From next generation gas turbines to scavenging waste heat from car exhausts, finding new materials with ultra-low thermal conductivity ($κ$) has the potential to lead to large gains in device efficiency. Crystal structures with inherently low $κ$ are consequently desirable, but candidate materials are rare and often difficult to make. Using first principles calculations and inelastic neutron scattering we have studied the pyrochlore La$_2$Zr$_2$O$_7$ which has been proposed as a next generation thermal barrier. We find that there is a highly anharmonic, approximately flat, vibrational mode associated with the kagome planes. Our results suggest that this mode is responsible for the low thermal conductivity observed in the pyrochlores and that kagome compounds will be a fruitful place to search for other low $κ$ materials.
Recently, a novel material with bilayer kagome lattice Ca$_{10}$Cr$_7$O$_{28}$ was proposed to be a gapless quantum spin liquid, due to the lack of long-range magnetic order and the observation of broad diffuse excitations. Here, we present the ultralow-temperature thermal conductivity measurements on single crystals of Ca$_{10}$Cr$_7$O$_{28}$ to detect its low-lying magnetic excitations. At finite temperatures, with increasing the magnetic fields, the thermal conductivity exhibits a clear dip around 6 T, which may correspond to a crossover in the magnetic ground state. At the zero-temperature…
Since the discovery of graphene, intensive efforts have been made in search of novel two-dimensional (2D) materials. Decreasing the materials dimensionality to their ultimate thinness is a promising route to unveil new physical phenomena, and potentially improve the performance of devices. Among recent 2D materials, analogs of graphene, the group IV elements have attracted much attention for their unexpected and tunable physical properties. Depending on the growth conditions and substrates, several structures of silicene, germanene, and stanene can be formed. Here, we report the synthesis of a…
Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion co…
Ultralow-Tensile Strain Enables Exciton Funneling and Energy Transfer to Boost MoSe2 Photoluminescence Quantum Yield
Faculty of Physics, University of Warsaw（华沙大学物理学院）；SPEC, CEA, CNRS, Université Paris-Saclay（法国原子能和替代能源委员会、国家科学研究中心、巴黎萨克雷大学SPEC实验室）；Université Paris-Saclay, ONERA, CNRS, Laboratoire d’étude des microstructures (LEM)（巴黎萨克雷大学、ONERA、国家科学研究中心微结构研究实验室）；School of Basic Sciences, Indian Institute of Technology Bhubaneswar（布巴内斯瓦尔印度理工学院基础科学学院）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（物质材料研究机构材料纳米架构研究中心）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（物质材料研究机构电子与光学材料研究中心）
The interesting properties of Kagome bands, consisting of Dirac bands and a flat band, have attracted extensive attention. However, the materials with only one Kagome band around the Fermi level cannot possess physical properties of Dirac fermions and strong correlated fermions simultaneously. Here, we propose a new type of band structure --- double Kagome bands, which can realize coexistence of the two kinds of fermions. Moreover, the new band structure is found to exist in a new two-dimensional material, phosphorus carbide P2C3. The carbide material shows good stability and unusual electroni…
Effect of superconductivity by Nb and V substitution in kagome CaPd5
Henan Normal University（河南师范大学）；Suzhou University of Science and Technology（苏州科技大学）；East China Normal University（华东师范大学）；Jinan University（暨南大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）
We present the results of first-principle calculations using the Vienna Ab-initio Simulation Package (VASP) for a new class of organometallics labeled TM3C6O6 (TM =Sc, Ti, V, Cr, Fe, Co, Ni and Cu) in the form of planar, two-dimensional, periodic free-standing layers. These materials, which can be produced by on-surface coordination on metallic surfaces, have a kagome lattice of TM ions. Calculating the structural properties, we show that all considered materials have local magnetic moments in the ground state, but four of them (with Fe, Co, Ni and Cu) show spin-crossover behavior by changing …
Analogous to the Hall effect, the Nernst effect is the generation of a transverse voltage due to a temperature gradient in the presence of a perpendicular magnetic field. The Nernst effect has promise for thermoelectric applications and as a probe of electronic structure. In magnetic materials, a so-called anomalous Nernst effect (ANE) is possible in zero magnetic field. Here we report a colossal ANE reaching 23 $μ$V/K in the ferromagnetic metal UCo$_{0.8}$Ru$_{0.2}$Al. Uranium's $5f$ electrons provide strong electronic correlations that lead to narrow bands, which are a known route to produci…
The macroscopic signature for Weyl nodes so far has been the negative longitudinal magnetoresistance arising from the chiral anomaly. However, negative longitudinal magnetoresistance is not unique to chiral anomaly and can arise due to completely different mechanisms such as current jetting or in ferromagnetic systems due to the suppression of scattering with magnons. Therefore, a macroscopic effect that can be uniquely attributed to the presence of Weyl nodes is desirable. Here we show that the planar Hall effect could be a hallmark for Weyl nodes. We investigated the anisotropic magnetoresis…
We have used soft x-ray photoemission electron microscopy to image the magnetization of single domain La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ nano-islands arranged in geometrically frustrated configurations such as square ice and kagome ice geometries. Upon thermal randomization, ensembles of nano-islands with strong inter-island magnetic coupling relax towards low-energy configurations. Statistical analysis shows that the likelihood of ensembles falling into low-energy configurations depends strongly on the annealing temperature. Annealing to just below the Curie temperature of the ferromagnetic film (…
Identifying and understanding the mechanisms behind strong phonon-phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice…
Synergistic doping and stabilization of magnetically tunable LnTi$_3$(Sb,Sn)$_4$ (Ln:Ce--Gd) kagome metals
Oak Ridge National Laboratory（橡树岭国家实验室）；Norfolk State University（诺福克州立大学）；University of Central Florida（中佛罗里达大学）；Gdansk University of Technology（格但斯克理工大学）
Accelerating Discovery of Extreme Lattice Thermal Conductivity by Crystal Attention Graph Neural Network (CATGNN) Using Chemical Bonding Intuitive Descriptors
