Person: He, Yang
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Publication Fermi Surface and Pseudogap Evolution in a Cuprate Superconductor(American Association for the Advancement of Science (AAAS), 2014) He, Yang; Yin, Yi; Zech, M.; Soumyanarayanan, Anjan; Yee, Michael Manchun; Williams, Tess Lawanna; Boyer, M. C.; Chatterjee, K.; Wise, W. D.; Zeljkovic, Ilija; Kondo, T.; Takeuchi, T.; Ikuta, H.; Mistark, P.; Markiewicz, R. S.; Bansil, A.; Sachdev, Subir; Hudson, E. W.; Hoffman, JennyThe unclear relationship between cuprate superconductivity and the pseudogap state remains an impediment to understanding the high transition temperature (Tc) superconducting mechanism. Here, we used magnetic field–dependent scanning tunneling microscopy to provide phase-sensitive proof that d-wave superconductivity coexists with the pseudogap on the antinodal Fermi surface of an overdoped cuprate. Furthermore, by tracking the hole-doping (p) dependence of the quasi-particle interference pattern within a single bismuth-based cuprate family, we observed a Fermi surface reconstruction slightly below optimal doping, indicating a zero-field quantum phase transition in notable proximity to the maximum superconducting Tc. Surprisingly, this major reorganization of the system’s underlying electronic structure has no effect on the smoothly evolving pseudogap.Publication Charge Order Driven by Fermi-Arc Instability in Bi2Sr2-xLaxCuO6+(American Association for the Advancement of Science (AAAS), 2013) Comin, R.; Frano, A.; Yee, Michael Manchun; Yoshida, Y.; Eisaki, H.; Schierle, E.; Weschke, E.; Sutarto, R.; He, F.; Soumyanarayanan, Anjan; He, Yang; Le Tacon, M.; Elfimov, I. S.; Hoffman, Jenny; Sawatzky, G. A.; Keimer, B.; Damascelli, A.The understanding of the origin of superconductivity in cuprates has been hindered by the apparent diversity of intertwining electronic orders in these materials. We combined resonant x-ray scattering (REXS), scanning-tunneling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) to observe a charge order that appears consistently in surface and bulk, and in momentum and real space within one cuprate family. The observed wave vectors rule out simple antinodal nesting in the single-particle limit but match well with a phenomenological model of a many-body instability of the Fermi arcs. Combined with earlier observations of electronic order in other cuprate families, these findings suggest the existence of a generic charge-ordered state in underdoped cuprates and uncover its intimate connection to the pseudogap regime.Publication Quantum phase transition from triangular to stripe charge order in NbSe2(Proceedings of the National Academy of Sciences, 2013) Soumyanarayanan, Anjan; Yee, Michael Manchun; He, Yang; van Wezel, Jasper; Rahn, Dirk J.; Rossnagel, Kai; Hudson, E. W.; Norman, Michael R.; Hoffman, JennyThe competition between proximate electronic phases produces a complex phenomenology in strongly correlated systems. In particular, fluctuations associated with periodic charge or spin modulations, known as density waves, may lead to exotic superconductivity in several correlated materials. However, density waves have been difficult to isolate in the presence of chemical disorder, and the suspected causal link between competing density wave orders and high-temperature superconductivity is not understood. Here we used scanning tunneling microscopy to image a previously unknown unidirectional (stripe) charge-density wave (CDW) smoothly interfacing with the familiar tridirectional (triangular) CDW on the surface of the stoichiometric superconductor NbSe2. Our low-temperature measurements rule out thermal fluctuations and point to local strain as the tuning parameter for this quantum phase transition. We use this quantum interface to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface nesting in the CDW phase of NbSe2. Our results highlight the importance of local strain in governing phase transitions and competing phenomena, and suggest a promising direction of inquiry for resolving similarly longstanding debates in cuprate superconductors and other strongly correlated materials.Publication Scanning Tunneling Microscopy Study on Strongly Correlated Materials(2015-12-01) He, Yang; Hoffman, Jennifer; Kim, Philip; Sachdev, SubirStrongly correlated electrons and spin-orbit interaction have been the two major research directions of condensed matter physics in recent years. The discovery of high temperature superconductors in 1986 not only brought excitement into the field but also challenged our theory on quantum materials. After almost three decades of extensive study, the underlying mechanism of high temperature superconductivity is still not fully understood, the reason for which is mainly a poor understanding of strongly correlated systems. The phase diagram of cuprate superconductors has become more complicated throughout the years as multiple novel electronic phases have been discovered, while few of them are fully understood. Topological insulators are a newly discovered family of materials bearing topological non-trivial quantum states as a result of spin-orbit coupling. The theoretically predicted topological Kondo insulators as strongly correlated systems with strong spin-orbital coupling make an ideal playground to test our theory of quantum materials. Scanning tunneling microscopy (STM) is a powerful technique to explore new phenomena in materials with exotic electronic states due to its high spacial resolution and high sensitivity to low energy electronic structures. Moreover, as a surface-sensitive technique, STM is an ideal tool to investigate the electronic properties of topological and non-topological surface states. In this thesis, I will describe experiments we performed on high temperature superconductors and topological Kondo insulators using STM. First, I will describe our experiments on a Bi-based high temperature superconductor $\mathrm{Bi_2Sr_2CuO_{6+\delta}}$. The quasiparticle interference technique uncovers a Fermi surface reconstruction. We also discovered the coexistence of Bogoliubov quasiparticle and pseudogap state at the antinodes. Afterwards, I will discuss our discovery of $d$-form factor density wave in the same material, showing the omnipresence of $d$ form factor density wave above and below the Fermi surface reconstruction. The relation between the $d$-form factor density wave and the pseudogap state is discussed. Second, I will describe our experiments on topological Kondo insulator $\mathrm{SmB_6}$ where spin-orbit coupling plays an important role in the strongly correlated electron system. I will present the spectroscopic evidence of Kondo hybridization based on a spectral decomposition technique. I will introduce a dimension reduction method in the fitting procedure to reduce the computation time by two order of magnitude. I will also discuss the possible quasiparticle interference patterns we discovered in $\mathrm{SmB_6}$.Publication Spin-Polarized Quantum Well States on Bi2-xFexSe3(American Physical Society (APS), 2015) Yee, Michael M.; Zhu, Z.-H.; Soumyanarayanan, Anjan; He, Yang; Song, Can-Li; Pomjakushina, Ekaterina; Salman, Zaher; Kanigel, Amit; Segawa, Kouji; Ando, Yoichi; Hoffman, JennyLow temperature scanning tunneling microscopy is used to image the doped topological insulator FexBi2−xSe3. Interstitial Fe defects allow the detection of quasiparticle interference (QPI), and the reconstruction of the empty state band structure. Quantitative comparison between measured data and density functional theory calculations reveals the unexpected coexistence of quantum well states (QWS) with topological surface states (TSS) on the atomically clean surface of FexBi2−xSe3. Spectroscopic measurements quantify the breakdown of linear dispersion due to hexagonal warping. Nonetheless, both QWS and TSS remain spin-polarized and protected from backscattering to almost 1 eV above the Dirac point, suggesting their utility for spin-based applications.Publication Imaging Emergent Heavy Dirac Fermions of a Topological Kondo Insulator(Springer Science and Business Media LLC, 2019-11-11) Soumyanarayanan, Anjan; He, Yang; Thompson, J. D.; Fisk, Z.; Wang, Xiangfeng; Paglione, Johnpierre; Morr, Dirk K.; Pirie, Harris; Liu, Yu; Chen, Pengcheng; Yee, M; Rosa, P; Hamidian, Mohammad; Hoffman, Jennifer; Kim, Dae-JeongThe interplay between strong electron interactions and band topology is a new frontier in the search for exotic quantum phases. The Kondo insulator SmB6 has emerged as a promising platform as its correlation-driven bulk gap is predicted to host topological surface modes entangled with f electrons, spawning heavy Dirac fermions . Unlike the conventional surface states of non-interacting topological insulators, heavy Dirac fermions are expected to harbor spontaneously generated quantum anomalous Hall states, non-Abelian quantum statistics, fractionalization, and topological order. However, the small energy scales required to probe heavy Dirac fermions have complicated their experimental realization. Here we use high-energy-resolution spectroscopic imaging in real and momentum space on SmB6. On cooling through 35 K, we observe the opening of an insulating gap that expands to 14 meV at 2 K. Within the gap, we image the formation of linearly dispersing surface states with effective masses reaching 410 ± 20 me. Our results demonstrate the presence of correlation-driven heavy surface states in SmB6, in agreement with theoretical predictions . Their high effective mass trans- lates to a large density of states near zero energy, which magnifies their susceptibility to the anticipated novel orders, and their potential utility.