Spin liquid in an almost ferromagnetic Kondo lattice

A theory of stabilization of a spin liquid in a Kondo lattice at temperatures close to the temperature of antiferromagnetic instability has been developed. Kondo exchange scattering of conduction electrons leads to emergence of a state of the spin liquid of the resonating valence bonds (RVB) type at T>T _K. Owing to this stabilization, low-energy processes of Kondo scattering with energies below T _K are frozen so that the “singlet” state of the Kondo lattice is not realized; instead a strongly correlated spin liquid with developed antiferromagnetic fluctuations occurs. A new version of the Feynman diagram technique has been developed to describe interaction between spin fluctuations and resonant valence bonds in a self-consistent manner. Emergence of a strongly anisotropic RVB spin liquid is discussed. Zh. éksp. Teor. Fiz. 112, 729–759 (August 1997)     

Spin freezing properties in LaNi5-xFex

We report magnetic and transport measurements in the metallic pseudobinary compound LaNi_5-xFe_x, which show a Fe spin freezing. The magnetic phase diagram in the x-T plane is derived in the range 0 ⩽ x 1.2, 20 K ⩽ T ⩽ 300 K. Above a critical Fe concentration x_c ≈ 0.3, the system undergoes a superparamagnetic to long range ferromagnetic ordering at a finite temperature T. At lower concentrations, no long range spin ordering is observed, which suggests that x is the percolation threshold. Instead, a transition to a phase characterized by strong irreversibilities is observed at temperature T_f(x). Very strong ferromagnetic coupling interactions between Fe atoms are observed at low concentrations (x < x_c), which contrast with small values of T observed for x ∼ x_c. The results, together with the nature of the spin freezing are discussed in relation to the 2d character of the magnetic lattice.     

Concentrated Kondo systems

This review considers the experimental and theoretical studies of concentrated Kondo systems (CKS), Kondo lattices, substitutional solid solutions and their transition from Kondo impurity to Kondo lattice, and ‘intermediate valence compounds’ which are, in fact, high T _K CKS (T _K is the Kondo temperature). The anomalous low temperature properties of CKS are related to the formation of the narrow (～k _B T _K) high-amplitude Abrikosov-Suhl resonance E _R in the vicinity of the Fermi level E _F. This resonance is situated exactly at E _F in low T _K CKS with T _K < Δ_CF and near E _F in high T _K CKS with T _K > Δ_CF (Δ_CF is the crystal field splitting). In low T _K ‘j=1/2’ CKS the condition E _R=E _F leads to an increase of the density of states at E _F, which is large enough to induce heavy fermion superconductivity in CeCu₂Si₂, UBe₁₃. We demonstrate that the transition from low T _K (E _R=E _F) to high T _K CKS (E _R≠E _F) might be what was formerly considered as a ‘Kondo-lattice-intermediate valence state’ transition. It appears that in many cases the essentially non-integer valence state of the rare-earth elements in metallic compounds is thermodynamically unstable with respect to a transition to an almost integer valence state, because it realizes the maximum gain in free energy from the Kondo condensation.     

Ising ferromagnet with biquadratic exchange

A spin one Ising system with biquadratic exchange, is investigated, using Green's function technique in random phase approximation (RPA). Transition temperature T_c and <(S^z)²> at T_c, are found to increase with biquadratic exchange parameter α for sc, bcc and fcc lattices. The variation of <(S^z)²> at T_c with α is found to be the same for the above lattices.     

稀磁合金中“电阻极大”现象的双杂质散射理论

本文基于s-d相互作用,考虑杂质之间存在RKKY相互作用,提出了一种新的双杂质散射模型。按照这个模型,当一个杂质作自旋翻转散射时,由于杂质之间存在着RKKY关联,它的自旋作为内部自由度会受到限制。由于这种关联,抑制了杂质的自旋翻转散射,结合Kondo的logT项,能形成电阻极大。本文计算了所有可能的“DIS”图(双杂质自能图),在Kondo电阻公式中加入了A/(T₀²—T²)这样的项。其中A是一个正常数。T₀是一个临界温度。当T≤T₀时,这个公式不再有意义。这个理论和已有的分子场理论在本质上是不同的.因为它并不依赖于合金中的磁有序.因此当T≥T_c时(T_c是磁有序转变温度),这种机制仍起作用,但分子场理论则不行.这是一种顺磁效应.我们和Cd-Mn(杂质浓度从0.01到0.1at./0)的实验曲线进行了比较,发现符合得很好.最后,我们认为即使在极低浓度下这种机制也是消除Kondo logT发散的主要原因. 关键词：     

Specific heat of a Kondo superconductor: (La, Ce) Al2

The specific heat of LaAl₂ and (La_1-xCe_x)Al₂ (x ? 0.0064) has been measured between 0.3 and 5 K, both in the superconducting and in the normal state. For all samples the same values for the Debye temperature as well as for the electronic specific heat coefficient have been determined. LaAl₂ shows an excellent BCS behavior. A remarkable excess specific heat at low temperatures due to the Kondo effect has been observed for all superconducting as well as for the normal conducting (La_1-xCe_x) Al₂ alloys. The specific heat jump ΔC at T_c depressed rapidly with increasing Ce concentration, allows the Kondo temperature T_K ? 1 K to be determined. ΔC vanishes at finite temperatures.     

Temperature dependence of the diffuse streak system in K2SnCl6

We have investigated the system of diffuse streaks in K₂SnCl₆ single crystals in the temperature interval from 4 to 640 K by the photographic registration of the scattered neutron intensity. The streak intensity decreases from T_{c¹ = 261 K} with increasing as well as with In the region of the phase transition at T_{c¹ = 261 K} and T_{c² = 255 K} the intensity decreases with falling temperature to one third. The high temperature behaviour is explained by a soft A_2g librational mode. Below T_{c² = 255 K} the intensity is proportional to ΔT. The streaks are detectable down to 230 K.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.     

Electrical resistivity and magnetoresistance of CeB6

Electrical resistivity and magnetoresistance of CeB₆ single crystal have been measured in the temperature range from 1.3 to 300 K under the magnetic field up to 85 kOe. Three characteristic phases are distinguished consistently with other measurements. The Kondo like behaviour in the resistivity observed in the high temperature phase is fitted by the conventional form for the dilute Kondo state with the Kondo temperature T_K = 5 ～ 10K and the unitarity limit resistivity $\text{?}{}_{\mathrm{u}}\text{? 110 μΩ}$ cm/Ce-atom. The negative magnetoresistance in the middle phase is stronger with increasing magnetic field and with decreasing temperature suggesting rapid destruction of the Kondo state. The magnetoresistance in the low temperature phase exhibits some anomalies suggesting sub-phases corresponding to several kinds of spin ordering.     

Kondo effect in spin glasses: Electrical resistivity

We investigate the influence of the Kondo effect on the electrical resistivity of spin glasses. Our approximation reduces in the limit of vanishing impurity concentrations to the Suhl-Nagaoka theory for the Kondo effect. The magnetic impurity interactions are taken into account in the form of time dependent two-spin correlation functions which can be measured by neutron scattering. The dynamics of the impurity spins leads to a partial destruction of the Kondo effect. For the resistivity this can be described by a temperature dependent effective spinS _eff ² (T) withS _eff ² 0 forT0 andS _eff ² S(S+1) forT, and by a reduction of the Kondo temperatureT _K. Sufficiently strong interactions lead toT _K=0. We obtain a resistivity maximum at a temperatureT _m due to the interplay of the Kondo effect and the spin dynamics;T _m depends onT _K and on the excitation spectrum, and therefore on parameters such as impurity concentration or pressure. The ratioT _m/T_f (T _f is the freezing temperature) is calculated for a single relaxation time and for a square density of relaxation modes and is compared with experimental data forAuCr,AuMn,AgMn, andCuMn. The influence of other possible modes on various spin glass properties is discussed.SFB 125 Aachen-Jülich-Köln     

Replica symmetry breaking in a fermionic cluster spin glass model in a transverse field

We analyze a fermionic Ising spin glass model in the presence of a transverse magnetic field Γ within a cluster mean field theory. The model considers a Sherrington-Kirkpatrick type interaction between magnetic moments of clusters with a ferromagnetic intra-cluster coupling J₀. The spin site operators are written as a bilinear combination of fermionic operators. In these quantum spin glass model, the inter-cluster disorder is treated by using a framework of one-step replica symmetry breaking within the static approximation. The effective intra-cluster interaction is then computed by means of an exact diagonalization method. Results for several cluster size n_s, values of Γ and J₀ are presented. For instance, the specific heat shows a broad maximum (for n_s>1) at a temperature above the freezing temperature T_f, which is characterized by the inter-cluster replica symmetry breaking. Phase diagrams T versus Γ show that the critical temperature T_f(Γ) decreases for any value of n_s when Γ increases until it reaches a quantum critical point at some value of Γ_c.     

A disordered ising model with internal degrees of freedom

A solvable Ising-system withnn-interactions as a basic system is considered. This system is assumed to undergo a phase transition so that the thermal quantityq=Σ _i> ₀ ² /N, which is not necessarily the order parameter of the basic system, is different from zero (q≠0) forT _c. The whole system consists of the basis {S _i} and an equal number of additional spins η _i which are coupled linearly with the basic spinsS _i through an exchange constantJ _i which is distributed at random. The pairs of spinsS _i ?η _i can be interpreted as clusters with an internal degree of freedom. The specific heat Δc and the susceptibility Δ_χ of the internal degree of freedom can be separated exactly and show typical spin glass properties: the specific heat Δc is an analytic function over the whole temperature range and the susceptibility Δ_χ shows a cusp at the transition temperatureT _c of the basic system.     

Pressure dependence of the electrical resistivity of Ag(Mn), both in the dilute limit and in the spin glass regime

The electrical resistivity of the dilute spin glass alloys Ag(1,540 ppm Mn) and Ag(1,015 ppm Mn) has been measured from 1.3–30 K at pressures up to 70 kbars. The temperature of the resistance maximum,T _m , is found to shift to higher temperatures, this shift being stronger for the more concentrated alloys.Additionally an Ag(20 ppm Mn) Kondo-alloy has been investigated. Application of pressure causes an increase of the Kondo temperatureT _K from which the volume dependence of the effective exchange constantJ is derived.With this information theT _m -shift in the spin glass alloys can be understood in a qualitative and quantitative manner within Larsen's model.     

Theory of high-T c superconductivity based on the fermion-condensation quantum phase transition

A theory of high-temperature superconductivity based on the combination of the fermion-condensation quantum phase transition and the conventional theory of superconductivity is presented. This theory describes maximum values of the superconducting gap, which can be as big as Δ₁～0.1ε _F , with ε _F being the Fermi level. We show that the critical temperature 2T _c ?Δ₁. If the pseudogap exists above T _c , then 2T*?Δ₁ and T* is the temperature at which the pseudogap vanishes. A discontinuity in the specific heat at T _c is calculated. The transition from conventional superconductors to high-T _c ones as a function of the doping level is investigated. The single-particle excitations and their lineshape are also considered     

Thermodynamic behavior of a water model with a liquid-liquid critical point

In this paper we calculate the constant pressure specific heat response of a water model with a liquid-liquid critical point. We show how, due to the existence of the critical point, there is a secondary maximum in the specific heat at some temperature T_*>T_h for any pressure P>P_c, being T_h the first order transition temperature between the high and the low density liquid phases and P_c the pressure of the critical point. This secondary maximum does not correspond to any long range correlated phase transition and does not show up in the temperature dependence of the isothermal compressibility.     

A new theoretical approach to study the effects of active molecules on lipid bilayer properties: The cholesterol problem

A model to study phospholipid bilayers with active molecules that greatly disturb molecular conformations of their neighboring lipids is proposed. In the particular case of cholesterol in DPPC bilayers, this method allows us to use the spin 1 Ising hamiltonian as a natural extension of the two state model of chain melting transition. Physical processes responsible for the decreasing of the total transition enthalpy Δ_tQ and temperature T_c, and for the phase separation, both below and above T_c, when cholesterol is added, are easily identified. From the details of the phase separation, the two peak specific heat data are explained. Similar models for the addition of other binding molecules to lipid bilayers are discussed.     

The Kondo temperature of gold

Precise measurements of the low temperature electrical resistivity were obtained for 7 samples of highly dilute AuFe alloys. The effective Fe concentrations ranged between 0.006 and 0.2 ppm. At these low concentrations no impurity-impurity effects could be detected and the data were in good agreement with the Hamann theory. Values of the impurity spin S and the Kondo temperature T_K were determined from the Hamann equation to be (0.15±0.01) and (1.5±0.1) K respectively.     

Observation of a Kondo effect for a dilute alloy containing Sm impurities

Measurements of the depressions of the superconducting transition temperature T_c with Sm impurity concentration and the specific heat jump at T_c as a function of T_c, and the temperature dependences of the normal state specific heat and magnetic susceptibility are reported for the matrix impurity system (LaSm)Sn₃. The results constitute the first definitive evidence of a Kondo effect for a dilute alloy containing Sm impurities.     

Estimates of the s-d interaction in CdCr from superconducting transition temperature and magnetic susceptibility

Cr has a well developed magnetic moment in a Cd host and the superconducting transition temperature, T_c, is strongly depressed. We estimate a spin, S = 1.5 leading to a depression rate - $\text{dT}{}_{\mathrm{c}}\text{dc}\text{= 140 K/at.\%}$. The s-d interaction strength is estimated to 0.5–0.7 eV leading to a Kondo temperature in the milliKelvin range.     

The incommensurate - commensurate phase transition in Rb2ZnCl4: confrontation of permittivity and birefringence data

Permittivity ϵ and birefringence Δn of the c-plates of Rb₂ZnCl₄ crystals have been measured simultaneously in the vicinity of the incommensurate (IC) - commensurate phase transition. Samples used are characterized by the maximum value ϵ = 150 occuring at a temperature T_m. Since ϵ in the IC phase is determined by the phase of the order parameter while Δn is sensitive to its modulus, confrontation of these data provides some insight into the transition process. The data show that i) the transition is essentially 1st order; ii) T_m marks the beginning of the transition process on cooling and its end on heating. Below T_m, both phases coexist within an interval ΔT_tr = 1.5K on cooling and 0.6K on heating. iii) There is no indication of any significant phase coexistence above T_m. An indirect conclusion is drawn that within ΔT_tr, the anomalous dielectric tail is due both to surviving solitons and to true domain wall contribution; below T_m−ΔT_tr domain walls take over.