559 research outputs found
Techniques and errors in measuring cross- correlation and cross-spectral density functions
Techniques and errors in measuring cross spectral density and cross correlation functions of stationary dynamic pressure dat
Strain-controlled band engineering and self-doping in ultrathin LaNiO films
We report on a systematic study of the temperature-dependent Hall coefficient
and thermoelectric power in ultra-thin metallic LaNiO films that reveal a
strain-induced, self-doping carrier transition that is inaccessible in the
bulk. As the film strain varies from compressive to tensile at fixed
composition and stoichiometry, the transport coefficients evolve in a manner
strikingly similar to those of bulk hole-doped superconducting cuprates with
varying doping level. Density functional calculations reveal that the
strain-induced changes in the transport properties are due to self-doping in
the low-energy electronic band structure. The results imply that thin-film
epitaxy can serve as a new means to achieve hole-doping in other (negative)
charge-transfer gap transition metal oxides without resorting to chemical
substitution
New Developments in Tellurite Glass Fibers
Recent developments on the manufacture of tellurite glass fibers are presented. Technical
issues related to glass synthesis, preform manufacturing and fiber drawing as well as prospective of
commercial exploitation are discussed
Evidence for the weakly coupled electron mechanism in an Anderson-Blount polar metal
Over 50 years ago, Anderson and Blount proposed that ferroelectric-like structural phase transitions may occur in metals, despite the expected screening of the Coulomb interactions that often drive polar transitions. Recently, theoretical treatments have suggested that such transitions require the itinerant electrons be decoupled from the soft transverse optical phonons responsible for polar order. However, this decoupled electron mechanism (DEM) has yet to be experimentally observed. Here we utilize ultrafast spectroscopy to uncover evidence of the DEM in LiOsO_3, the first known band metal to undergo a thermally driven polar phase transition (T_c ≈ 140 K). We demonstrate that intra-band photo-carriers relax by selectively coupling to only a subset of the phonon spectrum, leaving as much as 60% of the lattice heat capacity decoupled. This decoupled heat capacity is shown to be consistent with a previously undetected and partially displacive TO polar mode, indicating the DEM in LiOsO_3
Evidence for an extended critical fluctuation region above the polar ordering transition in LiOsO₃
Metallic
Li
Os
O
3
undergoes a continuous ferroelectric-like structural phase transition below
T
c
=
140
K
to realize a polar metal. To understand the microscopic interactions that drive this transition, we study its critical behavior above
T
c
via electromechanical coupling—distortions of the lattice induced by short-range dipole-dipole correlations arising from Li off-center displacements. By mapping the full angular distribution of second harmonic electric-quadrupole radiation from
Li
Os
O
3
and performing a simplified hyper-polarizable bond model analysis, we uncover subtle symmetry-preserving lattice distortions over a broad temperature range extending from
T
c
up to around 230 K, characterized by nonuniform changes in the short and long Li-O bond lengths. Such an extended region of critical fluctuations may explain anomalous features reported in specific heat and Raman scattering data and suggests the presence of competing interactions that are not accounted for in existing theoretical treatments. More broadly, our results showcase how electromechanical effects serve as a probe of critical behavior near inversion symmetry-breaking transitions in metals
Evidence for an extended critical fluctuation region above the polar ordering transition in LiOsO
Metallic LiOsO undergoes a continuous ferroelectric-like structural phase
transition below = 140 K to realize a polar metal. To understand the
microscopic interactions that drive this transition, we study its critical
behavior above via electromechanical coupling - distortions of the
lattice induced by short-range dipole-dipole correlations arising from Li
off-center displacements. By mapping the full angular distribution of second
harmonic electric-quadrupole radiation from LiOsO and performing a
simplified hyper-polarizable bond model analysis, we uncover subtle
symmetry-preserving lattice distortions over a broad temperature range
extending from up to around 230 K, characterized by non-uniform changes
in the short and long Li-O bond lengths. Such an extended region of critical
fluctuations may explain anomalous features reported in specific heat and Raman
scattering data, and suggests the presence of competing interactions that are
not accounted for in existing theoretical treatments. More broadly, our results
showcase how electromechanical effects serve as a probe of critical behavior
near inversion symmetry breaking transitions in metals.Comment: 6 pages main text, 4 figures, 10 pages supplementary informatio
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
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