Abstract The application of magnetic fields to layered cuprates suppresses their high-temperature superconducting behaviour and reveals competing ground states. In widely studied underdoped YBa 2 Cu 3 O 6+ x (YBCO), the microscopic nature of field-induced electronic and structural changes at low temperatures remains unclear. Here we report an X-ray study of the high-field charge density wave (CDW) in YBCO. For hole dopings ∼0.123, we find that a field ( B ∼10 T) induces additional CDW correlations along the CuO chain ( b -direction) only, leading to a three-dimensional (3D) ordered state along this direction at B ∼15 T. The CDW signal along the a -direction is also enhanced by field, but does not develop an additional pattern of correlations. Magnetic field modifies the coupling between the CuO 2 bilayers in the YBCO structure, and causes the sudden appearance of the 3D CDW order. The mirror symmetry of individual bilayers is broken by the CDW at low and high fields, allowing Fermi surface reconstruction, as recently suggested.
We present neutron scattering data on two single crystals of the high temperature superconductor La2-x(Ca,Sr)xCaCu2O6+delta. The Ca0.1-doped crystal exhibits a long-range antiferromagnetically ordered ground state. In contrast, the Sr0.15-doped crystal exhibits short-range antiferromagnetic order as well as weak superconductivity. In both crystals antiferromagnetic correlations are commensurate; however, some results on the Ca0.1-doped crystal resemble those on the spin-glass phase of La2-xSrxCuO4, where magnetic correlations became incommensurate. In addition, both crystals show a structural transition from tetragonal to orthorhombic symmetry. Quite remarkably, the temperature dependence and correlation length of the magnetic order is very similar to that of the orthorhombic distortion. We attribute this behavior to an orthorhombic strain-induced inter-bilayer magnetic coupling, which triggers the antiferromagnetic order. The large size of the crystals made it also possible to study the magnetic diffuse scattering along rods perpendicular to the CuO2 planes in more detail. For comparison we show X-ray diffraction and magnetization data. In particular, for the Ca0.1-doped crystal these measurements reveal valuable information on the spin-glass transition as well as a second anomaly associated with the Neel transition.
In those cases where charge-stripe order has been observed in cuprates, the crystal structure is such that the average rotational symmetry of the ${\mathrm{CuO}}_{2}$ planes is reduced from fourfold to twofold. As a result, one could argue that the reduced lattice symmetry is essential to the existence of stripe order. We use pressure to restore the average fourfold symmetry in a single crystal of ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_{4}$, and show by x-ray diffraction that charge-stripe order still occurs. Thus, electronically driven stripe order can spontaneously break the lattice symmetry.
Recent experiments on the original cuprate high-temperature superconductor, ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$, revealed a remarkable sequence of phase transitions. Here we investigate such crystals with the polar Kerr effect, which is sensitive to time-reversal-symmetry breaking. Concurrent birefringence measurements accurately locate the structural phase transitions from high-temperature tetragonal to low-temperature orthorhombic, and then to lower-temperature tetragonal, at which temperature strong Kerr signal onsets. Hysteretic behavior of the Kerr signal suggests that time-reversal symmetry is already broken well above room temperature, an effect that was previously observed in high quality ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ crystals.
The magnon thermal conductivity kappa(mag) of the hole-doped spin ladders in Sr14-xCaxCu24O41 has been investigated at low doping levels x. The analysis of kappa(mag) reveals a strong doping and temperature dependence of the magnon mean free path l(mag), which is a local probe for the interaction of magnons with the doped holes in the ladders. In particular, this novel approach to studying charge degrees of freedom via spin excitations shows that charge ordering of the holes in the ladders leads to a freezing out of magnon-hole scattering processes.
The Sr concentration dependence of the structural transition from the orthorhombic into the tetragonal low- temperature phase in rare-earth-doped $(R=\mathrm{Nd},\mathrm{Eu})$ ${\mathrm{La}}_{2\ensuremath{-}x\ensuremath{-}y}{\mathrm{Sr}}_{x}{R}_{y}{\mathrm{CuO}}_{4}$ has been studied in detail. Independently of the rare earth concentration the transition temperature is strongly reduced at $x\ensuremath{\sim}0.05.$ We give a qualitative argument that the effect can be attributed to the Coulomb repulsion between doped carriers. We find the enthalpy jump at the low-temperature transition to scale transition temperature between the high-temperature tetragonal and the orthorhombic phase. This effect can be understood in terms of a Landau expansion.
We present a detailed room-temperature x-ray powder diffraction study on ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{4+\ensuremath{\delta}}$ with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.12$ and $0\ensuremath{\leqslant}\ensuremath{\delta}\ensuremath{\leqslant}0.13$. For $x=0.02$, 0.04, and 0.06 the oxygen content phase diagrams of the Sr-doped samples show a similar sequence of pure phases and miscibility gaps as for pure ${\mathrm{La}}_{2}\mathrm{Ni}{\mathrm{O}}_{4+\ensuremath{\delta}}$. We find a weak Sr-doping dependence of the $\ensuremath{\delta}$ range for the pure low-temperature orthorhombic (LTO), low-temperature tetragonal, and high-temperature tetragonal (HTT) phases, but overall, the $\ensuremath{\delta}$ ranges of the different phases do not vary strongly for $x\ensuremath{\leqslant}0.06$. Drastic changes are observed for $x=0.08$ and 0.12, where miscibility gaps successively disappear. For $x=0.12$ all oxygen-doped samples are in the HTT phase. The mechanism responsible for the suppression of the phase separation seems to involves multiple factors, including the Coulomb interaction between Sr impurities and interstitial oxygens as well as the reduction of the $\mathrm{Ni}{\mathrm{O}}_{6}$ octahedral tilt angle. The doping dependence of the lattice parameters shows clear differences for pure Sr and pure O doping. With the exception of the LTO phase, the in-plane lattice parameters explicitly depend on the type of dopant, rather than the net hole content $p=x+2\ensuremath{\delta}$. In contrast, the orthorhombic strain in the LTO phase as well as the $c$-axis length appears to depend only on $p$, however, in the case of the $c$-axis length this ``universal'' behavior turns out to be coincidental. Our results also show that the chemical pressure of La-site dopants is highly anisotropic, whereas that of O interstitials appears to be more isotropic. In general, this study reveals that for an investigation of the intrinsic properties of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{4}$ with $x\ensuremath{\lesssim}0.12$, samples have to be annealed carefully to achieve $\ensuremath{\delta}=0$, since already an excess oxygen content as small as $\ensuremath{\delta}=0.01$ leads to phase separation.
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