Nuclear alignment in a Kondo system as a very low temperature thermometer

Cu doped with less than 1 ppm Mn⁵⁴ appears to offer a number of advantages as a -ray anisotropy thermometer in the temperature range 1 to 20 mK. In order to use such a thermometer it is necessary to know how the hyperfine field at the Mn⁵⁴ nuclei varies with temperature and with applied field. We have studied this -ray anisotropy and find that the hyperfine field is independent of temperature in the range 4 mK <T<10 mK. Further, we have determined the hyperfine field to ±5% as a function of applied field in the range 60 Oe<H _a<1200 Oe. The latter data may be used to determine a Kondo temperature of 64±2 mK for this system.Work performed under the auspices of the U.S. Atomic Energy Commission.     

Magnetic properties of highly dilutedPdFex andPtFex-alloys. Part II. Susceptibility at micro- and milli-kelvin temperatures

We have measured the 16 Hz susceptibility of the giant magnetic moments induced by Fe impurities in highly dilutedPdFe_x andPtFe_x samples with 2.5 ppm x 75 ppm in a wide temperature range, 30 K T 300 mK, and at static magnetic fields 0,01 mT B 25 mT. We find spin glass freezing at Tf(X)/X0,19mK/ppm Fe forPdFe_x and the larger value 0.26 mK/ppm Fe forPtFe_x. This is the first observation of spin glass freezing inPtFe_x. In the low-temperature range T 0.5T_f(x), the susceptibilities follow — ₀ T with small zero-temperature ₀ values forPdFe_X and vanishing ₀ values forPtFe_x. In the paramagnetic high-temperature range, we find (T — )^it-1 at T 10 mK independent of x forPdFe_x, and at T 2Tf(x) dependent of x forPtFe_x with vanishing values for both systems. The data compare well to the predictions of the Thouless-Anderson-Palmer TAP approach of the Sherrington-Kirkpatrick SK model for spin glasses.     

Heat capacity of titanium hydride at millikelvin temperatures

The heat capacity C of a TiH_1.98 bulk sample in zero and 72 mT magnetic fields has been measured at 8 mKT 600 mK. In zero magnetic field the sample shows metallic behavior. In a magnetic field the protons give an additional nuclear heat capacity contribution, whose value is in good agreement with the calculated one. From our data we also obtain an upper limit for the nuclear spin-lattice relaxation time ₁ and for the molecular hydrogen impurity concentration in the sample. ForT 10 mK, no superconducting or any other phase transition has been observed.     

A Holmium-in-gold nuclear orientation thermometer for use from 1 K to 30 mK

A new -ray anisotropy thermometer which may ultimately be capable of primary thermometry from 1 K down to ~30 mK is reported. The thermometer is a dilute solid solution, ~0.1 at %, of ^166mHo in gold. Two different sample preparation schemes and other relevant details are discussed. The anisotropy of the 712-keV -ray as a function of B and T is presented. The interpretation of the anisotropy in terms of the crystal field, electronic Zeeman, and hyperfine interaction is discussed. It is concluded that incorrect crystal field parameters for HoAu are the reason for the discrepancy between theory and experiment. Relaxation effects are briefly considered.Supported by the Australian Research Grants Committee and the Australian Institute of Nuclear Science and Engineering.     

Heat Inputs to Sub-mK Temperature Cryostats and Experiments from γ-Radiation and Cosmic Ray Muons

We measured the spectrum of energies deposited by -radiation, emanating from radioactive materials in the laboratory that houses our mK cryostat, and by cosmic ray muons. This allows us to quantify the heat input that adversely affects the lowest temperature accessible in sub-mK experiments. We use our nuclear stage, stage plate and experimental cell as a prototype model system, and calculate the power deposited due to low energy (below 2.65 MeV) background radiation quanta (~20 pW). This is significantly less than the power (~120 pW) deposited in the nuclear and experimental stages by muons. Installation of a 5 cm thick lead wall around the cryostat reduced the energy due to the flux of quanta by a factor of ~10 to ~2 pW, and the number of quanta by a factor of ~20. The lower energy, soft cosmic ray component was also affected by introducing the same thickness of lead, reducing the overall count of cosmic ray derived particles by ~15% and the heat leak to ~100 pW.     

The Ni3Al-Ni3Cr-Ni3Ru section of the Ni-Cr-Al-Ru system

An investigation is reported of the 75 at% nickel section of the Ni-Cr-Al-Ru system at 1523 and 1273 K. Constitutional data obtained by electron probe microanalysis, X-ray diffraction and microscopical examination are presented as partial isothermal sections. At 1523 K, the major part of the section consists of phase, while the aluminium-rich region contains a and+ region; the extent of the solid solution of chromium and ruthenium in totals ~ 4 at%. The ruthenium-rich corner of the section shows a two-phase region consisting of + ruthenium-rich solid solution. At 1273 K the,+ and + ruthenium regions increase in extent. The/ mismatch values in the equilibrated alloys studied lie in the range ~ –0.08 to –0.39%. Constitutional features of as-cast alloys are also reported.     

Temperature effects on the properties of Ge thin films

The effects of substrate temperature (T_s) on the properties of vacuum evaporated p-type Ge thin films have been investigated for 25s<400°C. Increase in the substrate temperature improves the crystallinity and increases the grain size resulting a gradual change from amorphous to polycrystalline structure which was attained above a substrate temperature of 225°C. Low resistive (1×10^–2 ohm-cm) and high mobility (280 cm²/V·s) films were obtained at T_s=400°C. It has been observed that the conduction mechanism in polycrystalline films was dominated successively by hopping, tunneling and thermionic emission as the sample temperature was increased from 40 to 400 K. In amorphous samples, conduction was described in terms of different hopping mechanisms.     

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.     

The dielectric tensor of magnetically ordered solid 3He

Solid (bcc) ³He undergoes a magnetic ordering transition at a temperature of about 1 mK. In the low-temperature ordered phase, two adjacent atomic planes of up nuclear spins are followed by two planes of down spins. A recent explanation by Guyer and Kumar depends on the spin-lattice coupling. Their theory predicts that in the magnetically ordered phase the lattice will be deformed. The deformed (tetragonal) lattice will exhibit dielectric anisotropy. The dielectric tensor is calculated, using a simple generalization of the Clausius-Mosotti theory.On sabbatical leave from Department of Physics, Technion—Israel Institute of Technology, Haifa, Israel.     

A novel X-ray technique for inspection of steel pipes

The corrosion-induced material loss in crude oil carrying steel pipelines was originally studied by making use of the backscatter X-ray technique. The steel thickness can be determined by studying the density profile of the backscatter intensity vs. the depth location of a voxel. There are, however, some practical limitations to the above method, and a new method for evaluating steel thickness, namely, the transcatter technique is described. This technique uses the intensity of the beam which is transmitted by the pipe wall and subsequently scattered by the hydrocarbon inside the pipeline. The thickness is evaluated using three techniques, namely, the sequential technique, the dual angle technique, and the reference technique. Of the three techniques, the sequential technique has been studied in detail. The mathematical equations and experimental results related to the transcatter technique show that the thickness can be measured with an accuracy of better than 10% for a nominal steel thickness of 8 mm with a measurement time of several minutes.     

Temperature-, concentration- and pressure-dependence of the viscosity of liquid3He-4He mixtures at low temperatures

We have investigated the viscosity of liquid³He-⁴He mixtures at various³He - concentrations (0.98%x9.5%) in the temperature range1 mK T 100 mK and at pressures 0 bar P 20 bar. At T10 mK the Fermi-liquid behaviour T² = const. as well as x^4/3 could be confirmed. However, there are significant deviations from theoretical predictions for the magnitude of the viscosity as well as for its pressure dependence.     

The process of heat and mass transport at the critical point of pure fluids

The effects of fast isentropic temperature propagation, called the piston effect, or critical speeding up, and slow mass diffusion, called critical slowing down, are investigated. A temperature propagation experiment in a spherical cell filled with pure SF₆ at critical density was performed during the Second German Spacelab Mission D2 in 1993. The results evidently confirm the presence of the piston effect both in the one-phase region and in the two-phase region. The numerical simulations are in remarkable good quantitative agreement with the experimental results.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Absence of Phase-Fluctuation Contribution to the Low-Frequency Optical Conductivity in d-Wave Superconductors at Zero Temperature

The behavior of the low-frequency optical conductivity _reg() in superconducting cuprates is, at the present, an open and interesting issue. In particular, since the zero-temperature and zero-frequency limit of _reg() attains a value much larger than the universal value expected within a self-consistent T-matrix calculation, an intriguing possibility is that the collective mode can also contribute to _reg(). By taking into account the effect of dissipation on the collective mode in a d-wave superconductor, we evaluate the phase-fluctuation contribution to _reg(0) within the formalism of the phase-only action. We show that even though the collective mode contributes to _reg() at finite frequencies, approaching the zero-temperature and zero-frequency regime the corrections at _reg() due to phase fluctuations vanish.     

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.     

Regularities of MgO single-crystal failure in microindentation

The peculiarities of the fracture of indentations on the (001) plane of MgO single crystals were investigated for two orientations of the Vickers indentor: (i) the indentation diagonals were parallel to the 100 direction, and (ii) the diagonals were parallel to 110. A brittleness anisotropy was revealed. A specific structure of square cracks was observed inside the indentations of the first orientation. Such a structure did not appear in the case of the second orientation. An explanation of the brittleness anisotropy was suggested using a model of material plastic flow in the indentation and considering the interaction of dislocations belonging to the active slip planes.     

Phase Transitions in Solid Oxygen as Thermometric Fixed Points

For more than thirty years behaviour of phase transitions in solid oxygen has been the subject of an interest to thermometry because oxygen is one of few substances having three phase transitions in the low temperature range. The triple point of oxygen is the primary fixed point of the International Temperature Scale of 1990 and – and – phase transitions are recommended as secondary fixed points. In this paper heat capacity of oxygen in the region of the – transition is investigated. The heat capacity peak, which has the total width of about 50 mK, has been observed to be composed, in reality, of two sharper peaks separeted by 20 mK. Some preliminary results of the – transition investigation are also reported.     

Abundance of 766.6 and 1001 keV γ-Quanta in the 238U Decay Chain

The abundances of 63.3, 185.7, 766.6, 1001 keV -quanta in the ²³⁸U decay chain, determinedby semiconductor -ray spectrometry of metallic uranium foil, are 4.2, 48, 0.39, and 1.12%, respectively.The abundance of 766.6 and 1001 keV -ray quanta is higher than the generally accepted values by a factor of 1.9.     

Pseudogap and Transport in HTS

Perpendicular transport is one of the key factors to HTS superconductivity, sampling the quasi-insulating blocking layer, separating the conducting CuO-planes, and driving the metal–insulator transition (MIT) that is induced by disorder and underdoping. Various measurements have been carried out to study the transport, the MIT, and the in-plane Fermi surface especially by surface methods via the blocking layer, and these depend sensitively on surface quality. ARXPS results on UHV cleaving show that at 300 K and 10^–10 Torr, a Bi hydroxide layer occurs in 30 min, followed by H₂O or C_y H_x OH chemisorption. Consequences of this result on STS, ARPES, perpendicular transport, Coulomb charging, and pseudogap are analyzed, yielding scenario for HTS superconductivity, where static and dynamic charge exchange via and with the blocking layer initiates plaques of preformed pairs of d-wave symmetry weakening the inplane Coulomb repulsion yielding by this plasmonic mechanism, finally, HTS. Consequences of this scenario on anisotropic transport with its strong Fermi velocity v_F anisotropy and its strong in-plane scattering rate (T) const. at (, 0) in k-space with pseudo gap kT^* _{_P}/3 and superconducting gap _S 3 kT_c maxima and the strongly decreasing rate (T) T at 0.4 (, ) with pseudo gap _{_p} (k) node and superconducting gap _s (k node are given.     

Kinetic interpretation ofα- andβ-Si3N4 formation from oxide-free high-purity silicon powder

A kinetic analysis of the isothermal nitridation of high-purity oxide-free silicon powder is described. The kinetic analysis suggests that the and polymorphs of Si₃N₄ are formed by separate and parallel reaction paths. This analysis provides for the decoupling and quantitative kinetic interpretation of- and-Si₃N₄ formation reactions. Consistent with existing microstructural and thermodynamic evidence, the-forming reaction is shown to obey a first-order rate law, whereas a phase-boundary controlled rate law describes the-forming reaction. A kinetic model employing these rate laws is developed and is used to predict the/ phase ratio as a function of isothermal reaction temperature and extent of reaction. The/ phase ratios so obtained are shown to be in good agreement with experimental observations made under a variety of reaction conditions.