Assessing the equilibrium morphologies of self-assembled heteroepitaxial quantum dots requires the estimation of their elastic (volumetric), surface, and edge energy contribution, all of them being shape dependent. Due to the size and multifaceted morphology of these islands, the estimation of the first term is typically a time-consuming or complicated task. A general rule to predict it from the sole morphologies would guarantee a precious advantage in this field. Here we present an interpolating function to fulfill this purpose for the prototypical systems of Ge/Si and InAs/GaAs. The trend is first extracted from a systematic analysis of realistic shapes observed on (001) substrates. It is then tested and corroborated for selected vicinal (tilted) substrates. Finally, the deviations due to intermixing and the underlying wetting layer are quantified. Of fundamental importance in this process is the identification of a morphological descriptor more accurate than the widely adopted aspect ratio, the limitations of which are discussed.

We present an approach to determine the onset of contact between a tip and a surface. The real contact area depending on the distance is calculated using Bader's quantum theory of atoms in molecules. The jump to contact, which is often observed in atomic force microscopy experiments, is used as an indicator for the initial point of contact, which in turn is defined by atomic relaxations and thus without the need of external parameters. Within our approach the contact area is estimated by evaluating the zero flux surfaces between the touching Bader atoms, where the necessary electronic density cutoff for the Bader partitioning is calculated to depend on the initial point of contact. Our proposed approach is therefore completely ab initio and we are able to define and calculate the real area of contact without imposing restrictions or free parameters. As a prototype system we choose a tip made of a ten-atom tungsten pyramid above a moiré layer of graphene on an fcc iridium (111) substrate. We find that the contact area depends exponentially on the effective distance between the tip apex and the surface atom directly below within the atomically relaxed nanosystem.

We show that recent spin-wave resonance studies of (Ga,Mn)As thin films performed by Liu et al. [Phys. Rev. B 75, 195220 (2007)] reveal a substantial increase in the in-plane uniaxial anisotropy at the film surface with respect to the bulk value (surface uniaxial anisotropy field $H_{2\parallel }^{\mathrm{surf}}=305$ Oe against the bulk value $H_{2\parallel }^{\mathrm{bulk}}=77$ Oe). At the same time, the cubic anisotropy at the film surface proves substantially lower than in the bulk ($H_{4\parallel }^{\mathrm{surf}}=39$ Oe against $H_{4\parallel }^{\mathrm{bulk}}=197$ Oe).

In this contribution we demonstrate structural control over a transport resonance in $\mathrm{HS}{\left({\mathrm{CH}}_2\right)}_n[1,4-{\mathrm{C}}_6{\mathrm{H}}_4]{\left({\mathrm{CH}}_2\right)}_n\mathrm{SH}$ $(n=1$, 3, 4, 6) metal-molecule-metal junctions, fabricated and tested using the scanning tunneling microscopy-based $I\left(z\right)$ method. The Breit-Wigner resonance originates from one of the arene $\pi$-bonding orbitals, which sharpens and moves closer to the contact Fermi energy as $n$ increases. Varying the number of methylene groups thus leads to a very shallow decay of the conductance with the length of the molecule. We demonstrate that the electrical behavior observed here can be straightforwardly rationalized by analyzing the effects caused by the electrostatic balance created at the metal-molecule interface. Such resonances offer future prospects in molecular electronics in terms of controlling charge transport over longer distances, and also in single-molecule conductance switching if the resonances can be externally gated.

The connectivity and tunability of superconducting qubits and resonators provide us with an appealing platform to study the many-body physics of microwave excitations. Here we present a multiconnected Jaynes-Cummings lattice model which is symmetric with respect to the nonlocal qubit-resonator couplings. Our calculation shows that this model exhibits a Mott insulator-superfluid-Mott insulator phase transition at commensurate fillings, featured by symmetric quantum critical points. Phase diagrams in the grand canonical ensemble are also derived, which confirm the incompressibility of the Mott insulator phase. Different from a general-purposed quantum computer, it only requires two operations to demonstrate this phase transition: the preparation and the detection of commensurate many-body ground state. We discuss the realization of these operations in a superconducting circuit.

We address the recently proposed orbital transition scenario in heavy fermion compounds exhibiting two superconducting domes. Our method of choice is nonresonant inelastic x-ray scattering (NIXS) which probes charge densities and can be modeled quantitatively. Our NIXS results of ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$, a model system of this class of materials, are well described by considering the same sequence of orbitals at all temperatures, thus putting into question whether orbital transitions are related to the formation of the second superconducting dome. The discrepancy between experimental findings and prediction could be due to the lack of accuracy in the theoretical description of the $4f$ states.

Motivated by the unconventional superconductivity observed in heavy-fermion metals, we investigate pairing susceptibilities near a continuous quantum phase transition of the Kondo-destruction type. We solve two-impurity Bose-Fermi Anderson models with Ising and Heisenberg forms of the interimpurity exchange interaction using continuous-time quantum Monte Carlo and numerical renormalization-group methods. Each model exhibits a Kondo-destruction quantum critical point separating Kondo-screened and local-moment phases. For antiferromagnetic interimpurity exchange interactions, singlet pairing is found to be enhanced in the vicinity of the transition. Implications of this result for heavy-fermion superconductivity are discussed.

We report on the modification of the $g$ factor by an in-plane electric field in an ${\mathrm{In}}_{0.026}{\mathrm{Ga}}_{0.974}\mathrm{As}$ epilayer. We performed external magnetic field scans of the Faraday rotation of the InGaAs film in order to independently determine the $g$ factor and the spin-orbit fields. The $g$ factor increases from $-0.4623\pm 0.0001$ at 0 V/cm to $-0.4570\pm 0.0001$ at 40 V/cm applied along the $\left[1\overline{1}0\right]$ crystal axis. The change in $g$ factor with electric field can have a large effect on the determination of the internal spin-orbit and nuclear fields from Larmor precession frequency measurements.

We show that the energy gap induced by ferromagnetically aligned magnetic impurities on the surface of a topological insulator can be filled, due to scattering off the nonmagnetic potential of the impurities. In both a continuum surface model and a three-dimensional tight-binding lattice model, we find that the energy gap disappears already at weak potential scattering as impurity resonances add spectral weight at the Dirac point. This can help explain seemingly contradictory experimental results as to the existence of a gap.

We combined high field magnetization and magneto-optical spectroscopy to investigate spin-charge coupling in Mn-substituted rare-earth indium oxides of chemical formula $R{\mathrm{In}}_{1-x}{\mathrm{Mn}}_x{\mathrm{O}}_3$ ($R=\text{Tb}$, Dy). The edge states, on-site ${\mathrm{Mn}}^{3+}d$ to $d$ excitations, and rare-earth $f$-manifold excitations all track the magnetization energy due to dominant Zeeman interactions. The field-induced modifications to the rare-earth excitations are quite large because spin-orbit coupling naturally mixes spin and charge, suggesting that the next logical step in the design strategy should be to bring spin-orbit coupling onto the trigonal bipyramidal chromophore site with a 4 or $5d$ center.

The physical properties of the single-crystalline samples of ${\mathrm{Lu}}_2{\mathrm{Ir}}_3{\mathrm{Si}}_5$ have been investigated by magnetic susceptibility, resistivity, and heat capacity studies. We observed multiple charge-density-wave (CDW) transitions in all the measurements. A strong thermal hysteresis at these transitions suggests a possible first order CDW ordering. In addition, the first order nature is ascertained by a very narrow and a huge cusp (62 J/mol K) in the zero field specific heat data which also suggests strong electron-phonon interchain coupling.

In this paper we present an atomic-resolution electron microscopy study of superlattices (SLs) where the colossal magnetoresistant manganite $\mathrm{L}{\mathrm{a}}_{0.67}\mathrm{C}{\mathrm{a}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ (LCMO) and the high critical temperature superconducting cuprate $\mathrm{L}{\mathrm{a}}_{1.85}\mathrm{S}{\mathrm{r}}_{0.15}\mathrm{Cu}{\mathrm{O}}_4$ (LSCO) are combined. Although good quality epitaxial growth can be achieved, both the choice of substrate and the relatively large lattice mismatch between these materials (around 2%) have a significant impact on the system properties [Phys. C 468, 991 (2008); Nature (London) 394, 453 (1998)]. Our samples, grown by pulsed laser deposition, are epitaxial and exhibit high structural quality. By means of cutting-edge electron microscopy and spectroscopy techniques we still find that the epitaxial strain is accommodated by a combination of defects, such as interface steps and antiphase boundaries in the manganite. These defects result in inhomogeneous strain fields through the samples. Also, some chemical inhomogeneities are detected, up to the point that novel phases nucleate. For example, at the LCMO/LSCO interface the $\mathrm{AB}{\mathrm{O}}_3$-type manganite adopts a tetragonal LSCO-like structure forming localized layers that locally resemble the composition of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{C}{\mathrm{a}}_{4/3}\mathrm{Mn}{\mathrm{O}}_4$. Structural distortions are detected in the cuprate as well, which may extend over lateral distances of several unit cells. Finally, we also analyze the influence of the substrate-induced strain by examining superlattices grown on two different substrates: ${\left(\mathrm{LaAl}{\mathrm{O}}_3\right)}_{0.3}{\left(\mathrm{S}{\mathrm{r}}_2\mathrm{AlTa}{\mathrm{O}}_6\right)}_{0.7}$ (LSAT) and $\mathrm{LaSrAl}{\mathrm{O}}_4$ (LSAO). We observe that SLs grown on LSAT, which are nonsuperconducting, present reduced values of the $c$ axis compared to superlattices grown on LSAO (which are fully superconducting). This finding points to the fact that the proper distance between copper planes in LSCO is essential in obtaining superconductivity in cuprates.

Recently the resistance saturation at low temperature in the Kondo insulator ${\mathrm{SmB}}_6$, a long-standing puzzle in condensed matter physics, was proposed to originate from a topological surface state. Here we systematically studied the magnetoresistance of ${\mathrm{SmB}}_6$ at low temperature up to 55 T. For temperature decreasing below 16 K, the temperature-dependent magnetoresistance exhibits a negative magnetoresistance, while the angular-dependent magnetoresistance shows a fourfold symmetry. Below 5 K, both temperature- and angular-dependent magnetoresistances show a similar crossover behavior in which the negative magnetoresistance is strongly suppressed and a twofold angular-dependent magnetoresistance appears. Furthermore, the angular-dependent magnetoresistance on a different crystal face confirms a two-dimensional surface state as the origin of magnetoresistance crossover below 5 K. Based on a two-channels model consisting of both surface and bulk states, the critical magnetic field $\left(H_c\right)$ of 125 T for field-dependent insulating behavior is extracted from our temperature-dependent resistance under different magnetic fields. Our results have important implications in understanding the novel low-temperature transport behavior in ${\mathrm{SmB}}_6$.

Topological insulators are a novel materials platform with high applications potential in fields ranging from spintronics to quantum computation. In the ongoing scientific effort to demonstrate controlled manipulation of their electronic structure by external means, i.e., the provision of knobs with which to tune properties, stoichiometric variation and surface decoration are two effective approaches that have been followed. In angle-resolved photoelectron spectroscopy (ARPES) experiments, both approaches are seen to lead to electronic band-structure changes. Most importantly, such approaches result in variations of the energy position of bulk and surface-related features and the creation of two-dimensional electron gases. The data presented here demonstrate that a third manipulation handle is accessible by utilizing the amount of super-band-gap light a topological insulator surface has been exposed to under typical ARPES experimental conditions. Our results show that this third knob acts on an equal footing with stoichiometry and surface decoration as a modifier of the electronic band structure, and that it is in continuous and direct competition with the latter. The data clearly point towards surface photovoltage and photoinduced desorption as the physical phenomena behind modifications of the electronic band structure under exposure to high-flux photons. We show that the interplay of these phenomena can minimize and even eliminate the adsorbate-related surface band bending on typical binary, ternary, and quaternary Bi-based topological insulators. Including the influence of the sample temperature, these data set up a detailed framework for the external control of the electronic band structure in topological insulator compounds in an ARPES setting. Four external knobs are available: bulk stoichiometry, surface decoration, temperature, and photon exposure. These knobs can be used in conjunction to fine tune the band energies near the surface and consequently influence the topological properties of the relevant electronic states.

To clarify the origin of a charge-density wave (CDW) phase in $1T-{\mathrm{TiSe}}_2$, we study the ground-state property of a half-filled two-band Hubbard model in a triangular lattice including electron-phonon interaction. By using the variational Monte Carlo method, the electronic and lattice degrees of freedom are both treated quantum mechanically on an equal footing beyond the mean-field approximation. We find that the cooperation between Coulomb interaction and electron-phonon interaction is essential to induce the CDW phase. We show that the “pure” exciton condensation without lattice distortion is difficult to realize under the poor nesting condition of the underlying Fermi surface. Furthermore, by systematically calculating the momentum-resolved hybridization between the two bands, we examine the character of electron-hole pairing from the viewpoint of BCS-BEC crossover within the CDW phase and find that the strong-coupling BEC-like pairing dominates. We therefore propose that the CDW phase observed in $1T-{\mathrm{TiSe}}_2$ originates from a BEC-like electron-hole pairing.

Dynamical phase transitions can occur in isolated quantum systems that are brought out of equilibrium by sudden parameter changes. We discuss the characterization of such dynamical phase transitions based on the statistics of produced excitations. We consider both the $O\left(N\right)$ model in the large-$N$ limit and a spin model with long-range interactions and show that the dynamical criticality of their prethermal steady states manifests most dramatically not in the average number of excitations but in their higher moments. We argue that the growth of defect fluctuations carries unique signatures of the dynamical criticality, irrespective of the precise details of the model. Our theoretical results should be relevant to quantum quench experiments with ultracold bosonic atoms in optical lattices.

We consider conductance mapping of systems based on the two-dimensional electron gas with scanning gate microscopy using two or more tips of an atomic force microscope. The paper contains the results of numerical simulations for a model tip potential. In addition, a few procedures are proposed for the extraction and manipulation of ballistic transport properties. In particular, we demonstrate that the multitip techniques can be used to obtain a readout of the Fermi wavelength, to detect potential defects, to filter specific transverse modes, and to tune the system into resonant conditions under which a stable map of the local density of states can be extracted from conductance maps using a third tip.