Magnetothermal Transport of Oriented Graphite at Low Temperatures

We have studied the magnetic field dependence of the thermal conductivity κ(T,B) of highly oriented pyrolytic graphite samples at temperatures 0.2K≤T < 10K and fields 0T≤ B ≤ 9T. The samples show clear deviations from the Wiedemann-Franz law with a kink behavior at fields B ～ 0.1 T near the metal-insulator transition observed in electrical resistivity measurements. We further show that the oscillations in the thermal conductivity at the quantum limit B > 1 T, which are correlated with the Landau quantization observed in Hall measurements, increase in amplitude with temperature following a ～ T³ law at T > 0.2 K and show a maximum at T ～ 6 K, suggesting that they are phonons mediated. PACS numbers: 72.20.My, 72.15.Eb, 73.43.-f     

Mechanism of Superconductivity in Graphite Intercalation Compounds Including CaC6

On the basis of the model that was successfully applied to KC₈, RbC₈, and CsC₈ about a quarter century ago, we have implemented a quantitative calculation of the superconducting transition temperature T _c to find that the observed values for T _c in both CaC₆ and YbC₆ are also reproduced in the model, indicating that the model is a unified one for superconductivity in the graphite intercalation compounds with T _c ranging three orders of magnitudes.     

Thermal Conductivity of Superconductor Sr2RuO4 at Low Temperatures and Magnetic Fields

The temperature dependence of thermal conductivity of Sr₂RuO₄ is investigated by the Boltzmann equation approach. We consider both the isotropic and anisotropic gap energy in superconducting state in the absence and presence of low magnetic field, and obtain the temperature dependence of the components of thermal conductivity in both cases. In the case of isotropic gap, it is proportional to T ^1/2, whereas in the case of anisotropic gap, K _xx ∝T ^?1,K _yy ∝T, and K _xy =K _yx =constant. Furthermore, we show that at a low magnetic field K behaves as in the case of the zero magnetic field.     

Electrical Spin Injection from Ferromagnetic Nanocontacts into Nondegenerated Silicon at Low Temperatures

We present results on the magnetoresistance of the system Ni/Al₂O₃/Si/Al₂O₃/Ni fabricated in lateral nanostructures. The substrate n-type Si is a nondegenerated semiconductor with a doping level of 10¹⁵ cm^?3. The results are presented between 11 and 30 K, where the electrical resistivity of the semiconductor varies about 4 orders of magnitude. The reduction of magnetoresistance at 30 K is consistent with the standard theory for spin injection between a metal and a semiconductor. By fitting the data with $e^{ - t_{N} / L_{\mathrm{SD}}}$ , the diffusion condition, as a function of the channel length t _N , where the magnetoresistance takes place, we deduced the values of spin diffusion length L _SD and spin lifetime τ _s .     

X-ray Diffraction Study of MgB2 at Low Temperatures

We have performed powder X-ray diffraction of MgB₂ superconductor between 10 and 300 K. Temperature dependence of integrated intensities of both (002) and (110) X-ray reflections shows a peak at around T_C, superconducting transition temperature. The integrated intensity of the X-ray reflection is related to the phonon frequency through a Debye--Waller factor. Lattice parameters a and c show negative thermal expansions at low temperatures. The negative thermal expansion might be due to an electronic origin and not directly related to the superconducting transition.     

Possible Phase Transition at Low mK Temperatures in Liquid Helium Mixture Films Adsorbed on Graphite

We report heat capacity and magnetisation measurements of ³He adsorbed on the surface of graphite plated with three atomic layers of ⁴He. For ³He coverages n ₃>4 nm^?2 the heat capacity corresponds to a 2D Fermi fluid. The inferred hydrodynamic mass of the ³He quasiparticles is 1.38±0.05 m₃. The ³He effective mass ratio increases with coverage to 2.4 at n ₃=4 nm^?2, due to Fermi liquid interactions. The heat capacity isotherm exhibits a steplike increase centred on n ₃=4.5 nm^?2, similar to that previously observed on four layers of ⁴He. This is associated with the population of the first excited Andreev surface bound state. However, in the present case, as n ₃ is increased through the step a pronounced anomalous feature develops in the temperature dependence of the heat capacity, around 10 mK. Below 5 mK the heat capacity is approximately linear in temperature. With n ₃=7 nm^?2, we find that this behaviour is very sensitive to small changes in the ⁴He third layer coverage, around the completed third layer. Measurements of the ³He magnetisation,, by continuous wave NMR methods, find a significant increase with decreasing temperature below around 20 mK. Together the data suggest that a phase transition takes place in the film at low mK temperatures.     

Coherent Spin Precession in Superfluid 3He-B Excited in a Field Minimum at Low Temperatures

No Heading The persistent precessing domain (PPD) is an isolated region of coherent spin precession which is observed in the B phase of superfluid ³He at the lowest achievable temperatures. It has many unusual properties and its free decay can exceed 1000s at the lowest temperatures. Previous observations of the PPD were highly irreproducible but we now find the PPD to be very reproducible when there is a field minimum along the cell axis. Here we discuss measurements of the PPD as we control the magnetic field profile, allowing the depth of the minimum to be adjusted.PACS numbers: 67.57.Lm, 67.57.Jj, 67.57.Fg     

细颗粒掺杂石墨高温热导率的研究

通过球磨分散方法制备了具有优良热机械性能的细颗粒掺杂石墨.该掺杂石墨材料具有致密的结构,细小的碳化物颗粒均匀分布在石墨基体中.研究表明;球磨分散方法制备的掺杂石墨比常规方法制备的石墨材料具有更优良的高温热性能以及抗热震性能.同时,本文对材料的性能与其微观结构进行关联.     

X-Ray Diffraction Study of Correlated Electron System at Low Temperatures

By using an ultra-low temperature X-ray diffractometer, we studied low temperature phase transitions of several materials. These phase transitions are classified into a ferro quadrupole ordered phase, that is, Jahn–Teller distortion (TmVO, ZnCrO), an antiferro quadrupole ordered phase (CeB, PrPb), and a superconducrivity (MgB). We also investigated unknown phases, such as the low temperature phase in PrPtBi and the 4th phase in CeLaB. Our low temperature X-ray measurement gives rise to a fruitful information about the phase transition. Not only a whole profile of the reflection peaks, which can be analyzed by the Rietveld method, we also made the precise measurement of the temperature dependence of the integrated intensity of the reflection peak, the full width at half maximum, and the lattice spacing for some fixed reflections. The temperature dependence of the integrated intensity (I.I.) can be expressed by the Debye–Waller expression. At low temperatures, the change of the phonon frequency can be obtained from the I.I. through the Debye–Waller factor. In MgB we observed the hardening of the phonon and the softening of phonon above and below the superconducting transition temperature, respsctively. This result suggests the relevant mechanisim of superconductivity in MgB is the electron–phonon interaction. From the precise measurement of the lattice constant, we found the negative thermal expansion at low temperatures in many correlated electron system. This negative thermal expansion can be explained by the Fermi liquid theory by Misawa.     

Cryogenic Processing: A Study of Materials at Low Temperatures

Cryogenics is an exciting, important and inexpensive technique that already has led to main discoveries and holds much future assurance. Cryogenic processing is the treatment of the materials at very low temperature around 77 K. This technique has been proven to be efficient in improving the physical and mechanical properties of the materials such as metals, alloys, plastics and composites. It improves the wear, abrasion, erosion and corrosion resistivity, durability and stabilizes the strength characteristics of various materials. Cryogenic refines and stabilizes the crystal lattice structure and distribute carbon particles throughout the material resulting a stronger and hence more durable material. In present paper, we have reviewed the effect of cryogenic treatment on some metals, alloys, plastics and composites.     

不同添加剂对石墨材料高温粘结性能的影响

在以酚醛树脂(ＰＦ)和碳化硼(Ｂ_４Ｃ)为原料的高温粘结剂基础上,Ａｌ_２Ｏ_３,Ｓｉ和白炭黑(超细ＳｉＯ_２)作为第二种添加剂分别加入到粘结剂中以制备新型高温粘结剂,并对石墨材料进行粘结．用新型高温粘结剂粘结的石墨部件分别在２００、８００、１５００℃下热处理,并在室温条件下测试其剪切强度．结果表明,复合添加剂对石墨粘结部件的粘结性能有明显的影响．其中,酚醛树脂＋Ｂ_４Ｃ＋白炭黑粘结剂具有较为理想的粘结性能,白炭黑的添加对提高石墨材料的高温粘结性能具有明显的作用．     

SQUID-based Technique to Study the Elastic Properties of Solids at Very Low Temperatures

Mechanical oscillators are widely used for measurements of the elastic properties of solids at low temperatures. In most of such experiments mechanical displacements are detected via capacitive changes. In order to improve the sensitivity we have developed a novel detection technique relying on the high sensitivity of a commercial dc SQUID. To demonstrate the performance of the new setup we have measured the variation of the sound velocity and the internal friction of vitreous silica and of the borosilicate glass BK7 in the temperature range from 5 mK to 1 K. Our results agree favorably with former experiments where the displacement was detected capacitively. Furthermore, our results confirm that deviations of the elastic behavior of amorphous solids from the predictions of the standard tunneling model exist at very low temperatures.      

Matrix Isolation of H Atoms at Low Temperatures

The recent history of the matrix isolation of atomic free radicals at low temperatures started with a research program at the US National Bureau of Standards and continued with the important breakthrough at Chernogolovka in Russia where a jet containing atomic free radicals was directed onto the surface of superfluid ⁴He. The samples collected consisted of gel-like substances made up of molecular nanoclusters, allowing the atomic free radicals to be isolated from one another and studied at 1.3 K. More recently, techniques were developed at Turku University which have been made the region T<1 K accessible for studies of H atoms entrapped in H₂ films. Very high concentrations of H atomic free radicals (??10¹⁸?C10¹⁹ cm^?3) have been attained using both the Turku and Chernogolovka methods. A discussion of the most recent experiments at Cornell and Turku will be given. Microwave and mm wave electron paramagnetic resonance techniques have been employed in these experiments. These techniques permitted studies of the exchange tunneling chemical reaction D+HD??H+D₂. Diffusion of H atoms through solid H₂ proceeds via the reaction H+H₂??H₂+H, leading to recombination (H+H??H₂). Quantum overlap of H atoms is thought to be responsible for exotic behavior of H atoms in solid H₂ films below 1 K, including a significant departure from the Boltzmann distribution of the relative populations of the two lowest hyperfine levels of atomic H.     

低温制备纳米α-Al2O3粉体

研究了以Al(NO3)3.9H2O和NH3．H2O溶液为原料,采用高纯氧化铝球磨原位引入晶种和添加ZnF2、AlF3的制备工艺,通过提高凝胶中晶种的均匀性和加速相转变时物质的扩散和传输,可降低过渡型氧化铝向α型氧化铝相转变温度,所制铝凝胶经900－920℃煅烧可获得平均粒径＜50nm的α型纳米氧化铝。     

低温制备纳米α-Al_2O_3粉体

研究了以 Ａｌ（ＮＯ３）３·９Ｈ２Ｏ和ＮＨ３·Ｈ２Ｏ溶液为原料,采用高纯氧化铝球磨原位引入晶种和添加 ＺｎＦ２、 ＡｌＦ３的制备工艺．通过提高凝胶中晶种的均匀性和加速相转变时物质的扩散和传输,可降低过渡型氧化铝向α型氧化铝相转变温度,所制铝凝胶经９００～９２０℃煅烧可获得平均粒径＜５０ｎｍ的α型纳米氧化铝．     

Models of Ballistic Propagation of Heat at Low Temperatures

Heat conduction at low temperatures shows several effects that cannot be described by the Fourier law. In this paper, the performance of various theories is compared in case of wave-like and ballistic propagation of heat pulses in NaF.