Low-temperature Tetragonal Phase Research Articles

Superconducting/non-superconducting phase boundary in the low temperature tetragonal phase of (La,RE)-Sr-Cu-O

From measurements of the structural, superconducting and normal state transport properties of rare earth doped La2−xSrxCuO4 we find a phase boundary between a superconducting and a non-superconducting state of the low temperature tetragonal phase. This phase boundary is determined by the magnitude of the buckling of the Cu-O-Cu bonds in the CuO2 layers, suggesting the importance of spin-orbit coupling for the electronic properties of these compounds.

Correlation between the rhombic distortion of the CuO 4 basal plane and Tc in La 1.6 − xNd 0.4Sr xCuO 4

The rhombic distortion of the CuO 4 basal plane in the tetragonal-low-temperature (TLT) phase (P4 2/ncm) of La 1.6 − xNdo .4Sr xCuO 4 has been investigated from high-resolution powder neutron diffraction at 10 K. It has been found that the rhombic distortion reaches a maximum at x = 0.115 in the TLT phase of 0.10 ≦ x ≦ 0.25. This correlates very much with the result that the local minimum of Tc as a function of x appears at x = 0.115. It is concluded that the suppression of superconductivity in the TLT phase depends not on the structural-phase-transition-temperature T d2 to the TLT phase but on the magnitude of the rhombic distortion of the CuO 4 basal plane.

Stability of the low-temperature tetragonal phase in La2?x Ba x CuO4 and related compounds

In La2−xBaxCuO4 (LBCO), the structural transition to a low-temperature tetragonal phase below 60 K and suppression of superconductivity are observed when the carrier density isp ∼ 1 /8 per copper. The replacement by divalent ions smaller than Ba2+ suppresses the static deformation of the lattice. We have found that the variationsTd2 and superconducting transition temperatureTc are quantitatively characterized by the averaged ionic radius at the La site or lattice parameters. This aspect of substitution could be regarded as the effect of chemical pressure, since similar variations have been reported on applying hydrostatic pressure. In La2−x−yNdy(Ba, Sr)xCuO4,Td2 increases with increasingy in a wide range ofp whileTc is suppressed only at p∼ l /8. The structural transition atTd2 here should be ascribed mainly to the crystallochemical origin.

Structural instabilities and superconductivity in La-214 compounds

In La2−xBa x CuO4 (LBCO) the transition to a low-temperature tetragonal phase and the suppression of superconductivity occur at the carrier concentration p≈ 1/8 per copper. We will discuss the roles of various material parameters that control this instability. An unusual lattice softening has been found by ultrasonic measurement on La2−xSr x CuO4 (LSCO). This softening is present only in an in-plane shearing mode and is ascribed to the growth of structural fluctuations in the normal state.

Structural transitions in La 2−y−xNd yBa xCuO 4 with x=0.125

The structural transition to a low-temperature tetragonal phase in La 2− x Ba x CuO 4 occurs at the hole concentration p∼ 1 8 . We have studied the structural and electrical properties of La 2− y− x Nd y Ba x CuO 4 with x=0.125 to clarify the effect of substitution by trivalent ions. The substitution by Nd 3+ enhances the structural transition temperature without changing of p. This enhancement is qualitatively explained by the crystallochemical origin.

Theory on the Structure of Twin Boundaries in La2−x,BaxCuO4

AbstractThe structure of the twin boundary in the low-temperature-orthorhombic phase of La2−xBaxCuO4 is studied using a Landau-type free energy model which reproduces the x − T phase diagram. The La2−xBaxCuO4 compound has a body-centered tetragonal (HTT) structure at high temperatures. Upon cooling it undergoes a structural phase transition to a low-temperature orthorhombic structure (LTO structure) which is caused by the tilting of the CuO6 octahedra about the (110) and (110) directions. Depending on the doping level x, the LTO structure may go through another phase transition at an even lower temperature to a low-temperature tetragonal phase (LTT phase). We find that the existence of LTT phase greatly changes the characteristics of the twin boundary in the LTO phase. LTT phase exists at twin boundaries of the LTO phase at temperatures well above the LTT to LTO transition temperature.

Anomalous composition dependence of Tc near Tcmax and microstructure of YBa 2Cu 3O 7−δ

Accurate measurements of T c ( δ) and ΔC p(δ) T c(δ) in the titled compound have shown sharp breaks in slope at or very near δ = δ 0 = 0.125. This behavior is explained by a supercell model which is similar to one previously proposed to explain the formation of the low-temperature tetragonal phase centered on x = x 0 = 0.125 in La 2− x (Ba,Sr) x CuO 4 alloys. Supercell ordering of resonant pinning centers is essential to produce coherent electron-phonon interactions mediated by such centers. Thus these data provide evidence for the crucial element in the quantum percolation model of high-temperature superconductivity.

Van Hove Jahn-Teller effect and high-T c superconductivity

It is demonstrated that the dominant nesting in the superconducting cuprates is associated with the van Hove singularities (vHs's). The associated structural instabilities are interpreted as band Jahn-Teller (JT) effects, with the two vHs's providing the electronic degeneracy. The low-temperature tetragonal phase of La 2− x Ba x CuO 4 is a static JT phase, while the low-temperature orthorhombic phase is modelled as a dynamic JT phase. The dynamic JT phase can be pictured as LTT-type domains separated by LTO domain walls (solitons).

Pressure effects on the structural phase transitions and superconductivity of La2-xBaxCuO4 (x=0.125).

Neutron-diffraction experiments have been performed to study the effects of hydrostatic pressure on the structural phase transitions of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ with x=0.125. The phase transition to the low-temperature tetragonal (LTT) structure is greatly suppressed by the application of pressure. The LTT transition temperature, observed at around 80 K at ambient pressure, decreases at the rate of about -135 K/GPa, and this LTT phase disappears under a pressure of around 0.6 GPa. The superconducting properties of this compound under high pressure have also been investigated in detail by measurments of the ac magnetic susceptibility. The bulk superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ remains constant at pressures of up to 0.5 GPa, and then increases with pressure at the rate of 10 K/GPa up to about 15 K at 1.5 GPa. From these results it is concluded that there is a clear correlation between the disappearance of the LTT phase and the enhancement of superconductivity. This conclusion supports earlier suggestions that there is a strong relationship between the crystal structure and superconductivity and that, in particular, the LTT phase is detrimental to the superconductivity in the ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ system.

Crystal structure and superconductivity of La2− xBaxCuO4 (0.03≤ x≤0.24)

High-resolution neutron powder diffraction has been performed to investigate the crystal structure of La 2− x Ba x CuO 4 as a function of x(0.03≤ x≤0.24). The crystal parameters at 115 K refined by profile analysis show that the Cu-planar O(1) bond lengths decrease by 0.008 Å when x is increased by 0.1. This result indicates that the overlap of Cu and planar O orbitals, and thus the charge transfer, increases with x. The length of the Cu-apical O(2) bonds decreases at a rate of about one half of that for Cu-O(1), suggesting that the apical oxygens O(2) also play an important role in the charge transfer process. The changes of the Cu-O(1) bond lengths of the compound with x=0.125 at 15 K are further studied in connection with the structural phase transition to the low-temperature tetragonal (LTT) phase. These results are discussed in terms of their implication for the electronic structure and superconductivity of this system.

Thermal expansion of martensitic In-Tl: Stress-induced microstructure and the premartensite state.

Thermal-expansion measurements in the temperature range 2--300 K have been performed for In--26.5 at. % Tl single crystals along the 〈100${\mathrm{〉}}_{\mathit{c}}$ directions, with the samples being prestressed uniaxially or biaxially. The different external stress fields forced the crystal to have different microstructural morphologies in the low-temperature tetragonal phase and consequently enabled the tetragonal c- or a-axis behavior to be measured. Anisotropy in the thermal expansion for directions corresponding to the different tetragonal axes commences at temperatures about 100\ifmmode^\circ\else\textdegree\fi{} above the transformation temperature, indicating premartensitic lattice distortion from the parent cubic phase. A similar anisotropy was observed between measurements along the [001${]}_{\mathit{c}}$ and [110${]}_{\mathit{c}}$ directions.

Erratum: Electronic and magnetic structures of cuprates with spin-orbit interaction

The effective Hamiltonian for the ${\mathrm{CuO}}_{2}$ plane of high-${\mathit{T}}_{\mathit{c}}$ cuprates is derived by taking into account the spin-orbit interaction. A finite-size exact-diagonalization technique is applied to the Hamiltonian, and the magnetic and electronic structures of the ${\mathrm{CuO}}_{2}$ plane in the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) phases of the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$-type crystals are examined. It is shown that the contribution from the in-plane oxygen 2${\mathit{p}}_{\mathit{z}}$ orbital perpendicular to the plane is essential for the emergence of the weak ferromagnetism in the LTO-phase ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. In the LTT phase, either the uniaxial antiferromagnetism or the weak ferromagnetism is induced, depending on how the 2${\mathit{p}}_{\mathit{z}}$ orbital contributes to the anisotropic superexchange interaction of the single Cu--O--Cu bond. We study the dynamics of a hole in an extended t-J model, and show that the lattice distortion works to change the symmetry of the electronic ground state via the spin-orbit coupling. The effect is, however, small and seems irrelevant to the anomalous properties of ${\mathrm{La}}_{1.88}$${\mathrm{Ba}}_{0.12}$${\mathrm{CuO}}_{4}$.

63Cu Nuclear Spin-Lattice Relaxation Study for Low-Temperature Structural Transition in (La1-xBax)2CuO4aroundx=0.06

The nuclear spin-lattice relaxation rate 1/ T 1 of 63 Cu was measured in the range from 1.5 to 300 K for x =0.0625 and x =0.08 in (La 1- x Ba x ) 2 CuO 4 . In the high-temperature tetragonal (HTT) and the low-temperature orthorhombic (LTO) phases in the normal state, 1/ T 1 T for both samples follows the Curie-Weiss law, C /( T +θ), suggesting that the two-dimensional (2D) antiferromagnetic (AF) fluctuation among Cu spins is dominant, similarly to the (La 1- x Sr x ) 2 CuO 4 system. In the HTT and LTO phases, no significant differences except for a slight difference in θ were observed between the two samples. In the low-temperature tetragonal (LTT) phase for x =0.0625, 1/ T 1 diverges at about 35 K, confirming the existence of magnetic ordering. The magnetic ordering at the finite temprature suggedsts that the dominant 2D-AF correlation of Cu spins in the HTT or LTO phase becomes the 3D-AF correlation derived from the some interactions along the c axis in the LTT phase.

Pressure effects on the structural phase transitions in La 2−rBa xCuO 4

Triple axis neutron scattering experiments with improved momentum transfer resolution were used to study the sequence of the structural phase transitions in La 2−xBa xCuO 4 under high pressure. For the composition x=0.12 and ordinary pressure we confirm the previously reported sequence of a high temperature tetragonal (HTT) phase (14/mmm) which, around 220 K, continuously changes into the low temperature orthorhombic (LTO) phase (Abma) and finally transforms almost completely into a low temperature tetragonal (LTT) phase (P4 3/ncm). Scanning the hydrostatic pressure up to 5 Kbars at given temperature chosen in the intermediate LTO phase we show that the orthorhombicity (a−b) (a+b) is continuously reduced at the rate of 1.310 −4/ Kbar compatible with a decrease of the HTT-LTO transition temperature at the rate of −6.5 K/ kbar. At low temperature we were able to study the pressure dependence of particular Bragg reflections and find that the LTO-LTT phase transition temperature is also depressed at the rate of −3 K/ kbar. Moreover we have determined the compressibility in the LTT phase. At 10 K we have found a compressibility of the order of 1.36 10 −3/ kbar while in the LTO we have measured a significantly smaller compressibility 4 10 −4/ kbar at about 80K. The evolution of the transition lines will be discussed within the framework of the electron-lattice coupling.

Low-temperature structural phase transitions in rare earths substituted calcium manganese oxides [Ca 2−xLn xMnO 4, where Ln=Nd, Sm-Lu and Y

The structural phase transitions of Ca 2−xLn xMnO 4 (Ln=Nd, Sm-Lu and Y) were investigated by powder X-ray diffraction in the temperature range 10–1000 K. The maximum amount of the substitution, x, for the formation of the single phase had a tendency to decrease with decreasing ionic radii of rare earths. The new type of orthorhombic phase with a distortion along [100] direction of the undistorted tetragonal phase was found for Ln=Nd, Sm, Lu and Y with x=0.2. The low temperature tetragonal phase was observed in the case of Ln=Sm with x=0.3.

Van Hove excitons and high-Tc superconductivity

The possible role of the van Hove singularity (vHs) in stabilizing the low-temperature orthorhombic (LTO) phase transition in La$_{2-x}$\-Sr$_x$\-CuO$_ 4$ (LSCO) is discussed. It is found that the vHs can drive a structural distortion in two different ways, either due to spin-orbit coupling or to dynamic Jahn-Teller (JT) effects. This paper discusses the latter effect in some detail. It is shown that a model Hamiltonian introduced earlier to describe the coupled electron -- octahedral tilt motions (`cageons') has a series of phase transitions, from a high-temperature disordered JT phase (similar to the high-temperature tetragonal phase of LSCO) to an intermediate temperature dynamic JT phase, of average orthorhombic symmetry (the LTO phase) to a low temperature static JT phase (the low temperature tetragonal phase). For some parameter values, the static JT phase is absent.

Phase stabilities in the AlCuRu system

Isothermal sections of the AlCuRu phase diagram at 500 and 800dgC have been determined for Al concentrations greater than 60 at.%. At 500°C, the icosahedral phase is not thermodynamically stable, whereas at 800°C it is stable over a fairly broad compositional region. The formation of the fci phase from the low temperature tetragonal phase is by a nucleation and growth process. The first-order transformation to the fci phase is endothermic, indicating that the fci phase is at a higher energy than the tetragonal phase.

Electronic and magnetic structures of cuprates with spin-orbit interaction.

The effective Hamiltonian for the ${\mathrm{CuO}}_{2}$ plane of high-${\mathit{T}}_{\mathit{c}}$ cuprates is derived by taking into account the spin-orbit interaction. A finite-size exact-diagonalization technique is applied to the Hamiltonian, and the magnetic and electronic structures of the ${\mathrm{CuO}}_{2}$ plane in the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) phases of the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$-type crystals are examined. It is shown that the contribution from the in-plane oxygen 2${\mathit{p}}_{\mathit{z}}$ orbital perpendicular to the plane is essential for the emergence of the weak ferromagnetism in the LTO-phase ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. In the LTT phase, either the uniaxial antiferromagnetism or the weak ferromagnetism is induced, depending on how the 2${\mathit{p}}_{\mathit{z}}$ orbital contributes to the anisotropic superexchange interaction of the single Cu--O--Cu bond. We study the dynamics of a hole in an extended t-J model, and show that the lattice distortion works to change the symmetry of the electronic ground state via the spin-orbit coupling. The effect is, however, small and seems irrelevant to the anomalous properties of ${\mathrm{La}}_{1.88}$${\mathrm{Ba}}_{0.12}$${\mathrm{CuO}}_{4}$.

Microstructural changes induced by lattice instabilities in La2− xBaxCuO4( x=0.125) and Nd 2NiO 4

We have studied the microstructural changes associated with the structural phase transition from a low temperature orthorhombic phase to a low-temperature tetragonal phase in metallic La 2− x Ba x CuO 4 and insulating Nd 2NiO 4. Employing electron microscopy, diffraction and dark field imaging the transition was found to be similar in both systems. Below the transition temperature a fine mixture exists of both tetragonal and orthorhombic domains. Details of microstructural changes of defect networks are compound specific. A simple model is also proposed to understand the microstructural changes observed in these two systems.

Bond-dependent symmetric and antisymmetric superexchange interactions in La2CuO4

The effective spin Hamiltonian that describes the magnetic properties of La${}_{2}$CuO${}_{4}$ is derived from the nearest-neighbor superexchange interactions. It is shown that the spin system of the CuO${}_{2}$ plane of the compound is frustrated; the principal axes of the symmetric parts of the one-bond anisotropy tensors are not the same for all the bonds. We also show that, due to a hidden symmetry of the one-bond anisotropic superexchange interactions, the symmetry of the lattice alone leads to the identification of the largest eigenvalue of the mean-field superexchange anisotropy tensor. The derived mean-field spin Hamiltonian is identical to that used previously on a phenomenological basis to account for the magnetic properties of La${}_{2}$CuO${}_{4}$ in the orthorhombic phase. Similar arguments explain the magnetic structure in the low-temperature tetragonal phase. These structures cannot be obtained without the inclusion of the symmetric part of the anisotropy tensor, neglected in other papers. Finally, we show how the addition of direct exchange may explain the observed spin-wave gaps.