Perspective on RBa2Cu3Ox materials from oxygen deficiency studies

We have measured the electrical resistivity, thermoelectric power, magnetic susceptibility, Hall coefficient and upper critical field near Tc in EuBa2Cu3Ox, where 6.09<x<7.0. These data suggest a Mott-Hubbard-like transition near a critical oxygen concentration xc∼6.4–6.5. Our results, combined with those reported elsewhere, show a striking similarity between the addition of oxygen to EuBa2Cu3O6 and the substitution of divalent atoms M for La in La2-yMyCuO4. We suggest that superconductivity in both systems is associated with the Cu-O planes and is driven electronically.


Introduction
The origin of superconductivity at temperatures exceeding 90 K in RBa,Cu30, (x2: 7) compounds has been the subject of some speculation.However, without substantial knowledge of the basic interactions present in the normal state of these materials, it is difficult to decide which of the several theoretical models [ 1 ] holds most promise.Soon after the discovery [ 2 ] of superconductivity in YBa2Cu30,, additional compounds were found [ 3 1, with R one of the rare-earth elements, which had transition temperatures T, nearly identical to that of non-magnetic Y.Those observations suggested immediately that superconductivity was due primarily to the Cu-0 planes/chains associated with the oxygen-defected structure of these layered perovskites [4].Subsequent work showed that transition metal substitutions [ 5 ] for Cu or oxygen removal [ 6 ] substantially depressed T,, in some cases completely suppressing superconductivity.
To date there have been several studies of both structural and physical properties of high-T, superconductors in which oxygen was removed intentionally.Neutron scattering measurements [ 71 as a function of oxygen content showed that oxygen is removed preferentially from the linear Cu-0 chain sites that run along the orthorhombic b-axis.For an oxygen content x= 6.5, the structure became tetragonal Although it is clear from these earlier studies that oxygen content plays a central role in determining the superconducting and structural properties, no investigation has been made that employs a variety of techniques to understand the normal state properties as the oxygen content is varied.Such experiments should provide additional insight into the underlying physical mechanisms that are important in these materials.To this end, we have studied the effect of varying oxygen content in EuBa2Cu30x through electrical resistivity, thermoelectric power S, dc magnetic susceptibility, Hall coefficient RH, and upper critical field Hc2 measurements.
To ensure consistency in these measurements, all experiments were performed on the same oxygen deficient samples which themselves were cut from a single large button having a density -80% of theoretical.

Experimental
For these experiments a large ( -1 g) button of EuBa2Cu30x was prepared by conventional ceramic techniques.Wet chemical analysis of pieces from this button gave an oxygen content X= 7.0?0.1.Direct current magnetization measurements on the starting material in 100 G showed a sharp onset of superconductivity at 95 + 1 K and shielding of magnetic flux at 7 K corresponding to 120% of perfect diamagnetism, not corrected for demagnetizing effects.To remove oxygen, sections of the button were subjected to vacuum and heated to between 300-600°C for 2 to 24 hours, depending on how much oxygen was to be removed.After the heat treatment, the sample was allowed to cool slowly to room temperature.Weight loss resulting from this treatment was assumed to arise solely from the removal of oxygen from the samples.The maximum relative weight loss produced a calculated oxygen content of x= 6.09, assuming x= 7.0 in the starting material.

Results
The effect of oxygen stoichiometry on the resistivity is summarized in fig. 1  ygen concentration for superconductivity of N 6.4, which is in the composition range where an orthorhombic-to-tetragonal transition is reported [ 7 ] to occur.X-ray analysis of our x= 6.33 sample confirmed that it was tetragonal and that there were less than 10% other phases present.
From Hall effect measurements on three samples (x= 7.0,6.79and 6.58 From these data we find that n at T, decreases by about a factor of 3.5 in going from x=7 to x=6.6.with T, scaling crudely with n(TkT,).
The upper critical field slope H& near T, was determined resistively in fields to 8 T on the Hall-effect samples.For these measurements the resistive midpoint was used to define Hc2 ( T).In agreement with earlier reports [ 191 we find HL2 = -3.1 T/K for x= 7.0.However, with decreasing oxygen content, H& is suppressed rapidly to values of -1.1 and -0.8 T/K for x= 6.8 and 6.6 respectively.
The low-field (100 G) magnetic susceptibility at 100 K varies non-monotonically as a function of x.Initial oxygen removal produces a decrease in x by more than 10% in going from x=7 to x=6.79 that is interrupted by a sudden increase for x= 6.64 and 6.68, signalling the appearance of the ordered oxygen defect phase.This variation in x versus x is qualitatively similar to that reported by Cava et al. [ lo] for YBa2Cus0,.Higher field (4000 G) susceptibility measurements on the x= 6.0 sample reveals a shoulder near 300 K that may be associated with ordering of copper moments, as recently detected in NQR [ 201 and neutron scattering [ 2 1 ] experiments.
For oxygen concentrations less than 6.3 a small "Curielike-tail" appears at temperatures below approximately 20 K that possibly is associated with the presence of a magnetic second phase; although, it could

Discussion
As a starting point for interpreting the data, it is useful to review what is known of the related system Laz_&Cu04, where M is a divalent atom.In undoped LazCuO, there is a finite electronic specific heat [22,23] at low temperatures

Magnetic susceptibility
[ 25 ] and neutron scattering experiments [ 26 ] show an antiferromagnetic transition in slightly oxygen deficient samples at TN -260 K which decreases as oxygen stoichiometry is approached [27].Upon substituting a divalent atom such as Ba or Sr for La, TN is suppressed to zero (for y< 0.06) and for larger y superconductivity appears, with T, reaching a maximum near y= 0.15 [ 28 1.The addition of Sr increases the number of hole carriers linearly with y for y<O.15 [ 24,291.In superconducting samples, y( T=O) is 3-5 mJ/mol K2 [ 22,301.An analysis [ 3 1 ] of the magnetic susceptibility and y( T,), determined from the specific heat jump at T,, shows that both are enhanced, indicating the presence of Coulomb correlations.
We believe that the addition of oxygen in RBa,Cu,O, is analogous to divalent atom substitution in La_MyCu04.
For strongly depleted (xc 6.4) samples, we observe a temperature dependent resistivity and thermopower that suggest transport by variable-range hopping.Because of both the sintered nature of the sample and the fact that the temperature interval over which reliable transport measurements can be made was limited, it is not possible to distinguish unambiguously whether the hopping is correlated or not.However, as argued previously [ 111, there is evidence for strong electron-electron interactions in x=7 samples that presumably persists upon removing oxygen.In the non-supercon-ducting regime (x<6.4),NQR [20] and neutron scattering [ 2 1 ] show antiferromagnetic ordering (of Cu 3d9 moments) at a transition temperature that depends strongly on oxygen content and goes to zero at a critical oxygen concentration (~~~6.4)above which superconductivity appears.Such behavior is consistent with a Mott-Hubbard-like transition near x,, with transport and magnetism arising from localized states (X < x,) and superconductivity (x> x,) from extended hole-like states produced by oxygen addition.
Recent measurements [ 32,331 of the low-temperature specific heat of YBazCusO, ( 6 < x < 7 ) crystals show that the electronic contribution changes little in going from the superconducting to non-superconducting states and that it remains large, 5-9 mJ/mol K2.In ref.The Pauli susceptibility ,yp of YBa$&O, is enhanced by electronic correlations, by over an order of magnitude relative to that calculated from the carrier density, to a value of N 6 X 1 O-' emu/g [ 111.
Assuming a comparable Xpin EuBazCu30, means that it contributes only about 7% of the measured susceptibility at 100 K.The change in carrier concentration between x=7.0 and 6.8, assuming the correlation enhancement of x remains constant in this interval, would give a reduction in xp (100 K) of 0.75 x lo-' emu/g; whereas, we observe a decrease in x ( 100 K) that is approximately one order-ofmagnitude larger.This implies that either a singleband description is inappropriate or electronic correlations are suppressed with decreased oxygen content.The former is implicated from the substantial difference in the temperature dependences of y1 for these two samples.[We note that a two-band model can be constructed that adequately accounts for the temperature dependence of p, n and S, in a self-consistent way, for EuBazCu30,.
This model assumes two conduction processes, one due to metallic-like carriers in parallel with an intrinsic, gaples semiconductor with temperature dependences characteristic of each process.The analogy here is to a half-filled Cu,,-O,, hybridized band and an almost completely filled OZp band.Reasonably good fits of the data to this model, shown by the dashed lines in fig.7, are obtained with physically reasonable parameters, one of which is a relatively large effective mass ( m* z 10 m,).]For oxygen concentrations less than 6.4,Ong et al.
[ 18 ] found an approximately linear decrease in n versus x for YBa,Cu,O,, such as was observed by Shafer et al. [29] for small Sr dopings in La,_,Sr,CuO, In that case it was argued that the linear dependence of n on Sr content was consistent with band filling of a single highly correlated Hubbard band.We suggest that similar arguments are appli- with a concommitant depression of T, to zero [ 8 1.More recent work [ 91, however, suggests that these observations depend sensitively on the way oxygen is removed.Depleting oxygen by a low-temperature gettering anneal presumably provides spatial ordering of the oxygen defects such that the structure remains orthorhombic to an oxygen content x26.0 with a corresponding weaker dependence of T, on x [ 91.Measurements of the electrical resistivity p and T, on these samples showed a minimum and a plateau respectively for 6.6 5 x6 6.8 which has been interpreted as evidence for a new ordered oxygen deficient structure having a transition temperature of 55-60 K.Magnetic susceptibility x measurements [ lo] on similarly prepared samples supported this interpretation.Considerable controversy persists, however, concerning what these observations imply for an understanding of high temperature superconductivity as well as the importance of Cu-0 planes relative to Cu-0 chains.

Fig. 1 . 1 Fig. 2 . 1 'Fig. 5 .
Fig. 1.Eletrical resistivity of sintered EuBa2CuS0, as a function of temperature for various values of x.For clarity only representative curves are shown for xz 6.48.
) a carrier concentration n was calculated assuming the single band expression n = 1 /R,ec, where RH is the measured Hall coefficient.For x=0.7, n increases linearly with T as ob-served before [ 111.However, for x= 6.8 and 6.6 a non-monotonic but weakly temperature dependent variation of n is observed, as shown in fig.6.Similar observations were made by Ong et al. [ 18 ] on sintered YBa2Cuj0,.

Fig. 6 .
Fig. 6.Carrier concentration n, calculated from the single band expression n= 1 /R,ec, where RH is the Hall coefficient, as a function of temperature for various EuBa2Cu,0,.
[y( T=O) -1 mJ/mol K*], a highly anisotropic resistivity [ 161 which in the Cu-0 planes has the form In pee T -'I* characteristic of correlated variable-range hopping, a small number of hole-like carriers [ 241, and a large positive thermoelectric power having a temperature dependence as expected from the resistivity [ 16 1.
[ 321 it  was argued that y( T=O) arises, at least in part, from extended or localized states at the Fermi energy.Such a conclusion is consistent with the results of this study.Interestingly, Ayache et al. [ 341 find a similar result for y( T=O) versus x but also find that the specific heat jump at T,[AC( T,) 1, which is clearly detectable for x=7, becomes undetectably small for ~~6.6 samples that show superconductivity as detected through magnetic susceptibility.Because AC is directly related to y( T,)(in BCS theory), these observations imply that y( T=O) and y( Tc) are not related directly.Assuming the dirty limit, we have calculated y( T,) from the critical field slope and resistivity near T, and find that y(T,) for x=6.6 should be approximately 30 times smaller than for x=7.0, consistent with the specific heat measurements [ 341 at T,.These results suggest that most of the electronic specific heat at T, is associated with the integrity of the Cu-0 chains, whereas that at T=O is dominated by the Cu-0 planes.

Fig. 7 .
Fig. 7. Temperature dependence of the electrical resistivity and thermoelectric power of EuBaZCu,O,.Dashed lines are a fit to the data assuming a two-band model for transport (see text).Such a model is also consistent with the temperature dependence of the Hall coefficient RH.We note that this model could not selfconsistently explain the measured temperature dependencies of p, Sand RH for x< 7.0.