Upper critical fields of quasi-low-dimensional superconductors with coexisting singlet and triplet pairing interactions in parallel magnetic fields

Quasi-low-dimensional type II superconductors in parallel magnetic fields are studied when singlet pairing interactions and relatively weak triplet pairing interactions coexist. Singlet and triplet components of order parameter are mixed at high fields, and at the same time an inhomogeneous superconducting state called a Fulde-Ferrell-Larkin-Ovchinnikov state occurs. As a result, the triplet pairing interactions enhance the upper critical field of superconductivity remarkably even at temperatures far above the transition temperature of parallel spin pairing. It is found that the enhancement is very large even when the triplet pairing interactions are so weak that a high field phase of parallel spin pairing may not be observed in practice. A possible relvance of the result in organic superconductors and a hybrid-ruthenate-cuprate superconductor is discussed.Comment: 4 pages, 5 figures, (Latex, revtex.sty, epsf.sty

Effects of short-range correlations on the Coulomb screening and the pairing interactions in electron-phonon systems - triplet pairing mediated by phonons

Effects of short-range correlations on the Coulomb screening, the phonons, and the pairing interactions are examined in electron-phonon systems. First, we derive a model Hamiltonian of Coulomb interactions which includes both the long-range part v_q and the short-range part U. It is found from the expression of the dielectric function that the strong on-site correlations weaken the Coulomb screening. Secondly, we examine the screened phonons and the interaction mediated by phonons. In a consistent picture, we derive an expression of the effective interaction which includes (1) the screened Coulomb interactions, (2) the pairing interactions mediated by phonons, and (3) the effective interactions mediated by spin and charge fluctuations. It is rewritten in a form of a summation of (a) the effective interactions of the pure Hubbard model without the long-range Coulomb interactions, and (b) the phonon-mediated interactions plus screened Coulomb interactions with corrections due to both U and v_q. Thirdly, we derive an effective Hamiltonian analogous to the BCS Hamiltonian. Fourthly, for some typical values of parameters, we obtain the ground state phase diagrams. It is found that spin-triplet superconductivity mediated by phonons occurs when the short-range electron correlations are sufficiently strong, and the Coulomb screening is sufficiently weak. We estimate the orders of the transition temperatures when the triplet superconductivity occurs. The obtained values are realistic for existing candidates of the triplet superconductors as the order of the magnitudes. The possible relevance of the phonon-mediated interactions to the heavy fermion superconductor UPt_3 and the layered superconductors such (TMTSF)_2X and Sr_2RuO_4 are briefly discussed.Comment: 19 pages, 16 figures, jpsj2.cl

Fulde-Ferrell-Larkin-Ovchinnikov State in Perpendicular Magnetic Fields in Strongly Pauli-Limited Quasi-Two-Dimensional Superconductors

We examine the Fermi-surface effect called the nesting effect for the FFLO state in strongly Pauli-limited Q2D superconductors, focusing on the effect of 3D factors, such as interlayer electron transfer, interlayer pairing, and off-plane magnetic fields including those perpendicular to the most conductive layers. We examine the systems with a large Maki parameter so that the orbital pair-breaking effect is negligible, except for the locking of the direction of the FFLO vector q in the field direction.It is known that the nesting effect for the FFLO state can be strong in QLD systems in which the orbital pair-breaking effect is suppressed by applying the mag. field parallel to the layers. Hence, it has sometimes been suggested that the nesting effect may hardly enhance the stability of the FFLO state for perpendicular fields. We illustrate that, contrary to this view, the nesting effect can strongly stabilize the FFLO state for perpendicular fields as well as for parallel fields when tz is small so that the Fermi surfaces are open in the kz-direction, where tz denotes the interlayer transfer energy. In particular, the nesting effect in perpendicular fields can be strong in interlayer states. For example, in systems with cylindrical Fermi surfaces warped by tz /= 0, interlayer states with Dlt_k prop sin k_z exhibit mu_e Hc=1.65 Dlt_a0 for perpendicular fields, which is much larger than typical values for parallel fields, such as mu_e Hc=Dlt_s0 of the s-wave state and mu_e Hc = 1.28 Delta_d0 of the d-wave state in cylindrical systems with tz=0. The present result could potentially provide a physical reason why the areas in the phase diagrams occupied by the high-field phases for the perpendicular and parallel fields are of the same order in CeCoIn5 and FeSe.Comment: 7 pages, 2 figures, 3 table

Vestigial Van Hove singularity and higher-temperature superconducting phase induced by perpendicular uniaxial pressures in quasi-two-dimensional superconductors

We examine quasi-two-dimensional superconductors near half-filling under uniaxial pressures perpendicular to conductive layers (hereafter called perpendicular pressures). It is a natural conjecture that the perpendicular pressure decreases Tc because it increases the interlayer electron hopping energy t_z, which weakens the logarithmic enhancement in the density of states due to the two-dimensional Van Hove singularity. It is shown that, contrary to this conjecture, the perpendicular pressure can significantly enhance Tc in systems off half-filling before it decreases Tc, and the strength of the enhancement significantly depends on the pairing symmetry. When the indices d, d', cz, and sz are defined for the basis functions cos k_x - cos k_y, sin k_x sin k_y, cos k_z, and sin k_z, respectively, it is shown that for s-, d-, cz-, and cz-d-wave pairing, Tc steeply increases with increasing t_z near a cusp at a certain value of t_z. On the other hand, for p-, cz-p-, sz-p-, and d'-wave pairing, Tc is almost unaffected by tz. For sz- and sz-d-wave pairing, Tc exhibits a broad and weak peak. The enhancement in Tc is the largest for this state and the second largest for the d-wave pairing and interlayer singlet (cz-wave) pairing. These results may explain recent observations in Sr2RuO4 under perpendicular pressures. A comparison between the theoretical and experimental results indicates that the p-, cz-p-, and sz-p-wave states, including chiral states, and the d'-wave state are the most likely candidates for the intrinsic 1.5-K phase, and the d-, cz-d-, and cz-wave states are the most likely candidates for the 3-K phase induced by the perpendicular pressure. The cz-p- and sz-p-wave states are interlayer triplet and interlayer singlet p-wave states with horizontal line nodes, respectively.Comment: 5 pages, 6 figure

Superconductivity in a Ferromagnetic Layered Compound

We examine superconductivity in layered systems with large Fermi-surface splitting due to coexisting ferromagnetic layers. In particular, the hybrid ruthenate-cuprate compound RuSr_2GdCu_2O_8 is examined on the coexistence of the superconductivity and the ferromagnetism, which has been observed recently. We calculate critical fields of the superconductivity taking into account the Fulde-Ferrell-Larkin-Ovchinnikov state in a model with Fermi-surfaces which shapes are similar to those obtained by a band calculation. It is shown that the critical field is enhanced remarkably due to a Fermi-surface effect, and can be high enough to make the coexistence possible in a microscopic scale. We also clarify the direction of the spatial oscillation of the order parameter, which may be observed by scanning tunneling microscope experiments.Comment: 4 pages, 4 figures, (Latex, revtex.sty, epsf.sty

Antiferromagnetic superconductors with effective mass anisotropy in magnetic fields

We derive critical field H_c2 equations for antiferromagnetic \textit{s}-wave, d_{x^2-y^2}-wave, and d_{xy}-wave superconductors with effective mass anisotropy in three dimensions, where we take into account (i) the Jaccarino-Peter mechanism of magnetic-field-induced superconductivity (FISC) at high fields, (ii) an extended Jaccarino-Peter mechanism that reduces the Pauli paramagnetic pair-breaking effect at low fields where superconductivity and an antiferromagnetic long-range order with a canted spin structure coexist, and (iii) the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) state. As an example, experimental phase diagrams observed in organic superconductor kappa-(BETS)_2FeBr_4 are theoretically reproduced. In particular, the upper critical field of low-field superconductivity is well reproduced without any additional fitting parameter other than those determined from the critical field curves of the FISC at high fields. Therefore, the extended Jaccarino-Peter mechanism seems to occur actually in the present compound. It is predicted that the FFLO state does not occur in the FISC at high fields in contrast to the compound lambda-(BETS)_2FeCl_4, but it may occur in low-field superconductivity for s-wave and d_{x^2-y^2}-wave pairings. We also briefly discuss a possibility of compounds that exhibit unconventional behaviors of upper critical fields.Comment: 11 pages, 9 figures, revtex

arXiv:cond-mat/0407281 v2 21 Sep 2005 Interlayer spin-singlet pairing induced by magnetic interactions in an antiferromagnetic superconductor

It is shown that interlayer spin-singlet Cooper pairing is induced by magnetic interactions in a metallic antiferromagnet of stacked conductive layers in which each layer is ferromagnetically polarized and they order antiferromagnetically in stacking direction. As a result, the antiferromagnetic long-range order and superconductivity coexist at low temperatures. It is shown that TAF > Tc except for in a very limited parameter region unless TAF = 0, where TAF and Tc denote the antiferromagnetic and superconducting transition temperatures, respectively. It is found that the exchange field caused by the spontaneous staggered magnetization does not affect superconductivity at all, even if it is very large. The resultant superconducting order parameter has a horizontal line node, and is isotropic in spin space in spite of the anisotropy of the background magnetic order. We discuss the possible relevance of the present mechanism to the antiferromagnetic heavy fermion superconductors UPd2Al3 and CePt3Si. PACS numbers: 74.20.Mn, 74.20.Rp 74.25.Ha In this paper, we show that interlayer spin-singlet Cooper pairing is induced by magnetic interactions in a certain kind of metallic antiferromagnet. We consider a layered system of itinerant electrons in which each layer is ferromagnetically polarized but the majority-spin alternates in stacking direction. Therefore, the magetic order is characterized by the wave vector Q = (0, 0, π/c), where we have assumed the a and b crystal axes to be parallel to the layers, and the c-axis in the stacking direction, and c denotes the c-axis lattice constant. It is also shown that the exchange field caused by spontaneous staggered magnetization does not influence superconductivity, however large it is. The heavy fermion superconductors, such as UPd 2 Al 3 and CePt 3 Si, can be candidates of the present mechanism. The antiferromagnetic long-range order is considered to be characterized by the wave vector Q = (0, 0, π/c), both in UPd 2 The order parameter of interlayer spin-singlet pairing has a horizontal line node. This also agrees with the experimental results in the compound UPd 2 Al 3 . The existence of the line node is suggested by the nuclear magnetic resonance (NMR) measurement Coexistence of superconductivity and magnetism has been studied in various models by many authors The magnetic structure mentioned above can be modeled most simply by the Hamiltonian with the kinetic energy term the on-site Coulomb interactions and the exchange interactions We have defined S i = 1 2 σσ ′ c † iσ σ σσ ′ c iσ ′ , n i = σ n iσ , and n iσ = c † iσ c iσ , where σ denotes the vector of Pauli matrices, and c kσ and c iσ denote the electron operators. We define J ij = J > 0 for R j = R i ±ĉ, J ij = −J < 0 for nearest neighbor sites (i, j) on the same layer, an