Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-T_(c) superconductors.Here,we report 17O nuclear magnetic resonance(NMR) studies on two ...Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-T_(c) superconductors.Here,we report 17O nuclear magnetic resonance(NMR) studies on two planar oxygen sites in stoichio?metric cuprate YBa_(2)Cu_(4)O_(8) to investigate the symmetry breaking inside the pseudogap phase.We observe that the Knight shifts of the two oxygen sites are identical at high temperatures but different below T_(nem) ~185 K,which is close to the pseudogap temperature T^(*).Our result provides a microscopic evidence for intra-unit-cell electronic nematicity.The difference in quadrupole resonance frequency between the two oxygen sites is unchanged below T_(nem),which suggests that the observed nematicity does not directly stem from the local charge density modulation.Furthermore,a short-range charge density wave(CDW) order is observed below T=150 K.The additional broadening in the 17O-NMR spectra because of this CDW order is determined to be inequivalent for the two oxygen sites,which is similar to that observed in case of nematicity.These results suggest a possible connection between nematicity,CDW order,and pseudogap.展开更多
We report^(75) As nuclear magnetic resonance(NMR)/nuclear quadrupole resonance(NQR) and transmission electron microscopy(TEM) studies on LaFeAsO_(1-x)F_x. There are two superconducting domes in this material. The firs...We report^(75) As nuclear magnetic resonance(NMR)/nuclear quadrupole resonance(NQR) and transmission electron microscopy(TEM) studies on LaFeAsO_(1-x)F_x. There are two superconducting domes in this material. The first one appears at 0.03 ≤ x ≤0.2 with T_c^(max) = 27 K, and the second one at 0.25 ≤x≤0.75 with T_c^(max) = 30 K. By NMR and TEM, we demonstrate that a C4-to-C2 structural phase transition(SPT) takes place above both domes, with the transition temperature T_s varying strongly with x. In the first dome, the SPT is followed by an antiferromagnetic(AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. By ^(75) As nuclear spin-lattice relaxation rate(1/T_1) measurements, we find that AF order and superconductivity coexist microscopically in LaFeAsO_(0.97) F_(0.03). In the coexisting region, 1/T_1 decreases at T_c but becomes proportional to T below 0.6 T_c, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for x ≤ 0.2 is quite similar to the doped BaFe_2As_2 system. The electrical resistivity p in the second dome can be fitted by ρ = ρ0 + AT^n with n = 1 and a maximal coefficient A at around xopt = 0.5-0.55 at which T_s extrapolates to zero and Tc is the maximal, which suggests the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO_(1-x)F_x, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.展开更多
基金supported by the National Natural Science Foundation of China (Grant Nos. 11974405, 11674377, and 11634015)the Ministry of Science and Technology of China (Grant Nos. 2016YFA0300502, and 2017YFA0302904)the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB33010100)。
文摘Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-T_(c) superconductors.Here,we report 17O nuclear magnetic resonance(NMR) studies on two planar oxygen sites in stoichio?metric cuprate YBa_(2)Cu_(4)O_(8) to investigate the symmetry breaking inside the pseudogap phase.We observe that the Knight shifts of the two oxygen sites are identical at high temperatures but different below T_(nem) ~185 K,which is close to the pseudogap temperature T^(*).Our result provides a microscopic evidence for intra-unit-cell electronic nematicity.The difference in quadrupole resonance frequency between the two oxygen sites is unchanged below T_(nem),which suggests that the observed nematicity does not directly stem from the local charge density modulation.Furthermore,a short-range charge density wave(CDW) order is observed below T=150 K.The additional broadening in the 17O-NMR spectra because of this CDW order is determined to be inequivalent for the two oxygen sites,which is similar to that observed in case of nematicity.These results suggest a possible connection between nematicity,CDW order,and pseudogap.
基金supported by the National Key Research and Development Program of China(Grant No.2017YFA0302904)the National Natural Science Foundation of China(Grant Nos.11674377,and 11634015)+2 种基金the Chinese Academy of Sciences(Grant No.XDB07020200)supported by research grants from Japan Society for the Promotion of Science(JSPS)(Grant No.16H0401618)supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences
文摘We report^(75) As nuclear magnetic resonance(NMR)/nuclear quadrupole resonance(NQR) and transmission electron microscopy(TEM) studies on LaFeAsO_(1-x)F_x. There are two superconducting domes in this material. The first one appears at 0.03 ≤ x ≤0.2 with T_c^(max) = 27 K, and the second one at 0.25 ≤x≤0.75 with T_c^(max) = 30 K. By NMR and TEM, we demonstrate that a C4-to-C2 structural phase transition(SPT) takes place above both domes, with the transition temperature T_s varying strongly with x. In the first dome, the SPT is followed by an antiferromagnetic(AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. By ^(75) As nuclear spin-lattice relaxation rate(1/T_1) measurements, we find that AF order and superconductivity coexist microscopically in LaFeAsO_(0.97) F_(0.03). In the coexisting region, 1/T_1 decreases at T_c but becomes proportional to T below 0.6 T_c, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for x ≤ 0.2 is quite similar to the doped BaFe_2As_2 system. The electrical resistivity p in the second dome can be fitted by ρ = ρ0 + AT^n with n = 1 and a maximal coefficient A at around xopt = 0.5-0.55 at which T_s extrapolates to zero and Tc is the maximal, which suggests the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO_(1-x)F_x, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.