Motivated by the recent discovery of a high-temperature superconducting phase in ${\mathrm{KFe}}_2{\mathrm{As}}_2$ at 16 GPa accompanied by an isostructural phase transition from the tetragonal to the compressed tetragonal phase, we extend the study of pressure effects to the whole $X{\mathrm{Fe}}_2{\mathrm{As}}_2$ family ($X=\mathrm{Na}$, K, Rb, Cs). We demonstrate that the ionic radius of the $X$ atom determines the transition to the compressed phase which induces relevant changes in electronic properties and the Fermi surface which can enhance the superconducting pairing. Based on these results and in analogy with ${\mathrm{KFe}}_2{\mathrm{As}}_2$, we theoretically propose ${\mathrm{Na}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$ as a possible new superconductor material in its compressed tetragonal phase which we predict to happen above 6 GPa.

The valley degree of freedom is intrinsic to spin qubits in Si/SiGe quantum dots. It has been viewed alternately as a hazard, especially when the lowest valley-orbit splitting is small compared to the thermal energy, or as an asset, most prominently in proposals to use the valley degree of freedom itself as a qubit. Here we present experiments in which microwave electric field driving induces transitions between both valley-orbit and spin states. We show that this system is highly nonlinear and can be understood through the use of dressed photon-orbital states, enabling a unified understanding of the six microwave resonance lines we observe. Some of these resonances are intervalley spin transitions that arise from a nonadiabatic process in which both the valley and the spin degree of freedom are excited simultaneously. For these transitions, involving a change in valley-orbit state, we find a tenfold increase in sensitivity to electric fields and electrical noise compared to pure spin transitions, strongly reducing the phase coherence when changes in valley-orbit index are incurred. In contrast to this nonadiabatic transition, the pure spin transitions, whether arising from harmonic or subharmonic generation, are shown to be adiabatic in the orbital sector. The nonlinearity of the system is most strikingly manifest in the observation of a dynamical anticrossing between a spin-flip, intervalley transition and a three-photon transition enabled by the strong nonlinearity we find in this seemly simple system.

Compounds having excess electrons from the formal valence viewpoint (electrides) are a new class of materials, which often take low-dimensional structures. We studied the (001) surface electronic structures of quasi-two-dimensional electrides ${\mathrm{Ca}}_2\mathrm{N}$ and ${\mathrm{Y}}_2\mathrm{C}$ by density functional theory using a slab model. Both materials were found to have a clean surface state well separated in energy from the bulk states. Furthermore, this state virtually floats above the surface and may be considered to be a hallmark of two-dimensional electrides. For ${\mathrm{Ca}}_2\mathrm{N}$, a tight-binding model in the Wannier representation was derived and analyzed, from which we concluded that the surface state, described by extra-surface $s$-like orbitals, is a Tamm state originating from an abrupt increase in potential energy at the surface.

Destructive quantum interference effect is implemented in large area molecular junctions to improve signatures of electron-phonon interaction. Vertical molecular junctions based on a cross-conjugated anthraquinone layer were fabricated and low-noise transport measurements were performed by acquiring simultaneously the current-voltage characteristics, its second derivative, and the differential conductance. Signatures of vibrational modes excited by inelastic events are present in the whole measured voltage range and superpose to the conductance suppression induced by destructive quantum interference. As a consequence vibrational modes have improved visibility in the low energy window $\left(<80\mathrm{meV}\right)$. Inelastic electron transport spectroscopy data are compared to infrared attenuated total reflection spectroscopy on Au/anthraquinone thin films. Common vibrational modes can be clearly identified, but inelastic electron tunneling spectroscopy reveals the existence of vibrational modes in a wider energy range $\left(0–400\mathrm{meV}\right)$ where infrared spectroscopy is lacking.

We present a theoretical study of the characteristics of the nonlinear spin-orbital angular momentum coupling induced by second-harmonic generation in plasmonic and dielectric nanostructures made of centrosymmetric materials. In particular, the connection between the phase singularities and polarization helicities in the longitudinal components of the fundamental and second-harmonic optical fields and the scatterer symmetry properties are discussed. By in-depth comparison between the interaction of structured optical beams with plasmonic and dielectric nanostructures, we have found that all-dielectric and plasmonic nanostructures that exhibit magnetic and electric resonances have comparable second-harmonic conversion efficiency. In addition, mechanisms for second-harmonic enhancement for single and chiral clusters of scatterers are unveiled and the relationships between the content of optical angular momentum of the incident optical beams and the enhancement of nonlinear light scattering is discussed. In particular, we formulate a general angular momenta conservation law for the nonlinear spin-orbital angular momentum interaction, which includes the quasi-angular-momentum of chiral structures with different-order rotational symmetry. As a key conclusion of our study relevant to nanophotonics, we argue that all-dielectric nanostructures provide a more suitable platform to investigate experimentally the nonlinear interaction between spin and orbital angular momenta, as compared to plasmonic ones, chiefly due to their narrower resonance peaks, lower intrinsic losses, and higher sustainable optical power.

In the context of friction we use atomistic molecular-dynamics simulations to investigate water confined between graphene sheets over a wide range of pressures. We find that thermal equilibration of the confined water is hindered at high pressures. We demonstrate that, under the right conditions, square ice can form in an asperity, and that it is similar to cubic ice VII and ice X. We simulate sliding of atomically flat graphite on the square ice and find extremely low friction due to structural superlubricity. The conditions needed for square ice to form correspond to low sliding speeds, and we suggest that the ice observed in experiments of friction on wet graphite is of this type.

We explore the stability of the ambient pressure zinc-blende polymorph (B3) structure of silicon carbide (SiC) at high pressures and temperatures where it transforms to the rocksalt (B1) structure. We find that the transition occurs ∼40 GPa lower than previously measured when heated to moderately high temperatures. A lower transition pressure is consistent with the transition pressures predicted in numerous ab initio computations. We find a large volume decrease across the transition of ∼17%, with the volume drop increasing at higher formation pressures, suggesting this transition is volume driven yielding a nearly pressure-independent Clapeyron slope. Such a dramatic density increase occurring at pressure is important to consider in applications where SiC is exposed to extreme conditions, such as in industrial applications or planetary interiors.

We performed a global search for possible Au-Si crystal structures using a genetic algorithm (GA) combined with density functional theory (DFT) calculations. Two Au-Si structures, $\mathrm{A}{\mathrm{u}}_8\mathrm{S}{\mathrm{i}}_8$ and $\mathrm{A}{\mathrm{u}}_{16}\mathrm{S}{\mathrm{i}}_8$, were found to be energetically stable and have no imaginary frequencies by phonon calculations. The formation energies of all the studied structures gave a convex hull of the Au-Si system, showing that the most stable composition was $\mathrm{Au}-\mathrm{Si}=1:1$ and that the Si-rich structures were much less stable than the Au-rich ones.

We explore the field-temperature phase diagram of the XY pyrochlore antiferromagnet ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ by means of magnetization and neutron diffraction experiments. Depending on the field strength and direction relative to the high symmetry cubic directions $\left[001\right],\left[1\overline{1}0\right]$, and $\left[111\right]$, the refined field-induced magnetic structures are derived from the zero field ${\psi }_2$ and ${\psi }_3$ states of the ${\mathrm{\Gamma }}_5$ irreducible representation which describes the ground state of XY pyrochlore antiferromagnets. At low field, domain selection effects are systematically at play. In addition, for $\left[001\right]$, a phase transition is reported towards a ${\psi }_3$ structure at a characteristic field $H_c^{001}=43$ mT. For $\left[1\overline{1}0\right]$ and $\left[111\right]$, the spins are continuously tilted by the field from the ${\psi }_2$ state, and no phase transition is found while domain selection gives rise to sharp anomalies in the field dependence of the Bragg peaks intensity. For $\left[1\overline{1}0\right]$, these results are confirmed by high resolution inelastic neutron scattering experiments, which in addition allow us to determine the field dependence of the spin gap. This study agrees qualitatively with the scenario proposed theoretically by Maryasin et al. [Phys. Rev. B 93, 100406(R) (2016)], yet the strength of the field-induced anisotropies is significantly different from theory.

We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6–19 nm). Using a combination of polarized neutron scattering and magnetometry, we were able to resolve previously observed peculiarities. Magnetometry, on the one hand, reveals a peak in the zero-field-cooled (ZFC) magnetization curves at low temperatures $(\sim 25$ K) but $no$ feature around the Néel temperature at 118 K. On the other hand, polarized neutron scattering shows the expected behavior of the AF order parameter vanishing around 118 K. Moreover, hysteresis curves measured at various temperatures reveal an exchange-bias effect, indicating a coupling of an AF core to a ferromagnetic (FM)-like shell. ZFC data measured at various fields exclude a purely superparamagnetic (SPM) scenario. We conclude that the magnetic behavior of MnO particles can be explained by a superposition of SPM-like thermal fluctuations of the AF-Néel vector inside the AF core and a magnetic coupling to a ferrimagnetic ${\mathrm{Mn}}_2{\mathrm{O}}_3$ or ${\mathrm{Mn}}_3{\mathrm{O}}_4$ shell. In addition, we have studied heterodimer (“Janus”) particles, where a FM FePt particle is attached to the AF MnO particle. Via the exchange-bias effect, the magnetic moment of the FePt subunit is stabilized by the MnO.

An exchange bias (EB) effect was observed in mixed valent $\mathrm{L}{\mathrm{a}}_x\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{Fe}{\mathrm{O}}_3$ ($x=0.125$, 0.25, 0.33) perovskites exhibiting the antiferromagnetic (AFM) helical order among $\mathrm{F}{\mathrm{e}}^{4+}$ ions coexisting with the ferromagnetic (FM) cluster phase in the ground state. The $\mathrm{L}{\mathrm{a}}^{3+}$ ions for $\mathrm{B}{\mathrm{a}}^{2+}$ site substitution, associated with increase in number of the AFM coupled $\mathrm{F}{\mathrm{e}}^{3+}$ - $\mathrm{F}{\mathrm{e}}^{4+}$ pairs as well as some $\mathrm{F}{\mathrm{e}}^{3+}$ - $\mathrm{F}{\mathrm{e}}^{3+}$ pairs, leads to strengthening of the AFM phase and consequently to the alteration of the EB characteristics, which depend on level of the La doping $x$. At low doping $x\le 0.25$, an abnormal dependence of the EB field, $H_{\mathrm{EB}}$, on the cooling field, $H_{\mathrm{cool}}$, was found. The $H_{\mathrm{EB}}$ increases rapidly with increasing cooling field at low $H_{\mathrm{cool}}$, but it falls suddenly at cooling fields higher than 20 kOe, reducing by an order of magnitude at 90 kOe. The suppression of EB is caused by the field-induced increased volume of the FM phase, due to the transformation of the AFM helical spin structure into the FM one. Thus, low-doped $\mathrm{L}{\mathrm{a}}_x\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{Fe}{\mathrm{O}}_3$ demonstrates a competition of two alternate cooling-field-induced effects, one of which leads to the EB anisotropy and another one to the enhanced ferromagnetism. In contrast, the $x=0.33$ sample, having a strong AFM constituent, shows no field-induced FM and no drop in the EB field. Accordingly, the $H_{\mathrm{EB}}$ vs $H_{\mathrm{cool}}$ dependence was found to be well explained in the framework of a model describing phase-separated AFM-FM systems, namely, the model assuming isolated FM clusters of size ∼4 nm embedded in the AFM matrix.

Barkhausen current noise is used to probe the slow field-driven conversion of the glassy relaxor ferroelectric state to an ordered ferroelectric state. The frequent presence of distinct micron-scale Barkhausen events well before the polarization current starts to speed up shows that the process is not a conventional nucleation-limited one. The prevalence of reverse switching events near the onset of the rapid part of the transition strongly indicates that electric dipole interactions play a key role. The combination of Barkhausen noise changes and changes in the complex dielectric response indicate that the process consists of an initial mixed-alignment domain formation stage followed by growth of the domains aligned with the applied field.

We present here a model of carrier distribution and transport in semiconductor alloys accounting for quantum localization effects in disordered materials. This model is based on the recent development of a mathematical theory of quantum localization which introduces for each type of carrier a spatial function called localization landscape. These landscapes allow us to predict the localization regions of electron and hole quantum states, their corresponding energies, and the local densities of states. We show how the various outputs of these landscapes can be directly implemented into a drift-diffusion model of carrier transport and into the calculation of absorption/emission transitions. This creates a new computational model which accounts for disorder localization effects while also capturing two major effects of quantum mechanics, namely, the reduction of barrier height (tunneling effect) and the raising of energy ground states (quantum confinement effect), without having to solve the Schrödinger equation. Finally, this model is applied to several one-dimensional structures such as single quantum wells, ordered and disordered superlattices, or multiquantum wells, where comparisons with exact Schrödinger calculations demonstrate the excellent accuracy of the approximation provided by the landscape theory.

Urbach tails in semiconductors are often associated to effects of compositional disorder. The Urbach tail observed in InGaN alloy quantum wells of solar cells and LEDs by biased photocurrent spectroscopy is shown to be characteristic of the ternary alloy disorder. The broadening of the absorption edge observed for quantum wells emitting from violet to green (indium content ranging from 0% to 28%) corresponds to a typical Urbach energy of 20 meV. A three-dimensional absorption model is developed based on a recent theory of disorder-induced localization which provides the effective potential seen by the localized carriers without having to resort to the solution of the Schrödinger equation in a disordered potential. This model incorporating compositional disorder accounts well for the experimental broadening of the Urbach tail of the absorption edge. For energies below the Urbach tail of the InGaN quantum wells, type-II well-to-barrier transitions are observed and modeled. This contribution to the below-band-gap absorption is particularly efficient in near-ultraviolet emitting quantum wells. When reverse biasing the device, the well-to-barrier below-band-gap absorption exhibits a red-shift, while the Urbach tail corresponding to the absorption within the quantum wells is blue-shifted, due to the partial compensation of the internal piezoelectric fields by the external bias. The good agreement between the measured Urbach tail and its modeling by the localization theory demonstrates the applicability of the latter to compositional disorder effects in nitride semiconductors.

This paper introduces a novel method to account for quantum disorder effects into the classical drift-diffusion model of semiconductor transport through the localization landscape theory. Quantum confinement and quantum tunneling in the disordered system change dramatically the energy barriers acting on the perpendicular transport of heterostructures. In addition, they lead to percolative transport through paths of minimal energy in the two-dimensional (2D) landscape of disordered energies of multiple 2D quantum wells. This model solves the carrier dynamics with quantum effects self-consistently and provides a computationally much faster solver when compared with the Schrödinger equation resolution. The theory also provides a good approximation to the density of states for the disordered system over the full range of energies required to account for transport at room temperature. The current-voltage characteristics modeled by three-dimensional simulation of a full nitride-based light emitting diode (LED) structure with compositional material fluctuations closely match the experimental behavior of high-quality blue LEDs. The model allows also a fine analysis of the quantum effects involved in carrier transport through such complex heterostructures. Finally, details of carrier population and recombination in the different quantum wells are given.

We present an exactly solvable model of a quantum spin liquid with Abelian anyons in $d=2$ spatial dimensions. With spins 1/2 on a triangular lattice and six-body interactions, our model has zero spin correlation length and localized elementary excitations like the toric codes of Kitaev and Wen. In contrast to those earlier models, it has more elementary particles—four bosons and three fermions—and higher topological degeneracy of 64 on a torus. Elementary excitations are boson-fermion pairs that come in 12 distinct flavors. We use string operators to expose the topological nature of the model.

Two-dimensional transition metal dichalcogenides entwine interaction, spin-orbit coupling, and topology. Hole-doped systems lack spin degeneracy: states are indexed with spin and valley specificity. This unique structure offers new possibilities for correlated phases and phenomena. We realize an unconventional superconducting pairing phase which is an equal mixture of a spin singlet and the $m=0$ spin triplet. It is stable against large in-plane magnetic fields, and its topology allows quasiparticle excitations of net nonzero Berry curvature via pair-breaking circularly polarized light.

Local $n$-particle vertex functions represent the crucial ingredient for diagrammatic extensions of dynamical mean field theory (DMFT). Hitherto their application has been restricted—with a few exceptions—to the $n=2$-particle vertex while higher-order vertices have been neglected. In this paper we derive a general analytical expression for the local $n$-particle (one-particle-reducible) vertex of the Falicov-Kimball model (FKM). We observe that the magnitude of such vertex functions itself strongly increases with the number of particles $n$. On the other hand, their effect on generic Feynman diagrams remains rather moderate due to the damping effect of the Green's functions present in such diagrams. Nevertheless, they yield important contributions to the self-energy corrections calculated in diagrammatic extensions of DMFT as we explicitly demonstrate using the example of dual-fermion calculations for the two-dimensional FKM at quarter filling of the stationary $f$ electrons. Here corrections to the self-energy obtained from the three-particle vertex are indeed comparable in magnitude to corresponding corrections stemming from the two-particle vertex.