7월 14일 (월)
Despite the long and crucial role of traditional solid-state physics in current silicon-based technologies, next-generation neuromorphic, non-volatile memory, and energy devices, which are key components in the era of the Internet of Things (IoT), require novel working principles that incorporate quantum physics emerging in low-dimensional materials. The primary research direction for future devices is to achieve ‘ultralow device operation energy’, ‘ultrahigh device operation speed’, and ‘large-scale device integration (up to 10^15)’, which necessitates exploring diverse quantum phenomena in low-dimensional device components. In this talk, I will present some of our recent efforts to establish new device physics for energy-intelligent devices, which could be a milestone for the promising future devices. In particular, dynamic convolutional neural networks, phase transitions, and other intriguing phenomena in two-dimensional (2D) materials will be discussed, along with applications in logic devices, neuromorphic computing, and energy devices.
Magic-angle twisted graphene superlattices have enabled the discovery of a plethora of correlated and topological phenomena, particularly superconductivity with unusual characteristics. However, understanding the nature of superconductivity in magic-angle graphene remains a significant challenge. A key difficulty lies in resolving the various energy scales in this strongly interacting system, especially the superconducting gap. Here, we report the first simultaneous tunneling spectroscopy and transport measurements of magic-angle graphene, introducing a novel approach to probing the superconducting state. This method reveals two coexisting V-shaped tunneling gaps with different energy scales: a distinct low-energy superconducting gap that disappears at the superconducting critical temperature and magnetic field, and a higher-energy pseudogap. The superconducting tunneling spectra exhibit a gap-filling behavior with temperature and magnetic field and display the Volovik effect, consistent with a nodal order parameter. Our findings highlight the unconventional nature of the superconducting gap in magic-angle graphene and establish an experimental framework for multidimensional investigations of tunable quantum materials.
The interplay between spontaneous symmetry breaking and topology can lead to exotic quantum states of matter. A prominent example is the quantum anomalous Hall (QAH) effect, characterized by an integer quantum Hall effect at zero magnetic field, arising from topologically nontrivial bands and intrinsic magnetism. Under strong electron-electron interactions, the fractional quantum anomalous Hall (FQAH) effect can emerge at zero magnetic field, serving as a lattice analog to the fractional quantum Hall effect but without Landau level formation. Here, I will first briefly outline the experimental observation of the FQAH effect in twisted bilayer MoTe2 using combined magneto-optical and magneto-transport measurements. Subsequently, I will introduce recent advancements, including observations of an abundant Jain sequence of fractional Chern insulator states, as well as ferromagnetism and twist-angle-dependent topology of higher-energy flat Chern bands. Additionally, I will discuss our latest results obtained with improved samples, demonstrating quantized fractional plateaus with vanishing longitudinal resistance. The direct observation of the FQAH effect and associated phenomena opens new avenues for investigating charge fractionalization and anyonic statistics at zero magnetic field.
Moiré twistronics has opened a new frontier in the study of emergent quantum phenomena in two-dimensional (2D) materials. Among these, marginal twist angle differences between stacked layers induce Moiré patterns that give rise to alternating polarization domains and dislocations. Solitonic domain walls, which form between these domains due to competing stacking orders and lattice relaxation in marginally twisted bilayers, have attracted particular interest. Recent studies in sliding ferroelectric systems such as 3R-MoS₂ have demonstrated undamped, relativistic-like motion of domain walls under external fields. However, the rigidity of the Moiré angle imposes significant constraints on the controllable formation and positioning of dislocations. To overcome these limitations, we propose a novel device concept that minimizes the formation energy of dislocations. In this platform, dislocations can be reversibly introduced and removed via applied electric fields, exhibiting clear hysteresis behavior. These findings not only provide a practical route to controlling polarization and Moiré twist-induced phenomena in van der Waals (vdW) materials but also open promising avenues for next-generation electronic device applications.
Through the creation of epitaxy between two distinct materials, it is possible to finely manage the misfit strain at their interface. Core-shell architectures are prominent examples of such strain-engineered materials, where adjusting the misfit strain at the interface allows for the tailored design of material properties. Achieving a comprehensive exploitation of the strain effects at interfaces necessitates an atomic-level insight into their 3D interface structures. In this study, we present a detailed atomic-level analysis of the 3D structure of Pd@Pt core-shell nanoparticles using atomic electron tomography. We obtained detailed 3D displacement and strain maps for these nanoparticles, unveiling a direct correlation between the strains at the surface and the interface. Additionally, we observed clear Poisson effects across the entire nanoparticle and within its local atomic bonds. The strain distribution exhibited notable shape-dependent anisotropy, a finding supported by molecular statics simulations. By analyzing the surface strains, we were able to predict the activities of the oxygen reduction reaction at the surface. These insights offer a profound understanding of the connections between structure and properties in core-shell configurations, indicating that both strain and catalytic surface properties can be meticulously adjusted through strategic core-shell design.
Light–matter interaction plays a crucial role in inducing phase transitions and creating non-equilibrium states that may not exist under equilibrium conditions. In this talk, I will present two studies conducted using time- and angle-resolved photoemission spectroscopy (trARPES): 1. Observation of Floquet–Bloch states in graphene [1] — We observed replica bands and their pump-polarization-dependent evolution in monolayer graphene under mid-infrared pump excitation. By comparing these features with those expected from Volkov states alone, we were able to demonstrate the existence of Floquet–Bloch states. 2. Discovery of transient topological crystalline order in optically driven SnSe [2] — We observed light-induced, linearly dispersive in-gap states near high-symmetry points in thin-film SnSe synthesized by molecular beam epitaxy (MBE). These states could originate from a structural phase transition to the Immm phase which is considered to emerge under light-induced non-equilibrium conditions [3].
[1] D. Choi*, M. Mogi*, et al., Nat. Phys., online (2025).
[2] M. Mogi*, D. Choi*, et al., axXiv:2502.14800 (2025).
[3] Y. Huang, et al., PRX 12, 011029 (2022).
The light-driven control of quantum materials is rapidly evolving into new stages with advances in more stable and powerful lasers. In this talk, we will present two optical driving protocols for guided material control: resonant excitation of collective modes and coherent off-resonant driving. In the first part, we investigate the photoinduced dynamics of the soft magnet Sr3Ir2O7—a system near a quantum critical point and recently proposed as an antiferromagnetic excitonic insulator—to determine its ground state and magnon dynamics. Upon photodoping, the magnon exhibits a dynamic energy blueshift, suggesting a non-thermal pathway for tuning electronic interactions. In the second part, we explore Floquet engineering under periodic driving as a deterministic and reversible approach for controlling correlated materials. We demonstrate how the driven dynamics on sub-cycle timescales—dubbed Floquet micromotion—manifest as a coherent electronic Raman continuum. The micromotion can also be manipulated to selectively break symmetries and interact with magnetic ordering in the low-frequency limit, suggesting new opportunities for micromotion-based material engineering. Broader insights into the light-driven control of correlated materials will also be discussed.
7월 15일 (화)
TBA
The quantum Gibbs state, which describes thermal equilibrium, plays a central role across various fields. In this work, we investigate bipartite quantum correlations in quantum Gibbs states of long-range interacting systems and present an efficient and accurate algorithm for constructing such states. We first identify the optimal condition under which bipartite information measures obey an area law—that is, they scale with the boundary size rather than the volume of subsystems—in systems with power-law decaying interactions [1]. Specifically, we demonstrate that the area law holds when the power-law exponent exceeds (D+1)/2, where D is the spatial dimension. By leveraging the clustering property of noncritical systems, our result extends the validity of the area law beyond the conventional bound of D+1, establishing its applicability even in regimes where thermodynamic extensivity breaks down. We then introduce an algorithm that constructs the matrix product operator of the quantum Gibbs state in one-dimensional long-range interacting systems while maintaining a controlled approximation error [2]. This algorithm is based on the renormalization group framework and ensures precision with a runtime that scales quasi-polynomially with system size.
[1] Donghoon Kim, Tomotaka Kuwahara, and Keiji Saito, Phys. Rev. Lett. 134, 020402 (2025)
[2] Rakesh Achutha, Donghoon Kim, Yusuke Kimura, and Tomotaka Kuwahara, Phys. Rev. Lett. 134, 190404 (2025)
Optical atomic clocks (OACs), utilizing optical transitions in atoms as a timebase, have achieved unprecedented precision and accuracy in scientific measurement, offering new frontiers in metrology and fundamental physics. The precision of state-of-the-art OACs is fundamentally limited by quantum projection noise in measurements on uncorrelated atoms, known as the standard quantum limit (SQL). Increasing atom numbers can suppress this noise statistically but introduces atomic interactions that compromise accuracy. Entanglement between atoms can overcome the SQL, and spin-squeezed states have demonstrated improved precision in various platforms. However, a quantum advantage in entanglement-enhanced clocks surpassing the best precision of conventional OACs has not yet been achieved.
Here, we demonstrate a spin-squeezed optical lattice clock that exceeds the SQL at the 10⁻¹⁸ level. Quantum nondemolition measurements via strong atom-cavity coupling squeeze the collective projection noise of 30,000 atoms by 7.1(1.0) dB. Improved motional control maintains coherence, resulting in a 5.1(1.0) dB metrological gain. A synchronous comparison between two independent spin-squeezed clock ensembles shows a 2.0(2) dB improvement beyond the SQL, reaching a fractional instability of 1.1×10⁻¹⁸. This work marks a milestone toward quantum-enhanced timekeeping and provides a promising platform to explore the interplay between gravity and quantum entanglement.
From high-temperature superconductivity in cuprates [1] to the experimental realization of strongly correlated, exotic physics in twisted heterostructures of graphene and other two-dimensional materials at their monolayer limit, [2,3] breakthroughs in condensed matter physics are often brought upon by the discovery of novel materials. Consequently, the scientific drive towards material discovery has been prevalent throughout the history of this field, as reflected more recently by the massive efforts towards the computational search and prediction of novel materials aided by the rapid progress in computational algorithms. [4,5] However, parallel advancements in experimental synthesis techniques are critically lacking, making the experimental verification and stabilization of theoretically predicted structures the rate-limiting step of the materials discovery process. From this perspective, developing synthesis techniques for novel material stabilization is of paramount importance for realizing further advancements in this field.
In this talk, I will propose that the combination of thin-film epitaxy with ‘soft-chemistry’ (de)intercalation can establish a unique platform conducive to the stabilization of novel materials that are not accessible by direct synthesis. As a prototypical example, I will discuss our work on the stabilization of the infinite-layer nickelates LnNiO2 (Ln = trivalent lanthanide) via metal-hydride-assisted soft-chemistry topotactic removal of oxygen from the perovskite nickelates LnNiO3, and the subsequent discovery of superconductivity in the hole-doped infinite-layer nickelates. [6,7] If time permits, I will also briefly discuss our very recent, preliminary efforts to expand this idea to different synthesis environments for promoting accelerated discovery of novel materials.
1. Bednorz, J. G. & Müller, K A., Z Phys. B Condens. Matter 64, 189–193 (1986).
2. Park, H. et al., Nature 622, 74–79 (2023).
3. Lu, Z. et al., Nature 626, 759–764 (2024).
4. Butler, K. T. et al., Nature 559, 547–555 (2018).
5. Merchant, A. et al., Nature 624, 80–85 (2023).
6. Li, D. et al., Nature 572, 624–627 (2019).
7. Lee, K. et al., Nature 619, 288–292 (2023).
Kagome materials offer a versatile platform for investigating a broad spectrum of emergent quantum phenomena. Their intrinsic geometrical frustration gives rise to electronic instabilities, such as flat bands and van Hove singularities, that are closely linked to various quantum phases including charge density wave (CDW), superconductivity, and quantum spin liquid. A notable recent example is the correlated kagome antiferromagnet FeGe with 2×2 CDW order which is unprecedented among kagome magnets. However, the microscopic mechanism underlying CDW formation in FeGe remains unclear.
To address this, we combined multiple experimental techniques to gain deeper insight into the nature of the CDW in this system. In the first part of this presentation, I will introduce our recent results on CDW in FeGe. Through a combination of angle-resolved photoemission spectroscopy (ARPES) and diffuse scattering studies, we provide a detailed description of the CDW in FeGe. Additionally, we track its evolution under high pressure using X-ray diffraction. In the second part of the presentation, I will present ARPES and diffuse scattering results on the new kagome compounds LuFe6Ge6 and ScNi6Ge6 which crystallize in space group 191. Although these compounds share similar crystal structure based on FeGe, they exhibit distinct characteristics compared to the parent compound FeGe. These findings underscore the "LEGO-like" modularity of kagome systems, where subtle changes in composition or stacking can lead to dramatically different emergent phenomena and open broad and tunable possibilities for future quantum materials research.
Magnons, the quanta of spin waves in magnetic materials, are promising for hybrid quantum systems due to their strong coupling with various excitations, such as microwave photons [1]. We present hybrid magnonic systems using superconducting Niobium Nitride (NbN) circuits and Yttrium Iron Garnet (YIG) spheres and films in an on-chip platform with strong magnon–microwave photon coupling. First, we demonstrate multiple-pulse interference of magnon excitations in a system of two YIG spheres mounted on a Si substrate, separated by 12 mm, and a shared coplanar waveguide (CPW) resonator made of superconducting NbN film [2]. The magnon modes of the two YIG spheres are strongly coupled via dispersive coupling with the microwave photon mode of the NbN resonator, exhibiting a coupling strength of approximately 15 MHz at 5.4 GHz. By injecting multiple microwave pulses and detecting the magnon amplitudes of each YIG sphere using a real-time detection technique [3], we observe coherent magnon exchange and interference between the YIG spheres, including arbitrary control of the hybrid magnonic state by tuning the carrier frequency and time delay [4]. Second, we report on-chip hybrid magnonic systems utilizing YSGG/YIG thin films, which enable high-Q magnon modes at temperatures below 4 K—previously unattainable with conventional GGG/YIG films. The magnon-photon coupling between a 100 nm-thick YIG film and an NbN resonator, integrated using the flip-chip technique, is observed as a clear band anticrossing, with a coupling strength of approximately 60 MHz at 3 GHz, resulting in a cooperativity of around 45. These advancements point to a promising direction for hybrid quantum systems leveraging magnons, which offer novel features such as wide-range frequency tunability and nonreciprocal propagation, while maintaining strong coupling and on-chip integrability. Altogether, these results open up a new field of on-chip quantum magnonics, where magnons can be coherently controlled in the quantum regime.
[1] Y. Li et al., Physical Review Letters 123, 107701 (2019)
[2] Y. Li et al., Physical Review Letters 128, 047701 (2022)
[3] M. Song et al., npj Spintronics 3, 12 (2025)
[4] M. Song et al., Nature Communications 16, 3649 (2025)
Bose–Einstein condensation (BEC), a macroscopic quantum phenomenon of bosons, represents a key platform for realizing coherent bosonic states in the quantum regime. BEC-driven coherent states exhibit superfluid-like properties, such as resistance-free flow against external potentials and behavior governed by a single wavefunction. Despite these advantages, the requirement for ultra-low temperatures of the BEC state has remained a significant barrier to practical applications. In contrast, magnon BECs can be realized at room temperature via parametric excitation, enabled by three-magnon scattering process above a threshold RF power [1, 2]. Moreover, magnon condensates can form in nanoscale magnetic structures, making them promising candidates for integrated spintronic circuits [3].
In this work, we demonstrate coherent magnon transport as a step toward realizing magnon BEC-based logic devices in a ferrimagnetic insulator, yttrium iron garnet (YIG) thin film. By measuring the DC spin pumping voltage at a platinum detector placed opposite a coplanar waveguide (CPW) antenna, we detect propagating magnons generated by microwave excitation. At low excitation power, the system exhibits conventional first-order magnon modes. However, as the RF power increases beyond the condensation threshold, second-order magnons associated with the condensate state emerge. Notably, these higher-order magnons exhibit anomalously longer propagation lengths that remain robust even under varying magnetic fields, in contrast to the primary mode. These findings highlight the potential of condensed magnon states and Bogoliubov excitations for implementing energy-efficient, coherent logic operations [4]. Building on this foundation, we aim to develop magnon BEC-based logic devices as a novel platform for wave-based and quantum-inspired information processing.
[1] A. Serga et al., Nat. Commun. 5, 1-8 (2014)
[2] M. Schneider et al., Nat. Nanotechnol. 15, 457-461 (2014)
[3] M. Schneider et al., Phys. Rev. Lett. 127, 237203 (2021)
[4] D. Bozhko et al., Nat. Commun. 10, 2460 (2019)