Session Y23: Superconductor-Insulator Transitions

Theory of Giant Magnetoresistance Peak in InO Superconducting Films

It has been suggested that the giant magnetoresistance peak seen on the insulating side of a superconductor-insulator transition in In$_2$O$_{3-x}$ films is a signature of remanant localized pairs which can participate in variable-range-hopping transport. A theory of pair-transport must necessarily take into account the bosonic nature of the pairs, as opposed to the fermionic nature of single-electron carriers. We show that this opposite statistical nature of the carriers alone can lead to opposite variations of the localization length with magnetic field and thus be a possible candidate for explaining the giant magnetoresistance peak. We further provide an explanation of the pairing mechanism in In$_2$O$_{3-x}$ films. We argue that the electron pairing mechanism here is non-BCS-type and is related instead to the local effect of electron pairs occupying Oxygen vacancies (with charge +2e). The neutral state of these defects may be more stable than the singly-charged defect, as supported by an ab initio analysis. This simple phenomenon leads naturally to a negative-U Hubbard model with strong disorder.   

Broadband microwave study of 2D superconductor-insulator quantum phase transition

Using our broadband microwave spectrometer, we investigate the complex AC conductance of disordered InO$_x$ films as a function of magnetic field through the 2D superconductor-insulator quantum phase transition. We have studied the behaviors of the frequency dependent complex response function of a particular InO$_x$ sample near the critical point in the limit of $\hbar \omega < K _{B} T$ and $\hbar \omega > K _{B} T$ and compare our results to theoretical models. We discuss the possibility for a novel insulating state on the insulating side of the transition through the frequency dependent conductance.   

Superconductor-insulator transition in amorphous indium oxide films: Role of in-plane magnetic field

We present experimental results from transport studies on amorphous indium oxide films that are driven through a superconductor-insulator transition by applying a pair-breaking magnetic field. The direction of the magnetic field is varied continuously from being perpendicular to the film plane to parallel to the film plane and we identify characteristically different transport regimes at different magnetic field values when the film is rotated in a magnetic field. We also study the evolution of these transport regimes as a function of disorder in these films. A distinctly clear magnetic field value, in the insulating phase, at which the sample resistance is independent of the angle and very weakly dependent on temperature and disorder is observed. Implications for our current understanding of the 2D superconductor - insulator transition will be presented.   

Evolution of magnetoresistance peak on disordered systems

Measurements of the electric and magnetic properties of amorphous indium oxide films are reported. The carrier densities of films have been tuned by employing electrostatic gating using electric double layer transistor (EDLT) configurations employing the ionic liquid DEME{\_}TFSI. We have observed the emergence of the magnetoresistance peak as films are tuned from the insulating to the superconducting regimes, and its disappearance as the system is doped further with additional carriers. The result also demonstrates the reversible tuning of the superconductor-insulator transition with low gate voltages   

Superconductor-Insulator transition in long Mo-Ge nanowires

We have studied transport properties of two series of long (1-25$\mu $m) and very narrow (9-20nm) homogenous amorphous Mo-Ge wires fabricated by advanced electron beam lithography. We observed that the wires undergo a superconducting-insulator transition that is controlled by the wire cross sectional area, i.e. by local physics. Reduction of mean-field critical temperature can be explained by the fermionic mechanism. We also observed an unusual zero-bias anomaly in the insulating state that has signatures of both Coulomb blockade and perturbative electron-electron interaction correction. In addition, some of our long superconducting wires appear to be localized superconductor, namely in these wires one-electron localization lengths is several times shorter than the length of a wires.   

The extent of the Cooper pair insulator phase in amorphous Pb$_{0.9}$Bi$_{0.1}$ nanohoneycomb films

Amorphous Bi nanohoneycomb (NHC) thin films, which contain a nanometer-scale array of holes and regular thickness undulations, exhibit an insulating phase made up of localized Cooper pairs (CPs) near their thickness-driven insulator to superconductor transition (IST). This Cooper pair insulator (CPI) phase includes a giant magnetoresistance peak, also observed in InO$_{x}$ and TiN. We have now produced NHC films of a new material, Pb$_{0.9}$Bi$_{0.1}$, that show a qualitatively similar CPI phase. We will show the evolution of this CPI phase from deep in the insulating state to the IST using transport measurements. Throughout this regime, we will track the appearance, growth, and range of the magnetoresistance oscillations (which indicate CP transport) and giant magnetoresistance peak. Considering these observations, we will discuss the likely extent of CP transport in these insulators.   

Superconductor-to-Insulator Transition in BaPb$_{1-x}$Bi$_x$O$_3$

BaBiO$_3$ is a charge density wave (CDW) insulator. Hole doping with Pb, i.e. BaPb$_{1-x}$Bi$_x$O$_3$, eventually suppresses the CDW, leading to superconductivity, with a maximum critical temperature Tc of 10 to 13K for $x\approx 0.25$. The origin of the CDW state, its doping dependence, the nature of the pairing mechanism and the effects introduced by disorder remain open questions. Here we investigate the normal state properties in the vicinity of the metal-semiconductor transition. Single crystalline samples of the solid solution BaPb$_{1-x}$Bi$_x$O$_3$, with $0 [Preview Abstract]

Electrostatic doping of high T$_{c}$ superconductors

The application of field effect transistor concepts to electrostatically doped strongly correlated electron systems has been the focus of intense research during the last years [C. H. Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)]. In this talk we will show our recent results on Electronic Double Layer Transistor (EDLT) techniques applied to high T$_{c}$ cuprates. The EDLT configuration, which employs ionic liquids as gate dielectrics, has succeeded in achieving unprecedented charge transfers, of the order of 10$^{15}$ carriers/cm$^{2}$. This large accumulation and depletion of carriers allows us to explore the phase diagram of YBa$_{2}$Cu$_{3}$O$_{7-x}$ and La$_{2}$CuO$_{4+\delta}$. We will focus on the physics of the superconductor to insulator transition [X. Leng et al., Phys. Rev. Lett. 107, 027001 (2011)] and discuss the magneto-transport properties of the underdoped and overdoped regions of the phase diagram [X. Leng, et al., arXiv:1108.0083v1].   

Dynamic charge order and superconductivity in lightly doped La$_{2-x}$Sr$_x$CuO$_4$ thin films

Recent experiments have demonstrated the existence of a charge glass state at very low temperatures deep within the spin glass phase ($T\ll T_{SG}$) in lightly doped, insulating La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) with $x = 0.03$. Here we use magnetotransport measurements in a series of high quality MBE-grown LSCO films ($x=0.03$, 0.05, 0.055, and 0.06) to investigate the doping dependence of this dynamically inhomogenous, charge ordered state as the superconductor-insulator transition (SIT) is approached. We show that the charge glassiness is suppressed with doping. Nevertheless, close enough to the SIT on the insulating side, it coexists and competes with superconducting fluctuations (SCFs), leading to a suppression of SCFs at low $T$.   

Two component Coulomb glass in disordered superconducting films

We propose a new two component Coulomb glass model which includes strong disorder, Coulomb interaction, and on-site electron pairing to investigate the effects of localized pairing in disordered films on the insulating side of a superconductor-insulator transition. In particular, we show how the density of states (DOS) changes with increasing on-site coupling between electrons, from an Efros-Shklovskii linear DOS for the electrons at weak coupling, to a strongly modified, non-monotonic DOS with nonuniversal Coulomb gap for electrons and on-site pairs at moderate coupling, and finally to an Efros-Shklovskii linear DOS for pairs at strong coupling. We discuss the effects of a spatially random coupling. We use a Miller Abrahams resistor network mapping to numerically calculate resistance for samples of this model, given temperature and localization length. With certain parameter choices, we can obtain a peak in resistance with respect to magnetic field, reminiscent of magnetoresistance peaks reported experimentally.   

Superconductor-insulator transition in Josephson junction arrays screened by a superconducting ground plane

We have studied quantum phase transitions in unconventional Josephson arrays with a large number of interacting nearest-neighbour islands (typically, 10). The range of inter-island interactions was controlled by the presence/absence of a superconducting ground plane placed in close proximity to the array. We have found that the superconductor-to-insulator transition occurs in the arrays with both short- and long-range interactions at $E_{J}/E_{C}\sim1$ . Here $E_{J}$ is the effective Josephson energy per island, $E_{C}$ is its charging energy. However, the critical resistance for the arrays with short-range interactions greatly exceeds the quantum resistance, in contrast to the case of arrays with long-range interactions. These experiments clearly show that the ratio $E_{J}/E_{C}$ is the only relevant (for the SIT) parameter, while the critical resistance can vary a great deal depending on the interaction range.   

Strong quantum interference in strongly disordered bosonic insulators

We study the model of variable-range hopping of bosons in an array of sites with short-range interactions and a large characteristic coordination number, which describes conduction in a broad class of insulators, such as strongly disordered superconductive films, systems close to the Anderson transition, Josephson networks, etc. Large coordination number leads to strong quantum interference phenomena yet allows for their analytical study. We develop a functional renormalization group scheme that repeatedly eliminates high-energy sites properly renormalizing the tunneling between the low-energy ones. Using this approach we determine the temperature and magnetic field dependence of the hopping conductivity and find a large positive magnetoresistance. With increasing magnetic field the behaviour of the conductivity crossovers from the Mott's law to an activational behaviour with the activation gap proportional to the magnetic field.   

Numerical identification of three distinct phases in a $(2+1)D$ array of quantum dissipative Josephson junctions

We have performed large-scale Monte-Carlo simulations on a model describing a $(2+1)D$ array of quantum dissipative Josephson junctions. With the superconducting phases as our fundamental degrees of freedom we are able to identify three distinct phases as function of Josephson coupling and dissipation strength. Apart from the fully superconducting state, where fluctuations in both space and time are at bay, and the normal phase, characterized by wild fluctuations, we find an additional phase featuring spatial phase coherence coinciding with temporal disorder.   

Metallic state in La-doped YBa$_2$Cu$_3$O$_y$ thin films with n-type charge carriers

Through substitution of La for Ba and reduction of oxygen, we successfully doped n- and p-type charge carriers into La-doped YBa$_2$Cu$_3$O$_y$ thin films synthetized by pulsed laser deposition technique. The n-type samples demonstrated metallic behaviors and in-plane resistivity exhibited a quadratic temperature dependence within the metallic regime, and then evolved into a ln T-dependence insulating-like state. Furthermore, the doping evolution of temperature with minimum resistivity (T$_{min}$) and electron-electron rate were investigated and showed asymmetry between p- and n-side. The present results suggest the potential of obtaining n-type superconductivity in La-doped YBa$_2$Cu$_3$O$_y$ and investigating n-p asymmetry (symmetry) in cuprates with the same crystallographic structure.