arXiv subjects

Tianyun Lin

Publications and source records attributed to Tianyun Lin.

14 recordsLinked to original sources

arXiv2026

Occupation Dynamics of Floquet-Volkov States and Spectral Sum Rule

Time-periodic light fields can dress electronic states in quantum materials, forming Floquet states whose dynamic occupation determines transient material properties. Here by using time- and angle-resolved photoemission spectroscopy (TrARPES), we reveal the transient occupation of Floquet-Volkov states in two semiconductors, black phosphorus and MoSe$_2$. While the occupation of the light-induced sidebands, directly reflected by TrARPES spectral weight, strongly depends on the driving field, we find that the total spectral weight obtained by summing up all sidebands is conserved upon below-gap driving. Our work provides critical insights into the Floquet population dynamics, which are essential for light-field tailoring of transient material properties.

cond-mat.mtrl-sci↗

arXiv2025

Floquet engineering enabled by charge density wave transition

Floquet engineering has emerged as a powerful approach for dynamically tailoring the electronic structures of quantum materials through time-periodic light fields generated by ultrafast laser pulses. The light fields can transiently dress Bloch electrons, creating novel electronic states inaccessible in equilibrium. While such temporal modulation provides dynamic control, spatially periodic modulations, such as those arising from charge density wave (CDW) order, can also dramatically reconstruct the band structure through real-space symmetry breaking. The interplay between these two distinct forms of modulation-temporal and spatial-opens a new frontier in electronic-phase-dependent Floquet engineering. Here we demonstrate this concept experimentally in the prototypical CDW material 1T-TiSe$_2$. Using time- and angle-resolved photoemission spectroscopy (TrARPES) with mid-infrared pumping, we observe a striking pump-induced instantaneous downshift of the valence band maximum (VBM), which is in sharp contrast to the subsequent upward shift on picosecond timescale associated with CDW melting. Most remarkably, the light-induced VBM downshift is observed exclusively in the CDW phase and only when the pump pulse is present, reaching maximum when pumping near resonance with the CDW gap. These observations unequivocally reveal the critical role of CDW in the Floquet engineering of TiSe$_2$. Our work demonstrates how time-periodic drives can synergistically couple to spatially periodic modulations to create non-equilibrium electronic states, establishing a new paradigm for Floquet engineering enabled by spontaneous symmetry breaking.

cond-mat.mtrl-sci↗

arXiv2025

High harmonic generation light source with polarization selectivity and sub-100-$\mu$m beam size for time- and angle-resolved photoemission spectroscopy

High-quality ultrafast light sources are critical for developing advanced time- and angle-resolved photoemission spectroscopy (TrARPES). While the application of high harmonic generation (HHG) light sources in TrARPES has increased significantly over the past decade, the optimization of the HHG probe beam size and selective control of the light polarization, which are important for TrARPES measurements, have been rarely explored. In this work, we report the implementation of high-quality HHG probe source with an optimum beam size down to 57 $\mu$m $\times$ 90 $\mu$m and selective light polarization control, together with mid-infrared (MIR) pumping source for TrARPES measurements using a 10 kHz amplifier laser. The selective polarization control of the HHG probe source allows to enhance bands with different orbital contributions or symmetries, as demonstrated by experimental data measured on a few representative transition metal dichalcogenide materials (TMDCs) as well as topological insulator Bi$_2$Se$_3$. Furthermore, by combining the HHG probe source with MIR pumping at 2 $\mu$m wavelength, TrARPES on a bilayer graphene shows a time resolution of 140 fs, allowing to distinguish two different relaxation processes in graphene. Such high-quality HHG probe source together with the MIR pumping expands the capability of TrARPES in revealing the ultrafast dynamics and light-induced emerging phenomena in quantum materials.

cond-mat.mes-hall↗

arXiv2025

Light-field dressing of transient photo-excited states above $E_F$

Time-periodic light-field provides an emerging pathway for dynamically engineering quantum materials by forming hybrid states between photons and Bloch electrons. So far, experimental progress on light-field dressed states has been mainly focused on the occupied states, however, it is unclear if the transient photo-excited states above the Fermi energy $E_F$ can also be dressed, leaving the dynamical interplay between photo-excitation and light-field dressing elusive. Here, we provide direct experimental evidence for light-field dressing of the transient photo-excited surface states above $E_F$, which exhibits distinct dynamics with a delay response as compared to light-field dressed states below $E_F$. Our work reveals the dual roles of the pump pulse in both photo-excitation and light-field dressing, providing a more comprehensive picture with new insights on the light-induced manipulation of transient electronic states.

cond-mat.mtrl-sci↗

arXiv2025

Floquet-Volkov interference in a semiconductor

Intense light-field can dress both Bloch electrons inside crystals and photo-emitted free electrons in the vacuum, dubbed as Floquet and Volkov states respectively. These quantum states can further interfere coherently, modulating light-field dressed states. Here, we report experimental evidence of the Floquet-Volkov interference in a semiconductor - black phosphorus. A highly asymmetric modulation of the spectral weight is observed for the Floquet-Volkov states, and such asymmetry can be further controlled by rotating the pump polarization. Our work reveals the quantum interference between different light-field dressed electronic states, providing insights for material engineering on the ultrafast timescale.

cond-mat.mes-hall↗

arXiv2024

Light-induced ultrafast glide-mirror symmetry breaking in black phosphorus

Symmetry breaking plays an important role in fields of physics, ranging from particle physics to condensed matter physics. In solid-state materials, phase transitions are deeply linked to the underlying symmetry breakings, resulting in a rich variety of emergent phases. Such symmetry breakings are often induced by controlling the chemical composition and temperature or applying an electric field and strain, etc. In this work, we demonstrate an ultrafast glide-mirror symmetry breaking in black phosphorus through Floquet engineering. Upon near-resonance pumping, a light-induced full gap opening is observed at the glide-mirror symmetry protected nodal ring, suggesting light-induced breaking of the glide-mirror symmetry. Moreover, the full gap is observed only in the presence of the light-field and disappears almost instantaneously ($\ll$100 fs) when the light-field is turned off, suggesting the ultrafast manipulation of the symmetry and its Floquet engineering origin. This work not only demonstrates light-matter interaction as an effective way to realize ultrafast symmetry breaking in solid-state materials, but also moves forward towards the long-sought Floquet topological phases.

cond-mat.mes-hall↗

arXiv2024

Manipulating the symmetry of photon-dressed electronic states

Strong light-matter interaction provides opportunities for tailoring the physical properties of quantum materials on the ultrafast timescale by forming photon-dressed electronic states, i.e., Floquet-Bloch states. While the light field can in principle imprint its symmetry properties onto the photon-dressed electronic states, so far, how to experimentally detect and further engineer the symmetry of photon-dressed electronic states remains elusive. Here by utilizing time- and angle-resolved photoemission spectroscopy (TrARPES) with polarization-dependent study, we directly visualize the parity symmetry of Floquet-Bloch states in black phosphorus. The photon-dressed sideband exhibits opposite photoemission intensity to the valence band at the $\Gamma$ point,suggesting a switch of the parity induced by the light field. Moreover, a "hot spot" with strong intensity confined near $\Gamma$ is observed, indicating a momentum-dependent modulation beyond the parity switch. Combining with theoretical calculations, we reveal the light-induced engineering of the wave function of the Floquet-Bloch states as a result of the hybridization between the conduction and valence bands with opposite parities, and show that the "hot spot" is intrinsically dictated by the symmetry properties of black phosphorus. Our work suggests TrARPES as a direct probe for the parity of the photon-dressed electronic states with energy- and momentum-resolved information, providing an example for engineering the wave function and symmetry of such photon-dressed electronic states via Floquet engineering.

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↗

arXiv2024

Hidden charge density wave induced shadow bands and ultrafast dynamics of CuTe investigated using time-resolved ARPES

Revealing the fine electronic structure is critical for understanding the underlying physics of low-dimensional materials. Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental technique for mapping out the experimental electronic structure. By reducing the photon energy (e.g. to 6 eV) using laser sources, a greatly improved momentum resolution can be achieved, thereby providing opportunities for ``zooming in'' the fine electronic structure and even revealing the previously unresolvable bands near the Brillouin zone center. Here, by using quasi-one-dimensional material CuTe as an example, we demonstrate the unique capability of laser-based ARPES in revealing the fine electronic structures of ``hidden'' charge density wave induced shadow bands near the Brillouin zone center, which are previously unresolvable using synchrotron sources. The observation of the shadow bands reveals the CDW phase from the aspect of band folding, and the unpredicted CDW band hybridization strongly modifies the electronic structure and Fermi surface, which suggests that such hybridization must be taken into account for studying the CDW transition. Moreover, the ultrafast non-equilibrium carrier dynamics are captured by time-resolved ARPES, revealing the relaxation dynamics through electron-phonon scattering. Our work demonstrates the advantages of laser-based ARPES in zooming in the fine electronic structures, as well as capturing the ultrafast dynamics of low-dimensional materials.

cond-mat.mes-hall↗

arXiv2024

Evolution of flat band and role of lattice relaxations in twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) exhibits correlated phenomena such as superconductivity and Mott insulating state related to the weakly dispersing flat band near the Fermi energy. Beyond its moir\'e period, such flat band is expected to be sensitive to lattice relaxations. Thus, clarifying the evolution of the electronic structure with twist angle is critical for understanding the physics of MATBG. Here, we combine nanospot angle-resolved photoemission spectroscopy and atomic force microscopy to resolve the fine electronic structure of the flat band and remote bands, and their evolution with twist angles from 1.07$^\circ$ to 2.60$^\circ$. Near the magic angle, dispersion is characterized by a flat band near the Fermi energy with a strongly reduced bandwidth. Moreover, near 1.07$^\circ$, we observe a spectral weight transfer between remote bands at higher binding energy and extract the modulated interlayer spacing near the magic angle. Our work provides direct spectroscopic information on flat band physics and highlights the role of lattice relaxations.

cond-mat.mes-hall↗

arXiv2023

Distinguishing and controlling Mottness in 1T-TaS$_2$ by ultrafast light

Distinguishing and controlling the extent of Mottness is important for materials where the energy scales of the onsite Coulomb repulsion U and the bandwidth W are comparable. Here we report the ultrafast electronic dynamics of 1T-TaS$_2$ by ultrafast time- and angle-resolved photoemission spectroscopy. A comparison of the electron dynamics for the newly-discovered intermediate phase (I-phase) as well as the low-temperature commensurate charge density wave (C-CDW) phase shows distinctive dynamics. While the I-phase is characterized by an instantaneous response and nearly time-resolution-limited fast relaxation (~200 fs), the C-CDW phase shows a delayed response and a slower relaxation (a few ps). Such distinctive dynamics refect the different relaxation mechanisms and provide nonequilibrium signatures to distinguish the Mott insulating I-phase from the C-CDW band insulating phase. Moreover, a light-induced bandwidth reduction is observed in the C-CDW phase, pushing it toward the Mott insulating phase. Our work demonstrates the power of ultrafast light-matter interaction in both distinguishing and controlling the extent of Mottness on the ultrafast timescale.

cond-mat.mes-hall↗

arXiv2023

Pseudospin-selective Floquet band engineering in black phosphorus

Time-periodic light field has emerged as a control knob for manipulating quantum states in solid-state materials, cold atoms and photonic systems via hybridization with photon-dressed Floquet states in the strong coupling limit, dubbed as Floquet engineering. Such interaction leads to tailored properties of quantum materials, for example, modifications of the topological properties of Dirac materials and modulation of the optical response. Despite extensive research interests over the past decade, there is no experimental evidence of momentum-resolved Floquet band engineering of semiconductors, which is a crucial step to extend Floquet engineering to a wide range of solid-state materials. Here, based on time- and angle-resolved photoemission spectroscopy measurements, we report experimental signatures of Floquet band engineering in a model semiconductor - black phosphorus. Upon near-resonance pumping at photon energy of 340 to 440 meV, a strong band renormalization is observed near the band edges. In particular, light-induced dynamical gap opening is resolved at the resonance points, which emerges simultaneously with the Floquet sidebands. Moreover, the band renormalization shows a strong selection rule favoring pump polarization along the armchair direction, suggesting pseudospin selectivity for the Floquet band engineering as enforced by the lattice symmetry. Our work demonstrates pseudospin-selective Floquet band engineering in black phosphorus, and provides important guiding principles for Floquet engineering of semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

A newly-designed femtosecond KBe$_2$BO$_3$F$_2$ device with pulse duration down to 55 fs for time- and angle-resolved photoemission spectroscopy

Developing a widely tunable vacuum ultraviolet (VUV) source with sub-100 femtoseconds (fs) pulse duration is critical for ultrafast pump-probe techniques such as time- and angle-resolved photoemission spectroscopy (TrARPES). While a tunable probe source with photon energy of 5.3 - 7.0 eV has been recently implemented for TrARPES by using a KBe$_2$BO$_3$F$_2$ (KBBF) device, the time resolution of 280 - 320 fs is still not ideal, which is mainly limited by the duration of the VUV probe pulse generated by the KBBF device. Here, by designing a new KBBF device which is specially optimized for fs applications, an optimum pulse duration of 55 fs is obtained after systematic diagnostics and optimization. More importantly, a high time resolution of 81 - 95 fs is achieved for TrARPES measurements covering the probe photon energy range of 5.3 - 7.0 eV, making it particularly useful for investigating the ultrafast dynamics of quantum materials. Our work extends the application of KBBF device to ultrafast pump-probe techniques with the advantages of both widely tunable VUV source and ultimate time resolution.

cond-mat.mes-hall↗

arXiv2022

Quasiparticle characteristics of the weakly ferromagnetic Hund's metal MnSi

Hund's metals are multi-orbital systems with $3d$ or $4d$ electrons exhibiting both itinerant character and local moments, and they feature Kondo-like screenings of local orbital and spin moments, with suppressed coherence temperature driven by Hund's coupling $J_H$. They often exhibit magnetic order at low temperature, but how the interaction between the Kondo-like screening and long-range magnetic order is manifested in the quasiparticle spectrum remains an open question. Here we present spectroscopic signature of such interaction in a Hund's metal candidate MnSi exhibiting weak ferromagnetism. Our photoemission measurements reveal renormalized quasiparticle bands near the Fermi level with strong momentum dependence: the ferromagnetism manifests through possibly exchange-split bands (Q1) below $T_C$ , while the spin/orbital screenings lead to gradual development of quasiparticles (Q2) upon cooling. Our results demonstrate how the characteristic spin/orbital coherence in a Hund's metal could coexist and compete with the magnetic order to form a weak itinerant ferromagnet, via quasiparticle bands that are well separated in momentum space and exhibit distinct temperature dependence. Our results imply that the competition between the spin/orbital screening and the magnetic order in a Hund's metal bears intriguing similarity to the Kondo lattice systems.

cond-mat.str-el↗