Guanghui Cheng

Vacuum fluctuations provide an important new way to control material properties noninvasively^1-6. Here, we present experimental evidence that they can enhance superconductivity. NbSe₂ is a layered transition-metal dichalcogenide with well-characterized superconducting behavior, providing a clear platform to reveal this effect. We have observed an increase in the critical temperature of superconducting NbSe₂ when it is embedded in a split-ring cavity resonator. Near the transition temperature, the critical current and critical field increase dramatically. Our observations are consistent with theoretical calculations showing that hybridization between electronic degrees of freedom and fluctuating cavity modes lowers the energy of the superconducting state. By providing a proof-of-principle demonstration of superconductivity enhancement via vacuum fluctuations, our work establishes a noninvasive technique for controlling the mainstay of quantum technology.

In recent years, lightwave has stood out as an ultrafast, non-contact control knob for developing compact superconducting circuitry. However, the modulation efficiency is limited by the low photoresponse of superconductors. Plasmons, with the advantages of strong light-matter interaction, present a promising route to overcome the limitations. Here we achieve effective modulation of superconductivity in thin-film NbSe₂ via near-field coupling to plasmons in gold nanoparticles. Upon resonant plasmon excitation, the superconductivity of NbSe₂ is substantially suppressed. The modulation factor exceeds 40% at a photon flux of 9.36 × 10¹³s⁻¹mm⁻², and the effect is significantly diminished for thicker NbSe₂ samples. Our observations can be theoretically interpreted by invoking the non-equilibrium electron distribution in NbSe₂ driven by the plasmon-associated evanescent field. Finally, a reversible plasmon-driven superconducting switch is realized in this system. These findings highlight plasmonic tailoring of quantum states as an innovative strategy for superconducting electronics.

Strain in two-dimensional (2D) materials has attracted particular attention because of the remarkable modification of electronic and optical properties. However, emergent electromechanical phenomena and hidden mechanisms, such as strain-superlattice-induced topological states or flexoelectricity under strain gradient, remain under debate. Here, using scanning photocurrent microscopy, we observe significant photo -current enhancement in hybrid vertical junction devices made of strained few-layer graphene and InGaN quantum dots. Optoelectronic response and photoluminescence measurements demonstrate a possible mechanism closely tied to the flexoelectric effect in few-layer graphene, where the strain can induce a lateral built-in electric field and assist the separation of electron-hole pairs. Photocurrent mapping reveals an unprecedentedly ordered hexagonal network, suggesting the potential to create a superlattice by strain engineering. Our work provides insights into optoelectronic phenomena in the presence of strain and paves the way for practical applications associated with strained 2D materials.

Monolayer transition metal dichalcogenides (ML-TMDCs) are a versatile platform to explore the transport dynamics of the tightly bound excitonic states. The diffusion of neutral excitons in various ML-TMDCs has been observed. However, the transport of charged excitons (trions), which can be driven by an in-plane electric field and facilitate the formation of an excitonic current, has yet been well investigated. Here, we report the direct observation of diffusion and drift of the trions in ML-WS₂ through spatially and time-resolved photoluminescence. An effective diffusion coefficient of 0.47 cm²/s was extracted from the broadening of spatial profiles of the trion emission. When an in-plane electric field is applied, the spatial shift of the trion emission profiles indicated a drift velocity of 7400 cm/s. Both the diffusion caused broadening and the drift caused shift of the emission profiles saturate because of the Coulomb interactions between trions and the background charges.

In this work, we demonstrated that p-type and n-type conduction could be induced in unintentionally doped pristine monolayer (ML) WS2 by forming a hybrid WS2/InGaN quantum dots (QDs) heterostructure, in which the ML-WS2 is partially covered by few-layer graphene. Under illumination, the photo-generated holes or electrons in the QDs were injected vertically into the ML-WS2 and then transported laterally therein. The polarity of the WS2 channel can be controlled by the bias applied to the graphene electrode. This work provides a potential approach to develop ambipolar devices of ML transition metal dichalcogenides through photocarrier doping.

An investigation of self-assembled polar InGaN quantum dots (QDs) on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD) is reported. The radiative exciton lifetime is measured by time-resolved photoluminescence at a low temperature of 18 K, where the non-radiative recombination can be negligible. A mono-exponential exciton decay with a radiative exciton lifetime of 480 ps for uncapped QDs is revealed. With an optimized GaN capping layer grown by a two-step method, a radiative exciton lifetime of 707 ps for the capped QDs is preserved. The short radiative exciton lifetime is much shorter than that for previously studied polar QDs and is even comparable with those grown along non-polar QDs, which is strong evidence of the reduction of built-in fields in these polar InGaN QDs.

Gallium nitride (GaN)-based light-emitting diodes (LEDs) are important for lighting and display applications. In this paper, we demonstrate green-emission (512 nm) InGaN quantum dot (QD) LEDs grown on a c-plane sapphire substrate by metal-organic chemical vapor deposition. A radiative lifetime of 707 ps for the uniform InGaN self-assembled QDs is obtained by time-resolved photoluminescence measurement at 18 K. The screening of the built-in fields in the QDs effectively improves the performance of QD LEDs. These high quantum efficiency and high temperature stability green QD LEDs are able to operate with negligible efficiency droop and with current density up to 106    A / cm 2 . Our results show that InGaN QDs may be a viable option as the active medium for stable LEDs.

Highly stable excited-state biexcitons are found in graphene/WS ₂ /BN heterostructures, which is attributed to the charge transfer and exciton dissociation at the hetero-interface.

Defect mediated nonradiative recombination limits the photoluminescence (PL) quantum yield of transition metal dichalcogenide monolayers (MLs). In this work, the enhancement of the PL intensity of ML WS₂ is reported in a van der Waals heterostructure of WS₂ ML and InGaN quantum dots (QDs) under excitation with photon energy larger than the bandgaps of both WS₂ and QDs. The mechanism of this PL enhancement is due to the aggregation of excitons in WS₂ ML toward the QD sites to form an interfacial bound state, which effectively mitigates the influence of defects. This exciton aggregation induced enhancement of PL intensity is more pronounced at low temperatures and under low power excitations. The lifetime and diffusion coefficient of the excitons in WS₂ ML are also measured to validate the aggregation scenario.