8,096 research outputs found
Gaugeon Formalism for Spin-3/2 Rarita-Schwinger Gauge Field
We provide a gauge covariant formalism of the canonically quantized theory of
spin-3/2 Rarita-Schwinger gauge field. The theory admits a quantum gauge
transformation by which we can shift the gauge fixing parameter. The quantum
gauge transformation does not change the BRST charge. Thus, the physical
Hilbert space is trivially independent of the gauge fixing parameter.Comment: 13 pages, no figures, LaTe
Fourier analyses of commensurability oscillations in Fibonacci lateral superlattices
Magnetotransport measurements have been performed on Fibonacci lateral
superlattices (FLSLs) -- two-dimensional electron gases subjected to a weak
potential modulation arranged in the Fibonacci sequence, LSLLSLS..., with
L/S=tau (the golden ratio). Complicated commensurability oscillation (CO) is
observed, which can be accounted for as a superposition of a series of COs each
arising from a sinusoidal modulation representing the characteristic length
scale of one of the self-similar generations in the Fibonacci sequence.
Individual CO components can be separated out from the magnetoresistance trace
by performing a numerical Fourier band-pass filter. From the analysis of the
amplitude of a single-component CO thus extracted, the magnitude of the
corresponding Fourier component in the potential modulation can be evaluated.
By examining all the Fourier contents observed in the magnetoresistance trace,
the profile of the modulated potential seen by the electrons can be
reconstructed with some remaining ambiguity about the interrelation of the
phase between different components.Comment: 11 pages, 10 figures, added references in Introduction, minor
revision
Modulation Induced Phase Transition from Fractional Quantum Hall to Stripe State at nu=5/3
We have investigated the effect of unidirectional periodic potential
modulation on the fractional quantum Hall (FQH) state at filling factors nu=5/3
and 4/3. For large enough modulation amplitude, we find that the resistivity
minimum at nu=5/3 gives way to a peak that grows with decreasing temperature.
Density matrix renormalization group calculation reveals that phase transition
from FQH state to unidirectional striped state having a period sim 4 l (with l
the magnetic length) takes place at nu=1/3 (equivalent to nu=5/3 by the
particle-hole symmetry) with the increase of the modulation amplitude,
suggesting that the observed peak is the manifestation of the stripe phase.Comment: 4 pages, 6 figures; minor revisio
Effect of Oscillating Landau Bandwidth on the Integer Quantum Hall Effect in a Unidirectional Lateral Superlattice
We have measured activation gaps for odd-integer quantum Hall states in a
unidirectional lateral superlattice (ULSL) -- a two-dimensional electron gas
(2DEG) subjected to a unidirectional periodic modulation of the electrostatic
potential. By comparing the activation gaps with those simultaneously measured
in the adjacent section of the same 2DEG sample without modulation, we find
that the gaps are reduced in the ULSL by an amount corresponding to the width
acquired by the Landau levels through the introduction of the modulation. The
decrement of the activation gap varies with the magnetic field following the
variation of the Landau bandwidth due to the commensurability effect. Notably,
the decrement vanishes at the flat band conditions.Comment: 7 pages, 6 figures, minor revisio
Origin of positive magnetoresistance in small-amplitude unidirectional lateral superlattices
We report quantitative analysis of positive magnetoresistance (PMR) for
unidirectional-lateral-superlattice samples with relatively small periods
(a=92-184 nm) and modulation amplitudes (V_0=0.015-0.25 meV). By comparing
observed PMR's with ones calculated using experimentally obtained mobilities,
quantum mobilities, and V_0's, it is shown that contribution from streaming
orbits (SO) accounts for only small fraction of the total PMR. For small V_0,
the limiting magnetic field B_e of SO can be identified as an inflection point
of the magnetoresistance trace. The major part of PMR is ascribed to drift
velocity arising from incompleted cyclotron orbits obstructed by scatterings.Comment: 12 pages, 9 figures, REVTe
Commensurability oscillations in the rf conductivity of unidirectional lateral superlattices: measurement of anisotropic conductivity by coplanar waveguide
We have measured the rf magnetoconductivity of unidirectional lateral
superlattices (ULSLs) by detecting the attenuation of microwave through a
coplanar waveguide placed on the surface. ULSL samples with the principal axis
of the modulation perpendicular (S_perp) and parallel (S_||) to the microwave
electric field are examined. For low microwave power, we observe expected
anisotropic behavior of the commensurability oscillations (CO), with CO in
samples S_perp and S_|| dominated by the diffusion and the collisional
contributions, respectively. Amplitude modulation of the Shubnikov-de Haas
oscillations is observed to be more prominent in sample S_||. The difference
between the two samples is washed out with the increase of the microwave power,
letting the diffusion contribution govern the CO in both samples. The failure
of the intended directional selectivity in the conductivity measured with high
microwave power is interpreted in terms of large-angle electron-phonon
scattering.Comment: 8 pages, 5 figure
Performance of Hybrid NbTiN-Al Microwave Kinetic Inductance Detectors as Direct Detectors for Sub-millimeter Astronomy
In the next decades millimeter and sub-mm astronomy requires large format
imaging arrays and broad-band spectrometers to complement the high spatial and
spectral resolution of the Atacama Large Millimeter/sub-millimeter Array. The
desired sensors for these instruments should have a background limited
sensitivity and a high optical efficiency and enable arrays thousands of pixels
in size. Hybrid microwave kinetic inductance detectors consisting of NbTiN and
Al have shown to satisfy these requirements. We present the second generation
hybrid NbTiN-Al MKIDs, which are photon noise limited in both phase and
amplitude readout for loading levels fW. Thanks to the
increased responsivity, the photon noise level achieved in phase allows us to
simultaneously read out approximately 8000 pixels using state-of-the-art
electronics. In addition, the choice of superconducting materials and the use
of a Si lens in combination with a planar antenna gives these resonators the
flexibility to operate within the frequency range THz. Given
these specifications, hybrid NbTiN-Al MKIDs will enable astronomically usable
kilopixel arrays for sub-mm imaging and moderate resolution spectroscopy.Comment: 7 pages, 3 figures. Presented at SPIE Astronomical Telescopes and
Instrumentation 2014: Millimeter, Submillimeter, and Far-Infrared Detectors
and Instrumentation for Astronomy VI
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