Using the Matrix Product State framework, we generalize the Affleck-Kennedy-Lieb-Tasaki (AKLT) construction to one-dimensional spin liquids with global color ${\rm SU}(N)$ symmetry, finite correlation lengths, and edge states that can belong to any self-conjugate irreducible representation of ${\rm SU}(N)$. Families of local parent Hamiltonians can be constructed and allow us to study the...
We investigate the topology of a periodically-driven two-dimensional Chern insulator hosting anomalous edge modes. Using a renormalization group approach on the stroboscopic Berry curvature, we obtain flow diagrams that clearly delineate all topological phase boundaries, therefore demonstrating that a detailed knowledge of the micromotion is not necessary to assess the appearance of the...
Model Hamiltonians with quasicrystalline order display a hierarchy of phenomena at different scales and are excellent starting points to explore unconventional effects not achievable in conventional periodic solids. In this talk, I will present a theoretical study of the model that interpolates between two well-known quasiperiodic examples: Aubry-André and Fibonacci model. In particular, I...
Topological insulators are a novel state of matter which, to date, have seen a variety of manifestations. All available realizations, however, share a common feature: their spectral bands are attributed with a nonlocal index that is quantized. This unique topological property commonly manifests through exotic bulk phenomena and robust boundary effects. In this talk, I will present a...
I will present our theoretical work that aims at revealing systematic trends and developing intuition across the entire family of 2D transition metal dichalcogenides (TMDs). I will address the relevance of the crystal and ligand fields in determining the relative stability of 1T and 1H polymorphs and introduce a unified picture of lattice instabilities (charge-density-wave and strong-coupling...
Topological quantum materials have become a ubiquitous topic in condensed matter physics over the past decade, but there is still plenty of room for new discoveries of exotic topological phases and improvements in device engineering. Here, I will report our achievements in thin film growth, soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES), and scanning tunnelling microscopy and...
Using muon spin-rotation experiments and density functional theory calculations, we present evidence for competing magnetic orders in a topological kagome magnet Co$_3$Sn$_2$S$_2$ [1]. Our results show that while the sample exhibits an out-of-plane ferromagnetic ground state, an in-plane antiferromagnetic state appears at temperatures above 90 K. Strikingly, the reduction of the anomalous Hall...
We present our recent results on the local magnetic and electronic properties at the topological insulator/ferromagnetic insulator interface EuS/Bi$_2$Se$_3$, which was previously reported to exhibit magnetic proximity persisting up to room temperature [1]. We use antiresonant ARPES at the Eu $M_5$ pre-edge to access the interface electronic band structure. Low energy muon spin rotation...
In the spin-chain compound CuGeO3, the relation between charge, spin, and lattice degrees of freedom, giving rise to the Spin-Peierls transition, is still unclear. In this system, Resonant Inelastic X-ray Scattering (RIXS) at the O K-edge is capable of detecting charge-transfer excitations, including the formation of a Zhang-Rice singlet. The probability for such a non-local process depends on...
Topological materials have many interesting properties and are the focus of intense theoretical and experimental research. The material PrAlGe has recently been predicted to be a Weyl semimetal with broken time reversal and inversion symmetries [1]. We present experimental results on various properties of PrAlGe single crystals such as magnetization, neutron diffraction, and electrical...
1T’-MoTe2 is a layered, van der Waals material, which has recently been proposed to host Weyl fermions, linked to the tilted Dirac cones in its band structure. In this transition metal dichalcogenide, very fine energy scales play an important role. However, the precise low-energy structure of these bands has so far been experimentally elusive, due to a complex coexistence of several electronic...
Fe3Sn2 is predicted to be a type-II Weyl semimetal which orders ferromagnetically below TC= 646 K. It undergoes a spin reorientation transition (SRT) between 300 K-100 K which together with recently shown coupling between its easy axis and the band structure paves the way of external control of its bulk properties. By probing anisotropic magnetoresistance, bulk magnetization and imaging the...
The low-energy electronic structure and topological nature of ZrTe$_5$ has recently been under debate with several contradictory results published. It consists of nearly linearly dispersing bands at the Gamma point with potentially a small band gap, making ZrTe$_5$ very sensitive to structural defects. However, only minor attention has been given to the influence of the sample growth method on...
We investigate two-dimensional spinful bulk insulating phases of matter that are protected by time-reversal and crystalline symmetries. In order to characterize these systems, we use the concept of corner charge fractionalization and show that charges are both quantized and remain stable as long as all symmetries are preserved. To define the topology, we employ symmetry indicators and Wilson...
Despite being predicted more than a decade ago, Hopf insulators still have no realistic candidate material realization. The problem with finding such material is two-fold: Most importantly, the corresponding topological invariant - integer-valued Hopf number - is only defined for a two-band system, while another source of the problem stems from the assumed absence of any symmetry required to...
A large-gap Quantum Spin Hall Insulating state was recently predicted to appear in the compound Pt2HgSe3. The lack of knowledge of phase equilibria in the Hg-Pt-Se system as well as the high-volatility and toxicity of the components render the conventional crystal growth methods unusable. Here we report on the successful growth of Pt2HgSe3 crystals at high pressure (1-3GPa), using a cubic...
Monolayer jacutingaite (Pt$_2$HgSe$_3$) was recently predicted to be a Kane-Mele Quantum Spin Hall insulator (QSHI), with a topological gap as large as 0.5 eV. We investigated the electronic band structure of bulk single crystals by angle-resolved photoemission. Surprisingly, on the (001) surface, we observed surface states dispersing over large areas of the Brillouin zone, which is unexpected...
Complex oxide thin films and heterostructures exhibit a wide variety of interesting functionalities at their interfaces, which are often not present in the corresponding bulk components. Here, we report on a metallic interface in multilayers of two Mott insulators, LaVO3 and LaTiO3, using a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We show that the...
Interplay between spin, charge, orbital and lattice degrees of freedom is extremely strong and at the origin of numerous phenomena in complex oxides [1]. The bulk 3d2 LaVO3 showcases an interesting phase diagram where the low temperature orbital and spin ordering are strongly dependent upon the A cations size [2]. The GdFeO3-type distortions remove the t2g degeneracy and modify the bandwidth...
Thin films of the transition-metal oxide LaNiO$_3$ (LNO) undergo a metal-insulator transition when their thickness is reduced to 2-3 unit cells. Here, we use a state-of-the-art laser-ARPES setup to map the electronic structure of LNO thin films with improved resolution. A series of high-quality films of thicknesses ranging from 19 to 2 unit cells is grown by sputter deposition and transferred...
Probing the local transport properties of two-dimensional electron systems (2DES) confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range. In this paper, we investigate the scattering-type scanning near field optical microscopy as a tool for studying the conducting LaAlO$_3$/SrTiO$_3$ interface from room temperature...
The interface between LaAlO3 and SrTiO3 hosts a conducting two-dimensional electron system (2DES) characterized by several interesting properties. When the 2DES is confined in-plane to realize structures with a lateral size comparable to the characteristic length-scales of the system, mesoscopic effects emerge in electronic transport. Here, we present the properties of nanowires...
We present novel quantum rings, which for example are fabricated from Rashba materials. These rings make use of the little explored interface between quantum mechanics and classical physics – their function is based on quantum collapses of electron wave packets combined with the coherent evolution of the quantum states. The devices feature fascinating properties such as unidirectional...
The recent progress in the assembly of 2D van der Waals heterostructures has shown that it is possible to stack virtually every material out of this class enabling a truly unprecedented potential to discover new physical phenomena or to engineer novel electronic functionalities.Despite the vast scope of possibilities enabled by vdW interfaces, a systematic microscopic understanding allowing...
The anomalous Hall effect (AHE) can arise even in systems without a net magnetization provided that certain common symmetries are absent. Here, we present experiments on the layered antiferromagnet Co1/3NbS2, which exhibits AHE below the Néel temperature TN=29 K in the bulk. Our transport measurements on micro-fabricated devices reveal a pronounced anisotropy in the resistivity –indicative of...
Twisted graphene bilayers provide a versatile platform to engineer metamaterials with novel emergent properties by exploiting the resulting geometric moiré superlattice. We show that tuning the twist angle to $\alpha^*\approx 0.8^\circ$ generates flat bands with triangular superlattice periodicity. When doped with $\pm 6$ electrons per moiré cell, these bands are half-filled and electronic...
We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling and reveals that its effective value is enhanced by a factor of about two, due to electronic correlations. The self-energies...
We have performed soft x-ray angle-resolved photoemission spectroscopy (ARPES) measurements on overdoped La-based cuprates La$_{2-x}$Sr$_x$CuO$_4$ and Eu$_{0.2}$La$_{1.8-x}$Sr$_x$CuO$_4$, and investigated the band structure in three-dimensional momentum space. While nodal part of the Fermi surface was $k_z$ independent, significant $k_z$-dispersion was unveiled in the antinodal portion. From...
Charge order (CO) and the connection to electron-phonon coupling (EPC) play crucial role in the low-energy regime of quasi-one-dimensional ladder materials. Characterizing the relevant excitations provides a direct tool to assess the underlying complex interactions. Resonant inelastic X-ray scattering (RIXS) is a powerful technique for probing phonons and its interplay with CO. We investigated...
Studying the prototypical ferromagnetic superconductor UGe$_2$ we demonstrate the potential of the Modulated IntEnsity by Zero Effort (MIEZE) technique—a novel neutron spectroscopy method with ultra-high energy resolution of at least 1 $\mu$eV—for the study of quantum matter. We reveal purely longitudinal spin fluctuations in UGe$_2$ with a dual nature arising from $5f$ electrons that are...
In 3D, only time-reversal ($\mathcal{T}$) and inversion ($\mathcal{I}$) symmetries are essential for superconductivity. We examine the 2D case and find that $\mathcal{T}$ and $\mathcal{I}$ are not required, and having a combination of either symmetry with a mirror operation ($M_z$) on the basal plane suffices. Combining energetic and topological arguments, we classify superconducting states...
HEAs are a new class of materials that consist of several principal elements arranged on simple lattices, stabilized by the high-configurational-entropy of the random mixing of the elements. In this presentation, we will show that the properties of this superconducting high-entropy alloy are strongly related to the valence electron count and that the superconducting transition temperatures...
The actinide superconductor ThFeAsN exhibits a $T_c$ of 30 K without doping or external pressure. Formally similar to LaFeAsO and predicted to be an antiferromagnet, surprisingly, the new material does not show any magnetic order.
Based on results of a series of ambient- and high-pressure experiments and DFT calculations [1,2], we show how ThFeAsN combines the peculiarities of unconventional...
