But because of the small spin quantum number (½) and the large energy scale over which spin fluctuations exist (hundreds of meV), large single crystals (>1 cm3) are needed. STM studies of high-temperature superconducting cuprates have revealed many unique and unexpected features in these materials.
Introduction
Subsolidus T-x phase diagram of the LaH 2 (metal)-LaH 3 (semiconductor) system based on differential scanning calorimetry and X-ray diffraction measurements. As we will discuss later, the oxygen non-stoichiometry of 123 results from the less stable square planar coordination of the Cul in the chains.
Lo Tcl
Oxygen content x
Average crystallographic structure
Unit cell of the average structure of tetragonal YBa2Cu306 (space group P4/m mm) with oxygen-depleted C u l - C u t chains along the b-axis. This destroys the planar character of the Cu2 planes and introduces the depression mentioned above.
- Nonequilibrium synthesis, quenching
OXYGEN IONSTOICHOMETRIC EFFECTS AND 1N YBa2Cu30 x 35 single-crystal RELLS above (Veal et al. 1990a). The difference from quenched samples (Jorgensen et al. 1990a) is clearly shown in the example of the parameter c in fig.
ZFC-100G I
Here we briefly discuss some possible reasons leading to the non-linearity of the a and b lattice parameters with oxygen content:. Cul site defects were observed and discussed in detail in single-crystal refinements of the double-chain compound 247-Ox (Y2Ba4Cu7014~) (Schwer et al.
Compared to the c-axis, the in-plane lattice parameters for the BAO and DO samples show a larger spread. Despite a greater spread of data, the changes in the slope can thus be clearly seen.
However, for optimally doped 123, the results of the PDF studies were quite ambiguous (Egami and Billinge 1996). The orthorhombic strain due to the different lengths of the a and b parameters is responsible for the atomic displacements of the 0 4 chain sites found by Francois et al. The authors found no evidence for splitting of the apical oxygen site.
To our knowledge, the existence of the Tc ~ 60 K phase was first reported by Tarascon et al. More details are given further below and in the discussion of the important points of the T - x phase diagram (section. The arrows indicate the displacements of the various atoms due to the formation of the superstructures.
Schematic illustration of the three sublattices Cul (solid squares), 04 (gray circles) and 05 (open circles) and the interaction potentials V used in the ASYNNNI model.
Oxygen concentration on basal plane
AEI 2
Note the changes in the signs of the a-axis at x ~ 6.75 and of the c-axis near optimal doping. The scales of the vertical axes are not the same and some data is shifted. The corresponding decrease in orthorhombicity indicates that, perhaps, already at x ~ 6.75 negative interactions begin, leading to the decrease of To in the overdoped phase (see also point c).
Liarokapis measured the Fano asymmetry of the in-phase Blg phonon as a function of the oxygen doping. However, there is a charge redistribution within the superconducting levels due to relative changes in the potentials of the Cu2, 0 2 and 03 ions. Of course, this ideal resolution of the Raman scattering can be reduced from case to case due to experimental limitations such as e.g. the laser beam cross-section.
The strengthening of the apex phonon is a consequence of the reduction of the apical bond by oxygen doping (cf.
This sudden change in the indentation indicates the existence of a structural phase transition at the beginning of the overdoping regime. 66a increasing Cu2-O2,O3 separation scales with increasing Tc, until the onset of the overdoped phase. In the overdoped region, the largest Raman shift as a function of non-stoichiometry occurs for the 0 2 , 0 3 mode of vibration in the phase (Ag phonon).
In the optimally doped and overdoped phase, the phonon frequency decreases by 8 cm from the total change). This work allowed a better understanding of the influence of Cu2 shifts on the spectra. The increase in orthorhombicity with oxygen doping is excessive (small a- large b-axis optimally doped).
The step at x = 6.95 corresponds to the onset of the overdoped phase and the martensitic transformation.
123-0, Emerson et al
The phase separation literature has been reviewed in a series of volumes edited by Mfiller and Benedek (1992), Sigmund and Mtiller (1994), Mihailovic et al. Most of this work is based on the proximity of superconducting and antiferromagnetic insulator states in the HTc cuprates. Historically, the first indication from materials research of the existence of an overdoped regime came with the finding of a maximum of Tc (Graf et al. 1990, Rusiecki et al. 1990) and with the c-axis anomalies discussed in sect.
However, in the earlier literature there are a series of indications for various anomalies of the oxygenated samples. Splitting of the specific heat peak for oxygen-rich samples was observed in long-annealed unquenched samples as early as the Interlaken conference, by Ishikawa et al. Work on single crystals by other groups also produced similar results (Inderhees et al. 1988, Butera 1988).
Phase separation into two orthorhombic phases with x ~ 7.0 and 6.7 but common orientation was found by high-resolution XRD scans in a triple axis spectrometer using Ge (111) crystals as monochromatic and analyzer (YouHoydoo et al. 1988 ).
Temperature (K)
A comprehensive study of the specific heat and a.c. susceptibility as a function of annealing temperature of polycrystalline samples was presented by Janod et al. With increasing annealing temperature, a crossing of Tel and To2 takes place in the samples of Janod et al., as shown in Fig. 82, but unfortunately we do not know exactly the corresponding change in the oxygen or carrier concentration. In fact, the same argument has been used later by Krfiger et al. 1997) to explain the changes of the lattice constants in the overdoped regime, but this could not be proved experimentally until now (sect.
Due to the strong anisotropy of the oxygen diffusion coefficient in 123-Ox, a one-dimensional bilateral diffusion model (Schmalzried 1981) was used to calculate D (Conder et al. 1993). This parameter was determined directly by thermogravimetric weight change measurements (Conder et al. 1993, Kr/iger et al. 1993) of exchange of oxygen isotopes 180 ~ 160 at very low temperatures (225-290 °C, thermobalance resolution 1 ~ tg). The splitting of the diamagnetic transition as a function of the exact oxygen content.” indication for phase separation.
The onset of the splitting coincides with the displacing martensitic transformation (sect. 6.4) at the onset of the.
The theory of the linear phase model is based on the theoretical work of Emery et al. The first experimental evidence for the existence of lines in La cuprates and possibly other HTc cuprates came with the ingenious work of Tranquad et al. 1995) and, starting from another point, the work of Bianconi and Missori (1994), who propose the existence of quantum wires in a Wigner polaronic crystal. Since this behavior should be reflected in the network properties of HTc cuprates, Sharma et al. 2000) used helium ion channeling MeV spectrometry to test the existence of incoherent lattice oscillations in 123-Ox at three different stoichiometries.
Other important parameters include, for example, the energy of the ions, the atomic numbers of the colliding and scattering ions, the interatomic distance and the electron shielding potential. 2000) discuss their results in conjunction with the stripe phase scenario (Emery et al., 1997). • The assignment of T1 to the banding temperature is true. At Tc = T3 we expect greater lattice coherence and this is reflected in an abrupt drop in ue×. The increase at T < T3 can be attributed to the spin-fluctuation dynamics, indicative of slowing boundary fluctuations that are more easily tracked by the lattice.
All these studies of Ca-doped 123-Ox in the literature were performed on samples that were not fully oxygenated, and none were supported by a direct chemical analysis of the oxygen content with a high-resolution method.
The oxygen content of the sample without Ca (overdoped with oxygen, O-O state) synthesized under the same conditions is significantly higher (6.994). The Ca content is unequivocally determined by these structural investigations (within 4%) and in accordance with the composition of the starting material. They can be detected due to the extreme accuracy of oxygen determination.
In fact, the linear scaling of the lattice parameters with the two oxygen groups. An influence of states A and B can be seen in the plot of apical connectivity vs . Due to the wider transition of the Ca-doped samples, To2 was not determined by the Shoenberg method (Zech et al.
The splitting of the diamagnetic transition in the Ca-doped phase, similar to the O-overdoped phase, strongly indicates physical phase separation.
Oxygen content x (average)
Tc2,o,~t is almost independent of the Ca doping, behaves similarly to oxygen-doped 123-O x and has approximately the same value (cf. fig. 85). The strong splitting of the diamagnetic transition of Ca- 123-O x indicates the existence of physical phase separation in the Ca-doped phase, similar to the O-doped phase.
NPD and XRD show that contraction of the apical bond ceases during this percolation. Other transitions appear in the determination of oxygen and linear scaling of the NPD and XRD data compared to the higher superstructures (ortho-V and ortho-VIII) found by XRD, only combinations of 2a0 and 3a0.
In a nao series of superstructures, this could be the range of the 2a0-3a0 miscibility gap (coexistence of the two superstructure phases) (Fig. 110, section). From here on, the stoichiometries of the nao superstructures do not exactly follow the order of the in-phase phases (Fig. 111) Local structure EXAFS (T =25 K): The pits abruptly decrease due to the shift of the O2,O3 plane to the Cu2 plane (fig. 76).
It is possible that this is a generic element of the HTC cuprates phase diagram.
La2.:rSrxCuO 4 Phase Diagram
La2CuO4+y Phase Diagram
Stage 4 Stage 3 Stage 2 Oxygen Doping C~' and Staging #)
3.2.2) clearly support the c-axis abrupt change and apical binding previously found by Cava et al. Deconvolution of the top-phonon width shows the coexistence of four phases in the non-stoichiometric range of 123. The physical phase separation is likely due to overdoped islands in a matrix of the optimally doped phase.
-EXAFS shows displacements of the 0 2 , 0 3 and Cu2 atoms parallel to the c-axis, which cause deformations of the dimple. We mentioned under (1) and (2) the physical phase separation at the two boundaries of the superconducting regime. Conder, K., 2000, Oxygen Exchange in the Superconductors of the YBaCuO Family, Habilitationsschrift (Department of Materials Sciences, ETH, Zurich).
Kaldis, 1997a, Studies of the local structure of the subdoped~ overdoped transition in YBa2Cu3Ox, in: Ten years after discovery, Proc.
