arXiv subjects

T. F. Nova

Publications and source records attributed to T. F. Nova.

3 recordsLinked to original sources

arXiv2025

Cavity-Driven Attractive Interactions in Quantum Materials

Many-body phenomena in quantum materials emerge from the interplay among a broad continuum of electronic states, and controlling these interactions is critical for engineering novel phases. One promising approach exploits fluctuations of the vacuum electromagnetic field confined within optical cavities to tailor electronic properties. Here, we demonstrate that cavity photons can mediate attractive interactions in a tunable van der Waals material and reorganize a continuum of electron-hole transitions into an exciton-like state. We introduce a broadband, sub-wavelength time-domain microscope that integrates exfoliated, dual-gated two-dimensional quantum materials into a terahertz cavity. This approach enables the first-ever measurement of the field-tunable bandgap of bilayer graphene at terahertz frequencies while revealing ultrastrong coupling with a vacuum Rabi frequency exceeding $Ω_{Rabi}/ω\approx 40\%$ of the bare photon energy. Crucially, we identify a novel cavity-induced resonance emerging from the interband continuum that resembles Coulomb-bound excitons and remains stable across a broad temperature range. By uniting longstanding theoretical predictions with advanced experimental techniques, our findings open new avenues for designing and probing unique light-matter states and realizing hybrid correlated phases in quantum materials.

cond-mat.mtrl-sci↗

arXiv2021

Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO$_3$

In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic, and chemical constraints. Here, we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO$_3$, a material that exhibits only partial orbital polarization, an unsaturated low-temperature magnetic moment, and a suppressed Curie temperature, $T_c$ = 27 K. The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to $T_{neq} >$ 80 K, nearly three times the thermodynamic transition temperature. First-principles and model calculations of the nonlinear phonon-orbital-spin coupling reveal that these effects originate from dynamical changes to the orbital polarization and the makeup of the lowest quasi-degenerate Ti $t_{2g}$ levels. Notably, light-induced high temperature ferromagnetism in YTiO$_3$ is found to be metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities.

cond-mat.str-el↗

arXiv2018

An effective magnetic field from optically driven phonons

Light fields at THz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the crystal lattice, has been shown to stimulate insulator-metal transitions, melt magnetic order, or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases. This nonlinear mode mixing drives rotations as well as displacements of the crystal-field atoms, mimicking the application of a magnetic field and resulting in the excitation of spin precession in the rare-earth orthoferrite $ErFeO_3$. Coherent control of lattice rotations may become applicable to other interesting problems in materials research, for example as a way to affect the topology of electronic phases.

cond-mat.mtrl-sci↗