Papers
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Magnetic Bulk Photovoltaic Effect in Bernal Bilayer Graphene
NOVEMBER 2025
The recent study published by Yuncheng Mao and Claudio Attaccalite explores how magnetic fields reshape the symmetry of bilayer graphene and unlock new photovoltaic responses. By examining both shift current and magnetic ballistic current under varying field strengths, the authors reveal striking roles played by edge states—from being nearly inactive under weak fields to becoming dominant contributors under strong fields governed by Landau levels.
This work highlights how time-reversal symmetry breaking and magnetic control can open fresh pathways for optoelectronic applications, quantum materials research, and the design of next-generation devices. The paper provides a comprehensive look at how bilayer graphene responds to magnetic environments, offering insights that bridge fundamental physics and applied nanotechnology.
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Long-lived coherence between incoherent excitons revealed by time-resolved angle-resolved photoemission spectroscopy: An exact solution
June 2025
We investigate the exciton dynamics in an exactly solvable two-band model for semiconductors. The model incorporates light-matter, electron-electron, and electron-phonon interactions, and captures exciton formation as well as the transition from the coherent to the incoherent regime. We analyze excitonic polarization, populations, and coherences, with special focus on their impact in time-resolved angle-resolved photoemission spectroscopy (TR-ARPES). For nonresonant pumping with below-gap photon energies, TR-ARPES spectra reveal distinct excitonic replica and quantum beats persisting in the incoherent regime. These are due to a coherence between different species of incoherent excitons. Such type of coherence is resistant to phonon dephasing, indicating that it follows different dynamics than those governing the coherences considered so far.
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Observation of Floquet–Bloch states in monolayer graphene
May 2025
Floquet engineering enables the manipulation of quantum phases of matter through periodic driving. It has been implemented across different platforms, ranging from photonic systems to optical lattices of ultracold atoms. In solids, coherent light–matter interaction induced by periodic driving leads to hybridization of Bloch electrons with photons, resulting in the formation of replica bands known as Floquet–Bloch states. These states have been observed in several materials, and their properties have been linked to a range of predicted phase transitions. However, direct energy and momentum-resolved observation of these states remains limited to a few. Here we report the direct observation of Floquet–Bloch states in monolayer epitaxial graphene. By using time-resolved and angle-resolved photoemission spectroscopy with mid-infrared pump excitation, we detected replicas of the Dirac cone. The dependence of these replica bands on pump polarization shows that they originate from the scattering between Floquet–Bloch states and photon-dressed free-electron-like photoemission final states, known as Volkov states. Our method can potentially be used to directly observe Floquet–Bloch states at large momenta in other quantum materials.
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Light-matter interactions in layered materials and heterostructures: from moiré physics and magneto-optical effects to ultrafast dynamics and hybrid meta-photonics
april 2025
Layered two-dimensional (2D) materials have revolutionized how we approach light–matter interactions, offering unprecedented optical and electronic properties with the potential for vertical heterostructures and manipulation of spin–valley degrees of freedom. The discovery of moiré physics in twisted heterostructures has further unlocked new possibilities for controlling the band structure of tailored semiconductor heterostructures. In parallel, the integration of 2D materials with hybrid photonic structures and ultrafast studies on their optical and spin–valley properties has revealed a wealth of novel physical phenomena. This perspective highlights the recent advances in our understanding of light–matter interactions in moiré and 2D systems, with a particular emphasis on ultrafast processes and the integration of these materials into photonic platforms. We explore the implications for optoelectronics and emerging photonic technologies, positioning 2D materials as a transformative tool for next-generation devices.
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Exact formula with two dynamically screened electron-phonon couplings for positive phonon-linewidths approximations
january 2025
In this paper, an exact formula for the phonon linewidths involving only dressed electron-phonon couplings and ensuring the positivity property is introduced. The formula is designed to account for both nonadiabatic and correlation effects, and it provides an alternative proof that density functional perturbation theory calculations of phonon linewidths are not affected by a double counting of screening diagrams. Furthermore, the treatment to nonequilibrium scenarios and offer a rigorous justification for employing the phononic Boltzmann equation is extended.
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Excitonic Bloch equations from first principles
january 2025
The ultrafast conversion of coherent excitons into incoherent excitons, as well as the subsequent exciton diffusion and thermalization, are central topics in current scientific research due to their relevance in optoelectronics, photovoltaics and photocatalysis. Current approaches to the exciton dynamics rely on model Hamiltonians that depend on already screened electron-electron and electron-phonon couplings. In this work, we subject the state-of-the-art methods to scrutiny using the ab initio Hamiltonian for electrons and phonons. We offer a rigorous and intuitive proof demonstrating that the exciton dynamics governed by model Hamiltonians is affected by an overscreening of the electron-phonon interaction. The introduction of an auxiliary exciton species, termed the irreducible exciton, enables us to formulate a theory free from overscreening and derive the excitonic Bloch equations. These equations describe the time-evolution of coherent, irreducible, and incoherent excitons during and after the optical excitation. They are applicable beyond the linear regime, and predict that the total number of excitons is preserved when the external fields are switched off.
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Theory of coherent phonons coupled to excitons
june 2024
The interaction of excitons with lattice vibrations underlies the scattering from bright to dark excitons as well as the coherent modulation of the exciton energy. Unlike the former mechanism, which involves phonons with finite momentum, the latter can be exclusively attributed to coherent phonons with zero momentum. We here lay down the microscopic theory of coherent phonons interacting with resonantly pumped bright excitons and provide the explicit expression of the corresponding coupling. The coupling notably resembles the exciton-phonon one, but with a crucial distinction: it contains the bare electron-phonon matrix elements rather than the screened ones. Our theory predicts that the exciton energy features a polaronic-like red-shift and monochromatic oscillations or beatings, depending on the number of coupled optical modes. Both the red-shift and the amplitude of the oscillations are proportional to the excitation density and to the square of the exciton-coherent-phonon coupling. We validate our analytical findings through comparisons with numerical simulations of time-resolved optical absorbance in resonantly pumped MoS2 monolayers.
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