Ion-size effects on the structure and superconductivity of La-214 compounds at doping

A structural transition to a low-temperature tetragonal (TLT) phase occurs below T_d2≈60 K in La_2−xBa_xCuO₄ only around x=0.125, accompanied by sharp suppression of superconductivity. In this study we investigated the effects of partial substitution of smaller divalent cations for Ba²⁺ ions (Ba²⁺ >Sr²⁺ >Ca²⁺) on these anomalous low-temperature properties. The suppression of superconductivity is strongly affected by the appearance of the TLT phase for this carrier concentration. We also show that the variations of T_d2 and T_c are quantitatively scaled by the averaged size of cations at the La site. This dependence can be interpreted as chemical effects acting effectively as applied pressure. We discuss how our observation is related to recent reports which claim that the TLT structure itself does not necessarily destroy the superconductivity.     

Temperature dependence of the Raman bandwidths and intensities of a lattice mode near the tricritical and second order phase transitions in NH4Cl

We calculate here the temperature dependence of the damping constant using the expressions derived from the Matsushita's theory and the temperature dependence of the order parameter from the molecular field theory for the tricritical (1.5?kbar) and second order (2.8?kbar) phase transitions in NH₄Cl. Our calculations are performed for the ν ₅ (174?cm^?1) Raman mode of NH₄Cl for the pressures studied. Predictions for the damping constant are in good agreement with our observed Raman bandwidths of this lattice mode for both pressures. The Raman intensities calculated from the molecular field theory by means of the order parameter are also in good agreement with our observed Raman intensities of the ν ₅ (174?cm^?1) mode for both tricritical (1.5?kbar) and second order (2.8?kbar) phase transitions in NH₄Cl. In this study our observed Raman intensities of this lattice mode are analysed using a power-law formula with the critical exponent β for the order parameter at those two pressures considered in NH₄Cl. From our analysis, the value of β?=?0.5 is obtained as the mean field value.     

<Emphasis Type="Italic">CP</Emphasis> violation due to multi-Froggatt–Nielsen fields

We study how to incorporate CP violation in the Froggatt–Nielsen (FN) mechanism. To this end, we introduce non-renormalizable interactions with a flavor democratic structure to the fermion mass generation sector. It is found that at least two iso-singlet scalar fields with a discrete symmetry imposed are necessary to generate CP violation due to the appearance of the relative phase between their vacuum expectation values. In the simplest model, the ratios of quark masses and the Cabibbo–Kobayashi–Maskawa (CKM) matrix including the CP violating phase are determined by the CKM element |V_us| and the ratio of two vacuum expectation values of FN fields, R=|R|e^iα (a magnitude and a phase). It is demonstrated how the angles φ_i (i=1,...,3) of the unitarity triangle and the CKM off-diagonal elements |V_ub| and |V_cb| are predicted as a function of |V_us|, |R| and α. Although the predicted value of the CP violating phase does not agree with the experimental data within the simplest model, the basic idea of our scenario would be promising if one wants to construct a more realistic model of flavor and CP violation. PACS 11.30.Er; 12.60.-i     

A polarized micro-Raman study of a 0.65PbMg1/3Nb2/3O3 - 0.35PbTiO3 single crystal

Polarized micro-Raman spectra of a 0.65PbMg_1/3Nb_2/3O₃-0.35PbTiO₃ (0.65PMN-0.35PT) single crystal poled in the [001] direction are obtained in a wide frequency range (50-2000 cm^-1) at different temperatures. The best fit to the Raman spectrum at 77 K is achieved using 17 Lorenzians to convolute into it, and this is proved to be a reasonable fit. According to the group theory and selection rules of overtone and combinational modes, apart from the seven Raman modes that are from first-order Raman scattering, the remaining ones are attributed to being from second-order Raman scattering. A comparison between the experimental results and theoretical predictions shows that they are in satisfactory agreement with each other. Our results indicate that at 77 K the sample belongs to the rhombohedral symmetry with the C_3v⁵ (R3m) space group (Z=1). In our study, on heating, the 0.65PMN-0.35PT single crystal undergoes a rhombohedral to tetragonal to cubic phase transition sequence. The two phase transitions occur at 340 and 440 K, which correspond to the disappearance of the soft mode near 106 cm^-1 recorded in VV polarization and the vanishing of the band around 780 cm^-1 in VH polarization, respectively.     

Influence of pressure on the electric-field-induced phase transitions in liquid crystals

Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T^*_AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the quadrupolar nematic ordering Q_ij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with pressure.     

Beauty,charm, and <Emphasis Type="Italic">F</Emphasis><Subscript><Emphasis Type="Italic">L</Emphasis></Subscript> at HERA: New data vs. Early predictions

One of the well-known effects of the asymptotic freedom is splitting of the leading-log BFKL pomeron into a series of isolated poles in complex angular momentum plane. Following our earlier works we explore the phenomenological consequences of the emerging BFKL-Regge factorized expansion for the small-x charm (F ₂ ^c ) and beauty (F ₂ ^b ) structure functions of the proton. As we found earlier, the colordipole approach to the BFKL dynamics predicts uniquely decoupling of subleading hard BFKL exchanges from F ₂ ^c at moderately large Q ². We predicted precocious BFKL asymptotics of F ₂ ^c (x,Q ²) with intercept of the rightmost BFKL pole α _P(0) − 1 = Δ_P ≈ 0.4. High-energy open beauty photo- and electroproduction probes the vacuum exchange at much smaller distances and detects significant corrections to the BFKL asymptotics coming from the subleading vacuum poles. In view of the accumulation of the experimental data on small −x F ₂ ^c and F ₂ ^b we extended our early predictions to the kinematical domain covered by new HERA measurements. Our structure functions obtained in 1999 agree well with the determination of both F ₂ ^c and F ₂ ^b by the H1 published in 2006 but contradict very recent (2008, preliminary)H1 results on F ₂ ^b . We present also comparison of our early predictions for the longitudinal structure function F _L with recent H1 data (2008) taken at very low Bjorken x. We comment on the electromagnetic corrections to the Okun-Pomeranchuk theorem.     

Reentrant Dipole Glass-Like Ordering in Weakly Concentrated KTaO3:Nb

Recently we reported that a very small admixture of Li^? dipole impurities into moderately concentrated KTaO₃:Nb leads to an unusual sequence of ferroelectric-type phase transitions with a reentrant glass phase formation at the lowest temperatures [1], Here, we present dielectric permittivity and Raman scattering study for nearly the same composition as used in Ref. [1], but without Li admixture, for KTa_0.982Nb_0.018,O₃ (KTN1.8). It is found that the presence of strong Li dipole defects is not an essential condition for the reentrant glass formation. Above T_c (27 K) KTN1.8 behaves as a conventional soft mode displacive type ferroelectric in paraelectric phase. However, below T_c a crossover to the order-disorder polar microregion dynamics and a state at which a reentrant glass phase coexists with long-range ordering occurs.     

超新星爆发环境核素56,57,59,60Co的电子俘获

基于壳模型与Random Phase Approximation理论, 利用Shell-Model Monte Carlo方法, 研究了超新星爆发环境核素^56,57,59,60Co的电子俘获与电子丰度变化率. 我们的结果与利用Aufderheide方法计算的结果进行了误差对比. 结果表明: 电子俘获率受温度和密度的影响大大增加, 甚至增加达6个数量级以上(如在ρ₇=0.43, Y_e=0.48核素^57,59,60Co). 另一方面, 随着温度和密度的增大, 电子丰度变化率大大降低, 甚至减小达5个数量级以上(如在ρ₇=5.86, Y_e=0.47核素⁵⁹Co). 通过对误差因子的分析表明, 在低温低密度环境二种结果误差较大; 而在高温高密度环境, 二种结果误差相对较小. 关键词： 电子俘获 超新星     

Temperature‐dependent vibrational studies of [N(C2H5)4]2MnCl4

Room‐temperature polarized Raman spectra of a single crystal and IR spectra of a polycrystalline sample were measured for [N(C₂H₅)₄]₂MnCl₄ and the assignment of the observed bands to the respective modes has been proposed. Temperature‐dependent Raman and far‐IR studies were also performed for the polycrystalline sample in order to obtain information on changes occurring in this material as a result of phase transitions at T₁ = 227 K and at T₂ = 199 K. These studies revealed that the higher‐temperature ferroelastic phase transition is associated with significant modification of vibrational properties due to ordering of tetraethylammonium groups. The lower‐temperature phase transition does not lead to any clear changes in the spectra. However, our results suggest that disorder of MnCl₄²⁻ ions decreases with decreasing temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Quantifying cross-correlations using local and global detrending approaches

In order to quantify the long-range cross-correlations between two time series qualitatively, we introduce a new cross-correlations test Q_CC(m), where m is the number of degrees of freedom. If there are no cross-correlations between two time series, the cross-correlation test agrees well with the χ²(m) distribution. If the cross-correlations test exceeds the critical value of the χ²(m) distribution, then we say that the cross-correlations are significant. We show that if a Fourier phase-randomization procedure is carried out on a power-law cross-correlated time series, the cross-correlations test is substantially reduced compared to the case before Fourier phase randomization. We also study the effect of periodic trends on systems with power-law cross-correlations. We find that periodic trends can severely affect the quantitative analysis of long-range correlations, leading to crossovers and other spurious deviations from power laws, implying both local and global detrending approaches should be applied to properly uncover long-range power-law auto-correlations and cross-correlations in the random part of the underlying stochastic process.     

High-purity Zirconium under Niobium ion implantation: possibility of a dynamic precipitation?

High-purity (6N purity) Zirconium was subjected to different Niobium ion (Nb⁺) fluences in a particle accelerator. Grazing incidence X-ray diffraction confirmed subsurface phase transformation. While an approximate scaling was noted between Nb⁺ fluence and quantum of phase transformation, the sample subjected to the highest ion irradiation also showed significant subsurface shear residual stresses (τ₁₃). Molecular dynamics simulations, considering momentum transfer, revealed a drop in τ₁₃ beyond a critical displacements per atom or Nb⁺ fluence. High-resolution cross-sectional transmission electron microscopy (HRXTEM) confirmed formation of bcc (body-centred cubic) β phase and also linked τ₁₃ with such transformation. HRXTEM revealed, at about 100 nm depth, presence of 10–15% β with 2–15 nm size. The β particles and the surrounding α also had significant microscopic shear strains. The dynamic nature of the Nb implantation is expected to create fluctuations in temperature, Nb concentration and relative lattice damage. Such fluctuation, on the other hand, is/was hypothesized to dynamically alter the critical nuclei size: a clear possibility of ‘dynamic precipitation’.     

Low Temperature Phase Diagrams¶of Fermionic Lattice Systems

We consider fermionic lattice systems with Hamiltonian H=H ^{(0)}+λH _Q , where H ^{(0)} is diagonal in the occupation number basis, while H _Q is a suitable “quantum perturbation”. We assume that H ^{(0)} is a finite range Hamiltonian with finitely many ground states and a suitable Peierls condition for excitations, while H _Q is a finite range or exponentially decaying Hamiltonian that can be written as a sum of even monomials in the fermionic creation and annihilation operators. Mapping the d dimensional quantum system onto a classical contour system on a d+1 dimensional lattice, we use standard Pirogov–Sinai theory to show that the low temperature phase diagram of the quantum system is a small perturbation of the zero temperature phase diagram of the classical system, provided λ is sufficiently small. Particular attention is paid to the sign problems arising from the fermionic nature of the quantum particles. As a simple application of our methods, we consider the Hubbard model with an additional nearest neighbor repulsion. For this model, we rigorously establish the existence of a paramagnetic phase with commensurate staggered charge order for the narrow band case at sufficiently low temperatures. Received: 23 December 1996/ Accepted: 7 April 1999     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.     

Stochastic fluctuations of persistent currents in mesoscopic rings

We investigate systematically the effect of the electronic Coulomb interaction on the stochastic fluctuations (from sample to sample) in the persistent currents I(Φ) of mesoscopic rings threaded by a magnetic flux Φ. In contrast to our recent publication [Europhys. Lett. 18 (1992) 457], we avoid separation of impurity and interaction scattering events and we present here a comprehensive view. By this alternative procedure, we are able to confirm our earlier result, namely that 〈I²〉^1/2 ～ ev_F/L, which is a much larger quantity than the average current 〈I〉 ～ ev_Fl/L².     

Spanning Forests and the q-State Potts Model in the Limit q →0

We study the q-state Potts model with nearest-neighbor coupling v=e^βJ−1 in the limit q,v → 0 with the ratio w = v/q held fixed. Combinatorially, this limit gives rise to the generating polynomial of spanning forests; physically, it provides information about the Potts-model phase diagram in the neighborhood of (q,v) = (0,0). We have studied this model on the square and triangular lattices, using a transfer-matrix approach at both real and complex values of w. For both lattices, we have computed the symbolic transfer matrices for cylindrical strips of widths 2≤ L ≤ 10, as well as the limiting curves B of partition-function zeros in the complex w-plane. For real w, we find two distinct phases separated by a transition point w=w₀, where w₀ =−1/4 (resp. w₀=−0.1753 ± 0.0002) for the square (resp. triangular) lattice. For w>w₀ we find a non-critical disordered phase that is compatible with the predicted asymptotic freedom as w → +∞. For w0 our results are compatible with a massless Berker–Kadanoff phase with central charge c=−2 and leading thermal scaling dimension x_T,1 = 2 (marginally irrelevant operator). At w=w₀ we find a “first-order critical point”: the first derivative of the free energy is discontinuous at w₀, while the correlation length diverges as w↓ w₀ (and is infinite at w=w₀). The critical ehavior at w=w₀ seems to be the same for both lattices and it differs from that of the Berker–Kadanoff phase: our results suggest that the central charge is c=−1, the leading thermal scaling dimension is x_T,1=0, and the critical exponents are ν=1/d=1/2 and α=1.     

Nonlinear dynamics of a two-dimensional lattice

The dynamics of the nonlinear excitations in a two-dimensional (2D) φ⁴-diatomic lattice, with nonlinear on-site electron-phonon coupling at the polarizable ion site has been presented, without considering the self consistent phonon approximation. One of the major results obtained from our calculations is in the understanding of continuous structural phase transition, where we have obtained the minimum in soft mode frequency at a soft mode temperatureT _s (>T _c), not at critical temperatureT _c. This occurs due to the anisotropy of such 2D systems.     

Quantum phase diagram for homogeneous Bose‐Einstein condensate

We calculate the quantum phase transition for a homogeneous Bose gas in the plane of s‐wave scattering length a_s and temperature T. This is done by improving a one‐loop result near the interaction‐free Bose‐Einstein critical temperature T_c⁽⁰⁾ with the help of recent high‐loop results on the shift of the critical temperature due to a weak atomic repulsion based on variational perturbation theory. The quantum phase diagram shows a nose above T_c⁽⁰⁾, so that we predict the existence of a reentrant transition above T_c⁽⁰⁾, where an increasing repulsion leads to the formation of a condensate.     

The BFKL-Regge factorization and F2b , F2c , FL at HERA: physics implications of nodal properties of the BFKL eigenfunctions

The asymptotic freedom is known to split the leading-log BFKL pomeron into a series of isolated poles in the complex angular momentum plane. One of our earlier findings was that the subleading hard BFKL exchanges decouple from such experimentally important observables as small-x charm F ₂ ^c , beauty F ₂ ^b and the longitudinal structure functions of the proton at moderately large Q ². For instance, we predicted precocious BFKL asymptotics of F ₂ ^c (x, Q ²) with intercept of the rightmost BFKL pole a _IP(0) − 1 = Δ_IP ≈ 0.4. On the other hand, the small-x open beauty photo- and electro-production probes the vacuum exchange for much smaller color dipoles which entails significant subleading vacuum pole corrections to the small-x behavior. In view of the accumulation of the experimental data on small-x F ₂ ^c and F ₂ ^b we extend our 1999 predictions to the kinematical domain covered by new HERA measurements. Our parameter-free results agree well with the determination of F ₂ ^c , F _L and published H1 results F ₂ ^b on but slightly overshoot the very recent (2008, preliminary) H1 results on F ₂ ^b .