Ultrafast dynamics of the Itinerant Antiferromagnet UNiGa 5
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0893-JJ05-08.1
Ultrafast dynamics of the Itinerant Antiferromagnet UNiGa5 Elbert E. M. Chia, H. J. Lee, Namjung Hur, E. D. Bauer, T. Durakiewicz, R. D. Averitt, J. L. Sarrao and A. J. Taylor Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. ABSTRACT Time-resolved photoinduced reflectivity data were measured for the itinerant antiferromagnet UNiGa5 (TN ≈ 85 K) from room temperature down to 10 K. The relaxation time τ increases sharply near TN, which we attribute to the opening of a spin gap. In addition, at the lowest temperatures τ increases with a T-1 dependence, which is similar to that shown by the heavy fermion YbAgCu4, but with no blocking of electron-phonon scattering within the DOS peak. The transient amplitude increase at TN with a temperature dependence that is consistent with the appearance of a spin gap.
INTRODUCTION There has been a great deal of interest in the “115” series of rare-earth and actinide compound, such as CeTIn5 (T=Co, Rh, Ir) [1-3], and PuT’Ga5 (T’=Co, Rh) [4,5], all of which exhibit unconventional superconductivity. Attention has been drawn to the uranium isomorphs UMGa5 (M=Ni, Pd, Pt) which are isostructural to the Ce and Pu counterparts, yet do not exhibit superconductivity at either ambient or high pressure. It has been suggested that this is due to the strong hybridization and itinerant character of the 5f levels, which leads to a relatively wide 5f band at EF and a lack of spin fluctuations [6]. Time-resolved photoinduced reflectivity measurements have been performed on heavy fermions, such as YbAgCu4 [7], and heavy-fermion superconductors such as CeCoIn5 [8]. Both display a divergence of the electron-phonon (e-p) relaxation time at the lowest temperatures, which can be explained by the Two-Temperature Model (TTM) [9]. In this model (which assumes a thermal electron distribution) τ varies as T-1 at low temperatures if there is no blocking of e-p scattering of heavy electrons within the density-of-states (DOS) peak, but varies more strongly than T-1, and has a larger magnitude, if there is e-p blocking within the DOS peak [10]. On the other hand, similar measurements have been also performed on materials with a gap in the quasiparticle spectrum including high-temperature superconductors like YBCO [11,12] and charge-density-wave materials like K0.3MoO3 [13]. In these materials the relaxation time diverges near the transition temperature wherein a gap opens in the single-particle DOS. The opening of a gap can lead to a relaxation bottleneck. For example, in superconductors it has been suggested that this bottleneck arises due to a competition between quasiparticle recombination and pair breaking by phonons [14]. UMGa5 (M=Ni, Pd, Pt) are 5f itinerant antiferromagnets with Néel temperatures TN ≈ 85 K (Ni), 26 K (Pt) and 40 K (Pd), and electronic specific heat coefficients γ = 30 mJ/mol.K2 (Ni), and 57 mJ/mol.K2 (Pt), respectively [15-17]. The moderate values of γ suggest that they are marginal heavy fermions. Photoemission data [18]
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