(Cu,C)Ba₂Ca₃Cu₄O_x–(LiF)_y: addition of LiF—an effective way to synthesize overdoped superconductor

Abstract

(Cu,C)Ba2Ca3Cu4Ox superconductor with addition of y mol LiF has been synthesized by a high-pressure method. For the same synthesis conditions it was found that (almost) single-phase Cu, C-1234 samples can be synthesized for yLiF = 0–0.1 if the amount of z mol AgO oxidizer is increased linearly from zAgO = 0.45 to 0.73 and for yLiF = 0.1–0.2 if zAgO = 0.73 = constant. Transport measurements (ρ(T) and room-temperature Seebeck coefficient) have shown that these samples are overdoped: LiF is an effective addition for synthesis of overdoped Cu, C-1234 with a controlled level of carriers. LiF addition continuously decreases Tc. The critical point at yLiF = 0.1 is discussed as the solubility limit of LiF and/or the point where the doping mechanism changes. It is proposed that the reason is the reaction of extra Li with C and O to form Li2CO3, inducing a lower concentration of C in Cu, C-1234/LiF crystals, and at the same time a possible substitution of Li not only for the Cu site but also for the Ca site, resulting in formation of a higher amount of residual Ca0.828CuO2 (for yLiF>0.1). LiF induces the formation of a liquid phase and acts as a flux promoting the formation of Cu,C-12 (n−1)n with . LiF modifies to some degree the grain growth from a 3D to a 2D type (thinner platelike grains have been observed in the LiF added samples).

Fish-Tail effect and irreversibility field of (Cu,C)Ba2Ca3Cu4O x -(LiF) y superconductor

“…Due to its radii produces some local shrinkage of the blocking blocks, which are able to depress the superconducting order parameter. However, despite the lack of precise information, our previous experiments [17] indicate that in the LiF-doped Cu,C-1234 samples, the donor effect of Li is compensated by the oxygen supplied from an AgO-source, which also wash out the effect of fluorine. We have also observed that for a given amount of LiF, deviation from the optimum concentration of oxygen, controlled through the addition of AgO, can produce biphasic samples [17].…”

“…However, despite the lack of precise information, our previous experiments [17] indicate that in the LiF-doped Cu,C-1234 samples, the donor effect of Li is compensated by the oxygen supplied from an AgO-source, which also wash out the effect of fluorine. We have also observed that for a given amount of LiF, deviation from the optimum concentration of oxygen, controlled through the addition of AgO, can produce biphasic samples [17]. Although our investigations will focus mainly on the single phase Cu,C-1234-LiF sample in comparison with the un-doped compound, some results on the biphasic systems will be also presented for a better understanding of the phenomena.…”

“…As-prepared bulk samples were characterized by X-ray diffraction, scanning electron microscopy (SEM), resistance R vs. temperature T (R(T)) and thermopower (Seebeck coefficient, S 293K ) measurements [17]. Noteworthy, R(T) dependence of all samples exhibits an outstanding linearity up to the room temperature with a negative residual resistivity, , ρ aT…”

Fish-Tail Effect and Irreversibility Field of (Cu, C)Ba2Ca3Cu4O x -(LiF) y Superconductor

“…This difference of the ionic radii produces some local shrinkage of the blocking blocks, which are able to depress the superconducting order parameter. However, despite the lack of precise information, our previous experiments [17] indicate that in the LiF-doped Cu,C-1234 samples, the donor effect of Li is compensated by the oxygen supplied from an AgO-source, which also wash out the effect of fluorine. We have also observed that for a given amount of LiF, deviation from the optimum concentration of oxygen, controlled through the addition of AgO, can produce biphasic samples [17].…”

Nano‐engineered pinning centres in YBCO superconducting films

“…A number of different methods have been successfully used to enhance J c in thin films in which heavy-ion and neutron irradiation proved to be very efficient, but less suitable than alternative routes for practical applications [4,5,6]. Another successful approach was through doping with various elements and compounds, like Zn, BaZrO 3 , LiF, Ca, etc., [7][8][9][10][11], that proved to be useful also for polycrystalline superconductors [12,13]. In superconducting films films, regular arrays of (sub)-micron defects (holes, metallic and magnetic dots) fabricated through photolithography were shown to pin the magnetic flux lines [14,15].…”