Antiferromagnetic order induced by an applied magnetic field in a high-temperature superconductor

Abstract

One view of the cuprate high-transition temperature (high-T c ) superconductors is that they are conventional superconductors where the pairing occurs between weakly interacting quasiparticles, which stand in one-to-one correspondence with 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 (e.g. charge and spin stripes) which cannot be mapped onto the electrons in ordinary metals. The phase diagram, a complex function of various parameters (temperature, doping and magnetic field), should then be approached using quantum field theories of objects such as textures and strings, rather than point-like electrons [2,3,4,5,6]. In an external magnetic field, magnetic flux penetrates type-II superconductors via vortices, each carrying one flux quantum [7]. The vortices form lattices of resistive material embedded in the non-resistive superconductor and can reveal the nature of the ground state -e.g. a conventional metal or an ordered, striped phase -which would have appeared had superconductivity not intervened. Knowledge of this ground state clearly provides the most appropriate starting point for a pairing theory. Here we report that for one high-T c superconductor, the applied field which imposes the vortex lattice, also induces antiferromagnetic order. Ordinary quasiparticle pictures cannot account for the nearly fieldindependent antiferromagnetic transition temperature revealed by our measurements.La 2-x Sr x CuO 4 , is the simplest high-T c superconductor. The undoped compound is an insulating antiferromagnet, where the spin moments on adjacent Cu 2+ ions are antiparallel [8]. Introduction of charge carriers via Sr doping reduces the ordered moment until it vanishes at x<0.13. In addition, for x>0.05 the commensurate antiferromagnetism is replaced by incommensurate order [2,3,9,10], where the repeat distance for the pattern of ordered moments is substantially larger than the spacing between neighbouring copper ions. La 2-x Sr x CuO 4 becomes a 2 superconductor for Sr dopings of 0.06<x<0.25; and for underdoped compounds (0.06<x<0.13), both superconductivity and magnetic order are present. Figure 1a shows the electrical resistance as a function of temperature T and magnetic field H, applied perpendicular to the superconducting CuO 2 planes, for a sample with x=0.10. The resistance decreases rapidly below the zero-field transition temperature T c (H=0T)=29K, and becomes zero below the irreversibility temperature T irr (H). The white circles locate T irr and show that it is a rapid function of applied field, so that even for fields much smaller than the upper critical field (H c2 (x=0.10)~45T [11]), perfect conductivity does not occur until the temperature is well below T c (H=0T).In our experiments we used magnetic neutron diffraction to measure the spin ordering in single crystals of underdoped La 2-x Sr x CuO 4 (x=0.10). The technique is analogous to mapping the positions of atoms in crystals using X-ray...

d-Wave superconductivity as a catalyst for antiferromagnetism in underdoped cuprates

“…Our results reproduce well the qualitative aspects of the experiment by Lake et al [2]. We find that some features depend on nonuniversal aspects of disorder, in particular the process of domain wall nucleation, and that while disorder-and magnetic-field-induced SDW order both add to the ordered moment, the interference of disorder and magnetic-field effects is quantitatively significant.…”

“…We consider this a reasonable qualitative approach, since the reported changes in the Fermi surface of these materials over the 'spin-glass' doping range are small [30] and it is therefore plausible that the primary effect on the electronic structure is due to the correlationinduced band narrowing [12]. The magnetically ordered phase can be enhanced by the increase in the correlation strength or stronger disorder potentials (see figures 1(c) and (d)).Our results reproduce well the qualitative aspects of the experiment by Lake et al [2]. We find that some features depend on nonuniversal aspects of disorder, in particular the process of domain wall nucleation, and that while disorder-and magnetic-field-induced SDW order both add to the ordered moment, the interference of disorder and magnetic-field effects is quantitatively significant.…”

“…Applying an external magnetic field strongly enhances the magnetization and reinforces incommensurate peaks at unambiguous wavevectors that are robust against variations in the impurity configurations ( figure 4(b)). Remarkably, the temperature dependence of the structure factor (figure 4(c)) closely resembles the neutron scattering data on LSCO by Lake et al [2]. For the results shown in figure 4, we have chosen a parameter set where the staggered magnetization in zero field has its onset at a temperature T g indistinguishable from T c .…”

“…Above a critical U c , a staggered spin pattern is nucleated in the vortex cores with a spatial extent reaching beyond the size of a vortex core ( figure 2(d)), as observed in experiment [2]. For the parameter set chosen, the core radius estimated from the area where the order parameter is suppressed is about one lattice spacing.…”

“…This state is characterized at low temperatures T by static local spin-density wave (SDW) order with an ordering wavevector Q near (π, π ), an order that is not present in the state above T c . This was first reported in elastic neutron scattering experiments [2] on La 2−x Sr x CuO 4 (LSCO), with a correlation length of several hundred Angstrom (Å), but has been confirmed in other underdoped cuprates as well [3][4][5][6]. An enhancement of incommensurate static order was observed on increasing the applied magnetic field up to 14 T [2].…”