1,378 research outputs found
Non-existence of the Luttinger-Ward functional and misleading convergence of skeleton diagrammatic series for Hubbard-like models
The Luttinger-Ward functional , which expresses the
thermodynamic grand potential in terms of the interacting single-particle
Green's function , is found to be ill-defined for fermionic models
with the Hubbard on-site interaction. In particular, we show that the
self-energy is not a single-valued functional of : in addition to
the physical solution for , there exists at least
one qualitatively distinct unphysical branch. This result is demonstrated for
several models: the Hubbard atom, the Anderson impurity model, and the full
two-dimensional Hubbard model. Despite this pathology, the skeleton Feynman
diagrammatic series for in terms of is found to
converge at least for moderately low temperatures. However, at strong
interactions, its convergence is to the unphysical branch. This reveals a new
scenario of breaking down of diagrammatic expansions. In contrast, the bare
series in terms of the non-interacting Green's function
converges to the correct physical branch of in all cases
currently accessible by diagrammatic Monte Carlo. Besides their conceptual
importance, these observations have important implications for techniques based
on the explicit summation of diagrammatic series.Comment: 5 pages, 5 figure
Orbital Polarization in Strained LaNiO: Structural Distortions and Correlation Effects
Transition-metal heterostructures offer the fascinating possibility of
controlling orbital degrees of freedom via strain. Here, we investigate
theoretically the degree of orbital polarization that can be induced by
epitaxial strain in LaNiO films. Using combined electronic structure and
dynamical mean-field theory methods we take into account both structural
distortions and electron correlations and discuss their relative influence. We
confirm that Hund's rule coupling tends to decrease the polarization and point
out that this applies to both the and local
configurations of the Ni ions. Our calculations are in good agreement with
recent experiments, which revealed sizable orbital polarization under tensile
strain. We discuss why full orbital polarization is hard to achieve in this
specific system and emphasize the general limitations that must be overcome to
achieve this goal.Comment: 13 pages, 13 figure
Dynamical behavior across the Mott transition of two bands with different bandwidths
We investigate the role of the bandwidth difference in the Mott
metal-insulator transition of a two-band Hubbard model in the limit of infinite
dimensions, by means of a Gutzwiller variational wave function as well as by
dynamical mean-field theory. The variational calculation predicts a two-stage
quenching of the charge degrees of freedom, in which the narrower band
undergoes a Mott transition before the wider one, both in the presence and in
the absence of a Hund's exchange coupling. However, this scenario is not fully
confirmed by the dynamical mean-field theory calculation, which shows that,
although the quasiparticle residue of the narrower band is zero within our
numerical accuracy, low-energy spectral weight still exists inside the
Mott-Hubbard gap, concentrated into two peaks symmetric around the chemical
potential. This spectral weight vanishes only when the wider band ceases to
conduct too. Although our results are compatible with several scenarios, e.g.,
a narrow gap semiconductor or a semimetal, we argue that the most plausible one
is that the two peaks coexist with a narrow resonance tied at the chemical
potential, with a spectral weight below our numerical accuracy. This
quasiparticle resonance is expected to vanish when the wider band undergoes the
Mott transition.Comment: 11 pages, 12 figure
TRIQS/CTHYB: A Continuous-Time Quantum Monte Carlo Hybridization Expansion Solver for Quantum Impurity Problems
We present TRIQS/CTHYB, a state-of-the art open-source implementation of the
continuous-time hybridisation expansion quantum impurity solver of the TRIQS
package. This code is mainly designed to be used with the TRIQS library in
order to solve the self-consistent quantum impurity problem in a multi-orbital
dynamical mean field theory approach to strongly-correlated electrons, in
particular in the context of realistic calculations. It is implemented in C++
for efficiency and is provided with a high-level Python interface. The code is
ships with a new partitioning algorithm that divides the local Hilbert space
without any user knowledge of the symmetries and quantum numbers of the
Hamiltonian. Furthermore, we implement higher-order configuration moves and
show that such moves are necessary to ensure ergodicity of the Monte Carlo in
common Hamiltonians even without symmetry-breaking.Comment: 19 pages, this is a companion article to that describing the TRIQS
librar
Theoretical prediction and spectroscopic fingerprints of an orbital transition in CeCu2Si2
We show that the heavy-fermion compound CeCu2Si2 undergoes a transition
between two regimes dominated by different crystal-field states. At low
pressure P and low temperature T the Ce 4f electron resides in the atomic
crystal-field ground state, while at high P or T the electron occupancy and
spectral weight is transferred to an excited crystal-field level that
hybridizes more strongly with itinerant states. These findings result from
first-principles dynamical-mean-field-theory calculations. We predict
experimental signatures of this orbital transition in X-ray spectroscopy. The
corresponding fluctuations may be responsible for the second high-pressure
superconducting dome observed in this and similar materials.Comment: 5 pages, 4 figures + 5 supplementary page
Simple predictors of in superconducting cuprates reveal role of interactions between effective Wannier orbitals in the 3-band model
At optimal doping, different cuprate compounds can exhibit vastly different
critical temperatures for superconductivity (), ranging from about 20 K to
about 135 K. The precise properties of the lattice that determine the magnitude
of the are currently unknown. In this paper, we investigate the
dependence of the optimal doping on the parameters of the Emery ()
model for the CuO planes in the cuprates. We show that the best scaling is
obtained not with the parameters of the model written in the real
(-orbital) space, but rather written in the space of effective Wannier
orbitals. In this basis, one obtains a model of three sublattices coupled
through all possible 4-point interactions. We identify multiple predictor
variables that fit the experimental to about K and that depend on
the coupling constants in the transformed Hamiltonian.Comment: 6 pages, 2 figures + supp. mat. 15 pages, 10 figure
How bad metals turn good: spectroscopic signatures of resilient quasiparticles
We investigate transport in strongly correlated metals. Within dynamical
mean-field theory, we calculate the resistivity, thermopower, optical
conductivity and thermodynamic properties of a hole-doped Mott insulator. Two
well-separated temperature scales are identified: T_FL below which Landau Fermi
liquid behavior applies, and T_MIR above which the resistivity exceeds the
Mott-Ioffe-Regel value and `bad-metal' behavior is found. We show that
quasiparticle excitations remain well-defined above T_FL and dominate transport
throughout the intermediate regime T_FL < T_MIR. The lifetime of these
`resilient quasiparticles' is longer for electron-like excitations, and this
pronounced particle-hole asymmetry has important consequences for the
thermopower. The crossover into the bad-metal regime corresponds to the
disappearance of these excitations, and has clear signatures in optical
spectroscopy.Comment: 5 pages + 4 supplementary pages; published versio
Dynamics of the quantum dimer model on the triangular lattice: Soft modes and local resonating valence-bond correlations
We report on an exhaustive investigation of the dynamical dimer-dimer
correlations in imaginary time for the quantum dimer model on the triangular
lattice using the Green's function Monte Carlo method. We show in particular
that soft modes develop upon reducing the dimer-dimer repulsion, indicating the
presence of a second-order phase transition into an ordered phase with broken
translational symmetry. We further investigate the nature of this ordered
phase, for which a 12-site unit cell has been previously proposed, with the
surprising result that significant Bragg peaks are only present at two of the
three high-symmetry points consistent with this unit cell. We attribute the
absence of a detectable peak to its small magnitude due to the nearly uniform
internal structure of the 12-site crystal cell.Comment: 6 pages, 8 figure
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