arXiv subjects

Omar Mehio

Publications and source records attributed to Omar Mehio.

5 recordsLinked to original sources

arXiv2026

Cavity-engineered ferroelectricity in SrTiO$_3$

Interfacing quantum materials with photonic cavity architectures is at the vanguard of materials engineering. The structured electromagnetic fluctuations of a dark cavity environment can renormalize fundamental thermodynamical material properties and energy scales, holding promise to create a rich ecosystem of cavity-induced ground states and functionalities absent in the bare material. However, direct experimental demonstrations that cavity coupling can stabilize or enhance genuinely new phases of matter have remained scarce. In this work, we establish the presence of cavity-induced and cavity-enhanced ferroelectric phases in free-standing SrTiO$_3$ membranes coupled to dark THz-frequency photonic crystal cavities. Using THz spectroscopy, we observe ultrastrong coupling (USC) between the ferroelectric soft mode in SrTiO$_3$ and the cavity resonance. The structural response of SrTiO$_3$ to this USC is measured via second harmonic generation imaging, which reveals a substantial (greater than 400%) enhancement of signal in the cavity environment and a distinct modification of the crystalline symmetry, indicating the onset and enhancement of ferroelectricity. We interpret these observations as resulting from renormalized piezoelectric and flexoelectric couplings, which in turn arise from the cavity-modified dielectric environment. This interpretation is supported with microscopic modeling via DFT-based multi-scale dynamics simulations and by macroscopic Ginzburg-Landau theory. Altogether, our experimental and theoretical results firmly confirm that cavity engineering can indeed be used to create new ground states in materials and enhance existing ones. This work establishes a new paradigm for materials engineering with direct applications for quantum transduction, quantum sensing, and nonlinear integrated photonics.

cond-mat.mtrl-sci↗

arXiv2026

Kolibri: A Sovereign European Model on the Pareto Frontier

This report introduces Kolibri, an English-German mixture-of-experts transformer with 78.1B total parameters and 3.46B active parameters per token, released as open weights under the Apache 2.0 licence. We built Kolibri for sovereign and specialised deployment, with a particular focus on German and on regulated domains such as public administration, industry, and aerospace. To support these settings, we control the full model-development process, including data, architecture, training infrastructure, post-training, and evaluation. We train Kolibri on 24T tokens across pre-training, mid-training, and long-context extension, with German accounting for more than 20% of the data mix by bytes. To reach this share, we complement German datasets with more than 2T German tokens that we curate from the web with our own pipeline and generate synthetically. Post-training combines supervised fine-tuning on curated German and English data with reinforcement learning on more than 1.2M internally curated tasks, which cover reasoning, tool use, instruction following, code, and retrieval. The model activates 4.4% of its parameters per token, and among the open models evaluated in this report, it lies on the Pareto frontier of quality and serving cost in English and German. Kolibri builds on Kolibri Origin, an earlier iteration that validated our full pipeline at scale. Both models come from our Model Factory, which connects data, experimentation, training, evaluation, and deployment through version-controlled workflows.

cs.CL↗

arXiv2025

A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator

The undoped antiferromagnetic Mott insulator naturally has one charge carrier per lattice site. When it is doped with additional carriers, they are unstable to spin fluctuation-mediated Cooper pairing as well as other unconventional types of charge, spin, and orbital current ordering. Photo-excitation can produce charge carriers in the form of empty (holons) and doubly occupied (doublons) sites that may also exhibit charge instabilities. There is evidence that antiferromagnetic correlations enhance attractive interactions between holons and doublons, which can then form bound pairs known as Hubbard excitons, and that these might self-organize into an insulating Hubbard exciton fluid. However, this out-of-equilibrium phenomenon has not been detected experimentally. Here, we report the transient formation of a Hubbard exciton fluid in the antiferromagnetic Mott insulator Sr$_{2}$IrO$_{4}$ using ultrafast terahertz conductivity. Following photo-excitation, we observe rapid spectral weight transfer from a Drude metallic response to an insulating response. The latter is characterized by a finite energy peak originating from intra-excitonic transitions, whose assignment is corroborated by our numerical simulations of an extended Hubbard model. The lifetime of the peak is short, approximately one picosecond, and scales exponentially with Mott gap size, implying extremely strong coupling to magnon modes.

cond-mat.str-el↗

arXiv2025

Observation of excitons bound by antiferromagnetic correlations

Two-dimensional Mott insulators host antiferromagnetic (AFM) correlations that are predicted to enhance the attractive interaction between empty (holons) and doubly occupied (doublons) sites, creating a novel pathway for exciton formation. However, experimental confirmation of this spin-mediated binding mechanism remains elusive. Leveraging the distinct magnetic critical properties of the Mott antiferromagnets Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$, we show using time-resolved THz spectroscopy that excitons only exist at temperatures below where short-range AFM correlation develops. The excitons remain stable up to photodoping densities approaching the predicted excitonic Mott insulator-to-metal transition, revealing a unique robustness against screening. Our results establish the viability of spin-bound excitons and introduce opportunities for excitonic control through magnetic degrees of freedom.

cond-mat.str-el↗

arXiv2022

Keldysh space control of charge dynamics in a strongly driven Mott insulator

The fate of a Mott insulator under strong low frequency optical driving conditions is a fundamental problem in quantum many-body dynamics. Using ultrafast broadband optical spectroscopy, we measured the transient electronic structure and charge dynamics of an off-resonantly pumped Mott insulator Ca$_2$RuO$_4$. We observe coherent bandwidth renormalization and nonlinear doublon-holon pair production occurring in rapid succession within a sub-100 femtosecond pump pulse duration. By sweeping the electric field amplitude, we demonstrate continuous bandwidth tuning and a Keldysh cross-over from a multi-photon absorption to quantum tunneling dominated pair production regime. Our results provide a procedure to control coherent and nonlinear heating processes in Mott insulators, facilitating the discovery of novel out-of-equilibrium phenomena in strongly correlated systems.

cond-mat.str-el↗