Search Results (60 total)

The temperature and magnetic field of the critical current density of four selected pure and C-doped MgB₂ samples have been investigated in detail and the flux pinning mechanism has been analyzed. It was found that the sintering temperature and the substitution of carbon can significantly modify the flux pinning mechanism. Below 30 K, the reduced field dependences of the reduced pinning force for all investigated samples were found to closely obey one scaling law, reflecting the presence of only one dominant pinning mechanism. A δT_c pinning mechanism was found to be mainly responsible in pure MgB₂ samples while the δl pinning mechanism becomes dominant for C-doped samples.

We report the observation of a novel phenomenon, the self-retracting motion of graphite, in which tiny flakes of graphite, after being displaced to various suspended positions from islands of highly orientated pyrolytic graphite, retract back onto the islands under no external influences. Reports of this phenomenon have not been found in the literature for single crystals of any kind. Models that include the van der Waals force, electrostatic force, and shear strengths were considered to explain the observed phenomenon. These findings may conduce to create nanoelectromechanical systems with a wide range of mechanical frequency from megahertz to gigahertz.

First-principles density functional theory calculations have been applied to study the electronic and optical properties of A-site substituted lead zirconate titanate (PZT), where group IIIA and group VB elements partially substitute Pb sites. The conduction band minima in the group IIIA element substituted PZT systems are found to be shared by Ti  3d and dopant states, which reduce the occupation on Ti  3d states by the electrons. Moreover, correlations between dopant electrons introduce the Mott-Hubbard band gap into PZT, which is intrinsically a charge-transfer insulator. This leads to a systematic reduction of energy and optical band gaps with increased atomic number of group IIIA substitutes. Similar chemical trend was found for group VB substitutes, which is, however, closely related to the electron bandwidth of Ti  3d states in the charge-transfer band gaps. All the trivalent substitutes are confirmed to effectively dilute the concentration of the oxygen vacancies under Pb-deficient conditions by our theoretical calculations.

The magnetotransport and magnetic property of the La_0.67Ca_0.33MnO₃ (LCMO) nanoring network (NRN) have been investigated. This NRN-LCMO shows a giant magnetoconductance at low temperatures with strong memory effect of magnetic field. Its high field magnetoconductance shows linear field dependence with an anomalously abrupt increase of slope at about 4 T. The magnetic field dependence of the resistance peak in the resistance vs temperature curve strongly depends on the grain size. The most interesting results are the exchange bias effect of magnetization and the resistance relaxation at low temperatures, which suggest the existence of two types of spin glass in the shell of LCMO grains. The magnetotransport behavior of NRN-LCMO was explained by considering the role of the spin-glass shell in the tunneling process with a core-shell model. We emphasized that the shell layers are magnetized by both the external magnetic field and the internal magnetic field induced by the ferromagnetic cores. This work also indicates that the electronic transport of NRN-LCMO can provide some important information on the magnetic state of the nanograin shell.

We report on transport measurements of an individual single-walled carbon nanotube (SWNT) coupled to a ferromagnetic and a nonmagnetic metal electrode. The low-temperature differential conductance shows a suppression of zero-bias conductance and Coulomb blockade oscillations, where the metallic SWNT behaves as a quantum dot. A marked hysteretic magnetoresistance is observed within the coercive region of the ferromagnetic metal film. The differential resistance has distinctly different values when the magnetization orientation is reversed. Our observed data suggest that the spin-split density of states lies within the SWNT quantum dot. We present an Anderson Hamiltonian to model the spin-polarized electron transport through the SWNT quantum dot with spin-split discrete levels, which supports the experimental observation.

We present a detailed Cu K-edge resonant inelastic x-ray scattering (RIXS) study of the Mott insulator La₂CuO₄ in the 1–7 eV energy loss range. As initially found for the high-temperature superconductor HgBa₂CuO_4+δ, the spectra exhibit a multiplet of weakly dispersive electron-hole excitations, which are revealed by utilizing the subtle dependence of the cross section on the incident photon energy. The close similarity between the fine structures for in-plane and out-of-plane polarizations is indicative of the central role played by the 1s core hole in inducing charge excitations within the CuO₂ planes. On the other hand, we observe a polarization dependence of the spectral weight, and careful analysis reveals two separate features near 2 eV that may be related to different charge-transfer processes. The polarization dependence indicates that the 4p electrons contribute significantly to the RIXS cross section. Third-order perturbation arguments and a shake-up of valence excitations are then applied to account for the final-energy resonance in the spectra. As an alternative scenario, we discuss fluorescence-like emission processes due to 1s→4p transitions into a narrow continuum 4p band.

The critical exponent β=0.17±0.01, where the quoted statistical error is from fits to the data, has been determined for the three-dimensional random-field Ising model (RFIM) order parameter upon zero-field cooling (ZFC) using extinction-free magnetic x-ray scattering techniques for Fe_0.85Zn_0.15F₂. This result is consistent with other exponents determined for the RFIM in that Rushbrooke scaling is satisfied. Nevertheless, there is poor agreement with equilibrium computer simulations, and the ZFC results do not agree with field-cooling results. We present details of hysteresis in Bragg scattering amplitudes and line shapes that help elucidate the effects of thermal cycling in the RFIM, as realized in dilute antiferromagnets in an applied field. We show that the ZFC critical-like behavior is consistent with a second-order phase transitions, albeit quasistationary rather than truly equilibrium in nature, as evident from the large thermal hysteresis observed near the transition.

We report that the conductance of macroscopic multiwall nanotube (MWNT) bundles under pressure shows power laws in temperature and voltage, as corresponding to a network of bulk-bulk connected Tomonaga-Luttinger liquids (LLs). Contrary to individual MWNTs, where the observed power laws are attributed to Coulomb blockade, the measured ratio for the end and bulk obtained exponents, ∼2.4, can be accounted for only by LL theory. At temperatures characteristic of interband separation, it increases due to thermal population of the conducting sheets unoccupied bands.

We calculate the new physics contributions to the rare semileptonic decay B→X_sl⁺l^- (l=e,μ) induced by the charged-Higgs loop diagrams appeared in the top quark two-Higgs doublet model (T2HDM). Within the considered parameter space, we found that (a) the effective Wilson coefficients C˜_i^eff(m_b) (i=7γ, 9V and 10A) in the T2HDM are always standard model-like; (b) the new physics contributions to C˜_7γ^eff and C˜_9V^eff can be significant in magnitude, but they tend to cancel each other; and (c) the T2HDM predictions for Br(B→X_sl⁺l^-) agree well with the measured value within 1 standard deviation.

We applied hydrostatic pressure (up to 10 GPa) to single-walled carbon nanotube bundles at low temperature (down to 2 K) to measure their magnetoresistance (MR) in a field up to 12 T. We found a pressure-induced transition in MR from positive to negative in the high-field regime. The onset of the transition occurs at ∼1.5  GPa, which correlates closely with the tube shape transitions. The characteristics of the high-pressure MR are consistent with a model of pressure-induced two-dimensional weak localization.

The asymmetry in the p̅ angular distribution in the sequential decay Ω̅ ⁺→Λ̅ K⁺→p̅ π⁺K⁺ has been measured to be α̅ _Ωα̅ _Λ=[+1.16±0.18(stat)±0.17(syst)]×10^-2 using 1.89×10⁶ unpolarized Ω̅ ⁺ decays recorded by the HyperCP (E871) experiment at Fermilab. Using the known value of α_Λ, and assuming that α̅ _Λ=-α_Λ, α̅ _Ω=[-1.81±0.28(stat)±0.26(syst)]×10^-2. A comparison between this measurement of α̅ _Ωα̅ _Λ and recent measurements of α_Ωα_Λ made by HyperCP shows no evidence of a violation of CP symmetry.

The atomic structure of the thin SiO₂ film on a Mo(112) substrate has been determined based on a combination of density functional theory calculations and high-quality experimental data obtained from scanning tunneling microscopy, infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. The film consists of a honeycomblike, two-dimensional network of corner-sharing [SiO₄] tetrahedra. One oxygen atom of each tetrahedron binds to the Mo(112) substrate and is located in a bridge position between Mo atoms located in rows protruding from the metal surface. The other three oxygen atoms form Si-O-Si bonds with the neighboring tetrahedra.

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2–8 eV range in the structurally simple compound HgBa₂CuO_4+δ at optimal doping (T_c=96.5  K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (<0.5  eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La₂CuO₄, demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.

The structure of a thin single crystalline SiO₂ film grown on Mo(112) has been studied by scanning tunneling microscopy, infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. In excellent agreement with the experimental results, density functional theory calculations show that the film consists of a two-dimensional network of corner sharing [SiO₄] tetrahedra, with one oxygen of each tetrahedron binding to the protruding Mo atoms of the Mo(112) surface.

A sensitive search for the lepton-number-violating decay Ξ^-→pμ^-μ^- has been performed using a sample of ∼10⁹  Ξ^- hyperons produced in 800  GeV/c p-Cu collisions. We obtain B(Ξ^-→pμ^-μ^-)<4.0×10^-8 at 90% confidence, improving on the best previous limit by 4 orders of magnitude.

A T-type three-terminal junction made of two as-grown semiconducting single-walled carbon nanotubes was fabricated with two electrodes attached to each terminal. It is found that the symmetric current-voltage characteristic along one nanotube can be turned to asymmetric by applying a voltage to the second nanotube acting as a local gate electrode—a property that can be used to form only nanotube-based transistors and amplifiers. Our analysis shows that an asymmetric geometry is essential for devices of this kind to exhibit such a function. More significant rectification behavior has also been observed across the end-bulk junction of the two nanotubes in the presence of a negative backgate voltage.

The interaction between impurities and solitary waves has been experimentally observed on the surface of a defective water layer subjected to vertical vibration. A slightly rugged surface on one sidewall of the water layer serves as the impurity, making the layer breadth at the defect slightly different elsewhere. A wide-breadth impurity will attract or pin not only the hydrodynamic breather at lower driving frequency, but also the kink at higher driving frequency, while a narrow-breadth one will repel them. Using a multiple scale expansion method, a nonlinear Schrödinger equation with an impurity term (NLSI) was derived from the basic hydrodynamic equation. Furthermore, we present numerical calculations that show good agreement between the NLSI-based theoretical model and the experimental results.

A sensitive search for the rare decays Ω^-→Λπ^- and Ξ⁰→pπ^- has been performed using data from the 1997 run of the HyperCP (Fermilab E871) experiment. Limits on other such processes do not exclude the possibility of observable rates for |ΔS|=2 nonleptonic hyperon decays, provided the decays occur through parity-odd operators. We obtain the branching-fraction limits B(Ω^-→Λπ^-)<2.9×10^-6 and B(Ξ⁰→pπ^-)<8.2×10^-6, both at 90% confidence level.

We have measured the α parameter of the Ω^-→ΛK^- decay using data collected with the HyperCP spectrometer during the 1997 fixed-target run at Fermilab. Analyzing a sample of 0.96×10⁶ Ω^-→ΛK^-, Λ→pπ^- decays, we obtain α_Ωα_Λ=[1.33±0.33(stat)±0.52(syst)]×10^-2. With the accepted value of α_Λ, α_Ω is found to be [2.07±0.51(stat)±0.81(syst)]×10^-2.

Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La_1−xSr_1+xMnO₄(0⩽x<0.7). Single crystals were grown by the floating-zone method at 18 La∕Sr concentrations. The low-temperature phase diagram can be understood by considering the strong coupling of the magnetic and orbital degrees of freedom, and it can be divided into three distinct regions: low (x<0.12), intermediate (0.12⩽x<0.45), and high (x⩾0.45) doping. LaSrMnO₄(x=0) is an antiferromagnetic Mott insulator, and its spin-wave spectrum is well described by linear spin-wave theory for the spin-2 square-lattice Heisenberg Hamiltonian with Ising anisotropy. Upon doping, as the e_g electron concentration (1−x) decreases, both the two-dimensional antiferromagnetic spin correlations in the paramagnetic phase and the low-temperature ordered moment decrease due to an increase of frustrating interactions, and Néel order disappears above x_c=0.115(10). The magnetic frustration is closely related to changes in the e_g orbital occupancies and the associated Jahn-Teller distortions. In the intermediate region, there exists neither long-range magnetic nor superstructural order. Short-range-correlated structural “nanopatches” begin to form above x∼0.25. At high doping (x⩾0.45), the ground state of La_1−xSr_1+xMnO₄ exhibits long-range superstructural order and a complex antiferromagnetic order, which differs from that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites, and for x=0.50 we conclude that it is of B2mm symmetry. For x>0.50, the superstructural order becomes incommensurate with the lattice, with a modulation wave vector ϵ that depends linearly on the e_g electron concentration: ϵ=2(1−x). On the other hand, the magnetic order remains commensurate, but loses its long-range coherence upon doping beyond x=0.50.

We report the first evidence for the decay Σ⁺→pμ⁺μ^- from data taken by the HyperCP (E871) experiment at Fermilab. Based on three observed events, the branching ratio is B(Σ⁺→pμ⁺μ^-)=[8.6_-5.4^+6.6(stat)±5.5(syst)]×10^-8. The narrow range of dimuon masses may indicate that the decay proceeds via a neutral intermediate state, Σ⁺→pP⁰,P⁰→μ⁺μ^- with a P⁰ mass of 214.3±0.5  MeV/c² and branching ratio B(Σ⁺→pP⁰,P⁰→μ⁺μ^-)=[3.1_-1.9^+2.4(stat)±1.5(syst)]×10^-8.

We have compared the p and p̅ angular distributions in 117×10⁶ Ξ^-→Λπ^-→pπ^-π^- and 41×10⁶ Ξ̅ ⁺→Λ̅ π⁺→p̅ π⁺π⁺ decays using a subset of the data from the HyperCP experiment (E871) at Fermilab. We find no evidence of CP violation, with the direct-CP-violating parameter A_ΞΛ≡(α_Ξα_Λ-α̅ _Ξα̅ _Λ)/(α_Ξα_Λ+α̅ _Ξα̅ _Λ)=[0.0±5.1(stat)±4.4(syst)]×10^-4.

We have searched for θ⁺(1.54)→K⁰p decays using data from the 1999 run of the HyperCP experiment at Fermilab. We see no evidence for a narrow peak in the K_S⁰p mass distribution near 1.54  GeV/c among 106 000 K_S⁰p candidates, and obtain an upper limit for the fraction of θ⁺(1.54) to K_S⁰p candidates of <0.3% at 90% confidence.

Extensive x-ray and neutron scattering experiments and additional transmission electron microscopy results reveal the partial decomposition of Nd_2−xCe_xCuO_4±δ (NCCO) in a low-oxygen-fugacity environment such as that typically realized during the annealing process required to create a superconducting state. Unlike a typical situation in which a disordered secondary phase results in diffuse powder scattering, a serendipitous match between the in-plane lattice constant of NCCO and the lattice constant of one of the decomposition products, (Nd,Ce)₂O₃, causes the secondary phase to form an oriented, quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from the secondary phase appear at rational positions (H,K,0) in the reciprocal space of NCCO. Additionally, because of neodymium paramagnetism, the application of a magnetic field increases the low-temperature intensity observed at these positions via neutron scattering. Such effects may mimic the formation of a structural superlattice or the strengthening of antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified through careful and systematic experimentation. For typical reduction conditions, the (Nd,Ce)₂O₃ volume fraction is approximately 1%, and the secondary-phase layers exhibit long-range order parallel to the NCCO CuO₂ sheets and have a typical thickness of approximately 100 Å. The presence of the secondary phase should also be taken into account in the analysis of other experiments on NCCO, such as transport measurements.