SO(4) theory of antiferromagnetism and superconductivity in Bechgaard salts

Motivated by recent experiments with Bechgaard salts, we investigate the competition between antiferromagnetism and triplet superconductivity in quasi-one-dimensional electron systems. We unify the two orders in an SO(4) symmetric framework, demonstrating the existence of such symmetry in one-dimensional Luttinger liquids. SO(4) symmetry strongly constrains the phase diagram, leading to coexistence regions of antiferromagnetic, superconducting, and normal phases, as observed in (TMTSF)(2)PF(6). We predict a sharp neutron scattering resonance in superconducting samples.     

Triplet superconductivity in a 1D itinerant electron system with transverse spin anisotropy

In this paper we study the ground state phase diagram of a one-dimensional t-J-U model away from half-filling. In the large-bandwidth limit and for ferromagnetic exchange with easy-plane anisotropy a phase with gapless charge and massive spin excitations, characterized by the coexistence of triplet superconducting and spin density wave instabilities is realized in the ground state. With increasing ferromagnetic exchange transitions into a ferrometallic and then a spin gapped triplet superconducting phase take place.     

重费米子超导与竞争序

与其他非常规超导系列相比, 重费米子超导体往往具有丰富多样的竞争序, 超导与各种竞争序相伴而生, 电子配对与反铁磁涨落、铁磁涨落、价态涨落、电四极矩涨落等量子临界涨落密切相关, 扩充了非常规超导的研究内容. 重费米子材料中的f电子往往同时参与超导与各种竞争序的形成, 表现出局域与巡游的二重性. 重费米子二流体理论为理解重费米子超导与竞争序的关系提供了新的思路.     

Pairing symmetry in monolayer of orthorhombic CoSb

Ferromagnetism and superconductivity are generally considered to be antagonistic phenomena in condensed matter physics. Here, we theoretically study the interplay between the ferromagnetic and superconducting orders in a recent discovered monolayered CoSb superconductor with an orthorhombic symmetry and net magnetization, and demonstrate the pairing symmetry of CoSb as a candidate of non-unitary superconductor with time-reversal symmetry breaking. By performing the group theory analysis and the first-principles calculations, the superconducting order parameter is suggested to be a triplet pairing with the irreducible representation of ³B_2u, which displays intriguing nodal points and non-zero periodic modulation of Cooper pair spin polarization on the Fermi surface topologies. These findings not only provide a significant theoretical insight into the coexistence of superconductivity and ferromagnetism, but also reveal the exotic spin polarized Cooper pairing driven by ferromagnetic spin fluctuations in a triplet superconductor.     

Stripes and superconductivity in a one-dimensional self-consistent model

We show that many observable properties of high-temperature superconductors can be obtained in the framework of a one-dimensional self-consistent model with included superconducting correlations. Analytical solutions for spin, charge, and superconductivity order parameters are found. The ground state of the model at low hole doping is a spin-charge solitonic superstructure. Increased doping leads to a transition to the superconducting phase. There is a region of doping where superconductivity, spin density wave, and charged stripe structure coexist. The charge density modulation appears in the vicinity of vortices (kinks in the 1D model) in the superconducting state.     

Superconductivity and magnetic order in CeRhIn5: spectra of coexistence

We discuss the fixed-point Hamiltonian and the spectrum of excitations of a quasi-bidimensional electronic system supporting simultaneously antiferromagnetic ordering and superconductivity. The coexistence of these two order parameters in a single phase is possible because the magnetic order is linked to the formation of a metallic spin density wave, and its order parameter is not associated to a spectral gap but to an energy shift of the paramagnetic bands. This peculiarity entails several distinct features in the phase diagram and the spectral properties of the model, which may have been observed in CeRhIn5. Apart from the coexistence, we find an abrupt suppression of the spin density wave when the superconducting and magnetic ordering temperatures are equal. The divergence of the cyclotron mass extracted from de Haas-van Alphen experiments is also analyzed in the same framework.     

Coexistence of superconductivity and density wave orders in a one-dimensional correlated system

By the field theory approach, we investigate a one-dimensional correlated electronic system modelled by the extended Hubbard Hamiltonian including a nearest-neighbor and a next-nearest-neighbor spin-exchange interactions with easy-axis anisotropy. At half filling, we obtain weak-coupling phase diagram. In addition to two insulating phases with transverse and longitudinal spin-density-wave orders, two metallic phases, characterized by the coexistence of singlet superconductivity and charge-density-wave orders and by the coexistence of triplet superconductivity and spin-density-wave orders, are realized in the ground state. Away from half filling, the degeneracy is split and the superconducting orders are favored.     

Novel coexistence of superconductivity with two distinct magnetic orders

The heavy fermion system exhibits properties that range from an incommensurate antiferromagnet for small to an exotic superconductor on the Ir-rich end of the phase diagram. At intermediate where antiferromagnetism coexists with superconductivity, two types of magnetic order are observed: the incommensurate one of and a new, commensurate antiferromagnetism that orders separately. The coexistence of -electron superconductivity with two distinct -electron magnetic orders is unique among unconventional superconductors, adding a new variety to the usual coexistence found in magnetic superconductors.     

Interplay of superconductivity and antiferromagnetism in SmRh4B4

A microscopic theory of interplay of superconductivity and antiferromagnetism in rare earth ternary systems is developed from first principles for less than half filledf atomic shells. Self consistent equations for the superconducting order parameter Δ and magnetic order parameter Γ, are derived using a Green’s function technique and equation of motion method. The theory is applied to explain the experimental results in the antiferromagnetic superconductor SmRh₄B₄. The present model explains true coexistence of superconductivity and antiferromagnetism and the suppression of superconductivity by antiferromagnetism. The behaviour of superconducting order parameter (Δ), magnetic order parameter (Γ), the specific heat, the density of states, free energy and critical field (H _c) is also studied for the system SmRh₄B₄.     

Phase diagram for a t-J bilayer: role of interlayer couplings

The phase diagram for a t-J bilayer as a function of interplanar hopping, t_⊥ and hole concentration, x is presented for a few different values of interplanar exchange, J_⊥ using variational Monte Carlo calculations. The phase diagram shows rich features, such as a coexistence of antiferromagnetism and superconductivity at underdoping, planar (d-wave) and interplanar (d_z-wave) superconducting correlations for small and large J_⊥, respectively at optimal and overdoping. Another unusual feature appears in the form of a dome shaped structure in the phase diagram where the superconducting correlations are initially assisted as interplanar hopping is enhanced for small t_⊥, while larger t_⊥ is found to be detrimental to superconductivity.     

Spinfluctuations versus phonons: The case of Heavy-Fermion superconductivity

Superconductivity of Heavy-Fermions, with an emphasis on hexagonal UPt₃, is investigated microscopically on the basis of the LNCA-approximation for strongly correlated electrons in a Kondo lattice and of Eliashberg theory for anisotropic singlet and triplet superconducting order. The effective interaction kernel incorporates exchange of spin fluctuations carried by strongly renormalized particle-hole excitations as well as exchange of phonons generated by the breathing mechanism. Particular care is taken to include realistic anisotropies in band structure and couplings and to expand interaction kernel and order parameter in appropriate Fermi surface harmonics. Separate interactions and corresponding gap equations are used for pseudo-spin-singlet and -triplet channel, with a restriction to the case of weak spin-orbit coupling. Our main results include a detailed study of phonon- and spin-fluctuation mechanisms as a possible source for superconductivity and, in combination, their mutual influence and competition. Furthermore, the questions regarding singlet or triplet order, conventional or unconventional symmetry, order parameter zeros and phase transitions between different superconducting states are addressed. We propose a possible scenario of Heavy-Fermion superconductivity.     

Triplet versus singlet superconductivity in quasi-one-dimensional conductors

Quasi-one-dimensional organic conductors are highly unconventional materials, which exhibit a wide variety of phenomena, including spin density waves, quantum Hall effect, and superconductivity. In this paper, we review some experimental and theoretical developments concerning the superconducting state of these systems, where a particular emphasis is placed on the possibility of triplet superconductivity. This possibility is supported by various experiments including upper critical field, Knight shift and NMR relaxation time measurements on the Bechgaard salt bistetramethyltetraselenafulvalene hexafluorophosphate [(TMTSF)₂PF₆]. However, similar NMR results are still lacking for another compound (TMTSF)₂ClO₄ and other members of the Bechgaard salts family. Furthermore, the pairing mechanism and order parameter symmetry are not yet fully known. Therefore, we include a discussion of both triplet and singlet pairing states, and analyse briefly the possibility that the symmetries of the superconducting order parameters are different for various compounds. Finally, we also discuss some open questions regarding the superconducting state of these systems.     

Tetracritical point and staggered vortex currents in superconducting state

A phase diagram reflecting the main features of the typical phase diagram of cuprate superconductors has been studied within the framework of the Ginzburg-Landau phenomenology in the vicinity of a tetracritical point, which appears as a result of the competition of the superconducting and insulating pairing channels. The superconducting pairing under repulsive interaction corresponds to a two-component order parameter, whose relative phase is related to the orbital antiferromagnetic insulating ordering. Under weak doping, the insulating order coexists with the superconductivity at temperatures below the superconducting phase transition temperature and is manifested as a weak pseudogap above this temperature. A part of the pseudogap region adjacent to the superconducting state corresponds to developed fluctuations of the order parameter in the form of quasi-stationary states of noncoherent superconducting pairs and can be interpreted as a strong pseudogap. As the doping level is increased, the system exhibits a phase transition from the region of coexistence of the superconductivity and the orbital antiferromagnetism to the usual superconducting state. In this state, a region of developed fluctuations of the order parameter in the form of quasi-stationary states of uncorrelated orbital circular currents exists near the phase transition line.     

Mott transition, antiferromagnetism, and d-wave superconductivity in two-dimensional organic conductors

We study the Mott transition, antiferromagnetism, and superconductivity in layered organic conductors using the cellular dynamical mean-field theory for the frustrated Hubbard model. A d-wave superconducting phase appears between an antiferromagnetic insulator and a metal for t'/t=0.3-0.7 or between a nonmagnetic Mott insulator (spin liquid) and a metal for t'/t>or=0.8, in agreement with experiments on layered organic conductors including kappa-(ET)2Cu2(CN)3. These phases are separated by a strong first-order transition. The phase diagram gives much insight into the mechanism for -wave superconductivity. Two predictions are made.     

Superconductivity of spin glasses

The coexistence of superconducting and spin glass phases is investigated. The dependence of the phase transition temperature in the spin glass state both on the impurity concentration and the superconductivity state is given. The influence of the spin glass phase on the renormalized frequency and the order parameter of the conduction electrons is determined.     

Two-Fermi-surface superconducting state and a nodal d-wave energy gap of the electron-doped Sm1.85Ce0.15CuO(4-δ) cuprate superconductor

We report on laser-excited angle-resolved photoemission spectroscopy in the electron-doped cuprate Sm1.85Ce0.15CuO(4-δ). The data show the existence of a nodal hole-pocket Fermi surface both in the normal and superconducting states. We prove that its origin is long-range antiferromagnetism by an analysis of the coherence factors in the main and folded bands. This coexistence of long-range antiferrmagnetism and superconductivity implies that electron-doped cuprates are two-Fermi-surface superconductors. The measured superconducting gap in the nodal hole pocket is compatible with a d-wave symmetry.     

FeSe基超导单晶与薄膜研究新进展:自旋向列序、电子相分离及高临界参数

FeSe基超导体的超导临界温度可大范围调控,物理现象丰富,是非常规超导机理研究的热点.由于较高的超导临界参数及易于加工等特点,FeSe基超导体在超导应用开发方面也日益受到重视.大尺寸高质量的单晶和薄膜形态的FeSe基超导材料,对于相关基础科学研究和应用开发都极为重要.作者近年来先后开发和发明了水热离子交换（ion-exchange）、离子脱插（ion-deintercalation）、基底辅助水热外延生长方法,成功解决了二元FeSe和插层（Li,Fe）OHFeSe超导体高质量单晶和薄膜的生长和物性调控难题.进而在相关物理问题的研究中取得新进展,包括发现二元FeSe中自旋向列序与超导电性密切相关,观测到（Li,Fe）OHFeSe中的电子相分离现象.此外,（Li,Fe）OHFeSe超导薄膜呈现很高的超导临界电流密度和上临界磁场,其应用前景值得关注.     

Coexistence of ferromagnetism and superconductivity close to a quantum phase transition: the Heisenberg- to Ising-type crossover

A microscopic mean-field theory of the phase coexistence between ferromagnetism and superconductivity in the weakly ferromagnetic itinerant electron system is constructed, while incorporating a realistic mechanism for superconducting pairing due to the exchange of critical spin fluctuations. The self-consistent solution of the resulting equations determines the superconducting transition temperature which is shown to depend strongly on the exchange splitting. The effect of phase crossover from isotropic (Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum phase transition is analyzed and the generic phase diagram is obtained. This scenario is then applied to the case of itinerant ferromagnet ZrZn2, which sheds light on the proposed phase diagram of this compound. A possible explanation of superconductivity in UGe2 is also discussed.     

Static and dynamical magnetic characters in doped Cu oxides

The discovery of the spin density wave (SDW) state defined as stripe in the doped Cu oxides becomes a central issue in the elucidation of the high-temperature superconducting mechanism. We present the coexistence of the stripe order and superconductivity, and the change of the stripe structure associated with the insultor–metal transition. Both static and dynamical SDW are robust of the high-temperature superconducting Cu oxides.     

Unusual nature of ferromagnetism coexisting with superconductivity in UGe2

We report the discovery of a jump in the magnetization of a macroscopic single crystalline sample of UGe2 that shows coexistence of ferromagnetism and superconductivity. In particular, we observe that the jump occurs at regular intervals of field and only at very low temperatures. This novel feature implies that the magnetic field induces a sudden change of the direction of the magnetization between two equivalent easy axes of magnetization even in a macroscopic sample. We ascribe it to a field-tuned resonant tunneling between quantum spin states, and we propose that the size of a magnetic domain is smaller than a superconducting coherence length.