The resistance to oxidizing environments exhibited by some Mn+1AXn(MAX)phases stems from the formation of stable and protective oxide layers at high operating temperatures.The MAX phases are hexagonally arranged layer...The resistance to oxidizing environments exhibited by some Mn+1AXn(MAX)phases stems from the formation of stable and protective oxide layers at high operating temperatures.The MAX phases are hexagonally arranged layered nitrides or carbides with general formula M_(n+1)AX_(n),n=1,2,3,where M is early transition elements,A is A block elements,and X is C/N.Previous attempts to model and assess oxide phase stability in these systems has been limited in scope due to higher computational costs.To address the issue,we developed a machine-learning driven high-throughput framework for the fast assessment of phase stability and oxygen reactivity of 211 chemistry MAX phase M_(2)AX.The proposed scheme combines a sure independence screening sparsifying operator-based machine-learning model in combination with grand-canonical linear programming to assess temperaturedependent Gibbs free energies,reaction products,and elemental chemical activity during the oxidation of MAX phases.The thermodynamic stability,and chemical activity of constituent elements of Ti_(2)AlC with respect to oxygen were fully assessed to understand the high-temperature oxidation behavior.The predictions are in good agreement with oxidation experiments performed on Ti_(2)AlC.We were also able to explain the metastability of Ti_(2)SiC,which could not be synthesized experimentally due to higher stability of competing phases.For generality of the proposed approach,we discuss the oxidation mechanism of Cr_(2)AlC.The insights of oxidation behavior will enable more efficient design and accelerated discovery of MAX phases with maintained performance in oxidizing environments at high temperatures。展开更多
The band structure in a kagome lattice can naturally exhibit flat band,Dirac cones,and van Hove singularity,enabling rich interplays between correlation and topology.However,the flat band is rarely detected just at th...The band structure in a kagome lattice can naturally exhibit flat band,Dirac cones,and van Hove singularity,enabling rich interplays between correlation and topology.However,the flat band is rarely detected just at the Fermi level in kagome materials,which would be crucial to realize emergent flat band physics.Here,combining angle-resolved photoemission spectroscopy,transport measurements and first-principles calculation,we observe a striking Fermi-level flat band in paramagnetic YCr_(6)Ge_(6)as a typical signature of electronic kagome lattice.We explicitly unveil that orbital character plays an essential role to realize electronic kagome lattice in crystals with transition-metal kagome layers.We further engineer this material with magnetic rare earth elements to break the time-reversal symmetry of the Fermi-level kagome flat band.Our work establishes a Fermi-level flat band in a kagome magnet as an exciting quantum platform.展开更多
基金We acknowledge support from National Science Foundation through grants no.(DMREF)CMMI-1729350.
文摘The resistance to oxidizing environments exhibited by some Mn+1AXn(MAX)phases stems from the formation of stable and protective oxide layers at high operating temperatures.The MAX phases are hexagonally arranged layered nitrides or carbides with general formula M_(n+1)AX_(n),n=1,2,3,where M is early transition elements,A is A block elements,and X is C/N.Previous attempts to model and assess oxide phase stability in these systems has been limited in scope due to higher computational costs.To address the issue,we developed a machine-learning driven high-throughput framework for the fast assessment of phase stability and oxygen reactivity of 211 chemistry MAX phase M_(2)AX.The proposed scheme combines a sure independence screening sparsifying operator-based machine-learning model in combination with grand-canonical linear programming to assess temperaturedependent Gibbs free energies,reaction products,and elemental chemical activity during the oxidation of MAX phases.The thermodynamic stability,and chemical activity of constituent elements of Ti_(2)AlC with respect to oxygen were fully assessed to understand the high-temperature oxidation behavior.The predictions are in good agreement with oxidation experiments performed on Ti_(2)AlC.We were also able to explain the metastability of Ti_(2)SiC,which could not be synthesized experimentally due to higher stability of competing phases.For generality of the proposed approach,we discuss the oxidation mechanism of Cr_(2)AlC.The insights of oxidation behavior will enable more efficient design and accelerated discovery of MAX phases with maintained performance in oxidizing environments at high temperatures。
基金supported by the Ministry of Science and Technology of China(Grants No.2018YFA0307000,and No.2018FYA0305800)the Innovation Program for Quantum Science and Technology(No.2021ZD0302802)+1 种基金the National Natural Science Foundation of China(Grants No.U2032128,and No.11874047)the Fundamental Research Funds for the Central Universities(Grant No.2042021kf0210).
文摘The band structure in a kagome lattice can naturally exhibit flat band,Dirac cones,and van Hove singularity,enabling rich interplays between correlation and topology.However,the flat band is rarely detected just at the Fermi level in kagome materials,which would be crucial to realize emergent flat band physics.Here,combining angle-resolved photoemission spectroscopy,transport measurements and first-principles calculation,we observe a striking Fermi-level flat band in paramagnetic YCr_(6)Ge_(6)as a typical signature of electronic kagome lattice.We explicitly unveil that orbital character plays an essential role to realize electronic kagome lattice in crystals with transition-metal kagome layers.We further engineer this material with magnetic rare earth elements to break the time-reversal symmetry of the Fermi-level kagome flat band.Our work establishes a Fermi-level flat band in a kagome magnet as an exciting quantum platform.
