598 research outputs found
First-principles investigation of organic photovoltaic materials C, C, [C]PCBM, and bis-[C]PCBM using a many-body -Lanczos approach
We present a first-principles investigation of the excited-state properties
of electron acceptors in organic photovoltaics including C, C,
[6,6]-phenyl-C-butyric-acid-methyl-ester ([C]PCBM), and
bis-[C]PCBM using many-body perturbation theory within the Hedin's
approximation and an efficient Lanczos approach. Calculated vertical
ionization potentials (VIP) and vertical electron affinities (VEA) of C
and C agree very well with experimental values measured in gas phase.
The density of states of all three molecules is also compared to photoemission
and inverse photoemission spectra measured on thin-films, exhibiting a close
agreement - a rigid energy-gap renormalization owing to intermolecular
interactions in the thin-films. In addition, it is shown that the low-lying
unoccupied states of [C]PCBM are all derived from the highest-occupied
molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene
C. The functional side group in [C]PCBM introduces a slight
electron transfer to the fullerene cage, resulting in small decreases of both
VIP and VEA. This small change of VEA provides a solid justification for the
increase of open-circuit voltage when replacing fullerene C with
[C]PCBM as the electron acceptor in bulk heterojunction polymer solar
cells.Comment: 9 pages, 4 figures, and 7 table
Topological Crystalline Insulator Nanomembrane with Strain-Tunable Band Gap
The ability to fine-tune band gap and band inversion in topological materials
is highly desirable for the development of novel functional devices. Here we
propose that the electronic properties of a free-standing nanomembrane of
topological crystalline insulator (TCI) SnTe and PbSn(Se,Te) are
highly tunable by engineering elastic strain and controlling membrane
thickness, resulting in tunable band gap and giant piezoconductivity. Membrane
thickness governs the hybridization of topological electronic states on
opposite surfaces, while elastic strain can further modulate the hybridization
strength by controlling the penetration length of surface states. We propose a
frequency-resolved infrared photodetector using force-concentration induced
inhomogeneous elastic strain in TCI nanomembrane with spatially varying width.
The predicted tunable band gap accompanied by strong spin-textured electronic
states will open up new avenues for fabricating piezoresistive devices,
thermoelectrics, infrared detectors and energy-efficient electronic and
optoelectronic devices based on TCI nanomembrane.Comment: 10 pages, 9 figure
Bridging Coherence Optics and Classical Mechanics -- A Universal Light Polarization-Entanglement Complementary Relation
While optics and mechanics are two distinct branches of physics, they are
connected. It is well known that geometrical/ray treatment of light has direct
analogies to mechanical descriptions of particle motion. However, connections
between coherence wave optics and classical mechanics are rarely reported. Here
we explore links of the two for an arbitrary light field by performing a
quantitative analysis of two optical coherence properties: polarization and
entanglement (implied by a wave picture of light due to Huygens and Fresnel). A
universal complementary identity relation is obtained. More surprisingly,
optical polarization, entanglement, and their identity relation are shown to be
quantitatively associated with mechanical concepts of center of mass and moment
of inertia through the Huygens-Steiner theorem for rigid body rotation. The
obtained result bridges coherence wave optics and classical mechanics through
the two theories of Huygens.Comment: 6 pages, 2 figure
Single-cluster dynamics for the random-cluster model
We formulate a single-cluster Monte Carlo algorithm for the simulation of the
random-cluster model. This algorithm is a generalization of the Wolff
single-cluster method for the -state Potts model to non-integer values
. Its results for static quantities are in a satisfactory agreement with
those of the existing Swendsen-Wang-Chayes-Machta (SWCM) algorithm, which
involves a full cluster decomposition of random-cluster configurations. We
explore the critical dynamics of this algorithm for several two-dimensional
Potts and random-cluster models. For integer , the single-cluster algorithm
can be reduced to the Wolff algorithm, for which case we find that the
autocorrelation functions decay almost purely exponentially, with dynamic
exponents , and for , and
4 respectively. For non-integer , the dynamical behavior of the
single-cluster algorithm appears to be very dissimilar to that of the SWCM
algorithm. For large critical systems, the autocorrelation function displays a
range of power-law behavior as a function of time. The dynamic exponents are
relatively large. We provide an explanation for this peculiar dynamic behavior.Comment: 7 figures, 4 table
Multivariable Scaling for the Anomalous Hall Effect
We derive a general scaling relation for the anomalous Hall effect in
ferromagnetic metals involving multiple competing scattering mechanisms,
described by a quadratic hypersurface in the space spanned by the partial
resistivities. We also present experimental findings, which show strong
deviation from previously found scaling forms when different scattering
mechanism compete in strength but can be nicely explained by our theory
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