Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 53

Eﬀects of columnar defects introduced by 2.6 GeV U-ion irradiation onJc characteristics of 2H-NbSe2
T. Tamegai,∗1 W. J. Li,∗1 S. Pyon,∗1 A. Takahashi,∗1 Y. Kobayashi,∗1 A. Yoshida,∗2 T. Kambara,∗2 andA. Ichinose∗3
One of the most salient features of superconductors
is the zero-resistance state. Although the presence ofsuch a state may be obvious in superconductors withouta magnetic ﬁeld, a motion of quantized vortices can destroy this useful state. The introduction of defects thatfunction as pinning centers for vortices is known to bea practical method to restore the ideal zero-resistancestate under strong magnetic ﬁelds, leading to the enhancement of the critical current density (Jc ) in superconductors. The irradiation of heavy ions on superconductors can create columnar defects (CDs) by destroying
the crystal structure of superconductors. CDs are ideal
pinning centers for vortices because they can trap vortices along their full length. The eﬀectiveness of CDs
in enhancing Jc was demonstrated in high-temperaturesuperconductors1) and also in iron-based superconductors.2) However, the eﬀect of heavy-ion irradiation onconventional superconductors has attracted much less attention to date. Only very recently, systematic work has
been conducted on 2H-NbSe2 irradiated at an angle of30◦ from the c-axis with a dose-equivalent matching ﬁeld(BΦ ) of 3 T. In this paper, the presence of the peak eﬀectof Jc is reported. We investigated the eﬀect of 2.6 GeV
U-ion irradiation on the Jc characteristics of 2H-NbSe2 .
Single crystals of 2H-NbSe2 with Tc ∼ 7.2 K weregrown using the iodine transport method. 2.6 GeV Uions were irradiated on 2H-NbSe2 at various angles, θCD ,from the c-axis and at various BΦ values up to 16 T. The
dependence of Jc on BΦ and θCD were studied.
Figure 1 shows scanning transmission electron microscope (STEM) image of 2H-NbSe2 irradiated by 2.6 GeV
U-ions from the directions of θCD = ±15◦ . It is clearthat continuous CDs with diameters of ∼7–10 nm werecreated.

100 nmFig. 1. Columnar defects created by 2.6 GeV U-ion irradiation ( θCD = ±15◦ ) in single-crystal 2H-NbSe2 .
∗1∗2∗3Department of Applied Physics, The University of Tokyo
RIKEN Nishina Center
Central Research Institute of Electric Power Industry

Fig. 2. Eﬀects of 2.6 GeV U-ion irradiation on Jc -H in 2HNbSe2 : (a) pristine and irradiated samples with BΦ =1, 2, 4, and 6 T (//c) and (b) samples irradiated with
θCD = 0◦ , ±10◦ , ±20◦ , and ±30◦ at BΦ = 4 T.

Figure 2(a) shows Jc as a function of the magneticﬁeld H(//c) at 4 K for a pristine sample of 2H-NbSe2and samples irradiated at BΦ = 1, 2, 4, and 6 T along
the c-axis. In all cases, Jc decreases monotonically withH. Compared with the pristine sample, Jc at 1 kOe
is enhanced by a factor of more than 10 even at BΦ =1 T. Figure 2(b) shows Jc as a function of H at 4 Kfor 2H-NbSe2 irradiated from the directions of θCD =±10◦ , ±20◦ , or ±30◦ at BΦ = 2 T + 2 T Shown as areference is the sample with BΦ = 4 T at θCD = 0◦ . In
all cases, the magnetic ﬁeld H is applied along the c-axis(the average direction of CDs). It is remarkable that,in samples irradiated with θCD = ±10◦ and ±20◦ , Jc
shows a broad peak at H = 2–3 kOe, which is less than
10% of the total BΦ . Such a peak eﬀect is similar to theone observed in a sample with tilted CDs.3) It should
also be noted that similar broad peaks were observed in(Ba, K)Fe2 As2 irradiated at θCD ∼ ±15◦ . In this case,however, the peak appears at H ∼ 1/3BΦ . At a larger|θCD |, the broad peak is suppressed and replaced by asharp drop of Jc below H ∼ 0.3 kOe. The origin of thebroad peak in Jc -H in 2H-NbSe2 with splayed CDs is
not known yet. More systematic studies on 2H-NbSe2
with diﬀerent conﬁgurations of CDs are required.
References

1) L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).
2) Y. Nakajima et al., Phys. Rev. B 80, 012510 (2009).
3) S. Eley et al., Sci. Rep. 8, 13162 (2018).
4) A. Park et al., Phys. Rev. B 97, 064516 (2018).

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