Theoretical uncertainty in baryon oscillations

We discuss the systematic uncertainties in the recovery of dark energy properties from the use of baryon acoustic oscillations as a standard ruler. We demonstrate that while unknown relativistic components in the universe prior to recombination would alter the sound speed, the inferences for dark energy from low-redshift surveys are unchanged so long as the microwave background anisotropies can measure the redshift of matter-radiation equality, which they can do to sufficient accuracy. The mismeasurement of the radiation and matter densities themselves (as opposed to their ratio) would manifest as an incorrect prediction for the Hubble constant at low redshift. In addition, these anomalies do produce subtle but detectable features in the microwave anisotropies.Comment: 4 pages, REVTeX, 1 figure. Submitted to PR

Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System

At total Landau level filling factor $\nu_{tot}=1$ a double layer two-dimensional electron system with small interlayer separation supports a collective state possessing spontaneous interlayer phase coherence. This state exhibits the quantized Hall effect when equal electrical currents flow in parallel through the two layers. In contrast, if the currents in the two layers are equal, but oppositely directed, both the longitudinal and Hall resistances of each layer vanish in the low temperature limit. This finding supports the prediction that the ground state at $\nu_{tot}=1$ is an excitonic superfluid.Comment: 4 pages, 4 figure

Evidence for a Goldstone Mode in a Double Layer Quantum Hall System

The tunneling conductance between two parallel 2D electron systems has been measured in a regime of strong interlayer Coulomb correlations. At total Landau level filling $\nu_T=1$ the tunnel spectrum changes qualitatively when the boundary separating the compressible phase from the ferromagnetic quantized Hall state is crossed. A huge resonant enhancement replaces the strongly suppressed equilibrium tunneling characteristic of weakly coupled layers. The possible relationship of this enhancement to the Goldstone mode of the broken symmetry ground state is discussed.Comment: 4 pages, 3 figures, 2 minor typeos fixe

Non-LTE spectral analyses of the lately discovered DB-gap white dwarfs from the SDSS

For a long time, no hydrogen-deficient white dwarfs have been known that have effective temperature between 30 kK and < 45 kK, i.e. exceeding those of DB white dwarfs and having lower ones than DO white dwarfs. Therefore, this temperature range was long known as the DB-gap. Only recently, the SDSS provided spectra of several candidate DB-gap stars. First analyses based on model spectra calculated under the assumption of local thermodynamic equilibrium (LTE) confirmed that these stars had 30 kK < Teff < 45 kK (Eisenstein et al. 2006). It has been shown for DO white dwarfs that the relaxation of LTE is necessary to account for non local effects in the atmosphere caused by the intense radiation field. Therefore, we calculated a non-LTE model grid and re-analysed the aforementioned set of SDSS spectra. Our results confirm the existence of DB-gap white dwarfs.Comment: 4 pages, 2 figures, to appear in: Proceedings of the 16th European Workshop on White Dwarf

Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO3_3/SrTiO3_3 interfaces

Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO$_3$/SrTiO$_3$ interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions.Comment: 4 figures in main text; 4 figures in the supplemen

Elastic theory of quantum Hall smectics: effects of disorder

We study the effect of disorder on quantum Hall smectics within the framework of an elastic theory. Based on a renormalization group calculation, we derive detailed results for the degrees of translational and orientational order of the stripe pattern at zero temperature and carefully map out the disorder and length-scale regimes in which the system effectively exhibits smectic, nematic, or isotropic behavior. We show that disorder always leads to a finite density of free dislocations and estimate the scale on which they begin to appear.Comment: 4 pages latex with 1 EPS figur

Redshift-Space Enhancement of Line-of-Sight Baryon Acoustic Oscillations in the SDSS Main-Galaxy Sample

We show that redshift-space distortions of galaxy correlations have a strong effect on correlation functions with distinct, localized features, like the signature of the baryon acoustic oscillations (BAO). Near the line of sight, the features become sharper as a result of redshift-space distortions. We demonstrate this effect by measuring the correlation function in Gaussian simulations and the Millennium Simulation. We also analyze the SDSS DR7 main-galaxy sample (MGS), splitting the sample into slices 2.5 degrees on the sky in various rotations. Measuring 2D correlation functions in each slice, we do see a sharp bump along the line of sight. Using Mexican-hat wavelets, we localize it to (110 +/- 10) Mpc/h. Averaging only along the line of sight, we estimate its significance at a particular wavelet scale and location at 2.2 sigma. In a flat angular weighting in the (pi,r_p) coordinate system, the noise level is suppressed, pushing the bump's significance to 4 sigma. We estimate that there is about a 0.2% chance of getting such a signal anywhere in the vicinity of the BAO scale from a power spectrum lacking a BAO feature. However, these estimates of the significances make some use of idealized Gaussian simulations, and thus are likely a bit optimistic.Comment: 17 pages, 27 figures. Minor changes to match final version accepted to Ap

The Droplet State and the Compressibility Anomaly in Dilute 2D Electron Systems

We investigate the space distribution of carrier density and the compressibility of two-dimensional (2D) electron systems by using the local density approximation. The strong correlation is simulated by the local exchange and correlation energies. A slowly varied disorder potential is applied to simulate the disorder effect. We show that the compressibility anomaly observed in 2D systems which accompanies the metal-insulator transition can be attributed to the formation of the droplet state due to disorder effect at low carrier densities.Comment: 4 pages, 3 figure