Critical current density (Jc) is one of the major limiting factors for high field applications of iron-based superconductors.Here, we report that Mn-ions are successfully incorporated into nontoxic superconducting (Li,Fe)OHFeSe films.Remarkably, the Jc is significantly enhanced from 0.03 to 0.32 MA/cm 2 under 33 T, and the vortex pinning force density monotonically increases up to 106 GN/m 3 , which is the highest record so far among all iron-based superconductors. Our results demonstrate that Mn incorporation is an effective method to optimize the performance of (Li,Fe)OHFeSe films, offering a promising candidate for high-field applications.Recently, elemental Mn has been incorporated into FeSe-11111 single crystals without obvious detriment to its Tc [20], which may provide an effective candidate. Moreover, iron-based superconductors in the form of films usually present a higher Jc than that of bulk samples [6,8].Therefore, it is worthy introducing transition metal ions into FeSe-11111 crystalline superconducting film for further optimization of their high-field performance.In this letter, we successfully introduced Mn-ions into a superconducting FeSe-11111 film synthesized through the so-called 2 matrix-assisted hydrothermal epitaxy (MHE) method [12]. A significant enhancement of Jc was observed in FeSe-11111 films by Mn-doping, increasing it tenfold from 0.03 to 0.32 MA/cm 2 under 33 T at 5 K. Remarkably, the vortex pinning force density (Fp) of Mndoped films monotonically increases to 106 GN/m 3 . To the best of our knowledge, this is the highest record so far among all iron-based superconducting systems. By analyzing Fp versus magnetic fields, we find the apparent enhancement of Jc in the Mn-doped FeSe-11111 film stems from the extra pinning centers induced by Mn doping. ExperimentsThe pure and Mn-doped FeSe-11111 films were synthesized via the MHE method that we developed [12]. The x-ray diffraction (XRD) experiments were carried out on a 9 kW Rigaku SmartLab X-ray diffractometer. The scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopy measurements were performed on a Hitachi SU5000. The electron energy loss spectroscopy (EELS) data were acquired using a transmission electron microscope (ARM200F, JEOL Inc.) equipped with a Gatan Quantum ER 965 Imaging Filter. Electrical transport measurements within 9T were collected with the standard four-probe method on a Quantum Design PPMS-9 system. The values of Jc were obtained using the criteria of 1 μV on I-V curves and the parameters of bridge were characterized by SEM. The high-field experiments up to 33 T were performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS.

Bulk Nb superconducting radio-frequency (SRF) cavities are widely used in accelerators, and their accelerating gradient and general performance are limited by the superheating field (B sh). To push the theoretical limit of the B sh, new multilayer structures are required. We fabricated FeSe-coated Nb films using pulsed laser deposition, performed structural characterizations, and measured the transport and magnetic properties for this superconductor-superconductor bilayer structure with smooth surface. Additionally, the measured B c1 of FeSe-coated Nb film is greatly enhanced, while the B sh of the FeSe layer is expected to be higher than that of bulk Nb, yet the superconducting transition temperature (T c) is less than 5 K. This work presents the first fabrication of a new coating layer: FeSe deposited on Nb, showing the possibility of using iron-based materials for multilayer structures in SRF cavities.

We studied the electronic Raman spectra of (Li1−xFex)OHFeSe as a function of light polarization and temperature. In the B1g spectra alone we observe the redistribution of spectral weight expected for a superconductor and two well-resolved peaks below Tc. The nearly resolution-limited peak at 110 cm −1 (13.6 meV) is identified as a collective mode. The peak at 190 cm −1 (23.6 meV) is presumably another collective mode since the line is symmetric and its energy is significantly below the gap energy observed by single-particle spectroscopies. Given the experimental band structure of (Li1−xFex)OHFeSe, the most plausible explanations include conventional spin-fluctuation pairing between the electron bands and the incipient hole band and pairing between the hybridized electron bands. The absence of gap features in A1g and B2g symmetry favors the second case. Thus, in spite of various differences between the pnictides and chalcogenides, this Letter demonstrates the proximity of pairing states and the importance of band structure effects in the Fe-based compounds.