Microrheology of the liquid-solid transition during gelation

The viscoelastic properties of physical and chemical polymer gels are characterized through the liquid-solid transition using particle tracking microrheology. Measurements of the probe particle mean-squared displacement are shifted as the extent of gelation increases to generate master curves. From the shift factors, we determine the gel point and critical scaling exponents. Both systems exhibit a critical relaxation exponent n approximately 0.6, where G' approximately G' approximately omega n for the incipient gel, consistent with the Rouse model of dynamic scaling in the percolation universality class.     

Collective mode in a superconductor with mixed-symmetry order parameter components

We consider a superconducting state with mixed-symmetry order parameter components, e.g., d+is or d+id(') with d(') = d(xy). We argue for the existence of a new orbital magnetization mode which corresponds to oscillations of relative phase straight phi between two components around an equilibrium value of straight phi = pi / 2. It is similar to the "clapping" mode in superfluid 3He-A. We estimate the frequency of this mode omega(0)(B,T) depending on the field and temperature for the specific case of magnetic field induced d(') = d(xy) state. This mode is tunable with a magnetic field with omega(0)(B,T) approximately BDelta(0), where Delta(0) is the magnitude of the d-wave order parameter. We also estimate the velocity s(B,T) of this mode.     

Wetting and molecular orientation of 8CB on silicon substrates

The wetting properties of 8CB ( 4(')-n-octyl-4-cyanobiphenyl) on silicon wafers have been studied with scanning polarization force microscopy (SPFM). Layer-by-layer spreading of 8CB droplets is observed. With the help of the surface potential mapping capability of SPFM, we found that the molecular dipole of the first monolayer of 8CB is parallel to the surface. A layer of nearly vertical molecular dimers on top of the monolayer has an associated surface potential of 40 mV, which is attributed to a distortion of the dimer. The dimer distortion propagates to the subsequent smectic bilayers, producing an additional 7 mV potential increase in the second layer, 2 mV on the third, and approximately 1 mV on the fourth.     

Phase transitions in 8CB liquid crystal confined to a controlled-pore glass: Deuteron NMR and small angle X-ray scattering studies

We present results of the deuteron nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) study of ordering and phase transition behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG) with nontreated and silanes-treated pore surfaces. The deuteron NMR spectra allowed to determine the degree of nematic liquid crystal ordering and also provided an indirect information on the confined 8CB smectic ordering via its influence on the nematic ordering. For the smectic phase these data are supplemented with measurements of the temperature dependence of the first-order SAXS diffraction pattern. The NMR results indicate that the average nematic and smectic order parameters of 8CB in the nontreated CPG are only weakly perturbed by the confinement. The SAXS data further suggest that in confined 8CB for both nontreated and silane-treated CPG a domainlike pattern appears in accordance with the Imry-Ma theorem.     

Study of exclusive radiative B meson decays

We have studied exclusive, radiative B meson decays to charmless mesons in 9.7x10(6) B&Bmacr; decays accumulated with the CLEO detector. We measure B(B0-->K(*0)(892)gamma) = (4.55(+0.72)(-0. 68)+/-0.34)x10(-5) and B(B+-->K(*+)(892)gamma) = (3.76(+0.89)(-0. 83)+/-0.28)x10(-5). We have searched for CP asymmetry in B-->K(*)(892)gamma decays and measure A(CP) = +0.08+/-0.13+/-0.03. We report the first observation of B-->K(*)(2)(1430)gamma decays with a branching fraction of (1.66(+0.59)(-0.53)+/-0.13)x10(-5). No evidence for the decays B-->rhogamma and B0-->omegagamma is found and we limit B(B-->(rho/omega)gamma)/B(B-->K(*)(892)gamma)<0.32 at 90% C.L.     

Electrophoretic microrheology in a dilute lamellar phase of a nonionic surfactant

We measured the complex electrophoretic mobility mu(*)(omega) of nanometer-sized particles dispersed in a lyotropic lamellar phase, and observed two relaxation processes corresponding to the two characteristic lengths of lamellar structure. Faster relaxation is caused by the distortion field of lamellar phase induced by the colloidal particles, and slower relaxation is presumably due to the defects in lamellar structure. Since the dynamic transport property is strongly influenced by the microscopic circumstances as shown in this paper, this method is referred to as electrophoretic microrheology.     

Oxygen-isotope effect on the In-plane penetration depth in underdoped La2-xSrxCuO4 single crystals

We report measurements of the oxygen-isotope effect (OIE) on the in-plane penetration depth lambda(ab)(0) in underdoped La2-xSrxCuO4 single crystals. A highly sensitive magnetic torque sensor with a resolution of Deltatau approximately 10(-12) N m was used for the magnetic measurements on microcrystals with a mass of approximately 10 &mgr;g. The OIE on lambda(-2)(ab)(0) is found to be -10(2)% for x = 0.080 and -8(1)% for x = 0.086. It arises mainly from the oxygen-mass dependence of the in-plane effective mass m(*)(ab). The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.     

Critical spin dynamics of the 2D quantum Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2

We report a neutron scattering study of the long-wavelength dynamic spin correlations in the model two-dimensional S = 1/2 square lattice Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2. The characteristic energy scale, omega(0)(T/J), is determined by measuring the quasielastic peak width in the paramagnetic phase over a wide range of temperature ( 0.2 less similarT/J less similar0.7). The obtained values for omega(0)(T/J) agree quantitatively between the two compounds and also with values deduced from quantum Monte Carlo simulations. The combined data show scaling behavior, omega approximately xi(-z), over the entire temperature range with z = 1.0(1), in agreement with dynamic scaling theory.     

Magneto-oscillations due to electron-electron interactions in the ac conductivity of a two-dimensional electron gas

Electron-electron interactions give rise to the correction, deltasigma(int)(omega), to the ac magnetoconductivity, sigma(omega), of a clean 2D electron gas that is periodic in omega_(c)(-1), where omega_(c) is the cyclotron frequency. Unlike conventional harmonics of the cyclotron resonance, which are periodic with omega, this correction is periodic with omega(3/2). Oscillations in deltasigma(int)(omega) develop at low magnetic fields, omega_(c)相似文献   

Rotational diffusion microrheology

Examining the rotational diffusion of a microparticle suspended in a soft material opens up exciting new opportunities for locally probing the frequency-dependent linear viscoelastic shear modulus, G*(omega). We study the one-dimensional rotational diffusion of a wax microdisk in an aqueous polymer entanglement network using light streak tracking. By measuring the disk's time-dependent mean square angular displacement, , we predict the polymer solution's G*(omega) using a rotational generalized Stokes-Einstein relation. The good agreement of the predicted modulus with mechanical measurements confirms this new microrheological approach.     

Anomalous peak in the superconducting condensate density of cuprate high- T(c) superconductors at a unique doping state

The doping dependence of the ratio of the superconducting condensate density to the effective mass, n(o)(s)/m(*)(ab), was studied in detail by muon-spin rotation for Y(0.8)Ca(0.2)Ba(2)(Cu(1-z)Zn(z))(3)O(7-delta) and Tl(0.5-y)Pb(0.5+y)Sr(2)Ca(1-x)Y(x)Cu(2)O(7). Our data show that n(o)(s)/m(*)(ab) exhibits a peak at a unique doping state in the overdoped regime. Its position coincides with the critical doping state, where the normal state pseudogap was reported to appear and to deplete the electronic density of states. This finding implies that the pseudogap primarily arises from a change in the electronic ground state rather than from thermal fluctuations.     

Excitations of one-dimensional Bose-Einstein condensates in a random potential

We examine bosons hopping on a one-dimensional lattice in the presence of a random potential at zero temperature. Bogoliubov excitations of the Bose-Einstein condensate formed under such conditions are localized, with the localization length diverging at low frequency as l(omega) approximately 1/omega(alpha). We show that the well-known result alpha=2 applies only for sufficiently weak random potential. As the random potential is increased beyond a certain strength, alpha starts decreasing. At a critical strength of the potential, when the system of bosons is at the transition from a superfluid to an insulator, alpha=1. This result is relevant for understanding the behavior of the atomic Bose-Einstein condensates in the presence of random potential, and of the disordered Josephson junction arrays.     

Dynamical response of quantum spin-glass models at t = 0

We study the behavior of two archetypal quantum spin glasses at T = 0 by exact diagonalization techniques: the random Ising model in a transverse field and the random Heisenberg model. The behavior of the dynamical spin response is obtained in the spin-glass ordered phase. In both models it is gapless and has the general form chi(")(omega) = qdelta(omega)+chi(")(reg)(omega), with chi(")(reg)(omega) approximately omega for the Ising and chi(")(reg)(omega) approximately const for the Heisenberg, at low frequencies. The method provides new insight to the physical nature of the low-lying excitations.     

Manifestation of random phase in a finite ensemble of a turbulent fluid

A field can be specified by a set of spectral coefficients, c(j)(t), relative to an expansion basis. In fluid turbulence, one often assumes the condition that if one were to extract data from an ensemble of realizations, one would find that the average, , would vanish unless i = i('), as in the case of a homogeneous ensemble in which the i's represent wave vectors. We analytically treat an extension to the case in which one has a larger number of coefficients than realizations, introducing intrinsic linear dependences that vitiate the stated condition.     

Structural investigations on an amorphous Se90Te10 alloy produced by mechanical alloying using EXAFS, cumulant expansion and RMC simulations

We investigated an amorphous Se(90)Te(10) alloy produced by mechanical alloying using two different approaches. First, we used extended x-ray absorption fine structure (EXAFS) spectroscopy and the cumulant expansion method using the Einstein model for the temperature dependence of the cumulants to obtain the cumulants C(*)(1), C(*)(2), and C(*)(3). From these, we found information about the structure of the alloy as well as the thermal and structural disorder, anharmonicity of the effective interatomic pair potentials, thermal expansion of the Se-Se and Se-Te bonds and asymmetry of the partial distribution functions g(Se-Se)(r) and g(Se-Te)(r). The cumulants C(*)(1), C(*)(2), and C(*)(3) also allowed us to reconstruct the g(EXAFS)(ij)(r,T) functions from EXAFS. Then, we made reverse Monte Carlo (RMC) simulations using the total structure factor S(K) obtained from synchrotron x-ray diffraction and the EXAFS oscillations χ(k) on the Se K edge as input data to obtain the g(RMC)(ij)(r) functions. Both methods furnished very similar g(ij)(r) functions, and the structural data obtained from them were also very similar. The results obtained from both methods showed the presence of Se-Te pairs indicating that there is alloying at the atomic level. In addition, we could not find any evidence of the presence of Te clusters in the alloy.     

Observation of in-medium modifications of the omega meson

The photoproduction of omega mesons on nuclei has been investigated using the Crystal Barrel/TAPS experiment at the ELSA tagged photon facility in Bonn. The aim is to study possible in-medium modifications of the omega meson via the reaction gamma + A --> omega + X --> pi(0)gamma + X('). Results obtained for Nb are compared to a reference measurement on a LH2 target. While for recoiling, long-lived mesons (pi(0), eta, and eta;(')), which decay outside of the nucleus, a difference in the line shape for the two data samples is not observed, we find a significant enhancement towards lower masses for omega mesons produced on the Nb target. For momenta less than 500 MeV/c an in-medium omega meson mass of M(medium) = [722(+4)(-4)(stat)+35-5(syst)] MeV/c(2) has been deduced at an estimated average nuclear density of 0.6rho(0).     

Superfluid transition in a rotating fermi gas with resonant interactions

We study a rotating atomic Fermi gas near a narrow s-wave Feshbach resonance in a uniaxial trap with frequencies Omega perpendicular, Omega z. We predict the upper-critical angular velocity, omega c2(delta,T), as a function of temperature T and detuning delta across the BEC-BCS crossover. The suppression of superfluidity at omega c2 is distinct in the BCS and BEC regimes, with the former controlled by depairing and the latter by the dilution of bosonic molecules. At low T and Omega z < Omega perpendicular, in the BCS and crossover regimes of 0 less similar delta less similar delta c, omega c2 is implicitly given by [formula: see text], vanishing as omega c2 approximately Omega perpendicular(1 - delta/delta c)(1/2) near [formula: see text] (with Delta the BCS gap and gamma the resonance width), and extending the bulk result variant Planck's over 2pi omega c2 approximately 2Delta2/epsilonF to a trap. In the BEC regime of delta < 0 we find omega c2-->Omega perpendicular-, where molecular superfluidity is destroyed only by large quantum fluctuations associated with comparable boson and vortex densities.     

Self-organized periodic photonic structure in a nonchiral liquid crystal

A hybrid-aligned cell of the smectic A liquid material 8CB gives two stable director configurations, one of which is periodic and gives strong diffraction of light. This photonic lattice director profile arises from "frustration" caused by conflicting constraints imposed by the boundary conditions and the constant amplitude smectic density wave. A model is proposed which accords well with the experiment results, predicting correctly the dependence of the periodicity on the cell thickness and reproducing optical polarization microscopy results.