Magnetotransport of lanthanum doped RuSr<Subscript>2</Subscript>GdCu<Subscript>2</Subscript>O<Subscript>8</Subscript> – the role of gadolinium

Strongly underdoped RuSr_1.9La_0.1GdCu₂O₈ has been comprehensively studied by dc magnetization, microwave measurements, magnetoresistivity and Hall resistivity in fields up to 9 T and temperatures down to 1.75 K. Electron doping by La reduces the hole concentration in the CuO₂ planes and completely suppresses superconductivity. Microwave absorption, dc resistivity and ordinary Hall effect data indicate that the carrier concentration is reduced and a semiconductor-like temperature dependence is observed. Two magnetic ordering transitions are observed. The ruthenium sublattice orders antiferromagnetically at 155 K in low applied magnetic fields, and the gadolinium sublattice orders antiferromagnetically at 2.8 K. The magnetoresistivity in this compound exhibits a complicated temperature dependence due to the occurence of the two magnetic orders and spin fluctuations. It is shown that the ruthenium magnetism influences the conductivity in the RuO₂ layers while the gadolinium magnetism influences the conductivity in the CuO₂ layers. The magnetoresistivity is isotropic above 4 K, but it becomes anisotropic close to the gadolinium antiferromagnetic order temperature.     

Vortex lattice accommodation on twin boundaries in studied by neutron diffraction

An extensive study of small angle neutron scattering was performed in twinned YBa₂Cu₃O₇ crystals in its superconducting state as a function of the angle between the c-axis of the crystal and the magnetic field. The half of the twin boundaries are oriented in the horizontal plane, which also contains the neutron beam and the magnetic field. Two different diffraction patterns are studied as a function of at 5 K and B = 0.5 T, one along the c-axis of the crystal, the other one along the applied field. These variations are interpreted in the model of accommodation of the vortices on the twin planes by zigzagging from these planes to the ab-planes of the crystal, in order to minimize their energy. Received: 9 March 1998 / Revised and Accepted: 12 June 1998     

Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes

A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (110), (100), (211) and (111) molybdenum crystal planes. Chemisorbed NH₃ causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, Δφ = ?2.25 eV on Mo(110) compared to Δφ = ?1.55 eV on Mo (111). The decomposition of NH₃ was followed by measuring the work function changes for stepwise heating of the Mo tip covered with NH₃ at 200 K. On the four studied planes NH₃ decomposition and H₂ desorption are completed at about 400 K. Δφ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH₃ adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH₃ decomposition and synthesis reactions.     

Phase coexistence in annealed CaMn7O12

The structural phase transition in annealed CaMn₇O₁₂ has been investigated by using high resolution synchrotron radiation powder diffraction. There is a phase coexistence phenomenon: two different crystallographic phases coexist in the material between 410 and 458 K. The first one is trigonal and it has a charge ordering (CO) of the Mn³⁺ and Mn⁴⁺ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from 22% at 418 K up to 100% at 468 K. Both phases have domains of at least 150 nm at each temperature. The annealing of CaMn₇O₁₂ relaxed a part of the strains in the lattice, but did not influence the phase coexistence phenomenon.     

Charge-density-wave instabilities and quantum transport in the monophosphate tungsten bronzes with m = 5 alternate structure

Resistivity, thermoelectric power and magnetotransport measurements have been performed on single crystals of the quasi two-dimensional monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m for m =5 with alternate structure, between 0.4 K and 500 K, in magnetic fields of up to 36 T. These compounds show one charge density instability (CDW) at 160 K and a possible second one at 30 K. Large positive magnetoresistance in the CDW state is observed. The anisotropic Shubnikov-de Haas and de Haas-van Alphen oscillations detected at low temperatures are attributed to the existence of small electron and hole pockets left by the CDW gap openings. Angular dependent magnetoresistance oscillations (AMRO) have been found at temperatures below 30 K. The results are discussed in terms of a weakly corrugated cylindrical Fermi surface. They are shown to be consistent with a change of the Fermi surface below 30 K. Received 23 November 1999 and Received in final form 23 March 2000     

Vibrational analysis of IR reflection-transmission from single crystal C60

We represent results of IR reflectivity and transmission measurements in the spectral range from 50 to 5000 cm^–1 for crystallographic {111} and {100} planes of C₆₀ single crystals between 80 K and 460 K. The spectra turned out to be highly anisotropic due to differences in geometric resonances. The geometries used allowed detailed simultaneous investigation of the four infrared activeF _1u fundamental modes together with some weak silent modes and the whole set of second- and third-order combination modes from an analysis of the reflection-transmission response. Photopolymerization is observed on the surface of the crystals after illumination in argon atmosphere. The orientational phase transition at 260 K is accompanied by a dramatic jump in the line widths within a fraction of a degree. This result is explained by a broadening mechanism due to collisions with librons and paralibrons for the low temperature phase and for the high temperature phase, respectively.     

Work function of adsorption systems potassium on tantalum and on molybdenum

Work functions φ_hkl of thermally annealed and potassium covered tantalum and molybdenum as a function of potassium surface density on (011), (112), (100) and (111) planes of these metals have been measured using a field emission microscope. The measurements have been performed at liquid nitrogen temperature (immobile layers). The work function decreases linearly at first, then more slowly, passes through a minimum, and then attains a constant value. Quantitative data on the dependence of φ_hklon surface density of potassium, N_hkl, for tantalum, molybdenum and tungsten have been compared. The principal results of the observations are: (i) for K on Ta, Mo, and W the work function minimum exhibits no distinct dependence on the type of substrate, however, it proves to depend on crystallographic direction of the latter; (ii) the values of the high coverage limit work function are approximately equal for one type of metal planes; (iii) the values of the high coverage limit surface densities of potassium adsorbed on Ta(011), Mo(011) and W(011) surfaces are approximately equal to the surface density of the (011) plane of bulk potassium crystal.     

Possible propagation of the Zhang-Rice singlet as a probable Cooper channel in the CuO2 planes

The issue of how superconductivity originate in the CuO₂ planes believed to be crucial to understanding the high Tc superconducting cuprates is still an going debate. In the wake of recent experimental observations of the Zhang-Rice singlet (ZRS), its formation and propagation need to be revisited especially by using a simple approach almost at a phenomenological level. Within a highly simplified correlated variational approach (HSCVA) in this Letter, a new formation of the ZRS as constituting the ground state of a single-band t-J model of the CuO₂ planes is developed. This formation is then used to demonstrate how the ZRS can be propagated as a probable Cooper channel in the CuO₂ planes.     

New picture of high-temperature superconductivity

The case for high-temperature superconductivity originating in SrO or BaO planes, or in interstitial regions, is made, including (i) four successfully predicted superconductors; (ii) evidence that the superconductivity of the major cuprates is associated with holes in these layers; (iii) data showing that Pr on one side of a cuprate-plane kills the superconductivity, but Pr on the other side does not; and (iv) evidence that doped Sr₂YRuO₆ has an onset of superconductivity at ∼45 K despite having no cuprate-planes.     

Study of the orientation of HNO3 molecules intercalated in graphite using nuclear techniques

Nuclear resonance scattering of photons from ¹⁵N has been used to study the orientation of the HNO₃ intercalant molecule with respect to the graphite planes. In the second-stage compound C₁₀(HNO₃) and the temperature range T = 15°K ? 300°, no orientational phase transition was observed and the NO₃ molecular plane was found to be oriented at an angle θ ∽ 82 ± 8° with respect to the graphite planes.     

Thermal faceting of the rutile TiO2(001) surface

Temperature dependent faceting of rutile TiO₂ surfaces cut to the (001) plane has been reported [Tait and Kasowski, Phys. Rev. B20 (1979) 5178]. By comparing LEED data to beam positions calculated for various sets of facet planes, the facet planes have been identified. The first ordered structure observed on annealing ion bombarded surfaces is composed of {011} facets with the facet planes in a (2 × 1) reconstruction. The high temperature structure produced on annealing above 1300K is best described as {114} facets; however, there are deviations of the observed LEED pattern from that calculated for {114} facets, possibly because of the presence of related planes. LEED data have now been obtained on the behavior of (110), (100), (011), (114), and (001) surfaces in UHV. The observed stability of TiO₂ surfaces can be related to the Ti ion coordination numbers in the surface plane as derived from stoichiometric terminations of the rutile lattice.     

The crystal structures and phase transition of methyl bromide

The detailed crystal structures of deuterated methyl bromide in both its α-and β-phases have been determined at 175 K and 146 K respectively by neutron powder profile measurements. The α-phase has space group Cmc2₁ while the β-phase has space group Pnma and both have a quasi-two-dimensional character. The major characteristic of the first order transition is a large change in the relative orientation of the two molecules lying in the mirror planes. The transition is successfully interpreted in terms of changes in the topology of the lattice energy surface calculated as a function of temperature from pair-wise interatomic potentials.     

Enhanced thermoelectric properties of bismuth intercalated compounds BixTiS2

The thermoelectric properties of Bi intercalated compounds Bi_xTiS₂ have been investigated at the temperatures from 5 to 310 K. The results indicate that Bi intercalation into TiS₂ leads to substantial decrease of its electrical resistivity (one order low for x=0.05 and two orders low for x=0.15, 0.25 at 300 K) and lattice thermal conductivity (22, 115 and 158% low at 300 K for x=0.05, 0.15 and 0.25, respectively). Specially, the figure of merit, ZT, of lightly intercalated compound Bi_0.05TiS₂ has been improved at all temperatures investigated, and specifically reaches 0.03 at 300 K, which is about twice as large as that of TiS₂.     

Structural phase transition in a perovskite-type NH3(CH2)3NH3CuCl4 crystal – X-ray and optical studies

X-ray powder studies and optical studies (polarized microscopic observation and linear birefringence studies) of the crystal NH₃(CH₂)₃NH₃CuCl₄ are presented. The X-ray powder studies revealed a change of symmetry from orthorhombic room-temperature phase to monoclinic phase above 434 K. A reversible phase transition of the first order at 434 K on heating and 432 K on cooling was observed in birefringence studies. Optical polarized microscopic observation revealed monodomain and multidomain states in the room-temperature orthorhombic phase with domain walls in (110) and (1-10) planes. The hypothetical prototypic phase is expected to be tetragonal. The change of symmetry from orthorhombic to monoclinic and expected domain structure was found above 434 K in the (010) plane.     

Structural characterization,thermal, vibrational properties and molecular motions in perovskite-type diaminopropanetetrachlorocadmate NH3(CH2)3NH3CdCl4 crystal

The work presents a detailed analysis of the sequencing of the structural phase transitions in NH₃(CH₂)₃NH₃CdCl₄ crystal by differential scanning calorimetry (DSC), X-ray, infrared, far infrared and Raman spectroscopy. DSC studies have shown that in analyzed crystal occurring one reversible continuous phase transition at 375/374 K (on heating/cooling). Observed in Nujol and Fluorolube mulls in the wide temperature range between 296 K and 413 K spectral changes through the structural phase transition can be attributed to an onset of motion of cations. An assignment of some bands due to internal modes has been also proposed.     

Vibrational study,phase transitions and electrical properties of 4-benzylpyridinium monohydrogenselenate

The title compound C₆H₅CH₂C₅H₄NH⁺·HSeO₄⁻ crystallizes in the orthorhombic system with the space group Pbca and the following unit cell dimensions: a=27.449(5) Å; b=10.821(6) Å and c=8.830(1) Å.The structure consists of infinite parallel two-dimensional planes built of HSeO₄⁻ anions and C₆H₅CH₂C₅H₄NH⁺ cations mutually.Differential scanning calorimetry study on 4-benzylpyridinium monohydrogen-selenate was carried out. A high temperature second order phase transition at 363 K was found and characterized by electric measurements. The Raman of polycrystalline sample has been recorded at different temperature between 297 and 373 K.The conductivity relaxation parameters associated with some H⁺ conduction have been determined from an analysis of the M′′/M′′_max spectrum measured in a wide temperature range. An appearance of the superionic phase transition in 4-BSe is closely related to a liberation or even a rotation increase of HSeO₄⁻ groups with heating.     

Synthesis, structure and magnetic properties of the new double perovskite Ca2CrSbO6

The title double perovskite has been synthesized by solid-state reaction in air. The crystal structure has been studied from powder X-ray diffraction data. Rietveld fits to the pattern show that this compound has a monoclinic symmetry [a=5.4932(3) Å, b=5.4081(3) Å, c=7.6901(5) Å, β=90.0022(1)°, at 300 K] defined in the space group P2₁/n, where the Cr and Sb cations are almost completely ordered in the B-sublattice of the perovskite structure. Magnetic susceptibility and magnetization measurements show that this compound behaves as a Curie-Weiss paramagnet at high temperatures with μ_eff=3.53(1) μ_B and θ_P=8 K, and exhibits a robust ferromagnetic component below the ordering temperature of T_C=13 K, with a saturation magnetization of 2.36 μ_B/f.u. at 5 K. To our knowledge, this is the first example of a ferromagnetic double perovskite containing a non-magnetic element, such as Sb, occupying one half of the B positions of the perovskite structure.     

First study of the B1g buckling phonon mode in optimally doped, de-twinned, YBa2Cu3O7−δ by inelastic X-ray scattering

We have measured the buckling, B_1g, phonon mode of optimally doped, de-twinned, YBa₂Cu₃O_7−δ using inelastic X-ray scattering (IXS) at BL35XU of SPring-8. Measurements of this mode, which has atomic motion transverse to the Cu-O planes, serve to demonstrate some of the advantages of the unique two-dimensional (2-D) analyzer array at BL35. Analysis based on fitting the entire spectra simultaneously at 10 and 100 K shows that the buckling mode is rather broad in the middle of the zone (near (0.3 0 0)). We see a consistent softening of the mode at 10 K as compared to 100 K for all measured wave-vectors (h 0 0), and a q-dependent softening as zone center is approached.     

Change of exciton-phonon interaction in layered ferroelastic crystals (Cs3Bi2I9)

The half-width of exciton absorption band (n=1) of Cs₃Bi₂I₉ layered ferroelastic crystals was studied carefully as function of temperature in the range from 5 to 300 K. For the first time, we have found a new physical effect: change of exciton-phonon interaction (from weak to strong) in the same sample as temperature increases. It was established that the temperature value T^*=150 K may be considered as characteristic one, below which a crystal loses the nature of layered substance. The effect is explained using a model that takes into account the reconstruction of the crystal lattice from non-layered to layered one.