| Superconductivity (SC) and ferromagnetism (FM), as the macroscopic manifestations of mi-croscopic quantum many-body physics, are the two quantum ground states in condensed matter. Orbital mechanism and for the spin-singlet SC, paramagnetic effect lead to the incompatible na-ture between SC and FM. However, with the development of theoretical and experimental studies of condensed matter physics, the coexisting phenomena between them are gradually unraveled and the materials with the coexistence of SC and FM are constantly discovered, which shows many exotic properties. For the meantime, exploring new superconducting materials and uncovering the novel mechanisms of Cooper pairing in new superconductors have always been one of the most important branches in condensed matter physics as the important significance of SC to human production and life.This dissertation mainly focuses on the studies of two aspects:One is about three supercon-ductors with the presence of ferromagnetically internal field, induced by doping EuFe2As2, which is one of the parent compounds in iron pnictides. As the transition temperature for the magnetic ordering is lower than that of SC, they are named as iron-based ferromagnetic superconductors. The presence of S-state Eu layers make the related systems show the interplay between localized moments and SC. The other is about the new layered superconductor we reported. The quasi-one-dimensional (quasi-1D) characters are endowed by the presence of PdTe2chains, and the ap-pearance of SC is extremely remarkable when considering the fertile physics the low-dimensional systems may have. To be specific, this thesis mainly covers three iron-based ferromagnetic super-conductors and a new low-dimensional superconductor:(1) By the isovalent doping of Ru at the Fe site in EuFe2As2, we obtained the iron-based ferromagnetic superconductor Eu(Feo.75Ruo.25)2As2crystals. We reported the novel anisotropic SC that the material shows and presents the evidences for the coexistence of SC and FM. The evolution of magnetic order and SC with the doping level in Eu(Fe1-xRux)2As2is as well discussed, and the anisotropic phase diagram is concluded;(2) We obtained the iron-based ferromagnetic superconductor Eu(Fe0.88Ir0.12)2As2crystals by Ir doping at the Fe site. The spin-reorientation proccess of Eu moments is expected to occur below the super-conducting transition, and the presence of the ferromagnetically internal field make the absence of Meissner state;(3) We obtained the iron-based ferromagnetic superconductor Eu(Fe0,91Rh0.09)2As2crystals by Rh doping at the Fe site. The conclusion about the vortex penetration at low temperature directly reflects the evidence of spontaneous vortex state (SVS), and we present the clear physical interpretation about the way for the coexistence between SC and FM in EuFe2As2-related system-s.(4) Superconducting and strong correlated electron properties in superconductor Ta4Pd3Te16crystal are discussed. The main conclusions are highlighted as following:(1) We have successfully grown the Ru-doped Eu(Fe0.75Ru0.25)2As2crystals. Although the resistance along ab plane and c axis both shows the superconducting transition at23K, the state for zero resistance along c axis is not achieved below23K. To ensure this phenomenon is indeed the intrinsic property, we measure the susceptibility, which shows the diamagnetic signal only ap-pears for external field along c axis, but no for field along ab plane. The finite resistance along c axis below the superconducting transition is consistent with the susceptibility results, which sug-gests the shielding current can not develop for field along ab plane. The hysteresis loops for H‖c at low temperature behave like an "ideal conductor" which shows zero resistance but does not have Meissner state, and for H‖ab are typical for a common ferromagnet. The initial magnetiza-tion curve for H‖ab is positive, consistent with the susceptibility results, further confirming the anisotropic SC in this material. Anisotropic density of vortex and pinning force are the possible reasons that result in the anisotropic SC. To understand the distinctiveness from other Eu-free iron-based superconductors with the same structure, we systematically investigate the phase diagram of Eu(Fe1-xRux)2As2. Within the suerconducting region of0.2≤x≤0.5, Ρab=0at low temper-ature only for the region of0.2≤x≤0.25, while no zero resistance for current along c axis is observable. Thus, the anisotropic SC is general for the whole superconducting region. The col-lapsed transition occur at the concentration, where the superconducting transition disappears. We also discuss the evolution of magnetic ordering of Eu layer with the doping level.(2)We have successfully grown the Ir-doped Eu(Fe0.88Ir0.12)2As2crystals through the pro-cess improvement of crystal growth. The resistance shows the superconducting transition at22K, and the anisotropy ratio γ=Habc2/Hc2c, derived from the measurement of MR under field a-long ab plane and c axis, is-1.3at low temperature, similar to other iron pinctides. According to Ginzburg-Landau theory, this ratio is typical for layered superconductors. However, the upper critical fields show anomalous anisotropy around20K, i.e.,γ<1, which we believe is asso-ciated with spin-reorientation process of the localized moment of Eu. The results of anisotropic susceptibility further confirm the reasonability of this physical picture, i.e., the S-state Eu2+-spin firstly lying along the ab plane followed by the tilting process towards c axis at lower temperature. Three characteristic temperatures are defined, which are the superconducting transition tempera-ture Tsc, the transition temperature Tmab for Eu2+-spin lying along ab plane and Tmtilt for Eu2+-spin tilting towards c axis. The low-field susceptibility suggests the absence of Meissner state at low temperature. Thus, the ferromagnetic component of the magnetic ordering of Eu layer places the system into the mixed state, based on which, we call it a ferromagnetic superconductor. Besides, the dominant superconducting part is successfully decomposed from the hysteresis loop for H‖c at low temperature, confirming the coexistence of SC and FM.(3) We have successfully grown the Rh-doped Eu(Feo.9iRho.o9)2As2crystals. After investi-gations of its transport and magnetic properties, the similar spin-reorientation process below the superconducting transition to that in Eu(Fe0.88Ir0.12)2As2is concluded. Besides, the re-entrant re-sistance around the temperature Tm for the magnetic ordering reminds us of the SVS, which is one of the ways for the coexistence of SC and FM and may be the underlying cause for this phe-nomenon. The low-field hysteresis loops shows the character of mixed state, and the behavior that the susceptibility is lower upon cooling than that upon heating in field-cooling (fc) mode is revealed around Tm. This kind of magnetic hysteresis is regarded as an effect induced by the vortex pinning in ferromagnetic superconductors. To further certify the presence of the SVS or vortex penetration, which lead to the crystal in the mixed state at low temperature, we carried out the measurement of ac susceptibility as Xac is related with the magnetic hysteresis. Our results confirm the crystal in the mixed state at low temperature, thus consistent with the physical picture, which is the SVS as the coexisting way between SC and FM in this system. Here, by the measurement techniques of ac and dc susceptibility, we firstly confirm the way for the coexistence of SC and FM in iron-based fer-romagnetic superconductors-spontaneous vortex phase. At last, a schematic diagram is concluded to vividly portray the picture about why and how SC originated from the FeAs layer coexists with FM originated from Eu layer in the form of the SVS in Eu(Fe0.91Rh0.09)2As2as a representative of iron-based ferromagnetic superconductors. The actual phase diagram derived from the data of the upper critical field is basically consistent with the schematic diagram, thus corroborating our physical interpretation of the interplay of SC and FM in iron-based ferromagnetic superconductors. (4) We have discovered a new superconductor Ta4Pd3Te16, and by our improved method for crystal growth, the crystal is successfully grown out. The new superconductor Ta4Pd3Te16has a layered structure with quasi-1D PdTe2chains, among which the weak coupling make the layered plane (-103) formed. The onset temperature for superconducting transition is4.62K, and by s-tudies of its physical properties, we found it is a typically type-Ⅱ superconductor. Although the dimensionless specific-heat jump AC/(γTC) agrees well with the BCS theory in the weak coupling scenario, the Sommerfeld coefficient～42.8mJ·K-2mol-1derived by fitting the specific-heat da-ta and the Fermi-Liquid (FL) behavior shown from the low-T resistivity both denote the strong electron-electron correlation in Ta4Pd3Te16. The Kadowaki-Woods ratio (△/γ2=0.211×10-5μΩ·cm·mol2·K2·mJ·2) even suggest this material is placed into the group of strongly-correlated electron system. Thus, possible unconventional mechanism of Cooper pairing and the (spin/charge) density wave due to the low dimensionality may appear in this quasi-1D layered material, which certainly deserve further investigations. |
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