Real-space anisotropy of the superconducting gap in the charge-density wave material 2H-NbSe2

We present a scanning tunneling microscopy (STM) and ab-initio study of the anisotropic superconductivity of 2H-NbSe2 in the charge-density-wave (CDW) phase. Differential-conductance spectra show a clear double-peak structure, which is well reproduced by density functional theory simulations enabling full k- and real-space resolution of the superconducting gap. The hollow-centered (HC) and chalcogen-centered (CC) CDW patterns observed in the experiment are mapped onto separate van der Waals layers with different electronic properties. We identify the CC layer as the high-gap region responsible for the main STM peak. Remarkably, this region belongs to the same Fermi surface sheet that is broken by the CDW gap opening. Simulations reveal a highly anisotropic distribution of the superconducting gap within single Fermi sheets, setting aside the proposed scenario of a two-gap superconductivity. Our results point to a spatially localized competition between superconductivity and CDW involving the HC regions of the crystal.

Pressure-dependent topological superconductivity on the surface of FeTe0.5Se0.5

FeTe1-xSex is a family of iron-based superconductors with its critical temperature (Tc) dependent on the composition of Se. A well-known Tc is 14.5 K for x = 0.45, which exhibits an s-wave superconducting gap between the topological superconducting surfaces states. Exchange interaction between the electrons has been proposed as the mechanism behind the formation of Cooper pairs for the sample of FeTe0.5Se0.5. In this article we provide further proof that exchange interaction, and hence the associated Tc, depends on the applied pressure on FeTe0.5Se0.5. Using density functional calculations for electrons and phonons and the Bardeen-Cooper-Schrieffer (BCS) theory for superconductivity, we found that Tc and superconducting gap for FeTe0.5Se0.5 soars under increasing compression, consistent with the results of experiment.

Hydrogen-impurity-induced conductance peaks in constriction type Josephson junctions

We studied hydrogen (H) and deuterium (D) impurity effects of superconducting Josephson current flowing through the superconductor-constriction-superconductor Josephson junctions (ScS-JJ). When H or D impurities are adsorbed on the surface of the ScS-JJ prepared by niobium (Nb) or lead (Pb), many spike-like peaks with almost the same spacing appear inside the superconducting gap in addition to anomalies owing to the multiple Andreev reflection in the differential conductance spectra. The spacing between the adjacent peaks is independent of the temperature variation. These indicate that H or D impurities adsorbed on the JJ are sources of noise for the Josephson current.

Investigations of proximity-induced superconductivity in the topological insulator Bi2Te3 by microRaman spectroscopy

We used the topological insulator (TI) Bi2Te3 and a high-temperature superconductor (HTSC) hybrid device for investigations of proximity-induced superconductivity (PS) in the TI. Application of the superconductor YBa2Cu3O7-δ (YBCO) enabled us to access higher temperature and energy scales for this phenomenon. The HTSC in the hybrid device exhibits emergence of a pseudogap state for T > Tc that converts into a superconducting state with a reduced gap for T < Tc. The conversion process has been reflected in Raman spectra collected from the TI. Complementary charge transport experiments revealed emergence of the proximity-induced superconducting gap in the TI and the reduced superconducting gap in the HTSC, but no signature of the pseudogap. This allowed us to conclude that Raman spectroscopy reveals formation of the pseudogap state but cannot distinguish the proximity-induced superconducting state in the TI from the superconducting state in the HTSC characterised by the reduced gap. Results of our experiments have shown that Raman spectroscopy is a complementary technique to classic charge transport experiments and is a powerful tool for investigation of the proximity-induced superconductivity in the Bi2Te3.

Superconducting Proximity Effect in Magnetic Molecules

<p>We studied the transport through magnetic molecules (MM) coupled to superconducting (S), ferromagnetic (F) and normal (N) leads, with the aim of investigating the interplay between the magnetism and the superconducting proximity effect. The magnetic molecules were modeled using the Anderson model with an exchange coupling between the electron spins and the spin of the molecule. We worked in the infinite superconducting gap limit and treated the coupling between the molecule and the superconducting lead exactly, via an effective Hamiltonian. For the F/N-MM-S systems we used a real-time diagrammatic perturbation theory to calculate the electronic transport properties of the systems to first order in the tunnel coupling to the normal or ferromagnetic lead and then analysed the properties with respect to the parameters of these models. For these systems we found that the current maps out the excitation energies of the eigenstates of the effective Hamiltonian and that various parameters in these systems can lead to a negative differential conductance. In the N-MM-S case the current had no overall spin dependence, but when the normal lead is instead ferromagnetic there was a spin dependence and both the electronic and molecular spin expectation values could take on non-zero values. We also found that the polarisation of the ferromagnetic lead suppresses the superconducting proximity effect. Furthermore in the N-MM-S case the Fano factor indicated a transition from Poissonian transport of single electrons to Poissonian transport of electron pairs as the superconducting proximity effect goes out of resonance, however in the F-MM-S case this did not occur. For the S-MM-S systems we calculated the equilibrium Josephson current and found that in the infinite superconducting gap limit no 0 − π transition was possible. Advantages of this study compared to related ones are that we allow for arbitrarily large Coulomb interactions and we take into account coupling to the superconducting lead non-perturbatively. This is however at the expense of working in the superconducting gap limit. Recently it has been possible to couple single molecules to superconducting leads. This study therefore aims to be indicative of the transport properties that will be observed in future experiments involving single magnetic molecules coupled to leads.</p>

Superconducting Proximity Effect in Magnetic Molecules

<p>We studied the transport through magnetic molecules (MM) coupled to superconducting (S), ferromagnetic (F) and normal (N) leads, with the aim of investigating the interplay between the magnetism and the superconducting proximity effect. The magnetic molecules were modeled using the Anderson model with an exchange coupling between the electron spins and the spin of the molecule. We worked in the infinite superconducting gap limit and treated the coupling between the molecule and the superconducting lead exactly, via an effective Hamiltonian. For the F/N-MM-S systems we used a real-time diagrammatic perturbation theory to calculate the electronic transport properties of the systems to first order in the tunnel coupling to the normal or ferromagnetic lead and then analysed the properties with respect to the parameters of these models. For these systems we found that the current maps out the excitation energies of the eigenstates of the effective Hamiltonian and that various parameters in these systems can lead to a negative differential conductance. In the N-MM-S case the current had no overall spin dependence, but when the normal lead is instead ferromagnetic there was a spin dependence and both the electronic and molecular spin expectation values could take on non-zero values. We also found that the polarisation of the ferromagnetic lead suppresses the superconducting proximity effect. Furthermore in the N-MM-S case the Fano factor indicated a transition from Poissonian transport of single electrons to Poissonian transport of electron pairs as the superconducting proximity effect goes out of resonance, however in the F-MM-S case this did not occur. For the S-MM-S systems we calculated the equilibrium Josephson current and found that in the infinite superconducting gap limit no 0 − π transition was possible. Advantages of this study compared to related ones are that we allow for arbitrarily large Coulomb interactions and we take into account coupling to the superconducting lead non-perturbatively. This is however at the expense of working in the superconducting gap limit. Recently it has been possible to couple single molecules to superconducting leads. This study therefore aims to be indicative of the transport properties that will be observed in future experiments involving single magnetic molecules coupled to leads.</p>

Color-flavor locked compact stars: An exact solution approach

We present an exact solution that could describe compact star composed of color-flavor locked (CFL) phase. Einstein’s field equations were solved through CFL equation of state (EoS) along with a specific form of [Formula: see text] metric potential. Further, to explore a generalized solution we have also included pressure anisotropy. The solution is then analyzed by varying the color superconducting gap [Formula: see text] and its effects on the physical parameters. The stability of the solution through various criteria is also analyzed. To show the physical validity of the obtained solution we have generated the [Formula: see text] curve and fitted three well-known compact stars. This work shows that the anisotropy of the pressure at the interior increases with the color superconducting gap leading to decrease in adiabatic index closer to the critical limit. Further, the fluctuating range of mass due to the density perturbation is larger for lower color superconducting gap leading to more stable configuration.

Preliminary demonstration of a persistent Josephson phase-slip memory cell with topological protection

Superconducting computing promises enhanced computational power in both classical and quantum approaches. Yet, scalable and fast superconducting memories are not implemented. Here, we propose a fully superconducting memory cell based on the hysteretic phase-slip transition existing in long aluminum nanowire Josephson junctions. Embraced by a superconducting ring, the memory cell codifies the logic state in the direction of the circulating persistent current, as commonly defined in flux-based superconducting memories. But, unlike the latter, the hysteresis here is a consequence of the phase-slip occurring in the long weak link and associated to the topological transition of its superconducting gap. This disentangles our memory scheme from the large-inductance constraint, thus enabling its miniaturization. Moreover, the strong activation energy for phase-slip nucleation provides a robust topological protection against stochastic phase-slips and magnetic-flux noise. These properties make the Josephson phase-slip memory a promising solution for advanced superconducting classical logic architectures or flux qubits.