26,608 research outputs found
Probing the Shape of a Graphene Nanobubble
Gas molecules trapped between graphene and various substrates in the form of
bubbles are observed experimentally. The study of these bubbles is useful in
determining the elastic and mechanical properties of graphene, adhesion energy
between graphene and substrate, and manipulating the electronic properties via
strain engineering. In our numerical simulations, we use a simple description
of elastic potential and adhesion energy to show that for small gas bubbles
( nm) the van der Waals pressure is in the order of 1 GPa. These
bubbles show universal shape behavior irrespective of their size, as observed
in recent experiments. With our results the shape and volume of the trapped gas
can be determined via the vibrational density of states (VDOS) using
experimental techniques such as inelastic tunneling and inelastic neutron
scattering. The elastic energy distribution in the graphene layer which traps
the nanobubble is homogeneous apart from its edge, but the strain depends on
the bubble size thus variation in bubble size allows control of the electronic
and optical properties.Comment: 5 Figures (Supplementary: 1 Figure), Accepted for publication in PCC
Activation gaps for the fractional quantum Hall effect: realistic treatment of transverse thickness
The activation gaps for fractional quantum Hall states at filling fractions
are computed for heterojunction, square quantum well, as well as
parabolic quantum well geometries, using an interaction potential calculated
from a self-consistent electronic structure calculation in the local density
approximation. The finite thickness is estimated to make 30% correction
to the gap in the heterojunction geometry for typical parameters, which
accounts for roughly half of the discrepancy between the experiment and
theoretical gaps computed for a pure two dimensional system. Certain model
interactions are also considered. It is found that the activation energies
behave qualitatively differently depending on whether the interaction is of
longer or shorter range than the Coulomb interaction; there are indications
that fractional Hall states close to the Fermi sea are destabilized for the
latter.Comment: 32 pages, 13 figure
Structural characterization of carbon nanotubes via the vibrational density of states
The electrical and chemical properties of carbon nanotubes vary significantly
with different chirality and diameter, making the experimental determination of
these structural properties important. Here, we show that the vibrational
density of states (VDOS) contains information on the structure of carbon
nanotubes, particularly at low frequencies. We show that the diameter and
chirality of the nanotubes can be determined from the characteristic low
frequency and modes in the VDOS. For zigzag nanotubes, the peak
splits into two peaks giving rise to another low energy peak. The
significant changes in the frequencies and relative intensities of these peaks
open up a route to distinguish among structurally different nanotubes. A close
study of different orientations of Stone-Wales defects with varying defect
density reveals that different structural defects also leave distinct
fingerprints in the VDOS, particularly in the and modes. With our
results, more structural information can be obtained from experiments which can
directly measure the VDOS, such as inelastic electron and inelastic neutron
spectroscopy.Comment: 5 Figures, Accepted for publication in Carbo
Density dependence of valley polarization energy for composite fermions
In two-dimensional electron systems confined to wide AlAs quantum wells,
composite fermions around the filling factor = 3/2 are fully spin
polarized but possess a valley degree of freedom. Here we measure the energy
needed to completely valley polarize these composite fermions as a function of
electron density. Comparing our results to the existing theory, we find overall
good quantitative agreement, but there is an unexpected trend: The measured
composite fermion valley polarization energy, normalized to the Coulomb energy,
decreases with decreasing density
Composite Fermions in Quantum Dots
We demonstrate the formation of composite fermions in two-dimensional quantum
dots under high magnetic fields. The composite fermion interpretation provides
a simple way to understand several qualitative and quantitative features of the
numerical results obtained earlier in exact diagonalization studies. In
particular, the ground states are recognized as compactly filled quasi-Landau
levels of composite fermions.Comment: Revtex. Postscript files of figures are appended the tex
Fall prevention in the community: what older people say they need
Original article can be found at: http://www.bjcn.co.uk/ Copyright MA HealthcareUptake of and adherence to fall prevention interventions is often poor and we know little about how older people’s perceptions of and beliefs about fall prevention interventions affect uptake. This study aimed to explore older people’s perceptions of the facilitators and barriers to participation in fall prevention interventions. We undertook a qualitative study with older people who had taken part in, declined to participate or adhere to fall prevention interventions using semi-structured interviews (n=65), and 17 focus groups (n=122) with older people (including 32 South Asian and 30 Chinese older people) in primary and community care settings in the South of England. A number of factors acted as either barriers or facilitators to uptake of interventions. Older people also made recommendations for improving access to interventions. Community nurses are ideally placed to screen older people, identify those at risk of falling and refer them to appropriate interventions as well as providing health promotion and education.Peer reviewe
Microscopic theory of the quantum Hall hierarchy
We solve the quantum Hall problem exactly in a limit and show that the ground
states can be organized in a fractal pattern consistent with the
Haldane-Halperin hierarchy, and with the global phase diagram. We present wave
functions for a large family of states, including those of Laughlin and Jain
and also for states recently observed by Pan {\it et. al.}, and show that they
coincide with the exact ones in the solvable limit. We submit that they
establish an adiabatic continuation of our exact results to the experimentally
accessible regime, thus providing a unified approach to the hierarchy states.Comment: 4 pages, 2 figures. Publishe
Composite Fermions in Negative Effective Magnetic Field: A Monte-Carlo Study
The method of Jain and Kamilla [PRB {\bf 55}, R4895 (1997)] allows numerical
generation of composite fermion trial wavefunctions for large numbers of
electrons in high magnetic fields at filling fractions of the form nu=p/(2mp+1)
with m and p positive integers. In the current paper we generalize this method
to the case where the composite fermions are in an effective (mean) field with
opposite sign from the actual physical field, i.e. when p is negative. We
examine both the ground state energies and the low energy neutral excitation
spectra of these states. Using particle-hole symmetry we can confirm the
correctness of our method by comparing results for the series m=1 with p>0
(previously calculated by others) to our results for the conjugate series m=1
with p <0. Finally, we present similar results for ground state energies and
low energy neutral excitations for the states with m=2 and p <0 which were not
previously addressable, comparing our results to the m=1 case and the p > 0,
m=2 cases.Comment: 11 page
Fermi-sea-like correlations in a partially filled Landau level
The pair distribution function and the static structure factor are computed
for composite fermions. Clear and robust evidence for a structure is
seen in a range of filling factors in the vicinity of the half-filled Landau
level. Surprisingly, it is found that filled Landau levels of composite
fermions, i.e. incompressible FQHE states, bear a stronger resemblance to a
Fermi sea than do filled Landau levels of electrons.Comment: 23 pages, revte
Study of Low Energy Spin Rotons in the Fractional Quantum Hall Effect
Motivated by the discovery of extremely low energy collective modes in the
fractional quantum Hall effect (Kang, Pinczuk {\em et al.}), with energies
below the Zeeman energy, we study theoretically the spin reversed excitations
for fractional quantum Hall states at and 3/7 and find qualitatively
different behavior than for . We find that a low-energy,
charge-neutral "spin roton," associated with spin reversed excitations that
involve a change in the composite-fermion Landau level index, has energy in
reasonable agreement with experiment.Comment: Postscript figures included. Accepted in Phys. Rev. B (Rapid
Communication
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