The anisotropy of Ti${}_2$AlC transport properties is investigated focusing on the Hall effect and resistivity vs temperature measurements performed on a highly (000l)-oriented thin film and a bulk polycrystalline sample. Experimental data are interpreted on the basis of density functional theory calculations including transport coefficients obtained with the Boltzmann semiclassical transport equation in the isotropic relaxation time approximation. It is shown that the Hall constant is independent of the temperature and that the charge-carrier sign depends on the investigated crystallographic orientation. Charge carriers exhibit a holelike character along the basal plane of the Ti${}_2$AlC, whereas the bulk sample Hall constant is negative. The resistivity anisotropy is also evidenced: using an effective medium approach, the room temperature basal plane resistivity is shown to be more than one order of magnitude lower than that along the $c$ axis. This very important anisotropy is shown to result from the anisotropy of the Fermi surface increased by electron-phonon interactions. These interactions are much more important along the $c$ axis than within the basal plane, a situation opposite to that observed in literature for Ti${}_2$GeC where resistivity was reported to be isotropic.

We study graphene antidot lattices (GALs) in magnetic fields. Using a tight-binding model and a recursive Green's function technique that we extend to deal with periodic structures, we calculate Hofstadter butterflies of GALs. We compare the results to those obtained in a simpler gapped graphene model. A crucial difference emerges in the behavior of the lowest Landau level, which in a gapped graphene model is independent of magnetic field. In stark contrast to this picture, we find that in GALs the band gap can be completely closed by applying a magnetic field. While our numerical simulations can only be performed on structures much smaller than can be experimentally realized, we find that the critical magnetic field for which the gap closes can be directly related to the ratio between the cyclotron radius and the neck width of the GAL. In this way, we obtain a simple scaling law for extrapolation of our results to more realistically sized structures and find resulting quenching magnetic fields that should be well within reach of experiments.

The zero-line mode can be produced in AB-stacking bilayer graphene under a spatially varying electric field. We investigate the transport properties of the zero-line mode in a six-terminal Hall bar system in the presence of a uniform magnetic field. We find that the Hall resistance is zero and the longitudinal resistance exhibits a plateau with $1/2(h/2e^2)$, when the Fermi level lies between the lowest conduction and highest valence Landau levels, which corresponds to the zero-line modes. Since the zero-line mode propagates along the domain wall separating opposite valley-Hall topologies, we also numerically measure the currents between source and drain and the conductance between two of the transversal terminals. Our finding shows that the current between source and drain is due to the existence of kink state, which can serve as a scheme to detect the zero-line mode. We further show that the zero-line mode under strong magnetic field is robust against disorders.

The interface electronic structure of correlated LaTiO${}_3$/SrTiO${}_3$ superlattices is investigated by means of the charge self-consistent combination of the local density approximation (LDA) to density functional theory with dynamical mean-field theory. Utilizing a pseudopotential technique together with a continuous-time quantum Monte Carlo approach, the resulting complex multiorbital electronic states are addressed in a coherent fashion beyond static mean field. General structural relaxations are taken into account on the LDA level and cooperate with the driving forces from strong electronic correlations. This alliance leads to a Ti($3d_{xy}$) dominated low-energy quasiparticle peak and a lower Hubbard band in line with photoemission studies. Furthermore correlation effects close to the band-insulating bulk SrTiO${}_3$ limit as well as the Mott-insulating bulk LaTiO${}_3$ limit are studied via realistic single-layer embeddings.

Epitaxial growth of Bi${}_2$Se${}_3$ layers usually takes place in the (0001) orientation due to the stability of this surface terminated by the van der Waals bonds. Here, we show that the layers grown on InP($11n$) substrates ($n=$ 3, 4, and 5) are not (0001) oriented. The approximate lattice match at the heterointerface leads to an alignment between the [0001] direction of Bi${}_2$Se${}_3$ and the [111] direction of InP. The consequential tilt of the Bi${}_2$Se${}_3$(0001) plane with respect to the surface of the high-index substrates gives rise to a formation of giant corrugations consisting of the (0001) and ($1\overline{1}00$) facets. We demonstrate critical influences of the in-plane polarization and miscut of the substrates which emerge owing to the strong overlayer-substrate interaction in the semicoherent heteroepitaxy: Twin domains are eliminated and the layers are strained to the extent that the lattice symmetry is altered. We examine the Dirac band structure under strain using density functional calculations. The Dirac point shifts away from the $\Gamma$ point and the spin degeneracy is lifted when the strain is in the Bi${}_2$Se${}_3$[$1\overline{1}00$] direction as the spatial inversion symmetry is removed.

The zone-folding method is a widely used technique in computing the electronic structure of carbon nanotubes. In this paper, curvature effects of boron and carbon nanotubes of different diameters and chiralities are systematically quantified using the density-functional-based tight-binding method. Here, the curvature effect in a nanotube is defined as the difference between the one-dimensional band structure calculated from the tubular atomic structure and the band structure calculated from the related two-dimensional sheet with the zone-folding method. For each nanotube, we quantify this difference by calculating the standard deviation of the band energies ${\sigma }_E$ and the maximal relative deviation between the derived ballistic currents $\delta I_{\max }$. For all considered nanotubes with diameters $d>2$ nm, the standard deviation ${\sigma }_E$ is below 60 meV and decreases only slowly, whereas $\delta I_{\max }$ is still as large as 8$\%$ and does not tend to zero for large $d$.

We study the static and dynamic behavior of transverse domain walls and 360${}^{\circ }$ domain walls in a thin film of isotropic material including pinning effects caused by geometric defects in the form of triangular antinotches. In terms of the static interaction, our model reduces the domain walls to sources of magnetic charge, allowing an electrostatic-like description of their interaction. Such a concept was applied to both of these magnetic textures allowing us to estimate the shortest distance between the antinotches at which the domains walls can be located, while still being pinned. Regarding the domain walls' dynamical behavior, accurate micromagnetic simulations of our system were performed, characterizing their recombination times as well as showing that triangular notches allow the coherent movement of single and arrays of 360${}^{\circ }$ domain walls by pulses of spin current. This behavior could not be observed in single transverse walls given the long-range interaction that they present, impeding coherent domain wall motion. These findings allow us to estimate the maximum 360${}^{\circ }$ domain wall density, observing an increase by a factor of four when compared to systems based on single transverse domain walls, which potentially gives our system important industrial applications.

An all-superconducting bijunction consists of a central superconductor contacted to two lateral superconductors, such that nonlocal crossed Andreev reflection is operating. Then new correlated transport channels for the Cooper pairs appear in addition to those of separated conventional Josephson junctions. We study this system in a configuration where the superconductors are connected through gate-controllable quantum dots. Multipair phase-coherent resonances and phase-dependent multiple Andreev reflections are both obtained when the voltages of the lateral superconductors are commensurate, and they add to the usual local dissipative transport due to quasiparticles. The two-pair resonance (quartets) as well as some other higher order multipair resonances are $\pi$ shifted at low voltage. Dot control can be used to dramatically enhance the multipair current when the voltages are resonant with the dot levels.

We present density functional theory (DFT) simulations and shock-reshock experiments for liquid carbon dioxide (CO${}_2$) in the range 100 to 800 GPa. The simulations support the previously suggested dissociation threshold around 50 GPa [W. J. Nellis et al., J. Chem. Phys. 95, 5268 (1991)] for shocked liquid CO${}_2$ and describe a very steep Hugoniot past dissociation. We performed the shock-reshock experiments using the Sandia Z machine. The Z machine magnetically accelerated aluminum flyer plates to shock compress cryogenic liquid CO${}_2$ to 550 GPa and attained reshock states up to 840 GPa. The plate impact experiments combined with well-characterized impedance matching standards and laser velocimetry results in high-accuracy measurements of the principal Hugoniot and reshock states of liquid CO${}_2$. The experimental results validated the DFT simulations at extreme conditions and the combination of experiment and DFT provide reliable data for evaluating existing and constructing future wide-range equations of state models for molecular compounds such as CO${}_2$.

In this paper, we investigate excited state dynamics in amorphous rubrene vacuum sublimed films. We report the direct observation of singlet fission in amorphous rubrene films. We have determined the fission rate to be >2.5×10${}^{12}$ s${}^{-1}$. Simultaneously, we observe strong polaron pair absorption and propose that polaron pair formation could be competing with singlet fission. Another possible conclusion from our experiments could be that two triplets from singlet fission might arise via polaron pairs. In either case, polaron pairs play an important role in singlet fission in an amorphous rubrene film. We also observe that triplets created by singlet fission fuse to regenerate a singlet, giving delayed fluorescence (DF) scaling linearly with initial laser energy (i.e., one singlet gives two triplets and two triplets give back one singlet). This is a strong evidence of $S_1^n\to 2T_1$. We did not observe substantial temperature dependence of DF decay curve shape, indicating that triplet migration in amorphous rubrene films is not hopping limited and that triplets undergo fusion before their migration.

We investigate the role of electron-electron interaction in a two-band Hubbard model based on the Bernevig-Hughes-Zhang Hamiltonian exhibiting the quantum spin Hall (QSH) effect. By means of dynamical mean-field theory, we find that a system with topologically trivial noninteracting parameters can be driven into a QSH phase at finite interaction strength by virtue of local dynamical fluctuations. For very strong interaction, the system reenters a trivial insulating phase by going through a Mott transition. We obtain the phase diagram of our model by direct calculation of the bulk topological invariant of the interacting system in terms of its single-particle Green's function.

Reflectance spectroscopy was performed for nonpolar and semipolar bulk aluminum nitride (AlN) substrates under uniaxial stress. The exciton-polariton theory was applied to interpret the reflectivity spectra, while the Bir-Pikus deformation potential theory was used to analyze the stress dependence of exciton transition energies. This approach allowed all the deformation potentials in AlN to be determined without the quasicubic approximation.

We study the potential for controlled population inversion in a coupled system comprised of a semiconductor quantum dot and a metal nanoparticle. We show that the widely used method of population inversion by a $\pi$ pulse can be modified for small interparticle distances. This modification depends strongly on the pulse duration. We also present analytical solutions of the nonlinear density matrix equations, for specific pulse envelopes, which lead to efficient excitonic population inversion in the quantum dot for several distances between the semiconductor quantum dot and the metal nanoparticle.

A balance-equation scheme is developed to investigate the magnetotransport in a dc current biased graphene. We examine the Shubnikov–de Haas oscillation under a nonzero bias current. With an increase in the current density, the oscillatory differential resistivity exhibits phase inversion, in agreement with recent experimental observation. In the presence of surface optical phonons, a second phase inversion may occur at higher dc bias due to the reduced influence of electron heating and the enhanced direct effect of current on differential magnetoresistivity. We also predict the appearance of current-induced magnetoresistance oscillation in suspended graphene at lower magnetic fields and larger current densities. For the graphene mobility currently available ($\approx 20$ m${}^2/$V s), the oscillatory behavior may be somewhat altered by magnetophonon resonance arising from intrinsic acoustic phonon under finite bias current conditions.

Double resonant sum-frequency generation (SFG) is demonstrated in Cu${}_2$O. The $1S$ yellow orthoexciton-polariton is resonantly excited by a single frequency dye laser (quadrupole transition) and coupled to the blue $1S$ exciton-polariton by a single frequency infrared laser (dipole transition). Sum-frequency (SF) spectra as a function of the energy of the infrared laser are measured for the two pump laser beams parallel and antiparallel. In the antiparallel configuration, pronounced oscillations are observed with a period scaling inversely with the crystal length. The origin of these oscillations is discussed in detail and attributed to a phase-matching effect. The appearance of phase matching can be understood if the blue polaritons—generated by SFG—move coherently through the crystal. This observation is very surprising, since blue exciton-polaritons excited in a one-photon transmission experiment exhibit an absorption length of only $100\text{nm}$ due to strong scattering.

The effects of steps and ordered defects on the surface states supported by Cu(110) terraces are investigated by a combination of reflection anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM). For several vicinal (110)-type surfaces, we measure the 2.1-eV RAS peak arising from transitions between surface states. A Poelsema-Comsa scattering model is used to relate the intensity of this RAS signal to the surface morphology (the distributions of terrace widths and step-edge roughness) observed by STM, providing a measure of the ability of the surface defects to scatter the Shockley-type surface state. A scattering cross section of area equivalent to 20 unit cells is obtained—a value consistent with previous results obtained from other types of defect for the Cu(110) surface. We find that the Poelsema-Comsa scattering model, originally developed for random distributions of defects, is also applicable to the modeling of RAS intensities of surfaces with ordered and partially ordered defects: specifically steps. Our results highlight the growing importance of the Poelsema-Comsa methodology in combination with RAS data for extracting topographic information associated with surface defects, particularly from surfaces in hostile environments, where RAS can access as a real-time in situ probe.

Low-temperature transport measurements on a $p$-GaAs quantum point contact are presented which reveal the presence of a conductance anomaly that is markedly different from the conventional “0.7 anomaly.” A lateral shift by asymmetric gating of the conducting channel is utilized to identify and separate different conductance anomalies of local and generic origins experimentally. While the more generic 0.7 anomaly is not directly affected by changing the gate configuration, a model is proposed which attributes the additional conductance features to a gate-dependent coupling of the propagating states to localized states emerging due to a nearby potential imperfection. Finite bias conductivity measurements reveal the interplay between the two anomalies consistently with a two-impurity Kondo model.

We present a first-principles approach for inelastic quantum transport calculations based on maximally localized Wannier functions. Electronic-structure properties are obtained from density-functional theory in a plane-wave basis, and electron-vibration coupling strengths and vibrational properties are determined with density-functional perturbation theory. Vibration-induced inelastic transport properties are calculated with nonequilibrium Green's function techniques; since these are based on a localized orbital representation we use maximally localized Wannier functions. Our formalism is applied first to investigate inelastic transport in a benzene molecular junction connected to monoatomic carbon chains. In this benchmark system the electron-vibration self-energy is calculated either in the self-consistent Born approximation or by lowest-order perturbation theory. It is observed that upward and downward conductance steps occur, which can be understood using multieigenchannel scattering theory and symmetry conditions. In a second example, where the monoatomic carbon chain electrode is replaced with a $(3,3)$ carbon nanotube, we focus on the nonequilibrium vibration populations driven by the conducting electrons using a semiclassical rate equation and highlight and discuss in detail the appearance of vibrational cooling as a function of bias and the importance of matching the vibrational density of states of the conductor and the leads to minimize joule heating and breakdown.

Using a general symmetry-based approach, we provide a classification of generic miniband structures for electrons in graphene placed on substrates with the hexagonal Bravais symmetry. In particular, we identify conditions at which the first moiré miniband is separated from the rest of the spectrum by either one or a group of three isolated mini Dirac points and is not obscured by dispersion surfaces coming from other minibands. In such cases, the Hall coefficient exhibits two distinct alternations of its sign as a function of charge carrier density.