arXiv subjects

C. Tejedor

Publications and source records attributed to C. Tejedor.

71 records · Page 4Linked to original sources

arXiv2000

Canted ground state in artificial molecules at high magnetic fields

We analyze the transitions that a magnetic field provokes in the ground state of an artificial homonuclear diatomic molecule. For that purpose, we have performed numerical diagonalizations for a double quantum dot around the regime of filling factor 2. We present phase diagrams in terms of tunneling and Zeeman couplings, and confinement strength. We identify a series of transitions from ferromagnetic to symmetric states through a set of canted states with antiferromagnetic couping between the two quantum dots.

cond-mat.mes-hall↗

arXiv1999

Exciton Beats in GaAs Quantum Wells: Bosonic Representation and Collective Effects

We discuss light-heavy hole beats observed in transient optical experiments in GaAs quantum wells in terms of a free-boson coherent state model. This approach is compared with descriptions based on few-level representations. Results lead to an interpretation of the beats as due to classical electromagnetic interference. The boson picture correctly describes photon excitation of extended states and accounts for experiments involving coherent control of the exciton density and Rayleigh scattering beating.

cond-mat.mes-hall↗

arXiv1999

Ultrafast Coherent Spectroscopy of the Fermi Edge Singularity

In this work we present a theoretical description of the transient response of the Fermi Edge Singularity (FES). We study the linear and the nonlinear response of an n-doped QW to laser pulses in the Coherent Control (CC) and Four Wave Mixing (FWM) Configurations. By means of a bosonization formalism we calculate the FWM signal emitted by the sample when it is excited by pulses spectrally peaked around the FES and we show that the long time behavior of the nonlinear signal is very similar to the linear case.

cond-mat.mes-hall↗

arXiv1999

Spin-isospin textured excitations in a double layer at filling factor $ν=2$

We study the charged excitations of a double layer at filling factor 2 in the ferromagnetic regime. In a wide range of Zeeman and tunneling splittings we find that the low energy charged excitations are spin-isospin textures with the charge mostly located in one of the layers. As tunneling increases, the parent spin texture in one layer becomes larger and it induces, in the other layer, a shadow spin texture antiferromagnetically coupled to the parent texture. These new quasiparticles should be observable by measuring the strong dependence of its spin on tunneling and Zeeman couplings.

cond-mat.mes-hall↗

arXiv1998

Exciton condensates in semiconductor quantum wells emit coherent light

We show that a quasi-two dimensional condensate of optically active excitons emits coherent light even in the absence of population inversion. This allows an unambiguous and clear experimental detection of the condensed phase. We prove that, due to the exciton-photon coupling, quantum and thermal fluctuations do not destroy condensation at finite temperature. Suitable conditions to achieve condensation are temperatures of a few K for typical exciton densities, and the use of a pulsed, and preferably circularly polarized, laser.

cond-mat↗

arXiv1998

Skyrmions in quantum Hall ferromagnets as spin-waves bound to unbalanced magnetic flux quanta

A microscopic description of (baby)skyrmions in quantum Hall ferromagnets is derived from a scattering theory of collective (neutral) spin modes by a bare quasiparticle. We start by mapping the low lying spectrum of spin waves in the uniform ferromagnet onto that of free moving spin excitons, and then we study their scattering by the defect of charge. In the presence of this disturbance, the local spin stiffness varies in space, and we translate it into an inhomogeneus metric in the Hilbert space supporting the excitons. An attractive potencial is then required to preserve the symmetry under global spin rotations, and it traps the excitons around the charged defect. The quasiparticle now carries a spin texture. Textures containing more than one exciton are described within a mean-field theory, the interaction among the excitons being taken into account through a new renormalization of the metric. The number of excitons actually bound depends on the Zeeman coupling, that plays the same role as a chemical potencial. For small Zeeman energies, the defect binds many excitons which condensate. As the bound excitons have a unit of angular momentum, provided by the quantum of magnetic flux left unbalanced by the defect of charge, the resulting texture turns out to be a topological excitation of charge 1. Its energy is that given by the non-linear sigma model for the ground state in this topological sector, i.e. the texture is a skyrmion.

cond-mat.mes-hall↗

arXiv1997

Spin degree of freedom in two dimensional exciton condensates

We present a theoretical analysis of a spin-dependent multicomponent condensate in two dimensions. The case of a condensate of resonantly photoexcited excitons having two different spin orientations is studied in detail. The energy and the chemical potentials of this system depend strongly on the spin polarization . When electrons and holes are located in two different planes, the condensate can be either totally spin polarized or spin unpolarized, a property that is measurable. The phase diagram in terms of the total density and electron-hole separation is discussed.

cond-mat.mes-hall↗

arXiv1996

Spin splitting in a polarized quasi-two-dimensional exciton gas

We have observed a large spin splitting between "spin" $+1$ and $-1$ heavy-hole excitons, having unbalanced populations, in undoped GaAs/AlAs quantum wells in the absence of any external magnetic field. Time-resolved photoluminescence spectroscopy, under excitation with circularly polarized light, reveals that, for high excitonic density and short times after the pulsed excitation, the emission from majority excitons lies above that of minority ones. The amount of the splitting, which can be as large as 50% of the binding energy, increases with excitonic density and presents a time evolution closely connected with the degree of polarization of the luminescence. Our results are interpreted on the light of a recently developed model, which shows that, while intra-excitonic exchange interaction is responsible for the spin relaxation processes, exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors.

cond-mat↗

arXiv1995

Polarized interacting exciton gas in quantum wells and bulk semiconductors

We develop a theory to calculate exciton binding energies of both two- and three-dimensional spin polarized exciton gases within a mean field approach. Our method allows the analysis of recent experiments showing the importance of the polarization and intensity of the excitation light on the exciton luminescence of GaAs quantum wells. We study the breaking of the spin degeneracy observed at high exciton density $(5 \ \ 10^{10} cm ^2)$. Energy level splitting betwen spin +1 and spin -1 is shown to be due to many-body inter-excitonic exchange while the spin relaxation time is controlled by intra-exciton exchange.

cond-mat↗

arXiv1995

Skyrmions and edge spin excitations in quantum Hall droplets

We present an analysis of spin-textures in Quantum Hall droplets, for filling factors $ν\simeq 1$. Analytical wavefunctions with well defined quantum numbers are given for the low-lying states of the system which result to be either bulk skyrmions or edge spin excitations. We compute dispersion relations and study how skyrmions become ground states of the Quantum Hall droplet at $ν\gtrsim 1$. A Hartree-Fock approximation is recovered and discussed for those spin textures.

cond-mat↗

arXiv1995

Composite fermions traversing a potential barrier

Using a composite fermion picture, we study the lateral transport between two two-dimensional electron gases, at filling factor 1/2, separated by a potential barrier. In the mean field approximation, composite fermions far from the barrier do not feel a magnetic field while in the barrier region the effective magnetic field is different from zero. This produces a cutoff in the conductance when represented as a function of the thickness and height of the barrier. There is a range of barrier heights for which an incompressible liquid, at $ν=1/3$, exists in the barrier region.

cond-mat↗

arXiv1994

Low-lying excitations of Quantum Hall Droplets

We present the adequate circular representation of charge density excitations in Quantum Hall droplets at filling factor 1. A novel set of operators describes magnetoexcitons with well defined both total and center-of-mass angular momentum. The accuracy of this description is shown by the high overlap of the magnetoexcitons with the exact eigenstates of a quantum dot in a magnetic field obtained from numerical calculations up to 20 electrons. From such magnetoexcitons we get a good understanding of total energies and spectral functions.

cond-mat↗

arXiv1994

Coherent and sequential photoassisted tunneling through a semiconductor double barrier structure

We have studied the problem of coherent and sequential tunneling through a double barrier structure, assisted by light considered to be present All over the structure, i,e emitter, well and collector as in the experimental evidence. By means of a canonical transformation and in the framework of the time dependent perturbation theory, we have calculated the transmission coefficient and the electronic resonant current. Our calculations have been compared with experimental results turning out to be in good agreement. Also the effect on the coherent tunneling of a magnetic field parallel to the current in the presence of light, has been considered.

cond-mat↗

arXiv1994

Capacitance spectroscopy in quantum dots: Addition spectra and decrease of tunneling rates

A theoretical study of single electron capacitance spectroscopy in quantum dots is presented. Exact diagonalizations and the unrestricted Hartree-Fock approximation have been used to shed light over some of the unresolved aspects. The addition spectra of up to 15 electrons is obtained and compared with the experiment. We show evidence for understanding the decrease of the single electron tunneling rates in terms of the behavior of the spectral weight function. (To appear in Phys. Rev. B (Rapid Comm.))

cond-mat↗

arXiv1993

Mode-Matching Technique for Transmission Calculations in Electron Waveguides at High Magnetic Fields

In this paper we present a mode-matching technique to study the transmission coefficient of mesoscopic devices such as electron waveguides in the presence of high magnetic fields for different situations. A detailed study of the difficulties rising due to the presence of the magnetic field is given and the differences with the zero magnetic field case are stressed. We apply this technique to calculate the transmission at non-zero magnetic field of two completely different systems: a) a quantum box built up on a quantum wire (or electron waveguide) by means of two barriers and b) a meandering quantum wire, {\em i.e.}, a wire with changes in the guiding direction. In the former case we analyze the so-called {\em Coulomb Blockade} and {\em Aharanov-Bohm} regimes and in the latter one we focus on the realistic case of soft, circular bends joining the different sections of the wire.

cond-mat↗

arXiv1993

Correlation Effects on Transport Through Few-Electrons Systems

We study lateral tunneling through a quantum box including electron-electron interactions in the presence of a magnetic field which breaks single particle degeneracies. The conductance at zero temperature as a function of the Fermi energy in the leads consists of a set of peaks related to changing by one the electron occupancy in the box. We find that the position and heights of the peaks are controlled by many-body effects. We compute the conductance up to 8 electrons for several cases where correlation effects dominate. In the range of intermediate fields spin selection rules quench some peaks. At low and high fields the behavior of the conductance as a function of the number of electrons is very different due to big changes in the many-body ground state wavefunctions.

cond-mat↗

arXiv1993

Phase Separation of Edge States in the Integer Quantum Hall Regime

Coulomb effects on the edge states of a two dimensional electron gas in the presence of a high magnetic field are studied for different widths of the boundaries. Schrödinger and Poisson equations are selfconsistently solved in the integer Quantum Hall regime. Regions of flat bands at the Fermi level appear for smooth interfaces in order to minimize the electrostatic energy related to the existence of dipoles induced by the magnetic field. These plateaus determine the phase separation in stripes of compressible and incompressible electron liquids.

cond-mat↗