13,292 research outputs found
Harmonic qualities in Debussy's "Les sons et les parfums tournent dans l'air du soir"
This analysis of the fourth piece from Debussy's Préludes Book I illustrates typical harmonic techniques of Debussy as manipulations of harmonic qualities. We quantify harmonic qualities via the magnitudes and squared-magnitudes of the coefficients of the discrete Fourier transform (DFT) of pitch class sets, following Ian Quinn. The principal activity of the piece occurs in the fourth and fifth coefficients, the octatonic and diatonic qualities, respectively. The development of harmonic ideas can therefore be mapped out in a two-dimensional octatonic/diatonic phase space. Whole-tone material, representative of the sixth coefficient of the DFT, also plays an important role. I discuss Debussy's motivic work, how features of tonality – diatonicity and harmonic function – relate to his musical language, and the significance of perfectly balanced set classes, which are a special case of nil DFT coefficients.Accepted manuscrip
Structure and electronic properties of transition-metal/Mg bimetallic clusters at realistic temperatures and oxygen partial pressures
Composition, atomic structure, and electronic properties of TMMgO
clusters (TM = Cr, Ni, Fe, Co, ) at realistic temperature and
partial oxygen pressure conditions are explored using the
{\em ab initio} atomistic thermodynamics approach. The low-energy isomers of
the different clusters are identified using a massively parallel cascade
genetic algorithm at the hybrid density-functional level of theory. On
analyzing a large set of data, we find that the fundamental gap E
of the thermodynamically stable clusters are strongly affected by the presence
of Mg-coordinated O moieties. In contrast, the nature of the transition
metal does not play a significant role in determining E. Using
E of a cluster as a descriptor of its redox properties, our
finding is against the conventional belief that the transition metal plays the
key role in determining the electronic and therefore chemical properties of the
clusters. High reactivity may be correlated more strongly with oxygen content
in the cluster than with any specific TM type.Comment: 7 pages, 5 figure
Equations of state and stability of MgSiO perovskite and post-perovskite phases from quantum Monte Carlo simulations
We have performed quantum Monte Carlo (QMC) simulations and density
functional theory (DFT) calculations to study the equations of state of
MgSiO perovskite (Pv) and post-perovskite (PPv), up to the pressure and
temperature conditions of the base of Earth's lower mantle. The ground state
energies were derived using QMC and the temperature dependent Helmholtz free
energies were calculated within the quasi-harmonic approximation and density
functional perturbation theory. The equations of state for both phases of
MgSiO agree well with experiments, and better than those from generalized
gradient approximation (GGA) calculations. The Pv-PPv phase boundary calculated
from our QMC equations of states is also consistent with experiments, and
better than previous LDA calculations. We discuss the implications for double
crossing of the Pv-PPv boundary in the Earth
Electronic Structure Shift of Deep Nanoscale Silicon by SiO- vs. SiN-Embedding as Alternative to Impurity Doping
Conventional impurity doping of deep nanoscale silicon (dns-Si) used in ultra
large scale integration (ULSI) faces serious challenges below the 14 nm
technology node. We report on a new fundamental effect in theory and
experiment, namely the electronic structure of dns-Si experiencing energy
offsets of ca. 1 eV as a function of SiO- vs. SiN-embedding with a
few monolayers (MLs). An interface charge transfer (ICT) from dns-Si specific
to the anion type of the dielectric is at the core of this effect and arguably
nested in quantum-chemical properties of oxygen (O) and nitrogen (N) vs. Si. We
investigate the size up to which this energy offset defines the electronic
structure of dns-Si by density functional theory (DFT), considering interface
orientation, embedding layer thickness, and approximants featuring two Si
nanocrystals (NCs); one embedded in SiO and the other in SiN.
Working with synchrotron ultraviolet photoelectron spectroscopy (UPS), we use
SiO- vs. SiN-embedded Si nanowells (NWells) to obtain their energy
of the top valence band states. These results confirm our theoretical findings
and gauge an analytic model for projecting maximum dns-Si sizes for NCs,
nanowires (NWires) and NWells where the energy offset reaches full scale,
yielding to a clear preference for electrons or holes as majority carriers in
dns-Si. Our findings can replace impurity doping for n/p-type dns-Si as used in
ultra-low power electronics and ULSI, eliminating dopant-related issues such as
inelastic carrier scattering, thermal ionization, clustering, out-diffusion and
defect generation. As far as majority carrier preference is concerned, the
elimination of those issues effectively shifts the lower size limit of Si-based
ULSI devices to the crystalization limit of Si of ca. 1.5 nm and enables them
to work also under cryogenic conditions.Comment: 14 pages, 17 Figures with a total 44 graph
High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations
Structural evolution of a prospective hydrogen storage material, ammonia
borane NH3BH3, has been studied at high pressures up to 12 GPa and at low
temperatures by synchrotron powder diffraction. At 293 K and above 1.1 GPa a
disordered I4mm structure reversibly transforms into a new ordered phase. Its
Cmc21 structure was solved from the diffraction data, the positions of N and B
atoms and the orientation of NH3 and BH3 groups were finally assigned with the
help of density functional theory calculations. Group-theoretical analysis
identifies a single two-component order parameter, combining ordering and
atomic displacement mechanisms, which link an orientationally disordered parent
phase I4mm with ordered distorted Cmc21, Pmn21 and P21 structures. We propose a
generic phase diagram for NH3BH3, mapping three experimentally found and one
predicted (P21) phases as a function of temperature and pressure, along with
the evolution of the corresponding structural distortions. Ammonia borane
belongs to the class of improper ferroelastics and we show that both
temperature- and pressure-induced phase transitions can be driven to be of the
second order. The role of N-H...H-B dihydrogen bonds and other intermolecular
interactions in the stability of NH3BH3 polymorphs is examined.Comment: 23 pages, 7 figure
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