In carbon nanotube and graphene gas sensing, the measured conductance change after the sensor is exposed to target molecules has been traditionally attributed to carrier density change due to charge transfer between the sample and the adsorbed molecule. However, this explanation has many problems when it is applied to graphene: The increased amount of Coulomb impurities should lead to decrease in carrier mobility which was not observed in many experiments, carrier density is controlled by the gate voltage in the experimental setup, and there are inconsistencies in the energetics of the charge transfer. In this paper we explore an alternative mechanism. Charged functional groups and dipolar molecules on the surface of graphene may counteract the effect of charged impurities on the substrate. Because scattering of electrons with these charged impurities has been shown to be the limiting factor in graphene conductivity, this leads to significant changes in the transport behavior. A model for the conductivity is established using the random phase approximation dielectric function of graphene and the first-order Born approximation for scattering. The model predicts optimal magnitudes for the charge and dipole moment which maximally screen a given charged impurity. The dipole screening is shown to be generally weaker than the charge screening although the former becomes more effective with higher gate voltage away from the charge neutrality point. The model also predicts that with increasing amount of adsorbates, the charge impurities eventually become saturated and additional adsorption always lead to decreasing conductivity.

Considerable efforts have recently been devoted to the experimental realization of a two-dimensional Chern insulator, i.e., a system displaying a quantum anomalous Hall effect. However, existing approaches such as those based on magnetic doping of topological-insulator thin films have resulted in small band gaps, restricting the effect to low temperatures. We use first-principles calculations to demonstrate that an interface between thin films of the topologically trivial ferromagnetic insulators EuO and GdN can result in a band inversion and a nonzero Chern number. Both materials are stoichiometric and the interface is nonpolar and lattice-matched, which should allow this interface to be achievable experimentally. We show that the band structure can be tuned by layer thickness or epitaxial strain, and can result in Chern insulators with gaps of over 0.1 eV.

We have measured the donor-bound electron spin dynamics in cubic GaN by time-resolved Kerr rotation experiments. The ensemble electron spin dephasing time in this quantum-dot-like system characterized by a Bohr radius of 2.5 nm is of the order of 1.5 ns as a result of the interaction with the fluctuating nuclear spins. It increases drastically when an external magnetic field as small as 10 mT is applied. We extract a dispersion of the nuclear hyperfine field $\delta {\mathrm{B}}_{\mathrm{n}}\sim 4\mathrm{mT},$ in agreement with calculations. We also demonstrate for the first time in GaN-based systems the optical pumping of nuclear spin yielding the buildup of a significant nuclear polarization.

We explore the lifetime of excitations in a dispersive Luttinger liquid. We perform a bosonization supplemented by a sequence of unitary transformations that allows us to treat the problem in terms of weakly interacting quasiparticles. The relaxation described by the resulting Hamiltonian is analyzed by bosonic and (after a refermionization) by fermionic perturbation theory. We show that the fermionic and bosonic formulations of the problem exhibit a remarkable strong-weak coupling duality. Specifically, the fermionic theory is characterized by a dimensionless coupling constant $\lambda =m^{*}{l}^{2}T$ and the bosonic theory by ${\lambda }^{-1}$, where $1/m^{*}$ and $l$ characterize the curvature of the fermionic and bosonic spectra, respectively, and $T$ is the temperature.

The combination of density functional theory and single-site dynamical mean-field theory, using both Hartree and full continuous-time quantum Monte Carlo impurity solvers, is used to study the metal-insulator phase diagram of perovskite transition-metal oxides of the form $AB{\mathrm{O}}_3$ with a rare-earth ion $A=\mathrm{Sr}$, La, Y and transition metal $B=\mathrm{Ti}$, V, Cr. The correlated subspace is constructed from atomiclike $d$ orbitals defined using maximally localized Wannier functions derived from the full $p-d$ manifold; for comparison, results obtained using a projector method are also given. Paramagnetic DFT + DMFT computations using full charge self-consistency along with the standard “fully localized limit” (FLL) double counting are shown to incorrectly predict that ${\mathrm{LaTiO}}_3$, ${\mathrm{YTiO}}_3$, ${\mathrm{LaVO}}_3$, and ${\mathrm{SrMnO}}_3$ are metals. A more general examination of the dependence of physical properties on the mean $p-d$ energy splitting, the occupancy of the correlated $d$ states, the double-counting correction, and the lattice structure demonstrates the importance of charge-transfer physics even in the early transition-metal oxides and elucidates the factors underlying the failure of the standard approximations. If the double counting is chosen to produce a $p-d$ splitting consistent with experimental spectra, single-site dynamical mean-field theory provides a reasonable account of the materials properties. The relation of the results to those obtained from “$d$-only” models in which the correlation problem is based on the frontier orbital $p-d$ antibonding bands is determined. It is found that if an effective interaction $U$ is properly chosen the $d$-only model provides a good account of the physics of the $d^1$ and $d^2$ materials.

We present an electrodynamic approach to the description of the linear response of solids to electromagnetic fields. For time and spatially varying applied fields we solve the dynamical equations satisfied by the gauge-invariant Green function and find microscopic charge and current densities that result in a form allowing for an easy construction of the multipole expansion of applied fields. Restricting ourselves to static and uniform electric and magnetic fields, we construct microscopic expressions for polarization and magnetization fields associated with each lattice site. The approach is in the spirit of the Power-Zienau-Wooley (PZW) treatment but generalized to account for the motion of the charge between lattice sites. We show that the macroscopic polarization and magnetization can be understood as the spatial average of the generalized PZW microscopic fields.

We report a detailed investigation of the electronic and magnetic properties of the transition metal (TM) doped two-dimensional (2D) ${\mathrm{MoS}}_2$ using ab initio calculations. The doping is achieved by substituting two or more Mo atoms by TM atoms of the $3d$ series. Additionally, the effect of codoping on the 2D ${\mathrm{MoS}}_2$ by cation-cation and cation-anion pairs is also investigated. Our results demonstrate that the TM doping of 2D ${\mathrm{MoS}}_2$ leads to a significant reduction of the energy gap and the appearance of magnetic features whose major characteristic is the ferromagnetic coupling of the TM dopants. The latter is found to be significantly enhanced by codoping as demonstrated by codoping with (Co,Cu), (Ni,Cu), (Mn,Cu), and (Mn,Sb) codopant pairs.

A general phenomenon of the Cherenkov radiation known in optics or acoustics of conventional materials is a formation of a forward cone of, respectively, photons or phonons emitted by a particle accelerated above the speed of light or sound in those materials. Here we suggest three-dimensional topological insulators as a unique platform to fundamentally explore and practically exploit the acoustic aspect of the Cherenkov effect. We demonstrate that by applying an in-plane magnetic field to a surface of a three-dimensional topological insulator one may suppress the forward Cherenkov sound up to zero at a critical magnetic field. Above the critical field the Cherenkov sound acquires pure backward nature with the polar distribution differing from the forward one generated below the critical field. Potential applications of this asymmetric Cherenkov reverse are in the design of low energy electronic devices such as acoustic ratchets or, in general, in low power design of electronic circuits with a magnetic field control of the direction and magnitude of the Cherenkov dissipation.

We have used photoemission spectroscopy in order to investigate the electronic states and chemical bonding related to Au induced atomic chains on the Ge(001) surface. Angle-resolved photoemission reveals two types of dispersions around the Fermi level whose intensities strongly depend on the incident photon energy. Around $h\nu =100\mathrm{eV}$, the band structure is dominated by an electronlike band of mainly one-dimensional (1D) character, which shows Tomonaga-Luttinger-like power-law behavior in the $k$-integrated spectral function. In contrast, lower photon energies reveal a metallic holelike dispersion which resembles the Ge bulk structure with its heavy-hole, light-hole, and split-off branches. The Au $4f$ core-level spectra show two doublets indicating two different Au bonding sites, whereas the Ge $3d$ core-level shows two surface components and one bulk component.

Combining experimental data, numerical transport calculations, and theoretical analysis, we study the temperature-dependent resistivity of high-mobility two-dimensional (2D) Si MOSFETs to search for signatures of weak localization induced quantum corrections in the effective metallic regime above the critical density of the so-called two-dimensional metal-insulator transition (2D MIT). The goal is to look for the effect of logarithmic insulating localization correction to the metallic temperature dependence in the 2D conductivity so as to distinguish between the 2D MIT being a true quantum phase transition versus being a finite-temperature crossover. We use the Boltzmann theory of resistivity including the temperature-dependent screening effect on charged impurities in the system to fit the data. We analyze weak perpendicular field magnetoresistance data taken in the vicinity of the transition and show that they are consistent with weak localization behavior in the strongly disordered regime $k_F\ell \gtrsim 1$. Therefore, we supplement the Boltzmann transport theory with a logarithmic in temperature quantum weak localization correction and analyze the competition of the insulating temperature dependence of this correction with the metallic temperature dependence of the Boltzmann conductivity. Using this minimal theoretical model, we find that the logarithmic insulating correction is masked by the metallic temperature dependence of the Boltzmann resistivity and therefore the insulating $lnT$ behavior may be apparent only at very low temperatures which are often beyond the range of temperatures accessible experimentally. Analyzing the low-$T$ experimental Si MOSFET transport data, we identify signatures of the putative insulating behavior at low temperature and density in the effective metallic phase.

Of fundamental interest in the field of spintronics is the mechanism of indirect exchange coupling between magnetic impurities embedded in metallic hosts. A range of physical features, such as magnetotransport and overall magnetic moment formation, are predicated upon this magnetic coupling, often referred to as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Recent theoretical studies on the RKKY in graphene have been motivated by possible spintronic applications of magnetically doped graphene systems. In this paper a combination of analytic and numerical techniques are used to examine the effects of defect dimensionality on such an interaction. We show, in a mathematically transparent manner, that moving from single magnetic impurities to extended lines of impurities effectively reduces the dimensionality of the system and increases the range of the interaction. This has important consequences for the spintronic application of magnetically-doped systems, and we illustrate this with a simple magnetoresistance device.

We show that bilayer graphene (BLG) exhibits magnetoelectric (ME) effects that are formally similar to those commonly seen in band insulators with broken inversion and time-reversal symmetries. Three unusual features characterize the ME responses exhibited by BLG: (i) unlike most other ME media, BLG is a conductor, (ii) BLG has a nonquantized ME coupling even though its electronic structure does not break parity and time-reversal symmetry, and (iii) the magnitude of the ME coupling in BLG is determined by the valley-isospin density, which can be manipulated experimentally. This last property also enables a purely electric measurement of valley-isospin densities. While our theoretical arguments use BLG as an example, they are generally valid for any material with similar symmetries.

The high-pressure behavior of andradite garnet, ${\mathrm{Ca}}_3{\mathrm{Fe}}_2{\mathrm{[SiO}}_4$]${}_3$, was studied at pressures up to 80 GPa using single-crystal synchrotron x-ray diffraction, Raman spectroscopy, and quantum mechanical calculations based on density functional theory. An isosymmetric phase transition was observed in the pressure range between 60 and 70 GPa, which is associated with a gradual high-spin to low-spin electronic transition in ${\mathrm{Fe}}^{3+}$. Experimental structural data before and after the phase transition are in excellent agreement with the theoretically predicted structural compression of the high-spin and low-spin phases, respectively. While the overall unit-cell volume is reduced by about $2.5\%$ across the phase transition, a collapse of about $10\%$ of the ${\mathrm{FeO}}_6$ octahedral volume is observed, attributed to the reduced Fe–O bond lengths associated with low-spin ${\mathrm{Fe}}^{3+}$. In combination with earlier data the present study shows that a spin collapse of ${\mathrm{Fe}}^{3+}$ in ${\mathrm{FeO}}_6$ octahedra will be triggered if the Fe–O bond length reaches a critical value of $d$(Fe–O) $<$ $\approx$ 1.9 Å. Earlier reported results for the compressibility of the cation coordination polyhedra are substantially revised, with $B_{{\mathrm{FeO}}_6}=195\left(2\right)$ GPa, $B_{{\mathrm{CaO}}_8}=104\left(2\right)$ GPa, and $B_{{\mathrm{SiO}}_4}=348\left(11\right)$ GPa for the high-spin phase. The mode Grüneisen parameters range between 0.61 and 1.34. The computed spin-pairing energy is $\approx 4.2$ eV at 0 GPa.

This paper shows by computer simulations that some crystalline systems have curves in their thermodynamic phase diagrams, so-called isomorphs, along which structure and dynamics in reduced units are invariant to a good approximation. The crystals are studied in a classical-mechanical framework, which is generally a good description except significantly below melting. The existence of isomorphs for crystals is validated by simulations of particles interacting via the Lennard-Jones pair potential arranged into a face-centered cubic (fcc) crystalline structure; the slow vacancy-jump dynamics of a defective fcc crystal is also shown to be isomorph invariant. In contrast, a NaCl crystal model does not exhibit isomorph invariances. Other systems simulated, though in less detail, are the Wahnström binary Lennard-Jones crystal with the ${\mathrm{MgZn}}_2$ Laves crystal structure, monatomic fcc crystals of particles interacting via the Buckingham pair potential and via a purely repulsive pair potential diverging at a finite separation, an ortho-terphenyl molecular model crystal, and SPC/E hexagonal ice. Except for NaCl and ice, the crystals simulated all have isomorphs. Based on previous simulations of liquid models, we conjecture that crystalline solids with isomorphs include most or all formed by atoms or molecules interacting via metallic or van der Waals forces, whereas covalently bonded or hydrogen-bonded crystals are not expected to have isomorphs; crystals of ions or dipolar molecules constitute a limiting case for which isomorphs are only expected when the Coulomb interactions are relatively weak. We briefly discuss the consequences of the findings for theories of melting and crystallization.

We present a first-principles investigation of the electronic structure and physical properties of doped lithium nitridometalates ${\mathrm{Li}}_2$(${\mathrm{Li}}_{1-x}{\mathrm{M}}_x$)N (LiMN) with M = Cr, Mn, Fe, Co, and Ni. The diverse properties include the equilibrium magnetic moments, magneto-crystalline anisotropy, magneto-optical Kerr spectra, and x-ray magnetic circular dichroism. We explain the colossal magnetic anisotropy in LiFeN by its unique electronic structure which ultimately leads to a series of unusual physical properties. The most unique property is a complete suppression of relativistic effects and freezing of orbital moments for in-plane orientation of the magnetization. This leads to the colossal spatial anisotropy of many magnetic properties including energy, Kerr, and dichroism effects. LiFeN is identified as an ultimate single-ion anisotropy system where a nearly insulating state can be produced by a spin orbital coupling alone. A very nontrivial strongly fluctuating and sign changing character of the magnetic anisotropy with electronic $3d$-atomic doping is predicted theoretically. A large and highly anisotropic Kerr effect due to the interband transitions between atomic-like Fe $3d$ bands is found for LiFeN. A giant anisotropy of the x-ray magnetic circular dichroism for the Fe $K$ spectrum and a very weak one for the Fe $L_{2,3}$ spectra in LiFeN are also predicted.

A phase which shows pressure-induced shuttlewise structural deformation between orthorhombic $Fddd$ and tetragonal $I{4}_1/amd$ structures has been predicted in solid atomic hydrogen by means of the first-principles calculations, including harmonic zero-point energy contributions of proton motions. The $Fddd$ structure is formed by shear distortion from the $I{4}_1/amd$ structure, and the angle specifying the distortion changes with pressure in the range $84–{96}^{\circ }$ around ${90}^{\circ }$, which corresponds to $I{4}_1/amd$. In the shuttlewise deforming phase, the electron-phonon interaction is enhanced owing to phonon softenings, which brings about superconductivity at elevated temperatures.

Low hole mobility currently limits the efficiency of amorphous silicon photovoltaic devices. We explore three possible phenomena contributing to this low mobility: coordination defects, self-trapping ionization displacement defects, and lattice expansion allowing for hole wave-function delocalization. Through a confluence of experimental and first-principles investigations, we demonstrate the fluidity of the relative prevalence of these defects as film stress and hydrogen content are modified, and that the mobility of a film is governed by an interplay between various defect types.

The question of the minimum energy required to change the information content of physical switches is attracting a growing interest because current computer technology is facing fundamental challenges, such as increased power dissipation, and rapidly approaching the limits of scaling. In this context, bistable nanomagnetic switches can be used to store bits of information, associating each logic state to a different equilibrium orientation of the magnetization. Here we present the results of virtual experiments implemented by micromagnetic simulations on elongated dots of Permalloy, performing a reset to 1 operation, also known as Landauer erasure. We show that for isolated, realistic, elliptical dots with sub-100 nm lateral dimension, the minimum-energy consumption turns out to be consistent with the value expected for an ideal bistable switch, i.e., $E=k_{B}T\times ln\left(2\right)$, known as the Landauer limit. The only condition to achieve the correct limit is that the adopted erasure protocol includes a proper time interval where the probability distribution of the magnetization has the same amplitude in the two possible states. However, it is shown that significant deviations from the theoretical limit can be found either increasing the lateral dot dimension above about 100 nm or considering a rectangular (rather than elliptical) dot shape, where the presence of straight edges promotes a nonuniform behavior of the dot magnetization. Finally, we show that if the magnetic switch is not isolated, as it happens in potential devices where a regular array of dots is present, the dipolar interaction among adjacent switches can lead to either a reduction or an increase of the minimum dissipated energy, depending on the specific geometric arrangement of the magnetic dots.

We discuss how to construct a tight-binding model Hamiltonan for the simplest possible solid, composed of hydrogen-like atoms. A single orbital per atom is not sufficient because the onsite electron-electron repulsion mixes in higher-energy orbitals. The essential physics is captured by a dynamic Hubbard model with one electronic orbital and an auxiliary spin degree of freedom per site. We point out that this physics can lead to a substantial shift in the position and width of electronic energy bands relative to what is predicted by conventional band-structure calculations.