We perform a magneto-optical study of a two-dimensional electron systems in the regime of the Stoner ferromagnetic instability for even quantum Hall filling factors on ${\mathrm{Mg}}_x{\mathrm{Zn}}_{1-x}\mathrm{O}/\mathrm{ZnO}$ heterostructures. Under conditions of Landau-level crossing, caused by enhanced spin susceptibility in combination with the tilting of the magnetic field, the transition between two rivaling phases, paramagnetic and ferromagnetic, is traced in terms of optical spectra reconstruction. Synchronous sharp transformations are observed both in the photoluminescence structure and parameters of collective excitations upon transition from paramagnetic to ferromagnetic ordering. Based on these measurements, a phase diagram is constructed in terms of the two-dimensional electron density and tilt angle of the magnetic field. Apart from stable paramagnetic and ferromagnetic phases, an instability region is found at intermediate parameters with the Stoner transition occurring at $\nu \approx 2$. The spin configuration in all cases is unambiguously determined by means of inelastic light scattering by spin-sensitive collective excitations. One indicator of the spin ordering is the intra-Landau-level spin exciton, which acquires a large spectral weight in the ferromagnetic phases. The other is an abrupt energy shift of the intersubband charge density excitation due to reconstruction of the many-particle energy contribution. From our analysis of photoluminescence and light scattering data, we estimate the ratio of surface areas occupied by the domains of the two phases in the vicinity of a transition point. In addition, the thermal smearing of a phase transition is characterized.

The spin-orbit interaction is investigated in a dual gated InAs/GaSb quantum well. Using an electric field, the quantum well can be tuned between a single-carrier regime with exclusively electrons as carriers and a two-carrier regime where electrons and holes coexist. The spin-orbit interaction in both regimes manifests itself as a beating in the Shubnikov–de Haas oscillations. In the single-carrier regime the linear Dresselhaus strength is characterized by $\beta =28.5$ meV Å and the Rashba coefficient $\alpha$ is tuned from 75 to 53 meV Å by changing the electric field. In the two-carrier regime a quenching of the spin splitting is observed and attributed to a crossing of spin bands.

We introduce a highly sensitive and relatively simple technique to observe magnetization motion in single Ni nanoparticles, based on charge sensing by electron tunneling at millidegrees Kelvin temperature. Sequential electron tunneling via the nanoparticle drives nonequilibrium magnetization dynamics, which induces an effective charge noise that we measure in real time. In the free spin diffusion regime, where the electrons and magnetization are in detailed balance, we observe that magnetic damping time exhibits a peak with the magnetic field, with a record long damping time of $\simeq 10$ ms.

By applying first-principles Peierls-Boltzmann transport calculations, the lattice thermal conductivities of type-I, type-II, and type-VIII silicon clathrates are investigated. The lattice thermal conductivity, spectral lattice thermal conductivity, and lattice thermal conductivity versus mean-free path (MFP) of phonons, relaxation times, and the anharmonicity of selected Si clathrates are calculated and compared. The microscopic origin of the slight difference in the lattice thermal conductivity of Si-I/II/VIII clathrates is discussed extensively. Our calculations uncover that the impact of phonon group velocity on the lattice thermal conductivity is far from enough to explain the discrepancies in ${\kappa }_{\text{lat}}$ of studied Si clathrates and it mainly is a consequence of the shorter lifetimes of the acoustic modes in type-VIII structure. Similarity between phase space for three-phonon anharmonic scattering processes of studied silicon clathrates highlights the influence of three-phonon coupling strength and the further calculation of the frequency-resolved final state spectra re-emphasizes that the subtle differences arise from the stronger three-phonon interaction coefficients.

The plastic deformation of crystalline materials is usually modeled as smoothly progressing in space and time, yet modern studies show intermittency in the deformation dynamics of single-crystals arising from avalanche behavior of dislocation ensembles under uniform applied loads. However, once the prism of the microstructure in polycrystalline materials disperses and redistributes the load on a grain-by-grain basis, additional length and time scales are involved. Thus, the question is open as to how deformation intermittency manifests for the nonuniform grain-scale internal driving forces interacting with the finer-scale dislocation ensemble behavior. In this work we track the evolution of elastic strain within individual grains of a creep-loaded titanium alloy, revealing widely varying internal strains that fluctuate over time. The findings provide direct evidence of how flow intermittency proceeds for an aggregate of $\sim 700$ grains while showing the influences of multiscale ensemble interactions and opening new avenues for advancing plasticity modeling.

Spin waves can induce domain wall motion in ferromagnets. We derive the equations of motion for a transverse domain wall driven by spin waves. Our calculations show that the magnonic spin-transfer torque does not cause rotation-induced Walker breakdown. The amplitude of spin waves that are excited by a localized microwave field depends on the spatial profile of the field and the excitation frequency. By taking this frequency dependence into account, we show that a simple one-dimensional model may reproduce much of the puzzling frequency dependence observed in early numerical studies.

We investigate the electronic band structure and Fermi surfaces (FSs) of $\mathrm{C}{\mathrm{r}}_2\mathrm{AlC}$ single crystals with angle-resolved photoemission spectroscopy. We evidence hole bands centered around the $M$ points and electron bands centered around the $\mathrm{\Gamma }$ point in reciprocal space. Electron and hole bands exhibit an open, tubular structure along the $c$ axis, confirming the quasi-two-dimensional character of this highly anisotropic, nanolamellar compound. Dependence of the photoionization cross sections on beam light polarization and orientation allows us to assess the orbital character of each observed band locally. Despite some differences, density functional theory calculations show a good agreement with experiment.

We present an efficient implementation of a surface Green's-function method for atomistic modeling of surfaces within the framework of density functional theory using a pseudopotential localized basis set approach. In this method, the system is described as a truly semi-infinite solid with a surface region coupled to an electron reservoir, thereby overcoming several fundamental drawbacks of the traditional slab approach. The versatility of the method is demonstrated with several applications to surface physics and chemistry problems that are inherently difficult to address properly with the slab method, including metal work function calculations, band alignment in thin-film semiconductor heterostructures, surface states in metals and topological insulators, and surfaces in external electrical fields. Results obtained with the surface Green's-function method are compared to experimental measurements and slab calculations to demonstrate the accuracy of the approach.

In quantum wires, such as metallic nanotubes, the optical absorption of the transverse polarization is controlled by the depolarization effect which stems from the redistribution of conduction electrons around the circumference of the system. The traditional electrostatics treatment of the depolarization effect relies on approximating the system by a cylinder with some effective dielectric permittivity. We demonstrate that this simple intuitive picture does not adequately describe optical absorption near its threshold, as the depolarization effect becomes dominated by many-body correlations which strongly modify the spectral dependence of absorption.

We introduce two-dimensional borane, a single-layered material of BH stoichiometry, with promising electronic properties. We show that, according to density functional theory calculations, two-dimensional borane is semimetallic, with two symmetry-related Dirac cones meeting right at the Fermi energy $E_f$. The curvature of the cones is lower than in graphene, thus closer to the ideal linear dispersion. Its structure, formed by a puckered trigonal boron network with hydrogen atoms connected to each boron atom, can be understood as distorted, hydrogenated borophene [Mannix et al., Science 350, 1513 (2015)]. Chemical bonding analysis reveals the boron layer in the network being bound by delocalized four-center two-electron $\sigma$ bonds. Finally, we suggest high pressure could be a feasible route to synthesize two-dimensional borane.

We investigate the unitary dynamics following a sudden increase $\mathrm{\Delta }U>0$ of repulsion in the paramagnetic sector of the half-filled Hubbard model on a Bethe lattice, by means of a variational approach that combines a Gutzwiller wave function with a partial Schrieffer-Wolff transformation, both defined through time-dependent variational parameters. Besides recovering at $\mathrm{\Delta }{U}_c$ the known dynamical transition linked to the equilibrium Mott transition, we find a pronounced dynamical anomaly at larger $\mathrm{\Delta }{U}_{*}>\mathrm{\Delta }{U}_c$ manifested in a singular behavior of the long-time average of double occupancy. Although the real-time dynamics of the variational parameters at $\mathrm{\Delta }{U}_{*}$ strongly resembles the one at $\mathrm{\Delta }{U}_c$, a careful frequency spectrum analysis suggests a dynamical crossover, instead of a dynamical transition, separating regions of a different behavior of the spin exchange.

Singlet-triplet qubits in lateral quantum dots in semiconductor heterostructures exhibit high-fidelity single-qubit gates via exchange interactions and magnetic field gradients. High-fidelity two-qubit entangling gates are challenging to generate since weak interqubit interactions result in slow gates that accumulate error in the presence of noise. However, the interqubit electrostatic interaction also produces a shift in the local double well detunings, effectively changing the dependence of exchange on the gate voltages. We consider an operating point where the effective exchange is first-order insensitive to charge fluctuations while maintaining nonzero interactions. This “sweet spot” exists only in the presence of interactions. We show that working at the interacting sweet spot can directly produce maximally entangling gates, and we simulate the gate evolution under realistic $1/f$ noise. We report theoretical two-qubit gate fidelities above 99% in GaAs and Si systems.

Iridates supply fertile ground for unconventional phenomena and exotic electronic phases. With respect to well-studied octahedrally coordinated iridates, we focus our attention on a rather unexplored iridate, ${\mathrm{Na}}_4{\mathrm{IrO}}_4$, showing an unusual square planar coordination. The latter is key to rationalizing the electronic structure and magnetic property of ${\mathrm{Na}}_4{\mathrm{IrO}}_4$, which is here explored by first-principles density functional theory calculations and Monte Carlo simulations. Due to the uncommon square planar crystal field, Ir $5d$ states adopt an intermediate-spin state with double occupation of the $d_{z^2}$ orbital, leading to a sizable local spin moment, at variance with many other iridates. The square planar crystal-field splitting is also crucial in opening a robust insulating gap in ${\mathrm{Na}}_4{\mathrm{IrO}}_4$, irrespective of the specific magnetic ordering or treatment of electronic correlations. Spin-orbit coupling plays a minor role in shaping the electronic structure, but leads to strong magnetocrystalline anisotropy. The easy axis perpendicular to the ${\mathrm{IrO}}_4$ plaquette, well explained using perturbation theory, is again closely related to the square planar coordination. Finally, the large single-ion anisotropy suppresses the spin frustration and stabilizes a collinear antiferromagnetic long-range magnetic ordering, as confirmed by Monte Carlo simulations predicting a quite low Néel temperature, expected from almost isolated ${\mathrm{IrO}}_4$ square planar units that act as crystalline building blocks.

In $5d$ transition-metal oxides, novel properties arise from the interplay of electron correlations and spin-orbit interactions. ${\mathrm{Na}}_4{\mathrm{IrO}}_4$, where the $5d$ transition-metal Ir atom occupies the center of the square-planar coordination environment, has attracted research interest. Based on density functional theory, we present a comprehensive investigation of electronic and magnetic properties of ${\mathrm{Na}}_4{\mathrm{IrO}}_4$. We propose the magnetic ground-state configuration, and find that the magnetic easy axis is perpendicular to the ${\mathrm{IrO}}_4$ plane. The magnetic anisotropy energy (MAE) of ${\mathrm{Na}}_4{\mathrm{IrO}}_4$ is found to be giant. We estimate the magnetic parameters in the generalized symmetry-allowed spin model, and find that the next-nearest-neighbor exchange interaction $J_2$ is much larger than other intersite exchange interactions and results in the magnetic ground-state configuration. The numerical results reveal that the anisotropy of interatomic spin-exchange interaction is quite small and the huge MAE comes from the single-ion anisotropy. This compound has a large spin gap but very narrow spin-wave dispersion, due to the large single-ion anisotropy and quite small intersite exchange couplings. We clarify that these remarkable magnetic features are originated from its highly isolated and low-symmetry ${\mathrm{IrO}}_4$ moiety. We also explore the possibility to further enhance the MAE.

The $GW$ self-energy method has long been recognized as the gold standard for quasiparticle (QP) calculations of solids in spite of the fact that the neglect of vertex corrections and the use of a density-functional theory starting point lack rigorous justification. In this work we remedy this situation by including a simple vertex correction that is consistent with a local-density approximation starting point. We analyze the effect of the self-energy by splitting it into short-range and long-range terms which are shown to govern, respectively, the center and size of the band gap. The vertex mainly improves the short-range correlations and therefore has a small effect on the band gap, while it shifts the band gap center up in energy by around 0.5 eV, in good agreement with experiments. Our analysis also explains how the relative importance of short- and long-range interactions in structures of different dimensionality is reflected in their QP energies. Inclusion of the vertex comes at practically no extra computational cost and even improves the basis set convergence compared to $GW$. Taken together, the method provides an efficient and rigorous improvement over the $GW$ approximation.

We report numerically accurate path integral Monte Carlo results for harmonically confined two-dimensional quantum dots containing up to $N=60$ interacting electrons. The finite-temperature values are extrapolated to 0 K and zero time step in order to provide precise upper-bound energies. The ground-state energies are compared against coupled-cluster and diffusion Monte Carlo results available in the literature for $N\le 20$. We also provide Padé fits for the energies as a function of $N$ for different strengths of the confining potential. The fits deviate less than $0.25\%$ from the path integral Monte Carlo data. Overall, our upper-bound estimates for the ground-state energies have lower values than previous diffusion Monte Carlo benchmarks due to the accurate nodal surface in our simulations. Hence, our results set a new numerical benchmark for two-dimensional (spin-unpolarized) quantum dots up to a large number of electrons.

For more than 20 years, observation of the nondissipative Hall viscosity in the quantum Hall effect has been impeded by the difficulty to probe directly the momentum of the two-dimensional electron gas. However, in three-dimensional systems such as superfluid ${}^3\mathrm{He}-\mathrm{B}$, the momentum density is readily probed through transverse acoustic waves. We show that in a three-dimensional elastic medium supporting transverse waves, a nonvanishing Hall viscosity induces circular birefringence. Such an effect has been observed in ${}^3\mathrm{He}-\mathrm{B}$ in the presence of a weak magnetic field, and is known as the acoustic Faraday effect. The acoustic Faraday effect has been understood in terms of the Zeeman splitting of the excited order parameter modes, which support the transverse wave propagation in the superfluid. We show that the Zeeman effect can generically lead to a nonzero Hall viscosity coefficient, and confirm this prediction using a simple phenomenological model for the ${}^3\mathrm{He}-\mathrm{B}$ collective modes. Therefore, we claim that the observation of the acoustic Faraday effect can be leveraged to make a direct observation of the Hall viscosity in superfluid ${}^3\mathrm{He}-\mathrm{B}$ in a magnetic field and other systems such as the crystalline ${\mathrm{Tb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ material.

An interface between ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ (LAO/STO) may host two-dimensional superconductivity, strong spin-orbit coupling, and magnetic ordering. There are indications that the interfacial superconducting layer is nonhomogeneous at the nanoscale. The presence of inhomogeneities may have strong implications on the superconducting transport properties and lead to formation of weak links between superconducting regions. In this paper, we report on the temperature and magnetic field dependence of superconducting transport in single nanowires and nanorings fabricated in a LAO/STO interface. The analysis of data proves that our nanostructures behave like uniform superconducting filaments carrying a critical current close to the theoretical Ginzburg-Landau depairing limit. Furthermore, we unexpectedly find an enhancement of the critical current of LAO/STO nanowires and nanorings at low temperature when a magnetic field is applied. Our results point towards the coexistence of a homogeneous interfacial superconductivity and spatially separated magnetism at the LAO/STO interface.