Cooperative Two-Quanta Phase Transitions in Quantum Optics and Superconductivity

The behavior of an electronic subsystem in strong interaction with phonon subsystem, or quantified electromagnetic field (QEF) is discussed. In this case the correlation effect between first and second order electron–phonon interaction (or atom–QEF interaction) takes place. It is shown that the temperature dependence of two-quanta exchange between Fermi sub-system and thermal reservoir gives a non-linear behavior of the order parameter in superconductivity and super-radiance, accompanied by an increase of the electron correlations with increasing temperature. The same effect is considered for two-quanta scattering processes, in which one quantum is absorbed and another is emitted. It is demonstrated, that the order parameter in such a system firstly increases with temperature achieving the maximal value. After that it decreases as in traditional phase transition effects.     

Phase Transitions and Phase Fluctuations in the Josephson-Junction Arrays

We study the effect of quantum and classical phase fluctuations on the phase transitions in the system of Josephson-junction arrays. We employed a variational method for calculating the Gaussian type fluctuation of the phase in the Josephson-junction array lattice systems without and with an external magnetic field. We investigate the spectrum of collective excitations and the effects of collective excitations on the transport properties of Josephson-junction arrays. We showed that the Hamiltonian for the lattice system of the Josephson junction is the same as the Hamiltonian for the classical or quantum two-dimensional interacting rotators. We also showed that the dynamics of fluctuations of the phase in the lattice system of Josephson junction is very similar to the lattice dynamics of the lattice in crystals. We also showed that in the lattice system of Josephson junctions there is the collective acoustic mode similar to the acoustic mode in the crystal lattice, and this mode may lead to the dissipation of the Josephson current in the superconducting array of Josephson junctions. The speed of sound of the collective acoustic mode of the phase fluctuation depends on the Josephson coupling energy and the Coulomb charging energy. The contribution of the collective acoustic mode to the low temperature specific heat is the same as the contribution of the acoustic phonons to the specific heat of crystals. We also discuss the future development of results and their application.     

Phase transformations and segregation in Fe–Ni alloys and nanoalloys

Ordering and segregation properties of Fe–Ni alloys and nanoalloys are investigated by means of Metropolis Monte Carlo (MMC) and molecular dynamics (MD) simulations. The model is based on an embedded atom potential which, according to thermodynamic integration, only stabilizes those phases that are observed experimentally. This stability is confirmed by MMC and the same phases are found stable in truncated octahedral nanoparticles containing no more than 201 atoms. At given composition, Ni segregates at {100} and nanoparticle surfaces on the Fe-rich side of the phase diagram, Fe segregates at intermediate compositions and no significant trend is predicted on the Ni-rich side. A BCC to L1₀ transition is observed to occur at a Ni fraction close to 0.32, both in bulk alloys and in nanoparticles. The transition gives rise to a change in the nanoparticle aspect ratio by a factor 2^1/2. Using MD, by varying temperature, it was possible to monitor a BCC to FCC transition in solid solution nanoparticles reversibly.     

Quantum Transport and Cyclotron Resonance Study of Ge/SiGe Quantum Wells in High Magnetic Fields

Shubnikov-de Haas oscillation and cyclotron resonance were studied for high mobility p-type Ge channels in strained Ge/Si_1?x Ge _x quantum wells, using pulsed high magnetic fields up to 50 T. Fine quantum oscillations were observed in ρ _xx . Reflecting the complex Landau level structure in the nearly degenerate valence bands, the Fourier transform of the oscillatory spectra consists of several peaks. Cyclotron resonance was measured at photon energies between 10 and 17 meV. Two well-defined resonance peaks were observed in two samples with different x, resulting in different strains. A large non-parabolicity and large strain dependence of the effective masses were observed.     

Quantum Oscillations and π-States in Multiply Connected Ferromagnet-Superconductor Hybrids

On the basis of Usadel equations, we consider superconductivity nucleation and Josephson current in multiply connected mesoscopic superconductor/ferromagnet (S/F) hybrids. We demonstrate that the exchange field can provoke an increase in the critical temperature T _c of the superconducting transition in the magnetic field. We study the Josephson effect in S/F composites and demonstrate that the negative sign of the critical current (π state) can be realized in such structures despite a dispersion of the distances between different segments of superconducting electrodes.     

Quantum Interference and Surface States Effects in Bismuth Nanowires

We report the observation of Aharonov-Bohm (AB) oscillations for single Bi nanowires with diameter d< 80 nm. The single nanowire samples with glass coating were prepared by the Ulitovsky technique; they were cylindrical single crystals with (10 $\bar{1}$ 1) orientation along the wire axis. The surface of Bi nanowire supports surface states which give rise to a significant population of charge carriers with high effective mass that form a highly conducting tube around the nanowire. The oscillations of longitudinal magnetoresistance (MR) of Bi nanowires with two periods ΔB ₁ and ΔB ₂ proportional to Φ₀ and Φ₀/2 were observed, where Φ₀=h/e is the flux quantum. From B～ 8 T down to B = 0 the extremums of Φ₀/2 oscillations are shifted up to 3π at B = 0 which is the manifestation of Berry phase shift due to electron moving in a nonuniform magnetic field. A derivative of MR was measured at various inclined angles. The observed angle variation of the periods is not in agreement with the theoretical dependence Δ(α)=Δ(0)/cos?α of the size effect oscillations of the “flux quantization” type. Moreover, the equidistant oscillations of MR exist in transverse magnetic fields under certain rotation angles. An interpretation of the MR oscillations is presented.     

Magnetic and Electronic Quantum Criticality in YbRh2Si2

The unconventional nature of the quantum criticality in YbRh₂Si₂ is highlighted on the basis of magnetoresistivity and susceptibility measurements. Results obtained under chemical pressure realized by isoelectronic substitution on the rhodium site are presented. These results illustrate the position of the T^?-line associated with a breakdown of the Kondo effect near the antiferromagnetic instability in the low-temperature phase diagram. Whereas at zero temperature the Kondo breakdown and the antiferromagnetic quantum critical point coincide in the proximity of the stoichiometric compound, they are seen to be detached under chemical pressure: For positive chemical pressure the magnetically ordered phase extends beyond the T ^?(B)-line. For sufficiently high negative pressure the T ^?(B)-line is separated from the magnetically ordered phase. From our detailed analysis we infer that the coincidence is retained at small iridium concentrations, i.e., at small negative chemical pressure. We outline further measurements which may help to clarify the detailed behavior of the two instabilities.     

Magnetotransport and THz-Optical Investigations at Devices with HgTe Quantum Wells

We investigated the magnetoconductivity and the Terahertz (THz) photo-conductivity of devices with HgTe quantum wells embedded in barrier layers of HgCdTe. For the photoconductivity measurements, a THz laser system (p-Ge-Laser) is applied. This laser uses transitions between Landau levels of light holes in Ge and emits laser pulses in the wavelength range 120 μm < λ < 180 μm. The THz laser radiation is used in order to excite charge carriers over the Landau-gap. The response of the sample to the laser impulses is in 2D-samples in the quantum Hall (QH) regime measured in order to receive data of the relaxation of the charge carriers. In this presentation we present photoconduction measurements of the HgTe-quantum well in Hall bar, Corbino as well as combined Corbino-Hall bar geometry in the QH-regime. The material system HgTe/HgCdTe is characterized by a small effective mass (compared to GaAs, in our case 0.023 m ₀) and accordingly smaller magnetic fields for the appropriate cyclotron energy. Thus, this material combination provides the opportunity to make THz detectors using magnetic fields below 2 T for operation.     

Phase transformation and structural characteristics of zinc-incorporated β-tricalcium phosphate

Biological performance of bioceramics such as calcium phosphate has been proved to be improved by substitution of different ions like Mg, Sr and Si. In this study, different amounts of Zn ions in nitrate form were incorporated into β-tricalcium phosphate in which various molar ratios of Ca:Zn were achieved: 3:0, 2.8:0.2, 2.6:0.4, 2.4:0.6, and 2.2:0.8. The mixtures were heated at different temperatures ranging from 800–1100 °C. The phase composition, amount of each phase and lattice parameters of β-tricalcium phosphate were determined by means of X-ray diffractometry and coupled software. Also, solubility of the heated mixtures was investigated by determining the amount of Ca and Zn released into a simulated body fluid during 120 h. The results revealed that only limited amount of Zn ions could be incorporated into β-tricalcium phosphate lattice and ZnO phase was formed when high content of zinc nitrate was introduced in initial mixture. Both a and c lattice parameters of β-tricalcium phosphate were reduced by adding Zn. The release rate of calcium ions into the simulated body fluid was approximately constant during 120 h while for Zn minor release was observed.     

Self reliance and Innovation of Qinshan Phase Ⅱ NPP Project

This article mainly describes the selfreliance and innovation of Qinshan nuclear power project of phase II,inbetween it contains new reactor core design, as well as related experimental and calculation analysis, especially for new reactor design produced fluidinduced vibration model test, theoretical analysis and testing inbuilt reactor; aiming at twoloop NSSS a series improvement made for safety systems and related safety analysis to enhance their reliability and redundancy; according to specialty of twoloop NSSS an optimization made for NPP parameters and design of related equipments, for the purpose to make the output of NPP maximal; design of main reactor building and TG building also improved according to characteristics of twoloop NSSS and site conditions. CRDM and refueling machine are researched and manufactured on base of selfreliance, their performance are better than design requirements, large portion of key equipments are localized through different way. In construction first time realized the integrated erection of containment dome. During the commissioning nonnuclear steam driving of TG set, as well as 500 kV high voltage rising using emergent diesel generator, etc. are carried out. In period of operation still continuous innovation and improvement are made, so that to keep the good record of operation.     

Conductance and Thermopower of a Quantum Dot with Fano–Rashba Effect

We investigate the conductance and thermopower of a Rashba quantum dot coupled to ferromagnetic leads. We show that the interference of localized electron states with resonant electron states leads to the appearance of the Fano–Rashba effect. This effect occurs due to the interference of bound levels of spin-polarized electrons with the continuum of electronic states with an opposite spin polarization. We obtain an important enhancement of the thermopower due to the Fano–Rashba effect.     

Phase Diagram and Luminescent Properties of CdTe–MnTe Solid Solutions

The T–x phase diagram of the CdTe–MnTe system is mapped out. The CdTe–MnTe join is shown to be nonpseudobinary. The system contains an extended series of CdTe-based solid solutions (0–71.4 mol % MnTe at 1070 K). At 1070 K, -MnTe dissolves up to 0.3 mol % CdTe. Data are presented on the composition dependences of lattice parameters for the solid solutions studied and the luminescent properties of Cd_{1 – x} Mn _x Te single crystals annealed in cadmium or tellurium vapor.     

Phase diagram and magnetovolume effect of pseudo-binary CrBFeB system

The Phase diagram of the pseudo-binary CrBFeB system was studied in the temperature range from 900 to 2200°C by using a piston-cylinder high pressure apparatus as a reaction vessel. The CrB-type phase was found to be unstable relative to the FeB-type phase at higher temperature. The magnetism and thermal expansion of the samples having the FeB-type structure were also studied in connection with their Invar properties.     

Phase stability in Zr–H and Ti–H systems

Abstract

Enthalpy calculations, based on Miedema's model for alloy formation, show the δ–hydride (ZrH_~1·6) to be the most stable hydride in the Zr–H system. The γ–hydride (ZrH) is the least stable and, as suggested in the literature, is quite probably metastable. In addition, the ε–hydride (ZrH_2±x) is shown to be the last hydride formed in the Zr–H system. TiH_1·38 is shown to be the most stable hydride in the Ti–H system corresponding to the γ–hydride phase on the equilibrium phase diagram.

MST/187     

Quantum Interference Effects in p-Si1−x Ge x Quantum Wells

Quantum interference effects, such as weak localization and electron-electron interaction (EEI), have been investigated in magnetic fields up to 11 T for hole gases in a set of Si_1?x Ge _x quantum wells with 0.13<x<0.95. The temperature dependence of the hole phase relaxation time has been extracted from the magneto-resistance between 35 mK and 10 K. The spin-orbit effects that can be described within the Rashba model were observed in low magnetic fields. A quadratic negative magneto-resistance was observed in strong magnetic fields, due to the EEI effect. The hole-phonon scattering time was determined from hole overheating in a strong magnetic field.     

Two-Photon Coherent Spin Flip and Polarization Rotation of Excitons in Quantum Dots

It is shown that the spin orientation and polarization of an exciton confined in a QD can be controlled via two-photon transitions, by virtually coupling the single-exciton states to the detuned ground and biexciton states. Furthermore, phonon-induced dephasing leading to loss of fidelity in the two-photon optical control schemes is studied. Exciton spin flip can be performed with errors as low as 10⁻³.     

Quantum Phases of Excitons and Their Detections in Electron-Hole Semiconductor Bilayer Systems

There have been extensive experimental search for possible exciton superfluid in semiconductor electron-hole bilayer (EHBL) systems below liquid Helium temperature. Here we construct a quantum Ginsburg-Landau theory to study the quantum phases and transitions in EHBL. We propose that in the dilute limit as distance is increased, there is a first order transition from the excitonic superfluid (ESF) to the excitonic supersolid (ESS) driven by the collapsing of a roton minimum, then a 2nd order transition from the ESS to excitonic normal solid. We show the latter transition is in the same universality class of superfluid to Mott transition in a rigid lattice. We then study novel elementary low energy excitations inside the ESS. We find that there are two “supersolidon” longitudinal modes (one upper branch and one lower branch) inside the ESS, while the transverse mode in the ESS stays the same as that inside a exciton normal solid (ENS). We also work out various experimental signatures of these novel elementary excitations by evaluating the Debye-Waller factor, density-density correlation, specific heat and vortex-vertex interactions. For the meta-stable supersolid generated by photon pumping, we show that several unique features of the photoluminescent can be used to detect the metastable ESS state generated by photon pumping without any ambiguity.     

Effect of Charge State in Nearby Quantum Dots on Quantum Hall Systems

Transport properties have been measured on two-dimensional electron system (2DES) with a nearby InAs self-assembled quantum dot layer, in order to clarify the anomalous behavior of magnetoresistance, ρ _xx , and Hall resistance, ρ _xy at $\nu=\frac{1}{3}$ , which was an unsolved problem in our previous paper (Takehana et al.: J. Phys. Soc. Jpn. 75, 114713, 2006). The significant suppression of both ρ _xx and ρ _xy was found to be reduced in the vicinity of $\nu=\frac{1}{3}$ , which indicates that the fractional quantum Hall effect prevent the spin-flip process between localized electrons and that extended 2DES, according to the discussion of the previous paper.