Talk

William Huddie – Entropic magnetic interactions

We study the interaction of an ensemble of large spins with two magnets whose magnetic moments are assumed to be so large as to be independent of temperature. For two different types of spin system we show that, under an appropriate range of parameters, the free energy of the spin system is dominated by entropy, causing a relative free energy difference between parallel and anti-parallel alignments of the magnetic moments and thus inducing a purely entropic interaction between the two large magnets. This interaction is more stable at increasing temperatures, though the range of stability is slightly different depending on the type of spin system.

Talk

Kevin Vonk – Structural and electronic properties of bismuth layers on Ge(111)

Bismuth is one of the most interesting topological materials. Among all stable materials, bismuth is known to exhibit the largest spin-orbit coupling. Recently, it has also been shown that bismuth can be classified as a higher-order topological insulator, which should exhibit so-called ‘hinge states’.

In this work, we will scrutinize the structural and electronic properties of different bismuth phases grown on Ge(111) using scanning tunnelling microscopy and spectroscopy. The first layer forms a (√3 x √3)R30° structure comprised of three-fold coordinated bismuth trimers that are located on Ge on-top sites. Our scanning tunnelling microscopy data indicate that the buckling of the underlying Ge(111) bilayer is strongly reduced. Currently, the group of Zeila Zanolli is performing total energy ab initio calculations to prove or disprove our conjecture. Furthermore, scanning tunnelling spectroscopy measurements reveal that the (√3 x √3)R30° bismuth layer has a well-defined trivial band gap. Finally, additional bismuth deposition yields the formation of Bi(110) islands with a rectangular symmetry, as well as Bi(111) islands with a hexagonal symmetry. We will show the preliminary results of our in-depth analysis on spatially-resolved scanning tunnelling spectra.

Talk

Chao Chen Ye – First-principles studies of fermiology in different topological phases of bulk ZrTe5

Topological insulators have been studied intensively over the last decades. Three-dimensional (3D) zirconium pentatelluride (ZrTe5) is an appropriate material for both theoretical and experimental investigations of distinct topological phases. In this work, we employ density functional theory and tight-binding calculations to study topological phases and Shubnikov-de Haas (SdH) oscillations of 3D bulk ZrTe5. We have uncovered the entire process of topological phase transitions in terms of electronic structures focusing on isoenergetic surfaces in the reciprocal space (i.e. isosurfaces). In addition, they can serve as a pattern for determining the corresponding topological phase without the need for calculating topological invariants. Our computed isosurfaces prove the predicted ones from experimental data and also provide an insight on how to experimentally vary the orientation of the magnetic field for SdH measurements. Notably, our SdH calculations reveal that symmetric Fermi pockets have the same frequencies, which means that they may not be experimentally detectable as different pockets.

Talk

Juan Daniel Torres – Flux-tunable Kitaev chain in a quantum dot array

Connecting quantum dots through Andreev bound states in a semiconductor-superconductor hybrid provides a platform to create a Kitaev chain. Interestingly, in a double quantum dot, a pair of poor man’s Majorana zero modes can emerge when the system is fine-tuned to a sweet spot, where superconducting and normal couplings are equal in magnitude. Control of the Andreev bound states is crucial for achieving this, usually implemented by varying its chemical potential. In this work, we propose using Andreev bound states in a narrow Josephson junction to mediate both types of couplings, with the ratio tunable by the phase difference across the junction. Now a minimal Kitaev chain can be easily tuned into the strong coupling regime by varying the phase and junction asymmetry, even without changing the dot-hybrid coupling strength. Furthermore, we identify an optimal sweet spot at π phase, enhancing the excitation gap and robustness against phase fluctuations. Our proposal introduces a new device platform and a new tuning method for realizing quantum-dot-based Kitaev chains.

Talk

Mazhar Ali – 2D Quantum Material Josephson Junctions

Josephson junctions are an important scientific and technological device where two superconductors are coupled together by a non-superconducting barrier, resulting in a sandwich-like heterostructure with superconducting properties which can modulated by the barrier or magnetic field through the barrier. Recently, great progress has been made in incorporating 2D quantum materials into these structures where their inherent properties can affect the tunneling superconductivity in novel ways. In this presentation we will discuss some of these results with particular focus on the incorporation of 2D Mott Insulators in between 2D Superconductors and the observation of magnetic field free Josephson diode behavoir and the route of non-reciprocal superconductivity toward technological use.

Talk

A. Mert Bozkurt – Tailoring arbitrary energy-phase relationships using Josephson tunnel junctions

Josephson tunnel junctions exhibit a simple current-phase relation, characterized by single harmonics.
Conversely, high-transparency Josephson junctions feature multiple harmonics, with the specific harmonics dependent on microscopic details of the junction, presenting a challenge for precise control. In this talk, I will illustrate that two Josephson tunnel junctions are connected in series, their energy-phase relationship is identical to a high-transparency Josephson junction. Based on this, I will demonstrate that by connecting multiple arms in parallel and introducing a magnetic flux, we can systematically engineer specific current-phase relationships. As an example, I will showcase a superconducting diode implementation with a high efficiency, a two-terminal device that controls supercurrent flow in one direction differently from the other. The resulting superconducting diode efficiency is robust against the imperfections in the design parameters, making it practical for real-world implementations. Beyond superconducting diodes, I will also showcase various other energy-phase relationships to demonstrate the versatility of the approach. This technique can be useful for engineering sophisticated energy-phase landscapes for advanced quantum computing systems.

Talk

Michiel Dubbelman – Superconductor/Mott insulator? Superconductor junctions and the field free superconducting diode effect

The superconducting diode effect allows a superconducting current to flow in one direction and a resistive current in the other, visible in the difference between positive and negative critical currents. Devices are categorized based on their magnetic field response: two categories show a crossing of critical currents at zero or finite magnetic fields, while the third, containing a single unexplained device shows no crossing. The single unexplained device is a van der Waals Josephson junction NbSe2/Nb3Br8/NbSe2. The niobium halides (Nb3X8, X=Cl, Br, I) have been predicted to have a decreasing U/t from the Cl to the I, going from Mott insulator to band insulator. The presentation will include data on the NbSe2/Nb3Cl8/NbSe2 Josephson diode and implications for the Nb3I8 version.

Talk

Kostas Vilkelis – Chiral adiabatic transmission protected by Fermi surface topology

We demonstrate that Andreev modes that propagate along a transparent Josephson junction have a perfect transmission at the point where three junctions meet. The chirality and
the number of quantized transmission channels is determined by the topology of the Fermi
surface and the vorticity of the superconducting phase differences at the trijunction. We
explain this chiral adiabatic transmission (CAT) as a consequence of the adiabatic evolution
of the scattering modes both in momentum and real space. We identify an effective energy
barrier that guarantees quantized transmission. We expect that CAT is observable in nonlocal conductance and thermal transport measurements. Furthermore, because it does not
rely on particle-hole symmetry, CAT is also possible to observe directly in metamaterials.

Talk

Rebecca Gharibaan – Towards microwave experiments in III-V 2DEGs

Microwave experiments in III-V 2DEGs are hindered by lossy microwave resonators when
fabricated on 2DEG substrates. The low quality factor of resonators on 2DEG substrates can
be attributed to dielectric losses. Flip-chip architecture allows us to fabricate resonators
on silicon and coupling them to 2DEG substrates by welding the chips with metallic contacts.
These metallic contacts can additionally carry current to the 2DEG substrate, essential for
certain microwave experiments. In this talk I will show the current progress towards optimizing
the galvanic connection between two chips in flip-chip architecture using indium bumps.

Talk

Antonio Manesco – A transport probe for valley phenomenon in bilayer graphene

Coherent control and detection of the valley degree of freedom is a cornerstone for valleytronics and valley-based quantum computation. However, access to valley-coherent phenomenon requires samples without short-range scattering. Recent fabrication advances on gated-defined bilayer graphene devices minimize the effects of inter-valley scattering. Consequently, a series of recent experiments reported control over valley-polarized states. Nevertheless, valley polarization is often probed indirectly due to the lack of a transport probe. We thus propose a gate-defined valley splitter as a valley-sensitive probing lead. We demonstrate the device operation with two example applications. First, inspired by the recent experiments with gate-defined quantum dots, we demonstrate how to read out valley polarization of dot levels. Secondly, we present a method to extract valley Hall voltage in a multiterminal setup.

Talk

Sergio Pierantoni – Electronic structure of the candidate FM-TI Mn1+xSb2-xTe4 by ARPES

Combining magnetism and nontrivial band topology can result in a variety of exotic quantum states (dissipationless edge states, gapped topological surface states..). In order to exploit the potential applications of these exotic quantum phases, it is crucial that these and further states are realized at high temperatures. Mn1Sb2Te4 has been proposed as an excellent platform to combine mangnetism and nontrivial band topology at high temperatures. In a recent study, MBE-grown samples of Mn rich Mn1Sb2Te4 have been suggested it to be a ferromagnetic topological insulator with -as yet- the highest Curie temperature of 45K. In this talk, I will present our ARPES dataset on MST single crystals with Tc as high as 70K. I will discuss the pro’s and con’s of using single crystals in the chase for a ferromagnetic topological insulator with very high transition (Curie) temperature, and introduce surface decoration with alkali metals and the use of circular dichroism in ARPES as powerful methods beyond conventional ARPES to sharpen the  determination of the topology of these materials

Poster

Bowy La Riviere – Extended chiral transition in a quantum loop ladder

Motivated by the recent discovery of a new chiral quantum phase transition, appearing in melting of period-four phases under the presence of a chiral perturbation, we investigate whether this chiral transition can be realized in quantum spin chains. For this we look at a quantum loop model – an effective model of a spin-1 ladder with a constrained Hilbert space limited to the singlet sector only. Through extensive density-matrix renormalization group simulations we show that the transition line between the plaquette (period-four) and the next-nearest-neighbor Haldane (disordered) phases originates in an Ashkin-Teller point. Beyond this point we report an extended interval of chiral transition.

Poster

Yoran Starmans – Towards electrical detection of spin-polarized surface states in Pb(1-x)Sn(x)Te nanowires

The topological crystalline insulator Pb(1-x)Sn(x)Te is a promising material system for applications in spintronics and topological quantum computing. Central to its potential lies the presence of robust surface states characterized by a helical spin-texture. However, a conclusive demonstration of these surface states via electrical transport remains absent. In this project, we aim to probe spin-polarization in our in-plane grown Pb(1-x)Sn(x)Te nanowires by using ferromagnetic contacts. Making use of the flexibility in both nanowire composition and device design, our objective is to relate this spin-polarization to topological surface states, while excluding influences of topologically trivial states and device artifacts. This project hence paves a path towards validating spin-polarized surface states in Pb(1-x)Sn(x)Te and successive Pb(1-x)Sn(x)Te-based computing devices.

Poster

Sebastian Miles – Poor man’s Majoranas through direct dot coupling

Can we improve excitation gaps and scaling of Poor man’s Majorana devices without compromising our ability to tune the device and recover the relevant physics? We and our collaborators have explored this question both theoretically and experimentally over the past year and came to the conclusion that, indeed, we can find and build a design delivering both. We remove the superconductor or hybrid section that mediates the coupling in state-of-the-art designs. Instead, we utilize Andreev bound states in a proximitized quantum dot to generate the relevant couplings. Theoretical analysis predicts that this design is able to host poor man’s Majoranas when tuning to the chemical potentials and barriers to certain, so called, sweet-spot values. The prediction is backed up by two experiments, one in a nanowire device, and the other in a 2D electron gas device. Both systems show signatures of poor man’s Majoranas when tuning the devices to a sweet-spot.

Poster

Cédric Cordero – Probing the crystal and optical anisotropies of 2D CrPS4 for polarization sensitive photodetection

Atomically thin van der Waals (vdW) materials display a variety of optical and electronic properties that are a result of their underlying symmetries, transport nature (metal, semiconductor or insulator), as well as thickness and inter/intralayer spin configuration, among others.
Amongst the spectrum of vdW materials, chromium thiophosphate CrPS4 is a layered antiferromagnetic semiconductor that displays a strong linear dichroism response, making it an interesting material for polarization-sensitive photodetection and high tunability of magnetic states through gating control.
In this work, 2D CrPS4 is exfoliated from a bulk crystal and a set of 12 diagonally opposite contacts (with a step of 30 degrees) are patterned on the flake. By measuring the photocurrent of the device as a function of the angle of incident linearly polarized light (proving the optical anisotropy) and changing the contact pair (probing the crystal anisotropy), we are able to distinguish the effect of both anisotropies over the photoresponse.
With this information, photodetectors can be engineered to display maximum or minimum signal by using the correct choice of polarization angle and contact sets. From our results, this leads to an 8-fold increase in photocurrent.
Future measurements in more complex heterostructures, including gate electrodes, could give insight into the mechanisms originating the photocurrent in these systems, which would be interesting to study coupled to its antiferromagnetic nature.

Poster

Yanliang Hou – Quantum transport in Dirac semimetal-based Josephson junctions

Topological Dirac semimetals have recently gained significant attention, since they possess exotic quantum states, particularly surface Fermi arc states. Here, we employ Dirac semimetal nanoflakes and nanowires-based Josephson junctions as a platform for searching Fermi-arc superconductivity. Then, we carry out experimental measurements related to finite-momentum Cooper pairing, in-plane orbital effect and 4π supercurrent-based ac Josephson effect. We have observed a large asymmetry in critical supercurrent, called superconducting diode effect caused by finite-momentum Cooper pairing. And we also have observed that the Ic exhibits oscillations with a parallel magnetic field, caused by in-plane orbital effect of surface states.

Poster

Julian Strik – Aging in the self-induced spin glass Nd(0001)

Elemental neodymium has been shown to be a self-induced spin glass, where glassy behaviour stems solely from the frustrated nature of the magnetic interactions [1]. This contrasts with traditional spin glasses, where the presence of disorder is essential toward realizing glassy behaviour. The magnetic state of Nd(0001) is characterized by a lack of long range order, but exhibits local non-collinear order (Q-states). Upon increasing the temperature, neodymium displays an unusual magnetic phase transition from a self-induced spin glass to a long-range ordered multi-Q phase [2]. Here, we explore the aging behaviour of Nd(0001) in its self-induced spin glass state using spin-polarized scanning tunneling microscopy in varying magnetic fields and variable temperature. We explore how the favourability of the Q-states evolves as we age the system and relate these changes to the preferred structure of the ordered phase. These observations indicate that neodymium may be a multi-well system, which deviates from the traditional energy landscape expected of spin glass systems, thus providing a new platform to study aging dynamics as well as dynamic heterogeneity.

[1] U. Kamber et al., Science 368 (2020).

[2] B. Verlhac et al., Nat. Phys. 18 (2022).

Poster

Padraig Maderson – Magnetism in Complex Rare-Earth Niobates

Poster

Viktoriia Radovskaia – Ultrafast light-induced switching in mixed van der Waals compounds

Controlling magnetic anisotropy, which defines the orientation of spins in space, provides a direct route to technologically relevant spin orientation switching. This control is particularly important in low-dimensional van der Waals (vdW) magnets, where it is believed to play a key role in maintaining long-range spin order. While conventional strategies such as strain engineering [1], doping [2] and electrical gating [3] allow continuous tuning of this important property in vdW magnets, they critically lack the speed required for rapid manipulation of the collective spins. Although recent studies have shown that ultrafast control of magnetic anisotropy in the antiferromagnetic phase of the vdW transition metal triphosphates MnPS₃ and NiPS₃ is possible using femtosecond laser pulses [4,5] no spin switching has been demonstrated so far.

In this study, we aim to consolidate continuous tuning and ultrafast control of magnetic anisotropy in intermetallic mixtures of MnPS₃ and NiPS₃, which are known to exhibit competing out- and in-plane magnetic anisotropies. By studying light-induced ultrafast spin dynamics in Mn_1-xNi_xPS₃, we aim to identify the time-domain fingerprints of spin switching. In particular, we show that at x = 0.1 and above a critical light fluence, the spin dynamics exhibit a rapid increase in amplitude and the appearance of anharmonicities, both of which are indicative of the switching phenomenon. Our approach not only enables ultrafast spin switching in vdW magnets, but also demonstrates a possible route towards the design and ultrafast control of competing magnetic states in vdW systems.

[1] Cenker, J., Sivakumar, S., Xie, K. et al. Reversible strain-induced magnetic phase transition in a van der Waals magnet. Nat. Nanotechnol. 17, 256–261 (2022).

[2] Chowdhury, R.R., DuttaGupta, S., Patra, C. et al. Unconventional Hall effect and its variation with Co-doping in van der Waals Fe3GeTe2. Sci Rep 11, 14121 (2021).

[3] Tang, M., Huang, J., Qin, F. et al. Continuous manipulation of magnetic anisotropy in a van der Waals ferromagnet via electrical gating. Nat Electron 6, 28–36 (2023).

[4] Mattias Matthiesen, Jorrit R. Hortensius, Samuel Mañas-Valero, et al. Phys. Rev. Lett. 130, 076702 (2023).

[5] Dmytro Afanasiev et al. Controlling the anisotropy of a van der Waals antiferromagnet with light. Sci. Adv.7, eabf3096(2021).

Poster

Zhiyuan Cheng – Magnetotransport measurements of a magnetic Kagome metal Yb0.5Co3Ge3

Kagome materials, characterized by their unique crystal structures, host an interesting band structure, where Dirac cones, van Hove singularity and flat bands coexist. As a result, these materials are a playground of rich physics at the interface of topology, correlation and magnetism. We are interested in the electronic properties of the Yb0.5Co3Ge3 – one of the 166 kagome metals. Previous studies in this material have revealed the presence of a charge density wave (CDW) state at 95 K [1] and the onset of a magnetic phase below 18-25 K [1, 2]. However, the origin of this magnetic phase is not completely understood.

In this work, we explore and analyze the electronic transport signatures of this magnetic phase in Yb0.5Co3Ge3 down to 2 K temperature and up to 8 T magnetic field. Our data shows a clear signature of Kondo effect in this Yb0.5Co3Ge3 below 23 K temperature. The angle-dependent magnetoresistance studied in these devices reveals the easy axis for the spins is along the c-axis of the crystal. We have further extended the understanding of kagome metals by performing magnetotransport study for pressures up to 2.8 GPa.

References

[1] Yaojia Wang et al., Chemistry of Materials 2022 34 (16), 7337-7343, DOI: 10.1021/acs.chemmater.2c01309

[2] Ashley Weiland et al., Crystal Growth & Design 2020 20 (10), 6715-6721, DOI: 10.1021/acs.cgd.0c00865

Poster

Dennis Klaassen – Topological Phase Transition in Germanene Nanoribbons from 1D to 0D Topological Modes

Investigating topological phases and phase transitions is pivotal for unveiling novel quantum states and propelling the development of advanced topological devices. However, the mechanisms underlying transitions between distinct topological phases, specifically from two-dimensional (2D) to one-dimensional (1D) systems, remain largely unexplored and poorly understood. Here, we fabricated arrays of zigzag terminated germanene nanoribbons with large topological gaps (100-150 meV) and metallic edge states. Their geometry facilitates a high density of parallel and straight 1D topological edge states. By systematically varying the nanoribbon width, we monitored the evolution of their topological characteristics, pinpointing a transition to a 1D topological insulator phase below a critical width of ∼2 nm. This transition is marked by the vanishing of the 1D edge states and the emergence of distinct zero-dimensional (0D) end states. We obtain theoretically and experimentally that the 0D topological behavior of (thin) germanene nanoribbons is rich and complex. The topological phase depends in a non-monotonic way on ribbon width, spin-orbit coupling, staggered mass, and termination.

Poster

Joost Aretz – Correlated Physics in Kagome Mott insulating heterostructure