2,079 research outputs found
Comparison of Power Dependence of Microwave Surface Resistance of Unpatterned and Patterned YBCO Thin Film
The effect of the patterning process on the nonlinearity of the microwave
surface resistance of YBCO thin films is investigated. With the use of a
sapphire dielectric resonator and a stripline resonator, the microwave of
YBCO thin films was measured before and after the patterning process, as a
function of temperature and the rf peak magnetic field in the film. The
microwave loss was also modeled, assuming a dependence of
on current density . Experimental and modeled results
show that the patterning has no observable effect on the microwave residual
or on the power dependence of .Comment: Submitted to IEEE Trans. MT
Phase Diagrams of Bi1-xSbx Thin Films with Different Growth Orientations
A closed-form model is developed to evaluate the band-edge shift caused by
quantum confinement for a two-dimensional non-parabolic carrier-pocket. Based
on this model, the symmetries and the band-shifts of different carrier-pockets
are evaluated for BiSb thin films that are grown along different crystalline
axes. The phase diagrams for the BiSb thin film systems with different growth
orientations are calculated and analyzed
Wood anomalies in resonant photonic quasicrystals
A theory of light diffraction from planar quasicrystalline lattice with
resonant scatterers is presented. Rich structure, absent in the periodic case,
is found in specular reflection spectra, and interpreted as a specific kind of
Wood anomalies, characteristic for quasicrystals. The theory is applied to
semiconductor quantum dots arranged in Penrose tiling.Comment: 6 pages, 3 figure
Disorder-induced double resonant Raman process in graphene
An analytical study is presented of the double resonant Raman scattering
process in graphene, responsible for the D and D features in the
Raman spectra. This work yields analytical expressions for the D and
D integrated Raman intensities that explicitly show the dependencies
on laser energy, defect concentration, and electronic lifetime. Good agreement
is obtained between the analytical results and experimental measurements on
samples with increasing defect concentrations and at various laser excitation
energies. The use of Raman spectroscopy to identify the nature of defects is
discussed. Comparison between the models for the edge-induced and the
disorder-induced D band intensity suggests that edges or grain boundaries can
be distinguished from disorder by the different dependence of their Raman
intensity on laser excitation energy. Similarly, the type of disorder can
potentially be identified not only by the intensity ratio
, but also by its laser energy
dependence. Also discussed is a quantitative analysis of quantum interference
effects of the graphene wavefunctions, which determine the most important
phonon wavevectors and scattering processes responsible for the D and
D bands.Comment: 10 pages, 4 figure
Probing the intrinsic state of a one-dimensional quantum well with a photon-assisted tunneling
The photon-assisted tunneling (PAT) through a single wall carbon nanotube
quantum well (QW) under influence an external electromagnetic field for probing
of the Tomonaga Luttinger liquid (TLL) state is suggested. The elementary TLL
excitations inside the quantum well are density () and spin
() bosons. The bosons populate the quantized energy levels
and where is the interlevel spacing, is an
integer number, is the tube length, is the TLL parameter. Since the
electromagnetic field acts on the bosons only while the neutral
and bosons remain unaffected, the PAT spectroscopy
is able of identifying the levels in the QW setup. The spin
boson levels in the same QW are recognized from Zeeman
splitting when applying a d.c. magnetic field field. Basic TLL
parameters are readily extracted from the differential conductivity curves.Comment: 10 pages, 5 figure
Quantum Size Effects and Transport Phenomena in PbSe Quantum Wells and PbSe/EuS Superlattices
It is established that the room-temperature dependences of transport properties on the total thickness of PbSe layers d in PbSe/EuS superlattices exhibit an oscillatory behavior. It is shown that the oscillation period Δd practically coincides with the period of the thickness oscillations observed earlier in single PbSe/EuS quantum well. The non-monotonic character of these dependences is attributed to quantum size effects. The theoretically estimated and experimentally determined Δd values are in good agreement
Nature and strength of bonding in a crystal of semiconducting nanotubes: van der Waals density functional calculations and analytical results
The dispersive interaction between nanotubes is investigated through ab
initio theory calculations and in an analytical approximation. A van der Waals
density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)] is used to
determine and compare the binding of a pair of nanotubes as well as in a
nanotube crystal. To analyze the interaction and determine the importance of
morphology, we furthermore compare results of our ab initio calculations with a
simple analytical result that we obtain for a pair of well-separated nanotubes.
In contrast to traditional density functional theory calculations, the vdW-DF
study predicts an intertube vdW bonding with a strength that is consistent with
recent observations for the interlayer binding in graphitics. It also produce a
nanotube wall-to-wall separation which is in very good agreement with
experiments. Moreover, we find that the vdW-DF result for the nanotube-crystal
binding energy can be approximated by a sum of nanotube-pair interactions when
these are calculated in vdW-DF. This observation suggests a framework for an
efficient implementation of quantum-physical modeling of the CNT bundling in
more general nanotube bundles, including nanotube yarn and rope structures.Comment: 10 pages, 4 figure
Electron properties of carbon nanotubes in a periodic potential
A periodic potential applied to a nanotube is shown to lock electrons into
incompressible states that can form a devil's staircase. Electron interactions
result in spectral gaps when the electron density (relative to a half-filled
Carbon pi-band) is a rational number per potential period, in contrast to the
single-particle case where only the integer-density gaps are allowed. When
electrons are weakly bound to the potential, incompressible states arise due to
Bragg diffraction in the Luttinger liquid. Charge gaps are enhanced due to
quantum fluctuations, whereas neutral excitations are governed by an effective
SU(4)~O(6) Gross-Neveu Lagrangian. In the opposite limit of the tightly bound
electrons, effects of exchange are unimportant, and the system behaves as a
single fermion mode that represents a Wigner crystal pinned by the external
potential, with the gaps dominated by the Coulomb repulsion. The phase diagram
is drawn using the effective spinless Dirac Hamiltonian derived in this limit.
Incompressible states can be detected in the adiabatic transport setup realized
by a slowly moving potential wave, with electron interactions providing the
possibility of pumping of a fraction of an electron per cycle (equivalently, in
pumping at a fraction of the base frequency).Comment: 21 pgs, 8 fig
Selection Rules for One- and Two-Photon Absorption by Excitons in Carbon Nanotubes
Recent optical absorption/emission experiments showed that the lower energy
optical transitions in carbon nanotubes are excitonic in nature, as predicted
by theory. These experiments were based on the symmetry aspects of free
electron-hole states and bound excitonic states. The present work shows,
however, that group theory does not predict the selection rules needed to
explain the two photon experiments. We obtain the symmetries and selection
rules for the optical transitions of excitons in single-wall carbon nanotubes
within the approach of the group of the wavevector, thus providing important
information for the interpretation of theoretical and experimental optical
spectra of these materials.Comment: 4 pages, 1 figure, 1 tabl
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