30 research outputs found
Hund nodal line semimetals: The case of twisted magnetic phase in the double-exchange model
We propose a class of topological metals, which we dub \emph{Hund nodal line
semimetals}, arising from the strong Coulomb interaction encoded in the Hund's
coupling between itinerant electrons and localized spins. We here consider a
particular twisted spin configuration, which is realized in the double exchange
model which describes the manganite oxides. The resulting effective tetragonal
lattice of electrons with hoppings tied to the local spin features an
antiunitary \emph{non-symmorphic} symmetry that in turn, together with another
non-symmorphic but unitary, glide mirror symmetry, protects crossings of a
double pair of bands along a high-symmetry line on the Brillouin zone boundary.
We also discuss symmetry breaking arising from various perturbations of the
twisted phase. Our results may motivate further studies of other realizations
of this state of matter, for instance in different spin backgrounds, properties
of its drumhead surface states, as well as its stability to disorder and
interactions among the itinerant electrons.Comment: 6 pages, 4 figures, published versio
Band gap prediction for large organic crystal structures with machine learning
Machine-learning models are capable of capturing the structure-property
relationship from a dataset of computationally demanding ab initio
calculations. Over the past two years, the Organic Materials Database (OMDB)
has hosted a growing number of calculated electronic properties of previously
synthesized organic crystal structures. The complexity of the organic crystals
contained within the OMDB, which have on average 82 atoms per unit cell, makes
this database a challenging platform for machine learning applications. In this
paper, the focus is on predicting the band gap which represents one of the
basic properties of a crystalline materials. With this aim, a consistent
dataset of 12 500 crystal structures and their corresponding DFT band gap are
released, freely available for download at https://omdb.mathub.io/dataset. An
ensemble of two state-of-the-art models reach a mean absolute error (MAE) of
0.388 eV, which corresponds to a percentage error of 13% for an average band
gap of 3.05 eV. Finally, the trained models are employed to predict the band
gap for 260 092 materials contained within the Crystallography Open Database
(COD) and made available online so that the predictions can be obtained for any
arbitrary crystal structure uploaded by a user.Comment: 10 pages, 6 figure
Coordination chemistry in molecular symmetry adapted spin space (mSASS)
Many areas of chemistry are devoted to the challenge of understanding,
predicting, and controlling the behavior of strongly localized electrons.
Examples include molecular magnetism and luminescence, color centers in
crystals, photochemistry and quantum sensing to name but a few. Over the years,
an amalgam of powerful quantum chemistry methods, simple intuitive models, and
phenomenological parameterizations have been developed, providing increasingly
complex and specialized methodologies. Even with increasing specialization, a
pervasive challenge remains that is surprisingly universal - the simultaneous
description of continuous symmetries (e.g. spin and orbital angular momenta)
and discrete symmetries (e.g. crystal field). Modeling behavior in these
complex systems is increasingly important for metal ions of unusual or
technologically relevant behavior. Additionally, development of broad-scope
models with physically-meaningful parameters carries the potential to
facilitate interdisciplinary collaboration and large-scale meta analysis. We
propose a generalized algorithmic approach, the molecular symmetry adapted spin
space (mSASS), to localized electronic structure. We derive the Hamiltonian in
symmetry-constrained matrix form with an exact account of free parameters and
examples. Although preliminary in its implementation, a fundamental benefit of
this approach is the treatment of spatial and spin-orbit symmetries without the
need for perturbative approximations. In general, the mSASS Hamiltonian is
large but finite and can be diagonalized numerically with high efficiency,
providing a basis for conceptual models of electronic structure that naturally
incorporates spin while leveraging the intuition and efficiency benefits of
crystallographic symmetry. For the generation of the mSASS Hamiltonian, we
provide an implementation into the Mathematica Software Package, GTPack.Comment: 10 pages, 4 figure
Ultrafast entropy production in pump-probe experiments
The ultrafast control of materials has opened the possibility to investigate
non-equilibrium states of matter with striking properties, such as transient
superconductivity and ferroelectricity, ultrafast magnetization and
demagnetization, as well as Floquet engineering. The characterization of the
ultrafast thermodynamic properties within the material is key for their control
and design. Here, we develop the ultrafast stochastic thermodynamics for
laser-excited phonons. We calculate the entropy production and heat absorbed
from experimental data for single phonon modes of driven materials from
time-resolved X-ray scattering experiments where the crystal is excited by a
laser pulse. The spectral entropy production is calculated for SrTiO and
KTaO for different temperatures and reveals a striking relation with the
power spectrum of the displacement-displacement correlation function by
inducing a broad peak beside the eigenmode-resonance.Comment: 11 pages, 6 figure
Online Search Tool for Graphical Patterns in Electronic Band Structures
We present an online graphical pattern search tool for electronic band
structure data contained within the Organic Materials Database (OMDB) available
at https://omdb.diracmaterials.org/search/pattern. The tool is capable of
finding user-specified graphical patterns in the collection of thousands of
band structures from high-throughput ab initio calculations in the online
regime. Using this tool, it only takes a few seconds to find an arbitrary
graphical pattern within the ten electronic bands near the Fermi level for
26,739 organic crystals. The tool can be used to find realizations of
functional materials characterized by a specific pattern in their electronic
structure, for example, Dirac materials, characterized by a linear crossing of
bands; topological insulators, characterized by a "Mexican hat" pattern or an
effectively free electron gas, characterized by a parabolic dispersion. The
source code of the developed tool is freely available at
https://github.com/OrganicMaterialsDatabase/EBS-search and can be transferred
to any other electronic band structure database. The approach allows for an
automatic online analysis of a large collection of band structures where the
amount of data makes its manual inspection impracticable.Comment: 8 pages, 8 figure