arXiv subjects

C. J. Lambert

Publications and source records attributed to C. J. Lambert.

At least 55 records · Page 3Linked to original sources

arXiv2005

Quantum-classical correspondence in the wavefunctions of Andreev billiards

We present a classical and quantum mechanical study of an Andreev billiard with a chaotic normal dot. We demonstrate that in general the classical dynamics of these normal-superconductor hybrid systems is mixed, thereby indicating the limitations of a widely used retracing approximation. We show that the mixed classical dynamics gives rise to a wealth of wavefunction phenomena, including periodic orbit scarring and localization of the wavefunction onto other classical phase space objects such as intermittent regions and quantized tori.

cond-mat.mes-hall↗

arXiv2005

Single channel conductance of H$_2$ molecules attached to platinum or palladium electrodes

We report a detailed theoretical study of the bonding and conduction properties of an hydrogen molecule joining either platinum or palladium electrodes. We show that an atomic arrangement where the molecule is placed perpendicular to the electrodes is unstable for all distances between electrodes. In contrast, the configuration where the molecule bridges the electrodes is stable in a wide range of distances. In this last case the bonding state of the molecule does not hybridize with the leads and remains localized within the junction. As a result, this state does not transmit charge so that electronic transport is carried only through the anti-bonding state. This fact leads to conductances of 1 $G_0$ at most, where $G_0=2e^2/h$. We indeed find that G is equal to 0.9 and 0.6 $G_0$ for Pt and Pd contacts respectively.

cond-mat.mtrl-sci↗

arXiv2004

First principles simulations of the magnetic and structural properties of Iron

We have implemented non-collinear GGA and a generalized Bloch's theorem to simulate unconmensurate spiral arrangements of spins in a Density Functional Theory code based on localized wave functions. We have subsequently performed a thorough study of the different states of bulk Iron. We determine the minimal basis set required to obtain reliable orderings of ground and excited states. We find that the most stable fcc phase is a spiral with an equilibrium lattice constant 3.56 A.

cond-mat.mtrl-sci↗

arXiv2003

Non-local current correlations in ferromagnet/superconductor nanojunctions

When two fully polarized ferromagnetic (F) wires with opposite polarizations make contact with a spin-singlet superconductor, a potential-induced current in wire 1 induces a non-local current of equal magnitude and sign in wire 2. The magnitude of this current has been studied in the tunneling limit and found to decay exponentially with the distance between the contact. In this paper we propose a new structure in which this novel non-local effect is increased by orders of magnitude. We study the spin-dependent electronic transport of a diffusive nanojunction and demonstrate that when a normal diffusive region is placed between the F leads and superconductor, the non-local initially increases with the separation between the F leads, achieving a maximum and decays as a power law with increasing separation.

cond-mat.mes-hall↗

arXiv2003

Effective Random Matrix Theory description of chaotic Andreev billiards

An effective random matrix theory description is developed for the universal gap fluctuations and the ensemble averaged density of states of chaotic Andreev billiards for finite Ehrenfest time. It yields a very good agreement with the numerical calculation for Sinai-Andreev billiards. A systematic linear decrease of the mean field gap with increasing Ehrenfest time $τ_E$ is observed but its derivative with respect to $τ_E$ is in between two competing theoretical predictions and close to that of the recent numerical calculations for Andreev map. The exponential tail of the density of states is interpreted semi-classically.

cond-mat.mes-hall↗

arXiv2003

Andreev bound states for superconducting-ferromagnetic box

Within the microscopic Bogoliubov--de Gennes (BdG) formalism an exact quantization condition for Andreev bound states of the ferromagnetic-superconducting hybrid systems of box geometry is derived and a semi-classical formula for the density of states is obtained. The semi-classical formula is shown to agree with the exact result, even when the exchange field $h$, is much larger than the superconductor order parameter, provided $h$ is small compared with the Fermi energy.

cond-mat.mes-hall↗

arXiv2003

Ring--Shaped Andreev Billiards in Quantizing Magnetic Fields

We present a detailed semiclassical study of a clean disk--shaped insulator--normal-metal--superconductor hybrid system in a magnetic field. It is based on an exact secular equation that we derived within the microscopic Bogoliubov--de Gennes (BdG) formalism. Results obtained from a classification of electron and hole orbits are in excellent agreement with those from an exact numerical diagonalization of the BdG equation. Our analysis opens up new possibilities for determining thermodynamic properties of mesoscopic hybrid systems.

cond-mat.mes-hall↗

arXiv2003

A logarithmic contribution to the density of states of rectangular Andreev billiards

We demonstrate that the exact quantum mechanical calculations are in good agreement with the semiclassical predictions for rectangular Andreev billiards and therefore for a large number of open channels it is sufficient to investigate the Bohr-Sommerfeld approximation of the density of states. We present exact calculations of the classical path length distribution $P(s)$ which is a non-differentiable function of $s$, but whose integral is a smooth function with logarithmically dependent asymptotic behavior. Consequently, the density of states of rectangular Andreev billiards has two contributions on the scale of the Thouless energy: one which is well-known and it is proportional to the energy, and the other which shows a logarithmic energy dependence. It is shown that the prefactors of both contributions depend on the geometry of the billiards but they have universal limiting values when the width of the superconductor tends to zero.

cond-mat.mes-hall↗

arXiv2002

Proximity-induced sub-gaps in Andreev billiards

We examine the density of states of an Andreev billiard and show that any billiard with a finite upper cut-off in the path length distribution $P(s)$ will possess an energy gap on the scale of the Thouless energy. An exact quantum mechanical calculation for different Andreev billiards gives good agreement with the semi-classical predictions when the energy dependent phase shift for Andreev reflections is properly taken into account. Based on this new semi-classical Bohr-Sommerfeld approximation of the density of states, we derive a simple formula for the energy gap. We show that the energy gap, in units of Thouless energy, may exceed the value predicted earlier from random matrix theory for chaotic billiards.

cond-mat.mes-hall↗

arXiv2001

A Hartree-Fock Study of Charge Redistribution in a 2D Mesoscopic Structure

In this paper, we investigate the ground state of two-dimensional disordered cylinders which contain spinless, interacting electrons using the Hartree-Fock approximation. Calculations of the deviation of the polarization from uniformity reveal a tendency of the charge to rearrange towards the ends of the system. The presence of disorder results in fluctuations of the deviation around its mean value, which are more pronounced when the disorder strength is of the order of the interaction between the electrons.

cond-mat↗

arXiv2000

Material-specific spin filtering in ferromagnet/superconductor ballistic nanojunctions

We study spin-dependent electronic transport across ferromagnet/superconductor ballistic junctions modeled using tight-binding Hamiltonians with s, p and d orbitals and material-specific parameters. We show that by accurately modeling the band structure of the bulk materials, one can reproduce the measured differential conductance of Cu/Pb nanocontacts. In contrast the differential conductance of Co/Pb contacts can only be reproduced if an enhanced magnetic moment is present at the interface.

cond-mat.mes-hall↗

arXiv2000

Transport across a normal-superconducting interface: a novel probe of electron-electron interactions in the normal metal

In this letter, we examine the effect of Coulomb interactions in the normal region of a normal-superconducting (N/S) mesoscopic structure, here the change from an attractive to a repulsive coulombic interaction, at the N/S interface, causes a shift in the order parameter phase. We show that this shift has a pronounced effect on Andreev bound states and demonstrate that the effect on Andreev scattering of non-zero order-parameter tails, can be used to probe the sign of the interaction in the normal region.

cond-mat.supr-con↗

arXiv2000

Weak localization correction to the FS interface resistance

The classical resistance of a contact between a ferromagnet (F) and a superconductor (S) acquires an additional contact term as compared to the contact between a ferromagnet and a normal metal. The necessity to match spin-polarized current in a ferromagnet to spin-less current in the superconductor results in the accumulation of non-equilibrium polarization near the F/S interface. In the present work, we show that the weak localization correction to the classical diffusion coefficient, $δD$, is dependent on the degree of polarization, with majority spins more likely to be reflected from the interface than minority spins. Taking into account the change in the spin polarized particle distribution in the F wire arising from $δD$, we calculate the weak localization correction to the F/S contact resistance.

cond-mat.mes-hall↗

arXiv2000

Enhancement of GMR due to spin-mixing in magnetic multilayers with a superconducting contact

We study the Giant Magnetoresistance (GMR) ratio in magnetic multiayers with a single superconducting contact in the presence of spin-mixing processes. It has been recently shown that the GMR ratio of magnetic multilayers is strongly suppressed by the presence of a superconducting contact when spin-flipping is not allowed. In this Letter we demonstrate that the GMR ratio can be dramatically enhanced by spin-orbit interaction and/or non-collinear magnetic moments. The system is described using a tight-binding model with either s-p-d or s-d atomic orbitals per site.

cond-mat.mes-hall↗

arXiv2000

Negative length orbits in normal-superconductor billiard systems

The Path-Length Spectra of mesoscopic systems including diffractive scatterers and connected to superconductor is studied theoretically. We show that the spectra differs fundamentally from that of normal systems due to the presence of Andreev reflection. It is shown that negative path-lengths should arise in the spectra as opposed to normal system. To highlight this effect we carried out both quantum mechanical and semiclassical calculations for the simplest possible diffractive scatterer. The most pronounced peaks in the Path-Length Spectra of the reflection amplitude are identified by the routes that the electron and/or hole travels.

cond-mat.mes-hall↗

arXiv1999

Phonon-mediated thermal conductance of mesoscopic wires with rough edges

We present an analysis of acoustic phonon propagation through long, free-standing, insulating wires with rough surfaces. Due to a crossover from ballistic propagation of the lowest-frequency phonon mode at $ω<ω_{1}=πc/W$ to a diffusive (or even localized) behavior upon the increase of phonon frequency, followed by re-entrance into the quasi-ballistic regime, the heat conductance of a wire acquires an intermediate tendency to saturate within the temperature range $T\sim \hbar ω_{1}/k_{B}$.

cond-mat↗

arXiv1999

Anomalous behaviour of S/N mesoscopic structures near T_c

We observe a maximum in the conductance of Aluminum / n-GaAs junctions at temperatures 20 mK lower than the superconducting transition temperature. This is the first observation of a non- monotonic behavior of the conductance near the superconducting transition in superconducting normal junctions. To accommodate this effect we calculate the full temperature dependence of the conductance of these structures, invoking quasiclassical Green's functions in the diffusive limit. In addition to the well-known low temperature peak at temperatures on the order of the Thouless energy, we find a maximum near the critical temperature. This peak has the same origin as the subgap conductance observed in superconducting-normal junctions at low temperatures. Its calculated magnitude and position are in good agreement with our experimental observation.

cond-mat.supr-con↗

arXiv1999

Correlations in optical phonon spectra of complex solids

Spectral correlations in the optical phonon spectrum of a solid with a complex unit cell are analysed using the Wigner-Dyson statistical approach. Despite the fact that all force constants are real, we find that the statistics are predominantly of the GUE type depending on the location within the Brillouin zone of a crystal and the unit cell symmetry. Analytic and numerical results for the crossover from GOE to GUE statistics are presented.

cond-mat.mes-hall↗