Zero temperature phase diagram of the ferromagnetic Kondo lattice

We study numerically the one-dimensional ferromagnetic Kondo lattice, a model widely used to describe nickel and manganese perovskites. Due to the competition between double and super-exchange, we find a region where the formation of magnetic islands induces a charge-ordered state. This ordering is present even in the absence of any inter-site Coulomb repulsion and presents an insulating gap associated to the charge structure. We study the metal–insulator transition induced by a magnetic field which removes simultaneously both charge and spin orderings. This new mechanism should be taken into account in theories of charge ordering involving spin degrees of freedom.     

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.     

压力环境下重费米子体系的物性探索

@@@@重费米子体系是近藤晶格中的强关联电子系统,它由于电、磁等相互作用的竞争呈现出丰富的电子基态,而且由于各种相互作用的能量尺度较小,它会对外界参量(压力、掺杂、磁场)的调控比较敏感。文章论述了重费米子体系的基本性质和它在压力环境下的物性演变以及相关的量子临界现象。超导态一般会出现在量子临界点附近,文章特别以“115”超导体系为例,讲述了超导态与其他长程序共存时可能出现的纹络化结构,介绍了如何运用压力下的谱学测量(如转角比热、软点接触隧道谱)来研究压力下重费米子体系的物性特别是超导序参量的对称性。     

压力环境下重费米子体系的物性探索

重费米子体系是近藤晶格中的强关联电子系统,它由于电、磁等相互作用的竞争呈现出丰富的电子基态,而且由于各种相互作用的能量尺度较小,它会对外界参量(压力、掺杂、磁场)的调控比较敏感。文章论述了重费米子体系的基本性质和它在压力环境下的物性演变以及相关的量子临界现象。超导态一般会出现在量子临界点附近,文章特别以“115”超导体系为例,讲述了超导态与其他长程序共存时可能出现的纹络化结构,介绍了如何运用压力下的谱学测量(如转角比热、软点接触隧道谱)来研究压力下重费米子体系的物性特别是超导序参量的对称性。     

Single crystal growth and superconductivity of Ca(Fe1−x Co x )2As2

We report the single crystal growth of Ca(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.082) from Sn flux. The temperature–composition phase diagram is mapped out based on the magnetic susceptibility and electrical transport measurements. The phase diagram of Ca(Fe_1?x Co _x )₂As₂ is qualitatively different from those of Sr and Ba; this could be due to both the charge doping and structural tuning effects associated with Co substitution.     

Superconducting properties of barium in three phases under high pressure from first principles

Electron-phonon coupling (EPC) in the three high-pressure phases of Ba is investigated using a pseudopotential plane-wave method based on density functional perturbation theory. The calculated values of superconducting critical temperature T c of Ba-I and Ba-II under pressure are consistent well with the trends observed experimentally. Moreover, Ba-V is found to be superconducting with a maximum T c exceeding 7.8 K at 45 GPa. With the increase of pressure, the values of T c increase in Ba I and Ba-II but the value of T c decreases in Ba-V. For Ba-I at pressures below 2 GPa, the increases of logarithmic average frequency ω log and electron-phonon coupling parameters λ both contribute to the enhancement of T c . For all the three phases at pressures above 2 GPa, T c is found to be primarily determined by λ . Further investigation reveals that for all the three phases, the change in λ with pressure can be explained mainly by change in the phonon frequency. Thus for Ba-II and Ba-V, although they exhibit completely different superconducting behaviors, their superconductivities have the same origin; the pressure dependence of T c is determined finally by the pressure dependence of phonon frequency.     

The Schrieffer-Wolff transformation for the underscreened Anderson lattice

We present a derivation of the Schrieffer-Wolff transformation for the Anderson Lattice Hamiltonian with a two-fold degenerate f-level in each site. The degeneracy of the f-electrons has been taken into account in order to describe uranium and other actinide magnetic compounds with a spin larger than , for example a total S=1 spin for the f-electrons. The transformed Hamiltonian has several terms as in the classical case, but we have obtained here both an exchange (Kondo) interaction between the S=1 f-spins and the spins of the conduction electrons, and also an effective f-band term. This f-band term describes better the underscreened Kondo lattice model which has been recently developed to explain the Kondo-ferromagnetism coexistence observed in uranium compounds such as UTe [N.B. Perkins, M.D. Nunez-Regueiro, J. R. Iglesias, B. Coqblin, Phys. Rev. B 76 (2007) 125101].     

Color superconductivity in dense quark matter

The possible structure of the QCD phase diagram is discussed focusing on color superconducting quark matter in the region of low temperatures and moderately large densities, which could be relevant for compact stars.     

Quantum phase transition of alkaline-earth fermionic atoms confined in an optical superlattice

Using the density matrix renormalization group method, we evaluate the spin and charge gaps of alkaline-earth fermionic atoms in a periodic one-dimensional optical superlattice. The number of delocalized atoms is equal to the lattice size and we consider an antiferromagnetic coupling between delocalized and localized atoms. We found a quantum phase transition from a Kondo insulator spin liquid state without confining potential to a charge-gapped antiferromagnetic state with nonzero potential. For each on-site coupling, there is a critical potential point for which the spin gap vanishes and its value increases linearly with the local interaction.     

Magnetocaloric effect in the ferromagnetic Kondo lattice model

The Kondo lattice model describes a lattice of localized spins S_i interacting with the conduction electrons via a local exchange coupling J. Assuming a ferromagnetic Hund's rule coupling J>0, the model can be used to describe some itinerant magnetocaloric materials such as Gd(Si_xGe_1-x)₄, La(Fe_1-xSi_x)₁₃, and LaCa_1-xMn_xO₃, which are important for magnetic refrigeration near room temperature. The localized magnetic moments are described in the model Hamiltonian by spin operators, and the conduction electrons by fermionic operators. To study the magnetocaloric effect, a uniform external magnetic field is added through a Zeeman term. By averaging the fermionic degrees of freedom, one obtains an indirect exchange coupling between spins at sites i and j, which corresponds to the RKKY interaction. The self-consistent mean value is evaluated in the effective Heisenberg Hamiltonian within the random phase approximation (RPA). The conduction electron magnetization for a given value of is obtained from the corresponding Green's functions through the equation of motion method. The pressure and doping dependence of the Curie temperature are taken into account in the evaluation of . The magnetocaloric effect is characterized by the isothermal entropy change ΔS and the adiabatic temperature change ΔT_ad upon magnetic field variations in the neighborhood of the ferromagnetic phase transition. The results are obtained for and compared to measurements with Gd compounds.     

新型FeSe基超导材料研究进展

FeSe基超导体作为铁基超导材料家族的重要组成部分,已经成为凝聚态物理研究的一个热点领域,对这类超导材料的探索和制备是研究其物理性质的基础.目前,对于FeSe基超导材料的探索主要集中于插层和外延单层FeSe薄膜.其中,通过插层方法获得的FeSe基超导材料具有独特的性质,且种类众多.本文介绍了近年来发现的一系列FeSe基高温超导材料,涵盖K_xFe₂Se₂,A_xNH₃FeSe,LiOHFeSe和有机分子插层FeSe等,并针对各种材料,简述了其性质及影响.     

Effect of Bond-Diluted on Spin-3/2 Transverse Ising Model with Crystal Field

The magnetic properties of the bond-diluted spin-3/2 transverse Ising model with the presence of a crystalfield on the honeycomb lattice are studied within the framework of the effective field theory with correlations. Theinteractions Jij are assumed to be independent random variables with distribution P(Jij) ＝ pδ(Jij - J) + (1 - P)δ(Jij).