243 research outputs found
Floquet Spectrum and Transport Through an Irradiated Graphene Ribbon
Graphene subject to a spatially uniform, circularly-polarized electric field
supports a Floquet spectrum with properties akin to those of a topological
insulator, including non-vanishing Chern numbers associated with bulk bands and
current-carrying edge states. Transport properties of this system however are
complicated by the non-equilibrium occupations of the Floquet states. We
address this by considering transport in a two-terminal ribbon geometry for
which the leads have well-defined chemical potentials, with an irradiated
central scattering region. We demonstrate the presence of edge states, which
for infinite mass boundary conditions may be associated with only one of the
two valleys. At low frequencies, the bulk DC conductivity near zero energy is
shown to be dominated by a series of states with very narrow anticrossings,
leading to super-diffusive behavior. For very long ribbons, a ballistic regime
emerges in which edge state transport dominates.Comment: 4.2 pages, 3 figure
Visibility of the Amplitude (Higgs) Mode in Condensed Matter
The amplitude mode is a ubiquitous collective excitation in condensed matter
systems with broken continuous symmetry. It is expected in antiferromagnets,
short coherence length superconductors, charge density waves, and lattice Bose
condensates. Its detection is a valuable test of the corresponding field
theory, and its mass gap measures the proximity to a quantum critical point.
However, since the amplitude mode can decay into low-energy Goldstone modes,
its experimental visibility has been questioned. Here we show that the
visibility depends on the symmetry of the measured susceptibility. The
longitudinal susceptibility diverges at low frequency as \chi_{\sigma\sigma} ~
i/\omega (d=2) or log(1/|\omega|) (d=3), which can completely obscure the
amplitude peak. In contrast, the scalar susceptibility is suppressed by four
extra powers of frequency, exposing the amplitude peak throughout the ordered
phase. We discuss experimental setups for measuring the scalar susceptibility.
The conductivity of the O(2) theory (relativistic superfluid) is a scalar
response and therefore exhibits suppressed absorption below the Higgs mass
threshold, \sigma ~ \omega^{2d+1}. In layered, short coherence length
superconductors, (relevant e.g. to cuprates) this threshold is raised by the
interlayer plasma frequency.Comment: 17 pages, 9 figure
Domain Patterns in the Microwave-Induced Zero-Resistance State
It has been proposed that the microwave-induced ``zero-resistance''
phenomenon, observed in a GaAs two-dimensional electron system at low
temperatures in moderate magnetic fields, results from a state with multiple
domains, in which a large local electric field \bE(\br) is oriented in
different directions. We explore here the questions of what may determine the
domain arrangement in a given sample, what do the domains look like in
representative cases, and what may be the consequences of domain-wall
localization on the macroscopic dc conductance. We consider both effects of
sample boundaries and effects of disorder, in a simple model, which has a
constant Hall conductivity, and is characterized by a Lyapunov functional.Comment: 19 pages, 5 figures; submitted to a special issue of Journal of
Statistical Physics, in honor of P. C. Hohenberg and J. S. Lange
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