arXiv subjects

Alon Ron

Publications and source records attributed to Alon Ron.

6 recordsLinked to original sources

arXiv2026

Non adiabatic dynamics of the ferroelectric soft mode

Most microscopic descriptions of structural dynamics assume the Born-Oppenheimer separation, where electrons adjust adiabatically to ionic motion. When this separation breaks down, electronic and lattice degrees of freedom can evolve on different timescales, giving rise to new physical phenomena beyond the adiabatic limit. Here we use time-resolved, phase-sensitive second-harmonic generation and pump-probe reflectivity to reshape the ferroelectric free-energy landscape of SnTe while separately tracking polar order and coherent lattice motion. When photoexcitation transiently suppresses the double-well barrier, polarization dynamics become strongly nonlinear, while the coherent phonon dynamics remain harmonic. This decoupling cannot be described by a single adiabatic coordinate for the electronic polarization and ionic positions. We provide a unifying physical description for the non adiabatic dynamics of the ferroelectric mode and the mixed displacive/order-disorder nature of SnTe based on a separation of scales for the renormalization of the ferroelectric stiffness.

cond-mat.mtrl-sci↗

arXiv2026

Polytype-Dependent Upconversion Photoluminescence in 3R-MoS2

Ferroelectric van der Waals materials offer switchable polarization states, yet optical readout of their stacking configurations remains challenging. Here, building on the resonant exciton-exciton annihilation (EEA) mechanism recently identified in 2H-phase TMDs, we report the first observation of upconversion photoluminescence (UPL) in rhombohedral MoS$_2$ and demonstrate that this many-body process is strongly polytype-dependent. Using low-temperature spectroscopy, we observe anti-Stokes emission with superlinear power dependence characteristic of EEA. Beyond serving as an accurate layer-number sensor due to discrete bandgap variations, UPL provides a sensitive probe of stacking order across thicknesses. The two neutral trilayer polytypes, which remain indistinguishable by surface potential measurements and second harmonic generation, exhibit markedly different UPL intensities. This sensitivity persists in thicker samples where multiple configurations coexist. First-principles calculations suggest that the intensity contrast originates primarily from the layer confinement of the annihilating excitons, while energy matching to the $\Gamma$ final-state manifold provides additional intensity selectivity. Power-dependent spectroscopy further disentangles two distinct annihilation channels originating from different dark exciton valleys, identified through their contrasting intensity scaling and opposite density-induced energy shifts. Crucially, the annihilation process doubles the energy separation of nearly degenerate dark excitons while converting their weak emission into bright signal, providing experimental access to valley-specific responses that are obscured in direct dark-exciton spectroscopy. Our findings demonstrate that ferroelectric configurations provide a new degree of freedom for controlling nonlinear optical processes.

cond-mat.mtrl-sci↗

arXiv2025

Coherent control through phonon anharmonicity

Anharmonic lattice vibrations play a key role in many physical phenomena. They govern the heat conductivity of solids, strongly affect the phonon spectra, play a prominent role in soft mode phase transitions, allow ultrafast engineering of material properties and more. The most direct evidence for anharmonicity is to measure the oscillation frequency changing as a function of the oscillation amplitude. For lattice vibrations, this is not a trivial task, and anharmonicity is probed indirectly through its effects on thermodynamic properties and spectral features or through coherent decay of one mode to another. However, measurement and control of the anharmonicity of a single Raman mode is still lacking. We show that ultrafast double pump-probe spectroscopy could be used to directly observe frequency shifts of Raman phonons as a function of the oscillation amplitude and disentangle the coherent contributions from quasi-harmonic sources such as temperature and changes to the carrier density in the thermoelectric materials SnTe and SnSe. Moreover, we show that coherent displacive phononic excitations in tandem with electron-phonon coupling is a pathway to dynamically control phonon anharmonicity. Our results have dramatic implications for the material engineering of future thermoelectrics. Moreover, our methodology could be used to isolate the basic mechanisms driving optically induced phase transitions and other nonlinear phenomena based on their unique timestamps.

cond-mat.mtrl-sci↗

arXiv2025

Photoluminescence Detection of Polytype Polarization in r-MoS2 Enabled by Asymmetric Dielectric Environments

The rhombohedral (r) polytypes of transition metal dichalcogenides (TMDs) constitute a novel class of two-dimensional ferroelectric materials, where lateral shifts between parallel layers induce reversible out-of-plane polarization. This emerging field, known as SlideTronics, holds significant potential for next-generation electronic and optoelectronic applications. While extensive studies have investigated the effects of electrical and chemical doping on excitonic signatures in 2H-TMDs, as well as the influence of dielectric environments on their optical properties, the impact of intrinsic polarization in asymmetric environments remains largely unexplored. Here, we demonstrate a striking polarization-dependent photoluminescence (PL) contrast of up to 400\% between ferroelectric domains in bilayer and trilayer rhombohedral molybdenum disulfide (r-MoS2). This pronounced contrast arises from an asymmetric dielectric environment, which induces polarization-dependent shifts in the Fermi energy, leading to a modulation of the exciton-trion population balance. A detailed temperature-dependent line shape analysis of the PL, conducted from 4K to room temperature, reveals domain-specific trends that further reinforce the connection between polarization states and excitonic properties. The persistence of these distinct optical signatures at room temperature establishes PL as a robust and non-invasive probe for ferroelectric domain characterization, particularly in fully encapsulated device architectures where conventional techniques, such as Kelvin probe force microscopy, become impractical.

cond-mat.mtrl-sci↗

arXiv2024

Pressure tuning of intrinsic and extrinsic sources to the anomalous Hall effect in CrGeTe$_3$

The integrated Berry curvature is a geometric property that has dramatic implications for material properties. This study investigates the integrated Berry curvature and other contributions to the anomalous Hall effect in CrGeTe$_3$ as a function of pressure. The anomalous Hall effect is absent in the insulating phase of CrGeTe$_3$ and evolves with pressure in a dome-like fashion as pressure is applied. The dome's edges are characterized by Fermi surface deformations, manifested as mixed electron and hole transport. We corroborate the presence of bipolar transport by ab-initio calculations which also predict a nonmonotonic behavior of the Berry curvature as a function of pressure. Quantitative discrepancies between our calculations and experimental results indicate that additional scattering mechanisms, which are also strongly tuned by pressure, contribute to the anomalous Hall effect in CrGeTe$_3$.

cond-mat.str-el↗

arXiv2020

Controlling ligand-mediated exchange interactions in periodically driven magnetic materials

A periodic drive could alter the effective exchange interactions in magnetic materials. Here, we explore how exchange pathways affect the effective interactions of periodically driven magnetic materials. Aiming to apply Floquet engineering methods to two-dimensional magnetic materials, we consider realistic models and discuss the effect of a periodic drive on ligand-mediated exchange interactions. We show that depending on bond angles and the number of ligand ions involved in the exchange process, drive-induced changes can be very different from those calculated from direct-hopping models considered earlier. We study these effects and find that the presence of ligand ions must be taken into account, especially for TMTCs where ligand ion mediated next-neighbor interactions play a crucial role in determining the magnetic ground state of the system.

cond-mat.str-el↗