Low-temperature phase and magnetic interactions in FCC Fe-Cr-Ni alloys

The low-temperature (5 K < T < 300 K) magnetic properties of a set of nine isostructural fcc Fe-Cr-Ni (Fe ≈ 68 at %, Cr ≈ 20 at%, Ni ≈ 9 at%) alloys were studied by SQUID magnetometry, neutron diffraction and ultrasonic techniques. Type-1 antiferromagnetic (AF) ordering was observed below the Néel temperature, T_N. The dc susceptibility, χ(T), did not exhibit a simple Curie-Weiss dependence. Above T_N, atemperature independent component χ₀ was observed, i.e., χ(T) = χ₀ + C / (itT + θ. T_N was systematically influenced by the lattice parameter, a, decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ_B obtained from neutron scattering was lower than the ≈ 1 μ_B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction (J₁) and ferromagnetic second-nearest-neighbors interaction (J₂) indicate that |J₂/J₁|≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.     

Phase transitions in the superionic protonic conductor CsHSeO4 investigated by Brillouin spectroscopy

A Brillouin investigation in CsHSeO₄ has been performed over the temperature range 20–165 °C which includes two phase transitions, in particular the transition to the superionic phase near T_s = 129 °C. We observed strong discontinuities for elastic constants C₁₁, C₂₂ and C₃₃ at T_s and a broadening of the Brillouin lines above T_s. The results are discussed on the basis of a linear coupling between strains and mobile protons.     

On the rotational partition function for tetrahedral molecules

A rigorous calculation of the quantum-mechanical rotational partition function for tetrahedral XY₄ molecules yields Q_r = ( )(2I_Y + 1)⁴π ^- exp(/4), where I_Y is the spin of the Y nucleus, and ≡ Bhc/kT. This result is accurate to 1 per cent or better for all values of B and T such that < .     

Effect of a magnetic field on MnAs0.88P0.12 below 80 K

MnAs_0.88P_0.12 has been studied by powder neutron diffraction in external magnetic fields up to 15.2 kOe and temperatures down to 4.2 K. MnAs_0.88P_0.12 takes the MnP type atomic arrangement and exists in para-, ferro- and different (essentially) helimagnetic states. The observation of a double 000^± satellite at 4.2 K < T 70 K adds further evidence to the chain of arguments for distinction between the helimagnetic states H'_a (4.2 K < T < T_S,1 ≈ 70 K) and H_a (T_S,2 ≈ 180 K < T < T_N = (243 ± 5) K). External magnetic fields at 4.2 K < T < 70 K evoke a new magnetic state, which is also characterized by a satellite doublet, and is tentatively designated H'_a,fan.     

The angular intensity correlation functions C(1) and C(10) for the scattering of light from randomly rough dielectric and metal surfaces

We study the statistical properties of the scattering matrix S(q|k) for the problem of the scattering of light of frequency ω from a randomly rough one-dimensional surface, defined by the equation x₃=ζ(x₁), where the surface profile function ζ(x₁) constitutes a zero-mean, stationary, Gaussian random process. This is done by studying the effects of S(q|k) on the angular intensity correlation function C(q,k|q',k')=〈I(q|k)I(q'|k')〉-〈I(q|k)〉〈I(q'|k')〉, where the intensity I(q|k) is defined in terms of S(q|k) by I(q|k)=L^-1₁(ω/c)|S(q|k)|², with L₁ the length of the x₁ axis covered by the random surface. We focus our attention on the C⁽¹⁾ and C⁽¹⁰⁾ correlation functions, which are the contributions to C(q,k|q',k') proportional to δ(q-k-q'+k') and δ(q-k+q'-k'), respectively. The existence of both of these correlation functions is consistent with the amplitude of the scattered field obeying complex Gaussian statistics in the limit of a long surface and in the presence of weak surface roughness. We show that the deviation of the statistics of the scattering matrix from complex circular Gaussian statistics and the C⁽¹⁰⁾ correlation function are determined by exactly the same statistical moment of S(q|k). As the random surface becomes rougher, the amplitude of the scattered field no longer obeys complex Gaussian statistics but obeys complex circular Gaussian statistics instead. In this case the C⁽¹⁰⁾ correlation function should therefore vanish. This result is confirmed by numerical simulation calculations.     

Influence of magnetization on lattice constants of 3d transition metal alloys

A simple relation is found in 3d transitional metal alloys between the lattice constant and the magnetization, which can be described with an equation: a(x) = a₀^A· (1 − x) + a₀^B. x + C μ(x). It is proposed that studies of lattice constants at high temperatures may serve as an experimental method to detect the existence of localized moments above T_c. The anomalous thermal expansion of the Invar alloy is explained as a result of the collapse of localized moments above T_c.     

Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

We have studied the stationary Josephson effect on YBa₂Cu₃O_7−δ (T_c=90 K) and Bi₂Sr₂Ca₁Cu₂ O₈ (T_c=80 K and 87 K for two samples of different origin) ceramic based junctions. The temperature dependence of the critical current near T_c has been found as I_c≈(T_c-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi₂Sr₂Ca₁Cu₂O₈, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/k_BT_c3.5.     

Narrowing of the TiBe2 CESR line at temperatures below 2 K: the case for thermal spontaneous ferromagnetism

A narrowing of the TiBe₂ CESR line has been found within the temperature range T≈1.55−1.8K at the frequency 595 MHz (field 220 G). This narrowing is shown to be mostly of the exchange type, and we propose that it is a manifestation of the weak thermal spontaneous ferromagnetism in the above mentioned T range with ΔT≈0.25 K with the possibility of two first order transitions. The theoretical and experimental basis for this conclusion and its implications for the understanding of the properties of TiBe_2−xCu_x, Sc₃In and ZrZn₂ itinerant magnets are discussed.     

Measurement of the slope parameter for the neutral pion spectrum in KL → πππ decay

The π⁰ spectrum in the K_L⁰ → 3π decay was measured using a wire chambers magnetic spectrometer. In the usual approximation, the matrix element can be expressed as: M² ≈ 1 + 2a₀(M_K/M_π²)(2T_π⁰−T_π⁰max) + a₁(M_K²/M_π⁴)(2T_π⁰−T_π⁰max)². We obtained a₀ = −0.282 ± 0.011 and a₁ consistent with zero.     

Entropy of planar random surfaces on the lattice

We study planar random surfaces on a hypercubic lattice in two and three dimensions by Monte Carlo techniques. Our data are consistent with the formula n₀(A;C) A^b₀A, where n₀(A;C) is the number of planar random surfaces with area A and boundary C. We find b₀ = −1.4 ± 0.2, = 5.31 ± 0.03 (for d = 2) and b₀ = −1.5 ± 0.2, = 7.13 ± 0.05 (for d = 3). The values of b₀ disagree with those obtained from the Polyakov string model.     

Influence of high magnetic fields on the low temperature heat capacity of the itinerant electron ferromagnet Sc3In

The low temperature (1.3–20 K) heat capacity of the weak itinerant electron ferromagnet Sc₃In was measured in magnetic fields up to 10 T. The measurements were made on three different samples containing 24.1, 24.3 and 24.4 at.% In with Curie temperatures at H = 0 of 5.5, 6.0 and 6.3 K, respectively. The heat capacity peak at T_c becomes smaller with increasing fields and at 9.98 T the magnetic entropies are only 11 to 19% of the zero field value. Above T_c the spin fluctuation contribution to heat capacity, which is enhanced by the magnetic field at low fields (<≈ 5 T), is quenched at high fields ( ≈ 5 T). Our results show that the spin fluctuations in Sc₃In are almost completely quenched by the magnetic fields of ≈ 10 T, and that the characteristics spin fluctuation temperature, T_s, of this itinerant electron ferromagnet is about the same as T_c.     

Generalized Einstein manifolds

A Finslerian manifold is called a generalized Einstein manifold (GEM) if the Ricci directional curvature R(u,u) is independent of the direction. Let F⁰(M, g_t) be a deformation of a compact n-dimensional Finslerian manifold preserving the volume of the unitary fibre bundle W(M). We prove that the critical points g₀ F⁰(g_t) of the integral I(g_t) on W(M) of the Finslerian scalar curvature (and certain functions of the scalar curvature) define a GEM. We give an estimate of the eigenvalues of Laplacian Δ defined on W(M) operating on the functions coming from the base when (M, g) is of minima fibration with a constant scalar curvature H admitting a conformal infinitesimal deformation (CID). We obtain λ ≥ H/(n − 1) (Δf = λf). If M is simply connected and λ = H/(n − 1), then (M, g) is Riemannian and is isometric to an n-sphere. We first calculate, in the general case, the formula of the second variationals of the integral I (g_t) for G = g₀, then for a CID we show that for certain Finslerian manifolds, I″(g₀) > 0. Applications to the gravitation and electromagnetism in general relativity are given. We prove that the spaces characterizing Einstein-Maxwell equations are GEMs.     

Temperature dependence of intensities and widths of N2-broadened lines in the 15 μm CO2 band from tunable laser measurements

Intensities and nitrogen-broadened widths of several low-J lines in the Q-branch of the 15 μm band of CO₂ have been determined over the temperature range 200–300 K. Measurements were made with a tunable infra-red diode laser spectrometer having a spectral resolution 10^-4 cm^-1. Measured intensities are uniformly about 7% lower than available calculations which were based on previous measurements of band intensity. Measured line widths are higher than available calculations and generally followed the relation

b_L⁰(T)=b_L⁰(T₀)(T₀|T)ⁿ

with n = 0.74 (standard deviation 0.08).     

Incommensurate antiferromagnetic structures in NdIn3

Among the AuCu₃-type RIn₃ series where R is a rare earth, the NdIn₃ compound presents a complex magnetic phase diagram. In this compound, which orders antiferromagnetically at T_N = 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 NdIn₃, 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 T_N T 5.5 K, τ₁ ≈ 0.037 2π/a and the structure is sine-wave modulated, for 5.5 K > T > 4.7 K, τ₂ = 0.017 2π/a and the structure squares up. Below 4.7 K the q = (1/2, 1/2, 0) commensurate structure is stabilized.     

Field-induced parameters of re-entrant magnets and concentrated spin glasses

We have used electron spin resonance measurements to derive the temperature and frequency dependences of the field-induced magnetization [M(T, f)] and anisotropy field [H_an (T)] in a number of amorphous alloys belonging to the series (Fe_pNi_1−p)₇₅P₁₆B₆Al₃. In re-entrant (p > p_c, the critical concentration for ferromagnetism) alloys at hi gh frequencies (f = 35 GHz, field ≈ 12 kOe) M reduces as T^3/2 at high T and as T below ≈ 40 K, the deviation from T^3/2 becoming more marked as p → p⁺_c. For p close to p_c, lowering the frequency first causes the T term to increase and ultimately ( ≈ 4 GHz) changes the variation of M with T to that discovered previously for concentrated spin glasses, namely M is constant at low T and drops linearly at high temperatures. For the re-entrants, the results are interpreted on the basis of a model which invokes an energy gap in the spin-wave spectrum, introduces a non-zero density of states of the gap energy and takes into consideration a low-q cut-off in the spin-wave integral in thelow-T (T) regime.In the concentrated spin glasses [M (0) - M (T)]/ M (0) is well represented by the function [exp (Δ / T) - 1]^-1, where Δ has values close to the corresponding Curie-Weiss temperatures θ_p but much larger than the respective spin glass transition temperatures T_SG. The temperature dependence of H_an is largely given by the function (1 - T/T^*), where T^* is equal to the zero-field freezing temperature for the re-entrants and T_SG for the spin glasses, respectively.     

Conductivity peaks broadening in the quantum Hall regime

We argue that it is the hopping transport that is responsible for broadening of the σ_xx peaks. Explicit expressions for the width Δν of a peak as a function of the temperature T, current J and frequency ω are found. It is shown that Δν grows with T as (T/T₁)^κ, where κ is the inverse localization-length exponent. The current J is shown to act like the effective temperature T_eff(J) ∝ J^1/2 if . Broadening of the ohmic ac-conductivity peaks with frequency ω is found to be determined by the effective temperature      

Intergranular properties of YBCO and BSCCO ceramic superconductors at low fields

Isothermal low-field AC susceptibility measurements have been used to analyze the intergranular critical current density J_c(T) on sintered, non-oriented YBa₂Cu₃O_7−δ and Bi₂Sr₂CaCu₂O_8+δ ceramic samples at zero field. Below the critical temperature, potential variations, J_c(T) ≈ (1−t_j)^m with t_j = T/T_j, have been found, T_j being the onset of grain's coherence, but with different exponents, supporting that different mechanisms limit the intergranular J_c values. Moreover, the effect of texture has been also considered on Bi₂Sr₂CaCu₂O_8+δ ceramics grown by the laser floating zone method, which have stronger intergranular junctions. Their high-temperature behaviour is limited by intrinsic effects, while at low temperatures the quality of the junctions is the limiting factor. The temperature dependence of the χ′(h₀) extrapolation at zero filed has also been correlated with the evolution of the intergranular penetration depth, λ_J(T).     

Effect of Pr and Ca substitution on superconductivity in Y(1−x−yPrxCay)Ba2Cu3O7−δ

Pr substituted at constant Ca concentration for Y in (Y_1−x−yPr_xCa_y)Ba₂Cu₃O_7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of T_c decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of T_c cannot be compensated by appropriate hole doping with Ca.     

Normal state magnetism of the high Tc cuprate superconductors

Magnetic measurements of various types have played an essential role in establishing the novel normal state characteristics of high transition temperature (T_c) superconductors with T_c > 23 K. Among these materials, the highest T_c's ( 125 K) are exhibited by the layered cuprates. In this paper, the normal state magnetic susceptibilities of the cuprates are reviewed and interpreted in the context of magnetic neutron scattering and other magnetic measurements, using the La_2−xM_xCuO₄-type and YBa₂Cu₃O_6+x-type materials as prototypical examples. The evolution of the magnetism upon doping the insulating antiferromagnetic “parent” compounds with x = 0 to form the high temperature superconductors is described. A recurrent property which differentiates these materials from conventional superconductors is the existence of strong antiferromagnetic correlations in the metallic state on the same sublattice of the structure in which the itinerant carriers reside.     

A study on the nonstoichiometry of tin oxides by coulometric titration

In this investigation, nonstoichiometries and defect structures of tin oxides were studied between 694 and 990 K by coulometric titration using solid state electrolyte (YSZ) cells. The relationship between nonstoichiometry of the oxide (x) and equilibrium oxygen partial pressure (P_o2) was expressed by the proportionality: xP_O2^−1/6. An intermediate oxide phase, Sn₃O₄ between Sn and SnO₂ was observed in the temperature range of 696–732 K. The standard Gibbs energy of formation of Sn₃O₄ via the reaction; was found to be ΔG^o_Sn3O4 = −1163960+417.36 T (J/mol). The standard Gibbs energy change for the defect formation reaction in SnO_2−x was calculated to be ΔG^o_SnO2−x = 3.05×10⁵−38.97 T (J/mol)).