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Register a new userDynamic transition in driven vortices across the peak effect in superconductors
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We study the zero-temperature dynamic transition from the disordered flow to an ordered flow state in driven vortices in type-II superconductors. The transition current $I_{p}$ is marked by a sharp kink in the $V(I)$ characteristic with a concomitant large increase in the defect concentration. On increasing magnetic field $B$, the $I_{p}(B)$ follows the behaviour of the critical current $I_{c}(B)$. Specifically, in the peak effect regime $I_{p}(B)$ increases rapidly along with $I_{c}$. We also discuss the effect of varying disorder strength on $I_{p}$.
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Magnetic hysteresis loops (MHLs) have been comparatively measured on both textured and single crystalline Sc5Ir4Si10 superconductors. Critical current densities and flux pinning forces are calculated from MHLs by Bean model. Three kinds of peaks of the flux pinning force are found at low fields near zero, intermediated fields, and high fields near the upper critical field, respectively. The characters and origins of these peaks are studied in detail.
We report a crucial experimental test of the present models of the peak effect in weakly disordered type-II superconductors. Our results favor the scenario in which the peak effect arises from a crossover between the Larkin pinning length and a rapidly falling elastic length in a vortex phase populated with thermally excited topological defects. A thickness dependence study of the onset of the peak effect at varying driving currents suggests that both screw and edge dislocations are involved in the vortex lattice disordering. The driven dynamics in 3D samples are drastically different from those in 2D samples. We suggest that this may be a consequence of the absence of a Peierls potential for screw dislocations in a vortex line lattice.
We analyze the structure of an $s-$wave superconducting gap in systems with electron-phonon attraction and electron-electron repulsion. Earlier works have found that superconductivity develops despite strong repulsion, but the gap, $Delta (omega_m)$, necessarily changes sign along the Matsubara axis. We analyze the sign-changing gap function from a topological perspective using the knowledge that a nodal point of $Delta (omega_m)$ is the center of dynamical vortex. We consider two models with different cutoffs for the repulsive interaction and trace the vortex positions along the Matsubara axis and in the upper frequency half plane upon changing the relative strength of the attractive and repulsive parts of the interaction. We discuss how the presence of dynamical vortices affects the gap structure along the real axis, detectable in ARPES experiments.
We start by showing that the most generic spin-singlet pairing in a superconducting Weyl/Dirac semimetal is specified by a $U(1)$ phase $e^{iphi}$ and $two~real~numbers$ $(Delta_s,Delta_5)$ that form a representation of complex algebra. Such a complex superconducting state realizes a $Z_2times U(1)$ symmetry breaking in the matter sector where $Z_2$ is associated with the chirality. The resulting effective XY theory of the fluctuations of the $U(1)$ phase $phi$ will be now augmented by coupling to another dynamical variable, the $chiral~angle$ $chi$ that defines the polar angle of the complex number $(Delta_s,Delta_5)$. We compute this coupling by considering a Josephson set up. Our energy functional of two phase variables $phi$ and $chi$ allows for the realization of a half-vortex (or double Cooper pair) state and its BKT transition. The half-vortex state is sharply characterized by a flux quantum which is half of the ordinary superconductors. Such a $pi$-periodic Josephson effect can be easily detected as doubled ac Josephson frequency. We further show that the Josephson current $I$ is always accompanied by a $chiral~Josephson~current$ $I_5$. Strain pseudo gauge fields that couple to the $chi$, destabilize the half-vortex state. We argue that our complex superconductor realizes an extension of XY model that supports confinement transition from half-vortex to full vortex excitations.
Superconductors can support large dissipation-free electrical currents only if vortex lines are effectively immobilized by material defects. Macroscopic critical currents depend on elemental interactions of vortices with individual pinning centers. Pinning mechanisms are nontrivial for large-size defects such as self-assembled nanoparticles. We investigate the problem of a vortex system interacting with an isolated defect using time-dependent Ginzburg-Landau simulations. In particular, we study the instability-limited depinning process and extract the dependence of the pin-breaking force on inclusion size and anisotropy for an emph{isolated vortex line}. In the case of a emph{vortex lattice} interacting with a large isolated defect, we find a series of first-order phase transitions at well-defined magnetic fields, when the number of vortex lines occupying the inclusion changes. The pin-breaking force has sharp local minima at those fields. As a consequence, in the case of isolated identical large-size defects, the field dependence of the critical current is composed of a series of peaks located in between the occupation-number transition points.
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