arXiv subjects

Yuki Nagai

Publications and source records attributed to Yuki Nagai.

97 records · Page 6Linked to original sources

arXiv2009

Surface Bound States in n-band Systems with Quasiclassical Approach

We discuss the tunneling spectroscopy at a surface in multi-band systems such as Fe-based superconductors with the use of the quasiclassical approach. We extend the single-band method by Matsumoto and Shiba [J. Phys. Soc. Jpn. 64, 1703 (1995)] into $n$-band systems ($n \geq 2$). We show that the appearance condition of the zero-bias conductance peak does not depend on details of the pair-potential anisotropy, but it depends on details of the normal state properties in the case of fully-gapped superconductors. The surface density of states in a two-band superconductor is presented as a simplest application. The quasiclassical approach enables us to calculate readily the surface-angular dependence of the tunneling spectroscopy.

cond-mat.supr-con↗

arXiv2009

Field angle dependence of the zero-energy density of states in unconventional superconductors: analysis of the borocarbide superconductor YNi2B2C

We investigate the field-angle-dependent zero-energy density of states for YNi2B2C with using realistic Fermi surfaces obtained by band calculations. Both the 17th and 18th bands are taken into account. For calculating the oscillating density of states, we adopt the Kramer-Pesch approximation, which is found to improve accuracy in the oscillation amplitude. We show that superconducting gap structure determined by analyzing STM experiments is consistent with thermal transport and heat capacity measurements.

cond-mat.supr-con↗

arXiv2008

Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario

We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al. [Phys. Rev. Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a fully-gapped anisotropic \pm s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like βFermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.

cond-mat.supr-con↗

arXiv2008

Kramer-Pesch approximation for analyzing field-angle-resolved measurements made in unconventional superconductors: A calculation of the zero-energy density of states

By measuring angular-oscillation behavior of the heat capacity with respect to the applied field direction, one can detect the details of the gap structure. We introduce the Kramer-Pesch approximation (KPA) as a new method to analyze the field-angle-dependent experiments quantitatively. We calculate the zero energy density of states for various combinations of typical Fermi surfaces and superconducting gaps. The KPA yields a merit that one can quantitatively compare theoretical calculations with experimental results without involving heavy numerical computations, even for complicated Fermi surfaces. We show an inadequacy of the frequently-used Doppler-shift technique, which is remedied by application of the KPA.

cond-mat.supr-con↗

arXiv2007

Positions of Point-Nodes in Borocarbide Superconductor YNi2B2C

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.

cond-mat.supr-con↗

arXiv2006

Analytical Formulation of the Local Density of States around a Vortex Core in Unconventional Superconductors

On the basis of the quasiclassical theory of superconductivity, we obtain a formula for the local density of states (LDOS) around a vortex core of superconductors with anisotropic pair-potential and Fermi surface in arbitrary directions of magnetic fields. Earlier results on the LDOS of d-wave superconductors and NbSe$_2$ are naturally interpreted within our theory geometrically; the region with high intensity of the LDOS observed in numerical calculations turns out to the enveloping curve of the trajectory of Andreev bound states. We discuss experimental results on YNi$_2$B$_2$C within the quasiclassical theory of superconductivity.

cond-mat.supr-con↗

arXiv2006

Analytical Result on Electronic States around a Vortex Core in a Noncentrosymmetric Superconductor

We analytically study electronic bound states around a single vortex core in a noncentrosymmetric superconductor such as ${\rm CePt_3Si}$ without mirror symmetry about the $ab$ plane. Considering a mixed spin-singlet-triplet Cooper pairing model, we obtain a formula about the local density of states (LDOS) around a vortex core in any directions of magnetic fields. The LDOS under a magnetic field perpendicular to the c axis are quite different from those of typical s-wave or d-wave superconductors. From the ellipticity of the spatial pattern of the LDOS around a vortex core, one can estimate experimentally the pairing symmetry of ${\rm CePt_3Si}$ such as the position of the gap nodes and the ratio of the singlet component to the triplet component in order parameter.

cond-mat.supr-con↗