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論文

Magnetic and transport properties of the pseudobrookite Al$$_{1-x}$$Ti$$_{2+x}$$O$$_{5}$$ single crystals

高濱 隆成*; 有薗 実駿*; 犬童 代梧*; 吉永 汰正*; 寺倉 千恵子*; 竹下 直*; 白崎 巧*; 野田 正亮*; 桑原 英樹*; 梶本 亮一; et al.

JPS Conference Proceedings (Internet), 38, p.011114_1 - 011114_6, 2023/05

We investigated the transport and magnetic properties of single crystals of the pseudobrookite Al$$_{1-x}$$Ti$$_{2+x}$$O$$_{5}$$ for $$0 le x le 1$$ grown using a floating zone. We found a correlation of spin-singlet Ti$$^{3+}$$-Ti$$^{3+}$$ dimers even in the conductive $$alpha$$ and $$lambda$$ phases which develops with increasing $$x$$. The development of the dimer correlation suppresses the magnetic susceptibility at the low temperature and enhances the electric conductivity perhaps due to the suppression of the magnetic scattering at the isolated Ti$$^{3+}$$ ions. The compound shows the best conductivity in the $$lambda$$ phase near the phase boundary between the $$lambda$$ and $$beta$$ phases where the dimer correlation is much developed. Some exotic conductive state may be realized under the background of fluctuation of the spin-singlet dimer in the $$lambda$$ phase near the phase boudary.

論文

Structural, magnetic, transport, and thermoelectric properties of the pseudobrookite AlTi$$_{2}$$O$$_{5}$$-Ti$$_{3}$$O$$_{5}$$ system

高濱 隆成*; 石井 透依*; 犬童 代梧*; 有薗 実駿*; 寺倉 千恵子*; 十倉 好紀*; 竹下 直*; 野田 正亮*; 桑原 英樹*; 斎木 琢夫*; et al.

Physical Review Materials (Internet), 4(7), p.074401_1 - 074401_11, 2020/07

 被引用回数:6 パーセンタイル:40.97(Materials Science, Multidisciplinary)

We investigated the structural, magnetic, transport, and high-temperature thermoelectric properties of single crystals of the pseudobrookite Al$$_{1-x}$$Ti$$_{2+x}$$O$$_{5}$$ for $$0 le x le 1$$ grown using a floating zone. We found a correlation of spin-singlet Ti$$^{3+}$$-Ti$$^{3+}$$ dimers coupled with the lattice even in the conductive $$alpha$$ and $$lambda$$ phases which develops with increasing $$x$$. This developing dimer correlation reduces the number of unpaired Ti$$^{3+}$$ ions, which makes the compound more conductive owing to the suppression of disorder for $$x$$ up to about 0.75. The dimer fluctuation causes a critical enhancement of the magnetic susceptibility at around 150 K in the $$lambda$$ phase near the boundary ($$x sim 0.9$$) between the $$lambda$$ and the $$beta$$ phases. Such a correlation of the spin-singlet Ti$$^{3+}$$-Ti$$^{3+}$$ dimers may produce a high Seebeck coefficient in the conductive $$alpha$$ and $$lambda$$ phases leading to a large thermoelectric power factor at high temperatures.

論文

Schottky specific heat of the lightly Mn-substituted electron-doped SrTiO$$_3$$

奥田 哲治*; 梶本 亮一; 野田 正亮*; 桑原 英樹*

AIP Advances (Internet), 8(10), p.101339_1 - 101339_5, 2018/10

 被引用回数:1 パーセンタイル:4.48(Nanoscience & Nanotechnology)

Large changes in the low-temperature specific heat (low-$$TC$$) by applying magnetic fields up to 9 Tesla were found in the lightly Mn-substituted electron-doped perovskites Sr$$_{1-x}$$La$$_{x}$$TiO$$_{3}$$. The changes in the low-$$TC$$ are qualitatively well explained by the Schottky specific heat ($$C_mathrm{Sch}$$) of localized spins of the Mn 3d electrons in weak internal magnetic fields via itinerant electrons. According to the analysis by using the conventional model, the doped Mn ions are apparently not Mn$$^{4+}$$ ($$S$$ = 3/2) ions, but are Jahn-Teller active Mn$$^{3+}$$ ($$S$$ = 2) ions or mixtures of Mn$$^{3+}$$ and Mn$$^{2+}$$ ions. However, the actual numbers of localized spins estimated from $$C_mathrm{Sch}$$ are about 30% smaller than the expected values. Part of the localized spins of the Mn 3d electrons may disappear due to Kondo coupling with the itinerant electrons, leading to the observed enhancement of the electronic specific heat coefficients.

論文

Elastic and dynamical structural properties of La and Mn-doped SrTiO$$_{3}$$ studied by neutron scattering and their relation with thermal conductivities

梶本 亮一; 中村 充孝; 村井 直樹; 社本 真一; 本田 孝志*; 池田 一貴*; 大友 季哉*; 畑 博人*; 江藤 貴弘*; 野田 正亮*; et al.

Scientific Reports (Internet), 8(1), p.9651_1 - 9651_8, 2018/06

 被引用回数:6 パーセンタイル:30.6(Multidisciplinary Sciences)

The electron-doped SrTiO$$_{3}$$ exhibits good thermoelectric properties, which makes this material a promising candidate of an n-type oxide thermoelectric device. Recent studies indicated that only a few percent co-doping of La and Mn in SrTiO$$_{3}$$ substantially reduces the thermal conductivity, thereby greatly improving the thermoelectric figure of merit at room temperature. Our time-of-flight neutron scattering studies revealed that by doping both La and Mn into SrTiO$$_{3}$$, the inelastic scattering spectrum shows a momentum-independent increase in the low-energy spectral weight approximately below 10 meV. The increase in the low-energy spectral weight exhibits a clear correlation with thermal conductivity. The correlation is attributed to dynamical and local structural fluctuations caused by the Jahn-Teller instability in Mn$$^{3+}$$ ions coupled with the incipient ferroelectric nature of SrTiO$$_{3}$$, as the origin of the low thermal conductivity.

口頭

Elastic and dynamical structural properties of La and Mn-doped SrTiO$$_{3}$$ studied by neutron scattering and their relation with thermal conductivities

梶本 亮一; 中村 充孝; 村井 直樹; 社本 真一; 本田 孝志*; 池田 一貴*; 大友 季哉*; 畑 博人*; 江藤 貴弘*; 野田 正亮*; et al.

no journal, , 

The electron-doped SrTiO$$_{3}$$ exhibits good thermoelectric properties. Recent studies indicated that only a few percent co-doping of La and Mn in SrTiO$$_{3}$$ substantially reduces the thermal conductivity. Then, we investigated the static as well as dynamical structural properties of La and Mn-doped SrTiO$$_{3}$$ by time-of-flight elastic and inelastic neutron scattering. We found that the La-Mn co-doped samples show tetragonal structures similar to the low-temperature structure of SrTiO$$_{3}$$, while the La-doped and Mn-doped samples maintain cubic structures with the same symmetry as the high-temperature structure of SrTiO$$_{3}$$. However, it is unlikely that it is the origin of the low thermal conductivity, since the structural transition in SrTiO$$_{3}$$ does not significantly change the thermal conductivity. On the other hand, we found that the inelastic scattering spectrum shows a momentum-independent increase in the low-energy spectral weight by the co-doping. Interestingly, the increase in the low-energy spectral weight exhibits a clear correlation with thermal conductivity. The substitution of Sr$$^{2+}$$ ions by La$$^{3+}$$ ions results in the electron doping, and further doping of the Mn ions are expected to create Jahn-Teller active Mn$$^{3+}$$ ions. We consider that dynamical and local structural fluctuations caused by the Jahn-Teller instability in Mn$$^{3+}$$ ions coupled with the incipient ferroelectric nature of SrTiO$$_{3}$$ induce additional low-energy scatterings and correspond to the origin of low thermal conductivity in the La and Mn co-doped compounds.

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