Spin fluctuations and superconductivity in Mo3Sb7

Temperature dependences of the magnetic susceptibility, specific heat, and electrical resistivity have been measured for the Mo(3)Sb(7) compound in the 0.6-350 K range. This compound exhibits bulk superconductivity occurring at 2.25 K and follows the Kadowaki-Woods relation, A/gamma(2)=1.0 x 10(-5) microOmega x cm(K x mol/mJ)(2), as a heavy-fermion system does. We show, from experimental evidence and theoretical argument, that Mo(3)Sb(7) can be classified as a coexistent superconductor-spin fluctuation system. The McMillan equation including paramagnon effects was found to give an accurate estimation of the transition temperature.     

Spin fluctuations in metals: Application to the actinides

We present here a review of the spin fluctuation theory and of its applications to transition and actinide systems, with a particular emphasis to the latter where some very anomalous properties find an explanation in terms of spin fluctuation effects. Firstly, we summarize the development of the spin fluctuation model which had been initially applied to transition metals and alloys such as palladium or Pd–Ni alloys. Then, we present the extension of the paramagnon model to nearly magnetic actinide systems by taking into account explicitly the temperature dependence of the Stoner susceptibility, because the 5f-band of actinides is much narrower than the d-band of transition metals. As a result the paramagnon contribution to the resistivity departs from the usual T ² and T power laws at temperatures higher than the spin fluctuation one and saturates at high temperatures, with eventually the presence of a maximum at intermediate temperatures. We present also the calculation of the other properties of actinide systems, namely the thermal resistivity, the thermoelectric power, the magnetic susceptibility, the specific heat capacity and the NMR relaxation rate, which are generally enhanced by the presence of paramagnons. Finally, we have introduced the concept of ‘antiferromagnetic-like’ spin fluctuations which have a maximum of the q-dependent susceptibility _χ(q) at a q value different from q =0, in contrast to the regular ferromagnetic spin fluctuations; both types of spin fluctuation give the same resistivity behaviour, while they yield a markedly different behaviour of the magnetic susceptibility, in agreement with experiment. The spin fluctuation theory is applied successfully to the different properties of neptunium and plutonium metals and of many nearly magnetic compounds such as UAl₂.     

Paramagnons in Wilson theory

Some amusing comparisons can be made between the ?⁴ Wilson theory and the uniform as well as local paramagnon problems.     

Les transitions de phases de Na5W3O9F5

We have performed susceptibility measurements on UAl₂ down to 80 mK. No magnetic ordering occur and the very low temperature behaviour shows that this alloy is a good example of a paramagnon system.     

Spin-fluctuation effects in Ce-Si system

Discussions are presented for the Ce-Si system within the framework of the paramagnon model. Data of both the specific heat and the magnetic susceptibility are examined. There are indications that the predictions of the theory manifest themselves in the present system, although there are unfavorable aspects also.     

Perturbation theory for the phase transition in the local paramagnon model

The vertex function, self-energy and the critical index for the 2 - ? dimensional local paramagnon model with the φ⁴ interaction are calculated for small values of ? by direct summation of the perturbation theory diagrams.     

Field-induced paramagnons at the metamagnetic transition of Ca1.8Sr0.2RuO4

The magnetic excitations in Ca1.8Sr0.2RuO4 were studied across the metamagnetic transition and as a function of temperature using inelastic neutron scattering. At low temperature and low magnetic field the magnetic response is dominated by a complex superposition of incommensurate antiferromagnetic fluctuations. Upon increasing the magnetic field across the metamagnetic transition, paramagnon and finally well-defined magnon scattering is induced, partially suppressing the incommensurate signals. The high-field phase in Ca1.8Sr0.2RuO4, therefore, has to be considered as an intrinsically ferromagnetic state stabilized by the magnetic field.     

Spin correlations in electron liquids: Analytical results for the local-field correction in two and three dimensions

We study the spin correlations in two- and three-dimensional electron liquids within the sum-rule version of the self-consistent field approach of Singwi, Tosi, Land, and Sj?lander. Analytic expressions for the spin-antisymmetric static structure factor and the corresponding local-field correction are obtained with density dependent coefficients. We calculate the spin-dependent pair-correlation functions, paramagnon dispersion, and static spin-response function within the present model, and discuss the spin-density wave instabilities in double-layer electron systems. Received: 22 September 1997 / Revised: 1 December 1997 / Accepted: 4 December 1997     

Non-linear spin fluctuations and phase transitions in itinerant electron magnets: CMR manganites

We present a review analyzing the effects of coupling of transverse magnons with longitudinal spin fluctuations in isotropic itinerant ferro- and antiferromagnets. It is shown that this coupling essentially changes the spectrum of longitudinal fluctuations. At low-temperatures their spectrum is dominated by the linear Landau relaxation, is purely quasielastic and described by a broad central peak of a paramagnon type. On approaching the critical temperature non-linear magnetic relaxation due to mode–mode couplings can dominate and lead to a rapid increase of the central peak and to a new mechanism of magnetic phase transitions governed by non-linear spin fluctuations. The formalism is applied to the CMR manganites where the observed quasielastic fluctuations can be viewed as non-linear spin-lattice fluctuations strongly affected by magnons.     

Specific heat of amorphous U−Fe,U−Co and U−Ni-alloys

Alloys of U-X with X=Fe, Co, Ni were prepared by rapid quenching in the concentration range between 50 and 83 at% U. The samples were obtained in the amorphous state as determined by X-ray diffraction. Parts of the samples were used to measure the specific heat by the relaxation method. The calculated density of states is twice as large as that of transition metals like amorphous Zr-alloys. Therefore these results are discussed within the frame of the paramagnon model by taking into account values of the magnetic susceptibility and the supercon-ducting transition temperature.     

Spin fluctuation contribution to the specific heat of strongly correlated fermions

We calculate the free energy of a system of fermions at low temperatures within the Hubbard model using a slave boson representation, which generalizes the approach of Kotliar and Ruckenstein. The mean field approximation is identical to Gutzwiller's solution. The one-loop corrections provide aT ³ lnT spin fluctuation contribution to the specific heat, which reduces for weak coupling to the result of paramagnon theory first derived by Brenig et al.Dedicated to Professor W. Brenig on the occasion of his 60th birthday     

Dynamic spin and charge susceptibilities in thet−J model

Based on the spin-rotation-invariant formulation of the Kotliar Ruckenstein slave-boson representation, the paramagnetic spin and charge susceptibilities in thet-J model are calculated. Analyzing the static spin susceptibility, the instability of the paraphase towards incommensurate magnetic order is in agreement, with the saddle-point phase diagram recently obtained by some of the authors. The spin dynamics at arbitrary frequencies, wave vectors and band fillings is calculated, where the Fermi-surface and correlation effects are studied. The magnetic instability region is investigated with respect to the formation of a collective spin-fluctuation mode. Near the transition point, a kinetic gap and a sharp peak in the spectral weight ((1,0) paramagnon) are obtained.     

Paramagnon satellites in photoemission spectra of chemisorbed hydrogen

A model for the chemisorption of hydrogen on transition metals is considered taking the intrametallic exchange forcesI and the extra-atomic exchangeJ into account. The extra-atomic exchange couples the single particle states of the adatom to the particle-hole states of the metal. This leads to dynamic effects similar to those found in soft X-ray spectra of metals and in the image force dynamics of chemisorbed atoms. Such dynamic effects can be expected to be particularly large near a ferromagnetic instability of the metal. The theoretical treatment of these effects is complicated because of Kondo type divergencies. ForJ large compared to the band widthB a simplified treatment is adopted avoiding explicit consideration of the Kondo effect. The intermetallic exchange effects then can be incorporated into the calculation of the dynamic effects. A simplified and crude estimate of the exchange enhancement of the dynamic effects near a ferromagnetic instability indicates strong paramagnon satellites in the photoemission spectrum leading to a broadening of the unperturbed spectrum.     

Spin fluctuations and properties of the thermoelectric power of nearly ferromagnetic iron monosilicide

The thermoelectric power of nearly ferromagnetic iron monosilicide, which passes through an electronic semiconductor-metal transition with increasing temperature, is investigated theoretically. The results of this investigation indicate that a sizable paramagnon-related increase in charge carriers can occur in nearly ferromagnetic semiconductors, and that spin fluctuations can modify the electronic spectrum and thereby renormalize the diffusion component of the thermoelectric power. The transition from semiconductor to metal decreases the paramagnon component sharply and the thermoelectric power changes sign, which agrees with experimental data for iron monosilicide. Fiz. Tverd. Tela (St. Petersburg) 41, 1054–1056 (June 1999)     

Critical temperature and enhanced isotope effect in the presence of paramagnons in phonon-mediated superconductors

We reconsider the long-standing problem of the effect of spin fluctuations on the critical temperature and isotope effect in a phonon-mediated superconductor. Although the general physics of the interplay between phonons and paramagnons has been rather well understood, the existing approximate formulas fail to describe the correct behavior of Tc for general phonon and paramagnon spectra. Using a controllable approximation, we derive an analytical formula for Tc which agrees well with exact numerical solutions of the Eliashberg equations for a broad range of parameters. Based on both numerical and analytical results, we predict a strong enhancement of the isotope effect when the frequencies of spin fluctuation and phonons are of the same order. This effect may have important consequences for near-magnetic superconductors such as MgCNi3.     

Long range force 1Rd+σ problems with d = σ

We suggest a similar critical behavior for all fluctuation problems with long range forces in 1/R^d+σ and fluctuation propagator (r + q^σ)^?1 when d = σ. The analogy between these problems may allow to better understand the d = 2 paramagnon problem in strongly interacting fermion systems.     

铁磁金属中自旋波散射引起的电阻率

铁磁金属中自旋向上电子与自旋向下电子的能量由于磁化而造成的分裂,在低温下对自旋波散射引起的电阻率(磁电阻率)有重要的影响。由于这个分裂,单带情况下低温磁电阻率总是衰减的;但两带情况下则有可能增强。考虑到自旋波谱中的能隙用单带模型所得的磁电阻率表达式与Tb和Dy的实验资料比较表明:不但低温电阻率的温度依赖性符合,数值上也与高温磁电阻测量结果一致。 关键词：     

Electrical property and phase transition of CdSe under high pressure

In situ electrical resistivity measurement of CdSe was performed under high pressure and moderate temperature using a diamond anvil cell equipped with a microcircuit. With the pressure increasing, a sharp drop in resistivity of over two orders of magnitude was observed at about 2.6 GPa, it was caused by the transition to the rock-salt CdSe. After that, the resistivity decreased linearly with pressure. However, in different pressure range, the decreasing degree was obviously different. This attributed to the different electron structures. By fitting to the curve of pressure dependence of resistivity in different pressure range, the relationship of the band gap to pressure was given and the metallization pressure was speculated to be in the range of 70-100 GPa. The temperature dependence of resistivity showed that in the experimental temperature and pressure range the resistivity had a positive temperature coefficient.     

Resistivity decrement in pre-twisted technically pure silver during creep deformation at room temperature

Annealed samples of technically pure silver were pre-twisted and subsequently creep tested in uniaxial tension at room temperature. In the initial stages of creep, it was found that a significant decrease in the electrical resistivity of the pre-twisted samples to a certain minimum value occurs. The magnitude of the resistivity decrease was sensitive to the type and amount of the twist given. The initial decrease in resistivity was followed by its linear increase with further creep time. Impurity segregation to the moving dislocations may be the reason for the observed initial resistivity decrease.     

替代式合金系统的电阻率

本文将文献[1]的无序晶态合金电阻率理论推广到包含长程有序的系统,从而建立了适用于晶态金属,无序及有序替代式合金的Ziman型电阻率理论。根据这个理论我们详细讨论了这类系统电阻率的温度依赖性。文中着重指出:合金系统结构因子的超结构峰对电阻率有重要贡献。这个贡献在低温下是一个T²项,它比电子-电子散射引起的T²项大得多,因而合金系统电阻率在T<<Θ(Θ是德拜温度)时有ρ≈ρ₀+ρ_a(T/Θ)²+ρ_i(T/Θ)⁵的形式。据此,许多A-15化合物正常态电阻率在低温下的反常行为很容易解释。作为例子,我们将低温电阻率的理论表达式与V₃Si的测量值作了比较,符合得很好。