Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.     

Polymorphism of anhydrous ferrous sulfate

The Mössbauer spectra of α-FeSO₄ and β-FeSO₄ have been measured in the temperature range 4.2 K ? T ? 30 K. The high temperature (β) phase is accompanied by a weak fraction of a third component. In analogy to CoSO₄, the third component of the three-fold polymorphism in anhydrous ferrous-sulfate is named the γ phase. X-ray powder measurements confirm this phase to be isomorphous to the corresponding γ-CoSO₄ structure. The parameters of the hyperfine interaction at 4.2 K of the three phases are evaluated. The magnetic ordering temperatures are T_N (α) = 24.0 K, T_N(β) = 17.0 ± 1.5 K and T_N(γ) = 19.5 ± 1.0 K.     

Phase diagram of the Ising antiferromagnet with nearest-neighbor and next-nearest-neighbor interactions on a square lattice

The phase diagram of the Ising model in the presence of nearest-neighbor (J₁) and next-nearest-neighbor (J₂) interactions on a square lattice is studied within the framework of the differential operator technique. The Hamiltonian is solved by effective-field theory in finite cluster (we have chosen N=4 spins). We have proposed a functional for the free energy (similar to Landau expansion) to obtain the phase diagram in the (T,α) space (α=J₂/J₁), where the transition line from the superantiferromagnetic (SAF) to the paramagnetic (P) phase is of first-order in the range 1/2<α<0.95 in contrast to previous study of CVM (Cluster Variational Method) that predict first-order transition for α=1.0. Our results for α=1.0 are in accordance with MC (Monte Carlo) simulations, that predict a second-order transition.     

High-resolution heat capacity of SnCl2·2H2O single crystal

The heat capacity of SnCl₂·2H₂O single crystal was measured in the close vicinity of the phase transition temperature, T_c = 217.994 ± 0.01 K. Its anomalous part ΔC could be expressed as ΔC = A_± ⊥ (T ? T_c)/T_c⊥^-α_±, where α₊ = 0.492 ± 0.02, α_- = 0.492 ± 0.02, A₊ = 1.148 JK^?mol^?, and A_- = 1.155 JK^?mol^?. A quasi-isothermal absorption of the enthalpy amounting to 34 J mol^? was observed at T_c.     

Compression dependence of αKT for ionic solids

In the present study, the compression dependence of αK_T is analyzed using Anderson-Isaak equation along with Stacey's equation of state for ionic solids such as NaCl and MgO. It is found that the value of αK_T decreases with increasing compression and attains a minimum, after which it increases with increasing compression along an isotherm. It is also found that αK_T tends to infinity as P→∞ or V→0. The presented nature of αK_T is valuable to understand the thermoelastic and thermodynamic properties of MgO and NaCl at high compressions and at high temperatures.     

Phase transition properties of the spin-1 Blume-Emery-Griffiths model with random transverse crystal field

Phase transition properties of a spin-1 Blume-Emery-Griffiths model (BEGM) with random transverse crystal field is studied by the effective field theory for a simple cubic lattice. In T−D_x space, we obtain the phase diagrams with the ratio α between the biquadratic interaction and the exchange interaction as well as a tunable parameter l of the transverse crystal field. The tricritical point (TCP) appears at α<0, which undergoes a crossover from positive to negative direction of the transverse crystal field when l<0. The TCP cannot be observed for α>0. The maximum critical temperature increases with the increase of α. The position of the peak value tends to the drift of negative or positive direction for a different magnitude or an imperfect (±) transverse crystal field distribution. In T−α space, the range of ordered phase is magnified when the ratio is changed from α<0 to α>0. The random transverse crystal field obviously affects the TCP.     

Internal conversion coefficients ofM4 transitions in125m, 127m, 129mTe decay

The internal conversion coefficients have been measured using a high resolution low energy Ge(Li) detector for the followingM4 transitions:¹²⁵Te: 109.27keV transitionα _T =357±11; RG method,¹²⁷Te: 88.26 keV transitionα _T =484±23; XPG method,¹²⁹Te: 105.50keV transitionα _T =213±10; XPG method. It is observed that these values are lower by 2.5–3.6% as compared with Hager and Seltzer's calculations. A comparison between experimental and theoreticalα _T andα _T values for elevenM4 transitions shows that the experimental values are systematically lower.     

Green function theory of Curie and order—order transitions in ferromagnetic systems

Dipolar critical temperatures in ferromagnetic systems with isotropic bilinear and biquadratic exchange are investigated by means of the Green function technique. Expressions are found for both the familiar Curie temperature, T_c, and the less well known order-order transition temperature, T_o, at which, under appropriate conditions, the magnetic ordering undergoes a change between fully aligned and canted ferromagnetism. At T = 0, a fully aligned state has <s_i^z = s for spin s and all lattice sites i, while a canted state has 〈s_i^z〉<s. It is shown independently of the Green function analysis that the T = 0 ground state is fully aligned if α, the ratio of biquadratic to bilinear exchange integrals, obeys ?[2s(s?1)]^?1<α< [2s²?2s+1]^?1. The region below the lower limit is identified as the range in which canted ferromagnetism can occur and is a range that does not appear to have been considered previously via the Green function formalism.The temperature dependence of the magnetic ordering is investigated by means of the double-time temperature-dependent Green function formalism. A new decoupling scheme is derived and used to reduce higher order Green functions to lowest order. It is found that a canted state, occuring at low temperatures, undergoes a transition to a fully aligned state at a temperature T₀ and subsequently becomes disordered at temperature T_c. Transitions to paramagnetism are found to be second order for α<α_c and first order for α?α_c where α_c is a critical value that depends on the atomic spin and weakly on the lattice structure. A phase diagram is given to illustrate the results, and a comparison is made with the corresponding results found in mean field theory.     

Charm photoproduction via fragmentation

The next-to-leading open charm production inγp collisions is calculated within the perturbative fragmentation functions formalism, to allow resummation ofα _slog(p _T ² /m ²) terms. In the largeP _T region (p _T>m) the result is consistent with the fixed order NLO calculation, small discrepancies being found for very largep _T and at the edge of phase space. The two approaches differ in the definition and the relative contribution of the direct and resolved terms, but essentially agree on their sum. The resummation is found to lead to a reduced sensitivity to the choice of the renormalization/factorization scale.     

Phase transition in a linear chain of classical spins with a logarithmic nearest neighbour pair potential

We present the complete calculation of the partition function and correlation functions of a linear array of classical spins coupled by a nearest neighbour logarithmic pair potential. In the case of a ferromagnetic coupling there occurs a phase transition at T_c > 0. The critical exponents of the specific heat C and the magnetic susceptibility χ (in the absence of an external field) are shown to have the non-classical value α = 2 and classical value γ = 1 respectively. The underlying mathematical mechanism of the phase transition is the complete degeneracy of all the eigenvalues of the corresponding integral equation (Kac's mechanism). Below T_c the partition function becomes complex. For antiferromagnetic coupling the free energy is analytic in the whole temperature range and so no phase transition occurs in this case.     

Critical properties and exchange interactions of the antiferromagnetic tetrahedral spinel

The high-temperature series expansion for the spin correlation functions of the A-spinel lattice has been derived. The development is extended to order 6 in β=1/k_BT with nearest-neighbour (nn) and next-nearest-neighbour (nnn) exchange couplings. The paramagnetic susceptibility and the correlation length are also given.The series is examined via the Padé approximants method and estimates for the transition temperature T_N are obtained for various values of the ratio α=J₂/|J₁|. It is found that the variation of critical temperature is well represented by T(α)=T(0)[1+3.0725α] for values of α in the range −0.2≤α. T(0) is the critical temperature of the nn model.The approach is applied to the experimental results of the particular system A-spinel CoCo₂O₄. The following estimates are obtained for the familiar critical exponents: γ=1.382 and v=0.701.     

The Stückelberg holographic superconductors with Weyl corrections

In this Letter we construct the Stückelberg holographic superconductor with Weyl corrections. Under such corrections, the Weyl coupling parameter γ plays an important role in the order of phase transitions and the critical exponents of second order phase transitions. So do the model parameters cα, α and c₄. Moreover, we show that the Weyl coupling parameter γ and the model parameters cα, α, c₄ which together control the size and strength of the conductivity coherence peak and the ratio of gap frequency over critical temperature ωg/Tc.     

Magnetic phase transition in KGd(WO4)2 double tungstate

Investigations of the specific heat of the potassium gadolinium double tungstate KGd(WO₄)₂ have been performed over the temperature range from 0.05 K up to 4 K in zero magnetic field. The λ -type anomaly observed at T = 0.42 K was interpreted as an indication of a second order magnetic phase transition within the Gadolinium sublattice. The critical exponent α was determined from experimental data.     

Shear viscosity of the Quark–Gluon Plasma from a virial expansion

We calculate the shear viscosity η in the quark–gluon plasma (QGP) phase within a virial expansion approach with particular interest in the ratio of η to the entropy density s, i.e. η/s. The virial expansion approach allows us to include the interactions between the partons in the deconfined phase and to evaluate the corrections to a single-particle partition function. In the latter approach we start with an effective interaction with parameters fixed to reproduce thermodynamical quantities of QCD such as energy and/or entropy density. We also directly extract the effective coupling α _V for the determination of η. Our numerical results give a ratio η/s≈0.097 at the critical temperature T _c, which is very close to the theoretical bound of 1/(4π). Furthermore, for temperatures T≤1.8T _c the ratio η/s is in the range of the present experimental estimates 0.1–0.3 at RHIC. When combining our results for η/s in the deconfined phase with those from chiral perturbation theory or the resonance-gas model in the confined phase we observe a pronounced minimum of η/s close to the critical temperature T _c.     

127I NQR and 1H NMR studies of 4-aminopyridinium tetraiodoantimonate(III); molecular motion and phase transition

The crystals of 4-NH₂PyHSbI₄ (Py = C₅H₄N) have been investigated by means of ¹²⁷I NQR, ¹H NMR T ₁ and DTA. The crystals can exist in two modifications of β and α(I) at room temperatures. The α(I)-phase is a metastable state which is obtained when the stable β form is heated. The α(I)-phase undergoes a first-order type phase transition of α(I) $\leftrightarrow \alpha $ (II) at 272 K (on heating), while the β-phase is stable down to 77 K. Four and two ¹²⁷I (m = ±1/2 $\leftrightarrow \pm $ 3/2) NQR lines have been found for the β- and α(II)-phases, respectively. One half of them is assignable to the terminal I atom(s) and the other to the bridging I atom(s) in each phase. All the resonance lines of the α(II)-phase underwent a disappearance above ca. 240 K and no resonance line was observed in the α(I)-phase. The second moment M ₂ value of ¹H NMR spectra with 8 G² at 290 K shows that the 4-NH₂PyH^?+? cations reside in the rigid lattice in the β-phase. In contrast, in the α(I)-phase the cation rotates about an axis more symmetric than pseudo threefold axis. The activation energy of 21 kJ mol^???1 was estimated for the reorientational motion in the α(I)-phase from the ¹H NMR T ₁ measurements. The nature of phase transitions in the 4-NH₂PyHSbI₄ is discussed in comparison with that in 4-NH₂PyHSbBr₄.     

Ni-Zr非晶合金晶化的透射电子显微镜研究(Ⅰ) ——Ni67Zr33晶化过程中的亚稳相

利用透射电子显微镜对Ni₆₇Zr₃₃非晶合金晶化的研究发现了两个新的亚稳相T₀与T₁0。其中T₁相为体心正交(准四方〕晶体,点阵常数a≌b=0.89nm,c=3.14nm,空间群为Iba2或Ibam。温度升高,T₁相转变为含有大量错排的A心正交Ni₁₀Zr₂相,用1/2(a+b)位移错排模型可以圆满地解释其电子衍射图中仅h+k为奇数的衍射斑沿c^*方向拉长的现象,晶化稳定相为Ni₁₀Zr,与Ni₂₁Zr₈(Ni₅Zr₂)相。 关键词：     

Novel superconducting phases in copper oxides and iron-oxypnictides: NMR studies

We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-T_c superconductivity (HTSC) for a disorder-free CuO₂ plane based on an evaluation of local hole density (p) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO₂ plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO₂ plane in an underdoped regime of HTSC. The T=0 phase diagram of AFM constructed here is in quantitative agreement with the theories in a strong correlation regime which is unchanged even with mobile holes. This Mott physics plays a vital role for mediating the Cooper pairs to make T_c of HTSC very high. By contrast, we address from extensive NMR studies on electron-doped iron-oxypnictides La1111 compounds that the increase in T_c is not due to the development of AFM spin fluctuations, but because the structural parameters, such as the bond angle α of the FeAs₄ tetrahedron and the a-axis length, approach each optimum value. Based on these results, we propose that a stronger correlation in HTSC than in FeAs-based superconductors may make T_c higher significantly.     

Effect of disorder and isotope on the properties of a two orbital double exchange system

A two-site single electron double exchange model incorporating orbital degeneracy, superexchange and electron-phonon (e-ph) interaction is studied using exact diagonalization method. The core spins are treated quantum mechanically. We study the ground state phase diagram as well as the magnetic susceptibility and the kinetic energy of the system as a function of temperature. Effect of difference in site energies, which mimics the role of site-diagonal disorder, is investigated. The susceptibility shows a peak at a characteristic temperature which we have referred to as T₀. The variation of T₀ with e-ph coupling and that of the isotope-shift exponent (α) with T₀ are obtained. We also investigate the field-induced change in the kinetic energy, which is related to the colossal magnetoresistance (CMR) of the system, and find that the disorder enhances the CMR even for the two-site system.