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Propagating Neutral Modes in an Intervalley Coherent State

Sat, 01 Feb 2025 00:00:00 +0000

Elementary excitations in quantum systems govern their responses to intrinsic and extrinsic stimuli, such as phase stabilities and device operations. Emergent collective excitations – magnons, spinons, Majorana modes etc. – are often the hallmark of exotic quantum states and can open up new device concepts with transformative performance. However, directly probing and isolating these excitations has been challenging because many of them do not couple to charge and therefore show weak responses in electrical measurements. Furthermore, they typically coexist with other conventional excitations that show stronger responses, such as charged quasiparticles, and are therefore eclipsed by the latter in steady-state measurements that measure total responses. Our mission was to develop an optical method to directly visualize charge-decoupled excitations in moiré materials.

To overcome these challenges, we developed a space-and-time-resolved pump-probe technique to image pure spin-valley modes in twisted WSe₂ moiré superlattice. First, we probe these excitations by encoding them into excitons and reading them out optically. This “exciton sensing” concept is very general, leveraging versatile interaction channels between exciton and collective excitations, such as dielectric screening or, in our case, phase-space filling. By shaping the pump laser to inject a localized packet of excitations and tracking its space-time evolution with a delayed wide-field probe, we captured non-equilibrium transport of these excitations. The combined spatial and temporal resolution enabled us to directly capture and separate multiple coexisting modes based on their distinct propagation behaviors.

Near the van Hove singularity (VHS), we discovered two new propagating collective modes. These modes travelled with drastically different velocities and, surprisingly, carried opposite spin-valley currents. One mode propagates at a large speed of >3 km/s and shows partial ballistic behavior, consistent with a gapless excitation; while the slower mode shows diffusive behavior and is likely a gapped excitation. Together, these behaviors can be naturally explained by phase (Goldstone) and amplitude (Higgs) mode of a spin-valley superfluid. Indeed, this spin-valley superfluid is predicted to appear in twisted WSe₂ moiré superlattices when electrons spontaneously break the U(1) valley rotation symmetry and enter an intervalley coherent state (IVC), i.e., a coherent superposition state of the K and K’ valleys.

The observation of new collective modes provides strong evidence of an IVC ground state. Recently, superconductivity has been discovered in adjacent regions of VHS. Studying these collective modes may help resolve the relation between the IVC and superconducting states, along with the underlying superconducting pairing mechanism. Furthermore, the IVC state, as a spin-valley superfluid, supports spin-valley supercurrent. Such new type of superfluid is a long-sought-after phenomenon for spin- and valleytronics but has remained elusive in experiments. Our results enable direct visualization of spin-valley currents, whose behaviors are consistent with a spin-valley superfluid. On the other hand, demonstrating truly dissipationless spin-valley transport will require more quantitative analysis, which relies on future improvements in both device fabrication and measurement capabilities. Finally, our space-time imaging technique can be readily applied to other collective modes in a broad range of material systems. One example is twisted MoTe₂, which is predicted to host exotic excitations including fractionalized excitations and magnetorotons in its fractional Chern insulator state. The non-equilibrium nature of our probe allows overcoming intrinsic limits of steady-state measurements and separating new excitations from conventional ones dynamically. In addition, by tracking the emergence and evolution of each collective mode in the phase diagram, our technique provides a powerful tool for uncovering hidden orders that were difficult to access. The IVC state observed here is one example. Similar concepts apply to a plethora of intriguing phases such as excitonic insulator and quantum spin liquid.

Dynamic Control of Metastable States in Two-Dimensional Zigzag Antiferromagnet

Wed, 01 Jan 2025 00:00:00 +0000

To be updated.

Optical imaging of flavor order in flat band graphene

Wed, 01 Jan 2025 00:00:00 +0000

We describe an optical technique that sensitively and selectively detects flavor textures via the exciton response of a proximal transition metal dichalcogenide layer (WSe₂). Through a systematic study of rhombohedral and rotationally faulted graphene bilayers and trilayers, we show that when the WSe₂ is in direct contact with the graphene, the exciton response is most sensitive to the large momentum rearrangement of the Fermi surface, and thus flavor orders. The wide-field imaging capability of optical probes allows us to obtain spatial maps of flavor orders with high throughput, and with broad temperature and device compatibility.

Tunable transient valley-pseudospin orders in a moiré Bose-Hubbard model

Mon, 01 Jan 2024 00:00:00 +0000

We demonstrate exciton-filling and magnetic field tunable exciton valley-pseudospin orders in a doped correlated insulator of excitons. We find evidence of an in-plane order of exciton ‘spin’ – here, valley pseudospin – around exciton filling vex = 1, which strongly suppresses the out-of-plane ‘spin’ polarization. Upon increasing vex or applying a small magnetic field of ~10 mT, it transitions into an out-of-plane ferromagnetic spin order that spontaneously enhances the ‘spin’ polarization, i.e., the circular helicity of emission light is higher than the excitation. The phase diagram is qualitatively captured by a spin-1/2 Bose–Hubbard model.

Correlated insulator of excitons in WSe2/WS2 moiré superlattices

Thu, 18 May 2023 00:00:00 +0000

We observed a bosonic correlated insulator in WSe₂/WS₂ moiré superlattices composed of excitons, tightly bound electron-hole pairs. In our experiment, we built a novel optical pump–probe technique, which is an optical counterpart of electrical capacitance measurement and distinctively different from conventional pump–probe concepts. We directly obtained exciton chemical potential depending on exciton density and identified an incompressible state of exciton at filling $\nu_{\text{ex}} = 1$, the hallmark of a bosonic correlated insulator. By further varying charge density with gate, we also observed a mixed correlated insulator involving both fermionic electrons and bosonic excitons. These observations are well captured by a Bose-Fermi-Hubbard model.

Gate tunability of the superconducting state at the EuO/KTaO3(111) interface

Fri, 01 Jan 2021 00:00:00 +0000