arXiv subjects

Clifford J. Allington

Publications and source records attributed to Clifford J. Allington.

5 recordsLinked to original sources

arXiv2026

Disentangling spin polarization from driven circular ionic motion in EuTiO$_3$

Coherently driven circular ionic motion has been reported to produce large helicity-dependent optical responses attributed to transient magnetization. However, the microscopic nature and magnitude of this phenomenon, sometimes referred to as dynamical multiferroicity, remain strongly debated. Here, we directly study the connection between ionic circulation induced by a high-field circular terahertz (THz) drive and its effect on the spin system of EuTiO$_3$ using X-rays. This material exhibits an optical response consistent with the putative magnetic signal observed in related non-magnetic materials. Furthermore, the presence of Eu$^{2+}$ ions enables the use of X-ray magnetic circular dichroism (XMCD) to test for the creation of transient spin polarization. Simultaneously, ultrafast X-ray diffraction (XRD) measures the circular ionic motion from which we determine the mechanical angular momentum. We find a classical ionic contribution of $3\times10^{-8}~μ_B$ from XRD and upper limits of $0.03~μ_B$ and $0.11~μ_B$ from the sensitivity of XMCD at the europium $M_5$ and $L_2$ edges, probing the $4f$ and $5d$ shells of Eu$^{2+}$, respectively. These observations show that large-amplitude circular ionic motion in this system is not accompanied by a detectable spin polarization despite the clear signature in the optical data, with the XMCD upper bounds and classical contribution differing by many orders of magnitude.

cond-mat.mtrl-sci↗

arXiv2026

Electric field switching of chiral phonons

Lattice vibrations carrying angular momentum, known as chiral phonons, have emerged as a promising route to control and understand complex material properties, yet their deterministic manipulation remains largely unexplored. Here we demonstrate electric-field switching of phonon angular momentum in the technologically relevant ferroelectric BaTiO3. Using circularly dichroic resonant inelastic X-ray scattering (CD-RIXS) at the oxygen K edge, we directly probe the phonon angular momentum and compare the measured dichroism with first-principles predictions of phonon-mode chirality. We find excellent agreement, revealing a momentum-dependent circular-dichroism contrast that exhibits a reversible gyroelectric effect, stable for at least 15 hours. Our results establish a robust mechanism for non-volatile control of chiral phonons and point towards new opportunities for phonon-based information and energy technologies.

cond-mat.mtrl-sci↗

arXiv2025

Visualizing a Terahertz Superfluid Plasmon in a Two-Dimensional Superconductor

The superconducting gap defines the fundamental energy scale for the emergence of dissipationless transport and collective phenomena in a superconductor. In layered high-temperature cuprate superconductors, where the Cooper pairs are confined to weakly coupled two-dimensional copper-oxygen planes, terahertz (THz) spectroscopy at sub-gap millielectronvolt energies has provided crucial insights into the collective superfluid response perpendicular to the superconducting layers. However, within the copper-oxygen planes the collective superfluid response manifests as plasmonic charge oscillations at energies far exceeding the superconducting gap, obscured by strong dissipation. Here, we present spectroscopic evidence of a below-gap, two-dimensional superfluid plasmon in few-layer Bi2Sr2CaCu2O8+x and spatially resolve its deeply sub-diffractive THz electrodynamics. By placing the superconductor in the near-field of a spintronic THz emitter, we reveal this distinct resonance-absent in bulk samples and observed only in the superconducting phase-and determine its plasmonic nature by mapping the geometric anisotropy and dispersion. Crucially, these measurements offer a direct view of the momentum- and frequency dependent superconducting transition in two dimensions. These results establish a new platform for investigating superfluid phenomena at finite momenta and THz frequencies, highlighting the potential to engineer and visualize superconducting devices operating at ultrafast THz rates.

cond-mat.supr-con↗

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↗

arXiv2021

Exciton-driven antiferromagnetic metal in a correlated van der Waals insulator

Collective excitations of bound electron-hole pairs -- known as excitons -- are ubiquitous in condensed matter, emerging in systems as diverse as band semiconductors, molecular crystals, and proteins. Recently, their existence in strongly correlated electron materials has attracted increasing interest due to the excitons' unique coupling to spin and orbital degrees of freedom. The non-equilibrium driving of such dressed quasiparticles offers a promising platform for realizing unconventional many-body phenomena and phases beyond thermodynamic equilibrium. Here, we achieve this in the van der Waals correlated insulator NiPS$_3$ by photoexciting its newly discovered spin-orbit-entangled excitons that arise from Zhang-Rice states. By monitoring the time evolution of the terahertz conductivity, we observe the coexistence of itinerant carriers produced by exciton dissociation and the long-wavelength antiferromagnetic magnon that coherently precesses in time. These results demonstrate the emergence of a transient metallic state that preserves long-range antiferromagnetism, a phase that cannot be reached by simply tuning the temperature. More broadly, our findings open an avenue toward the exciton-mediated optical manipulation of magnetism.

cond-mat.str-el↗