Paramagnetic Pair-breaking Effect Research Articles

Upper critical field and its anisotropy in RbCr3As3

The temperature dependence of the upper critical field $({H}_{c2})$ in ${\mathrm{RbCr}}_{3}{\mathrm{As}}_{3}$ single crystals $({T}_{c}\ensuremath{\approx}7.3$ K) has been determined by means of magnetoresistance measurements with temperature down to 0.35 K in static magnetic fields up to 38 T. The magnetic field was applied both for directions parallel $(H\ensuremath{\parallel}c,{H}_{c2}^{\ensuremath{\parallel}c})$ and perpendicular $(H\ensuremath{\perp}c,{H}_{c2}^{\ensuremath{\perp}c})$ to the Cr chains. The curves ${H}_{c2}^{\ensuremath{\parallel}c}(T)$ and ${H}_{c2}^{\ensuremath{\perp}c}(T)$ cross at $\ensuremath{\sim}5.5$ K. As a result, the anisotropy parameter $\ensuremath{\gamma}(T)={H}_{c2}^{\ensuremath{\perp}c}/{H}_{c2}^{\ensuremath{\parallel}c}(T)$ increases from 0.5 near ${T}_{c}$ to 1.6 at low temperature. Fitting with the Werthamer-Helfand-Hohenberg (WHH) model yields zero-temperature critical fields of ${\ensuremath{\mu}}_{0}{H}_{c2}^{\ensuremath{\parallel}c}(0)\ensuremath{\approx}27.2$ T and ${\ensuremath{\mu}}_{0}{H}_{c2}^{\ensuremath{\perp}c}(0)\ensuremath{\approx}43.4$ T, both exceeding the BCS weak-coupling Pauli limit ${\ensuremath{\mu}}_{0}{H}_{p}=1.84{T}_{c}=13.4$ T. The results indicate that the paramagnetic pair-breaking effect is strong for $H\ensuremath{\parallel}c$ but absent for $H\ensuremath{\perp}c$, which was further confirmed by the angle dependent magnetoresistance and ${H}_{c2}$ measurements.

Paramagnetic Pair-Breaking in Spin-Triplet Superconductors with Spin–Orbit Coupling: Application to Sr2RuO4

Paramagnetic pair-breaking effects by in-plane magnetic fields are studied in the presence of the spin–orbit coupling, based on the tight-binding three-orbital model for Sr2RuO4. The spin–orbit cou...

Coupled k-space structure ofd-wave superconducting and magnetic orders induced by paramagnetic pair-breaking effect

We theoretically investigate k-space structures of $d_{x^2-y^2}$-wave superconducting (SC) and spin-density-wave (SDW) orders in their coexistent phase induced by a paramagnetic pair-breaking (PPB) effect in relation to the high field and low temperature (HFLT) SC phase in CeCoIn$_5$. It is shown that, in k-space, the SDW order develops near the gap nodes where the SC order is suppressed by PPB, and the nesting condition for the SDW ordering is satisfied. By comparing the results in the $d_{x^2-y^2}$-wave SC model and those in an artificial model with no sign change of the gap function in k-space with each other, it is shown that the $d_{x^2-y^2}$-wave SC and SDW orders are enhanced altogether in k-space due to the sign change of the $d_{x^2-y^2}$-wave gap function there, and that this mutual enhancement largely stabilizes the coexistence of these orders in real space. It is also discussed that the field dependence of a SDW moment can be affected by the k-space structure of these orders, which is dependent on the curvature of the Fermi surface.

Details of coexistence of superconducting and antiferromagnetic orders induced by a paramagnetic pair-breaking effect

We theoretically investigate the k-space distributions of coexisting dx2-y2-wave superconducting (SC) and antiferromagnetic or spin-density-wave (SDW) orders induced by the paramagnetic pair-breaking (PPB) effect. It is shown that the SDW order develops in k- space regions near the SC nodes, where the SC order is suppressed by the PPB, and that the SC and SDW orders are enhanced with each other even in k-space owing to the sign change of the dx2-y2-wave SC gap function. The field dependence of the PPB-induced SDW moment in the case with a large Fermi surface curvature is discussed based on the distributions of the SC and SDW orders in k-space.

Microscopic picture on antiferromagnetic order induced by paramagnetic pair-breaking in d-wave superconductor

We theoretically investigate possible antiferromagnetic (AFM) orders in Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting phase with dx2-y2-wave paring induced by the paramagnetic pair-breaking effect. The AFM Q vector whose incommensurate part qAFM is parallel to [1,1,0] direction and independent of the field strength is obtained with the dispersion relation which have a remarkably inflected Fermi surface and the largest DOS along [1,1,0] direction. It is shown that, in a field parallel to [1,1,0], qAFM perpendicular to the field direction is stabilized as a result of the coupling between Q vector and the wavevector of FFLO modulation. These results are consistent with the observations in the high-field low-temperature phase of CeCoIn5.

강자성 초전도체의 연구동향과 전망

강자성 초전도체의 발견은 상극으로 알려진 강자성과 초전도성이 어떻게 상호작용하여 조화롭게 공존하는지에 대한 학문적인 연구뿐만 아니라 새로운 기술적인 응용을 위한 광범위한 탐구를 이끌고 있다. 본 해설논문에서는 강자성 초전도체에 대한 이해를 돕기 위하여, 먼저 초전도체의 쿠퍼쌍을 깨뜨리는 강한 자기장의 궤도 효과와 상자성 효과에 대하여 설명한다. 자기장의 이러한 효과 이외에도 초전도체/강자성체 복합 구조의 계면에서 발생하는 근접 효과에 의해 단일상 쿠퍼쌍은 강자성체를 지나가는 동안 불안정하여 아주 짧은 침투깊이를 가진다. 그러나 쿠퍼쌍이 홀-진동수 삼중상인 경우 안정되고 긴 유효길이를 가지게 되는데, 새로운 스핀 전자소자로서의 개발을 위해 그 연구의 중요성이 높아지고 있다. 마지막으로 다양한 강자성 초전도체와 양자구속효과에 의해 두 성질이 공존하는 저차원의 물질들을 소개한다. The findings of ferromagnetic superconductor have attracted much attention not only for fundamental research to investigate how the antagonistic properties of ferromagnetism and superconductivity coexist peacefully but also for potential technological applications. Firstly, in order to help for understanding the ferromagnetic superconductor, I have explained the orbital and paramagnetic pair-breaking effects of magnetic field, which breaks the superconducting Cooper pairs. In addition to such effects of magnetic field, the singlet Cooper pairs become unstable upon going through the ferromagnetic materials by the proximity effect. The proximity effect occurs at the interface of thin films composing of superconductor and ferromagnet and leads to have very short penetration depth of Cooper pairs. However, a type of odd-frequency triplet in comparison with the singlet could be very stable and has a longer effective depth. It needs to be explored for the innovative spintronic devices. Finally, various ferromagnetic superconductors coexist and the lower-dimensional materials under the Quantum confinement effect have been introduced.

Field Dependence of the Specific Heat in a Heavy-Fermion Superconductor CeIrIn5

We investigated temperature, field, and field-orientation dependences of the specific heat of CeIrIn 5 . The upper critical field H c2 was determined down to 80 mK from the specific heat measurements and exhibits a slight suppression at low temperatures. We revealed that the specific heat at 80 mK increases proportional to the square root of H up to nearly H c2 for both H ∥ a b and H ∥ c , implying the presence of line nodes and a weak contribution of the Pauli paramagnetic pair-breaking effect. Furthermore, we revealed that the specific heat shows a fourfold oscillation under a rotating magnetic field in the a b -plane. This result indicates that CeIrIn 5 has fourfold symmetry around the c -axis in its gap amplitude.

Magnetic and Superconducting Phase Diagram of Noncentrosymmetric CeRhSi3

The noncentrosymmetric heavy fermion superconductor CeRhSi 3 exhibits an extremely large superconducting upper critical field B c2 for the magnetic field along the c axis. This remarkable feature is ascribed to the noncentrosymmetric (Rashba-type) crystal structure which can avoid the paramagnetic pair-breaking effect. The pressure dependence of the B c2 ( T ) curve, however, is very complicated and remains as an unresolved issue. Recently, we found a bend in the B c2 ( T ) curve which emerges at a pressure where the antiferromagnetic order disappears. Furthermore, an anomaly in the electrical resistivity in the normal state emerges above the magnetic field of the bend point. The obtained magnetic and superconducting phase diagram thus appears to have complicated structure especially in the vicinity of the pressure where the antiferromagnetic order vanishes.

Competition and/or coexistence of antiferromagnetism and superconductivity in CeRhIn5 and CeCoIn5

AbstractThe Ce compounds CeCoIn5 and CeRhIn5 are ideal model systems to study the competition of antiferromagnetism (AF) and superconductivity (SC). Here we discuss the pressure–temperature and magnetic field phase diagrams of both compounds. In CeRhIn5 the interesting observation is that in zero magnetic field a coexistence AF + SC phase exist inside the AF phase below the critical pressure $p_{\rm c}^{\rm *} \approx 2$ GPa. Above $p_{\rm c}^{\rm *} $ AF is suppressed in zero field but can be re‐induced by applying a magnetic field. The collapse of AF under pressure coincides with the abrupt change of the Fermi surface. In CeCoIn5 a new phase appears at low temperatures and high magnetic field (LTHF) which vanishes at the upper critical field Hc2. In both compounds the paramagnetic pair breaking effect dominates at low temperature. We discuss the evolution of the upper critical field under high pressure of both compounds and propose a simple picture of the glue of reentrant magnetism to the upper critical field in order to explain the interplay of antiferromagnetic order and SC.

Novel superconducting properties on noncentrosymmetric heavy fermion CeRhSi3

We demonstrate an extremely high upper-critical-field, Bc2, in the noncentrosymmetric heavy-fermion superconductor CeRhSi3 for a magnetic field along the tetragonal c-axis. The robustness of the superconductivity over the magnetic field is outstanding among the known superconductors. The extremely large Bc2 is attributed to the absence of the paramagnetic pair-breaking effect characteristic of noncentrosymmetric superconductors.

Effects of two gaps and paramagnetic pair breaking on the upper critical field ofSmFeAsO0.85andSmFeAsO0.8F0.2single crystals

We investigated the temperature dependence of the upper critical field $[{H}_{c2}(T)]$ of fluorine-free ${\text{SmFeAsO}}_{0.85}$ and fluorine-doped ${\text{SmFeAsO}}_{0.8}{\text{F}}_{0.2}$ single crystals by measuring the resistive transition in low static magnetic fields and in pulsed fields up to 60 T. Both crystals show that ${H}_{c2}(T)$'s along the $c$ axis $[{H}_{c2}^{c}(T)]$ and in an $ab$-planar direction $[{H}_{c2}^{ab}(T)]$ exhibit a linear and a sublinear increase, respectively, with decreasing temperature below the superconducting transition. ${H}_{c2}(T)$'s in both directions deviate from the conventional one-gap Werthamer-Helfand-Hohenberg theoretical prediction at low temperatures. A two-gap nature and the paramagnetic pair-breaking effect are shown to be responsible for the temperature-dependent behavior of ${H}_{c2}^{c}$ and ${H}_{c2}^{ab}$, respectively.

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.

Antivortices due to competing orbital and paramagnetic pair-breaking effects

Thermodynamically stable vortex—antivortex structures in a quasi-twodimensional superconductor in a tilted magnetic field are predicted. For this geometry, both orbital and spin pair-breaking effects exist, with their relative strength depending on the tilt angle θ. The spectrum of possible states contains the ordinary vortex state (for large θ) and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state (for θ = 0) as limits. The quasi-classical equations are solved nearH c2 for arbitrary θ and it is shown that stable states with co-existing vortices and antivortices exist in a small interval close to θ= 0. The results are compared with recent predictions of antivortices in mesoscopic samples.

Reduction of Pauli Paramagnetic Pair-breaking Effect in Antiferromagnetic Superconductors

Antiferromagnetic superconductors in a magnetic field are studied. We examine a mechanism which significantly reduces the Pauli paramagnetic pair-breaking effect. The mechanism is realized even in the presence of the orbital pair-breaking effect. We illustrate it using a three-dimensional model with an intercalated magnetic subsystem. The upper critical field is calculated for various parameters. It is shown that the upper critical field can reach several times the pure Pauli paramagnetic limit. The possible relevance to the large upper critical field observed in the heavy fermion antiferromagnetic superconductor CePt_3Si discovered recently is briefly discussed. We try to understand the large upper critical field in the compound CePt_3Si and field-induced superconductivity in the compound CePb_3 within a unified framework.

Upper critical field and field-induced superconductivity in layered superconductors with antiferromagnetic subsystems

The phase diagrams on the temperature and magnetic field plane are examined in layered superconductors with antiferromagnetic subsystems in parallel magnetic fields. The field-induced superconductivity (FISC) occurs for strong Kondo coupling J K⪢ zJ by the Jaccarino–Peter mechansim, where J denotes the antiferromagnetic coupling constant of the subsystem. The experimental phase diagram of λ-(BETS) 2FeCl 4 is semi-quantitatively reproduced by taking into account the Fulde–Ferrell–Larkin–Ovchinnikov state with an order-parameter mixing effect. On the other hand, for weak Kondo coupling, an enhancement of the upper critical field occurs instead of the FISC. In particular, for J K∼ zJ⪢ Δ 0, the system is practically free from the paramagnetic pair-breaking effect, and very large areas of the superconductivity are obtained in the phase diagram.

Enhancement of the Upper Critical Field and a Field-Induced Superconductivity in Antiferromagnetic Conductors

We propose a mechanism by which the paramagnetic pair-breaking effect is largely reduced in superconductors with coexisting antiferromagnetic long- range and short-range orders. The mechanism is an extension of the Jaccarino and Peter mechanism to antiferromagnetic conductors, but the resultant phase diagram is quite different. In order to illustrate the mechanism, we examine a model which consists of mobile electrons and antiferromagnetically correlated localized spins with Kondo coupling between them. It is found that for weak Kondo coupling, the superconductivity occurs over an extraordinarily wide region of the magnetic field including zero field. The critical field exceeds the Chandrasekhar and Clogston limit, but there is no lower limit in contrast to the Jaccarino and Peter mechanism. On the other hand, for strong Kondo coupling, both the low-field superconductivity and a field-induced superconductivity occur. Possibilities in hybrid ruthenate cuprate superconductors and some organic superconductors are discussed.

Coexistence of Singlet and Triplet Attractive Channels in the Pairing Interactions Mediated by Antiferromagnetic Fluctuations

We propose a phase diagram of quasi-low-dimensional type II superconductors in parallel magnetic fields, when antiferromagnetic fluctuations contribute to the pairing interactions. We point out that pairing interactions mediated by antiferromagnetic fluctuations necessarily include both singlet channels and triplet channels as attractive interactions. Usually, a singlet pairing is favored at zero field, but a triplet pairing occurs at high fields where the singlet pairing is suppressed by the Pauli paramagnetic pair-breaking effect. As a result, the critical field increases divergently at low temperatures. A possible relation to experimental phase diagrams of a quasi-one-dimensional organic superconductor is briefly discussed. We also discuss a possibility that a triplet superconductivity is observed even at zero field.

Mixed state properties of organic superconductors

The physical properties of organic superconductors under a magnetic field are reviewed. With the aid of recent advances in theories for phenomenology, the superconductivity transitions of BEDT-TTF salts and A 3C 60 are analyzed by taking account of fluctuation, and thereby the physical parameters are reevaluated, while (TMTSF) 2X can be treated by an anisotropic effective mass tensor model. Other properties relevant to vortex dynamics and paramagnetic pair breaking effect are also mentioned.

A possible explanation of gap features in high-T c cuprates : Tunneling density of states and infrared conductivity

Within the Eliashberg theory, we have studied the paramagnetic pair-breaking effect on gap features in the tunneling density of states and infrared optical responses in high- T c cuprates. Especially, the dependence of this effect on the shape of α 2 F(ω) is examined by using two types of α 2 F(ω), each of which is supposed to be relevant to Bi2212 and YBCO, respectively. The feature of gap-filling and the reduction of both T c and the gap edge due to the pair-breaking effect are shown to depend significantly on the shape of α 2 F(ω). From these analyses, a possible explanation of the discrepancy between the gap ratios estimated from tunneling measurements and infrared measurements is proposed. Calculations of the optical responses with the inclusion of both paramagnetic and normal impurities of various concentrations indicate that infrared responses in these materials can be described well by the Eliashberg theory including both kinds of impurities of certain concentrations.