Research
Exploring electronic band structures of solid-state materials
Angle-resolved Photoemission Spectroscopy (ARPES)
Introduction of ARPES: https://www.youtube.com/watch?v=jdaqLhknhw8
Andrea Damascelli et al., Rev. Mod. Phys. 75, 473 (2003):
https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.75.473
Current Project
Construction of Nano-ARPES at TPS 39A2
We are constructing a Nano-scaled Angle-Resolved Photoemission Spectroscopy (ARPES) setup at the TPS BL39A2 end station. With a Nano-scaled synchrotron beam spot, this setup enables the investigation of the electronic structure of materials ranging in size from micrometers to nanometers, including heterostructures, multi-terminations at surfaces, and field-effect transistor devices, etc.
To achieve this, we utilize an elliptically polarized undulator (EPU) to provide synchrotron radiation (SR) light across a wide energy range of approximately 50-200eV, offering multiple polarization options (linear vertical, linear horizontal, left circular, and right circular polarization). A plane-mounted grating monochromator and two sets of Kirkpatrick-Baez (K-B) mirrors are employed to select the photon energy and refocus the beam spot to several micrometers. Finally, a fresnel zone plate and an Objective aperture (OSA) are utilized to focus the beam to the sub-micron scale further.
For sample manipulation and exploration of electronic structure in k-space, a 6-axis sample stage is employed to scan the sample. Electron kinetic energy and parallel momentum are recorded using a hemispherical analyzer with high energy and angular resolution.
Research Projects
Dirac Nodal Lines, Hourglass fermions in Semimetal Nb3SiTe6
keywords: topological dirac semimetals, hourglass fermion, nonsymmorphic symmetry, angle-resolved photoemission spectroscopy
R.-Y. Liu*, A. Huang, R. Sankar, J. A. Hlevyack, C.-C. Su, S.-C. Weng, M.-K. Lin, P. Chen, C.-M. Cheng, J. D. Denlinger, S.-K. Mo, A. V. Fedorov, C.-S. Chang, H.-T. Jeng*, T.-M. Chuang, T.-C. Chiang*, “Dirac Nodal Line in Hourglass Semimetal Nb3SiTe6”, Nano Lett. 23, 1, 380–388 (2023)
【研究成果】半金屬Nb3SiTe6材料中的沙漏費米子與狄拉克節點線 - 自然科學及永續研究推展中心 (ntu.edu.tw)
Hourglass fermion is a novel fermionic electronic state in Topological crystals. Researchers predicted its potential presence in two classes of materials with non-symmorphic symmetry in 2016. This electronic state, characterized by its distinctive hourglass-shaped energy band structure, also exhibits a unique three-dimensional extension of the quantum Hall spin effect. Our research team utilized various techniques such as Vacuum Ultraviolet (VUV) angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and X-ray diffraction to analyze the hourglass fermion electronic state in layered semimetal material(Nb3SiTe6). In addition to measuring the Dirac nodal lines of the hourglass fermion, they also observed the electronic band structure with spin-polarizations. We provided ample intuitive evidence to facilitate a deeper understanding of the physical properties of hourglass fermions.
Carrier dynamics in TMDC monolayer studied by HHG-based time-resolved ARPES
SPV relaxation in TMDCs by time-resolved XPS
keywords: transition metal dichalcogenides monolayers, time-resolved and angle-resolved photoemission spectroscopy, carrier dynamics, high-harmonic generation lasers, surface photovoltage effect, time-resolved x-ray photoemission spectroscopy, transition metal dichalcogenides
R.-Y. Liu, M.-K. Lin, P. Chen, T. Suzuki, P. C. J. Clark, N. K. Lewis, C. Cacho, E. Springate, C.-S. Chang, K. Okazaki, W. Flavell, I. Matsuda, T.-C. Chiang “Symmetry-breaking and spin-blockage effects on carrier dynamics in single-layer tungsten diselenide”, Phys. Rev. B, 100, 214309 (2019).
R.-Y. Liu, K. Ozawa, N. Terashima, Y. Natsui, B. Feng, S. Ito, W.-C. Chen, C.-M. Cheng, S. Yamamoto, H. Kato, T.-C. Chiang, I. Matsuda, “Controlling the surface photovoltage on WSe2 by surface chemical modification”, Appl. Phys. Lett., 112, 211603 (2018)
R.-Y. Liu, Y. Ogawa, P. Chen, K. Ozawa, T. Suzuki, M. Okada, T. Someya, Y. Ishida, K. Okazaki, S. Shin, T.-C. Chiang and I. Matsuda, “Femtosecond to picosecond transient effects in WSe2 observed by pump-probe angle-resolved photoemission spectroscopy”, Scientific Reports, 7, 15891 (2017).
Two-dimensional transition metal dichalcogenides (TMDCs) have garnered significant attention in the realms of valley-, spin-, and optoelectronics. Carrier dynamics of these materials are the key to applying TMDCs to emerging optical-electronic devices. However, comprehending the non-equilibrium electronic structure has remained elusive. Photoemission spectroscopy stands out as the most potent experimental technique for directly probing electronic states in materials. Recent advancements in time-resolved measurements now enable real-time tracking of these states. In this study, we devoted to ultrafast carrier dynamics at the surfaces of WSe2 crystals and monolayer WSe2 using time-resolved photoemission spectroscopy. Employing two light sources—the high-harmonic generation laser and highly brilliant synchrotron radiation—we chronologically trace the time evolution from femtoseconds to nanoseconds after optical pumping. The dynamical data are systematically discussed in terms of electronic structure.
In the femtosecond-time scale, various non-equilibrium states are observed in semiconducting WSe2 crystals, contingent upon the photon energy of the optical pulse. Floquet replica bands emerge when the pumping photon energy is below the bulk direct band gap. For photon energies exceeding the bulk band gap, photo-excitation to the bulk conduction band exhibits valley-dependent behavior, showing the exotic circular-polarization dependence. At the monolayer WSe2 surface, a notable band shift is observed in the femtosecond range by above-band-gap pump pulses. This shift is attributed to band gap renormalization due to population inversion and strong electronic interaction, decaying within picoseconds due to spin-selected carrier recombination.
In the subsequent picosecond-time scale, oscillations in the photoemission intensity of the bulk valence band reveal a frequency of 6.7 THz, related to coherent phonons. This observation indicates the formation of a two-phonon squeezed state.
In the nanosecond-time scale, relaxation of the surface photovoltage (SPV) effect, induced concurrently with photo-excitation, is apparent, reflecting carrier transfer between the surface and bulk regions. Formation of heterojunctions on WSe2 surfaces with donor (K)- or acceptor (C60)-adlayers modifies the generation and relaxation of SPV, suggesting potential optoelectronic regulation.
This study unveils temporal variations in the non-equilibrium electronic structure of WSe2 post-optical pulse using time-resolved photoemission spectroscopy. The observed dynamical phenomena occurring on various time scales are consistently described by light-matter interaction and carrier dynamics, offering insights for advancements in photo-science and optoelectronic devices.
Oscllations in ultra-thin metal films studied by ARPES
keywords: Metal thin films, Quantum well state, Angle-resolved Photoemission Spectroscopy, Molecular beam epitaxy growth, ultra high vacuum system
R.-Y. Liu, A. Huang, C.-C. Huang, C.-Y. Lee, C.-H. Lin, C.-M. Cheng, K.-D. Tsuei, H.-T. Jeng, I. Matsuda, and S.-J. Tang, “Deeper insight into phase relations in ultrathin Pb films”, Phys. Rev. B 92, 115415 (2015).
R.-Y. Liu,"Studies of work function, electron-phonon coupling constant, and superconductivity transition temperature of Pb thin films on Ge(111)", master degree thesis, NTHU (2013)
We conducted work function measurements on Pb thin films deposited on Ge(111) substrates ranging from 1 to 13 monolayers (ML). Our results revealed a bilayer oscillation in the work function as a function of thin film thickness. Additionally, we observed that the higher the highest occupied quantum well state (HOQWS), the higher the work function. A notable turning point in the bilayer oscillation was identified near 11 ML. Comparing the work function data with the thermal stability of the thin films, we discovered a phase difference of 1.62 between the two phenomena, which closely aligns with the 1.57 phase difference, or one-fourth of the periodicity, predicted by the free electron model. In another aspect of our study, we conducted microscopic four-point probes (micro 4PP) conductivity measurements on Pb thin films. We determined the superconductivity transition temperature (Tc) of 3 ML, 4 ML, and 8 ML films to be 1.60 K, 3.65 K, and 5.21 K, respectively. Additionally, electron-phonon coupling (EPC) measurements were performed on 6 ML and 8 ML Pb thin films using ARPES. The EPC values for the quantum well states (QWS) of the 6 ML and 8 ML Pb thin films were found to be 1.09 and 1.27, respectively. From the aforementioned results, we observed that both the Tc and EPC of the Pb films increase with film thickness, consistent with findings in similar systems. Finally, we conducted a comparative analysis and discussion of the superconductivity transition temperature obtained through micro 4PP measurements and predictions based on EPC.