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Register a new user2D XY Behavior observed in quasi-2D quantum Heisenberg antiferromagnets
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The magnetic properties of a new family of molecular-based quasi-two dimension $S=1/2$ Heisenberg antiferromagnets are reported. Three compounds, ($Cu(pz)_2(ClO_4)_2$, $Cu(pz)_2(BF_4)_2$, and $[Cu(pz)_2(NO_3)](PF_6)$) contain similar planes of Cu$^{2+}$ ions linked into magnetically square lattices by bridging pyrazine molecules (pz = $C_4H_4N_2$). The anions provide charge balance as well as isolation between the layers. Single crystal measurements of susceptibility and magnetization, as well as muon spin relaxation studies, reveal low ratios of N{e}el temperatures to exchange strengths ($4.25 / 17.5 = 0.243$, $3.80/15.3=0.248$, and $3.05/10.8=0.282$, respectively) while the ratio of the anisotropy fields $H_A$ (kOe) to the saturation field $H_mathrm{SAT}$ (kOe) are small ($2.6/490 = 5.3times10^{-3}$, $2.4/430=5.5times10^{-3}$, and $0.07/300=2.3times10^{-4}$, respectively), demonstrating close approximations to a 2D Heisenberg model. The susceptibilities of ClO$_4$ and BF$_4$ show evidence of an exchange anisotropy crossover (Heisenberg to $XY$) at low temperatures; their ordering transitions are primarily driven by the $XY$ behavior with the ultimate 3D transition appearing parasitically. The PF$_6$ compound remains Heisenberg-like at all temperatures, with its transition to the N{e}el state due to the interlayer interactions. Effects of field-induced anisotropy have been observed.
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Cu(pz)$_2$(ClO$_4$)$_2$ (with pz denoting pyrazine C$_4$H$_4$N$_2$) is a two-dimensional spin-1/2 square-lattice antiferromagnet with $T_{mathrm{N}}$ = 4.24 K. Due to a persisting focus on the low-temperature magnetic properties, its room-temperature structural and physical properties caught no attention up to now. Here we report a study of the structural features of Cu(pz)$_2$(ClO$_4$)$_2$ in the paramagnetic phase, up to 330 K. By employing magnetization, specific heat, $^{35}$Cl nuclear magnetic resonance, and neutron diffraction measurements, we provide evidence of a second-order phase transition at $T^{star}$ = 294 K, not reported before. The absence of a magnetic ordering across $T^{star}$ in the magnetization data, yet the presence of a sizable anomaly in the specific heat, suggest a structural order-to-disorder type transition. NMR and neutron-diffraction data corroborate our conjecture, by revealing subtle angular distortions of the pyrazine rings and of ClO$^-_4$ counteranion tetrahedra, shown to adopt a configuration of higher symmetry above the transition temperature.
High field specific heat, Cp, and magnetic susceptibility, c{hi}, measurements were performed on the quasi-two dimensional Heisenberg antiferromagnet [Cu(pyz)2(pyO)2](PF6)2. While no Cp anomaly is observed down to 0.5 K in zero magnetic field, the application of field parallel to the crystallographic ab-plane induces a lambda-like anomaly in Cp, consistent with Ising-type magnetic order. On the other hand, when the field is parallel to the c-axis, Cp and c{hi} show evidence of XY-type antiferromagnetism. We argue that it is a small but finite easy-plane anisotropy in quasi-two dimensional [Cu(pyz)2(pyO)2](PF6)2 that allows the unusual observation of field induced XY and Ising-type magnetic states.
Quantum spin liquids (QSLs) form an extremely unusual magnetic state in which the spins are highly correlated and fluctuate coherently down to the lowest temperatures, but without symmetry breaking and without the formation of any static long-range-ordered magnetism. Such intriguing phenomena are not only of great fundamental relevance in themselves, but also hold the promise for quantum computing and quantum information. Among different types of QSLs, the exactly solvable Kitaev model is attracting much attention, with most proposed candidate materials, e.g., RuCl$_3$ and Na$_2$IrO$_3$, having an effective $S$=1/2 spin value. Here, via extensive first-principle-based simulations, we report the investigation of the Kitaev physics and possible Kitaev QSL state in epitaxially strained Cr-based monolayers, such as CrSiTe$_3$, that rather possess a $S$=3/2 spin value. Our study thus extends the playground of Kitaev physics and QSLs to 3$d$ transition metal compounds.
We map the problem of the orbital excitation (orbiton) in a 2D antiferromagnetic and ferroorbital ground state onto a problem of a hole in 2D antiferromagnet. The orbiton turns out to be coupled to magnons and can only be mobile on a strongly renormalized scale by dressing with magnetic excitations. We show that this leads to a dispersion relation reflecting the two-site unit cell of the antiferromagnetic background, in contrast to the predictions based on a mean-field approximation and linear orbital-wave theory.
I present results from an extended Migdal-Eliashberg theory of electron-phonon interactions and superconductivity. The history of the electron-phonon problem is introduced, and then study of the intermediate parameter regime is justified from the energy scales in the cuprate superconductors. The Holstein model is detailed, and limiting cases are examined to demonstrate the need for an extended theory of superconductivity. Results of the extended approximation are shown, including spectral functions and phase diagrams. These are discussed with reference to Hohenbergs theorem, the Bardeen-Cooper-Schrieffer theory and Coulomb repulsion.
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