Abstract
Plasmons in two-dimensional (2D) materials beyond graphene have recently gained much attention. However, the experimental investigation is limited due to the lack of suitable materials. Here, we experimentally demonstrate localized plasmons in a correlated 2D charge-density-wave (CDW) material: 2H-TaSe2. The plasmon resonance can cover a broad spectral range from the terahertz (40 μm) to the telecom (1.55 μm) region, which is further tunable by changing thickness and dielectric environments. The plasmon dispersion flattens at large wave vectors, resulted from the universal screening effect of interband transitions. More interestingly, anomalous temperature dependence of plasmon resonances associated with CDW excitations is observed. In the CDW phase, the plasmon peak close to the CDW excitation frequency becomes wider and asymmetric, mimicking two coupled oscillators. Our study not only reveals the universal role of the intrinsic screening on 2D plasmons, but also opens an avenue for tunable plasmons in 2D correlated materials.
Highlights
Plasmons in two-dimensional (2D) materials beyond graphene have recently gained much attention
Recent theoretical studies suggest that the plasmon dispersion in real 2D materials flattens universally due to the intrinsic dielectric screening from interband transitions[13,14,15], which is inevitable for almost every crystal
2H-TaSe2 belongs to a transition metal dichalcogenide (TMDC), which attracts much attention due to the appearance of CDW orders[16]
Summary
Plasmons in two-dimensional (2D) materials beyond graphene have recently gained much attention. The plasmon dispersion flattens at large wave vectors, resulted from the universal screening effect of interband transitions. Recent theoretical studies suggest that the plasmon dispersion in real 2D materials flattens universally due to the intrinsic dielectric screening from interband transitions[13,14,15], which is inevitable for almost every crystal. Some later theoretical calculations and doping experiments revealed that the narrow d bands near the Fermi level may account for the negative dispersion[22,23,24] All these studies based on EELS are for bulk plasmons, while the coupling between 2D plasmons and CDW excitations in TaSe2 thin films is still unclear. We observe the coupling effects between TaSe2 plasmons and CDW excitations, which causes non-monotonic change of the peak height and linewidth when the temperature decreases. On the contrary to TaSe2, the plasmon peak of NbSe2 continually becomes sharper with decreasing temperature
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