Shapiro steps in charge-density-wave states driven by ultrasound

森 道康; 前川 禎通

Applied Physics Letters, 122(4), p.042202_1 - 042202_5, 2023/01

https://doi.org/10.1063/5.0132454

We show that ultrasound can induce the Shapiro steps (SS) in the charge-density-wave (CDW) state. When ultrasound with frequency and a dc voltage are applied, the SS occur at the current with integer . Even and odd multiples of SS are represented by two couplings between the CDW and ultrasound. Although an ac voltage bias with frequency induces the SS at , the ultrasound bias enhances the odd multiples more strongly than the even ones. This is the difference between the ultrasound and the ac voltage. Since the SS cause abrupt peaks in the , the extreme changes in the curve will be applied to a very sensitive ultrasound detector.

Half-integer Shapiro-steps in superconducting qubit with a -Josephson junction

森 道康; 前川 禎通

Applied Physics Express, 14(10), p.103001_1 - 103001_4, 2021/10

https://doi.org/10.35848/1882-0786/ac211d

A superconducting quantum interference device (SQUID) comprising 0- and -Josephson junctions (JJs), called -SQUID, is studied by the resistively shunted junction model. The -SQUID shows half-integer Shapiro-steps (SS) under microwave irradiation at the voltage = , with angular frequency and half-integer /2 in addition to integer . We show that the -SQUID can be a -qubit with spontaneous loop currents by which the half-integer SS are induced. Making the 0- and -JJs equivalent is a key for the half-integer SS and realizing the -qubit.

Spin excitations of the =1/2 one-dimensional Ising-like antiferromagnet BaCoVO in transverse magnetic fields

奥谷 顕*; 大西 弘明; 木村 尚次郎*; 竹内 徹也*; 木田 孝則*; 森 道康; 三宅 厚志*; 徳永 将史*; 金道 浩一*; 萩原 政幸*

Journal of the Physical Society of Japan, 90(4), p.044704_1 - 044704_9, 2021/04

https://doi.org/10.7566/JPSJ.90.044704

The high-field multifrequency electron spin resonance (ESR), the magnetization, and the specific heat of single crystals of BaCoVO (a quasi-one-dimensional Ising-like antiferromagnet with ) were measured in two different transverse magnetic fields: and . A softening of the lowest ESR mode was observed near the saturation field of the magnetization around 40 T in , whereas it occurs around 10 T in , far below the saturation field. These observations were reproduced by numerical calculations using the density-matrix renormalization group methods, including the staggered magnetic fields induced by a uniform external magnetic field and the interchain magnetic exchange interaction. We found that the directional dependence of the lowest mode originates mainly from the remarkable angular dependence of the induced magnetic field. The interchain interaction is also quantitatively important for reproducing the experimental results. The directional dependence of the lowest mode is demonstrated by the particle-hole excitation of spinons using the Jordan-Wigner transformation.

Dynamical density matrix renormalization group study of spin and charge excitations in the four-leg --

遠山 貴己; 森 道康; 曽田 繁利*

Physical Review B, 97(23), p.235137_1 - 235137_7, 2018/06

AA2018-0171.pdf:1.43MB

https://doi.org/10.1103/PhysRevB.97.235137

We investigate the dynamical spin and charge structure factors of the -- four-leg ladder by using dynamical density-matrix renormalization group. The dynamical charge structure factor along the momentum direction from to clearly shows low-energy excitations corresponding to the stripe order in hole doping. On the other hand, the stripe order weakens in electron doping, resulting in less low-energy excitations in the charge channel. In spin channel, we find incommensurate spin excitations near forming a hour-glass behavior in hole doping, while in electron doping we find clearly spin-wave-like dispersions starting from . Along the - direction, the spin excitations are strongly influenced by the stripes in hole doping, resulting in two branches that form a discontinuous behavior in the dispersion. In contrast, the electron-doped systems show a downward shift in energy toward .

Broad linewidth of antiferromagnetic spin wave due to electron correlation

森 道康

Journal of the Physical Society of Japan, 86(12), p.124705_1 - 124705_7, 2017/12

https://doi.org/10.7566/JPSJ.86.124705

We study magnetic excitations in a bilayer of antiferromagnetic (AF) insulator and correlated metal, in which the double occupancy is forbidden. Effective action of AF spinwave in the AF insulator is derived by using the path integral formula within the second order of an inter-plane coupling. The electrons correlation in the metallic plane is treated by the Gutzwiller approximation, which renormalizes the hopping integrals as proportional to hole density. Remarkable is that the AF magnons are broaden at low energies, only when the metallic planes includes correlation effect. The present results will be useful also to another bilayer of metal and ferrimagnet.

Magnetization dynamics and its scattering mechanism in thin CoFeB films with interfacial anisotropy

岡田 篤*; He, S.*; Gu, B.; 金井 駿*; Soumyanarayanan, A.*; Lim, S. T.*; Tran, M.*; 森 道康; 前川 禎通; 松倉 文礼*; et al.

Proceedings of the National Academy of Sciences of the United States of America, 144(15), p.3815 - 3820, 2017/04

http://doi.org/10.1073/pnas.1613864114

Studies of magnetization dynamics have incessantly facilitated the discovery of fundamentally novel physical phenomena, making steady headway in the development of magnetic and spintronics devices. The dynamics can be induced and detected electrically, offering new functionalities in advanced electronics at the nanoscale. However, its scattering mechanism is still disputed. Understanding the mechanism in thin films is especially important, because most spintronics devices are made from stacks of multilayers with nanometer thickness. The stacks are known to possess interfacial magnetic anisotropy, a central property for applications, whose influence on the dynamics remains unknown. Here, we investigate the impact of interfacial anisotropy by adopting CoFeB/MgO as a model system. Through systematic and complementary measurements of ferromagnetic resonance (FMR) on a series of thin films, we identify narrower FMR linewidths at higher temperatures. We explicitly rule out the temperature dependence of intrinsic damping as a possible cause, and it is also not expected from existing extrinsic scattering mechanisms for ferromagnets. We ascribe this observation to motional narrowing, an old concept so far neglected in the analyses of FMR spectra. The effect is confirmed to originate from interfacial anisotropy, impacting the practical technology of spin-based nanodevices up to room temperature.

Quasi-spin correlations in a frustrated quantum spin ladder

杉本 貴則*; 森 道康; 遠山 貴巳*; 前川 禎通

Physics Procedia, 75, p.861 - 867, 2015/12

https://doi.org/10.1016/j.phpro.2015.12.111

The quasi-spin correlations in a frustrated quantum spin ladder with one-half magnetization are theoretically studied by using the density-matrix renormalization-group method and the quasi- spin transformation. In this model, the frustration induces a gapless-to-gapful phase transition with a strong rung coupling. The gapful state is observed as the one-half magnetization plateau in the magnetization curve. In the magnetization-plateau state, we find that the quasi-spin dimer has a large expectation value with long-ranged correlations. This result does not only comes in useful to clarify the magnetization-plateau state, but gives a crucial information to understand the magnetization curve of the real compound BiCuPO, whose effective spin model corresponds to ours.

Magnetization plateaus by reconstructed quasispinons in a frustrated two-leg spin ladder under a magnetic field

杉本 貴則*; 森 道康; 遠山 貴巳*; 前川 禎通

Physical Review B, 92(12), p.125114_1 - 125114_6, 2015/09

AA2015-0451.pdf:0.43MB

https://doi.org/10.1103/PhysRevB.92.125114

The quantum phase transitions induced by a magnetic field are theoretically studied in a frustrated two-leg spin ladder. Using the density-matrix renormalization-group method, we find some magnetic phase transitions and plateaux in two different cases of strong and weak rung couplings. With the strong rung coupling, the three magnetization plateaux are found at 1/3, 1/2, and 2/3 due to the frustration. Those can be understood in terms of a quasi-spinon reconstructed from the singlet and the triplets of spins on a rung. The plateau at 1/2 corresponds to the valence bond solid of the quasispinons, while the plateaux at 1/3 and 2/3 can be associated with the array of quasi-spinons such as soliton lattice. This is different from the usual Bose-Einstein-condensation picture of triplons. Our results will be useful to understand magnetization curves in BiCuPO.

Enhanced spin Hall effect by electron correlations in CuBi alloys

Gu, B.; Xu, Z.; 森 道康; Ziman, T.*; 前川 禎通

Journal of Applied Physics, 117(17), p.17D503_1 - 17D503_4, 2015/05

https://doi.org/10.1063/1.4906443

We find that the SHA can be dramatically enhanced by Bi impurities close to the Cu surface. The mechanisms of this enhancement are two-fold. One is that the localized impurity state on surface has a decreased hybridization and combined with Coulomb correlation effect. The other comes from the low-dimensional state of conduction electrons on surface, which results in a further enhancement of skew scattering by impurities. Furthermore, we note that a discrepancy in sign of SHA between the experiment and previous theories is simply caused by different definitions of SHA. This re-establishes skew scattering as the essential mechanism underlying the spin Hall effect in CuBi alloys.

Sign change of the spin Hall effect due to electron correlation in nonmagnetic CuIr alloys

Xu, Z.; Gu, B.; 森 道康; Ziman, T.*; 前川 禎通

Physical Review Letters, 114(1), p.017202_1 - 017202_5, 2015/01

https://doi.org/10.1103/PhysRevLett.114.017202

Recently a positive spin Hall angle (SHA) of 0.021, was observed experimentally in nonmagnetic CuIr alloys and attributed predominantly to an extrinsic skew scattering mechanism, while a negative SHA was obtained from ab initio calculations, using consistent definitions of the SHA. We reconsider the SHA in CuIr alloys, with the effects of the local electron correlation U in 5d orbitals of Ir impurities, included by the quantum Monte Carlo method. We found that the SHA is negative if we ignore such local electron correlation, but becomes positive once U approaches a realistic value. This may open up a way to control the sign of the SHA by manipulating the occupation number of impurities.