arXiv subjects

J. J. Palacios

Publications and source records attributed to J. J. Palacios.

At least 73 records · Page 4Linked to original sources

arXiv2007

Critical fields for vortex expulsion from narrow superconducting strips

We calculate the critical magnetic fields for vortex expulsion for an infinitely long superconducting strip, using the Ginzburg-Landau formalism. Two critical fields can be defined associated with the disappearance of either the energetic stability or metastability of vortices in the center of the strip for decreasing magnetic fields. We compare the theoretical predictions for the critical fields in the London formalism with ours and with recently published experimental results. As expected, for narrow strips our results reproduce better the experimental findings.

cond-mat.supr-con↗

arXiv2007

Electronic Structure of gated graphene and graphene ribbons

We study the electronic structure of gated graphene sheets. We consider both infinite graphene and finite width ribbons. The effect of Coulomb interactions between the electrically injected carriers and the coupling to the external gate are computed self-consistently in the Hartree approximation. We compute the average density of extra carriers, $n_{2D}$, the number of occupied subbands and the density profiles as a function of the gate potential $V_g$. We discuss quantum corrections to the classical capacitance and we calculate the threshold $V_g$ above which semiconducting armchair ribbons conduct. We find that the ideal conductance of perfectly transmitting wide ribbons is proportional to the square root of the gate voltage.

cond-mat.mes-hall↗

arXiv2006

Formation of a Metallic Contact: Jump to Contact Revisited

The transition from tunneling to metallic contact between two surfaces does not always involve a jump, but can be smooth. We have observed that the configuration and material composition of the electrodes before contact largely determines the presence or absence of a jump. Moreover, when jumps are found preferential values of conductance have been identified. Through combination of experiments, molecular dynamics, and first-principles transport calculations these conductance values are identified with atomic contacts of either monomers, dimers or double-bond contacts.

cond-mat.mes-hall↗

arXiv2006

Electronic structure and transport properties of atomic NiO spinvalves

Ab-initio quantum transport calculations show that short NiO chains suspended in Ni nanocontacts present a very strong spin-polarization of the conductance. The generalized gradient approximation we use here predicts a similiar polarization of the conductance as the one previously computed with non-local exchange, confirming the robustness of the result. Their use as nanoscopic spinvalves is proposed.

cond-mat.mtrl-sci↗

arXiv2006

Emergence of half-metallicity in suspended NiO chains

Contrary to the antiferromagnetic and insulating character of bulk NiO, one-dimensional chains of this material can become half-metallic due to the lower coordination of their atoms. Here we present ab initio electronic structure and quantum transport calculations of ideal infinitely long NiO chains and of more realistic short ones suspended between Ni electrodes. While infinite chains are insulating, short suspended chains are half-metallic minority-spin conductors which display very large magnetoresistance and a spin-valve behaviour controlled by a single atom.

cond-mat.mtrl-sci↗

arXiv2006

Coherent transport in graphene nanoconstrictions

We study the effect of a structural nanoconstriction on the coherent transport properties of otherwise ideal zig-zag-edged infinitely long graphene ribbons. The electronic structure is calculated with the standard one-orbital tight-binding model and the linear conductance is obtained using the Landauer formula. We find that, since the zero-bias current is carried in the bulk of the ribbon, this is very robust with respect to a variety of constriction geometries and edge defects. In contrast, the curve of zero-bias conductance versus gate voltage departs from the $(2n+1) e^2/h$ staircase of the ideal case as soon as a single atom is removed from the sample. We also find that wedge-shaped constrictions can present non-conducting states fully localized in the constriction close to the Fermi energy. The interest of these localized states in regards the formation of quantum dots in graphene is discussed.

cond-mat.mes-hall↗

arXiv2006

Orbital eigenchannel analysis for ab-initio quantum transport calculations

We show how to extract the orbital contribution to the transport eigenchannels from a first-principles quantum transport calculation in a nanoscopic conductor. This is achieved by calculating and diagonalizing the first-principles transmission matrix reduced to selected scattering cross-sections. As an example, the orbital nature of the eigenchannels in the case of Ni nanocontacts is explored, stressing the difficulties inherent to the use of non-orthogonal basis sets and first-principles Hamiltonians.

cond-mat.mes-hall↗

arXiv2005

Transport in magnetically ordered Pt nanocontacts

Pt nanocontacts, like those formed in mechanically controlled break junctions, are shown to develop spontaneous local magnetic order. Our density functional calculations predict that a robust local magnetic order exists in the atoms presenting low coordination, i. e., those forming the atom-sized neck. In contrast to previous work, we thus find that the electronic transport can be spin-polarized, although the net value of the conductance still agrees with available experimental information. Experimental implications of the formation of this new type of nanomagnet are discussed.

cond-mat.mes-hall↗

arXiv2005

Coulomb blockade in electron transport through a C$_{60}$ molecule from first principles

We present results of spin-unrestricted first-principles quantum transport for a gated C$_{60}$ molecule weakly contacted to Al electrodes, making emphasis on the role played by the electronic localization and the spin degree of freedom. As expected, the conductance presents a series of peaks as a function of a gate voltage, demonstrating that transport in the Coulomb blockade regime can be properly treated within a first-principles scheme. A well-known manifestation of the interplay between Coulomb interaction and the spin degree of freedom in atoms and molecules, the Hund 's rule, determines the sequence of conductance peaks.

cond-mat.mtrl-sci↗

arXiv2005

Vortices in a rotating BEC under extreme elongation

We investigate a non-axisymmetric rotating BEC in a limit of rotation frequency for which the BEC transforms into a quasi-one-dimensional system. We compute the vortex lattice wavefunction by minimizing the Gross-Pitaevskii energy functional in the lowest Landau level approximation for different confinement potentials. The condensate typically presents a changing number of vortex rows as a function of the interaction strength or rotation-confinement ratio. More specifically, the vortex lattices can be classified into two classes according to their symmetry with respect to the longitudinal axis. These two classes correspond to different local minima of the energy functional and evolve independently as a function of the various parameters.

cond-mat.stat-mech↗

arXiv2005

Magnetic and orbital blocking in Ni nanocontacts

We address the fundamental question of whether magneto-resistance (MR) of atomic-sized contacts of Nickel is very large because of the formation of a domain wall (DW) at the neck. Using {\em ab initio} transport calculations we find that, as in the case of non-magnetic electrodes, transport in Ni nanocontacts depends very much on the orbital nature of the electrons. Our results are in agreement with several experiments in the average value of the conductance. On the other hand, contrary to existing claims, DW scattering does {\em not} account for large MR in Ni nanocontacts.

cond-mat.mtrl-sci↗

arXiv2004

Ballistic resistivity in aluminum nanocontacts

One of the major industrial challenges is to profit from some fascinating physical features present at the nanoscale. The production of dissipationless nanoswitches (or nanocontacts) is one of such attractive applications. Nevertheless, the lack of knowledge of the real efficiency of electronic ballistic/non dissipative transport limits future innovations. For multi-valent metallic nanosystems -where several transport channels per atom are involved- the only experimental technique available for statistical transport characterization is the conductance histogram. Unfortunately its interpretation is difficult because transport and mechanical properties are intrinsically interlaced. We perform a representative series of semiclassical molecular dynamics simulations of aluminum nanocontact breakages, coupled to full quantum conductance calculations, and put in evidence a linear relationship between the conductance and the contact minimum cross-section for the geometrically favored aluminum nanocontact configurations. Valid in a broad range of conductance values, such relation allows the definition of a transport parameter for nanomaterials, that represents the novel concept of ballistic resistivity.

cond-mat.mtrl-sci↗

arXiv2003

Electronic transport and vibrational modes in the smallest molecular bridge: H2 in Pt nanocontacts

We present a state-of-the-art first-principles analysis of electronic transport in a Pt nanocontact in the presence of H2 which has been recently reported by Smit et al. in Nature 419, 906 (2002). Our results indicate that at the last stages of the breaking of the Pt nanocontact two basic forms of bridge involving H can appear. Our claim is, in contrast to Smit et al.'s, that the main conductance histogram peak at G approx 2e^2/h is not due to molecular H2, but to a complex Pt2H2 where the H2 molecule dissociates. A first-principles vibrational analysis that compares favorably with the experimental one also supports our claim .

cond-mat.mtrl-sci↗

arXiv2003

Molecular electronics and first-principles methods

We discuss the key steps that have to be followed to calculate coherent quantum transport in molecular and atomic-scale systems, making emphasis on the ab-initio Gaussian Embedded Cluster Method recently developed by the authors. We present various results on a simple system such as a clean Au nanocontact and the same nanocontact in the presence of hydrogen that illustrate the applicability of this method in the study and interpretation of a large range of experiments in the field of molecular electronics.

cond-mat.mtrl-sci↗

arXiv2003

Fractional-quantum-Hall edge electrons and Fermi statistics

We address the quantum statistics of electrons created in the low-energy edge-state Hilbert space sector of incompressible fractional quantum Hall states, considering the possibility that they may not satisfy Fermi statistics. We argue that this property is not a priori obvious, and present numerical evidence based on finite-size exact-diagonalization calculations that it does not hold in general. We discuss different possible forms for the expression for the electron creation operator in terms of edge boson fields and show that none are consistent with our numerical results on finite-size filling-factor-2/5 states with short-range electron-electron interactions. Finally, we discuss the current body of experimental results on tunneling into quantum Hall edges in the context of this result.

cond-mat.mes-hall↗

arXiv2002

Implementing the Keldysh formalism into the ab initio Gaussian Embedded Cluster Method for the calculation of quantum transport

We discuss the key steps that have to be followed to calculate quantum transport out of equilibrium by means of the {\it ab initio} Gaussian Embedded Cluster Method recently developed by the authors. Our main aim is to emphasize through several examples that, if a sufficiently large portion of the electrodes is included in the {\it ab initio} calculation, which does also incorporate an electrochemical potential difference $μ_L-μ_R=eV$, there is no need to impose an electrostatic potential $V$ drop accross the system.

cond-mat↗

arXiv2002

Analysis of Scanning Tunneling Spectroscopy Experiments from First Principles: the Test Case of C60 Adsorbed on Au(111)

We report on the first principles determination of the conductance properties of Buckminster fullerene adsorbed on a gold surface, comparing them with recent Scanning Tunneling Microscopy/Spectroscopy (STM/S) experiments [J.Chem.Phys.116, 832 (2002)]. Our procedure has the two appealing features of being able to elucidate delicate aspects of STM/S results without resorting to ad hoc fittings or approximations and to provide a complete description of the phenomena involved when the tip approaches the adsorbate.

cond-mat.mtrl-sci↗

arXiv2002

First-principles phase-coherent transport in metallic nanotubes with realistic contacts

We present first-principles calculations of phase coherent electron transport in a carbon nanotube (CNT) with realistic contacts. We focus on the zero-bias response of open metallic CNT's considering two archetypal contact geometries (end and side) and three commonly used metals as electrodes (Al, Au, and Ti). Our ab-initio electrical transport calculations make, for the first time, quantitative predictions on the contact transparency and the transport properties of finite metallic CNT's. Al and Au turn out to make poor contacts while Ti is the best option of the three. Additional information on the CNT band mixing at the contacts is also obtained.

cond-mat.mtrl-sci↗