Structural influence of erbium centers on silicon nanocrystal phase transitions

Two different types of erbium-doped silicon nanocrystals, along with undoped, oxide-capped Si dots, are employed to probe the impact of the impurity center location on phase transition pressure. Using a combination of high pressure optical absorption, micro-Raman, and x-ray diffraction measurements in a diamond anvil cell, it is demonstrated that the magnitude of this phase transition elevation is strongly dictated by the average spatial location of impurity centers introduced into the nanocrystal along with the interfacial quality of the surrounding oxide.     

Using the phase transitions in Ni and Cu nanoclusters for data recording processes

Bulletin of the Russian Academy of Sciences: Physics - The possibility of using individual Cu and Ni nanoclusters as individual bits of information in memory devices constructed on the principle of...     

Surface phase transitions of silicon in the presence of a dense electron-hole plasma

A thermodynamic equation is given, that relates the surface phase transitions with phonon-defect- electron interactions. It permits the calculation of the variation of the phase transition temperatures, T_c, with the density of an electron-hole plasma in silicon. Under conditions met in short-pulse laser irradiation, it is shown that T_c might differ from thermal values. Also, temperature calculations derived from LEED experiments might be in error, due to electron-phonon couplings.     

Stability-instability transitions in silicon crystal growth

In order for a crystal to grow, source atoms must be incorporated into the underlying lattice. Typically, this process occurs on the surface in one of two modes: either through island nucleation or through step flow. However, a third, morphologically unstable growth mode has been predicted. Monitoring the surface of ultraflat substrates with an in situ scanning electron microscope, we prove that for the (111) face of silicon there is a transition from stable step flow to morphological instability and then to island nucleation.     

Confinement and phase transitions

Using a density dependent quark mass to express non-perturbative interaction effects, we calculate the critical density for a phase transition between nuclear and quark matter atT=0.     

Observation of two-electron-one-photon transitions in silicon

We report on the observation of Kαα X-rays of Si, produced in collisions of 15–28 MeV Si projectiles with various target atoms in the range Z = 6 to 29. Energy shifts of X-rays were measured and are compared with theoretical predictions. Cross section ratios for emission of Kαα and Kα radiation are given.     

Cosmic phase transitions

The sequence of phase transitions during the hot history of the universe is followed within a phenomenological framework. Particular emphasis is put on the QCD confinement transition, which is at reach under earth laboratory conditions. A tepid inflationary scenario on the GUT scale with bubble growth at moderate supercooling is discussed.     

量子相变

量子相变是一种发生在绝对零度,由量子涨落而非热涨落导致的相变现象,满足著名的海森堡不确定关系。通过零温量子临界点的研究,可获知物质系统更广泛范围的行为,包括稀土磁性绝缘体,高温超导体和二维电子气体等。     

Structural phase transitions ans some electronic properties of the (110) silicon surface

Work function, surface photovoltage and field effect kinetics were measured on a (110) Si surface under structural phase transitions of order-order type (4 × 5), (2 × 1), (4 × 5) controlled by LEED. Surface electronic state spectra appeared to have filled (donor) and empty (acceptor) surface bands separated by the gap. The biggest changes of characteristics mentioned were observed at transitions to the (2 × 1) structure. Energy diagrams are drawn for electron spectra at (4 × 5), (2 × 1) and (5 × 1) surface lattices. Some suggestions are made on the physical reasons of structural phase transitions and on geometrical models of the Si(110) surface structures.     

Structural phase transitions in ZnS

X-ray diffraction techniques have been used to investigate structural phase transitions in ZnS between 20 and 1200°C. These measurements imply that the transition from the cubic 3C structure to the hexagonal 2H structure is a first-order phase transition while transitions between the 2H, 4H, and the 6H(33) hexagonal structures were found to obey the symmetry rules of second-order phase transitions. Direct transitions from the cubic 3C structure to the 4 or 6H hexagonal structures are not observed.     

Pressure-induced phase transitions in gypsum

Abstract

We report high-pressure Raman scattering spectroscopy and energy dispersive X-ray diffraction investigations on gypsum, CaSO₄ · 2H₂O, at room temperature in a diamond cell. With increasing pressure, measurements indicate that CaSO₄ · 2H₂O undergoes two stages of crystalline-state phase transitions at 5 and 9 GPa, and then converts to a disordered phase above 11 GPa. The structures of the three high-pressure phases of gypsum have not been determined yet. These phases are tentatively named as “post-gypsum-I” (PG-I), “post-gypsum-II” (PG-II) and “disordered” according to the sequence of their appearance with pressure.

Gypsum shows anisotropic compressibility along three crystallographic axes with b > c > a below 5 GPa. The difference in the behavior of the two OH stretching modes in gypsum is attributed to the different reduction rate in the hydrogen bonding distances by the anisotropic axial compressibility.     

Relativistic two-body processes in axial-charge transitions

We study the contribution of two-body meson-exchange processes to axial-charge transitions using relativistic models of the nucleus. We conduct calculations at the mean-field and oneloop levels for nuclei in the lead region and also for tin and oxygen. The aim is to provide a comprehensive one-body plus two-body treatment within the relativistic framework. The results indicate that one- and two-body processes enhance the matrix elements of the axial-charge operator by some (70 ± 20) % in all three regions studied, in good qualitative agreement with the data and more conventional calculations. We also discuss the sensitivities of the calculation to the change in some of the couplings and the parameters involved.     

Intrinsic magnetostriction and phase transitions

A new possibility of first-order magnetic phase transitions in simple systems, based on the concept of intrinsic volume-magnetostriction Ω_s ≡ ? V_n/V_n, is suggested. The net volume V_n is defined as the difference between the total lattice volume and the sum of the individual atomic volumes. A magneto-volume equation of state for the net volume: Ω_s = Q₀η^s with s < 2, as well as the dependence of the effective exchange integral on the long-range order parameter η, are postulated. The value s ? 1.5 accounts well for the experimental values of the critical exponents β and δ, as well as for the deviation from linearity of the Arrott—Belov plots obtained for the ferromagnetic transition metals. The model leads to an order—order transition just below the order—disorder transition, thus providing an explanation for the two experimentally found temperatures, the forromagnetic Curie temperature and the paramagnetic Curie—Weiss temperature.     

Coarsening in fluid phase transitions

We review the understanding of the kinetics of fluid phase separation in various space dimensions. Morphological differences, percolating or disconnected domains, based on overall composition in a binary liquid or on density in a vapor–liquid system, are discussed. Depending upon the morphology, various possible mechanisms for domain growth are pointed out and discussions of corresponding theoretical predictions are provided. On the computational front, useful models and simulation methodologies are presented. Theoretically predicted growth laws have been tested via molecular dynamics simulations of vapor–liquid transitions. In the case of a disconnected structure, the mechanism has been confirmed directly.     

Singularities in first-order phase transitions

Several theories of phase transitions and their inter-relations have been criticized, focusing on the problem of whether z _c, the value of the fugacity corresponding to the point of condensation, is given by z _s, the smallest real positive singularity of the analytic function defined by the power series using volume-independent cluster integrals, or not. The present situation has been analysed and it is made clear that none of the existing theories can give the answer to this problem. Plausibility arguments for an affirmative or negative answer are discussed.     

Stimulated emission from donor transitions in silicon

The observation of far-infrared stimulated emission from shallow donor transitions in silicon is reported. Lasing with a wavelength of 59 &mgr;m due to the neutral donor intracenter 2p(0)-->1s(E) transition in Si:P pumped by CO2 laser radiation is obtained. Populations of D0 and D- center states and the balance of the radiation absorption and amplification are theoretically analyzed.     

Two-electron band to band transitions in silicon

Radiative two-electron band to band transitions are investigated experimentally in silicon. The question of phonon-participation is investigated. It turns out that at low temperature the phononless transitions dominate, whereas above room-temperature the phonon-assisted transitions become more important. A theoretical discussion leads to the conclusion that at least one participating phonon is an acoustical one.