Molecular dynamics simulations and lattice dynamics calculations are used to study the vibrational modes and thermal transport in Lennard-Jones superlattices with perfect and mixed interfaces. The secondary periodicity of the superlattices leads to a vibrational spectrum (i.e., dispersion relation) that is distinct from the bulk spectra of the constituent materials. The mode eigenvectors of the perfect superlattices are found to be good representations of the majority of the modes in the mixed superlattices for up to 20% interfacial mixing, allowing for extraction of phonon frequencies and lifetimes. Using the frequencies and lifetimes, the in-plane and cross-plane thermal conductivities are predicted using a solution of the Boltzmann transport equation (BTE), with agreement found with predictions from the Green-Kubo method for the perfect superlattices. For the mixed superlattices, the Green-Kubo and BTE predictions agree for the cross-plane direction, where thermal conductivity is dominated by low-frequency modes whose eigenvectors are not affected by the mixing. For the in-plane direction, mid-frequency modes that contribute to thermal transport are disrupted by the mixing, leading to an underprediction of thermal conductivity by the BTE. The results highlight the importance of using a dispersion relation that includes the secondary periodicity when predicting phonon properties in perfect superlattices and emphasize the challenges of estimating the effects of disorder on phonon properties.

We discuss the difficulties in discovering the Kondo effect in the resistivity of graphene. Similarly to the Kondo effect, electron-electron interaction effects and weak localization appear as logarithmic corrections to the resistance. In order to disentangle these contributions, a refined analysis of the magnetoconductance and the magnetoresistance is introduced. We present numerical simulations which display the discrimination of both effects. Further, we present experimental data of magnetotransport. When magnetic molecules are added to graphene, a logarithmic correction to the conductance occurs, which apparently suggests Kondo physics. Our thorough evaluation scheme, however, reveals that this interpretation is not conclusive: The data can be equally explained by electron-electron interaction corrections in an inhomogeneous sample. Despite our refined analysis, we conclude that additional spectroscopic information will be necessary to unambiguously identify the Kondo effect.

We demonstrate theoretically that an array of carbon nanoscrolls acts as a hyperbolic magnetic metamaterial in the terahertz regime with genuine subwavelength operation corresponding to a wavelength-to-structure ratio of about 200. Due to the low sheet resistance of graphene, the electromagnetic losses in an array of carbon nanoscrolls are almost negligible, offering a very sharp magnetic resonance of extreme positive and negative values of the effective magnetic permeability. The latter property leads to superior imaging properties for arrays of carbon nanoscrolls which can operate as magnetic endoscopes in the terahertz range where magnetic materials are scarce. Our optical modeling is supplemented with ab initio density functional calculations of the self-winding of a single layer of graphene onto a carbon nanotube so as to form a carbon nanoscroll. The latter process is viewed as a means to realize ordered arrays of carbon nanoscrolls in the laboratory based on arrays of aligned carbon nanotubes which are now routinely fabricated.

We investigate how a high current density perturbs the phonons in a biased graphene nanoconstriction coupled to semi-infinite electrodes. The coupling to electrode phonons, electrode electrons under bias, Joule heating, and current-induced forces is evaluated using first principles density functional theory and nonequilibrium Green's function calculations. We observe a strongly nonlinear heating of the phonons with bias and breakdown of the harmonic approximation when the Fermi level is tuned close to a resonance in the electronic structure of the constriction. This behavior is traced back to the presence of negatively damped phonons driven by the current. The effects may limit the stability and capacity of graphene nanoconstrictions to carry high currents.

First-principles Born-Oppenheimer molecular dynamics simulations of gallium clusters reproduce experimental specific heats to a high degree of accuracy, with no experimental input or fitting. Remaining systematic shifts of the melting temperature highlight important limitations of density-functional approximations. An analysis of the structural changes observed as a function of finite temperature provides evidence for the assignment of a melting temperature to a specific peak in the specific heat. The structural basis for features in the specific heat curves, in particular, the existence of multiple peaks, is demonstrated.

We have implemented the quasiparticle $GW$+dynamical mean field theory (DMFT) approach in the Vienna ab initio simulation package. To this end, a quasiparticle Hermitization of the $G_0{W}_0$ self-energy a lá Kotani-Schilfgaarde is employed, and the interaction values are obtained from the locally unscreened random phase approximation (RPA) using a projection onto Wannier orbitals. We compare quasiparticle $GW$+DMFT and local density approximation (LDA)+DMFT against each other and against experiment for SrVO${}_3$. We observe a partial compensation of stronger electronic correlations due to the reduced $GW$ bandwidth and weaker correlations due to a larger screening of the RPA interaction, so that the obtained spectra are quite similar and agree well with experiment. Noteworthy, the quasiparticle $GW$+DMFT better reproduces the position of the lower Hubbard side band.

GaN is a wide-band-gap semiconductor used in high-efficiency light-emitting diodes and solar cells. The solid is produced industrially at high chemical purities by deposition from a vapor phase, and oxygen may be included at this stage. Oxidation represents a potential path for tuning its properties without introducing more exotic elements or extreme processing conditions. In this work, ab initio computational methods are used to examine the energy potentials and electronic properties of different extents of oxidation in GaN. Solid-state vibrational properties of Ga, GaN, Ga${}_2$O${}_3$, and a single substitutional oxygen defect have been studied using the harmonic approximation with supercells. A thermodynamic model is outlined which combines the results of ab initio calculations with data from experimental literature. This model allows free energies to be predicted for arbitrary reaction conditions within a wide process envelope. It is shown that complete oxidation is favorable for all industrially relevant conditions, while the formation of defects can be opposed by the use of high temperatures and a high N${}_2$:O${}_2$ ratio.

In this work, we revisit the problem of predicting the surface segregation trends in binary transition metal alloys from the knowledge of the basic features of the pure component $d$-band electronic structure within tight-binding approximation. In contrast to previous trend studies, the present one includes, within the fourth-moment approximation (FMA) of the tight-binding scheme, both the difference in the average band energies (diagonal disorder) and the difference in the band widths (off-diagonal disorder) of the two components. We show that treating on the same footing these two effects is essential for a correct prediction of surface segregation. The presented study, giving a natural link between the electronic structure and mixing/segregation properties, is particularly interesting and useful in the context of the possibility of building efficient new energy models based on FMA for use in large (time) scale atomistic simulations.

To address the amount of disorder and interface diffusion induced by annealing, all-Heusler multilayer structures, consisting of ferromagnetic ${\text{Co}}_2\text{MnGe}$ and nonmagnetic Rh${}_2$CuSn layers of varying thicknesses, have been investigated by means of hard x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism. We find evidence for a 4 Å thick magnetically dead layer that, together with the identified interlayer diffusion, are likely reasons for the unexpectedly small magnetoresistance found for current-perpendicular-to-plane giant magnetoresistance devices based on this all-Heusler system. We find that diffusion begins already at comparably low temperatures between 200 and $250{}^{\circ }\mathrm{C}$, where Mn appears to be most prone to diffusion.

Er${}_2$Sn${}_2$O${}_7$  is considered, together with Er${}_2$Ti${}_2$O${}_7$, as a realization of the $XY$ antiferromagnet on the pyrochlore lattice. We present magnetization measurements confirming that Er${}_2$Sn${}_2$O${}_7$ does not order down to 100 mK but exhibits a freezing below 200 mK. Our neutron scattering experiments evidence the strong $XY$ character of the Er${}^{3+}$moment and point out the existence of short-range correlations in which the magnetic moments are in peculiar configurations, the Palmer-Chalker states, predicted theoretically for an $XY$ pyrochlore antiferromagnet with dipolar interactions. Our estimation of the Er${}_2$Sn${}_2$O${}_7$ parameters confirm the role of the latter interactions on top of relatively weak and isotropic exchange couplings.

Predicting the thermal stability of magnetic storage devices is an important and challenging task. Here, we demonstrate how the forward flux sampling method (FFS) can be used to determine the thermal stability of magnets with general microstructures for time scales ranging from picoseconds to years. To apply FFS to magnetic systems, we first use the nudged elastic band (NEB) method to determine a minimum energy path connecting the initial with the final state of the magnetic transition. Interfaces defined based on this minimum energy path then provide the basis for the FFS procedure in which dynamical trajectories are generated by integrating a stochastic version of the fundamental equation of motion of the magnetization (Landau-Lifshitz-Gilbert equation) at finite temperature. This approach allows to determine average lifetimes for incoherent reversal processes and it can be applied for any value of the damping constant. We validate the method for a single-grain particle by comparison with the results of direct Langevin simulations carried out and demonstrate its capabilities and efficiency by computing the lifetime of a graded media grain, a magnetic structure with a tailored magnetocrystalline spatial anisotropy profile.

We study $p$-wave pairing in a two-component Fermi system with unequal population across weak-coupling BCS to strong-coupling BEC regimes. We find a rich $m_s=0$ spin triplet $p$-wave superfluid (SF) ground state (GS) structure as a function of population imbalance. Under a phase stability condition, the “global” energy minimum is given by a multitude of “mixed” SF states formed of linear combinations of $m=\pm 1,0$ substates of the $\ell =1$ orbital angular momentum state. Except for the “pure” SF states ($\ell =1,m=\pm 1$), other states exhibit oscillation in energy with the relative phase between the constituent gap amplitudes. We also find states with “local” energy minimum that can be stable at higher polarizations, suggesting a quantum phase transition between the “global” and “local” minima phases driven by polarization. We relate the local and global minimum states to Morse and non-Morse critical points. We calculate the variations of the ground state energy with a relative phase angle, coupling, and polarization, and discuss possible consequences for experiments.

We have performed simultaneous measurements of magnetic chirality by using polarized neutrons and electric polarization along the $b$ axis of single crystals of YMn${}^{4+}$(Mn${}_{1-x}$Ga${}_x$)${}^{3+}$O${}_5$ with $x=0.047$ and $0.12$, in which nonmagnetic Ga ions dilute Mn${}^{3+}$ spins. The $x=0.047$ sample exhibits high-temperature incommensurate (HT-ICM), commensurate (CM), and low-temperature incommensurate (LT-ICM) magnetic phases in order of decreasing temperature, whereas the $x=0.12$ sample exhibits only HT-ICM and LT-ICM phases. Here, the CM and LT-ICM phases are ferroelectric and weak ferroelectric, respectively. Measurements conducted under zero-field heating after various field-cooling conditions evidence that the microscopic mechanisms of the spin-driven ferroelectricity in the CM and LT-ICM phases are different: The magnetic chirality of Mn${}^{4+}$ cycloidal spins plays a dominant role in the LT-ICM phase, whereas the magnetic exchange striction by the Mn${}^{4+}$-Mn${}^{3+}$ chain plays a dominant role in the CM phase. The polarization of YMn${}_2$O${}_5$ flips upon a CM to LT-ICM phase transition because the ferroelectricity driven by the magnetic chirality and the exchange striction provides opposite directions of polarization.

In contrast to the long-known fact that spin-flip hot spots, i.e., special $\mathbf{k}$ points on the Fermi surface showing a high spin-mixing parameter, do not occur in the bulk of monovalent (noble and alkali) metals, we found them on the surface Brillouin-zone boundary of ultrathin films of these metals. Density-functional calculations within the Korringa-Kohn-Rostoker Green function method for ultrathin (001) oriented Cu, Ag, and Au films of 10-layer thickness show that the region around the hot spots can have a substantial contribution, e.g., 52% in Au(001), to the integrated spin-mixing parameter, that could lead to a significant enhancement of the spin-relaxation rate or spin-Hall angle in thin films. Owing to the appearance of spin-flip hot spots, a large anisotropy of the Elliott-Yafet parameter [50% for Au(001)] is also found in these systems. The findings are important for spintronics applications in which noble metals are frequently used and in which the dimensionality of the sample is reduced.

We report an investigation of long-range ferromagnetic (FM) ordering in Mn-doped MoS${}_2$, MoSe${}_2$, MoTe${}_2$, and WS${}_2$ for Mn concentration less than 5$\%$ using density functional theory calculations. The long-range ferromagnetism of Mn spins is mediated by an antiferromagnetic (AFM) exchange between the localized Mn $d$ states and the delocalized $p$ states of the S, Se, and Te atoms. In contrast, transition metals like Fe, Co, and Ni show a FM exchange with the S, Se, and Te atoms, which results in a very weak FM (even slightly AFM) coupling for transition-metal defects with large separations. The Mn substitution at Mo or W sites is energetically favorable, thus making the Mn-doped dichalcogenides promising candidates for two-dimensional dilute magnetic semiconductors.

Magnon self-energy due to spin-charge coupling is calculated for the correlated ferromagnetic Kondo lattice model (FKLM) including the intraorbital Coulomb repulsion term $U$ for the band electrons using a diagrammatic expansion scheme. By systematically incorporating correlation effects in the form of self-energy and vertex corrections, the expansion scheme explicitly preserves the continuous spin-rotation symmetry and hence the Goldstone mode. Due to a near cancellation of the correlation-induced quantum correction terms at intermediate coupling and optimal band filling relevant for ferromagnetic manganites, the renormalized magnon energies for the correlated FKLM are nearly independent of correlation term. Even at higher-band fillings, despite exhibiting overall non-Heisenberg behavior, magnon dispersion in the $\Gamma$-$X$ direction retains nearly Heisenberg form. Therefore, the experimentally observed doping-dependent zone-boundary magnon softening must be ascribed to spin-orbital coupling effects.

(LaNiO${}_3$)${}_n$/(LaMnO${}_3$)${}_2$ superlattices were grown using ozone-assisted molecular beam epitaxy, where LaNiO${}_3$ is a paramagnetic metal and LaMnO${}_3$ is an antiferromagnetic insulator. The superlattices exhibit excellent crystallinity and interfacial roughness of less than one unit cell. X-ray spectroscopy and dichroism measurements indicate that electrons are transferred from the LaMnO${}_3$ to the LaNiO${}_3$, inducing magnetism in LaNiO${}_3$. Magnetotransport measurements reveal a transition from metallic to insulating behavior as the LaNiO${}_3$ layer thickness is reduced from five to two unit cells and suggest an inhomogeneous magnetic structure within LaNiO${}_3$.

Density functional theory calculations within both the local density approximation and the generalized gradient approximation are used to study Au-catalyzed growth under near-equilibrium conditions. We discuss both the chemical equilibrium of a GaAs nanowire with an As${}_2$ gas atmosphere and the mechanical equilibrium between the capillary forces at the nanowire tip. For the latter goal, the interface between the gold nanoparticle and the nanowire is modeled atomically within a slab approach, and the interface energies are evaluated from the total energies of the model systems. We discuss three growth regimes, one catalyzed by an (almost) pure Au particle, an intermediate alloy-catalyzed growth regime, and a Ga-catalyzed growth regime. Using the interface energies calculated from the atomic models, as well as the surface energies of the nanoparticle and the nanowire sidewalls, we determine the optimized geometry of the nanoparticle-capped nanowire by minimizing the free energy of a continuum model. Under typical experimental conditions of 10${}^{-4}$ Pa As${}_2$ and 700 K, our results in the local density approximation are insensitive to the Ga concentration in the nanoparticle. In these growth conditions, the energetically most favored interface has an interface energy of around 45 meV/Å${}^2$, and the correspondingly optimized droplet on top of a GaAs nanowire is somewhat larger than a hemisphere and forms a contact angle around 130${}^{\circ }$ for both pure Au and Au-Ga alloy nanoparticles.

We theoretically consider the carrier density tuned (apparent) two-dimensional (2D) metal-insulator transition (MIT) in semiconductor heterostructure-based 2D carrier systems as arising from a classical percolation phenomenon in the inhomogeneous density landscape created by the long-range potential fluctuations induced by random quenched charged impurities in the environment. The long-range Coulomb disorder inherent in semiconductors produces strong potential fluctuations in the 2D system where a fraction of the carriers gets trapped or classically localized, leading to a mixed two-component semiclassical transport behavior at intermediate densities where a fraction of the carriers is mobile and another fraction immobile. At high carrier density, all the carriers are essentially mobile, whereas at low carrier density, all the carriers are essentially trapped since there is no possible percolating transport path through the lake-and-mountain inhomogeneous potential landscape. The low-density situation with no percolation would mimic an insulator, whereas the high-density situation with allowed percolating paths through the lake-and-mountain energy landscape would mimic a metal with the system manifesting an apparent MIT in-between. We calculate the transport properties as a function of carrier density, impurity density, impurity location, and temperature using a two-component (trapped and mobile carriers) effective medium theory. Our theoretically calculated transport properties are in good qualitative agreement with the experimentally observed 2D MIT phenomenology in 2D electron and hole systems. We find a high- (low-) density metallic (insulating) temperature dependence of the 2D resistivity, and an intermediate-density crossover behavior which could be identified with the experimentally observed 2D MIT. The calculated density- and temperature-dependent resistivity in our theory mimics the phenomenology of 2D MIT experiments with reasonable parameter values for the background disorder.