arXiv subjects

C. A. Balseiro

Publications and source records attributed to C. A. Balseiro.

At least 55 records · Page 3Linked to original sources

arXiv2011

Electrical Control of the Chemical Bonding of Fluorine on Graphene

We study the electronic structure of diluted F atoms chemisorbed on graphene using density functional theory calculations. We show that the nature of the chemical bonding of a F atom adsorbed on top of a C atom in graphene strongly depends on carrier doping. In neutral samples the F impurities induce a sp^3-like bonding of the C atom below, generating a local distortion of the hexagonal lattice. As the graphene is electron-doped, the C atom retracts back to the graphene plane and for high doping (10^14 cm^-2) its electronic structure corresponds to a nearly pure sp^2 configuration. We interpret this sp^3-sp^2 doping-induced crossover in terms of a simple tight binding model and discuss the physical consequences of this change.

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arXiv2010

Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect

The ability to make electrical contact to single molecules creates opportunities to examine fundamental processes governing electron flow on the smallest possible length scales. We report experiments in which we controllably stretch individual cobalt complexes having spin S = 1, while simultaneously measuring current flow through the molecule. The molecule's spin states and magnetic anisotropy were manipulated in the absence of a magnetic field by modification of the molecular symmetry. This control enabled quantitative studies of the underscreened Kondo effect, in which conduction electrons only partially compensate the molecular spin. Our findings demonstrate a mechanism of spin control in single-molecule devices and establish that they can serve as model systems for making precision tests of correlated-electron theories.

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arXiv2010

Out of equilibrium transport through an Anderson impurity: Probing scaling laws within the equation of motion approach

We study non-equilibrium electron transport through a quantum impurity coupled to metallic leads using the equation of motion technique at finite temperature T. Assuming that the interactions are taking place solely in the impurity and focusing in the infinite Hubbard limit, we compute the out of equilibrium density of states and the differential conductance G_2(T,V) to test several scaling laws. We find that G_2(T,V)/G_2(T,0) is a universal function of both eV/T_K and T/T_K, being T_K the Kondo temperature. The effect of an in plane magnetic field on the splitting of the zero bias anomaly in the differential conductance is also analyzed. For a Zeeman splitting Δ, the computed differential conductance peak splitting depends only on Δ/T_K, and for large fields approaches the value of 2Δ. Besides the traditional two leads setup, we also consider other configurations that mimics recent experiments, namely, an impurity embedded in a mesoscopic wire and the presence of a third weakly coupled lead. In these cases, a double peak structure of the Kondo resonance is clearly obtained in the differential conductance while the amplitude of the highest peak is shown to decrease as \ln(eV/T_K). Several features of these results are in qualitative agreement with recent experimental observations reported on quantum dots.

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arXiv2008

Localized Spins on Graphene

The problem of a magnetic impurity, atomic or molecular, absorbed on top of a carbon atom in otherwise clean graphene is studied using the numerical renormalization group. The spectral, thermodynamic, and scattering properties of the impurity are described in detail. In the presence of a small magnetic field, the low energy electronic features of graphene make possible to inject spin polarized currents through the impurity using a scanning tunneling microscope (STM). Furthermore, the impurity scattering becomes strongly spin dependent and for a finite impurity concentration it leads to spin polarized bulk currents and a large magnetoresistance. In gated graphene the impurity spin is Kondo screened at low temperatures. However, at temperatures larger than the Kondo temperature, the anomalous magnetotransport properties are recovered.

cond-mat.str-el↗

arXiv2008

Magnetic breakdown of cyclotron orbits in systems with Rashba and Dresselhaus spin-orbit coupling

We study the effect of the interplay between the Rashba and the Dresselhaus spin-orbit couplings on the transverse electron focusing in two-dimensional electron gases. Depending on their relative magnitude, the presence of both couplings can result in the splitting of the first focusing peak into two or three. This splitting has information about the relative value of spin-orbit couplings and therefore about the shape of the Fermi surface. More interesting, the presence of the third peak is directly related to the tunneling probability ("magnetic breakdown") between orbits corresponding to the different sheets of the Fermi surface. In addition, destructive interference effects between paths that involve tunneling and those that do not can be observed in the second focusing condition. Such electron paths (orbits) could be experimentally detected using current techniques for imaging the electron flow opening the possibility to directly observe and characterize the magnetic breakdown effect in this system.

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arXiv2008

Anomalous Josephson Current in Junctions with Spin-Polarizing Quantum Point Contacts

We consider a ballistic Josephson junction with a quantum point contact in a two-dimensional electron gas with Rashba spin-orbit coupling. The point contact acts as a spin filter when embedded in a circuit with normal electrodes. We show that with an in-plane external magnetic field an anomalous supercurrent appears even for zero phase difference between the superconducting electrodes. In addition, the external field induces large critical current asymmetries between the two flow directions, leading to supercurrent rectifying effects.

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arXiv2007

Electronic Transport through Magnetic Molecules with Soft Vibrating Modes

The low-temperature transport properties of a molecule are studied in the field-effect transitor geometry. The molecule has an internal mechanical mode that modulates its electronic levels and renormalizes both the interactions and the coupling to the electrodes. For a soft mechanical mode the spin fluctuations in the molecule are dominated by the bare couplings while the valence changes are determined by the dressed energies. In this case, the transport properties present an anomalous behavior and the Kondo temperature has a weak gate voltage dependence. These observations are in agreement with recent experimental data.

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arXiv2007

Imaging transverse electron focusing in semiconducting heterostructures with spin-orbit coupling

Transverse electron focusing in two-dimensional electron gases (2DEGs) with strong spin-orbit coupling is revisited. The transverse focusing is related to the transmission between two contacts at the edge of a 2DEG when a perpendicular magnetic field is applied. Scanning probe microscopy imaging techniques can be used to study the electron flow in these systems. Using numerical techniques we simulate the images that could be obtained in such experiments. We show that hybrid edge states can be imaged and that the outgoing flux can be polarized if the microscope tip probe is placed in specific positions.

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arXiv2007

Slave boson theory for transport through magnetic molecules with vibronic states

We study the electron transport through a magnetic molecular transistor in the Kondo limit using the slave boson technique. We include the electron-phonon coupling and analyze the cases where the spin of the molecule is either S=1/2 or S=1. We use the Schrieffer-Wolff transformation to write down a low energy Hamiltonian for the system. In the presence of electron-phonon coupling, and for $S\smeq1$, the resulting Kondo Hamiltonian has two active channels. At low temperature, these two channels interfere destructively, leading to a zero conductance.

cond-mat.str-el↗

arXiv2007

Kondo effect with non collinear polarized leads: a numerical renormalization group analysis

The Kondo effect in quantum dots attached to ferromagnetic leads with general polarization directions is studied combining poor man scaling and Wilson's numerical renormalization group methods. We show that polarized electrodes will lead in general to a splitting of the Kondo resonance in the quantum dot density of states except for a small range of angles close to the antiparallel case. We also show that an external magnetic field is able to compensate this splitting and restore the unitary limit. Finally, we study the electronic transport through the device in various limiting cases.

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arXiv2006

Detection of spin polarized currents in quantum point contacts via transverse electron focusing

It has been predicted recently that an electron beam can be polarized when it flows adiabatically through a quantum point contact in a system with spin-orbit interaction. Here, we show that a simple transverse electron focusing setup can be used to detect such polarized current. It uses the amplitude's asymmetry of the spin-split transverse electron focusing peak to extract information about the electron's spin polarization. On the other hand, and depending on the quantum point contact geometry, including this one-body effect can be important when using the focusing setup to study many-body effects in quantum point contacts.

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arXiv2006

Electron-Phonon Correlation Effects in Molecular Transistors

The interplay of electron-electron and electron-phonon interactions is studied analytically in the Kondo regime. A Holstein electron-phonon coupling is shown to produce a weakening of the gate voltage dependence of the Kondo temperature and may explain the observed anomalies in some of these devices. A molecular center-of-mass mode opens a new channel for charge and spin fluctuations and in the antiadabatic limit the latter are described by an asymmetric two-channel Kondo model. Below the Kondo temperature the system develops a dynamical Jahn-Teller distortion and a low energy peak emerges in the phonon spectral density that could be observed in Raman microscopy experiments.

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arXiv2006

Universal Distribution of Kondo Temperatures in Dirty Metals

Kondo screening of diluted magnetic impurities in a disordered host is studied analytically and numerically in one, two and three dimensions. It is shown that in the T_K \to 0 limit the distribution of Kondo temperatures has a universal form, P(T_K) \sim T_K^{-α} that holds in the insulating phase and persists in the metallic phase close to the metal insulator transition. Moreover, the exponent αdepends only on the dimensionality. The most important consequence of this result is that the T-dependence of thermodynamic properties is smooth across the metal-insulator transition in three dimensional systems.

cond-mat.str-el↗

arXiv2005

Spin Hall effect in clean two dimensional electron gases with Rashba spin-orbit coupling

We study the spin polarization induced by a current flow in clean two dimensional electron gases with Rashba spin-orbit coupling. This geometric effect originates from special properties of the electron's scattering at the edges of the sample. In wide samples, the spin polarization has it largest value at low energies (close to the bottom of the band) and goes to zero at higher energies. In this case, the spin polarization is dominated by the presence of evanescent modes which have an explicit spin component outside the plane. In quantum wires, on the other hand, the spin polarization is dominated by interference effects induced by multiple scattering at the edges. Here, the spin polarization is quite sensitive to the value of the Fermi energy, especially close to the point where a new channel opens up. We analyzed different geometries and found that the spin polarization can be strongly enhanced.

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arXiv2005

Spin Accumulation and Equilibrium Currents at the Edge of 2DEGs with Spin-Orbit Coupling

We show that in a two dimensional electron gas (2DEG) the interplay between the Rashba spin-orbit coupling and a potential barrier (eg., the sample edge) generates interesting spin effects. In the presence of an external charge current, a spin accumulation is built up near the barrier while in equilibrium there is an inhomogeneous spin current which is localized at the barrier and flows parallel to it. When an in-plane magnetic field perpendicular to the barrier is applied, the system also develops an inhomogeneous charge current density. These effects originate purely from the structure of the eigenstates near the boundary.

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arXiv2005

Design and control of spin gates in two quantum dots arrays

We study the spin-spin interaction between quantum dots coupled through a two dimensional electron gas with spin-orbit interaction. We show that the interplay between transverse electron focusing and spin-orbit coupling allows to dynamically change the symmetry of the effective spin-spin Hamiltonian. That is, the interaction can be changed from Ising-like to Heisenberg-like and vice versa. The sign and magnitude of the coupling constant can also be tuned.

cond-mat.mes-hall↗

arXiv2005

Tuning the Non-local Spin-Spin Interaction between Quantum Dots with a Magnetic Field

We describe a device where the non-local spin-spin interaction between two quantum dots can be turned on and off and even changed sign with a very small magnetic field. The setup consists of two quantum dots at the edge of two two-dimensional electron gases (2DEGs). The quantum dots' spins are coupled through a RKKY-like interaction mediated by the electrons in the 2DEGs. A small magnetic field perpendicular to the plane of the 2DEG is used as a tuning parameter. When the cyclotron radius is commensurate with the interdot distance, the spin-spin interaction is amplified by a few orders of magnitude. The sign of the interaction is controlled by finely tuning the magnetic field. Our setup allows for several dots to be coupled in a linear arrangement and it is not restricted to nearest-neighbors interaction.

cond-mat.mes-hall↗

arXiv2004

Magnetic Moment Formation in Quantum Point Contacts

We study the formation of local magnetic moments in quantum point contacts. Using a Hubbard-like model to describe point contacts formed in a two dimensional system, we calculate the magnetic moment using the unrestricted Hartree approximation. We analyze different type of potentials to define the point contact, for a simple square potential we calculate a phase diagram in the parameter space (Coulomb repulsion - gate voltage). We also present an analytical calculation of the susceptibility to give explicit conditions for the occurrence of a local moment, we present a simple scaling argument to analyze how the stability of the magnetic moment depends on the point contact dimensions.

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