We present an atomistic theoretical study of the temperature dependence of the competition between Auger and radiative recombination in $c$-plane (In,Ga)N/GaN quantum wells with indium (In) contents of 10%, 15%, and 25%. The model accounts for random alloy fluctuations and the connected fluctuations in strain and built-in field. Our investigations reveal that the total Auger recombination rate exhibits a weak temperature dependence; at a temperature of 300 K and a carrier density of $n_{3D}=3.8\times {10}^{18}{\mathrm{cm}}^{-3}$, we find total Auger coefficients in the range of $\approx 6\times {10}^{-30}{\mathrm{cm}}^6/\mathrm{s}$ (10% In) to $\approx 3\times {10}^{-31}{\mathrm{cm}}^6/\mathrm{s}$ (25% In), thus large enough to significantly impact the efficiency in (In,Ga)N systems. Our calculations show that the hole-hole-electron Auger rate dominates the total rate for the three In contents studied; however, the relative difference between the hole-hole-electron and electron-electron-hole contributions decreases as the In content is increased to 25%. Our studies provide further insight into the origin of the “thermal droop” (i.e., the decrease in internal quantum efficiency with increasing temperature at a fixed carrier density) in (In,Ga)N-based light-emitting diodes. We find that the ratio of radiative to nonradiative (Auger) recombination increases in the temperature range relevant to the thermal droop ($\ge 300$ K), suggesting that the competition between these processes is not driving this droop effect in $c$-plane (In,Ga)N/GaN quantum wells. This finding is in line with recent experimental studies.

The effects of annealing on chemical vapor deposited graphene are investigated via a weak localization magnetoresistance measurement. Annealing at $300{}^{\circ }\mathrm{C}$ in inert gases, a common cleaning procedure for graphene devices, is found to raise the dephasing rate significantly above the rate from electron-electron interactions, which would otherwise be expected to dominate dephasing at 4 K and below. This extra dephasing is apparently induced by local magnetic moments activated by the annealing process, and depends strongly on the backgate voltage applied.

The structural complexity of surfaces of spinel-like oxides together with the existence of competing phases of different or mixed valencies have limited surface structure determinations so far. Only recently, the concept of subsurface cation vacancies (SCVs) could be demonstrated for an ${\mathrm{Fe}}_3{\mathrm{O}}_4$ single crystal surface. Here, we present the appearance of SCVs for ultrathin and thin films of ${\mathrm{Mn}}_3{\mathrm{O}}_4$(001) depending on film thickness. The structures of films grown by molecular beam epitaxy have been investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and near-edge x-ray absorption fine structure spectroscopy (NEXAFS). For coverage up to 4 (Mn ions)/(Ag atom) of ${\mathrm{Mn}}_3{\mathrm{O}}_4$ and an annealing temperature of 640 K, a $p(2\times 1)$ reconstruction has been observed in STM and LEED. Using NEXAFS, the identity of the manganese oxide has been narrowed down to the spinel ${\mathrm{Mn}}_3{\mathrm{O}}_4$ and its related vacancy compound $\gamma \text{−}{\mathrm{Mn}}_2{\mathrm{O}}_3$. Thus the $p(2\times 1)$ reconstruction corresponds to an ${\mathrm{Mn}}_3{\mathrm{O}}_4$(001) surface with $\mathrm{Mn}{\mathrm{O}}_2$ termination. For a lower annealing temperature of 450 K or thicker films [5 and 10 (Mn ions)/(Ag atom) ], additional $p(4\times 2)$ and $c(4\times 4)$ reconstructions have been observed. These extra reconstructions stem from lateral shifts of top-sublayer ions induced by SCVs.

We study the emergence of Majorana zero modes (MZMs) at the ends of a finite double zigzag honeycomb nanoribbon (zHNR), considering a minimal model composed of the first nearest neighbor hopping term, Rashba spin-orbit coupling (RSOC), $p$-wave superconducting pairing, and an applied external magnetic field (EMF). The energy spectrum profiles reveal regions with either spin up or down MZMs belonging to distinct topological phase transitions, which are characterized by their corresponding winding numbers and can be accessed by tuning the chemical potential of the nanoribbons. Hybrid systems constituted by zHNRs deposited on conventional $s$-wave superconductors are potential candidates for experimentally realizing our proposal. The spin discrimination of MZMs suggests a possible route for performing topological-conventional qubit operations using Majorana spintronics.

This work reports on the temperature dependence of the electron spin resonance (ESR) of ${\mathrm{Gd}}^{3+}$-doped ${\mathrm{SmB}}_6$ single crystals at X- and Q-band microwave frequencies in different crystallographic directions. We found an anomalous inhomogeneous broadening of the ${\mathrm{Gd}}^{3+}$ ESR linewidth ($\mathrm{\Delta }H$) within 5.3 K $\le T\le$ 12.0 K which is attributed to slow crystalline electric field (CEF) fluctuations, slower than the timescale of the used ESR microwave frequencies $(\sim 10\mathrm{GHz})$. This linewidth inhomogeneity may be associated to the coupling of the ${\mathrm{Gd}}^{3+}S$ states to the breathing mode of the ${\mathrm{SmB}}_6$ cage and can be simulated by a random distribution of the 4th CEF parameter, $b_4$, that strikingly takes negative and positive values. The temperature at which this inhomogeneity sets in is related to the onset of a continuous insulator-to-metal phase transition. In addition, based on the interconfigurational fluctuation relaxation model, the observed exponential $T$ dependence of $\mathrm{\Delta }H$ above $T\simeq$ 10 K gives rise to an excitation energy notably close to the hybridization gap of ${\mathrm{SmB}}_6$ ($\mathrm{\Delta }\simeq$ 60 K). This charge fluctuation scenario provides important ingredients to the physical properties of ${\mathrm{SmB}}_6$. We finally discuss the interplay between charge and valence fluctuations under the view of slow CEF fluctuations in ${\mathrm{SmB}}_6$ by coupling the ${\mathrm{Gd}}^{3+}$ ions to the breathing phonon mode via a dynamic Jahn-Teller-like mechanism.

As the bound state of two oppositely charged particles, excitons emerge from optically excited semiconductors as the electronic analog of a hydrogen atom. In the two-dimensional (2D) case, realized in either quantum well systems or truly 2D materials such as transition metal dichalcogenides, the relative motion of an exciton is described by two quantum numbers: the principal quantum number $n$, and a quantum number $j$ for the angular momentum along the perpendicular axis. Conservation of angular momentum demands that only the $j=0$ states of the excitons are optically active in a system illuminated by plane waves. Here, we consider the case for spatially structured light sources, specifically for twisted light beams with nonzero orbital angular momentum per photon. Under the so-called dipole approximation where the spatial variations of the light source occur on length scales much larger than the size of the semiconductor's unit cell, we show that the photon (linear and/or angular) momentum is coupled to the center-of-mass (linear and/or angular) momentum of the exciton. Thus our study establishes that the exciton spectrum is independent from the spatial structure of the light source.

Trilayer graphene is receiving an increasing level of attention due to its stacking-dependent magnetoelectric and optoelectric properties and its more robust ferromagnetism relative to monolayer and bilayer variants. Additionally, rhombohedral stacked trilayer graphene presents the possibility of easily opening a gap via either an external electric field perpendicular to the layers or the application of external strain. In this paper, we consider an external electric field to open a band gap in rhombohedral trilayer graphene and study the excitonic optical response of the system. This is done via the combination of a tight-binding model with the Bethe–Salpeter equation solved semianalytically and requiring only a simple numerical quadrature. We then discuss the valley-dependent optical selection rules followed by the computation of the excitonic linear optical conductivity for the case of a rhombohedral graphene trilayer encapsulated in hexagonal boron nitride. The tunability of the excitonic resonances via an external field is also discussed together with the increasing localization of the excitonic states as the field increases.

We examine room-temperature magnetic relaxation in polycrystalline Fe films. Out-of-plane ferromagnetic resonance measurements reveal Gilbert damping parameters of $\approx 0.0024$ for Fe films with thicknesses of 4–25 nm, regardless of their microstructural properties. This observation runs counter to the intuition that various film defects heavily influence Gilbert damping. The remarkable invariance with film microstructure suggests that room-temperature intrinsic Gilbert damping in the Fe films is mostly fixed by the bcc crystal structure within the bulk of nanoscale grains, with limited impact from grain boundaries and film roughness. By contrast, the in-plane FMR linewidths of the Fe films exhibit distinct nonlinear frequency dependencies, indicating the presence of strong extrinsic damping. To fit our in-plane FMR data, we have used a grain-to-grain two-magnon scattering model with two types of correlation functions aimed at describing the spatial distribution of inhomogeneities in the film. However, neither of the two correlation functions is able to reproduce the experimental data quantitatively with physically reasonable parameters. Our findings advance the fundamental understanding of intrinsic Gilbert damping in structurally disordered films, while demonstrating the need for a deeper examination of how microstructural disorder governs extrinsic damping.

Dependence of exchange bias (EB) on field-measuring direction is commonly studied to establish the relationship between unidirectional and magnetocrystalline anisotropies in ferromagnet/antiferromagnet (AFM) systems, and phenomenologically, a cosine series expansion with/without higher-order odd terms may be used to fit the experimental data. The role higher-order AFM anisotropy plays on EB and the reason for the existence of higher-order odd terms in the expansion are still ambiguous. Herein, the influence of AFM cubic anisotropy on EB and coercivity was studied in amorphous-CoFeB/epitaxially grown IrMn bilayers, based on experimental measurements and Monte Carlo simulations. The results show that EB is still unidirectionally symmetric with respect to the field-measuring direction, and the symmetric axis is parallel to the field-cooling (FC) direction. Monte Carlo simulation results evidence large enough AFM anisotropy capable of trapping the AFM spins in one of the easy-axis directions closest to the FC direction. These findings, on the one hand, reveal an EB mode with orthogonal major and minor axes by modifying the energy wells and thus patterning the spin alignments of the AFM IrMn layer by means of AFM in-plane fourfold anisotropy in layered thin films, external cooling, and measuring magnetic fields and, on the other hand, are an important step toward a complete microscopic understanding of EB.

The van der Waals oxide dichlorides $M\mathrm{O}{X}_2$ ($M=\mathrm{V}$, Ta, Nb, Ru, and Os; $X=\text{halogen}$ element), with different electronic densities, are attracting considerable attention. Ferroelectricity, spin-singlet formation, and orbital-selective Peierls phases were reported in this family with $d^1$ or $d^2$ electronic configurations, all believed to be caused by the strongly anisotropic electronic orbital degree of freedom. Here, using density functional theory and density matrix renormalization group methods, we investigate the electronic and magnetic properties of ${\mathrm{RuOCl}}_2$ and ${\mathrm{OsOCl}}_2$ with $d^4$ electronic configurations. Different from a previous study using ${\mathrm{VOI}}_2$ with $d^1$ configuration, these systems with $4d^4$ or $5d^4$ do not exhibit a ferroelectric instability along the $a$ axis. Due to the fully occupied $d_{xy}$ orbital in ${\mathrm{RuOCl}}_2$ and ${\mathrm{OsOCl}}_2$, the Peierls instability distortion disappears along the $b$ axis, leading to an undistorted $Immm$ phase (No. 71). Furthermore, we observe strongly anisotropic electronic and magnetic structures along the $a$ axis. For this reason, the materials of our focus can be regarded as “effective one-dimensional” systems even when they apparently have a dominant two-dimensional lattice geometry. The large crystal-field splitting energy (between $d_{xz/yz}$ and $d_{xy}$ orbitals) and large hopping between nearest-neighbor Ru and Os atoms suppresses the $J=0$ singlet state in $M{\mathrm{OCl}}_2$ ($M=\mathrm{Ru}$ or Os) with electronic density $n=4$, resulting in a spin-1 system. Moreover, we find staggered antiferromagnetic order with $\pi$ wave vector along the $M$-O chain direction ($a$ axis) while the magnetic coupling along the $b$ axis is weak. Based on Wannier functions from first-principles calculations, we calculated the relevant hopping amplitudes and crystal-field splitting energies of the $t_{2g}$ orbitals for the Os atoms to construct a multiorbital Hubbard model for the $M$-O chains. Staggered AFM with $\uparrow \text{−}\downarrow \text{−}\uparrow \text{−}\downarrow$ spin structure dominates in our density matrix renormalization group calculations, in agreement with density functional theory calculations. Our results for ${\mathrm{RuOCl}}_2$ and ${\mathrm{OsOCl}}_2$ provide guidance to experimentalists and theorists working on this interesting family of oxide dichlorides.

Topological semimetals, such as Dirac, Weyl, or line-node semimetals, are gapless states of matter characterized by their nodal band structures and surface states. In this work, we consider layered (topologically trivial) insulating systems in $D$ dimensions that are composed of coupled multilayers of $d$-dimensional topological semimetals. Despite being nominal bulk insulators, we show that crystal defects having codimension $(D-d)$ can harbor robust lower-dimensional topological semimetals embedded in a trivial insulating background. As an example we show that defect-bound topological semimetals can be localized on stacking faults and partial dislocations. Finally, we propose how an embedded topological Dirac semimetal can be identified in experiment by introducing a magnetic field and resolving the relativistic massless Dirac Landau level spectrum at low energies in an otherwise gapped system.

We report on the evolution with the magnetic field and the temperature of the vortex lattice (VL) in fully oxygenated ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ as studied by time-of-flight small-angle neutron scattering. Using the High Field Magnet/Extreme Environment Diractometer beamline at Helmholtz-Zentrum Berlin, we have obtained data up to 25.9 T—much higher than data available previously. Our VL structure results are consistent with the progressive suppression by the field of the superconductivity along the crystallographic $\mathbf{b}$ (CuO chain) direction and an accompanying shift of the nodal directions as the field is increased. The intensity of the diffracted signal reveals the spatial variation of magnetization caused by the VL (the “form factor”). Instead of a rapid falloff with the field, as seen in superconductors with smaller upper critical fields, we find that the form factor is almost constant with the field above $\sim 12$ T. We speculate that this is due to Pauli paramagnetic effects.

Chirality-induced spin selectivity (CISS) refers to an ability to induce a spin polarization of an electron transmitted through chiral materials. An important experimental observation is that incredibly large spin polarization is realized at room temperature even for organic molecules that have weak spin-orbit coupling (SOC), although SOC is the only interaction that can manipulate the electrons' spins in the setups. Therefore the mechanism of the CISS needs to be constructed in a way insensitive to or enhancing the magnitude of the SOC strength. In this paper, we describe a theoretical study of CISS with a model chiral molecule that belongs to the point group ${\mathrm{C}}_3$. In this molecule, electronic translational and rotational degrees of freedom for an injected electron are coupled to one another via the nuclear vibrational mode with a pseudo Jahn-Teller effect. By properly taking the molecular symmetry as well as the time-reversal symmetry into account and classifying the molecular ground states by their angular- and spin-momentum quantum numbers, we show that the chiral molecule can act as an efficient spin filter. The efficiency of this spin filtering can be nearly independent of the SOC strength in this model, while it well exceeds the spin polarization relying solely on the SOC. The nuclear vibrations turned out to have the role of not only mediating the translation-rotation coupling, but also enhancing the spin-filtering efficiency.

Chalcogenides in perovskite and the related layered Ruddlesden-Popper crystal structures (chalcogenide perovskites for brevity) are an exciting family of semiconductors but remain experimentally little studied. Chalcogenide perovskites share crystal structures and some physical properties with ionic compounds such as oxide and halide perovskites, but the metal-chalcogen bonds responsible for semiconducting behavior are substantially more covalent than in these more-studied perovskites. Here, we use complementary experimental and theoretical methods to study how the mixed ionic-covalent Zr-S bonds support the electronic structure and physical properties of perovskite ${\mathrm{BaZrS}}_3$ and Ruddlesden-Popper ${\mathrm{Ba}}_3{\mathrm{Zr}}_2{\mathrm{S}}_7$. We apply theoretical methods to assign features of experimentally measured x-ray absorption spectroscopy (XAS) to particular orbital transitions, enabling a clear physical interpretation of angle-dependent, polarized XAS data measured on single-crystal samples, and an atomistic view of the covalent bonding network that facilitates charge transport. Polarized Raman measurements identify signatures of crystalline anisotropy in ${\mathrm{Ba}}_3{\mathrm{Zr}}_2{\mathrm{S}}_7$ and enable the first assignments of mode symmetry in this material. Infrared reflectivity reveals electronic transport properties that augur well for the use of chalcogenide perovskites in optoelectronic and energy-conversion technologies.

Difference in $g$ factors in multidot structures can form the basis of dot-selective spin manipulation under global microwave irradiation. Employing electric dipole spin resonance facilitated by strong spin-orbit interaction (SOI), we observe differences in the extracted values of the single-hole effective $g$ factors of the constituent quantum dots of a GaAs/AlGaAs double quantum dot device at the level of $\sim 5\%–10\%$. We examine the continuous change in the hole $g$ factor with electrical detuning over a wide range of interdot tunnel couplings and for different out-of-plane magnetic fields. The observed tendency of the quantum dot effective $g$ factors to steadily increase on decreasing the interdot coupling or on increasing the magnetic field is attributed to the impact on the SOI of changing the dot confinement potential and heavy-hole light-hole mixing.

The contact resistance between two dissimilar semiconductors is determined by the carrier transmission through their interface. Despite the ubiquitous presence of interfaces, quantitative simulation of charge transport across such interfaces is difficult, limiting the understanding of interfacial charge transport. This work employs Green's functions to study the charge transport across representative Si/Ge interfaces. For perfect interfaces, it is found that the transmittance is small and the contact resistance is high, not only because the mismatch of carrier pockets makes it hard to meet the momentum conservation requirement, but also because of the incompatible symmetries of the Bloch wave functions of the two sides. In contrast, atomic mixing at the interface increases the carrier transmittance as the interface roughness opens many nonspecular transmission channels, which greatly reduces the contact resistance compared with the perfect interface. Specifically, we show that disordered interfaces with certain symmetries create more nonspecular transmission. The insights from our study will benefit the future design of high-performance heterostructures with low contact resistance.

Novel ternary platinum borides ${\mathrm{YbPt}}_5{\mathrm{B}}_2$ and ${\mathrm{LuPt}}_5{\mathrm{B}}_2$ have been synthesized by arc melting of constituent elements and subsequent annealing at 1020 K. The unit cell of ${\mathrm{YbPt}}_5{\mathrm{B}}_2 {[\mathrm{YbPt}}_5{\mathrm{B}}_2$ type, monoclinic space group $C2/m, a=15.4982\left(6\right)\AA , b=5.5288\left(3\right)\AA , c=5.5600\left(3\right)\AA , \beta =105.367{\left(3\right)}^{\circ }$, single-crystal x-ray diffraction data] is composed of two structural fragments alternating along the $a$ axis and extending infinitively along the $b$ axis: (i) columns of face-fused, boron-filled, and empty edge-connected trigonal metal prisms, and (ii) zigzag chains of B-filled trigonal prisms connected by common edges. ${\mathrm{LuPt}}_5{\mathrm{B}}_2$ was found to be isotypic (Rietveld refinement of powder x-ray diffraction data). ${\mathrm{YbPt}}_5{\mathrm{B}}_2$ exhibits two magnetic phase transitions at $T_{\mathrm{mag}1}\sim 8\mathrm{K}$ and $T_{\mathrm{mag}2}\sim 4\mathrm{K}$ as deduced from specific heat, magnetostriction, electric resistivity, and magnetization data. The Kadowaki-Woods ratio of $1.22\times {10}^{-5}\mu \mathrm{\Omega }\mathrm{cm}{\left(\mathrm{mol}\mathrm{K}/\mathrm{mJ}\right)}^2$ classifies ${\mathrm{YbPt}}_5{\mathrm{B}}_2$ as a heavy fermion system. A magnetic phase diagram was established, using magnetostriction and magnetoresistance data. Nonmagnetic ${\mathrm{LuPt}}_5{\mathrm{B}}_2$ is characterized by a simple metallic behavior.

Topological phases exhibit a plethora of striking phenomena, including disorder-robust localization and the propagation of waves of various nature. While this physics is actively explored in bosonic and fermionic cases, the topological phases of anyons — particles with fractional quantum statistics — are largely uncharted. Here, we unveil the topological transition mediated by the particles' quantum statistics that arises for two-anyon and three-anyon excitations in a one-dimensional array described by the extended Hubbard model. As we demonstrate, the interplay of two-particle interactions and tunneling processes enables topological edge states of anyon pairs whose existence and localization at one or another edge of the one-dimensional system is governed by the quantum statistics of particles. Since a direct realization of the proposed system is challenging, we develop a rigorous method to emulate the eigenmodes and eigenenergies of anyon pairs with resonant electrical circuits.

The system of an iron phthalocyanine molecule on the Au(111) surface has been studied recently due to its peculiar properties. In particular, several surprising results of scanning tunneling spectroscopy changing the position of the molecule and applying magnetic field can be explained by the non-Landau Fermi liquid state of a two-channel spin-1 Kondo model with anisotropy. The localized orbitals near the Fermi level are three, one of symmetry $z^2$ and two (nearly) degenerate $\pi$ orbitals of symmetry $xz$ and $yz$. Previous studies using the numerical renormalization group neglected one of these orbitals to render the problem tractable. Here we investigate, using a slave-boson mean-field approximation, if the splitting $S$ between $\pi$ orbitals caused by spin-orbit coupling (SOC) justifies this approximation. We obtain an abrupt transition from a three-band regime to a two-band one at a value of $S$, which is about 1/3 of the atomic SOC for Fe, justifying the two-band model for the system.

We derive the explicit analytical form for the charge-dipole and dipole-dipole interactions in two-dimensional (2D) configuration space. We demonstrate that the reduction of dimensionality can alter the charge-dipole and dipole-dipole interactions in the 2D case. The asymptotics of these interactions at long distances coincide to the charge-dipole and dipole-dipole interactions in three-dimensional configuration space. The obtained charge-dipole and dipole-dipole interactions will find wide application and contribute to the advancement of research on novel two-dimensional materials.