Thick Single Crystals Research Articles

Measurement of the linear polarization of Parametric X-radiation

The linear polarization of Parametric X-radiation (PXR) produced by 80.5 MeV electrons in a 13 μm thick silicon single crystal has been analyzed by means of a novel method of polarimetry exploiting directional information of the photoeffect in a charge coupled device consisting of 1.3×10 6 square pixels of 6.8 μm. The experiment was carried out at the Darmstadt superconducting linear accelerator S-DALINAC providing a low-emittance electron beam. The linear polarization of the (2 2 0) reflection observed in eight narrow angular bins between 20 ∘ and 21 ∘ with respect to the electron beam direction is consistent with complete local linear polarization. The orientation of the polarization plane, within measurement errors of typically 10 ∘, varies over the diffraction pattern in such a way as to be expected from kinematical theory. The result of this experiment is in contradiction to the only other PXR polarization measurement performed so far.

Analysis of the Angular Distribution and the Intensity of Parametric X-Ray Radiation in a Bragg Case

The angular distributions of PXR from 250 µm and 3 mm thick silicon single crystals were measured in a Bragg geometry and the numerical calculation for them were also carried out. In the calculation a Gaussian distribution was used for the angular divergence of the beam, which is due to the multiple scattering of electrons in the material. The intensity ratios of the higher order reflections to the fundamental one suggest that the absorption of X-rays in the crystal is small. The obtained angular distributions are also explained by including the effect of the anomalous transmission to the kinematical theory practically.

Selective Area Growth of GaN by MOVPE and HVPE

AbstractRecent successful results on the selective area growth (SAG) of GaN that has been done by MOVPE and HVPE are shown. The SAG were carried out on MOVPE-grown GaN (0001) / sapphire substrates with lined or dotted SiO2 masks. Sub-micron GaN dot and line structures are fabricated by the SAG in MOVPE, so that smoothly overgrown GaN layers are successfully realized using the epitaxially lateral overgrowth (ELO) technique. The ELO structures are confirmed to be good quality GaN single crystal with a smooth surface, no grain boundaries, and low-dislocation densities. In addition, thick GaN bulk single crystals without any cracks are grown by the SAG in HVPE. Crystalline and optical properties of the GaN bulk are much improved. The reduction in the thermal strain due to the growth on the limited area as well as the ELO are found to be effective to reduce crystalline defects of the GaN bulk single crystals.

Trajectory dependent charge-state distribution of planar channeled heavy ions

The exit charge state and exit angle distribution of 103.5 MeV oxygen ions transmitted through a 1 μm thick Si single crystal was measured as a function of the incident charge state and incident angle with respect to the (110) planar channeling direction. The aim of the present measurement was to investigate the dependence of electron loss and electron capture probabilities on the impact parameter. The conservation rule of the transverse energy allows us to determine the channeling trajectory of the ion from its incident and exit angles. A series of exit charge state and scattering angle distributions, Y( θ sc, q ex; θ in, q in, t), were measured by scanning the incident angle, θ in, with a step of 0.01° for incident charge states of q in = 6, 7 and 8 at path lengths of t = 9500 and 10500 A ̊ . Two parameter mappings of the yields as functions of θ in and θ sc clearly show characteristic features reflecting the combination of q in and q ex, which enabled us to deduce impact-parameter dependent electron transfer cross sections with an aid of a Monte Carlo simulation.

Angular dependence of the electronic energy loss for low energy heavy ions under channeling conditions

The electronic stopping of heavy ions with 2 ≤ Z 1 ≤ 36 in 〈110〉 direction of a 350 nm thick silicon single crystal was measured for velocities below Bohr velocity. Large oscillations in the electronic stopping power are observed for best-channeled ions of the same velocity, whereas for ions emerging from the foil some degrees to the channel axis the oscillations are strongly reduced.

Electronic structure of ultrathin ordered iron oxide films grown onto Pt(111).

It has recently been shown that well-ordered iron oxide films can be prepared epitaxially onto Pt(111) surfaces. We have investigated a one-monolayer-thick film with FeO stoichiometry and a ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$(111) multilayer. Moreover, we prepared well-ordered multilayers with \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ stoichiometry. Core and valence level photoemission data clearly reveal that the ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ and \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ multilayers on Pt(111) are chemically identical to thick single crystals and polycrystalline powder samples. The x-ray-absorption near-edge structures (XANES) at the O K edge arise from the hybridization between O 2p and Fe 3d states as well as Fe 4sp states, showing that the latter also are important for the bonding interaction of the oxides. Polarization-dependent XANES measurements at the O K edge reveal a FeO bond length in the monolayer of 2.23\ifmmode\pm\else\textpm\fi{}0.22 \AA{}, which almost corresponds to the bond length in the bulk FeO (2.15 \AA{}). The electronic structure of the iron oxides derived from the set of spectra presented will be described in terms of the ligand-field theory taking into consideration the exchange interaction between the iron 3d electrons.

Radiative electron capture by bare and H-like Si and Cl ions using the channeling technique and the associated solid-state effect.

Fully stripped and hydrogenlike Si and Cl ions, in the energy range 2--5 MeV/amu and channeled through a 0.17 \ensuremath{\mu}m thick Si single crystal, were used to study the radiative electron capture (REC) into the K shell of the ions. The associated effects due to the solid medium of the target were also investigated from the energy and the derived cross section of the REC photons. The K-shell REC cross sections were found to fall on a universal curve when plotted against the adiabaticity parameter of the collision system. The measured energy shifts in the REC photon energy and the higher yield of REC photons using crystalline targets, as compared to the available gas-target data, are indicative of an ``ion--solid-state effect'' caused by the electron-wake potential. The magnitude of these effects is shown to increase with Z/v of an ion with charge Z and velocity v, and are in qualitative agreement with recent calculations. The REC cross sections are, however, observed to be slightly smaller than the theoretical estimates after taking into account the effects associated with the solid medium. The widths of the REC peaks are compared with the theoretical models. The results of the present investigation using Si and Cl ions are presented coherently along with our earlier data using lighter ions.

Beam monitor for undulator white radiation in the hard-x-ray range

In this paper an instrument for white x-ray beam monitoring suitable for experiments with undulator beams at ESRF is presented. It is based on a 500 μm thick beryllium single crystal that is interposed in the beam. Information about the shape and position of the beam is obtained from the image of a Bragg reflection recorded by an x-ray camera. When the position is determined from a single video frame, the resolution is about 10 μm. The mosaic spread and the defects of the beryllium crystal degrade the fidelity of the beam images but do not affect the resolution of the instrument as a position monitor. The major limitation comes from the unavailability of large enough Be crystals of good quality.

The resolution function of a triple-crystal diffractometer for high-energy synchrotron radiation in nondispersive Laue geometry

The k-space resolution function of a triple-crystal diffractometer is calculated for an arrangement of three perfect silicon single crystals Bragg diffracting in nondispersive Laue geometry. A comparison is made with the results of measurements using synchrotron radiation in the energy range from 80 to 150 keV. In this case, absorption is very weak and according to dynamical theory the width of the diffraction pattern of thick perfect single crystals is proportional to the wavelength λ, whereas its Lorentzian tails are proportional to λ2. Together with the fact that the Bragg angles are only of the order of 2°, this leads to a concentration of the starlike k-space resolution function into a narrow band parallel to the reciprocal-lattice vector G. For diffraction of 80 keV synchrotron radiation at the silicon 111 reflection, the full width at half-maximum (FWHM) of the intensity distribution in the scattering plane is 1.1 × 10−5 Å−1 perpendicular to G and 2.2 × 10−4 Å−1 parallel to G. The observed differences in the contributions from monochromator and analyzer crystal to the resolution function are explained by the finite width of the electronic window of the detector counting chain and the non-Bragg scattering contribution from the crystals. If annealed Czochralski-grown silicon single crystals with a mosaicity of ~3′′ are used as monochromator and analyzer, the resolution is reduced by one order of magnitude, but for studies of imperfect samples or of diffuse scattering large gains in intensity can be accomplished this way.

Gamma-ray optics using57Co-doped iron single crystals

We have begun a search for “anomalous emission” and “anomalous temporal decay” of nuclear resonant radiation using internal sources located inside thick single crystals. Our samples are two iron single crystals of thicknesses 0.5 and 1.0 mm that have been doped on one side with57Co. From ordinary considerations, these samples are much too thick to allow any 14.4-keV radiation to pass through the crystal and out the other side. However, our results suggest that 14.4-keV radiation is able to be transmitted through the crystals. Furthermore, using coincidence techniques we examined the temporal decay of the 14.4-keV nuclear level under these circumstances.

Microwave properties perpendicular to the CuO 2 planes

The temperature dependence of the out-of-plane penetration depth λ c and the c-axis conductivity σ 1 c at 10 GHz was determined from the measurements of the surface impedance Z s in thick single crystals of La 2−xSr xCuO 4 ( x=0.09, 0.12, 0.15, 0.18, 0.19) and YBa 2Cu 3O 7−δ (δ≈0.1, 0.3). It is found that the temperature dependence of λ c is different from that of in-plane penetration depth λ ab in La 2−xSr xCuO 1 system. Also found is that the enhancement below T c is observed in δ 1 ab ( T) for all the crystals of YBa 2Cu 3O 7−δ, while the enhancement in δ 1( T) is drastically reduced with increasing δ. These results indicate that the out-of-plane electrodynamics in the superconducting state of high- T c cuprates is quite different from that in the planes.

Crystal growth and resitive upper critical field in Tl 2Ba 2CuO x and Tl 2Ba 2CaCu 2O y

Large thick single crystals of Tl 2Ba 2CuO x and Tl 2Ba 2CaCu 2O y have been successfully grown in a liquid coexisting with solid. It has been revealed that the resistive transition of Tl 2Ba 2CuO x shows essentially parallel shift upon application of magnetic filed while that of Tl 2Ba 2CaCu 2O y shows significant broadening. Temperature dependence of resistive upper critical field exhibits unusual upward curvature down to the lowest temperature.

Angular distributions of channeled particles

The paper describes angular distributions of channeled particles in thick single crystals. It is shown that the distribution of channeled particles over the angles may be calculated if the statistical equilibrium in the transverse phase space takes place. The results obtained in computer simulation are compared with analytical calculations for axially channeled electrons. It is shown, that due to the influence of the continuum string potential the angular distribution in the oriented crystal becomes essentially broader than one in an amorphous medium.

Growth and anisotropic superconducting properties of Tl 2Ba 2CuO 6 and Tl 2Ba 2CaCu 2O 8 single crystals

Large thick homogeneous single crystals of Tl 2Ba 2CuO 6 and Tl 2Ba 2CaCu 2O 8 superconductors have been successfully grown by the self-flux method under the liquid-solid coexisting condition. The anisotropy of the resistive superconducting transition in magnetic fields and the resistive H c2 have been studied using as-grown crystals. The temperature dependence of the resistive H c2 in Tl 2Ba 2CuO 6 is found to show a positive curvature down to the lowest temperature (∼0.06 T c(0)).

A large area cooled-CCD detector for electron microscopy

Large area cooled-CCDs are an excellent medium for (indirectly) recording electron images and electron diffraction patterns in real time and for use in electron tomography; real-time imaging is extremely useful in making rapid adjustments in the electron microscope. CCDs provide high sensitivity (useful for minimising dosage to radiation-sensitive biological specimen), good resolution, stable performance, excellent dynamic range and linearity and a reasonably fast readout. We have built an electron imaging device based on the EEV 1152 by 814 pixel CCD which is controlled from a unix based SUN Sparestation operating under X-Windows. The incident 100 kV electrons are converted to visible light in a 0.5 mm thick YAG single crystal which is imaged through a lens on to the CCD. The CCD electronics is designed to be as flexible as possible and allows a wide variation in the readout speed to cater for the relatively fast application where readout noise is less critical and low readout noise applications where the extra few seconds of readout time are not significant. The CCD electronics is built in VME format which is controlled through a S-bus to VME driver. With two parallel channels of readout the whole image can be read out in ∼ 1 s (using the faster readout speed) with 16 bit precision and the image is displayed under X-Windows in a few seconds. The present readout works at 500 kHz and has a noise of ∼ 30 e rms per pixel. With a Peltier cooling device we can operate the CCD at ∼ −40°C which reduces the dark current adequately to allow exposures of up to several minutes. Several examples of patterns collected with the system on a Philips CM12 microscope will be presented.

Some new aspects of the axial electron channeling process

The axial channeling of 1–10 GeV electrons in thick single crystals is investigated by means of Monte Carlo computer simulation. The dynamics of volume capture is discussed in detail. The partial and total lifetimes in the axial channel are calculated for the first time. The statistics of close nuclei collisions is analysed.

Bulk magnetic structure studies using polarized epithermal neutrons

Parity violating effects in the nucleus are normally very small, but in certain compound nuclear resonances such effects are enhanced to several percent, as in the 0.734 eV p-wave resonance in 139La. We have used this enhanced parity violation to measure the depolarization of neutrons traversing a thick single crystal of ferromagnetic holmium. The depolarization was dependent on the relative orientation of the neutron polarization and the holmium c crystalline axis. The effect was explained in terms of neutron precession in an array of highly correlated domains magnetized in the c axis direction. The results show the high degree of correlation of magnetic domains in the single crystal and demonstrate the usefulness of epithermal neutrons in measuring magnetic properties of thick crystals. The use of other parity violating resonances should allow polarization analysis of neutrons over a range of neutron energies.

Electromagnetic radiation of MeV electrons in thick aligned single crystals

Abstract Earlier published theoretical models for MeV electrons are generalized in this paper. Different theoretical predictions for planar channeling and accompanying electromagnetic radiation in thick crystals are presented. A comparative analysis of theoretical and experimental spectra of photon radiation is given. Thickness dependences of channeling quantum state populations, radiation line broadening and photon flux intensities are obtained. The existence of planar electron channeling at 54 MeV in a silicon single crystal with a thickness of several millimetres is shown.

Solution-grown single crystals of random terpolymer liquid crystal polymers: Chain folding, morphology, and effect of annealing

Abstract Thin (100–150 A thick) single crystals of large lateral dimensions have been grown from dilute solution in xylene of random terpolymer, thermotropic, liquid crystal polymers containing approximately equimolar amounts of C6 or C7 flexible segments, oxybenzoate, and dioxyphenyl. With both polymers having molecular lengths of 800 A or longer, electron diffraction patterns indicate chain folding. The crystal structure, hexagonal, is less perfect than found previously for thin films of the C7 (and C5) polymer crystallized from the nematic state by slow cooling (orthorhombic). Annealing of the crystals below the crystal-liquid crystal transition results in merging of overgrowths and rough crystal edges into the main crystal and a surface roughening, followed, at longer times and/or higher temperatures, by lamella thickening. Annealing in the nematic state results in the development of rosettes of lamellae. Implications of the results for crystallization mechanisms and morphology of the nematic state ar...

Depolarization of neutrons in ferromagnetic holmium by means of enhanced nuclear parity violation in 139La.

The depolarization of epithermal neutrons in a thick single crystal of ferromagnetic holmium has been measured and analyzed with a model of neutron precession through a highly correlated array of magnetic domains. The large parity violation in the 0.734-eV p-wave resonance in $^{139}\mathrm{La}$ was used to analyze the neutron polarization, and represents an application of parity violation in nuclear resonances to a measurement in condensed-matter physics.