Theory of Moiré Magnetism in Twisted Bilayer α‑RuCl₃

Motivated by the recent developments in moiré superlattices of van der Waals magnets and the desire to control the magnetic interactions of α-RuCl3, here we present a comprehensive theory of the long-range ordered magnetic phases of twisted bilayer α-RuCl3. Using a combination of first-principles calculations and atomistic simulations, we show that the stacking-dependent interlayer exchange gives rise to an array of magnetic phases that can be realized by controlling the twist angle. In particular, we discover a complex hexagonal domain structure in which multiple zigzag orders coexist. This multidomain order minimizes the interlayer energy while enduring the energy cost due to domain wall formation. Further, we show that quantum fluctuations can be enhanced across the phase transitions. Our results indicate that magnetic frustration due to stacking-dependent interlayer exchange in moiré superlattices can be exploited to tune quantum fluctuations and the magnetic ground state of α-RuCl3

Copper Vacancies and Heavy Holes in the Two-Dimensional Semiconductor KCu_3–<i>x</i>Se₂

The two-dimensional material KCu_3–<i>x</i>Se₂ was synthesized using both a K₂Se₃ flux and directly from the elements. It crystallizes in the CsAg₃S₂ structure (monoclinic space group C2/<i>m</i> with <i>a</i> = 15.417(3) Å, <i>b</i> = 4.0742(8) Å, <i>c</i> = 8.3190(17) Å, and β = 112.94(3)°), and single-crystal refinement revealed infinite copper-deficient [Cu_3–<i>x</i>Se₂]⁻ layers separated by K⁺ ions. Thermal analysis indicated that KCu_3–<i>x</i>Se₂ melts congruently at ∼755 °C. UV–vis spectroscopy showed an optical band gap of ∼1.35 eV that is direct in nature, as confirmed by electronic structure calculations. Electronic transport measurements on single crystals yielded an in-plane resistivity of ∼6 × 10^–1 Ω cm at 300 K that has a complex temperature dependence. The results of Seebeck coefficient measurements were consistent with a doped p-type semiconductor (<i>S</i> = +214 μV K^–1 at 300 K), with doping being attributed to copper vacancies. Transport is dominated by low-mobility (on the order of 1 cm² V^–1 s^–1) holes caused by relatively flat valence bands with substantial Cu 3d character and a significant concentration of Cu ion vacancy defects (<i>p</i> ∼ 10¹⁹ cm^–3) in this material. Electronic band structure calculations showed that electrons should be significantly more mobile in this structure type

Ag₂Se to KAg₃Se₂: Suppressing Order–Disorder Transitions via Reduced Dimensionality

We report an order–disorder phase transition in the 2D semiconductor KAg₃Se₂, which is a dimensionally reduced derivative of 3D Ag₂Se. At ∼695 K, the room temperature β-phase (CsAg₃S₂ structure type, monoclinic space group C2/<i>m</i>) transforms to the high temperature α-phase (new structure type, hexagonal space group <i>R</i>3̅<i>m</i>, <i>a</i> = 4.5638(5) Å, <i>c</i> = 25.4109(6) Å), as revealed by in situ temperature-dependent X-ray diffraction. Significant Ag⁺ ion disorder accompanies the phase transition, which resembles the low temperature (∼400 K) superionic transition in the 3D parent compound. Ultralow thermal conductivity of ∼0.4 W m^–1 K^–1 was measured in the “ordered” β-phase, suggesting anharmonic Ag motion efficiently impedes phonon transport even without extensive disordering. The optical and electronic properties of β-KAg₃Se₂ are modified as expected in the context of the dimensional reduction framework. UV–vis spectroscopy shows an optical band gap of ∼1 eV that is indirect in nature as confirmed by electronic structure calculations. Electronic transport measurements on β-KAg₃Se₂ yielded <i>n</i>-type behavior with a high electron mobility of ∼400 cm² V^–1 s^–1 at 300 K due to a highly disperse conduction band. Our results thus imply that dimensional reduction may be used as a design strategy to frustrate order–disorder phenomena while retaining desirable electronic and thermal properties

Ag₂Se to KAg₃Se₂: Suppressing Order–Disorder Transitions via Reduced Dimensionality

We report an order–disorder phase transition in the 2D semiconductor KAg₃Se₂, which is a dimensionally reduced derivative of 3D Ag₂Se. At ∼695 K, the room temperature β-phase (CsAg₃S₂ structure type, monoclinic space group C2/<i>m</i>) transforms to the high temperature α-phase (new structure type, hexagonal space group <i>R</i>3̅<i>m</i>, <i>a</i> = 4.5638(5) Å, <i>c</i> = 25.4109(6) Å), as revealed by in situ temperature-dependent X-ray diffraction. Significant Ag⁺ ion disorder accompanies the phase transition, which resembles the low temperature (∼400 K) superionic transition in the 3D parent compound. Ultralow thermal conductivity of ∼0.4 W m^–1 K^–1 was measured in the “ordered” β-phase, suggesting anharmonic Ag motion efficiently impedes phonon transport even without extensive disordering. The optical and electronic properties of β-KAg₃Se₂ are modified as expected in the context of the dimensional reduction framework. UV–vis spectroscopy shows an optical band gap of ∼1 eV that is indirect in nature as confirmed by electronic structure calculations. Electronic transport measurements on β-KAg₃Se₂ yielded <i>n</i>-type behavior with a high electron mobility of ∼400 cm² V^–1 s^–1 at 300 K due to a highly disperse conduction band. Our results thus imply that dimensional reduction may be used as a design strategy to frustrate order–disorder phenomena while retaining desirable electronic and thermal properties