AC Hopping Magnetotransport Across the Spin Flop Transition in Lightly Doped La_2CuO_4

The weak ferromagnetism present in insulating La_{2}CuO_4 at low doping leads to a spin flop transition, and to transverse (interplane) hopping of holes in a strong external magnetic field. This results in a dimensional crossover 2D $\to$ 3D for the in-plane transport, which in turn leads to an increase of the hole's localization length and increased conduction. We demonstrate theoretically that as a consequence of this mechanism, a frequency-dependent jump of the in-plane ac hopping conductivity occurs at the spin flop transition. We predict the value and the frequency dependence of the jump. Experimental studies of this effect would provide important confirmation of the emerging understanding of lightly doped insulating La_{2-x}Sr_xCuO_4.Comment: 4 pages, 1 figur

Half-skyrmion picture of single hole doped CuO_2 plane

Based on the Zhang-Rice singlet picture, it is argued that the half-skyrmion is created by the doped hole in the single hole doped high-T_c cuprates with N'eel ordering. The spin configuration around the Zhang-Rice singlet, which has the form of superposition of the two different d-orbital hole spin states, is studied within the non-linear \sigma model and the CP^1 model. The spin configurations associated with each hole spin state are obtained, and we find that the superposition of these spin configuration turns out to be the half-skyrmion that is characterized by a half of the topological charge. The excitation spectrum of the half-skyrmion is obtained by making use of Lorentz invariance of the effective theory and is qualitatively in good agreement with angle resolved photoemission spectroscopy on the parent compunds. Estimated values of the parameters contained in the excitation spectrum are in good agreement with experimentally obtained values. The half-skyrmion theory suggests a picture for the difference between the hole doped compounds and the electron doped compounds.Comment: 13 pages, 2 figures, to be published in Phys. Rev.

Partial spin freezing in the quasi-two-dimensional La2(Cu,Li)O4

In conventional spin glasses, the magnetic interaction is not strongly anisotropic and the entire spin system freezes at low temperature. In La2(Cu,Li)O4, for which the in-plane exchange interaction dominates the interplane one, only a fraction of spins with antiferromagnetic correlations extending to neighboring planes become spin-glass. The remaining spins with only in-plane antiferromagnetic correlations remain spin-liquid at low temperature. Such a novel partial spin freezing out of a spin-liquid observed in this cold neutron scattering study is likely due to a delicate balance between disorder and quantum fluctuations in the quasi-two dimensional S=1/2 Heisenberg system.Comment: 4 pages, 4 figure

Novel dynamic scaling regime in hole-doped La2CuO4

Only 3% hole doping by Li is sufficient to suppress the long-range 3-dimensional (3D) antiferromagnetic order in La2CuO4. The spin dynamics of such a 2D spin liquid state at T << J was investigated with measurements of the dynamic magnetic structure factor S(omega,q), using cold neutron spectroscopy, for single crystalline La2Cu0.94Li0.06O4. S(omega,q) peaks sharply at (pi,pi) and crosses over around 50K from omega/T scaling to a novel low temperature regime characterized by a constant energy scale. The possible connection to a crossover from the quantum critical to the quantum disordered regime of the 2D antiferromagnetic spin liquid is discussed.Comment: 4 pages, 4 figure