Defects play a critical role in determining the physical properties of quantum materials. These systems are highly sensitive to the presence and distribution of defects, which can disrupt their intrinsic properties or create entirely unexpected behaviors. As a result, precise defect control is essential for accurately studying and tailoring the fundamental properties of quantum materials.

The growth of high-quality single crystals with minimized and controlled defects is, therefore, a cornerstone for advancing research on quantum materials. By implementing advanced defect-engineering strategies during crystal growth, it becomes possible to manipulate material properties with unprecedented precision, enabling deeper insights into the underlying physics of quantum materials.

In this talk, I will discuss several examples of single crystal growth techniques aimed at reducing impurities and their significance in unveiling new opportunities for both fundamental research and technological applications.

Exciton, a bound state of photoexcited electrons and holes, has been an intriguingquasiparticle in both fields of condensed matter and optics. In this presentation, I will focus on excitons in Cu-based delafossite systems. Owing to the characteristic O-Cu-O dumbbell structures in these materials, Cu-based delafossites commonly exhibit a strong excitonic transition in their optical spectra. Our findings reveal that these excitons can not only serve as probes for detecting spin fluctuations in quantum magnets but also induce colossal optical anisotropy across the entire visible spectrum—an essential property for optics applications.

The concept of "geometrical frustration" provides a compelling framework for suppressing long-range magnetic order, potentially down to absolute zero temperature, enabling the realization of a quantum spin liquid state. While earlier studies predominantly investigated frustration in two-dimensional systems, certain three-dimensional lattices are predicted to exhibit even stronger effects. However, progress in this area has been hindered by the limited availability of suitable materials. In this talk, I will provide a brief overview of quantum magnetism in three-dimensional hyperkagome systems and present the synthesis and low-temperature characterization of Yb-based garnets. Bulk property measurements reveal no evidence of long-range order down to sub-Kelvin temperatures, suggesting the possible realization of a quantum spin liquid state in these materials. Additionally, by comparing specific heat data with theoretical models, we offer a preliminary estimate of the exchange interaction energy scale, which we aim to further investigate using neutron scattering experiments.

Quantum magnetism in quasiperiodic systems exhibits unique behaviors driven by the interplay of geometric constraints and nonperiodic order, giving rise to novel phases and phenomena. In this study, we investigate two intriguing phenomena in these systems. First, we explore the role of multipoles with unique point group symmetries and their associated frustration, revealing how these features stabilize the ground state with massive entanglement. Second, we show that localized magnetic moments, when coupled to critical states, enable the formation of robust and strong long-distance exchange interactions. These findings illuminate the underlying principles of quantum magnetism in quasiperiodic systems and open pathways for experimental exploration and applications in advanced quantum technologies.

The boundary state of a topological insulator can be gapped by symmetry-breaking perturbations. The gapped boundary itself can be again topological insulator, resulting in the topological boundary of topological boundary. This procedure, known as dimensional reduction, can be performed to obtain the topological states in lower dimensions. The dimensional reduction has been a concept for purely theoretical interest. Here, we report the experimental realization of the dimensional reduction in MoTe2.

The realization of Kitaev magnetism and quantum spin liquid phases in solid-state compounds has remained elusive due to the presence of various magnetic energy scales that reduce symmetries and stabilize long-range orders. Recently, layered honeycomb cobaltates have gathered significant attention following theoretical predictions of potential Kitaev exchange interactions from Co d^7 high-spin states. While early suggestions of Kitaev interactions in compounds like BaCo2(AsO4)2 have been challenged by first-principles electronic structure calculations, interest in honeycomb cobaltates persists due to experimental reports of field-induced paramagnetic phases, which are speculated to be potential spin liquid states. In this study, we present our recent investigations into two distinct types of layered honeycomb cobaltates, BaCo2(PnO4)2 (Pn = P, As, Sb)[1,2] and Cu3Co2SbO6[3]. Our results, based on dynamical mean-field theory calculations, suggest the presence of spin-orbit-entangled J_eff=1/2 moments. Further, we propose viable strategies, employing chemical substitutions and epitaxial strains, for tuning exchange interactions to enhance magnetic frustration in these systems.

[1] S. Samanta et al., Phys. Rev. B 106, 195136 (2022).

[2] S. Samanta, F. Cossu, H.-S. Kim, arXiv:2406.18003 (2024).

[3] G.-H. Kim et al., Sci. Adv. 10, eadn8694 (2024).

The Hubbard U, representing the Coulomb repulsion between electrons, is a fundamental parameter in the study of correlated system. It lies at the heart of the various phenomena such as superconductivity, Mott localization, Hund’s metallicity, etc. However, the correct estimation of the U is not an easy task, both experimentally and theoretically. This challenge is particularly critical in the application of the density functional theory-based approaches, where U plays a crucial role in providing correct explanation of the materials properties. In this talk, I will discuss ways to calculate the Coulomb interaction parameters and explore their implications.

The critical exponent, the power of critical behavior near phase transition and critical point, depends on global properties such as the space dimensionality, the symmetry of the order parameter, and the range of interaction rather than microscopic details. On the other hand, local geometry in the spin and orbital interaction plays an important role to coin an exotic magnetic state. There has been plenty of investigation into the magnetic excitation associated with space and spin dimensions of the low-dimensional magnet. Recently, we have found a new quasi-one-dimensional spin chain compound NiTe2O5 . Performing a comprehensive study of the magnetic properties and optical spectroscopy, we found that NiTe2O5 has intriguing antiferromagnetic state below T N = 30.5 K and has magnetic excitations. In this talk, I present comprehensive experimental results for the magnetic properties of NiTe 2 O 5 and discuss the physical mechanism associated with the local geometry.

[1] J. H. Lee et al., Phys. Rev. B 100,144441 (2019).
[2] S. H. Baek et al., Phys. Rev. B 104, 214431 (2021).

Entanglement is a manifestation of quantum non-locality, wherein actions performed on one part can instantaneously affect distant constituents. Our study unveils a highly entangled phase near a quantum metal-insulator transition, discovered through resonant inelastic x-ray scattering interferometry. This technique demonstrates that entanglement across atomic sites produces distinctive interference patterns, which our model calculations successfully reproduce, allowing us to extract an entanglement measure. These entanglement signatures enable multiple exotic symmetry-breaking orders to coexist, driving the metal to a correlated insulator. Complementary investigations using the anomalous Hall effect and Raman spectroscopy validate the presence of these hidden orders and their associated emergent excitations. This discovery underscores the profound impact of quantum entanglement on the emergence of unconventional orders, opening new avenues for investigating quantum materials. 

In this talk, we discus recent advances in quantum magnetism in terms of quantum entanglement. Many-body entanglement in quantum magnetism is difficult to be measured in condensed matter experiments, so its application to quantum science and technology is believed to be unreachable in near future. We discuss issues of the conventional approaches of our community and how to overcome their bottlenecks. 

From an experimental perspective, I will provide an overview of recent advances in quantum magnetism, with a particular focus on quantum spin liquids. My presentation will be divided into two main parts: novel research methodologies and the discovery of new materials from a variety of spin lattice systems.

In the first part, I will highlight recent developments in innovative techniques, including spin noise spectroscopy, the thermal Hall effect, and the magnetocaloric effect (MCE). In the second part, I will review groundbreaking observations from a variety of spin lattice systems:

1. Emergent photons and monopoles in dipolar-octapolar π-flux quantum spin ice within the pyrochlore lattice Ce2Zr2O7

2. The observation of spiral spin liquids in breathing Kagome bilayers Ca10Cr7O28

3. The identification of a spin liquid phase in the J1-J2 triangular antiferromagnet NaYbSe2 with J1/J2 ~ 0.07

4. The emergence of spin supersolids in S=1/2 XXZ triangular antiferromagnets Na2BaCo(PO4)2 and K2Co(SeO3)2

5. The observation of Dirac spinons and spin-entangled 1/9 magnetization plateaus in YCu3(OD)6+xBr3−x

6. Recent progress in Kitaev honeycomb materials and other spin lattice systems

By reviewing these recent developments, I aim to provide a foundation for discussing future research directions in the domestic quantum magnetism community.

The phase diagrams of quantum materials are often complex with multiple broken-symmetry phases. The complexity of the phase diagram is generally understood in terms of competing orders because a new phase emerges at the expense of the other via charge carrier doping, pressure, or magnetic field. On the other hand, the concept of intertwined order has been proposed to stress a cooperative nature of distinct orders. The vanadium-based kagome metals AV3Sb5 display charge-ordered phase and superconducting state which compete and intertwine with each other. In this talk, I will discuss the electronic structures of the pristine and the charge-ordered phases of the vanadium based kagome metals and their relationship with the superconductivity. 

Two distinct macroscopic quantum phenomena that arise from the collective behavior of electrons, superconductivity (SC) and charge-density waves (CDWs), commonly appear through the phase diagrams of various low-dimensional materials, such as transition-metal dichalcogenides, high-temperature copper oxide superconductors, and layered kagome metals. Since CDWs lead to the opening of a gap at the Fermi level, it has been widely accepted that SC emerges when these CDWs are suppressed. Although this competing interaction is a critical phenomenon in unconventional superconductivity, a direct correlation between the two has not yet been fully established. In this talk, I will focus on how scanning tunneling microscopy (STM) can reveal the order parameters associated with these broken symmetry states and their nanoscale variations. I will review recent STM works that visualize the spatial variations of CDWs and superconductivity under external control parameters, such as strain (or local structural deformation) and chemical doping. By correlating these variations, we can gain new insights into the competing behaviors of SC and CDW, thereby advancing our understanding of these complex systems.

Charge density wave (CDW) is one of the most ubiquitous electronic orders in quantum materials. While the essential ingredients of CDW order have been extensively investigated, a comprehensive microscopic understanding is yet to be reached. Recent research efforts on the CDW phenomena in two-dimensional (2D) materials provide a new pathway toward a deeper understanding of its complexity. In this talk, I will provide an overview of the CDW orders in 2D with atomically thin transition metal dichalcogenides as the materials platform and discuss the possible origins of the 2D CDW, novel quantum states coexisting with them, and exotic types of charge orders that can only be realized in the 2D limit. 

The recent compelling evidence has cast light on the unexpected aspect of CDW, additional symmetry breakings that accompany or occur concurrently with the CDW transition. In this talk, I will discuss various aspects of CDWs in different quantum materials, i) the additional symmetry breaking in CDW phase of 1T-TiSe2 and CsV3Sb5 systems, which are footprinted in the intensity of angle-resolved photoemission spectroscopy. 

Charge-density wave (CDW) phases naturally occur in low-dimensional metallic compounds. Oftentimes, they compete with other electronic instabilities, such as superconductivity, raising the question of the connection between these different phases and the underlying symmetry breaking processes. In this talk, I will review two layered CDW materials, the kagome metal CsV3Sb5 and the unidirectional CDW compound GdTe3. While the former is superconducting at low temperatures (TCDW >> TC), the latter can be tuned towards superconductivity under applied pressure.

In CsV3Sb5, our high-resolution polarization-dependent Raman data sheds light on the symmetry of CDW-related excitations and selected phonons, which allude to an emergent C2 symmetry within the CDW phase. This symmetry-breaking together with phonon anomalies and electronic Raman scattering at TCDW signifies the formation of a nematic phase through a concerted interplay of electronic correlations and electron-phonon coupling within the exotic CDW phase [1].

Meanwhile, for GdTe3, we uncover an unusually low symmetry of the primary CDW excitation, which can be interpreted as an axial Higgs-type excitation. This symmetry proves to be dramatically affected by magnetic fields, alluding to intrinsic magnetic degrees of freedom associated with the CDW formation. This raises the enticing possibility of observing exotic superconductivity with broken time-reversal symmetry in GdTe3 [2].

[1] D. Wulferding et al., Phys. Rev. Res. 4, 023215 (2022).

[2] D. Wulferding et al., arXiv:2411.08331 (2024).

In condensed matter physics, the interplay between electronic and magnetic topologies is crucial for understanding exotic electronic phenomena. Heavy-fermion compounds, characterized by Kondo lattices of localized magnetic moments interacting with itinerant electrons, exhibit a range of topological phases, including topological Kondo insulators, Weyl-Kondo semimetals, and topological spin textures. The magnetic Weyl semimetal candidate CeAlGe presents different magnetic topologies depending on the synthesis method. When synthesized via the flux method, it adopts a topologically trivial non-coplanar structure [1], while the floating-zone method yields a topologically non-trivial half-skyrmion lattice [2]. Despite varying antiferromagnetic ordering temperatures and Kondo coupling strengths [3], the underlying reasons for the distinct topological magnetism remain unclear. In this study, we synthesized high-quality CeAlGe single crystals using the floating-zone and flux methods. We investigated the tunability of topological magnetism by controlling the Kondo coupling strength through hydrostatic pressure, which enhances the hybridization between f-electrons and conduction electrons. We will discuss the results of electrical resistivity and Hall effect measurements under pressure.

[1] T. Suzuki et al., Science, 365, 6451 (2019).

[2] P. Puphal et al., Physical Review Letters 124, 017202 (2020).

[3] X. He et al., Science China Physics, Mechanics & Astronomy 66, 237011 (2023).

On the recent report of a field-induced first order transition in the superconducting state of CeRh 2 As 2 , which is a possible indication of a parity-switching transition of the superconductor, the microscopic physics is still under investigation. However, if two competing pairing channels of opposite parities do exist, a particle-particle collective mode referred to as the Bardasis-Schrieffer (BS) mode should generically exist below the pair-breaking continuum. The BS mode of the CeRh 2 As 2 superconductor can couple to the light, as it arises from a pairing channel with the parity opposite to that of the superconducting condensate. Here, by using a generic model Hamiltonian we carry out a qualitative investigation on the excitation energy of the BS mode with respect to the out-of-plane magnetic fields and its contribution to the optical conductivity [1]. Our findings indicate that the distinct coupling between the BS mode and the light can serve as evidence for the competing odd-parity channels of CeRh2As2 and other locally non-centrosymmetric superconductors.

[1] C. Lee and S. B. Chung, Commun. Phys. 6, 972 (2023).

I present an 75 As NMR/NQR study in a slightly off-stoichiometric LiFeAs phase, unexpectedly discovered in a heavily doped Li 0.5 Na 0.5 FeAs sample. Remarkably, our NMR/NQR results indicate the possible realization of triplet superconductivity in this material. We argue that the superconducting symmetry in LiFeAs is extremely sensitive to crystal off-stoichiometry, transitioning between single and triplet states, as previously proposed in our earlier work [1,2]. This finding sheds new light on the interplay between stoichiometry and unconventional superconductivity in iron-based materials, opening avenues for deeper exploration of novel superconducting states.

[1] S.-H. Baek et al., Eur. Phys. J. B 85, 159 (2012)

[2] S.-H. Baek et al., J. Phys.: Condens. Matter 25, 162204 (2013)

Pursuing topological superconductors (TSCs) and their elusive Majorana fermions remains a captivating frontier in condensed matter physics. TSCs, with a bulk pairing gap and gapless surface states hosting Majorana fermions offer significant potential for quantum computing. 

In this talk, we explore three types of TSCs and their corresponding Majorana fermions. First, we examine heterostructure-based TSCs, where proximity effects from superconductors to topological surfaces are proposed to induce superconductivity, analyzing both successes and limitations. Next, we investigate Majorana fermions in doped topological Dirac semimetals under lattice distortion. We demonstrate that lattice distortions and electron interactions can induce superconductivity with gapless surface Andreev-bound states and enhanced critical temperatures, suggesting pressure-induced topological superconductivity [1]. Finally, we focus on Majorana fermions in higher-order topological insulators, particularly MoTe₂. Surface superconductivity, driven by bulk-to-surface proximity-induced p-wave pairing, reveals higher-order topological phases. Analysis of the Bogoliubov-de-Gennes Hamiltonian shows the evolution of second-order topological hinge states into zero-energy Majorana hinge states [2]

[1] S. Cheon, K. H. Lee, S. B. Chung, and B. J. Yang, Sci. Rep. 11, 1-25 (2021).

[2] S. Lee, M. Kang, D. Kim, D. Y., J. Kim, S. Cho, S. Cheon, T. Park, (under review)

In this talk, I will review the current discussions on the basic physics of 1D and 2D charge density waves. I will introduce the fundamental concepts and a 1D model system we studied during last 20 years, In atomic wires on Si(111), as a test bed for such concepts. Then, I will review the current understanding of the unarguable 2D model systems such as NbSe2, TiSe2, and TaS2. Most of the fundamental issues will be discussed, which include the CDW energy gap, Fermi surface nesting, multi-band effect, structural distortions/degeneracy, chiral structures, interlayer coupling, and manybody interactions such as excitonic and Mott-Hubbard interactions. In particular, I will present the updated experimental studies during the last 5 years, including my own works, which have provided new insights into those classical topics. Due to the time limits, I will not deal with some advanced topics such as the competition with the superconductivity, topological excitations, collective excitations, and interactions with defects. 

CsV3Sb5 exhibits several emergent phenomena, including superconductivity (TC ~ 3.2 K), charge-density wave (T CDW ~ 98 K), and nematicity (Tnem ~ 36 K). Despite tremendous investigations for several years, the nature of the superconductivity and multiple electronic orders, as well as the relationship among them, remain elusive. In this study, we have investigated nematic susceptibility and thermal conductivity in a broad doping and temperature range in high-quality single crystals of Cs(V1-xTix)3Sb5 (x = 0 – 0.06) where a double-dome superconducting phase diagram is realized. In most doping ranges, the nematic susceptibility exhibits the Curie‒Weiss behavior above T nem and Ti doping systematically suppresses the Curie‒Weiss temperature from ~30 K for x = 0 to ~4 K for x=0.0075, resulting in a sign change at x = ~0.009, where the first superconducting dome exists. Furthermore, the Curie constant reaches a maximum at x = 0.01, suggesting a drastic enhancement of the nematic susceptibility near a putative nematic quantum critical point (NQCP) at x = ~0.009. Furthermore, we have investigated the temperature (T)- and magnetic field (H)-dependent thermal conductivity  of the Ti-substituted kagome superconductor Cs(V1-xTix)3Sb5 (x=0.0, 0.01, 0.015, and 0.06). In the samples belonging to the first superconducting dome (0 ≤ x ≤ 0.015), K(T) revealed a set of excitations that indicated the existence of a small full gap and monotonic suppression of the gap minimum with increasing x. In the same sample set, most quasiparticles, as inferred from K(H), were easily excited and delocalized when small H ≤~100 mT were applied along the c-axis, and the increasing behaviour of K(H) was similar to that of the nodal superconductors. Moreover, the electronic  reached the normal state values under H well below the upper critical fields, uncovering anomalous gapless superconducting states under H. All these results strongly support the presence of unconventional chiral d + id pairing inside the first superconducting dome, which develops a full gap that is quite fragile under time reversal symmetry breaking potential.
[1] Y. Sur at al Nat. Commun. 14, 3899 (2023)
[2] K. Nam et al. submitted (2024)

FeRh is a material known for its phase transition at approximately 370 K, transitioning from an antiferromagnetic (AFM) phase at low temperatures to a ferromagnetic (FM) phase at higher temperatures. By adjusting the stoichiometric ratio of FeRh, we achieved an AFM-FM mixed phase, where residual FM moments exist even in the AFM phase of FeRh. Thereby, one can expect intriguing phenomena coming from the two different exchange interactions of AFM-FM and FM-FM in FeRh/NiFe(FM) bilayer system. In this work, we investigated the magnetic properties of FeRh/NiFe bilayers and observed very asymmetric magnetic hysteresis loop with one-step switching in the field-cooled (positive) direction and two-step switching in the negative direction. This asymmetric behavior could be understood by the competition of exchange bias effect and exchange spring effect. When the field was swept from +7 T to -7 T, the FM moments of FeRh layer first switched and then the FM moments of NiFe layer switched, resulting in two-step switching behavior. This phenomenon is attributed to the strong exchange bias effect between AFM of FeRh layer and FM of NiFe layer. In contrast, as sweeping the field backward, only one-step switching behavior was observed, which can be interpreted as a consequence of the strong exchange spring effect between FM of FeRh layer and FM of NiFe layer. These findings highlight the complex interactions at the interface of magnetic bilayers, such as the exchange bias and exchange spring effects, and suggest a novel strategy for tailoring these properties to meet the specific requirements of potential applications in spintronic devices.

Van der Waals (vdW) materials exhibit great potential in the field of spintronics due to their excellent spin transport properties, substantial spin-orbit coupling, and high-quality interfaces. The recent discovery of Fe 3 GaTe 2 with T C above room temperature and a significant perpendicular magnetic anisotropy (PMA) holds importance for next-generation low-power magnetoelectronic and spintronic devices. In this talk, I present the electronic band structures of various room temperature vdW magnets including Fe 3 GaTe 2 using angle-resolved photoemission spectroscopy (ARPES), providing essential information for understanding and controlling these characteristics.

Systems having inherent structural asymmetry retain the Rashba-type spin-orbit interaction, which ties spin and momentum of electrons in the band structure leading to coupled spin and charge transport. This coupled spin-charge transports have been evidenced in various experimental platforms and could be utilized for various electronics. Recent discovery of altermagnet also provide unique spin-splitted Fermi surface which leads to coupled spin-charge transport implying novel spintronic applications. In this talk, I will present nonvolatile tuning of spin-charge conversion by employing ferroelectric film, which could be utilized for the next-generation of magnetic-memory devices [1]. For the second part of my talk, I will present the spin conversion and spin filtering effects observed in altermagnetic RuO2.

[1] J. Choi et al, Non-volatile Fermi level tuning for the control of spin-charge conversion at room temperature, Nat. Commun. 15, 8746 (2024).

Altermagnetism is a recently identified fundamental form of magnetism characterized by a vanishing net magnetization and a broken electronic structure with time-reversal symmetry. In this talk, we employ a combination of symmetry analysis and first-principle calculations to reveal that the crystallographic symmetry groups of numerous magnetic materials, featuring negligibly small relativistic spin-orbit coupling (SOC), are significantly larger than conventional magnetic groups. Consequently, a symmetry description incorporating partially decoupled spin and spatial rotations, termed the spin group, becomes essential. We establish the classifications of spin point groups that describe collinear magnetic structures, encompassing altermagnetic phases. Using MnTe as an example, we provide direct evidence for altermagnetism in MnTe.

Despite the promising properties of altermagnets for spintronic applications, controlling the Néel vector and thus achieving a single magnetic domain is commonly regarded as a challenging task. In this presentation, we will discuss the emergence of the staggered Dzyaloshinskii-Moriya interaction in an altermagnet [1], as derived by applying Moriya's rules. This staggered interaction enables the system to exhibit globally ordered weak ferromagnetism, paving the way for controlling the altermagnetic Néel vector using external fields. Additionally, we will present the reversal of the Néel vector and the hysteresis loop of an altermagnet based on the macrospin model with the staggered Dzyaloshinskii-Moriya interaction.

Recently, altermagnetic materials, which exhibit both the spin polarization of ferromagnetic materials and the antiferromagnetically coupled spin alignments of antiferromagnetic materials, have received significant attention [1]. In particular, altermagnetic materials are expected to overcome limitations in conventional ferromagnet-based spintronic devices by achieving both high tunneling magnetoresistance ratios [2] and ultrafast spin dynamics in the THz regime [1]. However, to utilize altermagnetic materials, it is crucial to control their magnetic domains. Although several pioneering research efforts have focused on manipulating altermagnetic domains using conventional spin currents and associated spin-orbit torques [3,4], these approaches face limitations because the ordering parameter of altermagnets is not spin. In this presentation, we discuss experimental methods for controlling and verifying altermagnetic domains.

[1] L. Šmejkal et al., Physical Review X 12, 040501 (2022).
[2] D.-F. Shao et al., npj Spintronics volume 2, 13 (2024).
[3] L. Han et al., Science Advances 10, adn0479 (2024).
[4] Z. Zhou et al., arXiv:2403.07396 (https://doi.org/10.48550/arXiv.2403.07396) 

Antiferromagnetic materials are rapidly attracting attention in spintronics for the next generation of picosecond and highly packed information technology, thanks to the absence of stray fields and ultrafast spin dynamics. The hexagonal D019-Mn3X (X = Ga, Ge, Sn) family is one of representatives with a non-collinear spin structure. Within this family, the magnetic Weyl semimetal Mn3Sn, characterized by its unique Kagome lattice spin structure, exhibits a range of exotic properties. Despite its negligible magnetization, Mn3Sn displays a considerable anomalous Hall effect (AHE), attributed to a non-zero net Berry curvature in bands near the Fermi level. It is observed that the magnetic properties of Mn3Sn are significantly influenced by various factors, including growth conditions and the presence of other phases. In this study, we investigated the structural and magnetic properties of Mn3Sn-based heterostructures, especially focusing on the effects of interfacial structure. The Mn3Sn thin films were deposited on MgO (110) substrates using co-sputtering techniques in an ultra-high vacuum magnetron sputtering system. The crystallinity of these samples was examined using X-ray diffraction and high-resolution transmission electron microscopy. Our results demonstrate that the epitaxial Mn3Sn thin films with the preferred [] growth direction exhibit a substantial AHE and a large binary switching of the Mn3Sn magnetic state (approximately 80%) induced by spin orbit torque. We find that the magnetic properties of Mn3Sn thin films are significantly influenced by the interfacial structure. Furthermore, temperature dependence of anomalous Hall conductivity also reveals the origins of the AHE observed in our Mn3Sn-based structures, contributing to the understanding of these complex magnetic systems.

The scalar spin chirality, which characterizes the fundamental unit of noncoplanar spin structures, plays an important role in rich chiral physics of magnetic materials. In particular, the intensive research efforts over the past two decades have demonstrated that the scalar spin chirality is the source of various novel Hall transports in solid-state systems, offering a primary route to bring about chiral phenomena in condensed matter physics. However, in all of the previous studies, the scalar spin chirality has been given as a stationary background, serving only a passive role in the transport properties of materials. It remains an open question whether or not the scalar spin chirality itself can exhibit a Hall-type transport. In this work, we show that the answer is yes: The scalar spin chirality is Hall-transported in Kagome ferromagnets and antiferromagnets under an external bias, engendering a phenomenon which we dub the scalar spin chirality Hall effect. Notably, this effect is present even in the absence of any spin-orbit coupling. The analytical theory for the scalar spin chirality Hall effect is corroborated by atomistic spin simulations. Our findings call for the need to lift the conventional assumption that the scalar spin chirality is a passive background in order to discover the active roles of the scalar spin chirality in transport properties.

Magnetic skyrmions, with their unique swirling spin textures, are rapidly emerging as exciting new carriers of information in spintronics [1]. Their stability, small size, and low energy requirements make them ideal candidates for next-generation technologies. Furthermore, quantum effects are expected to emerge in skyrmions when we shrink skyrmions to the nanoscale, opening up new possibilities for quantum technologies.

In this talk, I will explore the latest advances in skyrmion-based spintronics, beginning with the introduction of skyrmion-hosting materials, including thin-film systems [2] and 2D materials [3]. Then, I will present the dynamics of skyrmions driven by current and show key experimental demonstrations for skyrmion-based devices, such as electrical generation, deletion, and precise shifting of isolated skyrmions, alongside the manipulation of their motion.

Building on these operations, I will demonstrate proof-of-concept experiments for skyrmion racetrack memory [4], skyrmion transistors [5], and skyrmion neuromorphic computing devices. Moreover, I will discuss the cutting-edge research pushing skyrmions into the quantum regime and explore the potential for skyrmion-based quantum technologies

[1] A. Fert et al., Nat. Rev. Mater. 2, 17031 (2017).

[2] S. Yang et al., Nano Lett. 22, 8430 (2022).

[2] Y. Ji et al., Adv. Mater. 34, 2203275 (2024).

[3] M. Song et al., Adv. Mater. 34, 2203275 (2022).

[4] S. Yang et al., Adv. Mater. 33, 2104406 (2021).

[5] S. Yang et al., Adv. Mater. 35, 2208881 (2023).

This talk will review some of recent progresses reported by other speakers in the spintronics/orbitronics field and discuss possible future directions to pursue.

In the field of spintronics, domain wall (DW) motion-based devices have been a subject of extensive research for several decades. Initially, investigations focused on DW motion driven by spin transfer torque (STT). However, with the advent of spin orbit torque (SOT), it has been demonstrated that DW motion, facilitated by the combined effects of SOT and Dzyaloshinskii-Moriya Interaction (DMI), offers greater efficiency. In this study, we report the experimental observation of an unexpected DW motion that defies current theoretical explanations. Notably, the DW mobility, defined as the DW velocity per unit current density, exhibits variation with the ratio of the ferromagnetic to heavy metal layer widths. This finding suggests the presence of unexplored physics in DW dynamics. Additionally, we will briefly explore the potential applications and underlying physics of the Altermagnets.

In this talk, we present recent results on the quantum Hall effect in large-angle twisted trilayer graphene. Unlike the current research on magic-angle graphene and our previous studies on the integer and fractional quantum Hall effects in large-angle twisted bilayer graphene, which host Bose-Einstein condensation of excitons, the emergence of excitons in large-angle twisted trilayer graphene has to be questioned due to the nature of the three charge carriers. Instead, the quantum Hall behavior critically depends on the twisting orientation, such as alternating or chiral stacking.For example, in alternating twisted trilayer graphene, three counter-propagating edge modes emerge at zero filling factor, indicating that the three layers are effectively decoupled. In contrast, chiral twisted trilayer graphene behaves as a combination of large-angle twisted bilayer graphene and monolayer graphene. This distinction allows the quantum Hall signals from monolayer graphene and twisted bilayer graphene to be clearly discerned.

In addition, we will also show non-linear Hall effect in graphene. Contrary to the typical quantum Hall effect, which is observed in the linear response regime due to broken time-reversal symmetry from an external magnetic field or intrinsic magnetic orders as per Onsager’s reciprocal theorem, the nonlinear Hall effect—characterized by a Hall voltage that nonlinearly depends on perpendicular driving currents—stems from disrupted inversion symmetry and reduced crystal symmetries. This effect, deeply linked to the Berry curvature dipole moment, offers novel perspectives for exploring the topological properties of emergent quantum material phases. In our research, the nonlinear Hall effect is observed in graphene when symmetry is intentionally disrupted through proximity effects induced by molecular beam epitaxy-grown a-plane ZnO, a substance with inherent inversion symmetry breaking and a single mirror line along the crystal's a-axis. Our angle-resolved electrical measurements highlight that the second harmonic transverse Hall voltage reaches its maximum when the bias current aligns perpendicularly to the mirror axis and disappears when parallel.

In this talk, I am going to present our on-going theoretical works to understand various fractionalized states in twisted TMDCs like fractional Chern insulators and fractional quantum spin Hall effects within the minimal theoretical models. I will also highlight the importance of band mixing and accurate DFT band structures in conducting the research on this topic. 

The study of Josephson effects continues to illuminate critical aspects of superconducting quantum systems, offering deep theoretical insights and practical applications. The Josephson effect, first described in the 1950s, is often regarded as a well-understood phenomenon. However, it has largely been confined to equilibrium Josephson effects, such as those involving DC voltage or extremely small voltage drops. Consequently, while the Josephson effect has been experimentally realized in various superconductors and junctions over the past half-century, theoretical progress in nonequilibrium Josephson effects has remained limited. 

We explore nonequilibrium Josephson phenomena through a robust microscopic framework, addressing key challenges in the field [1-4]. We begin with an analytical solution to the longstanding issue of DC current-biased I-V characteristics in generic superconducting tunnel junctions [1]. Our results uncover novel subharmonic patterns and voltage staircases at DC current-biased Josephson junction with arbitrary transparencies [2].

Building on these insights, we reveal the emergence of a nonequilibrium fractional Josephson effect in superconductors hosting midgap states [3]. This effect arises from quantum interference between Cooper pair and quasiparticle tunneling, enabling single-electron charge transport. These findings are particularly pronounced in systems with Majorana bound states, demonstrating resilience to quasiparticle poisoning. Our study connects diverse nonequilibrium phenomena in Josephson junctions, offering a unified microscopic perspective that links foundational theory to emerging quantum effects in superconductivity.

[1] Sang-Jun Choi*, Björn Trauzettel, Phys. Rev. Lett. 128, 126801 (2022).
[2] Aritra Lahiri, Sang-Jun Choi*, Björn Trauzettel, arXiv:2412.09862 (2024).
[3] Aritra Lahiri, Sang-Jun Choi*, Björn Trauzettel, Phys. Rev. Lett. 131, 126301 (2023).
[4] Aritra Lahiri, Sang-Jun Choi, Björn Trauzettel, arXiv:2402.13074 (2024).

Many of fascinating quantum phenomena, such as high-temperature superconductivity, have been found in two-dimensional insulators doped by dopants. Even though these dopants seem to be randomly distributed, there is a short-range order of dopants. In most theoretical models, however, the presence of short-range order has been entirely neglected for the sake of brevity. In this talk, I will introduce our recent angle-resolved photoemission spectroscopy studies on the effect of short-range order to the electronic structure of doped two-dimensional insulators, focusing on how it leads to the formation of an unclear gap, namely, the pseudogap1,2.

On the other hand, there are many materials with pairs of sublattices in the primitive cell. This sublattice degree of freedom, the importance of which has been recognized in the study of graphene with the concept of pseudospin3, has also been neglected in the theoretical model. If time permits, I will also talk about the effect of sublattices to angle-resolved photoemission spectroscopy, focusing on how it can be used to describe anomalously disconnected segments in the Fermi surface of cuprates, namely, the Fermi arcs4. As one of the future directions, the same approach can be exploited to directly quantify the quantum geometric tensor in solids5.

[1] S. H. Ryu et al., Nature 596, 68 (2021).

[2] S. Park et al., Nature 634, 813 (2024).

[3] S. W. Jung et al., Nature Mater. 19, 277 (2020).

[4] Y. Chung et al., Nature Phys. 20, 1582 (2024).

[5] S. Kim et al., under review (2025).

When a classical photonic medium is driven by the time periodic perturbation of permittivity, a right-going mode with and a left-going Floquet mode with meet and open a momentum gap by the modulation strength of permittivity. In this talk, we present the analysis of the classical photonic temporal crystal (PTC) from the quantum mechanical perspective. The momentum gap transition can be understood from the localization-to-delocalization of photon eigen modes. In addition, a novel behavior of the Rabi oscillation in the quantum PTC will be introduced.

[1] JH Bae, KM Lee, B Min, KW Kim (in preparation)

The geometric characteristics of Bloch wavefunctions play crucial roles in the properties of electronic transport. The Hilbert-Schmidt quantum distance, one of the representative geometric quantities, has been shown to be crucial in Landau level structures[1,2] and bulk-

boundary correspondence[3,4]. In this talk, I will show that the Hilbert-Schmidt quantum distance is also critical in transport properties of quadratic band-touching systems including the singular flat band systems. First, we introduce the connection between the distribution of

quantum distance on the Fermi surface and the electronic transport scattering rate is established[5]. The general formulation is applied to isotropic quadratic band-touching semimetals, where one can concentrate on the role of quantum geometric effects other than the Berry curvature. It is verified that the thermoelectric power factor can be succinctly expressed in terms of the maximum quantum distance. Second, we demonstrate that in singular flat-band systems, the electric current responds instantaneously to an applied electric field, reaching a steady-state value without delay[6]. The rapid current generation and large current densities arise from inter-band coupling, which is geometrically characterized by the quantum distance. First-principles calculations reveal that the current generation in both cyclic graphene and V 3 F 8 is instantaneous, approaching the extreme speed limits of condensed matter systems, with operation frequencies even reaching the petahertz range (10 15 Hz).

[1] J.-W. Rhim et al., Nature 584, 59 (2020).

[2] Y. Hwang et al., Nature Communications 12, 6433 (2021).

[3] C.-g. Oh et al., Communications Physics 5, 320 (2022).

[4] H. Kim et al., Communications Physics 6, 305 (2023).

[5] C.-g. Oh et al., Advanced Science 11, 2411313 (2024).

[6] Y. Kim et al., to be submitted. (2025).

A Bilayer of semiconducting 2D electronic systems has long been a versatile platform to study electronic correlation with tunable interlayer tunneling, Coulomb interactions and layer imbalance. In the natural graphite bilayer, Bernal-stacked bilayer graphene (BBG), the Landau level gives rise to an intimate connection between the valley and layer. Adding a moiré superlattice potential enriches the BBG physics with the formation of topological minibands, potentially leading to tunable exotic quantum transports. Further increasing the number of layers is expected to rapidly expand the possible phase space one can explore to tune the interplay between the electronic correlation and band topology.

In this talk, I will present our recent magneto-transport measurements of a high-quality bilayer graphene-hexagonal boron nitride (hBN) heterostructure. The zero-degree alignment between the bilayer graphene and hBN generates a strong moiré superlattice potential for the electrons in BBG and the resulting Landau fan diagram of longitudinal and Hall resistance displays a Hofstadter butterfly pattern with an unprecedented level of detail. Our work demonstrates that the intricate relationship between valley and layer degrees of freedom controls the topology of moiré-induced bands, significantly influencing the energetics of interacting quantum phases in the BBG superlattice. We further observe signatures of field-induced correlated insulators and clear fractional quantization of interaction driven topological quantum phases. In the second part of the talk, I will discuss the important considerations in utilizing multilayer graphene heterostructures as ideal platforms to study the delicate interplay between topology and electron correlation. In particular, our recent results in helically stacked twisted trilayer graphenes will be presented as an example.

Magic-angle twisted bilayer graphene (MATBG) shows unconventional superconductivity in a sense that it shows a nodal superconducting gap in tunneling spectrum. However, unlike other unconventional superconductors, MATBG also exhibits clear experimental signatures of strong electron-phonon coupling in its transport, Raman and spectroscopic properties including recent observation of replica bands in angle-resolved photoemission spectroscopy. In this talk, we revisit theoretical studies of electron-phonon interaction (EPI) in MATBG. We discuss our atomistic calculations of strong electron-phonon coupling in MATBG, and possible pairing mechanisms from EPI, where intervalley K phonon could lead to non-s-wave pairing. We also discuss role of strong EPI in other graphene moire superlattices.

Twisted trilayer van der Waals crystals provides a unique platform for investigating the interplay of atomic periodicities, which can be modulated by varying the twist angles between its layers, resulting in complex moiré-of-moiré lattice structures. At small twist angles, the interlayer and intralayer interactions at two distinct interfaces in the twisted trilayer crystals give rise to an intricate network of domain structures that exhibit exotic electronic properties. In this study, we present a comprehensive structural phase diagram of twisted trilayer graphene (TTG) with atomic-scale lattice reconstruction. Using transmission electron microscopy in conjunction with advanced interatomic potential simulations, we reveal a variety of large-scale moiré lattices, including triangular, kagome, and hexagram-shaped domain patterns. Each domain is enclosed within a two-dimensional network of domain wall lattices. At small twist angles, competing rhombohedral and Bernal stacking orders, separated by a small energy difference, induce unconventional lattice reconstructions characterized by spontaneous symmetry breaking and nematic instability, emphasizing the role of long-range interactions across van der Waals layers. The tunable tessellation of diverse domain networks by adjusting the twist angles establishes TTG as a versatile platform for investigating the relationship between engineered nontrivial domain lattice structures and their emergent physical properties.

A few years ago, I contributed a section on twisted two-dimensional (2D) layered crystals for a roadmap paper on quantum materials [1]. Although the paper was published just three years ago, many of the topics discussed now seem outdated due to the rapid advancements in the field. In this brief talk, I will highlight some of the major challenges that remain to be addressed in the near future, and explore potential directions for future development that could have a lasting impact on both science and society.

[1] F. Giustino et al., The 2021 quantum materials roadmap, J. Phys. Mater. 3, 042006 (2021).

This review provides an overview of research activities related to topological and twisted materials, as well as their associated devices, conducted in South Korea. The aim is to set the stage for discussions on potential future research directions in this rapidly evolving field.

Under a strong optical excitation in solids, the electron-hole pairs are generated, accelerated, and eventually recombined to produce high harmonic generations (HHG) of the incident driving field. The polarimetry and ellipsometry of HHG is found to strongly depend on the electron-hole decoherence through the dephasing time T2 in two-dimensional semiconductors under the elliptically polarized driving field [1]. In the case, the dephasing time T2 is estimated to be just a few femtoseconds, which is so short that it is difficult to find comparable relevant time scales in the system. More extended scientific insights concerning the electron-hole decoherence would be available in an investigation of HHG of a strongly correlated electron system in a dissipative open quantum environment [2].

[1] Y. Kim et al., Nano Lett. 24, 1277 (2024).

[2] G. Bae, Y. Kim, J.D. Lee, unpublished (2025).

Light-matter interaction has been investigated through the estimation of response functions derived from perturbation theory, leading to a successful understanding of intriguing quantum phenomena. While perturbation theory has provided valuable insights into light-matter interactions, fully capturing the temporal dynamics of electrons and the lattice, particularly under strong external fields, remains a challenge. In this presentation, I will discuss the behavior of electrons subjected to external electric fields of varying strengths using time-dependent density functional theory (TDDFT), an increasingly important method for understanding the complex interplay between electrons and the lattice. Specifically, the underlying mechanism of giant shift currents in non-centrosymmetric low-dimensional systems will be presented. It has been found that the giant shift currents in WS₂ and Bi nanotubes originate from interchain interactions and a nontrivial electronic structure, respectively. The presence of orbital angular momentum induced by an external field in helical materials, and its optoelectronic manifestations, will be discussed.

Topological matter is one of the most notable examples of quantum materials. Although various classes of topological matter have been studied, such as topological insulators, Dirac and Weyl semimetals, and nodal-line semimetals, most of the research has focused on equilibrium physics. This talk will explore the possibility of generating a new type of topological matter under nonequilibrium conditions by periodically driving topologically trivial systems. Specifically, we will analyze the phase diagram of Floquet topological insulators in irradiated graphene exposed to circularly polarized light. Our analysis predicts the emergence of a negative resistance catastrophe, which could lead to a novel nonequilibrium zero-resistance state.

Optical pumping with an intense and ultrashort laser pulse can excite quasi-particles in solids, and disturb electron, spin or phonon sub-systems out of equilibrium states. This can provide us with valuable opportunities to investigate numerous intriguing phenomena realized in the photo-induced non-equilibrium states. In the talk, I am going to overview representative recent works revealing interactions among fundamental degrees of freedom and also ultrafast phase transitions in correlated electron systems or spin-charge-lattice coupled systems. 

The ultrafast optical manipulation of magnetic phenomena broadens our understanding of functional non-equilibrium states. These dynamics occur on extremely short timescales, necessitating four-dimensional (4D) views (3D-space plus 1D-time) to comprehensively track the evolving magnetic phenomena. We employ time-resolved resonant magnetic X-ray diffraction with an X-ray free electron laser to accomplish 4D visualization of light-induced coherent magnons in multiferroic hexaferrites [1,2]. In this presentation, I will discuss the discovery of a photoinduced effective magnetic field that persists long after photoirradiation, attributed to nonthermal inter-site electron transfer among magnetic ions. By directly quantifying this field for photoexcitation above the band gap, we observed a remarkable amplification of the photomagnetic effect, reaching an order of magnitude higher than below the gap with the same laser fluence. This result highlights a highly energy-efficient route to achieve substantial photoinduced fields in antiferromagnetic dielectrics, which is crucial for optospintronics applications. The X-ray methodology employed has no restriction on net magnetization, providing a powerful tool to uncover ultrafast magnetic phenomena across all classes of magnets.

[1] H. Ueda et al., Phys. Rev. Res. 4, 023007 (2022).

[2] H. Jang et al., Adv. Mater. 35, 2303032 (2023).

SrRuO3 is a well-known strongly correlated ferromagnetic metal of perovskite structure. It presents various interesting magnetic and electronic properties. We investigated the ultrafast dynamics of SrRuO3 and related materials using near infrared and X-ray pulses. We found that unusually strong magnon oscillations show up as reflectivity modulations in a 100 nm thick film while the oscillations appear much weaker in a 32 nm film and SrRuO3 based superlattices. The strong modulation of the electronic response to the spin precession is attributed to the magnetoelectric coupling caused by the triclinic phase that is formed in the intermediate thick SrRuO3 films. We also found that NIR pulse excitation modifies the lattice structure almost instantaneously. Our data demonstrate very strong coupling of the structural distortion to the electronic structure, details of which require further discussion. 

Recent advancements in femtosecond lasers and free-electron lasers have enabled the investigation of nonlinear regimes and lattice dynamics on ultrafast time scales. The time-resolved capabilities of ultrafast spectroscopy have become pivotal for investigating the quantum electronic and lattice band structures. In this study, we explore the charge density wave phases in (TaSe4)2I and TbTe3 using NIR-pump and hard-x-ray probe, where the goal is to monitor the order parameters and estimate the phenomenological free energy potential of the phase [1]. Using femtosecond light pulses, our approach provides novel insights into detecting elusive collective modes that were previously inaccessible. Additionally, we discuss the possible application of uniaxial strain to samples and outline plans for PAL-XFEL experiment on the magnetic Weyl semimetal Mn₃Sn.

[1] S. Kim et al., Rep. Prog. Phys. 87, 100501 (2024).

Quantum materials exhibit a rich variety of many-body and topological phenomena, holding promise for next generation applications. Ultrafast photoexcitation has emerged as a powerful approach to understand and control exotic quantum phases and interaction dynamics. The materials are excited via ultrashort, intense optical pulses and typically probed by optical techniques. Beyond the optical pump-probe techniques, X-rays have become a powerful probing tool due to advances in modern light sources, particularly X-ray free electron lasers (XFELs). With their high photon energy and strong penetration, X-rays are ideal for investigating the crystalline and electronic structures of materials. XFELs offer ultrashort (< 100 fs), high-intensity pulses and enable the study of ultrafast dynamics in quantum materials following photoexcitation.

In this presentation, I will introduce soft X-rays as a specialized ultrafast probe for quantum materials. Despite their limited penetration depth and longer wavelengths compared to hard X-rays, soft X-rays excel in directly probing valence electronic states, making them invaluable for studying electronic structures and complex ordering phenomena. I will present several examples demonstrating the utility of soft X-ray probes for studying ultrafast dynamics in quantum materials, particularly conducted at PAL-XFEL [1-6], and discuss future opportunities.

[1] Hoyoung Jang et al., Rev. Sci. Instrum. 91, 083904 (2020).

[2] Denitsa R. Baykusheva et al., Phys. Rev. X 12, 011013 (2022).

[3] Hoyoung Jang et al., Sci. Adv. 8, eabk0832 (2022).

[4] Hoyoung Jang et al., Adv. Mater. 35, 2303032 (2023).

[5] Martin Bluschke et al., Proc. Natl. Acad. Sci. U.S.A. 121, e2400727121 (2024).

[6] Seung-Phil Heo et al., arXiv: 2406.06913

2D spectroscopy utilizes sequential coherence generations for specific excitations and the time evolution of phase correlations among these coherent excitations. Such a nonlinear response function contains abundant information on mode couplings and many-body effects. In this talk, I will introduce the two-dimensional electron-phonon-coupling spectroscopy (2D EPC) that we have recently developed for directly extracting the electron-phonon-coupling strength in terms of a specific phonon mode- and electron energy. [1] I will describe how one could measure the electron-energy dependence of the EPC strength for individual phonon modes using this technique and, in turn, identify unique signatures distinguishing nonlocal Su-Schrieffer-Heeger (SSH) -type couplings from local Holstein-type couplings. Finally, I will discuss how this new 2D spectroscopy technique could be useful for quantum material studies with strong electron couplings.

[1] S. Qu et al., ArXiv:2310.03072