Specific features of the transformation of local levels into an impurity band of Ag<Subscript>1−<Emphasis Type="Italic">p</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">p</Emphasis></Subscript> disordered solid solutions

The phonon densities of states in disordered solid solutions of aluminum in silver are determined using the Jacobi matrix method. The transformation of discrete vibrational levels (local vibrational modes) into an impurity band with increasing concentration of impurity atoms is investigated. The formation of discrete vibrational levels is associated with the presence of individual aluminum impurity atoms in the lattice. It is demonstrated that, at relatively low (approximately 5–15%) aluminum concentrations, the broadening of the main local level is accompanied by the appearance of additional resonance peaks, which contain important information on the interatomic interactions in these solid solutions.     

Short-range-order lifetime and the "boson peak" in a metallic glass model

We extend the usual static view of short range order in metallic glasses to a dynamical model of local order. We use an atomistic simulation of a NiZr glass to investigate time-dependent fluctuations of the atomic environment. We show that, even in the "frozen" glass, the solute-centered clusters change their identities between distinct polyhedron types. The frequency spectrum of these transitions exhibits a characteristic peak which we show to be related to a universal vibrational anomaly of disordered solids: the controversial boson peak.     

Phonons and martensitic phase transitions in disordered Li−Mg alloys

We present a calculation of the phonon spectra and of the martensitic transition temperature for the bcc»hcp transformation in Li–Mg alloys. The treatment is based ona. pseudopotential-derived interatomic forces,b. a cluster relaxation technique for determining the static distortions of the mean crystal lattice in a substitutional alloy, andc. a recursion method for calculating the spectral functions and vibrational densities of state. We report that the observed variation of the martensitic temperature with concentration results from a delicate interplay of electronic and vibrational effects.This work was supported by the Jubiläumsfonds der Österreichischen Nationalbank under project no. 2141     

Vibrational contribution to the thermodynamics of nanosized precipitates: vacancy-copper clusters in bcc-Fe

The effects of lattice vibration on the thermodynamics of nanosized coherent clusters in bcc-Fe consisting of vacancies and/or copper are investigated within the harmonic approximation. A combination of on-lattice simulated annealing based on Metropolis Monte Carlo simulations and off-lattice relaxation by molecular dynamics is applied to obtain the most stable cluster configurations at T = 0 K. The most recent interatomic potential built within the framework of the embedded-atom method for the Fe-Cu system is used. The total free energy of pure bcc-Fe and fcc-Cu as well as the total formation free energy and the total binding free energy of the vacancy-copper clusters are determined for finite temperatures. Our results are compared with the available data from previous investigations performed using many-body interatomic potentials and first-principles methods. For further applications in rate theory and object kinetic Monte Carlo simulations, the vibrational effects evaluated in the present study are included in the previously developed analytical fitting formulae.     

Intrinsically minimal thermal conductivity in cubic I-V-VI2 semiconductors

We report measurements of the thermal conductivity of high-quality crystals of the cubic I-V-VI2 semiconductors AgSbTe2 and AgBiSe2. The thermal conductivity is temperature independent from 80 to 300 K at a value of approximately 0.70 W/mK. Heat conduction is dominated by the lattice term, which we show is limited by umklapp and normal phonon-phonon scattering processes to a value that corresponds to the minimum possible, where the phonon mean free path equals the interatomic distance. Minimum thermal conductivity in cubic I-V-VI2 semiconductors is due to an extreme anharmonicity of the lattice vibrational spectrum that gives rise to a high Grüneisen parameter and strong phonon-phonon interactions. Members of this family of compounds are therefore most promising for thermoelectric applications, particularly as p-type materials.     

Calculation of Vacancy Induced Properties in FCC Metals Using an Alternative Lattice Dynamical Model

Combining the conventional Kanzaki's lattice statics method with an alternative lattice dynamical model called decoupling transformation, a new approach is proposed to calculate the vacancy induced properties in FCC metals. This approach has the advantage over the traditional least-square fit approach in the way that all the model parameters are linearly independent and so it can avoid the problem of non-uniqueness in dynamical model parameters when interactions with more distant atoms are taken into account by fitting to the experimental phonon frequencies only. Numerical results on the lattice dynamical properties such as phonon dispersion curves and interatomic interaction force constants and the vacancy induced properties such as the atomic displacements, lattice relaxation energy, divacancy interaction energy and relaxation volume are obtained specifically for two similar FCC metals, to wit, Cu and Ni and are compared with previous calculations. It is concluded that the interatomic interactions up to the fourth nearest neighbours are already good enough for describing both the lattice vibrational and vacancy induced properties for both metals.     

Vibrational behaviour of self-interstitials infcc-metals

The vibrational properties of self-interstitials in the octahedral and 100-dumbbell configurations are studied for anfcc lattice with nearest neighbour interaction. Due to the local compression of the lattice in the vicinity of the interstitial both low-frequency resonant and high-frequency localized modes occur. The calculation of the local vibrational spectrum of the interstitial, with force constants fitted to the results of computer simulation, shows that the defect vibrates almost exclusively with resonant and localized frequencies and not with the typical eigenfrequencies of the ideal lattice. Analytical approximations for the resonant and localized modes are derived which show good agreement with the exact results. The low frequency resonances of the interstitial lead, even at low temperatures, to large thermal-displacements and to an remarkable increase of the specific heat.     

The vibrational modes of hydrogen adsorbed on Ni(110)

By studying the vibrational modes of H on Ni(110) as a function of coverage and temperature, a local picture of H site occupation is obtained in the lattice gas regime and on the (1 × 2) reconstructed surface at low temperature and for the irreversibly disordered surface formed by thermal conversion. Threefold sites are deduced from our data in both the lattice gas and the (1 × 2) reconstructed low temperature phases, with an additional loss in the latter phase ascribed to a second layer site. After thermal conversion, the threefold sites are depleted with a ? 0.5 monolayer (ML) transfer of H to second layer sites which appear to stabilize the surface and with ~ 0.5 ML H desorbing. Readsorption on the disordered surface indicates that a small amount of empty threefold Ni sites are still present after conversion. Various other models and site assignments are also discussed for comparison to the results of this study.     

The energy spectrum of disordered systems

A detailed report is given of the theoretical work carried out by the author during recent years on problems connected with the energy spectrum of a disordered solid. The treatment is general, with indications of applications to particular problems, such as the lattice vibrational spectrum of an imperfect crystal and the electronic levels associated with impurity atoms. Particular attention is given to the behaviour of the density of states (spectral density) of both the levels derived from those of the perfect crystal and the impurity levels. The effects of ordering of imperfections is considered, and some discussion of other approximations and models is given.     

The role of vibrations in thermodynamic properties of Cu-Ni alloys

We report results of a systematic study for vibrational thermodynamic functions of Cu-Ni alloys, in the harmonic approximation, using interaction potentials based on the embedded atom method with improved optimization techniques. The vibrational density of states of the systems is calculated using real space Green’s function method. From an investigation of local force fields we found that increasing Ni concentration in the alloy substantially stiffens the force experienced by Cu atoms compared to that of Ni atoms. Our calculations also reveal that vibrational entropy change between ordered and disordered crystals of Cu-Ni is negligible. However, the mixing entropy of the phonons and electronic states is found to be negative and favors un-mixing, and thus contributes to the miscibility gap.     

Vibrations of linear cobalt clusters on a stepped copper surface

The results of calculations of vibrational properties of cobalt dimers and trimers on the Cu(110) surface are reported. The local phonon density of states and their polarizations are analyzed. The calculations are performed using interatomic interaction potentials constructed in a tight-binding approximation. It was shown that the vibrational modes of the free standing dimer and trimer remain in the supported clusters but the revealed anisotropy of the local surface relaxation leads to deformation of the interatomic bonds and, as a result, to splitting and shifting of the phonon frequencies. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 67–73, January, 2009.     

Twist in an exactly solvable directed lattice ribbon

We investigate the transition between a twisted regime and a disordered regime in a directed ribbon model on a cubic lattice. A fugacity corresponding to an interaction which models half-twists in the ribbon is introduced and the interacting model is solved exactly. Our results suggest that conformational entropy and a local interaction which induces twist are key ingredients to model qualitatively the crossover behavior between a twisted (helical) regime and a denatured regime in duplex biopolymers such as DNA.     

Comparative study of vibrations in submonolayer structures of potassium on Pt(111)

We present results of a comparative study of the vibrational spectrum and local density of phonon states in ordered p(2 x 2) and (√3 x √3)R30° structures formed by potassium atoms on the Pt(111) surface. The calculations were performed with tight-binding interatomic interaction potentials. It was found that the mode associated with vertical displacements of K adatoms has an energy of about 20 meV in both K structures. The strength and energy of this mode slightly decreases with increasing coverage. This result is in good agreement with available experimental data. As in time-resolved second harmonic generation measurements, we observed low frequency modes for both structures considered, which are caused by the interaction of potassium with the second layer of the substrate.     

Anderson localization in a two-dimensional random gap model

We study the properties of the spinor wavefunction in a strongly disordered environment on a two-dimensional lattice. By employing a transfer-matrix calculation we find that there is a transition from delocalized to localized states at a critical value of the disorder strength. We prove that there exists an Anderson localized phase with exponentially decaying correlations for sufficiently strong scattering. Our results indicate that suppressed backscattering is not sufficient to prevent Anderson localization of surface states in topological insulators.     

Initial applications of the molecular model to compute defect vibrations of oxygen in silicon

Abstract

We have employed the molecular model introduced first by Jaswal to compute the vibrational spectra of oxygen bearing defects in a silicon crystal. This was done in the context of a silicon molecular cluster with outer valencies terminated by hydrogen. We employ the MINDO/3 semi-empirical electronic structure method to compute the total energy of the molecular cluster. We examine the conditions in applications of the molecular model required for accurate predictions of oxygen local-mode vibrational frequencies. We find that the oxygen atom and its nearest neighbor silicon atoms must be allowed to vibrate. The nearest-neighbor and next nearest-neighbor shells of silicon atoms must be allowed to relax from their lattice positions. The outermost relaxed shell of silicon atoms should be bonded to silicon atoms in their lattice positions. We apply the molecular model to three defects of crystalline silicon; interstitial oxygen, oxygen in a vacancy (the A-center), and two oxygen atoms in a vacancy. Comparison of our computed local-mode oxygen vibration frequencies with experiment shows the computed oxygen local-mode frequencies to be almost uniformly 10% greater than those observed. Isotope shifts fit experiment equally well. We conclude that the molecular model represents an accurate and efficient approach for the computation of defect local mode vibrational frequencies for oxygen and other defects in crystalline silicon.     

Strain-induced bond buckling and its role in insulating properties of Cr-doped V2O3

Structural transformations around both V and Cr atoms in (V1-xCrx)2O3 across its metal-insulator transition (MIT) at x approximately 0.01 are studied by extended x-ray absorption fine-structure technique. Our new results for Cr made possible by the use of a novel x-ray analyzer that we developed reveal the substitutional mechanism of Cr doping. We find that this system has a buckled structure with short Cr-V and long V-V bonds. This system of bonds is disordered around the average trigonal lattice ascertained by x-ray diffraction. Such local distortions can result in a long range strain field that sets in around dilute Cr atoms in microscopic regions. We suggest that such locally strained regions should be insulating even at small x. The possibility of local insulating regions within a metallic phase, first suggested by Rice and Brinkman in 1972, remains unaccounted for in modern MIT theories.     

Vibrational response of hydrogen-bonded interfacial water is dominated by intramolecular coupling

Using the surface-specific vibrational technique of vibrational sum-frequency generation, we reveal that the double-peaked structure in the vibrational spectrum of hydrogen-bonded interfacial water molecules originates from vibrational coupling between the stretch and bending overtone, rather than from structural effects. This is demonstrated by isotopic dilution experiments, which reveal a smooth transition from two peaks to one peak, as D2O is converted into HDO. Our results show that the water interface is structurally more homogeneous than previously thought.     

Experimental realization of the fuse model of crack formation

In this work, we present an experimental investigation of the fuse model. Our main goal was to study the influence of the disorder on the fracture process. The experimental apparatus used consisted of an L x L square lattice with fuses placed on each bond of the lattice. Two types of materials were used as fuses: copper and steel wool wires. The lattice composed only of copper wires varied from a weakly disordered system to a strongly disordered one. The lattice formed only by steel wool wires corresponded to a strongly disordered one. The experimental procedure consisted of applying a potential difference V to the lattice and measuring the respective current I. The characteristic function I(V) obtained was investigated in order to find the scaling law dependence of the voltage and the current on the system size L when the disorder was changed. Our results show that the scaling laws are only verified for the disordered regime.     

Spontaneous-emission control by local density of states of photonic crystal cavity

The local density of states (LDOS) of two-dimensional square lattice photonic crystal (PhC) defect cavity is studied.The results show that the LDOS in the centre is greatly reduced,while the LDOS at the point off the centre (for example,at the point (0.3a,0.4a),where a is the lattice constant) is extremely enhanced.Further,the disordered radii are introduced to imitate the real devices fabricated in our experiment,and then we study the LDOS of PhC cavity with configurations of different disordered radii.The results show that in the disordered cavity,the LDOS in the centre is still greatly reduced,while the LDOS at the point (0.3a,0.4a) is still extremely enhanced.It shows that the LDOS analysis is useful.When a laser is designed on the basis of the square lattice PhC rod cavity,in order to enhance the spontaneous emission,the active materials should not be inserted in the centre of the cavity,but located at positions off the centre.So LDOS method gives a guide to design the positions of the active materials (quantum dots) in the lasers.