452 research outputs found
Theory of interacting electrons on the honeycomb lattice
The low-energy theory of electrons interacting via repulsive short-range
interactions on graphene's honeycomb lattice at half filling is presented. The
exact symmetry of the Lagrangian with local quartic terms for the Dirac field
dictated by the lattice is D_2 x U_c(1) x (time reversal), where D_2 is the
dihedral group, and U_c(1) is a subgroup of the SU_c(2) "chiral" group of the
non-interacting Lagrangian, that represents translations in Dirac language. The
Lagrangian describing spinless particles respecting this symmetry is
parameterized by six independent coupling constants. We show how first imposing
the rotational, then Lorentz, and finally chiral symmetry to the quartic terms,
in conjunction with the Fierz transformations, eventually reduces the set of
couplings to just two, in the "maximally symmetric" local interacting theory.
We identify the two critical points in such a Lorentz and chirally symmetric
theory as describing metal-insulator transitions into the states with either
time-reversal or chiral symmetry being broken. In the site-localized limit of
the interacting Hamiltonian the low-energy theory describes the continuous
transitions into the insulator with either a finite Haldane's (circulating
currents) or Semenoff's (staggered density) masses, both in the universality
class of the Gross-Neveu model. The picture of the metal-insulator transition
on a honeycomb lattice emerges at which the residue of the quasiparticle pole
at the metallic and the mass-gap in the insulating phase both vanish
continuously as the critical point is approached. We argue that the Fermi
velocity is non-critical as a consequence of the dynamical exponent being fixed
to unity by the emergent Lorentz invariance. Effects of long-range interaction
and the critical behavior of specific heat and conductivity are discussed.Comment: 16 revtex pages, 4 figures; typos corrected, new and updated
references; published versio
High frequency dielectric and magnetic anomaly at the phase transition in NaV2O5
We found anomalies in the temperature dependence of the dielectric and the
magnetic susceptibiliy of NaV_2O_5 in the microwave and far infrared frequency
ranges. The anomalies occur at the phase transition temperature T_c, at which
the spin gap opens. The real parts of the dielectric constants epsilon_a and
epsilon_c decrease below T_c. The decrease of epsilon_a (except for the narrow
region close to T_c) is proportional to the intensity of the x-ray reflection
appearing at T_c. The dielectric constant anomaly can be explained by the
zigzag charge ordering in the ab-plane appearing below T_c. The anomaly of the
microwave magnetic losses is probably related to the coupling between the spin
and charge degrees of freedom in vanadium ladders.Comment: 3 PS-figures, LATEX-text, new experimental data added, typos
correcte
Charge-ordering phase transition and order-disorder effects in the Raman spectra of NaV2O5
In the ac polarized Raman spectra of NaV2O5 we have found anomalous phonon
broadening, and an energy shift of the low-frequency mode as a function of the
temperature. These effects are related to the breaking of translational
symmetry, caused by electrical disorder that originates from the fluctuating
nature of the V {4.5+} valence state of vanadium. The structural correlation
length, obtained from comparisons between the measured and calculated Raman
scattering spectra, diverges at T< 5 K, indicating the existence of the
long-range charge order at very low temperatures, probably at T=0 K.Comment: 8 pages, 4 figures, new version, to appear in PR
Two-dimensional magnetoexcitons in the presence of spin-orbit coupling
We study theoretically the effect of spin-orbit coupling on quantum well
excitons in a strong magnetic field. We show that, in the presence of an
in-plane field component, the excitonic absorption spectrum develops a
double-peak structure due to hybridization of bright and dark magnetoexcitons.
If the Rashba and Dresselhaus spin-orbit constants are comparable, the
magnitude of splitting can be tuned in a wide interval by varying the azimuthal
angle of the in-plane field. We also show that the interplay between spin-orbit
and Coulomb interactions leads to an anisotropy of exciton energy dispersion in
the momentum plane. The results suggest a way for direct optical measurements
of spin-orbit parameters.Comment: 9 pages, 6 figure
Crossover exponent in O(N) phi^4 theory at O(1/N^2)
The critical exponent phi_c, derived from the anomalous dimension of the
bilinear operator responsible for crossover behaviour in O(N) phi^4 theory, is
calculated at O(1/N^2) in a large N expansion in arbitrary space-time dimension
d = 4 - 2 epsilon. Its epsilon expansion agrees with the known O(epsilon^4)
perturbative expansion and new information on the structure of the five loop
exponent is provided. Estimates of phi_c and the related crossover exponents
beta_c and gamma_c, using Pade-Borel resummation, are provided for a range of N
in three dimensions.Comment: 8 latex page
Long-term variations in surface air pressure and surface air temperature in the Northern Hemisphere mid-latitudes
Ā© 2017, Allerton Press, Inc. The spatiotemporal variability of surface air pressure and surface air temperature in the Northern Hemisphere troposphere in 1990-2014 is described. In 2005 the low-frequency component (LFC) of average air temperature in January averaged over the latitude zone of 32.5Ā°-67.5Ā° N has stopped its increase that lasted for 35 years (from 1970). The LFC of air temperature in July has continued growing since 1975 (for 39 years). The anomalies of air pressure and air temperature for thirty-year periods and the dynamics of LFC of air temperature and air pressure in the atmospheric centers of action are analyzed
A multiloop improvement of non-singlet QCD evolution equations
An approach is elaborated for calculation of "all loop" contributions to the
non-singlet evolution kernels from the diagrams with renormalon chain
insertions. Closed expressions are obtained for sums of contributions to
kernels for the DGLAP equation and for the "nonforward" ER-BL
equation from these diagrams that dominate for a large value of , the
first -function coefficient. Calculations are performed in the covariant
-gauge in a MS-like scheme. It is established that a special choice of the
gauge parameter generalizes the standard "naive nonabelianization"
approximation. The solutions are obtained to the ER-BL evolution equation
(taken at the "all loop" improved kernel), which are in form similar to
one-loop solutions. A consequence for QCD descriptions of hard processes and
the benefits and incompleteness of the approach are briefly discussed.Comment: 13 pages, revtex, 2 figures are enclosed as eps-file, the text style
and figures are corrected following version, accepted for publication to
Phys. Rev.
Investigation of thermal and magnetic properties of defects in a spin-gap compound NaV2O5
The specific heat, magnetic susceptibility and ESR signals of a Na-deficient
vanadate Na_xV_2O_5 (x=1.00 - 0.90) were studied in the temperature range 0.07
- 10 K, well below the transition point to a spin-gap state. The contribution
of defects provided by sodium vacancies to the specific heat was observed. It
has a low temperature part which does not tend to zero till at least 0.3 K and
a high temperature power-like tail appears above 2 K. Such dependence may
correspond to the existence of local modes and correlations between defects in
V-O layers. The magnetic measurements and ESR data reveal S=1/2 degrees of
freedom for the defects, with their effective number increasing in temperature
and under magnetic field. The latter results in the nonsaturating magnetization
at low temperature. No long-range magnetic ordering in the system of defects
was found. A model for the defects based on electron jumps near vacancies is
proposed to explain the observed effects. The concept of a frustrated
two-dimensional correlated magnet induced by the defects is considered to be
responsible for the absence of magnetic ordering.Comment: 6 pages, 8 figure
Using electronic structure changes to map the H-T phase diagram of alpha'-NaV2O5
We report polarized optical reflectance studies of \alpha'-NaV2O5 as a
function of temperature (4-45 K) and magnetic field (0-60 T). Rung directed
electronic structure changes, as measured by near-infrared reflectance ratios
\Delta R(H)=R(H)/R(H=0 T), are especially sensitive to the phase boundaries. We
employ these changes to map out an H-T phase diagram. Topological highlights
include the observation of two phase boundaries slightly below T_{SG}, enhanced
curvature of the 34 K phase boundary above 35 T, and, surprisingly, strong
hysteresis effects of both transitions with applied field.Comment: 4 pages, 3 figures, PRB accepte
An improved \eps expansion for three-dimensional turbulence: summation of nearest dimensional singularities
An improved \eps expansion in the -dimensional () stochastic
theory of turbulence is constructed by taking into account pole singularities
at in coefficients of the \eps expansion of universal quantities.
Effectiveness of the method is illustrated by a two-loop calculation of the
Kolmogorov constant in three dimensions.Comment: 4 page
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