Visualization of an axion insulating state at the transition between 2 chiral quantum anomalous Hall states
Monica Allen, Yongtao Cui, Eric Yue Ma, Masataka Mogi, Minoru Kawamura, Ion Cosma Fulga, David Goldhaber-Gordon, Yoshinori Tokura, Zhi-Xun Shen
Magnetic topological insulators host chiral dissipationless edge modes, which mimic quantum Hall states but persist in the absence of a magnetic field. We use microwave impedance microscopy, which characterizes the local complex conductivity of a material, to provide direct visualization of these edge states and monitor their evolution across a magnetic-field–induced phase transition. The resulting images reveal an insulating state, which exhibits a distinct geometry of current flow, at the boundary between 2 quantum anomalous Hall (QAH) states with opposite chirality. Due to their immunity to backscattering, the edge currents present in the QAH regime provide a promising platform for future investigations of chiral Majorana modes, key building blocks for a topological quantum computer.
Rapid change of superconductivity and electron-phonon coupling through critical doping in Bi-2212
Y. He, M. Hashimoto, D. Song, S.-D. Chen, J. He, I. M. Vishik, B. Moritz, D.-H. Lee, N. Nagaosa, J. Zaanen, T. P. Devereaux, Y. Yoshida, H. Eisaki, D. H. Lu, Z.-X. Shen
Superconductivity happens when weakly interacting electrons develop an attraction towards each other and pair up. In classic theory of conventional superconductivity (Bardeen, Cooper and Schrieffer, 1957; Nobel prize 1972), it is the slowly vibrating lattice - or “phonons” when quantized - that provides this superglue. However, this mechanism has long been considered absent/insufficient in the hitherto strongest superconductor - high-temperature copper-based (cuprate) superconductors. Using angle-resolved photoemission spectroscopy, we recently discover that a particular type of lattice coupling can act in conjunction with increasing electron-electron correlation, consequently participating in a rapid increase of superconducting strength (energy x4) and change in superconducting character (gap-to-Tc ratio x2). This discovery explicitly links the lattice vibration to the otherwise purely electronically driven high-Tc superconductivity in the cuprates, and provides a promising avenue for multi-channel enhanced superconductivity for future superconductor engineering.
Quantum spin Hall state in monolayer 1T'-WTe2
Shujie Tang, Chaofan Zhang, Dillon Wong, Zahra Pedramrazi, Hsin-Zon Tsai, Chunjing Jia, Brian Moritz, Martin Claassen, Hyejin Ryu, Salman Kahn, Juan Jiang, Hao Yan, Makoto Hashimoto, Donghui Lu, Robert G. Moore, Chan-Cuk Hwang, Choongyu Hwang, Zahid Hussain, Yulin Chen, Miguel M. Ugeda, Zhi Liu, Xiaoming Xie, Thomas P. Devereaux, Michael F. Crommie, Sung-Kwan Mo & Zhi-Xun Shen
A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin–orbit coupling. By investigating the electronic structure of epitaxially grown monolayer 1T'-WTe2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulk bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe2 as a new class of QSH insulator with large bandgap in a robust two-dimensional materials family of transition metal dichalcogenides.
Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser
S. Gerber, S.-L. Yang, D. Zhu, H. Soifer, J. A. Sobota, S. Rebec, J. J. Lee, T. Jia, B. Moritz, C. Jia, A. Gauthier, Y. Li, D. Leuenberger, Y. Zhang, L. Chaix, W. Li, H. Jang, J.-S. Lee, M. Yi, G. L. Dakovski, S. Song, J. M. Glownia, S. Nelson, K. W. Kim, Y.-D. Chuang, Z. Hussain, R. G. Moore, T. P. Devereaux, W.-S. Lee, P. S. Kirchmann, Z.-X. Shen
The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a “coherent lock-in” measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.
Mobile metallic domain walls in an all-in-all-out magnetic insulator
Eric Yue Ma, Yong-Tao Cui, Kentaro Ueda, Shujie Tang, Kai Chen, Nobumichi Tamura, Phillip M. Wu, Jun Fujioka, Yoshinori Tokura, Zhi-Xun Shen
Magnetic domain walls are boundaries between regions with different configurations of the same magnetic order. In a magnetic insulator, where the magnetic order is tied to its bulk insulating property, it has been postulated that electrical properties are drastically different along the domain walls, where the order is inevitably disturbed. Here we report the discovery of highly conductive magnetic domain walls in a magnetic insulator, Nd2Ir2O7, that has an unusual all-in-all-out magnetic order, via transport and spatially resolved microwave impedance microscopy. The domain walls have a virtually temperature-independent sheet resistance of ~1 kilohm per square, show smooth morphology with no preferred orientation, are free from pinning by disorders, and have strong thermal and magnetic field responses that agree with expectations for all-in-all-out magnetic order.
Direct spectroscopic evidence for phase competition between the pseudogap and superconductivity in Bi2Sr2CaCu2O8+δ
M. Hashimoto, E. A. Nowadnick, R.-H. He, I. M. Vishik, B. Moritz, Y. He, K. Yanaka, R. G. Moore, D. Lu, Y. Yoshida, M. Ishikado, T. Sasagawa, K. Fuita, S. Ishida, S. Uchida, H. Eisaki, Z. Hussain, T. P. Devereaux, Z.-X. Shen
In the high-temperature (Tc) cuprate superconductors, increasing evidence suggests that the pseudogap, existing below the pseudogap temperature T*, has a distinct broken electronic symmetry from that of superconductivity. Particularly, recent scattering experiments on the underdoped cuprates have suggested that a charge ordering competes with superconductivity. However, no direct link of this physics and the important low-energy excitations has been identified. Here we report an antagonistic singularity at Tc in the spectral weight of Bi2Sr2CaCu2O8+δ as a compelling evidence for phase competition, which persists up to a high hole concentration p ~ 0.22. Comparison with a theoretical calculation confirms that the singularity is a signature of competition between the order parameters for the pseudogap and superconductivity. The observation of the spectroscopic singularity at finite temperatures over a wide doping range provides new insights into the nature of the competitive interplay between the two intertwined phases and the complex phase diagram near the pseudogap critical point.
Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3
J. J. Lee, F. T. Schmitt, R. G. Moore, S. Johnston, Y.-T. Cui, W. Li, M. Yi, Z. K. Liu, M. Hashimoto, Y. Zhang, D. H. Lu, T. P. Devereaux, D.-H. Lee, Z.-X. Shen
How Cooper pairs, the building blocks of superconductivity, can form at high temperatures in the cuprates and iron-based superconductors continues to pose a major intellectual challenge in physics. Recently a system composed of a single unit cell thick iron selenide film (1UC FeSe) grown on SrTiO3 (STO) substrate shows a superconducting-like energy gap at temperatures close to the boiling point of liquid nitrogen (77 K), a record for iron-based superconductors. Here, we report high resolution angle resolved photoemission spectroscopy (ARPES) results which reveal an unexpected and unique characteristic of the 1UC FeSe/STO system: each energy band of the FeSe film is almost exactly replicated at a fixed energy separation. Such a special shake off phenomenon suggests the presence of bosonic modes, most likely oxygen optical phonons in STO, which can only transmit anomalously small momentum to the electrons. Small-momentum-transfer electron-phonon coupling has the unusual benefit of helping superconductivity inmost channels, including those mediated by spin fluctuations. Our calculations suggest such coupling is responsible for raising the superconducting gap opening temperature in 1UC FeSe/STO. This discovery suggests a pathway to engineer high temperature superconductors.