arXiv subjects

Nobuhiko Hayashi

Publications and source records attributed to Nobuhiko Hayashi.

32 records · Page 2Linked to original sources

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.

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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.

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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.

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arXiv2002

Interplay between Vorticity and Chirality inside the Vortex Core in Chiral p-Wave Superconductors

The vortex core in chiral p-wave superconductors exhibits various properties owing to the interplay between the vorticity and chirality inside the vortex core. In the chiral p-wave superconductors, the site-selective nuclear spin-lattice relaxation rate T1^{-1} is theoretically studied inside the vortex core within the framework of the quasiclassical theory of superconductivity. T1^{-1} at the vortex center depends on the sense of the chirality relative to the sense of the magnetic field. The effect of a tilt of the magnetic field upon T1^{-1} is investigated. The effect of the anisotropy in the superconducting gap and the Fermi surface is then investigated. The result is expected to be experimentally observed as a sign of the chiral pairing state in a superconducting material Sr2RuO4.

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arXiv2002

Relation between Vortex Pinning Energy and Anderson's Theorem

We discuss the elementary vortex pinning in type-II superconductors in connection with the Anderson's theorem for nonmagnetic impurities. We address the following two issues. One is an enhancement of the vortex pinning energy in the unconventional superconductors. This enhancement comes from the pair-breaking effect of a nonmagnetic defect as the pinning center far away from the vortex core (i.e., the pair-breaking effect due to the non-applicability of the Anderson's theorem in the unconventional superconductors). The other is an effect of the chirality on the vortex pinning energy in a chiral p-wave superconductor. The vortex pinning energy depends on the chirality. This is related to the cancellation of the angular momentum between the vorticity and chirality in a chiral p-wave vortex core, resulting in local applicability of the Anderson's theorem (or local recovery of the Anderson's theorem) inside the vortex core.

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arXiv2002

Nuclear Magnetic Relaxation Rate in the Vortex State of a Chiral p-Wave Superconductor

The site-selective nuclear spin-lattice relaxation rate T1^{-1} is theoretically studied inside a vortex core in a chiral p-wave superconductor within the framework of the quasiclassical theory of superconductivity. It is found that T1^{-1} at the vortex center depends on the sense of the chirality relative to the sense of the magnetic field. Our numerical result shows a characteristic difference in T1^{-1} between the two chiral states, k_x + i k_y and k_x - i k_y under the magnetic field.

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arXiv2002

Numerical Study of Impurity Effects on Quasiparticles within S-wave and Chiral P-wave Vortices

The impurity problems within vortex cores of two-dimensional s-wave and chiral p-wave superconductors are studied numerically in the framework of the quasiclassical theory of superconductivity and self-consistent Born approximation under a trial form of the pair potential. The dispersion and impurity scattering rate (the inverse of the relaxation time) of the Andreev bound state localized in vortex cores are deduced from the angular-resoloved local density of states. The energy dependence of the impurity scattering rates depends on the pairing symmetry; particularly, in the chiral p-wave vortex core where chirality and vorticity have opposite sign and hence the total angular momentum is zero, the impurities are ineffective and the scattering rate is vanishingly small. Owing to the cancellation of angular momentum between chirality and vorticity, the chiral p-wave vortex core is similar to locally realized s-wave region and therefore non-magnetic impurity is harmless as a consequence of Anderson's theorem. The results of the present study confirm the previous results of analytical study (J. Phys. Soc. Jpn. {\bf 69} (2000) 3378) in the Born limit.

cond-mat.supr-con↗

arXiv2002

Elementary vortex pinning potential in a chiral p-wave superconductor

The elementary vortex pinning potential is studied in a chiral p-wave superconductor with a pairing d=z(k_x + i k_y) on the basis of the quasiclassical theory of superconductivity. An analytical investigation and numerical results are presented to show that the vortex pinning potential is dependent on whether the vorticity and chirality are parallel or antiparallel. Mutual cancellation of the vorticity and chirality around a vortex is physically crucial to the effect of the pinning center inside the vortex core.

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arXiv2001

Critical Temperature Tc versus Charging Energy Ec in Molecular-Intercalated Fullerenes

We study the recently discovered 117-Kelvin superconducting system C60/CHBr3 of the field-effect transistor and propose a possibility that the intercalation molecule CHBr3 plays a role of an electric capacitor in the C60 fullerene superconductor, which contrasts rather with an expectation that the intercalation molecule in that system acts as simple spacer molecule. Estimating the critical temperatures Tc for several C60/X (X: intercalation molecule), we suggest searching for intercalation molecules with large molecular polarizabilities, in order to attain higher Tc in the synthesis of fullerene superconductors and to more systematically develop high Tc superconducting electronic devices.

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arXiv2001

Critical Temperature T_c versus Charging Energy E_c in MgB2 and C60/CHBr3

The boride compounds MB_x related to the magnesium-boron stacking layered material MgB2 are discussed in terms of the B-B layers in the borides analogous to the Cu-O ones in the cuprates. We propose a possibility of superconducting materials which exhibit higher critical temperature T_c than 39 K of MgB2. We point out a role of interstitial ionic atoms M (e.g., Mg in MgB2) as capacitors, which reduce the condensation-energy loss due to the charging energy E_c between the B-B layers. In the viewpoint of the present model, the recently discovered 117-Kelvin superconductor C60/CHBr3 is also discussed in terms of the intercalation molecules CHBr3 as possible capacitors among the superconducting grains of C60 molecules.

cond-mat.supr-con↗

arXiv2001

Elementary vortex pinning potential in superconductors with unconventional order parameter

The elementary vortex pinning potential is studied in unconventional superconductors within the framework of the quasiclassical theory of superconductivity. Numerical results are presented for d-, anisotropic s-, and isotropic s-wave superconductors to show explicitly that in unconventional superconductors the vortex pinning potential is determined mainly by the loss of the condensation energy in bulk due to the presence of the pinning center, i.e., by the breakdown of the Anderson's theorem. It is found that the vortex pinning energy in the d-wave pairing case is 4 -- 13 times larger than those in the s-wave pairing cases. This means that an enhancement of pinning effect in unconventional superconductors occurs due to the breakdown of the Anderson's theorem. The case of a chiral p-wave superconductor is also investigated in terms of the vortex core states subject to the Andreev reflection, where important is whether the vorticity and chirality are parallel or antiparallel.

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arXiv2001

Kramer-Pesch effect in chiral p-wave superconductors

The pair-potential and current density around a single vortex of the two-dimensional chiral p-wave superconductor with ${\mib d}={\mib z}(p_x \pm \iu p_y)$ are determined self-consistently within the quasiclassical theory of superconductivity. Shrinking of the vortex core at low temperatures are considered numerically and analytically. Temperature-dependences of the spatial variation of pair-potential and circular current around the core and density of states at zero energy are the same as those in the isotropic s-wave case. When the senses of vorticity and chirality are opposite, however, we find two novel results; 1) the scattering rate due to non-magnetic impurities is considerably suppressed, compared to that in the s-wave vortex. From this observation, we expect that the chiral p-wave superconductors provide the best chance to observe the shrinking of the vortex ("Kramer-Pesch effect") experimentally. 2) The pair-potential of chiral p-wave superconductors inside vortex core recovers a combined time-reversal-Gauge symmetry, although this symmetry is broken in the region far from the vortex core. This local recovery of symmetry leads to the suppression of the impurity effect inside vortex core.

cond-mat.supr-con↗

arXiv1996

Local density of states in the vortex lattice in a type II superconductor

Local density of states (LDOS) in the triangular vortex lattice is investigated based on the quasi-classical Eilenberger theory. We consider the case of an isotropic s-wave superconductor with the material parameter appropriate to NbSe_2. At a weak magnetic field, the spatial variation of the LDOS shows cylindrical structure around a vortex core. On the other hand, at a high field where the core regions substantially overlap each other, the LDOS is sixfold star-shaped structure due to the vortex lattice effect. The orientation of the star coincides with the experimental data of the scanning tunneling microscopy. That is, the ray of the star extends toward the nearest-neighbor (next nearest-neighbor) vortex direction at higher (lower) energy.

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