Analogous to the keen interest in electron, hole, and exciton spin relaxation during the early days of semiconductor spintronics, measurements of valley relaxation in monolayer transition-metal dichalcogenide (TMD) semiconductors such as WSe2 are currently a focus of attention for potential applications in valleytronics. For many notional valleytronic devices, the important parameter is the intrinsic valley relaxation time of the resident electrons and holes that exist in n-type and p-type TMD monolayers. Using optical methods, the authors determine these timescales as a systematic function of carrier density, and study the (important) role of the underlying substrate. Microsecond-long valley relaxation of carriers is revealed at low densities.
Jing Li et al.
Phys. Rev. Materials 5, 044001 (2021)
The metal-insulator transition in NdNiO macroscopically manifests close-lying energy scales of lattice and electronic degrees of freedom. Hence, epitaxial heterostructures offer fascinating possibilities to manipulate these degrees of freedom. Here, the authors show that the metal-insulator transition in NdNiO epitaxial thin films grown on different facets of the same orthorhombic substrate varies over a wide temperature range. Authors’ combined results from electrical transport measurements, scanning transmission electron microscopy, and theory give detailed insights into the interplay of structural pinning, lattice mismatch, and electronic interactions promoting the complex facet and thickness dependence of the metal-insulator transition in NdNiO.
Y. E. Suyolcu et al.
Phys. Rev. Materials 5, 045001 (2021)
Many desirable material properties may be associated with disorder exhibiting local periodicity or correlations. However, few systems allow systematic studies into the effects of intrinsic crystallographic conflict on correlated disorder. The authors use epitaxial matching to stabilize an exemplar system: the pseudo-bcc UMo alloy, which exhibits a significant mismatch between the basis preferred symmetry and the global lattice. Employing diffuse and inelastic x-ray scattering techniques on 300-nm epitaxial films, combined with modeling, the authors discover a new form of correlated disorder which exhibits strong disorder-phonon coupling that dramatically suppresses phonon lifetimes. These findings have implications across a broad range of materials and could be exploited to develop future functional materials.
D. Chaney et al.
Phys. Rev. Materials 5, 035004 (2021)
Chirality, which is a fundamental property of symmetry, can produce unique electronic and magnetic properties. The problem is that many of the chiral inorganic compounds form a racemic mixture consisting of right- and left-handed enantiomers. Here, the authors succeeded in growing the enantiopure single crystal of Nd-based monoaxial chiral magnet, demonstrating the nontrivial magnetic phase diagram in this rare-earth chiral magnet with the DM interaction. This ternary rare-earth platinum boride system provides an attractive platform for examining chiral magnetism and the role of DM interaction.
Yoshiki J. Sato et al.
Phys. Rev. Materials 5, 034411 (2021)
The combination of an ultrawide band gap and controllable -type conductivity makes monoclinic gallium sesquioxide a promising material for high-power electronics. However, this technological development will require accurate knowledge about the identity and properties of prominent deep-level defects in the material. This work explores close-associate Ga-O divacancies. Owing to the low symmetry of the crystal structure, divacancies can potentially occur in a plethora of crystallographically inequivalent configurations. Hybrid functional calculations were performed to shed light on the relative stability of different divacancy configurations, the energy barriers for transformation between them, and trends in their electrical properties.
Y. K. Frodason et al.
Phys. Rev. Materials 5, 025402 (2021)
Chiral metamaterials can support chiral phonons leading to acoustical activity, the acoustical counterpart of optical activity. However, the properties of early metamaterial designs have been very highly anisotropic, and chiral acoustical phonons occurred only for selected high-symmetry directions. The authors propose a novel chiral metamaterial based on “twisting” a truncated octahedron in a simple-cubic unit cell. Not supported by crystal symmetry alone but rather by a tuned degeneracy, chiral phonons and large broadband acoustical activity are obtained for all phonon propagation directions in 3D. This result is notable because even isotropic achiral acoustical phonons are rare for crystalline materials.
Yi Chen et al.
Phys. Rev. Materials 5, 025201 (2021)
Dirac and Weyl semimetals are promising candidates for high-efficiency thermoelectrics due to their gapless spectrum and high mobility. Recent experiments have shown that the thermopower of Dirac and Weyl materials can be enormously enhanced by a magnetic field, especially when the material exhibits nearly-complete compensation of electron and hole carriers. Here the authors study theoretically the thermopower of compensated conductors, and they show that a strong increase of thermopower with magnetic field arises generically in compensated systems even at relatively low field values. The full field dependence exhibits a number of different regimes, which are mapped out in detail.
Xiaozhou Feng and Brian Skinner
Phys. Rev. Materials 5, 024202 (2021)