101 research outputs found
Non-perturbative approaches to magnetism in strongly correlated electron systems
The microscopic basis for the stability of itinerant ferromagnetism in
correlated electron systems is examined. To this end several routes to
ferromagnetism are explored, using both rigorous methods valid in arbitrary
spatial dimensions, as well as Quantum Monte Carlo investigations in the limit
of infinite dimensions (dynamical mean-field theory). In particular we discuss
the qualitative and quantitative importance of (i) the direct Heisenberg
exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a
high spectral density near the band edges caused by an appropriate lattice
structure and/or kinetic energy of the electrons. We furnish evidence of the
stability of itinerant ferromagnetism in the pure Hubbard model for appropriate
lattices at electronic densities not too close to half-filling and large enough
. Already a weak direct exchange interaction, as well as band degeneracy, is
found to reduce the critical value of above which ferromagnetism becomes
stable considerably. Using similar numerical techniques the Hubbard model with
an easy axis is studied to explain metamagnetism in strongly anisotropic
antiferromagnets from a unifying microscopic point of view.Comment: 11 pages, Latex, and 6 postscript figures; Z. Phys. B, in pres
The HSE hybrid functional within the FLAPW method and its application to GdN
We present an implementation of the Heyd-Scuseria-Ernzerhof (HSE) hybrid
functional within the full-potential linearized augmented-plane-wave (FLAPW)
method. Pivotal to the HSE functional is the screened electron-electron
interaction, which we separate into the bare Coulomb interaction and the
remainder, a slowly varying function in real space. Both terms give rise to
exchange potentials, which sum up to the screened nonlocal exchange potential
of HSE. We evaluate the former with the help of an auxiliary basis, defined in
such a way that the bare Coulomb matrix becomes sparse. The latter is computed
in reciprocal space, exploiting its fast convergence behavior in reciprocal
space. This approach is general and can be applied to a whole class of screened
hybrid functionals. We obtain excellent agreement of band gaps and lattice
constants for prototypical semiconductors and insulators with
electronic-structure calculations using plane-wave or Gaussian basis sets. We
apply the HSE hybrid functional to examine the ground-state properties of
rocksalt GdN, which have been controversially discussed in literature. Our
results indicate that there is a half-metal to insulator transition occurring
between the theoretically optimized lattice constant at 0 K and the
experimental lattice constant at room temperature. Overall, we attain good
agreement with experimental data for band transitions, magnetic moments, and
the Curie temperature.Comment: 13 pages, 4 figures, 6 table
Quantum Monte Carlo calculation of the finite temperature Mott-Hubbard transition
We present clear numerical evidence for the coexistence of metallic and
insulating dynamical mean field theory(DMFT) solutions in a half-filled
single-band Hubbard model with bare semicircular density of states at finite
temperatures. Quantum Monte Carlo(QMC) method is used to solve the DMFT
equations. We discuss important technical aspects of the DMFT-QMC which need to
be taken into account in order to obtain the reliable results near the
coexistence region. Among them are the critical slowing down of the iterative
solutions near phase boundaries, the convergence criteria for the DMFT
iterations, the interpolation of the discretized Green's function and the
reduction of QMC statistical and systematic errors. Comparison of our results
with those of other numerical methods is presented in a phase diagram.Comment: 4 pages, 5 figure
Landau Theory of the Finite Temperature Mott Transition
In the context of the dynamical mean-field theory of the Hubbard model, we
identify microscopically an order parameter for the finite temperature Mott
endpoint. We derive a Landau functional of the order parameter. We then use the
order parameter theory to elucidate the singular behavior of various physical
quantities which are experimentally accessible.Comment: 4 pages, 2 figure
Fluctuation-driven insulator-to-metal transition in an external magnetic field
We consider a model for a metal-insulator transition of correlated electrons
in an external magnetic field. We find a broad region in interaction and
magnetic field where metallic and insulating (fully magnetized) solutions
coexist and the system undergoes a first-order metal-insulator transition. A
global instability of the magnetically saturated solution precedes the local
ones and is caused by collective fluctuations due to poles in electron-hole
vertex functions.Comment: REVTeX 4 pages, 3 PS figure
A closer look into two-step perovskite conversion with X-ray scattering
Recently, hybrid perovskites have gathered much interest as alternative materials for the fabrication of highly efficient and cost-competitive solar cells; however, many questions regarding perovskite crystal formation and deposition methods remain. Here we have applied a two-step protocol where a crystalline PbI2 precursor film is converted to MAPbI3–xClx perovskite upon immersion in a mixed solution of methylammonium iodide and methylammonium chloride. We have investigated both films with grazing incidence small-angle X-ray scattering to probe the inner film morphology. Our results demonstrate a strong link between lateral crystal sizes in the films before and after conversion, which we attribute to laterally confined crystal growth. Additionally, we observe an accumulation of smaller grains within the bulk in contrast with the surface. Thus, our results help to elucidate the crystallization process of perovskite films deposited via a two-step technique that is crucial for controlled film formation, improved reproducibility, and high photovoltaic performance
The balancing act between high electronic and low ionic transport influenced by perovskite grain boundaries
\ua9 2024 The Royal Society of Chemistry.A better understanding of the materials\u27 fundamental physical processes is necessary to push hybrid perovskite photovoltaic devices towards their theoretical limits. The role of the perovskite grain boundaries is essential to optimise the system thoroughly. The influence of the perovskite grain size and crystal orientation on physical properties and their resulting photovoltaic performance is examined. We develop a novel, straightforward synthesis approach that yields crystals of a similar size but allows the tuning of their orientation to either the (200) or (002) facet alignment parallel to the substrate by manipulating dimethyl sulfoxide (DMSO) and tetrahydrothiophene-1-oxide (THTO) ratios. This decouples crystal orientation from grain size, allowing the study of charge carrier mobility, found to be improved with larger grain sizes, highlighting the importance of minimising crystal disorder to achieve efficient devices. However, devices incorporating crystals with the (200) facet exhibit an s-shape in the current density-voltage curve when standard scan rates are used, which typically signals an energetic interfacial barrier. Using the drift-diffusion simulations, we attribute this to slower-moving ions (mobility of 0.37
7 10-10 cm2 V-1 s-1) in combination with a lower density of mobile ions. This counterintuitive result highlights that reducing ion migration does not necessarily minimise hysteresis
Absence of hysteresis at the Mott-Hubbard metal-insulator transition in infinite dimensions
The nature of the Mott-Hubbard metal-insulator transition in the
infinite-dimensional Hubbard model is investigated by Quantum Monte Carlo
simulations down to temperature T=W/140 (W=bandwidth). Calculating with
significantly higher precision than in previous work, we show that the
hysteresis below T_{IPT}\simeq 0.022W, reported in earlier studies, disappears.
Hence the transition is found to be continuous rather than discontinuous down
to at least T=0.325T_{IPT}. We also study the changes in the density of states
across the transition, which illustrate that the Fermi liquid breaks down
before the gap opens.Comment: 4 pages, 4 eps-figures using epsf.st
Similarities between the Hubbard and Periodic Anderson Models at Finite Temperatures
The single band Hubbard and the two band Periodic Anderson Hamiltonians have
traditionally been applied to rather different physical problems - the Mott
transition and itinerant magnetism, and Kondo singlet formation and scattering
off localized magnetic states, respectively. In this paper, we compare the
magnetic and charge correlations, and spectral functions, of the two systems.
We show quantitatively that they exhibit remarkably similar behavior, including
a nearly identical topology of the finite temperature phase diagrams at
half-filling. We address potential implications of this for theories of the
rare earth ``volume collapse'' transition.Comment: 4 pages (RevTeX) including 4 figures in 7 eps files; as to appear in
Phys. Rev. Let
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