arXiv subjects

Runfa Feng

Publications and source records attributed to Runfa Feng.

3 recordsLinked to original sources

arXiv2026

Observation of Floquet-induced gap in graphene

Floquet engineering provides a powerful pathway for creating non-equilibrium phases of matter with tailored electronic structures and properties through time-periodic driving. As the original theoretical prototype, graphene established the framework in which the Floquet topological insulator with light-induced anomalous Hall effect was proposed. However, the defining spectroscopic signature of Floquet engineering in graphene--light-induced hybridization (avoided-crossing) gap at Floquet band crossings, has remained experimentally elusive. Here, we report direct observation of Floquet-induced hybridization gap in monolayer graphene under resonant driving by a strong light field. Time- and angle-resolved photoemission spectroscopy reveals gap opening at Floquet band crossings, accompanied by coherent Floquet sidebands. The gap exhibits pronounced momentum anisotropy, featuring two Dirac nodes protected by the spatiotemporal symmetry and tunable by light polarization. These results provide long-sought experimental demonstration of Floquet band engineering in graphene, opening up opportunities for light-field engineered quantum phases in graphene and related materials.

cond-mat.mes-hall↗

arXiv2025

Giant Rashba splitting in PtTe/PtTe$_2$ heterostructure

Achieving a large spin splitting is highly desirable for spintronic devices, which often requires breaking of the inversion symmetry. However, many atomically thin films are centrosymmetric, making them unsuitable for spintronic applications. Here, we report a strategy to achieve inversion symmetry breaking from a centrosymmetric transition metal dichalcogenide (TMDC) bilayer PtTe$_2$, leading to a giant Rashba spin splitting. Specifically, the thermal annealing turns one layer of PtTe$_2$ sample into a transition metal monochalcogenide (TMMC) PtTe through Te extraction, thus forming PtTe/PtTe$_2$ heterostructure with inversion symmetry breaking. In the naturally-formed PtTe/PtTe$_2$ heterostructure, we observe a giant Rashba spin splitting with Rashba coefficient of $\alpha_{R}$ = 1.8 eV$\cdot$$\AA$, as revealed by spin- and angle-resolved photoemission spectroscopy measurements. Our work demonstrates a convenient and effective pathway for achieving pronounced Rashba splitting in centrosymmetric TMDC thin films by creating TMMC/TMDC heterostructure, thereby extending their potential applications to spintronics.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast time- and angle-resolved photoemission spectroscopy with widely tunable probe photon energy of 5.3-7.0 eV for investigating dynamics of three-dimensional materials

Time- and angle-resolved photoemission spectroscopy (TrARPES) is a powerful technique for capturing the ultrafast dynamics of charge carriers and revealing photo-induced phase transitions in quantum materials. However, the lack of widely tunable probe photon energy, which is critical for accessing the dispersions at different out-of-plane momentum $k_z$ in TrARPES measurements, has hindered the ultrafast dynamics investigation of 3D quantum materials such as Dirac or Weyl semimetals. Here we report the development of a TrARPES system with a highly tunable probe photon energy from 5.3 to 7.0 eV. The tunable probe photon energy is generated by the fourth harmonic generation of a tunable wavelength femtosecond laser source by combining a $\beta$-BaB$_2$O$_4$ (BBO) crystal and a KBe$_2$BO$_3$F$_2$ (KBBF) crystal. High energy resolution of 29 - 48 meV and time resolution of 280 - 320 fs are demonstrated on 3D topological materials ZrTe$_5$ and Sb$_2$Te$_3$. Our work opens up new opportunities for exploring ultrafast dynamics in 3D quantum materials.

cond-mat.mes-hall↗