Microtechnology and Nanoscience

Title:&nbsp;<span style="font-size: 12.0pt; font-family: 'Aptos',sans-serif; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: Aptos; color: black; background: white; mso-ansi-language: EN-GB; mso-fareast-language: EN-GB; mso-bidi-language: AR-SA;">Impact of Ionic Doping on the Normal and Superconducting Properties of YBCO Thin Films and Nanowires<span style="font-size: 12.0pt; font-family: 'Aptos',sans-serif; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: Aptos; color: black; background: white; mso-ansi-language: EN-GB; mso-fareast-language: EN-GB; mso-bidi-language: AR-SA;"> 
Date and time: 16 December 2025, 09.30 Location: Kollektorn, MC2
Supervisor: Riccardo Arpaia Examiner: Floriana Lombardi&nbsp; 
Abstract: Despite almost 40 years of intense research, high-temperature superconductivity in cuprates remains one of the most intriguing unsolved problems in condensed matter physics. The phase diagram of these materials, which describes how their properties vary with temperature and doping, is extremely complex: even the normal state above the superconducting critical temperature is characterized by multiple regions and symmetry-breaking orders associated with charge, magnetic, lattice, and orbital excitations. The interplay and competition among these orders, which may be at the origin of the pairing mechanism, are still far from being fully understood.
In this thesis, to shed light on these phenomena and gain a clearer picture of normal-state orders and of their competition with superconductivity, we focus on YBa2Cu3O7−δ (YBCO), where doping is controlled by oxygen content, and we introduce a small fraction of Zn atoms substituting Cu. This chemical substitution modifies the CuO2 planes, which constitute the core of both the superconducting and normal-state properties of this family of materials. In particular, I optimized the growth of Zn-doped YBCO thin films on STO substrates via pulsed laser deposition. The partial substitution of Cu with non-magnetic Zn atoms effectively suppresses superconductivity, revealing the underlying normal-state properties hidden beneath the superconducting dome.
To explore different regions of the phase diagram and understand the effect of Zn, films were grown across a range of oxygen dopings, from underdoped to strongly overdoped regimes, and for two different Zn concentrations. The structure, morphology, and transport properties of these films were characterized: these measurements allowed me to precisely determine the doping and build the complete phase diagrams for both Zn levels. Our results reveal a clear suppression of Tc, an expansion of the insulating region, and a relative invariance of the pseudogap temperature. Finally, the films were patterned into Hall bars and nanowires to investigate transport properties via current-voltage characterization and transport measurements down to the nanoscale, providing information about material homogeneity at the nanodomain level.
This work contributes to the broader effort of disentangling the mechanisms behind unconventional superconductivity by accessing the normal state through chemical doping. Moreover, it establishes a new material platform that paves the way for future investigations in both transport and spectroscopic experiments.<span style="font-size: 12.0pt; font-family: 'Aptos',sans-serif; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: Aptos; color: black; mso-ansi-language: EN-GB; mso-fareast-language: EN-GB; mso-bidi-language: AR-SA;">

Title: Characterising Superfluid Weight and Dissipation in Cuprate Superconductors
Date and time:&nbsp;17 December 2025, 13.00 Location: Kollektorn, MC2
Supervisor: Floriana Lombardi Co-supervisor: Karn Rongrueangkul Examiner: Thilo Bauch
Abstract: High-temperature superconductors (HTS), despite being discovered over three decades ago, remain an unsolved puzzle that demands innovative approaches and advanced experimental techniques. The fundamental mechanism behind Cooper pair formation is still unknown, posing a major challenge to efforts aimed at increasing the critical temperature.&nbsp;
Recently, the observation of superconductivity in magic-angle twisted bilayer graphene (MATBG), driven by moiré physics and flat-band phenomena, has inspired new perspectives on HTS. This connection arises from striking similarities between cuprates and MATBG, both hosting multiple correlated electronic phases, including superconductivity, although at much lower critical temperature for MATBG. The increased density of states in flat bands naturally enhances the pairing critical temperature but simultaneously suppresses superfluid stiffness due to the diverging effective mass, hindering superconductivity. However, the quantum metric in multiband systems, such as MATBG, restores finite stiffness, enabling the occurrence of a superconducting state.&nbsp;
Analogous moiré-like effects can be induced in HTS thin films via substrate-driven superpotentials. For instance, quasi-periodic one-dimensional potentials from nanometer-scale facets on (110) MgO substrates significantly influence ultrathin YBCO films, promoting electronic nematicity, a precursor to flat-band physics.
In this thesis, we investigate superfluid stiffness, which is proportional to 1/𝜆2 (with&nbsp;𝜆&nbsp;being the London penetration depth), in YBCO films of varying thickness grown on different substrates. Our measurements reveal a pronounced increase in penetration depth for films on (110) MgO compared to LSAT substrates lacking surface nanostructures, indicating flatter electronic bands. These findings support the possibility of engineering band structures in YBCO thin films, paving the way toward tuning critical temperatures to higher values.
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Presentation of master thesis

Master and Bachelor thesis presentations at Microtechnology and Nanoscience

16 December 2025

09:30 Angelo Balistreri

17 December 2025

13:00 Oscar Fagrell Alkeskog, MPPHS