Despite their structural similarities, ${\mathrm{SrBi}}_2{\mathrm{Ta}}_2{\mathrm{O}}_9$ (SBT) and ${\mathrm{SrBi}}_2{\mathrm{Nb}}_2{\mathrm{O}}_9$ (SBN) undergo a different sequence of phase transitions. The phase diagram of SBT as a function of the temperature includes an intermediate phase between the high-temperature phase and the ferroelectric ground state, while in the niobium compound the intermediate phase is suppressed and a direct transition between the high- and low-temperature structures is observed. We present ab initio calculations that reveal the relevance of a trilinear coupling between three symmetry-adapted modes to stabilize the ground state in both compounds. This coupling is much stronger in SBN than in SBT. Within the framework of the phenomenological Landau theory, it is shown that, by solely increasing the strength of the trilinear coupling, the topology of the phase diagram of SBT can change enough to suppress the intermediate phase. Monte Carlo simulations on an idealized ${\varphi }^4$ Hamiltonian confirm that the trilinear coupling is the key parameter that determines the sequence of phase transitions, and that for higher dimensionality of the order parameters the stability region of the intermediate phase is narrower.

We construct a series of bbosonic-symmetry-protected topological (BSPT) states beyond group cohomology classification using “decorated defects” approach. This construction is based on topological defects of ordinary Landau order parameters, decorated with the bosonic short-range entangled (BSRE) states in $(4k+3)d$ and $(4k+5)d$ space-time (with $k$ being nonnegative integers), which do not need any symmetry. This approach not only gives these BSPT states an intuitive physical picture, it also allows us to derive the effective field theory for all these BSPT states beyond group cohomology.

The structural, magnetic, and magnetocaloric properties of epitaxial Ni-Co-Mn-Al thin films with different compositions have been studied. The films were deposited on MgO(001) substrates by co-sputtering on heated substrates. All films show a martensitic transformation, where the transformation temperatures are strongly dependent on the composition. The structure of the martensite phase is shown to be $14M$. The metamagnetic martensitic transformation occurs from strongly ferromagnetic austenite to weakly magnetic martensite. The structural properties of the films were investigated by atomic force microscopy and temperature dependent x-ray diffraction. Magnetic and magnetocaloric properties were analyzed using temperature dependent and isothermal magnetization measurements. We find that ${\mathrm{Ni}}_{41}{\mathrm{Co}}_{10.4}{\mathrm{Mn}}_{34.8}{\mathrm{Al}}_{13.8}$ films show giant inverse magnetocaloric effects with magnetic entropy change of 17.5 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for ${\mu }_0\Delta H=5$ T.

We consider the Rényi entanglement entropy of bosonic Tomonaga-Luttinger liquids under a particle bipartition and demonstrate that the leading order finite-size scaling is logarithmic in the system size with a prefactor equal to the inverse Luttinger parameter. While higher-order corrections involve a microscopic length scale, the leading-order scaling depends only on this sole dimensionless parameter which characterizes the low-energy quantum hydrodynamics. This result contrasts the leading entanglement entropy scaling under a spatial bipartition, for which the coefficient is universal and independent of the Luttinger parameter. Using quantum Monte Carlo calculations, we explicitly confirm the scaling predictions of Tomonaga-Luttinger liquid theory for the Lieb-Liniger model of $\delta$-function interacting bosons in the one-dimensional spatial continuum.

We present a comparative analysis of superconducting and charge-density-wave orders in the spin-fluctuation scenario for the cuprates. That spin-fluctuation exchange gives rise to $d$-wave superconductivity is well known. Several groups recently argued that the same spin-mediated interaction may also account for charge-density-wave order with momenta $(Q,0)$ or $(0,Q)$, detected in underdoped cuprates. This has been questioned on the basis that charge-density-wave channel mixes fermions from both nested and antinested regions on the Fermi surface, and fermions in the antinested region do not have a natural tendency to form a bound state, even if the interaction is attractive. We show that antinesting is not an obstacle for charge order, but to see this one needs to go beyond the conventional Eliashberg approximation. We show that in the perfect nesting/antinesting case, when the velocities of hot fermions are either parallel or antiparallel, the onset temperatures in superconducting and charge-density-wave channels are of comparable strength for any magnetic correlation length $\xi$. The superconducting $T_{\mathrm{sc}}$ is larger than $T_{\mathrm{cdw}}$, but only numerically. When the velocities of hot fermions are not strictly parallel/antiparallel, $T_{\mathrm{cdw}}$ progressively decreases as $\xi$ decreases and vanishes at some critical $\xi$.

We have performed high-resolution angle-resolved photoemission spectroscopy on layered polymorph series of $1H-, 5H-$, and $6H-\mathrm{CaAlSi}$, which exhibit superconductivity below 6.5, 5.7, and 7.7 K, respectively. While the overall band structure in the valence-band region is similar among these compounds, the volume of the Fermi surface at the $M$ point and the magnitude of the superconducting gap are markedly different from each other. Implications of such variation in the electronic structure due to the superstructure along the $c$ axis are discussed in relation to the physical properties of CaAlSi.

We determine the electronic structure and phase stability of paramagnetic ${\text{V}}_2{\text{O}}_3$ at the Mott-Hubbard metal-insulator transition (MIT) by employing a combination of an ab initio method for calculating band structures with dynamical mean-field theory. The structural transformation associated with the MIT occurs upon a slight expansion of the lattice volume by $\sim 1.5$%, in agreement with experiment. Our results show that the structural transition precedes the MIT, implying a complex interplay between electronic and lattice degrees of freedom. The MIT is found to be driven by a strong correlation-induced, orbital-selective renormalization of the V $t_{2g}$ bands. The effective electron mass of the $e_g^{\pi }$ orbitals diverges at the MIT. Our results show that full charge self-consistency is crucial for a correct description of the physical properties of ${\text{V}}_2{\text{O}}_3$.

We use the density matrix renormalization group method to study the interplay of the localized and itinerant behaviors in the one-dimensional Kondo-Heisenberg model. We find signatures of simultaneously localized and itinerant behaviors of the local spins and attribute this duality to their simultaneous entanglement within the spin chain and with conduction electrons due to incomplete hybridization. We propose a microscopic definition of the hybridization parameter that measures this “partial” itinerancy. Our results provide a microscopic support for the dual nature of $f$ electrons and the resulting two-fluid behavior widely observed in heavy electron materials.

The dielectric function and the effective conjugation length of an organic material determine the binding energy of an exciton formed after photon absorption. In ladder polymers the conjugation length is large and the size of the exciton is in principle not limited by its conjugation length, since it can delocalize along the backbone of the polymer. In this work, the anisotropic dielectric function of poly(benzimidazobenzophenanthroline) (BBL) in the UV-visible and infrared spectral range is obtained by DFT calculations and by modeling ellipsometric data, revealing both ionic and electronic contributions. For spin-coated BBL polymer thin films, we show uniaxial anisotropic behavior with high differences between the in-plane and out-of-plane dielectric function. We obtain a high dielectric constant of 8.3 in the direction of the polymer.

The precise adjustment of the polariton condensate flow under incoherent excitation conditions is an indispensable prerequisite for polariton-based logic gate operations. In this report, an all-optical approach for steering the motion of a polariton condensate using only nonresonant excitation is demonstrated. We create arbitrarily shaped functional potentials by means of a spatial light modulator, which allow for tailoring the condensate state and guiding a propagating condensate along reconfigurable pathways. Additional numerical simulations confirm the experimental observations and elucidate the interaction effects between background carriers and polariton condensates.

Using one of the methods of quantum nonequilibrium statistical physics, we have investigated the spin transport transverse to the normal metal/ferromagnetic insulator interface in hybrid nanostructures. An approximation of the effective parameters, when each of the interacting subsystems (electron spin, magnon, and phonon) is characterized by its own effective temperature, has been considered. The generalized Bloch equations which describe the spin-wave current propagation in the dielectric have been derived. Finally, two sides of the spin transport “coin” have been revealed: the diffusive nature of the magnon motion and magnon relaxation processes, responsible for the spin pumping, and the spin-torque effect.

Inducing magnetism onto a topological crystalline insulator (TCI) has been predicted to result in several novel quantum electromagnetic effects. This is a consequence of the highly strain-sensitive band topology of such symmetry-protected systems. We thus show that placing the TCI surface of SnTe in proximity to EuS—a ferromagnetic insulator—induces magnetism at the interface between SnTe and EuS, and thus breaks time-reversal symmetry in the TCI. Magnetotransport experiments on SnTe-EuS-SnTe trilayer devices reveal a hysteretic lowering of the resistance at the TCI surface that coincides with an increase in the density of magnetic domain walls. This additional conduction could be a signature of topologically protected states within domain walls. Additionally, a hysteretic anomalous Hall effect reveals that the usual in-plane magnetic moment of the EuS layer is canted towards a perpendicular direction at the interface. These results are evidence of induced magnetism at the SnTe-EuS interfaces, resulting in broken time-reversal symmetry in the TCI.

We theoretically investigate the thermoelectric properties of heat engines based on Mach-Zehnder interferometers. The energy dependence of the transmission amplitudes in such setups arises from a difference in the interferometer arm lengths. Any thermoelectric response is thus of purely quantum-mechanical origin. In addition to an experimentally established three-terminal setup, we also consider a two-terminal geometry as well as a four-terminal setup consisting of two interferometers. We find that Mach-Zehnder interferometers can be used as powerful and efficient heat engines which perform well under realistic conditions.

In this paper we clarify the nature of $\pi$ and $\pi +\sigma$ electron excitations in pristine graphene. We clearly demonstrate the continuous transition from single particle to collective character of such excitations and how screening modifies their dispersion relations. We prove that $\pi$ and $\pi +\sigma$ plasmons do exist in graphene, though occurring only for a particular range of wave vectors and with finite damping rate. Particular attention is paid to comparing the theoretical results with available EELS measurements in optical $\left(\mathrm{Q}\approx 0\right)$ and other $\left(\mathrm{Q}\ne 0\right)$ limits. The conclusions, based on microscopic numerical results, are confirmed in an approximate analytical approach.

We analyze the origin of spin-orbit coupling (SOC) in fluorinated graphene using density functional theory (DFT) and a tight-binding model for the relevant orbitals. As it turns out, the dominant source of SOC is the atomic spin-orbit of fluorine adatoms and not the impurity-induced SOC based on the distortion of the graphene plane as in hydrogenated graphene. More interestingly, our DFT calculations show that SOC is strongly affected by both the type and concentrations of the graphene's carriers, being enhanced by electron doping and reduced by hole doping. This effect is due to the charge transfer to the fluorine adatom and the consequent change in the fluorine-carbon bonding. Our simple tight-binding model, which includes the SOC of the $2p$ orbitals of F and effective parameters based on maximally localized Wannier functions, is able to account for the effect. The strong enhancement of the SOC induced by graphene doping opens the possibility to tune the spin relaxation in this material.

We construct the theory of a chiral Luttinger liquid that lives on the boundary of a Galilean invariant quantum Hall fluid. In contrast to previous studies, Galilean invariance of the total (bulk plus edge) theory is guaranteed. We consider electromagnetic response at the edge and calculate momentum- and frequency-dependent electric conductivity and argue that its experimental measurement can provide a new means to determine the “shift” and bulk Hall viscosity.

Exchange and Rashba spin-orbit interactions are expected to couple at ferromagnetic-metal/heavy-metal interfaces and influence their electronic structure. We examine these largely unexplored effects in Co(0001) films on W(110) by angle-resolved photoemission spectroscopy. We find breaking of inversion symmetry in the band dispersion of magnetic quantum well states along structurally equivalent directions of the Co films. Ab initio calculations ascribe this unusual band behavior to the relative orientation of coexisting exchange and Rashba fields. These findings elucidate fundamental interface electronic effects, which are associated to spin-orbit phenomena at ferromagnetic-metal/heavy-metal junctions.

We determine the effect of defects induced by ion bombardment on the Raman spectrum of single-layer molybdenum disulfide. The evolution of both the linewidths and frequency shifts of the first-order Raman bands with the density of defects is explained with a phonon confinement model, using density functional theory to calculate the phonon dispersion curves. We identify several defect-induced Raman scattering peaks arising from zone-edge phonon modes. Among these, the most prominent is the $\mathrm{LA}\left(M\right)$ peak at $\sim 227\mathrm{c}{\mathrm{m}}^{-1}$ and its intensity, relative to the one of first-order Raman bands, is found to be proportional to the density of defects. These results provide a practical route to quantify defects in single-layer $\mathrm{Mo}{\mathrm{S}}_2$ using Raman spectroscopy and highlight an analogy between the $\mathrm{LA}\left(M\right)$ peak in $\mathrm{Mo}{\mathrm{S}}_2$ and the $D$ peak in graphene.

We investigate both theoretically and experimentally the nonlinear propagation of surface-plasmon polaritons (SPP) on a single air/metal interface. Inspired by nonlinear dielectric waveguide theory, we analytically derive a model that describes the nonlinear propagation of SPPs, thus bridging the description of plasmonic and dielectric waveguides. The model, the numerical simulations, and the experiments, which are carried out in the 100 fs regime, reveal that the SPP undergoes strong ultrafast self-action which manifests itself through self-induced absorption. Our observations are consistent with a large, bulk, third-order nonlinear susceptibility $\left({\chi }^{\left(3\right)}\right)$ of gold and provide a self-consistent theory of self-action of SPPs at an air/metal interface. Experimentally, we find $\mathrm{Im}\left\{{\chi }^{-\left(3\right)}\right\}\sim 3\times {10}^{-16}{\mathrm{m}}^2/{\mathrm{V}}^2$. These findings have important implications in the nonlinear physics of plasmonics and metamaterials as they provide evidence that nonlinear absorption has a significant effect on the propagation of SPPs excited by intense optical pulses. This self-action is also expected to affect the anomalous absorption of light near subwavelength structures as well as the strength of desirable nonlinear processes such as third-harmonic generation and four-wave mixing, which will inevitably compete with nonlinear absorption.