2,556 research outputs found
Zero Modes and Conformal Anomaly in Liouville Vortices
The partition function of a two dimensional Abelian gauge model reproducing
magnetic vortices is discussed in the harmonic approximation. Classical
solutions exhibit conformal invariance, that is broken by statistical
fluctuations, apart from an exceptional case. The corresponding ``anomaly'' has
been evaluated. Zero modes of the thermal fluctuation operator have been
carefully discussed.Comment: RevTex, 14 pages, no figures. To appear on Nucl. Phys.
Spontaneous polarization and piezoelectricity in boron nitride nanotubes
Ab initio calculations of the spontaneous polarization and piezoelectric
properties of boron nitride nanotubes show that they are excellent
piezoelectric systems with response values larger than those of piezoelectric
polymers. The intrinsic chiral symmetry of the nanotubes induces an exact
cancellation of the total spontaneous polarization in ideal, isolated nanotubes
of arbitrary indices. Breaking of this symmetry by inter-tube interaction or
elastic deformations induces spontaneous polarization comparable to those of
wurtzite semiconductors.Comment: 5 pages in PRB double column format, 3 figure
Surface Polar Phonon Dominated Electron Transport in Graphene
The effects of surface polar phonons on electronic transport properties of
monolayer graphene are studied by using a Monte Carlo simulation. Specifically,
the low-field electron mobility and saturation velocity are examined for
different substrates (SiC, SiO2, and HfO2) in comparison to the intrinsic case.
While the results show that the low-field mobility can be substantially reduced
by the introduction of surface polar phonon scattering, corresponding
degradation of the saturation velocity is not observed for all three substrates
at room temperature. It is also found that surface polar phonons can influence
graphene electrical resistivity even at low temperature, leading potentially to
inaccurate estimation of the acoustic phonon deformation potential constant
Intrinsic Electrical Transport Properties of Monolayer Silicene and MoS2 from First Principles
The electron-phonon interaction and related transport properties are
investigated in monolayer silicene and MoS2 by using a density functional
theory calculation combined with a full-band Monte Carlo analysis. In the case
of silicene, the results illustrate that the out-of-plane acoustic phonon mode
may play the dominant role unlike its close relative - graphene. The small
energy of this phonon mode, originating from the weak sp2 bonding between Si
atoms, contributes to the high scattering rate and significant degradation in
electron transport. In MoS2, the longitudinal acoustic phonons show the
strongest interaction with electrons. The key factor in this material appears
to be the Q valleys located between the {\Gamma} and K points in the first
Brillouin zone as they introduce additional intervalley scattering. The
analysis also reveals the potential impact of extrinsic screening by other
carriers and/or adjacent materials. Subsequent decrease in the actual
scattering rate can be drastic, warranting careful consideration. Finally, the
effective deformation potential constants are extracted for all relevant
intrinsic electron-phonon scattering processes in both materials
AFLOW-QHA3P: Robust and automated method to compute thermodynamic properties of solids
Accelerating the calculations of finite-temperature thermodynamic properties is a major challenge for rational materials design. Reliable methods can be quite expensive, limiting their applicability in autonomous high-throughput workflows. Here, the three-phonon quasiharmonic approximation (QHA) method is introduced, requiring only three phonon calculations to obtain a thorough characterization of the material. Leveraging a Taylor expansion of the phonon frequencies around the equilibrium volume, the method efficiently resolves the volumetric thermal expansion coefficient, specific heat at constant pressure, the enthalpy, and bulk modulus. Results from the standard QHA and experiments corroborate the procedure, and additional comparisons are made with the recently developed self-consistent QHA. The three approaches—three-phonon, standard, and self-consistent QHAs—are all included within the open-source ab initio framework aflow, allowing the automated determination of properties with various implementations within the same framework
First principle theory of correlated transport through nano-junctions
We report the inclusion of electron-electron correlation in the calculation
of transport properties within an ab initio scheme. A key step is the
reformulation of Landauer's approach in terms of an effective transmittance for
the interacting electron system. We apply this framework to analyze the effect
of short range interactions on Pt atomic wires and discuss the coherent and
incoherent correction to the mean-field approach.Comment: 5 pages, 3 figure
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