We present results of precision measurements of magnetoresistance of isotopically pure Ho¹¹B₁₂ at low temperatures 2÷10 K in magnetic field up to 80 kOe of different orientation to the crystal axes. The data obtained revealed strong anisotropy of magnetoresistance and allowed us to reconstruct magnetic H-T phase diagrams for main crystallographic directions H || [001], [110], and [111]. Analysis of magnetoresistance derivatives allowed to conclude in favor of two main magnetoresistance contributions. Among of them the negative quadratic component is attributed to charge carriers scattering on a magnetic clusters of Ho³⁺ ions (4f component) and positive linear one may be explained in terms of scattering on a spin density waves (5d component).
We study the single impurity Anderson model in an external magnetic field. There are no exact results for the spectral function in this situation. Using a resummation of the diagrammatic expansion we demonstrate that the strong coupling regime in a weak magnetic field is Kondo-like with a quasiparticle resonant peak split into two. We find two exponentially small Kondo scales (temperatures), one for transverse and one for longitudinal spin fluctuations. We show that the salient features of the spectral function in the Kondo regime can be seen already within an extended random phase approximation. To reveal the dependence of the Kondo scales on the bare electron interaction, however, one has to employ a two-particle self-consistency with renormalized vertices. We use the parquet approach to derive the dependence of the Kondo scales on magnetic field.
The electron correlations in narrow energy bands are examined within the framework of the Hubbard model, generalized by taking into account the correlated hopping of electrons. Electronic conductivity and ferromagnetic ordering stabilization in the system with various forms of electronic density of states are studied. The influence of magnetic field, temperature and the form of density of states on concentration dependence of conductivity and magnetization is investigated. The correlated hopping is shown to cause the electron-hole asymmetry of transport and ferromagnetic properties of narrow band materials.
We apply the concept of statistical spin liquid, in which the doubly occupied quasimomentum configurations {|k⇅⟩} for quasiparticles are excluded from the Fock space, to a planar superconductor with real space pairing. The results compare very well with experimental data for the cuprates, namely (i) the condensed state appears only for the number of holes 0 < δ < 0.15-0.25; (ii) the temperature dependence of the gap is close to the BCS result, but the gap has systematically lower value and is of an extended s-wave form.
We provide a brief overview of recent theoretical results concerning the metallic and insulating states with very heavy quasiparticle masses and almost compensated magnetic moments, primarily in the antiferromagnetic state. The temperature dependence of the Kondo-insulator band gap is also discussed.
Two electrons on a lattice pose an interesting problem which can be solved exactly. When the effective interactions are strong enough, the bound states are formed. The properties of such bound pairs in an extended Hubbard model are examined for both signs of nearest neighbor hopping integral t. The stabilization of s*-wave pairing is found for t<0 and of f-wave for t>0. Phase diagrams for the existence of none, one or two s*-wave solutions are calculated.
One-dimensional atomic chain with a variable-range hopping is described within the extended Hubbard model. The Gutzwiller-ansatz approximation is used to determine the optimized single-particle (Wannier) wave functions in the correlated state. Hopping integral up to the third neighbors is taken into account and the results are compared with those for the infinite hopping range. Ground state energy of the system is compared with that making use of the rigorous Lieb-Wu solution with the optimized wave functions. The evolution of the properties as a function of interatomic distance is discussed.
Transport and magnetic properties of polycrystalline Tm_{0.03}Yb_{0.97}B₁₂ samples were investigated at temperatures 1.8-300 K in magnetic fields up to 9 T. The activated behavior of resistivity, the Hall coefficient and thermopower is described in terms of a narrow gap ε_g ≈ 16.6 meV, which controls the charge transport in Tm_{0.03}Yb_{0.97}B₁₂ at T>40 K. The maximum of magnetic susceptibility found at 50 K is shown to be induced by a spin gap Δ ≈ 4.7 meV being close to the half of the spin fluctuation energy in YbB₁₂. Large diffusive thermopower S=AT, A=-29.1 μV/K² and the Pauli susceptibility χ₀ ≈ 7.2×10¯³ emu/mol found below 20 K seem to be associated with the many-body resonance, which corresponds to states with an enhanced effective mass m* ≈ 250m₀ (m₀ - free electron mass). The effective parameters of magnetic centers and the analysis of anomalies favor the nonequivalent states of substitute Tm ions.
The electronic structure of the ternary RPtIn (R = La, Ce) compounds, which crystallize in the hexagonal ZrNiAl-type structure, was studied by X-ray photoelectron spectroscopy measurements and calculation using the ab initio methods (linear muffin-tin orbital in the atomic sphere approximation, full potential linear muffin-tin orbital, full potential linear orbital). The results showed that the valence band in these compounds is formed by the Pt 5d and In 5s and 5p states. The band calculations with spin-orbit coupling have shown that the Ce 4f peaks consist of two peaks above the Fermi level that correspond to the Ce 4 f_{7/2} and 4 f_{5/2} doublet and wide peaks corresponding to the La 4f states. The analysis of Ce 3d spectra on the basis of the Gunnarsson-Schönhammer model gives hybridization of 4f orbitals with the conduction electron band equal to 170 meV.
The spin-one-half Falicov-Kimball model with spin-dependent on-site interaction between localized (f) and itinerant (d) electrons is studied by small-cluster exact-diagonalization calculations and a well-controlled approximative method in two dimensions. The results obtained are used to categorize the ground-state configurations according to common features (charge and spin ordering) for all f and d electron concentrations (n_f and n_d) on finite square lattices. It is shown that only a few configuration types form the basic structure of the charge phase diagram in the n_f-n_d plane. In particular, the largest regions of stability correspond to the phase segregated configurations and configurations that can be considered as mixtures of chessboard configurations and the full (empty) lattice. Since the magnetic phase diagram is much richer than the charge phase diagram, the magnetic superstructures are examined only at selected values of f and d electron concentrations.
The extrapolation of small-cluster exact-diagonalization calculations and the Monte Carlo method is used to study the spin-one-half Falicov-Kimball model extended by the spin-dependent Coulomb interaction (J) between the localized f and itinerant d electrons as well as the on-site Coulomb interaction (U_{ff}) between the localized f electrons. It is shown that in the symmetric case the ground-state phase diagram of the model has an extremely simple structure that consists of only two phases, and namely, the charge-density-wave phase and the spin-density-wave phase. The nonzero temperature studies showed that these phases persist also at finite temperatures. The same calculations that we performed for unsymmetric case showed that charge and spin ordering can be destroyed simultaneously or consecutively.
The paper examines part of the ground state diagram of the extended Hubbard model, with the on-site attraction U<0 and intersite repulsion W>0 in the presence of charge density waves, superconducting andη-superconducting order parameters. We show the possibility of the stabilization of the mixed state, with all three nonzero order parameters, in the model with nearest neighbor interactions. The other result of the paper is application of the exact solution of the Schrödinger equation for the two-electron bound state, as an additional bound for the phase diagram of the model, resulting in the partial suppression of the superconducting state of the s-wave symmetry, in favor of the normal state phase.
We analyze the effects of a large spin-orbit coupling on the magnetic state of a d¹ transition-metal ion located in a tetrahedral environment. While in the ideal tetrahedral symmetry the spin-orbit coupling acts only as a perturbation on the atomic energy levels set by the crystal-field splitting, we demonstrate that its effects are strongly enhanced in the case of distorted geometries. In particular, we consider the specific case in which the tetrahedron is compressed along the z direction, and show that, by increasing the degree of flattening, a large spin-orbit interaction (i) can induce a substantial anisotropic, unquenched orbital momentum and (ii) can affect the hierarchy of the lowest energy levels that are involved in the magnetic exchange.
The tetragonal CePd_{2}P_{2} compound has been investigated by ac magnetic susceptibility and dc magnetization measurements. The experimental data reveal a ferromagnetic phase transition at T_{C}=28.4± 0.2 K. Using Arrot-Noakes plot and scaling laws for a second-order phase transition, critical exponents for the ferromagnetic transition in the system were obtained. The critical exponents are located between those of the mean field and Heisenberg model values.
The heat capacity in the applied magnetic field up to 9 T, susceptibility and magnetization of polycrystalline CeCu_{4}Ga are presented. Magnetic ordering was not observed down to 2 K. For temperature T < 200 K a Curie-Weiss behavior is observable giving an effective magnetic moment μ_{eff} =2.53 μ_{B}/f.u. The experimental value of μ_{eff} is close to the calculated one for a free Ce^{3+} ion (μ_{eff} = 2.54 μ_{B}/f.u.), thus indicating the presence of well localized magnetic moments carried by the stable Ce^{3+} ions. At low temperatures the electronic heat capacity coefficient value depends strongly on the temperature range used for the extrapolation and applied magnetic field. We observe a typical heavy fermion behavior with γ value of about 380 mJ mol^{-1} K^{-2} obtained from extrapolation to T = 0 K of the temperature range above 7 K. However, extrapolation of the lowest temperatures range yields the γ value of 3.3 J mol^{-1} K^{-2}.
A new compound NpPdSn was prepared and studied by X-ray diffraction, magnetization, heat capacity and electrical resistivity measurements, performed in the temperature range 2-300 K and under magnetic field up to 14 T. The crystal structure determined by single-crystal X-ray analysis is hexagonal with ZrNiAl-type (space group P\bar{6}2m). NpPdSn orders antiferromagnetically at 19 K and exhibits a Curie-Weiss behavior with μ_{eff}=2.66 μ_{B} and Θp_{p}=-47 K. Bulk properties show temperature variations similar to systems with strong electronic correlations with a large negative paramagnetic Curie temperature and an enhanced low-temperature specific heat (γ≈90 mJ/(mol K^{2})). It suggests that NpPdSn may be classified as a new Np-based antiferromagnetic Kondo lattice, one of the very few known amidst transuranium-based intermetallics.
We investigated the valence states of samarium ions in Sm deficient Sm_{1-x}B_{6} (x=0.03, 0.05, 0.08, 0.1 and 0.2) sintered samples, in single crystalline solid solutions Sm_{1-x}La_{x}B_{6} (x=0.16, 0.28, 0.4, 0.55 and 0.7) as well as in a SmB_{6} single crystal by X-ray absorption near edge structure (XANES) between 4.2 and 300 K. It was shown that the mixed valence state v_{Sm} of Sm-ions (v_{Sm} ≈ 2.51 for SmB_{6} at 4.2 K) in deficient samples is changed but maintained up to a concentration of 20% of vacancies and up to a concentration of 70% in samples doped with trivalent La-ions, and that in both cases it increases with temperature. On the other hand, the value of v_{Sm} increases with increasing vacancy level, whereas it decreases with increasing La^{3+} substitution. The obtained dependences of Sm-valence in SmB_{6} on vacancy as well as on La^{3+} ion concentration are in good agreement with susceptibility measurements and exact numerical calculation of the spinless Falicov-Kimball model in two dimensions.
In this brief overview we discuss the principal features of real space pairing as expressed via corresponding low-energy (t-J or periodic Anderson-Kondo) effective Hamiltonian, as well as consider concrete properties of those unconventional superconductors. We also rise the basic question of statistical consistency within the so-called renormalized mean-field theory. In particular, we provide the phase diagrams encompassing the stable magnetic and superconducting states. We interpret real space pairing as correlated motion of fermion pair coupled by short-range exchange interaction of magnitude J comparable to the particle renormalized band energy ≈ tx, where x is the carrier number per site. We also discuss briefly the difference between the real-space and the paramagnon-mediated sources of superconductivity. The paper concentrates both on recent novel results obtained in our research group, as well as puts the theoretical concepts in a conceptual as well as historical perspective. No slave-bosons are required to formulate the present approach.
The magnetization and electrical resistivity measurements on a CePdIn single crystal as well as its preparation and structural characterization are presented. The negative paramagnetic Curie temperatures indicate antiferromagnetic ground state, the anisotropy of the paramagnetic Curie temperature amounts 22.7 K. No ferromagnetic correlations were indicated. Powder neutron diffraction experiment performed at temperatures down to 0.4 K did not lead to observation of any magnetic peak in diffraction patterns. We estimate the magnetic moment on Ce atoms to be significantly lower than 0.5 μ_{B}. The temperature development of lattice parameters documents the standard thermal expansion of the unit cell; no signs of structural phase transition were observed.
Point-contact spectroscopy measurements have been performed on the R_3Pd_{20}X_6 (R = La, Ce; X = Si, Ge) cage compounds. In case of La the characteristic phonon energies have been obtained in agreement with that of the Raman scattering. In Ce_3Pd_{20}Si_6 we have observed asymmetric behaviour of dV/dI(V) dependences which was observed in non-Fermi liquid compound. In Ce_3Pd_{20}Ge_6 we have observed maxima at crystalline electric field energies, influenced by magnetic field. This is connected with quadrupolar ordering transition.
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