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２０２０年７月１４日（火）物 理 学 第 一 分 野

物理学第一分野ＤＣ３回生研究発表会
場所：理学研究科５号館 ５階・第四講義室
発表：２０分（別に質問１０分程度）
２０２０年７月１４日（火）１１：００～

目

開始

次

１． Light-driven modulation of liquid-crystalline order in nematic phase
CHIEN， CHIAO-YING（１１：００）
・・・・・ １
２．Non-Fermi liquid transport properties near the nematic quantum critical point of FeSe1-xSx
WENKAI HUANG（１１：３０）
・・・・・ ２

Light-driven modulation of liquid-crystalline orderin nematic phase
Soft Matter Physics Group

Chiao-Ying Chien

Abstract We immobilize azobenzene through polymerization, fabricating the liquid crystal gels withazobenzene-containing copolymer microstructure and study their photo-induced spatial variation ofbirefringence (∆n). We clarify that the surface anchoring of microstructure is crucial in modulationof photo-induced change of ∆n.
© 2020 Department of Physics, Kyoto University

Recently, Samitsu et al. proposed a principle of the molecular manipulator which transport polymer throughspatial variation of the scalar order parameter (S) in the nematic liquid crystal (LC) [1]. The spatial variation of S
was generated by stimulating trans-to-cis isomerization of azobenzene derivatives in the nematic phase. Under UVlight, the trans-azobenzene converts into cis form. The cis-azobenzene with a bent-like shape tends to destabilize
the liquid-crystalline order, resulting in a decrease of S and the birefringence (∆n) of the nematic phase [2]. Basedon this mechanism, the molecular manipulator provides a great potential to drive arbitrary nanometer-sized
impurities in the nematic phase. However, this method has a limitation. The natural diffusion of azobenzene mightcause poor spatial resolution of S, which obstructs the development of precisely-controlling the nm-sized impurities.
In this study, immobilization of azobenzene through polymerization is proposed to prevent the diffusion ofazobenzene in the nematic phase. We fabricate nematic LC gels with phase-separation of azobenzene-containing
copolymer microstructure. Unlike conventional photo-induced decrease of ∆n in the azobenzene/nematic mixture,
an increase of ∆n is newly observed under UV irradiation in the LC gels (Fig. 1) [3]. To elucidate the anomalous
increase of ∆n, the photo-induced change of ∆n and the morphology of the copolymer microstructure areinvestigated with the variant composition of LC gels. The shape of the copolymer microstructure, which have afinite anchoring strength on their surface, influence the LC director and result in a lower ∆n before the UV irradiation.
We find that the trans-to-cis isomerization of azobenzene weakens the surface anchoring of the copolymer
microstructure. Hence, the LC directors recover their uniform
distribution, and the increase of ∆n appears.
We tune the composition of LC gels to eliminate theanchoring effect of the copolymer microstructure [3] andevaluate the spatial resolution of the LC gels [4]. The photoinduced spatial variation of ∆n of the non-polymerized and thepolymerized samples are quantitatively compared throughmeasuring their full-width-at-half-maximum (FWHM). The
FWHM of the non-polymerized sample expands with time, Fig. 1. Photo-induced spatial variation of ∆n as awhile the polymerized sample remains the feature close to that function of UV exposure time.
of the stimulating light. The time evolution of ∆n clearly showsthat the photo-induced change of S is highly constrained in theirradiated area in the polymerized sample (Fig. 2). Based onthese results, we conclude that the LC material for modulating
the scalar order parameter with high spatial resolution has beenachieved by our method.
References[1] S. Samitsu et al., Nat. Mater. 9, 816 (2010).
[2] T. Ikeda, J. Mater. Chem. 13, 2037 (2003).
[3] C.-Y. Chien et al., submitted.
[4] C.-Y. Chien et al., submitted.

Fig. 2. Time evolution of photo-induced spatial
variation of ∆n in (a) non-polymerized and (b)polymerized samples, where red line is the UV lightdistribution.

Non-Fermi liquid transport properties near the nematicquantum critical point of FeSe1-xSx
Quantum Condensed Matter Group

Wen-Kai Huang

Abstract To study how the nematic fluctuations affect the normal state transport properties, wehave conducted charge transport measurements on FeSe1-xSx superconductors. We found that thequantum nematic fluctuations can cause similar transport phenomena that have been observed in
the case of antiferromagnetic fluctuations. Our results suggest that different types of quantum
fluctuations could result the same anomalous charge transport behaviors.
© 2020 Department of Physics, Kyoto University

Quantum phase transition is one of the central problems in strongly correlated materials. It has been discussed
that quantum fluctuations of order parameter can become large when the system is near a quantum critical point(QCP), a point where a zero temperature transition happens by controlling some nonthermal parameters. It isbelieved that these quantum fluctuations can give rise to the non-Fermi liquid (NFL) behaviors and areresponsible for unconventional superconductivity. Thus, understanding the physics associated with quantum
fluctuations is of primarily importance [1]. One canonical example of such QCP is an antiferromagnetic (AFM)
QCP, which has been realized in a variety of correlated materials, such as heavy fermion compounds and cuprates.
In this study, we focus on another type of QCP, i.e., a QCP of a nematic order that breaks the rotational symmetryof the underlying crystal lattice.
The iron-based superconductor FeSe1-xSx is a multi-band compound which has a nematic QCP without magnetic
order. For x = 0, a nematic order appears below Ts ~ 90 K. As the sulfur doping increases, the transition
temperature Ts decreases, and finally becomes zero at xc ~ 0.17. Around this nematic QCP, large nematicfluctuations have been observed by nematic susceptibility measurements, while AFM fluctuations are found to benegligible [2]. This indicates that FeSe1-xSx is suitable to study the impact of nematic fluctuations.
Here, to investigate how the nematic fluctuations affect the transport properties, we measured the dc-resistivity,
Hall effect and magnetoresistance at various doping levels across the nematic QCP of FeSe1-xSx. Near xc = 0.17,
we found that the resistivity shows linear dependence ontemperature at low temperatures, as shown in Fig.1, and the Hall
angle cotθH (= ρxy/ρxx ) is proportional to T2. Moreover, themagnetoresistance obeys modified Kohler’s rule (∆ρxx/ρxx ∝ tan2θH).
These transport properties near xc = 0.17 provide evidence for theNFL behaviors due to nematic QCP. It should be note that the AFM
fluctuations can also lead to the similar anomalous transport
behaviors [3]. Our results suggest that different types of quantum
fluctuations could result the same anomalous charge transport
behaviors [4].

References[1] H.v. Löhneysen et al., Rev. Mod. Phys. 79, 1015 (2007).
[2] P. Wiecki et al., Phys. Rev. B 98, 020507(R) (2018).
[3] H. Kontani, Rep. Prog. Phys. 71, 026501 (2008).
[4] W. K. Huang et al., submitted.

Fig. 1. Phase Diagram of FeSe1-xSx. Colors
represent exponent α given by formula ρxx= ρ0 + ATα. The green dot line represents
nematic transition temperature Ts. α ≈ 1near xc ≈ 0.17.
1