arXiv subjects

A. Tagliacozzo

Publications and source records attributed to A. Tagliacozzo.

32 records · Page 2Linked to original sources

arXiv2007

Coherent quasiparticle transport in grain boundary junctions employing high-Tc superconductors

Magneto-fluctuations of the normal resistance RN have been reproducibly observed in YBa2Cu3O7-d biepitaxial grain boundary junctions at low temperatures. We attribute them to mesoscopic transport in narrow channels across the grain boundary line, occurring in an unusual energy regime. The Thouless energy appears to be the relevant energy scale. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed.

cond-mat.mtrl-sci↗

arXiv2007

Quantum Rings with Rashba spin orbit coupling: a path integral approach

We employ a path integral real time approach to compute the DC conductance and spin polarization for electrons transported across a ballistic Quantum Ring with Rashba spin-orbit interaction. We use a piecewise semiclassical approximation for the particle orbital motion and solve the spin dynamics exactly, by accounting for both Zeeman coupling and spin-orbit interaction at the same time. Within our approach, we are able to study how the interplay between Berry phase, Ahronov Casher phase, Zeeman interaction and weak localization corrections influences the quantum interference in the conductance within a wide range of externally applied fields. Our results are helpful in inerpreting recent measurements on interferometric rings.

cond-mat.mes-hall↗

arXiv2006

Far Infrared absorption of non center of mass modes and optical sum rule in a few electron quantum dot with Rashba spin-orbit coupling

Spin-orbit interaction in a quantum dot couples far infrared radiation to non center of mass excitation modes, even for parabolic confinement and dipole approximation. The intensities of the absorption peaks satisfy the optical sum rule, giving direct information on the total number of electrons inside the dot. In the case of a circularly polarized radiation the sum rule is insensitive to the strength of a Rashba spin-orbit coupling due to an electric field orthogonal to the dot plane, but not to other sources of spin-orbit interaction, thus allowing to discriminate between the two.

cond-mat.str-el↗

arXiv2006

Observation of mesoscopic conductance fluctuations in YBaCuO grain boundary Josephson Junctions

Magneto-fluctuations of the normal resistance R_N have been reproducibly observed in high critical temp erature superconductor (HTS) grain boundary junctions, at low temperatures. We attribute them to mesoscopic transport in narrow channels across the grain boundary line. The Thouless energy appears to be the relevant energy scale. Our findings have significant implications on quasiparticle relaxation and coherent transport in HTS grain boundaries.

cond-mat.supr-con↗

arXiv2005

Quantum interference of electrons in a ring: tuning of the geometrical phase

We calculate the oscillations of the DC conductance across a mesoscopic ring, simultaneously tuned by applied magnetic and electric fields orthogonal to the ring. The oscillations depend on the Aharonov-Bohm flux and of the spin-orbit coupling. They result from mixing of the dynamical phase, including the Zeeman spin splitting, and of geometric phases. By changing the applied fields, the geometric phase contribution to the conductance oscillations can be tuned from the adiabatic (Berry) to the nonadiabatic (Ahronov-Anandan) regime. To model a realistic device, we also include nonzero backscattering at the connection between ring and contacts, and a random phase for electron wavefunction, accounting for dephasing due to disorder.

cond-mat.mes-hall↗

arXiv2005

Rashba-control for the spin excitation of a fully spin polarized vertical quantum dot

Far infrared radiation absorption of a quantum dot with few electrons in an orthogonal magnetic field could monitor the crossover to the fully spin polarized state. A Rashba spin-orbit coupling can tune the energy and the spin density of the first excited state which has a spin texture carrying one extra unit of angular momentum. The spin orbit coupling can squeeze a flipped spin density at the center of the dot and can increase the gap in the spectrum.

cond-mat.str-el↗

arXiv2004

Coherent response of a low T_c Josephson junction to an ultrafast laser pulse

By irradiating with a single ultrafast laser pulse a superconducting electrode of a Josephson junction it is possible to drive the quasiparticles (qp's) distribution strongly out of equilibrium. The behavior of the Josephson device can, thus, be modified on a fast time scale, shorter than the qp's relaxation time. This could be very useful, in that it allows fast control of Josephson charge qubits and, in general, of all Josephson devices. If the energy released to the top layer contact $S1$ of the junction is of the order of $\sim μJ$, the coherence is not degradated, because the perturbation is very fast. Within the framework of the quasiclassical Keldysh Green's function theory, we find that the order parameter of $S1$ decreases. We study the perturbed dynamics of the junction, when the current bias is close to the critical current, by integrating numerically its classical equation of motion. The optical ultrafast pulse can produce switchings of the junction from the Josephson state to the voltage state. The switches can be controlled by tuning the laser light intensity and the pulse duration of the Josephson junction.

cond-mat.supr-con↗

arXiv2003

Spin Exciton in quantum dot with spin orbit coupling in high magnetic field

Coulomb interactions of few ($ N $) electrons confined in a disk shaped quantum dot, with a large magnetic field $B=B^*$ applied in the z-direction (orthogonal to the dot), produce a fully spin polarized ground state. We numerically study the splitting of the levels corresponding to the multiplet of total spin $S=N/2$ (each labeled by a different total angular momentum $ J_z $) in presence of an electric field parallel to $ B $, coupled to $ S $ by a Rashba term. We find that the first excited state is a spin exciton with a reversed spin at the origin. This is reminiscent of the Quantum Hall Ferromagnet at filling one which has the skyrmion-like state as its first excited state. The spin exciton level can be tuned with the electric field and infrared radiation can provide energy and angular momentum to excite it.

cond-mat.mes-hall↗

arXiv2003

Adiabatic Control of the Electron Phase in a Quantum Dot

A Berry phase can be added to the wavefunction of an isolated quantum dot by adiabatically modulating a nonuniform electric field along a time-cycle. The dot is tuned close to a three-level degeneracy, which provides a wide range of possibilities of control. We propose to detect the accumulated phase by capacitively coupling the dot to a double-path inteferometer. The effective Hamiltonian for the phase-sensitive coupling is discussed in detail.

cond-mat.mes-hall↗

arXiv2001

Josephson Current in a Quantum Dot in the Kondo Regime Connected to Two Superconductors

We apply a Gutzwiller-like variational technique to study Josephson conduction across a quantum dot with an odd number of electrons connected to two superconducting leads. We show that, for small values of the superconducting gap, Kondo correlations and superconductivity cooperate to enhance the Josephson current. As the superconducting gap increases, the current changes sign and the system becomes a $π$-junction. The $π$-junction behavior sets in much before antiferromagnetic correlations at the dot can be treated perturbatively.

cond-mat.str-el↗

arXiv2000

Kondo ground state in a quantum dot with an even number of electrons

Kondo conduction has been observed in a quantum dot with an even number of electrons at the Triplet-Singlet degeneracy point produced by applying a small magnetic field $B$ orthogonal to the dot plane. At a much larger field $ B \sim B_*$, orbital effects induce the reversed transition from the Singlet to the Triplet state. We study the newly proposed Kondo behavior at this point. Here the Zeeman spin splitting cannot be neglected, what changes the nature of the Kondo coupling. On grounds of exact diagonalization results in a dot with cylindrical symmetry, we show that, at odds with what happens at the other crossing point, close to $B_*$, orbital and spin degrees of freedom are ``locked together'', so that the Kondo coupling involves a fictitious spin 1/2 only, which is fully compensated by conduction electrons under suitable conditions. In this sense, spin at the dot is fractionalized. We derive the scaling equation of the system by means of a nonperturbative variational approach. The approach is extended to the $B \neq B_*$-case and the residual magnetization on the dot is discussed.

cond-mat.mes-hall↗

arXiv1999

Sequential magnetotunneling in a vertical Quantum Dot tuned at the crossing to higher spin states

We have calculated the linear magnetoconductance across a vertical parabolic Quantum Dot with a magnetic field in the direction of the current. Gate voltage and magnetic field are tuned at the degeneracy point between the occupancies N=2 and N=3, close to the Singlet-Triplet transition for N=2. We find that the conductance is enhanced prior to the transition by nearby crossings of the levels of the 3 particle dot. Immediately after it is depressed by roughly 1/3, as long as the total spin S of the 3 electron ground state doesn't change from S=1/2 to S=3/2, due to spin selection rule. At low temperature this dip is very sharp, but the peak is recovered by increasing the temperature.

cond-mat.mes-hall↗

arXiv1998

Shell filling in non-linear magneto-tunneling spectroscopy of vertical quantum dots

We report on non-linear magneto-tunneling experiments carried out in single GaAs vertical quantum dots. We show that conduction at low voltage bias can be a spectroscopic tool for both the ground state and first excited states of few electrons on the dot. Increasing voltage a large resonant peak is observed and attributed to tunneling across the quasi-continuum of higher excited states.

cond-mat.mes-hall↗

arXiv1995

Andreev tunneling into a one-dimensional Josephson junction array

In this letter we consider Andreev tunneling between a normal metal and a one dimensional Josephson junction array with finite-range Coulomb energy. The $I-V$ characteristics strongly deviate from the classical linear Andreev current. We show that the non linear conductance possesses interesting scaling behavior when the chain undergoes a T=0 superconductor-insulator transition of Kosterlitz-Thouless-Berezinskii type. When the chain has quasi-long range order, the low lying excitation are gapless and the $I-V$ curves are power-law (the linear relation is recovered when charging energy can be disregarded). When the chain is in the insulating phase the Andreev current is blocked at a threshold which is proportional to the inverse correlation length in the chain (much lower than the Coulomb gap) and which vanishes at the transition point.

cond-mat↗