arXiv subjects

S. Frank

Publications and source records attributed to S. Frank.

28 records · Page 2Linked to original sources

arXiv2007

A Study of the Type Ia/IIn Supernova 2005gj from X-ray to the Infrared: Paper I

We present extensive ugrizYHJK photometry and optical spectroscopy of SN 2005gj obtained by the SDSS-II and CSP Supernova Projects, which give excellent coverage during the first 150 days after the time of explosion. These data show that SN 2005gj is the second clear case, after SN 2002ic, of a thermonuclear explosion in a dense circumstellar environment. Both the presence of singly and doubly ionized iron-peak elements (FeIII and weak SII, SiII) near maximum light as well as the spectral evolution show that SN 2002ic-like events are Type Ia explosions. Independent evidence comes from the exponential decay in luminosity of SN 2005gj, pointing to an exponential density distribution of the ejecta. The interaction of the supernova ejecta with the dense circumstellar medium is stronger than in SN 2002ic: (1) the supernova lines are weaker; (2) the Balmer emission lines are more luminous; and (3) the bolometric luminosity is higher close to maximum light. The velocity evolution of the Halpha components suggest that the CSM around SN 2005gj is clumpy and it has a flatter density distribution compared with the steady wind solution, in agreement with SN 2002ic. An early X-ray observation with Chandra gives an upper-limit on the mass loss rate from the companion of < 2x10^{-4} Msun/yr.

astro-ph↗

arXiv2007

The Peculiar SN 2005hk: Do Some Type Ia Supernovae Explode as Deflagrations?

We present extensive u'g'r'i'BVRIYJHKs photometry and optical spectroscopy of SN 2005hk. These data reveal that SN 2005hk was nearly identical in its observed properties to SN 2002cx, which has been called ``the most peculiar known type Ia supernova.'' Both supernovae exhibited high ionization SN 1991T-like pre-maximum spectra, yet low peak luminosities like SN 1991bg. The spectra reveal that SN 2005hk, like SN 2002cx, exhibited expansion velocities that were roughly half those of typical type Ia supernovae. The R and I light curves of both supernovae were also peculiar in not displaying the secondary maximum observed for normal type Ia supernovae. Our YJH photometry of SN 2005hk reveals the same peculiarity in the near-infrared. By combining our optical and near-infrared photometry of SN 2005hk with published ultraviolet light curves obtained with the Swift satellite, we are able to construct a bolometric light curve from ~10 days before to ~60 days after B maximum. The shape and unusually low peak luminosity of this light curve, plus the low expansion velocities and absence of a secondary maximum at red and near-infrared wavelengths, are all in reasonable agreement with model calculations of a 3D deflagration which produces ~0.25 M_sun of 56Ni.

astro-ph↗

arXiv2006

The Sources of the CDF-N : broadband spectral analyses, absorption measurements and the alpha_ox - L_UV relation

We present broadband properties of sources in the Chandra Deep Field-North with spectroscopic redshifts. The high luminosity and redshift bins are dominated by typical quasars. The intermediate redshift (z = 0.7-1.5) and luminosity ranges (L(0.5-8.0keV) = 10^(42.5-43.5) erg/s) show a mix of different source types, with the absorbed objects in the majority. At the faint flux limit of the CDF-N, a substantial fraction of the sources are identified as star-forming galaxies. The AGN in the sample can be divided into four classes based on their rest frame SEDs: luminous, unabsorbed QSOs; objects with reddened optical spectra and X-ray absorption; X-ray absorbed AGN with no signs of reddening in the optical spectra; and optically reddened sources with X-ray spectra indicative of obscuration. We argue that the AGN of higher luminosity tend to have a lower X-ray absorbing column density, but the ratio of X-ray absorbed to unabsorbed AGN remains constant with redshift. We find that the relations between UV and X-ray luminosities derived by Strateva et al. (2005) and Steffen et al.(2006) only hold for bright sources, and break down when faint objects in the sample are included.This is only partly owing to the fact that the majority of the faint sources are absorbed; several faint sources must also have intrinsically lower X-ray luminosity. A fit to the NIR-optical-UV broadband SEDs of the AGN allows us to constrain the absorption parameters independently from the X-ray analyses. We show that the N_H values derived by the X-ray and optical methods are not correlated.This may be because the X-ray absorption and optical attenuation do not originate at the same location, and/or the dust properties responsible for the optical attenuation in AGNs are very different from the locally known dust properties. (abridged)

astro-ph↗

arXiv2006

Effect of pressure on the polarized infrared optical response of quasi-one-dimensional LaTiO$_{3.41}$

The pressure-induced changes in the optical properties of the quasi-one-dimensional conductor LaTiO$_{3.41}$ were studied by polarization-dependent mid-infrared micro-spectroscopy at room temperature. For the polarization of the incident radiation parallel to the conducting direction, the optical conductivity spectrum shows a pronounced mid-infrared absorption band, exhibiting a shift to lower frequencies and an increase in oscillator strength with increasing pressure. On the basis of its pressure dependence, interpretations of the band in terms of electronic transitions and polaronic excitations are discussed. Discontinuous changes in the optical response near 15 GPa are in agreement with a recently reported pressure-induced structural phase transition and indicate the onset of a dimensional crossover in this highly anisotropic system.

cond-mat.mtrl-sci↗

arXiv2006

Pressure-induced insulator-to-metal transition in low-dimensional TiOCl

We studied the transmittance and reflectance of the low-dimensional Mott-Hubbard insulator TiOCl in the infrared and visible frequency range as a function of pressure. The strong suppression of the transmittance and the abrupt increase of the near-infrared reflectance above 12 GPa suggest a pressure-induced insulator-to-metal transition. The pressure-dependent frequency shifts of the orbital excitations, as well as the pressure dependences of the charge gap and the spectral weight of the optical conductivity above the phase transition are presented.

cond-mat.str-el↗

arXiv2005

Effect of annealing on the depth profile of hole concentration in (Ga,Mn)As

The effect of annealing at 250 C on the carrier depth profile, Mn distribution, electrical conductivity, and Curie temperature of (Ga,Mn)As layers with thicknesses > 200 nm, grown by molecular-beam epitaxy at low temperatures, is studied by a variety of analytical methods. The vertical gradient in hole concentration, revealed by electrochemical capacitance-voltage profiling, is shown to play a key role in the understanding of conductivity and magnetization data. The gradient, basically already present in as-grown samples, is strongly influenced by post-growth annealing. From secondary ion mass spectroscopy it can be concluded that, at least in thick layers, the change in carrier depth profile and thus in conductivity is not primarily due to out-diffusion of Mn interstitials during annealing. Two alternative possible models are discussed.

cond-mat.mtrl-sci↗

arXiv2003

Growth of GaMnAs under near-stoichiometric conditions

We studied the effect of the V/III flux ratio and substrate temperature on magnetotransport properties and lattice parameters of Ga0.96Mn0.04As grown by molecular beam epitaxy. For all the substrate temperatures, the conductivities and Curie temperatures of the layers were found to increase as the V/III flux ratio approaches 1. The Curie temperature as high as 95 K was achieved for the Ga0.96Mn0.04As samples grown at 240C and a V/III ratio of about 1.5. The lattice parameter of Ga0.96Mn0.04As increased with decreasing V/III ratio and/or increasing growth temperature. Possible reasons for the effect of V/III ratio on the magnetotransport properties and lattice parameter of GaMnAs are discussed.

cond-mat↗

arXiv2003

Enhancement of the Curie temperature in GaMnAs/InGaMnAs superlattices

We report on an enhancement of the Curie temperature in GaMnAs/InGaMnAs superlattices grown by low-temperature molecular beam epitaxy, which is due to thin InGaMnAs or InGaAs films embedded into the GaMnAs layers. The pronounced increase of the Curie temperature is strongly correlated to the In concentration in the embedded layers. Curie temperatures up to 110 K are observed in such structures compared to 60 K in GaMnAs single layers grown under the same conditions. A further increase in T$_C$ up to 130 K can be achieved using post-growth annealing at temperatures near the growth temperature. Pronounced thickness fringes in the high resolution X-ray diffraction spectra indicate good crystalline quality and sharp interfaces in the structures.

cond-mat.mtrl-sci↗

arXiv2003

Electronic and magnetic properties of GaMnAs: Annealing effects

The effect of short-time and long-time annealing at 250C on the conductivity, hole density, and Curie temperature of GaMnAs single layers and GaMnAs/InGaMnAs heterostructures is studied by in-situ conductivity measurements as well as Raman and SQUID measurements before and after annealing. Whereas the conductivity monotonously increases with increasing annealing time, the hole density and the Curie temperature show a saturation after annealing for 30 minutes. The incorporation of thin InGaMnAs layers drastically enhances the Curie temperature of the GaMnAs layers.

cond-mat↗

arXiv2002

Curie temperature and carrier concentration gradients in MBE grown GaMnAs layers

We report on detailed investigations of the electronic and magnetic properties of ferromagnetic GaMnAs layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the GaMnAs/GaAs interface. While high resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the GaMnAs/GaAs interface has been found by Raman spectroscopy as well as electrochemical capacitance-voltage profiling. The gradient in Curie temperature seems to be a general feature of GaMnAs layers grown at low-temperature. Possible explanations are discussed.

cond-mat.mtrl-sci↗