Site selectivities of alkali ions in CsRb2C60 and Cs2RbC60 superconductors

The temperature-dependent site selectivities of Cs and Rb ions in CsRb₂C₆₀ and Cs₂RbC₆₀ superconductors are computed using the free energy obtained from the configuration entropy and the total energy calculated using the ab initio pseudopotential density-functional theory. It is found that in CsRb₂C₆₀ the smaller Rb ions can occupy a considerable number of large octahedral interstitial sites at high temperatures, however, the transition to random occupation never takes place. For Cs₂RbC₆₀ the Cs occupancy of the large octahedral interstitial sites is almost always 100%, and, interestingly, the two tetrahedral interstitial sites are randomly occupied by Cs and Rb ions even at very low temperatures.     

Polymeric sheets in Mg4C60

We present preliminary results on the preparation and structure of Mg₄C₆₀, a new fulleride polymer. A series of Mg_xC₆₀ compositions (0<x<6) were prepared by solid state reaction. While most samples were multiphase, a single phase material was obtained at the nominal composition Mg₄C₆₀. The X-ray diffraction pattern of Mg₄C₆₀ was successfully indexed and fitted to a rhombohedral structure based on two-dimensional polymeric sheets of C₆₀ ions. This result extends the family of fulleride polymers to the salts of multiply charged cations and thus opens a new direction in this field.     

High pressure electrical transport measurements of Cs2MoS4 and KTbP2Se6 and X-ray diffraction study of Cs2MoS4

We report the results of electrical resistance measurements at high pressures on Cs₂MoS₄ and KTbP₂Se₆. The results of high pressure X-ray diffraction study of Cs₂MoS₄ are also presented. Interestingly, in the case of Cs₂MoS₄ the resistance vs. pressure follows the behavior of the absorption edge vs. pressure obtained from our optical measurements lending further support to a direct-indirect band crossing. In the case of KTbP₂Se₆,the phase transition at about 9.2 GPa is reflected in a sharp drop of the resistance. In addition we report the pressure dependence of the lattice constants as well as the equation of state of Cs₂MoS₄.     

Superconductivity of powder-in-tube Sr0.6K0.4Fe2As2 wires

Nb-sheathed Sr_0.6K_0.4Fe₂As₂ superconducting wires have been fabricated using the powder-in-tube (PIT) method for the first time and the superconducting properties of the wires have been investigated. The transition temperature (T_c) of the Sr_0.6K_0.4Fe₂As₂ wires is confirmed to be as high as 35.3 K. Most importantly, Sr_0.6K_0.4Fe₂As₂ wires exhibit a very weak J_c-field dependence behavior even the temperature is very close to T_c. The upper critical field H_c2(0) value can exceed 140 T, surpassing those of MgB₂ and all the low temperature superconductors. Such high H_c2 and superior J_c-field performance make the 122 phase SrKFeAs wire conductors a powerful competitor potentially useful in very high field applications.     

Superconducting and charge-density wave instabilities in ultrasmall-radius carbon nanotubes

We perform a detailed analysis of the band structure, phonon dispersion, and electron-phonon coupling of three types of small-radius carbon nanotubes (CNTs): (5,0), (6,0), and (5,5) with diameters 3.9, 4.7, and 6.8 Å respectively. The large curvature of the (5,0) CNTs makes them metallic with a large density of states at the Fermi energy. The density of states is also strongly enhanced for the (6,0) CNTs compared to the results obtained from the zone-folding method. For the (5,5) CNTs the electron-phonon interaction is dominated by the in-plane optical phonons, while for the ultrasmall (5,0) and (6,0) CNTs the main coupling is to the out-of-plane optical phonon modes. We calculate electron-phonon interaction strengths for all three types of CNTs and analyze possible instabilities toward superconducting and charge-density wave phases. For the smallest (5,0) nanotube, in the mean-field approximation and neglecting Coulomb interactions, we find that the charge-density wave transition temperature greatly exceeds the superconducting one. When we include a realistic model of the Coulomb interaction we find that the charge-density wave is suppressed to very low temperatures, making superconductivity dominant with the mean-field transition temperature around one K. For the (6,0) nanotube the charge-density wave dominates even with the inclusion of Coulomb interactions and we find the mean-field transition temperature to be around five Kelvin. We find that the larger radius (5,5) nanotube is stable against superconducting and charge-density wave orders at all realistic temperatures.     

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.     

Improved electron injection of OLEDs with a thin PBD layer at Alq3/Cs2CO3 interface

In this study, the effect of one oxadiazole derivative (PBD) using as an electron injection layer (EIL) at Alq₃/Cs₂CO₃ interface has been investigated. The present of PBD EIL was showed an interesting enhanced electron injection for OLEDs although the nominal electron injection barrier for PBD based OLEDs is much larger, because PBD owns an obvious higher intrinsic the Lowest Unoccupied Molecular Orbital level (2.3 eV) than that of Alq₃ (3.0 eV). For example, the current density of OLEDs at 8 V was increased from 54 mA/cm² to 168 mA/cm² when inserting a thin PBD layer (5 nm) at Alq₃/Cs₂CO₃ interface. Here the increased current is suggested associating with the changed electronic structure of PBD when it contacts with Cs₂CO₃.     

Conductance and EPR study of the endohedral fullerene Li@C60

Thin films and bulk samples of endohedral fullerenes Li@C₆₀ are studied using current-voltage (I-V) measurements and electron paramagnetic resonance (EPR). Electrical measurements show a linear behaviour for the I-V curves and give an average resistivity of ca. 1.5 kΩcm for thin Li@C₆₀ films deposited in vacuum, four orders of magnitude lower than C₆₀ samples. A drastic effect on the conductance, lowering it to the values typical for C₆₀, is observed when the Li@C₆₀ samples are exposed to ambient atmosphere. No additional paramagnetic centres (PCs) are found for the Li@C₆₀ compared to C₆₀ that can be related to the formation of dimers or trimers of the endohedral fullerene molecules. However, the presence of the Li atoms in the fullerene cages contributes to a change of the spin-spin and spin-lattice relaxation times. The spin-lattice relaxation time becomes four orders of magnitude longer compared to the spin-spin relaxation time.     

New features in the pressure dependence of photoluminescence spectra of C60 at room temperature

We present new results on the pressure dependence of the electronic band gap of molecular C₆₀ measured by photoluminescence spectroscopy up to 10 GPa at room temperature. In agreement with previous results, the energy gap decreases with increasing pressure up to about 6 GPa. For higher pressures, however, we observe an energy gap that is wider than that at 6 GPa.     

Magnetic susceptibility and magnetic resonance study of acceptor doped fullerenes C60(MF6)2 (M=As, Sb, P)

In order to check whether superconductivity occurs in the acceptor doped fullerenes C₆₀(MF₆)₂ (M=As, P, Sb) and to study their magnetic and structural properties, we have carried out magnetic, EPR and NMR measurements of these compounds. Temperature dependences of magnetic susceptibility down to 5 K and field dependences of magnetic moment at 5 K show no ‘bulk’ transition in superconducting state. Some reasons of the absence of superconductivity, such as insufficient charge transfer between C₆₀ and intercalated species and inhomogeneity of the compounds under study, are discussed.     

Scintillation and anomalous emission in elpasolite Cs2LiLuCl6:Ce

The luminescence and scintillation properties of Cs₂LiLuCl₆:0.5%Ce³⁺ are presented. Special attention is devoted to a 9.4 ns fast emission at 275 nm that can only be excited via the highest cubic field 5d_e state of Ce. Contrary to Cs₃LuCl₆ and Cs₂LiYCl₆, where the same type of fast emission was observed, the emission in Cs₂LiLuCl₆ is still observed at room temperature. Assuming that the 5d_e state is located inside the host conduction band (CB), we propose that the emission originates from a mixed state at or just below the bottom of the CB and ends at the 4f ground state of Ce³⁺. To proof this model we studied the thermal quenching of the anomalous luminescence and performed X-ray photoelectron spectroscopy. A model for a temperature-activated energy transfer from the anomalous state to the lowest 5d_t excited state of Ce³⁺ explains most of the results. Besides the 275 nm emission, the material shows 5d_t-4f Ce³⁺ emission at 370 and 406 nm and 2 ns fast core-valence luminescence when excited with 16-22 eV photons. The scintillation properties of Cs₂LiLuCl₆:Ce are briefly discussed.     

(T1u+T1g)⊗(hg+τ1u)vibronic interaction and superconductivity in C 60 n− fullerides

The closeness of low-lying T_1u and T_1g levels of C ₆₀ ⁻ could enable their mixing under an odd parity vibration of (T_{1 u} + T_{1 g} ⊗ (h_g + τ_{1 u})type. In addition, the two levels are susceptible to Jahn-Teller interaction due to five-fold degenerate h_g vibrations. This complex problem of (T_1u+T_1g)⊗(h_g+τ_1u) vibronic interaction is transformed to a form similar to T_2g ⊗ (ε_g + τ_2g) vibronic problem of octahedral symmetry. The problem is analysed in an infinite coupling model and compared with the experimental spectroscopic results for the C ₆₀ ⁻ radical. The resulting parameters are used to calculate the pair-binding energy and superconducting transition temperature in C ₆₀ ⁿ⁻ fullerides. Vibronic mixing with the T_1g level is found to be responsible for maximising the pair-binding energy at the doping level n=3. It is also found to be an important source of T_c enhancement.     

Singularities of magnetic and elastic characteristics of La2/3Ba1/3MnO3: Analysis of martensitic kinetics

A coordinated temperature behavior of magnetic susceptibility and internal friction has been observed in the La_2/3Ba_1/3MnO₃ manganite in the temperature region of the crystal phase separation 5–340 K. Stepwise temperature behavior of the susceptibility of the single crystal sample and corresponding singular behavior of the internal friction in the polycrystalline manganite have been found. These small-scale features of the temperature dependences of the susceptibility and the internal friction are considered to be a reflection of martensitic kinetics of the structural phase transformation R3¯c↔Imma in the 200 K temperature region.     

The effect of Fe and Ru substitution on the superconductivity in MgCNi3

We report the effects of partial substitutions of the VIIIa elements: Fe (3d) and Ru (4d) in the intermetallic perovskite superconductor MgCNi₃. In both the MgCNi_3−xRu_x and MgCNi_3−xFe_x systems, the superconducting critical temperature (T_C) decreases monotonically as x is increased. T_C decreases more slowly in the case of Ru, suggesting that the 3d electrons in Fe are magnetically pair breaking. This is supported by the normal state magnetic susceptibility measurements (χ), which shows that χ increases with Fe and decreases with Ru doping. No ferromagnetism is observed in either system.     

Growth and thermal stability of epitaxial BiFeO3 thin films

We have investigated the oxygen pressure and the temperature dependence on BiFeO₃ thin films deposited on SrTiO₃ substrates by pulsed laser deposition. Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction measurements indicate that high-quality epitaxial thin films are obtained for and T=650 °C. Outside of this pressure-temperature window, parasitic peaks attributed to β-Bi₂O₃ appear. We find an increase of the out-of-plane lattice parameter with oxygen pressure that we ascribe to Bi-deficiency due to its high volatility at low pressure. Ex-situ anneals have been performed and results show that as-grown single-phase BiFeO₃ thin films degrade after annealing, whereas as-grown BiFeO₃ containing impurity phases evolve toward a single-phase structure. These experiments demonstrate that parasitic phases can stabilize compounds which are usually unstable in air at elevated temperatures.     

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.     

Raman study of phase transformation of TiO2 rutile single crystal irradiated by infrared femtosecond laser

Titanium dioxide (TiO₂) rutile single crystal was irradiated by infrared femtosecond (fs) laser pulses with repetition rate of 250 kHz and phase transformation of rutile TiO₂ was observed. Micro-Raman spectra show that the intensity of E_g Raman vibrating mode of rutile phase increases and that of A_1g Raman vibrating mode decreases apparently within the ablation crater after fs laser irradiation. With increasing of irradiation time, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO₂ single crystal is partly transformed into anatase phase. The anatase phase content transformed from rutile phase increased to a constant with increasing of fs pulse laser irradiation time. The study indicates the more stable rutile phase is transformed into anatase phase by the high pressure produced by fs pulse laser irradiation.     

Reentrant high-conduction state in CuIr2S4 under pressure

The results of electrical resistance and angle dispersive X-ray diffraction measurements at high pressures and ambient temperature on the chalcogenide spinel, CuIr₂S₄ are reported. The resistance increases gradually and reaches around 12 GPa a value that is approximately forty times the initial value. Above 15 GPa, the resistance decreases up to 30 GPa and on further pressure increase tends to saturate at a value slightly above the ambient pressure value. Thus, the material exhibits a reentrant high conducting phase under pressure. The behaviour of the electrical resistance exhibits a close correlation with the structural evolution with pressure.     

First-principles calculations on the pressure induced zircon-type to scheelite-type phase transition of CaCrO4

The zircon-type and scheelite-type CaCrO₄ are investigated by first-principles calculations based on density-functional theory. The calculated zircon-type lattice parameters and the oxygen positions are in agreement with the experimental results and those of scheelite-type structure are studied for the first time in this work. The theoretical phase transition pressure of CaCrO₄ from zircon phase to scheelite phase is about 5.8 GPa, which is consistent with the experimental observation. From the density of states and the electronic band structures, CaCrO₄ is an insulator with a direct band gap (2.16 eV) for zircon-type structure and an indirect band gap (1.98 eV) for scheelite-type structure. The bulk moduli of the two phases are evaluated from the Murnaghan equation fit to the total energies as a function of the unit cell volume.     

Dependence of magnetic anisotropy of the La0.67Ca0.33MnO3 films on substrate and film thickness

Strain in the La_0.67Ca_0.33MnO₃ films has been tuned by varying substrate and film thickness, and its effects on magnetic anisotropy are studied based on the measurements of isothermal magnetization. Measuring the strain in the films by the out-of-plane lattice parameter (c), we found a strong dependence of the magnetic anisotropy constant (Ku) on strain. Ku decreases linearly from ∼−1.1×10⁶ erg/cm³ for c=0.763 nm to 1.2×10⁶ erg/cm³ for c=0.776 nm, corresponding to a change from tensile strain to compressive strain. Positive Ku signifies a uniaxial anisotropy with the easy axis perpendicular to the film plane, while negative Ku demonstrates an anisotropy of the easy plane character. Smaller or larger c leads a decrease or increase in Ku, which indicates the presence of other effects in addition to those associated with strain. Three distinctive processes for the magnetization are observed along the hard magnetic axis of the films on (001)SrTiO₃, suggesting a possibility of strain relaxation even in ultra-thin films.