Search Results (374 total)

Laser-based angle-resolved photoemission spectroscopy measurements have been carried out on the high energy electron dynamics in Bi₂Sr₂CaCu₂O₈ high temperature superconductor. Our superhigh resolution data, momentum-dependent measurements, and complete analysis provide important information to judge the nature of the high energy dispersion and kink. Our results rule out the possibility that the high energy dispersion from the momentum distribution curve (MDC) may represent the true bare band as believed in previous studies. We also rule out the possibility that the high energy kink represents electron coupling with some high energy modes as proposed before. Through detailed MDC and energy distribution curve analyses, we propose that the high energy MDC dispersion may not represent intrinsic band structure.

We report a new STAR measurement of the longitudinal double-spin asymmetry A_LL for inclusive jet production at midrapidity in polarized p+p collisions at a center-of-mass energy of sqrt[s]=200  GeV. The data, which cover jet transverse momenta 5<p_T<30  GeV/c, are substantially more precise than previous measurements. They provide significant new constraints on the gluon spin contribution to the nucleon spin through the comparison to predictions derived from one global fit to polarized deep-inelastic scattering measurements.

We present the first spin alignment measurements for the K^*0(892) and ϕ(1020) vector mesons produced at midrapidity with transverse momenta up to 5 GeV/c at sqrt[s_NN]=200 GeV at RHIC. The diagonal spin-density matrix elements with respect to the reaction plane in Au+Au collisions are ρ₀₀=0.32±0.04 (stat) ± 0.09 (syst) for the K^*0 (0.8<p_T<5.0 GeV/c) and ρ₀₀=0.34±0.02 (stat) ± 0.03 (syst) for the ϕ (0.4<p_T<5.0 GeV/c) and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector-meson spins. Spin alignments for K^*0 and ϕ in Au+Au collisions were also measured with respect to the particle's production plane. The ϕ result, ρ₀₀=0.41±0.02 (stat) ± 0.04 (syst), is consistent with that in p+p collisions, ρ₀₀=0.39±0.03 (stat) ± 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.

We present STAR results on the elliptic flow v₂ of charged hadrons, strange and multistrange particles from sqrt[s_NN]=200 GeV Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The detailed study of the centrality dependence of v₂ over a broad transverse momentum range is presented. Comparisons of different analysis methods are made in order to estimate systematic uncertainties. To discuss the nonflow effect, we have performed the first analysis of v₂ with the Lee-Yang zero method for K_S⁰ and Λ. In the relatively low p_T region, p_T≤2 GeV/c, a scaling with m_T-m is observed for identified hadrons in each centrality bin studied. However, we do not observe v₂(p_T) scaled by the participant eccentricity to be independent of centrality. At higher p_T,2≤p_T≤6 GeV/c,v₂ scales with quark number for all hadrons studied. For the multistrange hadron Ω, which does not suffer appreciable hadronic interactions, the values of v₂ are consistent with both m_T-m scaling at low p_T and number-of-quark scaling at intermediate p_T. As a function of collision centrality, an increase of p_T-integrated v₂ scaled by the participant eccentricity has been observed, indicating a stronger collective flow in more central Au+Au collisions.

We report on the observed differences in production rates of strange and multistrange baryons in Au+Au collisions at sqrt[s_NN]=200 GeV compared to p+p interactions at the same energy. The strange baryon yields in Au+Au collisions, when scaled down by the number of participating nucleons, are enhanced relative to those measured in p+p reactions. The enhancement observed increases with the strangeness content of the baryon, and it increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at the lower collision energy sqrt[s_NN]=17.3 GeV. The previous observations are for the bulk production, while at intermediate p_T,1<p_T<4 GeV/c, the strange baryons even exceed binary scaling from p+p yields.

We find an unconventional nucleation of a low-temperature paramagnetic metal phase with a monoclinic structure from the matrix of a high-temperature antiferromagnetic insulator phase with a tetragonal structure in a strongly correlated electronic system BaCo_0.9Ni_0.1S_1.97. Such unconventional nucleation leads to a decrease in resistivity by several orders with relaxation at a fixed temperature. The novel dynamical process could arise from the competition of strain fields, Coulomb interactions, magnetic correlations, and disorders. Such competition may frustrate the nucleation, giving rise to a slow, nonexponential relaxation and “physical aging” behavior.

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ⁰ and direct π⁺π^- photoproduction in ultraperipheral relativistic heavy ion collisions at sqrt[s_NN]=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ(AuAu→Au^*Au^*ρ⁰)=530±19(stat.)±57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ⁰ transverse momentum spectrum (p_T²) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σ_inc/σ_coh=0.29±0.03(stat.)±0.08(syst.). The ratio of direct π⁺π^- to ρ⁰ production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ⁰ spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

Laser-based angle-resolved photoemission measurements with superhigh resolution have been carried out on an optimally doped Bi₂Sr₂CaCu₂O₈ high temperature superconductor. New high energy features at ∼115  meV and ∼150  meV, in addition to the prominent ∼70  meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which cannot be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.

We study the possibility of performing perfect teleportation of unknown quantum states from multiple senders to a single receiver with a previously shared stabilizer state. In the model we considered, the utilized stabilizer state is partitioned into several subsystems and then each subsystem is distributed to a distinct party. We present two sufficient conditions for a stabilizer state to achieve a given nonzero teleportation capacity with respect to a given partition plan. The corresponding teleportation protocols are also explicitly given. Interestingly, we find that even mixed stabilizer states are also useful for perfect many-to-one teleportation. Finally, our work provides a perspective from stabilizer formalism to view the standard teleportation protocol and also suggests a technique for analyzing teleportation capability of multipartite entangled states.

We consider the possibility of using stabilizer states to perform deterministic dense coding among multiple senders and a single receiver. In the model we studied, the utilized stabilizer state is partitioned into several subsystems and then each subsystem is held by a distinct party. We present a sufficient condition for a stabilizer state to be useful for deterministic distributed dense coding with respect to a given partition plan. The corresponding protocol is also constructed. Furthermore, we propose a method to partially solve a more general problem of finding the set of achievable alphabet sizes for an arbitrary stabilizer state with respect to an arbitrary partition plan. Finally, our work provides a perspective from the stabilizer formalism to view the standard dense coding protocol and also unifies several previous results in a single framework.

We report the measurement of Λ and Λ̅ yields and inverse slope parameters in d+Au collisions at sqrt[s_NN]=200 GeV at forward and backward rapidities (y=±2.75), using data from the STAR forward time projection chambers. The contributions of different processes to baryon transport and particle production are probed exploiting the inherent asymmetry of the d+Au system. Comparisons to model calculations show that baryon transport on the deuteron side is consistent with multiple collisions of the deuteron nucleons with gold participants. On the gold side, HIJING-based models without a hadronic rescattering phase do not describe the measured particle yields, while models that include target remnants or hadronic rescattering do. The multichain model can provide a good description of the net baryon density in d+Au collisions at energies currently available at the BNL Relativistic Heavy Ion Collider, and the derived parameters of the model agree with those from nuclear collisions at lower energies.

The effect of metal-molecule contacts in molecular junctions is studied based on the analysis of a statistically significant number of devices and a proposed multibarrier tunneling (MBT) model, where a metal-molecule-metal junction is divided into three individual barriers: a molecular-chain body and metal-molecule contacts on either side of molecule. Using the MBT model with the statistical analysis, we could derive and distinguish decay coefficients for contact barriers (β₁,β₂), contact-dependent and contact-independent decay coefficients (β₀ vs β_body), and specific contact resistances in terms of different molecular length and different natures of metal-molecule contacts.

Photon propagation in biological tissue is commonly described by the radiative transfer equation, while the phase function in the equation represents the scattering characteristics of the medium and has significant influence on the precision of solution and the efficiency of computation. In this work, we present a generalized Delta-Eddington phase function to simplify the radiative transfer equation to an integral equation with respect to photon fluence rate. Comparing to the popular diffusion approximation model, the solution of the integral equation is highly accurate to model photon propagation in the biological tissue over a broad range of optical parameters. This methodology is validated by Monte Carlo simulation.

We determine rapidity asymmetry in the production of charged pions, protons, and antiprotons for large transverse momentum (p_T) for d+Au collisions at sqrt[s_NN]=200 GeV. The rapidity asymmetry is defined as the ratio of particle yields at backward rapidity (Au beam direction) to those at forward rapidity (d beam direction). The identified hadrons are measured in the rapidity regions |y|<0.5 and 0.5<|y|<1.0 for the p_T range 2.5<p_T<10  GeV/c. We observe significant rapidity asymmetry for charged pion and proton+antiproton production in both the rapidity regions. The asymmetry is larger for 0.5<|y|<1.0 than for |y|<0.5 and is almost independent of particle type. The measurements are compared to various model predictions employing multiple scattering, energy loss, nuclear shadowing, saturation effects, and recombination and also to a phenomenological parton model. We find that asymmetries are sensitive to model parameters and show model preference. The rapidity dependence of π^-/π⁺ and p̅ /p ratios in peripheral d+Au and forward neutron-tagged events are used to study the contributions of valence quarks and gluons to particle production at high p_T.

In this paper, we present a detailed investigation of the laser cooling and trapping of the Zn atom, and various schemes employing the ¹S₀-³P₀ transition, induced by nuclear magnetic moment or applied fields, as the clock transition. Using numerical simulations, the deceleration of Zn by a Zeeman slower and its capture by a magneto-optical trap (MOT) are analyzed, and the corresponding parameters are determined. The linear loss rate and the coefficient for two-body collisional loss in the MOT are discussed. To prove the feasibility of the intercombination line cooling, one-dimensional semiclassical Monte Carlo simulations are performed. Multiconfiguration Hartree-Fock and multiconfiguration Dirac-Fock approaches are employed to calculate the hyperfine-induced ¹S₀-³P₀ transition. Up to now, various schemes inducing the ¹S₀-³P₀ transition in bosonic isotopes have been proposed for alkaline-earth-metal atoms and Yb. Their applicability for Zn are investigated, and the corresponding parameters of Zn are calculated. Our results show that the Zn atom, either fermionic or bosonic, is a potential candidate for the quantum absorber used in laser-cooled neutral atomic optical frequency standard.

We report the first measurement of the opening angle distribution between pairs of jets produced in high-energy collisions of transversely polarized protons. The measurement probes (Sivers) correlations between the transverse spin orientation of a proton and the transverse momentum directions of its partons. With both beams polarized, the wide pseudorapidity (-1≤η≤+2) coverage for jets permits separation of Sivers functions for the valence and sea regions. The resulting asymmetries are all consistent with zero and considerably smaller than Sivers effects observed in semi-inclusive deep inelastic scattering. We discuss theoretical attempts to reconcile the new results with the sizable transverse spin effects seen in semi-inclusive deep inelastic scattering and forward hadron production in pp collisions.

We theoretically study the coherent interaction of a tripod-type atomic system with three laser fields, i.e., one probe and two coupling, or two probe and one coupling field. Our analytical and numerical results show that, when all three fields are on two-photon resonance, in addition to the Rabi frequencies of the coupling fields, the probe group velocities also depend on the spontaneous decay rates and the initial population distribution. So we can manipulate the probe pulses to achieve controllable ultraslow group velocities by preparing different initial atomic states. With an incoherent pump field applied, a narrow transparent window accompanied by abnormal dispersion may occur in the probe absorption spectrum, which then allows us to switch a probe pulse from ultraslow to superluminal propagation.

We present first measurements of the ϕ-meson elliptic flow (v₂(p_T)) and high-statistics p_T distributions for different centralities from sqrt[s_NN]=200  GeV Au+Au collisions at RHIC. In minimum bias collisions the v₂ of the ϕ meson is consistent with the trend observed for mesons. The ratio of the yields of the Ω to those of the ϕ as a function of transverse momentum is consistent with a model based on the recombination of thermal s quarks up to p_T∼4  GeV/c, but disagrees at higher momenta. The nuclear modification factor (R_CP) of ϕ follows the trend observed in the K_S⁰ mesons rather than in Λ baryons, supporting baryon-meson scaling. These data are consistent with ϕ mesons in central Au+Au collisions being created via coalescence of thermalized s quarks and the formation of a hot and dense matter with partonic collectivity at RHIC.

The system created in noncentral relativistic nucleus-nucleus collisions possesses large orbital angular momentum. Because of spin-orbit coupling, particles produced in such a system could become globally polarized along the direction of the system angular momentum. We present the results of Λ and Λ̅ hyperon global polarization measurements in Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV performed with the STAR detector at the BNL Relativistic Heavy Ion Collider (RHIC). The observed global polarization of Λ and Λ̅ hyperons in the STAR acceptance is consistent with zero within the precision of the measurements. The obtained upper limit, |P_Λ,Λ̅ |≤0.02, is compared with the theoretical values discussed recently in the literature.

A new method is presented for determining event-by-event fluctuations of elliptic flow, v₂, using first-order event planes. By studying the event-by-event distributions of v₂ observables and first-order event-plane observables, average flow 〈v₂〉 and event-by-event flow fluctuations can be separately determined, making appropriate allowance for the effects of finite multiplicity and nonflow. The method has been tested with Monte Carlo simulations. The connection between flow fluctuations and fluctuations of the initial-state participant eccentricity is discussed.

Wave-particle duality finds a natural application for electrons or light propagating in disordered media where coherent corrections to transport are given by two-wave interference. For scatterers with internal degrees of freedom, these corrections are observed to be much smaller than would be expected for structureless scatterers. By examining the basic example of the scattering of one photon by two spin-1/2 atoms—a case study for coherent backscattering—we demonstrate that the loss of interference strength is associated with which-path information stored by the scattering atoms.

The electronic structures of Bi₂Te₃ and Sb₂Te₃ crystals were calculated using the first-principles full-potential linearized augmented plane-wave method. We studied not only the unrelaxed crystals, which have the experimental lattice parameters and scaled atom coordinates, but also the relaxed crystals, which have the lattice parameters and scaled atom coordinates determined from theoretical structure optimizations. We found that Bi₂Te₃ has six highest valence-band edges and six lowest conduction-band edges regardless of relaxations. However, by varying structural parameters Sb₂Te₃ may undergo an electronic topological transition that the number of valence (and conduction) band edges changes between 6 and 12. Moreover, we presented the location of the band edges and the effective mass tenor parameters for electrons and holes associated with those band edges. Furthermore, we discussed the relation of the calculated electronic structures of the two crystals with the electrical properties of Bi₂Te₃∕Sb₂Te₃ superlattices.

We have searched for strangelets in a triggered sample of 61 million central (top 4%) Au+Au collisions at sqrt[s_NN]=200 GeV near beam rapidities at the STAR solenoidal tracker detector at the BNL Relativistic Heavy Ion Collider. We have sensitivity to metastable strangelets with lifetimes of order ≥0.1 ns, in contrast to limits over ten times longer in BNL Alternating Gradient Synchrotron (AGS) studies and longer still at the CERN Super Proton Synchrotron (SPS). Upper limits of a few 10^-6 to 10^-7 per central Au+Au collision are set for strangelets with mass ≳30 GeV/c².

Transport properties are systematically studied for single crystals of Cu_xTiSe₂ (0.015≤x≤0.110). Both the in-plane and out-of-plane resistivity show a wide peak due to charge density waves (CDWs) for single crystals with x≤0.025. After the CDW state is completely suppressed around x=0.055, the superconductivity is apparently enhanced by Cu doping. No superconducting transition is observed above 1.8  K for Cu_0.11TiSe₂. The anisotropy in the resistivity increases with increasing Cu content, and is nearly T independent. The CDW state has a strong effect on the Hall coefficient and thermopower. Large thermopower, comparable to that of the triangular lattice Na_xCoO₂, is observed in Cu_xTiSe₂. Intercalation of Cu induces a negative magnetoresistance due to the interaction between conducting carriers and localized magnetic moments.

The isothermal magnetoresistance (MR) with magnetic field (H) parallel and perpendicular to the ab plane is systematically studied on the single crystal Na_0.52CoO₂ with charge ordering at ∼50  K and an in-plane ferromagnetism below 25  K. The isothermal MR behavior with H‖ab plane and that with H⊥ab plane are quite different. When H‖ab plane, the MR is always negative and the in-plane ferromagnetic behavior is enhanced, while the MR with H⊥ab plane changes from negative to positive with decreasing temperature or increasing H, and the in-plane ferromagnetic behavior is suppressed. A striking feature is that the MR with H⊥ab plane shows a hysteresis behavior below 25  K, which is absent for the case of H‖ab plane. These results provide strong evidence for a spin-flop transition of small moments of Co^3.5−δ sites induced by H⊥ab plane, leading to a metamagnetic transition for small moments of Co^3.5−δ sites. Such complex magnetism suggests an unconventional superconductivity in Na_xCoO₂ system because the charge-ordering Na_xCoO₂ around x=0.5 has been considered to be the parent compound of superconductivity.