A geometrically frustrated (GF) magnet consists of localised magnetic moments, spins, whose orientation cannot be arranged to simultaneously minimise their interaction energies. Such materials may host novel fascinating phases of matter, such as fluid-like states called quantum spin-liquids. GF magnets have, like all solid-state systems, randomly located impurities whose magnetic moments may "freeze" at low temperatures, making the system enter a spin-glass state. We analyse the available data for spin-glass transitions in GF materials and find a surprising trend: the glass-transition temperature grows with decreasing impurity concentration and reaches a finite value in the impurity-free limit at a previously unidentified, "hidden", energy scale. We propose a scenario in which the interplay of interactions and entropy leads to a crossover in the permeability of the medium that assists glass freezing at low temperatures. This low-temperature, "eminuscent", phase may obscure or even destroy the widely-sought spin-liquid states in rather clean systems.

We have measured the specific heat of single-crystal YbBe13 from 0.4 to 25 K and compared it to Schottky anomalies based on two different previously reported Yb3+ crystal-field spectra. Qualitative agreement is found with the level scheme determined by inelastic neutron scattering, and small quantitative discrepancies are consistent with a strongly enhanced electronic contribution (30 mJ/mole K2). TN is determined to be 1.1150.016 K. © 1986 The American Physical Society.

The specific heat in the mixed phase of superconducting UBe13 is observed vary as the square of the applied magnetic field. This is shown to imply a Fermiliquid-type suppression of the exchange enhancement as the superconducting gap opens, similar to that seen in 3He. Consistent with this effect is the observed nonmonotonic temperature dependence of the Maki parameter K2 which is related the Pauli susceptibility. © 1999 Taylor & Francis Group, LLC.

The nonlinear susceptibility is discussed as a probe of multispin correlation functions. In the heavy-fermion compound URu2Si2, measurements of the leading nonlinear susceptibility (3) reveal a sharp anomaly at 17.3 K closely matching that of the specific heat at the same temperature. We attribute this result to the development of anisotropic triplet pairing in the particle-hole channel of the heavy electron fluid. © 1992 The American Physical Society.

Experimental studies are discussed which shed light on the nature of the heavy fermion normal state, the occurrence of heavy fermion superconductivity and anomalous properties of the superconducting state: i) Magnetization and magnetoresistance of the Kondo lattice UBe13 demonstrate the breakdown of a description in terms of a single energy scale TK. Here TK becomes temperature dependent below ≃ 10 K, and the canonical relationship between R(H) and M(H) does not hold. ii) Superconductivity in UPt3 occurs at the borderline to magnetism: spin-density wave-like instabilities are induced by a few percent of Th or Au, whereas mass enhancement and susceptibility are rapidly reduced by Ir substitution. This suggests that spin fluctuations, rather than phonons, are important in mediating superconducting pairing of heavy fermions in UPt3. iii) The possibilities of noncubic strain induced by anisotropic superconductivity in UBe13 is investigated by ultrasound experiments. The suggestion of tetragonal strain in (U, Th)Be13 is found to be not applicable, and observed frequency and amplitude dependent absorption anomalies at T ≪ Tc might reflect domain wall motion. © 1987.

We discuss the nonlinear susceptibility (χ3) as a probe of higher-order spin correlations in heavy fermion materials, with particular emphasis on U Ru2Si2 and U Be13. The sharp discontinuity of χ in U Ru2Si2 is contrasted with two other antiferromagnets U2Zn17 and U0.95Th0.05Pt3 with larger moments. Nonlinear magnetic properties of URu2Si2 can be understood simply in terms of Landau-Ginzberg theory for a large hidden, itinerant order parameter. In UBe13 we discuss how χ3 can be used to probe the presence of quadrupolar fluctuations. © 1994.

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li_2.15(3)Os_0.85(3)O₃ (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μ_B, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T^(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)-Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.