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Register a new userLow Energy Quasiparticle Excitation in the Vortex State of Borocarbide Superconductor YNi_2B_2C
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We have measured the low temperature heat capacity Cp and microwave surface impedance Zs in the vortex state of YNi_2B_2C. In contrast to conventional s-wave superconductors, Cp shows a nearly sqrt H-dependence. This sqrt H-dependence persists even after the introduction of the columnar defects which change the electronic structure of the vortex core regime dramatically and strongly disturb the regular vortex lattice. On the other hand, flux flow resistivity obtained from Zs is nearly proportional to H. Taken together, these results indicate that the vortex state of YNi_2B_2C is fundamentally different from the conventional s-wave counterparts, in that the delocalized quasiparticle states around the vortex core play a much more important role, similar to d-wave superconductors.
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To determine the superconducting gap function of a borocarbide superconductor YNi_2B_2C, the c-axis thermal conductivity kappa_zz was measured in a magnetic field rotated in various directions relative to the crystal axes. The angular variation of kappa_zz in H rotated within the ab-plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when H is rotated conically around the c-axis with a tilt angle of 45 degrees. Based on these results, we provide the first compelling evidence that the gap function of YNi_2B_2C has POINT NODES, which are located along the [100] and [010]-directions. This unprecedented gap structure challenges the current view on the pairing mechanism and on the unusual superconducting properties of borocarbide superconductors.
The effect of vortices on quasiparticle transport in cuprate superconductors was investigated by measuring the low temperature thermal conductivity of YBa_2Cu_3O_6.9 in magnetic fields up to 8 T. The residual linear term (as T to 0) is found to increase with field, directly reflecting the occupation of extended quasiparticle states. A study for different Zn impurity concentrations reveals a good agreement with recent calculations for a d-wave superconductor, thereby shedding light on the nature of scattering by both impurities and vortices. It also provides a quantitative measure of the gap near the nodes.
To determine the superconducting gap function of YNi2B2C, we calculate the local density of states (LDOS) around a single vortex core with the use of Eilenberger theory and the band structure calculated by local density approximation assuming various gap structures with point-nodes at different positions. We also calculate the angular-dependent heat capacity in the vortex state on the basis of the Doppler-Shift method. Comparing our results with the STM/STS experiment, the angular-dependent heat capacity and thermal conductivity, we propose the gap-structure of YNi2B2C, which has the point-nodes and gap minima along <110>. Our gap-structure is consistent with all results of angular-resolved experiments.
Topological superconductors as characterized by Majorana surface states has been actively searched for their significance in fundamental science and technological implication. The large spin-orbit coupling in Bi-Pd binaries has stimulated extensive investigations on the topological surface states in these superconducting compounds. Here we report a study of normal-state electronic structure in a centrosymmetric $alpha$-PdBi2 within density functional theory calculations. By investigating the electronic structure from the bulk to slab geometries in this system, we predict for the first time that $alpha$-PdBi2 can host orbital-dependent and asymmetric Rashba surface states near the Fermi energy. This study suggests that $alpha$-PdBi2 will be a good candidate to explore the relationship between superconductivity and topology in condensed matter physics.
We present a theory of quasiparticle Hall transport in strongly type-II superconductors within their vortex state. We establish the existence of integer quantum spin Hall effect in clean unconventional $d_{x^2-y^2}$ superconductors in the vortex state from a general analysis of the Bogoliubov-de Gennes equation. The spin Hall conductivity $sigma^s_{xy}$ is shown to be quantized in units of $frac{hbar}{8pi}$. This result does not rest on linearization of the BdG equations around Dirac nodes and therefore includes inter-nodal physics in its entirety. In addition, this result holds for a generic inversion-symmetric lattice of vortices as long as the magnetic field $B$ satisfies $H_{c1} ll B ll H_{c2}$. We then derive the Wiedemann-Franz law for the spin and thermal Hall conductivity in the vortex state. In the limit of $T to 0$, the thermal Hall conductivity satisfies $kappa_{x y}=frac{4pi^2}{3}(frac{k_B}{hbar})^2 T sigma^s_{xy}$. The transitions between different quantized values of $sigma^s_{xy}$ as well as relation to conventional superconductors are discussed.
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