arXiv subjects

O. Mehio

Publications and source records attributed to O. Mehio.

5 recordsLinked to original sources

arXiv2023

A coherent phonon-induced hidden quadrupolar ordered state in Ca$_2$RuO$_4$

Ultrafast laser excitation provides a means to transiently realize long-range ordered electronic states of matter that are hidden in thermal equilibrium. Recently, this approach has unveiled a variety of thermally inaccessible ordered states in strongly correlated materials, including charge density wave, ferroelectric, magnetic, and intertwined charge-orbital ordered states. However, more exotic hidden states exhibiting higher multipolar ordering remain elusive owing to the challenge of directly manipulating and detecting them with light. Here we demonstrate a method to induce a dynamical transition from a thermally allowed to a thermally forbidden spin-orbit entangled quadrupolar ordered state in Ca$_2$RuO$_4$ by coherently exciting a phonon that is strongly coupled to the order parameter. Combining probe photon energy-resolved coherent phonon spectroscopy measurements with model Hamiltonian calculations, we show that the dynamical transition is manifested through anomalies in the temperature, pump excitation fluence, and probe photon energy dependence of the strongly coupled phonon. With this procedure, we introduce a general pathway to uncover hidden multipolar ordered states and to control their re-orientation on ultrashort timescales.

cond-mat.str-el↗

arXiv2022

A light-induced Weyl semiconductor-to-metal transition mediated by Peierls instability

Elemental tellurium is a strongly spin-orbit coupled Peierls-distorted semiconductor whose band structure features topologically protected Weyl nodes. Using time-dependent density functional theory calculations, we show that impulsive optical excitation can be used to transiently control the amplitude of the Peierls distortion, realizing a mechanism to switch tellurium between three states: Weyl semiconductor, Weyl metal and non-Weyl metal. Further, we present experimental evidence of this inverse-Peierls distortion using time-resolved optical second harmonic generation measurements. These results provide a pathway to multifunctional ultrafast Weyl devices and introduce Peierls systems as viable hosts of light-induced topological transitions.

cond-mat.str-el↗

arXiv2020

Signatures of ultrafast reversal of excitonic order in Ta$_2$NiSe$_5$

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta$_2$NiSe$_5$ and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta$_2$NiSe$_5$ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our work expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

cond-mat.str-el↗

arXiv2018

Magnetic-Field Tuning of Light-Induced Superconductivity in Striped La$_{2-x}$Ba$_x$CuO$_4$

Optical excitation of stripe-ordered La$_{2-x}$Ba$_x$CuO$_4$ has been shown to transiently enhance superconducting tunneling between the CuO$_2$ planes. This effect was revealed by a blue-shift, or by the appearance of a Josephson Plasma Resonance in the terahertz-frequency optical properties. Here, we show that this photo-induced state can be strengthened by the application of high external magnetic fields oriented along the c-axis. For a 7-Tesla field, we observe up to a ten-fold enhancement in the transient interlayer phase correlation length, accompanied by a two-fold increase in the relaxation time of the photo-induced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically-enhanced interlayer coupling in La$_{2-x}$Ba$_x$CuO$_4$ does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photo-induced state may emerge from activated tunneling between optically-excited stripes in adjacent planes.

cond-mat.supr-con↗

arXiv2017

Hybridization gap in the heavy-fermion compound UPd$_2$Al$_3$ via quasiparticle scattering spectroscopy

We present results from point-contact spectroscopy of the antiferromagnetic heavy-fermion superconductor UPd$_2$Al$_3$: conductance spectra are taken from single crystals with two major surface orientations as a function of temperature and magnetic field, and analyzed using a theory of co-tunneling into an Anderson lattice. Spectroscopic signatures are clearly identified including the distinct asymmetric double-peak structure arising from the opening of a hybridization gap when a coherent heavy Fermi liquid is formed. Both the hybridization gap, found to be 7.2 $\pm$ 0.3 meV at 4 K, and the conductance enhancement above a flat background decrease upon increasing temperature. While the hybridization gap is extrapolated to remain finite up to $\sim$28 K, close to the temperature around which the magnetic susceptibility displays a broad peak, the conductance enhancement vanishes at $\sim$18 K, slightly above the antiferromagnetic transition temperature ($T_\textrm{N}$ $\approx$ 14 K). This rapid decrease of the conductance enhancement is understood as a consequence of the junction drifting away from the ballistic regime due to increased scattering off magnons associated with the localized U 5$f$ electrons. This shows that while the hybridization gap opening is not directly associated with the antiferromagnetic ordering, its visibility in the conductance is greatly affected by the temperature-dependent magnetic excitations. Our findings are not only consistent with the 5$f$ dual-nature picture in the literature but also shed new light on the interplay between the itinerant and localized electrons in UPd$_2$Al$_3$.

cond-mat.str-el↗