We study the dynamics of photoinduced charge carriers in semirealistic models of ${\mathrm{LaVO}}_3$ and ${\mathrm{YTiO}}_3$ polar heterostructures. It is shown that two types of impact ionization processes contribute to the carrier multiplication in these strongly correlated multiorbital systems: The first mechanism involves local spin state transitions, while the second mechanism involves the scattering of high-kinetic-energy carriers. Both processes act on the 10-fs timescale and play an important role in the harvesting of high-energy photons in solar cell applications. As a consequence, the optimal gap size for Mott solar cells is substantially smaller than for semiconductor devices.

Fracton phases of matter host fractionalized topological quasiparticles with restricted mobility. A wide variety of fracton models with Abelian excitations had been proposed and extensively studied while the candidates for non-Abelian fracton phases are less explored. In this paper, we investigate the effect of twisted defects in Abelian fracton models. The twisted defect is launched by introducing a branch cut line hosting anyon condensate. In particular, these twisted defects, which alter different types of quasiparticles, carry projective non-Abelian zero modes. En route, such defects can be engineered via strong on-site hybridization along a branch cut which provides wide tunability and flexibility in experiment platforms. The braiding of twisted defects with projective non-Abelian Berry phases renders an avenue toward fault-tolerant quantum computation.

In order to calculate x-ray emission spectroscopy (XES) spectra, we apply the $GW+$Bethe-Salpeter equation ($\mathrm{GW}+\mathrm{BSE}$) method on a basis of extended quasiparticle theory, which enables one to treat an arbitrary excited state as an initial state, because the initial state in the XES process is a highly excited state with a core hole. Compared to the preexisting experimental data of XES fluorescence photon energy, the calculated $\mathrm{GW}+\mathrm{BSE}$ results give values with about 1-eV accuracy, which is comparable to the previous results using the time-dependent density functional theory with the short-range corrected exchange-correlation functional, the equation of motion-coupled cluster single and double, and delta self-consistent field methods. Our $\mathrm{GW}+\mathrm{BSE}$ results reproduce corresponding experimental XES spectra without missing any peak. The method can assign the excitonic configuration of each peak in XES spectra with the quasiparticle levels. As a result, the analysis of excitonic structure for each peak gives obvious interpretation concerning the relation between excitonic states and valence states.

Motivated by the results of recent transport and optical conductivity studies, we propose a semi-infinite two-dimensional lattice model for interacting massive Dirac electrons in the pressurized organic conductor $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$, and address the problem of domain wall conductivity in a charge-ordered insulating phase under realistic experimental conditions. Using the extended Hubbard model at a mean field level, we present results of extensive numerical studies around the critical region of the model, reporting on the resistivity and optical conductivity calculated by means of the Nakano-Kubo formula. We find that the activation gap extracted from the resistivity data can be much smaller than the optical gap in the critical region, which is induced by metallic conduction along a one-dimensional domain wall emerging at the border of two charge-ordered ferroelectric regions with opposite polarizations. The data are consistent with the observed transport gap in real $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ samples that is reduced remarkably faster than the optical gap upon suppressing charge order with pressure. Our optical conductivity also reveals an additional shoulderlike structure at low energy inside the gap, which is argued to be directly relevant to the metallic bound states residing on the domain wall.

Graphene's electronic structure can be fundamentally altered when a substrate- or adatom-induced Kekulé superlattice couples the valley and isospin degrees of freedom. Here, we show that the band structure of Kekulé-textured graphene can be reengineered through layer stacking. We predict a family of Kekulé graphene bilayers that exhibit up to six-valley band structures, and room-temperature Dirac quasiparticles whose masses can be tuned electrostatically, constituting a versatile platform for valleytronics. Slower Fermi velocities than in pristine graphene make this system a good candidate to study many-body correlations of multiflavor models.

Screw dislocations are known to impede the thermal transport of homogeneous nanowires by reducing the phonon relaxation time without affecting the phonon group velocity. By using molecular dynamics simulations in this study, we show that the impact of screw dislocation on the thermal conductivity of the SiGe superlattice nanowires depends on the period length. The analysis of phonon transmission spectra and phonon mean free paths indicate that strong phonon-screw dislocation scatterings occur for phonons in the frequency range of 3–8 THz. The screw dislocations change the phonon scattering mechanisms, which is the main cause of the thermal conductivity reduction. Contrary to the case of homogeneous nanowires, a sizable decrease in the phonon group velocity is found in superlattices with screw dislocations. This phenomenon is attributed to the larger number of Si-Ge bonds in the vicinity of the interface due to the slipping of the atomic planes. In contrast to the decreased thermal conductivity, the phonon propagation in the interface region of the nanowires is enhanced by screw dislocations. Our findings provide critical insights into the understanding of dislocation-heat transfer relationship in materials, especially in heterostructures where interfaces are vital for thermal transport.

We study the low-temperature properties of the generalized Anderson impurity model in which two localized configurations, one with two doublets and the other with a triplet, are mixed by two degenerate conduction channels. By using the numerical renormalization group and the noncrossing approximation, we analyze the impurity entropy, its spectral density, and the equilibrium conductance for several values of the model parameters. Marked differences with respect to the conventional one-channel spin $s=1/2$ Anderson model, which can be traced as hallmarks of an impurity spin $S=1$, are found in the Kondo temperature, the width and position of the charge-transfer peak, and the temperature dependence of the equilibrium conductance. Furthermore, we analyze the rich effects of a single-ion magnetic anisotropy $D$ on the Kondo behavior. In particular, as shown before, for large enough positive $D$ the system behaves as a “non-Landau” Fermi liquid that cannot be adiabatically connected to a noninteracting system turning off the interactions. For negative $D$ the Kondo effect is strongly suppressed. While the model is suitable for the description of a single Ni impurity embedded in an O-doped Au chain, it is a generic one for $S=1$ and two channels and might be realized in other nanoscopic systems.

We explore the unusual nonreciprocal and diffractionless properties of surface plasmon polaritons propagating in drift-biased graphene-based metasurfaces. We show that applying a drift current on a graphene sheet leads to extremely asymmetric in-plane modal dispersions, from terahertz to infrared frequencies, associated with plasmons with low loss (high loss and ultrahigh confinement) traveling along (against) the bias. Strikingly, truly unidirectional wave propagation is prevented by the intrinsic nonlocal response of a graphene, a mechanism that shapes the energy flow over the surface. We also show that highly directive hyperbolic plasmons completely immune to backscattering propagate obliquely along the drift in nanostructured graphene. Finally, we discuss how spin-orbit interactions can be exploited in this platform to efficiently launch collimated plasmons along a single direction while maintaining giant nonreciprocal responses. Our findings offer a paradigm to excite, collimate, steer, and process surface plasmons over a broad frequency band.

We investigate the intrinsic magneto-optical properties of suspended single-walled carbon nanotubes of various chiralities, using a combination of spectrally and time-resolved spectroscopy of individual nanotubes at low temperature. This study elucidates the population and relaxation dynamics of the lowest-energy bright and dark singlet excitons that are mixed with the Aharonov-Bohm effect. The high-quality emission spectra reveal the residual oscillator strength of the dark exciton in zero field, as an optical signature of spin-orbit coupling. They also unveil an asymmetry in the bright exciton recombination line, attributed to the free nature of excitons in these unperturbed nanotubes.

We instigate the angle-dependent magnetoresistance (AMR) of the layered nodal-line Dirac semimetal ZrSiS for the in-plane and out-of-plane current directions. This material has recently revealed an intriguing butterfly-shaped in-plane AMR whose origin is not well understood. Our aim was to understand the mechanism behind this peculiar shape of AMR and also to probe AMR in the out-of-plane current direction. In contrast to the in-plane AMR, the polar out-of-plane AMR shows a surprisingly different response with a pronounced cusplike feature. The maximum of the cusplike anisotropy is reached when the magnetic field is oriented in the $a\text{−}b$ plane. Moreover, the AMR for the azimuthal out-of-plane current direction exhibits a very strong fourfold $a\text{−}b$ plane anisotropy. Combining the Fermi surfaces calculated from first principles with the Boltzmann's semiclassical transport theory, we reproduce all the prominent features of the unusual behavior of the in-plane and out-of-plane AMR. We can conclude that the dominant contribution the cusplike AMR lies in open orbits of the hole pocket and, in general, AMR is strongly influenced by charge compensation effect and the off-diagonal conductivity tensor elements, which give rise to peculiar butterfly-shaped AMR. Finally, the semiclassical model was also able to clarify the origin of strong nonsaturating (subquadratic) transverse magnetoresistance observed in this material, as an effect of imperfect charge-carrier compensation and open orbits.

We establish the double perovskite ${\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$ as a nearly ideal model system for $j=1/2$ moments, with resonant inelastic x-ray scattering indicating that the ideal $j=1/2$ state contributes by more than 99% to the ground-state wave function. The local $j=1/2$ moments form an fcc lattice and are found to order antiferromagnetically at $T_N=14\mathrm{K}$, more than an order of magnitude below the Curie-Weiss temperature. Model calculations show that the geometric frustration of the fcc Heisenberg antiferromagnet is further enhanced by a next-nearest neighbor exchange, and a significant size of the latter is indicated by ab initio theory. Our theoretical analysis shows that magnetic order is driven by a bond-directional Kitaev exchange and by local distortions via a strong magnetoelastic effect. Both, the suppression of frustration by Kitaev exchange and the strong magnetoelastic effect are typically not expected for $j=1/2$ compounds making ${\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$ a riveting example for the rich physics of spin-orbit entangled Mott insulators.

Electron solid phases of matter are revealed by characteristic vibrational resonances. Sufficiently large magnetic fields can overcome the effects of disorder, leading to a weakly pinned collective mode called the magnetophonon. Consequently, in this regime it is possible to develop a tightly constrained hydrodynamic theory of pinned magnetophonons. The behavior of the magnetophonon resonance across thermal and quantum melting transitions has been experimentally characterized in two-dimensional electron systems. Applying our theory to these transitions we explain several key features of the data. Firstly, violation of the Fukuyama-Lee sum rule as the transition is approached is shown to be a consequence of the non-Lorentzian form taken by the resonance. Secondly, this non-Lorentzian shape is shown to be caused by dissipative channels that become especially important close to melting: proliferating dislocations and uncondensed charge carriers.

We use high-resolution angle-resolved photoemission spectroscopy to investigate the electronic structure of the antiferromagnetic heavy fermion compound ${\mathrm{CePt}}_2{\mathrm{In}}_7$, which is a member of the ${\mathrm{CeIn}}_3$-derived heavy fermion material family. Weak hybridization among $4f$ electron states and conduction bands was identified in ${\mathrm{CePt}}_2{\mathrm{In}}_7$ at low temperature much weaker than that in the other heavy fermion compounds like ${\mathrm{CeIrIn}}_5$ and ${\mathrm{CeRhIn}}_5$. The Ce $4f$ spectrum shows fine structures near the Fermi energy, reflecting the crystal electric field splitting of the $4f_{5/2}^1$ and $4f_{7/2}^1$ states. Also, we find that the Fermi surface has a strongly three-dimensional topology, in agreement with density-functional theory calculations.

We have studied theoretically the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in silicene, two-dimensional silicon monolayer, in the presence of a perpendicular electric field $E_z$, in both undoped and doped cases. Both analytical and numerical results are presented to demonstrate this mechanism on the surface of a monolayer silicene. We show that due to spin-orbit coupling in silicene, rotation of the spin of itinerant electrons causes a twisted exchange interaction between adsorbed magnetic moments, which consists of three types of interactions, in terms of complex spin textures: Heisenberg, Dzyaloshinsky-Moriya, and Ising interactions. Furthermore, we explore the tunability of the RKKY terms using perpendicular electric field and Fermi energy. We find that the RKKY interaction can be written in an anisotropic Heisenberg form for the intrinsic case where the spin coupling could realize various spin models, such as the XXZ-like spin exchange. We obtain analytical expressions for the RKKY interaction in silicene. The analytic results show that the effective exchange interaction is described by a range function which decays exponentially with the distance between magnetic moments, in the limit of large distance between the magnetic impurities. Our findings provide insights into the electrical control of surface magnetic states in nanoelectronic and spintronic devices based on silicene.

Ferromagnetic resonance (FMR) is performed on kagome artificial spin ice (ASI) formed of disconnected ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanowires. Here we break the threefold angular symmetry of the kagome lattice by altering the coercive field of each sublattice via shape anisotropy modification. This allows for distinct high-frequency responses when a magnetic field is aligned along each sublattice and additionally enables simultaneous spin-wave resonances to be excited in all nanowire sublattices, unachievable in conventional kagome ASI. The different coercive field of each sublattice allows selective magnetic switching via global field, unlocking novel microstates inaccessible in homogeneous-nanowire ASI. The distinct spin-wave spectra of these states are detected experimentally via FMR and linked to underlying microstates using micromagnetic simulation.

We minimize the energy function of the classical Heisenberg model describing the frustrated wheel shape systems consisting of an odd number of spin vectors, where a single vector is located at the center and the remaining vectors occupy $n$ sites on a ring. Using the Lagrange manifold method developed recently, we find the exact geometrical configurations of the spin vectors corresponding to the global energy minima. We reveal two subsets of collinear $n/2$ spin vectors which are tilted by an angle $\psi$ in the opposite direction with respect to that fixed by the central vector. We prove that $\psi$ does not depend on $n$ and all the spin vectors are collinear, if the system is nonfrustrated or it is subject to weak frustration, otherwise the sublattices start to rotate. In this frustration region, the configurations are double degenerate and differ by chirality. Our findings confirm that the classification of spin frustration holds in the classical limit and allows us to discriminate different regions by the proper configurations. We demonstrate the correspondence between the total spin in the ground state of the quantum model and a component of the net total spin vector which can be exploited in analysis of the quantum models and physical complexes.

We introduce a planar embedding of the $k$-regular $k$-XORSAT problem, in which solutions are encoded in the ground state of a classical statistical mechanics model of reversible logic gates arranged on a square grid and acting on bits that represent the Boolean variables of the problem. The special feature of this embedding is that the resulting model lacks a finite-temperature phase transition, thus bypassing the first-order thermodynamic transition known to occur in the random graph representation of XORSAT. In spite of this attractive feature, the thermal relaxation into the ground state displays remarkably slow glassy behavior. The question addressed in this paper is whether this planar embedding can afford an efficient path to solution of $k$-regular $k$-XORSAT via quantum adiabatic annealing. We first show that our model bypasses an avoided level crossing and consequent exponentially small gap in the limit of small transverse fields. We then present quantum Monte Carlo results for our embedding of the $k$-regular $k$-XORSAT that strongly support a picture in which second-order and first-order transitions develop at a finite transverse field for $k=2$ and $k=3$, respectively. This translates into power-law and exponential dependences in the scaling of energy gaps with system size, corresponding to times-to-solution which are, respectively, polynomial and exponential in the number of variables. We conclude that neither classical nor quantum annealing can efficiently solve our reformulation of XORSAT, even though the original problem can be solved in polynomial time by Gaussian elimination.