Recent development in 2D materials beyond graphene

We discuss phenomena arising from the combined influence of electron correlation and spin-orbit coupling (SOC), with an emphasis on emergent quantum phases and transitions in heavy transition metal compounds with 4d and 5d elements. A common theme is the influence of spin-orbital entanglement produced by SOC, which influences the electronic and magnetic structure. In the weak-to-intermediate correlation regime, we show how nontrivial band-like topology leads to a plethora of phases related to topological insulators (TIs). We expound these ideas using the example of pyrochlore iridates, showing how many novel phases, such as the Weyl semimetal, axion insulator, topological Mott insulator, and TIs, may arise in this context. In the strong correlation regime, we argue that spin-orbital entanglement fully or partially removes orbital degeneracy, reducing or avoiding the normally ubiquitous Jahn-Teller effect. As we illustrate for the honeycomb-lattice iridates and double perovskites, this leads to enhanced qu...

Correlated Quantum Phenomena in the Strong Spin-Orbit Regime

Two-dimensional (2D) nanosheets, which possess atomic or molecular thickness and infinite planar lengths, are regarded as the thinnest functional nanomaterials. The recent development of methods for manipulating graphene (carbon nanosheet) has provided new possibilities and applications for 2D systems; many amazing functionalities such as high electron mobility and quantum Hall effects have been discovered. However, graphene is a conductor, and electronic technology also requires insulators, which are essential for many devices such as memories, capacitors, and gate dielectrics. Along with graphene, inorganic nanosheets have thus increasingly attracted fundamental research interest because they have the potential to be used as dielectric alternatives in next-generation nanoelectronics. Here, we review the progress made in the properties of dielectric nanosheets, highlighting emerging functionalities in electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanoelectronics.

Two‐Dimensional Dielectric Nanosheets: Novel Nanoelectronics From Nanocrystal Building Blocks

In the paragraph following eq 28, the average bond valence was incorrectly referred to as V/R. It has been corrected to V/N. The paper originally posted to the web on September 3, 2009, and was reposted on September 24, 2009.

http://pubs.acs.org/doi/pdf/10.1021/cr900053k

Recent developments in the methods and applications of the bond valence model.

A wide variety of cation-exchangeable layered transition metal oxides and their relatively rare counterparts, anion-exchangeable layered hydroxides, have been exfoliated into individual host layers, i.e., nanosheets. Exfoliation is generally achieved via a high degree of swelling, typically driven either by intercalation of bulky organic ions (quaternary ammonium cations, propylammonium cations, etc.) for the layered oxides or by solvation with organic solvents (formamide, butanol, etc.) for the hydroxides. Ultimate two-dimensional (2D) anisotropy for the nanosheets, with thickness of around one nanometer versus lateral size ranging from submicrometer to several tens of micrometers, allows them to serve either as an ideal quantum system for fundamental study or as a basic building block for functional assembly. The charge-bearing inorganic macromolecule-like nanosheets can be assembled or organized through various solution-based processing techniques (e.g., flocculation, electrostatic sequential deposition, or the Langmuir-Blodgett method) to produce a range of nanocomposites, multilayer nanofilms, and core-shell nanoarchitectures, which have great potential for electronic, magnetic, optical, photochemical, and catalytic applications.

Nanosheets of oxides and hydroxides: Ultimate 2D charge-bearing functional crystallites.

In the process of searching for the phases of the Ag-Ba-Ca-Cu-O system at 1200°C under 5 GPa, we found the new oxycarbonate superconductors (Cu0.5C0.5)Ba2Can-1CunO2n+3. Two members of the series with n=3 and n=4 were isolated as almost single phases; their structures are closely related to those of Tl (orHg) based 1223 and 1234, respectively. The Tl (Hg) sites are, in the present series, replaced by Cu and C. Disregarding ordering of Cu and C, the present compounds take tetragonal subcells; at=3.859(1), ct=14.766(5) A for n=3 and at=17.930(6) A for n=4. Electron microscopic observation indicated that Cu and C are, in both cases, located alternately along the a-axes of the subcells resulting in superstructures having as=2at, bs=bt and cs=2ct. The n=3 member showed a superconducting transition at 67 K, while the n=4 member at 117 K which is the highest Tc among the oxycarbonate superconductors found thus fur.

New oxycarbonate superconductors (Cu0.5C0.5)Ba2Can−1CunO2n+3 (n=3, 4) prepared at high pressure

We have investigated magnetic and magneto-optical properties of two-dimensional (2D) ${\mathrm{Ti}}_{0.8}{\mathrm{Co}}_{0.2}{\mathrm{O}}_{2}$ nanosheet, a new nanosized titanium oxide derived from layered titanates by exfoliation. Despite heavy doped 2D system in the dielectric ground, the multilayer films of ${\mathrm{Ti}}_{0.8}{\mathrm{Co}}_{0.2}{\mathrm{O}}_{2}$ nanosheets exhibit room-temperature ferromagnetism with magnetic moment of $1.4{\ensuremath{\mu}}_{B}∕\mathrm{Co}$. We also observe robust magnetic circular dichroism near the absorption edge at $4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, indicating a strong spin polarization in this 2D system. The first-principles study of ${\mathrm{Ti}}_{0.8}{\mathrm{Co}}_{0.2}{\mathrm{O}}_{2}$ nanosheet characterizes the ferromagnetic state with a spin-orbit-induced anisotropy.

Ferromagnetism in two-dimensionalTi0.8Co0.2O2nanosheets

Co NQR studies were performed in recently discovered superconductor Na$_x$CoO$_2$$\cdot y$H$_2$O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na$_x$CoO$_2$ were examined, SC bilayer-hydrate sample with $T_c \sim 4.7$ K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate $1/T_1$ in the SC sample, it was found that the coherence peak is absent just below $T_c$ and that $1/T_1$ is proportional to temperature far below $T_c$. These results, which are in qualitatively agreement with the previous result by Fujimoto {\it et al.}, suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, $1/T_1T$ of the SC sample shows gradual increase below 100K down to $T_c$, whereas $1/T_1T$ of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between $1/T_1T$ and $\chi_{\rm bulk}$ in the SC sample, the increase of $1/T_1T$ is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related with the unconventional superconductivity in this compound. The implication of this finding is discussed.

http://arxiv.org/pdf/cond-mat/0308506.pdf

Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in Na$_x$CoO$_2$$\cdot y$H$_2$O

A new form of LiMn2O4 is reported. The structure is the CaFe2O4-type and 6% denser than the spinel. The structure transformation was achieved by heating at 6 GPa. Analysis of the neutron diffraction pattern confirmed an average of the structure; the unit cell was orthorhombic at a = 8.8336(5) angstroms, b = 2.83387(18) angstroms, and c = 10.6535(7) angstroms (Pnma). Electron diffraction patterns indicated an order of superstructure 3a x b x c, which might be initiated by Li vacancies. The exact composition is estimated at Li(0.92)Mn2O4 from the structure analysis and quantity of intercalated Li. The polycrystalline CaFe2O4-type compound showed semiconducting-like characters over the studied range above 5 K. The activation energy was reduced to approximately 0.27 eV from approximately 0.40 eV at the spinel form, suggesting a possible enhancement of hopping mobility. Magnetic and specific-heat data indicated a magnetically glassy transition at approximately 10 K. As the CaFe2O4-type transition was observed for the mineral MgAl2O4, hence the new form of the lithium manganese oxide would provide valuable opportunities to study not only the magnetism of strongly correlated electrons but also the thermodynamics of the phase transition in the mantle.

E. Takayama-Muromachi

Papers

New oxycarbonate superconductors (Cu0.5C0.5)Ba2Can−1CunO2n+3 (n=3, 4) prepared at high pressure

Ferromagnetism in two-dimensionalTi0.8Co0.2O2nanosheets

Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in Na$_x$CoO$_2$$\cdot y$H$_2$O

Spinel-to-CaFe2O4-type structural transformation in LiMn2O4 under high pressure.

Publisher's Note: Continuous metal-insulator transition of the antiferromagnetic perovskite NaOsO3