arXiv subjects

Raquel Queiroz

Publications and source records attributed to Raquel Queiroz.

At least 37 records · Page 2Linked to original sources

arXiv2025

Roadmap for Photonics with 2D Materials

Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combination with layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary collection of those directions, where 2D materials contribute with polaritons of unique characteristics such as strong spatial confinement, large optical-field enhancement, long lifetimes, high sensitivity to external stimuli (e.g., electric and magnetic fields, heating, and strain), a broad spectral range from the far infrared to the ultraviolet, and hybridization with spin and momentum textures of electronic band structures. The explosion of photonics with 2D materials as a vibrant research area is producing breakthroughs, including the discovery and design of new materials and metasurfaces with unprecedented properties as well as applications in integrated photonics, light emission, optical sensing, and exciting prospects for applications in quantum information, and nanoscale thermal transport. This Roadmap summarizes the state of the art in the field, identifies challenges and opportunities, and discusses future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

cond-mat.mtrl-sci↗

arXiv2025

Local basis for interacting topological bands

The discovery of correlated states in moire materials has challenged the established methods of projecting interactions into a local Wannier basis due to topological obstructions that manifest in extended interactions. This difficulty can sometimes be evaded by decomposing the band into a basis of extended itinerant states and a lattice of local states, using the heavy fermion prescription. We revisit this framework by systematically identifying the dominant interaction channels guided by the eigenvalues of the projected density operator. This approach can be applied both to tight-binding and continuum models, allowing us to identify a hierarchy in interaction scales that can be universally used to reduce the Hilbert space dimension and determine an appropriate local basis for modeling electronic correlations in interacting topological materials.

cond-mat.str-el↗

arXiv2025

Quantum Geometry in Quantum Materials

Quantum geometry, characterized by the quantum geometric tensor, is pivotal in diverse physical phenomena in quantum materials. In condensed matter systems, quantum geometry refers to the geoemtric properties of Bloch states in the Brillouin zone. This pedagogical review provides an accessible introduction to the concept of quantum geometry, emphasizing its extensive implications across multiple domains. Specifically, we discuss the role of quantum geometry in optical responses, Landau levels, and fractional Chern insulators, as well as its influence on superfluid weight, spin stiffness, exciton condensates, electron-phonon coupling, etc. By integrating these topics, we underscore the pervasive significance of quantum geometry in understanding emergent behaviors in quantum materials. Finally, we present an outlook on open questions and potential future directions, highlighting the need for continued exploration in this rapidly developing field.

cond-mat.mes-hall↗

arXiv2025

Hidden States and Dynamics of Fractional Fillings in tMoTe2 Moiré Superlattices

The fractional quantum anomalous Hall (FQAH) effect was recently discovered in twisted MoTe2 bilayers (tMoTe2). Experiments to date have revealed Chern insulators from hole doping at v = -1, -2/3, -3/5, and -4/7 (per moiré unit cell). In parallel, theories predict that, between v = -1 and -3, there exist exotic quantum phases, such as the coveted fractional topological insulators (FTI), fractional quantum spin Hall (FQSH) states, and non-abelian fractional states. Here we employ transient optical spectroscopy on tMoTe2 to reveal nearly 20 hidden states at fractional fillings that are absent in static optical sensing or transport measurements. A pump pulse selectively excites charge across the correlated or pseudo gaps, leading to the disordering (melting) of correlated states. A probe pulse detects the subsequent melting and recovery dynamics via exciton and trion sensing. Besides the known states, we observe additional fractional fillings between v = 0 and -1 and a large number of states on the electron doping side (v > 0). Most importantly, we observe new states at fractional fillings of the Chern bands at v = -4/3, -3/2, -5/3, -7/3, -5/2, and -8/3. These states are potential candidates for the predicted exotic topological phases. Moreover, we show that melting of correlated states occurs on two distinct time scales, 2-4 ps and 180-270 ps, attributed to electronic and phonon mechanisms, respectively. We discuss the differing dynamics of the electron and hole doped states from the distinct moiré conduction and valence bands.

cond-mat.str-el↗

arXiv2025

Electronic and Structural Properties of Lanthanide-Doped MoS$_2$: Impact of Ionic Size and Orbital Configuration Mismatch

Single-photon emitters (SPEs) are crucial for quantum technologies such as quantum simulation, secure quantum communication, and precision measurements. Two-dimensional transition metal dichalcogenides (TMDCs) are promising SPE candidates due to their atomically thin nature and efficient photon extraction. However, their emission wavelengths limit compatibility with existing telecommunication technologies. Lanthanide doping in TMDCs, such as \defect{MoS}{2}, offers a potential solution by introducing sharp, $f$-orbital derived emissions in the infrared range. Yet, the feasibility of introducing these dopants remains uncertain due to their large ionic radii of the lanthanides. We employ density functional theory (DFT) calculations to investigate the structural and electronic properties of lanthanide-doped \defect{MoS}{2} monolayers (Ln=Ce, Er). By evaluating formation energies with up to three adjacent S vacancies, we assess how these vacancies mitigate lattice strain caused by the size mismatch of Ce and Er with Mo. Our results show that while \defect{Ln}{Mo} destabilizes the pristine lattice, S vacancies enhance thermodynamic stability. Charge state analysis indicates that defect states introduced by \defect{Ln}{Mo} localize near the valence band and remain stable across a wide Fermi energy range. Electronic structure analysis shows that Ce$^{4+}$ and Er$^{3+}$ maintain their oxidation states upon electron doping due to additional acceptor states from host-induced dangling bonds. These states arise from an orbital filling mismatch between dopants and Mo. Consequently, \defect{Ce}{Mo} is unlikely to exhibit infrared emissions due to its empty $f$-shell, whereas \defect{Er}{Mo} is expected to emit in the infrared. These findings demonstrate the potential of lanthanide-doped TMDCs as tunable SPEs and provide design strategies for optimizing their optical and electronic properties.

cond-mat.mtrl-sci↗

arXiv2024

Real-Space Imaging of the Band Topology of Transition Metal Dichalcogenides

The topological properties of Bloch bands are intimately tied to the structure of their electronic wavefunctions within the unit cell of a crystal. Here, we show that scanning tunneling microscopy (STM) measurements on the prototypical transition metal dichalcogenide (TMD) semiconductor WSe$_2$ can be used to unambiguously fix the location of the Wannier center of the valence band. Using site-specific substitutional doping, we first determine the position of the atomic sites within STM images, establishing that the maximum electronic density of states at the $K$-point lies between the atoms. In contrast, the maximum density of states at the $Γ$ point is at the atomic sites. This signifies that WSe$_2$ is a topologically obstructed atomic insulator, which cannot be adiabatically transformed to the trivial atomic insulator limit.

cond-mat.mtrl-sci↗

arXiv2024

Topologically protected flatness in chiral moiré heterostructures

The observation of delicate correlated phases in twisted heterostructures of graphene and transition metal dichalcogenides suggests that moiré flat bands are intrinsically resilient against certain types of disorder. Here, we investigate the robustness of moiré flat bands in the chiral limit of the Bistrizer-MacDonald model -- applicable to both platforms in certain limits -- and demonstrate drastic differences between the first magic angle and higher magic angles in response to chiral symmetric disorder that arise, for instance, from lattice relaxation. Using a hidden constant of motion, we decompose the non-abelian gauge field induced by interlayer tunnelings into two decoupled abelian ones, whose effective magnetic field splits into an anomalous contribution and a fluctuating part. The anomalous field maps the moiré flat bands onto a zeroth Dirac Landau level, whose flatness withstands any chiral symmetric perturbation due to a topological index theorem -- thereby underscoring a topological mechanism for band flatness. Only the first magic angle can fully harness this topological protection due to its weak fluctuating magnetic field. In higher magic angles, the amplitude of fluctuations largely exceeds the anomalous contribution, which we find results in an extremely large sensitivity to microscopic details. Through numerical simulations, we study various types of disorder and identify the processes that are enhanced or suppressed in the chiral limit. Interestingly, we find that the topological suppression of disorder broadening persists away from the chiral limit and is further accentuated by isolating a single sublattice polarized flat band in energy. Our analysis suggests the Berry curvature hotspot at the top of the $K$ and $K'$ valence band in the transition metal dichalcogenide monolayers is essential for the stability of its moiré flat bands and their correlated states.

cond-mat.mes-hall↗

arXiv2024

Semi-Dirac Fermions in a Topological Metal

Topological semimetals with massless Dirac and Weyl fermions represent the forefront of quantum materials research. In two dimensions (2D), a peculiar class of fermions that are massless in one direction and massive in the perpendicular direction was predicted sixteen years ago. These highly exotic quasiparticles - the semi-Dirac fermions - ignited intense theoretical and experimental interest but remain undetected. Using magneto-optical spectroscopy, we demonstrate the defining feature of semi-Dirac fermions - $B^{2/3}$ scaling of Landau levels - in a prototypical nodal-line metal ZrSiS. In topological metals, including ZrSiS, nodal-lines extend the band degeneracies from isolated points to lines, loops or even chains in the momentum space. With $\textit{ab initio}$ calculations and theoretical modeling, we pinpoint the observed semi-Dirac spectrum to the crossing points of nodal-lines in ZrSiS. Crossing nodal-lines exhibit a continuum absorption spectrum but with singularities that scale as $B^{2/3}$ at the crossing. Our work sheds light on the hidden quasiparticles emerging from the intricate topology of crossing nodal-lines and highlights the potential to explore quantum geometry with linear optical responses.

cond-mat.mtrl-sci↗

arXiv2024

Electronic Bound States in the Continuum in a 2D Metal

Bound states in the continuum (BICs) are quantum states that remain localized despite existing within a continuum of extended, delocalized states. They defy conventional wave theories and could be instrumental for quantum technologies that rely on the precise control of quantum states. While optical BICs have been realized in photonic systems, achieving electronic bound states in a metallic background remains an ongoing challenge. Here, we observe two defect states that remain localized within the metallic continuum of Pd5AlI2, a two-dimensional van der Waals metal. The emergence of these states is a manifestation of the hopping interference in the Pd5AlI2 lattice. This interference results in a (quasi) flat band and spatially localized eigenstates that are orthogonal to the metallic continuum thus avoiding hybridization with extended states.

cond-mat.mtrl-sci↗

arXiv2024

Measuring kinetic inductance and superfluid stiffness of two-dimensional superconductors using high-quality transmission-line resonators

The discovery of van der Waals superconductors in recent years has generated a lot of excitement for their potentially novel pairing mechanisms. However, their typical atomic-scale thickness and micrometer-scale lateral dimensions impose severe challenges to investigations of pairing symmetry by conventional methods. In this report we demonstrate a new technique that employs high-quality-factor superconducting resonators to measure the kinetic inductance -- up to a part per million -- and loss of a van der Waals superconductor. We analyze the equivalent circuit model to extract the kinetic inductance, superfluid stiffness, penetration depth, and ratio of imaginary and real parts of the complex conductivity. We validate the technique by measuring aluminum and finding excellent agreement in both the zero-temperature superconducting gap as well as the complex conductivity data when compared with BCS theory. We then demonstrate the utility of the technique by measuring the kinetic inductance of multi-layered niobium diselenide and discuss the limits to the accuracy of our technique when the transition temperature of the sample, NbSe$_2$ at 7.06 K, approaches our Nb probe resonator at 8.59 K. Our method will be useful for practitioners in the growing fields of superconducting physics, materials science, and quantum sensing, as a means of characterizing superconducting circuit components and studying pairing mechanisms of the novel superconducting states which arise in layered 2D materials and heterostructures.

cond-mat.supr-con↗

arXiv2024

Pomeranchuk Instability Induced by an Emergent Higher-Order van Hove Singularity on the Distorted Kagome Surface of Co$_3$Sn$_2$S$_2$

Materials hosting flat bands at the vicinity of the Fermi level promote exotic symmetry broken states. Common to many of these are van Hove singularities at saddle points of the dispersion or even higher-order van Hove singularities where the dispersion is flattened further. The band structure of kagome metals hosts both a flat band and two regular saddle points flanking a Dirac node. We investigate the kagome ferromagnetic metal Co$_3$Sn$_2$S$_2$ using scanning tunneling spectroscopy. We identify a new mechanism by which a triangular distortion on its kagome Co$_3$Sn surface termination considerably flattens the saddle point dispersion, and induces an isolated higher-order van Hove singularity (HOvHS) with algebraically divergent density of states pinned to the Fermi energy. The distortion-induced HOvHS precipitates a Pomeranchuk instability of the Fermi surface, resulting in the formation of a series of nematic electronic states. We visualize the nematic order across an energy shell of about 100 meV in both real-, reciprocal-, and momentum-spaces, as a cascade of wavefunction distributions which spontaneously break the remaining rotational symmetry of the underlying distorted kagome lattice, without generating any additional translational symmetry breaking. It signifies the spontaneous removal of a subset of saddle points from the Fermi energy to lower energies. By tracking the electronic wavefunction structure across the deformed Fermi surface we further identify a charge pumping-like evolution of the wavefunction center of mass. The mechanism we find for the generation of higher-order saddle points under a kagome distortion may be common to other kagome materials, and potentially other lattice structures, suggesting a generic new avenue for inducing unconventional electronic instabilities towards exotic states of matter.

cond-mat.str-el↗

arXiv2024

Frustrated hopping from orbital decoration of a primitive two-dimensional lattice

Materials hosting flat electronic bands are a central focus of condensed matter physics as promising venues for novel electronic ground states. Two-dimensional (2D) geometrically frustrated lattices such as the kagome, dice, and Lieb lattices are attractive targets in this direction, anticipated to realize perfectly flat bands. Synthesizing these special structures, however, poses a formidable challenge, exemplified by the absence of solid-state materials realizing the dice and Lieb lattices. An alternative route leverages atomic orbitals to create the characteristic electron hopping of geometrically frustrated lattices. This strategy promises to expand the list of candidate materials to simpler structures, but is yet to be demonstrated experimentally. Here, we report the realization of frustrated hopping in the van der Waals (vdW) intermetallic Pd$_5$AlI$_2$, emerging from orbital decoration of a primitive square lattice. Using angle-resolved photoemission spectroscopy and quantum oscillations measurements, we demonstrate that the band structure of Pd$_5$AlI$_2$ includes linear Dirac-like bands intersected at their crossing point by a flat band, essential characteristics of frustrated hopping in the Lieb and dice lattices. Moreover, Pd$_5$AlI$_2$ is exceptionally stable, with the unusual bulk band structure and metallicity persisting in ambient conditions down to the monolayer limit. Our ability to realize an electronic structure characteristic of geometrically frustrated lattices establishes orbital decoration of primitive lattices as a new approach towards electronic structures that remain elusive to prevailing lattice-centric searches.

cond-mat.str-el↗

arXiv2024

The quantum geometric origin of capacitance in insulators

In band insulators, where the Fermi surface is absent, adiabatic transport is allowed only due to the geometry of the Hilbert space. By driving the system at a small but finite frequency $ω$, transport is still expected to depend sensitively on the quantum geometry. Here we show that this expectation is correct and can be made precise by expressing the Kubo formula for conductivity as the variation of the \emph{time-dependent polarization} with respect to the applied field. In particular, a little appreciated effect is that at linear order in frequency, the longitudinal conductivity results from an intrinsic capacitance, determined by the ratio of the quantum metric and the spectral gap. We demonstrate that this intrinsic capacitance has a measurable effect in a wide range of insulators with non-negligible metric, including the electron gas in a quantizing magnetic field, the gapped bands of hBN-aligned twisted bilayer graphene, and obstructed atomic insulators such as diamond whose large refractive index has a topological origin. We also discuss the influence of quantum geometry on the dielectric constant.

cond-mat.mes-hall↗

arXiv2024

Electronic interactions in Dirac fluids visualized by nano-terahertz spacetime interference of electron-photon quasiparticles

Ultraclean graphene at charge neutrality hosts a quantum critical Dirac fluid of interacting electrons and holes. Interactions profoundly affect the charge dynamics of graphene, which is encoded in the properties of its electron-photon collective modes: surface plasmon polaritons (SPPs). Here we show that polaritonic interference patterns are particularly well suited to unveil the interactions in Dirac fluids by tracking polaritonic interference in time at temporal scales commensurate with the electronic scattering. Spacetime SPP interference patterns recorded in tera-hertz (THz) frequency range provided unobstructed readouts of the group velocity and lifetime of polariton that can be directly mapped onto the electronic spectral weight and the relaxation rate. Our data uncovered prominent departures of the electron dynamics from the predictions of the conventional Fermi-liquid theory. The deviations are particularly strong when the densities of electrons and holes are approximately equal. The proposed spacetime imaging methodology can be broadly applied to probe the electrodynamics of quantum materials.

cond-mat.str-el↗

arXiv2024

Quantum Metric in Step Response

Quantum geometry of Bloch wavefunctions has gained considerable interest with the discovery of moiré materials that exhibit bands flattened by quantum interference. The quantum metric, the symmetric part of the quantum geometric tensor, influences several observables, such as the dielectric constant, superfluid stiffness and optical spectral weight. However, a direct measurement of the metric itself has remained elusive so far. In linear response functions such as the conductivity, the matrix elements of the metric typically appear convoluted with energy prefactors, preventing finding an observable that is directly proportional to the total quantum metric. The only observable that may extract it is the integrated optical spectral weight weighted by the inverse frequency, a generalized sum rule known as the Souza-Wilkens-Martin (SWM) sum rule. However, the sum rule comes with experimental challenges, such as requiring a large spectrum of frequency resolution. In this work, we propose relaxation from constrained equilibrium as a method to directly measure the symmetric part of the time-dependent quantum geometric tensor (tQGT), which at $t=0$ is the quantum metric. Additionally, we comment on other geometric properties of insulators that are absent in the frequency expansions of conductivity in insulators but can, in principle, be revealed in step response.

cond-mat.mes-hall↗

arXiv2024

Ring states in topological materials

Ingap states are commonly observed in semiconductors and are often well characterized by a hydrogenic model within the effective mass approximation. However, when impurities are strong, they significantly perturb all momentum eigenstates, leading to deep-level bound states that reveal the global properties of the unperturbed band structure. In this work, we discover that the topology of band wavefunctions can impose zeros in the impurity-projected Green's function within topological gaps. These zeros can be interpreted as spectral attractors, defining the energy at which ingap states are pinned in the presence of infinitely strong local impurities. Their pinning energy is found by minimizing the level repulsion of band eigenstates onto the ingap state. We refer to these states as ring states, marked by a mixed band character and a node at the impurity site, guaranteeing their orthogonality to the bare impurity eigenstates and a weak energy dependence on the impurity strength. We show that the inability to construct symmetric and exponentially localized Wannier functions ensures topological protection of ring states. Linking ring states together, the edge or surface modes can be recovered for any topologically protected phase. Therefore, ring states can also be viewed as building blocks of boundary modes, offering a framework to understand bulk-boundary correspondence.

cond-mat.mes-hall↗

arXiv2024

Chiral limit and origin of topological flat bands in twisted transition metal dichalcogenide homobilayers

The observation of zero field fractional quantum Hall analogs in twisted transition metal dichalcogenides (TMDs) asks for a deeper understanding of what mechanisms lead to topological flat bands in two-dimensional heterostructures, and what makes TMDs an excellent platform for topologically ordered phases, surpassing twisted bilayer graphene. To this aim, we explore the chiral limits of massive Dirac theories applicable to $C_3$-symmetric moiré materials, and show their relevance for both bilayer graphene and TMD homobilayers. In the latter, the Berry curvature of valence bands leads to relativistic corrections of the moiré potential that promote band flattening, and permit a limit with exactly flat bands with nonzero Chern number. The relativistic corrections enter as a \emph{layer-orbit coupling}, analogous to spin-orbit coupling for relativistic Dirac fermions, which we show is non-negligible on the moiré scale. The Berry curvature of the TMD monolayers therefore plays an essential role in the flattening of moiré Chern bands in these heterostructures.

cond-mat.mes-hall↗

arXiv2024

Anderson Critical Metal Phase in Trivial States Protected by Average Magnetic Crystalline Symmetry

Transitions between distinct obstructed atomic insulators (OAIs) protected by crystalline symmetries, where electrons form molecular orbitals centering away from the atom positions, must go through an intermediate metallic phase. In this work, we find that the intermediate metals will become a scale-invariant critical metal phase (CMP) under certain types of quenched disorder that respect the magnetic crystalline symmetries on average. We explicitly construct models respecting average $C_{2z}T$, $m$, and $C_{4z}T$ and show their scale-invariance under chemical potential disorder by the finite-size scaling method. Conventional theories, such as weak anti-localization and topological phase transition, cannot explain the underlying mechanism. A quantitative mapping between lattice and network models shows that the CMP can be understood through a semi-classical percolation problem. Ultimately, we systematically classify all the OAI transitions protected by (magnetic) groups $Pm$, $P2'$, $P4'$, and $P6'$ with and without spin-orbit coupling, most of which can support CMP.

cond-mat.dis-nn↗