2009-present Lecturer, Osaka Prefecture University, Japan 2007-2008 CREST(JST) Postdoc Researcher, Japan Atomic Energy Agency, Japan 2006-2007 Postdoc Researcher, ETH-Zurich, Switzerland 2004-2006 JSPS Fellow for Research Abroad, ETH-Zurich, Switzerland 1999-2004 Research Associate, Computer Center, Okayama University, Japan 1999 Ph.D. (Physics), Okayama University, Japan 1998-1999 Research Fellow of the Japan Society for the Promotion of Science (JSPS), Japan
Abstract: The contribution of the vortex core has been taken into account properly in constructing a torque theory for multiband superconductors. We employ the prescription for describing the internal magnetic field in the vortex lattice by Hao et al. and by Yaouanc et al. to derive a torque formula as a natural extension of a preceding London theory. In marked contrast with the preceding model, our formula does not contain a phenomenological parameter \eta, which prevents us from obtaining a true upper critical field Hc2 by analyzing an experimental torque curve. The parameter \eta was originally introduced to take care of the uncertainty in determining the vortex core size \xi_v. Furthermore, we reveal that the \eta value is universally scaled by anisotropy \gamma, magnetic field B, and Hc2 due to field dependence of \xi_v. This may revitalize the single-band Kogan model in combination with a universal function \eta(\gamma,B,Hc2) instead of a constant \eta.
Abstract: Using the time-dependent Ginzburg-Landau equation with the complex relaxation time and the Maxwell equation, we systematically examine transverse motion of vortex dynamics in the presence of pinning disorders. Consequently, in the plastic flow phase in which moving and pinned vortices coexist, we find that the Hall voltage can generally change its sign. The origin of the sign change is ascribed to the fact that moving vortices are caused to strongly drift by the circular current of pinned vortices and the enforced transverse moving direction becomes opposite to that of the transport current. This suggests that the Hall sign change is a behavior common in all disordered type-II superconductors. In this paper, we discuss conditions to observe such an intrinsic effect and explain experimental results reported in the literature on the basis of this effect.
Abstract: We theoretically discuss the magnetic-field-angle dependence of the zero-energy density of states (ZEDOS) in superconductors. Point-node and line-node superconducting gaps on spherical and cylindrical Fermi surfaces are considered. The Doppler-shift (DS) method and the Kramer–Pesch approximation (KPA) are used to calculate the ZEDOS. Numerical results show that consequences of the DS method are corrected by the KPA.
Abstract: We briefly present three phenomenological theoretical works for superconducting properties in iron-based superconductors. The first is a suggestion of multi-band ±s-wave pairing model consistently explaining different measurement data, the second a proposal to detect ±s-wave signature, and the third an evaluation of the critical current in poly-crystalline samples by considering ±s-wave symmetry.
Abstract: The inhomogeneous 3-Kelvin (3K) phase of the eutectic Sr2RuO4 with Ru inclusions nucleates superconductivity at the interface between Ru and Sr2RuO4. The structure of the interface state and its physical properties are examined here. Two superconducting phases are identified between the transitions to the bulk phase at 1.5 K and to the 3K phase. The nucleation of the 3K phase results in a state conserving time reversal symmetry, which generates an intrinsically frustrated superconducting network in samples with many Ru inclusions. At a lower temperature (>1.5 K), a discontinuous (first order) transition to an interface state breaking time reversal symmetry is found leading to an unfrustrated network phase. It is shown that this phase transition located at a temperature between 1.5 and 3 K would yield the anomalous property showing that the critical current in such a network depends on the sign of the current, reproducing recent experimental observations.
Abstract: In order to consistently explain controversial experimental results on superconducting states observed by different probes in typical iron-based superconductors, we construct a realistic multiband s±-wave pairing model by combining the quasiclassical formalism with the first-principles calculation. The model successfully resolves the controversies in contrast to the fact that simplified models such as two-band s±-wave one fail to do. A key in the model is the existence of relatively small gaps which leads to material-dependent peculiarities.
Abstract: We demonstrate that a realistic multi-band model consistently explains the specific heat of typical iron-based superconductor. With density of states of each band obtained by first principle calculations, we evaluate multiple full-gap amplitudes from the angle resolved photoemission spectroscopy and successfully reproduce the specific heat. Consequently, it is found that the specific heat strongly depends density of states. In addition, the present calculations reveal that superconducting states of the iron-based pnictide are mainly characterized by two factors. One is an amplitude difference between large and small gaps, and the other is the distribution of their gap amplitudes.
Abstract: We discuss the surface Andreev bound states in Fe-based superconductors with the use of an effective five-band model and investigate the surface-angle dependence of the tunneling spectroscopy by a quasiclassical approach for an isotropic and an anisotropic ±s-wave gap superconductivity. We show that information on the normal state is important for the Andreev bound state and its peak positions do not depend on the gap amplitude anisotropy.
Abstract: The local density of states is studied theoretically in terms of the odd-frequency Cooper pairing induced around a vortex core. We find that a zero energy peak in the density of states at the vortex center is robust against nonmagnetic impurities in a chiral p-wave superconductor owing to an odd-frequency 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-frequency pair amplitudes inside vortex cores
Abstract: In order to explore a superconducting mechanism on iron-based superconductors, we numerically study a two-band minimal model considering two degenerate d_xz and d_yz orbitals on Fe atom. We perform exact diagonalization on a two-band and two-leg square ladder totally composed of 10 lattice sites, which is computationally equivalent to 4-leg 20-sites square-Hubbard-ladder. Consequently, we find that a robust pairing occurs in a wide parameter range when the intra-orbital repulsive interaction becomes smaller than the inter-orbital one. Moreover, the obtained binding energy can grow into much larger value than that obtained in the single band Hubbard model depending on the parameter range.
Abstract: We investigate the orientation of the vortex lattice driven by an applied current by means of numerical simulations based on the time-dependent Ginzburg–Landau (TDGL) theory. A lattice order is restored by a current driving of vortices under the influence of random vortex pinnings. The orientation of the moving vortex lattice is different between the presence and the absence of vortex pinnings. We show results of TDGL simulations for these phenomena.
Abstract: We perform first-principle phonon calculations for three typical iron-based superconductors, i.e., LaFeAsO,BaFe2As2, and FeSe. Though those crystals have different structures, we find that the optical modes associated with Fe vibration have almost similar characters. Moreover, we examine the pressure effect on phonons in FeSe. By increasing the external pressure, the phonon mode frequency related to Fe vibration effectively rises up and the electronic density of states at Fermi level also increases. These results may correlate to the critical temperature enhancement under high pressure.
Abstract: 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.
Abstract: 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.
Abstract: In order to resolve a discrepancy of the magnetic moment on Fe between the experimental and calculation results, we perform first-principle electronic structure calculations for iron-based superconductors LaFeAsO1-xFx in which x=0.0 and x=0.125 by using the LSDA + U framework. Consequently, we confirm in both the mother and doped compounds that negative U correction is crucial in matching the calculated magnetic moment with the observed one. A reason of the negative correction is that the Coulomb interaction on Fe orbitals is unexpectedly screened than LSDA’s expectation. We discuss which type of situation emerges when the negative U is a good correction in these compounds.
Abstract: By measuring the angular-oscillations 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 as a new method to analyze the field-angle-dependent experiments, which improves the previous Doppler-shift technique. We show that the Fermi-surface anisotropy is an indispensable factor for identifying the superconducting gap symmetry.
Abstract: Using the time-dependent Ginzburg-Landau equation with the complex relaxation rate and Maxwell equation, we perform the numerical simulation of vortex-pinning dynamics in two cases of metal-pinning and insulator-pinning. At the same time, the vortex-flow voltage induced by the vortex-motion and the Hall coefficient are calculated. Numerical simulations reveal that the trapped vortex at the pinning site disrupts the vortex-motion of non-trapped vortices and the Hall coefficient depends on that vortex-motion.
Abstract: We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al (2008 Phys. Rev. Lett. 101 087004) in Fe-based superconductors. We show that a fully gapped anisotropic ±s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like beta Fermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.
Abstract: We analyze the Josephson effect between a conventional and a non-centrosymmetric superconductor to examine characteristic features of such junctions and the symmetry of the superconducting phases. As a concrete example, we consider the non-centrosymmetric superconductor CePt3Si where Rashba spin–orbit coupling plays a crucial role and affects the Josephson pair tunneling. In this case, the Josephson coupling is composed of two parts, spin-singlet-like and spin-triplet-like components. The triplet-like component can lead to a Josephson coupling shifted by π relative to the singlet-like coupling. This has important implications on the interference effects and may explain some recent experimental results for the Al/CePt3Si junction.
Abstract: We study the Josephson effect between a conventional s-wave superconductor and a non-centrosymmetric superconductor with Rashba spin-orbit coupling. Rashba spin-orbit coupling affects the Josephson pair tunneling in a characteristic way. The Josephson coupling can be decomposed into two parts, a 'spin-singlet-like' and a 'spin-triplet-like' component. The latter component can lead to a shift of the Josephson phase by pi relative to the former coupling. This has important implications on interference effects and may explain some recent experimental results for the Al/CePt3Si junction. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy. Spatial profiles of superconductivity were obtained by conductance measurements at zero-bias voltage. Critical magnetic fields for vortex penetration and expulsion and for superconductivity breaking were measured for each island. The critical fields depending on the lateral size of the islands and existence of the minimum lateral size for vortex formation were observed.
Abstract: By means of numerical simulations based on Ginzburg–Landau theory, we study the vortex depinning from a columnar defect in a superconducting film. We evaluate the limiting thickness of the film, below which the depinning does not occur even under an application of the magnetic field perpendicular to the columnar defect. The limiting thickness is a measure of the pinning strength of the columnar defect. The dependence of this limiting thickness on the magnitude of the applied field is obtained for two types of columnar defects.
Abstract: We calculate the electronic structure for iron-based superconductors LaFeAsO1-xFx by using the LSDA+U framework. Consequently, we confirmed in the mother compound that the SDW and the orthorhombic structure becomes stable in a wide range of U. We show U dependence of the SDW magnetic moment and other properties. A highlight in this paper is that negative U correction is essential for matching the calculated magnetic moment with the observed one.
Abstract: To determine the superconducting gap function of YNi2B2C, we calculate the local density of states 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 scanning tunneling microscopy and spectroscopy 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.
Abstract: The tunneling characteristics of planar junctions between a normal metal and a noncentrosymmetric superconductor such as CePt3Si are examined. It is shown that the superconducting phase with mixed parity can give rise to characteristic zero-bias anomalies in certain junction directions. Andreev bound states at the interface are the origin of these zero-bias anomalies. The tunneling characteristics for different directions allow us to test the structure of the parity-mixed pairing state.
Abstract: We study charge transport in normal metal/s+p-wave superconductor (S+P) junctions with Rashba type spin - orbit coupling (RSOC). Applying the Blonder - Tinkham - Klapwijk theory, we calculate a bias (V) dependence of tunneling conductance, changing the strength of RSOC and Delta(p)/Delta(s). Here Delta(s(p)) is the absolute value of pair potential of S(P) component. For Delta(p)<Delta(s), a gap structure appears ranging from eV=0 to eV=vertical bar Delta(s) - Delta(p)vertical bar, and a coherent peak appears at eV=vertical bar Delta(s) + Delta(p)vertical bar. For Delta(p)>Delta(s), a zero bias conductance peak (ZBCP) is formed, a dip structure appears at vertical bar Delta(s) - Delta(p)vertical bar, and a weak coherent peak appears at eV=vertical bar Delta(s) + Delta(p)vertical bar. For large RSOC, a weak peak appears at eV=vertical bar Delta(s) - Delta(p)vertical bar because of RSOC. The bias dependence of conductance depends on the ratio Delta(p)/Delta(s) and changes qualitatively at Delta(p)/Delta(s)=1. For Delta(p)>Delta(s), vertical bar Delta(s) - Delta(p)vertical bar can be estimated from a width of ZBCP. For Delta s >Delta p, Delta s and Delta p can be estimated from the gap structure and the coherent peak. Thus, the magnitude of gaps can be determined from the experiment of scanning tunneling spectroscopy. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: Superconductivity in non-centrosymmetric materials can display various intriguing properties. Using the example of the CePt3Si a phenomenological description of this non-centrosymmetric superconductor is given, in an attempt to identify the symmetry of the Cooper pairing state. A short overview on other recently discovered non-centrosymmetric superconductors mainly, in strongly correlated electron systems, is given. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We briefly review the present status of our phenomenological study of superconductivity in non-centrosymmetric materials, with a strong focus on CePt3Si. The Anderson theorems are examined in the context of antisymmetric spin-orbit coupling. Then the behavior of superconductivity in high magnetic fields is discussed in particular in view of paramagnetic limiting and the presence of a so-called helical phase. Finally, various experimental data are discussed in order to identify the pairing symmetry as a parity-mixed "s-wave" phase.
Abstract: We numerically study the vortex core structure in a noncentrosymmetric superconductor such as CePt3Si without mirror symmetry about the xy plane. A single vortex along the z axis and a mixed singlet-triplet Cooper pairing model are considered. The spatial profiles of the pair potential, local density of states, supercurrent density, and radially-textured magnetic moment density around the vortex are obtained in the clean limit on the basis of the quasiclassical theory of superconductivity. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Noncentrosymmetric superconductors possess, in general, order parameters of mixed parity, i.e., the Cooper pairing state consists of spin-singlet and spin-triplet pairing components. We show that this property has important implications for the NMR and other measurable quantities in the heavy Fermion superconductor CePt3Si. The aspect of parity mixing explains the apparently contradicting observations of a Hebel-Slichter peak in the nuclear spin-lattice relaxation rate T-1(-1) and the presence of power law in the low-temperature behavior of certain physical quantities, indicating line nodes in the quasiparticle gap.
Abstract: For a noncentrosymmetric superconductor such as CePt3Si, we consider a Cooper pairing model with a two-component order parameter composed of spin-singlet and spin-triplet pairing components. We calculate the superfluid density tensor in the clean limit on the basis of the quasiclassical theory of superconductivity. We demonstrate that such a pairing model accounts for an experimentally observed feature of the temperature dependence of the London penetration depth in CePt3Si, i.e., line-node-gap behavior at low temperatures.
Abstract: We numerically study the vortex core structure in a noncentrosymmetric superconductor such as CePt3Si without mirror symmetry about the xy plane. A single vortex along the z axis and a mixed singlet-triplet Cooper pairing model are considered. The spatial profiles of the pair potential, local density of states, supercurrent density, and radially-textured magnetic moment density around the vortex are obtained in the clean limit on the basis of the quasiclassical theory of superconductivity. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: We numerically study the spatially resolved NMR around a single vortex in a noncentrosymmetric superconductor such as CePt3Si. The nuclear spin-lattice relaxation rate T-1(-1) under the influence of the vortex core states is calculated for an s + p-wave Cooper pairing state. The result is compared with that for an s-wave pairing state. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We investigate the order parameter of noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B via the behavior of the penetration depth lambda(T). The low-temperature penetration depth shows BCS-like behavior in Li2Pd3B, while in Li2Pt3B it follows a linear temperature dependence. We propose that broken inversion symmetry and the accompanying antisymmetric spin-orbit coupling, which admix spin-singlet and spin-triplet pairing, are responsible for this behavior. The triplet contribution is weak in Li2Pd3B, leading to a wholly open but anisotropic gap. The significantly larger spin-orbit coupling in Li2Pt3B allows the spin-triplet component to be larger in Li2Pt3B, producing line nodes in the energy gap as evidenced by the linear temperature dependence of lambda(T). The experimental data are in quantitative agreement with theory.
Abstract: 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 NbSe2 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 YNi2B2C within the quasiclassical theory of superconductivity.
Abstract: We numerically study the spatially resolved NMR around a single vortex in a noncentrosymmetric superconductor such as CePt3Si. The nuclear spin-lattice relaxation rate T-1(-1) under the influence of the vortex core states is calculated for an s + p-wave Cooper pairing state. The result is compared with that for an s-wave pairing state. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We analytically study electronic bound states around a single vortex core in a noncentrosymmetric superconductor such as CePt3Si 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 direction of the magnetic field. The LDOS under a magnetic field perpendicular to the c axis is quite different from that of typical s- or d-wave superconductors. From the ellipticity of the spatial pattern of the LDOS around a vortex core, one can experimentally estimate the pairing symmetry of CePt3Si, such as the position of the gap nodes and the ratio of the singlet component to the triplet component in the order parameter.
Abstract: The low-temperature shrinking of the vortex core (Kramer-Pesch effect) is studied for an isolated single vortex for chiral p-wave and s-wave superconducting phases. The effect of nonmagnetic impurities on the vortex core radius is numerically investigated in the Born limit by means of a quasiclassical approach. It is shown that in the chiral p-wave phase the Kramer-Pesch effect displays a certain robustness against impurities owing to a specific quantum effect, while the s-wave phase reacts more sensitively to impurity scattering. This suggests chiral p-wave superconductors as promising candidates for the experimental observation of the Kramer-Pesch effect.
Abstract: 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 T-1(-1) is theoretically studied inside the vortex core within the framework of the quasiclassical theory of superconductivity. T-1(-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 T-1(-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.
Abstract: We study pseudo-gap temperature T* of high-T-c superconductors by a Monte Carlo simulation of anisotropic 3D Josephson junction array (JJA) model based on the Ginzburg-Landau theory. We investigate T* both in the cases of zero external current and finite external current I in the JJA. It is found that, the external current I depresses only a little the pseudo-gap temperature T*, while the superconducting critical temperature T-c is much affected by I. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: The site-selective nuclear spin-lattice relaxation rate T-1(-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 T-1(-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 T-1(-1) between the two chiral states, (k) over bar (x) + i (k) over bar (y) and (k) over bar (x) - i (k) over bar (y) under the magnetic field. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: 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.
Abstract: We describe the essence of quantum effects inside s-wave and chiral p-wave vortices, without explicit use of quasiclassical Green function formalism. Physical quantities such as the impurity scattering rate and nuclear spin relaxation rate contain the coherence factor of the Andreev bound states in the matrix element of the transitions. The coherence factor of the Andreev bound state is different from that of a quasiparticle in bulk superconductors. Consequently, the physics within vortex core is different from that in normal state or that in spatially uniform superconducting state. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: 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. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: 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. 69 (2000) 3378] in the Born limit.
Abstract: The elementary vortex pinning potential is studied in a chiral p-wave superconductor with a pairing d=(z) over bar((k) over bar (x)+/-i (k) over bar (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.
Abstract: The boride compounds MBx 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 and B layers. In the viewpoint of the present model, the recently discovered 117-K superconductor C-60/CHBr3 is also discussed in terms of the intercalation molecules CHBr3 as possible capacitors among the superconducting grains of C-60 molecules. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The pair-potential and current, density around a single vortex of the two-dimensional chiral p-wave superconductor with d = (z) over cap (p(x) +/- ip(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 axe 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.
Abstract: The most popular description of superconductivity phenomenon in Sr2RuO4r is based on a so-called single-band (usually gamma -band) "isotropic p-wave order parameter". In a magnetic field parallel to the conducting planes, such triplet "isotropic p-wave phase" is not destroyed by the Clogston-Chaldrasekhar paramagnetic limiting field and can be destroyed only by the Meissner currents (i.e., the orbital effects). We analyze the orbital destructive effects against superconductivity for in-plane magnetic field (when electron orbits are open) and find that H-c2(//)(0) similar or equal to 0.75 T-c(dH(c2)(//)(T)/dT)(Tc) (which is a little bigger than the Werthamer-Helfand-Hohenberg value for an isotropic 3D case). We point out that the experimentally determined ratio H-c2(//)(0)/T-c(dH(c2)(//)(T)/dT)(Tc) similar or equal to 0.44 - 0.5 in Sr2RUO4 is significantly less than the calculated value 0.75. Since the upper critical field, H-c2(//)(T), is a well experimentally defined quantity in Sr2RUO4 (unlike high-T-c superconductors) we conclude that the single-band triplet "isotropic p-wave order parameter" seems to be inappropriate description of superconductivity in this material. Two possibilities are discussed: 1) Three-band nature of triplet superconductivity; 2) Singlet (d-wave) nature of superconducting pairing tin this case, the destructive actions of both the orbital effects and the Clogston-Chandrasekhar paramagnetic effects result in an agreement with the experimentally observed value of H-c2(//)(0)/Tc(dH(c2)(//)(T)/dT)(Tc)).
Abstract: The nondissipative transverse force acting an one moving vortex under the influence of another vortex is discussed in fermionic superfluid systems, where the relative velocity between the vortices is finite. On the basis of detailed numerical solutions of the Bogoliubov-de Gennes equation, the Berry phase for an adiabatic motion of the vortex line is examined for a two-vortex system. It is found that the detailed electronic structure of a vortex core can affect the transverse force, without abandoning the previous discussions of the robust Magnus force on a single vortex.
Abstract: Spatially inhomogeneous electron distribution around a single vortex is discussed on the basis of the Bogoliubov-de Gennes theory. The spatial structure and temperature dependence of the electron density around the vortex are presented. A relation between the vortex core charge and the vortex bound states (or the Caroli-de Gennes-Matricon states) is pointed out. Using the scanning tunneling microscope: information on the vortex core charge can he extracted through this relation.
Abstract: Focusing on a quantum-limit behavior, we study a single vortex in a clean s-wave type-II superconductor by self-consistently solving the Bogoliubov-de Gennes equation. The discrete energy levels of the vortex bound states in the quantum limit are discussed. The vortex core radius shrinks monotonically up to an atomic-scale length on lowering the temperature T, and the shrinkage stops to saturate at a lower T. The pair potential, supercurrent, and local density of states around the vortex exhibit Friedel-like oscillations. The local density of states has particle-hole asymmetry induced by the vortex. These are potentially observed directly by scanning tunneling microscopy.
Abstract: The local density of states (LDOS) in the triangular vortex lattice is investigated based on quasiclassical Eilenberger theory. We consider the case of an isotropic s-wave superconductor with the material parameter appropriate to NbSe2. At a weak magnetic field, the spatial Variation of the LDOS shows a 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 a sixfold star-shaped structure due to the vortex lattice effect. The orientation of the star coincides with the experimental data of scanning tunneling microscopy. That is, the ray of the star extends toward the nearest-neighbor (next-nearest-neighbor) vortex direction at higher (lower) energy.
Abstract: An isolated single vortex is considered within the framework of the quasiclassical theory. The local density of states around a vortex is calculated in a clean type-II superconductor with an anisotropy. The anisotropy of a superconducting energy gap is crucial for bound states around a vortex. A characteristic structure of the local density of states, observed in the layered hexagonal superconductor 2H-NbSe2 by scanning tunneling microscopy (STM), is well reproduced if one assumes an anisotropic s-wave gap in the hexagonal plane. The local density of states (or the bound states) around the vortex is interpreted in terms of quasiparticle trajectories to facilitate an understanding of the rich electronic structure observed in STM experiments. It is pointed out that further fine structures and extra peaks in the local density of states should be observed by STM.
Abstract: The vortex core structure in a d-wave superconductor is analyzed by solving the quasi-classical Eilenberger equation self-consistently. The pair function, current and magnetic field distributions around an isolated vortex are found to break circular symmetry and show four-fold symmetry, reflecting the internal degrees of freedom in d-wave pairing.
Abstract: The vortex structure of d(x2-y2)-wave superconductors is microscopically analyzed in the framework of the quasiclassical Eilenberger equations. If the pairing interaction contains an s-wave (d(xy)-wave) component in addition to a d(x2-y2)-wave component, the s-wave (d(xy)-wave) component of the order parameter is necessarily induced around a vortex in d(x2-y2)-wave superconductors. The spatial distribution of the induced s-wave and d(xy)-wave components is calculated. The s-wave component has an opposite winding number around the vortex near the d(x2-y2)-vortex core and its amplitude has the shape of a four-lobe clover. These are consistent with results based on the Ginzburg-Landau (GL) theory. The amplitude of the d(xy) component has the shape of an octofoil. The mixing of the d(xy) component cannot be explained by the GL theory, unless nonlocal correction terms are included.
Abstract: The vortex structure of pure d(x2-y2)-wave superconductors is microscopically analyzed in the framework of the quasiclassical Eilenberger equations. A self-consistent solution for the d-wave pair potential is obtained in the case of an isolated vortex. The vortex core structure, i.e., the pair potential, the supercurrent, and the magnetic field, is found to be fourfold symmetric even in the case that the mixing of the s-wave component is absent. The detailed temperature dependences of these quantities are calculated. The fourfold symmetry becomes clear when the temperature is decreased. The local density of states is calculated for the self-consistently obtained pair potential. From the results, we discuss the flow trajectory of the quasiparticles around a vortex, which is characteristic in d(x2-y2)-wave superconductors. The experimental relevance of our results to high-temperature superconductors is also given.
Abstract: The electronic structure of vortices in a type II superconductor is analyzed within the quasiclassical Eilenberger framework. The possible origin of a sixfold ''star'' shape of the local density of states, observed by scanning tunneling microscope (STM) experiments on NbSe2, is examined in the light of the three effects: the anisotropic pairing, the vortex lattice, and the anisotropic density of states at the Fermi surface. Outstanding features of split parallel rays of this star are well explained in terms of an anisotropic s-wave pairing. This reveals not only a rich internal electronic structure associated with a vortex core, but also unique ability of the STM spectroscopy.
Abstract: The vortex core structure in a d-wave superconductor is analyzed on the basis of the quasi-classical Eilenberger theory beyond the Ginzburg-Landau framework. The current and magnetic field distributions around an isolated vortex break the circular symmetry observed in s-wave pairing and show fourfold symmetry, reflecting, the internal degrees of freedom in d-wave pairing, i.e., (k) over cap(x)(2) - (k) over cap(y)(2) in reciprocal space through the low lying quasi-particle excitations. The peculiar orientation of the flux line lattice observed recently in a cuprate is discussed in light of the present theory.
Abstract: Electronic structure of a single vortex (or vortex core structure) in type-II superconductors is theoretically discussed in the present thesis.
Low-lying excited states in the superconductors due to the vortex, i.e., "vortex bound states," are examined in detail on the basis of numerical calculations.
Two points are focused on: the effect of superconducting gap anisotropy on a vortex and the property of a vortex in quantum-limit situation.
The electric charging of a vortex core is also discussed on the basis of the particle-hole asymmetry induced inside the vortex core.
