In the underdoped n-type cuprate Nd_2-xCe_xCuO₄, long-range antiferromagnetic order reconstructs the Fermi surface, resulting in a putative antiferromagnetic metal with small Fermi pockets. Using angle-resolved photoemission spectroscopy, we observe an anomalous energy gap, an order of magnitude smaller than the antiferromagnetic gap, in a wide portion of the underdoped regime and smoothly connecting to the superconducting gap at optimal doping. After considering all the known ordering tendencies in tandem with the phase diagram, we hypothesize that the normal-state gap in the underdoped n-type cuprates originates from Cooper pairing. The high temperature scale of the normal-state gap raises the prospect of engineering higher transition temperatures in the n-type cuprates comparable to those of the p-type cuprates.

Understanding unconventional superconductivity remains one of the most important unsolved problems in physics. A particularly noteworthy case is the copper-based high-temperature superconductor, which stands out due to its remarkably high transition temperature and relatively simple structure. These exceptional properties not only make the study of cuprates valuable for potential practical applications but also provide a prominent platform for deepened understanding of many-particle physics. In the realm of quantum materials, angle-resolved photoemission spectroscopy (ARPES) has emerged as an indispensable tool for examining the intricate electronic structure in momentum space. This methodology directly probes the single-particle spectral function and can uncover the underlying microscopic interactions. In recent years, technological advancements have enabled the development and implementation of time-resolved ARPES (tr-ARPES). tr-ARPES allows access to non-equilibrium transient states and provides valuable insights into the correlated dynamic properties. This thesis work is divided into two main parts. The first part focuses on the development of a high-resolution, high harmonic generation (HHG)-based tr-ARPES setup. The second part involves ARPES investigations of hole- and electron-doped cuprate superconductors.

The aim of developing the tr-ARPES setup was to have a light source with specific characteristics, including a narrow bandwidth, a wide range of photon energies (covering the entire first Brillouin zone), good temporal resolution (near the transform limit), and a high repetition rate (to mitigate the space charge effect). To meet these requirements, the chosen technical approach is the HHG method, driven by an unusually long laser pulse (~460 fs) and short wavelength (343 nm) from a frequency tripled Yb fiber laser. The selection of photon energy is achieved through switchable multilayer bandpass mirrors and thin film filters to prevent temporal broadening. As a result, we achieve an energy resolution of ΔE = 9, 14, 18, 111 meV for photon energies of hν = 10.8, 18.1, 25.3, 32.5 eV. For the pump pulse, a tunable range from 0.65 μm to 9 μm is provided by a two-stage optical-parametric amplifier. Further developments on the instrumental side is the exploration of using a spherical grating to select the HHG harmonics. This is realized by designing a very low-density grating so that the temporal broadening can be minimized. The spherical grating has been numerically calculated, fabricated, and experimentally characterized. This monochromator solution was compared with the mirror+filter configuration and has shown much higher efficiency (3.3 times higher for 10.8 eV and 12.9 times higher for 18.1 eV) with insignificant temporal broadening (6.8% increase for 18.1 eV). This experimental development provides a compact and efficient layout for ultrafast pulse extraction.

In the cuprate section, a thorough investigation was conducted on the optimally doped n-type cuprate (NdCeCuO) using static ARPES. The much-refined experimental conditions have enabled us to obtain unprecedented signal-to-noise ratio and detailed observations in this material. The results demonstrate two distinct sectors of states: the reconstructed main band, which exhibited a gap due to the antiferromagnetic (AF) interactions, and a remaining dispersion observed within this AF pseudogap. This in-gap dispersion forms a 'gossamer' Fermi surface that plays a crucial role in the electron pairing. Additionally, a replica band corresponding to the AF folding feature was observed, displaying a consistent energy difference (approximately 60 meV) in both momentum and temperature dependence, possibly suggesting a connection between the AF order and phonon coupling. Furthermore, the hole-doped cuprate (Bi-2212) was studied using the recently developed tr-ARPES system. Leveraging the use of time-of-flight detection, node-antinode information could be gathered from a single measurement. The dynamics of the in-gap states at the antinode exhibited disparities with the near-node region, potentially reflecting phenomena associated with the pseudogap.

Att förstå okonventionell supraledning är fortfarande ett av de viktigaste olösta problemen i modern fysik. Ett särskilt anmärkningsvärt fall är de kopparoxidbaserade högtemperatursupraledarna (kuprater), som sticker ut på grund av sina anmärkningsvärt höga övergångstemperaturer och relativt enkla struktur. Dessa exceptionella egenskaper gör inte bara studier av kuprater värdefulla för potentiella praktiska tillämpningar utan ger också en plattform för att studera mångpartikelfysik. Vinkelupplöst fotoemissionsspektroskopi (ARPES) har etablerat sig som ett oumbärligt verktyg för att studera elektronstrukturen hos kvantmaterial. Denna metod kan experimentellt mäta spektralfunktionen för ett kristalint material och därmed i princip fastställa alla enpartikelegenskaper hos materialet och underliggande mikroskopiska interaktioner. Under de senaste åren har tekniska framsteg möjliggjort utveckling och implementering av tidsupplöst ARPES (tr-ARPES). tr-ARPES ger tillgång till transienta icke-jämviktstillstånd och ger därmed värdefulla insikter i korrelerade systems dynamiska egenskaper. De arbeten som redovisas i denna avhandling är uppdelat i två huvuddelar. Den första delen fokuserar på utvecklingen av en högupplöst tr-ARPES ljuskälla baserad på övertonsgenerering (HHG). Den andra delen omfattar ARPES-undersökningar av hål- och elektrondopade kuprater.

Syftet med att utveckla tr-ARPES-ljuskällan var att ha åstadkomma en ljuskälla med specifika egenskaper såsom smal bandbredd, ett brett spektrum av fotonenergier (som täcker hela den första Brillouin-zonen), bra tidsupplösning (nära transformgränsen) och en hög repetitionsfrekvens (för att mildra rymdladdningseffekten). För att uppfylla dessa krav valdes HHG, driven av en ovanligt lång laserpuls (~460 fs) med kort våglängd (343 nm) från en frekvenstripplad Yb-fiberlaser. Valet av fotonenergi uppnås genom utbytbara flerskiktsbandpasspeglar och tunnfilmsfilter för att förhindra tidsmässig breddning. Med denna approach uppnår vi en energiupplösning på ΔE = 9, 14, 18, 111 meV för fotonenergier på hν = 10.8, 18.1, 25.3, 32.5 eV. En kontinuerligt valbar pumppuls tillhandahålls i området 0.65 μm till 9 μm av en tvåstegs optisk-parametrisk förstärkare. Fortsatt experimentellt utvecklingsarbete på den instrumentella sidan har gjorts för att utforska användandet av sfäriska gitter som ett alternativ för att välja HHG-övertoner. Detta har realiserats genom att utforma ett gitter med mycket låg linjedensitet så att den tidsmässiga breddningen kan minimeras. Det sfäriska gittret har beräknats numeriskt, tillverkats och experimentellt karakteriserats. En monokromator baserad på denna metod har jämförts med spegel+filter-konfigurationen och har visat mycket högre verkningsgrad (3.3 gånger högre för 10.8 eV och 12.9 gånger högre för 18.1 eV) med obetydlig tidsmässig breddning (6.8% ökning för 18.1 eV). Detta arbete ger därmed en kompakt och effektiv lösning för att extrahera ultrasnabba pulser från en HHG källa.

Inom ramen för arbetet på kuprater genomfördes en grundlig undersökning av den optimaldopade n-typ kupraten (NdCeCuO) med statisk ARPES. De mycket förfinade experimentella förhållandena har gjort det möjligt för oss att nå ettoöverträffat signal-brusförhållande. Resultaten har visat på två distinkta tillståndssektorer: den rekonstruerade Fermiytan, som uppvisar ett gap på grund av den antiferromagnetiska (AF) interaktionen, samt en kvarvarande spökfermiyta som kvarstår inom AF-pseudogapet. Denna spökfermiyta spelar en avgörande roll för bildandet av elektronpar. Dessutom visar experimenten ett replikband som uppträder som en kopia av det rekonstruerade bandet och som konsekvent är skiftat till högre bindningsenergi med ca. 60 meV. Denna energiskillnad tycks oberoende av både rörelsemängd och temperatur, vilket möjligen tyder på en koppling mellan AF-ordningen och elektron-phononinteraktionen. Utöver studierna av NdCuCuO har dessutom den håldopade kupraten (Bi-2212) studerats med det nyligen utvecklade tr-ARPES-systemet. Användandet av flygtidsdetektion har möjliggjort parallel mätning av dynamiken från nod till antinod. Dynamiken i tillstånden i gapet vid antinoden uppvisar skillnader med nära-nodregionen, vilket tentativt kan vara relaterat till pseudogapet vid antinoden.

Electron-doped cuprates consistently exhibit strong antiferromagnetic correlations, leading to the prevalent belief that antiferromagnetic spin fluctuations mediate Cooper pairing in these unconventional superconductors. However, early investigations showed that although antiferromagnetic spin fluctuations create the largest pseudogap at hot spots in momentum space, the superconducting gap is also maximized at these locations. This presented a paradox for spin-fluctuation-mediated pairing: Cooper pairing is strongest at momenta where the normal-state low-energy spectral weight is most suppressed. Here we investigate this paradox and find evidence that a gossamer—meaning very faint—Fermi surface can provide an explanation for these observations. We study Nd2–xCexCuO4 using angle-resolved photoemission spectroscopy and directly observe the Bogoliubov quasiparticles. First, we resolve the previously observed reconstructed main band and the states gapped by the antiferromagnetic pseudogap around the hot spots. Within the antiferromagnetic pseudogap, we also observe gossamer states with distinct dispersion, from which coherence peaks of Bogoliubov quasiparticles emerge below the superconducting critical temperature. Moreover, the direct observation of a Bogoliubov quasiparticle permits an accurate determination of the superconducting gap, yielding a maximum value an order of magnitude smaller than the pseudogap, establishing the distinct nature of these two gaps. We propose that orientation fluctuations in the antiferromagnetic order parameter are responsible for the gossamer states.

Ultrafast light sources have become an indispensable tool to access and understand transient phenomenon in material science. However, a simple and easy-to-implement method for harmonic selection, with high transmission efficiency and pulse duration conservation, is still a challenge. Here we showcase and compare two approaches for selecting the desired harmonic from a high harmonic generation source while achieving the above goals. The first approach is the combination of extreme ultraviolet spherical mirrors with transmission filters and the second approach uses a normal-incidence spherical grating. Both solutions target timeand angle-resolved photoemission spectroscopy with photon energies in the 10-20 eV range but are relevant for other experimental techniques as well. The two approaches for harmonic selection are characterized in terms of focusing quality, photon flux, and temporal broadening. It is demonstrated that a focusing grating is able to provide much higher transmission as compared to the mirror+filter approach (3.3 times higher for 10.8 eV and 12.9 times higher for 18.1 eV), with only a slight temporal broadening (6.8% increase) and a somewhat larger spot size (similar to 30% increase). Overall, our study establishes an experimental perspective on the trade-off between a single grating normal incidence monochromator design and the use of filters. As such, it provides a basis for selecting the most appropriate approach in various fields where an easy-to-implement harmonic selection from high harmonic generation is needed.

Condensed matter physics has often provided a platform for investigating the interplay between particles and fields in cases that have not been observed in high-energy physics. Here, using angle-resolved photoemission spectroscopy, we provide an example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the emergence of new Weyl fermions in the ferrimagnetic state. Our results establish NdAlSi as a magnetic Weyl semimetal and provide an experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.

2D materials provide a rich platform to study novel physical phenomena arising from quantum confinement of charge carriers. Many of these phenomena are discovered by surface sensitive techniques, such as photoemission spectroscopy, that work in ultra-high vacuum (UHV). Success in experimental studies of 2D materials, however, inherently relies on producing adsorbate-free, large-area, high-quality samples. The method that yields 2D materials of highest quality is mechanical exfoliation from bulk-grown samples. However, as this technique is traditionally performed in a dedicated environment, the transfer of samples into vacuum requires surface cleaning that might diminish the quality of the samples. In this article, a simple method for in situ exfoliation directly in UHV is reported, which yields large-area, single-layered films. Multiple metallic and semiconducting transition metal dichalcogenides are exfoliated in situ onto Au, Ag, and Ge. The exfoliated flakes are found to be of sub-millimeter size with excellent crystallinity and purity, as supported by angle-resolved photoemission spectroscopy, atomic force microscopy, and low-energy electron diffraction. The approach is well-suited for air-sensitive 2D materials, enabling the study of a new suite of electronic properties. In addition, the exfoliation of surface alloys and the possibility of controlling the substrate-2D material twist angle is demonstrated.

Here, we present a high repetition rate, narrow bandwidth, extreme ultraviolet photon source for time- and angle-resolved photoemission spectroscopy. The narrow bandwidth pulses Δ E = 9, 14, and 18 meV for photon energies h ν = 10.8, 18.1, and 25.3 eV are generated through high harmonic generation using ultra-violet drive pulses with relatively long pulse lengths (461 fs). The high harmonic generation setup employs an annular drive beam in tight focusing geometry at a repetition rate of 250 kHz. Photon energy selection is provided by a series of selectable multilayer bandpass mirrors and thin film filters, thus avoiding any time broadening introduced by single grating monochromators. A two stage optical-parametric amplifier provides < 100 fs tunable pump pulses from 0.65 μm to 9 μm. The narrow bandwidth performance of the light source is demonstrated through angle-resolved photoemission measurements on a series of quantum materials, including high-temperature superconductor Bi-2212, WSe2, and graphene. 

Materials with a kagome lattice structure display a wealth of intriguing magnetic properties due to their geometric frustration and intrinsically flat band structure. Recently, topological and superconducting states have also been observed in kagome systems. The kagome lattice may also host a "breathing" mode that leads to charge density wave (CDW) states, if there is strong electron-phonon coupling, electron-electron interaction, or external excitation of the material. This "breathing" mode can give rise to candidate distortions such as the star of David (SoD) or its inverse structure [trihexagonal (TrH)]. To date, in most materials, only a single type of distortion has been observed. Here, we present angle-resolved photoemission spectroscopy measurements on the kagome superconductor CsV3Sb5 at multiple temperatures and photon energies to reveal the nature of the CDW in this material. It is shown that CsV3Sb5 displays two intertwined CDW orders corresponding to the SoD and TrH distortions. These two distinct types of distortions are stacked along the c direction to form a three-dimensional CDW order where the two 2-fold CDWs are phase shifted along the c axis. The presented results provide not only key insights into the nature of the unconventional CDW order in CsV3Sb5, but also an important reference for further studies on the relationship between the CDW and superconducting order.