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We are trying to reduce the largest uncertainties in using white dwarf stars as Galactic chronometers by understanding the details of carbon crystalliazation that currently result in a 1–2 Gyr uncertainty in the ages of the oldest white dwarf stars. We expect the coolest white dwarf stars to have crystallized interiors, but theory also predicts hotter white dwarf stars, if they are massive enough, will also have some core crystallization. BPM 37093 is the first discovered of only a handful of known massive white dwarf stars that are also pulsating DAV, or ZZ Ceti, variables. Our approach is to use the pulsations to constrain the core composition and amount of crystallization. Here we report our analysis of 4 hours of continuous time series spectroscopy of BPM 37093 with Gemini South combined with simultaneous time-series photometry from Mt. John (New Zealand), SAAO, PROMPT, and Complejo Astronomico El Leoncito (CASLEO, Argentina).
Chemical conditions and mass transport properties of engineered barrier systems in TRU waste facilities would change with time due to the interaction of cement/bentonite materials. (‘TRU waste’ is one of categories of the radioactive wastes and contains a significant amount of alpha-emitting transuranic nuclides. In some countries, these wastes are classified into the Intermediate Level Waste (ILW).) Previous numerical model analyses to assess the long-term performance of engineered barrier systems in TRU waste repositories predicted to form Calcium Silicate Hydrate (C-S-H) species at the interface between the cementitious and bentonite materials. If C-S-H precipitates in the bentonite side of the boundary, mass transport in the bentonite buffer decreases and mineralogical alterations are expected to be restricted for a long period. The evidence of C-S-H precipitation in the bentonite side, however, still has not been identified in the former experimental studies. To improve the reliability of numerical analyses, immersion experiments were performed using contact samples of cementitious and bentonite materials, and X-ray absorption fine structure (XAFS) analysis was carried out to detect C-S-H precipitation at the contacting interface. Precipitation of C-S-H was confirmed from the obtained XAFS spectra. This result is one of the evidences to show the validity of the current numerical model analyses, which suggests that the bentonite buffer performance as an engineered barrier would be kept over a long period.
An outbreak of psittacosis related to a bird park occurred in Matsue City, Shimane Prefecture, Japan, during winter 2001. Seventeen cases of psittacosis (12 visitors, three staff, and two student interns) were confirmed. A cohort study was conducted among the park staff and students to determine the risk factors for the development of acute serologically confirmed psittacosis (SCP) infection. Being ‘bird staff’ had an increased risk of SCP infection (RR 3·96, 95% CI 1·48–10·58). Entering the staff building, where ill birds were maintained without proper isolation, was also associated with an increased risk of SCP infection (RR 3·61, 95% CI 1·03–12·6). Isolation of ill birds and quarantine measures were found to be insufficient. Dehumidifiers and a high-pressure water spray under a closed ventilation environment may have raised the concentration of Chlamydophila psittaci in the hothouses. Bird park staff and visitors should be educated about psittacosis.
Future microprocessor technologies will require interlayer dielectric (ILD) materials with a dielectric constant (κ-value) less than 2.5. Organosilicate glass (OSG) materials must be nanoporous to meet this demand. However, the introduction of nanopores creates many integration challenges. These challenges include 1) integrating nanoporous films with low mechanical strength into conventional process flows, 2) managing etch profiles, 3) processinduced damage to the nanoporous ILD, and 4) controlling the metal/nanoporous ILD interface. This paper reviews research to maximize mechanical strength by engineering optimal pore structures, controlling trench bottom roughness induced by etching and understanding its relationship to pore size, repairing plasma damage using silylation chemistry, and sealing a nanoporous surface for barrier metal (liner) deposition.
We report on magnetic properties of the GaN layers implanted with 3d transition metal ions. GaN layers grown by MOVPE on sapphire substrates, p- or n-doped, were implanted by Mn, Cr or V ions with a dose of 5×1016 cm−2 and implantation energy of 200 keV. Subsequently, a rapid thermal annealing in nitrogen atmosphere for 5 minutes at different temperatures (700°C – 1050°C) was performed. The magnetization as a function of magnetic field as well as the dependence on temperature revealed paramagnetic behavior for all samples. In addition, an antiferromagnetic coupling between implanted ions was found.
We use the self-interaction corrected (SIC) local spin-density (LSD) approximation to investigate the groundstate valency configuration of Mn impurities in p-type ZnO. In Zn1−xMnxO, we find the localized Mn2+ configuration to be preferred energetically. When codoping Zn1−xMnxO with N, we find that four d-states stay localized at the Mn site, while the remaining d-electron charge transfers into the hole states at the top of the valence bands. If the Mn concentration [Mn] is equal to the N concentration [N], this results in a scenario without carriers to mediate long range order. If on the other hand [N] is larger than [Mn], the N impurity band is not entirely filled, and carrier mediated ferromagnetism becomes theoretically possible.
We review our recent work on spin effects in low-dimensional electron gases studied using far-infrared photoconductivity technique. We measure the spin-orbit coupling parameter α via spectroscopy by detecting the combined resonance. Detailed filling-factor dependent study shows the collective nature of this excitation, in accordance to theoretical predictions that both Kohn and Larmor theorem are broken for long-wavelength excitations that changes both the Landau and spin quantum numbers. We find that the long spin-relaxation time of a two-dimensional electron gas results in a novel bolometric spin effect, which gives rise to a substantial photo resistance change by reversing the spin polarization of electrons at the Fermi-level.
Magnetization measurements were performed as a function of magnetic field H and temperature T on samples of nine different materials including clear fused quartz, cartridge brass, G-10 glass-reinforced epoxy, acetal homopolymer, glass-filled acetal, phenolic, and other plastics. A small yet distinct amount of ferromagnetic or paramagnetic impurities is observed in all the materials investigated in this study except quartz. In contrast, the magnetic response of quartz is typical of a diamagnet over the temperature range 5 K to 300 K. The volume susceptibility is equal to −4.4×10−7 (cgs) over the whole temperature range.
The Zn1−xMnxO thin films were grown on Al2O3 (0001) substrates by an r.f. magnetron sputtering method. The film grown with employing buffer layer shows mirror-like surface, while the film grown without buffer layer shows the columnar-structured configuration. The mirror-like Zn0.93Mn0.07O thin films have the single crystalline phase with (000ℓ) orientation normal to the substrate surface and show the UV emission originated from the near band-edge-emission for the measurements of x-ray diffraction and photoluminescence, respectively. The mirror-like Zn0.93Mn0.07O film clearly showed a hysteresis loop, which is obvious evidence of ferromagnetism, and the Curie temperature was determined to be 68 K for the characterization of the temperature-dependent magnetization.
We present a theoretical model to describe electrical spin injection from a ferromagnetic metal contact into a conjugated organic semiconductor. To achieve significant spin current, the organic semiconductor must be driven far out of local thermal equilibrium by an electric current. Effective spin injection therefore requires that equilibration between the conjugated organic semiconductor and the metallic contact be suppressed by an energy barrier to injection that may be due either to a large Schottky barrier or to an insulating tunnel barrier. The results are compared with simulations for a silicon based device structure. Detection of the injected spin current in the organic semiconductor is also addressed.
The static second hyperpolarizabilities (γ) of open-shell organic nonlinear optical (NLO) systems composed of singlet diradical molecules are investigated using ab initio molecular orbital (MO) and density functional theory (DFT) methods. It is found that neutral singlet diradical systems with intermediate diradical characters tend to enhance γ as compared to those with small and large diradical characters. This suggests that the diradical character is a novel control parameter of γ for singlet diradical systems.
Magnetic semiconductors are of interest for emerging spintronic applications, such as the integration of electronic information processing with magnetic data storage. We report on a new approach - furnace annealing under controlled ambients – aimed at increasing Mn incorporation and synthesizing new magnetic semiconductors with Tc greater than/around room temperature. These annealing treatments are hypothesized to reduce the effect of Mn interstitials. We have obtained preliminary SQUID magnetometry results which indicate ferromagnetic Curie temperatures of around 130 K in (In,Mn) Sb and 60 K in (In, Mn)P. X-ray diffraction was used to characterize phase homogeneity.
We report theoretical and experimental observation of photoexcitated hole spin selection in GaAs/GaAlAs n-i-n in resonant tunneling diodes. When subjected to magnetic and electric parallel fields, the spin splitted hole levels leads to several peak structure in the transmissivity. These experimental results are interpreted as an evidence of tunneling transport through spin polarized hole levels of non-magnetic diodes.
We present analytical expressions for the D'yakonov-Perel' spin relaxation rates under the combined action of bulk and structural inversion asymmetry for  zincblende heterostructures when terms up to linear and third order in k are included in the Hamiltonian. We see for  heterostructures that, under the right conditions, the lowest-order-in-k component of the spin relaxation tensor can be made to vanish for all spin components at the same time. We study how the inclusion of terms of higher order in k affects these results. We finally discuss a proposal for a resonant spin lifetime transistor (RSLT) using the spin lifetime tuning concepts presented above, where the characteristics of the  device give the designer an added degree of freedom on the direction of the injected spins.
We make a detailed analysis of each possible spin-orbit coupling of zincblende narrow-gap cylindrical quantum dots built in a two-dimensional electron gas. These couplings are related to both bulk (Dresselhaus) and structure (Rashba) inversion asymmetries. We study the competition between electron-electron and spin-orbit interactions on electronic properties of 2-electron quantum dots.
We report device concepts that exploit spin-orbit coupling for creating spin polarized current sources using nonmagnetic semiconductor resonant tunneling heterostructures, without external magnetic fields. The resonant interband tunneling spin filter exploits large valence band spin-orbit interaction to provide strong spin selectivity. The bi-directional spin pump induces the simultaneous flow of oppositely spin-polarized current components in opposite directions through spin-dependent resonant tunneling. The efficiency of resonant tunneling spin devices can be improved when the effects of structural inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) are combined properly, and incorporated into device design. The current spin polarizations of the proposed devices are electrically controllable, and potentially amenable to high-speed modulation. In principle, the electrically modulated spin-polarized current source could be integrated in optoelectronic devices for added functionality.
The galvanomagnetic effects in the new diluted magnetic semiconductors Pb1−xSnxTe:Yb were studied to determine the parameters of the electronic structure and to elucidate its influence on the magnetic properties. It was found that the temperature dependencies of the resistivity ρ and the Hall coefficient RH have a “metallic” character, however the RH changes in anomalous manner: its value increases more than by order of magnitude and then passes through maximum with increasing the temperature. Upon an increase of the ytterbium concentration the hole concentration decreases by more than order of magnitude. The results were explained assuming a formation of deep ytterbium-induced defect level in the valence band of the alloys, which moves up to its top with increasing the ytterbium concentration and pins the Fermi level within the valence band. The energy position of the Fermi level was calculated in the frame of two-band dispersion law and used to determine the position of Yb level in the alloys. The diagram of the charge carrier energy spectrum under varying the alloy composition was built.
The effect of Rashba spin-orbit coupling on the transport properties of InGaAs/InP quantum wire structures is investigated. The geometry of the wire structures was defined by selective wet chemical etching. For wires without a gate a clear beating pattern, due to the presence of the Rashba spin-orbit coupling, is observed for wires with a width down to 600 nm. For narrower wires no beating pattern is found. The experimental observations are explained by contribution of the Rashba spin-orbit coupling to the one-dimensional magnetosubbands. By depleting the one-dimensional conductor by means of a gate electrode the Rashba coupling strength could be controlled.
Transition metal doped (Mn, Fe and V) ZnO ceramics and their thin films were prepared by pulsed laser deposition on glass substrates. The ceramic targets did not show any additional phase formation from XRD measurement, except for high Fe concentrations. Optical absorption showed sub-band gap absorption for Mn doping. The band gap was shifted by 0.06 eV for V doped ZnO as concentration was increased to 10%. Micro Raman spectra showed some defect induced modes for all the transition metals doped ZnO ceramics. In V doped ZnO ceramics there was two phonon induced resonance Raman scattering with increase in dopant concentrations. Raman spectra for thin films did not show any significant additional modes for Mn and Fe. However V doped ZnO thin films showed an additional mode for concentration ≥5 %.
We are searching for pulsations in cool (< 6000 K) white dwarfs (WDs), hoping to apply asteroseismological techniques to improve our understanding of their structure and the physical processes inside them. This information is important as we use cool WDs to estimate the lower limit of the age of the Galactic disk. Within a spectroscopic and photometric survey of 110 cool WDs by Bergeron, Ruiz, & Legget, we find 28 candidates with appropriate effective temperatures, masses, and chemical compositions for possible pulsations in nonradial g modes with periods similar to those we observe in DAVs. So far, we have observed 4 candidates, but have found no evidence of large variation.