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Antiferromagnetically coupled S = 1/2 spins on an isotropic triangular lattice are the paradigm of frustrated quantum magnetism, but structurally ideal realizations are rare. Here, we investigate NaYbO2, which hosts an ideal triangular lattice of effective Jeff = 1/2 moments with no inherent site disorder. No signatures of conventional magnetic order appear down to 50 mK, strongly suggesting a quantum spin liquid ground state. We observe a two-peak specific heat and a nearly quadratic temperature dependence, in agreement with expectations for a two-dimensional Dirac spin liquid. Application of a magnetic field strongly perturbs the quantum disordered ground state and induces a clear transition into a collinear ordered state, consistent with a long-predicted up–up–down structure for a triangular-lattice XXZ Hamiltonian driven by quantum fluctuations. The observation of spin liquid signatures in zero field and quantum-induced ordering in intermediate fields in the same compound demonstrates an intrinsically quantum disordered ground state. We conclude that NaYbO2 is a model, versatile platform for exploring spin liquid physics with full tunability of field and temperature.

We investigate the crystal structure, magnetic properties, and crystalline-electric field of tetragonal, \\(I4_1/amd\\), NaCeO\\(_2\\). In this compound, Ce\\(^3+\\) ions form a tetragonally elongated diamond lattice coupled by antiferromagnetic interactions (\\(_CW = -7.69\\) K) that magnetically order below \\(T_N = 3.18\\) K. The Ce\\(^3+\\) \\(J = 5/2\\) crystalline-electric field-split multiplet is studied via inelastic neutron scattering to parameterize a \\(J_eff = 1/2\\) ground state doublet comprised of states possessing mixed \\(|m_z \\) character. Neutron powder diffraction data reveal the onset of \\(A\\)-type antiferromagnetism with \\(=0.57(2)\\) \\(_B\\) moments aligned along the \\(c\\)-axis. The magnetic structure is consistent with the expectations of a frustrated Heisenberg \\(J_1\\)-\\(J_2\\) model on the elongated diamond lattice with effective exchange values \\(J_1 > 4 J_2\\) and \\(J_1 > 0\\).

Here we present a neutron scattering-based study of magnetic excitations and magnetic order in NaYbO\\(_2\\) under the application of an external magnetic field. The crystal electric field-split \\(J = 7/2\\) multiplet structure is determined, revealing a mixed \\(|m_z>\\) ground state doublet and is consistent with a recent report Ding et al. [1]. Our measurements further suggest signatures of exchange effects in the crystal field spectrum, manifested by a small splitting in energy of the transition into the first excited doublet. The field-dependence of the low-energy magnetic excitations across the transition from the quantum disordered ground state into the fluctuation-driven ordered regime is analyzed. Signs of a first-order phase transition into a noncollinear ordered state are revealed at the upper-field phase boundary of the ordered regime, and higher order magnon scattering, suggestive of strong magnon-magnon interactions, is resolved within the previously reported \\(up-up-down\\) phase. Our results reveal a complex phase diagram of field-induced order and spin excitations within NaYbO\\(_2\\) and demonstrate the dominant role of quantum fluctuations cross a broad range of fields within its interlayer frustrated triangular lattice.

Antiferromagnetically coupled S=1/2 spins on an isotropic triangular lattice is the paradigm of frustrated quantum magnetism, but structurally ideal realizations are rare. Here we investigate NaYbO\\(_2\\), which hosts an ideal triangular lattice of \\(J_eff=1/2\\) moments with no inherent site disorder. No signatures of conventional magnetic order appear down to 50 mK, strongly suggesting a quantum spin liquid ground state. We observe a two-peak specific heat and a nearly quadratic temperature dependence in accord with expectations for a two-dimensional Dirac spin liquid. Application of a magnetic field strongly perturbs the quantum disordered ground state and induces a clear transition into a collinear ordered state consistent with a long-predicted up-up-down structure for a triangular lattice XXZ Hamiltonian driven by quantum fluctuations. The observation of spin liquid signatures in zero field and quantum-induced ordering in intermediate fields in the same compound demonstrate an intrinsically quantum disordered ground state. We conclude that NaYbO\\(_2\\) is a model, versatile platform for exploring spin liquid physics with full tunability of field and temperature.

Triangular lattice delafossite compounds built from magnetic lanthanide ions are a topic of recent interest due to their frustrated magnetism and realization of quantum disordered magnetic ground states. Here we report the evolution of the structure and electronic ground states of RbCe\\(X_2\\) compounds, built from a triangular lattice of Ce\\(^3+\\) ions, upon varying their anion character (\\(X_2\\)= O\\(_2\\), S\\(_2\\), SeS, Se\\(_2\\), TeSe, Te\\(_2\\)). This includes the discovery of a new member of this series, RbCeO\\(_2\\), that potentially realizes a quantum disordered ground state analogous to NaYbO\\(_2\\). Magnetization and susceptibility measurements reveal that all compounds manifest mean-field antiferromagnetic interactions and, with the exception of the oxide, possess signatures of magnetic correlations onset below 1 K. The crystalline electric field level scheme is explored via neutron scattering and ab initio calculations in order to model the intramultiplet splitting of the \\(J=5/2\\) multiplet. In addition to the two excited doublets expected within the \\(J=5/2\\) manifold, we observe one extra, local mode present across the sample series. This added mode shifts downward in energy with increasing anion mass and decreasing crystal field strength, suggesting a long-lived anomalous mode endemic to anion motion about the Ce\\(^3+\\) sites.

We investigate the magnetic properties of LiYbO\\(_2\\), containing a three-dimensionally frustrated, diamond-like lattice via neutron scattering, magnetization, and heat capacity measurements. The stretched diamond network of Yb\\(^3+\\) ions in LiYbO\\(_2\\) enters a long-range incommensurate, helical state with an ordering wave vector \\(k = (0.384, 0.384, 0)\\) that \"locks-in\" to a commensurate \\(k = (1/3, 1/3, 0)\\) phase under the application of a magnetic field. The spiral magnetic ground state of LiYbO\\(_2\\) can be understood in the framework of a Heisenberg \\(J_1-J_2\\) Hamiltonian on a stretched diamond lattice, where the propagation vector of the spiral is uniquely determined by the ratio of \\(J_2/|J_1|\\). The pure Heisenberg model, however, fails to account for the relative phasing between the Yb moments on the two sites of the bipartite lattice, and this detail as well as the presence of an intermediate, partially disordered, magnetic state below 1 K suggests interactions beyond the classical Heisenberg description of this material.

Significance The 2003 severe acute respiratory syndrome (SARS) epidemic and recent emergence of Middle East respiratory syndrome highlight the potential lethality of zoonotic coronavirus infections in humans. No specific antiviral treatment options are available. Coronaviruses possess the largest known RNA virus genomes and encode a complex replication machinery consisting of 16 viral nonstructural proteins (nsps). Our study reveals that the SARS-coronavirus RNA polymerase (nsp12) needs to associate with nsp7 and nsp8 to activate its capability to replicate long RNA. Moreover, this complex associates with nsp14, the proofreading subunit required to safeguard coronavirus replication fidelity. Our study thus defines the core of an RNA-synthesizing machinery that is unique in the RNA virus world and includes several key targets for antiviral drug development.