arXiv subjects

Angel Rubio

Publications and source records attributed to Angel Rubio.

At least 91 records · Page 5Linked to original sources

arXiv2025

Linear-Response Quantum-Electrodynamical Density Functional Theory Based on Two-Component X2C Hamiltonians

Linear-response quantum electrodynamical density functional theory (QEDFT) enables the description of molecular spectra under strong coupling to quantized photonic modes, such as those in optical cavities. Recently, this approach was extended to the relativistic domain using the four-component Dirac-Coulomb Hamiltonian. To provide a computationally efficient yet accurate alternative-particularly for modeling 2D spectra or collective coupling for large, heavy-element systems-this article introduces a two-component linear-response QEDFT method based on exact two-component (X2C) Hamiltonian models. We derive how the parent four-component Hamiltonian for coupled electron-photon systems undergoes the X2C transformation. Moreover, we show that, under common weak-field and dipole approximations, it suffices to apply the X2C transformation only during the ground-state self-consistent field procedure, with the subsequent calculations performed fully in the two-component regime using the same X2C decoupling matrix. The current implementation includes the atomic mean-field (amfX2C), extended atomic mean-field (eamfX2C), and molecular mean-field (mmfX2C) Hamiltonian models. Benchmark calculations demonstrate that the X2C approach closely reproduces reference four-component results, enabling us to efficiently tackle systems that would be otherwise computationally too expensive. As applications, we compute 2D spectra of a mercury porphyrin complex in a Fabry-Perot cavity, demonstrating off-resonant coupling and the appearance of multiple polaritonic branches. We also study a chain of AuH molecules, showing that collective coupling can locally modify chemical properties of a molecule with a perturbed bond length.

physics.chem-ph↗

arXiv2025

Terahertz field-induced metastable magnetization near criticality in FePS3

Controlling the functional properties of quantum materials with light has emerged as a frontier of condensed-matter physics, leading to the discovery of various light-induced phases of matter, such as superconductivity, ferroelectricity, magnetism and charge density waves. However, in most cases, the photoinduced phases return to equilibrium on ultrafast timescales after the light is turned off, limiting their practical applications. Here we use intense terahertz pulses to induce a metastable magnetization with a remarkably long lifetime of more than 2.5 milliseconds in the van der Waals antiferromagnet FePS3. The metastable state becomes increasingly robust as the temperature approaches the antiferromagnetic transition point, suggesting that critical order parameter fluctuations play an important part in facilitating the extended lifetime. By combining first-principles calculations with classical Monte Carlo and spin dynamics simulations, we find that the displacement of a specific phonon mode modulates the exchange couplings in a manner that favours a ground state with finite magnetization near the Néel temperature. This analysis also clarifies how the critical fluctuations of the dominant antiferromagnetic order can amplify both the magnitude and the lifetime of the new magnetic state. Our discovery demonstrates the efficient manipulation of the magnetic ground state in layered magnets through non-thermal pathways using terahertz light and establishes regions near critical points with enhanced order parameter fluctuations as promising areas to search for metastable hidden quantum states.

cond-mat.mtrl-sci↗

arXiv2025

Multiple Photon Field-induced Topological States in Bulk HgTe

Strong light-matter interactions can be exploited to modify properties of quantum materials both in and out of thermal equilibrium. Recent studies suggest electromagnetic fields in photonic structures can hybridize with condensed matter systems, resulting in photon field-dressed collective quantum states such as charge density waves, superconductivity, and ferroelectricity. Here, we show that photon fields in photonic structures, including optical cavities and waveguides, induce emergent topological phases in solids through polarization-mediated symmetry-breaking mechanisms. Using state-of-the-art quantum electrodynamic density functional theory (QEDFT) calculations, we demonstrate that strong light-matter coupling can reconfigure both the electronic and ionic structures of HgTe, driving the system into Weyl, nodal-line, or topological insulator phases. These phases depend on the relative orientation of the sample in the photonic structures, as well as the coupling strength. Unlike previously reported laser-driven phenomena with ultrashort lifetimes, the photon field-induced symmetry breaking arises from steady-state photon-matter hybridization, enabling multiple robust topological states to emerge. Our study demonstrates that vacuum fluctuations in photonic structures can be used to engineer material properties and realize rich topological phenomena in quantum materials on demand.

cond-mat.mtrl-sci↗

arXiv2025

Multi-plateau high-harmonic generation in liquids driven by off-site recombination

Non-perturbative high-harmonic generation (HHG) has recently been observed in the liquid phase, where it was demonstrated to have a different physical mechanism compared to gas and solid phases of matter. The currently best physical picture for liquid HHG eliminates scattered-electron contributions and identifies on-site recombination as the dominant contributor. This mechanism accurately predicts the cut-off energy and its independence of the driving laser wavelength and intensity. However, this implies that additional energy absorbed in the liquid as the driving laser intensity is increased does not result in higher-order non-linearities, which is in contrast to the conventional expectation from most nonlinear media. Here we experimentally observe the formation of a second plateau in HHG from multiple liquids (water, heavy water, propranol, and ethanol), thus explaining the conundrum of the missing higher-order response. We analyze this second plateau with a combination of experimental, state-of-the-art ab-initio numerical (in diverse systems of water, ammonia, and liquid methane), and semi-classical analytical, techniques, and elucidate its physical origin to electrons that recombine on neighboring water molecules rather than at the ionization site, leading to unique HHG ellipticity dependence. Remarkably, we find that the second plateau is dominated by electrons recombining at the second solvation shell, relying on wide hole delocalization. Theory also predicts the appearance of even higher plateaus, indicating a general trend. Our work establishes new physical phenomena in the highly non-linear optical response of liquids, paving the way to attosecond probing of electron dynamics in solutions.

physics.chem-ph↗

arXiv2025

Site-polarized Mott phases competing with a correlated metal in twisted WSe$_2$

Twisted WSe$_2$ hosts superconductivity, metal-insulator phase transitions, and field-controllable Fermi-liquid to non-Fermi-liquid transport properties. In this work, we use dynamical mean-field theory to provide a coherent understanding of the electronic correlations shaping the twisted WSe$_2$ phase diagram. We find a correlated metal competing with three distinct site-polarized correlated insulators; the competition is controlled by interlayer potential difference and interaction strength. The insulators are characterized by a strong differentiation between orbitals with respect to carrier concentration and effective correlation strength. Upon doping, a strong particle-hole asymmetry emerges, resulting from a Zaanen-Sawatzky-Allen-type charge-transfer mechanism. The associated charge-transfer physics and proximity to a van Hove singularity in the correlated metal sandwiched between two site-polarized insulators naturally explains the interlayer potential-driven metal-to-insulator transition, particle-hole asymmetry in transport, and the coherence-incoherence crossover in $3.65^\circ$ twisted WSe$_2$.

cond-mat.str-el↗

arXiv2025

Collectively-modified inter-molecular electron correlations: The connection of polaritonic chemistry and spin glass physics

Polaritonic chemistry has garnered increasing attention in recent years due to pioneering experimental results, which show that site- and bond-selective chemistry at room temperature is achievable through strong collective coupling to field fluctuations in optical cavities. Despite these notable experimental strides, the underlying theoretical mechanisms remain unclear. In this focus review, we highlight a fundamental theoretical link between the seemingly unrelated fields of polaritonic chemistry and spin glasses, exploring its profound implications for the theoretical framework of polaritonic chemistry. Specifically, we present a mapping of the dressed many-molecules electronic-structure problem under collective vibrational strong coupling to the spherical Sherrington-Kirkpatrick (SSK) model of a spin glass. This mapping uncovers a collectively induced instability of the intermolecular electron correlations, which could provide the long sought-after seed for significant local chemical modifications in polaritonic chemistry. Overall, the qualitative predictions made from the SSK model (e.g., dispersion effects, phase transitions, differently modified bulk and rare event properties, heating,...) agree well with available experimental observations. Our connection paves the way to incorporate, adjust and probe numerous spin glass concepts in polaritonic chemistry, such as modified fluctuation-dissipation relations, (non-equilibrium) aging dynamics, time-reversal symmetry breaking or stochastic resonances. Ultimately, the connection also offers fresh insights into the applicability of spin glass theory beyond condensed matter systems suggesting novel theoretical directions such as spin glasses with explicitly time-dependent (random) interactions.

physics.chem-ph↗

arXiv2025

Quantum melting of generalized electron crystal in twisted bilayer MoSe2

Electrons can form an ordered solid crystal phase ascribed to the interplay between Coulomb repulsion and kinetic energy. Tuning these energy scales can drive a phase transition from electron solid to liquid, i.e. melting of Wigner crystal. Generalized Wigner crystals (GWCs) pinned to moire superlattices have been reported by optical and scanning-probe-based methods. Using transport measurements to investigate GWCs is vital to a complete characterization, however, still poses a significant challenge due to difficulties in making reliable electrical contacts. Here, we report the electrical transport detection of GWCs at fractional fillings nu = 2/5, 1/2, 3/5, 2/3, 8/9, 10/9, and 4/3 in twisted bilayer MoSe2. We further observe that these GWCs undergo continuous quantum melting transitions to liquid phases by tuning doping density, magnetic and displacement fields, manifested by quantum critical scaling behaviors. Our findings establish twisted bilayer MoSe2 as a novel system to study strongly correlated states of matter and their quantum phase transitions.

cond-mat.str-el↗

arXiv2025

Cavity-Mediated Electron-Electron Interactions: Renormalizing Dirac States in Graphene

Embedding materials in optical cavities has emerged as a strategy for tuning material properties. Accurate simulations of electrons in materials interacting with quantum photon fluctuations of a cavity are crucial for understanding and predicting cavity-induced phenomena. In this article, we develop a non-perturbative quantum electrodynamical approach based on a photon-free self-consistent Hartree-Fock framework to model the coupling between electrons and cavity photons in crystalline materials. We apply this theoretical approach to investigate graphene coupled to the vacuum field fluctuations of cavity photon modes with different types of polarizations. The cavity photons introduce nonlocal electron-electron interactions, originating from the quantum nature of light, that lead to significant renormalization of the Dirac bands. In contrast to the case of graphene coupled to a classical circularly polarized light field, where a topological Dirac gap emerges, the nonlocal interactions induced by a quantum linearly polarized photon mode give rise to the formation of flat bands and the opening of a topologically trivial Dirac gap. When two symmetric cavity photon modes are introduced, Dirac cones remain gapless, but a Fermi velocity renormalization yet indicates the relevant role of nonlocal interactions. These effects disappear in the classical limit for coherent photon modes. This new self-consistent theoretical framework paves the way for the simulation of non-perturbative quantum effects in strongly coupled light-matter systems, and allows for a more comprehensive discovery of novel cavity-induced quantum phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Neural network distillation of orbital dependent density functional theory

Density functional theory (DFT) offers a desirable balance between quantitative accuracy and computational efficiency in practical many-electron calculations. Its central component, the exchange-correlation energy functional, has been approximated with increasing levels of complexity ranging from strictly local approximations to nonlocal and orbital-dependent expressions with many tuned parameters. In this paper, we formulate a general way of rewriting complex density functionals using deep neural networks in a way that allows for simplified computation of Kohn-Sham potentials as well as higher functional derivatives through automatic differentiation, enabling access to highly nonlinear response functions and forces. These goals are achieved by using a recently developed class of robust neural network models capable of modeling functionals, as opposed to functions, with explicitly enforced spatial symmetries. Functionals treated in this way are then called global density approximations and can be seamlessly integrated with existing DFT workflows. Tests are performed for a dataset featuring a large variety of molecular structures and popular meta-generalized gradient approximation density functionals, where we successfully eliminate orbital dependencies coming from the kinetic energy density, and discover a high degree of transferability to a variety of physical systems. The presented framework is general and could be extended to more complex orbital and energy dependent functionals as well as refined with specialized datasets.

physics.chem-ph↗

arXiv2025

Effective Equilibrium Theory of Quantum Light-Matter Interaction in Cavities: Extended Systems and the Long Wavelength Approximation

When light and matter interact strongly, the resulting hybrid system inherits properties from both constituents, allowing one to modify material behavior by engineering the surrounding electromagnetic environment. This concept underlies the emerging paradigm of cavity materials engineering, which aims at the control of material properties via tailored vacuum fluctuations of dark photonic environments. The theoretical description of such systems is challenging due to the combined complexity of extended electronic states and quantum electromagnetic fields. Here, we derive an effective, non-perturbative theory for low-dimensional crystals embedded in a Fabry-Pérot resonator, within the long-wavelength limit. Our approach incorporates the multimode and dispersive nature of the cavity field and reduces it to an effective single-mode description by imposing the condition of negligible momentum transfer from light to matter. Importantly, the resulting effective mode is characterized by a finite mode volume-even in the limit of extended cavities-which is directly linked to realistic cavity parameters. This ensures that the light-matter coupling remains finite in bulk systems. By explicitly accounting for the finite reflectivity of cavity mirrors, our theory also avoids double counting the contribution from free-space light-matter coupling. Overall, our results provide a robust and realistic framework for describing cavity-matter interactions at the Hamiltonian level, incorporating the electromagnetic environment beyond the idealized perfect-mirror approximation.

cond-mat.mes-hall↗

arXiv2025

Local-density correlation functional from the force-balance equation

The force-balance equation of time-dependent density-functional theory presents a promising route towards obtaining approximate functionals, however, so far, no practical correlation functionals have been derived this way. In this work, starting from a correlated wavefunction proposed originally by Colle and Salvetti [Theoret. Chim. Acta 37, 329 (1975)], we derive an analytical correlation-energy functional for the ground state based on the force-balance equation. The new functional is compared to the local-density correlation of the homogeneous electron gas and we find an increased performance for atomic systems, while it performs slightly worse on solids. From this point onward, the new force-based correlation functional can be systematically improved.

physics.chem-ph↗

arXiv2025

Equilibrium non-linear phononics by electric field fluctuations of terahertz cavities

Selective excitation of vibrational modes using strong laser pulses has emerged as a powerful material engineering paradigm. However, to realize deterministic control over material properties for device applications, it is desirable to have an analogous scheme without a drive, operating in thermal equilibrium. We here propose such an equilibrium analog of the light-driven paradigm, leveraging the strong coupling between lattice degrees of freedom and the quantum fluctuations of the electric field of a THz micro-cavity. We demonstrate this approach by showing, using ab initio data, how electric field fluctuations can induce a sub-dominant ferromagnetic order, on top of the dominant zig-zag antiferromagnet order, in FePS$_3$ close to its Néel temperature.

cond-mat.mtrl-sci↗

arXiv2025

High Harmonic Generation with Orbital Angular Momentum Beams: Beyond-dipole Corrections

We study the high harmonic generation with vortex beams beyond the dipole approximation. To do so we employ the full minimal coupling approach to account for multipolar coupling without truncation and describe the full spatio-temporal properties of the electromagnetic field. This allows us to investigate the beyond-dipole deviations in electron trajectories and the emitted power, where the influence of the orbital angular momentum contains both magnetic and quadrupolar effects. In contrast to the system driven by plane-wave light, we show that the non-linear dipole dynamics induced by the vortex beams are not confined to the polarization or propagation directions, but also have a component in the orthogonal direction. We identify the effects of the resulting symmetry breaking via increased beyond dipole corrections which are particularly apparent in even harmonics.

physics.atom-ph↗

arXiv2025

Quantum interference and occupation control in high harmonic generation from monolayer $WS_2$

Two-dimensional hexagonal materials such as transition metal dichalcogenides exhibit valley degrees of freedom, offering fascinating potential for valley-based quantum computing and optoelectronics. In nonlinear optics, the K and K' valleys provide excitation resonances that can be used for ultrafast control of excitons, Bloch oscillations, and Floquet physics. Under intense laser fields, however, the role of coherent carrier dynamics away from the K/K' valleys is largely unexplored. In this study, we observe quantum interferences in high harmonic generation from monolayer $WS_2$ as laser fields drive electrons from the valleys across the full Brillouin zone. In the perturbative regime, interband resonances at the valleys enhance high harmonic generation through multi-photon excitations. In the strong-field regime, the high harmonic spectrum is sensitively controlled by light-driven quantum interferences between the interband valley resonances and intraband currents originating from electrons occupying various points in the Brillouin zone, also away from K/K' valleys such as $Γ$ and M. Our experimental observations are in strong agreement with quantum simulations, validating their interpretation. This work proposes new routes for harnessing laser-driven quantum interference in two-dimensional hexagonal systems and all-optical techniques to occupy and read-out electronic structures in the full Brillouin zone via strong-field nonlinear optics, advancing quantum technologies.

physics.optics↗

arXiv2025

Phaseless auxiliary-field quantum Monte Carlo method for cavity-QED matter systems

We present a generalization of the phaseless auxiliary-field quantum Monte Carlo (AFQMC) method to cavity quantum-electrodynamical (QED) matter systems. The method can be formulated in both the Coulomb and the dipole gauge. We verify its accuracy by benchmarking calculations on a set of small molecules against full configuration interaction and state-of-the-art QED coupled cluster (QED-CCSD) calculations. Our results show that (i) gauge invariance can be achieved within correlation-consistent Gaussian basis sets, (ii) the accuracy of QED-CCSD can be enhanced significantly by adding the standard perturbative triples correction without light-matter coupling, and (iii) there is a straightforward way to evaluate the differential expression for the photon occupation number that works in any gauge. The high accuracy and favorable computational scaling of our AFQMC approach will enable a broad range of applications. Besides polaritonic chemistry, the method opens a way to simulate extended QED matter systems.

physics.chem-ph↗

arXiv2025

Imaging van Hove Singularity Heterogeneity in Overdoped Graphene

Tuning the chemical potential of a solid to the vicinity of a van Hove singularity (vHS) is a well-established route to discovering emergent quantum phases. In monolayer graphene, the use of electron-donating metal layers has recently emerged as a method to dope the chemical potential to the nearest vHS, as evidenced by Angle-Resolved Photoemission Spectroscopy (ARPES) measurements. In this work, we study the spatial uniformity of the doping from this process using spectroscopic imaging scanning tunneling microscopy (SI-STM). Using molecular beam epitaxy (MBE), we achieve electron doping of graphene on SiC using Ytterbium (Yb-Graphene). We show using in-situ ARPES that the chemical potential is shifted to within 250 meV of the vHS. Using in-situ SI-STM, we establish that there exists significant inhomogeneity in the vHS position in overdoped graphene. We find two separate reasons for this. First, the spatial inhomogeneity of the intercalated Yb leads to local variations in the doping, with a length scale of inhomogeneity set by the screening length of ~ 3 nm. Second, we observe the presence of substitutional Yb dopants in the graphene basal plane. These Yb dopants cause a strong local shift of the doping, along with a renormalization of the quasiparticle amplitude. Theoretical calculations confirm that the Yb impurities effectively change the local potential, thus energetically shifting the position of the van Hove singularity. Our results point to the importance of considering the spatial structure of doping and its inextricable link to electronic structure.

cond-mat.mes-hall↗

arXiv2025

Cavity engineering of solid-state materials without external driving

Confining electromagnetic fields inside an optical cavity can enhance the light-matter coupling between quantum materials embedded inside the cavity and the confined photon fields. When the interaction between the matter and the photon fields is strong enough, even the quantum vacuum field fluctuations of the photons confined in the cavity can alter the properties of the cavity-embedded solid-state materials at equilibrium and room temperature. This approach to engineering materials with light avoids fundamental issues of laser-induced transient matter states. To clearly differentiate this field from phenomena in driven systems, we call this emerging field cavity materials engineering. In this review, we first present theoretical frameworks, especially, ab initio methods, for describing light-matter interactions in solid-state materials embedded inside a realistic optical cavity. Next, we overview a few experimental breakthroughs in this domain, detailing how the ground state properties of materials can be altered within such confined photonic environments. Moreover, we discuss state-of-the-art theoretical proposals for tailoring material properties within cavities. Finally, we outline the key challenges and promising avenues for future research in this exciting field.

cond-mat.mtrl-sci↗

arXiv2025

Non-perturbative Mass Renormalization Effects in Non-relativistic Quantum Electrodynamics

In this work we investigate the effects that multi-mode photonic environments, e.g., optical cavities, have on the properties of quantum matter. We highlight the importance of the non-perturbative mass renormalization procedure for ab initio quantum electrodynamics simulations and how it connects to common approximations used in polaritonic chemistry and cavity materials engineering. We focus on one-dimensional systems which can be solved exactly for large number of photon modes. First, we apply mass renormalization to free particles. The value of the renormalized mass depends on the details of the photonic environment and on the number of particles. We then show how the multi-mode photon field influences various ground- and excited-state properties of atomic and molecular systems. For instance, we observe the enhancement of particle confinement in the binding potential for the atomic system, and the modification of the potential energy surfaces of the molecular dimer due to photon-mediated long-range interactions. We also highlight how these changes compare to the common free-space mass-renormalization approximation employed in electronic structure theory and quantum chemistry. Since such phenomena are enhanced under strong light-matter coupling in a cavity environment they will become relevant for the emerging fields of polaritonic chemistry and cavity materials engineering.

quant-ph↗