Magnetic and crystallographic transitions in the Cairo pentagonal magnet ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ are investigated by means of infrared synchrotron-based spectroscopy as a function of temperature (20–300 K) and pressure (0–15.5 GPa). One of the phonon modes is shown to exhibit an anomalous softening as a function of temperature in the antiferromagnetic phase below 240 K, highlighting spin-lattice coupling. Moreover, under applied pressure at 40 K, an even larger softening is observed through the pressure-induced structural transition. Lattice dynamical calculations reveal that this mode is indeed very peculiar as it involves a minimal bending of the strongest superexchange path in the pentagonal planes, as well as a decrease in the distances between second-neighbor irons. The latter confirms the hypothesis made by Friedrich et al., [J. Phys.: Condens. Matter 24, 145401 (2012)] about an increase in the oxygen coordination of irons being at the origin of the pressure-induced structural transition. As a consequence, one expects a new magnetic superexchange path that may alter the magnetic structure under pressure.

Superconducting quantum circuits is one of the leading candidates for a universal quantum computer. Designing novel qubit and multiqubit superconducting circuits requires the ability to simulate and analyze the properties of a general circuit. In particular, going outside the transmon approach, we cannot make assumptions on anharmonicity, thus precluding blackbox quantization approaches and necessitating the formal circuit quantization approach. We consider and solve two issues involved in simulating general superconducting circuits. One of the issues is the handling of free modes in the circuit, that is, circuit modes with no potential term in the Hamiltonian. Another issue is circuit size, namely, the challenge of simulating strongly coupled multimode circuits. The main mathematical tool we use to address these issues is the linear canonical transformation in the setting of quantum mechanics. We address the first issue by giving a provably correct algorithm for removing free modes by performing a linear canonical transformation to completely decouple the free modes from other circuit modes. We address the second by giving a series of different linear canonical transformations to reduce intermode couplings, thereby reducing the problem to the weakly coupled case and greatly mitigating the overhead for classical simulation. We benchmark our decoupling methods by applying them to the circuit of two inductively coupled fluxonium qubits, obtaining several orders of magnitude reduction in the size of the Hilbert space that needs to be simulated.

We present a numerical algorithm for the simulation of resonant x-ray magnetic scattering in reflection geometry for an arbitrary three-dimensional magnetization distribution over a multilayer sample utilizing the distorted-wave Born approximation. Our approach takes the medium boundary matrix approach for specular reflection, and the Born approximation typically used for off-specular reflection, combines the two, and adds the contribution from transmission through the sample before and after a reflection event. The algorithm is then tested on experimental data from an ${\mathrm{Al}}_2{\mathrm{O}}_3$/Co/Pt multilayer sample with hybrid Néel/Bloch/Néel domain walls at incidence angles at and near multilayer Bragg angles, and photon energies near the Co $L_3$ absorption edge, achieving high levels of agreement with experimental data. Incorporating the transmission components into the algorithm was found to explain the dichroism observed in scattering from Bloch-type domain walls, and uncovered the likely importance of diffuse scattering in transmission from the polycrystalline grain walls along the optical path of the x rays in the sample—a theme which deserves further investigation.

We have measured the angular dependence of ferromagnetic resonance (FMR) spectra for Fibonacci-distorted, kagome artificial spin ice. The number of strong modes in the FMR spectra depend on the orientation of the applied DC magnetic field. In addition, discontinuities observed in the FMR field-frequency dispersion curves also depend on DC field orientation, and signal a multistep DC magnetization reversal, which is caused by the reduced energy degeneracy of Fibonacci-distorted vertices. The results suggest that the orientation of applied magnetic field and severity of Fibonacci distortion constitute control variables for FMR modes and multistep reversal in future magnonic devices and magnetic switching systems.

We have used micromagnetic simulations to model backward-volume dipole-exchange spin waves in graded profiles of the bias magnetic field. We demonstrate spin-wave wavelength conversion upon the wave's reflection from turning points due to the two characteristic minima (“$U$ points”) occurring in their dispersion at finite (nonzero) wave vectors. As a result, backward-volume dipole-exchange spin waves confined in “spin-wave wells” either form Möbius modes, making multiple real-space turns for each reciprocal-space round trip, or split into pairs of degenerate modes in the valleys near the two $U$ points. The latter modes may therefore be assigned a pseudospin. We show that the pseudospin can be switched by scattering the spin wave from decreases of the bias magnetic field, while it is immune to scattering from field increases. Pseudospin creation and read-out can be accomplished using chiral spin-wave transducers, as described in Au et al. [Appl. Phys. Lett. 100, 182404 (2012)] and Au et al. [Appl. Phys. Lett. 100, 172408 (2012)], respectively. Taken together, the possibility of pseudospin creation, manipulation, and read-out suggests a path to development of a spin-wave version of valleytronics (“magnon valleytronics”), in which the pseudospin (rather than amplitude or phase) of spin waves would be used to encode data. Our results are not limited to graded bias magnetic field but can be generalized to other magnonic media with spatially varying characteristics, produced using the toolbox of graded index magnonics.

Searching for and investigating supersolids is a long-term outstanding problem in physics. In addition to the solid element ${}^4\mathrm{He}$ and cold atoms as potential candidates for the supersolid, the quantum system of light realized by circuit quantum electrodynamics is also a promising platform. In this paper, we propose a supersolid phase, i.e., a superradiant supersolid of light, where superradiant, superfluid, and solid orders coexist. We theoretically simulate the extended Jaynes-Cummings-Hubbard model describing the circuit quantum electrodynamics systems, mainly by the large-scale worm quantum Monte Carlo method, and find that a superradiant supersolid phase exists on triangular lattices due to the antiferromagnetic correlation between photons via light-atom coupling. We also confirm that the previous supersolid of light given by Bujnowski et al. [Phys. Rev. A 90, 043801 (2014)] is not stable. The phase transition between our superradiant supersolid phase and the superradiant solid phase can be continuous (first order) and above (below) the “symmetry point.” This is not the same as the pure Bose-Hubbard model on triangular lattices. The results herein could help in the search for a new superradiant supersolid phase in circuit quantum electrodynamic experiments and other light-matter coupling systems.

Topological superconductors are exotic gapped phases of matter hosting Majorana mid-gap states on their boundaries. In conventional three-dimensional topological superconductors, Majorana in-gap states appear in the form of spin-1/2 fermions with a quasirelativistic dispersion relation. Here, we show that unconventional Majorana states can emerge on the surface of three-dimensional topological superconductors protected by rotational symmetry. The unconventional Majorana surface states are classified into three different categories: a spin-$s$ Majorana fermion with $(2s+1)$-fold degeneracy $(s\ge 3/2)$, a Majorana Fermi line carrying two distinct topological charges, and a quartet of spin-1/2 Majorana fermions related by fourfold rotational symmetry. The spectral properties of the first two types, which go beyond conventional spin-1/2 fermions, are unique to topological superconductors and have no counterparts in topological insulators. We show that unconventional Majorana surface states can be obtained in the superconducting phase of doped $Z_2$ topological insulators or Dirac semimetals with rotational symmetry.

The sensitivity of density functional theory plus dynamical mean field theory calculations to different constructions of the correlated orbitals is investigated via a detailed comparison of results obtained for the quantum material ${\mathrm{NdNiO}}_2$ using different Wannier and projector methods to define the correlation problem. Using the same interaction parameters we find that the different methods produce different results for the orbital and band basis mass enhancements and for the orbital occupancies, with differing implications regarding the importance of multiorbital effects and charge transfer physics. Using interaction parameters derived from the constrained random phase approximation enhances the difference in results. For the isostructural cuprate ${\mathrm{CaCuO}}_2$, the different methods give quantitatively different mass enhancements but still result in the same qualitative physics.

We study the Kitaev spin-$1/2$ ladder, a model which exhibits self-localization due to fractionalization caused by exchange frustration. When a weak magnetic field is applied, the model is described by an effective fermionic Hamiltonian, with an additional time-reversal symmetry-breaking term. We show that this term alone is not capable of delocalizing the system but flux mobility is a prerequisite. For magnetic fields larger but comparable to the flux gap, fluxes become mobile and drive the system into a delocalized regime, featuring finite dc transport coefficients. Our findings are based on numerical techniques, exact diagonalization, and dynamical quantum typicality, from which we present results for the specific heat, the dynamical energy current correlation function, as well as the inverse participation ratio, contrasting the spin against the fermion representation.

The topology of electronic states in band insulators with mirror symmetry can be classified in two different ways. One is in terms of the mirror Chern number, an integer that counts the number of protected Dirac cones in the Brillouin zone of high-symmetry surfaces. The other is via a ${\mathbb{Z}}_2$ index that distinguishes between systems that have a nonzero quantized orbital magnetoelectric coupling (“axion-odd”), and those that do not (“axion-even”); this classification can also be induced by other symmetries in the magnetic point group, including time reversal and inversion. A systematic characterization of the axion ${\mathbb{Z}}_2$ topology has previously been obtained by representing the valence states in terms of hybrid Wannier functions localized along one chosen crystallographic direction, and inspecting the associated Wannier band structure. Here we focus on mirror symmetry, and extend that characterization to the mirror Chern number. We choose the direction orthogonal to the mirror plane as the Wannierization direction and show that the mirror Chern number can be determined from the winding numbers of the touching points between Wannier bands on mirror-invariant planes and from the Chern numbers of flat bands pinned to those planes. In this representation, the relation between the mirror Chern number and the axion ${\mathbb{Z}}_2$ index is readily established. The formalism is illustrated by means of ab initio calculations for SnTe in the monolayer and bulk forms, complemented by tight-binding calculations for a toy model.

Square-octagon lattice underlies the description of a family of two-dimensional materials such as tetragraphene. In the present paper we show that the tight-binding model of square-octagon lattice contains both conventional and high-order van Hove points. In particular, the spectrum of the model contains flat lines along some directions composed of high-order saddle points. Their role is analyzed by calculating the orbital susceptibility of electrons. We find that the presence of van Hove singularities (VHS) of different kinds in the density of states leads to strong responses: paramagnetic for ordinary singularities and more complicated for high-order singularities. It is shown that at doping level of high-order VHS the orbital susceptibility as a function of hoppings ratio $\alpha$ reveals the dia- to paramagnetic phase transition at $\alpha \approx 0.94$. This is due to the competition of paramagnetic contribution of high-order VHS and diamagnetic contribution of Dirac cones. The results for the tight-binding model are compared with the low-energy effective pseudospin-1 model near the three band touching point.

We study the topological properties of a spin-orbit coupled tight-binding model with flux on the kagome lattice. The model is time-reversal invariant and realizes a ${\mathbb{Z}}_2$ topological insulator as a result of artificial gauge fields. We develop topological arguments to describe this system showing three inequivalent sites in a unit cell and a flat band in its energy spectrum in addition to the topological dispersive energy bands. We show the stability of the topological phase towards spin-flip processes and different types of on-site potentials. In particular, we also address the situation where on-site energies may differ inside a unit cell. Moreover, a staggered potential on the lattice may realize topological phases for the half-filled situation. Another interesting result is the occurrence of a topological phase for large on-site energies. To describe topological properties of the system we use a numerical approach based on the twisted boundary conditions and we develop a mathematical approach, related to smooth fields.

Heterostructures comprising of polar and nonpolar perovskites, such as $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$, have been extensively studied because of the discovery of a two-dimensional electron (or hole) gas [2DEG (or 2DHG)] at the interface. $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ is chemically similar to $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ with polar/nonpolar interfaces. However, $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ exhibits spontaneous ferroelectric polarization, which leads to interesting effects in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$. Our theoretical simulations exhibit some different physical phenomena in this system, including interface-induced ferroelectric domains, monodirectional polarization, and 2DEGs (or 2DHGs) at the domain walls. Furthermore, it was identified that the observed 2DEGs (or 2DHGs) were spin polarized and exhibited ferromagnetism. The origin of the ferromagnetic 2DEGs (or 2DHGs) is discussed. These spin-polarized 2DEGs (or 2DHGs) observed in our $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ model would be important for fundamental research and applications.

Recent proof-of-principle experiments have demonstrated the implementation of neuromorphic computing using exciton-polaritons, making use of coherent classical states [D. Ballarini et al., Nano Lett. 20, 3506 (2020)]. At the same time, it is expected that nonlinear exciton polaritons can reach a quantum regime forming nonclassical states. Here we consider theoretically the quantum nature of exciton-polaritons, and we predict a superpolynomial quantum enhancement in image recognition tasks. This is achieved within experimentally accessible parameters.

We simulated electron transport across a binary nanocrystal solid (BNS) of PbSe NCs with diameters of 6.5 nm and 5.1 nm. We used our hierarchical nanoparticle transport simulator (HINTS) to model the transport in these BNSs. The mobility exhibits a minimum at a large-NC-fraction $f_{\mathrm{LNC}}=0.25$. The mobility minimum is deep at $T=80\mathrm{K}$ and partially smoothed at $T=300\mathrm{K}$. We explain this minimum as follows. As the LNC fraction $f_{\mathrm{LNC}}$ starts growing from zero, the few LNCs act as traps for the electrons traversing the BNS because their relevant energy level is lower. Therefore, increasing the $f_{\mathrm{LNC}}$ concentration of these traps decreases the mobility. As increasing $f_{\mathrm{LNC}}$ reaches the percolation threshold $f_{\mathrm{LNC}}$=$f_P$, the LNCs form sample-spanning networks that enable electrons to traverse the entire BNS via these percolating LNC networks. Transport through the growing percolating LNC networks drives the rapid growth of the mobility as $f_{\mathrm{LNC}}$ grows past $f_P$. Therefore, the electron mobility exhibits a pronounced minimum as a function of $f_{\mathrm{LNC}}$, centered at $f_{\mathrm{LNC}}=f_P$. The position of the mobility minimum shifts to larger LNC fractions as the electron density increases. We have studied the trends of this mobility minimum with temperature, electron density, charging energy, ligand length, and disorder. We account for the trends by a renormalized-trap model, in which capturing an electron renormalizes a deep LNC trap into a shallow trap or a kinetic obstacle, depending on the charging energy. We verified this physical picture by constructing and analyzing heat maps of the mobile electrons in the BNS.

Reading out Majorana bound states (MBSs) is essential both to verify their non-Abelian property and to realize topological quantum computation. Here, we construct a protocol to measure the parity of two MBSs in a Majorana island coupled to a double quantum dot (DQD). The parity information is mapped to the charge state of the DQD through Landau-Zener transition. The operation needed is sweeping the bias of the DQD, which is followed by charge sensing. In the case without fine-tuning, a single run of sweep and detection implements a weak measurement of the parity. We find that in general a sequence of about ten runs would completely project a superposition state to either parity, and the charge detection in each run records how the state of the MBSs collapses step by step. Remarkably, this readout protocol is a nondemolition measurement and robust to low-frequency charge fluctuation.

Determining the correct electronic structure of ${\mathrm{U}}_3{\mathrm{O}}_8$ remains a formidable experimental and theoretical challenge. In the low-temperature phase, two crystallographic U sites are separated into a distinct 2U(V)$+1\mathrm{U}$(VI) oxidation configuration. At low temperatures, the U(V) sites form a distorted honeycomb lattice, but the U(VI) sit on a triangular sublattice, suggesting potential for magnetic frustration effects. The spin configuration of the unpaired $f$ electrons on the U(V) sites is likely antiferromagnetic (AFM) from susceptibility measurements, but this has not been confirmed. Here, we present a neutron scattering investigation of the structure and dynamics of ${\mathrm{U}}_3{\mathrm{O}}_8$ from 1.7 to 600 K. We confirm static AFM ordering onset at between 22 and 25 K, which is present down to at least 1.7 K with AFM peaks corresponding to [0.5 1 1] and [0.5 2 2] in the orthorhombic phase. These measurements rule out static AFM order along the $a$ axis of the $Amm2$ phase, a configuration previously suggested by theory. Above 100 K a quasielastic scattering channel opens that we speculate arises from a lattice relaxation response to thermally activated electron hopping. This term does not conform to a magnetic form factor, so it is not related to spin relaxations. If correct, this mechanism stabilizes a continuous valence transition from $2\mathrm{U}\left(\mathrm{V}\right)+1\mathrm{U}\left(\mathrm{VI}\right)$ in the low-temperature ($T<600$ K) orthorhombic phase to the hexagonal phase that contains only one degenerate U site, wherein the U valence can be dynamically stabilized between $\mathrm{U}\left(\mathrm{V}\right)\leftrightarrow \mathrm{U}\left(\mathrm{VI}\right)$ by phonon-assisted electron hopping.

The near-edge fine structure in electron energy loss spectra is used to probe the electronic bonding environment of materials at high spatial resolution. Often, however, deviations from the ground state electronic properties are observed, due to the core hole created within the ionized atom. A method is proposed to remove core hole distortion from experimental electron energy loss spectra by calculating the electrodynamic work done in separating the moving, incident electron from the oppositely charged core hole. Dynamic screening of the core hole is modeled using the material dielectric properties. The resulting energy gain spectrum is deconvolved from the experimental measurement to give a “fully screened” spectrum that is free of core hole distortion. The method is tested on core loss edges in elemental Si, SiC, and ${\mathrm{SiO}}_x$. Despite assuming classical electrodynamics, the fundamental principle of an energy gain correction can be shown to be consistent with quantum mechanics, although shortcomings in some of the assumptions made on the nature of the core hole are also identified.

We present a coupled-wire construction of a model with chiral fracton topological order. The model combines the known construction of $\nu =1/m$ Laughlin fractional quantum Hall states with a planar p-string condensation mechanism. The bulk of the model supports gapped immobile fracton excitations that generate a hierarchy of mobile composite excitations. Open boundaries of the model are chiral and gapless, and can be used to demonstrate a fractional quantized Hall conductance where fracton composites act as charge carriers in the bulk. The planar p-string mechanism used to construct and analyze the model generalizes to a wide class of models including those based on layers supporting non-Abelian topological order. We describe this generalization and additionally provide concrete lattice-model realizations of the mechanism.

The influence of the surface running and surface localized plasmons on the optical properties (including Faraday and magneto-optical Kerr effects) is studied and compared for homogeneous as well as perforated metal-dielectric, sandwichlike structures. Closed-form exact expressions are derived for the macroscopic permittivity tensor of an assembly of metal-dielectric parallel flat slabs in the presence of an externally applied static magnetic field perpendicular to the slab plane. The structure-dependent resonance associated with running surface plasmon appears only in the perpendicular to the surface $zz$ macroscopic permittivity tensor component and does not depend on the perpendicularly applied magnetic field. The other diagonal tensor components vanish at the so-called “hybrid cyclotron-plasma frequency,” which depends on the slab thickness and the applied magnetic field. All analytical results are verified by numerical simulations.