Effect of antiferromagnetic spin correlations on lattice distortion and charge ordering in Pr0.5Ca1.5MnO4

The effect of orbital-lattice coupling on the electrical resistivity of YBaCuFeO5 investigated by X-ray absorption

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO5 (YBCFO). An onset of increase of resistivity at ~180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab-plane and is also observed with strong anisotropy between the ab-plane and the c-axis. The XLD spectra at the Fe L3,2-edge revealed a change in Fe 3d eg holes from the preferential [Formula: see text] orbital at high temperature (300-150 K) to the [Formula: see text] orbital at/below 125 K. The analysis of the Fe K-edge EXAFS data of YBCFO further revealed an unusual increase in the Debye-Waller factor of the nearest-neighbor Fe-O bond length at/below 125 K, suggesting phonon-softening behavior, resulting in the breaking of lattice symmetry, particularly in the ab-plane of Fe-related square pyramids. These findings demonstrate a close correlation between electrical resistivity and coupling of the preferred Fe 3d orbital with lattice distortion of a single crystal of YBCFO.

Manifestation of charge/orbital order and charge transfer in temperature-dependent optical conductivity of single-layered Pr0.5Ca1.5MnO4

Half-doped single-layered manganite, [Formula: see text] has shown a charge/orbital order of the e _g electrons and CE-type spin order of the t _2g electrons of the Mn ions. A previous experimental study on that system, supported by a simple modelling, has suggested that the charge/orbital ordering play an important role in governing the temperature dependence of optical conductivity of a broad peak around 0.7-0.8 eV. In addition, another peak around 3.5 eV, which is less sensitive to temperature, has been attributed to the charge transfer from O-p  to Mn-e _g orbitals. Nevertheless, the theoretical explanation was incomplete as the role of O-p  orbitals was not considered in the model. In this paper, we propose to improve the model by incorporating both Mn-e _g and O-p  orbitals. We assume the existence of charge/orbital ordering and investigate how this ordering as well as the charge-transfer phenomenon control the temperature dependence of the optical conductivity. Our results reveal the charge/orbital-ordering peak in the region 0.7-1.2 eV, which is blue-shifted with decreasing temperature, and the charge-transfer peak around 3.5 eV, which is less sensitive to temperature. The capability of our model to capture the general profile and temperature dependence of the optical conductivity suggests the validity of our theory.

Anisotropy in the magnetic interaction and lattice-orbital coupling of single crystal Ni3TeO6

This investigation reports on anisotropy in the magnetic interaction, lattice-orbital coupling and degree of phonon softening in single crystal Ni₃TeO₆ (NTO) using temperature- and polarization-dependent X-ray absorption spectroscopic techniques. The magnetic field-cooled and zero-field-cooled measurements and temperature-dependent Ni L_3,2-edge X-ray magnetic circular dichroism spectra of NTO reveal a weak Ni-Ni ferromagnetic interaction close to ~60 K (T_SO: temperature of the onset of spin ordering) with a net alignment of Ni spins (the uncompensated components of the Ni moments) along the crystallographic c-axis, which is absent from the ab-plane. Below the Néel temperature, T_N~ 52 K, NTO is stable in the antiferromagnetic state with its spin axis parallel to the c-axis. The Ni L_3,2-edge X-ray linear dichroism results indicate that above T_SO, the Ni 3d e_g electrons preferentially occupy the out-of-plane 3d_3z²_−r² orbitals and switch to the in-plane 3d_x²_−y² orbitals below T_SO. The inherent distortion of the NiO₆ octahedra and anisotropic nearest-neighbor Ni-O bond lengths between the c-axis and the ab-plane of NTO, followed by anomalous Debye-Waller factors and orbital-lattice in conjunction with spin-phonon couplings, stabilize the occupied out-of-plane (3d_3z²_−r²) and in-plane (3d_x²_−y²) Ni e_g orbitals above and below T_SO, respectively.

Anisotropy in the thermal hysteresis of resistivity and charge density wave nature of single crystal SrFeO3-δ: X-ray absorption and photoemission studies

The local electronic and atomic structures of the high-quality single crystal of SrFeO_3-δ (δ~0.19) were studied using temperature-dependent x-ray absorption and valence-band photoemission spectroscopy (VB-PES) to investigate the origin of anisotropic resistivity in the ab-plane and along the c-axis close to the region of thermal hysteresis (near temperature for susceptibility maximum, T_m~78 K). All experiments herein were conducted during warming and cooling processes. The Fe L_3,2-edge X-ray linear dichroism results show that during cooling from room temperature to below the transition temperature, the unoccupied Fe 3d e_g states remain in persistently out-of-plane 3d_3z²_-r² orbitals. In contrast, in the warming process below the transition temperature, they change from 3d_3z²_-r² to in-plane 3d_x²_-y² orbitals. The nearest-neighbor (NN) Fe-O bond lengths also exhibit anisotropic behavior in the ab-plane and along the c-axis below T_m. The anisotropic NN Fe-O bond lengths and Debye-Waller factors stabilize the in-plane Fe 3d_x²_-y² and out-of-plane 3d_3z²_-r² orbitals during warming and cooling, respectively. Additionally, a VB-PES study further confirms that a relative band gap opens at low temperature in both the ab-plane and along the c-axis, providing the clear evidence of the charge-density-wave nature of SrFeO_3-δ (δ~0.19) single crystal.

Electronic self-organization in the single-layer manganite Pr1-xCa1+xMnO4

We use neutron scattering to investigate the doping evolution of the magnetic correlations in the single-layer manganite Pr1-xCa1+xMnO4, away from the x=0.5 composition where the CE-type commensurate antiferromagnetic (AF) structure is stable. We find that short-range incommensurate spin correlations develop as the system is electron doped (x<0.5), which coexist with the CE-type AF order. This suggests that electron doping in this system induces an inhomogeneous electronic self-organization, where commensurate AF patches with x=0.5 are separated by electron-rich domain walls with short-range magnetic correlations. This behavior is strikingly different than for the perovskite Pr1-xCaxMnO3, where the long-range CE-type commensurate AF structure is stable over a wide range of electron or hole doping around x=0.5.