arXiv subjects

Stefano Sanvito

Publications and source records attributed to Stefano Sanvito.

At least 109 records · Page 6Linked to original sources

arXiv2014

Phenomenology of Andreev reflection from first-principles transport theory

We study Andreev reflection in normal metal-superconductor junctions by using an extended Blonder-Tinkham-Klapwijk model combined with transport calculations based on density functional theory. Starting from a parameter-free description of the underlying electronic structure, we perform a detailed investigation of normal metal-superconductor junctions, as the separation between the superconductor and the normal metal is varied. The results are interpreted by means of transverse momentum resolved calculations, which allow us to examine the contributions arising from different regions of the Brillouin zone. Furthermore we investigate the effect of a voltage bias on the normal metal-superconductor conductance spectra. Finally, we consider Andreev reflection in carbon nanotubes sandwiched between normal and superconducting electrodes.

cond-mat.mes-hall↗

arXiv2014

Stretching of BDT-gold molecular junctions: thiol or thiolate termination?

It is often assumed that the hydrogen atoms in the thiol groups of a benzene-1,4-dithiol dissociate when Au-benzene-1,4-dithiol-Au junctions are formed. We demonstrate, by stability and transport properties calculations, that this assumption can not be made. We show that the dissociative adsorption of methanethiol and benzene-1,4-dithiol molecules on a flat Au(111) surface is energetically unfavorable and that the activation barrier for this reaction is as high as 1 eV. For the molecule in the junction, our results show, for all electrode geometries studied, that the thiol junctions are energetically more stable than their thiolate counterparts. Due to the fact that density functional theory (DFT) within the local density approximation (LDA) underestimates the energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital by several electron-volts, and that it does not capture the renormalization of the energy levels due to the image charge effect, the conductance of the Au-benzene-1,4-dithiol-Au junctions is overestimated. After taking into account corrections due to image charge effects by means of constrained-DFT calculations and electrostatic classical models, we apply a scissor operator to correct the DFT energy levels positions, and calculate the transport properties of the thiol and thiolate molecular junctions as a function of the electrodes separation.

cond-mat.mes-hall↗

arXiv2014

Proximity induced topological state in graphene

The appearance of topologically protected states at the surface of an ordinary insulator is a rare occurrence and to date only a handful of materials are known for having this property. An intriguing question concerns the possibility of forming topologically protected interfaces between different materials. Here we propose that a topological phase can be transferred to graphene by proximity with the three-dimensional topological insulator Bi$_2$Se$_3$. By using density functional and transport theory we prove that, at the verge of the chemical bond formation, a hybrid state forms at the graphene/Bi$_2$Se$_3$ interface. The state has Dirac-cone-like dispersion at the $Γ$ point and a well-defined helical spin-texture, indicating its topologically protected nature. This demonstrates that proximity can transfer the topological phase from Bi$_2$Se$_3$ to graphene.

cond-mat.mes-hall↗

arXiv2014

Efficient spin injection and giant magnetoresistance in Fe/MoS$_2$/Fe junctions

We demonstrate giant magnetoresistance in Fe/MoS$_2$/Fe junctions by means of \textit{ab-initio} transport calculations. We show that junctions incorporating either a mono- or a bi-layer of MoS$_2$ are metallic and that Fe acts as an efficient spin injector into MoS$_2$ with an efficiency of about 45\%. This is the result of the strong coupling between the Fe and S atoms at the interface. For junctions of greater thickness a maximum magnetoresistance of $\sim$300\% is obtained, which remains robust with the applied bias as long as transport is in the tunneling limit. A general recipe for improving the magnetoresistance in spin valves incorporating layered transition metal dichalcogenides is proposed.

cond-mat.mes-hall↗

arXiv2014

Multiprobe quantum spin Hall bars

We analyze electron transport in multiprobe quantum spin Hall (QSH) bars using the Büttiker formalism and draw parallels with their quantum Hall (QH) counterparts. We find that in a QSH bar the measured resistance changes upon introducing side voltage probes, in contrast to the QH case. We also study four- and six-terminal geometries and derive the expressions for the resistances. For these our analysis is generalized from the single-channel to the multi-channel case and to the inclusion of backscattering originating from a constriction placed within the bar.

cond-mat.mes-hall↗

arXiv2013

Gate controlled spin pumping at a quantum spin Hall edge

We propose a four-terminal device designed to manipulate by all electrical means the spin of a magnetic adatom positioned at the edge of a quantum spin Hall insulator. We show that an electrical gate, able to tune the interface resistance between a quantum spin Hall insulator and the source and drain electrodes, can switch the device between two regimes: one where the system exhibits spin pumping and the other where the adatom remains in its ground state. This demonstrates an all-electrical route to control single spins by exploiting helical edge states of topological materials.

cond-mat.mes-hall↗

arXiv2013

Dimensionality driven charge density wave instability in TiS$_2$

Density functional theory and density functional perturbation theory are used to investigate the electronic and vibrational properties of TiS$_2$. Within the local density approximation the material is a semi-metal both in the bulk and in the monolayer form. Most interestingly we observe a Kohn anomaly in the bulk phonon dispersion, which turns into a charge density wave instability when TiS$_2$ is thinned to less than four monolayers. Such charge density wave phase can be tuned by compressive strain, which appears to be the control parameter of the instability.

cond-mat.mes-hall↗

arXiv2013

Electric Field Effects on Armchair MoS2 Nanoribbons

{\it Ab initio} density functional theory calculations are performed to investigate the electronic structure of MoS$_2$ armchair nanoribbons in the presence of an external static electric field. Such nanoribbons, which are nonmagnetic and semiconducting, exhibit a set of weakly interacting edge states whose energy position determines the band-gap of the system. We show that, by applying an external transverse electric field, $E_\mathrm{ext}$, the nanoribbons band-gap can be significantly reduced, leading to a metal-insulator transition beyond a certain critical value. Moreover, the presence of a sufficiently high density of states at the Fermi level in the vicinity of the metal-insulator transition leads to the onset of Stoner ferromagnetism that can be modulated, and even extinguished, by $E_\mathrm{ext}$. In the case of bi-layer nanoribbons we further show that the band-gap can be changed from indirect to direct by applying a transverse field, an effect which might be of significance for opto-electronics applications.

cond-mat.mes-hall↗

arXiv2013

Dynamical Exchange Interaction From Time-Dependent Spin Density Functional Theory

We report on {\it ab initio} time-dependent spin dynamics simulations for a two-center magnetic molecular complex based on time-dependent non-collinear spin density functional theory. In particular, we discuss how the dynamical behavior of the {\it ab initio} spin-density in the time-domain can be mapped onto a model Hamiltonian based on the classical Heisenberg spin-spin interaction $J\vcr{S}_1\cdot \vcr{S}_2$. By analyzing individual localized-spin trajectories, extracted from the spin-density evolution, we demonstrate a novel method for evaluating the effective Heisenberg exchange coupling constant, $J$, from first principles simulations. We find that $J$, extracted in such a new dynamical way, agrees quantitatively to that calculated by the standard density functional theory broken-symmetry scheme.

cond-mat.other↗

arXiv2013

Spin-pumping and inelastic electron tunneling spectroscopy in topological insulators

We demonstrate that a quantum spin Hall current, spontaneously generated at the edge of a two-dimensional topological insulator, acts as a source of spin-pumping for a magnetic impurity with uniaxial anisotropy. One can then manipulate the impurity spin direction by means of an electrical current without using either magnetic electrodes or an external magnetic field. Furthermore we show that the unique properties of the quantum spin Hall topological state have profound effects on the inelastic electron tunneling spectrum of the impurity. For low current densities inelastic spin-flip events do not contribute to the conductance. As a consequence the conductance steps, normally appearing at voltages corresponding to the spin excitations, are completely suppressed. In contrast an intense current leads to spin pumping and generates a transverse component of the impurity spin. This breaks the topological phase yielding to the conductance steps.

cond-mat.mes-hall↗

arXiv2013

Ab-initio study on the possible doping strategies for MoS$_2$ monolayers

Density functional theory is used to systematically study the electronic and magnetic properties of doped MoS$_2$ monolayers, where the dopants are incorporated both via S/Mo substitution or as adsorbates. Among the possible substitutional dopants at the Mo site, Nb is identified as suitable p-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS$_2$ conduction band, making the system n-type. Finally, the adsorption of charged molecules is considered, mimicking an ionic liquid environment. We find that molecules adsorption can lead to both n- and p-type conductivity, depending on the charge polarity of the adsorbed species.

cond-mat.mtrl-sci↗

arXiv2013

Detection of the electrostatic spin crossover effect in magnetic molecules

Scanning tunneling microscopy (STM) can be used to detect inelastic spin transitions in magnetic nano-structures comprising only a handful of atoms. Here we demonstrate that STM can uniquely identify the electrostatic spin crossover effect, whereby the exchange interaction between two magnetic centers in a magnetic molecule changes sign as a function of an external electric field. The fingerprint of such effect is a large drop in the differential conductance as the bias increases. Crucially in the case of a magnetic dimer the spin crossover transition inverts the order between the ground state and the first excited state, but does not change their symmetry. This means that at both sides of the conductance drop associated to the spin crossover transition there are two inelastic transition between the same states. The corresponding conductance steps split identically in a magnetic field and provide a unique way to identify the electrostatic spin crossover.

cond-mat.mes-hall↗

arXiv2013

Atomistic Simulations of Highly Conductive Molecular Transport Junctions Under Realistic Conditions

We report state-of-the-art atomistic simulations combined with high-fidelity conductance calculations to probe structure-conductance relationships in Au-benzenedithiolate(BDT)-Au junctions under elongation. Our results demonstrate that large increases in conductance are associated with the formation of monatomic chains (MACs) of Au atoms directly connected to BDT. An analysis of the electronic structure of the simulated junctions reveals that enhancement in the s-like states in Au MACs causes the increases in conductance. Other structures also result in increased conductance but are too short-lived to be detected in experiment, while MACs remain stable for long simulation times. Examinations of thermally evolved junctions with and without MACs show negligible overlap between conductance histograms, indicating that the increase in conductance is related to this unique structural change and not thermal fluctuation. These results, which provide an excellent explanation for a recently observed anomalous experimental result [Bruot et al., Nature Nanotech., 2012, 7, 35-40], should aid in the development of mechanically responsive molecular electronic devices.

cond-mat.mes-hall↗

arXiv2013

Structural Origins of Conductance Fluctuations in Gold-Thiolate Molecular Transport Junctions

We report detailed atomistic simulations combined with high-fidelity conductance calculations to probe the structural origins of conductance fluctuations in thermally evolving Au-benzene-1,4-dithiolate-Au junctions. We compare the behavior of structurally ideal junctions (electrodes with flat surfaces) to structurally realistic, experimentally representative junctions resulting from break junction simulations. The enhanced mobility of metal atoms in structurally realistic junctions results in significant changes to the magnitude and origin of the conductance fluctuations. Fluctuations are larger by a factor of 2-3 in realistic junctions compared to ideal junctions. Moreover, in junctions with highly deformed electrodes, the conductance fluctuations arise primarily from changes in the Au geometry, in contrast to results for junctions with non-deformed electrodes, where the conductance fluctuations are dominated by changes in the molecule geometry. These results provide important guidance to experimentalists developing strategies to control molecular conductance for device applications, and also to theoreticians invoking simplified structural models of junctions to predict their behavior.

cond-mat.mes-hall↗

arXiv2013

Spin-filtering efficiency of ferrimagnetic spinels CoFe2O4 and NiFe2O4

We assess the potential of the ferrimagnetic spinel ferrites CoFe2O4 and NiFe2O4 to act as spin filtering barriers in magnetic tunnel junctions. Our study is based on the electronic structure calculated by means of first-principles density functional theory within different approximations for the exchange correlation energy. We show that, in agreement with previous calculations, the densities of states suggest a lower tunneling barrier for minority spin electrons, and thus a negative spin-filter effect. However, a more detailed analysis based on the complex band-structure reveals that both signs for the spin-filtering efficiency are possible, depending on the band alignment between the electrode and the barrier materials and depending on the specific wave-function symmetry of the relevant bands within the electrode.

cond-mat.mtrl-sci↗

arXiv2013

Origin of the n-type and p-type conductivity of MoS2 monolayers on a SiO2 substrate

Ab-initio density functional theory calculations are performed to study the electronic properties of a MoS2 monolayer deposited over a SiO2 substrate in the presence of interface impurities and defects. When MoS2 is placed on a defect-free substrate the oxide plays an insignificant role, since the conduction band top and the valence band minimum of MoS2 are located approximately in the middle of the SiO2 band-gap. However, if Na impurities and O dangling bonds are introduced at the SiO2 surface, these lead to localized states, which modulate the conductivity of the MoS2 monolayer from n- to p-type. Our results show that the conductive properties of MoS2 deposited on SiO2 are mainly determined by the detailed structure of the MoS2 /SiO2 interface, and suggest that doping the substrate can represent a viable strategy for engineering MoS2 -based devices.

cond-mat.mtrl-sci↗

arXiv2012

Topological surface states scattering in Antimony

In this work we study the topologically protected states of the Sb(111) surface by using ab-initio transport theory. In the presence of a strong surface perturbation we obtain standing-wave states resulting from the superposition of spin-polarized surface states. By Fourier analysis, we identify the underlying two dimensional scattering processes and the spin texture. We find evidence of resonant transmission across surface barriers at quantum well states energies and evaluate their lifetimes. Our results are in excellent agreement with experimental findings. We also show that despite the presence of a step edge along a different high symmetry direction, not yet probed experimentally, the surface states exhibit unperturbed transmission around the Fermi energy for states with near to normal incidence.

cond-mat.mes-hall↗

arXiv2012

Andreev reflection in two-dimensional topological insulators with either conserved or broken time-reversal symmetry

We investigate Andreev reflection in two-dimensional heterojunctions formed by a superconductor in contact with a topological insulator ribbon either possessing or breaking time-reversal symmetry. Both classes of topological insulators exhibit perfect Andreev reflection, which is robust against disorder. This is assigned to topologically protected edge states. In the time-reversal symmetric case we show that doping one of the ribbon edges with magnetic impurities suppresses one Andreev channel, while no such suppression is seen in the broken symmetry situation. Based on this observation we suggest a tabletop transport experiment able to distinguish between the two types of topological insulators, which does not involve the direct measurement of the material band structure.

cond-mat.mes-hall↗