191,234 research outputs found
Measuring excitation-energy transfer with a real-time time-dependent density functional theory approach
We investigate the time an electronic excitation travels in a supermolecular
setup using a measurement process in an open quantum-system framework. The
approach is based on the stochastic Schr\"odinger equation and uses a
Hamiltonian from time-dependent density functional theory (TDDFT). It treats
electronic-structure properties and intermolecular coupling on the level of
TDDFT, while it opens a route to the description of dissipation and relaxation
via a bath operator that couples to the dipole moment of the density. Within
our study, we find that in supermolecular setups small deviations of the
electronic structure from the perfectly resonant case have only minor influence
on the pathways of excitation-energy transfer, thus lead to similar transfer
times. Yet, sizable defects cause notable slowdown of the energy spread
Detection of spin bias in four-terminal quantum-dot ring
In this work, we show that in a four-quantum-dot ring, via introducing a
local Rashba spin-orbit interaction the spin bias in the transverse terminals
can be detected by observing the charge currents in the longitudinal probes. It
is found that due to the Rashba interaction, the quantum interference in this
system becomes spin-dependent and the opposite-spin currents induced by the
spin bias can present different magnitudes, so charge currents emerge. Besides,
the charge currents rely on both the magnitude and spin polarization direction
of the spin bias. It is believed that this method provides an electrical but
practical scheme to detect the spin bias (or the spin current).Comment: 6 pages, 5 figure
Quantum Critical Environment Assisted Quantum Magnetometer
A central qubit coupled to an Ising ring of qubits, operating close to a
critical point is investigated as a potential precision quantum magnetometer
for estimating an applied transverse magnetic field. We compute the Quantum
Fisher information for the central, probe qubit with the Ising chain
initialized in its ground state or in a thermal state. The non-unitary
evolution of the central qubit due to its interaction with the surrounding
Ising ring enhances the accuracy of the magnetic field measurement. Near the
critical point of the ring, Heisenberg-like scaling of the precision in
estimating the magnetic field is obtained when the ring is initialized in its
ground state. However, for finite temperatures, the Heisenberg scaling is
limited to lower ranges of values.Comment: 10 pages, 9 figure
Topological effects in ring polymers: A computer simulation study
Unconcatenated, unknotted polymer rings in the melt are subject to strong
interactions with neighboring chains due to the presence of topological
constraints. We study this by computer simulation using the bond-fluctuation
algorithm for chains with up to N=512 statistical segments at a volume fraction
\Phi=0.5 and show that rings in the melt are more compact than gaussian chains.
A careful finite size analysis of the average ring size R \propto N^{\nu}
yields an exponent \nu \approx 0.39 \pm 0.03 in agreement with a Flory-like
argument for the topologica interactions. We show (using the same algorithm)
that the dynamics of molten rings is similar to that of linear chains of the
same mass, confirming recent experimental findings. The diffusion constant
varies effectively as D_{N} \propto N^{-1.22(3) and is slightly higher than
that of corresponding linear chains. For the ring sizes considered (up to 256
statistical segments) we find only one characteristic time scale \tau_{ee}
\propto N^{2.0(2); this is shown by the collapse of several mean-square
displacements and correlation functions onto corresponding master curves.
Because of the shrunken state of the chain, this scaling is not compatible with
simple Rouse motion. It applies for all sizes of ring studied and no sign of a
crossover to any entangled regime is found.Comment: 20 Pages,11 eps figures, Late
Conserved Linking in Single- and Double-Stranded Polymers
We demonstrate a variant of the Bond Fluctuation lattice Monte Carlo model in
which moves through cis conformations are forbidden. Ring polymers in this
model have a conserved quantity that amounts to a topological linking number.
Increased linking number reduces the radius of gyration mildly. A linking
number of order 0.2 per bond leads to an eight-percent reduction of the radius
for 128-bond chains. This percentage appears to rise with increasing chain
length, contrary to expectation. For ring chains evolving without the
conservation of linking number, we demonstrate a substantial anti-correlation
between the twist and writhe variables whose sum yields the linking number. We
raise the possibility that our observed anti-correlations may have counterparts
in the most important practical polymer that conserves linking number, DNA.Comment: Revised title, minor changes, updated references. 36 pages, including
14 figures. More formats available at
http://rainbow.uchicago.edu/~plewa/webpaper
Symmetry-enhanced supertransfer of delocalized quantum states
Coherent hopping of excitation rely on quantum coherence over physically
extended states. In this work, we consider simple models to examine the effect
of symmetries of delocalized multi-excitation states on the dynamical
timescales, including hopping rates, radiative decay, and environmental
interactions. While the decoherence (pure dephasing) rate of an extended state
over N sites is comparable to that of a non-extended state, superradiance leads
to a factor of N enhancement in decay and absorption rates. In addition to
superradiance, we illustrate how the multi-excitonic states exhibit
`supertransfer' in the far-field regime: hopping from a symmetrized state over
N sites to a symmetrized state over M sites at a rate proportional to MN. We
argue that such symmetries could play an operational role in physical systems
based on the competition between symmetry-enhanced interactions and localized
inhomogeneities and environmental interactions that destroy symmetry. As an
example, we propose that supertransfer and coherent hopping play a role in
recent observations of anomolously long diffusion lengths in nano-engineered
assembly of light-harvesting complexes.Comment: 6 page
- …