We studied the magnetoresistance of $R{\mathrm{TiO}}_3$ ($R=\text{Pr}$ and Ce) with offstoichiometry as a doped Mott-Hubbard system and found positive magnetoresistance proportional to the absolute value of the magnetic field in the antiferromagnetic phase of both insulating and metallic samples. This result indicates that not only the coherent state that appears with doping and is responsible for the metallic conduction, but also the incoherent state of the Mott-Hubbard insulator is affected by the spin configuration.

The magnetism of the oxygen deficient, $A$-site layered perovskite $\mathrm{TbBaM}{\mathrm{n}}_2{\mathrm{O}}_{5.75}$ has been studied by magnetic susceptibility vs temperature measurements and neutron powder diffraction. Four magnetic phases have been identified. Above 457 K a charge-melted state exists where ferromagnetic double-exchange interactions between the Mn ions dominate. Below 457 K charge ordering occurs and the exchange interactions become primarily antiferromagnetic. The magnetic moment in this phase is too large to be consistent with a charge ordering of $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ but could be explained by having two $e_g$ electrons being shared among three Mn centers, causing them to act as a single paramagnetic unit. This suggests that $S=11/2(\mathrm{M}{\mathrm{n}}^{3.33+}{)}_3$ polaron trimers may be present along with $\mathrm{M}{\mathrm{n}}^{3+}$. Below 203 K long-range ordering of the moments on the Mn ions occurs. Neutron powder diffraction data at 52 K reveal a magnetic structure containing an ↑↑↑↓ pattern of spins which is consistent with the formation of tripolarons. Below 11 K the $\mathrm{T}{\mathrm{b}}^{3+}$ ions also undergo long-range magnetic ordering.

In this project, a computational investigation utilizing density functional theory methods is carried out to elucidate the differences in stereochemical lone-pair activity of ${\mathrm{Pb}}^{2+}$ and ${\mathrm{Sn}}^{2+}$ A-site ions in epitaxial polar ${\mathrm{ATiO}}_3$ perovskites. The contrasting tendencies for the lead- and tin-based compounds to form different phases—$Amm2$ for the former versus $Cm$ for the latter—under biaxial tension are connected to the amount of charge concentrated within the lone pair lobes. Specifically, phases are energetically more preferable when as much charge as possible is dissipated out of the lobe, thus lowering the cost of Coulomb repulsions between the lone pair and the surrounding oxygen cage. Although a strong band-gap tuning was predicted in (fictitious) ${\mathrm{SnTiO}}_3$ during the tensile $P4mm\to Cm$ phase transformation [see W. D. Parker, J. M. Rondinelli, and S. M. Nakhmanson, Phys. Rev. B 84, 245126 (2011)], we find the same effect to be considerably weaker in ${\mathrm{PbTiO}}_3$. The insights gained about the electronic-level underpinnings of transitional behavior in such lone-pair-active epitaxial ferroelectrics may be used in the design of a new generation of more efficient electromechanical and electrooptical devices.

Dye-sensitized metal oxide polymer hybrid solar cells deliver a promising basis in organic solar cell development due to many conceptual advantages. Since the power conversion efficiency is still in a noncompetitive state, it has to be understood how the photocurrent contribution can be maximized (i.e., which dye-polymer properties are most beneficial for efficient charge generation in hybrid solar cells). By the comparison of three model systems for hybrid solar cells with $\mathrm{Ti}{\mathrm{O}}_2$-dye-polymer interfaces, this paper was aimed at elucidating the role of the exact mechanism of charge generation. In the exciton dissociation (ED) case, an exciton that is generated in the polymer is split at the dye-polymer interface. Alternatively, this exciton can be transferred to the dye via an energy transfer (ET), upon which charge separation occurs between dye and $\mathrm{Ti}{\mathrm{O}}_2$. For comparison, the third case is included in which the high lowest unoccupied molecular orbital of the dye does not allow exciton separation or ET from the dye to the polymer, so that the dye only is responsible for charge generation. To separate effects owing to differences in energy levels of the involved materials from the impact of local order and disorder in the polymer close to the interface, this paper further comprises a detailed analysis of the polymer crystallinity based on the H-aggregate model. While the massive impact of the poly(3-hexylthiophene) crystallinity on device function has been outlined for bare metal oxide-polymer interfaces, it has not been considered for hybrid solar cells with dye-sensitized $\mathrm{Ti}{\mathrm{O}}_2$. The results presented here indicate that all dye molecules in general influence the polymer morphology, which has to be taken into account for future optimization of hybrid solar cells. Apart from that, it can be suggested that ED on the polymer needs an additional driving force to work efficiently; thus, energy transfer seems to be currently the most promising strategy to increase the polymer photocurrent contribution.

For a III–V semiconductor quantum dot charged with a single hole, we investigate the feedback control of the nuclear spin bath through dynamical nuclear spin polarization. The scheme utilizes the hole-nuclear flip-flop by their anisotropic hyperfine interaction, where the flip direction of the nuclear spin can be conditioned on the sign of the overall hyperfine field through initialization processes that do not involve explicit measurement. We show that a negative feedback can be implemented to suppress the statistical fluctuations of the nuclear hyperfine field for enhancing the coherence time of the hole spin qubit. Positive feedback can prepare the nuclear spin ensemble into states where the nuclear hyperfine field distribution has two well separated peaks, realizing a quantum heat bath that cannot be described by a single effective temperature.

GaAs with an atomic monolayer of Cs is one of the best known photoemissive materials. The results of density functional theory calculations of Cs adsorption on the GaAs(100)-$\left(4\times 2\right)$ gallium-terminated reconstructed surface and the GaAs(110) surface are presented in this work. Coverage of up to 4 Cs ${\mathrm{atoms}/\mathrm{nm}}^2$ on GaAs surfaces has been studied to predict the work-function reduction and adsorption energies accurately. The high mobility of Cs atoms on the (110) surface allows formation of ordered structures, whereas the low mobility of Cs of the (100) surface causes amorphous growth.

We demonstrate efficient room-temperature photoluminescence and spectral tuning of epitaxially grown ZnSe/CdSe quantum well structures almost over the whole visible spectrum (470–600 nm wavelength). The key element to achieve the observed high quantum efficiency and enormous tuning range was the implementation of a special strain engineering technique, which allows us to suppress substantial lattice relaxation of CdSe on ZnSe. Previous studies indicated that a CdSe coverage exceeding 3 ML on ZnSe results in the formation of extensive lattice defects and complete quenching of the photoluminescence at low and room temperature. In contrast, our approach of strain engineering enables the deposition of planar CdSe quantum wells with a thickness ranging from 1 to 6 ML with excellent optical properties. We attribute the observed experimental features to a controllable strain compensation effect that is present in an alternating system of tensile and compressively strained epitaxial layers and supported this model by calculations of the transition energies of the ZnSe/CdSe quantum wells.

Recently, rhombohedral trilayer graphene (r-TLG) has attracted much attention because of its low-energy flat bands, which are predicted to result in many strongly correlated phenomena. However, there has been a need for more experimental evidence for these flat bands in the r-TLG, since the supporting substrates usually have strong destructive effects on the low-energy band structure of graphene systems. Here, we demonstrate that it is possible to directly probe the stacking order and electronic spectrum of the r-TLG on a graphite surface with scanning tunneling microscopy around a monoatomic step edge of the top graphene layer. The tunneling spectra of the r-TLG exhibit four adjacent peaks, which are generated by the low-energy flat bands, flanking the charge neutrality point. Based on these spectra, the true energy gap and the energy gap at the $K$ point of the r-TLG are determined as about 9 and 23 meV, respectively. The observed features are well reproduced by a low-energy effective Hamiltonian.

We determined experimentally the eigenvalues of two rectangular quantum billiards that contain circular scatterers forming a triangular grid, so-called Dirac billiards. For this we performed measurements of unprecedented accuracy using superconducting macroscopic-size microwave billiards that enclose a photonic crystal. The objective was the investigation of the peculiar features of the density of the eigenvalues (DOE), which resemble that of a graphene flake, and of their fluctuation properties. We identified in the measured resonance spectra Dirac points and in their adjacent bands the van Hove singularities (VHSs), that show up as sharp peaks in the DOE. The analysis of the experimental resonance frequencies and of the band structure, which was computed with a tight-binding model, revealed that the VHSs divide the associated band into regions where the system is governed by the nonrelativistic Schrödinger equation of the quantum billiard and the Dirac equation of the graphene billiard of corresponding shape, respectively. Furthermore, we demonstrate that Dirac billiards are most suitable for the modeling of idealized graphene. Indeed, the DOEs of both systems are well described by a finite tight-binding model which includes first-, second-, and third-nearest-neighbor couplings.

Structural transformations in a shallow surface region of a bulk Ag (001) target irradiated by a femtosecond laser pulse are investigated in large-scale atomistic simulations and experiments. The simulations reveal a complex interplay of fast laser melting, rapid resolidification, and dynamic relaxation of laser-induced stresses that leads to the formation of a subsurface porous region covered by a nanocrystalline surface layer. The generation of the porous region is consistent with the experimental observation of surface “swelling” occurring at laser fluences below the spallation/ablation threshold and may be related to the incubation effect in multipulse laser ablation of metals. The nanocrystalline layer is produced by massive nucleation of crystallites triggered by a deep undercooling of the melted surface region experiencing fast quenching at a rate on the order of ${10}^{11}\mathrm{K}/\mathrm{s}$. The predicted surface structure features random crystallographic orientation of nanograins and a high density of stacking faults, twins, and nanoscale twinned structural elements with fivefold symmetry, which suggests high hardness and possible enhancement of catalytic activity of the surface.

We develop a hydrodynamic description of transport properties in graphene-based systems, which we derive from the quantum kinetic equation. In the interaction-dominated regime, the collinear scattering singularity in the collision integral leads to fast unidirectional thermalization and allows us to describe the system in terms of three macroscopic currents carrying electric charge, energy, and quasiparticle imbalance. Within this “three-mode” approximation, we evaluate transport coefficients in monolayer graphene as well as in double-layer graphene-based structures. The resulting classical magnetoresistance is strongly sensitive to the interplay between the sample geometry and leading relaxation processes. In small, mesoscopic samples, the macroscopic currents are inhomogeneous, which leads to a linear magnetoresistance in classically strong fields. Applying our theory to double-layer graphene-based systems, we provide a microscopic foundation for a phenomenological description of giant magnetodrag at charge neutrality and find the magnetodrag and Hall drag in doped graphene.

Though the Fermi surface of surface states of a 3D topological insulator (TI) has zero magnetization, an arbitrary segment of the full Fermi surface has a unique magnetic moment consistent with the type of spin-momentum locking in hand. We propose a three-terminal set up, which directly couples to the magnetization of a chosen segment of a Fermi surface hence leading to a finite tunnel magnetoresistance (TMR) response of the nonmagnetic TI surface states, when coupled to spin polarized STM probe. This multiterminal TMR not only provides a unique signature of spin-momentum locking for a pristine TI but also provides a direct measure of momentum resolved out of plane polarization of hexagonally warped Fermi surfaces relevant for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, which could be as comprehensive as spin-resolved ARPES. Implication of this unconventional TMR is also discussed in the broader context of 2D spin-orbit (SO) materials.

We study the emergence of quasiparticles in Hund's metals with an $\mathrm{SU}\left(M\right)\times \mathrm{SU}\left(N\right)$-symmetric Kondo impurity model carrying both spin and orbital degrees of freedom. We show that the coupling of the impurity spin to the conduction electrons can be ferromagnetic, notably for hole-doped iron pnictides. We derive the weak-coupling renormalization group (RG) equations for arbitrary representations of $\mathrm{SU}\left(M\right)\times \mathrm{SU}\left(N\right)$. A ferromagnetic spin coupling results in a protracted RG flow, accounting for the surprising particle-hole asymmetry that is observed in the iron-pnictide systems. We establish the low coherence scale $T_{\mathrm{K}}$, which depends on the filling through the impurity representation. We also discuss the temperature dependence of the spin and orbital susceptibilities. Finally, we argue that this mechanism explains the strong valence dependence of the coherence scale observed in dilute transition-metal magnetic alloys.

${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is an intriguing material for which spin-orbit coupling plays a key role. Theoretical predictions have been made that the surface of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ should exhibit a clear signature of the quantum spin Hall effect. We studied the surface of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ using scanning tunneling microscopy and density-functional theory calculations. We observed atomic level resolution of the surface and two types of terminations with different surface periodicity and Na content. By comparing bias-dependent experimental topographic images to simulated images, we determined the detailed atomistic structure of both observed surfaces. One of these reveals a strong relaxation to the surface of Na atoms from the subsurface region two atomic layers below. Such dramatic structural changes well below the surface are highly unusual and cast doubt on any prediction of surface properties based on bulk electronic structure. Indeed, using spatially resolved tunneling spectroscopy, we found no indication of the predicted quantum spin Hall behavior.

Kinetic properties of a two-dimensional model of fermions interacting with antiferromagnetic spin excitations near the quantum critical point are considered. The temperature or doping are assumed to be sufficiently high, such that the pseudogap does not appear. In contrast to standard spin-fermion models, it is assumed that there are intrinsic inhomogeneities in the system suppressing space correlations of the antiferromagnetic excitations. It is argued that the inhomogeneities in the spin excitations in the “strange metal” phase can be a consequence of the existence of “$\pi$-shifted” domain walls in the doped antiferromagnetic phase. Averaging over the inhomogeneities and calculating physical quantities such as resistivity and some others one can explain unusual properties of cuprates unified under the name “marginal Fermi liquid.” The dependence of the slope of the linear temperature dependence of the resistivity on doping is compared with experimental data.

In this work, we demonstrate both theoretically and experimentally that nonperiodic metallic gratings can become transparent for broadband terahertz waves. It is shown that broadband high transmission appears in aperiodic metallic gratings (including quasiperiodic and disordered ones), which originates from the nonresonant excitations in the grating system. Quasiperiodic and disordered metallic gratings effectively weaken and even eliminate Wood's anomalies, which are the diffraction-related characters of periodic gratings. Consequently, both the transparence bandwidth and transmission efficiency are significantly increased due to the structural aperiodicity. An optimal condition is also achieved for broadband high transparency in aperiodic metallic gratings. Experimental measurements at the terahertz regime reasonably agree with both analytical analysis and numerical simulations. Furthermore, we show that for a specific light source, for example, a line source, a corresponding nonperiodic transparent grating can be also designed. We expect that our findings can be applied for transparent conducting panels, perfect white-beam polarizers, antireflective conducting solar cells, and beyond.

We report a systematic polarization-dependent angle-resolved photoemission spectroscopy study of the three-dimensional electronic structure of the recently discovered 112-type iron-based superconductor ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{FeAs}}_2$ $\left(x=0.1\right)$. Besides the commonly reported three holelike and two electronlike bands in iron-based superconductors, we resolve one additional holelike band around the zone center and one more fast-dispersing band near the $X$ point in the vicinity of the Fermi level. By tuning the polarization and the energy of incident photons, we are able to identify the specific orbital character and the $k_z$ dependence of all bands. Combining these results with band calculations, we find that As $4p_z$ and $4p_x\left(4p_y\right)$ orbitals contribute significantly to the additional three-dimensional holelike band and the narrow band, respectively. Also, there is considerable hybridization between the As $4p_z$ and Fe $3d$ orbitals in the additional holelike band, which suggests strong coupling between the unique arsenic zigzag bond layers and the FeAs layers therein. Our findings provide a comprehensive picture of the orbital character of the low-lying band structure of 112-type iron-based superconductors, which can be a starting point for the further understanding of their unconventional superconductivity.