8,665 research outputs found
Orbital-selective Mott-Hubbard transition in the two-band Hubbard model
Recent advances in the field of quantum Monte Carlo simulations for impurity
problems allow --within dynamical mean field theory-- for a more thorough
investigation of the two-band Hubbard model with narrow/wide band and
SU(2)-symmetric Hund's exchange. The nature of this transition has been
controversial, and we establish that an orbital-selective Mott-Hubbard
transition exists. Thereby, the wide band still shows metallic behavior after
the narrow band became insulating -not a pseudogap as for an Ising Hund's
exchange. The coexistence of two solutions with metallic wide band and
insulating or metallic narrow band indicates, in general, first-order
transitions.Comment: 4 pages, 3 figures; 2nd version as published in Phys. Rev. B (R);
minor corrections, putting more emphasis on differences in spectra when
comparing SU(2) and Ising Hund's exchang
Doped Mott insulator as the origin of heavy Fermion behavior in LiV2O4
We investigate the electronic structure of LiV2O4, for which heavy fermion
behavior has been observed in various experiments, by the combination of the
local density approximation and dynamical mean field theory. To obtain results
at zero temperature, we employ the projective quantum Monte Carlo method as an
impurity solver. Our results show that the strongly correlated a1g band is a
lightly doped Mott insulator which -at low temperatures- shows a sharp (heavy)
quasiparticle peak just above the Fermi level, which is consistent with recent
photoemission experiment by Shimoyamada et al. [Phys. Rev. Lett. 96 026403
(2006)].Comment: 4 pages, 5 figure
Reply to a Comment on ``Projective Quantum Monte Carlo Method for the Anderson Impurity Model and its Application to Dynamical Mean Field Theory''
In our reply, we show that the objections put forward in cond-mat/0508763
concerning our paper, Phys. Rev. Lett. 93, 136405 (2004), are not valid:
(i) There is no orthogonality catastrophe (OC) for our calculations, and it
is also generally not ``unpractical'' to avoid it.
(ii) The OC does not affect our results.Comment: 1 page, 1 figure, Phys. Rev. Lett. in print; also note
cond-mat/050944
Designing FeNiCr(CoCu) High Entropy Alloys Using Molecular Dynamics: A Study of the Enhanced Mechanical Properties of a Novel Group of Composites
High-Entropy Alloys (HEAs) are an emergent class of crystalline materials have exhibited unique mechanical properties and high-temperature functionality. Their unusual composition, having multiple principal elements in contrast to common alloys that use one principal element, results in a variety of useful, and in many cases unexpected characteristics. This project, in partnership with experimentalists at the Idaho National Laboratory and machine learning scientists at the University of Utah, explores these characteristics and composition-property relationships in HEAs. Utilizing Molecular Dynamics (MD) simulations, three HEAs, namely FeNiCr, FeNiCrCo, and FeNiCrCoCu, are studied in detail. The radial distribution function (RDF) and tensile strength (TS) are calculated for each alloy, and their values compared as a function of temperature, chemical content, and slip orientation. We used RDF analysis as a basis to examine the stability of HEAs; Fe- and Ni-dominant alloys consistently show five narrow RDF peaks, indicating a strong fit to the desired FCC lattice. We used this strategy to ensure the accuracy of further calculations as well as to demonstrate that MD offers a cost-effective tool for the qualitative analysis of materials. Additionally, we show that RDF analysis can be used as a basic predictive model for more complicated mechanical properties of a crystal, an important asset in material design and machine learning. Finally, we tested these predictions via study of stress-strain curves to evaluate TS of various alloys. We observe that Fe- and Ni-dominant alloys are strongest with peak values ranging from 23 to 26 GPa, while Co- and Cu-dominant alloys are weakest with peak values of 17 and 13 GPa respectively. We uncover fundamental relationships between TS and chemical composition in HEAs – especially the identity of the dominant element – as well as temperature and slip orientation, all of which are critical variables to consider in material design and functionality. Overall, the results in this work offer basic guidelines to design stable high temperature alloys and further highlight the importance of understanding composition-property relationships in HEAs. This work also underlines the power of MD, as it serves as a first step in establishing a high-throughput computational framework to study diverse alloys, with the ultimate goal of understanding the behavior of HEAs in its entirety
Realistic modeling of strongly correlated electron systems: An introduction to the LDA+DMFT approach
The LDA+DMFT approach merges conventional band structure theory in the local
density approximation (LDA) with a state-of-the-art many-body technique, the
dynamical mean-field theory (DMFT). This new computational scheme has recently
become a powerful tool for ab initio investigations of real materials with
strong electronic correlations. In this paper an introduction to the basic
ideas and the set-up of the LDA+DMFT approach is given. Results for the
photoemission spectra of the transition metal oxide La_{1-x}Sr_xTiO_3, obtained
by solving the DMFT-equations by quantum Monte-Carlo (QMC) simulations, are
presented and are found to be in very good agreement with experiment. The
numerically exact DMFT(QMC) solution is compared with results obtained by two
approximative solutions, i.e., the iterative perturbation theory and the
non-crossing approximation.Comment: 15 pages, 3 figures, SCES-Y2K Conference Proceeding
Quantum Monte Carlo study for multiorbital systems with preserved spin and orbital rotational symmetries
We propose to combine the Trotter decomposition and a series expansion of the
partition function for Hund's exchange coupling in a quantum Monte Carlo (QMC)
algorithm for multiorbital systems that preserves spin and orbital rotational
symmetries. This enables us to treat the Hund's (spin-flip and pair-hopping)
terms, which is difficult in the conventional QMC method. To demonstrate this,
we first apply the algorithm to study ferromagnetism in the two-orbital Hubbard
model within the dynamical mean-field theory (DMFT). The result reveals that
the preservation of the SU(2) symmetry in Hund's exchange is important, where
the Curie temperature is grossly overestimated when the symmetry is degraded,
as is often done, to Ising (Z). We then calculate the spectral
functions of SrRuO by a three-band DMFT calculation with tight-binding
parameters taken from the local density approximation with proper rotational
symmetry.Comment: 9 pages, 9 figures. Typos corrected, some comments and references
adde
Momentum-resolved spectral functions of SrVO calculated by LDA+DMFT
LDA+DMFT, the merger of density functional theory in the local density
approximation and dynamical mean-field theory, has been mostly employed to
calculate k-integrated spectra accessible by photoemission spectroscopy. In
this paper, we calculate k-resolved spectral functions by LDA+DMFT. To this
end, we employ the Nth order muffin-tin (NMTO) downfolding to set up an
effective low-energy Hamiltonian with three t_2g orbitals. This downfolded
Hamiltonian is solved by DMFT yielding k-dependent spectra. Our results show
renormalized quasiparticle bands over a broad energy range from -0.7 eV to +0.9
eV with small ``kinks'', discernible in the dispersion below the Fermi energy.Comment: 21 pages, 8 figure
Correlated electron tunneling through two separate quantum dot systems with strong capacitive interdot coupling
A system consisting of two independently contacted quantum dots with strong
electrostatic interaction shows interdot Coulomb blockade when the dots are
weakly tunnel coupled to their leads. It is studied experimentally how the
blockade can be overcome by correlated tunneling when tunnel coupling to the
leads increases. The experimental results are compared with numerical
renormalization group calculations using predefined (measured) parameters. Our
results indicate Kondo correlations due to the electrostatic interaction in
this double quantum dot system.Comment: 5 pages, 3 figures, published in Phys. Rev. Lett. Oct. 30t
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