Hole Doping In Low-dimensional Transition Metal Antifer- Romagnets And The Discovery Of A Superconductor K₂Cr₃As₃

Superconductivity is an important field in condensed matter physics and searching new su-perconductors is a significant direction of this field. After Onnes discovered superconductivity, BCS theory is successful in explaining conventional superconductors with electron-phonon cou-pling. However, the discovery of new unconventional superconductors such as heavy fermion, cuprates and iron based superconductors created a big challenge for the theory of superconductiv-ity for each system. Magnetism antagonizes superconductivity by breaking apart Cooper pair in conventional superconductors; however, the formation of Cooper pair is possibly related to mag-netism in unconventional superconductors. Although the mechanisms of superconductivity may not be the same for different unconventional superconductors they do have a similar phase dia-gram:superconductivity appears after magnetism is suppressed by some methods. Inspired by the similar phase diagrams of unconventional superconductors, this dissertation tries suppressing antiferromagnetism in a material by chemical doping to search for superconductivity.Following the current research background, this dissertation tries studying potassium doping effects in three antiferromagnets:BaFe2Se3, BaMn2As2 and BaCr2As2. The original achievements are shown as follows:(1) BaFe2Se3 is an antiferromagnetic insulator. All the samples with different potassium doping levels exhibit an insulating behavior. We did explicit characterizations of physical prop-erties, including anisotropic resistivity and magnetization as well as specific heat, for one dop-ing level Bao.6K0.4Fe2Se3. Bao.6Ko.4Fe2Se3 has a weak anisotropic resistivity and its conductivity obeys a variable-range-hopping behavior at low temperatures. We found that Bao.6Ko.4Fe2Se3 is a Heisenberg-like spin glass with some anisotropy.(2) After potassium doping in BaMn2As2, it changes from an insulator to a metal. However, the potassium doping level achieved can not completely suppress the G-type antiferromagnetism in BaMn2As2. No superconductivity has been realized in this system Ba1-xKxMn2As2. We did resistivity and anisotropic magnetization for x=0.19 and x=0.26. We first observed a weak ferromagnetism with a transition temperature of about 30-50 K. The easy magnetization axis of the weak ferromagnetism is along ab plane while the magnetic moment of G-type antiferromagnetism is along c axis. We found that magnetic susceptibility of ab plane for x=0.19 and x=0.26 obeyed an exotic Curie-Weiss behavior. The effective moment is equal to a spin-1/2 paramagnet. We performed first-principles calculations on parent compound and x=0.25. The 4p hole of As is the dominant part in the metallic conductivity. We proposed two possibilities for the weak ferromagnetism in Ba1-xKxMn2As2:one is the scenario of spin-polarized carriers, the other is antiferromagnetic spin pairs canted into ab plane.(3) Synthesizing potassium doping compounds of BaCr2As2 is not successful. However, we discovered two new quasi-one-dimensional compounds K2Cr3As3 and KCr3As3. They consist of one-dimensional (Cr3As3)∞ chains with potassium cations intercalated between them.We first discovered a new chromium arsenide compound K2Cr3As3 with a superconducting transition temperature 6.1 K at ambient pressure. It is an extreme type II superconductor. The resistivity of poly crystals has a linear temperature-dependence in the normal state, indicating a non-Fermi-liquid behavior. K2Cr3As3 has a very high upper critical field, exceeding Pauli paramagnetic limit (Hp=11 T) by 3-4 factors. So spin triplet pair in this compound is possible. It has a large normal electronic specific heat (γ=70-75 mJ K-2) and a high specific heat jump (γT/ΔC= 2.2-2.4), pointing to significant electron correlations and a strong coupling situation, respectively. All the results above strongly support an unconventional superconductivity in K2Cr3As3.We synthesized a new compound KCr3As3 by using ethanol to de-intercalate one potassium from K2Cr3As3. Resistivity, magnetization and specific heat measurements prove it is not super-conductive. KCr3As3 has a semiconduct-like resistivity at low temperatures but the resistivity is still small even at the lowest temperature. It obeys Curie-Weiss law at high temperatures and the effective moment of Cr is 0.68μB.Dc and ac magnetic susceptibility prove cluster spin glass state at low temperatures in KCr3As3. Although KCr3As3 is not superconductive, the comparative research on it is also a routine to fully understand the superconductivity in K2Cr3As3.In addition, some directions for further research are put forward in the last chapter of this dissertation.