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.     

Charge ordering in charge-compensated Na0.41CoO2 by oxonium ions

Charge ordering behavior is observed in the crystal prepared through the immersion of the Na_0.41CoO₂ crystal in distilled water. Discovery of the charge ordering in the crystal with Na content less than 0.5 indicates that the immersion in water brings about the reduction of the Na_0.41CoO₂. The formal valence of Co changes from +3.59 estimated from the Na content to +3.50, the same as that in Na_0.5CoO₂. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayer-hydrate [K. Takada, et al. J. Mater. Chem. 14 (2004) 1448]. The charge ordering is the same as that observed in Na_0.5CoO₂. It suggests that the Co valence of +3.50 is necessary for the charge ordering.     

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.     

Synthesis and properties of Mn1−XFeXS single-crystals at room temperature

Results of the first synthesis of Mn_1−XFe_XS single-crystals and its structural, electrical and magnetic properties at room temperature are presented. The Mn²⁺→Fe²⁺ substitution in Mn_1−XFe_XS solid solutions is accompanied by a compression of the NaCl lattice and a small deformation of the octahedral environments, and the concentration transition from dielectric to semimetal. Single-line Mossbauer spectra indicate the paramagnetic state Mn_1−XFe_XS sulfides at room temperature.     

Effect of 3d metal ion doping on the structure and superconductivity of (Tl0.5Pb0.5)Sr2CaCu2O7

(Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ system (M=Co or Ni), the superconducting critical temperature T_c decreases linearly with both Co and Ni concentrations and the rate of T_c decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl_0.5Pb_0.5)Sr₂Ca (Cu_2−xZn_x)O₇ system, the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses T_c much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in T_c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of T_c in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu d_x²−y² state due to the full d band of Zn ions.     

Temperature dependence of absorption edge anisotropy in 2H- MoSe2 layered semiconductors

The absorption-edge anisotropy of 2H- MoSe₂ was studied by photoconductivity (PC) measurements as a function of temperature in the range of 12-300 K. A significant shift towards lower energies has been observed in the PC spectra on the edge plane with respect to those corresponding to the van der Waals (VdW) plane. The parameters that describe the temperature dependence of the absorption edges are evaluated by the Bose-Einstein empirical expression. Effective phonon energy was estimated from the temperature dependence of the Urbach energy. The estimated effective phonon energy for the VdW and edge planes, respectively, can be correlated to the observed Raman active and modes.     

Frequency dependence of the resistive switching effect in Bi2Sr2CaCu2O8+y/Ag film heterocontacts

In this work we have performed the relaxation studies “in situ” of the electron instability effect (EIE) in the heterostructures based on BSCCO single crystals. The new effect of suppression of EIE or colossal electroresistance via application of an alternating low frequency electric field to the heterojunctions in the BSCCO-based single crystals has been found. It has been shown that the top possible frequencies for observation of the effect are of the order of 10³ Hz. This fact is interpreted as accumulation of the oxygen ions driven by the electric field to the interface. On the other hand, it has been shown that the switching events are limited by two time processes: t≈1 ms and about ten seconds. The first ones are caused by rearrangement of a charge net in the degraded surface at the electric field switching. The latter are caused by oxygen diffusion to vacancies under electric field above some threshold value. The considered experimental data confirm the correlation character of the HTSC properties as Mott systems, which appears in extreme sensitivity to the doping level, in the tendency to phase separation under external actions, in the hysteresis character of the metal-insulator transition.     

Calcium and oxygen doping in Y Ba2Cu3Oy

Both oxygen and calcium play important roles in inducing superconductivity in Y Ba₂Cu₃O_y (YBCO), which is an antiferromagnetic insulator at low O and Ca content. O induces superconductivity in Ca-free YBCO, while Ca does similarly in oxygen-deficient YBCO. For doping oxygen HgO was used as it decomposes at 476 ^°C into Hg, which escapes from the matrix leaving the crystal unaltered, and O, which provide a way to dope O in YBCO. Considering these facts, polycrystalline samples of Y _1−xCa_xBa₂Cu₃O_y with x=0, 0.1 and 0.2 with and without HgO addition were prepared through a solid-state reaction method. The samples were sintered at 950 ^°C in open atmosphere. These synthesized samples were characterized through using the X-ray diffraction technique (XRD) for phase evaluation, scanning electron microscopy (SEM) for grain morphology, energy dispersive X-ray analysis (EDX) for compositional analysis and the four-contact measurement technique for determining the superconducting transition temperature.     

High pressure effects on the electrical resistivity behavior of the Kondo lattice, YbPd2Si2

We report the influence of external high-pressure (P up to 8 GPa) on the temperature (T) dependence of electrical resistivity (ρ) of a Yb-based Kondo lattice, YbPd₂Si₂, which does not undergo magnetic ordering under ambient pressure condition. There are qualitative changes in the ρ(T) behavior due to the application of external pressure. While ρ is found to vary quadratically below 15 K (down to 45 mK) characteristic of Fermi-liquids, a drop is observed below 0.5 K for P=1 GPa, signaling the onset of magnetic ordering of Yb ions with the application of P. The T at which this fall occurs goes through a peak as a function of P (8 K for P=2 GPa and about 5 K at high pressures), mimicking Doniach's magnetic phase diagram. We infer that this compound is one of the very few Yb-based stoichiometric materials, in which one can traverse from valence fluctuation to magnetic ordering by the application of external pressure.     

X-ray-induced electrical conduction in the insulating phase of thiospinel CuIr2S4

Effects of X-ray irradiation on the crystal structure and the electrical resistance were examined at low temperatures for the insulating phase of spinel compound CuIr₂S₄. We found that the resistance decreases by more than five decades by irradiation at 8.5 K. The structural change from triclinic to tetragonal was observed at the same time. The X-ray-induced conductance is deduced to result from the destruction of Ir⁴⁺ dimers formed in the insulating phase. Slow relaxation of the resistance in the X-ray-induced state is also reported.     

Charge transport near pressure-induced antiferromagnetic quantum critical point in Magnéli-phase vanadium oxides

Resistivity (ρ) measurements on Magnéli phases V₇O₁₃ and V₈O₁₅ were performed under high pressures up to 3.5GPa. We have achieved a pressure-induced transition from an antiferromagnetic metal to a paramagnetic metal (PM) at critical pressures P_c≈3.4 and 3.3 GPa for V₇O₁₃ and V₈O₁₅, respectively. The critical behavior of ρ(T) near P_c turned out to be quite unusual in that no noticeable precursor effect was observed. This strongly contrasts with the canonical quantum critical point behavior observed in chemically modified systems such as Ni(S,Se)₂ and V₂O₃. We propose that the presence of two distinct Fermi surface segments is responsible for the observed unusual behaviors.     

Effect of Cr doping on the magnetic and electrical transport properties of charge-ordered Bi0.5Ca0.5MnO3

An experimental study on the magnetic and electrical transport properties of the manganites Bi_0.5Ca_0.5Mn_1−xCr_xO₃ (BCMCO) (0≤x≤0.12) is carried out. The results show that Cr doping can suppress the charge-ordering transition, favoring the ferromagnetic clusters. For x=0.12, the charge-ordering transition disappears but a very broad paramagnetic-ferromagnetic-like transition is detected at the Curie temperature T_C=72.6 K. It is caused by phase separation or coexistence of the charge-ordering and ferromagnetic phase. Moreover, the critical Cr content to destroy charge ordering phase in BCMCO does not match the general monotonous tendencies shown by Cr-doped Re_0.5Ca_0.5MnO₃ (Re-rare-earth). These differences are ascribed to the fact that the ground state in BCMCO differs markedly from the ferromagnetic metallic phase in Cr-doped Re_0.5Ca_0.5MnO₃ compounds.     

Effect of Zn substitution for Co on the transport properties of Y BaCo4O7

Electrical resistivity and Seebeck coefficients of Y BaCo_4−xZn_xO₇ (x=0.0,0.5,1.0,2.0) were investigated in the temperature range 350-1000 K. It was found that the electrical resistivity and activation energy increase with increasing Zn concentration, while Seebeck coefficients do not increase but decrease when electrical resistivity increases. We explained the increase of electrical resistivity and the drop of Seebeck coefficients for Zn-substituted samples by the decrease of carrier mobility, rather than of carrier concentration. The effect of oxygen absorption and desorption on the electrical resistivity and Seebeck coefficients was also investigated. An abrupt change of transport properties happens at about 650 K for x=0.0 and 0.5 samples measured in oxygen. For x=1.0 and 2.0 samples, however, such change disappears and the transport behavior in oxygen is almost same as that in nitrogen due to the significant suppression of oxygen diffusion caused by the higher Zn concentration in these samples.     

Structure and superconductivity of Mg1−xPbxB2

A series of polycrystalline samples of Mg_1−xPb_xB₂ (0≤x≤0.10) were prepared by a solid state reaction method and their structure, superconducting transition temperature and transport properties were investigated by means of X-ray diffraction (XRD) and resistivity measurements. Mg_1−xPb_xB₂ compounds were shown to adopt an isostructural AlB2-type hexagonal structure in a relatively small range of lead concentration, x≤0.01. The crystalline lattice constants were evaluated and were found to exhibit slight length compression as x increases. The superconducting transition temperature (T_c) steadily decreases with Pb doping. It is suggested that the mechanism of superconductivity reduction by lead doping can be attributed to the chemical pressure effect.     

Heat transport imaging in the spin-ladder compound Ca9La5Cu24O41

All-optical heat transport imaging on the spin-ladder compound Ca₉La₅Cu₂₄O₄₁ is presented. A ‘time-of-flight’ principle is discussed along with its experimental verification which can be used to measure the bulk thermal diffusion constant. The results of the thermal imaging experiments clearly demonstrate anisotropic magnon heat transport.     

Magnetic correlations in heavy fermion CeAl3 compound

To study the effect of hybridization on inter-ion correlations the measurements of the intensity of the quasielastic scattering on momentum transfer in Ce_0.9Y _0.1Al₃, Ce_0.9La_0.1Al₃ and CeAl₃ have been performed. Some hints that quasielastic magnetic scattering more sensitive to regularity in rare-earth sublattice than to the hybridization have been obtained. We also get some indications on the presence of ferromagnetic inter-ion correlations. In a contrast to quasielastic scattering the inelastic part of spectra is very sensitive to the hybridization.     

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.     

Study of photo-induced phenomena in VO2 by terahertz pump-probe spectroscopy

The ultrafast terahertz response to the photoexcitation in vanadium dioxide was investigated using the optical-pump terahertz-probe technique at room temperature. The optical excitation at 790 nm induced an ultrafast decrease of the transmittance of the terahertz pulses corresponding to the increase of the electronic conductivity within 0.7 ps, and then the transmittance decreases gradually up to 100 ps. This two-step behavior is very similar to the previous reports of the time resolved X-ray and electron diffractions. This fact indicates that the increase of the electronic conductivity and the change of the lattice structure proceed in parallel, and the photo-induced insulator-metal phase transition is of the Peierls type.     

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.     

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.