3,615 research outputs found
Instabilities on graphene's honeycomb lattice with electron-phonon interactions
We study the impact of electron-phonon interactions on the many-body
instabilities of electrons on the honeycomb lattice and their interplay with
repulsive local and non-local Coulomb interactions at charge neutrality. To
that end, we consider in-plane optical phonon modes with wavevectors close to
the point as well as to the points and calculate the effective
phonon-mediated electron-electron interaction by integrating out the phonon
modes. Ordering tendencies are studied by means of a momentum-resolved
functional renormalization group approach allowing for an unbiased
investigation of the appearing instabilities. In the case of an exclusive and
supercritical phonon-mediated interaction, we find a Kekul\'e and a nematic
bond ordering tendency being favored over the -wave superconducting state.
The competition between the different phonon-induced orderings clearly shows a
repulsive interaction between phonons at small and large wavevector transfers.
We further discuss the influence of phonon-mediated interactions on
electronically-driven instabilities induced by onsite, nearest neighbor and
next-to-nearest neighbor density-density interactions. We find an extension of
the parameter regime of the spin density wave order going along with an
increase of the critical scales where ordering occurs, and a suppression of
competing orders.Comment: 9 pages, 5 figure
Ladder-like optical conductivity in the spin-fermion model
In the nested limit of the spin-fermion model for the cuprates,
one-dimensional physics in the form of half-filled two-leg ladders emerges. We
show that the renormalization group flow of the corresponding ladder is towards
the d-Mott phase, a gapped spin-liquid with short-ranged d-wave pairing
correlations, and reveals an intermediate SO(5)SO(3) symmetry. We use
the results of the renormalization group in combination with a memory-function
approach to calculate the optical conductivity of the spin-fermion model in the
high-frequency regime, where processes within the hot spot region dominate the
transport. We argue that umklapp processes play a major role. For finite
temperatures, we determine the resistivity in the zero-frequency (dc) limit.
Our results show an approximate linear temperature dependence of the
resistivity and a conductivity that follows a non-universal power law. A
comparison to experimental data supports our assumption that the conductivity
is dominated by the antinodal contribution above the pseudogap.Comment: 11+2 pages, 8 figure
Fermion-induced quantum criticality in two-dimensional Dirac semimetals: Non-perturbative flow equations, fixed points and critical exponents
We establish a scenario where fluctuations of new degrees of freedom at a
quantum phase transition change the nature of a transition beyond the standard
Landau-Ginzburg paradigm. To this end we study the quantum phase transition of
gapless Dirac fermions coupled to a symmetric order parameter
within a Gross-Neveu-Yukawa model in 2+1 dimensions, appropriate for the
Kekul\'e transition in honeycomb lattice materials. For this model the standard
Landau-Ginzburg approach suggests a first order transition due to the
symmetry-allowed cubic terms in the action. At zero temperature, however,
quantum fluctuations of the massless Dirac fermions have to be included. We
show that they reduce the putative first-order character of the transition and
can even render it continuous, depending on the number of Dirac fermions .
A non-perturbative functional renormalization group approach is employed to
investigate the phase transition for a wide range of fermion numbers. For the
first time we obtain the critical , where the nature of the transition
changes. Furthermore, it is shown that for large the change from the
first to second order of the transition as a function of dimension occurs
exactly in the physical 2+1 dimensions. We compute the critical exponents and
predict sizable corrections to scaling for .Comment: 12+5 pages, 5 figure
High School Students\u27 Academic Achievement And Their Mothers\u27 Attitudes And Level Of Education
Early intervention for children with autism
The purpose of this study was to examine early intervention programs for children with autism to determine if the program will have a successful outcome. This study involved 34 children from birth to three years of age. Each child participated in an early childhood intervention program in Southern New Jersey. The children were divided into two groups. An independent sample t-test was conducted from the data to find a statistically significant difference within the children. It was not, however, significant in the children of the early intervention program
Ground state phase diagram of the half-filled bilayer Hubbard model
Employing a combination of functional renormalization group calculations and
projective determinantal quantum Monte Carlo simulations, we examine the
Hubbard model on the square lattice bilayer at half filling. From this combined
analysis, we obtain a comprehensive account on the ground state phase diagram
with respect to the extent of the system's metallic and (antiferromagnetically
ordered) Mott-insulating as well as band-insulating regions. By means of an
unbiased functional renormalization group approach, we exhibit the
antiferromagnetic Mott-insulating state as the relevant instability of the free
metallic state, induced by any weak finite onsite repulsion. Upon performing a
careful analysis of the quantum Monte Carlo data, we resolve the difficulty of
identifying this antiferromagnetic ground state for finite interlayer hopping
in the weak-coupling regime, where nonmonotonous finite-size corrections are
shown to relate to the two-sheeted Fermi surface structure of the metallic
phase. On the other hand, quantum Monte Carlo simulations are well suited to
identify the transition between the Mott-insulating phase and the band
insulator in the intermediate-to-strong coupling regime. Here, we compare our
numerical findings to indications for the transition region obtained from the
functional renormalization group procedure.Comment: 12 pages, 15 figure
Low temperature acoustic properties of amorphous silica and the Tunneling Model
Internal friction and speed of sound of a-SiO(2) was measured above 6 mK
using a torsional oscillator at 90 kHz, controlling for thermal decoupling,
non-linear effects, and clamping losses. Strain amplitudes e(A) = 10^{-8} mark
the transition between the linear and non-linear regime. In the linear regime,
excellent agreement with the Tunneling Model was observed for both the internal
friction and speed of sound, with a cut-off energy of E(min) = 6.6 mK. In the
non-linear regime, two different behaviors were observed. Above 10 mK the
behavior was typical for non-linear harmonic oscillators, while below 10 mK a
different behavior was found. Its origin is not understood.Comment: 1 tex file, 6 figure
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