Single-crystal neutron diffraction under high pressures: valence instabilities in Tm monochalcogenides

New high-pressure devices based on the use of sapphire anvils now allow single-crystal neutron diffraction experiments to be performed up to P=8–10 GPa. After giving a brief overview of the technique, we present its application to the study of pressure-induced valence instabilities in Tm monochalcogenides (TmX, X: S, Se, Te). A variety of new magnetic phases have been characterized, yielding a consistent picture of the evolution of magnetism through the series. The results indicate a striking interplay between magnetic order taking place at low temperature and different types of electronic ground states (classical semiconductor, narrow-gap Kondo insulator, metallic Kondo lattice, etc.) inferred from the transport properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamical magnetic susceptibility of intermediate valence Tm systems

We calculate the inelastic part of the magnetic neutron scattering cros section of a periodic model for valence fluctuations between two magnetic configurations. The calculations is restricted to the paramagnetic pahse where the coherent potential approximation is used.The resulting frequency (ω), wave vector (q) and temperature dependence agrees qualitatively with the experimental results in TmSe.We find that even for parameters for which there is no gap in the total density of states, an inelastic peak is present with a similar q dependence as the experimentally observed one.We discuss the effects of sample composition and compare the results with those corresponding to the impurity version of the model and the experimental results in dilute systems.     

Perturbative theory of a dilute mixed valence Tm system

The perturbative theory for a single mixed valence Tm-impurity system is presented. The ground state and the low temperature properties are studied.     

Blocking of valence fluctuations in europium oxynitrides

¹⁵¹Eu Mössbauer spectra of three compounds in the series EuO_1-xN_x with x = 0.05, 0.15 and 0.25 show well-resolved Eu²⁺ and Eu³⁺ absorbtion lines. The proportion of Eu³⁺ is just x. There is no evidence for electron hopping. Eu²⁺ appears to be unstable in environments with more than two nitrogen neighbours.     

Mössbauer studies of valence fluctuations

In the last ten years the Mössbauer technique has made a considerable contribution to the research of the phenomena of mixed valencies, valence instabilities, valence fluctuations and intermediate valencies of transition elements. The sensitivity of the hyperfine interaction parameters and in particular the isomer shift, to the valency of the Mössbauer ion, enabled the research of dynamics of valence fluctuations and the temperature, pressure and composition dependence of the ionic intermediate valence state. Studies of¹⁴⁹Sm,¹⁵²Sm and¹⁵³Eu in Sm_1?xR_xS contributed to the understanding of the outstanding insulator-metal phase transition that occurs in these systems. Studies of⁵⁷Fe and¹⁵¹Eu in mixed valent systems yield the charge fluctuation rates of the “hopping” mechanism, contributing to the conductivity in these systems. Studies of¹⁵¹Eu in EuRh₂, EuCu₂Si₂, EuPd₂Si₂, EuFe₄Al₈, and EuPd₆B₄ as a function of temperature and pressure reveal many aspects of the thermodynamics of intermediate valencies. Studies of systems like Eu_1?xLa_xRh₂, EuA_2?xB_x, EuA_5?xB_x reveal strong local environment dependence of the intermediate valency. Mössbauer spectra of¹⁶⁹Tm in TmSe,¹⁷⁰Yb in YbAl₃ and²³⁷Np in NpOs₂ also display phenomena associated with fluctuating valencies.     

A novel explanation of the phase transition with the valence change in cation-substituted samarium monochalcogenides

A new interpretation of the dilution-induced phase transition to the mixed valence (MV) state in the systems Sm_1−xMe_xS is proposed. The theory explains the absence of f-electron contribution to the low temperature conductivity of metallic MV phase. The clue point of the model is the proposal to distinguish the intercenter (interband) d.f.-hybridization existing in pure SmS from the intracenter d.f.-hybridization in Sm ions arising only due to the violation of the point crystal symmetry by the guest Me³⁺ ions. The competition of these two mechanisms results in formation of the double-well potential for the f-electrons, whose wave functions are poised between the wells in the MV state.     

Magnetic susceptibility and specific heat of intermediate valence Tm compounds

We calculate the static magnetic susceptibility and specific heat of a model periodic system, with valence fluctuations between two magnetic configurations, intended to describe intermediate valence Tm compounds.These properties are derived from an expression for the free energy obtained using the coherent potential approximation, which properly takes into account the correlation energy and the configurational entropy.The resulting properties are in reasonable agreement with the experimental data. Unlike the results found in the commonly studied Anderson model, we find divergences in the magnetic susceptibility that indicate the onset of ferromagnetic order for some values of the model parameters.     

Self-consistent perturbation theory for dynamics of valence fluctuations

Dynamical as well as equilibrium properties of model Ce systems are investigated in both the intermediate-valence and nearly integral-valence (Kondo) regimes at finite temperatures. With self-consistent account of hybridization effects between the conduction bands and the highly correlated 4f states, the 4f-electron density of states _4f () and the dynamical magnetic susceptibility () are derived. Equilibrium properties such as the static magnetic susceptibility and the averaged 4f-electron number are also computed within the same approximation scheme that neglects intersite interactions between different Ce ions. In the intermediate-valence regime the calculated line-shape of Im ()/ is close to the Lorentzian at high temperatures, but at low temperatures there appears an inelastic peak. In the Kondo regime it is shown that a sharp peak in _4f () develops at the Fermi level as the temperature decreases. The line-shape of Im ()/ is shown to be close to the Lorentzian at all temperatures. The half-width is considerably enhanced over the Korringa value expected for the local-moment system. The temperature dependence of the half-width agrees qualitatively with experimental results in Kondo compounds such as CeB₆, CeCu₂Si₂ and CeAl₃.     

Self-consistent perturbation theory for dynamics of valence fluctuations

Dynamical properties of valence-fluctuating systems are studied at absolute zero of temperature. The self-consistent perturbation theory developed for rare-earth impurity systems is used with some refinement. The theory takes account of the orthogonality catastrophe caused by hybridization of 4f and conduction electrons. Extensive numerical results are reported for the 4f-electron density of states _4f () and the dynamical magnetic susceptibility (). The results cover both the intermediate-valence and nearly integral-valence regimes of a model Ce impurity system. The present theory gives reasonable overall feature of dynamics including a sharp peak in _4f (0) in the Kondo regime. However, the low-frequency limit of dynamical quantities is not consistent with the Fermi-liquid property. It is shown that interpolation of the present results and those by the Fermi-liquid theory is possible. Hence accurate dynamical information is obtained over a wide excitation-frequency range.     

The role of fluctuations in mixed valence compounds

Previous renormalization group results for a simple spinless mixed valence model yielded large intersite ?-electron correlations if extended coherent hybridization mixed valence states are assumed. We simulate these correlations by an ?-level energy distribution and analyze its consequences. It is seen that a relatively small distribution width already smears the coherent hybridization gap leading to a lattice of essentially independent resonant levels.     

Self-consistent perturbation theory for dynamics of valence fluctuations

A perturbation-theoretic scheme is developed for dynamics of valence fluctuations in rare-earth systems with unstable 4f shells. The theory is formulated in close analogy to the standard Green-function method for many-body systems but without use of the linked-cluster theorem. This formulation regards hybridization between 4f and conduction-band states as perturbation and naturally incorporates the strong on-site 4f-electron correlation. Some favorable features are: (i) the approximation scheme automatically satisfies conservation laws required for response functions; (ii) realistic 4f-shell structures with crystalline-electric-field effects can be taken into account; (iii) the theory does not have divergence difficulties over the whole temperature range. In the lowest-order self-consistent approximation, explicit formulae for dynamical susceptibilities and 4f-electron density of states are presented. At high temperatures, the theory reproduces previous results obtained by the Mori method.     

Self-consistent perturbation theory for dynamics of valence fluctuations

Dynamical property of an intermediate-valence Tm ion in a metallic matrix is investigated by the use of the self-consistent perturbation theory. Both of the fluctuating valence states in Tm have nonzero angular momentum in contrast to Ce and Yb systems. It is shown that the participation of two valence states in the magnetic susceptibility leads to peculiar magnetic properties such as enhanced Korringa-type magnetic relaxation and a systematic deviation from the Curie-Weiss law of the susceptibility. The susceptibility diverges in the zero-temperature limit. The dynamical susceptibility can be fitted by the Lorentzian lineshape very well for a wide frequency and temperature range. The single-particle excitation spectrum is also derived from which the resistivity is calculated. A logarithmic increase of the resistivity with decreasing temperature is found.     

Zener double exchange from local valence fluctuations in magnetite

Magnetite (Fe3O4) is a mixed valent system where electronic conductivity occurs on the B site (octahedral) iron sublattice of the spinel structure. Below T(V)=123 K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B site sublattice (approximately 80 meV) are shifted upwards in energy above T_{V} due to the occurrence of B-B ferromagnetic double exchange in the mixed valent phase. The double exchange interaction affects only spin waves of Delta(5) symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by short-range electron correlations above T(V).     

The absence of valence fluctuations in SmNi2Sn2

In an earlier work [1] an anomaly in the lattice volume had been observed in SmNi₂Sn₂ which indicated possible valence fluctuations of Sm ions. Using a well characterised sample of SmNi₂Sn₂, we do not find any evidence of valence fluctuations from our susceptibility, resistivity, heat capacity and the lattice parameter data. SmNi₂Sn₂ orders magnetically below 8 K and all the Sm ions are trivalent.     

Electron-phonon coupling and monopolar charge fluctuations in intermediate valence TmSe

Polarized Raman spectra of intermediate valence Tm_xSe (x=0.95, 1.00, 1.01) have been measured. The scattering intensity is interpreted in terms of a one-phonon density of states weighted by the electron-phonon matrix element. The latter is described in a model of phonon-induced local, intraionic charge deformabilities. In the symmetry-analysed Raman intensities of A_1g character, two peaks near 8.8meV and 21.9meV are attributed to monopolar charge deformations of the valence-fluctuating Tm ions.     

Electrical resistivity of Sm monochalcogenides

The resistivity of mixed valence compounds has been computed in a two band Hubbard-like model. It is shown that there is a transition from a semiconductor to a metallic conductivity and that an intermediate valence can be found in both phases.     

Prediction of two-dimensional monochalcogenides: MoS and WS

Using density functional theory, we explore the possibility of two monolayer monochalcogenides, namely, MoS and WS. From the energetics, buckled and puckered structures are found to be more probable configurations. Our results on cohesive energy and phonon dispersion predict that the buckled structures of both MoS and WS are stable. On the other hand, while the puckered structure of WS clearly shows a dynamical instability, the same for MoS may have a stable configuration. Charge analyses predict ionic-like bonding in these systems. Density of states and band structure reveal a non-magnetic metallic nature for MoS in the stable configurations. However, for the buckled WS, our study predicts a non-magnetic semi-metallic nature. Further, semi-metal to indirect semiconductor transition has been observed for tensile strain of 5%, 6% and 8%.     

Thermodynamics of a model for valence fluctuations between two magnetic configurations

The thermodynamic Bethe-Ansatz equations of a model describing valence fluctuations between two magnetic configurations are derived. The model includes a spin-1/2 and a spin-1 configurations hybridized through the promotion of an electron to a conduction band.Thes–d like limits are reproduced. An expression for the specific heat at low temperatures is obtained.ConicetGuggenheim fellow