Comment on "Spatial line nodes and fractional vortex pairs in the Fulde-Ferrell-Larkin-Ovchinnikov phase"
This is a Comment on Phys.Rev.Lett. 100, 017001 (2008).
cond-mat.supr-con↗
arXiv subjects
Ryusuke Ikeda
Publications and source records attributed to Ryusuke Ikeda.
Superfluid $^3$He in globally isotropic random media
Recent theoretical and experimental studies of superfluid $^3$He in aerogels with a global anisotropy, e.g., due to an external stress, have definitely shown that the A-like phase with an equal spin pairing (ESP) in such aerogel samples is in the ABM (or, axial) pairing state. In this paper, the A-like phase of superfluid $^3$He in globally {\it isotropic} aerogel is studied in details by assuming a weakly disordered system in which singular topological defects are absent. Through calculation of the free energy, a disordered ABM state is found to be the best candidate of the pairing state of the globally isotropic A-like phase. Further, it is found through a one-loop renormalization group calculation that the coreless continuous vortices (or, vortex-skyrmions) are irrelevant to the long-distance behavior of the disorder-induced textures, and that the superfluidity is maintained in spite of lack of the conventional off-diagonal long range order. Therefore, the globally isotropic A-like phase at weak disorder is, like in the case with a global stretched anisotropy, a superfluid glass with the ABM pairing.
cond-mat.supr-con↗
Modulated vortex states in Rashba non-centrosymmetric superconductors
Vortex lattice structures to occur in Rashba non-centrosymmetric superconductors under a magnetic field parallel to the basal plane are studied by assuming a singlet Cooper pairing and taking account of both the paramagnetic and orbital depairings. A vortex lattice of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type with modulation perpendicular to the field is expected to occur in the limit of vanishing ${\tilde η}$, where ${\tilde η}$ is the spin-orbit coupling {\it normalized} by Fermi energy. As ${\tilde η}$ increases, however, this state tends to be replaced by another new vortex lattice with a modulation of the gap amplitude parallel to the helical phase modulation induced by the absence of inversion symmetry. Differences of the present results from those in the Pauli limit and a correlation of a structural transition with the $H_{c2}(T)$-curve are discussed in relation to possible experimental realization.
cond-mat.supr-con↗
Orbital-limiting and modulation induced by missing parity in non-centrosymmetric superconductors
We examine the depairing field $H_{c2}(T)$ of non-centrosymmetric superconductors with a spin-orbit coupling larger than the Zeeman energy at $H_{c2}(0)$ by taking account of the mixing of spin-singlet and triplet pairing states due to the missing parity. When the singlet and triplet pairing components are mixed with an equal weight in a cubic non-centrosymmetric system, the paramagnetic depairing effect is significantly suppressed so that $H_{c2}(T)$ approaches its orbital-limited value. A similar event also occurs in a quasi two-dimensional Rashba non-centrosymmetric system. The present results are relevant to the $H$-$T$ phase diagrams of CePt$_3$Si and the families of Li$_2$Pd$_{3-x}$Pt$_x$B.
cond-mat.supr-con↗
Instability of square vortex lattice in d-wave superconductors is due to paramagnetic depairing
Effects of the paramagnetic depairing on structural transitions between vortex lattices of a quasi two dimensional d-wave superconductor are examined. It is found that, in systems with Maki parameter $α_{\rm M}$ of order unity, a square lattice induced by a d-wave pairing is destabilized with {\it increasing} fields, and that a reentrant rhombic lattice occurs in higher fields. Further, a weak Fermi surface anisotropy competitive with the pairing symmetry induces another structural transition near $H_{c2}$. These results are consistent with the structure changes of the vortex lattice in CeCoIn$_5$ in ${\bf H} \parallel c$ determined from recent neutron scattering data.
cond-mat.supr-con↗
High field superconducting phase diagrams including Fulde-Ferrell-Larkin-Ovchinnikov vortex states
Motivated by a striking observation of a Fulde-Ferell-Larkin-Ovchinnikov (FFLO) vortex state in the heavy fermion material CeCoIn5 in fields {\it perpendicular} to the superconducting planes (${\bf H} \parallel c$), superconducting phase diagrams including an FFLO state of quasi two-dimensional (Q2D) superconductors are systematically studied. In the clean {\it limit}, the high field superconducting state in the low temperature limit should be not the FFLO state modulating along ${\bf H}$, appeared in CeCoIn5 in both ${\bf H} \parallel c$ and ${\bf H} \perp c$, but a different vortex state with a modulation, induced by the paramagnetism, perpendicular to the field. It is found that the presence of weak impurities is the origin of the absence in CeCoIn5 of the latter state and leads to the ${\bf H} \parallel c$ phase diagram, as seen in CeCoIn5, {\it apparently} different in character from that in ${\bf H} \perp c$.
cond-mat.supr-con↗
Strong Coupling Correction in Superfluid $^3$He in Aerogel
Effects of impurity scatterings on the strong coupling (SC) contribution, stabilizing the ABM (axial) pairing state, to the quartic term of the Ginzburg-Landau (GL) free energy of superfluid $^3$He are theoretically studied to examine recent observations suggestive of an anomalously small SC effect in superfluid $^3$He in aerogels. To study the SC corrections, two approaches are used. One is based on a perturbation in the short-range repulsive interaction, and the other is a phenomenological approach used previously for the bulk liquid by Sauls and Serene [Phys.Rev.B 24, 183 (1981)]. It is found that the impurity scattering favors the BW pairing state and shrinks the region of the ABM pairing state in the T-P phase diagram. In the phenomenological approach, the resulting shrinkage of the ABM region is especially substantial and, if assuming an anisotropy over a large scale in aerogel, leads to justifying the phase diagrams determined experimentally.
cond-mat.supr-con↗
Josephson vortex lattices with modulation perpendicular to an in-plane magnetic field in layered superconductor
In quasi low dimensional superconductors under {\it parallel} magnetic fields applied along a conducting direction, vortex lattices with a modulation of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type {\it perpendicular} to the field may occur due to an enhanced paramagnetic depairing. As the strength of an in-plane field is varied in a Q2D material, the Josephson vortex lattices accompanied by nodal planes are formed in higher Landau level (LL) modes of the superconducting (SC) order parameter and show field-induced structural transitions. A change of orientation of nodal planes induced by these transitions should be observed in transport measurements for an out-of-plane current in real superconductors with point disorder effective on the SC layers. Further, the $H_{c2}$-transition from this higher LL state to the normal phase is of second order for moderately strong paramagnetic effects but, in the case with a strong enough paramagnetic effect, becomes discontinuous as well as the transition between this modulated state and an ordinary Abrikosov vortex lattice in the lowest LL. Relevance of these results to recent observations in organic superconductors suggesting the presence of an FFLO state are discussed.
cond-mat.supr-con↗
Vortex tilt modulus in Fulde-Ferrell-Larkin-Ovchinnikov state
Vortex tilt response in Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice is theoretically examined as a probe reflecting the spatial structure of this state. In the FFLO state with nodal planes perpendicular to the magnetic field in a quasi 2D material under a parallel field, the tilt modulus E_{2} {\it of the nodal planes} decreases as the paramagnetic effect is effectively enhanced, and this reduction of E_{2} in turn reduces the vortex tilt modulus. This reduction of vortex tilt modulus, more remarkable in more anisotropic systems, accompanying the FFLO transition may be an origin of the monotonous reduction of sound velocity detected upon cooling in a ultrasound measurement for CeCoIn5.
cond-mat.supr-con↗
Effect of in-plane line defects on field-tuned superconductor-insulator transition behavior in homogeneous thin film
Field-tuned superconductor-insulator transition (FSIT) behavior in 2D isotropic and homogeneous thin films is usually accompanied by a nonvanishing critical resistance at low $T$. It is shown that, in a 2D film including line defects paralle to each other but with random positions perpendicular to them, the (apparent) critical resistance in low $T$ limit vanishes, as in the 1D quantum superconducting (SC) transition, under a current parallel to the line defects. This 1D-like critical resistive behavior is more clearly seen in systems with weaker point disorder and may be useful in clarifying whether the true origin of FSIT behavior in the parent superconductor is the glass fluctuation or the quantum SC fluctuation. As a by-product of the present calculation, it is also pointed out that, in 2D films with line-like defects with a long but {\it finite} correlation length parallel to the lines, a quantum metallic behavior intervening the insulating and SC ones appears in the resistivity curves.
cond-mat.supr-con↗
Stability of vortex state of Fulde-Ferrell-Larkin-Ovchinnikov type in magnetic fields perpendicular to superconducting layers
Motivated by recent NMR data suggesting the presence of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice in magnetic fields perpendicular to the superconducting plane in the heavy-fermion superconductor CeCoIn5, we examine here the stability of FFLO states in quasi two-dimensional (2D) superconductors under a perpendicular field. It is found that a 3D-like portion of Fermi surfaces peculiar to CeCoIn5 stabilizes a FFLO vortex state modulating along the field parallel to the c-axis and that the direction of modulation in a FFLO vortex lattice is related to the characters of the normal to FFLO and the Abrikosov to FFLO transitions.
cond-mat.supr-con↗
Pairing States of Superfluid 3He in Uniaxially Anisotropic Aerogel
Stable pairing states of superfluid 3He in aerogel are examined in the case with a global uniaxial anisotropy which may be created by applying an uniaxial stress to the aerogel. Due to such a global anisotropy, the stability region of an ABM pairing state becomes wider. In an uniaxially stretched aerogel, the pure polar pairing state with a horizontal line node is predicted to occur, as a 3d superfluid phase, over a measurable width just below the superfluid transition at T_c(P). A possible relevance of the present results to the case with no global anisotropy is also discussed.
cond-mat.dis-nn↗
Superconducting transition in disordered granular superconductors in magnetic fields
Motivated by a recent argument that the superconducting (SC) transition field of three-dimensional (3D) disordered superconductors with granular structure in a nonzero magnetic field should lie above $H_{c2}(0)$ in low $T$ limit, the glass transition (or, in 2D, crossover) curve $H_g(T)$ of disordered quantum Josephson junction arrays is examined by incorporating SC fluctuations. It is found that the glass transition or crossover in the granular materials can be described on the same footing as the vortex-glass (VG) transition in amorphous-like (i.e., nongranular) materials. In most of 3D granular systems, the vanishing of resistivity upon cooling should occur even above $H_{c2}(0)$, while the corresponding sharp drop of the resistivity in 2D case may appear only below $H_{c2}$ as a result of an enhanced quantum fluctuation.
cond-mat.supr-con↗
Comment on "Fluctuation-Driven First-Order Transition in Pauli-Limited $d$-Wave Superconductors"
This is a Comment on a recent Letter (Phys.Rev.Lett.93, 247002 (2004)) written by Dalidovich and Yang. It is pointed out that any Pauli-limited approach neglecting the vortices is inapplicable to describing the FFLO state in CeCoIn5.
cond-mat.supr-con↗
Equal-Spin Pairing State of Superfluid $^3$He in Aerogel
The equal-spin pairing (ESP) state, the so-called A-like phase, of superfluid $^3$He in aerogels is studied theoretically in the Ginzburg-Landau (GL) region by examining thermodynamics, and the resulting equilibrium phase diagram is mapped out. We find that, among the ABM, planar, and robust pairing states, the ABM state with presumably quasi long-ranged superfluid order is the best candidate of the A-like phase with a strange lowering of the polycritical point (PCP) observed experimentally.
cond-mat.supr-con↗
Suppression of the vortex glass transition due to correlated defects with a persistent direction perpendicular to an applied magnetic field
It is found on the basis of the lowest Landau level approach for the Ginzburg-Landau model that, in bulk type II superconductors with strong line disorder directed {\it perpendicularly} to an applied field, the continuous vortex-glass transition is depressed to the low $T$ limit in the limit of weak {\it point} disorder. An anomalous resistive broadening in twin-free YBCO with columnar defects in a field parallel to the layers is discussed based on this theoretical finding. Other phenomena which, we argue, arise indirectly from this mechanism in type II superconductors including correlated defects are also discussed.
cond-mat.supr-con↗
Modulated Vortex Lattice in High Fields and Gap Nodes
The mean field vortex phase diagram of a quasi two dimensional superconductor with a nodal d-wave pairing and with strong Pauli spin depairing is examined in the parallel field case in order to examine an effect of gap nodes (or minima) on the stability of a FFLO-like vortex lattice. We find through a heuristic argument and a model calculation with a four-fold anisotropic Fermi velocity that the FFLO-like state is relatively suppressed as the field approaches a nodal direction. When taking account of a couple of experimental results altogether, the present result strongly suggests that a node or gap minimum of the pairing function of CeCoIn$_5$ is located along the [100]-direction.
cond-mat.supr-con↗
Theoretical Description of Nearly Discontinuous Transition in Superconductors with Paramagnetic Depairing
Based on a theoretical argument and Monte Carlo simulations of a Ginzburg-Landau model derived microscopically, it is argued that, in type-II superconductors where {\it both} the paramagnetic {\it and} orbital depairings are important, a strong first-order transition (FOT) at $H_{c2}$ expected in the mean field (MF) approximation never occurs in real systems and changes due to the fluctuation into a crossover. The present result explains why a {\it nearly} discontinuous crossover at $H_{c2}$ with {\it no} intrinsic hysteresis is observed only in a clean superconducting material with a singlet pairing and a high condensation energy such as CeCoIn$_5$.
cond-mat.supr-con↗