Nature of the A Phase in CeCu 2 Si 2 O. Stockert, 1 E. Faulhaber, 2 G. Zwicknagl, 3 N. Stu ¨ßer, 4 H. S. Jeevan, 1 M. Deppe, 1 R. Borth, 1 R. Ku ¨chler, 1 M. Loewenhaupt, 2 C. Geibel, 1 and F. Steglich 1 1 Max-Planck-Institut CPfS, No ¨thnitzer Straße 40, D-01187 Dresden, Germany 2 Institut fu ¨r Festko ¨rperphysik, Technische Universita ¨t Dresden, D-01062 Dresden, Germany 3 Institut fu ¨ r Mathematische Physik, TU Braunschweig, D-38106 Braunschweig, Germany 4 Hahn-Meitner-Institut, Glienicker Straße 100, D-14109 Berlin, Germany (Received 24 October 2003; published 1 April 2004) Neutron diffraction experiments have been performed on a magnetically ordered CeCu 2 Si 2 single crystal exhibiting A-phase anomalies in specific heat and thermal expansion. Below T N  0:8K antiferromagnetic superstructure peaks have been detected. The propagation vector of the magnetic order appears to be determined by the topology of the Fermi surface of heavy quasiparticles as indicated by renormalized band-structure calculations. The observation of long-range incommensurate antiferromagnetic order as the nature of the A phase in CeCu 2 Si 2 suggests that a spin-density-wave instability is the origin of the quantum critical point in CeCu 2 Si 2 . DOI: 10.1103/PhysRevLett.92.136401 PACS numbers: 71.27.+a, 75.25.+z, 75.30.Mb The heavy-fermion metal CeCu 2 Si 2 is still of consid- erable interest for both theorists and experimentalists due to its intriguing low temperature properties. Since the observation of superconductivity in CeCu 2 Si 2 in 1979 [1], much work has been devoted to understand the un- usual properties of this prototypical heavy-fermion superconductor. For CeCu 2 Si 2 an unusual type of mag- netic order, the so-called ‘‘A phase’’ [2], discovered about ten years after the superconductivity by NMR [3] and muon spin rotation (SR) [4], has attracted much inter- est. Subsequent thermodynamic and transport measure- ments revealed a complex magnetic B; T phase diagram with different superconducting and magnetically ordered phases and a ground state depending very delicately on the actual stoichiometry [2]. The ground state can be either A phase, sometimes coexisting with superconduc- tivity, A=S where superconductivity expels the A phase, or only S(uperconducting) [5]. Doping experiments with Ge substituting the Si as well as experiments under hydro- static pressure strongly suggest that CeCu 2 Si 2 is located very close to a quantum critical point connected with the disappearance of the A phase. Non-Fermi-liquid behavior in the vicinity of the quantum critical point is observed, e.g., in the specific heat and the electrical resistivity [6]. For an understanding of this non-Fermi-liquid behavior at the quantum critical point in CeCu 2 Si 2 , it is of utmost importance to unravel the nature of the A phase. Although several attempts were made to detect magnetic order by neutron diffraction, all previous experiments failed in observing magnetic scattering intensity. However, clear signatures pointing to a spin-density wave (SDW) as origin of the A phase in CeCu 2 Si 2 were found in mea- surements of the electrical resistivity, showing the open- ing of a gap below the ordering temperature in certain directions [6], or in SR experiments, already giving a rough estimation of the ordered moment of 0:1 B [4]. Another possibility to gain more insight into the nature of the A phase in CeCu 2 Si 2 is to investigate the alloyed system CeCu 2 Si 1x Ge x  2 [7,8]. In particular, one can fol- low the magnetic order as a function of Ge doping. The advantage of the doped system lies in the fact that the ordering temperature is higher and the ordered moment enhanced. First neutron diffraction experiments were per- formed on CeCu 2 Ge 2 [9] and CeCu 2 Si 1x Ge x  2 [7,10] powder samples and a CeCu 2 Ge 2 single crystal [11]. They revealed an incommensurate antiferromagnetic or- der with a propagation vector  0:28 0:28 0:53 for CeCu 2 Ge 2 which is only weakly dependent on the Ge concentration. The magnetic moments of the sine- modulated structure are confined to the [110] plane [11]. However, no magnetic intensity was detected for Ge con- centrations x< 0:6. Later, single crystal neutron dif- fraction on CeCu 2 Si 1x Ge x  2 with 0:05  x  0:45 succeeded in observing the antiferromagnetic order [12]. The propagation vector remains roughly constant, i.e., in the vicinity of  0:25 0:25 0:5, but the ordered moment is strongly reduced compared to the alloys with higher Ge content. Recently, we were able to grow large single crystals of CeCu 2 Si 2 with well-defined properties [13]. Taking ad- vantage of the new knowledge on the magnetic structure in the Ge-doped system, we started a new attempt to solve the difficult, but fundamental question about the magnetic origin of the A phase by performing magnetic neutron diffraction measurements on these large single crystals. The experiments were performed on an A-phase CeCu 2 Si 2 single crystal. The single crystal was grown in an Al 2 O 3 crucible by a modified Bridgman tech- nique using Cu excess as flux medium. The largest crystal, used for the present study, has the dimensions 3  4  4 mm 3 (m  350 mg) and was oriented by x-ray PHYSICAL REVIEW LETTERS week ending 2 APRIL 2004 VOLUME 92, NUMBER 13 136401-1 0031-9007= 04=92(13)=136401(4)$22.50  2004 The American Physical Society 136401-1