Polaron Coherence as Origin of the Pseudogap Phase in High Temperature Superconducting Cuprates

Within a two component approach to high Tc copper oxides including polaronic couplings, we identify the pseudogap phase as the onset of polaron ordering. This ordering persists in the superconducting phase. A huge isotope effect on the pseudogap onset temperature is predicted and in agreement with experimental data. The anomalous temperature dependence of the mean square copper oxygen ion displacement observed above, at and below Tc stems from an s-wave superconducting component of the order parameter, whereas a pure d-wave order parameter alone can be excluded.Comment: 7 pages, 2 figure

Iron isotope effect on the superconducting transition temperature and the crystal structure of FeSe_1-x

The Fe isotope effect (Fe-IE) on the transition temperature T_c and the crystal structure was studied in the Fe chalcogenide superconductor FeSe_1-x by means of magnetization and neutron powder diffraction (NPD). The substitution of natural Fe (containing \simeq 92% of ^{56}Fe) by its lighter ^{54}Fe isotope leads to a shift of T_c of 0.22(5)K corresponding to an Fe-IE exponent of \alpha_Fe=0.81(15). Simultaneously, a small structural change with isotope substitution is observed by NDP which may contribute to the total Fe isotope shift of T_c.Comment: 4 pages, 3 figure

Polaron signatures in the phonon dispersion of high-temperature superconducting copper oxides

High-temperature superconducting copper oxides display a variety of both long-range and local lattice anomalies which are related to the onset of the pseudogap phase and / or the onset of superconductivity. Here we show that these anomalies demonstrate polaron formation where specifically the local character of the polarons plays an important role. We predict that unconventional isotope effects will appear in both the long-wavelength and local lattice effects

Isotope and interband effects in a multi-band model of superconductivity

Isotope effects (IEs) are essential in determining the pairing mechanism in superconductors. Whereas for Bardeen–Cooper–Schrieffer (BCS)-type superconductors, a clear consensus about IE exists, this is unknown in multiband superconductors (MBSs). We demonstrate here that for MBSs the IEs on the superconducting transition temperature can vary between the BCS value and zero as long as the intraband couplings are affected. It can, however, exceed the BCS value when interband effects are dominant. In both cases, a sign reversal is excluded. In addition, we show that interband coupling contributes substantially to enhancement of T c . The results are independent of the pairing symmetry and the system-specific band structure. Specifically, we do not address the IEs originating from the MBSs with respect to a specific superconductor, but rather study its emergence within this model and explore all possible sources within the weak coupling theory

Unconventional isotope effects as evidence for polaron formation in cuprates

Unconventional isotope effects (Phys. Rev. Lett., 92 (2004) 057602) as observed in high-temperature superconducting cuprates (HTSC), are explained by polaron formation which leads to a renormalization of the single-particle energies and the formation of density-density interactions. It is shown that the dominant contribution to these effects stems from the coupling to the quadrupolar Q2-type phonon mode

Isotope and interband effects in a multi-band model of superconductivity

Isotope effects (IEs) are essential in determining the pairing mechanism in superconductors. Whereas for Bardeen–Cooper–Schrieffer (BCS)-type superconductors, a clear consensus about IE exists, this is unknown in multiband superconductors (MBSs). We demonstrate here that for MBSs the IEs on the superconducting transition temperature can vary between the BCS value and zero as long as the intraband couplings are affected. It can, however, exceed the BCS value when interband effects are dominant. In both cases, a sign reversal is excluded. In addition, we show that interband coupling contributes substantially to enhancement of T c . The results are independent of the pairing symmetry and the system-specific band structure. Specifically, we do not address the IEs originating from the MBSs with respect to a specific superconductor, but rather study its emergence within this model and explore all possible sources within the weak coupling theory

Superconductivity in Fe and As based compounds: A bridge between MgB2 and cuprates

By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multigap superconductivity, it is shown that these systems strongly resemble MgB2, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similarly to MgB2 and in contrast to cuprates, the two superconducting gaps in the Fe-As based materials are isotropic s-wave gaps