Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity. The lack of direct evidence for electron-phonon ...coupling in the electron dynamics of the high-transition-temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle-resolved photoemission spectroscopy to probe electron dynamics-velocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80 meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity.
We report time- and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi2Se3. We observe oscillatory modulations of the electronic structure of both the bulk and ...surface states at a frequency of 2.23 THz due to coherent excitation of an A1g phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. Density functional theory calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A1g phonon mode coupling to the surface state in a topological insulator.
The nematic state, where a system is translationally invariant but breaks rotational symmetry, has drawn great attention recently due to the experimental observations of such a state in both cuprates ...and iron-based superconductors. The origin of nematicity and its possible tie to the pairing mechanism of high-T sub()c however, still remain controversial. Here, we study the electronic structure of a multilayer FeSe film using angle-resolved photoemission spectroscopy. The band reconstruction in the nematic state is clearly delineated. We find that the energy splitting between d sub()xzand d sub()yzbands shows a nonmonotonic distribution in momentum space. From the Brillouin zone center to the Brillouin zone corner, the magnitude of splitting first decreases, then increases, and finally reaches the maximum value of -70 meV. Moreover, besides the d sub()xzand d sub()yzbands, band splitting was also observed on the d sub()xybands with a comparable energy scale around 45 meV. Our results suggest that the electronic anisotropy in the nematic state cannot be explained by a simple on-site ferro-orbital order. Instead, strong anisotropy exists in the hopping of all d sub()xzd sub()yz and d sub()xyorbitals, the origin of which holds the key to a microscopic understanding of the nematicity in iron-based superconductors.
We used angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi₂Sr₂Ca₍₁₋x₎YxCu₂O₈ (Bi2212) to reveal the presence of two distinct energy gaps exhibiting ...different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increased with underdoping, a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near-nodal regime where a coherent peak in the spectrum can be observed, did not increase with less doping, a behavior not previously observed in the single particle spectra. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high-transition temperature superconductivity.
Self-organized electronically ordered phases are a recurring feature in correlated materials, resulting in, for example, fluctuating charge stripes whose role in high-TC superconductivity is under ...debate. However, the relevant cause-effect relations between real-space charge correlations and low-energy excitations remain hidden in time-averaged studies. Here we reveal ultrafast charge localization and lattice vibrational coupling as dynamic precursors of stripe formation in the model compound La(1.75)Sr(0.25)NiO4, using ultrafast and equilibrium mid-infrared spectroscopy. The opening of a pseudogap at a crossover temperature T* far above long-range stripe formation establishes the onset of electronic localization, which is accompanied by an enhanced Fano asymmetry of Ni-O stretch vibrations. Ultrafast excitation triggers a sub-picosecond dynamics exposing the synchronous modulation of electron-phonon coupling and charge localization. These results illuminate the role of localization in forming the pseudogap in nickelates, opening a path to understanding this mysterious phase in a broad class of complex oxides.
We explore manifestations of electron-phonon coupling on the electron spectral function for two phonon modes in the cuprates exhibiting strong renormalizations with temperature and doping. Applying ...simple symmetry considerations and kinematic constraints, we find that the out-of-plane, out-of-phase O buckling mode (B(1g)) involves small momentum transfers and couples strongly to electronic states near the antinode while the in-plane Cu-O breathing modes involve large momentum transfers and couples strongly to nodal electronic states. Band renormalization effects are found to be strongest in the superconducting state near the antinode, in full agreement with angle-resolved photoemission spectroscopy data.
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