Search Results (278 total)

Laser-based angle-resolved photoemission spectroscopy measurements have been carried out on the high energy electron dynamics in Bi₂Sr₂CaCu₂O₈ high temperature superconductor. Our superhigh resolution data, momentum-dependent measurements, and complete analysis provide important information to judge the nature of the high energy dispersion and kink. Our results rule out the possibility that the high energy dispersion from the momentum distribution curve (MDC) may represent the true bare band as believed in previous studies. We also rule out the possibility that the high energy kink represents electron coupling with some high energy modes as proposed before. Through detailed MDC and energy distribution curve analyses, we propose that the high energy MDC dispersion may not represent intrinsic band structure.

We present a detailed angle-resolved photoemission spectroscopy (ARPES) investigation of the RTe₃ family, which sets this system as an ideal “textbook” example for the formation of a nesting driven charge density wave (CDW). This family indeed exhibits the full range of phenomena that can be associated to CDW instabilities, from the opening of large gaps on the best nested parts of Fermi surface (up to 0.4 eV), to the existence of residual metallic pockets. ARPES is the best suited technique to characterize these features, thanks to its unique ability to resolve the electronic structure in k space. An additional advantage of RTe₃ is that the band structure can be very accurately described by a simple two dimensional tight-binding (TB) model, which allows one to understand and easily reproduce many characteristics of the CDW. In this paper, we first establish the main features of the electronic structure by comparing our ARPES measurements with the linear muffin-tin orbital band calculations. We use this to define the validity and limits of the TB model. We then present a complete description of the CDW properties and of their strong evolution as a function of R. Using simple models, we are able to reproduce perfectly the evolution of gaps in k space, the evolution of the CDW wave vector with R, and the shape of the residual metallic pockets. Finally, we give an estimation of the CDW interaction parameters and find that the change in the electronic density of states n(E_F), due to lattice expansion when different R ions are inserted, has the correct order of magnitude to explain the evolution of the CDW properties.

In this Brief Report, we prove that an analytically solvable two-qubit entanglement monotone introduced by Zhang [Phys. Rev. A 76, 032306 (2007)] is no greater than the square of concurrence. This proof is based on the equivalence between Zhang 's monotone and an observable entanglement measure for mixed states suggested by Mintert and Buchleitner [Phys. Rev. Lett. 98, 140505 (2007)]. Our result may clarify the connections between several recently defined entanglement measures and help understand their possible applications in quantum-information theory.

We investigated the two-dimensional electron momentum distributions of atomic negative ions in an intense laser field by solving the time-dependent Schrödinger equation (TDSE) and using the first- and second-order strong-field approximations (SFAs). We showed that photoelectron energy spectra and low-energy photoelectron momentum distributions predicted from SFAs are in reasonable agreement with the solutions from the TDSE. More importantly, we showed that accurate electron-atom elastic scattering cross sections can be retrieved directly from high-energy electron momentum spectra of atomic negative ions in the laser field. This opens up the possibility of measuring electron-atom and electron-molecule scattering cross sections from the photodetachment of atomic and molecular negative ions by intense short lasers, respectively, with temporal resolutions in the order of femtoseconds.

Manipulation of the quantum state by the Majorana transition in a spinor Bose-Einstein condensed system has been realized by altering the rotation frequency of the magnetic field direction. This kind of manipulation method has no limitation on the transition speed in principle, and the system is well closed. When this method is used on a pulsed atom laser, a multicomponent spinor atom laser is generated. We demonstrate that the experimental results agree with the theoretical prediction.

Popular modern generalized gradient approximations are biased toward the description of free-atom energies. Restoration of the first-principles gradient expansion for exchange over a wide range of density gradients eliminates this bias. We introduce a revised Perdew-Burke-Ernzerhof generalized gradient approximation that improves equilibrium properties of densely packed solids and their surfaces.

Laser-based angle-resolved photoemission measurements with superhigh resolution have been carried out on an optimally doped Bi₂Sr₂CaCu₂O₈ high temperature superconductor. New high energy features at ∼115  meV and ∼150  meV, in addition to the prominent ∼70  meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which cannot be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.

We use extreme-ultraviolet interferometry to measure the phase of high-order harmonic generation from transiently aligned CO₂ molecules. We unambiguously observe a reversal in phase of the high-order harmonic emission for higher harmonic orders with a sufficient degree of alignment. This results from molecular-scale quantum interferences between the molecular electronic wave function and the recolliding electron as it recombines with the molecule, and is consistent with a two-center model. Furthermore, using the combined harmonic intensity and phase information, we extract accurate information on the dispersion relation of the returning electron wave packet as a function of harmonic order. This analysis shows evidence of the effect of the molecular potential on the recolliding electron wave.

Analyses of temperature- and field-dependent ac susceptibility and magnetization data from a colossal magnetoresistive La_0.7Ba_0.3MnO₃ single crystal in terms of scaling behavior yield exponent values of δ=5.5±0.3 and γ=1.41±0.02 (both slightly larger than Heisenberg model predictions), with β=0.35±0.04 and a Curie temperature T_C=310±0.5  K. Detailed investigation of the low-field dc and ac susceptibilities reveals features consistent with the presence of a Griffiths phase (GP)—viz., an inverse susceptibility characterized by χ⁻¹∝(T−T_C^R)^1−λ with λ=0.67±0.05. These, combined with previous results, enable a phase diagram summarizing the evolution of the GP with composition in this system to be constructed, and in this context, the possible importance of the variation of the acoustic spin-wave stiffness D with composition is discussed.

The elastic wave transparency phenomenon of a solid sphere coated with metamaterials is investigated in a solid host medium having nonzero shear modulus. The first three scattering coefficients of the coated sphere are derived in the Rayleigh limit and expressed in terms of the effective parameters of the coated sphere assemblage. It is found that the effective bulk modulus, mass density, and shear modulus of the coated sphere system dominate the zeroth, first, and second order scattering effects, respectively. Quasistatic transparency conditions are obtained by setting these scattering coefficients to be zero. It is also shown that the obtained transparency conditions are the same as those derived from the neutral inclusion concept. Obtained results from full-wave analyses show that the given conditions can well predict the transparency induced by metamaterials even in the regime far beyond the Rayleigh limit.

We report measurements of both the phase and intensity of high-order harmonic emission from aligned CO₂ molecules, using a mixture of a molecular and an atomic gas. The molecules are transiently aligned using an ultrashort pulse, and a subsequent stronger pulse is used to generate high-order harmonics from a mixed Kr∕CO₂ sample. The high-order harmonic emission from the molecules interferes with the reference emission from the atoms. By monitoring the change in harmonic emission as a function of gas mixture and molecular alignment, we can retrieve the orientational dipole that has general features consistent with a two-center emission model.

Angle-resolved photoemission spectroscopy data for the bilayer manganite La_1.2Sr_1.8Mn₂O₇ show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic ground state emerges very rapidly with well-defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La_1.2Sr_1.8Mn₂O₇ can be regarded as a polaron coherence condensation process acting in concert with the double exchange interaction.

Detailed measurements of the magnetic and transport behavior of the two La_1-xCa_xMnO₃ single crystals exhibiting colossal magnetoresistance are summarized. The x=0.21 sample exhibits unusual exponents (δ=20±1, γ=1.71±0.1, β=0.09±0.01, T_C=182±1  K) and, more importantly, a Griffiths phase characterized by an exponent λ=0.70±0.2. By contrast, the x=0.20 specimen displays Heisenberg model behavior with no evidence of such a phase. Thus while a Griffiths phase accounts for the behavior of La_1-xCa_xMnO₃ near optimal doping, it does not appear to be a prerequisite for colossal magnetoresistance in this system.

Field-dependent ac susceptibility and low field (<150  Oe) magnetization measurements on single crystal La_0.73Ba_0.27MnO₃ reveal features consistent with the appearance of a Griffiths phase in the temperature interval between T_C=245 and 345  K. Such features, however, are suppressed by fields of only 150  Oe, a result consistent with reduced value of the susceptibility “exponent” (y) deduced from these data. Comparisons with the behavior of other doped manganites near optimal doping suggest that the proximity of the mean A-site radius ⟨r_A⟩ to its undistorted value r_A⁰ might provide an appropriate measure of the tendency of these systems to nucleate a Griffiths phase.

We have investigated the ionic relaxations, electronic structures, and optical properties for Cd_1−xZn_xTe alloys using density functional theory. The quasi-zinc-blende structure is used with special emphasis on the relaxation behaviors of Te²⁻ around either Cd²⁺ or Zn²⁺. Our calculations confirm that the relaxations of the anion rather than the cation contribute primarily to the alloying process as predicated by the experiments. The differences in the ionicity of Cd²⁺ and Zn²⁺ and their configurations around Te²⁻ are responsible for the different relaxation behaviors of Te²⁻. A striking result is the relevance of the relaxation behaviors of Te²⁻ with the alloying effect on the electronic states. This result supports the electronic features of Cd_0.5Zn_0.5Te alloy reported by the systematic analyses with quasirandom structure. The band structures obtained here are used to determine the optical functions. The comparison with the available experimental and theoretical results suggests an overall topological resemblance in the present dielectric function spectra when the band-gap correction is included.

Nd_0.7Sr_0.3MnO₃ (NSMO) films 7–300  nm thick have been grown on (001)(LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) and (001)SrTiO₃ (STO) substrates, with lattice mismatches of 0.33% and 1.29%, respectively. The strain state evolution was examined fully by x-ray reciprocal space maps, in order to clarify its impact on the thickness-dependent properties of the films. It was found that all NSMO/LSAT films are coherently strained, having almost the same Curie (T_C) and peak resistivity (T_p) temperatures at fixed thicknesses, while the NSMO/STO films evolve from being fully strained to relaxed, showing inhomogeneous magnetic transitions and lower or higher T_C than their counterparts at 7–60  nm. The results underline that T_C (T_p) and phase separation are all controlled by the strain states of the films.

To date, angle-resolved photoemission spectroscopy has been successful in identifying energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy. Here, at higher-energy scale, we present improved experimental data from four families of high-T_c superconductors over a wide doping range that reveal a hierarchy of many-body interaction scales focused on: the low-energy anomaly (“kink”) of 0.03–0.09 eV, a high-energy anomaly of 0.3–0.5 eV, and an anomalous enhancement of the width of the local-density-approximation-based CuO₂ band extending to energies of ≈2 eV. Besides their universal behavior over the families, we find that all of these three dispersion anomalies also show clear doping dependence over the doping range presented.

The tight-binding description of covalent bonding is used to propose a four-level, bond-order potential for elemental silicon. The potential addresses both the σ and π bonding and the valence of this sp-valent element. The interatomic potential is parametrized using ab initio and experimental data for the diamond cubic, simple cubic, face-centered-cubic, and body-centered-cubic phases of silicon. The bond-order potential for silicon is assessed by comparing the predicted values with other estimates of the cohesive energy, atomic volume, and bulk modulus for the β-Sn, bc8, st12, and 46 clathrate structures. The potential predicts a melting temperature of 1650±50 K in good agreement with the experimental value of 1687 K. The energetics of various high-symmetry point defect structures and the structure and energetics of small silicon clusters are investigated. The potential also provides a robust description of surface reconstructions; it notably predicts with high fidelity the surface formation energy of the (111) 7×7 dimer adatom stacking fault configuration.

By analogy with the electromagnetic wave, the acoustic transparency phenomenon is analyzed for a multilayered sphere with acoustic metamaterials. The neutral-inclusion concept is used to predict the transparency conditions in the quasistatic case, which are further confirmed by a full-wave analysis. The mechanism of the transparency is based on lowering the total-scattering cross section of the composite sphere. It is found that, to improve the transparency, the angle-dependent scattering cross section must also be minimized for all directions.

High-spin states in ¹⁸⁷Pt have been studied experimentally using the ¹⁷³Yb(¹⁸O, 4n) reaction at beam energies of 78 and 85 MeV. The previously known bands based on the νi_13/2,ν7/2^-[503], and νi_13/2²νj configurations have been extended to high-spin states, and new rotational bands associated with the ν3/2^-[512] and ν1/2^-[521] Nilsson orbits have been identified. The total Routhian surface calculations indicate that the transitional nucleus ¹⁸⁷Pt is very soft with respect to β and γ deformations. The band properties, such as level spacings, band crossing frequencies, alignment gains, and signature splittings, have been compared with the systematics observed in neighboring nuclei and have been interpreted within the framework of the cranked shell model. The rotational bands show different band crossing frequencies, which can be explained by the alignment either of i_13/2 neutrons or of h_9/2 protons. Importantly, evidence is presented for a πh_9/2 alignment at very low frequency in the ν7/2^-[503] band. The proton nature of the band crossing is strongly suggested by comparing the measured B(M1;I→I-1)/B(E2;I→I-2) ratios with the theoretical values from the semiclassical Dönau and Frauendof approach.

The homoepitaxial assembly of a (001) GaAs surface from atomic gallium and molecular As₂ vapor fluxes has been investigated with molecular dynamics simulations using a recently developed bond-order potential. The approach enables dynamic atomic assembly events to be observed as atoms condense to form thin film structures. During simulation of epitaxial growth, we observed a temperature-dependent arsenic solubility limit consistent with experimental results. The As₂ sticking probabilities and dynamic dimer-surface binding states for both gallium- and arsenic-terminated (001) surfaces were also explored. On gallium-terminated surfaces, significant switching between two weakly bound precursor states and an intermediate chemisorbed state was observed during the surface diffusion of arsenic dimers. The switching frequency was strongly temperature dependent. The arsenic dimers bound to arsenic-terminated surfaces were found to be more likely to desorb (instead of diffuse) when thermally perturbed from their adsorption sites. This sticking probability was strongly dependent on surface temperature, atomic adsorption site environment, and the orientation of the incoming dimer.

We demonstrate a room-temperature spin dynamo where the precession of electron spins in ferromagnets converts energy from microwaves to a bipolar current of electricity. The current/power ratio is at least 3 orders of magnitude larger than that found previously for spin-driven currents in semiconductors. The observed bipolar nature and intriguing symmetry are fully explained by the spin rectification effect via which the nonlinear combination of spin and charge dynamics creates dc currents.

The origin of the luminescence from Ga₂O₃ microcrystallites and nanostructures has been studied using time-resolved x-ray excited optical luminescence and x-ray absorption near-edge structure spectra in total electron yield and photoluminescence yield. The Ga₂O₃ nanostructures exhibit a crystalline β-Ga₂O₃ core and an amorphous oxide shell, an ultraviolet (UV) and a red emission with a shorter lifetime and a blue and a yellow emission with a longer lifetime when they are excited by x-ray. We suggest that the UV and red emissions from the Ga₂O₃ nanostructures are from the amorphous shell, while the blue and yellow emissions are from the crystalline β-Ga₂O₃ core; that the UV and red emissions are due to the presence of a subband of the amorphous Ga₂O₃, in the band gap. Our suggestion is in contrast to the previously proposed model, in which the UV emission from Ga₂O₃ is attributed to an intrinsic transition of crystalline β-Ga₂O₃. The implications of these observations are discussed.

Detailed field- and temperature-dependent ac susceptibility data on single-crystal La_0.73Ba_0.27MnO₃, while confirming the applicability of Heisenberg model exponents, indicate that the associated scaling law extends over a wide region of the (H,T) plane around T_c, with no evidence of crossover effects. From this it can be inferred that the system is magnetically homogeneous (at least in applied fields exceeding any anisotropy/coercive field), an unanticipated result in view of the existence of various sources of disorder (double- and superexchange coupling, for example), and reports of spontaneous electronic phase separation and Griffiths singularities in doped manganese perovskites.

While exact cloning of an unknown quantum state is prohibited by the linearity of quantum mechanics, approximate cloning is possible and has been used, e.g., to derive limits on the security of quantum communication protocols. In the case of asymmetric cloning, the information from the input state is distributed asymmetrically between the different output states. Here, we consider asymmetric phase-covariant cloning, where the goal is to optimally transfer the phase information from a single input qubit to different output qubits. We construct an optimal quantum cloning machine for two qubits that does not require ancilla qubits and implement it on an NMR quantum information processor.