G. G. Cabrera

Nodal liquid and s -wave superconductivity in transition metal dichalcogenides

We explore the physical properties of a unified microscopic theory for the coexistence of superconductivity and charge-density waves (CDWs) in two-dimensional transition-metal dichalcogenides. In the case of particle-hole symmetry, the elementary particles are Dirac fermions at the nodes of the charge density wave gap. When particle-hole symmetry is broken, electron (hole) pockets are formed around the Fermi surface. The superconducting ground state emerges from the pairing of nodal quasiparticles mediated by acoustic phonons via a piezoelectric coupling. We calculate several properties in the

Closed-form solutions for the highly anisotropic antiferromagnetic chain

s

Andreev tunneling through a double quantum-dot system coupled to a ferromagnet and a superconductor: Effects of mean-field electronic correlations

-wave superconducting phase, including specific heat, ultrasound absorption, nuclear magnetic relaxation (NMR), and thermal and optical conductivities. In the case with particle-hole symmetry, the specific-heat jump at the transition deviates strongly from ordinary superconductors. The NMR response shows an anomalous anisotropy due to the broken time-reversal symmetry of the superconducting gap, induced by the triple CDW state. The loss of the lattice inversion center in the CDW phase leads to anomalous coherence factors in the optical conductivity and to the appearance of an absorption edge at the optical gap energy. In addition, optical and thermal conductivities display anomalous peaks in the infrared when particle-hole symmetry is broken.

Coulomb gas approach to the anisotropic one-dimensional Kondo lattice model at arbitrary filling

Abstract We obtain the spectrum and derive closed-form solutions for the stationary states of one-dimensional antiferromagnetic Heisenberg Hamiltonian in the quasi-Ising asymptotic limit. Comparison with exact numerical results for the ground state shows however good accuracy in a wide range of the anisotropy parameter. Since our solutions are simple and in closed analytical form, a deep insight in the dynamics of linear antiferromagnets is obtained. The structure of the ground state presents similarities with the liquid-type disordered state proposed by Anderson.

Electromagnetic response of layered superconductors with broken lattice inversion symmetry

We study the transport properties of a hybrid nanostructure composed of a ferromagnet, two quantum dots, and a superconductor connected in series. By using the non-equilibrium Greens function approach, we have calculated the electric current, the differential conductance and the transmittance for energies within the superconductor gap. In this regime, the mechanism of charge transmission is the Andreev reflection, which allows for a control of the current through the ferromagnet polarization. We have also included interdot and intradot interactions, and have analyzed their influence through a mean field approximation. In the presence of interactions, Coulomb blockade tend to localized the electrons at the double-dot system, leading to an asymmetric pattern for the density of states at the dots, and thus reducing the transmission probability through the device. In particular, for non-zero polarization, the intradot interaction splits the spin degeneracy, reducing the maximum value of the current due to different spin-up and spin-down densities of states. Negative differential conductance (NDC) appears for some regions of the voltage bias, as a result of the interplay of the Andreev scattering with electronic correlations. By applying a gate voltage at the dots, one can tune the effect, changing the voltage region where this novel phenomenon appears. This mechanism to control the current may be of importance in technological applications.

Phase diagram of the anisotropic Kondo chain.

We establish a mapping of a general spin-fermion system in one dimension into a classical generalized Coulomb gas. This mapping allows a renormalization-group treatment of the anisotropic Kondo chain both at and away fromhalf-filling. We find that the phase diagram contains regions of paramagnetism, partial, and full ferromagnetic order. We also use the method to analyze the phases of the Ising-Kondo chain.

Spin-dependent Fermi surface effects and the GMR in magnetic multilayers

We investigate the macroscopic effects of charge-density waves (CDW) and superconductivity in layered superconducting systems with broken lattice inversion symmetry (allowing for piezoelectricity) such as two-dimensional transition metal dichalcogenides. We work with the low-temperature time-dependent Ginzburg-Landau theory and study the coupling of lattice distortions and low-energy CDW collective modes to the superconducting order parameter in the presence of electromagnetic fields. We show that superconductivity and piezoelectricity can coexist in these singular metals. Furthermore, our study indicates the nature of the quantum phase transition between a commensurate CDW phase and the stripe phase that has been observed as a function of applied pressure.

FINITE-SIZE CALCULATION OF ANISOTROPIC ANTIFERROMAGNETIC CHAINS AT NONZERO FIELD

We establish the phase diagram of the one-dimensional anisotropic Kondo lattice model at T = 0 using a generalized two-dimensional classical Coulomb gas description. We analyze the problem by means of a renormalization group treatment. We find that the phase diagram contains regions of paramagnetism, partial and full ferromagnetic order.

A VARIATIONAL BETHE-ANSATZ APPROACH TO THE HEISENBERG ANTIFERROMAGNET AT NON-ZERO FIELD

We study novel transport properties in metallic magnetic multilayers, to elucidate whether they can explain the giant magnetoresistance effect observed in those systems. Realistic Fermi surface topologies in layered ferromagnets are taken into account, with the possibilities of different types of orbits depending on the electron spin. Scattering from the spacer couples orbits of different topologies at both sides of the interface in a way similar to magnetic breakdown phenomena, with new interesting effects for the transport properties of multilayers.

Magnetic-size effects in ultrathin layers

The antiferromagnetic spin-1/2 Heisenberg model, with axial anisotropy and applied magnetic field parallel to the axis, is solved numerically by Lanczos diagonalization of finite systems. We devise an extremely accurate method that allows for a reliable extrapolation to the infinite size limit. In the above limit, our results are compared with exact analytical solutions obtained through the Bethe ansatz. As an important example, we probe the behavior of the Critical Magnetic Field in the whole anisotropic region. We find indications that suggest the presence of two different regimes for the scaling of size effects as a function of the anisotropy.