The eigenstates of an electron in the chiral two-dimensional electron gas (C2DEG) formed in an AB-stacked bilayer or an ABC-stacked trilayer graphene is a spinor with four or six components, respectively. These components give the amplitude of the wave function on the four or six carbon sites in the unit cell of the lattice. In the tight-binding approximation, the eigenenergies are thus found by diagonalizing a $4\times 4$ or a $6\times 6$ matrix. In the continuum approximation where the electron wave vector $k\ll 1/a_0,$ with $a_0$ the lattice constant of the graphene sheets, a common approximation is the two-component (or “two-band”) model1 where the eigenstates for the bilayer and trilayer systems are described by a two-component spinor that gives the amplitude of the wave function on the two sites with low energy $\left|E\right|\ll {\gamma }_1$ where ${\gamma }_1$ is the hopping energy between sites that are directly above one another in adjacent layers. The two-component model has been used extensively to study the phase diagram of the C2DEG in a magnetic field as well as its transport and optical properties. In this paper, we use a numerical approach to compute the eigenstates and Landau level energies of the full tight-binding model in the continuum approximation and compare them with the prediction of the two-component model when the magnetic field or an electrical bias between the outermost layers is varied. Our numerical analysis shows that the two-component model is a good approximation for bilayer graphene in a wide range of magnetic field and bias but mostly for Landau level $M=0.$ The applicability of the two-component model in trilayer graphene, even for level $M=0,$ is much more restricted. In this case, the two-component model fails to reproduce some of the level crossings that occur between the sublevels of $M=0.$

Here we discuss the possibility of modulating energy gaps of graphene nanoribbon bilayers, with zigzag edges, by applying electric fields. The system is disposed in the Bernal configuration and is described by a Hubbard Hamiltonian. We follow a Hartree-Fock mean-field theory to calculate the electronic properties of the system. Under the action of a transversal electric field, half-metallicity is found: One of the spin bands increases the gap energy as the intensity of the field is increased whereas the other decreases until achieving a null gap. For a particular electric field range, the system exhibits metallic and semiconducting features depending on the spin band. Half-metallicity is enhanced due to an extra effect for the bilayer system: The presence of a robust plateau-like in the gap versus field intensity diagram, for an intermediate energy gap value of the semiconducting band. The correlation of the gap plateau with local magnetizations and charge numbers in the two layers is investigated. Further applied gate voltages on the ribbons are considered to investigate the possibilities of getting new physical responses for tilted electric field configurations. Possible spintronic applications can be driven based on the differential spin-band features achieved.

Using first-principles calculations, we study the electronic structure of monolayer ${\mathrm{MoS}}_2$ under uniaxial strain. We show that the energy valleys drift far off the corners of the Brillouin zone (K points), about 12 times the amount observed in graphene. Therefore, it is essential to take this effect into consideration for a correct identification of the band gap. The system remains a direct band gap semiconductor up to 4% uniaxial strain, while the size of the band gap decreases from 1.73 to 1.54 eV. We also demonstrate that the splitting of the valence bands due to inversion symmetry breaking and spin-orbit coupling is not sensitive to strain.

The isotopes of hydrogen provide a unique exploratory laboratory for examining the role of zero point energy (ZPE) in determining the structural and dynamic features of the crystalline ices of water. There are two critical regions of high pressure: (i) near 1 TPa and (ii) near the predicted onset of metallization at around 5 TPa. At the lower pressure of the two, we see the expected small isotopic effects on phase transitions. Near metallization, however, the effects are much greater, leading to a situation where tritiated ice could skip almost entirely a phase available to the other isotopomers. For the higher pressure ices, we investigate in some detail the enthalpics of a dynamic proton sublattice, with the corresponding structures being quite ionic. The resistance toward diffusion of single protons in the ground state structures of high-pressure H${}_2$O is found to be large, in fact to the point that the ZPE reservoir cannot overcome these. However, the barriers toward a three-dimensional coherent or concerted motion of protons can be much lower, and the ensuing consequences are explored.

Defect complexes consisting of Cu substitutionals on Nb sites and oxygen vacancies in potassium niobate, KNbO${}_3$, are investigated with respect to their contribution to ferroelectric hardening by means of density-functional theory and classical atomistic simulations. We determine the easy and hard directions for the ferroelectric polarization created by these defect complexes, the energy differences between easy and hard directions, and upper limits for the energy barriers for switching the ferroelectric polarization between these directions. The ferroelectric polarization preferentially aligns with the defect complexes, which is expected to impede polarization switching and hence to contribute to ferroelectric hardening.

We theoretically investigate the applied magnetic field-angle dependence of the flux-flow resistivity ${\rho }_{\mathrm{f}}\left({\alpha }_{\mathrm{M}}\right)$ for a uniaxially anisotropic Fermi surface. ${\rho }_{\mathrm{f}}$ is related to the quasiparticle scattering rate inside a vortex core, which reflects the sign change in the superconducting pair potential. We find that ${\rho }_{\mathrm{f}}\left({\alpha }_{\mathrm{M}}\right)$ is sensitive to the sign change in the pair potential and has its maximum when the magnetic field is parallel to the gap-node direction. We propose the measurement of the field-angle dependent oscillation of ${\rho }_{\mathrm{f}}\left({\alpha }_{\mathrm{M}}\right)$ as a phase-sensitive field-angle resolved experiment.

We have performed extensive density matrix renormalization group (DMRG) studies of the Hubbard model on a honeycomb ladder. The band structure (with Hubbard $U=0$) exhibits an unusual quadratic band touching at half-filling, which is associated with a quantum Lifshitz transition from a band insulator to a metal. For one electron per site, nonzero $U$ drives the system into an insulating state in which there is no pair-binding between added electrons; this implies that superconductivity driven directly by the repulsive electron-electron interactions is unlikely in the regime of small doping, $x\ll 1$. However, the divergent density of states as $x\to 0$, the large values of the phonon frequencies, and an unusual correlation induced enhancement of the electron-phonon coupling imply that lightly doped polyacenes, which approximately realize this structure, are good candidates for high-temperature electron-phonon driven superconductivity.

We study the relationship between the nematic phases of Sr${}_3$Ru${}_2$O${}_7$ and quantum criticality. At ambient pressure, one nematic phase is associated with a metamagnetic quantum critical end point (QCEP) when the applied magnetic field is near the $c$ axis. We show, however, that this metamagnetic transition does not produce the same nematic signatures when the QCEP is reached by hydrostatic pressure with the field applied in the $ab$ plane. Moreover, a second nematic phase, that is seen for field applied in the $ab$ plane close to, but not right at, a second metamagnetic anomaly, persists with minimal change to the highest applied pressure, 16.55 kbar. Taken together our results suggest that metamagnetic quantum criticality may not be necessary for the formation of a nematic phase in Sr${}_3$Ru${}_2$O${}_7$.

Exciton polaritons can be created in semiconductor microcavities. These quasiparticles act as weakly interacting bosons with very light mass, of the order of ${10}^{-4}$ times the vacuum electron mass. Many experiments have shown effects which can be viewed as due to a Bose-Einstein condensate, or quasicondensate, of these particles. The lifetime of the particles in most of those experiments has been of the order of a few picoseconds, leading to significant nonequilibrium effects. By increasing the cavity quality, we have made samples with longer polariton lifetimes. With a photon lifetime on the order of 100–200 ps, polaritons in these structures can not only come closer to reaching true thermal equilibrium, a desired feature for many researchers working in this field, but they can also travel much longer distances. We observe the polaritons to ballistically travel on the order of 1 mm, and at higher densities we see transport of a coherent condensate, or quasicondensate, over comparable distances. In this paper we report a quantitative analysis of the flow of the polaritons both in a low-density, classical regime, and in the coherent regime at higher density. Our analysis gives us a measure of the intrinsic lifetime for photon decay from the microcavity and a measure of the strength of interactions of the polaritons.

A geometry-independent effective model for the contact self-energies is proposed to calculate the quantum conductance of patterned graphene devices using Green's functions. A Corbino disk, being the simplest device where the contacts cannot be modeled as semi-infinite ribbons, is chosen to illustrate this approach. This system's symmetry allows an analytical solution against which numerical calculations on the lattice can be benchmarked. The effective model perfectly describes the conductance of Corbino disks at low-to-moderate energies, and is robust against the size of the annular device region, the number of atoms on the edge, external magnetic fields, or electronic disorder. The contact model considered here affords an expedient, flexible, and geometry-agnostic approach that easily allows the consideration of device dimensions encompassing several million atoms, and realistic radial dimensions of a few hundreds of nanometers.

Thermoelectric effects in zigzag graphene nanoribbons with parallel alignment of the edge spin polarizations are investigated theoretically. Spin and charge thermopower, electrical and heat conductance, and charge and spin thermoelectric efficiency are calculated numerically for pristine nanoribbons as well as for nanoribbons with periodic one-dimensional lattice of structural defects in the form of antidots. It is shown that structural defects reduce thermal conductance due to phonons and open gaps in the corresponding electronic spectrum. This, in turn, leads to a significant enhancement of the Seebeck and spin Seebeck coefficients as well as of the thermoelectric efficiency. A giant enhancement appears in certain regions of chemical potential (controlled by doping or external gate) and survives at room temperatures.

The formation and oxidation reaction energies of 16 transition metal oxides (TMOs) are benchmarked against experiments with an increasing complexity of the exchange-correlation (xc) functionals: PBE, PBE + $U$ with a single $U$ for each transition metal element, PBE0 (25% exact exchange included), EXX (100% exact exchange), and EXX + RPA (random phase approximation for the correlation energy). Although rather challenging on standard CPU computing facilities, the RPA calculations were performed efficiently on graphic processing units (GPUs). For the formation energies, the PBE + $U$, PBE0, EXX + RPA improves significantly over PBE with mean absolute errors (MAE) of 0.83 (PBE), 0.39 (PBE + $U$), 0.34 (PBE0), and 0.39 (EXX + RPA) eV per oxygen. In addition, EXX + RPA improves over the other xc functionals on the oxidation reaction energies, with MAE of 0.27 (PBE), 0.28 (PBE + $U$), 0.30 (PBE0), to 0.13 (EXX + RPA) eV per oxygen. The distinct trend observed for the calculated oxidation reaction energies compared to the formation energies is due to that the errors in formation energies for PBE and EXX + RPA are systematic; while for PBE + $U$ and PBE0 the deviations have both signs, so that the error cancellations between different valence states work better for PBE and EXX + RPA. Finally, we compared the performance of the EXX + RPA for total energies and $G_0{W}_0$, which uses the random phase approximation in constructing the $W$ kernel, for band gaps, and discuss a few challenges for the EXX + RPA method on TMOs.

We derive the system of hydrodynamic equations governing the collective motion of massless fermions in graphene. The obtained equations demonstrate the lack of Galilean and Lorentz invariance and contain a variety of nonlinear terms due to the quasirelativistic nature of carriers. Using these equations, we show the possibility of soliton formation in an electron plasma of gated graphene. The quasirelativistic effects set an upper limit for soliton amplitude, which marks graphene out of conventional semiconductors. The mentioned noninvariance of the equations is revealed in spectra of plasma waves in the presence of steady flow, which no longer obey the Doppler shift. The feasibility of plasma-wave excitation by direct current in graphene channels is also discussed.

In this article first-principles DFT calculations and molecular dynamics simulations using empirical potentials have been used to study four different polymorphs of uranium dioxide that appear under high compressive and tensile deformations. It has been found, as expected, that the ground-state structure is the fluorite-type structure (space group $Fm\overline{3}m$). Under high compressive deformation urania transforms into cotunnite-type structure (space group $Pnma$), as already known experimentally. The calculated transition pressure is 28 GPa in agreement with the experimental data. Under tensile deformation urania transforms into either scrutinyite-type structure (space group $Pbcn$) or rutile-type (space group $P{4}_2/mnm$) structure. These two phases are almost energetically degenerate; hence it is impossible to distinguish which phase is the most favorable. The transition pressure for both phases is found to be equal to $-10$ GPa. Subsequently, assessment of four of the most used empirical potentials for ${\mathrm{UO}}_2$—Morelon, Arima, Basak, and Yakub—have been carried out comparing the equations of state with those found with DFT calculations. The Morelon potential has been found to be the most accurate to describe the different urania polymorphs. Using this empirical potential and a dedicated minimization procedure, complete transition pathways between the ground state ($Fm\overline{3}m$) and both tensile structures ($Pbcn$ or $P{4}_2/mnm$) are described. Finally, uniaxial tensile load molecular dynamics simulations have been performed. It has been found that for load in the $\langle 100\rangle$ direction urania transforms into the $Pbcn$ structure while for load in the $\langle 110\rangle$ direction it transits towards the $P{4}_2/mnm$ structure.

By investigating the magnetism of spins on a quasiperiodic lattice, we present an experimental study of static and dynamic magnetic properties, specific heat, and magnetic entropy of the Gd${}_3$Au${}_{13}$Sn${}_4$ quasicrystalline approximant. The magnetic sublattice of Gd${}_3$Au${}_{13}$Sn${}_4$ is a periodic arrangement of nonoverlapping spin clusters of almost perfect icosahedral symmetry, where gadolinium localized $f$ magnetic moments are distributed on equilateral triangles. The absence of disorder on the magnetic sublattice and the antiferromagnetic (AFM) interactions between the nearest-neighbor spins distributed on triangles result in geometrical frustration of spin-spin interactions. Thus, the Gd${}_3$Au${}_{13}$Sn${}_4$ phase can be viewed as a prototype site-ordered, geometrically frustrated spin system on icosahedral clusters. The zero-field-cooled and field-cooled magnetic susceptibilities, the alternating current susceptibility, the thermoremanent magnetization, the memory effect, the magnetic specific heat, and the magnetic entropy all show that the spin system undergoes at low temperatures a transition to a nonergodic state at the spin freezing temperature $T_f$ ≈ 2.8 K. Below this, the ergodicity is broken on the experimental timescale, because the thermally activated correlation times for the spin reorientations become macroscopically long. The magnetic state achieved at low temperatures by continuous cooling in low magnetic fields is likely a superposition of (1) metastable states with randomly frozen spins that have no long-range order yet undergo gradual spin-freezing dynamics and (2) an AFM-like magnetically ordered state with critical slowing dynamics. The magnetic properties of the site-ordered, geometrically frustrated Gd${}_3$Au${}_{13}$Sn${}_4$ system are discussed in comparison to site-disordered spin glasses that contain both randomness and frustration.

We demonstrate a finite spin polarization of photoelectrons emitted from GaAs(100) covered by a fractional Ag film. The photoemission yield shows a sharp maximum for intermediate coverage and the spin polarization increases with increasing laser intensity. Photoelectrons are excited by circularly polarized 100 fs laser pulses of 800 nm wavelength. We recorded the photoemitted electrons using a photoemission electron microscope combined with a Mott spin polarimeter. The spin polarization is analyzed in dependence on the excitation frequency and intensity and on the average thickness of the silver film. The results are explained by a model combining multiphoton photoemission and optical field emission. Our observation suggests a coherent excitation of circularly polarized plasmons and a transfer of angular momentum to the spin of photoemitted electrons.

Systems with a bulk first-order transition can display diverging correlation lengths close to a surface. This surface induced disordering yields a special type of surface criticality. Using extensive numerical simulations we study surface quantities in the two-dimensional Potts model with a large number of states $q$ which undergoes a discontinuous bulk transition. The surface critical exponents are thereby found to depend on the value of $q$, which is in contrast to prior claims that these exponents should be universal and independent of $q$. It follows that surface-induced disordering at first-order transitions is characterized by exponents that depend on the details of the model.

The possibility of tracking the average rotation sense of the magnetization vector of a ferromagnetic layer upon magnetization reversal by means of magnetotransport measurements is explored. It is demonstrated that the rotation sense of the ferromagnetic magnetization vector during a hysteresis loop can be determined for the polycrystalline Co/CoO exchange bias system by measuring the anisotropic magnetoresistance (AMR) with a specific choice of the measurement geometry. The AMR measurements reveal that the rotation direction of the magnetization vector can be reversed by performing an in-plane hysteresis loop with a magnetic field perpendicular to the cooling field. This reversal can be directly linked to the experimental fact that after training, i.e., after performing hysteresis loops with a field along the cooling field direction, the average orientation of the uncompensated magnetization of the granular CoO antiferromagnet can be largely rotated back to the initial orientation after field cooling by applying a perpendicular field with the appropriate amplitude and orientation.

In their ideal structure, double perovskites such as Sr${}_2$FeMoO${}_6$ have alternating Fe and Mo along each cubic axis, and a homogeneous ferromagnetic metallic ground state. Imperfect annealing leads to the formation of structural domains. The moments on mislocated Fe atoms that adjoin each other across the domain boundary have an antiferromagnetic coupling between them. This leads to a peculiar magnetic state, with ferromagnetic domains coupled antiferromagnetically. At a short distance the system exhibits ferromagnetic correlation while at large length scales the net moment is strongly suppressed due to interdomain cancellation. We provide a detailed description of the spin-wave excitations of this complex magnetic state, obtained within a $1/S$ expansion, for a progressively higher degree of mislocation, i.e., antisite disorder. At a given wave vector the magnons propagate at multiple energies, related, crudely, to “domain confined” modes with which they have a large overlap. We provide a qualitative understanding of the trend observed with growing antisite disorder, and contrast these results to the much broader spectrum that one obtains for uncorrelated antisites.

Detailed numerical simulations of the IV characteristics of a Josephson junction under external electromagnetic radiation show the devil's staircase within different bias current intervals. We have found that the observed steps form very precisely continued fractions. Increase of the amplitude of the radiation shifts the devil's staircase to higher Shapiro steps. An algorithm for the appearance and detection of subharmonics with increasing radiation amplitude is proposed. We demonstrate that the subharmonic steps registered in the well-known experiments by Dayem and Wiegand [Phys. Rev. 155, 419 (1967)] and Clarke [Phys. Rev. B 4, 2963 (1971)] also form continued fractions.