A plasmon is a collective excitation of electrons due to the Coulomb interaction. Both plasmons and single-particle excitations (SPEs) are eigenstates of bulk metallic systems and they are orthogonal to each other. In nontranslationally symmetric systems such as nanostructures, plasmons and SPEs coherently interact. It has been well discussed that the plasmons and SPEs, respectively, can couple with transverse (T) electric fields in such systems, and also that they are coupled with each other via longitudinal (L) field. However, there has been a missing link in the previous studies: the coherent coupling between the plasmons and SPEs mediated by the T field. Herein, we develop a theoretical framework to describe the self-consistent relationship between the plasmons and SPEs through both the L and T fields. The excitations are described in terms of the charge and current densities in a constitutive equation with a nonlocal susceptibility, where the densities include the L and T components. The electromagnetic fields originating from the densities are described in terms of Green's function in Maxwell's equations. The T field is generated from both densities, whereas the L component is attributed to the charge density only. We introduce a four-vector representation incorporating the vector and scalar potentials in the Coulomb gauge, in which the T and L fields are separated explicitly. The eigenvalues of the matrix for the self-consistent equations appear as the poles of the system excitations. A numerical demonstration of the excitation spectrum is performed for a rectangular nanorod. It indicates a non-negligible shift of the collective excitation and an enhancement of the energy transfer between the excitations by the T-field-mediated interaction. The developed formulation enables to approach unknown mechanisms for the enhancement of the coherent coupling between the plasmons and the hot carriers generated by radiative fields.

In this Letter, we investigate the effect of next-to-nearest atom hopping on Klein tunneling in graphene. An effective quantum dynamics equation is obtained based on an emergent generalized Dirac structure by analyzing the tight-binding model beyond the linear regime. We show that this structure has some interesting theoretical properties. First, it can be used to simplify quantum transport calculations used to characterize Klein tunneling; second, it is not chirally symmetric as hinted by previous work. Finally, it is reminiscent of theories on a space with a discrete topology. Exploiting these properties, we show that the discrete topology of the crystal lattice has an effect on the Klein tunneling, which can be experimentally probed by measuring the transmittance through $n\text{−}p\text{−}n$ junctions. We argue that this simulates some quantum gravity models using graphene and we propose an experiment to perform such measurements.

Layered transition-metal chalcogenide material (TMD) offers a platform to investigate two-dimensional (2D) superconductivity. Here, we report an electrical transport study of 2D centrosymmetric superconductor $1T^{\prime }\text{−}\mathrm{W}{\mathrm{S}}_2$. For a typical five-layer sample, the 2D superconductivity is revealed in Berezinskii-Kosterlitz-Thouless (BKT) transition with the characteristic temperature $T_{BKT}\sim 5\mathrm{K}$. The superconductivity is observed with a strong resist against the in-plane magnetic field. We find that the critical in-plane magnetic field is ∼3 times the Pauli paramagnetic limit $B_p$, with effective Zeeman-type spin-orbital coupling $\widetilde{{\beta }_{\mathrm{SO}}}\sim 2.8\mathrm{meV}$. In addition, we find the magnetoresistance trend to saturate at low temperature, suggesting anomalous quantum metallic states. These findings might stimulate further investigations on the correlation between superconductivity and symmetry of 2D superconductors, as well as the quantum metallic states at low temperature with finite magnetic field.

Oxide glasses have proven useful as bioactive materials, owing to their fast degradation kinetics and tunable properties. Hence, in recent years tailoring the properties of bioactive glasses through compositional design have become the subject of widespread interest for their use in medical application, e.g., tissue regeneration. Understanding the mixed alkali effect (MAE) in oxide glasses is of fundamental importance for tailoring the glass compositions to control the mobility of ions and, therefore, the glass properties that depend on it, such as ion release, glass transition temperature, and ionic conductivity. However, most of the previously designed bioactive glasses were based on trial-and-error, which is due to the complex glass structure that is nontrivial to analyze and, thus, the lack of a clear picture of the glass structure at short- and medium-range order. Accordingly, we use molecular dynamics simulations to study whether using the MAE can control the bioactivity and properties of 45S5 glass and its structural origins. We showed that the network connectivity, a structural parameter often used to access the bioactivity of silicate glasses, does not change with Na substitution with Li or K. On the contrary, the elastic moduli showed a strong dependence on the type of the modifier as they increased with increasing mean field strength. Similarly, the mobility of the glass elements was significantly affected by the type of modifier used to substitute Na. The change of the properties is further discussed and explained using changes at the short- and medium-range structure by giving evidence of previous experimental findings. Finally, we highlight the origin of the nonexistence of the MAE, the effect of the modifier on the bioactivity of the glasses, the importance of dynamical descriptors in predicting the bioactivity of oxide glasses, and we provide the necessary insights, at the atomic scale, needed for further development of bioactive glasses.

We investigate the structure of quasiparticle states localized in a core of an Abrikosov vortex in a clean layered superconductor in the presence of planar defects. It is shown that even a highly transparent defect opens a minigap at the Fermi energy. Its magnitude $E_g\sim \mathrm{\Delta }\sqrt{R}$ exceeds the mean level spacing for the chiral branch ${\omega }_0\sim {\mathrm{\Delta }}^2/E_F$, already for very small values of the reflection coefficient off the defect $R\ll 1$ ($\mathrm{\Delta }$ is the bulk gap). For $R\gtrsim \sqrt{\mathrm{\Delta }/E_F}$, formation of the minigap is accompanied by the appearance of subgap states localized along the defect, in accordance with A. V. Samokhvalov et al., Phys. Rev. B 102, 174501 (2020). The minigap takes its maximal value for the vortex located right at the defect, decreases with increasing the distance $b$ from the defect, and closes when $k_{F}b\sim (\mathrm{\Delta }/{\omega }_0)\sqrt{R}$. We also study various configurations of several planar defects (few crossing planes, stars, periodic structures). Although the minigap remains, a strong commensurability effect is observed. For two crossing planar defects, the magnitude of the minigap strongly depends on how close the intersection angle is to a rational fraction of $\pi$.

We theoretically demonstrate that a uniform static electric field distribution can be partially converted to radiation fields when a portion of the medium undergoes a temporal change of its permittivity. An in-depth theoretical investigation of this phenomenon is developed for a dielectric block with a steplike temporal change located inside a waveguide charged with a DC voltage source. Closed analytical expressions are derived for the radiated electric and magnetic fields. The exchange of energy between the electrostatic and electromagnetic fields is discussed. The reconciliation between the seemingly contradictory temporal and spatial boundary conditions for the electric and magnetic fields at the interface of the time-varying dielectric block is analyzed and elucidated. Our findings may provide an alternative solution for generating electromagnetic radiation based on time-varying media.

Rare-earth titanates are Mott insulators whose magnetic ground state—antiferromagnetic (AFM) or ferromagnetic (FM)—can be tuned by the radius of the rare-earth element. Here, we combine phenomenology and first-principles calculations to shed light on the generic magnetic phase diagram of a chemically substituted titanate on the rare-earth site that interpolates between an AFM and a FM state. Octahedral rotations present in these perovskites cause the AFM order to acquire a small FM component—and vice-versa—removing any multicritical point from the phase diagram. However, for a wide parameter range, a first-order metamagnetic transition line terminating at a critical endpoint survives inside the magnetically ordered phase. Like the liquid-gas transition, a Widom line emerges from the endpoint, characterized by enhanced fluctuations. In contrast to metallic FMs, this metamagnetic transition involves two symmetry-equivalent and insulating canted spin states. Moreover, instead of a magnetic field, we show that uniaxial strain can be used to tune this transition to zero temperature, inducing a quantum critical endpoint.

Multipole symmetries are of interest both as a window on fracton physics and as a crucial ingredient in realizing new universality classes for quantum dynamics. Here we address the question of whether and when multipole symmetries can be spontaneously broken, both in thermal equilibrium and at zero temperature. We derive generalized Mermin-Wagner arguments for the total or partial breaking of multipolar symmetry groups and generalized Imry-Ma arguments for the robustness of such multipolar symmetry breaking to disorder. We present both general results and explicit examples. Our results should be directly applicable to quantum dynamics with multipolar symmetries and also provide a useful stepping stone to understanding the robustness of fracton phases to thermal fluctuations, quantum fluctuations, and disorder.

Crystal geometry can greatly influence the emergent properties of quantum materials. As an example, the kagome lattice is an ideal platform to study the rich interplay between topology, magnetism, and electronic correlation. In this work, combining high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structure of $X{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ $(X=\mathrm{Dy},\mathrm{Tb},\mathrm{Gd},\mathrm{Y})$ family compounds. We observe the Dirac fermion and the flat band arising from the magnetic kagome lattice of Mn atoms. Interestingly, the flat band locates in the same energy region in all compounds studied, regardless of their different magnetic ground states and $4f$ electronic configurations. These observations suggest a robust Mn magnetic kagome lattice across the $X{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ family, thus providing an ideal platform for the search for, and investigation of, new emergent phenomena in magnetic topological materials.

We have computed the energy loss rate (ELR) of an intruding electron in three-dimensional tilting Dirac semimetals in the light of the excitation process. In contrast to two-dimensional electron gas, ELRs contributed by single-particle excitation (${\mathrm{ELRs}}^{\left(\mathrm{SP}\right)}$) show a nondecreasing tendency as the incidence velocity increases, and tend to replicate the behavior of the untilt intrinsic ${\mathrm{ELR}}^{\left(\mathrm{SP}\right)}$, which is described by a cubic polynomial coupling with a logarithm velocity term. In comparison, ELRs contributed by the plasmon excitation (${\mathrm{ELRs}}^{\left(\mathrm{P}\right)}$) first reach a maximum in the small velocity region followed by a slow discrepancy. As the contribution of plasmon excitation is restricted by the vanishing imaginary part of the dielectric function, the ${\mathrm{ELRs}}^{\left(\mathrm{P}\right)}$ also manifest as an opposite counterpart to the corresponding ${\mathrm{ELRs}}^{\left(\mathrm{SP}\right)}$ with different tilts. In addition, the threshold velocities in both ELRs are tilt and chemical potential dependent, and governed by the interplay of the Pauli exclusion constraints and the conservation law concealed in the energy loss function. Finally, we briefly discuss the inelastic mean scattering time. Our results may be verified by investigating the femtosecond electron dynamics in the time-resolved two-photon photoemission.

Plasmonics in topological semimetals offer exciting opportunities for fundamental physics exploration as well as for technological applications. Here, we investigate plasmons in the exemplar chiral crystal CoSi, which hosts a variety of multifold fermionic excitations. We show that CoSi hosts two distinct plasmon modes in the infrared regime at 0.1 and 1.1 eV in the long-wavelength limit. The 0.1 eV plasmon is found to be highly dispersive, and it originates from intraband collective oscillations associated with a double spin-1 excitation, while the 1.1 eV plasmon is dispersionless and it involves interband correlations. Both plasmon modes lie outside the particle-hole continuum and possess a long lifetime. Our study indicates that the CoSi class of materials will provide an interesting materials platform for exploring fundamental and technological aspects of topological plasmonics.

The effect of 50% Cu doping at the Au site in the topological Dirac semimetal CaAuAs is investigated through electronic band structure calculations, electrical resistivity, and magnetotransport measurements. Electronic structure calculations suggest a broken-symmetry-driven topological phase transition from the Dirac to triple-point state in CaAuAs via alloy engineering. The electrical resistivity of both the CaAuAs and ${\mathrm{CaAu}}_{0.5}{\mathrm{Cu}}_{0.5}\mathrm{As}$ compounds shows metallic behavior. Nonsaturating quasilinear magnetoresistance (MR) behavior is observed in CaAuAs. On the other hand, MR of the doped compound shows a pronounced cusplike feature in the low-field regime. Such behavior of MR in ${\mathrm{CaAu}}_{0.5}{\mathrm{Cu}}_{0.5}\mathrm{As}$ is attributed to the weak antilocalization (WAL) effect. The WAL effect is analyzed using different theoretical models, including the semiclassical $\sim \sqrt{B}$ one which accounts for the three-dimensional WAL and the modified Hikami-Larkin-Nagaoka model. A strong WAL effect is also observed in the longitudinal MR, which is well described by the generalized Altshuler-Aronov model. Our study suggests that the WAL effect originates from weak disorder and the spin-orbit coupled bulk state. Interestingly, we have also observed the signature of chiral anomaly in longitudinal MR when both the current and field are applied along the $c$ axis. The Hall resistivity measurements indicate that the charge conduction mechanism in these compounds is dominated by the holes with a concentration $\sim {10}^{20}{\mathrm{cm}}^{-3}$ and mobility $\sim {10}^2{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$.

Understanding electronic transport properties is important for designing devices for applications. Many studies rely on the semiclassical Boltzmann approach within the relaxation time approximation. This method delivers a graphic physical picture of the scattering process, but in some cases it lacks full quantum-mechanical effects. Here, we use a non-equilibrium Green's function Korringa-Kohn-Rostoker (KKR) method with phase-breaking scattering via virtual Büttiker terminals as a fully quantum mechanical approach to transport phenomena. With this, we assess the validity of the relation of the self-energy $\mathrm{\Sigma }$ to the scattering time $\tau$, often used in literature in the case of constant relaxation time approximation. We argue that the scattering time does not affect the thermopower in the Boltzmann approach and thus should take no effect either on the thermopower calculated via the Keldysh approach. We find a nearly linear relation for the transmission function $T_S(E_F,\mathrm{\Sigma })$ of free electrons and Cu with respect to $\frac{1}{\mathrm{\Sigma }}$. However, we find that this is not the case for Pd. We attribute this to neighboring states contributing due to the additional broadening via the self-energy $\mathrm{\Sigma }$. These findings suggest that a simple identification of scattering time and self-energy is not sufficient. Finally, we discuss the benefits and limits of the application of the virtual terminal approach.

Monolayer FeSe on ${\mathrm{SrTiO}}_3$ superconducts with reported $T_c$ as high as 100 K, but the dramatic interfacial $T_c$ enhancement remains poorly understood. Oxygen vacancies in ${\mathrm{SrTiO}}_3$ are known to enhance the interfacial electron doping, electron-phonon coupling, and superconducting gap, but the detailed mechanism is unclear. Here we apply scanning transmission electron microscopy and electron energy loss spectroscopy to FeSe/${\mathrm{SrTiO}}_3$ to image the diffusion of selenium into ${\mathrm{SrTiO}}_3$ to an unexpected depth of several unit cells, consistent with the simultaneously observed depth profile of oxygen vacancies. Our density functional theory calculations support the crucial role of oxygen vacancies in facilitating the thermally driven Se diffusion. In contrast to excess Se in the FeSe monolayer or FeSe/${\mathrm{SrTiO}}_3$ interface that is typically removed during postgrowth annealing, the diffused Se remains in the top few unit cells of the ${\mathrm{SrTiO}}_3$ bulk after the extended postgrowth annealing that is necessary to achieve superconductivity. Thus, the unexpected Se in ${\mathrm{SrTiO}}_3$ may contribute to the interfacial electron doping and electron-phonon coupling that enhance $T_c$, suggesting another important role for oxygen vacancies as facilitators of Se diffusion.

To explore the crystal structure of the recently claimed room-temperature superconducting carbonaceous sulfur hydrides, we searched for $\approx 250000$ structures over $\approx 800$ stoichiometries at 300 GPa via advanced crystal structure searching and cluster expansion method. Only several metastable high-temperature superconductors were identified, whose structures can be classified into hydrogen-rich molecular crystals and low-level carbon-doped ${\mathrm{H}}_3\mathrm{S}$-like structures by constructing the ternary phase diagram and simulating the electron-phonon interactions. The C–S–H molecular crystals are composed of ${\mathrm{CH}}_4, {\mathrm{SH}}_6$, and ${\mathrm{H}}_2$ molecules, where the superconductivity (with the highest superconducting critical temperature $T_c$ is 156 K found in ${\mathrm{CSH}}_{48}$ at 300 GPa) is mainly contributed to by ${\mathrm{H}}_2$ units, implying their $T_c$ values are unlikely to be higher than that of metallic molecular hydrogen ($T_c$ of 242 K at 450 GPa). The highest $T_c$ of low-level carbon-doped C–S–H compounds (up to 64 atoms in the primitive cell) at 300 GPa was estimated as 189 K for ${\mathrm{H}}_3{\mathrm{S}}_{0.917}{\mathrm{C}}_{0.083}$. Our results provide a comprehensive map between the crystal structure and superconductivity of carbonaceous sulfur hydride materials at high pressures.

We study superfluidity in the one-dimensional Bose-Hubbard model using a variational matrix product state technique. We determine the superfluid density as a function of the Hubbard parameters by calculating the energy cost of phase twists in the thermodynamic limit. As the system is critical, correlation functions decay as power laws and the entanglement entropy grows with the bond dimension of our variational state. We relate the resulting scaling laws to the superfluid density. We compare two different algorithms for optimizing the infinite matrix product state and develop a physical explanation why one of them (VUMPS) is more efficient than the other (iDMRG). Finally, we comment on finite-temperature superfluidity in one dimension and how our results can be realized in cold-atom experiments.

The Josephson equations predict remarkable effects concerning the phase state of a superconducting junction with an oscillating current induced by a static voltage. Whether the paradigm can be twisted by yielding an oscillating voltage without making use of harmonic drives is a fundamentally relevant problem yet not fully settled. Here, we demonstrate that a dynamical regime with an oscillating phase evolution is a general hallmark of driven Josephson systems exhibiting sign competition in the Josephson couplings. We show that in frustrated Josephson systems an oscillating phase dynamics gets switched on by driving the changeover among different ground states, which can be induced by varying the parameters that set the phase state. Remarkably, the character of the transitions in the Josephson phase space allows different types of dynamics, with few or several harmonics. This result sets out a characteristic mark of any superconducting system with frustrated Josephson couplings and can be exploited to disentangle the complexity of the underlying phases.

Free volume is one of the key structural concepts widely used to describe various glass phenomena. However, free volume is challenging to be rigorously defined in glass structures and directly determined by experiments. Here, we employed metallic glass as a simple glass model system and studied the structural evolution of an as-spun (hyperquenched) and another thermally annealed metallic glass with different amounts of free volume by in situ high-pressure synchrotron x-ray diffraction. Therefore, the effect of free volume on the compression behavior of metallic glasses can be clarified by comparing the two samples at identical experimental conditions. We find that a higher amount of free volume causes both lower density and bulk modulus as expected at ambient conditions. Surprisingly, during compression, the decrease of free volume has a constant rate up to $\sim 30\mathrm{GPa}$ and appears to be mostly reversible upon decompression without obvious permanent densification after pressure release. These results suggest an elastic process in metallic glasses under pressure. Implications of these results for the structural models based on the free-volume concept are discussed. The typically assumed glass structural model with isolated domains of “liquidlike” (free-volume rich) region embedded in a solidlike (free-volume poor) matrix seems unlikely. Instead, a more “homogeneous” model over a wide range of length scales such as a nested fractal network of liquid- and solidlike regions seems more consistent with the experimental observation.