Superconductivity In Weakly Correlated Non-centrosymmetric Compounds

Superconductivity is a fundamental physical phenomena arising from collective many body effect. Elucidation of the unconventional pairing mechanism as well as search for new superconductors with higher transition temperature or exotic properties is highly important and desired in condensed matter physics. Motivated by the discovery of the non-centrosymmetric heavy fermion superconductor CePt3Si in2004, superconductivity lacking inversion symmetry has become a subject of growing interest because of its conceptually different pairing state. An important issue is to reveal the novel pairing state and its relation to the antisymmetric spin-orbit coupling (ASOC) caused by the absence of inversion symmetry. Moreover, non-centrosymmetric superconductors were recently proposed as important candidates for studying topologic superconductivity.In this dissertation, I present a systematic study on the superconducting properties of the following weakly-correlated non-centrosymmetric superconductors: Y2C3, LaNiC2, NbxRe1-x and LaMSi (M=Rh, Ir); the latter three compounds were synthesized and characterized by myself. Particular attention is paid to the superconducting pairing state characterized by various thermodynamics properties and its connections with the antisymmetric spin-orbit coupling.This dissertation contains the following seven chapters:A brief introduction and history of superconductivity is given in Chapter1.Chapter2reviews the recent progress on the study of non-centrosymmetric superconductors (NCSs).Chapter3describes the experimental methods and instrument we used in this study, which mainly include the techniques for sample growth, e.g., solid state reaction, flux growth, arc-melting and Czochralski pulling, as well as the measurements of transport, magnetic and thermodynamic properties.In Chapter4, we study the superconducting properties of Y2C3by measuring the magnetic penetration depth△λ(T) and the upper critical field Bc2(T). It is found that the penetration depth△λ(T) and its corresponding superfluid density ρs(7) can be described in terms of a two-gap BCS model near Tc, but show linear temperature dependence at very low temperatures (T<<Tc), indicating the existence of line nodes in the superconducting energy gap. The upper critical field Bc2(T) of Y2C3demonstrates a weak upturn at low temperatures with a rather high value of Bc2(0)≈29T, which slightly exceeds the weak-coupling Pauli limit. We discuss the possible origins for these nontrivial superconducting properties, and argue that the nodal gap structure in Y2C3is likely attributed to the absence of inversion symmetry, which allows the admixture of spin-singlet and spin-triplet pairing states.In Chapter5, we show evidence of two-gap superconductivity for LaNiC2. The superconducting pairing state of LaNiC2is systematically investigated by measuring the London penetration depth△λ(T), the specific heat Cp(T,B) and the electrical resistivity p(T,B). Both△λ(T) and the electronic specific heat Ce(T)/T exhibit behavior at low temperatures that can be described in terms of a phenomenological two-gap BCS model. The residual Sommerfeld coefficient in the superconducting state, yo(B), shows a rapid increase at low fields and then an eventual saturation with further increasing magnetic field. A pronounced upturn curvature is observed in the upper critical field Bc2(T) near Tc. All these experimental observations support the existence of two-gap superconductivity in LaNiC2.Chapter6provides evidence of BCS superconductivity for NbxRe1-x. We synthesize the compounds NbxRe1-x(x=0.13-0.38) and study their superconducting properties by measuring the electrical resistivity p(7), magnetic susceptibility x(T), heat capacity CP(T) and London penetration depth△λ(T). It is found that the superconducting transition temperature, Tc, decreases monotonically with the increase of x. The upper critical field Bc2(T) for various x can be nicely scaled by its corresponding Tc. The electronic specific heat Ce(T)/T, the penetration depth△λ(T) and the superfluid density ρs(T) demonstrate exponential behavior at low temperatures and can be well fitted by a single-gap BCS model. The residual Sommerfeld coefficient yo(B) follows linear field dependence. All these properties suggest an5-wave type of superconductivity for NbxRe1-x (0.13≤x≤0.38).In Chapter7, we present a detailed study on the sample preparation of LaMSi (M=Rh, Ir) and the primary measurements of their superconducting properties. It is found that LaIrSi is a metastable superconductor:the as-grown samples are superconducting below Tc≈2.53K with a small volume fraction, while LaIr2Si2phase appears after annealing. The penetration depth△λ<T) of LaIrSi exhibits power-law behavior,△λ(T)～T1.7, at low temperatures, indicating a possible exotic pairing state. The LaRhSi samples exhibit bulk superconductivity below Tc≈4.4K, but always mixed with some impurity phases. The upper critical field Bc2(T) of LaIrSi and LaRhSi follow similar linear-temperature dependence, exceeding the weak-coupling WHH values in the low-temperature limit.A summary and perspective is presented at the end of the dissertation.