4,869 research outputs found

### Casimir effects in graphene systems: unexpected power laws

We present calculations of the zero-temperature Casimir interaction between
two freestanding graphene sheets as well as between a graphene sheet and a
substrate. Results are given for undoped graphene and for a set of doping
levels covering the range of experimentally accessible values. We describe
different approaches that can be used to derive the interaction. We point out
both the predicted power law for the interaction and the actual distance
dependence.Comment: 10 pages,5 figures, conferenc

### Light spin-1/2 or spin-0 Dark Matter particles

We recall and precise how light spin-0 particles could be acceptable Dark
Matter candidates, and extend this analysis to spin-1/2 particles. We evaluate
the (rather large) annihilation cross sections required, and show how they may
be induced by a new light neutral spin-1 boson U. If this one is vectorially
coupled to matter particles, the (spin-1/2 or spin-0) Dark Matter annihilation
cross section into e+e- automatically includes a v_dm^2 suppression factor at
threshold, as desirable to avoid an excessive production of gamma rays from
residual Dark Matter annihilations. We also relate Dark Matter annihilations
with production cross sections in e+e- scatterings. Annihilation cross sections
of spin-1/2 and spin-0 Dark Matter particles are given by exactly the same
expressions. Just as for spin-0, light spin-1/2 Dark Matter particles
annihilating into e+e- could be responsible for the bright 511 keV gamma ray
line observed by INTEGRAL from the galactic bulge.Comment: 10 page

### Casimir interactions in graphene systems

The non-retarded Casimir interaction (van der Waals interaction) between two
free standing graphene sheets as well as between a graphene sheet and a
substrate is determined. An exact analytical expression is given for the
dielectric function of graphene along the imaginary frequency axis within the
random phase approximation for arbitrary frequency, wave vector, and doping.Comment: 4 pages, 4 figure

### Spin dynamics in copper metaborate $CuB_2 O_4$ studied by muon spin relaxation

Copper metaborate CuB$_2$O$_{4}$ was studied by muon spin relaxation
measurements in order to clarify its static and dynamic magnetic properties.
The time spectra of muon spin depolarization suggest that the local fields at
the muon site contain both static and fluctuating components in all ordered
phases down to 0.3 K. In the weak ferromagnetic phase (20 K~$>T>$~9.3 K), the
static component is dominant. On the other hand, upon cooling the fluctuating
component becomes dominant in the incommensurate helix phase (9.3K > T > 1.4K).
The dynamical fluctuations of the local fields persist down to 0.3K, where a
new incommensurate phase (T < 1.4K) is expected to appear. This result suggests
that spins fluctuate even at T \to 0. We propose two possible origins of the
remnant dynamical spin fluctuations: frustration of the exchange interactions
and the dynamic behavior of the soliton lattice

### A theoretical and semiemprical correction to the long-range dispersion power law of stretched graphite

In recent years intercalated and pillared graphitic systems have come under
increasing scrutiny because of their potential for modern energy technologies.
While traditional \emph{ab initio} methods such as the LDA give accurate
geometries for graphite they are poorer at predicting physicial properties such
as cohesive energies and elastic constants perpendicular to the layers because
of the strong dependence on long-range dispersion forces. `Stretching' the
layers via pillars or intercalation further highlights these weaknesses. We use
the ideas developed by [J. F. Dobson et al, Phys. Rev. Lett. {\bf 96}, 073201
(2006)] as a starting point to show that the asymptotic $C_3 D^{-3}$ dependence
of the cohesive energy on layer spacing $D$ in bigraphene is universal to all
graphitic systems with evenly spaced layers. At spacings appropriate to
intercalates, this differs from and begins to dominate the $C_4 D^{-4}$ power
law for dispersion that has been widely used previously. The corrected power
law (and a calculated $C_3$ coefficient) is then unsuccesfully employed in the
semiempirical approach of [M. Hasegawa and K. Nishidate, Phys. Rev. B {\bf 70},
205431 (2004)] (HN). A modified, physicially motivated semiempirical method
including some $C_4 D^{-4}$ effects allows the HN method to be used
successfully and gives an absolute increase of about $2-3%$ to the predicted
cohesive energy, while still maintaining the correct $C_3 D^{-3}$ asymptotics

### Nuclear deformation and neutrinoless double-$\beta$ decay of $^{94,96}$Zr, $^{98,100}$Mo, $^{104}$Ru, $^{110}$Pd, $^{128,130}$Te and $^{150}$Nd nuclei in mass mechanism

The $(\beta ^{-}\beta ^{-})_{0\nu}$ decay of $^{94,96}$Zr, $^{98,100}$Mo,
$^{104}$Ru, $^{110}$Pd, $^{128,130}$Te and $^{150}$Nd isotopes for the
$0^{+}\to 0^{+}$ transition is studied in the Projected Hartree-Fock-Bogoliubov
framework. In our earlier work, the reliability of HFB intrinsic wave functions
participating in the $\beta ^{-}\beta ^{-}$ decay of the above mentioned nuclei
has been established by obtaining an overall agreement between the
theoretically calculated spectroscopic properties, namely yrast spectra,
reduced $B(E2$:$0^{+}\to 2^{+})$ transition probabilities, quadrupole moments
$Q(2^{+})$, gyromagnetic factors $g(2^{+})$ as well as half-lives
$T_{1/2}^{2\nu}$ for the $0^{+}\to 0^{+}$ transition and the available
experimental data. In the present work, we study the $(\beta ^{-}\beta
^{-})_{0\nu}$ decay for the $0^{+}\to 0^{+}$ transition in the mass mechanism
and extract limits on effective mass of light as well as heavy neutrinos from
the observed half-lives $T_{1/2}^{0\nu}(0^{+}\to 0^{+})$ using nuclear
transition matrix elements calculated with the same set of wave functions.
Further, the effect of deformation on the nuclear transition matrix elements
required to study the $(\beta ^{-}\beta ^{-})_{0\nu}$ decay in the mass
mechanism is investigated. It is noticed that the deformation effect on nuclear
transition matrix elements is of approximately same magnitude in $(\beta
^{-}\beta ^{-})_{2\nu}$ and $(\beta ^{-}\beta ^{-})_{0\nu}$ decay.Comment: 15 pages, 1 figur

### Investigations of fast neutron production by 190 GeV/c muon interactions on different targets

The production of fast neutrons (1 MeV - 1 GeV) in high energy muon-nucleus
interactions is poorly understood, yet it is fundamental to the understanding
of the background in many underground experiments. The aim of the present
experiment (CERN NA55) was to measure spallation neutrons produced by 190 GeV/c
muons scattering on carbon, copper and lead targets. We have investigated the
energy spectrum and angular distribution of spallation neutrons, and we report
the result of our measurement of the neutron production differential cross
section.Comment: 19 pages, 11 figures ep

### Determination of Strong-Interaction Widths and Shifts of Pionic X-Rays with a Crystal Spectrometer

Pionic 3d-2p atomic transitions in F, Na, and Mg have been studied using a bent crystal spectrometer. The pionic atoms were formed in the production target placed in the external proton beam of the Space Radiation Effects Laboratory synchrocyclotron. The observed energies and widths of the transitions are E=41679(3) eV and Î“=21(8) eV, E=62434(18) eV and Î“=22(80) eV, E=74389(9) eV and Î“=67(35) eV, in F, Na, and Mg, respectively. The results are compared with calculations based on a pion-nucleus optical potential

### PT-symmetry in honeycomb photonic lattices

We apply gain/loss to honeycomb photonic lattices and show that the
dispersion relation is identical to tachyons - particles with imaginary mass
that travel faster than the speed of light. This is accompanied by PT-symmetry
breaking in this structure. We further show that the PT-symmetry can be
restored by deforming the lattice

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