Study On Structure And Physical Properties Of The Spin 1/2 Ladder Compound Sr_14-XCa_xCu₂₄O₄₁

After the high-temperature superconductivity was discovered in doped two-dimensional antiferromagnetic copper oxides, one dimensional cuprates with the same Cu-O bonds have attracted much interest. Particularly, ladders with even legs are predicted to have a finite spin gap reminiscent of that in underdoped high-Tc cuprates, and the theoretical works suggest a possibility of superconductivity when doped with holes. The system Sr₁4-xCa_xCu₂4O₄1 we have studied is a kind of quasi-one-dimensional antiferromagnetic copper oxide, consists of two-legs ladders (Cu₂O₃), spin gap exists in both spin chains (CuO₂) and ladders (Cu₂O₃). In particular, superconductivity has been found in high Ca content doped compounds of this system. In this thesis, we exhibit several experimental investigations on this kind of ladder compounds.Firstly, we prepared two series of samples with the conventional solid state reaction method. And our systematic study was aimed at the first series, namely Sr₁4-xCa_xCu₂4O₄1 compounds with x (Ca) = 0,0.4, 3.5, 6, 7, 8.4.Secondly, we confirmed all the samples of the first series are of single phase by x-ray diffraction analysis and found the crystal grains have grown well by observed the surfaced pattern with scanning electronic microscope. The unit cell parameters of all the samples of the first series at room temperature were identified through peak-seeking and fitting program. We measured the Cu 2p ₃/2, O 1s,Ca 2p₃/2 and Sr 3d₅/2 photoelectron emission spectra and mainly discussed the valence of Cu ions. The results reveal that the valence of Cu ions in this system is mainly +2, Cu⁺3 ions is very few and the existence of O holes can explain for the average valence +2.25 of Cu ions.Thirdly, we measured the magnetic susceptibilities of the various samples of the first series (except for x(Ca)= 0.4, 7 ) under different magnetic intensities(H= 1T, 3T, 5T) within the temperature range of 10-300K. The effect of the doping of Ca and the magnetic field on the magnetic susceptibility, particularly the effect on spin gap related to the broad peak of the magnetic-field intensity around 60K is discussed. Furthermore, we fitted theexperimental data and discussed the connection between magnetic property and electronic state.Fourthly, we measured the specific heat of Sri4Cu24O4i and Srio^Ca3.5Cu2404i under different magnetic-field intensities (zero and 3 Tesla) within the temperature range of 2-12K and 50-90K respectively. The experimental data of the specific heat within 2-12K give out the coefficients of each term of specific heat (a > Y ^ ^). Base on these results we figured out the value of Debye temperature ?o ^ Fermi temperature TF and Fermilevel EF .We found the linear term of the specific heat is abnormally large and discussedthe effect on the specific heat by the doping of Ca and the magnetic-field intensities. Furthermore, we analysed the abnormality of specific heat within 50-90K.We connected this abnormality with the spin gap dued to the formation of dimer in the O1O2 chains and suggest that the doping of Ca and the magnetic intensities don't effect the open of spingap-Finally, we measured the dc-resistivity of Sri4Cu24O4i within the temperature range of 80-280K. The semiconductor behavior of Sri4Cu24O4i was confirmed. We identified the temperature Tp below which holes in the G12O3 ladders are localized. Accordingly, thereare two kinds of active energies: 82meV (belowTp) and 106meV (above Tp).