Effect of magnetic field on a spin-peierls cuprate,CuGeO3

We measured the static magnetization of polycrystalline CuGeO₃, which exhibits the spineierls transition around 14.0 K in the absence of the magnetic field. The field dependence of the magnetization below 14.0 K shows a characteristic nonlinear process associated with a phase transition, and phase boundaries between the dimerized phases and other phases were obtained. The magnetic phase diagram qualitatively agrees with that of an organic spin-Peierls material. The phase transition is of first order below 10.0 K, and of second order above 10.0 K. Above 10.0 K, the critical field decreases from 13 to 0 T with increasing temperature. On the other hand, it is nearly independent of temperature below 10.0 K.     

Hydrodynamic properties on superfluid3He in high magnetic fields up to 12 tesla

Using a torsional oscillator, superfluid fraction and viscosity of superfluid³He were measured in the A₁ and the a₂ phase up to 12 Tesla below 1 mK in the pressure of 29 bar. This is the first measurement in the high magnetic field region in a well defined geometry and in a controlled texture. The a₁ and the a₂ transition were seen clearly in the torsional oscillator frequency and amplitude. Two GL parameters, ₂₄₅ and ₂₄, are independent of field up to 12 Tesla. ₂₄₅ is 1.18 from the A₂ phase measurements, which agree with those from heat capacity measurement. While ₂₄ is 3.08 from the a₁ phase, which is smaller than that from recent spin entropy wave measurement.     

Specific heat of type-II superconductors in magnetic fields nearT c

The GLAG theory predicts that the value of the specific heatC _M in the mixed state just below the transition to the normal state, is a function of the Ginzburg—Landau (GL) parameter , even for vanishingly small magnetic fields(TT _c ).C _M is thus different from the specific heatC _S in the pure superconducting state(H=0) for all except 1.9. Experimental results of the authors generally confirm this paradoxical prediction which is not in contradiction with the thermodynamics of phase transitions. To experimentally clarify this behavior close toT _c , Nb₈₀Mo₂₀(4) and Pb₉₈In₂(0.8) specimens are studied in fields much smaller than those usually applied (down to17 Oe). A new method for measuring specific heat in decreasing as well as increasing temperature is introduced. Very good agreement with the theory is found for fields higher than 120 Oe while a deviation is observed close toT _c for lower fields where values of the Abrikosov lattice parameter would become larger than 1 µ. The sense and the magnitude of the deviation are such thatC _M andC _S would be equal for fields of 20 Oe and below. Possible explanations are discussed. It is suggested that a fundamental modification of the vortex state structure occurs in small fields close toT _c .     

Specific heat of A15 Nb3Si in fields to 7.1 T

The newly discovered high-T _c superconductor A15 Nb₃Si, with a lattice parameter of 5.091 å, is shown to have an anomalously low H _c2 of 12.5 ± 0.5 T, a Ginzburg-Landau of 16 ± 2, and an implied resistivity of 35 ± 5-cm. The question of whether poor ordering due to the explosive compression preparation is responsible for this low H _c2 is discussed.Work performed under the auspices of the U.S. Department of Energy.     

Specific heat of liquid cesium at temperatures up to 2000°K and pressures up to 12 MPa

We present and discuss measured values of the specific heat and resistivity of liquid cesium in the temperature range 1200–1950°K at pressures up to 12 MPa.     

Testing of strain-gauge pressure transducers up to 3.5 MPa at cryogenic temperatures and in magnetic fields up to 6 T

The characteristics of four commercial strain-gauge pressure transducers suitable for measurements in a temperature range from 4.2 to 293 K were determined in the presence of magnetic fields up to 6 T. The highest pressure measured was 3.5 MPa. Tests showed that if the transducers are used in these conditions and no corrections are made, uncertainties up to 19% of the transducer full scale may be inherent in measurements. When such high error levels are unacceptable, the transducers must be individually calibrated under the working conditions: in this way the uncertainties in pressure measurements can usually be kept below 1 % of the transducer full scale.     

Superconducting transition widths in applied magnetic fields

The width of superconducting-normal transitions for inhomogeneous ellipsoidal superconductors in an applied magnetic field is calculated. A simple model is used for the variation in transition temperature with position and the range of this variation is assumed to be larger than the Landau-Ginzburg coherence length. It is found that the decrease in transition width with increasing applied magnetic fields at low fields occurs because the boundaries between superconducting and normal regions begin to be determined less by the inhomogeneities and more by the loss of condensation energy between domains, and by sample surface effects. At higher fields, the transition width increases with applied field because of the finite demagnetization coefficient.Contribution of the National Bureau of Standards. Not subject to copyright.     

The squid picovoltmeter operating in magnetic fields up to 3.2 T

A method which enables the Squid dc voltmeter to be operated with a sample exposed to a significant external magnetic field is presented. A special superconducting shield with a finite time constant has been used to suppress time-dependent components of the field. With the shield, the Squid voltmeter has been able to work in magnetic fields up to 0.7 T in the flux-locked loop mode. The total voltage noise per unit bandwidth V_rms (referred to input) does not exceed the value of . To extend the range of performance to higher magnetic fields, simple low pass filters have been built into the input circuit to increase its time constant. With the signal coil of the Squid shunted by a resistor R_B = 3 × 10⁻⁶ω, the Squid voltmeter could be operated in fields up to 3.2 T with

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Specific heat and magnetization in dilute magnetic alloys

We have studied dilute alloys where interactions of the Ruderman-Kittel-Yosida type are assumed to exist between well-defined magnetic moments that are localized on impurities distributed at random in the matrix. Using a molecular field model, such as has been proposed by Blandin and Friedel, ¹² we show that, because of the geometrical characteristics of the interaction, simple correspondences are expected between the magnetic properties (specific heat and magnetization) of a given alloy for different values of the solute concentrationc. These quantities are expressed through concentration-independent functions of the reduced temperature,T/c, and the reduced field,h/c. The idea of reduced diagrams, where we plot both new and available data in terms ofT/c andh/c, follows from these functions. Generally good agreement is observed for the specific heat, the magnetization, the initial susceptibility, and the residual magnetization of a number of alloys. (The data used here are for theAu-Fe,Cu-Mn andAg-Mn systems.) Furthermore, discrepancies, due to other mechanisms, such as the Kondo effect, or the effect of a mean free path limitation may better appear in such diagrams.This work is part of a Thèse d'Etat that has been presented by Mr. Jean Souletie and is registered under n^o A.O. 1989, at the Centre de Documentation du C.N.R.S., France.     

Performance of transmission dynode photomultipliers in magnetic fields up to 1.5 T

The performance of commercially available photomultiplier tubes with 1, 2, 8 and 16 parallel transmission dynodes is examined in homogeneous magnetic fields B with strengths up to 1.5 T and axial as well as nonaxial orientations. The samples with 16 dynodes allow an operation with gains G ≥ 10³ and time resolutions σ ≤ 230 ps in such strong fields and for tilting angles θ ≤ 50°. The dependence of relative gain G/G(B = 0) and time resolution σ on B and θ is less expressed for the PMs with the smaller numbers of dynodes and a more compact design. The influence of B(θ) on the transit time is discussed qualitatively.     

Behaviour and accurate thermometry of carbon-glass resistance thermometers at low temperatures and in magnetic fields up to 7 T

The behaviour of carbon-glass resistance thermometers over a temperature range of 3.41-20 K and in magnetic fields of up to 7 T are investigated. It is shown that the percentage changes in resistance due to the application of a magnetic field, as a function of both temperature and field strength, can be described by the equation 100ΔR/R_o = [(1/T) − (1/20)] (AH² + BH). Using this equation, the temperature errors due to magneto resistance can be corrected to within 20 mK.     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-planeρ _ab (H, θ,T), which depends on the angleθ between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values ofρ _ab (H, θ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter γ≡(m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence ofρ _ab (H,θ,T).     

Specific heat of 5% and 3% 3He-4He solutions under pressure

Specific heat measurements have been made under pressure on a 5% ³He-⁴He solution between 10 and 93 mK, yielding values for the specific heat mass, m ^*, of 2.45 m ₃, 2.74 m ₃, and 3.07 m ₃ at pressures of 0, 10, and 20 atm, respectively; corresponding values of the zero-concentration effective mass m ₀ are 2.38 m ₃, 2.64 m ₃, and 2.90 m ₃ for the above pressures. Measurements were also made on a 3% solution. Above 35 mK, all the measurements show deviations from a free-Fermi-gas behavior.This work was partially supported by the U.S.-Israel Binational Science Foundation (BSF), Jerusalem.     

On the stability of carbon glass thermometers cycled at low temperatures under high magnetic fields

We report on the reliability of three carbon glass thermometers used in the temperature range 4.2 K – 50 K under high magnetic field conditions.In two of the three sensors we observed a serious lack of reproducibility (1.5 K at 25 K). These results indicate that in experiments requiring precise temperature measurements in the range 20 – 30 K, it is necessary to perform regular calibrations of carbon glass thermometers versus secondary standards such as platinum or germanium resistors.     

Ferrofluid flow under the influence of rotating magnetic fields

Experimental data are presented concerning the physical mechanism of the electromagnetically induced macroscopic rotation of ferrofluids. The rotating velocity distributions, determined using a DISA anemometer equipment and a tensometric transducer system, showed a very different behaviour of ferrofluids in a low frequency (0 + 10 Hz) rotating magnetic field in comparison with that observed in the higher frequency region ( >15 Hz). The data are compared with other experimental and theoretical results.     

The metal-dielectric transition in semimetals in extremely high magnetic fields. I

An analysis is given of a model of a semimetal with isotropic electron and hole spectra, separated in momentum space. The applied magnetic field is assumed to be strong enough to ensure that only one quantum level participates in each spectrum. The overlapping of levels is such that the Fermi energy is greater than the binding energy of the particle pairs. Weak point interaction is assumed to operate between the particles, which is valid for fields and dielectric constants that are not too high. The sign of the interaction can be arbitrary. Since the analysis is concerned with spinless fermions (degeneracy removed by the field), the system exhibits interactions between electrons and holes, but there is no interaction between particles of the same kind. It is shown that Cooper pairing of particles with the same charge is absent, but electron-hole excitons are formed, i.e., the transition to a dielectric takes place. The problem leads to parquet equations and is therefore solved only with logarithmic accuracy. A general method is developed for finding the anomalous averages corresponding to pairing, and the binding energies from asymmetric parquet vertices.     

Specific heat of aqueous HNO3 solutions at low temperatures (down to −196‡C)

An experimental method is shown by which the specific heat of aqueous HNO₃ solutions has been determined at low temperatures.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 4, pp. 669–674, April, 1973.