Low Temperature Physics at the Laboratoire National des Champs Magnétiques Pulsés in Toulouse

An overview over past and present activities and future developments at the Toulouse pulsed magnetic field facility is given, both as far as technical developments of the infrastructure, as well as low temperature physics performed at the LNCMP are concerned.     

Quantum Transport and Cyclotron Resonance Study of Ge/SiGe Quantum Wells in High Magnetic Fields

Shubnikov-de Haas oscillation and cyclotron resonance were studied for high mobility p-type Ge channels in strained Ge/Si_1?x Ge _x quantum wells, using pulsed high magnetic fields up to 50 T. Fine quantum oscillations were observed in ρ _xx . Reflecting the complex Landau level structure in the nearly degenerate valence bands, the Fourier transform of the oscillatory spectra consists of several peaks. Cyclotron resonance was measured at photon energies between 10 and 17 meV. Two well-defined resonance peaks were observed in two samples with different x, resulting in different strains. A large non-parabolicity and large strain dependence of the effective masses were observed.     

Cyclotron Resonance of Extremely Conductive 2D Holes in High Ge Content Strained Heterostructures

Cyclotron resonance has been observed in steady and pulsed magnetic fields from high conductivity holes in Ge quantum wells. The resonance positions, splittings and linewidths are compared to calculations of the hole Landau levels.     

Study of lifetimes and photoconductivity relaxation in heterostructures with Hg x Cd1 ? x Te/Cd y Hg1 ? y Te quantum wells

Carrier lifetimes in the continuum of the quantum well of a Hg _x Cd_{1 ? x} Te/Cd _y Hg_{1 ? y} Te hetero-structure were studied by terahertz pump-probe spectroscopy. It is found that the relaxation duration of the transmission signal is ??65 ps and is independent of the pump power. Such rapid relaxation in these structures is most likely determined by the interaction of holes with acoustic phonons due to a high density of states in the valence band and a larger effective mass compared with electrons. By the obtained data, the times of the interband nonradiative recombination of holes are determined. In this publication, we report the results of numerical calculation of the energy spectrum of the model structure, in which the possibility of obtaining population inversion at specified concentrations of nonequilibrium carriers is analyzed.     

The cyclotron resonance of holes in InGaAs/GaAs heterostructures with quantum wells in quantizing magnetic fields

The spectra of the cyclotron resonance of holes in the InGaAs/GaAs selectively doped heterostructures with quantum wells are studied in pulsed magnetic fields as high as 50 T at 4.2 K. The previously observed effect of the inverted (compared with the results of the single-particle calculation of the Landau levels) ratio of the spectral weight of two split components of the line of the cyclotron resonance, which is attributed to the effects of the exchange interaction of holes, is confirmed. It is found that the ratios of intensities of the components of the line of the cyclotron resonance profoundly differ on the ascending and descending branches of the magnetic field pulse, which may be associated with a long time of the spin relaxation of holes between the two lowest Landau levels, which constitute tens of milliseconds.     

Compact magnetospectrometer for pulsed magnets based on infrared quantum cascade lasers

In this paper we present a portable quantum cascade laser (QCL) based infrared magnetospectrometer covering the spectral range from 5 to 120 μm. The variation of the excitation wavelength is enabled by an easy change of the QCL plug-in modules, while the use of any other external source is also possible. The performance of the setup is illustrated via cyclotron-resonance studies under pulsed magnetic fields up to 60 T.     

The effect of high magnetic field on the photoresponse of Si:B structures with blocked conductivity in the impurity band

The dependence of photoconductivity on the magnetic field has been studied for Si:B blocked-impurity-band (BIB) structures with the boron impurity concentration of ～10¹⁸ cm^?3 in the active layer. Measurements were performed in the magnetic field B up to 30 T with the pulse length of 0.8 s in the temperature range T = 4.2?9 K, under irradiation of structures with the room-temperature background with ～10¹⁶ photons/(cm² s) intensity. It is established that, in the longitudinal configuration and with the magnetic field parallel to the electric field, the photocurrent decreases as B increases, mainly due to a decrease in the hole multiplication factor M in the magnetic field and/or an increase in the activation energy for hopping conductivity in the active layer. At T = 4.2 K, the photocurrent can drop by a factor of several tens. At the same time, at low bias voltage V _b , when M ≈ 1, and at higher temperatures, T ≈ 9 K, the photocurrent decreases no more than twofold in the field of ～30 T. It is found also that in the transverse configuration (with the magnetic field perpendicular to the electric field), the effect of the magnetic field on the photoresponse of a structure significantly increases (more than by an order of magnitude at T = 4.2 K). This fact can be explained by the accumulation of carriers in the blocking (undoped) layer of a BIB structure, which is related to increasing time of the hole flight across this layer due to high mobility of holes and strong bending of their trajectories in the transverse configuration.