This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

Spin glasses: Experimental facts, theoretical concepts, and open questions

Physical metallurgy of Ti–Ni-based shape memory alloys

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance. The metastability of these systems, which gives rise to various rearrangement processes at elevated temperatures, calls for an understanding of their diffusional behavior. From the fundamental point of view, these metallic glasses are the paradigm of dense random packing. Since the recent discovery of bulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid state and to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article the authors review experimental results and computer simulations on diffusion in metallic glasses and supercooled melts. They consider in detail the experimental techniques, the temperature dependence of diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, the isotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown that diffusion in metallic glasses is significantly different from diffusion in crystalline metals and involves thermally activated, highly collective atomic processes. These processes appear to be closely related to low-frequency excitations. Similar thermally activated collective processes were also found to mediate diffusion in the supercooled liquid state well above the caloric glass transition temperature. This strongly supports the mode-coupling scenario of the glass transition, which predicts an arrest of liquidlike flow already at a critical temperature well above the caloric glass transition temperature.

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Diffusion in Metallic Glasses and Supercooled Melts

Vibrational thermodynamics of materials

The physical properties of a topologically disordered amorphous material (glass), such as heat capacity and thermal conductivity, are markedly different from those of its ordered crystalline counterpart. The understanding of these phenomena is a notoriously complex problem. One of the universal features of disordered glasses is the 'boson peak', which is observed in neutron and Raman scattering experiments. The boson peak is typically ascribed to an excess density of vibrational states. Here, we study the nature of the boson peak, using numerical simulations of several glass-forming systems. We discovered evidence suggestive of the equality of the boson peak frequency to the Ioffe-Regel limit for 'transverse' phonons, above which transverse phonons no longer propagate. Our results indicate a possibility that the origin of the boson peak is transverse vibrational modes associated with defective soft structures in the disordered state. Furthermore, we suggest a possible link between slow structural relaxation and fast boson peak dynamics in glass-forming systems.

Universal link between the boson peak and transverse phonons in glass.

Small angle neutron time-of-flight spectroscopy and inelastic x-ray scattering results are reported on the acoustic vibrations of fused silica. An expression for the dynamical structure factor is given that successfully describes the scattering from these vibrations, including the boson peak. The data analysis indicates an acoustic localization edge at > 1 THz, in agreement with the position of the thermal conductivity plateau. Scattering results on another strong glass support the generality of our description. [S0031-9007(96)01538-4]

Scattering Investigation of Acoustic Localization in Fused Silica.

Experimental Investigation of the Dispersion of Collective Density Fluctuations near Q p in a Metallic Glass

Density fluctuations in liquid argon: Coherent dynamic structure factor along the 120-K isotherm obtained by neutron scattering.

The low momentum collective dynamics of heavy water has been investigated by means of neutron three-axis spectroscopy. Working at the resolution limit of this instrument, an energy resolution of 2.6 meV was achieved for a constant analyzer energy of 120 meV. This good resolution allowed us to establish definitely that the collective dynamics of water is dominated by the presence and interaction of opticlike and acousticlike branches, coupling at wave-vector transfers of 0.3 to 0.35 A(-1). The transition from slow to fast sound has been attributed to the interaction between opticlike modes and a solidlike fast acoustic mode.

High-resolution neutron scattering measurement of the dynamic structure factor of heavy water

A high resolution s1.5 meVd inelastic neutron scattering experiment was carried out, aiming at an accurateinvestigation of the high frequency and low momentum dynamic response in heavy water. The experimentaldata conﬁrm the existence of a dispersionless mode, besides the ordinary longitudinal collective dynamics. Asimpliﬁed model, based on the interaction of two vibrational branches, is proposed to interpret the observedfeatures of the dynamic spectra. The validity extent of this scheme is proved by applying it to room tempera-ture neutron and x-ray data, to temperature and pressure dependent x-ray data, and to room temperatureneutron data of vibrational density of states. The overall successfull results provided by this model, in con-junction with the combined analysis of the x-ray and neutron data on collective dynamics, enable a deeperinsight into the complex mechanisms of the water dynamics and provide a simple phenomenological explana-tion for the transition from ordinary to fast sound.DOI: 10.1103/PhysRevE.69.061203 PACS number(s): 47.10.1g, 61.12.Ex, 78.70.Nx, 63.50.1xI. INTRODUCTION

Jens-Boie Suck

Papers

Scattering Investigation of Acoustic Localization in Fused Silica.

Experimental Investigation of the Dispersion of Collective Density Fluctuations near Q p in a Metallic Glass

Density fluctuations in liquid argon: Coherent dynamic structure factor along the 120-K isotherm obtained by neutron scattering.

High-resolution neutron scattering measurement of the dynamic structure factor of heavy water

Brillouin neutron scattering in heavy water: evidence for two-mode collective dynamics.