Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

Personnel involved:
Semiconductors
Permanent: J. Leotin (Emeritus), J.M. Broto (E/C), B. Raquet (E/C), M. Goiran (E/C), J. Galibert (C),
H. Rakoto* (ITA), M. Nardone (ITA), S. George (ITA), O. Portugall (ITA)
* Deceased in September 2008.
Non permanent: M. Millot (Ph. D), JM. Poumirol (Ph. D ), N. Ubrig (Ph. D ).
Collaborations: M. Semtsiv, F. Hatami, T. Masselink (Institute of Physics, Humboldt Univ. Berlin), D. Smirnov
(NHMFL, Tallahassee), V.I.Gavrilenko, (Institute for Physics of Microstructures RAS, Nizhny Novgorod),
O.Drachenko, M. Helm (Forschungzentrum Dresden-Rossendorf, Germany), V. V. Rylkov (Kurchatov Institute,
Moscow), V.T Dolgopolov (Institute of Solid State Physics, Chernogolovka, Russia), A. Gold (CEMES,
Toulouse), H. Ohta (Molecular Photoscience Research Center, Kobe University, Japan), Alfredo Segura,
Universidad de Valencia (Spain).
The activity in the semiconductor domain covers essentially three different aspects: one is focussed on the
electronic structure of semiconductors, mainly low dimensional systems, through the investigation of high
magnetic field quantum effects on transport and spectroscopy in the Vis-FIR range, a second research axis is
devoted to the study of the correlations in strongly interacting 2D electron systems and the third one concerns the
properties of magnetic semiconductors. In the following section some results obtained in these different aspects
are presented.
Xt -Xl valleys crossover in AlP-GaP quantum wells
We have established by high magnetic field spectroscopy and magnetotransport on 2DEG confined in AlP/GaP
quantum wells (QW) that bulk AlP X-valleys are located exactly at the edge of the Brillouin zone and that in
many respects, the valley symmetry in AlP quantum wells (QW) with GaP barriers is similar to that of the AlAs
quantum wells with GaAs barriers. As expected for such systems, the valley degeneracy is lifted into a single Xzvalley
and a twofold X xy-valleys, due to on the one hand the valley-anisotropy confinement splitting and on the
other hand the biaxial strain splitting caused by the lattice mismatch between AlP and GaP. In the present study,
we have investigated the properties of quasi-two-dimensional electrons in a wide range of modulation-doped AlP
quantum wells (between 3 and 15 nm) with GaP barriers by measuring cyclotron resonance(CR), quantum Hall
effect, and Shubnikov de Haas oscillations. The experiments down to 1.55K were performed under high pulsed
magnetic field while static fields were used for the low temperature transport measurements down to 280mK.
Figure 1 summarizes the CR data, obtained at 4K on two different samples, one being a 15nm wide multi-QW
with 50 periods, and the other a 4nm wide single-QW.
Energy, meV
6
5
4
3
2
1
m*= (0.30�0.02 )m 0
m*= (0.52 �0.01�m 0
0
0 5 10 15 20 25 30
Magnetic field (T)
Fig. 1. Values of the resonant fields determined
from CR at different excitation energies for the
15nm MQW structure (full circles) and the 4nm
SQW (open circles).
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The two straight lines indicate parabolic conduction
bands in the investigated energy range for both Xz
and X xy valleys, the deduced cyclotron masses are
mc1=0.30�0.02m0 and mc2=0.52�0.01m0
respectively. The iso-energy surfaces are then
ellipsoids elongated along the �line of the BZ with
longitudinal and transverse effective masses: ml/m0
= 0.9�0.02 and m t/m 0 = 0.3�0.02. Regarding the
valley degeneracy, a direct determination is derived
for each QW from combination of longitudinal and
Hall resistances data.
Fig. 2 shows R xx(B) and R xy(B) for a 10nm wide QW at 1.55K and for a 3nm wide QW at 280 mK. In Fig2a,
standing for the wide QW having the Xxy-valley populated the sequence of integer filling factors is incremented
by 4 at low magnetic fields, high Landau quantum numbers, and then by one at low quantum numbers.