Nematicity is ubiquitous in electronic phases of high-Tc superconductors, particularly in the Febased systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length ξ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe0.97Co0.03)2As2 and optimally doped Ba(Fe0.94Co0.06)2As2. In all cases, we find that ξ is well described by a power law (T − T0) −1/2 extending over a wide temperature range. We attributed this mean-field behavior and the extended fluctuation regime to a sizable nemato-elastic coupling, which may be detrimental to superconductivity. arXiv:2001.01343v1 [cond-mat.supr-con] 6 Jan 2020

Angle-resolved photoemission (ARPES) experiments on copper oxide superconductors revealed enigmatic kinks in electronic dispersions near 10 meV presumably due to phonons or impuritites. We used inelastic neutron scattering to measure phonon branches below 15 meV in a large single crystal sample of optimally-doped Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO). The high quality dataset covered several Brilloiun zones with different final energies. In addition to acoustic branches, optic branches disperse from 4 meV and 7 meV zone center energies. The 4 meV branch interacts with acoustic phonons at small wavevectors, which destroys the LA character of the acoustic branch beyond 0.15 reciprocal lattice units. We propose a mechanism that explains the low energy electronic dispersion features based on this observation.

La1.67Sr0.33NiO4 develops charge and spin stripe orders at temperatures of roughly 200 K, with modulation wave vectors that are temperature independent. Various probes of spin and charge response have provided independent evidence for some sort of change below ∼ 50 K. In combination with a new set of neutron scattering measurements, we propose a unified interpretation of all of these observations in terms of a freezing of Ni-centered charges stripes, together with a glassy ordering of the spin stripes that shows up in neutron scattering as a slight rotation of the average spin direction.

Doped antiferromagnets host a vast array of physical properties and learning how to control them is one of the biggest challenges of condensed matter physics. La 1.67 Sr 0.33 NiO 4 (LSno) is a classic example of such a material. At low temperatures holes introduced via substitution of La by Sr segregate into lines to form boundaries between magnetically ordered domains in the form of stripes. the stripes become dynamic at high temperatures, but LSno remains insulating presumably because an interplay between magnetic correlations and electron-phonon coupling localizes charge carriers. Magnetic degrees of freedom have been extensively investigated in this system, but phonons are almost completely unexplored. We searched for electron-phonon anomalies in LSno by inelastic neutron scattering. Giant renormalization of plane ni-o bond-stretching modes that modulate the volume around ni appears on entering the dynamic charge stripe phase. other phonons are a lot less sensitive to stripe melting. Dramatic overdamping of the breathing modes indicates that dynamic stripe phase may host small polarons. We argue that this feature sets electron-phonon coupling in nickelates apart from that in cuprates where breathing phonons are not overdamped and point out remarkable similarities with the colossal magnetoresistance manganites. Mott insulators should become metallic when extra charge carriers are introduced by doping. However, many of them remain insulating or become very poor metals with large electrical resistivity and incoherent or diffusive transport 1,2. This behavior is particularly common in transition metal oxides that have the potential to realize novel electronic phases with interesting and exotic properties from nontrivial topologies to superconductivity 3. Poor electrical conductivity is typically associated with charge carrier localization arising from interactions between different quasiparticles 4. Learning how to control these interactions is challenging, especially in the presence of strong electron-electron correlations. Electron-phonon coupling is often involved in localization of charge carriers in crystalline materials. For example, in the case of polaron formation, the carriers locally distort the atomic lattice and the distortions trap the carriers when the electron-phonon coupling strength is large enough 5. A detailed understanding of both electronic and phonon channels is necessary to accurately account for such phenomena. A lot of research focused on the former 4,6 , but the latter is poorly characterized in many interesting materials. Time-of-flight neutron scattering instruments can map the phonon spectra over hundreds of Brillouin zones, but comprehensive analysis of these datasets is extremely difficult and time-consuming. For example, small peaks in the background can be assigned to phonons. A broad peak may arise from a superposition of two or more closely-spaced peaks. Some phonons can be overlooked since most of them have appreciable structure factor only in a few zones, etc. Recently we deve...

We investigated long-wavelength nematic fluctuations in an Fe-based superconductor LiFeAs near q=(0.05,0,0) by measuring temperature-dependent renormalization of acoustic phonons through inelastic neutron scattering. We found that the phonons have conventional behavior, as would be expected in the absence of electronic nematic fluctuations. This observation implies that either electronphonon coupling is too weak to see any effect or that nematic fluctuations are not present. Keywords: pnictide, superconductor, nematic, INSJust like the cuprates, iron pnictides and iron chalcogenites become superconducting when antiferromagnetic "parent" compounds are doped 1 . The parent compounds as well as low-doped superconducting ones undergo magnetic and structural phase transitions at temperatures T N and T s respectively. As a function of decreasing temperature, the system first becomes paramagnetic orthorhombic phase below T S , and then orders antiferromagnetically below T N with T N ≤T S . A lot of recent research focused on the role of electronic nematicity, where electronic properties of the system occur preferentially along one of two otherwise degenerate perpendicular directions in the paramagnetic orthorhombic state.2 Spin nematic order occurs when magnetic fluctuations break the C4 rotational symmetry but magnetic order does not form 3 . Magnetoelastic coupling 4,5 of the lattice to nematic fluctuations is responsible for orthorhombic distortion at zero and low doping. This spin-phonon coupling changes some phonon frequencies across the magnetic ordering transition, but the effect is much smaller than predicted.