arXiv subjects

Lorenzo Graziotto

Publications and source records attributed to Lorenzo Graziotto.

10 recordsLinked to original sources

arXiv2026

Absence of a Superradiant Phase Transition in Dirac Landau Polaritons

One of the most striking predictions in cavity quantum electrodynamics is the condensation of photons into a macroscopically populated ground state, the so-called superradiant phase transition (SRPT). SRPTs are theorized to occur in light-matter coupled systems above a critical coupling strength, yet have not been experimentally realized in equilibrium. On the contrary, the very existence of SRPTs has been largely disputed by No-Go theorems. In cavity-coupled electronic systems with Dirac dispersion, the diamagnetic $\vec{A}^2$-term crucial to No-go theorems is not present at leading order, making graphene Landau level transitions ultrastrongly coupled to terahertz cavities good candidates for SRPTs. In this work, we present the first terahertz spectroscopic measurements of an hBN-encapsulated monolayer graphene flake coupled to a highly sub-wavelength resonator mode. By tuning the graphene carrier density, we drive the resulting Landau polaritons into the ultrastrong coupling regime, with the normalized coupling reaching $\approx 40 \%$, approaching criticality. In this regime, the continuous SRPT would lead to a unique spectroscopic polariton softening, which we consistently rule out. The full polariton dispersion is instead quantitatively reproduced by a Hopfield Hamiltonian using a quasistatic near-field model that accounts for the sub-wavelength character of the cavity.

cond-mat.mes-hall↗

arXiv2026

Fluctuation engineering in cavity quantum materials

Coupling tailored electromagnetic fluctuations to materials provides a resource for controlling correlated quantum matter. By structuring the frequency, spatial, and modal distribution of fluctuations through a new generation of cavity quantum materials, vacuum and thermal spectra can shift phase boundaries and stabilize or suppress orders. This review organizes the field around a fluctuation-focused perspective, surveying a practical design toolbox and recent milestones, and outlining theory-experiment challenges in realistic, multimode, beyond-long-wavelength regimes. We highlight photonic observables and map opportunities for equilibrium and driven control across superconducting, magnetic, moire, and topological platforms.

cond-mat.mes-hall↗

arXiv2025

Casimir Stabilization of Fluctuating Electronic Nematic Order

Vacuum cavity control of quantum materials is the engineering of quantum materials systems through electromagnetic zero-point fluctuations. In this work we articulate a generic mechanism for vacuum optical control of correlated electronic order: Casimir control, where the zero-point energy of the electromagnetic continuum, the Casimir energy, depends on the properties of the material system. To assess the experimental viability of this mechanism we focus on the Casimir stabilization of fluctuating nematic order. In nematic Fermi liquids, different orientations of the electronic order are often energetically degenerate. Thus, while local domains of fixed orientation may form, thermal disordering inhibits long range order. By engineering the electromagnetic environment of the electronic system, however, we show that the Casimir energy can be used as a tool to preferentially stabilize particular orientations of the nematic order. As a concrete example, we examine the interplay between a birefringent crystal -- which sources an anisotropic electromagnetic environment -- and a quantum Hall stripe system, an archetypal nematic Fermi fluid. We show that for experimentally feasible setups, the anisotropy induced by the orientation dependent Casimir energy can be $10^4$ times larger than other mechanisms known to stabilize quantum Hall stripes. This finding convincingly implies that our setting may be realized with currently available experimental technology. Having demonstrated that the Casimir energy can be used to stabilize fluctuating nematic order, we close by discussing the implications for recent terahertz cavity experiments on quantum Hall stripes, as well as pave the road towards broader Casimir control of competing correlated electronic phases.

cond-mat.str-el↗

arXiv2025

Probing enhanced electron-phonon coupling in graphene by infrared resonance Raman spectroscopy

We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at $\mathbf{K}$, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D$^\prime$ peaks with respect to that measured in graphite. Comparing with fully \textit{ab initio} theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.

cond-mat.mes-hall↗

arXiv2025

Terahertz chiral sub-wavelength cavities breaking time-reversal symmetry via ultra-strong light-matter interaction

We demonstrate terahertz chiral sub-wavelength cavities that break time-reversal symmetry by coupling the degenerate linearly polarized modes of two orthogonal sets of nano-antenna arrays using the inter-Landau level transition of a two-dimensional electron gas in a perpendicular magnetic field, realizing normalized light-matter coupling rates up to $Ω_R/ω_{\mathrm{cav}} = 0.78$ with a dispersion that is modified by the parasitic capacitive coupling between the orthogonal antennas. The deep sub-wavelength confinement of the nano-antennas means that the ultra-strong coupling regime can be reached even with a small number of carriers compared to Fabry-Perot cavities, making it viable to be used with a variety of 2D materials. The non-degenerate circularly polarized ground state was only obtained after carefully optimizing the optical design to minimize the parasitic coupling to linearly polarized light.

cond-mat.mes-hall↗

arXiv2025

Cavity QED Control of Quantum Hall Stripes

Controlling quantum phases of materials with vacuum field fluctuations in engineered cavities is a novel route towards the optical control of emergent phenomena. We demonstrate, using magnetotransport measurements of a high-mobility two-dimensional electron gas, striking cavity-induced anisotropies in the electronic transport, including the suppression of the longitudinal resistance well below the resistivity at zero magnetic field. Our cavity-induced effects occur at ultra-low temperatures (< 200 mK) when the magnetic field lies between quantized Hall plateaus. We interpret our results as arising from the stabilization of thermally-disordered quantum Hall stripes. Our work presents a clear demonstration of the cavity QED control of a correlated electronic phase.

cond-mat.mes-hall↗

arXiv2024

Enhanced fractional quantum Hall gaps in a two-dimensional electron gas coupled to a hovering split-ring resonator

The magnetotransport of a high-mobility two-dimensional electron gas coupled to a hovering split-ring resonator with controllable distance is studied in the quantum Hall regime. The measurements reveal an enhancement by more than a factor 2 of the quantum Hall energy gaps at the fractional filling factors 4/3, 5/3, and 7/5, alongside a concurrent reduction in exchange splitting at odd integer filling factors. Theoretically, we show the strength of both effects to be quantitatively compatible with the emergence of an effective electron-electron long-range attractive interaction mediated by the exchange of virtual cavity photons in the presence of significant spatial gradients of the cavity electric vacuum fields. These results unveil a compelling interplay between cavity quantum electrodynamics and electronic correlations in two-dimensional systems, with profound implications for the manipulation and control of quantum phases in 2D materials.

cond-mat.mes-hall↗

arXiv2024

Infrared resonance Raman of bilayer graphene: signatures of massive fermions and band structure on the 2D peak

Few-layer graphene possesses low-energy carriers which behave as massive fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes which are resonant in the visible, and induces a clearer frequency-separation of the sub-structures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each sub-structure and, comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron-phonon matrix elements.

cond-mat.mes-hall↗

arXiv2024

Theory of infrared double-resonance Raman spectrum in graphene: the role of the zone-boundary electron-phonon enhancement

We theoretically investigate the double-resonance Raman spectrum of monolayer graphene down to infrared laser excitation energies. By using first-principles density functional theory calculations, we improve upon previous theoretical predictions based on conical models or tight-binding approximations, and rigorously justify the evaluation of the electron-phonon enhancement found in Ref. [Venanzi, T., Graziotto, L. et al., Phys. Rev. Lett. 130, 256901 (2023)]. We proceed to discuss the effects of such enhancement on the room temperature graphene resistivity, hinting towards a possible reconciliation of theoretical and experimental discrepancies.

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arXiv2023

Testing the Renormalization of the von Klitzing Constant by Cavity Vacuum Fields

In light of recent developments demonstrating the impact of cavity vacuum fields inducing the breakdown of topological protection in the integer quantum Hall effect, a compelling question arises: what effects might cavity vacuum fields have on fundamental constants in solid-state systems? In this work we present an experiment that assesses the possibility of the von Klitzing constant itself being modified. By employing a Wheatstone bridge, we precisely measure the difference between the quantized Hall resistance of a cavity-embedded Hall bar and the resistance standard, achieving an accuracy down to 1 part in 105 for the lowest Landau level. While our results do not suggest any deviation that could imply a modified Hall resistance, our work represents pioneering efforts in exploring the fundamental implications of vacuum fields in solid-state systems.

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