212 research outputs found
Entanglement Spectra of Interacting Fermions in Quantum Monte Carlo Simulations
In a recent article T. Grover [Phys. Rev. Lett. 111, 130402 (2013)]
introduced a simple method to compute Renyi entanglement entropies in the realm
of the auxiliary field quantum Monte Carlo algorithm. Here, we further develop
this approach and provide a stabilization scheme to compute higher order Renyi
entropies and an extension to access the entanglement spectrum. The method is
tested on systems of correlated topological insulators.Comment: 7+ pages, 5 figure
Spontaneous particle-hole symmetry breaking of correlated fermions on the Lieb lattice
We study spinless fermions with nearest-neighbor repulsive interactions
(- model) on the two-dimensional three-band Lieb lattice. At
half-filling, the free electronic band structure consists of a flat band at
zero energy and a single cone with linear dispersion. The flat band is expected
to be unstable upon inclusion of electronic correlations, and a natural channel
is charge order. However, due to the three-orbital unit cell, commensurate
charge order implies an imbalance of electron and hole densities and therefore
doping away from half-filling. Our numerical results show that below a
finite-temperature Ising transition a charge density wave with one electron and
two holes per unit cell and its partner under particle-hole transformation are
spontaneously generated. Our calculations are based on recent advances in
auxiliary-field and continuous-time quantum Monte Carlo simulations that allow
sign-free simulations of spinless fermions at half-filling. It is argued that
particle-hole symmetry breaking provides a route to access levels of finite
doping, without introducing a sign problem.Comment: 9 pages, 6 figures, added data for strong Coulomb repulsion and
classical Ising-limi
Dimerized Solids and Resonating Plaquette Order in SU(N)-Dirac Fermions
We study the quantum phases of fermions with an explicit SU(N)-symmetric,
Heisenberg-like nearest-neighbor flavor exchange interaction on the honeycomb
lattice at half-filling. Employing projective (zero temperature) quantum Monte
Carlo simulations for even values of N, we explore the evolution from a
weak-coupling semimetal into the strong-coupling, insulating regime.
Furthermore, we compare our numerical results to a saddle-point approximation
in the large-N limit. From the large-N regime down to the SU(6) case, the
insulating state is found to be a columnar valence bond crystal, with a direct
transition to the semimetal at weak, finite coupling, in agreement with the
mean-field result in the large-N limit. At SU(4) however, the insulator
exhibits a subtly different valence bond crystal structure, stabilized by
resonating valence bond plaquettes. In the SU(2) limit, our results support a
direct transition between the semimetal and an antiferromagnetic insulator.Comment: 5 pages, 6 figure
Exact diagonalization study of the tunable edge magnetism in graphene
The tunable magnetism at graphene edges with lengths of up to 48 unit cells
is analyzed by an exact diagonalization technique. For this we use a
generalized interacting one-dimensional model which can be tuned continuously
from a limit describing graphene zigzag edge states with a ferromagnetic phase,
to a limit equivalent to a Hubbard chain, which does not allow ferromagnetism.
This analysis sheds light onto the question why the edge states have a
ferromagnetic ground state, while a usual one-dimensional metal does not.
Essentially we find that there are two important features of edge states: (a)
umklapp processes are completely forbidden for edge states; this allows a
spin-polarized ground state. (b) the strong momentum dependence of the
effective interaction vertex for edge states gives rise to a regime of partial
spin-polarization and a second order phase transition between a standard
paramagnetic Luttinger liquid and ferromagnetic Luttinger liquid.Comment: 11 pages, 8 figure
Dynamical Signatures of Edge-State Magnetism on Graphene Nanoribbons
We investigate the edge-state magnetism of graphene nanoribbons using
projective quantum Monte Carlo simulations and a self-consistent mean-field
approximation of the Hubbard model. The static magnetic correlations are found
to be short ranged. Nevertheless, the correlation length increases with the
width of the ribbon such that already for ribbons of moderate widths we observe
a strong trend towards mean-field-type ferromagnetic correlations at a zigzag
edge. These correlations are accompanied by a dominant low-energy peak in the
local spectral function and we propose that this can be used to detect
edge-state magnetism by scanning tunneling microscopy. The dynamic spin
structure factor at the edge of a ribbon exhibits an approximately linearly
dispersing collective magnonlike mode at low energies that decays into Stoner
modes beyond the energy scale where it merges into the particle-hole continuum.Comment: 4+ pages including 4 figure
Periodic Anderson model with electron-phonon correlated conduction band
This paper reports dynamical mean field calculations for the periodic
Anderson model in which the conduction band is coupled to phonons. Motivated in
part by recent attention to the role of phonons in the -
transition in Ce, this model yields a rich and unexpected phase diagram which
is of intrinsic interest. Specifically, above a critical value of the
electron-phonon interaction, a first order transition with two coexisting
phases develops in the temperature-hybridization plane, which terminates at a
second order critical point. The coexisting phases display the familiar Kondo
screened and local moment character, yet they also exhibit pronounced polaronic
and bipolaronic properties, respectively.Comment: 4 pages, 6 figure
Antiferromagnetism in the Hubbard Model on the Bernal-stacked Honeycomb Bilayer
Using a combination of quantum Monte Carlo simulations, functional
renormalization group calculations and mean-field theory, we study the Hubbard
model on the Bernal-stacked honeycomb bilayer at half-filling as a model system
for bilayer graphene. The free bands consisting of two Fermi points with
quadratic dispersions lead to a finite density of states at the Fermi level,
which triggers an antiferromagnetic instability that spontaneously breaks
sublattice and spin rotational symmetry once local Coulomb repulsions are
introduced. Our results reveal an inhomogeneous participation of the spin
moments in the ordered ground state, with enhanced moments at the three-fold
coordinated sites. Furthermore, we find the antiferromagnetic ground state to
be robust with respect to enhanced interlayer couplings and extended Coulomb
interactions.Comment: 4+ pages, 4 figures; final versio
ОБОСНОВАНИЕ МЕТОДОВ РАСЧЕТОВ ПАРАМЕТРОВ ГИДРОТРАНСПОРТА ВыСОКОКОНЦЕНТРИРОВАННыХ ГИДРОСМЕСЕЙ
Днепропетровская область – крупный промышленный регион, в котором сосредоточены предприятия металлургической, химической и машинострои-тельной отраслей промышленности, расположенный на пересечении водных путей, железнодорожных и автомобильных магистралей, направленных к ме-сторождениям каменных углей и железной руды. Металлургические заводы, коксохимические комбинаты и тепловые электростанции являются крупней-шими потребителями воды, основного экологического ресурса планеты, а также основными источниками загрязнения водного бассейна региона. Несмотря на использование на таких предприятиях замкнутых циклов водоснабжения, про-блема аккумулирования жидких отходов остается для них актуальной и острой. Особенно актуальным это является для горно-обогатительных комбинатов (ГОК) Кривбасса, большая часть которых заканчивает эксплуатацию хранилищ отходов, построенных в 60-70 годы прошлого столетия. Дальнейшее развитие региона во многом определяется промышленным потенциалом этих предпри-ятий, поскольку они, с одной стороны, являются потребителями угля, железной руды и прочих ресурсов, чем обеспечивают работоспособность отечественных предприятий, а с другой стороны, выпускают продукцию, потребителями кото-рой являются многие предприятия страны
Single hole dynamics in the t-J model on a square lattice
We present quantum Monte Carlo (QMC) simulations for a single hole in a t-J
model from J=0.4t to J=4t on square lattices with up to 24 x 24 sites. The
lower edge of the spectrum is directly extracted from the imaginary time
Green's function. In agreement with earlier calculations, we find flat bands
around , and the minimum of the dispersion at
. For small J both self-consistent Born approximation and
series expansions give a bandwidth for the lower edge of the spectrum in
agreement with the simulations, whereas for J/t > 1, only series expansions
agree quantitatively with our QMC results. This band corresponds to a coherent
quasiparticle. This is shown by a finite size scaling of the quasiparticle
weight that leads to a finite result in the thermodynamic limit for
the considered values of . The spectral function is
obtained from the imaginary time Green's function via the maximum entropy
method. Resonances above the lowest edge of the spectrum are identified, whose
J-dependence is quantitatively described by string excitations up to J/t=2
Critical Role of IRF-5 in the Development of T helper 1 responses to Leishmania donovani infection
The transcription factor Interferon Regulatory Factor 5 (IRF-5) has been shown to be involved in the induction of proinflammatory cytokines in response to viral infections and TLR activation and to play an essential role in the innate inflammatory response. In this study, we used the experimental model of visceral leishmaniasis to investigate the role of IRF-5 in the generation of Th1 responses and in the formation of Th1-type liver granulomas in Leishmania donovani infected mice. We show that TLR7-mediated activation of IRF-5 is essential for the development of Th1 responses to L. donovani in the spleen during chronic infection. We also demonstrate that IRF-5 deficiency leads to the incapacity to control L. donovani infection in the liver and to the formation of smaller granulomas. Granulomas in Irf5-/- mice are characterized by an increased IL-4 and IL-10 response and concomitant low iNOS expression. Collectively, these results identify IRF-5 as a critical molecular switch for the development of Th1 immune responses following L. donovani infections and reveal an indirect role of IRF-5 in the regulation of iNOS expression
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