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Register a new userQuantum fluctuations in superconducting dots at finite temperature
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We study the thermodynamics of ultrasmall metallic grains with level spacing $delta$ comparable or smaller than the pairing correlation energy, at finite temperatures, $T gsim delta$. We describe a method which allows to find quantum corrections to the effect of classical fluctuations. We present results for thermodynamic quantities in ordered grains and for the reentrant odd susceptibility in disordered grains.
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We study the thermodynamics of ultrasmall metallic grains with the mean level spacing comparable or larger than the pairing correlation energy in the whole range of temperatures. A complete picture of the thermodynamics in such systems is given taking into account the effects of disorder, parity and classical and quantum fluctuations. Both spin susceptibility and specific heat turn out to be sensitive probes to detect superconducting correlations in such samples.
We present the real-time renormalization group (RTRG) method as a method to describe the stationary state current through generic multi-level quantum dots with a complex setup in nonequilibrium. The employed approach consists of a very rudiment approximation for the RG equations which neglects all vertex corrections while it provides a means to compute the effective dot Liouvillian self-consistently. Being based on a weak-coupling expansion in the tunneling between dot and reservoirs, the RTRG approach turns out to reliably describe charge fluctuations in and out of equilibrium for arbitrary coupling strength, even at zero temperature. We confirm this in the linear response regime with a benchmark against highly-accurate numerically renormalization group data in the exemplary case of three-level quantum dots. For small to intermediate bias voltages and weak Coulomb interactions, we find an excellent agreement between RTRG and functional renormalization group data, which can be expected to be accurate in this regime. As a consequence, we advertise the presented RTRG approach as an efficient and versatile tool to describe charge fluctuations theoretically in quantum dot systems.
A single spin in a Josephson junction can reverse the flow of the supercurrent. At mesoscopic length scales, such $pi$-junctions are employed in various instances from finding the pairing symmetry to quantum computing. In Yu-Shiba-Rusinov (YSR) states, the atomic scale counterpart of a single spin in a superconducting tunnel junction, the supercurrent reversal so far has remained elusive. Using scanning tunneling microscopy (STM), we demonstrate such a 0 to $pi$ transition of a Josephson junction through a YSR state as we continuously change the impurity-superconductor coupling. We detect the sign change in the critical current by exploiting a second transport channel as reference in analogy to a superconducting quantum interference device (SQUID), which provides the STM with the required phase sensitivity. The measured change in the Josephson current is a signature of the quantum phase transition and allows its characterization with unprecedented resolution.
It is known that entanglement can be converted to work in quantum composite systems. In this paper we consider a quench protocol for two initially independent reservoirs $A$ and $B$ described by the quantum thermal states. For a free fermion model at low temperatures, the von Neumann entropy of each reservoir increases once the reservoirs are coupled. At the moment of decoupling there is an energy transfer to the system in the amount set by the von Neumann entropy accumulated during joint evolution of $A$ and $B$. This energy transfer appears as work produced by the quench to decouple the reservoirs. Once the reservoirs are disconnected, the information about their mutual correlations $-$ von Neumann entropy $-$ is stored in the energy increment of each reservoir. This result provides a possibility of a direct readout of quantum correlations at low temperature.
Fluctuating superconductivity - vestigial Cooper pairing in the resistive state of a material - is usually associated with low dimensionality, strong disorder or low carrier density. Here, we report single particle spectroscopic, thermodynamic and magnetic evidence for persistent superconducting fluctuations in heavily hole-doped cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ ($T_c$ = 66~K) despite the high carrier density. With a sign-problem free quantum Monte Carlo calculation, we show how a partially flat band at ($pi$,0) can help enhance superconducting phase fluctuations. Finally, we discuss the implications of an anisotropic band structure on the phase-coherence-limited superconductivity in overdoped cuprates and other superconductors.
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