Synthesis And Physical Properties Of Quasi-One-Dimensional Chromium Pnictide

After Kamerlingh Onnes discovered superconductivity in pure mercury,superconductivity has always been a hot topic in condensed matter physics.The revolutionary discovery of high-temperature SC in cuprates in the 1980s has inspired the unprecedented enthusiasm to explore new superconductors,especially in transition-metal compounds.The continuous efforts over a quarter century have led to the discoveries of unconventional SC in a few classes of materials including iron pnictides,which bear two fundamental similarities to the cuprates:strong electron correlations and quasi two dimensionality.Superconductivity(SC)rarely appears in quasi-one-dimensional(Q1D)crystalline materials primarily because of the Peierls instability towards a state of the charge-density wave.So it is meaningful to explore possible unconventional SC in a Q1D material with significant electron correlations.Following the discovery of superconductivity at 6.1 K in the ternary Cr-based arsenides K2Cr3As3.This dissertation tries studying the element substitution effect of the K2Cr3As3.The original results are shown as follows:(1)Bulk superconductivity emerges at 4.8 K for Rb2Cr3As3.The crystal structure is character-ized by[(Cr3As3)-2]∞ double-walled subnanotubes,above Tc which the polycrystalline sample’s normal-state resistivity shows a linear temperature dependence up to 35 K.The estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit by a factor of 2.Further-more,the electronic specific-heat coefficient extrapolated to zero temperature in the mixed state increases with(?),suggesting the existence of nodes in the superconducting energy gap.Through the electrical resistance measurements under magnetic fields,we find the Rb2Cr3As3 single crystal has anisotropic Hc2(T)behaviours in a broad temperature.The normal-state magnetoresistance is small and negative for H⊥c,which could be related to spin fluctuations.The Hc2 anisotropy at Tc is about twice of that in the sister compound K2Cr3As3,consistent with the weaker interchain coupling expected from the longer interchain distance in Rb2Cr3As3.The curvatures of Hc2‖(T)and Hc2⊥(T)are characteristic of paramagnetically and orbitally limited,respectively.The zero-temperature upper critical fields are extrapolated to be μ0Hc2‖(0)= 17.5 T and μ0Hc2⊥(0)=29 T.The latter is over three times of the Pauli paramagnetic limit.The anisotropy reversal as well as the linear increase of Hc2⊥(T)down to 0.075Tc suggest dominant spin-triplet superconductivity in Rb2Cr3As3.(2)Cs2Cr3As3 possesses the largest interchain distance,bulk superconductivity appears below 2.2 K.Similar to the former sister compound Rb2Cr3As3,which crystallizes in a hexagonal lattice and exhibits a non-Fermi liquid behavior with a linear temperature dependence of resistivity in the normal state,and a high upper critical field beyond the Pauli limit as well,suggesting a common unconventional SC in the Q1D Cr-based material.(3)Polycrystalline samples of RbCr3As3 and CsCr3As3 were synthesized by reacting Rb2Cr3As3 and Cs2Cr:3As3 polycrystals with absolute ethanol,the crystal structure also remains the characteristic quasi-one-dimensional[(C3rAs3)-2]∞ chains.Both materials are not supercon-ductors,but show a cluster spin glass state below 5-6 K.Importantly,the Cr localized moment increases with the expansion of interchain distance.This further confirms the existence of Cr local-moment components in the "A133" system,which implies the relevance of spins to the novel superconductivity in "A233" materials.