In the present work we discuss the propagation of excitons across a one-dimensional Su-Schrieffer-Heeger lattice, which possesses both harmonic oscillations and weak quartic anharmonicities. When quantizing these vibrational degrees of freedom we identify several phonon-conserving nonlinearities, each one with a different impact on the excitonic transport. Our analysis identifies a dominant nonlinear correction to the phonon hopping which leads to a strong enhancement of exciton conduction compared to a purely linear vibrational dynamics. Thus quartic lattice nonlinearities can be exploited to induce transitions from localized to delocalized transport, even for very weak amplitudes.

Here we study both static and time-resolved dynamic magnetic properties of the compensated ferrimagnet ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ from room temperature down to 10 K, thus crossing the magnetic compensation temperature $T_M$. The behavior is analyzed with a model of a simple collinear ferrimagnet with uniaxial anisotropy and site-specific gyromagnetic ratios. We find a maximum zero-applied-field resonance frequency of $\sim 160\mathrm{GHz}$ and a low intrinsic Gilbert damping $\alpha \sim 0.02$, making it a very attractive candidate for various spintronic applications.

We report on a study on a representative set of FeP-based MnFePSi samples by means of Mn NMR in both zero and applied magnetic field. The first-order nature of the magnetic transition is demonstrated by truncated order parameter curves with a large value of the local ordered moment at the Curie point, even at compositions where the transition appears second order from magnetic measurements. No weak ferromagnetic order could be detected at Si-poor compositions showing the kinetic arrest phenomenon, but rather the phase separation of fully ferromagnetic domains from volume fractions where Mn spins are fluctuating. The more pronounced decrease of the ordered moment at the $3f$ sites than at the $3g$ sites on approaching $T_C$, characteristic of the mixed magnetism of these materials, is shown to be driven by the drop of the $3f$ spin density instead of enhanced spin fluctuations. The temperature-driven $3f$ spin extinction is demonstrated to evolve into a truly nonmagnetic state of the $3f$ Mn ions well above $T_C$, in agreement with theoretical models and in contrast with previous experiments who detected just a partial moment quenching. Besides “normal” $3f$ Mn ions undergoing a magnetic to spinless state transition, NMR in Mn-rich compositions detects the disproportionation at $3f$ sites of a significant minority Mn fraction with negligible hyperfine couplings, which retains its diamagnetic character independent of temperature. Such a diamagnetic fraction qualitatively accounts for the reduced average $3f$ moment previously reported at large Mn concentrations.

The coupling of spin and orbital degrees of freedom in the trilayer ${\mathrm{Sr}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ sets a long-standing puzzle due to the peculiar anisotropic coexistence of out-of-plane ferromagnetism and in-plane metamagnetism. Recently, the induced magnetic structure by in-plane applied fields was investigated by means of spin-polarized neutron diffraction, which allowed the extraction of a substantial orbital component of the magnetic densities at Ru sites. It has been argued that the latter is at the origin of the evident layer-dependent magnetic anisotropy, where the inner layers carry larger magnetic momenta than the outer ones. We present a spin-polarized neutron diffraction study in order to characterize the nature of the ferromagnetic state of ${\mathrm{Sr}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ in the presence of a magnetic field applied along the $c$ axis. The components of the magnetic densities at the Ru sites reveal a vanishing contribution of the orbital magnetic moment which is unexpected for a material system where orbital and spin degeneracies are lifted by spin-orbit coupling and ferromagnetism. We employ a model that includes the Coulomb interaction and spin-orbit coupling at the Ru site to address the origin of the suppression of the orbital magnetic moment. The emerging scenario is that of nonlocal orbital degrees of freedom playing a significant role in the ferromagnetic phase, with a Coulomb interaction that is crucial to making an antialigned orbital moment at short distance, resulting in a ground state with vanishing local orbital moments.

Deterministic current-induced spin-orbit torque (SOT) switching of magnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic heterostructure with the ferromagnetic layer being perpendicularly magnetized typically requires an externally applied in-plane field to break the switching symmetry. We show that by inserting an in-plane magnetized ferromagnetic layer CoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure, deterministic SOT switching of the perpendicularly magnetized top CoFeB layer can be realized without the need of in-plane bias field. Kerr imaging study further unveils that the observed switching is mainly dominated by domain nucleation and domain wall motion, which might limit the potentiality of using this type of multilayer stack design for nanoscale SOT-MRAM application. Comparison of the experimental switching behavior with micromagnetic simulations reveals that the deterministic switching in our devices cannot be explained by the stray field contribution of the in-plane magnetized layer, and the roughness-caused Néel coupling effect might play a more important role in achieving the observed field-free deterministic switching.

An effective way to enhance the critical temperature ($T_c$) of bulk superconductivity (SC) is employing external pressure accompanied with or without phase transitions. However, the variations of $T_c$ are not always “positively” proportional to the pressure even in the same superconducting phase. There exist a class of bulk superconductors such as $\beta \text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$, whose $T_c$ will be suppressed by pressure starting from the atmospheric pressure. Then the pressure approach of enhancing SC becomes invalid because it is impossible to apply a negative pressure. Here we demonstrate that quantum electronic stress (QES) induced by electronic doping affords a general approach to further enhance $T_c$ of such kinds of bulk superconductors without causing phase transition. Our first-principles calculations show that electron doping gives rise to a negative QES acting as an effective “negative” pressure, which expands the lattice of $\beta \text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$. Consequently, it leads to an increase of electronic density of states at the Fermi level and softening of phonon vibration modes, which in turn enhances the strength of electron-phonon coupling. Conversely, the same concept also explains the experimental reports of pressure and hole-type substitution suppressed SC of bulk $\beta \text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$, which unifies the effects of pressure and chemical substitution into the same theoretical framework of QES tuning SC. We also envision that surface QES induced by quantum confinement may play an important role in affecting the $T_c$ of epitaxial superconductors, such as the experimentally reported $\beta \text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$ thin films.

We investigate the Bardeen-Cooper-Schrieffer–Bose-Einstein condensation crossover in a two-band superfluid Fermi gas with an energy shift between the bands. When the intraband coupling in the cold (first) band is fixed as weak, we find that in the case of vanishing pair-exchange interband coupling and in the strong-coupling limit of the hot (second) band the system undergoes a transition to a single-component configuration with a full suppression of the first energy gap and a full redistribution of particles between bands. For nonvanishing pair-exchange interband coupling we reveal a nonmonotonic dependence of the energy gap in the first band vs intraband coupling in the second band, with the presence of a hump. In the case of weak interband coupling, the system shows a significant amplification of the intrapair correlation length of the condensate in the first band in the strong-coupling regime of the second band, which clearly indicates the coexistence of giant Cooper pairs and a bosonic condensate even for nonzero temperatures. This can lead to a nonmonotonic temperature dependence of the second energy gap with a peak. The here predicted coexistence of giant Cooper pairs and bosonic molecules can be verified by means of the visualization of vortex cores in two-component atomic condensates as well as in some iron-based superconductors.

The conducting states, recently discovered at the surface of two special class of insulators—topological insulators and topological crystalline insulators—are distinguished by their insensitivity to local and nonmagnetic surface defects at a level of disorder, sufficiently small to be described within the perturbation theory. However, the behavior of the surface states in case of nonlocal macroscopic imperfections is not clear. Here, we propose a systematic study of the topological surface states on vicinal planes (deviations from perfect surface cleavage) in a topological crystalline insulator of the tin telluride family, by using realistic first-principles-derived tight-binding models. The theoretical framework proposed is quite general and easily permits the extensions to other topological insulator families.

The presence of orbital degree of freedom in strongly correlated systems leads to unusual orderings. In this paper we use a combination of density-functional theory calculations and various experimental investigations to reveal a unique ground state for a ${\mathrm{Ti}}^{3+}$ containing spinel oxide, ${\mathrm{MnTi}}_2{\mathrm{O}}_4$, which hosts an exotic combination of a rare tetramer orbital (associated with ${\mathrm{Ti}}^{3+}3d^1$ electron) ordering along equivalent $\langle 111\rangle$ directions involving all three $t_{2\mathrm{g}}$ orbitals, a ferrimagnetic Mn-Ti, and a ferromagnetic Ti-lattice spin ordering. A combination of spin-orbital superexchange and Jahn-Teller-effect related strain-energy optimization provides a microscopic understanding for the stabilization of the unique ground state, which is found to be also electrically polar. The tetramer orbital ordering induces Ti-Ti bond length modulations and the short and long Ti-Ti bonds form helices around the crystallographic $c$ axis with a particular winding direction, causing the ground state structure to become spatial chiral.

$\mathrm{SrIr}{\mathrm{O}}_3$ is a semimetallic complex oxide of interest for spintronic applications due to the large spin-orbit coupling arising from iridium. It has unusual charge transport properties derived from a complex multiband electronic structure, with electron and hole pockets both contributing to conductivity. We report ferromagnetic resonance of $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ and $\mathrm{SrIr}{\mathrm{O}}_3$ epitaxial bilayer films on ${\left(\mathrm{LaAl}{\mathrm{O}}_3\right)}_{0.3}{\left(\mathrm{S}{\mathrm{r}}_2\mathrm{AlTa}{\mathrm{O}}_6\right)}_{0.7}$ substrates. Anomalous trends in the out-of-plane magnetic anisotropy and Landé $g$ factor suggest that orbital magnetism is modified by proximity of $\mathrm{SrIr}{\mathrm{O}}_3$ at low temperatures, likely contributing to large (∼fivefold) enhancements in Gilbert damping. However, enhanced Gilbert damping due to spin pumping is also apparent in the temperature range 250–300 K. The effective spin-mixing conductance is evaluated to be $G_{\uparrow \downarrow }^{\mathrm{eff}}\sim 0.5\times {10}^{14}{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-2}$, and the spin scattering length scale of $\mathrm{SrIr}{\mathrm{O}}_3$ is of the order of ∼1 nm. Our work demonstrates the delicate interplay of pure spin current with interfacially mediated spin-orbit effects in a complex oxide heterostructure, exploiting temperature as a control parameter, and should be of interest for both spin pumping and understanding the electronic structure of thin film iridates.

We study the dynamics of incoherent exciton formation under interband photoexcitation in $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ using time-and angle-resolved photoemission spectroscopy at 90 K. Hot electrons injected by allowed optical transitions with 3.40-eV photons show ultrafast relaxation to the conduction-band minimum (CBM), surviving up to 500 fs after excitation. Whereas hot-electron states with high excess energy show a rapid population decay of ∼25 fs, an abrupt increase to 130 fs is observed for states with excess energies of 0.15 eV. This latter is interpreted in terms of phonon bottleneck dynamics characteristic of longitudinal-optical phonon mediated energy relaxation. Excitons, having a small binding energy of 60 meV, are formed below the CBM quasi-instantaneously and subsequently relax to the 1S-exciton state of the yellow series within 1.5 ps. We find that, together with possible plasma screening for electron-hole interaction, the cooling of exciton center-of-mass motion plays an important role in the exciton relaxation dynamics. The characteristic features of exciton photoionization process are critically discussed based on the detailed analysis of angle-resolved photoemission results for the 1S excitons formed 1.5 ps after excitation.

We study a generic spin-fermion model, where a two-level system (spin) is coupled to two metallic leads with different chemical potentials, in the presence of monochromatic driving fields. The real-time dynamics of the system is simulated beyond the Markovian limit by an iterative numerically exact influence functional path integral method. Our results show that although both system-bath coupling and chemical potential difference contribute to dissipation, their effects are distinct. In particular, under certain drivings the asymptotic Floquet states of the system exhibit robustness against a range of system-bath coupling strength: the asymptotic behaviors of the system are insensitive to different system-bath coupling strength, while they are highly tunable by the chemical potential difference of baths. Further simulations show that such robustness may be essentially a result of the interplay between driving, bath electronic structure, and system-bath coupling. Therefore, the robustness could break down depending on the characteristics of the interplay. In addition, under fast linearly polarized driving the quantum stochastic resonance is demonstrated that stronger system-bath coupling (stronger dissipation) enhances rather than suppresses the amplitude of coherent oscillations of the system.

We establish a theory of maximal energy efficiency and output power for three-terminal thermoelectric engines which have two independent output electric currents and one input heat current. This setup goes beyond conventional heat engines with only one output electric current. We derive the maximal energy efficiency and output power and their trade-off for three-terminal thermoelectric engines with and without time-reversal symmetry. This formalism goes beyond the known results for conventional thermoelectric engines and shows some interesting features. A concrete example of a quantum-dot three-terminal thermoelectric engine is studied to demonstrate that for the same system, our setup can substantially enlarge the physical parameter region with high efficiency and power, when compared with previous setups with only one output electric current. Therefore, the setup with two output electric currents offers a promising pathway toward high-performance thermoelectric devices. Our theoretical framework also applies for thermoelectric heat engines with multiple output electric currents, providing a formalism for the study of maximal efficiency and power in complex thermoelectric materials and devices.

Band gap engineering of graphene has been intensively studied, but it is still difficult to be realized effectively in practice, which limits its applications in optoelectronics. Here, by using first-principles methods, we predicate that the band gap of graphene can be obviously enlarged up to 0.83 eV and type-I band alignment is also realized in the $\text{graphene}/\mathrm{BN}/M{X}_2$ (M = Mo, W; X = S, Se) heterotrilayers when the interlayer distance is compressed. Moreover, the electrons and holes can be confined inside the gap opened graphene layer, and the staggered band alignment can be formed under the presence of electric field as well as the vertical pressure in the $\text{graphene}/\text{BN}/M{X}_2$ heterotrilayers. Our results may pay an effective route to tune the gap of graphene and broaden its applications in optoelectronic fields.

Electron transport in anatase ${\mathrm{TiO}}_2$, which has important applications in oxide electronics and photocatalysis, is still poorly understood. We investigate the electron mobility in anatase ${\mathrm{TiO}}_2$ by performing first-principles calculations of electron and phonon spectra as well as electron-phonon coupling. The formation of large polarons (quasiparticles formed by electrons interacting with phonons in a polar medium) leads to a renormalization of the electronic band structure, which we address using many-body perturbation theory. We correlate the lowering of the mobility of these quasiparticles to the renormalization of band velocities due to the electron-phonon interaction. These results explain why the mobility decreases with increasing temperature, as observed in experiments.

Highly intense electric field pulses can move the electronic momentum occupation in correlated metals over large portions of the Brillouin zone, leading to phenomena such as dynamic Bloch oscillations. Using the nonequilibrium fluctuation-exchange approximation for the two-dimensional Hubbard model, we study how such nonthermal electron distributions drive collective spin and charge fluctuations. Suitable pulses can induce a highly anisotropic modification of the occupied momenta, and the corresponding spin dynamics results in a transient change from antiferromagnetic to anisotropic ferromagnetic correlations. To good approximation this behavior is understood in terms of an instantaneous response of the spin correlations to the single-particle properties.

An ensemble of resonators arranged on a subwavelength scale is usually considered as a bulk effective medium, known as a metamaterial, and can offer unusual macroscopic properties. Here, we take a different approach and limit ourselves to the study of only a few number of such elementary components and demonstrate that they still offer uncommon opportunities. Typically, owing to the multiple scattering and the phase shift that the resonances offer, we observe fields that vary at scales completely independent of the wavelength in free space. By smartly tuning the resonance frequencies, we can design at will the complex current distribution in those resonators. This way, we design a superdirective antenna, i.e., an antenna that is surprisingly more directive than its size would foreshadow. This approach is verified numerically and experimentally in the context of microwaves, but this applies to any wave field where subwavelength resonators exist.

In two-dimensional topological insulators and superconductors, it is intuitive to imagine a magnetic monopole inside the compact reciprocal space, carrying a quantized charge corresponding to a topological invariant. However, a quantitative realization of such a physically appealing picture has proven challenging. Here, we point out that the well-known analytical “branch points” of the band structure are exactly the natural magnetic monopoles that correspond to a topological invariant. We then illustrate the evolution of the magnetic monopoles across the topological phase transition and use it to provide natural descriptions of (1) the discontinuous jump of integer topological invariants, (2) the semimetallicity on the phase boundary, (3) an extension of localized edge states into bulk states (thus the smallest physical size of the sample allowed), and (4) how a change in global topology can be induced via a local change in reciprocal space. This generic approach alone offers a comprehensive physical understanding of all aspects of topological properties, and is applicable to all topological states of matter.

Exotic ordered ground states driven by electronic correlations are expected to be induced in monolayer graphene when doped to the Van Hove singularity. Such doping levels are reached by intercalating Gd in graphene on SiC(0001), resulting in a strong homogeneity and stability. The electronic spectrum now exhibits severe renormalizations. Flat bands develop which are driven by electronic correlations according to our theoretical studies. Due to strong electron-phonon coupling in this regime, polaron replica bands develop. Thus, interesting ordered ground states should be made accessible.

In the pursuit of accurate descriptions of strongly correlated quantum many-body systems, dynamical mean field theory (DMFT) has been an invaluable tool for elucidating the spectral properties and quantum phases of both phenomenological models and ab initio descriptions of real materials. Key to the DMFT process is the self-consistent map of the original system into an Anderson impurity model, the ground state of which is computed using an impurity solver. The power of the method is thus limited by the complexity of the impurity model the solver can handle. Simulating realistic systems generally requires many correlated sites. By adapting the recently proposed adaptive sampling configuration interaction (ASCI) method as an impurity solver, we enable much more efficient zero-temperature DMFT simulations. The key feature of the ASCI method is that it selects only the most relevant Hilbert space degrees of freedom to describe the ground state. This reduces the numerical complexity of the calculation, which will allow us to pursue future DMFT simulations with more correlated impurity sites than in previous works. Here, we present the ASCI-DMFT method and example calculations on the one- and two-dimensional Hubbard models that exemplify its efficient convergence and timing properties. We show that the ASCI approach is several orders of magnitude faster than the current best published ground-state DMFT simulations, which allows us to study the bath discretization error in simulations with small clusters, as well as to address cluster sizes beyond the current state of the art. Our approach can also be adapted for other embedding methods such as density matrix embedding theory and self-energy embedding theory.