301 research outputs found
Nanoscale imaging of the electronic and structural transitions in vanadium dioxide
We investigate the electronic and structural changes at the nanoscale in
vanadium dioxide (VO2) in the vicinity of its thermally driven phase
transition. Both electronic and structural changes exhibit phase coexistence
leading to percolation. In addition, we observe a dichotomy between the local
electronic and structural transitions. Nanoscale x-ray diffraction reveals
local, non-monotonic switching of the lattice structure, a phenomenon that is
not seen in the electronic insulator-to-metal transition mapped by near-field
infrared microscopy.Comment: 23 pages including 7 figure
Nanoscale layering of antiferromagnetic and superconducting phases in Rb2Fe4Se5
We studied phase separation in a single-crystalline antiferromagnetic
superconductor Rb2Fe4Se5 (RFS) using a combination of scattering-type scanning
near-field optical microscopy (s-SNOM) and low-energy muon spin rotation
(LE-\mu SR). We demonstrate that the antiferromagnetic and superconducting
phases segregate into nanometer-thick layers perpendicular to the iron-selenide
planes, while the characteristic in-plane size of the metallic domains reaches
10 \mu m. By means of LE-\mu SR we further show that in a 40-nm thick surface
layer the ordered antiferromagnetic moment is drastically reduced, while the
volume fraction of the paramagnetic phase is significantly enhanced over its
bulk value. Self-organization into a quasiregular heterostructure indicates an
intimate connection between the modulated superconducting and antiferromagnetic
phases.Comment: 5 pages, 2 figures. Updated version published in Phys. Rev. Lett. on
5 July 201
Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material
Uniaxial materials whose axial and tangential permittivities have opposite
signs are referred to as indefinite or hyperbolic media. In such materials
light propagation is unusual, leading to novel and often non-intuitive optical
phenomena. Here we report infrared nano-imaging experiments demonstrating that
crystals of hexagonal boron nitride (hBN), a natural mid-infrared hyperbolic
material, can act as a "hyper-focusing lens" and as a multi-mode waveguide. The
lensing is manifested by subdiffractional focusing of phonon-polaritons
launched by metallic disks underneath the hBN crystal. The waveguiding is
revealed through the modal analysis of the periodic patterns observed around
such launchers and near the sample edges. Our work opens new opportunities for
anisotropic layered insulators in infrared nanophotonics complementing and
potentially surpassing concurrent artificial hyperbolic materials with lower
losses and higher optical localization.Comment: 25 pages, 5 figure
Label-free infrared spectroscopy and imaging of single phospholipid bilayers with nanoscale resolution
Mid-infrared
absorption spectroscopy has been used extensively
to study the molecular properties of cell membranes and model systems.
Most of these studies have been carried out on macroscopic samples
or on samples a few micrometers in size, due to constraints on sensitivity
and spatial resolution with conventional instruments that rely on
far-field optics. Properties of membranes on the scale of nanometers,
such as in-plane heterogeneity, have to date eluded investigation
by this technique. In the present work, we demonstrate the capability
to study single bilayers of phospholipids with near-field mid-infrared
spectroscopy and imaging and achieve a spatial resolution of at least
40 nm, corresponding to a sample size of the order of a thousand molecules.
The quality of the data and the observed spectral features are consistent
with those reported from measurements of macroscopic samples and allow
detailed analysis of molecular properties, including orientation and
ordering of phospholipids. The work opens the way to the nanoscale
characterization of the biological membranes for which phospholipid
bilayers serve as a model
Transmission of light through periodic arrays of square holes: From a metallic wire mesh to an array of tiny holes
J. Bravo-Abad, L. Martín-Moreno, F. J. García-Vidal, Euan Hendry, and J. Gómez Rivas, Physical Review B, Vol. 76, article 241102(R) (2007). "Copyright © 2007 by the American Physical Society."A complete landscape is presented of the electromagnetic coupling between square holes forming a two-dimensional periodic array in a metallic film. By combining both experimental and theoretical results along with a first-principles Fano model, we study the crossover between the physics of metallic wire meshes (when holes occupy most of the unit cell) and the phenomenon of extraordinary optical transmission, which appears when the size of the holes is very small in comparison with the period of the array
Electrostatic modification of infrared response in gated structures based on VO2
We investigate the changes in the infrared response due to charge carriers
introduced by electrostatic doping of the correlated insulator vanadium dioxide
(VO2) integrated in the architecture of the field effect transistor.
Accumulation of holes at the VO2 interface with the gate dielectric leads to an
increase in infrared absorption. This phenomenon is observed only in the
insulator-to-metal transition regime of VO2 with coexisting metallic and
insulating regions. We postulate that doped holes lead to the growth of the
metallic islands thereby promoting percolation, an effect that persists upon
removal of the applied gate voltage.Comment: 14 pages, including 4 figure
Simulating quantum statistics with entangled photons: a continuous transition from bosons to fermions
In contrast to classical physics, quantum mechanics divides particles into
two classes-bosons and fermions-whose exchange statistics dictate the dynamics
of systems at a fundamental level. In two dimensions quasi-particles known as
'anyons' exhibit fractional exchange statistics intermediate between these two
classes. The ability to simulate and observe behaviour associated to
fundamentally different quantum particles is important for simulating complex
quantum systems. Here we use the symmetry and quantum correlations of entangled
photons subjected to multiple copies of a quantum process to directly simulate
quantum interference of fermions, bosons and a continuum of fractional
behaviour exhibited by anyons. We observe an average similarity of 93.6\pm0.2%
between an ideal model and experimental observation. The approach generalises
to an arbitrary number of particles and is independent of the statistics of the
particles used, indicating application with other quantum systems and large
scale application.Comment: 10 pages, 5 figure
Near-field spectroscopy of silicon dioxide thin films
We analyze the results of scanning near-field infrared spectroscopy performed
on thin films of a-SiO2 on Si substrate. The measured near-field signal
exhibits surface-phonon resonances whose strength has a strong thickness
dependence in the range from 2 to 300 {nm}. These observations are compared
with calculations in which the tip of the near-field infrared spectrometer is
modeled either as a point dipole or an elongated spheroid. The latter model
accounts for the antenna effect of the tip and gives a better agreement with
the experiment. Possible applications of the near-field technique for depth
profiling of layered nanostructures are discussed.Comment: 8 pages, 6 figure
Infrared nanoscopy of Dirac plasmons at the graphene-SiO2 interface
We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac
fermions in graphene. This is achieved by confining mid-IR radiation at the
apex of a nanoscale tip: an approach yielding two orders of magnitude increase
in the value of in-plane component of incident wavevector q compared to free
space propagation. At these high wavevectors, the Dirac plasmon is found to
dramatically enhance the near-field interaction with mid-IR surface phonons of
SiO2 substrate. Our data augmented by detailed modeling establish graphene as a
new medium supporting plasmonic effects that can be controlled by gate voltage.Comment: 12 pages, 4 figure
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