Polycrystalline Mg1-xHgxB2 samples with x=0, 1%, 2.5%, 5%, 7.5%, and 10% have been synthesized by solid-state reaction. Different from the substitution effect of Al, C, Li, etc. on crystal structure of MgB2, Hg substi...Polycrystalline Mg1-xHgxB2 samples with x=0, 1%, 2.5%, 5%, 7.5%, and 10% have been synthesized by solid-state reaction. Different from the substitution effect of Al, C, Li, etc. on crystal structure of MgB2, Hg substitution for Mg results in an increase of the lattice constant in both a and c directions. The super- conductivity of MgB2 is also suppressed by Hg substitution. The observed suppression of super- conductivity by Hg substitution is discussed in terms of the interband impurity scattering effect in two-band superconductors.展开更多
The electronic structure and superconducting gap structure are prerequisites to establish microscopic theories in understanding the superconductivity mechanism of iron-based superconductors.However,even for the most e...The electronic structure and superconducting gap structure are prerequisites to establish microscopic theories in understanding the superconductivity mechanism of iron-based superconductors.However,even for the most extensively studied optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2),there remain outstanding controversies on its electronic structure and superconducting gap structure.Here we resolve these issues by carrying out high-resolution angle-resolved photoemission spectroscopy(ARPES)measurements on the optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2)superconductor using both Helium lamp and laser light sources.Our results indicate the‘‘flat band"feature observed around the Brillouin zone center in the superconducting state originates from the combined effect of the superconductivity-induced band back-bending and the folding of a band from the zone corner to the center.We found direct evidence of the band folding between the zone corner and the center in both the normal and superconducting state.Our resolution of the origin of the flat band makes it possible to assign the three hole-like bands around the zone center and determine their superconducting gap correctly.Around the zone corner,we observe a tiny electronlike band and an M-shaped band simultaneously in both the normal and superconducting states.The obtained gap size for the bands around the zone corner(~5.5 meV)is significantly smaller than all the previous ARPES measurements.Our results establish a new superconducting gap structure around the zone corner and resolve a number of prominent controversies concerning the electronic structure and superconducting gap structure in the optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2).They provide new insights in examining and establishing theories in understanding superconductivity mechanism in iron-based superconductors.展开更多
Electron density differences resulting from atom displacement patterns aligned with phonon modes in MgB2 have been calculated using density functional theory (DFT). The extent of phonon anomalies, identified as indica...Electron density differences resulting from atom displacement patterns aligned with phonon modes in MgB2 have been calculated using density functional theory (DFT). The extent of phonon anomalies, identified as indicators of the superconducting transition temperature, Tc, under a range of conditions in AlB2-type structures, reduce as boron atoms are displaced from their equilibrium positions along E2g mode directions. The Fermi energy for displacements along the directions of the E2g phonon mode accounts for changes in the covalent B-B bond electronic charge density. We applied differential atom displacements to show that the shifted σ band structure associated with the light effective mass became tangential to the Fermi level and that the Fermi surface undergoes a topological transition at a critical relative displacement of ~0.6% of the boron atoms from equilibrium. The difference in Fermi energies at this critical displacement and at the equilibrium position correspond to the superconducting energy gap. The net volume between tubular σ surfaces in reciprocal space correlated with the depth of the phonon anomaly and, by inference, it is a key to an understanding of superconductivity. This ab initioapproach offers a phenomenological understanding of the factors that determine Tc based on knowledge of the crystal structure.展开更多
We study the Be-C doped MgB2 system by the first principles method based on density functional theory. The compensation effect between electron type doping and hole type doping is shown in the total density of states ...We study the Be-C doped MgB2 system by the first principles method based on density functional theory. The compensation effect between electron type doping and hole type doping is shown in the total density of states on the Fermi level, the real part of optical conductivity, and the number of effective carriers. The compensation mechanisms are discussed. The critical temperatures for different systems are calculated.展开更多
基金This work was supported by the National Science Foundation of China under Grant No. 50372052, 50588201the National Basic Research Program of China (973 program) under Grant No. 2007CB616906+1 种基金the Program for Changjiang Scholars and Innovative Research Team in UniversityAustralian Research Council under Grant No. DP0559872, DP0881739.
文摘Polycrystalline Mg1-xHgxB2 samples with x=0, 1%, 2.5%, 5%, 7.5%, and 10% have been synthesized by solid-state reaction. Different from the substitution effect of Al, C, Li, etc. on crystal structure of MgB2, Hg substitution for Mg results in an increase of the lattice constant in both a and c directions. The super- conductivity of MgB2 is also suppressed by Hg substitution. The observed suppression of super- conductivity by Hg substitution is discussed in terms of the interband impurity scattering effect in two-band superconductors.
基金financial support from the National Key Research and Development Program of China(2016YFA0300300,2017YFA0302900,2018YFA0704200 and 2019YFA0308000)the National Natural Science Foundation of China(11888101,11922414,11874405,and 62022089)+2 种基金the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(XDB25000000 and XDB33000000)the Youth Innovation Promotion Association of CAS(2017013 and 2019007)the Research Program of Beijing Academy of Quantum Information Sciences(Y18G06)。
文摘The electronic structure and superconducting gap structure are prerequisites to establish microscopic theories in understanding the superconductivity mechanism of iron-based superconductors.However,even for the most extensively studied optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2),there remain outstanding controversies on its electronic structure and superconducting gap structure.Here we resolve these issues by carrying out high-resolution angle-resolved photoemission spectroscopy(ARPES)measurements on the optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2)superconductor using both Helium lamp and laser light sources.Our results indicate the‘‘flat band"feature observed around the Brillouin zone center in the superconducting state originates from the combined effect of the superconductivity-induced band back-bending and the folding of a band from the zone corner to the center.We found direct evidence of the band folding between the zone corner and the center in both the normal and superconducting state.Our resolution of the origin of the flat band makes it possible to assign the three hole-like bands around the zone center and determine their superconducting gap correctly.Around the zone corner,we observe a tiny electronlike band and an M-shaped band simultaneously in both the normal and superconducting states.The obtained gap size for the bands around the zone corner(~5.5 meV)is significantly smaller than all the previous ARPES measurements.Our results establish a new superconducting gap structure around the zone corner and resolve a number of prominent controversies concerning the electronic structure and superconducting gap structure in the optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2).They provide new insights in examining and establishing theories in understanding superconductivity mechanism in iron-based superconductors.
文摘Electron density differences resulting from atom displacement patterns aligned with phonon modes in MgB2 have been calculated using density functional theory (DFT). The extent of phonon anomalies, identified as indicators of the superconducting transition temperature, Tc, under a range of conditions in AlB2-type structures, reduce as boron atoms are displaced from their equilibrium positions along E2g mode directions. The Fermi energy for displacements along the directions of the E2g phonon mode accounts for changes in the covalent B-B bond electronic charge density. We applied differential atom displacements to show that the shifted σ band structure associated with the light effective mass became tangential to the Fermi level and that the Fermi surface undergoes a topological transition at a critical relative displacement of ~0.6% of the boron atoms from equilibrium. The difference in Fermi energies at this critical displacement and at the equilibrium position correspond to the superconducting energy gap. The net volume between tubular σ surfaces in reciprocal space correlated with the depth of the phonon anomaly and, by inference, it is a key to an understanding of superconductivity. This ab initioapproach offers a phenomenological understanding of the factors that determine Tc based on knowledge of the crystal structure.
基金Project supported by the National Natural Science Foundation of China (No.10775088)the Key Program of Theoretical Physics of Shandong Province
文摘We study the Be-C doped MgB2 system by the first principles method based on density functional theory. The compensation effect between electron type doping and hole type doping is shown in the total density of states on the Fermi level, the real part of optical conductivity, and the number of effective carriers. The compensation mechanisms are discussed. The critical temperatures for different systems are calculated.
