119 research outputs found
Doping-Induced Spectral Shifts in Two Dimensional Metal Oxides
Doping of strongly layered ionic oxides is an established paradigm for
creating novel electronic behavior. This is nowhere more apparent than in
superconductivity, where doping gives rise to high temperature
superconductivity in cuprates (hole-doped) and to surprisingly high Tc in HfNCl
(Tc=25.5K, electron-doped). First principles calculations of hole-doping of the
layered delafossite CuAlO2 reveal unexpectedly large doping-induced shifts in
spectral density, strongly in opposition to the rigid band picture that is
widely used as an accepted guideline. These spectral shifts, of similar origin
as the charge transfer used to produce negative electron affinity surfaces and
adjust Schottky barrier heights, drastically alter the character of the Fermi
level carriers, leading in this material to an O-Cu-O molecule-based carrier
(or polaron, at low doping) rather than a nearly pure-Cu hole as in a rigid
band picture. First principles linear response electron-phonon coupling (EPC)
calculations reveal, as a consequence, net weak EPC and no superconductivity
rather than the high Tc obtained previously using rigid band expectations.
These specifically two-dimensional dipole-layer driven spectral shifts provides
new insights into materials design in layered materials foe functionalities
besides superconductivity.Comment: 6 pages, 8 figures,1 tabl
Orbital order, stacking defects and spin-fluctuations in the -electron molecular solid RbO
We examine magnon and orbiton behavior in localized O anti-bonding
molecular orbitals using an effective Kugel-Khomskii Hamiltonian
derived from a two band Hubbard model with hopping parameters taken from {\em
ab initio} density functional calculations. The considerable difference between
intraband and interband hoppings leads to a strong coupling between the spin
wave dispersion and the orbital ground state, providing a straightforward way
of experimentally determining the orbital ground state from the measured magnon
dispersion. The near degeneracy of different orbital ordered states leads to
stacking defects which further modulate spin-fluctuation spectra. Proliferation
of orbital domains disrupts long-range magnetic order, thus causing a
significant reduction in the observed N\'eel temperature.Comment: 5 pages, 2 figure
Effect of Local Electron-Electron Correlation in Hydrogen-like Impurities in Ge
We have studied the electronic and local magnetic structure of the hydrogen
interstitial impurity at the tetrahedral site in diamond-structure Ge, using an
empirical tight binding + dynamical mean field theory approach because within
the local density approximation (LDA) Ge has no gap. We first establish that
within LDA the 1s spectral density bifurcates due to entanglement with the four
neighboring sp3 antibonding orbitals, providing an unanticipated richness of
behavior in determining under what conditions a local moment hyperdeep donor or
Anderson impurity will result, or on the other hand a gap state might appear.
Using a supercell approach, we show that the spectrum, the occupation, and the
local moment of the impurity state displays a strong dependence on the strength
of the local on-site Coulomb interaction U, the H-Ge hopping amplitude, the
depth of the bare 1s energy level epsilon_H, and we address to some extent the
impurity concentration dependence. In the isolated impurity, strong interaction
regime a local moment emerges over most of the parameter ranges indicating
magnetic activity, and spectral density structure very near (or in) the gap
suggests possible electrical activity in this regime.Comment: 9 pages, 5 figure
NaAlSi: a self-doped semimetallic superconductor with free electrons and covalent holes
The layered ternary sp conductor NaAlSi, possessing the iron-pnictide "111"
crystal structure, superconducts at 7 K. Using density functional methods, we
show that this compound is an intrinsic (self-doped) low-carrier-density
semimetal with a number of unusual features. Covalent Al-Si valence bands
provide the holes, and free-electron-like Al 3s bands, which propagate in the
channel between the neighboring Si layers, dip just below the Fermi level to
create the electron carriers. The Fermi level (and therefore the
superconducting carriers) lies in a narrow and sharp peak within a pseudogap in
the density of states. The small peak arises from valence bands which are
nearly of pure Si, quasi-two-dimensional, flat, and coupled to Al conduction
bands. Isostructural NaAlGe, which is not superconducting above 1.6 K, has
almost exactly the same band structure except for one missing piece of small
Fermi surface. Certain deformation potentials induced by Si and Na
displacements along the c-axis are calculated and discussed. It seems likely
that the mechanism of pairing is related to that of several other lightly doped
two-dimensional nonmagnetic semiconductors (TiNCl, ZrNCl, HfNCl), which is not
well understood but apparently not of phonon origin.Comment: 9 pages, 7 figures, 1 tabl
First-Principles Study of Electronic and Vibrational Properties of BaHfN
The transition metal nitride BaHfN, which consists of weakly bonded
neutral slabs of closed shell ions, has structural and chemical similarities to
other layered nitrides which have impressive superconducting T when
electron doped: AHfNCl, AZrNCl, ATiNCl, with ,
and K, respectively for appropriate donor (A) concentrations . These
similarities suggest the possibility of BaHfN being another relatively high
T nitride upon doping, with effects of structure and the role of specific
transition metal ions yet to be understood. We report first-principles
electronic structure calculations for stoichiometric BaHfN using density
functional theory with plane-wave basis sets and separable dual-space Gaussian
pseudopotentials. An indirect band gap of 0.8 eV was obtained and the lowest
conduction band is primarily of Hf 5 character, similar to
-ZrNCl and -TiNCl. The two N sites, one in the Hf layer and
another one in the Ba layer, were found to have very anisotropic Born effective
charges (BEC):deviations from the formal charge (-3) are opposite for the two
sites, and opposite for the two orientations (in-plane, out of plane). LO-TO
splittings and comparison of BECs and dielectric constant tensors to those of
related compounds are discussed, and the effect of electron doping on the
zone-center phonons is reported.Comment: 11 pages, 5 figure
First Principles Study of the Electronic and Vibrational Properties of LiNbO2
In the layered transition metal oxide LiNbO the Nb () ion is
trigonal-prismatically coordinated with O ions, with the resulting crystal
field leading to a single band system for low energy properties. A
tight-binding representation shows that intraplanar second neighbor hopping
meV dominates the first neighbor interaction ( meV). The
first and third neighbor couplings are strongly modified by oxygen
displacements of the symmetric Raman-active vibrational mode, and
electron-phonon coupling to this motion may provide the coupling mechanism for
superconductivity in Li-deficient samples (where K). We calculate all
zone-center phonon modes, identify infrared (IR) and Raman active modes, and
report LO-TO splitting of the IR modes. The Born effective charges for the
metal ions are found to have considerable anisotropy reflecting the degree to
which the ions participate in interlayer coupling and covalent bonding. Insight
into the microscopic origin of the valence band density, composed of Nb
states with some mixing of O states, is obtained from examining
Wannier functions for these bands.Comment: 12 pages, 7 figures; Updated with reviewer comments; Updated
reference
The crucial importance of the -- hybridization in transition metal oxides
We studied the influence of the trigonal distortion of the regular octahedron
along the (111) direction, found in the layers. Under such a
distortion the orbitals split into one and two degenerated
orbitals. We focused on the relative order of these orbitals.
Using quantum chemical calculations of embedded clusters at different levels of
theory, we analyzed the influence of the different effects not taken into
account in the crystalline field theory; that is metal-ligand hybridization,
long-range crystalline field, screening effects and orbital relaxation. We
found that none of them are responsible for the relative order of the
orbitals. In fact, the trigonal distortion allows a mixing of the and
orbitals of the metallic atom. This hybridization is at the origin of the
-- relative order and of the incorrect prediction of the
crystalline field theory
Electronic Structure Calculation by First Principles for Strongly Correlated Electron Systems
Recent trends of ab initio studies and progress in methodologies for
electronic structure calculations of strongly correlated electron systems are
discussed. The interest for developing efficient methods is motivated by recent
discoveries and characterizations of strongly correlated electron materials and
by requirements for understanding mechanisms of intriguing phenomena beyond a
single-particle picture. A three-stage scheme is developed as renormalized
multi-scale solvers (RMS) utilizing the hierarchical electronic structure in
the energy space. It provides us with an ab initio downfolding of the global
band structure into low-energy effective models followed by low-energy solvers
for the models. The RMS method is illustrated with examples of several
materials. In particular, we overview cases such as dynamics of semiconductors,
transition metals and its compounds including iron-based superconductors and
perovskite oxides, as well as organic conductors of kappa-ET type.Comment: 44 pages including 38 figures, to appear in J. Phys. Soc. Jpn. as an
invited review pape
"They brought you back to the fact you're not the same": Sense of self after traumatic brain injury
This paper considers contexts following traumatic brain injury, exploring what may be at stake when dominant expectations predict a ‘lost’ or ‘broken’ self. I explore stories co-constructed with one young man and his mother to illustrate their personal and intersubjective understandings of identity, at times conflicting, within family interactions and when encountering normative practices of neurorehabilitation clinicians. The ower relations portrayed confront this man’s narrative attempts to align his present and pre-injury self, including standard assessments delineating change, administered by healthcare professionals. I consider a need for greater attention to interaction-generated disruption to sense of self, wthin contemporary conceptualisations of ‘person-centred care’
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