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Register a new userField-angle Resolved Flux-flow Resistivity as a Phase-sensitive Probe of Unconventional Cooper Pairing
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We theoretically investigate the applied magnetic field-angle dependence of the flux-flow resistivity $rho_{rm f}(alpha_{rm M})$ for an uniaxially anisotropic Fermi surface. $rho_{rm f}$ is related to the quasiparticle scattering rate $varGamma$ inside a vortex core, which reflects the sign change in the superconducting pair potential. We find that $rho_{rm f}(alpha_{rm M})$ is sensitive to the sign-change in the pair potential and has its maximum when the magnetic field is parallel to the gap-node direction. We propose the measurement of the field-angle dependent oscillation of $rho_{rm f}(alpha_{rm M})$ as a phase-sensitive field-angle resolved experiment.
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We theoretically investigate the magnetic-field-angle dependence of the flux-flow resistivity $rho_{rm f}$ in unconventional superconductors. Two contributions to $rho_{rm f}$ are considered: one is the quasiparticle (QP) relaxation time $tau(bm{k}_{rm F})$ and the other is $omega_0(bm{k}_{rm F})$, which is a counterpart to the interlevel spacing of the QP bound states in the quasiclassical approach. Here, $bm{k}_{rm F}$ denotes the position on a Fermi surface. Numerical calculations are conducted for a line-node s-wave and a d-wave pair potential with the same anisotropy of their amplitudes, but with a sign change only for a d-wave one. We show that the field-angle dependence of $rho_{rm f}$ differs prominently between s-wave and d-wave pairs, reflecting the phase of the pair potentials. We also discuss the case where $tau$ is constant and compare it with the more general case where $tau$ depends on $bm{k}_{rm F}$.
Fresnel single aperture diffraction (FSAD) is proposed as a phase-sensitive probe for pairing symmetry and Fermi surface of a superconductor. We consider electrons injected, through a small aperture, into a thin superconducting (SC) layer. It is shown that in case of SC gap symmetry $Delta(-k_x,mathbf{k}_parallel)=Delta(k_x,mathbf{k}_parallel)$ with $k_x$ and $mathbf{k}_parallel$ respectively the normal and parallel component of electron Fermi wavevector, quasiparticle FSAD pattern developed at the image plane is zeroth-order minimum if $k_x x=npi$ ($n$ is an integer and $x$ is SC layer thickness). In contrast, if $Delta(-k_x,mathbf{k}_parallel)=-Delta(k_x, mathbf{k}_parallel)$, the corresponding FSAD pattern is zeroth-order maximum. Observable consequences are discussed for iron-based superconductors of complex multi-band pairings.
Measurements of the current-voltage characteristics were performed on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with doping level $0.044 leq x leq 0.1$. An unconventional increase in the flux-flow resistivity $rho_{rm ff}$ with decreasing magnetic field was observed across this doping range. Such an abnormal field dependence of flux-flow resistivity is in contrast with the linear field dependence of $rho_{rm ff}$ in conventional type-II superconductors, but is similar to the behavior recently observed in the heavy-fermion superconductor CeCoIn$_5$. A significantly enhanced $rho_{rm ff}$ was found for the x=0.06 single crystals, implying a strong single-particle energy dissipation around the vortex cores. At different temperatures and fields and for a given doping concentration, the normalized $rho_{rm ff}$ scales with normalized field and temperature. The doping level dependence of these parameters strongly suggests that the abnormal upturn flux-flow resisitivity is likely related to the enhancement of spin fluctuations around the vortex cores of the optimally doped samples.
We numerically investigate the effect of in-plane anisotropic Fermi surface (FS) on the flux-flow resistivity $rho_{rm f}$ under rotating magnetic field on the basis of the quasiclassical Greens function method. We demonstrate that one can detect the phase in pairing potential of Cooper pair through the field-angular dependence of $rho_{rm f}$ even if the FS has in-plane anisotropy. In addition, we point out one can detect the gap-node directions irrespective of the FS anisotropy by measuring $rho_{rm f}$ under rotating field.
When both two-electron textit{and} two-hole Cooper-pairing are treated on an equal footing in the ladder approximation to the Bethe-Salpeter (BS) equation, the zero-total-momentum Cooper-pair energy is found to have two textit{real} solutions $mathcal{E}_{0}^{BS}=pm 2hbar omega_{{D}%}/sqrt{{e}^{2/lambda }+{1}}$ which coincide with the zero-temperature BCS energy gap $Delta =hbar omega_{D}/sinh (1/lambda) $ in the weak coupling limit. Here, $hbar omega_{D}$ is the Debye energy and $lambda geq 0$ the BCS model interaction coupling parameter. The interpretation of the BCS energy gap as the binding energy of a Cooper-pair is often claimed in the literature but, to our knowledge, never substantiated even in weak-coupling as we find here. In addition, we confirm the two purely-textit{imaginary} solutions assumed since at least the late 1950s as the textit{only} solutions, namely, $mathcal{E}_{0}^{BS}=pm i2hbar omega_{{D}}/sqrt{{e}^{2/lambda}{-1}}.$
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