arXiv subjects

Arturo Tagliacozzo

Publications and source records attributed to Arturo Tagliacozzo.

15 recordsLinked to original sources

arXiv2022

Quantum zero point electromagnetic energy difference between the superconducting and the normal phase in a HTc superconducting metal bulk sample

We provide a novel methodological approach to the estimate of the change of the Quantum Vacuum electromagnetic energy density in a High critical Temperature superconducting metal bulk sample, when it undergoes the transition in temperature, from the superconducting to the normal phase. The various contributions to the Casimir energy in the two phases are highlighted and compared. While the TM polarization of the vacuum mode allows for a macroscopic description of the superconducting transition, the changes in the TE vacuum mode induced by the superconductive correlations are introduced within a microscopic model, which does not explicitly take into account the anisotropic structure of the material.

cond-mat.supr-con↗

arXiv2016

Junction of three off-critical quantum Ising chains and two-channel Kondo effect in a superconductor

We show that a junction of three off-critical quantum Ising chains can be regarded as a quantum spin chain realization of the two-channel spin-1/2 overscreened Kondo effect with two superconducting leads. We prove that, as long as the Kondo temperature is larger than the superconducting gap, the equivalent Kondo model flows towards the 2 channel Kondo fixed point. We argue that our system provides the first controlled realization of 2 channel Kondo effect with superconducting leads. This, besides its the theoretical interest, is of importance for potential applications to a number of context, including the analysis of the quantum entanglement properties of a Kondo system.

cond-mat.str-el↗

arXiv2016

High critical temperature nodal superconductors as building block for time-reversal invariant topological superconductivity

We study possible applications of high critical temperature nodal superconductors for the search for Majorana bound states in the DIII class. We propose a microscopic analysis of the proximity effect induced by d-wave superconductors on a semiconductor wire with strong spin-orbit coupling. We characterize the induced superconductivity on the wire employing a numerical self-consistent tight-binding Bogoliubov-de Gennes approach, and analytical considerations on the Green's function. The order parameter induced on the wire, the pair correlation function, and the renormalization of the Fermi points are analyzed in detail, as well as the topological phase diagram in the case of weak coupling. We highlight optimal Hamiltonian parameters to access the nontrivial topological phase which could display time-reversal invariant Majorana doublets at the boundaries of the wire.

cond-mat.supr-con↗

arXiv2016

From four- to two-channel Kondo effect in junctions of XY spin chains

We consider the Kondo effect in Y-junctions of anisotropic XY models in an applied magnetic field along the critical lines characterized by a gapless excitation spectrum. We find that, while the boundary interaction Hamiltonian describing the junction can be recasted in the form of a four-channel, spin-1/2 antiferromagnetic Kondo Hamiltonian, the number of channels effectively participating in the Kondo effect depends on the chain parameters, as well as on the boundary couplings at the junction. The system evolves from an effective four-channel topological Kondo effect for a junction of XX-chains with symmetric boundary couplings into a two-channel one at a junction of three quantum critical Ising chains. The effective number of Kondo channels depends on the properties of the boundary and of the bulk. The XX-line is a "critical" line, where a four-channel topological Kondo effect can be recovered by fine-tuning the boundary parameter, while along the line in parameter space connecting the extreme regimes, XX-line and the critical Ising point the junction is effectively equivalent to a two-channel topological Kondo Hamiltonian. Using a renormalization group approach, we determine the flow of the boundary couplings, which allows us to define and estimate the critical couplings and Kondo temperatures of the different Kondo (pair) channels. Finally, we study the local transverse magnetization in the center of the Y-junction, eventually arguing that it provides an effective tool to monitor the onset of the two-channel Kondo effect.

cond-mat.str-el↗

arXiv2013

Electron-phonon interaction on the surface of a 3D topological insulator

We analyze the role of a Rayleigh surface phonon mode in the electron-phonon interaction at the surface of a 3D topological insulator. A strong renormalization of the phonon dispersion, leading eventually to the disappearance of Rayleigh phonons, is ruled out in the ideal case of a continuum long-wavelength limit, which is only justified if the surface is clean and defect free. The absence of backscattering for Dirac electrons at the Fermi surface is partly responsible for the reduced influence of the electron-phonon interaction. A pole in the dielectric response function due to the Rayleigh phonon dispersion could drive the electron-electron interaction attractive at low frequencies. However, the average pairing interaction within the weak coupling approach is found to be too small to induce a surface superconducting instability.

cond-mat.mes-hall↗

arXiv2010

Suppression of Kondo-assisted co-tunneling in a spin-1 quantum dot with Spin-Orbit interaction

Kondo-type zero-bias anomalies have been frequently observed in quantum dots occupied by two electrons and attributed to a spin-triplet configuration that may become stable under particular circumstances. Conversely, zero-bias anomalies have been so far quite elusive when quantum dots are occupied by an even number of electrons greater than two, even though a spin-triplet configuration is more likely to be stabilized there than for two electrons. We propose as an origin of this phenomenon the spin-orbit interaction, and we show how it profoundly alters the conventional Kondo screening scenario in the simple case of a laterally confined quantum dot with four electrons.

cond-mat.mes-hall↗

arXiv2005

Charge dynamics effects in conductance through a large semi-open quantum dot

Fano lineshapes in resonant transmission in a quantum dot imply interference between localized and extended states. The influence of the charge accumulated at the localized levels, which screens the external gate voltage acting on the conduction channel is investigated. The modified Fano q parameter and the resonant conduction is derived starting from a microscopic Hamiltonian. The latest experiments on "charge sensing" and ``Coulomb modified Fano sensing `` compare well with the results of the present model.

cond-mat.mes-hall↗

arXiv2004

Fano versus Kondo Resonances in a Multilevel "Semi-Open" Quantum Dot

Linear conductance across a large quantum dot via a single level e_0 with large hybridization to the contacts is strongly sensitive to quasi-bound states localized in the dot and weakly coupled to e_0. It oscillates with the gate voltage due to interference of the Fano type. At low temperature and Coulomb blockade, Kondo correlations damp the oscillations on an extended range of gate voltage values, by freezing the occupancy of the e_0 level itself. As a consequence, antiresonances of Fano origin are washed out. The results are in good correspondence with experimental data for a large quantum dot in the semi-open regime.

cond-mat.mes-hall↗

arXiv2004

Hamiltonian theory of the strongly-coupled limit of the Kondo problem in the overscreened case

By properly generalizing Nozie`res' Fermi liquid theory, we construct an Hamiltonian approach to the scattering of conduction electrons off a spin-1/2 impurity in the ovescreneed Kondo regime, as T -> 0. We derive the S-matrix at the interacting fixed point, and the corresponding phase shifts, together with leading energy corrections to the unitary limit. We apply our results to obtain the low-temperature dependence of the 2-channel Kondo conductance, and we relate it to possible transport experiments in a Quantum Dot

cond-mat.mes-hall↗

arXiv2004

Josephson versus Kondo coupling in a quantum dot 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. Our method projects out all states on the dot but the Kondo singlet and is valid when Kondo correlations are dominant and no Andreev bound states localized at the dot are available for Kondo screening. In these conditions superconducting pairing is a competing effect and the junction is $π-$like, to optimize antiferromagnetic correlations on the dot. As the superconducting gap increases, the Josephson current also increases, but its phase dependence becomes strongly non sinusoidal.

cond-mat.str-el↗

arXiv2004

Linear Kondo conductance in a quantum dot

In a tunneling experiment across a quantum dot it is possible to change the coupling between the dot and the contacts at will, by properly tuning the trasparency of the barriers and the temperature. Gate voltages allow for changes of the relative position of the dot addition energies and the Fermi level of the leads. Here we discuss the two limiting cases: weak and strong coupling in the tunneling Hamiltonian. In the latter case Kondo resonant conductance can emerge at low temperature in a Coulomb blockade valley. We give a pedagogical approach to the single-channel Kondo physics at equilibrium and review the Nozieres scattering picture of the correlated fixed point. We emphasize the effect of an applied magnetic field and show how an orbital Kondo effect can take place in vertical quantum dots tuned both to an even and to an odd number of electrons at a level crossing. We extend the approach to the two-channel overscreened Kondo case and discuss recent proposals for detecting the non-Fermi liquid fixed point which could be reached at strong coupling.

cond-mat.str-el↗

arXiv2002

Possibility of two-channel orbital Kondo "spin" 1/2 conductance in a quantum dot

We propose that two-channel orbital Kondo ``spin'' 1/2 conductance can be measured in a quantum dot at Coulomb Blockade with an odd number of electrons with contacts in a pillar configuration, if an orthogonal magnetic field induces an appropriate level crossing. At the zero-temperature strong coupling fixed point the conductance reaches the unitarity limit with a non-Fermi liquid sqrt(T)-law.

cond-mat.mes-hall↗

arXiv2001

Compensation of the spin of a quantum dot at Coulomb blockade

We discuss a new entangled state that has been observed in the conduction across a quantum dot. At Coulomb blockade, electrons from the contacts correlate strongly to those localized in the dot, due to cotunneling processes. Because of the strong Coulomb repulsion on the dot, its electron number is unchanged w.r.to the dot in isolation, but the total spin is fully or partly compensated. In a dot with N=even at the singlet-triplet crossing, which occurs in large magnetic field, Kondo correlations lead to a total spin S=1/2.

cond-mat.mes-hall↗

arXiv1999

Spin fractionalization of an even number of electrons in a Quantum dot

An experiment is proposed of non perturbative tunneling in a Quantum dot connected to leads in a pillar configuration, which would shed light on the physics of the mesoscopic Kondo problem. We propose for the first time that what is coupled to the leads in the case of even number of electrons on the dot is not just the electrons on the QD, with their own spins, but the very total spin of the dot macroscopic state itself, that displays features of the Kondo physics, that leads to fractionalization of the spin in the dot, that is to a "spinon box".

cond-mat↗

arXiv1997

Addition energies of a Quantum Dot with harmonic electron-electron interactions

We study a two dimensional electron system in a parabolic confining potential and constant magnetic field for the case of harmonic electron-electron interaction. We present analytic results for the electrochemical potential versus magnetic field and discuss the effects of correlation in connection with the addition energy of a Quantum Dot with few electrons.

cond-mat.mes-hall↗