arXiv subjects

N. Stefanakis

Publications and source records attributed to N. Stefanakis.

At least 19 recordsLinked to original sources

arXiv2025

Qubit States Based on Fractional Vortices in $0-\pi$ Josephson Junctions

We investigate the static properties of $0-\pi$ Josephson junctions and their potential application as superconducting qubits. We show that fractional vortex and fractional antivortex configurations give rise to a bistable free-energy landscape with stable minima corresponding to topologically distinct fractional-vortex states. In the long-junction limit the two configurations correspond to stable equilibrium states, whereas in the short-junction limit the reduced barrier suggests enhanced quantum tunneling and hybridization between them. These results suggest that $0-\pi$ Josephson junctions may provide a physically meaningful platform for qubit-like two-level dynamics, provided that a finite tunneling amplitude exists between the fractional-vortex and fractional-antivortex states.

cond-mat.mes-hall↗

arXiv2014

Elastic vs brittle behaviour of carbon nanotubes, investigated through molecular dynamics simulations

The objective of the present paper is to investigate the mechanical properties of carbon nanotubes. We use classical molecular dynamics simulation in order to study the effect of external compression, bending and torsion to nanotubes. We find that in the low temperature limit, the nanotubes are resilient, sustaining extreme strain without signs of brittleness or plasticity. For high temperatures and under tension the nanotubes show brittle or plastic behavior.

cond-mat.mes-hall↗

arXiv2014

Detection efficiency of a BEGe detector using the Monte Carlo method and a comparison to other calibration methods

In this paper we model by using the Monte Carlo simulation code PENELOPE [1, 2] a Broad Energy Germanium (BEGe) detector and determine its efficiency. The simulated geometry consists of a point source located close to the detector as well as volume sources with cylindrical geometry. A comparison of the simulation is made to experimental results as well as to analytical calculations.

physics.ins-det↗

arXiv2005

Interface effects and termination of finite length nanotubes

The objective of the present paper is to investigate interface effects in carbon nanotubes. We use both real and k-space tight binding method. We study in detail the effect of wrapping vector on the electronic properties. We analyze the effect of the curvature in closed nanotube to the electronic properties. The finite length of the nanotube affects the electronic properties.

physics.comp-ph↗

arXiv2004

Fluxon-semifluxon interaction in an annular long Josephson 0-pi-junction

We investigate theoretically the interaction between integer and half-integer Josephson vortices (fluxons and semifluxons) in an annular Josephson junction. Semifluxons usually appear at the 0-$π$-boundary where there is a $π$-discontinuity of the Josephson phase. We study the simplest, but the most interesting case of one $π$-discontinuity in a loop, which can be created only artificially. We show that measuring the current-voltage characteristic after injection of an integer fluxon, one can determine the polarity of a semifluxon. Depending on the relative polarity of fluxon and semifluxon the static configuration may be stable or unstable, but in the dynamic state both configurations are stable. We also calculate the depinning current of $N$ fluxons pinned by an arbitrary fractional vortex.

cond-mat.supr-con↗

arXiv2004

Interface effects in superconductor-nanotubes hybrid structures

The objective of the present paper is to investigate the proximity effect in junctions of superconductors with carbon nanotubes. The method is the lattice BdG equations within the Hubbard model. The proximity effect depends sensitively on the connection giving the possibility to control the proximity effect by performing simple geometrical changes in the hybrid system

cond-mat.mes-hall↗

arXiv2003

Magnetic field effect in hybrid nanostructures

We examine the effect of the magnetic field on the proximity effect in nanostructures, self consistently using the Bogoliubov-deGennes formalism within the two dimensional extended Hubbard model. We calculate the local density of states and the pair amplitude. We study several nanostructures: superconductor - two dimensional electron gas, superconductor - ferromagnet. In these structures the magnetic field can be considered as a modulation parameter for the proximity effect.

cond-mat.mes-hall↗

arXiv2003

BdG equations within lattice Hubbard model for the description of $π$-states in nanoscale S-FF-S junctions

We calculate the persistent currents in $S-F_{\uparrow}F_{\uparrow[\downarrow]}-S$ structures (S denotes the superconductor in a closed ring geometry, F the ferromagnet in a two dimensional geometry, and the arrows the alignment of exchange field) as a function of the applied magnetic flux, self consistently by solving the Bogoliubov-de Gennes equations within the two dimensional Hubbard model. The local current shows a sign change i.e. a 0 to $π$ transition in the parallel alignment of the magnetizations and it does not change sign in antiparallel alignment.

cond-mat.supr-con↗

arXiv2003

Proximity effect in superconductor Aharonov Bohm loop hybrid structures

We study the proximity effect in superconductor - metallic or ferromagnetic Aharonov Bohm loop hybrid structures self consistently using the Bogoliubov-deGennes formalism within the two dimensional extended Hubbard model. We calculate the local density of states, the pair amplitude, and persistent currents as a function of several modulation parameters, the position around the loop, the exchange field, the magnetic flux through the loop, the loop size, and the chemical potential in the loop atoms. We find that the parameters above can modulate the proximity effect.

cond-mat.supr-con↗

arXiv2003

Transport properties of ferromagnet/d-wave superconductor/ferromagnet double junctions

We investigate transport properties of a trilayer made of a d-wave superconductor connected to two ferromagnetic electrodes. Using Keldysh formalism we show that crossed Andreev reflection and elastic cotunneling exist also with d-wave superconductors. Their properties are controlled by the existence of zero energy states due to the anisotropy of the d-wave pair potential.

cond-mat.supr-con↗

arXiv2002

Proximity effect in ferromagnet-superconductor hybrid structures: role of the pairing symmetry

The spatial variations of the pair amplitude, and the local density of states in d-wave or s-wave superconductor-ferromagnet hybrid structures are calculated self consistently using the Bogoliubov-deGennes formalism within the two dimensional extended Hubbard model. We describe the proximity effect in superconductor / ferromagnet (SF) bilayers, FSF trilayers, and interfaces between a superconductor and a ferromagnetic domain wall. We investigate in detail the role played by the pairing symmetry, the exchange field, interfacial scattering and crossed Andreev reflection.

cond-mat.mes-hall↗

arXiv2002

Proximity effect in multiterminal hybrid structures

We consider the proximity effect in multiterminal ferromagnet superconductor (FSF) hybrid structures in which two or three electrodes are connected to a superconductor. We show that two competing effects take place in these systems: (i) pair breaking effects due to the response to the exchange field induced in the superconductor; (ii) a reduction of the \SOP at the interface that takes place already in NS junctions. We focus on this second effect that dominates if the thickness of the S layer is small enough. We consider several single-channel electrodes connected to the same site. We calculate the superconducting order parameter and the local density of state (LDOS). With two ferromagnetic electrodes connected to a superconductor we find that the superconducting order parameter in the ferromagnetic alignment is larger than the superconducting order parameter in the antiferromagnetic alignment ($Δ_{\rm F} >Δ_{\rm AF}$), in agreement with [Eur. Phys. J. B {\bf 25}, 373 (2002)]. If a third spin polarized electrode is connected to a superconductor we find that $Δ_{\rm F}-Δ_{\rm AF}$ can change sign as the transparency of the third electrode increases. This can be understood from the fact that the superconducting order parameter is reduced if pair correlations among the ferromagnetic electrodes increase. If the two ferromagnetic electrodes are within a finite distance we find Friedel oscillations in the Gorkov function but we still obtain $Δ_{\rm F} > Δ_{\rm AF}$.

cond-mat.mes-hall↗

arXiv2002

Resonant propagation of fluxons in corner junctions with triplet pairing symmetry

We present the numerical solutions for the I-V characteristics, and describe the motion of fluxons in a frustrated Josephson junction made of an unconventional triplet superconductor and an s-wave superconductor. In the inline geometry, and long length limit the moving integer fluxon interacts with the bound fractional fluxon but is not able to change its position or polarity. We observe different modes of multifluxon propagation. In the small length limit the moving fluxon is a combination of the stable solutions that exist in the static case and additional steps are introduced in the I-V diagram.

cond-mat.supr-con↗

arXiv2002

Resonant flux motion and I-V -characteristics in frustrated Josephson junctions

We describe the dynamics of fluxons moving in a frustrated Josephson junction with p, d, and f-wave symmetry and calculate the I-V characteristics. The behavior of fluxons is quite distinct in the long and short length junction limit. For long junctions the intrinsic flux is bound at the center and the moving integer fluxon or antifluxon interacts with it only when it approaches the junction's center. For small junctions the intrinsic flux can move as a bunched type fluxon introducing additional steps in the I-V characteristics. Possible realization in quantum computation is presented.

nlin.PS↗

arXiv2002

Tunneling spectra for quasi-one-dimensional organic superconductors

The tunneling conductance spectra of a normal-metal/insulator/quasi-one-dimensional superconductor is calculated by using the Blonder-Tinkham-Klapwijk formulation. The pairing symmetry of the superconductor is assumed to be $p$, $d$, and $f$-wave. It is found that there is a well defined zero energy peak in electron tunneling along the direction parallel to the chains or normal to these when the transmitted quasiparticles feel different sign of the pair potential. The actual line shape of the spectra is sensitive to the nodes of the pair potential on the Fermi surface.

cond-mat.supr-con↗

arXiv2002

Josephson effect test for triplet pairing symmetry

The critical current modulation and the spontaneous flux of the vortex states in corner Josephson junctions between Sr$_2$RuO$_4$ and a conventional s-wave superconductor are calculated as a function of the crystal orientation, and the magnetic field. For Sr$_2$RuO$_4$ we assume two nodeless p-wave pairing states. Also we use the nodal $f$-wave states $B_{1g}\times E_u$ and $B_{2g} \times E_u$, and one special p-wave state having line nodes. It is seen that the critical current depends solely on the topology of the gap.

cond-mat.supr-con↗

arXiv2002

Tunneling current in triplet f-wave superconductors with horizontal lines of nodes

We calculate the tunneling conductance spectra of a normal-metal/insulator/triplet superconductor using the Blonder-Tinkham-Klapwijk (BTK) formulation. Possible states for the superconductor are considered with horizontal lines of nodes, breaking the time reversal symmetry. These results would be useful to discriminate between pairing states in superonductor Sr$_2$RuO$_4$ and also in UPt$_3$.

cond-mat.supr-con↗

arXiv2001

Local density of states for the corner geometry interface of d-wave superconductors, within the extended Hubbard model

The spatial variations of the order parameter, and the local density of states (LDOS) on the corner of s-wave or $d_{x^2-y^2}$-wave superconductors, as well as in superconductor-insulator-normal metal interfaces, are calculated self consistently using the Bogoliubov-deGennes formalism within the two dimensional extended Hubbard model. The exact diagonalization method is used. Due to the suppression of the dominant d-wave order parameter, the extended s-wave order parameter is induced near the surface, that alternates its sign for the topmost sites at adjacent edges of the lattice and decays to zero in the bulk. The presence of surface roughness results into the appearance of the zero band conduction peak (ZBCP) near the corner surface which lacks from the predictions of the quasiclassical theory.

cond-mat.supr-con↗