Spin-crossover (SCO) solids have been studied for their fascinating properties, exhibiting first-order phase transitions and macroscopic bistabilities, accompanied by significant magnetic, structural, and optical changes. These exceptional properties make these materials promising for applications as high-density information storage and optical switches. Recently, the critical progress made in chemistry allowed the design of spin-crossover nanocomposites, combining the properties of two types of spin-crossover solids having different properties, like different lattice parameters, bulk moduli, transition temperatures, ligand fields, etc. In this paper, we consider a microscopic electroelastic description of a SCO nanostructure made of a SCO active core surrounded by a SCO active shell, for which we impose an unconventional elastic frustration at the core-shell interface. The detailed examination of the thermodynamic properties of such a nanocomposite, as a function of the lattice parameter misfit between the two constituents, revealed that the frustration causes unexpected behaviors on the thermal dependence of the average bond lengths, such as the emergence of two- or three-step spin transitions, with self-organization of the spin states in the plateau regions. These results highlight the nontrivial character of the magnetoelastic properties in switchable SCO nanoparticles.

We demonstrate a tenfold resonant enhancement of magneto-optical effects in perpendicularly magnetized ${[\mathrm{Co}/\mathrm{Pt}]}_N$ nanodots mediated by the excitation of optimized plasmon modes. Two magnetoplasmonic systems are considered: square arrays of ${[\mathrm{Co}/\mathrm{Pt}]}_N$ nanodots on glass and identical arrays on a $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_2$ bilayer. On glass, the optical and magneto-optical spectra of the nanodot arrays are dominated by the excitation of a surface lattice resonance (SLR), whereas on $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_2$, a narrow surface plasmon polariton (SPP) resonance tailors the spectra further. Both the SLR and SPP modes are magneto-optically active leading to an enhancement of the Kerr angle. We detail the dependence of optical and magneto-optical spectra on the number of Co/Pt bilayer repetitions, the nanodot diameter, and the array period, offering design rules on how to maximize and spectrally tune the magneto-optical response of perpendicularly magnetized ${[\mathrm{Co}/\mathrm{Pt}]}_N$ nanodots.

The heating of electrons in graphene by laser irradiation, and its effects on the lattice structure, are studied. Values for the temperature of the electron system in realistic situations are obtained. For sufficiently high electron temperatures, the occupancy of the states in the $\sigma$ band of graphene is modified. The strength of the carbon-carbon bonds changes, leading to the emergence of strains, and to buckling in suspended samples. While most applications of “strain engineering” in two-dimensional materials focus on the effects of strains on electronic properties, the effect studied here leads to alterations of the structure induced by light. This optomechanical coupling can induce deflections in the order of $\sim 50$ nm in micron-sized samples.

Defect spins in silicon carbide have become promising platforms with respect to quantum information processing and quantum sensing. Indeed, the optically detected magnetic resonance (ODMR) of defect spins is the cornerstone of the applications. In this work, we systematically investigate the contrast and linewidth of laser and microwave power-dependent ODMR with respect to ensemble-divacancy spins in silicon carbide at room temperature. The results suggest that magnetic field sensing sensitivity can be greatly improved for the optimized laser and microwave power range. The experiment will be useful for the applications of silicon carbide defects in quantum information processing and ODMR-dependent quantum sensing.

We study theoretically the localization properties of two distinct one-dimensional quasiperiodic lattice models with a single-particle mobility edge (SPME) separating extended and localized states in the energy spectrum. The first one is the familiar Soukoulis-Economou trichromatic potential model with two incommensurate potentials, and the second is a system consisting of two coupled 1D Aubry-André chains each containing one incommensurate potential. We show that as a function of the Hamiltonian model parameters, both models have a wide single-particle intermediate phase, defined as the regime where localized and extended single-particle states coexist in the spectrum, leading to a behavior intermediate between purely extended or purely localized when the system is dynamically quenched from a generic initial state. Our results thus suggest that both systems could serve as interesting experimental platforms for studying the interplay between localized and extended states, and may provide insight into the role of the coupling of small baths to localized systems. We also calculate the Lyapunov (or localization) exponent for several incommensurate 1D models exhibiting SPME, finding that such localization critical exponents for quasiperiodic potential induced localization are nonuniversal and depend on the microscopic details of the Hamiltonian.

Simultaneous coexistence of room-temperature ferromagnetism and ferroelectricity in Fe-doped ${\mathrm{BaTiO}}_3$ (BTO) is intriguing, as such Fe doping into tetragonal BTO, a room-temperature ferroelectric, results in the stabilization of its hexagonal polymorph which is ferroelectric only below $\sim 80\mathrm{K}$. Here, we investigate its origin and show that Fe-doped BTO has a mixed-phase room-temperature multiferroicity, where the ferromagnetism comes from the majority hexagonal phase and a minority tetragonal phase gives rise to the observed weak ferroelectricity. In order to achieve majority tetragonal phase (responsible for room-temperature ferroelectricity) in Fe-doped BTO, we investigate the role of different parameters which primarily control the paraelectric hexagonal phase stability over the ferroelectric tetragonal one and identify three major factors, namely the effect of ionic size, Jahn-Teller (JT) distortions, and oxygen-vacancies, to be primarily responsible. The effect of ionic size which can be qualitatively represented using the Goldschmidt's tolerance factor seems to be the major dictating factor for the hexagonal phase stability. The understanding of these factors not only enables us to control them but also to achieve a suitable codoped BTO compound with enhanced room-temperature multiferroic properties.

Neutron scattering measurements have demonstrated that the heavily Cu doped ${\mathrm{NaFe}}_{1-x}{\mathrm{Cu}}_x\mathrm{As}$ compound behaves like a Mott insulator exhibiting both real-space Fe-Cu stripes and antiferromagnetism below a Néel temperature for $x\lesssim 0.5$. We have investigated the evolution of structural and magnetic ordering using ${}^{23}\mathrm{Na}$ and ${}^{75}\mathrm{As}$ NMR for single crystals ($x$ = 0.39 and 0.48), confirming antiferromagnetism in the form of magnetic stripes. We show that end-chain defects in these stripes are the principal source of magnetic disorder and are responsible for cluster spin-glass transitions in both compounds, in the latter case coexistent with antiferromagnetism. Aided by our numerical simulation of the ${}^{75}\mathrm{As}$ spectra, we show that a staggered magnetization at the Fe sites is induced by nonmagnetic Cu dopants.

We study a Josephson junction involving a Weyl and a multi-Weyl semimetal separated by a barrier region of width $d$ created by putting a gate voltage $U_0$ over the Weyl semimetal. The topological winding number of such a junction changes across the barrier. We show that $I_c{R}_N$ for such junctions, where $I_c$ is the critical current and $R_N$ the normal-state resistance, in the thin-barrier limit, has a universal value independent of the barrier potential. We provide an analytical expression of the Andreev bound states and use it to demonstrate that the universal value of $I_c{R}_N$ is a consequence of change in topological winding number across the junction. We also study the AC Josephson effect in such a junction in the presence of an external microwave radiation, chart out its current-voltage characteristics, and show that the change in the winding number across the junction shapes the properties of its Shapiro steps. We discuss the effect of increasing barrier thickness $d$ on the above-mentioned properties and chart-out experiments which may test our theory.

The electron-nuclei hyperfine interaction of electrons in indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment has been experimentally studied by measuring the polarization degree of the photoluminescence in a transverse magnetic field (Hanle effect) and the polarization recovery in a longitudinal magnetic field. The different symmetries of the $X$ valley electron Bloch amplitudes at the As, In, and Al nuclei strongly affect the hyperfine interaction. The hyperfine constants corresponding to these nuclei have been determined.

Within a tight-binding approach, we investigate the effect of twisting angle on the energy levels of circular bilayer graphene (BLG) quantum dots (QDs) in both the absence and presence of a perpendicular magnetic field. The QDs are defined by an infinite-mass potential, so that the specific edge effects are not present. In the absence of magnetic field (or when the magnetic length is larger than the moiré length), we show that the low-energy states in twisted BLG QDs are completely affected by the formation of moiré patterns, with a strong localization at AA-stacked regions. When magnetic field increases, the energy gap of an untwisted BLG QD closes with the edge states, localized at the boundaries between the AA- and AB-stacked spots in a twisted BLG QD. Our observation of the spatial localization of the electrons in twisted BLG QDs can be experimentally probed by low-bias scanning tunneling microscopy measurements.

We study the combined effects of spin transfer torque, voltage modulation of interlayer exchange coupling, and magnetic anisotropy on the switching behavior of perpendicular magnetic tunnel junctions (p-MTJs). In asymmetric p-MTJs, a linear-in-voltage dependence of interlayer exchange coupling enables the effective perpendicular anisotropy barrier to be lowered for both voltage polarities. This mechanism is shown to reduce the critical switching current and effective activation energy. Finally, we analyze the possibility of having switching via interlayer exchange coupling only.

Electronic structure calculations based on density functional theory (DFT) give quantitatively accurate predictions of properties of most materials containing light elements. For heavy materials, and in particular for $f$-electron systems, DFT based methods can fail both qualitatively and quantitatively for two distinct reasons: their failure to describe confinement effects arising from localized $f$-electron behavior and their incomplete or approximate treatment of relativity. In addition, different methods for incorporating relativistic effects, which give identical results in most light materials, can give different predictions in heavy elements. In order to develop a quantitative capability for calculating the properties of these materials, it is essential to separate the predictions of the underlying equations from the uncertainty introduced in approximations used in computation. Working toward that goal, we have developed a code, called dirac-fp, which is based directly on solving the Dirac-Kohn-Sham equations and uses the full-potential linear muffin-tin orbital (FP-LMTO) approach to electronic structure. In order to assess the performance of dirac-fp, we perform calculations on three different face-centered cubic materials using different approximate treatments of relativity: the scalar relativistic (SR) approach commonly used in most solid-state DFT codes, the scalar relativistic plus spin-orbit coupling corrections (SR+SO) approach which includes spin-orbit coupling self-consistently using the SR states inside the muffin tins, and the Dirac-Kohn-Sham (Dirac) approach implemented in dirac-fp. Performing calculations on thorium, in which relativistic effects should be strong, aluminum, in which relativistic effects should be negligible, and gold, in which relativistic effects play an intermediate role, we find that the Dirac approach is able to provide theoretically consistent results in the electronic structure and ground-state properties across all three materials.

We present a theoretical study of the interplay between topological $p$-wave superconductivity, orbital magnetic fields, and quantum Hall phases in coupled wire systems. First, we calculate the phase diagram and physical observables of a fermionic ladder made of two coupled Kitaev chains and discuss the presence of two and four Majorana zero modes. Second, we analyze hybrid systems consisting of a Kitaev chain coupled to a Luttinger liquid. By tuning the magnetic field and the carrier density, we identify quantum Hall and charge density wave phases, as well as regimes in which superconductivity is induced in the second chain by the proximity effect. Finally, we consider two-dimensional systems made of weakly coupled ladders. There we engineer a $p+ip$ superconductor and describe a generalization of the $\nu =1/2$ fractional quantum Hall phase. These phases might be realized in solid-state or cold-atom nanowires.

Point defects in two-dimensional (2D) materials hold great promise for optoelectronic and quantum technologies. Their properties depend sensitively on the dielectric environment and number of 2D layers, but this has remained a challenge to include in first-principles calculations on account of the high computational cost. Recent first-principles techniques facilitate efficient prediction of substrate effects on defect charge transition levels in 2D materials, replacing the substrate by a continuum dielectric model. Here, we show that analogous continuum treatment of defect-free layers in multilayer 2D materials accurately predicts defect energies compared to explicit multilayer calculations, but at a small fraction of the computational cost. Applications of this technique to one- to five-layer and bulk hexagonal boron nitride reveal that defect ionization energies systematically decrease with an increasing number of layers and for defects in inner layers due to increased dielectric screening. Our results highlight the dominant role of electrostatic screening in the effect of the environment and the feasibility of tuning defect levels in 2D materials using material thickness and defect location within the material.

The important influence of the relative refractive index of scattering inclusions on energy transport of classical waves through disordered media is clearly demonstrated through ultrasonic experiments on monodisperse emulsions. Our ultrasonic techniques measure both the transmitted average wave field and the multiply scattered diffusive intensity, enabling a full characterization of wave transport through the media and the measurement of both the group and energy velocities over a wide range of frequencies. The emulsions were fabricated using microfluidic techniques that permit accurate control of droplet size and concentration, for droplet inclusions with very different acoustic properties relative to the yield stress fluids in which the droplets were immersed. Thus we have been able to investigate emulsions containing either “slow” fluorinated oil droplets (sound speed $v_1$ less than $v_0$ of the surrounding fluid) or “fast” liquid metallic droplets $\left(v_1>v_0\right)$. We find that the energy velocity that describes the transport of energy by the dominant diffusive waves is mainly governed by the sound speed within the scatterers, and can be either much slower or faster than any of the other wave velocities. The possibility that the energy velocity could be faster than any other wave velocity when $n_{\text{rel}}=v_0/v_1<1$ was not anticipated in previous work. These observations are successfully explained by theories that are valid for scalar waves in media containing a low concentration of scatterers, and are directly applicable to our dilute “all-fluid” emulsions. The role of droplet resonances on the behavior of the energy velocity is also demonstrated, and the mechanism leading to the large differences in the energy velocity in the two emulsion systems is elucidated through calculations of the energy density inside the droplets relative to the incident energy density.

Extended x-ray absorption fine structure spectroscopy of the light-harvesting formamidinium lead bromide $\left({\mathrm{FAPbBr}}_3\right)$ perovskite, a system with attractive optoelectronic performance, shows anomalously large variance in Pb-Br bond length, some 50% larger than in its inorganic ${\mathrm{CsPbBr}}_3$ counterpart. Using first-principles molecular dynamics simulations, we find a significant contribution to this variance coming from the FA cation, and show that the FA does not just tumble in its cuboctahedral ${\mathrm{Br}}_{12}$ cage, but instead stochastically sticks to, and detaches from one of the 12 nearest Br atoms after another, leading to the large variance in Pb-Br bond length. Our results demonstrate dynamic coupling between the FA-Br moiety and perovskite cage vibrations, and that tunability in dynamics can be achieved by changing the cation type and perovskite lattice parameter. Thus, our results provide information that needs to be considered in any of the intensely debated models of electron-phonon coupling in lead halide perovskites.

The influence of hydrostatic pressure on the Curie temperature $T_C$ of thin ferromagnetic (Ga,Mn)As layers is studied. New experimental data unambiguously point to both positive and negative pressure-induced changes of Curie temperature. The positive pressure coefficient is observed for samples with relatively high values of $T_C$ and can be quantitatively described by the $p\text{−}d$ Zener model of carrier-mediated ferromagnetism within the six-band $k·p$ formalism and the ab initio approach. First-principles calculations of structural, electronic, and magnetic properties of (Ga,Mn)As show that antiferromagnetic coupling of substitutional Mn atoms with interstitial ones may account for a decrease of $T_C$ under pressure in samples having a substantial concentration of interstitial Mn.