课题组博士生谢兴在顶级期刊Advanced Materials 上发表论文

      近日,课题组博士生谢兴同学在刘老师的指导下，以第一作者在国际材料领域顶级期刊Advanced Materials(影响因子32.08)上发表题为“Interlayer Valley Coupling and Hybridized Excitons in Twisted van der Waals Bilayers with Broken Translational Symmetry”(具有破缺平移对称性的转角范德华双层中的层间谷耦合与杂化激子的研究)的实验论文。

图：具有破缺平移对称性的转角范德华双层中的层间谷耦合与杂化激子的研究

      扭转范德华同质双层和异质结构为探索强关联态和奇异光子学效应提供了一个极具前景的平台。实现这些丰富物理现象的关键因素在于相邻原子层之间的相对旋转，这种旋转会产生长波长的莫尔图案。莫尔超晶格能够调制层间电子耦合并引入周期性势场，从而在小扭转角下形成近乎平坦的电子能带。平带会显著抑制电子的动能，进而放大库仑相互作用的影响，并诱导关联绝缘态、超导态以及非平庸拓扑相的出现。在扭转双层石墨烯中，这类平带对扭转角极为敏感，通常只出现在接近所谓“魔角”（约 1.1°）的狭窄角度范围内，此时层间杂化与能带位移达到最佳平衡。 这一限制削弱了扭转石墨烯体系中关联现象的可调控性和鲁棒性。相比之下，过渡金属硫族化合物（TMDs）展现出不同的物理图景。其直接带隙、强自旋–轨道耦合以及对外加位移场较弱的依赖，使得莫尔势可以随扭转角实现更加连续的调控。 因此，TMD 双层中的莫尔效应可在显著更宽的角度范围内被观察到，通常包括 R 型堆垛中的 0° 至 5°，以及 H 型堆垛中的 55° 至 60°。

      在较大扭转角下，莫尔波长变短，由于空间周期性减弱以及层间调制变弱，相关的平带现象会逐渐消失。因此，中等或大扭转角双层体系在实验和理论上仍然相对缺乏研究，尽管这些角度区域可能蕴含发现新型电子行为的潜在机会。值得注意的是，当扭转角为非公度角时，体系不具备长程平移对称性，从而无法形成传统意义上的电子超晶格。在这类准周期结构中，周期性的破缺可能诱导出与周期性莫尔晶体本质不同的多体效应。例如，近期关于 30° 扭转双层石墨烯的研究揭示了具有十二重旋转对称性的多个狄拉克锥的形成，这是准晶体系中特有的 Umklapp 散射过程的重要标志。 此类结构中的强层间耦合能够显著重构能带结构，产生诸如价带顶附近的小能隙²⁵以及电场可调的层间 K–Q 谷耦合等特征；后者最近已在 WSe₂ 莫尔准晶中得到报道。理论上，预计在平移对称性破缺的扭转双层体系中会出现多种谷杂化和散射机制。这些机制包括不同布里渊区点之间的谷间耦合、自旋–谷锁定的调制以及非平庸拓扑效应。然而，尽管人们对这类准周期莫尔体系的兴趣不断增加，相关实验实现仍然较少，许多理论预测的现象尚未得到验证，这也进一步推动了该方向的深入研究。阐明莫尔准晶、层间耦合与涌现电子态之间的相互作用，已成为凝聚态物理和材料科学中一个极具吸引力的前沿方向。

      在这里，我们报道了在平移对称性破缺的扭转 MoS₂ 双层中发现高能杂化层间激子，这些激子出现在 11° 和 30° 扭转角样品中。该类杂化激子源于 K 谷空穴与 −K 谷电子之间的耦合，并通过 K–(−K) 层间耦合进一步与层内激子发生杂化。值得注意的是，这些激子在磁场中表现出几乎可以忽略的谷塞曼效应，其 g 因子约为 −0.6，并且该值取决于层内激子与层间激子之间的耦合强度。此外，在升高温度时，温度诱导的失谐效应会使层间激子的能量逐渐接近层内激子的能量。这些发现凸显了非周期莫尔晶体在揭示新型莫尔光子学和量子现象方面的巨大潜力。

论文链接：http://doi.org/10.1002/adma.73772

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Broken Symmetry Brings Hidden Valley Excitons into View

Twisted two-dimensional crystals are usually celebrated for the periodic moiré landscapes they create at small angles. A new study shows that when translational symmetry is broken at large or incommensurate twist angles, hidden interlayer valley coupling can emerge, producing high-energy hybridized excitons with unusual magnetic and thermal responses.

Twistronics has reshaped the study of quantum materials. By simply rotating one atomically thin layer relative to another, researchers can engineer long-period moiré patterns, flatten electronic bands and access correlated electronic phases, unconventional superconductivity and moiré excitons. Much of this field has focused on small twist angles, where the moiré wavelength is long and the resulting periodic potential can be treated as a superlattice. Yet this emphasis leaves a broad and less explored regime: large-angle and incommensurate twisted bilayers, where conventional translational symmetry is lost and the rules governing electronic and excitonic states become fundamentally different.

A recent study of twisted MoS₂ bilayers now shows that broken translational symmetry is not simply a complication, but a powerful route to new excitonic physics. In these aperiodic twisted structures, Umklapp scattering can connect electronic valleys that are normally momentum mismatched, enabling interlayer coupling between the K and −K valleys. This unconventional valley coupling gives rise to high-energy hybridized interlayer excitons that are absent in commensurate twisted MoS₂ homobilayers. The results reveal a distinct regime of moiré exciton physics in which aperiodicity, rather than periodicity, creates the essential quantum coupling.

The central idea is that large-angle twisted bilayers do not necessarily form a conventional moiré mini-Brillouin zone. In a 30° twisted MoS₂ bilayer, for example, the structure displays dodecagonal quasicrystalline order and lacks long-range translational periodicity. Under these conditions, reciprocal lattice vectors from the two layers can mediate Umklapp processes that bring otherwise separated valley momenta into alignment. In particular, the K valley in one layer can couple to the −K valley in the other layer, a process that is generally forbidden or strongly suppressed in natural bilayers and conventional commensurate stacks.

This mechanism provides a pathway for forming hybridized interlayer excitons. In a simple picture, an electron and a hole residing in different layers form an interlayer exciton. In the present twisted MoS₂ system, however, this interlayer exciton further hybridizes with intralayer A or B excitons through Umklapp-enabled K–(−K) interlayer valley coupling. The resulting quasiparticles inherit mixed intralayer and interlayer character: they are optically bright, spatially indirect in part, and sensitive to both valley mixing and interlayer momentum coupling.

Experimentally, the evidence is compelling. In 30° twisted MoS₂ bilayers, the authors observe two distinct high-energy photoluminescence peaks at approximately 1.938 and 2.090 eV. These peaks appear in both photoluminescence and reflectance contrast spectra, but are absent in monolayer MoS₂ and in 0° and 60° aligned MoS₂ bilayers. Their energies lie above the conventional intralayer A and B excitons, consistent with reduced binding energy associated with partial spatial separation between electrons and holes. Time-resolved photoluminescence further supports their interlayer character: one of the hybridized excitons exhibits a lifetime of about 1.0 ns, markedly longer than typical intralayer exciton lifetimes in transition-metal dichalcogenides.

The magnetic response provides an even sharper fingerprint of the hybridized nature of these excitons. In monolayer and bilayer transition-metal dichalcogenides, valley Zeeman splitting is a powerful probe of spin, orbital and valley magnetic moments. Conventional intralayer and interlayer excitons usually exhibit distinct g-factors because their constituent carriers occupy different valley and layer configurations. In the twisted MoS₂ quasicrystal, however, the high-energy hybridized exciton displays an almost vanishing valley Zeeman splitting, with an effective g-factor of about −0.6. This unusually small value indicates strong compensation between intralayer-like and interlayer-like magnetic responses, reflecting robust hybridization between the underlying excitonic states.

This near-cancellation of the Zeeman response is not a trivial reduction in signal. Rather, it reveals the internal composition of the quasiparticle. A hybrid exciton can be viewed as a coherent mixture of excitonic states with different magnetic moments. When the interlayer and intralayer components contribute Zeeman shifts of opposite sign or different magnitude, their mixing can strongly suppress the net splitting. In this sense, the almost vanishing g-factor acts as a spectroscopic signature of the hidden valley coupling enabled by broken translational symmetry.

The work also uncovers unusual magnetic-field-dependent valley polarization. The hybridized A-like and B-like excitons exhibit different field responses, reflecting their distinct spin configurations and valley-conserving or valley-flipping pathways. One hybridized exciton shows an “X”-shaped dependence of the degree of circular polarization on magnetic field, whereas the other displays a more moderate “V”-like behavior. These contrasting trends suggest that the magnetic field tunes the relative energy alignment of valleys in different layers and thereby modifies the balance between valley-preserving recombination, intervalley scattering and interlayer tunneling.

Importantly, the phenomenon is not restricted to the 30° quasicrystal. The authors further examine twisted MoS₂ homobilayers at several twist angles, including 11°, 13.2° and 21.8°. High-energy hybridized interlayer excitons are prominently observed in the incommensurate 11° and 30° samples, whereas they are not seen in the same way in the commensurate twisted structures. The 11° sample also exhibits negligible Zeeman splitting with a similarly small effective g-factor, supporting the conclusion that broken translational symmetry plays a key role in enabling the observed interlayer valley coupling.

Temperature-dependent measurements add another layer to this picture. The hybridized excitons persist up to room temperature, but their energies shift more rapidly with increasing temperature than those of conventional intralayer excitons. As temperature rises from cryogenic conditions to 300 K, the energy separation between the hybridized excitons and the corresponding intralayer excitons decreases. This behavior suggests temperature-induced detuning: the interlayer component gradually shifts closer to the intralayer exciton energy, modifying the degree of hybridization. At elevated temperatures, the valley polarization of the hybridized excitons also decreases and approaches that of the intralayer excitons, consistent with increased phonon-assisted intervalley scattering and reduced hybridization-induced valley protection.

The broader significance of this work lies in how it reframes the role of aperiodicity in twisted materials. In conventional moiré physics, periodicity is often viewed as the organizing principle that creates minibands, moiré potentials and trapped excitons. Here, however, the lack of translational symmetry is the key ingredient. It permits momentum-space coupling channels that would otherwise be inaccessible, allowing K and −K valleys in different layers to communicate through Umklapp processes. This expands the conceptual space of twistronics from periodic moiré superlattices to aperiodic and quasicrystalline systems where symmetry breaking itself becomes a functional design tool.

Several implications follow. First, large-angle twisted bilayers should not be treated as featureless or weakly coupled systems simply because their moiré wavelength is short. Instead, they can host unconventional valley coupling and hybridized excitonic states that are invisible in conventional small-angle moiré frameworks. Second, exciton spectroscopy provides a sensitive probe of hidden interlayer momentum coupling, particularly when combined with magnetic-field and temperature-dependent measurements. Third, aperiodic twist may offer new routes to valleytronic and photonic functionalities by engineering the internal composition, lifetime, oscillator strength and magnetic response of excitons.

Future work will likely focus on microscopic visualization and control. Momentum-resolved optical spectroscopy, nanoscale mapping of local stacking environments and more detailed many-body calculations could clarify how specific Umklapp channels contribute to exciton formation. Electrical gating, pressure, dielectric engineering or strain could provide additional knobs for tuning the detuning and coupling strength between intralayer and interlayer excitons. Extending this approach to other transition-metal dichalcogenides and heterobilayers may reveal a broader family of aperiodic excitonic states, including valley couplings beyond K–(−K), such as K–Q or Γ-related channels.

The study therefore points to a new direction in moiré materials research. Rather than seeking only highly ordered superlattices, researchers can exploit broken translational symmetry as an active quantum design principle. In twisted MoS₂ bilayers, this principle brings hidden valley excitons into optical view, producing bright high-energy hybridized states, suppressed valley Zeeman splitting and temperature-tunable detuning. These findings suggest that the next frontier of twistronics may lie not only in perfect moiré crystals, but also in the rich and less charted physics of aperiodic quantum matter.