{2}}$ atmosphere, it recovers SC. These strongly support the underlying mechanism is oxygen migration, instead of electrostatically induced carriers. The R$_{\mathrm{c}}$ is extracted to be about 5320$\Omega $, suggestive of quantum phase fluctuation. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V25.00012: Phase Transitions in a Two-dimensional Electron System at Oxide Interface with Dual Gate Tuning Zhuoyu Chen, Hisashi Inoue, Hyeok Yoon, Di Lu, Tyler Merz, Seung Sae Hong, Adrian Swartz, Yanwu Xie, Hongtao Yuan, Yasuyuki Hikita, Harold Hwang The ground state of a two-dimensional (2D) electron system can be controlled by parameters including disorder, carrier density, and magnetic field. Using the conducting channel formed at the LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ (001) heterointerface, we performed magnetotransport measurements with simultaneous electric field effect gating from both the top epitaxial LaAlO$_{\mathrm{3}}$ layer and the back SrTiO$_{\mathrm{3}}$ (001) substrate. Besides conventional carrier density tuning, the structural asymmetry inherent to the dual-gate device also enables independent modulation of the disorder level in the conduction channel probed through carrier mobility. Under different top and back gate voltages and magnetic field combinations, the interface channel showed strikingly different conducting states including zero resistance (superconductor), saturating small finite resistance (``metal''), and increasing resistance (insulator), when approaching zero temperature. These results provide a unique opportunity for understanding the quantum phase transitions in 2D superconducting systems with continuously tunable parameters. [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V25.00013: Behavior of an Amorphous Superconducting Thin Film in a Tunable Dissipative Environment Ilana Percher, Allen Goldman It has been shown that a dissipative electrical environment supports superconductivity. This is particularly true for low-dimensional systems of disordered and granular superconductors close the insulating state, where Ohmic dissipation can damp the order parameter fluctuations that would otherwise destroy global phase coherence. We will present the latest results from experiments in which a homogeneously disordered indium oxide film is placed in very close proximity to a two dimensional electron gas (2DEG) within a gallium arsenide/aluminum gallium arsenide heterostructure. The dissipation provided by the 2DEG depends on its carrier concentration, which is varied electrostatically by means of a back gate. We are grateful to the groups of Prof. Rachel Goldman at University of Michigan and Prof. Aviad Frydman at Bar Ilan University for growing heterostructures and superconducting films (respectively) for this experiment. [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V25.00014: The role of disorder and electron-electron interactions in the superconductor-insulator transition of molybdenum disulfide Joseph Prestigiacomo, Anindya Nath, Anthony Boyd, Qingfeng Liu, Judy Wu, Thomas Sutto, Michael Osofsky The 2D layered transition-metal dichalcogenide, MoS2, first garnered interest over 40 years ago when it was discovered that it becomes a superconductor (SC) after electrochemical intercalation with alkali- or alkali-earth metals. Recently, however, a superconductor-insulator (SI) transition was observed in MoS2 by electric-field gating with ionic liquid (IL) dielectrics, substances that enable induced charge-carrier concentrations (n) much larger than are possible using conventional solid-state dielectric gate barriers. Despite this feat, detailed studies of gate-tuned metal-insulator transitions in MoS2 have mainly focused on the understanding the various mobility-reducing scattering mechanisms thought to contribute to the smaller than predicted on/off ratios observed in MoS2-based FETs. In this presentation, we discuss the results of an investigation into the role of disorder and electron-electron interactions in the SI transition of mechanically-exfoliated multilayer and CVD-grown few-layer MoS2 probed by carefully examining their low temperature magneto-transport properties as a function of charge carrier-concentration via IL-gating. [Preview Abstract] |
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