arXiv subjects

P. D. Sacramento

Publications and source records attributed to P. D. Sacramento.

At least 55 records · Page 3Linked to original sources

arXiv2011

Fidelity spectrum and phase transitions of quantum systems

Quantum fidelity between two density matrices, $F(ρ_1,ρ_2)$ is usually defined as the trace of the operator ${\cal F}=\sqrt{\sqrt{ρ_1} ρ_2 \sqrt{ρ_1}}$. We study the logarithmic spectrum of this operator, which we denote by {\it fidelity spectrum}, in the cases of the $XX$ spin chain in a magnetic field, a magnetic impurity inserted in a conventional superconductor and a bulk superconductor at finite temperature. When the density matrices are equal, $ρ_1=ρ_2$, the fidelity spectrum reduces to the entanglement spectrum. We find that the fidelity spectrum can be a useful tool in giving a detailed characterization of different phases of many-body quantum systems.

quant-ph↗

arXiv2011

Enhancement of the critical temperature in iron-pnictide superconductors by finite size effects

Recent experiments have shown that, in agreement with previous theoretical predictions, superconductivity in metallic nanostructures can be enhanced with respect to the bulk limit. Motivated by these results we study finite size effects (FSE) in an iron-pnictide superconductor. For realistic values of the bulk critical temperature Tc ~ 20-50K, we find that, in the nanoscale region L ~ 10 nm, Tc(L) has a complicated oscillating pattern as a function of the system size L. A substantial enhancement of Tc with respect to the bulk limit is observed for different boundary conditions, geometries and two microscopic models of superconductivity. Thermal fluctuations, which break long range order, are still small in this region. Finally we show that the differential conductance, an experimental observable, is also very sensitive to FSE.

cond-mat.supr-con↗

arXiv2011

Finite energy spectral function of an anisotropic 2D system of coupled Hubbard chains

We study the crossover from the one-dimensional to the two-dimensional Hubbard model in the photoemission spectra of weakly coupled chains. The chains with on-site repulsion are treated using the spin-charge factorized wave function, that is known to provide an essentially exact description of the chain in the strong coupling limit. The hoppings between the chains are considered as a perturbation. We calculate the dynamical spectral function at all energies in the random-phase approximation, by resuming an infinite set of diagrams. Even though the hoppings drive the system from a fractionalized Luttinger-liquid-like system to a Fermi-liquid-like system at low energies, significant characteristics of the one-dimensional system remain in the two-dimensional system. Furthermore, we find that introducing (frustrating) hoppings beyond the nearest neighbor one, the interference effects increase the energy and momentum range of the one--dimensional character.

cond-mat.str-el↗

arXiv2010

U(1) slave-particle study of the finite-temperature doped Hubbard model in one and two dimensions

One-dimensional systems have unusual properties such as fractionalization of degrees of freedom. Possible extensions to higher dimensional systems have been considered in the literature. In this work we construct a mean field theory of the Hubbard model taking into account a separation of the degrees of freedom inspired by the one-dimensional case and study the finite-temperature phase diagram for the Hubbard chain and square lattice. The mean field variables are defined along the links of the underlying lattice. We obtain the spectral function and identify the regions of higher spectral weight with the fractionalized fermionic (spin) and bosonic (charge) excitations.

cond-mat.str-el↗

arXiv2009

Single vortex structure in two models of iron pnictide $s^\pm$ superconductivity

The structure of a single vortex in a FeAs superconductor is studied in the framework of two formulations of superconductivity for the recently proposed sign-reversed $s$ wave ($s^\pm$) scenario: {\it (i)} a continuum model taking into account the existence of an electron and a hole band with a repulsive local interaction between the two; {\it (ii)} a lattice tight-binding model with two orbitals per unit cell and a next-nearest-neighbour attractive interaction. In the first model, the local density of states (LDOS) at the vortex centre, as a function of energy, exhibits a peak at the Fermi level, while in the second model such LDOS peak is deviated from the Fermi level and its energy depends on band filling. An impurity located outside the vortex core has little effect on the LDOS peak, but an impurity close to the vortex core can almost suppress it and modify its position.

cond-mat.supr-con↗

arXiv2009

Theory of Andreev reflection in a two-orbital model of iron-pnictide superconductors

A recently developed theory for the problem of Andreev reflection between a normal metal (N) and a multiband superconductor (MBS) assumes that the incident wave from the normal metal is coherently transmitted through several bands inside the superconductor. Such splitting of the probability amplitude into several channels is the analogue of a quantum waveguide. Thus, the appropriate matching conditions for the wave function at the N/MBS interface are derived from an extension of quantum waveguide theory. Interference effects between the transmitted waves inside the superconductor manifest themselves in the conductance. We provide results for a FeAs superconductor, in the framework of a recently proposed effective two-band model and two recently proposed gap symmetries: in the sign-reversed s-wave ($Δ\cos(k_x)\cos(k_y)$) scenario resonant transmission through surface Andreev bound states (ABS) at nonzero energy is found as well as destructive interference effects that produce zeros in the conductance; in the extended s-wave ($Δ[\cos(k_x)+\cos(k_y)]$) scenario no ABS at finite energy are found.

cond-mat.supr-con↗

arXiv2009

Quantum waveguide theory of Andreev reflection in multiband superconductors: the case of Fe-pnictides

The problem of Andreev reflection between a normal metal and a multiband superconductor is addressed. The appropriate matching conditions for the wave function at the interface are established on the basis of an extension of quantum waveguide theory to these systems. Interference effects between different bands of the superconductor manifest themselves in the conductance and the case of FeAs superconductors is specifically considered, in the framework of a recently proposed effective two-band model, in the sign-reversed s-wave pairing scenario. Resonant transmission through surface Andreev bound states is found as well as destructive interference effects that produce zeros in the conductance at normal incidence. Both these effects occur at nonzero bias voltage.

cond-mat.supr-con↗

arXiv2008

Electron transmission in normal/heavy-fermion superconductor junctions

The Andreev reflection between a normal metal (N) and a heavy-fermion superconductor (HFS) is studied and the boundary conditions for the electron's wave function in the two systems are established in the framework of a two band model for the HFS. Hence we show in a simple and explicit way that the mass enhancement factors in the heavy-fermion (HF) metal do not cause impedance at the N/HFS interface, in accordance with arguments previously presented. We also present an extension of the theory to a two-fluid model for the heavy-fermion, as possibly applicable to, e.g., CeCoIn_5.

cond-mat.supr-con↗

arXiv2007

The TTF finite-energy spectral features in photoemission of TTF-TCNQ: The Hubbard-chain description

A dynamical theory which accounts for all microscopic one-electron processes is used to study the spectral function of the 1D Hubbard model for the whole $(k, ω)$-plane, beyond previous studies which focused on the weight distribution in the vicinity of the singular branch lines only. While our predictions agree with those of the latter studies concerning the tetracyanoquinodimethane (TCNQ) related singular features in photoemission of the organic compound tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) metallic phase, the generalized theory also leads to quantitative agreement concerning the tetrathiafulvalene (TTF) related finite-energy spectral features, which are found to correspond to a value of the on-site repulsion $U$ larger than for TCNQ. Our study reveals the microscopic mechanisms behind the unusual spectral features of TTF-TCNQ and provides a good overall description of those features for the whole $(k, ω)$-plane.

cond-mat.str-el↗

arXiv2007

Fidelity Between Partial States as Signature of Quantum Phase Transitions

We introduce a partial state fidelity approach to quantum phase transitions. We consider a superconducting lattice with a magnetic impurity inserted at its centre, and look at the fidelity between partial (either one-site or two-site) quantum states. In the vicinity of the point of the quantum phase transition, we observe a sudden drop of the fidelity between two one-site partial states corresponding to the impurity location and its close vicinity. In the case of two-site states, the fidelity reveals the transition point as long as one of the two electron sites is located at the impurity, while the other lies elsewhere in the lattice. We also determine the Uhlmann mixed state geometric phase, recently introduced in the study of the structural change of the system state eigenvectors in the vicinity of the lines of thermal phase transitions, and find it to be trivial, both for one- and two-site partial states, except when an electron site is at the impurity. This means that the system partial state eigenvectors do not contribute significantly to the enhanced state distinguishability around the point of this quantum phase transition. Finally, we use the fidelity to analyze the total amount of correlations contained within a composite system, showing that, even for the smallest two-site states, it features an abrupt quantitative change in the vicinity of the point of the quantum phase transition.

quant-ph↗

arXiv2007

Entanglement signatures of the quantum phase transition induced by a magnetic impurity in a superconductor

The insertion of a magnetic impurity in a superconductor induces a first order quantum phase transition as the coupling to the electronic spin density increases. As the transition is crossed, a discontinuity is exhibited by various quantities, like the total spin density, the total gap function and the gap function at the impurity location. The location of other quantum phase transitions have been detected by singularities in entanglement measures of the system. In this work we show that the single-site and two-site von Neumann entropies, the mutual information and the Meyer-Wallach measure show discontinuities at the quantum phase transition. The negativity is less sensitive to the transition. We study in detail these quantities as a function of spin coupling and distance from the impurity center.

cond-mat.supr-con↗

arXiv2007

Magnetic impurities in a superconductor: Effect of domain walls and interference

We consider the effect of magnetic impurities, modeled by classical spins, in a conventional superconductor. We study their effect on the quasiparticles, specifically on the spin density and local density of states (LDOS). As previously emphasized, the impurities induce multiple scatterings of the quasiparticle wave functions leading to complex interference phenomena. Also, the impurities induce quantum phase transitions in the many-body system. Previous authors studied the effect of either a small number of impurities (from one to three) or a finite concentration of impurities, typically in a disordered distribution. In this work we assume a regular set of spins distributed inside the superconductor in such a way that the spins are oriented, forming different types of domain walls, assumed stable. This situation may be particularly interesting in the context of spin transfer due to polarized currents traversing the material.

cond-mat.supr-con↗

arXiv2006

Pseudofermion scattering theory

In this paper we study the scattering theory associated with the pseudofermion dynamical theory for the Hubbard chain. In terms of pseudofermions the spectral properties are controlled by zero-momentum forward scattering only. The pseudofermion $S$ matrix is expressed as a commutative product of $S$ matrices, each corresponding to an elementary two-pseudofermion scattering event. This commutative factorization is stronger than the usual factorization associated with Yang-Baxter Equation for the original spin 1/2 electron bare $S$ matrix. Our results reveal the scattering mechanisms which control the exotic finite-energy spectral properties of the low-dimensional complex materials and correlated systems of cold fermionic atoms on an optical lattice. Importantly, the exotic scatterers and scattering centers predicted by the theory were observed by angle-resolved photoelectron spectroscopy in low-dimensional organic metals.

cond-mat.str-el↗

arXiv2006

The Hubbard model description of the photoemission TCNQ singular features

In this paper we use the pseudofermion dynamical theory (PDT) in the study of the one-electron removal singular spectral features the one-dimensional Hubbard model. The PDT reveals that in the whole $(k, ω)$-plane such features are of power-law type and correspond to well defined lines of three types: the charge singular branch lines, the spin singular branch lines, and the border lines. One of our goals is the study of the momentum and energy dependence of the spectral-weight distribution in the vicinity of such lines. We find that the charge and spin branch lines correspond to the main tetracyanoquinodimethane (TCNQ) peak dispersions observed by angle-resolved photoelectron spectroscopy in the quasi-1D organic conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). Our expressions refer to all values of the electronic density and on-site repulsion $U$. The weight distribution in the vicinity of the singular spectral lines is fully controlled by the overall pseudofermion phase shifts. Moreover, the shape of these lines is determined by the bare-momentum dependence of the pseudofermion energy dispersions.

cond-mat.str-el↗

arXiv2006

Effects of disorder on the vortex charge

We study the influence of disorder on the vortex charge, both due to random pinning of the vortices and due to scattering off non-magnetic impurities. In the case when there are no impurities present, but the vortices are randomly distributed, the effect is very small, except when two or more vortices are close by. When impurities are present, they have a noticeable effect on the vortex charge. This, together with the effect of temperature, changes appreciably the vortex charge. In the case of an attractive impurity potential the sign of the charge naturally changes.

cond-mat.supr-con↗

arXiv2005

Current-induced motion of a domain wall in magnetic nanowires

The dynamics of current-induced motion of a magnetic domain wall in a quasi-one-dimensional ferromagnet with both easy-axis and easy-plane anisotropy, is studied. We pay a special attention to the case of a sharp domain wall, and calculate the spin torque created by the electric current. The torque has two components, one of which is acting as a driving force for the motion of the domain wall while the other distorts its shape, forcing thus the magnetic moments to deviate from the easy plane.

cond-mat.mtrl-sci↗

arXiv2005

Local density of states of a strongly type-II d-wave superconductor: The binary alloy model in a magnetic field

We calculate self-consistently the local density of states (LDOS) of a d-wave superconductor considering the scattering of the quasiparticles off randomly distributed impurities and off externally induced vortices. The impurities and the vortices are randomly distributed but the vortices are preferably located near the impurities. The increase of either the impurity repulsive potential or the mpurity density only affects the density of states (DOS) slightly. The dominant effect is due to the vortex scattering. The results for the LDOS agree qualitatively with experimental results considering that most vortices are pinned at the impurities.

cond-mat.supr-con↗

arXiv2004

Superconductivity Driven by Chain Coupling and Electronic Correlations

We present an analysis of a system of weakly coupled Hubbard chains based on combining an exact study of spectral functions of the uncoupled chain system with a renormalization group method for the coupled chains. For low values of the onsite repulsion $U$ and of the doping $δ$, the leading instability is towards a superconducting state. The process includes excited states above a small correlation pseudogap. Similar features appear in extended Hubbard models in the vicinity of commensurate fillings. Our theoretical predictions are consistent with the phase diagram observed in the (TMTTF)$_2$X and (TMTSF)$_2$X series of organic compounds.

cond-mat.str-el↗