Diffusion of phonons through (along and across) the ultrathin crystalline films

Instead of usual approach, applying displacement-displacement Green's functions, the momentum-momentum Green's functions will be used to calculate the diffusion tensor. With this type of Green's function we have calculated and analyzed dispersion law in film-structures. A small number of phonon energy levels along the direction of boundary surfaces joint of the film are discrete-ones and in this case standing waves could occur. This is consequence of quantum size effects. These Green's functions enter into Kubo's formula defining diffusion properties of the system and possible heat transfer direction through observed structures. Calculation of the diffusion tensor for phonons in film structure requires solving of the system of difference equations. Boundary conditions are included into mentioned system through the Hamiltonian of the film-structure. It has been shown that the diagonal elements of the diffusion tensor express discrete behavior of the dispersion law of elementary excitations. More important result is that they are temperature independent and that their values are much higher comparing with bulk structures. This result favors better heat conduction of the film, but in direction which is perpendicular to boundary film surface. In the same time this significantly favors appearance 2D superconducting surfaces inside the ultra-thin crystal structure, which are parallel to the boundary surface.This is the peer-reviewed version of the article: Šetrajčić, Jovan P., Stevo Jaćimovski, and Siniša M. Vučenović. 2017. ‘Diffusion of Phonons through (along and across) the Ultrathin Crystalline Films’. Physica A-Statistical Mechanics and Its Applications 486: 839–48. [https://doi.org/10.1016/j.physa.2017.06.003

Moguća stanja nosilaca naelektrisanja u tankim višeslojnim superprovodnim keramikama

This paper analyzes behavior (spectra and states) of the elementary charge carriers in anisotropic perovskite structures, such as modern superconducting ceramics. Translational symmetry of the atom (ion) distribution of the electron (or hole) system is broken by atomic/ionic/molecular sputtering and doping, as well as due to existence of two boundary surfaces. This is a charge carrier's model of high-temperature superconductors in which the observed symmetry breaking orthogonal to CuO planes was treated as a perturbation. The single-particle fermion's wave functions and the possible energies of charge carriers were determined.U radu je analizirano ponašanje (spektri i stanja) elementarnih nosilaca naelektrisanja u anizotropnim perovskitnim strukturama, kakve su savremene superprovodne keramike. Translaciona simetrija atomskih (jonskih) rasporeda sistema elektrona (ili šupljina) je narušena atomskim/jonskim/molekulskim rasprašivanjem (spaterovanjem) i dopiranjem, kao i postojanjem dveju graničnih površi. Ovo je model nosilaca naelektrisanja kod visoko-temperaturskih superprovodnika u kojem se posmatrano narušenje simetrije normalno na CuO ravni tretira kao perturbacija. Određene su jedno-čestične fermionske talasne funkcije i mogu e energije nosilaca naelektrisanja

Phonon Contribution in Thermodynamics of Nano-Crystalline Films and Wires

Spectra of possible phonon states, as well as thermodynamic characteristics of nanocrystals (ultrathin film and quantum wire) of simple cubic crystalline structure are analyzed in this paper, using the method of two-time dependent Green functions. From energy spectra and internal energy of the system the thermal capacitance of these structures in low temperature region is found. The temperature behavior of specific heat is compared to that of corresponding bulk structure. It is shown that at extremely low temperatures thermal capacitance of quantum wire is considerably lower than the thermal capacitance 4 film as well as the bulk sample. Consequences of this fact are discussed in detail and its influence to thermodynamic properties of materials is estimated

Discrete resonant absorption in molecular nanofilms

We have analysed the significant different optical absorption of molecular nanofilm crystalline formations. The main reason for this behaviour is in the existence of boundaries on the film structures and we needed to investigate these properties theoretically. In this process, we have done some combinations of (where possible) analytical calculations and (in other cases) numerical calculations. We have calculated the allowed energies of exciton (quasi)particles, and their appearance in nanofilms but in particular direction - perpendicular to the surface of the film. We have analysed why surface exciton localization appears in a film, and after that we calculated dielectric permittivity and optical indices (absorption and refraction). We have established that boundary parameters greatly influence optical phenomena. This influence is resonant, i.e. frequency-related and selectable (depends on what layer is calculated). We even found some particular conditions for the appearance of one (single) absorption line

NANOTEHNOLOGIJE U GRAĐEVINSKIM MATERIJALIMA

Nanotehnologije su u stanju pružiti velike mogućnosti u unapređivanju, prvenstveno termodinamičkih i mehaničkih, osobina građevinskih materijala, najčešće betona i betonskih konstrukcija. Nanomaterijali poboljšavaju održivost, izdržljivost, ali i neka druga svojstva kao što je, npr., mogućnost samočišćenja. Naučna istraživanja nanostruktura su u stalnom porastu, pružajući nova saznanja o potencijalu njihove primjene u građivinarstvu, čime se otvaraju mnogobrojne mogućnosti u poboljšanju karakteristika, ali i smanjenju troškova proizvodnje i ugradnje građevinskih materijala. Ovaj rad će dati pregled nekih osnovnih definicija nanotehnologija i karakterizacije nanostruktura, kao i nanoaditiva koji su danas najznačajniji u unapređenju građevinskih (betonskih) proizvoda

New thermoelectrics in functionofattenuation of heat dissipation

U radu su predstavljeni rezultati istraživanja termoelektričnih svojstava pniktida rijetkih zemalja i monoslojnog grafena. Oba materijala pokazuju izuzetnu sklonost i efikasnost u transformaciji toplote u električnu struju, te su perspektivni za primjenu kao „čistači“ toplotnog otpada.We have introduced in this paper results of research of thermoelectric properties of the rear earth pnictides and monolayer graphene. Both of these materials show exceptional tendency and efficiency in transformation from heat to electricity, and have perspective application in heat harvesting