We discuss the conditions under which the predicted (but not yet observed) zero-field inter-layer excitonic condensation in double layer graphene has a critical temperature high enough to allow detection. Crucially, disorder arising from charged impurities and corrugation in the lattice structure—invariably present in all real samples—affects the formation of the condensate via the induced charge inhomogeneity. In the former case, we use a numerical Thomas-Fermi-Dirac theory to describe the local fluctuations in the electronic density in double layer graphene devices and estimate the effect these realistic fluctuations have on the formation of the condensate. To make this estimate, we calculate the critical temperature for the interlayer excitonic superfluid transition within the mean-field BCS theory for both optimistic (unscreened) and conservative (statically screened) approximations for the screening of the interlayer Coulomb interaction. We also estimate the effect of allowing dynamic contributions to the interlayer screening. We then conduct similar calculations for double quadratic bilayer graphene, showing that the quadratic nature of the low-energy bands produces pairing with critical temperature of the same order of magnitude as the linear bands of double monolayer graphene.

Plasmons in two-dimensional electron systems with nonparabolic bands, such as graphene, feature strong dependence on electron-electron interactions. We use a many-body approach to relate plasmon dispersion at long wavelengths to Landau Fermi-liquid interactions and quasiparticle velocity. An identical renormalization is shown to arise for the magnetoplasmon resonance. For a model with $N\gg 1$ fermion species, this approach predicts a power-law dependence for plasmon frequency vs carrier concentration, valid in a wide range of doping densities, both high and low. Gate tunability of plasmons in graphene can be exploited to directly probe the effects of electron-electron interaction.

We study the effect of electron-electron interactions in a spinful Chern insulator. For weak on-site repulsive interactions at half-filling, the system is a weakly correlated Chern insulator adiabatically connected to the noninteracting ground state, while in the limit of infinitely strong repulsion the system is described by an effective spin model recently predicted to exhibit a chiral spin liquid ground state. In the regime of large but finite repulsion, we find an exotic gapped phase with characteristics partaking of both the noninteracting Chern insulator and the chiral spin liquid. This phase has an integer quantized Hall conductivity $2e^2/h$ and quasiparticles with electric charges that are integer multiples of the electron charge $e$, but the ground state on the torus is fourfold degenerate and quasiparticles have fractional statistics. We discuss how these unusual properties affect the outcome of a charge pumping experiment and, by deriving the topological field theory, elucidate that the topological order is of the exotic ${\mathbb{Z}}_2$ double-semion type.

By propagating the many-body Schrödinger equation, we determine the exact time-dependent Kohn-Sham potential for a system of strongly correlated electrons which undergo field-induced tunneling. Numerous features are entirely absent from the approximations commonly used in time-dependent density-functional theory. The self-interaction correction is strong and time dependent, owing to electron localization, and prominent dynamic spatial potential steps arise from minima in the charge density, as modified by the Coulomb interaction experienced by the partially tunneled electron.

We consider an effect of weak impurities on the electronic properties of graphene within the functional renormalization-group approach. The energy dependences of the electronic self-energy and density of states near the neutrality point are discussed. Depending on the symmetry of the impurities, the electronic damping $\Gamma$ and density of states $\rho$ can deviate substantially from those given by the self-consistent Born approximation. We investigate the crossover from the results of the self-consistent Born approximation, which are valid far from the neutrality point to the strong-coupling (diffusive) regime near the neutrality point. For impurities, which are diagonal in both valley and sublattice indices, we obtain a finite density of states at the Fermi level with values which are much bigger than the result of the self-consistent Born approximation.

We study theoretically the nature of correlations in space and time of the current in a one-dimensional bilinear array of tunnel junctions in the normal conduction limit, using the kinetic Monte Carlo method. The bilinear array consists of two parallel rows of tunnel junctions, capacitively coupled in a ladder configuration. The electrostatic potential landscape and the charge-charge interaction length both depend on the circuit capacitances, which in turn influence transport and charge correlations in the array. We observe the formation of stationary charge states when only one rail is voltage biased. When a symmetric bias is applied to both rails, the site at which the positive and negative charge carriers recombine can drift throughout the array. We also calculate charge densities and auto- and cross-correlation functions.

We derive the gradient expansion for the exchange energy of a spin-polarized electron gas by perturbing the uniformly spin-polarized state and thus inducing a small noncollinearity that is slowly varying in space. We show that the contribution due to the induced longitudinal gradient of the spin polarization to the exchange energy differs from the contribution due to the transverse gradient. The difference is present at any nonvanishing spin polarization and becomes larger with increasing spin polarization. We argue that improved generalized gradient approximations of spin-density-functional theory must account for the difference between the longitudinal- and transverse-spin stiffness.

Monolayer graphene grown on Ru(0001) surfaces forms a superstructure with periodic modulations in its geometry and electronic structure. The large dimension and inhomogeneous features of this superstructure make its description and subsequent analysis a challenge for theoretical modeling based on density functional theory. In this work, we compare two different approaches to describe the same physical properties of this surface, focusing on the geometry and the electronic states confined at the surface. In the more complex approach, the actual moiré structure is taken into account by means of large unit cells, whereas in the simplest one, the graphene moiré is completely neglected by representing the system as a stretched graphene layer that adapts pseudomorphically to Ru(0001). As shown in previous work, the more complex model provides an accurate description of the existing experimental observations. More interestingly, we show that the simplified stretched models, which are computationally inexpensive, reproduce qualitatively the main features of the surface electronic structure. They also provide a simple and comprehensive picture of the observed electronic structure, thus making them particularly useful for the analysis of these and maybe other complex interfaces.

Arrays of twisted carbon nanotubes and nanotube ropes are equivalent to a torsional spring capable of storing energy. The advantage of carbon nanotubes over a twisted rubber band, which is used to store energy in popular toys, is their unprecedented toughness. Using ab initio and parametrized density functional calculations, we determine the elastic range and energy storage capacity of twisted carbon nanotubes and nanotube ropes. We find that a twisted nanotube rope may reversibly store energy by twisting, stretching, bending, and compressing constituent nanotubes. We find that in the elastic regime, the interior of a twisted rope encounters hydrostatic pressures of up to tens of GPa. We examine the limits of reversible energy storage and identify structural deformations beyond the elastic limit, where irreversibility is associated with breaking and forming new covalent bonds. Under optimum conditions, the calculated reversible mechanical energy storage capacity of twisted carbon nanotube ropes surpasses that of advanced Li-ion batteries by up to a factor of 4 to 10.

Raman scattering and photoluminescence (PL) emission are used to investigate a single layer of tungsten disulfide (WS${}_2$) obtained by exfoliating $n$-type bulk crystals. Direct gap emission with both neutral and charged exciton recombination is observed in the low temperature PL spectra. The ratio between the trion and exciton emission can be tuned simply by varying the excitation power. Moreover, the intensity of the trion emission can be independently tuned using additional subband gap laser excitation.

We investigate the quantum tunneling of electrons in an AA-stacked bilayer graphene (BLG) $n$-$p$ junction and $n$-$p$-$n$ junction. We show that Klein tunneling of an electron can occur in this system. The quasiparticles are not only chiral but are additionally described by a “cone index.” Due to the orthogonality of states with different cone indexes, electron transport across a potential barrier must strictly conserve the cone index, and this leads to the protected cone transport which is unique in AA-stacked BLG. Together with the negative refraction of electrons, electrons residing in different cones can be spatially separated according to their cone index when transmitted across an $n$-$p$ junction. This suggests the possibility of “cone-tronic” devices based on AA-stacked BLG. Finally, we calculate the junction conductance of the system.

We report the results of neutron elastic-scattering measurements made between −250 °C and 620 °C on the lead-free relaxor (Na${}_{1/2}$Bi${}_{1/2}$)TiO${}_3$ (NBT). Strong, anisotropic, elastic diffuse scattering intensity decorates the (100), (110), (111), (200), (210), and (220) Bragg peaks at room temperature. The wave-vector dependence of this diffuse scattering is compared to that in the lead-based relaxor Pb(Mg${}_{1/3}$Nb${}_{2/3}$)O${}_3$ (PMN) to determine if any features might be common to relaxors. Prominent ridges in the elastic diffuse scattering intensity contours that extend along ⟨110⟩ are seen that exhibit the same zone dependence as those observed in PMN and other lead-based relaxors. These ridges disappear gradually on heating above the cubic-to-tetragonal phase transition temperature T${}_{\mathrm{CT}}$ $=$ 523 °C, which is also near the temperature at which the dielectric permittivity begins to deviate from Curie-Weiss behavior. We thus identify the ⟨110⟩-oriented ridges as a relaxor-specific property. The diffuse scattering contours also display narrower ridges oriented along ⟨100⟩ that are consistent with the x-ray results of Kreisel et al. [Phys. Rev. B 68, 014113 (2003)]; these vanish near 320 °C, indicating that they have a different physical origin. The ⟨100⟩-oriented ridges are not observed in PMN. We observe no equivalent relaxor-specific elastic diffuse scattering from the homovalent relaxor analogues K${}_{0.95}$Li${}_{0.05}$TiO${}_3$ ($A$-site disordered) and KTa${}_{0.95}$Nb${}_{0.05}$O${}_3$ ($B$-site disordered). This suggests that the ⟨110⟩-oriented diffuse scattering ridges are correlated with the presence of strong random electric fields and invites a reassessment of what defines the relaxor phase. We find that doping NBT with 5.6$\%$ BaTiO${}_3$, a composition close to the morphotropic phase boundary with enhanced piezoelectric properties, increases the room-temperature correlation length along $\left[1\overline{1}0\right]$ from 40 to 60 Å while doubling the associated integrated diffuse scattering. Similar behavior was reported by Matsuura et al. [Phys. Rev. B 74, 144107 (2006)] for morphotropic compositions of PMN doped with PbTiO${}_3$. Finally, we comment on the recent observation of monoclinicity in NBT at room temperature by placing a strict bound on the strength of the (½½½) superlattice reflection associated with the $Cc$ space group based on the atomic coordinates published in the x-ray study by Aksel et al. [Appl. Phys. Lett. 98, 152901 (2011)] for NBT. We show that a skin effect, analogous to that reported in the relaxors PZN-xPT and PMN-xPT, can reconcile our neutron single-crystal data with the x-ray powder data of Aksel et al. [Appl. Phys. Lett. 98, 152901 (2011)]. Our finding of a skin effect in a lead-free, $A$-site disordered, heterovalent relaxor supports the idea that it arises in the presence of strong random electric fields.

Using first-principles density functional theory based calculations, we study systematically the effect of medium to large Cu substitution at the Mn, Ga as well as Ni sites on the geometric, bulk mechanical, electronic, and magnetic properties of Ni${}_2$MnGa and Mn${}_2$NiGa. The calculations have been carried out for possible austenite and martensite phases using a supercell approach. Partial Cu substitutions at Mn and Ga sites show promises in terms of the electronic and magnetic properties for both Ni${}_2$MnGa and Mn${}_2$NiGa alloys from an application point of view. Our calculations predict that for certain partial substitutions, the austenite to martensite transition temperature is likely to increase and the system remains magnetic in nature. On the other hand, a significantly large amount of Cu substitution at the Ni site seems to stabilize the austenite phase in both of these alloy systems rendering a martensite transition unlikely. Interestingly, the overall trend in the changes in the structural, bulk mechanical, electronic, and magnetic properties of these two different types of alloy systems, Ni${}_2$MnGa and Mn${}_2$NiGa, as a result of substantial Cu substitution in all the three different sites, is found to be the same.

Ab initio alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to determine the elastic properties of Ni-Fe alloys with Fe:Ni ratio 1:3. The interplay between magnetic and chemical effects is investigated by computing the lattice parameters and the single- and polycrystal elastic moduli for different partially ordered structures in the ferro- and paramagnetic states. It is found that the influence of long-range chemical order on the bulk properties strongly depends on the magnetic state. The largest magnetic-order-induced changes are obtained for the chemically ordered $L$1${}_2$ phase. The ferromagnetic $L$1${}_2$ system possesses $\sim$$5.4\%$ larger elastic Debye temperature than the paramagnetic $L$1${}_2$ phase, which in turn has a similar ${\Theta }_D$ as the chemically disordered face-centered cubic phase in either the ferro- or paramagnetic state. It is concluded that magnetic ordering has a substantially larger impact on the bulk parameters of Ni${}_3$Fe than chemical ordering. The calculated trends are explained based on the electronic structure of nonmagnetic, ferromagnetic, and paramagnetic ordered and disordered phases.

Currently, significant research efforts are geared towards using carbon-based materials for electronic applications. Here we report the observation of magnetic ordering of implanted Mn in HOPG substrates. Superconducting quantum interference device measurements show higher moments for Mn-doped samples and also the existence of double and inverted hysteresis in both undoped and doped samples. High-resolution transmission electron microscopy shows the presence of nanocrystals in implanted samples. Grazing incidence synchrotron x-ray diffraction studies show the presence of three stable manganese carbides, including antiferromagnetic Mn${}_{23}$C${}_6$. X-ray magnetic circular dichroism (XMCD) measurements show ferromagnetic ordering of Mn moments at temperatures below ∼100 K. However, a very weak XMCD signal indicates that only about 1$\%$ of Mn atoms are ferromagnetically ordered. We conclude that the observed Mn ferromagnetic ordering is caused by uncompensated Mn moments on the surface of antiferromagnetic Mn${}_{23}$C${}_6$ nanocrystals that are aligned by the local magnetic field.

First-principles calculations reveal a large cooperative coupling of Jahn-Teller (JT) distortion to oxygen-octahedron rotations in perovskite LaMnO${}_3$. The combination of the two distortions is responsible for stabilizing the strongly orthorhombic $A$-AFM insulating ($I$) $ePbnm$ ground state relative to a metallic ferromagnetic (FM-$M$) phase. However, epitaxial strain due to coherent matching to a crystalline substrate can change the relative stability of the two states. In particular, coherent matching to a square-lattice substrate favors the less orthorhombic FM-$M$ phase, with the $A$-AFM phase stabilized at higher values of tensile epitaxial strain due to its larger volume per formula unit, resulting in a coupled magnetic and metal-insulator transition at a critical strain close to 1%. At the phase boundary, a very large magnetoresistance is expected. Tensile epitaxial strain enhances the JT distortion and opens the band gap in the $A$-AFM-$I$ $c$-$ePbnm$ phase, offering the opportunity for band-gap engineering. Compressive epitaxial strain induces a transition within the FM-$M$ phase from the $c$-$ePbnm$ orientation to the $ab$-$ePbnm$ orientation with a change in the direction of the magnetic easy axis relative to the substrate, yielding strain-controlled magnetization at the phase boundary. Similar behavior is expected in other JT active $Pbnm$ perovskites.

Neutron and synchrotron resonant x-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in a CeFeAsO single crystal. The RXMS of magnetic reflections at the Ce $L_{\mathrm{II}}$ edge shows a magnetic transition that is specific to the Ce antiferromagnetic long-range ordering at $T_{\mathrm{Ce}}\approx$ 4 K with short-range Ce ordering above $T_{\mathrm{Ce}}$, whereas neutron diffraction measurements of a few magnetic reflections indicate a transition at $T^{*}\approx$ 12 K with an unusual order parameter. Detailed order-parameter measurements on several magnetic reflections by neutrons show a weak anomaly at 4 K that we associate with the Ce ordering. The successive transitions at $T_{\mathrm{Ce}}$ and $T^{*}$ can also be clearly identified by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at $T^{*}\approx$ 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the $ab$ plane. The Fe spin reorientation transition and short-range Ce ordering above $T_{\mathrm{Ce}}$ reflect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through $T^{*}$ and $T_{\mathrm{Ce}}$ is proposed.

Using realistic multiorbital tight-binding Hamiltonians and the $T$-matrix formalism, we explore the effects of a nonmagnetic impurity on the local density of states in Fe-based compounds. We show that scanning tunneling spectroscopy (STS) has very specific anisotropic signatures that track the evolution of orbital splitting (OS) and antiferromagnetic gaps. Both anisotropies exhibit two patterns that split in energy with decreasing temperature, but for OS these two patterns map onto each other under ${90}^{\circ }$ rotation. STS experiments that observe these signatures should expose the underlying magnetic and orbital order as a function of temperature across various phase transitions.

We study the transport properties of the Kondo insulator SmB${}_6$ with a specialized configuration designed to distinguish bulk-dominated conduction from surface-dominated conduction. We find that as the material is cooled below 4 K, it exhibits a crossover from bulk to surface conduction with a fully insulating bulk. We take the robustness and magnitude of the surface conductivity, as is manifest in the literature of SmB${}_6$, to be strong evidence for the topological insulator metallic surface states recently predicted for this material.

Superconducting metamaterials are utilized to study the approach to the plasmonic limit simply by tuning temperature to modify the superfluid density, and thus the superfluid plasma frequency. We examine the persistence of artificial magnetism in a metamaterial made with superconductors in the plasmonic limit, and compare to the electromagnetic behavior of normal metals as a function of frequency as the plasma frequency is approached from below. Spiral-shaped Nb thin film meta-atoms of scaled dimensions are employed to explore the plasmonic behavior in these superconducting metamaterials, and the scaling condition allows extraction of the temperature dependent superfluid density, which is found to be in good agreement with expectations.