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تسجيل مستخدم جديدFreezing of stripes in lightly-doped La_{2-x}Sr_{x}CuO_{4} as manifested in magnetic and transport properties of untwinned single crystals
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Resistivity and magnetization measurements are used for studying the transverse sliding of AF domain boundaries in lightly doped La_{2-x}Sr_{x}CuO_{4}. We discuss that it is the freezing of the transverse boundary motion that is responsible for the appearance of ``spin-glass features at low temperatures.
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We study long wavelength magnetic excitations in lightly doped La_{2-x}Sr_{x}CuO_{4} (x < 0.03) detwinned crystals. The lowest energy magnetic anisotropy induced gap can be understood in terms of the antisymmetric spin interaction inside the antiferr
omagnetic (AF) phase. The second magnetic resonace, analyzed in terms of in-plane spin anisotropy, shows unconventional behavior within the AF state and led to the discovery of collective spin excitations pertaining to a field induced magnetically ordered state. This state persists in a 9 T field to more than 100 K above the N{e}el temperature in x = 0.01.
We present a study of the magnetic susceptibility in carefully detwinned La_{2-x}Sr_{x}CuO_4 single crystals in the lightly-doped region (x=0-0.03), which demonstrates a remarkable in-plane anisotropy of the spin system. This anisotropy is found to p
ersist after the long-range antiferromagnetic (AF) order is destroyed by hole doping, suggesting that doped holes break the AF order into domains in which the spin alignment is kept essentially intact. It turns out that the freezing of the spins taking place at low temperatures is also notably anisotropic, implying that the spin-glass feature is governed by the domain structure as well.
Superstructure reflections due to the ordering of holes into stripes in La_(1.45)Nd_(0.4)Sr_(0.15)CuO_4 have been studied with high energy x-ray diffraction. These reflections have been observed clearly for the first time in a sample which is superco
nducting at low temperatures (T_c = 10 K). The stripe peaks vanish above 62(5) K whereas the magnetic signal of the stripe ordering which has been seen with neutrons before is already suppressed at 45 K. Our results confirm that the ordering of spins and holes is driven by the charges as it is found in the case of La_(1.6-x)Nd_(0.4)Sr_(x)CuO_(4) at the doping level of x = 0.12.
We have performed zero-field muon spin rotation measurements on single crystals of La_{2-x}Sr_{x}CuO_{4} to search for spontaneous currents in the pseudo-gap state. By comparing measurements on materials across the phase diagram, we put strict upper
limits on any possible time-reversal symmetry breaking fields that could be associated with the pseudo-gap. Comparison between experimental limits and proposed circulating current states effectively eliminates the possibility that such states exist in this family of materials.
To investigate the validity of the Wiedemann-Franz (WF) law in disordered but metallic cuprates, the low-temperature charge and heat transport properties are carefully studied for a series of impurity-substituted and carrier-overdoped La_{1.8}Sr_{0.2
}Cu_{1-z}M_zO_4 (M = Zn or Mg) single crystals. With moderate impurity substitution concentrations of z = 0.049 and 0.082 (M = Zn), the resistivity shows a clear metallic behavior at low temperature and the WF law is confirmed to be valid. With increasing impurity concentration to z = 0.13 (M = Zn) or 0.15 (M = Mg), the resistivity shows a low-T upturn but its temperature dependence indicates a finite conductivity in the T to 0 limit. In this weakly-localized metallic state that is intentionally achieved in the overdoped regime, a {it negative} departure from the WF law is found, which is opposite to the theoretical expectation.
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