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Register a new userQuantum criticality at the superconductor insulator transition probed by the Nernst effect
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The superconductor-insulator transition (SIT) is an excellent example for a quantum phase transition at zero temperature, dominated by quantum fluctuations. These are expected to be very prominent close to the quantum critical point. So far most of the experimental study of the SIT has concentrated on transport properties and tunneling experiments which provide indirect information on criticality close to the transition. Here we present an experiment uniquely designed to study the evolution of quantum fluctuations through the quantum critical point. We utilize the Nernst effect, which has been shown to be effective in probing superconducting fluctuation. We measure the Nernst coefficient in amorphous indium oxide films tuned through the SIT and find a large signal on both the superconducting and the insulating sides which peaks close to the critical point. The transverse Peltier coefficient, $alpha_{xy}$ which is the thermodynamic quantity extracted from these measurements, follows quantum critical scaling with critical exponents $ u sim 0.7$ and $z sim 1$ which is consistent with a clean XY model in 2+1 dimensions.
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The superconductor-insulator transition (SIT) is considered an excellent example of a quantum phase transition which is driven by quantum fluctuations at zero temperature. The quantum critical point is characterized by a diverging correlation length and a vanishing energy scale. Low energy fluctuations near quantum criticality may be experimentally detected by specific heat, $c_{rm p}$, measurements. Here, we use a unique highly sensitive experiment to measure $c_{rm p}$ of two-dimensional granular Pb films through the SIT. The specific heat shows the usual jump at the mean field superconducting transition temperature $T_{rm c}^{rm {mf}}$ marking the onset of Cooper pairs formation. As the film thickness is tuned toward the SIT, $T_{rm c}^{rm {mf}}$ is relatively unchanged, while the magnitude of the jump and low temperature specific heat increase significantly. This behaviour is taken as the thermodynamic fingerprint of quantum criticality in the vicinity of a quantum phase transition.
Superconductivity at the interface between the insulators LaAlO3 and SrTiO3 has been tuned with the electric field effect. The data provide evidence for a two dimensional quantum superconductor to insulator (2D-QSI) transition. Here we explore the compatibility of this phase transition line with Berezinskii-Kosterlitz-Thouless (BKT) behavior and a 2D-QSI transition. In an intermediate regime, limited by a finite size effect, we uncover remarkable consistency with BKT- criticality, weak localization in the insulating state and non-Drude behavior in the normal state. Our estimates for the critical exponents of the 2D-QSI-transition, z =1 and nu=3, suggest that it belongs to the 3D-xy universality class.
Recently superconductivity at the interface between the insulators LaAlO3 and SrTiO3 has been tuned with the electric field effect to an unprecedented range of transition temperatures. Here we perform a detailed finite size scaling analysis to explore the compatibility of the phase transition line with Berezinskii-Kosterlitz-Thouless (BKT) behavior and a 2D-quantum phase(QP)-transition. In an intermediate regime, limited by a gate voltage dependent limiting length, we uncover remarkable consistency with a BKT-critical line ending at a metallic quantum critical point, separating a weakly localized insulator from the superconducting phase. Our estimates for the critical exponents of the 2D-QP-transition, z=1 and nu=0.66, suggest that it belongs to the 3D-xy universality class.
We isolated flux disorder effects on the transport at the critical point of the quantum magnetic field tuned Superconductor to Insulator transition (BSIT). The experiments employed films patterned into geometrically disordered hexagonal arrays. Spatial variations in the flux per unit cell, which grow in a perpendicular magnetic field, constitute flux disorder. The growth of flux disorder with magnetic field limited the number of BSITs exhibited by a single film due to flux matching effects. The critical metallic resistance at successive BSITs grew with flux disorder contrary to predictions of its universality. These results open the door for controlled studies of disorder effects on the universality class of an ubiquitous quantum phase transition.
The effect of an electric field on the conductance of ultrathin films of metals deposited on substrates coated with a thin layer of amorphous Ge was investigated. A contribution to the conductance modulation symmetric with respect to the polarity of the applied electric field was found in regimes in which there was no sign of glassy behavior. For films with thicknesses that put them on the insulating side of the superconductor-insulator transition, the conductance increased with electric field, whereas for films that were becoming superconducting it decreased. Application of magnetic fields to the latter, which reduce the transition temperature and ultimately quench superconductivity, changed the sign of the reponse of the conductance to electric field back to that found for insulators. We propose that this symmetric response to capacitive charging is a consequence of changes in the conductance of the a-Ge layer, and is not a fundamental property of the physics of the superconductor-insulator transition as previously suggested.
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