Infrared study of spin-Peierls compound alpha'-NaV2O5

Infrared reflectance of alpha'-NaV2O5 single crystals in the frequency range from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In addition to phonon modes identification, for the a-polarised spectrum a broad continuum absorption in the range of 1D magnetic excitation energies was found. The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with vanishing intensity in c-polarisation. Activation of new phonons due to the lattice dimerisation were detected below 35K as well as pretransitional structural fluctuations up to 65K.Comment: 3 pages, 2 figures, 1 table. Contributed paper for the SCES'98 (15-18 July 1998, Paris). To be published in Physica

Intersubband magnetophonon resonances in quantum cascade structures

We report on our magnetotransport measurements of GaAs/GaAlAs quantum cascade structures in a magnetic field of up to 62 T. We observe novel quantum oscillations in tunneling current that are periodic in reciprocal magnetic field. We explain these oscillations as intersubband magnetophonon resonance due to electron relaxation by emission of either single optical or acoustic phonons. Our work also provides a non-optical in situ measurement of intersubband separations in quantum cascade structures.Comment: 5 pages, 4 figure

Study of spin-Peierls transition in alpha'--NaV2O5 by infrared reflectivity

Polarized infrared reflectivity measurements have been performed on single crystals of the spin-Peierls compound alpha'-NaV2O5 in the temperature range 20-300 K. Pronounced spectral features associated with the formation of the dimerized phase were detected both in the a- and b-polarizations (perpendicular and parallel to the spin-1/2 chains, respectively). The temperature dependence of a salient spectral line at 718 cm^-1 sharply rising below the transition temperature T_SP obeys a (1-T/T_SP)^(2beta) law with T_SP \simeq 34.3$K and beta \simeq 0.25. In addition, a continuum signal is observed in the whole temperature range in the a-polarized optical conductivity spectra. In order to interpret these results, calculations of the static dimerization and of the optical conductivity based on a mean-field and a dynamical treatment of the lattice respectively are proposed.Comment: 4 pages, 5 PostScript figures, Minor change

Carrier thermalization dynamics in single zincblende and wurtzite InP nanowires

Using transient Rayleigh scattering (TRS) measurements, we obtain photoexcited carrier thermalization dynamics for both zincblende (ZB) and wurtzite (WZ) InP single nanowires (NW) with picosecond resolution. A phenomenological fitting model based on direct band-to-band transition theory is developed to extract the electron-hole–plasma density and temperature as a function of time from TRS measurements of single nanowires, which have complex valence band structures. We find that the thermalization dynamics of hot carriers depends strongly on material (GaAs NW vs InP NW) and less strongly on crystal structure (ZB vs WZ). The thermalization dynamics of ZB and WZ InP NWs are similar. But a comparison of the thermalization dynamics in ZB and WZ InP NWs with ZB GaAs NWs reveals more than an order of magnitude slower relaxation for the InP NWs. We interpret these results as reflecting their distinctive phonon band structures that lead to different hot phonon effects. Knowledge of hot carrier thermalization dynamics is an essential component for effective incorporation of nanowire materials into electronic devices

Dispositif de compensation pour la mise en évidence d'effets oscillatoires de la magnétorésistance en champ magnétique pulsé

A device is described which allows us to keep out the monotonic part of the magnetoresistance near oscillations, when using a pulsed high magnetic field with exponential decay.Un dispositif électronique permet d'éliminer la partie monotone de la magnétorésistance au voisinage d'oscillations, pour des mesures effectuées à l'aide d'un champ magnétique pulsé à décroissance exponentielle

Caractérisation par magnétotransport d'une couche électronique bidimensionnelle dans une structure GaAs à dopage Si dans un plan (100)

n-type-δ-doping layers in GaAs prepared by molecular beam epitaxy are characterized by magnetotransport measurements at 2 K up to 43 T. Shubnikov-de Haas measurements on the magnetoresistance give evidence of three electric subbands, having respectively the populations 3.6 ; 1.18 and 0.48 × 1012 cm-2. The measured electronic concentrations account for 90 % of the total Silicon donors introduced during the growth. In addition, the doping layer thickness is estimated equal to 30 Å. Low field magnetotransport measurement enable to derive the subbands mobility values found respectively equal to 780 ; 5 200 and 8 200 cm2/V.s. Finally, the quantum Hall effect is shown for the first time in a 2 DEG having a mobility value below 1 000 cm2/V.s.Un gaz bidimensionnel d'électrons, obtenu par dopage au silicium dans le plan (100) durant la croissance épitaxiale par jet moléculaire de GaAs, est caractérisé à 2 K par l'étude de l'effet Hall et de la magnétorésistance jusqu'à 43 T. Lorsque le champ magnétique est perpendiculaire au plan (100), trois sous-bandes électriques, avec des concentrations respectives de 3,6 ; 1,18 et 0,48 x 1012 cm-2 sont révélées expérimentalement. Elles rendent compte de 90 % de la population des atomes de silicium et présentent des mobilités de 780 ; 5 200 et 8 200 cm2/V.s. Une épaisseur de la couche égale à 30 Å a pu être calculée. Lorsque le champ magnétique est parallèle au plan de la couche, la magnétorésistance donne une signature des sous-bandes électriques qui révèle une quatrième sous-bande peuplée. Enfin, l'effet Hall quantique est mis en évidence pour la première fois dans un système électronique de mobilité inférieure à 1 000 cm2/V.s

Step-scan time-domain terahertz magneto-spectroscopy

We present a novel approach for terahertz time-domain spectroscopy of magneto-optic phenomena. The setup used in this work combines a tabletop pulsed magnet and a standard terahertz time-domain spectroscopy system. The approach is based on repetitive operation of the pulsed magnet and step-wise increment of the delay time of the time-domain spectroscopy system. The method is demonstrated by plotting the magneto-transmission spectra of linearly polarized THz pulses through the hole gas of a Ge sample and the electron gas of GaAs, InSb and InAs samples. Cyclotron resonance spectra are displayed in the frequency range from 200 GHz to 2 THz and for a magnetic field up to 6 T. The GaAs spectra are analyzed in more detail using simulations based on the Drude model

Spin gap state in α'-NaV2O5 studied by far infrared spectroscopy

We present far infrared transmission measurements of α'-NaV2O5 single crystals. This material is known to undergo a spin-Peierls like transition at TSP=35K. Below the transition temperature we observed together with expected new sharp phonon lines a strong gap-like depression of absorption occurring in the low energy part of a continuum likely connected with two-magnon excitations. The optical gap temperature dependence and its value closely match the spin gap values measured by neutron scattering experiments. Taking the optical gap as a fingerprint of the spin gap, we found that the relationship between the spin gap value and the dimerisation lattice parameter do not follow the scaling relation expected for the SP systems