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We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl3 through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.Although quantum fluctuations of both spin and charge degrees of freedom are the key to the essential physics of many challenging problems in condensed matter systems, the microscopic control of zero-point fluctuations has to date remained largely a theoretical abstraction. However, full control over the interaction parameters can now be effected in cold atomic condensates through the standing-wave amplitudes of the optical lattice. Similarly, in quantum magnets the exchange interactions can be controlled by the application of pressure, altering the effect of spin fluctuations. We follow this approach to investigate the physics of a quantum system whose fluctuations are "tuned" in a continuous way.The most dramatic manifestation of such control is the driving of a quantum phase transition [QPT, Fig. 1(a)] between two different ground states [1]. Structurally dimerized S = 1/2 spin systems offer a particularly clean realization both of the magnetic field-induced QPT, which has been studied extensively in a number of materials [2], and of the qualitatively different magnetic QPT driven by hydrostatic pressure [3]. The Hamiltonian

The strong-leg S=1/2 Heisenberg spin ladder system (C(7)H(10)N)(2)CuBr(4) is investigated using density matrix renormalization group calculations, inelastic neutron scattering, and bulk magnetothermodynamic measurements. Measurements showed qualitative differences compared to the strong-rung case. A long-lived two-triplon bound state is confirmed to persist across most of the Brillouin zone in a zero field. In applied fields, in the Tomonaga-Luttinger spin-liquid phase, elementary excitations are attractive, rather than repulsive. In the presence of weak interladder interactions, the strong-leg system is considerably more prone to three-dimensional ordering.

The superconducting and magnetic properties of Fe y Se 0.25 Te 0.75 single crystals ͑0.9Յ y Յ 1.1͒ were studied by means of x-ray diffraction, superconducting quantum interference device magnetometry, muon-spin rotation, and elastic neutron diffraction. The samples with y Ͻ 1 exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for y Ն 1 but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe y Se 1−x Te x can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.

The low energy part of the vibration spectrum in PbMg 1/3 Nb 2/3 O 3 (PMN) relaxor ferroelectric has been studied by means of neutron scattering above and below the Burns temperature, T d . The transverse acoustic and the lowest transverse optic phonons are strongly coupled and we have obtained a model for this coupling. We observe that the lowest optic branch is always underdamped. A resolution-limited central peak and quasi-elastic scattering appear in the vicinity of the Burns temperature. It is shown that it is unlikely that the quasielastic scattering originates from the combined effects of coupling between TA and TO phonons with an increase of the damping of the TO phonon below T d . The quasi-elastic scattering has a peak as a function of temperature close to the peak in the dielectric constant while the intensity of the central peak scattering increases strongly below this temperature. These results are discussed in terms of a random field model for relaxors.

The state with a giant permittivity ͑Ј ϳ 10 4 ͒ and ferromagnetism have been observed above 185 K ͑including room temperature͒ in single crystals of diluted semiconductor manganite-multiferroic Eu 0.8 Ce 0.2 Mn 2 O 5 in the investigations of x-ray diffraction, heat capacity, dielectric and magnetic properties, conductivity, and Raman light-scattering spectra of this material. X-ray diffraction study has revealed a layered superstructure along the c axis at room temperature. A model of the state with a giant Ј including as-grown two-dimensional layers with doping impurities, charge carriers, and double-exchange-coupled Mn 3+-Mn 4+ ion pairs is suggested. At low temperatures these layers form isolated electrically neutral small-size one-dimensional superlattices, in which de Haas-van Alphen oscillations were observed. As temperature grows and hopping conductivity increases, the charge carrier self-organization in the crystal causes formation of a layered superstructure consisting of charged layers ͑with an excess Mn 3+ concentration͒ alternating with dielectric layers of the initial crystal-the ferroelectricity due to charge-ordering state. Ferromagnetism results from double exchange between Mn 3+ and Mn 4+ ions by means of charge carriers in the charged layers. Temperature evolution of frequency shifts of A g modes and quasielastic scattering in Raman-scattering spectra agree with the pattern of phase transitions in ECMO suggested.

We have used inelastic neutron scattering and muon-spin rotation to compare the low energy magnetic excitations in single crystals of superconducting Fe(1.01)Se(0.50)Te(0.50) and non-superconducting Fe(1.10)Se(0.25)Te(0.75). We confirm the existence of a spin resonance in the superconducting phase of Fe(1.01)Se(0.50)Te(0.50), at an energy of 7 meV and a wavevector of (1/2, 1/2, 0). The non-superconducting sample exhibits two incommensurate magnetic excitations at (1/2, 1/2, 0) ± (0.18, - 0.18, 0) which rise steeply in energy, but no resonance is observed at low energies. A strongly dispersive low energy magnetic excitation is also observed in Fe(1.10)Se(0.25)Te(0.75) close to the commensurate antiferromagnetic ordering wavevector (1/2 - δ, 0, 1/2), where δ≈0.03. The magnetic correlations in both samples are found to be quasi-two-dimensional in character and persist well above the magnetic (Fe(1.10)Se(0.25)Te(0.75)) and superconducting (Fe(1.01)Se(0.50)Te(0.50)) transition temperatures.