Showing papers on "Absorption (logic) published in 2012"

Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides

Abstract: Quasiparticle band structures and optical properties of MoS${}_{2}$, MoSe${}_{2}$, MoTe${}_{2}$, WS${}_{2}$, and WSe${}_{2}$ monolayers are studied using the GW approximation in conjunction with the Bethe-Salpeter equation (BSE). The inclusion of two-particle excitations in the BSE approach reveals the presence of two strongly bound excitons ($A$ and $B$) below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band at the $K$ point of the Brillouin zone. The transition energies for monolayer MoS${}_{2}$, in particular, are shown to be in excellent agreement with available absorption and photoluminescence measurements. Excitation energies for the remaining monolayers are predicted to lie in the range of 1--2 eV. Systematic trends are identified for quasiparticle band gaps, transition energies, and exciton binding energies within as well as across the Mo and W families of dichalcogenides. Overall, the results suggest that quantum confinement of carriers within monolayers can be exploited in conjunction with chemical composition to tune the optoelectronic properties of layered transition-metal dichalcogenides at the nanoscale.

The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still Be Missing

Abstract: Although galaxies, groups, and clusters contain {approx}10% of the baryons, many more reside in the photoionized and shocked-heated intergalactic medium (IGM) and in the circumgalactic medium (CGM). We update the baryon census in the (H I) Ly{alpha} forest and warm-hot IGM (WHIM) at 10{sup 5-6} K traced by O VI {lambda}1032, 1038 absorption. From Enzo cosmological simulations of heating, cooling, and metal transport, we improve the H I and O VI baryon surveys using spatially averaged corrections for metallicity (Z/Z {sub Sun }) and ionization fractions (f {sub HI}, f {sub OVI}). Statistically, the O VI correction product correlates with column density, (Z/Z {sub Sun })f {sub OVI} Almost-Equal-To (0.015)(N {sub OVI}/10{sup 14} cm{sup -2}){sup 0.70}, with an N {sub OVI}-weighted mean of 0.01, which doubles previous estimates of WHIM baryon content. We also update the Ly{alpha} forest contribution to baryon density out to z = 0.4, correcting for the (1 + z){sup 3} increase in absorber density, the (1 + z){sup 4.4} rise in photoionizing background, and cosmological proper length dl/dz. We find substantial baryon fractions in the photoionized Ly{alpha} forest (28% {+-} 11%) and WHIM traced by O VI and broad-Ly{alpha} absorbers (25% {+-} 8%). The collapsed phasemore » (galaxies, groups, clusters, CGM) contains 18% {+-} 4%, leaving an apparent baryon shortfall of 29% {+-} 13%. Our simulations suggest that {approx}15% reside in hotter WHIM (T {>=} 10{sup 6} K). Additional baryons could be detected in weaker Ly{alpha} and O VI absorbers. Further progress requires higher-precision baryon surveys of weak absorbers, down to minimum column densities N {sub HI} {>=} 10{sup 12.0} cm{sup -2}, N {sub OVI} {>=} 10{sup 12.5} cm{sup -2}, N {sub OVII} {>=} 10{sup 14.5} cm{sup -2}, using high signal-to-noise data from high-resolution UV and X-ray spectrographs.« less

Dense electron-positron plasmas and ultraintense γ rays from laser-irradiated solids.

Abstract: In simulations of a 10 PW laser striking a solid, we demonstrate the possibility of producing a pure electron-positron plasma by the same processes as those thought to operate in high-energy astrophysical environments. A maximum positron density of ${10}^{26}\text{ }\text{ }{\mathrm{m}}^{\ensuremath{-}3}$ can be achieved, 7 orders of magnitude greater than achieved in previous experiments. Additionally, $35%$ of the laser energy is converted to a burst of $\ensuremath{\gamma}$ rays of intensity ${10}^{22}\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$, potentially the most intense $\ensuremath{\gamma}$-ray source available in the laboratory. This absorption results in a strong feedback between both pair and $\ensuremath{\gamma}$-ray production and classical plasma physics in the new ``QED-plasma'' regime.

Quantum absorption refrigerator.

Abstract: A quantum absorption refrigerator driven by noise is studied with the purpose of determining the limitations of cooling to absolute zero. The model consists of a working medium coupled simultaneously to hot, cold, and noise baths. Explicit expressions for the cooling power are obtained for Gaussian and Poisson white noise. The quantum model is consistent with the first and second laws of thermodynamics. The third law is quantified; the cooling power ${\mathcal{J}}_{c}$ vanishes as ${\mathcal{J}}_{c}\ensuremath{\propto}{T}_{c}^{\ensuremath{\alpha}}$, when ${T}_{c}\ensuremath{\rightarrow}0$, where $\ensuremath{\alpha}=d+1$ for dissipation by emission and absorption of quanta described by a linear coupling to a thermal bosonic field, where $d$ is the dimension of the bath.

All-optical magnetization reversal by circularly polarized laser pulses: Experiment and multiscale modeling

Abstract: We present results of detailed experimental and theoretical studies of all-optical magnetization reversal by single circularly-polarized laser pulses in ferrimagnetic rare earth---transition metal (RE--TM) alloys Gd${}_{x}$Fe${}_{90\ensuremath{-}x}$Co${}_{10}$ ($20%lxl28%$). Using single-shot time-resolved magneto-optical microscopy and multiscale simulations, we identified and described the unconventional path followed by the magnetization during the reversal process. This reversal does not involve precessional motion of magnetization but is governed by the longitudinal relaxation and thus has a linear character. We demonstrate that this all-optically driven linear reversal can be modeled as a result of a two-fold impact of the laser pulse on the medium. First, due to absorption of the light and ultrafast laser-induced heating, the medium is brought to a highly nonequilibrium state. Simultaneously, due to the ultrafast inverse Faraday effect the circularly polarized laser pulse acts as an effective magnetic field of the amplitude up to $\ensuremath{\sim}$20 T. We show that the polarization-dependent reversal triggered by the circularly polarized light is feasible only in a narrow range (below 10%) of laser fluences. The duration of the laser pulse required for the reversal can be varied from $\ensuremath{\sim}$40 fs up to at least $\ensuremath{\sim}$1700 fs. We also investigate experimentally the role of the ferrimagnetic properties of GdFeCo in the all-optical reversal. In particular, the optimal conditions for the all-optical reversal are achieved just below the ferrimagnetic compensation temperature, where the magnetic information can be all-optically written by a laser pulse of minimal fluence and read out within just 30 ps. We argue that this is the fastest write-read event demonstrated for magnetic recording so far.

Time-dependent density functional theory for strong electromagnetic fields in crystalline solids

Abstract: We apply the coupled dynamics of time-dependent density functional theory and Maxwell equations to the interaction of intense laser pulses with crystalline silicon. As a function of electromagnetic field intensity, we see several regions in the response. At the lowest intensities, the pulse is reflected and transmitted in accord with the dielectric response, and the characteristics of the energy deposition are consistent with two-photon absorption. The absorption process begins to deviate from that at laser intensities of $\ensuremath{\sim}$${10}^{13}$ W/cm${}^{2}$, where the energy deposited is of the order of 1 eV per atom. Changes in the reflectivity are seen as a function of intensity. When it passes a threshold of about $3\ifmmode\times\else\texttimes\fi{}{10}^{12}$ W/cm${}^{2}$, there is a small decrease. At higher intensities, above $2\ifmmode\times\else\texttimes\fi{}{10}^{13}$ W/cm${}^{2}$, the reflectivity increases strongly. This behavior can be understood qualitatively in a model treating the excited electron-hole pairs as a plasma.

Coherent perfect absorption in epsilon-near-zero metamaterials

Abstract: In conventional materials, strong absorption usually requires that the material have either high loss or a large thickness-to-wavelength ratio ($d/\ensuremath{\lambda}\ensuremath{\gg}1$). We find the situation to be vastly different for bilayer structures composed of a metallic substrate and an anisotropic epsilon-near-zero (ENZ) metamaterial, where the permittivity in the direction perpendicular to its surface, ${\ensuremath{\epsilon}}_{z}$, vanishes. Remarkably, perfect absorption can occur in situations where the metamaterial is arbitrarily thin ($d/\ensuremath{\lambda}\ensuremath{\ll}1$) and arbitrarily low loss. Our numerical and analytical solutions reveal that under the conditions ${\ensuremath{\epsilon}}_{z}\ensuremath{\rightarrow}0$ and $\ensuremath{\mathfrak{I}}({\ensuremath{\epsilon}}_{z})\ensuremath{\gg}\ensuremath{\mathfrak{R}}({\ensuremath{\epsilon}}_{z})$, at perfect absorption there is a linear relationship between the thickness and the loss, which means the thickness of the absorber can be pushed to zero by reducing the material loss to zero. This counterintuitive behavior is explained in terms of coherent perfect absorption (or stimulated absorption) via critical coupling to a fast wave propagating along the ENZ layer.

Strict comparison and Z-absorption of nuclear C∗-algebras

Abstract: For any unital separable simple infinite-dimensional nuclear C∗-algebra with finitely many extremal traces, we prove that \( \mathcal{Z} \)-absorption, strict comparison and property (SI) are equivalent. We also show that any unital separable simple nuclear C∗-algebra with tracial rank zero is approximately divisible, and hence is \( \mathcal{Z} \)-absorbing.

Neutrino spectra evolution during protoneutron star deleptonization

Abstract: The neutrino-driven wind, which occurs after the onset of a core-collapse supernova explosion, has long been considered as the possible site for the synthesis of heavy $r$-process elements in the Universe. Only recently, it has been possible to simulate supernova explosions up to $\ensuremath{\sim}10\text{ }\text{ }\mathrm{seconds}$, based on three-flavor Boltzmann neutrino transport. These simulations show that the neutrino luminosities and spectra of all flavors are very similar and their difference even decreases during the deleptonization of the proto-neutron star. As a consequence, the ejecta are always proton rich which rules out the possible production of heavy $r$-process elements ($Zg56$). We perform a detailed analysis of the different weak processes that determine the neutrino spectra. Nonelectron flavor (anti)neutrinos are produced and interact only via neutral-current processes, while electron (anti)neutrinos have additional contributions from charge-current processes. The latter are dominated by ${\ensuremath{ u}}_{e}$-absorption on neutrons and ${\overline{\ensuremath{ u}}}_{e}$-absorption on protons. At early times, charge-current processes are responsible for spectral differences between ${\ensuremath{ u}}_{e}$, ${\overline{\ensuremath{ u}}}_{e}$ and ${\ensuremath{ u}}_{\ensuremath{\mu}/\ensuremath{\tau}}$. However, as the region of neutrino decoupling moves to higher densities during deleptonization, charge-current reactions are suppressed by final state Pauli blocking. ${\overline{\ensuremath{ u}}}_{e}$ absorption on protons is suppressed due to the continuously increasing chemical potential of the neutrons. ${\ensuremath{ u}}_{e}$ absorption on neutrons is blocked by the increasing degeneracy of the electrons. These effects result in negligible contributions from charge-current reactions on time scales on the order of tens of seconds, depending on the progenitor star. Hence, the neutrino spectra are mainly determined from neutral-current processes which do not distinguish between the different flavors and result in the convergence of the spectra. These findings are independent of the charge-current reaction rates used. It rules out the possibility of neutron-rich ejecta at late times and the production of heavy $r$-process elements from nonrotating and not magnetized proto-neutron stars.

Search for Cosmic Neutrino Point Sources with Four Years of Data from the ANTARES Telescope

Abstract: In this paper, a time integrated search for point sources of cosmic neutrinos is presented using the data collected from 2007 to 2010 by the ANTARES neutrino telescope. No statistically significant signal has been found and upper limits on the neutrino flux have been obtained. Assuming an $E_{ u}^{-2}$ spectrum, these flux limits are at $1-10\times10^{-8}$ GeV cm$^{-2}$ s$^{-1}$ for declinations ranging from $-90^{\circ}$ to 40$^{\circ}$. Limits for specific models of RX J1713.7-3946 and Vela X, which include information on the source morphology and spectrum, are also given.

Pressure effects on the electronic and optical properties of A WO 4 wolframites ( A = Cd, Mg, Mn, and Zn): The distinctive behavior of multiferroic MnWO 4

Abstract: The electronic band-structure and band-gap dependence on the $d$ character of ${A}^{2+}$ cation in $A$WO${}_{4}$ wolframite-type oxides is investigated for different compounds ($A$ $=$ Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled $d$ levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled $d$ electron shells, give rise to direct and wide band gaps, Mn, with a half-filled $d$ shell, is found to have an indirect band gap that is more than 1.6 eV smaller than for the other wolframites. In addition, the band gaps of MgWO${}_{4}$, ZnWO${}_{4}$, and CdWO${}_{4}$ blue-shift linearly with pressure, with a pressure coefficient of approximately 13 eV/GPa. However, the band gap of multiferroic MnWO${}_{4}$ red-shifts at \ensuremath{-}22 meV/GPa. Finally, in MnWO${}_{4}$, absorption bands are observed at lower energy than the band gap and followed with pressure based on the Tanabe-Sugano diagram. This study allows us to estimate the crystal-field variation with pressure for the MnO${}_{6}$ complexes and how it affects their band-gap closure.

Ultrafast x-ray scattering of xenon nanoparticles: imaging transient states of matter.

Abstract: Femtosecond x-ray laser flashes with power densities of up to ${10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ at 13.7 nm wavelength were scattered by single xenon clusters in the gas phase. Similar to light scattering from atmospheric microparticles, the x-ray diffraction patterns carry information about the optical constants of the objects. However, the high flux of the x-ray laser induces severe transient changes of the electronic configuration, resulting in a tenfold increase of absorption in the developing nanoplasma. The modification in opaqueness can be correlated to strong atomic charging of the particle leading to excitation of ${\mathrm{Xe}}^{4+}$. It is shown that single-shot single-particle scattering on femtosecond time scales yields insight into ultrafast processes in highly excited systems where conventional spectroscopy techniques are inherently blind.

High signal-to-noise ratio laser technique for accurate measurements of spectral line parameters

Abstract: An experiment enabling extremely high signal-to-noise ratios in the measurement of spectral line shapes is described. This approach, which combines high-bandwidth locking of a continuous wave probe laser and the frequency-stabilized cavity ring-down spectroscopy technique, enables long-term signal averaging and yields high-resolution spectra with a relatively wide dynamic range and low detection limit. By probing rovibronic transitions of the ${}^{16}$O${}_{2}$ $B$ band near $\ensuremath{\lambda}=689$ nm, exceptionally precise measurements of absorption line shape and line position are demonstrated. A signal-to-noise ratio of $220\phantom{\rule{0.16em}{0ex}}000$ and a minimum detectable absorption coefficient of $2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ is reported, which corresponds to the lowest line intensity measurable by this setup of approximately $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}/(\mathrm{molecule}$ ${\mathrm{cm}}^{\ensuremath{-}2})$. Careful analysis of the data revealed a subtle line-shape asymmetry that could be explained by the speed dependence of the collisional shift. The demonstrated measurement precision enables the quantification of systematic line-shape deviations, which were approximately 1 part in 80 000 of the peak absorption. The influence of slowly drifting etaloning effects on the precision of the line-shape analysis is discussed. How this method can enable experiments that address a number of fundamental physical problems including the accurate optical measurement of the Boltzmann constant and tests of the symmetrization postulate is also discussed.

Optical Conductivity of Bismuth-Based Topological Insulators

Abstract: The optical conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ and the spectral weight $SW$ of four topological insulators with increasing chemical compensation (${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3},\phantom{\rule{0.28em}{0ex}}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{2}\mathrm{Te},\phantom{\rule{0.28em}{0ex}}{\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Se}}_{3}$, and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$) have been measured from 5 to 300 K and from subterahertz to visible frequencies. The effect of compensation is clearly observed in the infrared spectra through the suppression of an extrinsic Drude term and the appearance of strong absorption peaks that we assign to electronic transitions among localized states. From the far-infrared spectral weight $SW$ of the most compensated sample (${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$), one can estimate a density of charge carriers on the order of ${10}^{17}/{\mathrm{cm}}^{3}$ in good agreement with transport data. Those results demonstrate that the low-energy electrodynamics in single crystals of topological insulators, even at the highest degree of compensation presently achieved, is still influenced by three-dimensional charge excitations.

High-signal-to-noise-ratio laser technique for accurate measurements of spectral line parameters

Abstract: An experiment enabling extremely high signal-to-noise ratios in the measurement of spectral line shapes is described. This approach, which combines high-bandwidth locking of a continuous wave probe laser and the frequency-stabilized cavity ring-down spectroscopy technique, enables long-term signal averaging and yields high-resolution spectra with a relatively wide dynamic range and low detection limit. By probing rovibronic transitions of the ${}^{16}$O${}_{2}$ $B$ band near $\ensuremath{\lambda}=689$ nm, exceptionally precise measurements of absorption line shape and line position are demonstrated. A signal-to-noise ratio of $220\phantom{\rule{0.16em}{0ex}}000$ and a minimum detectable absorption coefficient of $2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ is reported, which corresponds to the lowest line intensity measurable by this setup of approximately $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}/(\mathrm{molecule}$ ${\mathrm{cm}}^{\ensuremath{-}2})$. Careful analysis of the data revealed a subtle line-shape asymmetry that could be explained by the speed dependence of the collisional shift. The demonstrated measurement precision enables the quantification of systematic line-shape deviations, which were approximately 1 part in 80 000 of the peak absorption. The influence of slowly drifting etaloning effects on the precision of the line-shape analysis is discussed. How this method can enable experiments that address a number of fundamental physical problems including the accurate optical measurement of the Boltzmann constant and tests of the symmetrization postulate is also discussed.

Cation off-stoichiometry leads to high p -type conductivity and enhanced transparency in Co 2 ZnO 4 and Co 2 NiO 4 thin films

Abstract: We explore the effects of cation off-stoichiometry on structural, electrical, optical, and electronic properties of Co${}_{2}$ZnO${}_{4}$ normal spinel and Co${}_{2}$NiO${}_{4}$ inverse spinel using theoretic and experimental (combinatorial and conventional) techniques, both at thermodynamic equilibrium and in the metastable regime. Theory predicts that nonequilibrium substitution of divalent Zn on nominally trivalent octahedral sites increases net hole density in Co${}_{2}$ZnO${}_{4}$. Experiment confirms high conductivity and high work function in Co${}_{2}$NiO${}_{4}$ and Zn-rich Co${}_{2}$ZnO${}_{4}$ thin films grown by nonequilibrium physical vapor deposition techniques. High $p$-type conductivities of Co${}_{2}$ZnO${}_{4}$ (up to 5 S/cm) and Co${}_{2}$NiO${}_{4}$ (up to 204 S/cm) are found over a broad compositional range, they are only weakly sensitive to oxygen partial pressure and quite tolerant to a wide range of processing temperatures. In addition, off-stoichiometry caused by nonequilibrium growth decreases the optical absorption of Co${}_{2}$ZnO${}_{4}$ and Co${}_{2}$NiO${}_{4}$ thin films, although the 500-nm thin films still have rather limited transparency. All these properties as well as high work functions make Co${}_{2}$ZnO${}_{4}$ and Co${}_{2}$NiO${}_{4}$ thin films attractive for technological applications, such as hole transport layers in organic photovoltaic devices or $p$-type buffer layers in inorganic solar cells.

Generating energetic electrons through staged acceleration in the two-plasmon-decay instability in inertial confinement fusion

Abstract: A new hot-electron generation mechanism in two-plasmon-decay instabilities is described based on a series of 2D, long-term ($\ensuremath{\sim}10\text{ }\text{ }\mathrm{ps}$) particle-in-cell and fluid simulations under parameters relevant to inertial confinement fusion. The simulations show that significant laser absorption and hot-electron generation occur in the nonlinear stage. The hot electrons are stage accelerated from the low-density region to the high-density region. New modes with small phase velocities develop in the low-density region in the nonlinear stage and form the first stage for electron acceleration. Electron-ion collisions are shown to significantly reduce the efficiency of this acceleration mechanism.

Fingerprints of order and disorder in the electronic and optical properties of crystalline and amorphous TiO 2

Abstract: We have investigated the structural and electronic properties as well as the linear optical response of amorphous TiO${}_{2}$ within density functional theory and a numerically efficient density functional based tight-binding approach as well as many-body perturbation theory. The disordered TiO${}_{2}$ phase is modeled by molecular dynamics. The equivalence to experimentally characterized amorphous phases is demonstrated by atomic structure factors and radial pair-distribution functions. By density functional theory calculations, using both the semilocal Perdew-Burke-Ernzerhof functional and the nonlocal Heyd-Scuseria-Ernzerhof screened hybrid functional, the electronic energy gap is found to be larger than in the crystalline TiO${}_{2}$ phases rutile and brookite but close to the anatase band gap. The quasiparticle energy gap of amorphous TiO${}_{2}$ is determined to be $\ensuremath{\gtrsim}$3.7 eV, while the optical gap is estimated to $\ensuremath{\lesssim}$3.5 eV. The disorder-induced formation of localized electronic states has been analyzed by the information entropy of the charge density distributions. The frequency-dependent optical constants, calculated from the complex dielectric function, have been determined in independent particle approximation. Besides similar absorption characteristics between the most common crystalline phases and amorphous TiO${}_{2}$, we find distinct differences in the optical spectra in the energy region between 5 eV and 8 eV. These differences can be assigned to the loss of symmetry in the local atomic structure of the disordered material. While the composition of the crystalline phases rutile, anatase, and brookite is well described by periodic arrangements of distorted TiO${}_{6}$ octahedra building blocks, the amorphous phase is characterized by partial loss of this octahedral coordination and the disorder-induced formation of under- and over-coordinated Ti ions. This leads to the absence of the characteristic crystal-field splitting of unoccupied Ti${}_{3d}$ states into ${e}_{g}$ and ${t}_{2g}$ like subbands. The optical characteristics of the amorphous phase are interpreted as a superposition of optical transitions that reflect the various local symmetries of the manifold of synthesizable crystalline TiO${}_{2}$ phases. The linear optical properties, calculated within the independent-particle approximation, are found to be in good agreement with the available experimental data.

Fully ordered Sr 2 CrReO 6 epitaxial films: A high-temperature ferrimagnetic semiconductor

Abstract: We report growth of Sr${}_{2}$CrReO${}_{6}$ epitaxial films with 99$%$ Cr/Re ordering and crystalline perfection comparable to those of high-quality semiconductor films. The Sr${}_{2}$CrReO${}_{6}$ films show a Curie temperature of 508 K and a saturation magnetization of 1.29 ${\ensuremath{\mu}}_{\mathrm{B}}$ per formula unit, which confirms the presence of strong spin-orbit coupling. Unexpectedly, electrical transport and optical absorption measurements indicate that Sr${}_{2}$CrReO${}_{6}$ is a semiconductor with a band gap of 0.21 eV. The unique combination of high temperature ferrimagnetism and semiconductivity is both scientifically interesting and highly attractive for applications.

Resonant Kα spectroscopy of solid-density aluminum plasmas.

Abstract: The x-ray intensities made available by x-ray free electron lasers (FEL) open up new x-ray matter interaction channels not accessible with previous sources. We report here on the resonant generation of $K\ensuremath{\alpha}$ emission, that is to say the production of copious $K\ensuremath{\alpha}$ radiation by tuning the x-ray FEL pulse to photon energies below that of the $K$ edge of a solid aluminum sample. The sequential absorption of multiple photons in the same atom during the 80 fs pulse, with photons creating $L$-shell holes and then one resonantly exciting a $K$-shell electron into one of these holes, opens up a channel for the $K\ensuremath{\alpha}$ production, as well as the absorption of further photons. We demonstrate rich spectra of such channels, and investigate the emission produced by tuning the FEL energy to the $K\mathrm{\text{\ensuremath{-}}}L$ transitions of those highly charged ions that have transition energies below the $K$ edge of the cold material. The spectra are sensitive to x-ray intensity dependent opacity effects, with ions containing $L$-shell holes readily reabsorbing the $K\ensuremath{\alpha}$ radiation.

Optical properties of cubic GaN from 1 to 20 eV

Abstract: We present a comprehensive overview of the optical properties of zinc-blende GaN. By a variety of different methods, such as temperature-dependent photoluminescence, photoluminescence excitation spectroscopy, photoreflectance, and ellipsometry, we investigate its emission and absorption related characteristics. The sample under study is a nearly strain-free epitaxial layer grown on freestanding cubic SiC. The light-hole/heavy-hole exciton was found at $3.271\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ at $5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, shifting to $3.208\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ at $295\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The split-off exciton transition was detected to be $21\phantom{\rule{0.222222em}{0ex}}\mathrm{meV}$ higher in energy. Taking the difference in the exciton binding energies into account, this yields a spin-orbit energy of ${\ensuremath{\Delta}}_{\mathrm{so}}=15\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{0.222222em}{0ex}}\mathrm{meV}$. Donor and acceptor binding energies could be estimated by photoluminescence to be 30 and $130\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, respectively. By synchrotron-based spectroscopic ellipsometry the complex dielectric function up to an energy of $20\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ could be determined. Comparison with ab initio calculations allows an assignment of high-energy features to the peculiarities of the band structure.

Effect of pressure on the band gap and the local FeO 6 environment in BiFeO 3

Abstract: BiFeO${}_{3}$ exhibits a complex phase-transition sequence under pressure associated with changes in octahedron tilts and displacements of Bi${}^{3+}$ and Fe${}^{3+}$ cations. Here, we investigate the local structure of Fe${}^{3+}$ as a function of pressure through absorption crystal-field spectroscopy in the 0--18 GPa range. We focus on the influence of phase transitions on the Fe${}^{3+}$ off-center displacement through the energy ($E$) and oscillator strength (${f}_{d\ensuremath{-}d}$) of the ${}^{4}$T${}_{1}$ and ${}^{4}$T${}_{2}$ Fe${}^{3+}$ (3${d}^{5}$) bands observed below the band gap (${E}_{\mathrm{gap}}$ $=$ 2.49 eV) at 1.39 and 1.92 eV, respectively, at ambient conditions. Pressure induces linear redshift of both ${}^{4}$T${}_{1}$ and ${}^{4}$T${}_{2}$ bands, consistent with the compression of the FeO${}_{6}$ octahedron under pressure. On the other hand, the transition oscillator strength (${f}_{d\ensuremath{-}d}=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$), enabled by both the exchange mechanism and the off-center Fe${}^{3+}$ distortion, slightly increases with pressure. The absence of notable anomalies in the variation of $E$($P$) and ${f}_{d\ensuremath{-}d}$($P$) through the phase sequence from the ferroelectric rhombohedral $R$3$c$ phase to the nonpolar orthorhombic Pnma phase suggests a persisting off-center position of the Fe${}^{3+}$. While this local polarity is correlated and expected in the ferroelectric $R$3$c$ phase, its presence in the high-pressure nonpolar Pnma phase indicates the presence of local polar instabilities.

Nuclear bound state of η ′ ( 958 ) and partial restoration of chiral symmetry in the η ′ mass

Abstract: The in-medium mass of the ${\ensuremath{\eta}}^{\ensuremath{'}}$ meson is discussed in a context of partial restoration of chiral symmetry in nuclear medium. The ${\ensuremath{\eta}}^{\ensuremath{'}}$ mass is expected to be reduced by an order of 100 MeV at the saturation density. The reduction is a consequence of the suppression of the anomaly effect on the ${\ensuremath{\eta}}^{\ensuremath{'}}$ mass induced by partial restoration of chiral symmetry. This strong attraction in ${\ensuremath{\eta}}^{\ensuremath{'}}$ nuclear systems does not accompany large absorption of ${\ensuremath{\eta}}^{\ensuremath{'}}$ into nuclear matter. This leads to the possibility of so narrow bound states of the ${\ensuremath{\eta}}^{\ensuremath{'}}$ meson in nuclei to be seen in hadronic reactions with light nuclear targets.

Electronic structure and optical, mechanical, and transport properties of the pure, electron-doped, and hole-doped Heusler compound CoTiSb

Abstract: The Heusler compound CoTiSb was synthesized and investigated theoretically and experimentally with respect to electronic structure and optical, mechanical, and vibrational properties. The optical properties were investigated in a wide spectral range from 10 meV to 6.5 eV and compared with ab initio calculations. The optical spectra confirm the semiconducting nature of CoTiSb, with a strong exciton absorption at 1.83 eV. The calculated phonon dispersion as well as elastic constants verify the mechanical stability of CoTiSb in the cubic $C{1}_{b}$ system. Furthermore, solid solution series of CoTi${}_{1\ensuremath{-}x}$${M}_{x}$Sb ($M=\text{Sc}$, V and $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.2$) were synthesized and investigated. The transport properties were calculated by all-electron ab initio methods and compared to the measurements. The thermoelectric properties were investigated by measuring the temperature dependence of electrical resistivity, Seebeck coefficient, and thermal conductivity. The thermal conductivity of the substituted compounds was significantly reduced. Sc substitution resulted in a $p$-type behavior with a high Seebeck coefficient of +177.8 $\ensuremath{\mu}$V/K (350 K) at 5$%$ Sc substitution. This value is in good agreement with the calculations. Fully relativistic Korringa--Kohn--Rostoker calculations in combination with the coherent potential approximation clarify the different contribution of states in the (001) plane of the Fermi surface for Sc- or V-substituted compounds CoTi${}_{0.95}$${M}_{x}$Sb ($M=\text{Sc}$, V).

Constraining fundamental constant evolution with HI and OH lines

Abstract: We report deep Green Bank Telescope spectroscopy in the redshifted HI 21cm and OH 18cm lines from the $z = 0.765$ absorption system towards PMN J0134-0931. A comparison between the "satellite" OH 18cm line redshifts, or between the redshifts of the HI 21cm and "main" OH 18cm lines, is sensitive to changes in different combinations of three fundamental constants, the fine structure constant $\alpha$, the proton-electron mass ratio $\mu \equiv m_p/m_e$ and the proton g-factor $g_p$. We find that the satellite OH 18cm lines are not perfectly conjugate, with both different line shapes and stronger 1612 MHz absorption than 1720 MHz emission. This implies that the satellite lines of this absorber are not suitable to probe fundamental constant evolution. A comparison between the redshifts of the HI 21cm and OH 18cm lines, via a multi-Gaussian fit, yields the strong constraint $[\Delta F/F] = [-5.2 \pm 4.3] \times 10^{-6}$, where $F \equiv g_p [\mu \alpha^2]^{1.57}$ and the error budget includes contributions from both statistical and systematic errors. We thus find no evidence for a change in the constants between $z = 0.765$ and the present epoch. Incorporating the constraint $[\Delta \mu/\mu ] < 3.6 \times 10^{-7}$ from another absorber at a similar redshift and assuming that fractional changes in $g_p$ are much smaller than those in $\alpha$, we obtain $[\Delta \alpha/\alpha ] = (-1.7 \pm 1.4) \times 10^{-6}$ over a lookback time of 6.7 Gyrs.

Structural, electrical, and optical properties of hydrogen-doped ZnO films

Abstract: Hydrogen doped ZnO thin films were deposited by radio frequency magnetron sputtering from a ceramic target on $c$-plane sapphire and fused silica using H${}_{2}$ and O${}_{2}$ as reactive gases. Structural analysis revealed that all films are polycrystalline with the $c$ axis oriented perpendicularly to the substrate surface. The lateral grain size was strongly affected by the oxygen content of the sputtering gas and decreased dramatically above a critical content of 4.5 $%$. We were able to adjust the carrier density of the films by the deposition parameters to any value between 10${}^{14}$ and 2 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{20}$ cm${}^{\ensuremath{-}3}$. Using temperature-dependent Hall-effect measurements we identified thermionic emission over Coulomb-barriers created by surface trap states at the grain boundaries and tunneling effects to dominate the carrier transport. Preparing and thoroughly characterizing the films is a prerequisite for our investigation of the dependence of the optical band gap energy on the carrier density. We use results from experiment as well as first-principles calculations (including Burstein-Moss shift, band gap renormalization, and excitonic effects) in order to understand the mechanisms that determine how free electrons influence the energy position of the optical absorption onset.

Condensates and quasiparticles in inflationary cosmology: Mass generation and decay widths

Abstract: During de Sitter inflation massless particles of minimally coupled scalar fields acquire a mass and a decay width thereby becoming quasiparticles. For bare massless particles nonperturbative infrared radiative corrections lead to a self-consistent generation of mass, for a quartic self-interaction $M\ensuremath{\propto}{\ensuremath{\lambda}}^{1/4}H$, and for a cubic self-interaction the mass is induced by the formation of a nonperturbative condensate leading to $M\ensuremath{\propto}{\ensuremath{\lambda}}^{1/3}{H}^{2/3}$. These radiatively generated masses restore de Sitter invariance and result in anomalous scaling dimensions of superhorizon fluctuations. We introduce a generalization of the nonperturbative Wigner-Weisskopf method to obtain the time evolution of quantum states that include the self-consistent generation of mass and regulate the infrared behavior. The infrared divergences are manifest as poles in $\ensuremath{\Delta}={M}^{2}/3{H}^{2}$ in the single particle self-energies, leading to a rearrangement of the perturbative series nonanalytic in the couplings. A set of simple rules that yield the leading order infrared contributions to the decay width are obtained and implemented. The lack of kinematic thresholds entail that all particle states acquire a decay width, dominated by the emission and absorption of superhorizon quanta $\ensuremath{\propto}(\ensuremath{\lambda}/H{)}^{4/3}[H/{k}_{ph}(\ensuremath{\eta}){]}^{6}$; $\ensuremath{\lambda}[H/{k}_{ph}(\ensuremath{\eta}){]}^{6}$ for cubic and quartic couplings respectively to leading order in $M/H$. The decay of single particle quantum states hastens as their wave vectors cross the Hubble radius and their width is related to the highly squeezed limit of the bi- or trispectrum of scalar fluctuations respectively.

Power-law dependence of the optical conductivity observed in the quantum spin-liquid compound κ-(BEDT-TTF) 2 Cu 2 (CN) 3

Abstract: The Mott-insulator $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$ is the prime candidate of a quantum spin-liquid with puzzling magnetic properties. Our terahertz and infrared investigations reveal that also the charge dynamics does not follow the expectations for a Mott insulator. We observe a large in-gap absorption where the excess conductivity exhibits a power-law behavior ${\ensuremath{\sigma}}_{1}^{\mathrm{exc}}(\ensuremath{\omega})\ensuremath{\propto}{\ensuremath{\omega}}^{n}$ that grows stronger as the temperature decreases and extends all the way through the far infrared. With $n\ensuremath{\approx}0.8$ to 1.5, the exponent is significantly smaller than predicted by Ng and Lee [T.-K. Ng and P. A. Lee, Phys. Rev. Lett. 99, 156402 (2007)] for spinon contributions to the optical conductivity. We suggest fluctuations become important in the spin-liquid state and couple to the electrodynamic properties differently compared to the antiferromagnetic Mott insulator $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu[N(CN)${}_{2}$]Cl. We discuss the various possibilities of how charge fluctuations are influenced by the presence or absence of magnetic order.

Charging of Heated Colloidal Particles Using the Electrolyte Seebeck Effect

Abstract: We propose a novel actuation mechanism for colloids, which is based on the Seebeck effect of the electrolyte solution: Laser heating of a nonionic particle accumulates in its vicinity a net charge $Q$, which is proportional to the excess temperature at the particle surface The corresponding long-range thermoelectric field $E\ensuremath{\propto}1/{r}^{2}$ provides a tool for controlled interactions with nearby beads or with additional molecular solutes An external field ${E}_{\mathrm{ext}}$ drags the thermocharged particle at a velocity that depends on its size and absorption properties; the latter point could be particularly relevant for separating carbon nanotubes according to their electronic band structure

Cores in Infra-Red Dark Clouds (IRDCs) seen in the Hi-GAL survey between l = 300{\deg} and l = 330{\deg}

Abstract: We have used data taken as part of the Herschel infrared Galactic Plane survey (Hi-GAL) to study 3171 infrared-dark cloud (IRDC) candidates that were identified in the mid-infrared (8 {\mu}m) by Spitzer (we refer to these as 'Spitzer-dark' regions). They all lie in the range l=300 - 330 \circ and |b| 6 1 \circ. Of these, only 1205 were seen in emission in the far-infrared (250-500 {\mu}m) by Herschel (we call these 'Herschel-bright' clouds). It is predicted that a dense cloud will not only be seen in absorption in the mid-infrared, but will also be seen in emission in the far-infrared at the longest Herschel wavebands (250-500 {\mu}m). If a region is dark at all wavelengths throughout the mid-infrared and far-infrared, then it is most likely to be simply a region of lower background infrared emission (a 'hole in the sky'). Hence, it appears that previous surveys, based on Spitzer and other mid-infrared data alone, may have over-estimated the total IRDC population by a factor of 2. This has implications for estimates of the star formation rate in IRDCs in the Galaxy.We studied the 1205 Herschel-bright IRDCs at 250 {\mu}m, and found that 972 of them had at least one clearly defined 250-{\mu}m peak, indicating that they contained one or more dense cores. Of these, 653 (67 per cent) contained an 8-{\mu}m point source somewhere within the cloud, 149 (15 per cent) contained a 24-{\mu}m point source but no 8-{\mu}m source, and 170 (18 per cent) contained no 24-{\mu}m or 8-{\mu}m point sources. We use these statistics to make inferences about the lifetimes of the various evolutionary stages of IRDCs.