In the talk, I will focus on two members of the ZrNiAl structure type with twisted magnetic kagome layers. HoAgGe represents the first crystalline realization of kagome spin ice, evidenced by neutron scattering, and displays striking fractionalized plateau states in magnetic and transport experiments [1-3].
CrNiAs is a kagome metal in which the ferromagnetic order of Cr moments can be...
Kagome metals are known for their unique electronic band structure containing flat bands and Dirac cones with topological character. This has elevated interest in kagome metals as an adaptable system to study the interplay of band topology with superconductivity, itinerant magnetism, and other charge instabilities that are driven by electronic correlations. In the RT6Sn6 kagome metals,...
The Shastry-Sutherland lattice, also known as the orthogonal dimer lattice, is a paradigmatic geometry of frustrated magnetism both for classical and quantum spins. In this talk, I will report on extensive tensor network simulations of that model that have allowed us to understand the remarkably rich phase diagram of the spin-1/2 Shastry-Sutherland SrCu2(BO3)2 under pressure and magnetic...
Kagome antiferromagnets have garnered significant attention in condensed matter physics owing to their potential to host a diverse range of quantum spin liquids (QSLs) and fractional magnetization plateaus. In particular, the S=1/2 kagome antiferromagnet has been regarded as a holy grail for realizing QSLs, with candidates ranging from gapped Z2 to U(1) Dirac QSLs. Theoretical studies further...
In the layered transition-metal dichalcogenide 1T-TaSe$_2$ the formation of a star-of-David charge density wave results in a half-filled band at the Fermi surface that is sensitive to electron correlations. Interestingly, while the bulk material remains a metal, the surface displays a mix of different phases, ranging from insulating to metallic, all with the same in-plane charge ordering.
We...
Given the small binding between layers, stacking of van der Waals materials is a powerful tool for exploring the physics of quantum condensed matter. However, exploitation for engineering will require taking advantage of thicker defective stacks, or an ability to control the stacking order via external stimuli, such as electrical or optical pulses.
I will present X-ray diffraction data of...
Magnetically intercalated transition metal dichalcogenides (TMDs) provide a versatile three-dimensional (3D) material platform to explore quantum phenomena and functionalities that emerge from an intricate interplay between magnetism, band structure, and correlations.
I shall describe the observation of a nearly magnetization-free anomalous Hall effect (AHE) in V1/3NbS2. Our single-crystal...
Hexagonal boron nitride (hBN), a polar wide gap insulator, is the gate dielectric of choice in the field of 2D materials. However, surprisingly little is known about how hBN affects the electronic properties of 2D materials of interest. Here, we use nano-ARPES to study the prototypical systems of monolayer transition metal dichalcogenide semiconductors on hBN. Our data show two replica bands...
Two-dimensional materials and heterostructures exhibit correlated phenomena, whose electronic structures are fundamental to the underlying physics. In this talk, I will present our experimental progress on engineering the electronic structure of quantum materials using moiré superlattice potential and time-periodic light fields. In particular, topological flat bands in rhombohedral graphene...
Understanding how quasiparticle renormalization influences thermodynamic properties is a central question of Fermi-liquid theory. In this work, we analyze the mass enhancement arising from the reduction of quasiparticle weight and its impact on the electronic specific heat. A straightforward microscopic evaluation of the Migdal–Galitskii expression, using a $T$-independent spectral function,...
The interplay between magnetism and superconductivity is one of the central topics of condensed matter physics, which has recently been put into new light by the discovery of altermagnets. Here, we study this interplay from a fundamental symmetry perspective using irreducible co-representations of the altermagnetic spin-point groups. We construct and tabulate all symmetry-allowed pairing...
Distorted kagome metals have emerged as fertile platforms for investigating the interplay between geometrical frustration, magnetoelastic coupling, and exotic electronic states. Rare-earth based kagome metal RAgGe (R = Tb, Dy) crystallize in a hexagonal structure that hosts a distorted kagome network of magnetic ions, providing an intriguing platform to study field-induced spin reorientation...
Angle-resolved photoemission spectroscopy (ARPES) provides direct momentum-resolved access to correlation effects and orbital textures in quantum materials. In this talk, I will show how a combined experimental and theoretical approach allows us to extract these key electronic properties in van der Waals and Kagome-related systems.
First, I will discuss our ARPES and...
Superconductivity in graphene can arise when 2,3 or 4 layers are twisted with respect to each other at the so-called magic angle. We experimentally demonstrate Josephson Junctions and SQUIDS that behave like devices fabricated from conventional superconductors and insulators. We also observe time-resolved vortex trapping and detect quantum tunneling of vortices at low temperatures. For samples...
Kagome lattice provides a rich platform for exploring novel quantum states, emerging from the interplay between its frustrated corner-sharing triangular geometry and intriguing electronic structure. Co3Sn2S2 is a kagome lattice ferromagnet, exhibiting a unique interplay between its electronic wavefunction topology and magnetic spin configuration. This interaction results in several intriguing...
In a first part I will summarise results of an experimental study to unravel the magnetic ground state and interactions in Co$_3$Sn$_2$S$_2$, conducted with colleagues from PSI (Yona Soh) and the ILL (David Tam). By exploring the magnetic interactions to moderately high energies and using non-invasive probes to measure the magnetic ground state, we discover that the spin wave spectrum and...
I will discuss recent theoretical progress in understanding the role played by the electron-electron interactions in orbital magnetism, and highlight results obtained for the flat bands of the magic angle twisted bilayer graphene, as well as for some twisted transition metal dichalcogenides. I will also discuss the effect of the external out-of-plane magnetic field on the correlated ground...
This talk will review kinetic magnetism in the Fermi Hubbard model in triangular lattices. Focus will be on the regime of strong interactions, $t<<U$. In such systems for densities close to $n=1$ dominant magnetic interactions arise from magnetic polaron dressing of charge carrier propagating in a spin incoherent Mott insulator. In the case of hole doping, antiferromagnetic polarons originate...
Thin films under strain and with low interfacial adhesion are known to buckle and delaminate from their substrates [1,2]. While buckling has previously been reported in MoS₂ films grown on x-cut SiO₂, there are few reports of this phenomenon in other TMDCs or in films only a few tens of nanometers thick. Here, we present the first observation of buckling in MBE-grown NbSe₂ thin films (10–15...
Superconductivity often emerges as a dome around a quantum critical point (QCP) where long- range order is suppressed to zero temperature, e.g., in high-temperature superconductors. However, the presence of a putative charge-density-wave (CDW) QCP and its impact on emerging superconductivity received increased scientific attention only after the discovery of CDW order in copper oxides. By now,...
NbSe$_2$ is a transition metal dichalcogenide (TMDC) well known for having the highest superconducting transition temperature among all pristine TMDCs. While bulk NbSe$_2$ is only found in its 2H superconducting phase, it has been shown that 1T-NbSe$_2$ can be stabilized on a substrate through molecular beam epitaxy growth, with extremely different properties from the 2H phase. 1T-NbSe$_2$ is...
Understanding magnetism in two-dimensional (2D) van der Waals (vdW) magnets requires nanoscale local probes. Scanning SQUID microscopy (SSM), with ~100 nm resolution, enables direct imaging of magnetic structures in mono- and few-layer materials. Using SSM, we investigate the magnetic behavior of CrPS₄, a weakly anisotropic vdW interlayer antiferromagnet. Monolayer CrPS₄ shows no remanence and...
I will describe recent experiments where we used optical pumping techniques to manipulate the Chern number of integer and fractional Chern insulators in twisted MoTe2 homobilayer.
The Quantum Twisting Microscope (QTM) is a groundbreaking instrument that enables energy- and momentum-resolved measurements of quantum phases via tunneling spectroscopy across twistable van der Waals heterostructures. In this work, we significantly enhance the QTMs resolution and extend its measurement capabilities to higher energies and twist angles by incorporating hexagonal boron nitride...
Kagome metals host van Hove points and flat bands, with the resulting nesting effects potentially leading to unconventional CDW states. Using inelastic X-ray scattering, we show that the CDW in KV3Sb5 forms via a continuous softening of phonons to zero energy, similar to transition metal dichalcogenides (such as NbSe2). The soft phonons exhibit a prominent in-plane anisotropy that mirrors the...
The CsCr3Sb5 exhibits superconductivity in close proximity to a density-wave (DW) like ground state at ambient pressure, however details of the DW are still elusive. Using first-principles density-functional calculations, we found its ground state to be a 4×2 altermagnetic spin-density-wave (SDW) at ambient pressure. The magnetic long range order is coupled to the lattice, generating 4a0...
The question of time-reversal symmetry breaking in CsV3Sb5 remains an open and debated topic, with no definitive consensus established to date. Conventional magnetic field-sensitive probes have not conclusively confirmed the presence of time-reversal symmetry-breaking fields. This presentation will provide new insights into this crucial aspect through a study into the optical coherent phonon...
Two-dimensional materials (2DM) and their heterostructures offer tunable electrical and optical properties, primarily modifiable through electrostatic gating and twisting. While electrostatic gating is a well-established method for manipulating 2DM, achieving real-time control over interfacial properties remains a frontier in exploring 2DM physics and advanced quantum device technology....
CsV3Sb5 exhibits several emergent phenomena, including superconductivity (TC ~ 3.2 K), charge-density wave (TCDW ~ 98 K), and nematicity (Tnem ~ 36 K). Despite tremendous investigations for several years, the nature of the superconductivity and multiple electronic orders, as well as the relationship among them, remain elusive. In this study, we have investigated nematic susceptibility and...
Unconventional superconductivity often emerges in complex materials in which competing orders and complicated band structure obscure its origin. In contrast, $\kappa$-organics are geometrically frustrated quasi-2D single-band systems in which superconductivity arises near the bandwidth-tuned Mott metal-insulator transition in the absence of other orders. We show that in chemically substituted...
As a kagome Weyl semimetal with a high Néel temperature, Mn3Ga has attracted interest due to its large anomalous Hall and topological transport properties. We performed angle-dependent magnetoresistance measurements on the Mn3Ga thin films and observed a distinct anomaly in the temperature evolution and angular dependence of the resistivity. The change in the angular profile indicates a...
Accessing milli-Kelvin temperatures is a prerequisite for quantum-matter research and applications in quantum technologies. Adiabatic demagnetization refrigeration (ADR) is a simple and sustainable alternative to 3He/4He dilution refrigeration. Geometrically frustrated triangular and kagome rare earth oxides feature important advantages compared to the traditionally utilized hydrated...
While correlated phenomena of flat bands have been extensively studied in twisted systems, the ordered states that emerge from interactions in the intrinsic flat bands of kagome lattice materials remain largely unexplored. The newly discovered kagome metal CsCr3Sb5 offers a unique and rich platform for this research, as its multi-orbital flat bands at the Fermi surface result in a complex...
There are long-standing ideas and experiments concerning the emergence of unconventional quasiparticles in strongly interacting Fermi systems. The most dramatic are the fractional states originally observed for the two-dimensional electron gases in semiconductor heterostructures subjected to perpendicular magnetic fields, and explained shortly thereafter by Laughlin. Others found in zero field...
Flat bands emerge in a diverse array of materials, spanning twisted heterostructures and compounds with geometrically-frustrated lattices. They feature strong correlations as well as non-trivial topology. Recent experiments on kagome and pyrochlore metals have uncovered a rich variety of strong-correlation phenomena [1], and the 2024 discovery of superconductivity in TMD moiré systems has...
Edge states of chiral topologically ordered phases are commonly described by chiral Luttinger liquids, an effective theory that is exact only in the conformal limit; but in crystalline systems, deviations from simple power-law scaling of correlators generally emerge. Motivated by recent bulk observations of fractional Chern insulators in two-dimensional materials, we revisit this framework on...
Wigner crystals are extremely fragile, which is shown to result from very strong geometric frustration germane to long-range Coulomb interactions. Physically, this is manifested by a very small characteristic energy scale for sheer density fluctuations, which are gapless excitations in a translationally invariant system. The presence of disorder, however, breaks translational invariance, thus...
The band-structure picture of metals is very successful in many materials where the electron correlations are weak. On the other extreme, when correlations are very strong, one expects interaction-induced insulators – due to Mott localization or symmetry breaking. However, the intermediate regime where correlations are strong but the material remains gapless, harbors many open questions in our...
While the effect of vacuum fields on atomic system (such as through the Lamb shift) is well known, there is a recent interest in controlling the many-body phases of solid-state systems using vacuum fluctuations strongly coupled to such a system inside a microcavity. In such a system, the strength of the electric field caused by the vacuum fluctuations, to which the strength of the...
I introduce the concept of quantum printing [1]-- the imprinting of quantum states from photons and phonons onto quantum matter. The discussion is focusing on charged fluids (metals, superconductors, Hall fluids) and neutral systems (magnets, excitons). I demonstrate how structured light can generate topological excitations, including vortices in superconductors and skyrmions in magnets. I...
The self-assembly of matter in the aftermath of a phase transition results in emergent states that are the origin of everything around us. Here we investigate the dynamics of emergent mesoscopic topologically non-trivial domain states formed after an polaronic Wigner crystal is quenched by an external perturbation using a fast STM. In particular, we study the detailed single-electron (polaron)...
The unusual properties of superconductivity in magic-angle twisted bilayer graphene (MATBG) have sparked enormous research interest. However, despite the dedication of intensive experimental efforts and the proposal of several possible pairing mechanisms, the origin of its superconductivity remains elusive. In this talk, using angle-resolved photoemission spectroscopy with micrometer spatial...