595 research outputs found
Ward identities for the Anderson impurity model: derivation via functional methods and the exact renormalization group
Using functional methods and the exact renormalization group we derive Ward
identities for the Anderson impurity model. In particular, we present a
non-perturbative proof of the Yamada-Yosida identities relating certain
coefficients in the low-energy expansion of the self-energy to thermodynamic
particle number and spin susceptibilities of the impurity. Our proof underlines
the relation of the Yamada-Yosida identities to the U(1) x U(1) symmetry
associated with particle number and spin conservation in a magnetic field.Comment: 8 pages, corrected statements about infintite flatband limi
Spectral function of the Anderson impurity model at finite temperatures
Using the functional renormalization group (FRG) and the numerical
renormalization group (NRG), we calculate the spectral function of the Anderson
impurity model at zero and finite temperatures. In our FRG scheme spin
fluctuations are treated non-perturbatively via a suitable Hubbard-Stratonovich
field, but vertex corrections are neglected. A comparison with our highly
accurate NRG results shows that this FRG scheme gives a quantitatively good
description of the spectral line-shape at zero and finite temperatures both in
the weak and strong coupling regimes, although at zero temperature the FRG is
not able to reproduce the known exponential narrowing of the Kondo resonance at
strong coupling.Comment: 6 pages, 3 figures; new references adde
Magnetic Skyrmion Lattice by Fourier Transform Method
We demonstrate a fast numerical method of theoretical studies of skyrmion
lattice or spiral order in magnetic materials with Dzyaloshinsky-Moriya
interaction. The method is based on the Fourier expansion of the magnetization
combined with a minimization of the free energy functional of the magnetic
material in Fourier space, yielding the optimal configuration of the system for
any given set of parameters. We employ a Lagrange multiplier technique in order
to satisfy micromagnetic constraints. We apply this method to a system that
exhibits, depending on the parameter choice, ferromagnetic, skyrmion lattice,
or spiral (helical) order. Known critical fields corresponding to the
helical-skyrmion as well as the skyrmion-ferromagnet phase transitions are
reproduced with high precision. Using this numerical method we predict new
types of excited (metastable) states of the skyrmion lattice, which may be
stabilized by coupling the skyrmion lattice with a superconducting vortex
lattice. The method can be readily adapted to other micromagnetic systems.Comment: 12 pages, 8 figure
Quantum criticality of dipolar spin chains
We show that a chain of Heisenberg spins interacting with long-range dipolar
forces in a magnetic field h perpendicular to the chain exhibits a quantum
critical point belonging to the two-dimensional Ising universality class.
Within linear spin-wave theory the magnon dispersion for small momenta k is
[Delta^2 + v_k^2 k^2]^{1/2}, where Delta^2 \propto |h - h_c| and v_k^2 \propto
|ln k|. For fields close to h_c linear spin-wave theory breaks down and we
investigate the system using density-matrix and functional renormalization
group methods. The Ginzburg regime where non-Gaussian fluctuations are
important is found to be rather narrow on the ordered side of the transition,
and very broad on the disordered side.Comment: 6 pages, 5 figure
Charge Disproportionation, Mixed Valence, and Janus Effect in Multiorbital Systems: A Tale of Two Insulators
Multiorbital Hubbard models host strongly correlated "Hund's metals" even for interactions much stronger than the bandwidth. We characterize this interaction-resilient metal as a mixed-valence state. In particular, it can be pictured as a bridge between two strongly correlated insulators: a high-spin Mott insulator and a charge-disproportionated insulator which is stabilized by a very large Hund's coupling. This picture is confirmed comparing models with negative and positive Hund's coupling for different fillings. Our results provide a characterization of the Hund's metal state and connect its presence with charge disproportionation, which has indeed been observed in chromates and proposed to play a role in iron-based superconductors
Measurement of the top quark forward-backward production asymmetry and the anomalous chromoelectric and chromomagnetic moments in pp collisions at √s = 13 TeV
Abstract The parton-level top quark (t) forward-backward asymmetry and the anomalous chromoelectric (d̂ t) and chromomagnetic (μ̂ t) moments have been measured using LHC pp collisions at a center-of-mass energy of 13 TeV, collected in the CMS detector in a data sample corresponding to an integrated luminosity of 35.9 fb−1. The linearized variable AFB(1) is used to approximate the asymmetry. Candidate t t ¯ events decaying to a muon or electron and jets in final states with low and high Lorentz boosts are selected and reconstructed using a fit of the kinematic distributions of the decay products to those expected for t t ¯ final states. The values found for the parameters are AFB(1)=0.048−0.087+0.095(stat)−0.029+0.020(syst),μ̂t=−0.024−0.009+0.013(stat)−0.011+0.016(syst), and a limit is placed on the magnitude of | d̂ t| < 0.03 at 95% confidence level. [Figure not available: see fulltext.
- …