26,834 research outputs found
Surface Impedance and Bulk Band Geometric Phases in One-Dimensional Systems
Surface impedance is an important concept in classical wave systems such as
photonic crystals (PCs). For example, the condition of an interface state
formation in the interfacial region of two different one-dimensional PCs is
simply Z_SL +Z_SR=0, where Z_SL (Z_SR)is the surface impedance of the
semi-infinite PC on the left- (right-) hand side of the interface. Here, we
also show a rigorous relation between the surface impedance of a
one-dimensional PC and its bulk properties through the geometrical (Zak) phases
of the bulk bands, which can be used to determine the existence or
non-existence of interface states at the interface of the two PCs in a
particular band gap. Our results hold for any PCs with inversion symmetry,
independent of the frequency of the gap and the symmetry point where the gap
lies in the Brillouin Zone. Our results provide new insights on the
relationship between surface scattering properties, the bulk band properties
and the formation of interface states, which in turn can enable the design of
systems with interface states in a rational manner
Optical interface states protected by synthetic Weyl points
Weyl fermions have not been found in nature as elementary particles, but they
emerge as nodal points in the band structure of electronic and classical wave
crystals. Novel phenomena such as Fermi arcs and chiral anomaly have fueled the
interest in these topological points which are frequently perceived as
monopoles in momentum space. Here we report the experimental observation of
generalized optical Weyl points inside the parameter space of a photonic
crystal with a specially designed four-layer unit cell. The reflection at the
surface of a truncated photonic crystal exhibits phase vortexes due to the
synthetic Weyl points, which in turn guarantees the existence of interface
states between photonic crystals and any reflecting substrates. The reflection
phase vortexes have been confirmed for the first time in our experiments which
serve as an experimental signature of the generalized Weyl points. The
existence of these interface states is protected by the topological properties
of the Weyl points and the trajectories of these states in the parameter space
resembles those of Weyl semimetal "Fermi arcs surface states" in momentum
space. Tracing the origin of interface states to the topological character of
the parameter space paves the way for a rational design of strongly localized
states with enhanced local field.Comment: 36 pages, 9 figures. arXiv admin note: text overlap with
arXiv:1610.0434
Applications of Bound States in the Continuum in Photonics
The intriguing properties of bound states in the continuum (BICs) have
attracted a lot of attention in photonics. Besides being effective in confining
light in a counter-intuitive way, the correspondence between the near-field
mode pattern and the far-field radiation of BICs manifests the fascinating
topological characteristics of light. Early works on photonic BICs were mainly
focused on designing artificial structures to facilitate their realization,
while recent advances have shifted to exploring their exceptional properties in
applications. In this review, we survey important breakthroughs and recent
advances in this field. We detail the unique properties of BICs, including
light confinement enhancement, sharp Fano resonances, and topological
characteristics. We provide insights into the unique phenomena derived from
BICs and the impact of BICs on various applications. We also discuss the
paradigm shift enabled or facilitated by BICs in several emerging research
frontiers, such as parity-time symmetric systems, higher-order topology,
exciton-photon coupling, and moir\'e superlattices
Quantum phase transitions in the Kane-Mele-Hubbard model
We study the two-dimensional Kane-Mele-Hubbard model at half filling by means
of quantum Monte Carlo simulations. We present a refined phase boundary for the
quantum spin liquid. The topological insulator at finite Hubbard interaction
strength is adiabatically connected to the groundstate of the Kane-Mele model.
In the presence of spin-orbit coupling, magnetic order at large Hubbard U is
restricted to the transverse direction. The transition from the topological
band insulator to the antiferromagnetic Mott insulator is in the universality
class of the three-dimensional XY model. The numerical data suggest that the
spin liquid to topological insulator and spin liquid to Mott insulator
transitions are both continuous.Comment: 13 pages, 10 figures; final version; new Figs. 4(b) and 8(b
Pseudospin symmetry and its approximation in real nuclei
The origin of pseudospin symmetry and its broken in real nuclei are discussed
in the relativistic mean field theory. In the exact pseudospin symmetry, even
the usual intruder orbits have degenerate partners. In real nuclei, pseudospin
symmetry is approximate, and the partners of the usual intruder orbits will
disappear. The difference is mainly due to the pseudo spin-orbit potential and
the transition between them is discussed in details. The contribution of
pseudospin-orbit potential for intruder orbits is quite large, compared with
that for pseudospin doublets. The disappearance of the pseudospin partner for
the intruder orbit can be understood from the properties of its wave function.Comment: 10 pages, 3 figure
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