The first family of rare-earth-based single chain magnets is presented. Compounds of general formula [M(hfac)3(NITPhOPh)], where M = Eu, Gd, Tb, Dy, Ho, Er, or Yb, and PhOPh is the nitronyl-nitroxide radical (2,4'-benzoxo-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), have been structurally characterized and found to be isostructural. The characterization of both static and dynamic magnetic properties of the whole family is reported. Dy, Tb, and Ho compounds display slow relaxation of the magnetization, and ac susceptibility shows a thermally activated regime with energy barriers of 69, 45, and 34 K for Dy, Tb, and Ho compounds, respectively, while only a frequency-dependent susceptibility is observed for Er below 2.0 K. In Gd and Yb derivatives, antiferromagnetic interactions dominate. The pre-exponential factors differ by about 4 orders of magnitude. Finite size effects, due to naturally occurring defects, affect the static and dynamic properties of the compounds differently.

A Family of Rare-Earth-Based Single Chain Magnets: Playing with Anisotropy

Describing all aspects of the physics of transition metal compounds, this book provides a comprehensive overview of this unique and diverse class of solids. Beginning with the basic concepts of the physics of strongly correlated electron systems, the structure of transition metal ions, and the behaviours of transition metal ions in crystals, it goes on to cover more advanced topics such as metal-insulator transitions, orbital ordering, and novel phenomena such as multiferroics, systems with oxygen holes, and high-Tc superconductivity. Each chapter concludes with a summary of key facts and concepts, presenting all the most important information in a consistent and concise manner. Set within a modern conceptual framework, and providing a complete treatment of the fundamental factors and mechanisms that determine the properties of transition metal compounds, this is an invaluable resource for graduate students, researchers and industrial practitioners in solid state physics and chemistry, materials science, and inorganic chemistry.

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Transition Metal Compounds

The effect of adding high pressure as a control parameter in solids, liquids, and gases expands opportunities to observe unexpected novel phenomena and understand matter in extreme environments. This review on high pressure science highlights subjects ranging from quantum criticality to Earth science. State-of-the-art experimental methods at megabar pressures are also discussed. The proliferation of pressure-induced phases illustrate promising new directions for this field of research.

https://link.aps.org/accepted/10.1103/RevModPhys.90.015007

Solids, liquids, and gases under high pressure

Compass models are theories of matter in which the couplings between the internal spin (or other relevant field) components are inherently spatially (typically, direction) dependent. A simple illustrative example is furnished by the 90\ifmmode^\circ\else\textdegree\fi{} compass model on a square lattice in which only couplings of the form ${\ensuremath{\tau}}_{i}^{x}{\ensuremath{\tau}}_{j}^{x}$ (where ${{\ensuremath{\tau}}_{i}^{a}{}}_{a}$ denote Pauli operators at site $i$) are associated with nearest-neighbor sites $i$ and $j$ separated along the $x$ axis of the lattice while ${\ensuremath{\tau}}_{i}^{y}{\ensuremath{\tau}}_{j}^{y}$ couplings appear for sites separated by a lattice constant along the $y$ axis. Similar compass-type interactions can appear in diverse physical systems. For instance, compass models describe Mott insulators with orbital degrees of freedom where interactions sensitively depend on the spatial orientation of the orbitals involved as well as the low-energy effective theories of frustrated quantum magnets, and a host of other systems such as vacancy centers, and cold atomic gases. The fundamental interdependence between internal (spin, orbital, or other) and external (i.e., spatial) degrees of freedom which underlies compass models generally leads to very rich behaviors, including the frustration of (semi-)classical ordered states on nonfrustrated lattices, and to enhanced quantum effects, prompting, in certain cases, the appearance of zero-temperature quantum spin liquids. As a consequence of these frustrations, new types of symmetries and their associated degeneracies may appear. These intermediate symmetries lie midway between the extremes of global symmetries and local gauge symmetries and lead to effective dimensional reductions. In this article, compass models are reviewed in a unified manner, paying close attention to exact consequences of these symmetries and to thermal and quantum fluctuations that stabilize orders via order-out-of-disorder effects. This is complemented by a survey of numerical results. In addition to reviewing past works, a number of other models are introduced and new results established. In particular, a general link between flat bands and symmetries is detailed.

https://link.aps.org/accepted/10.1103/RevModPhys.87.1

Compass models: Theory and physical motivations

The magnetic structure and magnetization dynamics of systems of plane frustrated Ising chain lattices are reviewed for three groups of compounds: , , and . The available experimental data are analyzed and compared in detail. It is shown that a high-temperature magnetic phase on a triangle lattice is normally and universally a partially disordered antiferromagnetic (PDA) structure. The diversity of low-temperature phases results from weak interactions that lift the degeneracy of a 2D antiferromagnetic Ising model on the triangle lattice. Mean-field models, Monte Carlo simulation results on the static magnetization curve, and results on slow magnetization dynamics obtained with Glauber's theory are discussed in detail.

Frustrated lattices of Ising chains

The magnetic behavior of the Ca3Co2O6 spin chain compound is characterized by a large Ising-like character of its ferromagnetic chains, set on a triangular lattice, that are antiferromagnetically coupled. At low temperature, T < 7 K, the 3D antiferromagnetic state evolves towards a spin frozen state. In this temperature range, magnetic field driven magnetization of single crystals (H
				// chains) exhibits stepped variations. The occurrence of these steps at regular intervals of the applied magnetic field, Hstep
				= 1.2 T, is reminiscent of the quantum tunneling of the magnetization (QTM) of molecular based magnets. Magnetization relaxation experiments also strongly support the occurrence of this quantum phenomenon. This first observation of QTM in a magnetic oxide belonging to the large family of A3BB′O6 compounds opens new opportunities to study a quantum effect in a very different class of materials from molecular magnets.

Quantum tunneling of the magnetization in the Ising chain compound Ca3Co2O6

1. Some basic notions of the classical and quantum statistical physics 2. General theory of phase transitions 3. Bose- and Fermi-statistics 4. Phonons in crystals 5. General Bose-systems: Bose-condensation 6. Magnetism 7. Electrons in metals 8. Interacting electrons: Green functions and Feynman diagrams (methods of the field theory in many-particle physics) 9. Electrons with Coulomb interaction 10. Fermi-liquid theory and its possible generalizations 11. Instabilities and phase transitions in electronic systems 12. Strongly correlated electrons 13. Magnetic impurities in metals, Kondo effect, heavy fermions and mixed valence References Index.

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Basic Aspects of the Quantum Theory of Solids: Order and Elementary Excitations

We discuss the ground state of the spin-orbital model for spin-one ions with partially filled ${t}_{2g}$ levels on a honeycomb lattice We find that the orbital degrees of freedom induce a spontaneous dimerization of spins and drive them into nonmagnetic manifold spanned by hard-core dimer (spin-singlet) coverings of the lattice The cooperative ``dimer Jahn-Teller'' effect is introduced through a magnetoelastic coupling and is shown to lift the orientational degeneracy of dimers leading to a peculiar valence bond crystal pattern The present theory provides a theoretical explanation of nonmagnetic dimerized superstructure experimentally seen in ${\mathrm{Li}}_{2}{\mathrm{RuO}}_{3}$ compound at low temperatures

Classical Dimers and Dimerized Superstructure in an Orbitally Degenerate Honeycomb Antiferromagnet

Basic Aspects of the Quantum Theory of Solids

When an electronic system has strong correlations and a large spin-orbit interaction, it often exhibits a plethora of mutually competing quantum phases. How a particular quantum ground state is selected out of several possibilities is a very interesting question. However, equally fascinating is how such a quantum entangled state breaks up due to perturbation. This important question has relevance in very diverse fields of science from strongly correlated electron physics to quantum information. Here we report that a quantum entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows nontrivial doping dependence as we perturb the Ru honeycomb lattice by replacing Ru with Li. Through extensive experimental studies, we demonstrate that the VBC phase melts into a valence bond liquid phase of the RVB (resonating valence bond) type. This system offers an interesting playground where one can test and refine our current understanding of the quantum competing phases in a single compound.

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Daniel I. Khomskii

Papers

Quantum tunneling of the magnetization in the Ising chain compound Ca3Co2O6

Basic Aspects of the Quantum Theory of Solids: Order and Elementary Excitations

Classical Dimers and Dimerized Superstructure in an Orbitally Degenerate Honeycomb Antiferromagnet

Basic Aspects of the Quantum Theory of Solids

Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of Li2RuO3