We have developed a nuclear magnetic resonance (NMR) system that uses spin injection from a highly polarized spin source. Efficient spin injection into GaAs from a half-metallic spin source of Mn-rich $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$ enabled an efficient dynamic nuclear polarization of Ga and As nuclei in GaAs and a sensitive detection of NMR signals. Moreover, coherent control of nuclear spins, or the Rabi oscillation between two quantum levels formed at Ga nuclei, induced by a pulsed NMR has been demonstrated at a relatively low magnetic field of ∼0.1 T. This provides a novel all-electrical solid-state NMR system with the high spatial resolution and high sensitivity needed to implement scalable nuclear-spin-based qubits.

In order to study the variation of a microscopic internal magnetic field with the Ca content $\left(x\right)$, we have systematically measured muon-spin rotation and relaxation $\left({\mu }^{+}\mathrm{SR}\right)$ spectra for ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2 \left(0\le x\le 1\right)$ powder samples mainly in a zero external field. As $x$ increases from 0, a Pauli-paramagnetic phase is observed even at the lowest $T$ measured (1.8 K) until $x=0.45$; then, a short-range antiferromagnetic (AF) ordered phase appears for $0.48\le x\le 0.75$, and finally a long-range AF ordered phase is stabilized for $x\ge 0.75$. The evolution of the magnetic order is connected to the shrinking of the $c$-axis length as a function of $x$, which naturally enhances the AF interaction between the two adjacent Co planes.

We present detailed investigations of the magnetic properties of an Fe monolayer on W and $\mathrm{Ta}\left(110\right)$ surfaces based on the ab initio screened Korringa-Kohn-Rostoker method. By calculating tensorial exchange coupling coefficients, the ground states of the systems are determined using atomistic spin dynamics simulations. Different types of ground states are found in the systems as a function of relaxation of the Fe layer. In the case of the $\mathrm{W}\left(110\right)$ substrate this is reflected in a reorientation of the easy axis from in-plane to out-of-plane. For $\mathrm{Ta}\left(110\right)$ a switching appears from the ferromagnetic state to a cycloidal spin spiral state, then to another spin spiral state with a larger wave vector, and for large relaxations, a rotation of the normal vector of the spin spiral is found. Classical Monte Carlo simulations indicate temperature-induced transitions between the different magnetic phases observed in the $\mathrm{Fe}/\mathrm{Ta}\left(110\right)$ system. These phase transitions are analyzed both quantitatively and qualitatively by finite-temperature spin wave theory.

Recent studies of dynamical screening of the electronic Coulomb interactions in solids have revived interest in lattice models of correlated fermions coupled to bosonic degrees of freedom (Hubbard-Holstein-type models). We propose a new dynamical mean-field-based approach to dynamically screened Coulomb interactions. In the effective Anderson-Holstein model, a transformation to slave rotors [S. Florens and A. Georges, Phys. Rev. B 66, 165111 (2002)] is performed to decouple the dynamical part of the interaction. This transformation allows for a systematic derivation and analysis of recently introduced approximate schemes for the solution of dynamical impurity problems, in particular, the Bose factor ansatz within the dynamic atomic limit approximation (DALA) with and without Lang-Firsov correction. More importantly still, it suggests an optimized choice for a Bose factor in the sense of the variational principle of Feynman and Peierls. We demonstrate the accuracy of our scheme and present a comparison to calculations within the DALA.

A platform for constructing microscopic Hamiltonians describing bosonic symmetry-protected topological (SPT) states is presented. The Hamiltonians we consider are examples of frustration-free Rokhsar-Kivelson models, which are known to be in one-to-one correspondence with classical stochastic systems in the same spatial dimensionality. By exploring this classical-quantum mapping, we are able to construct a large class of microscopic models which, in a closed manifold, have a nondegenerate gapped symmetric ground state describing the universal properties of SPT states. Examples of one- and two-dimensional SPT states which illustrate our approach are discussed.

We discuss the evolution of topological states and their orbital textures in the mixed-valence compounds ${\text{SmB}}_6$ and ${\text{YbB}}_6$ within the framework of the generalized gradient approximation plus onsite Coulomb interaction $(\mathrm{GGA}\hspace{0.17em}+\hspace{0.17em}U)$ scheme for a wide range of values of $U$. In ${\text{SmB}}_6$, the topological Kondo insulator (TKI) gap is found to be insensitive to the value of $U$, but in sharp contrast, Kondo physics in isostructural ${\text{YbB}}_6$ displays a surprising sensitivity to $U$. In particular, as $U$ is increased in ${\text{YbB}}_6$, the correlated TKI state in the weak-coupling regime transforms into a $d-p$-type topological insulator phase with a band inversion between $\text{Yb-5}d$ and $\text{B-2}p$ orbitals in the intermediate coupling range, without closing the insulating energy gap throughout this process. Our theoretical predictions related to the TKI and non-TKI phases in ${\text{SmB}}_6$ and ${\text{YbB}}_6$ are in substantial accord with recent angle-resolved photoemission spectroscopy experiments.

We show theoretically that the strong interaction of a two-dimensional electron gas (2DEG) with a dressing electromagnetic field drastically changes its transport properties. Particularly, the dressing field leads to a giant increase of conductivity (which can reach thousands of percents), resulting in nontrivial oscillating dependence of conductivity on the field intensity, and suppressing the weak localization of 2DEG. As a consequence, the developed theory opens an unexplored way to control transport properties of 2DEG by a strong high-frequency electromagnetic field. From an experimental viewpoint, this theory is applicable directly to quantum wells exposed to a laser-generated electromagnetic wave.

The linear and two-dimensional coherent optical spectra of Coulomb-coupled quantum emitters are discussed with respect to the underlying coupling processes. We present a theoretical analysis of the two different resonance energy transfer mechanisms between coupled nanostructures: Förster and Dexter interaction. Our investigation shows that the features visible in optical spectra of coupled quantum dots can be traced back to the nature of the underlying coupling mechanism (Förster or Dexter). Therefore, we discuss how the excitation transfer pathways can be controlled by choosing particular laser polarizations and mutual orientations of the quantum emitters in coherent two-dimensional spectroscopy. In this context, we analyze to what extent the delocalized double-excitonic states are bound to the optical selection rules of the uncoupled system.

The phonon-induced flip of the exciton spin in single flat semiconductor quantum dots with a light-hole exciton ground state is studied. The corresponding quartet, split by the exchange interaction, consists of three bright states and a dark state located energetically below the bright exciton. The two in-plane polarized bright states contribute to single-phonon transitions to the dark state and also to the upper bright state polarized in the $z$ growth direction of the dot. For these processes, the presented analytical results are calculated for the relaxation driven by the spin-orbit interaction in the conduction and the light-hole valence subbands. The estimated spin-relaxation times at low temperature are (at least) one order of magnitude lower than the bright exciton lifetime. Two other possible transitions, within the in-plane polarized doublet and between the $z$-polarized bright and dark states as well, proceed via intermediate states with a contribution from two acoustic phonons. These processes are strongly suppressed at low temperature, whereas they appear to be of the same intensity as single-phonon transitions at high enough temperatures.

We show theoretically that a weakly interacting gas of spin-polarized exciton-polaritons in a semiconductor microcavity supports propagation of spin waves. The spin waves are characterized by a parabolic dispersion at small wave vectors which is governed by the polariton-polariton interaction constant. Due to spin anisotropy of polariton-polariton interactions the dispersion of spin waves depends on the orientation of the total polariton spin. For the same reason, the frequency of homogeneous spin precession/polariton spin resonance depends on their polarization degree.

We present a practical scheme for creating topological polaritons in garden-variety systems based, for example, on zinc-blende semiconductor quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as an appealing alternative for topological states in exciton-based systems. Topological polaritons and indirect excitons open a new frontier for topological states in solid-state systems, which can be directly probed and manipulated while offering a system with nonlinear interactions.

We discuss how to locate critical points in the Berezinskii-Kosterlitz-Thouless (BKT) universality class by means of gap-scaling analyses. While accurately determining such points using gap extrapolation procedures is usually challenging and inaccurate due to the exponentially small value of the gap in the vicinity of the critical point, we show that a generic gap-scaling analysis, including the effects of logarithmic corrections, provides very accurate estimates of BKT transition points in a variety of spin and fermionic models. As a first example, we show how the scaling procedure, combined with density-matrix-renormalization-group simulations, performs extremely well in a nonintegrable spin-$3/2$ XXZ model, which is known to exhibit strong finite-size effects. We then analyze the extended Hubbard model, whose BKT transition has been debated, finding results that are consistent with previous studies based on the scaling of the Luttinger-liquid parameter. Finally, we investigate an anisotropic extended Hubbard model, for which we present the first estimates of the BKT transition line based on large-scale density-matrix-renormalization-group simulations. Our work demonstrates how gap-scaling analyses can help to locate accurately and efficiently BKT critical points, without relying on model-dependent scaling assumptions.

The mechanism to realize the peculiar flat bands generally existing in $R{\mathrm{Co}}_5$ ($R=\mathrm{rare}$ earth) compounds is clarified by analyzing the first-principles band structures and the tight-binding model. These flat bands are constructed from the localized eigenstates, the existence of which is guaranteed by the destructive interference of the intersite hopping among the $\mathrm{Co}\text{−}3d$ states at the kagome sites and those between the kagome and honeycomb sites. Their relative positions to other bands can be controlled by varying the lattice parameters keeping their dispersion almost flat, which suggests the possibility of flat-band engineering.

Interatomic Coulombic decay (ICD) is a relaxation process induced by electronic correlation. In this work we study the ICD process in a two coupled quantum wells (QWs) nanostructure. We study a simple one-dimensional effective potential using experimental parameters of the semiconductor QW layers, i.e., using the single-band effective-mass approximation. In our calculations we consider the discontinuity of the effective mass of the electron in each of the QW layers. We control the ICD lifetime by changing the distance between the two wells. The expected overall trend is a decrease of ICD lifetime with a decrease in the distance between the wells. We show that the distance can be tuned such that the emitted ICD electron is trapped in a metastable state in the con-tinuum, i.e., a one-electron resonance state. This causes the lifetime of the ICD to be an order of magnitude smaller even at very long distances, and improves the efficiency of the ICD process. For the ICD to be the dominant decay mechanism it must prevail over all other possible competitive decay processes. We have found that the lifetime of the ICD is on the time scale of picoseconds. Therefore, based on our results we can design an experiment that will observe the ICD phenomenon in QWs nanostructure. This work can lead to a design of a wavelength-sensitive detector which is efficient even at low intensities.

Several experiments have shown qubit coherence decay of the form $exp[-{(t/T_2)}^{\alpha }]$ due to environmental charge-noise fluctuations. We present a microscopic description for temperature dependencies of the parameters $T_2$ and $\alpha$. Our description is appropriate to qubits in semiconductors interacting with spurious two-level charge fluctuators coupled to a thermal bath. We find distinct power-law dependencies of $T_2$ and $\alpha$ on temperature depending on the nature of the interaction of the fluctuators with the associated bath. We consider fluctuator dynamics induced by first- and second-order tunneling with a continuum of delocalized electron states. We also study one- and two-phonon processes for fluctuators in either GaAs or Si. These results can be used to identify dominant charge-dephasing mechanisms and suppress them.

We introduce a concise formalism to characterize nanometer-sized tori based on carbon nanotubes and to determine their stability by combining ab initio density functional calculations with a continuum elasticity theory approach that requires only shape information. We find that the high strain energy in nanotori containing only hexagonal rings is significantly reduced in nanotori containing also other polygons. Our approach allows to determine local curvature and link it to local strain energy, which is correlated with local stability and chemical reactivity.

We investigate the electronic properties of boron-carbon-nitrogen (B-C-N) analogues of a recently proposed carbon kagome lattice [Chen et al., Phys. Rev. Lett. 113, 085501 (2014)]. The B-C-N kagome lattices are constructed by replacing the carbon zigzag chains of the carbon kagome lattice with boron nitride zigzag chains. We use calculations of phonon dispersion curves to demonstrate the thermodynamic stabilities of the BCN and ${\mathrm{BC}}_4\mathrm{N}$ kagome lattices. The B-C-N kagome lattices are wide-band gap semiconductors although the band gaps of the BCN and ${\mathrm{BC}}_4\mathrm{N}$ kagome lattices are increased and reduced, respectively, compared with the carbon case. The reduction of the band gap is found to be caused by a direct to indirect gap transition in the ${\mathrm{BC}}_4\mathrm{N}$ kagome lattice.

Scattering of normally incident longitudinal and transverse acoustic waves by a randomly rough surface of an elastically isotropic solid is analyzed within the small-perturbation approach. In the limiting case of a large correlation length $L$ compared with the acoustic wavelength, the specularity reduction is given by $4{\eta }^2{k}^2$, where $\eta$ is the rms roughness and $k$ is the acoustic wave vector, which is in agreement with the well-known Kirchhoff approximation result often referred to as Ziman's equation [J. M. Ziman, Electrons and Phonons (Clarendon Press, Oxford, 1960)]. In the opposite limiting case of a small correlation length, the specularity reduction is found to be proportional to ${\eta }^2{k}^4{L}^2$, with the fourth power dependence on frequency as in Rayleigh scattering. Numerical calculations for a Gaussian autocorrelation function of surface roughness connect these limiting cases and reveal a maximum of diffuse scattering at an intermediate value of $L$. This maximum becomes increasingly pronounced for the incident longitudinal wave as the Poisson's ratio of the medium approaches 1/2 as a result of increased scattering into transverse and Rayleigh surface waves. The results indicate that thermal transport models using Ziman's formula are likely to overestimate the heat flux dissipation due to boundary scattering, whereas modeling interface roughness as atomic disorder is likely to underestimate scattering.