Showing papers on "Absorption (logic) published in 1994"
TL;DR: It is shown that one-electron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulomb-correlated model.
Abstract: We show that one-electron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulomb-correlated model. The lowest optical state is an exciton, whose binding energy is estimated theoretically to be 0.90\ifmmode\pm\else\textpm\fi{}0.15 eV. Measurements of the photoconductivity quantum efficiency in poly[2-methoxy,5-(${2}^{\mathcal{'}}$-ethyl-hexyloxy)-1,4 phenylenevinylene] reveal a binding energy in good agreement with the theoretical value.
197 citations
TL;DR: In this article, a quantitative analysis of the reflectivity spectra using a multilayer response function revealed the line positions, linewidths and the oscillator strengths for the charged phonon modes.
Abstract: In situ IR reflectivity was used to study the doping of thin ${\mathrm{C}}_{60}$ films with K and Rb by a load and equilibrate procedure. Nearly pure phases of ${\mathit{A}}_{\mathit{x}}$${\mathrm{C}}_{60}$ for x=1, 3, 4, and 6 were obtained in this way and the IR spectra were characterized. A quantitative analysis of the reflectivity spectra using a multilayer response function revealed the line positions, linewidths and the oscillator strengths for the ${\mathit{F}}_{1\mathit{u}}$ modes. The second-lowest mode and the highest mode showed good agreement with the charged phonon model with respect to line shift and absorption intensity and were thus assigned as charged phonon modes. The coupling constants for these modes to the virtual ${\mathit{t}}_{1\mathit{u}}$\ensuremath{\rightarrow}${\mathit{t}}_{1\mathit{g}}$ transition were found to be of the order 0.01. This value is smaller but not negligible compared to the coupling of the gerade modes with ${\mathit{A}}_{\mathit{g}}$ and ${\mathit{H}}_{\mathit{g}}$ symmetry. Thus, the coupling from the virtual ${\mathit{t}}_{1\mathit{u}}$\ensuremath{\rightarrow}${\mathit{t}}_{1\mathit{g}}$ transition may contribute as well to the pairing process in the superconducting phase.
127 citations
TL;DR: The interactions of intense, p-polarized, femtosecond laser pulses with solid-density plasma targets is studied with particle-in-cell simulation, eventually leading to efficient absorption over a wide range of laser irradiance.
Abstract: The interactions of intense, p-polarized, femtosecond laser pulses with solid-density plasma targets is studied with particle-in-cell simulation. At large angles of incidence the density profile is rapidly modified by a ``push-pull'' combination of ponderomotive force and the space-charge effect of electrons held outside the solid, eventually leading to efficient absorption over a wide range of laser irradiance. For I${\ensuremath{\lambda}}^{2}$g${10}^{17}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ \ensuremath{\mu}${\mathrm{m}}^{2}$, the final absorption rate peaks at 45\ifmmode^\circ\else\textdegree\fi{}--50\ifmmode^\circ\else\textdegree\fi{} and is largely independent of intensity; the average absorption fraction and the shape of the fast electron distribution both depend on the ratio of the pulse length to the ion shelf expansion time ${\mathit{t}}_{\mathit{s}}$\ensuremath{\propto}${\mathrm{\ensuremath{\omega}}}_{0}^{\mathrm{\ensuremath{-}}1}$(${\mathit{m}}_{\mathit{i}}$/${\mathit{Zm}}_{\mathit{e}}$${)}^{1/2}$.
102 citations
TL;DR: It is shown that the oxygen vacancy in corundum crystals is able also to trap a third electron; experimental evidence for this is discussed and calculated absorption and luminescence energies obtained are in good agreement with experimental data.
Abstract: The semiempirical intermediate neglect of differential overlap method was used for calculating optical properties of ${\mathit{F}}^{+}$ and F centers (oxygen vacancy trapped one and two electrons, respectively) embedded into large quantum clusters, ${\mathrm{Al}}_{26}$${\mathrm{O}}_{39}$. The geometry was optimized for both the ground and excited states of defects. Calculated absorption and luminescence energies obtained for ${\mathit{F}}^{+}$ and F centers are in good agreement with experimental data. Their energy levels lie in the gap between the upper valence band and conduction band, like for similar centers in MgO and alkali halides. It is shown that the oxygen vacancy in corundum crystals is able also to trap a third electron; experimental evidence for this is discussed.
101 citations
TL;DR: The asymmetric line shapes are calculated using a resonance scattering expression, proportional to a free-bound Franck-Condon factor, that is justified by full quantum scattering calculations of a collision in a radiation field.
Abstract: High resolution photoassociation spectra of colliding ultracold trapped atoms contain detailed information about ground and excited state interactions. We calculate the asymmetric line shapes for ultracold ${\mathrm{Na}}_{2}$ absorption using a resonance scattering expression, proportional to a free-bound Franck-Condon factor, that is justified by full quantum scattering calculations of a collision in a radiation field. The line shapes illustrate Wigner threshold law behavior, which is characteristic of the quantum limit as $T\ensuremath{\rightarrow}0$. Using an adiabatic hyperfine analysis, we calculate and compare a model spectra for $J=1,2,3, \mathrm{and} 4$ features of the ${\mathrm{Na}}_{2}$ $^{1}\ensuremath{\Pi}_{g}(\ensuremath{
u}=48)$ state to recent high resolution experimental data.
91 citations
TL;DR: In this article, the authors measured the electrical resistivity, polarized x-ray absorption, and magnetic neutron scattering for a Haldane chain compound with Zn and Ca doping and found that Zn doping introduces holes, residing mainly in the orbital of the oxygens in the NiO chains.
Abstract: We have measured the electrical resistivity, polarized x-ray absorption, and magnetic neutron scattering for ${\mathrm{Y}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Ba}{\mathrm{Ni}}_{1\ensuremath{-}y}{\mathrm{Zn}}_{y}{\mathrm{O}}_{5}$ to determine how doping affects the charge and spin dynamics of a Haldane chain compound. While Zn doping, which severs the NiO chains, increases the resistivity beyond that of the pure material, Ca doping introduces holes, residing mainly in the ${2p}_{z}$, orbital of the oxygens in the NiO chains. Both dopants lead to simple finite size effects above the Haldane gap. In addition, we have discovered that Ca doping yields substantial magnetic states below the Haldane gap.
91 citations
01 Dec 1994
TL;DR: This work has shown that Cs3Lu2Br9:1%Er3+ is an ideal system to study NIR-to-VIS upconversion dynamics and is one of a family of dimer compounds which show intense VIS upconverted luminescence when excited in the NIR.
Abstract: Single crystals of ${\mathrm{Cs}}_{3}$${\mathrm{Lu}}_{2}$${\mathrm{Br}}_{9}$:1%${\mathrm{Er}}^{3+}$ were grown using the Bridgman technique. The low phonon energy of \ensuremath{\le}190 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ strongly suppresses multiphonon relaxation in this host. For a series of excited states radiative relaxation is found to be dominant. As a consequence, ${\mathrm{Cs}}_{3}$${\mathrm{Lu}}_{2}$${\mathrm{Br}}_{9}$:1%${\mathrm{Er}}^{3+}$ shows strong visible upconversion luminescence from $^{2}$${\mathit{H}}_{9/2}$ and $^{4}$${\mathit{F}}_{7/2}$ when excited into the $^{4}$${\mathit{I}}_{9/2}$ and $^{4}$${\mathit{I}}_{11/2}$ multiplets in the near infrared, respectively. The upconversion dynamics for both excitations was studied in detail using time-resolved spectroscopy. Both excited-state absorption (ESA) and energy-transfer upconversion (ETU) were found to be active and could be identified on the basis of the upconversion transients. For the population of the $^{4}$${\mathit{F}}_{7/2}$ and $^{4}$${\mathit{S}}_{3/2}$ states both ESA and ETU can be observed simultaneously. The relative efficiency of ESA and ETU is determined by the energetic resonance of the respective processes. Whereas nonresonant ESA during the 10 ns laser pulse is strongly suppressed for energy mismatches g12 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, ETU is still efficient. An analysis of the upconversion transients shows that energy migration is dominant even at 1% ${\mathrm{Er}}^{3+}$ dotation. The excitations have excitonic character leading to a characteristic prolongation of the decays in the upconversion transients.
81 citations
TL;DR: In this paper, the authors discussed the absorption structure due to conduction electrons in trivalent rare-earth hexaborides (R${\mathrm{B}}_{6}$; R=La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, and Y).
Abstract: Reflectivity spectra of trivalent rare-earth hexaborides (R${\mathrm{B}}_{6}$; R=La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, and Y) and mixed-valent ${\mathrm{SmB}}_{6}$ have been measured in the photon energy region from 1 meV to 40 eV at 300 and 9 K. We discuss the absorption structure due to conduction electrons. Even in ${\mathrm{LaB}}_{6}$ and ${\mathrm{YB}}_{6}$, which have no 4f electrons, the optical conductivity spectra were not fitted by a simple Drude model and a frequency-dependent relaxation time was observed. This was attributed to the electron-phonon and electron-electron scattering. In R${\mathrm{B}}_{6}$ with 4f electrons, a common absorption structure was observed at the energy position of about 0.6 eV. The intensity was found to be almost proportional to the 4f spin angular momentum. From the f-sum rule, the absorption was found to be due to the conduction electrons. The origin was assigned to be due to the transition or to the relevant exciton absorption from the saddle points of ${\mathrm{\ensuremath{\Sigma}}}_{1}$ at the neck point to the saddle points of ${\mathrm{\ensuremath{\Gamma}}}_{12}$ and ${\mathrm{\ensuremath{\Gamma}}}_{25}$ assisted by the scattering of the intra-atomic 5d-4f Coulomb exchange interaction, in particular enhanced in the heavy R${\mathrm{B}}_{6}$ compounds by the lattice instability. In ${\mathrm{SmB}}_{6}$, the absorption that was seen in trivalent R${\mathrm{B}}_{6}$'s was also observed in addition to absorptions from 4f states of ${\mathrm{Sm}}^{2+}$ to 5d states and from 5d to 4f in ${\mathrm{Sm}}^{3+}$. This is thought to mean that the Fermi level of ${\mathrm{SmB}}_{6}$ is located at the same energy position in the band structure as that of the trivalent R${\mathrm{B}}_{6}$.
76 citations
TL;DR: In this article, the authors measured the optical reflectivity over a broad spectral range of single-phase compounds and evaluated the optical conductivity both below and above the superconducting transition temperature.
Abstract: We have measured the optical reflectivity over a broad spectral range of single-phase ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ compounds, and have evaluated the optical conductivity both below and above the superconducting transition temperature. We identify various features in the excitation spectrum of the normal state: a Drude contribution at low frequency, a midinfrared absorption, and several interband transitions. In the superconducting state our results are in full agreement with a BCS singlet ground state, with the measured gap values consistent with the weak-coupling limit. We also evaluate several intrinsic parameters characterizing both the normal and superconducting state, such as the plasma frequency, the relaxation scattering rate, and the mean free path, besides the superconducting gaps and the penetration depth. We compare these quantities with similar ones from other experiments and find satisfactory agreement. Moreover, we present our calculation of the electrodynamic response above and below ${\mathit{T}}_{\mathit{c}}$ with the standard Eliashberg electron-phonon theory of superconductivity, which strongly supports a pairing mechanism mediated by high-frequency intramolecular phonon modes.
76 citations
TL;DR: In this paper, a completely metallic far-infrared $c$-axis response was reported for a fully oxygenated crystal, where the normal state is determined by a temperature and frequency dependent scattering rate of the charge carriers.
Abstract: We report on a completely metallic far-infrared $c$-axis response of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ for a fully oxygenated crystal. The normal state is determined by a temperature and frequency dependent scattering rate of the charge carriers. At $T\ensuremath{\ll}{T}_{c}$ the spectra are characterized by the onset of an absorption process in the energy range of (4-5) ${k}_{B}{T}_{c}$ and by the possible coexistence of paired and unpaired carriers. The estimated anisotropy ratio in the penetration depth (\ensuremath{\cong}8) suggests a unique superconductivity property, dirty-limit-like superconductivity in the $c$ direction and clean limit in the $a$ direction.
75 citations
TL;DR: In this paper, an f-f transition intensity model was proposed to analyze the crystal-field splittings of several anomalous multiplets, and the resulting 144-level data set was analyzed in terms of a model Hamiltonian that assumes site symmetry for the ions in Nd:YAG.
Abstract: Results obtained from optical absorption measurements on ${\mathrm{Nd}}^{3+}$:${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ (Nd:YAG) at 10 and 29 K are reported and analyzed. The low-temperature (10 K) absorption spectrum shows transitions from the lowest Stark component of the $^{4}$${\mathit{I}}_{9/2}$ (ground) multiplet to 133 of the 155 crystal-field (Stark) levels predicted to be located between 3900 and 40 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, spanning 35 excited multiplet manifolds of ${\mathrm{Nd}}^{3+}$(4${\mathit{f}}^{3}$). Among the 133 transitions observed in the 10-K absorption spectrum, 97 are sufficiently well resolved to permit quantitative determination of transition line strengths. Energy levels for the $^{4}$${\mathit{I}}_{9/2}$ and $^{4}$${\mathit{I}}_{11/2}$ multiplets are taken from previously obtained optical emission measurements, and the resulting 144-level data set is analyzed in terms of a model Hamiltonian that assumes ${\mathrm{D}}_{2}$ site symmetry for the ${\mathrm{Nd}}^{3+}$ ions in Nd:YAG. Inclusion of two-electron correlation crystal-field (CCF) interaction terms in the model Hamiltonian explains the crystal-field splittings of several anomalous multiplets, and reduces the rms deviation between calculated and observed energies (for 144 levels) from 28 to 14 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The optical line-strength data obtained in this study are analyzed in terms of an f-f transition intensity model developed by us in previous work. This model has broad applicability in analyses of f-f intensity data for transitions between Stark levels. Emission branching ratios for transitions from the $^{4}$${\mathit{F}}_{3/2}$ multiplet are calculated and compared with literature values.
TL;DR: In this article, the optical properties of trivalent chromium ions in a typical crystal of the Ca-gallogermanate family were investigated by means of absorption, emission, excitation, and lifetime measurements.
Abstract: The optical properties of trivalent chromium ions in ${\mathrm{La}}_{3}$${\mathrm{Ga}}_{5}$${\mathrm{SiO}}_{14}$ crystals have been investigated by means of absorption, emission, excitation, and lifetime measurements The particular intermediate value of the octahedral crystal field Dq/B\ensuremath{\sim}25 causes the overlapping of the $^{4}$${\mathit{T}}_{2}$ with the $^{2}$E and $^{2}$${\mathit{T}}_{1}$ levels This overlap is the origin of Fano resonances in the absorption spectrum and of the simultaneous presence of broadband and narrow-line emissions Polarized measurements have allowed the determinations of second-order effects such as the tetragonal or orthorhomic distortion of the octahedron of oxygen ions around the chromium ion Moreover the temperature shift of the peak and the nonexponential decay of the broad emission band are indications of the disorder present in the crystal lattice All these results produce a fairly complete picture of the energy levels of ${\mathrm{Cr}}^{3+}$ ions in a typical crystal of the Ca-gallogermanate family
TL;DR: In this article, absorption and magnetic circular x-ray dichroism (MCXD) measurements were performed at the L 2,3, and K 3,3 edges of the Ce and the K edge of the iron, respectively.
Abstract: Hydrogen insertion in the ${\mathrm{Ce}}_{2}$${\mathrm{Fe}}_{17}$${\mathrm{H}}_{\mathit{x}}$ system leads to spectacular changes of the physical properties. A relative increase of the Curie temperature of almost 100% is observed as well as the highest volume increase. In addition to previous neutron diffraction measurements which pointed out an increase of the local iron moments, we present in this paper absorption and magnetic circular x-ray dichroism (MCXD) experiments performed at the ${\mathit{L}}_{2,3}$ edges of Ce and the K edge of iron. These measurements allow us to study the evolution of the Ce electronic structure upon hydrogen insertion. Values for the Ce valence have been derived from both absorption and MCXD signals. The MCXD data are analyzed for their shape, intensity, and sign, and compared to the macroscopic magnetic measurements. It is shown that in these compounds the Ce atoms bear a 5d magnetic moment, which is coupled antiferromagnetically to that on iron. There is no orbital contribution to this 5d moment which is thus solely of spin origin. The values of the 5d moment are deduced as a function of each composition in the ${\mathrm{Ce}}_{2}$${\mathrm{Fe}}_{17}$${\mathrm{H}}_{\mathit{x}}$ system at low and room temperature. At the K edge of iron, the shape and intensity of the dichroic signal have been related to the filling of the majority 3d band.
TL;DR: In this article, a unified picture, namely, thermalization within the inhomogeneously broadened exciton band, was used to explain the excitonic photoluminescence (PL) spectra in quantum wells independently of the presence, or lack thereof, of the low-energy shift of the PL line with respect to the absorption peak (Stokes shift).
Abstract: We show that a unified picture, namely, thermalization within the inhomogeneously broadened exciton band, can be used to explain the excitonic photoluminescence (PL) spectra in quantum wells independently of the presence, or lack thereof, of the low-energy shift of the PL line with respect to the absorption peak (Stokes shift). The Stokes shift itself turns out to be a mere consequence of the thermal equilibrium with a quadratic dependence on the absorption linewidth and a linear dependence on the inverse of the excitonic temperature. The predictions are found to be in excellent agreement with careful measurements in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As single-quantum-well structures.
TL;DR: In this paper, ground-based measurements of Zn~II, Cr~II and Fe~II absorption lines along the line of sight to PKS 0454+0356 were used to identify the galaxy which most probably gives rise to the damped Lyman alpha system.
Abstract: We present {\it Hubble Space Telescope} and ground--based data on the $z_{abs}=0.8596$ metal line absorption system along the line of sight to PKS 0454+0356. The system is a moderate redshift damped Lyman alpha system, with ${\rm N(HI)}=(5.7\pm0.3)\times10^{20}$~cm$^{-2}$ as measured from the {\it Faint Object Spectrograph} spectrum. We also present ground--based images which we use to identify the galaxy which most probably gives rise to the damped system; the most likely candidate is relatively underluminous by QSO absorber standards ($M_B \sim -19.0$ for $q_0=0.5$ and $H_0=50$ \kms Mpc$^{-1}$), and lies $\sim 8.5h^{-1}$ kpc in projection from the QSO sightline. Ground--based measurements of Zn~II, Cr~II, and Fe~II absorption lines from this system allow us to infer abundances of [Zn/H]=$-1.1$, [Cr/H]=$-1.2$, and [Fe/H]=$-1.2$, indicating overall metallicity similar to damped systems at $z >2$, and that the depletion of Cr and Fe onto dust grains may be even {\it less} important than in many of the high redshift systems of comparable metallicity. Limits previously placed on the 21-cm optical depth in the $z=0.8596$ system, together with our new N(H~I) measurement, suggest a very high spin temperature for the H~I, $T_S >> 580$ K.
TL;DR: Bifurcation theory provides organizing principles for understanding this proliferation and for interpreting the data, which interpret as ``recurrence strength,'' as a function of the classical action of the closed orbit.
Abstract: Measurements were made of the absorption spectrum of hydrogen atoms to levels near the ionization threshold in a strong magnetic field. Taking advantage of a classical scaling law, we varied the photon energy and the magnetic-field strength simultaneously, and measured absorption versus ${\mathit{B}}^{\mathrm{\ensuremath{-}}1/3}$ at fixed scaled energy, \ensuremath{\varepsilon}=E/(B/${\mathit{B}}_{\mathrm{o}}$${)}^{2/3}$. The absorption rate has sinusoidal fluctuations which are correlated with closed classical orbits of the electron. Fourier transformation of this signal gives peaks, which we interpret as ``recurrence strength,'' as a function of the classical action of the closed orbit. Closed-orbit theory gives formulas for these recurrence strengths. We find that the formulas are in good agreement with the measurements. As the scaled energy is increased, observed recurrences proliferate, consistent with a change from orderly to chaotic motion of the electron. Bifurcation theory provides organizing principles for understanding this proliferation and for interpreting the data. New ``exotic'' orbits suddenly appear through saddle-node bifurcations. The ``main sequence'' of orbits is produced from an orbit parallel to B through a sequence of pitchfork and period-doubling bifurcations. Other recurrences are created by period-tripling and higher-order bifurcations of existing orbits. These bifurcations can have ``generic'' structure, or sometimes the structures are modified by symmetries of the system. Focusing effects associated with these bifurcations cause some recurrences to be particularly strong.
TL;DR: Results confirm that the higher $T_c$ of the $\langle{12}\rangle$rim particles arises from finite-size scaling.
Abstract: $^{57}Fe$ Mossbauer measurements show that the $T_c$ of $\langle{12}\rangle$-nm particles of $MnFe_2O_4$ is higher than that of the $\langle{33}\rangle$-nm particles, which exhibit the bulk $T_c$ value (573 K). The $^{57}Fe$ isomer shift as well as the internal field are, however, independent of the particle size, as is the cation distribution determined by Mn and Fe extended x-ray absorption fine structure and Mn x-ray absorption near-edge structure measurements at different temperatures. The results confirm that the higher $T_c$ of the $\langle{12}\rangle$rim particles arises from finite-size scaling.
TL;DR: In this paper, the frequency dependence of the absorption constant and the enhancement of the refractive index in the region of interband transitions between confined quasi-two-dimensional states of the valence and conduction bands of PbTe/PbTe/${\mathrm{Pb}}_{1\mathrm{\ensuremath{-]--}}\mathit{x}}$${\mathm{Eu}}_{\math it{x}$Te multiple-quantum-well (MQW) structures is studied experimentally as well as theoretically.
Abstract: The frequency dependence of the absorption constant \ensuremath{\alpha}(\ensuremath{\omega}) and the enhancement of the refractive index n(\ensuremath{\omega}) in the region of interband transitions between confined quasi-two-dimensional states of the valence and conduction bands of PbTe/${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Eu}}_{\mathit{x}}$Te multiple-quantum-well (MQW) structures is studied experimentally as well as theoretically. Transmission, reflectivity, and photoconductivity spectra of several MQW's with PbTe well widths from 62 to 118 \AA{} and ${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Eu}}_{\mathit{x}}$Te barrier widths from 486 to 621 \AA{} (x\ensuremath{\approxeq}3%) are compared with calculated spectra. The experimental data are compared to calculations of the transmission and reflectivity of the multilayer structures based on frequency-dependent dielectric functions \ensuremath{\epsilon}(\ensuremath{\omega}) of the buffer and MQW layers. For the buffer layer experimentally determined values for \ensuremath{\epsilon}(\ensuremath{\omega}) are used, whereas for the MQW system the sum of a background dielectric function and an additional term, which takes into account the electronic contribution due to interband transitions between electric subbands in the valence band (VB) and conduction band (CB), are used.For calculating the second term two approaches were followed. The conventional approach is based on treating these interband transitions as originating from confined states within the PbTe wells only. The second, more realistic, approach is based on a complete calculation of the band structure of the MQW systems using a k\ensuremath{\cdot}p envelope function approximation which yields proper energy eigenstates, eigenfunctions, and oscillator strengths. The absorption constant \ensuremath{\alpha}(\ensuremath{\omega}) is then obtained from an integration in k space. The electronic contribution \ensuremath{\delta}${\mathrm{\ensuremath{\epsilon}}}^{\mathrm{VB}\mathrm{\ensuremath{-}}\mathrm{CB}}$(\ensuremath{\omega}) to the total dielectric function is independent of the position z along the growth direction in the MQW structure, i.e., in this respect the MQW is treated as being optically homogeneous. The calculated transmission and reflectivity data fit the experimental data very well. The steplike changes in \ensuremath{\alpha}(\ensuremath{\omega}) result in cusplike changes of the refractive index n(\ensuremath{\omega}). From a comparison of the experimentally determined interband transition energies with the calculated values on several samples, the conduction-band offset is determined to be \ensuremath{\Delta}${\mathit{E}}_{\mathit{c}}$/\ensuremath{\Delta}${\mathit{E}}_{\mathit{g}}$=0.55\ifmmode\pm\else\textpm\fi{}0.2.
TL;DR: In this article, electron paramagnetic resonance (EPR), photo-EPR, optical absorption, and photoluminescence measurements have been made on photorefractive vanadium-doped CdTe.
Abstract: Electron paramagnetic resonance (EPR), photo-EPR, optical absorption, and photoluminescence measurements have been made on photorefractive vanadium-doped CdTe This EPR observation of the ${\mathrm{V}}^{3+}$ ion in CdTe, along with the observed features assigned to ${\mathrm{V}}^{2+}$ in the optical-absorption spectrum, provide direct evidence for the substitutional incorporation of vanadium Photo-EPR measurements were also conducted and provide direct evidence for the optical activity of ${\mathrm{V}}^{3+}$ under sub-band-gap illumination A qualitative discussion linking these observations to photoionization channels is presented
TL;DR: In this paper, the photoabsorption coefficient of Mg, Al, and Si from 25 eV up to the absorption edge was measured using a laser-produced plasma source, where the surface oxide was corrected for by taking the ratio of the absorption for different film thicknesses.
Abstract: Absolute measurements of the photoabsorption coefficient of Mg, Al, and Si from 25 eV up to the ${\mathit{L}}_{3}$ absorption edge are presented. Transmission measurements were performed on free-standing thin films using a laser-produced plasma source. The surface oxide is corrected for by taking the ratio of the absorption for different film thicknesses. The values so obtained are, in general, lower than have been reported in the literature. Structure below the edge is observed for Al and Si. Despite the fact that the absorption below the ${\mathit{L}}_{3}$ edge is due to the valence or conduction electrons, the magnitude of the absorption coefficient for the solid is much higher than is predicted by a simple Drude model and is close to that expected from calculations for the free atom.
TL;DR: In this article, the infrared-to-blue upconversion in yttrium aluminum garnet and yttium lithium fluoride crystals has been studied at temperatures between 4.4 and 300 K for different activator concentrations.
Abstract: The infrared-to-blue upconversion in ${\mathrm{Pr}}^{3+}$-doped yttrium aluminum garnet and yttrium lithium fluoride crystals has been studied at temperatures between 4.4 and 300 K for different activator concentrations. The excitation spectra and decay-time measurements indicated the sequential absorption of two photons between levels of the 4${\mathit{f}}^{2}$ electronic configuration. The first photon is absorbed nonresonantly by a weak phonon band associated with the $^{3}$${\mathit{H}}_{4}$${\ensuremath{\rightarrow}}^{1}$${\mathit{G}}_{4}$ transition; the second resonant absorption step is from the $^{1}$${\mathit{G}}_{4}$(1) to one of the $^{1}$${\mathit{I}}_{6}$ levels that relax to the $^{3}$${\mathit{P}}_{0}$ state. Transitions from the excited $^{1}$${\mathit{G}}_{4}$ state obeyed the spin and point symmetry selection rules for electric-dipole transitions. Excited-state-absorption spectra revealed structure not observed in one photon absorption and excitation spectra.
TL;DR: In this article, a detailed low energy electron diffraction analysis was performed to investigate the adsorption of potassium on Al(111) at 90 K and at 300 K, respectively, and the obtained interatomic distances and surface relaxations were discussed in terms of an interplay of the electrostatic repulsion between adatoms and substrate screening.
Abstract: A detailed low energy electron diffraction analysis has been performed to investigate the adsorption of potassium on Al(111) at 90 K and at 300 K. At each temperature a (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} structure is formed, however, the adsorbate positions are different. For absorption at 90 K the adatoms occupy on-top sites and at 300 K they occupy substitutional sites, both geometries have hitherto been regarded as very unusual. An irreversible phase transition from the former to the latter stable structure occurs on warming to 300 K. The obtained interatomic distances and surface relaxations are discussed in terms of an interplay of the electrostatic repulsion between adatoms and substrate screening as well as coordination number. An approximate scheme for including the effect of anisotropic atomic motions under the influence of thermal excitation was applied. It is found that the R factors for the on-top site are significantly improved whereas no improvement is found for the substitutional site.
TL;DR: The origin of the induced absorption by pumping pulses has not necessarily been resolved, but the in-gap states induced by the photodoping effect is suggested as a reason.
Abstract: We have investigated the transient absorption spectra of insulating ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ and ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$ thin films by the pump and probe method from 1.3 to 2.8 eV with a time resolution of 100 fs. For both insulating samples, a bleaching signal with a 0.6--0.9 ps initial decay time was observed in the spectral range of the ${\mathrm{O}}_{2\mathit{p}}$ to ${\mathrm{Cu}}_{3\mathit{d}}$ charge-transfer transition at around 1.8 eV. This initial decay can be explained as a nonradiative transition through the successive emission of magnons having a comparably large energy quantum. In the photon-energy region lower than the bleaching signal, an induced absorption signal is observed. The origin of the induced absorption by pumping pulses has not necessarily been resolved, but the in-gap states induced by the photodoping effect is suggested as a reason.
TL;DR: In this article, the ground and lowest excited-state potential energy surfaces of a diisocyanomenthane complex were characterized using single-crystal polarized absorption, emission, polarized excitation, resonance Raman, fluorescence, phosphorescence lifetime data, and MM2 calculations.
Abstract: Rh{sub 2}(dimen){sub 4}{sup 2+} (dimen = 1,8-diisocyanomenthane) exhibits very long Rh-Rh bond distances in the solid state (varying from 4.48 {angstrom} for the PF{sub 6}{sup {minus}}salt to 3.861 {angstrom} for the B(C{sub 6}H{sub 5}){sub 4}{sup {minus}}salt), but the lowest d{sigma}* {yields} p{sigma} excitation produces an excited state with a considerably shorter Rh-Rh distance ({approximately}3.2 {angstrom}). Several techniques (absorption, single-crystal polarized absorption, emission, polarized excitation, resonance Raman, fluorescence, phosphorescence lifetime data, and MM2 calculations) help describe the ground- and lowest excited-state potential energy surfaces of this complex.
TL;DR: In this article, the Coulomb interaction terms are treated exactly in their full three-dimensional form throughout the calculation, especially in the case of the biexciton, which is a more physically realistic procedure than was employed in previous calculations which used effective one-dimensional potentials.
Abstract: Exciton and biexciton binding energies, wave functions, and oscillator strengths are calculated variationally for rectangular GaAs quantum-well wires in an effective-mass approximation. The Coulomb interaction terms are treated exactly in their full three-dimensional form throughout the calculation, especially in the case of the biexciton, which is a more physically realistic procedure than was employed in previous calculations which used effective one-dimensional potentials. Our treatment is unique in the use of a two-dimensional Fourier expansion of the Coulomb potential, which removes the numerical difficulty with the 1/r singularity and considerably reduces the computational effort. As an intermediate step, we calculate the linear absorption coefficients at the exciton resonance as a function of the wire dimensions. The results compare favorably with those of Suemune and Coldren [IEEE J. Quantum Electron. QE-24, 1778 (1988)]. Using the results of the exciton and biexciton calculation, we then estimate the third-order nonlinear optical susceptibility for near-resonant excitonic absorption by using the approach of Ishihara [Phys. Status Solidi 159, 371 (1990)]. We obtain ${\mathrm{\ensuremath{\chi}}}^{(3)}$'s on the order of ${10}^{\mathrm{\ensuremath{-}}1}$--1 esu for various wire dimensions.
TL;DR: In this article, the authors used a laser light pulse projected through a semitransparent grid pattern to excite a sinusoidal excess carrier grating within the sample, and the resulting Fourier spectrum shows a peak for a frequency corresponding to the grating period and the decay of the amplitude of this spatialfrequency component represents a characteristic grating erasure time, which is related to the carrier diffusivity.
Abstract: Carrier-diffusion measurements in silicon using a newly developed Fourier-transient-grating technique is presented. The method uses a laser light pulse projected through a semitransparent grid pattern to excite a sinusoidal excess carrier grating within the sample. The interdiffusion of carriers is monitored by free-carrier absorption of a focused infrared probe beam. The grating is scanned across the probe beam and the spatial Fourier transform is calculated at each sampling time following the excitation pulse. The resulting Fourier spectrum shows a peak for a frequency corresponding to the grating period and the decay of the amplitude of this spatial-frequency component represents a characteristic grating erasure time, which is related to the carrier diffusivity. This Fourier-transient-grating method allows sensitive measurements of the carrier diffusivity over a broad range of injection levels, both in the minority carrier regime as well as for high-injection conditions including the transition between the two regimes. Here, measurement data are presented for silicon samples of various doping concentration and types for excess carrier injections in the range \ensuremath{\sim}${10}^{12}$--${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. At low densities of injected carriers, our measurement data are in agreement with generally accepted low-injection lattice-scattering mobility values showing the transition to the high-injection range according to ambipolar theory. However, at excess carrier concentrations exceeding ${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, the diffusion coefficient is clearly reduced with respect to the ambipolar diffusivity (using constant electron and hole diffusivities) due to carrier-carrier scattering effects. This reduction is stronger above an injected carrier density of ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ than that predicted by many-body quantum theory [J. F. Young and H. M. van Driel, Phys. Rev. B 26, 2147 (1982)]. The diffusivity data, converted to mobilities using Einstein's relation, have also been compared to recent semiempiric drift-mobility models used for semiconductor device simulation.
TL;DR: In this article, the formation of mixtures of superposition states of the coherent states, also known as Schrodinger-cat states, from a competitive process involving a twophoton parametric process and two-photon absorption, was studied.
Abstract: We study the formation of mixtures of superposition states of the coherent states \ensuremath{\Vert}\ifmmode\pm\else\textpm\fi{}\ensuremath{\alpha}〉, also known as Schr\"odinger-cat states, from a competitive process involving a two-photon parametric process and two-photon absorption. Using the fact that photons are created or destroyed in pairs, we deduce the long-time steady-state form of the density operator. We numerically study the evolution at all times and show that generally the field is in a mixed state. We also examine nonclassical properties of the field such as squeezing and sub-Poissonian statistics, and also the evolution of the Q function.
TL;DR: In this paper, the X-ray-absorption near-edge structure (XANES) spectra at the Pr 4f/O 2p covalency with pressure were studied for tetravalent XANES spectra.
Abstract: X-ray-absorption near-edge structure (XANES) spectra at the Pr ${\mathit{L}}_{\mathrm{III}}$ threshold were studied for tetravalent ${\mathrm{PrO}}_{2}$ as a function of external pressure up to \ensuremath{\simeq}117 kbar The observed spectral changes of the double-peaked white line reflect an increase of the Pr 4f/O 2p covalency with pressure, in analogy to previous results for tetravalent ${\mathrm{CeO}}_{2}$ and ${\mathrm{CeF}}_{4}$ A splitting of the white-line component at the highest energy into two peaks, revealed in the second derivatives of the measured XANES spectra, is interpreted as arising from crystal-field interaction of the 5d states
TL;DR: No strong correlation was found between measured 3[ital d]- occupancy changes and heats of formation of the alloys, consistent with expectations that outer [ital s] electrons play an important role in the alloying of Cu with Pt, Pd, and Au.
Abstract: We used electron-energy-loss spectrometry to measure the intensities of the Cu white lines found at the onsets of the ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ absorption edges of rapidly quenched CuZr, CuTi, CuPd, CuPt, and CuAu. Enhancements of the white lines in the alloys as compared to elemental Cu were correlated with changes in the 3d occupancy of Cu atoms upon alloying. We found that the Cu atoms lost 0.2\ifmmode\pm\else\textpm\fi{}0.06 d electrons when alloyed with Ti or Zr, 0.${0}_{\mathrm{\ensuremath{-}}0.0}^{+0.06}$ d electrons when alloyed with Au or Pd, and 0.${03}_{\mathrm{\ensuremath{-}}0.03}^{+0.06}$d electrons when alloyed with Pt. We found no strong correlation between measured 3d- occupancy changes and heats of formation of the alloys, consistent with expectations that outer s electrons play an important role in the alloying of Cu with Pt, Pd, and Au.
TL;DR: The numerical renormalization group is used to calculate the auxiliary spectral functions of the U=\infty Anderson impurity model, finding that Vertex corrections in the convolution formulae for physical Green's functions are singular at the threshold and may not be neglected in the Fermi liquid regime.
Abstract: We use the numerical renormalization group to calculate the auxiliary spectral functions of the $U=\ensuremath{\infty}$ Anderson impurity model. The slave-boson and pseudo-fermion spectral functions diverge at the threshold with exponents ${\ensuremath{\alpha}}_{b}$ and ${\ensuremath{\alpha}}_{f}$ given by the x-ray photoemission and the x-ray absorption exponents, respectively. In contrast to the noncrossing approximation, the exact exponents obtained here depend on the impurity occupation number. In general, vertex corrections in the convolution formulas for physical Green's functions are singular at the threshold and may not be neglected in the Fermi liquid regime.