Switching materials in channels of nonlinear optics (NLOs) are of particular interest in NLO material science. Numerous crystalline NLO switches based on structural phase transition have emerged, but ...most of them reveal a single‐step switch between two different second‐harmonic‐generation (SHG) states, and only very rare cases involve three or more SHG states. Herein, we report a new organic‐inorganic hybrid salt, (Me3NNH2)2CdI4, which is an unprecedented case of a reversible three‐step NLO switch between SHG‐silent, ‐medium, ‐low, and ‐high states, with high contrasts of 25.5/4.3/9.2 in a temperature range of 213–303 K. By using the combined techniques of variable‐temperature X‐ray single‐crystal structural analyses, dielectric constants, solid‐state 13C nuclear magnetic resonance spectroscopy, and Hirshfeld surface analyses, we disclose that this four‐state switchable SHG behavior is highly associated with the stepwise‐changed molecular dynamics of the polar organic cations. This finding demonstrates well the complexity of molecular dynamics in simple hybrid salts and their potential in designing new advanced multistep switching materials.
A new simple hybrid salt has been synthesized that exhibits reversible multistep phase transitions and an unprecedented thermally induced three‐step “silent‐medium‐low‐high” second‐harmonic‐generation switching behavior. This behavior arises from complex and stepwise molecular dynamic changes of the polar organic cations.
Precise control of solid-state elastic waves' mode content and coherence is of great use nowadays in reinforcing mechanical energy harvesting/storage, nondestructive material testing, wave-matter ...interaction, high sensitivity sensing, and information processing, etc. Its efficacy is highly dependent on having elastic transmission channels with lower loss and higher degree of freedom. Here, we demonstrate experimentally an elastic analog of the quantum spin Hall effects in a monolithically scalable configuration, which opens up a route in manipulating elastic waves represented by elastic pseudospins with spin-momentum locking. Their unique features including robustness and negligible propagation loss may enhance elastic planar-integrated circuit-level and system-level performance. Our approach promotes topological materials that can interact with solid-state phonons in both static and time-dependent regimes. It thus can be immediately applied to multifarious chip-scale topological phononic devices, such as path-arbitrary elastic wave-guiding, elastic splitters and elastic resonators with high-quality factors.
Recent explorations of topology in physical systems have led to a new paradigm of condensed matters characterized by topologically protected states and phase transition, for example, topologically ...protected photonic crystals enabled by magneto-optical effects. However, in other wave systems such as acoustics, topological states cannot be simply reproduced due to the absence of similar magnetics-related sound-matter interactions in naturally available materials. Here, we propose an acoustic topological structure by creating an effective gauge magnetic field for sound using circularly flowing air in the designed acoustic ring resonators. The created gauge magnetic field breaks the time-reversal symmetry, and therefore topological properties can be designed to be nontrivial with non-zero Chern numbers and thus to enable a topological sonic crystal, in which the topologically protected acoustic edge-state transport is observed, featuring robust one-way propagation characteristics against a variety of topological defects and impurities. Our results open a new venue to non-magnetic topological structures and promise a unique approach to effective manipulation of acoustic interfacial transport at will.
We report a new method to promote the conductivities of metal–organic frameworks (MOFs) by 5 to 7 magnitudes, thus their potential in electrochemical applications can be fully revealed. This method ...combines the polarity and porosity advantages of MOFs with the conductive feature of conductive polymers, in this case, polypyrrole (ppy), to construct ppy‐MOF compartments for the confinement of sulfur in Li–S batteries. The performances of these ppy‐S‐in‐MOF electrodes exceed those of their MOF and ppy counterparts, especially at high charge–discharge rates. For the first time, the critical role of ion diffusion to the high rate performance was elucidated by comparing ppy‐MOF compartments with different pore geometries. The ppy‐S‐in‐PCN‐224 electrode with cross‐linked pores and tunnels stood out, with a high capacity of 670 and 440 mAh g−1 at 10.0 C after 200 and 1000 cycles, respectively, representing a new benchmark for long‐cycle performance at high rate in Li–S batteries.
MOF host S: The electrical conductivity of metal–organic frameworks (MOFs) was promoted by the construction of polymer–MOF composites. Using MOF‐based sulfur hosts, the critical role of porosity at high charge–discharge rates in Li–S batteries was elucidated. MOFs with short ion transfer pathways and large pore apertures were identified as the most suitable for long‐term cycling at extremely high rates.
An extremely rare non‐Kramers holmium(III) single‐ion magnet (SIM) is reported to be stabilized in the pentagonal‐bipyramidal geometry by a phosphine oxide with a high energy barrier of 237(4) cm−1. ...The suppression of the quantum tunneling of magnetization (QTM) at zero field and the hyperfine structures originating from field‐induced QTMs can be observed even from the field‐dependent alternating‐current magnetic susceptibility in addition to single‐crystal hysteresis loops. These dramatic dynamics were attributed to the combination of the favorable crystal‐field environment and the hyperfine interactions arising from 165Ho (I=7/2) with a natural abundance of 100 %.
An extremely rare non‐Kramers holmium(III) single‐ion magnet is reported. The suppression of the quantum tunneling of magnetization at zero field and the hyperfine structures were observed in AC magnetic susceptibility measurements, and were attributed to the combination of a favorable crystal‐field environment and the hyperfine interactions arising from 165Ho (I=7/2) with a natural abundance of 100 %.
Stable and efficient guided waves are essential for information transmission and processing. Recently, topological valley-contrasting materials in condensed matter systems have been revealed as ...promising infrastructures for guiding classical waves, for they can provide broadband, non-dispersive and reflection-free electromagnetic/mechanical wave transport with a high degree of freedom. In this work, by designing and manufacturing miniaturized phononic crystals on a semi-infinite substrate, we experimentally realized a valley-locked edge transport for surface acoustic waves (SAWs). Critically, original one-dimensional edge transports could be extended to quasi-two-dimensional ones by doping SAW Dirac "semimetal" layers at the boundaries. We demonstrate that SAWs in the extended topological valley-locked edges are robust against bending and wavelength-scaled defects. Also, this mechanism is configurable and robust depending on the doping, offering various on-chip acoustic manipulation, e.g., SAW routing, focusing, splitting, and converging, all flexible and high-flow. This work may promote future hybrid phononic circuits for acoustic information processing, sensing, and manipulation.
Hierarchical porosity and functionalization help to fully make use of metal–organic frameworks (MOFs) for their diverse applications. Herein, a simple strategy is reported to construct hierarchically ...porous MOFs through a competitive coordination method using tetrafluoroborate (M(BF4)x, where M is metal site) as both functional sites and etching agents. The resulting MOFs have in situ formed defect‐mesopores and functional sites without sacrificing their structure stability. The formation mechanism of the defect‐mesopores is elucidated by a combination of experimental and first‐principles calculation method, indicating the general feasibility of this new approach. Compared with the original microporous counterparts, the new hierarchical MOFs exhibit superior adsorption for the bulky dye molecules and catalytic performance for the CO2 conversion attributed to their specific hierarchical pore structures.
A simple and novel strategy is reported to fabricate a series of hierarchically porous metal–organic frameworks through the competitive coordination method. The formation mechanism of defect‐mesopores is elucidated by a combination of experimental and first‐principles calculation methods. Furthermore, the adsorption and catalytic advantage over the original microporous counterparts is also demonstrated attributed to their specific hierarchical pore structures.
Abstract
Conventional ultrafine-grains can generate high strength in Mg alloys, but significant tradeoff of corrosion resistance due to inclusion of a large number of non-equilibrium grain ...boundaries. Herein, an ultrafine-grain structure consisting of dense ultrafine twins is prepared, yielding a high strength up to 469 MPa and decreasing the corrosion rate by one order of magnitude. Generally, the formation of dense ultrafine twins in Mg alloys is rather difficult, but a carefully designed multi-directional compression treatment effectively stimulates twinning nucleation within twins and refines grain size down to 300 nm after 12-passes compressions. Grain-refinement by low-energy twins not only circumvents the detrimental effects of non-equilibrium grain boundaries on corrosion resistance, but also alters both the morphology and distribution of precipitates. Consequently, micro-galvanic corrosion tendency decreases, and severe localized corrosion is suppressed completely. This technique has a high commercial viability as it can be readily implemented in industrial production.
A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a ...symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron’s spin-1/2 (fermionic) time-reversal symmetry
T
f
2
=
−
1
. However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon’s spin-1 (bosonic) time-reversal symmetry
T
b
2
=
−
1
. In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry,
T
p
(
T
p
2
=
−
1
)
, due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp
rather than by the bosonic time-reversal symmetry Tb
. This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators.
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