Possible strong electron-lattice interaction and giant magneto-elastic effects in Fe-pnictides

The possibility for an appreciable many-body contribution to the electron-phonon interaction (EPI) in Fe-pnictides is discussed in the model where EPI is due to the electronic polarization of As- ions. The EPI-pol coupling ismuch larger than the one obtained in the LDA band structure calculations. It contributes significantly to the intra-band s-wave pairing and an appreciable positive As-isotope effect in the superconducting critical temperature is expected. In the Fe-breathing mode the linear (in the Fe-displacements) EPI-pol coupling vanishes, while the non-linear (quadratic) one is very strong. The part of the EPI-pol coupling, which is due to the "potential" energy (the Hubbard U) changes, is responsible for the giant magneto-elastic effects in MFe_{2}As_{2}, M=Ca, Sr, Ba since it gives much larger contribution to the magnetic pressure than the band structure effects do. This mechanism is contrary to the LDA prediction where the magneto-elastic effects are due to the "kinetic" energy effects, i.e. the changes in the density of states by the magneto-elastic effects. The proposed $EPI-pol is expected to be operative (and strong) in other Fe-based superconductors with electronically polarizable ions such as Se, Te, S etc., and in high-temperature superconductors due to the polarizability of the O-ions.Comment: 6 pages, 2 figures; new References are added, text improved, typos correcte

Phononic filter effect of rattling phonons in the thermoelectric clathrate Ba8_8Ge40+x_{40+x}Ni6−x_{6-x}

One of the key requirements for good thermoelectric materials is a low lattice thermal conductivity. Here we present a combined neutron scattering and theoretical investigation of the lattice dynamics in the type I clathrate system Ba-Ge-Ni, which fulfills this requirement. We observe a strong hybridization between phonons of the Ba guest atoms and acoustic phonons of the Ge-Ni host structure over a wide region of the Brillouin zone which is in contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host cages. It occurs without a strong decrease of the acoustic phonon lifetime which contradicts the usual assumption of strong anharmonic phonon--phonon scattering processes. Within the framework of ab-intio density functional theory calculations we interpret these hybridizations as a series of an ti-crossings which act as a low pass filter, preventing the propagation of acoustic phonons. To highlight the effect of such a phononic low pass filter on the thermal transport, we compute the contribution of acoustic phonons to the thermal conductivity of Ba$_8$Ge$_{40}$Ni$_{6}$ and compare it to those of pure Ge and a Ge$_{46}$ empty-cage model system.Comment: 10 pages, 10 figure

Unraveling the exciton binding energy and the dielectric constant in single crystal methylammonium lead tri-iodide perovskite

We have accurately determined the exciton binding energy and reduced mass of single crystals of methylammonium lead tri-iodide using magneto-reflectivity at very high magnetic fields. The single crystal has excellent optical properties with a narrow line width of $\sim 3$meV for the excitonic transitions and a 2s transition which is clearly visible even at zero magnetic field. The exciton binding energy of $16 \pm 2$meV in the low temperature orthorhombic phase is almost identical to the value found in polycrystalline samples, crucially ruling out any possibility that the exciton binding energy depends on the grain size. In the room temperature tetragonal phase, an upper limit for the exciton binding energy of $12 \pm 4$ meV is estimated from the evolution of 1s-2s splitting at high magnetic field.Comment: 5 pages, 4 figure

Magnon-Phonon coupling in Fe3_3GeTe2_2

We study the dynamic coupling of magnons and phonons in single crystals of Fe3GeTe2 (FGT) using inelastic scanning tunneling spectroscopy (ISTS) with an ultra-low temperature scanning tunneling microscope. Inelastic scattering of hot carriers off phonons or magnons has been widely studied using ISTS, and we use it to demonstrate strong magnon-phonon coupling in FGT. We show a strong interaction between magnons and acoustic phonons which leads to formation of van Hove singularities originating in avoided level crossings and hybridization between the magnonic and phononic bands in this material. We identify these additional hybridization points in experiments and compare their energy with density functional theory calculations. Our findings provide a platform for designing the properties of dynamic magnon-phonon coupling in two-dimensional materials.Comment: 6 pages, 3 figure

Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe

FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes, while ist monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16 T and temperatures down to 2 K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. The observation of the vortex-antivortex unbinding transition in different flakes provide a direct signature of a dominant two-dimensional pairing channel. However, the upper critical field reflects the evolution of the multi-band nature of superconductivity in FeSe becoming highly two-dimensional and strongly anisotropic only in the thin limit. Our study provides detailed insights into the evolution of the superconducting properties of a multi-band superconductor FeSe in the thin limit in the absence of a dopant substrate

Physical properties of single-crystalline Ba 8 Ni 3.5 Ge 42.1 h 0.4

Clathrates are candidate materials for thermoelectric applications because of a number of unique properties. The clathrate I phases in the Ba-Ni-Ge ternary system allow controlled variation of the charge carrier concentration by adjusting the Ni content. Depending on the Ni content, the physical properties vary from metal-like to insulator-like and show a transition from p-type to n-type conduction. Here we present first results on the characterization of millimeter-sized single crystals grown by the Bridgman technique. Single crystals with a composition of Ba8Ni3.5Ge42.1h0.4 show metallic behavior (dp/dT > 0) albeit with high resistivity at room temperature [p (300 K) = 1 mOhm cm]. The charge carrier concentration at 300 K, as determined from Hall-effect measurements, is 2.3 e-/unit cell. The dimensionless thermoelectric figure of merit estimated at 680 K is ZT ~ 0.2. Keywords Clathrates - thermoelectric material - intermetallic compound - nicke

Revealing the single electron pocket of FeSe in a single orthorhombic domain

Authors acknowledge Diamond Light Source for time on beamline I05-ARPES under Proposal SI23890. L.C.R. acknowledges funding from the Royal Commission for the Exhibition of 1851.We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by an angle-resolved photoemission spectroscopy beamline with a highly focused beam spot (nano-ARPES), we identify clear stripelike orthorhombic domains in FeSe with a length scale of approximately 1-5 μm. Our photoemission measurements of the Fermi surface and band structure within individual domains reveal a single electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron-pocket electronic structure of FeSe, observed without the application of uniaxial strain, and calls for further theoretical insight into this unusual Fermi surface topology. Our results also showcase the potential of nano-ARPES for the study of correlated materials with local domain structures.Publisher PDFPeer reviewe