Synthesis and physical properties of LaO1-xFxFeAs

We have prepared the newly discovered Fe-based superconducting material LaO_1-xF_xFeAs (0 ≤ x ≤ 0.2) in polycrystalline form and have investigated the samples by means of structural, thermodynamic and transport measurements. Our investigations reveal a non superconducting phase at 0 ≤ x 0.04 which for x = 0 is characterized by a structural transition towards an orthorhombic distortion at T_s ≈ 160 K and antiferromagnetic spin order at T_N ≈ 138 K. Both transitions lead to strong anomalies in various transport properties as well as in magnetization and in specific heat. Remarkably, the transition temperatures are only weakly doping dependent up to x ≈ 0.04. However, the transitions are abruptly suppressed at x ≥ 0.05 in favour of a superconducting phase with a critical temperature T_c 20 K. Upon further increasing the F-doping T_c increases up to a maximum of T_c = 26.8 K at x = 0.1 which is followed by a decrease down to T_c ≈ 10 K at x ≥ 0.15.     

Crystal and magnetic structure of single-crystal La<Subscript>1-x</Subscript><Emphasis Type="Bold">Sr</Emphasis><Subscript>x</Subscript><Emphasis Type="Bold">MnO</Emphasis><Subscript>3</Subscript><Emphasis Type="Bold">(x ≈ 1/8)</Emphasis>

A neutron powder diffraction (NPD) study on the crystal and magnetic structure of a crushed La_1-xSr_xMnO₃ (x ≈ 1/8) single crystal has been performed. The sample belongs to orthorhombic (Pnma, O) above the Jahn-Teller (JT) transition temperature (T_JT) and monoclinic (P12₁/c1, M^′) in the JT regime. We have also refined the NPD data below the charge/orbital ordering (CO/OO) temperature (T_CO/OO) with a monoclinic (P12₁/c1, M^′′) model because the experimental resolution was insufficient to clearly identify a triclinic structure. The refined lattice parameters show an obvious breathing-mode distortion between T_CO/OO and T_JT, accompanied by a large deviation of the monoclinic angle β from 90°, signifying a very strong cooperative JT distortion. A ferromagnetic (FM) moment of 3.43(5)μ _B/Mn besides an A-type antiferromagnetic (A-AFM) moment of 0.54(2) μ _B/Mn is directed mainly along the b axis in P12₁/c1 symmetry at 5 K. With increasing temperature, the A-AFM domains transform into FM ones above ~100 K and the FM spin orientation turns from the b to the c axis in crystallographic b-c plane below T_c = 187(1) K. The magnetization measurements show typical anomalies around T_CO/OO and T_JT. The measured saturation moment of 3.9(1)μ _B/Mn at 70 kOe and 5 K is well consistent with the sum 3.97(5)μ _B/Mn of the refined FM and A-AFM moments at 5 K, implying the A-AFM spins are aligned in field direction at 70 kOe. The applied magnetic field can affect the paramagnetic insulating (PMI) state in the range of magnetic polarons. Based on the size of JT distortion and the bond-valence sums (BVS’s), the CO/OO phenomenon is being discussed.     

Modified fragmentation function in heavy ion collisions at RHIC via direct <Emphasis Type="Italic">γ</Emphasis>-jet measurements

The presented results are the first measurements at RHIC for direct γ-charged hadron azimuthal correlations in heavy ion collisions. We use these correlations to study the color charge density of the medium through the medium-induced modification of high-p _T parton fragmentation. Azimuthal correlations of direct photons at high transverse energy (8<p _T <16 GeV) with away-side charged hadrons of transverse momentum (3<p _T <6 GeV/c) have been measured over a broad range of centrality for Au+Au collisions and p+p collisions at  GeV in the STAR experiment. A transverse shower shape analysis in the STAR Barrel Electromagnetic Calorimeter Shower Maximum Detector is used to discriminate between the direct photons and photons from the decays of high p _T π ⁰. The per-trigger away-side yield of direct γ is smaller than from π ⁰ trigger at the same centrality class. Within the current uncertainty the I _CP of direct γ and π ⁰ are similar.     

Magnetic ground state and the spin-state transitions in YBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript>

The crystal and magnetic structure of the perovskite-like, oxygen deficient cobalt oxide YBaCo₂O_5.5 has been studied by means of neutron and X-ray diffraction in the 10–300 K temperature range. The magnetic ground state is characterized by a coexistence of two distinct antiferromagnetic phases. In the first one, the ionic moments of high-spin Co³⁺ ions in the pyramidal sites are ordered in a spiral arrangement, while octahedral sites are non-magnetic due to presence of low-spin Co³⁺ ions. The arrangement in the second phase is collinear of the G-type, with non-zero moments both in pyramidal (high-spin Co³⁺ ions) and octahedral sites (presumably a mixture of the low- and high-spin states). With increasing temperature, at 260–300 K, the system develops a gradual structural transformation, which is associated with appearance of spontaneous magnetic moment. This process is related to a thermally induced reversion of low- and high-spin states at the octahedral sites to the intermediate-spin Co³⁺ states, resulting in an insulator-metal transition at T_C ≈ T_IM ≈ 295 K.     

High pressure study of the organic compound (TMTTF)<Subscript>2</Subscript>BF<Subscript>4</Subscript>

High pressure resistivity measurements of the organic compound (TMTTF)₂BF₄ have been performed in a newly developped Bridgman cell providing good pressure conditions on a wide pressure range. For the first time in this compound a zero resistance superconducting state is observed between 3 and 4 GPa. At temperatures above the superconducting transition, the resistivities of the two high quality samples show a different behavior. One sample, provides indications for a magnetic quantum critical point at the maximum of T_c, whereas in the other antiferromagnetic spin-fluctuations are present above T_c.     

Effect of annealing on structure and magnetic properties of Fe-N and Fe-Ti-N thin films

The structure and magnetic properties of Fe-N and Fe-Ti-N films have been studied as a function of annealing temperature T_a with a transmission electron microscope and a vibrating sample magnetometer. The as-prepared Fe-N films consist of the γ^′-Fe₄N and α^′′-Fe₁₆N₂ phases, and the Fe-Ti-N films are composed of the γ^′-Fe₄N, α^′′-Fe₁₆N₂, and TiN phases. The structural changes with annealing temperature in the Fe-N films are distinct. The α^′′-Fe₁₆N₂ decomposes into α+γ^′ phases in the Fe-N film annealed at about 300 °C, and it disappears in the film annealed at 350 °C. Annealing of the Fe-Ti-N films shows no structural changes between room temperature (RT) and 500 °C. The saturation magnetization 4πM_S and coercivity H_c of the Fe-N films change drastically with the annealing temperature T_a, whereas those of the Fe-Ti-N films do not change with T_a up to 500 °C. These results indicate that the additon of Ti may improve the thermal stability of Fe-N films. Recieved: 6 Juli 1998 / Accepted: 19 Oktober 1998 / Published online: 10 March 1999     

Observation of the muon inner bremsstrahlung at?LEP1

Muon bremsstrahlung photons converted in front of the DELPHI main tracker (TPC) in dimuon events at LEP1 were studied in two photon kinematic ranges: 0.2<E _γ ≤1 GeV and transverse momentum with respect to the parent muon p _T <40 MeV/c, and 1<E _γ ≤10 GeV and p _T <80 MeV/c. A good agreement of the observed photon rate with predictions from QED for the muon inner bremsstrahlung was found, contrary to the anomalous soft photon excess that has been observed recently in hadronic Z ⁰ decays. The obtained ratios of the observed signal to the predicted level of the muon bremsstrahlung are 1.06±0.12±0.07 in the photon energy range 0.2<E _γ ≤1 GeV and 1.04±0.09±0.12 in the photon energy range 1<E _γ ≤10 GeV. The bremsstrahlung dead cone is observed for the first time in the direct photon production at LEP. Deceased.     

Numerical study of the directed polymer in a 1 + 3 dimensional random medium

The directed polymer in a 1+3 dimensional random medium is known to present a disorder-induced phase transition. For a polymer of length L, the high temperature phase is characterized by a diffusive behavior for the end-point displacement R² ∼L and by free-energy fluctuations of order ΔF(L) ∼O(1). The low-temperature phase is characterized by an anomalous wandering exponent R²/L ∼L^ω and by free-energy fluctuations of order ΔF(L) ∼L^ω where ω∼0.18. In this paper, we first study the scaling behavior of various properties to localize the critical temperature T_c. Our results concerning R²/L and ΔF(L) point towards 0.76 < T_c ≤T₂=0.79, so our conclusion is that T_c is equal or very close to the upper bound T₂ derived by Derrida and coworkers (T₂ corresponds to the temperature above which the ratio remains finite as L ↦ ∞). We then present histograms for the free-energy, energy and entropy over disorder samples. For T ≫T_c, the free-energy distribution is found to be Gaussian. For T ≪T_c, the free-energy distribution coincides with the ground state energy distribution, in agreement with the zero-temperature fixed point picture. Moreover the entropy fluctuations are of order ΔS ∼L^1/2 and follow a Gaussian distribution, in agreement with the droplet predictions, where the free-energy term ΔF ∼L^ω is a near cancellation of energy and entropy contributions of order L^1/2.     

Electric-field dependence of dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 ceramics

The effect of a dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr_0.2Ti_0.8)O₃ (BZT) ceramics are investigated. Diffuse phase transitions were observed in BZT ceramics and the Curie–Weiss exponent (CWE) was γ∼2.0. The dielectric constant versus temperature characteristics and the γ in the modified Curie–Weiss law, ε ⁻¹=ε _m ⁻¹[1+(T−T _m ) ^γ /C₁](1≤γ≤2), as a function of the dc bias field was obtained for BZT ceramics. The results indicated that γ is a function of dc bias field, and the γ value decreased from 2.04 to 1.73 with dc bias field increasing from 0 to 20 kV/cm. The dielectric constant decreases with increasing dc bias field, indicating a field-induced phase transition. The dc bias field has a strong effect on the position of the dielectric peak and affects the magnitude of the dielectric properties over a rather wide temperature range. The peak temperature of the dielectric loss does not coincide with the dielectric peak and an obvious minimum value for the dielectric loss at the temperature of the dielectric peaks is observed. At room temperature, 300 K, the high tunability (K=80%), the low loss tangent (≈0.01) and the large FOM (74), clearly imply that these ceramics are promising materials for tunable capacitor-device applications.     

Evidence for an isomeric 3/2- state in 53Co

The fragmentation of a 550MeV/u primary beam of ⁵⁸Ni on a ⁹Be target has been used to measure time- and energy-correlated γ decays following the implantation of event-by-event discriminated secondary fragments into a ⁹Be stopper plate. A new isomeric γ decay with T _1/2 = 14( ) ns and E _γ = 646.2(2) keV is observed and attributed to the decay of the yrast 3/2^- state in ⁵³ ₂₇Co₂₆ . This short-lived isomeric state has been populated by means of nuclear reactions during the stopping process of the secondary fragments. The experimental findings are discussed in the framework of large-scale spherical shell model calculations in conjunction with isospin symmetry-breaking residual interactions for the A = 53 , T _z = ±1/2 mirror nuclei ⁵³Co and ⁵³Fe .     

Simulation study of χc→J/Ψ+γ detection with J/Ψ→e+e? in pp collisions in the ALICE experiment at LHC

We present Monte Carlo preliminary results about the feasibility to detect the χ _c family in p-p collisions at 14 TeV in the ALICE Central Barrel at CERN LHC. The χ _c1 and χ _c2 were forced to decay in the radiative channel J/Ψ+γ→e ⁺ e ⁻+γ and were merged with a proton-proton non-biased collision. After Monte Carlo transport and simulation of the detector response, the e ⁺, e ⁻ and converted γ were reconstructed and identified in the ALICE ITS, TPC and TRD detectors. Separate signals corresponding to γ from χ _c1 and from χ _c2 were observed. The position and relative weight of the fit to gaussians agreed with the input values within the statistical limits. An erratum to this article can be found at     

Physical property characterization of bulk MgB<Subscript>2</Subscript> superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity κ(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB₂, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3–4 nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB₂. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm^-1. Superconductivity is observed at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power S(T), and thermal conductivity κ(T) measurements. The power law fitting of ρ(T) give rise to Debye temperature (Θ_D) at 1400 K which is found consistent with the theoretical fitting of S(T), exhibiting Θ _D of 1410 K and carrier density of 3.81 × 10²⁸/m³. Thermal conductivity κ(T) shows a jump at 38 K, i.e., at T_c, which was missing in some earlier reports. Critical current density (J_c) of up to 10⁵ A/cm² in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20 K respectively. The superconducting performance parameters viz. irreversibility field (H_irr) and critical current density J_c(H) of the studied MgB₂ are improved profoundly with addition of nano-SiC and nano-diamond. The physical property parameters measured for polycrystalline MgB₂ are compared with earlier reports and a consolidated insight of various physical properties is presented.     

The phase diagram of (NH4I)x(KI)1−x

The χ,T phase diagram of (NH₄I)_x(KI)_1−x has been determined using neutron diffraction experiments and dielectric spectroscopy. At low temperatures and with decreasing χ, the sequence γ, β, ε and glass phase has been detected. The critical concentration χ_c≈0.55 separates the glassy phase with frozen-in orientational disorder from the ε phase which reveals long-range orientational order. Close to χ_c our experiments reveal evidence for two subsequent glass transitions.     

Effect of γ irradiation on the hysteresis phenomena at the incommensurate-commensurate phase transition in Rb2ZnBr4

The effect of γ irradiation on the temperature hysteresis in dielectric permittivity ɛ and loss tangent tan δ of crystalline Rb₂ZnBr₄ has been studied in the vicinity of the incommensurate-commensurate phase transition. The ɛ(T) and tan δ(T) curves were found to exhibit anomalies in the form of maxima. Hysteresis was observed in the measured properties, including the transition temperature T _c (ΔT=T _c ^h −T _c ^c ), in both unirradiated and irradiated samples. It is shown that, as the radiation dose increases the extent of the hysteresis ΔT increases, the values of ɛ _max and tan δ _max at the transition point decrease, and the anomalies wash out. Fiz. Tverd. Tela (St. Petersburg) 40, 1911–1914 (October 1998)     

Search for resonant absorption of solar axions emitted in M1 transition in <Superscript>57</Superscript>Fe nuclei

A search for resonant absorption of 14.4 keV solar axions by a ⁵⁷Fe target was performed. The Si(Li) detector placed inside the low-background setup was used to detect the γ-quanta appearing in the deexcitation of the 14.4 keV nuclear level: A+⁵⁷Fe→⁵⁷Fe^*→⁵⁷Fe+γ. The new upper limit for the hadronic axion mass has been obtained of m _A ≤159 eV (95% c.l.) (S=0.5, z=0.56).     

The 130Nd↦130Pr↦130Ce decay chain revisited

An investigation of low-lying energy levels of ¹³⁰Pr and ¹³⁰Ce has been made via β-decay. The radioactive parent nuclei were produced via ^94,96Mo + ⁴⁰Ca reactions at 6.8 MeV/nucleon and transported by a He-jet system. Gamma-γ-t, γ-X-t, γ-e^--t coincidence measurements were performed. Internal conversion coefficients have been measured with a magnetic spectrometer. The new β-decay half-life of ¹³⁰Nd has been determined to be 21(3) s. A partial level scheme of ¹³⁰Pr at low-spin has been constructed. The β-decay of ¹³⁰Pr to ¹³⁰Ce suggests the existence of three β-instable isomeric states in ¹³⁰Pr, while only one half-life T _1/2 = 40(4) s has been measured. Based on the present work and low-lying excited states in neighbouring odd-mass nuclei, the spins and parities of isomeric states in ¹³⁰Pr are proposed to be 2⁺, (4,5)⁺ and (7,8^±). The ground state in ¹³⁰Pr is still undetermined. Received: 20 August 2001 / Accepted: 6 November 2001     

Level structure of 123Cs observed from 123Ba decay and described using the IBFM and CQPC models

The decay of ¹²³Ba to ¹²³Cs has been studied with mass-separated sources. Singles spectra of γ-rays, X-rays and conversion electrons as well as γ - γ - t, γ - e ⁻ - t, and X - e ⁻ - t coincidences were recorded. A level scheme of ¹²³Cs has been constructed including 26 new excited states and 82 transitions. The existence of a I ^π = 11/2⁻, T _1/2 = 1.7 s isomer, bandhead of the h _11/2 band, is confirmed. Its excitation energy equals 156.3 keV. A new T _1/2 = 114 ns isomer has been established at 232 keV using the in-beam recoil catcher technique. It is assigned to the I ^π = 9/2⁺ bandhead of the gpg _9/2 band. Collective bands based on the 1/2⁺ ground state and first 3/2⁺ excited state are proposed. The level structure is described in the frame of the interacting boson-fermion and core-quasiparticle coupling models.     

High resolution diffraction and small angle scattering neutron investigations of LaCo<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>O<Subscript>3+</Subscript><Subscript>δ</Subscript>: effect of oxygen content

We have investigated LaCo_0.5Mn_0.5O_{3+ δ} compounds with different oxygen content by means of magnetization, high resolution and small-angle neutron diffraction measurements. Oxygen content decrease down to stoichiometric composition leads to an essential increase of T_C and magnetic moment while Co/Mn ionic ordering degree is kept almost constant. It is assumed that upon oxygen reduction Co³⁺ ions change their valence state down to 2+ one that leads to dominating of Co²⁺-Mn⁴⁺ ferromagnetic interactions as well as T_C increase. Magnetic properties can be explained in terms of coexistence of long-range ferromagnetic order and short-range clusters with antiferromagnetic interactions prevailing. Size distribution of the mentioned short-range magnetic inhomogeneities is rather mild within the samples but it is strongly temperature dependent.     

Pressure variation of Luttinger liquid parameters in single wall carbon nanotubes networks

We measure electrical transport on networks of single wall nanotube of different origin as a function of temperature T, voltage V and pressure P. We observe Luttinger liquid (LL) behavior, a conductance ∝T^α and a dynamic conductance ∝V^α. We observe a sample dependent P variation of the α parameters, interpreted as Fermi level changes due to pressure induced charge transfer. We show how, through standard four-leads and crossed configuration methods, it is possible to determine α_bulk and α_end, respectively. We study and discuss the pressure and doping level dependences of the number of channels N, the LL parameter g and the intra-rope tube-tube coupling constant U within a phenomenological model.     

Competition between Jahn-Teller effect and spin-orbit coupling in Td Au20 ±γ, γ = 1,2,3

In this work we explored the competition between spin-orbit and Jahn-Teller effects as decisive influences in the ground states of the T_d Au₂₀ ^±γ (γ=1,2,3). All our electronic calculations were based in the relativistic density functional theory in the zeroth-order regular approximation (ZORA) to the Dirac equation. Calculations were done in both versions of ZORA: scalar relativistic and full-relativistic including the spin-orbit interaction. We concluded that for the Au₂₀ ^-3 ion is necessary to use a full-relativistic theory in order to predict a symmetry-lowering from tetrahedral. We predict a trigonal C_3v isomeric form for this trianion due to a Jahn-Teller distortion of its parent T_d. For the rest of ions we found a tendency to conserve their pristine tetrahedral configurations. In these cases we found one of both possibilities: a quenching of the vibronic interaction by spin-orbit coupling or simply a weak Jahn-Teller effect which is not enough to distort appreciably the cluster.