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1. Depletion-driven antiferromagnetic, paramagnetic, and ferromagnetic behavior in quasi-two-dimensional buckled colloidal solids

   https://doi.org/10.1063/5.0146155  

   Hill, Analisa; Tanaka, Michio; Aptowicz, Kevin B.; Mishra, Chandan K.; Yodh, A. G.; Ma, Xiaoguang (May 2023, The Journal of Chemical Physics)

   We investigate quasi-two-dimensional buckled colloidal monolayers on a triangular lattice with tunable depletion interactions. Without depletion attraction, the experimental system provides a colloidal analog of the well-known geometrically frustrated Ising antiferromagnet [Y. Han et al., Nature 456, 898–903 (2008)]. In this contribution, we show that the added depletion attraction can influence both the magnitude and sign of an Ising spin coupling constant. As a result, the nearest-neighbor Ising “spin” interactions can be made to vary from antiferromagnetic to para- and ferromagnetic. Using a simple theory, we compute an effective Ising nearest-neighbor coupling constant, and we show how competition between entropic effects permits for the modification of the coupling constant. We then experimentally demonstrate depletion-induced modification of the coupling constant, including its sign, and other behaviors. Depletion interactions are induced by rod-like surfactant micelles that change length with temperature and thus offer means for tuning the depletion attraction in situ. Buckled colloidal suspensions exhibit a crossover from an Ising antiferromagnetic to paramagnetic phase as a function of increasing depletion attraction. Additional dynamical experiments reveal structural arrest in various regimes of the coupling-constant, driven by different mechanisms. In total, this work introduces novel colloidal matter with “magnetic” features and complex dynamics rarely observed in traditional spin systems. 

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2. Ferromagnetism in 2D organic iron hemoglobin crystals based on nitrogenated conjugated micropore materials

   https://doi.org/10.1039/c9cp04509k  

   Pimachev, Artem; Nielsen, Robert D.; Karanovich, Anri; Dahnovsky, Yuri (November 2019, Physical Chemistry Chemical Physics)

   In this work we study a low-cost two-dimensional ferromagnetic semiconductor with possible applications in biomedicine, solar cells, spintronics, and energy and hydrogen storage. From first principle calculations we describe the unique electronic, transport, optical, and magnetic properties of a π-conjugated micropore polymer (CMP) with three iron atoms placed in the middle of an isolated pore locally resembling heme complexes. This material exhibits strong Fe-localized d z2 bands. The bandgap is direct and equal to 0.28 eV. The valence band is doubly degenerate at the Γ -point and for larger k -wavevectors the HOMO band becomes flat with low contribution to charge mobility. The absorption coefficient is roughly isotropic. The conductivity is also isotropic with the nonzero contribution in the energy range 0.3–8 eV. The xy -component of the imaginary part of the dielectric tensor determines the magneto-optical Faraday and Kerr rotation. Nonvanishing rotation is observed in the interval of 0.5–5.0 eV. This material is found to be a ferromagnet of an Ising type with long-range exchange interactions with a very high magnetic moment per unit cell, m = 6 μ B . The exchange integral is calculated by two independent methods: (a) from the energy difference between ferromagnetic and antiferromagnetic states and (b) from a magnon dispersion curve. In the former case J nn = 27 μeV. In the latter case the magnon dispersion is fitted by the Ising model with the nearest and next-nearest neighbor spin interactions. From these estimations we find that J nn = 19.5 μeV and J nnn = −3 μeV. Despite the different nature of the calculations, the exchange integrals are only within 28% difference. 

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3. The disordered and correlated insulator Bi2CrAl3O9

   https://doi.org/https://doi-org.ezproxy.library.ubc.ca/10.1103/PhysRevB.100.104425  

   Umana, J. C.; Baccarella, A. M.; Steinke, L; Geritano, A; Janssen, Y.; Aronson, M. C; Simonson, J. W. (September 2019, PRB)

   The 3d transition metal insulator Bi2CrAl3O9 forms with a quasi-one-dimensional structure characterized by linear chains of edge-sharing, Cr-and Al-centered, distorted octahedra. The UV/Vis spectrum of high-quality single crystals is marked by broad absorption edges corresponding to direct transitions across a 1.36-eV insulating gap. Measurements of dc magnetic susceptibility χ reveal a fluctuating moment of 2.60±0.01μB/Cr—reduced from the 3.87μB/Cr expected for Cr3+, while the Weiss temperature ΘW=−21±1 K implies that the prevailing local moment interactions are weakly antiferromagnetic in nature. Some 10% of the fluctuating moment is quenched, presumably due to the onset of an antiferromagnetic or spin glass phase at temperature T★=98±3 K, while measurements of magnetization versus field H at T≤10 K scale as H/T0.68(4), suggesting the presence of quantum fluctuations associated with a disordered phase. Density functional theory calculations carried out within the generalized gradient approximation are in excellent agreement with experimental results, asserting that short-range magnetic interactions remnant above T★ stabilize the insulating state 

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4. Quantum phases in the honeycomb-lattice $J_1$ – $J_3$ ferro-antiferromagnetic model

   https://doi.org/10.1103/PhysRevB.108.L180406  

   Jiang, Shengtao; White, Steven R; Chernyshev, A L (November 2023, Physical Review B)

   Using large-scale density-matrix renormalzation group calculations and minimally augmented spin-wave theory, we demonstrate that the phase diagram of the quantum 𝑆=1/2 𝐽1–𝐽3 ferro-antiferromagnetic model on the honeycomb lattice differs dramatically from the classical one. It hosts the double-zigzag and Ising-𝑧 phases as unexpected intermediaries between ferromagnetic and zigzag states that are also extended beyond their classical regions of stability. In broad agreement with quantum order-by-disorder arguments, these collinear phases replace the classical spiral state. 

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5. Randomizing of Oligopeptide Conformations by Nearest Neighbor Interactions between Amino Acid Residues

   https://doi.org/10.3390/biom12050684  

   Schweitzer-Stenner, Reinhard; Milorey, Bridget; Schwalbe, Harald (May 2022, Biomolecules)

   Flory’s random coil model assumes that conformational fluctuations of amino acid residues in unfolded poly(oligo)peptides and proteins are uncorrelated (isolated pair hypothesis, IPH). This implies that conformational energies, entropies and solvation free energies are all additive. Nearly 25 years ago, analyses of coil libraries cast some doubt on this notion, in that they revealed that aromatic, but also β-branched side chains, could change the 3J(HNHCα) coupling of their neighbors. Since then, multiple bioinformatical, computational and experimental studies have revealed that conformational propensities of amino acids in unfolded peptides and proteins depend on their nearest neighbors. We used recently reported and newly obtained Ramachandran plots of tetra- and pentapeptides with non-terminal homo- and heterosequences of amino acid residues to quantitatively determine nearest neighbor coupling between them with a Ising type model. Results reveal that, depending on the choice of amino acid residue pairs, nearest neighbor interactions either stabilize or destabilize pairs of polyproline II and β-strand conformations. This leads to a redistribution of population between these conformations and a reduction in conformational entropy. Interactions between residues in polyproline II and turn(helix)-forming conformations seem to be cooperative in most cases, but the respective interaction parameters are subject to large statistical errors. 

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