arXiv subjects

Y. Tanuma

Publications and source records attributed to Y. Tanuma.

16 recordsLinked to original sources

arXiv2008

Model for Vortex-core tunneling spectroscopy of chiral p-wave superconductors via odd-frequency pairing states

The local density of states (LDOS) around a vortex core is studied theoretically in terms of the odd-frequency (odd-omega) Cooper pairing induced around the center of the vortex core. We find a robust zero-energy peak in the LDOS at a vortex core in a chiral p-wave superconductor originating from an odd-omega s-wave pair amplitude. We suggest how to discriminate a spin-triplet pairing symmetry and spatial chiral-domain structure by scanning tunneling spectroscopy via odd-omega pair amplitudes inside vortex cores.

cond-mat.supr-con↗

arXiv2007

Odd-frequency pairing in normal metal/superconductor junctions

We study the induced odd-frequency pairing states in ballistic normal metal/superconductor (N/S) junctions where a superconductor has even-frequency symmetry in the bulk and a normal metal layer has an arbitrary length. Using the quasiclassical Green's function formalism, we demonstrate that, quite generally, the pair amplitude in the junction has an admixture of an odd-frequency component due to the breakdown of translational invariance near the N/S interface where the pair potential acquires spatial dependence. If a superconductor has even-parity pair potential (spin-singlet s-wave state), the odd-frequency pairing component with odd-parity is induced near the N/S interface, while in the case of odd-parity pair potential (spin-triplet $p_{x}$-wave or spin-singlet $d_{xy}$-wave) the odd-frequency component with even-parity is generated. We show that in conventional s-wave junctions, the amplitude of the odd-frequency pairing state is enhanced at energies corresponding to the peaks in the local density of states (LDOS). In $p_x$- and $d_{xy}$-wave junctions, the amplitude of the odd-frequency component on the S side of the N/S interface is enhanced at zero energy where the midgap Andreev resonant state (MARS) appears due to the sign change of the pair potential. The odd-frequency component extends into the N region and exceeds the even-frequency component at energies corresponding to the LDOS peak positions, including the MARS.

cond-mat.supr-con↗

arXiv2004

Theory of proximity effect in normal metal/$d_{x^2-y^2}$-wave superconductor interface in the presence of subdominant components of the pair potentials

Superconducting proximity effect in normal metal (N) / $d_{x^2-y^2}$-wave superconductor (D) junctions in the presence of attractive interelectron potentials which can induce subdominant s-wave pair potentials both in N and D sides, is studied based on the quasiclassical Green's function theory, where spatial dependencies of the pair potentials are determined self-consistently. In the N/D junctions with orientational angle with $θ= 0$, the s-wave component is induced in the N side by the proximity effect only for high transparent case, where the induced s-wave components in both the N and D sides do not break the time reversal symmetry (TRS). For fully transparent case, the resulting local density of states has a very sharp zero-energy peak (ZEP), the origin of which is the sign change of the pair potentials felt by the quasiparticles between the s-wave component in the N side and $d_{x^2-y^2}$-wave dominant component in the D side through Andreev reflections. On the other hands, for $θ= π/4$, the subdominant s-wave component which breaks the TRS appears near the interface. Besides, for lower transparent cases, the subdominant imaginary s-wave component is also induced near the interface in the N side. The proximity induced s-wave component in the N side does not enhance the magnitude of the s-wave component of the pair potential which break the TRS in the D side. The resulting LDOS at the interface has the ZEP or its splitting depending on the transparency of the junction.

cond-mat.supr-con↗

arXiv2003

Josephson effect in quasi one-dimensional unconventional superconductors

Josephson effect in junctions of quasi one-dimensional triangular lattice superconductors is discussed, where the theoretical model corresponds to organic superconductors (TMTSF)_2PF_6. We assume the quarter-filling electron band and p, d and f wave like pairing symmetries in organic superconductors. To realize the electronic structures in organic superconductors, we introduce the asymmetric hopping integral, (t') among second nearest lattice sites. At t'=0, the Josephson current in the d wave symmetry saturates in low temperatures, whereas those in the p and the f wave symmetries show the low-temperature anomaly due to the zero-energy state at the junction interfaces. The low-temperature anomaly appears even in the d wave symmetry in the presence of t', whereas the anomaly is suppressed in the f wave symmetry. The shape of the Fermi surface is an important factor for the formation of the ZES in the quarter-filling electron systems.

cond-mat.supr-con↗

arXiv2003

Zero-bias conductance peak splitting due to multiband effect in tunneling spectroscopy

We study how the multiplicity of the Fermi surface affects the zero-bias peak in conductance spectra of tunneling spectroscopy. As case studies, we consider models for organic superconductors $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$ and (TMTSF)$_2$ClO$_4$. We find that multiplicity of the Fermi surfaces can lead to a splitting of the zero-bias conductance peak (ZBCP). We propose that the presence/absence of the ZBCP splitting is used as a probe to distinguish the pairing symmetry in $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$.

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arXiv2003

Josephson effect in d-wave superconductor junctions in a lattice model

Josephson current between two d-wave superconductors is calculated by using a lattice model. Here we consider two types of junctions, $i.e.$, the parallel junction and the mirror-type junction. The maximum Josephson current $(J_{c})$ shows a wide variety of temperature ($T$) dependence depending on the misorientation angles and the types of junctions. When the misorientation angles are not zero, the Josephson current shows the low-temperature anomaly because of a zero energy state (ZES) at the interfaces. In the case of mirror-type junctions, $J_c$ has a non monotonic temperature dependence. These results are consistent with the previous results based on the quasiclassical theory. [Y. Tanaka and S. Kashiwaya: Phys. Rev. B \textbf{56} (1997) 892.] On the other hand, we find that the ZES disappears in several junctions because of the Freidel oscillations of the wave function, which is peculiar to the lattice model. In such junctions, the temperature dependence of $J_{c}$ is close to the Ambegaokar-Baratoff relation.

cond-mat.supr-con↗

arXiv2003

Influence of impurity-scattering on tunneling conductance in d-wave superconductors with broken time reversal symmetry

Effects of impurity scattering on tunneling conductance in dirty normal-metal/insulator/superconductor junctions are studied based on the Kubo formula and the recursive Green function method. The zero-bias conductance peak (ZBCP) is a consequence of the unconventional pairing symmetry in superconductors. The impurity scattering in normal metals suppresses the amplitude of the ZBCP. The degree of the suppression agrees well with results of the quasiclassical Green function theory. When superconductors have $d$+is-wave pairing symmetry, the time-reversal symmetry is broken in superconductors and the ZBCP splits into two peaks. The random impurity scattering reduces the height of the two splitting peaks. The position of the splitting peaks, however, almost remains unchanged even in the presence of the strong impurity scattering. Thus the two splitting peaks never merge into a single ZBCP.

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arXiv2003

Possible anomalous Doppler shift effect in superconductor Sr$_{2}$RuO$_{4}$

The effect of the Doppler shift is studied in a model for the $α$-$β$ bands of Sr$_2$RuO$_4$ consisting of two hybridized 1D bands. Assuming a superconducting gap with nodes in the diagonal directions, we examine the oscillation of the surface density of states and the thermal conductivity under a rotating magnetic field. Upon varying the strength of the hybridization, the oscillation in these quantities is found to exhibit 2D to 1D crossover. In the crossover regime, which corresponds to the actual Sr$_2$RuO$_4$, the thermal conductivity exhibits a two-fold-symmetry oscillation, while the four-fold-symmetry component in the oscillation is barely detectable.

cond-mat.supr-con↗

arXiv2002

Magnetotunneling spectroscopy as a probe for pairing symmetry determination in quasi-2D anisotropic superconductors

As a probe to determine the pairing symmetry of quasi-two-dimensional anisotropic superconductors, we propose tunneling spectroscopy in the presence of magnetic field, where the magnetic field is parallel to the two dimensional planes and rotated. As a case study, we apply this idea to the models of high-$T_{C}$ cuprates and organic superconductors $κ$-(ET)$_2X$. The surface density of states at the Fermi energy exhibits a characteristic oscillation upon rotating the direction of the magnetic field due to the Doppler shift of the energy of quasiparticles. The surface density of states has a minimum when the applied magnetic field is parallel to the node direction of the pair potential independent of the detailed shape of the Fermi surface. The amplitude of the oscillation is sensitively affected by the shape of Fermi surface.

cond-mat.supr-con↗

arXiv2002

Tunneling conductance in normal metal - high $T_{C}$ cuprate junctions in the presence of magnetic field

The magnetic field responses of the zero-bias conductance peak (ZBCP) in tunneling spectra of high $T_{C}$ cuprate junctions are studied theoretically. Our calculation is based on the lattice Green's function method and takes the realistic electronic structure of the high $T_{C}$ cuprate into account. In marked contrast to previous works, it is shown that the critical magnetic field strength $H_{C}$ exists for the splittings of the ZBCP's to be discernible. $H_{C}$ is almost proportional to the product of the magnitude of pair potential, transmissivity of the junction, and the inverse of the Fermi velocity parallel to the interface (1/$v_{Fy}$). The calculated $H_{C}$'s for the hole-doped superconductors are higher than those for electron-doped ones because of relatively large magnitude of pair potential and the small magnitude of $v_{Fy}$ originating from peculiar shape of the Fermi surface.

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arXiv2002

Theory of magnetotunneling spectroscopy in spin triplet p-wave superconductors

We study the influence of a magnetic field $H$ on the zero-bias conductance peak (ZBCP) due to zero-energy Andreev bound state (ZES) in normal metal / unconventional superconductor. For p-wave junctions, ZBCP does not split into two by $H$ even for sufficiently low transparent junctions, where ZBCP clearly splits for d-wave. This unique property originates from the fact that for p-wave superconductors, perpendicularly injected quasiparticle form ZES, which contribute most dominantly on the tunneling conductance. In addition, we show that for $p_{x}$+i$p_{y}$-wave superconductor junctions, the height of ZBCP is sensitive to $H$ due to the formation of broken time reversal symmetry state. We propose that tunneling spectroscopy in the presence of magnetic field, $i.e.$, $magnetotunneling$, is an promising method to determine the pairing symmetry of unconventional superconductors.

cond-mat.supr-con↗

arXiv2002

Tomography of pairing symmetry from magnetotunneling spectroscopy -- a case study for quasi-1D organic superconductors

We propose that anisotropic $p$-, $d$-, or $f$-wave pairing symmetries can be distinguished from a tunneling spectroscopy in the presence of magnetic fields, which is exemplified here for a model organic superconductor ${(TMTSF)}_{2}X$. The shape of the Fermi surface (quasi-one-dimensional in this example) affects sensitively the pairing symmetry, which in turn affects the shape (U or V) of the gap along with the presence/absence of the zero-bias peak in the tunneling in a subtle manner. Yet, an application of a magnetic field enables us to identify the symmetry, which is interpreted as an effect of the Doppler shift in Andreev bound states.

cond-mat.supr-con↗

arXiv2001

Tunneling conductance of normal metal / $d_{x^{2}-y^{2}}$-wave superconductor junctions in the presence of broken time reversal symmetry states near interfaces

In order to clarify the influence of (the presence of) the broken time-reversal symmetry state (BTRSS) induced near the interface, tunneling conductance spectra in normal metal / $d_{x^{2}-y^{2}}$-wave superconductor junctions are calculated on the basis of the quasiclassical Green's function method. The spatial dependence of the pair potential in the superconductor side is determined self-consistently. We discuss two types of the symmetry on the BTRSS; i) $d_{x^{2}-y^{2}}$+is-wave state and ii) $d_{x^{2}-y^{2}}$+i$d_{xy}$-wave state. It is shown that the amplitude of the subdominant component (is-wave or i$d_{xy}$-wave) is quite sensitive to the transmission coefficient of the junction. As the results, the splitting of the zero-bias conductance peak due to the BTRSS inducement is detectable only at junctions with small transmission coefficients for both cases. When the transmission coefficients are relatively large, the explicit peak splitting does not occur and the difference in the two cases appears in the height of the zero-bias peaks.

cond-mat.supr-con↗

arXiv2001

Theoretical study of quasiparticle states near the surface of a quasi-one-dimensional organic superconductor $({TMTSF})_{2}{PF}_6$

Quasiparticle states near the surface of a quasi-one-dimensional organic superconductor $({TMTSF})_{2}{PF}_6$ are studied based on an extended Hubbard model on a quasi-one dimensional lattice at quarter-filling. Three types of pairing symmetries, (i) p-wave, (ii) d-wave, or (iii) $f$-wave are assumed. The resulting surface density of states has characteristic features for each pairing symmetry: i) a zero-energy peak (ZEP) in a U-shaped structure, ii) a V-shaped structure without ZEP, and iii) a ZEP in a V-shaped structure. From these results, we propose that the tunneling spectroscopy serves as a strong method to identify the the pairing symmetry in $({TMTSF})_{2}{PF}_6$.

cond-mat.supr-con↗

arXiv2001

Influence of impurity scattering on tunneling conductance in normal metal- d -wave superconductor junctions

Tunneling conductance spectra between a normal metal / d-wave superconductor junction under the presence of bulk impurities in the superconductor are studied. The quasiclassical theory has been applied to calculate the spatial variation of the pair potential and the effect of impurity scattering has been introduced by t-matrix approximation. The magnitude of a subdominant s-wave component at the interface is shown to robust against the impurity scattering while that for a subdominant $d_{xy}$-wave component is largely suppressed with the increase of the impurity scattering rate. The zero-bias conductance peak due to the zero-energy Andreev bound states is significantly broadened for the case of Born limit impurity compared with that of unitary limit impurity.

cond-mat.supr-con↗

arXiv1998

Theory of Local Density of States of d-Wave Superconducting State Near the Surfaces of the t-J Model

Spatial dependencies of the pair potential and the local density of states near the surfaces of $d_{x^{2}-y^{2}}$-wave superconductors are studied theoretically. The calculation is based on the t-J model within a mean-field theory with Gutzwiller approximation. Various types of surface geometries are considered. Similar to our result in the extended Hubbard model, it is found that the formation of zero-energy states strongly depends on the surface geometry. In addition to this feature, the zero-energy states give peak splitting for the (110) surfaces when the super-exchange interaction $J$ is large. This is due to the induced s-wave component near the surface. The present result explains the microscopic origin of the spontaneous time- reversal symmetry breaking at the surfaces of high-$T_{c}$ superconductors.

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