We present recent developments using the temperature dependent effective potential technique (TDEP) to model vibrational properties of materials. The technique relies on explicitly temperature-dependent effective Hamiltonians, determined from first principles, to capture all orders of non-harmonic effects. We will present applications to inelastic neutron scattering and Raman scattering. In addition, we will present a newly formed theory for light-matter interaction in the strong coupling regime with strong implications for the vibrational properties of polar materials.

Recently, topological phases of matter have attracted considerable attention due to their unique and robust properties. In particular, the possibility of finding or inducing superconductivity in a topological phase (known as topological superconductivity) has been an active area of research in recent years, for instance in quantum engineering where such a phase would be useful for quantum computing.

In this seminar, I will present a charge transport study of nanostructures of trigonal-PtBi₂, a noncentrosymmetric crystal with very strong spin-orbit coupling. In recent works,^[1][2][3] we evidenced the superconducting properties of this material and we predict it to be a Weyl- and nodal-line semimetal. I will focus on two main results: The discovery of 2-dimensional superconductivity at sub-kelvin temperatures, and the discovery of an anomalous planar Hall effect (APHE) in the normal phase, robust up to room temperature.

While superconductivity has already been reported under pressure in t-PtBi₂ above 2K^[4][5], we found that single crystals of t-PtBi₂ also display superconductivity at ambient pressure, with a critical temperature Tc ∼ 600mK^[1]. When thinning down the crystals with mechanical exfoliation, the superconductivity becomes two-dimensional below t ∼ 70nm. Remarkably, even at such large thicknesses, nanostructures show clear Berezinskii–Kosterlitz–Thouless (BKT) transitions, a usually very fragile transition only evidenced yet in nearly-atomically thin superconducting films. This unusual feature might be related to the recent discovery in ARPES that the superconductivity occurs on the Fermi arcs, the topological surface states of Weyl semimetals.

At higher temperature (in the normal state), we discovered a large planar Hall effect^[3] – the appearance under an external in-plane magnetic field of a transverse voltage dependent on the relative orientations of the electric (current) and magnetic fields – which is a signature of Weyl physics in non-magnetic materials^[6][7]. Additionally, we characterized an anomalous planar Hall response^[8], which we attribute to a mechanism where topological nodal-lines – 1d band touchings in k-space, which we predict in t-PtBi₂ – get converted into Weyl nodes under even infinitesimal magnetic fields. Both effects, which are signatures of topological phases, are robust up to room temperature^[2].

^[1] Veyrat et al. in Nano Letters (2023)
^[2] Veyrat et al. accepted at Nature Communications (2024)
^[3] Veyrat et al. under review at SciPost Physics (2024)
^[4] Wang et al. in Physical Review B 103, 1–6 (2021)
^[5] Bashlakov et al. in Low Temperature Physics 48, 747–754 and on arXiv 2205.06610 (2022)
^[6] Burkov in Physical Review B 96, 041110 (2017)
^[7] Nandy et al. in Physical Review Letters 119, 1–6 (2017)
^[8] Battilomo et al. in Physical Review Research 3, 1–6 (2021)

Intercalation of layered iron chalcogenide superconductors with guest species gives access to a gallery of layered phases with enhanced superconducting properties. While ntercalation isolates the electronically active FeSe layers (cf. large interlayer separation, d), it also mediates the geometry and strength of electronic correlations within the basic building FeSe₄ block, enquiring about its role in facilitating high T_c.

Here, the experimental platform Li_x(C₅H₅N)_yFe_2-zSe₂ is derived by intercalating [Li-C₅H₅N] adducts in the van der Waals gap of β-FeSe. Through low-temperature solvothermal techniques, two polytypes of Li_x(C₅H₅N)_yFe_2-zSe₂ with different d (~16,3 Å vs. ~11,5 Å) and T_c (44 K vs. 39 K) are made available, offering good testing ground to deliniate the conditions that foster high T_c in such expanded lattice systems. Probing the systems with advanced experimental methods available at synchrotron X-ray and neutron facilities tells that intercalation acts as a tuning knob of both the atomic and electronic structure, mediating the materials' correlations across a wide range of length and time scales.

The outcomes highlight that intercalation-mediated carrier-doping and variations in the local geometry of the FeSe₄ building blocks offer a means to optimize T_c at the frontier of large interlayer separation.

When materials are patterned in three dimensions, opportunities exist to tailor and create functionalities associated with an increase in complexity, the breaking of symmetries, and the introduction of curvature and non-trivial topologies. For superconducting nanostructures, the extension to the third dimension may trigger the emergence of new physical phenomena and technological advances.^[1][2][3]

In my talk, I will introduce a three-dimensional (3D) nanopatterning technique^[4] that allows fabrication and control of the emergent properties of a 3D superconducting nanostructure. I will discuss the experimental results that show a strong geometrical anisotropy of the critical field. Through this, it is possible to achieve the reconfigurable coexistence of superconducting and normal states in 3D superconducting architecture and the local definition of weak links. This insight into the influence of 3D geometries on superconducting properties offers a route to local reconfigurable control for future computing devices, sensors, and quantum technologies.

^[1] Fomin et al. in Nano Letters 12, 1282 (2012)
^[2] Córdoba et al. in Nano Letters 19, 8597 (2019)
^[3] M. Hayashi et al. in Physical Review B 72, 024505 (2005)
^[4] Skoric et al. in Nano Letters 20, 184 (2020)

Molecular systems are promising candidates for realizing quantum technologies, especially in quantum sensing and quantum biotechnology, due to their reproducible nature and chemical tailorability. Among the many molecular systems studied for their potential application in quantum technologies, photogenerated spin qubits are of particular interest as they form highly spin-polarized initial states, and under the right electronic environment have long coherence times and allow for optical read-out strategies to be implemented.

The first class of organic materials that are exceptionally promising spin qubit candidates are those based on bright mesitylated trityl (TTM) radicals. I present recent spin-resonance-based results on a TTM-based diradical, bridged via a carbazole moiety, exhibiting a ground state room temperature Tm time of ~ 1 µs, ground state spin polarization persisting beyond 200 µs after laser flash at 200 K, and optical addressability at 100 K^[1]. TTM-based diradicals have enormous potential as molecular spin qubits as they have the required properties at elevated temperatures compared to other molecular color centers, and could be engineered to form NV center analogs.

The second class of organic materials promising as spin qubits are organic semiconductors that undergo singlet fission (SF) after photoexcitation. SF can lead to the formation of highly spin-polarized, triplet (S=1) and quintet (S=2) excitons. These excitons can be optically detected^[2] and can be used for quantum sensing through changes in coherence times via interaction with analytes and in quantum biotechnology be useful for dynamic nuclear polarization^[3].

I will present recent work on a series of pentacene dimer systems with different linkers studied using electron paramagnetic resonance (EPR) spectroscopy.^[4][5] I will show how in one dimer system different excitation wavelengths result in different quintet sublevel populations. While other dimers exhibit long-lived (tens of µs) quintet states in transient EPR and allow for pulsed EPR experiments (including nutation experiments) under frozen glass conditions. Understanding the relationship between spin properties and molecular structure will enable realizing SF systems for quantum technologies.

Another architecture for realizing spin qubits comes in 2D materials. In recent years graphene and hexagonal boron nitride have been explored in this regard. However, they utilize either the chemical attachment of paramagnetic groups or the creation of defect states, and not the intrinsic magnetism of the 2D material. In a recent publication,^[6] I have shown that phosphorene nanoribbons exhibit room temperature paramagnetic and ferromagnetic signals, and through collaboration, it was revealed that upon optical excitation energy is funneled to the edge state. Therefore, this material has intrinsic edge state magnetism and an optical band gap, making phosphorene nanoribbons a unique playground for exploring the interplay between spin, magnetism, and the semiconducting ground states, providing a stepping-stone towards low-dimensional nanomaterials in quantum electronics.

In the last part, I will discuss my research vision and goals for the future, utilizing both molecular and 2D material-based spin qubits, which can be manipulated, initialized, and readout, through microwave, optical, and electrical strategies. Here I will focus on scalability and even hybrid spin qubit architectures.

^[1] R. Chowdhury, P. Murto, N. A. Panjwani et al. on arXiv 2406.03365 (2024)
^[2] P. J. Budden, L. R. Weiss, M. Müller, N. A. Panjwani et al. in Nature Communications 12, 1527 (2021)
^[3] Y. Kawashima, T. Hamachi et al in Nature Communications 14, 1056 (2023)
^[4] W. Kim, N. A. Panjwani et al. in Cell Reports Physical Science 5, 102045 (2024)
^[5] K. Majumder, S. Mukherjee, N. A. Panjwani et al. in Journal of the American Chemical Society 145, 20883 (2023)
^[6] A. Ashoka, A. J. Clancy, N. A. Panjwani et al. on arXiv 2211.11374 (2022)

Electrons at the border of localization generate exotic states of matter across all classes of strongly correlated electron materials and many other quantum materials with emergent functionality. Heavy electron metals are a model example, in which magnetic interactions arise from the opposing limits of localized and itinerant electrons. This remarkable duality is intimately related to the emergence of a plethora of novel quantum matter states such as unconventional superconductivity, electronic-nematic states, hidden order and most recently topological states of matter such as topological Kondo insulators and Kondo semimetals and putative chiral superconductors. The outstanding challenge is that the archetypal Kondo lattice model that captures the underlying electronic dichotomy is notoriously difficult to solve for real materials. Here, I will present a microscopic model for the heavy-fermion antiferromagnet CeIn₃. As pointed out in this seminar, we succeeded – for the first time – to design an ab-initio theory with quantitative power. The underlying multi-orbital periodic Anderson model of CeIn₃ embedded with input from ab-initio band structure calculations was perturbatively reduced to a simple Kondo-Heisenberg model, which captures the magnetic interactions quantitatively. This tractable Hamiltonian was validated via high-resolution neutron spectroscopy that reproduces accurately the magnetic soft modes in CeIn₃, which are believed to mediate unconventional superconductivity. The presented study paves the way for a quantitative understanding of metallic quantum states such as unconventional superconductivity, not only in heavy-fermion materials, but in all kinds of strongly correlated materials.

cf. W. J. Simeth, Z. Wang, E. A. Ghioldi, D. M Fobes, A. Podlesnyak, N. H. Sung, E. D. Bauer, J. Lass, S. Flury, J. Vonka, D. G. Mazzone, C. Niedermayer, Y. Nomura, R. Arita, C. D. Batista, P. Ronning & M. Janoschek in Nature Communications 14, 8239 (2023)

A major discovery in condensed matter physics is the discovery of High-Temperature Superconductivity in cuprates (copper oxides), which still hold the record for the highest critical temperature at ambient pressure. They feature layers of CuO₂ planes, believed to be responsible for their electronic properties, involving strong electronic correlations. Doping gives rise to a very complex phase diagram, going from an antiferromagnetic insulating phase to a pseudo-gap, superconductivity, and a strange metal phase, along with coexisting and/or competing orders of charge and spin. However, a comprehensive theoretical explanation of the Cooper pairing mechanism in these materials is still a debated subject, in which magnetic excitation (paramagnons) are promising candidates. The hole-doped Ca₂CuO₂Cl₂ copper oxychloride serves as an excellent compound to investigate all these phases on common ground. Its stable and simple I4/mmm 1-layer structure and strong 2D character make it very suitable for theoretical calculations, allowing direct comparison with experimental work.

In this talk I will discuss the magnetic excitation measured by Resonant Inelastic X-ray scattering (RIXS) up to the optimal doping. The paramagnon exhibits a similar dispersion with doping, along the (h,0) direction, similar to all cuprates, and a softening along the (h,h) direction, as also measured in other cuprates. Along the (h,h) direction, the bimagnon weakens in the underdoped phase, while a charge continuum seems to arise at higher doping. Raman spectroscopy confirm that the bimagnon become weaker with doping. The paramagnon band-with have the same energy as a waterfall feature in the electronic bands, as measured in Angle resolved Photo-Emission Spectroscopy (ARPES), suggesting a link between the two phenomena. This is indeed supported by cluster-DMFT calculations, which suggest a spin-polaron band emerge at such energy scale.