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２０２４年７月９日（火）物 理 学 第 一 分 野

物理学第一分野ＤＣ３回生研究発表会
場所：理学研究科５号館 ５階・第四講義室
発表：２０分（別に質問１０分程度）
２０２４年７月９日（火）９：００～

開始

目次

１． Neural Network Quantum States and Quantum Skyrmions
Ashish Joshi（９：００）
・・・・・ １
２． Coherent spin switching driven by THz-magnetic-field-modified potential energy
Zhenya Zhang（９：３０）・・・・・ ２

Neural Network Quantum States and Quantum Skyrmions
Condensed Matter Theory Group

Ashish Joshi

Abstract We study the ground state properties and time dynamics of quantum skyrmions using neural network
quantum states as variational wave functions. We show that the spins in the quantum skyrmion ground states areentangled. We also provide a way to move quantum skyrmions using a magnetic field.
© 2024 Department of Physics, Kyoto University

Magnetic skyrmions are topologically protected spin structures with vortex-like configurations, discovered in avariety of materials, including MnSi, FeCoSi, FeGe, and hold potential as denser memory devices and qubits [1].
The size of these quasiparticles can range from micrometers to a few times the atomic lattice spacing, wherequantum effects become important. Most research in skyrmions has been done only in the classical regime, and it
is not clear if that is applicable to these ‘quantum skyrmions’. Thus, a purely quantum mechanical analysis isneeded to study quantum skyrmions. However, the wave function of interacting quantum systems scales
exponentially with the system size, which makes its exact calculation impossible for more than a few particles andapproximation methods are needed to study systems with large number of particles.
In this research, we study the ground state properties and time dynamics of quantum skyrmions using variational
Monte Carlo with artificial neural networks as the trial state. We consider a two-dimensional quantum Heisenberg
model with Dzyaloshinskii-Moriya interaction (DMI). First, we show that the ground state accommodates aquantum skyrmion for a large range of Hamiltonian parameters [2]. The spins in these quantum skyrmions are
weakly entangled, and the entanglement increases with decreasing DMI. We also find that the central spin iscompletely disentangled from the rest of the lattice, establishing a non-destructive way of detecting this type ofskyrmion by local magnetization measurements. Then, we study the dynamics of quantum skyrmions under anexternal magnetic field gradient using the time-dependent variational principle [3]. We show that field gradientsare an effective way of manipulating and moving quantum skyrmions, which is necessary for realizing theirapplications. Our research shows that artificial neural networks offer a promising way of studying quantum
magnetic systems that are outside the realm of exact diagonalization.

Fig. 1 Representing the quantum skyrmion ground state using an artificial neural network. The input to the network x
are quantum spin configurations. The output of the network is the wave function which it learns by minimizing theenergy. The spins in the quantum skyrmion are entangled as shown by the heatmap plotting the Renyi entropy.

References[1] N. Nagaosa and Y. Tokura, Nat. Nanotechnol. 8, 899 (2013).
[2] A. Joshi, R. Peters, and T. Posske, Physical Review B 108, 094410 (2023).
[3] A. Joshi, R. Peters, and T. Posske, arXiv:2403.08184 (2024).

Coherent spin switching driven byTHz-magnetic-field-modified potential energy
Optical Materials Science GroupZhenya Zhang

Abstract We studied the coherent control of magnetization in the canted antiferromagnet Sm0.7Er0.3FeO3.
We observed spin switching after the first half cycle of the quasi-ferromagnetic mode and found that thisphenomenon is driven by the dynamical modification of the potential energy of magnetization induced by
the strong multicycle THz magnetic fields.
© 2024 Department of Physics, Kyoto University

A good understanding of spin dynamics is essential for the design and development of spintronic devices for
information transport, processing, and recording. In contrast to ferromagnets, antiferromagnets have thehigh-speed spin precession at terahertz (THz) frequencies as a result of the robust antiferromagnetic exchangeinteraction. Thus, the ultrafast control of antiferromagnetic spins has attracted considerable interest. In previousstudies, optical pulses in the range from visible to mid-infrared frequencies have been demonstrated accessible forthe coherent spin excitation. However, these methods often lead to significant heating, because the spin excitationin these studies are mediated by the excitation of either electrons or phonons. THz pulses has emerged as anexcellent tool for coherent spin excitation that does not involve a strong excitation of electrons or phonons due tothe low photon energies. The Zeeman torque applied by the magnetic components of THz pulses can coherently
excite the spin dynamics which are available in various antiferromagnets and temperature ranges [1]. However,
due to the relatively weak magnetic field of THz pulses propagating in free space, many experiments on nonlinear
coherent spin dynamics in antiferromagnets can only investigate a restricted range of phenomena.
In this study, we investigated the coherent spin switching in a canted antiferromagnet Sm0.7Er0.3FeO3 near room
temperature using Tesla-class THz magnetic fields. To generate a Tesla-class THz magnetic field in the sample,
we designed a spiral-shaped microresonator and fabricated it on the sample surface by electron-beam lithography[2-4]. When the microresonator is irradiated with a THz pulse, an oscillating current is induced that generates astrong multicycle THz magnetic near-field at about 0.46 THz perpendicular to the sample surface. The coherent
spin dynamics initiated by the THz magnetic near-field are detected by measuring the change in the ellipticity
angle () of an 800-nm probe pulse due to the magneto-optic effect.
The time-resolved signal  exhibited an oscillation of the quasi-ferromagnetic (q-FM) mode (<0.05 THz). As
the field strength is increased beyond a critical value, a long-lived offset emerges after the first half cycle of theq-FM mode, and the amplitude of this offset exhibits a steep dependence on the field strength. The threshold
behavior indicates that spin switching occurs after sufficiently strong excitation, that is, the magnetic orderparameter overcomes the barrier and reaches the second minimum of a potential energy. We derived and solved
the sine-Gordon equation based on the Landau–Lifshitz–Gilbert equation to reproduce the q-FM-mode dynamics
and the spin switching. To explain why the offset emerges after the first half q-FM-mode cycle, we analyticallysolved the sine-Gordon equation by replacing the expression for the potential modified by the fast-oscillating
magnetic field with a “time-averaged” effective potential that varies with the envelope of the field. We found thatthe effective potential initially drives the magnetic order parameter away from the barrier, and when the potentialmodification vanishes, the magnetic order parameter starts to approach the barrier and overcomes it due to theinertia effect. These results clarify the nonlinearities of the q-FM mode in this antiferromagnet [5].
References[1] T. Kampfrath et al., Nat. Photon. 5, 31 (2011).
[2] Z. Zhang et al., Phys. Rev. Res. 3, L032021 (2021).
[3] Z. Zhang et al., Nat. Commun. 14, 1795 (2023).
[4] K. Maruyama, Z. Zhang et al., Appl. Phys. Express 17, 022004 (2024).
[5] Z. Zhang et al., submitted, under review (2024).