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Register a new userPr-magnetism in the quasi-skutterudite compound PrFe$_2$Al$_8$
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The intermetallic compound PFA that possesses a three-dimensional network structure of Al polyhedra centered at the transition metal element Fe and the rare earth Pr is investigated through neutron powder diffraction and inelastic neutron scattering in order to elucidate the magnetic ground state of Pr and Fe and the crystal field effects of Pr. Our neutron diffraction study confirms long-range magnetic order of Pr below $T_N $ = 4.5~K in this compound. Subsequent magnetic structure estimation reveals a magnetic propagation vector $k$ = ($frac{1}{2}$~0~$frac{1}{2}$) with a magnetic moment value of 2.5~$mu_mathrm{B}$/Pr along the orthorhombic $c$-axis and evidence the lack of ordering in the Fe sublattice. The inelastic neutron scattering study reveals one crystalline electric field excitation near 19~meV at 5~K in PFA. The energy-integrated intensity of the 19~meV excitation as a function of |$Q$| $(A^{-1})$ follows the square of the magnetic form factor of Pr$^{3+}$ thereby confirming that the inelastic excitation belongs to the Pr sublattice. The second sum rule applied to the dynamic structure factor indicates only 1.6(2)~$mu_mathrm{B}$ evolving at the 19~meV peak compared to the 3.58~$mu_mathrm{B}$ for free Pr$^{3+}$, indicating that the crystal field ground state is magnetic and the missing moment is associated with the resolution limited quasi-elastic line. The magnetic order occurring in Pr in PFA is counter-intuitive to the symmetry-allowed crystal field level scheme, hence, is suggestive of exchange-mediated mechanisms of ordering stemming from the magnetic ground state of the crystal field levels.
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We report $^{51}$V nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) measurements on a quasi-1D antiferromagnet BaCo$_2$V$_2$O$_8$ under transverse field along the [010] direction. The scaling behavior of the spin-lattice relaxation rate above the N{e}el temperatures unveils a 1D quantum critical point (QCP) at $H_c^{1D}approx 4.7$ T, which is masked by the 3D magnetic order. With the aid of accurate analytical analysis and numerical calculations, we show that the zone center INS spectrum at $H_c^{1D}$ is precisely described by the pattern of the 1D quantum Ising model in a magnetic field, a class of universality described in terms of the exceptional $E_8$ Lie algebra. These excitations keep to be non-diffusive over a certain field range when the system is away from the 1D QCP. Our results provide an unambiguous experimental realization of the massive $E_8$ phase in the compound, and open new experimental route for exploring the dynamics of quantum integrable systems as well as physics beyond integrability.
We revisited the anisotropy of the heavy-fermion material CeCo$_2$Ga$_8$ by measuring the electrical resistivity and magnetic susceptibility along all the principal $mathbf{a}$-, $mathbf{b}$- and $mathbf{c}$-axes. Resistivity along $mathbf{c}$-axis ($rho_c$) shows clear Kondo coherence below about 17 K, while both $rho_{a}$ and $rho_{b}$ remain incoherent down to 2 K. The magnetic anisotropy is well understood within the theoretical frame of crystalline electric field effect in combination with magnetic exchange interactions. We found the anisotropy ratio of these magnetic exchange interactions, $|J_{ex}^c/J_{ex}^{a,b}|$, reaches a large value of 4-5. We, therefore, firmly demonstrate that CeCo$_2$Ga$_8$ is a quasi-one-dimensional heavy-fermion compound both electrically and magnetically, and thus provide a realistic example of textit{Kondo chain}.
In a quantum spin liquid, the magnetic moments of the constituent electron spins evade classical long-range order to form an exotic state that is quantum entangled and coherent over macroscopic length scales [1-2]. Such phases offer promising perspectives for device applications in quantum information technologies, and their study can reveal fundamentally novel physics in quantum matter. Quantum spin ice is an appealing proposal of one such state, in which the fundamental ground state properties and excitations are described by an emergent U(1) lattice gauge theory [3-7]. This quantum-coherent regime has quasiparticles that are predicted to behave like magnetic and electric monopoles, along with a gauge boson playing the role of an artificial photon. However, this emergent lattice quantum electrodynamics has proved elusive in experiments. Here we report neutron scattering measurements of the rare-earth pyrochlore magnet Pr$_2$Hf$_2$O$_7$ that provide evidence for a quantum spin ice ground state. We find a quasi-elastic structure factor with pinch points - a signature of a classical spin ice - that are partially suppressed, as expected in the quantum-coherent regime of the lattice field theory at finite temperature. Our result allows an estimate for the speed of light associated with magnetic photon excitations. We also reveal a continuum of inelastic spin excitations, which resemble predictions for the fractionalized, topological excitations of a quantum spin ice. Taken together, these two signatures suggest that the low-energy physics of Pr$_2$Hf$_2$O$_7$ can be described by emergent quantum electrodynamics. If confirmed, the observation of a quantum spin ice ground state would constitute a concrete example of a three-dimensional quantum spin liquid - a topical state of matter which has so far mostly been explored in lower dimensionalities.
The magnetic, thermodynamic and electrical/thermal transport properties of the caged-structure quasi-skutterudite Gd$_3$Ir$_4$Sn$_{13}$ are re-investigated. The magnetization $M(T)$, specific heat $C_p(T)$ and the resistivity $rho(T)$ reveal a double-phase transition -- at $T_{N1}sim$ 10~K and at $T_{N2}sim$ 8.8~K -- which was not observed in the previous report on this compound. The antiferromagnetic transition is also visible in the thermal transport data, thereby suggesting a close connection between the electronic and lattice degrees of freedom in this Sn-based quasi-skutterudite. The temperature dependence of $rho(T)$ is analyzed in terms of a power-law for resistivity pertinent to Fermi liquid picture. Giant, positive magnetoresistance (MR) $approx$ 80$%$ is observed in Gd$_3$Ir$_4$Sn$_{13}$ at 2~K with the application of 9~T. The giant MR and the double magnetic transition can be attributed to the quasi-cages and layered antiferromagnetic structure of Gd$_3$Ir$_4$Sn$_{13}$ vulnerable to structural distortions and/or dipolar or spin-reorientation effects. The giant value of MR observed in this class of 3:4:13 type alloys, especially in a Gd-compound, is the highlight of this work.
Magnetic materials with pyrochlore crystal structure form exotic magnetic states due to the high lattice frustration. In this work we follow the effects of coupling of the lattice and electronic and magnetic degrees of freedom in two Praseodymium-based pyrochlores Pr$_2$Zr$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$. In both materials the presence of magnetic interactions does not lead to magnetically ordered low temperature states, however their electronic properties are different. A comparison of Raman phonon spectra of Pr$_2$Zr$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$ allows us to identify magneto-elastic coupling in Pr$_2$Zr$_2$O$_7$ that elucidates its magnetic properties at intermediate temperatures, and allows us to characterize phonon-electron coupling in the semimetallic Pr$_2$Ir$_2$O$_7$. We also show that the effects of random disorder on the Raman phonon spectra is small.
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