Metamagnetic-like transition in CeRu2Si2?

Extensive magnetization and magnetoresistivity experiments on CeRu₂Si₂ are described, with special emphasis on the metamagnetic-like transition that occurs atH*=80 kOe. By contrast to the low-field susceptibility, the differential susceptibility atH* almost diverges atT0. It is suggested that high magnetic fields (H>H*) restore a situation where the interactions between particles have collapsed. Comparison is made with other well-known examples: TiBe₂, UPt₃, TmSe, and³He.     

Transport properties and determination of the basic parameters of superconductors with overlapping bands

The thermoconductivity and ultrasonic attenuation in non-single-band super-conductors are investigated. It is proved that the ratio ₂/₁ atT T _c is equal to that atT=0. It is also shown that ultrasonic measurements and data on the phonon thermoconductivity can be used very effectively for definition of the basic parameters of the superconductors with overlapping bands.     

Mutual friction and vortex motion near the superfluid transition

The mutual friction parametersB and B for a moving vortex are calculated near the superfluid transition. They are proportional to the kinetic coefficient associated with the order parameter and, asT , diverge as (T – T)^–1/3, in agreement with experiment. The nonlinear couplings between the order parameter and the entropym, both the reversible one and the one in the free energy, are found to be crucial in the mutual friction near the point. These couplings were neglected in a previous paper by the author. First, the reversible coupling in the dynamic equations makes the chemical potential deviation long-ranged and causes the dissipation to take place only near the vortex core. Second,B can diverge asT T only in the presence of the coupling of the formm||² in the free energy. Thus theE model of Halperin et al., where the latter coupling is absent, cannot explain the critical anomaly ofB. The helical mode of a single vertex line is also investigated and its dispersion relation is found to be quite different from that at low temperatures. This mode has the same time scale as that of the second-sound mode when the wave vectors are of the order of the inverse correlation length, thus obeying the usual dynamic scaling law. The time correlation functions of the displacement fluctuations of a vortex line are also obtained. The force acting on a moving vortex is calculated and is found to be equal to the classical Magnus force.     

Electron spin resonance and optical absorption spectra of Cu2+ ions in Na2SO4-ZnSO4 glasses

Electron spin resonance (ESR) and optical absorption spectra of Cu²⁺ ions in Na₂SO₄-ZnSO₄ glasses have been studied. The ESR spectra of Cu²⁺ ion-doped glasses exhibited a pronounced peak atg=2.07 and a shallow quadruplet atg=2.35, the latter arising from the hyperfine splitting of g. ESR spectra of Cu²⁺ ion-doped glasses were also studied by varying the concentration of Cu²⁺ions, temperature and composition of the glasses. The optical absorption spectra exhibited a broad absorption band in the near infrared region, which is attributed to²B_1g²B_2g transition. By correlating the ESR and optical absorption data, the bonding orbital coefficients ² and ₁ ² for Cu²⁺ ions have been evaluated.     

Transport in the low-temperature limit of a Landau Fermi liquid

The Boltzmann equation for Landau quasiparticles is solved for T 0 by a specialization of a method discussed by Sykes and Brooker. The quasiparticle distribution function is expanded in Legendre polynomials, assuming a boundary condition which imposes axial symmetry, and even-order terms are assumed to relax together with relaxation time _e , odd-order terms with relaxation time _o . By letting wavelength , with finite, one obtains a first-sound solution, and by lettingT 0, and then , one obtains a zero-sound solution. When these solutions are used to calculate the pressure, it is found that the first-sound solution is consistent with hydrodynamics, exhibiting viscosity = _s , while the zero-sound velocityc ₁=[^–1(B₁+4/3_s)]^1/2, so that phenomenologically zero-sound propagates like a longitudinal elastic wave in a glass. A higher zero-sound mode is also predicted, but is heavily damped. The heat flux is calculated and found to obey Vernotte's equation, which contains an intertial term, added to Fourier's law, that becomes significant asT 0.     

Influence of the technological conditions on the luminescence of Eu3+ ions in Sr2SnO4

Different synthesis conditions have been used to prepare the stoichiometric compounds of the Sr2–xEuxSnO4 with x=0.00, 0.01, 0.02 and 0.03. As the 5D07F0 transition is forbidden by a selection rule of the Judd–Ofelt theory, the J=0J=0 level gains some intensity from the mixing of J states in the Sr2–xEuxKxSnO4 and Sr1–xEuxSnO4 samples. For Sr2–xEuxSnO4 samples prepared under a slightly reducing atmosphere, the J=0J=0 transition is split into two peaks, which confirms the substitution of Eu3+ ions for different crystallographic sites. © 1998 Chapman & Hall     

Defects on TiBiTe2 single crystals

Single crystals of rhombohedral TiBiTe₂ examined by transmission electron microscopy consist of large areas separated by {100}- and {110}-type twins. By measuring the rhombohedral angle from the splitting of diffraction patterns in the twin areas, we calculated the residual strain which remained after the transformation O_h D _3d, ranging from 1.2 to 12.2 %. Dislocations with Burgers vector (a/2) 1¯10 were mobile and the main slip system was (111) 1¯10. Planar defects with complex - fringe contrast were also observed.     

Optical and esr spectra of titanium (III) in Na2O-B2O3 and Na2O-P2O5 glasses

The optical absorption and the esr spectra of titanium(III) in binary Na₂O-B₂O₃ and Na₂O-P₂O₅ glasses have been studied. Titanium(III) produces two optical absorption bands around 20 000 and 14 000 cm^–1 which are assigned to the²B_2g ²B_1g and²B_2g ²A_1g transitions respectively of Ti³⁺ in a tetragonally distorted octahedral environment. The absorption bands in phosphate glasses are narrower and absorption coefficients higher than those in borate glasses. The esr spectrum of titanium(III) in all the glasses consists of a broad asymmetric line withg 1.94 in borate glasses, andg 1.92 in phosphate glasses; no hyperfine structure has been observed.     

A superconducting vibrating reed applied to flux-line pinning. I. Theory

A theoretical treatment is given of a superconducting reed clamped at one end and performing flexural vibrations in a homogeneous longitudinal magnetic fieldB _a. When the flux lines are rigidly pinned the reed behaves like an ideal diamagnet whose bending distorts the external field. This generates a magnetic restoring force (line tension) B _a ² which is independent of the reed thicknessd, whereas the mechanical restoring force (stiffness) is d ³. Therefore, the resonance frequency /2 of a thin superconducting reed increases drastically when a fieldB _a is applied, or for a givenB _a, when the reed is cooled below its critical temperatureT _c. With decreasing pinning strength (characterized by Labusch's parameter ) the resonance frequency decreases, ²²– _pin ² where _pin ^{2 –1}, and an attenuation _v ^–2 occurs due to the viscous motion of flux lines. For larger vibration amplitudes an additional, amplitude-dependent damping _h ^–3 occurs due to the hysteretic losses caused by elastic instabilities during flux motion.On leave from Centro Atómico, Bariloche, Argentina.     

Solutions to the dispersion equation for transverse sound in a Fermi liquid with nonzero LandauF 2

The Boltzmann equation for the Fermi-liquid quasiparticle distribution on the Fermi surface is expanded in spherical harmonics of symmetry appropriate to transverse sound, of which all higher terms relax together with relaxation time . Assuming a solution exp[i(kz – t)], one obtains a dispersion equation for complex /k. In the zero-sound limit,T 0 and , real solutions exist for /k v _F when the LandauF ₁>6 as quoted experimentally andF ₂<0, supporting a previous theory of thermal conduction, where a fit to experiment also gaveF ₂<0. Undamped modes can also exist forF ₁<6,F ₂/s>0, which were not found by Abrikosov and Khalatnikov and Lea and co-authors, who neglectedF ₂. In the hydrodynamic limit, isO(k ²) and imaginary, indicating relaxational behavior for transverse first sound in agreement with earlier treatments. Existence of highly damped modes atT=0 is examined, and it is found most can be ruled out by the principle that LandauF _n parameters decrease in magnitude withn.     

Free convective flow in an enclosure with a cooled inclined upper surface

Natural convective air flow in an enclosure with a horizontal lower wall, vertical side-walls and a straight inclined top wall has been numerically studied. The lower wall is at a uniform high temperature while the top wall is cooled to a uniform lower temperature. The temperature of the side-walls varies in a prescribed way between the temperatures of the bottom and top walls. This flow situation is related to that occurring in a proposed system for use in developing countries for drying crops such as corn and rice. It has been assumed that the flow is steady, laminar, and two-dimensional and that the fluid properties are constant except for the density change with temperature that gives rise to the buoyancy forces. The governing equations have been expressed in terms of stream function and vorticity and written in dimensionless form. The finite element method has been used to obtain the solution to these dimensionless equations. Results have been obtained for enclosures with aspect ratios of between 0.25 and 1 for top surface angles of inclination of between 0 and 45° and for Rayleigh numbers, based on the enclosure width, of between 1000 and 10⁷. The results have been used to study the effect of changes in the governing parameters on the flow pattern in the enclosure and on the mean heat transfer rate to the upper surface.List of symbols A aspect ratio, h/w - h mean height of cavity - k thermal conductivity - Nu local Nusselt number based on w - mean Nusselt number for top surface based on w - n n/w - n coordinate measured normal to boundary surface considered - p pressure - Pr Prandtl number - q local dimensionless heat transfer rate - mean heat transfer rate to top surface - Ra Rayleigh number based on w - T dimensionless temperature - T temperature - T _H temperature of bottom surface - T _C temperature of top surface - u velocity component in x direction - v velocity component in y direction - w width of cavity - x dimensionless x coordinate - x horizontal coordinate position - y dimensionless y coordinate - y vertical coordinate position - thermal diffusivity This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada.     

Amplifier based on a field-effect triode with negative current feedback

Conclusions A field-effect triode amplifier with series negative current feedback allows a voltage gain of the order of 200–300 to be obtained for a load resistance R_s1 M. The coefficient K_u begins to decrease noticeably only for a feedback resistance above 500 .The current gain reaches (8–10)·10³. Increasing the resistances R_s and R_L to hundreds of ohms has practically no effect on K_i. For a further increase of R_s and R_L the coefficient K_i decreases.The power gain reaches its maximum value (of the order of 10⁴ or more) for R_s100 and R_L=10–100 k. An increase in R_s leads to a reduction of K_pmax and to a shift of the extremum of the function K_p=f(R_L) into the range of higher values of R_L.A large input resistance of the amplifier (tens of megohms and higher) is obtained when R_s increases to 10–100 M. The maximum input resistance is obtained for R_L and R_s and may exceed values of from hundreds of megohms to several gigaohms. The minimum input resistance is hundreds of kilohms for R_L and R_s0.The minimum input resistance (5–10 k or less) is ensured for R_g and R_L0. An increase of the output resistance to hundreds of megohms or higher occurs for R_g and R_s.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 67–70, September, 1971.     

Microstructure and properties of supersaturated ZA27 alloy during natural ageing

The decomposition of the supersaturated solid solution of ZA27 alloy at room temperature has been investigated by X-ray diffraction, TEM and mechanical properties testing. Based on the results obtained, both continuous precipitation and cellular reaction occur during the decomposition process. The continuous precipitation follows the sequence: ₁ + spherical GP zones ₂ + elliptical GP zones ₃ + R + . The cellular reaction can be written: + + . The properties of the alloy depend on the microstructure. After 1 month of ageing, a series of changes of microstructure have taken place. The properties of the alloy are: _b=500 MPa, =13%,H _v=148.     

Ionic conductivity and crystal structure relationships in Ti/Cu substituted Bi4V2O11

Crystal structure and oxide ion conductivity of a series of Ti and Ti-Cu double substituted Bi₄V₂O₁₁ compounds, Bi₂V_(1–x)Ti _x O_(11–x)/2 (0.085 x 0.15), and Bi₂V_0.9Cu_(0.1–x)TixO_5.35+x (0 x 0.1), were investigated using X-ray powder diffraction and ac impedance spectroscopy in the temperature and frequency range of 100–700°C and 10^–2–10⁷ Hz, respectively.Structural phase transitions, and , occur as a function of composition in Ti substituted compounds for which the is evidenced to be stable at room temperature when x exceeds 0.125. For all Ti-Cu double substituted compounds studied, the room temperature phase was identified to be phase.The required amount of Ti for phase stabilization at room temperature was significantly reduced and the conductivity improved when Cu substituted a part of Ti. Therefore, for the Bi₂V_0.9Cu_(0.1–x)Ti _x O_5.35+x (0 x 0.075) compounds the ionic conductivity increased and activation energy decreased with decreasing x. At low temperature, the highest ion conductivity was obtained for Bi₂V_0.9Cu_0.1O_5.35. At high temperature (T>500°C), a different behavior was observed. The total conductivity increased at first with decreasing x values down to x = 0.05 and then decreased. The maximum conductivity was obtained for Bi₂V_0.9Cu_0.05Ti_0.05O_5.4, and the activation energy decreased with decreasing x values, such as what happened at low temperature.     

Spatial Evolution of High Frequency Phonons in Superfluid 4He from a Pulse-Heated Metal Film

An almost monochromatic spectrum of high frequency (/k _B 10 K) phonons in superfluid ⁴ He is created by a short ( 0.1s) pulse of Joule-heating in a metal film submerged in the liquid at saturated vapour pressure (svp). These phonons have lifetimes that tend to infinity as T 0, and are the ones effective in quantum evaporation experiments. Most of these high frequency (hf) phonons are not injected into the liquid ⁴ He across the metal—liquid interface, but are created in the liquid by energy-increasing interactions which begin with the injected phonons of much lower energy (/k _B 1 K). These hf phonons are created up to 5 millimetres in front of the heater, hence the time of flight from a heater to a detector only gives an approximate value (lower bound) of their energies. Here we present measurements at svp of phonon energy fluxes in liquid ⁴ He at various distances from a pulse-heated metal film. Analysis of these signals gives an improved determination of the hf phonon spectrum (peaked at 10.20 ± 0.05 K with HWHM 0.2 K on the high energy side).     

Magnetic field and temperature dependence of the Knight shift in superconductors of small dimensions

The magnetic field dependence of the paramagnetic Knight shift in weak-coupling superconductors is evaluated within the framework of the BCS-Gor'kov theory for samples of small dimensions. The behavior of the monotonic and oscillatory parts of the Knight shift are examined in detail in the regimesT0 andTT _c ^– .     

High-field magnetostriction in the pseudo-metamagnetic heavy-fermion system CeRu2Si2

Low-temperature magnetostriction experiments have been performed on CeRu₂Si₂ single crystals in magnetic fields up to 15T. WhenH is applied parallel to the tetragonalc-axis, a very large expansion is observed in all lattice directions. A pronounced anomaly occurs at a field close to the metamagnetic threshold. The results are discussed with use of thermodynamic relations and support a very simple scaling behavior for the pressure dependence of the magnetization.Laboratoire associé à l'Université Scientifique, Technologique et Médicale de Grenoble.     

Absorption and Circular Dichroism Spectra of Bi12SiO20<Nd> Crystals

The absorption and circular dichroism spectra of Nd-doped Bi₁₂SiO₂₀ crystals were measured in the range 10000 to 20000 cm^–1. The spectra showed electronic transitions related to only one type of Nd-related center: Nd substituting for Bi in position with symmetry C ₁. The bands due to the transitions from the first Stark component of the ground state to Stark components of excites states (⁴ I _9/2 ⁴ G _5/2, ⁴ I _9/2 ² G _7/2, and ⁴ I _9/2 ⁴ G _7/2) were analyzed in detail. The dipole strength D _0k , rotatory power R _0k , and anisotropy factor G _0k of these transitions were calculated. The intensities of transitions to Stark components were shown to vary by more than one order of magnitude within an excited-state multiplet. The anisotropy factors of the ⁴ I _9/2 ² G _7/2 and ⁴ I _9/2 ⁴ G _7/2 transitions, allowed in the magnetic dipole approximation, are, on the average, larger than that of the ⁴ I _9/2 ⁴ G _5/2 transition, which points to a significant contribution of the magnetic moment (<>²) to the total intensity of the transition.     

Spontaneous nuclear ferromagnetic ordering of in nuclei in AuIn2 Part I. Nuclear specific heat and nuclear susceptibility

We have measured the nuclear specific heat C_n and nuclear susceptibility _n of In nuclei (I=9/2, =5.5 _n) in the cubic intermetallic compound AuIn₂ (Korringa constant =0.11 Ksec) in the normal conducting state at 30K10mK and 2mTB115 mT. Our data show a positive nuclear Weiss temperature =+ 43 K and that the In nuclei undergo a nuclear ferromagnetic transition at T_c=35 K. The In nuclei experience an internal field of about 10 mT (obtained from C_n at T>T_c ). The nuclear ordering temperature T_c and the internal field increase with applied magnetic field. From the data we deduce exchange constants for the investigated system. The critical entropy reduction S(T_c)/S_max=8.6% and critical enthalpy E=0.28 RT_c are in reasonable agreement with the measured ordering temperature T_c,applying the Heisenberg model for a simple cubic I=9/2 spin system. The nuclear spin relaxation time calculated from the real and imaginary parts of _n is 10 msec at T>50 K, but drops to <1msec at T_c.This is the first observation of a spontaneous nuclear magnetic ordering transition in a not-hyperfine-enhanced metal at thermal equilibrium, i.e. at T _nuclear =T _electron .     

Electrical properties of Si4+ substituted copper ferrite

The electrical resistivity, , and Seebeck coefficient, , for the system Cu_1+xSi_xFe_2–2xO₄ (where x = 0.05, 0.1, 0.15, 0.2 and 0.3) have been studied as a function of temperature. Temperature variation of the resistivity exhibits two breaks. Each break is associated with a change in activation energy. The conduction process at low temperature is governed by the reaction Cu _A ¹⁺ + Cu _A ²⁺ Cu _A ²⁺ + Cu _A ¹⁺ . However, at higher temperature, it is due to intersite cation exchange and reoxidation such as Cu _A ²⁺ + Fe _B ³⁺ Cu _B ²⁺ + Fe _A ³⁺ . Measurement of the Seebeck coefficient, , from room temperature to 800 K reveals n-type conduction for the sample with x= 0.05, while the measurements for other samples show p-type conduction for lower temperatures and n-type conduction at higher temperatures. The activation energies in the paramagnetic region are found to be less than those in the ferrimagnetic region.