Efimov effect in quantum magnets

Physics is said to be universal when it emerges regardless of the underlying microscopic details. A prominent example is the Efimov effect, which predicts the emergence of an infinite tower of three-body bound states obeying discrete scale invariance when the particles interact resonantly. Because of its universality and peculiarity, the Efimov effect has been the subject of extensive research in chemical, atomic, nuclear and particle physics for decades. Here we employ an anisotropic Heisenberg model to show that collective excitations in quantum magnets (magnons) also exhibit the Efimov effect. We locate anisotropy-induced two-magnon resonances, compute binding energies of three magnons and find that they fit into the universal scaling law. We propose several approaches to experimentally realize the Efimov effect in quantum magnets, where the emergent Efimov states of magnons can be observed with commonly used spectroscopic measurements. Our study thus opens up new avenues for universal few-body physics in condensed matter systems.Comment: 7 pages, 5 figures; published versio

mu+SR as a probe of anisotropy in low-dimensional molecular magnets

We describe how muon-spin rotation (mu+SR) can be particularly effective in determining the onset temperature of 3D ordering in low-dimensional molecular magnets, even when the low dimensionality means that this transition is masked in data from bulk thermodynamic probes such as heat capacity measurements. We illustrate the use of mu+SR with various copper-based magnets, including copper pyrazine dinitrate, Cu(C4H4N2)(NO3)(2), which orders below 0. 107 K, and also for the organic radical-ion salt DEOCC-TCNQF(4) which appears to be one of the most ideal examples of 1D S = 1/2 Heisenberg anti ferromagnets yet discovered. (c) 2007 Elsevier Ltd. All rights reserved

Magnetic order in the S=1/2 two-dimensional molecular antiferromagnet copper pyrazine perchlorate Cu(Pz)(2)(ClO4)(2)

We present an investigation of magnetic ordering in the two-dimensional S= 2 quantum magnet Cu (Pz)2 (Cl O4) 2 using specific heat and zero-field muon-spin relaxation (μ+ SR). The magnetic contribution to the specific heat is consistent with an exchange strength of 17.7 (3) K. We find unambiguous evidence for a transition to a state of three-dimensional long-range order below a critical temperature TN =4.21 (1) K using μ+ SR even though there is no feature in the specific heat at that temperature. The absence of a specific heat anomaly at TN is consistent with recent theoretical predictions. The ratio of TN J=0.24 corresponds to a ratio of intralayer to interlayer exchange constants of J′ J=6.8× 10-4, indicative of excellent two-dimensional isolation. The scaled magnetic specific heat of [Cu (Pz)2 (H F2)] B F4, a compound with an analogous structure, is very similar to that of Cu (Pz)2 (Cl O4) 2 although both differ slightly from the predicted value for an ideal 2D S= 2 Heisenberg antiferromagnet. © 2007 The American Physical Society

Importance of Halogen···Halogen Contacts for the Structural and Magnetic Properties of CuX ₂ (pyrazine- <i>N</i> , <i>N</i> '-dioxide)(H ₂ O) ₂ (X = Cl and Br)

The structural and magnetic properties of the newly crystallized CuX(2)(pyzO)(H(2)O)(2) (X = Cl, Br; pyzO = pyrazine-N,N'-dioxide) coordination polymers are reported. These isostructural compounds crystallize in the monoclinic space group C2/c with, at 150 K, a = 17.0515(7) Å, b = 5.5560(2) Å, c = 10.4254(5) Å, β = 115.400(2)°, and V = 892.21(7) Å(3) for X = Cl and a = 17.3457(8) Å, b = 5.6766(3) Å, c = 10.6979(5) Å, β = 115.593(2)°, and V = 950.01(8) Å(3) for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu(2+) ions linked through bidentate pyzO ligands. These chains are joined together through OH···O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu-X···X-Cu contacts. Bulk magnetic susceptibility measurements at ambient pressure show a broad maximum at 7 (Cl) and 28 K (Br) that is indicative of short-range magnetic correlations. The dominant spin exchange is the Cu-X···X-Cu supersuperexchange because the magnetic orbital of the Cu(2+) ion is contained in the CuX(2)(H(2)O)(2) plane and the X···X contact distances are short. The magnetic data were fitted to a Heisenberg 1D uniform antiferromagnetic chain model with J(1D)/k(B) = -11.1(1) (Cl) and -45.9(1) K (Br). Magnetization saturates at fields of 16.1(3) (Cl) and 66.7(5) T (Br), from which J(1D) is determined to be -11.5(2) (Cl) and -46.4(5) K (Br). For the Br analog the pressure dependence of the magnetic susceptibility indicates a gradual increase in the magnitude of J(1D)/k(B) up to -51.2 K at 0.84 GPa, suggesting a shortening of the Br···Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at ∼3.5 GPa. Muon-spin relaxation measurements indicate that CuCl(2)(pyzO)(H(2)O)(2) is magnetically ordered with T(N) = 1.06(1) K, while the signature for long-range magnetic order in CuBr(2)(pyzO)(H(2)O)(2) was much less definitive down to 0.26 K. The results for the CuX(2)(pyzO)(H(2)O)(2) complexes are compared to the related CuX(2)(pyrazine) materials