2,728 research outputs found
Correlated spinless fermions on the honeycomb lattice revisited
We investigate the quantum many-body instabilities of the extended Hubbard
model for spinless fermions on the honeycomb lattice with repulsive
nearest-neighbor and 2nd nearest-neighbor density-density interactions. Recent
exact diagonalization and infinite density matrix renormalization group results
suggest that a putative topological Mott insulator phase driven by the 2nd
nearest-neighbor repulsion is suppressed, while other numerically exact
approaches support the topological Mott insulator scenario. In the present
work, we employ the functional renormalization group (fRG) for correlated
fermionic systems. Our fRG results hint at a strong suppression of the
scattering processes stabilizing the topological Mott insulator. From analyzing
the effects of fermionic fluctuations, we obtain a phase diagram which is the
result of the competition of various charge ordering instabilities.Comment: 9 pages, 8 figure
International trade and competition policy
With the completion of the Uruguay Round of international trade negotiations, attention turns to plausible next steps. One question on the agenda of possibilities is the adoption of competition policies that complement or substitute for the remedies traditionally used to deal with international trade distortions. This paper examines three cases --industrial diamonds, potash from Saskatchewan, and cement from Greece
International competition policy and economic development
During the past half century many nations have adopted policies whose function is to discourage cartels and other restrictive practices. Industrialized nations led the movement toward pro-competition policies, but more recently, developing nations have begun to join the parade. Initial steps have also been taken toward the implementation of competition policies spanning national borders, and proposals for their extension have been made. This paper analyzes the consequences national and international competition policies would have for developing nations. Topics covered include the dependence of LDCs on cartelized commodity exports, the terms on which intermediate goods and technology are imported by LDCs, access to the markets of industrialized nations, the consequences of substituting predatory pricing standards for the criteria traditionally used to combat dumping in international trade, and the links between domestic and international market structure and the absorption of advanced technology. --
The size distribution of profits from innovation
The research reported in this paper seeks to determine how skewed the distribution of profits from technological innovation is --i.e., whether it conforms most closely to the Paretian, log normal, or some other distribution. The question is important, because high skewness makes it difficult to pursue risk-hedging portfolio strategies. This paper examines data from several sources
Bond-ordered states and -wave pairing of spinless fermions on the honeycomb lattice
Spinless fermions on the honeycomb lattice with repulsive nearest-neighbor
interactions are known to harbour a quantum critical point at half-filling,
with critical behaviour in the Gross-Neveu (chiral Ising) universality class.
The critical interaction strength separates a weak-coupling semimetallic regime
from a commensurate charge-density-wave phase. The phase diagram of this basic
model of correlated fermions on the honeycomb lattice beyond half-filling is,
however, less well established. Here, we perform an analysis of its many-body
instabilities using the functional renormalization group method with a basic
Fermi surface patching scheme, which allows us to treat instabilities in
competing channels on equal footing also away from half-filling. Between
half-filling and the van-Hove filling, the free Fermi surface is hole-like and
we again find a charge-density wave instability to be dominant at large
interactions. Moreover, its characteristics are those of the half-filled case.
Directly at the van-Hove filling the nesting property of the free Fermi surface
stabilizes a dimerized bond-order phase. At lower filling the free Fermi
surface becomes electron-like and a superconducting instability with -wave
symmetry is found to emerge from the interplay of intra-unitcell repulsion and
collective fluctuations in the proximity to the charge-density wave
instability. We estimate the extent of the various phases and extract the
corresponding order parameters from the effective low-energy Hamiltonians.Comment: 11 pages, 11 figure
Unconventional pairing and electronic dimerization instabilities in the doped Kitaev-Heisenberg model
We study the quantum many-body instabilities of the Kitaev-Heisenberg Hamiltonian on the honeycomb lattice as a
minimal model for a doped spin-orbit Mott insulator. This spin- model is
believed to describe the magnetic properties of the layered transition-metal
oxide NaIrO. We determine the ground-state of the system with finite
charge-carrier density from the functional renormalization group (fRG) for
correlated fermionic systems. To this end, we derive fRG flow-equations adapted
to the lack of full spin-rotational invariance in the fermionic interactions,
here represented by the highly frustrated and anisotropic Kitaev exchange term.
Additionally employing a set of Ward identities for the Kitaev-Heisenberg
model, the numerical solution of the flow equations suggests a rich phase
diagram emerging upon doping charge carriers into the ground-state manifold
( quantum spin liquids and magnetically ordered phases). We
corroborate superconducting triplet -wave instabilities driven by
ferromagnetic exchange and various singlet pairing phases. For filling , the -wave pairing gives rise to a topological state with protected
Majorana edge-modes. For antiferromagnetic Kitaev and ferromagnetic Heisenberg
exchange we obtain bond-order instabilities at van Hove filling supported by
nesting and density-of-states enhancement, yielding dimerization patterns of
the electronic degrees of freedom on the honeycomb lattice. Further, our flow
equations are applicable to a wider class of model Hamiltonians.Comment: 24 pages, 18 figures, corresponds to journal versio
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
Instabilities of interacting electrons on the honeycomb bilayer
We investigate the instabilities of interacting electrons on the honeycomb
bilayer by means of the functional renormalization group for a range of
interactions up to the third-nearest neighbor. Besides a novel instability
toward a gapless charge-density wave we find that using interaction parameters
as determined by ab-initio calculations for graphene and graphite puts the
system close to the boundary between antiferromagnetic and quantum spin Hall
instabilities. Importantly, the energy scales for these instabilities are large
such that imperfections and deviations from the basic model are expected to
play a major role in real bilayer graphene, where interaction effects seem to
be seen only at smaller scales. We therefore analyze how reducing the critical
scale and small doping of the layers affect the instabilities.Comment: 5 pages, 4 figure
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