arXiv subjects

Ryusuke Ikeda

Publications and source records attributed to Ryusuke Ikeda.

At least 37 records · Page 2Linked to original sources

arXiv2015

Quasiclassical analysis of vortex lattice states in Rashba noncentrosymmetric superconductors

Vortex lattice states occurring in noncentrosymmetric superconductors with the spin-orbit coupling of Rashba type under a magnetic field parallel to the symmetry plane are examined by assuming the $s$-wave pairing case and in an approach combining the quasiclassical theory with the Landau level expansion of the superconducting order parameter. The resulting field-temperature phase diagrams include not only a discontinuous transition but a continuous crossover between different vortex lattice structures, and, further, a critical end point of a structural transition line is found in an intermediate field and at a low temperature in the present approach. It is pointed out that the strange field dependence of the vortex lattice structure is a consequence of that of an anisotropy stemming from the Rashba spin-orbit coupling, and that the critical end point is related to the helical phase modulation peculiar to these materials in the ideal Pauli-limited case. Further, calculation results on the local density of states detectable in STM experiments are also presented.

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arXiv2015

Possible Field-Temperature Phase Diagrams of Two-Band Superconductors with Paramagnetic Pair-Breaking

Possible field-temperature superconducting (SC) phase diagrams in two-band quasi-two-dimensional materials with a strong paramagnetic pair-breaking (PPB) are considered theoretically. Attention is paid to the case under a magnetic field $perpendicular$ to the SC layers and to essential differences from the counterpart in the ordinary single-band material. It is found by examining the $H_{c2}(T)$ curve and the vortex lattices to be realized close to $H_{c2}$ that a PPB-induced SC phase with a spatial modulation parallel to the field tends to occur more easily than in the single-band case, and that a crisscrossing vortex lattice proposed previously can occur in place of the conventional Fulde-Ferrell-Larkin-Ovchinnikov state within a parameter range. The relevance of the obtained results to FeSe and other iron-based superconductors is discussed.

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arXiv2015

Antiferromagnetic Order oriented by Fulde-Ferrell-Larkin-Ovchinnikov Superconducting Order

Resolving the high-field superconducting phase (HFSP), often called as the Q-phase, and the antiferromagnetic or spin-density-wave (SDW) order appearing in the phase remains a crucial issue on the superconductor CeCoIn$_5$. It is shown that a switching of the SDW domain due to a tiny change of the magnetic field direction in HFSP, reported and interpreted as an evidence of the presence of a $π$-triplet pairing inducing the SDW order [S. Gerber et al., Nature Physics 10, 126 (2014)], can be explained with no triplet pairing component if the $d$-wave superconducting order in HFSP includes the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulation parallel to the field. This result corroborates the picture that HFSP of CeCoIn$_5$ and the SDW order found only in the phase are consequences of the strong paramagnetic pair-breaking in this $d$-wave superconductor.

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arXiv2015

Anisotropic Strong Coupling Effects on Superfluid 3He in Aerogels - Conventional Spin-Fluctuation Approach -

Motivated by recent experiments on liquid $^3$He reporting emergence of novel superfluid phases in globally anisotropic aerogels, our previous theory on superfluid $^3$He in globally anisotropic aerogels is extended to incorporate effects of an anisotropy of the quasiparticle scattering cross section on the strong coupling (SC) contributions to the Ginzburg-Landau (GL) free energy on the basis of the spin-fluctuation (paramagnon) approach to the SC contributions developed by Brinkman et al. [Phys. Rev. A {\bf 10}, 2386 (1974)]. In the globally isotropic case, impurity effects on the SC correction destabilize the A-phase even at higher pressures of about 30 (bar) and make the B-phase the only state in equilibrium, while SC contributions accompanied by a global stretched anisotropy to the GL quartic terms generally tend to broaden the stability region of the A-phase compared with that of the B-phase. In particular, in contrast to the cases in bulk and in the isotropic aerogel, the SC corrections to the GL {\it quadratic} terms are not negligible in the globally anisotropic case but may change the sign of the apparent anisotropy depending on the magnitude of the frequency cutoff of the normal paramagnon propagator. Based on this sign change of the apparent anisotropy, we discuss different strange observations on superfluid $^3$He in porous media such as disappearance of the polar superfluid phase at higher pressures seen in nematically-ordered aerogels and absence of the B-phase and of the A-phase with planar ${\hat {\bf l}}$-vector in a stretched aerogel.

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arXiv2013

Strong-coupling approach to antiferromagnetic ordering driven by paramagnetic pair-breaking in d-wave superconducting phase

The field-induced antiferromagnetic (AFM) ordering in the $d_{x^2-y^2}$-paired superconducting phase, which has been recently found in the weak-coupling approach as a basic mechanism due to the Pauli paramagnetic pair-breaking (PPB) in relation to the high-field behaviors in CeCoIn_5, is studied in the strong-coupling approach taking account of the electron correlation. Applying the fluctuation-exchange (FLEX) approximation to the two-dimensional Hubbard model including the Zeeman term, it is shown that the PPB-induced AFM ordering in the superconducting (SC) phase and the first order SC transition on H_{c2}(T) are realized in the strong-coupling approach as well as those in the weak-coupling model, and that the AFM ordering is affected by the quasiparticle renormalization and the amplitude of the SC order parameter. This AFM ordering may appear in a wide range of materials close to an AFM quantum critical point (QCP).

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arXiv2013

Angular dependence of antiferromagnetic order induced by paramagnetism in d-wave superconductor

Antiferromagnetic (AFM) order and a spatial order peculiar to Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, previously indicated in the quasi two-dimensional d-wave superconductors CeCoIn_5 with strong paramagnetic pair breaking (PPB) in a magnetic-field parallel to the basal plane, are considered in the field configurations tilted from the basal plane within an approach assuming that the wavelength of the FFLO modulation is relatively long. It is demonstrated that, with increasing the tilt angle, both the AFM and FFLO orders are gradually suppressed, and that disappearance of the AFM order in zero temperature limit occurs at a lower angle than that of the FFLO state. Consequently, a nonmagnetic FFLO-ordered high field SC phase is realized in an intermediate range of the tilt angle even at low enough temperatures. As the perpendicular field configuration (${\bf H} \parallel c$) is approached by the field-tilt, the AFM order in real space is found close to the FFLO nodal planes in contrast to the high field behavior in ${\bf H} \perp c$ case. Further, in the field v.s. temperature (H-T) phase diagram, the AFM order reduces, at a higher angle, to an AFM quantum critical point (QCP) lying at a lower field than H_{c2}(0) as a consequence of competition between the field dependences of the nesting condition and of PPB. These features of the AFM order and the resulting H-T phase diagram strikingly coincide with those seen in a recent NMR measurement on CeCoIn_5 in tilted field configurations.

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arXiv2011

Fluctuation and Order of Antiferromagnetism induced by Paramagnetic Pair-Breaking in Superconducting Vortex Lattice

Effects of the strong Pauli-paramagnetic pair-breaking (PPB) on the vortex lattice in d-wave superconductors are theoretically studied by putting emphasis on consequences of the PPB-induced antiferromagnetic (AFM) ordering in the spatial modulation in the vortex lattice. It is shown that the PPB-induced AFM fluctuation in the superconducting state leads to an enhancement of the vortex lattice form factor which is a measure of spatial variations of the internal magnetic field and that the enhancement becomes more remarkable as an AFM instability is approached. It is also demonstrated that the PPB-induced AFM ordering is assisted by the vortex-lattice modulation, and thus, that the resulting AFM order is spatially modulated, while it is not localized in the vortex cores but coexistent with the nonvanishing superconducting order parameter. These results are discussed in connection with two phenomena observed in CeCoIn5, the anomalous field dependence of the vortex lattice form factor and the AFM order appearing inside the high-field and low-temperature superconducting phase.

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arXiv2011

Emergent Antiferromagnetism in D-wave Superconductor with Strong Paramagnetic Pair-Breaking

It is theoretically shown that, in the four-fold symmetric d-wave superconducting phase, a paramagnetic pair-breaking (PPB) enhanced sufficiently by increasing the applied magnetic field induces not only the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state but also an incommensurate antiferromagnetic (AFM) order with Q-vector parallel to a gap node. This AFM ordering tends to occur only below H_{c2} at low temperatures, i.e., in the presence of a nonvanishing superconducting energy gap $Δ$ rather than in the normal phase. Through a detailed study on the resulting AFM order and its interplay with the FFLO spatial modulation of $Δ$, it is argued that the strange high field and low temperature (HFLT) superconducting phase of CeCoIn_5 is a coexisting phase of the FFLO and incommensurate AFM orders, and that this PPB mechanism of an AFM ordering is also the origin of the AFM quantum critical fluctuation which has occurred close to H_{c2}(0) in several unconventional superconductors including CeCoIn_5.

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arXiv2011

Vortex-lattice melting in two-dimensional superconductors in intermediate fields

To examine the field dependence of the vortex lattice melting transition in two-dimensional (2D) superconductors, Monte Carlo simulations of the 2D Ginzburg-Landau (GL) model are performed by extending the conventional lowest Landau level (LL) approximation to include several {\it higher} LL modes of the superconducting order parameter with LL indices up to six. It is found that a nearly vertical melting line in lower fields, which is familiar within the elastic theory, is reached just by including higher LL modes with LL indices less than five, and that the first order character of the melting transition in higher fields is significantly weakened with decreasing the field. Nevertheless, a genuine crossover to the consecutive continuous melting picture intervened by a hexatic liquid is not found within the use of the GL model.

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arXiv2010

Diamagnetism above Tc in underdoped Bi2.2Sr1.8Ca2Cu3O10+d

Single crystals of ${\rm Bi}_{2+x}{\rm Sr}_{2-x}{\rm Ca}_{2}{\rm Cu}_{3}{\rm O}_{10+δ}$(Bi2223) with $x=0.2$ were grown by a traveling solvent floating zone method in order to investigate the superconducting properties of highly underdoped Bi2223.Grown crystals were characterized by X-ray diffraction, DC susceptibility and resistivity measurements, confirming Bi2223 to be the main phase.The crystals were annealed under various oxygen partial pressures to adjust their carrier densities from optimally doped to highly underdoped.The fluctuation diamagnetic component above the superconducting transition temperature $T_{\rm c}$ extracted from the anisotropic normal state susceptibilities $χ_{ab}(T)$ ($H\perp c$) and $χ_{c}(T)$ ($H\parallel c$) was found to increase with underdoping, suggesting a decrease in the superconducting dimensionality and/or increase in the fluctuating vortex liquid region.

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arXiv2010

Vortex core transitions in superfluid 3He in globally anisotropic aerogels

Core structures of a single vortex in A-like and B-like phases of superfluid 3He in uniaxially compressed and stretched aerogels are studied by numerically solving Ginzburg-Landau equations derived microscopically. It is found that, although any uniaxial deformation leads to a wider A-like phase with the axial pairing in the pressure-temperature phase diagram, the vortex core states in the two phases in aerogel depend highly on the type of deformation. In a compressed aerogel, the first-order vortex core transition (VCT) previously seen in the bulk B phase appears at any pressure in the B-like phase while no strange vortex core is expected in the corresponding A-like phase. By contrast, in a stretched aerogel, the VCT in the B-like phase is lost while another VCT is expected to occur between a nonunitary core and a polar one in the A-like phase. Experimental search for these results is hoped to understand correlation between superfluid 3He and aerogel structure.

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arXiv2010

Anisotropy-induced Vortex Core Transitions in Superfluid 3He in Aerogel

Core structures of a single vortex in A-like and B-like phases of superfluid 3He in uniaxially compressed and stretched aerogels are studied by numerically solving Ginzburg-Landau equations derived microscopically. It is found that, although the A-like phase with axial pairing is extended by any uniaxial deformation, the vortex core states in the two phases depend highly on the type of deformation in aerogel: In a compressed aerogel, the first order vortex core transition (VCT) previously seen in the bulk B phase appears at any pressure in B-like phase, while no strange vortex core is expected in A-like phase. By contrast, in a stretched aerogel, the VCT in B-like phase is lost, while another VCT is expected to occur between a nonunitary core and the polar one in A-like phase. An experimental search for these results is hoped to understand correlation between superfluid $^3$He and aerogel structures.

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arXiv2010

Coexistence of d-wave superconductivity and antiferromagnetism induced by paramagnetic depairing

It is shown theoretically that, in the superconducting state with $d_{x^2-y^2}$-pairing, a strong Pauli paramagnetic depairing (PD) induces not only the modulated Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state but also an incommensurate antiferromagnetic (AFM) (or spin-density wave) ordering with ${\bf Q}$-vector nearly paralle to a gap node. This intrinsic mechanism of field-induced coexistence of the $d$-wave superconducting and AFM orders does not need to assume a pair-density wave but is enhanced by the FFLO ordering. It is argued that this is the common origin of {\it both} the coexistent FFLO and AFM phases of CeCoIn$_5$ {\it and} the AFM quantum critical behavior around the superconducting $H_{c2}(0)$ seen in several heavy fermion superconductors and overdoped cuprates.

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arXiv2010

Antiferromagnetic ordering induced by paramagnetic depairing in unconventional superconductors

Antiferromagnetic (AFM) (or spin-density wave) quantum critical fluctuation enhanced just below H_c2(0) have been often observed in d-wave superconductors with a strong Pauli paramagnetic depairing (PD) including CeCoIn_5. It is shown here that such a tendency of field-induced AFM ordering is a consequence of strong PD and should appear particularly in superconductors with a gap node along the AFM modulation. Two phenomena seen in CeCoIn_5, the anomalous vortex lattice form factor and the AFM order in the Fulde-Ferrell-Larkin-Ovchinnikov state, are explained based on this peculiar PD effect.

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arXiv2010

Impurity-induced broadening of the transition to a Fulde-Ferrell-Larkin-Ovchinnikov phase

Recent study on doping effects in the heavy fermion superconductor CeCoIn$_5$ has shown that a small amount of doping induces unexpectedly large broadening of the second order transition into the high field and low temperature (HFLT) phase of this material. Motivated by this observation, effects of quenched disorder on the second order transition into a longitudinal Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state are examined. The observed large broadening of the transition is naturally explained as a consequence of softness of each FFLO nodal plane. The present results strongly support the scenario identifying the HFLT phase of CeCoIn$_5$ with a longitudinal FFLO vortex state.

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arXiv2010

Roles of Antiferromagnetic Fluctuation in High Field Phase Diagram of Superconductors with Strong Paramagnetic Depairing

The high field phase diagram and magnetic properties of CeCoIn_5 below H_c2(0) are examined from the picture regarding the high field and low temperature (HFLT) phase as a possible Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice. Crucial roles of antiferromagnetic (AFM) fluctuations enhanced close to H_c2(0) are stressed. The FFLO vortex lattice with a longitudinal modulation parallel to the field is stabilized compared with those with lateral modulations as a consequence of the presence of AFM fluctuations. Further, an unusual field-induced enhancement of the flux distribution is argued to be a consequence of interplay between the paramagnetic depairing and AFM fluctuation, both of which are enhanced with increasing field.

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arXiv2009

Vortex lattice structure dependent on pairing symmetry in Rashba superconductors

Vortex lattice structures in Rashba noncentrosymmetric superconductors in magnetic fields parallel to the basal plane (${\bf H} \perp c$) are examined based on the BCS-like Hamiltonian and the resulting Ginzburg-Landau functional. Due to the momentum dependent anisotropy of the Zeeman effect induced by the broken inversion symmetry, the vortex lattice in higher fields generally shows some unidirectional modulation of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type orienting in the plane perpendicular to ${\bf H}$. However, the direction of the modulation and the lattice structure depend significantly on the underlying pairing symmetry: When the mixing between spin singlet and triplet pairing components is negligible, the resulting modulated structure tends to have reflection symmetry, while the vortex lattice in systems with a significant singlet-triplet mixing has no reflection symmetry in most cases. The latter result implying the presence in {\it real} materials of two degenerate orientations of the lattice structure separated by domain walls may be relevant to the extremely low magnetic decay rate observed in CePt$_3$Si.

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arXiv2009

Conductivity in glass phases of disordered granular superconductors in magnetic fields

The electric conductivities in glass phases of three-dimensional (3D) granular superconductors in magnetic fields are examined based on a quantum disordered Josephson-junction array. A correct inclusion of the Ohmic dissipative dynamics leads to glass phases with divergent dc conductivity. In contrast, with no dissipative term, a metallic response in a glass phase is obtained irrespective of the range of correlation of quenched disorder, i.e., even in the so-called Bose-glass phase with experimentally vanishing resistivity.

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