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Abstract Pt-Ni octahedral nanocrystals with exposed {111} facets are highly active oxygen reduction reaction (ORR) catalysts, yet achieving precise control over their particle size remains challenging. In this work, three Pt-Ni octahedral nanocatalysts with tunable sizes were synthesized using a one-pot colloidal method by adjusting the amount of NiCl₂·6H₂O precursor. Interestingly, both decreasing and increasing the Ni precursor amount relative to the standard value of 0.05 mmol resulted in smaller octahedra. At low precursor concentrations, limited Ni precursor supply restricts crystal growth, whereas at high concentrations, accelerated nucleation and Cl⁻-induced stabilization of the (111) facets inhibit further growth. Electrochemical measurements show that the catalyst exhibiting the smallest particle size delivers the highest ORR mass activity (1.98 A/mg_Pt), likely due to its large specific surface area. This work highlights the crucial role of precursor concentration in tailoring nanoparticle dimensions and enhancing the electrochemical activity of Pt-Ni catalysts for the ORR. Graphical abstract 

As a type of energy carrier, polaritons can play a dominant role in the thermal conductivity of nano/microstructures. Here, we report an asymmetric thermal conductivity mediated by polaritons that break the Lorentz reciprocity. In contrast to existing approaches that rely on nonlinearity or time modulation, we leverage nonreciprocal polaritons induced by magnetic effects. We show that nonreciprocal surface plasmon polaritons in time-reversal symmetry-breaking systems, including magneto-optical materials and magnetic Weyl semimetals, can alter the symmetry of thermal conductivity in systems like thin films, resulting in direction-dependent thermal conductivity. Thermal conductivities of reciprocal material systems can also be made asymmetric through near-field coupling with these material systems. In accompaniment with the surging interest in nonreciprocal thermal radiation by polaritons, we extend the role of nonreciprocal polaritons from radiative heat transfer systems to conduction systems, paving the way for next-generation thermal devices and efficient energy management solutions. 

Controlling photon-mediated energy flow is central to the future of communications, thermal management, and energy harvesting technologies. Recent breakthroughs have revealed that many-body systems violating Lorentz reciprocity can sustain persistent photon heat current at thermal equilibrium, hinting at a paradigm of heat flow akin to superconductivity. Yet, the behavior of such systems far from equilibrium remains largely unexplored. In this work, we uncover the rich physics of radiative heat transfer in nonequilibrium, far-field many-body systems composed of thermal emitters that break Lorentz reciprocity. We show that the total heat flow naturally decomposes into two distinct components: an equilibrium term, which generates a persistent circulating heat current within the system, and a nonequilibrium term, which governs energy exchange with the environment. Remarkably, while the internal persistent heat current is ever-present, the nonequilibrium contribution can be precisely engineered to achieve perfect heat rectification and circulation. Our results open a route toward designing thermal systems with unprecedented control—unlocking the potential for lossless heat circulation and one-way thermal devices. This fundamentally shifts the landscape for next-generation thermal logic, energy conversion, and photonic heat engines. 

Abstract Electromagnetic hyperbolicity has driven key functionalities in nanophotonics, including super-resolution imaging, efficient energy control, and extreme light manipulation. Central to these advances are hyperbolic polaritons—nanometer-scale light-matter waves—spanning multiple energy-momentum dispersion orders with distinct mode profiles and incrementally high optical momenta. In this work, we report the mode conversion of hyperbolic polaritons across different dispersion orders by breaking the structure symmetry in engineered step-shape van der Waals (vdW) terraces. The mode conversion from the fundamental to high-order hyperbolic polaritons is imaged using scattering-type scanning near-field optical microscopy (s-SNOM) on both hexagonal boron nitride (hBN) and alpha-phase molybdenum trioxide (α-MoO₃) vdW terraces. Our s-SNOM data, augmented with electromagnetics simulations, further demonstrate the alteration of polariton mode conversion by varying the step size of vdW terraces. The mode conversion reported here offers a practical approach toward integrating previously independent different-order hyperbolic polaritons with ultra-high momenta, paving the way for promising applications in nano-optical circuits, sensing, computation, information processing, and super-resolution imaging.