Program

DAY 1 (July 1, 2021)

Session Chair: Jung Hoon Han

Talk 1 [9:30 AM - 10:15 AM]

Speaker: Yong-il Shin (Seoul National University)

Title: Wall-Vortex Composite Defects in a Spinor Bose-Einstein Condensate

Abstract: Spinor Bose-Einstein condensates (BECs) of atomic gases are a continuous ordered system with multiple symmetry breaking, providing an experimental platform for studying the physics of topological defects. In this talk, I will report our recent observation of spin domain walls bounded by half-quantum vortices (HQVs) in a spin-1 BEC with antiferromagnetic interactions. A spinor condensate was initially prepared in the easy-plane polar phase, and then, suddenly quenched into the easy-axis polar phase. Domain walls were created via the spontaneous Z2 symmetry breaking in the phase transition and the walls dynamically split into composite defects due to snake instability. We identified the end points of the defects as HQVs for the polar order parameter and demonstrated the mass supercurrent in their proximity using Bragg scattering. In a strong quench regime, we observed that singly charged quantum vortices are formed with the relaxation of free wall-vortex composite defects. Our results demonstrate a nucleation mechanism for composite defects via phase transition dynamics.

Talk 2 [10:20 AM - 11:05 AM]

Speaker: Minchul Lee (Kyung Hee University)

Title: Majorana signature in dissipation via chiral topological superconductors

Abstract: This talk presents the studies on dissipation in a topological resistor–capacitor (RC) circuit consisting of a quantum dot coupled to a Majorana edge mode formed around a chiral topological superconductor. The dissipation is measured in terms of the relaxation resistance R_q whose frequency dependence is investigated in both the linear and non-linear response regimes. The nature of Majorana fermions, which are their own anti-particles, is found to lead to substantially enhanced relaxation resistance. Furthermore, a destructive interference completely suppresses the low-frequency relaxation resistance so that R_q \propto \Omega^2 where \Omega is the frequency of the ac driving on the dot. Interestingly, when the time-dependent dot level crosses the Fermi level, R_q \propto \Omega^{-1/3} diverges as \Omega \to 0, exhibiting a universal scaling law for the dissipative part of the ac power. The latter is attributed to the δ-peak in the dot density of states due to a uncoupled dot Majorana mode at the dot resonance condition.

Talk 3 [11:10 AM - 11:55 AM]

Speaker: Kab-Jin Kim (KAIST)

Title: Dynamics of magnetic solitons and their applications

Abstract: Magnetic solitons, such as domain wall, vortices, Bloch lines and skyrmions have started to play an important role in modern magnetism because of their extraordinary stability, which can be exploited in the production of memory and logic devices. Recently, a type of antisymmetric exchange interaction, namely the Dzyaloshinskii–Moriya interaction (DMI), has been uncovered and found to influence the formation of topological defects. Exploring how the DMI affects the dynamics of topological defects is therefore an important task. In this talk, I will briefly review our recent processes on the study of dynamic magnetic solitons. I will also present a new concept of memory and logic devices by utilizing the topological solitons.

Session Chair: Se Kwon Kim

Talk 4 [2:00 PM - 2:45 PM]

Speaker: Gil-Ho Lee (POSTECH)

Title: Evidence of Higher Order Topology in Multilayer WTe₂ from Josephson Coupling through Anisotropic Hinge States

Abstract: Non-centrosymmetric Td-WTe₂, a type-II Weyl semimetal, is also expected to have higher order topological phases with topologically protected, helical one-dimensional (1D) hinge states when its Weyl points are annihilated. However, the detection of these hinge states is difficult due to the semimetallic behaviour of its bulk. Here, we spatially resolve hinge states by analysing the magnetic field interference of supercurrent in Nb-WTe₂-Nb proximity Josephson junctions. The Josephson current along the a-axis of the WTe₂ crystal, but not along the b-axis, shows sharp enhancements at the edges of the junction; the amount of enhanced Josephson current was comparable to the upper limits of a single 1D helical channel. Our experimental observations suggest a higher order topological phase in WTe₂ and its corresponding anisotropic topological hinge states, in agreement with theoretical calculations.

Talk 5 [2:45 PM - 3:30 PM]

Speaker: Sangmo Cheon (Hanyang University)

Title: Topological Solitons in Su-Schrieffer-Heeger model and Beyond

Abstract: Topology and symmetry are fundamental principles in searching for new topological science [1]. Here, we will discuss the topological solitons and related phenomena based on the topology and symmetry in 1D electronic systems.

A topological soliton is a highly stable object that exists in various systems, including but not limited to optical fibers, magnetic materials (skyrmions), and topological insulators (chiral edge state). The striking features of such topological solitons and topological edge states are its topological charge and particle-antiparticle symmetry, which are robust even under perturbation. Topological solitons can maintain their shapes and be created and annihilated pairwise. Therefore, topological solitons are expected to open a pathway to future topological information science and technology. As one of the famous quantum topological solitons in the electronic system, Su-Schrieffer-Heeger (SSH) solitons showed many impressive quantum phenomena such as Thouless topological charge pumping, fractional fermion charge (e/2), and spin-charge separation [2]. Such a soliton pair is proved to satisfy the particle-antiparticle symmetry via chiral symmetry by Jackiw and Rebbi in the old days [3]. However, there have been few limited studies about the fractionalization and particle-antiparticle relations beyond the SSH model.

In this talk, we will discuss the topological solitons' new features by generalizing the Su-Schrieffer-Heeger model. 1^st, we reported the emergence of chiral solitons in a 1D charge density wave (CDW) system of indium atomic wires self-assembled on a silicone wire [4, 5]. Similar to the chiral edge state of 2D quantum Hall effect, the chirality of chiral soliton comes from the Chern number of dimensionally extended systems. 2^nd, we discovered the topological chiral solitons could possess particle-antiparticle symmetry even in the presence of chiral symmetry breaking [7, 8]. Hence, this finding supports pair creation and annihilation of topological chiral solitons with opposite quantum numbers, one of the fundamental elements for the topological operation in topological devices. 3^rd, we generalized the fractionalization concept and found the second fractionalization of chiral soliton [7,8]. 4^th, we found the topological quaternary operation using chirally extended Z₄ solitons [6]. Finally, by extending the SSH model into 2D electronic system, we suggested the chiral order in a CDW system [9]. We believe that our findings will inspire researchers seeking feasible and promising topological systems.

[1] H. M. Zahid, C. L. Kane, "Colloquium: topological insulators." Reviews of Modern Physics 82, 4 (2010)

[2] W. P. Su, J. R. Schrieffer, A. J. Heeger, “Solitons in Polyacetylene.” Phys. Rev. Lett. 42, 1698 (1979).

[3] R. Jackiw, C. Rebbi, "Solitons with fermion number ½." Phys. Rev. D 13, 3398–3409 (1976).

[4] T.H. Kim, H.W. Yeom. "Topological Solitons versus Nonsolitonic Phase Defects in a Quasi-One-Dimensional Charge-Density Wave." Phys. Rev. Lett. 109, 246802 (2012).

[5] S. Cheon, T.H. Kim, S.H. Lee, HW Yeom, "Chiral solitons in a coupled double Peierls chain," Science 350, 6257 (2015).

[6] T.H. Kim, S. Cheon, H.W. Yeom, "Switching Chiral Solitons for toward Z₄ Algebraic Operation", Nature Physics, 13.5, 444 (2017).

[7] S.-H. Han, S.-G. Jeong, T.-H. Kim*, S. Cheon*, "Topological features of groundstates and topological solitons in generalized Su-Schrieffer-Heeger models using generalized time-reversal, particle-hole, and chiral symmetries," Phys. Rev. B 102, 235411 (2020)

[8] C-G. Oh, S.-H. Han, S.-G. Jeong, T.-H. Kim*, S. Cheon*, "Particle-Antiparticle Duality and Fractionalization of Topological Chiral Solitons," Scientific Reports, 11, 1013 (2021)

[9] S.-W. Kim, H.-J. Kim, S. Cheon*, T.-H. Kim*, "Two-dimensional chiral stacking orders in quasi-one-dimensional charge density waves," Phys. Rev. B 102, 121408(R) (2020)

Session Chair: Suk Bum Chung

Talk 6 [3:45 PM - 4:30 PM]

Speaker: Chanyong Hwang (KRISS)

Title: Magnetic skyrmion: topological soliton for next generation spin device

Abstract: Magnetic skyrmion is a topological soliton, where its spin textures are determined by many different parameters. One of these parameters is the Dzyaloshinskii-Moriya interaction(DMi). This DMi originates from the spin-orbit interaction, where its functional form is defined by the second order perturbation. So that its effect is small in magnitude and not quite clearly separable from other magnetoelectric parameters. The measurement and control of DMi becomes a challenging issue in spintronics. The actual size of this topological soliton and its border, domain wall, is determined by the parameters such as DMi and anisotropy constant of the magnetic materials. Unfortunately, we have very limited situation to control these parameters so that the window to allow the magnetic skyrmion is very narrow. So far, most people rely on the perturbative method to twist the spin structure starting from the winding number 0. We have shown the microscopic origin of the formation of magnetic skyrmions starting from the stripe phase and presented the way to generate it with any kind of magnetic material with perpendicular magnetic anisotropy[1]. Then with this notion in mind, we will show very intuitive way to generate/delete the magnetic skyrmion for device application. At the end, we will discuss the expected quantum effect of the collection of magnetic skyrmions.

[1] NPG Asia Materials 13, 20 (2021)

Talk 7 [4:30 PM - 5:15 PM]

Speaker: Kyoung-Whan Kim (KIST)

Title: Quantum transport of spin and orbital angular momenta of electrons

Abstract: Dynamics of solitons in condensed matters are typically induced by spin and orbital currents generated by an external perturbation. It is therefore important to understand fundamental nature of spin and orbital transport of electrons. Previous works are based on semiclassical transport theories, which focus on transport of eigenstates. Recent studies, however, imply importance of transport of quantum coherent states, such as transverse spin currents in ferromagnets and orbital currents in normal metals. In this talk, we present how to develop an effective diffusion formalism for transport of quantum states and discuss novel features of their transport. We start from the quantum Boltzmann equation which allows for quantum coherence and adopt reasonable assumptions to obtain an effective drift-diffusion equation which can be solved analytically. After solving the drift-diffusion equation, we found novel phenomena like self-generated spin torque, oscillatory decaying nature, generation of orbital-rotation current, and so on.

DAY 2 (July 2, 2021)

Session Chair: Se Kwon Kim

Talk 1 [9:30 AM - 10:15 AM]

Speaker: Yong-Joo Doh (GIST)

Title: 4π-Periodic Fractional Josephson Effect in Topological Insulators

Abstract: Topological insulators (TIs) are bulk insulators including metallic (and topological) surface states, which are topologically protected by time-reversal symmetry. TI nanoribbon combined with conventional s-wave superconductor is expected to provide the topological superconducting state hosting the Majorana fermion, which is essential for the topological quantum computation. Here, we report our recent experimental results of topological supercurrent through surface states in (Bi_0.81Sb_0.19)₂Se₃ TI nanoribbon in contact with PbIn superconducting electrodes.

Below the superconducting transition temperature, the Josephson junction of TI nanoribbon exhibits supercurrent up to I_c = 1.5 μA at T = 7 mK. When we apply the axial magnetic field along the nanoribbon axis, the critical current oscillates with the periodicity corresponding to the magnetic flux quantum, 𝛟₀ = h/2e, where h is Planck’s constant and e is the electric charge. This periodicity is consistent with the Altshuler-Aronov-Spivak oscillations and half of the Aharonov-Bohm oscillations in the TI nanoribbon. Under the application of microwave with the frequency of f_MW = 1.6 GHz, Shapiro steps are observed except the first one. The first-step missing is attributed to a 4p-periodic current-phase relation in TI nanoribbon-based Josephson junction due to Majorana bound states. Our observations are consistent with the numerical calculations based on modified resistively and capacitively shunted junction model including 4p-periodic supercurrent. To the best of our knowledge, our experimental results are for the first time to demonstrate the fractional Josephson effect in topological insulator nanoribbon. These results indicate that TI nanoribbon-based superconducting junctions would be a promising building block for the development of topological quantum information devices.

Talk 2 [10:20 AM - 11:05 AM]

Speaker: Jeehoon Kim (POSTECH)

Title: Superconducting disoliton as a linear potential system

Abstract: A disoliton as a bounded soliton system occurs as a universal phenomenon at all length scales. Despite the discovery of several disoliton systems-initially in a swimming pool as a half vortex ring- the underlying physics, such as the nature of the interaction potential, remains elusive due to the lack of a proper model system. In this talk we report on a superconducting disoliton (SDS) created as a U-shape quantum flux tube in a superconducting Nb film. A systematic control on a single SDS reveals the purported 1D linear potential, i.e. distance-independent force. SDS will provide a novel platform for exploring 1D phase transition, quantum chiral transport, and non-Abelian statistics.​

Talk 3 [11:10 AM - 11:55 AM]

Speaker: Gyungchoon Go (KAIST)

Title: Magnetic waveguides by using magnetic solitons

Abstract:The magnon spintronics has gathered great attention because they are not only fundamentally interesting but also provide access to wave-based information devices with low energy dissipation [1-3]. One of the most promising candidates is the magnonic crystal waveguide which provides a possible scheme to manipulate the spin wave (magnon) characteristics [4, 5]. The allowed and forbidden spin wave mode of the magnonic crystal is determined by the magnon band structure. Thus, the spin wave manipulation in the magnonic crystal requires a variation of the structural configuration or material parameters. In this talk, I suggest alternative waveguides of the spin wave by utilizing magnetic vortices (skyrmions) [6] and domain walls.

[1] V. V. Kruglyak, S. O. Demokritov, and D. Grundler, J. Phys. D. Appl. Phys. 43, 264001 (2010).

[2] B. Lenk, H. Ulrichs, F. Garbs, and M. Münzenberg, Phys. Rep. 507, 107–136 (2011).

[3] A. V. Chumak, V. I. Vasyuchka, A. A. Serga, and B. Hillebrands, Nat. Phys. 11, 453 (2015).

[4] C. Elachi, IEEE Trans. Magn. 11, 36 (1975).

[5] S. K. Kim, K. S. Lee, and D. S. Han, Appl. Phys. Lett. 95, 082507 (2009).

[6] G. Go, I.-S. Hong, S.-W. Lee, S. K. Kim, and K.-J. Lee, Phys. Rev. B 101, 134423 (2020).

Session Chair: Jung Hoon Han

Talk 4 [2:00 PM - 2:45 PM]

Speaker: Sug-Bong Choe (Seoul National University)

Title: Dzyaloshinskii–Moriya interaction at metallic interfaces

Abstract: The spin-orbit coupling across the interfaces provides an antisymmetric exchange interaction, so-called Dzyaloshinskii–Moriya interaction (DMI). Due to its chiral nature, the DMI prefers chiral spin alignments and thus, generates chiral magnetic structures such as chiral magnetic domain walls (DWs) and magnetic skyrmions. These chiral structures provide rich physical phenomena and also, prospective technological opportunities. Here, we present our recent development of a measurement scheme for the DMI strength from a single metallic interface. The measurement scheme consists of a vacuum chamber, electromagnet, and optical microscope. The first experimental results will be discussed. Also, from series of a-few-atomic-layer-thick magnetic films, we demonstrate experimentally that interfacial phenomena require finite thickness for their full emergence. The layer thickness dependences revealed that the interfacial DMI begins to appear with thickness and emerges completely in the thickness of 2 to 3 atomic layers, at which the magnitude is maximized. The existence of the threshold thickness indicates the need to refine conventional perspectives on interfacial phenomena and imposes the lowest structural bound and optimum thickness to maximize interfacial effects for technological applications.

Talk 5 [2:45 PM - 3:30 PM]

Speaker: Hyeonsik Cheong (Sogang University)

Title: Optical spectroscopy of 2-dimensional van der Waals antiferromagnets

Abstract: Two-dimensional magnetic van der Waals materials have attracted much interest recently. Magnetism in low dimensional systems is an interesting topic for the fundamental physics, and atomically thin magnetic materials are promising candidates for novel spintronic devices. Antiferromagnetic 2-dimensional materials are particularly interesting both for fundamental physics and also for antiferromagnetism-based spintronic devices. However, traditional research tools such as neutron scattering to probe antiferromagnetic ordering cannot be employed for atomically thin materials due to the small sample volume. Although magneto-optical Kerr effect measurements can be used to monitor the magnetic ordering in ferromagnetic materials, the lack of net magnetization precludes the use of the Kerr effect in probing antiferromagnetic ordering. Optical techniques such as Raman spectroscopy or second-harmonic generation (SHG) are becoming increasingly important for the study of antiferromagnetic 2-dimensional materials. Raman spectroscopy, for example, can be an invaluable tool to probe the magnetic transition in antiferromagnetic van der Waals materials that show spectroscopic features that correlate with magnetic ordering [1]. Furthermore, recent spectroscopic studies revealed a novel coherent state in some of these materials stabilized by the antiferromagnetic ordering. In this presentation, I will review recent achievements in the study of antiferromagnetism in 2 dimensions using optical spectroscopy. FePS₃ exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for a 2-dimensional order-disorder transition [2]. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering. On the other hand, NiPS₃, which shows an XXZ-type antiferromagnetic ordering in bulk, exhibits antiferromagnetic ordering down to 2 layers with a slight decrease in the transition temperature, but the magnetic ordering is suppressed in the monolayer limit [3]. Furthermore, an almost resolution-limited, sharp optical transition was observed in photoluminescence and optical absorption measurements on NiPS₃, which is interpreted as being due to a novel excitonic state coupled with the antiferromagnetic ordering [4]. Furthermore, a Heisenberg-type antiferromagnet MnPS₃ also exhibits ordering down to 2 layers [5] with a weak dependence of the Néel temperature on the thickness [6].

[1] Kim K, Lee J-U, Cheong H, Nanotechnology 30, 452001 (2019).

[2] Lee J-U, et al., Nano Letters 16, 7433 (2016).

[3] Kim K, et al., Nature Communications 10, 345 (2019).

[4] Kang S, et al., Nature 583, 785 (2020).

[5] Kim K, et al., 2D Materials 6, 041001 (2019).

[6] Lim S, et al., Current Applied Physics 21, 1 (2021).

Session Chair: Suk Bum Chung

Talk 6 [3:45 PM - 4:30 PM]

Speaker: Chan-Ho Yang (KAIST)

Title: Strongly Correlative Ionics

Abstract: Long-range correlated fluctuations in order parameters occur at phase transition points, revealing emergent functionalities such as the morphotropic-phase-boundary driven high-k dielectrics, large piezoelectricity, and quantum paraelectricity. Likewise, the correlation length of thermal and quantum fluctuations of atomic positions can be divergently large at the critical point, leading to rapid and coherent ionic conduction. However, the lack of a lattice system that exhibits competing defect orderings has hindered study of the feasibility of this general thermodynamic wisdom. Here, we demonstrate that the activation energy associated with oxygen transport is sharply lowered in Ca-substituted bismuth ferrite (Bi_1-xCa_xFeO_3-d) thin films. Our demonstration relies on the finding that a compositional phase boundary between two isostructural phases with oxygen vacancy channel orderings competitively along the <100> or <110> crystal axes emerges at a chemical doping ratio. From first-principle calculations, we clarify that the effective short-range attraction between two positively charged oxygen vacancies sharing lattice deformation is the driving force of the long-range order and collective diffusion in the system. Our findings open a new avenue into strongly correlative ionics.

Talk 7 [4:30 PM - 5:15 PM]

Speaker: Tae-Hwan Kim (POSTECH)

Title: Chiral solitons in quasi-one-dimensional charge-density waves

Abstract: Chirality or handedness is one of topological features, which manifests in various forms in nature and plays a significant role in all branches of the natural sciences including chemistry, biology, mathematics, and physics. In spin-ordered states, chirality manifests magnetic chiral solitons in chiral magnets, vortexes or skyrmions in thin magnetic layers, and topological monopoles in Weyl semimetals. Such topological particle-like excitations can be used as next-generation information carriers in non-volatile memory and logic devices on the basis of their topological robustness.

1D charge density wave (CDW) of indium atomic wires self-assembled on a silicon surface was known to have topological solitons with unexpected chirality. The indium wire is described by a coupled double Peierls-dimerized atomic chain with four degenerate ground phases. The interchain coupling induces dynamical breaking of the sublattice symmetry and results in three types of topological solitons—right-chiral, left-chiral, and achiral solitons—each having distinct soliton states. As a result, chiral solitons exhibit topological edge states bridging two degenerate but topologically distinct ground states.

Chiral solitons including a ground state (null state) can carry quaternary-digit information in terms of topological charges. Furthermore, due to their unique four-fold degeneracy (or the Z₄ symmetry), Z₄ algebraic operations would be possible in principle among these solitons and a null state. Thus, these topological chiral solitons can be utilized not only for multi-digit memories in electronic systems, but also for logic devices using topological excitations. In contrast to the other logic operation schemes, the chiral soliton operation does not need sophisticated engineering since it is intrinsic to the system’s Z₄ symmetry.

In this talk, I will show various local phase defects of In atomic wires on Si(111) in their quasi-1D CDW states. I will discuss the solitonic characteristics including their chirality, electronic states, and mobility. Then, I will demonstrate that two solitons with different chirality can merge into another chiral soliton. This work will open a door towards logic devices using chiral topological excitations. Chiral solitons could be a platform for storage and operation of robust topological multi-digit information.