H.-D. Zhou

Jahn-Teller effect and stability of the charge-ordered state in La1-xCaxMnO3 (0.5 ≤ x ≤ 0.9) manganites

The longitudinal ultrasonic sound velocity and attenuation, the resistivity, and lattice parameters were studied as a function of temperature from 30 K to 300 K in La 1 − x Ca x MnO3 (0.5 ≤ x ≤ 0.9). For all the samples, a dramatic stiffening of the sound velocity below the charge ordering transition temperature TCO was directly driven by distinct changes of the lattice parameters due to the formation of long-range ordering of Jahn-Teller–distorted MnO 6 octahedra. The relative change of the sound velocity (ΔV/V) below TCO depends on the Ca concentration x and reaches the maximum at x = 0.75, implying that the effective strength of the electron-lattice interaction with the Jahn-Teller distortion is the strongest at x = 0.75 and hence the charge-ordered state is mostly stabilized near x = 0.75 and insensitive to the application of a magnetic field, which is supported by the charge transport properties under high magnetic fields up to 14 T.

Energy dissipation in Bi2Sr2CaCu2O8+δ single crystal

Abstract The in-plane resistivity of Bi 2 Sr 2 CaCu 2 O 8+ δ single crystal in different magnetic fields was measured for magnetic field H parallel and perpendicular to the CuO 2 plane. The irreversibility line H irr ⊥ c ( T ) for H parallel to the CuO 2 plane locates in higher field and higher temperature region in the H – T phase diagram than H irr ∥ c ( T ) for H perpendicular to the CuO 2 plane. H irr ∥ c ( T ) is dominated by surface barrier pinning, while H irr ⊥ c ( T ) shows different temperature dependencies above and below 2 T, which may indicate different energy dissipation above and below the field. The field dependence of activation energy above 2 T in H ⊥ c configuration could be explained by vortex–antivortex expansion in CuO 2 plane.

Transport and magnetic properties in La0.7Ca0.3Mn1−xCuxO3

Abstract The resistivity, ac susceptibility, electron spin resonance (ESR), and X-ray photoemission spectroscopy (XPS) were investigated in La0.7Ca0.3Mn1−xCuxO3 (x=0, 0.1, and 0.15). It was found that the resistivity becomes larger and the paramagnetic to ferromagnetic transition temperature TC shifts to lower temperatures with increasing Cu-doping level. However, the insulator–metal and magnetic transition temperatures of the system are still high even when the content of Cu is equal to 0.15, which is also confirmed by the characterizations of magnetization and resistivity of La0.7Ca0.3Mn0.85Cu0.15O3 epitaxial film. Our analysis shows that the main effect arises from the proportion variation of Mn4+ ions. Taking Cu 2p3/2 core level spectra into consideration, we pointed out that there are both Cu3+ and Cu2+ ions in the system.

Magnetic Field Dependence of Penetration Depth of Optimally Doped Bi2Sr2CaCu2O8+δ Single Crystal

The behavior of the in-plane penetration depth λ(T) extracted from the reversible magnetization in different magnetic fields for Bi 2 Sr 2 CaCu 2 O 8+δ single crystal is investigated. It is found that the temperature dependence of penetration depth in different fields follows a quadratic function between 40 and 60 K. The magnetic field dependence of penetration depth at different temperatures obeys a power law. The observed temperature dependence of λ(T) in different fields is similar to the predicted behaviors of the nonlocal-nonlinear London theory.