Thermal aging effects on surface of 2.5 Me V Fe ion-irradiated Fe–0.6%Cu alloy were investigated using positron annihilation techniques. The samples were irradiated at 573 K to a dose of 0.1 dpa. Their thermal aging ...Thermal aging effects on surface of 2.5 Me V Fe ion-irradiated Fe–0.6%Cu alloy were investigated using positron annihilation techniques. The samples were irradiated at 573 K to a dose of 0.1 dpa. Their thermal aging was performed at 573 K for 5, 50 and 100 h. From the results of Doppler broadening measurement, an obvious trough could be seen in near-surface region from the S parameters and inflection point form at S–W curves. This indicates changes in the annihilation mechanism of positrons in surface region after thermal aging. Coincident Doppler broadening indicates that the density of Cu precipitates in the thermal aged samples decreased, due to recovery of the vacancies.展开更多
Weintroduce a general class of orbital-free density functionals(OF-DFT)decomposed into a local part in coordinate space and a local part in reciprocal space.As a demonstration of principle,we choose for the former the...Weintroduce a general class of orbital-free density functionals(OF-DFT)decomposed into a local part in coordinate space and a local part in reciprocal space.As a demonstration of principle,we choose for the former the Thomas-Fermi-von Weizsäcker(TFW)kinetic energy density functional(KEDF)and for the latter a form derived from the Lindhard function,but with the two system-dependent adjustable parameters.These parameters are machine-learned from Kohn-Sham data using Bayesian linear regression with a kernel method,which employs moments of the Fourier components of the electronic density as the descriptor.Through a number of representative cases,we demonstrate that our machine-learned model provides more than an order-of-magnitude improvement in the accuracy of the frozen-phonon energies compared to theTFWKEDF,with negligible increase in the computational cost.Overall,this work opens an avenue for the construction of accurate KEDFs for OF-DFT.展开更多
Ising superconductivity,observed in NbSe2 and similar materials,has generated tremendous interest.Recently,attention was called to the possible role that spin fluctuations(SF)play in this phenomenon,in addition to the...Ising superconductivity,observed in NbSe2 and similar materials,has generated tremendous interest.Recently,attention was called to the possible role that spin fluctuations(SF)play in this phenomenon,in addition to the dominant electron–phonon coupling(EPC);the possibility of a predominantly triplet state was discussed and led to a conjecture of viable singlet–triplet Leggett oscillations.However,these hypotheses have not been put to a quantitative test.In this paper,we report first principle calculations of the EPC and also estimate coupling with SF,including full momentum dependence.We find that:(1)EPC is strongly anisotropic,largely coming from the K-K0 scattering,and therefore excludes triplet symmetry even as an excited state;(2)superconductivity is substantially weakened by SF,but anisotropy remains as above;and,(3)we do find the possibility of a Leggett mode,not in a singlet–triplet but in an s_(++)–s_(±)channel.展开更多
We investigate the magnetism of a previously unexplored distorted spin-1/2 kagome model consisting of three symmetry-inequivalent nearest-neighbor antiferromagnetic Heisenberg couplings J_(■),J,and J′,and uncover a ...We investigate the magnetism of a previously unexplored distorted spin-1/2 kagome model consisting of three symmetry-inequivalent nearest-neighbor antiferromagnetic Heisenberg couplings J_(■),J,and J′,and uncover a rich ground state phase diagram even at the classical level.Using analytical arguments and numerical techniques we identify a collinear Q^(→)=0 magnetic phase,two unusual non-collinear coplanar Q^(→)=(1/3,1/3)phases and a classical spin liquid phase with a degenerate manifold of non-coplanar ground states,resembling the jammed spin liquid phase found in the context of a bond-disordered kagome antiferromagnet.We further show with density functional theory calculations that the recently synthesized Y-kapellasite Y_(3)Cu_(9)(OH)_(19)C_(l8) is a realization of this model and predict its ground state to lie in the region of Q^(→)=(1/3,1/3)order,which remains stable even after the inclusion of quantum fluctuation effects within variational Monte Carlo and pseudofermion functional renormalization group.The presented model opens a new direction in the study of kagome antiferromagnets.展开更多
Density functional theory (DFT) and its extensions, such as DFT+U and DFT+dynamical mean-field theory, are invaluable for studying magnetic properties in solids. However, rare-earth (R) materials remain challenging du...Density functional theory (DFT) and its extensions, such as DFT+U and DFT+dynamical mean-field theory, are invaluable for studying magnetic properties in solids. However, rare-earth (R) materials remain challenging due to self-interaction errors and the lack of proper orbital polarization. We show how the orbital dependence of self-interaction error contradicts Hund’s rules and plagues magnetocrystalline anisotropy (MA) calculations, and how analyzing DFT states that respect Hund’s rules can mitigate this issue. We benchmark MA in RCo_(5), R_(2)Fe_(14)B, and RFe1_(2), extending prior work on RMn_(6)Sn_(6), achieving excellent agreement with experiments. Additionally, we illustrate a semi-analytical perturbation approach that treats crystal fields as a perturbation in the large spin-orbit coupling limit. Using Gd-4f crystal-field splitting, this method provides a microscopic understanding of MA and enables rapid screening of high-MA materials.展开更多
基金supported by the National Natural Science Foundation of China(Nos.91026006,91226103,11475193,11475197,11575205and 11505192)Beijing Natural Science Foundation(No.1164017)
文摘Thermal aging effects on surface of 2.5 Me V Fe ion-irradiated Fe–0.6%Cu alloy were investigated using positron annihilation techniques. The samples were irradiated at 573 K to a dose of 0.1 dpa. Their thermal aging was performed at 573 K for 5, 50 and 100 h. From the results of Doppler broadening measurement, an obvious trough could be seen in near-surface region from the S parameters and inflection point form at S–W curves. This indicates changes in the annihilation mechanism of positrons in surface region after thermal aging. Coincident Doppler broadening indicates that the density of Cu precipitates in the thermal aged samples decreased, due to recovery of the vacancies.
基金the support of the Quantum Science and Engineering Center(QSEC)at George Mason UniversityP.S.gratefully acknowledges funding from the U.S.Department of Energy,Office of Science,under Grant No.DE-SC0023445+1 种基金T.O.acknowledges support provided by the George Mason University Provost Dissertation Completion GrantM.E.has been partially supported by the Simons Foundation.
文摘Weintroduce a general class of orbital-free density functionals(OF-DFT)decomposed into a local part in coordinate space and a local part in reciprocal space.As a demonstration of principle,we choose for the former the Thomas-Fermi-von Weizsäcker(TFW)kinetic energy density functional(KEDF)and for the latter a form derived from the Lindhard function,but with the two system-dependent adjustable parameters.These parameters are machine-learned from Kohn-Sham data using Bayesian linear regression with a kernel method,which employs moments of the Fourier components of the electronic density as the descriptor.Through a number of representative cases,we demonstrate that our machine-learned model provides more than an order-of-magnitude improvement in the accuracy of the frozen-phonon energies compared to theTFWKEDF,with negligible increase in the computational cost.Overall,this work opens an avenue for the construction of accurate KEDFs for OF-DFT.
基金The work at GMU(S.D.and I.I.M.)was supported by Office of Naval Research through grant N00014-20-1-2345H.P.and E.R.M.acknowledge support from the National Science Foundation(NSF)under Grant No.DMR-2035518 and Grant No.OAC-2103991 for method development,superconductivity analysis,and code develop-ment.D.F.A+2 种基金was supported by the U.S.Department of Energy,Office of Basic Energy Sciences,Division of Materials Sciences and Engineering,under Award No.DE-SC0021971This work used the Extreme Science and Engineering Discovery Environment(XSEDE)70 which is supported by NSF Award No.ACI-1548562Specifically,this work used Comet at the San Diego Supercomputer Center through allocation TG-DMR180071.
文摘Ising superconductivity,observed in NbSe2 and similar materials,has generated tremendous interest.Recently,attention was called to the possible role that spin fluctuations(SF)play in this phenomenon,in addition to the dominant electron–phonon coupling(EPC);the possibility of a predominantly triplet state was discussed and led to a conjecture of viable singlet–triplet Leggett oscillations.However,these hypotheses have not been put to a quantitative test.In this paper,we report first principle calculations of the EPC and also estimate coupling with SF,including full momentum dependence.We find that:(1)EPC is strongly anisotropic,largely coming from the K-K0 scattering,and therefore excludes triplet symmetry even as an excited state;(2)superconductivity is substantially weakened by SF,but anisotropy remains as above;and,(3)we do find the possibility of a Leggett mode,not in a singlet–triplet but in an s_(++)–s_(±)channel.
基金F.F.acknowledges support from the Alexander von Humboldt Foundation through a postdoctoral Humboldt fellowship.A.R.,R.V.,and J.R.acknowledge support by the Deutsche Forschungsgemeinschaft(DFG,German Research Foundation)for funding through TRR 288-422213477(projects A05,B05)(A.R.and R.V.)CRC 183(project A04)(J.R.).I.I.M.acknowledges support from the U.S.Department of Energy through Grant no.DE-SC0021089 and from the Wilhelm and Else Heraeus Foundation.
文摘We investigate the magnetism of a previously unexplored distorted spin-1/2 kagome model consisting of three symmetry-inequivalent nearest-neighbor antiferromagnetic Heisenberg couplings J_(■),J,and J′,and uncover a rich ground state phase diagram even at the classical level.Using analytical arguments and numerical techniques we identify a collinear Q^(→)=0 magnetic phase,two unusual non-collinear coplanar Q^(→)=(1/3,1/3)phases and a classical spin liquid phase with a degenerate manifold of non-coplanar ground states,resembling the jammed spin liquid phase found in the context of a bond-disordered kagome antiferromagnet.We further show with density functional theory calculations that the recently synthesized Y-kapellasite Y_(3)Cu_(9)(OH)_(19)C_(l8) is a realization of this model and predict its ground state to lie in the region of Q^(→)=(1/3,1/3)order,which remains stable even after the inclusion of quantum fluctuation effects within variational Monte Carlo and pseudofermion functional renormalization group.The presented model opens a new direction in the study of kagome antiferromagnets.
基金supported by the U.S. Department of Energy (USDOE), Office of Basic Energy Sciences, Division of Materials Sciences and EngineeringThe initial work by L.K. were supported by the USDOE Early Career Research Program+1 种基金Ames National Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358IIM acknowledges support from the National Science Foundation under Award No. DMR-2403804.
文摘Density functional theory (DFT) and its extensions, such as DFT+U and DFT+dynamical mean-field theory, are invaluable for studying magnetic properties in solids. However, rare-earth (R) materials remain challenging due to self-interaction errors and the lack of proper orbital polarization. We show how the orbital dependence of self-interaction error contradicts Hund’s rules and plagues magnetocrystalline anisotropy (MA) calculations, and how analyzing DFT states that respect Hund’s rules can mitigate this issue. We benchmark MA in RCo_(5), R_(2)Fe_(14)B, and RFe1_(2), extending prior work on RMn_(6)Sn_(6), achieving excellent agreement with experiments. Additionally, we illustrate a semi-analytical perturbation approach that treats crystal fields as a perturbation in the large spin-orbit coupling limit. Using Gd-4f crystal-field splitting, this method provides a microscopic understanding of MA and enables rapid screening of high-MA materials.
