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Bosonic topological insulator intermediate state in the superconductor-insulator transition
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Local spectroscopies across the superconductor-insulator transition
We explore the manifestation of quantum fluctuations across the superconductor-insulator transition (SIT) in three different local measurements: the two-particle density of states P(r,ω), compressibility κ(r), and diamagnetic susceptibility χ(r) revealed through their local maps. The map of χ(r) displays growing fluctuations upon increasing temperature as well as upon tuning the quantum tuning parameter g , as expected, but remarkably, these fluctuations persist well below T c as the SIT is approached, indicating the quantum nature of these fluctuations. Here, the map of κ(r) paints a similar picture when tuned via g, but in contrast to χ(r), we find a fundamental difference in its evolution with temperature, providing a complementary local probe to χ(r). P(r,ω), obtained using maximum entropy analytic continuation of Monte Carlo simulations on 2D quantum Josephson junction arrays, shows strongly diminished zero-energy spectral weight in nearly-insulating islands, correlating with regions of suppressed κ(r). We discuss the experimental implications of our results for scanning Josephson spectroscopy, compressibility, and scanning SQUID measurements in the context of quantum fluctuations.
Superconductor-Insulator Transitions in Exfoliated Bi 2 Sr 2 CaCu 2 O 8+δ Flakes
The recently developed solid electrolyte not only allows for substantial charge carrier modulation, which is crucial for realizing emergent quantum phenomena, but also offers a facile platform to integrate with surface sensitive probes, such as tunneling spectroscopy. Here, we apply this unique technique to address the superconductor-insulator transition (SIT) in mechanically exfoliated Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) by transport and tunneling measurements. Back-gating via the solid electrolyte engenders intercalation of lithium ions into BSCCO, inducing a reversible SIT. Through tunneling spectroscopy across a graphite (G)/BSCCO heterojunction, we further observe that a symmetric pseudo-gap persists deep in the insulating regime, indicative of a percolative transition. The combination of the field effect modulation and spectroscopic study is applicable to a large variety of two-dimensional (2D) materials for gaining deeper insights into the many-body physics.
Nature of superconductor-insulator transition at LaAlO{sub 3}/SrTiO{sub 3} interface
The two-dimensional electron liquid, at the interface between two band insulators LaAlO{sub 3} and SrTiO{sub 3}, exhibits novel, unconventional superconductivity below 200 mK. One of the remarkable properties of the two-dimensional superconductor is its fantastic tunability by external parameters such as gate-voltage or magnetic field. We study the superconductor to insulator transition induced by gate-voltage by employing a self-consistent, mean-field Bogoliubov-de Gennes treatment based on an effective model. We show that the non-monotonic behaviour of the superconductivity with respect to gate-voltage is intrinsically due to the Rashba spin-orbit coupling. With increasing gate-voltage both the electron concentration and Rashba spin-orbit splitting increases. Elevated electron filling boosts superconductivity whereas enhanced spin-orbit splitting annihilates electron-pairing. The non-monotonicity is a result of this competition. The device application of the superconductor-insulator transition in this interface is discussed.
Universal relation between doping content and normal-state resistance in gate voltage tuned ultrathin Bi 2 Sr 2 CaCu 2 O 8 + x flakes
We report gate voltage tunable ultra-thin high-T c cuprates supply a unique platform to investigate the electronic phase diagram and superconductor-insulator transition. One of the challenges in this field is the precise determination of the doping content in the underdoped non-superconducting region. Here we report the discovery of a universal relation between the doping content p and the normal-state resistance at a fixed temperature R(T f ), p = α + β ln[1/R(T f )], in the ultra-thin Bi 2 Sr 2 CaCu 2 O 8+x flakes. The in-depth analysis shows that the evolution of carrier scattering probability with doping content and the change of effective mass caused by superconductor-insulator transition are two key factors leading to this logarithmic relation. Based on our finding, the more precise electronic phase diagram can be established. In addition, the superconductor-insulator transition is verified to be a quantum phase transition using a finite size scaling analysis. The scaling exponent z v is found to have a close correlation with the disorder levels. The present result provides an important foundation to investigate the fascinating electronic states in the ultra-thin cuprates.
The Superconductor-Superinsulator Transition: S-duality and the QCD on the Desktop
We show that the nature of quantum phases around the superconductor-insulator transition (SIT) is controlled by charge-vortex topological interactions, and does not depend on the details of material parameters and disorder. We find three distinct phases, superconductor, superinsulator, and bosonic topological insulator. The superinsulator is a state of matter with infinite resistance in a finite temperature range, which is the S-dual of the superconductor and in which charge transport is prevented by electric strings binding charges of opposite sign. The electric strings ensuring linear confinement of charges are generated by instantons and are dual to superconducting Abrikosov vortices. Material parameters and disorder enter the London penetration depth of the superconductor, the string tension of the superinsulator and the quantum fluctuation parameter driving the transition between them. They are entirely encoded in four phenomenological parameters of a topological gauge theory of the SIT. Finally, we point out that, in the context of strong coupling gauge theories, the many-body localization phenomenon that is often referred to as an underlying mechanism for superinsulation is a mere transcription of the well-known phenomenon of confinement into solid-state physics language and is entirely driven by endogenous disorder embodied by instantons with no need of exogenous disorder.
Charge Berezinskii-Kosterlitz-Thouless transition in superconducting NbTiN films
Three decades after the prediction of charge-vortex duality in the critical vicinity of the two-dimensional superconductor-insulator transition (SIT), one of the fundamental implications of this duality-the charge Berezinskii-Kosterlitz-Thouless (BKT) transition that should occur on the insulating side of the SIT-has remained unobserved. The dual picture of the process points to the existence of a superinsulating state endowed with zero conductance at finite temperature. Here, we report the observation of the charge BKT transition on the insulating side of the SIT in 10 nm thick NbTiN films, identified by the BKT critical behavior of the temperature and magnetic field dependent resistance, and map out the magnetic-field dependence of the critical temperature of the charge BKT transition. Lastly, we ascertain the effects of the finite electrostatic screening length and its divergence at the magnetic field-tuned approach to the superconductor-insulator transition.
Dimension Effects in Insulating NbTiN Disordered Films and the Asymptotic Freedom of Cooper Pairs
This study investigates the evolution of the electronic transport in disordered NbTiN films at the insulating side of the superconductor-insulator transition as function of the effective system size along the current direction. We demonstrate that both, the threshold voltage characterizing low-temperature superinsulating phase, and the Berezinskii-Kosterlitz-Thouless criticality of the conductance above the superinsulator critical temperature, vanish upon increasing disorder taking the system far from the disorder-driven superconductor-insulator transition. We find that decrease in the system's effective size suppresses its insulating and superinsulating properties and that threshold voltage drops linearly with the diminishing system size. We demonstrate that this linear dependence can be used for experimental measurement of the linear tension of the electric string binding Cooper pairs and anti-Cooper pairs in a superinsulator.
Superconducting phase transitions in disordered NbTiN films
Suppression of superconductivity in disordered systems is a fundamental problem of condensed matter physics. Here we investigate superconducting niobium-titanium-nitride (Nb 1-x Ti x N) thin films grown by the atomic layer deposition (ALD) with slightly different growth process parameters. We observe a smooth crossover from the disorder-driven superconductor-normal metal transition (SMT) to the superconductor-insulator transition (SIT) via the intermediate Bose metal state detected by the low-temperature saturation of the temperature dependence of the sheet resistance. We demonstrate that the SIT via the intervening Bose metal state occurs if the sheet resistance of the film in the maximum, R max prior to the superconducting drop of R(T), exceeds R q = h/4e 2 .
Superconductor–Insulator Transition in NbTiN Films
Experimental results indicating a direct disorder-induced superconductor-insulator transition in NbTiN thin films have been reported. It has been shown that an increase in the resistance per square in the normal state is accompanied by the suppression of the critical temperature of the superconducting transition T c according to the fermion mechanism of suppression of superconductivity by disorder. At the same time, the temperature of the Berezinskii-Kosterlitz-Thouless transition is completely suppressed at a nonzero critical temperature and, then, the ground state changes to insulating, which is characteristic of the boson model of suppression of superconductivity by disorder. Furthermore it has been shown that the temperature dependences of the resistance of insulating films follow the Arrhenius activation law.
High-temperature superconductivity in monolayer Bi 2 Sr 2 CaCu 2 O 8+δ
Atomically-thin layered van der Waals crystals represent ideal material systems in the ultimate two-dimensional (2D) limit. The reduction in dimensionality fundamentally alters the electronic structure of the materials, often with profound consequences as best exemplified by the emergence of Dirac fermion in graphene. Vast opportunities exist in extending this top-down approach to other material systems. Among all layered materials, high-temperature superconductors stand out for their fundamental importance in material research, and for their potential impact on future technology. Experimental efforts obtaining intrinsic monolayers from bulk high-temperature superconductors, however, are faced with tremendous difficulties. Here, we developed a new fabrication process that enabled us to produce intrinsic monolayer crystals of high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212; here monolayer refers to a half unit cell that contains two CuO 2 planes). We discovered that the highest superconducting transition temperature, T c , of monolayer Bi-2212 is as high as that of optimally doped bulk. The lack of dimensionality effect on T c defies expectations from Mermin–Wagner theorem; it also sharply contrasts with much reduced T c in conventional superconductors in 2D limit. Being atomically thin, monolayer Bi-2212 offers unprecedented tunability over its doping and disorder level across the superconductor-insulator transition (SIT). Finite size scaling analysis at SIT reveals that the system belongs to two distinct universality classes, and provides a unified picture for disparate observations in cuprate systems. Finally, our results establish monolayer Bi-2212 as an ideal platform for studying high-temperature superconductivity (HTS) and other strongly correlated phenomena in 2D.
Electronic transport in two-dimensional high dielectric constant nanosystems
There has been remarkable recent progress in engineering high-dielectric constant two dimensional (2D) materials, which are being actively pursued for applications in nanoelectronics in capacitor and memory devices, energy storage, and high-frequency modulation in communication devices. Yet many of the unique properties of these systems are poorly understood and remain unexplored. Here we report a numerical study of hopping conductivity of the lateral network of capacitors, which models two-dimensional insulators, and demonstrate that 2D long-range Coulomb interactions lead to peculiar size effects. We find that the characteristic energy governing electronic transport scales logarithmically with either system size or electrostatic screening length depending on which one is shorter. Our results are relevant well beyond their immediate context, explaining, for example, recent experimental observations of logarithmic size dependence of electric conductivity of thin superconducting films in the critical vicinity of superconductor-insulator transition where a giant dielectric constant develops. Our findings mark a radical departure from the orthodox view of conductivity in 2D systems as a local characteristic of materials and establish its macroscopic global character as a generic property of high-dielectric constant 2D nanomaterials.
Disordered Berezinskii-Kosterlitz-Thouless transition and superinsulation
Here, we investigate the critical Berezinskii-Kosterlitz-Thouless (BKT) behavior of disordered two-dimensional Josephson-junction arrays (JJA) on the insulating side of the superconductor-insulator transition (SIT) taking into account the effect of hitherto ignored residual random dipole moments of the superconducting grains. We show that for weak Josephson coupling the model is equivalent to a Coulomb gas subjected to a disorder potential with logarithmic correlations. We demonstrate that strong enough disorder transforms the BKT divergence of the correlation length, ξ BKT ∝ exp (const / √T-T BKT ), characterizing the average distance between the unbound topological excitations of the opposite signs, into a more singular Vogel-Fulcher-Tamman (VFT) behavior, ξ VFT ∝ exp [const / (T-T VFT )] , which is viewed as a hallmark of glass transitions in glass-forming materials. We further show that the VFT criticality is a precursor of the transition into a nonergodic superinsulating state, while the BKT critical behavior implies freezing into an ergodic confined BKT state. Finally, our finding sheds light on the yet unresolved problem of the origin of the VFT criticality.
Transient superconductivity without superconductivity
Recent experiments on K 3 C 60 and layered copper-oxide materials have reported substantial changes in the optical response following application of an intense THz pulse. These data have been interpreted as the stimulation of a transient superconducting state even at temperatures well above the equilibrium transition temperature. We propose an alternative phenomenology based on the assumption that the pulse creates a nonsuperconducting, though nonequilibrium, situation in which the linear response conductivity is negative. The negative conductivity implies that the spatially uniform prepulse state is unstable and evolves to a new state with a spontaneous electric polarization. Furthermore, this state exhibits coupled oscillations of entropy and electric charge whose coupling to incident probe radiation modifies the reflectivity, leading to an apparently superconductinglike response that resembles the data. Dependencies of the reflectivity on polarization and angle of incidence of the probe are predicted and other experimental consequences are discussed.
Exact Boson-Fermion Duality on a 3D Euclidean Lattice
The idea of statistical transmutation plays a crucial role in descriptions of the fractional quantum Hall effect. However, a recently conjectured duality between a critical boson and a massless two-component Dirac fermion extends this notion to gapless systems. This duality sheds light on highly nontrivial problems such as the half-filled Landau level, the superconductor-insulator transition, and surface states of strongly coupled topological insulators. Although this boson-fermion duality has undergone many consistency checks, it has remained unproven. Here, we describe the duality in a nonperturbative fashion using an exact UV mapping of partition functions on a 3D Euclidean lattice.
Charge density wave and superconductivity in the disordered Holstein model
The interplay between electron-electron correlations and disorder has been a central theme of condensed matter physics over the past several decades, with particular interest in the possibility that interactions might cause delocalization of an Anderson insulator into a metallic state, and the disrupting effects of randomness on magnetic order and the Mott phase. In this work, we extend this physics to explore electron-phonon interactions and show, via exact quantum Monte Carlo simulations, that the suppression of the charge density wave correlations in the half-filled Holstein model by disorder can stabilize a superconducting phase. Our simulations thus capture qualitatively the suppression of charge ordered phases and emergent superconductivity recently seen experimentally.
Spin-polarized tunneling in critically disordered Be-Al bilayers
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