Simultaneous neutron scattering and thermal expansion measurements on the heavy-fermion superconductor URu 2 Si 2 under hydrostatic pressure of 0.67 GPa have been performed in order to detect the successive paramagnetic, hidden order, and large moment antiferromagnetic phases on cooling. The temperature dependence of the sharp low energy excitation at the wave vector Q 0 = ͑1,0,0͒ shows clearly that this resonance is a signature of the hidden order state. In the antiferromagnetic phase, this resonance disappears. The higher energy excitation at the incommensurate wave vector Q 1 = ͑1.4,0,0͒ persists in the antiferromagnetic phase but increases in energy.The elucidation of the nature of a hidden order in exotic materials, which belong often to the rich class of strongly correlated electronic systems, is a hot subject as it can lead to the discovery of unexpected new order parameters. Debates exist on quite different proposals such as orbital hidden order in the heavy fermion system URu 2 Si 2 , 1 multipolar ordering in rare earth skutterudites 2 or "spin order accompanying loop current" in cuprate superconductors. 3 Due to the dual character of the 5f electrons in URu 2 Si 2 between localized ͑leading to the possibility of multipolar ordering͒ and itinerant ͑possibility of large Fermi surface instabilities͒, this compound has been the subject of a large variety of experiments. 4 At zero pressure, a phase transition occurs from the paramagnetic ͑PM͒ phase to a so-called hidden order ͑HO͒ phase at a temperature T 0 ϳ 17.5 K. The hidden order label reflects the fact that this order may not be of dipolar origin. The order parameter is not yet determined: spin or charge density wave, 5-7 multipolar ordering, [8][9][10][11] orbital antiferromagnetism, 1 chiral spin state, 12 and helicity order 13 have been proposed. The long standing debate on the occurrence of a tiny ordered moment M 0 ϳ 0.02 B per U atom at T → 0 K for the antiferromagnetic ͑AF͒ wave vector Q AF = ͑0,0,1͒ seems to converge now toward an extrinsic origin directly related to the high sensitivity of URu 2 Si 2 to pressure and stress ͑low critical pressure P x ϳ 0.5 GPa͒. 4,[14][15][16] Pressure studies 4,17-19 reveal an interesting phase diagram ͑Fig. 1͒. At T → 0 K, neutron scattering experiments 4 show that the hidden-order ground state switches at P x to a large moment antiferromagnetic ͑AF͒ state of sublattice magnetization M 0 near 0.3 B / U with a propagation vector Q AF . The HO-AF boundary T x ͑P͒ meets the T 0 ͑P͒ line at the tricritical point ͑T ‫ء‬ ϳ 19.3 K, P ‫ء‬ ϳ 1.36 GPa͒; 19 above P ‫ء‬ , a unique ordered phase ͑AF͒ is established below T N ͑P͒. Previous nuclear magnetic resonance ͑NMR͒ experiments, 14,20 as well as transport measurements, 5,19 indicate clearly that nesting occurs at T 0 , as well as at T N , indicating also that the Fermi surface is not deeply modified through the transition line T x .The interest in URu 2 Si 2 is reinforced by the appearance of unconventional superconductivity at T sc ϳ 1.2 K for P =0 ͑Ref. 21͒, which d...

Much recent attention has been devoted towards unraveling the microscopic optoelectronic properties of hybrid organic-inorganic perovskites. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr_{3}, FAPbBr_{3}, MAPbI_{3}, and α-FAPbI_{3} (MA: methylammonium, FA: formamidinium). We report a complete set of elastic constants characterized by a very soft shear modulus C_{44}. Further, a tendency towards an incipient ferroelastic transition is observed in FAPbBr_{3}. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α-FAPbI_{3}.

New inelastic neutron scattering experiments have been performed on URu 2 Si 2 with special focus on the response at Q 0 =(1,0,0), which is a clear signature of the hidden order (HO) phase of the compound. With polarized inelastic neutron experiments, it is clearly shown that below the HO temperature (T 0 = 17.8 K) a collective excitation (the magnetic resonance at E 0 ≃ 1.7 meV) as well as a magnetic continuum co-exist. Careful measurements of the temperature dependence of the resonance lead to the observation that its position shifts abruptly in temperature with an activation law governed by the partial gap opening and that its integrated intensity has a BCS-type temperature dependence. Discussion with respect to recent theoretical development is made.

Since the seminal ideas of Berezinskii, Kosterlitz and Thouless, topological excitations are at the heart of our understanding of a whole novel class of phase transitions. In most of the cases, those transitions are controlled by a single type of topological objects. There are however some situations, still poorly understood, where two dual topological excitations fight to control the phase diagram and the transition. Finding experimental realization of such cases is thus of considerable interest. We show here that this situation occurs in BaCo 2 V 2 O 8 , a spin-1/2 Ising-like quasione dimensional antiferromagnet when subjected to a uniform magnetic field transverse to the Ising axis. Using neutron scattering experiments, we measure a drastic modification of the quantum excitations beyond a critical value of the magnetic field. This quantum phase transition is identified, through a comparison with theoretical calculations, to be a transition between two different types of solitonic topological objects, which are captured by different components of the dynamical structure factor.The pioneering work of Berezinskii, Kosterlitz and Thouless (BKT) 1,2 has enlightened the role played by topological excitations in the two dimensional classical XY model. Since then, the topological aspects have been found to be crucial not only to a host of two dimensional classical systems 3 , but also in a spectacular way in the one dimensional quantum world 13 with in particular the remarkable case of spin-1 chains 5 . Such concepts have allowed to understand important aspects of the physics of materials such as the quantum hall effect 6 and even predict new classes of systems such as topological insulators 7 . Identifying and understanding the topological aspects of matter has thus become a major focus in condensed matter physics and quantum optics, where topo-logical phases such as the Haldane model 8 have been remarkably realized 9 .For classical and quantum critical phenomena, we have by now a good understanding of the prototypical topological phase transition in which only a single topological entity controls the transition. This was the case in the original BKT work, where vortex-antivortex excitations deconfine in a similar way than electrical charges in the two dimensional Coulomb gas 10 . In the quantum world, this situation is described by the celebrated sine-Gordon model, which also plays a central role in quantum field theory 11 . However, a richer and more difficult to understand class of topological transitions was rapidly pointed out to also play a major role for several systems 12,13 . This situation arises when two conjugate fields, subjected to the Heisenberg uncertainty principle and plunged into different potentials, compete with each other. The phase diagram is thus controlled by the confinement/deconfinement of the corresponding dual topological exc itations. These situations are considerably more difficult to analyse 12,13 and need much more sophisticated field theory descriptions such as the so-called dual-fie...

Entanglement is a concept that has defied common sense since the discovery of quantum mechanics. Two particles are said to be entangled when the quantum state of each particle cannot be described independently, no matter how far apart in space and time the two particles are. We demonstrate experimentally that unpaired spins separated by several hundred ångström entangle through a collection of spin singlets made up of antiferromagnetic spin-1/2 chains in a bulk material. Low-temperature magnetization and specific heat studies as a function of magnetic field reveal the occurrence of very dilute spin dimers and at least two quantum phase transitions related to the breaking of excited local triplets. The mechanism at the origin of the unpaired spins inside the quantum chains is the inter-modulation potential between two sublattices, and may be replicated using well-designed synthetic multilayers. E ntanglement is an essential feature of the macroscopic world 1 , despite the fact that human perception has difficulty accepting such strange predictions. A very fruitful area of research today is quantum computing technology, where entanglement is being harnessed for eventual use in quantum computation and communications 2,3 . Scientists are also developing new areas of research where entanglement could lie at the origin of the interactions in ever larger objects, ranging from photons to clusters of atoms to molecules-and even to cosmological objects.Most of the protocols for quantum communication rely on entangled photons 4 , as they are weakly interacting and can be easily manipulated using existing optical fibre technology. However, it is not always convenient to use photons to exchange information as, in particular, it is not straightforward to convert information between physically different qubits. Another approach described in seminal papers by Bose 5,6 suggested the use of spin chains as quantum channels for short-or mid-range communication, showing that, by means of the magnetic interaction between the spins that compose the chain, the transfer of information arises naturally as the system evolves dynamically, without the requirement for any external control. Transferring quantum information between distant qubits through spin chains would be highly desirable. However, this procedure often requires the repeated application of swapping gates and is, consequently, very demanding experimentally. Moreover, the features that characterize the transmission, such as teleportation, fidelity, transfer quality and speed, depend on the properties of the underlying quantum system 2,3 .Other developments have explored the conditions for longdistance entanglement without the need to perform operations and measurements. Among the many possible spin configurations it seems that antiferromagnetic spin arrays characterized by spin gaps above the ground state can exhibit true entanglement over long distances 7 . Furthermore, the entanglement is a slowly decreasing function of distance, allowing robust teleportation across finite di...

The interpretation of the magnetic phase diagrams of strongly correlated electron systems remains controversial. In particular, the physics of quantum phase transitions, which occur at zero temperature, is still enigmatic. Heavyfermion compounds aretextbook examples of quantum criticality, as doping, or the application of pressure or a magnetic field can lead to a quantum phase transition between a magnetically ordered state and a paramagnetic regime. A central question concerns the microscopic nature of the critical quantum fluctuations. Are they antiferromagnetic or of local origin? Here we demonstrate, using inelastic neutron scattering experiments, that the quantum phase transition in the heavy-fermion system Ce1-xLaxRu2Si2 is controlled by fluctuations of the antiferromagnetic order parameter. At least for this heavyfermion family, the Hertz-Millis-Moriya spin fluctuation approach seems to be a sound basis for describing the quantum antiferromagnetic-paramagnetic instability.

Abstract.Inelastic neutron scattering experiments were performed on single crystals of the heavy-fermion compound CeIn 3 for temperatures below and above the Néel temperature, T N . In the antiferromagnetically ordered phase, well-defined spinwave excitations with a bandwidth of 2 meV are observed. The spin waves coexist with quasielastic (QE) Kondo-type spin-fluctuations and broadened crystal-field (CF) excitations below T N . Above T N , only the QE and CF excitations persist, with a weak temperature dependence.

Inelastic neutron scattering measurements have been performed on single crystals of the heavy fermion compound Ce0.925La0.075Ru2Si2 in broad energy [0.1, 9.5 meV] and temperature [40 mK, 294 K] ranges in order to address the question of scaling behavior of the dynamical spin susceptibility at the quantum critical point of an itinerant magnetic system. For two wavevectors Q corresponding to uncorrelated and antiferromagnetically correlated spin fluctuations, it is found that the dynamical spin susceptibility χ ′′ (Q, E, T ) is independent of temperature below a cut-off temperature TQ: the spin fluctuation amplitude saturates at low temperatures contrarily to its expected divergence at a quantum critical point. Above TQ, a Q-dependent scaling behavior of the form T χ ′′ (Q, E, T ) = CQf [E/(aQT β Q )] with βQ < 1 is obtained. This scaling does not enter the general framework of quantum phase transition theories, since it is obtained in a high temperature range, where Kondo spin fluctuations depend strongly on temperature.