Researchers at Columbia University have successfully synthesized the first 2D heavy fermion material. They introduce the new material, a layered intermetallic crystal composed of cerium, silicon, and iodine (CeSiI), in a research article published in Nature.

Heavy fermion compounds are a class of materials with electrons that are up to 1,000 times heavier than usual. In these materials, electrons get tangled up with magnetic spins that slow them down and increase their effective mass. Such interactions are thought to play important roles in a number of enigmatic quantum phenomena, including superconductivity, the movement of electrical current with zero resistance.

Researchers have been exploring heavy fermions for decades, but in the form of bulky, 3D crystals. The new material synthesized by Ph.D. student Victoria Posey in the lab of Columbia chemist Xavier Roy will allow researchers to drop a dimension.

We study the competing effects of collective generalized measurements and interaction-induced scrambling in the dynamics of an ensemble of spin-1/2 particles at the level of quantum trajectories. This setup can be considered as analogous to the one leading to measurement-induced transitions in quantum circuits. We show that the interplay between collective unitary dynamics and measurements leads to three regimes of the average Quantum Fisher Information (QFI), which is a witness of multipartite entanglement, as a function of the monitoring strength. While both weak and strong measurements lead to extensive QFI density (i.e., individual quantum trajectories yield states displaying Heisenberg scaling), an intermediate regime of classical-like states emerges for all system sizes where the measurement effectively competes with the scrambling dynamics and precludes the development of quantum correlations, leading to sub-Heisenberg-limited states. We characterize these regimes and the crossovers between them using numerical and analytical tools, and discuss the connections between our findings, entanglement phases in monitored many-body systems, and the quantum-to-classical transition.

While interactions within a many-body quantum system tend to generate highly correlated states, performing local measurements will typically tend to disentangle the different subsystems. When combined, the interplay between these two effects often lead to measurement-induced transitions, which separate two distinct stable phases: one interaction-driven, where entanglement is high, and another measurement-driven, where entanglement is low. However, different types of measurements can lead to other scenarios, and often also generate entanglement themselves. In this work we study quantum many-body systems where both interactions and measurements take place collectively and thus generate a high degree of entanglement if acting separately. We show that nontrivial competition between these two actors emerges, leading to configurations with very low entanglement.

In 1960, Luttinger proposed a universal principle connecting the total capacity of a system for particles with its response to low-energy excitations. Although easily confirmed in systems with independent particles, this theorem remains applicable in correlated quantum systems characterized by intense inter-particle interactions.

However, and quite surprisingly, Luttinger’s theorem has been shown to fail in very specific and exotic instances of strongly correlated phases of matter. The failure of Luttinger’s theorem and its consequences on the behavior of quantum matter are at the core of intense research in condensed matter physics.