共查询到20条相似文献,搜索用时 31 毫秒
1.
Magnetic phases in PrCo 2Si 2 have been studied by measurements of magnetization, neutron diffraction and electrical resistivity. For <9 K, the magnetic structure with a propagation vector k = (0,0,1) [2π/ c] is stable. Incommensurate structures k = (0,0,0.926) and (0,0,0.777) appear for 9 K < T <17 K and 17 K < T <30 K, respectively. 相似文献
2.
The dynamic properties of a stochastic n-vector model are investigated for T < Tc in d=4−ε dimensions. Besides the non-conserved order parameter the model involves also the conserved densities of generators of the symmetry group O( n). We calculate the excitation spectra of those conserved densities and the transverse fluctuations of the order parameter to linear order in ε in the hydrodynamic region kξ1. The propagating modes have linear dispersion and quadratic damping in accordance with the phenomenological theory. The relaxing modes, however, exhibit non-hydrodynamic wavenumber dependence with a relaxation rate ω k ∞ kd/2. 相似文献
3.
We have measured the resistivity of textured Bi 1.84Pb 0.4Sr 2Ca 2Cu 3O y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10 3 A/cm 2 and magnetic field up to 0.3 T. We find that the effective activation energy Ue follows Ue( T, J, H)= U0(1− T/ Tp) mln( Jc0/ J) H− with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm 2, where Tp is a function of applied magnetic field and current density. This result suggests the effective activation energy Ue be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed. 相似文献
4.
Magnetic measurements on hexagonal single crystals of M 2Mo 3O 8 (M = Mn, Fe, Co and Ni) are reported. The Mn compound orders ferrimagnetically at Tc = 41.5 K; the Fe and Co compounds antiferrimagnetically at TN = 59.5 K and 40.8 K, respectively. No magnetic ordering was found in the Ni compound down to 2 K. All the compounds showed strong magnetic anisotropy when ordered, the results indicating that the magnetic spins' preferred orientation is along the hexagonal axis. Mn 2Mo 3O 8 shows a temperature dependence of the spontaneous magnetization in the ferrimagnetic regime which is rarely observed: as T → 0, Ms → 0. 相似文献
5.
The polarized absorption and fluorescence spectra of Pr 3+ in single crystals of LiYF 4 having the scheelite structure have been investigated and assignments made on the basis of S 4 site symmetry. Strong vibronic coupling associated with the 3H 4→ 3H 5, 1D 2 transitions in absorption and the 3P 0→ 3H 4,6 transitions in fluorescence was observed. Using selection rules for vibronic coupling and the known k = 0 phonons, these “extra” features of the spectrum can be accounted for in polarization and frequency. 相似文献
6.
Among the AuCu 3-type RIn 3 series where R is a rare earth, the NdIn 3 compound presents a complex magnetic phase diagram. In this compound, which orders antiferromagnetically at TN = 5.9 K, three magnetic phases are separated by two sharp first-order transitions. The phase diagrams, determined for the main crystallographic directions, by magnetization measurements in fields up to 7 T show a strong dependence of the transition temperatures. In order to determine the actual magnetic structures in NdIn 3, a neutron diffraction experiment has been performed on a single crystal under magnetic field. It shows that the magnetic structures are collinear (single q) with the magnetic moments aligned along the fourfold axis. They clearly evidence the existence of two incommensurate phases with q = (1/2, 1/2, τ): for TN T 5.5 K, τ 1 ≈ 0.037 2π/ a and the structure is sine-wave modulated, for 5.5 K > T > 4.7 K, τ 2 = 0.017 2π/ a and the structure squares up. Below 4.7 K the q = (1/2, 1/2, 0) commensurate structure is stabilized. 相似文献
7.
The magnetic structure of MnPd 3 has been refined using improved crystal structure data for the I4/ mmm space group. The magnetic moment on Pd atoms coupled antiparallel to the Mn moments of 4.1 ± 0.3 μ B was found by neutron diffraction to be 0.15 ± 0.03 μ B in the 4( c) and 4( e) positions. The Pd moments on the 4( d) sites are assumed to be zero on symmetry grounds. 相似文献
9.
We consider supersymmetric extensions of the standard model with two pairs of Higgs doublets. We study the possibility of spontaneous CP violation in these scenarios and present a model where the origin of CP violation is soft, with all the complex phases in the Lagrangian derived from complex masses and vacuum expectation values (VEVs) of the Higgs fields. The main ingredient of the model is an approximate global symmetry, which determines the order of magnitude of Yukawa couplings and scalar VEVs. We assume that the terms violating this symmetry are suppressed by powers of the small parameter ε PQ = O( mb/ mt). The tree-level flavor-changing interactions are small due to a combination of this global symmetry and a flavor symmetry, but they can be the dominant source of CP violation. All CP-violating effects occur at order ε PQ2 as the result of exchange of almost decoupled extra Higgs bosons and/or through the usual mechanisms with an almost real CKM matrix. On dimensional grounds, the model gives ε K ≈ ε PQ2 and predicts for the neutron electric dipole moment (and possibly also for ε K1) a suppression of order ε PQ2 with respect to the values obtained in standard and minimal supersymmetric scenarios. The predicted CP asymmetries in B decays are generically too small to be seen in the near future. The mass of the lightest neutral scalar, the strong CP problem, and possible contributions to the Z decay into b quarks (the Rb puzzle) are also briefly addressed in the framework of this model. 相似文献
10.
Polycrystalline samples of R 3Cu 4Si 4 (R=Dy, Ho, Er) intermetallics were studied with neutron diffraction methods. All of them crystallize in the orthorhombic structure of Gd 3Cu 4Ge 4-type and order antiferromagnetically at low temperatures. Magnetic moments localized at the rare earth atoms, that occupy two non-equivalent 2d and 4e sublattices, order simultaneously in Dy 3Cu 4Si 4. The order is described by the propagation vector
accompanied by
, δ=0.025(2). In Ho 3Cu 4Si 4 two propagation vectors are needed to model the magnetic order. These are:
for the 4e sublattice, which disorders as the first when the temperature increases, and
for the 2d sublattice. A similar situation is observed for Er 3Cu 4Si 4 where the propagation vectors are: k=(0,1−δ,0), δ=0.097(2) for the 4e sublattice, which disorders as the first with increasing temperature, and
, δ=0.0015(6) for the 2d sublattice. 相似文献
11.
For a coupled partial wave with a bound state, the entire T-matrix is determined by the bound-state energy and wave function, the on-shell mixing parameter and a real symmetric matrix function σ( k2, 2 k11) of the continuous momentum variables k and the discrete eigenchannel labels . The on-shell part of σ is given by the eigenphases, the arbitrariness of the T-matrix is the off-shell continuation of this phase-shift matrix σ( k2, k1) into two dimensions. Three possible techniques are suggested to go from to the entire T-matrix. This construction procedure for T avoids the usual complications of a potential fit. 相似文献
12.
以SrO和CrO 2为原料, 在高温高压的条件下直接反应生成纯相的K 2NiF 4结构的Sr 2CrO 4多晶样品. 结构用粉末X射线衍射及GSAS精修表征. 磁化率测试显示样品存在一个弱的反铁磁相变, 奈尔温度为 TN=95 K. 在奈尔温度以上, 磁化率随温度的变化遵循居里-外斯定律. 对样品进行了电阻测试, 结果显示了样品的绝缘特性. 相似文献
13.
The ultrasonic speeds, u and viscosities, η of binary mixtures of formamide (FA) with ethanol, 1-propanol, 1,2-ethanediol, and 1,2-propanediol, including those of pure liquids, over the entire composition range were measured at 293.15, 298.15, 303.15, 308.15, 313.15, and 318.15 K. From the experimental values of u and η, the deviations in isentropic compressibility, Δ ks, in ultrasonic speed, Δ u, and in viscosity, Δ η were calculated. The variation of these parameters with composition and temperature of the mixtures are discussed in terms of molecular interaction in these mixtures. The observed trends in Δ ks values indicate the presence of specific interactions between FA and alkanol molecules. The Δ ks values follow the order: ethanol < 1-propanol < 1,2-propanediol < 1,2-ethanediol. It is observed that the Δ ks values depend upon the number of hydroxyl groups and alkyl chain length in these alkanol molecules. Furthermore, the free energies, Δ G, enthalpies, Δ H and entropies, Δ S of activation of viscous flow have also been obtained by using Eyring viscosity equation and their dependence on composition of the mixtures have been discussed. 相似文献
14.
An organic molecule isomerizes in viscous solvents when appropriate cavities are formed around it in the course of slow diffusive thermal fluctuations of solvent molecules. The isomerization occurs when fast twisting (vibrational) fluctuations around a bond get to have large amplitudes in such cavities. This situation can be described by the two-reaction-coordinate model of Sumi and Marcus originally proposed for electron transfer reactions. In fact, the rate constant derived from this model fits nicely to that observed for thermal Z→E isomerization of substituted azobenzenes and N-benzylideneanilines. The rate constant is influenced by slow speeds of diffusive motions of solvent molecules, whose relaxation time τ is usually proportional to the solvent viscosity η. It has a form of k = 1/( kTST−1+ kf−1), where kTST, independent of τ, represents the rate constant expected from the transition state theory (TST), while kf ∝ τ − with 0 < ≤ 1 represents the part controlled by solvent fluctuations. An analytic expression of for the isomerization reactions is given in terms of physical parameters underlying the reaction mechanism with cavity formation. This rate-constant formula is a general one applicable widely also to other solution reactions, covering from the TST-validated regime for a small τ to the TST-invalidated one for a large τ. In the former, k approaches kTST since kf kTST, while in the latter, k approaches kf since kf kTST, becoming decreasing with a decrease in the typical speed (∝ τ−1) of solvent fluctuations. The dependence of k ≈ kf ∝ η− in the non-TST regime has often been observed also in biological reactions such as enzymatic ones. In this case, it is not appropriate to say that reactions are controlled by slow speeds of solvent fluctuations, but we should rather say that enzymes utilize this situation, which has been called conformational gating, in the course of solvent-fluctuation-driven conformational fluctuations of proteins. It has important meanings in protein functions. 相似文献
15.
An attempt is made to synthesize Nd 2Co 14C compound by mechanical alloying Nd 16Co 76B 8−xC x (0 x8) alloys and subsequent annealing. Phase formation and magnetic properties of Nd 2Fe 14B-type Nd 16Co 76B 8−xC x alloys and their hydrides are investigated. The Nd 2Co 14(B,C) phase with Nd 2Fe 14B-type structure is formed for Nd 16Co 76B 8−xC x (0 x7) alloys, while NdCo 7C δ phase with TbCu 7-type structure is observed in Nd 16Co 76C 8 alloy. The lattice parameter c of the Nd 2Co 14(B,C) phase decreases with increasing the carbon content. A limit volume of the unit cell to form the Nd 2Fe 14B-type structure is estimated to be 0.870 nm 3. The spin-reorientation temperature TSR increases with increasing the carbon content, due to an enhancement of magnetocrystalline anisotropy caused by carbon substitution for boron. After hydrogenation, the lattice expansion is observed for Nd 16Co 76B 8−xC x (0 x7) alloys. The spin-reorientation temperature of Nd 16Co 76B 8−xC xH y (0 x7) is much lower than that of the host alloys. Some structural and magnetic properties of hypothetic Nd 2Co 14C and Nd 2Co 14CH y compounds are estimated by extrapolation. 相似文献
16.
The magnetic ordering in the tetragonal ternary compound U 4Cu 4P 7 has been studied by neutron diffraction. It orders below TN = 146 K with an antiferromagnetic structure of wave vector k = (001). The magnetic ordering corresponds to a stacking of ferromagnetic (001) uranium planes according to the sequences m1, m1, m2, - m2, - m1, - m1, - m2, m2 where m1 and m2 represent the magnetic moment, directed along the tetragonal axis of the two uranium sites U(1) (0,0,± z 1) and U(2) (0,0, ± z2) respectively. The magnetic moments on these two sites have different temperature dependencies as well as well as they reach the different values of 1.1 and 2.2.μ B for the U(1) and U(2) sites, respectively. 相似文献
17.
Let (Γ, d) be the 3 D-calculus or the 4 D±-calculus on the quantum group SUq (2). We describe all pairs (π, F) of a *-representation π of
( SUq(2)) and of a symmetric operator F on the representation space satisfying a technical condition concerning its domain such that there exist a homomorphism of first order differential calculi which maps d x into the commutator [i F, π( x)] for x ε
( SUq (2)). As an application commutator representations of the two-dimensional left-covariant calculus on Podles quantum 2-sphere Sqc2 with c = 0 are given. 相似文献
18.
A stack of annular detectors made of high-purity germanium was used to measure simultaneously pd → 3H π + and pd → 3He π 0 differential cross sections at beam momenta of 750 MeV/ c, 800 MeV/ c, and 850 MeV/ c over a large angular range. The extracted total cross sections for the pd → 3He π 0 reactions bridge a gap between near threshold data and those in the resonance region. The ratio of the cross sections for the two reaction channels taken at the same η= pcmπ/ mπ yields 2.11±0.08 indicating that a deviation from isospin symmetry is very small. 相似文献
19.
The structural, electrical and magnetic properties of the superconducting ferromagnets, Gd 1.4−xDy xCe 0.6Sr 2RuCu 2O 10 ( x=0–0.6) are systematically investigated as a function of Dy doping and temperature. These compounds are characterized by high temperature superconductivity ( Tc ranging from 20 to 40 K depending upon the Dy content) co-existing with weak ferromagnetism with two magnetic transitions ( TM2 ranging from 95 to 106 K and TM1 around 120 K). Doping with Dy gives no significant structural changes except for a minor change in the c/ a ratio. However the superconducting transition temperature is significantly suppressed and magnetic ordering temperature enhanced on Dy doping. These effects are described and discussed. 相似文献
20.
Bi 2Te 3 films were prepared by thermal evaporation technique. X-ray diffraction analysis for as-deposited and annealed films in vacuum at 150 °C were polycrystalline with rhombohedral structure. The crystallite size is found to increase as the film thickness increases and has values in the range 67–162 nm. The optical constants (the refractive index, n, and absorption index, k) were determined using transmittance and reflectance data in the spectral range 2.5–10 μm for Bi 2Te 3 films with different thicknesses (25–99.5 nm). Both n and k are independent on the film thickness in the investigated range. It was also found that Bi 2Te 3 is a high refractive index material ( n has values of 4.7–8.8 in the wavelength range 2.5–10 μm). The allowed optical transitions were found to be direct optical transitions with energy gap eV. The optical conductivities σ1 = ƒ( hν) and σ2 = f( hν) show distinct peaks at about 0.13 and 0.3 eV, respectively. These two peaks can be attributed to optical interband transitions. 相似文献
|