Near-edge x-ray absorption fine structure (NEXAFS) and resonant inelastic x-ray scattering (RIXS) measurements at the oxygen $K$ edge were combined with theoretical spectrum simulations, based on periodic density functional theory and nuclear quantum dynamics, to investigate the electronic structure and chemical bonding in kaolinite ${\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_5({\mathrm{OH})}_4$. We simulated NEXAFS spectra of all crystallographically inequivalent oxygen atoms in the crystal and RIXS spectra of the hydroxyl groups. Detailed insight into the ground-state potential energy surface of the electronic states involved in the RIXS process were accessed by analyzing the vibrational excitations, induced by the core excitation, in quasielastic scattering back to the electronic ground state. In particular, we find that the NEXAFS pre-edge is dominated by features related to OH groups within the silica and alumina sheets, and that the vibrational progression in RIXS can be used to selectively probe vibrational modes of this subclass of OH groups. The signal is dominated by the OH stretching mode, but also other lower vibrational degrees of freedom, mainly hindered rotational modes, contribute to the RIXS signal.

We found new two-dimensional (2D) quantum ($S=1/2$) antiferromagnetic systems: $\mathrm{Cu}R{E}_2\mathrm{G}{\mathrm{e}}_2{\mathrm{O}}_8$ ($RE=\mathrm{Y}$ and La). According to our analysis of high-resolution x-ray and neutron diffraction experiments, the Cu network of $\mathrm{Cu}R{E}_2\mathrm{G}{\mathrm{e}}_2{\mathrm{O}}_8$ ($RE=\mathrm{Y}$ and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular $b$ axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of $E_{\mathrm{g}}\cong 2\mathrm{eV}$. Taken together, our observations make $\mathrm{Cu}R{E}_2\mathrm{G}{\mathrm{e}}_2{\mathrm{O}}_8$ ($RE=\mathrm{Y}$ and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering.

The staggered quantum walk is a model defined by the product of local operators associated with two or more independent graph tessellations. This model is versatile, encompasses several well-known discrete-time quantum walks, and inherits interesting features of the continuous-time quantum walk. We propose an implementation of the staggered quantum walk model with superconducting microwave resonators, where the required local operations are provided by the nearest neighbor interaction of the resonators coupled through superconducting quantum interference devices. The tunability of the interactions makes this system an excellent toolbox for this class of quantum walks. We focus on the one-dimensional case and discuss its generalization to a more general class known as triangle-free graphs.

We develop the theoretical framework for calculating magnetic noise from conducting two-dimensional (2D) materials. We describe how local measurements of this noise can directly probe the wave-vector dependent transport properties of the material over a broad range of length scales, thus providing new insight into a range of correlated phenomena in 2D electronic systems. As an example, we demonstrate how transport in the hydrodynamic regime in an electronic system exhibits a unique signature in the magnetic noise profile that distinguishes it from diffusive and ballistic transport and how it can be used to measure the viscosity of the electronic fluid. We employ a Boltzmann approach in a two-time relaxation-time approximation to compute the conductivity of graphene and quantitatively illustrate these transport regimes and the experimental feasibility of observing them. Next, we discuss signatures of isolated impurities lodged inside the conducting 2D material. The noise near an impurity is found to be suppressed compared to the background by an amount that is directly proportional to the cross-section of electrons/holes scattering off of the impurity. We use these results to outline an experimental proposal to measure the temperature dependent level shift and linewidth of the resonance associated with an Anderson impurity.

We study the exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted-magnetic-semiconductor quantum wells by using time-resolved photoluminescence (PL). The magnetic-field and temperature dependencies of this dynamics allow us to separate the nonmagnetic and magnetic contributions to the exciton localization. We deduce the EMP energy of 14 meV, which is in agreement with time-integrated measurements based on selective excitation and the magnetic-field dependence of the PL circular polarization degree. The polaron formation time of 500 ps is significantly longer than the corresponding values reported earlier. We propose that this behavior is related to strong self-localization of the EMP, accompanied with a squeezing of the heavy-hole envelope wave function. This conclusion is also supported by the decrease of the exciton lifetime from 600 ps to 200–400 ps with increasing magnetic field and temperature.

We study experimentally the oscillations of a nonstationary transient signal of a semiconductor microcavity with embedded InGaAs quantum wells. The oscillations occur as a result of quantum beats between the upper and lower polariton modes due to strong exciton-photon coupling in the microcavity sample (Rabi oscillations). The detection of a spectrally resolved signal has allowed for a separate observation of oscillations at the eigenfrequencies of two polariton modes. Surprisingly, the observed oscillations measured at the lower and upper polariton modes have opposite phases. We demonstrate theoretically that opposite-phase oscillations are caused by a pump-induced modification of polariton Hopfield coefficients, which govern the ratio of exciton and photon components in each of the polariton modes. Such behavior is a fundamental feature of the quantum beats of coupled light-matter states. In contrast, the reference pump-probe experiment performed for pure excitonic states in a quantum well heterostructure with no microcavity revealed in-phase oscillations of the pump-probe signals measured at different excitonic levels.

New physical phenomena and exciting material properties may be introduced by interfaces present in multilayers. Here, interplanar spacing oscillations in cubic CrN/AlN multilayers were experimentally observed by using spherical aberration-corrected high-resolution transmission electron microscopy, and corroborated by first-principles calculations. These oscillations are closely related to changes in the electronic structure. Electron spectroscopy and microscopy were employed to obtain generalized relationships between the electronic structure on the one hand, and (non-)stoichiometry or strains in the strained multilayers on the other hand, which are successfully interpreted by means of theoretical calculations. The present study may provide atomic-scale clues on the mechanism of extraordinary strength pertaining to the CrN/AlN multilayers.

The results of a detailed study of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ surface-state energy structure in the vicinity of surface steps using scanning tunneling microscopy and spectroscopy methods are presented. An increase in the chemical potential level $\mu$ near the step edge is observed. The value of the increase $\delta \mu \sim 0.1$ eV is found to correlate with the step height. The effect is caused by redistribution of electron wave functions between the outer and inner edges of surface steps, as known for normal metals. The smaller value of the chemical potential shift and its larger characteristic length of $\sim 10$ nm reflect specifics of the helical surface states. This increase is accompanied by enlargement of the normalized differential tunneling conductance in the helical surface-state energy region and thereby produces the illusion of the appearance of edge states. We show that the enlargement is reproduced in the framework of the tunneling model taking into account the tunneling gap transparency change when the chemical potential moves away from the Dirac point.

Our predictions, based on density-functional calculations, reveal that surface doping of ZnO nanowires with Bi leads to a linear-in-$k$ splitting of the conduction-band states, through spin-orbit interaction, due to the lowering of the symmetry in the presence of the dopant. This finding implies that spin polarization of the conduction electrons in Bi-doped ZnO nanowires could be controlled with applied electric (as opposed to magnetic) fields, making them candidate materials for spin-orbitronic applications. Our findings also show that the degree of spin splitting could be tuned by adjusting the dopant concentration. Defect calculations and ab initio molecular dynamics simulations indicate that stable doping configurations exhibiting the foregoing linear-in-$k$ splitting could be realized under reasonable thermodynamic conditions.

Finding microscopic models for metallic states that exhibit quantum critical properties is a major theoretical challenge. We calculate the dynamical local spin susceptibility $\chi (T,\omega )$ for a Hubbard model at half-filling using dynamical mean-field theory, which is exact in infinite dimensions. Qualitatively distinct behavior is found in the different regions of the phase diagram: Mott insulator, Fermi liquid metal, bad metal, and a quantum critical region above the finite-temperature critical point. The signature of the latter is $\omega /T$ scaling, where $\omega$ is the frequency and $T$ is the temperature. Our results are consistent with previous results showing scaling of the dc electrical conductivity, and they are relevant to experiments on organic charge-transfer salts.

We formulate a microscopic linear response theory of magnon pumping applicable to multiple-magnonic-band uniform ferromagnets with Dzyaloshinskii-Moriya interactions. From the linear response theory, we identify the extrinsic and intrinsic contributions where the latter is expressed via the Berry curvature of magnonic bands. We observe that in the presence of a time-dependent magnetization Dzyaloshinskii-Moriya interactions can act as fictitious electric fields acting on magnons. We study various current responses to this fictitious field and analyze the role of Berry curvature. In particular, we obtain an analog of the Hall-like response in systems with nontrivial Berry curvature of magnon bands. After identifying the magnon-mediated contribution to the equilibrium Dzyaloshinskii-Moriya interaction, we also establish the Onsager reciprocity between the magnon mediated thermal torques and heat pumping. We apply our theory to the magnonic heat pumping and torque responses in honeycomb and kagome lattice ferromagnets.

Several experimental groups have reported superconductivity in single unit cell layers of FeSe on ${\mathrm{SrTiO}}_3$ and a few other substrates, with critical temperature $T_c$ reports ranging up to 100 K, and a variety of theoretical work has been done. Here we examine more closely the interaction of a single FeSe layer with a ${\mathrm{TiO}}_2$ terminated ${\mathrm{SrTiO}}_3$(001) (STO) substrate. Several situations are analyzed: the underlying ideal interface, the effect of $z$-polarized longitudinal optic $\left[\mathrm{LO}\right(\widehat{z}$)] phonons in STO, electron doping of the STO substrate, substitution of Se by the bordering chalcogenides S and Te, and doping by adsorption of K on the FeSe surface. These results complement earlier studies of O and Se vacancies. The O $p_y,p_y$ surface band of STO persists at the interface, and by sharing holes with the hole pocket of FeSe it plays a part in the behavior around the interface, initially by determining the Fe Fermi level lineup with the STO band gap. The $\mathrm{LO}\left(\widehat{z}\right)$ mode causes strong band shifts around the interface but the strength of coupling to the Fe bands cannot be obtained with our methods. Adsorption of 25% K (one K per four Fe) fills the small O interface hole pocket and donates the rest of the electrons to the Fe hole pockets, filling them.

Increasing the collection efficiency from solid-state emitters is an important step towards achieving robust single-photon sources, as well as optically connecting different nodes of quantum hardware. A metallic substrate may be the most basic method of improving the collection of photons from quantum dots, with predicted collection efficiency increases of up to 50%. The established “method-of-images” approach models the effects of a reflective surface for atomic and molecular emitters by replacing the metal surface with a second fictitious emitter which ensures appropriate electromagnetic boundary conditions. Here, we extend the approach to the case of driven solid-state emitters, where exciton-phonon interactions play a key role in determining the optical properties of the system. We derive an intuitive polaron master equation and demonstrate its agreement with the complementary half-sided cavity formulation of the same problem. Our extended image approach offers a straightforward route towards studying the dynamics of multiple solid-state emitters near a metallic surface.

We extend a recently developed Fermi liquid (FL) theory for the asymmetric single-impurity Anderson model [C. Mora et al., Phys. Rev. B 92, 075120 (2015)] to the case of an arbitrary local magnetic field. To describe the system's low-lying quasiparticle excitations for arbitrary values of the bare Hamiltonian's model parameters, we construct an effective low-energy FL Hamiltonian whose FL parameters are expressed in terms of the local level's spin-dependent ground-state occupations and their derivatives with respect to level energy and local magnetic field. These quantities are calculable with excellent accuracy from the Bethe ansatz solution of the Anderson model. Applying this effective model to a quantum dot in a nonequilibrium setting, we obtain exact results for the curvature of the spectral function, $c_A$, describing its leading $\sim {\varepsilon }^2$ term, and the transport coefficients $c_V$ and $c_T$, describing the leading $\sim V^2$ and $\sim T^2$ terms in the nonlinear differential conductance. A sign change in $c_A$ or $c_V$ is indicative of a change from a local maximum to a local minimum in the spectral function or nonlinear conductance, respectively, as is expected to occur when an increasing magnetic field causes the Kondo resonance to split into two subpeaks. Surprisingly, we find that the fields $B_A$ and $B_V$ at which $c_A$ and $c_V$ change sign are parametrically different, with $B_A$ of order $T_K$ but $B_V$ much larger. In fact, in the Kondo limit $c_V$ never vanishes, implying that the conductance retains a (very weak) zero-bias maximum even for strong magnetic field and that the two pronounced finite-bias conductance side peaks caused by the Zeeman splitting of the local level do not emerge from zero-bias voltage.

We investigate real-space localization in the few-particle regime of the XXZ spin-$1/2$ chain with a random magnetic field. Our investigation focuses on the time evolution of the spatial variance of nonequilibrium densities, as resulting for a specific class of initial states, namely, pure product states of densely packed particles. Varying the strength of both particle-particle interactions and disorder, we numerically calculate the long-time evolution of the spatial variance $\sigma \left(t\right)$. For the two-particle case, the saturation of this variance yields an increased but finite localization length, with a parameter scaling different to known results for bosons. We find that this interaction-induced increase is stronger the more particles are taken into account in the initial condition. We further find that our nonequilibrium dynamics are clearly inconsistent with normal diffusion and instead point to subdiffusive dynamics with $\sigma \left(t\right)\propto t^{1/4}$.

Thermal transport in low-dimensional materials possesses various novel features since anomalous energy carriers may heavily contribute. Black phosphorene exhibits excellent thermal properties and thus it attracts attention about the possible existence of anomalous energy carriers. In this paper, we find that shockwave appears as a dominant energy carrier in the zigzag direction of black phosphorene when a supersonic thermal excitation above a critical strength is exerted. Comparing with the diffusive thermal transport, shockwave carries a considerable amount of excitation energy and propagates faster than the local acoustic speed. It leads to a strong anisotropic enhancement in the transport speed of thermal energy in the zigzag direction, up to a factor of twofold compared with no shockwave. The linear increase of velocity with intensity and exponential decay of intensity with time of the shockwave are observed. Unlike solitary waves, the collision of shockwaves exhibits no phase shift in the spatial-temporal trajectory. Moreover, it shows that shockwave velocity decreases with tensile strain, which is favorable for modulation. Our results reveal a possible anomalous energy transport process and may help in designing black phosphorene-based thermal devices.

The anisotropy of magnetic properties commonly is introduced in textbooks using the case of an antiferromagnetic system with Ising type anisotropy. This model presents huge anisotropic magnetization and a pronounced metamagnetic transition and is well-known and well-documented both in experiments and theory. In contrast, the case of an antiferromagnetic $X\text{−}Y$ system with weak in-plane anisotropy is only poorly documented. We studied the anisotropic magnetization of the compound ${\mathrm{GdRh}}_2{\mathrm{Si}}_2$ and found that it is a perfect model system for such a weak-anisotropy setting because the ${\mathrm{Gd}}^{3+}$ ions in ${\mathrm{GdRh}}_2{\mathrm{Si}}_2$ have a pure spin moment of $S=7/2$, which orders in a simple AFM structure with $\mathbf{Q}=\left(001\right)$. We observed experimentally in $M\left(B\right)$ a continuous spin-flop transition and domain effects for a field applied along the $\left[100\right]$ and the $\left[110\right]$ direction, respectively. We applied a mean-field model for the free energy to describe our data and combine it with an Ising chain model to account for domain effects. Our calculations reproduce the experimental data very well. In addition, we performed magnetic x-ray scattering and x-ray magnetic circular dichroism measurements, which confirm the AFM propagation vector to be $\mathbf{Q}=\left(001\right)$ and indicate the absence of polarization on the rhodium atoms.

We have studied the effect of the Dzyaloshinskii-Moriya interaction on kagome Heisenberg antiferromagnet using the Schwinger boson mean field theory (SBMFT). Within SBMFT framework, Messio et al. had argued that the ground state of the kagome antiferromagnet is possibly a chiral topological spin liquid [Phys. Rev. Lett. 108, 207204 (2012)]. Thus, we have computed a zero-temperature ground-state phase diagram considering the time-reversal symmetry breaking states as well as fully symmetric Ansätze. We discuss the relevance of these results in experiments and other studies. Finally, we have computed the static and dynamic spin structure factors in relevant phases.