Search results for "lek"
showing 10 items of 2808 documents
Topological polarization, dual invariants, and surface flat band in crystalline insulators
2020
We describe a three-dimensional crystalline topological insulator (TI) phase of matter that exhibits spontaneous polarization. This polarization results from the presence of (approximately) flat bands on the surface of such TIs. These flat bands are a consequence of the bulk-boundary correspondence of polarized topological media, and contrary to related nodal line semimetal phases also containing surface flat bands, they span the entire surface Brillouin zone. We also present an example Hamiltonian exhibiting a Lifshitz transition from the nodal line phase to the TI phase with polarization. Utilizing elasticity tetrads, we show a complete classification of 3D crystalline TI phases and invar…
Time-dependent quantum transport in nanosystems : a nonequilibrium Green's function approach
2016
A time-dependent extension to the Landauer–Büttiker approach to study transient quantum transport in arbitrary junctions composed of leads and conducting devices is developed. The nonequilibrium Green’s function approach is employed for describing the charge and heat transport dynamics. The importance of the developed method is that it provides a closed formula for the time-dependent density matrix in both electronic and phononic systems. In the electronic case the nonequilibrium conditions are due to a switch-on of a bias voltage in the leads or a perturbation in the junction whereas in the phononic case the central region of interest is coupled to reservoirs of di erent temperatures. In b…
Electron-phonon interaction in flat-band superconductivity
2017
Parhaiten tunnettu suprajohtavuuden syntymekanismi perustuu fononien välittämään vetovoimaan elektronien välillä. Tässä tutkielmassa tutkin fononien välittämää suprajohtavuutta systeemeissä, jossa elektronivyöt ovat tasomaisia. Tasovyöllä elektronien dispersio on erittäin heikko, jolloin tilatiheys on tavanomaista suurempi. Tämän takia suprajohtavuus tasovyöllä on tavanomaista voimakkaampi silloin kun elektronien välinen vetovoima on heikko. Eliashbergin teoria on elektroni–fononi-suprajohtavuuden teoria, joka ottaa luonnollisella tavalla fononien äärellisen nopeuden huomioon elektronien välisessä vuorovaikutuksessa. Pohjustuksena tasovyösuprajohtavuuteen perehdyn Eliashbergin teoriaan ensi…
Selective Formation of S4- and T-Symmetric Supramolecular Tetrahedral Cages and Helicates in Polar Media Assembled via Cooperative Action of Coordina…
2020
We report on the synthesis and self-assembly study of novel supramolecular monomers encompassing quadruple hydrogen-bonding motifs and metal coordinating 2,2’-bipyridine units. When mixed with metal ions such as Fe2+ or Zn2+, the tetrahedron cage complexes are formed in quantitative yields and full diastereoselectivity, even in highly polar acetonitrile or methanol solvents. The symmetry of the complexes obtained has been shown to depend critically on the flexibility of the ligand. Restriction of the rotation of the hydrogen-bonding unit with respect to the metal coordinating site results in a T-symmetric cage, whereas by introducing flexibility either through a methylene linker or rotating…
Inorganic Anion-Mediated Supramolecular Entities of 4-Amino-3,5-Bis(4-Pyridyl)-1,2,4-Triazole Salts Assisted by the Interplay of Noncovalent Interact…
2023
The development of new families of synthetic molecular systems projecting neutral, bi-, or multi-H-bonding donor units is significant to acquire the desired selectivity within the fascinating area of anion recognition. Here, we illustrate the reaction between a neutral 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole ligand (L) with acidic solutions containing either chloride, bromide, nitrate, phosphate, iodide, sulfate, hexafluorosilicate, fluoride, tetrafluoroborate or perchlorate anions, yielding 16 new anion-mediated supramolecular entities, H2LCl2 (1), H2LBr2 (2), H2L(NO3)2 (3), HL(H2PO4) (4), H2L(H2PO4)2 (5), [H2L]2I4 (6), H2L(NO3)2 (7), H2L(SO4)·H2O (8), H2LSiF6 (9), H2LSiF6·2H2O (10), H2L…
Influencing the self‐sorting behavior of [2.2]paracyclophane based ligands by introducing isostructural binding motifs
2020
Two isostructural ligands with either nitrile ( L nit ) or isonitrile ( L iso ) moieties directly connected to a [2.2]paracyclophane backbone with pseudo‐meta substitution pattern have been synthesized. The ligand itself ( L nit ) or its precursors ( L iso ) were resolved via HPLC on a chiral stationary phase and the absolute configuration of the isolated enantiomers was assigned by XRD analysis and/or by comparison of quantum‐chemical simulated and experimental ECD‐spectra. Surprisingly, the resulting metallosupramolecular aggregates formed in solution upon coordination of [(dppp)Pd(OTf) 2 ] differ in their composition: whereas L nit forms dinuclear complexes L iso exclusively forms trinuc…
Self-assembly of M4L4 tetrahedral cages incorporating pendant P=S and P=Se functionalised ligands
2019
Herein, the synthesis of metal–organic tetrahedral cages featuring flexible thio- and selenophosphate-based ligands is described. The cages were prepared by sub-component self-assembly of A=P(OC6H4NH2-4)3 (A = S, Se) or S=P(SC6H4NH2-4)3,2-pyridinecarboxaldehyde, and either Fe[BF4]2 or Co[BF4]2. Preliminary host–guest studies into the ability of the pendant PQS and PQSe groups to interact with suitable substrates will be discussed. peerReviewed
Kā mēs runājam par ēdienu. Ēdienkartes latviešu valodā
2019
Ēdienkartes latviešu valodā ir bieži sastopams teksta veids, tomēr valodniecībā tās līdz šim maz pētītas. Pētījuma mērķis ir apskatīt ēdienkaršu vēsturi, aplūkot ēdienkaršu stilistisko piederību, struktūru un leksiku, īpašu uzmanību veltot lietvārdiem un aizgūtu vārdu lietojumam, īpašības vārdiem, darbības vārdiem un apstākļa vārdiem. Pētījuma materiālu veido 50 ēdienkartes latviešu valodā, kas publiski pieejamas ēdināšanas uzņēmumu mājaslapās vai sociālajos tīklos un ievāktas laika posmā no 2018. gada novembra līdz 2019. gada janvārim.
Svešvārdu semantiskās atbilsmes latviešu un itāliešu valodā
2015
Maģistra darbā „Svešvārdu semantiskās atbilsmes latviešu un itāliešu valodā” tiek salīdzināti un analizēti līdzīga skanējuma, bet atšķirīgas nozīmes vai nozīmes nianses svešvārdi – īpašības vārdi latviešu un itāliešu valodā. Teorētiskajā daļā tiek aprakstīta un skaidrota svešvārdu izpratne latviešu un itāliešu valodniecībā, kā arī tiek sniegts ieskats itāliešu valodas leksiskas attīstībā, kas līdz šim latviešu valodā vēl nav ticis publicēts. Praktiskajā daļā tiek skatītas un analizētas konkrētu svešvārdu nozīmes abās valodās, balstoties uz vārdnīcās, kā arī valodas korpusos pieejamo informāciju. Maģistra darba mērķis ir konstatēt un analizēt svešvārdu semantiku latviešu un itāliešu valodā. …
Improved Quadratic Time-frequency Distributions for Detecting Inter-turn Short Circuits of PMSMs in Transient States
2020
This paper aims to improve quadratic time-frequency distributions to adapt condition monitoring of electrical machines in transient states. Short-Time Fourier transform (STFT) has been a baseline signal processing technique for detecting fault characteristic frequencies. However, limits of window sizes due to loss of frequency- or time-resolution, make it hard to capture rapid changes in frequencies. Within this study, Choi-Williams and Wigner-Ville distributions are proposed to effectively detect peaks at characteristic frequencies while still maintaining low computation time. The improved quadratic time-frequency distributions allow for generating spectrograms of a longer lasting data sig…