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Register a new userMicroscopic model for multiple flux transitions in mesoscopic superconducting loops
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A microscopic model is constructed which is able to describe multiple magnetic flux transitions as observed in recent ultra-low temperature tunnel experiments on an aluminum superconducting ring with normal metal - insulator - superconductor junctions [Phys. Rev. B textbf{70}, 064514 (2004)]. The unusual multiple flux quantum transitions are explained by the formation of metastable states with large vorticity. Essential in our description is the modification of the pairing potential and the superconducting density of states by a sub-critical value of the persistent current which modulates the measured tunnel current. We also speculate on the importance of the injected non-equilibrium quasiparticles on the stability of these metastable states.
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We solve the Ginzburg-Landau equation (GLE) for the mesoscopic superconducting thin film of the square shape in the magnetic field for the wide range of the Ginzburg-Landau parameter $0.05<kappa_{eff}<infty $. We found that the phase with the antivortex exists in the broad range of parameters. When the coherence length decreases the topological phase transition to the phase with the same total vorticity and a reduced symmetry takes place. The giant vortex with the vorticity $m=3$ is found to be unstable for any field, $xi /a$ and $kappa_{eff}ge 0.1$. Reduction of $ kappa _{eff}$ does not make the phase with antivortex more stable contrary to the case of the cylindric sample of the type I superconductor.
We study the magnetic field driven Quantum Phase Transition (QPT) in electrostatically gated superconducting LaTiO3/SrTiO3 interfaces. Through finite size scaling analysis, we show that it belongs to the (2+1)D XY model universality class. The system can be described as a disordered array of superconducting islands coupled by a two dimensional electron gas (2DEG). Depending on the 2DEG conductance tuned by the gate voltage, the QPT is single (corresponding to the long range phase coherence in the whole array) or double (one related to local phase coherence, the other one to the array). By retrieving the coherence length critical exponent u, we show that the QPT can be clean or dirty according to the Harris criteria, depending on whether the phase coherence length is smaller or larger than the island size. The overall behaviour is well described by a theoretical approach of Spivak et al., in the framework of the fermionic scenario of 2D superconducting QPT.
Gray tin, also known as {alpha}-Sn, has been attracting research interest recent years due to its topological nontrivial properties predicted theoretically. The Dirac linear band dispersion has been proved experimentally by angle resolved photoemission spectroscopy. We have grown a series of {alpha}-Sn thin film samples in two types with different substrates and thicknesses by molecular beam epitaxy. To explore the possible exotic physical properties related to the topological band structures, we have measured the electrical transport properties of our {alpha}-Sn thin film samples and observed multiple superconducting transitions. We have identified the transitions above 4.5 K, besides the transition maybe related to the b{eta} phase around 3.7 K. The changes of the superconducting properties over time reflect the aging effects in our samples. We have also confirmed the strain effects on the superconducting transitions through altering the relative thickness of our samples.
The magnetic flux periodicity of superconducting loops as well as flux quantization itself are a manifestation of macroscopic quantum phenomena with far reaching implications. They provide the key to the understanding of many fundamental properties of superconductors and are the basis for most bulk and device applications of these materials. In superconducting rings the electrical current has been known to periodically respond to a magnetic flux with a periodicity of $bm{h/2e}$. Here, the ratio of Plancks constant and the elementary charge defines the magnetic flux quantum $bm{h/e}$. The well-known $bm{h/2e}$ periodicity is viewed to be a hallmark for electronic pairing in superconductors and is considered evidence for the existence of Cooper pairs. Here we show that in contrast to this long-term belief, rings of many superconductor bear an $bm{h/e}$ periodicity. These superconductors include the high-$bm{T_c}$ cuprates, Sr$_2$RuO$_4$, the heavy-fermion superconductors, as well as all other unconventional superconductors with nodes in the energy gap functions, and s-wave superconductors with small gaps or states in the gap. As we show, the 50-year-old Bardeen--Cooper--Schrieffer theory of superconductivity implies that for multiply connected paths of such superconductors the ground-state energies and consequently also the supercurrents are generically $bm{h/e}$ periodic. The origin of this periodicity is a magnetic-field driven reconstruction of the condensate and a concomitant Doppler-shifted energy spectrum. The robust, flux induced reconstruction of the condensate will be an important aspect to understand the magnetic properties of mesoscopic unconventional superconductors.
We show that the three-junction SQUID device designed for the Josephson flux qubit can be used to study quantum chaos when operated at high energies. In the parameter region where the system is classically chaotic we analyze the spectral statistics. The nearest neighbor distributions $P(s)$ are well fitted by the Berry Robnik theory employing as free parameters the pure classical measures of the chaotic and regular regions of phase space in the different energy regions. The phase space representation of the wave functions is obtained via the Husimi distributions and the localization of the states on classical structures is analyzed.
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