The presence of ferroelectricity in hexagonal InMnO${}_3$ has been highly under debate. The results of our comprehensive experiments of low-temperature ($T$) polarization, transmission electron microscopy, and high-angle annular dark-field scanning TEM on well-controlled InMnO${}_3$ reveal that the ground state is ferroelectric with $P$6${}_3$$\mathit{cm}$ symmetry, but a nonferroelectric $P$$\overline{3}$$c$1 state exists at high $T$ and can be quenched to room temperature. We found that the competing ferroelectric and antiferroelectric phases coexist in mesoscopic scales and can be deliberately controlled by varying thermal treatments.

We report superconductivity in the ternary half-Heusler compound LuPtBi, with $T_c=1.0$ K and $H_{c2}=1.6$ T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. The combination of strong spin-orbit coupling and noncentrosymmetric crystal structure make LuPtBi a strong candidate for 3D topological superconductivity.

Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated by superfluid density measurements for two-dimensional (2D) disordered NbN films with disorder level very close to a superconductor-insulator transition (SIT). Our data show a robust BKT transition even near this 2D disorder-tuned quantum critical point. This observation is in direct contrast with previous data on deeply underdoped quasi-2D cuprates near the SIT. As our NbN films approach the quantum critical point, the vortex core energy, an important energy scale in the BKT transition, scales with the superconducting gap, not with the superfluid density, as expected within the standard 2D-$XY$ model description of BKT physics.

We have investigated magnetic-field ($H$∥[100]) effect on a spin-frustrated tetragonally distorted spinel Mn${}_3$O${}_4$ by measurements of magnetization, dielectric constant, and strain as well as synchrotron x-ray and neutron diffraction. In two low-temperature phases below 40 K, long-wavelength lattice modulations coupled with magnetic order have been confirmed, revealing spin-lattice coupling caused by the exchange striction in the geometrically frustrated Mn${}^{3+}$ spin moments on the distorted pyrochlore lattice. A magnetic-field-induced phase transition has been found at low temperatures, which involves an abrupt change in both structural and magnetic configurations. The magnetostructural phase transition by the application of a magnetic field $H$∥[100] is discussed in terms of interplay among spin, orbital, and lattice degrees of freedom.

The emergence of magnetic reconstructions at the interfaces of oxide heterostructures are often explained via subtle modifications in the electronic densities, exchange couplings, or strain. Here, an additional possible route for induced magnetism is studied in the context of the (LaNiO${}_3$)${}_n$/(LaMnO${}_3$)${}_n$ superlattices using a hybrid tight-binding model. In the LaNiO${}_3$ region, the induced magnetizations decouple from the intensity of charge leakage from Mn to Ni, but originate from the spin-filtered quantum confinement present in these nanostructures. In general, the induced magnetization is the largest for the (111)-stacking and the weakest for the (001)-stacking superlattices; results compatible with the exchange bias effects reported by Gibert et al. [Nat. Mater. 11, 195 (2012)].

We analyze an effective model on a square lattice with two types of forward scattering interactions, which, respectively, drive ferromagnetism (FM) and electronic nematic order via a $d$-wave Pomeranchuk instability ($d$PI). The FM and $d$PI in general compete with each other and they are typically separated by a first-order phase boundary in the plane of the chemical potential and temperature. Nevertheless, there is a parameter region where the $d$PI occurs inside the FM phase, leading to their coexistence. We also study the effect of a magnetic field by choosing a chemical potential where the ground state is paramagnetic without a field. In this case, instead of FM, the $d$PI competes with a metamagnetic instability. The latter occurs above a threshold strength of the FM interaction and otherwise the $d$PI is stabilized with a dome-shaped phase diagram in the plane of a magnetic field and temperature. The FM interaction shifts the center of the dome to a lower field, accompanied by a substantial reduction of the field range where the $d$PI is stabilized and by an extension of the first-order part of the transition line, although the maximal critical temperature does not change. Our results indicate that proximity to the FM instability can be important to understand the experimental phase diagram observed in the bilayer ruthenate Sr${}_3$Ru${}_2$O${}_7$.

We investigate the electronic and magnetic properties of Mn-doped monolayer MoS${}_2$ using a combination of first-principles density functional theory (DFT) calculations and Monte Carlo simulations. Mn dopants that are substitutionally inserted at Mo sites are shown to couple ferromagnetically via a double-exchange mechanism. This interaction is relatively short ranged, making percolation a key factor in controlling long-range magnetic order. The DFT results are parameterized using an empirical model to facilitate Monte Carlo studies of concentration- and temperature-dependent ordering in these systems, through which we obtain Curie temperatures in excess of room temperature for Mn doping in the range of 10–15$\%$. Our studies demonstrate the potential for engineering a new class of atomically thin dilute magnetic semiconductors based on Mn-doped MoS${}_2$ monolayers.

We present magnetotransport and ${}^{209}$Bi nuclear magnetic resonance (NMR) data on a series of single crystals of Bi${}_2$Se${}_3$, Bi${}_2$Te${}_2$Se, and Cu${}_x$Bi${}_2$Se${}_3$ with varying carrier concentrations. The Knight shift of the bulk nuclei is strongly correlated with the carrier concentration via a hyperfine coupling of 27 $\mu$eV, which may have important consequences for scattering of the protected surface states. Surprisingly we find that the NMR linewidths and the spin-lattice relaxation rate appear to be dominated by the presence of localized spins, which may be related to the presence of Se vacancies.

We report on the synthesis and low-temperature transport properties of Ce${}_{0.13}$Co${}_4$Ge${}_6$Se${}_6$ and Yb${}_{0.14}$Co${}_4$Ge${}_6$Se${}_6$. Reitveld refinement was used to identify the structure and stoichiometry of the two compositions. The compounds are skutterudite derivatives with rhombohedral symmetry. The thermal conductivity data were investigated by employing the Debye model with different phonon-scattering parameters. The structural properties have a significant effect on the electrical and thermal properties. This work is part of the continuing effort to explore different skutterudite compositions and structure types, as this class of materials continues to be of interest for thermoelectrics applications.

We present a complete photonic logic gate architecture in a compact solid-state system, making use of the nonlinear and spintronic properties of exciton polaritons in semiconductor microcavities. The dynamics of the system is modeled using the spinor Gross-Pitaevskii equations, and it is shown that the proposal fulfills all the necessary criteria for fully functioning information processing devices without the use of any external electric fields.

The atomic step edges on a transition-metal (TM) surface play an important role in surface catalysis due to their distinct reactivities from that of the terrace. The nonuniform growth of graphene on Ni(111) is just such a case. Using first-principles calculations, we show that the step edges of Ni(111) can be passivated in varying degrees by surface alloying with a series of carefully chosen TMs. The alloyed surface can be more uniform between the edges and the terrace, and able to facilitate large-area and high-quality growth of graphene.

It has recently been realized that zero modes with projective non-Abelian statistics, generalizing the notion of Majorana bound states, may exist at the interface between a superconductor and a ferromagnet along the edge of a fractional topological insulator (FTI). Here, we study two-dimensional architectures of these non-Abelian zero modes, whose interactions are generated by the charging and Josephson energies of the superconductors. We derive low-energy Hamiltonians for two different arrays of FTIs on the plane, revealing an interesting interplay between the real-space geometry of the system and its topological properties. On the one hand, in a geometry where the length of the FTI edges is independent on the system size, the array has a topologically ordered phase, giving rise to a qudit toric code Hamiltonian in perturbation theory. On the other hand, in a geometry where the length of the edges scales with system size, we find an exact duality to an Abelian lattice gauge theory and no topological order.

We present a scattering-state description for the nonequilibrium multichannel charge transport in the presence of electron-vibration couplings. It is based on an expansion of scattering orders of eigenchannel states. Examining charge transitions between scattering states, we clarify competing inelastic and elastic scattering processes and compare with the interpretation based on the nonequilibrium Green's functions formalism. We also derive a general expression for conductance variations in single-channel systems that provides a comprehensive picture for the variation, including the well-known result, the 0.5 rule, from the aspect of interplay between elastic and inelastic scattering processes.

We study plasmon excitations in infinitely long graphene nanoribbons using a quasistatic approach, where one-dimensional coupled equations for electrostatic potential and excited charge density are derived in the transverse direction. By incorporating a hydrodynamical description of the excited charge density, we investigate nonlocal effects in plasmon excitations. Moreover, the method presented here provides means to look into the nonlocal plasmon response in more complex graphene nanostructures such as wedges. We find that the plasmon frequencies are lowered by interband transitions and raised due to nonlocal effects. The frequency shifts depend monotonically on the dielectric constant of the surrounding medium. Most importantly, the nonlocal effects can strongly affect the excited charge density at the edges. Finally, we show that a larger increase of dipolar plasmon frequencies occurs in smaller graphene nanoribbons as expected.

We fabricate free-standing graphene structures from epitaxial graphene on silicon carbide using a photoelectrochemical (PEC) etching process. A combination of Raman spectroscopy and magnetotransport measurements was used to investigate multiterminal devices in various geometries. From the analysis of Raman data and Shubnikov–de Haas oscillations, we conclude that the buffer layer is converted into a graphene layer and, thus, monolayer graphene on SiC gets converted to a free-standing AB-stacked bilayer. The bilayer exhibits inversion-symmetry breaking because of differential doping between the layers. Additionally, lateral inhomogeneities exist in the form of domains with nonuniform mobility. The same PEC process on a pure buffer layer, however, does not yield monolayer graphene.

Exciton-polariton condensates are considered as a deterministic source of bright, coherent non-Gaussian light. Exciton-polariton condensates emit coherent light via photoluminescence through the microcavity mirrors due to the spontaneous formation of coherence. Unlike conventional lasers which emit coherent Gaussian light, polaritons possess a natural nonlinearity due to the interaction of the excitonic component. This produces light with a negative component to the Wigner function at steady-state operation when the phase is stabilized via a suitable method such as injection locking. In contrast to many other proposals for sources of non-Gaussian light, in our case, the light typically has an average photon number exceeding one and emerges as a continuous wave. Such a source may have uses in continuous-variable quantum information and communication.

The nonlinear optical properties of few-layer MoS${}_2$ two-dimensional crystals are studied using femtosecond laser pulses. We observed highly efficient second-harmonic generation from the odd-layer crystals, which shows a polarization intensity dependence that directly reveals the underlying symmetry and orientation of the crystal. Additionally, the measured second-order susceptibility spectra provide information about the electronic structure of the material. Our results open up opportunities for studying the nonlinear optical properties in these two-dimensional crystals.

Spin-crossover molecules have recently emerged as a family of compounds potentially useful for implementing molecular spintronics devices. The calculations of the electronic properties of such molecules is a formidable theoretical challenge, as one has to describe the spin ground state of a transition metal as the ligand field changes. The problem is dominated by the interplay between strong electron correlation at the transition-metal site and charge delocalization over the ligands, and thus it fits into a class of problems where density functional theory may be inadequate. Furthermore, the crossover activity is extremely sensitive to environmental conditions, which are difficult to fully characterize. Here we discuss the phase transition of a prototypical spin-crossover molecule as obtained with diffusion Monte Carlo simulations. We demonstrate that the ground state changes depending on whether the molecule is in the gas or in the solid phase. As our calculation provides a solid benchmark for the theory, we then assess the performances of density functional theory. We find that the low-spin state is always overstabilized, not only by the (semi-)local functionals, but even by the most commonly used hybrids (such as B3LYP and PBE0). We then propose that reliable results can be obtained by using hybrid functionals containing about $50\%$ of exact exchange.

The electronic properties of the FeO and CoO single layer thin films on Pt(111) were examined by core-level soft x-ray spectroscopy. These oxygen terminated films are bilayer-thick and isostructural Pt, Fe-Co, and O atoms stacked with small lateral shifts due an incommensurate relation with Pt(111). Probing occupied and unoccupied states projected on the oxygen atoms revealed states at the Fermi level, suggesting orbital mixing with the metal substrate, and also indicated an anisotropy in transition metal-oxygen bonding geometry. The differences in the core-level spectral features were attributed to the substrate induced modification of the charge transfer energy and with an additional valence electron on Co.