The Nematic Character of a Hidden Ordered State revealed by Magnetic Fields.
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The Nematic Character of a Hidden Ordered State revealed by Magnetic Fields. Peter S.Riseborough1, S. G. Magalhaes2 and E.J. Calegari3 1 Dept of Physics, Temple University, Phialdelphia, PA 19122, U.S.A. 2 Inst. Fis., Univ. Fed. Fluminense, Niteroi, Rio de Janeiro, 24210346 Brazil 3 Dept. Fis. Lab. Teoria Mat. Condensada, Univ. Fed. Santa Maria, Santa Maria, RS 97105900, Brazil ABSTRACT We examine a novel phase of the underscreened Anderson lattice Model that might pertain to the ”Hidden Ordered” phase of URu2Si2. We show that the system breaks spinrotational invariance below the critical temperature and spontaneously selects a preferred axis of spin quantization. As a result, the low temperature phase exhibits a magnetic anisotropy, where the electronic properties depend not only on the magnitude of the magnetic field but also on the orientation of the applied field relative to the axis of quantization. The results are discussed in the context of recent experimental findings on URu2Si2. INTRODUCTION URu2Si2 exhibits a lambda anomaly in the specific heat at 17.5 K[1]. The resistivity and susceptibility measurements are consistent with the transition producing a partial gapping of 40% of the Fermi-surface area, where the gap has a magnitude of the order 7 to 10 meV [2]. The gap has been observed directly by optical conductivity [3] and tunneling measurements [4]. Despite the nearly 30 years that have elapsed since the transition was first discovered, the nature of the order parameter remains unknown. Recently, it has become evident form photoemission [5,6] and de Haas - van Alphen experiments [7,8,9] that, at the transition, the Brillouin zone folds through the nesting wave vector Q=(1,0,0). Thus, it appears as though the electronic structure changes at the transition, going from body centered tetragonal to simple tetragonal. However, this transformation is entirely electronic as it does not involve any changes in the atomic positions. Further clues to the nature of the transition were reported by Okazaki et al. [10], who performed measurements of the magnetic torque. They found that the magnetic susceptibility in the a-b plane had a four-fold symmetry above the transition, but they also found that the smallest single crystals developed a two-fold anisotropy in the magnetic susceptibility below the transition temperature. The symmetry-breaking was not apparent in larger crystals, presumably because the effect averages out in multi-domain crystals. High precision x-ray measurements were performed in order to search for evidence of a structural nematic transition. However, the results are equivocal. Cyclotron resonance measurements were reported [11] as giving evidence of the breaking of the four-fold in plane symmetry. The current picture suggests that, if the transition involves nematicity, it is not present in the crystal structure but only involves the electrons. Furthermore, the electronic nematicity only has been observed in the presence of a magnetic field. Here we examine the magnetic susceptibility of a novel
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