Unperturbed Lattice Research Articles

An FTIR-ATR Study of the Incorporation of the Peptide Gramicidin D into DPPA Self-Assembled Multilayers

Self-assembled multilayer films (n ≃ 2-45) of the phospholipid dipalmitoyl L-α-phosphatidic acid (DPPA) have been shown, by polarized FTIR-ATR spectroscopy, to be well-ordered and packed in an all-trans configuration in a hexagonal crystalline lattice. Incorporation of the peptide gramicidin D results in a largely unperturbed lipid lattice (on the infrared length scale). This implies strongly that peptide aggregation occurs, as was also found for the same peptide in Langmuir-Blodgett multilayers of the same lipid.

The magnetic properties of planar ferromagnets with different types of structural disorder

The magnetic properties of planar ferromagnets are investigated in the frame of the integral operator technique. The various kinds of structural disorder described by solid dilution and amorphisation are examined. For the calculation of the phase diagrams, a simple comparative method is proposed. This method can be applied to such disordered systems where, for the unperturbed lattice, exact solutions exist.

Alkali-doped (CH) x and derivative systems: Resonant raman scattering experimental spectra and theoretical analysis

We present new experimental Raman spectra of 13C enriched trans(CH) x and trans(CD) x doped chemically with potassium at a high level (15%). Results are compared to the previous investigations carried out on standard trans(CH) x doped identically. The isotopic effects which are observed on the Raman bands are studied as a function of the excitation wavelength in the wide range from the near U.V. to the near I.R.. As previously proposed in the case of n-doped trans(CH) x, the doping induced features are interpreted in the frame ofthe perturbed Green function method. The new vibrational Raman modes are calculated by taking into account in the lattice dynamics of the conjugated segments, the perturbation induced by the trapped charges on the polymeric chains. This is expressed by the parameter Λ which is the change of the force constants with respect to the unperturbed lattice, due to the modifications of the electron-electron and electron-vibrations interactions. Good agreement is found between the calculated and experimental Raman active frequencies in K-doped ( 13CH) x and (CD) x, as well as in standard (CH) x.

Characterization of Cd implanted and annealed GaAs and InP by perturbed angular correlation (PAC) spectroscopy

Investigations of 111mCd implanted GaAs and InP crystals using the microscopically sensitive perturbed angular correlation technique show that the implanted Cd is incorporated on unperturbed substitutional lattice sites during rapid thermal annealing at significant lower temperatures than for electrical activation is required. In GaAs the higher implantation temperature at 473 K did not show any influence on this annealing stage, whereas as higher implantation dose hinders the annealing. We conclude that not only the local environment of the implant but also the long-range lattice perfection has to be restored for the electrical activation of implants in III-V compound semiconductors.

N-doped transpolyacetylene Raman spectra: Analysis of the doping induced bands

In n-doped transpolyacetylene samples, additional Raman bands are observed experimentally in the Raman spectra for a dopant concentration y ⪢ 5%. These new features in trans (CH) x are peaked at ω ⋍ 1270 cm −1 and ω ⋍ 1560–1600 cm −1 , while in trans(CD) x at ω ⋍ 1075 cm −1 and ω ⋍ 1550 cm −1 , respectively. In this paper, a model is proposed to give an interpretation of these new Raman bands in the frame of the perturbed Green formalism. In this model, the new vibrational Raman modes are calculated by taking into account in the lattice dynamics of the conjugated segments the perturbation induced by the trapped charges on the polymeric chains. This is expressed by ⋀ the change of the force constants with respect to the unperturbed lattice, due to the modifications of the electron-electron and electron-vibration interactions. Good agreement is found between the experimental and calculated Raman frequencies in both the n-doped(CH) x and (CD) x systems.

Diffusion of iron in copper studied by Mössbauer spectroscopy on single crystals.

The broadening of the $^{57}\mathrm{Fe}$ M\"ossbauer line due to diffusion jumps of iron atoms in a copper single crystal has been measured as a function of crystal orientation and temperature between 1060 and 1313 K. The results are the following: (i) The anisotropy of the broadening agrees with what is expected for 〈110〉 jumps into nearest-neighbor vacancies. (ii) The deviation of the measured anisotropy from the curve predicted for self-diffusion permits the determination of the vacancy jump frequencies in the neighborhood of the iron impurity. The best fit indicates that association and dissociation jumps of the vacancy with or from the impurity are slower, but internal jumps in the vacancy-impurity complex are faster than vacancy jumps in the unperturbed copper lattice. (iii) From the temperature dependence of the resonant intensity (the area of the M\"ossbauer line) it can be concluded that there is no significant impurity-vacancy binding. (iv) The diffusion coefficient for iron in copper and its temperature dependence are in good agreement with tracer results. At 1313 K, D=(4.2\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{m}}^{2}$/s.

Colour centres and luminescence in KCl:Ni crystals

AbstractOptical absorption, thermoluminescence (TL), and monochromatic thermoluminescence (MTL) glow curves, TL and X‐ray luminescence spectra of KCl:Ni crystal are measured. Strong absorption bands appear at 218 and 275 nm due to the transition of an electron from adjacent Cl− ion to Ni++ ion. The reduction of these bands upon X‐irradiation shows the trapping of electrons by the divalent impurity ions. Only the first peak at 90 °C appears in the TL and MTL glow curve. F bleaching neither produces Z centres nor reduces the V band appreciably but decreases the TL glow curve heavily. These results are also interpreted in terms of electron trapping by impurity ions. Emissions due to decay of STE (Cl ion trapping an electron in the excited state) near free cation vacancy (445 nm) or unperturbed lattice sites (545 nm) are very weak while those near Ni++ ions, free (585 nm) or associated with cation vacancies (380 nm), are very strong. This is due to the high mobility of STE and Cl centres towards Ni++ ions.

In-beam studies of defect cascades in metals

The PAD method allows the investigation of recoil-induced defect cascades during the first 10 −9−10 −5 s after their production by nuclear reactions. A short review of experimental results and their interpretation is given. Recent very accurate measurements of 69Ge in a 66Zn single crystal performed in the temperature range between 25 K and 580 K allow one to draw conclusions concerning the dynamical rearrangement of the radiation damage in the vicinity of the primary recoil atom. The measured spectra for temperatures below 500 K deviate from the modulation pattern expected for static defect distributions around the probe atoms. This proves that the temperature dependence of the fraction of probe atoms located at unperturbed substitutional lattice sites has to be explained in terms of trapping and detrapping of defects. No distinct defect induced quadrupole interaction frequency could be resolved. The increase in the observed anisotropy between 400 K and 500 K is analyzed in the framework of a new extended model for detrapping processes in noncubic metals. This model reproduces the observed temperature dependence of the different harmonics of the perturbation function. It is shown that the center of the efg distribution caused by the trapped defects is shifted to a value close to zero. It thus deviates significantly from the value of the substitutional lattice site. The dissolution energy of the probe-defect complex was deduced to be 0.54(5) eV.

Mechanism of the tubular diffusion of helium

In fact, in conditions of prolonged annealing, the solution of vacancies in the lattice is in equilibrium with the dislocations, and hence the chemical potential of the vacancies is zero. In consequence, in the absence of helium, the pores are in conditions of solution (infinitely large critical dimension), regardless of their positions with respect to the dislocations. In the presence of helium in the lattice, however, its admission into the pores is a stimulatory cause of their development. In this case the pore position with respect to the dislocations is significant - the contribution of the tubular-diffusion mechanism of helium considerably increases its intake into the pores at dislocations. It is known that the vicinity of the dislocation core is characterized by elevated mobility of point defects, impurity atoms, and gas in the direction along the line of the dislocation [2]. The energy of migration along the dislocation lines, according to the estimate of [3], is approximately half the energy of migration in the unperturbed lattice. With regard to motion in the direction normal to the dislocations, these regions are characterized by considerably smaller mobility of the diffusing agent and, in addition, may be separated from the unperturbed lattice by a small potential barrier [4]. This difference in the absolute values and symmetry of mobility for a regular lattice and the regions adjacent to dislocations leads to the appearance of the tubular-diffusion mechanism: The diffusing atoms incident in the region adjacent to a dislocation are held there to a certain extent by the potential barrier and migTate with increased velocity along the dislocation. The path of the diffusing atom along the dislocation may amount to many thousands of lattice constants before the atom returns to the unperturbed lattice, if the chemical potential of the diffusing atom is changed here (e.g., the dislocation leaves the surface, or intersects apore), there arises a direction flux along the dislocation line sustained by the influx of atoms from the lattice to the dislocation. In experiments on the annealing of samples preliminarily irradiated with helium, the pores exist in a two-component solution of vacancies (of concentration CV) and helium atoms (of concentration Cg). The development of a pore consisting of n(t) vacancies and x(t) helium atoms at time t is expediently considered in the plane (n, x) [5] (Fig. 1) with the equations of motion

Interaction of Si, Ge, and Be solutes with vacancies and interstitials in Ni

The solute-vacancy and the solute-interstitial binding energies for Si, Ge, and Be solutes in Ni matrix have been calculated, using the results of an augmented-plane-wave calculation of the electronic structure of the solute and the solvent atoms. The electronic wave function of the solute is expressed in terms of the wave functions of the unperturbed host lattice. This allows the partial densities of states (DOS) of different angular momentum type at the solute site to be written in terms of those at the solvent atom site and their energy-dependent phase shifts. It is found that the $d$ electron DOS which is large and dominant at the Ni site is rather small at the solute site for all the solutes studied here. At the Si and Ge sites the $p$ DOS is dominant while at the Be site both $s$ and $p$ components are important. The excess charge density displaced by the solute and the potential associated with it show the oscillatory behavior. The $3s$ and $4s$ electrons of Si and Ge, respectively, form bound states below the bottom of the Ni conduction band. These states influence in a profound manner the interaction of these solutes with both vacancies and interstitials. No bound state is formed in the case of Be, where the scattering states in the conduction band are the sole contributors to the interaction energy. We have found that Ge, even though oversized, has a positive binding energy of 0.28 eV with the interstitial in the mixed dumbbell configuration. The undersized Si and Be form mixed dumbbells with binding energies of 0.90 and 0.58 eV, respectively. Contrary to usual expectations the vacancy-solute binding is calculated to be rather strong, 0.73 and 0.55 eV, respectively, for Si and Ge in the nearest-neighbor vacancy-solute configuration. The corresponding interaction with Be is found to be repulsive with a binding energy of -0.32 eV.

On the Interaction between a Vacancy and a Dislocation in Metals

AbstractThe interaction ΔEvd between a vacancy and a (screw or an edge) dislocation in metals is investigated using a tight‐binding‐type electronic theory. The repulsive core‐core interaction energies at short distances are simulated by the Born‐Mayer potential. It is shown that there are systematic differences in the ΔEvd between the present results and those obtained by using a pair potential method. Furthermore, it is found that the atomic configuration around a vacancy in the dislocation core region is essentially different from that in the unperturbed lattice.

Density of states determination of mixed semiconductors by neutron diffraction

The phonon density of states of mixed crystals has been determined by neutron diffraction in powders. In the “two mode type” mixed crystal ZnSxSe1-x, the great features of the host lattice density of states, are not modified up to an “impurity” concentration of 25%. On the other hand, the presence of Te in ZnSexTe1-x with x = 0.25, give rise to a “one-mode type” mixed crystal, the density of states of which is an average of that of the two components.These results justify that the eigenfrequencies of a two modes type mixed crystal can be calculated using the unperturbed host lattice density of states (additivity of Green's function).

The influence of ω-phase on the superconductivity of Ti-Mo alloys

Abstract In Ti-Mo alloys the maxima in the low-temperature electronic specific heat coefficient (λ) and the superconducting transition temperature ( T c ) which occur near 20 at.% Mo ( e / a ∼ 4.4) are induced by omega-phase precipitates in the lower concentration alloys, which decrease λ and T c below the values expected for an unperturbed bcc lattice.

Lattice Vibrations of Solid Solutions; Infrared Absorption Spectra of Nitrate Ions in Alkali Halides

Infrared-absorption spectra of alkali halide single crystals with nitrate-ion impurities show a fine structure on the internal vibrational bands of the nitrate ion that must be ascribed to combinations with the lattice modes of the crystal. A part of these combinations arises from lattice modes that are practically the same as for the unperturbed lattice. Specific heats calculated from the observed spectra are in excellent agreement with the calorimetrically determined values for the pure alkali halide crystals. Other features in the experimentally obtained spectra must be ascribed to combinations with local modes. Group-theoretical considerations lead to selection rules for first-order transitions involving these lattice modes, as well as for absorption due to combinations with the internal vibrational or electronic transitions. A model calculation gives the frequencies of the local modes in terms of the eigenvectors and eigenfrequencies of the unperturbed lattice with the ratio of the masses of the substituted ion and the impurity ion as parameter. Calculated local-mode frequencies are compared with the observed frequencies and with frequencies that have been reported in the literature.

Localized Vibrational Modes at Plane Faces of Defects in Crystal Lattices

AbstractThe localized modes of vibrations are dicussed for the planes (001), (011), and (111) of impurity atoms in an infinitely extended simple cubic lattice. The frequencies show a band structure according to the translational invariance of the lattice in the impurity plane. It is found that optical and accoustical bands exist, the former lying completely, the latter only partially in the band of the unperturbed lattice. A free surface can be regarded as a special case of these models. Choosing appropriate coupling parameters one obtains localized surface states which have to coincide with the well‐known Rayleigh waves in the limiting case of long waves.

Degenerate Germanium. II. Band Gap and Carrier Recombination

The data bearing on the band structure of degenerate germanium have been abstracted from the literature and intercompared. A variety of effects are described in terms of a voltage characteristic of each. It is found that all the voltages have essentially the same value and show a striking independence of the carrier concentrations. The conclusion is drawn that this voltage closely represents the band gap of degenerate germanium. It is shown that the thermal current at low temperature must be carried by recombination in the junction, which gives the proper barrier if the recombination centers lie near the band edge. This model of recombination explains the thermal current, the minority carrier lifetime, the excess current, and the emission spectrum. The reflectivity peak at 2.2 ev is also consistent if the bands at the [111] edge are roughly parallel to each other. The barrier found from the transition capacitance is not understood. The recombination centers lie close to the band edge. Assuming they are donors and acceptors, their capture cross section at room temperature is about ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ per neutral atom. On the basis of this model, the thermal gap of highly degenerate germanium is about 30 mv less than for the unperturbed lattice. The shrinkage is independent of temperature.

The edge emission of ZnS, CdS and ZnO and its relation to the lattice vibrations of these solids

It is shown that the vibrational structure appearing in the luminescent emission close to the fundamental absorption bands of ZnS, CdS and ZnO corresponds to the frequency of the longitudinal lattice vibrations of the optical branch, either alone or in combination with the transverse vibrations of this branch. This proves that the edge emission is a property of the pure, unperturbed lattice. In connection with observations by Klick as well as from fundamental theoretical reasoning it is concluded that the fluorescence is due to excitons. Weak bands, appearing at shorter wavelengths are attributed to band-band transitions or to higher exciton states. The appearance of double peaks in the infra red absorption spectrum of ZnO and BeO is explained by a process which involves phonons of the optical and acoustical branch according to a theory given by Born and Blackman.