Novel physical properties emerge when the thickness of charge density wave(CDW)materials is reduced to the atomic level,owing to the significant modification of the electronic band structure and correlation effects.He...Novel physical properties emerge when the thickness of charge density wave(CDW)materials is reduced to the atomic level,owing to the significant modification of the electronic band structure and correlation effects.Here,we investigate the layer-dependent CDW phase transition and evolution of the nonequilibrium state of 1T-TaS_(2)nanoflakes using pump-probe spectroscopy.Both the low-energy single-particle and collective excitation relaxations exhibit sharp changes at〜210 K,indicating a phase transition from commensurate CDW to nearly commensurate CDW state.The single particle process reveals that the phase transition stiffness(PTS)is thickness-dependent.Moreover,a small PTS is observed in thin nanoflakes,which is attributed to the reduced thickness that increases the fluctuation and inhibits the nucleation and growth of discommensurations.In addition,the phase mode vanishes when the discommensuration network appears.Our results suggest that the carrier dynamics could be an efficient operational approach to measuring the quantum phase transition in correlated materials.展开更多
Questions hovering over the modulation of bandgap size and excitonic effect on nonlinear absorption in twodimensional transition metal dichalcogenides (TMDCs) have restricted their application in micro/nano optical mo...Questions hovering over the modulation of bandgap size and excitonic effect on nonlinear absorption in twodimensional transition metal dichalcogenides (TMDCs) have restricted their application in micro/nano optical modulator, optical switching, and beam shaping devices. Here, degenerate two-photon absorption (TPA) in the near-infrared region was studied experimentally in mechanically exfoliated MoS2 from single layer to multilayer. The layer-dependent TPA coefficients were significantly modulated by the detuning of the excitonic dark state (2p). The shift of the quasiparticle bandgap and the decreasing of exciton binding energy with layers were deduced, combined with the non-hydrogen model of excitons in TMDCs and the scaling rule of semiconductors. Our work clearly demonstrates the layer modulation of nonlinear absorption in TMDCs and provides support for layer-dependent nonlinear optical devices, such as optical limiters and optical switches.展开更多
基金We thank Prof.Yong Wang(Nankai University)for the valued discussions.We acknowledge financial support from the National Key Research and Development Program of China(Nos.2017YFA0205000,2017YFA0303600,2016YFA0200701)the National Natural Science Foundation of China(Nos.21425310,21790353,21721002,21822502,21673058)+1 种基金Strategic Priority Research Program of Chinese Academy of Sciences(Nos.XDB36000000,XDB30000000)the Key Research Program of Frontier Sciences of CAS(No.QYZDB-SSW-SYS031).
文摘Novel physical properties emerge when the thickness of charge density wave(CDW)materials is reduced to the atomic level,owing to the significant modification of the electronic band structure and correlation effects.Here,we investigate the layer-dependent CDW phase transition and evolution of the nonequilibrium state of 1T-TaS_(2)nanoflakes using pump-probe spectroscopy.Both the low-energy single-particle and collective excitation relaxations exhibit sharp changes at〜210 K,indicating a phase transition from commensurate CDW to nearly commensurate CDW state.The single particle process reveals that the phase transition stiffness(PTS)is thickness-dependent.Moreover,a small PTS is observed in thin nanoflakes,which is attributed to the reduced thickness that increases the fluctuation and inhibits the nucleation and growth of discommensurations.In addition,the phase mode vanishes when the discommensuration network appears.Our results suggest that the carrier dynamics could be an efficient operational approach to measuring the quantum phase transition in correlated materials.
基金Strategic Priority Research Program of CAS(XDB16030700)Program of Shanghai Academic Research Leader(17XD1403900)+7 种基金Natural Science Foundation of Shanghai(18ZR1444700)Youth Innovation Promotion Association of the Chinese Academy of SciencesNational Natural Science Foundation of China(NSFC)(11874370,61675217,61875213)Shanghai Rising-Star Program(19QA1410000)CAS Interdisciplinary Innovation Team and National Natural Science Foundation of China(11535010)CAS Center for Excellence in Ultra-Intense Laser ScienceKey Research Program of Frontier Science of CAS(QYZDB-SSW-JSC041)President’s International Fellowship Initiative(PIFI)of CAS(2017VTA0010,2017VTB0006,2018VTB0007)
文摘Questions hovering over the modulation of bandgap size and excitonic effect on nonlinear absorption in twodimensional transition metal dichalcogenides (TMDCs) have restricted their application in micro/nano optical modulator, optical switching, and beam shaping devices. Here, degenerate two-photon absorption (TPA) in the near-infrared region was studied experimentally in mechanically exfoliated MoS2 from single layer to multilayer. The layer-dependent TPA coefficients were significantly modulated by the detuning of the excitonic dark state (2p). The shift of the quasiparticle bandgap and the decreasing of exciton binding energy with layers were deduced, combined with the non-hydrogen model of excitons in TMDCs and the scaling rule of semiconductors. Our work clearly demonstrates the layer modulation of nonlinear absorption in TMDCs and provides support for layer-dependent nonlinear optical devices, such as optical limiters and optical switches.
