We give a further step in the quantum mechanical description of engineered atomic spin structures by deriving a master equation of the Redfield type that governs the dynamics of the atomic spin density matrix. By generalizing this approach to charge-specific density matrices, we are able to describe magnetic transport quantities, such as the average inelastic current and the shot noise, accessible by tunneling spectroscopy. Our method suitably describes moderate lead-atom coupling regimes where quantum coherence effects cannot be disregarded. We contrast our approach with the existing descriptions in terms of rate equations and show examples where coherence effects are crucial to understand the physics of spin-polarized tunnel current through spin structures.

Single-ion lanthanide-organic complexes can provide stable magnetic moments with well-defined orientation for spintronic applications on the atomic level. Here, we show by a combined experimental approach of scanning tunneling microscopy and x-ray absorption spectroscopy that dysprosium-tris(1,1,1-trifluoro-4-(2-thienyl)-2,4-butanedionate) $\left(\mathrm{Dy}{\left(\mathrm{tta}\right)}_3\right)$ complexes deposited on a Au(111) surface undergo a molecular distortion, resulting in distinct crystal field symmetry imposed on the Dy ion. This leads to an easy-axis magnetization direction in the ligand plane. Furthermore, we show that tunneling electrons hardly couple to the spin excitations, which we ascribe to the shielded nature of the $4f$ electrons.

The charge dynamics of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{TM}}_x{)}_2{\mathrm{As}}_2$ (TM = Mn and Cr) has been investigated by optical spectroscopy. It was found that in addition to the strong suppression of the coherent charge transport, the magnetic impurity induces a novel spectral feature related to the carrier localization in the far-infrared region above the antiferromagnetic transition temperature. We attribute it to the cooperative effect between conduction electrons and local magnetic order induced by magnetic impurities. The present results demonstrate that Mn and Cr are not conventional magnetic pair breakers in iron pnictides.

The effects of doubly ionized oxygen vacancies $\left({V_{\mathrm{O}}}^{2+}\right)$ on the electronic structure and charge distribution in rutile $\mathrm{Ti}{\mathrm{O}}_2$ are studied by combining first-principles calculations based on density functional theory and experimental results from x-ray photoelectron and x-ray absorption measurements carried out in synchrotron facilities on rutile $\mathrm{Ti}{\mathrm{O}}_2$ nanoparticles. The generalized gradient approximation of the Perdew-Burke-Ernzerhof functional has demonstrated its suitability for the analysis of the ${V_{\mathrm{O}}}^{2+}$ defects in rutile $\mathrm{Ti}{\mathrm{O}}_2$. It has been found that the presence of empty electronic states at the conduction band shifted $\sim 1\mathrm{eV}$ from $t_{2g}$ and $e_g$ states can be associated with local distortions induced by ${V_{\mathrm{O}}}^{2+}$ defects, in good agreement with Gauss-Lorentzian band deconvolution of experimental O $K$-edge spectra. The asymmetry of $t_{2g}$ and $e_g$ bands at the O-$K$ edge has been associated with ${V_{\mathrm{O}}}^{2+}$, which can enrich the understanding of studies where the presence of these defects plays a key role, as in the case of doped $\mathrm{Ti}{\mathrm{O}}_2$.

We present results on the acoustoelectric current driven through a quantum point contact (QPC) placed on a suspended nanobridge, which is subject to surface acoustic waves (SAWs). The magnitude of this current is much larger than that of a two-dimensional gas, and the system has enhanced sensitivity to perturbations. In particular, the current oscillations as a function of magnetic field exhibit additional features associated with the spin splitting. Furthermore, the negative voltage applied to the QPC gates induces the oscillations revealing the subband structure not seen in transport measurements at the elevated temperatures of our experiment. Near the pinch off conditions, the acoustoelectric current becomes negative which we attribute to the enhanced backscattering caused by the SAW-phonons’ absorption and emission.

The carrier mobility of anisotropic two-dimensional semiconductors under longitudinal acoustic phonon scattering was theoretically studied using deformation potential theory. Based on the Boltzmann equation with the relaxation time approximation, an analytic formula of intrinsic anisotropic mobility was derived, showing that the influence of effective mass on mobility anisotropy is larger than those of deformation potential constant or elastic modulus. Parameters were collected for various anisotropic two-dimensional materials (black phosphorus, Hittorf's phosphorus, ${\mathrm{BC}}_2\mathrm{N}$, MXene, ${\mathrm{TiS}}_3$, and ${\mathrm{GeCH}}_3$) to calculate their mobility anisotropy. It was revealed that the anisotropic ratio is overestimated by the previously described method.

The gapless and linear electronic band structure of graphene opens up Auger scattering channels bridging the valence and the conduction band and changing the charge carrier density. Here, we reveal experimentally accessible signatures of Auger scattering in optically excited graphene. To be able to focus on signatures of Auger scattering, we apply a low excitation energy, weak pump fluences, and a cryostatic temperature, so that all relevant processes lie energetically below the optical phonon threshold. In this regime, carrier-phonon scattering is strongly suppressed and Coulomb processes govern the carrier dynamics. Depending on the excitation regime, we find an accumulation or depletion of the carrier occupation close to the Dirac point. This reflects well the behavior predicted from Auger-dominated carrier dynamics. Based on this observation, we propose a multicolor pump-probe experiment to uncover the extreme importance of Auger channels for the nonequilibrium dynamics in graphene.

We develop a quantum-mechanical theory for Landau damping of surface plasmons in metal nanostructures of arbitrary shape. We show that the electron surface scattering, which facilitates plasmon decay in small nanostructures, can be incorporated into the metal dielectric function on par with phonon and impurity scattering. The derived surface scattering rate is determined by the local field polarization relative to the metal-dielectric interface and is highly sensitive to the system geometry. We illustrate our model by providing analytical results for surface scattering rate in some common shape nanostructures. Our results can be used for calculations of hot carrier generation rates in photovoltaics and photochemistry applications.

Shadowing effects during the growth of nano- and microstructures are crucial for the realization of several technological applications. They are given by the shielding of the incoming material flux provided by the growing structures themselves. Their features have been deeply investigated by theoretical approaches, revealing important information to support experimental activities. However, comprehensive investigations able to follow every stage of the growth processes as a whole, particularly useful to design and understand targeted experiments, are still challenging. In this work, we study the thin-film growth dynamics by means of a diffuse interface approach accounting for both deposition with shadowing effects and surface diffusion driven by the minimization of the surface energy. In particular, we introduce the coupling between a phase-field model and the detailed calculation of the incoming material flux at the surface deposited from vacuum or vapor phase in the ballistic regime. This allows us to finely reproduce the realistic morphological evolution during the growth on nonflat substrates, also accounting for different flux distributions. A general assessment of the method, focusing on two-dimensional profiles, is provided thanks to the comparison with a sharp-interface approach for the evolution of the early stages. Then, the long-time-scale dynamics is shown in two and three dimensions, providing a general overview of the features observed during deposition on corrugated surfaces involving flattening, increasing of surface roughness with the growth of columnar structures, and voids formation.

We develop a theoretical model to describe the dissipative dynamics of singlet-triplet qubits in GaAs quantum dots. Using the concurrence experimentally obtained [Shulman et al., Science 336, 202 (2012)] as a guide, we assume that each logical qubit fluctuates under the action of a random telegraph noise (RTN) that simulates the $1/f^{\alpha }$ noise. We also study the dynamics of concurrence as a function of the amplitude of the RTN, the correlation time of the RTN, the preparation time of states, and the two-qubit coupling. Furthermore, we show that the two-qubit coupling together with the preparation time strongly affect the entanglement dissipative dynamics and both physical quantities can be employed to enhance the entanglement between singlet-triplet qubits.

The combination of photoelectron diffraction, density functional theory, and multiple scattering calculations is used for a quantitative analysis of the widely debated structure of ${\mathrm{TiO}}_2$ (011). The modeling of the diffraction patterns for different surface reconstructions allows one to discriminate the key structural constraints required or, on the contrary, prohibited in the reconstruction of this termination. In particular, photodiffraction rules out previously proposed reconstructions while it evidences the key feature of the ${\mathrm{TiO}}_2$ (011) termination: an oxygen splitting induced by missing rows.

The clathrate compound ${\mathrm{Ce}}_3{\mathrm{Pd}}_{20}{\mathrm{Si}}_6$ is a heavy-fermion metal that exhibits magnetically hidden order at low temperatures. Reputedly, this exotic type of magnetic ground state, known as “phase II”, could be associated with the ordering of Ce $4f$ quadrupolar moments. In contrast to conventional (dipolar) order, it has vanishing Bragg intensity in zero magnetic field and, as a result, has escaped direct observation by neutron scattering until now. Here we report the observation of diffuse magnetic neutron scattering induced by an application of magnetic field along either the $\left[1\overline{1}0\right]$ or the $\left[001\right]$ direction within phase II. The broad elastic magnetic signal that surrounds the (111) structural Bragg peak can be attributed to a short-range $\mathrm{G}$-type antiferromagnetic arrangement of field-induced dipoles modulated by the underlying multipolar order on the simple-cubic sublattice of Ce ions occupying the $8c$ Wyckoff site. In addition, for magnetic fields applied along the $\left[001\right]$ direction, the diffuse magnetic peaks in ${\mathrm{Ce}}_3{\mathrm{Pd}}_{20}{\mathrm{Si}}_6$ become incommensurate, suggesting a more complex modulated structure of the underlying multipolar order that can be continuously tuned by a magnetic field.

Spin-liquid behavior was recently suggested experimentally in the moderately one-dimensional organic compound $\kappa \text{−}{\mathrm{H}}_3{\left(\text{Cat-EDT-TTF}\right)}_2$. This compound can be modeled by the one-band Hubbard model on the anisotropic triangular lattice with $t^{\prime }/t\simeq 1.5$, where $t^{\prime }$ is the minority hopping. It thus becomes important to extend previous studies, that were performed in the range $0\le t^{\prime }/t\le 1.2$, to find out whether there is a regime where Mott insulating behavior can be found without long-range magnetic order. To this end, we study the above model in the range $1.2\le t^{\prime }/t\le 2$ using cluster dynamical mean-field theory (CDMFT). We argue that it is important to choose a symmetry-preserving cluster rather than a quasi-one-dimensional cluster. We find that, upon increasing $t^{\prime }/t$ beyond $t^{\prime }/t\approx 1.3$, the Mott transition at zero temperature is replaced by a first-order transition separating a metallic state from a collinear magnetic insulating state excluding the possibility to find a quantum spin liquid for the physically relevant value $t^{\prime }/t\simeq 1.5$. The phase diagram obtained in this study can provide a working basis for moderately one-dimensional compounds on the anisotropic triangular lattice.

We present a tight-binding (TB) model and $\mathbf{k}·\mathbf{p}$ theory for electrons in monolayer and few-layer InSe. The model is constructed from a basis of all $s$ and $p$ valence orbitals on both indium and selenium atoms, with tight-binding parameters obtained from fitting to independently computed density functional theory (DFT) band structures for mono- and bilayer InSe. For the valence and conduction band edges of few-layer InSe, which appear to be in the vicinity of the $\mathrm{\Gamma }$ point, we calculate the absorption coefficient for the principal optical transitions as a function of the number of layers, $N$. We find a strong dependence on $N$ of the principal optical transition energies, selection rules, and optical oscillation strengths, in agreement with recent observations [D. A. Bandurin et al., Nat. Nanotechnol. (2016)]. Also, we find that the conduction band electrons are relatively light ($m\propto 0.14–0.18m_e$), in contrast to an almost flat, and slightly inverted, dispersion of valence band holes near the $\mathrm{\Gamma }$ point, which is found for up to $N\propto 6$.

A modal expansion approach is developed and employed to investigate and elucidate the nonlinear mechanism behind the multistability and formation of coupled multimode polariton solitons in microcavity wires. With pump switched on and realistic dissipation parameters, truncating the expansion up to the second-order wire mode, our model predicts two distinct coupled soliton branches: stable and unstable. Modulational stability of the stationary homogeneous solution and soliton branches stability are studied. Our simplified 1D model is in remarkably good agreement with the full 2D mean-field Gross-Pitaevskii model, reproducing correctly the soliton existence domain upon variation of pump amplitude and the onset of multistability.

On a square lattice, the Hubbard model at half filling reduces to the Heisenberg model and exhibits antiferromagnetism. When doped away from half filling this model gives rise to $d$-wave superconductivity. This behavior is reminiscent of the phenomenology of the cuprate family with their high $T_{c}d$-wave superconductivity and their antiferromagnetic parent compound. It is therefore interesting to study an extension of the Hubbard model which includes spin orbit coupling. We have previously studied this model away from half filling [see, for example, Farrell and Pereg-Barnea, Phys. Rev. B 89, 035112 (2014)] and found that the addition of spin-orbit coupling and Zeeman field leads to topological superconductivity with $d+id$ pairing function. In this paper we are interested in the ‘parent compound’ of this state. Namely, we study the half filling, strong coupling limit of the square lattice Hubbard model with spin orbit coupling and Zeeman field. The strong coupling expansion of the model is a spin model which contains compass anisotropy and Dzyaloshinsky-Moriya interaction on top of the usual Heisenberg term. We analyze this spin model classically and find an incommensurate spin density wave (ISDW) for low Zeeman fields. This ISDW has a wave vector $\vec{Q}$ which deviates from $(\pi ,\pi )$ by an amount which is proportional to the spin-orbit coupling and can therefore serve as a signature. We study the stability of the ISDW phase using spin wave theory and find a stable and an unstable region. At higher but moderate Zeeman fields we find a tilted antiferromagnet and a ferromagnet at high Zeeman fields.

The stability of a single Belavin-Polyakov (BP) skyrmion in an isotropic Heisenberg ferromagnet is studied. Such a skyrmion is higher in energy than the uniform ferromagnetic state and is thus metastable. Starting from the lattice model in two spatial dimensions and using Maleyev-Dyson representation for spin operators, we examine the effects of magnon-magnon interaction for two quantities at $T=0$. First, we discuss self-energy corrections to the magnon energy. Second, we analyze the two-particle Green's function and possible bound states of two magnons. The simplicity of the model makes possible full analytic treatment of all relevant processes. We found that the magnons remain well-defined quasiparticles with a finite lifetime. The bound states of two magnons are suppressed near the skyrmion, although they are not excluded far away from it. A resonance for magnons of the dilational mode in the vicinity of the BP skyrmion is also found, which leads to a redistribution of the spectral weight. We conclude that the BP skyrmion as the classical topological object is not destroyed by quantum fluctuations.

We investigate the evolution of spin and orbital order in undoped ${\mathrm{LaMnO}}_3$ under increasing temperature with a model including both superexchange and Jahn-Teller interactions. We used several cluster mean field calculation schemes and find coexisting $A$-type antiferromagnetic and $C$-type alternating orbital order at low temperature. The value of the Jahn-Teller coupling between strongly correlated $e_g$ orbitals is estimated from the orbital transition temperature at $T_{\mathrm{OO}}\simeq 780$ K. By a careful analysis of onsite and on-bond correlations, we demonstrate that spin-orbital entanglement is rather weak. We have verified that the magnetic transition temperature is influenced by entangled spin-orbital operators as well as by entangled orbital operators on the bonds, but the errors introduced by decoupling such operators partly compensate each other. Altogether, these results justify why the commonly used disentangled spin-orbital model is so successful in describing the magnetic properties and the temperature dependence of the optical spectral weights for ${\mathrm{LaMnO}}_3$.