Heisenberg Ising-Kondo necklace model with transverse field for the heavy fermion compound URu2Si2.

In this work, we introduce the Heisenberg Ising-Kondo necklace with transverse field as a possible model to describe the heavy-fermion compound URu2Si2. The physics of this compound presents many open questions, like the transition to the hidden order (HO) phase at T 0 = 17.5 K. Our Hamiltonian includes elements that come from crystal field processes and Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The main idea is to allow the transverse field, that a priori acts only in the localized moments, to influence the conduction-electrons via the RKKY interaction. Our results, obtained using a spin-wave approach for a one-dimensional lattice system, reveal a re-entrance behavior on the zero temperature phase diagram. This is an interesting result since combined with the magnetization values of the canted AF phase allow to link this phase with the mixed hidden order/large moment antiferromagnetic (HO/LMAF) phase observed in URu2Si2.

Tail-like regime and BCS-BEC crossover due to hybridization in a two-band superconductor.

Superconductivity in strongly correlated systems is a remarkable phenomenon that attracts huge interest. The study of this problem is relevant for materials such as the high T c oxides, pnictides and heavy fermions. These systems also have in common the existence of electrons of several orbitals that coexist at a common Fermi surface. In this paper we study the effect of pressure, chemical or applied on multi-band superconductivity. Pressure varies the atomic distances and consequently the overlap of the wave-functions in the crystal. This rearranges the electronic structure that we model including a pressure dependent hybridization between the bands. We consider the case of two-dimensional systems in a square lattice with inverted bands. We study the conditions for obtaining a pressure induced superconductor quantum critical point and show that hybridization, i.e. pressure can induce a Bardeen-Cooper-Schrieffer-Bose-Einstein condensation crossover in multi-band systems even for moderate interactions. We found a tail-like superconductor regime and briefly discuss the influence of the symmetry of the order parameter in the results.

Casimir amplitudes in topological quantum phase transitions.

Topological phase transitions constitute a new class of quantum critical phenomena. They cannot be described within the usual framework of the Landau theory since, in general, the different phases cannot be distinguished by an order parameter, neither can they be related to different symmetries. In most cases, however, one can identify a diverging length at these topological transitions. This allows us to describe them using a scaling approach and to introduce a set of critical exponents that characterize their universality class. Here we consider some relevant models of quantum topological transitions associated with well-defined critical exponents that are related by a quantum hyperscaling relation. We extend to these models a finite-size scaling approach based on techniques for calculating the Casimir force in electromagnetism. This procedure allows us to obtain universal Casimir amplitudes at their quantum critical points. Our results verify the validity of finite-size scaling in these systems and confirm the values of the critical exponents obtained previously.

Multiband superconductivity in BiS2-based layered compounds.

A mean-field treatment is presented of a square lattice two-orbital-model for [Formula: see text] taking into account intra- and inter-orbital superconductivity. A rich phase diagram involving both types of superconductivity is presented as a function of the ratio between the couplings of electrons in the same and different orbitals ([Formula: see text]) and electron doping x. With the help of a quantity we call orbital-mixing ratio, denoted as [Formula: see text], the phase diagram is analyzed using a simple and intuitive picture based on how [Formula: see text] varies as electron doping increases. The predictive power of [Formula: see text] suggests that it could be a useful tool in qualitatively (or even semi-quantitatively) analyzing multiband superconductivity in BCS-like superconductors.

Non-linear conduction due to depinning of charge order domains in Fe3O2BO3.

The oxyborate Fe3O2BO3 presents a charge density wave (CDW) transition close to room temperature. As we show here, this is associated with a well defined anomaly in the specific heat. Below this transition, when applying in a single crystal of Fe3O2BO3 a DC voltage above a temperature dependent threshold, a high current is liberated in this material. We study the conduction in single crystals of Fe3O2BO3 with voltage applied parallel and perpendicular to the crystallographic c axis direction. The observed currents are attributed to the depinning of charge ordered domains above a threshold voltage V T2 that gives rise to a collective conduction due to coherent domains. Compliance limited DC data shows that above a lower threshold voltage depinning is smooth and follows a power law scaling. Similar depinning with power law scaling is also revealed in the AC conductivity.

Fermi points and topological quantum phase transitions in a multi-band superconductor.

The importance of models with an exact solution for the study of materials with non-trivial topological properties has been extensively demonstrated. The Kitaev model plays a guiding role in the search for Majorana modes in condensed matter systems. Also, the sp-chain with an anti-symmetric mixing among the s and p bands is a paradigmatic example of a topological insulator with well understood properties. Interestingly, these models share the same universality class for their topological quantum phase transitions. In this work we study a two-band model of spinless fermions with attractive inter-band interactions. We obtain its zero temperature phase diagram, which presents a rich variety of phases including a Weyl superconductor and a topological insulator. The transition from the topological to the trivial superconducting phase has critical exponents different from those of Kitaev's model.

Quantum-critical spin dynamics in quasi-one-dimensional antiferromagnets.

By means of nuclear spin-lattice relaxation rate T(1)(-1), we follow the spin dynamics as a function of the applied magnetic field in two gapped quasi-one-dimensional quantum antiferromagnets: the anisotropic spin-chain system NiCl(2)-4SC(NH(2))(2) and the spin-ladder system (C(5)H(12)N)(2)CuBr(4). In both systems, spin excitations are confirmed to evolve from magnons in the gapped state to spinons in the gapless Tomonaga-Luttinger-liquid state. In between, T(1)(-1) exhibits a pronounced, continuous variation, which is shown to scale in accordance with quantum criticality. We extract the critical exponent for T(1)(-1), compare it to the theory, and show that this behavior is identical in both studied systems, thus demonstrating the universality of quantum-critical behavior.

Fluctuations in a superconducting quantum critical point of multi-band metals.

In multi-band metals quasi-particles arising from different atomic orbitals coexist at a common Fermi surface. Superconductivity in these materials may appear due to interactions within a band (intra-band) or among the distinct metallic bands (inter-band). Here we consider the suppression of superconductivity in the intra-band case due to hybridization. The fluctuations at the superconducting quantum critical point (SQCP) are obtained by calculating the response of the system to a fictitious space- and time-dependent field, which couples to the superconducting order parameter. The appearance of superconductivity is related to the divergence of a generalized susceptibility. For a single-band superconductor this coincides with the Thouless criterion. For fixed chemical potential and large hybridization, the superconducting state has many features in common with breached pair superconductivity with unpaired electrons at the Fermi surface. The T = 0 phase transition from the superconductor to the normal state is in the universality class of the density-driven Bose-Einstein condensation. For a fixed number of particles and in the strong coupling limit, the system still has an instability to the normal state with increasing hybridization.

Superconducting quantum critical point in CeCoIn(5-x)Sn(x).

We report a combined pressure-doping study in the nearly two-dimensional heavy fermion superconductor CeCoIn5 as its superconducting phase is driven to the normal state by Sn doping and/or applied pressure. Temperature-pressure-dependent electrical resistivity measurements were performed at the vicinity of a superconducting quantum critical point where Tc→0. A universal plot of the concentration- and pressure-dependent phase diagram suggests that for the concentrations studied a single mechanism is responsible for reducing Tc and bringing the system to the superconducting quantum critical point. A two-band model with hybridization controlled by pressure and doping provides a consistent description of the phase diagram and the suppression of the d-wave superconductivity in this material.

Pressure induced FFLO instability in multi-band superconductors.

Multi-band systems such as inter-metallic and heavy fermion compounds have quasi-particles arising from different orbitals at their Fermi surface. Since these quasi-particles have different masses or densities, there is a natural mismatch of the Fermi wavevectors associated with different orbitals. This makes these materials potential candidates to observe exotic superconducting phases as Sarma or FFLO phases, even in the absence of an external magnetic field. The distinct orbitals coexisting at the Fermi surface are generally hybridized and their degree of mixing can be controlled by external pressure. In this work we investigate the existence of an FFLO type of phase in a two-band BCS superconductor controlled by hybridization. At zero temperature, as hybridization (pressure) increases we find that the BCS state becomes unstable with respect to an inhomogeneous superconducting state characterized by a single wavevector q.

Dimensional crossover in anisotropic Kondo lattices.

Dimensional crossover in the Kondo necklace model is analyzed using the bond-operator method at zero and finite temperatures. Explicit relations describing quasi-two-dimensional properties are obtained by asymptotically solving the resulting equations. The crossover from two dimensions (2d) to three dimensions (3d) is investigated, turning on the electronic hopping ([Formula: see text]) of conduction electrons between different planes. In order to give continuity to our analysis, both cases of crossover, quasi-three-dimensional (q3d) and quasi-one-dimensional (q1d), are also investigated. The phase diagram as a function of temperature T, [Formula: see text] and [Formula: see text], where [Formula: see text] is the hopping within the planes, is calculated. Unusual reentrant behavior in the temperature-dependent antiferromagnetic critical line is found close to two dimensions. Near the isotropic three-dimensional quantum critical point the critical line is described by a standard power law with a square root dependence on the distance to the quantum critical point.

Theoretical investigation of the spin exchange interactions and magnetic properties of the homometallic ludwigite Fe(3)O(2)BO(3).

The homometallic ludwigite Fe(3)O(2)BO(3) has a complex structure made up of corner- and edge-sharing FeO(6) octahedra and exhibits a number of apparently puzzling magnetic properties. The reasons for these properties were probed by examining the trends in the spin exchange interactions of Fe(3)O(2)BO(3). To analyze the relative strengths of spin exchange interactions in such a complex magnetic solid, we first generalized the method of spin dimer analysis and then employed the resulting formulation to investigate how the magnetic properties of Fe(3)O(2)BO(3) are related to its reported crystal structures. The spin-orbital interaction energies calculated for various spin dimers of Fe(3)O(2)BO(3) provide estimates for the relative strengths of the associated spin exchange interactions, which in turn account for the observed magnetic properties of Fe(3)O(2)BO(3).

Structural transition and pair formation in Fe3O2BO3.

We observe for the first time a structural phase transition in the oxyborate Fe3O2BO3 which occurs along three leg ladders present in this material. X-ray diffraction shows that this transition at 283 K is associated with a new phase where atomic displacements occur in alternate directions perpendicular to the axis and within the plane of the ladders. Magnetic data show that these displacements lead to the formation of singlet pairs which dissociate close to the structural transition. Anomalies in the transport properties also occur close to 283 K showing that the structural transition is related to a charge ordering phenomenon in a low dimensional structure.