Quantum criticality at the superconductor-insulator transition revealed by specific heat measurements (original) (raw)
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Physical Review B, 2007
A microscopic analysis of the superconducting quantum critical point realized via a pair-breaking quantum phase transition is presented. Finite temperature crossovers are derived for the electrical conductivity, which is a key probe of superconducting fluctuations. By using the diagrammatic formalism for disordered systems, we are able to incorporate the interplay between fluctuating Cooper pairs and electrons, that is outside the scope of a time-dependent Ginzburg Landau or effective bosonic action formalism. It is essential to go beyond the standard approximation in order to capture the zero temperature correction which results purely from the (dynamic) quantum fluctuations and dictates the behavior of the conductivity in an entire low temperature quantum regime. All dynamic contributions are of the same order and conspire to add up to a negative total, thereby inhibiting the conductivity as a result of superconducting fluctuations. On the contrary, the classical and the intermediate regimes are dominated by the positive bosonic channel. Our theory is applicable in one, two and three dimensions and is relevant for experiments on superconducting nanowires, doubly-connected cylinders, thin films and bulk in the presence of magnetic impurities, magnetic field or other pair-breakers. A window of non-monotonic behavior is predicted to exist as either the temperature or the pair-breaking parameter is swept.
Investigation of specific heat in ultrathin two-dimensional superconducting Pb
Physical Review B
The Berezinskii-Kosterlitz-Thouless (BKT) transition is expected to have a clear signature on the specific heat. The singularity at the transition temperature TBKT is predicted to be immeasurable, and a broad non-universal peak is expected at T > TBKT. Up to date this has not been observed in two-dimensional superconductors. We use a unique highly sensitive technique to measure the specific heat of ultrathin Pb films. We find that thick films exhibit a specific heat jump at TC that is consistent with BCS theory. As the film thickness is reduced below the superconducting coherence length and the systems enters the 2D limit the specific heat reveals BKT-like behavior. We discuss these observations in the framework of the continuous BCS-BKT crossover as a function of film thickness.
Quantum-critical region of the disorder-driven superconductor–insulator transition
Physica C: Superconductivity, 2008
We investigate low temperature transport properties of thin TiN superconducting films, differing by the degree of disorder. At zero magnetic field we find an extremely sharp separation between the superconducting-and insulating phases, indicating a direct superconductor-insulator transition without an intermediate metallic phase. We show that in the critical region of the transition a peculiar highly inhomogeneous insulating state with superconducting correlations forms. The insulating films exhibit thermally activated conductivity and huge positive magnetoresistance at low magnetic fields. A sharp depinning transition at some voltage V T is observed in the I-V curves at very low temperatures. We propose a percolation type of depinning with the threshold voltage determined by the Coulomb blockade energy for the Cooper pairs between neighboring self-induced superconducting islands, with V T being the total voltage along the first conduction path. The observed hysteretic behavior of the threshold and steps on the dI/dV vs. V curves support this percolation picture of the depinning transition.
Quantum superconductor–insulator transition: implications of BKT critical behavior
Journal of Physics: Condensed Matter, 2013
We explore the implications of Berezinskii-Kosterlitz-Thouless (BKT) critical behavior on the two-dimensional (2D) quantum superconductor-insulator (QSI) transition driven by the tuning parameter x. Concentrating on the sheet resistance R(x, T) BKT behavior implies: an explicit quantum scaling function for R(x, T) along the superconducting branch ending at the nonuniversal critical value
Quantum fluctuations and dissipative phase transition in granular superconductors
The zero temperature phase diagram of d-dimensional granular superconductors is considered. For small values of Josephson coupling Vcollective long-range quantum fluctuations play the leading role in the destruction of global phase coherence. At V-+0 the critical value of the dimensionless ohmic conductance o, diverges for d< 2 and it is equal to (Y,Z 0.82 for d= 3. For not very small values of V superconductivity may be destroyed by quantum phase slips on inidividual grains. In this case global phase coherence is restored only due to dissipative phase transition. The value of the critical resistance is found to be universal for strongly disordered granular films.
Role of surface Cooper pair interactions on critical temperature of ultra thin film superconductors
Physics Letters A, 2008
The superconductivity mechanism of Pb thin film on a Si substrate in the weak interaction regime is investigated. A discrete Fermi surface is constructed depend on the film thickness and electron density and crystallographic orientation. We consider two types of Cooper pair interactions, Cooper pair interaction at thin film surfaces and Cooper pair in the thin film volume. We have chosen the surface Cooper pair interaction, of ultra thin films superconductor proportional to the inverse of thin film thickness, while the volume Cooper pair interaction has been considered as a constant. By these assumptions, we have found oscillation feature of critical temperature T c and energy gap in terms of thin film thickness similar to the experimental results for Pb/Si(111) thin film superconductor. However, by increasing number of Pb layers, the thin film T c goes to bulk T c. In contrast to the previous claimed constant value for the b (T = 0)/k B T c in bulk, we have found oscillation of this parameter in terms of thin film thickness similar to the T c oscillation.
Physical Review E, 2016
We report on a study of the superconducting order parameter thermodynamic fluctuations in YBa 2 Cu 3 O 7−δ , Bi 2 Sr 2 CaCu 2 O 8+δ and KOs 2 O 6 compounds. A non-perturbative technique within the framework of the renormalized Gaussian approach is proposed. The essential features are reported (analytically and numerically) through Ginzburg-Landau (GL) model-based calculations which take into account both the dimension and the microscopic parameters of the system. By presenting a self-consistent approach (SCA) improvement on the GL theory, a technique for obtaining corrections to the asymptotic critical behavior in terms of non universal parameters is developed. Therefore, corrections to the specific heat and the critical transition temperature for one-, two-and three-dimensional samples are found taking into account the fact that fluctuations occur at all length scales as the critical point of a system is approached. The GL model in the free-field approximation and the 3D-XY model are suitable for describing the weak and strong fluctuation regimes respectively. However, with a modified quadratic coefficient, the renormalized GL model is able to explain certain experimental observations including the specific heat of complicated systems, such as the cuprate superconductors and the β-pyrochlore oxides. It is clearly shown that the enhancement, suppression or rounding of the specific heat jump of high-T c cuprate superconductors at the transition are indicative of the order parameter thermodynamic fluctuations according to the dimension and the nature of interactions.