Radiation-induced resistance oscillation and instability in GaAS/AlGaAS heterostructure under transverse magnetic fields

This paper studies dynamics of a modulation-doped GaAs/AlGaAs heterostructure under transverse magnetic fields and microwave radiations. It finds that negative differential conductivity, due to the real-space electron transfer and delayed dielectric relaxation of the interface potential barrier, can lead to complex behaviours when a relatively small magnetic field is applied. Quasiperiodicity, frequency-locking and the routes from period-doubling to chaos are found. Under a large magnetic field, however, two time-independent homogeneous steady states exist; and the longitudinal resistance of the system shows an interesting oscillation with period tuned by the ratio of microwave radiation frequency w to the cyclotron frequency Wc and local minima at ω/ωc = integer ＋ 1/4.     

Beating standard quantum limit via two-axis magnetic susceptibility measurement

We report a metrology scheme which measures the magnetic susceptibility of an atomic spin ensemble along the $x$ and $z$ directions and produces parameter estimation with precision beating the standard quantum limit. The atomic ensemble is initialized via one-axis spin squeezing with optimized squeezing time and parameter $\phi$ (to be estimated) assumed as uniformly distributed between 0 and $2\pi$ while fixed in each estimation. One estimation of $\phi$ can be produced with every two magnetic susceptibility data measured along the two axes respectively, which has an imprecision scaling $({1.43\pm0.02})/N^{0.687\pm0.003}$ with respect to the number $N$ of the atomic spins. The measurement scheme is easy to implement and is robust against the measurement fluctuation caused by environment noise and measurement defects.     

Topology of toroidal helical fields in non-circular cross-sectional tokamaks

The ordinary differential magnetic field line equations are solved numerically; the tokamak magnetic structure is studied on Hefei Tokamak-7 Upgrade （HT-TU） when the equilibrium field with a monotonic q-profile is perturbed by a helical magnetic field. We find that a single mode （m, n） helical perturbation can cause the formation of islands on rational surfaces with q=m/n and q=（m±1,±2,±3,...）/n due to the toroidicity and plasma shape （i.e. elongation and triangularity）, while there are many undestroyed magnetic surfaces called Kolmogorov-Arnold-Moser （KAM） barriers on irrational surfaces. The islands on the same rational surface do not have the same size. When the ratio between the perturbing magnetic field Br（r） and the toroidal magnetic field amplitude Bφ0 is large enough, the magnetic island chains on different rational surfaces will overlap and chaotic orbits appear in the overlapping area, and the magnetic field becomes stochastic. It is remarkable that the stochastic layer appears first in the plasma edge region.     

Double Diffractive Process Factorization from ηc and γ Associated Production at Tevatron and LHC

We present a study of associated ηc and γ double diffractive production in pp^- collision based on Ingelman-Schlein model, and the framework of non-relativistic QOD factorization formalism for quarkonia production. The prediction of ηc and γ is more reliable than J/ψ production, because the associated ηc and γ production is a pure color-octet process, and the dominant contribution comes from color octet ^1S0^(8) subprocess, which is related to the color octet matrix element of ^1S0(8) of J/ψ by the heavy quark spin symmetry and the large PT J/ψ production data. We find that the ratio of diffractive to inclusive cross sections is independent of the values of color octet matrix elements, but is sensitive to the gluon factor of the Pomeron and renormalized Pomeron flux factors. So experimental measurement of this ratio can give us more information of the nature of Pomeron and test the assumption of hard diffractive factorization in hadron-hadron collisions.     

Searching a systematics for nonfactorizable contributions to hadronic decays ofD 0 andD + mesons

We investigate nonfactorizable contributions to charm meson decays in $D \to \bar K\pi /\bar K\rho /\bar K*\pi /\bar Ka_1 /\bar K*\rho $ modes. Obtaining the contributions from spectator-quark diagrams forN _c=3, we determine nonfactorizable isospin 1/2 and 3/2 amplitudes required to explain the data for these modes. We observe that ratio of these amplitudes seem to follow a universal value.     

Mesoscopic transport through toroidal carbon nanotubes threaded with a THz magnetic flux

We have investigated the quantum transport through mesoscopic systems with a toroidal carbon nanotube coupled with two metal leads (N-TCN-N) threaded with an ac magnetic flux. The energy shifting takes place by applying the magnetic flux, and this shifting arises from both the dc and ac components of magnetic flux. The dc magnetic flux induces the periodic variation of energy gap E _g of the TCN, and the ac magnetic flux component always increases the energy gap. As the photon energy is larger than the energy gap , the electrons in the valence band can jump to the conductance band at zero temperature, and the tunneling current appears for , ( ). The differential conductance and tunneling current display clear effect of ac flux by modifying the current oscillation structures. The photon-assisted tunneling current exhibits stair-like I-V characteristics, and it shows different behaviors for different TCN systems. The magnitude of the current is suppressed by the applied ac flux. We also present the time-dependent current evolution, which is contributed by the oscillating current components.Received: 31 May 2004, Published online: 3 August 2004PACS: 73.40.-c Electronic transport in interface structures - 73.63.Fg Nanotubes - 73.61.Wp Fullerenes and related materials - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals     

Study of the FCNH Coupling with Boosted Higgs at LHC*

We present a study about the flavor changing coupling of the top quark with the Higgs boson through the channel $pp\to H t/\bar{t}$ with $H\to b\bar{b}$ at LHC. The final states considered for the such process are $l^\pm+\mathbb{E}_{T}+3b$. We focus on the boosted region in the phase space of the Higgs boson. The backgrounds and events are simulated and analyzed. The sensitivities for the FCNH couplings are estimated. It is found that it is more sensitive for $y_{\rm tu}$ than $y_{\rm tq}$ at LHC. The upper limits of the FCNH couplings can be set at LHC with 3000 ${\rm fb}^{-1}$ integrated luminosity as $\vert y_{\rm tu}\vert^2=1.1\times10^{-3}$ and $\vert y_{\rm tc}\vert^2=7.2\times 10^{-3}$ at 95% C.L.     

Lowest- Q2 measurement of the γp → Δ reaction: Probing the pionic contribution

To determine nonspherical angular-momentum amplitudes in hadrons at long ranges (low Q²), data were taken for the p(ˉe, e'p)π⁰ reaction in the Δ region at Q ² = 0.060 (GeV/c)² utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios at W = 1232 MeV are , Re( )%, and Re( )%. These disagree with predictions of constituent quark models but are in reasonable agreement with lattice calculations with nonlinear (chiral) pion mass extrapolations, with chiral effective field theory, and with dynamical models with pion cloud effects. These results confirm the dominance, and general Q² variation, of the pionic contribution at large distances.     

Dynamics of a two-level system coupled to Ohmic bath: a perturbation approach

The quantum dynamics, both non-equilibrium and equilibrium, of the dissipative two-level system is studied by means of the perturbation approach based on a unitary transformation. It works well for the whole parameter range and and our main results are: the coherence-incoherence transition is at ; for the non-equilibrium correlation ; the susceptibility is of a double peak structure for and the Shibas relation is exactly satisfied; at the transition point the equilibrium correlation in the long time limit.Received: 14 October 2003, Published online: 8 June 2004PACS: 72.20.Dp General theory, scattering mechanisms - 05.30.-d Quantum statistical mechanics     

Kinetic Ising model in a time-dependent oscillating external magnetic field: effective-field theory

Recently,Shi et al.[2008 Phys.Lett.A 372 5922] have studied the dynamical response of the kinetic Ising model in the presence of a sinusoidal oscillating field and presented the dynamic phase diagrams by using an effective-field theory(EFT) and a mean-field theory(MFT).The MFT results are in conflict with those of the earlier work of Tom’e and de Oliveira,[1990 Phys.Rev.A 41 4251].We calculate the dynamic phase diagrams and find that our results are similar to those of the earlier work of Tom’e and de Oliveira;hence the dynamic phase diagrams calculated by Shi et al.are incomplete within both theories,except the low values of frequencies for the MFT calculation.We also investigate the influence of external field frequency(ω) and static external field amplitude(h0) for both MFT and EFT calculations.We find that the behaviour of the system strongly depends on the values of ω and h0.     

Production properties of K^\star(892)^\pm vector mesons and their spin alignment as measured in the NOMAD experiment

First measurements of mesons production properties and their spin alignment in charged current (CC) and neutral current (NC) interactions are presented. The analysis of the full data sample of the NOMAD experiment is performed in different kinematic regions. For and mesons produced in CC interactions and decaying into we have found the following yields per event: and respectively, while for the and mesons produced in interactions the corresponding yields per event are: and . The results obtained for the parameter, and for and produced in CC interactions, are compared to theoretical predictions tuned on LEP measurements in annihilation at the pole. For mesons produced in interactions the measured parameter is . PACS 13.15.+g, 13.60.Le, 13.87.Fh, 13.88.+e, 14.40.Ev     

Phonon-assisted tunneling in the quantum regime of Mn12 acetate

Longitudinal or transverse magnetic fields applied on a crystal of Mn12 acetate allows one to observe independent tunnel transitions between m = -S+p and m = S-n-p ( n = 6-10, p = 0-2 in longitudinal field and n = p = 0 in transverse field). We observe a smooth transition (in longitudinal) from coherent ground-state to thermally activated tunneling. Furthermore, two ground-state relaxation regimes show a crossover between quantum spin relaxation far from equilibrium and near equilibrium, when the environment destroys multimolecule correlations. Finally, we stress that the complete Hamiltonian of Mn12 should contain odd spin operators of low order.     

外磁场与带轴夹角对非晶FeSiB/Cu/FeSiB三明治薄带巨磁阻抗特性的影响

采用层间胶合方法制备了淬态非晶FeSiB/Cu/FeSiB三明治薄带,研究了同尺寸单层薄带和三明治薄带的巨磁阻抗(giant magneto-impedance, GMI)随外磁场与带轴夹角β的变化特性.结果表明,FeSiB单层薄带在7.0 MHz最佳响应频率下,GMI仅约30%,外磁场与带轴夹角对单层薄带GMI几乎没有影响;三明治薄带的GMI效应则十分显著,在0.6 MHz最佳响应频率下,纵、横向GMI比分别达到272%和464%, GMI随β的增大而增强;所有β角的三明治薄带GMI曲线都出现各向异性峰,各向异性峰随β的增大而展宽.根据磁畴转动模型推导了薄带横向磁导率与各向异性场及β之间的函数关系式.结果显示,三明治薄带GMI随夹角β变化的特性与理论推算的横向磁导率变化有较好的一致性,而单层薄带则不然.该磁畴转动模型能定性解释三明治薄带GMI随外磁场方向变化特性.     

ELECTRONIC AND OPTICAL PROPERTIES OF WURTZITE GaN:——A THEORETICAL APPROACH

The band structures of wurtzite GaN(α-GaN) are studied using the nearest and second-nearest neighbour semi-empirical tight-hinding method in sp³ s^* model. The calculated direct fundamental gap of α-GaN is 3.45 eV, which is in good agreement with the experimental data. The density of states and the imaginary pert of dielectric function (ε₂ (ω)) are evaluated to he in the regions - 10.0 - 12 eV and (1.0 - 10.0 eV, respectively. There are mainly three peaks at 6,4, 7,5, 8.4 eV, dominating the ε₂(ω) spectrum. The two components of the ε₂(ω) (i. e. ε_2xy(ω) and ε_2z(ω) ) are also calculated; and the real prat of dielectric function, reflectivity, absorption coefficient, and refractive index are all studied.     

Ionization of the hydrogen atom: from weak to strong static electric fields

A quantum dynamical treatment of the S-G effect, to the leading order in for the electron, where is the fine-structure constant, and for spin 1/2 charged particles (e.g., the proton), in general, leads to a unitary expression for the probability density on the observation screen, where the magnetic field has a controllable longitudinal uniform component along the initial average direction of propagation of the particle, in addition to a non-uniform, almost longitudinal, magnetic field lying in the plane defined by the quantization axis, in question, of the spin and the initial average direction of propagation.Received: 3 April 2003, Published online: 22 July 2003PACS: 03.65.-w Quantum mechanics - 03.65.Nk Scattering theory - 24.70.+s Polarization phenomena in reactions     

Masses and couplings of vector mesonsfrom the pion electromagnetic,weak, and $\pi\gamma$ transition form factors

We analyze the pion electromagnetic, charged-current, and transition form factors at timelike momentum transfers q, GeV², using a dispersion approach. We discuss in detail the propagator matrix of the photon-vector meson system and define certain reduced amplitudes, or vertex functions, describing the coupling of this system to final states. We then apply the derived analytic expressions to the analysis of the recent , , and data. We find the reduced amplitudes for the coupling of the photon and vector mesons to two pseudoscalars to be constant, independent of s, in the range considered, indicating a freezing of the amplitudes for GeV. The fit to the form factor data leads to the following values of the Breit-Wigner resonance masses MeV, MeV and MeV, where the errors are only statistical.Received: 8 December 2003, Published online: 23 March 2004T. Paulus: Now at Philips     

Quantum interference effects in a multidriven transition Fg=3←→Fe=2

We have theoretically and experimentally studied the quantum coherence effects of a degenerate transition Fg =3←→Fe=2 system interacting with a weak linearly polarized （with σ＋ components） probe light and a strong linearly polarized （with σ＋ components） coupling field. Due to the competition between the drive Rabi frequency and the Zeeman splitting, electromagnetically induced transparency （EIT） and electromagnetically induced absorption （EIA） are present at the different values of applied magnetic field in the case where the Zeeman splitting of excited state Δe is larger than the Zeeman splitting of ground state Δg （i.e.Δe 〉 Δg）.     

Scattering amplitudes with massive fermions using BCFW recursion

We study the QCD scattering amplitudes for and where q is a massive fermion. Using a particular choice of massive fermion spinor we derive compact expressions for the partial spin amplitudes for the 2→2 process. We then investigate the corresponding 2→3 amplitudes using the BCFW recursion technique. For the helicity conserving partial amplitudes we present new expressions, but we were unable to treat the helicity-flip amplitudes recursively, except for the case where all the gluon helicities are the same. We therefore evaluate the remaining partial amplitudes using standard Feynman diagram techniques.     

Probing the CP nature of the Higgs bosons by $t\overline{t}$ productionat photon linear colliders

We study the effects of heavy Higgs bosons on the production process at photon linear colliders. The interference patterns between the resonant Higgs production amplitudes and the continuum QED amplitudes are examined. The patterns tell us not only the CP nature of the Higgs bosons but also the phase of the -Higgs vertex which gives new information about the Higgs couplings to new charged particles. We point out that it is necessary to use circularly polarized photon beams to produce efficiently heavy Higgs bosons whose masses exceed the electron beam energy, and show that the above interference patterns of the production amplitudes can be studied by observing t and decay angular distributions. Analytic expressions for the helicity amplitudes for the sequential process are presented in terms of the generic production amplitudes.Received: 2 June 2003, Published online: 24 October 2003     

CP-sensitive observables in chargino production with transverse $e^{\pm}$ beam polarization

We consider the process at a linear collider with transverse beam polarization. We investigate the influence of the CP phases on azimuthal asymmetries in with subsequent two-body decays and . We show that triple product correlations involving the transverse beam polarization vanish if at least one subsequent chargino decay is not observed. We derive this result within the minimal supersymmetric standard model (MSSM) with complex parameters; however, it holds also in the general MSSM with SUSY flavor violation.Received: 29 March 2004, Published online: 30 July 2004