The stacking order in trilayer graphene plays a critical role in determining the existence of an electric field tunable band gap. We present spatially resolved tunneling spectroscopy measurements of dual gated Bernal (ABA) and rhombohedral (ABC) stacked trilayer graphene devices. We demonstrate that while ABA trilayer graphene remains metallic, ABC trilayer graphene exhibits a widely tunable band gap as a function of electric field. However, we find that charged impurities in the underlying substrate cause substantial spatial fluctuations of the gap size. Our work elucidates the microscopic behavior of trilayer graphene and its consequences for macroscopic devices.

The atomic and electronic properties of the Cs/SrTiO${}_3$ heterostructure are investigated using density functional theory. Our results suggest that an antiferrodistortive (AFD) or a ferroelectric (FE) like distortion could be induced in SrTiO${}_3$ (STO) by a Cs layer, depending on the interface bonding geometry between the metal and the oxide. Furthermore, while the AFD distortion extends throughout the STO region, the FE-like distortion decays within two layers of STO. Independent of the interface geometry, we find a large $e_g$ orbital splitting (orbital order) around the interface, leading to an $e_g$-based two-dimensional electron gas (2DEG) confined within the two layers of STO, which is fundamentally different from the conventional $t_{2g}$-based 2DEG in this material. This unique feature can be understood in terms of the local orbital mixing at the interface. We discuss the relation between the orbital order and structural distortion.

We report diffusion Monte Carlo calculations on the phase diagrams of para-H${}_2$ and ortho-D${}_2$ adsorbed on top of a first layer of the same substances on graphene. We found that the ground state of the second layer is a triangular incommensurate solid for both isotopes. The densities for promotion to a second layer and for the onset of a two-dimensional solid on that second layer compare favorably with available experimental data in both cases.

¡¡We report on the electronic transport of graphene nanoribbon (GNR) arrays fabricated by a chemical unzipping of well-aligned single-walled carbon nanotubes. The high quality of narrow GNRs is implied by the existence of high-field current saturation and the relatively low intensity of disorder peak in parallel polarized Raman spectra. We find the zero-bias anomaly and the power-law behavior in differential conductance as a function of bias voltage and temperature in the one-dimensional GNRs, which can be well described as Luttinger liquid. Furthermore, this Luttinger-liquid behavior can be tuned by changing the gate voltage.

The structural and electronic properties of Fe atoms in the subsurface region of hydrogen-passivated Si(111) surfaces are investigated by scanning tunneling microscopy (STM). In the STM images, Fe atoms show a characteristic triangular structure with orientation and dimensions that match the Si host lattice. Comparison between atomically resolved microscopy images and the crystalline structure of the H/Si(111) surface indicate that Fe atoms occupy substitutional sites which are located in the third Si layer underneath the surface.

We propose and study a renormalization group transformation that can be used also for models with strong quenched disorder, like spin glasses. The method is based on a mapping between disorder distributions, chosen such as to keep some physical properties (e.g., the ratio of correlations averaged over the ensemble) invariant under the transformation. We validate this ensemble renormalization group by applying it to the hierarchical model (both the diluted ferromagnetic version and the spin glass version), finding results in agreement with Monte Carlo simulations.

The magnetic and crystal structures of the title materials have been investigated using high-resolution neutron and synchrotron x-ray powder diffraction. Ferromagnetic Ru dimers are observed in the spin-ordered ground states of all three materials, evidencing an intradimer double exchange interaction. Antiferromagnetic order between dimers breaks the degeneracy of the triangular lattice but no distortion of the $B^{\prime }=\mathrm{La}$ and Y structures is observed. The magnetic ordering transition temperature increases with the radius of the $B^{\prime }$ cation. An anomalous 120 K structural transition in Ba${}_3$NdRu${}_2$O${}_9$ is attributed to Jahn-Teller distortion, evidenced by large-amplitude ${\Gamma }_6$ deformations of the NdO${}_6$ octahedra. Further lattice anomalies accompany magnetic ordering transitions at 24 and 18 K in Ba${}_3$NdRu${}_2$O${}_9$. Nd spins order ferromagnetically and a substantial magnetic anisotropy leads to large coercivity and $\frac{1}{3}$-magnetization steps in the magnetic hysteresis loop.

There has been an interesting debate on the primary source of chiral $p$-wave superconductivity in Sr${}_2$RuO${}_4$. We present a comparative study on the vortex structure between a single two-dimensional (2D) band and quasi-1D band model by using the Bogoliubov-de Gennes theory. The pattern of the local density of states at zero bias around a vortex core has a diamond shape in the quasi-1D model and is much more isotropic in the 2D model. The spin-lattice relaxation rate well below the superconducting transition temperature is greatly enhanced in the vortex state in the 2D model, but not in the quasi-1D model. These features can be tested by using a scanning tunneling microscope and nuclear magnetic resonance or nuclear quadrupole resonance to distinguish the models for the superconductivity in Sr${}_2$RuO${}_4$.

The conductance mismatch problem limits the spin-injection efficiency significantly, and spin injection into graphene usually requires high-quality tunnel barriers to circumvent the conductance mismatch. We introduce an approach which enables the generation of pure spin current into single-layer graphene (SLG) that is free from electrical conductance mismatch by using dynamical spin injection. An experimental demonstration of spin-pumping-induced spin current generation and spin transport in SLG at room temperature was successfully achieved, and the spin coherence length was estimated to be 1.36 $\mu$m by using a conventional theoretical model based on the Landau-Lifshitz-Gilbert equation. The spin coherence length is proportional to the quality of SLG, which indicates that spin relaxation in SLG is governed by the Elliot-Yafet mechanism, as was reported.

For certain crystalline systems, most notably the organic compound EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$, experimental evidence has accumulated of an insulating state with a high density of gapless neutral excitations that produce Fermi-liquid-like power laws in thermodynamic quantities and thermal transport. This has been taken as evidence of a fractionalized spin-liquid state. In this Rapid Communication, we argue that if the experiments are taken at face value, the most promising spin-liquid candidates are a $Z_4$ spin liquid with a spinon-Fermi surface and no broken symmetries, or a $Z_2$ spin liquid with a spinon-Fermi surface and at least one of the following spontaneously broken: (a) time-reversal and inversion, (b) translation, or (c) certain point-group symmetries. We present a solvable model on the triangular lattice with an (a) type $Z_2$ spin-liquid ground state.

We propose theoretically an active material for electromagnetic radiation with frequency of GHz by use of spin-torque oscillators. The origin of the amplification is the energy supplied to the magnetization by the injected current. We show that close to a resonance with current-driven magnetization, the imaginary part of magnetic permeability becomes indeed negative for either of the two circular polarizations, resulting in a negative imaginary part of the refractive index. Besides, the real part of the refractive index is also manipulated by the current. Our system thus realizes an active filter to obtain circular polarized radiation and/or an electromagnetic metamaterial having negative refractive index, both controlled electrically.

We investigate the high-temperature phase of layered manganites, and demonstrate that the charge-orbital phase transition without magnetic order in La${}_{0.5}$Sr${}_{1.5}$MnO${}_4$ can be understood in terms of the density-wave instability. The orbital ordering is found to be induced by the nesting between segments of Fermi surface with different orbital characters. The simultaneous charge and orbital orderings are elaborated with a mean-field theory. The ordered orbitals are shown to be $d_{x^2-y^2}\pm d_{3z^2-r^2}$.

We report an Eu $L_3$-edge resonant inelastic x-ray scattering (RIXS) investigation of the electronic structure of EuB${}_6$. We observe that the RIXS spectral weight around 1.1 eV increases dramatically when the system is cooled below the ferromagnetic ordering temperature and follows the magnetic order parameter. This spectral feature is attributed to the intersite excitation from the local 4$f$ orbital to the spin-split 5$d$ orbital on the neighboring site, illustrating the essential role of exchange splitting of the conducting electrons. Based on our density functional theory calculations and the RIXS data, EuB${}_6$ at low temperature can be consistently described with a semimetallic electronic structure with incomplete spin polarization. We propose that half-metallicity in EuB${}_6$ can be achieved utilizing the strong tunability of the electronic structure against gate voltage, strain, and magnetic field.

We have performed systematic angle-resolved photoemission spectroscopy of the topological crystalline insulator (TCI) Pb${}_{1-x}$Sn${}_x$Te to elucidate the evolution of its electronic states across the topological phase transition. As previously reported, the band structure of SnTe ($x=1.0$) measured on the (001) surface possesses a pair of Dirac-cone surface states located symmetrically across the $\overline{X}$ point in the (110) mirror plane. Upon approaching the topological phase transition into the trivial phase at $x_c\approx 0.25$, we discovered that Dirac cones gradually move toward the $\overline{X}$ point with its spectral weight gradually reduced with decreasing $x$. In samples with $x\le 0.2$, the Dirac-cone surface state is completely gone, confirming the occurrence of the topological phase transition. Also, the evolution of the valence band feature is found to be consistent with the bulk band inversion taking place at $x_c$. The tunability of the location of the Dirac cones in the Brillouin zone would be useful for applications requiring Fermi-surface matching with other materials, such as spin injection.

Continuous-time quantum Monte Carlo method combined with dynamical mean field theory is used to calculate both periodic Anderson model (PAM) and Kondo lattice model (KLM). Different parameter sets of both models are connected by the Schrieffer-Wolff transformation. For degeneracy $N=2$, a special particle-hole symmetric case of PAM at half filling which always fixes one electron per impurity site is compared with the results of the KLM. We find a good mapping between PAM and KLM in the limit of large on-site Hubbard interaction $U$ for different properties like self-energy, quasiparticle residue and susceptibility. This allows us to extract quasiparticle mass renormalizations for the $f$ electrons directly from KLM. The method is further applied to higher degenerate case and to realistic heavy fermion system CeRh${\text{In}}_5$ in which the estimate of the Sommerfeld coefficient is proven to be close to the experimental value.

We study macroscopically aligned single-wall carbon nanotube arrays with uniform lengths via polarization-dependent terahertz and infrared transmission spectroscopy. Polarization anisotropy is extreme at frequencies less than $\sim$100 cm${}^{-1}$ with no sign of attenuation when the polarization is perpendicular to the alignment direction. The attenuation for both parallel and perpendicular polarizations increases with increasing frequency, exhibiting a pronounced and broad peak around 450 cm${}^{-1}$ in the parallel case. We model the electromagnetic response of the sample by taking into account both radiative scattering and absorption losses. We show that our sample acts as an effective antenna due to the high degree of alignment, exhibiting much larger radiative scattering than absorption in the mid/far-infrared range. Our calculated attenuation spectrum clearly shows a non-Drude peak at $\sim$450 cm${}^{-1}$ in agreement with the experiment.

We use a recently developed ab initio approach to calculate the lattice thermal conductivities of compound semiconductors. An exact numerical solution of the phonon Boltzmann transport equation is implemented, which uses harmonic and anharmonic interatomic force constants determined from density functional theory as inputs. We discuss the method for calculating the anharmonic interatomic force constants in some detail, and we describe their role in providing accurate thermal conductivities in a range of systems. This first-principles approach obtains good agreement with experimental results for well-characterized systems (Si, Ge, and GaAs). We determine the intrinsic upper bound to the thermal conductivities of cubic aluminum-V, gallium-V, and indium-V compounds as limited by anharmonic phonon scattering. The effects of phonon-isotope scattering on the thermal conductivities are examined in these materials and compared to available experimental data. We also obtain the lattice thermal conductivities of other technologically important materials, AlN and SiC. For most materials, good agreement with the experimental lattice thermal conductivities for naturally occurring isotopic compositions is found. We show that the overall frequency scale of the acoustic phonons and the size of the gap between acoustic and optic phonons play important roles in determining the lattice thermal conductivity of each system. The first-principles approach used here can provide quantitative predictions of thermal transport in a wide range of systems.

Ab initio density functional theory calculations are performed to study the electronic properties of a MoS${}_2$ monolayer deposited over a SiO${}_2$ substrate in the presence of interface impurities and defects. When MoS${}_2$ is placed on a defect-free substrate, the oxide plays an insignificant role since the conduction band top and the valence band minimum of MoS${}_2$ are located approximately in the middle of the SiO${}_2$ band gap. However, if Na impurities and O dangling bonds are introduced at the SiO${}_2$ surface, these lead to localized states, which modulate the conductivity of the MoS${}_2$ monolayer from $n$- to $p$-type. Our results show that the conductive properties of MoS${}_2$ deposited on SiO${}_2$ are mainly determined by the detailed structure of the MoS${}_2$/SiO${}_2$ interface, and suggest that doping the substrate can represent a viable strategy for engineering MoS${}_2$-based devices.

The effect of quantum well states (QWS) formed in a Cu overlayer on the magnetic anisotropy of Fe and of Co films underneath is investigated at low temperatures. An oscillatory magnetic anisotropy is observed below 50 K, with a period of $L_{Cu}=5.8$ ML, and it is the same for Cu/Fe and Cu/Co. We attribute this effect to QWS from $\mathit{sp}$ electrons in Cu which hybridize to $d$ electrons at the Fermi level in Fe (and Co) and thus modify the magnetic anisotropy at low temperatures. This is why the effect was not so pronounced in the experiments reported 15 years ago showing the low-amplitude oscillations detected at 170 K [Ch. Würsch et al., Nature (London) 389, 937 (1997)].