Functional Transition Metal Perovskite Oxides with 6s2 Lone Pair Activity Stabilized by High-Pressure Synthesis

Perovskite ABO3 oxides that have Bi and Pb at the A site and transition metals at the B site, when stabilized by high-pressure synthesis at several gigapascals, provide a rich parameter space of fascinating properties. Stereochemical 6 s2 lone pairs of Bi3+ and Pb2+ induce polar or antipolar distortions. 6 s2 and 6 s0 (Bi5+ and Pb4+) charge degree of freedom enable intermetallic charge transfer transitions. The structural distortion and the charge degree of freedom are coupled with magnetism of transition metals, resulting in various functionalities. In particular, we highlight magnetization reversal by electric field and polarization rotation in BiFe1− xCo xO3, negative thermal expansion in modified BiNiO3 and PbVO3, and systematic charge distribution changes in Pb MO3 ( M = 3 d transition metal). Expected final online publication date for the Annual Review of Materials Science, Volume 51 is August 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Twist engineering of the two-dimensional magnetism in double bilayer chromium triiodide homostructures

Twist engineering, or the alignment of two-dimensional (2D) crystalline layers with desired orientations, has led to tremendous success in modulating the charge degree of freedom in hetero- and homo-structures, in particular, in achieving novel correlated and topological electronic phases in moiré electronic crystals. However, although pioneering theoretical efforts have predicted nontrivial magnetism and magnons out of twisting 2D magnets, experimental realization of twist engineering spin degree of freedom remains elusive. Here, we leverage the archetypal 2D Ising magnet chromium triiodide (CrI3) to fabricate twisted double bilayer homostructures with tunable twist angles and demonstrate the successful twist engineering of 2D magnetism in them. Using linear and circular polarization-resolved Raman spectroscopy, we identify magneto-Raman signatures of a new magnetic ground state that is sharply distinct from those in natural bilayer (2L) and four-layer (4L) CrI3. With careful magnetic field and twist angle dependence, we reveal that, for a very small twist angle (~ 0.5 degree), this emergent magnetism can be well-approximated by a weighted linear superposition of those of 2L and 4L CI3 whereas, for a relatively large twist angle (~ 5 degree), it mostly resembles that of isolated 2L CrI3. Remarkably, at an intermediate twist angle (~ 1.1 degree), its magnetism cannot be simply inferred from the 2L and 4L cases, because it lacks sharp spin-flip transitions that are present in 2L and 4L CrI3 and features a dramatic Raman circular dichroism that is absent in natural 2L and 4L ones. Our results demonstrate the possibility of designing and controlling the spin degree of freedom in 2D magnets using twist engineering.

Deconfinement of electric charges in hydrogen-bonded ferroelectrics

In addition to the gauge charges, a new charge degree of freedom is identified in the deconfined phase in the lattice Ising gauge theory. While applying to the hydrogen-bonded ferroelectrics, the new charge is essentially the electric charge, leading to the divergent dielectric susceptibility. The new degree of freedom paves an experimentally accessible way to identify the deconfined phase in the lattice Ising gauge theory.

Magnetic Properties of Spinel Cobaltite A Co2O4 (A=Zn and Li)

We synthesized polycrystals of cobaltite spinels LiCo2O4, ZnCo2O4, and their mixed crystal (Li1-xZnx)Co2O4, and performed the magnetic susceptibility and specific heat measurements. LiCo2O4 with the Weiss temperature ΘW ~ -114 K exhibits an antiferromagnetic transition at TN ~ 30 K. On the other hand, ZnCo2O4 with the Weiss temperature ΘW ~ -90 K exhibits the absence of magnetic phase transition down to low temperature (2 K), indicating the presence of strong frustration. Taking into account the absence of magnetic phase transition in the orbital-degenerate ZnCo2O4, it is suggested that the antiferromagnetic transition in the charge-orbital-degenerate LiCo2O4 is driven by the charge degree of freedom. Furthermore, the magnetic properties of (Li1-xZnx)Co2O4 suggest that the antiferromagnetic transition in LiCo2O4 is sensitively suppressed by diluting the charge degeneracy.

New directions in spintronics

Conventional microelectronics exploits only the charge degree of freedom of the electron. Bringing the spin degree of freedom to bear on sensing, radio frequency, memory and logic applications opens up new possibilities for ‘more than Moore’ devices incorporating magnetic components that can couple to an external field, store a bit of data or represent a Boolean state. Moreover, the electron spin is an archetypal two-state quantum system that is an excellent candidate for a solid-state realization of a qubit.

Instant-form Hamiltonian and Becchi–Rouet–Stora–Tyutin formulations of the Nielsen–Olesen model in the broken symmetry phase

The usual instant-form (equal-time) Hamiltonian and Becchi–Rouet–Stora–Tyutin formulations of the Nielsen–Olesen model are investigated in two-space one-time dimension in the broken (frozen) symmetry phase, where the phase ϕ(xμ) of the complex matter field Φ(xμ) carries the charge degree of freedom of the complex matter field and is, in fact, akin to the Goldstone boson. PACS No.: 11.10.Ef

The front-form Hamiltonian and BRST formulations of the Nielsen–Olesen model in the broken symmetry phase

The front-form Hamiltonian and BRST formulations of the Nielsen–Olesen model are investigated in two-space one-time dimension in the broken (frozen) symmetry phase, where the phase ϕ(xμ) of the complex matter field Φ(xμ) carries the charge degree of freedom of the complex matter field and is, in fact, akin to the Goldstone Boson.PACS No.: 11.15.–q

GAUGE INVARIANT DRESSED HOLON AND SPINON IN DOPED CUPRATES

We develop a partial charge-spin separation fermion-spin theory implemented by the gauge invariant dressed holon and spinon. In this novel approach, the physical electron is decoupled as the gauge invariant dressed holon and spinon, with the dressed holon behaviors like a spinful fermion, and represents the charge degree of freedom together with the phase part of the spin degree of freedom, while the dressed spinon is a hard-core boson, and represents the amplitude part of the spin degree of freedom, then the electron single occupancy local constraint is satisfied. Within this approach, the charge transport and spin response of the underdoped cuprates is studied. It is shown that the charge transport is mainly governed by the scattering from the dressed holons due to the dressed spinon fluctuation, while the scattering from the dressed spinons due to the dressed holon fluctuation dominates the spin response.