Antiferromagnetic transition in Bi2Sr2Ca1−xYxGu2O8+y
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We systematically studied the transport properties of single crystals of $Eu_{1-x}Sr_xFe_{2-y}$Co$_{y}As_2$. Co doping can suppress the spin-density wave (SDW) ordering and induces a superconducting transition, but a resistivity reentrance due to the antiferromagnetic ordering of $Eu^{2+}$ spins is observed, indicating the competition between antiferromagnetism (AFM) and superconductivity. It is striking that the resistivity reentrance can be completely suppressed by external magnetic field (H) because a metamagnetic transition from antiferromagnetism to ferromagnetism for $Eu^{2+}$ spins is induced by magnetic field. Superconductivity without resistivity reentrance shows up by partial substitution of Eu$^{2+}$ with non-magnetic Sr$^{2+}$ to completely destroy the AFM ordering of $Eu^{2+}$ spins. These results suggest that the antiferromagnetism destroys the superconductivity, while the ferromagnetism can coexist with the superconductivity in the iron-based high-$T_c$ superconductors.
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Magnetism
Mott insulator
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We have measured the magnetic susceptibilities of single crystals of ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{GeO}}_{3}$ with extremely low Zn concentration $(x)$ lower than $x=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at very low temperatures to investigate the spin-Peierls and antiferromagnetic transitions. The results show that the undoped ${\mathrm{CuGeO}}_{3}$ has no antiferromagnetic phase down to 12 mK and there exists an antiferromagnetic long-range order with the easy axis along the $c$ axis for $x$ down to $1.12(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}.$ The minimum observed N\'eel temperature was 0.0285 K for the $x=1.12(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ sample. From the concentration dependence of the N\'eel temperature it is concluded that there is no critical concentration for the occurrence of the antiferromagnetic long-range order. This indicates that the dimerization sustains the coherence of the antiferromagnetic phase of the spin polarization in impurity-doped systems and is consistent with the theory of the impurity-doped spin-Peierls system. The temperature dependence of the susceptibilities at $T>{T}_{N}$ of all samples indicates that the magnetic correlations between localized spins are enhanced by a relatively large interchain interaction of ${\mathrm{CuGeO}}_{3}.$
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The origin of a high Neel temperature in a 5$d$ oxide, NaOsO$_3$, has been analyzed within the mean-field limit of a multiband Hubbard model and compared with the analogous 4$d$ oxide, SrTcO$_3$. Our analysis shows that there are a lot of similarities in both these oxides on the dependence of the the effective exchange interaction strength ($J_0$) on the electron-electron interaction strength ($U$). However, the relevant value of $U$ in each system puts them in different portions of the parameter space. Although the Neel temperature for NaOsO$_3$ is less than that for SrTcO$_3$, our results suggest that there could be examples among other 5$d$ oxides which have a higher Neel temperature. We have also examined the stability of the G-type antiferromagnetic state found in NaOsO$_3$ as a function of electron doping within GGA+U calculations and find a robust G-type antiferromagnetic metallic state stabilized. The most surprising aspect of the doped results is the rigid band-like evolution of the electronic structure which indicates that the magnetism in NaOsO$_3$ is not driven by fermi surface nesting.
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Based on recent experimental results for electron-doped cuprate oxides and ferromagnetic superconductors, it is shown that antiferromagnetic fluctuations always develop in the superconducting phase of both low- and high-temperature superconductors. The relation between the magnitude of the antiferromagnetic pseudogap and the characteristic temperature of the antiferromagnetic pseudogap opening is obtained. The characteristic temperature of the antiferromagnetic pseudogap opening for metal superconductors is estimated.
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The magnetic susceptibility of stoichiometric iron ditelluride has been measured between 15° and 100°K. The material appears to be antiferromagnetic with a Néel temperature at about 85°K. The curve is anomalous in that the susceptibility at 15°K is greater than that at the Néel temperature. This behaviour can be explained by a simple model of an antiferromagnetic containing some non-magnetic ions, provided that the concentration of the latter changes as a result of the ordering process.
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We studied the effect of doping on the antiferromagnetism.The Neel temperature TN decays rapidly with the increasing of the doping concentration x.And the Neel temperature will get to zero when it reach the critical concentration.It means the antiferromagnetic long range order is destroyed completely.We can find the effect of doping has connection with the number of the layer.We will study it theoretically.
Néel temperature
Atmospheric temperature range
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