422 research outputs found
Flow to strong coupling in the two-dimensional Hubbard model
We extend the analysis of the renormalization group flow in the
two-dimensional Hubbard model close to half-filling using the recently
developed temperature flow formalism. We investigate the interplay of d-density
wave and Fermi surface deformation tendencies with those towards d-wave pairing
and antiferromagnetism. For a ratio of next nearest to nearest neighbor
hoppings, t'/t=-0.25, and band fillings where the Fermi surface is inside the
Umklapp surface, only the d-pairing susceptibility diverges at low
temperatures. When the Fermi surface intersects the Umklapp surface close to
the saddle points, d-wave pairing, d-density wave, antiferromagnetic and, to a
weaker extent, d-wave Fermi surface deformation susceptibilities grow together
when the interactions flow to strong coupling. We interpret these findings as
indications for a non-trivial strongly coupled phase with short-ranged
superconducting and antiferromagnetic correlations, in close analogy with the
spin liquid ground state in the well-understood two-leg Hubbard ladder.Comment: 8 pages, to appear in European Physical Journal
Chiral d-wave superconductivity in doped graphene
A highly unconventional superconducting state with a spin-singlet
-wave, or chiral d-wave, symmetry has recently been
proposed to emerge from electron-electron interactions in doped graphene.
Especially graphene doped to the van Hove singularity at 1/4 doping, where the
density of states diverges, has been argued to likely be a chiral d-wave
superconductor. In this review we summarize the currently mounting theoretical
evidence for the existence of a chiral d-wave superconducting state in
graphene, obtained with methods ranging from mean-field studies of effective
Hamiltonians to angle-resolved renormalization group calculations. We further
discuss multiple distinctive properties of the chiral d-wave superconducting
state in graphene, as well as its stability in the presence of disorder. We
also review means of enhancing the chiral d-wave state using proximity-induced
superconductivity. The appearance of chiral d-wave superconductivity is
intimately linked to the hexagonal crystal lattice and we also offer a brief
overview of other materials which have also been proposed to be chiral d-wave
superconductors.Comment: 51 pages, 8 figures. Invited topical review in J. Phys.:Condens.
Matte
Orbital Dependence of Quasiparticle Lifetimes in Sr2RuO4
Using a phenomenological Hamiltonian, we investigate the quasiparticle
lifetimes and dispersions in the three low energy bands, gamma, beta, and alpha
of Sr2RuO4. Couplings in the Hamiltonian are fixed so as to produce the mass
renormalization as measured in magneto-oscillation experiments. We thus find
reasonable agreement in all bands between our computed lifetimes and those
measured in ARPES experiments by Kidd et al. [1] and Ingle et al. [2]. In
comparing computed to measured quasiparticle dispersions, we however find good
agreement in the alpha-band alone.Comment: 7 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
Instabilities at [110] Surfaces of d_{x^2-y^2} Superconductors
We compare different scenarios for the low temperature splitting of the
zero-energy peak in the local density of states at (110) surfaces of
d_{x^2-y^2}-wave superconductors, observed by Covington et al.
(Phys.Rev.Lett.79 (1997), 277). Using a tight binding model in the
Bogolyubov-de Gennes treatment we find a surface phase transition towards a
time-reversal symmetry breaking surface state carrying spontaneous currents and
an s+id-wave state. Alternatively, we show that electron correlation leads to a
surface phase transition towards a magnetic state corresponding to a local spin
density wave state.Comment: 4 pages, 5 figure
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
Landau-Fermi liquid analysis of the 2D t-t' Hubbard model
We calculate the Landau interaction function f(k,k') for the two-dimensional
t-t' Hubbard model on the square lattice using second and higher order
perturbation theory. Within the Landau-Fermi liquid framework we discuss the
behavior of spin and charge susceptibilities as function of the onsite
interaction and band filling. In particular we analyze the role of elastic
umklapp processes as driving force for the anisotropic reduction of the
compressibility on parts of the Fermi surface.Comment: 10 pages, 16 figure
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
Staggered flux vortices and the superconducting transition in the layered cuprates
We propose an effective model for the superconducting transition in the
high-T_c cuprates motivated by the SU(2) gauge theory approach. In addition to
variations of the superconducting phase we allow for local admixture of
staggered flux order. This leads to an unbinding transition of vortices with
staggered flux core that are energetically preferable to conventional vortices.
Based on parameter estimates for the two-dimensional t-J model we argue that
the staggered flux vortices provide a way to understand a phase with a moderate
density of mobile vortices over a large temperature range above T_c that yet
exhibits otherwise normal transport properties. This picture is consistent with
the large Nernst signal observed in this region.Comment: 4 pages, 3 figure
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