Formation of pure two-electron triplet states in weakly coupled quantum dots attached to ferromagnetic leads

Weakly coupled quantum dots in the Pauli spin blockade regime are considered with respect to spin-dependent transport. By attaching one half-metallic and one non-magnetic lead, the Pauli spin blockade if formed by a pure triplet state with spin moment $S_z=1$ or -1. Furthermore, additional spin blockade regimes emerge because of full occupation in states with opposite spin to that of the half-metallic lead.Comment: 6 pages, 2 figures, 1 table, minor changes to appear as publishe

Detection of spin reversal and nutations through current measurements

The dynamics of a single spin embedded in a the tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. Moment reversal of the local spin induced by the bias voltage across the junction is shown to have a measurable effect on the tunneling current. Furthermore, the frequency of a harmonic bias voltage is picked up by the local spin dynamics and transferred back to the current generating a double frequency component.Comment: 5 pages, 5 figures; published version (with minor corrections

Detection of exchange interaction in STM measurements through Fanolike interference effects

We address Fano-like interference effects in scanning tunneling microscopy (STM) measurements of nanoscale systems, e.g. two-level systems. Common for these systems is that second order tunneling contributions give rise to interference effects that cause suppressed transmission through the system for certain energies. The suppressed transmission is measurable either in the differential conductance or in the bias voltage derivative thereof.Comment: 9 pages, 3 figures, submitted: in addition to the results published in Phys. Rev. B, 75, 153309 (2007), this paper contains a more thorough discussion on the used transport formalism, studies of asymmetric couplings to the substrate, and discussion of non-resonant levels. The non-resonant case is related to spin-dependent tunnelin

Lithiation of InSb and Cu2_2Sb : A Theoretical Investigation

In this work the mechanism of Li insertion/intercalation in the anode materials InSb and Cu$_2$Sb is investigated by means of the first principles total energy calculations. The total charge densities for the lithiated products of the two compounds are presented. Based on these results the change in the bonding character on lithiation is discussed. Further, the isomer shift for InSb and Cu$_2$Sb and there various lithiated products is reported. The average insertion/intercalation voltage and volume expansion for transitions from InSb to Li$_2$InSb and Cu$_2$Sb to Li$_2$CuSb are calculated and found to be in good agreement with the experimental values. These findings help to resolve the controversy regarding the lithiation mechanism in InSb.Comment: 5 pages 3 figure

Adaptive Optics Discovery of Supernova 2004ip in the Nuclear Regions of the Luminous Infrared Galaxy IRAS 18293-3413

We report a supernova discovery in Ks-band images from the NAOS CONICA adaptive optics (AO) system on the ESO Very Large Telescope (VLT). The images were obtained as part of a near-infrared search for highly-obscured supernovae in the nuclear regions of luminous and ultraluminous infrared galaxies. SN 2004ip is located within a circumnuclear starburst at 1.4 arcsec (or 500 pc) projected distance from the K-band nucleus of the luminous infrared galaxy IRAS 18293-3413. The supernova luminosity and light curve are consistent with a core-collapse event suffering from a host galaxy extinction of up to about 40 magnitudes in V-band which is as expected for a circumnuclear starburst environment. This is the first supernova to be discovered making use of AO correction and demonstrates the potential of the current 8-meter class telescopes equipped with AO in discovering supernovae from the innermost nuclear regions of luminous and ultraluminous infrared galaxies.Comment: 4 pages, 3 figures, ApJ Letters (accepted

Modeling the Radio and X-ray Emission of SN 1993J and SN 2002ap

Modeling of radio and X-ray observations of supernovae interacting with their circumstellar media are discussed, with special application to SN 1993J and SN 2002ap. We emphasize the importance of including all relevant physical mechanisms, especially for the modeling of the radio light curves. The different conclusions for the absorption mechanism (free-free or synchrotron self-absorption), as well as departures from an $\rho \propto r^{-2}$ CSM, as inferred by some authors, are discussed in detail. We conclude that the evidence for a variation in the mass loss rate with time is very weak. The results regarding the efficiencies of magnetic field generation and relativistic particle acceleration are summarized.Comment: 10 pages, 2 figures. Uses svmult.cls. To appear in proceedings of IAU Colloquium 192 "Supernovae (10 years of SN 1993J)", April 2003, Valencia, Spain, eds. J. M. Marcaide and K. W. Weile

Supernova 1998bw - The final phases

The probable association with GRB 980425 immediately put SN 1998bw at the forefront of supernova research. Here, we present revised late-time BVRI light curves of the supernova, based on template images taken at the VLT. To follow the supernova to the very last observable phases we have used HST/STIS. Deep images taken in June and November 2000 are compared to images taken in August 2001. The identification of the supernova is firmly established. This allows us to measure the light curve to about 1000 days past explosion. The main features are a rapid decline up to more than 500 days after explosion, with no sign of complete positron trapping from the Cobolt-56 decay. Thereafter, the light curve flattens out significantly. One possible explanation is powering by more long lived radioactive isotopes, if they are abundantly formed in this energetic supernova.Comment: 13 pages, 5 figures, A&A, In pres

The tunnel magnetoresistance in chains of quantum dots weakly coupled to external leads

We analyze numerically the spin-dependent transport through coherent chains of three coupled quantum dots weakly connected to external magnetic leads. In particular, using the diagrammatic technique on the Keldysh contour, we calculate the conductance, shot noise and tunnel magnetoresistance (TMR) in the sequential and cotunneling regimes. We show that transport characteristics greatly depend on the strength of the interdot Coulomb correlations, which determines the spacial distribution of electron wave function in the chain. When the correlations are relatively strong, depending on the transport regime, we find both negative TMR as well as TMR enhanced above the Julliere value, accompanied with negative differential conductance (NDC) and super-Poissonian shot noise. This nontrivial behavior of tunnel magnetoresistance is associated with selection rules that govern tunneling processes and various high-spin states of the chain that are relevant for transport. For weak interdot correlations, on the other hand, the TMR is always positive and not larger than the Julliere TMR, although super-Poissonian shot noise and NDC can still be observed

Eleven years of radio monitoring of the Type IIn supernova SN 1995N

We present radio observations of the optically bright Type IIn supernova SN 1995N. We observed the SN at radio wavelengths with the Very Large Array (VLA) for 11 years. We also observed it at low radio frequencies with the Giant Metrewave Radio Telescope (GMRT) at various epochs within $6.5-10$ years since explosion. Although there are indications of an early optically thick phase, most of the data are in the optically thin regime so it is difficult to distinguish between synchrotron self absorption (SSA) and free-free absorption (FFA) mechanisms. However, the information from other wavelengths indicates that the FFA is the dominant absorption process. Model fits of radio emission with the FFA give reasonable physical parameters. Making use of X-ray and optical observations, we derive the physical conditions of the shocked ejecta and the shocked CSM.Comment: 22 pages, 2 tables, 13 figures, Accepted for publication in Astrophysical Journa

The Orbit of the Eclipsing X-ray Pulsar EXO 1722-363

With recent and archival Rossi X-Ray Timing Explorer (RXTE) X-ray measurements of the heavily obscured X-ray pulsar EXO 1722-363 (IGR J17252-3616), we carried out a pulse timing analysis to determine the orbital solution for the first time. The binary system is characterized by a_x sin(i) = 101 +/- 3 lt-s and P_orb = 9.7403 +/- 0.0004 days (90% confidence), with the precision of the orbital period being obtained by connecting datasets separated by more than 7 years (272 orbital cycles). The orbit is consistent with circular, and e < 0.19 at the 90% confidence level. The mass function is 11.7 +/- 1.2 M_sun and confirms that this source is a High Mass X-ray Binary (HMXB) system. The orbital period, along with the previously known ~414 s pulse period, places this system in the part of the Corbet diagram populated by supergiant wind accretors. Using previous eclipse time measurements by Corbet et al. and our orbital solution, combined with the assumption that the primary underfills its Roche lobe, we find i > 61 degrees at the 99% confidence level, the radius of the primary is between 21 R_sun and 37 R_sun, and its mass is less than about 22 M_sun. The acceptable range of radius and mass shows that the primary is probably a supergiant of spectral type B0I-B5I. Photometric measurements of its likely counterpart are consistent with the spectral type and luminosity if the distance to the system is between 5.3 kpc and 8.7 kpc. Spectral analysis of the pulsar as a function of orbital phase reveals an evolution of the hydrogen column density suggestive of dense filaments of gas in the downstream wake of the pulsar, with higher levels of absorption seen at orbital phases 0.5-1.0, as well as a variable Fe K_alpha line.Comment: Submitted to ApJ, 11 pages, 11 figure