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PubMed2004

Shallow donors in diamond: chalcogens, pnictogens, and their hydrogen complexes.

The utility of diamond as an electronic material is compromised by the lack of a suitable shallow donor. Here, ab initio theory is used to investigate the donor levels of substitutional pnictogen (N, P, As, and Sb) and chalcogen (S, Se, and Te) impurities and chalcogen-hydrogen defects in diamond. Substitutional S is found to be a deep donor, while As and Sb possess donor levels significantly shallower than P, which so far is the most effective shallow donor found by experiment.

Journal Article↗
PubMed2004

Shock compressing diamond to a conducting fluid.

Laser generated shock reflectance data show that diamond undergoes a continuous transition from optically absorbing to reflecting between Hugoniot pressures 600<P(H)<1000 GPa. The data are consistent with diamond having a thermal population of carriers at P(H) approximately 600 GPa, undergoing band overlap metallization at P(H) approximately 1000 GPa and melting at 800<P(H)<1000 GPa. The results agree well with an equation of state model that predicts that elemental carbon remains solid throughout the interior of Neptune.

Journal Article↗
PubMed2004

Three-dimensional MgB2-type superconductivity in hole-doped diamond.

We substantiate by numerical and analytical calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB2, albeit in three dimensions. Holes at the top of the zone-centered, degenerate sigma-bonding valence-band couple strongly to the optical bond-stretching modes. The increase from two to three dimensions reduces the mode softening crucial for T(c) reaching 40 K in MgB2. Even if diamond had the same bare coupling constant as MgB2, which could be achieved with 10% doping, T(c) would be only 25 K. Superconductivity above 1 K in Si (Ge) requires hole doping beyond 5% (10%).

Journal Article↗
PubMed2004

Superconductivity in boron-doped diamond.

Superconductivity of boron-doped diamond, reported recently at T(c)=4 K, is investigated exploiting its electronic and vibrational analogies to MgB2. The deformation potential of the hole states arising from the C-C bond-stretch mode is 60% larger than the corresponding quantity in MgB2 that drives its high T(c), leading to very large electron-phonon matrix elements. The calculated coupling strength lambda approximately 0.5 leads to T(c) in the 5-10 K range and makes phonon coupling the likely mechanism. Higher doping should increase T(c) somewhat, but the effects of three dimensionality primarily on the density of states keep doped diamond from having a T(c) closer to that of MgB2.

Journal Article↗
PubMed2004

Stacking-fault based microscopic model for platelets in diamond.

We propose a new model for {001} platelets in diamond based on the formation of a metastable stacking-fault. The core of the defect is a double layer of threefold coordinated sp2 carbon atoms embedded in the sp3 diamond matrix. The properties of the model were determined using ab initio calculations. All significant experimental signatures attributed to the platelets are fully accounted for. The model is also very appealing from the point of view of kinetics, since naturally occurring shearing processes will lead to the formation of the metastable fault.

Journal Article↗
PubMed2005

Initial stages of Ti growth on diamond (100) surfaces: from single adatom diffusion to quantum wire formation.

Using first-principles total energy calculations within density functional theory, we investigate the energetics, kinetics, and transport properties of Ti on clean and hydrogen-terminated diamond (100)-2x1 surfaces at increasing Ti coverages. On a clean surface, an isolated Ti adatom prefers to adsorb on top of a C-C dimer row, and also diffuses faster along the dimer row direction. As the Ti coverage increases, the preferred adsorption site converts from an atop site to a site located between the dimer rows. Passivation of the surface at the monohydride coverage not only greatly enhances the Ti mobility, but also weakens the diffusion isotropy. Based on these energetic and kinetic characteristics, we propose a viable approach to fabricate ideal Ti quantum wires on hydrogen-terminated diamond substrates.

Journal Article↗
PubMed2005

Importance of quantum tunneling in vacancy-hydrogen complexes in diamond.

Our ab initio calculations of the hyperfine parameters for negatively charged vacancy-hydrogen and nitrogen-vacancy-hydrogen complexes in diamond compare static defect models and models which account for the quantum tunneling behavior of hydrogen. The static models give rise to hyperfine splittings that are inconsistent with the experimental electron paramagnetic resonance data. In contrast, the hyperfine parameters for the quantum dynamical models are in agreement with the experimental observations. We show that the quantum motion of the proton is crucial to the prediction of symmetry and hyperfine constants for two simple defect centers in diamond. Static a priori methods fail for these systems.

Journal Article↗
PubMed2006

Low-energy electrodynamics of superconducting diamond.

Heavily boron-doped, diamond films can become superconducting with critical temperatures Tc well above 4 K. Here we first measure the reflectivity of such a film down to 5 cm(-1), by also using coherent synchrotron radiation. We thus determine the optical gap 2Delta, the field penetration depth lambda, the range of action of the Ferrell-Glover-Tinkham sum rule, and the electron-phonon spectral function alpha2F(omega). We conclude that diamond behaves as a dirty BCS superconductor.

Journal Article↗
PubMed2003

Diamond planar refractive lenses for third- and fourth-generation X-ray sources.

The fabrication and testing of planar refractive hard X-ray lenses made from bulk CVD diamond substrates is reported. The lens structures were generated by electron-beam lithography and transferred by reactive-ion etching into the diamond. Various lens designs were fabricated and tested at 12.4 and 17.5 keV photon energy. Efficiencies of up to 71% and gains of up to 26 were achieved. A line focus of 3.2 micro m (FWHM) was measured. These lenses should be able to withstand the extreme flux densities expected at the planned fourth-generation X-ray sources.

Journal Article↗
PubMed2005

Diamond thermal expansion measurement using transmitted X-ray back-diffraction.

The linear thermal expansion coefficient of diamond has been measured using forward-diffracted profiles in X-ray backscattering. This experimental technique is presented as an alternative way of measuring thermal expansion coefficients of solids in the high-resolution Bragg backscattering geometry without the intrinsic difficulty of detecting the reflected beam. The temperature dependence of the lattice parameter is obtained from the high sensitivity of the transmitted profiles to the Bragg angle variation with temperature. The large angular width of the backscattering profiles allows the application of this technique to mosaic crystals with high resolution. As an application of this technique the thermal expansion coefficient of a synthetic type-Ib diamond (110) single crystal was measured from 10 to 300 K. Extremely low values (of the order of 1 x 10(-7) +/- 5 x 10(-7)) for the linear thermal expansion coefficient in the temperature range from 30 to 90 K are in good agreement with other reported measurements.

Journal Article↗
PubMed2005

X-ray emission spectroscopy with a laser-heated diamond anvil cell: a new experimental probe of the spin state of iron in the Earth's interior.

Synchrotron-based X-ray emission spectroscopy (XES) is well suited to probing the local electronic structure of 3d transition metals such as Fe and Mn in their host phases. The laser-heated diamond anvil cell technique is uniquely capable of generating ultra-high static pressures and temperatures in excess of 100 GPa and 3000 K. Here X-ray emission spectroscopy and X-ray diffraction have been interfaced with the laser-heated diamond cell for studying the electronic spin states of iron in magnesiowüstite-(Mg0.75,Fe0.25)O and its crystal structure under lower-mantle conditions. X-ray emission spectra of the ferrous iron in a single crystal of magnesiowüstite-(Mg0.75,Fe0.25)O indicate that a high-spin to low-spin transition of ferrous iron occurs at 54 to 67 GPa and 300 K and the ferrous iron remains in the high-spin state up to 47 GPa and 1300 K. This pilot study points to the unique capability of the synchrotron-based XES and X-ray diffraction techniques for addressing the issue of electronic spin transition or crossover in 3d transition metals and compounds under extreme high-P-T conditions.

Journal Article↗
PubMed2005

Strategies for reducing preferred orientation and strain in powder samples for high-pressure synchrotron X-ray diffraction in diamond-anvil cells.

Among the many problems associated with high-pressure X-ray diffraction from polycrystalline samples in the diamond-anvil cell are strain and preferred orientation. A method is presented for efficiently reducing preferred orientation of powder samples compressed in diamond-anvil cells to pressures in excess of 20 GPa. This method may be successfully applied to samples of yield strength higher than alkalihalides. In addition, the problem of strain is discussed using ice-VII as an example and as an illustration of the importance of laser heating as a method of minimizing strain.

Journal Article↗
PubMed1996

Applications of synchrotron radiation to defect characterization and pendellösung fringe-spacing measurement in a natural diamond.

A small polished rectangular block of natural diamond unusually free from impurity zoning was examined by birefringence and synchrotron X-ray topography to map internal defects and assess effects of some accidental mechanical damage. ;Orthogonal-view' projection topographs facilitated comparisons between optical and X-ray images. Speedy and precise positioning of crystal sections cut in synchrotron X-ray section topographic studies was aided by radiographic and fluoroscopic procedures, which are described. Several X-ray reflection and refraction phenomena are illustrated, including high-contrast refraction images of crystal edges, chipping and crack outcrops. It was demonstrated that by combining optical and X-ray goniometry it was possible to record Pendellösung fringe systems employing precisely selected X-ray wavelengths in well defined crystal volumes. Valuable gain in fringe visibility resulted from the use of pure sigma-mode polarized synchrotron radiation. Absolute values of the diamond 004 structure factor were derived from five fringe systems measured, the findings (in upward order) being 11.7, 11.7, 12.0, 12.5, 12.6. Their scatter greatly exceeds the uncertainty in the measurement procedure (believed to be within +/-1%) and is attributed to different strain distributions within the five volumes sampled.

Journal Article↗
PubMed1997

Diamond beam-position monitor for undulator radiation and tests at the Tristan Super Light Facility.

A photon beam-position monitor using a diamond foil as a position-sensitive device has been developed for use on insertion-device beamlines of third-generation synchrotron radiation facilities such as SPring-8, and was tested on the undulator beamline of the Tristan Super Light Facility at KEK. The beam test results show that the diamond monitor can be operated in a photoconductive mode like a semiconductor detector. It has a linear working range of about +/-1 mm and a position sensitivity of less than 3 mum. The stability of the monitor was confirmed by continuous operation under low photon beam intensity conditions.

Journal Article↗
PubMed1998

X-ray parametric scattering by a diamond crystal.

Spontaneous X-ray parametric scattering from a diamond single crystal has been observed at the Photon Factory. The high perfection of the diamond single crystal and the small angular divergence of synchrotron radiation has enabled down-converted X-ray photon pairs to scatter into two small solid angles which satisfy the phase-matching condition. High-efficiency noise reduction was performed using avalanche photodiode detectors with good time resolution.

Journal Article↗
PubMed1988

Elevated red cell adenosine deaminase activity: a marker of disordered erythropoiesis in Diamond-Blackfan anaemia and other haematologic diseases.

Red-cell adenosine deaminase (ADA) activity in children with Diamond-Blackfan anaemia is significantly increased (1.91 +/- 0.90 U/g Hb) compared to that seen in transient erythroblastopenia of childhood (0.80 +/- 0.16 U/g Hb) or normal individuals (0.61 +/- 0.13 U/g). These data thus further support that measurement of this purine metabolic enzyme is useful in diagnosing the cause of pure RBC aplasia in children. Of interest, however, elevated RBC-ADA activity also is seen in some children with acute leukaemia and other haematologic disorders. In children with acute lymphoblastic leukaemia (ALL), the increase in RBC-ADA activity is proportional to the degree of anaemia. However, the elevated RBC-ADA activity in this leukaemic population is not related to the fetal haemoglobin concentration. These data suggest increased RBC-ADA activity may be a non-specific manifestation of abnormal erythroid stem cell function, an alteration distinct from that seen with reactivation of fetal erythropoiesis. However, since almost all patients with Diamond-Blackfan anaemia manifest elevated RBC-ADA activity, this chemical alteration yet may reflect the specific erythroid differentiation lesion in this disorder.

Acute Disease↗
PubMed2005

An abutment screw loosening study of a Diamond Like Carbon-coated CP titanium implant.

The aim of this study was to quantify the extent of abutment screw loosening and thus understand the role of frictional and wear factors in abutment screw loosening by using a cyclic loading device to compare Diamond Like Carbon (DLC)-coated and non-coated implants. The properties of DLC films, including hardness, wear resistance, chemical stability, and biocompatibility, are similar to those of real diamond materials. In this study, a 1-mum thick DLC film served to protect and lubricate a layer of commercially-pure titanium affixed to the top of a dental implant (external hexagon-shaped implant). A cyclic loading force was then applied to the top of the prosthetic portion of the implants in order to determine the difference in looseness of the titanium abutment screw between ten DLC-coated implants and ten non-coated implants. The abutment screw loosening tests were performed with 100 N of force at a frequency of 20 Hz. Data indicate that implants with a DLC coating are more resistant to an applied force (P = 0.002) than are those without the coating. We hope these results will be useful for preventing implant abutment screw loosening.

Carbon↗
PubMed1985

Diamond-Blackfan syndrome: a possible role of cellular factors for erythropoietic suppression.

CFU-E growth from fractionated bone marrow cells of 5 children with Diamond-Blackfan syndrome was studied. In all patients, CFU-E growth was reduced in mononuclear cell-rich fraction. In 2 of the 5 patients, CFU-E growth was returned to normal by the depletion of E-rosette forming cells or monocytes from mononuclear cell-rich fraction. In the patient whose CFU-E growth returned to normal by the depletion of E-rosette forming cells, co-cultivation between bone marrow buffy coat cells and autologous bone marrow E-rosette forming cells resulted in a significant decrease of CFU-E growth, and there was a significant increase in CFU-E growth by treatment with monoclonal antiserum to OKT4. We concluded that immunologic causes such as cellular factors may play a role, at least in part, in the pathogenesis of Diamond-Blackfan syndrome.

Anemia, Aplastic↗