Modeling diffusion of adsorbed polymer with explicit solvent

Computer simulations of a polymer chain of length N strongly adsorbed at the solid-liquid interface in the presence of explicit solvent are used to delineate the factors affecting the N dependence of the polymer lateral diffusion coefficient, D(||). We find that surface roughness has a large influence, and D(||) scales as D(||) approximately N(-x), with x approximately 3/4 and x approximately 1 for ideal smooth and corrugated surfaces, respectively. The first result is consistent with the hydrodynamics of a "particle" of radius of gyration R(G) approximately N(nu) (nu=0.75) translating parallel to a planar interface, while the second implies that the friction of the adsorbed chains dominates. These results are discussed in the context of recent measurements.     

Two-flux composite fermion series of the fractional quantum Hall states in strained Si

Magnetotransport properties are investigated in a high-mobility two-dimensional electron system in the strained Si quantum well of a (100) Si(0.75)Ge(0.25)/Si/Si(0.75)Ge0.25 heterostructure, at temperatures down to 30 mK and in magnetic fields up to 45 T. We observe around nu=1/2 the two-flux composite fermion (CF) series of the fractional quantum Hall effect (FQHE) at nu=2/3, 3/5, 4/7, and at nu=4/9, 2/5, 1/3. Among these FQHE states, the nu=1/3, 4/7, and 4/9 states are seen for the first time in the Si/SiGe system. Interestingly, of the CF series, the 3/5 state is weaker than the nearby 4/7 state and the 3/7 state is conspicuously missing, resembling the observation in the IQHE regime that the nu=3 is weaker than the nearby nu=4 state. Our results can be quantitatively understood in the picture of CF's with the valley degree of freedom.     

Spectroscopic evidence for the localization of Skyrmions near nu = 1 as T --> 0

Optically pumped nuclear magnetic resonance measurements of 71Ga spectra were carried out in an n-doped GaAs/Al(0.1)Ga0.9As multiple quantum well sample near the integer quantum Hall ground state nu = 1. As the temperature is lowered (down to T approximately 0.3 K), a "tilted plateau" emerges in the Knight shift data, which is a novel experimental signature of quasiparticle localization. The dependence of the spectra on both T and nu suggests that the localization is a collective process. The frozen limit spectra appear to rule out a 2D lattice of conventional Skyrmions.     

Criticality in the (2+1)-dimensional compact Higgs model and fractionalized insulators

We use a novel method of computing the third moment M3 of the action of the (2+1)-dimensional compact Higgs model in the adjoint representation with q=2 to extract correlation length and specific heat exponents nu and alpha without invoking hyperscaling. Finite-size scaling analysis of M3 yields the ratios (1+alpha)/nu and 1/nu separately. We find that alpha and nu vary along the critical line of the theory, which however exhibits a remarkable resilience of Z2 criticality. We propose this novel universality class to be that of the quantum phase transition from a Mott-Hubbard insulator to a charge-fractionalized insulator in two spatial dimensions.     

63Cu nuclear relaxation in the quantum critical point system CeCu5.9Au0.1

63Cu NQR measurements of the 63Cu T1 are reported for the quantum critical point system CeCu5.9Au0.1 over temperatures ranging from 0.1 up to 4.2 K. Below approximately 1 K the magnetization recovery exhibits a stable, nonexponential decay function which we believe signals the onset of 2D quantum critical fluctuations, as has been noted in the literature. We find T1(-1) is proportional to T0.75 for the region T < 1 K. The observed temperature dependence is in agreement with a phenomenological model of non-Fermi liquid behavior based on the uniform susceptibility but is inconsistent with calculations based on susceptibility peaks identified via neutron scattering experiments.     

Spin-liquid phase in a spin-1/2 quantum magnet on the kagome lattice

We study a model of hard-core bosons with short-range repulsive interactions at half filling on the kagome lattice. Using quantum Monte Carlo numerics, we find that this model shows a continuous superfluid-insulator quantum phase transition, with exponents z=1 and nu approximately 0.67(5). The insulator, I*, exhibits short-ranged density and bond correlations, topological order, and exponentially decaying spatial vison correlations, all of which point to a Z2 fractionalized phase. We estimate the vison gap in I* from the temperature dependence of the energy. Our results, together with the equivalence between hard-core bosons and S=1/2 spins, provide compelling evidence for a spin-liquid phase in an easy-axis spin-1/2 model with no special conservation laws.     

Improved limits on nu(e) emission from mu+ decay

We investigated mu(+) decays at rest produced at the ISIS beam stop target. Lepton flavor (LF) conservation has been tested by searching for nu(e) via the detection reaction p(nu(e),e(+))n. No nu(e) signal from LF violating mu(+) decays was identified. We extract upper limits of the branching ratio (BR) for the LF violating decay mu(+)-->e(+)+nu(e)+nu(-) compared to the standard model (SM) mu(+)-->e(+)+nu(e)+nu(mu) decay: BR<0.9(1.7) x 10(-3) (90% C.L.) depending on the spectral distribution of nu(e) characterized by the Michel parameter rho=0.75(0.0). These results improve earlier limits by one order of magnitude and restrict extensions of the SM in which nu(e) emission from mu(+) decay is allowed with considerable strength. The decay mu(+)-->e(+)+nu(e)+nu(mu) often proposed as a potential source for the nu(e) signal observed in the LSND experiment can be excluded.     

Microwave resonance of the bubble phases in 1/4 and 3/4 filled high Landau levels

We have measured the diagonal conductivity, sigma(xx), in the microwave regime of an ultrahigh mobility two dimensional electron system. We find a sharp resonance in Re[sigma(xx)] versus frequency when nu>4 and the partial filling of the highest Landau level, nu(*), is approximately 1/4 or 3/4 and temperatures <0.1 K. The resonance appears for a range of nu(*) from 0.20 to 0.38 and again from 0.64 to 0.80. The peak frequency f(pk) changes from approximately 500 to approximately 150 MHz as nu(*)=1/2 is approached. This range of f(pk) shows no dependence on nu where the resonance is observed. The quality factor, Q, of the resonance is maximum at about nu(*)=0.25 and 0.74. We interpret the resonance as due to a pinning mode of the bubble phase crystal.     

New upper limit on the total neutrino mass from the 2 degree field galaxy redshift survey

We constrain f(nu) identical with Omega(nu)/Omega(m), the fractional contribution of neutrinos to the total mass density in the Universe, by comparing the power spectrum of fluctuations derived from the 2 Degree Field Galaxy Redshift Survey with power spectra for models with four components: baryons, cold dark matter, massive neutrinos, and a cosmological constant. Adding constraints from independent cosmological probes we find f(nu)<0.13 (at 95% confidence) for a prior of 0.1相似文献   

Quantum phase transitions of hard-core bosons in background potentials

We study the zero temperature phase diagram of hard-core bosons in two dimensions subjected to three types of background potentials: staggered, uniform, and random. In all three cases there is a quantum phase transition from a superfluid (at small potential) to a normal phase (at large potential), but with different universality classes. As expected, the staggered case belongs to the XY universality, while the uniform potential induces a mean field transition. The disorder driven transition is clearly different from both; in particular, we find z approximately 1.4, nu approximately 1, and beta approximately 0.6.     

Absorption spectrum around nu=1: evidence for a small-size Skyrmion

We measure the absorption spectrum of a two-dimensional electron system (2DES) in a GaAs quantum well in the presence of a perpendicular magnetic field. We focus on the absorption spectrum into the lowest Landau level around nu=1. We find that the spectrum consists of bound electron-hole complexes, trionlike and excitonlike. We show that their oscillator strength is a powerful probe of the 2DES spatial correlations. We find that near nu=1 the 2DES ground state consists of Skyrmions of small size (a few magnetic lengths).     

Can a non-symmetric metric mimic NCQFT in $e^ + e^- \to \gamma \gamma$?

In the non-symmetric gravitational theory (NGT) the space-time metric departs from the flat-space Minkowski form in such a way that it is no longer symmetric, i.e. . We find that in the most conservative such scenario coupled to quantum field theory, which we call minimally non-symmetric quantum field theory (MNQFT), there are experimentally measurable consequences similar to those from non-commutative quantum field theory (NCQFT). This can be expected from the Seiberg-Witten map which has recently been interpreted as equating gauge theories on non-commutative space-times with those in a field-dependent gravitational background. In particular, in scattering processes such as the pair annihilation , both theories make the same striking prediction that the azimuthal cross section oscillates in . However the predicted number of oscillations differs in the two theories: MNQFT predicts between one and four, whereas NCQFT has no such restriction.Received: 17 May 2004, Revised: 10 September 2004, Published online: 18 November 2004This work was supported by the Department of Physics at Tsinghua University and the Chinese Academy of Sciences.     

Reorientation of anisotropy in a square well quantum hall sample

We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are isotropic in perpendicular magnetic field but become strongly anisotropic at nu = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, B(ip) approximately 2.5 T, with the easy-current direction parallel to B(ip) but rotates by 90 degrees at B(ip) approximately 10 T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model.     

New phase transition between partially and fully polarized quantum Hall states with charge and spin gaps at nu = 2/3

The average electron spin polarization Rho of a two-dimensional electron gas confined in GaAs/GaAlAs multiple quantum wells was measured by NMR near the fractional quantum Hall state with filling factor nu = 2/3. Above this filling factor (2/3< or = nu < 0.85), a strong depolarization is observed corresponding to two spin flips per additional flux quantum. The most remarkable behavior of the polarization is observed at nu = 2/3, where a quantum phase transition from a partially polarized (Rho approximately 3/4) to a fully polarized (Rho = 1) state can be driven by increasing the ratio between the Zeeman and the Coulomb energy above a critical value eta(c) = Delta(Z)/Delta(C) = 0.0185.     

KamLAND and solar antineutrino spectrum

We use the recent KamLAND observations to predict the solar antineutrino spectrum at some confidence limits. We find a scaling of the antineutrino probability with respect to the magnetic field profile—in the sense that the same probability function can be reproduced by any profile with a suitable peak field value—that can be utilized to obtain the general shape ofthe solar antineutrino spectrum. This scaling and the upper bound on the solar antineutrino event rate, which can be derived from the data, lead to: 1) an upper bound on the solar antineutrino flux and 2) the prediction of their energy spectrum. We get \(\phi _{\bar \nu } < 3.8 \times 10^{ - 3} \phi (^8 B)\) or \(\phi _{\bar \nu } < 5.5 \times 10^{ - 3} \phi (^8 B)\) at 95% C.L., assuming Gaussian or Poissonian statistics, respectively. For 90% C.L., these become \(\phi _{\bar \nu } < 3.4 \times 10^{ - 3} \phi (^8 B)\) and \(\phi _{\bar \nu } < 4.9 \times 10^{ - 3} \phi (^8 B)\). This provides an improvement by a factor of 3–5 with respect to the existing bounds. These limits are quite general and independent of the detailed structure of the magnetic field in the solar interior.     

Evidence for two different solid phases of two-dimensional electrons in high magnetic fields

We have observed two different rf resonances in the frequency dependent real diagonal conductivity of very high quality two-dimensional electron systems in the high magnetic field insulating phase and interpret them as coming from two different pinned electron solid phases (labeled as "A" and "B"). The "A" resonance is observable for Landau level filling nu<2/9 [reentrant around the nu=1/5 fractional quantum Hall effect (FQHE)] and then crosses over to the different "B" resonance which dominates at sufficiently low nu. Moreover, the "A" resonance is found to show dispersion with respect to the size of the transmission line, indicating that the "A" phase has a large correlation length. We suggest that quantum correlations such as those responsible for FQHE may play an important role in giving rise to such different solids.     

Proposal for a chaotic ratchet using cold atoms in optical lattices

We investigate a new type of quantum ratchet which may be realized by cold atoms in a double-well optical lattice, pulsed with unequal periods. The classical dynamics is chaotic and we find the classical diffusion rate D is asymmetric in momentum up to a finite time t(r). The quantum behavior produces a corresponding asymmetry in the momentum distribution which is "frozen-in" by dynamical localization provided the break time t(*)>or=t(r). We conclude that the cold atom ratchets require Db/ variant Planck's over 2pi approximately 1, where b is a small deviation from period-one pulses.     

Exotic versus conventional scaling and universality in a disordered bilayer quantum heisenberg antiferromagnet

We present Monte Carlo simulations of a two-dimensional bilayer quantum Heisenberg antiferromagnet with random dimer dilution. In contrast with exotic scaling scenarios found in other random quantum systems, the quantum phase transition in this system is characterized by a finite-disorder fixed point with power-law scaling. After accounting for corrections to scaling, with a leading irrelevant exponent of omega approximately 0.48, we find universal critical exponents z=1.310(6) and nu=1.16(3). We discuss the consequences of these findings and suggest new experiments.     

Microscopic origin of the next-generation fractional quantum Hall effect

Most of the fractions observed to date belong to the sequences nu=n/(2pn+/-1) and nu=1-n/(2pn+/-1), n and p integers, understood as the familiar integral quantum Hall effect of composite fermions. These sequences fail to accommodate, however, many fractions such as nu=4/11 and 5/13, discovered recently in ultrahigh mobility samples at very low temperatures. We show that these "next generation" fractional quantum Hall states are accurately described as the fractional quantum Hall effect of composite fermions.     

Effective spectral function for quasielastic scattering on nuclei

Spectral functions that are used in neutrino event, generators to model quasielastic (QE) scattering from nuclear targets include Fermi gas, Local Thomas Fermi gas (LTF), Bodek-Ritchie Fermi gas with high momentum tail, and the Benhar-Fantoni two dimensional spectral function. We find that the \(\nu \) dependence of predictions of these spectral functions for the QE differential cross sections ( \({d^2\sigma }/{dQ^2 d\nu }\) ) are in disagreement with the prediction of the \(\psi '\) superscaling function which is extracted from fits to quasielastic electron scattering data on nuclear targets. It is known that spectral functions do not fully describe quasielastic scattering because they only model the initial state. Final state interactions distort the shape of the differential cross section at the peak and increase the cross section at the tails of the distribution. We show that the kinematic distributions predicted by the \(\psi '\) superscaling formalism can be well described with a modified effective spectral function (ESF). By construction, models using ESF in combination with the transverse enhancement contribution correctly predict electron QE scattering data.