332 research outputs found
Phonon Dispersion Effects and the Thermal Conductivity Reduction in GaAs/AlAs Superlattices
The experimentally observed order-of-magnitude reduction in the thermal
conductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n)
superlattices is investigated theoretically for (2x2), (3x3) and (6x6)
structures using an accurate model of the lattice dynamics. The modification of
the phonon dispersion relation due to the superlattice geometry leads to
flattening of the phonon branches and hence to lower phonon velocities. This
effect is shown to account for a factor-of-three reduction in the thermal
conductivity with respect to bulk GaAs along the growth direction; the
remainder is attributable to a reduction in the phonon lifetime. The
dispersion-related reduction is relatively insensitive to temperature (100 < T
< 300K) and n. The phonon lifetime reduction is largest for the (2x2)
structures and consistent with greater interface scattering. The thermal
conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs
force constant differences than to those associated with molecular masses.Comment: 5 figure
The thermal conductivity reduction in HgTe/CdTe superlattices
The techniques used previously to calculate the three-fold thermal
conductivity reduction due to phonon dispersion in GaAs/AlAs superlattices
(SLs) are applied to HgTe/CdTe SLs. The reduction factor is approximately the
same, indicating that this SL may be applicable both as a photodetector and a
thermoelectric cooler.Comment: 5 pages, 2 figures; to be published in Journal of Applied Physic
Fractional quantum Hall effect in CdTe
The fractional quantum Hall (FQH) effect is reported in a high mobility CdTe
quantum well at mK temperatures. Fully-developed FQH states are observed at
filling factor 4/3 and 5/3 and are found to be both spin-polarized ground state
for which the lowest energy excitation is not a spin-flip. This can be
accounted for by the relatively high intrinsic Zeeman energy in this single
valley 2D electron gas. FQH minima are also observed in the first excited (N=1)
Landau level at filling factor 7/3 and 8/3 for intermediate temperatures.Comment: Submitte
Acoustic signalling reflects personality in a social mammal
Social interactions among individuals are often mediated through acoustic signals. If acoustic signals are consistent and related to an individual's personality, these consistent individual differences in signalling may be an important driver in social interactions. However, few studies in non-human mammals have investigated the relationship between acoustic signalling and personality. Here we show that acoustic signalling rate is repeatable and strongly related to personality in a highly social mammal, the domestic pig (Sus scrofa domestica). Furthermore, acoustic signalling varied between environments of differing quality, with males from a poor-quality environment having a reduced vocalization rate compared with females and males from an enriched environment. Such differences may be mediated by personality with pigs from a poor-quality environment having more reactive and more extreme personality scores compared with pigs from an enriched environment. Our results add to the evidence that acoustic signalling reflects personality in a non-human mammal. Signals reflecting personalities may have far reaching consequences in shaping the evolution of social behaviours as acoustic communication forms an integral part of animal societies
Magneto-resistance quantum oscillations in a magnetic two-dimensional electron gas
Magneto-transport measurements of Shubnikov-de Haas (SdH) oscillations have
been performed on two-dimensional electron gases (2DEGs) confined in CdTe and
CdMnTe quantum wells. The quantum oscillations in CdMnTe, where the 2DEG
interacts with magnetic Mn ions, can be described by incorporating the
electron-Mn exchange interaction into the traditional Lifshitz-Kosevich
formalism. The modified spin splitting leads to characteristic beating pattern
in the SdH oscillations, the study of which indicates the formation of Mn
clusters resulting in direct anti-ferromagnetic Mn-Mn interaction. The Landau
level broadening in this system shows a peculiar decrease with increasing
temperature, which could be related to statistical fluctuations of the Mn
concentration.Comment: 8 pages, 6 figure
Wafer bonding solution to epitaxial graphene - silicon integration
The development of graphene electronics requires the integration of graphene
devices with Si-CMOS technology. Most strategies involve the transfer of
graphene sheets onto silicon, with the inherent difficulties of clean transfer
and subsequent graphene nano-patterning that degrades considerably the
electronic mobility of nanopatterned graphene. Epitaxial graphene (EG) by
contrast is grown on an essentially perfect crystalline (semi-insulating)
surface, and graphene nanostructures with exceptional properties have been
realized by a selective growth process on tailored SiC surface that requires no
graphene patterning. However, the temperatures required in this structured
growth process are too high for silicon technology. Here we demonstrate a new
graphene to Si integration strategy, with a bonded and interconnected compact
double-wafer structure. Using silicon-on-insulator technology (SOI) a thin
monocrystalline silicon layer ready for CMOS processing is applied on top of
epitaxial graphene on SiC. The parallel Si and graphene platforms are
interconnected by metal vias. This method inspired by the industrial
development of 3d hyper-integration stacking thin-film electronic devices
preserves the advantages of epitaxial graphene and enables the full spectrum of
CMOS processing.Comment: 15 pages, 7 figure
Enhancement of the spin-gap in fully occupied two-dimensional Landau levels
Polarization-resolved magneto-luminescence, together with simultaneous
magneto-transport measurements, have been performed on a two-dimensional
electron gas (2DEG) confined in CdTe quantum well in order to determine the
spin-splitting of fully occupied electronic Landau levels, as a function of the
magnetic field (arbitrary Landau level filling factors) and temperature. The
spin splitting, extracted from the energy separation of the \sigma+ and \sigma-
transitions, is composed of the ordinary Zeeman term and a many-body
contribution which is shown to be driven by the spin-polarization of the 2DEG.
It is argued that both these contributions result in a simple, rigid shift of
Landau level ladders with opposite spins.Comment: 4 pages, 3 figure
{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers
The magnetization distribution, its energetic characterization by the
interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt
multilayers are investigated using density functional theory combined with the
direct method to determine phonon frequencies. It is found that ferromagnetic
order between consecutive Fe layers is favoured, with the enhanced magnetic
moments at the interface. The bilinear and biquadratic coupling coefficients
between Fe layers are shown to saturate fast with increasing thickness of
nonmagnetic Pt layers which separate them. The phonon calculations demonstrate
a rather strong dependence of partial iron phonon densities of states on the
actual position of Fe monolayer in the multilayer structure.Comment: 7 pages, 8 figure
- âŠ