HTSC Compounds Research Articles

Cluster structure of superconducting phase and the nature of peaks in the doping dependences of the London penetration depth in iron pnictides

The mechanism of formation of dome-like phase diagrams and the features of magnetic field penetration in iron pnictides with hetero- and isovalent doping are considered within the framework of the previously proposed model, which assumes the local character of doping and the cluster structure of resulted superconducting phase. It is shown that the proposed model, despite its simplicity and neglect of the features of the electronic structure, makes it possible not only to accurately calculate the positions of superconducting domes on the phase diagrams of specific HTSC compounds, but also to explain the nature and position of sharp peaks in the London penetration depth depending on the doping level.

Structural and electroresistive properties of layered compounds based on the 1-2-3 HTSC system and transition metal dichalcogenides under extreme external influences (review)

The problem of the influence of extreme external influences (high pressure, sharp temperature drops, structural relaxation, and strong magnetic fields) on various mechanisms of electric transport of HTSC compounds Re1Ba2Cu3O7-δ (Re = Y or another rare-earth ion) and dichalcogenides of transition metals are considered. The features of the crystal structure and the effect of structural defects of various morphologies on the electrical conductivity of these compounds in the normal, pseudogap, and superconducting states are discussed. A review of the experimental data obtained in the study of the effect of high hydrostatic pressure and other extreme effects on various mechanisms of electric transport of Re1Ba2Cu3O7-δ compounds of various compositions and transition metal dichalcogenides of various technological backgrounds is carried out. Various theoretical models devoted to the effect of high pressure on the electrical conductivity of HTSC compounds of the 1-2-3 system and transition metal dichalcogenides are discussed, and a comprehensive comparative analysis of their magnetoresistive characteristics under extreme external influences is performed. In particular, it was shown, that the relatively weak effect of pressure on the Tc value of optimally doped samples can be explained within the framework of a model assuming the presence of a Van Hove singularity in the spectrum of charge carriers which is characteristic of strongly coupled lattices. This is confirmed by the observation similar features of the behavior of the baric derivatives dTc/dP depending on the change composition in NbSe2 single crystals, which also belong to systems of two-dimensional lattices and have a similar anisotropy parameter. Nevertheless, it is still possible to formulate a number of questions that have not yet found a final experimental and theoretical solution. Namely, what is the role of the crystal lattice and structural defects and, in particular, twinning planes? What is the reason for the broadening of the resistive transitions of HTSC compounds into the superconducting state under pressure, and what is the relationship between this broadening and charge transfer and the nature of the redistribution of the vacancy subsystem? What is the role of phase separation in the implementation of different modes of longitudinal and transverse transport? Obviously, more research, both experimental and theoretical, is needed to answer these questions.

Electric transport and the pseudogap in the 1-2-3 HTSC system, under all-around compression (Review Article)

The problem of how high pressure impacts various electric transport mechanisms in HTSC compounds REBa2Cu3O7-δ (RE = Y, or another rare-earth ion) is considered. The features of the crystal structure and the effects that structural defects of different morphologies have on the electrical conductivity of these compounds in the normal, pseudogap, and superconducting states, are discussed. A review of the experimental data obtained in studies on the effect of high hydrostatic pressure on the various electric transport mechanisms of REBa2Cu3O7-δ compounds, with varying composition and technical prehistory, is conducted. Different theoretical models devoted to the subject of how high pressure impacts the electrical conductivity of 1-2-3 HTSC system compounds are discussed.

Superconducting phase diagrams of cuprates and pnictides as a key to understanding the HTSC mechanism

This paper reviews experimental phase diagrams of cuprates and pnictides to demonstrate that specific features of the superconducting phase diagrams in both HTSC families can be understood within the framework of the proposed approach, which assumes the formation, under heterovalent doping, of localized trion complexes consisting of a doped carrier and charge transfer (CT) excitons. The geometry of such cells containing CT excitons (CT plaquettes) in the basal plane of the crystal is determined by its crystal structure and the type of dopant, so that the dopant concentration range corresponding to the existence of a percolation cluster of CT plaquettes can be readily determined for each particular compound. These dopant concentration ranges coincide with good accuracy with the experimental ranges of superconducting domes in the phase diagrams of the HTSC compounds considered. The generation of free carriers and the mechanism of superconducting pairing in this pattern is related to biexciton complexes (Heitler–London centers) emerging in neighboring CT plaquettes.

Oxygen doping of HTSC and resistive switching in HTSC-based heterostructures

The studies of the bipolar resistive switching effect in thin film heterojunctions (YBa2Cu3O7−δ/Ag) and (Nd 2−xCexCuO4−y/Ag) have exhibited the role of oxygen as a doping element in hole- and electron-doped HTSC compounds.

Spin state of iron—the control parameter of iron-containing HTSC: Dependence of ground state energy, phonon energies and atom positions on the spin state of iron ion in FeTe

The density functional theory was applied to calculate the electronic structure, ground state energy, phonon frequencies at the center and at the boundary of the Brillouin zone, as well as the distribution of magnetic moment over d-orbitals of iron in the magnetically ordered phase of FeTe as functions of the magnetic moment—spin state of iron. It was shown that the ground state energy minimum is achieved at a certain distribution of the magnetic moment over d-orbitals and at a certain spin state of the iron ion. The impact of the new type of interaction (“the spin state–lattice” interaction) on the phonon subsystem was studied numerically. It was found that the high-frequency part of the phonon spectra undergoes the most significant changes upon changing the spin state. To describe the properties of iron-containing HTSC compounds, an integral parameter (spin state of iron ion) was suggested.

Effect of low-temperature treatment and subsequent high-temperature annealing on the critical current density of YBa2Cu3O y

The field dependences of the critical current density of the HTSC compound YBa2Cu3O y recovered at T = 920–950°C after the low-temperature treatment have been investigated. At T = 200°C, structural defects are formed in a wet environment, which are capable of initiating pinning of magnetic vortices. A short-term (1–3 h) recovery annealing performed at T = 930–950°C leaves in the samples a fairly large amount of structural defects formed during the low-temperature treatment, which results in a substantial increase in the critical current density in magnetic fields of ∼2 T as compared to the ceramics not subjected to double annealing. A longer high-temperature treatment removes the structural defects formed and brings the electrophysical properties of YBa2Cu3O y to the level characteristic of the ceramics produced by standard technology.

Negative thermoelectric power of over-doped Bi2Sr2CaCu2O8 superconductor

The thermoelectric power S(T) studies of Bi2Sr2CaCu2O8 (Bi2212) samples, including as-processed Partially Melted (Bi2212-PM) and 800°C O2-annealed (Bi2212-O2) are reported here. While Bi2212-PM sample is near optimum doping regime, Bi2212-O2 is an over-doped sample. TcS=0 values obtained through S(T) data match very well with those deduced from the R(T). Interestingly, S(T) behavior of the optimally-doped Bi2212-PM sample is seen to be positive in whole temperature range, it is found negative for the over-doped Bi2212-O2 sample above TcS=0. The observed negative S(T) dependence of the over-doped Bi2212-O2 is in accord with the recent band structure calculations and is in line with the theoretical investigations on a van Hove singularity in electronic density of states in HTSc compounds.

Crystal structure of Hg,Tl-1223 HTSC compound under high pressure and low temperature

Powder diffraction data for high temperature superconducting compound Hg0.8Tl0.2Ba2Ca2Cu3O8+δ at pressure range 0–20 GPa and temperature interval 100–300 K are presented. Unit cell parameters and atomic coordinates are calculated. It is found, that Ba-atoms do not shift towards HgOx-planes and splitting of BaO-planes do not decrease with external pressure. This result contradicts with evident charge transfer between HgOx- and CuO2-planes. State of buckling of CuO2 planes changes with pressure from slightly positive (for (zO2-zCu)value) at ambient pressure to strongly negative at P≥1.56 GPa. A growth of superconducting transition temperature as a function of the external pressure even for overdoped Hg,Tl-compounds up to P ≈ 15 GPa we explain as a result of an interplay between a sequence of pressure induced charge transfer and observed change of crystal structure.

Possible nature of the pseudogap anomalies in HTSC

An HTSC model, in which the interaction of valence-band electrons with diatomic negative U centers is assumed to be responsible for the anomalous properties of HTSC compounds, is proposed and used to explain the nature of the pseudogap and pseudogap anomalies (including the giant Nernst effect, the anomalous diamagnetism above Tc, the “transfer” of the optical spectral weight). For YBa2Cu3O6 + δ, the pseudogap opening temperature T* and Tc are calculated as functions of the degree of doping δ. The calculated dependences agree quantitatively with the experimental dependences without using scale fitting parameters. The good agreement between the calculated and experimental results can serve as an argument for the proposed HTSC model.

New experimental criterion of the Berezinskii-Kosterlitz-Thouless transition

Based on the experimental study of YBa2Cu3O7−x single crystal nonlinearity near T c , a new criterion for observing the Berezinskii-Kosterlitz-Thouless (BKT) transition in cuprate HTSC compounds was proposed. The criterion consists in the existence of the maximum in the dependence of the voltage of the second harmonic of the sample response signal excited by alternating current on the direct bias current. It was shown that the position of the second harmonic maximum is independent of temperatures near T c , which is consistent with the BKT transition independence of the measuring current.

Possibilities of Tc enhancement for already known HTSC compounds

Enhancement of T c was attempted for RE123 and Bi(Pb)2223 superconductors, because these materials have been well-developed for practical applications, while their T c’s are lower compared to other compounds having similar microstructures, such as Hg1212 and Tl2223, respectively. For the RE123 system, many factors, such as selection of RE element, partial RE substitution for Ba and incorporation of impurities including C, affect T c mainly through the change of local crystal structure, however, effective methods for large enhancement of T c were not found. On the other hand, T c of Bi(Pb)2223 was successfully enhanced up to 116 K by post-annealing at ∼700 °C for a long time. Change in cation composition during annealing was suggested to contribute the T c enhancement of Bi(Pb)2223.

Phase diagram of the Ba2YCu3O6-Ba2YCu3O7 system below 400°C

X-ray diffraction analysis and transmission electron microscopy have been used to study the low-temperature decomposition of the nonstoichiometric (in oxygen) HTSC compound Ba2YCu3O7 − δ. The phase diagram of the Ba2YCu3O6-Ba2YCu3O7 system below ≤400°C has been constructed. The temperature range corresponding to phase separation has been found to be divided into two portions. At T > 250°C, two orthorhombic phases characterized by different oxygen contents are formed; at the higher temperatures, the phase separation of the compound into a tetragonal and an orthorhombic phase takes place. The separation was also found to observe at T = 100°C; this indicates the possibility of natural aging for the Ba2YCu3O7 − δ compound at room temperature.

On the characterization of BiMO2NO3 (M=Pb, Ca, Sr, Ba) materials related with the Sillén X1 structure

The compounds BiMO2NO3, with M=Pb, Ca, Sr, and Ba, were obtained as single-phase products from solid-state reactions in an atmosphere of nitrous gases. The oxide nitrates with Pb and Ca crystallize in the tetragonal space group I4/mmm with two formula units per unit cell; the oxide nitrates with Sr and Ba crystallize in the orthorhombic space group Cmmm with four formula units per unit cell. Lattice parameters at room temperature are a=397.199(4), c=1482.57(2)pm for M=Pb; a=396.337(5), c=1412.83(3)pm for M=Ca; a=1448.76(3), b=567.62(1), c=582.40(1)pm for M=Sr and a=1536.50(8), b=571.67(3), c=597.55(3)pm for M=Ba. The structures, which were refined by powder X-ray diffraction, consist of alternating [BiMO2]+ and [NO3]− layers stacked along the direction of the long axis. IR and thermogravimetric data are also given. The various M2+ cations in BiMO2NO3 are compatible with each other; therefore and because of their layer-type structure, these compounds are interesting precursors for oxide materials, e.g., the HTSC compounds (Bi,Pb)2Sr2Can−1CunOx.

Nanosized Pinning Sites in HTSC Compounds

Due to the small coherence lengths of the high-Tc compounds, effective pinning sites are defects or particles of nanometer size according to ξ3. Integral magnetic measurements of the magnetization as a function of temperature in large applied magnetic fields (up to 7 T) have revealed that practically all high-Tc compounds are spatially inhomogeneous, which can be caused by oxygen deficiency (YBCO), solid solutions of Nd/Ba (NdBCO and light rare earth 123-type compounds), intergrowths (Bi-based superconductors), and doping by pair-breaking dopants like Zn, Pr, etc. Various irradiation experiments by neutrons, protons, and heavy-ions have enabled the artificial introduction of effective pinning sites into the high-Tc samples, thus creating many different observations in the magnetic data. From all these observations, we construct a pinning diagram explaining many features observed in high-Tc samples.

Fluctuation-induced pseudogap in high- Tc cuprates

The pseudogap in HTSC is treated in the framework of the model assuming that the interaction of band electrons with negative-U centers (NUC) is responsible for anomalous properties of HTSC compounds. It is shown that the pseudogap is the superconducting gap arising in under- and optimum-doped HTSC at T> T c due to the large fluctuations of NUC occupation in small clusters. The temperature of pseudogap opening T ∗ is calculated for YBa 2Cu 3O 6+ δ as a function of doping. The results are in a good agreement with experiment.

Microscopic origin of [formula omitted] anomaly in La 2− x− y(Nd,Eu) y(Ba,Sr) xCuO 4

We adopt a t 1– t 2– t 3– J– G model for explanation of x≈ 1 8 anomaly in HTSC compounds. The calculated charge susceptibility has a maximum near ( π, π) leading to charge density wave (CDW) formation with the CDW amplitude e Q = x in very narrow band regime at intermediate temperatures and near ( π, π/2) as temperature approaches zero. The in-phase domain structure as a candidate for “stripe” picture is proposed. We propose an experiment to apply pressure, and predict fast increase of T c at anomalous value x≈ 1 8 in the presence of CDW. The enhancement of T c will decline when CDW are suppressed by pressure.

Stripes and superconductivity in the HTSC copper oxides

Powder diffraction using synchrotron radiation and high-pressure cell has been performed for the HTSC compound Hg0.8Tl0.2Ba2Ca2Cu3O8.15 in the temperature range 100–300 K and external pressure 0–35 GPa. Observed structural anomalies at T2∼240 K and T1∼160 K have been attributed with stripe domain structure. A negative thermal expansion coefficient in a wide temperature range was observed at P=1 GPa. Analysis of crystal structure shows a suppression of T1 and T2 features by pressure. As external pressure leads to increase of Tc for investigated material, we suppose that superconductivity and stripe structure may have the same origin, but their interactions are competing. EXAFS (CuK-edge) measurements support this conclusion.

On the specific features and transformation of the band structure of mercury-based HTSC compounds

A systematic analysis of the temperature dependences of the thermopower S(T) for different phases of the HgBa2Can−1Cu n O2n+2+δ family (n=1, 2, 3) at different doping levels is performed in the framework of a narrow-band phenomenological model. Quantitative estimates of the main parameters of the band responsible for conduction in the normal phase of HgBa2Can−1Cu n O2n+2+δ are given for optimally doped samples. The character of the variation in these parameters with an increasing number n of the copper-oxygen layers is discussed. A trend toward broadening of the conduction band with increasing n is revealed, which can be due to the increase of the density-of-states (DOS) peak near the Fermi level with an increasing number of the CuO2 layers responsible for the formation of the conduction band. It is found that an increase in the number n leads to an increase in the fraction of localized carriers in the band owing to a more defective structure observed in the more complex phases of HgBa2Can−1Cu n O2n+2+δ. The variations in the band-structure parameters in going from under-to overdoped compositions in the HgBa2Can−1Cu n O2n+2+δ family are also discussed.

Evolution of the band structure of quasiparticles with doping in copper oxides on the basis of a generalized tight-binding method

Two methods for stabilizing the two-hole 3B1g state as the ground state instead of the Zhang-Rice singlet are determined on the basis of an orthogonal cellular basis for a realistic multiband pd model of a CuO2 layer and the dispersion relations for the valence band top in undoped and doped cases are calculated. In the undoped case, aside from the valence band, qualitatively corresponding to the experimental ARPES data for Sr2CuO2Cl2 and the results obtained on the basis of the t-t′-J model, the calculations give a zero-dispersion virtual level at the valence band top itself. Because of the zero amplitude of transitions forming the virtual level the response corresponding to it is absent in the spectral density function. In consequence, the experimental ARPES data do not reproduce its presence in this antiferromagnetic undoped dielectric. A calculation of the doped case showed that the virtual level transforms into an impurity-type band and acquires dispersion on account of the nonzero occupation number of the two-hole states and therefore should be detected in ARPES experiments as a high-energy peak in the spectral density. The computed dispersion dependence for the valence band top is identical to the dispersion obtained by the Monte Carlo method, and the ARPES data for optimally doped Bi2Sr2CaCu2O8+δ samples. The data obtained also make it possible to explain the presence of an energy pseudogap at the symmetric X point of the Brillouin band of HTSC compounds.