Xun Shi

Light–matter interactions in frustrated Kagome metals provide a platform for investigating hidden quantum states, while the microscopic origin of non-equilibrium symmetry breaking remains under discussion. Here, spin–lattice coupling in RbV₃Sb₅ is found to induce rotational and time-reversal symmetry breaking through enhancement of a single-Q_M phonon mode. The resulting anisotropic lattice distortion lifts geometric frustration and stabilizes a non-equilibrium ferrimagnetic phase accompanied by an intrinsic anomalous Hall effect. The calculations further indicate that coherent phonon excitation can modulate symmetry under strong optical fields without requiring orbital antiferromagnetism or extrinsic perturbations. The induced spin polarization, together with spin–orbit coupling, generates finite Berry curvature in momentum space and modifies the topological electronic structure. These results provide insight into the interplay among spin, lattice, and charge degrees of freedom in non-equilibrium correlated states of Kagome materials and may be relevant for optically controlled spintronic functionalities.

Nonvolatile control of the Mott transition is a central goal in correlated-electron physics, offering access to fascinating emergent states and great potential for technological applications. Compared to chemical or mechanical approaches, ultrafast optical excitation further promises a path to create and manipulate novel non-equilibrium phases with ultimate spatiotemporal precision. However, achieving a truly nonvolatile electronic phase transition in laser-excited Mott systems remains an elusive challenge. Here, we present a highly robust and reversible method for optical control of the Mott state in van der Waals systems. Specifically, using angle-resolved photoemission spectroscopy, we observe a nonvolatile Mott-to-metallic transition in the ultrafast laser-excited charge density wave (CDW) material 1T-TaSe2. Complementary theoretical calculations reveal that this transition originates from a rearrangement of the interlayer CDW stacking. This new stacking order, formed following the ultrafast quenching of the CDW, circumvents the need for large-scale atomic sliding. Intriguingly, it introduces a significant in-plane component to the electron hopping and effectively reduces the ratio of on-site Coulomb interaction to bandwidth, thereby suppressing the Mott state and stabilizing a metallic phase. Our results establish optical-control of interlayer stacking as a versatile strategy for inducing nonvolatile phase transitions, opening a new route to tailor correlated electronic phases and realize reconfigurable high-frequency devices.

In the recently discovered kagome superconductor A V ₃ Sb ₅ ( A = K, Rb, and Cs), superconductivity is intertwined with an unconventional charge density wave order. The pairing symmetry remains elusive owing to the lack of direct measurement of the superconducting gap in the momentum space. Here, utilizing laser-based ultra-high-resolution and low-temperature angle-resolved photoemission spectroscopy, we observe Fermi-surface-dependent anisotropic Cooper pairing in kagome superconductor CsV ₃ Sb ₅ . We detect a highly anisotropic superconducting gap structure with anisotropy exceeding 80% and a gap maximum along the V-V bond direction on a Fermi surface originating from the 3 d -orbital electrons of the V kagome lattice. This is in stark contrast to the isotropic superconducting gap structure on the Fermi surface occupied by Sb 5 p -orbital electrons. Our direct observation of the Fermi-surface-dependent anisotropic pairing in CsV ₃ Sb ₅ is fundamental for understanding the intertwined orders in the ground state of kagome superconductors.

Nonvolatile optical manipulation of material properties on demand is a highly sought-after feature in the advancement of future optoelectronic applications. While the discovery of such metastable transition in various materials holds good promise for achieving this goal, their practical implementation is still in the nascent stage. Here, we unravel the nature of the ultrafast laser-induced hidden state in 1T-TaS2 by systematically characterizing the electronic structure evolution throughout the reversible transition cycle. We identify it as a mixed-stacking state involving two similarly low-energy interlayer orders, which is manifested as the charge density wave phase disruption. Furthermore, our comparative experiments utilizing the single-pulse writing, pulse-train erasing and pulse-pair control explicitly reveal the distinct mechanism of the bidirectional transformations -- the ultrafast formation of the hidden state is initiated by a coherent phonon which triggers a competition of interlayer stacking orders, while its recovery to the initial state is governed by the progressive domain coarsening. Our work highlights the deterministic role of the competing interlayer orders in the nonvolatile phase transition in the layered material 1T-TaS2, and promises the coherent control of the phase transition and switching speed. More importantly, these results establish all-optical engineering of stacking orders in low-dimensional materials as a viable strategy for achieving desirable nonvolatile electronic devices.

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order and lattice geometry^{1,2,3,4,5,6,7,8,9}. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive^{10,11,12,13,14,15,16,17}. In particular, consensus on the electron pairing symmetry has not been achieved so far^18,19,20, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic and orbital-independent superconducting gap in the momentum space of two exemplary CsV₃Sb₅-derived kagome superconductors—Cs(V_0.93Nb_0.07)₃Sb₅ and Cs(V_0.86Ta_0.14)₃Sb₅—using ultrahigh-resolution and low-temperature angle-resolved photoemission spectroscopy. Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Our comprehensive characterizations of the superconducting gap provide indispensable information on the electron pairing symmetry of kagome superconductors, and advance our understanding of the superconductivity and intertwined electronic orders in quantum materials.

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic, and orbital-independent superconducting gap in the momentum space of two exemplary CsV$_3$Sb$_5$-derived kagome superconductors -- Cs(V$_{0.93}$Nb$_{0.07}$)$_3$Sb$_5$ and Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking CDW in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.

The newly discovered kagome superconductors represent a promising platform for investigating the quantum interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic and orbital-independent superconducting gap in the momentum space of two exemplary CsV3Sb5-derived kagome superconductors — Cs(V0.93Nb0.07)3Sb5 and Cs(V0.86Ta0.14)3Sb5, using ultrahigh resolution and low temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, we find that such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, the scaling ratio of gap versus superconducting transition temperature is close to the Bardeen-Cooper-Schrieffer (BCS) value, indicating a pairing in the weak-coupling regime. Our direct observation of the superconducting gap structure points to a persistent s-wave pairing in such kagome superconductors.

Charge density wave (CDW) order is an emergent quantum phase that is characterized by periodic lattice distortion and charge density modulation, often present near superconducting transitions. Here, we uncover a novel inverted CDW state by using a femtosecond laser to coherently reverse the star-of-David lattice distortion in 1T-TaSe2. We track the signature of this novel CDW state using time- and angle-resolved photoemission spectroscopy and the time-dependent density functional theory to validate that it is associated with a unique lattice and charge arrangement never before realized. The dynamic electronic structure further reveals its novel properties that are characterized by an increased density of states near the Fermi level, high metallicity, and altered electron–phonon couplings. Our results demonstrate how ultrafast lasers can be used to create unique states in materials by manipulating charge-lattice orders and couplings.