67 research outputs found
Negative thermal expansion in single-component systems with isotropic interactions
We have devised an isotropic interaction potential that gives rise to
negative thermal expansion (NTE) behavior in equilibrium many-particle systems
in both two and three dimensions over a wide temperature and pressure range
(including zero pressure). An optimization procedure is used in order to find a
potential that yields a strong NTE effect. A key feature of the potential that
gives rise to this behavior is the softened interior of its basin of
attraction. Although such anomalous behavior is well known in material systems
with directional interactions (e.g., zirconium tungstate), to our knowledge
this is the first time that NTE behavior has been established to occur in
single-component many-particle systems for isotropic interactions. Using
constant-pressure Monte Carlo simulations, we show that as the temperature is
increased, the system exhibits negative, zero and then positive thermal
expansion before melting (for both two- and three-dimensional systems). The
behavior is explicitly compared to that of a Lennard-Jones system, which
exhibits typical expansion upon heating for all temperatures and pressures.Comment: 21 pages, 13 figure
Topological Photonic Quasicrystals: Fractal Topological Spectrum and Protected Transport
We show that it is possible to have a topological phase in two-dimensional
quasicrystals without any magnetic field applied, but instead introducing an
artificial gauge field via dynamic modulation. This topological quasicrystal
exhibits scatter-free unidirectional edge states that are extended along the
system's perimeter, contrary to the states of an ordinary quasicrystal system,
which are characterized by power-law decay. We find that the spectrum of this
Floquet topological quasicrystal exhibits a rich fractal (self-similar)
structure of topological "minigaps," manifesting an entirely new phenomenon:
fractal topological systems. These topological minigaps form only when the
system size is sufficiently large because their gapless edge states penetrate
deep into the bulk. Hence, the topological structure emerges as a function of
the system size, contrary to periodic systems where the topological phase can
be completely characterized by the unit cell. We demonstrate the existence of
this topological phase both by using a topological index (Bott index) and by
studying the unidirectional transport of the gapless edge states and its
robustness in the presence of defects. Our specific model is a Penrose lattice
of helical optical waveguides - a photonic Floquet quasicrystal; however, we
expect this new topological quasicrystal phase to be universal.Comment: 12 pages, 8 figure
Topological crystalline protection in a photonic system
Topological crystalline insulators are a class of materials with a bulk
energy gap and edge or surface modes, which are protected by crystalline
symmetry, at their boundaries. They have been realized in electronic systems:
in particular, in SnTe. In this work, we propose a mechanism to realize
photonic boundary states topologically protected by crystalline symmetry. We
map this one-dimensional system to a two-dimensional lattice model with
opposite magnetic fields, as well as opposite Chern numbers in its even and odd
mirror parity subspaces, thus corresponding to a topological mirror insulator.
Furthermore, we test how sensitive and robust edge modes depend on their mirror
parity by performing time dependent evolution simulation of edge modes in a
photonic setting with realistic experimental parameters.Comment: 10 pages, 7 figure
Period-doubled Floquet Solitons
We propose and experimentally demonstrate a family of Floquet solitons in the
bulk of a photonic topological insulator that have double the period of the
drive. Our experimental system consists of a periodically-modulated honeycomb
lattice of optical waveguides fabricated by femtosecond laser writing. We
employ a Kerr nonlinearity in which self-focusing gives rise to spatial lattice
solitons. Our photonic system constitutes a powerful platform where the
interplay of time-periodic driving, topology and nonlinearity can be probed in
a highly tunable way.Comment: 5 pages, 3 figures, Supplementary Informatio
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