Thermal conductivity of superconducting UPt3

The thermal conductivity of the heavy-fermion superconductor UPt₃ has been measured on two single crystals of different quality as a function of temperature and applied magnetic field for different orientations of the heat current relative to both the field and the crystalline axes. The temperature dependence of is far from exponential and nearly the same for both crystals, in which the heat current is, respectively, parallel and perpendicular to the hexagonalc axis, suggesting a gap structure with nodes in the basal plane and normal to it. The field dependence of is strongly anisotropic. In the best sample at low fields, where the scattering of heat carriers by vortices is thought to be important, (H) depends on the relative orientation of field and current. On the other hand, at high fields nearH _c2 (in both samples), (H) depends on the relative orientation of field and crystalline axes, reflecting an anisotropy in the gap structure and in the Fermi velocities.     

Superconducting phase diagram of UPt3 studied by thermal expansion and specific heat

The specific heat and thermal expansion of UPt₃ were measured in the vicinity of the superconducting transition for magnetic fields up to 10 kOe, applied along the hexagonalc axis. Both thermodynamic techniques show clear evidence of two transitions in zero field, which merge at a critical field of approximately 8 kOe. The upper critical field curve determined by the resistivity for the upper transition exhibits a small but abrupt change of slope at a comparable value of the field. The phase diagram that emerges forHc is therefore qualitatively similar to that previously found forHc, this time with a critical point (or region) aroundH*=8 kOe,T*=0.36 K. The implications of a nearly isotropic phase diagram are discussed in connection with recent theories.     

Mo-Cu合金的制备及导热模型

采用QM-1SP4-CL行星式球磨机将Mo、Cu粉简单混合和机械合金化后进行压片,在1250℃液相烧结1.5h后获得致密的Mo-30Cu合金;添加1.5%硬脂酸作造孔剂的Mo、Cu混合粉压片在1050℃固相烧结1.5h后获得孔隙率为33.186%的Mo-30Cu合金。利用激光热导仪测定所制备的Mo-30Cu合金的热导率。采用Maxwell模型、Hasselman and Johnson模型、单元结构模型和多相系统的传导性计算Mo-30Cu合金的理论热导率值。通过Mo-30Cu合金的热导率实测值与理论值的比较,得出适用于不同工艺状态的Mo-30Cu合金的热导率模型。     

用于材料导热系数瞬态测量的数学模型

本文介绍了用于材料导热系数瞬态测量的两种数学模型。它们考虑到在实验过程中材料膨胀的影响。两个微分方程均适用于由大脉冲电流直接加热的细长型管状样品的瞬态实验。在第二个方程中 ,时空变量完全保持独立。它对于进行计算机模拟特别有效。     

Thermal conductivity and gap structure of the superconducting phases of UPt3

We present new measurements of the thermal conductivity (κ) of UPt₃ down to very low temperatures (16 mK) and under magnetic fields (up to 4 T) which cover all the superconducting phases of UPt₃. The measurements in zero field are compared with recent theoretical predictions for the thermal conductivity, which is dominated by impurity states at the lowest temperatures studied. The measurements under magnetic field at low temperatures are surprising since they don’t show the expected low field square root dependence, . The discontinuity ofdκ/dT atT _c changes drastically when passing from the high field low temperatureC phase to the low field high temperatureA phase: this is related to the change of the symmetry of the superconducting order parameter when crossing theA→C phase transition.     

Thermal conductivity of superconducting UPt3 at low temperatures

We study the thermal conductivity within the E_1g and E_2u models for superconductivity in UPt₃ and compare the theoretical results for electronic heat transport with recently measured results reported by Lussier, Ellman and Taillefer. The existing data down to T/T_c 0.1 provides convincing evidence for the presence of both line and point nodes in the gap, but the data can be accounted for either by an E_1g or E_2u order parameter. We discuss the features of the pairing symmetry, Fermi surface, and excitation spectrum that are reflected in the thermal conductivity at very low temperatures. Significant differences between the E_1g and E_2u models are predicted to develop at excitation energies below the bandwidth of the impurity-induced Andreev bound states. The zero-temperature limit of the axis thermal conductivity, lim_T0 k_c/T, isuniversal for the E_2u model, but non-universal for the E_1g model. Thus, impurity concentration studies at very low temperatures should differentiate between the nodal structures of the E_2u and E_1g models.     

The Angular Dependence of the Critical Current in UPt3 and the Symmetry of the Order Parameter

The superconducting critical current along the c-axis of the unconventional superconductor UPt₃ has been determined as a function of magnetic field direction in the basal (a-a*) plane in the three (A, B and C) phases of the mixed state. The critical current has maxima for field directions parallel to both the a- and a*-axis in the superconducting C phase, but has maxima only parallel to the a-axis in the A and B phases. Our experiments thus show that the anisotropy of the superconducting critical current in UPt₃ changes at the same field at which the symmetry of the order parameter changes. This demonstrates for the first time that the critical current is sensitive to the symmetry of the superconducting order parameter.     

聚合物基复合材料导热模型的研究现状及应用

综述了描述聚合物基复合材料导热性能的经典模型和新模型,详细列举了各模型的特点,比较了模型之间的区别,具体分析了影响复合体系导热性能的各种因素.结合对碳纳米管复合材料的研究给出了一些常用模型的应用建议,同时提出了当前研究中存在的模型通用性差的问题,指出今后聚合物基导热复合材料的研究趋势应为开发新的高导热纳米填充物和纳米复合技术以使填充物在基体中能形成有效的导热通路,从而改善复合体系的导热性能.     

Thermal Conductivity of n-Aluminium Nitride-Added MgB2 Superconductor in Normal and Superconducting State

Thermal conductivity (k) of 0, 5, 10, and 15 wt.% aluminium nitride (n-AlN)-added polycrystalline MgB ₂ superconductors, synthesized by solid reaction is discussed both in the normal and superconducting states between 20 and 300 K. The prepared samples are characterized using X-ray diffraction (XRD) and field electron gun scanning electron microscope (FEG–SEM). Resistivity measurement confirms a decrease in superconducting transition temperature of MgB ₂ (T _c=38.5 K) with n-AlN addition and decreases to ～35 K in case of 15 wt.% n-AlN-added MgB ₂ sample. Thermal conductivity of both MgB ₂ and n-AlN-added MgB ₂ pellets does not show any hump around T _c, and the absolute values of k decrease with increasing n-AlN in MgB ₂. Temperature dependence of the thermal conductivity of MgB ₂ and n-AlN-added MgB ₂ has been analyzed, assuming the role of both electrons and phonons. The Wiedemann–Franz law does not work well for the present samples, which indicates inelastic scattering (L _eff < L ₀). Thermal conductivity of MgB ₂ and n-AlN-added MgB ₂ pellets is explained by assuming effective Lorentz number, L _eff= 0.1 L ₀. Electronic thermal conductivity in superconducting state ( \(k_{\text {el}}^{\mathrm {s}} )\) follows “two-gap model” and has been used to estimate the values of band gaps, relative contribution of each band in thermal transport, and intraband scattering relaxation time. The estimated values are fairly consistent with the previously reported results for MgB ₂. We further confirm that n-AlN addition in MgB ₂ introduces disorders in π bands, which reduce the π band gapsand intraband relaxation time ( \(\tau _{\pi }^{\text {im}})\) . The lattice contribution of thermal conductivity in both normal and superconducting states is analyzed in the terms of Callaway’s model, assuming various phonon scatterings. Our analysis indicates that the lattice thermal conductivity of MgB ₂ is dominated by phonon-sheet-like fault scattering. Addition of n-AlN in MgB ₂ enhances the phonon scattering from sheet-like faults, and dislocations induced strain field scattering by >7 times compared to that for pure MgB ₂ pellets.     

Controlled Creation of Structural Defects in the Heavy Fermion Superconductor UPt3 and Its Influence on the Superconducting Properties

The superconducting properties of the heavy fermion UPt ₃ have been changed by irradiation with high energy electrons which creates point defects in a reproducible and controled way. Measurements of the residual resistivity, critical temperature, upper critical field and thermal conductivity have been realized on these irradiated samples. The strong suppresion of superconductivity with increasing defect concentration is in agreement with the theory of unconventional superconductivity. However, our thermal conductivity data contradicts the simple predictions derived from the most popular modelsE _1g and E _2u) of the superconducting order parameter in UPt ₃ .     

Superconducting Gap Structure of YNi2B2C Determined by Thermal Conductivity

In order to determine the superconducting gap structure of the borocarbide superconductor YNi₂B₂C, we have measured the angular variation of the c-axis thermal conductivity _zz in magnetic field rotated within the ab-planes. A clear fourfold symmetry with narrow cusps was observed in the angular variation of _zz . These results provide a strong evidence that the gap function has point nodes which are located along the a- and b-axes of the crystal.     

Two Effective Thermal Conductivity Models for Porous Media with Hollow Spherical Agglomerates

Based on the microstructure features of xonotlite-type micro-pore calcium silicate, two unit cell models, the point-contact hollow spherical model and the surface-contact hollow cubic model, are developed. As one of several excellent insulation materials, xonotlite is represented as porous media with hollow spherical agglomerates. By one-dimensional heat conduction analysis using theunit cell, the effective thermal conductivity of xonotlite is determined. The results show that both of the models are in agreement with experimental data. The algebraic expressions based on the unit cell models can be used to calculate the effective thermal conductivity of porous media that have similar structure features as xonotlite.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China     

Microwave Conductivity of Superconducting and Insulating Fullerides

Superconductivity of A₃C₆₀ (A=K and Rb) is destroyed by the intercalation of neutral ammonia molecules, and is replaced by the antiferromagnetic ground state. These properties suggest the importance of the electronic correlation in fullerides, which have been believed to be simple BCS superconductors. By using a microwave cavity perturbation technique, we confirmed that the conductivity of the antiferromagnet (NH₃)K_3–x Rb _x C₆₀ at 200 K is already 3–4 orders of magnitude smaller than those of superconductors, K₃C₆₀ and (NR₃) _x NaRb₂C₆₀, and that the antiferromagnetic compounds are insulators without metal-insulator transition at the Néel temperature. These results indicate that the Mott–Hubbard transition in the A₃C₆₀ systems is driven by a reduction of lattice symmetry from face-centered-cubic (fcc) to face-centered-orthorhombic (fco), rather than by the magnetic ordering.     

A theory for the superconducting phases of UPt3

I discuss a phenomenological theory for the multiple superconducting phases of UPt₃ that is based on an order parameter belonging to an orbital 2D representation of the hexagonal point group which is coupled to a weak symmetry breaking field. I show that (1) the existing H-T and P-T phase diagrams (including an apparent tetracritical point in the H-T plane for all field orientations), (2) the anisotropy of the upper critical field over the full temperature range, (3) the correlation between superconductivity and basal plane antiferromagnetism and (4) low-temperature power laws in the transport and thermodynamic properties can be explained qualitatively, and in many respects quantitatively, by an odd-parity, E_2u order parameter with a pair spin projection of zero along the -axis. AFM ordering in the basal plane, which couples to the superconducting order parameter, acts as the symmetry breaking field that is responsible for both the apparent tetracritical point and the zero-field double transition.     

Thermodynamic and transport properties of UPt3

In the normal phase of UPt₃, the magnetoresistivity is large and positive with the striking feature of a quasi-independence of the temperature and magnetic field terms; the thermoelectric power has aT ² dependence and the susceptibility is almost constant up to 4 K. The superconducting transition is broadened and the specific heat jump is weak due to the strong anisotropy of its normal phase. FromT _c to 146 mK (the lowest experimental temperature), a largeT ² contribution is observed in the specific heat, but the thermal conductivity has the same dependence only below 150 mK. These results are compared with the predictions given for polar odd-pairing superconductivity.     

Thermal Conductivity of Normal Liquid 3He in Aerogel

We report thermal conductivity measurements for normal phase liquid ³He inside 98 and 95% aerogels at dilution refrigerator temperatures (5 to 50 mK). In our measurements we use cylindrical samples of aerogel which have been fabricated inside quartz tubes. Each tube is enclosed at one end by a small chamber to which a known heat input is applied at a measured temperature. The other end is in an open volume of helium which is heat sunk to the refrigerator. Vibrating wire thermometry is used to measure the helium temperatures. The thermal conductivity measurements show clearly the effects of quasiparticle-impurity scattering, from which values of the mean free path are derived. These are 92 nm for the 98% aerogel and 56 nm for the 95% sample.     

3D Anisotropic Thermal Conductivity of Exfoliated Rhenium Disulfide

ReS₂ represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single‐crystalline ReS₂ in a distorted 1T phase is determined at room temperature for the in‐plane directions parallel and perpendicular to the Re‐chains, and the through‐plane direction using time‐domain thermoreflectance. ReS₂ is prepared in the form of flakes having thicknesses of 60–450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in‐plane thermal conductivity is higher along the Re‐chains, (70 ± 18) W m^?1 K^?1, as compared to transverse to the chains, (50 ± 13) W m^?1 K^?1. As expected from the weak interlayer bonding, the through‐plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m^?1 K^?1, resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in‐plane directions. The thermal conductivity and interface thermal conductance of ReS₂ are discussed relative to the other 2D materials.     

Enlarged Photonic Band Gap in Heterostructure of Metallic Photonic and Superconducting Photonic Crystals

We show theoretically that the frequency range of photonic band gap of a hetero-structure which is made of a metallic photonic and superconducting photonic crystal can be enlarged due to the combination of the reflection band properties of the superconductor–dielectric (PC1) and metallic–dielectric (PC2) periodic structures. The transmittance and band structure of the considered structures are calculated using simple transfer matrix method and the Bloch theorem. Beside this, we have also calculated the transmittance of the superconducting photonic structure (PC1), metallic photonic structure (PC2) and heterostructure of metallic photonic and superconductor photonic crystals (PC1/PC2) for TE and TM-mode at the different angles of incidence.