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Recent Highlights

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

Science 357, 71 (2017)

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

Science 350, 538 (2015)

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

Nature Materials 14, 37 (2015)

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

Nature 515, 245 (2014)

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.