On the frequency dependence of the transition temperature in spin glasses

For the first time, the frequency dependence of T_f (temperature of the maximum of the a.c. susceptibility of spin-glasses) is shown to obey a Fulcher law $\text{τ = τ}{}_{\mathrm{o}}\text{exp}\text{[}\text{E}{}_{\mathrm{a}}\text{k(}\text{T}{}_{\mathrm{f}}\text{?}\text{T}{}_{\mathrm{f}}\text{)}\text{]}$. This is observed as well in the case of dilute alloys (or R.K.K.Y. spin-glasses : $\text{Cu}$Mn, $\text{Au}$Fe, …) as for frustrated systems (Eu_1?xGd_xS, Eu_xSr_1?xS …). For R.K.K.Y. spin-glasses, only in the case of a very small amplitude, V_o of the R.K.K.Y. interaction, this time dependence approaches an Arrhenius law. In the case of “frustrated” spin-glasses the concentration is the main parameter to determine the kind of frequency dependence of T_f. These properties are evidence for a glass-like phase transition in spin-glasses. The scaling of the frequency dependence of T_f with V_o is justified for R.K.K.Y. spin-glasses from present data.     

Light scattering study of dispersion and attenuation of hypersound in adamantane

The Brillouin scattering techniques have been used to measure the velocity dispersion of hypersonic acoustic waves in the “high temperature” disordered cubic phase of adamantane. Shear waves, characteristic of the C₄₄ elastic constant, show no significant dispersion. Longitudinal waves propagating in the (001) plane show strong velocity dispersion. The measures have been performed at the same temperature T = 295.7 K. Using a classical single relaxation time model for the dispersion as a function of frequency at temperature T, the L-mode data have been correctly fitted.The importance of the dispersion $\text{(C}{}_{\mathrm{\infty }}\text{? C}{}_{\mathrm{o}}\text{)}\text{C}{}_0$ for the elastic constants is 20% for C₁₁, 51% for C₁₂ #1% for C₄₄ and ?2.8% for $\text{(C}{}_{11}\text{? C}{}_{12}\text{)}\text{2}$. The fitted relaxation time is τ ? 9 × 10^?11 sec.     

NMR study of one-dimensional ionic conductor with hollandite-type structure. II. Frequency dependence of spin-lattice relaxation time of 27Al

Frequency dependence of spin-lattice relaxation time T₁ of ²⁷Al in one-dimensional K⁺ ion conductor, K-Al-priderite, was measured at 45 K in the frequency range from 10.1 MHz to 55 MHz. It is found that T₁ is proportional to ω^1.49±0.05 and agrres well with the $\text{ω}{}^{\mathrm{<mtext>3</mtext><mtext>4</mtext>}}$ dependence derived by the continuum diffusion model. The intrinsic activation energy is determined to be 0.058 eV by doubling the slope E_NMR=0.029 eV of the d(ln T₁)/dT curve in the low temperature region. The frequency dependence of T₁ in the high temperature region measured in the frequency range from 11.5 MHz to 20.8 MHz shows a tendency that the frequency dependence becomes smaller than the $\text{ω}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence as temperature is raised above 450 K.     

Pulsed excitation of luminescence in dihydrated oxalic acid

Luminescence has been excited by an a.c. electric field in oxalic acid dihydrate at temperatures 273, 283 and 293 K. The voltage and frequency dependence of the emitted light flux has been studied. The brightness (B) has been observed to increase with voltage (V_rms) and frequency (v). The relation b = b₀ exp(-b/V^{$\text{1}\text{2}$_rms)} describes the voltage dependence of brightness quite accurately at frequencies greater than 500 Hz. The brightness wave forms at different voltages and frequencies of the applied field have also been studied. One primary and one secondary peak have been observed in each half cycle of the applied sinusoidal voltage.     

Johnson noise at low temperature

It has long been accepted that Johnson noise in metals is proportional to absolute temperature, and is materials-independent. This is a semi-classical result, based on the equipartition of energy. Johnson noise in copper below 10 mK suggests a departure from this relation. The data have been fitted empirically to T_J = T_o coth $\text{T}{}_{\mathrm{<mtext>o</mtext>}}\text{T}$, where T_J is the “Johnson noise” temperature, T is measured by CMN, and T_o is chosen for best fit and is about 2 mK. T_o is now identifiable with a zero-point frequency of electronic charge imbalance, which moves with average Fermi velocity ν from end to end of the resistor of length l. $\text{T}{}_{\mathrm{<mtext>o</mtext><mtext>\; =\; </mtext><mtext>h\nu </mtext><mtext>2kl\; \approx \; 2\; </mtext><mtext>mK</mtext>}}$, and so is materials- and geometry-dependent.     

Optical studies of the isotropic point in (CnH2n+1NH3)2ZnX4 crystals with 10⩽n⩽14 and X4Cl4,Cl2Br2

The dispersion of the optic axial angle of the (C_nH_2n+1NH₃)₂ZnX₄ crystals with 10?n?14 and X₄Cl₄,Cl₂Br₂ has been investegated between 273 K and 363 K in the spectral interval from 470 nm. to 650 nm. The experimental results show, together with a high anomalous optical dispersion, the axistence of an isotropic point. At this point the optical plane rotates 90$\text{o}$^- around the acute bisectrix. A dependence of the isotropic point temperature with n, X, and the wave length is also found.     

Heat capacity of [Ni(OCH3)(acac)(CH3OH)]4 from 0.4 to 285 K: Spin interaction and tunnel-splitting of internal rotation of methyl group

The heat capacity of the tetranuclear nickel cluster complex, tetrakis [μ₃-methoxo-2,4-pentanedionato (methanol) nickel (2)], has been measured from 0.4 to 285 K. Contrary to the previous prediction by Bertrand et al.[6], that this complex exhibits an intercluster ferromagnetic spin coupling, the present heat capacity measurement shows no indication of the spin-ordering effect caused by the intercluster interaction at least down to 0.4 K. Instead, a heat capacity anomaly centered around 1.5 K with a shoulder at 0.5 K has been observed. This anomaly is well accounted for in terms of both a level splitting of the ground S = 4 state due to a uni-axial crystalline anisotropy and a tunnel-splitting of the rotational levels of methyl groups. The intracluster spin exchange constant J and the single-ion zero-field splitting parameter D are determined to be $\text{J}\text{hc}\text{= 7.0 cm}{}^{\mathrm{?1}}$ and $\text{D}\text{hc}\text{= 3.65 cm}{}^{\mathrm{?1}}$ (h being the Planck constant and c the speed of light in a vacuum). The temperature dependence of the effective Bohr magneton [6] is also satisfactorily accounted for on the basis of this model. The tunnel-splitting δ of the lowest rotational level of the four methyl groups belonging to methoxides is estimated to be $\text{δ}\text{hc}\text{= 0.4–0.7 cm}{}^{\mathrm{?1}}$ and the corresponding potential barrier V_o is found to be V_o = 1.9–1.5 kJmol^?1.     

Raman study of charge-density-wave phase transition in quasi-one-dimensional conductor (TaSe4)2I

Polarized Raman spectra were obtained in the quasi-one-dimensional conductor (TaSe₄)₂I above and below the charge-density-wave (CDW) transition temperature (T_c=263 K). The Raman intensities of many peaks become intenser and two of the phonon peaks shift to higher frequency with decreasing temperature. Moreover a new broad peak at about 90 cm^?1 and a new peak around 166 cm^?1 appear in the low-temperature phase. The polarization characteristic shows that the former is assigned to totally symmetric mode. The damping constant of the phonon at 90 cm^?1 increases markedly with increasing temperature. The frequency shifts to higher frequency as the temperature increases and the coupling coefficient is approximately proportional to (T_c?T)^{$\text{1}\text{2}$}. This peak becomes Raman active owing to the CDW phase transition. The temperature dependence of the damping constant and the frequency shift may have a relation to the dynamical properties of the CDW phase transition.     

A thermodynamic study of nonstoichiometric cerium dioxide

Thermogravimetric measurements were performed on nonstoichiometric CeO_2?x in the temperature range 750–1500°C and from oxygen pressures of 10^?2 to 10^?26 atm. From this data the deviation from stoichiometry x = x(T, P_o2) was determined. The thermodynamic quantities Δ$\text{H}$_o2 and Δ$\text{S}$_o2 were calculated in the region 0.001? x ? 0.3 and found to be independent of temperature.In the composition region 0.001< x < 0.006, the variation of Δ$\text{S}$_o2 with x is consistent with a defect model involving randomly distributed doubly ionized oxygen vacancies. The experimental dependence of x and σ (electrical conductivity) is shown to be consistent with this model as Δ$\text{H}$_o2 (≈ -10 eV) exhibits a slight dependence on x. It is postulated that the variation in Δ$\text{H}$_o2 may result from lattice parameter increases with x, while the defects remain essentially randomly distributed.In the composition region 0.006 < x < 0.1, x ∝ with 1 < n < 5, and in the region 0.1 < x < 0.3, x ∝ with n increasing rapidly with x to n? 30. This behavior is believed to result from increasing defect interaction with increasing departures from stoichiometry. It is interesting to note that the ordered phase observed by Bevan and Kordis between CeO_1·72 and CeO_1·70 was not observed in this study at temperatures between 1300° and 1500°C.     

The frequency dependence of the ac susceptibility of the PdMn spin glass

We have measured the frequency dependence of the ac susceptibility _X(T) for three alloys of $\text{Pd}$Mn in the spin-glass regime. A strong dependence of _X(T) on frequency is observed near and below the freezing temperature in contrast to the archetypal $\text{Cu}$Mn spin glass. We discuss possible reasons for these differences.     

Pressure dependence of the electric field gradient and knight shift tensors of single crystal gallium

Nuclear pure quadrupole resonance has been observed in single crystal gallium as a function of hydrostatic pressure up to 7 kbar at temperatures of 77, 198 and 273 K. The resonance frequency v_Q increases linearly with pressure and the slope ($\text{δv}{}_{\mathrm{O}}\text{δP}$)_T is 13.7, 15.5 and 16.3 Hz/bar at 77, 198 and 273 K respectively. The asymmetry parameter decreases nonlinearly with pressure. Using compressibility and thermal expansion data, the volume dependence of the major principle component of the electric field gradient was deduced. The principal components K_x, K_y and K_z of the Knight shift tensor have been measured as a function of pressure to 6 kbar at 77 K. The isotropic and anisotropic components of the Knight shift were deduced as a function of pressure, and K_iso is found to vary with volume as V^{4.95 ± 0.80}.     

Electrical conduction and 1ƒ noise in Li-DOPED MnO

Electrical resistivity, thermoelectric power and current noise were measured on Li-doped MnO single crystals in the temperature range from 300 to 1000 K. Below 700 K the crystals are p-type and the activation energy of the resistivity is 0.75 eV. Around 700 K the activation energy changes from 0.75 to 1.25 eV owing to a change from p- to n-type conduction. The depth of the Li acceptor is found to be 0.65 eV. From resistivity and thermoelectric power data it is concluded that the bandgap in first approximation can be written as E_s(T) = E_o ? γT between 750 and 1000 K, with E_o = 1.9 eV and γ = 6 × 10^?4 eV/K. The current noise spectra show $\text{1}\text{?}$ noise. The magnitude of the $\text{1}\text{?}$ noise is strongly temperature dependent. From the noise data it is deduced that E_o = 2.2 eV and γ = 10^?3 eV/K in the temperature range 430–700 K.     

Exchange energy for conduction electrons in (ZnMn)Se derived from spin-flip Raman scattering and magnetization

Spin-flip Raman scattering, magnetization, and susceptibility data for Zn_0.97Mn_0.03Se are reported. The exchange energy N_oα = 243 ± 10 meV for the conduction electrons is obtained from an analysis of the Raman and magnetization data. At large magnetic fields (H > 60 kOe), the spin-flip energy ΔE saturates at 14 meV. At low fields ΔE does not extrapolate to zero as H → 0, which is characteristic of scattering from donor-bound electrons. The low temperature magnetization curves are fit to a modified Brillouin function. The fit gives $\text{x}\text{?}\text{/x}\text{= 0.67}$ as the fraction of active magnetic ions, and an effective temperature T_eff = T + T_o with T_o = 1.1 K. The magnetic susceptibility follows a Curie-Weiss law between T = 150 and 280 K with a Curie-Weiss temperature θ = ?33 K.     

Conductivite electrique totale du protoxyde de cuivre en fonction de sa composition definie par des electrodes d'alliage or-cuivre

The total electrical conductivity of cuprous oxide has been measured from 800 to 1100°K as a function of its composition, especially in the range of very low oxygen pressures up to the limit of equilibrium with copper. These measurements have been carried out by means of a new method using copper-gold alloy electrodes in order to have given activities of components.The results indicate for cuprous oxide a p semi-conduction throughout its stability range.The conductivity σ (ohm^?1. cm^?1) is given as a function of the temperature T (K) and the partial pressure P_o2 (torr) of oxygen in equilibrium with the oxide by the following equation:

     

Energy dependence of the reactions KLop→KSop,π+Λ, π+Σo from 1540 to 1610 MeV

The reactions K_L^op→K_S^op, π⁺Λ, π⁺Σ^o have been measured for center-of-mass energies from 1540 to 1610 MeV. Channel cross sections and coefficients of the Legendre polynomial expansion of the differential cross sections and hyperon polarizations are presented. We see no evidence in the πΛ channel for the suggested $\text{3}\text{2}{}^{\mathrm{?}}$ resonance at 1580 MeV. The cross section for the K_S^op channel shows an energy dependence which is not predicted by the existing phase shift solutions based on charged kaon data.     

The anomalously high dielectric constant in narrow-gap semiconductor Cd0.17Hg0.83Te

The measurements performed at 4.2 K with a Rayleigh type UHF interferometer at a frequency ω/2π = 36.0 GHz, using circular polarization, have revealed the existence of an anomaly in the dielectric constant of Cd_0.17Hg_0.83Te (namely, æ ? 175) and demonstrated but a slight dependence of the value upon the magnetic field for $\text{h}\text{?}$ω_H～E_F.     

Inelastic neutron scattering study of magnetic excitations in the helimagnetic and antiferromagnetic phases of NiBr2

The spin wave dispersion in NiBr₂ has been studied by medium and long wavelength inelastic neutron scattering in the [1 1 0], [1 0 0] and [0 0 1] directions at 4.2 and 30 K, i.e. in the incommensurate helical and collinear antiferromagnetic phases. The values of the intralayer Heisenberg exchange constant J_ij and XY anisotropy constant D at 4.2(30) K are J₀₁ 0.379(1)(0.379(1)), J₀₂ 0.0036(50)(0.0036(50)), J₀₃ - 0.105(5) (?0.105(5)), J′ - 0.0423(50)(?0.389(50))D 0.0364(50)(0.0290(50)), where J′; is the interlayer exchange constant. In fitting the 4.2 K data account is taken of the co-existence of three equivalent domains and of intensity arising from $\text{ω(}\text{q}\text{)}$ and $\text{ω(}\text{q ± k}{}_0\text{)}$ where $\text{k}{}_0$ is the wavevector of the helix. In the low frequency region of the dispersion curve such peaks are resolved. The results reinforce the hypothesis that in zero-field the commensurate-incommensurate phase transition is driven by fluctuations.     

Spin waves in an Invar alloy (Fe0.65Ni0.35)

The spin wave dispersion relation in an Invar alloy Fe_0.65Ni_0.35 has been measured at 4.2 K in the [111] direction by neutron inelastic scattering.Well defined magnon groups have been observed up to an energy transfer of about 80 meV. The spin wave dispersion is well described by ?ω=Dq²(1?βq²) with $\text{D=143}\text{meV}\text{A}\text{?}$ and $\text{β=0.12}\text{A}\text{?}{}^2$. The value of D is in accord with the value extrapolated from other neutron scattering results at higher contents of Ni and disagrees with spin wave resonance results.No trace of γ-iron type antiferromagnetic order could be detected at 4.2 K in this alloy by elastic neutron scattering measurements.     

Raman scattering by two LO-phonons near Γ in GaAs

Near resonance the second order phonon spectra of semiconductors exhibit peaks which correspond to 2 LO phonons with wave-vector $\text{q}\text{→}\text{?o}$ (2 LO(Γ)). The frequencies of these 2 LO(Γ)-peaks are studied for photon energies varying across the E_o + Δ_o gap of GaAs. These peaks are shown to be slightly shifted to lower frequencies compared with the exact 2 LO(Γ) frequency. The peak positions when plotted as a function of the incident photon energy display an oscillatory behaviour which corresponds to the fact that LO-phonons of different wave vectors are created in the Raman process as the photon energy is tuned across the E_o+Δ_o resonance.