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Nicolás Lorente

Publications and source records attributed to Nicolás Lorente.

At least 37 records · Page 2Linked to original sources
arXiv2020

Spin dependent transmission of Nickelocene-Cu contacts probed with shot noise

The current $I$ through nickelocene molecules and its noise are measured with a low temperature scanning tunneling microscope on a Cu(100) substrate. Density functional theory calculations and many-body modeling are used to analyze the data. During contact formation, two types of current evolution are observed, an abrupt jump to contact and a smooth transition. These data along with conductance spectra ($dI/dV$) recorded deep in the contact range are interpreted in terms of a transition from a spin-1 to a spin-1/2 state that is Kondo screened. Many-body calculations show that the smooth transition is also consistent with a renormalization of spin excitations of a spin-1 molecule by Kondo exchange coupling. The shot noise is significantly reduced compared to the Schottky value of $2eI$ but no influence of the Kondo effect or spin excitations are resolved. The noise can be described in the Landauer picture in terms of spin-polarized transmission of $\approx$35% through two degenerate $d_π$-orbitals of the Nickelocene molecule.

cond-mat.mes-hall↗
arXiv2017

Probing magnetic interactions between Cr adatoms on the $β$-Bi$_2$Pd superconductor

We show that the magnetic ordering of coupled atomic dimers on a superconductor is revealed by their intra-gap spectral features. Chromium atoms on the superconductor $β$-Bi$_2$Pd surface display Yu-Shiba-Rusinov bound states, detected as pairs of intra-gap excitations in the tunneling spectra. We formed Cr dimers by atomic manipulation and found that their intra-gap features appear either shifted or split with respect to single atoms. The spectral variations reveal that the magnetic coupling of the dimer changes between ferromagnetic and antiferromagnetic depending on its disposition on the surface, in good agreement with density functional theory simulations. These results prove that superconducting intra-gap state spectroscopy is an accurate tool to detect the magnetic ordering of atomic scale structures.

cond-mat.mes-hall↗
arXiv2016

Mapping the orbital structure of impurity bound states in a superconductor

A magnetic atomic impurity inside a superconductor locally distorts superconductivity. They scatter Cooper pairs as a potential with broken time-reversal symmetry, what leads to localized bound states with subgap excitation energies, named hereon Shiba states. Most conventional approaches to study Shiba states treat magnetic impurities as point scatterers with an isotropic exchange interaction, while the complex internal structure of magnetic impurities is usually neglected. Here, we show that the number and the shape of Shiba states are correlated to the spin-polarized atomic orbitals of the impurity, hybridized with the superconducting host, as supported by Density Functional Theory simulations. Using high-resolution scanning tunneling spectroscopy, we spatially map the five Shiba excitations found on sub-surface chromium atoms in Pb(111), resolving both their particle and hole components. While the maps of particle components resemble the \textit{d} orbitals of embedded Cr atoms, the hole components differ strongly from them. The orbital fingerprints of Shiba states thus unveil the magnetic ground state of the impurity, and identify scattering channels and interactions, all valuable tools for designing atomic-scale superconducting devices.

cond-mat.supr-con↗
arXiv2016

Improvements on non-equilibrium and transport Green function techniques: the next-generation transiesta

We present novel methods implemented within the non-equilibrium Green function code (NEGF) transiesta based on density functional theory (DFT). Our flexible, next-generation DFT-NEGF code handles devices with one or multiple electrodes ($N_e\ge1$) with individual chemical potentials and electronic temperatures. We describe its novel methods for electrostatic gating, contour opti- mizations, and assertion of charge conservation, as well as the newly implemented algorithms for optimized and scalable matrix inversion, performance-critical pivoting, and hybrid parallellization. Additionally, a generic NEGF post-processing code (tbtrans/phtrans) for electron and phonon transport is presented with several novelties such as Hamiltonian interpolations, $N_e\ge1$ electrode capability, bond-currents, generalized interface for user-defined tight-binding transport, transmission projection using eigenstates of a projected Hamiltonian, and fast inversion algorithms for large-scale simulations easily exceeding $10^6$ atoms on workstation computers. The new features of both codes are demonstrated and bench-marked for relevant test systems.

cond-mat.mes-hall↗
arXiv2016

Electronic structure of atomic manganese chains supported on Cu$_2$N / Cu (100)

Scanning tunnelling microscopy and density functional theory studies of manganese chains adsorbed on Cu$_2$N/Cu (100) reveal an unsuspected electronic edge state at $\sim 1$ eV above the Fermi energy. This Tamm-like state is strongly localised to the last Mn atom of the chain and fully spin polarised. However, no equivalence is found for occupied states, and the electronic structure at $\sim -1$ eV is mainly spin unpolarised due to the extended $p$-states of the N atoms that mediate the coupling between the Mn atoms in the chain. Odd-numbered Mn chains present an exponentially decreasing direct coupling with distance between the two edges, leading to a vanishing bonding/anti-bonding splitting of states while even-numbered Mn chains present perfect decoupling of both edges due to the the antiferromagnetic ordering of Mn chains.

cond-mat.mes-hall↗
arXiv2015

Entanglement-induced Kondo screening in atomic spin chains

Quantum entanglement permeates the complex ground states of correlated electron materials defying single-particle descriptions. Coupled magnetic atoms have potential as model systems for entanglement in condensed matter giving the opportunity to create artificial many-body states which can be controlled by tuning the underlying interactions. They provide an avenue to unravel the complexities of correlated-electron materials. Here we use low temperature scanning tunnelling microscopy (STM) and atomic manipulation to tune entanglement in chains of magnetic atoms. We find that a Kondo singlet state can emerge from this entanglement. The many electron Kondo state is based on the screening of the entangled spin ground state of the chain by substrate electrons and can be engineered to envelop at least ten magnetic atoms. The concomitant Kondo resonance measured in the differential conductance enables the electric read-out of entanglement. By tuning composition and coupling strength within atomic chains it is possible to create model spin chains with defined entanglement. This lays the foundation for a new class of experiments to construct exotic correlated-electron materials atom by atom.

cond-mat.mes-hall↗
arXiv2014

Spin transport in dangling-bond wires on doped H-passivated Si(100)

New advances in single-atom manipulation are leading to the creation of atomic structures on H passivated Si surfaces with functionalities important for the development of atomic and molecular based technologies. We perform total-energy and electron-transport calculations to reveal the properties and understand the features of atomic wires crafted by H removal from the surface. The presence of dopants radically change the wire properties. Our calculations show that dopants have a tendency to approach the dangling-bond wires, and in these conditions, transport is enhanced and spin selective. These results have important implications in the development of atomic-scale spintronics showing that boron, and to a lesser extent phosphorous, convert the wires in high-quality spin filters.

cond-mat.mtrl-sci↗
arXiv2014

Spontaneous spin switching via substrate-induced decoherence

We analyze the spontaneous switching between the two Néel states of Fe chains on Cu$_2$N/Cu(100), experimentally studied by Loth et al. [Science 335, 196 (2012)]. We show that, in the experimental low-temperature regime (T ~ 1K), decoherence induced by substrate electrons deeply modifies the dynamics of the chain magnetization. The Rabi oscillations associated to quantum tunneling of the isolated chain are replaced by an irreversible decay of considerably longer characteristic time. The computed switching rate is small, rapidly decreasing with chain length, with a 1/T behavior and in good agreement with the experiment. Quantum tunneling is only recovered as the switching mechanism at extremely low temperatures, the corresponding cross-over moving rapidly to further low temperatures as the chain length is increased.

cond-mat.mes-hall↗
arXiv2013

Electron transport signature of H$_2$ dissociation on atomic gold wires

Non-equilibrium Green's functions calculations based on density functional theory show a direct link between the initial stages of H$_2$ dissociation on a gold atomic wire and the electronic current supported by the gold wire. The simulations reveal that for biases below the stability threshold of the wire, the minimum-energy path for H$_2$ dissociation is not affected. However, the electronic current presents a dramatic drop when the molecule initiates its dissociation. This current drop is traced back to quantum interference between electron paths when the molecule starts interacting with the gold wire.

cond-mat.mes-hall↗
arXiv2013

Adsorption site determination of a molecular monolayer via inelastic tunneling

We have combined scanning tunneling microscopy with inelastic electron tunneling spectroscopy (IETS) and density functional theory (DFT) to study a tetracyanoethylene monolayer on Ag(100). Images show that the molecules arrange in locally ordered patterns with three non-equivalent, but undeterminable, adsorption sites. While scanning tunneling spectroscopy only shows subtle variations of the local electronic structure at the three different positions, we find that vibrational modes are very sensitive to the local atomic environment. IETS detects sizeable mode frequency shifts of the molecules located at the three topographically detected sites, which permits us to determine the molecular adsorption sites through identification with DFT calculations.

cond-mat.mtrl-sci↗
arXiv2012

Elastic transport through dangling-bond silicon wires on H passivated Si(100)

We evaluate the electron transmission through a dangling-bond wire on Si(100)-H (2x1). Finite wires are modelled by decoupling semi-infinite Si electrodes from the dangling-bond wire with passivating H atoms. The calculations are performed using density functional theory in a non-periodic geometry along the conduction direction. We also use Wannier functions to analyze our results and to build an effective tight-binding Hamiltonian that gives us enhanced insight in the electron scattering processes. We evaluate the transmission to the different solutions that are possible for the dangling-bond wires: Jahn-Teller distorted ones, as well as antiferromagnetic and ferromagnetic ones. The discretization of the electronic structure of the wires due to their finite size leads to interesting transmission properties that are fingerprints of the wire nature.

cond-mat.mtrl-sci↗
arXiv2012

Energetics and stability of dangling-bond silicon wires on H passivated Si(100)

We evaluate the electronic, geometric and energetic properties of quasi 1-D wires formed by dangling bonds on Si(100)-H (2 x 1). The calculations are performed with density functional theory (DFT). Infinite wires are found to be insulating and Peierls distorted, however finite wires develop localized electronic states that can be of great use for molecular-based devices. The ground state solution of finite wires does not correspond to a geometrical distortion but rather to an antiferromagnetic ordering. For the stability of wires, the presence of abundant H atoms in nearby Si atoms can be a problem. We have evaluated the energy barriers for intradimer and intrarow diffusion finding all of them about 1 eV or larger, even in the case where a H impurity is already sitting on the wire. These results are encouraging for using dangling-bond wires in future devices.

cond-mat.mtrl-sci↗
arXiv2011

Many-body effects in magnetic inelastic electron tunneling spectroscopy

Magnetic inelastic electron tunneling spectroscopy (IETS) shows sharp increases in conductance when a new conductance channel associated to a change in magnetic structure is open. Typically, the magnetic moment carried by an adsorbate can be changed by collision with a tunneling electron; in this process the spin of the electron can flip or not. A previous one-electron theory [Phys. Rev. Lett. {\bf 103}, 176601 (2009)] successfully explained both the conductance thresholds and the magnitude of the conductance variation. The elastic spin flip of conduction electrons by a magnetic impurity leads to the well known Kondo effect. In the present work, we compare the theoretical predictions for inelastic magnetic tunneling obtained with a one-electron approach and with a many-body theory including Kondo-like phenomena. We apply our theories to a singlet-triplet transition model system that contains most of the characteristics revealed in magnetic IETS. We use two self-consistent treatments (non-crossing approximation and self-consistent ladder approximation). We show that, although the one-electron limit is properly recovered, new intrinsic many-body features appear. In particular, sharp peaks appear close to the inelastic thresholds; these are not localized exactly at thresholds and could influence the determination of magnetic structures from IETS experiments.Analysis of the evolution with temperature reveals that these many-body features involve an energy scale different from that of the usual Kondo peaks. Indeed, the many-body features perdure at temperatures much larger than the one given by the Kondo energy scale of the system.

cond-mat.mes-hall↗
arXiv2011

Multi-orbital Non-Crossing Approximation from maximally localized Wannier functions: the Kondo signature of copper phthalocyanine on Ag (100)

We have developed a multi-orbital approach to compute the electronic structure of a quantum impurity using the non-crossing approximation. The calculation starts with a mean-field evaluation of the system's electronic structure using a standard quantum chemistry code. Here we use density functional theory (DFT). We transformed the one-electron structure into an impurity Hamiltonian by using maximally localized Wannier functions (MLWF). Hence, we have developed a method to study the Kondo effect in systems based on an initial one-electron calculation. We have applied our methodology to a copper phthalocyanine molecule chemisorbed on Ag (100), and we have described its spectral function for three different cases where the molecule presents a single spin or two spins with ferro- and anti-ferromagnetic exchange couplings. We find that the use of broken-symmetry mean-field theories such as Kohn-Sham DFT cannot deal with the complexity of the spin of open-shell molecules on metal surfaces and extra modeling is needed.

cond-mat.mes-hall↗
arXiv2010

Theoretical Study of Magnetic Moments Induced by Defects at the SiC (110) Surface

The effect of different surface defects on the atomic and electronic structures of cubic $β$-SiC(110) surface are studied by means of a first principles calculation based on Density Functional Theory using the SIESTA code. In the calculations, the possibility of different spin population at each atom is allowed. We find that while adsorption of oxygen or nitrogen and adsorption of hydrogen at the C surface carbon atoms do not induce magnetic moments on SiC(110); Si vacancies, substitutional C at the Si site and H or F adsorbed at the silicon surface sites induce localized magnetic moments as large as 0.7 $μ_{B}$ at the carbon atoms close to the defect. The local magnetic moment arrange ment varies from ferromagnetic in the case of H adsorption to antiferromagnetic in the Si vacancy and substitutional C cases. The case of H adsorption at the Si surface atoms is discussed in detail. It is concluded that magnetism is mainly due to the local character of the C valence orbitals.

cond-mat.mtrl-sci↗
arXiv2010

Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100)

The lifetimes of spin excitations of Mn adsorbates on CuN/Cu(100) are computed from first-principles. The theory is based on a strong-coupling T-matrix approach that evaluates the decay of a spin excitation due to electron-hole pair creation. Using a previously developed theory [Phys. Rev. Lett. {\bf 103}, 176601 (2009) and Phys. Rev. B {\bf 81}, 165423 (2010)], we compute the excitation rates by a tunneling current for all the Mn spin states. A rate equation approach permits us to simulate the experimental results by Loth and co-workers [Nat. Phys. {\bf 6}, 340 (2010)] for large tunnelling currents, taking into account the finite population of excited states. Our simulations give us insight into the spin dynamics, in particular in the way polarized electrons can reveal the existence of an excited state population. In addition, it reveals that the excitation process occurs in a way very different from the deexcitation one. Indeed, while excitation by tunnelling electrons proceeds via the s and p electrons of the adsorbate, deexcitation mainly involves the d electrons.

cond-mat.mes-hall↗
arXiv2010

Band selection and disentanglement using maximally-localized Wannier functions: the cases of Co impurities in bulk copper and the Cu (111) surface

We have adapted the maximally-localized Wannier function approach of [I. Souza, N. Marzari and D. Vanderbilt, Phys. Rev. B 65, 035109 (2002)] to the density functional theory based Siesta method [J. M. Soler et al., J. Phys.: Cond. Mat. 14, 2745 (2002)] and applied it to the study of Co substitutional impurities in bulk copper as well as to the Cu (111) surface. In the Co impurity case, we have reduced the problem to the Co d-electrons and the Cu sp-band, permitting us to obtain an Anderson-like Hamiltonian from well defined density functional parameters in a fully orthonormal basis set. In order to test the quality of the Wannier approach to surfaces, we have studied the electronic structure of the Cu (111) surface by again transforming the density functional problem into the Wannier representation. An excellent description of the Shockley surface state is attained, permitting us to be confident in the application of this method to future studies of magnetic adsorbates in the presence of an extended surface state.

cond-mat.str-el↗