Kang Wang

Abstract:

First-principles computational methods can predict the superconducting critical temperature T c of conventional superconductors through the electron-phonon spectral function. Full convergence of this quantity requires Brillouin-zone integration on very dense grids, presenting a bottleneck to high-throughput screening for high-T c systems. In this work, we show that an electron-phonon spectral function calculated at low cost on a coarse grid yields accurate T c predictions, provided the function is rescaled to correct for the inaccurate value of the density of states at the Fermi energy on coarser grids. Compared to standard approaches, the method converges rapidly and improves the accuracy of predictions for systems with sharp features in the density of states. This approach can be directly integrated into existing materials screening workflows, enabling the rapid identification of promising candidates that might otherwise be overlooked.

Abstract:

Identifying high-temperature unconventional charge order and superconductivity in kagome systems is crucial for understanding frustrated, correlated electrons and enabling future quantum technologies. Here, we report that the kagome superconductor YRu₃Si₂ hosts an exceptional interplay of charge order, magnetism, and superconductivity, revealed through a comprehensive suite of muon spin rotation (μSR), magnetotransport, X-ray diffraction, and density functional theory (DFT). We identify a high-temperature charge-ordered state with propagation vector (1/2,0,0) and a record onset temperature of 800 K, unprecedented in kagome systems and quantum materials more broadly. μSR measurements further reveal time-reversal symmetry-breaking below 25 K and field-induced magnetism near 90 K, features mirrored in the magnetoresistance, which reaches 45% at low temperatures. Band-structure calculations show two van Hove singularities near the Fermi level, including one within a flat band. At low temperatures, YRu₃Si₂ becomes superconducting below T_c = 3.4 K with either two full isotropic gaps or an anisotropic nodeless gap. These results establish YRu₃Si₂ as a prime platform for studying correlated kagome physics.

Abstract:

The high-temperature phase of the hexagonal halide perovskite KNiCl₃ is investigated using time-of-flight single crystal neutron diffraction at 633 K (360 °C). Phonons are captured through thermal diffuse scattering, integrated in energy but resolved in momentum. Harmonic phonon calculations based on density functional theory yield imaginary phonon frequencies for this phase, indicating the presence of structural instabilities at this level of theory. It is shown that the inclusion of anharmonic phonon-phonon interactions removes these instabilities, leading to good qualitative agreement with the experimental diffuse scattering. These results demonstrate that the high-temperature phase of KNiCl₃ is stabilized by anharmonic phonon-phonon interactions.

Abstract:

Understanding competing charge density wave (CDW) orders in the bilayer kagome metal ScV₆Sn₆ remains challenging. Experimentally, upon cooling, short-range order with wave vector q 2 = ( 1 3 , 1 3 , 1 2 ) forms, which is subsequently suppressed by the condensation of long-range q 3 = ( 1 3 , 1 3 , 1 3 ) CDW order at lower temperature. Theoretically, however, the q₂ CDW is predicted as the ground state, leaving the CDW mechanism elusive. Here, using anharmonic phonon-phonon calculations combined with density functional theory, we predict a temperature-driven structural phase transitions from the high-temperature pristine phase to the q₂ CDW, followed by the low-temperature q₃ CDW, explaining experimental observations. We demonstrate that semi-core electron states stabilize the q₃ CDW over the q₂ CDW. Furthermore, we find that the out-of-plane lattice parameter controls the competing CDWs, motivating us to propose compressive bi-axial strain as an experimental protocol to stabilize the q₂ CDW. Finally, we suggest Ge or Pb doping at the Sn site as another potential avenue to control CDW instabilities. Our work provides a full theory of CDWs in ScV₆Sn₆, rationalizing experimental observations and resolving earlier discrepancies between theory and experiment.

Abstract:

Ultrathin bismuth exhibits rich physics including strong spin-orbit coupling, ferroelectricity, nontrivial topology, and light-induced structural dynamics. We use ab initio calculations to show that light can induce structural transitions to four transient phases in bismuth monolayers. These light-induced phases exhibit nontrivial topological character, which we illustrate using the recently introduced concept of spin bands and spin-resolved Wilson loops. Specifically, we find that the topology changes via the closing of the electron and spin band gaps during photoinduced structural phase transitions, leading to distinct edge states. Our study provides strategies to tailor electronic and spin topology via ultrafast control of photoexcited carriers and associated structural dynamics.