arXiv subjects

Edoardo Baldini

Publications and source records attributed to Edoardo Baldini.

At least 19 recordsLinked to original sources

arXiv2026

Emergent Trion Resonance Driven by Lattice Reconstruction in a Moiré Superlattice

We investigate how many-electron excited states emerge in twisted MoSe2 homobilayers when the lattice reconstructions evolve. Notably, we identify a new trion resonance that arises in the transition regime of lattice reconstruction, where gradual changes in atomic alignment between the layers occur. Magnetic field-dependent measurements, supported by first-principles calculations, indicate that the exciton forms at the K valley while the doped hole resides in the Gamma valley. First-principles calculations further indicate that two nearly degenerate exciton resonances can arise, localized at different sites within the moiré supercell. We propose that the new trion resonance is a "charge-transfer" trion, in which the electron-hole pair is spatially separated from the doped hole. The emergence of these complex excited states stems from the distinct moiré potentials acting on holes and excitons, resulting in their different spatial distribution within the superlattice.

cond-mat.mes-hall↗

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

Nonresonant Raman control of ferroelectric polarization

Important advances have recently been made in the search for materials with complex multi-phase landscapes that host photoinduced metastable collective states with exotic functionalities. In almost all cases so far, the desired phases are accessed by exploiting light-matter interactions via the imaginary part of the dielectric function through above-bandgap or resonant mode excitation. Nonresonant Raman excitation of coherent modes has been experimentally observed and proposed for dynamic material control, but the resulting atomic excursion has been limited to perturbative levels. Here, this challenge is overcome by employing nonresonant ultrashort pulses with low photon energies well below the bandgap. Using mid-infrared pulses, ferroelectric reversal is induced in lithium niobate, and the large-amplitude mode displacements are characterized through femtosecond stimulated Raman scattering and second harmonic generation. This approach, validated by first-principle calculations, defines a novel method for synthesizing hidden phases with unique functional properties and manipulating complex energy landscapes at reduced energy consumption and ultrafast speeds.

physics.optics↗

arXiv2025

Electromagnon signatures of a metastable multiferroic state

Magnetoelectric multiferroic materials, particularly type-II multiferroics where ferroelectric polarizations arise from magnetic order, offer significant potential for the simultaneous control of magnetic and electric properties. However, it remains an open question as to how the multiferroic ground states are stabilized on the free-energy landscape in the presence of intricate competition between the magnetoelectric coupling and thermal fluctuations. In this work, by using terahertz time-domain spectroscopy in combination with an applied magnetic field, photoexcitation, and single-shot detection, we reveal the spectroscopic signatures of a magnetic-field-induced metastable multiferroic state in a hexaferrite. This state remains robust until thermal influences cause the sample to revert to the original paraelectric state. Our findings shed light on the emergence of metastable multiferroicity and its interplay with thermal dynamics.

cond-mat.str-el↗

arXiv2025

Distinct Optical Excitation Mechanisms of a Coherent Magnon in a van der Waals Antiferromagnet

The control of antiferromagnets with ultrashort optical pulses has emerged as a prominent field of research. Tailored laser excitation can launch coherent spin waves at terahertz frequencies, yet a comprehensive description of their generation mechanisms is still lacking despite extensive efforts. Using terahertz emission spectroscopy, we investigate the generation of a coherent magnon mode in the van der Waals antiferromagnet NiPS$_3$ under a range of photoexcitation conditions. By tuning the pump photon energy from transparency to resonant with a $d$-$d$ transition, we reveal a striking change in the coherent magnon's dependence on the pump polarization, indicating two distinct excitation mechanisms. Our findings provide a strategy for the manipulation of magnetic modes via photoexcitation around sub-gap electronic states.

cond-mat.str-el↗

arXiv2024

Terahertz stimulated parametric downconversion of a magnon mode in an antiferromagnet

In condensed matter systems, interactions between collective modes offer avenues for nonlinear coherent manipulation of coupled excitations and quantum phases. Antiferromagnets, with their inherently coupled magnon modes, provide a promising platform for nonlinear control of microscopic spin waves and macroscopic magnetization. However, nonlinear magnon-magnon interactions have been only partially elaborated, leaving key gaps in the prospects for potential ultrahigh-bandwidth magnonic signal processing. Here, we use a pair of intense terahertz pulses to sequentially excite two distinct coherent magnon modes in an antiferromagnet and find that the magnon mode with a lower frequency undergoes amplification when the higher-frequency mode is driven. We unveil the nonlinear excitation pathways of this stimulated parametric downconversion process by using polarization-selective two-dimensional terahertz spectroscopy. Our work provides fundamental insights into nonlinear magnonics in antiferromagnets, laying the groundwork for forthcoming spintronic and magnonic devices based on nonlinear magnon-magnon interactions.

cond-mat.mtrl-sci↗

arXiv2024

Terahertz field-induced nonlinear coupling of two magnon modes in an antiferromagnet

Magnons are quantized collective spin-wave excitations in magnetically ordered materials. Revealing their interactions among these collective modes is crucial for the understanding of fundamental many-body effects in such systems and the development of high-speed information transport and processing devices based on them. Nevertheless, identifying couplings between individual magnon modes remains a long-standing challenge. Here, we demonstrate spectroscopic fingerprints of anharmonic coupling between distinct magnon modes in an antiferromagnet, as evidenced by coherent photon emission at the sum and difference frequencies of the two modes. This discovery is enabled by driving two magnon modes coherently with a pair of tailored terahertz fields and then disentangling a mixture of nonlinear responses with different origins. Our approach provides a route for generating nonlinear magnon-magnon mixing.

cond-mat.mtrl-sci↗

arXiv2024

Keldysh tuning of photoluminescence in a lead halide perovskite crystal

In 1964, Keldysh laid the groundwork for strong-field physics in atomic, molecular, and solid-state systems by delineating a ubiquitous transition from multiphoton absorption to quantum electron tunneling under intense AC driving forces. While both processes in semiconductors can generate carriers and result in photon emission through electron-hole recombination, the low quantum yields in most materials have hindered direct observation of the Keldysh crossover. Leveraging the large quantum yields of photoluminescence in lead halide perovskites, we show that we can not only induce bright light emission from extreme sub-bandgap light excitation but also distinguish between photon-induced and electric-field-induced processes. Our results are rationalized by the Landau-Dykhne formalism, providing insights into the non-equilibrium dynamics of strong-field light-matter interactions. These findings open new avenues for light upconversion and sub-bandgap photon detection, highlighting the potential of lead halide perovskites in advanced optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Coexistence of interacting charge density waves in a layered semiconductor

Coexisting orders are key features of strongly correlated materials and underlie many intriguing phenomena from unconventional superconductivity to topological orders. Here, we report the coexistence of two interacting charge-density-wave (CDW) orders in EuTe4, a layered crystal that has drawn considerable attention owing to its anomalous thermal hysteresis and a semiconducting CDW state despite the absence of perfect FS nesting. By accessing unoccupied conduction bands with time- and angle-resolved photoemission measurements, we find that mono- and bi-layers of Te in the unit cell host different CDWs that are associated with distinct energy gaps. The two gaps display dichotomous evolutions following photoexcitation, where the larger bilayer CDW gap exhibits less renormalization and faster recovery. Surprisingly, the CDW in the Te monolayer displays an additional momentum-dependent gap renormalization that cannot be captured by density-functional theory calculations. This phenomenon is attributed to interlayer interactions between the two CDW orders, which account for the semiconducting nature of the equilibrium state. Our findings not only offer microscopic insights into the correlated ground state of EuTe4 but also provide a general non-equilibrium approach to understand coexisting, layer-dependent orders in a complex system.

cond-mat.str-el↗

arXiv2023

Probing charge order of monolayer NbSe$_2$ within a bulk crystal

Atomically thin transition metal dichalcogenides can exhibit markedly different electronic properties compared to their bulk counterparts. In the case of NbSe$_2$, the question of whether its charge density wave (CDW) phase is enhanced in the monolayer limit has been the subject of intense debate, primarily due to the difficulty of decoupling this order from its environment. Here, we address this challenge by using a misfit crystal that comprises NbSe$_2$ monolayers separated by SnSe rock-salt spacers, a structure that allows us to investigate a monolayer crystal embedded in a bulk matrix. We establish an effective monolayer electronic behavior of the misfit crystal by studying its transport properties and visualizing its electronic structure by angle-resolved photoemission measurements. We then investigate the emergence of the CDW by tracking the temperature dependence of its collective modes. Our findings reveal a nearly sixfold enhancement in the CDW transition temperature, providing compelling evidence for the profound impact of dimensionality on charge order formation in NbSe$_2$.

cond-mat.mtrl-sci↗

arXiv2023

Terahertz field-driven magnon upconversion in an antiferromagnet

Tailored light excitation and nonlinear control of lattice vibrations have emerged as powerful strategies to manipulate the properties of quantum materials out of equilibrium. Generalizing the use of coherent phonon-phonon interactions to nonlinear couplings among other types of collective modes would open unprecedented opportunities in the design of novel dynamic functionalities in solids. For example, the collective excitations of magnetic order -- magnons -- can carry information with little energy dissipation, and their coherent and nonlinear control would provide an attractive route to achieve collective-mode-based information processing and storage in forthcoming spintronics and magnonics. Here, we discover that intense terahertz (THz) fields can initiate processes of magnon upconversion mediated by an intermediate magnetic resonance. By using a suite of advanced spectroscopic tools, including a newly demonstrated two-dimensional (2D) THz polarimetry technique enabled by single-shot detection, we unveil the unidirectional nature of coupling between distinct magnon modes of a canted antiferromagnet. Calculations of spin dynamics further suggest that this coupling is a universal feature of antiferromagnets with canted magnetic moments. These results demonstrate a route to inducing desirable energy transfer pathways and THz-induced coupling between coherent magnons in solids and pave the way for a new era in the development of ultrafast control of magnetism.

cond-mat.mtrl-sci↗

arXiv2023

Spin-correlated exciton-polaritons in a van der Waals magnet

Strong coupling between light and elementary excitations is emerging as a powerful tool to engineer the properties of solid-state systems. Spin-correlated excitations that couple strongly to optical cavities promise control over collective quantum phenomena such as magnetic phase transitions, but their suitable electronic resonances have yet to be found. Here we report strong light-matter coupling in $\textrm{NiPS}_3$, a van der Waals antiferromagnet with highly correlated electronic degrees of freedom. A previously unobserved class of polaritonic quasiparticles emerges from the strong coupling between its spin-correlated excitons and the photons inside a microcavity. Detailed spectroscopic analysis in conjunction with a microscopic theory provides unique insights into the origin and interactions of these exotic magnetically coupled excitations. Our work introduces van der Waals magnets to the field of strong light-matter physics and provides a path towards the design and control of correlated electron systems via cavity quantum electrodynamics.

cond-mat.mtrl-sci↗

arXiv2023

Light-induced insulator-metal transition in Sr$_2$IrO$_4$ reveals the nature of the insulating ground state

Sr$_2$IrO$_4$ has attracted a lot of attention due to its structural and electronic similarities to La$_2$CuO$_4$ which is the parent compound of high-T$_c$ superconducting cuprates. It was proposed to be a strong spin-orbit coupled J$_{eff}$ = 1/2 Mott insulator, but the Mott nature of its insulating ground state and the origin of the gap have not been conclusively established. Here, we use ultrafast laser pulses to realize an insulator-metal transition in Sr$_2$IrO$_4$ and probe the resulting dynamics using time- and angle-resolved photoemission spectroscopy. We observe a closing of the gap and the formation of weakly-renormalized electronic bands in the gap region. Comparing these observations to the expected temperature and doping evolution of Mott gaps and Hubbard bands provides clear evidence that the insulating state does not originate from Mott correlations. We instead propose a correlated band insulator picture, where antiferromagnetic correlations play a key role in the opening of the gap. More broadly, our results demonstrate that energy-momentum resolved nonequilibrium dynamics can be used to clarify the nature of equilibrium states in correlated materials.

cond-mat.str-el↗

arXiv2023

Discovery of enhanced lattice dynamics in a single-layered hybrid perovskite

Layered hybrid perovskites have attracted much attention in recent years due to their emergent physical properties and exceptional functional performances, but the coexistence of lattice order and structural disorder severely hinders our understanding of these materials. One unsolved problem regards how the lattice dynamics are affected by the dimensional engineering of the inorganic frameworks and the interaction with the molecular moieties. Here, we address this question by using a combination of high-resolution spontaneous Raman scattering, high-field terahertz spectroscopy, and molecular dynamics simulations. This approach enables us to reveal the structural vibrations and disorder in and out of equilibrium and provides surprising observables that differentiate single- and double-layered perovskites. While no distinct vibrational coherence is observed in double-layer perovskites, we discover that an off-resonant terahertz pulse can selectively drive a long-lived coherent phonon mode through a two-photon process in the single-layered system. This difference highlights the dramatic change in the lattice environment as the dimension is reduced. The present findings pave the way for the ultrafast structural engineering of hybrid lattices as well as for developing high-speed optical modulators based on layered perovskites.

cond-mat.mtrl-sci↗

arXiv2022

Intrinsic 1T' phase induced in atomically thin 2H-MoTe$_2$ by a single terahertz pulse

Polymorphic transitions in layered transition metal dichalcogenides provide an excellent platform for discovering exotic phenomena associated with metastable states, ranging from topological phase transitions to enhanced superconductivity. In particular, the transition from 2H to 1T'-MoTe$_2$, which was thought to be induced by high-energy photon irradiation among many other means, has been intensely studied for its technological relevance in nanoscale transistors. Despite the remarkable electrical performance arising from this 2H-to-1T' transition, it remains controversial whether a crystalline 1T' phase is produced because optical signatures of this putative transition are found to be associated with the formation of elemental Te clusters instead. Here, we demonstrate the creation of an intrinsic 1T' lattice after irradiating a mono- or few-layer 2H-MoTe$_2$ with a single field-enhanced terahertz pulse, whose low photon energy limits possible structural damage by optical pulses. To visualize the temporal evolution of this irreversible transition, we further develop a single-shot terahertz pump-second harmonic probe technique, and we find that the transition out of the 2H phase occurs within 10 ns after photoexcitation. Our results not only resolve the long-standing debate over the light-induced polymorphic transition in MoTe$_2$, they also highlight the unique capability of strong-field terahertz pulses in manipulating the structure of quantum materials.

cond-mat.mes-hall↗

arXiv2022

Simulating terahertz field-induced ferroelectricity in quantum paraelectric SrTiO$_3$

Recent experiments have demonstrated that light can induce a transition from the quantum paraelectric to the ferroelectric phase of SrTiO$_3$. Here, we investigate this terahertz field-induced ferroelectric phase transition by solving the time-dependent lattice Schrödinger equation based on first-principles calculations. We find that ferroelectricity originates from a light-induced mixing between ground and first excited lattice states in the quantum paraelectric phase. In agreement with the experimental findings, our study shows that the non-oscillatory second harmonic generation signal can be evidence of ferroelectricity in SrTiO$_3$. We reveal the microscopic details of this exotic phase transition and highlight that this phenomenon is a unique behavior of the quantum paraelectric phase.

cond-mat.mtrl-sci↗

arXiv2022

Snapshots of a light-induced metastable hidden phase driven by the collapse of charge order

Nonequilibrium hidden states, both transient and long-lived, provide a unique window into thermally inaccessible regimes of strong coupling between microscopic degrees of freedom in quantum materials. Understanding the physical origin of these states is of both fundamental and practical significance, allowing the exploration of far-from-equilibrium thermodynamics and the development of optoelectronic devices with on-demand photoresponses. However, mapping the ultrafast formation of a long-lived hidden phase remains a long-standing challenge in physics since the initial state of the system is not recovered rapidly and conventional pump-probe methods are thus not applicable. Here, using a suite of state-of-the-art single-shot spectroscopy techniques, we present a direct ultrafast visualization of the photoinduced phase transition to both transient and long-lived hidden states in an electronic crystal, 1T-TaS2. Capturing the dynamics of this complex phase transformation in a single-shot fashion demonstrates a commonality in microscopic pathways, driven by the collapse of charge order, that the system undergoes to enter the hidden state and provides unambiguous spectral fingerprints that distinguish such state from thermally accessible phases. We present a theory of fluctuation-dominated process that explains both the dynamics and the nature of the metastable state. Our results settle the debate around the origin of this elusive metastable state and pave the way for the discovery of new quantum phases of matter.

cond-mat.str-el↗

arXiv2021

Terahertz Field-Induced Reemergence of Quenched Photoluminescence in Quantum Dots

Continuous and concerted development of colloidal quantum-dot light-emitting diodes over the past two decades has established them as a bedrock technology for the next generation of displays. However, a fundamental issue that limits the performance of these devices is the quenching of photoluminescence due to excess charges from conductive charge transport layers. Although device designs have leveraged various workarounds, doing so often comes at the cost of limiting efficient charge injection. Here we demonstrate that high-field terahertz (THz) pulses can dramatically brighten quenched QDs on metallic surfaces, an effect which persists for minutes after THz irradiation. This phenomenon is attributed to the ability of the THz field to remove excess charges, thereby reducing trion and non-radiative Auger recombination. Our findings show that THz technologies can be used to suppress and control such undesired non-radiative decay, potentially in a variety of luminescent materials for future device applications.

cond-mat.mes-hall↗