Superconducting anisotropy and vortex pinning in CaKFe_4As_4 and KCa_2Fe_4As_4F_2

The vortex pinning determining the current carrying capacity of a superconductor is an important property to the applications of superconducting materials.For layered superconductors,the vortex pinning can be enhanced by a strong interlayer interaction in accompany with a suppression of superconducting anisotropy,which remains to be investigated in iron based superconductors(FeSCs)with the layered structure.Here,based on the transport and magnetic torque measurements,we experimentally investigate the vortex pinning in two bilayer FeSCs,CaKFe₄As₄(Fe1144)and KCa₂Fe₄As4F₂(Fe12442),and compare their superconducting anisotropyγ.While the anisotropyγ≈3 for Fe1144 is much smaller thanγ≈15 in Fe12442 around Tc,a higher flux pinning energy as evidenced by a higher critical current density is found in Fe1144,as compared with the case of Fe12442.In combination with the literature data of Ba_0.72K_0.28Fe2As₂ and Nd Fe As_O0.82F_0.18,we reveal an anti-correlation between the pinning energy and the superconducting anisotropy in these Fe SCs.Our results thus suggest that the interlayer interaction can not be neglected when considering the vortex pinning in Fe SCs.     

Low temperature specific heat of 12442-type KCa_2Fe_4As_4F_2 single crystals

Low-temperature specific heat(SH) is measured for the 12442-type KCa_2Fe_4As_4F_2 single crystal under different magnetic fields.A clear SH jump with the height of ?C/T|_(T_c)= 130 mJ mol~(-1) K~(-2) is observed at the superconducting transition temperature T_c.It is found that the electronic SH coefficient ?γ(H) quickly increases when the field is in the low-field region below 3T and then considerably slows down the increase with a further increase in the field, which indicates a rather strong anisotropy or multi-gap feature with a small minimum in the superconducting gap(s). The temperature-dependent SH data indicate the presence of the T~2 term, which supplies further information and supports the picture with a line-nodal gap structure. Moreover, the onset point of the SH transition remains almost unchanged under the field as high as 9 T, which is similar to that observed in cuprates, and places this system in the middle between the BCS limit and the Bose-Einstein condensation.     

NMR and NQR studies on superconducting Sr2RuO4

We review our nuclear-magnetic resonance (NMR) and nuclear-quadrupole-resonance (NQR) studies in superconducting Sr₂RuO₄, which have been performed in order to investigate the gap structure and the pairing symmetry in the superconducting state and magnetic fluctuations in the normal state. The spin-lattice relaxation rate (1/T₁) of a high-quality sample with shows a sharp decrease without a coherence peak just below T_c, followed by a T³ behavior down to 0.15 K. This result indicates that the superconducting gap in pure Sr₂RuO₄ is a highly anisotropic character with a line-node gap. The Knight shift, which is related to the spin susceptibility, is unchanged in the superconducting state irrespective of the direction of the applied fields and various magnitude of the field. This result strongly suggests that the superconducting pairs are in the spin-triplet state, and the spin direction of the triplet pairs is considered to be changed by small fields of several hundred Oe.     

Coexistence of Polaronic States and Superconductivity in Iron-Pnictide Compound Ba_2Ti_2Fe_2As_4O

The electronic structure of iron-pnictide compound superconductor Ba_2Ti_2Fe_2As_4O, which has metallic intermediate Ti_2O layers, is studied using angle-resolved photoemission spectroscopy. The Ti-related bands show a‘peak-dip-hump' line shape with two branches of dispersion associated with the polaronic states at temperatures below around 120 K. This change in the spectra occurs along with the resistivity anomaly that was not clearly understood in a previous study. Moreover, an energy gap induced by the superconducting proximity effect opens in the polaronic bands at temperatures below T_c(~21 K). Our study provides the spectroscopic evidence that superconductivity coexists with polarons in the same bands near the Fermi level, which provides a suitable platform to study interactions between charge, lattice and spin freedoms in a correlated system.     

Novel rubidium polyfluorides with F_3, F_4, and F_5 species

Pressure has an important effect on chemical bonds and their chemical properties. The atypical compounds NaCl_3 and CsF_3 are predicted to be stable at high pressure and show unique physical and chemical properties. By using ab initio random structure searching and density functional theory calculations, we predicted multiple thermodynamically stable atypical compounds, which are RbF_2, RbF_3, RbF_4, and RbF_5 in the pressure range of 0–300 GPa. In these stable compounds, homonuclear bondings of F_3, F_4, and F_5 species are easily formed. The electron structure calculation showed that except for Fd-3 m phase of RbF_2, these stable compounds are insulators and F 5 p orbitals play an important role in the Fermi level. It is interesting that the compounds RbF_5 could be stable at nearly ambient pressure and 0 K which will stimulate experimental studies in the future.     

Comparing Y−Ba−Cu−O superconductors by Cu,O and Ba NMR/NQR

This paper discusses various NMR/NQR studies performed by the resonance group at the Physik-Institut on the structurally related compounds YBa₂Cu₃O₇ and YBa₂Cu₄O₈. Employing different isotopes and comparing similar measurements offers the possibility to gain new insights into these compounds. The discussion addresses the following topics: Electric field gradients and their temperature and pressure dependence;T _c enhancement by Ca doping; Cu and O Knight shifts; Ba relaxation. The wide range of applicability of the resonance methods in superconductivity research is demonstrated.     

NQR, NMR and Crystal Structure Studies of [C(NH2)3]2HgX4 (X = Br, I)

The crystal structure of [C(NH₂)₃]₂HgBr₄ has been determined at room temperature: monoclinic, space group C2/c, with a = 10.035(2), b = 11.164(2), c = 13.358(3) Å, β = 111.67(3)°, and Z = 4. The crystal consists of planar [C(NH₂)₃]⁺ and distorted tetrahedral [HgBr₄]^2? ions. The Hg atom is located on a two-fold axis such that two sets of inequivalent Br atoms exist in an [HgBr₄]^2? ion. In accordance with the crystal structure, two ⁸¹Br NQR lines widely separated in frequency were observed between 77 and ca. 380 K. [C(NH₂)₃]₂HgI₄ yielded four ¹²⁷I NQR lines ascribable to m = ±1/2 ? ±3/2 transitions, indicating that its crystal structure is different from the bromide complex. The ¹H NMR T ₁ measurements showed a single minimum for the bromide but two minima for the iodide. The analyses based on the C₃ reorientations of the planar [C(NH₂)₃]⁺ ions gave the activation energies of 29.8 kJ mol^?1 for the bromide, and 30.2 and 40.0 kJ mol^?1 for the iodide.     

NQR, NMR and Crystal Structure Studies of [C(NH2)3]3Sb2Br9

The crystal structure of [C(NH₂)₃]₃Sb₂Br₉ was determined at 143 K: monoclinic, space group C2/c, Z = 4, a = 15.695 (3), b = 9.039(2), c = 18.364(3) Å, β = 96.94(1)°. The structure consists of two crystallographically independent guanidinium ions and two-dimensional corrugated sheets of (Sb₂Br₉ ^3?) _n , in which SbBr₆ octahedra are connected through three bridging Br atoms each other. One of the cations situates in a cavity of the (Sb₂Br₉ ^3?) _n layer with statistical disorder, while the other situates between the layers without disorder. Three ⁸¹Br NQR resonance lines were assignable to terminal Br atoms, while only one line was found for two inequivalent bridging Br atoms. All the ⁸¹Br NQR resonance lines were subjected to fade-out at low temperatures. The temperature dependence curve of ¹H NMR T ₁ showed well defined two minima, which were explained by postulating the C₃ reorientations of two types of cations with very different activation energies. The DTA (DSC) measurement revealed a phase transition of a first-order type at 444 K.     

NMR and NQR studies in high-T c oxides: YBa2Cu3O y (6.0≤y≤6.91)

The NQR and NMR techniques have been utilized to characterize the local oxygen coordination of inequivalent Cu sites and the electronic properties in both the normal and superconducting states of YBa₂Cu₃O_y (6.0<-y≤6.91). The distinct NQR lines associated with the different oxygen-coordinated Cu sites, hence the locally differentiated charged states, have been observed. The degree of charge differentiation at the Cu(2) plane sites was found to be increased with decreasingy from 6.91, which might be related with the decrease ofT _c. An anomalous temperature dependence of Cu nuclear spin-lattice relaxation timeT ₁ has been observed for both the Cu(1) chain and Cu(2) plane sites fory=6.91 and it is discussed in connection with antiferromagnetic spin fluctuations in the normal state.     

127I NQR and 1H NMR studies of 4-aminopyridinium tetraiodoantimonate(III); molecular motion and phase transition

The crystals of 4-NH₂PyHSbI₄ (Py = C₅H₄N) have been investigated by means of ¹²⁷I NQR, ¹H NMR T ₁ and DTA. The crystals can exist in two modifications of β and α(I) at room temperatures. The α(I)-phase is a metastable state which is obtained when the stable β form is heated. The α(I)-phase undergoes a first-order type phase transition of α(I) $\leftrightarrow \alpha $ (II) at 272 K (on heating), while the β-phase is stable down to 77 K. Four and two ¹²⁷I (m = ±1/2 $\leftrightarrow \pm $ 3/2) NQR lines have been found for the β- and α(II)-phases, respectively. One half of them is assignable to the terminal I atom(s) and the other to the bridging I atom(s) in each phase. All the resonance lines of the α(II)-phase underwent a disappearance above ca. 240 K and no resonance line was observed in the α(I)-phase. The second moment M ₂ value of ¹H NMR spectra with 8 G² at 290 K shows that the 4-NH₂PyH^?+? cations reside in the rigid lattice in the β-phase. In contrast, in the α(I)-phase the cation rotates about an axis more symmetric than pseudo threefold axis. The activation energy of 21 kJ mol^???1 was estimated for the reorientational motion in the α(I)-phase from the ¹H NMR T ₁ measurements. The nature of phase transitions in the 4-NH₂PyHSbI₄ is discussed in comparison with that in 4-NH₂PyHSbBr₄.     

Electronic properties of YBa2Cu4O8 and Y2Ba4Cu7O15 as probed by NMR/NQR

Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) allow the investigation of electronic properties at the atomic level. We will report on such studies in typical members of the Y-Ba-Cu-O family of high-temperature superconductors, namely YBa₂Cu₄O₈ and Y₂Ba₄Cu₇O₁₅. We will deal with temperature and orientational dependences of Cu and O Knight shifts and spin-lattice relaxation. Topics to be discussed are: (1) Existence of a common electronic spin-susceptibility (a single-spin fluid) in the planes and in the chains of YBa₂Cu₄O₈. (2) Evidence for the existence of a pseudo spin-gap in the anti-ferromagnetic fluctuations in YBa₂Cu₄O₈ and Y₂Ba₄Cu₇O₁₅. (3) Evidence for non-s symmetry of the pair wave function in YBa₂Cu₄O₈. (4) First NQR observation of a strong coupling of inequivalent Cu-O planes in Y₂Ba₄Cu₇O₁₅. (5) Alternative explanations of relaxation and Knight shift.     

过渡金属Tc及其氮化物TcN,TcN2,TcN3与TcN4低压缩性的第一性原理计算研究

采用基于密度泛函理论的第一性原理计算方法研究了Tc及其氮化物的弹性性质、电子结构、原子布局数等,并分析和计算了它的氮化物的理论硬度.结果表明随着Tc中氮的掺入量的渐增,出现层状结构,它们的弹性模量并非单调增加,其中TcN的体弹模量最大而剪切模量最小;层状结构的TcN3的剪切模量最大而体弹模量最小,TcN4的理论计算硬度最大.说明在Tc的氮化物中,其体弹模量与材料的晶体结构、平均每个原子上的电荷密度和材料的质量密度有关;化学键的共价性结构和氮元素的含量对理论计算硬度有正作用;而剪切模量的极大值则与其层状结构及体系中一定量的方向基本一致的N-N键相关.     

A reexamination of19F NMR in selected solids,the conductor Ag2F and Reference Insulators for studies of YBa2Cu3O7-type superconductors

NMR signals of¹⁹F have been measured in the polycrystalline inorganic conductor Ag₂F and in the polycrystalline insulators AgF, YOF, EuOF, YF₃, CuF₂, BaF₂ and KF to compare them to the signals found in the so-called «1-2-3-type» compounds with claimed formulas RBa₂Cu₃O_7-xF_x (R=Y and Eu). No evidence for a Knight-shifted, built-in fluorine-signal was found in the 1-2-3-type superconductors, whereas Ag₂F shows a clearly downfield shifted¹⁹F peak with reference to AgF.