A.S. Mishchenko

Raman scattering of extra vibrational modes in mixed-valence compounds

Polarized Raman spectra of mixed valence compounds SmB 6 and Sm(Y)S were measured and analyzed within the framework of an excitonic theory to elucidate the connection between 4f charge fluctuations and the appearance of additional modes. In SmB 6 the resonance mode or bound state is a first-order contribution with Γ + 25 , symmetry. In Sm(Y)S the resonance mode has monopolar (Γ + 1 ) symmetry. A transition to a coherent electron-vibrational state is observed

THERMODYNAMICS OF CENISN AT LOW TEMPERATURE AND IN WEAK MAGNETIC FIELD

Abstract Experimental and theoretical description of low-temperature spin-liquid contribution to the thermal expansion and magnetostriction of Kondo-lattice compound CeNiSn is presented. Together with the previously published interpretation of inelastic neutron scattering spectra and low-temperature specific heat, these studies give the consistent picture of low-T thermodynamics of a Kondo lattice whose properties are determined by the interplay between the spin-liquid excitations and the soft crystal-field excitations.

Paramagnetic labeling as a method for the soft spectroscopy of electronic states

A self-consistent microscopic theory of the relaxation of the crystal-field levels of an impurity ion in a state with an integer valence implanted in a normal metal is devised. A microscopic approach based on the Coqblin-Schrieffer-Cooper approach, rather than the formal model of the sf exchange interaction, makes it possible to take into account the specific details of both the crystal-field states of the impurity ion and the electronic band spectrum of the metal. A new method for the soft spectroscopy of electronic states based on measurements of the temperature dependence of the width ΓMM′(T) of transitions between the crystal-field states |M〉 of a paramagnetic ion implanted in the compound being studied is proposed. To make specific use of this method in neutron and optical spectroscopy, a classification of the types of temperature dependence of the natural relaxation width γM(T) of the levels is devised, and procedures for possible experimental methods are proposed. A nonzero value of the natural relaxation width γG(T) of the crystal-field ground state | G〉 of an impurity ion at zero temperature is obtained within the proposed self-consistent model, but is beyond the scope of perturbation theory. It is shown that the widely accepted estimate of the characteristic temperature of Kondo systems T*=ΓG(T=0)/2 from the quasielastic scattering width at zero temperature ΓG(T=0)/2 is incorrect in the case of strong relaxation in a system with soft crystal fields. The proposed model is applied to the quantitative analysis of the relaxation of the crystal-field levels of paramagnetic Pr3+ ions implanted in CeAl3 and LaAl3. The results of the calculations are in quantitative agreement with the experimental data.

Stabilization of spin liquid in Kondo lattice: High temperature regime

Abstract The mechanism explaining the key role of AFM correlations in formation of the heavy fermion state is offered. It is shown that in the case of (T ∗ −T N )/T N ⪡ 1 , the critical spin fluctuations transform the mean-field second-order transition to RVB state into the crossover from high-temperature paramagnetic behavior of localized spins to strongly correlated spin liquid with quasi itinerant character of susceptibility. Thus the spin-liquid state by its origin is close to magnetic instability, so either short-range or long-range AFM order should arise at low T.The mechanism explaining the key role of AFM correlations in formation of the heavy fermion state is offered. It is shown that in the case of

Spin liquid in an almost ferromagnetic Kondo lattice

(T^*-T_N)/T_N<<1

Self-interaction corrected band structure of black-phase SmS

the critical spin fluctuations transform the mean-field second-order transition to RVB state into the crossover from high-temperature paramagnetic behavior of localized spins to strongly correlated spin liquid with quasi itinerant character of susceptibility. Thus the spin liquid state {\it by its origin} is close to magnetic instability, so either short-range or long-range AFM order should arise at low T.

On the interplay between heavy-fermion and soft crystal field excitations in Kondo lattices

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>TK. Owing to this stabilization, low-energy processes of Kondo scattering with energies below TK 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.

On the influence of soft crystal field excitations on the spectrum of spin excitations in CeNiSn-type Kondo lattices

Abstract We extend a local density (LDA) calculation with self-interaction corrections (SIC) applied to black-phase SmS with six 4f-states assumed localized to the case where the valency of the Sm ion becomes a non-integer. We propose the construction of localized molecular orbital (MO) states of correct symmetry by superposition of 4 f 5 2 states with a linear combination of 5 d 3 2 states at the adjacent atoms. These orbitals are more extended than pure 4f-states and, therefore, yield a smaller SIC in dependence of the 5d weight. Simultaneously, this parameter determines the 4f occupancy. Our self-consistent SIC-LDA calculation shows that the correlation between the MO level position and the 4f occupancy is consistent with experiment if only one of the six 4f states is treated in this way. If the MO level reaches the bottom of the conduction band a reconstruction of the electronic structure should take place.

On the spin origin of heavy fermions in rare earth intermetallides

Abstract On the grounds of the microscopic theory of heavy-fermion spin-liquids a novel description of low-energy excitation spectra in CeNiSn and related compounds is offered. The anomalous properties of orthorhombic CeNiSn and related materials are explained by the interplay between the fermi-type spinon excitations with the energy scale T ∗ ≈ T K and the one-site crystal field excitations with the energy Δ CF ∗ . The theory gives quantitative description of inelastic neutron scattering spectra. It resolves also the apparent contradiction between metallic conductivity and gap-wise behavior of thermodynamic properties and spin response at low temperatures.

Erratum: Spin liquid in an almost ferromagnetic Kondo lattice [JETP 85, 399–414 (August 1997)]

Abstract On the grounds of the microscopic theory of heavy-fermion spin liquids a novel description of low-energy excitation spectra in CeNiSn and related compounds is offered. The anomalous properties of orthorhombic CeNiSn and related materials are explained by the interplay between the fermi-type spinon excitations with the energy scale T ∗ ≈ T K and the one-site crystal field excitations with the energy Δ CF ∗ . The theory gives both quantitative and qualitative description of inelastic neutron scattering spectra and low-temperature thermodynamics. It resolves also the apparent contradiction between metallic conductivity and gap-wise behavior of thermodynamic properties and spin response at low temperatures.