176 research outputs found
Phonon anharmonicities in graphite and graphene
We determine from first-principles the finite-temperature
properties--linewidths, line shifts, and lifetimes--of the key vibrational
modes that dominate inelastic losses in graphitic materials. In graphite, the
phonon linewidth of the Raman-active E2g mode is found to decrease with
temperature; such anomalous behavior is driven entirely by electron-phonon
interactions, and does not appear in the nearly-degenerate infrared-active E1u
mode. In graphene, the phonon anharmonic lifetimes and decay channels of the
A'1 mode at K dominate over E2g at G and couple strongly with acoustic phonons,
highlighting how ballistic transport in carbon-based interconnects requires
careful engineering of phonon decays and thermalization.Comment: 5 pages, 4 figures; typos corrected and reference adde
Bulk Aluminum at High Pressure: A First-Principles Study
The behavior of metals at high pressure is of great importance to the fields
of shock physics, geophysics, astrophysics, and nuclear materials. In order to
further understand the properties of metals at high pressures we studied the
equation of state of aluminum using first-principles techniques up to 2500 GPa,
pressures within reach of the planned L.L.N.L. National Ignition Facility. Our
simulations use density-functional theory and density-functional perturbation
theory in the generalized gradient approximation at 0K. We found core overlaps
to become relevant beyond pressures of 1200 GPa. The equations of state for
three phases (fcc, bcc, and hcp) were calculated predicting the fcc-hcp,
fcc-bcc, and hcp-bcc transitions to occur at 215 GPa, 307 GPa, and 435 GPa
respectively. From the phonon dispersions at increasing pressure, we predict a
softening of the lowest transverse acoustic vibrational mode along the [110]
direction, which corresponds to a Born instability of the fcc phase at 725 GPa.Comment: 4 pages, 5 figures, accepted to Phys. Rev. B as a Brief Report. This
version has update many figures. Moreover we provided updated and more
accurate numbers based on further in-depth analyses of potential
computational error
Electron-Phonon Interactions and the Intrinsic Electrical Resistivity of Graphene
We present a first-principles study of the temperature- and density-dependent
intrinsic electrical resistivity of graphene. We use density-functional theory
and density-functional perturbation theory together with very accurate Wannier
interpolations to compute all electronic and vibrational properties and
electron-phonon coupling matrix elements; the phonon-limited resistivity is
then calculated within a Boltzmann-transport approach. An effective
tight-binding model, validated against first-principles results, is also used
to study the role of electron-electron interactions at the level of many-body
perturbation theory. The results found are in excellent agreement with recent
experimental data on graphene samples at high carrier densities and elucidate
the role of the different phonon modes in limiting electron mobility. Moreover,
we find that the resistivity arising from scattering with transverse acoustic
phonons is 2.5 times higher than that from longitudinal acoustic phonons. Last,
high-energy, optical, and zone-boundary phonons contribute as much as acoustic
phonons to the intrinsic electrical resistivity even at room temperature and
become dominant at higher temperatures.Comment: 7 pages 5 figure
Calculating DMFT forces in ab-initio ultrasoft pseudopotential formalism
In this paper, we show how to calculate analytical atomic forces within
self-consistent density functional theory + dynamical mean-field theory
(DFT+DMFT) approach in the case when ultra-soft or norm-conserving
pseudopotentials are used. We show how to treat the non-local projection terms
arising within the pseudopotential formalism and circumvent the problem of
non-orthogonality of the Kohn-Sham eigenvectors. Our approach is, in principle,
independent of the DMFT solver employed, and here was tested with the Hubbard I
solver. We benchmark our formalism by comparing against the forces calculated
in CeO and PrO by numerical differentiation of the total free
energy, as well as by comparing the energy profiles against the numerically
integrated analytical forces.Comment: 12 pages, 3 figure
Exploring the Effect of the Number of Hydrogen Atoms on the Properties of Lanthanide Hydrides by DMFT
Lanthanide hydrogen-rich materials have long been considered as one of the
candidates with high-temperature superconducting properties in condensed matter
physics, and have attracted great interest. Attempts to investigate the effects
of different compositions of lanthanide hydrogen-rich materials are ongoing,
with predictions and experimental studies in recent years having shown that
substances such as LaH 10 , CeH 9 , and LaH 16 exhibit extremely high
superconducting temperatures between 150-250 GPa. In particular, researchers
have noted that in those materials an increase in the f character at the Fermi
level leads to an increase in the superconducting temperature. Here, we further
elaborate on the effect of the ratios of lanthanide to hydrogen in these
substances with the aim to bring more clarity to the study of superhydrides in
these extreme cases by comparing a variety of lanthanide hydrogen-rich
materials with different ratios using the DMFT method, and provide ideas for
later structural predictions and material property studies.Comment: 7pages, 5figure
Mouse and rat ultrasonic vocalizations in neuroscience and neuropharmacology: State of the art and future applications
Mice and rats emit ultrasonic vocalizations (USVs), which may express their arousal and emotional states, to communicate with each other. There is continued scientific effort to better understand the functions of USVs as a central element of the rodent behavioral repertoire. However, studying USVs is not only important because of their ethological relevance, but also because they are widely applied as a behavioral readout in various fields of biomedical research. In mice and rats, a large number of experimental models of brain disorders exist and studying the emission of USVs in these models can provide valuable information about the health status of the animals and the effectiveness of possible interventions, both environmental and pharmacological. This review (i) provides an updated overview of the contexts in which ultrasonic calling behaviour of mice and rats has particularly high translational value, and (ii) gives some examples of novel approaches and tools used for the analysis of USVs in mice and rats, combining qualitative and quantitative methods. The relevance of age and sex differences as well as the importance of longitudinal evaluations of calling and non-calling behaviour is also discussed. Finally, the importance of assessing the communicative impact of USVs in the receiver, that is, through playback studies, is highlighted
Use of a Plasma-Sprayed Titanium-Hydroxyapatite Femoral Stem in Hip Arthroplasty in Patients Older than 70 Years. Is Cementless Fixation a Reliable Option in the Elderly?
Background: Although cementless implants are increasing in popularity, the use of cementless femoral stems for total hip arthroplasty (THA) and hip hemiarthroplasty (HH) in elderly patients remains controversial. The aim of this study was to report the outcomes of a cementless stem used in a large multicentric cohort of elderly patients receiving elective THA and HH for displaced femoral neck fracture. Methods: A total of 293 patients (301 hips) aged 70 years or older (mean age, 78 years; range, 70–93) who received the same cementless plasma-sprayed porous titanium–hydroxyapatite stem were retrospectively evaluated after primary THA and HH to investigate stem survival, complications, and clinical and radiographic results. Results: Cumulative stem survival was 98.5% (95% CI, 96.4–99.4%; 91 hips at risks) with revision due to any reason as the end-point at 10-year follow-up (mean 8.6 years, range 4–12). No stem was revised due to aseptic loosening. The mean Forgotten Joint Score was 98.7. Radiographically, the implants showed complete osseointegration, with slight stress-shieling signs in less than 10% of the hips. Conclusion: The use of cementless stems was proven to be a reliable and versatile option even in elderly patients for elective THA and HH for femoral neck fracture
Customising excitation properties of polycyclic aromatic hydrocarbons by rational positional heteroatom doping: the peri-xanthenoxanthene (PXX) case
In this paper we tackle the challenge of gaining control of the photophysical properties of PAHs through a site-specific N-doping within the structural aromatic framework. By developing a simple predictive tool that identifies C(sp2)-positions that if substituted with a heteroatom would tailor the changes in the absorption and emission spectral envelopes, we predict optimal substitutional patterns for the model peri-xanthenoxanthene (PXX) PAH. Specifically, TDDFT calculations of the electron density difference between the S1 excited state and S0 ground state of PXX allowed us to identify the subtleties in the role of sites i.e., electron donating or withdrawing character on excitation. The replacement of two C(sp2)-atoms with two N-atoms, in either electron donating or withdrawing positions, shifts the electronic transitions either to low or high energy, respectively. This consequently shifts the PXX absorption spectral envelop bathochromically or hypsochromically, as demonstrated by steady-state absorption spectroscopic measurements. Within the series of synthesised N-doped PXX, we tune the optical band gap within an interval of ∼0.4 eV, in full agreement with the theoretical predictions. Relatedly, measurements show the more blueshifted the absorption/emission energies, the greater the fluorescence quantum yield value (from ∼45% to ∼75%). On the other hand, electrochemical investigations suggested that the N-pattern has a limited influence on the redox properties. Lastly, depending on the N-pattern, different supramolecular organisations could be obtained at the solid-state, with the 1,7-pattern PXX molecule forming multi-layered, graphene-like, supramolecular sheets through a combination of weak H-bonding and π–π stacking interactions. Supramolecular striped patterned sheets could also be formed with the 3,9- and 4,10-congeners when co-crystallized with a halogen-bond donor molecule
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