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The existence of quantum tricriticality and exotic phases are found in a tricritical Dicke triangle (TDT) where three cavities, each one containing an ensemble of three-level atoms, are connected to each other through the action of an artificial magnetic field. The conventional superradiant phase (SR) is connected to the normal phase through first- and second-order boundaries, with tricritical points located at the intersection of such boundaries. Apart from the SR phase, a chiral superradiant (CSR) phase is found by tuning the artificial magnetic field. This phase is characterized by a nonzero photon current and its boundary presents chiral tricritical points (CTCPs). Through the study of different critical exponents, we are able to differentiate the universality class of the CTCP and TCP from that of second-order critical points, as well as find distinctive critical behavior among the two different superradiant phases. The TDT can be implemented in various systems, including atoms in optical cavities as well as the circuit QED system, allowing the exploration of a great variety of critical manifolds.

 

Single-entity electrochemistry is of fundamental importance and shows promise for ultrasensitive biosensing applications. Recently, we have demonstrated that various charged nanoparticles can be detected individually based on the non-redox open-circuit potential (OCP) changes induced by their collision events on a floating carbon nanoelectrode (CNE). Unlike the widely used amperometry approach, the potentiometric method provides the label-free detection of individual nanoscale entities without redox mediators in the solution. However, the CNE lacks specificity for molecular recognition during the collision events because of the limited methods of surface functionalization for carbon surfaces. Herein, we used surface-functionalized gold nanoelectrode (GNE) to overcome this limitation of CNE. The GNE modified with Raman reporter molecule also enabled surface-enhanced Raman spectroscopy (SERS) measurements. By using simultaneous time-resolved OCP and SERS measurements, both the OCP and SERS signals induced by the “hit-n-run” type of gold nanoparticle (GNP) collision events can be better understood. Also, by introducing a zwitterionic molecule, we formed near “stealth” surface and demonstrated that the non-specific adsorptions of GNPs to the surface of GNE have been suppressed, allowing continuous detection of hit-n-run events for over 30 min. 

Plasmonic metal nanostructures are essential for plasmon‐mediated chemical reactions (PMCRs) and surface‐enhanced Raman spectroscopy (SERS). The nanostructures are commonly made from the coinage metals gold and silver. Copper (Cu) is less used mainly due to the difficulties in fabricating stable nanostructures. However, Cu is an attractive option with its strong plasmonic properties, high catalytic activities, and relatively cheap price. Herein, we fabricated tunable, reliable, and efficient Cu nanoelectrodes (CuNEs). Using time‐resolved electrochemical SERS, we have comprehensively studied the reversible chemical transformations between aromatic amine and nitro groups modified on the CuNE surface. Their PMCRs are well‐controlled by changing the surface roughness, the oxidation states of Cu, and the applied electrode potential. We thus demonstrate that the Cu nanostructures enable better investigations in the interplays between PMCR, electrochemistry, and Cu catalysis.