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1. An iron ketimide single-molecule magnet [Fe ₄ (NCPh ₂ ) ₆ ] with suppressed through-barrier relaxation

   https://doi.org/10.1039/D0SC01578D  

   Cook, Andrew W.; Bocarsly, Joshua D.; Lewis, Richard A.; Touchton, Alexander J.; Morochnik, Simona; Hayton, Trevor W. (January 2020, Chemical Science)

   Reaction of FeBr 2 with 1.5 equiv. of LiNCPh 2 and 2 equiv. of Zn, in THF, results in the formation of the tetrametallic iron ketimide cluster [Fe 4 (NCPh 2 ) 6 ] ( 1 ) in moderate yield. Formally, two Fe centers in 1 are Fe( i ) and two are Fe( ii ); however, Mössbauer spectroscopy and SQUID magnetometry suggests that the [Fe 4 ] 6+ core of 1 exhibits complete valence electron delocalization, with a thermally-persistent spin ground state of S = 7. AC and DC SQUID magnetometry reveals the presence of slow magnetic relaxation in 1 , indicative of single-molecule magnetic (SMM) behaviour with a relaxation barrier of U eff = 29 cm −1 . Remarkably, very little quantum tunnelling or Raman relaxation is observed down to 1.8 K, which leads to an open hysteresis loop and long relaxation times (up to 34 s at 1.8 K and zero field and 440 s at 1.67 kOe). These results suggest that transition metal ketimide clusters represent a promising avenue to create long-lifetime single molecule magnets. 

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2. Aromatic character of [Au ₁₃ ] ⁵⁺ and [MAu ₁₂ ] ^4+/6+ (M = Pd, Pt) cores in ligand protected gold nanoclusters – interplay between spherical and planar σ-aromatics

   https://doi.org/10.1039/C9CP04477A  

   Fedik, Nikita; Boldyrev, Alexander I.; Muñoz-Castro, Alvaro (November 2019, Physical Chemistry Chemical Physics)

   The most characteristic feature of planar π-aromatics is the ability to sustain a long-range shielding cone under a magnetic field oriented in a specific direction. In this article, we showed that similar magnetic responses can be found in σ-aromatic and spherical aromatic systems. For [Au 13 ] 5+ , long-range characteristics of the induced magnetic field in the bare icosahedral core are revealed, which are also found in the ligand protected [Au 25 (SH) 18 ] − model, proving its spherical aromatic properties, also supported by the AdNDP analysis. Such properties are given by the 8-ve of the structural core satisfying the Hirsch 2( N + 1) 2 rule, which is also found in the isoelectronic [M@Au 12 ] 4+ core, a part of the [MAu 24 (SR) 18 ] 2− (M = Pd, Pt) cluster. This contrasts with the [M@Au 12 ] 6+ core in [MAu 24 (SR) 18 ] 0 (M = Pd, Pt), representing 6-ve superatoms, which exhibit characteristics of planar σ-aromatics. Our results support the spherical aromatic character of stable superatoms, whereas the 6-ve intermediate electron counts satisfy the 4 N + 2 rule (applicable for both π- and σ-aromatics), showing the reversable and controlled interplay between 3D spherical and 2D σ-aromatic clusters. 

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3. Two-dimensional Dirac spin-gapless semiconductors with tunable perpendicular magnetic anisotropy and a robust quantum anomalous Hall effect

   https://doi.org/10.1039/d0mh00396d  

   Sun, Qilong; Ma, Yandong; Kioussis, Nicholas (August 2020, Materials Horizons)

   null (Ed.)

   A major recent breakthrough in materials science is the emergence of intrinsic magnetism in two-dimensional (2D) crystals, which opens the door to more cutting-edge fields in the 2D family and could eventually lead to novel data-storage and information devices with further miniaturization. Herein we propose an experimentally feasible 2D material, Fe 2 I 2 , which is an intrinsic room-temperature ferromagnet exhibiting perpendicular magnetic anisotropy (PMA). Using first-principles calculations, we demonstrate that single-layer (SL) Fe 2 I 2 is a spin-gapless semiconductor with a spin-polarized Dirac cone and linear energy dispersion in one spin channel, exhibiting promising dissipation-less transport properties with a Fermi velocity up to 6.39 × 10 5 m s −1 . Our results reveal that both strain and ferroelectric polarization switching could induce an out-of- to in-plane spin reorientation in the 2D Fe 2 I 2 layer, revealing its advantage in assembling spintronic devices. In addition, spin–orbit coupling (SOC) triggers a topologically nontrivial band gap of 301 meV with a nonzero Chern number (| C | = 2), giving rise to a robust quantum anomalous Hall (QAH) state. The 2D crystal also exhibits high carrier mobilites of 0.452 × 10 3 and 0.201 × 10 3 cm 2 V −1 s −1 for the electrons and holes, respectively. The combination of these unique properties renders the 2D Fe 2 I 2 ferromagnet a promising platform for high efficiency multi-functional spintronic applications. 

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4. Imaging quantum spin Hall edges in monolayer WTe ₂

   https://doi.org/10.1126/sciadv.aat8799  

   Shi, Yanmeng; Kahn, Joshua; Niu, Ben; Fei, Zaiyao; Sun, Bosong; Cai, Xinghan; Francisco, Brian A.; Wu, Di; Shen, Zhi-Xun; Xu, Xiaodong; et al (February 2019, Science Advances)

   A two-dimensional (2D) topological insulator exhibits the quantum spin Hall (QSH) effect, in which topologically protected conducting channels exist at the sample edges. Experimental signatures of the QSH effect have recently been reported in an atomically thin material, monolayer WTe 2 . Here, we directly image the local conductivity of monolayer WTe 2 using microwave impedance microscopy, establishing beyond doubt that conduction is indeed strongly localized to the physical edges at temperatures up to 77 K and above. The edge conductivity shows no gap as a function of gate voltage, and is suppressed by magnetic field as expected. We observe additional conducting features which can be explained by edge states following boundaries between topologically trivial and nontrivial regions. These observations will be critical for interpreting and improving the properties of devices incorporating WTe 2 . Meanwhile, they reveal the robustness of the QSH channels and the potential to engineer them in the monolayer material platform. 

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5. Cu ₄ MnGe ₂ S ₇ and Cu ₂ MnGeS ₄ : two polar thiogermanates exhibiting second harmonic generation in the infrared and structures derived from hexagonal diamond

   https://doi.org/10.1039/D1DT02535J  

   Glenn, Jennifer R.; Cho, Jeong Bin; Wang, Yiqun; Craig, Andrew J.; Zhang, Jian-Han; Cribbs, Marvene; Stoyko, Stanislav S.; Rosello, Kate E.; Barton, Christopher; Bonnoni, Allyson; et al (December 2021, Dalton Transactions)

   The new, quaternary diamond-like semiconductor (DLS) Cu 4 MnGe 2 S 7 was prepared at high-temperature from a stoichiometric reaction of the elements under vacuum. Single crystal X-ray diffraction data were used to solve and refine the structure in the polar space group Cc. Cu 4 MnGe 2 S 7 features [Ge 2 S 7 ] 6− units and adopts the Cu 5 Si 2 S 7 structure type that can be considered a derivative of the hexagonal diamond structure. The DLS Cu 2 MnGeS 4 with the wurtz-stannite structure was similarly prepared at a lower temperature. The achievement of relatively phase-pure samples, confirmed by X-ray powder diffraction data, was nontrival as differential thermal analysis shows an incongruent melting behaviour for both compounds at relatively high temperature. The dark red Cu 2 MnGeS 4 and Cu 4 MnGe 2 S 7 compounds exhibit direct optical bandgaps of 2.21 and 1.98 eV, respectively. The infrared (IR) spectra indicate potentially wide windows of optical transparency up to 25 μm for both materials. Using the Kurtz–Perry powder method, the second-order nonlinear optical susceptibility, χ (2) , values for Cu 2 MnGeS 4 and Cu 4 MnGe 2 S 7 were estimated to be 16.9 ± 2.0 pm V −1 and 2.33 ± 0.86 pm V −1 , respectively, by comparing with an optical-quality standard reference material, AgGaSe 2 (AGSe). Cu 2 MnGeS 4 was found to be phase matchable at λ = 3100 nm, whereas Cu 4 MnGe 2 S 7 was determined to be non-phase matchable at λ = 1600 nm. The weak SHG response of Cu 4 MnGe 2 S 7 precluded phase-matching studies at longer wavelengths. The laser-induced damage threshold (LIDT) for Cu 2 MnGeS 4 was estimated to be ∼0.1 GW cm −2 at λ = 1064 nm (pulse width: τ = 30 ps), while the LIDT for Cu 4 MnGe 2 S 7 could not be ascertained due to its weak response. The significant variance in NLO properties can be reasoned using the results from electronic structure calculations. 

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