Ultrafast carrier dynamics in GeSn thin film based on time-resolved terahertz spectroscopy

We measure the time-resolved terahertz spectroscopy of GeSn thin film and studied the ultrafast dynamics of its photo-generated carriers.The experimental results show that there are photo-generated carriers in GeSn under femtosecond laser excitation at 2500 nm,and its pump-induced photoconductivity can be explained by the Drude–Smith model.The carrier recombination process is mainly dominated by defect-assisted Auger processes and defect capture.The firstand second-order recombination rates are obtained by the rate equation fitting,which are(2.6±1.1)×10^(-2)ps^(-1)and(6.6±1.8)×10^(-19)cm^(3)·ps^(-1),respectively.Meanwhile,we also obtain the diffusion length of photo-generated carriers in GeSn,which is about 0.4μm,and it changes with the pump delay time.These results are important for the GeSn-based infrared optoelectronic devices,and demonstrate that Ge Sn materials can be applied to high-speed optoelectronic detectors and other applications.

Hybridization-mediated quasiparticle and phonon dynamics in single crystal cerium films

We report the ultrafast optical pump-probe spectroscopy measurements on the single-crystal cerium films.Our experimental results of temperature-dependent quasiparticle dynamics reveal development of the hybridization between localized f moments and conduction electrons,i.e.,evolving from fluctuating hybridization to collective hybridization.Exotic phonon renormalization is discovered to appear at the emerging temperature(T^(+))of fluctuating hybridization apart from its known presence at the coherent temperature(T^(*)),and can hardly be explained by the mean-field theory.The quasiparticle relaxation at high temperatures indicates coexistence of nonthermal electron-electron scattering and inelastic Kondo scattering,while its behavior at low temperatures suggests onset ofαphase in theγ-phase dominated film.We also extract the indirect gap below T^(*),representing the appearance of collective hybridization.Our findings provide novel information about the hybridization and phase evolution in the heavy fermion systems.

Terahertz Emission in Quantum Materials

The study of the interaction between terahertz(THz)radiation and quantum materials has been an active area of research due to its potential for understanding fundamental physics and the development of novel technologies.In this review,we focus on the time-dependent photocurrents,behind which are the up-to-date understanding of the physical processes.We provide the recent advancements in revealing the unique properties of quantum materials via the THz emission spectroscopy.Because the theoretical interpretation of some new experimental results is still evolving,this review is intended to inspire further research in this exciting and rapidly growing field.

Screen time among school-aged children of aged 6-14:a systematic review Screen time among school-aged children of aged 6-14:a systematic review

Background Screen time refers to the time an individual spends using electronic or digital media devices such as televisions,smart phones,tablets or computers.The purpose of this study was to conduct systematic review to analyze the relevant studies on the length and use of screen time of school-aged children,in order to provide scientific basis for designing screen time interventions and perfecting the screen use guidelines for school-aged children.Methods Screen time related studies were searched on PubMed,EMBASE,Clinical Trials,Controlled Trials,The WHO International Clinical Trials Registry Platform,the Cochrane Central Register of Controlled Trials,CNKI,and Whipple Journal databases from January 1,2016 to October 31,2021.Two researchers independently screened the literature and extracted the data,and adopted a qualitative analysis method to evaluate the research status of the length and usage of screen time of school-aged students.Results Fifty-three articles were included.Sixteen articles studied screen time length in the form of continuous variables.Thirty-seven articles studied screen time in the form of grouped variables.The average screen time of schoolchildren aged 6 to 14 was 2.77 h per day,and 46.4%of them had an average screen time≥2 h per day.A growth trend could be roughly seen by comparing studies in the same countries and regions before and after the COVID-19 outbreak.The average rates of school-aged children who had screen time within the range of≥2 h per day,were 41.3%and 59.4%respectively before and after January 2020.The main types of screen time before January 2020 were watching TV(20 literatures),using computers(16 literature),using mobile phones/tablets(4 literatures).The mainly uses of screens before January 2020 were entertainment(15 literatures),learning(5 literatures)and socializing(3 literatures).The types and mainly uses of screen time after January 2020 remained the same as the results before January 2020.Conclusions Excessive screen time has become a common behavior among children and adolescents around the world.Intervention measures to control children’s screen use should be explored in combination with different uses to reduce the proportion of non-essential uses.

Extremely low-energy collective modes in a quasi-one-dimensional topological system

We have investigated the quasiparticle dynamics and collective excitations in the quasi-one-dimensional material ZrTe_(5) using ultrafast optical pump-probe spectroscopy.Our time-domain results reveal two coherent oscillations having extremely low energies of hω_(1)~0.33 me V(0.08 THz)and hω_(2)-1.9 me V(0.45 THz),which are softened as the temperature approaches two different critical temperatures(~54 K and~135 K).We attribute these two collective excitations to the amplitude mode of photoinduced dynamic charge density waves in ZrTe_(5) with tremendously small nesting wave vectors.Furthermore,a peculiar quasiparticle decay process associated with the hω_(2) mode with a timescale of~1-2 ps is found below the transition temperature T*(~135 K).Our findings provide pivotal information for studying the fluctuating order parameters and their associated quasiparticle dynamics in various low-dimensional topological systems and other materials.