About Young Scientist Award of the Physical Society of Japan (2022)



【Introduction】
    The 16th (2022) Young Scientist Award of the Physical Society of Japan (JPS) has been announced.(Here)

・ Kosuke Karube (RIKEN) 
・ Takashi Kurumaji (The University of Tokyo)
・ Yasuyuki Shimura (Hiroshima University)
・ Masamichi Nakajima (Osaka University)

Congratulations to all the award winners!
    
    The award was established with the following objectives in mind.
”The JPS has established the Young Scientist Award for young members to encourage the research of outstanding young scientists who will lead the future of physics and to further revitalize the society.”
    To be eligible for the award, you must be a member of the Physical Society of Japan. You must be 39 years old or younger on April 1 of the year following the year you receive the award. For the 16th award, the recipient must have been born on or after April 2, 1983. However, the age limit may be relaxed in the case of circumstances such as interruption of research due to childbirth or childcare. 

    By the way, you are interested in Japan's leading young physicists and their research, aren't you?
    
    In this blog article, I would like to introduce the contents of the award-winning papers in area 8 (Strongly correlated electron system), in which I am particularly interested.

【Method】
    Read and summarize each paper (mainly the abstract and intro section).
Citation counts are from Google Scolar or JPSJ. (As of 10/26/2021)

Contents】

Kosuke Karube (RIKEN)
”Exploration of the stability of magnetic skyrmion and development of new materials”

"Robust metastable skyrmions and their triangular-square lattice structural transition in a high-temperature chiral magnet", Nature Materials 15, 1237 (2016).
Citations:157 times
Summary:
    Skyrmion is an attractive spin structure for future spintronic device applications, due to their topologically protected nature. Among the materials exhibiting this spin structure, the Co-Zn-Mn alloy Co8Zn8Mn4 has attracted much attention because it exhibits a triangular lattice skyrmion phase near room temperature. However, the equilibrium state of the skyrmion phase exists only in a narrow magnetic field region near room temperature, which poses a challenge for applications. 
    In this paper, he uses cooling in a magnetic field to realize metastable skyrmion phases over a wide range of temperatures and magnetic fields, and show from small-angle neutron scattering and AC susceptibility measurements that the novel square lattice skyrmion phase can be realized at low temperatures.
Figure 1, Metastable skyrmion phase of Co8Zn8Mn4.

"Disordered skyrmion phase stabilized by magnetic frustration in a chiral magnet", Science Advances 4, eaar7043 (2018).
Citations:48 times
Summary:
    Co-Zn-Mn alloys with β-Mn structure show various magnetic states depending on their composition. While β-Mn itself does not show the long-range order due to its geometrical frustration and exhibits spin-liquid properties, the intermediate composition Co7Zn7Mn6 exhibits ferromagnetic and spin glass transitions. 
    In this paper, the existence of different kinds of skyrmion phases in Co7Zn7Mn6 near the ferromagnetic and spin glass transition temperatures, respectively, has been revealed by small-angle neutron scattering, magnetic susceptibility, and Lorentzian TEM measurements.
Figure 2, Magnetic phase transition and disordered skyrmion structure in Co7Zn7Mn6.

"Room-temperature antiskyrmions and sawtooth surface textures in a non-centrosymmetric magnet with S4 symmetry", Nature Materials 20, 335 (2021).
Citations:3 times
Summary:
    Skyrmion is a spin structure that has attracted much attention in both fundamental research and spintronics applications. Among the various skyrmion structures, the antiskyrmion structure has been predicted to occur when the Dzyaloshinsky-Moriya interaction in the crystal is anisotropic with respect to the crystal axis, and the crystal structure has D2d or S4 symmetry with 4-fold rotation symmetry.
    In this paper, he has shown for the first time that antiskyrmion occurs above room temperature in Fe1.9Ni0.9Pd0.2P with S4 symmetry, and that the transition from antiskyrmion to skyrmion depends on the magnetic field and sample thickness, as revealed by Lorentz TEM.
Figure 3, Antiskyrmions in S4 symmetric materials.

Takashi Kurumaji (The University of Tokyo)
”Development of new skyrmion materials and Study of the relationship between lattice inversion symmetry and the mechanism”

"Skyrmion lattice with a giant topological Hall effect in a frustrated triangular-lattice magnet", Science 365, 914 (2019).
Citations:183 times
Summary:
    Skyrmion, which has attracted attention from both basic research and industrial applications, were thought to require the Dzyaloshinsky-Moriya interaction that occurs at interfaces with broken inversion symmetry or non-centrosymmetric structures. On the other hand, theoretically, it has been predicted that skyrmion can be formed even in centrosymmetric structures due to magnetic frustration, and verification of this prediction has been awaited.
    In this paper, he has shown from topological Hall effect measurements and resonant X-ray scattering experiments that magnetic frustration of the triangular lattice created by Gd atoms in the centrosymmetric metal Gd2PdSi3 leads to the appearance of skyrmion.
Figure 4, Skyrmion phase in the center-symmetric material Gd2PdSi3.

"Néel-type skyrmion lattice in the tetragonal polar magnet VOSe2O5", Physical Review Letters 119, 237201 (2017).
citations:79 times
Summary:
    Skyrmion has been observed in various materials, and a spin structure called Bloch-type skyrmion has been observed in chiral magnetic materials such as B20 alloy, multiferroic Cu2OSeO3, and Co-Zn-Mn alloy. On the other hand, in magnetic thin films, the Dzyaloshinsky-Moriya interaction is modulated by inversion symmetry breaking, and a spin structure called Néel-type skyrmion has been observed. For further verification of the Néel-type skyrmion structure, it was desired to find a material that exhibits the structure in bulk.
    In this paper, he has shown from magnetic susceptibility and small-angle neutron scattering measurements that Néel-type skyrmions occur in the bulk material, the tetragonal polar magnet VOSe2O5.
Figure 5, Néel-type skyrmion structure in the tetragonal polar magnet VOSe2O5.

"Direct observation of Néel-type spin modulation in VOSe2O5", Journal of the Physical Society of Japan 90, 024705 (2021).
Citations:2 times
Summary:
    In distinguishing the various skyrmions, it is necessary to determine the moment direction of the spins that make up the skyrmion. Lorentz TEM, spin-polarized STM, spin-polarized LEEM, and NV-centered microscopy have been developed as means to study the direction. In particular, Lorentzian TEM has become a major experimental tool for observing Bloch-type skyrmions, while it is difficult to observe Néel-type skyrmions  due to geometrical limitations.
    In this paper, he has succeeded in determining the spin structure of the Néel-type skyrmion phase and the surrounding competing magnetic phases of the tetragonal polar magnet VOSe2O5 by polarized small-angle neutron scattering.
Fig. 6, Spin structure of VOSe2O5 determined by polarized small-angle neutron scattering.

Yasuyuki Shimura (Hiroshima University)
"Correlation of hidden degrees of freedom in 4f electron systems discovered from cryogenic magnetic field response"

"Low temperature magnetization of Yb2Pt2Pb with the Shastry-Sutherland type lattice and a high-rank multipole interaction", J. Phys. Soc. Jpn. 81, 103601 (2012).
(Arxiv)
Citations:13 times
Summary:
    The tetragonal metal compound Yb2Pt2Pb is a material in which Yb atoms form a Shastry-Sutherland lattice and is known to exhibit various magnetic phases depending on the direction of the applied magnetic field.  To explain this complex magnetic state, a model called the orthogonal Ising model has been proposed. 
    In this paper, by measuring the temperature and field dependence of magnetization down to cryogenic temperatures of T=0.08K, he has revealed the influence of higher-order multipole interactions of rank 7 on the formation of complex magnetic states.
Fig. 7, Temperature field phase diagram as a function of field direction for Yb2Pt2Pb.

"Giant anisotropic magnetoresistance due to purely orbital rearrangement in the quadrupolar heavy fermion superconductor PrV2Al20", Phys. Rev. Lett. 122, 256601 (2019).
citations:4 times
Summary:
     From the viewpoint of industrial applications, much attention has been paid to the charge and spin degrees of freedom of electrons, and expectations have been placed on the orbital degrees of freedom as a further degree of freedom. 
    The orbital order originating from orbital degrees of freedom appears in 3d-electron system, Mn oxides, and is the cause of the giant magnetoresistance. However, the orbital degrees of freedom in 3d electron systems are strongly coupled to the spin degrees of freedom, making it difficult to exploit the contribution from the pure orbital component. 
    In this paper, he focuses on the 4f-electron system, PrV2Al20, which exhibits nonmagnetic antiferroquadrupolar ordering below T=0.7 K. By conducting transport measurements under a high magnetic field at cryogenic temperatures, he has succeeded in observing the giant magnetoresistance and the anisotropic magnetoresistance originating from Fermi surface reconstruction due to antiferroquadrupolar ordering.
Figure 8, Temperature field phase diagram and magnetoresistance of PrV2Al20.

"Antiferromagnetic correlations in strongly valence fluctuating CeIrSn", Phys. Rev. Lett. 126, 217202 (2021).
Citations:0 times
Summary:
    Ce- and Yb-based metal compounds exhibit heavy electron behavior due to the coupling of conduction electrons with the localized moment of 4f electrons. In general, magnetic Ce3+ has a larger ionic radius than non-magnetic Ce4+, so valence fluctuation is known to cause positive thermal expansion and magnetostriction.
    In this paper, he has confirmed by HAXPES that CeIrSn, a 4f-electron metal compound in which Ce atoms form a quasi-Kagome lattice, is a valence-fluctuating material that exhibits a Kondo temperature of TK=480K. Furthermore, magnetostriction measurements down to T=0.05K and thermal expansion measurements down to T=0.5K have revealed that this material exhibits negative thermal expansion and magnetostriction. Moreover, μSR measurements down to T=0.1 K have revealed that the reason for this unusual behaviors is the development of antiferromagnetic correlations in the temperature region two orders of magnitude lower than the Kondo temperature.
Figure 9, HAXPES measurement of the valence fluctuation state of CeIrSn.


Masamichi Nakajima (Osaka University)
"Study of electronic states in iron-based superconductors by optical spectroscopy"

"Unprecedented anisotropic metallic state in BaFe2As2 revealed by optical spectroscopy", Proc. Natl. Acad. Sci. U.S.A. 108, 12238 (2011).
Citations:208 times
Summary:
    In copper oxides and iron arsenide high-temperature superconductors, superconductivity phase occurs in the vicinity of the symmetry-breaking electronic phase, and this relationship is of great interest. 
    BaFe2As2, one of the parent materials of iron-based superconductors, is known to undergo a transition from the tetragonal-paramagnetic phase at high temperatures to the orthorhombic-antiferromagnetic phase at low temperatures. It was reported by STM and ARPES that an in-plane anisotropic electronic state is realized in the low-temperature phase of this material, but these measurements were performed on samples in the twin state with mixed crystal axis directions. 
    In this paper, bulk-sensitive and energy-resolved optical conductivity measurements are performed on a sample of annealed high-quality and detwinned single crystal by uniaxial pressure to reveal that a truly anisotropic electronic state is realized in this system.
Figure 10, In-plane anisotropy of optical conductivity of BaFe2As2.


"Normal-state charge dynamics in doped BaFe2As2: Roles of doping and necessary ingredients for superconductivity", Sci. Rep. 4, 5873 (2014).
Citations:51 times
Summary:
    In BaFe2As2, one of the iron arsenide superconductor parent materials, superconductivity is induced by K, Co, and P substitution of each element. These correspond to hole doping, electron doping, and chemical pressure application. This behavior is different from that of cuprate superconductors, in which superconductivity occurs only by doping one type of carrier for one material. 
    The deformation of the Fermi surface due to elemental substitution in iron arsenide superconductors has been investigated by ARPES, but the effect on the charge dynamics including superconductivity has not been clarified.
    In this paper, optical conductivity measurements of the normal state of BaFe2As2 with each element substituted have revealed that the presence of a certain incoherent carrier component is essential for the development of superconductivity.
Figure 11, Compositional dependence of optical conductivity components in element-substituted BaFe2As2.


"Evolution of charge dynamics in FeSe1-xTex: Effects of electronic correlations and nematicity", Phys. Rev. B 104, 024512 (2021).
Citations:0 times
Summary:
    One of the iron-based superconductors, FeSe, exhibits a structural phase transition called the nematic transition at Ts=90K, which originates from the electronic system, and then a superconducting transition at Tc=8K. 
    Especially in Fe(Se,Te)/CaF2, where Te is substituted into FeSe thin film on CaF2 substrate, Tc increases with the suppression of nematic transition, and the maximum Tc=23K is shown. On the other hand, it is theoretically known that high-Tc superconductivity occurs in the vicinity of the Mott insulator phase, which has strong electron correlation, and in fact it has been reported that Te substitution strengthens the electron correlation in the system. 
    In this paper, systematic optical conductivity measurements of Fe(Se,Te)/CaF2 reveal that the increase of coherent conduction component due to the suppression of nematic transition by Te substitution contributes to the increase of Tc, and the further Te substitution causes the Tc to decrease due to too strong electronic correlation.
Figure 12, Composition dependence of optical conductivity of Fe(Se,Te)/CaF2

【Conclusion】
    In this article, I looked into the achievements of the recipients of the JPS Young Scientist Award (2022).
    
    They are producing overwhelming results.

    I have to do my best, too!

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