Far-infrared Transmission Research Articles

Far-infrared ab plane reflectance and transmission of Bi2Sr2CaCu2O8

We review recent experimental data on the infrared thin film transmission and single crystal reflection spectra of Bi2Sr2CaCu2O8 (Tc ∼ 85 K). Both show that the low frequency response can be described by a Drude absorption with a relaxation rate for the carriers of the order of 100 cm−1. This places the materials in the clean limit and no superconducting gap structure is evident. A temperature independent midinfrared absorption is present with a sharp onset at 300 cm−1.

Lattice-mode coupling to the charge-density wave in (TMTSF)2ReO4 (where TMTSF is bis-tetramethyltetraselenafulvalene).

We report high-resolution far-infrared reflectivity and transmission spectra of protonated and deuterated bis-(tetramethyltetraselenafulvalene) perrhenate [(TMTSF${)}_{2}$${\mathrm{ReO}}_{4}$] at 25 K and higher temperatures. This is the region in which the infrared-active lattice modes are to be found. We observe for E?a a very strong lattice mode with at least seven components. We speculate on the nature of this mode and show how the zone folding leads to new modes which couple directly to the low-temperature-phase charge-density wave. Fewer but similar splittings are observed for radiation polarized with E?b'.

Far-infrared studies of the phase transitions in beta -quinol-methanol and methanol-D4 clathrates.

Low-temperature far-infrared transmission studies of \ensuremath{\beta}-quinol-methanol and \ensuremath{\beta}-quinol-methanol-D4 clathrates have been performed. Far-infrared transmission spectra in the region of 30 to 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at 4.6 K reveal the presence of lattice lines that are not present in \ensuremath{\beta}-quinol clathrates with small guest inclusions and which shift and broaden as the temperature is increased. In order to further investigate these effects, a cw far-infrared laser has been employed to interrogate the behavior of these lattice lines as a function of both temperature and density of filled cages. Utilizing the coincidence between the 54.25-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ emission from a ${\mathrm{CH}}_{2}$${\mathrm{F}}_{2}$ far-infrared laser and a guest-induced 55.2-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ lattice line, we find the absorption coefficient exhibits an abrupt change in slope at the same temperatures at which heat capacity and low-frequency dielectric absorption anomalies have been reported. The temperature at which the absorption coefficient anomaly occurs increases linearly from 22 to 67 K for methanol densities of 40% to 95%, respectively. We believe these effects can be attributed to cooperative behavior due to long-range strain-mediated interactions.

Far-infrared transmission of Bi2Sr2CaCu2O8 films.

The far-infrared transmission spectra for high-conductivity Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} films with the radiation polarized in the {ital ab} plane have been measured for temperatures above and below {ital T}{sub {ital c}}. The normal-state properties show a Drude-like response. The transmission spectrum can be used to derive both the dc conductivity (90 {mu}{Omega} cm at {ital T}{sub {ital c}}) and the relaxation rate (165 cm{sup {minus}1} at {ital T}{sub {ital c}}). If an energy gap at 3.5{ital kT}{sub {ital c}} or higher is assumed then the film is in the clean limit. In the superconducting state the transmission drops at low frequency but remains finite.

Cyclotron resonance of two-dimensional holes in strained-layer quantum well structure of (100)In0.20Ga0.80As/GaAs

Cyclotron resonance of the two-dimensional hole gas (2DHG) in the strained-layer quantum well structure of In0.20Ga0.80As/GaAs is observed in far-infrared transmission measurements made at 4.2 K. The cyclotron mass of the 2DHG in the In0.20Ga0.80As channel is (0.191±0.008)me for a 2D hole density p2D =8.5×1011/cm2.

Far-infrared transmission spectra of doped amorphous germanium chalcogenide semiconductors

Optical absorption spectra of amorphous compositions in the systems GeSBi, GeSeBi and GeSeSb have been studied in the spectral region 50 to 600 cm −1 at room temperature. The spectra exhibit bismuth/antimony induced changes in the glass network. Interpretation of the results indicates some differences in the structural built-up of the different categories of amorphous semiconductors.

Long-wavelength optical phonons in Zn1-xMnxSe.

The far-infrared reflection spectra and transmission spectra of ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se (x=0.18, 0.31) have been measured. Anomalous frequencies shifts of the MnSe-like optical phonons with the increase of composition x have been observed. The force constants of Mn-Se and Zn-Se have been obtained by fitting the experimental spectra with the random-element-isodisplacement model.

Far-infrared response of one-dimensional electronic systems in single- and two-layered quantum wires

The far-infrared (FIR) response of arrays of periodic single- and two-layered quantum wires has been investigated. The wire structures have been prepared by ultrafine deep-mesa etching of modulation-doped $\frac{{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}}{\mathrm{GaAs}}$ heterostructures and two-layered quantum-well systems. Due to narrow geometrical dimensions (500 nm), quantum confinement arises and leads to the formation of one-dimensional electronic subbands with a typical energy separation of 1-3 meV. The FIR transmission spectra of the single- and two-layered quantum wire structures show one and two resonances, respectively. The resonance frequencies are observed at significantly higher energies compared to the one-dimensional subband separation. This implies that collective interactions have a strong influence on the excited transitions and leads to the conclusion that the FIR resonances in the one-dimensional electronic systems have predominantly the character of layer-coupled local plasmon modes.

Far-infrared spectra and a new incommensurate phase transition in ferroelectric guanidine sulphate hexahydrate at low temperature

The behaviour of guanidine aluminium sulphate hexahydrate (GASH) at low temperatures is similar to that of triglycine sulphate (TGS). At 80 K, all GASH samples with thickness 20-8500 mu m appear to be homogeneous from careful far-infrared transmission measurements. At 4 K, for thickness below 200 mu m, the GASH samples are still homogeneous and keep their 80 K absorption coefficient values. For the thickest samples, at 4 K, a surface layer model has to be considered in a first approximation, with a quite transparent bulk and a surface layer that also maintains the 80 K absorption coefficient values. An explanation is suggested for both TGS and GASH in terms of reduction to one formula unit per primitive cell at some temperature T0. The transition should only occur if the sample dimensions are large enough, and it should start from the centre of the sample. From extensive measurements, they have found far-infrared spectroscopic evidence at temperatures T below the transition temperature T0 that both GASH and TGS single crystals have a superstructure where the electric dipole associated with one formula unit is slightly rotated when the primitive cell is translated from the centre to the surface of the sample.

Far-infrared spectroscopic investigation of the incommensurate and commensurate phases in ferroelectric betaine calcium chloride dihydrate (BCCD)

Abstract We have measured the far-infrared transmission spectra of BCCD, ((CH3)3NCH2 COO CaCl2 2H2O), polarized in the (ferroelectric) y-axis in the wavenumber interval 10 cm−1≤ ν ≤ 110 cm−1 and in the temperature range 20 K≤ T ≤300 K. The temperature dependence of new bands arising in the various commensurate phases and of hard-core modes have been studied. The spectra are compared with Raman spectra. A type of symmetry for the soft mode has been proposed based on the obvious selection rules obeyed by the spectra.

Zone centre phonons in high TcBa2YCu3O7

Abstract The zone centre phonons have been investigated in Ba2YCu3O7 using an angular force model. The theoretical values are in reasonable agreement with the available single crystal micro Raman and far-infrared transmission spectra measurements. It indicates that the observed transmission minima are due to phonon absorption.

RAMAN AND INFRARED STUDY OF YBa2 Cu3 O7

We have measured Raman and far-infrared spectra of YBa2 Cu3 O7 as a function of temperature. No significant changes are observed in the Raman spectra as with temperature. Far-infrared transmission measurements show a band at 100 cm −1 broadening at Tc .

Far-infrared transmission spectra of high T c (Y 0.6Ba 0.4) 2CuO 4−y superconductors

We have measured the far-infrared transmission spectra of two samples of the YBaCuO superconductor which have transition temperatures of 98K and 86K respectively. Both samples showed absorption peaks at 110 cm -1, 141 cm -1, 173 cm -1, 209 cm -1, 234 cm -1, 273 cm -1, and 328 cm -1, in the frequency range of 50 – 340 cm -1, in both the normal and the superconducting states. The peak frequencies did not vary with temperature. The linewidths were smaller for the sample with T c = 98 K. For one sample it was estimated from the temperature dependence of the transmission spectra that the superconducting energy gap was 2δ ≅ 3.3 kT c.

Magnetic excitons in space-charge layers in GaAs.

We have studied the high-frequency conductivity of multilayer two-dimensional electron systems on GaAs in perpendicular magnetic fields with far-infrared transmission spectroscopy. In addition to cyclotron excitation, new resonances with energies close to the cyclotron frequency are observed. Their number and dependence on the magnetic field strength and charge density are consistent with those theoretically predicted for two-dimensional magnetic excitons.

Far-infrared measurement of the energy gap of La1.8Sr0.2CuO4.

We report the far-infrared transmission of the high-T/sub c/ superconductor La/sub 1.8/Sr/sub 0.2/CuO/sub 4/ at temperatures above and below T/sub c/ and for magnetic fields up to 10 T. At 7 K our data is best described by a distribution of superconducting energy gaps with a representative value of about 50 cm/sup -2/ (i.e., 72 K).

Plasmon and magnetoplasmon excitation in two-dimensional electron space-charge layers on GaAs.

The excitation of two-dimensional (2D) plasmons in ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As-GaAs heterostructures has been investigated with far-infrared transmission spectroscopy. Grating couplers of high efficiency and samples of high mobility (\ensuremath{\mu}\ensuremath{\approxeq}1.8\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{2}$/V? make it possible to study the 2D plasmon dispersion up to plasmon wave vectors q\ensuremath{\gtrsim}2\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Thus we can test the theoretically predicted second-order q corrections to the dispersion due to nonlocal effects, finite thickness of the space-charge layer, and correlations. In perpendicular magnetic fields, a strong nonlocal interaction of the 2D magnetoplasmon resonance with harmonics of the cyclotron resonance n${\ensuremath{\omega}}_{c}$ (n=2,3,4,. . .) is observed. Excellent agreement is found with predictions of a classical nonlocal theory.

Far-infrared absorption and transmission of ultrathin superconducting aluminium films

The FIR absorption and transmission have been measured of ultrathin superconducting Al films. The absorption of films with a thickness of the order of 10 mm is in excellent agreement with weak-coupling calculations. Ultrathin films with a thickness less than 5 nm show interesting deviations from the weak-coupling theory which are interpreted in terms of an enhanced electron-phonon coupling induced by a softening of the lattice.

Far-Infrared Transmission Measurements of N-Methyl-N-Ethyl-Morpholinium-Ditetracyanoquinodimethanide [MEM(TCNQ)2] at Temperatures above and below Spin-Peierls Transition

Far-infrared transmission spectra of a single crystal N-methyl-N-ethyl-morpholinium-ditetracyanoquinodimethanide, MEM(TCNQ) 2 , was measured for the first time over the range from 15 to 200 cm -1 at 1.5 K (below spin-Peierls transition temperature T sp =17.4 K) and 23 K (above T sp ). We found that the absorption lines 130.5 and 150.5 cm -1 at 23 K shift to 126 and 156 cm -1 at 1.5 K. With an aid of calculations of phonons above and below T sp , it is concluded that the 126, 150.5 and 156 cm -1 lines are phonons and the 150.5 cm -1 line at 23 K changes to the 156 cm -1 line at 1.5 K. The 126 cm -1 line at 1.5 K is due to the lattice distortion caused by the spin-Peierls transition and can be interpreted by the folding mechanism. The assignment of the 130.5 cm -1 line at 23 K is remained in the present stage.

Infrared Reflectivity Spectrum and Lattice Vibration of Spinel-Type Compound HgIn2S4

Indium thiospinel compound with normal spinel structure HgIn2S4 single crystal was synthesized by the Bridgman method, and far-infrared reflection and transmission spectra were measured in the wavelength region from 20 to 1000 cm-1. The dispersion parameters are calculated by dispersion analysis using classical oscillators. The strength of the four kinds of short-range force constant is calculated using the dynamical matrices proposed by Bruesch et al., and the effective charges for each ion are obtained under several assumptions.

Far-infrared properties of lattice resonant modes. VII. Excited states and paraelectric pairs.

Far-infrared transmission measurements have been made on single-crystal KBr:${\mathrm{Li}}^{+}$ and NaCl:${\mathrm{Cu}}^{+}$ samples up to 16 cm in length. At 1.2 K, weak satellite lines are observed at frequencies around the strong lattice-resonant-mode transition. With increasing temperature the strengths of the resonant mode and the satellite lines decrease and excited-state transitions increase in strength. Although we assume that these temperature-activated lines are excited-state transitions to the overtone levels of the resonant mode, no model potential has been found which is completely consistent with all of the infrared and Raman data. We demonstrate experimentally that these excited-state transitions can be used to monitor the mean impurity concentration of these long, inhomogeneously doped crystals. Then, using this new technique, we show that most of the satellite modes with frequencies below the resonant-mode frequency are due to impurity pairs while weak modes with frequencies near the resonant-mode frequency are associated with isolated impurities. For KBr:${\mathrm{Li}}^{+}$, a 50%-50% isotopic mixture of the dopant is used to show that the low-frequency paraelectric modes reported by Bridges arise from a diatomic Li center. Whether the center is a ${\mathrm{Li}}_{2}$${\mathrm{}}^{+}$ ion or two ${\mathrm{Li}}^{+}$ ions in next-nearest-neighbor sites has not been resolved.