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An electron paramagnetic resonance (EPR) study of type-II silicon (Si) clathrate with low sodium (Na) guest concentration provides insights into the properties of this cagelike Si allotrope and into Na as an $n$-type dopant. An EPR line arising from unterminated Si bonds in a disordered Si phase is identified in the system. The presence of this amorphouslike component helps explain the thermally activated, hopping-related conductivity that has long been observed in the system but which has been poorly understood. This study also identifies ``superhyperfine'' lines surrounding each Na hyperfine line due to interactions of the donor electron with a $^{29}\mathrm{Si}$ nucleus on the cage and confirms the identification of hyperfine lines due to interactions of two neighboring Na nuclei. Analysis of the strength of these interactions shows that the electron wave function associated with the Na donor extends onto surrounding cages consistent with identification of Na as a shallow donor, and provides quantitative evidence of Na clustering.

Polarization-dependent photon spectroscopy (dichroism) using signal-enhancing superchiral beams is shown to be sensitive to magnetic properties of the sample, whereas previous investigations explored charge-like electronic properties of chiral samples. In the process of unveiling the potential to observe magnetic circular dichroism (MCD), we underline an affinity between spectroscopies using the Borrmann effect, twisted beams and superchiral beams. Use of an effective wavevector in a quantum-mechanical theory unites the aforementioned spectroscopies and vastly improves our understanding of their advantages. Exploiting an effective wavevector for superchiral beams, natural circular dichroism (NCD) is derived from electric dipole - magnetic dipole (E1-M1) and electric dipole - electric quadrupole (E1-E2) absorption events, and MCD is derived from electric quadrupole-electric quadrupole (E2-E2) absorption. Signal enhancement by superchiral beams is a straightforward gain for the user because NCD and MCD are otherwise precisely the same as for a circularly polarized beam, according to our calculations. Our analysis shows that enhancement of E2-E2 is superior to that available for parity-odd events under consideration. Electronic degrees of freedom in all dichroic signals are encapsulated in atomic multipoles that are frequently used in theoretical interpretations of several established experimental techniques.

Nanocrystals encapsulated within an amorphous matrix are computationally analyzed to quantify the degree to which the matrix modifies the nature of their quantum-confinement power---i.e., the relationship between nanocrystal size and the gap between valence- and conduction-band edges. A special geometry allows exactly the same amorphous matrix to be applied to nanocrystals of increasing size to precisely quantify changes in confinement without the noise typically associated with encapsulating structures that are different for each nanocrystal. The results both explain and quantify the degree to which amorphous matrices redshift the character of quantum confinement. The character of this confinement depends on both the type of encapsulating material and the separation distance between the nanocrystals within it. Surprisingly, the analysis also identifies a critical nanocrystal threshold below which quantum confinement is not possible---a feature unique to amorphous encapsulation. Although applied to silicon nanocrystals within an amorphous silicon matrix, the methodology can be used to accurately analyze the confinement softening of other amorphous systems as well.

We report on the ultrafast carrier dynamics in hydrogenated nanocrystalline silicon (nc$\ensuremath{-}$Si:H) using time-resolved terahertz spectroscopy. Photoexcitation at 407 nm primarily produces charge carriers in the $a\ensuremath{-}$Si phase, but they undergo a rapid electron transfer to the $c\ensuremath{-}$Si phase prior to complete thermalization into the band-tail states of $a\ensuremath{-}$Si. We studied the carrier dynamics on a range of nc$\ensuremath{-}$Si:H samples with varying crystalline volume fractions (${X}_{c}$) and mapped out the carrier dynamics with sub-ps resolution. Our measurements are consistent with a model in which electrons are first trapped at interface states at the $a\ensuremath{-}$Si--$c\ensuremath{-}$Si boundary prior to being thermally emitted into the $c\ensuremath{-}$Si phase. Wavelength and temperature dependent measurements are consistent with our model. The phenomena observed here have implications toward solar cell structures that utilize an amorphous material as an absorber layer, previously thought to have a mobility value too low to attain effective charge transport in a device.

Steady-state- and time-resolved-photoluminescence (PL) and cathodoluminescence (CL) studies have been carried out on several synthetic type-IIb boron-doped semiconducting diamond samples. The behavior of the 1.84-eV red donor-acceptor pair band is compared directly with that of the well-known green band-A emission near 2.3 eV. Time-dependent CL studies show conclusively that the green band is also a donor-acceptor pair emission, thus confirming at least a part of the model introduced by Dean 30 years ago. Time-dependent PL measurements also indicate that the green and red emissions are in direct competition, thus suggesting a model in which pair emission occurs from two different donors to the neutral boron acceptor.

Compared with direct-gap semiconductors, the valley degeneracy of silicon and germanium opens up new channels for spin relaxation that counteract the spin degeneracy of the inversion-symmetric system. Here the symmetries of the electron-phonon interaction for silicon and germanium are identified and the resulting spin lifetimes are calculated. Room-temperature spin lifetimes of electrons in silicon are found to be comparable to those in gallium arsenide, however, the spin lifetimes in silicon or germanium can be tuned by reducing the valley degeneracy through strain or quantum confinement. The tunable range is limited to slightly over an order of magnitude by intravalley processes., 7 pages, 3 figures, 13 tables

Measurements of the reflectivity of ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$, for x=0 to 0.4, in the frequency range from \ensuremath{\simeq}250 to 25 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are used to obtain the conductivity as a function of frequency and composition. The end-member compound, ${\mathrm{BaBiO}}_{3}$, has a half-filled electronic band, and a charge-density-wave (CDW) ground state, while at x\ensuremath{\simeq}0.4 this material is a superconductor with ${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}30 K. Excitations across the CDW energy gap produce a large peak in the optical conductivity at about 16 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (2 eV), which shifts to lower energy and broadens as the band filling is reduced by doping (xg0). For compositions near x=0.4 a broad contribution to the conductivity extending from \ensuremath{\simeq}3000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to 10 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which appears to have evolved from the CDW peak, is still observed. These observations suggest that a remnant CDW-like order may still be present even in the composition range where superconductivity occurs. They also highlight fundamental differences in the electronic properties of bismuthate and cuprate materials.

It is established that vacancies in diamond migrate, during annealing, primarily in their neutral charge state, with an activation energy of 2.3\ifmmode\pm\else\textpm\fi{}0.3 eV. Negative vacancies are destroyed by first converting to neutral centers in a reversible charge transfer process. In relatively pure diamonds (type IIa) and in diamonds (type I) containing large concentrations of nitrogen, effectively all the vacancies in the samples after irradiation can be accounted for in their neutral (${\mathit{V}}^{0}$) and negative (${\mathit{V}}^{\mathrm{\ensuremath{-}}}$) charge states. In nitrogen-rich diamonds, the vacancies are predominantly trapped during annealing at the nitrogen. From the annealing data we derive the relative oscillator strengths of the main absorption bands of ${\mathit{V}}^{0}$, ${\mathit{V}}^{\mathrm{\ensuremath{-}}}$, and of one vacancy combined either with a single N atom, a pair of N atoms, or the larger ``B'' aggregate of nitrogen. In the absence of the intrinsic nonradiative decay channels of luminescence from ${\mathit{V}}^{0}$, we show that the radiative decay time would be 35\ifmmode\pm\else\textpm\fi{}7 ns. In common natural (``type IaA'') diamonds, variations of absorption linewidths during annealing imply that about 40% of the vacancies are created within a few atomic sites of the nitrogen impurity, and direct observation confirms that vacancy production is enhanced in these diamonds. About half the vacancies, including those created near the nitrogen, anneal at each temperature about 12 times faster than those vacancies whose creation is not correlated with the nitrogen.

Elastic-neutron-scattering experiments, performed on a single crystal of Tl{sub 0.3}MoO{sub 3}, show the formation of a periodic distortion below the metal-semiconductor transition at about 170 K. Similar to the isostructural blue bronzes K{sub 0.3}MoO{sub 3} and Rb{sub 0.3}MoO{sub 3}, this periodic distortion is characterized by a temperature-dependent wave vector {bold q}=(0,{ital q}{sub {ital b}}({ital T}),0.5) referring to the high-temperature space group {ital C}2/{ital m}. The incommensurate component {ital q}{sub {ital b}}({ital T}) decreases continuously from 0.267 at the transition temperature to 0.253 at about 100 K, below which it remains constant down to 10 K. Unlike the K and Rb analogs for which there is still some debate on whether there is a commensurate-incommensurate transition at 100 K, the Tl blue bronze is clearly incommensurate down to low temperatures. While the magnetic susceptibility shows a substantial anomaly around 50 K, detailed neutron-scattering studies in this temperature region reveal no significant effect on the charge-density wave.

Measurements of the temperature dependence of the infrared conductivity of the CuO{sub 2} planes in YBa{sub 2}Cu{sub 3}O{sub 7} in the superconducting state are reported. Remarkably, we find that the temperature dependence of {sigma}{sub 1}({omega},{ital T}) is independent of {omega} for {omega}{approx lt}500 cm{sup {minus}1}, and that it is the same as that of the nuclear Korringa product, 1/{ital T}{sub 1}{ital T}. These observations indicate that for the best-characterized high-temperature superconductor, YBa{sub 2}Cu{sub 3}O{sub 7}, the phenomenology of dynamic properties is fundamentally different from that of conventional superconductors.

We discuss the form of the infrared conductivity and reflectivity in strong-coupling superconductors with a finite mean free path. We restrict attention to the local limit, in which the London penetration depth is much larger than the coherence length. We further assume that the fluctuations responsible for electronic pairing may be treated within a conventional Eliashberg approximation. The conductivity typically exhibits two onsets: one at 2${\mathrm{\ensuremath{\Delta}}}_{0}$ and another at 2${\mathrm{\ensuremath{\Delta}}}_{0}$+${\mathrm{\ensuremath{\omega}}}_{0}$, with ${\mathrm{\ensuremath{\omega}}}_{0}$ a typical fluctuation energy. The strength of the latter onset relative to the one at 2${\mathrm{\ensuremath{\Delta}}}_{0}$ increases as the ratio of the mean free path to the coherence length increases, and in an extremely clean system (2${\mathrm{\ensuremath{\Delta}}}_{0}$\ensuremath{\gg}${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$) becomes the dominant feature in both the conductivity and reflectivity.

A detailed study of infrared properties (reflectivity, conductivity, and dielectric response), emphasizing reproducible results from fully oxygenated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ crystals (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}93 K) and films, is presented. The extrapolated values of ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}) at low frequency are roughly consistent with the measured temperature-dependent dc resistivity. Although not well understood, this infrared conductivity can be interpreted in terms of a frequency-dependent scattering rate of \ensuremath{\sim}kT+\ensuremath{\Elzxh}\ensuremath{\omega}, with a low-frequency mass enhancement of roughly 2 to 4 associated with a carrier-spin related interaction. Infrared measurements polarized along the c axis suggest a conductivity anisotropy of roughly 40:1 near ${\mathit{T}}_{\mathit{c}}$ in the normal state. In the superconducting state an energy scale of 2${\mathrm{\ensuremath{\Delta}}}_{\mathit{c}}$\ensuremath{\simeq}3${\mathit{kT}}_{\mathit{c}}$ is suggested by c-axis polarized measurements, while a much larger characteristic energy of 2${\mathrm{\ensuremath{\Delta}}}_{\mathit{a}\mathrm{\ensuremath{-}}\mathit{b}}$\ensuremath{\simeq}8${\mathit{kT}}_{\mathit{c}}$ is evident in the (a-b)-plane conductivity. From the area missing from the conductivity up to this very large gap, a reasonable estimate (\ensuremath{\simeq}1700 \AA{}) for the (a-b)-plane penetration depth is obtained. Evidence for non-BCS temperature dependence, strong pair breaking scattering, and possible fluctuation effects is discussed. A comparison to infrared data from ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8\mathrm{\ensuremath{-}}\mathit{y}}$ shows a similarly large energy scale, 2${\mathrm{\ensuremath{\Delta}}}_{\mathit{a}\mathrm{\ensuremath{-}}\mathit{b}}$\ensuremath{\simeq}8${\mathit{kT}}_{\mathit{c}}$; for the cubic ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ superconductor a more conventional energy scale, 2\ensuremath{\Delta}\ensuremath{\simeq}4${\mathit{kT}}_{\mathit{c}}$ is observed. The unusually large energy scale obtained from the (a-b)-plane measurements of the layered cuprates lies far beyond the range of previously studied superconducting energy gaps (2\ensuremath{\Delta}\ensuremath{\simeq}3 to 5${\mathit{kT}}_{\mathit{c}}$).

It is argued that the model advanced by Lenef and Rand [Phys. Rev. B 53, 13 441 (1996)] for the nitrogen-vacancy center in diamond, exhibiting the 1.945-eV luminescence is incorrect. Lenef and Rand argue that the electronic ground state consists of two electrons occupying ${a}_{1}$ states localized on N and C, respectively, and are decoupled from the other three electrons occupying gap states. This model is inconsistent with experiments and other theoretical works. The existence of a Jahn-Teller state $46 {\mathrm{cm}}^{\ensuremath{-}1}$ above the excited state of the defect is also in conflict with previous experiments.

We report the first measurement of the energy gap of the high-temperature bismuth oxide superconductor ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ (${T}_{c}$\ensuremath{\simeq}30 K). With temperature- and magnetic-field-dependent infrared reflectivity measurements, we obtain a reduced energy gap of 2\ensuremath{\Delta}/${\mathrm{kT}}_{c}$\ensuremath{\simeq}3.5\ifmmode\pm\else\textpm\fi{}0.5, consistent with a BCS-type mechanism with moderate or weak coupling. In the normal state a broad peak in the infrared conductivity is observed near 5000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This unusual infrared behavior is common to both the ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ and the ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{K}}_{\mathrm{x}}$${\mathrm{BiO}}_{3}$ systems and may provide a fundamental clue to the mechanism of superconductivity in the cubic bismuth oxides.

We obtain the a-b plane infrared conductivity of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ from temperature-dependent reflectivity spectra of crystals and films with ${T}_{c}$\ensuremath{\simeq}92 K. The normal state is characterized by a free-carrier band with an enhanced low-frequency mass and a frequency-dependent scattering rate, which indicate coupling to an excitation spectrum with a characteristic frequency scale of ${\ensuremath{\omega}}_{c}$\ensuremath{\lesssim}700 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and coupling strength of \ensuremath{\lambda}\ensuremath{\simeq}2 to 3.

We measure the reflectivity of pure and doped polycrystalline La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ in the gap region and up to 40,000 cm/sup -1/(approx. =5 eV). For the doped sample we analyze the reflectivity in terms of a superposition of a metallic a-b plane component and a nearly insulating c-axis component with ordinary phonons. This approach removes several unphysical results obtained assuming a homogeneous surface and allows agreement between the measured infrared and dc properties, which has not been previously obtained. This approach further indicates that our infrared gap signature is dominated by the c-axis contribution and insensitive to the gap in the a-b plane. We estimate the c-axis gap to be 2..delta..approx. =2k/sub B/T/sub c/, which is small compared to both the isotropic BCS prediction and tunnelling measurements.

We have investigated the low-frequency complex conductivity of the charge-density-wave condensate in ${\mathrm{Tl}}_{0.3}$${\mathrm{MoO}}_{3}$, in the temperature range 40\char21{}90 K, by the measurement of admittance sampled in the frequency interval 5 Hz\char21{}13 MHz. The observed response can be characterized in terms of a simple Debye relaxation model with a distribution of relaxation times by analogy with the reported behavior of its isostructural analog ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$. Despite qualitative similarities with the general trends observed in ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$, the relaxational response in ${\mathrm{Tl}}_{0.3}$${\mathrm{MoO}}_{3}$ differed significantly in detail. Both the mean relaxation times (${\ensuremath{\tau}}_{0}$) and static dielectric constants (${\ensuremath{\epsilon}}_{0}$) are shown to have Arrhenius temperature dependence with activation energies of 743 and 152 K, respectively. For applied dc biases above the threshold field (${E}_{T}$) for nonlinear conduction, the response shows structure at frequencies that resemble ``washboard'' characteristics of a moving charge condensate. From the values of the high-frequency real and imaginary parts of the dielectric constants, the existence of yet another relaxation process is proposed.

We present measurements of the properties of epitaxial thin films of superconducting YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/, with T/sub c/ about 90 K, transition widths of approx.2.5 K, and critical currents in excess of 10/sup 5/A/cm/sup 2/ at 77 K. The temperature dependence of the upper critical field exceeds 4T/K for fields parallel to the film (i.e., Hperpendicular to the c axis) and the anisotropy of the upper critical field ranged between two and three. Our tunneling gaps (..delta..approx.15--21 meV) are similar to previous results on ceramic and single-crystal samples of YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/. Infrared measurements of the gap energy suggest a larger value than previous measurements on polycrystalline material. The temperature dependence of the Hall coefficient is similar to that for the bulk.

We have made infrared reflectivity measurements on single crystals of ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ and ${\mathrm{La}}_{1.92}$${\mathrm{Sr}}_{0.08}$Cu${\mathrm{O}}_{4}$ with the infrared radiation polarized parallel and perpendicular to the copper-oxygen planes. In ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ we observe nearly insulating spectra, from which we easily identify the dominant in-plane and out-of-plane infrared-active phonons. When ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ is doped with Sr, we observe a dramatic increase in the electronic contribution to the $a\ensuremath{-}b$ plane conductivity, which tends to screen the in-plane phonons. In contrast, the out-of-plane response is essentially unchanged by doping. This leads to a conductivity anisotropy which we estimate to be at least 50 to 1 for our ${\mathrm{La}}_{1.92}$${\mathrm{Sr}}_{0.08}$Cu${\mathrm{O}}_{4}$ crystal. Our observations on single crystals resolve some of the controversies which have arisen over the interpretation of reflectivity measurements on polycrystalline samples.

We report the first infrared measurement of the superconducting energy gap of $\mathrm{Ba}{\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{O}}_{3}$. In our polycrystalline samples with ${T}_{c}\ensuremath{\simeq}9.5$ K ($x\ensuremath{\simeq}0.2$) we obtain $2\ensuremath{\Delta}\ensuremath{\simeq}3.2k{T}_{c}$, roughly in agreement with the weak-coupling Bardeen-Cooper-Schrieffer prediction, $2\ensuremath{\Delta}\ensuremath{\simeq}3.2k{T}_{c}$, and with tunneling measurements of the gap. We do not observe any structure above the gap energy associated with strong coupling.

We report the first observation of antiferromagnetic resonance in La/sub 2-//sub x/CuO/sub 4-//sub y/. Transmitting far-infrared radiation through our polycrystalline samples we observe a resonance at 9 cm/sup -1/ from which we extract an anisotropy energy of approx. =0.02 cm/sup -1/. This is more than an order of magnitude smaller than in the quadratic-layer spin-1 antiferromagnet K/sub 2/NiF/sub 4/, in which anisotropy has been shown to be responsible for the observed Neel transition. Nevertheless, using the same analysis we find that, because of the very large intraplanar exchange coupling, the small anisotropy that we measure is sufficient to account for the observed Neel temperatures of up to 300 K in La/sub 2-//sub x/CuO/sub 4-//sub y/.

We have studied the infrared reflectivity of normal and superconducting La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ up to 4000 cm/sup -1/. For x = 0.15 we observe an electronic excitation at about 60 meV (450 cm/sup -1/), and, at essentially the same energy, an optic phonon. As we move away from the optimal high critical temperature (T/sub c/) value of x = 0.15, we find that these features become much weaker. We propose that this 60-meV electronic excitation and the phonon in resonance with it provide the mediating interaction which gives rise to superconductivity at such high temperatures in this class of materials.