CeCu2Si2和UBe13的超导理论(Ⅱ)——热力学量的计算

根据文献[1]中给出的模型和能隙方程,计算了超导态的热力学临界磁场,T_c处的比热跃变以及T_c以下的比热行为。结果表明:有关热力学量的临界值与实验结果定性相符;T_c以下的比热行为在整个温区与CeCu₂Si₂和UBe₁₃的实验结果符合。 关键词：     

一个可能的P波型重费密子超导体——UBe13

在朗道的费密液体理论框架下,我们解释了重费密子超导体UBe₁₃的超导临界温度随压力变化的实验结果。所得结论是:PBe₁₃可能是一个P波型的重费密子超导体。同时也发现由于加压使得系统的f能级与费密能级的相对移动和f电子与导带电子杂化的增加对S波超导有利,而对P波超导不利。 关键词：     

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe₁₃, URu₂Si₂ and U₆Fe. The magnetic field penetration depth in the superconducting state of UBe₁₃ is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowT _c . A sharp increase in the precession frequency is seen, starting atT _c . This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U₆Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atT _c=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.     

Phenomenological model of superconductivity in U1−xThxBe13

A phenomenological model taking into account the interaction between superconductivity and the coherence of Kondo screening is introduced. This model describes the main experimental data on UBe₁₃, including the behaviour of T_c in U_1−xTh_xBe₁₃ under ambient and elevated pressures.     

Structural differences between single crystal and polycrystalline UBe13

We report on observations of structural and chemical differences between samples of UBe₁₃ that were synthesised using two different methods. Unexplained discrepancies in properties between samples with differing synthesis had previously been found in this heavy fermion superconductor. A polycrystalline UBe₁₃ sample was made by arc-melting the constituents. Single crystals were grown using an aluminium flux and had a consistently slightly larger lattice parameter than the polycrystals, which merited further study. Neutron diffraction data were collected at the Lujan Center at LANSCE on the HIPPO diffractometer. Aluminium was detected by inductively coupled plasma mass spectrometry (ICP-MS) in the flux-grown single crystal (0.803 wt%), and small amounts (~0.2 wt%) of thorium were detected in the UBe₁₃ polycrystalline sample. In order to probe the implications of the presence of Al, calculations by spin-polarised DFT-GGA+U show that the incorporation of Al onto the 96i site (the lowest symmetry site in the structure) is energetically more favourable than on other sites. In general, the trends calculated by DFT for bond lengths and lattice parameter increases are consistent with bond lengths experimentally observed by neutron diffraction, but specific percentage changes with aluminium incorporation may be obscured by the unexpected thorium in the polycrystalline sample. The aggregate of our initial observations suggests that incorporation of aluminium from the flux into single crystal UBe₁₃ is significant.     

三重态双极化子的超导A相与B相

本文应用对称破缺的自洽Hartree近似讨论了三重态双极化子的超导A相和B相。在弱耦合的情况下,通过自由能的计算得到B相总比A相稳定;在强耦合的情况下,加入涨落反馈项可使得A相的自由能比B相的自由能低,或者发生A相到B相的转变,这和³He超流的情形类似。本文还精确计算了超导稳定相的热力学临界场H_c随温度的变化,并与重费密子超导体UBe₁₃,UPt₃和URi₂Si₂的上临界场作了比较,发现H_c能定性解释所有这些超导体的上临界场的反常温度特性。 关键词：     

Thermodynamic properties of the superconducting state in YC2

We characterise the superconducting state of the binary carbide YC₂ by means of magnetisation and specific heat measurements. The thermodynamic properties reveal weak coupling superconductivity close to the type I limit (κ_GL ≈ 0.8). The specific heat data in the superconducting state, as well as the¹²C/¹³C-isotope shift are in excellent agreement with weak coupling BCS predictions.     

Anisotropy effects in cesium flouroxide tungsten bronze

The upper critical field H_c2(T) and the specific heat jump ΔC(T_c) are calculated for Cs_0.1WO_2.9F_0.1 using a two-band model. The model parameters are obtained by adjusting the theoretical H_c2(T) values to experimental results. The model calculation predicts an anomalous specific heat jump ΔC as a function of the inverse relation T_c(H).     

Physicochemical,dielectric, and piezoelectric properties and conductivity of LiNbO<Subscript>3</Subscript>: ZnO crystals (4.02–8.91 mol %)

The physicochemical characteristics of the crystal–melt system during the growth of LiNbO₃: ZnO crystals have been investigated in the range of impurity concentration [ZnO] in the melt of 4.02–8.91 mol %. The threshold impurity concentration corresponding to a significant change in the formation conditions and structure of LiNbO₃: ZnO crystals is refined ([ZnO] = 6.76 mol % in the melt). The dielectric and piezoelectric properties and conductivity of multidomain LiNbO₃: ZnO crystals have been analyzed. The occurrence of a significant spontaneous increase in the unipolarity upon high-temperature annealing has only been shown to be typical of LiNbO₃: ZnO crystals grown from melts in the near-threshold concentration range (~5.4 < [ZnO] ≤ 6.76 mol % in the melt). This effect is accompanied by a large and reproducible increase in the static piezoelectric coefficient d ₃₃₃. The value of the piezoelectric-coefficient jump Δd ₃₃₃ linearly increases with an increase in the specific-conductivity jump Δσ near the temperature T* ≈ 800 K.     

Muon Knight shift and zero-field relaxation in (U,Th)Be13

We report μ⁺ zero-field relaxation and Knight shift studies of the heavy-fermion superconductors U_1−xTh_xBe₁₃, x=0 and 0.033. The Knight shift in UBe₁₃ shows a strong decrease as the temperature is reduced in the superconducting state, unlike U_0.967Th_0.033Be₁₃ in which the shift remains at about the normal state value. If the superconducting state in UBe₁₃ has odd-parity, the decrease of K_μ below T_c suggests that the order parameter is pinned to the lattice. Either spin-orbit scattering or the existence of two distinct superconducting states with different spin susceptibilities in the (U,Th)Be₁₃ system would explain the differences observed in the Th-doped and pure UBe₁₃ materials. The latter hypothesis would exclude conventional BCS superconductivity. No evidence for magnetic order is seen in the zero-field relaxation rate for either material down to 0.3 K.     

Investigations of UBe13 of other compounds of Be and U in normal and superconductive states

During the last few years superconductive systems of heavy fermions with highly large values ofm* and electronic heat capacityγT have been thoroughly investigated. The following compounds viz CeCu₂Si₂ (T _c=0.6 K;γ=1100*), UBe₁₃(T _c=0.95 K;γ=1000) and UPt₃ (T _c=0.5 K;γ=450)(I) may be referred to such systems as well as the U and Ce compounds: U₂Pt C₂ (T _c=1.47 K;γ=75), U₆Fe (T _c=3.86 K;γ=25), U₆Co (T _c=2.3 K;γ=21), URu₂Si₂ (T _c=0.68 K,γ=17.6), as well asα-U (T _c=2.1;γ=12), CeRu₃Si₂ (T _c=1 K;γ=39), CeOs₂ (T _c=1,1 K;γ=22), CeRu₂ (T _c=6 K;γ=23.3) and α-Ce (T _c?2K;γ=14) (Alekseevskii and Homskiy 1985). It should be noted that there exists a class of U and Ce compounds with a similar structure as those given above, which undergo transition to superconductive state, but are not characterized by abnormal values ofm* (Alekseevskii 1984). Many authors considered superconductivity of heavy fermion systems as unusually anisotropic where charge carrier coupling occurs in P-state (Stewart 1984). On the other hand such a view does not agree with many experimental results, e.g. lack of anisotropy Hc₂ for UBe₁₃ (Alekseevskiiet al 1985) as predicted by Gorkov (1984) and the results of investigation of the Josephson effect. The Hall-effect investigations for UBe₁₃ in a wide range of fields and temperatures (Alekseevskii 1984) make it possible to consider systems with two types of carriers—heavy and light. The unique properties of the above systems in a number of cases are possibly caused by these two types of carriers and the peculiarity of interaction between them.     

Specific heat jump for superconducting virtual bound state alloys

Using Anderson model in the non-magnetic limit, we have calculated the jump in specific heat at T_c, (ΔC), for superconducting virtual bound state alloys. It is found that the normalized (ΔC) vs normalized T_c curves deviate considerably from the BCS value. A more significant result of the present study is that the initial slope of such a curve has a maximum value of 3.638. It is the highest value reported in literature for the impurity problem and is much larger than obtained even in a large pair breaking situation like Kondo effect (2.481).     

Localized spin fluctuations in superconducting alloys

The effects due to localized spin fluctuations (LSF) in dilute superconducting alloys are investigated. Expressions for the critical temperature T_c as function of impurity concentration n_I and the specific heat jump at the transition ΔC(T_c) are obtained, both in the weak and strong magnetic limits. The results are discussed in the light of available experimental data.     

Critical field and specific heat of the anisotropic superconductor Nb3S4

We report on the first measurements of the critical field B_c2 and the specific heat of Nb₃S₄, which is a linear structured compound with a superconducting transition temperature of 3.65 K. The angular dependence of B_c2 is well described by the effective-mass model. The ratio of the critical fields parallel and perpendicular to the c-axis gives a value of 4.6. The small value of the specific heat jump at $\text{T}{}_{\mathrm{c}}\text{(}\text{Δc}\text{γT}{}_{\mathrm{c}}\text{= 0.95)}$ can be explained with an anisotropic gap function.     

Specific heat of a transforming V3Si crystal

The specific heat of a transforming V₃Si crystal in the normal, mixed, and superconducting states has been measured from 6 to 30K in zero and 52.3 kG magnetic fields. An analysis has been carried out in a self-consistent way based on the second-order phase transition from the normal to the superconducting state in zero magnetic field. Various physical parameters characterizing the superconducting and normal states are derived from the thermodynamics and the BCS weak coupling theory. The most important parameter obtained in this analysis is 2Δ(0)/kT_c = 3.46, which indicates a weak coupling in our V₃Si sample.     

DC current effect on the magnetic phase and transport properties of polycrystalline La0.7Ca0.3MnO3

The influence of DC current on the resistivity and phase transition of polycrystalline La_0.7Ca_0.3MnO₃ has been investigated. The specific heat measurement found that charge carriers and ferromagnetic spin-wave contributions were changed after applied DC current. Applying high electric fields leads to the formation of ferromagnetic regions. The resistivity drops abruptly once the percolating current path is established. As current through the sample disappears, the larger ferromagnetic (FM) clusters, however, remain and are frozen in giving a measurable contribution to the specific heat of the system. The larger clusters should give rise to the value of spin-wave stiffness constant (D), as it is expected to increase the strength of the ferromagnetic coupling. The metallic ferromagnetic regions would make the charge carrier delocalization and attribute to specific heat linear term γ.     

Specific heat and critical behavior of Tl0.3MoO3 near the Peierls phase transition

The specific heat of the quasi-one-dimensional charge-density-wave (CDW) compound Tl_0.3MoO₃ has been measured using an adiabatic continuous heating method from 100 to 220 K. A specific heat jump associated with the Peierls phase transition occurs at 172.3 K. A good scaling relation between the excess specific heat and the susceptibility is found between 140 K and 190 K. Further analysis indicates that the width of the critical region of Tl_0.3MoO₃ is about 10 K and the specific-heat critical behavior can be well described by the three-dimensional XY model.     

Superconductivity of triplet bipolarons

Superconductivity of triplet bipolarons has been investigated in a phase analogous to the superfluid A-phase of ³He by use of the broken-symmetry Hartree approximation. The transition temperature T_c, the chemical potential and the order-parameters have been obtained, and the specific heat and the thermodynamic critical field have also been calculated. It has been found that there exist two energy gaps in the superconducting state. In the normal phase there also exists a gap, whose magnitude is consistent with the recent experimental evidence of the heavy-fermion superconductor (HFS), UPt₃. Temperature dependence of the critical field of another HFS UBe₁₃, which is not yet understood, can also be qualitatively accounted for by this model. These show the possibility that triplet bipolarons might be formed in the HFSs.     

A model for the heat of transport in ionic conductors

The ion flow caused by a temperature gradient originates the ionic thermopower which is quantified by the heat of transport. Experimentally, it is known that in superionic conductors, the heat of transport Q is nearly equal to the activation energy for ion transport E_a. In the present paper, a model for the heat of transport in ionic conductors has been proposed based on a lattice dynamical theory of diffusion. We have shown that the relationship between Q and E_a is determined by the participation degree of different phonon modes, in particular the short wavelength phonons to the atomic jump processes. The implication of this finding to the transport properties of superionic conductors has been discussed, and it is suggested that the degree of the collective motion in ionic conductors increases with the increase in Q/E_a. The model predicts that good ionic conductors will show large value of Q/E_a. The importance of the acoustic phonons in the ion transport processes has been also pointed out.     

NMR and other magnetic resonance techniques in unstable magnets

This paper reviews and compares the use of nuclear magnetic resonance (NMR) and related hyperfine techniques [muon spin rotation (μSR) and, to a lesser extent, other methods] in the study of 4f and 5f magnetism in “unstable magnets”, i.e., intermediate-valent and heavy-fermion materials. In both NMR and μSR the features of interest are the spectral shape, the frequency shiftK (Knight shift in metals) and the spin-lattice relaxation rate 1/T ₁. For temperatures below the characteristic or “Kondo” temperatureT ₀ these experiments given evidence for (1) modification of the transferred hyperfine field [nonlinearK(χ)]. (2) spin fluctuations with a characteristic fluctuation rate ∼k _B T ₀/h, (3) strong energy-gap anisotropy (zeros of the gap along lines on the Fermi surface) in heavy-fermion superconductors, (4) spin-singlet Cooper pairing from the change in muon Knight shift in superconducting UBe₁₃, and (5) very weak static magnetism (10⁻¹–10⁻³ μ_B/f atom) in CeAl₃, CeCu₂Si₂, U_1−x Th _x Be₁₃ (x=0.033), and UPt₃. There is some controversy concerning the interpretation of 1/T ₁ well aboveT ₀ in UBe₁₃; the situation is reviewed.