Unraveling the mechanism of superconductivity strong against magnetic fieldrealized in uranium compound
T. Hattori, H. Sakai, Y. Tokunaga, S. Kambe, T. D. Matsuda† and Y. Haga
R. G. for Materials Physics for Heavy Element Systems, ASRC, JAEA

Superconductivity is a practical phenomenon expected to be
utilized as a wide range of application, such as superconducting(SC) power transmission and a linear motor car, because electric
resistance goes to completely zero and a superconductor
completely repel a magnetic field. However, Superconductivity
requires a very low temperature. Moreover superconductivity isusually weak against magnetic field and easily destroyed by amagnetic field. These properties are obstacles in practicalapplication of superconductivity in a wider field. For that reason,
many researches are still exploring superconductors thatpossesses a higher transition temperature and a capability towithstand a stronger magnetic field.
Superconductivity of the uranium compound URu2Si2, on
which we are actively researching recent years, has the property
of being extremely strong to a magnetic field, and therefore toclarify its mechanism has been an important subject. However,handling this compound is strictly restricted due to its radio
activity, and the experiments with sufficient accuracy has notbeen done so far. Therefore, by utilizing the facility of JAEA,
we synthesized the ultra-pure single crystal of URu2Si2 andformed it into a shape suitable for measurement. As a result, we
successfully measured the nuclear magnetic resonance in the SC
state of uranium compounds at cryogenic temperatures with thehighest precision in the world. Nuclear magnetic resonance is atechnique for microscopically examining electrons surrounding
nuclei through magnet-like property of nuclei (nuclear spins),
and enables us to explore electronic states of superconductivity.
Electrons in material have a magnet-like property called anelectron spin, and each electron spin is randomly oriented in
the normal state. When an attractive force dominates between
two electrons at low temperatures, the electrons make pairs
called the Cooper pairs with their spins coupled to be
antiparallel directions and become SC states by creating aligned
wave-like states (Fig. 1). When a strong magnetic field isapplied to Cooper pairs, it is known that electron spin reorientsto the direction of the magnetic field, so that superconductivity
breaks accompanied by breaking of Cooper pairs (Fig. 2 a). In
the URu2Si2 case, however, by using the high resolution nuclear
magnetic resonance measurements we have demonstrated thatthe electron spin of URu2Si2 is uniaxially oriented and its
uniaxial orientation is robust with respect to the magnetic field(strong spin anisotropy). This strong spin anisotropy of URu2Si2in SC state was experimentally probed for the first time (Fig. 2b).
This achievement is expected to deepen the understanding ofsuperconductivity and provides guidelines for exploring morepractical superconductors. In addition, such an exotic electronic
state of the very strong spin anisotropy is thought to originatesfrom uranium, which contains many electrons, and thus ourfindings are expected to contribute to the progress offundamental material physics and of fundamental nuclear
science.
This work was supported by JSPS KAKENHI Grants No.
JP15K05152, No. JP16K17757, No. JP15KK0174, No.
JP15H05745, and No. JP15H05884 (J-Physics).

Fig.1 Schematic image of superconductivity. Electrons (yellowspheres) in material have a magnet-like character called electronspin (black arrow). A pair called Cooper pair is formed by directing
this electron spin in antiparallel direction, and it flows like a wave,
resulting superconducting state.

(a)

(b)

Fig.2 Cooper pairs’ behaviours when magnetic field (blue solidline) is applied.
(a) Normally, electron spin reorients to the magnetic field
direction, and the Cooper pair breaks.
(b) In the uranium compound URu2Si2, electron spin is aligned toone direction, and it does not tilt in the magnetic field, soCooper pairs are maintained even under a strong magnetic field.
† Present address: Tokyo Metropolitan Univ.

References[1] T. Hattori et al., J. Phys. Soc. Jpn. 85, 073711 (2016).
[2] T. Hattori et al., Phys. Rev. Lett. 120, 027001 (2018).