1.Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2.School of Physical sciences, University of Chinese Academy of Sciences, Beijing 100049, China 3.Strong-Field and Ultrafast Photonics Lab, Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China 4.Songshan Lake Materials Laboratory, Dongguan 523808, China
Abstract:We employ the time resolved pump probe experiment to investigate the ultrafast dynamics in a topological semimetal molybdenum phosphide (MoP), which exhibits triple degenerate points in the momentum space. Two relaxation processes with the lifetime of 0.3 and 150 ps have been observed. We attribute the fast component to the electron-phonon scattering and the slow component to the phonon-phonon scattering, respectively. Temperature dependence investigation shows that both the lifetimes of the fast and slow components enhance slightly with increasing temperature. We also successfully generate and detect a thermal-stress-induced coherent acoustic phonon mode with a frequency of 0.033 THz, which does not vary with temperature. Our ultrafast spectroscopy investigation of the quasiparticle dynamics and the coherent phonon in MoP provides useful experimental facts and information about the overall excited state dynamics and the temperature dependence of electron-phonon coupling. Keywords:ultrafast spectroscopy/ triple degenerate fermions/ electron-phonon coupling/ topological semimetal
3.结果与讨论样品大小约为1 mm × 1 mm. 图1(b)展示了固定在铜托上样品的扫描电子显微镜(SEM)图片, 由于MoP单晶是绝缘体, 进行SEM测量时电子会富集在样品表面, 因此其SEM图片非常明亮. SEM结果表明我们合成的样品表面平整光滑, 适合进行光学实验探测. MoP的晶体具有Pm2(编号187)空间群结构, 该结构具有旋转对称性C3 z和镜像对称性My和Mz[7]. 图1(c)和图1(d)分别展示了不同角度下的晶格结构, 其中蓝色实心球为Mo原子, 红色实心球为P原子. 在MoP组成的基本六面体结构单元中, P原子位于(0, 0, 0)的顶点位置, Mo原子位于(1/3, 2/3, 1/2)的体内位置[7]. 图 1 MoP的时间分辨超快动力学过程 (a) 温度从7 K到290 K变化的?R/R0曲线; (b)进行泵浦探测实验所用MoP样品的SEM图片; (c)和(d)分别为MoP样品在不同角度下的晶格结构. 蓝色和红色小球分别代表Mo原子和P原子 Figure1. Time-resolved pump-probe spectroscopy showing the ultrafast dynamics of MoP: (a) The ?R/R0 of MoP at several typical temperatures from 7 to 290 K; (b) SEM image of our sample; (c) and (d) Schematic lattice structures of MoP. Blue and red balls: Mo and P atoms, respectively.
其中Afast和Aslow分别表示两个弛豫过程的幅值; τfast和τslow分别表示两个弛豫过程的寿命; f, A0, φ, 和τph分别表示相干态声子的频率、振幅、初相位以及寿命. 图3展示了7 K温度下利用(1)式对?R/R0信号进行拟合的结果, 其中黑色圆圈代表原始数据, 蓝色曲线为拟合结果, 从中可以看出拟合结果与原始数据符合得较好. 据此获得了在7 K温度下, 两个准粒子弛豫过程的寿命分别为0.16 ps和128 ps, 相干态声子的频率为0.03 THz, 初相位为–0.16π, 声子寿命大约为4.7 ps. 由图1(a)易见, 动力学行为在各个温度下相似, 采用(1)式对各个温度均进行了拟合, 拟合结果也相似, 此处不再展示其它温度结果. 在图3的插图中展示相干态声子的频率对温度的依赖关系, 声子频率稳定在0.03 THz附近, 基本不随温度变化[15]. 图 3 温度为7 K的?R/R0的拟合结果, 其中空心圆圈代表原始实验数据, 蓝色实线代表拟合曲线. 插图为激发的相干态声学支声子的频率对温度的依赖, 在整个温区均为0.033 THz Figure3. Fitting of the ?R/R0 at 7 K, where the black circles represent the raw data and the blue curve represents the fitting result, respectively. The inset illustrates the temperature dependence of the frequency of the coherent acoustic phonon, which stays 0.033 THz for the whole temperature range.
进一步利用(1)式对不同温度下的?R/R0信号进行了拟合, 拟合参数对温度的依赖汇总在图4中. 图4(a)和图4(b)分别展示了快分量的幅值与寿命对温度的依赖, 该过程的特征寿命为0.18—0.42 ps, 该时间尺度下反应的物理过程一般为电子-声子散射弛豫通道. 图4(a)和图4(b)中的红色虚线表征了振幅和寿命随温度的变化趋势, 这两者都随着温度增加而增加. 图 4 光激发载流子的弛豫过程对温度的依赖 (a)Afast, (b)τfast, (c)Afast和(d)τslow分别表示快分量和慢分量的幅值和寿命随温度的变化.红色和蓝色分别代表快分量和慢分量 Figure4. Temperature dependence of the amplitudes and lifetimes: (a)Afast, (b)τfast, (c)Afast和(d)τslow. The red and blue linesdenote the fast and slow components, respectively.
图 1 MoP的时间分辨超快动力学过程 (a) 温度从7 K到290 K变化的?R/R0曲线; (b)进行泵浦探测实验所用MoP样品的SEM图片; (c)和(d)分别为MoP样品在不同角度下的晶格结构. 蓝色和红色小球分别代表Mo原子和P原子
图 2 温度依赖的动力学二维彩图
图 3 温度为7 K的?R/R0的拟合结果, 其中空心圆圈代表原始实验数据, 蓝色实线代表拟合曲线. 插图为激发的相干态声学支声子的频率对温度的依赖, 在整个温区均为0.033 THz
图 4 光激发载流子的弛豫过程对温度的依赖 (a)Afast, (b)τfast, (c)Afast和(d)τslow分别表示快分量和慢分量的幅值和寿命随温度的变化.红色和蓝色分别代表快分量和慢分量