Low-temperature Tetragonal Research Articles

Structural transitions and related fluctuations in La1.875-xSmxSr0.125CuO4.

Features of both structural transitions and related fluctuations in La 1.875-x Sm x Sr 0.125 CuO 4 have been examined by means of electron diffraction in a temperature range between about 600 K and about 20 K. As the temperature is lowered, a structural phase transition from a high-temperature tetragonal (HTT) structure to a low-temperature orthorhombic (LTO) one takes place in 0≤x≤0.2 and HTT→LTO→Pccn→LTT successive transitions in 0.3≤x≤0.4 where the Pccn and low-temperature tetragonal (LTT) structures are characterized as a rotation of an oxygen octahedron about two (110) directions

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.

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.

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}$.

Ionic conductivities of Na 4MgP 2O 8 and partially substituted Na 4+ xMgP 2− xSi xO 8 solid solutions

Ionic conductivity measurement of pure Na 4MgP 2O 8 shows a phase transition similar to that of Na 3PO 4. The tetragonal low temperature α-phase of Na 4MgP 2O 8 prepared by solid state reaction has only moderate ionic conductivity, ∼7.2×10 −5 S/cm at 300°C. The high temperature cubic γ-form of Na 4MgP 2O 8 has a conductivity of 3.6×10 −3 S/cm at 400°C. Substitution of Si IV for P V stabilizes the cubic γ-phase of Na 4+ x MgP 2− x Si x O 8 for 0.0⩽x⩽0.3 and enhances the ionic conductivity. The conductivity of γ-Na 4+ x MgP 2− x Si x O 8 increases with Si content to a maximum of σ 300°C∼2.9×10 −3 S/ cm at x=0.2.

Competition of magnetic order and superconductivity investigated by positive muon spin rotation in La2?x M x CuO4 (M = Ba, Sr) aroundx=0.12

Muon spin rotation (μSR) is applied to La2−xBa x CuO4 aroundx=0.12 where the superconductivity (SC) is suppressed remarkably. The magnetic ordering of Cu-moments appears below 35 K in the narrow range ofx where a lattice instability from the low temperature orthorhombic (LTO) to the low-temperature tetragonal (LTT) structure exists. The present study suggests strongly that the magnetic ordering of Cu-moments is an important factor in the suppression of high-Tc superconductivity aroundx=0.12 in La2−xBa x CuO4. Similar results are obtained for the La2−xSr x CuO4 and La2−x−ySr x Nd y CuO4 systems.

Magnetic Order of Cu-Moments in La2-xBaxCuO4aroundx=0.12 Observed by Positive Muon Spin Rotation

Positive muon spin rotation (µ + SR) is applied to La 2- x Ba x CuO 4 around x =0.12, where superconductivity is suppressed remarkably. The magnetic ordering of Cu-moments appears below 35 K in the narrow range of 0.11≦ x ≦0.135, where the transition from the low-temperature orthorhombic (LTO) to the low-temperature tetragonal (LTT) structure exists. The present study suggests strongly that the magnetic ordering of Cu-moments plays an important role in the suppression of the high- T c superconductivity around x =0.12 in La 2- x Ba x CuO 4 .

Anisotropic transport properties of single-crystal La2-x-yNdySrxCuO4: Effect of the structural phase transition.

We have successfully grown single crystals of La 2-x-y Nd y Dr x CuO 4 (x=0,0.12,0.20;y∼0.4) by the traveling-solvent floating-zone method. All of these crystals show a tlattice instability to the low-temperature tetragonal (LTT) phase and enable us to investigate the anisotropic transport properties of the LTT phase. For x=0.12, various transport coefficient show a discontinuous jump at the phase transition (evidence for a first-order transition), and superconductivity is suppressed almost completely, while the effects are much reduced for x=0.20, which show a bulk superconductivity with reduced T c

Anomalous Cu-NQR Spectral Change Due to Low-Temperature Structural Transition aroundx=0.06in (La1-xBax)2CuO4

In (La 1- x Ba x ) 2 CuO 4 , the Cu-NQR spectra for x =0.0625 and x =0.08 were observed in each phase of the crystal structure. The temperature dependence of the spin echo decay rate, T 2 -1 , was also measured from 1.5 K to 300 K for both samples. The two well-separated sets of NQR spectra were observed in the high-temperature tetragonal (HTT) and low-temperature orthorhombic (LTO) phases in both samples. The spectrum at 1.5 K in the low-temperature tetragonal (LTT) phase for x =0.0625 is abnormally broadened compared to the spectra in the HTT and LTO phases, and T 2 -1 shows strong divergence at about 35 K. These two abnormal static and dynamical results indicate that the electronic state of the Cu atom changes from a nonmagnetic to magnetic one in the LTO → LTT transition, and some magnetic phase transition occurs at about 35 K which is lower than the LTT transition temperature, 60 K.

Van Hove excitons and high- Tc superconductivity VIIIA : Valence bond density waves

It has recently been shown that an important role of correlation effects is to renormalize the band structure to be more ‘Hubbard-like’ at half-filling, driving a Mott transition. Using this insight, the density wave (DW)-superconductivity competition in the high- T c oxides is reexamined. It is found that both the low-temperature orthorhombic (LTO) and the low-temperature tetragonal (LTT) transitions in La 2− x Ba x CuO 4 (LBCO) and related compounds are driven by the strong nesting of the van Hove singularity - the LTT as a static and the LTO a dynamic Jahn-Teller effect. A mean-field theory of the LTO phase is presented, in terms of a coherent superposition of charge DWs or a ‘valence bond density wave’. The resulting DW-superconductivity phase diagram strongly resembles that found in LBCO. Connections of the present model to the magnetic pseudogap and to one-dimensionality are discussed.

Mode—softening in orthorhombic La2CuO4

Abstract The low—temperature orthorhombic (LTO) phase of high—Tc superconducting La2-xBaxCuO4 having two formula units per primitive cell may exhibit a further structural transition into a low—temperature tetragonal (LTT) phase with four La2CuO4 units depending on the Ba—doping, 0·05 500 K, we have investigated mode—softening in the LTO phase of the possible sequence of transformations: HTT → LTO → LTT. Estimates for non—universal parameters of an appropriate Ginzburg—Landau (GL) theory for a two—component order—parameter field of these transformations, as being obtained from a comparison with experiments, a...

Cooperative Pseudo Jahn-Teller Effect as Microscopic Mechanism of (OMT)-(LTT) Structural Phase Transition in La2−xBaxCuO4Superconductors

ABSTRACTA microscopic mechanism for the structural phase transition from the orthorhombic mediate temperature (OMT) phase into the low temperature tetragonal (LTT) one is suggested on the basis of the cooperative pseudo Jahn-Teller effect. The local distortions mixing the ground and the first excited electronic states are ordered antiferrodistortively and are connected in part, with the oxygen octahedra rotations around the [100] axis. The results are in agreement, with the neutron scattering experiments data.

Variations of the second harmonic intensity at the low temperature phase transition IN K1-xLixTaO3under electric field

Abstract Optical second harmonic generation (SHG) of Nd:YAG laser light in K1-xLixTaO3 with x = 0.016 and x = 0.022 has been studied under zero-field cooling (ZFC) and by cooling under electric field (FC). A variation of the phase transition temperature and of the width of the thermal hysteresis with field is observed. The strong increase of the SHG-intensity under field cooling and the accompanying small changes of birefringence1 can be interpreted as an increase of the mean size of ordered regions from antiparallel to parallel polarization. A comparison with the behavior of crystals with Li-concentration above the critical limit xc = 0.022 for two samples with x = 0.036 and x = 0.063 is presented. In addition SHG-data of nominally pure KTaO3 under the same electric field and cooling procedures indicate the existence of microregions and the influence of a lattice polarisation under field. The components of the nonlinear susceptibility tensor reveal a polar tetragonal low temperature symmetry in the Li co...

Lattice instabilities, isotope effect, and high-Tc superconductivity in La2-xBaxCuO4.

Unlike ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$, ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$, x\ensuremath{\sim}${\mathit{x}}_{\mathrm{cr}}$=0.12, undergoes a structural change to a low-temperature tetragonal (LTT) phase below 60 K, and this transformation is known to suppress ${\mathit{T}}_{\mathit{c}}$ drastically. We present self-consistent local-density-functional calculations for this system that indicate the ${\mathrm{CuO}}_{6}$ octahedrons are unstable to an X-point tilt in any direction, thereby leading to strongly anharmonic X-point vibrations. The LTT phase is most favored energetically, consistent with experiment. The effect of the LTT distortion on the electronic structure accounts for the suppression of ${\mathit{T}}_{\mathit{c}}$ in the LTT phase, and for the enhanced isotope effect at x=${\mathit{x}}_{\mathrm{cr}}$.

Lattice instability and low-temperature phase transition in Pr2NiO4.

Some layered perovskites with general formulas {ital R}{sub 2}{ital M}O{sub 4} ({ital R}=rare earth; {ital M}=Cu, Ni or Co) undergo a sequence of structural phase transitions. The high-temperature tetragonal phase (HTT) is transformed to the low-temperature orthorhombic phase (LTO) at {ital T}{sub {ital c}2}, after further cooling, there is a new low-temperature tetragonal (LTT) phase at {ital T}{sub {ital c}1}. The soft mode behavior at {ital T}{sub {ital c}1} is observed with inelastic neutron scattering in this family of compounds for Pr{sub 2}NiO{sub 4}. One of the two branches, of the doubly degenerate {ital X}-point phonon of the HTT phase, becomes softer at {ital T}{sub {ital c}1}. In the LTT phase, this phonon branch is stiffer and appears at higher energy. The orthorhombic strain and the soft-mode frequency undergo a discontinuous jump at the transition temperature, indicating a clear first-order character of this phase transformation. From these data, the dynamics of the LTO{r arrow}LTT phase transformation in the insulator Pr{sub 2}NiO{sub 4+{delta}} ({delta}=0) can be well characterized. For La{sub 2{minus}{ital x}}Ba{sub {ital x}}CuO{sub 4} ({ital x}=1/8) the same kind of transition is observed, which coincides with the suppression of the superconducting state, but the presence of free charge carriers maymore » complicate the interpretation of the results in this case.« less

Low-temperature structural phase transition in La2NiO4+ delta.

La{sub 2}NiO{sub 4+{delta}} ({delta}=0) has a high-temperature tetragonal (HTT) phase ({ital T}{gt}680 K), a low-temperature orthorhombic (LTO) phase, and a low-temperature tetragonal (LTT) phase ({ital T}{lt}70 K); that is, HTT{r arrow}LTO{r arrow}(LTT). These phases are isomorphic to those of La{sub 2{minus}{ital x}}Ba{sub {ital x}}CuO{sub 4} ({ital x}=0.12). Using Raman spectroscopy, we have measured the {ital c}-axis-polarized phonons. The {sup 2}{ital A}{sub 1{ital g}} modes of the HTT phase occur in all three phases at 155 and 445 cm{sup {minus}1}, with energies and widths only weakly dependent on temperature. The low-energy NiO{sub 6}-octahedra tilting modes (which drive the phase transitions) have been observed and clearly show the first-order nature of the LTO{r arrow}LTT phase transition. The lowest-energy Raman mode observed (at 70 cm{sup {minus}1} with {ital B}{sub 1{ital g}} symmetry), allowed only in the LTT phase, involves rocking motion of the undisplaced in-plane oxygen atoms.

Second harmonic generation study of field-induced changes in the phase transition of K1-xLixTaO3

Abstract The phase transition in K1−xLixTaO3 with x=0.036 and x=0.063 has been investigated using optical second harmonic generation (SHG). The application of an electric field during cooling and heating shifts the phase transition to higher temperatures and increases the intensity of the frequency doubled light drastically. This can be interpreted as an increased mean size of ordered regions in the low-temperature domain state by the electric field. The shape of the tensor of the nonlinear susceptibility corresponds to a polar tetragonal low temperature symmetry. The temporal dependence of the SHG intensity upon application of an electric field yields an activation energy similar to that for the Li reorientation.