Publications & News
Unconventional spectral signature of Tc in a pure d-wave superconductor
Su-Di Chen, Makoto Hashimoto, Yu He, Dongjoon Song, Jun-Feng He, Ying-Fei Li, Shigeyuki Ishida, Hiroshi Eisaki, Jan Zaanen, Thomas P. Devereaux, Dung-Hai Lee, Dong-Hui Lu & Zhi-Xun Shen
In conventional superconductors, the superconducting phase transition into a zero-resistance, perfectly diamagnetic state is accompanied by a jump in the specific heat and the opening of a spectral gap. The transport, magnetic, and thermodynamic signatures of this transition in unconventional, high-transition temperature (Tc) cuprates have been known since the 1980s, yet its associated spectroscopic singularity remained unknown. Here we resolve this long-standing puzzle with a high-precision angle-resolved photoemission spectroscopy (ARPES) study on overdoped (Bi,Pb)2Sr2CaCu2O8+δ (Bi2212).
SLAC and Stanford researchers reveal the fourth signature of the superconducting transition in cuprates
The results cap 15 years of detective work aimed at understanding how these materials transition into a superconducting state where they can conduct electricity with no loss.
After 20 Years of Trying, Scientists Succeed in Doping a 1D Atomic Chain of Cuprates
The chemically controlled chains reveal an ultrastrong attraction between electrons that may help cuprate superconductors carry electrical current with no loss at relatively high temperatures.
Incoherent strange metal sharply bounded by a critical doping in Bi2212
Su-Di Chen, Makoto Hashimoto, Yu He, Dongjoon Song, Ke-Jun Xu, Jun-Feng He, Thomas P. Devereaux, Hiroshi Eisaki, Dong-Hui Lu, Jan Zaanen, Zhi-Xun Shen
In normal metals, macroscopic properties are understood using Fermi liquid theory and the concept of quasiparticles. In the cuprate high-temperature superconductors, the metallic state at doping levels producing the highest transition temperature is anomalous, and is known as a 'strange metal'. It does not conform to Fermi liquid theory, and is characterized by an incoherent spectral function. We studied this state using angle-resolved photoemission spectroscopy. When doping is increased across a temperature-independent critical doping pc ~ 0.19, we observed that near the Brillouin zone boundary, the strange metal abruptly reconstructs into a more conventional metal with quasiparticles. Above the temperature of superconducting fluctuations, we found that the pseudogap also discontinuously collapses at the very same value of pc. These observations suggest that the incoherent strange metal is a distinct state and a prerequisite for the pseudogap, incompatible with existing scenarios relating the pseudogap to a quantum critical point.
Visualization of an axion insulating state at the transition between two 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 directly visualize 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 two quantum anomalous Hall (QAH) states with opposite chirality. Due to their immunity to backscattering, edge currents present in the QAH regime provide a promising platform for future investigation of chiral Majorana modes, key building blocks for a topological quantum computer.
Rapid change of superconductivity and electron-phonon coupling through critical doping in Bi2212
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 the 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 pairing glue. However, this mechanism has long been considered insufficient or even absent in the high-temperature copper-based (cuprate) superconductors. Using angle-resolved photoemission spectroscopy, we recently discovered that a particular type of lattice coupling can cooperate with increasing electron-electron correlation, producing in a rapid increase of superconducting strength (×4 energy) and change in superconducting character (×2 gap-to-Tc ratio). 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. This is a result of topologically-protected dissipationless edge states which 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 using first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening — 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, consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe2, part of the two-dimensional family of transition metal dichalcogenides, as a new class of QSH insulator with a large bandgap.
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 iron 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 kiloohm 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.