Orbital-dependent electron dynamics in Fe-pnictide superconductors

We report on orbital-dependent quasiparticle dynamics in EuFe$_2$As$_2$, a parent compound of Fe-based superconductors and a novel way to experimentally identify this behavior, using time- and angle-resolved photoelectron spectroscopy across the spin density wave transition. We observe two different relaxation time scales for photo-excited d$_x$$_z$/d$_y$$_z$ and d$_x$$_y$ electrons. While d$_x$$_z$/d$_y$$_z$ electrons relax faster through the electron-electron scattering channel, showing an itinerant character, d$_x$$_y$ electrons form a quasi-equilibrium state with the lattice due to their localized character, and the state decays slowly. Our findings suggest that electron correlation in Fe-pnictides is an important property, which should be taken into careful account when describing the electronic properties of both parent and electron-doped compounds, and therefore establish a strong connection with cuprates

Metalloporphyrins on Oxygen-Passivated Iron: Conformation and Order Beyond the First Layer

On-surface metal porphyrins can undergo electronic and conformational changes that play a crucial role in determining the chemical reactivity of the molecular layer. Therefore, understanding those properties is pivotal for the design and implementation of organic-based devices. Here, by means of photoemission orbital tomography supported by density functional theory calculations, we investigate the electronic and geometrical structure of two metallated tetraphenyl porphyrins (MTPPs), namely ZnTPP and NiTPP, adsorbed on the oxygen-passivated Fe(100)-p(1x1)O surface. Both molecules weakly interact with the surface as no charge transfer is observed. In the case of ZnTPP our data correspond to those of moderately distorted molecules, while NiTPP exhibits a severe saddle-shape deformation. From additional experiments on NiTPP multilayer films, we conclude that this distortion is a consequence of the interaction with the substrate, as the NiTPP macrocycle of the second layer turns out to be flat. We further find that distortions in the MTPP macrocycle are accompanied by an increasing energy gap between the highest occupied molecular orbitals (HOMO and HOMO-1). Our results demonstrate that photoemission orbital tomography can simultaneously probe the energy level alignment, the azimuthal orientation, and the adsorption geometry of complex aromatic molecules even in the multilayer regime

KEMIČNA IN STRUKTURNA RAZISKAVA MOLEKULE KOBALTOVEGA FTALOCIANINA

In the last two decades, studies on organic molecules mimicking substances of fundamental importance in nature, like chlorophyll or hemoglobin, have attracted researchers’ attention. These molecules are building blocks for a family of materials also referred to as “organic semiconductors”. Such compounds can be implemented in numerous applications, ranging from data-storage to light harvesting. Some of their fundamental advantages include low cost, light weight, relatively easy engineering and mechanical flexibility, compatible with bending plastic substrates. In this thesis work we investigated the chemical, structural and electronic properties of cobalt phthalocyanines (CoPc). These molecules have promising applications in the field of magnetic data storage and spintronics in general, due to the ferromagnetic properties of the cobalt atom. Several techniques like photoemission core-level spectroscopy and valence band spectroscopy, together with X-ray absorption, have been used in order to determine the CoPc properties in gaseous phase, i.e. in the absence of interaction with the surrounding environment. Another set of experiments was devoted to the commissioning of the CITIUS time-resolved photoemission setup, that will be used in future studies of CoPc molecules on surfaces.Organske molekule, ki so po sestavi podobne ključnim molekulam v naravi, kot sta klorofil ter hemoglobin, so v zadnjih letih pritegnile pozornost raziskovalcev. Omenjene molekule so gradniki materialov, ki jim pravimo tudi organski polprevodniki. Njihova uporaba sega na mnoga področja, npr. od naprav za shranjevanje podatkov do sončnih celic. Nekatere izmed prednosti the materialov so nizka cena, nizka teža, preprost inženiring in mehanska upogljivost, ki omogočajo njihovo uporabo na upogljivih plastičnih substratih. V magistrski nalogi bodo predstavljeni rezultati raziskav kemičnih, strukturnih in elektronskih lastnosti kobaltovega ftalocianinata molekula je zaradi feromagnetnih lastnosti kobaltovega atoma še posebej zanimiva za uporabo na področjih magnetnega shranjevanja podatkov in spintronike. Pri raziskavah lastnosti kobaltovega ftalocianina v plinastem stanju, t. j. v odsotnosti interakcij z okolico, so uporabljene tehnike, kot sta fotoemisijska spektroskopija notranjih in valenčnih elektronskih stanj ter rentgenska absorpcijska spektroskopija. Lastnosti molekul so bile preučevane v visokem vakuumu. Preostali poskusi, ki so predstavljeni v nalogi, so bili namenjeni karakterizaciji novega svetlobnega vira CITIUS, ki ga bomo v prihodnjih raziskavah uporabljali za časovno odvisne fotoemisijske meritve na molekulah kobaltovega ftalocianina na površinah

Prednosti in slabosti experimentov z ultrakratkimi dvobarvnimi bliski

Advances in the development of lasers have led to a new class of radiation sources generating coherent, tunable, ultrashort light pulses in the spectral region ranging from infrared to soft X-rays. This includes high-order harmonics generation in gas (HHG), on which relies the CITIUS facility at University of Nova Gorica (Slovenia), and free-electron lasers (FELs), such as the facility FERMI at Elettra-Sincrotrone Trieste (Italy). The distinctive structure of HHG and FEL radiation paved the way to time-resolved experiments, which are performed to investigate events occurring on a short, or very short, temporal scale, from picoseconds to femtoseconds. This work focuses on the advantages and disadvantages of some experimental techniques based on using these novel light sources to investigate the microscopic and/or ultrafast dynamics of matter samples, which have been previously driven out of equilibrium. Advantages rely on the implementation of various applications based on two-color schemes and, more specifically, include the possibility of acquiring two-dimensional frequency maps, measuring electrons’ effective masses, or investigating electronic properties decoupled from the influence of the lattice. Particular focus will be put on experimental methods relying on photoelectric effect and photoelectron spectroscopy. In all experiments, we took advantage of one or more specific properties of HHG and FEL sources, such as controllable chirp, to study laser dressed states in helium, variable polarization, to study electronic properties of iron-based pnictides and ultrashort pulses (< 10 fs) to study the purely electronic dynamics in transition metal dichalcogenides. On the other hand, the study of the interface between a molecule and a topological insulator revealed some intrinsic limitations and physical drawbacks of the technique, such as spurious effects originating from the high power pulses, like multiphoton absorption and the space charge effect, or the reduction of experimental resolution when pushing for shorter and shorter pulse durations. Some disadvantages are also connected to the current state-of-the-art in the field of ultrashort laser systems, where a trade-off needs to be found between repetition rate and laser power. Finally, state-of-the-art experiments based on the ability to generate ultrashort pulses carrying orbital angular momentum in visible, near-infrared as well as extreme UV range will be presented. The use of these pulses opens the door to the investigation of new physical phenomena, such as probing magnetic vortices using extreme ultraviolet light from a free-electron laser or imprinting the spatial distribution of an ultrashort infrared pulse carrying orbital angular momentum onto a photoelectron wave packet.Napredki v razvoju laserjev so pripeljali do nove vrste svetlobnih virov, ki lahko generirajo koherentne in po valovni dolžini nastavljive ultra kratke svetlobne sunke v spektralnem območju vse od infrardeče svetlobe pa do mehkih rentgenskih žarkov. V to spektralno območje spadajo tudi harmoniki visokega reda generirani v plinu (ang. high-harmonic generation [HHG]), na katerih temelji svetlobni vir CITIUS na Univerzi v Novi Gorici, ter laserji na proste elektrone (ang., free-electron laser [FEL]), kot je svetlobni vir FERMI v laboratoriju Elettra v Trstu. HHG in FEL izvori svetlobe so omogočili izvajanje časovno ločljivih poskusov na časovnih skalah reda velikosti nekaj pikosekund pa vse do nekaj femtosekund. Raziskovalno delo predstavljeno v tej nalogi se osredotoča na prednosti in slabosti nekaterih eksperimentalnih tehnik, ki slonijo na uporabi omenjenih svetlobnih virov, z namenom raziskav ultrahitre dinamike materialov izven termodinamičnega ravnovesja. Prednosti HHG in FEL svetlobnih izvorov so predvsem v možnosti implementacije t.i. dvobarvnih poskusov, ki se lahko uporabljajo npr. za zajemanje dvodimenzionalnih frekvenčnih map ali za merjenje efektivnih mas elektronov ločeno od vplivov kristalne mreže. V ospredju bodo eksperimenti, ki temeljijo na fotoefektu in fotoelektronski spektroskopiji. Pri vseh poskusih je bila uporabljena vsaj ena od posebnih lastnosti HHG ali FEL svetlobnih virov, kot so npr. nadzorovan “chirp” (spreminjanje frekvence svetlobnega sunka s časom) za preučevanje lasersko pogojenih stanj v atomih helija, spremenljiva polarizacija za preučevanje lastnosti železovih pniktidov, ter zelo kratki pulzi (< 10 fs) za preiskovanje izolirane elektronske dinamike v dihalkogenidih prehodnih kovin. Po drugi strani pa smo pri izvajanju dvobarvnih študij na vmesnih plasteh med molekulami in topološkim izolatorjem naleteli na nekaj omejitev ter fizikalnih slabosti, npr. na nezaželene pojave, ki izvirajo iz uporabe svetlobnih sunkov visoke moči, kot so večfotonska absorbcija, prostorski naboj, ter zmanjšanje energijske ločljivosti pri uporabi zelo kratkih svetlobnih sunkov. Nekatere omejitve so povezane tudi s trenutnim stanjem tehnike na področju ultrahitrih laserskih sistemov, kjer je potrebno najti pravi kompromis v ravnovesju med frekvenco svetlobnih sunkov, ter močjo laserja. Na koncu bodo predstavljeni novi napredni poskusi, ki temeljijo na generiranju ultrahitrih svetlobnih bliskov v bližnjem infrardečem ter ekstremnem ultravijoličnem (UV) spektralnem območju, ki nosijo tirno vrtilno količino (ang. orbital angular momentum [OAM]). Uporaba takšnih bliskov bo omogočila preučevanje novih fizikalnih pojavov, kot sta npr. opazovanje magnetnih vrtincev s pomočjo XUV sunkov, ter prenos tirne vrtilne količine s svetlobe na proste elektrone

Orbital selective dynamics in Fe-pnictides triggered by polarized pump pulse excitations

Quantum materials display exotic behaviours related to the interplay between temperature-driven phase transitions. Here, we study the electron dynamics in one such material, CaFe$_2$As$_2$, a parent Fe-based superconductor, employing time and angle-resolved photoemission spectroscopy. CaFe$_2$As$_2$ exhibits concomitant transition to spin density wave state and tetragonal to orthorhombic structure below 170 K. The Fermi surface of this material consists of three hole pockets ($alpha$, $beta$ and $gamma$) around $Gamma$-point and two electron pockets around $X$-point. The hole pockets have $d_{xy}$, $d_{yz}$ and $d_{zx}$ orbital symmetries. The $beta$ band constituted by $d_{xz}$/$d_{yz}$ orbitals exhibit a gap across the magnetic phase transition. We discover that polarized pump pulses can induce excitations of electrons of a selected symmetry. More specifically, while $s$-polarized light (polarization vector perpendicular to the $xz$-plane) excites electrons corresponding to all the three hole bands, $p$-polarized light excites electrons essentially from ($alpha$,$beta$) bands which are responsible for magnetic order. Interestingly, within the magnetically ordered phase, the excitation due to the $p$-polarized pump pulses occur at a time scale of 50 fs, which is significantly faster than the excitation induced by $s$-polarized light ($sim$ 200 fs). These results suggest that the relaxation of different ordered phases occurs at different time scales and this method can be used to achieve selective excitations to disentangle complexity in the study of quantum materials

Dirac Bands in the Topological Insulator Bi2Se3 Mapped by Time‐Resolved Momentum Microscopy

Abstract The energy dispersion of the unoccupied Dirac bands of the topological insulator Bi2Se3 has been studied up to large parallel momenta and intermediate state energies using a setup for laser‐based time‐resolved momentum microscopy with 6 eV probe‐photons. A strongly momentum‐dependent evolution of the topologically protected Dirac states into a conduction band resonance is observed, highlighting the anisotropy dictated by the symmetry of the surface. The results are in remarkable agreement with the theoretical surface spectrum obtained from a GW‐corrected tight‐binding model, suggesting the validity of the approach in the prediction of the quasiparticle excitation spectrum of large systems with non‐trivial topology. After photoexcitation with 0.97 eV photons, assigned to a bulk valence band‐conduction band transition, the out‐of‐equilibrium population of the surface state evolves on a multi‐picosecond time scale, in agreement with a simple thermodynamical model with a fixed number of particles, suggesting a significant decoupling between bulk and surface states

Dissecting Mott and charge-density wave dynamics in the photoinduced phase of 1T-TaS[sub]2

The two-dimensional transition-metal dichalcogenide 1T−TaS2 is a complex material standing out for its puzzling low temperature phase marked by signatures amenable to both Mott-insulating and charge-density wave states. Electronic Mott states, coupled to a lattice, respond to coherent optical excitations via a modulation of the lower (valence) Hubbard band. Such dynamics is driven by strong electron-phonon coupling and typically lasts for tens of picoseconds, mimicking coherent structural distortions. Instead, the response occurring at the much faster timescale, mainly dominated by electronic many-body effects, is still a matter of intense research. By performing time- and angle-resolved photoemission spectroscopy, we investigated the photoinduced phase of 1T−TaS2 and found out that its lower Hubbard band promptly reacts to coherent optical excitations by shifting its binding energy towards a slightly larger value. This process lasts for a time comparable to the optical pump pulse length, mirroring a transient change of the onsite Coulomb repulsion energy (U). Such an observation suggests that the correction to the bare value of U, ascribed to the phonon-mediated screening which slightly opposes the Hubbard repulsion, is lost within an interval of a few tens of femtoseconds and can be understood as a fingerprint of electronic states largely decoupled from the lattice. Additionally, these results enforce the hypothesis, envisaged in the current literature, that the transient photoinduced states belong to a sort of crossover phase instead of an equilibrium metallic one

Study of ultraviolet-visible fluorescence emission following resonant Auger decay of the 2p⁻¹ nl core-excited states of argon atoms

Abstract We have studied the excitation and relaxation of Ar⁺ ions populated in resonant Auger decay from the Ar 2p⁻¹ nl core-excited states by using ultraviolet-visible fluorescence spectroscopy and photon-photon delayed coincidence technique. Fluorescence emission yields were measured in the photon energy range of 240–255 eV for the 3s²3p⁴(¹D)5s(²D) → 3s²3p⁴(¹D)4p(²F) (393 nm) and 3s²3p⁴(¹D)4d(²F) → 3s²3p⁴(¹D)4p(²F) (335 nm) transitions as well as for the 380–500 nm wideband emission. Delayed coincidence photon decay curves for the cascade transitions of the 335/459 nm and 393/459 nm lines were measured at the four most intense Ar 2p core excitations and the coincidence yields for both studied cascade channels were obtained at these Ar 2p excitations