Physics

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Vertically aligned few-layered graphene (FLG) nanoflakes were synthesized on bare silicon (Si) substrates by a microwave plasma enhanced chemical vapor deposition method. In situ nitrogen (N2) plasma treatment was carried out using electron cyclotron resonance plasma, resulting in various nitrogen functionalities being grafted to the FLG surface. Compared with pristine FLGs, the N2 plasma-treated FLGs showed significant improvement in field emission characteristics by lowering the turn-on field (defined at 10 ?A/cm 2) from 1.94 to 1.0 V/?m. Accordingly, the field emission current increased from 17 ?A/cm2 at 2.16 V/?m for pristine FLGs to about 103 ?A/cm2 at 1.45 V/?m for N-doped FLGs. Furthermore, N-doped FLG samples retained 94% of the starting current over a period of 10?000 s, during which the fluctuations were of the order of ±10.7% only. The field emission behavior of pristine and N2 plasma-treated FLGs is explained in terms of change in the effective microstructure as well as a reduction in the work function as probed by X-ray photoelectron valence band spectroscopy. © 2011 American Chemical Society.

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(SoNS) School of Natural Sciences

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Journal of Physical Chemistry C

Enhanced and stable field emission from in situ nitrogen-doped few-layered graphene nanoflakes

We report thermal annealing and 100 MeV Si8+ swift heavy ion irradiation effects on the structural and magnetic properties of Ni-implanted HfO2 thin films. At low Ni doping concentration (∼1%), HfO2 thin films show ferromagnetic behavior. We clearly demonstrate the cluster free nature of our film using cross-sectional high resolution transmission microscopy and magnetization vs. temperature data. Rutherford backscattering spectrometry is used to estimate the film thickness and to establish that Ni-ions are placed in the HfO2 matrix. By comparing the results for the annealed and swift heavy ion irradiated samples, it is concluded that the enhancement in magnetic signal is closely related to the dispersion/diffusion of implanted Ni and defect creation such as oxygen vacancies. The results of magnetic force microscopy supported the observation of room temperature ferromagnetism in Ni-implanted HfO2 films. © 2010 Elsevier B.V. All rights reserved.

Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

Enhancement of ferromagnetism in Ni-implanted HfO2 dielectric thin films

The effect of the substitution of one Cu2+ ion (S=1/2) by Mg2+ ion (S=0) on the magnetic property of the linear-chain spin trimer compound Ca3 Cu3 (PO4) 4 was studied using dc magnetic susceptibility in Ca3 Cu2 Mg (PO4) 4 together with those in Ca3 Cu3 (PO4) 4 and Ca3 Cu2 Ni (PO4) 4 for comparison. A remarkable difference is observed among the results obtained in the Mg and the Ni substituted compounds. A clear signature of the reduction in the net spin of the trimers in Ca3 Cu2 Mg (PO4) 4 was found below 65 K, which is compatible with the presence of the Cu2-Cu1-Mg trimers in the singlet ground state, together with those of the type Cu2-Cu1-Cu2 and Mg-Cu1-Mg in the doublet state. However, no signature of the reduction in the net spin of the trimers was observed in Ca3 Cu2 Ni (PO4) 4. This finding suggests that the type of trimers present in Ca3 Cu2 Mg (PO4) 4 and Ca3 Cu2 Ni (PO4) 4 are different. Furthermore, a singlet ground state is partially realized in a quantum spin trimer system Ca3 Cu2 Mg (PO4) 4. {\textcopyright} 2010 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Magnetic properties of the spin trimer compound Ca3 Cu 2 Mg ( PO4 ) 4 from susceptibility measurements

P 31 nuclear-magnetic-resonance (NMR) studies have been performed in trimer spin chain compound Ca3 CuNi2 (PO4) 4, in the temperature range 4-300 K. In the range 16-300 K, the spectrum corresponds to a typical overlap of two powder patterns, consistent with two inequivalent phosphorous sites, having different shift parameters. A comparison of the isotropic hyperfine field (H hf iso) and the axial part (H hf ax) obtained in the paramagnetic phase of Ca3 CuNi2 (PO4) 4, with those obtained in Ca3 Cu3 (PO4) 4, suggest a stronger interchain exchange interaction in Ca3 CuNi2 (PO4) 4, which is possibly the reason for the higher antiferromagnetic transition temperature in Ca3 CuNi2 (PO4) 4. The temperature dependence of the spin-lattice relaxation rate (1/ T1) shows a clear signature of long-range magnetic order in Ca3 CuNi2 (PO4) 4 below 16 K (TN). TN agrees quite satisfactorily with that obtained from the derivative of the molar susceptibility versus T plot. The NMR line shape below TN shows the signature of a considerable distribution of the internal magnetic field in the ordered state with the existence of two magnetic sublattices with opposite direction of polarization with respect to the direction of the external magnetic field. A comparison of the behavior of the 1/ T1 data in Ca3 CuNi2 (PO4) 4 with those in Sr3 Cu3 (PO4) 4 clearly suggests that the mechanism of the relaxation changes from a two magnon-mediated Raman process in the former to a three magnon-mediated process in the latter. {\textcopyright} 2010 The American Physical Society.

NMR study of a magnetic phase transition in Ca3 CuNi2 ( PO4 ) 4: A spin trimer compound

Physical Review B

Energetics and electronic structure of phenyl-disubstituted polyacetylene: A first-principles study

The PT-symmetric complexi?ed Scarf II potential V(x) =-V 1sech2 x + iV2sechx tanh x, V1 &gt; 0, V2 ≠= 0, is revisited to study the interplay among its coupling parameters. The existence of an isolated real and positive energy level that has recently been identfied as a spectral singularity or zero-width resonance is here demonstrated through the behaviour of the corresponding wavefunctions and some property of the associated pseudo-norms is pointed out. We also construct four different rationally extended supersymmetric partners to V(x), which are PT-symmetric or complex non-PT-symmetric according to the coupling parameters range. A detailed study of one of these partners reveals that SUSY preserves the V(x) spectral singularity existence. © 2010 IOP Publishing Ltd.

Journal of Physics A: Mathematical and Theoretical

An update on the PT-symmetric complexified Scarf II potential, spectral singularities and some remarks on the rationally extended supersymmetric partners

We have used in situ low-energy electron microscopy (LEEM) to correlate the atomic and electronic structure of graphene films on polycrystalline Ni with nm-scale spatial resolution. Spatially resolved electron scattering measurements show that graphene monolayers formed by carbon segregation do not support the π-plasmon of graphene, indicating strong covalent bonding to the Ni. Graphene bilayers have the Bernal stacking characteristic of graphite and show the expected plasmon loss at 6.5 eV. The experimental results, in agreement with first-principles calculations, show that the π-band structure of free-standing graphene appears only in films with a thickness of at least two layers and demonstrate the sensitivity of the plasmon loss to the electronic structure. © 2010 American Chemical Society.

ACS Nano

Spatially-resolved structure and electronic properties of graphene on polycrystalline Ni

Surface erosion of a strained Si0.8 Ge0.2 epilayer by 500 eV Ar+ ions with a fluence of 4×1017 ions/cm2 and the transformation from crystalline ripples to elongated Ge islands are reported by aligning the beam from 69° to 50° from the surface normal. Crystalline nature and the near surface Ge enrichment in such ripple structures are revealed by transmission electron microscopy. Moreover, ion beam induced decomposition of the SiGe network and the appearance of dislocation bands by suppressing the near-bandgap emission are manifested by μ -Raman and photoluminescence studies, respectively. © 2010 American Institute of Physics.

Journal of Applied Physics

Crystalline ripples at the surface of ion eroded strained Si0.8 Ge0.2 epilayers

We address the challenging issue of how CP violation is realized in higher dimensional gauge theories without higher dimensional elementary scalar fields. In such theories interactions are basically governed by a gauge principle and therefore to get CP violating phases is a nontrivial task. It is demonstrated that CP violation is achieved as the result of compactification of extra dimensions, which is incompatible with the 4-dimensional CP transformation. As a simple example we adopt a 6-dimensional U(1) model compactified on a 2-dimensional orbifold T2/Z4. We argue that the 4-dimensional CP transformation is related to the complex structure of the extra space and show how the Z4 orbifolding leads to CP violation. We confirm by explicit calculation of the interaction vertices that CP violating phases remain even after the rephasing of relevant fields. For completeness, we derive a rephasing invariant CP violating quantity, following a similar argument in the Kobayashi-Maskawa model which led to the Jarlskog parameter. As an example of a CP violating observable we briefly comment on the electric dipole moment of the electron. © 2010 The American Physical Society.

Physical Review D - Particles, Fields, Gravitation and Cosmology

CP violation due to compactification

On the basis of various examples we provide evidence that noncommutative spacetime involving position-dependent structure constants will give rise to deformed oscillator algebras. In turn, starting from some q-deformations of these algebras in a two-dimensional space for which the entire deformed Fock space can be constructed explicitly, we derive the commutation relations for the dynamical variables in noncommutative spacetime. We compute minimal areas resulting from these relations, i.e. finitely extended regions for which it is impossible to resolve any substructure in form of measurable knowledge. The size of the regions we find is determined by the noncommutative constant and the deformation parameter q. Any object in this type of spacetime structure has to be of membrane type or in certain limits of string type. © 2010 IOP Publishing Ltd.