JOPSS:検索結果一覧

検索結果：　82 件中 1件目～20件目を表示

発表形式

Initialising ...

選択項目を絞り込む

掲載資料名

Initialising ...

発表会議名

Initialising ...

筆頭著者名

Initialising ...

キーワード

Initialising ...

発表言語

Initialising ...

発行年

Initialising ...

開催年

Initialising ...

論文

Singular continuous and nonreciprocal phonons in quasicrystal AlPdMn

松浦直人*; Zhang, J.*; 上村祥史*; 古府麻衣子; 枝川圭一*

Physical Review Letters, 133(13), p.136101_1 - 136101_5, 2024/09

https://doi.org/10.1103/PhysRevLett.133.136101

被引用回数：1 パーセンタイル：48.32(Physics, Multidisciplinary)

In quasicrystals lacking translational symmetry but having highly ordered structures, understanding how phonons propagate in their aperiodic lattices remains an unsolved issue. We present an inelastic neutron scattering study on acoustic phonon modes of icosahedral quasicrystal AlPdMn, revealing hierarchical pseudo-gap structure in low-energy acoustic modes. Additionally, phonon intensities are asymmetric in energy and wave vectors with respect to the Bragg peak, indicating characteristic nonreciprocal phonon propagation in quasicrystals.

論文

Quasielastic neutron scattering probing H $^{-}$ dynamics in the H $^{-}$ conductors LaH $_{3-2x}$ O $_{x}$

玉造博夢; 福井慧賀*; 飯村壮史*; 本田孝志*; 多田朋史*; 村上洋一*; 山浦淳一*; 倉本義夫*; 佐賀山基*; 山田武*; et al.

Physical Review B, 107(18), p.184114_1 - 184114_8, 2023/05

https://doi.org/10.1103/PhysRevB.107.184114

被引用回数：1 パーセンタイル：13.37(Materials Science, Multidisciplinary)

Using an incoherent quasi-elastic neutron scattering (QENS) technique, we investigate H $^{-}$ dynamics in a series of oxyhydrides LaH $_{3-2x}$ O $_{x}$ that exhibit characteristic high H $^{-}$ conductivity. In the end member LaH $_{3}$ ( = 0), two kinds of H $^{-}$ dynamics are identified: the jump diffusion and the localized motion. The jump length in the jump diffusion mode increases with increasing . The localized motion is identified as a jump between the two inequivalent sites. These dynamics are corroborated by our molecular dynamical simulations. Our QENS data suggest that similar H $^{-}$ dynamics occurs also in oxyhydrides LaH $_{3-2x}$ O $_{x}$ ( 0), whose H $^{-}$ concentration dependence is consistent with the previous measurement of ionic conductivity. We also discuss the possibility that LaH $_{3-2x}$ O $_{x}$ is an example of H $^{-}$ ion conductors governed by the concerted migration mechanism. The identified H $^{-}$ dynamics is a key to understanding the anomalous hydrogen concentration dependence of the diffusion coefficient in lanthanum hydrides, which has been a longstanding mystery in this compound.

論文

The Translational, rotational, and phonon dynamics of water in ZrO $_{2}$ /water nanofluid

吉田亨次*; 真田雄介*; 山口敏男*; 松浦直人*; 玉造博夢; 内山裕士*

Journal of Molecular Liquids, 366, p.120218_1 - 120218_9, 2022/11

https://doi.org/10.1016/j.molliq.2022.120218

被引用回数：1 パーセンタイル：6.73(Chemistry, Physical)

The translational, rotational, and phonon dynamics of water in zirconium oxide (ZrO $_{2}$ ) nanofluid were investigated by quasi-elastic neutron scattering, $^{17}$ O NMR relaxation, and inelastic X-ray scattering, respectively. The particle size of ZrO $_{2}$ is 3 100 nm, and the concentration of ZrO $_{2}$ nanoparticles is 5, 10, 20, and 30 wt%. The translational and rotational motions of water molecules become retarded with increasing the concentration of ZrO $_{2}$ . This indicates that there is an attractive interaction between water molecules and the surface of the nanoparticles. The high-frequency sound velocity of the nanofluid obtained from phonon dynamics measurements is slightly larger than that of pure water although it is not much difference compared to the case of the translation and rotation. These findings indicate that water dynamics in the nanofluid is different from that of the bulk. It might be reasons for the enhancement of the thermal conductivity of the nanofluid.

論文

Two-step Mott transition in Ni(S,Se); SR studies and charge-spin percolation model

Sheng, Q.*; 金子竜也*; Yamakawa, Kohtaro*; Guguchia, Z.*; Gong, Z.*; Zhao, G.*; Dai, G.*; Jin, C.*; Guo, S.*; Fu, L.*; et al.

Physical Review Research (Internet), 4(3), p.033172_1 - 033172_14, 2022/09

https://doi.org/10.1103/PhysRevResearch.4.033172

A pyrite system NiS $_{2-x}$ Se exhibits a bandwidth controlled Mott transition via (S,Se) substitutions in a two-step process: the antiferromagnetic insulator (AFI) to antiferromagnetic metal (AFM) transition at 0.45 followed by the AFM to paramagnetic metal (PMM) transition at 1.0. Among a few other Mott systems which exhibit similar two-step transitions, Ni(S,Se) is of particular interest because a large intermediate AFM region in the phase diagram would provide unique opportunities to study the interplay between the spin and charge order. By comparing and combining our muon spin relaxation studies and previous neutron scattering studies, here we propose a picture where the spin order is maintained by the percolation of "nonmetallic" localized and dangling Ni moments surrounded by S, while the charge transition from AFI to AFM is caused by the percolation of the conducting paths generated by the Ni-Se-Ni bonds.

論文

Revealing the ion dynamics in Li $_{10}$ GeP $_{2}$ S $_{12}$ by quasi-elastic neutron scattering measurements

堀智*; 菅野了次*; Kwon, O.*; 加藤祐樹*; 山田武*; 松浦直人*; 米村雅雄*; 神山崇*; 柴田薫; 川北至信

Journal of Physical Chemistry C, 126(22), p.9518 - 9527, 2022/06

https://doi.org/10.1021/acs.jpcc.2c01748

被引用回数：11 パーセンタイル：59.11(Chemistry, Physical)

Understanding Li-ion conduction in superionic conductors accelerates the development of new solid electrolytes to enhance the charge-discharge performances of all-solid-state batteries. We performed a quasi-elastic neutron scattering study on a model superionic conductor (Li $_{10+x}$ Ge $_{1+x}$ P $_{2-x}$ S $_{12}$ , LGPS), to reveal its ion dynamics on an angstrom-scale spatial range and a pico-to-nanosecond temporal range. The observation of spectra at 298 K confirmed the high lithium diffusivity. The obtained diffusion coefficient was in the order of 10 $^{-6}$ cm $^{2}$ s $^{-1}$ at temperatures 338 K and was higher than the reported diffusion coefficient over a longer time scale, as determined by the pulse-field gradient nuclear magnetic resonance method. This difference indicates that there are impediments to ionic motion over a longer time scale. The dynamic behavior of the Li ions was compared with that observed for the Li $_{9}$ P $_{3}$ S $_{9}$ O $_{3}$ phase, which possesses the same crystal structure type, but a lower ionic conductivity. The LGPS phase possessed a high lithium mobility over a distance of 10 , as well as a larger fraction of mobile Li ions, thereby indicating that these features enhance lithium conduction over a longer spatial scale, which is important in all-solidstate batteries.

論文

Hyperfine splitting and nuclear spin polarization in NdPdAl and NdPd $_{20}$ Ge

目時直人; 柴田薫; 松浦直人*; 北澤英明*; 鈴木博之*; 山内宏樹; 萩原雅人; Frontzek, M. D.*; 松田雅昌*

Journal of the Physical Society of Japan, 91(5), p.054710_1 - 054710_6, 2022/05

https://doi.org/10.7566/JPSJ.91.054710

被引用回数：0 パーセンタイル：0.00(Physics, Multidisciplinary)

NdPdAlにおけるNdの超微細相互作用分裂を高分解能の中性子非弾性散乱によって研究した。Nd核の核スピン(=7/2)と、の軌道が主成分の基底状態を示す4電子との間の超微細相互作用によって、磁気秩序相において生じた超微細相互作用分裂に起因する中性子非弾性散乱ピークを、 = 3eVのエネルギーに観察した。低温では、Nd核スピンの低エネルギー状態の占拠確率が増加することによりNd核スピンが偏極し、反強磁性散乱ピーク強度に寄与するが、その際、中性子の非スピン反転過程の寄与が最も大きい。この現象の応用として反強磁性散乱ピーク強度の温度変化から、NdPd $_{20}$ GeのNd磁気モーメント及び超微細相互作用分裂の大きさを見積もることに成功した。

論文

Magnetic Bragg peak enhancement under ultrasound injection

社本真一*; 赤津光洋*; 松浦直人*; 河村聖子; 針井一哉*; 小野正雄*; Chang, L.-J.*; 伊藤孝; 根本祐一*; 家田淳一

Physical Review Research (Internet), 4(1), p.013245_1 - 013245_7, 2022/03

https://doi.org/10.1103/PhysRevResearch.4.013245

イットリウム鉄ガーネットの磁気ブラッグピークに対する超音波注入効果を、準弾性中性子散乱によって研究した。磁気ブラッグピークは、温度の低下とともに大幅に強化さる。エネルギー幅は、超音波注入によって引き起こされる試料温度上昇の平方根に比例して増加した。磁気ブラッグピークは格子振動によって増強されることから、この増強はスピン-格子結合と密接に関連していると予想される。縦波モードで100Kを超えると観測された急激な散乱信号の低下は、スピン-格子結合の劣化を示唆している。これは、温度が100Kを超えるとスピンゼーベック効果が低下することと一致しており、スピン-格子結合による劣化メカニズムに証明を与えている。

論文

Microscopic dynamics of lithium diffusion in single crystal of the solid-state electrolyte La $_{2/3-x}$ Li $_{3x}$ TiO $_{3}$ (x = 0.13) studied by quasielastic neutron scattering

松浦直人*; 藤原靖幸*; 森分博紀*; 尾原幸治*; 川北至信

Physical Review B, 104(9), p.094305_1 - 094305_7, 2021/09

https://doi.org/10.1103/PhysRevB.104.094305

被引用回数：8 パーセンタイル：42.22(Materials Science, Multidisciplinary)

Quasielastic neutron scattering (QENS) measurements combined with first principles based molecular dynamics calculations were conducted to study the dynamics of Li $^{+}$ ions in a solid-state electrolyte La $_{2/3-x}$ Li $_{3x}$ TiO $_{3}$ (LLTO) with . By using a large $^{7}$ Li-enriched single crystal sample, a QENS signal was clearly observed along the three principal axes [110], [111], and [001] at a temperature () of 600 K. Wave vector dependence of the linewidth of the QENS signal along each direction was explained well using the Chudley-Elliot model for jumps between the A sites of the perovskite lattice through the bottleneck square, which was also supported by molecular dynamics calculations. The estimated self-diffusion coefficient of Li $^{+}$ ( $D_{rm Li}$ ) in the ab plane was slightly larger than that along the c axis, suggesting quasi-isotropic diffusion, that is, the three-dimensional diffusion of Li $^{+}$ ions. The decrease in $D_{rm Li}$ with decreasing was reasonably explained by a thermal activation process with the activation energy determined from ionic-conductivity measurements. Furthermore, the estimated values of the self-diffusion coefficient are comparable to those in the sulfide-based Li $^{+}$ ion conductor, Li $_{7}$ P $_{3}$ S $_{11}$ , with 10 times larger ionic conductivity, which clarifies how to understand the Li conduction mechanism in LLTO and Li $_{7}$ P $_{3}$ S $_{11}$ in a unified manner.

論文

Spin glass behavior and magnetic boson peak in a structural glass of a magnetic ionic liquid

古府麻衣子; 綿貫竜太*; 榊原俊郎*; 河村聖子; 中島健次; 松浦直人*; 上木岳士*; 阿久津和宏*; 山室修*

Scientific Reports (Internet), 11(1), p.12098_1 - 12098_8, 2021/06

https://doi.org/10.1038/s41598-021-91619-z

被引用回数：8 パーセンタイル：58.52(Multidisciplinary Sciences)

Glassy magnetic behavior has been observed in a wide range of crystalline magnetic materials called spin glass. Here, we report spin glass behavior in a structural glass of a magnetic ionic liquid, C4mimFeCl $_{4}$ . Magnetization measurements demonstrate that an antiferromagnetic ordering occurs at $T_{rm N}$ = 2.3 K in the crystalline state, while a spin glass transition occurs at $T_{rm SG}$ = 0.4 K in the structural glass state. In addition, localized magnetic excitations were found in the spin glass state by inelastic neutron scattering, in contrast to spin-wave excitations in the ordered phase of the crystalline sample. The localized excitation was scaled by the Bose population factor below $T_{rm SG}$ and gradually disappeared above $T_{rm SG}$ . This feature is highly reminiscent of boson peaks commonly observed in structural glasses. We suggest the "magnetic" boson peak to be one of the inherent dynamics of a spin glass state.

論文

Position dependency of the scattered intensity in the time-of-flight backscattering spectrometer DNA

松浦直人*; 山田武*; 富永大輝*; 小林誠*; 中川洋; 川北至信

JPS Conference Proceedings (Internet), 33, p.011068_1 - 011068_6, 2021/03

https://doi.org/10.7566/JPSCP.33.011068

飛行時間分析型後方散乱分光器DNAにおける散乱強度の位置依存性を調査した。垂直方向(位置敏感一次元検出器(PSD)のピクセル方向)と水平方向(PSD毎)の両方で周期的な構造が見られた。DNAに設置されたソラースリットとアナライザー結晶の設計値を超えた過度な湾曲が、強度分布の起源であると考えられる。我々は、強度の位置依存性と弾性散乱のエネルギーオフセットを系統的に補正できるソフトウェアを開発した。このソフトウェアにより、本来の散乱強度からのずれが補正でき、エネルギー分解能などのデータの質も向上させることができる。

論文

Position-encoded automatic cell elevator for BL02, J-PARC MLF

富永大輝*; 小林誠*; 山田武*; 松浦直人*; 川北至信; 笠井聡*

JPS Conference Proceedings (Internet), 33, p.011095_1 - 011095_5, 2021/03

BB2019-1955.pdf:1.33MB

https://doi.org/10.7566/JPSCP.33.011095

トップローディング型冷凍機用の垂直移動型試料交換機が、J-PARC MLFのBL02に設置された中性子分光器の利用支援のために開発された。PEACEと名付けられたこの試料交換機は、中性子照射位置での試料位置の再現性を、PEEKと呼ぶ物質でできたガイドを利用して制御している。3つの試料位置でのバックグラウンド散乱のプロファイルの変化は1.6%以内であることが分かった。この結果は、垂直軸からの試料位置の偏差が0.3mmであることを考えると妥当である。

論文

Magnetism induced by interlayer electrons in the quasi-two-dimensional electride Y $_{2}$ C; Inelastic neutron scattering study

玉造博夢; 村上洋一*; 倉本義夫*; 佐賀山基*; 松浦直人*; 川北至信; 松石聡*; 鷲尾康仁*; 井下猛*; 浜田典昭*; et al.

Physical Review B, 102(22), p.224406_1 - 224406_5, 2020/12

https://doi.org/10.1103/PhysRevB.102.224406

被引用回数：9 パーセンタイル：43.41(Materials Science, Multidisciplinary)

Magnetic excitations in layered electride Y $_{2}$ C have been found by inelastic neutron scattering. We have observed weak but clear magnetic scattering around the wave number , but no magnetic order down to the lowest temperature measured (7 K). The imaginary part of the dynamical susceptibility deduced is well described by the Lorentz function of energy for each momentum . The width of the Lorentzian is proportional to $Q(Q^2+kappa^{2})$ with $kappa^{-1} sim 4{rm AA}$ at K. We have also found that with increasing the magnetic form factor decays faster than that of a electron in a single Y atom, which indicates a more extended magnetic moment in Y $_{2}$ C. These results provide experimental evidence that the itinerant magnetism in Y $_{2}$ C originates from the anionic electrons that reside in the interlayers. The Curie-Weiss-like behavior of the magnetic susceptibility reported in Y $_{2}$ C is ascribed to the mode coupling effects of spin fluctuations.

論文

Ultralow-energy magnon anomaly in yttrium iron garnet

社本真一; 安井幸夫*; 松浦直人*; 赤津光洋*; 小林義明*; 根本祐一*; 家田淳一

Physical Review Research (Internet), 2(3), p.033235_1 - 033235_6, 2020/08

https://doi.org/10.1103/PhysRevResearch.2.033235

イットリウム鉄ガーネット結晶の超低エネルギーマグノンを、非弾性中性子散乱によって45eV以下のエネルギー範囲で調べたところ、[111]方向に約0.1Tの磁場が印加した場合に、10Kで超低エネルギーのマグノン異常を発見した。

論文

Status of neutron spectrometers at J-PARC

梶本亮一; 横尾哲也*; 中村充孝; 川北至信; 松浦直人*; 遠藤仁*; 瀬戸秀紀*; 伊藤晋一*; 中島健次; 河村聖子

Physica B; Condensed Matter, 562, p.148 - 154, 2019/06

BB2018-0526.pdf:2.45MB

https://doi.org/10.1016/j.physb.2018.11.061

被引用回数：17 パーセンタイル：58.46(Physics, Condensed Matter)

Seven time-of-flight quasielastic/inelastic neutron scattering instruments are installed on six beamlines in the Materials and Life Science Experimental Facility (MLF) at the Japan Proton Accelerator Research Complex (J-PARC). Four of these instruments are chopper-type direct-geometry spectrometers, one is a near-backscattering indirect-geometry spectrometer, and the other two are spin-echo-type spectrometers. This paper reviews the characteristic features, recent scientific outcomes, and progress in development of MLF spectrometers.

報告書

Proceedings of the User Group Meeting on MLF Spectrometers (DIRECTION 2018); October 11, 2018, Ibaraki Quantum Beam Research Center (Tokai, Ibaraki, Japan)

梶本亮一; 横尾哲也*; 松浦直人*

JAEA-Review 2018-024, 78 Pages, 2019/01

JAEA-Review-2018-024.pdf:76.52MB

平成30年10月11日、MLF中性子分光器による合同のユーザーミーティング「DIRECTION 2018」がJ-PARC MLF、高エネルギー加速器研究機構物質構造科学研究所、総合科学研究機構の共催で開催された。本ミーティングは6つの分光器、すなわちBL01(4SEASONS)、BL02(DNA)、BL06(VIN ROSE)、BL12(HRC)、BL14(AMATERAS)、BL23(POLANO)におけるユーザー利用の活性化およびユーザーと装置の協力関係の構築を目指して開催されたものである。研究会では14件の口頭発表および21件のポスター発表が行われ、各装置を利用して行われた研究成果の紹介、各装置や試料環境に関する現状の報告がなされるとともに、装置や試料環境に関する要望や今後の展開等について密接な議論が交わされた。本報告書はこのミーティングの講演要旨および講演で使用された発表資料を収録したものである。

論文

Neutron scattering study of yttrium iron garnet

社本真一; 伊藤孝; 大西弘明; 山内宏樹; 稲村泰弘; 松浦直人*; 赤津光洋*; 樹神克明; 中尾朗子*; 茂吉武人*; et al.

Physical Review B, 97(5), p.054429_1 - 054429_9, 2018/02

https://doi.org/10.1103/PhysRevB.97.054429

被引用回数：21 パーセンタイル：65.26(Materials Science, Multidisciplinary)

イットリウム鉄ガーネットの核および磁気構造と全マグノン分散を中性子散乱により調べた。低エネルギーの分散は強磁性マグノンで期待されるように14meVまで2次関数の分散を示した。積分した動的磁化率"()の虚部は低エネルギーで平方根のエネルギー依存性を示した。"()から絶対値でマグノン状態密度を求めた。その値は理論的に予想されるマグノン分散でひとつのカラリティーモードに対応する。

論文

Polar rotor scattering as atomic-level origin of low mobility and thermal conductivity of perovskite CH $_{3}$ NH $_{3}$ PbI $_{3}$

Li, B.; 川北至信; Liu, Y.*; Wang, M.*; 松浦直人*; 柴田薫; 河村聖子; 山田武*; Lin, S.*; 中島健次; et al.

Nature Communications (Internet), 8, p.16086_1 - 16086_9, 2017/06

https://doi.org/10.1038/ncomms16086

被引用回数：83 パーセンタイル：90.21(Multidisciplinary Sciences)

Perovskite CH $_{3}$ NH $_{3}$ PbI $_{3}$ exhibits outstanding photovoltaic performances, but the understanding of the atomic motions remains inadequate even though they take a fundamental role in transport properties. Here, we present a complete atomic dynamic picture consisting of molecular jumping rotational modes and phonons, which is established by carrying out high-resolution time-of-flight quasi-elastic and inelastic neutron scattering measurements in a wide energy window ranging from 0.0036 to 54 meV on a large single crystal sample, respectively. The ultrafast orientational disorder of molecular dipoles, activated at approximately 165 K, acts as an additional scattering source for optical phonons as well as for charge carriers. It is revealed that acoustic phonons dominate the thermal transport, rather than optical phonons due to sub-picosecond lifetimes. These microscopic insights provide a solid standing point, on which perovskite solar cells can be understood more accurately and their performances are perhaps further optimized.

論文

Development of spin-wave-like dispersive excitations below the pseudogap temperature in the high-temperature superconductor La $_{2-x}$ Sr $_{x}$ CuO $_{4}$

松浦直人*; 川村奨*; 藤田全基*; 梶本亮一; 山田和芳*

Physical Review B, 95(2), p.024504_1 - 024504_6, 2017/01

https://doi.org/10.1103/PhysRevB.95.024504

被引用回数：10 パーセンタイル：42.20(Materials Science, Multidisciplinary)

Inelastic neutron scattering measurements of magnetic excitations in hole-doped high- cuprates La $_{2-x}$ Sr $_{x}$ CuO $_{4}$ ( and 0.075) have been performed over a wide temperature range of 5 500 K extending beyond the pseudogap temperature of 360 K for . For , the two-dimensional spin-wave excitations at low temperatures are thermally robust and the dispersion is slightly softened on heating up to K. For , the spin-wave-like excitations in the upper part of the hourglass-shaped dispersion remain at K. However, further heating above K induces a broad ridge centered at ; thereby the hourglass-shaped dispersion becomes blurred above the pseudogap temperature. The localized spin nature of magnetic excitations below suggests that the pseudogap is related to a proximity to a Mott insulator rather than being a precursor of Cooper pairs.

論文

Negative correlation between electrical response and domain size in a Ti-composition-gradient Pb[(Mg $_{1/3}$ Nb $_{2/3}$ ) $_{1-x}$ Ti $_{x}$ ]O $_{3}$ crystal near the morphotropic phase boundary

清水大輔*; 塚田慎也*; 松浦直人*; 坂本潤哉*; 小島誠治*; 並河一道*; 水木純一郎; 大和田謙二

Physical Review B, 92(17), p.174121_1 - 174121_5, 2015/11

https://doi.org/10.1103/PhysRevB.92.174121

被引用回数：16 パーセンタイル：53.90(Materials Science, Multidisciplinary)

Ti組成が傾斜した単結晶試料Pb[(Mg $_{1/3}$ Nb $_{2/3}$ ) $_{1-x}$ Ti $_{x}$ ]O $_{3}$ を用いてモルフォトロピック相境界(MPB)近傍の相図ならびに電気応答と結晶コヒーレンス長の相関を調べた。結晶は29mol%と34.7mol%付近に相境界がありそれらは単斜晶相MB-単斜晶相MC,単斜晶相MC-正方晶相境界と対応する。結晶コヒーレンス長のTi濃度依存性は電気応答と強い負の相関を示しており、これらはMPB近傍におけるドメインサイズ効果として説明可能である。

論文

Neutron scattering study of spin fluctuations in La $_{2-x}$ Sr $_{x}$ CuO $_{4}$ by four-dimensional mapping of neutron cross-sections using the 4SEASONS chopper spectrometer

脇本秀一; 池内和彦*; 新井正敏; 藤田全基*; 梶本亮一; 川村奨*; 松浦直人*; 中島健次; 山田和芳*

JPS Conference Proceedings (Internet), 8, p.034013_1 - 034013_6, 2015/09

https://doi.org/10.7566/JPSCP.8.034013

Previous neutron results of La $_{2-x}$ Sr $_{x}$ CuO $_{4}$ (LSCO) by TOF measurements revealed that magnetic dynamical structure factor has a maximum at 18 meV. To clarify the origin of the maximum, we carried out 4 dimensional mapping of dynamical structure factor to see details of both magnons and phonons. We found that = 0 shows ordinary spin waves while = 0.08 shows a maximum at 18 meV where -independent phonon branch crosses. This imply that in superconducting compositions phonon may also indirectly related to the superconductivity.