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3. ANTIFERROMAGNETISM

ANTIFERROMAGNETIC ORDER INDUCED BY AN APPLIED MAGNETIC FIELD IN A HIGH-TEMPERATURE SUPERCONDUCTOR

The following points are made by B. Lake et al (Nature 2002 415:299):

1) One view of the high-transition-temperature (high-Tc) copper oxide superconductors is that they are conventional superconductors where the pairing occurs between weakly interacting quasiparticles (corresponding to the electrons in ordinary metals), although the theory has to be pushed to its limit(1). An alternative view is that the electrons organize into collective textures (for example, charge and spin stripes) which cannot be "mapped" onto the electrons in ordinary metals. Understanding the properties of the material would then need quantum field theories of objects such as textures and strings, rather than point-like electrons(2-5).

2) In an external magnetic field, magnetic flux penetrates type II superconductors via vortices, each carrying one flux quantum. The vortices form lattices of resistive material embedded in the non-resistive superconductor, and can reveal the nature of the ground state -- for example, a conventional metal or an ordered, striped phase -- which would have appeared had superconductivity not intervened, and which provides the best starting point for a pairing theory.

3) The authors report that for one high-Tc superconductor, the applied field that imposes the vortex lattice also induces "striped" antiferromagnetic order. Ordinary quasiparticle models can account for neither the strength of the order nor the nearly field-independent antiferromagnetic transition temperature observed in these measurements.

4) The authors suggest that their data provide clear evidence for intrinsic antiferromagnetism coexisting with superconductivity in the same sample. The authors also suggest that their data taken together with resistivity measurements strongly indicate that in a magnetic field large enough to destroy superconductivity, an underdoped copper oxide would become an incommensurate, antiferromagnetic insulator. This means that if we want to construct a theory of superconductivity at T = 0 in underdoped copper oxides, the most obvious starting point is this antiferromagnetic insulator, and not -- as in conventional theories based on a Bardeen–Cooper–Schrieffer approach -- a metallic Fermi liquid of weakly interacting quasiparticles.

References (abridged):

1. Monthoux, P., Balatsky, A. V. & Pines, D. Weak-coupling theory of high-temperature superconductivity in the antiferromagnetically correlated copper oxides. Phys. Rev. B 46, 14803-14817 (1992)

2. Zaanen, J. & Gunnarson, O. Charged magnetic domain lines and the magnetism of high-Tc oxides. Phys. Rev. B 40, 7391-7394 (1989)

3. Tranquada, J. M. et al. Evidence for stripe correlations of spins and holes in copper-oxide superconductors. Nature 375, 561-563 (1995)

4. Kivelson, S. A., Fradkin, E. & Emery, V. J. Electronic liquid-crystal phases of a doped Mott insulator. Nature 393, 550-553 (1998)

5. Zaanen, J. Stripes defeat the Fermi liquid. Nature 404, 714-715 (2000)

Related Material:

ANTIFERROMAGNETIC ORDER AS THE COMPETING GROUND STATE IN ELECTRON-DOPED Nd1.85Ce0.15CuO4

The following points are made by H.J. KANG et al (Nature 2003 423:522):

1) Superconductivity in the high-transition-temperature (high-Tc) copper oxides competes with other possible ground states(1,2). The physical explanation for superconductivity can be constrained by determining the nature of the closest competing ground state, and establishing if that state is universal among the high-Tc materials. Antiferromagnetism has been theoretically predicted(3,4) to be the competing ground state. A competing ground state is revealed when superconductivity is destroyed by the application of a magnetic field, and antiferromagnetism has been observed in hole-doped materials under the influence of modest fields(5). None of the previous experiments have revealed the quantum phase transition from the superconducting state to the antiferromagnetic state, because they failed to reach the upper critical field Bc2.

2) The authors report the results of transport and neutron-scattering experiments on electron-doped Nd1.85Ce0.15CuO4, where Bc2 can be reached. The applied field reveals a static, commensurate, anomalously conducting long-range ordered antiferromagnetic state, in which the induced moment scales approximately linearly with the field strength until it saturates at Bc2. This and previous experiments on the hole-doped materials therefore establishes antiferromagnetic order as a competing ground state in the high-Tc copper oxide materials, irrespective of electron or hole doping.

References (abridged):

1. Levi, B. G. Magnetism and superconductivity fight for control in high-Tc superconductors. Phys. Today 55, 14-15 (2002)

2. Sachdev, S. & Zhang, S. C. Tuning order in the cuprate superconductors by a magnetic field. Science 295, 452-454 (2002)

3. Zhang, S. C. A unified theory based on SO(5) symmetry of superconductivity and antiferromagnetism. Science 275, 1089-1096 (1997)

4. Arovas, D. P., Berlinsky, A. J., Kallin, C. & Zhang, S. C. Superconducting vortex with antiferromagnetic core. Phys. Rev. Lett. 79, 2871-2874 (1997)

5. Katano, S., Sato, M., Yamada, K., Suzuki, T. & Fukase, T. Enhancement of static antiferromagnetic correlations by magnetic field in a superconductor La2-xSrxCuO4 with x = 0.12. Phys. Rev. B 62, R14677-R14680 (2000)

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