Zeeman coupling and Dzyaloshinskii-Moriya interaction driven by electric current vorticity

藤本 純治*; 小椎八重 航*; 松尾 衛; 前川 禎通

Physical Review B, 103(22), p.L220402_1 - L220402_5, 2021/06

https://doi.org/10.1103/PhysRevB.103.L220402

The Dzyaloshinskii-Moriya interaction, i.e., antisymmetric exchange interaction, combined with a Zeeman magnetic field gives rise to various magnetic states such as chiral, helical, and skyrmionic states. This interaction conventionally originates from spin-orbit coupling and thus needs somewhat heavy elements. In contrast, we here show a Dzyaloshinskii-Moriya interaction which is driven by electric current vorticity. We also find that the vorticity acts on localized spins as a Zeeman field, which may explain a recent experiment on current-driven magnetic skyrmion creation and annihilation without an external magnetic field in a device with a notch structure in FeGe. The theory explains the control of skyrmion creation and annihilation by current direction and opens different possibilities for studies of magnetic textures by using structural settings.

Possible method to observe the breathing mode of a magnetic domain wall in the Josephson junction

森 道康; 小椎八重 航*; 挽野 真一*; 前川 禎通

Journal of Physics; Condensed Matter, 26(25), p.255702_1 - 255702_5, 2014/06

AA2014-0149.pdf:0.28MB

https://doi.org/10.1088/0953-8984/26/25/255702

A magnetic domain wall (DW) behaves as a massive particle with elasticity. Sliding and oscillation of the DW have been observed experimentally, whereas vibration of a width in the DW, "breathing mode", has not been measured so far. We theoretically propose how to observe the breathing mode by the Josephson junction having a ferromagnetic layer between superconducting electrodes. The current-voltage (-) curve is calculated by an equivalent circuit of the resistively shunted junction model. The breathing mode is identified by stepwise structures in the - curve, which appear at the voltages =) with the fundamental constant , integer number , and the frequency of the breathing mode .

Towards precise measurement of oscillatory domain wall by ferromagnetic Josephson junction

挽野 真一*; 森 道康; 小椎八重 航*; 前川 禎通

Applied Physics Letters, 100(15), p.152402_1 - 152402_3, 2012/04

https://doi.org/10.1063/1.3701782

We theoretically propose a principle for precise measurement of oscillatory domain wall (DW) by a ferromagnetic Josephson junction, which is composed of a ferromagnetic wire with DW and two superconducting electrodes. The current-voltage curve exhibits stepwise structures, only when DW oscillates in the ferromagnetic wire. The voltage step appears at = (/2) with the fundamental constant /, integer number , and the DW frequency . Since can be determined in the order of 10 accuracy, the oscillatory DW will be measured more precisely than present status by conventional method.

Thermoelectric response in the incoherent transport region near Mott transition; The Case study of LaSrVO

打田 正輝*; 大石 晃史*; 松尾 まり; 小椎八重 航*; 小野瀬 佳文*; 森 道康; 藤岡 淳*; 宮坂 茂樹*; 前川 禎通; 十倉 好紀*

Physical Review B, 83(16), p.165127_1 - 165127_5, 2011/04

https://doi.org/10.1103/PhysRevB.83.165127

We report a systematic investigation on the high-temperature thermoelectric response in a typical filling-control Mott transition system LaSrVO. In the vicinity of the Mott transition, incoherent charge transport appears with increasing temperature and the thermopower undergoes two essential crossovers, asymptotically approaching the limit values expected from the entropy consideration, as known as Heikes formula. By comparison with the results of the dynamical mean field theory, we show that the thermopower in the Mott critical state mainly measures the entropy per charge carrier that depends on electronic degrees of freedom available at the measurement temperature. Our findings verify that the Heikes formula is indeed applicable to the real correlated electron systems at practical temperatures (T200K).

Magnetization dynamics and Josephson effect

森 道康; 小椎八重 航*; 挽野 真一*; 前川 禎通

no journal, , 

We theoretically propose how to observe the breathing mode by the Josephson junction having a ferromagnetic layer between superconducting electrodes. The current-voltage (I-V) curve is calculated by an equivalent circuit of the resistively shunted junction model. The breathing mode is identified by stepwise structures in the I-V curve, which appear at the voltages V=n(/2e)w with the fundamental constant /2e, integer number n, and the frequency of the breathing mode w. This is the most feasible method to observe the breathing mode of the DW due to the sensitivity of the Josephson junction. In principle, magnetic resonance methods are also possible to measure the breathing mode. However, owing to their low sensitivity, theresonance measurements would be limited to a sample containing huge numbers of DWs or a specific structure of a ferromagnet. Although such ameasurement is realized in the Skyrmion lattice, in which the rotational and the breathing modes are observed, it has not been done on the breathing mode of the DW so far. Our method is accessible to the breathing mode of a single DW due to the sensitivity of the Josephson junction.

Theory of Josephson diode effect

森 道康; 小椎八重 航*; 前川 禎通*

no journal, , 

Josephson diode effect between two singlet superconductors separated by a Rashba system is studied theoretically. The coupling energy of two superconductors is analytically calculated using a tunneling Hamiltonian with a one-dimensional model. By applying a magnetic field perpendicular to a current bias, we obtain a coupling energy proportional to the current bias and the magnetic field. This makes the critical current asymmetric with respect to the current bias or the magnetic field, i.e., Josephson diode effect. In addition, depending on a distance between the superconducting electrodes , the Josephson diode effect changes its sign and magnitude. The magnitude is inversely proportional to a band split caused by the spin-orbit interaction. Since is experimentally controllable, the Josephson diode effect can be optimized by tuning of .

軌道縮退をもつ強相関電子系における熱起電力

松尾 まり; 岡本 敏史*; 小椎八重 航*; 森 道康; 前川 禎通

no journal, , 

コバルト酸化物NaCoOは大きな熱起電力を持つことで知られる物質である。その巨大な熱起電力の起源として、電子相関の強い系では、電子の運ぶスピン・軌道自由度によるエントロピー輸送の熱起電力への寄与が提案されており、2つの高温極限においてドーピングとスピン・軌道自由度に依存した熱起電力の式が示されている。本研究では、強い電子相関に着目し、ハバード模型をもちいて熱電応答係数の温度依存性を広い温度領域にわたって調べてきた。計算には動的平均場理論(DMFT)を用いた。DMFTは電子相関を非摂動的に取り込みながら、コヒーレントとインコヒーレントの両部分を扱うことができる手法である。1サイト不純物問題のソルバーとしては、強結合極限からの展開であるNon-crossing approximation(NCA)を用いている。前回の学会では、1軌道ハバード模型における熱起電力が温度とともに非単調に振る舞うことを示し、2つの高温極限の式によって与えられる値にそれぞれ漸近する様子を示した。今回は、ハバード模型に軌道自由度を取り込み、軌道の効果が与える熱電応答の温度依存性への寄与について議論を行う。

Josephson effect involving magnetic domain wall motion

森 道康; 挽野 真一*; 小椎八重 航*; 前川 禎通

no journal, , 

Nano-scale magnetic materials for spintronics devices are extensively studied due to many advantages such as enhanced operationspeed, low power consumption, and high integration of memory cell. Non-volatile memory using a magnetic domain wall (DW) is one example of such devices, and many studies are devoted to control DW. Among those studies, the oscillatory DW is experimentally observed and is examined toward applications, e.g. high sensitive magnetic sensor, nano-scale telecommunication, rf-assisted writing of magnetic bit, and microwave generator. Once such devices are realized in a microscopic circuit, one needs to measure the DW frequency more precisely. We theoretically propose a principle for precise measurement of oscillatory domain wall (DW) by a ferromagnetic Josephson junction,which is composed of a ferromagnetic wire with DW and two superconducting electrodes. The current-voltage curve exhibits stepwisestructures, only when DW oscillates in the ferromagnetic wire. The voltage step appears at V=n(h/2e)f with the fundamental constant h/e,integer number n, and the DW frequency f. Since V can be determined in the order of 10 accuracy, the oscillatory DW will be measured more precisely than present status by conventional method.

動的平均場理論を用いた多軌道ハバード模型の熱起電力の研究

松尾 まり; 岡本 敏史*; 小椎八重 航*; 森 道康; 前川 禎通

no journal, , 

遷移金属酸化物に代表される強相関電子系は、高効率熱電材料の探索に新たな可能性を与えるものとして、注目を集めている。強相関電子系が示す巨大な熱起電力の起源として、電荷・スピン・軌道自由度が与えるエントロピー輸送への寄与が提案されている。この理論は、LaSrVO に観測される熱起電力の非単調な温度依存性に見られる高温側の漸近的な振る舞いをよく説明する。本研究では、これまでの計算を拡張し、多軌道ハバード模型における熱電応答係数の温度依存性を広い温度領域にわたって調べた。解析手法として動的平均場理論を用いた。この手法は電子相関を非摂動的に取り込みながら、コヒーレントそしてインコヒーレントな電子状態の両部分を同時に取り扱うことができる手法である。また1サイト不純物問題のソルバーとして、強結合極限からの展開であり複雑な相互作用に対応することが可能な非交差近似を用いた。講演では、熱電応答の温度依存性にみる、軌道間の交換相互作用並びに結晶場分裂の役割について議論する。

Temperature dependence of thermopower in strongly correlated multiorbital systems

森 道康; 関野 まり*; 岡本 敏史*; 小椎八重 航*; 前川 禎通

no journal, , 

The thermopower in strongly correlated electron systems attracts much attention. The thermopower is the entropy flow by the electric current, so that the spin and orbital degrees of freedom are of importance to enhance the thermopower. For the strongly correlated systems, the thermopower was shown to exhibit the non-monotonic temperature dependence. Furthermore, in the multiorbital correlated system, the electronic state reflects the crystal field splitting and the Hund coupling. It is worth examining the effect of the crystal field splitting and the Hund coupling on the temperature dependence of the thermopower in multiorbital correlated systems. We study temperature dependence of the thermopower in the two- and three-orbital Hubbard models by using the dynamical mean-field theory with the non-crossing approximation as impurity solver. It is clarified how the Hund coupling, the crystal field splitting and the Coulomb interaction produce the non-monotonic temperature behavior of the thermopower. It is also found that the sign of thermopower is changed by temperature and electron density. The entropy consideration at high temperatures, i.e., Heikes formula, consistently explains the effect of the crystal field splitting and the Hund coupling on the thermopower. Several materials are discussed in the light of our theory.