Electrical transport measurements of the heavy-fermion compound YbFe${}_2$Zn${}_{20}$ were carried out under pressures up to 8.23 GPa and down to temperatures of nearly 0.3 K. The pressure dependence of the low-temperature Fermi-liquid state was assessed by fitting $\rho \left(T\right)={\rho }_0+A{T}^n$ with $n=2$ for $T<T_{\mathrm{FL}}$. Power-law analysis of the low-temperature resistivities indicates $n=2$ over a broad temperature range for $P\lesssim 5$ GPa. However, at higher pressures, the quadratic temperature dependence is only seen at the very lowest temperatures, and instead shows a wider range of $n<2$ power-law behavior in the low-temperature resistivities. As pressure was increased, $T_{\mathrm{FL}}$ diminished from $\sim$11 K at ambient pressure to $\sim$0.6 K at 8.23 GPa. Over the same pressure range, the $A$ parameter increased dramatically with a functional form of $A\propto {(P-P_{\mathrm{c}})}^{-2}$ with $P_{\mathrm{c}}\simeq 9.8$ GPa being the critical pressure for a possible quantum critical point.

Using advanced ab initio calculations, we describe the formation and confinement of a two-dimensional electron gas in short-period ($\simeq$4-nm) Nb-doped SrTiO${}_3$ superlattices as a function of Nb doping. We predict complete two-dimensional confinement for doping concentrations higher than 70$\%$. In agreement with previous observations, we find a large thermopower enhancement at room temperature. However, this effect is primarily determined by dilution of the mobile charge over a multitude of weakly occupied bands. As a general rule, we conclude that thermopower in similar heterostructures will be more enhanced by weak, rather than tight, spatial confinement.

The pressure evolution of the Raman spectrum of graphene grown by chemical vapor deposition on polycrystalline copper is investigated with the use of polar and nonpolar pressure transmitting media (PTM). The $G$ and 2$D$ Raman bands exhibit similar pressure slopes for both PTM irrespective of any unintentional initial doping and/or strain of the samples. Our analysis suggests that any pressure-induced charge transfer effects are too small to influence the pressure response of graphene; rather, it is determined by the compressibility of the substrate and the graphene interaction with the substrate and the PTM. For the nonpolar PTM, a peculiar pressure behavior of graphene is observed in the PTM solidification regime, which resembles that of freestanding graphene.

We investigate the effect of electronic correlations on the transmission phase of quantum coherent scatterers, considering quantum dots in the Coulomb blockade regime connected to two single-channel leads. We focus on transmission zeros and the associated $\pi$-phase lapses that have been observed in interferometric experiments. We numerically explore two types of models for quantum dots: (i) lattice models with up to eight sites, and (ii) resonant level models with up to six levels. We identify different regimes of parameters where the presence of electronic correlations is responsible for the increase or the decrease of the number of transmission zeros versus electrochemical potential on the dot. We show that within the two models considered, interaction effects do not reproduce the universal behavior of alternating resonances and phase lapses, experimentally observed in many-electron Coulomb blockaded dots.

We identify two elemental interatomic couplings that control the collaborative (as opposed to competing) interaction between the O${}_6$ octahedral rotations (usually called antiferrodistortive or AFD modes) and the antiferroelectric (AFE) displacement patterns of the $A$-site cations in oxides with the $\mathit{AB}$O${}_3$ perovskite structure. Straightforward analytical derivations allow us to reproduce and explain the origin of various long-range AFE orders that have been previously found in different phases of several perovskite compounds, all possessing simple or even complex long-range AFD patterns. Our analysis also leads to the prediction of original peculiar combinations of AFD and AFE orders that are awaiting to be observed.

We treat the ground state, elementary excitations, and neutron scattering cross section for a system of trimers consisting of three tightly bound spins $\frac{1}{2}$ on a distorted kagome lattice, subject to isotropic nearest-neighbor (usually antiferromagnetic) Heisenberg interactions. The interactions between trimers are assumed to be weak compared to the intratrimer interactions. We compare the spin-wave excitation spectrum of trimers with that obtained from standard spin-wave theory and attribute the differences at low energy to the fact that the trimer formulation includes exactly the effects of intratrimer zero-point motion.

The quantum critical antiferromagnetic (AFM) fluctuation spectra measured by inelastic neutron scattering recently in two heavy-fermion superconductors are used together with their other measured properties to calculate their $d$-wave superconducting transition temperatures $T_{\mathrm{c}}$. To this end, the linearized Eliashberg equations for $d$-wave superconductivity induced by AFM fluctuations are solved in models of fermions with various levels of nesting. The results for the ratio of $T_{\mathrm{c}}$ to the characteristic spin-fluctuation energy are well parametrized by a dimensionless coupling constant and the AFM correlation length. Comparing the results with experiments suggests that one may reasonably conclude that superconductivity in these compounds is indeed caused by AFM fluctuations. This conclusion is strengthened by a calculation with the same parameters of the measured coefficient of the normal-state quantum-critical resistivity $\propto T^{3/2}$ characteristic of Gaussian AFM quantum-critical fluctuations. The calculations give details of the superconducting coupling as a function of the correlation length and the integrated fluctuation spectra useful in other compounds.

An emerging area in condensed matter physics is the use of multilayered heterostructures to enhance ferroelectricity in complex oxides. Here we demonstrate that optically pumping carriers across the interface between thin films of a ferroelectric (FE) insulator and a ferromagnetic metal can significantly enhance the FE polarization. The photoinduced FE state remains stable at low temperatures for over one day. This occurs through screening of the internal electric field by the photoexcited carriers, leading to a larger, more stable polarization state that may be suitable for applications in areas such as data and energy storage.

The contribution of a built-in electric field to ferroelectric phase transition in asymmetric ferroelectric tunnel junctions is studied using a multiscale thermodynamic model. It is demonstrated in detail that there exists a critical thickness at which an unusual ferroelectric-“polar nonferroelectric” phase transition occurs in asymmetric ferroelectric tunnel junctions. In the “polar nonferroelectric” phase, there is only one nonswitchable polarization which is caused by the competition between the depolarizing field and the built-in field, and closurelike domains are proposed to form so as to minimize the system energy. The transition temperature is found to decrease monotonically as the ferroelectric barrier thickness is decreased and the reduction becomes more significant for the thinner ferroelectric layers. As a matter of fact, the built-in electric field not only results in smearing of the phase transition, but also forces the transition to take place at a reduced temperature. Such findings may impose a fundamental limit on the work temperature and thus should be further taken into account in the future ferroelectric-tunnel-junction–type or ferroelectric-capacitor–type devices.

The formation and field-induced switching of defect dipoles in acceptor doped lead titanate is described by a kinetic model representing an extension of the well established Arlt-Neumann model [Ferroelectrics 76, 303 (1987)]. Energy barriers for defect association and reorientation of oxygen vacancy-dopant (Cu and Fe) complexes are obtained from first-principles calculations and serve as input for kinetic coefficients of the rate equation model. The numerical solution of the model describes the time evolution of the oxygen vacancy distribution at different temperatures and dopant concentrations in the presence or absence of an alternating external field. We predict the characteristic time scale for the alignment of all defect dipoles with the spontaneous polarization of the surrounding matrix. In this state the defect dipoles act as obstacles for domain wall motion and contribute to the experimentally observed aging. Under cycling conditions the fully aligned configuration is perturbed and a dynamic equilibrium is established with defect dipoles in parallel and antiparallel orientation relative to the spontaneous polarization. This process can be related to the deaging behavior of piezoelectric ceramics.

The magnetic skyrmions as derived from the proper screw spin state in chiral-lattice magnets (e.g., MnSi) show the intrinsic spin swirling direction or the magnetic helicity. The crystal chirality (${\Gamma }_{\mathrm{c}}$) and skyrmion helicity (${\gamma }_{\mathrm{s}}$) were simultaneously determined at the identical specimen areas of MnSi and Fe${}_{1-x}$Co${}_x$Si ($x=0.2,0.5$) by using convergent-beam electron diffraction and Lorentz transmission electron microscopy techniques, respectively. The reversal of the ${\Gamma }_{\mathrm{c}}$-${\gamma }_{\mathrm{s}}$ correlation is observed between MnSi and Fe${}_{1-x}$Co${}_x$Si and ascribed to the different sign of the Dzyaloshinskii-Moriya interaction between both compounds.

Heavy fermion materials naturally combine strong spin-orbit interactions and electronic correlations. When there is precisely one conduction electron per impurity spin, the coherent heavy fermion state is insulating. This Kondo insulating state has recently been argued to belong to the class of quantum spin Hall states. Motivated by this conjecture and a very recent experimental realization of this state, we investigate a model for Kondo insulators with spin-orbit coupling. Using dynamical mean-field theory we observe an interaction-driven transition between two distinct topological states, indicated by a closing of the bulk gap and a simultaneous change of topological invariants as obtained in a classification which takes into account lattice symmetries. At large interaction strength we find a topological heavy fermion state, characterized by strongly renormalized heavy bulk bands, hosting a pair of zero-energy edge modes. The model allows a detailed understanding of the temperature dependence of the single particle spectral function and in particular the energy scales at which one observes the appearance of edge states within the bulk gap.

We study the Gutzwiller method for the spinless fermion model in one dimension, which is one of the simplest models that incorporates the intersite Coulomb interaction. The Gutzwiller solution of this model has been studied in the literature but with differing results. We obtain the Gutzwiller solution of the problem by a careful enumeration of the many-particle configurations and explain the origin of the discrepancy in the literature to be due to the neglect of the correlation between the neighboring bond occupancies. The correct implementation of the Gutzwiller approach yields results different from the slave-boson mean-field theory, unlike for the Hubbard model with on-site interaction, where both methods are known to be equivalent. The slave-boson and the Gutzwiller results are compared to the exact solution, available for the half-filled case, and to the numerical exact diagonalization results for the case of general filling.

The valence band offsets of the CuInSe${}_2$/CdS and CuInSe${}_2$/ZnS (110) interfaces are obtained based on various definitions using first-principles calculations in the framework of hybrid density functional theory. Both the strained band offset and the unstrained, or natural, band offset are investigated, where the two phases share and do not share in-plane lattice parameters perpendicular to the stacking direction, respectively. The valence band offset is determined by first obtaining the difference between the reference levels of two phases in the regions far from the interface and then adding the difference between the valence band maximum and the reference levels of bulk for the two phases. The nonfaceted (110) interface and a number of (112)/(11$\overline{2}$) faceted interfaces, some containing ordered point defects in the CuInSe${}_2$ (CIS) region, are considered. The excess energies of CIS/CdS and CIS/ZnS interfaces are lower when there are no ordered point defects, in contrast to the CIS surfaces that stabilize with ordered defect formation. The valence band offset is not significantly dependent on the atomic configurations at the interface as long as there are no charged layers. Surface calculations suggest that the reference level, which is determined by the average electrostatic potential at the atomic site, is not strongly dependent on lattice strain. A definition of the natural valence band offset that assumes a strain-invariant difference in the reference levels of the two phases provides values almost independent of the in-plane lattice parameters used in the interface calculation, which are about $-$1.2 and $-$1.3 eV with respect to CIS for the CIS/CdS and CIS/ZnS interfaces that contain no charged layers, respectively. The ionization potential difference can differ from the natural valence band offset by up to 0.3 eV without any consistent tendency to overestimate or underestimate, showing that the ionization potential difference is not necessarily a reasonable measure of the natural valence band offset.

Low-temperature transport experiments on a $p$-type GaAs quantum dot capacitively coupled to a quantum point contact are presented. The time-averaged as well as time-resolved detection of charging events of the dot are demonstrated and they are used to extract the tunneling rates into and out of the quantum dot. The extracted rates exhibit a super-linear enhancement with the bias applied across the dot, which is interpreted in terms of a dense spectrum of excited states contributing to the transport, characteristic for heavy hole systems. The full counting statistics of charge transfer events and the effect of back action is studied. The normal cumulants as well as the recently proposed factorial cumulants are calculated and discussed in view of their importance for interacting systems.

To adjust the charging state of a molecular metal complex in the context of a density functional theory description of coherent electron transport through single-molecule junctions, we correct for self-interaction effects by fixing the charge on a counterion, which in our calculations mimics the effect of the gate in an electrochemical scanning tunneling microscope setup, with two competing methods, namely, the generalized $\Delta$self consistent field ($\Delta$SCF) technique and screening with solvation shells. One would expect a transmission peak to be pinned at the Fermi energy for a nominal charge of $+$1 on the molecule in the junction, but we find a more complex situation in this multicomponent system defined by the complex, the leads, the counterion, and the solvent. In particular the equilibrium charge transfer between the molecule and the leads plays an important role, which we investigate in relation to the total external charge in the context of electronegativity theory.

We report the observation of an abnormal carrier transport phenomenon in polycrystalline semiconducting graphene grown by solid carbon source molecular beam epitaxy. At the lowest temperatures in samples with small grain size, the conduction does not obey the two-dimensional Mott-type variable-range hopping (VRH) conduction often reported in semiconducting graphene. The hopping exponent $p$ is found to deviate from the 1/3 value expected for Mott VRH with several samples exhibiting a $p=2/5$ dependence. We also show that the maximum energy difference between hopping sites is larger than the activation energy for nearest-neighbor hopping, violating the assumptions of the Mott model. The 2/5 dependence more closely agrees with the quasi-one-dimensional VRH model proposed by Fogler, Teber, and Shklovskii (FTS). In the FTS model, conduction occurs by tunneling between neighboring metallic wires. We suggest that metallic edge states and conductive grain boundaries play the role of the metallic wires in the FTS model.

Periodic repetitions of nonaligned and finite-sized graphene nanoribbon domains, known as graphene nanowiggles, can be synthesized using a surface-assisted bottom-up approach. They have been predicted to possess unusual properties such as tunable band gaps and versatile magnetic behaviors. Here, a first-principles many-body Green's function approach within the $GW$ approximation is used to accurately compute their band gaps, which are in the range 0.00–3.65 eV depending on geometry and magnetism. We also perform spin-polarized density functional theory calculations to demonstrate that the previously predicted complex spin states for free-standing nanowiggles are not significantly altered by the presence of the gold substrate on which they are synthesized.

We investigate the polarization dependence of localization length in alternating isotropic-birefringent stacks with uncorrelated thickness disorder. The birefringent layers can be positive uniaxial, negative uniaxial, or a mixture of both. Stacks which contain a mixture are shown to suppress the Brewster delocalization anomalies and, over all incident angles, exhibit $p$-polarization localization length maxima that are of similar magnitude to normal incidence. Furthermore, we propose a parameter set that enables the $p$-polarization localization length to monotonically decrease with angle of incidence. This investigation was inspired by weakly polarizing mirrors on the sides of silvery fish and provides a generic means to produce polarization-insensitive, broadband reflections from a random, all-dielectric layered medium.

To understand the origin of the nodal gap structure realized in BaFe${}_2$(As,P)${}_2$, we study the three-dimensional gap structure based on the three-dimensional ten-orbital Hubbard model with quadrupole interaction. In this model, strong spin and orbital fluctuations develop when the random-phase approximation is used. By solving the Eliashberg gap equation, we obtain the fully gapped $s$-wave state with (without) sign reversal between holelike and electronlike Fermi surfaces due to strong spin (orbital) fluctuations, the so-called $s_{\pm }$-wave ($s_{++}$-wave) state. When both spin and orbital fluctuations develop strongly, which will be realized near the orthorhombic phase, we obtain a nodal $s$-wave state in the crossover region between the $s_{++}$-wave and $s_{\pm }$-wave states. The nodal $s$-wave state obtained possesses loop-shaped nodes on electronlike Fermi surfaces, due to the competition between attractive and repulsive interactions in $\mathit{k}$ space. In contrast, the superconducting gaps on the holelike Fermi surfaces are fully gapped due to orbital fluctuations. The present study explains the main characteristics of the anisotropic gap structure in BaFe${}_2$(As,P)${}_2$ observed experimentally.