Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 50 (2017)

Eﬀects of 2.6 GeV uranium irradiation on FeSe†
Y. Sun,∗1 A. Park,∗1 S. Pyon,∗1 T. Tamegai,∗1 A. Ichinose,∗2 A. Yoshida,∗3 and T. Kambara∗3
The introduction of nonmagnetic disorders has been
proven to be an eﬀective method to identify the gap
structures of novel superconductors.1) For point defects introduced by light-particle irradiations, such as
electrons and protons, a clear suppression of Tc wasobserved in both cuprates and iron pnictides, whichwas attributed to the sign change of the order parameter in d-wave and s± (or competition between s± ands++ via inter- and intra-band scatterings), respectively. In the case of correlated disorders created by heavyion irradiation, such as columnar defects, an obvious
suppression of Tc together with the increase in thenormal-state resistivity has been reported in cuprates.
However, the Tc shows only a small or nondetectablechange in iron pnictides. The penetration depth in Coor K-substituted BaFe2 As2 was found to change after
heavy-ion irradiation, which is consistent with the s±
scenario. On the other hand, the columnar defects created by heavy-ion irradiation are also strong pinning
centers due to the geometrical similarity with vortices,which have already been proven to be eﬀective for theenhancement of critical current density Jc . Their controllability is also advantageous in the study of vortex
physics. Thus, studies of the eﬀects of heavy-ion irradiation on FeSe are crucial to the understanding of
its pairing mechanism and vortex physics, which is important for the application of this material.
In this report, the eﬀects of heavy-ion irradiation onFeSe single crystals are studied in detail by irradiating 2.6-GeV uranium up to a dose-equivalent matching ﬁeld of Bφ = 16 T. FeSe single crystals were grownby the vapor transport method. The obtained crystals were of high quality with a sharp superconducting
transition width ΔTc < 0.5 K from susceptibility measurements and a large residual resistivity ratio of ∼33
as reported in our previous publications.2–4) Before theirradiation, single crystals were cleaved to thin plateswith thickness ∼20 μm along the c-axis, which is muchless than the projected range of 2.6-GeV uranium forFeSe of ∼60 μm, calculated by “Stopping and Rangeof Ions in Matter-2008 (SRIM-2008)”. The 2.6 GeV
uranium beam was irradiated parallel to the c-axis of
the crystal at room temperature. The uranium irradiation was performed at the RI Beam Factory operated
by RIKEN Nishina Center and CNS, the University ofTokyo.
The columnar defects were conﬁrmed by highresolution transmission electron microscopy observa†∗1∗2∗3Fig. 1. Normalized Tc (Tc /Tc0 , where Tc0 is the value ofTc for the pristine sample) and the self-ﬁeld Jc at 2 K
as a function of the matching ﬁeld Bφ (bottom axis)
and damaged area (top axis) for the 2.6-GeV uraniumirradiated FeSe.

tion, and they are almost continuous along the c-axis
with a diameter of ∼10 nm, which is much greater thanthat of 2-5 nm observed in iron pnictides. As shownin Fig. 1, the value of Tc is found to be gradually suppressed by introducing columnar defects at a rate ofdTc /dBφ ∼ -0.29 KT−1 , which is greater than thatobserved in iron pnitides.5) The quick suppression ofTc suggests a unique pairing mechanism of FeSe.
The critical current density is ﬁrst dramatically enhanced with irradiation reaching a value over ∼2×105
A/cm2 at 2 K (self-ﬁeld) for Bφ = 2 T, and then gradually suppressed with further increase in Bφ . The
evolution of Tc and Jc with irradiation dose is explained by the large diameter of columnar defects.
The nature of the quick enhancement of Jc by a small dose of heavy-ion irradiation in FeSe is also advantageous for real application. In addition, vortex
dynamics study shows that the δl-pinning associated with charge-carrier mean-free-path ﬂuctuations andthe δTc -pinning associated with spatial ﬂuctuations of
the transition temperature are found to coexist in pristine FeSe, while the irradiation increases the δl-pinningand makes it dominant above Bφ = 4 T.
References
1) H. Alloul et al., Phy. Mod. Rev. 81, 45 (2009).
2) Y. Sun et al., Phy. Rev. B 92, 144509 (2015).
3) Y. Sun et al., Phy. Rev. B 93, 104502 (2016).
4) Y. Sun et al., Phy. Rev. B 94, 134505 (2016).
5) F. Ohtake et al., Physica C 518, 47 (2015).

Condensed from Phys. Rev. B 95, 104514 (2017)
Department of Applied Physics, The University of Tokyo
Central Research Institute of Electric Power Industry, Electric Power Engineering Research Laboratory
RIKEN Nishina Center- 21 -