Ultrafast Carrier Relaxation Dynamics in a Nodal-Line Semimetal PtSn4.

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 PtSn4, 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 PtSn4 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.

Association Between Park Use and Moderate-to-Vigorous Activity During COVID-19 Years among a Cohort of Low-Income Youth.

Neighborhood parks are important venues to support moderate-to-vigorous (MVPA) activity. There has been a noticeable increase promoting physical activity among youth in neighborhood parks. This paper aims to assess the association between park use and MVPA among low-income youth in a large urban area. We recruited a cohort of 434 youth participants during the COVID pandemic years (2020-2022) from low-income households in Washington, D.C. We collected multiple data components: accelerometry, survey, and electronic health record data. We explored the bivariate relationship between the accelerometer-measured daily MVPA time outcome and survey-based park use measures. A mixed-effect model was fitted to adjust the effect estimate for participant-level and time-varying confounders. The overall average daily MVPA time is 16.0 min (SD = 12.7). The unadjusted bivariate relation between daily MVPA time and frequency of park visit is 1.3 min of daily MVPA time per one day with park visits (p < 0.0001). The model-adjusted estimate is 0.7 daily MVPA minutes for 1 day with park visit (p = 0.04). The duration of a typical park visit is not a significant predictor to daily MVPA time with or without adjustments. The initial COVID outbreak in 2020 resulted in a significant decline in daily MVPA time (- 4.7 min for 2020 versus 2022, p < 0.0001). Park visit frequency is a significant predictor to low-income youth's daily MVPA time with considerable absolute effect sizes compared with other barriers and facilitators. Promoting more frequent park use may be a useful means to improve low-income youth's MVPA outcome.

Floquet Engineering of Black Phosphorus upon Below-Gap Pumping.

Time-periodic light field can dress the electronic states and lead to light-induced emergent properties in quantum materials. While below-gap pumping is regarded favorable for Floquet engineering, so far direct experimental evidence of momentum-resolved band renormalization still remains missing. Here, we report experimental evidence of light-induced band renormalization in black phosphorus by pumping at photon energy of 160 meV, which is far below the band gap, and the distinction between below-gap pumping and near-resonance pumping is revealed. Our Letter demonstrates light-induced band engineering upon below-gap pumping, and provides insights for extending Floquet engineering to more quantum materials.

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 materials1-3, cold atoms4 and photonic systems5 through hybridization with photon-dressed Floquet states6 in the strong-coupling limit, dubbed Floquet engineering. Such interaction leads to tailored properties of quantum materials7-11, for example, modifications of the topological properties of Dirac materials12,13 and modulation of the optical response14-16. Despite extensive research interests over the past decade3,8,17-20, 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, on the basis of time and angle-resolved photoemission spectroscopy measurements, we report experimental signatures of Floquet band engineering in a model semiconductor, black phosphorus. On near-resonance pumping at a photon energy of 340-440 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 favouring pump polarization along the armchair direction, suggesting pseudospin selectivity for the Floquetband 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.

A newly designed femtosecond KBe2BO3F2 device with pulse duration down to 55 fs for time- and angle-resolved photoemission spectroscopy.

Developing a widely tunable vacuum ultraviolet (VUV) source with a sub-100 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 a photon energy of 5.3-7.0 eV has been recently implemented for TrARPES by using a KBe2BO3F2 (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 the KBBF device to ultrafast pump-probe techniques with the advantages of both a widely tunable VUV source and ultimate time resolution.

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 kz 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 ß-BaB2O4 crystal and a KBe2BO3F2 crystal. A high energy resolution of 29-48 meV and time resolution of 280-320 fs are demonstrated on 3D topological materials ZrTe5 and Sb2Te3. Our work opens up new opportunities for exploring ultrafast dynamics in 3D quantum materials.

Population Inversion and Dirac Fermion Cooling in 3D Dirac Semimetal Cd3As2.

Revealing the ultrafast dynamics of three-dimensional (3D) Dirac fermions is critical for both fundamental science and device applications. So far, how the cooling of 3D Dirac fermions differs from that of two-dimensional (2D) and whether there is population inversion are fundamental questions to be answered. Here we reveal the ultrafast dynamics of Dirac fermions in a model 3D Dirac semimetal Cd3As2 by time- and angle-resolved photoemission spectroscopy with a tunable probe photon energy. The energy- and momentum-resolved relaxation rate shows a linear dependence on the energy, suggesting Dirac fermion cooling through intraband relaxation. Moreover, a population inversion is reported based on the observation of accumulated photoexcited carriers in the conduction band with a lifetime of 3.0 ps. Our work provides direct experimental evidence for a long-lived population inversion in a 3D Dirac semimetal, which is in contrast to 2D graphene with a much shorter lifetime.

Light-Tunable Surface State and Hybridization Gap in Magnetic Topological Insulator MnBi8Te13.

MnBi8Te13 is an intrinsic ferromagnetic (FM) topological insulator with different complex surface terminations. Resolving the electronic structures of different termination surfaces and manipulation of the electronic state are important. Here, by using micrometer spot time- and angle-resolved photoemission spectroscopy (µ-TrARPES), we resolve the electronic structures and reveal the ultrafast dynamics upon photoexcitation. Photoinduced filling of the surface state hybridization gap is observed for the Bi2Te3 quintuple layer directly above MnBi2Te4 accompanied by a nontrivial shift of the surface state, suggesting light-tunable interlayer interaction. Relaxation of photoexcited electrons and holes is observed within 1-2 ps. Our work reveals photoexcitation as a potential control knob for tailoring the interlayer interaction and surface state of MnBi8Te13.

Complete convergence and complete moment convergence for weighted sums of extended negatively dependent random variables under sub-linear expectation.

In this paper, we study the complete convergence and complete moment convergence for weighted sums of extended negatively dependent (END) random variables under sub-linear expectations space with the condition of [Formula: see text], further [Formula: see text], [Formula: see text] ([Formula: see text] is a slow varying and monotone nondecreasing function). As an application, the Baum-Katz type result for weighted sums of extended negatively dependent random variables is established under sub-linear expectations space. The results obtained in the article are the extensions of the complete convergence and complete moment convergence under classical linear expectation space.