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At least 793 records · Page 44Linked to original sources
PubMed2000

Electrical and mechanical properties of C70 fullerene and graphite under high pressures studied using designer diamond anvils.

We compare electrical and mechanical properties of C70 fullerene with high purity graphite to 48 GPa at room temperature using designer diamond anvils with embedded electrical microprobes. The electrical resistance of C70 shows a minimum at 20 GPa with transformation to an amorphous insulating phase complete above 35 GPa, while graphite remains conducting. Nanoindentation shows hardness values 220 times larger for the pressure quenched amorphous phase than for similarly treated graphite. Our studies establish that the amorphous carbon phase produced from C70 has unique properties not attainable from graphite.

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PubMed2001

Origin of the different reconstructions of diamond, Si, and Ge(111) surfaces.

Ab initio calculations of the 2x1, c(2x8), and 7x7 reconstructions of the diamond, Si, and Ge(111) surfaces are reported. The pi-bonded chain, adatom, and dimer-adatom-stacking fault models are studied to understand the driving forces for a certain reconstruction. The resulting energetics, geometries, and band structures are compared for the elemental semiconductors with different atomic sizes, and chemical trends are derived. We show why the lowest-energy reconstructions are different for the group-IV materials considered.

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PubMed2002

Diamond patterns in the cellular front of an overdriven detonation.

A nonlinear integral-differential equation describing the cellular front of an overdriven detonation is obtained by an analysis carried out in the neighborhood of the instability threshold. The analysis reveals both an unusual mean streaming motion, resulting from the rotational part of the oscillatory flow, and pressure bursts generated by the crossover of cusps representative of Mach stems propagating on the detonation front. A numerical study of the nonlinear equation exhibits the "diamond" patterns observed in experiments. An overall physical understanding is provided.

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PubMed2002

Formation of a high T(c) electron-hole liquid in diamond.

We report on the observation of electron-hole ( e-h) liquid (EHL) in diamond by time-resolved luminescence measurements under an intense femtosecond photoexcitation above the band gap. The EHL luminescence band is observed below the e-h plasma band, showing a finite rise time of several tens of picoseconds. The rise time, which corresponds to the nucleation and the growth of the e-h droplets, plummets on approaching the EHL critical temperature. Time-resolved spectral shape analysis reveals a very high carrier density of 1x10(20) cm(-3) and very high critical temperature of T(c) = 165 K of EHL.

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PubMed2003

Hydrogen incorporation in diamond: the nitrogen-vacancy-hydrogen complex.

We report the identification of the nitrogen-vacancy-hydrogen complex in a freestanding nitrogen-doped isotopically engineered single crystal diamond synthesized by chemical vapor deposition. The hydrogen atom is located in the vacancy of a nearest-neighbor nitrogen-vacancy defect and appears to be bonded to the nitrogen atom maintaining the trigonal symmetry of the center. The defect is observed by electron paramagnetic resonance in the negative charge state in samples containing a suitable electron donor (e.g., substitutional nitrogen N(0)(S)).

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PubMed2003

Design of shallow donor levels in diamond by isovalent-donor coupling.

Using the first-principles pseudopotential method, we have studied simultaneous isovalent and n-type doping in diamond. We show that Si induces fully occupied isovalent levels near the valence band maximum. The Si levels interact with N donor levels, making them much shallower. The donor transition energy level of the N + 4Si defect complexes is found to be 0.09 eV below the conduction band minimum, which is the shallowest level found thus far for this system. The binding energy of the N + 4Si complex is also large enough to insure its stability.

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PubMed2004

Hydrogen incorporation in diamond: the vacancy-hydrogen complex.

We report the identification of the vacancy-hydrogen complex in single crystal diamond synthesized by chemical vapor deposition. The S=1 defect is observed by electron paramagnetic resonance in the negative charge state. The hydrogen atom is bonded to one of the carbon atoms neighboring the vacancy. Unlike the analogous defect in silicon, no symmetry lowering reconstruction occurs between the three remaining carbon dangling orbitals. The very small measured hydrogen hyperfine interaction is explained by dipolar coupling between the hydrogen and the unpaired electron probability density delocalized on the three equivalent carbon neighbors.

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PubMed2004

Superhard cubic BC2N compared to diamond.

Recent experiments claimed successful synthesis of cubic boron-carbonitride compounds BC2N with an extreme hardness second only to diamond. In the present Letter, we examine the ideal strength of cubic BC2N using first-principles calculations. Our results reveal that, despite the large elastic parameters, compositional anisotropy and strain dependent bonding character impose limitation on their strength. Consequently, the hardness of the optimal BC2N structure is predicted to be lower than that of cubic BN, the second hardest material known. The measured extreme hardness of BC2N nanocomposites is most likely due to the nanocrystalline size effect and the bonding to the surrounding amorphous carbon matrix. This may prove to be a general rule useful in the quest for new superhard covalent materials.

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PubMed2004

Role of the dopant in the superconductivity of diamond.

We present an ab initio study of the recently discovered superconductivity of boron doped diamond within the framework of a phonon-mediated pairing mechanism. The role of the dopant, in substitutional position, is unconventional in that half of the coupling parameter lambda originates in strongly localized defect-related vibrational modes, yielding a very peaked Eliashberg alpha2F(omega) function. The electron-phonon coupling potential is found to be extremely large, and T(C) is limited by the low value of the density of states at the Fermi level. The effect of boron isotope substitution is explored.

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PubMed2004

Dependence of the superconducting transition temperature on the doping level in single-crystalline diamond films.

Homoepitaxial diamond layers doped with boron in the 10(20)-10(21) cm(-3) range are shown to be type II superconductors with sharp transitions (approximately 0.2 K) at temperatures increasing from 0 to 2.1 K with boron contents. The critical concentration for the onset of superconductivity in those 001-oriented single-crystalline films is about 5-7 10(20) cm(-3). The H-T phase diagram has been obtained from transport and ac-susceptibility measurements down to 300 mK.

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PubMed2005

Reflectance anisotropy spectra of the diamond (100)-(2x1) surface: evidence of strongly bound surface state excitons.

We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.

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PubMed2005

Three-dimensional nonlinear lattices: from oblique vortices and octupoles to discrete diamonds and vortex cubes.

We construct a variety of novel localized topological structures in the 3D discrete nonlinear Schrödinger equation. The states can be created in Bose-Einstein condensates trapped in strong optical lattices and crystals built of microresonators. These new structures, most of which have no counterparts in lower dimensions, range from multipole patterns and diagonal vortices to vortex "cubes" (stack of two quasiplanar vortices) and "diamonds" (formed by two orthogonal vortices).

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PubMed2005

Experimental observation of the 1/3 magnetization plateau in the diamond-chain compound Cu3(CO3)2(OH)2.

The magnetic susceptibility, high field magnetization, and specific heat measurements of Cu3(CO3)2(OH)2, which is a model substance for the frustrating diamond spin chain model, have been performed using single crystals. Two broad peaks are observed at around 20 and 5 K in both magnetic susceptibility and specific heat results. The magnetization curve has a clear plateau at one third of the saturation magnetization. The experimental results are examined in terms of theoretical expectations based on exact diagonalization and density matrix renormalization group methods. An origin of magnetic anisotropy is also discussed.

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PubMed2005

Na2V3O7: a frustrated nanotubular system with spin-1/2 diamond ring geometry.

Following the recent discussion on the puzzling nature of the interactions in the nanotubular system Na(2)V(3)O(7), we present a detailed ab initio microscopic analysis of its electronic and magnetic properties. By means of a nontrivial downfolding study we propose an effective model in terms of tubes of nine-site rings with the geometry of a spin-diamond necklace with frustrated inter-ring interactions. We show that this model provides a quantitative account of the observed magnetic behavior.

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PubMed2005

Molecular limits to the quantum confinement model in diamond clusters.

The electronic structure of monodispersed, hydrogen-passivated diamond clusters (diamondoids) in the gas phase has been studied with x-ray absorption spectroscopy. The data show that the bulk-related unoccupied states do not exhibit any quantum confinement. Additionally, density of states below the bulk absorption edge appears, consisting of features correlated to CH and CH2 hydrogen surface termination, resulting in an effective redshift of the lowest unoccupied states. The results contradict the commonly used and very successful quantum confinement model for semiconductors, which predicts increasing band edge blueshifts with decreasing particle size. Our findings indicate that in the ultimate size limit for nanocrystals a more molecular description is necessary.

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PubMed2006

Tunneling spectroscopy and vortex imaging in boron-doped diamond.

We present the first scanning tunneling spectroscopy study of single-crystalline boron-doped diamond. The measurements were performed below 100 mK with a low temperature scanning tunneling microscope. The tunneling density of states displays a clear superconducting gap. The temperature evolution of the order parameter follows the weak-coupling BCS law with Delta(0)/kBTc approximately 1.74. Vortex imaging at low magnetic field also reveals localized states inside the vortex core that are unexpected for such a dirty superconductor.

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PubMed2006

Stark shift control of single optical centers in diamond.

Lifetime-limited optical excitation lines of single nitrogen-vacancy (NV) defect centers in diamond have been observed at liquid helium temperature. They display unprecedented spectral stability over many seconds and excitation cycles. Spectral tuning of the spin-selective optical resonances was performed via the application of an external electric field (i.e., the Stark shift). A rich variety of Stark shifts were observed including linear as well as quadratic components. The ability to tune the excitation lines of single NV centers has potential applications in quantum information processing.

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PubMed2006

Polarization and readout of coupled single spins in diamond.

We study the coupling of a single nitrogen-vacancy center in diamond to a nearby single nitrogen defect at room temperature. The magnetic dipolar coupling leads to a splitting in the electron spin resonance frequency of the nitrogen-vacancy center, allowing readout of the state of a single nitrogen electron spin. At magnetic fields where the spin splitting of the two centers is the same, we observe a strong polarization of the nitrogen electron spin. The amount of polarization can be controlled by the optical excitation power. We combine the polarization and the readout in time-resolved pump-probe measurements to determine the spin relaxation time of a single nitrogen electron spin. Finally, we discuss indications for hyperfine-induced polarization of the nitrogen nuclear spin.

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