We use an unbiased, continuous-time quantum Monte Carlo method to address the possibility of a zero-temperature phase without charge-density-wave (CDW) order in the Holstein and, by extension, the Holstein-Hubbard model on the half-filled square lattice. In particular, we present results spanning the whole range of phonon frequencies, allowing us to use the well understood adiabatic and antiadiabatic limits as reference points. For all parameters considered, our data suggest that CDW correlations are stronger than pairing correlations even at very low temperatures. These findings are compatible with a CDW ground state that is also suggested by theoretical arguments.

We study random close-packed systems of magnetic spheres by Monte Carlo simulations in order to estimate their phase diagram. The uniaxial anisotropy of the spheres makes each of them behave as a single Ising dipole along a fixed easy axis. We explore the phase diagram in terms of the temperature and the degree of alignment (or texturation) among the easy axes of all spheres. This degree of alignment ranges from the textured case (all easy axes pointing along a common direction) to the nontextured case (randomly distributed easy axes). In the former case, we find long-range ferromagnetic order at low temperature but, as the degree of alignment is diminished below a certain threshold, the ferromagnetic phase gives way to a spin-glass phase. This spin-glass phase is similar to the one previously found in other dipolar systems with strong frozen disorder. The transition between ferromagnetism and spin glass passes through a narrow intermediate phase with quasi-long-range ferromagnetic order.

The spin-admixture parameter $\gamma$ is key to understanding spin dynamics in molecules. It measures the influence of spin-orbit coupling on spin dynamics in virtually all models for molecular materials. However, the predictive quality of its current first-principles electronic structure theory formulation [Z. G. Yu, Phys. Rev. B 85, 115201 (2012)] is limited by multiple approximations. Consequently, current literature is lacking further first-principles predictions of $\gamma$ beyond those of the original paper, and the application of such in computational studies of the role of spin mixing in organic and molecular spintronics. Here, we generalize the previous formulation of $\gamma$ to remove all approximations limiting its predictive accuracy. The result is dramatic qualitative improvements in several model systems for organic spintronics. The significantly increased transferability of our method also allows its application to a range of heavier, metal-center “molecular qubit” molecules without modification. For these molecules, we identify molecular spin-orbit coupling as the key to their spin relaxation, with unprecedented accuracy. The accuracy and computational robustness of our reformulated method makes it ideal for the kind of large-scale, high-throughput computational exploration of chemical spaces used in several other areas of molecular nanotechnology.

Several aspects of phonon scattering in the transmission electron microscope are investigated with focus on the physically distinct effects of the thermal average over initial states as well as the excitation or annihilation of phonons. Coupled-channel calculations for the model system of a single atom in a harmonic potential are presented and interpreted in terms of inelastic scattering and thermal fluctuations. Furthermore, common approximations for inelastic scattering calculations are compared to this model. Additionally, the description of phonon scattering by means of an $S$-matrix approach is investigated. The $S$-matrix approach allows differentiation between scattering events and propagation of scattered waves within the specimen. Within this approach, it is rigorously proven that by suitable integration over calculations with static atomic coordinates inelastic phonon scattering can be approximated for any initial object state. The limits of this approximation are discussed.

Based on density functional theory calculations, we show how symmetry mismatch at interface induces perpendicular magnetic anisotropy (PMA) for the perovskite/brownmillerite-typed $\mathrm{L}{\mathrm{a}}_{1/2}\mathrm{S}{\mathrm{r}}_{1/2}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{LaCo}{\mathrm{O}}_{2.5}$ superlattices in different strain states. We found strong interfacial reconstructions, which result in considerable orbital hybridization due to distortion/tilting of the $\mathrm{Mn}{\mathrm{O}}_6$ octahedron and $\mathrm{Co}{\mathrm{O}}_4$ tetrahedron. We identified the orbital pairs that strongly affect magnetic anisotropy, showing that tuning the degree of orbital hybridization by lattice strain is an effective approach to tune magnetic anisotropy. Remarkably, not only octahedron-coordinated Mn ions but also tetrahedron-coordinated Co ions contribute to PMA. This work presents a guidance for tuning spin orientation by interface engineering.

We have conducted experiments to probe how the dynamics of nanocontact vortex oscillators can be modulated by an external signal. We explore the phase-locking properties in both the commensurate and chaotic regimes, where chaos appears to impede phase-locking while a more standard behavior is seen in the commensurate phase. These different regimes correspond to how the periodicity of the vortex core reversal relates to the frequency of core gyration around the nanocontact; a commensurate phase appears when the reversal rate is an integer fraction of the gyration frequency, while a chaotic state appears when this ratio is irrational. External modulation where the power spectral density exhibits rich features appears due to the modulation between the external source frequency, gyration frequency, and core-reversal frequency. We explain these features with first- or second-order modulation between the three frequencies. Phase-locking is also visible between the external source frequency and internal vortex modes (gyration and core-reversal modes).

We analyze the role of boundary geometry in viscous electronic hydrodynamics. We address the twin questions of how boundary geometry impacts flow profiles, and how one can engineer boundary conditions—in particular the effective slip parameter—to manipulate the flow in a controlled way. We first propose a micropatterned geometry involving finned barriers, for which we show by an explicit solution that one can obtain effectively no-slip boundary conditions regardless of the detailed microscopic nature of the channel surface. Next we analyze the role of mesoscopic boundary curvature on the effective slip length, in particular its impact on the Gurzhi effect. Finally we investigate a hydrodynamic flow through a circular junction, providing a solution which suggests an experimental setup for determining the slip parameter. We find that its transport properties differ qualitatively from the case of ballistic conduction, and thus presents a promising setting for distinguishing the two.

The alteration of the electronic band gap induced by axial compression of single-wall carbon nanotubes was investigated using molecular dynamics simulation. Beyond a critical level of compressive strain, three classes of buckling modes take place depending on the aspect ratio of the system. Among these three classes, two different shell-buckling modes are found to yield contrasting band-gap modulation behavior as a response to axial compression near the buckling threshold. The potential advantage of using shell buckling to induce gap modulations when designing nanotube-based electromechanical devices is also discussed.

We propose a two-dimensional non-Hermitian Chern insulator with inversion symmetry, which is anisotropic and has staggered gain and loss in both $x$ and $y$ directions. In this system, conventional bulk-boundary correspondence holds. The Chern number is a topological invariant that accurately predicts the topological phase transition and the existence of helical edge states in the topologically nontrivial gapped phase. In the gapless phase, the band touching points are isolated and protected by the symmetry. The degenerate points alter the system topology, and the exceptional points can destroy the existence of helical edge states. Topologically protected helical edge states exist in the gapless phase for the system under open boundary condition in one direction, which are predicted by the winding number associated with the vector field of average values of Pauli matrices. The winding number also identifies the detaching points between the edge states and the bulk states in the energy bands. The non-Hermiticity also supports a topological phase with zero Chern number, where a pair of in-gap helical edge states exists. Our findings provide insights into the symmetry protected non-Hermitian topological insulators.

Plasmonic devices with feature sizes of a few nanometers exhibit effects which can be described by the nonlocal hydrodynamic Drude model. We demonstrate how to exploit contour integral methods for computing eigenfrequencies and resonant states of such systems. We propose an approach for deriving the modal expansion of relevant physical observables. We use the methods to perform a modal analysis for a metal nanowire. All complex eigenfrequencies in a large frequency range and the corresponding resonant states are computed. We identify those resonant states which are relevant for the extinction cross section of the nanowire.

Based on first-principles calculations and $\mathit{k}·\mathit{p}$ model analyses, we uncover the coexistence and coupling of Rashba spin splitting with electronic ferroelectricity in bilayer transition metal dichalcogenides $M{X}_2$ ($M=\text{Mo,W}$; $X=\text{S,Se,Te}$) with certain stacking configurations. The reversible spontaneous ferroelectric polarization, along the out-of-plane direction (the preferred direction for applications), totally arises from the interlayer charge transfer, rather than being governed by the ionic displacement as found in conventional ferroelectrics. The spin texture related to the Rashba spin splitting can be reversed upon inversion of the ferroelectric polarization. In particular, by applying a small in-plane compressive strain, the magnitude of Rashba band splitting can be tuned to be as large as 100 meV. These results would open up possibilities for exploring two-dimensional multiferroic physics and developing electrically controlled nanoscale spintronic devices.

We consider a class of one-dimensional non-Hermitian models with a special type of a chiral symmetry which is related to pseudo-Hermiticity. We show that the topology of a Hamiltonian belonging to this symmetry class is determined by a hidden Chern number described by an effective two-dimensional Hermitian Hamiltonian $H^{\mathrm{eff}}(k,\eta )$, where $\eta$ is the imaginary part of the energy. This Chern number manifests itself as topologically protected in-gap end states at zero real part of the energy. We show that the bulk-boundary correspondence coming from the hidden Chern number is robust and immune to the non-Hermitian skin effect. We introduce a minimal model Hamiltonian supporting topologically nontrivial phases in this symmetry class, derive its topological phase diagram, and calculate the end states originating from the hidden Chern number.

It is known that three-dimensional magnetic systems with glide symmetry can be characterized by a $Z_2$ topological invariant together with the Chern number associated with the normal vector of the glide plane, and they are expressed in terms of integrals of the Berry curvature. In the present paper, we study the fate of this topological invariant when inversion symmetry is added while time-reversal symmetry is not enforced. There are two ways to add inversion symmetry, leading to space group Nos. 13 and 14. In space group No. 13, we find that the glide-$Z_2$ invariant is expressed solely from the irreducible representations at high-symmetry points in $k$ space. It constitutes the ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry-based indicator for this space group, together with another ${\mathbb{Z}}_2$ representing the Chern number modulo 2. In space group No. 14, we find that the symmetry-based indicator ${\mathbb{Z}}_2$ is given by a combination of the glide-$Z_2$ invariant and the Chern number. Thus, in space group No. 14, from the irreducible representations at high-symmetry points we can only know possible combinations of the glide-$Z_2$ invariant and the Chern number, but in order to know each value of these topological numbers, we should calculate integrals of the Berry curvature. Finally, we show that in both cases, the symmetry-based indicator ${\mathbb{Z}}_4$ for inversion symmetric systems leading to the higher-order topological insulators is directly related with the glide-$Z_2$ invariant and the Chern number. As an independent approach to these results, we also construct all invariants from the layer construction for these space groups, and we show complete agreement with the above results for the topological invariants constructed from $k$-space topology.

Analogous to circular spin current in an isolated quantum loop, bias-induced circular spin current can also be generated under certain physical conditions in a nanojunction having single- and/or multiple-loop geometries which we propose considering a magnetic quantum system. The key aspect of our work is the development of a suitable theory for defining and analyzing circular spin currents in presence of environmental dephasing and impurities. Unlike a transport current in a conducting junction, a circular current may enhance significantly in presence of disorder and phase randomizing processes. Our analysis provides a spin-dependent phenomenon, and can give important signatures in designing suitable spintronic devices as well as selective spin regulations.

Alongside a vast number of two-dimensional (2D) materials, $g\text{−}{\mathrm{SiC}}_7$ siligraphene has already been introduced to the 2D nanomaterial family with extraordinary sunlight absorption. In this work, we exploit the concept of the in-plane biaxial strain-induced electro-optical engineering in siligraphene from ab initio many-body perturbation theory calculations (GW + Bethe-Salpeter equation). While the semiconductor nature of the system is robust for both tensile and compressive strains, the direct GW band gap of 1.35 eV is decreased (increased) linearly with tensile (compressive) strain. This is accompanied by a variation in the optical transitions. The tensile strain in $g\text{−}{\mathrm{SiC}}_7$ shifts the optical absorption peaks to the lower photon energies, the so-called red shift, whereas the compressive strain causes a shift to the higher frequencies, the so-called blue shift. Eventually, the exciton binding energy trend and the real-space exciton distribution for the bright bound state is studied with strain, resulting in a alteration of the spatial distribution radius of exciton wave functions from tensile to compressive. Our findings extend the logic applications of siligraphene in photovoltaic devices, especially in tailoring the power conversion efficiency of excitonic solar cells.

This paper addresses the so-called inverse problem which consists in searching for (possibly multiple) parent target Hamiltonian(s), given a single quantum state as input. Starting from ${\mathrm{\Psi }}_0$, an eigenstate of a given local Hamiltonian ${\mathcal{H}}_0$, we ask whether or not there exists another parent Hamiltonian ${\mathcal{H}}_{\mathrm{P}}$ for ${\mathrm{\Psi }}_0$, with the same local form as ${\mathcal{H}}_0$. Focusing on one-dimensional quantum disordered systems, we extend the recent results obtained for Bose-glass ground states [M. Dupont and N. Laflorencie, Phys. Rev. B 99, 020202(R) (2019)] to Anderson localization, and the many-body localization (MBL) physics occurring at high energy. We generically find that any localized eigenstate is a very good approximation for an eigenstate of a distinct parent Hamiltonian, with an energy variance ${\sigma }_{\mathrm{P}}^2\left(L\right)={〈{\mathcal{H}}_{\mathrm{P}}^2〉}_{{\mathrm{\Psi }}_0}-{\langle {\mathcal{H}}_{\mathrm{P}}\rangle }_{{\mathrm{\Psi }}_0}^2$ vanishing as a power law of system size $L$. This decay is microscopically related to a chain-breaking mechanism, also signaled by bottlenecks of vanishing entanglement entropy. A similar phenomenology is observed for both Anderson and MBL. In contrast, delocalized ergodic many-body eigenstates uniquely encode the Hamiltonian in the sense that ${\sigma }_{\mathrm{P}}^2\left(L\right)$ remains finite at the thermodynamic limit, i.e., $L\to +\infty$. As a direct consequence, the ergodic-MBL transition can be very well captured from the scaling of ${\sigma }_{\mathrm{P}}^2\left(L\right)$.

In this work the thermal expansion of $\mathrm{G}{\mathrm{d}}_5\mathrm{S}{\mathrm{i}}_{1.3}\mathrm{G}{\mathrm{e}}_{2.7}$ magnetic nanogranules was studied. A crossover from positive (macro- and microscale in the literature) towards a negative thermal expansion (at nanoscale) is observed. The negative thermal expansion (NTE) behavior was found in two temperature windows: 90–160 K $({\beta }^{\mathrm{LT}}\sim -32.2\mathrm{ppm}{\mathrm{K}}^{-1})$, and within the room temperature 255–340 K, where a giant NTE was observed $(-69\mathrm{ppm}{\mathrm{K}}^{-1})$. The key atomic mechanism driving the NTE is identified as an atomic triplet-chain flexibility. The results suggest that the NTE behavior emerges as a size-reduction effect caused by the intrinsic nanoparticles surface pressure (estimated at 11 kbar), leading to the positive thermal expansion–NTE crossover at the nanoscale.

The magnetic and structural properties of polycrystalline ${\mathrm{Co}}_{4-x}{\mathrm{Ni}}_x{\mathrm{Nb}}_2{\mathrm{O}}_9$ ($x=1,2$) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For $x=1$, the compound crystallizes in the trigonal $P\overline{3}c1$ space group. Below $T_N=31$ K it develops a weakly noncollinear antiferromagnetic structure with magnetic moments in the $ab$ plane. The compound with $x=2$ has crystal structure of the orthorhombic $Pbcn$ space group and shows a hard ferrimagnetic behavior below $T_C=47$ K. For this compound a weakly noncollinear ferrimagnetic structure with two possible configurations in the $ab$ plane was derived from neutron diffraction study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provides further information on the magnetic structure of the compounds.