arXiv subjects

E. Lipparini

Publications and source records attributed to E. Lipparini.

15 recordsLinked to original sources

arXiv2010

The Effect of Interactions on the Conductance of Graphene Nanoribbons

We study the effects of the interaction between electrons and holes on the conductance G of quasi-one-dimensional graphene systems. We first consider as a benchmark the limit in which all interactions are negligible, recovering the predictions of the tight-binding approximation for the spectrum of the system, and the well-known result G=4 e^2/h for the lowest conductance quantum. Then we consider an exactly solvable field theoretical model in which the electro-magnetic interactions are effectively local. Finally, we use the effective field theory formalism to develop an exactly solvable model in which we also include the effect of non-local interactions. We find that such interactions turn the nominally metallic armchair graphene nanoribbon into a semi-conductor, while the short-range interactions lead to a correction to the G=4 e^2/h formula.

cond-mat.mes-hall↗

arXiv2010

Quantum Monte Carlo study of circular quantum dots in presence of Rashba interaction

We present the numerical Quantum Monte Carlo results for the ground state energy of circular quantum dots in which Rashba spin-orbit iteraction is present. Diffusion Monte Carlo with spin propagation is applied in order to treat the spin-orbit interaction correctly, following previous work done in the fieldof the two-dimensional electron gas. Together with ground state energies, also numerical results for density and spin-density profiles are given.

cond-mat.supr-con↗

arXiv2010

Spin-orbit excitations of quantum wells

Confinement asymmetry effects on the photoabsorption of a quantum well are discussed by means of a sum-rules approach using a Hamiltonian including a Rashba spin-orbt coupling. We show that while the strength of the excitation is zero when the spin-orbit coupling is neglected, the inclusion of the spin-orbit interaction gives rise to a non zero strength and mean excitation energy in the far-infrared region. A simple expression for these quantities up to the second order in the Rashba parameter was derived. The effect of two-body Coulomb interaction is then studied by means of a Quantum Monte Carlo calculation, showing that electron-electron correlations induce only a small deviation from the independent particle model result.

cond-mat.supr-con↗

arXiv2009

Effective Field Theory for the Quantum Electrodynamics of a Graphene Wire

We study the low-energy quantum electrodynamics of electrons and holes, in a thin graphene wire. We develop an effective field theory (EFT) based on an expansion in p/p_T, where p_T is the typical momentum of electrons and holes in the transverse direction, while p are the momenta in the longitudinal direction. We show that, to the lowest-order in (p/p_T), our EFT theory is formally equivalent to the exactly solvable Schwinger model. By exploiting such an analogy, we find that the ground state of the quantum wire contains a condensate of electron-hole pairs. The excitation spectrum is saturated by electron-hole collective bound-states, and we calculate the dispersion law of such modes. We also compute the DC conductivity per unit length at zero chemical potential and find g_s =e^2/h, where g_s=4 is the degeneracy factor.

cond-mat.mes-hall↗

arXiv2009

Quantum Monte Carlo study of the two-dimensional electron gas in presence of Rashba interaction

We introduce a variant to the Diffusion Monte Carlo algorithm that can be employed to study the effects of the Rashba interaction in a many electron systems. Because of the spin--orbit nature of Rashba interaction a standard algorithm cannot be applied and therefore a specific imaginary time spin dependent propagator has been developed and implemented following previous work developed in the framework of nuclear physics. We computed the ground state energy of the 2D electron gas at different densities for several values of the Rashba interaction strength as a function of "Rashba spin states" polarization. Comparison is given with analytically known Hartree-Fock results and for the system in absence of Coulomb interaction.

cond-mat.str-el↗

arXiv2006

Brueckner-Hartree-Fock study of circular quantum dots

We calculate ground state energies in the Brueckner-Hartree-Fock theory for $N$ electrons (with $N\le 20$) confined to a circular quantum dot and in presence of a static magnetic field. Comparison with the predictions of Hartree-Fock, local-spin-density and exact configuration-interaction theories is made. We find that the correlations taken into account in Brueckner-Hartree-Fock calculations give an important contribution to the ground state energies, specially in strongly confined dots. In this high-density range, corresponding in practice to self-assembled quantum dots, the results of Brueckner-Hartree-Fock calculations are close to the exact values and better than those obtained in the local-spin-density approximation.

cond-mat.str-el↗

arXiv2003

Erratum: Diffusion Monte Carlo study of circular quantum dots

Several of the energies in Tables I and II of our previous paper are incorrect. Most of the errors are due to incorrectly inputting the symmetry of some of the states, others are due to incorrectly converting or transcribing the energies. In particular, for N=4 we had a near degeneracy and a violation of Hund's first rule. The corrected result has $|L=0,S=1>$ as the ground state, as predicted by Hund's rule. Hund's first rule is satisfied for all $N$ for the confining potential used. We present corrected versions of Tables I and II.

cond-mat↗

arXiv2001

Multipole modes and spin features in the Raman spectrum of nanoscopic quantum rings

We present a systematic study of ground state and spectroscopic properties of many-electron nanoscopic quantum rings. Addition energies at zero magnetic field (B) and electro-chemical potentials as a function of $B$ are given for a ring hosting up to 24 electrons. We find discontinuities in the excitation energies of multipole spin and charge density modes, and a coupling between the charge and spin density responses that allow to identify the formation of ferromagnetic ground states in narrow magnetic field regions. These effects can be observed in Raman experiments, and are related to the fractional Aharonov-Bohm oscillations of the energy and of the persistent current in the ring.

cond-mat↗

arXiv2000

Diffusion Monte Carlo study of circular quantum dots

We present ground and excited state energies obtained from Diffusion Monte Carlo (DMC) calculations, using accurate multiconfiguration wave functions, for $N$ electrons ($N\le13$) confined to a circular quantum dot. We analyze the electron-electron pair correlation functions and compare the density and correlation energies to the predictions of local spin density approximation theory (LSDA). The DMC estimated change in electrochemical potential as function of the number of electrons in the dot is compared to that from LSDA and Hartree-Fock (HF) calculations.

cond-mat↗

arXiv2000

Far-infrared excitations in a quantum antidot at finite magnetic fields

We have investigated the far-infrared dipole modes of a quantum antidot submitted to a perpendicularly applied magnetic field B. The ground state of the antidot is described within local spin-density functional theory, and the spectrum within time-dependent local spin-density functional theory. The results are compared with those corresponding to a quantum dot of similar electronic surface density. The method is able to reproduce two of the more salient experimental features, namely that main bulk and edge modes have the same circular polarization, and that the negative B dispersion edge branch oscillates, having minima at the B values corresponding to fully occupied Landau levels. It fails, however, to yield the unique feature of short-period antidot lattices that the energy of the edge magnetoplasmon approaches the cyclotron frequency for small B. The existence of anticyclotron polarized bulk modes is discussed, and a detailed account of the dipole spin mode is presented.

cond-mat↗

arXiv1999

Transverse dipole spin modes in quantum dots

We have carried out a systematic analysis of the transverse dipole spin response of a large size quantum dot within time-dependent current density functional theory. Results for magnetic fields corresponding to integer filling factors are reported, as well as a comparison with the longitudinal dipole spin response. As in the two dimensional electron gas, the spin response at high spin magnetization is dominated by a low energy transverse mode.

cond-mat↗

arXiv1999

Multipole excitations in quantum dots

We have employed time-dependent local-spin density theory to analyze the multipole spin and charge density excitations recently found in GaAs-AlGaAs quatum dots [C. Schüller et al, Phys. Rev. Lett {\bf 80}, 2673 (1998)]. The overall agreement between theory and experiment is good, identifying the angular momentum of the modes observed in the experiment. We have found that high multipolarity spin density edge modes originate from interband transitions instead that from intraband transitions, as it happens in the dipole case.

cond-mat↗

arXiv1999

Magnetoplasmons in quantum rings

We have studied the structure and dipole charge density response of nanorings as a function of the magnetic field using local-spin density functional theory. Two small rings consisting of 12 and 22 electrons confined by a positively charged background are used to represent the cases of a narrow and a wide ring. The results are qualitatively compared with experimental data existing on microrings and on antidots. A smaller ring containing 5 electrons is also analyzed to allow for a closer comparison with a recent experiment on a two electron quantum ring.

cond-mat↗

arXiv1998

Spin and density longitudinal response of quantum dots in time-dependent local-spin-density approximation

The longitudinal dipole response of a quantum dot has been calculated in the far-infrared regime using local spin density functional theory. We have studied the coupling between the collective spin and density modes as a function of the magnetic field. We have found that the spin dipole mode and single particle excitations have a sizeable overlap, and that the magnetoplasmon modes can be excited by the dipole spin operator if the dot is spin polarized. The frequency of the dipole spin edge mode presents an oscillation which is clearly filling factor ($ν$) related. We have found that the spin dipole mode is especially soft for even $ν$ values, becoming unstable for magnetic fields in the region $1 < ν\leq 2$. Results for selected number of electrons and confining potentials are discussed. An analytical model which reproduces the main features of the microscopic spectra has been developed.

cond-mat↗

arXiv1997

Far-infrared edge modes in quantum dots

We have investigated edge modes of different multipolarity sustained by quantum dots submitted to external magnetic fields. We present a microscopic description based on a variational solution of the equation of motion for any axially symmetric confining potential and multipole mode. Numerical results for dots with different number of electrons whose ground-state is described within a local Current Density Functional Theory are discussed. Two sum rules, which are exact within this theory, are derived. In the limit of a large neutral dot at B=0, we have shown that the classical hydrodynamic dispersion law for edge waves ω(q) \sim \sqrt{q \ln (q_0/q)} holds when quantum and finite size effects are taken into account.

cond-mat↗