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Register a new userDetermination of the Antiferroquadrupolar Order Parameters in UPd3
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By combining accurate heat capacity and X-ray resonant scattering results we have resolved the long standing question regarding the nature of the quadrupolar ordered phases in UPd_3. The order parameter of the highest temperature quadrupolar phase has been uniquely determined to be antiphase Q_{zx} in contrast to the previous conjecture of Q_{x^2-y^2} . The azimuthal dependence of the X-ray scattering intensity from the quadrupolar superlattice reflections indicates that the lower temperature phases are described by a superposition of order parameters. The heat capacity features associated with each of the phase transitions characterize their order, which imposes restrictions on the matrix elements of the quadrupolar operators.
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The nature of multipolar order and hyperfine-enhanced (HE) $^{141}$Pr nuclear spin dynamics in PrV$_2$Al$_{20}$ was investigated using the muon spin relaxation technique. No explicit sign of time-reversal symmetry breaking was found below the multipolar order temperature $T_Qsim 0.6$ K in a zero applied field as anticipated on the basis of the antiferroquadrupolar (AFQ) order picture proposed by Sakai and Nakatsuji [J. Phys. Soc. Jpn. 80, 063701 (2011)]. Further evidence of the nonmagnetic ground state was obtained from the observation of HE $^{141}$Pr nuclear spin fluctuations in the MHz scale. A marked increase in the muon spin-lattice relaxation rate (1/$T_{rm 1,mu}$) was observed below 1 K with decreasing temperature, which was attributed to the perturbation on the HE $^{141}$Pr nuclear spin dynamics associated with the development of AFQ correlations. The longitudinal field dependence of 1/$T_{rm 1,mu}$ revealed that the enhanced $^{141}$Pr nuclear spin accidentally has an effective gyromagnetic ratio close to that of the muon.
We have employed resonant x-ray magnetic scattering to specifically probe the magnetic order of the rare-earth ions in multiferroic $mathrm{TbMn_2O_5}$. Two energy resonances were observed, one originated from the E1-E1 dipolar transition and the other from the E2-E2 quadrupolar transition. These resonances directly probe the valence 5d band and the partially occupied 4f band, respectively. First, full polarization analysis, which is a measurement of the scattered polarization as a function of incident polarization, confirmed a spin polarization of the terbium valence states (probed by the E1-E1 transition) by the $mathrm{Mn^{4+}}$ spin density in the commensurate phase. Second, full polarization analysis data were collected in the low-temperature incommensurate and commensurate phases when tuned to the E2-E2 resonance. By employing a least-squares fitting procedure, the spin orientations of the terbium ion sublattice were refined.
In complex materials observed electronic phases and transitions between them often involves coupling between many degrees of freedom whose entanglement convolutes understanding of the instigating mechanism. Metal-insulator transitions are one such problem where coupling to the structural, orbital, charge, and magnetic order parameters frequently obscures the underlying physics. Here, we demonstrate a way to unravel this conundrum by heterostructuring a prototypical multi-ordered complex oxide NdNiO3 in ultra thin geometry, which preserves the metal-to-insulator transition and bulk-like magnetic order parameter, but entirely suppresses the symmetry lowering and charge order parameter. These findings illustrate the utility of heterointerfaces as a powerful method for removing competing order parameters to gain greater insight into the nature of the transition, here revealing that the magnetic order generates the transition independently, leading to a purely electronic Mott metal-insulator transition.
Superconducting and antiferroquadrupolar (AFQ) transitions in a Pr-based compound PrRh2Zn20 have been found to occur simultaneously at Tc=TQ=0.06 K. The superconducting transition manifests itself by zero resistance and large diamagnetic susceptibility. The specific heat exhibits a Schottky anomaly peaking at 14 K and magnetization curves measured at 2 K show anisotropic behaviors. The analysis of these data indicates that the crystalline electric field (CEF) ground state of the trivalent Pr ion is the non-Kramers Gamma3 doublet with the quadrupolar degrees of freedom. A sharp peak in the specific heat at 0.06 K has been attributed not to the superconducting transition but to the AFQ transition because the ordering temperature TQ decreases in B || [100] but increases in B || [110] and B || [111] with increasing B up to 6 T. This anisotropic behavior of TQ(B) can be well explained by a two-sublattice mean-field calculation, which corroborates the AFQ ordered state below TQ. The entropy release at TQ is only 10% of Rln2 expected for the Gamma3 doublet, suggesting possible interplay between the quadrupolar degrees of freedom and the superconductivity.
Spin-1/2 Heisenberg antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to Cs$_2$CuCl$_4$, yielding $J/k_B=4.7(2)$ K, $J/k_B=1.42(7)$ K [$J/Jsimeq 0.30$] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs$_2$CuBr$_4$, we obtain $J/k_B=14.9(7)$ K, $J/k_B=6.1(3)$ K, [$J/Jsimeq 0.41$], providing exact and conclusive information on the exchange couplings in this frustrated spin system.
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