Xiaolong Chen

The spatial modulation of electron density into a wave-like pattern, known as charge density wave (CDW), represents a fundamental quantum state that often coexists with superconductivity, quantum Hall states and axion insulating phases. Conventional CDWs are mediated by longitudinal acoustic phonons, exhibit picometer-scale lattice distortions (10⁻¹²−10⁻¹¹m), and typically vanish upon approaching the atomic limit. Here, we report a series of anomalous CDW behaviors in the 2D superatomic superconductor Au₆Te₁₂Se₈. Remarkably, its CDW is governed by transverse phonons and exhibits a real-space displacement of ~ 4 Ångström, which is an order of magnitude larger than that in conventional CDW. Furthermore, we observe a dimensional response persisting up to micrometer-scale thickness, a regime where other materials are already considered as bulk. Through liquid helium-temperature transmission electron microscopy, ultrafast pump-probe spectroscopy and transport measurements, we demonstrate a prominent enhancement of the CDW transition temperature (T_CDW) from < 2 K in the bulk to 110 K upon approaching the “superatomic limit”. Our findings not only reveal anomalous facets of both CDW and superatomic materials, but the competition between this anomalous CDW and superconductivity opens avenues for exploring unconventional electron-phonon interactions.

The charge density wave (CDW) transition in the PrSb2 compound has been systematically investigated through variable-temperature Raman scattering, x-ray diffraction, and electrical resistivity measurements. A non-monotonic anomaly in resistivity is observed near 100 K. Below this temperature, an amplitude mode appears in Raman spectra—a well-established fingerprint of CDW order—that redshifts and weakens upon warming. X-ray diffraction reveals a change in lattice parameter slope at 100 K, evidencing electron-lattice coupling and confirming CDW instability without structural phase transition. Notably, no anomalies were found in magnetization or specific heat near 100 K. Furthermore, at T = 1.8 K and with H ⊥ c axis, a series of fractional magnetization plateaus are observed, resembling the “devil's staircase.” These findings show that PrSb2 hosts coexisting CDW instability and antiferromagnetic order with strong magneto-transport responses, making it a rare platform for studying quantum order interactions in correlated electron systems.

The Boson peak (BP)─an excess of low-frequency vibrational states─is long regarded as a definitive signature of structural disorder. Its recent observation in nominally ordered crystals challenges this view, yet a conclusive understanding is impeded by complexities such as phase transitions, orientational glassiness, dipole disorder, or strong anharmonicity in previous model systems. Here, we introduce the layered organic–inorganic hybrids DAPCdX₄ (DAP = 1,3-diaminopropane; X = Cl, Br) as a clean platform free of phase transitions, glassiness, and any form of disorder. Low-temperature specific heat reveals a pronounced boson-peak-like anomaly with tunneling-state contributions down to 0.5 K. Crucially, the anomaly’s temperature and intensity are directly modulated by hydrogen-bond strength, tuned via halide substitution (Cl → Br). Combined Raman spectroscopy and first-principles calculations trace its origin to an excess of low-frequency vibrations in the inorganic framework, induced by hydrogen-bond-mediated coupling between low-energy optical and acoustic phonons. Our work demonstrates that the boson peak can emerge intrinsically in a harmonic crystal lattice, providing a definitive structure–property relationship for controlling ∼1 THz vibrations via noncovalent interactions.

Superatomic compounds, composed of atomic clusters interwoven by weak chemical bonds exhibit large anharmonicity vibrations, are excellent candidates for ultralow thermal conductivity (κ) materials. However, growing bulk superatomic single crystals is challenging due to complex chemical composition and chemical bonds, and studies on their intrinsic thermal property are scarce. Here, we grew high-quality superatomic single crystals of Re6Se8Te7 and Re6Te15, both of which are narrow-bandgap semiconductors that change into metals under external physical pressure. At room temperature, κ is 0.32 ± 0.02 and 0.53 ± 0.02 W m−1 K−1 in Re6Se8Te7 and Re6Te15, respectively, ranking among the lowest values reported in all-inorganic bulk crystals. It is mainly attributed to the large Grüneisen parameter (1.93) and low average sound speed (<1482 m/s), which are due to soft Te7 nets weakly embedded among the rigid Re6Se8 (Re6Te8) quasi-cubic clusters. The appearance of boson peak, i.e., a hump of C(T)/T3, verifies the existence of disordered phonon transports. In addition, the temperature dependence of κ can be described by the classic Debye–Callaway model. Notably, above 350 K, the κ values of Re6Se8Te7 and Re6Te15 are remarkably close to the upper limit derived from a glassy-like diffusion model. This finding sets the superatomic compounds as a promising family for searching ultralow-κ and energy management materials.

Binary stoichiometry FeSe (s-FeSe) is a well-known parent of high-temperature unconventional superconductors owing to its charge-neutral layer, highly tunable structure and electronic properties, and rich interplays among multiple electronic phases. Yet the s-FeSe synthesized via high-temperature equilibrium reactions bears the notorious interstitial Fe,where merely 3% of them is sufficient to kill the superconductivity. Here, we successfully synthesized a new non-stoichiometric Fe1.11Se single crystal with a superconducting onset temperature (Tconset) of 30.4 K through a hydrothermal ion-exchange and de-intercalation route. 11% interstitial Fe ions exceed the equilibrium phase diagram limit. Intriguingly, under physical pressure, the Tconset of exhibits a "V"-shaped evolution with a minimum at 2-2.6 GPa, and then upturning into a second superconducting region, reminiscent of the behaviors in FeSe intercalates. Furthermore, a pressure-induced possible magnetic order, previously only observed in pressurized s-FeSe, shows up. These results offer fresh insights into the role of interstitial Fe in governing superconducting and transport properties under non-equilibrium synthesis and tuning strategies.

In layered transition metal dichalcogenides such as TaSe ₂ , Cu intercalation enhances superconductivity and suppresses charge density wave order by electron doping, demonstrating an effective strategy for tuning competing quantum phases.