The elastoresistivity tensor $m_{ij,kl}$ characterizes changes in a material's resistivity due to strain. As a fourth-rank tensor, elastoresistivity can be a uniquely useful probe of the symmetries and character of the electronic state of a solid. We present a symmetry analysis of $m_{ij,kl}$ (both in the presence and absence of a magnetic field) based on the crystalline point group, focusing for pedagogic purposes on the $D_{4h}$ point group (of relevance to several materials of current interest). We also discuss the relation between $m_{ij,kl}$ and various thermodynamic susceptibilities, particularly where they are sensitive to critical fluctuations proximate to a critical point at which a point-group symmetry is spontaneously broken.

Topological insulators in the presence of a strong Coulomb interaction constitute novel phases of matter. Transitions between these phases can be driven by single-particle or many-body effects. On the basis of ab initio calculations, we identify a concrete material, i.e., ${\mathrm{Ca}}_2{\mathrm{PtO}}_4$, that turns out to be a hole-doped weak topological insulator. Interestingly, the Pt $d$ orbitals in this material are relevant for the band inversion that gives rise to the topological phase. Therefore, Coulomb interactions should be of importance in ${\mathrm{Ca}}_2{\mathrm{PtO}}_4$. To study the influence of interactions on the weak topological insulating phase, we look at a toy model corresponding to a layer-stacked three-dimensional version of the Bernevig-Hughes-Zhang model with local interactions. For a low to intermediate interaction strength, we discover novel interaction-driven topological phase transitions between the weak topological insulator and two Dirac semimetal phases. The latter correspond to gapless topological phases. For strong interactions, the system eventually becomes a Mott insulator.

We discuss how to analytically obtain an essentially infinite matrix product state (MPS) representation of the ground state of the XY model. On one hand this allows us to illustrate how the Ornstein-Zernike form of the correlation function emerges in the exact case using standard MPS language. On the other hand we study the consequences of truncating the bond dimension of the exact MPS, which is also part of many tensor network algorithms, and analyze how the truncated MPS transfer matrix is representing the dominant part of the exact quantum transfer matrix. In the gapped phase we observe that the correlation length obtained from a truncated MPS approaches the exact value following a power law in effective bond dimension. In the gapless phase we find a good match between a state obtained numerically from standard MPS techniques with finite bond dimension and a state obtained by effective finite imaginary time evolution in our framework. This provides a direct hint for a geometric interpretation of finite entanglement scaling at the critical point in this case. Finally, by analyzing the spectra of transfer matrices, we support the interpretation put forward by V. Zauner et al. [New J. Phys. 17, 053002 (2015)] that the MPS transfer matrix emerges from the quantum transfer matrix though the application of Wilson's numerical renormalization group along the imaginary-time direction.

We clarify the relation between two kinds of statistics for particle excitations in planar systems: the braid statistics of anyons and the Haldane exclusion statistics (HES). It is shown nonperturbatively that the HES exists for incompressible anyon liquid in the presence of a Hall response. We also study the statistical properties of a specific quantum anomalous Hall model with Chern-Simons term by perturbation in both compressible and incompressible regimes, where the crucial role of edge states to the HES is shown.

A two-dimensional network model representing an array of coupled orbitals of nonzero orbital momenta has been used to illustrate the origin of the topological phase transformation between insulators of different Chern number. The model is based on the standard tight-binding approach, improved by inclusion of wave-function phases of orbital states. For a coupling stronger than a critical value the existence of chiral edge states has been observed. The corresponding anomalous Hall conductivity becomes quantized, as experimentally observed in magnetic topological insulators.

We present an analysis of the polymorphic energy ordering and properties of the rocksalt and zinc-blende structures of manganese oxide using fixed node diffusion Monte Carlo (DMC). Manganese oxide is a correlated, antiferromagnetic material that has proven to be challenging to model from first principles across a variety of approaches. Unlike conventional density functional theory and some hybrid functionals, fixed node diffusion Monte Carlo finds the rocksalt structure to be more stable than the zinc-blende structure, and thus recovers the correct energy ordering. Analysis of the site-resolved charge fluctuations of the wave functions according to DMC and other electronic structure descriptions gives insights into elements that are missing in other theories. While the calculated band gaps within DMC are in agreement with predictions that the zinc-blende polymorph has a lower band gap, the gaps themselves overestimate reported experimental values.

Using a deterministic single P donor placed with atomic precision accuracy next to a nanoscale silicon quantum dot, we present a way to analyze the energy spectrum of small quantum dots in silicon by tunnel-coupled transport measurements. The energy-level structure of the quantum dot is observed as resonance features within the transport bias triangles when the donor chemical potential is aligned with states within the quantum dot as confirmed by a numeric rate equation solver SIMON. This technique allows us to independently extract the quantum dot level structure irrespective of the density of states in the leads. Such a method is useful for the investigation of silicon quantum dots in the few-electron regime where the level structure is governed by an intricate interplay between the spin- and the valley-orbit degrees of freedom.

Snake states are open trajectories for charged particles propagating in two dimensions under the influence of a spatially varying perpendicular magnetic field. In the quantum limit they are protected edge modes that separate topologically inequivalent ground states and can also occur when the particle density rather than the field is made nonuniform. We examine the correspondence of snake trajectories in single-layer graphene in the quantum limit for two families of domain walls: (a) a uniform doped carrier density in an antisymmetric field profile and (b) antisymmetric carrier distribution in a uniform field. These families support different internal symmetries but the same pattern of boundary and interface currents. We demonstrate that these physically different situations are gauge equivalent when rewritten in a Nambu doubled formulation of the two limiting problems. Using gauge transformations in particle-hole space to connect these problems, we map the protected interfacial modes to the Bogoliubov quasiparticles of an interfacial one-dimensional $p$-wave paired state. A variational model is introduced to interpret the interfacial solutions of both domain wall problems.

In this study, we analyze the interaction between excitons and longitudinal-optical phonons (LO phonons) in single, wurtzite GaN quantum dots (QDs) by means of micro-photoluminescence $\left(\mu \mathrm{PL}\right)$ spectroscopy. We report on Stokes-shifted emission lines measured for hundreds of single QDs. A decrease of the Huang-Rhys factor $\left(\sim 0.5\text{–}0.01\right)$ is observed with increasing QD emission energy that can be modeled in an adiabatic approximation applying two-particle eight-band $\mathbf{k}·\mathbf{p}$ wave functions. In order to obtain the QD dimensions and shape needed for these calculations, we conduct a scanning transmission electron microscope (STEM) analysis, not only focusing on the QD dimensions, but also on alloying effects. The QD height is identified as the most detrimental parameter for the exciton-LO-phonon interaction in strongly polar QD systems based on nitrides. Additionally, we extract the LO-phonon energy for a significant number of individual QDs from our $\mu \mathrm{PL}$ data set scaling in-between the bulk values for the QD (GaN) and the matrix material (AlN). Such a large variation of the LO-phonon energy cannot be explained by the alloying effects attested by our STEM analysis. Hence, the exciton-LO-phonon interaction resides in a volume that encloses the QD in the growth direction and a fraction of the matrix material depending on the QD height. We approximate this exciton-LO-phonon interaction volume by a sphere with a constant diameter of $2.6\pm 0.2$ nm.

We show that any heat definition expressed as an energy change in the reservoir energy plus any fraction of the system-reservoir interaction is not an exact differential when evaluated along reversible isothermal transformations, except when that fraction is zero. Even in that latter case the reversible heat divided by temperature, namely entropy, does not satisfy the third law of thermodynamics and diverges in the low temperature limit. These results are found within the framework of nonequilibrium Green functions (NEGF) using a single level quantum dot strongly coupled to fermionic reservoirs and subjected to a time-dependent protocol modulating the dot energy as well as the dot-reservoir coupling strength.

We measure the dissipation and frequency shift of a magnetically coupled cantilever in the vicinity of a silicon chip, down to 25 mK. The dissipation and frequency shift originates from the interaction with the unpaired electrons, associated with the dangling bonds in the native oxide layer of the silicon, which form a two-dimensional system of electron spins. We approach the sample with a $3.43 \mu \mathrm{m}$-diameter magnetic particle attached to an ultrasoft cantilever and measure the frequency shift and quality factor as a function of temperature and the distance. Using a recent theoretical analysis [J. M. de Voogd et al., arXiv:1508.07972] of the dynamics of a system consisting of a spin and a magnetic resonator, we are able to fit the data and extract the relaxation time $T_1=0.39\pm 0.08$ ms and spin density $\sigma =0.14\pm 0.01$ spins per $\mathrm{nm}{}^2$. Our analysis shows that at temperatures $\le 500$ mK magnetic dissipation is an important source of noncontact friction.

The effects of the bonding mechanism and band alignment in a ferroelectric (FE) ${\mathrm{BaTiO}}_3$/ferromagnetic ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_3$ heterostructure are studied using x-ray photoelectron spectroscopy and first-principles calculations. The band lineup at the interface is determined by a combination of band bending and polarization-induced modification of core-hole screening. A Schottky barrier height for electrons of $1.22\pm 0.17$ eV is obtained in the case of downwards FE polarization of the top layer. The symmetry of the bonding states is emphasized by integrating the local density of states $\pm 0.2$ eV around the Fermi level, and strong dependence on the FE polarization is found: upwards, polarization stabilizes Ti $t_{2g}\left(xy\right)$ orbitals, while downwards, polarization favors Ti $t_{2g}\left(yz\right)$ symmetry. It is predicted that the abrupt $(\text{La},\text{Sr})|{\mathrm{TiO}}_2$ interface is magnetoelectrically active, leading to a A-type antiferromagnetic coupling of the first ${\mathrm{TiO}}_2$ interface layer with the underlying manganite layer through a superexchange mechanism.

In the past, current-distance spectroscopy has been widely applied to determine variations of the work function at surfaces. While for homogeneous sample areas this technique is commonly accepted to yield at least qualitative results, its applicability to atomic-scale variations has not been proven neither right nor wrong. Here we benchmark measurements of the current-distance decay constant against the well established Kelvin probe force spectroscopy for four distinctly different cases with atomic-scale variations of the local contact potential. The two techniques yield quite different results. Whereas the maps of the current-distance decay constant are consistent with being topographical artifacts, the Kelvin probe force spectroscopy maps show variations of the local contact potential difference in agreement with expected surface dipoles. This comparison clarifies that maps of the current-distance decay constant are not suited to directly characterize contact potential variations at surfaces on atomic length scales.

It is shown that the Weyl semimetal TaAs can have a significant polar vector contribution to its optical activity. This is quantified by ab initio calculations of the resonant x-ray diffraction at the Ta $L1$ edge. For the Bragg vector (400), this polar vector contribution to the circular intensity differential between left and right polarized x rays is predicted to be comparable to that arising from linear dichroism, which could be tested by future experiments. Implications this result has in regards to optical effects predicted for topological Weyl semimetals are discussed.

The effect of a polarized nuclear spin bath on the dynamical behavior of a single electronic spin is studied theoretically and experimentally. The polarization of a single nuclear spin modifies the spin-echo signal of its neighboring electronic spin. When the electronic spin is surrounded by a bath of polarized nuclei, the spin-echo signals manifest a characteristic frequency related only to the nuclear spins abundance and their collective polarization. This frequency is proposed as an indicator for the local nuclear bath polarization. We quantify the realistic experimental regimes at which the scheme is efficient. Our proposal has potential applications for quantum sensing schemes, and opens a route for a systematic study of polarized mesoscopical systems.

Hybrid graphene–topological insulator (TI) devices were fabricated using a mechanical transfer method and studied via electronic transport. Devices consisting of bilayer graphene (BLG) under the TI ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ exhibit differential conductance characteristics which appear to be dominated by tunneling, roughly reproducing the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ density of states. Similar results were obtained for BLG on top of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, with tenfold greater conductance consistent with a larger contact area due to better surface conformity. The devices further show evidence of inelastic phonon-assisted tunneling processes involving both ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and graphene phonons. These processes favor phonons which compensate for momentum mismatch between the TI $\mathrm{\Gamma }$ and graphene $K,K^{\prime }$ points. Finally, the utility of these tunnel junctions is demonstrated on a density-tunable BLG device, where the charge neutrality point is traced along the energy-density trajectory. This trajectory is used as a measure of the ground-state density of states.

We present a self-consistent analysis of the topological superconductivity arising from the interaction between self-ordered localized magnetic moments and electrons in one-dimensional conductors in contact with a superconductor. We show that, due to a gain in entropy, there exists a magnetically ordered yet nontopological phase at finite temperatures that is relevant for systems of magnetic adatom chains on a superconductor. The spin-orbit interaction is taken into account, and we show that it causes a modification of the magnetic order, yet without affecting the topological properties.