The natural mineral azurite Cu(3)(CO(3))(2)(OH)(2) is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. Motivated by the lack of a unified description for this system, we perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material.

We present a specific heat and inelastic neutron scattering study in magnetic fields up into the 1/3 magnetization plateau phase of the diamond chain compound azurite Cu3(CO3)2(OH)2. We establish that the magnetization plateau is a dimer-monomer state, i.e., consisting of a chain of S = 1/2 monomers, which are separated by S = 0 dimers on the diamond chain backbone. The effective spin couplings Jmono/kB = 10.1(2) K and J dimer /kB = 1.8(1) K are derived from the monomer and dimer dispersions. They are associated to microscopic couplings J1/kB = 1(2) K, J2/kB = 55(5) K and a ferromagnetic J3/kB = −20(5) K, possibly as result of d z 2 orbitals in the Cu-O bonds providing the superexchange pathways.PACS numbers: 75.30. Et, 75.10.Pq, 75.45.+j Great interest has surrounded the observation of a 1/3 magnetization plateau in azurite CuThis material, famous as a painting pigment of deepblue colour, has been proposed as a realisation of the exotic diamond-chain Hamiltonian of coupled spin-1/2 moments, written aŝHere, J 2 is the magnetic coupling of the diamond backbone, while J 1 and J 3 represent the coupling of the monomers along the chain [3, 4, 5] (Fig. 3). Depending on the relative coupling strengths J 1 , J 2 , J 3 , this model affords a host of exotic phases and quantum phase transitions, including possibly M = 1/3 fractionalisation [6] or exotic dimer phases [4]. However, determining the magnetic exchange couplings in azurite has proved difficult, yielding controversial results. While a susceptibility χ study claims, subsequent numerical studies of χ dispute this claim, proposing a ferromagnetic (FM) J 3 , and thus a non-frustrated scenario [2]. The general issue underlying these starkly contrasting interpretations of the same experimental data is that of the nature of magnetic coupling in low-dimensional (low-D) quantum magnets. In azurite Cu 3 (CO 3 ) 2 (OH) 2 , the Cu 2+ ions (S = 1/2) are in a square-planar coordination on two inequivalent sites [7]. The system has a monoclinic crystal structure (space group P2 1 /c, lattice parameters a = 5.

Discontinuous changes of the lattice parameters at the Mott metal-insulator transition are detected by high-resolution dilatometry on deuterated crystals of the layered organic conductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. The uniaxial expansivities uncover a striking and unexpected anisotropy, notably a zero effect along the in-plane c axis along which the electronic interactions are relatively strong. A huge thermal expansion anomaly is observed near the end point of the first-order transition line enabling us to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent alpha approximately 0.8+/-0.15 at odds with the novel criticality recently proposed for these materials [Kagawa et al., Nature (London) 436, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.

The presence of a quantum-critical point (QCP) can significantly affect the thermodynamic properties of a material at finite temperatures T . This is reflected, e.g., in the entropy landscape SðT,rÞ in the vicinity of a QCP, yielding particularly strong variations for varying the tuning parameter r such as pressure or magnetic field B. Here we report on the determination of the critical enhancement of ∂S∕∂B near a B-induced QCP via absolute measurements of the magnetocaloric effect (MCE), ð∂T ∕∂BÞ S and demonstrate that the accumulation of entropy around the QCP can be used for efficient low-temperature magnetic cooling. Our proof of principle is based on measurements and theoretical calculations of the MCE and the cooling performance for a Cu 2þ -containing coordination polymer, which is a very good realization of a spin-½ antiferromagnetic Heisenberg chain-one of the simplest quantum-critical systems.quantum criticality | quantum magnetism | low-dimensional spin systems | magnetothermal effect T he magnetocaloric effect (MCE), i.e., a temperature change in response to an adiabatic change of the magnetic field, has been widely used for refrigeration. Although up until now applications have focused on cryogenic temperatures (1-3), possible extensions to room temperature have been discussed (4). The MCE is an intrinsic property of all magnetic materials in which the entropy S changes with magnetic field B. Paramagnetic salts have been the materials of choice for low-temperature refrigeration (1), including space applications (5-7), with an area of operation ranging from about one or two degrees Kelvin down to some hundredths or even thousandths degree Kelvin. Owing to their large ΔS∕ΔB values, the ease of operation, and the applicability under microgravity conditions, paramagnets have matured to a valuable alternative to 3 He-4 He dilution refrigerators, the standard cooling technology for reaching sub-Kelvin temperatures.A large MCE also characterizes a distinctly different class of materials, where the low-temperature properties are governed by pronounced quantum many-body effects. These materials exhibit a B-induced quantum-critical point (QCP)-a zero-temperature phase transition-and the MCE has been used to study their quantum criticality (8)(9)(10)(11)(12)(13)(14) or to determine their B-T phase diagrams (15)(16)(17)(18)(19). The aim of the present work is to provide an accurate determination of the enhanced MCE upon approaching a B-induced QCP both as a function of B and T and to explore the potential of this effect for magnetic cooling.Materials in the vicinity of a QCP have been of particular current interest, as their properties reflect critical behavior arising from quantum fluctuations instead of thermal fluctuations that govern classical critical points (20). Prominent examples of findings made here include the intriguing low-temperature behaviors encountered in some heavy-fermion metals, itinerant transition metal magnets (21 and references cited therein, 22), or magnetic insulators (23, 24) and the ...

Strain-order-parameter coupling and phase diagrams in superconducting UPt3Bruls, G.; Weber, D.M.; Wolf, B.; Thalmeier, P.; Lüthi, B.; Visser, A. de; Menovsky, A.

We investigate the structural, electronic and magnetic properties of the newly synthesized mineral barlowite Cu4(OH)6FBr which contains Cu 2+ ions in a perfect kagome arrangement. In contrast to the spin-liquid candidate herbertsmithite ZnCu3(OH)6Cl2, kagome layers in barlowite are perfectly aligned due to the different bonding environments adopted by F − and Br − compared to Cl − . We perform density functional theory calculations to obtain the Heisenberg Hamiltonian parameters of Cu4(OH)6FBr which has a Cu 2+ site coupling the kagome layers. The 3D network of exchange couplings together with a substantial Dzyaloshinskii-Moriya coupling lead to canted antiferromagnetic ordering of this compound at TN = 15 K as observed by magnetic susceptibility measurements on single crystals.

A highly modular synthesis of BNB- and BOB-doped phenalenyls is presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid anhydride 1 with LiAlH4/Me3SiCl afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent transformations. Upon addition of MesMgBr/Me3SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through the reaction of 2 with (Me3Si)2NR′ or 5 with H2NR′ or H2O (R′ = H, Me, p-Tol). A helically twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed by a Grignard-mediated double ring-closure reaction. All compounds devoid of B–H bonds show favorable optoelectronic properties, such as luminescence and reversible reduction behavior. In the case of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion salt K­[7 •] was generated through chemical reduction with K metal and characterized by EPR spectroscopy. K­[7 •] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation of the diamagnetic BNB-doped 1H-phenalene K­[7H].

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