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

Critical phenomena and renormalization-group theory

Valence fluctuation phenomena occur in rare-earth compounds in which the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration (valence) and/or of the magnetic moment. The authors review the experimental results observed in the subset of such systems for which the 4f ions form a lattice with identical valence on each site. The discussion includes key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering. This is followed by a review of existing theoretical work concerning both the ground states and the isomorphic phase transitions which occur in such compounds; the emphasis is on those aspects which make valence fluctuation phenomena such a challenging many-body problem.

https://iopscience.iop.org/article/10.1088/0034-4885/44/1/001/pdf

Valence fluctuation phenomena

Static critical phenomena in ferromagnets with quenched disorder

Colossal-magnetoresistance materials: manganites and conventional ferromagnetic semiconductors

Large-scale computing system characteristics vary by application class, but power and energy use has become a major problem for all classes. Superconducting computing may be able to serve the needs of these systems significantly better than conventional technology. Recent developments in single flux quantum circuit technology for digital logic include variants with greatly improved energy efficiency. Concepts were investigated for computing systems capable of performance in the range from 1 to 1000 PFLOP/s. The concept systems were constrained to use existing commercial cryogenic refrigerators and Nb superconducting technology. In order to meet the performance goals, cache and main memory capable of operating at cryogenic temperatures will be required. Superconducting computing is shown to be potentially competitive on the basis of power and energy efficiency if key component technologies can meet specific goals. Potential advantages of superconducting computing are identified as well as areas requiring further development.

Energy-Efficient Superconducting Computing—Power Budgets and Requirements

The ultimate goal of the Intelligence Advanced Research Projects Activity (IARPA)'s Cryogenic Computing Complexity (C3) program is to demonstrate a complete superconducting computer including processing units and cryogenic memory. IARPA expects that the C3 program will be a five-year two-phase program. Phase one, which encompasses the first three years, primarily serves to develop the technologies that are required to separately demonstrate a small superconducting processor and memory units. Phase two, which is for the final two years, will integrate those new technologies into a small-scale working model of a superconducting computer. Program goals are presented, and the approaches of the phase-one teams are reviewed.

Cryogenic Computing Complexity Program: Phase 1 Introduction

Magnetic phase transition in La 0.7 Sr 0.3 MnO 3 : Microwave absorption studies

Reentrant magnetism - a low field study

We report a systematic study of the temperature dependences of microwave absorption and low field dc magnetization in powder samples of the superconductor Y 1 Ba 2 Cu 3 O 6.9 . Both measurements indicate a transition temperature near 90 K. The temperature dependence of the diamagnetic susceptibility, as well as its value at T ⪡ T c are close to that of a conventional BCS superconductor.

M. A. Manheimer

Papers

Energy-Efficient Superconducting Computing—Power Budgets and Requirements

Cryogenic Computing Complexity Program: Phase 1 Introduction

Magnetic phase transition in La 0.7 Sr 0.3 MnO 3 : Microwave absorption studies

Reentrant magnetism - a low field study

Initial susceptibility and microwave absorption in powder samples of Y1Ba2Cu3O6.9