13,274 research outputs found
Coupled charge and valley excitations in graphene quantum Hall ferromagnets
Graphene is a two-dimensional carbon material with a honeycomb lattice and
Dirac-type low-energy spectrum. In a strong magnetic field, where Coulomb
interactions dominate against disorder broadening, quantum Hall ferromagnetic
states realize at integer fillings. Extending the quantum Hall ferromagnetism
to the fractional filling case of massless Dirac fermions, we study the
elementally charge excitations which couple with the valley degrees of freedom
(so-called valley skyrmions). With the use of the density matrix renomalization
group (DMRG) method, the excitation gaps are calculated and extrapolated to the
thermodynamic limit. These results exhibit numerical evidences and criterions
of the skyrmion excitations in graphene.Comment: 5 pages, 5 figure
Correlation functions for time-dependent calculation of linear-response functions
We emphasize the importance of choosing an appropriate correlation function
to reduce numerical errors in calculating the linear-response function as a
Fourier transformation of a time-dependent correlation function. As an example
we take dielectric functions of silicon crystal calculated with a
time-dependent method proposed by Iitaka et al. [Phys. Rev. E 56, 1222 (1997)].Comment: to be published in Phys.Rev.E 01 Dec 1997, 2 pages, 4 figures, more
information at http://espero.riken.go.jp
Ground-State Phase Diagram of the XXZ Model on a Railroad-Trestle Lattice with Asymmetric Leg Interactions
Using the bosonization and level spectroscopy methods, we study the
ground-state phase diagram of a XXZ antiferromagnet on a railroad-trestle
lattice with asymmetric leg interactions. It is shown that the asymmetry does
not change the dimer/Neel transition line significantly, which agrees with the
expectation based on a naive bosonization procedure, but it does change the
dimer/spin-fluid transition line. To understand this observation, we analyze
eigenvectors of the ground state, dimer excitation, doublet excitation and Neel
excitation, and find that only the doublet excitation is affected by the
asymmetric interaction.Comment: 6 pages, 11 Postscript figures, use jpsj2.cl
Evidence for the Multiverse in the Standard Model and Beyond
In any theory it is unnatural if the observed parameters lie very close to
special values that determine the existence of complex structures necessary for
observers. A naturalness probability, P, is introduced to numerically evaluate
the unnaturalness. If P is small in all known theories, there is an observer
naturalness problem. In addition to the well-known case of the cosmological
constant, we argue that nuclear stability and electroweak symmetry breaking
(EWSB) represent significant observer naturalness problems. The naturalness
probability associated with nuclear stability is conservatively estimated as
P_nuc < 10^{-(3-2)}, and for simple EWSB theories P_EWSB < 10^{-(2-1)}. This
pattern of unnaturalness in three different arenas, cosmology, nuclear physics,
and EWSB, provides evidence for the multiverse. In the nuclear case the problem
is largely solved even with a flat multiverse distribution, and with nontrivial
distributions it is possible to understand both the proximity to neutron
stability and the values of m_e and m_d - m_u in terms of the electromagnetic
contribution to the proton mass. It is reasonable that multiverse distributions
are strong functions of Lagrangian parameters due to their dependence on
various factors. In any EWSB theory, strongly varying distributions typically
lead to a little or large hierarchy, and in certain multiverses the size of the
little hierarchy is enhanced by a loop factor. Since the correct theory of EWSB
is unknown, our estimate for P_EWSB is theoretical. The LHC will determine
P_EWSB more robustly, which may remove or strengthen the observer naturalness
problem of EWSB. For each of the three arenas, the discovery of a natural
theory would eliminate the evidence for the multiverse; but in the absence of
such a theory, the multiverse provides a provisional understanding of the data.Comment: 79 pages, 23 figure
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