The Interplay of Electronic and Nuclear Magnetism in PtFe x at Milli-, Micro-, and Nanokelvin Temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFe _x samples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mTB248 mT. The susceptibility data have been measured at temperatures of 0.3 KT100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 K. The nuclear heat capacity data taken at 1.4 KT10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 K. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 KT0.5 mK and 2.5 mTB22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 KT0.5 mK and at 5.4 mTB200 mT. Spin-lattice and effective spin-spin relaxation times ₁and ₂ ^* of ¹⁹⁵ Pt strongly depend on x and on the external magnetic field. No temperature dependence of ₁and ₂ ^* could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 K.     

Nuclear ferromagnetic ordering of141Pr in the diluted Van Vleck paramagnets Pr1−x Y x Ni5

We report on the first investigation of the effect of magnetic dilution on nuclear magnetic spin interactions in metals; we studied this effect in the diluted Van Vleck paramagnets Pr_1–xY_xNi₅. In addition, we investigated the electronic magnetic properties of these intermetallic compounds. For this purpose, we have measured the nuclear and electronic susceptibilities of Pr_1–xY_xNi5 with x = 0 to 0.2 at 50K T 8 mK and at 2.2 K T 300 K. We observe a linear decrease of the electronic Van Vleck susceptibility and of the hyperfine enhancement factor by 13% (per mole Pr³⁺). The hyperfine-enhanced nuclear susceptibility decreases by about a factor of three when going fromx = 0 tox = 0.2. The samples show nuclear ferromagnetic transitions of¹⁴¹Pr with nuclear Curie temperatures T_c decreasing from 370 K to 100 K and Weiss temperatures decreasing from 218 K to 13 K in this concentration range. These data are compared with the results of mean-field calculations. In addition, we report on measurements of the nuclear spin relaxation time of these compounds, for which we find values of a few msec at millikelvin temperatures and a critical speeding-up at T_c. Our data give hints that for x > 0.2 the character of the nuclear magnetic transition may change, possibly to a nuclear spin glass freezing.     

Surface study of liquid3He using surface state electrons

We have measured the mobility of surface state electrons (SSE) on liquid³He, 3, aiming to study the elementary surface excitations of the Fermi liquid. A gradual increase of 3 below 300 mK is attributed to the scattering of electrons by ripplons. Ripplons do exist in³He down to 100 mK. We observe an abrupt decrease of 3, due to the transition to the Wigner solid (WS). The dependences of the WS conductivity and mobility on temperature and magnetic field differ from the SSE behavior on liquid⁴He.     

Upper Critical Field Hc2 on c Axis of Ag/Bi-2223 Polycrystal Tape

The upper critical field H _c2, based on Ginzburg–Landau theory at medial magnetic fields, can be calculated from the relationship of the magnetization intensity M(H) versus magnetic field H if M is linear with ln₀ H. For Ag/Bi-2223 tapes, the measured M(H) was found to be linear with ln₀ H. In this case, the values of H _c2(T) may in principle be determined. To do this, we will meet another problem in that the obliquity of crystal planes is not equal for different grains in tapes, and values of grain-oblique angle will appear in the calculated H _c2. Obviously, for Ag/Bi-2223 tapes is an uncertain parameter and hard to determine. To avoid the effects of , we only study the H _c2 in the c axis direction and the projections of H onto the c axis is taken as the actual applied fields. Thus, the effects of grain-oblique angle may be counteracted when measuring critical current density J, if the external magnetic fields are applied to the tape along both the parallel to and perpendicular to the c axes directions (on the narrow surfaces of the tape). On medial fields (H = 0 – 3 T), the upper critical fields H _c2(T) on c axis for the Ag/Bi-2223 tape are obtained and fitted as ₀ H _c2(T) = 830 e ^–0.07T . The average slope d[₀ H _c2(T)]/dT – 8T/K is found to be as large as that of Bi-2212. On extrapolating the relation to T = 0 K, the value of ₀ H _c2(0) 800T. The coherence length _ab is determined from H _c2(c), and _ab (0) = 0.63 nm at T = 0 K.     

Magnetic Ordering in 3He Nano-Clusters

Simultaneous measurements of magnetic susceptibility from 0.5 to 10 mK and pressure from 2.88 to 3.54 MPa have been made in ³ He nano-clusters embedded in a ⁴ He matrix, following phase separation. The susceptibility of the 3.54 MPa, all-solid sample behaves similarly to that of bulk ³ He for v=21.3 cm ³ /mole, with a Weiss constant =–250K. For the 2.88 MPa, liquid-droplet sample, =140K, indicating a ferromagnetic tendency, similar to 2-D films at some coverages. At intermediate pressures, has a peak near 1.05 mK, but without a discontinuity. For all samples, had a solid-like contribution to the lowest temperatures. Magnetic ordering in nano-clusters appears to be different than the U2D2 phase of bulk ³ He.     

Plastic dilution refrigerators

A new design for conventional,³He re-circulating, dilution refrigerators has been developed and tested. The units are made out of plastic and can be designed to have very small diameters (<15mm). These characteristics make them ideal to cool samples below 100 mK in high or time-varying magnetic fields. Furthermore, they are inexpensive, reliable and easily constructed. The best refrigerators reach continuous temperatures of around 10 mK at low circulation rates ( 100 mol/s). The cooling power at high temperatures is limited by the speed of the circulation pump and not by the refrigerator itself. The basic design and construction methods are discussed.     

Magnetic Impurities in Glass and Silver Powder at Milli- and Microkelvin Temperatures

In search of an ideal paramagnet for thermometry at very low temperatures, we have studied the magnetic behaviour of impurity moments (localized Fe³⁺, impurity concentration x = (180±10) ppm) dissolved in a structural borosilicate glass, in the temperature range 0.07 T 300 mK by means of dynamic ac susceptibility, and at 1.6T 300 K by static dc magnetization. In order to improve the thermal coupling of the insulating glass at the lowest temperatures, it was pulverized and mixed with silver powder of submicron grain size; the composite was subsequently compacted to a cylindrical sample by applying a pressure of a few kbar. This contact method which is applicable to other materials with bad thermal conductivity as well improved the accessible minimum temperature for the glass down to 0.1 mK. At low temperatures, we observe a Curie Weiss law for the dynamic susceptibility of the magnetic Fe³⁺ impurities in the glass down to 0.6 mK, a broad maximum of at T 0.38 mK and a decrease towards even lower temperatures. Compared to the frequently used, highly diluted metallic spin glasses, the magnetic behaviour of the glass makes inductance thermometry applicable in a much larger temperature range. In addition, we have investigated the low temperature magnetic properties of a sample of compacted silver powder. The small amount of paramagnetic impurities in the Ag particles (x = (4 ± 1) ppm) exhibits a low temperature susceptibility which can be described by the Kondo effect with an unexpectedly small Kondo temperature of T _K 1 mK.     

Switching and Hysteresis on the I-V Curves of Submicron BSCCO (2212) Bridges

No Heading In thin BSCCO (2212) whisker-based superconducting bridges with width and length 1 m we observe random telegraph noise at discrete values of current. We show that the noise is associated with spontaneous processes of addition and removal of discrete vortex trains and reflects the regular structure of steps on the I–V curves. The average lifetime of the trains falls down with increasing temperature T in a complex way: intervals of steep drop ( an order of magnitude per Kelvin) are separated with a plateau of (T) with 10^–2 s. The 70 GHz irradiation with 100 W power results in the growth of the average switching frequency by 5 orders of magnitude; thus the HTSC bridges could be efficient detectors of microwave radiation. We discuss the peculiar features of (T).PACS Numbers: 74.25.Fy, 74.40.+k, 74.60.Ge, 74.72.Hs     

3He melting pressure temperature scale below 25 mK

Using⁶⁰Co -ray anisotropy radiation as a primary thermometer, with a Pt NMR susceptibility secondary thermometer, we have made high-precision measurements of the³He melting pressure versus temperature from 500 K to 25 mK. Temperatures obtained for the fixed points on the melting curve are: the superfluid A transition T_A = 2.505mK, the A-B transition T_AB = 1.948 mK, and the solid ordering temperature T_N = 0.934 mK. We provide a functional form for P(T), which, with the fixed points, constitutes a convenient temperature scale, based on a primary thermometer, usable to well below 1 mK.     

Experimental Evaluation of a Single Stage Superfluid Stirling Refrigerator Using a Large Plastic Recuperator

This paper describes the low and high temperature experimental performance of the second single stage superfluid Stirling refrigerator (SSR) to use a plastic recuperator. This SSR has a total internal volume of 96.6 cm³ and uses a Kapton recuperator which has 12.10 cm³ devoted to recuperative heat transfer. Operating from a high temperature of 1.0 K and with 1.5% and 3.0% ³He–⁴He mixtures, this SSR achieves a low temperature of 291 mK and delivers net cooling powers of 3705 W at 750 mK, 977 W at 500 mK, and 409 W at 400 mK. Cooling power versus cold piston temperature for various frequencies of operation and for two piston stroke configurations are also provided. These results are non-dimensionalized and compared to the Schmidt model of a regenerative Stirling refrigerator and the adiabatic model of a recuperative Stirling refrigerator. The SSR was also operated from high temperatures between 1.0 K and 2.0 K. This SSR achieves low temperatures of 412 mK, 620 mK, 1.069 K, and 1.459 K operating from high temperatures of 1.2 K, 1.4 K, 1.6 K, and 1.8 K respectively. This high temperature performance is compared to the theoretical performance of the SSR using a phonon–roton gas model.     

Chebyshev approximation for the roots of the equation BiWГ(μ)=μVГ(μ)

A minimax approximation, uniform for Bi [0, ), is developed for the roots of the equation BiW()=V(), by means of Chebyshev polynomials.Translated from Inzhenero-Fizicheskii Zhurnal, Vol. 28, No. 4, pp. 710–715, April, 1975.     

Fermi liquid behaviour of the viscosity of3He-4He mixtures

We have investigated³He-⁴He mixtures at³He-concentrations 0.98%x9.5% by the vibrating wire technique in the temperature range 1 mKT 100 mK and at pressures 0 bar p 20 bar. In the degenerate regime of the mixtures the Landau theory of Fermi liquids predicts a temperature dependence of the viscosity proportionalT ^–2. We report on the first observation of this behaviour at 3 mKT 10 mK for all investigated concentrations and pressures. At temperatures below about 20 mK slip corrections had to be taken into account due to the increase of the quasiparticle mean free path at very low temperatures. The low-temperature cut-off in T ² = constant indicates the transition into the ballistic regime of the mixtures, where the mean free path of the quasiparticles exceeds the radius of the vibrating wire. Our results for the pressure dependence of the viscosity as well as for its magnitude show substantial differences from predictions based on pseudopotential theory. However, a calculation of with the quasiparticle interaction potential of recent solubility measurements in mixtures agrees well with our experimental data, in particular the pressure independence of .     

Electron-phonon coupling as a mechanism for high-temperature superconductivity

We have solved the Eliashberg gap equations which are valid for arbitrary phonon frequency, (_ph), electron-phonon coupling constant, (), and screened Coulomb interaction, ( ^*). We have used values of (_ph),, and ( ^*) appropriate to the cuprate superconductors, and calculated the density of states, the pair potential., and the value of the gap at T=0 K. Using the linearized Eliashberg equations in the matrix representation, we have calculatedT _c and 2/k T _c . We have found that we can account for the highT _c 's in the cuprates with reasonable values for, ^*, and _ph.     

Sintering of aluminium nitride: Particle size dependence of sintering kinetics

The effect of particle size (0.78 4.4 m) on the sintering kinetics of AIN powder was investigated in the temperature range from 1600 to 2000° C and the results were analysed on the basis of vacancy diffusion models. The mechanisms of sintering are discussed.Fractional shrinkage is proportional to the nth power of soaking time with n = 0.20 for 4.4 m and 1.5 m powders and 0.33 for 0.78 m powder. For the 0.78 m powder at 1900° C, however, n decreases gradually as grain growth proceeds. The experimental activation energy for sintering is between 92 kcal/mole for 4.4 m and 129 kcal/mole for 0.78 m powder. Unlike this activated energy, the rate of sintering and the diffusion constant calculated from it increase drastically with decrease of particle size; the derived diffusion constant for 1.5 m powder is 10¹ to 10² times larger than that of 4.4 m powder, and for 0.78 m powder the diffusion constant is estimated to be still higher.The particle-size dependence of parameter n and the diffusion constant seems to be caused by a variation in predominant diffusion mechanisms; namely, bulk diffusion in coarse powder and surface or grain-boundary diffusion in fine powder.     

Measurement of the Micrometer Diameters of Focused Laser-Beam Radiation with a Thin-Wire Bolometer

The profile distributions of a focused Gaussian beam of 63-m laser radiation for E- and H-polarizations were recorded by a thin-wire bolometer 3 m in diameter. The profiles were used to determine the beam diameters, which were D ^E = (8.16 ± 0.12) m and D ^H = (8.00 ± 0.12) m upon elimination of the dominant systematic errors.     

The Superconductive Critical Magnetic Field of Beryllium

We have measured the temperature dependence of the superconductive critical magnetic field of three samples of beryllium from three different sources. The irregular geometric shape of one sample prevented a complete analysis of the data, but the data for the other two samples were well represented by the BCS model of superconductivity. The values of T _c and H _c (0) are found to be sample dependent. For the purest sample studied, T _c =24.34±0.02 mK, H _c (0)=107.7±0.2 T and (/V)=32.9±0.3 J/cm³ K². By a very small extrapolation of the data as a function of purity, it is inferred that the critical parameters for pure Be are: T _c =24.38±0.02 mK, H _c (0)=107.9 T±0.2 T and (/V)=33.0±0.3 J/cm³ K².     

The Impact of Nuclear Magnetism on Superconductivity in a Metal with Nuclear Electric Quadrupole Splitting: Indium

We report the first investigation of the impact of nuclear magnetism on superconductivity in the tetragonal metal indium. We have measured the superconducting critical field B_c(T) and in its vicinity the nuclear magnetic heat capacity at ultralow temperatures, 170 KT200 mK. We compare the measured quantities with calculations which consider the nuclear magnetic Zeeman and the dominating nuclear electric quadrupole interaction in indium. The heat capacity data support the occurence of a positive sign of the electrical field gradient at nuclear sites and in consequence the existence of a nuclear low spin ground state. Surprisingly, at lowest investigated temperatures, 170 KT1 mK, the reduction of the critical field B_c(T) clearly exceeds the size of the calculated magnetization ₀ M(B_c, T) which is limited by the nuclear low spin ground state. In all other materials the interplay of nuclear magnetism and superconductivity has been studied so far (Al, AuAl ₂ , AuIn ₂ , Rh, and Sn), the bare nuclear magnetization appeared as an upper limit of the reduction of the critical field.     

The effect of sheet thickness on the magnetic properties of 77 wt% Ni permalloys

The effect of sheet thickness,t, on the magnetic properties of some Ni-Fe alloys of approximate composition 77% Ni-14% Fe-5% Cu-4 wt% Mo for various heat treatments in the temperature range 300 to 1250° C has been investigated. The range of sheet thickness used was 50 to 375m and was obtained by cold-rolling without interstage annealing. It was found that permeability increases with decreasing sheet thickness, attains a maximum att75m and starts to fall as the thickness is further reduced. The increase in permeability with decreasing sheet thickness (t>75m) is thought to be due to a decrease in eddy current losses, changes in texture formation and possibly the degree of short range order (SRO) developed in the material. The permeability obeys a 1/t ² relationship with sheet thickness in the range 75 to 375m. Below a sheet thickness of 75m permeability starts to decrease with decreasing sheet thickness. The reason for this fall in permeability is thought to be due to a thickness dependent magnetostatic energy contribution to the wall energy associated with free poles along the domain wall and also to the loss ratio,, which increases rapidly at small thicknesses. This rapid increase in is due to factors such as the ratio of domain wall spacing to sheet thickness, which varies with sheet thickness, the spin relaxation term, which has a 1/t ² dependence on sheet thickness, and the increase in the wall energy which increases the domain wall spacing. Finally a decrease in permeability could be caused by a different degree of SRO in the thinner specimens due to different cooling rates which could lead to an increase in first anisotropy constant,K ₁, and saturation magnetostriction, _s which then produces a decrease in _i att<75m when the degree of SRO in the specimens is not optimum.     

Experimental Performance of a Single Stage Superfluid Stirling Refrigerator Using a Small Plastic Recuperator

The experimental performance of the first superfluid Stirling refrigerator (SSR) to use a plastic recuperator is reported. This SSR is a single stage machine, has a total internal volume of 83 cm ³ , and uses a 3 cm ³ Kapton heat exchanger. Operating from a high temperature of 1.0 K and with a 1.5% ³ He– ⁴ He mixture, this SSR achieves a low temperature of 344 mK and delivers net cooling powers of 1.86 mW at 750 mK, 358 W at 500 mK, and 97 W at 400 mK. Cooling power versus cold piston temperature for various frequencies of operation and for two piston stroke configurations are provided.     

Carrier Mobility in GaSb–V2Ga5and GaSb–GaV3Sb5 Eutectic Alloys

Experimental evidence is presented that semiconductor–metal eutectics with a low content of the metallic phase (4 vol %) are similar in electronic structure to inhomogeneous semiconductors. The microstructure of undoped and Te-doped GaSb–V₂Ga₅ and GaSb–GaV₃Sb₅ eutectic alloys is examined, and the Hall mobility of carriers in these alloys is determined. The anomalous temperature variation of Hall mobility in GaSb–V₂Ga₅ ( T ²) and GaSb–GaV₃Sb₅ ( T ⁵) is interpreted in terms of infinite clusters ofn-type metallic inclusions embedded in a p-type semiconductor matrix and interconnected through overlapping inhomogeneous interfaces. It seems likely that the difference in conductivity type between the semiconductor matrix and the infinite clusters gives rise to a random large-scale potential relief. Te compensation of GaSb in the eutectic alloys causes the Hall mobility to vary more rapidly with temperature, T ³ to T ¹⁰, which is interpreted as due to an increase in the amplitude of the random large-scale potential relief, the formation of infinite clusters, and partial compensation of unintentional acceptor doping in the semiconductor matrix.