Search Results (162 total)

Making use of the master equation and effective Hamiltonian approach, we investigate the steady-state entanglement in a three-qubit XX model. Both symmetric and nonsymmetric qubit-qubit couplings are considered. The system (the three qubits) is coupled to two bosonic baths at different temperatures. We calculate the steady state by the effective Hamiltonian approach and discuss the dependence of the steady-state entanglement on the temperatures and couplings. The results show that for symmetric qubit-qubit couplings, the entanglements between the nearest neighbors are equal, independent of the temperatures of the two baths. The maximum of the entanglement arrives at T_L=T_R. For nonsymmetric qubit-qubit couplings, however, the situation is totally different. The baths at different temperatures would benefit the entanglement and the entanglements between the nearest neighbors are no longer equal. By examining the probability distribution of each eigenstate in the steady state, we present an explanation for these observations. These results suggest that the steady entanglement can be controlled by the temperature of the two baths.

We present a scheme to drive a finite-dimensional quantum system into the decoherence-free subspaces (DFS) by Lyapunov control. Control fields are established by Lyapunov function. This proposal works well for both closed and open quantum systems, with replacing the DFS by desired subspaces for closed systems. An example which consists of a four-level system with three degenerate states driven by three lasers is presented to gain further insight on the scheme. Numerical simulations for the dynamics of the system are performed and the results are good.

We report highly sensitive de Haas-van Alphen (dHvA) effect measurements on a high-mobility two-dimensional electron system in an AlAs quantum well. Here two valleys are occupied forming a pseudospin system. At 400 mK, the dHvA effect shows pronounced oscillations at filling factors ν=1 to four. In the quantum limit at ν=1 the data are consistent with an interaction-enhanced valley splitting, which exceeds the Zeeman spin splitting in a perpendicular field B. When tilting B the energy gap ΔE at ν=1 shows first an unexpectedly strong angular dependence and then remains constant. This suggests a crossover in the energy gap, most likely from a spin to a pseudospin gap. We attribute the strong initial dependence of ΔE on the tilt angle to skyrmion-type spin excitations. Surprisingly, the dHvA oscillation amplitudes do not display coincidence phenomena at higher filling factors. This is explained by the large valley splitting and avoided crossings of energy levels.

We report the first image of an intact, frozen hydrated eukaryotic cell using x-ray diffraction microscopy, or coherent x-ray diffraction imaging. By plunge freezing the specimen in liquid ethane and maintaining it below -170 °C, artifacts due to dehydration, ice crystallization, and radiation damage are greatly reduced. In this example, coherent diffraction data using 520 eV x rays were recorded and reconstructed to reveal a budding yeast cell at a resolution better than 25 nm. This demonstration represents an important step towards high resolution imaging of cells in their natural, hydrated state, without limitations imposed by x-ray optics.

Recently, waves propagating with negative phase velocity [simply called antiwaves (AWs)] have attracted great attention in the area of nonlinear oscillatory systems. In the present work we investigate the parameter conditions for AWs. So far AWs have been revealed from systems slightly beyond Hopf bifurcation or some other instabilities, and from some wave sources with certain restricted frequencies. Here we study general oscillatory media (including generalized complex Ginzburg-Landau systems and Brusselator model) and specify the parameter conditions of AWs by certain characteristic behaviors of the dispersion relation of the systems. Moreover, we predict that AWs and NWs (normal waves with positive phase velocity) can be realized at a same intrinsic parameter values but different pacing frequencies in parameter regions where the dispersion relation exhibits a maximum or minimum. All numerical simulations are perfectly consistent with these theoretical predictions where the oscillatory systems are driven by external periodic pacings with 1:1 frequency locking responses.

Three independent searches for an electric dipole moment (EDM) of the positive and negative muons have been performed, using spin precession data from the muon g-2 storage ring at Brookhaven National Laboratory. Details on the experimental apparatus and the three analyses are presented. Since the individual results on the positive and negative muons, as well as the combined result, d_μ=(0.0±0.9)×10^-19e cm, are all consistent with zero, we set a new muon EDM limit, |d_μ|<1.8×10^-19e cm (95% C.L.). This represents a factor of 5 improvement over the previous best limit on the muon EDM.

Using a sample of 58 million J/ψ events collected with the BESII detector at the BEPC, more than 100 000 J/ψ→pp̅ π⁰ events are selected, and a detailed partial wave analysis is performed. The branching fraction is determined to be Br(J/ψ→pp̅ π⁰)=(1.33±0.02±0.11)×10^-3. A long-sought missing N^*, first observed in J/ψ→pn̅ π^-, is observed in this decay too, with mass and width of 2040_-4⁺³±25   MeV/c² and 230_-8⁺⁸±52   MeV/c², respectively. Its spin-parity favors 3 / 2⁺. The masses, widths, and spin parities of other N^* states are obtained as well.

We generalize the standard Born-Oppenheimer approximation to the case of open quantum systems. We define the zeroth-order Born-Oppenheimer approximation for an open quantum system as the regime in which its effective Hamiltonian can be diagonalized with fixed slowly changing variables. We then establish validity and invalidity conditions for this approximation for two types of dissipations—the spin relaxation and the dissipation of center-of-mass motion. As an example, the Born-Oppenheimer approximation for a two-level open system is analyzed.

We present the design, characterization, and modeling of a specific optical metamaterial, and employ it to manipulate the light polarizations at optical frequencies. Experimental results reveal that the maximum polarization conversion efficiency, i.e., the energy portion converted from s to p polarization after reflection, can be as high as 96% at the wavelength of ∼685 nm. Simulations and analytical results, which are in reasonable agreements with the experimental results, reveal that the underlying physics are governed by the particular electric and magnetic resonances in the optical metamaterial.

The Bardeen/Cooper/Schrieffer–Bose-Einstein condensation (BCS-BEC) crossover and phase diagram for asymmetric nuclear superfluid with pairings in isospin I=0 and I=1 channels are investigated at the mean-field level by using a density-dependent nucleon-nucleon potential. Induced by the in-medium nucleon mass and density-dependent coupling constants, neutron-proton Cooper pairs could be in BEC state at sufficiently low density, but there is no chance for the BEC formation of neutron-neutron and proton-proton pairs at any density and asymmetry. We calculate the phase diagram in asymmetry-temperature plane for weakly interacting nuclear superfluid and find that including the I=1 channel changes significantly the phase structure at low temperature. There appears a new phase with both I=0 and I=1 pairings at low temperature and low asymmetry, and the gapless state in any phase with I=1 pairing is washed out and all excited nucleons are fully gapped.

In previous work on the quantum mechanics of an atom freely falling in a general curved background spacetime, the metric was taken to be sufficiently slowly varying on time scales relevant to atomic transitions that time derivatives of the metric in the vicinity of the atom could be neglected. However, when the time dependence of the metric cannot be neglected, it was shown that the Hamiltonian used there was not Hermitian with respect to the conserved scalar product. This Hamiltonian was obtained directly from the Dirac equation in curved spacetime. This raises the paradox of how it is possible for this Hamiltonian to be non-Hermitian. Here, we show that this non-Hermiticity results from a time dependence of the position eigenstates that enter into the Schrödinger wave function, and we write the expression for the Hamiltonian that is Hermitian for a general metric when the time dependence of the metric is not neglected.

Quasiequilibrium models of rapidly rotating triaxially deformed stars are computed in general relativistic gravity, assuming a conformally flat spatial geometry (Isenberg-Wilson-Mathews formulation) and a polytropic equation of state. Highly deformed solutions are calculated on the initial slice covered by spherical coordinate grids, centered at the source, in all angular directions up to a large truncation radius. Constant rest mass sequences are calculated from nearly axisymmetric to maximally deformed triaxial configurations. Selected parameters are to model (proto-) neutron stars; the compactness is M/R=0.001, 0.1, 0.14, and 0.2 for polytropic index n=0.3 and M/R=0.001, 0.1, 0.12, and 0.14 for n=0.5, where M/R refers to that of a nonrotating spherical star having the same rest mass. We confirmed that the triaxial solutions exist for these parameters as in the case of Newtonian polytropes. However, it is also found that the triaxial sequences become shorter for higher compactness, and those disappear at a certain large compactness for the n=0.5 case. In the scenario of the contraction of proto-neutron stars being subject to strong viscosity and rapid cooling, it is plausible that, once the viscosity driven secular instability sets in during the contraction, the proto-neutron stars are always maximally deformed triaxial configurations, as long as the compactness and the equation of state parameters allow such triaxial sequences. Detection of gravitational waves from such sources may be used as another probe for the nuclear equation of state.

By using the effective Hamiltonian approach, we present a self-consistent framework for the analysis of geometric phases and dynamically stable decoherence-free subspaces in open systems. Comparisons to the earlier works are made. This effective Hamiltonian approach is then extended to a non-Markovian case with the generalized Lindblad master equation. Based on this extended effective Hamiltonian approach, the non-Markovian master equation describing a dissipative two-level system is solved, an adiabatic evolution is defined, and the corresponding adiabatic condition is given.

Exchange bias phenomena are observed in La_0.25Ca_0.75MnO₃ nanoparticles with average sizes ranging from 40  to  1000  nm. It is found that the magnetic hysteresis loops display horizontal and vertical shifts in field-cooled processes. The variations of the exchange bias field (H_EB) and the coercivity (H_c) with particle size follow nonmonotonic dependencies and show maxima for particles with diameter around 80  nm at T=5  K, which can be mainly ascribed to the changes in uncompensated surface spins with nanoparticle size. The peak position for H_c shifts to larger particle size at higher temperature while that for H_EB is unconspicuous. The linear relationship between H_EB and vertical magnetization shift (M_EB) further indicates that the characteristics of uncompensated spins play an important role in the variations of H_EB for the manganite.

We characterize the impurity state responsible for current flow in zinc-doped indium phosphide nanocrystals through first-principles calculations based on a real-space implementation of density-functional theory and pseudopotentials. We found the activation energy of the acceptor state to range from the value of the acceptor state in the bulk (0.03 eV) to up to values of ∼2.5 eV in the smaller nanocrystals as a result of the three-dimensional quantum confinement. This maximum value for the nanocrystals is an order of magnitude bigger than the maximum value found for one-dimensional nanomaterials (nanowires) within the same theoretical approach (∼0.2 eV). Our results show that the progressive reduced dimensionality in p-type indium phosphide materials strongly reduces the capability of the materials to generate free carriers.

Based on 58×10⁶ J/ψ events collected with the BESII detector at the Beijing Electron-Positron Collider, the baryon pair processes J/ψ→Σ⁺Σ̅ ^- and J/ψ→Ξ⁰Ξ̅ ⁰ are observed for the first time. The branching fractions are measured to be B(J/ψ→Σ⁺Σ̅ ^-)=(1.50±0.10±0.22)×10^-3 and B(J/ψ→Ξ⁰Ξ̅ ⁰)=(1.20±0.12±0.21)×10^-3, where the first errors are statistical and the second ones are systematic.

We show that it is possible to modify the stationary state by a feedback control in a two-level dissipative quantum system. Based on the geometric control theory, we also analyze the effect of the feedback on the time-optimal control in the dissipative system governed by the Lindblad master equation. These effects are reflected in the functions Δ_A(x⃗) and Δ_B(x⃗), which characterize the optimal trajectories, as well as the switching functions Φ(t) and θ(t), which characterize the switching point in time for the time-optimal trajectory.

The Monte Carlo wave function method or the quantum-trajectory–jump approach is a powerful tool to study dissipative dynamics governed by the Markovian master equation, in particular for high-dimensional systems and when it is difficult to simulate directly. We extend this method to the non-Markovian case described by the generalized Lindblad master equation. Two examples to illustrate the method are presented and discussed. The results show that the method can correctly reproduce the dissipative dynamics for the system. The difference between this method and the traditional Markovian jump approach and the computational efficiency of this method is also discussed.

We observe an obvious anomalous line shape of the e⁺e^-→ hadrons total cross sections in the energy region between 3.700 and 3.872 GeV. It is inconsistent with the explanation for only one simple ψ(3770) resonance with a statistical significance of 7σ. The anomalous line shape may be explained by two possible enhancements of the inclusive hadron production near the center-of-mass energies of 3.764 and 3.779 GeV, indicating that either there is likely a new structure in addition to the ψ(3770) resonance around 3.773 GeV, or there are some physics effects reflecting the DD̅ production dynamics.

We investigate the Hubbard chain at half band filling with additional nearest-neighbor and next-nearest-neighbor spin exchange J₁ and J₂ using bosonization and the density-matrix renormalization group. For J₂=0 we find a spin-density-wave phase for all positive values of the Hubbard interaction U and the Heisenberg exchange J₁. A frustrating spin exchange J₂ induces a bond-order-wave phase. For some values of J₁, J₂, and U, we observe a spin-gapped metallic Luther-Emery phase.

We define the condition of the quasiclassical state of free particles, which is useful in the approximate treatment of quantum systems. Then we introduce classical pure ensembles. Their states are represented using distribution functions on phase space. We compare distribution functions of classical pure ensembles and Wigner distribution functions of quasiclassical states for free particles and draw two conclusions: (i) A wave function does not describe an individual particle but a classical pure ensemble. (ii) Given a quasiclassical wave function, we can tell which classical pure ensemble is described by it.

In this paper we study nonlinear oscillatory systems consisting of two media, one supporting forward propagating waves and the other inwardly propagating waves, separated by an interface. We find that the interface can select the type of wave. Under certain well-defined parameter condition, these waves propagate in two different media with the same frequency and same wave number; the interface of the two media is transparent to these waves. The frequency and wave number of these interface-selected waves (ISWs) are predicted explicitly. When parameters are varied from this parameter set, the wave numbers of the two domains become different, and the difference increases from zero continuously as the distance between the given parameters and this parameter set increases from zero. It is found that ISWs can play crucial roles in practical problems of wave competition, e.g., ISWs can suppress spirals and antispirals.

Isotropic negative permeability resulting from Mie resonance is demonstrated in a three-dimensional (3D) dielectric composite consisting of an array of dielectric cubes. A strong subwavelength magnetic resonance, corresponding to the first Mie resonance, was excited in dielectric cubes by electromagnetic wave. Negative permeability is verified in the magnetic resonance area via microwave measurement and the dispersion properties. The resonance relies on the size and permittivity of the cubes. It is promising for construction of novel isotropic 3D left-handed materials with a simple structure.

We extend the previously established mode-expansion theory to study the eigenmodes of metallic ring systems made by thin wires possessing filmlike rectangular cross sections. Applications of the theory to a single split-ring resonator (SRR) yield essentially the same results with finite-difference-time-domain (FDTD) simulations on realistic structures, which justify some basic assumptions adopted in the theory. We then apply the theory to study the resonance properties of a broadside coupled split-ring resonator and show that such a planar resonator exhibits magnetic responses along all three dimensions under different conditions. FDTD simulations on realistic structures are performed to successfully verify the predictions based on the extended mode-expansion theory and reveal that mutual-SRR interactions in a periodic SRR array may lead to a significant change of the eigenmode properties, including a reversal of the frequency sequence for two resonance modes. We finally employ FDTD simulations to design a realistic layered metamaterial that exhibits magnetic responses along all three dimensions.

Using density-functional theory within the local spin-density approximation, SrRuO₃ and CaRuO₃ are investigated under epitaxial and uniaxial strains. Strains of the order of 2%–3% are predicted to radically alter the magnetic properties of SrRuO₃ and CaRuO₃ ultrathin films, indicating a large magnetostructural coupling in these systems. In particular, SrRuO₃ is shown to become nonmagnetic for sufficient tensile epitaxial and compressive uniaxial strains; in contrast, CaRuO₃ is predicted to become ferromagnetic for tensile epitaxial strains. These results suggest routes for the manipulation of the magnetic order of perovskite oxides ultrathin films via coherent epitaxial growth.