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METALS, SUPERCONDUCTORS... 2009Magnetoresistance anisotropy and AMRO in the electron dopedsuperconducting cuprate Nd 2−x Ce x CuO 4Recent discoveries of magnetic quantum oscillations inhole- and electron-doped cuprate superconductors clearlydemonstrate the importance of high-field magnetotransportexperiments for exploring the Fermi surface inthese materials. Besides quantum oscillations, semiclassicalangle-dependent magnetoresistance oscillations(AMRO) are known to be a very efficient method forFermi surface studies of layered systems, such as organicconductors [Kartsovnik, Chem . Rev. 104, 5737(2004)]. AMRO have also been observed in hole-overdopedTl 2 Ba 2 CuO 6+δ [Hussey et al., Nature 425, 814 (2003)].Aiming to find AMRO in the electron-doped superconductorNd 2−x Ce x CuO 4 (NCCO), we have performed detailedstudies of the angular dependence of its interlayer magnetoresistance.The experiments were performed on crystalswith doping levels in the range 0.13 ≤ x ≤ 0.17, in a 28 Tresistive magnet. The samples were mounted on a homemadetwo-axes rotating stage allowing an in situ rotation ata fixed B, at temperatures down to 1.4 K. The interlayer resistanceR c was measured as a function of angle θ betweenthe field direction and [001] axis of the crystal, at differentfixed angles ϕ, see inset in Fig. 77(a).0.17. Hence, the relevant Fermi surfaces should be identicalfor all these compositions. On the other hand, ShubnikovdeHaas data [Helm et al., Phys. Rev. Lett. 103,157002(2009)] suggest a reconstruction of the Fermi surface to occurbetween x = 0.16 and 0.17. The apparent discrepancy isresolved by taking into account a possible magnetic breakdownbetween the hole- and electron-like parts of the reconstructedFermi surface. In this case, one should consider apossibility that the Fermi surface is reconstructed over theentire superconducting doping range. Further experimentson AMRO and magnetoquantum oscillations at fields above30 T are essential to clarify the details of the Fermi surfaceevolution in overdoped NCCO and its implications for superconductivity.As shown in figure 77(a), underdoped NCCO crystals displayan anomalous dome-like shape of R c (θ), which is mostlikely associated with spin dependent scattering in a magneticallyordered system. Notably, this behavior is observedeven for superconducting compositions, up to the optimaldoping level. The presence of ordered spins readjusted atchanging the magnetic field orientation is manifested in ahysteresis between up and down θ-sweeps.At increasing Ce concentration to the optimal and, further,to the overdoped regime, the anomalous contribution tomagnetoresistance weakens, giving way to the conventionalmechanism associated with the orbital effect of magneticfield on charge carriers. This gives rise to the positive slopeof the angular dependence, dρ c /d|θ| > 0 in an extendedangular range, 30 ◦ < |θ| 80 ◦ , as shown in Fig. 77(b) fora sample with x = 0.165. The most interesting feature inthis range is a shallow hump superposed on the monotonicslope around ±52 ◦ . The same feature has been found onsamples with x = 0.16 and 0.17 (the latter being the highestaccessible doping level for NCCO). The position of thehump stays constant at changing temperature or magneticfield strength. Such a behavior is characteristic of AMRO.At this stage, a rigorous quantitative analysis of the AMROfeatures is difficult due to their small amplitude. Nevertheless,already from the existing data one can draw an importantqualitative conclusion. The positions of the AMROturn out to be the same, at all ϕs, for x = 0.16,0.165, andFigure 77: Angle-dependent interlayer magnetoresistance ofNd 2−x Ce x CuO 4 at B = 28 T, T = 1.4 K. (a) Underdoped sample,x = 0.13, exhibits an anomalous dome-like shape with a hysteresisbetween up and down θ-sweeps. At |θ| > 70 ◦ the resistancesharply drops due to the onset of superconductivity. Inset showsthe geometry of the experiment. (b) For the overdoped sample,x = 0.165, a conventional magnetoresistance dominates at least at30 ◦ < |θ| < 70 ◦ . Arrows point to AMRO features whose positionsdepend on azimuthal angle ϕ.I. SheikinM. V. Kartsovnik and T. Helm (Walther-Meissner-Institut, Garching, Germany)58