Band Jahn-Teller Research Articles

Charge ordering and heavy fermions in the semimetal Yb 4As 3

We present a model for Yb 4As 3 which can explain semiquantitatively a number of its unusual properties. Based on the view of the Yb-sites as being aligned on chains, the structural phase transition at T c ⋍ 300 K is described by a band Jahn-Teller effect of correlated electrons being driven by a charge ordering of the Yb-ions. The qualitative behavior of the corresponding elastic constant c 44 is explained. The heavy-fermion behavior on a scale of T ∗ ⋍ 50 K is understood within the framework of a slave-boson approximation to the effective low-temperature Hamiltonian.

Magnetism in Body-Centered Tetragonal FeRh1-xPdxAlloys (II) Band Structure

First-principle spin-polarized band calculations were performed for the body-centered tetragonal FeRh 1- x Pd x alloys with x =0, 0.5 and 1 as a function of the axial ratio c / a using the LMTO method. The energetically competing ferromagnetic and antiferromagnetic states were found to exist in bcc FeRh and bct FeRh 0.5 Pd 0.5 , as experimentally expected. The substitution of Pd for Rh hardly changes the shape of the density of states but simply raises the Fermi level when the axial ratio is fixed at a constant value. The tetragonal lattice elongation removes the degeneracy of the Fe d -band, and a large peak of DOS in the minority spin band for the bcc structure splits into two peaks for the bct structure, resulting in a reduction of the Fe magnetic moment. The large tetragonal elongation in the FeRh 1- x Pd x system is induced by the band Jahn-Teller effect.

Transport properties of CeAu2−xCuxIn

Abstract Measurements on the electrical resistivity ϱ, the thermopower S and the specific heat C have been done on CeAu 2− x Cu x In samples (0 ⩽ x ⩽ 0.5) having a cubic Heusler-type crystal. A crystal-phase transition possibly originating from a band Jahn-Teller effect was evidenced from sudden changes of ϱ and S . Neel temperature T N of the samples was found to be 1.2 K and did not depend on the value of λ. Structures in ϱ are considered to originate from a Kondo interaction. However, a small and negative value of S and a realization of a full entropy value of R ln 2 slightly above T N indicate that the Kondo interaction is weak.

Van Hove Jahn-Teller effect and high- Tc superconductivity

A new model is introduced, in which the structural instabilities associated with the van Hove singularities are reinterpreted in terms of band Jahn-Teller (JT) effects. This allows a model of the low-temperature phases of La 2 − χBa χCuO 4 in terms of static (the low-T tetragonal) or dynamic (the orthorhombic) JT phases. A solitonic model of the dynamic JT phase is presented.

超高圧下における混合原子価錯体の原子価転移と相図

Cs2Au2X6 (X=Cl, Br, I) are mixed valence complexes with Perovskite-type structure. Recently, we have elucidated the structural P-T phase diagram of these complexes under high pressures and high temperatures for the first time. When the pressure is applied at room temperature, Cs2Au2X6 (X=Cl, Br, I) undergo a tetragonal-to-tetragonal phase transition at 11 GPa, 9 GPa and 5. 5 GPa, respectively. This transition is regarded as a band Jahn-Teller transition driven by the AuI, III→AuII transition. The cubic perovskite structure appears under high pressures and high temperatures for all the three complexes. The cubic phase is obtained as a metastable state at room temperature and ambient pressure. The Au valence states in the second tetragonal phase and the cubic phase are considered to be AuII.

Van Hove Jahn-Teller effect and high-T c superconductivity

It is demonstrated that the dominant nesting in the superconducting cuprates is associated with the van Hove singularities (vHs's). The associated structural instabilities are interpreted as band Jahn-Teller (JT) effects, with the two vHs's providing the electronic degeneracy. The low-temperature tetragonal phase of La 2− x Ba x CuO 4 is a static JT phase, while the low-temperature orthorhombic phase is modelled as a dynamic JT phase. The dynamic JT phase can be pictured as LTT-type domains separated by LTO domain walls (solitons).

Electrical conductivity of the perovskite-type mixed-valence compound Cs2Au2I6 under high pressures and low temperature

We have investigated the electrical conductivity of single crystal Cs2Au2I6 under high pressures (0–8 GPa) and low temperature (4.2–300 K). We have observed strongly pressure-dependent resistivity anomalies under hydrostatic pressure condition. We have also observed drastic resistance jump around 5 GPa which may be associated with the “band Jahn-Teller” transition or the CDW transition.

A structural phase transition and magnetic properties in a Heusler alloy Ni 2MnGa

Magnetization and NMR measurements have been made for a ferromagnetic Heusler alloy Ni 2MnGa. The following results were obtained, (1) a magnetic moment on the Ni site was determined for the first time, (2) the magnitude of Ni moment which is induced by the molecular field from local Mn moments depends on structure and also on temperature. (3) These results are consistent with recent theoretical results discussing the band Jahn-Teller effect.

Metallization and phase transitions of the three-dimensional halogen-bridge mixed-valence complex Cs 2Au 2I 6 under high pressure

We have recently shown that insulating mixed-valence complex Cs 2Au 2I 6 transforms gradually toa metallic state between ambient pressure and 4.5 GPa at room temperature. Also shown that Cs 2Au 2I 6 undergoes a metal-to-metal transition in a process of increasing temperature at 6.5 GPa. The present energy-dispersive X-ray diffraction study has revealed that Cs 2Au 2I 6 undergoes a pressure-induced first-order tetragonal-to-tetragonal phase transition at about 5.5 GPa and room temperature. At the transition pressure, the Au-Au distance elongates along the c-axis, which is due to the ‘band Jahn-Teller’ transition. The Au 11 valence state is considered to be realized at this pressure. This phase transition would relate to the minimum in the pressure-resistance curve near 5.5 GPa, previously reported. It was also found that Cs 2Au 2I 6 undergoes a first-order tetragonal-to-cubic phase transition at 6.8 GPa and T ∾ 350 K.

Electronic Structure and Lattice Transformation in Ni2MnGa and Co2NbSn

It is known experimentally that Heusler alloys, Ni 2 MnGa and Co 2 NbSn undergo the cubic-to-tetragonal lattice transformation, in a ferromagnetic state and a paramagnetic one, respectively. We calculated electronic structures of Co 2 NbSn and Ni 2 MnGa for both cubic and tetragonal structures by KKR method. Comparing the density-of-state of the cubic and tetragonal structures for each alloy, it is expected that the band Jahn-Teller effect cause the lattice transformation in both alloys.

Electronic structure of RhTi phases

The phase transition of the intermetallic compound RhTi (cubic → tetragonal) was studied using band structure calculations. It was found that a band Jahn-Teller effect arises from the 25% filled Ti 3d e g-like band. We calculated that the energy of the compound is lowered by 0.04 eV (unit cell) −1by the tetragonal distortion.

Temperature dependence of the elastic constants of La3S4

A theoretical model is presented to explain the temperature dependence of the elastic constants of Le3S4. This model is based on the band Jahn-Teller mechanism and explains the behaviour of c' and c44 consistently. The theory has been fitted to experimental results and the physical meanings of the parameters obtained are discussed.

Conduction Property of LaAg1-xInx

The electrical resistivity ρ and the Hall resistivity ρ H of LaAg 1- x In x (0< x <0.7) have been measured and analyzed on the basis of two-band model. The calculated temperature dependence of ρ and ρ H agree well with the experimental ones. The s electrons are about 0.2 per unit cell for all x and change little at the band Jahn-Teller transformation temperature T M . The d -hole is in a range of 0.6–0.9 per unit cell and its number changes at T M . The total s -electron conductivity is about 30% of all and is very large compared with YZn and heavy rare earth RAg 1- x In x (R=Gd, Tb and Dy).

Interference between Superconductivity and Martensitic Transition in A15 Compounds

An interference effect between the high- T c superconductivity and martensitic transition in A15 compounds such as V 3 Si and Nb 3 Sn has been studied. It is found from the measurements of the sound velocity, sound attenuation, thermal dilation and the electrical resistance, that an occurrence of superconductivity interferes drastically with the developing tetragonal distortions caused by the martensitic transformation, but the high- T c superconductivity in these compounds is only weakly affected by the presence of the martensitic transition. This unilateral interference effect is qualitatively consistent with the second-order band Jahn-Teller mechanism which means the Fermi level crossing the upper split band of the doubly degenerate \(\varGamma_{12}\) subbands. Discussions on T c are given on the basis of the Fermi surface effect of \(\varGamma_{12}\) subbands.

Behaviour of the A15 compounds under non-hydrostatic stress

The effect of non-hydrostatic stress on the cubic-to-tetragonal (C-T) transition and the superconductivity (SC) in the A15 compounds has been studied theoretically. The author has assumed that the C-T transition is driven by the band Jahn-Teller effect involving the electronic band which evolves from the doubly degenerate Gamma 12 state at the centre of the cubic Brillouin zone. The SC is also assumed to pertain to this band. He has shown that both the C-T transition and the SC should be affected by the Gamma 12 symmetry-adapted stress ( sigma 12+ sigma 22)1/2, where sigma 1=(s1-s2)/21/2, sigma 2=(2s3-s1-s2)/61/2 and si are the stress components in the Voigt notation. Apart from explaining several experimental results, he has pointed out that the origin of some seemingly different anomalies in the A15 compounds is due to the presence of stress in the samples.

Structural and Magnetic Properties of RAg (R=La, Ce, Pr and Nd) at Hydrostatic Pressures up to 37 kbar

Pressure-induced structural transitions and associated changes in magnetism of the light RAg have been thoroughly investigated from measurements of electrical resistivity ρ, ac magnetic susceptibility up to 37 kbar, magnetization σ up to 15 kbar and compressibility up to 3 kbar in temperatures from 1.5 K to 300 K. A cubic to tetragonal structural transition T M is found to be induced at pressure p c =6, 2, 10 and 30 kbar for R=La, Ce, Pr and Nd, respectively, and to increase with further increasing ρ at about the same rate of +13∼+16 K/kbar. The appearance of T M and the different p c values for the RAg are discussed on the basis of the band Jahn-Teller model. T M is accompanied by a shift of the magnetic ordering temperature T mag . For CeAg, another structural transition T M is also found at ρ=20 kbar. The remarkably larger pressure effects of T map , ρ and σ are briefly discussed with recent theories of the Kondo lattice.

Phonon softening in ferromagnetic BCC iron

The temperature dependence of the phonon frequencies in ferromagnetic BCC iron is discussed using nearest- and second-nearest-neighbour force constants, which are evaluated from elastic moduli obtained previously by the spin-fluctuation theory with the tight-binding model. The simple calculation accounts semi-quantitatively for the essential features in the observed phonon spectrum. In particular, the marked softening of the transverse (110) T1 mode is shown to be due to the softening in the second-nearest-neighbour force constant arising from the magnetic band Jahn-Teller effect.

Transport Properties of LaAg1-xInx, CeAg1-xInx and RAg (R=Pr, Nd, Gd and Y)

The temperature dependences of the electrical resistivity ρ and the thermoelectric power S of LaAg 1- x In x , CeAg 1- x In x and RAg (R=Pr, Nd, Gd and Y) compounds have been investigated. In addition, the Hall coefficient R 0 of LaAg 1- x In x has been measured. The cubic to tetragonal transition at the temperature, T M , is found to have strong effects on ρ, S and R 0 for LaAg 1- x In x , while for CeAg 1- x In x , no change appears in S at T M . The In concentration dependences of ρ, S , R 0 and T M for LaAg 1- x In x and CeAg 1- x In x are discussed on the basis of the band Jahn-Teller model and the s - d interband scattering model. The variations in ρ and S for RAg with R are discussed in relation to the structural instabilities in RAg.

Effect of structural transformation on electronic properties of near-equiatomic V-Ru alloys

The change in electronic structure caused by the transformation from cubic to tetragonal phase has been investigated in near-equiatomic V-Ru and V-M-Ru (M=Ti, Ta) alloys through measurements of electrical resistivity, Hall coefficient and Knight shift of 51V. The absolute values of these properties in a specimen increase with the transformation, which means a decrease in the electronic density of states. The values in all the specimens are universally dependent on the electron-atom ratio Z in both cubic and tetragonal phases. This universal dependence on Z implies the validity of the rigid-band approximation. The increase in Z lowers the values in the cubic phase, while raising them in the tetragonal phase. These results suggest that the transformation is caused by the band Jahn-Teller effect.

Electron-phonon interaction in LaAg

Magnetoacoustic de Haas-van Alphen data on LaAg are compared with a new relativistic band structure calculation including La-4f states. We find convincing agreement of the measured and calculated dHvA frequencies. The deformation of the Fermi surface under strain is determined from the magnetoacoustic dHvA amplitude. The strongest deformation potential coupling is found for three equivalent pockets which become inequivalent under orthorhombic strain. The lifting of their degeneracy leads to the softening of theT3 elastic constant. This is different from the useual band Jahn-Teller effect which is based on the lifting of subband degeneracy at a single point ink-space.