Charge and orbital ordering in Na0.5CoO2

The ground state of Na_0.5CoO₂ has been calculated using the full potential local orbital method and local density approximation plus Hubbard U (); the results demonstrate that charge and orbital ordering evidently exist in the present system in association with the antiferromagnetic state. Notable structural features observed between 300 and 30 K have been carefully examined using in situ TEM investigations, a superstructure with a wave vector of Q1=a^∗/2, becoming commonly visible below , can be interpreted as the charge/orbital ordering on the Co1 and Co2 sites. Moreover, we have also observed another notable superstructure with Q2=a^∗/4 below the phase transition of , which suggests a more complicated orbital ordered state existing at lower temperatures.     

Effect of Ti doping on the electrical transport and magnetic properties of layered compound Na0.8CoO2

Effect of Ti doping on the electrical transport and magnetic properties of layered Na_0.8Co_1−xTi_xO₂ compounds has been investigated. The lattice parameters a and c increase with x. A minor amount of Ti doping results in a metal-insulator transition at low temperatures. For samples with x>0.03, the variable-range hopping process dominates the transport behavior above a certain temperature. The temperature dependence of magnetization of all the samples is found to obey the Curie-Weiss law. The mechanism of the doping effect is discussed.     

Praseodymium silicide formation at the Pr2O3/Si interface

The interface and layer structure of praseodymium (Pr) oxide layers grown on Si(0 0 1) from a high-temperature effusion cell are studied using grazing incidence X-ray diffraction. Due to the interdiffusion of praseodymium and silicon atoms, Pr silicide forms in the layers. We find that Pr silicide is the favorable structure under oxygen deficient growth conditions in the Pr oxide layer. To avoid the silicidation, additional oxygen must be supplied. The formation of Pr silicide is suppressed for layers grown with an oxygen partial pressure of 10⁻⁷ mbar at a substrate temperature of 700 °C.     

RHEED intensity oscillation of C60 growth on GaAs substrates

Intensity oscillation of reflection high-energy electron diffraction (RHEED) is observed during C₆₀ layer epitaxial growth on GaAs (1 1 1)B and (1 1 1)A substrates. The frequency of the oscillation coincides well with growth rate of C₆₀ layers, suggesting that C₆₀ layers grow with repeating nucleation and a step flow growth as with GaAs and other semiconductor materials. Unusual oscillation is observed in the initial C₆₀ layer growth on GaAs (1 1 1)B substrates with (2 × 2) reconstruction. The initial layer growth is completed at approximately half monolayer coverage by C₆₀ molecules. This phenomenon is explained by the model that C₆₀ absorption sites are limited due to As-trimers absorbed on (1 1 1)B surfaces. This model is strongly supported by the fact that no such effect is observed on GaAs (1 1 1)A substrates where no As-trimer is absorbed.     

Charge ordering in amorphous WOx films

We report on the observation of highly anisotropic viscous electronic conducting phase in amorphous WO_1.55 films that occurs below a current (I)- and frequency (f  )-dependent temperature T^∗(I,f)$T^{\ast }(I,f)$. At T<T^∗(I,f)$T<T^{\ast }(I,f)$ the rotational symmetry of randomly disordered electronic background is broken leading to the appearance of mutually perpendicular metallic- and insulating-like states. A rich dynamic behavior of the electronic matter occurring at T<T^∗(I,f)$T<T^{\ast }(I,f)$ provides evidence for an interplay between pinning effects and electron–electron interactions. The results suggest a dynamic crystallization of the disordered electronic matter, viz. formation of sliding Wigner crystal, as well as the occurrence of quantum liquid-like crystal or stripe phase at low drives.     

Pressure-induced phase transition of BaH2 : Post Ni2In phase

Up to now a Ni₂In structure is a final step in the structural sequence of ionic AX₂ compounds under high pressure. Powder X-ray diffraction experiments on BaH₂ were performed at room temperature and high pressures up to 69 GPa. Successive phase transformations were observed to occur in two stages. The first was from the cotunnite to the Ni₂In structure at 2.5 GPa. The second transition commenced at pressures around 50 GPa and was completed at 65 GPa. At the transition the arrangement of a cation sublattice changes from an hcp to a simple hexagonal lattice. This is the first observation of the post Ni₂In phase.     

Structural phase transitions and sodium ordering in Na0.5CoO2: a combined electron diffraction and Raman spectroscopy study

The nonstoichiometric Na_xCoO₂ system exhibits extraordinary physical properties that correlate with temperature and Na concentration in its layered lattice without evident long-range structure modification when conventional crystallographic techniques are applied. For instance, Na_0.7CoO₂, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na_0.33CoO₂·1.3H₂O is a newly discovered superconductor with T_c∼4 K, and Na_0.5CoO₂ exhibits an unexpected charge ordering transition at around T_co∼55 K. Recent studies suggest that the transport and magnetic properties in the Na_xCoO₂ system strongly depend on the charge carrier density and local structural properties. Here we report a combined variable temperature transmission electron microscopy and Raman scattering investigation on structural transformations in Na_0.5CoO₂ single crystals. A series of structural phase transitions in the temperature range from 80 to 1000 K are directly identified and the observed superstructures and modulated phases can be interpreted by Na-ordering. The Raman scattering measurements reveal phase separation and a systematic evolution of active modes along with phase transitions. Our work demonstrates that the high mobility and ordering of sodium cations among the CoO₂ layers are a key factor for the presence of complex structural properties in Na_xCoO₂ materials, and also demonstrate that the combination of electron diffraction and Raman spectroscopy measurements is an efficient way for studying the cation ordering and phase transitions in related systems.     

Modification of the charge ordering transition in the quasi-one-dimensional conductor (TMTTF)2SbF6 under pressure

We have measured the temperature dependences of the conductance G and the dielectric permittivity ε′ of the (TMTTF)₂SbF₆ compound under a moderate pressure. The maximum of G(T) associated with the Mott-Hubbard localization disappears under pressure. With increasing pressure the peak in ε′(T), corresponding to the charge ordering (CO) phase transition, shifts to lower temperatures and broadens. At pressures above 0.24 GPa, ε′(T) becomes strongly frequency dependent. These modifications are explained in the frame of the extended Hubbard model and a slowing down behavior.     

Early-stage ordering in in-situ annealed Fe51Pt49 films

We evidenced an early-stage ordering (ESO) in Fe₅₁Pt₄₉ film before the appearance of superlattice diffraction (long-range-order, LRO) using 40-nm-thick films prepared by magnetron sputtering onto quartz substrate. The appearance of L1₀ phase for samples deposited at substrate temperatures (T_s) 400 °C and higher was verified by X-ray diffraction. Surface roughness of Fe₅₁Pt₄₉ films, obtained via X-ray specular reflectivity with computational fitting, increases from 3.8 to 11 Å as T_s is increased from 25 to 275 °C. As further increase of T_s to 375 °C, the roughness drops to 3.2 Å and then increases again to 38 Å with T_s up to 700 °C. Measurement on residual strain demonstrates that it is initially compressive at T_s<400 °C. Thereafter the strain transfers to a tensile one and increases in magnitude as increasing T_s up to 700 °C corresponding to LRO transformation. Local atomic rearrangement is observed for samples deposited at T_s>250 °C by using extended X-ray absorption fine structure. Coercivity of films increases from 10 to 460 Oe as T_s increase from 25 to 375 °C (ESO) and then from 460 to 10,700 Oe with T_s 375-700 °C (normal LRO). The worked out quantitative estimation of ESO engages with that of LRO before T_s 400 °C.     

Hidden order, collective excitations, and entropy loss in the heavy fermion superconductor URu2Si2

We have clarified the ‘hidden order’ of URu₂Si₂, and the spin density wave, and calculated the large entropy loss on Kondo channel transition to the one channel Kondo fixed point of the crystal. The three collective excitations, particularly the one discovered recently, have been either derived or explained. These and other findings, in excellent agreement with outstanding data, are of general applicability to the analogues of URu₂Si₂ including the nonstoichiometric cuprates. The results show the presence of multichannel Kondo effect in these crystals.     

Ferromagnetism and metallicity in the modulated Sr1−xThxCoO3−δ oxygen deficient perovskites with x∼0.1

The Th⁴⁺ for Sr²⁺ substitution in SrCoO_3−δ is found to be an efficient way to stabilize an oxygen deficient perovskite. The structural study reveals a Th⁴⁺ solubility limited to ∼10% so that the chemical formula, Sr_0.9Th_0.1CoO_2.79, is obtained. The comparison to the Y³⁺ for Sr²⁺ substitution shows that a lower oxygen content is obtained for the former according to the Sr_0.75Y_0.25CoO_2.55 formula. The higher oxygen content reached with Th⁴⁺ has a strong impact on the physical properties: Sr_0.9Th_0.1CoO_2.79 is an itinerant ferromagnet (T_C∼200 K, ρ_5 K=2 mΩ cm) whereas Sr_0.75Y_0.25CoO_2.55 is a robust insulating antiferromagnet (T_N∼320 K, ρ_5 K=10⁵ Ω cm). Clearly, the substitution of a tetravalent cation for Sr²⁺ appears to be a promising route to synthesize SrCoO_3−δ itinerant ferromagnets without the use of high oxygen pressure nor electrochemical oxidation.     

Polarized SSXANES study of spin ordering in ferromagnetic and paramagnetic phases of La1.2Sr1.65Ca0.15Mn2O7

Spin-selected polarized X-ray absorption near-edge structures (SSXANES) at the Mn K-edge from a bilayer La_1.2Sr_1.65Ca_0.15Mn₂O₇ single crystal have been studied with high resolution, both in the ferromagnetic (15 K) as well as paramagnetic phase (300 K). The orientation-dependent SSXANES spectra show unmistakable temperature dependence as the system makes the ferromagnetic to paramagnetic phase transition. The pre-edge structures are too intense to be ascribed to weak quadrupole transitions and are interpreted in terms of hybridization of Mn 3d orbitals with O2p and Mn 4p orbitals over and above similar onsite hybridization. The results also indicate possible existence of a small local (time-frozen) ferromagnetic ordering in the macroscopically disordered state. Need for further experimental and theoretical work on the SSXANES spectra from the bilayer system is emphasized.     

Electrochemical de-intercalation, oxygen non-stoichiometry, and crystal growth of NaxCoO2−δ

We report a detailed study of de-intercalation of Na from the compound Na_xCoO_2−δ using an electrochemical technique. We find evidence for stable phases with Na contents near the fractions x?1/3, 1/2, 5/8, 2/3, and 3/4. Details regarding the floating-zone crystal growth of Na_0.75CoO₂ single crystals are discussed as well as results from magnetic susceptibility measurements. We observe the presence of significant oxygen deficiencies in powder samples of Na_0.75CoO_2−δ prepared in air, but not in single crystal samples prepared in an oxygen atmosphere. The oxygen deficiencies in a Na_0.75CoO_2−δ sample with δ∼0.08 remain even after electrochemically de-intercalating to Na_0.3CoO_2−δ.     

The effect of Gd-doping on the charge ordering state of Bi0.3−xGdxCa0.7MnO3 (0≤x≤0.30)

The effect of Gd-doping on the charge ordering (CO) state in perovskite-type manganates Bi_0.3−xGd_xCa_0.7MnO₃ with x=0, 0.02, 0.05, 0.1, 0.3 has been investigated by transport and magnetic property measurements. It is found that CO temperature (T_CO) and antiferromagnetic (AFM) ordering temperature T_N occurring below T_CO decrease obviously with increasing Gd-doping level. Accompanying the variation of T_CO, the increased magnetization and the decreased resistivity are observed. In addition, the increased magnetic inhomogeneity has been also observed in the samples based on the difference between the zero-field-cooling (ZFC) magnetization M_ZFC and field-cooling (FC) magnetization M_FC, which is ascribed to the competition between ferromagnetic (FM) phase induced by Gd-doping and CO AFM phase. The experimental results indicate that the Bi³⁺ lone pair electron with 6s² character plays a dominating role on the CO state of Bi_0.3Ca_0.7MnO₃.     

Electronic structure of dimerized spinel ZnV2O4

Electronic structure calculations were performed for ZnV₂O₄, a material close to a metal-insulator transition. Structural optimization leads to the formation of V-V dimers along the off-plane chains. A strong spin-lattice coupling is expected close to the transition to itinerancy. No orbital ordering is observed in such a structure, and the experimentally found magnetic structure is naturally explained.     

Numerical study of charge ordering in NaxCoO2

A simple microscopic model of charge ordering in the Na_xCoO₂ system is presented. The model takes into account the interplane interactions between the ordered Na ions and d electrons from the CoO₂ layers as well as the nearest-neighbor intraplane Coulomb interactions between d electrons. It is shown that a driving force of charge ordering in the CoO₂ layers is the interplane interaction that alone is able to describe various types of inhomogeneous charge ordering (e.g., the striped phases) as well as to predict correctly the conducting properties of the system.     

Extended X-ray absorption fine-structure (EXAFS) of a complex oxide structure: a full multiple scattering analysis of the Au L3-edge EXAFS of Au2O3

Crystalline Au₂O₃ was obtained by hydrothermal synthesis at 3000 atm and its extended X-ray absorption fine-structure (EXAFS) at the Au L₃-edge was measured at room temperature. A detailed full multiple scattering (MS) analysis using FEFF8 theory shows that only a small number of scattering paths contribute significantly to the EXAFS of Au₂O₃. Because of the complex unit cell (low local symmetry) of the Au₂O₃ structure, contributions of MS paths are almost negligible. The results indicate that FEFF8 theory provides a good reference for the analysis of Au-O phases.     

Specific heat of (Ca1−xSrx)3Ru2O7 single crystals

We have measured the specific heat of crystals of (Ca_1−xSr_x)₃Ru₂O₇ using ac- and relaxation-time calorimetry. Special emphasis was placed on the characterization of the Néel () and structural () phase transitions in the pure, x=0 material. While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L∼0.3R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-field-like step at T_N, but its magnitude () is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with the full ordering of localized spins. The T_N transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the T_c transition decreases rapidly with Sr substitution, and is not observable for our x=0.58 sample.     

Metastable structure and properties of (La0.73Bi0.27)0.67Ca0.33MnO3

The electrical transport and magnetic properties of high Bi doped (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃ are studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T. Significant temperature and magnetic field hystereses are observed in both resistivity and magnetization measurements. Meanwhile, an enhanced magnetoresistance effect, within a wide temperature window, is obtained in the (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃. The hysteresis and enhanced magnetoresistance are discussed based on an inhomogeneous metastable structure related to the Bi dopant.     

Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.