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Abstract: Superfluid helium droplets represent a unique state of matter, which are large clusters of helium typically containing approximately 103-1011 helium atoms and exhibit remarkable properties such as superfluidity, a very low temperature (0.37 K) and high thermal conductivity. This PhD project investigates two novel aspects of superfluid helium droplets: the use of superfluid helium nanodroplets as the nanoreactors to grow magnetic nanoparticles and the generation and exploration of quantum vortices in a controlled manner. In the first part, we exploit the very low temperature and rapid cooling to develop a new approach for fabricating magnetic nanoparticles. For the very low temperature and the ultrahigh thermal conductivity, superfluid helium can suppress thermal effects during the atom-by-atom growth of magnetic nanoparticles, making the relatively weak exchange interactions (compared with metallic bonding) the driving force. As a result, the atomic spins align for ferromagnetic elements and thus the magnetic moments of nanoparticles are maximized. In particular, we focus on iron nanoparticles coated by a gold shell and investigate their properties by electron microscopy (for structural investigation) and x-ray circular dichroism (XMCD), for magnetic property measurement) at the Diamond Light Source. We first study mass spectrometry of small iron clusters and observed abnormal behaviours. Unlike other molecular clusters formed in helium droplets such as water, gold and silver, which typically follow a Poisson distribution, Fe+ channel was found to be far greater than that of FeN+ (N = 2-8) clusters. We postulate this as an indicator for the formation of high-spin iron clusters inside superfluid helium and attempt to provide an interpretation based on DFT calculations. However, XMCD showed an expected low magnetisation for Fe/Au core-shell nanoparticles which is even lower than iron oxide nanoparticles, indicating that the neutral Fe atoms are oxidized into Fe2+ within the nanoparticles which is magnetically inert. This is accounted by the very high electron negativity of Au atoms and the alloying effect during the growth of nanoparticles, which dismisses the magnetic properties. Our work shows that the choice of protective shell is important to maintain the magnetic properties of iron nanoparticles and points the direction for the next-step research. The second part presents a breakthrough in quantum vortex research. We demonstrate a novel method for generating controlled vortex arrays in superfluid helium droplets through collisions with cesium ions. Subsequential addition of metal atoms (Ag and Au) to helium droplets and the formation of nanodroplets allow the vortex lattices to be imaged after the nanoparticles are deposited onto a solid surface. By this approach we have revealed a record-high vortex density of 5.6×10¹⁴ m⁻², exceeding previous observations in bulk superfluid helium by more than six orders of magnitude. This unprecedented vortex density opens new possibilities for studying quantum hydrodynamics at extreme angular momenta and investigating quantum turbulence in previously inaccessible regimes. Through detailed theoretical analysis and experimental characterization, this work establishes superfluid helium droplets as a versatile platform for both materials synthesis and fundamental research. Our findings not only advance the understanding of superfluidity but also provide a new pathway for developing high-performance magnetic nanomaterials that can potentially revolutionize biomedical science and technologies.

Open Access

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Abstract: The 4Hb polytype of TaS2 is a natural heterostructure of H and T-type layers. Intriguing recent evidence points towards a possibly chiral superconducting ground state, unlike the superconductivity found in other polytypes where the T layers are absent, requiring understanding of the possible contributions of electrons from the T layers. Here we use micro-focused angle resolved photoemission spectroscopy to reveal that the T termination of the 4Hb structure is metallic, but a subsurface T layer - seen below an H termination and thus more representative of the bulk case - is gapped. The results imply a complete charge transfer of 1 electron per 13 Ta from the T to adjacent H layers in the bulk, but an incomplete charge transfer at the T termination, yielding a metallic Fermi surface with a planar-chiral character. A similar metallic state is found in an anomalous region with likely T-H-H’ stacking at the surface. Our results exclude cluster Mott localisation in either the bulk or surface of 4Hb-TaS2 and point to a scenario of superconductivity arising from Josephson-like tunneling between the H layers.

Open Access

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Abstract: Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel LiB2O4 materials. On the B-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi2O4 and heavy fermion behavior in LiV2O4. To date, however, LiTi2O4 has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic ground states. Here, we present a multimodal investigation of LiTi2O4, combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi2O4 from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi2O4. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

Open Access

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Abstract: Exchange bias fields at antiferromagnet/ferromagnet (AFM/FM) interfaces play a crucial role in the performance of spintronic devices. Despite extensive research, the physical origin of exchange bias remains incompletely understood. In this study, we conduct a detailed investigation of a prototype AFM/FM interface widely used in spintronic applications, i.e., the IrMn/CoFeB interface. High-resolution synchrotron X-ray measurements reveal the existence of uncompensated Mn spins at the interface. While most of these spins are strongly coupled to the adjacent CoFeB layer, a small fraction remains pinned to the underlying IrMn underlayer. Element-specific X-ray magnetic circular dichroism hysteresis loops show that these pinned spins can be switched by increasing the annealing magnetic field. Furthermore, micromagnetic simulations indicate that an imbalance in the quantity of antiparallel pinned spins contributes to the observed variation in exchange bias. Overall, these findings offer important insights into the microscopic mechanisms governing exchange bias and its tunability.

Open Access

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Abstract: Moiré heterostructures, created by stacking 2D materials together with a finite lattice mismatch or rotational twist, represent a new frontier of designer quantum materials. Typically, however, this requires the painstaking manual assembly of heterostructures formed from exfoliated materials. Here, clear spectroscopic signatures of moiré lattice formation in epitaxial heterostructures of monolayer (ML) NbSe2 grown on graphite substrates are observed. Angle-resolved photoemission measurements and theoretical calculations of the resulting electronic structure reveal moiré replicas of the graphite π states forming pairs of interlocking Dirac cones. Interestingly, these intersect the NbSe2 Fermi surface at the -space locations where NbSe2's charge-density wave (CDW) gap is maximal in the bulk. This provides a natural route to understand the lack of CDW enhancement for ML-NbSe2/graphene as compared to a more than fourfold enhancement for NbSe2 on insulating support substrates, and opens new prospects for using moiré engineering for controlling the collective states of 2D materials.