Recently it has been shown that surface magnetic doping of topological insulators induces backscattering of Dirac states which are usually protected by time-reversal symmetry [Sessi et al., Nat. Commun. 5, 5349 (2014)]. Here we report on quasiparticle interference measurements where, by improved Fermi level tuning, strongly focused interference patterns on surface Mn-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ could be directly observed by means of scanning tunneling microscopy at 4 K. Ab initio and model calculations reveal that their mesoscopic coherence relies on two prerequisites: (i) a hexagonal Fermi surface with large parallel segments (nesting) and (ii) magnetic dopants which couple to a high-spin state. Indeed, x-ray magnetic circular dichroism shows superparamagnetism even at very dilute Mn concentrations. Our findings provide evidence of strongly anisotropic Dirac-fermion-mediated interactions and demonstrate how spin information can be transmitted over long distances, allowing the design of experiments and devices based on coherent quantum effects in topological insulators.

We show that the axial component of the magnetic permeability tensor is resonant for a wire medium consisting of high-index epsilon-positive nanowires, and its real part changes the sign at a certain frequency. At this frequency the medium experiences the topological phase transition between the elliptic and hyperbolic type of dispersion. We show that the transition regime is characterized by an extremely strong dependence of the permeability on the wave vector. This implies very high density of electromagnetic states that results in the filamentary pattern and noticeable Purcell factor for a transversely oriented magnetic dipole.

Motivated by the recent resonant inelastic x-ray scattering (RIXS) experiment for the electron-doped cuprates ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ with $x\approx 0.15$, we compute the density-density correlation function in the $t-J$ model on a square lattice by including interlayer hopping and the long-range Coulomb interaction. We find that collective charge excitations are realized not inside the particle-hole continuum, but above the continuum as plasmons. The plasmon mode has a rather flat dispersion near the in-plane momentum ${\mathbf{q}}_{\parallel }=(0,0)$ with a typical excitation energy of the order of the intralayer hopping $t$ when the out-of-plane momentum $q_z$ is zero. However, when $q_z$ becomes finite, the plasmon dispersion changes drastically near ${\mathbf{q}}_{\parallel }=(0,0)$, leading to a strong dispersive feature with an excitation gap scaled by the interlayer hopping $t_z$. We discuss the mode recently observed by RIXS near ${\mathbf{q}}_{\parallel }=(0,0)$ in terms of the plasmon mode with a finite $q_z$.

We perform a theoretical and experimental study of the optical properties of a $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ perovskite prepared by a vapor-assisted solution process, motivated in part by very high photovoltaic cell efficiencies. Several widespread theoretical approaches are used in an attempt to determine the most appropriate approach which would reproduce the experimental electronic structure and optical properties of the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ perovskite. We compare a semilocal approximation to the density functional theory with hybrid functionals and time-dependent hybrid functional calculations, evaluating the effects of exchange tuning and spin-orbit coupling. Using these methods we calculate the electronic structure, optical absorption spectrum, and frequency-dependent dielectric function of the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ perovskite. The results are compared to the experimentally obtained dielectric functions acquired from ellipsometry measurements. We demonstrate that inclusion of spin-orbit coupling in theoretical calculations is critical in describing the electronic and optical properties of the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ perovskite. Good agreement with experimental data is achieved when the optical spectra are computed using time-dependent hybrid density functional theory with spin-orbit coupling.

Polarization-dependent two-dimensional Fourier-transform spectroscopy (2DFTS) is performed on excitons in strained bulk GaAs layers, probing the coherent response for differing amounts of strain. Uniaxial tensile strain lifts the degeneracy of heavy-hole (HH) and light-hole (LH) valence states, leading to an observed splitting of the associated excitons at low temperature. Increasing the strain increases the magnitude of the HH/LH exciton peak splitting, induces an asymmetry in the off-diagonal interaction coherences, increases the difference in the HH and LH exciton homogenous linewidths, and increases the inhomogeneous broadening of both exciton species. All results arise from strain-induced variations in the local electronic environment, which is not uniform along the growth direction of the thin layers. For cross-linear polarized excitation, wherein excitonic signals give way to biexcitonic signals, the high-strain sample shows evidence of bound LH, HH, and mixed biexcitons.

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) and Ce $4d-4f$ resonant photoemission spectroscopy (PES) on single crystals of ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$ for $x=0.0$ and 0.5 in order to investigate the Ce $4f$ electronic states. In Ce $L_3$-edge XAS, a mixed valence of Ce was found in the $x=0.0$ sample, and F doping suppressed it, which is consistent with the results on polycrystalline samples. As for resonant PES, we found that the Ce $4f$ electrons in both $x=0.0$ and 0.5 systems respectively formed a flat band at 1.0 and 1.4 eV below the Fermi level and there was no contribution to the Fermi surfaces. Interestingly, Ce valence in ${\mathrm{CeOBiS}}_2$ deviates from ${\mathrm{Ce}}^{3+}$ even though Ce $4f$ electrons are localized, indicating the Ce valence is not in a typical valence fluctuation regime. We assume that localized Ce $4f$ in ${\mathrm{CeOBiS}}_2$ is mixed with unoccupied Bi $6p_z$, which is consistent with a previous local structural study. Based on the analysis of the Ce $L_3$-edge XAS spectra using Anderson's impurity model calculation, we found that the transfer integral becomes smaller, increasing the number of Ce $4f$ electrons upon the F substitution for O.

A central feature of the periodic Anderson model is the competition between antiferromagnetism, mediated by the Ruderman-Kittel-Kasuya-Yosida interaction at small conduction electron-local electron hybridization $V$, and singlet formation at large $V$. At zero temperature, and in dimension $d>1$, these two phases are separated by a quantum critical point $V_c$. We use quantum Monte Carlo (QMC) simulations to explore the effect of impurities which have a local hybridization $V_{*}<V_c$ in the antiferromagnetic regime which are embedded in a bulk singlet phase with $V>V_c$. We measure the suppression of singlet correlations and the antiferromagnetic correlations which form around the impurity, as well as the size of the resulting domain. Exact diagonalization calculations for linear chains allow us to verify that the qualitative features obtained at intermediate coupling and finite $T$ persist to strong coupling and $T=0$, regimes which are difficult to access with QMC. Our calculations agree qualitatively with NMR measurements in ${\mathrm{CeCoIn}}_{5-x}{\mathrm{Cd}}_x$.

A fundamental theory is developed for describing laser-driven resonance energy transfer (RET) in dimensionally constrained nanostructures within the framework of quantum electrodynamics. The process of RET communicates electronic excitation between suitably disposed emitter and detector particles in close proximity, activated by the initial excitation of the emitter. Here, we demonstrate that the transfer rate can be significantly increased by propagation of an auxiliary laser beam through a pair of nanostructure particles. This is due to the higher order perturbative contribution to the Förster-type RET, in which laser field is applied to stimulate the energy transfer process. We construct a detailed picture of how excitation energy transfer is affected by an off-resonant radiation field, which includes the derivation of second and fourth order quantum amplitudes. The analysis delivers detailed results for the dependence of the transfer rates on orientational, distance, and laser intensity factor, providing a comprehensive fundamental understanding of laser-driven RET in nanostructures. The results of the derivations demonstrate that the geometry of the system exercises considerable control over the laser-assisted RET mechanism. Thus, under favorable conformational conditions and relative spacing of donor-acceptor nanostructures, the effect of the auxiliary laser beam is shown to produce up to 70% enhancement in the energy migration rate. This degree of control allows optical switching applications to be identified.

We present first-principles total-energy electronic-structure calculations that provide the microscopic mechanism of the adatom interchange reaction on the Sn- and Pb-covered Ge(111)-($2\times 8$) and the Sb-covered Si(111)-($7\times 7$) surfaces with and without the tip of the atomic force microscope (AFM). We find that, without the presence of the AFM tip on the Ge surface, the adatom interchange occurs through the migration of the adatom, the spontaneous formation of the dimer structures of the two adatoms, the dimer-dimer structural transitions that induce the exchange of the positions of the two adatoms, and then the backward migration of the adatom. We also find that the dimer structure is unfeasible at room temperature on the Si surface and the adatom interchange are hereby unlikely. With the presence of the tip, we find that the reaction pathways are essentially the same for the Ge surface but that the energy barriers of the migration and the exchange processes are substantially reduced by the AFM tip. We further find that the AFM tip induces the spontaneous formation of the dimer structure even on the Si surface, hereby opening a channel of the interchange of the adatoms. Our calculations show that the bond formation between the AFM tip atom and the surface adatom is essential for the atom manipulation using the AFM tip.

Coherent phonons are an excellent tool to investigate the interplay between electronic and structural dynamics. The displacive excitation of coherent phonons in elemental bismuth is one of the most widely studied processes for this purpose. We employ time-resolved photoelectron diffraction to access the structural dynamics by recording the photoemission intensity from one initial state as a function of emission angle. In comparison with tight-binding and single-scattering cluster calculations, this allows electronic and structural effects to be disentangled. Hence, the full dynamics of the hot electron gas and of coherently excited phonons can be accessed in a single experiment. As a major result the phase lag between the coherent phonons and the modulation of the electronic structure can be determined with high precision. The phonon phase lag with respect to the modulation of the electronic structure is about $2.85\pm 0.21$ rad, thus significantly smaller than $\pi$. The difference is not due to phonon decay by energy dissipation into low-energy modes, but rather caused by the very early evolution of the highly excited electron distribution.

We measure the spin Hall angle in Au layers of 5–100 nm thicknesses by spin pumping from ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ epitaxial films grown by ultrahigh vacuum, off-axis sputtering. We observe a striking increase in the spin Hall angle for Au layers thinner than the measured spin diffusion length of 12.6 nm. In particular, the 5 nm Au layer shows a large spin Hall angle of 0.087, compared to those of 0.016 and 0.017 for the 50 and 100 nm Au layers, respectively, suggesting that the top surface plays a dominant role in spin Hall physics when the spin current is able to reach it. Other spin pumping related parameters, including Gilbert damping enhancement, interfacial spin mixing conductance, and spin current are also determined for Au layers of various thicknesses. Given the pervasive role of ultrathin films in electrical and spin transport applications, this result emphasizes the importance of considering the impact of the top surface and reveals the possibility of tuning critical spin parameters by film thickness.

Computationally-guided material discovery is being increasingly employed using a descriptor-based screening through the calculation of a few properties of interest. A precise understanding of the uncertainty associated with first-principles density functional theory calculated property values is important for the success of descriptor-based screening. The Bayesian error estimation approach has been built in to several recently developed exchange-correlation functionals, which allows an estimate of the uncertainty associated with properties related to the ground state energy, for example, adsorption energies. Here, we propose a robust and computationally efficient method for quantifying uncertainty in mechanical properties, which depend on the derivatives of the energy. The procedure involves calculating energies around the equilibrium cell volume with different strains and fitting the obtained energies to the corresponding energy-strain relationship. At each strain, we use instead of a single energy, an ensemble of energies, giving us an ensemble of fits and thereby, an ensemble of mechanical properties associated with each fit, whose spread can be used to quantify its uncertainty. The generation of ensemble of energies is only a post-processing step involving a perturbation of parameters of the exchange-correlation functional and solving for the energy non-self-consistently. The proposed method is computationally efficient and provides a more robust uncertainty estimate compared to the approach of self-consistent calculations employing several different exchange-correlation functionals. We demonstrate the method by calculating the uncertainty bounds for several materials belonging to different classes and having different structures using the developed method. We show that the calculated uncertainty bounds the property values obtained using three different GGA functionals: PBE, PBEsol, and RPBE. Finally, we apply the approach to calculate the uncertainty associated with the DFT-calculated elastic properties of solid state Li-ion and Na-ion conductors.

The Raman response in a superconductor is a powerful probe to investigate the symmetry of the superconducting gap. Here we show that in a multiband superconductor it also offers the unique opportunity to establish whether the driving pairing interaction has an intraband or interband character. In the model with one hole and one electron band the full gauge-invariant Raman response, obtained by accounting for the fluctuations of both the density and superconducting phase degrees of freedom, is always dominated by the Leggett mode, regardless its nature. However, while in the case of intraband-dominated pairing the Josephson-like phase fluctuations of the two condensates identify a well-defined peak, as observed in ${\text{MgB}}_2$, for dominant interband pairing the Leggett resonance is pushed at twice the largest gap, resembling apparently a pair-breaking peak. The latter case is in very good agreement with experimental data in iron-based superconductors, suggesting that an interband pairing mechanism should be at play in these systems. These results have also interesting implications for the nonlinear optical response probed by means of intense THz fields.

We study periodically driven closed quantum systems where two parameters of the system Hamiltonian are varied periodically in time with frequencies ${\omega }_1$ and ${\omega }_2=r{\omega }_1$. We show that such drives may be used to tune towards dynamics-induced freezing where the wave function of the state of the system after a drive cycle at time $T=2\pi /{\omega }_1$ has almost perfect overlap with the initial state. We locate regions in the $({\omega }_1,r)$ plane where the freezing is near exact for a class of integrable models and a specific nonintegrable model. The integrable models that we study encompass Ising and $\mathit{XY}$ models in $d=1$, Kitaev model in $d=2$, and Dirac fermions in graphene and atop a topological insulator surface, whereas the nonintegrable model studied involves the experimentally realized one-dimensional tilted Bose-Hubbard model in an optical lattice. In addition, we compute the relevant correlation functions of such driven systems and describe their characteristics in the region of the $({\omega }_1,r)$ plane where the freezing is near exact. We supplement our numerical analysis with semianalytic results for integrable driven systems within adiabatic-impulse approximation and discuss experiments which may test our theory.

The localized-itinerant nature of $\mathrm{Ce}\text{−}4f$ valence electrons in heavy fermion compound ${\mathrm{CeIn}}_3$ under pressure is studied thoroughly by means of the combination of density functional theory and single-site dynamical mean-field theory. The detailed evolutions of electronic structures of ${\mathrm{CeIn}}_3$, including total and partial density of states, momentum-resolved spectral functions, and valence state histograms, are calculated in a wide pressure range where the corresponding volume compression $V/V_0\in [0.6,1.0]$ (here $V_0$ is the experimental crystal volume) at $T\cong 116$ K. Upon increasing pressure, two strong peaks associated with the $\mathrm{Ce}\text{−}4f$ states emerge near the Fermi level, and the $c\text{−}f$ hybridization and valence state fluctuation are enhanced remarkably. Moreover, the kinetic and potential energies rise, while the occupancy, total angular momentum, and low-energy scattering rate of the $\mathrm{Ce}\text{−}4f$ electrons decline with respect to pressure. All the physical observables considered here exhibit prominent kinks or fluctuations in $V/V_0\in [0.80,0.90]$, which are probably the desired fingerprints for the $\mathrm{Ce}\text{−}4f$ localized-itinerant crossover.

Topological insulators are states of matter distinguished by the presence of symmetry-protected metallic boundary states. These edge modes have been characterized in terms of transport and spectroscopic measurements, but a thermodynamic description has been lacking. The challenge arises because in conventional thermodynamics the potentials are required to scale linearly with extensive variables such as volume, which does not allow for a general treatment of boundary effects. In this paper, we overcome this challenge with Hill thermodynamics. In this extension of the thermodynamic formalism, the grand potential is split into an extensive, conventional contribution, and the subdivision potential, which is the central construct of Hill's theory. For topologically nontrivial electronic matter, the subdivision potential captures measurable contributions to the density of states and the heat capacity: it is the thermodynamic manifestation of the topological edge structure. Furthermore, the subdivision potential reveals phase transitions of the edge even when they are not manifested in the bulk, thus opening a variety of possibilities for investigating, manipulating, and characterizing topological quantum matter solely in terms of equilibrium boundary physics.

Orbital degrees of freedom in condensed matter could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In $4f$-electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focused on a purely quadrupole-active system ${\mathrm{PrIr}}_2{\mathrm{Zn}}_{20}$ showing superconductivity in the presence of an antiferroquadrupole order at $T_{\mathrm{Q}}=0.11$ K. We observed non-Fermi-liquid (NFL) behaviors emerging in the electrical resistivity $\rho$ and the $4f$ contribution to the specific heat, $C_{4f}$, in the paramagnetic state at $T > T_{\mathrm{Q}}$. Moreover, in magnetic fields $B\le 6$ T, all data sets of $\rho \left(T\right)$ and $C_{4f}\left(T\right)$ are well scaled with characteristic temperatures $T_0$'s. This observation of the NFL state in the nonmagnetic quadrupole-active system has an origin intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons with an energy scale of $k_{\mathrm{B}}{T}_0$. At $T\le 0.13$ K, $\rho \left(T\right)$ and $C_{4f}\left(T\right)$ exhibit anomalies as $B$ approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.

The anomalous Hall effect (AHE) is investigated for a ferromagnetic ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ single crystal with a geometrically frustrated kagome bilayer of Fe. The scaling behavior between anomalous Hall resistivity ${\rho }_{xy}^A$ and longitudinal resistivity ${\rho }_{xx}$ is quadratic and further analysis implies that the AHE in the ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ single crystal should be dominated by the intrinsic Karplus-Luttinger mechanism rather than extrinsic skew-scattering or side-jump mechanisms. Moreover, there is a sudden jump of anomalous Hall conductivity ${\sigma }_{xy}^A$ appearing at about 100 K where the spin-reorientation transition from the $c$ axis to the $ab$ plane is completed. This change of ${\sigma }_{xy}^A$ might be related to the evolution of the Fermi surface induced by the spin-reorientation transition.

Motivated by the observation of multiphoton electric dipole spin-resonance processes in InAs nanowires, we study theoretically the transport dynamics of a periodically driven five-level system, modeling the level structure of a two-electron double quantum dot. We show that the observed multiphoton resonances, which are dominant near interdot charge transitions, are due to multilevel Landau-Zener-Stückelberg-Majorana interference. Here, a third energy level serves as a shuttle that transfers population between the two resonant spin states. By numerically integrating the master equation, we replicate the main features observed in the experiments: multiphoton resonances (as large as eight photons), a robust odd-even dependence, and oscillations in the electric dipole spin-resonance signal as a function of energy-level detuning.