Suppression of Hidden Order and Emergence of Ferromagnetism in URu 2-x Re x Si 2
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1104-NN07-04
Suppression of Hidden Order and Emergence of Ferromagnetism in URu2-xRexSi2 Nicholas P. Butch, Todd A. Sayles, Benjamin T. Yukich, and M. Brian Maple Department of Physics and Institute for Pure and Applied Physical Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA, 92093 ABSTRACT Measurements on URu2-xRexSi2 single crystals indicate that substitution of Re for Ru in URu2Si2 reduces the transition temperature of the hidden order state and quickly destroys superconductivity. At intermediate Re concentrations, weak ferromagnetism emerges and non Fermi liquid (NFL) behavior is observed in the low-temperature specific heat and electrical resistivity. A scaled Arrott analysis of the magnetization indicates the onset of ferromagnetism at x = 0.15, where the hidden order disappears, and that the quantum phase transition is associated with novel critical exponents. INTRODUCTION The hidden order (HO) phase of the heavy-fermion superconductor URu2Si2 has kept this unusual compound at the focus of condensed matter physics research for over two decades since its discovery [1-3]. To this day, the nature of the order parameter in the HO phase remains a matter of controversy. The phase transition temperature of roughly 17 K is marked by a large specific heat anomaly, but only a small antiferromagnetic staggered moment is observed in the ordered phase, too small to account for the entropy liberated at the transition [4]. The many subsequent experiments and theoretical studies have been unable to generate a consensus, although experimental evidence suggests that a Fermi-surface instability is involved [2,5,6] and that an opening in a spin-excitation spectrum at incoherent wavevectors can account for the entropy at the transition [7]. Initial studies of transition metal substitution for Ru found that both the HO and superconducting phases are suppressed by less than 10% substitution [8-10] and the existence of ferromagnetism at moderate Re concentration [8-12]. Non-Fermi liquid (NFL) behavior, often associated with quantum fluctuations in the vicinity of an ordering temperature driven to 0 K, was found far in the ferromagnetic (FM) phase [13]. Here, recent measurements on single crystals of URu2-xRexSi2 are reported, extending earlier results [14]. EXPERIMENT Single crystals of URu2-xRexSi2 were pulled via the Czochralski technique in a tri-arc furnace and annealed in an Ar atmosphere at 900 oC for 7 days. Powder x-ray diffraction indicated single-phase material, while clean back-reflection Laue patterns confirmed welloriented single crystals (Fig. 1). Electrical resistivity ρ measurements as a function of temperature T were performed in a liquid 4He cryostat and a 3He-4He dilution refrigerator. Measurements of magnetization M were made using a Quantum Design Magnetic Properties Measurement System (MPMS) in magnetic fields H as high as 5.5 T. Measurements of the specific heat C were performed in a 3He refrigerator utilizing a semi-adiabatic heat pulse technique.
Figure 1. a) Single crystal
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