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Journal Articles

Optomagnonic Josephson effect in antiferromagnets

Nakata, Koki

Physical Review B, 104(10), p.104402_1 - 104402_12, 2021/09

 Times Cited Count:0 Percentile:0(Materials Science, Multidisciplinary)

Combining advanced technologies of optics and antiferromagnetic spintronics, we present a method to realize ultrafast Josephson spin currents through the optomagnonic Barnett effect by using macroscopic coherence of magnon condensates.

Journal Articles

Topological Hall effects of magnons in ferrimagnets

Nakata, Koki; Kim, S. K.*

Journal of the Physical Society of Japan, 90(8), p.081004_1 - 081004_10, 2021/08

 Times Cited Count:0 Percentile:0(Physics, Multidisciplinary)

This article reviews the past works on topological Hall effects of magnons in insulating magnets with a particular focus on ferrimagnets.

Journal Articles

Magnonic Wiedemann-Franz law

Nakata, Koki

Kotai Butsuri, 56(8), p.429 - 439, 2021/08

We review the thermomagnetic relation of bosons, the Wiedemann-Franz law for magnon transport, and discuss the universality.

Journal Articles

Magnonic thermal transport using the quantum Boltzmann equation

Nakata, Koki; Onuma, Yuichi*

Physical Review B, 104(6), p.064408_1 - 064408_8, 2021/08

 Times Cited Count:0 Percentile:0(Materials Science, Multidisciplinary)

Using the quantum Boltzmann equation for Bose systems, we have developed the formula for thermal transport of bosons and found a difference from the conventional, with a particular focus on magnons.

Journal Articles

Optomagnonic Barnett effect

Nakata, Koki; Takayoshi, Shintaro*

Physical Review B, 102(9), p.094417_1 - 094417_11, 2020/09

 Times Cited Count:4 Percentile:68.88(Materials Science, Multidisciplinary)

Combining the technologies of quantum optics and magnonics [A. Rebei and J. Hohlfeld, Phys. Lett. A 372, 1915 (2008); J. Appl. Phys. 103, 07B118 (2008)] [S. Takayoshi et al., Phys. Rev. B 90, 214413 (2014); S. Takayoshi et al., Phys. Rev. B 90, 085150 (2014)], we find that the circularly polarized laser can dynamically realize the quasiequilibrium magnon Bose-Einstein condensates in insulating ferrimagnets. This provides an access to coherent magnons in the high frequency regime of the order of terahertz.

Journal Articles

Universal $$frac{1}{3}$$-suppression of magnonic shot noise in diffusive insulating magnets

Nakata, Koki; Onuma, Yuichi*; Matsuo, Mamoru*

Physical Review B, 100(1), p.014406_1 - 014406_8, 2019/07

 Times Cited Count:1 Percentile:10.83(Materials Science, Multidisciplinary)

Extending a Boltzmann-Langevin theory to magnons, we show a universality of current-noise suppression in diffusive systems against the difference of quantum-statistical properties of bosons and fermions.

Journal Articles

Laser control of magnonic topological phases in antiferromagnets

Nakata, Koki; Kim, S. K.*; Takayoshi, Shintaro*

Physical Review B, 100(1), p.014421_1 - 014421_9, 2019/07

 Times Cited Count:5 Percentile:51.19(Materials Science, Multidisciplinary)

We study the laser control of magnon topological phases induced by the Aharonov-Casher effect in insulating antiferromagnets (AFs). Using Floquet formalism, we study nonequilibrium magnon dynamics away from the adiabatic limit and its effect on topological phenomena. Our results provide a handle to control and design magnon topological properties in the insulating AF, and thus open a new door to "photonic magnonics".

Journal Articles

Asymmetric quantum shot noise in magnon transport

Nakata, Koki; Onuma, Yuichi*; Matsuo, Mamoru*

Physical Review B, 99(13), p.134403_1 - 134403_7, 2019/04

 Times Cited Count:2 Percentile:22.92(Materials Science, Multidisciplinary)

We study a frequency-dependent noise-to-current ratio for asymmetric, symmetric, and noncommutative current noise in a ferromagnetic insulating junction, and extract quantum-mechanical properties of magnon transport at low temperatures. We demonstrate that the noncommutative noise, which vanishes in the dc limit (i.e., a classical regime), increases monotonically as a function of frequency, and show that the noncommutative noise associated directly with quantum fluctuations of magnon currents breaks through the classical upper limit determined by the symmetric noise and realizes asymmetric quantum shot noise. Finally, we show that our theoretical predictions are within experimental reach with current device and measurement schemes while exploiting low temperatures. Our work provides a platform toward experimental access to quantum fluctuations of magnon currents.

Journal Articles

Tunable magnonic thermal Hall effect in skyrmion crystal phases of ferrimagnets

Kim, S. K.*; Nakata, Koki; Loss, D.*; Tserkovnyak, Y.*

Physical Review Letters, 122(5), p.057204_1 - 057204_6, 2019/02

 Times Cited Count:24 Percentile:93.68(Physics, Multidisciplinary)

We theoretically study the thermal Hall effect by magnons in skyrmion crystal phases of ferrimagnets in the vicinity of the angular momentum compensation point (CP). For an experimental proposal, we predict that themagnonic thermal Hall conductivity changes its sign when the ferrimagnet is tuned across the CP, providing a way to control heat flux in spin-caloritronic devices on the one hand and a feasible way todetect the CP of ferrimagnets on the other hand.

Journal Articles

Magnonic noise and Wiedemann-Franz law

Nakata, Koki; Onuma, Yuichi*; Matsuo, Mamoru*

Physical Review B, 98(9), p.094430_1 - 094430_8, 2018/09

 Times Cited Count:9 Percentile:59.67(Materials Science, Multidisciplinary)

We theoretically establish mutual relations among magnetic momentum, heat, and fluctuations of propagating magnons in a ferromagnetic insulating junction in terms of noise and the bosonic Wiedemann-Franz (WF) law. Using the Schwinger-Keldysh formalism, we calculate all transport coefficients of a noise spectrum for both magnonic spin and heat currents, and establish Onsager relations between the thermomagnetic currents and the zero-frequency noise. Making use of the magnonic WF law and the Seebeck coefficient in the low-temperature limit, we theoretically discover universal relations, i.e. being independent of material parameters, for both the nonequilibrium and equilibrium noise, and show that each noise is described solely in terms of thermal conductance.

Journal Articles

Research at University of Basel in Switzerland

Nakata, Koki

Kiso Kagaku Noto, 24(1), p.37 - 39, 2018/03

We explain the research achievement at the university of Basel and discuss the research at JAEA.

Journal Articles

Cross-checking groundwater age by $$^{4}$$He and $$^{14}$$C dating in a granite, Tono area, central Japan

Hasegawa, Takuma*; Nakata, Kotaro*; Tomioka, Yuichi*; Goto, Kazuyuki*; Kashiwaya, Koki*; Hama, Katsuhiro; Iwatsuki, Teruki; Kunimaru, Takanori*; Takeda, Masaki

Geochimica et Cosmochimica Acta, 192, p.166 - 185, 2016/11

 Times Cited Count:5 Percentile:29.75(Geochemistry & Geophysics)

Groundwater dating was performed simultaneously by the $$^{4}$$He and $$^{14}$$C methods in granite of the Tono area in central Japan. Groundwater was sampled at 30 packed-off sections of six 1000-m boreholes. $$^{4}$$He concentrations increased and $$^{14}$$C concentrations decreased along a groundwater flow path on a topographic gradient. $$^{4}$$He ages were calculated by using the in situ $$^{4}$$He production rate derived from the porosity, density, and U and Th content of the rock, neglecting external flux. The linear relation between the $$^{4}$$He ages and the noncorrected $$^{14}$$C ages, except in the discharge area. Simultaneous measurements make it feasible to estimate the accumulation rate of $$^{4}$$He and initial dilution of $$^{14}$$C, which cannot be done with a single method. Cross-checking groundwater dating has the potential to provide more reliable groundwater ages.

Oral presentation

Central Research Institute of Electric Power Industry - Japan Atomic Energy Agency Joint Research; A Study of the hydraulic properties and transport characteristics around the Mizunami Underground Laboratory Project

Hasegawa, Takuma*; Nakata, Kotaro*; Tomioka, Yuichi*; Goto, Kazuyuki*; Kashiwaya, Koki*; Hama, Katsuhiro

no journal, , 

The groundwater flow velocity is one of the important items in the safety assessment for the HLW disposal. Generally, it is too difficult to measure the groundwater flow velocity directly. Therefore, to determine the groundwater age by radio active isotopes are effective. However, there are some problems to apply the metrology to the natural samples. For example, because the ratio of Carbon isotope changes by dissolving of the carbonate mineral and resolving the organic materials, the development of the technique for correcting this is needed. In this study, the groundwater ages are measured by $$^{4}$$He and $$^{14}$$C, etc. and compared these results as a joint research with Central Research Institute of Electric Power Industry.

Oral presentation

Magnonic spin current-noise in insulating bulk magnets

Nakata, Koki; Onuma, Yuichi*; Matsuo, Mamoru*

no journal, , 

Extending a Boltzmann-Langevin theory to magnons, we show a universality of current-noise suppression in diffusive systems against the difference of quantum-statistical properties of bosons and fermions. We theoretically discover that compared with a Poissonian shot noise of magnons in an insulating ferromagnetic junction, the magnonic shot noise is suppressed in the diffusive insulating bulk magnet and noise-to-current ratio (Fano factor) at low temperatures exhibits a universal behavior, i.e., the same suppression 1/3 as the one for electron transport in diffusive conductors, despite the difference of quantum-statistical properties.

Oral presentation

Quantum spin-current noise of magnons

Nakata, Koki; Onuma, Yuichi*; Matsuo, Mamoru*

no journal, , 

Using the Keldysh formalism, we calculate a magnonic current-noise and compare it with the current-noise of electrically charged electrons. We thus discuss the universality of current-noise against the difference of quantum-statistical properties of bosons and fermions.

Oral presentation

Laser control of magnonic topological phases in antiferromagnets

Nakata, Koki; Kim, S. K.*; Takayoshi, Shintaro*

no journal, , 

We study the laser control of magnon topological phases induced by the Aharonov-Casher effect in insulating antiferromagnets (AFs). Our results provide a handle to control and design magnon topological properties in the insulating AF.

Oral presentation

Optomagnonic Barnett effect

Nakata, Koki

no journal, , 

Combining the technologies of quantum optics and magnonics, we find that the circularly polarized laser can dynamically realize the quasiequilibrium magnon Bose-Einstein condensates (BEC). The Zeeman coupling between the laser and spins generates the optical Barnett field. We show that the optical Barnett field develops the total magnetization in insulating ferrimagnets with reversing the local magnetization, which leads to the quasiequilibrium magnon BEC. This laser-induced magnon BEC transition through optical Barnett effect, dubbed the optomagnonic Barnett effect, provides an access to coherent magnons in the high frequency regime of the order of terahertz. The optomagnonic Barnett effect is a key ingredient for the application to spintronics (e.g., spin Josephson effect).

Oral presentation

Optomagnonic Barnett effect

Nakata, Koki

no journal, , 

Combining the technologies of quantum optics and magnonics, we find that the circularly polarized laser can dynamically realize the quasiequilibrium magnon Bose-Einstein condensates (BEC). The Zeeman coupling between the laser and spins generates the optical Barnett field. We show that the optical Barnett field develops the total magnetization in insulating ferrimagnets with reversing the local magnetization, which leads to the quasiequilibrium magnon BEC. This laser-induced magnon BEC transition through optical Barnett effect, dubbed the optomagnonic Barnett effect, provides an access to coherent magnons in the high frequency regime of the order of terahertz. The optomagnonic Barnett effect is a key ingredient for the application to spintronics (e.g., spin Josephson effect).

Oral presentation

Laser control of magnonic topological phases in antiferromagnets

Nakata, Koki; Kim, S. K.*; Takayoshi, Shintaro*

no journal, , 

We study the laser control of magnon topological phases induced by the Aharonov-Casher effect in insulating antiferromagnets (AFs). Since the laser electric field can be considered as a time-periodic perturbation, we apply the Floquet theory and perform the inverse frequency expansion by focusing on the high frequency region. Using the obtained effective Floquet Hamiltonian, we study nonequilibrium magnon dynamics away from the adiabatic limit and its effect on topological phenomena. This work enables us to control and design magnonic topological properties away from equilibrium.

Oral presentation

Numerical modeling of pore water migration and water quality evolution due to compaction and diagenesis processes of sedimentary rock

Komura, Yuto*; Kashiwaya, Koki*; Miyakawa, Kazuya; Nakata, Kotaro*; Koike, Katsuaki*

no journal, , 

no abstracts in English

Oral presentation

A Numerical modeling of groundwater evolution due to compaction and diagenesis of sedimentary rock at Horonobe area

Komura, Yuto*; Kashiwaya, Koki*; Miyakawa, Kazuya; Nakata, Kotaro*; Koike, Katsuaki*

no journal, , 

no abstracts in English

21 (Records 1-20 displayed on this page)