L. A. de Vaulchier

Linear temperature variation of the penetration depth in YBa2Cu3O7 − δ thin films

We have measured the penetration depth λ(T) on YBa2Cu3O7 thin films from transmission at 120, 330 and 510 GHz, between 5 and 50 K. Our data yield simultaneously the absolute value and the temperature dependence of λ(T). In high-quality films λ(T) exhibits the same linear temperature dependence as single crystals, showing its intrinsic nature, and λ(0) = 1570 A. In a lower quality one, the more usual T2-dependence is found, and λ(0) = 3400 A. This suggests that the T2 variations is of extrinsic origin. Our results put the d-wave-like interpretation in a much better position.

Inter-Landau level scattering and LO-phonon emission in terahertz quantum cascade laser

A terahertz quantum cascade laser (QCL) structure based on a bound to continuum and LO-phonon extraction stage is studied under a strong magnetic field. Two series of power oscillations as a function of magnetic field are observed. Comprehensive simulations of the lifetimes allow the first series to be assigned to interface roughness (elastic) and the second to LO-phonon scattering (inelastic) of hot carriers in an excited Landau level, previously unobserved in terahertz QCL. The authors demonstrate that for the latter, the thermal electron distribution in the upper laser state leads to a breaking of the population inversion and hence laser action.

YBa2Cu3O7 films epitaxially grown on MgO, LaAlO3, SrLaAlO4 and Al2O3 substrates structural and superconducting properties in correlation with the microwave surface resistance and the far-infrared transmittance

YBa2Cu3O7 epitaxial films have been grown by laser ablation on MgO, LaAlO3, SrLaAlO4 and Al2O3 substrates. Films on LaAlO3 have excellent structural, superconducting and microwave properties, reflected by surface-resistance (RS) values systematically in the range 0.5–1 mΩ (10 GHz, 77 K). On MgO substrates, such low RS (<0.50 mΩ) values can be reached on both fully “normal-0°” and “45°” in-plane oriented YBa2Cu3O7 films, whereas in mixed (0°–45°) films the presence of the high-angle grain boundaries leads to a surface resistance larger than 100 mΩ. On SrLaAlO4 and bare Al2O3 substrates, despite good superconducting properties, obtained for sapphire due to the specific substrate surface treatment, the structural quality and surface resistance are moderate. The latter results are in complete agreement with the correlation we have demonstrated between structural quality and surface resistance as well as far-infrared transmittance. We also point out the importance of a RS determination as an intrinsic superconducting parameter: the absence of a direct correlation with low-frequency measurements shows that the latter are not sufficient to fully characterize the superconducting thin films.

Evidence for excitonic polarons inInAs∕GaAsquantum dots

We investigate the interband transitions in several ensembles of self-assembled InAs/GaAs quantum dots by using photoluminescence excitation spectroscopy under strong magnetic fields up to 28 telsa. Well defined resonances are observed in the spectra. The magnetic field dependence of the resonance energies allows an unambiguous assignment of the interband transitions which involve both discrete states of the quantum dots and wetting layer states. A strong anticrossing between two transitions is observed in all samples, which cannot be accounted for by a purely excitonic model. The coupling between the mixed exciton-LO phonon states is calculated using the Frohlich Hamiltonian. The excitonic polaron energies as well as the oscillator strengths of the interband transitions are determined. An anticrossing is predicted when two exciton-LO phonon states have close enough energies with phonon occupations which differ by one. A good agreement is found between the calculations and the experimental data evidencing the existence of excitonic polarons.

Space charge mediated negative differential resistance in terahertz quantum well detectors

In terahertz quantum well infrared photodetectors, a built-in-charge-mediated regime transition of the electronic transport is thoroughly investigated. The very strong current discontinuity and negative differential resistivity behavior are explained in terms of band structure reorganizations. The analysis of bias versus current measurements reveals that the transition occurs when the first two wells of the structure become partially drained, and the second well enters the ionized regime before the first one. Both many-body effects and a careful model of the contact have to be considered to account for these features. The source of the built-in charge is identified as intersubband impact ionization. The regime transition is one of its few experimental proofs, and provides an original approach to investigate hot electron kinetics in multi-quantum-well structures.

Intersubband impact ionization in THz QWIPs: shaping band structure reorganizations to design novel detectors

Electronic transport in AlGaAs/GaAs THz Quantum Wells Intersubband Photodetectors (QWIPs) exhibits two different regimes separated by huge discontinuities (up to five orders of magnitude) in the resistivity. They are interpreted in terms of band structure reorganizations triggered by intersubband impact ionization. We will analyze and model their in uence on the electronic transport. The magnitude of the transport modifications is explained by the small transition energy and the sharpness of the electrons distribution at stake in THz QWIPs. Measurements under magnetic field or temperature show that the broadening of the electron distribution damps the effects of impact ionization. Some experimental features of the electronic transport of shorter wavelength detectors are then reproduced. The use of intersubband impact ionization in THz QWIPs to design high gain and fast novel detectors is discussed.

THz inter-Landau level emission in a quantum cascade structure

A THz quantum cascade structure has been designed to allow population relaxation between crossing Landau levels from different subbands under perpendicular magnetic field. Electroluminescence measurements show intersubband luminescence as well as inter-Landau level emission (cyclotron emission).

Impact ionization in THz QWIPs

Recently phase transitions in THz Quantum Wells Inter-subband Photodetectors (QWIP) were observed. Very large current discontinuities - up to five orders of magnitude - were obtained, at low temperature [1]. Quantum well impact ionization shifts the structure from a resistive “down” state, where the current flows through inter-well quantum tunneling to a highly conductive “up” state. In this paper, thanks to the analysis of the bias versus current “s” curve, an accurate investigation of impact ionization processes in quantum wells is analyzed. The use of these phenomena for high sensitivity quantum well THz detection will be presented.

Magneto‐optical interband transitions in semiconductor quantum dots : evidence for excitonic polarons

We investigate the interband transitions of self‐assembled InAs/GaAs quantum dots by using resonant photoluminescence spectroscopy under strong magnetic field. Well defined resonances are observed in the spectra. A strong anti crossing between two transitions is observed, which cannot be accounted for by a purely excitonic model. The coupling between the mixed exciton‐LO phonon states is calculated using the Frohlich Hamiltonian. The excitonic polaron energies as well as the oscillator strengths of the interband transitions are determined. An anticrossing is predicted when two exciton‐LO phonon states have close enough energies with phonon occupations which differ by one. A good agreement is found between the calculations and the experimental data evidencing the existence of excitonic polarons.

Far-infrared probe of size dispersion and population fluctuations in doped self-assembled quantum dots

Abstract We investigate the FIR magneto-optical transitions in doped self-assembled InAs quantum dots with a filling ranging from 1 to 6 electrons per dot. Significant changes in the line width and the dispersion of the transitions are observed as the doping level is varied, in agreement with our theoretical calculations. We show that our technique is a very effective tool to obtain informations regarding the dot size dispersion and the electron filling fluctuations.