Search for: All records

Kagome metals have emerged as a frontier in condensed matter physics due to their potential to host exotic quantum states. Among these, CsV₃Sb₅has attracted significant attention for the unusual coexistence of charge density wave (CDW) order and unconventional superconductivity, presenting an ideal system for exploring the emergent phenomena from the interplay of phonons, electronic fluctuations, and topological effects. The nature of CDW formation in CsV₃Sb₅is unconventional and has sparked considerable debate. In this study, we examine the origin of the CDW state via ab initio finite-temperature simulations of the lattice dynamics. Through a comparative study of CsV₃Sb₅and 2H-NbSe₂, we demonstrate that the experimental absence of phonon softening—a hallmark of conventional CDW transition—in CsV₃Sb₅along with the presence of a weakly first-order transition, can be attributed to quantum zero-point atomic motion. This zero-point motion smears the free energy landscape of CDW, effectively stabilizing the pristine structure even below the CDW transition temperature. We argue that this surprising behavior could cause coexistence of pristine and CDW structures across the transition and lead to a weak first-order transition. Our predicted lattice dynamical behavior is supported by coherent phonon spectroscopy in single-crystalline CsV₃Sb₅. Our results provide crucial insights into the formation mechanism of CDW materials that exhibit little to no phonon softening, including cuprates, and highlight the surprising role of quantum effects in emergent properties of relatively heavy-element materials like CsV₃Sb₅. 

Abstract The class ofAV₃Sb₅(A=K, Rb, Cs) kagome metals hosts unconventional charge density wave states seemingly intertwined with their low temperature superconducting phases. The nature of the coupling between these two states and the potential presence of nearby, competing charge instabilities however remain open questions. This phenomenology is strikingly highlighted by the formation of two ‘domes’ in the superconducting transition temperature upon hole-doping CsV₃Sb₅. Here we track the evolution of charge correlations upon the suppression of long-range charge density wave order in the first dome and into the second of the hole-doped kagome superconductor CsV₃Sb_5−xSn_x. Initially, hole-doping drives interlayer charge correlations to become short-ranged with their periodicity diminished along the interlayer direction. Beyond the peak of the first superconducting dome, the parent charge density wave state vanishes and incommensurate, quasi-1D charge correlations are stabilized in its place. These competing, unidirectional charge correlations demonstrate an inherent electronic rotational symmetry breaking in CsV₃Sb₅, and reveal a complex landscape of charge correlations within its electronic phase diagram. Our data suggest an inherent 2k_fcharge instability and competing charge orders in theAV₃Sb₅class of kagome superconductors. 

Abstract Vacancy‐ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of Ru^IVhalides are presented;A₂RuCl₆andA₂RuBr₆, whereAis K, NH₄, Rb or Cs. We show that the optical properties and spin‐orbit coupling (SOC) behavior can be tuned through changing theAcation and the halide. Within a series, the energy of the ligand‐to‐metal charge transfer increases as the unit cell expands with the largerAcation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non‐magnetic ground state withJ_eff=0 caused by SOC. Ru‐Xcovalency, and consequently, the delocalization of metald‐electrons, result in systematic trends of the SOC constants due to variations in theAcation and the halide anion.