Jeffery L. Tallon

General trends in oxygen stoichiometry effects on Tc in Bi and Tl superconductors

Abstract We have previously shown that Tc for Bi2 (Sr, Ca)n+1CunO2n+4+δ (n=1, 2 and 3) varies with oxygen stoichiometry δ determined by annealing in a variety of oxygen partial pressures and temperatures. Annealing results are now also presented for several Tl-superconductors in the series Tlm(Ba, Sr)2Can−1CunO2n+m+2+δ, for both m=1 and 2, revealing two general results for this class of Bi or TI superconductors: (1) effects on Tc due to labile oxygen occur for all m=2 compounds but are absent in m = 1 compounds with Sr. m=1 compounds with Ba do have variable oxygen but the effects on the c-axis are the opposite to m=2 compounds. This suggests for the latter that the labile oxygen resides in interstitial sites within the Bi2O2 or Tl2O2 bi layers; (2) the hole concentration per Cu decreases in progressing from n = 1 to 2 to 3 in either class so that the parent n = 3 and n = 2 compounds lie respectively on the low- and high-hole concentration sides of the peak in Tc while the n = 1 compounds extend out into the non-superconducting domain at very high hole concentration.

The relationship between bond-valence sums and Tc in cuprate superconductors

Abstract The recently demonstrated correlation of T c with copper or oxygen bond valence sums for a wide range of superconducting cuprates is shown to contradict the behaviour observed for YBa 2 CU 3 O x as x is varied from 6 to 7. This conflict is removed by introducing two independent variables, namely, the sum of and difference of the copper and oxygen bond valence sums. The latter is a measure of the hole density on the CuO 2 plane and as this increases T c passes through a maximum T c (max). The former indicates the relative distribution of these holes on the oxygen or copper sites for a given compound. T c (max) correlates closely with the preference for hole distribution on the oxygen atoms. A general phase-diagram using these bond sum parameters is presented.

Reversible Switching of a Cobalt Complex by Thermal, Pressure, and Electrochemical Stimuli: Abrupt, Complete, Hysteretic Spin Crossover

Triply switchable [Co(II)(dpzca)(2)] shows an abrupt, reversible, and hysteretic spin crossover (T(1/2)↓ = 168 K, T(1/2)↑ = 179 K, and ΔT(1/2) = 11 K) between the high-spin (HS) and low-spin (LS) states of cobalt(II), both of which have been structurally characterized. The spin transition is also reversibly triggered by pressure changes. Moreover, in a third reversible switching mechanism for this complex, the magnetic properties can be switched between HS cobalt(II) and LS cobalt(III) by redox.

Evidence on the pseudogap and condensate from the electronic specific heat

Abstract Evidence on the nature and hole density (p) dependence of the normal state pseudogap is obtained from the electronic specific heat of several cuprate series, including new data for Bi:2212. The pseudogap energy decreases as Eg(p)∼J(1−p/pcrit) for p pcrit, where pcrit∼0.19 is close to optimal Tc. This is correlated with an abrupt crossover from weak to strong superconductivity at pcrit. We conclude that the pseudogap is an intrinsic feature of the normal state density of states that competes with the condensate for low energy spectral weight and is not related to precursor superconducting fluctuations. The linear p-dependencies of the entropy, the superfluid density and the states-non-conserving pseudogap are all interrelated, and appear to reflect strong correlations in the doped Mott insulator.

Ca- and La-substitution in YBa2Cu3O7−ξ, Y2Ba4Cu7O15−ξ and YBa2Cu4O8

Abstract We report the preparation and characterization of the homologous series of high temperature superconductors, Y 2 Ba 4 Cu 6+ n O 14+ n −ξ with n =0 (123), n =1 (247) and n =2 (124) as a function of Ca and La substitution through measurement of T c , lattice parameters, selected Rietveld refinements and thermogravimetric analysis. Solubility limits of Ca and La are investigated and found to be widely different for each member. Data is presented showing that Ca substitutes on both the Y- and Ba- sites and as the Ca concentration increases the relative fraction on the Y site decreases. Systematic trends indicate that for n =0 123 is slightly overdoped, 247 is slightly underdoped and 124 is well underdoped relative to T c(max) such that 0.06 La substitution, 0.04 Ca substitution and 0.15 Ca substitution is required to maximise T c in 123, 247 and 124, respectively. Differences in the orthorhombicity between the members of the series as a function of substitution are interpreted in terms of differences in hole doping and in oxygen stoichoimetry for each member. For instance, the observed rapid decrease in the orthorhombicity in 124 at a high La content is interpreted as evidence that oxygen loads into the interchain site. This destabilises the ribbons leading to the formation of tetragonal 123.

Calcium-substituted superconducting RBa2Cu4O8 with Tc ∼ 90 K prepared at one atmosphere

Abstract Calcium substituted RBa 2 Cu 4 O 8 with R = Y and Er has been prepared between 805°C and 830°C under oxygen at one atmosphere. The zero-resistance T c is displaced up from 79 K (unsubstituted) to 87 K at 10% substitution. This compound has the benefit of a high- T c while circumventing the problems of oxygen-loading and associated microcracking. The substitution level of 10% lies at the solubility limit under the preparation conditions used and neither prereaction at higher temperatures, nor the use of co-substitution with La assisted in obtaining higher substitution levels.

Single-phase Pb-substituted Bi2+yCan−1Sr2CunO2n+4+δ, n=2 and 3: Structure, Tc and effects of oxygen stoichiometry

Abstract The effects of Pb-substitution for Bi in the homologous superconducting series Bi2+yCan−1Sr2CunO2n+4+δ are investigated for nearly single phase n = 2 and 3. Under our reaction conditions a solid solution exists for up to 16% substitution above which the n = 1 phase begins to appear and the composition Bi1.9Pb0.35Can−1Sr2CunO2n+4 remains stable for a range of Pb-rich start ing compositions for both n=2 and n=3. The c-axis satisfies the rule c = 18.2+6.3 n A and two b-axis and c-axis superstructures are evident from electron diffraction which are identical in n = 2 and n = 3. The effects of Tc of anneals at different temperatures and oxygen partial pressures are explored and the conditions for maximising Tc at 93 K for n = 2 and 106 K for n = 3 are found.

Evidence for a generic quantum transition in high-T c cuprates

We study the low-energy spin fluctuations and superfluid density of a series of pure and Zn-substituted high-T(c) superconductors (HTS) using the muon spin relaxation and ac-susceptibility techniques. At a critical doping state, p(c), we find (i) simultaneous abrupt changes in the magnetic spectrum and in the superconducting ground state and (ii) that the slowing down of spin fluctuations becomes singular at T=0. These results provide experimental evidence for a quantum transition that separates the superconducting phase diagram of HTS into two distinct ground states.

Effect of counteranion X on the spin crossover properties of a family of Diiron(II) Triazole complexes [FeII2(PMAT)2](X)4

Seven diiron(II) complexes, [Fe(II)(2)(PMAT)(2)](X)(4), varying only in the anion X, have been prepared, where PMAT is 4-amino-3,5-bis{[(2-pyridylmethyl)-amino]methyl}-4H-1,2,4-triazole and X = BF(4)(-) (1), Cl(-) (2), PF(6)(-) (3), SbF(6)(-) (4), CF(3)SO(3)(-) (5), B(PhF)(4)(-) (6), and C(16)H(33)SO(3)(-) (7). Most were isolated as solvates, and the microcrystalline ([3], [4]·2H(2)O, [5]·H(2)O, and [6]·½MeCN) or powder ([2]·4H(2)O, and [7]·2H(2)O) samples obtained were studied by variable-temperature magnetic susceptibility and Mössbauer methods. A structure determination on a crystal of [2]·2MeOH·H(2)O, revealed it to be a [LS-HS] mixed low spin (LS)-high spin (HS) state dinuclear complex at 90 K, but fully high spin, [HS-HS], at 293 K. In contrast, structures of both [5]·¾IPA·H(2)O and [7]·1.6MeOH·0.4H(2)O showed them to be [HS-HS] at 90 K, whereas magnetic and Mössbauer studies on [5]·H(2)O and [7]·2H(2)O revealed a different spin state, [LS-HS], at 90 K, presumably because of the difference in solvation. None of these complexes undergo thermal spin crossover (SCO) to the fully LS form, [LS-LS]. The PF(6)(-) and SbF(6)(-) complexes, 3 and [4]·2H(2)O, appear to be a mixture of [HS-LS] and [HS-HS] at low temperature, and undergo gradual SCO to [HS-HS] on warming. The CF(3)SO(3)(-) complex [5]·H(2)O undergoes gradual, partial SCO from [HS-LS] to a mixture of [HS-LS] and [HS-HS] at T(1/2) ≈ 180 K. The B(PhF)(4)(-) and C(16)H(33)SO(3)(-) complexes, [6]·(1)/(2)MeCN and [7]·2H(2)O, are approximately [LS-HS] at all temperatures, with an onset of gradual SCO with T(1/2) > 300 K.

Room-temperature spin crossover and Langmuir-Blodgett film formation of an iron(II) triazole complex featuring a long alkyl chain substituent : the tail that wags the dog

[Fe(II)(C(16)dpt)(2)(NCS)(2)].(2/3)H(2)O displays temperature-mediated spin crossover (SCO) with T((1/2)) = 290 K and the long alkyl chain substituent on the dipyridyltriazole ligand facilitates the formation of a stable Langmuir-Blodgett film at an air-water interface.