50,026 research outputs found
Correlated electrons systems on the Apollonian network
Strongly correlated electrons on an Apollonian network are studied using the
Hubbard model. Ground-state and thermodynamic properties, including specific
heat, magnetic susceptibility, spin-spin correlation function, double occupancy
and one-electron transfer, are evaluated applying direct diagonalization and
quantum Monte Carlo. The results support several types of magnetic behavior. In
the strong-coupling limit, the quantum anisotropic spin 1/2 Heisenberg model is
used and the phase diagram is discussed using the renormalization group method.
For ferromagnetic coupling, we always observe the existence of long-range
order. For antiferromagnetic coupling, we find a paramagnetic phase for all
finite temperatures.Comment: 7 pages, 8 figure
Structural and magnetic transition in CeFeAsO: separated or connected?
Using an adapted Sn-flux growth technique we obtained comparatively large
CeFeAsO single crystals of better quality than previously reported polycrystals
or single crystals, as evidenced by much sharper anomalies at the structural
and magnetic phase transitions as well as a much higher residual resistivity
ratio of 12. In the magnetically ordered phase we observe a very pronounced
metallic behavior of the in-plane resistivity, which excludes a Mott insulator
regime at low temperature. The separation Delta_T = T_0 - T_N between
structural and magnetic ordering temperatures decreases with increasing sample
quality, from 18 K in the initial reports to 6 K in the present single
crystals, demonstrating that this separation is not an intrinsic property of
the RFeAsO systems. Our results indicate that the coupling between magnetic
ordering and structural distortion is very similar in AFe2As2 and RFeAsO type
of compounds, much more similar than previously thought. The implications of
our experimental results give arguments both in favor and against the nematic
phase model.Comment: published in PRB with the title 'Coupling between the structural and
magnetic transition in CeFeAsO
Generalizing the Planck distribution
Along the lines of nonextensive statistical mechanics, based on the entropy
, and Beck-Cohen
superstatistics, we heuristically generalize Planck's statistical law for the
black-body radiation. The procedure is based on the discussion of the
differential equation (with ),
whose particular case leads to the celebrated law, as originally shown by
Planck himself in his October 1900 paper. Although the present generalization
is mathematically simple and elegant, we have unfortunately no physical
application of it at the present moment. It opens nevertheless the door to a
type of approach that might be of some interest in more complex, possibly
out-of-equilibrium, phenomena.Comment: 6 pages, including 2 figures. To appear in {\it Complexity,
Metastability and Nonextensivity}, Proc. 31st Workshop of the International
School of Solid State Physics (20-26 July 2004, Erice-Italy), eds. C. Beck,
A. Rapisarda and C. Tsallis (World Scientific, Singapore, 2005
Bare LO-Phonon Peak in THz-Emission Signals: a Dielectric-Function Analysis
We present a normal-mode analysis of coupled photocarrier-phonon dynamics in
Te. We consider a dielectric function which accounts for LO phonons and the
electron-hole gas within the Debye-Huckel model and RPA. Our main finding is
the existence of a bare LO phonon mode in the system even at high carrier
density. This oscillation is an unscreened L- mode arising from ineffective
screening at large wave vectors. This mode is consistent with the bare
LO-phonon peak in recent THz-emission spectra of Te.Comment: 3 pages, 1 figure, Special Issue: Proceedings of the 10th Brazilian
Workshop on Semiconductor Physics, Guaruja/SP, April/200
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