Interplay Among Stripe Phase And Transport Properties In La_1.6-xNd_0.4Sr_xCuo₄and K_0.8Fe_1.65Se₂Superconductors

Experimental and theoretical researches have verified that the intrinsic electron inhomogeneity plays a dominant role in the cuprates superconductors, which leads to the appearance of the stripe phases. In the stripe phase, the doped carriers accommodate in striped pattern to reduce their kinetic energy and separate the antiferromagnetic domains in the CuO2plane. The existence of the stripe phase in cuprates superconductors has been confirmed by many experimental results. It is believed that the stripe phase plays a key role in the mechanism of high temperature superconductivity and has become one of the most interesting hot topics in condensed matter physics.In this dissertation, the Hall effect, the Seebeck effect and the specific heat were studied in the La1.6-xNd0.4SrxCuO4single crystals with static stripe phase and the influence of the stripe phase on these properties were discussed. Furthermore, the anisotropy of the K0.8Fe1.65Se2single crystal which contains prominent stripe structures was also studied.In chapter one, the picture and the properties of the stripe phase were introduced. The influence of the stripe phase on the Fermi surface and the specific heat of the high temperature superconductors were discussed. Moreover, the phase separation in K0.8Fe1.65Se2superconductor was also introduced, and the relationship between the phase separation and superconductivity was analyzed. In chapter two, the crystal growth process by optical floating-zone single crystal furnace was firstly introduced. Then, the Hall coefficient (RH) and the Seebeck coefficient (S) of La1.6-xNd0.4SrxCuO4(LNSCO) single crystals were studied. It is found that above the charge ordering temperature (Td), the RH and S change a little as the temperature changes. While they drop abruptly from Td and eventually change sign from positive to negative at low temperatures, indicating the presence of electron pockets in the Fermi surface due to the Fermi surface reconstruction. The RH and S for La2-xSrxCuO4single crystal without static stripe phase do not change sign. With increasing Sr doping level, the charge stripe order state in LNSCO weakens and correspondingly the negative RH gets small. These results indicate that the static charge stripe order plays an important role in the appearance of the negative RH and the Fermi surface reconstruction is closely to the static charge stripe order. The stability of the charge stripe order can also be tuned by the epitaxial strain induced by the mismatch between the thin film and the substrate. In the thinner film, the static charge stripe is greatly suppressed by the strong epitaxial strain, and the negative RH disappears. While for a strain released thicker film the negative RH recovers. These results again indicate the key role of the static charge stripe in Fermi surface reconstruction in the system.In chapter three, the specific heat of La1.6-xNdo.4SrxCu04(x=0.10,0.12,0.15) single crystals were systematically studied. It is found that the biggest entropy change associated with the structural phase transition appears in sample x=0.12with the most stable charge order and the smallest one in sample x=0.15among the three samples, indicating that the stability of the charge order could be characterized by the entropy change. At low temperatures, a large Schottky anomaly specific heat appears because of the existence of magnetic rare-earth ions Nd3+. The peak of Schottky anomaly shifts gradually to higher temperatures with increasing magnetic fields, and can be fitted in terms of a model associated with the splitting of the Nd3+ground-state doublet in magnetic fields. Furthermore, the phonon spectrum of the system is modified by the replacement of La with Sr and a higher frequency vibration mode appears which results in the enhancement of the lattice specific heat in the system.In chapter four, the in-plane anisotropy of K0.8Fe1.65Se2single crystal was studied through measuring the angular dependencies of out-of-plane resistivity pc. It is found that the upper critical field Hc2shows a fourfold symmetry which is closely related to the fourfold symmetric superconducting stripes arising from the phase separation in K0.8Fe1.65Se2system. When the magnetic field is applied along the superconducting stripes, it can penetrate the stripes heavily due to the small width of the stripes. The penetration of the magnetic field can dramatically lower the free energy of the superconducting stripes, so the upper critical field Hc2110parallel to the stripe is higher than that in other directions. Therefore, Hc2(φ) will display a fourfold symmetry. A scaling approach was proposed to analyze the in-plane fourfold symmetry of Hc2, which fits the experimental data very well. Furthermore, the out-of-plane resistivity pc(φ) also shows a fourfold symmetry below Tc which is also related to the fourfold symmetric superconducting stripes. The fourfold symmetric behaviors of pc(φ) in flux flow region were described by a modified Bardeen-Stephen equation after considering the stripes. The vortex dynamics of K0.8Fe1.65Se2system are also affected by the fourfold symmetric superconducting stripes, which is manifested by the fourfold symmetry in the irreversibility field Hirr and the activation energy U for flux motion. Moreover, pc(φ) shows a twofold symmetry above the superconducting transition temperature Tc. The twofold symmetry gradually weakens as the temperature approaches the antiferromagnetic order (AFM) temperature, so the twofold symmetry probably originates from the AFM order.In chapter five, the anisotropy between the in-plane and out-of-plane in K0.8Fe1.65Se2system was studied. The upper critical field Hc2and the activation energy U for flux motion were obtained by measuring the in-plane resistive transition curves. It is found that the anisotropy parameter Γ≈3.35in K0.8Fe1.65Se2system which is larger than that of the cuprates superconductors, indicating the potentially application of K0.8Fe1.65Se2superconductor. The critical current density Jc was found to be smaller than that in other Fe-based superconductors, which may be related to the phase separation in K0.8Fe1.65Se2system.In chapter six, the nanoparticles of La2-xSrxCuO4(0.10<x<0.30) superconductors were synthesized by the sol-gel method and the magnetic properties of the nanoparticles were studied. It is found that the magnetic susceptibility of the nanoparticles shows an upturn in low temperature, which arises from the weak ferromagnetism indicated by the ferromagnetic hysteresis loop above Tc. The Electron Spin Resonance (ESR) spectra for the nanoparticles all show an absorption peak, while it is absent in the polycrystalline samples. The results indicate that there are lots of uncompensated surface Cu2+spins in the nanoparticles. The coexistence of the free electrons and the local magnetic moments in the nanoparticles make the RKKY interaction possible and the origin of the weak ferromagnetism may be due to the enhancement of RKKY ferromagnetic coupling between uncompensated surface Cu2+spins.