Tl-based Cuprate Research Articles

Stable fluoride based sputtering target for Tl-based cuprate superconducting thin film fabrication

Thin films of thallium based superconductors were prepared successfully using sputtering target fabricated as a mixture of barium fluoride, calcium fluoride and copper oxide. Such a target is stable, does not degrade and does not involve any special handling (vacuum or argon atmosphere storage). Precursor films sputtered from this target contained less than 5 at.% of fluorine so no fluorine content reduction is needed. Depending on the substrate, film thickness and synthesis conditions, Tl-2212 phase or a mixture of Tl-2212 and Tl-2223 phases was prepared. Although Tl-2212 phase has epitaxial character, Tl-2223 phase, growing on the top of Tl-2212, is only c-axis oriented. The highest values of TC0 were obtained using 300 nm thick precursor films thallinated at 860 °C. After the thallination the films contained a mixture of Tl-2212 and Tl-2223 phase with a critical temperature of TC0 = 106 K and a critical current density (at 77 K) up to 6 × 104 A/cm2. Lower TC0 and JC values suggest that further optimization of the film synthesis is needed; however, the first results show that the stable sputtering target prepared from fluorides is a good candidate for high quality Tl-based thin film fabrication.

Superconducting interaction charge in thallium-based high-TC cuprates: Roles of cation oxidation state and electronegativity

Superconductivity in the Tl-based cuprates encompasses a notably broad range of measured optimal transition temperatures TC0, ranging from lowest in the charge-depleted Tl-1201 compounds (Tl1–x(Ba/Sr)1+yLa1–yCuO5–δ), such as Tl0.7LaSrCuO5 (37K) and TlBa1.2La0.8CuO5 (45.4K), to highest in the Tl-1223 compound TlBa2Ca2Cu3O9±δ (133.5K). Seven Tl-based cuprates are considered and compared using the model of superconductive pairing via electronic interactions between two physically separated charge reservoirs, where TC0∝(ση/A)1/2ζ−1 is determined by the superconducting interaction charge fraction σ, the number η of CuO2 layers, and the basal-plane area A, each per formula unit, and the transverse distance ζ between interacting layers. Herein it is demonstrated that σ follows from the elemental electronegativity and the oxidation state of Tl, and other structurally analogous cations. The comparatively lower elemental electronegativity of Tl, in conjunction with its oxidation state, explains the higher σ and TC0 values in the Tl-based compounds relative to their Bi-based cuprate homologs. A derivation of σ is introduced for the optimal Tl2Ba2Caη–1CuηO2η+4 (for η=1, 2, 3) compounds, which exhibit a Tl oxidation state at or near +3, obtaining the fundamental value σ0=0.228 previously established for YBa2Cu3O6.92. Also reported is the marked enhancement in σ associated with Tl+1 and analogous inner-layer cations relative to higher-valence cations. For a model proposition of σ=σ0, the fractional Tl+1 content of the mixed-valence compound, TlBa2Ca2Cu3O9±δ, is predicted to be 1/3 at optimization, in agreement with existing data. Charge depletion is illustrated for the two Tl-1201 compounds, where σ<σ0 values are determined according to substitution of Ba+2 or Sr+2 by La+3, and/or Tl depletion. Additionally, statistical analysis of calculated and experimental transition temperatures of 48 optimal superconductors shows an absence of bias in determining σ, A, and ζ.

Patterning of Tl-based superconducting films using new etching solution

a b s t r a c t Tl-based cuprate superconducting films were prepared in a two-step process by RF magnetron sputtering of an amorphous precursor and ex situ thallination in open system. The films prepared on LaAlO3 and CeO2 buffered R-plane sapphire substrates consisted from c-axis oriented Tl-2212 superconducting phase. The zero resistance critical temperature TC0 exhibited values up to 94 K. Subsequently, superconducting structures were prepared from the Tl-based thin films using photolithography process and wet etching. A new etchant based on potassium iodide was used for the Tl-based film patterning. The prepared structures had sharp edges, unchanged phase composition and critical temperature values. Such a way of the Tlbased film patterning is very simple, fast and easy to realize. © 2014 Elsevier B.V. All rights reserved.

Angular Dependence of the Fluctuation Magnetization Vector Above the Superconducting Transition of a Highly Anisotropic High-T c Cuprate

In highly anisotropic cuprate superconductors it is generally accepted that the reversible magnetization vector, M, is essentially perpendicular to the superconducting CuO2 layers in a wide range of crystal orientations with respect to the applied magnetic field, H. In a recent work [J. Mosqueira et al., Phys. Rev. B 84, 134504 (2011)] it is shown, however, that the dependence of M on the H orientation in the reversible mixed state of a high-quality Tl-based cuprate presents a notable deviation from this behavior. Here we extend these measurements to the fluctuation region above Tc, in order to check whether the above mentioned effect is also present.

Nuclear Quadrupole Resonance Frequency in Multilayered Cuprates

The 63 Cu nuclear quadrupole resonance (NQR) frequency ν Q in the multilayered cuprates is calculated in the cluster model by the exact diagonalization method. The charge imbalance between the outer CuO 2 plane (OP) with apical oxygen O A and the inner plane (IP) without O A in three-layered Tl 2 Ba 2 Ca 2 Cu 3 O 10 is estimated by comparing our results with the experimental ν Q . In Tl-based cuprates with more than three layers, we predict a large enhancement of the splitting of ν Q between OP and IP by taking into account the reduction in bond length between Cu and O A and the resulting increase in charge transfer energy. Our results show that the NQR frequency is a useful quantity for estimating the charge imbalance in the multilayered cuprates.

Mode coupling effect in infrared spectra of Tl-based cuprates

We study the mode coupling effect in infrared spectra of Tl-based cuprates. For samples near optimal doping, the reflectance roughly has a linear-frequency dependence in the normal state, but displays a pronounced knee structure followed by a dip-like feature at higher frequency below T c. We illustrate that the structure could not be caused solely by the strong coupling effect of electrons with a bosonic mode. A gap in the density of states has to be taken into account. The spectral evolution with doping for Tl-2201 crystals in the overdoped side was also investigated. Similar to Bi-2212 system, the mode coupling feature weakens dramatically with overdoping and tends to vanish in the heavily overdoped sample.

Out-of-plane temperature-dependent resistivity studies on Tl-based superconductors

Temperature-dependent resistivities in theab-planeand c-axis of Tl-based cuprates have been measured. Unlike theab-plane properties,which are metallic, c-axis transport is semiconductor-like in the normal state forTl2Ba2Ca2Cu3Ox (Tl-2223) andTl2Ba2CaCu2Ox (Tl-2212). Incontrast, for Tl2Ba2CuOx (Tl-2201), transport is metal-like in both the in-plane and thec-axis. For multi-layered cuprates, transport properties along thec-axis could be described by a tunnelling model, whereas for single-layered compoundTl-2201 it would be easier for the out-of-plane transport behaviour to be coherentsince the there are no insulating Ca layers in its structure. Moreover, combiningthe studies on Bi-2201, which has an insulating behaviour for the out-of-planeresistivity, we suggest that the Tl–O layers in Tl-based superconductors couldbe conducting, unlike the weakly correlated Bi–O layers in Bi-based cuprates.

Mode coupling effect in infrared spectra of Tl-based cuprates

We study the mode coupling effect in infrared spectra of Tl-based cuprates. For samples near optimal doping, the reflectance roughly has a linear-frequency dependence in the normal state, but displays a pronounced knee structure followed by a dip-like feature at higher frequency below Tc. We illustrate that the structure could not be caused solely by the strong coupling effect of electrons with a bosonic mode. A gap in the density of states has to be taken into account. The spectral evolution with doping for Tl-2201 crystals in the overdoped side was also investigated. Similar to Bi-2212 system, the mode coupling feature weakens dramatically with overdoping and tends to vanish in the heavily overdoped sample. We elaborate that the observed trend does not favor a phonon origin of the mode, but could be reconciled with coupling to magnetic excitations.

Growth and upper critical field of heavily overdoped Tl 2Ba 2CuO x single crystals

Growth technique for single crystals of Tl-based cuprate superconductors are established using a perfectly sealed gold crucible. Single crystals of Tl 2Ba 2CuO x can be easily obtained. Upper critical fields ( H c2) of heavily overdoped Tl 2Ba 2CuO x crystals ( T c=9.7, 15.7 K) have been estimated by measuring high frequency (28.5 GHz) microwave surface resistance. It is found that the temperature dependence of H c2 is not divergent but tends to be saturated toward 0 K. This is extremely different from temperature dependence of resistive superconducting transition point of direct current resistance in magnetic fields reported before. The extrapolated upper critical fields H c2(0) of the crystals are 0.85 T ( T c=9.7 K) and 1.9 T ( T c=15.7 K), respectively.

Crystal growth techniques for Tl-based cuprate superconductors

New techniques for the crystal growth of Tl-based cuprate superconductors have been developed. It is found that a gold crucible and closed atmosphere are crucial for their crystal growth by a so-called self-flux method for a long time. As an example, crystals of Tl 2Ba 2CuO x , which is most important among the Tl-based cuprate superconductors because of its wide T c-change and anomalous resistive critical fields, have been grown. Thus, mm-sized single crystals of Tl 2Ba 2CuO x in a high quality can be obtained. It should be noted that the value of the resistivity ratio R(300 K)/R(T c ) (RRR) of one of the as-grown crystals is 18.3. This high RRR value indicates that the newly developed techniques are useful to obtain extremely high-quality single crystals of not only Tl 2Ba 2CuO x but also other Tl-based cuprate superconductors.

Preparation and properties of high- Tc superconducting Tl-1223 thick films obtained by screen-printing method

Tl-based cuprate Tl 0.5Pb 0.5Ba 0.4Sr 1.6Ca 2Cu 3O x superconducting 20–60 μm thick films, were obtained using the screen-printing technique with a two-step method comprising deposition of a precursor film on polycrystalline 8 mol% Y 2O 3-doped ZrO 2 substrates followed by Tl-vapour annealing at 910°C. The resistance measurements yielded T c(0) values of about 110–115 K with transition widths of 2–4 K. X-ray diffraction confirmed the presence of the superconducting Tl-1223 major phase. Scanning electron microscopy in combination with EDS investigations showed that besides the superconducting crystallites also Ca-Sr-Cu-O compounds of different composition were present as secondary phases. The distribution of the secondary phases and the interaction of superconducting films with YSZ substrates is discussed.

Systematics ofc-axis phonons in the thallium- and bismuth-based cuprate superconductors

We present grazing incidence reflectivity measurements in the far-infrared region at temperatures above and below ${T}_{c}$ for a series of thallium- $({\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6},$ ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8})$ and bismuth- $({\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6},$ ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8},$ and ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8})$ based cuprate superconductors. From the spectra, which are dominated by the $c$-axis phonons, longitudinal optical (LO) frequencies are directly obtained. The reflectivity curves are well fitted by a series of Lorentz oscillators. In this way the transverse optical (TO) phonon frequencies were accurately determined. On the basis of the comparative study of the Bi- and Tl-based cuprates with a different number of ${\mathrm{CuO}}_{2}$ layers per unit cell, we suggest modifications of the assignment of the main oxygen modes. We compare the LO frequencies in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ and ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10}$ obtained from intrinsic Josephson junction characteristics with our measurements, and explain the discrepancy in LO frequencies obtained by the two different methods.

Doping dependence of the in-plane penetration depth and fishtail inBi2Sr2Ca1−xYxCu2O8+δsingle crystals

We report on the London penetration depth ${\ensuremath{\lambda}}_{\mathrm{ab}}(0),$ measurements, and second peak $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{H}$ determination in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi-2212) crystals with different doping levels. By combining cationic substitutions and oxygen pressure annealings, crystals with a doping state going from the underdoped to overdoped regime have been obtained. The corresponding ${\ensuremath{\lambda}}_{\mathrm{ab}}(0)$ values, deduced from reversible magnetization measurements have allowed us to evidence a ``boomerang'' shape for the ${T}_{c}[1/{\ensuremath{\lambda}}_{\mathrm{ab}}^{2}(0)]$ curve in the $0.8l~{T}_{c}{/T}_{c}^{\mathrm{max}}l~1$ region. Moreover, other ${T}_{c}[1/{\ensuremath{\lambda}}_{\mathrm{ab}}^{2}(0)]$ values for superconducting crystals of Hg- and Tl-based cuprates are found to fit well with this boomerang path, demonstrating its universal nature. The overdoped side of the boomerang for the Bi-2212 crystals clearly shows that $1/{\ensuremath{\lambda}}_{\mathrm{ab}}^{2}(0)$ decreases as the doping level is increased. Since by the same time the characteristic field of the fishtail $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{H}$ monotonously increases, we have thus established that $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{H}$ increases with $1/{\ensuremath{\lambda}}_{\mathrm{ab}}^{2}(0)$ decreasing. Consequently, the linear relation $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{H}={\ensuremath{\Phi}}_{0}/{\ensuremath{\lambda}}_{\mathrm{ab}}^{2}(0)$ predicted by the electromagnetic model cannot account alone for the present experimental results in the overdoped regime.

Thick films of the Tl-based superconductor fabricated via the sol-gel technique

Superconducting thick films of Tl-based cuprate were grown via the sol-gel technique with a two-step method: deposition of a Tl-free precursor film followed by Tl-vapor annealing. A zero resistance superconducting transition temperature ( T c) of 120 K on polycrystalline MgO substrates for Tl-1223 films and of 124 K on (100) LaAlO 3 substrates and polycrystalline MgO substrates for Tl-2223 films was obtained. The intragranular critical current density at 5 K of these films is greater than 10 6 A/cm 2 for Tl-1223 and Tl-2223. The microstructure of these films was examined with X-ray diffraction and scanning electron microscopy.

Oxidation State of Cu and Superconductivity in Bilayer Tl-Based Cuprates

The oxidation state of Cu in the homologous Tl2Ba2Can-1CunO2n+4+δ (n = 1 - 4) series has been analyzed in terms of the bond valence sums. It is found that the totally mobile hole concentration nH,t of the type-I CuO2 planes at the optimal level increases with increasing number of CuO2 layers per unit cell from 1 to 3, and the largest nH,t is observed to be 0.474 when n = 3, which just corresponds to the highest Tc in the bilayer Tl-based materials, and then it decreases with the increase of n. It is indicated that the correlation between the maximum value of Tc and n is similar to that between nH,t and n.

Scaling behavior and mixed-state Hall effect in epitaxialHgBa2CaCu2O6+δsthin films

We have measured the mixed-state Hall effect in superconducting ${\mathrm{HgBa}}_{2}$ ${\mathrm{CaCu}}_{2}$ ${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ thin films. We found that the Hall resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathrm{xy}}$ shows a double sign reversal in low fields, then shifts to positive without changing the general shape in higher fields. At the higher field region, the Hall conductivity ${\mathrm{\ensuremath{\rho}}}_{\mathrm{xy}}$ is observed to be the sum of two terms, ${\mathrm{C}}_{1}$ /H and ${\mathrm{C}}_{2}$ H, where ${\mathrm{C}}_{1}$ and ${\mathrm{C}}_{2}$ are field independent. The scaling behavior between ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{xy}}$ and ${\mathrm{\ensuremath{\rho}}}_{\mathrm{xx}}$ (longitudinal resistivity) shows a strong dependence on fields. The scaling exponent \ensuremath{\mathrm{B}} in ${\mathrm{\ensuremath{\rho}}}_{\mathrm{xy}}$ =A${\mathrm{\ensuremath{\rho}}}_{\mathrm{xx}}^{\mathrm{\ensuremath{\mathrm{B}}}g}$ increases from 1.5\ifmmode\pm\else\textpm\fi{}0.1 to 1.9\ifmmode\pm\else\textpm\fi{}0.1 as field increases from 1 to 5.5 T. The field dependence of tangent of the Hall angle is linear only in the flux-flow regime, different from Bi- and Tl-based cuprates.

Determination of oxygen content and cation valences in superconducting Tl-based cuprates by a wet chemical method.

A wet chemical route based on constant-potential electrolysis and different reactivities of ${\mathrm{Cu}}^{3+}$ and ${\mathrm{Tl}}^{3+}$ with water is proposed to distinguish the average valences of Cu and Tl in superconducting thallium cuprates. The validity of the method is demonstrated using ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$, ${\mathrm{Tl}}_{2}$${\mathrm{O}}_{3}$, and ${\mathrm{Tl}}_{2}$O as reference materials for ${\mathrm{Cu}}^{3+}$, ${\mathrm{Tl}}^{3+}$, and ${\mathrm{Tl}}^{+}$, respectively. Furthermore, the oxygen contents and the cation valences have been determined for various Tl-22(n-1)n phases (where n=1--4). With increasing n value the Cu valence is found to decrease, while the valence of thallium remains rather constant at ca. +3, regardless of the value of n. Based on these results, the possible carrier doping mechanisms in the Tl-22(n-1)n phases are discussed.

Determination of oxygen contents and cation valences in the superconducting bismuth and thallium cuprates

The oxygen contents and valences of cations in various superconducting cuprates such as Bi2Sr2(Ca, Y)Cu2Oy, Tl2Ba2Can−1CunOy, etc. which contain two kinds of multi-valence metals have been determined by novel wet chemical methods. The wet chemical methods are based on the differences in oxidation power of each cation and at the last step an electrochemical method is employed for measuring the amount of reducing agent, i.e., CuCl. For superconducting Bi-based cuprates, a modified constant-current coulometry has been developed. For the superconducting Tl-based cuprates a constant-potential electrolysis method which makes it easy to control the electrolysis potential has been applied. The results obtained for Bi2Sr2(Ca, Y)Cu2Oy have shown that both Bi- and Cu-valences decrease with increasing Y, which is consistent with the results of earlier works. The results obtained for the double Tl-O layer superconductors show that the Tl valence is about +3 in all the thallium cuprates and different from the value obtained from XPS measurements of earlier works. The reason for this discrepancy is discussed.

Weak link behavior of grain boundaries in Nd-, Bi-, and Tl-based cuprate superconductors

We have studied single-grain-boundary junctions in the neodymium-, bismuth-, and thallium-based cuprate superconductors and find that they behave as weak links, qualitatively similar to the YBa2Cu3O7−δ superconductor. In general, the grain boundary critical current is determined by flux flow for small misorientation angles and by Josephson junctionlike coupling for large angles. The latter is verified by the observation of voltage oscillations with an external magnetic field in superconducting quantum interference devices built using single-grain-boundary junctions of these materials. The commonality of behavior of grain boundaries in all of the high temperature cuprate superconductors suggests that the weak link is most likely associated with the structure of the grain boundary and the evidence points increasingly to dislocations, which describe the topology of the boundary.

A method for doping Tl-based cuprate superconductors with holes

We previously demonstrated that the T c for a Tl-2223 superconductor was increased to above 125 K when the sample was encapsulated in an evaluated quartz tube and post-annealed at 750°C for 250 h. It was indicated by thermo-electric power measurements that the post-annealed Tl-2223 sample had a hole concentration higher than an as-sintered Tl-2223 sample. Moreover, when the same encapsulation-and-post-annealing (ECPA) technique was applied to a Tl-2212 phase and a Tl-2234 sample, the T c decreased in the case of Tl-2212 while it increased in the case of TL-2234 phase. This demonstrated that the ECPA technique was affective in increasing the hole concentration in both over-doped (i.e. Tl-2212) and under-doped (i.e. Tl-2223 and Tl-2234) phases. Judging from the changes in the lattice constants and the chemical composition, it was likely that the increases in hole concentration and the subsequent changes in T c were due to decreases in the Tl content during ECPA processes.