Magnetic Stress-Driven Metal-Insulator Transition in Strongly Correlated Antiferromagnetic CrN.

Traditionally, the Coulomb repulsion or Peierls instability causes the metal-insulator phase transitions in strongly correlated quantum materials. In comparison, magnetic stress is predicted to drive the metal-insulator transition in materials exhibiting strong spin-lattice coupling. However, this mechanism lacks experimental validation and an in-depth understanding. Here we demonstrate the existence of the magnetic stress-driven metal-insulator transition in an archetypal material, chromium nitride. Structural, magnetic, electronic transport characterization, and first-principles modeling analysis show that the phase transition temperature in CrN is directly proportional to the strain-controlled anisotropic magnetic stress. The compressive strain increases the magnetic stress, leading to the much-coveted room-temperature transition. In contrast, tensile strain and the inclusion of nonmagnetic cations weaken the magnetic stress and reduce the transition temperature. This discovery of a new physical origin of metal-insulator phase transition that unifies spin, charge, and lattice degrees of freedom in correlated materials marks a new paradigm and could lead to novel device functionalities.

Polar Semiconducting Scandium Nitride as an Infrared Plasmon and Phonon-Polaritonic Material.

The interaction of light with collective charge oscillations, called plasmon-polariton, and with polar lattice vibrations, called phonon-polariton, are essential for confining light at deep subwavelength dimensions and achieving strong resonances. Traditionally, doped-semiconductors and conducting metal oxides (CMO) are used to achieve plasmon-polaritons in the near-to-mid infrared (IR), while polar dielectrics are utilized for realizing phonon-polaritons in the long-wavelength IR (LWIR) spectral regions. However, demonstrating low-loss plasmon- and phonon-polaritons in one host material will make it attractive for practical applications. Here, we demonstrate high-quality tunable short-wavelength IR (SWIR) plasmon-polariton and LWIR phonon-polariton in complementary metal-oxide-semiconductor compatible group III-V polar semiconducting scandium nitride (ScN) thin films. We achieve both resonances by utilizing n-type (oxygen) and p-type (magnesium) doping in ScN that allows modulation of carrier concentration from 5 × 1018 to 1.6 × 1021 cm-3. Our work enables infrared nanophotonics with an epitaxial group III semiconducting nitride, opening the possibility for practical applications.