Time-periodic light fields can dress electronic states in quantum materials, forming Floquet states whose dynamic occupation determines the 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 MoSe2. 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 the light-field tailoring of transient material properties.

Floquet engineering provides an emerging pathway for tailoring the electronic states of quantum materials through time-periodic drive. A critical step along this direction is achieving light-induced modifications of the dynamical electronic structure, such as avoided-crossing gap at the Floquet Brillouin zone boundary, via efficient coupling of electrons with the coherent light-field. Here, we report robust Floquet-induced gap in bulk graphite that persists despite the presence of interlayer coupling and photo-excitation. Using time- and angle-resolved photoemission spectroscopy with intense mid-infrared pumping, we directly reveal Floquet-induced gaps at resonance points both in the valence and conduction bands, accompanied by coherent Floquet sidebands. The gap and sidebands coexist with photo-excited carriers, yet their distinct timescales allow us to disentangle their origins. Our demonstration of robust Floquet-induced gaps establishes graphite as a platform for coherent manipulation of Dirac fermions and realization of light-engineered quantum phases.

Challenges associated with discharging hydrogen gas from various storage systems for fuel cell usage.

Floquet engineering has emerged as a powerful approach for dynamically tailoring the electronic structures of quantum materials through time-periodic light fields. The light fields generated by ultrafast laser pulses can transiently dress Bloch electrons, creating novel electronic states inaccessible in equilibrium. While such temporal modulation provides a dynamic control, spatially periodic modulations, such as those arising from charge density wave (CDW) order, can also dramatically reconstruct the electronic structure through real-space symmetry breaking. The interplay between these two distinct forms of modulation-temporal and spatial-opens a frontier in phase-selective Floquet engineering. Here we demonstrate this concept experimentally in the prototypical CDW material 1T-TiSe2. Using time- and angle-resolved photoemission spectroscopy with mid-infrared pumping, we observe a striking momentum-dependent 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. 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 enabling the phase-selective Floquet engineering of TiSe2. Our work demonstrates how time-periodic drives can synergistically couple to spatially periodic modulations, establishing a paradigm for phase-selective Floquet engineering enabled by spontaneous symmetry breaking.

Rhombohedral graphene exhibits an exceptionally diverse array of correlated phases that depend sensitively on the displacement field. Compiling reported phases into a unified phase diagram reveals a pronounced field-dependent electron-hole asymmetry: correlated states on the hole-doped side emerge at small displacement fields, whereas the fractional quantum anomalous Hall effect (FQAHE) is observed exclusively on the electron-doped side under large displacement fields. This stark asymmetry highlights the need to understand how flat bands evolve with displacement fields. Here, we directly visualize the field-induced electron-hole asymmetric band flattening in rhombohedral pentalayer graphene (R5G) using nanospot angle-resolved photoemission spectroscopy with electrostatic gating. Beyond gap opening and spectral weight redistribution indicative of layer polarization, the gating field drives a strongly asymmetric modification of the flat bands: the flat valence band (FVB) evolves into an M-shaped dispersion at high field, whereas the flat conduction band (FCB) progressively flattens with increasing field. Comparison with calculations identifies critical parameters governing the band curvature of R5G, from which the resulting finite Berry curvature and near-ideal quantum geometry support the emergence of topological phases under electron doping at large fields. These results establish a direct link between the asymmetric phase diagram, band structure evolution, and quantum geometry, providing a microscopic framework for understanding correlated and topological phases in rhombohedral 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 the 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 the direct observation of a Floquet-induced hybridization gap in monolayer graphene under resonant driving by a strong light field. Time- and angle-resolved photoemission spectroscopy reveals a gap opening at Floquet band crossings, accompanied by coherent Floquet sidebands. The gap exhibits pronounced momentum anisotropy, featuring two Dirac nodes protected by spatiotemporal symmetry and tunable by light polarization. These results provide the long-sought experimental demonstration of Floquet band engineering in graphene, opening up opportunities for light-field-engineered quantum phases in graphene and related materials.

Twisted MoTe_2 hosts intriguing correlated quantum phenomena including the fractional quantum anomalous Hall effect in twisted bilayer (t-BL) MoTe_2 near 3.7^∘, which is sensitive to the twist angle and moiré superlattices. Here, we directly visualize the twist-angle-modulated electronic structure of t-BL and twisted double-bilayer (t-DBL) near this critical angle. We find that the moiré superlattice not only modifies the relative energy between Γ and K valleys in t-BL MoTe_2, but also strongly reconstructs the Γ valley for both t-BL and t-DBL. Specifically, the deep p_z-derived band at Γ exhibits a distinct splitting that systematically varies with increasing twist angle. Theoretical analysis suggests that this modulation arises from the twist-angle-dependent lattice relaxation, especially interfacial corrugations. Our work directly visualizes the moiré-modulated electronic structure and provides key spectroscopic information of lattice relaxation and interlayer interactions underlying the physics of twisted MoTe_2.

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.

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.

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 photoexcited states above the Fermi energy E_{F} can also be dressed, leaving the dynamical interplay between photoexcitation and light-field dressing elusive. Here, we provide direct experimental evidence for light-field dressing of the transient photoexcited 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 photoexcitation and light-field dressing, providing a more comprehensive picture with new insights on the light-induced manipulation of transient electronic states.

The fractional quantum anomalous Hall effect (FQAHE) is a fascinating emergent quantum state characterized by fractionally charged excitations in the absence of a magnetic field. Recently, the FQAHE has been observed in aligned rhombohedral pentalayer graphene on BN (aligned R5G/BN)1 with moiré potential. Intriguingly, the FQAHE preferably emerges when carriers are displaced away from the moiré interface1-3, raising debates about the role of moiré potential4-17. Here, by performing nanospot angle-resolved photoemission spectroscopy, we directly visualize the topological flat band in both aligned and non-aligned R5G/BN. The moiré potential in the aligned sample generates moiré bands and enhances the topological flat band as compared to non-aligned sample. Combined with theoretical calculations, we propose that the moiré bands on the top surface arise through the interlayer Coulomb interaction with the moiré-modulated bottom layer. Our results provide direct experimental evidence for the role of moiré potential in aligned rhombohedral graphene, and establish a foundation for understanding its emergent quantum phenomena.

Capturing ultrafast dynamics over a large momentum space is critical for revealing the relationship between the electronic and structural modulations in quantum materials. Here, by performing time- and angle-resolved photoemission spectroscopy measurements at the Synergetic Extreme Condition User Facility (SECUF) equipped with an extreme ultraviolet light source, we reveal the ultrafast dynamics of a charge-density wave (CDW) material, 1T-TiSe2, upon photoexcitation. Pump-induced CDW melting is revealed from two aspects: gap closing of the CDW at the Brillouin zone (BZ) center and weakening of the CDW folded band at the BZ boundary. By comparing the transient electronic structure and spectral weight over a large momentum space, we further reveal the carrier redistribution involving the excitation of electrons from the Gamma point to the M point. This study provides a comprehensive picture of the physics and ultrafast dynamics of a CDW material across the entire BZ.

Phosphorene has attracted extensive research interest due to its anisotropic properties with rich tunabilities such as strongly layer-dependent and field-tunable band gaps. Revealing how the electronic structure evolves down to monolayer is critical for understanding its anisotropic properties and field tunability. Here, we report the hitherto unexplored electronic structure of monolayer phosphorene, as well as field tunability in few-layer phosphorene. A strongly enhanced mass anisotropy is observed in monolayer phosphorene, which clearly distinguishes it from bilayer and thicker flakes. Moreover, by introducing an external electric field upon doping, modifications of the mass anisotropy and energy spacing between subbands in few-layer phosphorene are further revealed. Our work reveals the unique electronic structure of monolayer phosphorene, highlighting the critical roles of interlayer coupling as well as external electric field in tailoring the band parameters in phosphorene.

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.

A nonlinearly enhanced electrical reactance Im Z under a large AC current was measured to explore emergent inductors, which constitute a new class of inductors based on the spin-transfer torque effect. A nonlinear Im Z was observed in conducting magnets that contain noncollinear magnetic textures and interpreted as the realization of an inductance due to current-induced spin dynamics. However, curious behavior was concomitantly observed. For instance, the nonlinear Im Z always has a cutoff frequency of 100-104 Hz, which is much lower than the resonance frequency of a ferromagnetic domain wall, similar to 107 Hz. Furthermore, the magnitude of Im Z is much greater than that theoretically expected, and the temperature and magnetic field dependences are complicated. This behavior appears to be difficult to understand in terms of the current-induced spin dynamics, and therefore, the earlier interpretation of the nonlinear Im Z should be further verified. Here, we theoretically and experimentally show that time-varying Joule heating and its impact on the AC electrical response can naturally explain these observations. In the experimental approach, we investigate the nonlinear AC electrical response of two conducting materials that exhibit no magnetic order, CuIr2S4 and 1T '-MoTe2. Under time-varying Joule heating, a nonlinearly enhanced Im Z is observed in both systems, verifying the concept of the Joule-heating-induced AC electrical response. We reconsider the nonlinear emergent inductance reported thus far and discover that the Joule-heating-induced AC electrical response approximately reproduces the temperature and magnetic field dependences, cutoff frequency, and magnitude of Im Z. Our study implies that the nonlinear Im Z previously observed in conducting magnets that contain noncollinear magnetic textures includes a considerable contribution of the Joule-heating-induced apparent AC impedance.

We report here an expanded porphyrinoid, cyclo[2]pyridine[8]pyrrole, 1, that can exist at three closed-shell oxidation levels. Macrocycle 1 was synthesized via the oxidative coupling of two open chain precursors and fully characterized by means of NMR and UV-vis spectroscopies, MS, and X-ray crystallography. Reduction of the fully oxidized form (1, blue) with NaBH4 produced either the half-oxidized (2, teal) or fully reduced forms (3, pale yellow), depending on the amount of reducing agent used and the presence or absence of air. Reduced products 2 or 3 can be oxidized to 1 by various oxidants (quinones, FeCl3, and AgPF6). Macrocycle 1 also undergoes proton-coupled reductions with I-, Br-, Cl-, SO32-, or S2O32- in the presence of an acid. Certain thiol-containing compounds likewise reduce 1 to 2 or 3. This conversion is accompanied by a readily discernible color change, making cyclo[2]pyridine[8]pyrrole 1 able to differentiate biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH).

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 Γ point, suggesting a switch of the parity induced by the light field. Moreover, a "hot spot" with strong intensity confined near Γ 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.

Ultrashort laser pulses can be used to manipulate quantum materials and probe the ultrafast dynamics, where the pump photon energy can strongly affect the light-matter coupling mechanisms. Here, by pumping holedoped Bi2Te3 using a low photon energy covering 135-330 meV, we reveal different excitations and relaxation dynamics upon above-gap and below-gap pumping by time- and angle-resolved photoemission spectroscopy. The above-gap pumping mainly leads to the injection of photoexcited carriers into the bulk conduction band and unoccupied surface states, while below-gap pumping leads to multiple sets of light-field dressed sidebands. In addition, the excited surface states above the Fermi energy also exhibit distinctive relaxation dynamics due to different relaxation channels involving intervalley, interband, and intraband scatterings. Our work reveals the essential role of pump photon energy on the light-matter coupling mechanisms, evolving from above-gap carrier injection to below-gap light-field dressing with distinct relaxation dynamics.

N-doped carbon-coated SnP2O7 anodes anchored on P-doped carbon framework (SPO/C-P@C-N) have been successfully synthesized via a novel and environmentally friendly method, using a green raw material of phytic acid as phosphorus and carbon sources, and dimethylimidazole as the second carbon source and nitrogen source. N-doped carbon coating and P-doped carbon framework inhibit the volume shrinkage and expansion, improve the electronic conductivity and diffusion coefficient of Li+ ions as well as capacitive contribution ratio, reduce the charge transfer resistance and the particle size, and then enhance the electrochemical performance of SPO/CP@C-N. SPO/C-P@C-N-1.2 with the carbon content of 18.5 wt% exhibits excellent electrochemical performance. The discharge specific capacity is 416.6 mAh/g at 0.5 A/g after 200 cycles. 234.3 mAh/g is kept at 2 A/g for the 1000th cycle. In addition, the discharge specific capacity of the LiNi0.5Mn1.5O4//SPO/C-P@C-N-1.2 full cell is 206.6 mAh/g for the 200th cycle at 0.5C in 1.4-4.4 V. The full cell can power green, red, blue, and orange light emitting diode (LED) bulbs, as well as LED small light strings and heart-shaped circuit boards. It is the first report on SnP2O7 successfully applied to full cells, demonstrating the potential application.