Research On Ultra-low Temperature Transport Properties Of Unconventional Superconductors

In most element and alloy superconductors, the electrons with equal but opposite momentum and spin can bind Copper pairs in virtue of an attractive interaction. The weak attractive interaction arises from the virtual exchange of phonons and the screened Coulomb repulsion between electrons. In analogy to quantum mechanics, the pairing is described by an isotropic wave function, and denoted as s-wave pairing. These are labeled conventional superconductors for their superconducting order only break the U(1) gauge symmetry and superconducting mechanics can be interpreted within the BCS theory. In contrast, superconductivity is denoted unconventional, if below the transition temperature Tc additional symmetries are broken besides the gauge symmetry, or if the pairing mechanism is mediated by non-phononic interactions, such as magnetic interactions.In the past thirties years, two novel classes of strongly correlated materials are widely studied--the heavy fermion compounds and cuprate high-temperature superconductors. The former are intermetallics containing rare earth ions and the latter are highly anisotropic superconductors after doping their parent compounds. Superconductivity discovered in iron pnictides announced the era of unconventional superconductivity to a wider community. These iron pnictides are the second-highest-temperature superconducting material family known to date and might offer the hope of finally unveiling the secret of high-temperature superconductivity. Superconductivity discovered in noncentrosymmetric materials also fall outside the BCS paradigm of electron-phonon mediated pairing with an isotropic gap. For noncentrosymmetric superconductors, pairing function on the same Fermi surface sheet becomes a mixture of spin singlet and triplet. Spin triplet components in the pairing function favor anisotropic or nodal gap structure giving rise to unusual behaviors. In this dissertation, we report some progresses on studying the superconducting gap structures of several types of unconventional superconductors by means of thermal transport methods. The corresponding results are listed as follows:1. We measured the thermal conductivity of electrons overdoped FeAs-based superconductor BaFe1.73Co0.27As2 (Tc=8.1 K), where the normal state can be described within the picture of Fermi liquid. In zero field, the residual linear term ko/T is negligible, suggesting a nodeless superconducting gap in the ab plane. In magnetic fields, ko/T increase rapidly, very different from that of conventional s-wave superconductors. This anomalous ko/T(H) may reveal an exotic gap structure:the vanishing hole (?) pocket has a much larger gap than the electron (y and 6) pockets which contain most of the carriers. Such an exotic gap structure is an evidence for superconducting state induced by interband interactions, in which the band with the smaller density of states has a larger gap.2. The in-plane resistivity of extremely overdoped superconductor KFe2As2 (Tc= 3.0 K) single crystal were measured in dilution fridge down to 50 mK. We observe non-Fermi-liquid behavior p?T1.5 at Hc2= 5 T, and the development of a Fermi liquid state when further increasing the field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field Hc2.In zero field, there is a large residual linear term ko/T, and the field dependence of Ko/T(H) mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe2As2 have nodes. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.3. We measured thermal conductivity of the iron selenide superconductor FeSex (Tc= 8.8 K) down to 120 mK and up to 14.5 T ((?)3/4Hc2). In zero field, the residual linear term ko/T at T?0 is only about 16?W K-2 cm-1, less than 4% of its normal-state value. Such a small ko/T does not support the existence of nodes in the superconducting gap. More importantly, the field dependence of Ko/T in FeSex is very similar to that in NbSe2, a typical multigap s-wave superconductor. We consider our data as strong evidence for multigap nodeless (at least in ab plane) superconductivity in FeSex.4. For the first time, we report field-induced quantum critical point and nodal superconductivity in the heavy-fermion superconductor Ce2Pdln8 (Tc= 0.68 K). From resistivity measurement, a field-induced quantum critical point occurring at the upper critical field Hc2, is demonstrated from p?T near Hc2 and p?T2 when further increasing field. Large residual linear term ko/T at zero field and the rapid increase of k(H)/T at low field give evidences for nodal superconductivity in Ce2Pdln8. The sharp jump of?(H)/T near Hc2 suggests a first-order-like phase transition at low temperature.5. We have successfully grown pyrochlore oxide superconductor Cd2Re207 (Tc= 1.38 K) by chemical vapor transport method. The absence of inversion symmetry after the structure transition at T= 200 K yields an asymmetric spin-orbit interaction that breaks parity. The absence of residual linear term Ko/T is strong evidence for nodeless superconducting gap. In magnetic fields, Ko/T grows steadily with fields and finally saturates at H= 0.4 T, indicating the upper critical field Hc2 of Cd2Re2O7. The exotic field dependence of normalized residual linear term(k0/T)/(kNO/T) indicates a slight anisotropic gap structure which may be due to the weak spin-triplet components in the pairing function.6. We have successfully grown spin ice material Ho2Ti2O7 by optical floating zone method. Thermal transport measurements show a transition occurring at about 250 mK, which is characterized by the true freezing temperature (Tf) of Ho2Ti207. Phonons hardly feel any magnetic scattering below Tf, contrary to the case above Tf where thermal conductivity is suppressed by magnetic excitations. Both freezing temperatures and thermal conductivity above Tf decrease obviously with applying a magnetic field parallel to [111] direction. The restoration of them was founded when applying an even higher magnetic field than H= 1.2 T.