Riccardo Reho – Proximity induced superconductivity in PbTe/Pb via first principles

Session 1A

Semiconductor-superconductor hybrid devices have been proposed as promising platforms for detecting and analyzing Majorana zero modes.
Due to their robustness against local perturbations, Majorana zero modes finds applications in the realm of topological quantum computing.
The emergence of Majorana zero modes depends on a delicate balance between the semiconductor’s properties such as: strong spin-orbit coupling, large Landé g-factor, and, the superconductor’s pairing potential. Equally important is the interaction between the superconductor and semiconductor states, which affects their relative energy alignment and gives rise to the proximity-induced superconductivity effect.
However, it remains uncertain whether all these conditions can be satisfied in a given system. Hence, a quantitative analysis based on textit{first principles} methods is desirable.
We apply the SIESTA–BdG method to solve the Bogoliubov–de Gennes equations for a PbTe/Pb heterostructure. We observe the emergence of a soft BCS-like superconducting gap, suggesting weak hybridization between PbTe and Pb states.
We find a significant difference in the average electrostatic potential of ~1.2 eV at the interface, which poses a challenge to the emergence of MZMs, as it prevents proper energy alignment between the semiconductor Zeeman gap and the superconductor gap.
We resolve a proximity-induced superconducting gap on the PbTe side of the heterostructure, which is most pronounced near the interface, alongside a softening of the superconducting gap on the Pb side.
Furthermore, we show that the pairing potential is anisotropic.
Our results suggest that both the normal and superconducting properties are robust against lattice deformations (strain) and electrostatic perturbations (electric field).

Puhua Wan – Inversion Symmetry Breaking in the Orbital Fulde-Ferrell State

Session 1A

An unconventional finite has been realized by coupling the orbital effect of the magnetic field and the spin-orbit coupling, now known as orbital FFLO, in multilayer Ising superconductor NbSe2. The possible configuration of the non-zero Cooper pair momentum can either break or preserve the inversion symmetry of superconductivity, dividing the superconducting phase into the orbital Fulde-Ferrell (FF) and Larkin–Ovchinnikov (LO) states. In this talk, we will show the phase transition from the orbital FF to the LO state and the associated symmetry transition.

Feike van Veen – A Transport Study on the Surface Hybridization in 3D Topological Insulators

Session 1A

The increasing demand for computation power inspires the search for novel systems that show potential for revolutionizing chip design and computational methods. The quantum spin Hall effect (QSHE) was predicted to contribute to both, as it exhibits helical edge channels allowing for a quantized conductance that can be utilized in the development of quantum computing. The QSHE has been realized in HgTe quantum wells and other 2D systems [1], however, these materials oppose many fabrication challenges and other platforms possibly showing the QSHE remain of significant interest.

Three dimensional topological insulators (3D TIs) form a class of materials that show significant avail due to their numerous applications and, therefore, their fabrication methods are studied and optimized thoroughly [2]. In the ultrathin of 3D TIs, i.e. when the thickness is of the same order as the surface state penetration depth, a hybridization gap is expected to open at the Dirac point wherein quantum spin Hall states can reside [3, 4].

We experimentally show that a hybridization opens a gap at the Dirac point in ultrathin 3D TIs. We successfully fabricated Josephson junctions and investigate the spatial current distribution in these devices. Furthermore, we show that we can neatly control the chemical potential in our devices, imparting us with promising outlook for next generation devices.

 

[1] König, M. et al., Science 318, 766–770 (2007)

[2] Hasan, M. Z.  et al., Rev. Mod. Phys. 82, 3045–3067 (2010)

[3] Asmar, M. M. et al, Phys. Rev. B 97, 075419 (2018)

[4] Moes, J. R. et al. Nano Lett. 24, 5110-5116 (2024)

Marieke Altena – Pushing transport to the edge in WTe2

Session 1A

WTe2 is a promising material to study quantum effects relevant for quantum computing. In WTe2 the surface and edge states behave differently in transport measurements from its bulk character. This difference is caused by the topological nature of WTe2 as a type-II Weyl semimetal and a higher-order topological state, which means this transport behaviour is intrinsically embedded in the band structure of WTe2. The surface and edge states can form an interesting platform to study Majorana-physics particularly when superconductivity is induced in these states.

 

We measured the electronic transport properties of thin, nanometer sized WTe2 devices at low temperatures. Superconductivity was successfully induced into the WTe2 flakes by using superconducting Nb-contacts. While the entire flake exhibited superconducting behavior, the topological nature of WTe2 is expected to enhance the supercurrent at the edges. By analyzing the field dependence of the critical current, we extract the current distribution within the WTe2 flake-based Josephson junction. Our measurements on these structures show a clear current enhancement at the edge of the flake compared to the bulk supercurrent. Interestingly, this enhancement is also observed at step edges caused by thickness variations within the flake.

Lumen Eek – tba

Session 1B

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Diego Felipe Muñoz Arboleda – Thermodynamics and entanglement entropy of the non-Hermitian SSH model

Session 1B

Topological phase transitions are found in a variety of systems and were shown to be deeply related with a thermodynamic description through scaling relations. Here we investigate the entanglement entropy, which is a quantity that captures the central charge of a critical model and the thermodynamics of the nonreciprocal Su-Schrieffer-Heeger (SSH) model. Although this model has been widely studied, the thermodynamic properties reveal interesting physics not explored so far. In order to analyze the boundary effects of the model, we use Hill’s thermodynamics to split the grand potential in two contributions: the extensive one, related to the bulk, and the subdivision one, related to the boundaries. Then, we derive the thermodynamic entropy for both the edges and the bulk, and the heat capacity for the bulk at the topological phase transitions. The latter is related to the central charge when the underlying theory is a conformal field theory, whereas the first reveals the resilience of the topological edge states to finite temperatures. The phase transition between phases that are adiabatically connected with the Hermitian SSH model display the well-known behavior of systems within the Dirac universality class, but the transition between phases with complex energies shows an unexpected critical behavior that signals the emergence of an imaginary time crystal.

Anouar Moustaj – Topological phase of the interacting SSH model

Session 1B

The interacting SSH model provides an ideal ground to study the interplay between topologically insulating phases and electron-electron interactions. We study the polarization density as a topological invariant and provide an analytic treatment of its behavior in the low-energy sector of the one-dimensional interacting SSH model. By formulating the topological invariant in terms of Green’s functions, we use the low-energy field theory of the model, the Thirring model, to derive the behavior of the polarization density. We show that the polarization density in the continuum theory describes the usual topological insulating phases, but it contains an extra factor coming from the scaling dimensions of the fields in the low-energy quantum field theory. We interpret this as a measure of the modified charge of the new excitations in the system. We find two distinct contributions: a renormalization of the electronic charge $e$ of a Fermi liquid, because of quasiparticle smearing, and an additional contribution coming from the topological charge of the soliton arising in the bosonized version of the Thirring model, the sine-Gordon model.

Bowy La Riviere – $mathbb{Z}_4$ transitions in quantum loop models on a zig-zag ladder

Session 1B

We study the nature of quantum phase transitions out of $mathbb{Z}_4$ ordered phases in quantum loop models on a zig-zag ladder.  We report very rich critical behavior that includes a pair of Ising transitions, a multi-critical Ashkin-Teller point and a remarkably extended interval of a chiral transition. Although plaquette states turn out to be essential to realize chiral transitions, we demonstrate that critical regimes can be manipulated by deforming the model as to increase the presence of leg-dimerized states. This can be done to the point where the chiral transition turns into first order, we argue that this is associated with the emergence of a critical end point.

Lucas Maisel Licerán – High-temperature unconventional excitonic insulators from band geometry

Session 2A

We theoretically study the excitonic insulator phase in two recently proposed two-dimensional candidate materials with nontrivial band geometry and topology. Contrary to previous works on the topic, we consider interaction channels beyond the usual intraband electron-hole scattering, which due to the strong band hybridization are on equal footing with the latter. Furthermore, they break the conservation of the individual electron and hole numbers and are thus crucial for the determination of the excitonic pairing symmetry. By performing mean-field calculations at zero and finite temperatures we find that the excitonic order parameter is of the chiral d-wave type. We discuss the nontrivial topology of this unconventional phase and analyze its superfluid properties. In particular, the BKT crossover temperature lies around 100 K on realistic substrates, which is almost two orders of magnitude larger than that obtained under the assumption of intraband scattering alone. Our findings suggest an important competition between many-body effects in materials with nontrivial geometry and topology, and motivate the search of exotic phases of matter in such systems.

Cedric Cordero Silis – Van der Waals Materials for Polarization-Sensitive All-2D Photodetectors

Session 2A

Light detectors are key components in photonic circuits, converting light information into electronic signals. Van der Waals (vdWs) materials provide an excellent platform for photonics, with strong light-matter interaction.

In this presentation, I will show the use of phase-engineered two-dimensional (2D) transition-metal dichalcogenides (TMDs) for optoelectronic applications. Using laser-induced phase engineering, we modify the crystal structure of MoTe2 from the semiconducting 2H phase to the semi-metallic 1T’. Our photocurrent measurements show that the photogenerated carriers come from the 1T’-2H junction with the Schottky barrier between the materials playing a key role in the photon-to-charge current conversion.

Additionally, I will show that the low-symmetry CrPS4 vdWs crystal can be used as a photodetector with high polarization sensitivity. Probing the photocurrent along different light polarizations and crystallographic directions, we demonstrate how the linear dichroism influences the photoresponse of our devices. Finally, we correlate the photoresponse to the excitation of specific states, relating their symmetry to the electronic transport anisotropy.

These results highlight the versatility of vdWs materials for designing tunable, high-performance optoelectronic devices and demonstrate their potential for highly sensitive all-2D photonic detectors.

William Huddie – Entropic magnetic interactions

Session 2A

Entropic interactions are well-known and studied in a variety of systems in statistical physics. In this talk, we give a possible method for realising entropic interactions in a system of artificial spin ice (which at low temperatures is exactly described by the six-vertex model, a purely entropic model of water ice), coupled to two macroscopic magnets. We exhibit a non-zero entropy difference between parallel and anti-parallel configurations of the large magnets, and give some outlook and possible applications of this result.

Thijs Roskamp – Corner lithography: A route to a SQUID on a cantilever

Session 2B

Superconducting quantum interference devices (SQUIDs) are the most sensitive magnetic flux sensors and are used in scanning SQUID microscopy (SSM) to spatially resolve and map magnetism. Conventional SQUID probes used in SSM make use of planar silicon substrates which limit their spatial resolution to several micrometers due to an increased sample-pickup area spacing. In order to increase both the probes spatial resolution and magnetic sensitivity the SQUID pickup area must be brought in closer proximity to the surface.

Recent advances in the fabrication of SQUID probes for SSM have employed ideas from other scanning probe microscopy techniques like atomic force microscopy (AFM) and scanning tunneling microscopy (STM) to move the SQUID to the apex of a sharp tip and increase the spatial resolution to several tens of nanometers [1, 2]. Although these SQUID-on-tip (SOT) probes have greatly advanced the field of SSM, there are still challenges in the fabrication process such as limited flexibility and one-by-one fabrication.

We have used the principles of corner lithography [3] and molding in silicon wafers to create freestanding superconducting wireframe probes on the wafer scale.  By controlling the initial mold size loop diameters from sub-200 nm to several micrometers can be realized. With a focused ion beam (FIB) we aim to pattern superconducting weak links at the apex of the fabricated probe to create a SQUID on a cantilever.

[1] Finkler et al. Nano Letters 10, 1046-1049 (2010)
[2] Wyss et al. Phys. Rev. App. 17, 034002 (2022)
[3] Berenschot et al. Small 8, 3823-3831 (2012)

Sergio Barquero – Topological or not? An ARPES answer on the candidate high-Tc FM TI Mn1+xSb2-xTe4

Session 2B

The combination of nontrivial band topology & magnetism results in a wide variety of exotic electronic phases that -if realised at high temperatures- could revolutionise fields like spintronics or low-power consumption electronics. The new, second-generation ferrimagnetic compound Mn1+xSb2-xTe4 (0.1 ≤ x ≤ 1, abbreviated here as MST) promises to host the quantum anomalous Hall effect (QAHE) and other topological phases at higher temperatures than any of its predecessors, with TCurie’s up to 73K. Their long-range magnetic order is confirmed and thus the big question in these samples is “are they topologically non-trivial?”. As I showed in my previous QuMat talk (Delft pillar meeting), MST crystals are significantly p-type doped, meaning the Dirac point of the putative TSS’s is well above EF. Nevertheless I will aim to persuade you that our temperature-dependent ARPES data clearly argue that the answer to this question is “yes”.

Joost Aretz – Nb3X8: From Weakly Correlated Obstructed Atomic Insulators to Strongly Correlated Mott Insulators

Session 2B

Nb3X8 (X is Cl,Br or I) are a class of layered van der Waals materials formed on a breathing-mode kagome lattice, which show a gapped electronic structure. Motivated by our previous work demonstrating that monolayer Nb3Cl8 is a Mott insulator (1), we focus here on electronic correlations in all bulk Nb3X8 compounds. We study the electronic structure via downfolded many-body models derived from first principles using the constrained Random Phase Approximation (cRPA) and solved within Cluster Dynamical Mean Field Theory (CDMFT). Our results show that the Coulomb interaction either renormalizes the band gap or drives the system into a strongly correlated Mott insulating regime, depending on the compound. These findings are further supported by our doping analysis. This work establishes Nb3X8 as a promising platform for investigating the transition from weak to strong electronic correlations.

(1) S. Grytsiuk, M. I. Katsnelson, E. G. C. P. van Loon, and M. Rösner. Nb3Cl8 a prototypical layered Mott-Hubbard insulator. npj Quantum Mater., January 2024.

Matthieu Verstraete – First-principles transport including magnetic and spin-orbit effects

Session 3

Topology is the next frontier in materials science, opening possibilities for ultra low power devices, exquisite sensing capabilities, and synergies with quantum computing through the protection of state coherence.

We will showcase recent advances in the theory and applications of first-principles transport calculations, to include magnetism and spin-orbit interactions, and what will be needed to tackle the most complex topological materials, which contain both effects. Using spinor wave functions and SOC naturally incorporates spin-flip processes in electron-phonon scattering. Comparisons to experiments on “simple” 3d ferromagnetic metals require the inclusion of electron-magnon scattering, both in resistivity and in magnon drag.

As a next step into topology, we have calculated the first-principles conductivity of Weyl semi-metals, in particular TaAs for which our calculations are in very good agreement with available experiments. Finally, magnetoelectrically-active 2D Nickel Iodide (NiI2) can host topological magnetic states, and we find that it shows strong sensitivity to pressure. Combining experimental characterization with first and second principles simulations, we determine the pressure dependency (up to 20 GPa) of the electronic band structure, magnetic phase transition, and spinwave dispersion.

– X Ma et al. New Journal of Physics 25, 043022 (2023)

– G Allemand and MJ Verstraete, unpublished (2024)

– J Kapeghian et al. Physical Review B 109 (1), 014403 (2024)

– C Occhialini et al. arxiv.org/abs/2306.11720

Hai Wang – How Excitons Get Charged: Tracking Trion Formation Dynamics in Monolayer Semiconductors via Ultrafast THz Photoconductivity Studies

Session 3

The conversion of light into electrical currents is a fundamental process underlying the operation of varied optoelectronic devices including photovoltaics and photodetectors. Understanding the underlying photophysics, e.g. generation and transport of charge carriers in the photoactive materials following photoexcitations, is crucial for improving the energy conversion efficiency of devices. Layered two-dimensional (2D) materials are emerging building blocks for the next generation electronics and optoelectronics. Since the discovery of graphene, the family of the 2D materials have been largely expanded, including the metallic (e.g. graphene), semiconducting (e.g. transition metal dichalcogenides, TMDs) and insulating phases (e.g. hexagonal boron nitride).

Owing to the reduced charge screening and confinement effects in monolayer semiconductors, the photogenerated electron and hole are subjected to strong coulomb interactions, leading to formation of bound electron-hole pairs, so called excitons with binding energy of 100s of meV. These charge neutral states can be further charged to form “charged” excitons or “trions”, with binding energies on the order of the thermal excitation at room temperature. While the static dynamics of the trion has been well studied, its formation dynamics and transport effects have remained unexplored. I would like to present our ongoing effort in tuning and monitoring charge exciton formation dynamics in a model monolayer semiconductor, WS2,  by combining electrochemical gating and ultrafast THz spectroscopy. Our results unveil an intriguing interplay between the background gating charge density and the photogenerated exciton density on the trion formation probability. I will finish my talk with potential research collaborations that I would like to develop inside the consortium based on these first results.

Zeila Zanolli – Density functional Bogoliubov-de Gennes theory for superconductors implemented in the SIESTA code

Session 3

We present SIESTA-BdG, an implementation of the simultaneous solution of the Bogoliubov-de Gennes (BdG) and density functional theory (DFT) problem in SIESTA, a first-principles method and code for material simulations which uses pseudopotentials and a localized basis set. This unified approach describes both conventional and unconventional superconducting states, and enables a description of inhomogeneous superconductors and heterostructures. We demonstrate the validity, accuracy, and efficiency of SIESTA-BdG by computing physically relevant quantities (superconducting charge density, band structure, superconducting gap features, density of states) for conventional singlet (Nb, Pb) and unconventional (FeSe) superconductors. We find excellent agreement with experiments and results obtained within the KKR-BdG computational framework. SIESTA-BdG forms the basis for modeling quantum transport in superconducting devices and including—in an approximate fashion—the superconducting DFT potential of Oliveira, Gross, and Kohn.

 

[1] R. Reho, N. Wittemeier, A. H. Kole, P. Ordejón, and Z. Zanolli, Phys. Rev. B 110, 134505 [2024]

Auke Vlasblom – Simultaneous Atomic-Scale Imaging and Electronic Characterization of Wet-Chemically Prepared Bi2Se3 Nanoplatelets

Session 4A

Colloidal semiconductor nanoparticles are of great interest for various optoelectronic applications, such as integration in displays, solar cells and electronics. For applications, the surface of nanoparticles is of critical importance. However, until now, no technique exists to simultaneously investigate the atomic structure (e.g.  the presence of defects) and the electronic properties of a nanoparticle—foremost limited by the presence of ligands that prevent direct access to the surface with a local probe. Here, we present a new and widely applicable procedure that allows investigation of the surface of a nanoparticle with a local probe. Using this method, nanoparticles are transferred to an atomically clean substrate under ultra–high vacuum conditions. We demonstrate the procedure for topological two-dimensional Bi2Se3 nanoplatelets deposited on Au(111). We reveal the atomic and electronic structure of the surface of colloidally synthesised Bi2Se3 nanoplatelets with scanning tunnelling microscopy and spectroscopy measurements. In particular, we studied the various types of defects that occur, and determine their influence on the electronic structure.

Kevin Vonk – Bismuthene on Bi(110)/Ge(111): a nanometer sized happy accident

Session 4A

Bismuth has long been touted as a great candidate for investigating topological effects at the edge. The monolayer unbuckled bismuth (111) phase, known as bismuthene, has shown the potential of bismuth as a two-dimensional topological insulator [1]. Furthermore, the buckled (111) phase also hints at being a 2D topological insulator, although only observed for multilayer islands thus-far [2, 3]. Initially, we investigated the structural and electronic properties of various bismuth phases on top of Ge(111) substrates. We have found several Bi phases on the Ge(111) surface: a (√3 x √3)R30° wetting layer, the rectangular Bi(110) phase and the hexagonal Bi(111) phase. Now, we present a fourth phase, which consists of a flat honeycomb structure with a lattice constant that is slightly larger than bulk bismuth. As such, we identify this phase as bismuthene. This is the first time that bismuthene has been reported on a substrate other than SiC. Notwithstanding any quantum size effects, we also observe an enhancement of the density of states near the island edges, hinting towards the topological effects present in bismuthene.

Julian Strik – A quantum simulator to study electronic structures in the Hofstadter limit

Session 4A

Quantum simulators are a pathway to study novel physical phenomena which are difficult to predict or observe in synthesized materials. To this end, the physical behavior of materials ranging from gasses to superconducting qubits has been used to emulate Hamiltonians [1,2]. The most iconic of which is the Hubbard model, where previously unobserved phenomena were seen [3]. To date, there is still a lack of viable platforms for quantum simulation to study confined electrons in strong magnetic fields, including controlling orbital and lattice symmetries as well as the long-range nature of the coupling. With such a platform, it would be possible to study the so-called Hofstadter limit. Reaching this limit requires magnetic fields that induce electron orbits on the same length scale as the periodicity of the lattice in question. For typical crystals, this corresponds to field strengths that are unattainable in a laboratory.

In this talk I will discuss a new quantum simulator to study electronic structure in the Hofstadter limit, which is based on using Cs atoms on the semiconducting surface of InSb [4]. We begin by patterning Cs atoms on the surface with scanning tunneling microscopy, sculpting confinement potentials on the 2DEG which act as artificial atoms (i.e. quantum dots). I will show the response of these artificial atoms to strong magnetic fields, where they exhibit Fock-Darwin states [5]. Furthermore, I will show how these artificial atoms can be coupled into molecular structures and what the response of their resultant electronic structure is to these magnetic fields. I will link this to the Hofstadter picture and further comment on perspectives to use this platform to study the role of spin-orbit coupling and topology.

[1] Bloch, I. et al., Nature Physics 8, 267–276 (2012)
[2] Houck, A. A. et al., Nature Physics 8, 292–299 (2012)
[3] J.P. Dehollain, et al, Nature, 579, 528 (2020)
[4] E. Sierda, et al, Science 380, 1048 (2023).
[5] L. P. Kouwenhoven et al., Rep. Prog. Phys. 64, 6 (2001).

Robert Cañellas Núñez – Topological edge and corner states in bismuth fractal nanostructures

Session 4B

Topological materials hosting metallic edges characterized by integer-quantized conductivity in an insulating bulk have revolutionized our understanding of transport in matter.
The topological protection of these edge states is based on symmetries and dimensionality.
While integer-dimensional effects on topological properties have been studied extensively, the interplay of topology and fractals, which may have a non-integer dimension, remains largely unexplored. Here we demonstrate that topological edge and corner modes arise in fractals formed upon depositing thin layers of bismuth on an indium antimonide substrate. Our scanning tunnelling microscopy results and theoretical calculations reveal the appearance and stability of nearly zero-energy modes at the corners of Sierpiński triangles, as well as the formation of outer and inner edge modes at higher energies. This work opens the perspective to extend electronic device applications in real materials at non-integer dimensions with robust and protected topological states.

Marcus Bäcklund – Topological Classification of Multifold Exceptional Points

Session 4B

Multifold exceptional points (EPs) are generic features in a large variety of non-Hermitian (NH) systems realized in, e.g., photonics, mechanical metamaterials and ultra-cold atomics systems. Despite their ubiquity, their topological nature has just recently been systematically classified using the so called resultant winding number [1]. This topological classification circumvents critial difficulties in NH systems as n-fold EPs (EPns) generically exists on intersections of lower-dimensional manifolds of EP(n-1)s, which makes every non-trivial domain of integration around EPns defective and hence ill-defined. Furthermore, the classification scheme allows for a connection to the classification of vector bundles, further unraveling the topological (and geometrical) nature of these objects [2]. In this talk, I will present this classification, explain why it is important, and, if time allows, also tease on the connection to vector bundles and the fundamental mathematical construction. I aim to do this in a semi-pedestrian way (from a mathematical perspective) to make sure not to lose the connection to real physical systems.

[1] T. Yoshida, J.L.K. König, L. Rødland, E.J. Bergholtz, and M. Stålhammar, Winding Topology of Multifold Exceptional Points, arXiv:2409.09153.

[2] M. Stålhammar, Work in progress.

Montserrat Navarro Espino – First-principles study of Bi(trimer) on a Ge(111) surface

Session 4B

In recent years, the study of proximity effects at the nanoscale has gained significant attention in materials physics, as stacking materials can be used to tune their properties. One example involves Bi (a higher order topological insulator) grown on top of Ge(111), with the first layer (wetting layer) forming a Bi trimer on the substrate. STM topography experiments have shown that the presence of the wetting layer alters the buckling in the top layer of the Ge(111) surface. However, the surface reconstruction and the consequences in electronic and topological properties of the heterostructure remain unclear. To address this, we characterized the structural and electronic properties of the Bi(trimer)/Ge(111) heterostructure using the DFT implementation in the SIESTA code. Our results show that the presence of the wetting layer leads to an increase in the buckling of atoms at the trimer center, while buckling is reduced at other surface sites with respect to the bulk material. Additionally, the band structure calculations show the opening of an energy gap, Rashba splitting and band inversion, indicating a non-trivial topology in the states of the system. Our future work aims to understand the topological features of the heterostructure by computing the topological invariant Z2.