We theoretically investigate the single- and few-electron states of HgTe topological insulator quantum dots using the configuration interaction method and find distinguishing features for both cases. For a single electron, the kinetic energy of the edge states follows an “edge” quantization rule and is robust against the deformation of the circular quantum dot into various elliptic ones, in contrast to the bulk states of the quantum dot. For two electrons, the ratio between the kinetic energy and the Coulomb energy is insensitive to the dot size when the dot is circular, but can be tuned over a relatively wide range by deforming the circular dot into elliptic ones, in contrast to conventional quantum dots, where the kinetic (Coulomb) energy dominates for small (large) dots. For a few electrons, the electrons first fill the edge states in the bulk band gap, and the addition energy and ground state spin exhibit universal even-odd oscillation due to the shape-independent two-fold degeneracy of the edge states. The size of this edge shell can be controlled by tuning the dot size or the band gap of the HgTe quantum well via lateral or vertical electric gating, respectively.

We exploit the flexibility offered by an (In,Ga)As/GaAs quantum dot spin ensemble to demonstrate that complex dynamic evolutions can be excited in the ensemble magnetization and accessed by tailored pulsed laser protocols. The modes for spin precession about a magnetic field are adapted to the periodic excitation protocol such that at specific times the magnetization can effectively be decomposed in two, three, or four equal components with angles of $\pi ,2\pi /3$, or $\pi /2$ between them. Optical orientation of these components by an additional laser pulse leads to the generation of higher harmonics in the spin precession, as evidenced by time-resolved ellipticity measurements.

We present a method for improving measurements of the entanglement Rényi entropies in quantum Monte Carlo simulations by relating them with measurements of participation Rényi entropies. Exploiting the capability of building improved estimators for the latter makes it possible to obtain very good estimates for entanglement Rényi entropies. When considering a full system instead of a bipartition, the method can be further ameliorated, providing access to the thermodynamic Rényi entropies with high accuracy. We also explore a recently proposed method for the reconstruction of the entanglement spectrum from entanglement Rényi entropies and finally show how potential entanglement Hamiltonians may be tested for their validity using a comparison with thermal Rényi entropies.

We investigate sudden quenches across the critical point in the transverse field Ising chain with a perturbing nonintegrable next-nearest-neighbor interaction. Expressions for the return (Loschmidt) amplitude and associated rate function are derived to linear order in the next-nearest-neighbor coupling. In the thermodynamic limit these quantities exhibit nonanalytic behavior at a set of critical times, a phenomenon referred to as a dynamical quantum phase transition. We quantify the effect of the integrability breaking perturbation on the location and shape of these nonanalyticities. Our results agree with those of earlier numerical studies and offer further support for the assertion that the dynamical quantum phase transitions exhibited by this model are a generic feature of its postquench dynamics and are robust with respect to the inclusion of nonintegrable perturbations.

We present a theoretical and experimental study of photonic and electronic transport properties of a voltage biased InAs semiconductor double quantum dot (DQD) that is dipole coupled to a superconducting transmission line resonator. We obtain the master equation for the reduced density matrix of the coupled system of cavity photons and DQD electrons accounting systematically for both the presence of phonons and the effect of leads at finite voltage bias. We subsequently derive analytical expressions for transmission, phase response, photon number, and the nonequilibrium steady-state electron current. We show that the coupled system under finite bias realizes an unconventional version of a single-atom laser and analyze the spectrum and the statistics of the photon flux leaving the cavity. In the transmission mode, the system behaves as a saturable single-atom amplifier for the incoming photon flux. Finally, we show that the back action of the photon emission on the steady-state current can be substantial. Our analytical results are compared to exact master equation results establishing regimes of validity of various analytical models. We compare our findings to available experimental measurements.

Interfaces between dissimilar polar materials may provide a pathway to new device functionality, including high carrier mobility layers at the interface. The development of these materials has proven challenging, in part because of the high energy cost of forming polar surfaces. Our density functional theory calculations explore the mechanisms by which a real material satisfies the electrostatic criteria for stability imposed by a polar surface. The consequences of polarity are studied by comparing the formation energies, charge distribution, and electronic structure of a number of low-index surfaces of rocksalt MgO and CaO. These surfaces are explored both in their bare, undecorated form as well as with surface reconstructions and adsorbed foreign species. Our ground-state surface energies are extended to relevant environmental conditions by use of ab initio thermodynamics. We find that the high energy of bare polar surfaces is the result of the significant charge redistribution that arises to compensate the polarity and pushes electronic states into the forbidden band gap. Other mechanisms of polarity compensation (reconstruction or foreign species adsorption) are therefore seen more frequently. We explain the experimental observations of surface roughness during growth in the [111] direction. In typical epitaxial growth conditions, there is preferential formation of an octopolar reconstruction of the {111} surface, which exposes {001}-type nanofacets. The low energy of the {001} surface likely causes these facets to grow, leading to a rough surface morphology. Our results indicate that when water vapor is present during growth, a smooth, polar surface can be stabilized by the formation of a hydroxyl layer.

We report on the development of electron holographic tomography towards a versatile potential measurement technique, overcoming several limitations, such as a limited tilt range, previously hampering a reproducible and accurate electrostatic potential reconstruction in three dimensions. Most notably, tomographic reconstruction is performed on optimally sampled polar grids taking into account symmetry and other spatial constraints of the nanostructure. Furthermore, holographic tilt series acquisition and alignment have been automated and adapted to three dimensions. We demonstrate 6 nm spatial and 0.2 V signal resolution by reconstructing various, previously hidden, potential details of a GaAs/AlGaAs core-shell nanowire. The improved tomographic reconstruction opens pathways towards the detection of minute potentials in nanostructures and an increase in speed and accuracy in related techniques such as x-ray tomography.

The diversity of mesostructures formed in steel at cooling from a high-temperature austenite (γ) phase is determined by the interplay of shear reconstructions of crystal lattice and diffusion of carbon. Combining first-principles calculations with large-scale phase-field simulations we demonstrate a decisive role of magnetic degrees of freedom in the formation of energy relief along the Bain path of γ-α transformation and, thus, in this interplay. We show that there is the main factor, namely, the magnetic state of iron and its evolution with temperature which controls the change in character of the transformation. Based on the computational results we propose a simple model which reproduces, in good agreement with experiment, the most important curves of the phase transformation in Fe-C, namely, the lines relevant to a start of ferrite, bainite, and martensite transformations. Phase-field simulations within the model describe qualitatively typical patterns at these transformations.

The high-temperature phase transition between the tetragonal (scheelite) and monoclinic (fergusonite) forms of yttrium tantalite (${\mathrm{YTaO}}_4$ ) has been studied using a combination of first-principles calculations and a Landau free-energy expansion. Calculations of the Gibbs free energies show that the monoclinic phase is stable at room temperature and transforms to the tetragonal phase at 1430 °C, close to the experimental value of $1426\pm 7$ °C. Analysis of the phonon modes as a function of temperature indicate that the transformation is driven by softening of transverse acoustic modes with symmetry $E_u$ in the Brillouin zone center rather than the Raman-active $B_g$ mode. Landau free-energy expansions demonstrate that the transition is second order and, based on the fitting to experimental and calculated lattice parameters, it is found that the transition is a proper rather than a pseudoproper type. Together these findings are consistent with the transition being ferroelastic.

We study the connection between the cumulants of a time-integrated observable of a quantum system and the $\mathcal{P}\mathcal{T}$-symmetry properties of the non-Hermitian deformation of the Hamiltonian from which the generating function of these cumulants is obtained. This non-Hermitian Hamiltonian can display regimes of broken and of unbroken $\mathcal{P}\mathcal{T}$ symmetry, depending on the parameters of the problem and on the counting field that sets the strength of the non-Hermitian perturbation. This in turn determines the analytic structure of the long-time cumulant generating function (CGF) for the time-integrated observable. We consider in particular the case of the time-integrated (longitudinal) magnetization in the one-dimensional Ising model in a transverse field. We show that its long-time CGF is singular on a curve in the magnetic field/counting field plane that delimits a regime where $\mathcal{P}\mathcal{T}$ symmetry is spontaneously broken (which includes the static ferromagnetic phase), from one where it is preserved (which includes the static paramagnetic phase). In the paramagnetic phase, conservation of $\mathcal{P}\mathcal{T}$ symmetry implies that all cumulants are sublinear in time, a behavior usually associated with the absence of decorrelation.

Surface acoustic waves (SAW) were generated on a thin layer of the ferromagnetic semiconductor (Ga,Mn)(As,P). The out-of-plane uniaxial magnetic anisotropy of this dilute magnetic semiconductor is very sensitive to the strain of the layer, making it an ideal test material for the dynamic control of magnetization via magnetostriction. The amplitude and phase of the transmitted SAW during magnetic field sweeps showed a clear resonant behavior at a field close to the one calculated to give a precession frequency equal to the SAW frequency. A resonance was observed from 5 to 85 K, just below the Curie temperature of the layer. A full analytical treatment of the coupled magnetization/acoustic dynamics showed that the magnetostrictive coupling modifies the elastic constants of the material and accordingly the wave-vector solution to the elastic wave equation. The shape and position of the resonance were well reproduced by the calculations, in particular the fact that velocity (phase) variations resonated at lower fields than the acoustic attenuation variations. We suggest one reinterpret SAW-driven ferromagnetic resonance as a form of resonant, dynamic, delta-$E$ effect, a concept usually reserved for static magnetoelastic phenomena.

Using the Landau-Lifshitz-Bloch (LLB) equation for ferromagnetic materials, we derive analytic expressions for temperature-dependent absorption spectra as probed by ferromagnetic resonance. By analyzing the resulting expressions, we can predict the variation of the resonance frequency and damping with temperature and coupling to the thermal bath. We base our calculations on the technologically relevant ${\mathrm{L}1}_0\mathrm{FePt}$, parametrized from atomistic spin dynamics simulations, with the Hamiltonian mapped from ab initio parameters. By constructing a multimacrospin model based on the LLB equation and exploiting GPU acceleration, we extend the study to investigate the effects on the damping and resonance frequency in $\mu \mathrm{m}$-sized structures.

Antiferromagnetic chains with an odd number of spins are known to undergo a transition from an antiparallel to a spin-flop configuration when subjected to an increasing magnetic field. We show that in the presence of an anisotropy favoring alignment perpendicular to the field, the spin-flop state appears for both weak and strong fields, the antiparallel state appearing for intermediate fields. Both transitions are second order, the configuration varying continuously with the field intensity. Such reentrant transition is robust with respect to quantum fluctuations and it might be observed in different types of nanomagnets.

This paper revisits the weakly fourth-order anisotropic Ginzburg-Landau (GL) theory of freezing (also known as the Landau-Brazowskii model or theory of weak crystallization) by comparing it to a recent density functional approach, the phase-field crystal (PFC) model. First we study the critical behavior of a generalized PFC model and show that (i) the so-called one-mode approximation is exact in the leading order, and (ii) the direct correlation function has no contribution to the phase diagram near the critical point. Next, we calculate the anisotropy of the crystal-liquid interfacial free energy in the phase-field crystal (PFC) model analytically. For comparison, we also determine the anisotropy numerically and show that no range of parameters can be found for which the phase-field crystal equation can quantitatively model anisotropy for metallic materials. Finally, we derive the leading order PFC amplitude model and show that it coincides with the weakly fourth-order anisotropic GL theory, as a consequence of the assumptions of the GL theory being inherent in the PFC model. We also propose a way to calibrate the anisotropy in the Ginzburg-Landau theory via a generalized gradient operator emerging from the direct correlation function appearing in the generating PFC free energy functional.

Synchrotron-based infrared and Raman spectroscopies were brought together with diamond anvil cell techniques and an analysis of the magnetic properties to investigate the pressure-induced high $\to$ low spin transition in $\left[{\mathrm{Ru}}_2{\left({\mathrm{O}}_2\mathrm{CMe}\right)}_4{]}_3{\left[\mathrm{Cr}\right(\mathrm{CN})}_6\right]$. The extended nature of the diruthenium wave function combined with coupling to chromium-related local lattice distortions changes the relative energies of the ${\pi }^{*}$ and ${\delta }^{*}$ orbitals and drives the high $\to$ low spin transition on the mixed-valence diruthenium complex. This is a rare example of an externally controlled metamagnetic transition in which both spin-orbit and spin-lattice interactions contribute to the mechanism.

Electron-magnon spin-flip scattering in thin films was studied by investigating the thickness dependence of the anisotropic magnetoresistance (AMR) effect and spin-wave stiffness. The absolute resistivity change due to the AMR effect ($\Delta \rho$) in Ni, Ni:V, and Ni:Cr doped films reduced with film thickness. This loss of AMR is due to enhanced spin-flip scattering, dropping at the same thickness irrespective of dopant. The spin-wave stiffness reduced at the same thickness, confirming enhanced electron-magnon spin-flip scattering. The AMR ratio was fitted with a simple model, in which thickness dependence was included in a spin mixing resistivity term. This analysis gives insight into the fundamental contribution of magnon scattering to the resistivity in thin films, which ultimately has relevance to spin coherence in spintronic devices.

We report the results of direct measurement of remanent hysteresis loops on nanochains of ${\mathrm{BiFeO}}_3$ at room temperature under zero and $\sim 20$ kOe magnetic field. We noticed a suppression of remanent polarization by nearly $\sim 40\%$ under the magnetic field. The powder neutron diffraction data reveal significant ion displacements under a magnetic field which seems to be the origin of the suppression of polarization. The isolated nanoparticles, comprising the chains, exhibit evolution of ferroelectric domains under dc electric field and complete ${180}^{\circ }$ switching in switching-spectroscopy piezoresponse force microscopy. They also exhibit stronger ferromagnetism with nearly an order of magnitude higher saturation magnetization than that of the bulk sample. These results show that the nanoscale ${\mathrm{BiFeO}}_3$ exhibits coexistence of ferroelectric and ferromagnetic order and a strong magnetoelectric multiferroic coupling at room temperature comparable to what some of the type-II multiferroics show at a very low temperature.

The element-specific magnetization and ordering in trilayers consisting of 0.3–1.4 monolayer (ML) thick Fe layers embedded in Pd(001) has been determined using x-ray resonant magnetic scattering. The proximity to Fe induces a large moment in the Pd which extends $\sim 2\mathrm{nm}$ from the interfaces. The magnetization as a function of temperature is found to differ significantly for the Fe and Pd sublattices: The Pd signal resembles the results obtained by magneto-optical techniques with an $\mathit{apparent}$ three-dimensional (3D) to two-dimensional (2D) transition in spatial dimensionality for Fe thickness below $\sim 1\mathrm{ML}$. In stark contrast, the Fe data exhibits a $2\mathrm{D}$ behavior. No ferromagnetic signal is obtained from Fe below the $2\mathrm{D}$ percolation limit in Fe coverage $(\sim 0.7\mathrm{ML})$, while Pd shows a ferromagnetic response for all samples. The results are attributed to the temperature dependence of the susceptibility of Pd and a profound local anisotropy of submonolayered Fe.

The equilibrium physics of quantum impurities frequently involves a universal crossover from weak to strong reservoir-impurity coupling, characterized by single-parameter scaling and an energy scale $T_K$ (Kondo temperature) that breaks scale invariance. For the noninteracting resonant level model, the nonequilibrium time evolution of the Loschmidt echo after a local quantum quench was recently computed explicitly [R. Vasseur, K. Trinh, S. Haas, and H. Saleur, Phys. Rev. Lett. 110, 240601 (2013)]. It shows single-parameter scaling with variable $T_{K}t$. Here, we scrutinize whether similar universal dynamics can be observed in various interacting quantum impurity systems. Using density matrix and functional renormalization group approaches, we analyze the time evolution resulting from abruptly coupling two noninteracting Fermi or interacting Luttinger liquid leads via a quantum dot or a direct link. We also consider the case of a single Luttinger liquid lead suddenly coupled to a quantum dot. We investigate whether the field-theory predictions for the universal scaling as well as for the large-time behavior successfully describe the time evolution of the Loschmidt echo and the entanglement entropy of microscopic models. Our study shows that for the considered local quench protocols the above quantum impurity models fall into a class of problems for which the nonequilibrium dynamics can largely be understood based on the knowledge of the corresponding equilibrium physics.