We used resonant laser spectroscopy of multiple InGaAs quantum dots to spatially locate charge fluctuators in the surrounding semiconductor matrix. By mapping out the resonance condition between a narrow-band laser and the neutral exciton transitions of individual dots in a field effect device, we identified spectral discontinuities as arising from charging and discharging events that take place within the volume adjacent to the quantum dots. Our analysis suggests that residual carbon dopants are a major source of charge-fluctuating traps in quantum dot heterostructures.

A resurgence in the use of the donor-acceptor approach in synthesizing conjugated polymers has resulted in a family of high-mobility ambipolar systems with exceptionally narrow energy bandgaps below 1 eV. The ability to transport both electrons and holes is critical for device applications such as organic light-emitting diodes and transistors. Infrared spectroscopy offers direct access to the low-energy excitations associated with injected charge carriers. Here we use a diffraction-limited IR microscope to probe the spectroscopic signatures of electron and hole injection in the conduction channel of an organic field-effect transistor based on an ambipolar DA polymer polydiketopyrrolopyrrole-benzobisthiadiazole. We observe distinct polaronic absorptions for both electrons and holes and spatially map the carrier distribution from the source to drain electrodes for both unipolar and ambipolar biasing regimes. For ambipolar device configurations, we observe the spatial evolution of hole-induced to electron-induced polaron absorptions throughout the transport path. Our work provides a platform for combined transport and infrared studies of organic semiconductors on micron length scales relevant to functional devices.

Spin-transfer torque in a class of magnetic tunnel junctions with noncollinear magnetizations, referred to as spin filter tunnel junctions, is studied within the tight-binding model using the nonequilibrium Green's function technique within Keldysh formalism. These junctions consist of one ferromagnet (FM) adjacent to a magnetic insulator (MI) or two FM separated by a MI. We find that the presence of the magnetic insulator dramatically enhances the magnitude of the spin-torque components compared to conventional magnetic tunnel junctions. The fieldlike torque is driven by the spin-dependent reflection at the MI/FM interface, which results in a small reduction of its amplitude when an insulating spacer (S) is inserted to decouple MI and FM layers. Meanwhile, the dampinglike torque is dominated by the tunneling electrons that experience the lowest barrier height. We propose a device of the form FM/(S)/MI/(S)/FM that takes advantage of these characteristics and allows for tuning the spin-torque magnitudes over a wide range just by rotation of the magnetization of the insulating layer.

Terahertz emission between exciton-polariton branches in semiconductor microcavities is expected to be strongly stimulated in the polariton laser regime due to the high density of particles in the lower state (final-state stimulation effect). However, nonradiative scattering processes depopulate the upper state and greatly hinder the efficiency of such terahertz sources. In this work, we suggest a scheme using multiple microcavities and exploiting the transition between two interband polariton branches located below the exciton level. We compare the nonradiative processes loss rates in single and double cavity devices and we show that a dramatic reduction can be achieved in the latter, enhancing the efficiency of the terahertz emission.

We show how to engineer enhanced skew scattering and gaplike openings in the surface state of three-dimensional topological insulators using only nonmagnetic impurities. Enhanced skew scattering off nonmagnetic impurities is obtained as a finite size effect of the scattering potential. Intimately related to the generated skew scattering is the emergence of a gaplike density of electron states locally around the impurities and surrounded by sharp resonances, with an extended energy gap appearing in engineered impurity structures.

Applying the correlated electronic structure method based on density functional theory plus the Hubbard $U$ interaction, we have investigated the tetragonal scheelite structure Mott insulator ${\mathrm{KOsO}}_4$, whose $e_g^1$ configuration should be affected only slightly by spin-orbit coupling (SOC). The method reproduces the observed antiferromagnetic Mott-insulating state, populating the Os $d_{z^2}$ majority orbital. The quarter-filled $e_g$ manifold is characterized by a symmetry breaking due to the tetragonal structure, and the Os ion shows a crystal field splitting ${\Delta }_{\text{cf}}=1.7$ eV from the $t_{2g}$ complex, which is relatively small considering the high formal oxidation state $\mathrm{Os}{}^{7+}$. The small magnetocrystalline anisotropy before including correlation (i.e., in the metallic state) is increased by more than an order of magnitude in the Mott-insulating state, a result of a strong interplay between large SOC and a strong correlation. In contrast to conventional wisdom that the $e_g$ complex will not support orbital magnetism, we find that for the easy axis [100] direction the substantial Os orbital moment $M_L\approx -0.2{\mu }_B$ compensates half of the Os spin moment $M_S=0.4{\mu }_B$. The origin of the orbital moment is analyzed and understood in terms of additional spin-orbital lowering of symmetry, and beyond that due to structural distortion, for magnetization along [100]. Further interpretation is assisted by analysis of the spin density and the Wannier function with SOC included.

The Fermi level of a material is a fundamental quantity that determines its electronic properties. Thus, the ability to tune Fermi levels is important for developing electronic device materials. However, for most materials, the Fermi level is limited to a certain range in the band gap due to the existence of certain intrinsic compensating defects. Here we demonstrate that quenching can be used as an effective way to overcome this limit, allowing the Fermi levels to be tuned in a much wider range. Taking a photovoltaic material CdTe as a prototype example, we analyzed the physical origin of Fermi level pinning and explained why growing the sample at high temperature followed by rapid quenching to room temperature can overcome the self-compensation limit. We further show that for CdTe, quenching can increase the Fermi level range from about 0.6 to 1.1 eV, which has a great potential in improving CdTe solar cell performance. Our proposed strategy of tuning Fermi level positions beyond the intrinsic equilibrium doping limit is general and can be applied to other semiconductor systems.

We study the impact that local strain effects have on the spatial distribution of In in coherent ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ grown epitaxially on GaN(0001) using an effective crystal growth modeling technique that combines a semi-grand-canonical Monte Carlo simulation with an ab initio parametrized empirical force field. Our calculations show that ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ epitaxial layers exhibit a strong tendency towards ordering, as highlighted by the formation of a vertical stack of the $\sqrt{3}\times \sqrt{3}$ patterned layers along the $\langle c\rangle$ direction. The ordering phenomena are identified as a key factor that determines lateral phase separation in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ epitaxial layers at the nanometer scale. Consequences of this nanophase separation for the enhanced radiative emission through carrier localization in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ of $x<1/3$ are discussed.

The electrical current through an arbitrary junction connecting quantum wires of spinless interacting fermions is calculated in fermionic representation. The wires are adiabatically attached to two reservoirs at chemical potentials differing by the applied voltage bias. The relevant scale-dependent contributions in perturbation theory in the interaction up to infinite order are evaluated and summed up. The result allows one to construct renormalization group equations for the conductance as a function of voltage (or temperature, wire length). There are two fixed points at which the conductance follows a power law in terms of a scaling variable $\Lambda$, which equals the bias voltage $V$, if $V$ is the largest energy scale compared to temperature $T$ and inverse wire length $L^{-1}$, and interpolates between these quantities in the crossover regimes.

By changing the size and the shape of ferroelectric nanoparticles, one can govern their polar properties, including their improvement in comparison with the bulk material. The shift of the ferroelectric transition temperature can reach hundreds of degrees Kelvin. A phenomenological description of these effects was proposed in the framework of Landau-Ginsburg-Devonshire (LGD) theory using the concepts of surface tension and surface bond contraction. However, this description contains a series of poorly defined parameters, and the physical nature is ambiguous. It appears that the size and shape dependences of the phase transition temperature, along with all polar properties, are defined by the nature of the size effect. Existing LGD-type models do not take into account that defect concentration strongly increases near the particle surface. In order to develop an adequate phenomenological description of size effects in ferroelectric nanoparticles, one should consider Vegard strains (local lattice deformations) originating from defect accumulation near the surface. In this paper, we propose a theoretical model that takes into account Vegard strains and performs a detailed quantitative comparison of the theoretical results with experimental ones for quasispherical ${\mathrm{KTa}}_{1\text{−}x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ nanoparticles (average radius 25 nm), which reveal the essential (about 100 K) increase of the transition temperature in spherical nanoparticles in comparison with bulk crystals. From the comparison between the theory and experiment, we established the leading contribution of Vegard strains to the extrinsic size effects in ferroelectric nanoparticles. We determined the dependence of Vegard strains on the content of Nb and reconstructed the Curie temperature dependence on the content of Nb using this dependence. The dependence of the Curie temperature on the Nb content becomes a nonmonotonic one for the small (<20 nm) elongated ${\mathrm{KTa}}_{1\text{−}x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ nanoparticles. We established that the accumulation of intrinsic and extrinsic defects near the surface can play a key role in the physical origin of extrinsic size effects in ferroelectric nanoparticles and govern its main features.

In spite of its practical interest, the Al-Cu system remains largely unexplored, especially on its Cu-rich side. In order to improve the knowledge of this system, we perform a thorough ab initio study of fcc-based Al-Cu energetics, using the recently proposed M2BCE reciprocal-space cluster expansion approach. We demonstrate the existence of two clearly distinct composition domains, revealing complex ground-state properties. Below 50% Cu, the $\mathrm{GP}2\text{−}\mathrm{A}{\mathrm{l}}_3\mathrm{Cu}$ compound appears as highly favored, in agreement with the well-documented transformation sequence in Al-based alloys. Conversely, the domain between 50% and 80% Cu displays a much shallower landscape, characterized by the existence of a wealth of compounds undergoing $\mathrm{fcc}\to \mathrm{bcc}$ structural instabilities. While such “Bain paths” have been identified for a long time in iron-based alloys, our work gives evidence for their existence in the Al-Cu system. As a striking application, these instabilities provide plausible athermal mechanisms for the formation of Cu-rich phases, in particular for the unexpected emergence of ${\gamma }_1\text{−}\mathrm{A}{\mathrm{l}}_4\mathrm{C}{\mathrm{u}}_9$, a Hume-Rothery compound observed in various nonequilibrium conditions.

We report a detailed study of the magnetic properties of the buckled kagome compound ${\mathrm{Cu}}_3{\mathrm{Y}(\mathrm{SeO}}_3{)}_2{\mathrm{O}}_2\mathrm{Cl}$ using heat capacity, magnetization, powder neutron diffraction, electron spin resonance, and first-principles calculations. The crystal structure is confirmed to be isotypic with the mineral francisite, with orthorhombic space group symmetry Pmmn throughout the temperature range 5–300 K. Magnetization, heat capacity, and neutron diffraction confirm long range magnetic order below $T_N=35$ K. The electron spin resonance spectra reveal the presence of two modes corresponding to two different crystallographic Cu positions. The principal $g$ values of the $g$ tensor of Cu1 sites were found to be $g_1=2.18\left(4\right),g_2=2.10\left(6\right)$, and $g_3$ = 2.05(9), while the effective $g$ factor of Cu2 sites is almost isotropic and is on average $g=2.09\left(5\right)$. At low temperatures, ${\mathrm{Cu}}_3{\mathrm{Y}(\mathrm{SeO}}_3{)}_2{\mathrm{O}}_2\mathrm{Cl}$ undergoes a metamagnetic transition, with a critical field $B_C=2.6$ T at 2 K, due to the suppression of the interplane exchange interactions and saturates in modest magnetic field $B_S$ ⩽ 8 T. The first-principles calculations allow an estimation of both intraplane and interplane exchange interactions. The weakness of the interplane exchange interaction results in low values of the critical fields for the metamagnetic transition, while the competition between intraplane exchange interactions of different signs results in a similarly low value of the saturation field.

We theoretically investigate electronic transport through a junction where a quantum dot (QD) is tunnel coupled on both sides to semiconductor nanowires with strong spin-orbit interaction and proximity-induced superconductivity. The results are presented as stability diagrams, i.e., the differential conductance as a function of the bias voltage applied across the junction and the gate voltage used to control the electrostatic potential on the QD. A small applied magnetic field splits and modifies the resonances due to the Zeeman splitting of the QD level. Above a critical field strength, Majorana bound states (MBS) appear at the interfaces between the two superconducting nanowires and the QD, resulting in a qualitative change of the entire stability diagram, suggesting this setup as a promising platform to identify MBS. Our calculations are based on a nonequilibrium Green's function description and is exact when Coulomb interactions on the QD can be neglected. In addition, we develop a simple pictorial view of the involved transport processes, which is equivalent to a description in terms of multiple Andreev reflections, but provides an alternative way to understand the role of the QD level in enhancing transport for certain gate and bias voltages. We believe that this description will be useful in future studies of interacting QDs coupled to superconducting leads (with or without MBS), where it can be used to develop a perturbation expansion in the tunnel coupling.

The discovery of superconductivity in alkali-ammonia cointercalated FeSe has generated intensive interest because of their highest $T_{\mathrm{c}}$ (∼45 K) among iron-chalcogenide superconductors with bulk form. Here, we report the phase diagrams of two series of Li/Na-ammonia cointercalated $\mathrm{FeS}{\mathrm{e}}_{1-z}\mathrm{T}{\mathrm{e}}_z$. When superconductivity is suppressed by Te doping, the magnetic ordering states are emergent. Moreover, a novel phase $\mathrm{L}{\mathrm{i}}_x{\left(\mathrm{N}{\mathrm{H}}_3\right)}_y\mathrm{F}{\mathrm{e}}_{2-\delta }\mathrm{T}{\mathrm{e}}_2$ with possible antiferromagnetism is discovered. This strongly indicates the intimate relation between superconductivity and magnetism in these materials, like in other iron-based superconductors. On the other hand, comparative structure analysis with $\mathrm{FeS}{\mathrm{e}}_{1-z}\mathrm{T}{\mathrm{e}}_z$ suggests that although there is remarkable similarity in phase diagrams for both iron-chalcogenide and iron-pnictide superconductors, the different types of anions with different charges lead to the dissimilar controlling factors in superconductivity for both classes of materials. This opens up an opportunity to tune the superconductivity in iron-chalcogenide superconductors in more ways than just one.

Investigations of $\mathrm{Mg}{\mathrm{B}}_2$ and Fe-based superconductors in recent years have revealed many unusual effects of multiband superconductivity, but manifestations of anisotropic multiband effects in the critical current density $J_c$ have not been addressed experimentally, mostly because of the difficulties in measuring $J_c$ along the $c$ axis. To investigate the effect of very different intrinsic anisotropies of σ and π electron bands in $\mathrm{Mg}{\mathrm{B}}_2$ on current transport, we grew epitaxial films with a tilted $c$ axis (θ ∼ 19.5°), which enabled us to measure the components of $J_c$ along both the $ab$ plane and the $c$ axis using magneto-optical and transport techniques. These measurements were combined with scanning and transmission electron microscopy, which revealed terraced steps on the surface of the tilted-$c$-axis films. The measured field and temperature dependencies of the anisotropic $J_c\left(H\right)$ show that $J_{c,\mathrm{L}}$ parallel to the terraced steps is higher than $J_{c,\mathrm{T}}$ perpendicular to the terraced steps, and $J_c$ of thinner films (50 nm) obtained from transport experiments at 0.1 T reaches ∼10% of the depairing current density $J_d$ in the $ab$ plane, while magneto-optical imaging revealed much higher $J_c$ at lower fields. To analyze the experimental data we developed a model of anisotropic vortex pinning which accounts for the observed behavior of $J_c$ in the tilted-$c$-axis films and suggests that the apparent anisotropy of $J_c$ is affected by current pairbreaking effects in the weaker π band. Our results indicate that the out-of-plane current transport mediated by the π band could set the ultimate limit of $J_c$ in $\mathrm{Mg}{\mathrm{B}}_2$ polycrystals.

While the Born elastic stability criteria are well known for cubic crystals, there is some confusion in the literature about the form they should take for lower-symmetry crystal classes. Here we present closed form necessary and sufficient conditions for elastic stability in all crystal classes, as a concise and pedagogical reference to stability criteria in noncubic materials.