Xian Hui Chen

Since the discovery of charge density wave (CDW) and superconductivity, kagome metal AV 3 Sb 5 ( A = K , Rb, Cs) provides a new platform for exploring novel many-body quantum phenomena. In CsV 3 Sb 5 , a stripelike CDW with commensurate wave vector q = 3 / 8 was observed under moderate pressures, which leads to a peculiar superconducting double-dome behavior in pressure-dependent phase diagram. Previous density functional theory (DFT) calculations indicate that the pressure-induced stripelike CDW is beyond conventional phonon softening scenario, suggesting a nontrivial role of electronic correlations. However, an in-depth understanding for the pressure-induced unconventional CDW remains elusive. Here, we performed pressure-dependent V 51 nuclear magnetic resonance (NMR) measurements on KV 3 Sb 5 and RbV 3 Sb 5 . Although the superconducting double-dome behavior is absent in pressurized KV 3 Sb 5 and RbV 3 Sb 5 , a pressure-induced CDW phase, ascribed to a possible incommensurate triple- Q CDW, is identified by NMR spectra in both materials, indicating that the pressure-induced unconventional CDW beyond DFT calculations is a common feature for kagome metal AV 3 Sb 5 . In contrast to the stripelike CDW, the pressure-induced incommensurate triple- Q CDW does not strongly suppress the superconducting temperature ( T c ) but coincides with an almost plateau behavior at intermediate pressure regime in the pressure-dependent superconducting phase diagram. Furthermore, by systematically analyzing the Korringa relation between Knight shift and nuclear spin-lattice relaxation rate in AV 3 Sb 5 , Van Hove singularity-driven electronic fluctuations are revealed as an effective knob for the pressure-induced unconventional CDW. Finally, our present findings underscore the pressure-induced unconventional CDW as a novel correlated quantum state in kagome metal AV 3 Sb 5 .

Intriguing many-body ground states often emerge from quantum Hall systems, driven by electron–electron interactions. Among them, states breaking translational and rotational symmetries─manifested by anisotropic magnetotransport─can be stabilized by intrinsic band anisotropy. Here, we fabricate few-layer black phosphorus devices hosting a high-mobility two-dimensional hole gas with large effective-mass anisotropy. These exceptional properties enable us to observe pronounced anisotropic transport in the quantum Hall regime. Unlike conventional anisotropic stripe states, these emergent states remain robust up to T ≈ 4 K, and their corresponding partial filling factors (ṽ) deviate from the conventional value of 1/2: ṽ approaches 1/4 in the N = 2 Landau level and increases to approximately 0.4 with increasing Landau-level index. Supported by theoretical calculations, our results point to the emergence of anisotropic quantum Hall states consistent with stripe-like order. Our work establishes few-layer black phosphorus as a unique platform for exploring emergent quantum phenomena arising from anisotropic correlated electronic states.

Kagome superconductors AV3Sb5 (A = K, Rb, Cs) exhibit intertwined symmetry-breaking orders, including charge density wave (CDW), superconductivity, and time-reversal symmetry breaking (TRSB), yet their relationship, nature, and experimental detections remain under intense debate. At the heart of these controversies lies a critical yet often overlooked factor: the surface effect, which may engineer distinct technique-dependent observations. By combining density functional theory calculations and ARPES measurements, we unveil the important role of surface effect on the CDW instability and electronic reconstruction in CsV3Sb5. The surface CDW instability is much reduced on the Cs and Sb terminations but well-preserved on half-Cs termination. On the Cs-terminated surface, the interlayer expansion induces quantum confinement and two-dimensional quantum well states. These results explain the diversity of surface band structures observed in AV3Sb5, potentially reconcile conflicting observations of superconductivity and TRSB between bulk- and surface-sensitive probes, and offer a new perspective on studying exotic phenomena in AV3Sb5.

Mott physics lies at the heart of many intriguing quantum phenomena. Recently, van der Waals materials emerge as a powerful platform to explore these correlated electronic states, yet tracking how such a Mott state develops and interacts with neighboring phases remains an experimental challenge. Here we use scanning tunnelling spectroscopy to study a filling-controlled Mott state and its interplay with superconductivity in 4Hb-TaS₂, which interleaves strongly correlated 1T-TaS₂ layers and superconducting 1H-TaS₂ layers. By resolving the electronic spectral function across different filling levels driven by interlayer charge transfer, we quantitatively capture the continuous evolution of the Mott state, characterized by the emergence of Mott-Hubbard bands and the systematic suppression of the central quasiparticle peak. Moreover, while the 1H layers exhibit a uniform, fully opened superconducting gap, the heavily correlated electrons in the 1T layers act destructively against pairing, creating nanoscale, non-superconducting paramagnetic puddles. Our findings provide a quintessential visualization of the iconic Hubbard model and establish layered heterostructures as an ideal playground for designing exotic correlated quantum phases.

Van der Waals layered transition metal thiophosphate CuInP₂S₆ has attracted significant research interest due to its intriguing ferroelectric and optoelectronic properties. In this work, by combining high-pressure Raman spectroscopy, optical absorption measurements, and ab initio calculations, we uncover a cascade of pressure-induced phase transitions in CuInP₂S₆. Remarkably, electrical transport measurements reveal the emergence of superconductivity above 58 GPa. The superconducting transition temperature T_c increases progressively with pressure, reaching 15 K at 215 GPa─the highest T_c reported to date among the metal phosphorus trichalcogenide (MPX₃) family. We attribute the origin of this high T_c in CuInP₂S₆ to a unique pairing mechanism characterized by dynamic carrier exchange between flat bands and highly dispersive bands dominated by sulfur vibrations. Our findings enrich the understanding of layered MPX₃ materials and highlight pressure as a powerful tool for exploring high-T_c superconductivity in sulfur-based systems.

Quantum fluctuations endow spin systems with surprisingly enriched magnetic phase diagrams. In frustrated magnets, strong quantum fluctuations boosted by either geometrical incompatibility or competitive exchange interactions stabilize cascades of unusual phases of matter. Here we reveal the presence of multiple quantum phases in the honeycomb antiferromagnet Na₃Ni₂BiO₆, both inside and beyond its field-induced one-third magnetization plateau. Comprehensive measurements of thermodynamic quantities demonstrate that the one-third plateau comprises at least three distinct spin states with nearly-degenerate net magnetization, separated by first-order transitions that likely involve sequential spin reconfiguration. Upon further increasing the magnetic field, the system evolves across a myriad of peculiar phases before reaching full polarization; these intermediate phases possess copious low-energy excitations, manifested by anomalous upturns of specific heat at ultralow temperatures — probably hinting at the development of “hidden” ordered ground states. The complex magnetic phase diagram of Na₃Ni₂BiO₆ underlines the preponderant impact of quantum fluctuations on a honeycomb spin lattice with strong exchange frustration.