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.

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Exploring cuprate chains Superconductivity in cuprates takes place in their two-dimensional (2D) layers but solving even the simplest model of interacting fermions in 2D is a challenge. The theory problem simplifies in 1D, with experiment becoming the tricky part. Chen et al . synthesized a cuprate that consists of parallel chains and behaves like a 1D system. Crucially, the material could be doped over a wide range of hole concentrations. The researchers showed that including a near-neighbor attractive interaction in a 1D model of interacting fermions was necessary to explain their photoemission measurements. —JS

The observation of replica bands in single-unit-cell FeSe on SrTiO3 (STO)(001) by angle-resolved photoemission spectroscopy (ARPES) has led to the conjecture that the coupling between FeSe electrons and the STO phonons are responsible for the enhancement of Tc over other FeSe-based superconductors. However the recent observation of a similar superconducting gap in single-unit-cell FeSe/STO(110) raised the question of whether a similar mechanism applies. Here we report the ARPES study of the electronic structure of FeSe/STO(110). Similar to the results in FeSe/STO(001), clear replica bands are observed. We also present a comparative study of STO(001) and STO(110) bare surfaces, and observe similar replica bands separated by approximately the same energy, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on different STO surface terminations are likely enhanced by a common mechanism.

We developed a table-top vacuum ultraviolet (VUV) laser with 113.778 nm wavelength (10.897 eV) and demonstrated its viability as a photon source for high resolution angle-resolved photoemission spectroscopy (ARPES). This sub-nanosecond pulsed VUV laser operates at a repetition rate of 10 MHz, provides a flux of 2 × 10(12) photons/s, and enables photoemission with energy and momentum resolutions better than 2 meV and 0.012 Å(-1), respectively. Space-charge induced energy shifts and spectral broadenings can be reduced below 2 meV. The setup reaches electron momenta up to 1.2 Å(-1), granting full access to the first Brillouin zone of most materials. Control over the linear polarization, repetition rate, and photon flux of the VUV source facilitates ARPES investigations of a broad range of quantum materials, bridging the application gap between contemporary low energy laser-based ARPES and synchrotron-based ARPES. We describe the principles and operational characteristics of this source and showcase its performance for rare earth metal tritellurides, high temperature cuprate superconductors, and iron-based superconductors.

The experimental realization of the quantum anomalous Hall (QAH) effect in magnetically-doped (Bi, Sb)2Te3 films stands out as a landmark of modern condensed matter physics. However, ultra-low temperatures down to few tens of mK are needed to reach the quantization of Hall resistance, which is two orders of magnitude lower than the ferromagnetic phase transition temperature of the films. Here, we systematically study the band structure of V-doped (Bi, Sb)2Te3 thin films by angle-resolved photoemission spectroscopy (ARPES) and show unambiguously that the bulk valence band (BVB) maximum lies higher in energy than the surface state Dirac point. Our results demonstrate clear evidence that localization of BVB carriers plays an active role and can account for the temperature discrepancy.

Oxide materials are important candidates for the next generation of electronics due to a wide array of desired properties which they can exhibit alone or when combined with other materials. While SrTiO 3 (STO) is often considered a prototypical oxide, it too hosts a wide array of unusual properties including a two dimensional electron gas (2DEG) which can form at the surface when exposed to UV light. Using layer-by-layer growth of high quality STO films, we show that the 2DEG only forms with the SrO termination and not with the TiO 2 termination, contrary to expectation. This behavior is similarly seen in BaTiO 3 (BTO), in which the 2DEG is only observed for BaO terminated films. These results will allow for a deeper understanding, and better control, of the electronic structure of titanate films, substrates and 1 arXiv:1811.12652v1 [cond-mat.str-el] 30 Nov 2018

Here we report that TiSe 2 thin films can be epitaxially grown on TiO 2 substrates despite different lattice symmetry between the two materials. The TiSe 2 thin films can be prepared on TiO 2 via molecular beam epitaxy (MBE) in two ways: by conventional co-deposition using selenium and titanium sources, and by evaporating just selenium on reconstructed surfaces of TiO 2 . Both growth methods yield crystalline thin films with similar electronic band structures. TiSe 2 films on TiO 2 substrates exhibit large electron doping and a lack of charge density wave (CDW) order, which is different from both bulk single crystal TiSe 2 and TiSe 2 thin films on graphene. These phenomena can be explained by selenium vacancies in the TiSe 2 films, which naturally occur when these films are grown on TiO 2 substrates. Our successful growth of transition metal dichalcogenide (TMDC) films on a transition metal oxide (TMO) substrate provides a platform to further tune the electrical and optical properties of TMDC thin films. LETTER RECEIVED