Magnetic-field-induced valence transition in rare-earth systems

The magnetic-field-induced valence transition in rare-earth systems has been investigated using the periodic Anderson model supplemented by the Falicov-Kimball term. This model has been solved by first decoupling the Falicov-Kimball term as proposed by Khomskii and Koharjan and then taking the limit of infinite intra-site Coulomb repulsion. The valence transition both in the absence and in the presence of magnetic field as a function of temperature is studied. It has been found that the system makes transition from non-magnetic to magnetic state when the magnetic field increases beyond a critical value H _c. The phase boundary defined in terms of reduced field H _c(T)/H _c(0) and reduced temperature T/T _v (T _v being the valence transition temperature in the absence of field) is almost independent of the position of the localized level. The results are in qualitative agreement with experimental observations in Yb- and Eu-compounds.     

Valence variations in the monolayer regime of Sm on the Nb(110) surface

Sm overlayers in the monolayer regime, deposited on a Nb(110) single-crystal surface at room temperature, have been studied by means of high-resolution photoelectron spectroscopy of the Nb 3d_5/2 core level and the Sm valence band. In the submonolayer regime, the Sm valence varies from mainly divalent for very low coverages to essentially trivalent close to a complete monolayer. Above 1 ML, a new divalent component appears in the Sm 4f spectra, corresonding to divalent Sm in the second layer. The mixed valence in this overlayer system is concluded to be heterogeneous (all Sm atoms have integer but site-dependent valence). Sm forms ordered overlayers on Nb(110) and the Sm growth is consistent with a layer-by-layer growth mode (Frank–van der Merwe growth).     

Influence of parity-forbiddenness in the onset of mixed valence state in Sm1−x GdxS

The electronic structure of Sm and Gd in Sm_1−x Gd_xS has been experimentally studied by the x-ray shift method. Besides the well known strong increase of Sm valence at x≈0.15 (associated with the electronic transition to the mixed valence state), a decrease of Sm valence for x⩾0.9 has been observed for the first time. This phenomenon is explained as due to the 4f-5d hybridization on neighboring Sm atoms involved in the onset of the mixed valence state, and is interpreted as a manifestation of parity violation forbiddenness. Fiz. Tverd. Tela (St. Petersburg) 39, 1017–1019 (June 1997)     

Valence band variation in Si (110) nanowire induced by a covered insulator

In this work, we investigate strain effects induced by the deposition of gate dielectrics on the valence band structures in Si (110) nanowire via the simulation of strain distribution and the calculation of a generalized 6 × 6k$\cdot$p strained valence band. The nanowire is surrounded by the gate dielectric. Our simulation indicates that the strain of the amorphous SiO₂ insulator is negligible without considering temperature factors. On the other hand, the thermal residual strain in a nanowire with amorphous SiO₂ insulator which has negligible lattice misfit strain pushes the valence subbands upwards by chemical vapour deposition and downwards by thermal oxidation treatment. In contrast with the strain of the amorphous SiO₂ insulator, the strain of the HfO₂ gate insulator in Si (110) nanowire pushes the valence subbands upwards remarkably. The thermal residual strain by HfO₂ insulator contributes to the up-shifting tendency. Our simulation results for valence band shifting and warping in Si nanowires can provide useful guidance for further nanowire device design.     

The amplification of far-infrared radiation on optically excited p-type germanium

The far-infrared spectrum of cyclotron resonance absorption in the valence band of p-type germanium has been calculated using 6 × 6 k*p Hamiltonian for degenerate valence band for magnetic field 20T and lattice temperature 77K. That has been shown that excitation by pulseCO ₂ laser radiation can lead to the amplification on light hole cyclotron resonance.     

Valence transition in the periodic Anderson model

A very rich phase diagram has recently been found in CeCu₂Si₂ from high pressure experiments where, in particular, a transition between an intermediate valence configuration and an integral valent heavy fermion state has been observed. We show that such a valence transition can be understood in the framework of the periodic Anderson model. In particular, our results show a breakdown of a mixed-valence state which is accompanied by a drastic change in the f occupation in agreement with experiment. This valence transition can possibly be interpreted as a collapse of the large Fermi surface of the heavy fermion state which incorporates not only the conduction electrons but also the localized f electrons. The theoretical approach used in this paper is based on the novel projector-based renormalization method (PRM). With respect to the periodic Anderson model, the method was before only employed in combination with the basic approximations of the well-known slave-boson mean-field theory. In this paper, the PRM treatment is performed in a more sophisticated manner where both mixed as well as integral valent solutions have been obtained. Furthermore, we argue that the presented PRM approach might be a promising starting point to study the competing interactions in CeCu₂Si₂ and related compounds.     

Overlap valence quarks on a twisted mass sea: A case study for mixed action lattice QCD

We discuss a lattice QCD mixed action investigation employing Wilson maximally twisted mass sea and overlap valence fermions. Using four values of the lattice spacing, we demonstrate that the overlap Dirac operator assumes a point-like locality in the continuum limit. We also show that by adopting suitable matching conditions for the sea and valence theories a consistent continuum limit for the pion decay constant and light baryon masses can be obtained. Finally, we confront results for sea–valence mixed meson masses and the valence scalar correlator with corresponding expressions of chiral perturbation theory. This allows us to extract low energy constants of mixed action chiral perturbation which characterize the strength of unitarity violations in our mixed action setup.     

Exact solution for a degenerate Anderson impurity in the limit embedded into a correlated host

We consider the one-dimensional t - J model, which consists of electrons with spin S on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the lattice sites and spin-exchange J between neighboring sites. The model is integrable at the supersymmetric point, J = t. Without spoiling the integrability we introduce an Anderson-like impurity of spin S (degenerate Anderson model in the limit), which interacts with the correlated conduction states of the host. The lattice model is defined by the scattering matrices via the Quantum Inverse Scattering Method. We discuss the general form of the interaction Hamiltonian between the impurity and the itinerant electrons on the lattice and explicitly construct it in the continuum limit. The discrete Bethe ansatz equations diagonalizing the host with impurity are derived, and the thermodynamic Bethe ansatz equations are obtained using the string hypothesis for arbitrary band filling as a function of temperature and external magnetic field. The properties of the impurity depend on one coupling parameter related to the Kondo exchange coupling. The impurity can localize up to one itinerant electron and has in general mixed valent properties. Groundstate properties of the impurity, such as the energy, valence, magnetic susceptibility and the specific heat coefficient, are discussed. In the integer valent limit the model reduces to a Coqblin-Schrieffer impurity. Received: 31 December 1997 / Accepted: 17 March 1998     

Temperature induced valence instabilities in ternary Eu-pnictides: a comprehensive view

We report on measurements of the lattice constants, magnetic susceptibility, L_III X-ray absorption and Mössbauer-effect on EuNiP and EuPdP, which crystallize in the hexagonal layered Ni₂In structure. In both compounds the Eu valence above 510 K is 2.33. With decreasing temperature they show one (EuPdP) or two (EuNiP) first order phase transitions with a valence increase of about 0.16. At the same time thec-axis shrinks while the a-axis even increases. From Mössbauer measurements we show that the nature of the valence mixing is static. In contrast, the valence mixing in isostructural EuPtP is static at low temperatures, too, but it becomes homogeneous valent above a first order phase transition at 235 K. The behaviour of these compounds (as well as that of EuPdAs) is explained in a new model of electrostatically charged layers. In this model we can explain the temperature dependence of the lattice constants, the static valence mixing and the occurrence of preferred valences of the order 2 6/n. Together with the compression shift model of Hirst we can also understand the mechanism of the phase transitions. A comparison with EuT₂Si₂ compounds in ThCr₂Si₂ structure shows that in EuTX compounds only the electronic structure of the transition elements is relevant for the occurrence of mixed valency.     

Domain population in SrTiO3 below the cubic-to-tetragonal phase transition

High-angle double-crystal X-ray diffractometry (HADOX) has been applied to measure the intensity around three reciprocal lattice points: 400, 040 and 004 of SrTiO₃. Each point splits into two in the tetragonal phase according to the appearance of three kinds of domains; the temperature dependence of the intensity of component peaks at these points was measured under identical conditions. The results show that the domain population varies with temperature in a range of about 20 K below the transition. It is found, however, that the total intensities of 400 and 004 of the tetragonal phase are constant; this is observed by summing up the relevant intensity at the three reciprocal lattice points.     

Effect of pressure on the Yb valency in YbS and YbTe

Abstract

We have measured L_III-edge x-ray absorption spectra (L_III-XA) of Yb in YbS and YbTe and optical reflectivity spectra of YbTe at pressures up to 34 GPa. In both materials the Yb ion undergoes a continuous valence change. In the case of YbS, the pressure-induced variation of the Yb mean valence v(I_III) agrees well with previous optical and lattice parameter studies. For YbTe, however, the XA data indicate the onset of a valence transition at a lower pressure (～10 GPa) than observed in optical spectra (～16 GPa). This behavior is explained by larger hybridization effects between the chalcogen p-bands and 4f states in YbTe as compared to YbS.     

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.     

应变Si/(001)Si1-xGex空穴有效质量各向异性

基于应变Si/（001）Si_1-xGe_x材料价带E（k）-k关系模型,研究获得了其沿不同晶向的空穴有效质量.结果表明,与弛豫材料相比,应变Si/（001）Si_1-xGe_x材料价带带边（重空穴带）、亚带边（轻空穴带）空穴有效质量在某些k矢方向变化显著,各向异性更加明显.价带空穴有效质量与迁移率密切相关,该研究成 关键词： 应变Si 价带 空穴有效质量     

High pressure synthesis of the mixed valent and nonsuperconducting ternary YbRhxSny compound

The ternary compound YbRh_1.4Sn_4.6 with the phase I structure (simple cubic) when subjected to a pressure of 40 kbar at 800°C is found to transform to phase III structure (f.c.c.) with the composition YbRh_1.1Sn₃. The latter compound has a lattice parameter of $\text{a = 13.735}\text{A}\text{?}$ which suggests that the Yb is in an intermediate valence state. The temperature dependence of magnetic susceptibility suggests that the Yb is in a homogeneously mixed valence state in the pressure synthesized product. In the phase I structure YbRh_1.1Sn_4.6 is superconducting at 8.6°K, but in the phase III structure the compound YbRh_1.1Sn₃ is not superconducting down to 0.9°K. It is suggested that superconductivity and mixed valence are incompatible.     

Effects of the temperature and the pressure on the intermediate valence of Ce(In1−xSnx)3

The curve of the lattice constant of samples Ce(In_1-xSn_x)₃ plotted as a function of the composition x deviates from the linear Vegard's law; the deviation is found to be much more eminent at liquid N₂ temperature than at room temperature. Effects of the pressure on the electrical resistivity was found to be very large for a sample with a composition x = 0.5 which is near to the critical value of the occurence of the intermediate valence state of Ce atoms. The atomic volume of Ce atoms is seen to play an important role for the occurence of the intermediate valence state.     

First-principles calculations on structural, elastic, electronic, optical and thermal properties of CsPbCl3 perovskite

The structural, elastic, electronic, optical and thermal properties of the semiconductor perovskite CsPbCl₃ were investigated using the pseudo-potential plane wave (PP-PW) scheme in the frame of generalized gradient approximation (GGA) and local density approximation (LDA). The computed lattice constant agrees reasonably with experimental and theoretical ones. The CsPbCl₃ crystal behaves as ductile material. The valence bands are separated from the conduction bands by a direct band gap R-R. We distinguished hybridization between Pb-p states and Cl-p states in the valence bonding region. Under compression at P=30 GPa, this material will have a metallic character. The thermal effect on the lattice constant, bulk modulus, Debye temperature and heat capacity C_V was predicted using the quasi-harmonic Debye model. To the author's knowledge, most of the studied properties are reported for the first time.     

Electronic and hole minibands in quantum wire arrays of different crystallographic structure

We consider semiconductor heterostructures consisting of GaAs rods embedded in Al_xGa_1 − xAs and disposed in sites of a square or triangular lattice. The electronic and hole spectra around the conduction band bottom and the valence band top are examined for electrons and holes propagating in plane of periodicity, versus geometry of the lattice formed by the rods, concentration of Al in the matrix material, and structural parameters including the filling fraction and the lattice constant. Our calculations use the envelope function and are based on the effective-mass approximation. We show that the electronic and hole spectra resulting from the periodicity of the heterostructure, depend on the factors considered and that the effect of lattice geometry varies substantially with lattice constant. For low lattice constant values the minigaps are significantly wider in the case of triangular lattice, while for high lattice constant values only slightly thiner minigaps occur in the square lattice-based arrays. We discuss the consequences of our findings for the efficiency of solar cells.     

Bethe-ansatz solution of a model for a mixed valent impurity with two magnetic configurations: II. Numerical solution of the thermodynamic equations

A mixed valence impurity with two magnetic configurations of total angular momentumJ ₂ andJ ₁=J ₂+1/2, respectively, coupled by conduction electrons with total angular momentum 1/2 via a hybridization matrix element is considered. The thermodynamic Bethe-ansatz equations derived previously are solved numerically for various values ofJ ₂. Thef-level occupation, the entropy, the magnetic susceptibility and the specific heat are obtained as a function of temperature for variousf-level positions. The magnetic field dependence is also discussed in the limit of integer valence (exchange model).Supported by the CONICET, Argentina     

Temperature and spatial diffusion of atoms cooled in a 3D lin ⊥lin bright optical lattice

We present a detailed experimental study of a three-dimensional lin⊥lin bright optical lattice. Measurements of the atomic temperature and spatial diffusion coefficients are reported for different angles between the lattice beams, i.e. for different lattice constants. The experimental findings are interpreted with the help of numerical simulations. In particular we show, both experimentally and theoretically, that the temperature is independent of the lattice constant. Received 5 July 2001 and Received in final form 13 August 2001     

Mixed-Spin Ising Model in an Oscillating Magnetic Field and Compensation Temperature

Godoy et al. (Phys. Rev. B 69, 054428, 2004) presented a study of the magnetic properties of a mixed spin (1/2,1), Ising ferrimagnetic model on a hexagonal lattice without an oscillating magnetic field. They employed dynamic mean-field calculations and Monte Carlo simulations to find the compensation point of the model and to present the phase diagrams. It has been found that the N-type compensation temperature appears only when the intrasublattice interaction between spins in the σ sublattice is ferromagnetic. Moreover, the system only undergoes a second-order phase transition. In this work, we extend the study a dynamic compensation temperature of a mixed spin-1/2 and spin-1 Ising ferrimagnetic system on a hexagonal lattice in the presence of oscillating magnetic field within the framework of dynamic mean-field calculations. We find that the system displays the N-type compensation temperature. We also calculate dynamic phase diagrams in which contain the paramagnetic, ferrimagnetic, nonmagnetic fundamental phases and two different mixed phases, depending on the interaction parameters and oscillating magnetic field. The system also exhibits tricritical and reentrant behaviors.