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Register a new userChemical Substitution and High Pressure Effects on Superconductors in the $Ln$OBiS$_{2}$ ($Ln$ = La-Nd) System
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A large number of compounds which contain BiS$_{2}$ layers exhibit enhanced superconductivity upon electron doping. Much interest and research effort has been focused on BiS$_{2}$-based compounds which provide new opportunities for exploring the nature of superconductivity. Important to the study of BiS$_{2}$-based superconductors is the relation between structure and superconductivity. By modifying either the superconducting BiS$_2$ layers or the blocking layers in these layered compounds, one can effectively tune the lattice parameters, local atomic environment, electronic structure, and other physical properties of these materials. In this article, we will review some of the recent progress on research of the effects of chemical substitution in BiS$_{2}$-based compounds, with special attention given to the compounds in the $Ln$OBiS$_{2}$ ($Ln$ = La-Nd) system. Strategies which are reported to be essential in optimizing superconductivity of these materials will also be discussed.
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The upper critical field $H_{c2}$ of polycrystalline samples of $Ln$O$_{0.5}$F$_{0.5}$BiS$_{2}$ ($Ln$ = La, Nd) at ambient pressure (tetragonal structure) and high pressure (HP) (monoclinic structure) have been investigated via electrical resistivity measurements at various magnetic fields up to 8.5 T. The $H_{c2}$($T$) curves for all the samples show an uncharacteristic concave upward curvature at temperatures below $T_c$, which cannot be described by the conventional one-band Werthamer-Helfand-Hohenberg theory. For the LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ sample under HP, as temperature is decreased, the upper critical field $H_{onset}$, estimated from the onset of the superconducting transitions, increases slowly between 4.9 and 5.8 T compared with the slope of $H_{onset}$($T$) below 4.9 T and above 5.8 T. This anomalous behavior reveals a remarkable similarity in superconductivity between LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ samples measured under HP and synthesized under HP, although the crystal structures of the two samples were reported to be different. The experimental results support the idea that local atomic environment, which can be tuned by applying external pressure and can be quenched to ambient pressure via high temperature-pressure annealing, is possibly more essential to the enhancement of $T_c$ for BiS$_2$-based superconductors than the structural phase transition. On the other hand, such anomalous behavior is very subtle in the case of NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ under HP, suggesting that the anisotropy of the upper critical field in the $ab$-plane and the possible lattice deformation induced by external pressure is weak. This explains why the pressure-induced enhancement of $T_c$ for NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ is not as large as that for LaO$_{0.5}$F$_{0.5}$BiS$_{2}$.
We prepared the samples K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ (Ln=Sm, Nd and La) with ThCr$_2$Si$_2$-type structure. These samples were characterized by X-ray diffraction, resistivity, susceptibility and thermoelectric power (TEP). Substitution of Ln (Ln=La, Nd and Sm) for K in K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ system raises the superconducting transition temperature to 34-36 K. The TEP measurements indicate that the TEP of K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ is positive, being similar to the case of the Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ system with p-type carrier. In the K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ system, the superconducting $KFe_2As_2$ with $T_csim 3$ K is the parent compound, and no structural and spin-density wave instabilities exist in this system.
We have synthesized a series of the Ruddlesden-Popper nickelate solid solution Ln4-xLnxNi3O10 (Ln and Ln = La, Pr and Nd; x = 0, 1, 2 and 3) via the citrate precursor method at different reacting atmospheres. Both the electronic-transport and magnetization measurements on these samples show well-defined phase transitions at temperatures between 135 K and 165 K. These transition temperatures, the room-temperature resistivities, as well as the changes of the Pauli-paramagnetic susceptibilities at the respective phase transitions, strongly correlate with the Goldschmidt tolerance factor t, irrespective of the combination of the magnetic rare-earth ions with unmagnetic La3+. We conclude that these changes of the electronic properties are mostly related to the distortion of the NiO6 octahedra at the phase transition which is strongly correlated with the tolerance factor t, but are rather insensitive to the magnetism of the rare-earth ions Ln3+ and Ln3+.
Electrical resistivity measurements as a function of temperature between 1 K and 300 K were performed at various pressures up to 3 GPa on the superconducting layered compounds Ln(O0.5F0.5)BiS2 (Ln = La, Ce). At atmospheric pressure, La(O0.5F0.5)BiS2 and Ce(O0.5F0.5)BiS2 have superconducting critical temperatures, Tc, of 3.3 K and 2.3 K, respectively. For both compounds, the superconducting critical temperature Tc initially increases, reaches a maximum value of 10.1 K for La(O0.5F0.5)BiS2 and 6.7 K for CeO(0.5F0.5)BiS2, and then gradually decreases with increasing pressure. Both samples also exhibit transient behavior in the region between the lower Tc phase near atmospheric pressure and the higher Tc phase. This region is characterized by a broadening of the superconducting transition, in which Tc and the transition width, delta Tc, are reversible with increasing and decreasing pressure. There is also an appreciable pressure-induced and hysteretic suppression of semiconducting behavior up to the pressure at which the maximum value of Tc is found. At pressures above the value at which the maximum in Tc occurs, there is a gradual decrease of Tc and further suppression of the semiconducting behavior with pressure, both of which are reversible.
Layered textit{Ln}OBiS$_2$ compounds with textit{Ln} = La, Ce, Pr, Nd, and Yb can be rendered conducting and superconducting via two routes, substitution of F for O or the tetravalent ions Ti, Zr, Hf, and Th for trivalent textit{Ln} ions. Electrical resistivity measurements on non-fluorinated La$_{0.80}$Ti$_{0.20}$OBiS$_2$ and La$_{0.85}$Th$_{0.15}$OBiS$_2$ superconductors were performed between $sim$1.5 K and 300 K and under pressure up to 2.4 GPa. For both compounds, the superconducting transition temperature $T_c$, which is $sim$2.9 K at ambient pressure, gradually increases with pressure to 3.2-3.7 K at $sim$1 GPa, above which it is suppressed and the superconducting transitions become very broad. Measurements of the normal state electrical resistivity of the two compounds reveal discontinuous changes of the resistivity as a function of pressure at $sim$0.6 GPa. Surprisingly, above 1.3 GPa, semiconducting-like behavior reappears in La$_{0.80}$Ti$_{0.20}$OBiS$_2$. This study reveals a new high-pressure phase of La$_{1-x}$$M$$_x$OBiS$_2$ containing the tetravalent ions $M$ = Ti, Th which does not favor superconductivity. In contrast, application of pressure to fluorinated LaO$_0.5$F$_0.5$BiS$_2$ produces an abrupt tetragonal-monoclinic transition to a metallic phase with an enhanced $T_c$. These results demonstrate that the response of the normal and superconducting properties of LaOBiS$_2$-based compounds depends strongly on the atomic site where the electron donor ions are substituted.
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