arXiv subjects

V. M. García-Suárez

Publications and source records attributed to V. M. García-Suárez.

17 recordsLinked to original sources

arXiv2020

SIESTA: recent developments and applications

A review of the present status, recent enhancements, and applicability of the SIESTA program is presented. Since its debut in the mid-nineties, SIESTA's flexibility, efficiency and free distribution has given advanced materials simulation capabilities to many groups worldwide. The core methodological scheme of SIESTA combines finite-support pseudo-atomic orbitals as basis sets, norm-conserving pseudopotentials, and a real-space grid for the representation of charge density and potentials and the computation of their associated matrix elements. Here we describe the more recent implementations on top of that core scheme, which include: full spin-orbit interaction, non-repeated and multiple-contact ballistic electron transport, DFT+U and hybrid functionals, time-dependent DFT, novel reduced-scaling solvers, density-functional perturbation theory, efficient Van der Waals non-local density functionals, and enhanced molecular-dynamics options. In addition, a substantial effort has been made in enhancing interoperability and interfacing with other codes and utilities, such as Wannier90 and the second-principles modelling it can be used for, an AiiDA plugin for workflow automatization, interface to Lua for steering SIESTA runs, and various postprocessing utilities. SIESTA has also been engaged in the Electronic Structure Library effort from its inception, which has allowed the sharing of various low level libraries, as well as data standards and support for them, in particular the PSML definition and library for transferable pseudopotentials, and the interface to the ELSI library of solvers. Code sharing is made easier by the new open-source licensing model of the program. This review also presents examples of application of the capabilities of the code, as well as a view of on-going and future developments.

physics.comp-ph↗

arXiv2020

Electronics without bridging components

We propose a new paradigm of electronic devices based only on two electrodes separated by a gap, i.e. without any functional element bridging them. We use a tight-binding model to show that, depending on the type of material of the electrodes and its structure, several electronic functionalities can be achieved: ohmic behaviour, rectification, negative differential resistance, spin-filtering and magnetoresistance. In particular, we show that it is possible to deliver a given functionality by changing the coupling between the surface and bulk states and between the surface states across the gap, which dramatically changes the current-voltage characteristics. These results prove that it is possible to have functional electronic and spintronic elements on the nanoscale without having physical components bridging the electrodes.

cond-mat.mes-hall↗

arXiv2013

Impact of Fano and Breit-Wigner resonances in the thermoelectric properties of nanoscale junctions

We show that the thermoelectric properties of nanoscale junctions featuring states near the Fermi level strongly depend on the type of resonance generated by such states, which can be either Fano or Breit-Wigner-like. We give general expressions for the thermoelectric coefficients generated by the two types of resonances and calculate the thermoelectric properties of these systems, which encompass most nanoelectronics junctions. We include simulations of real junctions where metalloporphyrin molecules bridge gold electrodes and prove that for some metallic elements the thermoelectric properties show a large variability. We find that the thermopower and figure of merit are largely enhanced when the resonance gets close to the Fermi level and reach values much higher than typical values found in other nanoscale junctions. The specific value and temperature dependence are determined by a series of factors such as the strength of the coupling between the state and other molecular states, the symmetry of the state, the strength of the coupling between the molecule and the leads and the spin filtering behavior of the junction.

cond-mat.mes-hall↗

arXiv2013

Universality in the transport response of molecular wires physisorbed onto graphene electrodes

We analyze the low-voltage transport response of large molecular wires bridging graphene electrodes, where the molecules are physisorbed onto the graphene sheets by planar anchor groups. In our study, the sheets are pulled away to vary the gap length and the relative atomic positions. The molecular wires are also translated in directions parallel and perpendicular to the sheets. We show that the energy position of the Breit-Wigner molecular resonances is universal for a given molecule, in the sense that it is independent of the details of the graphene edges, gaps lengths or of the molecule positions. We discuss the need to converge carefully the k-sampling to provide reasonable values of the conductance.

cond-mat.mes-hall↗

arXiv2012

Non-equilibrium transport response from equilibrium transport theory

We propose a simple scheme that describes accurately essential non-equilibrium effects in nanoscale electronics devices using equilibrium transport theory. The scheme, which is based on the alignment and dealignment of the junction molecular orbitals with the shifted Fermi levels of the electrodes, simplifies drastically the calculation of current-voltage characteristics compared to typical non-equilibrium algorithms. We probe that the scheme captures a number of non-trivial transport phenomena such as the negative differential resistance and rectification effects. It applies to those atomic-scale junctions whose relevant states for transport are spatially placed on the contact atoms or near the electrodes.

cond-mat.mes-hall↗

arXiv2012

Impact of edge shape on the functionalities of graphene-based single-molecule electronics devices

We present an ab-initio analysis of the impact of edge shape and graphene-molecule anchor coupling on the electronic and transport functionalities of graphene-based molecular electronics devices. We analyze how Fano-like resonances, spin filtering and negative differential resistance effects may or may not arise by modifying suitably the edge shapes and the terminating groups of simple organic molecules. We show that the spin filtering effect is a consequence of the magnetic behavior of zigzag-terminated edges, which is enhanced by furnishing these with a wedge shape. The negative differential resistance effect is originated by the presence of two degenerate electronic states localized at each of the atoms coupling the molecule to graphene which are strongly affected by a bias voltage. The effect could thus be tailored by a suitable choice of the molecule and contact atoms if edge shape could be controlled with atomic precision.

cond-mat.mes-hall↗

arXiv2011

Structure and electronic properties of molybdenum monoatomic wires encapsulated in carbon nanotubes

Monoatomic chains of molybdenum encapsulated in single walled carbon nanotubes of different chiralities are investigated using density functional theory. We determine the optimal size of the carbon nanotube for encapsulating a single atomic wire, as well as the most stable atomic arrangement adopted by the wire. We also study the transport properties in the ballistic regime by computing the transmission coefficients and tracing them back to electronic conduction channels of the wire and the host. We predict that carbon nanotubes of appropriate radii encapsulating a Mo wire have metallic behavior, even if both the nanotube and the wire are insulators. Therefore, encapsulating Mo wires in CNT is a way to create conductive quasi one-dimensional hybrid nanostructures.

cond-mat.mes-hall↗

arXiv2011

Quantum Interference in Single Molecule Electronic Systems

We present a general analytical formula and an ab initio study of quantum interference in multi-branch molecules. Ab initio calculations are used to investigate quantum interference in a benzene-1,2-dithiolate (BDT) molecule sandwiched between gold electrodes and through oligoynes of various lengths. We show that when a point charge is located in the plane of a BDT molecule and its position varied, the electrical conductance exhibits a clear interference effect, whereas when the charge approaches a BDT molecule along a line normal to the plane of the molecule and passing through the centre of the phenyl ring, interference effects are negligible. In the case of olygoynes, quantum interference leads to the appearance of a critical energy $E_c$, at which the electron transmission coefficient $T(E)$ of chains with even or odd numbers of atoms is independent of length. To illustrate the underlying physics, we derive a general analytical formula for electron transport through multi-branch structures and demonstrate the versatility of the formula by comparing it with the above ab-initio simulations. We also employ the analytical formula to investigate the current inside the molecule and demonstrate that large counter currents can occur within a ring-like molecule such as BDT, when the point charge is located in the plane of the molecule. The formula can be used to describe quantum interference and Fano resonances in structures with branches containing arbitrary elastic scattering regions connected to nodal sites.

cond-mat.mes-hall↗

arXiv2010

Impact of dimerization and stretching on the transport properties of molybdenum atomic wires

We study the electrical and transport properties of monoatomic Mo wires with different structural characteristics. We consider first periodic wires with inter-atomic distances ranging between the dimerized wire to that formed by equidistant atoms. We find that the dimerized case has a gap in the electronic structure which makes it insulating, as opposed to the equidistant or near-equidistant cases which are metallic. We also simulate two conducting one-dimensional Mo electrodes separated by a scattering region which contains a number of dimers between 1 and 6. The $I-V$ characteristics strongly depend on the number of dimers and vary from ohmic to tunneling, with the presence of different gaps. We also find that stretched chains are ferromagnetic.

cond-mat.mes-hall↗

arXiv2008

Giant thermopower and figure of merit in single-molecule devices

We present a study of the thermopower $S$ and the dimensionless figure of merit $ZT$ in molecules sandwiched between gold electrodes. We show that for molecules with side groups, the shape of the transmission coefficient can be dramatically modified by Fano resonances near the Fermi energy, which can be tuned to produce huge increases in $S$ and $ZT$. This shows that molecules exhibiting Fano resonances have a high efficiency of thermoelectric cooling which is not present for conventional un-gated molecules with only delocalized states along their backbone.

cond-mat.mtrl-sci↗

arXiv2008

Tailoring the Fermi level of the leads in molecular-electronic devices

The dependence of the transport properties on the specific location of the Fermi level in molecular electronics devices is studied by using electrodes of different materials. The zero-bias transport properties are shown to depend dramatically on the elemental composition of the electrodes, even though the shape of the transmission coefficients is very similar. By using alkaline materials it is possible to move the Fermi level from the HOMO-LUMO gap to the LUMO resonance and change dramatically the length dependence of the conductance of molecular wires, which opens the possibility of using molecules with different lengths and very similar conductances in nanoscale circuits. This method shows how to dramatically increase the conductance of molecular devices and alter qualitatively and quantitatively their electronic and transport properties.

cond-mat.mes-hall↗

arXiv2008

Non-trivial length dependence of the conductance and negative differential resistance in atomic molecular wires

We study the electronic and transport properties of two novel molecular wires made of atomic chains of carbon atoms (polyynes) capped with either, benzene-thiols or pyridines. While both molecules are structurally similar, the electrical conductance of benzene-thiol-capped chains attached to gold electrodes is found to be much higher than that of pyridine-capped chains. We predict that the conductance is almost independent of molecular length, which suggests that these molecules could be ideal molecular wires for sub-10 nm circuitry. Both systems exhibit negative differential resistance (NDR) but its origin and characteristics depend on the type of molecule. We find a novel type of NDR mechanism produced by the movement of the LUMO resonance with bias. We also show that by gating the pyridine-capped molecules it is possible to make the NDR disappear and dramatically modify the $I$-$V$ characteristics and the length dependence.

cond-mat.mes-hall↗

arXiv2007

Conformation dependence of molecular conductance: chemistry versus geometry

Recent experiments by Venkatamaran {\em et al.} [Nature (London) {\bf 442}, 904 (2006)] on a series of molecular wires with varying chemical compositions, revealed a linear dependence of the conductance on $\mathrm{cos}^2θ$, where $θ$ is the angle of twist between neighboring aromatic rings. To investigate whether or not this dependence has a more general applicability, we present a first principles theoretical study of the transport properties of this family of molecules as a function of the chemical composition, conformation and the contact atom and geometry. If the Fermi energy $E_\mathrm{F}$ lies within the HOMO-LUMO gap, then we reproduce the above experimental results. More generally, however, if $E_\mathrm{F}$ is located within either the LUMO or HOMO states, the presence of resonances destroys the linear dependence of the conductance on $\mathrm{cos}^2θ$ and gives rise to non-monotonic behaviour associated with the level structure of the different molecules. Our results suggest that the above experiments provide a novel method for extracting spectroscopic information about molecules contacted to electrodes.

cond-mat.mtrl-sci↗

arXiv2007

Strongly correlated electron physics in nanotube-encapsulated metallocene chains

The structural, electronic and transport properties of metallocene molecules (MCp$_2$) and isolated or nanotube-encapsulated chains are studied by using a combination of density functional theory and non-equilibrium Green's functions. The analysis first discusses the whole series of isolated MCp$_2$ molecules, where M = V, Cr, Mn, Fe, Co, Ni, Ru, and Os. The series presents a rich range of electronic and magnetic behaviors due to the interplay between the crystal field interaction and Hund's rules, as the occupation of the $d$ shell increases. The article then shows how many of these interesting properties can also be seen when MCp$_2$ molecules are linked together to form periodic chains. It is argued that encapsulation of the chains inside carbon nanotubes, that is exothermic for radii larger than 4.5 Å, provides the missing mechanical stability and electrical isolation. The structural stability, charge transfer, magnetic and electronic behavior of the ensuing chains, as well as the modification of the electrostatic potential in the nanotube wall produced by the metallocenes as thoroughly discussed. We argue that the full devices can be characterized by two doped, strongly correlated Hubbard models whose mutual hybridization is almost negligible. The charge transferred from the chains produces a strong modification of the electrostatic potential in the nanotube walls, which is amplified in case of semiconducting and endothermic nanotubes. The transport properties of isolated metallocenes between semi-infinite nanotubes are also analyzed and shown to lead to important changes in the transmission coefficients of clean nanotubes for high energies.

cond-mat.mtrl-sci↗

arXiv2007

Electronic Properties of Alkali- and Alkaline-Earth-Intercalated Silicon Nanowires

We present a first-principles study of the electronic properties of silicon clathrate nanowires intercalated with various types of alkali or alkaline-earth atoms. We find that the band structure of the nanowires can be tuned by varying the impurity atom within the nanowire. The electronic character of the resulting systems can vary from metallic to semiconducting with direct band gaps. These properties make the nanowires specially suitable for electrical and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2005

The Smeagol method for spin- and molecular-electronics

{\it Ab initio} computational methods for electronic transport in nanoscaled systems are an invaluable tool for the design of quantum devices. We have developed a flexible and efficient algorithm for evaluating $I$-$V$ characteristics of atomic junctions, which integrates the non-equilibrium Green's function method with density functional theory. This is currently implemented in the package {\it Smeagol}. The heart of {\it Smeagol} is our novel scheme for constructing the surface Green's functions describing the current/voltage probes. It consists of a direct summation of both open and closed scattering channels together with a regularization procedure of the Hamiltonian, and provides great improvements over standard recursive methods. In particular it allows us to tackle material systems with complicated electronic structures, such as magnetic transition metals. Here we present a detailed description of {\it Smeagol} together with an extensive range of applications relevant for the two burgeoning fields of spin and molecular-electronics.

cond-mat.mes-hall↗

arXiv2005

Non-universal behavior of the parity effect in monovalent atomic wires

We propose a mixed analytical-ab-initio method for the accurate calculation of the conductance in monovalent atomic wires. The method relies on the most general formula for ballistic transport through a monovalent wire, whose parameters can be determined from first-principles calculations. Our central result is the demonstration of the highly non-universal behavior of the conductance, which depends on the fine details of the contacts to the leads. We are therefore able to reconcile a large number of the apparently contradictory results that have recently appeared in the literature.

cond-mat.mes-hall↗