Modified Bridgman anvils for high pressure synthesis and neutron scattering

ABSTRACT

A simple modified Bridgman design for large volume pressure anvils usable in the Paris-Edinburgh (PE) press has been demonstrated at Oak Ridge National Laboratory Spallation Neutron Source. The design shows advantages over the toroidal anvils typically used in the PE press, mainly rapid compression/decompression rates, complete absence of blow-outs upon drastic phase transitions, simplified cooling, high reliability, and relative low loads (～40 tons) corresponding to relatively high pressures (～20?GPa). It also shows advantages over existing large-volume diamond cells as sample volumes of ～2–3?mm³ can be easily and rapidly synthesized. The anvils thus allow sample sizes sufficient for in situ neutron diffraction as well as rapid synthesis of adequate amounts of new materials for ex situ analysis via total neutron scattering and neutron spectroscopy.     

Photochromism in single nitrogen-vacancy defect in diamond

Photochromism in single nitrogen-vacancy optical centers in diamond is demonstrated. Time-resolved optical spectroscopy shows that intense irradiation at 514 nm switches the nitrogen-vacancy defects to the negative form. This defect state relaxes back to the neutral form under dark conditions. Temporal anticorrelation of photons emitted by the different charge states of the optical center unambiguously indicates that the nitrogen-vacancy defect accounts for both 575 nm and 638 nm emission bands. Possible mechanism of photochromism involving nitrogen donors is discussed. PACS 61.72.-y;61.72.Ji;03.67.-a     

A quantum computer based on NV centers in diamond: Optically detected nutations of single electron and nuclear spins

In an effort to realize a two-bit processor for a quantum computer on the basis of single nitrogen-vacancy defect centers (NV centers) in diamond, the optically detected nutations of the electron spin of a single NV center in the ground state and of the nuclear spin of a ¹³C atom located at a diamond lattice site nearest to the NV center are studied. The photodynamics of NV and NV + ¹³C centers under different temperatures and optical excitation conditions is discussed. A seven-level model of a center excited by radiation from an Ar⁺ laser at room temperature is proposed. On the basis of this model, the experimental spectra of optically detected electron paramagnetic and electron-nuclear double resonances of single NV and NV + ¹³C centers in diamond nanocrystals, as well as experimental data on the optically detected nutations of the electron and nuclear spins of these centers caused by the actions of pulsed microwave and radiofrequency fields, respectively, are interpreted.     

Polarized light propagating in a magnetic field as a probe for millicharged fermions

Possible extensions of the standard model of particle physics suggest the existence of particles with small, unquantized electric charge. Photon-initiated pair production of millicharged fermions in a magnetic field would manifest itself as a vacuum magnetic (VM) dichroism. We show that laser polarization experiments searching for this effect yield, in the mass range below 0.1 eV, much stronger constraints on millicharged fermions than previous laboratory searches. VM birefringence due to virtual pair production gives a slightly better constraint for masses between 0.1 and a few eV. We comment on the possibility that the VM dichroism observed by PVLAS arises from pair production of such millicharged fermions rather than from single production of axionlike particles. Such a scenario can be confirmed or firmly excluded by a search for invisible decays of orthopositronium with a branching-fraction sensitivity of about 10(-9).     

Optical biochemical sensor for determining hydroperoxides in nonpolar organic liquids as archetype for sensors consisting of amphiphilic conetworks as immobilisation matrices

This paper reports the successful design of a prototype of an optical biochemical sensor for the determination of hydroperoxides in nonpolar organic liquids. The sensor consists of a matrix of an amphiphilic polymer conetwork (APCN), a novel class of very promising polymeric materials for easy preparation of biochemical sensor matrices. APCNs are characterised by nanoscopic phase separation between the hydrophilic and the hydrophobic phases. For medium ratios of conetwork composition, the domains of both phases are interconnected both on the surface of the conetworks and throughout the bulk. The APCNs have peculiar swelling properties—the hydrophilic phase swells in hydrophilic media and the hydrophobic phase swells in hydrophobic media. In both types of media dissolved reagents can diffuse from the solution into the swollen phase of the polymeric conetwork. This enables loading of the hydrophilic phase of the APCNs with enzymes and indicator reagents by simple impregnation. Hydrophobic analytes can diffuse into the polymeric conetwork via its hydrophobic phase and react with indicator reagents immobilised in the hydrophilic phase at the huge internal interface between the two opposite phases. To prepare the described hydroperoxide-sensitive biosensors, we used APCN films consisting of 58% (w/w) poly(2-hydroxyethyl acrylate) (PHEA) as hydrophilic chains and 42% (w/w) polydimethylsiloxane (PDMS) as hydrophobic linkers. Horseradish peroxidase (HRP) and diammonium 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) as indicator reagent were co-immobilised in this optically clear and transparent matrix. In this feasibility study the conditions investigated were principally those relevant to characterisation of the innovative matrix material and the disposable biosensor produced from it; the biosensor was not optimised. Sensitivity toward tert-butylhydroperoxide (tBuOOH) dissolved in n-heptane was acceptable, between approximately 1 and at least 50 mmol L⁻¹, even in the dry state. The response time was 1.7 to 5.0 min. No leaching of immobilised reagents was observed during a period of at least one hour. Pre-swelling the sensors with water increased the reaction rate and the total turnover number of the enzyme. In a dry atmosphere at 4 °C the sensors were found to be stable for at least two weeks.     

Quantum registers based on single NV + <Emphasis Type="Italic">n</Emphasis><Superscript>13</Superscript>C centers in diamond: II. Spin characteristics of registers and spectra of optically detected magnetic resonance

The spin Hamiltonian method in combination with ab initio calculations of the spin characteristics of quantum registers that include an electron spin S = 1 of a single NV center in the ground electronic state and nuclear spins I = 1/2 of several atoms ¹³C located at different lattice sites near the vacancy of the NV center is applied to find eigenvalues and eigenfunctions of spin systems NV + n ¹³C for cases where the lattice sites nearest to the vacancy of the NV center contain one, two, or three ¹³C nuclear spins, as well as for cases where ¹³C atoms are located at sites more distant from the vacancy. For these single spin NV + n ¹³C systems, the spectra of optically detected magnetic resonance (ODMR) are calculated, which agree well with available experimental data.