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At least 1,009 records · Page 56Linked to original sources
PubMed2002

Limits on extra dimensions and new particle production in the exclusive photon and missing energy signature in pp collisions at square root [s]=1.8 TeV.

The exclusive gammaE(T) signal has a small standard model cross section and is thus a channel sensitive to new physics. This signature is predicted by models with a superlight gravitino or with large extra spatial dimensions. We search for such signals at the Collider Detector at Fermilab, using 87 pb(-1) of data at square root [s]=1.8 TeV, and extract 95% C.L. limits on these processes. A limit of 221 GeV is set on the scale |F|(1/2) in supersymmetric models. For 4, 6, and 8 extra dimensions, model-dependent limits on the fundamental mass scale M(D) of 0.55, 0.58, and 0.60 TeV, respectively, are found. We also specify a "pseudo-model-independent" method of comparing the results to theoretical predictions.

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PubMed2003

First-principles calculation of vibrational Raman spectra in large systems: signature of small rings in crystalline SiO2.

We present an approach for the efficient calculation of vibrational Raman intensities in periodic systems within density functional theory. The Raman intensities are computed from the second order derivative of the electronic density matrix with respect to a uniform electric field. In contrast to previous approaches, the computational effort required by our method for the evaluation of the intensities is negligible compared to that required for the calculation of vibrational frequencies. As a first application, we study the signature of 3- and 4-membered rings in the Raman spectra of several polymorphs of SiO2, including a zeolite (H-ZSM-18) having 102 atoms per unit cell.

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PubMed2003

Polarized x-ray emission from magnetized neutron stars: signature of strong-field vacuum polarization.

In the atmospheric plasma of a strongly magnetized neutron star, vacuum polarization can induce a Mikheyev-Smirnov-Wolfenstein type resonance across which an x-ray photon may (depending on its energy) convert from one mode into the other, with significant changes in opacities and polarizations. We show that this vacuum resonance effect gives rise to a unique energy-dependent polarization signature in the surface emission from neutron stars. The detection of polarized x rays from neutron stars can provide a direct probe of strong-field quantum electrodynamics and constrain the neutron star magnetic field and geometry.

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PubMed2003

Quantum signatures in laser-driven relativistic multiple scattering.

The dynamics of an electronic Dirac wave packet evolving under the influence of an ultraintense laser pulse and an ensemble of highly charged ions is investigated numerically. Special emphasis is placed on the evolution of quantum signatures from single to multiple scattering events. We quantify the occurrence of quantum relativistic interference fringes in various situations and stress their significance in multiple-particle systems, even in the relativistic range of laser-matter interaction.

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PubMed2004

Scanning tunneling microscopy chemical signature of point defects on the MoS2(0001) surface.

Using ab initio calculations, we have studied the modification of the electronic structure of the MoS2(0001) surface by several point defects: a surface S vacancy and different transition metal atoms substituting a S atom (Pd, Au, Fe, and V). With a S vacancy, a gap state appears with weight mostly on the Mo and S atoms surrounding the vacancy. The substitutional atoms of complete d band (Pd and Au) do not present magnetic polarization and slightly modify the DOS near the Fermi energy. On the other hand, the incomplete d band atoms (Fe and V) present spin polarization and modify significantly the states near the band edges. From calculated STM images and STS curves, we show that this chemical signature can be measured and used to characterize the surface defects of the substrate which are suitable nucleation centers for nanocluster growth.

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PubMed2004

Topological signature of deterministic chaos in short nonstationary signals from an optical parametric oscillator.

Although deterministic chaos has been predicted to occur in the triply resonant optical parametric oscillator (TROPO) 15 years ago, experimental evidence of chaotic behavior in this system has been lacking so far, in marked contrast with most nonlinear systems, where chaos has been actively tracked and found. This situation is probably linked to the high sensitivity of the TROPO to perturbations, which adversely affects stationary operation at high power. We report the experimental observation in this system of a burst of irregular behavior of duration 80 micros. Although the system is highly nonstationary over this time interval, a topological analysis allows us to extract a clear-cut signature of deterministic chaos from a time series segment of only nine base cycles (3 micros). This result suggests that nonstationarity is not necessarily an obstacle to the characterization of chaos.

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PubMed2004

Quantum signatures of breather-breather interactions.

The spectrum of the quantum discrete nonlinear Schrödinger equation, or boson Hubbard Hamiltonian, on a periodic 1D lattice shows some interesting detailed band structure, which may be interpreted as the quantum signature of a two-breather interaction in the classical case. This fine structure is studied using degenerate perturbation theory. We also present a modification to this model, which increases the mobility of bound states.

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PubMed2004

Dynamical signatures of self-phase-locking in a triply resonant optical parametric oscillator.

We report on specific signatures of self-phase-locking in a cw frequency divide-by-three optical parametric oscillator subject to two competing chi(2) nonlinear processes that couple the signal and idler subharmonic waves. The self-phase-locked pair appears as a broad fringe dip within the mode-pair cluster. We have also observed Hopf instabilities of the zero-detuning case at approximately 4 x the pump threshold. These results open the path to experimental investigation of quantum entanglement and phase-locked transverse mode structures in this novel class of parametric oscillators.

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PubMed2005

Signatures of homoclinic motion in quantum chaos.

Homoclinic motion plays a key role in the organization of classical chaos in Hamiltonian systems. In this Letter, we show that it also imprints a clear signature in the corresponding quantum spectra. By numerically studying the fluctuations of the widths of wave functions localized along periodic orbits we reveal the existence of an oscillatory behavior that is explained solely in terms of the primary homoclinic motion. Furthermore, our results indicate that it survives the semiclassical limit.

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PubMed2005

Signature of Mott-insulator transition with ultracold fermions in a one-dimensional optical lattice.

Using the exact Bethe ansatz solution of the Hubbard model and Luttinger liquid theory, we investigate the density profiles and collective modes of one-dimensional ultracold fermions confined in an optical lattice with a harmonic trapping potential. We determine a generic phase diagram in terms of a characteristic filling factor and a dimensionless coupling constant. The collective oscillations of the atomic mass density, a technique that is commonly used in experiments, provide a signature of the quantum phase transition from the metallic phase to the Mott-insulator phase. A detailed experimental implementation is proposed.

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PubMed2005

Pattern formation as a signature of quantum degeneracy in a cold exciton system.

The development of a Turing instability to a spatially modulated state in a photoexcited electron-hole system is proposed as a novel signature of exciton Bose statistics. We show that such an instability, driven by kinetics of exciton formation, can result from stimulated processes that build up near quantum degeneracy. The stability of an electron-hole interface which describes recently observed exciton rings is analyzed. Interface instability occurs below a critical temperature, with a periodic 1D pattern developing via a continuous (type II) transition, in a qualitative agreement with observations.

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PubMed2005

Signatures of fractional statistics in noise experiments in quantum Hall fluids.

The elementary excitations of fractional quantum Hall (FQH) fluids are vortices with fractional statistics. Yet, this fundamental prediction has remained an open experimental challenge. Here we show that the cross-current noise in a three-terminal tunneling experiment of a two dimensional electron gas in the FQH regime can be used to detect directly the statistical angle of the excitations of these topological quantum fluids. We show that the noise also reveals signatures of exclusion statistics and of fractional charge. The vortices of Laughlin states should exhibit a bunching effect, while for higher states in the Jain sequences they should exhibit an "antibunching" effect.

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PubMed2005

Intermolecular dynamical charge fluctuations in water: a signature of the H-bond network.

We report a simulation of deuterated water using a Car-Parrinello approach based on maximally localized Wannier functions. This provides local information on the dynamics of the hydrogen-bond network and on the origin of the low-frequency infrared activity. The oscillator strength of the translational modes, peaked around approximately 200 cm-1, is anisotropic and originates from intermolecular--not intramolecular--charge fluctuations. These fluctuations are a signature of a tetrahedral hydrogen-bonding environment.

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PubMed2006

Magnetic anisotropy of Co2+ as signature of intrinsic ferromagnetism in ZnO:Co.

We report on the magnetic properties of thoroughly characterized Zn(1-x)Co(x)O epitaxial thin films, with low Co concentration, x = 0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.

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PubMed2006

Local tunneling spectroscopy as a signature of the Fulde-Ferrell-Larkin-Ovchinnikov state in s- and d-wave superconductors.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states for two-dimensional s- and d-wave superconductors (s- and d-SCs) are self-consistently studied under an in-plane magnetic field. While the stripe solution of the order parameter is found to have lower free energy in s-SCs, a square lattice solution appears to be energetically more favorable in the case of d-SCs. At certain symmetric sites, we find that the features in the local density of states (LDOS) can be ascribed to two types of bound states. We also show that the LDOS maps for d-SCs exhibit bias-energy-dependent checkerboard patterns. These characteristics can serve as signatures of the FFLO states.

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PubMed2006

Signatures for multi-alpha-condensed states.

An experimental way of testing Bose-Einstein condensation of alpha clusters in the atomic nucleus is reported. The enhancement of cluster emission and the multiplicity partition of possible emitted clusters could be direct signatures for the condensed states. The barrier for the emission of clusters, such as (8)Be and (12)C(O(+)(2)), is calculated and compared with the barrier for the sequential emission of 2 or 3alpha particles from the compound nucleus. For the calculations, a simple approach using a folded Woods-Saxon potential is used.

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PubMed2006

Signatures of quantum behavior in single-qubit weak measurements.

With the recent surge of interest in quantum computation, it has become very important to develop clear experimental tests for "quantum behavior" in a system. This issue has been addressed in the past in the form of the inequalities due to Bell and those due to Leggett and Garg. These inequalities concern the results of ideal projective measurements, however, which are experimentally difficult to perform in many proposed qubit designs, especially in many solid-state qubit systems. Here, we show that weak continuous measurements, which are often practical to implement experimentally, can yield particularly clear signatures of quantum coherence, both in the measured correlation functions and in the measured power spectrum.

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PubMed2006

Signatures of a noise-induced quantum phase transition in a mesoscopic metal ring.

We study a mesoscopic ring with an inline quantum dot threaded by an Aharonov-Bohm flux. Zero-point fluctuations of the electromagnetic environment capacitively coupled to the ring, with omega(s) spectral density, can suppress tunneling through the dot, resulting in a quantum phase transition from an unpolarized to a polarized phase. We show that robust signatures of such a transition can be found in the response of the persistent current in the ring to the external flux as well as to the bias between the dot and the arm. Particular attention is paid to the experimentally relevant cases of Ohmic (s = 1) and sub-Ohmic (s = 1/2) noise.

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