Many electronic systems (e.g., the cuprate superconductors and heavy fermions) exhibit striking features in their dynamical response over a prominent range of experimental parameters. While there are some empirical suggestions of particular increasing length scales that accompany such transitions in some cases, this identification is not universal and in numerous instances no large correlation length is evident. To better understand, as a matter of principle, such behavior in quantum systems, we extend a known mapping (earlier studied in stochastic or supersymmetric quantum mechanics) between finite temperature classical Fokker-Planck systems and related quantum systems at zero temperature to include general nonequilibrium dynamics. Unlike Feynman mappings or stochastic quantization methods in field theories (as well as more recent holographic type dualities), the classical systems that we consider and their quantum duals reside in the same number of space-time dimensions. The upshot of our very broad and rigorous result is that a Wick rotation exactly relates (i) the dynamics in general finite temperature classical dissipative systems to (ii) zero temperature dynamics in the corresponding dual many-body quantum systems. Using this correspondence, we illustrate that, even in the absence of imposed disorder, many continuum quantum fluid systems (and possible lattice counterparts) may exhibit a zero-point “quantum dynamical heterogeneity” wherein the dynamics, at a given instant, is spatially nonuniform. While the static length scales accompanying this phenomenon do not seem to exhibit a clear divergence in standard correlation functions, the length scale of the dynamical heterogeneities can increase dramatically. We further study “quantum jamming” and illustrate how a hard-core bosonic system can undergo a zero temperature quantum critical metal-to-insulator-type transition with an extremely large effective dynamical exponent $z>4$ that is consistent with length scales that increase far more slowly than the relaxation time as a putative critical transition is approached. Similar results may hold for spin-liquid-type as well as interacting electronic systems. We suggest ways to analyze experimental data in order to adduce such phenomena. Our approach may be used to analyze other quenched quantum systems.

First-principles density functional theory DFT$+U$ calculations and experimental neutron diffraction structure analyses were used to determine the low-temperature crystallographic and magnetic structure of bixbyite $\alpha$-Mn${}_2$O${}_3$. The energies of various magnetic arrangements, calculated from first principles, were fit to a cluster-expansion model using a Bayesian method that overcomes a problem of underfitting caused by the limited number of input magnetic configurations. The model was used to predict the lowest-energy magnetic states. Experimental determination of magnetic structure benefited from an optimized sample synthesis, which produced crystallite sizes large enough to yield a clear splitting of peaks in the neutron powder diffraction patterns, thereby enabling magnetic-structure refinements under the correct orthorhombic symmetry. The refinements employed group theory to constrain magnetic models. Computational and experimental analyses independently converged to similar ground states, with identical antiferromagnetic ordering along a principal magnetic axis and secondary ordering along a single orthogonal axis, differing only by a phase factor in the modulation patterns. The lowest-energy magnetic states are compromise solutions to frustrated antiferromagnetic interactions between certain corner-sharing [MnO${}_6$] octahedra.

We present quantum Monte Carlo calculations that probe the paramagnet-ferromagnet phase transition in a two-dimensional Stoner Hamiltonian. With a screened Coulomb interaction we observe a first-order ferromagnetic transition for short screening lengths, and a second-order transition with a longer screening length, accompanied by a rising critical interaction strength. Finally, we discuss the consequences of our results for an ultracold atomic gas with finite ranged interactions.

Inelastic electron scattering is applied to investigate the impact of potassium intercalation on the charge-carrier plasmon energy and dispersion in the charge-density wave (CDW) bearing compound $2H$-TaSe${}_2$. We observe an unususal doping dependence of the plasmon dispersion, which even changes sign on alkali addition. In contrast to the continous energy shift of the plasmon position on doping at lowest momentum transfer, its dispersion changes in a rather discontinuous manner. We argue that the observed dynamics can be explained only in a picture, where complex phenomena are taken into account, including the suppression of the CDW on doping as well as the interplay of the CDW and the plasma resonance.

We have studied the electronic and local magnetic structure of the hydrogen interstitial impurity at the tetrahedral site in diamond-structure Ge, using an empirical tight binding $+$ dynamical mean-field-theory approach because within the local-density approximation (LDA) Ge has no gap. We first establish that within LDA the $1s$ spectral density bifurcates due to entanglement with the four neighboring $s{p}^3$ antibonding orbitals, providing an unanticipated richness of behavior in determining under what conditions a local moment hyperdeep donor or Anderson impurity will result, or on the other hand, a gap state might appear. Using a supercell approach, we show that the spectrum, the occupation, and the local moment of the impurity state displays a strong dependence on the strength of the local on-site Coulomb interaction $U$, the H-Ge hopping amplitude, the depth of the bare $1s$ energy level ${\epsilon }_H$, and we address to some extent the impurity concentration dependence. In the isolated impurity, strong interaction regime a local moment emerges over most of the parameter ranges, indicating magnetic activity, and spectral density structure very near (or in) the gap suggests possible electrical activity in this regime.

We present a systematic study of the impurity effect on $T_{\mathrm{c}}$ in Fe-based superconductors, assuming that the sign-reversal $s$-wave state due to interpocket repulsion (the $s_{\pm }$-wave state) is realized. For this purpose, we introduce several realistic impurity models with nonlocal modifications of potentials and hopping integrals around the impurity site. When we use the impurity model parameters for $3d$- and $4d$-impurity atoms derived from the recent first-principles study by Nakamura et al. [Phys. Rev. B 83, 144512 (2011)], we find that the $s_{\pm }$-wave state is very fragile against impurities: The superconductivity without impurities $T_{\mathrm{c}0}=30\text{K}$ is destroyed by introducing small residual resistivity ${\rho }_0^{\mathrm{cr}}=5z^{-1}–10z^{-1}\mu \Omega \mathrm{cm}$ ($z^{-1}=m^{*}/m$, being the mass-enhancement factor), consistent with the previous theoretical study for the on-site impurity model by Onari and Kontani [Phys. Rev. Lett. 103, 177001 (2009)]. This result is essentially unchanged for different nonlocal impurity models with realistic parameters. We also discuss the effect of the impurity-induced non-local-orbital order on the superconducting state.

We investigate physical properties of an Anderson impurity embedded in the bulk of a topological insulator. The slave-boson mean-field approximation is used to account for the strong electron correlation at the impurity. Different from the results of a quantum impurity on the surface of a topological insulator, we find for the band-inverted case that a Kondo resonant peak and in-gap bound states can be produced simultaneously. However, only one of them appears for the normal case. It is shown that the mixed-valence regime is much broader in the band-inverted case, while it shrinks to a very narrow regime in the normal case. Furthermore, a self-screening of the Kondo effect may appear when the interaction between the bound-state spin and impurity spin is taken into account.

Electronic band-structure calculations combined with the Boltzmann transport approach are implemented to determine temperature-dependent electron-transport properties such as electrical conductivity $\sigma \left(T\right)$, thermopower $S\left(T\right)$, electronic thermal conductivity ${\kappa }_e\left(T\right)$, and related quantities such as the Lorenz number $L\left(T\right)$ and the power factor $PF\left(T\right)$. The Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) is employed to study the electronic structure of a chemically disordered system and also to explore the Fermi surface. This allows for the determination of the velocity and lifetime of an electron. The aforementioned procedure was then applied to investigate promising $n$-doped Mg${}_2$Si${}_{1-x}$Sn${}_x$ thermoelectric material. Using different approaches for relaxation time as well as treating the lattice thermal conductivity ${\kappa }_l$ as an adjustable parameter, the thermoelectric figure of merit, $ZT$, was calculated in terms of temperature $T$ and carrier concentration $n$. The $ZT(n,T)$ map for Mg${}_2$Si${}_{1-x}$Sn${}_x$ clearly shows that, for an experimental value of ${\kappa }_l\sim 1.25$ W m${}^{-1}$ K${}^{-1}$, the strongest thermoelectric properties are expected for $x\sim 0.6$ composition, when two conduction bands tend to degenerate near the $X$ point. On the whole, the KKR-CPA results well correspond to experimental findings upon employing the constant-relaxation-time approach, especially concerning thermopower $S\left(T\right)$. The above-mentioned method was also used to estimate the figure of merit of a hypothetical nanograin material with low thermal conductivity (${\kappa }_l\sim 0.5$ W m${}^{-1}$ K${}^{-1}$), yielding $ZT\sim 2$.

We use the Bloch-Redfield-Wangsness theory to calculate the effects of acoustic phonons in coherent control experiments where quantum-dot excitons are driven by shaped laser pulses. This theory yields a generalized Lindblad equation for the density operator of the dot, with time-dependent damping and decoherence due to phonon transitions between the instantaneous dressed states. It captures similar physics to the form recently applied to Rabi oscillation experiments [Ramsay et al., Phys. Rev. Lett. 104, 017402 (2010)] but guarantees positivity of the density operator. At sufficiently low temperatures, it gives results equivalent to those of fully non-Markovian approaches [Lüker et al., Phys. Rev. B 85, 121302 (2012)] but is significantly simpler to simulate. Several applications of this theory are discussed. We apply it to adiabatic rapid passage experiments and show how the pulses can be shaped to maximize the probability of creating a single exciton using a frequency-swept laser pulse. We also use this theory to propose and analyze methods to determine the phonon density of states experimentally, i.e., phonon spectroscopy, by exploring the dependence of the effective damping rates on the driving field.

We perform advanced radiation leakage microscopy of routing dielectric-loaded plasmonic waveguiding structures. By direct-plane imaging and momentum-space spectroscopy, we analyze the energy transfer between coupled waveguides as a function of gap distance and reveal the momentum distribution of curved geometries. Specifically, we observed a clear degeneracy lift of the effective indices for strongly interacting waveguides in agreement with coupled-mode theory. We use momentum-space representations to discuss the effect of curvature on dielectric-loaded waveguides. The experimental images are successfully reproduced by a numerical and an analytical model of the mode propagating in a curved plasmonic waveguide.

We report the doping effect of a magnetic Fe ion in Bi${}_2$Te${}_3$, that is, Bi${}_{2-x}$Fe${}_x$Te${}_3$ ($x$ $=$ 0, 0.08, 0.15, 0.2, 0.25, and 0.3). The paramagnetic magnetization data reveal that the Fe ions are doped in the divalent form. The Fe${}^{2+}$ state substituted for Bi${}^{3+}$ can create hole donors, which compensate for the electron dopants of Bi${}_2$Te${}_3$ with a small amount of Te excess. This causes the $n$-type carrier density to be decreased with increasing $x$, and finally be changed to $p$ type at $x$ $=$ 0.3, where the carrier mobility suddenly drops and the electrical resistivity abruptly increases. These results are consistent with angle-resolved photoemission spectroscopy experiments. The Fermi level shifts downward with increasing $x$. Furthermore, we find a larger spin polarization for the Fe-doped Bi${}_2$Te${}_3$ samples, which is crucial for future spintronics applications.

We introduce the concept of furtive quantum sensing, demonstrating the possibility of concealing quantum objects from matter waves while maintaining their ability to interact and to get excited by the impinging particles. This is obtained by cloaking, with tailored homogeneous metamaterial layers, quantum systems having internal degrees of freedom. The effect is a low-observable quantum sensor with drastically reduced elastic scattering and optimized absorption levels, a concept that opens interesting venues in particle detection, high-efficiency electrical pumping, and quantum information processing.

One-way excitation of spoof surface plasmons (SPs) and strongly pronounced diodelike extraordinary transmission of linearly polarized waves in the beaming regime can be obtained by combining spoof SPs and cross-polarization conversion resonances. The reciprocal composite structure that is suggested to realize this mechanism consists of a symmetric metallic grating with a subwavelength slit and a metamaterial based ultrathin 90${}^{\circ }$ polarization rotator and, therefore, shows the broken spatial inversion symmetry. In contrast to the earlier studies of SP inspired transmission through subwavelength slits, asymmetric (one-way) beaming is demonstrated at normal incidence and for both $s$- and $p$-polarized incident waves. Furthermore, as an implication of Lorentz reciprocity, the studied diodelike mechanism is reversible, which manifests itself in that transmission is significant for one of the two opposite illumination directions at $s$ polarization and for the other direction at $p$ polarization. The obtained numerical and experimental results verify the general idea and enable us to select the optimal operation regimes.

We inspect the initial and the long-time evolution of excitations in Fermi liquids by analyzing the time structure of the electron spectral function. Focusing on the short-time limit we study the electron-boson model for the homogeneous electron gas and apply the first-order (in boson propagator) cumulant expansion of the electron Green's function. In addition to a quadratic decay in time upon triggering the excitation, we identify nonanalytic terms in the time expansion similar to those found in the Fermi edge singularity phenomenon. We also demonstrate that the exponential decay in time in the long-time limit is inconsistent with the $\mathit{GW}$ approximation for the self-energy. The background for this is the Paley-Wiener theorem of complex analysis. To reconcile with the Fermi liquid behavior an inclusion of higher order diagrams (in the screened Coulomb interaction) is required.

A comprehensive investigation of the luminescent properties of carbon nitride polymers, based on tri-$s$-triazine units, has been conducted. Steady-state temperature- and excitation-power-dependent as well as time-resolved measurements with near-UV excitation ($\lambda =325$ nm and 405 nm) yield strong photoluminescence, covering the visible spectrum. The spectral, thermal, and temporal features of the photoluminescence can be satisfactorily described by the excitation and radiative recombination of molecular excitons, localized at single tri-$s$-triazine units. The discussed model is in accordance with the recently reported absorption features of carbon nitride polymers. Thus, from the point of view of optical spectroscopy, the material effectively behaves as a monomer.

We examine a continuum effect of a dynamic Wannier-Stark ladder (DWSL) driven by a cw laser—with $F_{ac}$ and $\omega$ as amplitude and frequency, respectively—by means of an excess density of states (DOS), ${\rho }^{\left(ex\right)}\left(E\right)$, closely related to the more familiar DOS and proportional to the lifetime of a resonance state. It is mathematically shown that ${\rho }^{\left(ex\right)}\left(E\right)$ is governed by three different physical mechanisms: the single-channel resonance mechanism, the multichannel nonresonance mechanism, and the multichannel resonance mechanism. The last mechanism becomes more important with the increase in $F_{ac}$. The effect of the interchannel interaction is maximized when the ratio of a Bloch frequency to $\omega$, represented as $\eta$, equals unity. In the actual calculations based on the $R$-matrix Floquet theory, it is revealed that, in a large-$F_{ac}$ region, ${\rho }^{\left(ex\right)}\left(E\right)$ for $\eta =1$ shows a complicated spectral structure composed of a couple of newly growing peaks, in contrast to ${\rho }^{\left(ex\right)}\left(E\right)$ for $\eta =3$ which just shows a monotonic change of a single spectral peak. It is speculated that the pronounced feature of the former spectra is attributed to the Fano-like multichannel resonance mechanism, whereas the feature of the latter case is attributed to the multichannel nonresonance mechanism.

We present a detailed study of many-body effects associated with the intraband $1s$-$2p$ transition in two- and three-dimensional photoexcited semiconductors. We employ a previously developed excitonic model to treat effects of exchange and phase space filling (PSF). In this work, we extend the model to include intraband transitions and static free-carrier screening. The exciton transition energies are renormalized by many-body interactions, and the excitonic dynamical equations provide simple expressions for the individual contributions of screening, PSF and exchange. The excitonic model correctly predicts the blue shift and bleaching of the $1s$ exciton resonance due to exchange and PSF. Free-carrier screening is found to enhance these effects by lowering the binding energy of the $1s$ exciton. In contrast, the effects of free-carrier screening on the $1s$-$2p$ transition energy are subtler. For a coherent exciton system, in the absence of free-carrier screening, exchange and PSF lead to a blue shift of the transition energy. However, screening decreases the $1s$ binding energy faster than the $2p$ binding energy, which in turn decreases the transition energy. Thus screening effects oppose exchange and PSF, and the overall magnitude and sign of the $1s$-$2p$ transition energy shift depends on the free-carrier density. Specifically, for low to moderate excitation densities, exchange and PSF can be dominated by screening, leading to a net redshift of the transition energy. The results for two- and three-dimensional systems are qualitatively similar, although the magnitude of the shift is much smaller in three dimensions.

We present an atomistic study of the behavior and interactions of hydrogen and vacancies in body centered cubic (bcc) iron, using both ab initio and classical molecular dynamics methods. Hydrogen causes damage to materials through embrittlement, hardening, and swelling; we investigate the role of vacancies in these processes. Hydrogen, which normally diffuses with a very small barrier, is strongly trapped at monovacancies and vacancy clusters, resulting in changes to its electronic structure. Following saturation of the surface of a vacancy cluster, the formation of H${}_2$ molecules is possible, at variance with the situation in the bulk. High local concentrations of hydrogen increase the likelihood of vacancy formation and stabilize vacancy clusters. Small hydrogen-vacancy clusters generally tend to diffuse by dissociation, but the trivacancy is shown to be capable of dragging hydrogen while migrating. We describe the structure of clusters of vacancies with varying hydrogen concentrations, finding that compact or spherical bubbles are generally lower energy than planar or linear configurations. Comparison with other bcc metals and with experiment is provided. For systems involving light elements such as hydrogen, corrections for zero-point energy are very important; we include these in our calculations and discuss their importance for different properties.