Resonant states in GaAs/Ga1-xAlxAs Multi-Quantum-Wells

The effect of buffer layers on resonant states in a Multi-Quantum-Well (MQW) sandwiched between two substrates is investigated here theoretically. These resonances appear as well-defined peaks in the density of states (DOS). The local and total densities of states are obtained from an analytical determination of the Green functions. Due to the substrate/buffer layer/ MQW /substrate interaction, different kinds of resonant states are found and their properties are investigated. We show in particular that an incident electron in the left-hand side substrate is transmitted in the right hand side substrate of the structure with large time delays in the phase time. The peaks in the phase time associated with the transmission coefficient are found to be similar to those corresponding to the DOS. The intensity of these peaks associated with extended states in MQW’s and Tamm like states lying at the MQW/buffer layer interface, strongly depends on the width of the buffer layer.The effect of buffer layers on resonant states in a Multi-Quantum-Well (MQW) sandwiched between two substrates is investigated here theoretically. These resonances appear as well-defined peaks in the density of states (DOS). The local and total densities of states are obtained from an analytical determination of the Green functions. Due to the substrate/buffer layer/ MQW /substrate interaction, different kinds of resonant states are found and their properties are investigated. We show in particular that an incident electron in the left-hand side substrate is transmitted in the right hand side substrate of the structure with large time delays in the phase time. The peaks in the phase time associated with the transmission coefficient are found to be similar to those corresponding to the DOS. The intensity of these peaks associated with extended states in MQW’s and Tamm like states lying at the MQW/buffer layer interface, strongly depends on the width of the buffer layer

Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as \sim 8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than one order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano and micro-electromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.Comment: 5 page

Dose finding and O6-alkylguanine-DNA alkyltransferase study of cisplatin combined with temozolomide in paediatric solid malignancies

Cisplatin may have additive activity with temozolomide due to ablation of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (MGMT). This phase I/II study determined recommended combination doses using the Continual Reassessment Method, toxicities and antitumour activity in paediatric patients, and evaluated MGMT in peripheral blood mononuclear cells (PBMCs) in order to correlate with haematological toxicity. In total, 39 patients with refractory or recurrent solid tumours (median age ∼13 years; 14 pretreated with high-dose chemotherapy, craniospinal irradiation, or having bone marrow involvement) were treated with cisplatin, followed the next day by oral temozolomide for 5 days every 4 weeks at dose levels 80 mg m−2/150 mg m−2 day−1, 80/200, and 100/200, respectively. A total of 38 patients receiving 113 cycles (median 2, range 1–7) were evaluable for toxicity. Dose-limiting toxicity was haematological in all but one case. Treatment-related toxicities were thrombocytopenia, neutropenia, nausea-vomiting, asthenia. Hearing loss was experienced in five patients with prior irradiation to the brain stem or posterior fossa. Partial responses were observed in two malignant glioma, one brain stem glioma, and two neuroblastoma. Median MGMT activity in PBMCs decreased after 5 days of temozolomide treatment: low MGMT activity correlated with increased severity of thrombocytopenia. Cisplatin–temozolomide combinations are well tolerated without additional toxicity to single-agent treatments; the recommended phase II dosage is 80 mg m−2 cisplatin and 150 mg m−2 × 5 temozolomide in heavily treated, and 200 mg m−2 × 5 temozolomide in less-heavily pretreated children

Magnons et surstructures magnétiques d'interface

We study, in a simple model, the localized magnons at an interface between two Heisenberg ferromagnets and also the ferromagnetic stability at the interface. We consider simple cubic crystals having the same lattice parameter and the same spin value in the fundamental state on each site, but different exchange integrals between first and second nearest neighbours. We define an interface by coupling two semi-infinite crystals having the same crystallographic surface. We first study the conditions for the existence of localized magnons at (001) interfaces as well as the dispersion curves of localized and resonant magnons in the high symmetry directions of the Brillouin zone. We also determine the effect of the interface interactions on these modes. Then, we show that magnetic superstructures may exist at (110) interfaces. Such an instability is given by the existence of a soft localized mode at the interface.Nous étudions, dans un modèle simple, les magnons localisés à l'interface de deux ferromagnétiques de Heisenberg ainsi que la stabilité ferromagnétique de l'interface. Nous considérons des cristaux cubiques simples ayant la même distance intératomique et la même valeur de spin sur chaque site dans l'état fondamental mais différant par les valeurs de leurs intégrales d'échange entre atomes premiers et seconds voisins. Une interface s'obtient par le couplage de deux cristaux semi-infinis possédant la même surface cristallographique. Nous étudions d'abord sur les interfaces (001) la condition d'existence des magnons localisés ainsi que les lois de dispersion des modes localisés et résonnants dans les directions de haute symétrie de la zone de Brillouin. Nous déterminons ainsi l'effet des interactions à l'interface sur l'évolution de tels modes. Ensuite, nous montrons la possibilité d'existence de surstructures magnétiques sur les interfaces (110). Une telle instabilité se traduit par l'existence d'un mode mou de magnon localisé à l'interface

Contribution vibrationnelle aux propriétés thermodynamiques d'une monocouche physisorbée

Using a simple model for an adsorbed monolayer, we derive analytically the T3 vibrational contribution to the low temperature specific heat and compute numerically the vibrational specific heat and entropy of a physisorbed layer for all temperatures. The specific heat exhibits a maximum at a certain temperature.A l'aide d'un modèle simple, nous calculons analytiquement l'expression en T3 de la chaleur spécifique vibrationnelle à basse température T d'une monocouche adsorbée et numériquement les variations de la chaleur spécifique et de l'entropie vibrationnelle à toute température dans le cas d'une couche faiblement liée au substrat. La chaleur spécifique présente un maximum pour une température donnée

Vibrational contribution to the surface specific heat of a fluid

We report analytical results for the vibrational contributions to the low temperature surface specific heat ΔCV(T) of a non viscous liquid It is shown by a Green's function method that when capillary waves exist on the surface of such a liquid ΔCV( T) = MT 4/3 + NT2 + PT 8/3 + QT3 + O(T10/3). The coefficients M, N, P and Q are given in closed form as functions of the density of the liquid, the surface tension and the velocity of the longitudinal bulk waves.Nous donnons des résultats analytiques pour la contribution vibrationnelle à la chaleur spécifique ΔCV(T) de basse température d'un liquide non visqueux. Il est montré par une méthode de fonctions de Green qu'en présence d'ondes capillaires de surface ΔCV( T) = MT4/3 + NT2 + PT 8/3 + QT3 + O(T10/3). Les coefficients M, N, P et Q sont donnés analytiquement en fonction de la densité du liquide, de la tension superficielle et de la vitesse longitudinale du son dans le volume du liquide

LATTICE DYNAMICAL ASPECTS OF PHASE TRANSITIONS IN ADSORBED MONOLAYERS

Nous présentons quelques aspects des transitions de phase du second ordre liés à l'existence possible de modes mous de vibration. La théorie tient compte de la variation avec la température des constantes de force entre tous les atomes et spécialement entre l'adsorbat et le substrat. Cette variation peut faire apparaître à une température donnée un phonon mou.We present some aspects of second ordrer phase transitions in connection with the possible existence of soft surface modes. The theory takes due account of the variation with temperature of the force constants bounding all the atoms, especially the adatoms to the substrate. This variation may enable a phonon to become soft at a given temperature

Magnons dans les super-réseaux ferromagnétiques à 3 - couches

A brief study of bulk and surface magnons in 3-layer Heisenberg ferromagnetic superlattices is presented. Each layer is modelled by a simple-cubic lattice of spins coupled via nearestneighbor exchange interactions. The superlattice has a larger periodicity in the direction perpendicular to the slabs and therefore many magnon branches in the folded Brillouin zone. In the gaps existing between these magnon branches appear the surface-localized magnons. We give closed-form expressions for bulk and surface magnons in 3-layer superlattices, together with the corresponding dispersion curves for one spectific case. An application is also done for a Fe/Co superlattice.Une brève étude des magnons de volume et de surface dans les super-réseaux à 3-couches est présentée. Chaque couche est modélisée par un réseau cubique simple de spins couplés par des interactions d'échange entre premiers voisins. Le super-réseau possède une grande périodicité dans la direction perpendiculaire aux couches et en conséquence, il existe plusieurs branches de magnons dans la zone de Brillouin réduite. Dans les gaps existants entre ces branches de magnons apparaissent des magnons localisés à la surface. Nous donnerons sous forme analytique les relations de dispersion des magnons de volume et de surface d'un super-réseau a 3 couches que nous illustrons par un exemple. Une application est aussi donnée pour un super-réseau Fe/Co

Resonant Phonons in Adsorbed Slabs

Resonant phonons, sometimes also called leaky waves, are phonons associated with a crystal defect, a surface, an adsorbed layer and whose frequencies fall inside the bulk crystal band. Such resonant phonons were studied experimentally and theoretically before for clean surfaces and for adsorbed monolayers. We present here a study of resonant phonons associated with the adsorption of a slab of L monolayers on a substrate. With the help of a simple atomic model, we obtained a closed form expression giving the variation of the transverse phonon density of states associated with the adsorption of the slab. An application which qualitatively simulates the adsorption of L monolayers of Ge on Si shows the existence of well-defined resonant phonons within the bulk acoustic band of the substrate