Search

We present the results of a Hubble Space Telescope WFC3/F160W SNAPSHOT sur- vey of the host galaxies of 39 long-duration gamma-ray bursts (LGRBs) at z < 3. We have non-detections of hosts at the locations of 4 bursts. Sufficient accuracy to as- trometrically align optical afterglow images and determine the location of the LGRB within its host was possible for 31/35 detected hosts. In agreement with other work, we find the luminosity distribution of LGRB hosts is significantly fainter than that of a star formation rate-weighted field galaxy sample over the same redshift range, indicating LGRBs are not unbiasedly tracing the star formation rate. Morphologi- cally, the sample of LGRB hosts are dominated by spiral-like or irregular galaxies. We find evidence for evolution of the population of LGRB hosts towards lower-luminosity, higher concentrated hosts at lower redshifts. Their half-light radii are consistent with other LGRB host samples where measurements were made on rest-frame UV obser- vations. In agreement with recent work, we find their 80 per cent enclosed flux radii distribution to be more extended than previously thought, making them intermedi- ate between core-collapse supernova (CCSN) and super-luminous supernova (SLSN) hosts. The galactocentric projected-offset distribution confirms LGRBs as centrally concentrated, much more so than CCSNe and similar to SLSNe. LGRBs are strongly biased towards the brighter regions in their host light distributions, regardless of their offset. We find a correlation between the luminosity of the LGRB explosion site and the intrinsic column density, N_H , towards the burst., Comment: 26 pages, accepted to MNRAS

It is known that for a non-relativistic quantum particle traveling freely on the $x$-axis, the positional probability can flow in the opposite direction to the particle's velocity. The maximum possible amount of such backflow that can occur over any time interval has been determined previously as the largest positive eigenvalue of a certain hermitian observable, with the value $0.0384517\dots$, or about $4\%$ of the total probability on the line. The eigenvalue problem is now considered numerically in the more general case of states with momentum restricted to the range $p_0

We consider the time evolution of nearly degenerate two-state systems with an external interaction. Conditions in which full population transfer is possible when the two-states become degenerate are considered. A new variation of the coupled differential equations for the probability amplitudes suggests a possible singularity in the time evolution of the system. The singularity occurs at the point in time where full control in the degenerate limit is achieved. We solve this new variation to derive more information about non-degenerate population control. The new results of numerical calculations for occupational probabilities of non-degenerate systems are presented and interpreted. These results are used to understand how population control breaks down as the photon energy and the energy of the system are changed. Applications are discussed, with an emphasis on interactions of twisted-vortex and plane-wave photons with a variety of targets, including macroscopic gas cells of atoms, of large single crystals, and of molecules., Comment: 9 pages, 4 figures

Long-duration Gamma-Ray Bursts (GRBs) allow us to pinpoint and study star-forming galaxies in the early universe, thanks to their orders of magnitude brighter peak luminosities compared to other astrophysical sources, and their association with deaths of massive stars. We present Hubble Space Telescope Wide Field Camera 3 detections of three Swift GRB host galaxies lying at redshifts $z = 5.913$ (GRB 130606A), $z = 6.295$ (GRB 050904), and $z = 6.327$ (GRB 140515A) in the F140W (wide-$JH$ band, $\lambda_{\rm{obs}}\sim1.4\,\mu m$) filter. The hosts have magnitudes (corrected for Galactic extinction) of $m_{\rm{\lambda_{obs},AB}}= 26.34^{+0.14}_{-0.16}, 27.56^{+0.18}_{-0.22},$ and $28.30^{+0.25}_{-0.33}$ respectively. In all three cases the probability of chance coincidence of lower redshift galaxies is $\lesssim2\,\%$, indicating that the detected galaxies are most likely the GRB hosts. These are the first detections of high redshift ($z > 5$) GRB host galaxies in emission. The galaxies have luminosities in the range $0.1-0.6\,L^{*}_{z=6}$ (with $M_{1600}^{*}=-20.95\pm0.12$), and half-light radii in the range $0.6-0.9\,\rm{kpc}$. Both their half-light radii and luminosities are consistent with existing samples of Lyman-break galaxies at $z\sim6$. Spectroscopic analysis of the GRB afterglows indicate low metallicities ($[\rm{M/H}]\lesssim-1$) and low dust extinction ($A_{\rm{V}}\lesssim0.1$) along the line of sight. Using stellar population synthesis models, we explore the implications of each galaxy's luminosity for its possible star formation history, and consider the potential for emission-line metallicity determination with the upcoming James Webb Space Telescope., Comment: Published in ApJ

Scattering of beams of light and matter from multi-electron atomic targets is formulated in the position representation of quantum mechanics. This yields expressions for the probability amplitude, a(b), for a wide variety of processes. Here the spatial parameter b is the distance of closest approach of incoming particles traveling on a straight line with the center of the atomic target. The correlated probability amplitude, a(b), reduces to a relatively simple product of single electron probability amplitudes in the widely used independent electron approximation limit, where the correlation effects of the Coulomb interactions between the atomic electrons disappear. As an example in which a(b} has an explicit dependence on b}, we consider transversely finite vortex beams of twisted photons that lack the translational invariance of infinite plane-wave beams. Some experimental considerations and future applications are briefly considered., Comment: 20 pages, 5 figures

The relatively new atomic form factor for twisted (vortex) beams, which carry orbital angular momentum (OAM), is considered and compared to the conventional atomic form factor for plane wave beams that carry only spin angular momentum (SAM). Since the vortex symmetry of a twisted photon is more complex that that of a plane-wave, evaluation of the atomic form factor is also more complex for twisted photons. On the other hand, the twisted photon has additional parameters, including the OAM quantum number, $\ell$, the nodal radial number, $p$, and the Rayleigh range, $z_R$ that determines the cone angle of the vortex. This Rayleigh range may be used as a variable parameter to control, in new ways, the interaction of twisted photons with matter. Here we address: i) normalization of the vortex atomic form factor, ii) displacement of target atoms away from the center of the beam vortex, and iii) formulation of transition probabilities for a variety of photon-atom processes. We attend to features related to new experiments that can test the range of validity and accuracy of calculations of these variations of the atomic form factor. Using the absolute square of the form factor for vortex beams, we introduce a vortex factor that can be directly measured., Comment: 22 pages, 2 figures

We obtain an expression for the matrix element for a twisted (Laguerre-Gaussian profile) photon scattering from a hydrogen atom. We consider photons incoming with an orbital angular momentum (OAM) of $\ell \hbar$, carried by a factor of $e^{i \ell \phi}$ not present in a plane-wave or pure Gaussian profile beam. The nature of the transfer of $+2\ell$ units of OAM from the photon to the azimuthal atomic quantum number of the atom is investigated. We obtain simple formulae for these OAM flip transitions for elastic forward scattering of twisted photons when the photon wavelength $\lambda$ is large compared with the atomic target size $a$, and small compared the Rayleigh range $z_R$, which characterizes the collimation length of the twisted photon beam., Comment: 16 pages

Time ordering may be defined by first defining the limit of no time ordering (NTO) in terms of a time average of an external interaction, V(t). Previously, time correlation was defined in terms of a similar limit called the independent time approximation (ITA). Experimental evidence for time correlation has not yet been distinguished from experimental evidence for time ordering.

The behavior of simply pulsed qubits (quantum systems with two linearly independent states) may be characterized by the energy difference $\Delta E$ between the two states of the qubit and by an external stimulating potential $V(t)$ that causes transitions between them. Thus, the operation of such quantum mechanical systems may be categorized in various regions that explicitly depend on $\Delta E$ and $V(t)$. Limiting cases of degenerate, perturbative, and adiabatic regions are discussed. A comprehensive and illustrative map for simply pulsed qubits is presented that can be used as a visual tool for students. Furthermore, analytic solutions may be obtained when the interaction $V(t)$ is proportional to $\delta(t-t_k)$, namely when a fast interaction, called a kick, is used., Comment: 20 pages, 1 figure

Analytic solutions are developed for two-state systems (e.g. qubits) strongly perturbed by a series of rapidly changing pulses, called `kicks'. The evolution matrix may be expressed as a time ordered product of evolution matrices for single kicks. Single, double, and triple kicks are explicitly considered, and the onset of observability of time ordering is examined. The effects of different order of kicks on the dynamics of the system are studied and compared with effects of time ordering in general. To determine the range of validity of this approach, the effect of using pulses of finite widths for 2s-2p transitions in atomic hydrogen is examined numerically., Comment: 22 pages, 7 figures

We examine time ordering effects in strongly, suddenly perturbed two-state quantum systems (kicked qubits) by comparing results with time ordering to results without time ordering. Simple analytic expressions are given for state occupation amplitudes and probabilities for singly and multiply kicked qubits. We investigate the limit of no time ordering, which can differ in different representations., Comment: 26 pages, 5 figures

We describe a new means for electrically creating spin polarization in semiconductors. In contrast to spin injection of electrons by tunneling through a reverse-biased Schottky barrier, we observe spin accumulation at the metal/semiconductor interface of forward-biased ferromagnetic Schottky diodes, which is consistent with a theory of spin-dependent reflection off the interface. Spatiotemporal Kerr microscopy is used to image the electron spin and the resulting dynamic nuclear polarization that arises from the non equilibrium carrier polarization., Comment: 13 pages, 4 figures, submitted for publication

We examine how time ordering works in quantum mechanics and in classical mechanics., Comment: 13 pages, 3 figures

The Bethe roots describing the ground state energy of the integrable 1D model of interacting bosons with weakly repulsive two-body delta interactions are seen to satisfy the set of Richardson equations appearing in the strong coupling limit of an integrable BCS pairing model. The BCS model describes boson-boson interactions with zero centre of mass momentum of pairs. It follows that the Bethe roots of the weakly interacting boson model are given by the zeros of Laguerre polynomials. The ground state energy and the lowest excitation are obtained explicitly via the Bethe roots. A direct link has thus been established, in the context of integrable 1D models, between bosons interacting via weakly repulsive two-body delta-interactions and strongly interacting Cooper pairs of bosons., Comment: 9 pages, 1 figure. This revised version makes contact with earlier work of Gaudin and clarifies the link to the BCS pairing model

Novel spin transport behavior is theoretically shown to result from replacing the usual metal (or poly-silicon) gate in a silicon field-effect transistor with a ferromagnet, separated from the semiconductor by an ultra-thin oxide. The spin-dependent interplay between the drift current (due to a source-drain bias) and the diffusion current (due to carrier leakage into the ferromagnetic gate) results in a rich variety of spin dependence in the current that flows through such a device. We examine two cases of particular interest: (1) creating a 100% spin-polarized electrical current and (2) creating a pure spin current without a net electrical current. A spin-valve consisting of two sequential ferromagnetic gates is shown to exhibit magnetoresistance dependent upon the relative orientations of the magnetization of the two ferromagnets. The magnetoresistance ratio grows to arbitrarily large values in the regime of low source-drain bias, and is limited only by the spin-flip time in the channel., Comment: 6 pages, 4 figures

A theory of the proximity effects of the exchange splitting in a ferromagnetic metal on a two dimensional electron gas (2DEG) in a semiconductor is presented. The resulting spin-dependent energy and lifetime in the 2DEG create a marked spin-splitting in the driven in-plane current. The theory of the planar transport allows for current leakage into the ferromagnetic layer through the interface, which leads to a competition between drift and diffusion. The spin-dependent in-plane conductivity of the 2DEG may be exploited to provide a new paradigm for spintronics devices based on planar devices in a field-effect transistor configuration. An illustrative example is provided through the transport theory of a proposed spin-valve which consists of a field-effect transistor configuration with two ferromagnetic gates. Results are provided for two experimentally accessible systems: the silicon inversion layer and the naturally-formed InAs accumulation layer., Comment: 14 pages, 14 figures

We find conditions required to achieve complete population transfer, via coherent population trapping, from an initial state to a designated final state at a designated time in a degenerate $n$-state atom, where transitions are caused by an external interaction. In systems with degenerate states there is no time ordering. Analytic expressions have been found for transition probabilities in a degenerate $n$-state atom interacting with a strong external field that gives a common time dependence to all of the transition matrix elements. Except for solving a simple $n^{th}$ order equation to determine eigenvalues of dressed states, the method is entirely analytic. These expressions may be used to control electron populations in degenerate $n$-state atoms. Examples are given for $n=2$ and $n=3$., Comment: 24 pages, 6 figures

We consider the time evolution of the occupation probabilities for the 2s-2p transition in a hydrogen atom interacting with an external field, V(t). A two-state model and a dipole approximation are used. In the case of degenerate energy levels an analytical solution of the time-dependent Shroedinger equation for the probability amplitudes exists. The form of the solution allows one to choose the ratio of the field amplitude to its frequency that leads to temporal trapping of electrons in specific states. The analytic solution is valid when the separation of the energy levels is small compared to the energy of the interacting radiation., Comment: 6 pages, 3 figures

We find conditions required to achieve complete population transfer, via coherent population trapping, from an initial state to a designated final state at a designated time in a degenerate 3-level atom, where transitions are caused by an external interaction. Complete population transfer from an initially occupied state 1 to a designated state 2 occurs under two conditions. First, there is a constraint on the ratios of the transition matrix elements of the external interaction. Second, there is a constraint on the action integral over the interaction, or "area", corresponding to the phase shift induced by the external interaction. Both conditions may be expressed in terms of simple odd integers., Comment: 22 pages, 4 figures

We find a set of conditions to achieve complete population transfer, via coherent population trapping, from an initial state to a designated final state at a designated time in a degenerate n-state atom, where the transitions are caused by an external interaction., Comment: 4 pages, 1 figure

A quantum theory of the spin-dependent scattering of semiconductor electrons by a Schottky barrier at an interface with a ferromagnet is presented. The reflection of unpolarized non-equilibrium carriers produces spontaneous spin-polarization in the semiconductor. If a net spin-polarization pre-exists in the semiconductor, the combination of the ferromagnet magnetization and the incident carrier polarization combine to tilt the reflected polarization in the semiconductor. The spin reflection properties are investigated as functions of the system characteristics: the Schottky barrier height, semiconductor doping and applied bias. The effect on reflection due to the variation of the barrier width with electron energy is contrasted for two means of excitation: optical or electrical. Optically excited electrons have a wider energy spread than the near-equilibrium excitation from non-magnetic ohmic contacts., Comment: 9 pages, 7 figures, submitted to PRB

We report on the results of optical studies of the newly discovered superconductor Cd$_2$Re$_2$O$_7$ in the normal state. We show that the compound has an exotic metallic state at low temperature. The optical conductivity spectrum exhibits two distinct features: a sharp renormalized resonance mode at zero frequency and a broad mid-infrared excitation band. Detailed analysis reveals a moderate enhancement of the effective mass at low temperature and low frequency., Comment: 5 pages, 4 figures. to be published in Phys. Rev. B, scheduled issue: 01 August 2002

A theoretical prediction of the spin-dependent electron self-energy and in-plane transport of a two-dimensional electron gas in proximity with a ferromagnetic gate is presented. The application of the predicted spin-dependent properties is illustrated by the proposal of a device configuration with two neighboring ferromagnetic gates which produces a magnetoresistance effect on the channel current generated by nonmagnetic source and drain contacts. Specific results are shown for a silicon inversion layer with iron gates. The gate leakage current is found to be beneficial to the spin effects., Comment: 3 pages, 2 figures, Replaced with revised version

We present a theory of the imprinting of the electron spin coherence and population in an n-doped semiconductor which forms a junction with a ferromagnet. The reflection of non-equilibrium semiconductor electrons at the interface provides a mechanism to manipulate the spin polarization vector. In the case of unpolarized excitation, this ballistic effect produces spontaneous electron spin coherence and nuclear polarization in the semiconductor, as recently observed by time-resolved Faraday rotation experiments. We investigate the dependence of the spin reflection on the Schottky barrier height and the doping concentration in the semiconductor and suggest control mechanisms for possible device applications., Comment: 4 pages with 2 figures

We present optical conductivity data for Lu(Ni$_{1-x}$Co$_x$)$_2$B$_2$C over a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials show evidence of being good Drude metals with the infrared data in reasonable agreement with dc resistivity measurements at low frequencies. An absorption threshold is seen at approximately 700 cm-1. In the cobalt-doped material we see a superconducting gap in the conductivity spectrum with an absorption onset at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong coupling. The pure material is in the clean limit and no gap can be seen. We discuss the data in terms of the electron-phonon interaction and find that it can be fit below 600 cm-1 with a plasma frequency of 3.3 eV and an electron-phonon coupling constant lambda_{tr}=0.33 using an alpha^{2}F(omega) spectrum fit to the resistivity., Comment: 10 pages with 10 embedded figures, submitted to PRB

The interplane optical spectrum of the organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br was investigated in the frequency range from 40 to 40,000 cm-1. The optical conductivity was obtained by Kramers-Kronig analysis of the reflectance. The absence of a Drude peak at low frequency is consistent with incoherent conductivity but in apparent contradiction to the metallic temperature dependence of the DC resistivity. We set an upper limit to the interplane transfer integral of tb = 0.1 meV. A model of defect-assisted interplane transport can account for this discrepancy. We also assign the phonon lines in the conductivity to the asymmetric modes of the ET molecule., Comment: 7 pages with embedded figures, submitted to PRB

We show that a central $1/r^n$ singular potential (with $n\geq 2$) is renormalized by a one-parameter square-well counterterm; low-energy observables are made independent of the square-well width by adjusting the square-well strength. We find a closed form expression for the renormalization-group evolution of the square-well counterterm., Comment: 15 pages LaTex, 5 eps figures, error in figures and text corrected

The ab-plane optical spectra of one underdoped and one nearly optimally doped single crystal of HgBa_2Ca_2Cu_3O_{8+delta} were investigated in the frequency range from 40 to 40,000 cm^-1. The frequency dependent scattering rate was obtained by Kramers Kronig analysis of the reflectance. Both crystals have a scattering rate gap of about 1000 cm^-1 which is much larger than the 700 cm^-1 gap seen in optical studies of several cuprates with maximum Tc around 93 K. There appears to be a universal scaling between scattering rate gap and maximum Tc for the cuprate superconductors., Comment: 4 pages with 5 embedded figures. Submitted to PRL