Search Results (476 total)

This research focuses on the electrokinetic motion of fullerenes suspended in liquid crystal host medium, which are investigated in the homogeneously aligned nematic liquid crystal cells driven by in-plane field. We investigated the effect of electrophoretic and dielectrophoretic forces and related parameters of the colloidal fullerenes in liquid crystals. The electrophoretic mobility, zeta potential, and critical voltage have been evaluated. Fullerenes suspended in liquid crystal medium migrated toward the positive electrode, but were pulled back in the opposite direction when the polarity was reversed especially at low frequency range (<5 Hz). At higher electric field and higher frequency ranges, the net displacement of fullerenes has been observed. We demonstrate that the dielectrophoretic force dominated the motion in the colloidal fullerenes by a proper analysis of different electrophoretic parameters. In addition, the electrodynamics of fullerenes was explained by applying the theory of the dielectrophoresis and Schwarz’s formula. We propose a model to estimate the density of fullerenes suspended in liquid crystal medium.

An anomalous abrupt drop in the electrical conductivity has been observed at the ferroelastic phase transition of a proton-irradiated system of hydrogen-bonded TlH₂PO₄. As a result of the high-resolution ³¹P NMR chemical-shift measurements, distinct changes in the atomic displacements due to the irradiation were identified in the ferroelastic and paraelastic phases. Besides, ¹H NMR spin-spin relaxation measurements revealed a change due to the irradiation in the proton dynamics at the ferroelastic phase transition, apparently accounting for the much-reduced electrical conductivity in the paraelastic phase of the irradiated system.

Some important features of the graphene physics can be reproduced by loading ultracold fermionic atoms in a two-dimensional optical lattice with honeycomb symmetry and we address here its experimental feasibility. We analyze in great detail the optical lattice generated by the coherent superposition of three coplanar running laser waves with respective angles 2π/3. The corresponding band structure displays Dirac cones located at the corners of the Brillouin zone and close to half-filling this system is well described by massless Dirac fermions. We characterize their properties by accurately deriving the nearest-neighbor hopping parameter t₀ as a function of the optical lattice parameters. Our semiclassical instanton method proves in excellent agreement with an exact numerical diagonalization of the full Hamilton operator in the tight-binding regime. We conclude that the temperature range needed to access the Dirac fermions regime is within experimental reach. We also analyze imperfections in the laser configuration as they lead to optical lattice distortions which affect the Dirac fermions. We show that the Dirac cones do survive up to some critical intensity or angle mismatches which are easily controlled in actual experiments. In the tight-binding regime, we predict, and numerically confirm, that these critical mismatches are inversely proportional to the square root of the optical potential strength. We also briefly discuss the interesting possibility of fine tuning the mass of the Dirac fermions by controlling the laser phase in an optical lattice generated by the incoherent superposition of three coplanar independent standing waves with respective angles 2π/3.

In contrast to the previous reports that the divalent perovskite SrCrO₃ was believed to be cubic structure and nonmagnetic metal, recent measurements suggest coexistence of majority tetragonally distorted weak antiferromagnetic phase and minority nonmagnetic cubic phase. Within the local (spin) density approximation [L(S)DA] our calculations confirm that a slightly tetragonally distorted phase indeed is energetically favored. Using the correlated band theory method (LDA+Hubbard U) as seems to be justified by the unusual behavior observed in SrCrO₃, above the critical value U_c=4 eV only the distorted phase undergoes an orbital-ordering transition, resulting in t_2g²→d_xy¹(d_xzd_yz)¹ corresponding to the filling of the d_xy orbital but leaving the other two degenerate. The Fermi surfaces of the cubic phase are simple with nesting features, although the nesting wave vectors do not correlate with known data. This is not uncommon in perovskites; the strongly directional d−d bonding often leads to boxlike Fermi surfaces and either the nesting is not strong enough or the matrix elements are not large enough to promote instabilities. Fixed spin moment calculations indicate the cubic structure is just beyond a ferromagnetic Stoner instability [IN(0)≈1.1] in L(S)DA and that the energy is unusually weakly dependent on the moment out to 1.5μ_B/Cr (varying only by 11 meV/Cr), reflecting low-energy long-wavelength magnetic fluctuations. We observe that this system shows strong magnetophonon coupling (change in Cr local moment is ∼7.3μ_B/Å) for breathing phonon modes.

Dür [Phys. Rev. Lett. 87, 230402 (2001)] constructed N-qubit bound entangled states which violate a Bell inequality for N≥8, and his result was recently improved by showing that there exists an N-qubit bound entangled state violating the same Bell inequality if and only if N≥6 [Phys. Rev. A 79, 032309 (2009)]. On the other hand, it has been also shown that the states which Dür considered violate Bell inequalities different from the inequality for N≥6. In this paper, by employing different forms of Bell inequalities, in particular, a specific form of Bell inequalities with M settings of the measuring apparatus for sufficiently large M, we prove that there exists an N-qubit bound entangled state violating the M-setting Bell inequality if and only if N≥4.

We study the cyclotron maser instability (CMI) driven by an energetic ring-beam distribution by a particle simulation to explain possible generation mechanisms of intense radiation phenomena observed in space. The main objective is to understand the nonlinear processes that control saturation of the emission process. Our study reveals new issues that have been overlooked in past literature. It is found that electrostatic wave modes excited by the electron beam instability compete with the electromagnetic waves excited by the CMI. Nonlinear effects of these electrostatic modes tend to redistribute the energy of the energetic electrons and make the physics more complicated. The CMI can be much less effective in a realistic case than it is anticipated theoretically.

We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (γ-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I_AA, in central Au+Au collisions, is 0.32±0.12^stat±0.09^syst for hadrons of 3<p_T^h<5 in coincidence with photons of 5<p_T^γ<15 GeV/c. The suppression is comparable to that observed for high-p_T single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z_T≡p_T^h/p_T^γ, as expected for a measurement of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

The momentum distribution of electrons from semileptonic decays of charm and bottom quarks for midrapidity |y|<0.35 in p+p collisions at sqrt[s]=200  GeV is measured by the PHENIX experiment at the Relativistic Heavy Ion Collider over the transverse momentum range 2<p_T<7  GeV/c. The ratio of the yield of electrons from bottom to that from charm is presented. The ratio is determined using partial D/D̅ →e^±K^∓X (K unidentified) reconstruction. It is found that the yield of electrons from bottom becomes significant above 4  GeV/c in p_T. A fixed-order-plus-next-to-leading-log perturbative quantum chromodynamics calculation agrees with the data within the theoretical and experimental uncertainties. The extracted total bottom production cross section at this energy is σ_bb̅ =3.2_-1.1^+1.2(stat)_-1.3^+1.4(syst)μb.

Although superresolution can be successfully obtained by negative index materials, the unavoidable losses ultimately limits image resolution. Using the near-field active phase-correction method, we theoretically predict the significant enhancement of both superresolution and transmission in a realistic lossy superlens system. In a SiC superlens system, resolvable separation between two slits significantly improves from λ/4.2 to λ/14.2 and the transmission increases 30.5 times compared to the conventional index match method.

X-ray diffraction of sphingomyelin-dihydrocholesterol (SM-DChol) monolayers revealed short-ranged (∼25  Å) 2D ordering. These nanoclusters show two distinct regions: below the cusp point of the phase diagram (35 mol% DChol), a constant d spacing was observed; above the cusp, the d spacing increases linearly with DChol in accordance to Vegard’s law for binary alloys. The components in this lipidic alloy are thus a 65∶35 SM-DChol entity and excess DChol. Reflectivity data further support the emergence above the cusp of an uncomplexed DChol population with greater vertical mobility.

The double helicity asymmetry in neutral pion production for p_T=1 to 12  GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, ΔG, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of μ²=4  GeV² a next to leading order QCD analysis gives ΔG^[0.02,0.3]=0.2, with a constraint of -0.7<ΔG^[0.02,0.3]<0.5 at Δχ²=9 (∼3σ) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the ΔG constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.

In three-dimensional cluster morphology multilayer Si/SiGe nanostructures, we find an anticorrelation between photoluminescence (PL) originating from SiGe clusters and the PL associated with electron-hole droplets (EHDs) localized within nanometer-thick Si separating layers. We show that Auger processes eject holes from SiGe clusters and facilitate the exciton/EHD phase transition in the Si layers. An unusual N-shaped SiGe cluster PL decay curve is attributed to the reverse EHD/electron-hole gas phase transition and carrier redistribution between Si spacer layers and SiGe clusters.

Recently, it was shown that in inclusive B→X_sℓ⁺ℓ^- decay, an angular decomposition provides three independent (q² dependent) observables. A strategy was formulated to extract all measurable Wilson coefficients in B→X_sℓ⁺ℓ^- from a few simple integrals of these observables in the low q² region. The experimental measurements in the low q² region require a cut on the hadronic invariant mass, which introduces a dependence on nonperturbative b quark distribution functions. The associated hadronic uncertainties could potentially limit the sensitivity of these decays to new physics. We compute the nonperturbative corrections to all three observables at leading and subleading order in the power expansion in Λ_QCD/m_b. We find that the subleading power corrections give sizeable corrections, of order -5% to -10% depending on the observable and the precise value of the hadronic mass cut. They cause a shift of order -0.05  GeV² to -0.1  GeV² in the zero of the forward-backward asymmetry.

In the standard slow-roll inflationary cosmology, quantum fluctuations in a single field, the inflaton, generate approximately Gaussian primordial density perturbations. At present, the bispectrum and trispectrum of the density perturbations have not been observed and the probability distribution for these perturbations is consistent with Gaussianity. However, Planck satellite data will bring a new level of precision to bear on this issue, and it is possible that evidence for non-Gaussian effects in the primordial distribution will be discovered. One possibility is that a trispectrum will be observed without evidence for a nonzero bispectrum. It is not difficult for this to occur in inflationary models where quantum fluctuations in a field other than the inflaton contribute to the density perturbations. A natural question to ask is whether such an observation would rule out the standard scenarios. We explore this issue and find that it is possible to construct single-field models in which inflaton-generated primordial density perturbations have an observable trispectrum, but a bispectrum that is too small to be observed by the Planck satellite. However, an awkward fine-tuning seems to be unavoidable.

We study the pattern evolution of pre-rippled Au(001) during sputtering by an ion beam that is incident perpendicular to the initial ripple in azimuth at a grazing angle. Prepatterned ripples decay exponentially with time and new ripples develop only after extended flat areas form along the crossing-ion beams. Hence, the superposition of the initial and new ripple patterns does not occur. The kinetic behaviors of new ripples growing on pre-rippled Au(001) by the crossing-ion beams are distinct from those on initially flat Au(001). When comparing the pre-rippled surface to the initially flat surface, the morphological evolution is substantially influenced by enhanced nonlinear effects such as redeposition.

Using high-resolution synchrotron x-ray powder diffraction we have investigated the structural phase transitions and equations of state of titanium dioxide (TiO₂) under high pressure before and after heating at high temperature. The phase sequence we observe experimentally is as follows: rutile (RT)→columbite (CB)→baddeleyite (MI)→orthorhombic I (OI)→orthorhombic II (OII). The equations of state as determined from our experiments are consistent with previous measurements and computations. The only exception is the OII phase for which we find a significantly lower room-pressure bulk modulus (K₀) of 312 (±34) GPa and room-pressure volume (V₀) of 25.28 (±0.35) ų as compared to previous experiments. We find that the volume decreases across the OI→OII phase transition at room temperature by ∼8.3%, in very good agreement with our static first-principles calculations which predict volume changes of 8.2% and 7.6% for local-density approximation and generalized gradient approximation, respectively. This volume collapse is significantly higher than previously determined but consistent with the volume decrease observed in other dioxides across this transition.

Cisplatin has been known as an anticancer drug for a long time. It is therapeutically active upon binding to DNA. A double-bound cisplatin bends DNA into a localized kink. We model the elastic properties of cisplatin-DNA adducts at moderate tension (<6 pN). It is shown that from the mechanical point of view the action of cisplatin can be revealed by reduced persistence length. We derived two expressions for the persistence length, which apply in the linear-response and the strong-force regimes, respectively. Experimental data for DNA adducts stretched by magnetic tweezers are consistently fitted by these expressions. This allows us to estimate the degree of platination at various salt concentrations.

We demonstrate accurate single-qubit control in an ensemble of atomic qubits trapped in an optical lattice. The qubits are driven with microwave radiation, and their dynamics tracked by optical probe polarimetry. Real-time diagnostics is crucial to minimize systematic errors and optimize the performance of single-qubit gates, leading to fidelities of 0.99 for single-qubit π rotations. We show that increased robustness to large, deliberately introduced errors can be achieved through the use of composite rotations. However, during normal operation the combination of very small intrinsic errors and additional decoherence during the longer pulse sequences precludes any significant performance gain in our current experiment.

The PHENIX experiment presents results from the RHIC 2006 run with polarized p+p collisions at sqrt[s]=62.4  GeV, for inclusive π⁰ production at midrapidity. Unpolarized cross section results are measured for transverse momenta p_T=0.5 to 7  GeV/c. Next-to-leading order perturbative quantum chromodynamics calculations are compared with the data, and while the calculations are consistent with the measurements, next-to-leading logarithmic corrections improve the agreement. Double helicity asymmetries A_LL are presented for p_T=1 to 4  GeV/c and probe the higher range of Bjorken x of the gluon (x_g) with better statistical precision than our previous measurements at sqrt[s]=200  GeV. These measurements are sensitive to the gluon polarization in the proton for 0.06<x_g<0.4.

The ability to store multiple optical modes in a quantum memory allows for increased efficiency of quantum communication and computation. Here we compute the multimode capacity of a variety of quantum memory protocols based on light storage in ensembles of atoms. We find that adding a controlled inhomogeneous broadening improves this capacity significantly.

Cisplatin was incidentally discovered to suppress cell division and became one of the most successful antitumor drugs. It is therapeutically active upon binding to DNA and locally kinking it. We demonstrate that after a bimodal modeling, the degree of platination of a single DNA molecule can be consistently and reliably estimated from elasticity measurements performed with magnetic tweezers. We predicted and measured for the first time two separate persistence lengths of kinked DNA at high and low tensions. We also directly observed that the degree of platination of DNA strongly depends on the concentration of sodium chloride as required for cisplatin’s intracellular activity. Our study shows that micromanipulation techniques accurately reveal the degree of chemical modification of DNA which can be used for a new type of structure-sensitive biosensors.

Proton beams generated from thin aluminum and Mylar foil targets that are irradiated by a 30  fs Ti:sapphire laser pulse with an intensity of 2.2×10¹⁸  W∕cm² were investigated. Protons from the Mylar targets were observed to have an energy higher by a factor of 2 and were higher in number by an order of magnitude as compared with those generated from the aluminum targets. The maximum proton energy of 1.3±0.12  MeV obtained from the Mylar target was found to be similar with previous observations that used laser pulses with different intensities. To address the anomalous behavior of the maximum proton energy for plastic targets, an acceleration model is proposed. In this model, the protons are accelerated by a resistively induced electric field in the front of the target, which can account for the experimental observations.

To study the microscopic electronic and magnetic interactions in the substoichiometric iron chalcogenide FeSe_1−x which is observed to superconduct at x≈1 / 8 up to T_c=27 K, we use first principles methods to study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a ferrimagnetic cluster of eight Fe atoms, which for the ordered x=1 / 8 alloy leads to half-metallic conduction. Similar magnetic clusters are obtained for FeTe_1−x and for BaFe₂As₂ with an As vacancy although neither of these are half-metallic. Based on fixed spin-density results, we suggest the low-energy excitations in FeSe_1−x are antiparamagnonlike with short correlation length.