4,929 research outputs found
Quantum Hall ferromagnetism in graphene: a SU(4) bosonization approach
We study the quantum Hall effect in graphene at filling factors \nu = 0 and
\nu = \pm, concentrating on the quantum Hall ferromagnetic regime, within a
non-perturbative bosonization formalism. We start by developing a bosonization
scheme for electrons with two discrete degrees of freedom (spin-1/2 and
pseudospin-1/2) restricted to the lowest Landau level. Three distinct phases
are considered, namely the so-called spin-pseudospin, spin, and pseudospin
phases. The first corresponds to a quarter-filled (\nu =-1) while the others to
a half-filled (\nu = 0) lowest Landau level. In each case, we show that the
elementary neutral excitations can be treated approximately as a set of
n-independent kinds of boson excitations. The boson representation of the
projected electron density, the spin, pseudospin, and mixed spin-pseudospin
density operators are derived. We then apply the developed formalism to the
effective continuous model, which includes SU(4) symmetry breaking terms,
recently proposed by Alicea and Fisher. For each quantum Hall state, an
effective interacting boson model is derived and the dispersion relations of
the elementary excitations are analytically calculated. We propose that the
charged excitations (quantum Hall skyrmions) can be described as a coherent
state of bosons. We calculate the semiclassical limit of the boson model
derived from the SU(4) invariant part of the original fermionic Hamiltonian and
show that it agrees with the results of Arovas and co-workers for SU(N) quantum
Hall skyrmions. We briefly discuss the influence of the SU(4) symmetry breaking
terms in the skyrmion energy.Comment: 16 pages, 4 figures, final version, extended discussion about the
boson-boson interaction and its relation with quantum Hall skyrmion
Momentum Space Regularizations and the Indeterminacy in the Schwinger Model
We revisited the problem of the presence of finite indeterminacies that
appear in the calculations of a Quantum Field Theory. We investigate the
occurrence of undetermined mathematical quantities in the evaluation of the
Schwinger model in several regularization scenarios. We show that the
undetermined character of the divergent part of the vacuum polarization tensor
of the model, introduced as an {\it ansatz} in previous works, can be obtained
mathematically if one introduces a set of two parameters in the evaluation of
these quantities. The formal mathematical properties of this tensor and their
violations are discussed. The analysis is carried out in both analytical and
sharp cutoff regularization procedures. We also show how the Pauli Villars
regularization scheme eliminates the indeterminacy, giving a gauge invariant
result in the vector Schwinger model.Comment: 10 pages, no figure
Competing impurities and reentrant magnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) revisited. The role of the Dzyaloshinskii-Moriya and XY anisotropies
We study the order-from-disorder transition and reentrant magnetism in
La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) within the framework of a long-wavelength
nonlinear sigma model that properly incorporates the Dzyaloshinskii-Moriya and
XY anisotropies. Doping with nonmagnetic impurities, such as Zn, is considered
according to classical percolation theory, whereas the effect of Sr, which
introduces charge carriers into the CuO(2) planes, is described as a dipolar
frustration of the antiferromagnetic order. We calculate several magnetic,
thermodynamic, and spectral properties of the system, such as the
antiferromagnetic order parameter, the Neel temperature, the spin-stiffness,
and the anisotropy gaps, as well as their evolution with both Zn and Sr doping.
We explain the nonmonotonic and reentrant behavior experimentally observed for
T_N by Hucker et al. in Phys. Rev. B 59, R725 (1999), as resulting from the
reduction, due to the nonmagnetic impurities, of the dipolar frustration
induced by the charge carriers (order-from-disorder). Furthermore, we find a
similar nonmonotonic and reentrant behavior for all the other observables
studied. Most remarkably, our results show that while for x=2% and z=0 the
Dzyaloshinskii-Moriya gap \Delta_{DM}=0, for z=15% it is approximately
\Delta_{DM} = 7.5 cm^(-1). The later is larger than the lowest low-frequency
cutoff for Raman spectroscopy (~ 5 cm^(-1)), and could thus be observed in
one-magnon Raman scattering.Comment: 13 pages, 10 figure
Phase Transition and Monopoles Densities in a Nearest Neighbors Two-Dimensional Spin Ice Model
In this work, we show that, due to the alternating orientation of the spins
in the ground state of the artificial square spin ice, the influence of a set
of spins at a certain distance of a reference spin decreases faster than the
expected result for the long range dipolar interaction, justifying the use of
the nearest neighbor two dimensional square spin ice model as an effective
model. Using an extension of the model presented in ref. [Scientific Reports 5,
15875 (2015)], considering the influence of the eight nearest neighbors of each
spin on the lattice, we analyze the thermodynamics of the model and study the
monopoles and string densities dependence as a function of the temperature.Comment: 11 pages, 8 figure
Low-field microwave absorption and magnetoresistance in iron nanostructures grown by electrodeposition on n-type lightly-doped silicon substrates
In this study we investigate magnetic properties, surface morphology and
crystal structure in iron nanoclusters electrodeposited on lightly-doped (100)
n-type silicon substrates. Our goal is to investigate the spin injection and
detection in the Fe/Si lateral structures. The samples obtained under electric
percolation were characterized by magnetoresistive and magnetic resonance
measurements with cycling the sweeping applied field in order to understand the
spin dynamics in the as-produced samples. The observed hysteresis in the
magnetic resonance spectra, plus the presence of a broad peak in the
non-saturated regime confirming the low field microwave absorption (LFMA), were
correlated to the peaks and slopes found in the magnetoresistance curves. The
results suggest long range spin injection and detection in low resistive
silicon and the magnetic resonance technique is herein introduced as a
promising tool for analysis of electric contactless magnetoresistive samples.Comment: 12 pages, 5 figure
Spin g-factor due to electronic interactions in graphene
The gyromagnetic factor is an important physical quantity relating the
magnetic-dipole moment of a particle to its spin. The electron spin g-factor in
vacuo is one of the best model-based theoretical predictions ever made, showing
agreement with the measured value up to ten parts per trillion. However, for
electrons in a material the g-factor is modified with respect to its value in
vacuo because of environment interactions. Here, we show how interaction
effects lead to the spin g-factor correction in graphene by considering the
full electromagnetic interaction in the framework of pseudo-QED. We compare our
theoretical prediction with experiments performed on graphene deposited on SiO2
and SiC, and we find a very good agreement between them.Comment: Improved version of the manuscript; valley g-factor part has been
remove
Magnetic susceptibility anisotropies in a two-dimensional quantum Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions
The magnetic and thermodynamic properties of the two-dimensional quantum
Heisenberg antiferromagnet that incorporates both a Dzyaloshinskii-Moriya and
pseudo-dipolar interactions are studied within the framework of a generalized
nonlinear sigma model (NLSM). We calculate the static uniform susceptibility
and sublattice magnetization as a function of temperature and we show that: i)
the magnetic-response is anisotropic and differs qualitatively from the
expected behavior of a conventional easy-axis QHAF; ii) the Neel second-order
phase transition becomes a crossover, for a magnetic field B perpendicular to
the CuO(2) layers. We provide a simple and clear explanation for all the
recently reported unusual magnetic anisotropies in the low-field susceptibility
of La(2)CuO(4), L. N. Lavrov et al., Phys. Rev. Lett. 87, 017007 (2001), and we
demonstrate explicitly why La(2)CuO(4) can not be classified as an ordinary
easy-axis antiferromagnet.Comment: 6 pages, 3 figures, Revtex4, accepted for publication in Phys. Rev.
Dynamics of topological defects in a spiral: a scenario for the spin-glass phase of cuprates
We propose that the dissipative dynamics of topological defects in a spiral
state is responsible for the transport properties in the spin-glass phase of
cuprates. Using the collective-coordinate method, we show that topological
defects are coupled to a bath of magnetic excitations. By integrating out the
bath degrees of freedom, we find that the dynamical properties of the
topological defects are dissipative. The calculated damping matrix is related
to the in-plane resistivity, which exhibits an anisotropy and linear
temperature dependence in agreement with experimental data.Comment: 4 pages, as publishe
Derivation of the generalized Non Linear Sigma Model in the presence of the Dzyaloshinskii-Moriya interaction
We derive the long-wavelength non-linear sigma model for a two-dimensional
Heisenberg system in the presence of the Dzyaloshinskii-Moriya and
pseudodipolar interactions. We show that the system is a non-conventional
easy-axis antiferromagnet, displaying an anomalous coupling between the
magnetic field and the staggered order parameter. Our results are in good
agreement with recent experimental data for undoped La2CuO4 compounds.Comment: Proceedings of SCES05, to appear on Physica
de Broglie-Proca and Bopp-Podolsky massive photon gases in cosmology
We investigate the influence of massive photons on the evolution of the
expanding universe. Two particular models for generalized electrodynamics are
considered, namely de Broglie-Proca and Bopp-Podolsky electrodynamics. We
obtain the equation of state (EOS) for each case using
dispersion relations derived from both theories. The EOS are inputted into the
Friedmann equations of a homogeneous and isotropic space-time to determine the
cosmic scale factor . It is shown that the photon non-null mass does not
significantly alter the result valid for a massless photon
gas; this is true either in de Broglie-Proca's case (where the photon mass
is extremely small) or in Bopp-Podolsky theory (for which is extremely
large).Comment: 8 pages, 2 figures; v2 matches the published versio
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