Addendum to the paper "Dead-time free measurement of dipole-dipole interactions between electron spins" by M. Pannier, S. Veit, A. Godt, G. Jeschke, and H.W. Spiess [J. Magn. Reson. 142 (2000) 331-340

The development of four-pulse DEER as described, which has been published in the Journal of Magnetic Resonance more than 10 years ago. The corresponding paper is an example where a slight advance, such as adding a refocusing pulse, which in retrospect looks so simple, can have a remarkable impact on an entire field of science. In our case it offered a simple way to exact measurements of distances between defined species in the nanometer range. The current applications are mainly in determining structures of proteins and nucleic acids.     

An NMR thermometer for cryogenic magic-angle spinning NMR: the spin-lattice relaxation of (127)I in cesium iodide

The accurate temperature measurement of solid samples under magic-angle spinning (MAS) is difficult in the cryogenic regime. It has been demonstrated by Thurber et al. (J. Magn. Reson., 196 (2009) 84-87) [10] that the temperature dependent spin-lattice relaxation time constant of ⁷⁹Br in KBr powder can be useful for measuring sample temperature under MAS over a wide temperature range (20–296 K). However the value of T₁ exceeds 3 min at temperatures below 20 K, which is inconveniently long. In this communication, we show that the spin-lattice relaxation time constant of ¹²⁷I in CsI powder can be used to accurately measure sample temperature under MAS within a reasonable experimental time down to 10 K.     

Bunch length measurement at Tsinghua Thomson scattering X-ray source

The length of electron beam from a photocathode RF gun is determined by a spectrometer, according to the relative energy spread induced by the bunch length during the acceleration in a linac. For a photocathode RF gun, different laser injected phase and beam charge are studied. The compression is changed for the different laser phases, as from 10° to 30°, and the bunch length is lengthened due to the strong longitudinal space charge force, caused by the increased charge.     

Efficient preparation of a four-photon cluster state with homodyne measurement via cross-Kerr nonlinearity

A scheme is proposed for generating a multiphoton entangled cluster state among four modes. The scheme only uses Kerr medium, beam splitter and homodyne measurements on coherent light fields, which can be efficiently made in quantum optical laboratories. The photon in the signal mode is prepared in a superposition state of the vacuum state and one-photon state while the probe beam is initially set in a coherent state superposition. The strong probe mode interacts successively with multiple signal-mode photo...     

Evaluating practical measurements of fire-induced vent flows with stereoscopic PIV

Stereoscopic particle image velocimetry (SPIV) was applied to a fire-induced doorway flow to provide the velocity field for computations of the mass flow rate of air into an enclosure. For a flow of uniform temperature and concentration, the technique met all of the requirements to provide the best estimate of the mass flow rate. Simultaneous measurements of vertical distributions of velocity and temperature were also conducted with conventional vent flow techniques, bi-directional probes and thermocouples. Correction factors for mass flow rate computations using the conventional techniques were determined based on comparisons to the SPIV results. An average correction factor of unity was determined for the bi-directional probe technique thus further confirming the utilization of velocity distributions acquired using the technique in mass flow rate computations. An average correction factor of 0.69 was determined for mass flow rates computed from vertical temperature distributions inside and outside the enclosure. This agrees with average correction factors determined in previous studies. The results from the present study suggest that the conventional techniques, which are practical and affordable for routine fire testing, may be applied with greater confidence for fires up to 500 kW.     

Eu-activated Ba2Mg(BO3)2 yellow-emitting phosphors for near ultraviolet-based light-emitting diodes

A series of Eu²⁺-activated Ba₂Mg(BO₃)₂ yellow phosphors were prepared by a high temperature solid-state reaction. The phosphor emits intense yellow light under near ultraviolet excitation. Large Stokes shift can be attributed to the asymmetric nature of the Eu site and the lack of rigidity in the host. The concentration self-quenching mechanism of Ba₂Mg(BO₃)₂:Eu²⁺ is d-d interaction and the critical transfer distance is calculated to be about 12.29 Å. Prototype light-emitting diodes were fabricated by coating the Ba₂Mg(BO₃)₂:0.07Eu²⁺ phosphor onto ∼370 nm-emitting InGaN chips. The LEDs exhibit intense yellow-emitting under a forward bias of 20 mA. The results indicate that Eu²⁺-activated Ba₂Mg(BO₃)₂ is a candidate as a yellow component for fabrication of near-UV white light-emitting diodes.     

Facile synthesis of thiol-stabilized CdSexTe1−x nanocrystals

CdSe_xTe_1−x nanocrystals (x=0.25, 0.40, 0.50, 0.60 and 0.75) were synthesized using thioglycerol as a stabilizing agent. The composition of the CdSe_xTe_1−x nanocrystals was precisely controlled by tuning the precursor (Se/Te) ratio. The structural, morphological and optical properties of the nanocrystals were analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), diffused reflectance spectroscopy (DRS) and photoluminescence (PL) measurements. It is found that the Se/Te ratio significantly affects the properties of the resultant CdSe_xTe_1−x nanocrystals. XRD pattern of the CdSe_xTe_1−x nanocrystals revealed cubic, hexagonal and mixed phases depending on the ratio of Se:Te. Surface morphology of the CdSe_xTe_1−x nanocrystals showed nanoclusters of sizes ∼50 nm, with the adjacent cluster interlinking each other. DRS revealed the size dependence band gap energy prevailing in the CdSe_xTe_1−x nanocrystals from 1.52 to 2.66 eV due to the optical bowing effect. PL measurements exhibited band edge emission in the visible spectral region, and are red shifted with increase in Se concentration. The facile route employed in the present work to synthesis the CdSe_xTe_1−x nanocrystals in an aqueous medium is simple and controllable, and the strategy presented will be handy in preparing diverse semiconducting nanocrystals.     

Structural stabilities, electronic, elastic and optical properties of SrTe under pressure: A first-principles study

The structural, electronic, elastic and optical properties as well as phase transition under pressure of SrTe have been systematically investigated by first-principles pesudopotential calculations. Five possible phases of SrTe have been considered. Our results show that SrTe undergoes a phase transition from NaCl-type (B1) to CsCl-type (B2) structure at 10.9 GPa with a volume collapse of 9.43%, and no further transition is found. We find that SrTe prefer h-MgO instead of wurtzite (B4) structure for its metastable phase because that the ionic compound prefers a high coordination. The elastic moduli, energy band structures, real and imaginary parts of the dielectric functions have been calculated for all considered phases, and we find that a smaller energy gap yields a larger high-frequency dielectric constant. Our calculated results are discussed and compared with the available experimental and theoretical data.     

Optical properties of superhard BC5: First-principles calculations

BC₅ is a newly synthesized superhard material. We present a systematic investigation of optical properties of BC₅ in P3m1 and I-4m2 phases at ambient and high pressure in the framework of density functional theory with the generalized gradient approximation (GGA) in this paper. Optical properties such as dielectric function, refractive index, absorption, reflectivity and electron energy-loss spectrum are obtained successfully. The feature in the spectra of the optical parameters is discussed. Through calculation, we find BC₅ is optically anisotropic. Moreover, the dielectric function exhibits a large change at 70 GPa pressure for P3m1 BC₅ phase, but I-4m2 phase not, indicating the stable electronic structure that the I-4m2 phase possesses.