The spin relaxation of a two-dimensional electron system (2DES) formed in a symmetric quantum well is studied theoretically when the quantum well is parallel to the (110) plane of the zinc-blende structure, the spin polarization is perpendicular to the well, and electrons occupy only the ground subband. The spin-relaxation rate is calculated as a function of the distribution of donor impurities which are placed in the well layer. Considered processes of the spin relaxation are (1) the intrasubband process by impurity-potential-induced spin-orbit interaction (SOI), which is the Elliott-Yafet mechanism in the 2DES, and (2) virtual intersubband processes consisting of a spin flip by (2a) well-potential-induced SOI or (2b) the Dresselhaus SOI, and a scattering from an impurity. It is shown that all of the above processes disappear when all impurities are located on the center plane of the well. Even if impurities are distributed over three (110) atomic layers, the spin-relaxation rate is two orders of magnitude lower than that for the uniform distribution over the well width of 7.5 nm. In GaAs/AlGaAs type-I quantum wells, the processes (1) and (2a) interfere constructively, being dominant over (2b) for the well width of $\sim$10 nm, while in some type-II quantum wells, they can interfere destructively.

Nanocrystals encapsulated within an amorphous matrix are computationally analyzed to quantify the degree to which the matrix modifies the nature of their quantum-confinement power—i.e., the relationship between nanocrystal size and the gap between valence- and conduction-band edges. A special geometry allows exactly the same amorphous matrix to be applied to nanocrystals of increasing size to precisely quantify changes in confinement without the noise typically associated with encapsulating structures that are different for each nanocrystal. The results both explain and quantify the degree to which amorphous matrices redshift the character of quantum confinement. The character of this confinement depends on both the type of encapsulating material and the separation distance between the nanocrystals within it. Surprisingly, the analysis also identifies a critical nanocrystal threshold below which quantum confinement is not possible—a feature unique to amorphous encapsulation. Although applied to silicon nanocrystals within an amorphous silicon matrix, the methodology can be used to accurately analyze the confinement softening of other amorphous systems as well.

In this paper, we explore the localization transition and the scaling properties of both quasi-one-dimensional and two-dimensional quasiperiodic systems, which are constituted from coupling several Aubry-André (AA) chains along the transverse direction, in the presence of next-nearest-neighbor (NNN) hopping. The localization length, two-terminal conductance, and participation ratio are calculated within the tight-binding Hamiltonian. Our results reveal that a metal-insulator transition could be driven in these systems not only by changing the NNN hopping integral but also by the dimensionality effects. These results are general and hold by coupling distinct AA chains with various model parameters. Furthermore, we show from finite-size scaling that the transport properties of the two-dimensional quasiperiodic system can be described by a single parameter and the scaling function can reach the value 1, contrary to the scaling theory of localization of disordered systems. The underlying physical mechanism is discussed.

The axial magneticeffect, i.e., the generation of an energy current parallel to an axial magnetic field coupling with opposite signs to left- and right-handed fermions, is a nondissipative transport phenomenon intimately related to the gravitational contribution to the axial anomaly. An axial magnetic field emerges naturally in condensed matter in so-called Weyl semimetals. We present a measurable implementation of the axial magnetic effect. We show that the edge states of a Weyl semimetal at finite temperature possess a temperature dependent angular momentum in the direction of the vector potential intrinsic to the system. Such a realization provides a plausible context for the experimental confirmation of the elusive gravitational anomaly.

Excitons, quasiparticles consisting of electron-hole pairs bound by the Coulomb interaction, are a potential medium for the processing of photonic information in the solid state. Information processing via excitons requires efficient techniques for the transport and manipulation of these uncharged particles. We have carried out a detailed investigation of the transport of excitons in GaAs quantum wells by surface acoustic waves. Based on these results, we introduce here a concept for the interconnection of multiple remote exciton systems based on the long-range transport of dipolar excitons by a network of configurable interconnects driven by acoustic wave beams. By combining this network with electrostatic gates, we demonstrate an integrated exciton multiplexer capable of interconnecting, gating, and routing exciton systems separated by millimeter distances. The multiplexer provides a scalable platform for the manipulation of exciton fluids with potential applications in information processing.

The physics picture on scanning tunneling spectroscopy of single-wall carbon nanotubes (SWCNTs) was revisited recently [H. Lin et al., Nat. Mater. 9, 235 (2010)] with an image potential model under the framework of the many-body theory whose description is different from that of conventional one-particle tight-binding theory. The model is explored further in the present study of SWCNTs with an ultrahigh-vacuum scanning tunneling microscope. In the experiments, two types of samples were measured. In one sample, the nanotubes were in intimate contact with the gold surface and the observed tunneling gaps of semiconductor nanotubes fit the prediction of the one-particle model. In the other sample, the nanotubes were isolated by a thin polymer (4-vinylpyridine) layer from the gold surface. The semiconducting SWCNTs in the latter sample show tunneling gaps several hundreds of milli–electron volts larger than the prediction of the one-particle model. The results can, however, be interpreted by the modified image potential model, which takes into account the surface dielectric mechanism. The consistent picture of the tunneling gaps of the different samples provides insight into the scanning tunneling spectroscopy of SWCNTs from the standpoint of many-body theory.

We investigate integer and fractional quantum Hall states in quantum point contacts (QPCs) of different geometries, defined in AlGaAs/GaAs heterostructures employing different doping and screening techniques. We find that even in the highest mobility samples, interference and localization strongly influence the transport properties. We propose microscopic models for these effects, based on single- and many-electron physics. For integer quantum Hall states, transport is modulated due to the self-consistent formation of compressible regions of enhanced or reduced density in the incompressible region of the constriction. In the fractional quantum Hall regime, we observe the localization of fractionally charged quasiparticles in the constriction and an interplay of single- and many-electron physics. At low electron densities and in comparatively weak magnetic fields, single-electron interference dominates transport. Utilizing optimized growth and gating techniques, the $\nu =5/2$ state can be observed in a QPC, conserving the bulk properties in an unprecedented quality. Our results might improve the understanding of the influence of localization on the transmission properties of QPCs, which is necessary for the interpretation of interference experiments employing QPCs, especially at $\nu =5/2$.

We report on the influence of the interfacial roughness on the thermal boundary conductance between two solids, using molecular dynamics. We show evidence of a transition between two regimes, depending on the interfacial roughness: When the roughness is small, the boundary conductance is constant, taking values close to the conductance of the corresponding planar interface. When the roughness is larger, the conductance becomes larger than the planar interface conductance and the relative increase is found to be close to the increase of the interfacial area. The cross-plane conductivity of a superlattice with rough interfaces is found to increase in a comparable amount, suggesting that heat transport in superlattices is mainly controlled by the boundary conductance. These observations are interpreted using the wave characteristics of the energy carriers. We characterize also the effect of the angle of the asperities and find that the boundary conductance displayed by interfaces having steep slopes may become important if the lateral period characterizing the interfacial profile is large enough. As a result, triangular-shaped interfaces may be used to enhance the conductance of planar interfaces by a factor greater than three. Finally, we consider the effect of the shape of the interfaces and show that the sinusoidal interface displays the highest conductance because of its large true interfacial area. All of these considerations are relevant to the optimization of nanoscale interfacial energy transport.

We describe an ab initio disordered local moment theory for materials with quenched static compositional disorder traversing first-order magnetic phase transitions. It accounts quantitatively for metamagnetic changes and the magnetocaloric effect. For perfect stoichiometric B2-ordered FeRh, we calculate the transition temperature of the ferromagnetic-antiferromagnetic transition to be $T_t=$ 495 K and a maximum isothermal entropy change in 2 T of $\left|\Delta S\right|=21.1$ J K${}^{-1}$ kg${}^{-1}$. A large (40%) component of $\left|\Delta S\right|$ is electronic. The transition results from a fine balance of competing electronic effects which is disturbed by small compositional changes; e.g., swapping just 2% Fe of “defects” onto the Rh sublattice makes $T_t$ drop by 290 K. This hypersensitivity explains the narrow compositional range of the transition and impurity doping effects.

Infrared reflectivity spectra of cubic ${\text{SrMnO}}_3$ ceramics reveal 18% stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at $T_N=233$ K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first-principles calculations. Moreover, polar phonons become Raman active below $T_N$, although their activation is forbidden by symmetry in the $Pm\overline{3}m$ space group. This gives evidence that the cubic $Pm\overline{3}m$ symmetry is locally broken below $T_N$ due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small nonstoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers $n_e=3.6\times {10}^{20}$ cm${}^{-3}$ and the sample composition Sr${}^{2+}$Mn${}_{0.98}^{4+}$Mn${}_{0.02}^{3+}$O${}_{2.99}^{2-}$. The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundaries.

Systematic ${}^{31}$P-NMR studies on LaFe(${\text{As}}_{1-x}$${\text{P}}_x$)(${\text{O}}_{1-y}$${\text{F}}_y$) with $y=0.05$ and 0.1 have revealed that the antiferromagnetic spin fluctuations (AFMSFs) at low energies are markedly enhanced around $x=0.6$ and 0.4, respectively, and as a result, $T_c$ exhibits respective peaks at 24 and 27 K against the P substitution for As. This result demonstrates that the AFMSFs are responsible for the increase in $T_c$ for LaFe(${\text{As}}_{1-x}$${\text{P}}_x$)(${\text{O}}_{1-y}$${\text{F}}_y$) as a primary mediator of the Cooper pairing. From a systematic comparison of AFMSFs with a series of (${\text{La}}_{1-z}$${\text{Y}}_z$)${\text{FeAsO}}_{\delta }$ compounds in which $T_c$ reaches 50 K for $z=0.95$, we remark that a moderate development of AFMSFs causes $T_c$ to increase up to 50 K under the condition that the local lattice parameters of the FeAs tetrahedron approach those of the regular tetrahedron. We propose that $T_c$ of Fe-pnictides exceeding 50 K is maximized under an intimate collaboration of the AFMSFs and other factors originating from the optimization of the local structure.

We have used low energy electron microscopy (LEEM) to characterize the structure and stability of Bi phases on Cu{111}. As a function of temperature we find that the Cu{111}($\sqrt{3}\times \sqrt{3}$)R${30}^{\circ }$-Bi surface alloy phase gradually dealloys and is fully depleted from Bi at a temperature of 803 K. The dealloying leads to a defect induced change of its elastic properties. The Bi surface alloy phase coexists with a Bi overlayer phase that exhibits a sharp decrease in density in a narrow temperature interval just below the temperature where the surface alloy phase has fully dealloyed. LEEM is used to directly evaluate the structure as well as the entropic contributions that determine the stability of each of the phases.

We consider electrostatically coupled quantum dots in topological insulators, otherwise confined and gapped by a magnetic texture. By numerically solving the (2 + 1) Dirac equation for the wave packet dynamics, we extract the energy spectrum of the coupled dots as a function of bias-controlled coupling and an external perpendicular magnetic field. We show that the tunneling energy can be controlled to a large extent by the electrostatic barrier potential. Particularly interesting is the coupling via Klein tunneling through a resonant valence state of the barrier. The effective three-level system nicely maps to a model Hamiltonian, from which we extract the Klein coupling between the confined conduction and valence dots levels. For large enough magnetic fields Klein tunneling can be completely blocked due to the enhanced localization of the degenerate Landau levels formed in the quantum dots.

Density functional theory calculations are used to show that it is possible to dope semiconducting transition-metal dichalcogenides (TMD) such as MoS${}_2$ and WS${}_2$ with electrons and/or holes either by chemical substitution or by adsorption on the sulfur layer. Notably, the activation energies of lithium and phosphorus, a shallow donor and a shallow acceptor, respectively, are smaller than 0.1 eV. Substitutional halogens are also proposed as alternative donors adequate for different temperature regimes. All dopants proposed result in very little lattice relaxation and, hence, are expected to lead to minor scattering of the charge carriers. Doped MoS${}_2$ and WS${}_2$ monolayers are extrinsic in a much wider temperature range than 3D semiconductors, making them superior for high temperature electronic and optoelectronic applications.

Using first principles, we calculate the lattice thermal conductivity of Bi, Sb, and Bi-Sb alloys, which are of great importance for thermoelectric and thermomagnetic cooling applications. Our calculation reveals that the ninth-neighbor harmonic and anharmonic force constants are significant; accordingly, they largely affect the lattice thermal conductivity. Several features of the thermal transport in these materials are studied: (1) the relative contributions from phonons and electrons to the total thermal conductivity as a function of temperature are estimated by comparing the calculated lattice thermal conductivity to the measured total thermal conductivity, (2) the anisotropy of the lattice thermal conductivity is calculated and compared to that of the electronic contribution in Bi, and (3) the phonon mean free path distributions, which are useful for developing nanostructures to reduce the lattice thermal conductivity, are calculated. The phonon mean free paths are found to range from 10 to 100 nm for Bi at 100 K.

We use conventional and aberration-corrected transmission electron microscopy (TEM) and ab initio calculations to investigate the structural and electronic properties of β-FeSi${}_2$ nanoparticles, which are a promising material for photovoltaic applications due to a band gap of <1 eV and a high absorption coefficient. The nanoparticles have average sizes of ∼20 nm, form aggregates, and are prepared by gas-phase synthesis. Amorphous SiO${}_x$ shells with thicknesses of ∼1.7 nm around β-FeSi${}_2$ cores are identified on individual nanoparticles using electron energy-loss spectroscopy, while stacking fault domains in the nanoparticles are observed using high-resolution TEM, nanobeam electron diffraction, and automated diffraction tomography. Ab initio calculations indicate only minor changes in band structure in the faulted structure when compared to perfect β-FeSi${}_2$. The optical properties of imperfect β-FeSi${}_2$ nanoparticles are therefore expected to be the same as those of the perfect structure, suggesting that β-FeSi${}_2$ nanoparticles are suitable candidates for use in optical absorber layers in thin film solar cells.

In this article we present a method based on ab initio calculations to predict compositions at morphotropic phase boundaries in lead-free perovskite solid solutions. This method utilizes the concept of flat free energy surfaces and involves the monitoring of pressure-induced phase transitions as a function of composition. As model systems, solid solutions of ${\text{Na}}_{1/2}$${\text{Bi}}_{1/2}$${\text{TiO}}_3$ with the alkali substituted ${\text{Li}}_{1/2}$${\text{Bi}}_{1/2}$${\text{TiO}}_3$ and ${\text{K}}_{1/2}$${\text{Bi}}_{1/2}$${\text{TiO}}_3$ and the alkaline earth substituted ${\text{CaTiO}}_3$ and ${\text{BaTiO}}_3$ are chosen. The morphotropic compositions are identified by determining the composition at which the phase transition pressure equals zero. In addition, we discuss the different effects of hydrostatic pressure (compression and tension) and chemical substitution on the antiphase tilts about the [111] axis ($a^{-}{a}^{-}{a}^{-}$) present in pure ${\text{Na}}_{1/2}$${\text{Bi}}_{1/2}$${\text{TiO}}_3$ and how they develop in the two solid solutions ${\text{Na}}_{1/2}$${\text{Bi}}_{1/2}$${\text{TiO}}_3$–${\text{CaTiO}}_3$ and ${\text{Na}}_{1/2}$${\text{Bi}}_{1/2}$${\text{TiO}}_3$–${\text{BaTiO}}_3$. Finally, we discuss the advantages and shortcomings of this simple computational approach.