arXiv subjects

Emil Viñas Boström

Publications and source records attributed to Emil Viñas Boström.

At least 19 recordsLinked to original sources

arXiv2026

Optical observation of interlayer spin correlation

Spin correlations govern numerous collective behaviors of quantum materials, underpinning exotic phenomena such as unconventional superconductivity and topological magnetism. In layered materials, the interlayer spin correlation is important because it characterizes the magnetic ground state and determines spin transport across the interface. Yet interlayer spin correlations have remained hard to measure directly, leaving one of the most basic quantities of two-dimensional magnetism out of experimental reach. Here we provide the first direct optical probe of interlayer spin correlations, using two-magnon Raman scattering in the van der Waals antiferromagnet (AFM) CrSBr, in remarkable agreement with a microscopic spin-wave model without any fitting parameter. Moreover, the two-magnon channel switches on only in the AFM state and vanishes when a magnetic field takes the crystal to a ferromagnetic state. We furthermore establish an exciton-mediated variant of the technique, where tuning the laser near the exciton resonance enhances the signal roughly tenfold due to the exciton's large oscillator strength. Magnon-pair Raman spectroscopy thus opens a direct optical window into interlayer spin correlations in van der Waals magnets, extendable to twisted bilayers and proximity-coupled heterostructures.

cond-mat.mes-hall↗

arXiv2026

Antiferromagnetism-altered plasmon dynamics

The interaction between plasmons and magnons is a long-sought phenomenon with implications for fundamental physics and spintronics applications. In three-dimensional systems, this coupling is suppressed by the large mismatch in energy scales, but two-dimensional (2D) plasmons with gapless dispersion can overlap with magnons over a broad spectral range. Despite numerous theoretical predictions, experimental observation of magnon-plasmon interaction has remained elusive. In this work, we study a first-of-its-kind hybrid plasmon-magnon platform based on 2D materials. By deploying scattering-type scanning near-field optical microscopy (s-SNOM) with terahertz radiation, we image propagating plasmon wavepackets at a graphene/NiPS$_3$ interface and track their dynamics across the antiferromagnetic transition of NiPS$_3$. We observe a clear renormalization of the plasmon-polariton dispersion concurrent with the onset of antiferromagnetic order. With complementary Raman scattering and nano-terahertz spectroscopy, we unveil spectral weight redistribution and dielectric screening changes, potentially associated with the multi-magnon continuum, as the underlying mechanism. These results provide solid evidence of coupling between plasmon and antiferromagnetic order, marking a cornerstone for a potential platform for hybrid magnon-plasmon interactions in 2D materials, opening avenues for coherent spin-plasmon devices and tunable terahertz spintronic components.

cond-mat.str-el↗

arXiv2026

Few-Photon Fluorescence from Ultracold Bosons in an Optical Cavity

Within a cavity quantum electrodynamics framework, we analyse the fluorescent spectrum of ultracold bosons in resonant and second-harmonic generation (SHG) regimes. Two settings are considered: (i) (few) bosons in an optical lattice and (ii) a trapped two-component dilute Bose-Einstein condensate (2BEC) within the Bogoliubov scheme. Atom-photon dynamics are treated exactly via time-dependent configuration interaction, while cavity leakage is modelled via classical oscillator baths. For optical lattices, bosons in short Bose-Hubbard chains with two levels per site show spectral intensity that increases with atom number at weak interactions but decreases and redshifts at strong interactions once atoms outnumber sites. In the 2BEC case, spectra largely scale with particle number when 2BEC-cavity and inter-particle couplings are rescaled, while strong correlations induce SHG redshifts. These results reveal some generic features of fluorescence in ultracold bosons in optical cavities, and hint at possible experiments and further theoretical developments.

cond-mat.quant-gas↗

arXiv2026

Cavity Control of Strongly Correlated Electrons Beyond Resonant Coupling

Interfacing materials with electromagnetic cavities offers a route to modify equilibrium properties through structured vacuum fluctuations. The coupling between light and correlated electrons lacks a characteristic energy scale, making vacuum induced ground state modifications of such systems inherently off-resonant and sensitive to the full photon mode structure. We develop a consistent cavity-QED formalism for dispersive electromagnetic environments in the Coulomb gauge, capturing both dynamical dressing via the vector potential and static screening by the dielectric. With this formalism, we perform a non-perturbative study of the cavity-induced modification of magnetism in the Hubbard model close to half-filling, including all cavity modes and with parameters determined from first principles. At half-filling, we show that the modification of the magnetic exchange interaction $J$ is controlled by a generalized Purcell factor, proportional to the frequency integrated photonic density of states relative to free space. This result identifies polaritonic surface cavities as promising platforms to modify correlated systems. For the surface cavity a competition between static screening and dynamical dressing via the vector potential leads to a net enhancement of $J$, directly observable in two-magnon Raman spectroscopy. The inclusion of screening is essential to obtain even qualitatively correct results. At weak doping, a variational exact diagonalization of the cavity-coupled $t$-$J$ polaron reveals that a low-frequency surface cavity can reverse the nodal-antinodal dichotomy of the bare polaron dispersion. This effect lies beyond mean field theory and is observable via ARPES measurements. Our framework establishes a concrete design principle linking cavity geometry to material response in the off-resonant regime, which will guide future experimental and theoretical explorations.

quant-ph↗

arXiv2026

Light-Matter-Coupling formalism for magnons: probing quantum geometry with light

Nontrivial quantum geometry is a key feature of the wavefunctions of collective magnetic excitations in topological systems, but accessing it experimentally remains an open challenge. While Raman circular dichroism (RCD) has emerged as a promising probe, the fundamental link between the RCD and magnon quantum geometry has remained unsettled, and complicated by the fact that magnons are charge neutral. Here, we identify when and why this link exists. We show that, under broad conditions, the Fleury-Loudon Raman vertex can be obtained directly from a light-matter coupling expansion of the effective magnon Hamiltonian, bypassing the conventional microscopic derivation based on virtual electronic processes. This yields an analytical connection between the RCD and the Berry curvature of magnon bands. Applied to monolayer CrI\textsubscript{3}, our theory predicts finite temperature signatures of topological magnons in the RCD. These results establish a general route to quantum-geometry sensitive optical probes in magnonic systems.

cond-mat.mes-hall↗

arXiv2026

Cavity control of multiferroic order in single-layer NiI$_2$

Controlling materials through their interactions with electromagnetic vacuum fluctuations is an emergent frontier in material engineering. Although recent experiments have demonstrated dark cavity effects for electronic material phases, like superconductivity, ferroelectricity and charge density waves, a smoking gun experiment for magnetic systems is lacking. Largely, this comes from the focus on phase transitions, where a large critical light-matter coupling is needed to observe cavity modifications. Here, we propose spiral magnets, where even a small cavity-mediated change in magnetic interactions is reflected in a change of the spiral wavelength, as a promising platform to observe cavity effects. We focus on the single-layer multiferroic NiI$_2$, interacting with electric field fluctuations from surface phonon polaritons of the paraelectric substrate SrTiO$_3$. With decreasing substrate-material distance, the ratio of nearest and third nearest neighbor exchange interactions reduces, leading to an increase of the spiral wavelength and an eventual transition into a ferromagnetic state. Our work identifies a realistic platform to observe cavity vacuum renormalization effects in magnetic systems.

cond-mat.mtrl-sci↗

arXiv2025

Anyonic Chern insulator in graphene induced by surface electromagnon vacuum fluctuations

Sub-wavelength cavities have emerged as a promising platform to realize strong light-matter coupling in condensed matter systems. Previous studies are limited to dielectric sub-wavelength cavities, which preserve time-reversal symmetry. Here, we lift this constraint by proposing a cavity system based on magneto-electric materials, which host surface electromagnons with non-orthogonal electric field and magnetic field components. The quantum fluctuations of the surface electromagnons drive a nearby graphene monolayer into an anyonic Chern insulator, characterized by anyonic quasi-particles and a topological gap that decays polynomially with the graphene-substrate distance. Our work opens a path to controllably break time-reversal symmetry and induce exotic quantum states through cavity vacuum fluctuations.

cond-mat.mes-hall↗

arXiv2025

Engineering 2D square lattice Hubbard models in 90$^\circ$ twisted Ge/SnX (X=S, Se) moiré supperlattices

Due to the large-period superlattices emerging in moire two-dimensional (2D) materials, electronic states in such systems exhibit low energy flat bands that can be used to simulate strongly correlated physics in a highly tunable setup. While many investigations have thus far focused on moire flat bands and emergent correlated electron physics in triangular, honeycomb and quasi-one-dimensional lattices, tunable moire realizations of square lattices subject to strong correlations remain elusive. Here we propose a feasible scheme to construct moire square lattice systems by twisting two or more layers of 2D materials in a rectangular lattice by 90 degrees. We demonstrate the concept with twisted GeX/SnX (X=S, Se) moire superlattices and calculate their electronic structures from first principles. We show that the lowest conduction flat band in these systems can be described by a square lattice Hubbard model with parameters which can be controlled by varying the choice of host materials, number of layers, and external electric fields. In particular, twisted double bilayer GeSe realizes a square lattice Hubbard model with strong frustration due to the next nearest neighbour hopping that could host unconventional superconductivity{, in close analogy to the Hubbard model for copper-oxygen planes of cuprate high-temperature superconductors}. The presented scheme uses 90-degree twisted 2D materials with rectangular unit cells as a promising platform for realizing the physical phenomena of square lattice Hubbard models, establishing a new route for studying its rich phase diagram of magnetism, charge order, and unconventional superconductivity in a highly tunable setting.

cond-mat.str-el↗

arXiv2025

Cavity-altered superconductivity

Is it feasible to alter the ground state properties of a material by engineering its electromagnetic environment? Inspired by theoretical predictions, experimental realizations of such cavity-controlled properties without optical excitation are beginning to emerge. Here, we devised and implemented a novel platform to realize cavity-altered materials. Single crystals of hyperbolic van der Waals (vdW) compounds provide a resonant electromagnetic environment with enhanced density of photonic states and prominent mode confinement. We interfaced hexagonal boron nitride (hBN) with the molecular superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br ($κ$-ET). The frequencies of infrared (IR) hyperbolic modes of hBN match the IR-active carbon-carbon stretching molecular resonance of ($κ$-ET) implicated in superconductivity. Nano-optical data supported by first-principles molecular Langevin dynamics simulations confirm the presence of resonant coupling between the hBN hyperbolic cavity modes and the carbon-carbon stretching mode in ($κ$-ET). Meissner effect measurements via magnetic force microscopy demonstrate a strong suppression of superfluid density near the hBN/($κ$-ET) interface. Non-resonant control heterostructures, including RuCl$_3$/($κ$-ET) and hBN/$\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_{8+x}$, do not display the superfluid suppression. These observations suggest that hBN/($κ$-ET) realizes a cavity-altered superconducting ground state. Our work highlights the potential of dark cavities devoid of external photons for engineering electronic ground state properties of complex quantum materials.

cond-mat.supr-con↗

arXiv2025

Visualizing a Terahertz Superfluid Plasmon in a Two-Dimensional Superconductor

The superconducting gap defines the fundamental energy scale for the emergence of dissipationless transport and collective phenomena in a superconductor. In layered high-temperature cuprate superconductors, where the Cooper pairs are confined to weakly coupled two-dimensional copper-oxygen planes, terahertz (THz) spectroscopy at sub-gap millielectronvolt energies has provided crucial insights into the collective superfluid response perpendicular to the superconducting layers. However, within the copper-oxygen planes the collective superfluid response manifests as plasmonic charge oscillations at energies far exceeding the superconducting gap, obscured by strong dissipation. Here, we present spectroscopic evidence of a below-gap, two-dimensional superfluid plasmon in few-layer Bi2Sr2CaCu2O8+x and spatially resolve its deeply sub-diffractive THz electrodynamics. By placing the superconductor in the near-field of a spintronic THz emitter, we reveal this distinct resonance-absent in bulk samples and observed only in the superconducting phase-and determine its plasmonic nature by mapping the geometric anisotropy and dispersion. Crucially, these measurements offer a direct view of the momentum- and frequency dependent superconducting transition in two dimensions. These results establish a new platform for investigating superfluid phenomena at finite momenta and THz frequencies, highlighting the potential to engineer and visualize superconducting devices operating at ultrafast THz rates.

cond-mat.supr-con↗

arXiv2025

Dynamic Interfacial Quantum Dipoles in Charge Transfer Heterostructures

Hysteretic gate responses of two-dimensional material heterostructures serve as sensitive probes of the underlying electronic states and hold significant promise for the development of novel nanoelectronic devices. Here we identify a new mechanism of hysteretic behavior in graphene/$h$BN/$α$-$\mathrm{RuCl_3}$ charge transfer field effect devices. The hysteresis loop exhibits a sharp onset under low temperatures and evolves symmetrically relative to the charge transfer equilibrium. Unlike conventional flash memory devices, the charge transfer heterostructure features a transparent tunneling barrier and its hysteretic gate response is induced by the dynamic tuning of interfacial dipoles originating from quantum exchange interactions. The system acts effectively as a ferroelectric and gives rise to remarkable tunability of the hysteretic gate response under external electrical bias. Our work unveils a novel mechanism for engineering hysteretic behaviors via dynamic interfacial quantum dipoles.

cond-mat.mes-hall↗

arXiv2025

Chiral superconductivity in a semiconducting wire induced by helical magnetic order

Chiral superconductors are sought after for their promising but elusive Majorana zero modes. We show that a one-dimensional semiconductor in proximity to a conventional superconductor and a helical magnet can exhibit chiral superconductivity, without the need for external magnetic fields or intrinsic spin-orbit coupling. The effective proximity-induced gap and the triplet gap arising from magnon fluctuations can be made to interfere constructively. The heterostructure can be tuned into a topological regime with Majorana zero modes at its ends, over a range of chemical potentials proportional to the spin-electron coupling.

cond-mat.supr-con↗

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

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↗

arXiv2024

Origin of the insulating and superconducting phases in molecular solid $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$

Recent studies of organic molecular solids are highlighted by their complex phase diagram and light-induced phenomena, such as Mott insulator, spin liquid phase, and superconductivity. However, a discrepancy between experimental observation and first-principle calculation on the $κ$-(BEDT-TTF)$_2$X family inhibits understanding their properties. Here, we revisit the electronic structure of $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ with the recently developed DFT+GOU method to correct the energy level of molecular orbital states in the molecular solid. Our work reveals that the insulating electronic structure of $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ originates from the energy gap between the highest occupied and the lowest unoccupied molecular orbital states of the BEDT-TTF dimers, that are the periodic unit of the molecular solid. We verify that our calculation result provides consistent band gap, optical conductivity, and evolution of the metal-insulator transition as a function of pressure with experimental observations. Especially, the superconducting dome of $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, which originates from the flat band state at the Fermi level, is reproduced. Additionally, we constructed a new low-energy lattice model based on the ability of electronic structure data that can be used to address many-body physics, such as quantum spin liquid and double-holon dynamics. Our provides a deeper understanding of the complex phase diagram and various light-induced phenomena in the $κ$-(BEDT-TTF)$_2$X family and the other complex organic molecular solids.

cond-mat.str-el↗

arXiv2024

Cavity Spectroscopy for Strongly Correlated Systems

Embedding materials in optical cavities has emerged as an intriguing perspective for controlling quantum materials, but a key challenge lies in measuring properties of the embedded matter. Here, we propose a framework for probing strongly correlated cavity-embedded materials through direct measurements of cavity photons. We derive general relations between photon and matter observables inside the cavity, and show how these can be measured via the emitted photons. As an example, we demonstrate how the entanglement phase transition of an embedded H$_2$ molecule can be accessed by measuring the cavity photon occupation, and showcase how dynamical spin correlation functions can be accessed by measuring dynamical photon correlation functions. Our framework provides an all-optical method to measure static and dynamic properties of cavity-embedded materials.

cond-mat.str-el↗

arXiv2024

Spontaneous emergence of phonon angular momentum through hybridization with magnons

Chirality, the breaking of improper rotational symmetry, is a fundamental concept spanning diverse scientific domains. In condensed matter physics, chiral phonons, originating from circular atomic motions that carry angular momentum, have sparked intense interest due to their coupling to magnetic degrees of freedom, enabling potential phonon-controlled spintronics. However, modes and their counter-rotating counterparts are typically degenerate at the Brillouin zone center. Selective excitation of a single-handed circulating phonon requires external stimuli that break the degeneracy. Whether energetically nondegenerate circularly polarized phonons can appear spontaneously without structural or external symmetry breaking remains an open question. Here, we demonstrate that nondegenerate elliptically polarized phonon pairs can be induced by coupling to magnons with same helicity in the van der Waals antiferromagnet $\mathrm{FePSe_3}$. We confirm the presence of magnon-phonon hybrids, also known as magnon polarons, which exhibit inherent elliptical polarization with opposite helicities and distinct energies. This nondegeneracy enables their coherent excitation with linearly polarized terahertz pulses, which also endows these rotating modes with chirality. By tuning the polarization of the terahertz drive and measuring phase-resolved polarimetry of the resulting coherent oscillations, we determine the ellipticity and map the trajectory of these hybrid quasiparticles. Our findings establish a general approach to search for intrinsically nondegenerate phonons with angular momentum at the center of the Brillouin zone and introduce a new methodology for characterizing their ellipticity, outlining a roadmap towards chiral-phonon-controlled spintronic functionalities.

cond-mat.mtrl-sci↗

arXiv2024

Terahertz Control of Linear and Nonlinear Magno-Phononics

Coherent manipulation of magnetism through the lattice provides unprecedented opportunities for controlling spintronic functionalities on the ultrafast timescale. Such nonthermal control conventionally involves nonlinear excitation of Raman-active phonons which are coupled to the magnetic order. Linear excitation, in contrast, holds potential for more efficient and selective modulation of magnetic properties. However, the linear channel remains uncharted, since it is conventionally considered forbidden in inversion symmetric quantum materials. Here, we harness strong coupling between magnons and Raman-active phonons to achieve both linear and quadratic excitation regimes of magnon-polarons, magnon-phonon hybrid quasiparticles. We demonstrate this by driving magnon-polarons with an intense terahertz pulse in the van der Waals antiferromagnet $\mathrm{FePS_3}$. Such excitation behavior enables a unique way to coherently control the amplitude of magnon-polaron oscillations by tuning the terahertz field strength and its polarization. The polarimetry of the resulting coherent oscillation amplitude breaks the crystallographic $C_2$ symmetry due to strong interference between different excitation channels. Our findings unlock a wide range of possibilities to manipulate material properties, including modulation of exchange interactions by phonon-Floquet engineering.

cond-mat.mtrl-sci↗