B_{1g}-Phonon Anomaly Driven by Fermi Surface Instability at Intermediate Temperature in YBa_{2}Cu_{3}O_{7-δ}.

We performed temperature- and doping-dependent high-resolution Raman spectroscopy experiments on YBa_{2}Cu_{3}O_{7-δ} to study B_{1g} phonons. The temperature dependence of the real part of the phonon self-energy shows a distinct kink at T=T_{B1g} above T_{c} due to softening, in addition to the one due to the onset of the superconductivity. T_{B1g} is clearly different from the pseudogap temperature with a maximum in the underdoped region and resembles charge density wave onset temperature, T_{CDW}.

Quantum Phase Transition of Correlated Iron-Based Superconductivity in LiFe_{1-x}Co_{x}As.

The interplay between unconventional Cooper pairing and quantum states associated with atomic scale defects is a frontier of research with many open questions. So far, only a few of the high-temperature superconductors allow this intricate physics to be studied in a widely tunable way. We use scanning tunneling microscopy to image the electronic impact of Co atoms on the ground state of the LiFe_{1-x}Co_{x}As system.

Phonon Boost Effect on the S-wave Superconductor with Incipient Band.

We showed that the all phonons - not only forward-scattering phonon but also local (all-momentum-scattering) phonon - contribute to boosting Tc of the S-wave pairing state in the incipient band model. In particular, when the incipient band sinks deeper, the phonon boost effect of the local phonon increases and becomes as effective as the one of the forward-scattering phonon. Our finding implies that all interface phonons from the SrTiO substrate - not only the 90 meV Fuchs-Kliewer (F-K) phonon but also the 60 meV F-K phonon - as well as all intrinsic phonons of the FeSe monolayer itself, regardless of their degree of "forwardness", should contribute to increase Tc of the FeSe/STO monolayer system.

Switching Magnetism and Superconductivity with Spin-Polarized Current in Iron-Based Superconductor.

We explore a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single-crystal Sr_{2}VO_{3}FeAs shows that a spin-polarized tunneling current can switch the Fe-layer magnetism into a nontrivial C_{4} (2×2) order, which cannot be achieved by thermal excitation with an unpolarized current. Our tunneling spectroscopy study shows that the induced C_{4} (2×2) order has characteristics of plaquette antiferromagnetic order in the Fe layer and strongly suppresses superconductivity.

Frustration-driven C symmetric order in a naturally-heterostructured superconductor SrVOFeAs.

A subtle balance between competing interactions in iron-based superconductors (FeSCs) can be tipped by additional interfacial interactions in a heterostructure, often inducing exotic phases with unprecedented properties. Particularly when the proximity-coupled layer is magnetically active, rich phase diagrams are expected in FeSCs, but this has not been explored yet. Here, using high-accuracy As and V nuclear magnetic resonance measurements, we investigate an electronic phase that emerges in the FeAs layer below T  ~ 155 K of SrVOFeAs, a naturally assembled heterostructure of an FeSC and a Mott-insulating vanadium oxide.

Correlation of Fe-Based Superconductivity and Electron-Phonon Coupling in an FeAs/Oxide Heterostructure.

Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial phonons in an iron-based superconductor Sr_{2}VO_{3}FeAs (T_{c}≈33  K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in the VO_{2} layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.

Superconducting properties of the-wave state: Fe-based superconductors.

Although the pairing mechanism of Fe-based superconductors (FeSCs) has not yet been settled with consensus with regard to the pairing symmetry and the superconducting (SC) gap function, the vast majority of experiments support the existence of spin-singlet sign-changing-wave SC gaps on multi-bands (s±-wave state). This multi-bands±-wave state is a very unique gap stateand displays numerous unexpected novel SC properties, such as a strong reduction of the coherence peak, non-trivial impurity effects, nodal-gap-like nuclear magnetic resonance signals, various Volovik effects in the specific heat (SH) and thermal conductivity, and anomalous scaling behaviors with a SH jump and condensation energy versus, etc. In particular, many of these non-trivial SC properties can easily be mistaken as evidence for a nodal-gap state such as a-wave gap.

Volovik effect in the ±s-wave state for the iron-based superconductors.

We studied the field dependencies of specific heat coefficient γ(H)=lim(T→0)C(T,H)/T and thermal conductivity coefficient lim(T→0)κ(T,H)/T of the ±s-wave state in the mixed state. It is a generic feature of the two band s-wave state with the unequal sizes of gaps, small ΔS and large ΔL, that Doppler shift of the quasiparticle excitations (Volovik effect) creates a finite density of states, on the extended states outside of vortex cores, proportional to H in contrast to the square root(H) dependence of the d-wave state. The impurity scattering effect makes this generic H-linear dependence sublinear approaching to the square root(H) behavior.