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Register a new userMagnetic extrema in electronic susceptibility and heat capacity of mesoscopic systems
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Oscillating behaviour of the susceptibility and heat capacity is considered for normal and superconducting mesoscopic systems (nanoclusters and quantum dots). It is proved that at low temperature an increasing magnetic field applied to a mesoscopic system generates local extrema of the susceptibility and heat capacity. A maximum for the susceptibility and a minimum for heat capacity simultaneously arise in those points of the field where crossings of quantum levels of the normal and superconducting mesoscopic systems take place.
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Phase transitions in superconducting mesoscopic disks have been studied over the H-T phase diagram through heat capacity measurement of an array of independent aluminium disks. These disks exhibit non periodic modulations versus H of the height of the heat capacity jump at the superconducting to normal transition. This behaviour is attributed to giant vortex states characterized by their vorticity L. A crossover from a bulk-like to a mesoscopic behaviour is demonstrated. $C_{rm p}$ versus H plots exhibit cascades of phase transitions as L increases or decreases by one unity, with a strong hysteresis. Phase diagrams of giant vortex states inside the superconducting region are drawn in the vortex penetration and expulsion regimes and phase transitions driven by temperature between vortex states are thus predicted in the zero field cooled regime before being experimentally evidenced.
We study the influence of superconducting correlations on the electronic specific heat in a diffusive superconductor-normal metal-superconductor Josephson junction. We present a description of this system in the framework of the diffusive-limit Greens function theory, taking into account finite temperatures, phase difference as well as junction parameters. We find that proximity effect may lead to a substantial deviation of the specific heat as compared to that in the normal state, and that it can be largely tuned in magnitude by changing the phase difference between the superconductors. A measurement setup to confirm these predictions is also suggested.
We have measured the electronic heat capacity of thin film nanowires of copper and silver at temperatures 0.1 - 0.3 K; the films were deposited by standard electron-beam evaporation. The specific heat of the Ag films of sub-100 nm thickness agrees with the bulk value and the free-electron estimate, whereas that of similar Cu films exceeds the corresponding reference values by one order of magnitude. The origin of the anomalously high heat capacity of copper films remains unknown for the moment. Based on the low heat capacity and the possibility to devise a tunnel probe thermometer on it, the Ag films form a promising absorber material, e.g., for micro-wave photon calorimetry.
The method of the quasiclassical Greens function is used to determine the equilibrium properties of one-dimensional (1D) interacting Fermi systems, in particular, the bulk and the local (near a hard wall) density of states. While this is a novel approach to 1D systems, our findings do agree with standard results for Luttinger liquids obtained with the bosonization method. Analogies to the so-called $P(E)$ theory of tunneling through ultrasmall junctions are pointed out and are exploited. Further applications of the Greens function method for 1D systems are discussed.
The ability to transport energy is a fundamental property of the two-dimensional Dirac fermions in graphene. Electronic thermal transport in this system is relatively unexplored and is expected to show unique fundamental properties and to play an important role in future applications of graphene, including opto-electronics, plasmonics, and ultra-sensitive bolometry. Here we present measurements of bipolar, electron-diffusion and electron-phonon thermal conductances, and infer the electronic specific heat, with a minimum value of 10 $k_{rm{B}}$ ($10^{-22}$ JK$^{-1}$) per square micron. We test the validity of the Wiedemann-Franz law and find the Lorenz number equals $1.32times(pi^2/3)(k_{rm{B}}/e)^2$. The electron-phonon thermal conductance has a temperature power law $T^2$ at high doping levels, and the coupling parameter is consistent with recent theory, indicating its enhancement by impurity scattering. We demonstrate control of the thermal conductance by electrical gating and by suppressing the diffusion channel using superconducting electrodes, which sets the stage for future graphene-based single microwave photon detection.
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