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Neutron scattering measurements on single-crystal samples of the mineral herbertsmithite, which is a spin-1/2 kagome-lattice antiferromagnet, provide evidence of fractionalized spin excitations at low temperatures, indicating that the ground state of herbertsmithite may be a quantum spin liquid. Creating a quantum spin liquid Quantum spin liquids are exotic states of matter with atomic magnetic moments that are highly correlated but resist ordering even when cooled to absolute zero. They display remarkable collective behaviour, of potential relevance for understanding high T c superconductivity, and host exotic excitations with fractional quantum numbers. On the downside, conclusive evidence for their existence is still missing. Tian-Heng Han et al . now report an exciting result from neutron scattering measurements on large single crystals of 'herbertsmithite', a two-dimensional frustrated antiferromagnet. Specifically, they observe the emergence of fractional spin excitations at low temperature, which is a hallmark signature of quantum spin liquids. Fractional spin excitations have so far only been seen in one-dimensional systems. The experimental realization of quantum spin liquids is a long-sought goal in physics, as they represent new states of matter. Quantum spin liquids cannot be described by the broken symmetries associated with conventional ground states. In fact, the interacting magnetic moments in these systems do not order, but are highly entangled with one another over long ranges 1 . Spin liquids have a prominent role in theories describing high-transition-temperature superconductors 2 , 3 , and the topological properties of these states may have applications in quantum information 4 . A key feature of spin liquids is that they support exotic spin excitations carrying fractional quantum numbers. However, detailed measurements of these ‘fractionalized excitations’ have been lacking. Here we report neutron scattering measurements on single-crystal samples of the spin-1/2 kagome-lattice antiferromagnet ZnCu 3 (OD) 6 Cl 2 (also called herbertsmithite), which provide striking evidence for this characteristic feature of spin liquids. At low temperatures, we find that the spin excitations form a continuum, in contrast to the conventional spin waves expected in ordered antiferromagnets. The observation of such a continuum is noteworthy because, so far, this signature of fractional spin excitations has been observed only in one-dimensional systems. The results also serve as a hallmark of the quantum spin-liquid state in herbertsmithite.

Magnetism plays a key role in modern technology and stimulates research in several branches of condensed matter physics. Although the theory of classical magnetism is well developed, the demonstration of a widely tunable experimental system has remained an elusive goal. Here, we present the realization of a large-scale simulator for classical magnetism on a triangular lattice by exploiting the particular properties of a quantum system. We use the motional degrees of freedom of atoms trapped in an optical lattice to simulate a large variety of magnetic phases: ferromagnetic, antiferromagnetic, and even frustrated spin configurations. A rich phase diagram is revealed with different types of phase transitions. Our results provide a route to study highly debated phases like spin-liquids as well as the dynamics of quantum phase transitions.

Bound states of elementary spin waves (magnons) have been predicted to occur in one-dimensional quantum magnets; the observation of two-magnon bound states in a system of ultracold bosonic atoms in an optical lattice is now reported. Quanta at the double Two papers published in this issue of Nature show that the propagation of energy quanta in very different physical systems can exhibit the same, unusual dynamics, where bound pairs of quanta become dominant. Ofer Firstenberg et al . realize coherent interactions between individual photons — quanta of light — which are massless and do not usually interact. They achieve this using a quantum nonlinear medium inside which individual photons pair up and travel as massive particles with strong mutual attraction. Potential applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light. The second paper deals with magnons, the quanta that carry energy in magnets. More than eighty years ago, Hans Bethe predicted the existence of bound states of elementary spin waves (magnons) in one-dimensional quantum magnets. Experimental observation of the phenomenon remained elusive, but now Takeshi Fukuhara et al . have observed two-magnon bound states in a system of ultracold bosonic atoms in an optical lattice. The results provide a new way of studying fundamental properties of quantum magnets. In an accompanying News and Views, Sougato Bose puts these two independent findings into a general context of quantum many-body dynamics. The existence of bound states of elementary spin waves (magnons) in one-dimensional quantum magnets was predicted almost 80 years ago 1 . Identifying signatures of magnon bound states has so far remained the subject of intense theoretical research 2 , 3 , 4 , 5 , and their detection has proved challenging for experiments. Ultracold atoms offer an ideal setting in which to find such bound states by tracking the spin dynamics with single-spin and single-site resolution 6 , 7 following a local excitation 8 . Here we use in situ correlation measurements to observe two-magnon bound states directly in a one-dimensional Heisenberg spin chain comprising ultracold bosonic atoms in an optical lattice. We observe the quantum dynamics of free and bound magnon states through time-resolved measurements of two spin impurities. The increased effective mass of the compound magnon state results in slower spin dynamics as compared to single-magnon excitations. We also determine the decay time of bound magnons, which is probably limited by scattering on thermal fluctuations in the system. Our results provide a new way of studying fundamental properties of quantum magnets and, more generally, properties of interacting impurities in quantum many-body systems.

Magnetic bistability, as manifested in the magnetization of ferromagnetic materials or spin crossover in transition metal complexes, has essentially been restricted to either bulk materials or to very low temperatures. We now present a molecular spin switch that is bistable at room temperature in homogeneous solution. Irradiation of a carefully designed nickel complex with blue-green light (500 nanometers) induces coordination of a tethered pyridine ligand and concomitant electronic rearrangement from a diamagnetic to a paramagnetic state in up to 75% of the ensemble. The process is fully reversible on irradiation with violet-blue light (435 nanometers). No fatigue or degradation is observed after several thousand cycles at room temperature under air. Preliminary data show promise for applications in magnetic resonance imaging.

The ability to make electrical contact to single molecules creates opportunities to examine fundamental processes governing electron flow on the smallest possible length scales. We report experiments in which we controllably stretched individual cobalt complexes having spin S = 1, while simultaneously measuring current flow through the molecule. The molecule's spin states and magnetic anisotropy were manipulated in the absence of a magnetic field by modification of the molecular symmetry. This control enabled quantitative studies of the underscreened Kondo effect, in which conduction electrons only partially compensate the molecular spin. Our findings demonstrate a mechanism of spin control in single-molecule devices and establish that they can serve as model systems for making precision tests of correlated-electron theories.

A thaw in the spin ice Chiral spin liquids are a long-sought hypothetical class of spin liquids — spin systems in frustrated magnets that neither freeze nor order even at T = 0 — in which time-reversal symmetry is macroscopically broken even in the absence of an applied magnetic field or any magnetic dipole long-range order. Machida et al . report an investigation of the magnetic and transport properties of the metallic frustrated magnet Pr 2 Ir 2 O 7 , observing a spontaneous Hall effect in the absence of uniform magnetization and at zero magnetic field. The data and analysis suggests that a chiral spin-liquid phase is induced by melting of a spin ice and the formation of chiral spin textures. Chiral spin liquids are a hypothetical class of spin liquids in which time-reversal symmetry is macroscopically broken even in the absence of an applied magnetic field or any magnetic dipole long-range order. Although such spin-liquid states were proposed more than two decades ago, they remain elusive. Here, evidence is presented that the time-reversal symmetry can be broken spontaneously on a macroscopic scale in the absence of magnetic dipole long-range order, suggesting the emergence of a chiral spin liquid. Spin liquids are magnetically frustrated systems, in which spins are prevented from ordering or freezing, owing to quantum or thermal fluctuations among degenerate states induced by the frustration. Chiral spin liquids are a hypothetical class of spin liquids in which the time-reversal symmetry is macroscopically broken in the absence of an applied magnetic field or any magnetic dipole long-range order. Even though such chiral spin-liquid states were proposed more than two decades ago 1 , 2 , 3 , an experimental realization and observation of such states has remained a challenge. One of the characteristic order parameters in such systems is a macroscopic average of the scalar spin chirality, a solid angle subtended by three nearby spins. In previous experimental reports, however, the spin chirality was only parasitic to the non-coplanar spin structure associated with a magnetic dipole long-range order or induced by the applied magnetic field 4 , 5 , 6 , 7 , 8 , 9 , 10 , and thus the chiral spin-liquid state has never been found. Here, we report empirical evidence that the time-reversal symmetry can be broken spontaneously on a macroscopic scale in the absence of magnetic dipole long-range order. In particular, we employ the anomalous Hall effect 4 , 11 to directly probe the broken time-reversal symmetry for the metallic frustrated magnet Pr 2 Ir 2 O 7 . An onset of the Hall effect is observed at zero field in the absence of uniform magnetization, within the experimental accuracy, suggesting an emergence of a chiral spin liquid. The origin of this spontaneous Hall effect is ascribed to chiral spin textures 4 , 5 , 12 , 13 , which are inferred from the magnetic measurements indicating the spin ice-rule formation 14 , 15 .

A second-order phase transition is characterized by spontaneous symmetry breaking. The nature of the broken symmetry in the so-called \"hidden-order\" phase transition in the heavy-fermion compound URu(2)Si(2), at transition temperature T(h) = 17.5 K, has posed a long-standing mystery. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below T(h), which breaks the four-fold rotational symmetry of the tetragonal URu(2)Si(2). Two-fold oscillations in the magnetic torque under in-plane field rotation were sensitively detected in small pure crystals. Our findings suggest that the hidden-order phase is an electronic \"nematic\" phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry.

The development of collective long-range order by means of phase transitions occurs by the spontaneous breaking of fundamental symmetries. Magnetism is a consequence of broken time-reversal symmetry, whereas superfluidity results from broken gauge invariance. The broken symmetry that develops below 17.5 kelvin in the heavy-fermion compound URu 2 Si 2 has long eluded such identification. Here we show that the recent observation of Ising quasiparticles in URu 2 Si 2 results from a spinor order parameter that breaks double time-reversal symmetry, mixing states of integer and half-integer spin. Such ‘hastatic’ order hybridizes uranium-atom conduction electrons with Ising 5 f 2 states to produce Ising quasiparticles; it accounts for the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. Hastatic order predicts a tiny transverse moment in the conduction-electron ‘sea’, a colossal Ising anisotropy in the nonlinear susceptibility anomaly and a resonant, energy-dependent nematicity in the tunnelling density of states. The formation of Ising quasiparticles in URu 2 Si 2 results from ‘hastatic’ order, which breaks double time-reversal symmetry, mixing states of integer and half-integer spin, and accounts for the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. Novel 'hastatic' order in condensed matter At temperatures below 17.5 K, the heavy-fermion uranium compound URu 2 Si 2 exists in a mysterious 'hidden-order' phase that has eluded characterization for 25 years. In this study the authors have used various pieces of experimental evidence to establish the nature of the spontaneous phase transition that takes place at 17.5 K, and conclude that both single and double time-reversal symmetry occur, resulting in mixing of itinerant conduction electrons and localized 'Ising' states in the 5 f 2 orbital of uranium atoms. This is a fundamentally new type of order, which the authors dub hastatic (from the Latin hasta , a spear), and which they say could be a phenomenon that applies to other systems in which mixing with f -orbital states takes place.

The magnetic properties of the ground state of a low-density free-electron gas in three dimensions have been the subject of theoretical speculation and controversy for seven decades 1 . Not only is this a difficult theoretical problem to solve, it is also a problem which has not hitherto been directly addressed experimentally. Here we report measurements on electron-doped calcium hexaboride (CaB 6 ) which, we argue, show that—at a density of 7× 10 19  electrons cm −3 —the ground state is ferromagnetically polarized with a saturation moment of 0.07 µ B per electron. Surprisingly, the magnetic ordering temperature of this itinerant ferromagnet is 600 K, of the order of the Fermi temperature of the electron gas.

Transition metal takes charge The introduction of 'foreign' elements into transition-metal oxides (called chemical doping) can change the valence state of the metal's cations and hence modify the physical properties of the material as a whole. These changes can be dramatic, for example causing high-temperature superconductivity in copper oxides and colossal magnetoresistance in manganese oxides. Youwen Long et al . have identified an oxide system, the perovskite LaCu 3 Fe 4 O 12 , in which changes in valence state occur when charge is shuffled between different cations (iron and copper) in the host structure, rather than via doping. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe 3.75+ state (partnered with fairly common Cu 2+ ions) to one possessing rare Cu 3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, the material contracts slightly on being warmed through the transition. The temperature sensitivity of this effect makes it a strong candidate for technological applications. This paper identifies an oxide system where changes in valence state occur as a result of charge being shuffled between different cations in the host structure, rather than via doping, this charge transfer being sensitive to temperature. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe3.75+ state to one possessing rare Cu3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, are accompanied by negative thermal expansion. Changes of valence states in transition-metal oxides often cause significant changes in their structural and physical properties 1 , 2 . Chemical doping is the conventional way of modulating these valence states. In ABO 3 perovskite and/or perovskite-like oxides, chemical doping at the A site can introduce holes or electrons at the B site, giving rise to exotic physical properties like high-transition-temperature superconductivity and colossal magnetoresistance 3 , 4 . When valence-variable transition metals at two different atomic sites are involved simultaneously, we expect to be able to induce charge transfer—and, hence, valence changes—by using a small external stimulus rather than by introducing a doping element. Materials showing this type of charge transfer are very rare, however, and such externally induced valence changes have been observed only under extreme conditions like high pressure 5 , 6 . Here we report unusual temperature-induced valence changes at the A and B sites in the A-site-ordered double perovskite LaCu 3 Fe 4 O 12 ; the underlying intersite charge transfer is accompanied by considerable changes in the material’s structural, magnetic and transport properties. When cooled, the compound shows a first-order, reversible transition at 393 K from LaCu 2+ 3 Fe 3.75+ 4 O 12 with Fe 3.75+ ions at the B site to LaCu 3+ 3 Fe 3+ 4 O 12 with rare Cu 3+ ions at the A site. Intersite charge transfer between the A-site Cu and B-site Fe ions leads to paramagnetism-to-antiferromagnetism and metal-to-insulator isostructural phase transitions. What is more interesting in relation to technological applications is that this above-room-temperature transition is associated with a large negative thermal expansion.