45,004 research outputs found
Low temperature terahertz spectroscopy of n-InSb through a magnetic field driven metal-insulator transition
We use fiber-coupled photoconductive emitters and detectors to perform
terahertz (THz) spectroscopy of lightly-doped n-InSb directly in the cryogenic
(1.5 K) bore of a high-field superconducting magnet. We measure transmission
spectra from 0.1-1.1 THz as the sample is driven through a metal-insulator
transition (MIT) by applied magnetic field. In the low-field metallic state,
the data directly reveal the plasma edge and magneto-plasmon modes. With
increasing field, a surprisingly broad band (0.3-0.8 THz) of low transmission
appears at the onset of the MIT. This band subsequently collapses and evolves
into the sharp 1s -> 2p- transition of electrons `frozen' onto isolated donors
in the insulating state.Comment: 4 pages, 3 figure
Strongly Enhanced Hole-Phonon Coupling in the Metallic State of the Dilute Two-Dimensional Hole Gas
We have studied the temperature dependent phonon emission rate () of a
strongly interacting (22) dilute 2D GaAs hole system using a standard
carrier heating technique. In the still poorly understood metallic state, we
observe that () changes from () to ()
above 100mK, indicating a crossover from screened piezoelectric(PZ) coupling to
screened deformation potential(DP) coupling for hole-phonon scattering.
Quantitative comparison with theory shows that the long range PZ coupling
between holes and phonons has the expected magnitude; however, in the metallic
state, the short range DP coupling between holes and phonons is {\it almost
twenty times stronger} than expected from theory. The density dependence of
() shows that it is {\it easier} to cool low density 2D holes in GaAs
than higher density 2D hole systems.Comment: To appear in Phys. Rev. Let
Temperature and Magnetic Field Enhanced Hall Slope of a Dilute 2D Hole System in the Ballistic Regime
We report the temperature() and perpendicular magnetic field()
dependence of the Hall resistivity of dilute metallic
two-dimensional(2D) holes in GaAs over a broad range of
temperature(0.02-1.25K). The low Hall coefficient, , is found to be
enhanced when decreases. Strong magnetic fields further enhance the slope
of at all temperatures studied. Coulomb interaction corrections
of a Fermi liquid(FL) in the ballistic regime can not explain the enhancement
of which occurs in the same regime as the anomalous metallic
longitudinal conductivity. In particular, although the metallic conductivity in
2D systems has been attributed to electron interactions in a FL, these same
interactions should reduce, {\it not enhance} the slope of as
decreases and/or increases.Comment: Accepted for publication in Physical Review Letter
Suppression of weak localization effects in low-density metallic 2D holes
We have measured the conductivity in a gated high-mobility GaAs two
dimensional hole sample with densities in the range (7-17)x10^9 cm^-2 and at
hole temperatures down to 5x10^-3 E_F. We measure the weak localization
corrections to the conductivity g=G/(e^2/h) as a function of magnetic field
(Delta g=0.019 +/- 0.006 at g=1.5 and T=9 mK) and temperature (d ln g/dT<0.0058
and 0.0084 at g=1.56 and 2.8). These values are less than a few percent of the
value 1/pi predicted by standard weak localization theory for a disordered 2D
Fermi liqui
Primordial perturbations and non-Gaussianities in Ho\v{r}ava-Lifshitz gravity
We investigate primordial perturbations and non-gaussianities in the
Ho\v{r}ava-Lifshitz theory of gravitation. In the UV limit, the scalar
perturbation in the Ho\v{r}ava theory is naturally scale-invariant, ignoring
the details of the expansion of the Universe. One may thus relax the
exponential inflation and the slow-roll conditions for the inflaton field. As a
result, it is possible that the primordial non-gaussianities, which are
``slow-roll suppressed'' in the standard scenarios, become large. We calculate
the non-gaussianities from the bispectrum of the perturbation and find that the
equilateral-type non-gaussianity is of the order of unity, while the local-type
non-gaussianity remains small, as in the usual single-field slow-roll inflation
model in general relativity. Our result is a new constraint on the the
Ho\v{r}ava-Lifshitz gravity.Comment: 12 pages, no figure, v5, matching the CPC versio
Measurement of atmospheric nitrous acid at Blodgett Forest during BEARPEX2007
Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH) in the lower troposphere. Understanding HONO chemistry, particularly its sources and contribution to HO_x (=OH+HO_2) production, is very important for understanding atmospheric oxidation processes. A highly sensitive instrument for detecting atmospheric HONO based on wet chemistry followed by liquid waveguide long path absorption photometry was deployed in the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at Blodgett Forest, California in late summer 2007. The median diurnal variation shows minimum HONO levels of about 20–30 pptv during the day and maximum levels of about 60–70 pptv at night, a diurnal pattern quite different from the results at various other forested sites. Measured HONO/NO_2 ratios for a 24-h period ranged from 0.05 to 0.13 with a mean ratio of 0.07. Speciation of reactive nitrogen compounds (NO_y) indicates that HONO accounted for only ~3% of total NO_y. However, due to the fast HONO loss through photolysis, a strong HONO source (1.59 ppbv day^(−1)) existed in this environment in order to sustain the observed HONO levels, indicating the significant role of HONO in NO_y cycling. The wet chemistry HONO measurements were compared to the HONO measurements made with a Chemical Ionization Mass Spectrometer (CIMS) over a three-day period. Good agreement was obtained between the measurements from the two different techniques. Using the expansive suite of photochemical and meteorological measurements, the contribution of HONO photolysis to HO_x budget was calculated to be relatively small (6%) compared to results from other forested sites. The lower HONO mixing ratio and thus its smaller contribution to HO_x production are attributed to the unique meteorological conditions and low acid precipitation at Blodgett Forest. Further studies of HONO in this kind of environment are needed to test this hypothesis and to improve our understanding of atmospheric oxidation and nitrogen budget
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