编译 | 未玖

Science, 27 OCT 2023, VOL 382, ISSUE 6669

《科学》2023年10月27日，第382卷，6669期

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物理学Physics

Observation and control of hybrid spin-wave–Meissner-current transport modes

自旋波—迈斯纳电流混合输运模式的观测与控制

▲ 作者：M. Borst, P. H. Vree, A. Lowther, A. Teepe, S. Kurdi, I. Bertelli, et al.

▲ 链接：https://www.science.org/doi/full/10.1126/science.adj7576

▲ 摘要：

超导体是一种零电阻率、可排斥磁场的材料，这被称为迈斯纳效应。超导体的无耗散抗磁响应是磁悬浮和量子干涉装置等电路的核心。

研究组在薄膜磁铁中使用超导抗磁性来塑造控制自旋波（即在磁体中的集体自旋激发，是颇有前景的片上信号载体）输运的磁环境。基于金刚石的磁成像技术，实验观察到具有强烈变化、温度可调波长的自旋波—迈斯纳电流混合输运模式，然后用聚焦激光演示了自旋波折射的局部控制。

该研究结果证明了超导体操控自旋波输运的多功能性，并在自旋波光栅、滤波器、晶体和腔中具有潜在的应用。

▲ Abstract：

Superconductors are materials with zero electrical resistivity and the ability to expel magnetic fields, which is known as the Meissner effect. Their dissipationless diamagnetic response is central to magnetic levitation and circuits such as quantum interference devices. In this work, we used superconducting diamagnetism to shape the magnetic environment governing the transport of spin waves—collective spin excitations in magnets that are promising on-chip signal carriers—in a thin-film magnet. Using diamond-based magnetic imaging, we observed hybridized spin-wave–Meissner-current transport modes with strongly altered, temperature-tunable wavelengths and then demonstrated local control of spin-wave refraction using a focused laser. Our results demonstrate the versatility of superconductor-manipulated spin-wave transport and have potential applications in spin-wave gratings, filters, crystals, and cavities.

Quantum walk comb in a fast gain laser

快速增益激光器中的量子行走梳

▲ 作者：Ina Heckelmann, Mathieu Bertrand, Alexander Dikopoltsev, Mattias Beck, Giacomo Scalari & Jérôme Faist

▲ 链接：https://www.science.org/doi/full/10.1126/science.adj3858

▲ 摘要：

光子学中的合成晶格使人们能在新维度上探索光的状态，这超越了仅在物理空间中常见的现象。

研究组提出并演示了一种在合成频率空间内通过外部调制具有超快恢复时间的环形半导体激光器所形成的量子行走梳。初始弹道量子行走不会消散至合成晶格的低超模态；相反，这种状态稳定在一个宽频梳中，释放了合成频率晶格的全部潜力。

该设备产生低噪声、近平坦的宽带梳（达到每厘米带宽100），并为产生宽带、可调谐和稳定的频率梳提供了一个颇有前景的平台。

▲ Abstract：

Synthetic lattices in photonics enable the exploration of light states in new dimensions, transcending phenomena common only to physical space. We propose and demonstrate a quantum walk comb in synthetic frequency space formed by externally modulating a ring-shaped semiconductor laser with ultrafast recovery times. The initially ballistic quantum walk does not dissipate into low supermode states of the synthetic lattice; instead, the state stabilizes in a broad frequency comb, unlocking the full potential of the synthetic frequency lattice. Our device produces a low-noise, nearly flat broadband comb (reaching 100 per centimeter bandwidth) and offers a promising platform to generate broadband, tunable, and stable frequency combs.

Giant lattice softening at a Lifshitz transition in Sr2RuO4

Sr2RuO4中Lifshitz相变处的巨大晶格软化

▲ 作者：H. M. L. Noad, K. Ishida, Y.-S. Li, E. Gati, V. Stangier, N. Kikugawa, et al.

▲ 链接：https://www.science.org/doi/full/10.1126/science.adf3348

▲ 摘要：

固体中电子自由度和结构自由度的相互作用是一个研究热点。60多年前，Lifshitz讨论了一种违反直觉的可能性：在拓扑费米面跃迁时由传导电子驱动的晶格软化。但他预测该影响很小，至今尚无确切的实验证据。

在测量应力—应变关系的同时，利用基于压电的单轴压力传感器对超纯金属钌酸锶进行调谐，研究组揭示了二维费米面在Lifshitz相变处杨氏模量的巨大软化，并表明其确实完全由相关能带的传导电子驱动。

▲ Abstract：

The interplay of electronic and structural degrees of freedom in solids is a topic of intense research. More than 60 years ago, Lifshitz discussed a counterintuitive possibility: lattice softening driven by conduction electrons at topological Fermi surface transitions. The effect that he predicted, however, was small and has not been convincingly observed. Using a piezo-based uniaxial pressure cell to tune the ultraclean metal strontium ruthenate while measuring the stress-strain relationship, we reveal a huge softening of the Young’s modulus at a Lifshitz transition of a two-dimensional Fermi surface and show that it is indeed driven entirely by the conduction electrons of the relevant energy band.

Universal scaling of the dynamic BKT transition in quenched 2D Bose gases

二维玻色气体中动态BKT相变的普遍标度

▲ 作者：Shinichi Sunami, Vijay Pal Singh, David Garrick, Abel Beregi, Adam J. Barker, Kathrin Luksch, et al.

▲ 链接：https://www.science.org/doi/full/10.1126/science.abq6753

▲ 摘要：

对多体量子系统非平衡动力学的理解是统计物理学的基础，探索这些系统普遍特性的实验可解决这些基本问题。

研究组报道了二维（2D）玻色气体中经Berezinskii-Kosterlitz-Thouless（BKT）相变由超流体到正常态的突变引发的普适动力学的测量。通过将2D玻色气体一分为二来降低密度，实现了跨越临界点的突变。随后用物质波干涉法探测弛豫动力学，以测量局部相位波动。

结果表明，相位相关函数和涡旋密度的时间演化都遵循普遍标度律。该结论与经典场模拟的结果一致，并通过实时重整化群论进行了阐释。

▲ Abstract：

The understanding of nonequilibrium dynamics in many-body quantum systems is a fundamental issue in statistical physics. Experiments that probe universal properties of these systems can address such foundational questions. In this study, we report the measurement of universal dynamics triggered by a quench from the superfluid to normal phase across the Berezinskii-Kosterlitz-Thouless transition in a two-dimensional (2D) Bose gas. We reduced the density by splitting the 2D gas in two, realizing a quench across the critical point. The subsequent relaxation dynamics were probed with matter-wave interferometry to measure the local phase fluctuations. We show that the time evolution of both the phase correlation function and vortex density obeys universal scaling laws. This conclusion is supported by classical-field simulations and interpreted by means of real-time renormalization group theory.

材料科学Materials Science

Room-temperature wavelike exciton transport in a van der Waals superatomic semiconductor

范德华超原子半导体中的室温波状激子输运

▲ 作者：Jakhangirkhodja A. Tulyagankhodjaev, Petra Shih, Jessica Yu, Jake C. Russell, Daniel G. Chica, Michelle E. Reynoso, et al.

▲ 链接：https://www.science.org/doi/full/10.1126/science.adf2698

▲ 摘要：

半导体中能量和信息的传输受限于电子载流子和晶格声子之间的散射，导致扩散和损耗传输，这限制了所有半导体技术。

利用范德华（vdW）超原子半导体Re6Se8Cl2，研究组证明了声激子—极化子的形成，这是一种屏蔽声子散射的电子准粒子。他们在室温下直接成像了Re6Se8Cl2中的极化子输运，揭示了持续一纳秒和几微米的准弹道波状传播。

屏蔽极化子输运导致电子能量传播长度比其他vdW半导体大几个数量级，甚至在一纳秒内超过硅。该研究与直觉相反，准平坦电子能带和强激子—声学声子耦合共同决定Re6Se8Cl2的输运性质，建立了一条通往弹道室温半导体的途径。

▲ Abstract：

The transport of energy and information in semiconductors is limited by scattering between electronic carriers and lattice phonons, resulting in diffusive and lossy transport that curtails all semiconductor technologies. Using Re6Se8Cl2, a van der Waals (vdW) superatomic semiconductor, we demonstrate the formation of acoustic exciton-polarons, an electronic quasiparticle shielded from phonon scattering. We directly imaged polaron transport in Re6Se8Cl2 at room temperature, revealing quasi-ballistic, wavelike propagation sustained for a nanosecond and several micrometers. Shielded polaron transport leads to electronic energy propagation lengths orders of magnitude greater than in other vdW semiconductors, exceeding even silicon over a nanosecond. We propose that, counterintuitively, quasi-flat electronic bands and strong exciton–acoustic phonon coupling are together responsible for the transport properties of Re6Se8Cl2, establishing a path to ballistic room-temperature semiconductors.

化学Chemistry

Multiplicative enhancement of stereoenrichment by a single catalyst for deracemization of alcohols

单一催化剂对醇去消旋化立体富集的倍增效应

▲ 作者：Lu Wen, Jia Ding, Lingfei Duan, Shun Wang, Qing An, Hexiang Wang, et al.

▲ 链接：https://www.science.org/doi/full/10.1126/science.adj0040

▲ 摘要：

外消旋混合物的去消旋化立体化学富集必须克服不利的熵效应和微观可逆性原理；最近，由高能光输入揭示的光化学反应途径启发了这方面的创新，最常见的是通过消除立体C（sp3）-H键。

研究组报道了一种光化学驱动的去消旋化方案，其中单手性催化剂通过C-C键裂解和C-C键形成这两个机制不同的步骤，来实现立体诱导的倍增效应，从而导致高水平的立体选择性。

由手性磷酸或双恶唑啉配位的钛催化剂的配体—金属电荷转移激发有效地富集了具有相邻和完全取代立体中心的外消旋醇，对映体比率高达99：1。

机理研究支持通过一种常见前手性中间体进行顺序自由基介导的键断裂和键形成途径，并表明，尽管总体立体富集程度很高，但每个单独步骤的选择性并不高。

▲ Abstract：

Stereochemical enrichment of a racemic mixture by deracemization must overcome unfavorable entropic effects as well as the principle of microscopic reversibility; recently, photochemical reaction pathways unveiled by the energetic input of light have led to innovations toward this end, most often by ablation of a stereogenic C(sp3)–H bond. We report a photochemically driven deracemization protocol in which a single chiral catalyst effects two mechanistically different steps, C–C bond cleavage and C–C bond formation, to achieve multiplicative enhancement of stereoinduction, which leads to high levels of stereoselectivity. Ligand-to-metal charge transfer excitation of a titanium catalyst coordinated by a chiral phosphoric acid or bisoxazoline efficiently enriches racemic alcohols that feature adjacent and fully substituted stereogenic centers to enantiomeric ratios up to 99:1. Mechanistic investigations support a pathway of sequential radical-mediated bond scission and bond formation through a common prochiral intermediate and reveal that, although the overall stereoenrichment is high, the selectivity in each individual step is moderate.