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Terahertz vortex beams with different superposition of the orbital angular momentum $l={\pm}1$, ${\pm}2$, ${\pm}3$, and ${\pm}4$ and spin angular momentum ${\sigma}={\pm}1$ were used to study antiferromagnetic (AFM) resonances in TbFe$_3$(BO$_3$)$_4$ and Ni$_3$TeO$_6$ single crystals. In both materials we observed a strong vortex beam dichroism for the AFM resonances that are split in external magnetic field. The magnitude of the vortex dichroism is comparable to that for conventional circular dichroism due to ${\sigma}$. The selection rules at the AFM resonances are governed by the total angular momentum of the vortex beam: $j={\sigma}+l$. In particular, for $l={\pm}2$, ${\pm}3$, and ${\pm}4$ the sign of $l$ is shown to dominate over that for conventional circular polarization ${\sigma}$., Comment: 12 pages, 4 figures; Supplemental Material: 10 pages, 8 figures; Accepted for publication in Physical Review Letters

Optical spectroscopy, x-ray diffraction measurements, density functional theory (DFT), density functional theory + embedded dynamical mean-field theory ($\mathrm{DFT}+eDMFT$), and crystal-field calculations have been used to characterize structural and electronic properties of hexagonal ${M}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ $(M=\mathrm{Fe},\mathrm{Mn})$ polar magnets. Our experimental data are consistent with the room-temperature structure belonging to the space group $P{6}_{3}mc$ for both compounds. The experimental structural and electronic properties at room temperature are well reproduced within $\mathrm{DFT}+eDMFT$ method, thus establishing its predictive power in the paramagnetic phase. With decreasing temperature, both compounds undergo a magnetic phase transition, and we argue that this transition is concurrent with a structural phase transition (symmetry change from $P{6}_{3}mc$ to $P{6}_{3}$) in the Fe compound and an isostructural transition (no symmetry change from $P{6}_{3}mc$) in the Mn compound.

Circularly polarized light with spin angular momentum is one of the most valuable probes of magnetism. We demonstrate that light beams with orbital angular momentum (OAM), or vortex beams, can also couple to magnetism exhibiting dichroisms in a magnetized medium. Resonant optical absorption in a ferrimagnetic crystal depends strongly on both the handedness of the vortex and the direction of the beam propagation with respect to the sample magnetization. This effect exceeds the conventional dichroism for circularly polarized light. Our results demonstrate the high potential of the vortex beams with OAM as a new spectroscopic probe of magnetism in matter.

High-resolution far-infrared reflection and polarized ellipsometry, as well as Raman scattering temperature-dependent measurements are used to study spin-phonon and electron-phonon interactions in rare-earth (RE) iron borates with the R32 structure of a natural mineral huntite, namely, in RFe3(BO3)4 with R ¼Pr, Nd, and Sm. Pronounced peculiarities in the ω(T) dependences at the Neel temperature TN E32 K are observed for all the compounds studied and the origin of these peculiarities is discussed. A coupling between lattice phonons and crystal-field excitations of a RE ion manifests itself by a renormalization of frequencies and intensities of coupled modes. Modeling of the spectra has revealed the value of about 15 cm � 1 for the electron–phonon coupling constant in PrFe3(BO3)4.

Transmittance spectra of $\mathrm{TbFe}{\mathrm{O}}_{3}$ single crystals have been studied in the far-infrared spectral range $15\ensuremath{-}120\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ in the external magnetic fields up to 9 T and at low temperatures down to 1.5 K. The temperature and magnetic-field dependencies of the antiferromagnetic resonance (AFMR) modes have been measured below and above the magnetic ordering temperature of Tb moments ${T}_{N}^{\mathrm{Tb}}=3.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Both the quasiferromagnetic and quasiantiferromagnetic modes of AFMR demonstrate hardening at $Tl{T}_{N}^{\mathrm{Tb}}$. The quasiantiferromagnetic mode gains electric-dipole activity along the $c$ axis below ${T}_{N}^{\mathrm{Tb}}$ and thus behaves as a hybrid, i.e., both electric- and magnetic-dipole active, mode. In addition to AFMR modes, below ${T}_{N}^{\mathrm{Tb}}$ we observed the appearance of electromagnon excitation at about $27\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, which is electric-dipole active along the $c$ axis. The electromagnon is optically active only in a narrow temperature range, $2.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl{T}_{N}^{\mathrm{Tb}}$ at $H=0$, and in a narrow range of magnetic fields 2.4--2.7 T applied along the $b$ axis at $T=1.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We argue that this electromagnon appears in magnetic phases, which are compatible with a spontaneous electric polarization along the $b$ axis.

We report on a new effect caused by the electron-phonon coupling in a stoichiometric rare-earth antiferromagnetic crystal subjected to an external magnetic field, namely, the appearance of a nonzero gap in the spectrum of electronic excitations in an arbitrarily small field. The effect was registered in the low-temperature far-infrared (terahertz) reflection spectra of an easy-axis antiferromagnet PrFe_{3}(BO_{3})_{4} in magnetic fields B_{ext}∥c. Both paramagnetic and magnetically ordered phases (including a spin-flop one) were studied in magnetic fields up to 30 T, and two bifurcation points were observed. We show that the field behavior of the coupled modes can be successfully explained and modeled on the basis of the equation derived in the framework of the theory of coupled electron-phonon modes, with the same field-independent electron-phonon interaction constant |W|=14.8 cm^{-1}.

Optical properties and lattice dynamics of hexagonal $2H\ensuremath{-}\mathrm{BaMn}{\mathrm{O}}_{3}$ single crystals are studied experimentally in a wide temperature range by means of rotating analyzer ellipsometry and Raman scattering. The magnitude of the direct electronic band gap is found to be ${E}_{g}=3.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. At room temperature the far-infrared (IR) ellipsometry spectra reveal six IR-active phonons; two of them are polarized along the $c$ axis and four are polarized within the a-b plane. Seven phonon modes are identified in the Raman scattering experiments. Group theoretical mode analysis and complementary density functional theory lattice dynamics calculations are consistent with the $2H\ensuremath{-}\mathrm{BaMn}{\mathrm{O}}_{3}$ structure belonging to the polar $P{6}_{3}mc$ space group at room temperature. All observed vibrational modes are assigned to specific eigenmodes of the lattice. The neutron diffraction measurements reveal a structural phase transition upon cooling below ${T}_{C}=130\ifmmode\pm\else\textpm\fi{}5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is accompanied by a lattice symmetry change from $P{6}_{3}mc$ to $P{6}_{3}cm$. Simultaneously, at temperatures below ${T}_{C}$ several additional IR- and Raman-active modes are detected in experimental spectra. This confirms the occurrence of a structural transition, which is possibly associated with the appearance of electrical polarization along the $c$ axis and a previously known tripling of the primitive cell volume at low temperatures.

An unusual phase of carbon with the fcc crystal structure, the lattice constant of which is very close to that of diamond, was reported before in a number of publications. In all the published works, the existence of fcc carbon could not be unambiguously established, as only insignificant amounts of fcc carbon in mixture with other carbon modifications were obtained so far. The present work provides for the first time clear evidence for the existence of the carbon modification with the fcc crystal structure. Thin films consisting exceptionally of fcc carbon were obtained by various methods, namely by hot-filament CVD, plasma-assisted CVD and treatment of the diamond surface in hydrogen plasmas. Results of electron diffraction, Raman spectroscopy, AES, XPS, EELS and HRTEM of the films consisting of fcc carbon clearly indicate its unique crystal and electron structure. The micro-hardness of the films of fcc carbon was found to be equal to HV0.01 = 250 which is quite different from the hardness of all the known carbon modifications.

We present a new approach to determine the site incorporation of p-type impurities in the doped layers of microelectronic device structures based on X-ray standing wave (XSW) technique. Activation behavior of Zn in InP epitaxial layers grown by metal organic vapor phase epitaxy (MOVPE) on InP(1 0 0) substrates has been studied. The XSW experiments were performed at the A2 beamline of the Cornell high-energy synchrotron source (CHESS). Angular dependences of the Zn–K fluorescence intensity excited by the XSW field inside the InP layer for symmetrical (4 0 0) reflection have been measured along with the Indium L-edge and phosphorus K-edge fluorescence and X-ray reflectivity. Analysis of the XSW data based on the dynamical diffraction theory in layered crystal structures allowed us to determine the fractions of both Zn atoms incorporated into crystal lattice and interstitial Zn. In our example, a 1 lm thickMOCVD-grown InP layer with the nominal concentration of Zn atoms of (2.4 ± 0.2) · 10 18 cm � 3 has the fraction of the substitutional Zn of (65 ± 5)%. This result is in a good agreement with electrical activation of Zn measured with a combination of SIMS and CV profilometry. The accurate knowledge of the interstitial-to-substitutional ratio as a function of growth conditions is required to optimize electrical activation of Zn and to control its diffusion in the device structures. � 2004 Elsevier B.V. All rights reserved.

Infrared-active optical phonons were studied in olivine LiFePO4 oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm−1. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO4 crystal were measured from the delithiated ab-surface of the LiFePO4 crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theory calculations for the phonon modes in both LiFePO4 and FePO4.

Far-infrared reflection and ellipsometry measurements on multiferroic ${\mathrm{PrFe}}_{3}({\mathrm{BO}}_{3}){}_{4}$ and ${\mathrm{SmFe}}_{3}({\mathrm{BO}}_{3}){}_{4}$ single crystals were used to investigate the interaction between lattice phonons and electronic excitations associated with $4f$ crystal-field transitions. A temperature-dependent interference between two types of excitations was observed in ${\mathrm{PrFe}}_{3}({\mathrm{BO}}_{3}){}_{4}$ in which the frequency of $4f$ crystal-field electronic excitation of ${\mathrm{Pr}}^{3+}$ falls into the TO-LO frequency interval of the optical phonon mode near 50 ${\mathrm{cm}}^{\ensuremath{-}1}$ (1.5 THz). Experimental data were explained on the basis of a theoretical model of coupled electron-phonon modes. The fitting procedure revealed the value 14.8 ${\mathrm{cm}}^{\ensuremath{-}1}$ for the electron-phonon coupling constant. This rather large value points to an essential role played by the electron-phonon interaction in the physics of multiferroics.

Spectroscopic ellipsometry study of Na3Ir3O8 suggests thatelectron-phonon interactions play a prominent role in its electronicstructure.

The high brightness, broad spectral coverage and pulsed characteristics of infrared synchrotron radiation enable time-resolved spectroscopy under throughput-limited optical systems, as can occur with the high-field magnet cryostat systems used to study electron dynamics and cyclotron resonance by far-infrared techniques. A natural extension for magnetospectroscopy is to sense circular dichroism, i.e. the difference in a material’s optical response for left and right circularly polarized light. A key component for spectroscopic circular dichroism is an achromatic 1 4 wave retarder functioning over the spectral range of interest. We report here the development of an in-line retarder using total internal reflection in high-resistivity silicon. We demonstrate its performance by distinguishing electronic excitations of differing handedness for GaAs in a magnetic field. This 1 4 wave retarder is expected to be useful for far-infrared spectroscopy of circular dichroism in many materials.

Electrochemical doping and in situ Raman scattering were used to study charge transfer in K- and Li-doped single wall carbon nanotubes (SWNT) as a function of alkali concentration. An 8 cm ˇ1 downshift was observed for the tangential phonon mode of SWNT doped to stoichiometries of KC24 and Li1:25C6. The shift in both systems is reversible upon de-doping despite an irreversible loss of crystallinity. These results indicate that the tangential mode shifts result from electron transfer from alkali dopants to the SWNT, and that these modes are only weakly aAected by long-range order within the ropes. ” 2001 Elsevier Science B.V. All rights reserved.

Understanding the behaviour of the dielectric constant in ferroelectric thin films remains a challenging problem. These ferroelectric materials have high static dielectric constants, and so are important for their applications in high-storage-density capacitor structures such as dynamic random access memory (DRAM). But the dielectric constant tends to be significantly reduced in thin films, thereby limiting the potential benefit of ferroelectrics for memory devices. Extensive studies have shown that this phenomenon could be caused by a 'dead layer' of very low dielectric constant between the ferroeletric film and the electrode. And, although very few direct measurements are in fact available, it has been recognized that the lattice dynamical properties in the thin films should also play a key role in the reduction of the dielectric constant. Here we report far-infrared ellipsometry and low-frequency dielectric measurements in SrTiO3 thin films, which demonstrate that the Lyddane-Sachs-Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material. This indicates that the dramatic reduction of the dielectric constant is a consequence of a profound change of the lattice dynamical properties, in particular of the reduced softening of its lowest optical-phonon mode. Our results therefore provide a better understanding of the fundamental limitations of the dielectric constant values in ferroelectric thin films.

Far-infrared spectra of ferrimagnetic Dy3Fe5O12 single crystals have been studied in the spectral range between 12 and 700 cm(-1) and in a wide temperature range between 5 K and 300 K using transmission spectroscopy and rotating analyzer ellipsometry. In the temperature range below T-C = 16 K for the magnetic ordering of Dy3+ spins, a number of ligand field and Kaplan-Kittel (KK) exchange resonance modes have been found. Temperature dependencies of their frequencies allowed us to estimate the ratio between the Fe-Dy and Dy-Dy exchange constants. We found that variation of the oscillator strength of the KK modes correlates with the magnetic-field-induced changes of magnetic permeability. The low-temperature peak in the static value of the dielectric function at T-C = 16 K may be related to the observed temperature-induced variation of the oscillator strength of the optical phonons.

Far-infrared optical transmission spectra of the antiferromagnetic resonances, or magnons, and crystal-field (CF) transitions have been studied in hexagonal $R$MnO${}_{3}$ ($R$ $=$ Er, Tm, Yb, Lu) single crystals. The magnon and CF frequencies, their oscillator strengths, and effective $g$ factors have been measured using external magnetic fields up to 9 T in the temperature range between 1.5 and 100 K. The magnon frequency increases systematically with a decrease of the $R$ ion radius. The magnetic ordering of $R$ ions ($R$ $=$ Er, Tm, Yb) was observed at low temperatures $Tl3.5$ K and in strong external magnetic fields. The observed effects are analyzed taking into account the main magnetic interactions in the system including exchange of the Mn${}^{3+}$ spins with ${R}^{3+}$ paramagnetic moments.

Neutron diffraction is used to show that small (∼7 MPa, or 70 bar) uniaxial pressure produces significant changes in the populations of magnetic domains in a single crystal of 2% Nd-doped bismuth ferrite. The magnetic easy plane of the domains converted by the pressure is rotated 60° relative to its original position. These results demonstrate extreme sensitivity of the magnetic properties of multiferroic bismuth ferrite to tiny (less than 10(-4)) elastic strain, as well as weakness of the forces pinning the domain walls between the cycloidal magnetic domains in this material.

Far-infrared spectra of magnetodielectric Dy3Fe5O12 garnet were studied using a combination of transmittance, reflectivity, and rotating analyzer ellipsometry. In addition to purely dielectric and magnetic modes, we observed several hybrid modes with a mixed magnetic and electric dipole activity. Using 4 x 4 matrix formalism for materials with mu(omega) 1, we modeled the experimental optical spectra and determined the far-infrared dielectric and magnetic permeability functions. The matching condition mu(omega_h)Se = epsilon(omega_h)Sm for the oscillator strengths Se/m explains the observed vanishing of certain hybrid modes at omega_h in reflectivity.

Forward models for the Mueller Matrix (MM) components of materials with relative magnetic permeability tensor {\mu} \neq 1 are studied. 4x4 matrix formalism is employed to produce general solutions for the complex reflection coefficients and MMs of dielectric-magnetic materials having arbitrary crystal symmetry and arbitrary laboratory orientation. For certain orientations of materials with simultaneously diagonalizable {\epsilon} and {\mu} tensors (with coincident principal axes), analytic solutions to the Berreman equation are available. For the single layer thin film configuration of these materials, analytic formulas for the complex reflection and transmission coefficients are derived for orthorhombic or higher crystal symmetry. The separation of the magnetic and dielectric contributions to the optical properties of a material are demonstrated using measurements of the MM at varying angles of incidence., Comment: presented at ICSE-V, May 2010, submitted to Thin Solid Films presented at the International Conference on Spectroscopic Ellipsometry (ICSE-V, May 2010), submitted to Thin Solid Films

Linearly polarized spectra of far-infrared (IR) transmission in ${\text{HoMn}}_{2}{\text{O}}_{5}$ multiferroic single crystals have been studied in the frequency range between 8.5 and $105\text{ }{\text{cm}}^{\ensuremath{-}1}$ and for temperatures between 5 and 300 K. Polarization of IR-active excitations depends on the crystallographic directions in ${\text{HoMn}}_{2}{\text{O}}_{5}$ and is sensitive to the magnetic phase transitions. We attribute some of the infrared-active excitations to electric-dipole transitions between ligand-field (LF) split states of ${\text{Ho}}^{3+}$ ions. For light polarization along crystalline $b$ axis, the oscillator strength of electric dipoles at low frequencies (10.5, 13, and $18\text{ }{\text{cm}}^{\ensuremath{-}1}$) changes significantly at the commensurate-incommensurate antiferromagnetic phase transition at ${T}_{3}=19\text{ }\text{K}$. This effect shows a strong correlation with the pronounced steps of the $b$-directional static dielectric function. We propose that the LF on ${\text{Ho}}^{3+}$ connects the magnetism and dielectric properties of this compound through coupling with the Mn spin structure. We comment on the possibility for composite excitations of magnons and excited LF states.

The intriguing structural properties of a ${\text{ZnMnGaO}}_{4}$ film epitaxially grown on MgO (001) substrate have been investigated using synchrotron-radiation-based x-ray diffraction. The ${\text{ZnMnGaO}}_{4}$ film consisted of a self-assembled checkerboard (CB) structure with highly aligned and regularly spaced vertical nanorods. The lattice parameters of the orthorhombic and rotated tetragonal phases of the CB structure were analyzed by measuring $H\text{\ensuremath{-}}K$, $H\text{\ensuremath{-}}L$, and $K\text{\ensuremath{-}}L$ cross-sectional reciprocal space maps. We demonstrate that symmetry of lattice distortions at the phase boundaries provides means for the coherent coexistence of two domain types within the film volume.

Thin films of GaN with the V/III≈10 ratio were grown by low-pressure metal organic vapour phase epitaxy (LP-MOVPE) using N2 and Dimethylhydrazine (DMHy) as a carrier gas and nitrogen precursor, respectively. For the growth temperatures in the range from 550 to 690 ∘C the GaN layers exhibited good surface morphology. In the temperature range from approximately 550 to 610 ∘C, the growth rate increases with increasing temperature, characteristic of the process limited by surface kinetics with the activation energy of approximately 36 kcal/mol. For the temperatures between 620 and 690 ∘C, the growth rate was nearly independent of temperature, which is indicative of a mass transport limited growth. The activation energy was about 4.6 kcal/mol. Micro Raman spectroscopy revealed a significant relaxation of the selection rules for the scattering by the optical phonons in the films grown at lower temperatures. Variation of the intensity ratio for E 2 H and E 1 phonon modes has been attributed to the changes in the structural quality of the films grown at different temperatures.

Optically detected cyclotron resonance of two-dimensional electrons has been studied in nominally undoped CdTe/(Cd,Mn)Te quantum wells. The enhancement of carrier quantum confinement results in an increase of the electron cyclotron mass from 0.099$m_0 $ to 0.112$m_0 $ with well width decreasing from 30 down to 3.6 nm. Model calculations of the electron effective mass have been performed for this material system and good agreement with experimental data is achieved for an electron-phonon coupling constant $\alpha $=0.32.

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We discovered that perovskite (Ba,La)SnO3 can have excellent carrier mobility even though its band gap is large. The Hall mobility of Ba0.98La0.02SnO3 crystals with the n-type carrier concentration of \sim 8-10\times10 19 cm-3 is found to be \sim 103 cm2 V-1s-1 at room temperature, and the precise measurement of the band gap \Delta of a BaSnO3 crystal shows \Delta=4.05 eV, which is significantly larger than those of other transparent conductive oxides. The high mobility with a wide band gap indicates that (Ba,La)SnO3 is a promising candidate for transparent conductor applications and also epitaxial all-perovskite multilayer devices.

A novel 40 Gbit/s optical receiver is presented. The device consists of a waveguide photodiode integrated with a semiconductor optical amplifier. The receiver has a small signal bandwidth of 39 GHz and a conversion gain of more than 800 V/W. A back-to-back sensitivity of -17 dBm has been demonstrated for non-return to zero data transmission at 40 Gbit/s. A sensitivity of -32dBm or 110 photons/bit has been realised for 40 Gbit/s return to zero transmission with an erbium doped fibre preamplifier and an optical filter.