454 research outputs found
Antiferromagnetic order and spin dynamics in iron-based superconductors
High-transition temperature (high-) superconductivity in the iron
pnictides/chalcogenides emerges from the suppression of the static
antiferromagnetic order in their parent compounds, similar to copper oxides
superconductors. This raises a fundamental question concerning the role of
magnetism in the superconductivity of these materials. Neutron scattering, a
powerful probe to study the magnetic order and spin dynamics, plays an
essential role in determining the relationship between magnetism and
superconductivity in high- superconductors. The rapid development of
modern neutron time-of-flight spectrometers allows a direct determination of
the spin dynamical properties of iron-based superconductors throughout the
entire Brillouin zone. In this review, we present an overview of the neutron
scattering results on iron-based superconductors, focusing on the evolution of
spin excitation spectra as a function of electron/hole-doping and isoelectronic
substitution. We compare spin dynamical properties of iron-based
superconductors with those of copper oxide and heavy fermion superconductors,
and discuss the common features of spin excitations in these three families of
unconventional superconductors and their relationship with superconductivity.Comment: 48 pages, 41 figures. Accepted by Reviews of Modern Physic
Magnetic Frustration and Iron-Vacancy Ordering in Iron-Chalcogenide
We show that the magnetic and vacancy orders in the 122
iron-chalcogenides can be naturally derived from the
model with being the ferromagnetic (FM) nearest neighbor
exchange coupling and being the antiferromagnetic (AFM) next and
third nearest neighbor ones respectively, previously proposed to describe the
magnetism in the 11(FeTe/Se) systems. In the 11 systems, the magnetic exchange
couplings are extremely frustrated in the ordered bi-collinear
antiferromagnetic state so that the magnetic transition temperature is low. In
the 122 systems, the formation of iron vacancy order reduces the magnetic
frustration and significantly increases the magnetic transition temperature and
the ordered magnetic moment. The pattern of the 245 iron-vacancy order
() observed in experiments is correlated to the
maximum reduction of magnetic frustration. The nature of the iron-vacancy
ordering may hence be electronically driven. We explore other possible vacancy
patterns and magnetic orders associated with them. We also calculate the spin
wave excitations and their novel features to test our model.Comment: Figures are modified and more discussion is adde
The effect of Cr impurity to superconductivity in electron-doped BaFe2-xNixAs2
We use transport and magnetization measurements to study the effect of
Cr-doping to the phase diagram of the electron-doped superconducting
BaFe2-xNixAs2 iron pnictides. In principle, adding Cr to electron-doped
BaFe2-xNixAs2 should be equivalent to the effect of hole-doping. However, we
find that Cr doping suppresses superconductivity via impurity effect, while not
affecting the normal state resistivity above 100 K. We establish the phase
diagram of Cr-doped BaFe2-x-yNixCryAs2 iron pnictides, and demonstrate that
Cr-doping near optimal superconductivity restore the long-range
antiferromagnetic order suppressed by superconductivity.Comment: 10 pages, 5 figure
Observation of Magnetic Moments in the Superconducting State of YBa₂Cu₃O₆.₆
Neutron scattering measurements for YBa2Cu3O6.6 have identified small magnetic moments that increase in strength as the temperature is reduced below T* and further increase below Tc. An analysis of the data shows the moments are antiferromagnetic between the Cu-O planes with a correlation length of longer than 195 Ã… in the a-b plane and about 35 Ã… along the c axis. The origin of the moments is unknown, and their properties are discusssed both in terms of Cu spin magnetism and orbital bond currents
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