1,079 research outputs found
Analytical approximation for single-impurity Anderson model
We have applied the recently developed dual fermion technique to the spectral
properties of single-band Anderson impurity problem (SIAM). In our approach a
series expansion is constructed in vertices of the corresponding atomic
Hamiltonian problem. This expansion contains a small parameter in two limiting
cases: in the weak coupling case (), due to the smallness of the
irreducible vertices, and near the atomic limit (), when bare
propagators are small. Reasonable results are obtained also for the most
interesting case of strong correlations (). The atomic problem of
the Anderson impurity model has a degenerate ground state, so the application
of the perturbation theory is not straightforward. We construct a special
approach dealing with symmetry-broken ground state of the renormalized atomic
problem. Formulae for the first-order dual diagram correction are obtained
analytically in the real-time domain. Most of the Kondo-physics is reproduced:
logarithmic contributions to the self energy arise, Kondo-like peak at the
Fermi level appears, and the Friedel sum rule is fulfilled. Our approach
describes also renormalization of atomic resonances due to hybridization with a
conduction band. A generalization of the proposed scheme to a multi-orbital
case can be important for the realistic description of correlated solids.Comment: 6 pages, 5 figure
Cracking the Quantum Advantage threshold for Gaussian Boson Sampling
Scientists in quantum technology aspire to quantum advantage: a computational
result unattainable with classical computers. Gaussian boson sampling
experiment has been already claimed to achieve this goal. In this setup
squeezed light states interfere in a mid-sized linear optical network. The
exact simulation of the counting statistics of detectors is far beyond the
possibilities of modern supercomputers once exceeds . We challenge
quantum advantage for a mid-sized Gaussian boson sampling setup and propose the
approximate algorithm to obtain the probability of any specific measurement
outcome. For an idealized 70-mode device, our approximation achieves accuracy
competitive with the experimental one.Comment: The supplementary material is uploaded as a separate fil
Electron energy spectrum of the spin-liquid state in a frustrated Hubbard model
Non-local correlation effects in the half-filled Hubbard model on an
isotropic triangular lattice are studied within a spin polarized extension of
the dual fermion approach. A competition between the antiferromagnetic
non-collinear and the spin liquid states is strongly enhanced by an
incorporation of a k-dependent self-energy beyond the local dynamical
mean-field theory. The dual fermion correc- tions drastically decrease the
energy of a spin liquid state while leaving the non-collinear magnetic states
almost non-affected. This makes the spin liquid to become a preferable state in
a certain interval of interaction strength of an order of the magnitude of a
bandwidth. The spectral function of the spin-liquid Mott insulator is
determined by a formation of local singlets which results in the energy gap of
about twice larger than that of the 120 degrees antiferromagnetic Neel state.Comment: 6 pages, 4 figure
Superperturbation solver for quantum impurity models
We present a very efficient solver for the general Anderson impurity problem.
It is based on the perturbation around a solution obtained from exact
diagonalization using a small number of bath sites. We formulate a perturbation
theory which is valid for both weak and strong coupling and interpolates
between these limits. Good agreement with numerically exact quantum Monte-Carlo
results is found for a single bath site over a wide range of parameters. In
particular, the Kondo resonance in the intermediate coupling regime is well
reproduced for a single bath site and the lowest order correction. The method
is particularly suited for low temperatures and alleviates analytical
continuation of imaginary time data due to the absence of statistical noise
compared to quantum Monte-Carlo impurity solvers.Comment: 6 pages, 5 figure
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