MTI Nonconventional Insulators Workshop, 2012

Abstracts (texts only)

Please see the booklet for full abstracts.

Y. Asano
Majorana Fermions and Odd-frequency Cooper Pairs

Majorana fermion (MF) has been an exciting object since the original prediction by Majorana. Finding of MFs and controlling of Majorana bound states (MBSs) are hot research issues in condensed matter physics from the view of potential application of MBS to the topological quantum computation. To date, we have known several promising systems hosting MFs such as spin-triplet p-wave superconductors, topological insulator /superconductor heterostructures, semiconductor / superconductor junctions with strong spin-orbit coupling, helical superconductors, and superconducting topological insulators. Most attracting among them is the semiconductor nano wire fabricated on top of a superconductor because of its easy tunability of MBS by changing the chemical potential in the nano wireand by applying the Zeeman field onto it[1,2].
We discuss a strong relationship between Majorana fermions and odd-frequency Cooper pairs which appear at a disordered normal (N) nano wire attached to a topologically nontrivial superconducting (S) one. The transport properties in superconducting nano wire junctions show universal behaviors irrespective of the degree of disorder: the quantized zero-bias differential conductance at 2e2/h in NS junction and the fractional current-phase relationship of the Josephson effect in SNS junction. Such behaviors are exactly the same as those in the anomalous proximity effect of odd-parity spin-triplet superconductors.[3,4] We show that odd-frequency Cooper pairs support the universal transport properties. The odd-frequency pairs exist wherever the Majorana fermions stay.[5]

Tatyana I. Baturina
Evolution of Superconductivity with Increasing Disorder in Two Dimensions

The interplay between superconductivity and localization is a phenomenon of fundamental interest, and the question of the nature of superconductivity and its evolution in two-dimensional disordered systems continues to receive a great deal of theoretical and experimental attention. The answers will shed a light on the nature of the ground states of two dimensional electronic systems.
I will review experimental data for various materials and discuss how they relate to two possible ways of terminating superconductivity in two dimensions, fermionic and bosonic, which correspond to two different bordering states, metal, where the Cooper pairing completely vanishes, and Bose-insulator, where Cooper pairing still survives in a form of localized Cooper pairs.
I will focus on the question to which degree the transport and magnetoresistive data can be conclusive in favoring one or another scenario and which criteria for the proper choice are available at present. In particular, I will touch upon the following issues: (i) the role of the superconducting fluctuations and their enhancement caused by increasing disorder; (ii) pseudogap-like behavior and its connection with preformed localized Cooper pairs; (iii) the meaning of scaling behavior.

Alexey Bezryadin
Understanding quantum phase slips at high bias currents in thin wires

In many existing models of superconductor-insulator transitions (SIT) the key element is so-called quantum phase slip (QPS). Thus a quantitative understanding of the QPS is probably necessary to finally achieve a generally accepted picture of the SIT in thin superconducting wires. We measure quantum and thermal phase-slip rates using the standard deviation of the switching current in superconducting nanowires. Our rigorous quantitative analysis provides firm evidence for the presence of quantum phase slips (QPSs) in homogeneous nanowires at high bias currents. We observe that as temperature is lowered, thermal fluctuations freeze at a characteristic crossover temperature T*, below which the dispersion of the switching current saturates to a constant value, indicating the presence of QPSs. The scaling of the crossover temperature T* with the critical temperature Tc is linear, T* \propto Tc, which is consistent with the theory of macroscopic quantum tunneling. We can convert the wires from the initial amorphous phase to a single-crystal phase, in situ, by applying calibrated voltage pulses. This technique allows us to probe directly the effects of the wire resistance, critical temperature, and morphology on thermal and quantum phase slips.

V. Chandrasekhar
Charge-Vortex Duality at the LAO/STO Interface

The conducting gas that forms at the interface between the two insulating oxides LaAlO3 (LAO) and SrTiO3 (STO) shows a rich variety of behavior, including an electric-field dependent conductivity, superconductivity, gate-controlled superconductor-to-insulator transition, and ferromagnetism. Recently, we showed for the first time the coexistence of superconductivity and ferromagnetism at the LAO/STO interface, which manifests itself as a superconducting phase diagram that is hysteretic in the applied magnetic field. We show that the coexistence of ferromagnetism and superconductivity enables a novel demonstration of the concept of charge-vortex duality in the superconductor-to-insulator transition, which shows up as a magnetic field sweep-rate dependence of the magnetoresistance in the superconducting and insulating regimes.

C. Chapelier
Tunneling spectroscopy of fluctuating and localized preformed Coopers pairs in highly disordered superconducting films

We have performed tunneling spectroscopy on superconducting titanium nitride and indium oxide films in the vicinity of the disorder-driven superconductor-insulator transition (SIT). Tunnelling spectroscopy highlights a rather unusual superconducting state with a pseudogap regime above the critical temperature Tc [1]. We demonstrated that this pseudogap is the signature of short lived Cooper pairs that are preformed above Tc. It evolves at low temperature into an inhomogeneous superconducting system due to spatial fluctuations of the disorder at the mesoscopic scale in both materials [2,3]. However, the SIT in TiN and InO films display different characters. Ultrathin TiN films remain bad metals with dominating two-dimensional thermodynamic fluctuations when disorder is increased. In this case Tc goes to zero at the critical disorder of the SIT, whereas in InO, Tc remains above 1 K on the superconducting side of the SIT. In this latter situation, localization takes over and the preformed Cooper pairs above Tc can locally remain localized at zero temperature. We showed that the absence of BCS coherence peaks at the gap edges in the local one particle density of states is the signature of these localized Cooper pairs. Besides, using our STM, we have continuously analyzed the local conductance between the tunneling regime and the point-contact regime. In the latter, Andreev spectroscopy reveals a new energy scale related to the quantum coherence energy and independent from spatial fluctuations of the pairing energy [4].

Yuval Gefen

Detecting the passage of an interfering particle through one of the interferometer's arms, known as “which path" measurement, gives rise to interference visibility degradation (dephasing). We have considered [1] a detector at equilibrium. At finite temperature dephasing is caused by thermal fluctuations of the detector. More interestingly, in the zero temperature limit, equilibrium quantum fluctuations of the detector give rise to dephasing of the out-of-equilibrium interferometer. This dephasing is a manifestation of an orthogonality catastrophe which differs qualitatively from Anderson's. Its magnitude is directly related to the Friedel sum rule. Our analysis, originally addressing an electronic Mach-Zehnder interferometer, has been generalized to the case of a setup which includes a closed quantum dot embedded in one of the interferometer’s arms.

A. Golubov
Odd-frequency pairing in superconducting hybrid systems

Theory of odd-frequency pairing in superconducting hybrid structures is presented, where quite generally an odd-frequency order parameter is induced near interfaces. General description of superconducting proximity effect in a normal metal or a ferromagnet attached to an unconventional superconductor (S) is given for various types of symmetry in S. Experiments are proposed to detect odd-frequency pairing using various types of junctions with unconventional superconductors.

Gian Guzman-Verri
Theory of Relaxor Ferroelectrics

Relaxor ferroelectric crystals are characterized by critical fluctuations of polarization that extend over a wide region of temperatures with unusual line shapes. Despite more than 50 years of intense research, a satisfactory theory of relaxor ferroelectricity has remained elusive. We present a theory of the fluctuations of relaxors with a model of polarizable unit cells with dipolar forces, local anharmonic forces, and local random fields. We find that (i) arbitrarily small compositional disorder together with dipolar forces extend the region of critical fluctuations down to absolute zero temperature and (ii) the correlation functions of polarization are highly anisotropic and slowly varying with a power law component. We compare our results to the elastic diffuse scattering measured by neutron experiments and frequency dependent dielectric constant measurements.

M. Zahid Hasan
Topological Surface States in Topological Insulators, Superconductors and Beyond

Bulk Topological Insulators are a new phase of electronic matter which realizes a non-quantum-Hall-like topological state in the bulk matter and unlike the quantum Hall liquids can be turned into superconductors. In this talk, I will first briefly review the basic theory and experimental probes that reveal topological order. I will then discuss experimental results that demonstrate the fundamental properties of topological insulators such as spin-momentum locking, non-trivial Berry’s phases, mirror Chern number, absence of backscattering or no U-turn rule, protection by time-reversal symmetry and the existence of room temperature topological order (at the level of M.Z.H. and C.L. Kane, Rev. of Mod. Phys., 82, 3045 (2010)). I will then discuss the possible exotic roles of broken symmetry phases such as superconductivity and magnetism in doped topological insulators and their potential device applications in connection to our recent results as well as briefly outline the emerging research frontiers of the field as a whole. If time permits, I will discuss experimental progress in finding topological insulators beyond Kane-Mele Z2 paradigm.

Arthur F. Hebard
Emerging granularity and the anomalous Hall insulator in disorder-tuned thin-film ferromagnets

We report on an emergence of granularity when disorder in polycrystalline magnetic films is tuned through the metal-insulator transition. Disorder, measured by the sheet resistance R0 at T = 5 K, is tuned either by fabricating separate samples with different thicknesses or by room temperature anneals of a given sample. All studies are performed in situ with no exposure of the samples to air. For Fe films with R0 < 3000 Ohm, weak localization corrections to the anomalous Hall conductivity become manifest because of a high rate of spin-conserving inelastic scattering off spin-wave excitations[1]. For R0 > 3000 Ohm the scaling relations that define the localization corrections begin to break down and granularity becomes important. In this granular region the transverse resistance Rxy = Lxy/Lxx2 is constant near 100 Ohm over a large range of disorder strengths (3000 Ohm < R0 < 106 Ohm) implying an unusual anomalous Hall insulator (AHI) in which the two-dimensional (2D) conductivities Lxx and Lxy both approach zero in such a way that the ratio Lxy/Lxx2 remains constant. At high disorder strengths (R0 > 70 kOhm) intergranular tunneling dominates the longitudinal transport while Hall transport is still controlled by scattering processes within a single grain. We provide evidence that this AHI behavior, also seen in Co films, breaks down when dipole interactions cause the superparamagnetic grains to align antiferromagnetically. One might call this 2D insulating system a super-antiferromagnetic state, similar to the three-dimensional (3D) superferromagnetic state that has been previously proposed by theory[2]. In weakly disordered two-dimensional Gd films (R0 < 3000 Ohm), which have larger and more strongly coupled magnetic moments, we also observe quantum corrections to the conductivity. However in the region of slightly higher disorder strengths, 15 kOhm < R0 < 31 kOhm (where Fe still shows logarithmic corrections to the conductivity) the conductivity data for Gd obey a fractional power-law temperature dependence that collapses onto two separate scaling curves for metallic and insulating regimes respectively[3]. The observation of a metal-insulator transition implies that for high disorder strength the phase relaxation rate caused by scattering of quasiparticles off spin-wave excitations is sufficiently large to render the dephasing length less than the film thickness, making the film effectively three-dimensional. AHI behavior is not seen in these Gd films.

Manuel Houzet
Detecting Majorana states by driving a Josephson junction out of equilibrium

We study Josephson junctions between superconductors connected through the helical edge states of a two-dimensional topological insulator in the presence of a magnetic barrier. In equilibrium, the Andreev bound states of the junction are 4-periodic in the superconducting phase difference [1]. It was speculated that, at finite dc bias voltage, the junction exhibits a fractional ac Josephson effect with half the Josephson frequency [2,3], as well as a parity effect in the Shapiro steps in the presence of an additional microwave irradiation [3]. We show that signatures of these effects can be seen in the current noise.

Lev Ioffe
Superinductors: a novel type of Josephson ladders implementing quantum critical Ising model

Implementation of superinductor, the element that is characterized by a purely inductive response and impedance much larger than quantum (6.5 kOm), is a very long standing challenge. Recently, we have designed and implemented it in very special Josephson ladders with tunable frustration. The same ladders allows one an experimental realization of the one dimensional phi^4 theory with the mass that changes sign with magnetic field. Close to the critical point the low energy excitations in this theory can be described by Ising model in transverse field. The solution of the latter shows that these excitations are Majorana fermions. I argue that these fermionic excitations can be observed and perhaps were already observed in Josephson ladders.

Ying Jia
Catalyst-Free Growth of Millimeter-Long Topological Insulator Bi2Se3 Nanoribbons and the Observation of pi-Berry Phase

We synthesized nanoribbons of the topological insulator Bi2Se3 with a catalyst-free physical vapor deposition method. With better sample quality comparing with the Au-catalyzed VLS-grown Bi2Se3 nanoribbons, strong SdH quantum oscillations with up to 11 cycles can be identified in the magnetoresistance in intermediate magnetic fields (< 9 Tesla). Magnetic field orientation dependent measurements enabled us to identify the topological surface states as the dominant contributor to the SdH quantum oscillations. The fan diagram of the Landau levels and in particular, the fitting of the oscillatory resistance with the Lifshitz-Kosevich theory yield a Berry phase with a phase quantity (1\pm 0.1) pi, indicating the existence of ideal Dirac fermions in the topological insulator Bi2Se3.

Effects of Disorder in Doped Topological Insulators

In the last few years there has been an explosive development in materials science – the theoretical prediction of a new class of insulators in three dimensions (3D) having a fully developed electronic gap in the bulk but with metallic Dirac states on their surfaces which can carry spin/charge currents topologically protected against (time-invariant) disorder and perturbations. Novel electronic excitations of the surface states have been predicted, among them the elusive Majorana fermions that may emerge as the p-wave superconductivity is induced in a topological insulator (TI) by a superconducting proximity effect.  Recently, it has been shown that intercalation of Cu into Bi2Se3 turns this material superconducting with Cooper pairing occurring at ~ 4K. Bi2Se3 is a (2nd generation) TI, and suggestions have been made that superconductivity in Bi2Se3 is of topological origin. Unmodified Bi2Se3 is an `intrinsically` n-type narrow-band semiconductor where adding more electrons by Cu doping could promote electron correlations leading to superconductivity. Doping an `intrinsically` p-type TI with holes may in principle lead to the correlation effects in the hole channel.  In this talk I will report on our recent work on the presumed hole injection into Sb2Te3, an intrinsically p-type TI, by doping it with iodine. In the undoped system, large diamagnetic susceptibility observed at high temperatures and high magnetic fields is consistent with large spin-orbit coupling and Dirac dispersion. The diamagnetism is suppressed at low fields, with the uniquely singular behavior of magnetic susceptibility in the zero-field limit. The singularity and the anomalous orbital magnetism is consistent with the disorder-damped diamagnetism predicted for an ungapped Dirac system (Fukuyama effect)  and can be seen as arising from  the vacancies and other defects in the crystal structure of the TI.  Our experiments show that extrinsic doping strongly affects the orbital response, however the singular Fukuyama damping appears robust. I will discuss this in the context of separation of charge-compensation and disorder, topological protection of Dirac states, and correlation effects and possible superconductivity in both n- and p-type TIs.

N. Mason
Proximity Effects and Novel States in Mesoscopic Superconductor-Normal Metal-Superconductor Arrays

In this talk, I will discuss our experiments on arrays of superconducting islands patterned on normal metal films. The underlying normal metal can become superconducting due to the proximity effect; thus, by changing the size and spacing of the superconducting islands, we can control the superconducting, metallic, and even insulating properties of the metal film. I will discuss electrical transport measurements of these systems, including characterization of the superconducting transitions, vortex dynamics in a finite magnetic-field, and evidence that the system approaches unusual metallic ground states as the island spacing is increased. Controlling the spacing and clustering of the islands also gives insight into the insulating states that appear in 2D superconducting systems.

K. A. Matveev
Equilibration of a spinless Luttinger liquid

We study how a Luttinger liquid of spinless particles in one dimension approaches thermal equilibrium. Full equilibration requires processes of backscattering of excitations, which occur at energies of the order of the bandwidth. Such processes are not accounted for by the Luttinger-liquid theory. We treat the high-energy excitations as mobile impurities and derive an expression for the equilibration rate in terms of their spectrum. Our results apply at any interaction strength and can be tested by studying temperature dependence of the conductance of quantum wires.

Y. Meir
Superconductor-Insulator Transition in Disordered Thin Films

The interplay of disorder and superconductivity is one of the outstanding questions in condensed matter physics, especially in low dimensions, where fluctuations play a significant role. In this talk I will concentrate on the nature of the superconductor-insulator transition in disordered thin films. First, I will show how dimensional crossover can cause a transition as a function of film thickness, and second I will employ a novel formulation to calculate transport through disordered superconductors to calculate the magnetoresistance, shedding light of some of the puzzling observations in these samples.

Miguel Ortuno
Hopping Transport in Two-Dimensional Coulomb Glass

We study hopping transport in two-dimensional arrays of random capacitors comprising a square lattice. The charge at each node is a sum of the charges located at the capacitor plates connected to this node and assumes the values 1/2 or -1/2 measured in the units of electronic charge. We find that the interaction between two charges depends logarithmically upon the distance between them. The density of the energy states accounting for the electrostatic potential appears to be exponential, in accord with that predicted by an extension of Efros-Shklovskii arguments for the Coulomb gap, however the specific parameters characterizing the distribution noticeably deviate from the Efros-Shklovskii-like predictions. We investigate the temperature dependence of hopping conductivity and reveal the cusp characteristic to the Berezinskii-Kosterlitz-Thouless (BKT) transition. As the temperature decreases, the BKT behavior transforms into the Arrhenius dependence with the characteristic energy being proportional to the logarithm of the system size.

David Pekker
Strong disorder real space renormalization group perspective on insulators and localization

The real space renormalization group (RSRG) is a powerful tool for analyzing phase transitions in strongly disordered systems. Traditionally RSRG has enjoyed most success in exploring properties of the ground state of one-dimensional systems. We apply the RSRG in three new settings: (a) the disordered stack of 2D superfluids, in which disorder smears the superfluid-normal transition and leaves behind an anomalous sliding phase; (b) the 2D disordered quantum rotor model, in which we study the superfluid to Mott glass transition; (c) the many-body spectrum and consequent many-body localization of disordered spin chains.

Pratap Raychaudhuri
Non-ergodicity, universal scaling and phase fluctuations in a strongly disordered s-wave superconductor

In the past few years, the observation of a pseudogapped state in scanning tunneling spectroscopy measurements in strongly disordered s-wave superconductors (NbN, TiN, InOx) has opened a new paradigm in our understanding of conventional superconductors in the presence of strong disorder [1,2,3]. It has been conjectured that the superconducting transition in these materials is driven by phase fluctuations rather than vanishing of the amplitude of the superconducting order parameter. The pseudogapped state is thus attributed to the existence of phase incoherent "Cooper pairs" above Tc even after the global superconducting state is destroyed. To verify this conjecture we have measured of the complex conductivity in strongly disordered NbN films at frequencies ranging from 60 kHz to 20 GHz. The low frequency measurement (60 kHz) was performed using a two coil mutual inductance technique. Measurements in the range 0.5-20 GHz was performed using a broadband microwave setup. Below Tc, the superfluid density is frequency independent in the entire frequency range of our measurements. However, above Tc the superfluid density (ns) becomes strongly frequency dependent. At the lowest frequency, ns drops to zero at Tc. As the probing frequency is increased ns remains finite for T>Tc and vanishes at a higher temperature. Complementary measurements using scanning tunneling spectroscopy reveals that the superconducting order parameter becomes strongly inhomogeneous, with the formation of domains where the superconducting order parameter is large, separated by regions where the order parameter is suppressed. In this talk I will discuss the implication of these results and argue that these provide definite evidence of phase fluctuations between domains that “spontaneously” form at high disorder, and which strongly influence the superconducting transition in a strongly disordered s-wave superconductor.

Thomas Proslier
Atomic layer deposition of thin superconducting films and multilayer heterostructure

We report the use of atomic layer deposition (ALD) to synthesize thin superconducting films and multilayer superconductor-insulator (S-I) heterostructures. The ALD technique applied to superconducting films opens the way for a variety of applications, including improving the performance and decreasing the cost of high energy particle accelerators, superconducting wires for energy storage, and bolometers for radiation detection. Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and associated phenomena in homogeneous layers in the ultra-thin film limit. In this respect, we will present results of ALD-grown transition metal-based superconductors, including nitrides, carbides, and silicides of niobium, nitrides of molybdenum and titanium, and Nb1-xTixN/AlN-based S-I heterostructures.

Nayana Shah
Out-of-equilibrium transport signatures of Majorana fermions in a p-wave topological superconducting wire

Recently there has been a lot of excitement generated by the possibility of realizing and detecting Majorana fermions within the arena of condensed matter physics and its potential implication for topological quantum computing.
In the pursuit of identifying and understanding the signatures of Majorana fermions in realistic systems, we go beyond the low-energy effective-model descriptions of Majorana bound states to derive non-equilibrium transport properties of a topological superconducting wire in the presence of arbitrarily large applied voltages. By virtue of a microscopic calculation we are able to model the tunnel coupling between the superconducting wire and the metallic leads realistically, study the role of high-energy non-topological excitations, predict how the behavior compares for an increasing number of odd versus even number of sites, and study the evolution across the topological quantum phase transition. Our results have concrete implications for the experimental search and study of Majorana fermions.

Dan Shahar
Little-Parks Oscillations in an Insulator

When the disorder of a superconducting material is high enough it can undergo a transition into an insulating state. Paradoxically, this insulating state has been suggested to arise from superconductivity itself. We have conducted a study of a highly disordered InO films that were patterned with an array of holes. With the right treatment, the films could be driven across the superconductor-insulator transition. We found that the Little-Parks oscillations in the superconducting state persisted, virtually uninterrupted, into the insulator, supporting the role played by superconducting correlations in the insulating phase.

C. Strunk
Size-dependent Conduction near the Superconductor/Insulator Transition in thin TiN-Films

Recently, a transition to a highly insulating state with an unmeasurably small linear conductance was observed in strongly disordered TiN thin films [1]. It was suggested that this transition may be interpreted as the charge analog of the familiar vortex Berezinskii-Kosterlitz-Thouless (BKT) transition, resulting from a granularity within the electronic system [2]. The necessary condition for the BKT-transition is the presence of a long-ranged electrostatic interaction, which depends logarithmically on distance between charge carriers. Such an interaction should manifest itself in a thermally activated behavior of the conductance, with an activation energy depending logarithmically on the size of the sample.
We investigate the linear and non-linear conduction properties of square TiN films experimentally, and find that a strong dependence on their lateral size L, when L varies between 0.5 and 500mm. Three ranges temperature have to be distinguished: between ~150mK and 1K the conductance G(T) is thermally activated and the activation energy ?c grows logarithmically with L; between 50mK and 150mK a size dependent saturation of G occurs, and below 50mK the I(V)-curves become non-linear for the largest samples. In the latter case we observe power laws I~V? in the dc IV-characteristics with a strongly temperature dependent exponent ?, which is a hallmark of the binding of charge/anti-charge pairs in presence of long-ranged interactions. These observations point towards the importance of long-ranged two-dimensional Coulomb interactions in our films. A strong size dependence occurs also in the magnetoresistance and the I(V)-characteristics.

H. Suderov
Scanning tunneling conductance at very low temperatures in superconducting and insulating phases of ultra-thin TiN films

I will show large scale tunneling conductance maps made at 100 mK in ultra thin superconducting TiN films close to the superconductor to insulator transition. We find sizable insulating regions surrounded by superconducting regions. The insulating regions have a V-shaped density of states without quasiparticle peaks. Application of a magnetic field leads to a homogeneous phase with a decreased zero bias conductance. Conductance maps reveal that the superconducting state emerges out of a strongly energy dependent electronic spectrum with practically zero states at the Fermi level. Energy scales of superconductor and insulator are the same, suggesting that the transition occurs in spatially separated regions.

Yukio Tanaka
Topological aspects of Andreev bound state in superconductivity

It is well know from the history of superconductivity, interference between electron and hole generates special bound state specifict to superconductivity so called Andreev bound state [1]. It is known that surface Andreev bound state is an important ingredient to identify unconventional superconductors [2]. Up to now, there have been several types of Andreev bound states stemming from their topological origins [3]. It can be classified into i)dispersionless flat band type realized in cuprate, ii)linear dispersion type realized in chiral superconductor like Sr2RuO4, iii)helical dispersion type realized in non-centrosymmetric superconductor and iv)cone type in the surface state on B-phase of superfluid 3He . The common feature in these systems is the presence of anisotropic pair potential changes sign on the Fermi surface. On the other hand, current direction is to produce Andreev bound state using conventional spin-singlet pairing. One of the key point is the spin-orbit coupling to reduce the electronÅfs degree of freedom [4]. For example, both in the presence of spin-orbit coupling and Zeeman field, since it is possible to generate spinless fermion system, the linear dispersion edge state is generated like chiral p-wave superconductor. In this talk we focus on i)superconductor / ferromagnet hetero structures on the topological insulator [5-6] , ii) evolution of edge states and critical phenomena in the Rashba superconductor with magnetization [7], iii)topological superconductivity in bilayer Rashba system [8], and iv)superconducting topological insulator [9-11].

Nandini Trivedi
Prediction of a novel insulator across the disorder-driven superconductor-insulator transition

I will show how emergent meso-scale granularity in a disordered s-wave superconductor eventually destroys superconductivity and produces an insulator of Cooper pairs with a hard gap in the single particle spectral function. This insulator sustains large diamagnetic currents and has a Josephson energy scale that vanishes as the transition is approached from the insulator.

James Valles
Experimental Insights into the Insulators that Form at the Superconductor to Insulator Transition

Superconductors of a range of materials including simple metals, cuprates, organics and grapheme-metal composites can be driven through a quantum superconductor to insulator transition. While significant microscopic features of the superconducting state, such as the existence of Cooper pairs, are known, much less is known about the nature of the insulator or insulators that can develop at the SIT. Ultrathin films of conventional superconducting materials, which can be driven through a Superconductor to Insulator Transition (SIT) by increasing their disorder, applying a strong magnetic field or gating have been useful for probing the insulator. It is well established that in a number of systems the superconducting phase gives way to a Cooper Pair Insulator (CPI) state. This remarkable phase consists of localized Cooper pairs that transport currents through thermal activated incoherent tunneling. It exhibits a positive magnetoresistance, which greatly exceeds that exhibited by any single electron transport dominated system. How the Cooper pairs become localized and what determines the energy barrier controlling their transport are questions currently under investigation. I will describe how our experiments on elemental films patterned with a nanoscale honeycomb array of holes provide unique insight into the CPI phase. In particular, I will present evidence that electronic inhomogeneities created by structural features are essential for the localization of Cooper pairs in these films. More generally, our results imply that at least two distinct insulators can form at the SIT: one with localized Cooper pairs and the other with localized unpaired electrons.

J. Vanacken
The superconductor-insulator transition in high Tc superconducting La2-x(Sr/Ce)xCuO4 & Low Tc Superconducting Diamond:B thin films

In the early days of superconductivity, looking for superconductivity in a non-metal was considered rather foolish. Nowadays, however, several superconductors have been discovered, which are, essentially, doped insulators. In this work, we will focus on only a couple examples, namely the high temperature superconducting copper-oxides and the low temperature superconducting boron doped diamond. We will present the systematic evolution with doping of the normal state properties (high field resistivity, Hall effect, local STS measurements of the order parameter / topography) and the superconducting parameters of La2-xSrxCuO4 thin film cuprates as well as B:Diamond thin films. We will describe the evolution from insulator to superconductor, and propose a novel mechanism describing the origin of the pseudogap. This mechanism might even become a pairing mechanism under the right circumstances.

Peng Xiong
Magnetic Field Induced Super-insulating State and Enhancement of Superconductivity in Ultrathin Pb Films

We have carried out a systematic and detailed examination of the superconductivity and superconductor-insulator quantum phase transitions (SITs) in ultrathin amorphous Pb films as functions of disorder (thickness), magnetic field, and paramagnetic pair-breaking. The Pb films are grown via quench-condensation in a modified dilution refrigerator under ultrahigh vacuum at low temperature, and all the electrical measurements are performed in situ. Here we present two sets of observations from these experiments: 1) A perpendicular magnetic field induces an SIT with transport behavior resembling those in the SIT of a granular film [1], such as reentrance and double-reentrance in R(T) and a peak in the magnetoresistance [2]. The observations are suggestive of mesoscale phase separation near Hc, and an insulating state with localized superconductivity in the field-tuned SIT. In contrast, the disorder and magnetic impurity driven SITs show no sign of phase separation and inhomogeneous insulating states. 2) In the same films, a parallel magnetic field is found to enhance superconductivity, increasing the mean-field TC by as much as 13% at 8 T [3]. The TC enhancement exhibits a non-monotonic dependence on the film thickness, initially increases and then decreases with increasing thickness, and vanishes in the 3D limit. Incremental deposition of paramagnetic impurity on a film progressively suppresses and eventually eliminates the effect of parallel field enhancement of superconductivity.

© 2012 Materials Theory Institute, MSD, ANL, USA