Microstructures of an MTG YBa₂Cu₃O_ysuperconductor

Abstract

The superfluid density ρs(T) ≡ 1/λ 2 (T) has been measured at 2.64 GHz in highly underdoped YBa2Cu3O6+y, at 37 dopings with Tc between 3 K and 17 K. Within limits set by the transition width ∆Tc ≈ 0.4 K, ρs(T) shows no evidence of critical fluctuations as T → Tc, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that ρs displays the behaviour expected for a quantum phase transition in the (3 + 1)-dimensional XY universality class, with ρs0 ∝ (p − pc), Tc ∝ (p − pc) 1/2 and ρs(T) ∝ (Tc − T) 1 as T → Tc. PACS numbers: 74.72.Bk, 74.25.Nf, 74.25.Bt, 74.25.Ha Current research on high temperature superconductiv-ity focuses on the underdoped cuprates, in a region of the phase diagram where d-wave superconductivity gives way to antiferromagnetism [1]. One proposal for this regime is that at temperatures up to about 100 K above T c , superconductivity persists locally, with long-range phase coherence suppressed by fluctuations in the phase of the superconducting order parameter [2, 3, 4, 5, 6, 7, 8]. Early results showing a linear relation between T c and the superfluid density ρ s (T = 0) [9] provided the original motivation for this point of view, suggesting that T c is low in underdoped materials because the phase stiffness is low. Further support for this idea has come from measurements showing a finite phase stiffness above T c at terahertz frequencies [10], and from experiments that appear to detect the phase-slip voltage of thermally diffusing vortices in the normal state [11]. If the physics of the underdoped cuprates is indeed that of a fluctuating d-wave superconductor there should be a regime where quantum fluctuations come into play as T c falls to zero with decreasing doping. Here we test this idea with a detailed study of the doping dependence of the superfluid density in the vicinity of the critical doping for supercon-ductivity. High homogeneity of T c is particularly difficult to achieve in the underdoped cuprates, where the control parameter is chemical doping and the materials are well away from plateaus or turning points in T c (y). The YBa 2 Cu 3 O 6+y system has two advantages in this doping range: with careful work, there can be sufficient control of doping homogeneity to produce samples with sharp superconducting transitions [12, 13]; and the process of CuO-chain ordering can be harnessed to provide continuous tunability of the carrier density in a single sample, with no change in cation disorder [14]. This is possible because the loosely held chain oxygen atoms in these high quality samples remain mobile at room temperature and gradual ordering into CuO-chain structures slowly pulls electrons from the CuO 2 planes, smoothly increasing hole doping over time [15, 16]. For this experiment, single crystals of YBa 2 Cu 3 O 6+y were grown in barium zirconate crucibles and have high purity and low defect levels, with cation disorder at the 10 −4 level [17]. A crystal 0.3 mm thick was cut and polished with Al 2 O 3 abrasive into an ellipsoid of revolution about ...

Microstructure of melt-textured Y - Ba - Cu - O oxides with addition and the formation mechanism of the Ba - Cu - O platelet structure

“…A similar result was reported for the melt-textured Y123 system obtained from a mixture of Y123 and Y211 powder with PtO 2 addition; in that case the Y211 anisotropy was reduced by way of the control of fabrication parameters [37]. If one considers the flux pinning as being either due to the Y211/Y123 interface [7,16] or due to the microdefects around the Y211 [8,11,[13][14][15], the spherical Y211 particles may be seen as an improvement on the needle-like Y211 particles, because they have more surface area per unit volume and a higher defect density around the Y211 than do the plate-like or needle-like particles. Details of the formation characteristics of the stacking fault will be explained later.…”

“…Therefore, the Y123/Y211 interface was proposed as a flux-pinning site for this material and this proposal was supported by another group [16]. In contrast, other microstructural investigations [8,11,[13][14][15] revealed that many crystallographic and chemical inhomogeneities were present at the Y123/Y211 interface. In the Y123 phase region around the Y211, yttrium was enriched but barium was depleted [8].…”

“…The characteristics of the stacking fault and its possible formation mechanisms have been reported in the literature [8,9,[13][14][15][16]. Until now, the stacking faults have been believed to be formed during the high-temperature peritectic reaction [16].…”

“…Since the development of melt texture growth (MTG) processing [1] which is widely used for fabrication of Y-Ba-Cu-O superconductors with critical current density (J c ) exceeding 10 4 cm −2 at several tesla [1][2][3][4][5], the microstructure of melt-processed Y-Ba-Cu-O has been intensively studied with an aim of understanding the fluxpinning mechanism of the oxide superconductor [6][7][8][9][10][11][12][13][14][15][16]. As a result of these efforts, various microstructural defects such as transformation twins [12], stacking faults [8,[14][15][16], dislocations [11,13] and oxygen deficiencies [17] have been observed within the directionally grown Y123 domain and some of them have been proposed as flux-pinning sites for this material. In addition, incorporation of the non-superconducting Y 2 BaCuO 5 (Y211) phase into meltprocessed YBa 2 Cu 3 O 7−y (Y123) has been reported to be effective in further increasing J c for this material [18,19].…”

Microstructure and local composition of yttrium-bearing metal oxides obtained by zone melting

“…The growth of the critical current is associated with the formation of coarse grains and their matched orientation after such a treatment, which diminishes the effects of the weak bonds that serve as tunnel barriers for the current carriers on imperfect intergrain contacts in polycrystalline HTSC. The high critical current density is preserved in the melt-textured specimens in a strong magnetic field because the recrystallized matrix superconducting phase in them always contains inclusions of other phases (for example, Y2BaCuO s in yttrium-bearing YBa2Cu30 x cuprate) that play, directly or indirectly, the role of pinning centers of the magnetic flux [4,5].…”