A prominent characteristic of two-dimensional higher-order topological insulators (2D-HOTIs) is the topologically protected corner modes associated with gapped edge states. Opening the gap of bulk and edge states is a key to realizing 2D-HOTIs. In this paper, we theoretically propose that 2D-HOTIs can be realized on porous network models. We consider two porous network models, the graphenylenelike and the porous-honeycomb models, which respectively contain 12 and 18 atoms in supercells. These superstructures can open the bulk gap as well as the edge gap and induce HOTIs. Experimentally, graphene nanomesh, other 2D porous materials, and organic molecules naturally host supercells with regular nanoholes, which can be candidates to realizing HOTIs. Our studies reveal that 2D-HOTIs can be realized on some 2D porous network models and provide a promising route to explore HOTI states in real materials.

Programmable materials hold great potential for many applications such as deployable structures, soft robotics, and wave control; however, the presence of instability and disorder might hinder their utilization. Through a combination of analytical, numerical, and experimental analyses, we harness the interplay between instabilities, geometric frustration, and mechanical deformations to control the propagation of sound waves within self-assembled soft materials. We consider levitated magnetic disks confined by a magnetic boundary in-plane. The assemblies can be either ordered or disordered depending on the intrinsic disk symmetry. By applying an external load to the assembly, we observe the nucleation and propagation of different topological defects within the lattices. In the presence of instabilities, the defect propagation gives rise to time-independent localized transition waves. Surprisingly, in the presence of frustration, the applied load briefly introduces deformation-induced order to the material. By further deforming the lattices, new patterns emerge across all disk symmetries. We utilize these patterns to tune sound propagation through the material. Our findings could open new possibilities for designing exotic materials with potential applications ranging from sound control to soft robotics.

It has recently been shown that the polarization of a ferroelectric can be strongly affected by light pumping or even transiently reversed by mid-IR pumping within a record short subpicosecond timescale. It was then suggested that the reversal of polarization is most likely prevented by the depolarizing field that appears during the process. To minimize the effect of the depolarizing field, I propose to create an elongated domain by choosing a corresponding pump spot shape. I calculate the polarization dynamics for a particular experimental setup with mid-IR pumping and show that the polarization reversal is possible in this case.

This work introduces a paradigm for acoustic imaging in which a metasurface converts the acoustic waves scattered by remote scenes into coherent light focused into images by conventional optical cameras. The metasurface is composed of acousto-optical unit cells that sense the local acoustic pressure and use the resulting signal to modulate the amplitude of the electric field produced by a laser on an optical aperture. We derive the general design requirements for the image reconstruction in the optical domain and validate the concept through acoustic field measurements of the ultrasound scattered from an object submerged in water followed by numerical simulations predicting how this field is processed by the metasurface and camera. We show that this approach has two main advantages compared to traditional acoustic imaging systems. First, the acoustic-to-optic wavelength down-conversion leads to effective acoustical apertures very large compared to the physical size. Second, the unit cells are not synchronized electronically and thus the complexity of the metasurface increases only linearly with the number of unit cells, which is a significantly slower increase compared to conventional synchronized arrays. This work shows that these advantages lead to compact acoustic cameras providing image resolutions higher than possible with conventional acoustic imaging methods.

The strong band edge exciton-phonon interactions in metal halide perovskite quantum dots (QDs) offer a unique platform to explore many-body phenomena. Employing $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_3$ QDs as a perovskite model system, we report the observation of spin-selective polaronic biexcitons using collective excitations of two circularly polarized ultrafast lasers of a duration that is two orders of magnitude shorter than the exciton lifetime and one order of magnitude shorter than the spin relaxation time. The intervalley polaron pairing of charge carriers determines the anomalously strong exciton-exciton interactions, where the Haynes factor is an order of magnitude larger than the bulk and five times larger than the two-dimensional and quantum well semiconductors, demonstrating a very robust correlation of excitons. Our findings reveal a mechanism of generating highly stable biexciton states even at room temperature to realize higher-order correlations of charge carriers such as quantum droplets and Bose-Einstein condensates.

Synthetic antiferromagnets (SAFs) with perpendicular magnetic anisotropy (PMA) have recently attracted intensive attention for their potential applications in domain-wall-based racetrack memory. In this paper, swift heavy Fe ion irradiation is introduced to tune the magnetic properties of Pt/Co/Pt/Ru/Pt/Co/Ta SAFs from spin-flip to spin-flop transition. The evolutions of magnetic domain and domain wall motion are investigated to obtain the microscopic insight into the change of magnetic states. Both the experimental and the simulated results demonstrate that different responses of the effective PMA constant and the interlayer-exchange-coupling strength to ion irradiation determine the magnetic-phase transitions. The structure evolution from experiments and theoretical simulations suggests that the ion-irradiation-induced damage mainly happens at the Pt/Co interface. The damage leads to the different decrease rates of the effective PMA constant and the interlayer-exchange-coupling strength due to their intrinsic direct and Ruderman-Kittel-Kasuya-Yosida interaction nature, respectively. A quite good thermal stability of samples is obtained with ion fluence $<1.5\times {10}^{14}\mathrm{ions}/\mathrm{c}{\mathrm{m}}^2$. In this paper, we demonstrate that ion irradiation is a promising method to tailor magnetic properties of SAFs. Additionally, the ion-irradiation-induced well-controlled antiferromagnetically coupled domains could show potential applications in spintronic devices.

We investigate the evolution of spin polarization, spontaneous Hall angle (SHA), saturation magnetization, and Curie temperature of $B2$-ordered ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ thin films under varying antisite disorder, induced by ${\mathrm{Ne}}^{+}$-ion irradiation. The spin polarization increases monotonically as a function of ion fluence. A relatively high polarization of 46% and a SHA of 3.1% are achieved on 40 nm films irradiated with $2\times {10}^{16}$ $\mathrm{ions}/{\mathrm{cm}}^2$ at 30 keV. An interesting divergence in the trends of the magnetization and SHA is observed for low disorder concentrations. The high spin polarization and its broad tunability range make ion-irradiated ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ a promising material for application in spin electronic devices.

In the context of the modified Becke-Johnson (mBJ) potential, we recently underlined that $\overline{g}$, the average of $\left|\nabla \rho \right|/\rho$ in the unit cell, has markedly different values in transition-metal oxides and pure transition metals [Tran et al., J. Appl. Phys. 126, 110902 (2019)]. However, since $\overline{g}$ is a constant it is not able to provide local information about a particular atom in the system. Furthermore, while $\overline{g}$ can be used only for periodic bulk solids, a local (i.e., position-dependent) version would allow us to consider also low-dimensional systems and interfaces. Such a local function has been proposed by Rauch et al. [J. Chem. Theory Comput. 16, 2654 (2020)] for the local mBJ potential. Actually, a local version of $\overline{g}$, or of another similar quantity like the reduced density gradient $\overline{s}$, could also be used in the framework of other methods. Here, we explored the idea to use such a local function $\tilde{g}$ (or $\tilde{s}$), defined as the average of $g$ (or $s$) over a certain region around a transition-metal atom, to estimate the degree of on-site correlation on this atom. We found a large difference in our correlation estimators between noncorrelated and correlated materials, proving its usefulness and reliability. Our estimators can subsequently be used to determine whether or not a Hubbard $U$ on-site correction in the density functional theory (DFT)$+U$ method should be applied to a particular atom. This is particularly interesting in cases where the degree of correlation of the transition-metal atoms is not clear, like interfaces between correlated and noncorrelated materials or oxygen-covered metal surfaces. In such cases, our estimators could also be used for an interpolation of $U$ between correlated and noncorrelated atoms.

Resolving the heavy fermion band in the conduction electron momentum resolved spectral function of the Kondo lattice model is challenging since, in the weak coupling limit, its spectral weight is exponentially small. In this article we consider a composite fermion operator, consisting of a conduction electron dressed by spin fluctuations that shares the same quantum numbers as the electron operator. Using approximation free auxiliary field quantum Monte Carlo simulations we show that, for the SU(2) spin-symmetric model on the square lattice at half filling, the composite fermion acts as a magnifying glass for the heavy fermion band. In comparison to the conduction electron residue that scales as $e^{-W/J_k}$ with $W$ the bandwidth and $J_k$ the Kondo coupling, the residue of the composite fermion tracks $J_k$. This result holds down to $J_k/W=0.05$ and confirms the point of view that magnetic ordering, present below $J_k/W=0.18$, does not destroy the heavy quasiparticle. We furthermore investigate the spectral function of the composite fermion in the ground state and at finite temperatures, for $\mathrm{SU}\left(N\right)$ generalizations of the Kondo lattice model, as well as for ferromagnetic Kondo couplings, and compare our results to analytical calculations in the limit of high temperatures, large-$N$, large-$S$, and large $J_k$. Based on these calculations, we conjecture that the composite fermion operator provides a unique tool to study the destruction of the heavy fermion quasiparticle in Kondo breakdown transitions. The relation of our results to scanning tunneling spectroscopy and photoemission experiments is discussed.

We study the flat-band ferromagnetic phase of the Haldane-Hubbard model on a honeycomb lattice within a bosonization scheme for flat-band Chern insulators, focusing on the calculation of the spin-wave excitation spectrum. We consider the Haldane-Hubbard model with the noninteracting lower bands in a nearly flat band limit, previously determined for the spinless model, and at $1/4$ filling of its corresponding noninteracting limit. Within the bosonization scheme, the Haldane-Hubbard model is mapped onto an effective interacting boson model, whose quadratic term allows us to determine the spin-wave spectrum at the harmonic approximation. We show that the excitation spectrum has two branches with a Goldstone mode and Dirac points at center and at the $K$ and $K^{\prime }$ points of the first Brillouin zone, respectively. We also consider the effects on the spin-wave spectrum due to an energy offset in the on-site Hubbard repulsion energies and due to the presence of a staggered on-site energy term, both quantities associated with the two triangular sublattices. In both cases, we find that an energy gap opens at the $K$ and $K^{\prime }$ points. Moreover, we also find some evidence for an instability of the flat-band ferromagnetic phase in the presence of the staggered on-site energy term. We provide some additional results for the square lattice topological Hubbard model previously studied within the bosonization formalism and comment on the differences between the bosonization scheme implementation for the correlated Chern insulators on both square and honeycomb lattices.

We study longitudinal magnetotransport in disorder-free cylindrical Weyl semimetal nanowires. Our theory includes a magnetic flux $\mathrm{\Phi }$ piercing the nanowire and captures the finite curvature of the Fermi arc in the surface Brillouin zone through a boundary angle $\alpha$. Electron backscattering by acoustic phonons via the deformation potential causes a finite resistivity which we evaluate by means of the semiclassical Boltzmann approach. We find that low-energy transport is dominated by surface states, where transport observables are highly sensitive to the angle $\alpha$ and to Aharonov-Bohm phases due to $\mathrm{\Phi }$. A generic subband dispersion relation allows for either one or two pairs of Fermi points. In the latter case, intranode backscattering is possible and implies a parametrically larger resistivity than for a single Fermi point pair. As a consequence, large and abrupt resistivity changes take place across the transition points separating parameter regions with a different number of Fermi point pairs in a given subband.

Quantum transport simulations are rapidly evolving and now encompass well-controlled tensor network techniques for many-body limits. One powerful approach combines matrix product states with extended reservoirs. In this method, continuous reservoirs are represented by explicit, discretized counterparts and a chemical potential or temperature drop is maintained by external relaxation. Currents are strongly influenced by relaxation when it is very weak or strong, resulting in a simulation analog of Kramers turnover for solution-phase chemical reactions. At intermediate relaxation, the intrinsic conductance, that given by the Landauer or Meir-Wingreen expressions, moderates the current. We demonstrate that strong impurity scattering (i.e., a small steady-state current) reveals anomalous transport regimes within this methodology at weak-to-moderate and moderate-to-strong relaxation. The former is due to virtual transitions and the latter to unphysical broadening of the populated density of states. Thus, the turnover analog has five standard transport regimes, further constraining the parameters that lead to recovery of the intrinsic conductance. In the worst case, the common strategy of choosing a relaxation strength proportional to the reservoir level spacing can prevent convergence to the continuum limit. This advocates a simulation strategy where one utilizes the current versus relaxation turnover profiles to identify simulation parameters that most efficiently reproduce the intrinsic physical behavior.

In this paper, we demonstrate that it is possible to combine two computational approaches, usually used in very different contexts, namely, the Green-Kubo formalism and statistical approaches, to solve the Boltzmann transport equation to calculate the thermal conductivity of semiconductors. In this framework, first the phonon transport is solved by the Monte Carlo method according to the Debye-Callaway or Holland formalisms, then the flux autocorrelation is calculated. This new approach has been implemented to calculate the properties of Si, Ge, GaN, and C (diamond) in an extended temperature range, from 50 to 600 K. The simulation results are compared with those given by experimentation, with very good agreement.

Elastic diffraction of fast atoms at crystal surfaces under grazing incidence $\theta \approx 1$° has strong similarities with atomic diffraction at thermal energies discovered almost hundred years ago. Here, we focus on the polar scattering profile, which does not exhibit diffraction features but shows well-defined elastic and inelastic components that are found to be essentially independent of the crystallographic axis. The width ${\sigma }_{\theta }$ of the inelastic component is very sensitive to the weak attractive forces responsible for the physisorption. This effect is visible on an energy range almost ten times larger than the depth $D$ of the physisorption well. Experimental data are analyzed using a binary collision model with a Morse potential where the width ${\sigma }_{\theta }$ of the scattering profile is connected to the classical energy loss and is governed by the surface stiffness, defined as the logarithmic derivative of the interaction potential along the surface normal. The main outcome is that the weak attractive forces make the mean surface potential almost twice harder at low energy.

Characterizing the many-body localization (MBL) transition in strongly disordered and interacting quantum systems is an important issue in the field of condensed matter physics. We study the single-particle Green's functions for a disordered interacting system in one dimension using exact diagnonalization in the infinite temperature limit and provide strong evidence that single-particle excitations carry signatures of delocalization to MBL transition. In the delocalized phase, the typical values of the local density of states and the scattering rate are finite while in the MBL phase, the typical values for both the quantities become vanishingly small. The probability distribution functions of the local density of states and the scattering rate are broad log-normal distributions in the delocalized phase while the distributions become very narrow and sharply peaked close to zero in the MBL phase. We also study the eigenstate Green's function for all the many-body eigenstates and demonstrate that both, the energy-resolved typical scattering rate and the typical local density of states, can track the many-body mobility edges.

The transfer of energy through a network of nodes is fundamental to how both nature and current technology operate. Traditionally, we think of the nodes in a network being coupled to channels that connect them, in which energy is passed from node to channel to node until it reaches its targeted site. Here we introduce an alternate approach to this, where our channels are replaced by collective environments (or, actually, reservoirs) which interact with pairs of nodes. We show how energy initially located at a specific node can arrive at a target node—even though that environment may be at zero temperature. Further, we show that such a migration occurs on much faster timescales than the damping rate associated with a single spin coupled to the reservoir. Our approach shows the power of being able to tailor both the system and environment and the symmetries associated with them to provide new directions for future quantum technologies.

We report the results of muon-spin relaxation and magnetometry investigations of bulk ${\mathrm{Mn}}_{1.4}{\mathrm{Pt}}_{0.9}{\mathrm{Pd}}_{0.1}\mathrm{Sn}$ and MnNiGa, two materials that have been proposed to host topological magnetic states in thin lamellae (antiskyrmions for ${\mathrm{Mn}}_{1.4}{\mathrm{Pt}}_{0.9}{\mathrm{Pd}}_{0.1}\mathrm{Sn}$ and biskyrmions for MnNiGa), and show spin reorientation transitions in the bulk. Our measurements reveal dynamic fluctuations surrounding the magnetic phase transitions in each material. In particular, we demonstrate that the behavior approaching the higher-temperature transitions reflects a decrease in the frequency of dynamics with increasing temperature. At low temperatures the two systems both show spin dynamics over a broad range of frequencies that persist below the respective spin reorientation transitions.

We consider a three-dimensional model of coupled Su-Schrieffer-Heeger (SSH) chains. The analytically soluble model discussed here reliably reproduces the features of the band structure of crystalline polyacetylene as obtained from density-functional theory. We show that when a certain interchain coupling is sufficiently increased, the system develops a ring of Weyl nodes. We argue that such an increase could be achieved experimentally by intercalation or extreme pressure. With the addition of a simple intraorbital pairing term we find that the system supports an exotic superconducting state with drumhead surface states and annular Majorana states localized on the surface. In addition to suggesting a real material realization of a nodal ring semimetal and possibly topological superconductivity, our results provide a different perspective on the SSH model, demonstrating that a simple extension of this broadly impacting model can once again provide fundamental insights on the topological behavior of condensed matter systems.