The dielectric properties of Pb(Zn1/3Nb2/3)O3-PbZrO3-PbTiO3 (PZN-PZ-PT) system near the rhombohedral/tetragonal morphotropic phase boundary (MPB) are carefully studied in this paper.It is found that,for all samples,th...The dielectric properties of Pb(Zn1/3Nb2/3)O3-PbZrO3-PbTiO3 (PZN-PZ-PT) system near the rhombohedral/tetragonal morphotropic phase boundary (MPB) are carefully studied in this paper.It is found that,for all samples,the curves around the temperatures of dielectric permittivity peak show the characteristics of diffuse phase transition.The change in PbZrO3/PbTiO3 ratio has much influence on the dielectric properties of the samples.The extent of diffuse phase transition increases with the increasing Zr/Ti ratio.The samples in rhombohedral region have typical diffuse phase transition in the temperature range measured.However,for the samples with tetragonal symmetry,a spontaneous normal ferroelectric-relaxor phase transition exists at temperature lower than that of permittivity peak.This normal ferroelectric-relaxor phase transition is confirmed by the experiment of thermally driven current.The analysis of TEM reveals that the samples in tetragonal region show a 90° macrodomain structure,while the samples in rhombohedral region have the configuration of microdomain structure.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
Cr_2Ge_2Te_6is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure,thus represents a promising material for novel electronic and spintronic devices.Here we combine scanning tunneling mic...Cr_2Ge_2Te_6is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure,thus represents a promising material for novel electronic and spintronic devices.Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr_2Ge_2Te_6.Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition,as well as a peculiar double-peak electronic state on the Cr-site defect.These features can be quantitatively explained by density functional theory calculations,which uncover a close relationship between the electronic structure and magnetic order.These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr_2Ge_2Te_6.展开更多
Problems of thermodynamic phase transition originate inherently in solidification, combustion and various other significant fields. If the transition region among two locally stable phases is adequately narrow, the dy...Problems of thermodynamic phase transition originate inherently in solidification, combustion and various other significant fields. If the transition region among two locally stable phases is adequately narrow, the dynamics can be modeled by an interface motion. This paper is devoted to exhaustive analysis of the invariant solutions for a modified Kuramoto-Sivashinsky equation in two spatial and one temporal dimensions is presented. This nonlinear partial differential equation asymptotically characterizes near planar interfaces, which are marginally long-wave unstable. For this purpose, by applying the classical symmetry method for this model the classical symmetry operators are attained.Moreover, the structure of the Lie algebra of symmetries is discussed and the optimal system of subalgebras, which yields the preliminary classification of group invariant solutions is constructed. Mainly, the Lie invariants corresponding to the infinitesimal symmetry generators as well as associated similarity reduced equations are also pointed out. Furthermore,the nonclassical symmetries of this nonlinear PDE are also comprehensively investigated.展开更多
文摘The dielectric properties of Pb(Zn1/3Nb2/3)O3-PbZrO3-PbTiO3 (PZN-PZ-PT) system near the rhombohedral/tetragonal morphotropic phase boundary (MPB) are carefully studied in this paper.It is found that,for all samples,the curves around the temperatures of dielectric permittivity peak show the characteristics of diffuse phase transition.The change in PbZrO3/PbTiO3 ratio has much influence on the dielectric properties of the samples.The extent of diffuse phase transition increases with the increasing Zr/Ti ratio.The samples in rhombohedral region have typical diffuse phase transition in the temperature range measured.However,for the samples with tetragonal symmetry,a spontaneous normal ferroelectric-relaxor phase transition exists at temperature lower than that of permittivity peak.This normal ferroelectric-relaxor phase transition is confirmed by the experiment of thermally driven current.The analysis of TEM reveals that the samples in tetragonal region show a 90° macrodomain structure,while the samples in rhombohedral region have the configuration of microdomain structure.
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.
基金supported by the Basic Science Center Project of NSFC(51788104)the MOST of China(2015CB921000)+6 种基金the support from Tsinghua University Initiative Scientific Research Program and NSFC(11774196)S.H.Z.is supported by the National Postdoctoral Program for Innovative Talents of China(BX201600091)the China Postdoctoral Science Foundation(2017M610858)the support of the National Key Research and Development Program(2016YFA0300404)NSFC Grant(11674326)the Joint Funds of NSFC and the Chinese Academy of Sciences’Large-Scale Scientific Facility(U1432139)supported in part by the Beijing Advanced Innovation Center for Future Chip(ICFC)
文摘Cr_2Ge_2Te_6is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure,thus represents a promising material for novel electronic and spintronic devices.Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr_2Ge_2Te_6.Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition,as well as a peculiar double-peak electronic state on the Cr-site defect.These features can be quantitatively explained by density functional theory calculations,which uncover a close relationship between the electronic structure and magnetic order.These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr_2Ge_2Te_6.
文摘Problems of thermodynamic phase transition originate inherently in solidification, combustion and various other significant fields. If the transition region among two locally stable phases is adequately narrow, the dynamics can be modeled by an interface motion. This paper is devoted to exhaustive analysis of the invariant solutions for a modified Kuramoto-Sivashinsky equation in two spatial and one temporal dimensions is presented. This nonlinear partial differential equation asymptotically characterizes near planar interfaces, which are marginally long-wave unstable. For this purpose, by applying the classical symmetry method for this model the classical symmetry operators are attained.Moreover, the structure of the Lie algebra of symmetries is discussed and the optimal system of subalgebras, which yields the preliminary classification of group invariant solutions is constructed. Mainly, the Lie invariants corresponding to the infinitesimal symmetry generators as well as associated similarity reduced equations are also pointed out. Furthermore,the nonclassical symmetries of this nonlinear PDE are also comprehensively investigated.
