Evaluation of Crack Propagation and Post-cracking Hinge-type Behavior in the Flexural Response of Steel Fiber Reinforced Concrete

An experimental evaluation of crack propagation and post-cracking behavior in steel fiber reinforced concrete (SFRC) beams, using full-field displacements obtained from the digital image correlation technique is presented. Surface displacements and strains during the fracture test of notched SFRC beams with volume fractions (Vf) of steel fibers equal to 0.5 and 0.75% are analyzed. An analysis procedure for determining the crack opening width over the depth of the beam during crack propagation in the flexure test is presented. The crack opening width is established as a function of the crack tip opening displacement and the residual flexural strength of SFRC beams. The softening in the post-peak load response is associated with the rapid surface crack propagation for small increases in crack tip opening displacement. The load recovery in the flexural response of SFRC is associated with a hinge-type behavior in the beam. For the stress gradient produced by flexure, the hinge is established before load recovery is initiated. The resistance provided by the fibers to the opening of the hinge produces the load recovery in the flexural response

Theory of Spin-Conserving Excitation of the N−V−N-V^- Center in Diamond

The negatively charged nitrogen-vacancy defect ($N-V^-$ center) in diamond is an important atomic-scale structure that can be used as a qubit in quantum computing and as a marker in biomedical applications. Its usefulness relies on the ability to optically excite electrons between well-defined gap states, which requires clear and detailed understanding of the relevant states and excitation processes. Here we show that by using hybrid density-functional-theory calculations in a large supercell we can reproduce the zero-phonon line and the Stokes and anti-Stokes shifts, yielding a complete picture of the spin-conserving excitation of this defect.Comment: 4 pages, 2 figure

All-optical hyperpolarization of electron and nuclear spins in diamond

Low thermal polarization of nuclear spins is a primary sensitivity limitation for nuclear magnetic resonance. Here we demonstrate optically pumped (microwave-free) nuclear spin polarization of $^{13}\mathrm{C}$ and $^{15}\mathrm{N}$ in $^{15}\mathrm{N}$-doped diamond. $^{15}\mathrm{N}$ polarization enhancements up to $-2000$ above thermal equilibrium are observed in the paramagnetic system $\mathrm{N_s}^{0}$. Nuclear spin polarization is shown to diffuse to bulk $^{13}\mathrm{C}$ with NMR enhancements of $-200$ at room temperature and $-500$ at $\mathrm{240~K}$, enabling a route to microwave-free high-sensitivity NMR study of biological samples in ambient conditions.Comment: 5 pages, 5 figure

Theoretical model of the dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide

Dynamic nuclear spin polarization (DNP) mediated by paramagnetic point defects in semiconductors is a key resource for both initializing nuclear quantum memories and producing nuclear hyperpolarization. DNP is therefore an important process in the field of quantum-information processing, sensitivity-enhanced nuclear magnetic resonance, and nuclear-spin-based spintronics. DNP based on optical pumping of point defects has been demonstrated by using the electron spin of nitrogen-vacancy (NV) center in diamond, and more recently, by using divacancy and related defect spins in hexagonal silicon carbide (SiC). Here, we describe a general model for these optical DNP processes that allows the effects of many microscopic processes to be integrated. Applying this theory, we gain a deeper insight into dynamic nuclear spin polarization and the physics of diamond and SiC defects. Our results are in good agreement with experimental observations and provide a detailed and unified understanding. In particular, our findings show that the defects' electron spin coherence times and excited state lifetimes are crucial factors in the entire DNP process

Carbon antisite clusters in SiC: a possible pathway to the D_{II} center

The photoluminescence center D_{II} is a persistent intrinsic defect which is common in all SiC polytypes. Its fingerprints are the characteristic phonon replicas in luminescence spectra. We perform ab-initio calculations of vibrational spectra for various defect complexes and find that carbon antisite clusters exhibit vibrational modes in the frequency range of the D_{II} spectrum. The clusters possess very high binding energies which guarantee their thermal stability--a known feature of the D_{II} center. The di-carbon antisite (C_{2})_{Si} (two carbon atoms sharing a silicon site) is an important building block of these clusters.Comment: RevTeX 4, 6 pages, 3 figures Changes in version 2: Section headings, footnote included in text, vibrational data now given for neutral split-interstitial, extended discussion of the [(C_2)_Si]_2 defect incl. figure Changes version 3: Correction of binding energy for 3rd and 4th carbon atom at antisite; correction of typo

Characterization of the nitrogen split interstitial defect in wurtzite aluminum nitride using density functional theory

We carried out Heyd-Scuseria-Ernzerhof hybrid density functional theory plane wave supercell calculations in wurtzite aluminum nitride in order to characterize the geometry, formation energies, transition levels and hyperfine tensors of the nitrogen split interstitial defect. The calculated hyperfine tensors may provide useful fingerprint of this defect for electron paramagnetic resonance measurement.Comment: 5 pages, 3 figure

The Principles of Social Order. Selected Essays of Lon L. Fuller, edited With an introduction by Kenneth I. Winston

The electron spins of semiconductor defects can have complex interactions with their host, particularly in polar materials like SiC where electrical and mechanical variables are intertwined. By combining pulsed spin resonance with ab initio simulations, we show that spin-spin interactions in 4H-SiC neutral divacancies give rise to spin states with a strong Stark effect, sub-10(-6) strain sensitivity, and highly spin-dependent photoluminescence with intensity contrasts of 15%-36%. These results establish SiC color centers as compelling systems for sensing nanoscale electric and strain fields

Engineering chromium related single photon emitters in single crystal diamond

Color centers in diamond as single photon emitters, are leading candidates for future quantum devices due to their room temperature operation and photostability. The recently discovered chromium related centers are particularly attractive since they possess narrow bandwidth emission and a very short lifetime. In this paper we investigate the fabrication methodologies to engineer these centers in monolithic diamond. We show that the emitters can be successfully fabricated by ion implantation of chromium in conjunction with oxygen or sulfur. Furthermore, our results indicate that the background nitrogen concentration is an important parameter, which governs the probability of success to generate these centers.Comment: 14 pages, 5 figure