Competition between hidden order and antiferromagnetism inURu2Si2under uniaxial stress studied by neutron scattering

Abstract

We have performed elastic neutron scattering experiments under uniaxial stress σ applied along the tetragonal [100], [110] and [001] directions for the heavy electron compound URu2Si2. We found that antiferromagnetic (AF) order with large moment is developed with σ along the [100] and [110] directions. If the order is assumed to be homogeneous, the staggered ordered moment µo continuously increases from 0.02 µB/U (σ = 0) to 0.22 µB/U (0.25 GPa). The rate of increase ∂µo/∂σ is ∼ 1.0 µB/GPa, which is four times larger than that for the hydrostatic pressure (∂µo/∂P ∼ 0.25 µB/GPa). Above 0.25 GPa, µo shows a tendency to saturate, similar to the hydrostatic pressure behavior. For σ || [001], µo shows only a slight increase to 0.028 µB/U (σ = 0.46 GPa) with a rate of ∼ 0.02 µB/GPa, indicating that the development of the AF state highly depends on the direction of σ. We have also found a clear hysteresis loop in the isothermal µo(σ) curve obtained for σ || [110] under the zero-stress-cooled condition at 1.4 K. This strongly suggests that the σ-induced AF phase is metastable, and separated from the "hidden order" phase by a first-order phase transition. We discuss these experimental results on the basis of crystalline strain effects and elastic energy calculations, and show that the c/a ratio plays a key role in the competition between these two phases.

Group theory and octupolar order inURu2Si2

“…The present experimental status is that the intrinsic low-pressure behavior of URu 2 Si 2 is purely non-magnetic. Furthermore, a number of recent experiments proves that the hidden order breaks time reversal invariance, so it cannot be quadrupolar [7,20,38]. In particular, Yokoyama et al [7] found that uniaxial stress (which is time reversal invariant) induces large-amplitude antiferromagnetism, which breaks time reversal invariance.…”

“…Yokoyama et al [7] carried out magnetic neutron scattering measurement in the presence of uniaxial stress applied to a single crystal sample either along, or perpendicular to the tetragonal axis. Stress σ (001) does not produce significant change in magnetic moments.…”

“…Under D 2h , A tetr 2u → B orth 1u and B tetr 2u → B orth 1u , so the order parameters T β z and J z become mixed (Table IV). A state with spontaneous T β z octupolar order carries J z magnetic dipole moments as well [23], accounting for the observations [7].…”

“…As we are going to describe, theoretical progress has long been held up by the ambiguity of experimental findings on apparently heterogeneous samples. However, crucial recent experiments [6,7] allow to infer what the equilibrium properties of ideal samples of URu 2 Si 2 would be.…”

Local suppression of the hidden-order phase by impurities in URu2Si2

“…Moreover recent neutron scattering experiments evidence that this small moment is not an intrinsic feature of the HO, but a spurious effect due to local strains induced by crystal defects in the sample 6 . Nevertheless magnetic ordering is not completely extraneous to URu 2 Si 2 : an antiferromagnetic phase with large moment can be stabilized by applying pressure or strain 5,[7][8][9] . Since 1985 several theories have been proposed to identify the nature of the hidden order parameter.…”