Phase diagram of electron-hole systems: Interplay between exciton Mott transition and quantum pair condensation

Quasi-thermal-equilibrium states of electron-hole (e-h) systems in photoexcited insulators are studied from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type pair-condensed state, and the exciton Mott transition between an insulating exciton/biexciton gas phase and a metallic e-h plasma phase. We determine the quasi-equilibrium phase diagram of the e-h system at zero and finite temperatures with applying the dynamical mean-field theory (DMFT) to the e-h Hubbard model with both repulsive and attractive on-site interactions. Effects of inter-site interactions on the exciton Mott transition are also clarified with applying the extended DMFT to the extended e-h Hubbard model.     

High pressure effect on the electrical properties of NiS2

A semiconductor-metal transition in the electrical resistance of NiS₂, which has been suggested to be a Mott transition, is observed with decreasing temperature under pressure up to 44 kbar. The transition temperature increases with pressure with a slope of $\text{dT}\text{dP}\text{= 6 ± 1 K/kbar}$. The activation energy in a semiconducting region is found to decrease with increasing pressure and to vanish at about 46 kbar. The critical pressure and temperature are predicted to be 46 ± 2 kbar and 350 ± 20 K.     

Magnetic susceptibility study of magnetic properties in low stage FeCl3 intercalated graphite

Measurement of the ac susceptibility of well characterized ferric chloride intercalated graphite compounds both parallel and perpendicular to the basal plane, shows that the stage 1 and 2 compounds undergo a magnetic phase transition at 4.3 ± 0.2 K and 1.3 ± 0.2K respectively.     

Effect of strontium substitution on the structural and magnetic properties of La1.8Sr0.2MMnO6 (M = Ni,Co)-layered manganites

Sr-substituted perovskites, La_1.8Sr_0.2MMnO₆ (M = Ni, Co), were synthesized using the solid-state reaction technique to present a systematic study on their morphological, structural and magnetic properties. The average grain size of the as-prepared La_1.8Sr_0.2NiMnO₆ samples are in the range of 0.2–0.7 µm and those for La_1.8Sr_0.2CoMnO₆ manganites are 0.1–2.8 μm, which is significantly less than that of unsubstituted La₂NiMnO₆ (LNMO) and La₂CoMnO₆ (LCMO) manganites. The XPS analysis enlightened about phase purity, binding energy and oxygen vacancy of La_1.8Sr_0.2MMnO₆ manganites. The Sr-substituted LNMO has revealed a sharp ferromagnetic to paramagnetic phase transition at 160 ± 2 K, which is about 120 K less than that of parent LNMO. The Sr-substituted LCMO exhibited such a transition at 220 ± 2 K, which is 8 K less than that of parent LCMO. The temperature-dependent magnetization measurements suggest that the effect of Sr on the transition temperature in LNMO is more significant than that of LCMO.     

非晶态Fe13Ni67.2P4.5B15.3合金的临界现象

本文报道非晶态Fe₁₃Ni_67.2P_4.5B_15.3合金的磁化强度与温度和磁场关系的测量结果。在居里温度附近样品的磁特性符合二级相变规律,得到临界指数β=0.39±0.02,γ=1.56±0.06,δ=5.20±0.1,样品的居里温度T_c=(180.4±0.2)K。在实验误差范围内,临界指数β,γ,δ满足γ=β(δ-1)关系,在168—192K温度范围,实验数据满足二级相变的磁状态方程。当T>270K时,样品顺磁磁化率服从居里-外斯定律,由居里-外斯常数c计算出有效顺磁磁矩P_eff=3.19 μ_B。 关键词：     

Oxygen diffusion in uranium dioxide in the temperature range of phase transitions

The structure of and oxygen diffusion in UO₂ are studied by the molecular dynamics method in the range of transition to the superionic state (melting of the oxygen sublattice) and near the melting point of UO₂. The temperature dependence of the diffusion coefficient of a doubly charged oxygen ion in UO₂ is constructed. In the crystalline state at temperatures between 1800 and 2600 K, this dependence is described by an exponential dependence with a diffusion activation energy of 2.6±0.2 eV. In the superionic state (2600–3100 K), the activation energy of diffusion of an oxygen anion decreases to 1.88±0.13 eV. In melt (3100–3600 K), the exponential dependence of the diffusion coefficient of O^2- persists but the activation energy of diffusion decreases still further, to 0.8±0.2 eV. Our experimental results agree (within the limits of experimental error) with data on oxygen diffusion in the crystalline phase obtained by other researchers.     

Orientational and positional disorder of ions and its freezing in the CsNO2-TlNO2 system2

The heat capacities of Cs_0.695Tl_0.305NO₂ (Specimen I) and Cs_0.385Tl_0.615NO₂ (Specimen II) have been measured between 14 and 350 K. Specimen I underwent a phase transition at (197.7 ± 0.1) K, with ΔS = (19.2 ± 1.5) JK^?mol^?, and specimen II at (214.5 ± 0.2) K, with ΔS = (5.4 ± 1.0) JK^?1mol^?1, respectively. Above the phase transition, an exothermic temperature drift due to phase separation was observed. Annealing of the sample at 203 K for 300 hr brought about complete phase separation. The solid solution system annealed at 203 K gave two heat capacity peaks at (203.3 ± 0.1) K, with ΔS = (13.8 ± 0.8) JK^?1 mol^?1, and (242.4 ± 0.2) K, with ΔS = (10.6 ± 1.3) JK^?1 for Specimen I, and at (203.0 ± 0.1) K with ΔS = (6.7 ± 0.5) JK^?1 mol^?1, and (257.5 ± 0.2) K with ΔS = (17.9 ± 1.7) JK^?1 mol^?1 for Specimen II. The phase diagram of the CsNO₂-TlNO₂ binary system was constructed on the basis of DTA, heat capacity and dielectric measurements. In the metastable phase, the existence of a residual entropy due to the freezing of a random distribution of Cs⁺₁ and Tl⁺ cations in addition to the orientational disorder of the NO₂^?1 ion was confirmed by a comparison of entropies of the stable and the metastable phases.     

Specific heat of single phase Nb3Ge

For the first time, the specific heat of single phase, stoichiometric, high transition temperature (21.8 K) A-15 Nb₃Ge has been measured. From the data between 4 and 29 K, the linear term coefficient, γ, of the specific heat is found to be 30.3±1. mJ/mole-K² and the Debye temperature, ?_D, is 302±2 K. The bulk energy gap parameter, 2Δ/kT_c, is found to be 4.2±0.2, in agreement with tunneling measurements.     

非晶态Co70Cr20Zr10合金在居里温度附近的磁性

本文研究非晶态Co₇₀Cr₂₀Zr₁₀合金在居里温度附近的磁性,样品的磁特性符合二级相变规律。得到临界指数β=0.45±0.02,γ=1.9±0.1,δ=5.13±0.05。Co₇₀Cr₂₀Zr₁₀合金的居里温度为T_c=(186.7±0.2)K。临界指数β,γ,δ满足γ=β(δ-1)关系,但临界指数值都偏离三维Heisenberg模型的理论值,这种行为可能起源于非晶态合金磁的不均匀性。讨论了Kouvel-Fisher(K-F)参数γ(T)对温度T的依赖关系。 关键词：     

Ferromagnetism in the Mott insulator Ba2NaOsO6

Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d1 ferromagnetic Mott insulator with an ordered moment of approximately 0.2microB per formula unit and TC=6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet ground state anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.     

Gamma radiation in the decay of249Cf

Gamma radiation following alpha decay of²⁴⁹Cf was investigated with high resolution Ge(Li) detectors in singles and coincidence arrangements. Nine new γ-rays were identified and placed in the previously-known decay scheme. Their energies in keV and intensities relative to the 388 keV (100) γ-ray are: 37.5±0.1 (0.026), 65.87±0.1 (0.017), 121.5±0.4 (0.070), 229.2±0.2 (0.072), 255.7±0.2 (0.065), 390.5±0.2 (0.025), 405.9±0.1 (0.014), 588.8±0.1 (0.003), and 643.5±0.1 (0.007). Coincidence measurements revealed that the intensity of the 54.7 keV γ-ray is shared by three intraband transitions and that the 121.5 keV crossover transition is present in the ground-state band.     

Variation of properties of superconducting Nb3Ge prepared by chemical vapor deposition

We discuss how some of the properties of chemically vapor deposited Nb₃Ge depend on deposition conditions. Strong variations of transition temperature and A-15 lattice parameter occur with changes in gas composition. Rather broad transition onsets up to 23.0 ± 0.2°K have been observed.     

Nb3Sn正常-超导转变时的比热反常

在16.0°K—20.3°K之间测量了Nb₃Sn样品的热容量。Nb₃Sn在临界温度附近的比热跳跃值ΔC=2.21(±5％)焦耳/克分子·度。样品的临界温度T_c=17.88°K,转变宽度ΔT_c≈0.2°K。ΔC值利用热力学关系式确定了Nb₃Sn在0°K时的热力学临界场H₀=5300奥斯特。利用本文的结果和文献上关于热膨胀系数的跳跃值Δα及?T/?P值验证了热力学关系式。扼要地描述了比热测量装置.     

Mott transition of the electron-hole liquid in Ge

A treatment is given of the Mott transition of a gas of excitons in equilibrium with electron-hole droplets in Ge. We find that transition occurs near 4K. We can come to no firm conclusion whether the Mott transition is a first-order transition separate from the liquid-gas transition, as has been suggested.     

Mott transition in modulated lattices and parent insulator of (K,Tl)(y)Fe(x)Se(2) superconductors

The degree of electron correlations remains a central issue in the iron-based superconductors. The parent iron pnictides are antiferromagnetic, and their bad-metal behavior has been interpreted in terms of proximity to a Mott transition. We study such a transition in multiorbital models on modulated lattices containing an ordered pattern of iron vacancies, using a slave-rotor method. We show that the ordered vacancies lead to a band narrowing, which pushes the system to the Mott insulator side. This effect is proposed to underlie the insulating behavior observed in the parent compounds of the newly discovered (K,Tl)(y)Fe(x)Se(2) superconductors.     

CdTe的状态方程及其高压相变

 用金刚石压砧高压X光衍射技术研究了Ⅱ-Ⅵ族化合物CdTe的室温状态方程和室温高压相变。实验的最高压力达39.2 GPa。实验中发现CdTe从(3.3±0.1)GPa开始从闪锌矿结构相相NaCl结构相转变，相变时体积收缩15.8%；从(10.3±0.2)GPa开始从NaCl相向β-Sn结构相转变，相变时无体积突变；在(12.2±0.2)GPa由β-Sn相向正交结构相转变，相变时也无体积突变。CdTe的压缩数据用最小二乘法以Bridgman状态方程和Murnaghan状态方程拟合，得到其零压时合相变压力时各个相的体弹模量及体弹模量的压力微商，并与其它的实验合理论结果进行比较。     

Specific heat of A-15 Nb3Si produced by explosive compression

The specific heat from 1.2 to 23 K has been measured on a new high T_c superconductor, A-15 Nb₃Si. The sample was prepared by explosive compression and has an onset of bulk superconductivity at 18.0 K, with a transition width of 0.7 K. The density of states for pure A-15 Nb₃Si implied from the specific heat data is 0.94 ± 0.20 states/eV-atom, ΔC/γ T_c is 2.0 ± 0.2.     

Superconductivity with onset above 23° K in NbGe sputtered films

Sputtered films of nominal composition Nb₃Ge deposited onto hot substrates, of the type recently found by Gavaler to have superconductivity onsets at 22.3° K, have now been obtained with a maximum onset temperature of 23.2 ± 0.2° K and a transition width of ～ 1.2° K. The sputtering conditions necessary to achieve these results are discussed and suggest that the high T_c phase may be detrimentally affected by the presence of high energy particles present under normal sputtering conditions.