We introduce a block Lanczos (BL) recursive technique to construct quasi-one-dimensional models, suitable for density-matrix renormalization group (DMRG) calculations, from single- as well as multiple-impurity Anderson models in any spatial dimensions. This new scheme, named BL-DMRG method, allows us to calculate not only local but also spatially dependent static and dynamical quantities of the ground state for general Anderson impurity models without losing elaborate geometrical information of the lattice. We show that the BL-DMRG method can be easily extended to treat a multiorbital Anderson impurity model where not only inter- and intraorbital Coulomb interactions but also Hund's coupling and pair hopping interactions are included. We also show that the symmetry adapted BL bases can be utilized, when it is appropriate, to reduce the computational cost. As a demonstration, we apply the BL-DMRG method to three different models for graphene with a structural defect and with a single hydrogen or fluorine absorbed, where a single Anderson impurity is coupled to conduction electrons in the honeycomb lattice. These models include (i) a single adatom on the honeycomb lattice, (ii) a substitutional impurity in the honeycomb lattice, and (iii) an effective model for a single carbon vacancy in graphene. Our analysis of the local dynamical magnetic susceptibility and the local density of states at the impurity site reveals that, for the particle-hole symmetric case at half-filling of electron density, the ground state of model (i) behaves as an isolated magnetic impurity with no Kondo screening, while the ground state of the other two models forms a spin-singlet state where the impurity moment is screened by the conduction electrons. We also calculate the real-space dependence of the spin-spin correlation functions between the impurity site and the conduction sites for these three models. Our results clearly show that, reflecting the presence or absence of unscreened magnetic moment at the impurity site, the spin-spin correlation functions decay as $\propto$ $r^{-3}$, differently from the noninteracting limit ($\propto$ $r^{-2}$), for model (i) and as $\propto$ $r^{-4}$, exactly the same as the noninteracting limit, for models (ii) and (iii) in the asymptotic $r$, where $r$ is the distance between the impurity site and the conduction site. Finally, based on our results, we shed light on recent experiments on graphene where the formation of local magnetic moments as well as the Kondo-like behavior have been observed.

The first-principles linear response method is used within the local-density approximation to calculate the full phonon band structures and phonon density of states (DOS) of $\mathrm{CsSn}{X}_3$ ($X=\text{Cl}$, Br, or I) in different phases. The relations between soft phonon modes and phase transitions are investigated. We find soft phonon modes only in the cubic and tetragonal phases, not in the orthorhombic and monoclinic phases. A dispersionless soft phonon branch spreads from the k point $M$ to $R$ in the Brillouin zone of the cubic phase. The lower symmetry tetragonal phase results from the condensation of the soft phonon mode at the k point $M$. Furthermore, the condensation of the soft phonon mode at the k point $Z$ in the Brillouin zone of tetragonal phase results in the orthorhombic $\gamma$ phase. To facilitate comparison with experimental data, we calculate infrared spectra for the cubic phase. At this point only a limited comparison with experimental data is possible. We find that the calculated modes agree with the available experimental data when we assign the second and third calculated modes to the experimental first and second modes. The lowest calculated mode is at a frequency where the phonon DOS has a maximum value. So the strong phonon-phonon interaction results in short phonon lifetime or strong broadening, which could explain why this mode has not been observed. Our first-principles calculated IR spectra show that the third observed mode in IR absorption is actually the highest longitudinal optical (LO) rather than transverse optical mode. We show, furthermore, that a strong LO-plasmon coupling may be expected in these materials and can explain observed Raman data for ${\mathrm{CsSnI}}_3$.

We study the transport properties of a quantum dot-Majorana hybrid system, in which each of the paired Majorana bound states is connected to one quantum dot. With the help of the nonequilibrium Green's function method, we obtain an exact solution of the Green's functions and calculate the currents through the quantum dots and nonlocal noise cross correlation between the currents. As a function of dot energy levels ${\epsilon }_1$ and ${\epsilon }_2$, we find that for the symmetric level configuration ${\epsilon }_1={\epsilon }_2$, the noise cross correlation is negative in the low lead voltage regime, while it becomes positive with the increase of the lead voltages. Due to the particle-hole symmetry, the cross correlation is always positive in the antisymmetric case ${\epsilon }_1=-{\epsilon }_2$. In contrast, the cross correlation of non-Majorana setups is always positive. For comparison, we also perform the diagonalized master equation calculation to check its applicability. It is found that the diagonalized master equations work well in most regimes of system parameters. Nevertheless, it shows an obvious deviation from the exact solution by the nonequilibrium Green's function method when all eigenenergies of the dot-Majorana hybrid system and simultaneously the energy intervals are comparable to the dot-lead coupling strength.

We present atomic-scale force interaction data acquired on an insulating low-symmetry substrate, namely the calcite$\left(10.4\right)$ surface. A pronounced asymmetry of the imaged species is apparent along the vertical axis, which will be identified as a tilted band of purely attractive short-range interaction forces. An independent determination of the absolute surface orientation by an optical method suggests that the observed asymmetry is related to the tilt of the carbonate groups within the surface, which, in turn, is linked to the surface orientation. Thus our noncontact atomic force microscopy based work presents the determination of the absolute orientation for a low-symmetry surface.

We observe that Cu clusters grow on surface terraces of graphite as a result of physical vapor deposition in ultrahigh vacuum. We show that the observation is incompatible with a variety of models incorporating homogeneous nucleation and calculations of atomic-scale energetics. An alternative explanation, ion-mediated heterogeneous nucleation, is proposed and validated, both with theory and experiment. This serves as a case study in identifying when and whether the simple, common observation of metal clusters on carbon-rich surfaces can be interpreted in terms of homogeneous nucleation. We describe a general approach for making system-specific and laboratory-specific predictions.

We investigate atomic and electronic structures of the intriguing low-temperature phase of ${\mathrm{IrTe}}_2$ using high-resolution scanning tunneling microscopy and spectroscopy. We confirm various stripe superstructures such as $\times 3$, $\times 5$, and $\times 8$. The strong vertical and lateral distortions of the lattice for the stripe structures are observed in agreement with recent calculations. The spatial modulations of electronic density of states are clearly identified as separated from the structural distortions. These structural and spectroscopic characteristics are not consistent with the charge-density wave and soliton lattice model proposed recently. Instead, we show that the Ir (Te) dimerization together with the Ir $5d$ charge ordering can explain these superstructures, supporting the Ir dimerization mechanism of the phase transition.

For the $3d$ ferromagnets iron, cobalt, and nickel we compute the spin-dependent inelastic electronic lifetimes due to carrier-carrier Coulomb interactions including spin-orbit coupling. We find that the spin-dependent density of states at the Fermi energy does not, in general, determine the spin dependence of the lifetimes because of the effective spin-flip transitions allowed by the spin mixing. The majority and minority electron lifetimes computed including spin-orbit coupling for these three $3d$ ferromagnets do not differ by more than a factor of 2, and agree with experimental results.

Using pulsed electroluminescence detected magnetic resonance (PELDMR), we study the dynamics of spin processes in ${\mathrm{Alq}}_3$ based light-emitting diodes. The transitions induced by magnetic resonance are found to be much faster than the space charge reaction time that is measured by looking at the electroluminescence frequency response to an ac bias voltage. This observation excludes a change in the equilibrium space charge distributions as the cause of PELDMR, in favor of a change of the electron-hole recombination rate. At low temperatures the effect of electron spin resonance on the electroluminescence changes sign and lasts longer. The postpulse electroluminescence recovery is well fitted by a biexponential function characterized by two very different time scales, which are consistent with a detailed balance for the singlet and triplet states, in conformity with the electron-hole pair model.

Recent theoretical work on time-periodically kicked Hofstadter model found robust counterpropagating edge modes. It remains unclear how ubiquitously such anomalous modes can appear, and what dictates their robustness against disorder. Here we shed further light on the nature of these modes by analyzing a simple type of periodic driving where the hopping along one spatial direction is modulated sinusoidally with time while the hopping along the other spatial direction is kept constant. We obtain the phase diagram for the quasienergy spectrum at flux 1/3 as the driving frequency $\omega$ and the hopping anisotropy are varied. A series of topologically distinct phases with counterpropagating edge modes appear due to the harmonic driving, similar to the case of a periodically kicked system studied earlier. We analyze the time dependence of the pair of Floquet edge states localized at the same edge and compare their Fourier components in the frequency domain. In the limit of small modulation, one of the Floquet edge mode within the pair can be viewed as the edge mode originally living in the other energy gap shifted in quasienergy by $\hslash \omega$, i.e., by absorption or emission of a “photon” of frequency $\omega$. Our result suggests that counterpropagating Floquet edge modes are generic features of periodically driven integer quantum Hall systems, and not tied to any particular driving protocol. It also suggests that the Floquet edge modes would remain robust to any static perturbations that do not destroy the chiral edge modes of static quantum Hall states.

We investigate the zero-temperature phase diagram of the fully frustrated transverse-field Ising model on the square lattice both in the classical limit and in the presence of quantum fluctuations. At the classical level (the limit of infinite spin $S$), we find that upon decreasing the transverse field $\Gamma$ this model exhibits a phase transition from the fully polarized state into an eightfold degenerate translational symmetry breaking state. This phase can be identified to correspond to plaquette order in the dimer language and remains the lowest-energy state in the entire range of fields below the critical one, ${\Gamma }_c$. The eightfold degenerate solution that corresponds to columnar order in the dimer language is a saddle point of the classical energy. It is degenerate with the plaquette solution at $\Gamma =0$ and is only slightly higher in energy in the whole interval $0<\Gamma <{\Gamma }_c$. The effect of quantum fluctuations is investigated in the context of a large S expansion both for the plaquette and columnar structures. For this purpose we employ an approximate method allowing to estimate from above the fluctuation-induced correction to the energy of a configuration which at the classical level is a saddle point of the energy, not a local minimum, and find that the harmonic quantum fluctuations show a clear tendency to overcome the energy difference between the two states. For relatively high fields, the transition from the plaquette to the columnar state takes place at values of $S$ large enough to be in the domain of validity of the harmonic approximation.

The operation of blue light-emitting diodes based on polyspirobifluorene with a varying number of $N,N,N^{\prime },N^{\prime }$ tetraaryldiamino biphenyl (TAD) hole-transport units (HTUs) is investigated. Assuming that the electron transport is not affected by the incorporation of TAD units, model calculations predict that a concentration of 5% HTU leads to an optimal efficiency for this blue-emitting polymer. However, experimentally an optimum performance is achieved for 10% TAD HTUs. Analysis of the transport and recombination shows that polymer light-emitting diodes with 5%, 7.5%, and 12.5% TAD units follow the predicted behavior. The enhanced performance of the polymer with 10% TAD originates from a decrease in the number of electron traps, which is typically a factor of three lower than the universal value found in many polymers. This reduced number of traps leads to a reduction of nonradiative recombination and exciton quenching at the cathode.

Understanding the growth of organic semiconducting molecules with shape anisotropy is of high relevance to the processing of optoelectronic devices. This work provides insight into the growth of thin films of the prototypical rodlike organic semiconductor diindenoperylene on a microscopic level by analyzing in detail the film morphology. We model our data, which were obtained by high-resolution grazing incidence small-angle x-ray scattering, using a theoretical description from small-angle scattering theory derived for simple liquids. Based on form-factor calculations for different object types, we determine how the island shapes change in the respective layers. Atomic force microscopy measurements approve our findings.

When graphene is deformed in a dynamical manner, a time-dependent potential is induced for the electrons. The potential is antisymmetric with respect to valleys and has far-reaching effects. In this paper we find some straightforward applications to Raman spectroscopy: a valley-antisymmetric potential broadens the Raman $D$ band but does not affect the $2D$ band, which is already observed by recent experiments. The space derivative of the valley-antisymmetric potential gives a force field that accelerates intervalley phonons, while it corresponds to the longitudinal component of the previously discussed pseudoelectric field acting on the electrons. Effects of a pseudoelectric field on the electron is quite difficult to observe due to the valley-antisymmetric coupling constant, on the other hand, such obstacle is absent for intervalley phonons with $A_{1g}$ symmetry that constitute the $D$ and $2D$ bands.

We study coherent dynamics in a system of dipolar coupled spin qubits diluted in a solid and subjected to a driving microwave field. In the case of rare earth ions, an anisotropic crystal background results in anisotropic $g$ tensor and thus modifies the dipolar coupling. We develop a microscopic theory of spin relaxation in a transient regime for the frequently encountered case of axially symmetric crystal field. The calculated decoherence rate is nonlinear in the Rabi frequency. We show that the direction of a static magnetic field that corresponds to the highest spin $g$ factor is preferable in order to obtain a higher number of coherent qubit operations. The results of calculations are in excellent agreement with our experimental data on Rabi oscillations recorded for a series of $\mathrm{CaW}{\mathrm{O}}_4$ crystals with different concentrations of $\mathrm{N}{\mathrm{d}}^{3+}$ ions.

We study the ground state phase diagram of a frustrated spin-1/2 four-leg spin tube in an external magnetic field. We explore the parameter space of this model in the regime of all-antiferromagnetic exchange couplings by means of three different approaches: analysis of low-energy effective Hamiltonian, a Hartree variational approach, and density matrix renormalization group for finite clusters. We find that in the limit of weakly interacting plaquettes, low-energy singlet, triplet, and quintuplet states play an important role in the formation of fractional magnetization plateaus. We study the transition regions numerically and analytically, and find that they are described, at first order in a strong-coupling expansion, by an $XXZ$ spin-1/2 chain in a magnetic field; the second-order terms give corrections to the $XXZ$ model. All techniques provide consistent results which allow us to predict the existence of fractional plateaus in an important region in the space of parameters of the model.

We performed positron annihilation spectroscopy measurements on uranium dioxide irradiated with 45 MeV $\alpha$ particles. The positron lifetime was measured as a function of the temperature in the 15–300 K range. The experimental results were combined with electronic structure calculations of positron lifetimes of vacancies and vacancy clusters in ${\mathrm{UO}}_2$. Neutral and charged defects consisting of from one to six vacancies were studied computationally using the $\mathrm{DFT}+U$ method to take into account strong correlations between the $5f$ electrons of uranium. The two-component density functional theory with two different fully self-consistent schemes was used to calculate the positron lifetimes. All defects were relaxed taking into account the forces due to the creation of defects and the positron localized in the vacancy. The interpretation of the experimental observations in the light of the DFT + $U$ results and the positron trapping model indicates that neutral ${\mathrm{V}}_{\text{U}}+2{\mathrm{V}}_{\text{O}}$ trivacancies (bound Schottky defects) are the predominant defects detected in the 45 MeV $\alpha$ irradiated ${\mathrm{UO}}_2$ samples. Our results show that the coupling of a precise experimental work and calculations using carefully chosen assumptions is an effective method to bring further insight into the subject of irradiation induced defects in ${\mathrm{UO}}_2$.

We report an anomalous enhancement of the critical temperature $\left(T_{\mathrm{c}}\right)$ when the thickness $\left(d\right)$ of the magnetic layer is decreased from 60 to 20 Å in amorphous FeZr/AlZr multilayers. Further reduction of the thickness causes a decrease of $T_{\mathrm{c}}$, as expected by finite-size scaling, while the magnetic moment decreases monotonically for all values of $d$. The increase of the critical temperature is attributed to a reversed interface effect where local changes in the nearest-neighbor distance and coordination number gives a higher effective magnetic coupling at the interfaces compared to the interior of the layer. We have successfully described the results within a model where such interface effects are combined with finite-size scaling.