Identification of Ultrafast Photophysical Pathways in Photoexcited Organic Heterojunctions

The exciton dissociation and charge separation occurring on subpicosecond time scales following the photoexcitation are studied in a model donor/acceptor heterojunction using a fully quantum approach. Higher-than-LUMO acceptor orbitals which are energetically aligned with the donor LUMO orbital participate in the ultrafast interfacial dynamics by creating photon-absorbing charge-bridging states in which charges are spatially separated and which can be directly photoexcited. Along with the states brought about by single-particle resonances, the two-particle (exciton) mixing gives rise to bridge states in which charges are delocalized. Bridge states open up a number of photophysical pathways that indirectly connect the initial donor states with states of spatially separated charges and compete with the efficient progressive deexcitation within the manifold of donor states. The diversity and efficiency of these photophysical pathways depend on a number of factors, such as the precise energy alignment of exciton states, the central frequency of the excitation, and the strength of carrier-phonon interaction.Comment: Final, published versio

Overlapping fragments method for electronic structure calculation of large systems

We present a method for the calculation of electronic structure of systems that contain tens of thousands of atoms. The method is based on the division of the system into mutually overlapping fragments and the representation of the single-particle Hamiltonian in the basis of eigenstates of these fragments. In practice, for the range of system size that we studied (up to tens of thousands of atoms), {the dominant part of the calculation scales} linearly with the size of the system when all the states within a fixed energy interval are required. The method is highly suitable for making good use of parallel computing architectures. We illustrate the method by applying it to diagonalize the single-particle Hamiltonian obtained using the density functional theory based charge patching method in the case of amorphous alkane and polythiophene polymers.Comment: 9 pages, 10 figures, the version accepted in J. Chem. Phy

Coherent direct position estimation in distributed massive MIMO systems

Disertacija se bavi problemom direktne koherentne lokalizacije izvora xirokopojasnih radio signala pomou masivnih vixeantenskih sistemima u prostorno koherentnom scenariju LOS (Line-OfSight) komponenti. Ovaj scenario je tipiqan za male elije u milimetarskom opsegu u petoj generaciji 5G elijskih sistema. Lokalizacija se zasniva na obradi signala sa distribuiranih antenskih nizova koji mogu imati podnizove sa faziranim antenskim rexetkama. Ideja ove disertacije je da se infrastruktura buduih beiqnih sistema pete generacije (5G) iskoristi, pored komunikacije, i za lokalizaciju koja je predmet disertacije. Cilj je da se ostvari prezicnost procene pozicije za 2 do 3 reda veliqine bolju od talasne duine nosioca, xto klasiqne metode za dvokoraqnu i jednokoraqnu (direktnu) lokalizaciju ne omoguavaju. Da bi se to postiglo, koriste se koherentne metode { one koje pored pomaka anvelopa koriste i informacije sadrane u fazama nosioca LOS komponenti. Da bi se to moglo iskoristiti, potrebno je da u eliji postoji prostorna koherencija LOS komponenti. Zbog toga se istraivanje pre svega oslanja na male elije (sa LOS uslovima) i milimetarski (mmWave) opseg (koji ima povoljne uslove prostiranja), ali nije ograniqeno na njih dok god je prethodni uslov zadovoljen. Korixene su sledee metode istraivanja. Matematiqki je modelovan prostorno koherentni scenario i za njega su izvedene teorijske granice preciznosti lokalizacije. Zatim su predloene metode lokalizacije. Njihove performanse su analizirane simulacijama i eksperimentalno. Za eksperimente je korixen hardver koji je napravljen u sklopu istraivanja. Izvedene su Kramer-Raove granice preciznosti lokalizacije za model signala u prostorno koherentnom scenariju i pokazano je da su obrnuto srazmerne kvadratu frekvencije nosioca. Predloeno je vixe tipova metoda za lokalizaciju { nekoherentne, polukoherentne i koherentne; metode za poznatu sekvencu (kooperativan predajnik/korisnik) i za nepoznatu (nekooperativan predajnik); metode maksimalne verodostojnosti izvedene za jednokorisniqki potpuni LOS sluqaj i potprostorne metode. Predloene koherentne metode su statistiqki efikasne (njihova preciznost dostie izvedene teorijske granice) i ostvaruju preciznost za 2 do 3 reda veliqine bolju od talasne duine nosioca. Analiziran je i sluqaj vixestrukog prostiranja, a metode i tada ostvaruju preciznost za 2 reda veliqine bolju od talasne duine. Takoe funkcionixu u scenariju sa vixe (i to prostorno bliskih) predajnika pribline snage. Pritom, potprostorne metode funkcionixu i kad je interferirajui predajnik za 30 dB vee snage od ciljanog. Svi ovi rezultati su ostvareni za razumne vrednosti sistemskih parametara, kao xto su odnosi signal-xum i broj odbiraka. Pored toga, algoritmi funkcionixu i pri malim odnosima signal-xum, zahvaljujui tome xto direktno na osnovu sirovih signala procenjuju poziciju, za razliku od klasiqnih metoda koje gube informacije pravei meuprocene. Dodatna povoljnost direktne lokalizacije je xto se izbegava numeriqki zahtevan problem asocijacije. Algoritmi funkcionixu i za kratke opservacione intervale, zahvaljujui zdruenoj obradi svih spektralnih komponenti signala.The thesis deals with the problem of direct coherent localization of wideband radio signal sources, using massive MIMO (Multiple-Input-MultipleOutput) antenna systems in scenarios with spatially coherent LOS (Line-Of-Sight) signal components. These scenarios are typically found in small cells in the mmWave (millimeter wave) range in the fifth generation (5G) cellular systems. The localization is based on the processing of signals received by a distributed antenna array which may include phased antenna subarrays. In this thesis, the idea is to use the infrastructure of future 5G cellular systems for the localization in the thesis and for communication as well. The goal is to achieve position estimation accuracy by 2 to 3 orders of magnitude better than the carrier wavelength, which cannot be achieved by classical methods for two-step and one-step (direct) localization. In order to achieve that, coherent methods are used { those that, in addition to the information contained in envelope shifts, also use the information contained in carrier phase shifts of the LOS components. Spatial coherence of LOS components in the given cell is required to allow this information to be used. Thus, the research deals mostly with small cells (in LOS conditions) and the mmWave range (which has suitable propagation conditions), but is not limited to them as long as the previous condition is satisfied. The following research methods were used. A spatially coherent scenario was mathematically modelled and theoretical localization accuracy bounds were derived for it. Then, appropriate localization methods were proposed. Their performance was analyzed by simulations and experimentally. A hardware platform built as a part of the research was used in the experiments. Cram´er-Rao bounds on the localization accuracy have been derived for the signal model for the spatially coherent scenario and it has been shown that they are inversely proportional to the squared carrier frequency. Different types of localization methods have been proposed { non-coherent, semi-coherent and coherent; knownsequence methods (cooperative transmitter/user) and unknown-sequence methods (noncooperative transmitter); maximum-likelihood methods derived for the singleuser LOS-only scenario and subspace-based methods. The coherent methods are statistically efficient (their accuracy approaches the derived theoretical bounds) and an accuracy by 2 to 3 orders of magnitude better than the carrier wavelength isachieved. Multipath propagation is also analyzed and the methods achieve an accuracy by 2 orders of magnitude better than the carrier wavelength even in that case. They also perform well in a scenario with multiple (even spatially close) transmitters with comparable powers. Additionally, the subspace-based methods perform well even if the power of an interfering transmitter is 30 dB higher than that of the selected one. All of these results are achieved for reasonable values of the system parameters, such as the signal-to-noise ratio (SNR) and the number of used samples. Also, the algorithms perform well even at low SNRs, thanks to the fact that they estimate the position directly based on the raw signals, as opposed to classical methods which reduce the amount of useful information by making intermediate estimates. Another advantage of direct localization is the fact that the numerically complex data association problem is evaded. The algorithms also perform well even for short observation intervals, owing to the joint processing of all the spectral signal components

Electron-phonon coupling in crystalline organic semiconductors: Microscopic evidence for nonpolaronic charge carriers

We consider electron(hole)-phonon coupling in crystalline organic semiconductors, using naphthalene for our case study. Employing a first-principles approach, we compute the changes in the self-consistent Kohn-Sham potential corresponding to different phonon modes and go on to obtain the carrier-phonon coupling matrix elements (vertex functions). We then evaluate perturbatively the quasiparticle spectral residues for electrons at the bottom of the lowest-unoccupied- (LUMO) and holes at the top of the highest-occupied (HOMO) band, respectively obtaining $Z_e\approx 0.74$ and $Z_h\approx 0.78$. Along with the widely accepted notion that the carrier-phonon coupling strengths in polyacenes decrease with increasing molecular size, our results provide a strong microscopic evidence for the previously conjectured nonpolaronic nature of band-like carriers in these systems.Comment: final, published versio

Cumulant expansion in the Holstein model: Spectral functions and mobility

We examine the range of validity of the second-order cumulant expansion (CE) for the calculation of spectral functions, quasiparticle properties, and mobility of the Holstein polaron. We devise an efficient numerical implementation that allows us to make comparisons in a broad interval of temperature, electron-phonon coupling, and phonon frequency. For a benchmark, we use the dynamical mean-field theory (DMFT) which gives, as we have recently shown, rather accurate spectral functions in the whole parameter space even in low dimension. We find that in one dimension the CE resolves well both the quasiparticle and the first satellite peak in a regime of intermediate coupling. At high temperatures, the charge mobility assumes a power law $\mu\propto T^{-2}$ in the limit of weak coupling and $\mu\propto T^{-3/2}$ for stronger coupling. We find that, for stronger coupling, the CE gives slightly better results than the self-consistent Migdal approximation (SCMA), while the one-shot Migdal approximation is appropriate only for a very weak electron-phonon interaction. We also analyze the atomic limit and the spectral sum rules. We derive an analytical expression for the moments in CE and find that they are exact up to the fourth order, as opposed to the SCMA where they are exact to the third order. Finally, we analyze the results in higher dimensions.Comment: 22 pages, 14 figures + supp.mat. 12 pages, 9 figure

Symmetry of k·p Hamiltonian in pyramidal InAs/GaAs quantum dots: Application to the calculation of electronic structure

A method for the calculation of the electronic structure of pyramidal self-assembled InAs/GaAs quantum dots is presented. The method is based on exploiting the C-4 symmetry of the 8-band k·p Hamiltonian with the strain taken into account via the continuum mechanical model. The operators representing symmetry group elements were represented in the plane wave basis and the group projectors were used to find the symmetry adapted basis in which the corresponding Hamiltonian matrix is block diagonal with four blocks of approximately equal size. The quantum number of total quasiangular momentum is introduced and the states are classified according to its value. Selection rules for interaction with electromagnetic field in the dipole approximation are derived. The method was applied to calculate electron and hole quasibound states in a periodic array of vertically stacked pyramidal self-assembled InAs/GaAs quantum dots for different values of the distance between the dots and external axial magnetic field. As the distance between the dots in an array is varied, an interesting effect of simultaneous change of ground hole state symmetry, type, and the sign of miniband effective mass is predicted. This effect is explained in terms of the change of biaxial strain. It is also found that the magnetic field splitting of Kramer's double degenerate states is most prominent for the first and second excited state in the conduction band and that the magnetic field can both separate otherwise overlapping minibands and concatenate otherwise nonoverlapping minibands

Quantum transport in semiconductor quantum dot superlattices: electron-phonon resonances and polaron effects

Electron transport in periodic quantum dot arrays in the presence of interactions with phonons was investigated using the formalism of nonequilibrium Green's functions. The self-consistent Born approximation was used to model the self-energies. Its validity was checked by comparison with the results obtained by direct diagonalization of the Hamiltonian of interacting electrons and longitudinal optical phonons. The nature of charge transport at electron -- phonon resonances was investigated in detail and contributions from scattering and coherent tunnelling to the current were identified. It was found that at larger values of the structure period the main peak in the current -- field characteristics exhibits a doublet structure which was shown to be a transport signature of polaron effects. At smaller values of the period, electron -- phonon resonances cause multiple peaks in the characteristics. A phenomenological model for treatment of nonuniformities of a realistic quantum dot ensemble was also introduced to estimate the influence of nonuniformities on current -- field characteristics