$\beta$-${\text{CaCr}}_2$${\text{O}}_4$ has a cycloidal magnetic structure and isostructural ${\text{NaCr}}_2$${\text{O}}_4$ shows unconventional colossal magnetoresistance below a canted antiferromagnetic (AFM) ordering temperature of $T_{\mathrm{N}}=125$ K. In this paper, magnetic and electronic properties of the solid-solution system ${\text{Ca}}_{1-x}$${\text{Na}}_x$${\text{Cr}}_2$${\text{O}}_4$, are investigated. The $T_{\mathrm{N}}$ value decreases with decreasing $x$ from $x=1$ to $\frac{2}{3}$. A spin-glass behavior appears for $0.1\le x\le 0.6$. Interestingly, the spin-glass character is detected even for $\frac{2}{3}\le x\le 0.95$ coexisting with the AFM ordering. Curie constant increases and Weiss temperature decreases with decreasing $x$. Curie constant is significantly suppressed from theoretical value, suggesting large quantum fluctuations and strong hybridization between Cr and O ions for higher-$x$ values. Substitution of Ca for Na introduces the ligand holes. The decrease in Weiss temperature points out that a single ligand hole mediates ferromagnetic interaction of approximately 400 K by the double-exchange mechanism. Saturation magnetization and magnetic susceptibility at 2 K show unusual $x$ dependencies, which suggests that not only the canting angle but also the periodicity of magnetic ordering depend on $x$. Electrical resistivity for $0.2\le x\le 0.95$ follows the variable-range hopping mechanism, suggesting Anderson localization at the $x$ range.

We explore the propagation of a single hole in the quantum compass model, whose nematic ground state is given by mutually decoupled antiferromagnetic chains. The compass model can be seen as the strong-coupling limit of a spinless two-band Hubbard model, which we study here using mean-field theory and the variational cluster approach. Due to the symmetries of the compass model, the inherent disorder along one lattice direction turns out not to affect hole motion, and doping a hole consequently does not lift the subextensive degeneracy of the nematic phase. To broaden and deepen understanding, we derive a generalized itinerant model and address the transition to two-dimensional Ising order. We observe coherent hole motion in both the nematic and the antiferromagnetic phases, also in the presence of quantum fluctuations away from pure Ising exchange. In addition to quantum fluctuations and interorbital hopping, three-site hopping is found to play an important role and to dominate propagation in the two-dimensional Ising limit as well as along the antiferromagnetic chains in the nematic order which forms in the compass model.

Several variants of the recently proposed density matrix embedding theory (DMET) [G. Knizia and G. K-L. Chan, Phys. Rev. Lett. 109, 186404 (2012)] are formulated and tested. We show that spin symmetry breaking of the lattice mean-field allows precise control of the lattice and fragment filling while providing very good agreement between predicted properties and exact results. We present a rigorous proof that at convergence this method is guaranteed to preserve lattice and fragment filling. Differences arising from fitting the fragment one-particle density matrix alone versus fitting fragment plus bath are scrutinized. We argue that it is important to restrict the density matrix fitting to solely the fragment. Furthermore, in the proposed broken symmetry formalism, it is possible to substantially simplify the embedding procedure without sacrificing its accuracy by resorting to density instead of density matrix fitting. This simplified density embedding theory (DET) greatly improves the convergence properties of the algorithm.

In this paper, we study the influence of quasiresonant and nonresonant excitation on the interference properties of single photons emitted from quantum dots (QDs). The quasiresonant excitation scheme leads to an increase of interference visibility of photons emitted from the same QD to 69% compared to 12% for nonresonant excitation. Furthermore, we demonstrate quantum interference of photons emitted from separate QDs which are simultaneously excited into their $p$ shell. We can readily extract a two-photon interference visibility as high as (39 ± 2)% for nonpostselected coincidences exceeding the predicted value based on coherence and radiative decay times of the quantum dot emission ($\sim$25%). We account for this observation by treating the emission of both quantum dots as inhomogeneously broadened ensembles of Fourier-limited photons and observe good congruence between experiment and model.

We obtain the quasiparticle band structure of ABA and ABC-stacked graphene trilayers through ab initio density-functional theory (DFT) and many-body quasiparticle calculations within the $GW$ approximation. To interpret our results, we fit the DFT and $GW$ $\pi$ bands to a low-energy tight-binding model, which is found to reproduce very well the observed features near the $K$ point. The values of the extracted hopping parameters are reported and compared with available theoretical and experimental data. For both stackings, the self-energy corrections lead to a renormalization of the Fermi velocity, an effect also observed in previous calculations on monolayer graphene. They also increase the separation between the higher-energy bands, which is proportional to the nearest-neighbor interlayer hopping parameter ${\gamma }_1$. Both features are brought to closer agreement with experiment through the self-energy corrections. Finally, other effects, such as trigonal warping, electron-hole asymmetry, and energy gaps, are discussed in terms of the associated parameters.

We demonstrate strong acoustic gain in electric-field biased piezoelectric semiconductors at frequencies near the plasmon frequency in the terahertz range. When the electron drift velocity produced by an external electric field is higher than the speed of sound, Cherenkov radiation of phonons generates amplification of sound. It is demonstrated that this effect is particularly effective at $\varepsilon$-near-zero response, leading to giant levels of acoustic gain. Operating at conditions with strong acoustic amplification, we predict unprecedented enhancement of the scattered sound field radiated from an electrically controlled piezoelectric slab waveguide. This extreme sound field enhancement in an active piezo material shows potential for acoustic sensing and loss compensation in metamaterials and nonlinear devices.

The nonlinear dynamics of electrons in multilayer epitaxial graphene is investigated by time-resolved terahertz (THz) spectroscopy in a regime where the interaction of electrons with the external field dominates over scattering processes. The predominantly coherent electron response to the THz field involves both intra- and interband currents, leading to coherently driven interband transitions of carriers and to the generation of higher harmonics of the THz carrier frequency. The overall behavior of the graphene layers is always absorptive, even after generation of an initial electron-hole distribution by femtosecond midinfrared excitation. The results are in agreement with theoretical calculations of the nonperturbative THz response.

Based on first-principles calculations, we predict Bi${}_2$TeI, a stoichiometric compound that is synthesized, to be a weak topological insulator (TI) in layered subvalent bismuth telluroiodides. Within a bulk energy gap of 80 meV, two Dirac-cone-like topological surface states exist on the side surface perpendicular to the BiTeI layer plane. These Dirac cones are relatively isotropic due to the strong interlayer coupling, distinguished from those of previously reported weak TI candidates. Moreover, with chemically stable cladding layers, the BiTeI-Bi${}_2$-BiTeI sandwiched structure is a robust quantum spin Hall system, which can be obtained by simply cleaving the bulk Bi${}_2$TeI.

We have performed spin- and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)${}_2$Te${}_4$ and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators [Pb(Bi,Sb)${}_2$Te${}_4$, Bi${}_2$Te${}_3$, Bi${}_2$Se${}_3$, and TlBiSe${}_2$] and elucidated that the out-of-plane spin polarization $P_z$ is systematically correlated with the warping strength. However, the magnitude of $P_z$ is found to be only half that expected from the $k·p$ theory when the warping is strong. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators.

Scaling of surface fluctuations of polycrystalline CdTe/Si(100) films grown by hot-wall epitaxy are studied. The growth exponent of surface roughness and the dynamic exponent of the autocorrelation function in the mound growth regime agree with the values of the Kardar-Parisi-Zhang (KPZ) class. The scaled distributions of heights, local roughness, and extremal heights show remarkable collapse with those of the KPZ class, giving an experimental observation of KPZ distributions in $2+1$ dimensions. Deviations from KPZ values in the long-time estimates of dynamic and roughness exponents are explained by spurious effects of multipeaked coalescing mounds and by effects of grain shapes. Thus, this scheme for investigating universality classes of growing films advances over the simple comparison of scaling exponents.

We used spatially resolved photoluminescence (PL) and resonant Raman spectroscopy to study the electronic structure of single GaAs nanowires (NWs) consisting of zinc-blende (ZB) and wurtzite (WZ) segments. For narrow ZB segments and stacking faults the energy range of the observed PL peak positions is found to deviate from that of the maxima in resonance Raman profiles. These different energy ranges reflect the fact that the PL recombination is dominated by spatially indirect transitions whereas the resonance enhancement of Raman scattering is caused by direct transitions. Our results provide evidence for the type II band alignment between ZB and WZ GaAs and a coherent picture of all near-band-gap transition energies in GaAs NWs.

In this paper, we have experimentally and numerically studied the nonradiative intersubband (ISB) relaxation in $n$-type Ge/SiGe quantum well (QW) systems. Relaxation times have been probed by means of pump-probe experiments. An energy balance model has been used to interpret the experimental differential transmission spectra and to assess the relevance in the nonradiative relaxation dynamics of both electron and lattice temperature as well as of the carrier density. The comparison between experimental data and theoretical simulation allowed us to calibrate the interaction parameters which describe the electron-optical phonon scattering in two-dimensional (2D) Ge systems. Characteristic relaxation times has been calculated and compared with those of GaAs QWs as a function of the 2D electron density, of the subband energy separation, and of the lattice and electronic temperature. We found that ISB relaxation times for the Ge/SiGe systems are generally shorter than that previously calculated when the electron distribution was neglected. Nonetheless, our main result is that the relaxation time in Ge/SiGe QW systems is longer than 10 ps, also for transition energies above the Ge optical phonon energy, up to 300 K. Furthermore, we obtained that the relaxation times are at least one order of magnitude longer than in GaAs-based systems.

We investigate theoretically the nonlinear dynamics of the most fundamental component of a bistable coupled oscillator. Under a weak radio frequency excitation, the resonator is parametrically tuned into self-sustained oscillatory regimes. The transfer of electrons from one contact to the other is then mechanically assisted, generating a rectified current. The direction of the rectified current is, in general, determined by the phase shift between the mechanical oscillations and the signal. However, we locate intriguing parametrical regions of unidirectional rectified current, resulting from spontaneous parity-symmetry breaking. In these regions, a dynamical symmetry breaking is induced by the nonlinear coupling of the mechanical and electrical degrees of freedom. The achieved unstable regime favors then a unidirectional response, resulting in a direct, positive electric current. When operating within the Coulomb blockade limit, the charge balance in the oscillators perturbs drastically the mechanical motion, causing large accelerations that further enhance the shuttle response. Our results suggest a practical scheme for the realization of a self-powered device in the nanoscale.

The multipole ordered phase in Ce${}_{0.7}$La${}_{0.3}$B${}_6$, emerging below 1.5 K and named phase IV, has been studied by resonant x-ray diffraction in magnetic fields. By utilizing diamond x-ray phase plates to rotate the incident linear polarization and a conventional crystal analyzer system, full linear polarization analysis has been performed to identify the order parameters. The analysis shows that the ${\Gamma }_{5g}$ ($O_{yz}$, $O_{zx}$, $O_{xy}$) quadrupoles are more induced by the field than the ${\Gamma }_{3g}$ ($O_{20}$ and $O_{22}$) quadrupoles on the ${\Gamma }_{5u}$ ($T_{x+y+z}^{\beta }$) antiferro-octupole order in phase IV. The problem is that this result is contradictory to a mean-field calculation, which inevitably gives the ${\Gamma }_{3g}$ quadrupole as the main induced moment. This result indicates that the ${\Gamma }_{5g}$ quadrupole order is close in energy. We consider that a large fluctuation of the ${\Gamma }_{5g}$ quadrupole is hidden behind the primary ordering of the ${\Gamma }_{5u}$ octupole and that the multipolar fluctuation significantly affects the ordering phenomenon.

The search for realistic one-dimensional (1D) models that exhibit dominant superconducting (SC) fluctuations effects has a long history. In these 1D systems, the effects of commensurate band fillings—strongest at half-filling—and electronic repulsions typically lead to a finite charge gap and the favoring of insulating density wave ordering over superconductivity. Accordingly, recent proposals suggesting a gapless metallic state in the Holstein-Hubbard (HH) model, possibly superconducting, have generated considerable interest and controversy, with the most recent work demonstrating that the putative dominant superconducting state likely does not exist. In this paper we study a model with nonlocal electron-phonon interactions, in addition to electron-electron interactions. This model unambiguously possesses dominant superconducting fluctuations at half filling in a large region of parameter space. Using both the numerical multi-scale functional renormalization group (MFRG) for the full model and an analytic conventional renormalization group for a bosonized version of the model, we demonstrate the existence of these dominant SC fluctuations and show that they arise because the spin-charge coupling at high energies is weakened by the nonlocal electron-phonon interaction and the charge gap is destroyed by the resultant suppression of the Umklapp process. The existence of the dominant SC pairing instability in this half-filled 1D system suggests that nonlocal boson-mediated interactions may be important in the superconductivity observed in the organic superconductors.

Using multicomponent Ginzburg-Landau simulations, we show a plethora of vortex states possible in mesoscopic three-band superconductors. We find that mesoscopic confinement stabilizes chiral states, with nontrivial phase differences between the band condensates, as the ground state of the system. As a consequence, we report the broken-symmetry vortex states, the chiral states where vortex cores in different band condensates do not coincide (split-core vortices), as well as fractional-flux vortex states with broken time-reversal symmetry.

Despite the popularity of doping silicon nanocrystals (Si NCs) embedded in silicon dioxide (SiO${}_2$), theoretical understanding on the doping of Si NCs has been nearly all directed to hydrogen-passivated freestanding Si NCs. Now we simulate the doping of Si NCs embedded in SiO${}_2$ by constructing Si@SiO${}_2$ models in which Si NCs are completely covered by a thin layer of SiO${}_2$. In the framework of density functional theory, we investigate the locations of P, the binding energies of P, the energy-level schemes, and the radiative recombination for P-doped Si NCs embedded in SiO${}_2$. It is found that a dangling bond at the Si/SiO${}_2$ interface makes a difference for the doping of a Si NC with P. The P doping of Si NCs embedded in SiO${}_2$ vastly differs from that of Si NCs passivated by hydrogen.

The adsorption of pentacene on the Cu(110) surface has been investigated by reflectance difference spectroscopy (RDS) and spot profile analysis low energy electron diffraction (SPA-LEED). The sensitivity of RDS to electronic transitions involving both surface states and intramolecular transitions allows a close monitoring of the formation of interfacial electronic states and structural phase transitions of the pentacene monolayer on Cu(110).

A site-specific analysis scheme based on density functional theory is used to investigate the static polarizability response of Na${}_N$ clusters for $N$ up to 80. The cluster structures used in the study stem from extensive searches for the respective global minima. The analysis involves partitioning the total cluster polarizability exactly into site (or atomic) contributions; it also results in the decomposition of the polarizability into local (or dipole) and charge transfer contributions. The computed total polarizabilities are found to be in excellent agreement with recent experimental measurements up to a small overall shift. The site analysis provides clear evidence that interior atoms in sodium clusters are strongly screened from an applied external field by the charge induced at the cluster surface. In addition, cluster size trends in the local and charge transfer contributions are shown to be reproduced very well by a simple metal sphere model. The overall picture is that of clusters exhibiting metallic behavior down to the smallest sizes.

We study the tunneling density of states (TDOS) of a disordered electronic system with Coulomb interaction on the insulating side of the Anderson localization transition. The average TDOS shows a critical behavior at high energies, with a crossover to a soft Coulomb gap $\Delta$ at low energies. We demonstrate that the single-particle excitations experience a localization transition (which belongs to the noninteracting universality class) at an energy $E=\pm E_c$. The mobility edge $E_c$ scales with the distance ${\mu }_c-\mu$ from the interacting critical point according to $E_c\propto {({\mu }_c-\mu )}^{\nu z}$, where $\nu$ and $z$ are the localization-length and the dynamical critical exponents. Local TDOS shows strong fluctuations and long-range correlations which reflect the multifractality associated with interacting and noninteracting fixed points as well the localization of low-energy excitations.