Specific heat of the non-centrosymmetric superconductor Re3W

The alloys of non-centrosymmetric superconductor, Re 3 W, which were reported to have an α-Mn structure [P. Greenfield and P. A. Beck, J. Metals, N. Y. 8, 265 (1959)] with T c = 9 K, are prepared by arc melting. The values of ac susceptibility and the low-temperature specific heat of these alloys are measured. It is found that there are two superconducting phases coexisting in the samples with T c1 ≈ 9 K and T c2 ≈ 7 K, which are both non-centrosymmetric in structure as reported previously. By analysing the specific heat data measured in various magnetic fields down to a temperature of 1.8 K, we find that the absence of the inversion symmetry does not lead to an obvious deviation from an s-wave pairing symmetry in Re 3 W.     

Collisional-radiative volume recombination and ionization coefficients for quasi-stationary helium plasmas

Collisional-radiative recombination and ionization coefficients, α andS, have been calculated on the basis of a non-hydrogen-like collisional-radiative model for atomic helium. The singlet and triplet systems have been considered as two individual systems coupled to each other through a set of elementary processes for collisions and radiation. Auto-ionization states have not been taken into account. Therefore the computations of the coefficient α have been limited to the range 125≦T _e [°K]≦64000. (AboveT _e=64 000 °K the auto-ionizing states considerably influence the recombination rates (di-electronic recombination).) The results show that the α- andS-coefficients for helium are more sensitive to radiative trapping than those for hydrogen under comparable absorption conditions. Concerning the α-values: For low electron densities one obtains approximately α_helium≈2α_hydrogen, whereas at high electron densities one finds approximately α_helium≈1/2α_hydrogen (to 1/10 α_hydrogen at high electron temperatures). Collisional-radiative ionization coefficients for atomic helium have not yet been published in the past. In the present paper one finds for the first time a complete set ofS-coefficients for different degrees of radiative trapping.     

Isotope effect in the linewidth distribution of single-molecule spectra in doped toluene at 2 K

The low-temperature dynamics of two different varieties of a low-molecular glass—protonated toluene (C₆H₅CH₃) and perdeuterated toluene (C₆D₅CD₃)—weakly doped with tetra-tert-butylterrylene (TBT) molecules was investigated with single-molecule spectroscopy. In both glassy matrices the distributions of the spectral widths were measured for a large number of single TBT molecules at T=2 K. The marked shift of the distribution upon deuteration of matrix was observed and attributed to an isotope effect which is qualitatively analyzed. It was found that at this temperature the tunneling dynamics of the matrix dominates the broadening behavior and hydrogen/deuterium atoms are involved in the tunneling motions.     

Anomalous low-temperature behavior of the thermal characteristics of MgB<Subscript>2</Subscript>

We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB₂) in the vicinity of T_c and at lower temperatures. It was established that MgB₂, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB₂ is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at T_c≈40 K, MgB₂ exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB₂ becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB₂ and to an increase in the density of the Bose condensate corresponding to these carriers at T_c2≈10–12 K.     

Wing profile measurements of the resonance emission lines of Na (5890/96Å) and Sr (4607Å) in flames

The blue and red wing intensity distributions of the Na 5890Åand 5896Åresonance lines were measured in emission in a pre-mixed, laminar, shielded H₂/O₂ flame at 1 atm with Ar or N₂ as diluent gases (T≈2000 K). The wavelength range scanned amounted to about 20Åfrom line center. In addition, we measured the wing intensity distributions of the resonance line of Sr (4607Å) in CO/N₂O flames at 1 atm (T≈2800 K) in a wavelength range of about 10Åfrom line center. The scanning monochromator used in these emission experiments had a spectral bandwidth of 0.30±0.04Å.For the resonance lines of Na and of Sr, the wavelength dependence of each wing was derived from 25 repeated scans and found to differ from theoretical predictions based on binary quasistatic theory.The red wing of the D₁ line of Na was investigated for the occurrence of satellites. We discovered a structure resembling a “smoothed” satellite at about 12Åfrom line centre. A satellite-like structure was also found in the blue wing of the resonance emission line of Sr at about 5Åfrom line center.     

1H NMR study of the α phase of Mg2NiHx system

Hydrogen behavior in the α phase of Mg₂NiH_x system was studied by ¹H NMR. ¹H NMR spectra and spin-lattice relaxation times, T₁ and T_1ρ, of Mg₂NiH_0.22 were measured in the temperature range between 100 and 480 K. The drastic change in the linewidth is observed between 170 and 340 K, and ¹H rigid lattice is observed below 170 K, from which it is deduced that the hydrogen atoms are randomly distributed in α-Mg₂NiH_x. The relaxation mechanism for t₁ is the paramagnetic one, while the T_1ρ value is determined partially by hydrogen diffusion. The hydrogen diffusion rate has been determined from the linewidth and the T_1ρ value. The paramagnetic relaxations observed in T₁ and T_1ρ have been discussed relating to the hydrogen diffusion.     

Ultrasonic attenuation anomalies at the two transitions in antiferromagnetic FeGe2

The attenuation of longitudinal and shear sound waves is measured through the Néel temperature, T_N≈286 K, and the lower transition temperature, T_K≈265 K, of FeGe₂. Longitudinal sound with wavevector q along the [100] axis of this tetragonal antiferromagnetic metal shows an attenuation peak at T_N, which is reversibly suppressed by compressive uniaxial stress σ along [010]. The estimated pressure dependence of T_N is dT_N/dp=(?2.8±0.3) mK bar^?1. The peak at T_N shows thermal and stress hysteresis, which suggests that it is associated with domain wall motion and that this transition is first order.     

Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Correlated vortex pinning in Si-nanoparticle doped MgB2

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB₂ tapes have been measured at temperatures T≥28 K in magnetic fields B≤0.9 T. The irreversibility line B_irr(T) for doped tape shows a stepwise variation with a kink around 0.3 T. Such B_irr(T) variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field B_?) and is very different from a smooth B_irr(T) variation in undoped MgB₂ samples. The microstructure studies of nanoparticle doped MgB₂ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.     

Second harmonic of magnetization and Cu magnetic system behavior at phase separation in moderately oxygenated single crystals of La2CuO4+x

The results on the nonlinear longitudinal response (second order uniform dynamic susceptibility χ₂(ω)) of two moderately doped La₂CuO_4+x single crystals in the temperature region of the phase separation of extra oxygen are reported. Large 2D-fragments (I≈7·10³ Å) of stoichiometric La₂CuO₄ are shown to be responsible for the signal in sample withT _ps>-T _N≈243 K. The signal of the crystal withT _ps<T _N≈272 K is due to oxygen-rich phase. Its peculiarities are related either to change and destruction of magnetic ordering in the course of the phase separation or to the ordering of the extra oxygen.     

Fluoride glasses in the InF3-GaF3-YF3-PbF2-CaF2-ZnF2 system

New glasses have been synthesized in a multicomponent system based on indium fluoride. Samples of a few mm in thickness were obtained. They are transparent and homogeneous. Main physical properties such as density, characteristic temperatures, density, thermal expansion and refractive index have been measured. The evolution versus composition is reported for samples with the formula: (35−x) InF₃-xGaF₃-10YF₃-25PbF₂-15CaF₂-15ZnF₂. T_g lies between 260 and 296 °C while melting starts around 480 °C. Glass samples are stable at room temperature. By comparison with other standard fluoride glasses, they exhibit higher refractive index and density.     

Experimental Stark widths and shifts of two ultraviolet C(II) lines

Photoelectric measurements of Stark widths and shifts of the components of the C(II) multiplet UV13 at 2837 Å were made with a wall-stabilized arc in a gas mixture of argon, hydrogen and carbon dioxide. Plasma diagnostics have been carried out to determine the electron density and the temperature. For experiments performed under two different physical conditions, the values n_e(1) = 4.95 × 10¹⁶ cm^?3, T(1) = 11,430 K and n_e(2) = 6.78 × 10¹⁶ cm^?3, T(2) = 12,080 K were obtained. Agreement between our widths and experimental data of Plati?a et al. is within error limits; a discrepancy of a factor of 4 appears in a comparison of our data with those of Kusch.     

Magnetotransport properties of ferromagnetic LaMnO3+δ nano-sized crystals

Transport and magnetic properties of LaMnO_3+δ nanoparticles with average size of 18 nm have been investigated. The ensemble of nanoparticles exhibits a paramagnetic to ferromagnetic (FM) transition at T_C～246 K, while the spontaneous magnetization disappears at T≈270 K. It was found that the blocking temperature lies slightly below T_C. The temperature dependence of the resistivity shows a metal–insulator transition at T≈192 K and low-temperature upturn at T<50 K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. The low temperature I–V characteristics are well described by indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates.     

CVM study of FCC and BCC Ising magnets with generalised spin number

Calculations using the cluster variation method (CVM) in the tetrahedron approximation have been applied to multicomponent FCC and BCC Ising systems with spin number up to s=7/2. The magnetic specific heat capacity (C_H) of ferro- and antiferromagnetic FCC and BCC systems has been calculated in the first neighbour approximation and compared with data obtained with other methods. A concavity in the C_H curve at T≈T_C/2 develops with increasing spin number.     

Effect of pressure and anionic substitution on the electrical properties of HgTeS crystals

The resistivity ρ and the Hall constant R for the HgTe_1?x S_x (0.04≤x≤0.6) crystals have been investigated in the temperature range 4.2–350 K in the magnetic fields B up to 14 T. The pressure dependences of the resistivity ρ have been measured at the pressures P as high as 1 GPa at temperature T=77–300 K and magnetic field B=0–2 T. It is found that the samples with x≤0.20 exhibit a decreasing dependence ρ(T) typical of zero-gap semiconductors, whereas the samples with x≥0.27 show the dependence ρ(T) characteristic of semimetals. For the semiconducting crystals with x≈0.20 and x≈0.14, the temperature coefficient of ρ(T) changes sign at T=265 and T>300 K, respectively. Under a pressure of ≈1 GPa, the temperature of the sign inversion decreases by ≈30 K. An increase in the magnetic field B and a rise in the temperature T lead to a change in the sign of the Hall constant R for the semiconducting samples, but do not affect the electronic sign of R for the semimetallic samples. The behavior of R and ρ correlates with the thermoemf data obtained at the quasi-hydro-static pressure P up to 3 GPa. It is demonstrated that the substitution of sulfur atoms for tellurium atoms brings about an increase in the concentration of electrons and a decrease in their mobility. The transition to the wide-gap semiconductor phase is observed at P>1–1.5 GPa. The conclusion is drawn that the semimetallic crystals HgTe_1?x S_x with x≥0.27 and HgSe are similar in properties.     

Acoustic perturbations in a pulsedRF-plasma

The response of a stationary weakly ionized plasma to a density perturbation in the neutral gas component was studied in a neon plasma with the following typical properties: electron density ¯N _e≈8×10¹² cm^?3, electron temperature on the axis of the vesselT _e0≈3.0 eV; neutral gas densityN _n≈1×10¹⁷cm^?3 and neutral gas temperatureT _n0≈600 °K. A neutral density perturbation, generated 50 cm apart from the plasma, produces a fluctuation in the ion density and a sharp spike in the differential voltage of a floating double probe. The experimental observations demonstrate the propagation of an ion sheath and of an electric field perturbation together with the neutral density perturbation. An interpretation of the plasma response to acoustic wave pulses has been proposed by Ingard and Schulz in a theory on acoustic wave modes in a weakly ionized gas. The experimental results are in good agreement with the theoretical expectations.     

Effect of field-induced magnetic ordering on the electrical resistance of the frustrated magnet Au80Co20

Au₈₀Co₂₀ platelets were produced by solid-state quenching followed by furnace annealing and characterized by X-ray diffraction. Magnetization and magnetoresistance (MR) were measured from 2 to 265 K. Minima of the zero-field electrical resistance R₀(T) were observed at low temperatures (T_m≈20 K) in both as-quenched and annealed samples. These minima disappear upon applying a magnetic field. At low T, a negative MR is associated to strong frustration and a very short magnetic coherence length; at higher temperatures, a positive MR is also observed.     

Zur Asymmetrie der Reaktion139La(n,α)136Cs mit polarisierten Neutronen

Experimental and theoretical investigations have been performed to determine the thermal conductivity of hydrogen in the temperature range between 2000 and 7000 °K. For this purpose the radial temperature distributions for various currents and theE-I-characteristic of a low current wall-stabilized hydrogen arc have been measured. In the dark region of the arc outside the bright core the temperature and the thermal conductivity between 2000 and 4500 °K were found by means of the schlieren technique. The electron temperature in the core of the arc results from spectroscopic measurements. The gas temperature has been calculated with a formula, derived from the kinetic theory of gases. Assuming a constant collision integralQ _eH ¹¹ the radial distribution of electric conductivity has been calculated according to Langevin's formula. The valueQ _eH ¹¹ =30·10^?16 cm₂ results by comparing the integrated conductance with the measured one. Since now the radial distribution of power input and the temperatures are known, the thermal conductivity between 4500 and 7000 °K can be determined as well. The total course of the heat conductivity shows a strong peak at the temperature of 3740 °K characteristic for the dissociation process.     

Observation of the antiferromagnetic transition in the linear chain compound KFeS2 by magnetic susceptibility and heat capacity measurements

Magnetic susceptibility (ξ) measurements and the first heat capacity measurements on KFeS₂ are reported. The data were obtained on a specially purified sample over the temperature ranges 10–360 K and 225–296 K, respectively. Both measurements revealed small but distinct singularities at the Néel temperature T_N≈253 K; extensive short range ordering above T_N was also evident from these data. The present work shows that the intrinsic ξ(T) of KFeS₂ was largely masked by that from magnetic impurities in previous ξ studies of this compound.     

Messung von Temperaturverteilungen im H2-Kaskadenbogen bis 27000°K

On a stationary, flowless, and cylindrical hydrogen arc, the radial temperature distributions are measured spectroscopically at a pressure of 1 at and up to a current of 150 amps. By reducing the diameter of the improved high power cascade arc chamber (4) to 2 mm, the instabilities of this can be overcome. For the measurements the hydrogen lineH _β is used, relative overall intensity of which is derived from the line profile by means of a new method. As the line emission coefficient ofH _β has a maximum at approximately 16,000 °K, the temperature can be determined using the Larenz method which yields quite reliable results by normalizing the relative line intensity measured in the arc to that maximum. For the measurement, the “end-on” observation is adopted, since at high currents, i.e. at axial temperatures in the arc of more than 16,000 °K, the intensity in the axis of the arc is considerably lower than outside the axis, i.e. in the edge of the arc. To avoid any perspective distortions of the arc image at the slit, a telecentric optical array is used. The error caused by the inhomogeneous layers of the electrode regions is eliminated by determining the intensity difference ofH _β on two arcs of very different lengths. For the employed current range, radial temperature profiles in the range of 11,000–27,000 °K with ionization degrees of up to 98% are obtained.