Search

The cubic perovskite ScF exhibits pronounced isotropic negative thermal expansion and a structural quantum phase transition. We present measurements of the thermal expansion from 200 K to about 1.6 K. We find evidence for stress effects at both high and low temperatures. The coefficient of volumetric thermal expansion and the Grüneisen parameter exhibit power-law temperature dependences above 12 K. The Grüneisen parameter changes sign near 2.4 K and reaches by 1.6 K. Our results are consistent with predictions of Grüneisen divergence on the ordered side of a quantum critical point. [ABSTRACT FROM AUTHOR]

The family of rare earth molybdenum bronzes, reduced ternary molybdates of composition Ln Mo 16 O 44, was synthesized and a detailed structural study carried out. Bond valence sum (BVS) calculations clearly show that the molybdenum ions in tetrahedral coordination are hexavalent while the electron count in the primitive unit cell is odd. Yet, measurements show that the phases are semiconductors. The temperature dependence of the magnetic susceptibility of samples containing several different rare earth elements was measured. These measurements verified the presence of a 6.5 K magnetic phase transition not arising from the rare earth constituent, but likely associated with the unique isolated ReO 3 -type Mo 8 O 36 structural subunits in this phase. To better understand the behavior of these materials, electronic structure calculations were performed within density functional theory. Results suggest a magnetic state in which these structural moieties have an internal ferromagnetic arrangement, with small ~1/8 μ B moments on each Mo. We suggest that the Mo 8 O 36 units behave like pseudoatoms with spin 1/2 derived from a single hole distributed over the eight Mo atoms that are strongly hybridized with the O atoms of the subunit. Interestingly, while the compound is antiferromagnetic, our calculations suggest that a field-stabilized ferromagnetic state, if achievable, will be a narrow band half-metal. [ABSTRACT FROM AUTHOR]

Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices. [ABSTRACT FROM AUTHOR]

Single-crystal X-ray diffraction studies indicate that the compound Ba27Fe16Ti33O117 crystallizes in the rhombohedral space group R3m, with a hexagonal unit cell a=5.7400(8), c=127.11(3) A˚; Z=1.5. The arrangement may be described as a 54-layer (54L) close-packed structure (stacking sequence (cchcchhhhcchhhhcch)3) built from oxygen and {Ba,O} layers, with Ti4+ occupying octahedra and Fe3+ occupying both octahedral and tetrahedral interstices. The 54L structure contains hexagonal 6L BaTiO3-type (cch)2 units via a 9-fold repeat of the 6L stacking sequence, with iron preferentially occupying layers centered around z=1/6. Ba27Fe16Ti33O117 melts incongruently at 1270 °C and is difficult to purify in polycrystalline form, although crystals are easily obtained from partial melts. The new compound is a member of a family of ternary Ba&z.sbnd;Fe&z.sbnd;Ti&z.sbnd;O phases that may be considered as dielectric–magnetic hybrids of barium-polytitanate and barium-hexaferrite crystal chemistries. [ABSTRACT FROM AUTHOR]

Reports on the crystallographical determination of the structure of the molecular cluster compound Cr6Te8(PEt3)6. Reaction of the TePEt3 with either (Et3P)2Cr(allyl)2 or Cr(2,4-dimethylpentadienyl)2; Conversion of the compound to the extended inorganic solid state compound; Concentric formation of the molecule.

The authors present a systematic study on the growth of the ternary compound InxAl1-xN by molecular beam epitaxy. This work concentrates on In mole fractions x around 0.17, as this composition is in-plane lattice matched to GaN. At a growth temperature of 540 °C, high quality material was obtained using a total metal to nitrogen flux ratio of ∼1. Using these growth parameters, high quality GaN/InAlN superlattices were obtained without growth interruptions. [ABSTRACT FROM AUTHOR]

Copper phthalocyanine (Cu–Pc) single crystals were grown by physical vapor transport and field-effect transistors (FETs) on the surface of these crystals were prepared. These FETs function as p-channel accumulation-mode devices. Charge carrier mobilities of up to 1 cm2/V s combined with a low field-effect threshold were obtained. These remarkable FET characteristics, along with the highly stable chemical nature of Cu–Pc, make it an attractive candidate for device applications. [ABSTRACT FROM AUTHOR]

Reports on the crystal structure of the superconductor lutetium-nickel-boron-carbon or LuNi(2)B(2)C and its related phase LuNiBC. Derivation of LuNi(2)B(2)C and LuNiBC from the Lu-Ni-B-C system; Use of electronic diffraction on crystals of LuNi(2)B(2)C and LuNiBC; Atomic positions and isotropic temperature factors for LuNi(2)B(2)C and LuNiBC.

Epitaxial films of CdTe and (Cd,Mn)Te were grown on (001) GaAs substrates in a vacuum evaporator. Conventional Bragg diffraction indicated that the films had a single orientation, either (a) CdTe(001) || GaAs(001), or (b) CdTe(111) || GaAs(001). Grazing-incidence x-ray diffraction showed that most of the films had both orientations with (a) CdTe[110] || GaAs[110] and (b) CdTe[112] || GaAs[110]. Only one of the two possible domains was found in case (b). A mosaic of well-crystallized islands appears to accommodate the large film strain of -12.8% due to the lattice mismatch between CdTe and GaAs. [ABSTRACT FROM AUTHOR]

The Zeeman splitting of the conduction band in the magnetic semiconductor Cd1-xMnxTe with x<0.25 has been inferred from measurements of Fowler–Nordheim tunneling characteristics in a metal-semiconductor-metal structure. The large g value of the band electrons leads to a decrease of the barrier height in a magnetic field and, consequently, to an exponential increase in the tunneling current. [ABSTRACT FROM AUTHOR]

The compounds LnMo16O44 (Ln: Y, La-Lu) are synthesized from stoichiometric mixtures of Ln2O3, Mo, and MoO3 (silica tubes, 700 °C, 5 d) and characterized by powder and single crystal XRD, magnetic measurements, and DFT electronic structure calculations. [ABSTRACT FROM AUTHOR]

Demonstrates the growth of YBa[sub 2]Cu[sub 3]O[sub 7-delta] thin films in pure ozone. Application of molecular beam epitaxy; Efficiency of the thermally driven ozone-oxidation process; Enhancement of pure-ozone oxidation by ultraviolet irradiation; Extension of thin film stability limit to lower growth pressures.

Presents a study which examined heavy fermion state in a magnetic system. Details of the magnetic and magnetotransport measurements of (U[subx]Ce[sub1-x])2Zn17 alloys; Description on an effective paramagnetic moment of the alloys; Indication of resistivity measurements.

Transport, specific heat, and magnetic measurements have been performed on three alkali hollandites: KRu4O8, RbRu4O8, and a newly synthesized Cs analog, Cs0.8Li0.2Ru4O8, which was determined to have space group I4/m (#87) and lattice parameters, a = 10.0850(4) and c = 3.12180(20). In contrast to the ruthenium perovskites, which display a wide range of electrical and magnetic behavior, the alkali hollandites are simple paramagnetic metals. [Copyright &y& Elsevier]

The structure of the mixed-valent Ba5Fe5O14 (BaFeO2.8), prepared using a molten KOH–Ba(OH)2 flux, has been determined using single crystal X-ray diffraction. Ba5Fe5O14 forms twinned crystals with the orthorhombic space group Cmcm, a=5.7615(8), b=9.9792(14) and c=24.347(3) A˚, Z=4. The structure, which is closely related to the 10H BaFeO2.65 perovskite, is composed of two oxygen-deficient BaO2 layers and eight BaO3 layers with a (hchhc)2 stacking sequence, where h and c denote hexagonal or cubic layers. A displacement of barium and oxygen atoms in the BaO2 layers from hexagonal special positions lowers the symmetry from hexagonal to orthorhombic. This combination of stacking and vacancies creates trimers of face-sharing FeO6 octahedra pillared by dimers of corner-sharing FeO4 tetrahedra. The Fe4+ atoms are located in the center of the trimer and in the tetrahedral sites. The magnetism of Ba5Fe5O14, investigated using SQUID magnetometry, is characteristic of a strongly coupled antiferromagnet. [Copyright &y& Elsevier]

Subsolidus phase relations have been determined for the BaO:TiO2:Ta2O5 system by X-ray diffraction analysis of ∼100 specimens prepared in air at temperatures near but below the solidus (1275 °C to 1500 °C). For the BaO:Ta2O5 subsystem, the formation of all three polymorphs of BaTa2O6 (orthorhombic, tetragonal, and hexagonal) was confirmed. The present study found no stable phases forming between Ba5Ta4O15 and BaO. In the ternary system, along the line BaTiO3–Ba5Ta4O15, the previously reported 8L and 10L hexagonal perovskite derivatives were confirmed, albeit with somewhat wider homogeneity ranges; BaTiO3 dissolves Ta5+ up to BaTi0.64Ta0.29O3, which contains 7% B-cation vacancies and exhibits a dilated cubic unit cell (a=4.0505(2) A˚). The formation of Ba3Ti4Ta4O21, a member of the hexagonal A3M8O21-type ternary oxides, was confirmed as well as its solid solution, for which a different mechanism of formation is suggested. Several new compounds have been found, including four members of the orthorhombic (space group Cmcm) “rutile-slab” homologous series, BaTinTa4O11+2n, with n-values 3,5,7,9; the unit cells for these compounds are given. Three ternary phases with close-packed [Ba,O] layer structures related to that of 6L Ba4Ti13O30 were found: 13L Ba18Ti53Ta2O129, 7L Ba10Ti27Ta2O69, and 8L Ba6Ti14Ta2O39. The crystal structures of the 13L and 7L phases were determined by single-crystal X-ray diffraction (13L Ba18Ti53Ta2O129: cchcchcchcchc, C2/m, a=9.859(2) A˚, b=17.067(5) A˚, c=30.618(8) A˚, β=96.11(2)°, Z=2; 7L Ba10Ti27Ta2O69: cchcchc, C2/m, a=9.855(3) A˚, b=17.081(7) A˚, c=16.719(7) A˚, β=101.18(3)°; Z=2), and are described and compared with those of the 6L and 8L members of this family. Phases with tetragonal tungsten bronze (TTB) related structures occur over large compositional ranges in the BaO:TiO2:Ta2O5 system, both within the ternary (Ba6−xTi2−2xTa8+2xO30, x=0→0.723), and along the BaO–Ta2O5 binary (from 43 to 26 mol% BaO), which in turn dissolves up to ∼12 mol% TiO2 to form extensive single-phase fields with different TTB superstructures. X-ray powder diffraction data are given for three binary BaO:Ta2O5 compounds exhibiting the TTB structure and two superstructures. The large compositional ranges of the TTB-type phases are accompanied by only small changes in specific volume. The TTB-type regions within the ternary and those emanating from the BaO–Ta2O5 binary do not co-exist in equilibrium, which likely reflects fundamental differences in the structural mechanisms that accommodate the variable stoichiometries. The similarities and differences between the BaO:TiO2:Ta2O5 and BaO:TiO2:Nb2O5 systems are described. [Copyright &y& Elsevier]

Nonpolar a-plane (1120) MgxZn1-xO (a-MgxZn1-xO) films are deposited on (0112) r-sapphire substrates using metalorganic chemical vapor deposition with varying Mg composition (x from 0 to 0.25). Unit cell parameters with Mg composition are determined by high-resolution triple-axis x-ray diffraction. In-plane strain along the c-axis [0001] and m-axis [1100] in the films is anisotropic and increases with increasing Mg composition. The in-plane strain anisotropy changes with Mg composition in a-MgxZn1-xO. Calculations are carried out to determine the influence of Mg content on the residual interfacial strain. [ABSTRACT FROM AUTHOR]

We deposited [formula] nonpolar a-plane ZnO (a-ZnO) films on (0112) r-sapphire substrates using metalorganic chemical vapor deposition. Unit cell deformation due to interfacial strain in films was determined by triple-axis x-ray diffraction. Due to low symmetry of a-plane, anisotropic strain is observed along the [0001] (c-axis) and [1100] (m-axis) in-plane axes. Out-of-plane strain along [1120] (a-axis) is tensile and relaxes for film thickness >=2 μm. The in-plane m-axis is under tensile strain and c-axis is under compressive strain. Increase in film thickness increases in-plane strain anisotropy due to faster relaxation along the m-axis. Thermal and lattice mismatch strains are separated by curve fitting. [ABSTRACT FROM AUTHOR]

Reports on the observation of superconductivity in single crystals of electric-field doped fullerene C[sub 70]. Maximum transition temperature achieved by the sample; Resistance of a field-effect doped C[sub 70] single crystal as a function of temperature.

Investigates the magnetic anisotropy of La0.7Sr0.3MnO3 (LSMO) thin films and single crystals using vibrating sample and torque magnetometry. How strain anisotropy dominates the magnetic anisotropy of thin films; Behavior of the magnetic anisotropy of LSMO single crystals subject to no external stress; Examination of several different types of LSMO samples; Results of the study.

The temperature (T) as a function of pressure (P) phase diagram is reported for the cubic Laves phase compound Au2Pb, which was recently proposed to support linearly dispersing topological bands, together with conventional quadratic bands. At ambient pressure, Au2Pb exhibits several structural phase transitions at T1 = 97 K, T2 = 51 K, and T3 = 40 K with superconductivity below Tc = 1.2 K. Applied pressure results in a rich phase diagram where T1, T2, and T3 evolve strongly with P and a possible new phase is stabilized for P > 0.64 GPa that also supports superconductivity below 1.1 K. These observations suggest that Au2Pb is an ideal system in which to investigate the relationship between structural degrees of freedom, band topology, and resulting anomalous behaviors. [ABSTRACT FROM AUTHOR]

A facility for X-ray diffraction has been developed at the National High Magnetic Field Laboratory. It brings diffraction capability to the 25 T Florida split coil magnet and implements temperature control in a range of 15-295 K using a cold finger helium cryostat. This instrument represents an alternative to pulsed magnetic field systems, and it exceeds the static magnetic fields currently available at synchrotron facilities. Magnetic field compatibility of an X-ray source and detectors with the sizable magnetic fringe fields emanating from the magnet constrained the design of the diffractometer. [ABSTRACT FROM AUTHOR]

Single crystals of LnMg 2 Bi 2 (Ln=Yb, Eu, Sm) and SmMg 2 Sb 2 were synthesized using Mg–Bi metal and Mg–Sb metal fluxes, respectively. The crystal structures are of the CaAl 2 Si 2 type with space group P 3 ̅ m1 (#164, Z =1): SmMg 2 Bi 2 ( a =4.7745(1) Å, c =7.8490(2) Å), EuMg 2 Bi 2 ( a =4.7702(1) Å, c =7.8457(2) Å), YbMg 2 Bi 2 ( a =4.7317(2) Å, c =7.6524(3) Å), and SmMg 2 Sb 2 ( a =4.6861(1) Å, c =7.7192(2) Å). Heat capacity, electrical transport, and magnetization of all bismuth containing phases were measured. The materials behave as “poor metals” with resistivity between 2 and 10 mΩ cm. Temperature independent Van Vleck paramagnetism is observed in SmMg 2 Bi 2 indicative of divalent samarium (Sm 2+ ) ions. [ABSTRACT FROM AUTHOR]

Single crystals of the new semiconducting oxytelluride phase, Ba 2 TeO, were synthesized from barium oxide powder and elemental tellurium in a molten barium metal flux. Ba 2 TeO crystallizes in tetragonal symmetry with space group P 4/ nmm (#129), a =5.0337(1) Å, c =9.9437(4) Å, Z =2. The crystals were characterized by single crystal x-ray diffraction, heat capacity and optical measurements. The optical measurements along with electronic band structure calculations indicate semiconductor behavior with a band gap of 2.93 eV. Resistivity measurements show that Ba 2 TeO is highly insulating. [ABSTRACT FROM AUTHOR]

Single-crystal x-ray diffraction, magnetic susceptibility, magnetization, heat capacity, and electrical resistivity measurements are reported for the new tetragonal compound Ce2PdGe3, which forms in the space group P42/mmc (No. 131)--a relative of the a-ThSi2-type structure. Measurements reveal a two-part antiferromagnetic phase transition at Tn,1 = 10.7 K and TN,2 = 9.6 K and subsequent ferromagnetlike ordering near Tc = 2.25 K. The ordered ground state emerges from a lattice of Ce ions that are hybridized with the conduction electrons, as revealed by the enhanced electronic coefficient of the specific heat γ ≈ 50 mJ/mol-Ce-K2 (extrapolated to T = 0 for T < Tc ). Electronic structure calculations reveal a Fermi surface that includes sheets with distinct nesting vectors. Chemical/structural disorder also plays an important role, as evidenced by results from single-crystal x-ray diffraction, the width of the peaks in the heat capacity at 7k and Tc, and the small residual resistivity ratio RRR = ρ300K/ρ0 = 1-8. [ABSTRACT FROM AUTHOR]

The lattice parameter changes in three types of Nb3Sn superconducting wires during uniaxial stress–strain measurements at 4.2 K have been measured by high-energy synchrotron x-ray diffraction. The nearly-stress-free Nb3Sn lattice parameter has been determined using extracted filaments, and the elastic strain in the axial and transverse wire directions in the different wire phases has been calculated. The mechanical properties of the PIT and RRP wire are mainly determined by the properties of Nb3Sn and unreacted Nb. This is in contrast to the bronze route wire, where the matrix can carry substantial loads. In straight wires the axial Nb3Sn pre-strain is strongest in the bronze route wire, its value being smaller in the PIT and RRP wires. A strong reduction of the non-Cu elastic modulus of about 30% is observed during cool-down from ambient temperature to 4.2 K. The Nb3Sn Poisson ratio at 4.2 K measured in the untwisted bronze route wire is 0.35. The present study also shows that the process route has a strong influence on the Nb3Sn texture. [ABSTRACT FROM AUTHOR]

The stoichiometric 114-layered material YBaCo4O7 exhibits long-range antiferromagnetic order below a Néel temperature of 106 K. Nonstoichiometric YBaCo4O7.1, which contains a relatively small amount (1.4%) of interstitial oxygen, has recently been shown to have drastically modified magnetic properties compared to the parent compound. The present experiments have used magnetization, ac susceptibility, and zero applied field NMR to study the spin configuration and spin dynamics in a single crystal of YBaCo4O7.1 as a function of temperature below 100 K. Evidence has been obtained for a magnetic transition at 80 K corresponding to some form of spin freeze-out. Based on previous results for the stoichiometric material, it is likely that the freezing process involves spins in the triangular layers in this frustrated antiferromagnet. At lower temperatures, dynamic effects persist and below 50 K a fraction of the spins, located primarily in the kagome layers, constitute what may be termed a viscous spin liquid component. For T < 10 K, a disordered or glasslike spin structure, with a large distribution of spin correlation times, emerges as the low-temperature state of the spin system. [ABSTRACT FROM AUTHOR]

The texture of Nb3Sn in recent multifilamentary composite wires has been studied by neutron diffraction, synchrotron x-ray diffraction and electron backscatter diffraction. In powder-in-tube (PIT) type superconductors the Nb precursor filaments exhibit a strong 〈110〉 fiber texture as a consequence of the severe cold drawing process, and a 〈110〉 texture is also observed in the Nb3Sn. In the Nb–Ta precursor of the restacked rod process (RRP) strand there is an additional texture component, and in both Ta-alloyed and Ti-alloyed RRP type conductors the Nb3Sn grains grow with a preferential 〈100〉 orientation. [ABSTRACT FROM AUTHOR]

LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which find a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin fluctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase diagram is indeed observed in the isostructural NaFeAs compound. Previous angle-resolved photoemission (ARPES) experiments revealed the existence of both hole and electron-like surfaces, but with rather distinct cross-sectional areas and an absence of the nesting that is thought to underpin both magnetic order and superconductivity in the pnictide family of superconductors. These ARPES observations were challenged by subsequent de Haas–van Alphen (dHvA) measurements which detected a few, electron-like Fermi surface sheets in rough agreement with the original band calculations. Here, we show a detailed dHvA study unveiling additional, small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which ought to correspond to hole-like orbits, as previously observed by ARPES. Therefore, our results reconcile the apparent discrepancy between ARPES and the previous dHvA results. The small size of these Fermi surface pockets suggests a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor an s-wave pairing symmetry for LiFeAs. But similar electron-like Fermi surfaces combined with very different hole pockets between LiFeAs and LiFeP suggest that the nodes in the gap function of LiFeP might be located on the hole pockets. This would be difficult to reconcile with the current understanding of the s± scenario. [ABSTRACT FROM AUTHOR]

We report the discovery of superconductivity in Nb3PdxSe7 with an x-dependent superconducting transition temperature as high as Tc ≃2.1 K for x ≃ 0.7 (middle point of the resistive transition). Needlelike single crystals display anisotropic upper-critical fields with an anisotropy y = Hc2b/c2a as large as 6 between fields applied along their needle axis (or b axis) or along the a axis. As for the Fe based superconductors γ is temperature-dependent, suggesting that Nb3Pd0.7Se7 is a multiband superconductor. This is supported by band structure calculations which reveal a Fermi surface composed of quasi-one-dimensional and quasi-two-dimensional sheets of hole character, as well as three-dimensional sheets of both hole and electron character. Remarkably, Hc2b is observed to saturate at Hc2b(T → 0 K) ≃ 14.1 T which is 4.26 × Hp where Hp is the Pauli-limiting field in the weak-coupling regime. The synthesis procedure yields additional crystals belonging to the Nb2PdxSe5 phase which also becomes superconducting when the fraction of Pd is varied. For both phases we find that superconductivity condenses out of an anomalous metallic state, i.e., displaying ∂ρ/∂T < 0 above Tc similarly to what is observed in the pseudogap phase of the underdoped cuprates. An anomalous metallic state, low-dimensionality, multiband character, extremely high and anisotropic Hc2's are all ingredients for unconventional superconductivity. [ABSTRACT FROM AUTHOR]

The new oxytelluride Ba3Yb2O5Te was obtained from an alkaline earth flux. Ba3Yb2O5Te crystallizes in the tetragonal space group P4/mmm (#123), with a=4.3615(3)Å and c=11.7596(11)Å, Z=1. The structure combines two distinct building blocks, a Ba2Yb2O5 perovskite-like double layer with square bipyramidal coordination of the ytterbium ions, and a CsCl-type BaTe layer. Short range magnetic order is apparent at below 5K, with the magnetic behavior above this temperature dominated by crystal field effects. The structure may be considered as an analog to the Ruddlesden–Popper phases, where the NaCl-type layer has been replaced by the CsCl-type layer. The two-dimensional magnetic behavior is expected based on the highly anisotropic nature of the structure. [ABSTRACT FROM AUTHOR]

The spin structure and spin dynamics in the extended kagome frustrated antiferromagnet YBaCo4O7 have been investigated using zero field and low applied field 59Co NMR. The YBaCo4O7 lattice is made up of bipyramid Co-ion units that form alternating planes of edge-sharing spin triangles and corner-sharing kagome spin triangles in an unusual exchange topology. Our low-temperature spin configuration results, based on hyperfine field orientations, are consistent with those from neutron scattering for the triangle spins which order antiferromagnetically below 106 K. For the kagome spins at low temperatures the static hyperfine fields are found to be oriented orthogonal to those of the triangle spins in a spin-flop configuration that is in disagreement with the neutron findings. Nuclear relaxation rate measurements made as a function of temperature show that inhomogeneous dynamic spin disorder occurs in kagome planes well below the Néel point. [ABSTRACT FROM AUTHOR]

The magnetic, transport, and structural properties of cold-pressed Ba3CoRu2O9 show (i) an antiferromagnetic transition, (ii) a semiconductor-semiconductor electronic transition where the resistivity is dominated by the electron hopping between the Ru25+O9 dimers with itinerant Ru electrons at high temperatures, and (iii) a hexagonal to orthorhombic structural phase transition. All three transitions occur at 93 K and are related to the Ru5+ ions in the Ru2O9 dimers. The Ru-O bond distortion below 93 K further indicates a possible orbital ordering for Ru5+ ions in isolated Ru2O9 dimers, which accounts for the strong orbital, charge, and spin couplings. [ABSTRACT FROM AUTHOR]

We present the low-temperature XRD, susceptibility, specific heat, and resistivity results on single crystal Mn1-xCox V2O4. With increasing Co doping, the chemical pressure related to the decreasing V-V distance drives the system toward the itinerant electron limit, accompanied with the increase of the ferrimagnetic transition temperature and the suppression of the structural distortion. These effects are compared to pressure effects, and show that the V-V distance is the critical parameter controlling the properties of AV2O4. [ABSTRACT FROM AUTHOR]

We compare the superconducting phase-diagram under high magnetic fields (up to H = 45 T) of Fe1+ySe0.4Te0.6 single crystals originally grown by the Bridgman-Stockbarger (BRST) technique, which were annealed to display narrow superconducting transitions and the optimal transition temperature Tc ≳ 14 K, with the diagram for samples of similar stoichiometry grown by the traveling-solvent floating-zone technique as well as with the phase diagram reported for crystals grown by a sell-flux method. We find that the so-annealed samples tend to display higher ratios Hc2/Tc, particularly for fields applied along the interplanar direction, where the upper critical field Hc2(T) exhibits a pronounced concave down curvature followed by saturation at lower temperatures T. This last observation is consistent with previous studies indicating that this system is Pauli limited. An analysis of our Hc2(T) data using a multiband theory suggests the emergence of the Fulde-Ferrel-Larkin-Ovchnikov state at low temperatures. A detailed structural x-ray analysis, reveals no impurity phases but an appreciable degree of mosaicity in as-grown BRST single crystals that remains unaffected by the annealing process. Energy-dispersive x-ray analysis showed that the annealed samples have a more homogeneous stoichiometric distribution of both Fe and Se with virtually the same content of interstitial Fe as the nonannealed ones. Thus we conclude that the excess of Fe, in contrast to structural disorder, contributes to the decrease of the superconducting upper-critical fields of this series. Finally, a scaling analysis of the fluctuation conductivity in the superconducting critical regime, suggests that the superconducting fluctuations have a two-dimensional character in this system. [ABSTRACT FROM AUTHOR]

Abstract: Single crystals of the spin dimer compound Pb2V3O9 were grown for the first time by the floating zone method. Room temperature single-crystal X-ray diffraction measurements confirm their crystallinity and structure. The crystal growth process is discussed in detail and high temperature differential scanning calorimetry measurements that show a structural phase transition from a monoclinic to a triclinic unit cell at 759(1)K are presented. [Copyright &y& Elsevier]

Materials that combine coupled electric and magnetic dipole order are termed ‘magnetoelectric multiferroics’. In the past few years, a new class of such materials, ‘induced-multiferroics’, has been discovered, wherein non-collinear spiral magnetic order breaks inversion symmetry, thus inducing ferroelectricity. Spiral magnetic order often arises from the existence of competing magnetic interactions that reduce the ordering temperature of a more conventional collinear phase. Hence, spiral-phase-induced ferroelectricity tends to exist only at temperatures lower than ∼40 K. Here, we propose that copper(II) oxides (containing Cu2+ ions) having large magnetic superexchange interactions can be good candidates for induced-multiferroics with high Curie temperature (TC). In fact, we demonstrate ferroelectricity with TC=230 K in cupric oxide, CuO (tenorite), which is known as a starting material for the synthesis of high-Tc (critical temperature) superconductors. Our result provides an important contribution to the search for high-temperature magnetoelectric multiferroics. [ABSTRACT FROM AUTHOR]

Presents a retraction by 'Nature' magazine on the study on superconductivity in single crystals of the fullerene C[sub 70] as a result of an independent investigation from conducted at the behest of the Bell Laboratories. Committee's concerns related to the validity of data associated with the device measurements described in the paper; Legitimate ideas and contributions included in the paper.

Abstract: Assessing the fundamental limits of the charge carrier mobilities in organic semiconductors is important for the development of organic electronics. Although devices such as organic field effect transistors (OFETs), organic thin film transistors (OTFTs) and organic light emitting diodes (OLEDs) are already used in commercial applications, a complete understanding of the ultimate limitations of performance and stability in these devices is still lacking at this time. Crucial to the determination of electronic properties in organic semiconductors is the ability to grow ultra-pure, fully ordered molecular crystals for measurements of intrinsic charge transport. Likewise, sensitive tools are needed to evaluate crystalline quality. We present a high-resolution X-ray diffraction and X-ray topography analysis of single-crystals of rubrene that are of the quality being reported to show mobilities as high as amorphous silicon. We show that dislocations and grain boundaries, which may limit charge transfer, are prominent in these crystals. [Copyright &y& Elsevier]

Abstract: The crystal structures, synthesis and physical properties of ruthenium hollandites ALi2Ru6O12 (A=Na, K) with a new pseudo-hexagonal structure type are described. Analogous to tetragonal hollandites, the framework is made of MO6 octahedra in double chains that share corner oxygens with each other to create interstitial tunnels. The tunnels are either hexagonal or triangular in cross-section. Magnetic susceptibilities, low temperature specific heat, and electrical resistivities are reported. The data indicate that these materials are normal, low density of states metals. This new structure type can be extended from A=Group I to A=Group II ions with the synthesis of CaLi2Ru6O12 and SrLi2Ru6O12. [Copyright &y& Elsevier]

Determining the nature of the electronic phases that compete with superconductivity in high-transition-temperature (high- Tc) superconductors is one of the deepest problems in condensed matter physics. One candidate is the `stripe' phase, in which the charge carriers (holes) condense into rivers of charge that separate regions of antiferro magnetism. A related but lesser known system is the `spin ladder', which consists of two coupled chains of magnetic ions forming an array of rungs. A doped ladder can be thought of as a high-Tc material with lower dimensionality, and has been predicted to exhibit both superconductivity and an insulating `hole crystal' phase in which the carriers are localized through many-body interactions. The competition between the two resembles that believed to operate between stripes and superconductivity in high-Tc materials. Here we report the existence of a hole crystal in the doped spin ladder of Sr14Cu24O41 using a resonant X-ray scattering technique. This phase exists without a detectable distortion in the structural lattice, indicating that it arises from many-body electronic effects. Our measurements confirm theoretical predictions, and support the picture that proximity to charge ordered states is a general property of superconductivity in copper oxides. [ABSTRACT FROM AUTHOR]

We report the preparation, crystallization, and solid-state characterization of a benzyl-substituted spirobiphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state, with the molecules packed in an unusual one-dimensional (1 -D) fashion that we refer to as a π-step stack. This particular mode of 1 -D stacking is forced on the lattice arrangement by the presence of the orthogonal phenalenyl units that were specifically incorporated to prevent the crystallization of low-dimensional structures. The structure shows that this strategy is effective, and neighboring molecules in the stack can only interact via the overlap of one pair of active (spin-bearing) carbon atoms per phenalenyl unit, leading to the π-step structure in which the remaining four active carbon atoms per phenalenyl unit do not interact with nearest neighbor molecules. The magnetic susceptibility data in the temperature range 4-360 K may be fit to an antiferromagnetic Heisenberg S = 1/2 linear chain model with intrachain spin coupling J = -52.3 cm-1. Despite the uniform stacking, the material has a room temperature conductivity of 1.4 x 10-3 S/cm and is best described as a Mott insulator. [ABSTRACT FROM AUTHOR]

High quality epitaxial MgxZn1−xO films are essential for the fabrication of ZnO/MgxZn1−xO heterostructures. In this work, MgxZn1−xO (0&les;x&les;0.33) thin films are epitaxially grown on (0 1 1¯ 2) r-sapphire substrates by metalorganic chemical vapor deposition. A thin ZnO buffer layer in the order of 50 A˚ is found to be critical for the growth of single crystal MgxZn1−xO (0&les;x&les;0.33) films with a wurtzite-type structure. The energy bandgap of MgxZn1−xO films as a function of Mg composition was deduced using a UV-Visible spectrophotometer at room temperature in conjunction with Rutherford backscattering spectroscopy measurements. The epitaxial relationship between the wurtzite-type MgxZn1−xO films and r-sapphire substrates is determined to be (1 1 2¯ 0) MgxZn1−xO&par;(0 1 1¯ 2) Al2O3, and [0 0 0 1] MgxZn1−xO&par;[0 1¯ 1 1] Al2O3. In-plane X-ray diffraction reflections show the lower lattice mismatch along the c-axis of the MgxZn1−xO films on r-sapphire substrates. High resolution transmission electron microscopy analysis of the film/substrate interface indicates the region in the film near the interface is crystalline though strained. [Copyright &y& Elsevier]

We have synthesized a new compound with formula Re4As6S3 and characterized its crystal structure by Rietveld powder diffraction methods. Re4As6S3 crystallizes in an face-centered cubic unit cell, space group F4¯3m (no. 216), with lattice constant a=9.8608(1) A˚ and Z=4. The rhenium atoms form tetrahedral clusters linked via tetrahedral arsenic clusters to produce an NaCl-type arrangement. The oxidation state of rhenium is IV and the number of electrons shared by the rhenium atoms in the cluster is 12. The structure is based on an ordered defect thio-spinel A(1−x)B2X4 where the B-type atoms form tetrahedral clusters. [Copyright &y& Elsevier]

Resonant optical excitation of certain molecular vibrations in κ - (BEDT - TTF) 2 Cu [N (CN) 2] Br has been shown to induce transient superconductinglike optical properties at temperatures far above equilibrium T c. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator κ - (BEDT - TTF) 2 Cu [N (CN) 2] Cl and the metallic compound κ - (BEDT - TTF) 2 Cu (NCS) 2. We find nonequilibrium photoinduced superconductivity only in κ - (BEDT - TTF) 2 Cu [N (CN) 2] Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are prerequisites for this effect. [ABSTRACT FROM AUTHOR]

Fe ions of dose 5×1016 cm-2 were implanted at 200 keV into a-plane ZnO epitaxial films. The epitaxial quality of the postannealed samples was verified by x-ray diffraction ω-rocking curves and φ scans, whereas x-ray absorption spectroscopy identified the presence of both Fe2+ and Fe3+ ions, as well as changes in their relative concentration during postannealing. Superconducting quantum interference device measurements show that the as-implanted and postannealed films are ferromagnetic at room temperature. The saturation magnetization reduces during annealing possibly due to the decrease in the number of oxygen vacancies. [ABSTRACT FROM AUTHOR]

We demonstrate high-reflectivity crack-free Al0.18Ga0.82N/Al0.8Ga0.2N distributed Bragg reflectors (DBR) for the spectral region around 350 nm grown by molecular-beam epitaxy on thick GaN templates. The structural quality of the DBR layers is maintained by compensating the compressive and tensile stress in each λ/4 pair. This approach results in the lowest elastic strain energy and allows the growth of thick coherently strained DBRs. A 25 period mirror provides a 26 nm wide stop band centered at 347 nm with the maximum reflectivity higher than 99%. [ABSTRACT FROM AUTHOR]

We have investigated the crystallization of amorphous oxides that are considered likely candidates to replace amorphous SiO[sub 2] as the gate dielectric in advanced field-effect transistors. To avoid crystallization, the mole fraction of main-group oxide in the Zr–Si–O, Zr–Al–O, and Hf–Si–O systems must be greater than 83%, 65%, and 78%, respectively, leading to a maximum useful dielectric constant of only 6.9, 12.7, and 6.6, respectively. We conclude that the silicate systems are not likely to be useful as replacements for SiO[sub 2], while aluminates are more promising. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mg[sub x]Zn[sub 1-x]O (0

Interprets the metalorganic chemical vapor deposition technique called selective area growth (SAG) using a three-dimensional vapor phase model. Profiles of thickness predicted by the model; Long range effects of the thickness composition of metals; Uses of the SAG process; Key idea behind SAG.