Ultralow-Frequency Epsilon-Near-Zero States in 3D-Printed High-Entropy Alloy Metacomposites for Ultra-Thin Perfect RF Absorption.

Epsilon-near-zero (ENZ) materials with radio-frequency perfect absorption are pivotal for next-generation electromagnetic stealth, 5G (fifth-generation mobile networks) signal integrity, and IoT (internet of things) security. Here, 3D-printed metacomposites achieving low-frequency ultra-thin ENZ absorption (>90%, 55-110 MHz, d/△λ<1/2455) are realized by confining high-entropy alloy (HEA) nanoparticles within hierarchically ordered porous carbon (HOPC). This hierarchical design leverages HEA's flattened band structures to maximize electron effective mass, while interfacial electron redistribution at HEA-carbon boundaries delocalizes charges and reduces carrier concentration. These dual effects synergistically suppress plasma frequency to 72.4 MHz, converting strong negative permittivity into near-zero states. A cocktail effect is discovered for reducing the plasma frequency with increasing the entropy. Concurrently, resonant enhancement from three complementary mechanisms-surface plasmons at HEA@graphitic core-shell interfaces, interfacial polarization in PU/HOPC heterojunctions, and hierarchical pore-cavity modes-boosts positive permittivity. Engineered cancellation of weakened negative permittivity and reinforced positive permittivity enables an ultra-broadband |ε'|<1 response spanning 55-110 MHz. The ENZ-mode perfect absorption of ultra-thin thickness, ultralow frequency, angle robustness, and broad band is achieved eventually. This work establishes a new paradigm for breaking the Rozanov limit via material-genesis ENZ engineering, bypassing artificial metamaterial arrays.