Dielectric/piezoelectric properties and temperature dependence of domain structure evolution in lead free single crystal

(K_0.5Na_0.5)NbO₃ (KNN) single crystals were grown using a high temperature flux method. The dielectric permittivity was measured as a function of temperature for [001]-oriented KNN single crystals. The ferroelectric phase transition temperatures, including the rhombohedral–orthorhombic T_R−O, orthorhombic–tetragonal T_O−T and tetragonal–cubic T_C were found to be located at −149  C, 205 C and 393 C, respectively. The domain structure evolution with an increasing temperature in [001]-oriented KNN single crystal was observed using polarized light microscopy (PLM), where three distinguished changes of the domain structures were found to occur at −150  C, 213 C and 400 C, corresponding to the three phase transition temperatures.     

Search for the rare decays , , and

Rare decay modes , J/ψ→D⁻π⁺+c.c., and are searched for using events collected with the BESII detector at the BEPC. No signal above background is observed. We present upper limits on the branching fractions of , B(J/ψ→D⁻π⁺)<7.5×10⁻⁵, and at the 90% confidence level.     

Tree-level contributions to the rare decays , , and in the Standard Model

The tree-level contributions to the rare decays , , and are analyzed and compared to those occurring in , , and . It is shown that these purely long-distance contributions, arising from the exchange of a charged lepton, can be significant in B⁺ decays for an intermediate τ, potentially blurring the distinction between the modes used to extract B⁺→τ⁺ν_τ and those used to probe the genuine short-distance and FCNC transitions. Numerically, the tree-level contributions are found to account for 98%, 12% and 14% of the total , , and rates, respectively.     

Measurement of at

We present a measurement of the cross section for Z boson production times the branching fraction to tau lepton pairs in collisions at . The measurement is performed in the channel in which one tau lepton decays into a muon and neutrinos, and the other tau lepton decays hadronically or into an electron and neutrinos. The data sample corresponds to an integrated luminosity of 1.0 fb⁻¹ collected with the DØ detector at the Fermilab Tevatron Collider. The sample contains 1511 candidate events with an estimated 20% background from jets or muons misidentified as tau leptons. We obtain , which is consistent with the standard model prediction.     

Search for pair production of first-generation leptoquarks in collisions at

A search for pair production of first-generation leptoquarks (LQ) is performed with data collected by the DØ experiment in collisions at at the Fermilab Tevatron Collider. In a sample of data corresponding to 1 fb⁻¹ the search has been performed on the final states with two electrons and two jets or one electron, two jets and missing transverse energy. We find our data consistent with standard model expectations. The results are combined with those found in a previous analysis of events with two jets and missing transverse energy to obtain scalar LQ mass limits. We set 95% C.L. lower limits on a scalar LQ mass of 299 GeV, 284 GeV and 216 GeV for β=1, β=0.5 and β=0.02 respectively, where β is the LQ branching ratio in the eq channel. This improves the results obtained with a lower luminosity sample from Run II of the Tevatron. Lower limits on vector LQ masses with different couplings from 357 GeV to 464 GeV for β=0.5 are also set using this analysis.     

Power factor enhancement in Pr-substituted alloys prepared by high-pressure sintering

Bi₈₅Sb_15−xPr_x (x=0,1,2,3) alloys with partial substitution of Pr for Sb were synthesized by mechanical alloying followed by high-pressure sintering. The crystal structure was characterized by X-ray diffraction. The electrical conductivity and Seebeck coefficient were measured in the temperature range of 80–300 K. The results show that the electrical conductivity and Seebeck coefficient of Pr-substituted samples are both larger than those of the reference sample, Bi₈₅Sb₁₅, in the whole measurement temperature range. The power factor of Bi₈₅Sb₁₃Pr₂ reaches a maximum value of 3.83×10⁻³ W K⁻² m⁻¹ at 235 K, which is about four times larger than that of the reference sample, Bi₈₅Sb₁₅, at the same temperature.     

New CP phase of – mixing on T violation in

A large CP-violating phase uncovered recently by CDF and DØ Collaborations in the time-dependent CP asymmetry (CPA) of the B_s→J/Ψ decay clearly indicates that a non-Cabibbo–Kobayashi–Maskawa (CKM) phase has to be brought into the b→s transition. We find that the model with SU(2)_L singlet exotic quarks cannot only provide the new phase induced from the Z-mediated flavor changing neutral current (FCNC) at tree level, but also strongly relate the B_s– mixing, B_q→V_qℓ⁺ℓ⁻ (V_d[s]=K^*[]) and B_s→μ⁺μ⁻ together. In particular, we show that the new CP phase can be unambiguously exposed by the large statistical significances of T violating observables in B_q→V_qℓ⁺ℓ⁻, while the branching ratio of B_s→μ⁺μ⁻ can be enhanced to be O(10⁻⁸).     

Magnetic transitions and magnetoresistance of -type () compounds

Magnetic transitions and magnetotransport properties of polycrystalline Tb_1−xGd_xMn₆Ge₆ (x=0.2–1.0) compounds have been investigated by magnetic property and resistivity measurements in an applied magnetic field up to 50 kOe. The cell parameter a,c and cell volume V of compounds (x=0.2–1.0) increase with an increasing Gd content. The compounds (x=0.2–1.0) show a rich variety of magnetic behavior, such as antiferromagnetic, ferrimagnetic and paramagnetic state with increasing temperature. Their Curie temperatures increase almost linearly with an increasing Gd content from 460 K for x=0.2 to 484 K for x=1.0. The compounds (x=0.2–1.0) display the field induced metamagnetic transitions, and the threshold fields first increase and then decrease with an increasing Gd content. The magnetoresistance curves of the Tb_0.4Gd_0.6Mn₆Ge₆ compounds in an applied magnetic field up to 50 kOe are presented and the magnetoresistance effects are related to the metamagnetic transitions.     

Lattice, magnetic and transport properties in antiperovskite compounds

The temperature-dependent magnetization, lattice, and transport properties of Mn₃Sn_1−xGe_xC (0≤x≤0.5) compounds are systematically investigated. The Mn–Mn atomic distance decreases as Ge content is increased, and the transition temperature from ferromagnetic (or ferrimagnetic) to paramagnetic state decreases too. Mn₃SnC has a large magnetovolume effect (MVE). However, Ge-doping in Mn₃SnC gradually reduces the MVE, till the MVE disappears. Whether there is an abnormal lattice change or not, there always exists an anomalous increase in resistivity near the magnetic phase transition point with decreasing temperature.     

Atomically flat GaMnN by diffusion of Mn into GaN()

The interaction of Mn thin films on atomically flat GaN() has been investigated by Scanning Tunneling Microscopy (STM) and Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). GaN surfaces prepared by repeated sputtering/annealing cycles show various reconstructions but essentially the 6×6 one. A high density of small islands (height 0.6 nm, diameter 5 nm) nucleates upon deposition of 0.3 ML Mn. Upon annealing at 575 C, these islands coalesce into larger islands (height 4 nm, diameter 50 nm) between which the GaN surface is visible. Atomically resolved STM images between the islands show the (3×3) reconstruction of GaN. Annealing the sample further to 675 C leads to the restoration of a bare GaN surface. ToF-SIMS reveals that Mn diffused into the bulk of the sample.     

First-principles study of pressure effects on and

We have performed first-principles calculations to investigate the pressure effects on CaFe₂As₂ and BaFe₂As₂. Our calculations show that in CaFe₂As₂, the orthorhombic structure transforms to a collapsed tetragonal structure at 0.4 GPa with a volume collapse of 9.5%, which is in agreement with experiments. Together with the structural phase transition, CaFe₂As₂ undergoes a magnetic transition from the stripe antiferromagnetic ordering to the nonmagnetic state. For BaFe₂As₂, we predict that the orthorhombic structure transforms to the tetragonal structure at 9.4 GPa. Unlike CaFe₂As₂, the magnetic moments of Fe ions in BaFe₂As₂ are not zero and the stripe antiferromagnetic ordering transforms to the checkerboard antiferromagnetic ordering together with the structural phase transition. The stability of the orthorhombic structure up to 9.4 GPa suggests that superconductivity and magnetism coexist in BaFe₂As₂.     

Far-infrared dielectric functions and phonon spectra of alloys determined by spectroscopic ellipsometry

We have used spectroscopic ellipsometry to determine the complex dielectric function of a series of ternary Be_xZn_1−xTe thin films grown by molecular beam epitaxy. The II–VI semiconductor alloys were grown on InP substrates that had an InGaAs buffer layer. After the growth, X-ray diffraction experiments were performed in order to determine the alloy concentration. A standard inversion technique was used to obtain the dielectric functions from the measured ellipsometric spectra, obtained between 2000 nm (5000 cm⁻¹) and 40,000 nm (250 cm⁻¹). By modelling the dielectric function as a collection of oscillators, representing longitudinal and transverse optical phonons of the Be_xZn_1−xTe lattice, we were able to recover the phonon spectra for this alloy system. It is argued that the additional phonon modes that are obtained from ellipsometry are best understood from the recently-proposed percolation model.     

High-rate characteristic of F-substitution cathode materials for Li–ion batteries

The effect of fluorine substitution on the electrochemical properties of Li₃V ₂(PO₄)₃ cathode materials was studied. Samples with stoichiometric proportions of Li₃V ₂(PO₄)_3−xF_x (x=0,0.05,0.10,0.15) were prepared by adding LiF in the starting materials of Li₃V ₂(PO₄)₃. XRD studies showed that the F-substituted Li₃V ₂(PO₄)₃ had the same monoclinic structure as the un-substituted Li₃V ₂(PO₄)₃. SEM images showed that F-substitution Li₃V ₂(PO₄)₃ had a regular and uniform particles. The results of electrochemical measurement showed that F-substitution can improve the rate capability of these cathode materials. The Li₃V ₂(PO₄)_2.90F_0.10 sample showed the best high rate performance. Its discharge capacity at 10 C rate was 117 mA h g⁻¹ with 30th capacity retention of about 90.60%. The electrode reaction reversibility and electronic conductivity was enhanced, and the charge transfer resistance was decreased through F-substitution. The improved electrochemical performance of F-substitution Li₃V ₂(PO₄)₃ cathode materials were attributed to the above factors.     

Search for squarks and gluinos in events with jets and missing transverse energy using 2.1 fb of collision data at

A data sample corresponding to an integrated luminosity of 2.1 fb⁻¹ collected by the DØ detector at the Fermilab Tevatron Collider was analyzed to search for squarks and gluinos produced in collisions at a center-of-mass energy of 1.96 TeV. No evidence for the production of such particles was observed in topologies involving jets and missing transverse energy, and 95% C.L. lower limits of 379 GeV and 308 GeV were set on the squark and gluino masses, respectively, within the framework of minimal supergravity with tanβ=3, A₀=0, and μ<0. The corresponding previous limits are improved by 54 GeV and 67 GeV.     

Search for scalar leptoquarks in the acoplanar jet topology in collisions at

A search for leptoquarks has been performed in 310 pb⁻¹ of data from collisions at a center-of-mass energy of 1.96 TeV, collected by the D0 detector at the Fermilab Tevatron Collider. The topology analyzed consists of acoplanar jets with missing transverse energy. The data show good agreement with standard model expectations, and a lower mass limit of 136 GeV has been set at the 95% C.L. for a scalar leptoquark decaying exclusively into a quark and a neutrino.     

Influence of oxygen partial pressure on structural, transport and magnetic properties of Co doped films

Pulse laser deposited (PLD) thin films of Co doped TiO₂ on silicon and quartz substrates are investigated. A mixture (1:1) of argon and oxygen with various total pressures (6.6 mPa to 53 Pa) is used to vary the oxygen content in the samples. The crystal structure and transport/ magnetic properties of Co_xTi_1−xO_2−δ (x=0.01, 0.03, 0.06) thin films are found to have strong dependence on oxygen stoichiometry. X-ray diffraction (XRD) data reveal mixed phase material containing both anatase and rutile. However, the stability of each phase depends on the amount of oxygen present in the chamber during the growth of the films. X-ray Photoelectron Spectroscopy (XPS) shows the incorporation of Co in TiO₂ and is in the 2+ oxidation state. There occurs an enhancement in electrical conductivity and magnetization due to the off stoichiometric oxygen. The resistivity data follow a simple thermal activation model, giving carriers’ activation energies in the range of 20 to 140 meV. A bound magnetic polaron model is adopted to explain the observed magnetic behavior of the films.     

Counting BPS operators in SYM

The free field partition function for a generic U(N) gauge theory, where the fundamental fields transform in the adjoint representation, is analysed in terms of symmetric polynomial techniques. It is shown by these means how this is related to the cycle polynomial for the symmetric group and how the large N result may be easily recovered. Higher order corrections for finite N are also discussed in terms of symmetric group characters. For finite N, the partition function involving a single bosonic fundamental field is recovered and explicit counting of multi-trace quarter BPS operators in free super-Yang–Mills discussed, including a general result for large N. The partition function for quarter BPS operators in the chiral ring of super-Yang–Mills is analysed in terms of plane partitions. Asymptotic counting of BPS primary operators with differing R-symmetry charges is discussed in both free super-Yang–Mills and in the chiral ring. Also, general and explicit expressions are derived for SU(2) gauge theory partition functions, when the fundamental fields transform in the adjoint, for free field theory.     

Fabrication of polyaniline/ heterojunction structure using plasma enhanced polymerization technique

The work reports on the fabrication of a p–n heterojunction structure comprised of polyaniline (PANI) and TiO₂ nanoparticles. PANI was deposited by plasma enhanced polymerization on TiO₂ thin film substrates. The structural and the crystalline properties demonstrated the coherence and the substantive interaction of the plasma polymerized PANI molecules with the TiO₂ nanoparticle thin film. The UV–Vis studies of PANI/TiO₂ thin film supported the internalization of PANI with TiO₂ nanoparticles due to π–π^* transition of the phenyl rings with the lone pair electrons () of the nitrogen atom present in the PANI molecules. The I–V characteristics of the PANI/TiO₂ heterojunction structure were obtained in the forward and the reverse biased at applied voltage ranging from −1 V to +1 V with a scan rate of 2 mV/s. The proficient current in the PANI/TiO₂ heterojunction structure was attributed to the well penetration of PANI molecules into the pores of the TiO₂ nanoparticle thin film. The I–V characteristics ensured an efficient charge movement at the junction of PANI/TiO₂ interface and thus, behaved as a typical ohmic system.