arXiv subjects

Yongkang Ju

Publications and source records attributed to Yongkang Ju.

5 recordsLinked to original sources

arXiv2026

Coherent seeding and control of dynamical ferroelectricity by phonon anharmonicity

Optical control of quantum materials has progressed along two separate directions: creating non-equilibrium states inaccessible at equilibrium, and coherently controlling ultrafast dynamics with multi-pulse protocols. Ferroelectricity is especially attractive in this context because its order parameter, macroscopic polarization, directly links inversion-symmetry breaking to functional response. Yet light-induced ferroelectricity has so far been confined to quantum paraelectrics near the ferroelectric instability, where critical fluctuations obscure the formation of a homogeneous ferroelectric state and complicate its deterministic coherent control. Unifying these capabilities -- preparing a symmetry-broken state and then coherently steering its functionality -- remains a central challenge. Here we show that intense terahertz excitation of a soft phonon mode induces a ferroelectric state in centrosymmetric PbTe, a thermoelectric material with strong lattice anharmonicity but no ferroelectric transition at finite temperature. The light-induced symmetry-broken state can be realized up to about 100 K, without relying on local dipolar fluctuations. Experiment and theory together reveal that terahertz-driven anharmonic coupling between degenerate transverse optical phonons underlies this ferroelectric induction. Furthermore, we demonstrate coherent amplification and suppression of the induced polarization via a double-pulse-excitation protocol. These results establish terahertz-driven anharmonic mode coupling as a general strategy for controlling mode-mediated functionalities in quantum materials, opening a route to ultrafast information processing.

cond-mat.mtrl-sci↗

arXiv2025

Lattice-decoupled rotatable stripe-like charge order within the strange metal phase of 2M-WS2

In quantum materials, charge orders typically stabilize in specific crystallographic orientations, though their formation mechanisms may vary. Here, using low-temperature scanning tunneling microscopy (STM), we discover a lattice-decoupled rotatable stripe-like charge order coexisting with superconductivity in 2M-WS2. The charge order manifests five distinct orientations across different sample regions, yet maintains an identical wavelength. This directional decoupling from host lattice challenges existing paradigms. First-principles calculations of phonon spectra and nesting function fail to explain the ordering mechanism. Intriguingly, the transition temperature of the charge orders exhibits spatial variations (21-46 K), coinciding with the temperature range of the recently reported strange metal phase in this material. This correlation suggests that the interplay between strong electronic correlations and electron-phonon coupling must be critically evaluated to elucidate the emergence of this unconventional charge order.

cond-mat.supr-con↗

arXiv2025

Strain and twist angle driven electronic structure evolution in twisted bilayer graphene

In twisted bilayer graphene (TBG) devices, local strains frequently coexist and intertwine with the twist-angle-dependent moiré superlattice, significantly influencing the electronic properties of TBG, yet their combined effects remain incompletely understood. Here, using low-temperature scanning tunneling microscopy, we study a TBG device exhibiting both a continuous twist-angle gradient from 0.35° to 1.30° and spatially varying strain fields, spanning the first (1.1°), second (0.5°) and third (0.3°) magic angles. We visualize the evolution of flat and remote bands in energy and real space with atomic resolution. Near the first magic angle, we discover an anomalous spectral weight transfer between the two flat band peaks, signifying the role of strain and electronic correlations, as further evidenced by an unusual spatial dispersion of these peaks within a moiré unit cell. In contrast, remote band peak energy offers a strain-insensitive indicator of the local twist angle. Structural analysis further reveals non-negligible shear strain across the sample. All observations are quantitatively reproduced by a continuum model that incorporates heterostrain and a self-consistent Hartree potential, revealing the critical but unexplored role of shear strain in shaping the low-energy electronic landscape of TBG.

cond-mat.mes-hall↗

arXiv2024

Ultrafast Carrier Relaxation Dynamics in a Nodal-Line Semimetal PtSn$_4$

Topological Dirac nodal-line semimetals host topologically nontrivial electronic structure with nodal-line crossings around the Fermi level, which could affect the photocarrier dynamics and lead to novel relaxation mechanisms. Herein, by using time- and angle-resolved photoemission spectroscopy, we reveal the previously-inaccessible linear dispersions of the bulk conduction bands above the Fermi level in a Dirac nodal-line semimetal PtSn$_4$, as well as the momentum and temporal evolution of the gapless nodal lines. A surprisingly ultrafast relaxation dynamics within a few hundred femtoseconds is revealed for photoexcited carriers in the nodal line. Theoretical calculations suggest that such ultrafast carrier relaxation is attributed to the multichannel scatterings among the complex metallic bands of PtSn$_4$ via electron-phonon coupling. In addition, a unique dynamic relaxation mechanism contributed by the highly anisotropic Dirac nodal-line electronic structure is also identified. Our work provides a comprehensive understanding of the ultrafast carrier dynamics in a Dirac nodal-line semimetal.

cond-mat.mes-hall↗

arXiv2023

Stripe charge order driven manipulation of Majorana bound states in 2M-WS2 topological superconductor

Majorana bound states (MBSs) are building blocks for topological quantum computing. They can be generated via the combination of electronic topology and superconductivity. To achieve logic operations via Majorana braiding, positional control of the MBS must be established. To this end, exotic co-existing phases or collective modes in an intrinsic topological superconductor can provide a tuning knob to manipulate the MBS. Here we report the observation of a striped surface charge order coexisting with superconductivity and its controllable tuning of the MBS in the topological superconductor 2M-WS2 using low-temperature scanning tunneling microscopy. By applying an out-of-plane magnetic field, we observe that MBS is absent in vortices in the region with strong stripe order. This is in contrast to adjacent underlaying layers without charge order where vortex-bound MBSs are observed. Via theoretical simulations, we show that the surface stripe order does not destroy the bulk topology, but it can effectively modify the spatial distribution of MBS, i.e., it pushes them downward away from the 2M-WS2 surface. Our findings demonstrate that the interplay of charge order and topological superconductivity can be used to manipulate the positions of the MBS, and to explore of new states of matter.

cond-mat.supr-con↗