Search Results (169 total)

We investigate surface plasmon-polariton (SPP) propagation in a metal-semiconductor-metal structure where semiconductor is highly excited to have an optical gain. We show that near the SPP resonance, the imaginary part of the propagation wave vector changes from positive to hugely negative, corresponding to an amplified SPP propagation. The SPP mode experiences an unexpected giant modal gain that is 1000 times of material gain in the excited semiconductor, a phenomenon not known to exist in any other system. We show that the physical origin of such giant gain is the slowing down of average energy propagation in the structure.

Ionic current through a 3 nm in diameter nanopore has been investigated using molecular dynamics. Results indicate that the ionic current increases linearly as the electrolyte concentration increases from 0.4 to 0.9 M, beyond which the ionic current increases at a slower rate. In contradiction to the expectation that higher surface charge density will lead to more ions in the nanopore, and therefore, higher ionic current, the ionic current shows an increase-decrease profile as the surface charge density increases. These unusual observations are attributed to the fact that ions close to the wall experience large viscous force, leading to low mobility.

Impurity effects on the stability of a charge-ordered antiferromagnetic state in electron-doped manganite La_0.4Ca_0.6MnO₃ are investigated by doping Ru for Mn. Our results show that Ru doping at a tiny level of ∼2% is sufficient to suppress significantly the charge-ordered insulated state and generate a ferromagnetic metallic one. The blocked metastable state and the first-order metal-insulator transitions observed in the slightly doped samples can be attributed to the phase-separated state where ferromagnetic domains are embedded in the charge-ordered matrix. The doping effect is further investigated theoretically based on the two-orbital model. The calculation suggests that two factors are vitally important for the Ru-doping driven strong ferromagnetic tendency: (1) the ferromagnetic coupling between Ru and Mn spins and (2) the enhancement of e_g electron density while the topological defects always contribute to the instability of charge-ordered state.

Using the random matrix theory, we investigate the ensemble statistics of edge transport of a quantum spin Hall insulator with multiple edge states in the presence of quenched disorder. Dorokhov-Mello-Pereyra-Kumar equation applicable for such a system is established. It is found that a two-dimensional quantum spin Hall insulator is effectively a new type of one-dimensional (1D) quantum conductor with the different ensemble statistics from that of the ordinary 1D quantum conductor or the insulator with an even number of Kramers edge pairs. The ensemble statistics provides a physical manifestation of the Z₂ classification for the time-reversal invariant insulators.

The variance of an observable in a quantum state is usually used to describe Heisenberg uncertainty relation. For mixed states, the variance includes quantum and classical uncertainties. By means of the skew information and the decomposition of the variance, a stronger uncertainty relation was presented by Luo [Phys. Rev. A 72, 042110 (2005)]. In this paper, by using Wigner-Yanase-Dyson information which is a generalization of the skew information, we propose a general uncertainty relation of mixed states.

We propose a Hamiltonian of ultracold spinless atom in optical lattices including the two-body interaction of nearest neighbors, which reduces to the Bose-Hubbard model in weak-interaction limit. An atom-pair hopping term appearing in the Hamiltonian explains naturally the recent experimental observation of correlated tunneling in a double-well trap with strong atom-atom interactions and moreover leads to a dynamic process of atom-pair tunneling where strongly interacting atoms can tunnel back and forth as a fragmented pair. Finally a dynamics of oscillations induced by the atom-pair tunneling is found in the strong interaction regime, where the Bose-Hubbard model gives rise to the insulator state with fixed time-averaged value of atom-occupation number only. Quantum phase transitions between two quantum phases characterized by degenerate and nondegenerate ground states are shown to be coinciding with the Landau second-order phase-transition theory. In the system of finite atom number the degeneracy of ground states can be removed by quantum tunneling for the even number of atoms but not for the odd number.

Neutrino factory and muon collider cooling lattices require both high gradient rf cavities and strong focusing solenoids. Experiments have shown that there may be serious problems operating rf in the required magnetic fields. Experimental observations using vacuum rf cavities in magnetic fields are discussed, current published models of breakdown with and without magnetic fields are briefly summarized, and some of their predictions compared with observations. A new theory of magnetic field dependent breakdown is presented. It is proposed that electrons emitted by field emission on asperities on one side of a cavity are focused by the magnetic field to the other side where they induce mechanical fatigue leading to cavity surface damage in small spots. Metal is then electrostatically drawn from the molten spots, becomes vaporized and ionized by field emission from the remaining damage, and causes breakdown. The theory is fitted to existing 805 MHz data and predictions are made for performance at 201 MHz. The model predicts breakdown gradients significantly below those specified for either the International Scoping Study neutrino factory or a muon collider. Possible solutions to these problems are discussed, including designs for magnetically insulated rf in which the cavity walls are designed to be parallel to chosen magnetic field contour lines and consequently damage from field emission is expected to be suppressed. An experimental program that could study these problems and their possible solution is outlined. We also mention the use of high pressure gas as an alternative possible solution.

We demonstrate reversible movement of 1 / 2[11̅ 0](110) dislocation loops generated from nanodisturbances in a β-titanium alloy. High resolution transmission electron microscope observations during an in situ tensile test found three reversible deformation mechanisms, nanodisturbances, dislocation loops and martensitic transformation, that are triggered in turn with increasing applied stress. All three mechanisms contribute to the nonlinear elasticity of the alloy. The experiments also revealed the evolution of the dislocation loops to disclination dipoles that cause severe local lattice rotations.

We show that a complex network of phase oscillators may display interfaces between domains (clusters) of synchronized oscillations. The emergence and dynamics of these interfaces are studied for graphs composed of either dynamical domains (influenced by different forcing processes), or structural domains (modular networks). The obtained results allow us to give a functional definition of overlapping structures in modular networks, and suggest a practical method able to give information on overlapping clusters in both artificially constructed and real world modular networks.

We report a series of new surface reconstructions on BaTiO₃(001) as a function of environmental conditions, determined via scanning tunneling microscopy and low energy electron diffraction. Using density functional theory calculations and thermodynamic modeling, we construct a surface phase diagram and determine the atomic structures of the thermodynamically stable phases. Excellent agreement is found between the predicted phase diagram and experiment. The results enable prediction of surface structures and properties under the entire range of accessible environmental conditions.

In spectroscopy, it is conventional to treat pulses much stronger than the linewidth as delta functions. In NMR, this assumption leads to the prediction that π pulses do not refocus the dipolar coupling. However, NMR spin echo measurements in dipolar solids defy these conventional expectations when more than one π pulse is used. Observed effects include a long tail in the CPMG echo train for short delays between π pulses, an even-odd asymmetry in the echo amplitudes for long delays, an unusual fingerprint pattern for intermediate delays, and a strong sensitivity to π-pulse phase. Experiments that set limits on possible extrinsic causes for the phenomena are reported. We find that the action of the system’s internal Hamiltonian during any real pulse is sufficient to cause the effects. Exact numerical calculations, combined with average Hamiltonian theory, identify terms that are sensitive to parameters such as pulse phase, dipolar coupling, and system size. Visualization of the entire density matrix shows a unique flow of quantum coherence from nonobservable to observable channels when applying repeated π pulses.

The tiny difference between hard π pulses and their delta-function approximation can be exploited to control coherence. Variants on the magic echo that work despite a large spread in resonance offsets are demonstrated using the zeroth- and first-order average Hamiltonian terms, for ¹³C NMR in ⁶⁰C. The ²⁹Si NMR linewidth of silicon has been reduced by a factor of about 70 000 using this approach, which also has potential applications in magnetic resonance microscopy and imaging of solids.

In D. Li, X. Li, H. Huang, and X. Li [Phys. Rev. A 76, 032304 (2007)], we defined the residual entanglement for odd n qubits. In this reply, we improve the definition by averaging the original residual entanglement with respect to each qubit, which makes the defined quantity invariant under any permutation of all the qubits.

In this paper, we present a stochastic model for the mixed-feedback loop (MFL), a motif found in integrated cellular networks of transcription regulation and protein-protein interaction. Previous bifurcation analysis indicates that this motif can serve as a bistable switch or a clock. We investigate how extrinsic and intrinsic noise affects its dynamic behaviors systematically. We find that this motif can exploit noise to enrich its dynamical performance. When the MFL is in the bistable region, under fluctuation of extrinsic noise, the MFL system can switch from one steady state to the other and meanwhile one protein’s production is amplified for more than three orders of magnitude. Further, from an engineering perspective, this noise-based switch and amplifier for gene expression is very easy to control. Without extrinsic noise, spontaneous transition between states occurs as the consequence of intrinsic noise. Such a switch is controlled by the parameters and system size. On the other hand, intrinsic noise can induce sustained stochastic oscillation when the corresponding deterministic system does not oscillate. Such stochastic oscillation shows the best performance at an optimal noise level, indicating the occurrence of intrinsic noise stochastic resonance which can contribute to the robustness of this oscillator. When considering the effects of extrinsic noise near bifurcation points, a similar phenomenon of extrinsic noise stochastic resonance is unveiled.

We show there are at least 28 distinct true stochastic local operations and classical communication (SLOCC) entanglement classes for four qubits by means of SLOCC invariant and semi-invariants and derive the number of degenerate SLOCC classes for n qubits.

We investigate the enhancement of spin polarization in a quantum wire in the presence of a constriction and a spin-orbit coupling segment. It is shown that the spin-filtering effect is significantly heightened in comparison with the configuration without the constriction. It is understood in the studies that the constriction structure plays a critical role in enhancing the spin filtering by means of confining the incident electrons to occupy one channel only while the outgoing electrons occupy two channels. The enhancement of spin filtering has also been analyzed within the perturbation theory. Because the spin polarization arises mainly from the scattering between the constriction and the segment with spin-orbit coupling, the subband mixing induced by spin-orbit interaction in the scattering process and the inferences result in higher spin-filtering effect.

The nanocapsules with crystalline cores of GdAl₂ compound and shells of amorphous Al₂O₃ were prepared by evaporating Gd_xAl_100−x (x=50, 60, 70, 80, and 90) alloys using a modified arc-discharge technique. The morphologies, average sizes, lattice constants, and surface characteristics of GdAl₂∕Al₂O₃ nanocapsules were studied in detail by means of x-ray diffraction, energy dispersive spectroscopy, x-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The formation mechanism of the nanocapsules was analyzed in detail. The differences in Curie temperatures and anisotropy constants of these nanocapsules were discussed with respect to their different structural characteristics. From 180  to  5  K, the magnetic entropy change of the GdAl₂∕Al₂O₃ nanocapsules continuously increases with decreasing temperature T and rapidly enhances when the temperature tends to 5  K. The largest entropy change −ΔS at 7.5  K can respectively reach 18.02, 18.71, and 31.01  J  kg⁻¹  K⁻¹ by varying the magnetic field from 7  to  1  T for the nanocapsules synthesized by arc-discharging Gd₇₀Al₃₀, Gd₈₀Al₂₀, and Gd₉₀Al₁₀ alloys. The appearance of a large entropy change at low temperatures was ascribed to a lower anisotropy energy barrier and a high magnetic-moment density of the nanocapsules. The linear relation between the magnetic entropy change and the reciprocal of the temperature (1∕T) was discussed in terms of superparamagnetism and magnetocaloric theory.

The magnetic, transport and magnetotransport properties of antiferromagnetic ε-(Mn_1−xFe_x)_3.25Ge (x=0.17 and 0.2) compounds are investigated systematically. For both compounds, a large positive magnetoresistance (MR) is observed in the temperature regime below the temperature (T_t) for a transition from a triangular to a collinear antiferromagnetic spin configuration, while a negative magnetoresistance appears between T_t and the Néel temperature T_N, and then the MR ratio turns to be positive again above T_N. Below T_t, the positive MR reaches a maximum ratio of 6.1% at 5  K at an applied magnetic field of 5  T for the ε-(Mn_0.83Fe_0.17)_3.25Ge compound. For the ε-(Mn_0.83Fe_0.2)_3.25Ge compound, the positive MR ratio reaches its maximum at 120  K and nearly vanishes at 5  K. The origin of the anomalous positive and negative MR effects is discussed in terms of the shrinkage of the orbits, the spin fluctuations, the spin configurations, and the magnetic transitions.

We find an invariant for n qubits and propose residual entanglement for n qubits by means of the invariant. Thus, we establish a relation between stochastic local operations and classical communication (SLOCC) entanglement and residual entanglement. Invariant and residual entanglement can be used for SLOCC entanglement classification of n qubits.

NMR spin echo measurements of ¹³C in C₆₀, ⁸⁹Y in Y₂O₃, and ²⁹Si in silicon are shown to defy conventional expectations when more than one π pulse is used. Multiple π-pulse echo trains may either freeze out or accelerate the decay of the signal, depending on the π-pulse phase. Average Hamiltonian theory, combined with exact quantum calculations, reveals an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse.

In this work, we have both theoretically and experimentally investigated the photonic transmission in a one-dimensional random n-mer (RN) dielectric system. Due to the positional correlation in the RN structure, the localization-delocalization transitions of photons happen at expected frequencies of photons. Multiple resonant transmissions are found in the photonic band gap. At each resonant mode, zero-Lyapunov exponent and undecayed field distribution of electromagnetic waves have been found through the whole system. Furthermore, the channel is opened for photonic transport at the resonant frequency, and the density of states of photons increases step by step as frequency increases. The theoretical results are experimentally demonstrated in RN dielectric multilayer films of SiO₂∕TiO₂ for visible and near infrared light.

We study the oxygen doping dependence of the equilibrium first-order melting and second-order glass transitions of vortices in Bi₂Sr₂CaCu₂O_8+δ. Doping affects both anisotropy and disorder. Anisotropy scaling is shown to collapse the melting lines only where thermal fluctuations are dominant. Yet, in the region where disorder breaks that scaling, the glass lines are still collapsed. A quantitative fit to melting and replica symmetry-breaking lines of a 2D Ginzburg-Landau model further reveals that disorder amplitude weakens with doping, but to a lesser degree than thermal fluctuations, enhancing the relative role of disorder.

The first-order quantum corrections to the static structure factor of vortex lattices in a rapidly rotating quasi-two-dimensional Bose-Einstein condensate are calculated. Furthermore, we estimate the melting filling fraction by using a criterion similar to that used in thermal melting related to the Debye-Waller factor of the smallest reciprocal lattice vector.

The magnetic transitions and magnetic and magnetostrictive properties of Tb_xDy_1−x(Fe_0.8Co_0.2)₂ (0.20⩽x⩽0.40) compounds have been investigated. The spin-reorientation temperature T_SR decreases from above to below room temperature, when x is increased from 0.25 to 0.40. The easy magnetization direction at room temperature of the Laves phase lies along the ⟨100⟩ axis in compounds with 0.20⩽x⩽0.27, while it lies along the ⟨111⟩ axis as 0.30⩽x⩽0.40. The magnetocrystalline anisotropy constant K₁ at room temperature reaches a minimum value at x=0.30, indicating it is near the composition for anisotropy compensation. The large polycrystalline saturation magnetostriction λ_s≈980 ppm is observed for x=0.30, which can be ascribed to the large magnetostriction coefficients λ₁₁₁ and λ₁₀₀. λ₁₀₀ has a value larger than 600 ppm for the compounds with 0.30⩽x⩽0.35, which can be attributed to the change of the filling of the 3d band due to Co substitution for Fe. Tb_0.30Dy_0.70(Fe_0.8Co_0.2)₂ with a high magnetostriction and a low anisotropy is found to be a good candidate material for magnetostriction applications. A detailed spin configuration diagram for Tb_xDy_1−x(Fe_0.8Co_0.2)₂ Laves phase around the composition for the anisotropy compensation is given, which should be a guide to develop novel magnetostrictive materials for applications in this series.