arXiv subjects

Jocelyn L. Mendes

Publications and source records attributed to Jocelyn L. Mendes.

4 recordsLinked to original sources

arXiv2026

Experimentally Mapping the Phase Diagrams of Photoexcited Small Polarons

Understanding the fundamental properties that dictate photoexcited polarons in materials is critical to tuning their properties. Theoretical models of polarons have only recently been extended to the excited state. Experimental measurements of polaron formation and transport have been widely undertaken across a range of materials, from photocatalysts and superconductors to soft conducting polymers. Here, we map thermalized excited state experimental measurements of quantities such as polaron strength onto phase diagrams of the Holstein, Hubbard-Holstein, and t-J-Holstein models. This work demonstrates that tuning electron-phonon coupling strength, electron localization, and spin exchange can be leveraged to suppress or control polarons in transition metal oxides. We find that the t-J-Holstein model provides the clearest qualitative description of electron polarons in the measured iron oxides and could be generally applied to a wide range of systems that exhibit polaron formation in the excited state. This work combines experimental data with ground state models to provide a qualitative parameter space for informing photoexcited electron polaron design, under which excited state polaronic behavior can be classified within ground-state calculable models.

cond-mat.mtrl-sci↗

arXiv2025

Coherent and Dynamic Small Polaron Delocalization in CuFeO$_{2}$

Small polarons remain a significant bottleneck in the realization of efficient devices using transition metal oxides. Routes to engineer small polaron coupling to electronic states and lattice modes to control carrier localization remain unclear. Here, we measure the formation of small polarons in CuFeO$_{2}$ using transient extreme ultraviolet reflection spectroscopy and compare it to theoretical predictions in realistically parameterized Holstein models, demonstrating that polaron localization depends on its coupling to the high-frequency versus low-frequency components of the phonon bath. We measure that small polaron formation occurs on a comparable ~100 fs timescale to other Fe(III) compounds. After formation, a dynamic delocalization of the small polaron occurs through a coherent lattice expansion between Fe-O layers and charge-sharing with surrounding Fe(IV) states. Our simulations of polaron formation dynamics reveal that two major factors dictate polaron formation timescales: phonon density and reorganization energy distributions between acoustic and optical modes, matching experimental findings. Our work provides a detailed, real-time observation of how electronic-structural coupling in a polaron-host material can be leveraged to suppress polaronic effects for various applications.

cond-mat.mtrl-sci↗

arXiv2025

Dynamic Competition Between Hubbard and Superexchange Interactions Selectively Localizes Electrons and Holes Through Polarons

Controlling the effects of photoexcited polarons in transition metal oxides can enable the long timescale charge separation necessary for renewable energy applications as well as controlling new quantum phases through dynamically tunable electron-phonon coupling. In previously studied transition metal oxides, polaron formation is facilitated by a photoexcited ligand-to-metal charge transfer (LMCT). When the polaron is formed, oxygen atoms move away from iron centers, which increases carrier localization at the metal center and decreases charge hopping. Studies of yttrium iron garnet and erbium iron oxide have suggested that strong electron and spin correlations can modulate photoexcited polaron formation. To understand the interplay between strong spin and electronic correlations in highly polar materials, we studied gadolinium iron oxide (GdFeO3), which selectively forms photoexcited polarons through an Fe-O-Fe superexchange inter-action. Excitation-wavelength-dependent transient extreme ultraviolet (XUV) spectroscopy selectively excites LMCT and metal-to-metal charge transfer transitions (MMCT). The LMCT transition suppresses photoexcited polaron formation due to the balance between superexchange and Hubbard interactions, while MMCT transitions result in photoexcited polaron formation within 250+/-40 fs. Ab initio theory demonstrates that electron and hole polarons localize on iron centers following MMCT. In addition to understanding how strong electronic and spin correlations can control strong electron-phonon coupling, these experiments separately measure electron and hole polaron interactions on neighboring metal centers for the first time, providing insight into a large range of charge-transfer and Mott-Hubbard insulators.

cond-mat.mtrl-sci↗

arXiv2023

Antiadiabatic Small Polaron Formation in the Charge Transfer Insulator ErFeO3

Small polaron formation is dominant across a range of condensed matter systems. Small polarons are usually studied in terms of ground-state transport and thermal fluctuations, but small polarons can also be created impulsively by photoexcitation. The temporal response of the lattice and local electron correlations can then be separated, such as with transient XUV spectroscopy. To date, photoexcited small polaron formation has only been measured to be adiabatic. The reorganization energy of the polar lattice is large enough that the first electron-optical phonon scattering event creates a small polaron without significant carrier thermalization. Here, we use transient XUV spectroscopy to measure antiadiabatic polaron formation by frustrating the iron-centered octahedra in a rare-earth orthoferrite lattice. The small polaron is measured to take several picoseconds to form over multiple coherent charge hopping events between neighboring Fe3+-Fe2+ sites, a timescale that is more than an order of magnitude longer compared to previous materials. The measured interplay between optical phonons, electron correlations, and on-site lattice deformation give a clear picture of how antiadiabatic small polaron transport would occur in the material. The measurements also confirm the prediction of the Holstein and Hubbard-Holstein model that the electron hopping integral must be larger than the reorganization energy to achieve antiadiabaticity. Moreover, the measurements emphasize the importance of considering dynamical electron correlations, and not just changes in the lattice geometry, for controlling small polarons in transport or photoexcited applications.

cond-mat.mtrl-sci↗