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Emergence of charge order from the vortex state of a high-temperature superconductor


There are many materials in which the electron density spontaneously varies in space instead of being uniform. This is called charge order or charge-density-wave order. Although already observed under various forms in some cuprate superconductors at high-temperature, this phenomenon had not, until recently, been considered as important or universal in these materials. In 2011, however, the discovery by researchers at LNCMI Grenoble of charge order in YBa2Cu3Oy has given a new twist to the field by suggesting that charge ordering is actually a generic trend in these superconductors.

Using nuclear magnetic resonance, the same team has now measured in collaboration with colleagues from the NHMFL at Tallahassee how this charge order evolves as a function of magnetic field in YBa2Cu3Oy.

The yellow tubes represent the vortex cores whose radius is defined by the superconducting coherence length xSC. Black & white halos represent areas where the charge density is modulated. The charge-density modulation might develop over a long distance in the superconductor when these halos, the density of which increases with the magnetic field, begin to overlap.

They have observed the existence of a threshold field at low temperature, which depends on the number of charge carriers in the system.
At this critical field, the material is in a state where the superconducting fluid creates vortices, at the center of which superconductivity is suppressed. The researchers have shown that the existence of a threshold field may be explained if the charge order is rejected by the superconducting state and thus forced to take refuge around the vortex cores (see the figure). Increasing the magnetic field increases the density of vortices, which would, beyond a critical density, lead to the formation of a charge density wave ordered over a long distance.
Such a nucleation of charge order around vortices in the superconducting state had previously been suggested by studies of Bi-2212, another cuprate superconductor, using scanning tunneling microscopy at low magnetic fields. Therefore, the results substantiate the hypothesis of a fundamental and universal competition between superconductivity and charge order in cuprates. Although the troubling proximity between these two orders can be explained by new theoretical models directly associating the two effects, the researchers involved in this high-field work point out that the existence of a link between the two phases does not necessarily imply that one is causing the other. Therefore, despite tremendous recent progress in understanding the unique properties of these cuprates, the origin of their superconductivity remains under debate.

Emergence of charge order from the vortex state of a high temperature superconductor. T. Wu, H. Mayaffre, S. Krämer, M. Horvatić, C. Berthier, P.L. Kuhns, A.P. Reyes, W.N. Hardy, R. Liang, D.A. Bonn, and M.-H. Julien, Nature Communications 4, 2113 (2013).

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