Papers by Guillermo Cabrera
Magnon anomalous interactions from gauge symmetry: Are magnons ‘charged’ under spin?
Physica B: Condensed Matter, 2010
The Pauli–Schrödinger non-relativistic theory of spin-12 charged fermions requires invariance und... more The Pauli–Schrödinger non-relativistic theory of spin-12 charged fermions requires invariance under the U(1)×SU(2) gauge group. The U(1) symmetry responds for the usual coupling of a charged particle to the electromagnetic gauge potentials Aμ, while the spin interactions with the electromagnetic fields are obtained by changing the status of the global SU(2) rotational invariance of the Pauli–Schrödinger Lagrangian density to a
Physics Letters A, 2010
The non-relativistic Pauli-Schrödinger theory has a richer gauge structure than usually expected,... more The non-relativistic Pauli-Schrödinger theory has a richer gauge structure than usually expected, being invariant under the U (1) × SU(2) gauge group, which allows to define spin-current density vectors and obtains the relevant conserved quantities from Noether's theorem. The electromagnetic fields E and B play the role of the gauge potentials for the SU(2) sector of the gauge group and can possibly contribute with a corresponding invariant curvature self-energy term in the Lagrangian density. From the dynamics of the U (1) and SU(2) gauge fields we show that electric fields can be induced by spin-currents originated from the SU(2) gauge symmetry.
Quantum transport in a ferromagnetic nanowire: conductance and MR effect
Physics Letters A, 2005
In this Letter we analyze the general features of quantum transport in a ferromagnetic nanowire b... more In this Letter we analyze the general features of quantum transport in a ferromagnetic nanowire based on a simple model. The local spin distribution, provided by well localized d-electrons, is governed by the Heisenberg–Ising ferromagnetic Hamiltonian while for the s-like conduction electron we have considered the Anderson model. The interaction between local spin and itinerant electrons is given by HI=−Δ∑iS→i⋅σ→ δ(x−xi).
Physica E: Low-dimensional Systems and Nanostructures, 2007
In this paper we investigate the origin of the giant magnetoresistance effect in atomic-sized mag... more In this paper we investigate the origin of the giant magnetoresistance effect in atomic-sized magnetic contacts within the framework of Landauer's theory, which relates scattering to conductance in a direct way. It is believed that in magnetic contacts the scattering mechanism is spin-dependent, provided by the exchange interaction between local spin and itinerant electrons. In our model the local spin distribution is explained in terms of localized d-electrons obeying a Heisenberg Hamiltonian while the electronic transport is taken into account by itinerant s-like electrons according to the Anderson model. As the main result we show that the MR effect is strongly dependent on spin-flipping probability of conduction electrons.
Can we explain why leptonic electroweak interactions are chiral?
EPL (Europhysics Letters), 2013
ABSTRACT One of the fundamental ingredients in the standard model of electroweak interactions bet... more ABSTRACT One of the fundamental ingredients in the standard model of electroweak interactions between fundamental particles is chirality. Currently there are no hints about why nature has chosen the chiral way. The answer to such a fundamental question could be the extension of the whole theory to the superluminal sector of the Lorentz-Poincaré group. Restricting ourselves to leptons, we postulate that the neutrinos are a priori described by a tachyonic Dirac Lagrangian. It could provide a simple explanation for the parity violation in weak interactions and why electroweak theory has a chiral aspect, leading to invariance under a SUL(2) × UY (1) gauge group. Right-handed neutrino becomes sterile and decoupled from the other particles quite naturally.
Spin Hall effect induced by a gravitational field
Annals of Physics, 2010
The experiment by Collela et al. (1975) [1] evidenced in a striking manner how the gravitational ... more The experiment by Collela et al. (1975) [1] evidenced in a striking manner how the gravitational field appears in quantum mechanics. Within the modern framework of gauge theories, one can ascribe such effect as due to gauge fields originated from fundamental symmetries of spacetime: local transformations of the Lorentz–Poincaré group. When this gauge principle is applied to the Dirac equation,
International Journal of Modern Physics B, 1999
Electronic properties of disordered binary alloys are studied via the calculation of the average ... more Electronic properties of disordered binary alloys are studied via the calculation of the average Density of States (DOS) in two and three dimensions. We propose a new approximate scheme that allows for the inclusion of local order effects in finite geometries and extrapolates the behavior of infinite systems following finite-size scaling ideas. We particularly investigate the limit of the Quantum Site Percolation regime described by a tight-binding Hamiltonian. This limit was chosen to probe the role of short range order (SRO) properties under extreme conditions. The method is numerically highly efficient and asymptotically exact in important limits, predicting the correct DOS structure as a function of the SRO parameters. Magnetic field effects can also be included in our model to study the interplay of local order and the shifted quantum interference driven by the field. The average DOS is highly sensitive to changes in the SRO properties and striking effects are observed when a m...
Physical Review A, 1987
The large principal-quantum-number limit is investigated. Several concrete examples are worked ou... more The large principal-quantum-number limit is investigated. Several concrete examples are worked out and the correspondence principle is reexamined under the light shed by these calculations. In particular, it is shown that a simple linear combination of three one-dimensional harmonic-oscillator eigenstates leads to results at variance with classical physics, in the high quantum-number limit. The possibility of actually observing these phenomena is also brieAy discussed.
Physical Review B, 2013
Gauge symmetries have been identified in graphene and associated with specific physical propertie... more Gauge symmetries have been identified in graphene and associated with specific physical properties. For instance, the U (1) gauge group is related to electrodynamics in (1 + 2)-dimensional [(1 + 2)D] space-time and non-Abelian gauge groups can describe curvature and torsion. Here we demonstrate that the Dirac Lagrangian for massless electrons near the Dirac points is also invariant under the group SU (2) related to local spin rotations, leading to the correct spin-orbit interactions and a rigorous definition for the spin-current density. Furthermore, we computed the charge and spin conductivity within the framework of Kubo linear response theory, using the algebra of relativistic Dirac spinors in (1 + 2)D space-time. The minimal value of electrical conductivity is predicted to be πq 2 /h, in agreement with typical experimental findings.
Journal of Magnetism and Magnetic Materials, 1992
The Heisenberg-Ising Hamiltonian has been exactly solved using the Lanczos method, for chain of u... more The Heisenberg-Ising Hamiltonian has been exactly solved using the Lanczos method, for chain of up to N = 28 spins. In the whole anisotropic region, the mass gap displays a dominant exponential behavior with size. The extrapolated ground state is doubly degenerate, and presents long-range order with dominant components of the Néel type. Quantum fluctuations become more important as long as the anisotropy is reduced, leading to a disordered singlet ground state for the infinite system at the isotropic point. Low-dimensional magnetism is a fascinating subject. Experimentally, quasi
Physical Review B, 2004
We investigate the macroscopic effects of charge density waves (CDW) and superconductivity in lay... more 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 (2D) transition metal dichalcogenides (TMD). 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.
Arxiv preprint arXiv:0901.4230, 2009
A toy model for the electroweak interactions(without chirality) is proposed in a six dimensional ... more A toy model for the electroweak interactions(without chirality) is proposed in a six dimensional spacetime with 3 timelike and 3 spacelike coordinates. The spacetime interval ds 2 = dxµdx µ is left invariant under the symmetry group SO(3, 3). We obtain the six-dimensional version of the Dirac gamma matrices, Γµ, and write down a Dirac-like lagrangian density, L = iψΓ µ ∇µψ. The spinor ψ is decomposed into two Dirac spinors, ψ1 and ψ2, which we interpret as the electron and neutrino fields, respectively. In six-dimensional spacetime the electron and neutrino fields are then merged in a natural manner. The SO(3, 3) Lorentz symmetry group must be locally broken to the observable SO(1, 3) Lorentz group, with only one observable time component, tz. The tz-axis may not be the same at all points of the spacetime and the effect of breaking the SO(3, 3) spacetime symmetry group locally to an SO(1, 3) Lorentz group is perceived by the observers as the existence of the gauge fields. The origin of mass may be attributed to the remaining two hidden timelike dimensions. We interpret the origin of mass and gauge interactions as a consequence of extra time dimensions, without the need of the so-called Higgs mechanism for the generation of mass. Further, we are able to give a geometric meaning to the electromagnetic and non-abelian gauge symmetries.
Le Journal de Physique Colloques, 1988
We study thin ferro and antiferromagnetic films for the Heisenberg model using a modified version... more We study thin ferro and antiferromagnetic films for the Heisenberg model using a modified version of the Onsager reaction field approximation adapted for finite-size layered systems. No phase transition is obtained for the finite system, but spin-spin correlations are enhanced when the number of layers increases.
Physical Review B, 1991
A recently devised trial wave function for the Heisenberg-Ising model is tested against linear sp... more A recently devised trial wave function for the Heisenberg-Ising model is tested against linear spin waves in the whole anisotropic region. The above wave function is asymptotically exact in the limit of high anisotropy, and is constructed using locah'zed spin fluctuations over Neel states. The structure of this trial state is compared with the one presented by the zero-point state of the linear-spinwave theory. Striking similarities are found between both, despite that spin waves represent Auctuations that encompass the whole system. A crossover between both approaches near the isotropic point is attributed to a delocalization of spin fluctuations. We present results for one and two dimensions, and compare with exact results or numerical simulations.
physica status solidi (b), 2010
We give an extensive treatment of the pairing symmetry in the ferromagnetic superconductor U Ge2.... more We give an extensive treatment of the pairing symmetry in the ferromagnetic superconductor U Ge2. We show that one can draw important conclusions concerning the superconducting state, considering only the transformation properties of the pairing function, without assumptions about the form of the pairing amplitudes.
Physical Review B, 2010
We study the transport properties of a hybrid nanostructure composed of a ferromagnet, two quantu... more 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 Green's 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.
Arxiv preprint cond-mat/0109499, 2001
The distribution of magnetic moments in finite ferromagnetic bodies was first investigated by Lan... more The distribution of magnetic moments in finite ferromagnetic bodies was first investigated by Landau and Lifshitz in a famous paper [Phys. Z. Soviet Union, 8, 153 (1935)], where they obtained the domain structure of a ferromagnetic crystal at low temperatures, in the regime of saturated magnetization. In this article, we investigate the general properties of ferromagnetic domain walls of uniaxial crystals from the view point of the Landau free energy. We present the basic ideas at an introductory level, for non-experts. Extending the formalism to the vicinity of the Curie temperature, where a general qualitative description by the Landau theory of phase transitions can be applied, we find that domain walls tend to suppress the layers, leading to a continuous vanishing of the domain structure with anomalous critical exponents. In the saturated regime, we discuss the role of domain walls in mesoscopic systems and ferromagnetic nanojunctions, relating the observed magnetoresistance with promising applications in the recent area of spintronics.
Physical Review B, 1991
The electronic density of states {DOS) of disordered binary alloys is studied for a tight-binding... more The electronic density of states {DOS) of disordered binary alloys is studied for a tight-binding model Hamiltonian, using a local approach based on a continued-fraction representation for the Green function. In addition to a single-site probability, a short-range-order parameter is introduced to monitor nearest-neighbor correlations. Our method employs a termination that allows for local fluctuations of both species of atoms, according to the configuration produced. The final DOS is obtained as a weighted average over all the local spectra generated in the way described above. We probe our approach in the one-dimensional case, and excellent agreement with Monte Carlo simulations is found.
Physical Review B, 1979
Using a simple tight-binding model and the transfer matrix approach, we have calculated the spect... more Using a simple tight-binding model and the transfer matrix approach, we have calculated the spectral density of states (SDOS) of a rare-earth metal in the presence of a surface for different magnetic arrangements (such as ferromagnetic, antiferromagnetic, and conical orderings). The local density of states (LDOS) has also been calculated for some examples, integrating the SDOS over the Brillouin zone. The main effect observed deals with the absence of Van Hove's singularities in the surface LDOS, a fact that appears to be an intrinsic property of the surface, Finally the relaxation of the overlap parameters at the surface is discussed and some numerical examples are shown.
The Journal of Chemical Physics, 1997
The thermally activated desorption of dissociated diatomic species from a metallic surface is des... more The thermally activated desorption of dissociated diatomic species from a metallic surface is described as a lattice-gas problem on a square lattice with nearest-and next-nearest neighbor interactions between the adsorbates and investigated within dynamic Monte Carlo simulations. In the limit of fast diffusion with respect to desorption, it can be shown that the desorption rate depends directly on the local order induced by the interactions within the adsorbate layer. Therefore, by employing an appropriate quasi-equilibrium cluster approximation for the local order ͑beyond the quasi-chemical approximation͒, a differential equation can be derived that depends on self-consistently calculated structure forms, reproducing quantitatively the temperature-programmed desorption spectra simulated with the Monte Carlo procedure. In this way it can be shown that the time evolution obtained from the dynamic Monte Carlo algorithm is indeed ''correct,'' and on the other hand, that it can be successfully substituted by a ''cheaper'' cluster approximation.
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Papers by Guillermo Cabrera