We investigate the magnetic couplings in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ in the Mott-insulating picture, combining density-functional theory, dynamical mean-field theory, and many-body perturbation theory. We first determine the form of the $j_{\mathrm{eff}}=1/2$ pseudospin via the local-density-approximation $+$ dynamical mean-field theory approach. Next we study the magnetic interactions in the strong-to-intermediate coupling regime. To this end, we calculate the superexchange pseudospin tensors ${\mathrm{\Gamma }}_1, {\mathrm{\Gamma }}_2$, and ${\mathrm{\Gamma }}_3$ up to fourth order and analyze their dependence on the screened Coulomb interaction integrals $U$ and $J$. We show that, due to term cancellations, the experimental nearest-neighbor coupling ${\mathrm{\Gamma }}_1$ is reasonably well reproduced for a whole range of realistic $(U,J)$ values. We show that increasing the Hund's rule coupling $J$ (within the window of realistic values) can lead to large fourth-order contributions, which could explain the ferromagnetic next-nearest-neighbor coupling ${\mathrm{\Gamma }}_2$ extracted from the spin-wave dispersion. This regime is characterized by a sizable ring exchange $K$. For $(U,J)$ values that yield a Mott insulator with a half-filled $j_{\mathrm{eff}}=1/2$ state, however, fourth-order terms remain minor even if the gap is small. For no realistic parameters, we find a sizable next-next-nearest-neighbor coupling ${\mathrm{\Gamma }}_3\sim \left|{\mathrm{\Gamma }}_2\right|$. Possible implications are discussed.

The honeycomb antiferromagnet under a triaxial strain is studied using the quantum Monte Carlo simulation. The strain dimerizes the exchange couplings near the corners, thus destructs the antiferromagnetic order therein. The antiferromagnetic region is continuously reduced by the strain. For the same strain strength, the exact numerical results give a much smaller antiferromagnetic region than the linear spin-wave theory. We then study the strained $XY$ antiferromagnet, where the magnon pseudomagnetic field behaves quite differently. The $0\mathrm{th}$ Landau level appears in the middle of the spectrum, and the quantized energies above (below) it are proportional to $n^{\frac{1}{3}}\left(n^{\frac{2}{3}}\right)$, which is in great contrast to the equally spaced ones in the Heisenberg case. Besides, we find the antiferromagnetic order of the $XY$ model is much more robust to the dimerization than the Heisenberg one. The local susceptibility of the Heisenberg case is extracted by the numerical analytical continuation, and no sign of the pseudo-Landau levels is resolved. It is still not certain whether the result is due to the intrinsic problem of the numerical analytical continuation. Thus, the existence of the magnon pseudo-Landau levels in the spin-$\frac{1}{2}$ strained Heisenberg Hamiltonian remains an open question. Our results are closely related to the two-dimensional van der Waals quantum antiferromagnets, and may be realized experimentally.

We study particle-hole symmetry at the integer quantum Hall plateau transition using composite fermion mean-field theory. Because this theory implicitly includes some electron-electron interactions, it also has applications to certain fractional quantum Hall plateau transitions. Previous work [P. Kumar et al., Phys. Rev. B 100, 235124 (2019)] using this approach showed that the diffusive quantum criticality of this transition is described by a nonlinear sigma model with topological $\theta =\pi$ term. This result, which holds for both the Dirac and Halperin, Lee, and Read composite fermion theories, signifies an emergent particle-hole (reflection) symmetry of the integer (fractional) quantum Hall transition. Here we consider the stability of this result to various particle-hole symmetry-violating perturbations. In the presence of quenched disorder that preserves particle-hole symmetry, we find that finite longitudinal conductivity at this transition requires the vanishing of a symmetry-violating composite fermion effective mass, which if present would generally lead to $\theta \ne \pi$ and a corresponding violation of particle-hole symmetric electrical transport ${\sigma }_{xy}\ne \frac{1}{2}\frac{e^2}{h}$. When the disorder does not preserve particle-hole symmetry, we find that $\theta$ can vary continuously within the diffusive regime. Our results call for further study of the universality of the quantum Hall plateau transition.

Recent developments in experiments with vibrational electron energy loss spectroscopy (EELS) have revealed spectral shape variations at spatial resolutions down to the atomic scale. Interpretation in terms of local phonon density of states enables their qualitative understanding, yet a more detailed analysis is calling for advances in theoretical methods. Recently, we have presented a frequency resolved frozen phonon multislice method for simulations of vibrational EELS [P. M. Zeiger and J. Rusz, Phys. Rev. Lett. 124, 025501 (2020).]. Detailed simulations for a plane-wave electron beam scattering on vibrating hexagonal boron nitride are presented in a companion paper [P. M. Zeiger and J. Rusz, Phys. Rev. B 104, 104301 (2021)]. Here we present simulations of vibrational EELS assuming a convergent electron probe of nanometer size and atomic size on a hexagonal boron nitride structure model with a planar defect. With a nanometer beam we observe spectral shape modifications in the presence of the defect, which are correlated with local changes of the phonon density of states. With an atomic-sized electron beam, we observe the same, although with better contrast. In addition, we observe atomic-level contrast and atomic scale spectral shape modifications, which are particularly strong for small detector collection angles.

We clarify the interplay and competition between disorder and flat band in the Creutz ladder with interparticle interactions focusing on the system's dynamics. Without disorder, the Creutz ladder exhibits flat-band many-body localization (FMBL). In this work, we find that disorder generates drastic effects on the system, i.e., addition of it induces a thermal phase first and further increase of it leads the system to the conventional many-body-localized (MBL) phase. The competition gives novel localization properties and unconventional quench dynamics to the system. We first draw the global sketch of the localization phase diagram by focusing on the two-particle system. The thermal phase intrudes between the FMBL and MBL phases, the regime of which depends on the strength of disorder and interactions. Based on the two-particle phase diagram and the properties of the quench dynamics, we further investigate finite-filling cases in detail. At finite-filling fractions, we again find that the interplay and competition between the interactions and disorder in the original flat-band Creutz ladder induces a thermal phase, which separates the FMBL and MBL phases. We also verify that the time evolution of the system coincides with the static phase diagrams. For suitable fillings, the conservation of the initial-state information and low-growth entanglement dynamics are also observed. These properties depend on the strength of disorder and interactions.

Of all parent compounds of iron-based high-temperature superconductors, ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ exhibits by far the largest magnetostructural coupling due to the sizable biquadratic interaction between Eu and Fe moments. While the coupling between Eu antiferromagnetic (AFM) order and Fe structural/AFM domains enables rapid field detwinning, this prevents simple magnetometry measurements from extracting the critical fields of the Eu metamagnetic transition. Here, we measure these critical fields by combining x-ray magnetic circular dichroism spectroscopy with in situ tunable uniaxial stress and applied magnetic field. The combination of two tuning knobs allows us to separate the stress detwinning of structural domains from the field-induced reorientation of Eu moments. Intriguingly, we find a spin-flip transition which can only result from a strongly anisotropic interaction between Eu planes. We argue that this anisotropic exchange is a consequence of the strong anisotropy in the magnetically ordered Fe layer, which presents a form of higher-order coupling between Eu and Fe magnetism.

We report the experimental observation of half-integer Shapiro steps in the strong ferromagnetic Josephson junction (Nb-NiFe-Nb) by investigating the current-phase relation under radio-frequency microwave excitation. The half-integer Shapiro steps are robust in a wide temperature range from $T=4$ to 7 K. The half-integer Shapiro steps could be attributed to the coexistence of 0 and $\pi$ states in the strong ferromagnetic NiFe Josephson junctions with the spatial variation of the NiFe thickness. This scenario is also supported by the high-resolution transmission electron microscopy characterization of the Nb/NiFe/Nb junction.

This study investigated the electronic structure of $\mathrm{Sr}{\mathrm{Ti}}_{1\text{−}x}{\mathrm{V}}_x{\mathrm{O}}_3$ (STVO) thin films, which are solid solutions of strongly correlated transparent conductive oxide (TCO) $\mathrm{SrV}{\mathrm{O}}_3$ and oxide semiconductor $\mathrm{SrTi}{\mathrm{O}}_3$, using in situ photoemission spectroscopy. STVO is one of the most promising candidates for correlated-metal TCO because it has the capability of optimizing the performance of transparent electrodes by varying $x$. Systematic and significant spectral changes were found near the Fermi level ($E_{\mathrm{F}}$) as a function of $x$, while the overall electronic structure of STVO is in good agreement with the prediction of band-structure calculations. As $x$ decreases from 1.0, spectral weight transfer occurs from the coherent band near $E_{\mathrm{F}}$ to the incoherent states (lower Hubbard band) around 1.0–1.5 eV. Simultaneously, a pseudogap is formed at $E_{\mathrm{F}}$, indicating a significant reduction in quasiparticle spectral weight within close vicinity of $E_{\mathrm{F}}$. This pseudogap seems to evolve into an energy gap at $x=0.4$, suggesting the occurrence of a composition-driven metal-insulator transition. From angle-resolved photoemission spectroscopic results, the carrier concentration $n$ changes proportionally as a function of $x$ in the metallic range of $x=0.6–1.0$. In contrast, the mass enhancement factor, which is proportional to the effective mass ($m^{*}$), does not change significantly with varying $x$. These results suggest that the key factor of $n/m^{*}$ in optimizing the performance of correlated-metal TCO is tuned by $x$, highlighting the potential of STVO to achieve the desired TCO performance in the metallic region.

The recently discovered kagome magnet $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ ($R$ = Y and lanthanides) features a variety of magnetic ground states and exhibits topological quantum phases. Here we report on a combined angle-resolved photoemission spectroscopy and first-principles calculations study of the electronic structure in $\mathrm{Gd}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$, which contains Mn kagome lattice with in-plane ferrimagnetism. Typical kagome electronic dispersions are found around the Fermi energy ($E_{\mathrm{F}}$), i.e., Dirac dispersions, flat bands (FBs), and saddle points. The Dirac dispersions with $d_{z2}$ orbital characters at $E_{\mathrm{F}}$ are sensitive to adding onsite Hubbard $U$ on the Mn $3d$ atoms, revealing that electronic correlation effectively modulates the Dirac dispersions of a kagome magnet $\mathrm{Gd}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$. On the other hand, to match the observed FBs with $d_{xy}/d_{x2-y2}$ orbital characters around 0.4 eV below $E_{\mathrm{F}}$, the calculated bands are still needed to be overall renormalized by a factor of about 2 after adding a small $U$ for matching the Dirac dispersions at $E_{\mathrm{F}}$. The different electronic correlations, which are directly reflected by the Hubbard $U$ and the renormalization factor, could be associated with the orbital-selective kagome bands at the different binding energies. Our findings could have an instructive significance to tuning the individual kagome feature via the electronic correlations.

Electrons in $\alpha \text{−}{T}_3$ lattices behave as condensed-matter analogies of integer-spin Dirac fermions. The three atoms making up the unit cell bestow the energy spectrum with an additional energy band that is completely flat, providing unique electronic properties. The interatomic hopping term, $\alpha$, is known to strongly affect the electronic spectrum of the two-dimensional (2D) lattice, allowing it to continuously morph from graphenelike responses to the behavior of fermions in a dice lattice. For pristine lattice structures the energy bands are gapless, but small deviations in the atomic equivalence of the three sublattices will introduce gaps in the spectrum. It is unknown how these affect transport and electronic properties such as the energy spectrum of superlattice minibands. Here we investigate the dependency of these properties on the parameter $\alpha$ accounting for different symmetry-breaking terms, and we show how it affects band-gap formation. Furthermore, we find that superlattices can force band gaps to close and shift in energy. Our results demonstrate that $\alpha \text{−}{T}_3$ superlattices provide a versatile material for 2D band-gap engineering purposes.

In this paper, we design and predict the structural, electronic, transport, and optical properties of two-dimensional Janus $\mathrm{GaIn}X\mathrm{O}$ ($X=\mathrm{S}$, Se, Te) monolayers by using first-principles calculations. The stability of all six possible configurations of $\mathrm{GaIn}X\mathrm{O}$ was examined through the analysis of their phonon spectra. It is found that, except for the OGaInTe monolayer, all the other configurations are dynamically stable. Further, the mechanical stability has been also investigated via calculations for the elastic constants. Depending on the stacking configuration, $\mathrm{GaIn}X\mathrm{O}$ can be either semiconductor (with a direct or indirect band gap) or metallic. Interestingly, the carrier mobilities of the investigated systems are found to be highly directional isotropic; however, the mobility of electrons is much higher than that of holes, suggesting that these monolayers can be potential materials for applications in nanoscale electronics. Finally, strong optical absorption in a wide light region extending from the infrared to the ultraviolet region is also predicted in the Janus $\mathrm{GaIn}X\mathrm{O}$ depending on its stacking configuration. The findings not only get insights into the fundamental properties of the Janus materials based on group III monochalcogenide monolayers but also suggest them as potential candidates for applications in the next-generation optoelectronic nanodevices.

It is widely accepted that both backscattering and dissipation cannot occur in topological systems because of the topological protection. Here we show that thermal dissipation can occur in the quantum Hall regime in graphene in the presence of dissipation sources, although the Hall plateaus and the zero longitudinal resistance still survive. Dissipation appears along the downstream chiral flow direction of the constriction in the Hall plateau regime, but it occurs mainly in the bulk in the Hall plateau transition regime. In addition, dissipation processes are accompanied with the evolution of the energy distribution from nonequilibrium to equilibrium. This indicates that topology neither prohibits the appearance of dissipation nor prohibits entropy increasing, which opens a new topic on the dissipation in topological systems.

We generate the perturbative expansion of the single particle Green's function and related self-energy for a half-filled single-band Hubbard model on a square lattice. We invoke algorithmic Matsubara integration to evaluate single-particle quantities for real and Matsubara frequencies and verify results through comparison to existing data on the Matsubara axis. With low-order expansions at weak coupling we observe a number of outcomes expected at higher orders: the opening of a gap, pseudogap behavior, and Fermi-surface reconstruction. Based on low-order perturbations, we consider the phase diagram that arises from truncated expansions of the self-energy and Green's function and their relation via the Dyson equation. From Matsubara axis data, we observe insulating behavior in direct expansions of the Green's function, whereas the same order of truncation of the self-energy produces metallic behavior. This observation is supported by additional calculations for real frequencies. We attribute this difference to the order in which diagrams are implicitly summed in the Dyson series. By separating the reducible and irreducible contributions at each order we show that the reducible diagrams implicitly summed in the Dyson equation lead to incorrect physics in the half-filled Hubbard model. Our observations for this particular case lead us to question the utility of the Dyson equation for any problem that shows a disparity between reducible and irreducible contributions to the expansion of the Green's function.

A formalism to describe steady-state electronic and thermal transport in the framework of density functional theory is presented. A one-to-one correspondence is proven between the three basic variables of the theory, i.e., the density on as well as the electrical and heat currents through the junction, and the three basic potentials, i.e., the local potential in as well as the DC bias and thermal gradient across the junction. Consequently, the Kohn-Sham system of the theory requires three exchange-correlations potentials. In linear response, the formalism leads to exact expressions for the many-body transport coefficients (both electrical and thermal conductances and Seebeck coefficient) in terms of both the corresponding Kohn-Sham coefficients and derivatives of the exchange-correlations potentials. The theory is applied to the single impurity Anderson model and an accurate analytic parametrization for these derivatives in the Coulomb blockade regime is constructed through reverse engineering.

The indirect band gap of the diamondlike silicon does not allow direct travel of electrons between valence band and conduction band edges, which has limited its application and performance in optoelectronic devices. Searching new silicon allotropes with highly tunable or direct band gap becomes more and more urgent with the increasing demand on clean energy. Here, we predict a silicon allotrope, $h10$-Si, which is an indirect-gap semiconductor with a band gap of about 0.96 eV. Furthermore, the electronic and optical properties exhibit a strong dependence on strain. In particular, the indirect band gap of the predicted silicon allotrope can be switched to a direct band gap at a uniaxial tensile strain of about 8% and the light absorption can be tuned continuously in a wide range of photon energy. Besides, the single-layer counterpart of this allotrope is confirmed to be dynamically stable and more stable in energy than silicene, endowing it with potential applications in nanoscale devices.

Manipulation of polarization states with metasurfaces is a compelling approach for on-chip photonics and portable information processing. Yet it remains challenging to generate different types of polarization states with a single piece of a metasurface. This paper demonstrates a metasurface to resolve this issue, which is made of L-shaped resonators with different geometrical sizes. Each resonator diffracts a right- or left-handed circularly polarized state with an additional geometrical-scaling-induced phase. The type of polarization state of each diffracted beam is determined by the enantiomorphism, size, and spatial sequence of the resonators in the unit cell. The number of the beams is modulated by the geometry and separation of the resonators. We provide examples to illustrate how to achieve the specific number of diffracted beams with the desired polarization states in experiments. We suggest that this strategy can be applied for integrated photonics and portable quantum information processing.