The nonlinear optical response of organic polaritonic matter has received increasing attention due to their enhanced and controllable nonlinear response and their potential for novel optical devices such as compact photon sources and optical and quantum information devices. Using $z$ scans at different wavelengths and incident powers we have studied the nonlinear optical dispersion of ultrastrongly coupled organic cavity polaritons near the lower polariton band. We show that the up to 150-fold enhancement of the nonlinear response compared to a cavityless organic film arises from an intensity-dependent polaritonic resonant frequency shift (“blueshift”). Consequently, we find that these $z$-scan data can only be described by several terms of a power series expansion in intensity whose respective contributions depend on power broadening and detuning from the lower polariton band. We further show that the nonlinear response can be quantitatively described by a semiclassical three-level molecular model coupled to the cavity in which saturation reduces the Rabi splitting, thus accounting for the lower polariton band's observed blueshift.

We present a theoretical study on the influence of the flexoelectric coupling on the spatial distribution and temperature behavior of the spontaneous polarization for several types of stable domain structure in thin ferroelectric films, such as stripe domains and vortices. Finite-element modeling (FEM) for ${\mathrm{BaTiO}}_3$ films and analytical calculations within the Landau-Ginzburg-Devonshire approach reveals that an out-of-plane polarization component can be very sensitive to the flexoelectric coupling for periodic quasi-2D stripe domains and 3D vortex-antivortex structures. However, the influence is rather different for these structures. The flexoelectric coupling increases significantly the amplitude of a small out-of-plane polarization component in the stripe domains, and the “up” or “down” direction of the component is defined by the sign of the flexoelectric coefficients. Concerning the vortex-antivortex pairs, their antivortices with in-plane antiсirculation have smooth wide dipolar cores through the entire film, whose shape and other features are almost insensitive to the coupling. The vortices with in-plane vorticity have spikelike cores with an out-of-plane quadrupolar moment induced by the flexoelectric coupling. The cores are located near the film–dead-layer interfaces. FEM results corroborated by analytical calculations prove that a change of the flexoelectric coefficient sign leads to a reorientation of the core axial polarization, making the flexosensitive 3D vortices similar to the recently introduced “flexons” in cylindrical nanoparticles. The relatively wide temperature range (from 200 to 400 K) of the flexosensitive vortices' existence gives us confidence that they can be observed experimentally in thin ferroelectric films by scanning-probe and nonlinear optical microscopy methods.

The effective medium representation is fundamental in providing a performance-to-design approach for many devices based on metamaterials. While there are recent works in extending the effective medium concept into the temporal domain, experimental implementation remains an open challenge. Here, we construct an acoustic metamaterial dynamically switching between two different resonance strengths with a time-varying convolution kernel, which can now incorporate both frequency dispersion of metamaterials and temporal modulation. We establish the effective medium formula in temporally averaging the compressibilities, densities, and even Willis coupling parameters of the two configurations. A phase delay between the modulation of different atoms is found to have a negligible effect on the effective medium. Our realization enables a high-level description of metamaterials in the spatiotemporal domain, making many recent proposals, such as magnet-free nonreciprocity, inverse prism, and parametric amplification using space-time metamaterials, possible for implementation in the future.

Systems with strong electron-phonon couplings typically exhibit various forms of charge order, while strong electron-electron interactions lead to magnetism. We use determinant quantum Monte Carlo calculations to solve a model on a square lattice with a caricature of these interactions. In the limit where electron-electron interactions dominate it has antiferromagnetic order, while where electron-phonon coupling dominates there is columnar valence-bond-solid (VBS) order. We find an intervening phase that hosts coexisting nematic and antiferromagnetic orders. We have also found evidence of a Landau-forbidden continuous quantum phase transition with an emergent $O\left(4\right)$ symmetry between the VBS and the nematic antiferromagnetic phases.

Electrical conductance measurements have a limited scope in identifying Andreev edge states (AESs), which form the basis for realizing various topological excitations in quantum Hall (QH)–superconductor (SC) junctions. To unambiguously detect AESs, we measure shot noise along with electrical conductance in a graphene-based QH-SC junction at integer filling $\nu =2$. Remarkably, we find that the Fano factor of shot noise approaches half when the bias energy is less than the superconducting gap ($2\mathrm{\Delta }$), whereas it is close to zero above $2\mathrm{\Delta }$. This is striking, given that, at the same time, the electrical conductance remains quantized at $2e^2/h$ within and above $2\mathrm{\Delta }$. A quantized conductance is expected to produce zero-shot noise due to its dissipationless flow. However, at a QH-SC interface, AESs carry the current in the zero-bias limit and an equal mixing of electron- and holelike states produces half of the Poissonian shot noise with quantized conductance. The observed results are in accord with our detailed theoretical calculations of electrical conductance and shot noise based on the nonequilibrium Green's function method in the presence of disorder. Our results pave the way in using shot noise as a detection tool in the search for exotic topological excitations in QH-SC hybrids.

Graphite, as a well-known carbon-based solid, is a paradigmatic example of the so-called van der Waals layered materials, which display a large anisotropy in their physical properties. Here we study quantum effects in structural and elastic properties of graphite by using path-integral molecular dynamics simulations in the temperature range from 50 to 1500 K. This method takes into account quantization and anharmonicity of vibrational modes in the material. Our results are compared with those found by using classical molecular dynamics simulations. We analyze the volume and in-plane area as functions of temperature and external stress. The quantum motion is essential to correctly describe the in-plane and out-of-plane thermal expansion. Quantum effects cause also changes in the elastic properties of graphite with respect to a classical model. At low temperature we find an appreciable decrease in the linear elastic constants, mainly in $C_{12}$ and $C_{44}$. Quantum corrections in stiffness constants can be in some cases even larger than 20%. The bulk modulus and Poisson's ratio are reduced by 4% and 19%, respectively, due to zero-point motion of the C atoms. These quantum effects in structural and elastic properties of graphite are non-negligible up to temperatures higher than 300 K.

Muonium is a combination of first- and second-generation matter formed by the electrostatic interaction between an electron and an antimuon (${\mu }^{+}$). Although a well-known physical system, their ability to form collective excitations in molecules had not been observed. Here, we give evidence for the detection of a muonium state that propagates in a molecular semiconductor lattice via thermally activated dynamics: a muonium polaron. By measuring the temperature dependence of the depolarization of the muonium state in ${\mathrm{C}}_{60}$, we observe a thermal narrowing of the hyperfine distribution that we attribute to the dynamics of the muonium between molecular sites. As a result of the time scale for muonium decay, the energies involved, charge and spin selectivity, this quasiparticle is a widely applicable experimental tool. It is an excellent probe of emerging electronic, dynamic, and magnetic states at interfaces and in low dimensional systems, where direct spatial probing is an experimental challenge owing to the buried interface, nanoscale elements providing the functionality localization and small magnitude of the effects.

In this work, we investigate the evolution and settling of magnon condensation in the spin-1/2 dimer system ${\mathrm{Sr}}_3{\mathrm{Cr}}_2{\mathrm{O}}_8$ using a combination of magnetostriction in pulsed fields and inelastic neutron scattering in a continuous magnetic field. The magnetic structure in the Bose-Einstein condensation phase was probed by neutron diffraction in pulsed magnetic fields up to 39 T. The magnetic structure in this phase was confirmed to be an $\mathit{XY}$-antiferromagnetic structure validated by irreducible representational analysis. The magnetic phase diagram as a function of an applied magnetic field for this system is presented. Furthermore, zero-field neutron diffraction results indicate that dimerization plays an important role in stabilizing the low-temperature crystal structure.

Spin reorientation and magnetization reversal are two important features of the rare-earth orthorhombic perovskites ($RM{\mathrm{O}}_3$) that have attracted a lot of attention, though their exact microscopic origin has eluded researchers. Here, using density functional theory and classical atomistic spin dynamics we build a general Heisenberg magnetic model that allows to explore the whole phase diagram of the chromite and ferrite compounds and to scrutinize the microscopic mechanism responsible for spin reorientations and magnetization reversals. We show that the occurrence of a magnetization reversal transition depends on the relative strength and sign of two interactions between rare-earth and transition-metal atoms: superexchange and Dzyaloshinskii-Moriya. We also conclude that the presence of a smooth spin reorientation transition between the so-called ${\mathrm{\Gamma }}_4$ and the ${\mathrm{\Gamma }}_2$ phases through a coexisting region, and the temperature range in which it occurs, depends on subtle balance of metal-metal (superexchange and Dzyaloshinskii-Moriya) and metal–rare-earth (Dzyaloshinsky-Moriya) couplings. In particular, we show that the intermediate coexistence region occurs because the spin sublattices rotate at different rates.

Heterostructures made from topological and magnetic insulators promise to form excellent platforms for new electronic and spintronic functionalities mediated by interfacial effects. We report the results of a first-principles density functional theory study of the geometric, electronic structure, and magnetic properties of the $\mathrm{EuS}\left(111\right)/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ interface, including van der Waals and relativistic spin-orbit effects. In contrast to previous theoretical studies, we find no appreciable magnetic anisotropy in such a heterostructure. We also do not see additional induced magnetization at the interface or the magnetic proximity effect on the topological states. This is due to the localized nature of Eu moments and because of a partial charge transfer of $\sim 0.5$ electron from Eu to Se. The formation of the surface dipole shifts the Dirac cone about 0.4 eV below the chemical potential, and the associated electrostatic screening moves the topological state from the first to the second quintuple layer of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$.

We study the chiral magnetic effect (CME) in tilted multi-Weyl semimetals (mWSMs) employing a two-band lattice model. We focus on the type-II phase of mWSMs, introduced by incorporating a Lorentz symmetry violating tilt term. Our work adds to the current understanding of the CME (and also the anomalous Hall effect) in the type-II phase of mWSMs and near the Lifshitz transition by varying tilt. Like the elementary Weyl semimetal case, our results also indicate that the Berry curvature drives the CME for higher monopole charges. We find a peak in the CME at the transition point and discuss its significance using the density of states. Along the way we also examine both observables as a function of the energy separation of the Weyl points.

Whether ${\mathrm{SmB}}_6$ is a topological insulator remains hotly debated. Our density functional theory plus dynamical mean field theory calculations are in excellent agreement with a large range of experiments, from the $4f^{5.5}$ intermediate valency to x-ray and photoemission spectra. We show that ${\mathrm{SmB}}_6$ is a strongly correlated topological insulator, albeit not a Kondo insulator, using a symmetry analysis of the “pole extended” Hamiltonian, which fully captures the effect of the self-energy arising from the strong electron-electron correlation within the Sm $4f$ orbitals. The topological surface states are analyzed, addressing conflicting interpretations of photoemission data.

In graphene moiré superlattices, electronic interactions between layers are mostly hidden as band structures get crowded because of folding, making their interpretation cumbersome. Here, the evolution of the electronic band structure as a function of the interlayer rotation angle is studied using density functional theory followed by unfolding bands and then comparing them with their corresponding individual components. We observe interactions in regions not theoretically elucidated so far, where for small interlayer angles, gaps turn into discretelike states that are evenly spaced in energy. We find that $V_{pp\sigma }$ attractive interactions between out-of-plane orbitals from different layers are responsible for the discretization. Furthermore, when the interlayer angle becomes small, these discrete, evenly spaced states have energy differences comparable to graphene phonons. Thus they might be relevant to explain electron-phonon-assisted effects, which have been experimentally observed in graphene moiré superlattices.

The complex behavior of magnetoconductance of Bi films grown epitaxially on Si(111) with a thickness of 20–100 bilayers (BL) was measured at $T$= 9 K in magnetic fields up to $B=4\mathrm{T}$, oriented in-plane parallel and perpendicular to the electric dc current $I$. Contributions to magnetoconductance (MC) by diffuse scattering, by weak localization (WL) as well as by weak antilocalization (WAL) were identified. All these components to MC turned out to be isotropic in two dimensions, i.e., no dependence on angle between $B$ and $I$ within the surface plane was found. Only for $B\perp I$ an increase of MC was detected that is, to first approximation, $\propto B^2$. It is ascribed to ballistic scattering between the Rashba-split interfaces that allow Umklapp scattering without spin flip. While MC within the surface states, dominant at small thicknesses, $d$, shows negligible diffuse scattering under the chosen geometry, their quantum corrections are characterized by WAL with $\alpha =-0.3$ and a coupling strength that decays $\propto 1/d$ with layer thickness. The admixing of quantized bulk states, which dominates MC above 50 BL, not only increases diffuse scattering, it introduces WL in combination with WAL. Presumably due to hybridization with the surface states, it also modifies strongly the WAL component for $d>60$ BL. Thus our findings suggest an intriguing interplay in magnetotransport between 2D and quantized 3D states at the Fermi surface of ultrathin bismuth quantum films and provide further deep insight into the electronic transport in quantized and partly spin split bands.

We study the impact of classical short-range nonlinear interactions on transport in lattices with no dispersion. The single-particle band structure of these lattices contains flat bands only, and cages noninteracting particles into compact localized eigenstates. We demonstrate that there always exist local unitary transformations that detangle such lattices into decoupled sites in dimension 1. Starting from a detangled representation and inverting the unitary transformations, we arrive at the all-bands-flat generator for single-particle Hamiltonians in one dimension, which is also straightforwardly generalized to higher dimensions. The entangling unitary transformations are parametrized by sets of angles. For a given member of the set of all-bands-flat, additional short-range nonlinear interactions destroy caging in general, and induce transport. However, fine-tuned subsets of the unitary transformations allow caging to be completely restored. We derive the necessary and sufficient fine-tuning conditions for nonlinear caging, and we provide computational evidence of our conclusions for one-dimensional systems.

We consider translationally invariant tight-binding all-bands-flat networks which lack dispersion. In a recent work [C. Danieli et al., Phys. Rev. B 104, 085131 (2021)], we identified the subset of these networks that shows nonlinear caging in the presence of Kerr-like local nonlinearities, i.e., it preserves nonexcited network sites and therefore keeps compact excitations compact. Here we replace nonlinear terms by Bose-Hubbard interactions and study quantum caging. We identify the quantum caging conditions that are related to the nonlinear caging conditions and that guarantee the existence of an extensive set of conserved quantities in any lattice dimension, as first revealed in Tovmasyan et al. [Phys. Rev. B 98, 134513 (2018)] for a set of specific networks. Consequently, transport is realized through moving pairs of interacting particles that break the single-particle caging. We further prove the existence of degenerate energy renormalized compact states for any finite number $M$ of participating particles in one- and higher-dimensional lattice—results that explain and generalize previous observations for two particles on a diamond chain [Vidal et al., Phys. Rev. Lett. 85, 3906 (2000)].

Motivated by the recent synthesis of two-dimensional monolayer AlSb, we theoretically investigate its ground-state and electronic properties using the first-principles calculations coupled with Bethe-Salpeter equation. An excitonic instability is revealed as a result of larger exciton binding energy than the corresponding one-electron energy gap by $\sim 0.1$ eV, which is indicative of a many-body ground state accompanied by spontaneous exciton generation. Spin-orbit coupling is proven to play a vital role in the prediction of the ground state. At room temperature, the two-dimensional monolayer AlSb is expected to transform into a direct gap semiconductor with phonon-limited electron and hole mobilities both around 1700 ${\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$. These results show that monolayer AlSb may provide a promising platform for realization of the excitonic insulator and for applications in the next-generation electronic devices.

Control of magnetism by an applied electric field is a desirable technique for the functionalization of magnetic materials. Motivated by recent experiments, we study the electric field control of the interfacial magnetism of ${\mathrm{CaRuO}}_3/{\mathrm{CaMnO}}_3$ (CRO/CMO) (001), a prototype interface between a nonmagnetic metal and an antiferromagnetic insulator. Even without the electric field, the interfacial CMO layer acquires a ferromagnetic moment due to a spin-canted state, caused by the Anderson-Hasegawa double exchange (DEX) between the Mn moments and the leaked electrons from the CRO side. An electric field would alter the carrier density at the interface, leading to the possibility of controlling the magnetism, since DEX is sensitive to the carrier density. We study this effect quantitatively using density-functional calculations in the slab geometry. We find a textbook-like dielectric screening of the electric field, which introduces polarization charges at the interfaces and the surfaces. The extra charge at the interface enhances the ferromagnetism via the DEX interaction, while away from the interface the original antiferromagnetic (AFM) state of the Mn layers remains unchanged. The effect could have potential application in spintronics devices.