Division of Physics

Photonic time crystals (PTCs) are materials whose dielectric permittivity is periodically modulated in time. Akin to conventional photonic crystals, PTCs give rise to momentum bandgap as well as to topological insulators in the time domain. Here, we experimentally demonstrate the properties of a k gap in a PTC with real dimensions, realized in a dynamically modulated microwave transmission-line metamaterial. Wave amplification is observed within the k gap, leading to a narrowing of the initial power spectrum and its shifting toward the gap. To probe the mid-gap topological state, we introduce a temporal interface separating two PTCs with distinct topological phases. The phase difference measured between time-reflected and time-refracted waves at the interface, together with the temporal confinement of the topological state, proves the realization of nontrivial temporal topology. By integrating k gap amplification with time-domain topological features, our work opens new avenues for light generation and manipulation in time-varying photonic materials.

We present a comprehensive first-principles investigation of a series of proposed two-dimensional Janus MBNS2 (M = Ti, Zr, and Hf) monolayers using density functional theory. The study systematically explores their structural stability, mechanical properties, Raman spectra, piezoelectric behavior, electronic band structures, and carrier transport characteristics. The results indicate that all proposed materials are semiconductors with dynamically and mechanically stable crystal structures. The calculated band gaps of the Janus MBNS2 monolayers range from 1.41 to 1.71 eV. Notably, the Janus HfBNS2 monolayer exhibits a large Zeeman spin-splitting of 225.2 meV due to strong spin–orbit coupling, highlighting its potential for spintronic applications. Carrier mobility in these materials is found to be intrinsically low, primarily limited by electron–phonon interactions. Detailed scattering analysis reveals that acoustic deformation potential scattering is the dominant mechanism limiting charge transport, while other scattering mechanisms contribute less significantly. The proposed Janus MBNS2 monolayers also display considerable mechanical robustness, with in-plane Young’s moduli reaching up to 206.29 N/m. Moreover, the vertically asymmetric crystal structures of these compounds give rise to both in-plane and out-of-plane piezoelectric responses, enhancing their potential for use in piezotronic and electromechanical devices. These findings not only extend the family of functional 2D Janus semiconductors but also provide valuable insights into the fundamental properties, particularly transport limitations arising from electron–phonon interactions, offering guidance for future materials engineering and optimization.

Great technical advances have been achieved since the first atom-trap trace analysis (ATTA) -based radiokrypton application in Egypt, where 1 Myr old groundwater was discovered. Beyond advances in ATTA measurement capabilities, including reduction in sample size, analysis duration, and analytical uncertainty, major progress has been achieved over the past two decades in the sample collection and preparation techniques. These advances paved the expansion of ATTA-based noble gas applications to many other aquifers worldwide, illuminating the nature and flow pattern of deep groundwater systems. While the potential of this new analytical technique for old groundwater dating is well recognized, another important aspect yet to be examined is the reproducibility of radiokrypton in aquifers over time, i.e., how representative is a discrete groundwater sample, collected at a specific time and location, for the natural groundwater system? The likelihood of a negative answer is increased by flow-field disturbance in aquifers following massive groundwater abstraction. Here, we present repeated 81Kr sampling and measurements in twenty-one sites over Israel, mostly of deep (up to 1 km) wells tapping confined aquifers in the arid to hyperarid Negev desert. The results demonstrate that radiokrypton measurements are indeed reproducible, even in cases where samples were collected as long as nine years apart and from highly productive (∼1 Mm3/yr order) pumping wells. Furthermore, many of the repeated measurements in this study (17 out of the 21 sites) were conducted with different ATTA Instruments in two different laboratories using slightly different sampling, preparation, and analysis techniques, yet with an overall good agreement. The consistency in the ATTA-based 81Kr-dating results over time highlights the robustness of this state-of-the-art technique as a tool to unravel groundwater flow patterns and encourages further applications to many other yet-to-be-explored deep aquifers.

The intrinsic structural lability and limited processability of inorganic halide perovskites heavily hamper their commercial applications. Herein, we develop a facile and scalable room-temperature (RT) water-based procedure for the synthesis of Na–Cu-codoped CsPbIxBr3–x-embedded Pb(OH)I (denoted as the Na–Cu–CsPbIxBr3–x@Pb(OH)I) composite to overcome the above-mentioned bottlenecks. Compared to Na doping, the codoping with Na and Cu leads to a slight shift of the photoluminescence (PL) peak position from ∼612 to 622 nm, increases the PL intensity by 32.5 times, and improves the PL quantum yield from 1% to 5%. Notably, the optimized Na–Cu–CsPbIxBr3–x@Pb(OH)I composite displays outstanding stability under various harsh environments such as soaking in RT water or hot water (up to 90 °C) and annealing at high temperature, which indicates that Pb(OH)I can act as an effective water-blocking and protective encapsulation layer. It is more interesting that the photoluminescence (PL) intensity of Na–Cu–CsPbIxBr3–x@Pb(OH)I can be significantly increased to 2.17 and 1.29 times after soaking the sample in RT water for 35 and 105 days, respectively, and 1.25 times even after immersing in hot water (90 °C) for 6 h, indicating an improved optical property. Meanwhile, the crystal structure and emission wavelength are maintained. The insight of this work is quite beneficial for designing highly stable inorganic perovskite-based nanomaterials for realization in light-emitting devices and other optoelectronic devices.

Magnetic higher-order topological insulators have attracted enormous research attention due to their intriguing hinge or corner modes and quantized magnetoelectric effect. Notably, Zintl compounds are prominent examples of hosting higher-order topological and Axion phases. However, materials that facilitate a higher-order topological phase in three-dimensional (3D) bulk and two-dimensional (2D) monolayer forms are yet to be proposed. In this work, utilizing first-principles calculations, we identify EuA2X2 (A: In, Sn; X: Sb, Bi) compounds as promising magnetic topological materials that host higher-order topological states in both 3D and 2D forms, except for the EuIn2Sb2 monolayer. Moreover, all the investigated 3D bulk materials exhibit an Axion phase. The higher-order topological and Axion phases were examined by the parity-based topological invariant, Z4, due to their centrosymmetric nature. Interestingly, the EuSn2Sb2 monolayer exhibits a higher-order topological insulating phase with a significant band gap of 0.4 eV in both ferromagnetic (FM) and antiferromagnetic (AFM) configurations. Meanwhile, the FM-EuIn2Sb2 monolayer exhibits a Chern topological feature. For brevity, topological surface, edge, and corner states are demonstrated for the 3D bulk and monolayer of EuSn2Sb2. The tiny gap in the surface states confirms the presence of a higher-order topological phase in 3D. Similarly, gapped edges and degenerate in-gap corner modes support the higher-order topological nature in the monolayer. Our study reveals the diverse topological phases in Eu-based layered compounds, highlighting their potential for future applications in spintronics and topological devices.

Topologically nontrivial systems can be protected by band topology in momentum space, as seen in topological insulators and semimetals, or real-space topology, such as in lattice deformations known as topological disclinations (TDs). TDs, with inherent chiral symmetry, can support localized states pinned spectrally to the middle of the topological gap, preventing hybridization with bulk bands, and making them promising for topological lasers. Here, we experimentally realize a C4v symmetric TD laser based on perovskite exciton-polariton lattices at room temperature. Protected by the chiral and point group symmetries of the lattice, the TD state emerges in the middle of the gap and at the core of the perovskite lattice. Under a non-resonant pulsed excitation, coherent polariton lasing occurs precisely at the TD state with a low threshold of 9.5 μJ/cm2, as confirmed by momentum space and real space spectra measurements. This study not only introduces a class of symmetry-protected topological lasers, but also expands the landscape for exploring exciton-polariton light-matter interactions with novel topological structures.

Understanding the mechanism of the insulator-metal transition (IMT) in VO2 is a necessary step in optimising this material's properties for a range of functional applications. Here, Rietveld refinement of synchrotron X-ray powder diffraction patterns is performed on thermochromic V1-xWxO2 (0.0 ≤ x ≤ 0.02) nanorod aggregates over the temperature range 100 ≤ T ≤ 400 K to examine the effect of doping on the structure and properties of the insulating monoclinic (M1) phase and metallic rutile (R) phase. Precise measurement of the lattice constants of the M1 and R phases enabled the onset (Ton) and endset (Tend) temperatures of the IMT to be determined accurately for different dopant levels. First-principles calculations reveal that the observed decrease in both Ton and Tend with increasing W content is a result of Peierls type V-O-V dimers being replaced by linear W-O-V dimers with a narrowing of the band gap. The results are interpreted in terms of the bandwidth-controlled Mott-Hubbard IMT model, providing a more detailed understanding of the underlying physical mechanisms driving the IMT as well as a guide to optimising properties of VO2-based materials for specific applications.

Controlled heating experiments in an inert environment have been performed on WS2 monolayers, in order to clarify the conflicting reports on the high-temperature photoluminescent response of 2D TMDs. We find that in contrast to some previous results on both WS2 and MoS2, the photoluminescent intensity shows a consistent reduction above room temperature. This is accompanied by an almost linear redshift of the peak maximum, and a nearly linear increase in the peak width, which is attributed to an enhanced interaction with optical phonons. Moreover, by fitting the photoluminescence integral intensity with an Arrhenius type dependence, we demonstrate that the center of the WS2 monolayer flake starts to undergo irreversible degradation at a temperature of 573 K in an inert environment. Regions close to flake edges in contrast, with a more intense room temperature PL response, remain stable. The macroscopic PL signal is largely recovered in these regions following subsequent cooling to room temperature.

Abstract A comprehensive set of level energies is provided for the valence and K-vacancy states of the ions Fe xvii, Fe xviii, Fe xix, Fe xx, Fe xxi, Fe xxii, Fe xxiii, and Fe xxiv. Level energies were calculated with the relativistic Multi-Reference Møller–Plesset Perturbation Theory method (MR-MP). The data set includes level energies coming from the configurations , , , , , and , where 1 ≤ q ≤ 8, n ≤ 5, and l ≤ 3. The results have been compared with data from the National Institute of Standards and Technology (NIST) online database and with previous calculations. In general, the deviation from the NIST-recommended values is below 0.6 and 1.0 eV for valence and K-vacancy level energies, respectively. However, we identify numerous outliers among the NIST-recommended values. The extensive data set presented here greatly augments the amount of available reference level energies in the NIST database for L-shell ions of Fe, and we expect our data set to ease line identification and confirm level designations.