Temperature Dependence Of Bands Research Articles

A note on temperature-dependent band narrowing in oligo-acene crystals

We present a theoretical description of polaron band narrowing in oligo-acene crystals dueto electron–lattice interaction. The analysis is based on a model which takes both local andnonlocal contributions to the electron–phonon coupling into account. Differentapproximation schemes are discussed and compared. The theory is supplemented byquantitative ab initio calculations of the temperature dependence of polaron bandwidths inoligo-acene crystals which show the important role of in-plane nonlocal electron–phononcoupling.

Temperature-dependent quasiparticle band structure of an antiferromagnetic two-layer EuTe film

The temperature-dependent quasiparticle band structure of an antiferromagnetic two layer EuTe film is calculated using the s–f model. The one electron part required for the many body evaluation was obtained from TB-LMTO band structure calculation. A suitable supercell with five layers of empty spheres was used in the band structure calculation to obtain the Bloch energies of the film. The quasiparticle band structure of the (1 0 0) film is calculated at different temperatures and we find significant correlation effects in the spectrum. Some bands exhibit wave-vector-dependent splitting and the magnitude of the split-gap increases with increasing temperature. It is clearly seen as the combined effects of correlation and temperature. We find a red-shift of the conduction band edge of this EuTe film with respect to temperature whereas the bulk EuTe exhibits blue shift.

Tunable photonic crystals fabricated in III-V semiconductor slab waveguides using infiltrated liquid crystals

Microcavity structures formed by two-dimensional photonic crystal mirrors with triangular lattice and a crystal period of 280 nm suitable for transmission experiments at about 1 μm were fabricated in III-V semiconductor planar waveguides. The photonic crystals were filled with a liquid crystal of type E7. The wavelength of the resonance peak can be shifted by the temperature-dependent refractive index of the liquid crystal. The temperature shift follows the typical refractive index behavior of liquid crystals, with a jump in wavelength at the clearing point (about 60 °C) and a continuous shift below. The wavelength jump is about 4 nm within a few Kelvin, while the total shift amounts to 9 nm between 20 and 70 °C. The experimental results agree well with the calculated temperature dependent photonic band structure by assuming a preferential alignment of the liquid-crystal molecules parallel to the holes.

S-shaped behaviour of the temperature-dependent energy band gap in dilute nitrides

We have investigated the temperature dependence of the band gap energy in GaInNAs, GaNAs and InGaAs quantum wells. In the structures containing nitrogen the well-known S-shaped characteristic was observed at low temperatures. We explain this anomalous temperature behaviour by strong carrier localization in potential fluctuations at low temperatures. In the nitrogen free samples, there was no S-shaped behaviour and the empirical Varshni dependence was followed.

Indication of charge-density-wave formation in Bi(111).

Photoemission spectroscopy of Bi(111) reveals a small hexagonal two-dimensional Fermi surface (FS) associated with an electron band centered in the surface Brillouin zone. Along the hexagon the Fermi momentum k(F) ranges from 0.053 to 0.061 A(-1). Temperature dependent valence band spectra show an anisotropic energy gap Delta near the Fermi level. We find a transition temperature of about 75 K. At 11 K, the gap is Delta=4 meV at the corner and Delta=7.5 meV at the side of the hexagon. Arguments based on susceptibility chi(--> q) calculations of a hexagonal FS are used to discuss an incommensurate charge-density-wave (CDW) formation associated with a q(CDW)=0.106 A(-1).

Different role of filled and empty surface states in a polyfunctional molecule adsorption: Geranyl acetone on Si(111)7×7

New perspectives in molecular electronics are opening up through controlled surface molecular synthesis. The first step of such a synthesis implies the adsorption of a polyfunctional molecule, which might use one functional group for the surface adsorption, and the other one(s) for further reactions. Here we present an adsorption study of Si(111)7×7 of geranyl-acetone (C13H22O, E-5,9 undecadien-one) characterized by a ketone and two unconjugated double bonds. The study has been performed by temperature and coverage dependent valence band photoemission and room temperature scanning tunneling microscopy. The use of these combined techniques allows us to infer that the interaction between the geranyl acetone and the silicon surface occurs selectively through the rest atom and the carbonyl group, most likely through the oxygen atom. The geranyl acetone does not undergo any fragmentation upon adsorption Si(111)7×7, as has been observed for smaller molecules on the same surface {acetaldehyde [Y. Bu, J. Breslin, M. C. Lin, J. Phys. Chem. B 101, 1872 (1997)] for instance}. The interaction of the chain with the surface is weak and is characterized in the STM images as a darkening of one adatom in positive bias, around the reacted rest atom.

Optical properties of SrSe thin films grown by molecular beam epitaxy

Absorption, reflection and Raman scattering measurements have been carried out for the optical properties of a SrSe thin film grown by molecular beam epitaxy on a BaF2 (111) substrate. The film quality and lattice constant have been assessed by x-ray diffraction, where the good quality of the SrSe thin film is evident in a full width at half maximum value of 310 arcsec. Temperature-dependent indirect band gaps are obtained with the aid of temperature-dependent absorption measurements from 16 to 300 K on the SrSe thin film. By fitting the far-infrared reflection spectra, transverse optical phonon reflection bands in the binary SrSe thin film have been assigned, which are further verified by temperature-dependent Raman scattering measurements.

Analysis of the Solvent- and Temperature-Dependent Raman Spectral Changes of S1trans-Stilbene and the Mechanism of the trans to cis Isomerization: Dynamic Polarization Model of Vibrational Dephasing and the CC Double-Bond Rotation

Solvent- and temperature-dependent band shape changes of the olefinic CC stretch Raman band of S1 trans-stilbene have been analyzed on the basis of the dynamic polarization model. The analysis has shown that the solvent-induced dynamic polarization gives rise to the dephasing of the CC stretch vibration and, concomitantly, triggers and facilitates the rotation around the CC bond leading to the trans to perpendicular (and eventually to cis) isomerization. Picosecond time-resolved Raman spectra have been measured in the three alkane solvents, hexane, octane, and decane at a number of different temperatures ranging from 268 to 338 K and a total of 40 peak positions and the bandwidths have been determined for the CC stretch band. The correlation plot of the bandwidth against peak position shows a clear linear relationship that is predicted by the dynamic polarization model. Picosecond time-resolved fluorescence measurements have been performed in the same three alkane solvents at five different temperatures f...

Band gaps, effective masses and refractive indices of PbSrSe thin films: Key properties for mid-infrared optoelectronic device applications

Key properties of PbSrSe thin films grown by molecular beam epitaxy have been studied for mid-infrared optoelectronic device applications. Detailed knowledge of the material parameters for the device design is required. The material parameters considered are: temperature-dependent band gaps, composition (or band gap)-dependent effective masses, and energy-dependent refractive indices. The study has been carried out by a combination of temperature-dependent photoluminescence and absorption measurements with the theoretical models on PbSrSe thin films of Sr compositions of as high as 0.276. The derived empirical equations for band gaps, effective masses, and refractive indices have been employed successfully in PbSe/Pb0.934Sr0.066Se multiple quantum well mid-infrared laser systems, for studying the band offsets and subband behavior. We have shown that the derived material parameters clearly promise of being applied to other PbSrSe thin films and PbSe/PbSrSe heterostructure systems for their optoelectronic applications.

Composition and temperature dependence of electron band structure in ZnSe 1− xS x

Using the empirical pseudo-potential method combined with the improved virtual crystal approximation where compositional disorder is included as an effective potential, we report on the results of composition and temperature-dependent optical band gaps, of the ZnSe 1− x S x (0≤ x≤1) type alloys. The obtained results compare reasonably well with the available experimental data. The fundamental band gap is found to vary non-linearly with composition, showing a bowing. Similar bowing is also observed from the temperature-dependent fundamental band gap. This bowing decreases with increasing temperature over the range 0–200 K, but it increases gradually on going from 200 to 300 K. The expression E g( x, T)=2.828+0.45 x+0.53 x 2+ T(−3.45 x 2+3.22 x−3.88)×10 −4 was obtained from the data can be used to obtain the energy gap as a function of x and T over a large range of these variables.

Study of band structure in PbSe/PbSrSe quantum wells for midinfrared laser applications

The electronic states in PbSe/Pb0.934Sr0.066Se multiple-quantum-well structures (MQWs) grown by molecular-beam epitaxy have been investigated both theoretically and experimentally for the midinfrared laser applications. With the aid of combined temperature-dependent photoluminescence and absorption measurements on a Pb0.934Sr0.066Se thin film for the effective masses and temperature-dependent band gaps, we find that the PbSe/PbSrSe MQWs have type-I band alignment and the conduction band offset ratio is Qc=0.82±0.03. The calculation, taking into account the strain, carrier confinements, and the multivalley band structure, can well explain both the observed luminescence peak energies and the temperature coefficient of the luminescence peaks as a function of well thickness.

Temperature-dependent valence band offset and band-gap energies of pseudomorphic GaAsSb on GaAs

The band-gap (Egp) and valence band offset (ΔEv) energies of pseudomorphic GaAsSb layers on GaAs substrate are determined from temperature-dependent photoluminescence measurements on GaAsSb/GaAs and GaAsSb/GaAlAs quantum wells grown by molecular beam epitaxy. A clear evidence of staggered type-II band alignment of GaAsSb relative to GaAs and a value of 1.05 for the valence band offset ratio (Qv) are proposed. Finally, through a detailed comparison of these values with those published previously, we have shown that the scatter in Qv found in the literature (<1 to 2.1) is closely dependent on the exact determination of Egp. Particularly, we have shown that the strain dependence of the deformation potential is important in the calculation of the strain energy contribution to Egp.

Temperature-dependent electronic structure of EuO films

We present calculations for the temperature-dependent band structure of ferromagnetic EuO films based on a parametrization of the d–f model Hamiltonian using results of first-principles TB-LMTO band structure calculations. The combination of first-principles calculation and many-body model calculation enables us to determine the temperature-dependent bandstructure of up to 20-layer EuO-films over the entire temperature range.

Optical properties of atomic boron in rare gas matrices: An ultraviolet-absorption/laser induced fluorescence study with ab initio and diatomics-in-molecules molecular dynamics analysis

Boron atoms have been produced by laser ablation and trapped in solid Ar, Kr, and Xe matrices. In addition to stable doublet absorptions at 210–230 nm, broad temperature dependent bands less in blue have been recorded. With the aid of laser induced fluorescence measurements, ab initio pair potential calculations, and diatomics-in-molecules simulations these novel broad lines at 241 nm in Ar, 247 nm in Kr, and 254 nm in Xe were assigned to boron 3s(2S)←2p(2P) transitions. The thermal behavior of the broad absorption bands is similar to what was reported for boron atoms detected by electron paramagnetic resonance [J. Chem. Phys. 114, 7144 (2001)]. The revised interpretation based on the new spectroscopic observations is in good agreement with theoretical predictions.

Temperature-dependent band structure of bulk EuO

We present calculations for the temperature-dependent electronic structure of bulk ferromagnetic EuO based on a parametrization of the d–f model Hamiltonian using results of first-principles TB-LMTO band-structure calculations. The presented method avoids the problem of double counting of relevant interactions and takes into account the symmetry of the atomic orbitals. It enables us to determine the temperature-dependent band structure of EuO over the entire temperature range.

Temperature dependence of the optical energy gap and Urbach–Martienssen’s tail in the absorption spectra of the layered semiconductor Tl 2GaInSe 4

The effect of temperature on the optical energy gap for Tl 2GaInSe 4 layered crystals were investigated in the temperature range of 10–330 K. The variation of the optical absorption coefficient with energy near the band edge was analysed by Elliot’s model. From the analysis of the temperature dependent band gap with Manoogian and Woolley’s semi-empirical model, the Debye temperature, the dielectric constant and the effective masses of free excitons, electrons and holes were estimated as (373±13) K, 9.2±0.3, (0.31±0.06) m 0, (0.42±0.08) m 0 and (1.16±0.22) m 0, respectively. Effective phonon energy was obtained as hν p=56±1 meV from the temperature dependence of the Urbach’s energy. This relatively large hν p value was associated with the electronic distortion in addition to the influence of electron/exciton–phonon interaction. The electronic distortion results from the existence of multiple structural defects associated with two dimensional dislocations or stacking faults, presumably originated from voids and precipitates of the component elements, in the Tl 2GaInSe 4 lattice.

Electronic Properties of Bismuth Nanowires

ABSTRACTThe pressure filling of anodic alumina templates with molten bismuth has been used to synthesize single crystalline bismuth nanowires with diameters ranging from 7 to 200nm and lengths of 50μm. The nanowires are separated by dissolving the template, and electrodes are affixed to single Bi nanowires on Si substrates. A focused ion beam (FIB) technique is used first to sputter off the oxide from the nanowires with a Ga ion beam and then to deposit Pt without breaking vacuum. The resistivity of a 200nm diameter Bi nanowire is found to be only slightly greater than the bulk value, while preliminary measurements indicate that the resistivity of a 100nm diameter nanowire is significantly larger than bulk. The temperature dependence of the resistivity of a 100nm nanowire is modeled by considering the temperature dependent band parameters and the quantized band structure of the nanowires. This theoretical model is consistent with the experimental results.

Temperature-dependent quasiparticle conduction bandstructure of the EuS monolayer

The temperature-dependent quasiparticle band structure of aferromagnetic EuS monolayer is calculated using the s-f model whichcombines the one-electron band structure and a many-body modelevaluation. The one-electron part required for the many-bodycalculation was obtained from a TB-LMTO (tight-binding linear muffin-tin orbital) band-structure calculation.A suitable supercell with five layers of empty spheres was used inthe band-structure calculations to obtain the Bloch energies of themonolayer. We find striking correlation effects in thequasiparticle spectrum induced by the s-f exchange interaction andalso observe significant temperature effects in the spectrum. Wehave further calculated the quasiparticle spectrum of the bulk EuSusing the same s-f model in order to compare the results with thosefor the monolayer and to analyse the influences of the lowdimensionality on the quasiparticle spectrum of this system. Thebulk spectrum also exhibits significant correlation and temperatureeffects. The conduction bands of the monolayer as well as the bulkEuS show red-shift with respect to temperature and the magnitude ofthe red-shift is found to be larger for some bands in the bulk EuScompared to that of the monolayer.

Ground-State Proton-Transfer Tautomer of the Salicylate Anion

Solutions of sodium salicylate in anhydrous polar solvents exhibit a weak, temperature-dependent absorption band (λmax ≈ 325 nm) lying in the Stokes gap between the main absorption (296 nm) and the fluorescence band (396 nm, acetonitrile). This weak, longer wavelength absorption band is hardly observable in aqueous solution, but its intensity increases with temperature and increases with polarity in anhydrous organic solvents in the order of ethanol < acetonitrile < dimethyl sulfoxide at room temperature. After correction for solvent thermal contraction, the temperature-dependent absorption spectrum of salicylate in acetonitrile solutions reveals a clear isosbestic point (e310 = 2000 M-1 cm-1) characteristic of an equilibrium between two salicylate species with band-maximum extinction coefficients of e325 = 3400 M-1 cm-1 and e296 = 3586 M-1 cm-1. In acetonitrile at room temperature (298 K) the concentration equilibrium constant (minor/major) for the interconversion reaction between the two species is K298...

Temperature dependence of electronic optical absorption in (NMe 4++ · TCNQ −·) 2TCNQ

The temperature dependence of the linearly polarized optical absorption spectrum has been measured in single crystals of (NMe 4 +·TCNQ −·) 2TCNQ, where NMe 4 and TCNQ are tetramethylammonium and tetracyanoqui nodi methane, respectively, in the temperature range between −100 and +150 °C. In addition to intense TCNQ −· dimer bands, strongly temperature-dependent satellite bands appear at around 1.3 eV and 1.9 eV. The singular behavior of the intensity and peak position of the 1.3 eV band shows that a second order phase transition takes place at 126 °C. The origin of this band is discussed in relation to the mechanism of the phase transition.