Recent work has extended topological band theory to open, non-Hermitian Hamiltonians, yet little is understood about how non-Hermiticity alters the topological quantization of associated observables. We address this problem by studying the quantum anomalous Hall effect (QAHE) generated in the Dirac surface states of a 3D time-reversal-invariant topological insulator (TI) that is proximity-coupled to a metallic ferromagnet. By constructing a contact self-energy for the ferromagnet, we show that in addition to generating a mass gap in the surface spectrum, the ferromagnet can introduce a non-Hermitian broadening term, which can obscure the mass gap in the spectral function. We calculate the Hall conductivity for the effective non-Hermitian Hamiltonian describing the heterostructure and show that it is no longer quantized despite being classified as a Chern insulator based on non-Hermitian topological band theory. Our results indicate that the QAHE will be challenging to experimentally observe in ferromagnet-TI heterostructures due to the finite lifetime of quasi-particles at the interface.

Topology plays a central role in nearly all disciplines of physics, yet its applications have so far been restricted to closed, lossless systems in thermodynamic equilibrium. Given that many physical systems are open and may include gain and loss mechanisms, there is an eminent need to reexamine topology within the context of non-Hermitian theories that describe open, lossy systems. The recent generalization of the Chern number to non-Hermitian Hamiltonians initiated this reexamination; however, there is so far no established connection between a non-Hermitian topological invariant and the quantization of an observable. In this work, we show that no such relationship exists between the Chern number of non-Hermitian bands and the quantization of the Hall conductivity. Using field theoretical techniques, we derive an exact expression for the non-quantized Hall conductivity of a generic two-level non-Hermitian Hamiltonian. Furthermore, we calculate the longitudinal and Hall conductivities of a non-Hermitian Hamiltonian with a finite Chern number to explicitly demonstrate the disconnect between the Hall conductivity and the Chern number. These results demonstrate that the Chern number does not provide a physically meaningful classification of non-Hermitian Hamiltonians.arXiv:1901.09961v2 [cond-mat.mes-hall]

As experimental probes have matured to observe ultrafast transient and high frequency responses of materials and devices, so to have the theoretical methods to numerically and analytically simulate time-and frequency-resolved transport. In this review article, we discuss recent progress in the development of the time-dependent and frequency-dependent nonequilibrium Green function (NEGF) technique. We begin with an overview of the theoretical underpinnings of the underlying Kadanoff-Baym equations and derive the fundamental NEGF equations in the time and frequency domains. We discuss how these methods have been applied to a variety of condensed matter systems such as molecular electronics, nanoscale transistors, and superconductors. In addition, we survey the application of NEGF in fields beyond condensed matter, where it has been used to study thermalization in ultra-cold atoms and to understand leptogenesis in the early universe. Throughout, we pay special attention to the challenges of incorporating contacts and interactions, as the NEGF method is uniquely capable of accounting for such features.

Phase-change memory devices have found applications in in-memory computing where the physical attributes of these devices are exploited to compute in places without the need to shuttle data between memory and processing units. However, nonidealities such as temporal variations in the electrical resistance have a detrimental impact on the achievable computational precision. To address this, a promising approach is projecting the phase configuration of phase change material onto some stable element within the device. Here, the projection mechanism in a prominent phase-change memory device architecture, namely mushroom-type phase-change memory, is investigated. Using nanoscale projected Ge 2 Sb 2 Te 5 devices, the key attributes of state-dependent resistance, drift coefficients, and phase configurations are studied, and using them how these devices fundamentally work is understood.

As the semiconductor industry turns to alternate conductors to replace Cu for future interconnect nodes, much attention has been focused on evaluating the electrical performance of Ru. The typical hexagonal close-packed (hcp) phase has been extensively studied, but relatively little attention has been paid to the face-centered cubic (fcc) phase, which has been shown to nucleate in confined structures and may be present in tight-pitch interconnects. Using ab initio techniques, we benchmark the performance of fcc Ru. We find that the phonon-limited bulk resistivity of the fcc Ru is less than half of that of hcp Ru, a feature we trace back to the stronger electron-phonon coupling elements in hcp Ru that are geometrically inherited from the modified Fermi surface shape of the fcc crystal. Despite this benefit of the fcc phase, high grain boundary scattering results in increased resistivity compared to Cu-based interconnects with similar average grain size. We find, however, that the line resistance of fcc Ru is lower than that of Cu below 21 nm line width due to the conductor volume lost to adhesion and wetting liners. In addition to studying bulk transport properties, we evaluate the performance of adhesion liners for fcc Ru. We find that it is energetically more favorable for fcc Ru to bind directly to silicon dioxide than through conventional adhesion liners such as TaN and TiN. In the case that a thin liner is necessary for the Ru deposition technique, we find that the vertical resistance penalty of a liner for fcc Ru can be up to eight times lower than that calculated for conventional liners used for Cu interconnects. Our calculations, therefore, suggest that the formation of the fcc phase of Ru may be a beneficial for advanced, low-resistance interconnects. arXiv:2001.02216v3 [cond-mat.mtrl-sci] a) fcc b) hcp