30 research outputs found
Cu-doped Pb(PO)O, and V doped SrTiO -- a tutorial on electron-crystal lattice coupling in insulating materials with transition metal dopants
We provide two pedagogical examples to understand the mechanisms at play in
stabilizing an insulating state, and an impurity level in the bandgap in two
insulators using DFT+U: Cu-doped Pb(PO)O and V-doped SrTiO
transition metal-doped insulators. We find both to be insulating, with isolated
impurity (flat) bands, regardless of doping location. In both cases, the
electron degeneracy and crystal lattice symmetry are broken, leading to an
insulating state, and a magnetically and orbitally polarized impurity state
within the gap, clearly separated from the valence and conduction bands. We
also resolve previously noticed inconsistencies between DFT and experiment
regarding doping site energetics, crystal structure, and transparency in
Cu-doped phosphate lead apatite 'LK-99'. Doping one of each type of site in the
same unit cell ( doping) also simply leads to two spin-polarized
impurity bands in the material's gap. The local transition metal ion sites may
behave like color centers (or f-centers), possibly giving color at low
temperatures to what we predict to be a transparent, insulating material in the
recently synthesized LK-99. Weak ferromagnetism may be possible due to the
relatively delocalized unpaired spins in the valence band. Further work should
take into account the possibility of further changes in stoichiometry,
different doping locations, supercell effects, and quantification of magnetic
exchange interactions
Trigonal Symmetry Breaking and its Electronic Effects in Two-Dimensional Dihalides and Trihalides
We study the consequences of the approximately trigonal () point
symmetry of the transition metal (M) site in two-dimensional van der Waals
MX dihalides and MX trihalides. The trigonal symmetry leads to a 2-2-1
orbital splitting of the transition metal shell, which may be tuned by the
interlayer distance, and changes in the ligand-ligand bond lengths. Orbital
order coupled to various lower symmetry lattice modes may lift the remaining
orbital degeneracies, and we explain how these may support unique electronic
states using ZrI and CuCl as examples, and offer a brief overview of
possible electronic configurations in this class of materials. By building and
analysing Wannier models adapted to the appropriate symmetry we examine how the
interplay among trigonal symmetry, electronic correlation effects, and -
orbital charge transfer leads to insulating, orbitally polarized magnetic
and/or orbital-selective Mott states. Our work establishes a rigorous framework
to understand, control, and tune the electronic states in low-dimensional
correlated halides. Our analysis shows that trigonal symmetry and its breaking
is a key feature of the 2D halides that needs to be accounted for in search of
novel electronic states in materials ranging from CrI to -RuCl
High-resolution spatial mapping of the temperature distribution of a Joule self-heated graphene nanoribbon
We investigate the temperature distributions of Joule self-heated graphene
nanoribbons (GNRs) with a spatial resolution finer than 100 nm by scanning
thermal microscopy (SThM). The SThM probe is calibrated using the Raman G mode
Stokes/anti-Stokes intensity ratio as a function of electric power applied to
the GNR devices. From a spatial map of the temperature distribution, heat
dissipation and transport pathways are investigated. By combining SThM and
scanning gate microscopy data from a defected GNR, we observe hot spot
formation at well-defined, localized sites.Comment: 4 pages, 3 figures, accepted on App. Phys. Let
Database, Features, and Machine Learning Model to Identify Thermally Driven Metal-Insulator Transition Compounds
Metal-insulator transition (MIT) compounds are materials that may exhibit
insulating or metallic behavior, depending on the physical conditions, and are
of immense fundamental interest owing to their potential applications in
emerging microelectronics. There is a dearth of thermally-driven MIT materials,
however, which makes delineating these compounds from those that are
exclusively insulating or metallic challenging. Here we report a material
database comprising temperature-controlled MITs (and metals and insulators with
similar chemical composition and stoichiometries to the MIT compounds) from
high quality experimental literature, built through a combination of
materials-domain knowledge and natural language processing. We featurize the
dataset using compositional, structural, and energetic descriptors, including
two MIT relevant energy scales, an estimated Hubbard interaction and the charge
transfer energy, as well as the structure-bond-stress metric referred to as the
global-instability index (GII). We then perform supervised classification,
constructing three electronic-state classifiers: metal vs non-metal (M),
insulator vs non-insulator (I), and MIT vs non-MIT (T). We identify two
important descriptors that separate metals, insulators, and MIT materials in a
2D feature space: the average deviation of the covalent radius and the range of
the Mendeleev number. We further elaborate on other important features (GII and
Ewald energy), and examine how they affect classification of binary vanadium
and titanium oxides. We discuss the relationship of these atomic features to
the physical interactions underlying MITs in the rare-earth nickelate family.
Last, we implement an online version of the classifiers, enabling quick
probabilistic class predictions by uploading a crystallographic structure file