Research Group for Nanoscale Structure and Function of Advanced Materials

Group Leader : FUKUTANI Katsuyuki
Members : HIGEMOTO Wataru, FUKAYA Yuki, ITO Takashi, YASUDA Satoshi, YANO
Masahiro, YAMAKAWA Koichiro, TERASAWA Tomoo, UETA Hirokazu, SHAMOTO Shinichi, MACHIDA Shinichi, ODA Yuto

A macroscopic matter is assembly of microscopic ones and thefunctional materials whose characteristics are exposed byinvestigation of both local and whole state. An experimental
technique that can be used to selectively investigate a local state
can extend the field of materials research. Existing technicaldevelopments enable us to efficiently produce high-intensity
beams of muons, positrons, neutrons, and ions, which are usefulfor studying various atomic structures and dynamics offunctional materials at the length scale of advanced beams. In
addition, experiments that can obey local states at surface,interface, or around impurity have been developed as well, andthey can be used to obtain new scientific insights. Our
objectives are to study the nanoscale structures and functions ofmaterials, and develop advanced measurement technologies.
Polaronic nature of a hydrogen-related paramagnetic centerin SrTiO3 revealed by muon spin spectroscopy [1]Electron-doped strontium titanates (SrTiO3) are promisingmaterials for emerging oxide electronics, which show
exceptionally high electron mobilities for transition metal oxides
with d-like conduction bands. Metallic conductivity of thesecompounds indicates that the excess electrons predominantly
have an itinerant character. On the other hand, a deep singleparticle state in the band gap together with a coherent state near
the conduction band minimum for H-doped [2], Nb-doped, andLa-doped n-type sample, is detected by photoemission
spectroscopy (PES). This suggests that a localized electron statecoexists with the itinerant state in electron-doped SrTiO3.
However, the origin of the localized state remains unclear.
We performed muon spin rotation (µ+SR) measurements on
SrTiO3 to elucidate the nature of the localized state in H-dopedSrTiO3. Positive muons (µ+) implanted into SrTiO3 behave as aradioactive isotope of H at an interstitial site in the dilute doping
limit and provide microscopic insight into excess electrons
brought in with the H-like µ+ dopants. We observed aparamagnetic signal at cryogenic temperatures (Fig. 1), whichindicates localization of an electron near interstitial µ+. Based ona dipolar character of hyperfine interactions between µ+ and the
localized electron, we concluded that the electron stays at the Ti
site adjacent to µ+ to form a µ+-Ti3+ complex. This result
suggests that the in-gap state observed in H-doped SrTiO3 may
be associated with a Ti3+ polaron stabilized near an ionized H+.
High-temperature short-range order in Mn3RhSi [3]Conventional phase transitions are well understood in termsof the order parameter, based on the Landau–Ginzburg–Wilson
theory. However, the unconventional magnetic orders ofconduction electrons in the metallic phase has been observed forhigh temperature superconductors and heavy fermion
compounds. These unconventional magnetic orders have been
limited to relatively low temperatures as quantum phasetransitions. Here high-temperature magnetic short-range order isobserved as one of the unconventional magnetic orders attemperatures up to 720 K in a noncentrosymmetric intermetallic
antiferromagnet Mn3RhSi with a well-ordered lattice assummarized in Fig. 2. This temperature is the highest record forthe magnetic short-range order so far to our knowledge. The
magnetic Mn ions form a hyperkagome network of cornersharing triangles, where the spins are geometrically frustrated.
The spin network is equivalent to that of a spin liquid and nonFermi-liquid material, β-Mn. Our observation indicates that ametallic phase with magnetic short-range order exists at hightemperatures.

Fig. 2 Phase diagram of the family of compounds with the β-Mn
structure as a function of the cubic lattice parameter, in addition
to Mn3RhSi (open and closed diamonds).

Fig. 1 µ+SR frequency spectrum at 5 K in a field of 0.5 T appliedalong the [001] direction. The satellite peaks indicated byarrows correspond to a µ+-Ti3+ complex.

References[1] T. U. Ito et al., Appl. Phys. Lett. 115, 192103 (2019).
[2] M. D’Angelo et al., Phys. Rev. Lett. 108, 116802 (2012).
[3] H. Yamauchi et al., Commun. Mat. 1, 43 (2020).