arXiv subjects

Jaime Ferrer

Publications and source records attributed to Jaime Ferrer.

35 records · Page 2Linked to original sources

arXiv2016

Structural phase transition and material properties of few-layer monochalcogenides

GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a temperature $T_c$ well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above $T_c$. As the in-plane lattice transforms from a rectangle onto a square at $T_c$, the electronic, spin, optical, and piezo-electric properties dramatically depart from earlier predictions. Indeed, the $Y-$ and $X-$points in the Brillouin zone become effectively equivalent at $T_c$, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at $T_c$. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement has been drawn by theoretical predictions of giant piezo-electricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about $3\times 10^{-12}$ $C/K m$ here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides

cond-mat.mtrl-sci↗

arXiv2015

The Hubbard Dimer: A density functional case study of a many-body problem

This review explains the relationship between density functional theory and strongly correlated models using the simplest possible example, the two-site Hubbard model. The relationship to traditional quantum chemistry is included. Even in this elementary example, where the exact ground-state energy and site occupations can be found analytically, there is much to be explained in terms of the underlying logic and aims of Density Functional Theory. Although the usual solution is analytic, the density functional is given only implicitly. We overcome this difficulty using the Levy-Lieb construction to create a parametrization of the exact function with negligible errors. The symmetric case is most commonly studied, but we find a rich variation in behavior by including asymmetry, as strong correlation physics vies with charge-transfer effects. We explore the behavior of the gap and the many-body Green's function, demonstrating the `failure' of the Kohn-Sham method to reproduce the fundamental gap. We perform benchmark calculations of the occupation and components of the KS potentials, the correlation kinetic energies, and the adiabatic connection. We test several approximate functionals (restricted and unrestricted Hartree-Fock and Bethe Ansatz Local Density Approximation) to show their successes and limitations. We also discuss and illustrate the concept of the derivative discontinuity. Useful appendices include analytic expressions for Density Functional energy components, several limits of the exact functional (weak- and strong-coupling, symmetric and asymmetric), the Kohn-Sham hopping energy functional for 3 sites, various adiabatic connection results, proofs of exact conditions for this model, and the origin of the Hubbard model from a minimal basis model for stretched H$_2$.

cond-mat.str-el↗

arXiv2014

Stability and properties of high-buckled two-dimensional tin and lead

In realizing practical non-trivial topological electronic phases stable structures need to be determined first. Tin and lead do stabilize an optimal two-dimensional high-buckled phase --a hexagonal-close packed bilayer structure with nine-fold atomic coordination-- and they do not stabilize topological fullerenes, as demonstrated by energetics, phonon dispersion curves, and the structural optimization of finite-size samples. The high-buckled phases are metallic due to their high atomic coordination. The optimal structure of fluorinated tin lacks three-fold symmetry and it stabilizes small samples too. It develops two oblate conical valleys on the first Brillouin zone coupling valley, sublattice, and spin degrees of freedom with a novel $τ_zσ_xs_x$ term, thus making it a new 2D platform for valleytronics.

cond-mat.mes-hall↗

arXiv2012

Symmetry-induced interference effects in metalloporphyrin wires

Organo-metallic molecular structures where a single metallic atom is embedded in the organic backbone are ideal systems to study the effect of strong correlations on their electronic structure. In this work we calculate the electronic and transport properties of a series of metalloporphyrin molecules sandwiched by gold electrodes using a combination of density functional theory and scattering theory. The impact of strong correlations at the central metallic atom is gauged by comparing our results obtained using conventional DFT and DFT+U approaches. The zero bias transport properties may or may not show spin-filtering behavior, depending on the nature of the d state closest to the Fermi energy. The type of d state depends on the metallic atom and gives rise to interference effects that produce different Fano features. The inclusion of the U term opens a gap between the d states and changes qualitatively the conductance and spin-filtering behavior in some of the molecules. We explain the origin of the quantum interference effects found as due to the symmetry-dependent coupling between the d states and other molecular orbitals and propose the use of these systems as nanoscale chemical sensors. We also demonstrate that an adequate treatment of strong correlations is really necessary to correctly describe the transport properties of metalloporphyrins and similar molecular magnets.

cond-mat.mes-hall↗

arXiv2011

A Molecular Study of CaCO$_3$ cluster configurations

Equilibrium relationships involving solids are based on bulk thermodynamic properties that concern ideal crystals of infinite size. However, real processes towards equilibrium imply development of finite molecular-scale entities. The configuration of these early-stage clusters and the estimation of their excess energies with respect to the ideal crystal are keys to understanding the macroscopic behaviour of a given system. As nucleation events are difficult to study experimentally, both because they occur spontaneously and because the nucleus size is very small, atomistic simulations are a suitable tool for understanding the early stages of crystallisation. Here, starting from the ideal atomic positions in calcite and aragonite, the relaxation in vacuum of finite clusters of CaCO$_3$ is explored. Nucleation and growth of calcium carbonate phases constitute a very important subject of research in a wide variety of fields. A complete study of CaCO$_3$ should include many different aspects: size and shape of the critical nuclei under diverse conditions, possibility of nucleation from precursor phases, nucleus energy and nucleus surface energy, relationship nucleus-substrate in heterogeneous nucleation...We present a preliminary study of nucleation of calcium carbonate where nuclei are considered to be isolated from any previous phase or substrate. Even when this situation does in no way represent realistic conditions, it can be a helpful first approach to more complex studies.

cond-mat.mtrl-sci↗

arXiv2010

Effects of bonding type and interface geometry on coherent transport through the single-molecule magnet Mn12

We examine theoretically coherent electron transport through the single-molecule magnet Mn$_{12}$, bridged between Au(111) electrodes, using the non-equilibrium Green's function method and the density-functional theory. We analyze the effects of bonding type, molecular orientation, and geometry relaxation on the electronic properties and charge and spin transport across the single-molecule junction. We consider nine interface geometries leading to five bonding mechanisms and two molecular orientations: (i) Au-C bonding, (ii) Au-Au bonding, (iii) Au-S bonding, (iv) Au-H bonding, and (v) physisorption via van der Waals forces. The two molecular orientations of Mn$_{12}$ correspond to the magnetic easy axis of the molecule aligned perpendicular [hereafter denoted as orientation (1)] or parallel [orientation (2)] to the direction of electron transport. We find that the electron transport is carried by the lowest unoccupied molecular orbital (LUMO) level in all the cases that we have simulated. Relaxation of the junction geometries mainly shifts the relevant occupied molecular levels toward the Fermi energy as well as slightly reduces the broadening of the LUMO level. As a result, the current slightly decreases at low bias voltage. Our calculations also show that placing the molecule in the orientation (1) broadens the LUMO level much more than in the orientation (2), due to the internal structure of the Mn$_{12}$. Consequently, junctions with the former orientation yield a higher current than those with the latter. Among all of the bonding types considered, the Au-C bonding gives rise to the highest current (about one order of magnitude higher than the Au-S bonding), for a given distance between the electrodes. The current through the junction with other bonding types decreases in the order of Au-Au, Au-S, and Au-H. Importantly, the spin-filtering effect in all the nine geometries stays robust and their ratios of the majority-spin to the minority-spin transmission coefficients are in the range of 10$^3$ to 10$^8$. The general trend in transport among the different bonding types and molecular orientations obtained from this study may be applied to other single-molecular magnets.

cond-mat.mes-hall↗

arXiv2009

First-principles study of electron transport through the single-molecule magnet Mn12

We examine electron transport through a single-molecule magnet Mn12 bridged between Au electrodes using the first-principles method. We find crucial features which were inaccessible in model Hamiltonian studies: spin filtering and a strong dependence of charge distribution on local environments. The spin filtering remains robust with different molecular geometries and interfaces, and strong electron correlations, while the charge distribution over the Mn12 strongly depends on them. We point out a qualitative difference between locally charged and free-electron charged Mn12.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic properties of small Pt-capped Fe, Co and Ni clusters: A density functional theory study

Theoretical studies on M$_{13}$ (M = Fe, Co, Ni) and M$_{13}$Pt$_n$ (for $n$ = 3, 4, 5, 20) clusters including the spin-orbit coupling are done using density functional theory. The magnetic anisotropy energy (MAE) along with the spin and orbital moments are calculated for M$_{13}$ icosahedral clusters. The angle-dependent energy differences are modelled using an extended classical Heisenberg model with local anisotropies. From our studies, the MAE for Jahn-Teller distorted Fe$_{13}$, Mackay distorted Fe$_{13}$ and nearly undistorted Co$_{13}$ clusters are found to be 322, 60 and 5 $μ$eV/atom, respectively, and are large relative to the corresponding bulk values, (which are 1.4 and 1.3 $μ$eV/atom for bcc Fe and fcc Co, respectively.) However, for Ni$_{13}$ (which practically does not show relaxation tendencies), the calculated value of MAE is found to be 0.64 $μ$eV/atom, which is approximately four times smaller compared to the bulk fcc Ni (2.7 $μ$eV/atom). In addition, MAE of the capped cluster (Fe$_{13}$Pt$_4$) is enhanced compared to the uncapped Jahn-Teller distorted Fe$_{13}$ cluster.

cond-mat.mes-hall↗

arXiv2009

Unconventional magnetism in small gold organic molecules

We present a theoretical study of the magnetic properties of dicyclopentadienyl metallocene and phthalocyanine molecules, that contain the transition metal atoms M = Fe, Co, Ni, Cu, Zn, Ir, Pt and Au. Our most important prediction is that gold and copper molecules are magnetic. We find that the magnetism of these molecules is fairly unconventional: the gold atom itself is weakly magnetic or even non-magnetic. Its role is rather to induce magnetism in the surrounding carbon and nitrogen atoms, producing a sort of spin density wave.

cond-mat.mtrl-sci↗

arXiv2009

Spin-filtering effect in the transport through a single-molecule magnet Mn$_{12}$ bridged between metallic electrodes

Electronic transport through a single-molecule magnet Mn$_{12}$ in a two-terminal set up is calculated using the non-equilibrium Green's function method in conjunction with density-functional theory. A single-molecule magnet Mn$_{12}$ is bridged between Au(111) electrodes via thiol group and alkane chains such that its magnetic easy axis is normal to the transport direction. A computed spin-polarized transmission coefficient in zero-bias reveals that resonant tunneling near the Fermi level occurs through some molecular orbitals of majority spin only. Thus, for low bias voltages, a spin-filtering effect such as only one spin component contributing to the conductance, is expected. This effect would persist even with inclusion of additional electron correlations.

cond-mat.mtrl-sci↗

arXiv2007

On-site approximation for spin-orbit coupling in LCAO density functional methods

We propose a computational method that simplifies drastically the inclusion of spin-orbit interaction in density functional theory implemented on localised atomic orbital basis sets. Our method is based on a well-known procedure for obtaining pseudopotentials from atomic relativistic 'ab initio' calculations and on an on-site approximation for the spin-orbit matrix elements. We have implemented the technique in the SIESTA code, and we show that it provides accurate results for the overall band structure and splittings of group IV and III-IV semiconductors as well as for 5d metals.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic anisotropies of late transition metal atomic clusters

We analyze the impact of the magnetic anisotropy on the geometric structure and magnetic ordering of small atomic clusters of palladium, iridium, platinum and gold, using Density Functional Theory. Our results highlight the absolute need to include self-consistently the spin orbit interaction in any simulation of the magnetic properties of small atomic clusters, and a complete lack of universality in the magnetic anisotropy of small-sized atomic clusters.

cond-mat.mtrl-sci↗

arXiv2006

Predictions for the formation of new atomic chains in Mechanically Controllable Break Junction experiments

We analyze the stability and magnetic properties of infinite zigzag atomic chains of a large number of late third, fourth and fifth-row transition metal atoms, as well as of the Group IV elements Si, Ge, Sn and Pb. We find that zigzag chains of third- and fourth-row elements are not stable, while those made of Si, Ge, Sn, Pb, W, Os, Ir, Pt and Au are. These results correlate well with known data in Mechanically Controllable Break Junction experiemnts (MCBJE). We therefore conjecture that the stability of an infinite chain is at least a necessary condition for the formation of a finite sized one in MCBJE. We therefore predict that Sn and Os, and possibly W and Pb chains may be found in those experiments. We also find that the bonds in Hg chains are extremely soft. We finally show that the magnetic moments and anisotropies of Ir and Pt chains show a non-trivial behavior.

cond-mat.mtrl-sci↗

arXiv1994

Crossover from Spin-Density-Wave to Neel-like Ground state

The characterization and evolution of a Spin Density Wave into the Quantum Neel ground state is considered in the context of a weak coupling theory of the half-filled Hubbard model. Magnetic properties obtained from this weak coupling approach in one dimension compare favorably with exact results from Bethe ansatz (BA). A study of the evolution of several length scales from weak to strong coupling is also presented.

cond-mat↗

arXiv1994

Bosonization on the lattice: the emergence of the higher harmonics

A general and transparent procedure to bosonize fermions placed on a lattice is presented. Harmonics higher than $k_F$ are shown to appear in the one-paticle Green function, due to the compact character of real electron bands. Quantitative estimations of the role of these higher harmonics are made possible by the bosonization technique presented here.

cond-mat↗

arXiv1993

Crossover from the Josephson effect to bulk superconducting flow

The crossover between ideal Josephson behavior and uniform superconducting flow is studied by solving exactly the Ginzburg-Landau equation for a one-dimensional superconductor in the presence of an effective delta function potential of arbitrary strength. As the effective scattering is turned off, the pairs of Josephson solutions with equal current evolve into a uniform and a solitonic solution with nonzero phase offset. It is also argued that a microscopic description of the crossover must satisfy the self-consistency condition, which is shown to guarantee current conservation. The adiabatic response to an external bias is briefly described. The ac Josephson effect is shown to break down when the external voltage is applied at points which are sufficiently far from the junction.

cond-mat↗

arXiv1992

Spin liquid phase for the frustrated Quantum Heisenberg Antiferromagnet on a square lattice

The existance of a spin disordered ground state for the frustrated Quantum Heisenberg Antiferromagnet on a square lattice is reconsidered. It is argued that there is a unique action which is continuous through the whole phase diagram, except at the Lifshitz point, so that the Neel and helicoidal states can not coexist and there has to be an intermediate spin liquid state. To show it, a detailed study combining Spin-Wave theory, Schwinger Bosons Mean Field Theory and a scaling analysis of the appropriate hydrodynamic action is performed. When done carefully, all these theories agree and strongly support the existance of the spin liquThe manuscript has eight figures, which are available upon request to the author. e-mail address is ferrer@physics.rutgers.edu

cond-mat↗