Comparison of the scintillation noise above different observatories measured with MASS instruments

Scintillation noise is a major limitation of ground base photometric precision. An extensive dataset of stellar scintillation collected at 11 astronomical sites world-wide with MASS instruments was used to estimate the scintillation noise of large telescopes in the case of fast photometry and traditional long-exposure regime. Statistical distributions of the corresponding parameters are given. The scintillation noise is mostly determined by turbulence and wind in the upper atmosphere and comparable at all sites, with slightly smaller values at Mauna Kea and largest noise at Tolonchar in Chile. We show that the classical Young's formula under-estimates the scintillation noise.The temporal variations of the scintillation noise are also similar at all sites, showing short-term variability at time scales of 1 -- 2 hours and slower variations, including marked seasonal trends (stronger scintillation and less clear sky during local winter). Some correlation was found between nearby observatories.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 11 figure

Laser-induced persistent photovoltage on the surface of a ternary topological insulator at room temperature

Using time- and angle-resolved photoemission, we investigate the ultrafast response of excited electrons in the ternary topological insulator (Bi$_{1 x}$Sb$_{x}$)$_2$Te$_3$ to fs-infrared pulses. We demonstrate that at the critical concentration $x$=0.55, where the system becomes bulk insulating, a surface voltage can be driven at room temperature through the topological surface state solely by optical means. We further show that such a photovoltage persists over a time scale that exceeds $\sim$6 $\mu$s, i.e, much longer than the characteristic relaxation times of bulk states. We attribute the origin of the photovoltage to a laser-induced band-bending effect which emerges near the surface region on ultrafast time scales. The photovoltage is also accompanied by a remarkable increase in the relaxation times of excited states as compared to undoped topological insulators. Our findings are relevant in the context of applications of topological surface states in future optical devices.Comment: 5 pages, 4 figure

First results of site testing program at Mt. Shatdzhatmaz in 2007 - 2009

We present the first results of the site testing performed at Mt.~Shatdzhatmaz at Northern Caucasus, where the new Sternberg astronomical institute 2.5-m telescope will be installed. An automatic site monitor instrumentation and functionality are described together with the methods of measurement of the basic astroclimate and weather parameters. The clear night sky time derived on the basis of 2006 -- 2009 data amounts to 1340 hours per year. Principle attention is given to the measurement of the optical turbulence altitude distribution which is the most important characteristic affecting optical telescopes performance. For the period from November 2007 to October 2009 more than 85\,000 turbulence profiles were collected using the combined MASS/DIMM instrument. The statistical properties of turbulent atmosphere above the summit are derived and the median values for seeing $\beta_0 = 0.93$~arcsec and free-atmosphere seeing $\beta_{free} = 0.51$~arcsec are determined. Together with the estimations of isoplanatic angle $\theta_0 = 2.07$~arcsec and time constant \tau_0 = 2.58 \mbox{ ms}, these are the first representative results obtained for Russian sites which are necessary for development of modern astronomical observation techniques like adaptive optics.Comment: Accepted for publication in MNRAS, 17 pages, 15 figure

Observation of Mixed Fermionic-Bosonic Helium Clusters by Transmission Grating Diffraction

Small weakly bound boson-fermion ⁴Hem³Hen clusters formed in a free jet expansion are identified using nondestructive transmission grating diffraction. The observations confirm the existence of more than 11 very tenuous complexes including the three-body halo molecule ⁴He₂³He and the pseudo-Borromean complex ⁴He₂³He₂. Effective cluster formation temperatures, extracted from a sudden freeze model for cluster growth using theoretical binding energies, increase smoothly with cluster size, thereby confirming the calculations with the possible exception of ⁴He₂³He₂

Ultrafast spin polarization control of Dirac fermions in topological insulators

Three-dimensional topological insulators (TIs) are characterized by spin-polarized Dirac-cone surface states that are protected from backscattering by time-reversal symmetry. Control of the spin polarization of topological surface states (TSSs) using femtosecond light pulses opens novel perspectives for the generation and manipulation of dissipationless surface spin currents on ultrafast timescales. Using time-, spin-, and angle-resolved spectroscopy, we directly monitor for the first time the ultrafast response of the spin polarization of photoexcited TSSs to circularly-polarized femtosecond pulses of infrared light. We achieve all-optical switching of the transient out-of-plane spin polarization, which relaxes in about 1.2 ps. Our observations establish the feasibility of ultrafast optical control of spin-polarized Dirac fermions in TIs and pave the way for novel optospintronic applications at ultimate speeds.Comment: 9 pages, 4 figure