arXiv subjects

Benedetta Flebus

Publications and source records attributed to Benedetta Flebus.

At least 19 recordsLinked to original sources

arXiv2026

Hidden Frustration in Collinear Altermagnets: Pairing Vortices and Equilibrium Spin Current Loops

We show that a magnet can remain perfectly collinear while its quantum vacuum circulates. In a centrosymmetric altermagnet, a symmetry-allowed locally staggered Dzyaloshinskii-Moriya coupling imprints a gauge-irremovable vortex-antivortex pair into the anomalous magnon pair correlations at high-symmetry points in momentum space. In real space, these hidden vortices produce an antiferrochiral array of equilibrium spin currents circulating oppositely around neighboring plaquettes. Gauge-irremovable frustration therefore survives despite classical collinearity: it exists entirely in the quantum correlations. Our results show that complex pairing, gauge-invariant fluxes, and equilibrium loop currents - structures encountered across electronic flux phases, spin liquids, and frustrated quantum magnets - can be encoded in the squeezed vacuum of a collinear magnet.

cond-mat.mes-hall↗

arXiv2026

Asymmetric Floquet-Engineered Mode Coupling in Hybrid Magnonics

In hybrid magnonic systems, linear magnon--photon hybridization inherently produces symmetric, reciprocal interactions, precluding asymmetric mode coupling. Floquet driving can tailor mode coupling strengths but, with single-tone modulation, inevitably generates a symmetric interaction that preserves this reciprocity. Here we introduce dual-tone Floquet modulation to unlock a new degree of freedom in hybrid magnonic systems, where the relative phase $\theta$ of two commensurate drives continuously controls the asymmetry of the Floquet-engineered interaction, enabling asymmetric mode coupling absent in existing hybrid magnonic systems. We demonstrate this in a strongly coupled cavity magnonic device, where tuning $\theta$ reversibly switches single-sided Autler--Townes splitting between the two hybrid modes---a direct spectroscopic signature of phase-programmable asymmetric coupling. This approach opens a new path toward controllable nonreciprocal and topological functionalities in hybrid magnonic systems, with broad implications for advanced quantum and classical signal processing.

cond-mat.mes-hall↗

arXiv2026

Field-induced first order transitions and phase coexistence in the Kitaev quantum spin liquid candidate BaCo2(AsO4)2

BaCo2(AsO4)2 (BCAO) is an insulating Kitaev quantum spin liquid candidate with a rich low-temperature phase diagram. Below 5 K, it exhibits double-zigzag magnetic order. Upon application of an in-plane magnetic field, the magnetic structure first transforms into an up-up-down (UUD) state near 0.12 T and then enters a fully spin-polarized ferromagnetic (FM) state near 0.5 T. In the narrow field regime close to the polarized phase, a finite residual thermal conductivity has been reported, suggesting a possible field-induced quantum spin liquid phase. Using neutron scattering, we show that the field-induced double-zigzag-to-UUD transition near 0.15 T is accompanied by the emergence of a cluster of localized UUD spin excitations that coexist with conventional spin waves. Upon further increasing field, BCAO undergoes a first-order UUD-to-FM transition with coexistence of UUD and FM phases, accompanied by a sharp response in the spin Seebeck coefficient. These results do not support a quantum spin liquid scenario near the UUD-to-FM critical field. Instead, modeling indicates that the broad excitations arise from bound spin-flip pairs, while a low-lying dispersive branch near the FM phase boundary carries the same sign of magnetization as the FM order. These excitations naturally account for the observed sign of the spin Seebeck response and are likely relevant to the thermal conductivity near the critical field.

cond-mat.str-el↗

arXiv2026

Experimental Realization of Synthetic Magnonic Lattice via Floquet Engineering

Magnonic systems, which exploit spin-wave excitations in magnetic materials, offer a promising platform for coherent information processing due to their low dissipation, strong nonlinearities, and intrinsic nonreciprocity. However, scaling magnonic circuits remains challenging, particularly with low-loss insulators such as yttrium iron garnet (YIG), which are difficult to pattern. Here, we experimentally realize a synthetic dimension in a magnonic system by coupling multimode magnon resonances in the frequency domain using time-periodic Floquet modulation. This approach enables electronically tunable interactions between discrete modes within a single YIG device, forming a reconfigurable mode-space lattice that supports functionalities such as Bloch oscillation. Our results demonstrate that high-dimensional magnonic dynamics can be achieved without increasing device footprint, establishing synthetic dimensions as a scalable and programmable route for integrated magnonic technologies. This advancement positions magnonic systems as promising platforms for engineering emergent phenomena that are inaccessible at equilibrium.

cond-mat.mes-hall↗

arXiv2026

Observation and Control of Spontaneous Magnon Emission from Spin Ensembles in 2D Hexagonal Boron Nitride

Hybrid systems consisting of color centers and magnetic materials provide an appealing solid-state platform for advancing the burgeoning quantum technological revolution. Exploring novel coupling mechanisms between optically active spin defects and quantum degrees of freedom is directly relevant in this context. Here, we report observation and control of spontaneous magnon emission from boron-vacancy centers in 2D hexagonal boron nitride (hBN), an unconventional qubit-magnon dipole coupling channel that dominates in the near-zero temperature limit. The spontaneous magnon emission process starts to be overshadowed by thermal magnon effect as temperature increases, reflecting the crossover from an emission-dominated, effectively cold magnon reservoir to a thermally occupied spin bath where absorption and stimulated processes restore balance. By increasing the spin defect density, we further present that spontaneous magnon emission into a common spin bath could help establish quantum correlations in dense hBN spin ensembles. Our results are quantitatively captured by detailed theoretical modeling, bringing insights into understanding qubit-magnon coupling, correlated spin dynamics, and many-body physics of color centers in the quantum regime.

cond-mat.mes-hall↗

arXiv2026

Dynamical metastability and transient topological magnons in interacting driven-dissipative magnetic systems

Metastability, i.e., partial relaxation to long-lived, quasi-stationary states before true asymptotic equilibrium sets in, emerges ubiquitously in classical and quantum dynamical systems as a result of timescales separation. In open quantum systems, an intrinsically nonequilibrium analogue, dynamical metastability, can originate from the spectral geometry of a non-Hermitian operator. In noninteracting models, this mechanism produces boundary-sensitive anomalous relaxation, transient amplification, and topologically mandated long-lived edge modes, all of which are enhanced as system size grows. Here we extend dynamical metastability into the nonlinear, interacting regime and identify magnetic heterostructures as a natural platform for its exploration. We introduce an interacting spin Lindbladian whose linearized magnon dynamics map onto a dynamically metastable Hatano-Nelson chain, and show that dynamical metastability in the noninteracting limit seeds genuinely nonlinear phenomena, including size-dependent spin dipping and anomalous attraction to unstable equilibria. Long-lived edge states associated to topologically mandated Dirac bosons persist under nonlinearities and disorder. We further analyze the magnetization dynamics in magnetic multilayers within the classical Landau-Lifshitz-Gilbert-Slonczewski framework, identifying Dzyaloshinskii-Moriya interaction, nonlocal damping, and spin-transfer torque as control parameters governing bulk-boundary stability mismatch and band topology. While all the distinctive dynamical phenomena previously identified reappear in this experimentally relevant setting, the LLGS framework also supports multistability and limit cycles that are absent in the quantum model. Our results constitute the first systematic study of dynamical metastability in nonlinear dynamics, directly relevant to spin-torque oscillator arrays, magnonic devices, and beyond.

cond-mat.mes-hall↗

arXiv2025

Chemical potential of magnon polarons

Using a rotationally invariant formulation of spin-lattice coupling, we derive a rigorous definition of the chemical potential for magnon-polaron quasiparticles in collinear ferromagnets (FMs) and antiferromagnets (AFMs), valid when magnetoelastic scattering equilibrates magnons and acoustic phonons on timescales much shorter than those associated with quasiparticle-nonconserving relaxation processes. While our microscopic framework applies to generic magnon-phonon interactions, here we focus on high-symmetry crystals where the two transverse acoustic modes form a degenerate doublet. This doublet can combine into circularly polarized phonons, making the chiral selectivity of the coupling manifest: the FM magnon mode hybridizes only with the co-rotating phonon, whereas in collinear AFMs each magnon branch of opposite handedness couples to the phonon of the same chirality. We show that, in both FM and AFM systems, the nonequilibrium magnon-polaron gas is governed by a single chemical potential conjugate to the conserved axial angular momentum. In FMs, the two hybrid branches in the co-rotating sector share this chemical potential, weighted by their magnonic fractions; in AFMs, the four magnon-polaron branches split into two chiral sectors that carry opposite angular momenta and couple with opposite sign to the same chemical potential. Building on this microscopic thermodynamic framework, we formulate a Boltzmann transport theory for magnon-polarons and derive compact expressions for angular-momentum and heat currents that interpolate continuously to the decoupled regime and reproduce the phenomenological magnon-polaron transport framework underlying previous spin Seebeck analyses.

cond-mat.mes-hall↗

arXiv2025

Perspective: Magnon-magnon coupling in hybrid magnonics

The internal coupling of magnetic excitations (magnons) with themselves has created a new research sub-field in hybrid magnonics, i.e., magnon-magnon coupling, which focuses on materials discovery and engineering for probing and controlling magnons in a coherent manner. This is enabled by, one, the abundant mechanisms of introducing magnetic interactions, with examples of exchange coupling, dipolar coupling, RKKY coupling, and DMI coupling, and two, the vast knowledge of how to control magnon band structure, including field and wavelength dependences of frequencies, for determining the degeneracy of magnon modes with different symmetries. In particular, we discuss how magnon-magnon coupling is implemented in various materials systems, with examples of magnetic bilayers, synthetic antiferromagnets, nanomagnetic arrays, layered van der Waals magnets, and (DMI SOT materials) in magnetic multilayers. We then introduce new concept of applications for these hybrid magnonic materials systems, with examples of frequency up/down conversion and magnon-exciton coupling, and discuss what properties are desired for achieving those applications.

cond-mat.mtrl-sci↗

arXiv2025

Emergent spatiotemporal order and nonreciprocity in driven-dissipative nonlinear magnetic systems

The identification of platforms with independently tunable nonlinearity and non-Hermiticity promises a quantitative route to far-from-equilibrium universality across many-body systems. Here we show that a conventional ferromagnetic multilayer realizes this paradigm: balancing a dc drive against Gilbert damping stabilizes a self-organized, current-carrying nonequilibrium condensate that spontaneously breaks spacetime-translation symmetry. The chirality of this spin superfluid limit cycle state generates an inherently nonreciprocal flow: long-wavelength magnons of opposite chirality acquire asymmetric dispersions and propagate direction-selectively, realizing a spin superfluid diode. This asymmetry is flow-borne - it reflects broken Galilean invariance and requires neither structural asymmetry nor finely tuned gain-loss balance. Linearized dynamics in the comoving superfluid frame are intrinsically pseudo-Hermitian and, in the long-wavelength sector, can be mapped to a (1+1)D wave equation on curved spacetime. Spatial modulation of the drive enables the generation of sonic horizons that parametrically amplify magnon pairs and produce Hawking-like particle-hole emission. Our results establish a tabletop route from nonlinear dissipative-driven magnetization dynamics to nonreciprocal transport, nonequilibrium phase transitions, and analogue-gravity kinematics.

cond-mat.mes-hall↗

arXiv2025

Coupling Magnons to an Opto-Electronic Parametric Oscillator

Hybrid magnonic systems have emerged as versatile modular components for quantum signal transduction and sensing applications owing to their capability of connecting distinct quantum platforms. To date, the majority of the magnonic systems have been explored in a local, near-field scheme, due to the close proximity required for realizing a strong coupling between magnons and other excitations. This constraint greatly limits the applicability of magnons in developing remotely-coupled, distributed quantum network systems. On the contrary, opto-electronic architectures hosting self-sustained oscillations has been a unique platform for longhaul signal transmission and processing. Here, we integrated an opto-electronic oscillator with a magnonic oscillator consisting of a microwave waveguide and a Y3Fe5O12(YIG) sphere, and demonstrated strong and coherent coupling between YIG's magnon modes and the opto-electronic oscillator's characteristic photon modes - revealing the hallmark anti-crossing gap in the measured spectrum. In particular, the photon mode is produced on-demand via a nonlinear, parametric process as stipulated by an external seed pump. Both the internal cavity phase and the external pump phase can be precisely tuned to stabilize either degenerate or nondegenerate auto-oscillations. Our result lays out a new, hybrid platform for investigating long-distance coupling and nonlinearity in coherent magnonic phenomena, which may be find useful in constructing future distributed hybrid magnonic systems.

cond-mat.mes-hall↗

arXiv2025

Generating single- and many-body quantum magnonic states

The growing interest in quantum magnonics is driving the development of advanced techniques for generating, controlling, and detecting non-classical magnonic states. Here, we explore the potential of an ensemble of solid-state spin defects coupled to a shared magnetic bath as a source of such states. We establish a theoretical framework to characterize the quantum correlations among magnons emitted by the ensemble into the bath and investigate how these correlations depend on experimentally tunable parameters. Our findings show that the emitted magnons retain the quantum correlations inherent to the solid-state emitters, paving the way for the deterministic generation of quantum many-body magnonic states.

cond-mat.mes-hall↗

arXiv2025

Angular Momentum Fluctuations in the Phonon Vacuum of Symmetric Crystals

Although time-reversal and inversion symmetry constrain the angular momentum of each phonon mode to vanish, we show that the vacuum state of crystals with such symmetries can nevertheless exhibit finite angular momentum fluctuations, which persist at finite temperature. These fluctuations arise from quantum coherence between nondegenerate modes with noncollinear polarizations and are encoded in the off-diagonal components of the angular momentum operator. Their origin lies in the noncommutativity between the phonon Hamiltonian and angular momentum, which enables time-dependent rotational dynamics even in symmetric vacua. Using a minimal model, we provide an intuitive picture of this phenomenon in terms of beating between linearly polarized modes, which generates a finite instantaneous angular momentum while remaining symmetry-forbidden in the mean. We further show that these vacuum fluctuations give rise to distinct finite-frequency spectral signatures and outline a concrete route for their detection using time-resolved spectroscopic probes sensitive to lattice polarization and symmetry. Our results identify a previously unexplored regime of lattice dynamics, revealing that even the symmetric phonon vacuum can harbor structured, dynamical angular-momentum correlations.

cond-mat.mtrl-sci↗

arXiv2024

Engineering long-lived entanglement through dissipation in quantum hybrid solid-state platforms

Spin squeezing, a form of many-body entanglement, is a crucial resource in quantum metrology and information processing. While experimentally viable protocols for generating stable spin squeezing have been proposed in quantum optics setups, there is growing interest in quantum hybrid solid-state systems as alternative platforms for both engineering and exploring many-body quantum phenomena. In this work, we propose a scheme to generate long-lived spin squeezing in an ensemble of solid-state qubits interacting with electromagnetic noise emitted by a squeezed solid-state bath. We identify the conditions under which quantum correlations within the bath can be transferred to the qubit array, driving it into an entangled state independently of its initial configuration. To assess the experimental feasibility of our approach, we analyze the dynamics of an array of solid-state spin defects coupled to a common ferromagnetic bath, which is driven into a non-equilibrium squeezed state through its interaction with a surface acoustic wave mode. Our results demonstrate that the ensemble can exhibit steady-state spin squeezing under suitable conditions, opening new pathways for the generation of robust many-body entanglement in solid-state spin ensembles.

quant-ph↗

arXiv2024

Cooperative non-reciprocal emission and quantum sensing of symmetry breaking

Non-reciprocal propagation of energy and information is fundamental to a wide range of quantum technology applications. In this work, we explore the quantum many-body dynamics of a qubit ensemble coupled to a shared bath that mediates coherent and dissipative inter-qubit interactions with both symmetric and anti-symmetric components. We find that the interplay between anti-symmetric (symmetric) coherent and symmetric (anti-symmetric) dissipative interactions results in non-reciprocal couplings, which, in turn, generate a spatially asymmetric emission pattern. We demonstrate that this pattern arises from non-reciprocal interactions coupling different quantum many-body states within a specific excitation manifold. Focusing on solid-state baths, we show that their lack of time-reversal and inversion symmetry is a key ingredient for generating non-reciprocal dynamics in the qubit ensemble. With the plethora of quantum materials that exhibit this symmetry breaking at equilibrium, our approach paves the way for realizing cooperative non-reciprocal transport in qubit ensembles without requiring time-modulated external drives or complex engineering. Using an ensemble of nitrogen-vacancy (NV) centers coupled to a generic non-centrosymmetric ferromagnetic bath as a concrete example, we demonstrate that our predictions can be tested in near-future experiments. As the spatial asymmetry in the relaxation dynamics of the qubit ensemble is a direct probe of symmetry breaking in the solid-state bath, our work also opens the door to developing model-agnostic quantum sensing schemes capable of detecting bath properties invisible to current state-of-the-art protocols, which operate solid-state defects as single-qubit sensors.

quant-ph↗

arXiv2024

The role of excitation vector fields and all-polarisation state control of cavity magnonics

Recently the field of cavity magnonics, a field focused on controlling the interaction between magnons and confined microwave photons within microwave resonators, has drawn significant attention as it offers a platform for enabling advancements in quantum- and spin-based technologies. Here, we introduce excitation vector fields, whose polarisation and profile can be easily tuned in a two-port cavity setup, thus acting as an effective experimental knob to explore the coupled dynamics of cavity magnon-polaritons. Moreover, we develop theoretical models that accurately predict and reproduce the experimental results for any polarisation state and field profile within the cavity resonator. This versatile experimental platform offers a new avenue for controlling spin-photon interactions and as such also delivering a mechanism to readily control the exchange of information between hybrid systems.

quant-ph↗

arXiv2024

Magnetic structure of EuCd$_2$Sb$_2$ single-crystal thin-film

We investigate the magnetic order in single crystalline EuCd$_2$Sb$_2$ thin films using a combined theoretical and experimental approach. Resonant elastic x-ray scattering experiments reveal a sharp magnetic peak at $q = (0, 0, \frac{1}{2})$ below $T_N = 7.2$ K, indicative of interlayer antiferromagnetic ordering. Additionally, we observe a weak diffuse magnetic signal centered at $q = (0, 0, 1)$ that persists above $T_N$, up to $T_C \sim 11$ K. Our Monte-Carlo simulations of a classical spin model approximation of the Eu magnetic sublattice demonstrate that the diffuse signal can arise from ferromagnetic coupling in the top few layers due to surface oxidation. On the other hand, the bulk of the sample exhibits antiferromagnetic coupling between layers. Finally, our fit of the model parameters to the magnetic ordering temperatures, shed light on the exchange couplings that are key in stabilizing the observed composite magnetic order.

cond-mat.mtrl-sci↗

arXiv2023

Magnon-Photon Coupling in an Opto-Electro-Magnonic Oscillator

The opto-electronic oscillators (OEOs) hosting self-sustained oscillations by a time delayed mechanism are of particular interest in long-haul signal transmission and processing. On the other hand, owing to their unique tunability and compatibility, magnons - as elementary excitations of spin waves - are advantageous carriers for coherent signal transduction across different platforms. In this work, we integrated an opto-electronic oscillator with a magnonic oscillator consisting of a microwave waveguide and a yttrium iron garnet sphere. We find that, in the presence of the magnetic sphere, the oscillator power spectrum exhibits sidebands flanking the fundamental OEO modes. The measured waveguide transmission reveals anti-crossing gaps, a hallmark of the coupling between the opto-electronic oscillator modes and the Walker modes of the sphere. Experimental results are well reproduced by a coupled-mode theory that accounts for nonlinear magnetostrictive interactions mediated by the magnetic sphere. Leveraging the advanced fiber-optic technologies in opto-electronics, this work lays out a new, hybrid platform for investigating long distance coupling and nonlinearity in coherent magnonic phenomena.

physics.optics↗

arXiv2023

Solid-state platform for cooperative quantum dynamics driven by correlated emission

While traditionally regarded as an obstacle to quantum coherence, recent breakthroughs in quantum optics have shown that the dissipative interaction of a qubit with its environment can be leveraged to protect quantum states and synthesize many-body entanglement. Inspired by this progress, here we set the stage for the -- yet uncharted -- exploration of analogous cooperative phenomena in hybrid solid-state platforms. We develop a comprehensive formalism for the quantum many-body dynamics of an ensemble of solid-state spin defects interacting with the magnetic field fluctuations of a common solid-state reservoir. Our framework applies to any solid-state reservoir whose fluctuating spin, pseudospin, or charge degrees of freedom generate magnetic fields. To understand whether correlations induced by dissipative processes can play a relevant role in a realistic experimental setup, we apply our model to a qubit array interacting via the spin fluctuations of a ferromagnetic bath. Our results show that the low-temperature collective relaxation rates of the qubit ensemble can display clear signatures of super- and subradiance, i.e., forms of cooperative dynamics traditionally achieved in atomic ensembles. We find that the solid-state analog of these cooperative phenomena is robust against spatial disorder in the qubit ensemble and thermal fluctuations of the magnetic reservoir, providing a route for their feasibility in near-term experiments. Our work lays the foundation for a multi-qubit approach to quantum sensing of solid-state systems and the direct generation of many-body entanglement in spin-defect ensembles. Furthermore, we discuss how the tunability of solid-state reservoirs opens up novel pathways for exploring cooperative phenomena in regimes beyond the reach of conventional quantum optics setups.

quant-ph↗