Tl-O Layers Research Articles

Universal phonon softening in the pseudogap state of Tl2Ba2Can−1CunO2n+4+δ

Exploring the origin of the pseudogap is important for the understanding of superconductivity in cuprates. Here we report a systematical experimental study on the phonon vibrational properties of ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{n\ensuremath{-}1}{\mathrm{Cu}}_{n}{\mathrm{O}}_{2n+4+\ensuremath{\delta}}$ ($n=1,2,3$) single crystals based on the Raman scattering measurements over the temperature range from 10--300 K. The temperature evolution of the frequency and linewidth of the observed phonon modes in each member of this family does not follow the expected self-energy effect when entering the superconducting state. The anomalies are observed for the phonon modes involving the vibrations of the atoms in the Tl-O layer and the apical oxygen at the temperature around 150 K, which is higher above the superconducting transition. The phonon mode of the apical oxygen exhibits a pronounced universal softening behavior. From the comparison with the existing experimental data for various orders, we find that the observed starting temperature for the phonon softening corresponds to the onset opening temperature of the pseudogap. This finding indicates a large lattice effect in the pseudogap state and the non-negligible spin-phonon coupling for such a phonon softening.

Preparation and Physical Properties of Tl0.65Hg0.35Ba2Can-1CunO2n+3-/delta(n = 1, 2, 3) Superconductors

Multilayered cuprates of Tl0.65Hg0.35Ba2Can-1CunO2n+3-/delta, with n = 1–3 have been synthesized by using solid state reaction method in order to investigate the variation of transition temperature Tc. The temperature dependence of resistivity showed that the Tc depends on n. The c-lattice constants of the observed phases obey, the c-axis rule of c = 3.117n+6.5467 for Tl-O layer structures. With an increase in Cu-O layers, the superconducting transition temperature is determined by electrical resistivity-temperature dependency and rises reaching a maximum value of 121 K at n = 3.

ChemInform Abstract: Determination of the Local Structure and Electronic States of High-Tc Superconductors by Scanning Tunneling Microscopy

To date, scanning tunneling microscopy (STM) has significantly advanced the understanding of the local structure, electronic properties, and reactivity of semiconductor surfaces. Less recognized is the fact that STM can also elucidate the local structural and electronic properties of low-dimensional materials. Herein, the authors demonstrate that STM can provide new and essential insight into the physical complexities of high-temperature copper oxide superconductors that is unavailable from conventional studies. After reviewing the basic theoretical concepts needed to evaluate STM data, the authors discuss several specific cases that illustrate the unique information that STM provides. These examples include (1) the atomic level nature of structural disorder in the BiO and TlO layers of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} and the low-energy electronic states associated with these structural features, (2) the local structure and electronic consequences of metal substitution and oxygen doping in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}, and (3) low-temperature tunneling spectroscopy measurements of the superconducting energy gap. Lastly, the authors summarize exciting future avenues to pursue in STM studies of these and other materials. 57 refs., 15 figs.

Tl2Ba2CuO6+δ brings spectroscopic probes deep into the overdoped regime of the high-Tc cuprates

Single-particle spectroscopic probes, such as scanning tunnelling and angle-resolved photoemission spectroscopy (ARPES), have provided us with crucial insights into the complex electronic structure of the high-Tc cuprates, in particular for the under- and optimally-doped regimes where high-quality crystals suitable for surface-sensitive experiments are available. Conversely, the elementary excitations on the heavily overdoped side of the phase diagram remain largely unexplored. Important breakthroughs could come from the study of Tl2Ba2CuO6+δ (Tl2201), a structurally simple system whose doping level can be tuned from optimal to extreme overdoping by varying the oxygen content. Using a self-flux method and encapsulation, we have grown single crystals of Tl2201, which were then carefully annealed under controlled oxygen partial pressures. Their high quality and homogeneity are demonstrated by narrow rocking curves and superconducting transition widths. For higher dopings, the crystals are orthorhombic, a lattice distortion stabilized by O interstitials in the TlO layer. These crystals have enabled the first successful ARPES study of both normal and superconducting-state electronic structure in Tl2201, allowing a direct comparison with the Fermi surface from magnetoresistance and the gap from thermal conductivity experiments. This establishes Tl2201 as the first high-Tc cuprate for which a surface-sensitive single-particle spectroscopy and a comparable bulk transport technique have arrived at quantitative agreement on a major feature such as the normal state Fermi surface. The momentum dependence of the ARPES lineshape reveals, however, an unexpected phenomenology: in contrast to the case of under- and optimally-doped cuprates, quasiparticles are sharp near (π,0), the antinodal region where the gap is maximum, and broad at (π/2, π/2), the nodal region where the gap vanishes. This reversed quasiparticle anisotropy past optimal doping, and its relevance to scattering, many-body, and quantum-critical phenomena in the high-Tc cuprates, is discussed.

Unsubstituted Tl-1223: a possible candidate for high current applications of superconductivity

The Tl–Ba–Ca–Cu–O system has been investigated. We report on the superconducting properties and the pathway for the formation of the Tl-1223 phase and on equilibrium phase relations. The formation of Tl-1223 implies the double TlO layer phases and occurs at high temperature (890–910°C) by a progressive de-intercalation of TlO layers from Tl-2223. As a consequence of this non-equilibrium process, transient intergrowth phases may be formed and have been unambiguously identified. This path of formation results in a platelet-like morphology for Tl-1223 grains which, in the view of the fabrication of tapes, is much more interesting than the globular shape observed for (Tl,Pb)(Sr,Ba) 2Ca 2Cu 3O z . Moreover, the superconducting properties ( T c and irreversibility line) of the unsubstituted Tl-1223 phase compare well with those of (Tl,Pb)(Sr,Ba) 2Ca 2Cu 3O z . With regard to phase equilibria, we show the existence of a three phase field Tl 2Ca 3O 6–(Tl-2212)–(Ca,Tl) 1− x CuO y which prevents the formation of Tl-2223 for Tl contents higher than two.

Hole Confinement to CuO2 Layers in High-Tc Cuprate Superconductors due to Crystal Potential

The distribution of positive holes in high-Tc cuprate superconductors is discussed in terms of the electrostatic potential in crystals. The maps of the crystal potential are drawn for Tl2Ba2Can-1CunO2n+4 (n=1–4) homologous series of high-Tc superconductors. Close to the CuO2 layer, there are always negative regions, which extend over the entire crystal. Moreover, the potential distribution at a plane of an apical oxygen becomes positive almost everywhere. It is thus realized that a supercurrent carried by holes will be confined within the conduction layer between the planes of apical-oxygen. Furthermore, the potential of the TlO layer, which is considered as a charge reservoir, is far positive. According to the potential gradient, electrons will move out from the CuO2 (conduction) layer to the TlO layer, while holes will move in the opposite direction.

High-temperature in situ neutron powder diffraction study of monoclinic Tl2Ba2CuO6+δ

The monoclinic form of the Tl2Ba2CuO6+δ superconductor (F112/m symmetry) has been studied using high-resolution high-temperature in situ time-of-flight neutron powder diffraction. The amount of excess oxygen controls both the crystal structure and electronic properties: the as-prepared material (a=5.4412 Å, b=5.5026 Å, c=23.1523 Å, γ=90.127 °, δ=0.15) is non-superconducting, whereas the 700 K vacuum-annealed sample (a=5.4874 Å, b=5.5288 Å, c=23.3076 Å, γ=90.034 °, δ=0.11) is superconducting below Tc=73 K. Thermal variations of atomic c-axial displacements and bond valence sums demonstrate an electronic charge transfer from the TlO layers to the CuO2 planes. The peak widths of the Bragg reflections indicate microscopic strains in the c-axial direction, due to disordering of interstitial oxygen in the Tl2O2+δ bilayer, and in the ab-plane, due to the monoclinic and orthorhombic distortions, or static displacements of the Tl and O atoms in the TlO layers. Superstructural diffraction peaks due to an incommensurate structural modulation decrease in intensity with oxygen loss over the monoclinic-to-orthorhombic progression.

Synthesis and Structure of a New Family of Nickelates: Tl(Ln2Sr2)Ni2O9, Ln=La, Pr, Nd, Sm, Eu, and Gd

A novel series of nickelates described by the formula Tl(Ln2Sr2)Ni2O9, Ln=La, Pr, Nd, Sm, Eu, and Gd, have been synthesized and their structures determined using both powder X-ray diffraction and powder neutron diffraction. The materials are isostructural with the superconducting cuprate, Tl(La2.4Ba1.6)Cu2O9, crystallizing in a tetragonal unit cell, space group I4/mmm, lattice parameters a=3.8069–3.7667(1) Å and c=30.055–29.390(1) Å. The compounds adopt the 1201–0201 intergrowth structure, consisting of layers of apex-sharing stoichiometric NiO6 octahedra connected by a TlO layer. A decrease in the cell dimensions is observed across the series, consistent with the lanthanide contraction and partial ordering of the strontium and lanthanide ions on the A cationic sites is observed. Magnetic susceptibility data reveal no evidence of long-range magnetic ordering for two of the compounds, Tl(La2Sr2)Ni2O9 and Tl(Gd2Sr2)Ni2O9.

Valences of Cu and Tl and chemical bond parameters in Tl xBa 2Cu yO 6+ δ, Tl 2Ba 2CaCu 2O 8, and Tl 2Ba 2Ca 2Cu 3O 10

The valences of Cu and Tl and chemical bond parameters, that is, bond covalency and macroscopic linear susceptibility in Tl x Ba 2Cu y O 6+ δ , Tl 2Ba 2CaCu 2O 8, and Tl 2Ba 2Ca 2Cu 3O 10 have been calculated using a semi-empirical method. For Tl x Ba 2Cu y O 6+ δ , the results indicate that the average valences of Tl and Cu are between +2.0 and +3.0, indicating that part of Cu and Tl is in a state of +3.0. With the increase of oxygen content and Tl/Cu ( x/ y) ratios, the valence of Tl increases, while for the valence of Cu this trend is only observed for fixed Tl/Cu ratio samples. The valence of Tl changes more rapidly than that of Cu. For samples where both oxygen content and Tl/Cu ratio change, there exists an optimal valence of Cu and Tl for the highest T c. On the other hand, for fixed Tl/Cu ratio samples, the smaller the valences of Cu and Tl, the higher the T c. The bond covalencies of Ba–O, Cu–O (in CuO 2 layer), and Tl–O (the bond of Tl with the inserted oxygen between double TlO layers) are sensitive only to the Tl/Cu ratios, while those of the remaining Tl–O bonds and Cu–O (outside CuO 2 layer) are influenced by both. The macroscopic linear susceptibility tends to decrease with the increase of oxygen content and Tl/Cu ratios. Finally, Tl 2Ba 2CaCu 2O 8 and Tl 2Ba 2Ca 2Cu 3O 10 have also been investigated.

Superconductivity in the chromium-doped Tl-1212 phase (Tl0.8M0.2)Sr2(Ca1-xCrx)Cu2O7 (M=Bi and In)

The single TlO layer TlSr2Can-1CunO2n+3 with n = 2, often called Tl-1212 phase can be extensively subjected to elemental substitutions forming numerous superconducting materials in the family. In this paper, materials with nominal starting compositions (TI0.8M0.2)Sr2(Ca1-x Crx)Cu2O7 with M=Bi and In have been prepared and investigated by the d.c. electrical resistance method and powder X-ray diffraction analyses. Most of the samples showed the Tl-1212 as the major phase and 1201 as the minor phase. The Bi-substituted samples showed a higher onset temperature and higher 1212 phase volume fraction compared to the In-substituted materials. The importance of ionic radius of the substituted elements and the effect of melting/volatility point of the starting components on the onset critical transition temperature (Tc,onset) and phase formation is discussed.

A defective structure for Tl-deficient Tl2-xBa2CaCu2Oysuperconductors

Abstract Considerable Tl deficiency has been detected in the Tl2Ba2CaCu2Oy (Tl-2212) superconducting phase formed in thin films and polycrystals. Transmission electron microscopy was used to characterize the structure of this Tl-deficient phase. It has been found that the space group changes to be I4mm which is apparently different from I4/mmm as usually reported for the Tl-2212 phase. The change in the space group is due to a lack of the (001) mirrors in the defective Tl-2212 structure which can be directly imaged by high-resolution electron microscopy. Image simulations confirmed a proposed structure model in which inhomogeneously distributed Tl vacancies and Tl disorder in the Tl-O layers are considered to be responsible for the symmetry change. The stability of such a defective Tl-2212 phase will be discussed.

A defective structure for Tl-deficient Tl2-xBa2CaCu2Oy superconductors

Abstract Considerable Tl deficiency has been detected in the Tl2Ba2CaCu2Oy (Tl-2212) superconducting phase formed in thin films and polycrystals. Transmission electron microscopy was used to characterize the structure of this Tl-deficient phase. It has been found that the space group changes to be I4mm which is apparently different from I4/mmm as usually reported for the Tl-2212 phase. The change in the space group is due to a lack of the (001) mirrors in the defective Tl-2212 structure which can be directly imaged by high-resolution electron microscopy. Image simulations confirmed a proposed structure model in which inhomogeneously distributed Tl vacancies and Tl disorder in the Tl-O layers are considered to be responsible for the symmetry change. The stability of such a defective Tl-2212 phase will be discussed.

Synthesis and properties of fluorine-doped Tl(1223): bulk materials and Ag-sheathed tapes

Fluorine-doped Tl(1223) superconductors were prepared starting from mixtures of oxides (carbonates) and fluorides (in particular TlF) under moderately high gas pressure (50 bar). The thermodynamics of the phase formation were found to be modified so that the melting point was lowered and the region where a liquid phase coexists enlarged. 90% phase purity was obtained for bulk materials with the composition , prepared by a two-step synthesis including TlF among the starting materials. The critical temperature remained approximately the same as for F-free samples , but a remarkable increase of the irreversibility line was observed below 75 K. The samples showed superconducting properties even for the highest amount of fluorine introduced (about one atom per formula unit). Ag-sheathed tapes were prepared by the conventional PIT method, using ex situ reacted Tl(1223) powders or producing the reaction in situ. Transport critical current densities of about were measured. This value is slightly lower than generally observed for F-free samples, but the magnetic-field dependence was slightly improved. The tape anisotropy could be increased to a factor of 2.5 by performing cycles of mechanical pressing and heating above the formation temperature. The results (in particular neutron scattering diffraction) indicate that, with high-pressure synthesis, F remains in the sample and substitutes for O in the TlO layers in the Tl(1223) phase.

The influence of pressure-induced relaxation effects on Tc in TlSr 2CaCu 2O 7− δ

High pressure experiments on polycrystalline TlSr 2CaCu 2O 7− δ (Tl-1212) reveal relaxation effects in T c that vary strongly with oxygen content. Unlike Tl 2Ba 2CuO 6+ δ , where distinct low- and high-temperature effects are found, relaxation in Tl-1212 occurs only for temperatures above 210 K. This relaxation is believed to originate from the ordering of oxygen vacancies in the TlO layer.

The synthesis and structure of Tl(Sr1.4La2.6)Ni2O9; a direct structural analogue of a superconducting cuprate

The nickelate Tl(Sr 1.4 La 2.6 )Ni 2 O 9 , synthesised by the reaction of Tl 2 O 3 , La 2 O 3 and Sr 2 Ni 2 O 5 , is isostructural with the equivalent superconducting cuprate phase, consisting of layers of apex-sharing, stoichiometric NiO 6 octahedra separated by a TlO layer.

THE MECHANISM OF SURFACE DECOMPOSITION OF TL- AND HG-CONTAINING SUPERCONDUCTORS

The surface chemistry of various Tl- and Hg-containing superconducting oxides has been studied by high resolution transmission electron microscopy. The mechanism of surface decomposition of these materials is discussed. High chemical reactivity of the Tl-O and Hg-O layers seem to be the main course of the decomposition. It is proposed that a surface substitution of Tl and Hg may stabilise the structures without losing superconducting properties.

Charge transfer process in Tl 2Ba 2Ca 2Cu 3O 10 and Tl 2Ba 2CaCu 2O 8 thin films probed by polarized X-ray absorption spectroscopy

We report high-resolution polarized O 1s X-ray absorption spectra of highly c-axis-oriented Tl 2Ba 2Ca 2Cu 3O 10 (Tl-2223) and Tl 2Ba 2CaCu 2O 8 (Tl-2212) superconducting thin films. Near the O 1s edge, the low-energy pre-edge peak at 528.3 eV has mainly O 2p xy symmetry, while the pre-edge peak at 529.4 eV has predominantly O 2p z character. The spectral weight of the pre-edge peak at 528.3 eV, originating from the CuO 2 planes, increases ∼ 60% from the Tl-2212 to Tl-2223 thin films. Conversely, the high-energy pre-edge peak at 530.3 eV, originating from the TlO planes, is lower in spectral intensity by ∼ 30% in Tl-2223 as compared to the Tl-2212 thin film. The experimental results clearly demonstrate the pictures of the self doping due to the charge transfer from the CuO 2 layers to the TlO layers.

On Tl-1212 superconductor single crystals: structure and magnetism changes on annealing

The complete crystal structure of several single crystals with nominal stoichiometry TlBa 2CaCu 2O 7 (Tl-1212) has been determined by least-squares refinement of Mo K α X-ray diffraction data before and after sequential anneals in flowing argon and/or oxygen. Prior work, primarily on TlBa 2Ca 2Cu 3O x , suggested that both Tl 1+ and Tl 3+ are present in the charge reservoir Tl-O layer(s). The relative amounts could be reversibly altered by mild oxidation or reduction anneals without significant changes in the superconducting onset temperature, T c, consistent with maintaining a hole concentration near the optimum doping. These anneals produced small reversible atomic displacements within the unit cell. In contrast, the atomic positional changes in the Tl-1212 crystals upon such annealing are just at the detectability level, and although all the crystals studied belong to this structure type, the mean values in some of these parameters grouped by starting growth compositions result in differences at the 95% confidence level. Meissner data show sharpening of the transition of the as-grown crystals with annealing, but T c changes are much smaller than observed in Tl-1212 powders.

Electronic structure ofTlBa2CaCu2O7−δ

The core levels of ${\mathrm{TlBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (Tl-1212) epitaxial films have been measured with x-ray photoelectron spectroscopy (XPS). The valence electronic structure has been determined using the full-potential linear muffin-tin-orbital band-structure method and measured with XPS. The calculations show that a van Hove singularity (VHS) lies above the Fermi level (${\mathrm{E}}_{\mathrm{F}}$) for the stoichiometric compound (\ensuremath{\delta}=0), while for 50% oxygen vacancies in the Tl-O layer (\ensuremath{\delta}=0.5) ${\mathrm{E}}_{\mathrm{F}}$ is in close proximity to the VHS. Samples annealed in nitrogen (to reduce the hole overdoping by the removal of oxygen) exhibit higher core-level binding energies and a higher ${\mathrm{T}}_{\mathrm{c}}$, consistent with a shift of ${\mathrm{E}}_{\mathrm{F}}$ closer to the VHS. Comparisons are made to the core levels and valence bands of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Tl-2212) and ${\mathrm{HgBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ (Hg-1212). The similarity of the Cu ${2\mathrm{p}}_{3\mathrm{/}2}$ spectra for Tl-1212 and Tl-2212 indicates that the number of Tl-O layers has little effect on the Cu-O bonding. However, the Tl-1212 and Hg-1212 Cu ${2\mathrm{p}}_{3\mathrm{/}2}$ signals exhibit differences which suggest that the replacement of ${\mathrm{Tl}}^{3+}$ with ${\mathrm{Hg}}^{2+}$ results in a decrease in the O 2p\ensuremath{\rightarrow}Cu 3d charge-transfer energy and differences in the probabilities of planar vs apical oxygen charge transfer and/or Zhang-Rice singlet-state formation. Differences between the Tl-1212 and the Tl-2212 and Hg-1212 measured valence bands are consistent with the calculated Cu 3d and (Tl,Hg) 6s/5d partial densities of states.

Process optimization for the fabrication of Tl2Ba2Ca2Cu3O10 thin films

A process has been developed for the fabrication of nearly single phase superconducting Tl2Ba2Ca2Cu3O10 thin films on (100) LaAlO3 substrates with a superconducting transition temperature Tc of 120 K and low microwave surface resistance at temperatures up to 110 K. Amorphous BaCaCuO precursor films were first deposited by rf magnetron sputtering and then thallinated at elevated temperatures. The double TlO layer phases (Tl2Ba2Ca2Cu3O10 + Tl2Ba2CaCu2O8) formed preferentially over the single TlO layer phases (TlBa2Ca2Cu3O9 + TlBa2CaCu2O7) at high Tl2O partial pressures. Thin films containing Tl2Ba2Ca2Cu3O10 and a small amount of CuO were prepared from Cu-rich precursor films (Cu/Ba > 1.7), while lower Cu content led to the formation of Tl2Ba2CaCu2O8 as a secondary phase. Tl2Ba2Ca2Cu3O10 film epitaxy was enhanced by carrying out the thallination in reduced oxygen partial pressures of 0.01–0.05 atm. Following the thallination step, the Tl2Ba2Ca2Cu3O10 thin films had a superconducting transition temperature Tc of only 106 ± 4 K. An additional 62 h anneal at 800 °C or an 8 h anneal at 850 °C in a Tl2O/O2 atmosphere increased the Tc to 120 K. The increase in Tc was accompanied by a decrease in the c-axis lattice constant, an enhancement in the long-range order in the c-direction, and the formation of a small amount of Tl2Ba2CaCu2O8 as a secondary phase. Minimization of surface resistance at high temperature (95–110 K) requires that the fraction of Tl2Ba2CaCu2O8 secondary phase in the films be kept low. Process routes are also described for the formation of nearly single phase TlBa2Ca2Cu3O9 and TlBa2CaCu2O7 thin films and the formation of a new ordered intergrowth phase, Tl4Ba4Ca3Cu5O18, which consists of alternating Tl2Ba2CaCu2O8 and Tl2Ba2Ca2Cu3O10 layers.