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Register a new userEvidence for bosonic mode coupling in electron dynamics of LiFeAs superconductor
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Super-high resolution laser-based angle-resolved photoemission measurements are carried out on LiFeAs superconductor to investigate its electron dynamics. Three energy scales at $sim$20 meV, $sim$34 meV and $sim$55 meV are revealed for the first time in the electron self-energy both in the superconducting state and normal state. The $sim$20 meV and $sim$34 meV scales can be attributed to the coupling of electrons with sharp bosonic modes which are most likely phonons. These observations provide definitive evidence on the existence of mode coupling in iron-based superconductors.
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We performed scanning tunneling spectroscopic experiments on hole-doped NdBa$_2$Cu$_3$O$_{7-delta}$. The d$I$/d$V$ curves obtained at 4.2 K are asymmetric with clear peak-dip and hump structures. Energy derivatives of these curves show peaks at energies beyond the dip features. Highly precise full potential bandstructure calculations confirm a featureless electronic density of states in that energy region. Our results indicate that tunneling electrons couple to a collective mode in the CuO$_2$ plane.
The superconducting compound, LiFeAs, is studied by scanning tunneling microscopy and spectroscopy. A gap map of the unreconstructed surface indicates a high degree of homogeneity in this system. Spectra at 2 K show two nodeless superconducting gaps with $Delta_1=5.3pm0.1$ meV and $Delta_2=2.5pm0.2$ meV. The gaps close as the temperature is increased to the bulk $T_c$ indicating that the surface accurately represents the bulk. A dip-hump structure is observed below $T_c$ with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering.
We analyze optical spectroscopy data of the electron-doped superconductor (Pr$_{2-x}$Ce$_x$)CuO$_4$ (PCCO) to investigate the coupling of the charge carriers to bosonic modes. The method of analysis is the inversion of the optical scattering rate $tau^{-1}_{rm op}(omega,T)$ at different temperatures $T$ by means of maximum entropy technique combined with Eliashberg theory. We find that in the superconducting state the charge carriers couple to two dominant modes one at $sim 10 $meV and a second one at $sim 45 $meV. The low energy mode shows a strong temperature dependence and disappears at or slightly above the critical temperature $T_c$. The high energy mode exists above $T_c$ and moves towards higher energies with increasing temperatures. It also becomes less prominent at temperatures $> 100 $K above which it evolves into a typical spin-fluctuation background. In contrast to the hole-doped High-$T_c$ superconductors PCCO proves to be a superconductor close to the dirty limit.
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 50-80meV, 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 specific heat capacity measurements on a LiFeAs single crystal at temperatures down to 400 mK and magnetic fields up to 9 Tesla. A small specific heat jump at Tc and finite residual density of states at T=0 K in the superconducting (SC) state indicate that there are strong unitary scatterers that lead to states within the SC gap. A sub-linear magnetic field dependence of the Sommerfeld coefficient gamma(H) at T=0 K is equally well fitted by both a nodal d-wave gap as well as a sign changing multiband pm s-wave gap. When impurity effects are taken into account, however, the linear temperature dependence of the electronic specific heat C_{el}/T at low temperatures argues against a nodal d-wave superconducting gap. We conclude that the SC state of LiFeAs is most compatible with the multiband pm s-wave SC state with the gap values Delta_{small}=0.46 Delta_{large}.
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