To clarify the origin of the nontrivial thermoelectric properties observed in the delafossite oxide ${\mathrm{CuRhO}}_2$ polycrystals, we performed the systematic transport measurements on the single-crystalline ${\mathrm{CuRhO}}_2$ samples. In the parent compound, we found a pronounced peak structure due to a phonon-drag effect in the temperature dependence of the Seebeck coefficient, which was also confirmed by the size effect experiments. In the Mg-substituted crystals, in contrast to the results of the polycrystals, both the resistivity and the Seebeck coefficient decreased with increasing Mg content $y$. In particular, the coefficient $A$ for the $T^2$ term of the resistivity and the $T$-linear coefficient for the Seebeck coefficient at low temperatures are well described within a simple relationship expected for metals, which is also applicable to the correlated materials with low carrier densities.

Nonreciprocal transport phenomena indicate that the forward and backward flows differ and are attributed to broken inversion symmetry. In this paper, we study the nonreciprocity of the thermal and thermoelectric transport of electronic systems resulting from inversion-symmetry-broken crystal structures. The nonlinear electric, thermoelectric, and thermal conductivities are derived up to the second order in an electric field and a temperature gradient by using the Boltzmann equation with the relaxation time approximation. All the second-order conductivities appearing in this paper are described by two functions and their derivatives, and they are related to each other in the same way that linear conductivities are, e.g., via the Wiedemann-Franz law. We found that nonvanishing thermal-transport coefficients in the zero-temperature limit appear in nonlinear conductivities, which dominate the thermal transport at a sufficiently low temperature. The nonlinear conductivities and possible observable quantities are estimated in a $1H$ monolayer of the transition-metal dichalcogenide ${\mathrm{MoS}}_2$ and a polar semiconductor $\mathrm{BiTe}X$ ($X=\mathrm{I},\mathrm{Br}$).

Auger recombination (AR) being electron-hole annihilation with energy-momentum transfer to another carrier is believed to speed up in materials with small band gap. We theoretically show that this rule is violated in gapless three-dimensional materials with ultrarelativistic electron-hole dispersion, Weyl semimetals (WSMs). Namely, AR is prohibited by energy-momentum conservation laws in prototypical WSM with a single Weyl node, even in the presence of anisotropy and tilt. In real multinode WSMs, the geometric dissimilarity of nodal dispersions enables weak internode AR, which is further suppressed by strong screening due to large number of nodes. While partial AR rates between the nodes of the same node group are mutually equal, the intergroup processes are nonreciprocal, so that one of the groups is geometrically protected from AR. This geometrical protection prolongs AR lifetime up to two orders of magnitude, to the level of nanoseconds.

We report a theoretical study of the spin-dependent transport properties in heterostructures containing a three-dimensional topological insulator (TI) thin film and ferromagnetic normal insulator (FMNI) slab. Within the framework of a continual approach for the FMNI/TI/FMNI trilayer model, we reveal how the magnetic proximity effect at the TI/FMNI interface can influence an intrinsic Hall response of the system. We predict that the FMNI/TI/FMNI trilayer undergoes a transition into the quantum anomalous Hall phase either from the topologically trivial phase or from the quantum spin Hall phase, which is controlled by tuning the proximity-induced exchange field, the TI film thickness, and the TI band structure parameters. We draw the corresponding phase diagram of the FMNI/TI/FMNI trilayer. Moreover, we argue that the roughness at the TI/FMNI interfaces can cause the decomposition of the TI film into topologically distinct domains, which affects the Hall conductivity. We discuss the specifics of manifestation and complexities of observation of quantized conductivity in realistic TI/FMNI heterostructures.

The formation of band offset (BO) at isovalent semiconductor heterojunctions has been branded “bulklike” because of an insensitivity of the BO to the interface orientation and atomic structure and a transitivity in BOs. Even though tunability and nontransitivity of BO are frequently found for heterovalent interfaces, empirical theories with built-in bulklike characteristics have thus far dominated the explanation of experimental BOs. Presently, the distribution of charge density and the formation of BO at a large number of interfaces between lattice-matched perovskite oxides are studied in detail using density functional theory. Ionic screening is found to dominate the formation of the BO, as a sharp dependence of the (apparently tunable) BO on atomic structure for unrelaxed interfaces is essentially washed out upon lattice relaxation. Numerical experimentation with deliberate embedding of dipolar layers in perovskite oxides and their interfaces corroborates the effectiveness of ionic screening. The relaxed, converged (bulklike) BOs are found to be in good agreement with the prediction of the neutral polyhedra theory (NPT). The success of the NPT, presently for ionic interfaces and previously for covalent zinc-blende interfaces, unmasks a possible connection between the partition into neutral symmetric cells and the energy-minimization requirement on the interface charge distribution. The independence of the BO on interface specifics, i.e., bulklike behavior, is shown to stem directly from such a property of charge distribution with minimized electrostatic energy. As energy minimization governs the formation of charge distribution in general, NPT is expected to describe the band offset of a wide variety of material interfaces.

Bosonic condensates of exciton-polaritons are characterized by a well-defined pseudospin, which makes them attractive for quantum information schemes and spintronic applications, as well as the exploration of synthetic spin-orbit coupling. However, precise polarization control of coherent polariton condensates under nonresonant injection, the most important ingredient for such advanced studies, still remains a core challenge. Here, we address this problem and demonstrate unprecedented control of the pseudospin of an exciton-polariton condensate. The ultrafast stimulated scattering process allows the observation of completely spin-polarized condensates under highly nonresonant, circularly polarized excitation. This conservation of spin population translates, in the case of linearly polarized excitation, into an elliptically polarized emission. The degree of ellipticity can be controlled by varying the exciton-photon detuning and condensate density. Additionally, cavity engineering allows us to generate completely linearly polarized condensates with a deterministically chosen orientation. Our findings are of fundamental importance for the engineering and design of polaritonic devices that harness the spinor degree of freedom, such as chiral lasers, spin switches, and polaritonic topological insulator circuits.

Combining atomic-resolution scanning transmission electron microscopy (STEM) images and electron-energy-loss spectroscopy spectra in aberration-corrected STEM, first-principle calculations, and spectra simulations, we investigate the origin of the surface state in a $\mathrm{LaCo}{\mathrm{O}}_3$ nanoparticle. We show that a $\mathrm{LaCo}{\mathrm{O}}_3$ nanoparticle appears as the core-shell structure, where the core has the mixture of low- and high-spin states and the shell has increased concentration of the high-spin state. Based on experimental observations and theoretical calculations, we propose that it is neither the oxygen vacancy nor strain but the suppression of oxygen octahedral tilting due to surface reconstruction that plays a dominant role in the spin-state crossover to the higher-spin state at the surface. These results provide great insights into understanding the origin of ferromagnetism as well as the enhanced electrocatalytic activity in this compound.

By means of high-resolution electron energy loss spectroscopy, we investigate the low-energy excitation spectrum of transition-metal monopnictides hosting Weyl fermions. We observe gapped plasmonic modes in (001)-oriented surfaces of single crystals of NbAs and TaAs at 66 and 68 meV, respectively. Our findings are consistent with theory and we estimate an effective Coulomb interaction strength ${\alpha }_{\mathrm{eff}}\approx 0.41$ for both samples. We also demonstrate that the modification of the surface of transition-metal monopnictides by the adsorption of chemical species (in our case, oxygen and hydrocarbon fragments) changes the frequency of the plasmonic excitations, with a subsequent modification of the effective interaction strength in the 0.30–0.48 range. The remarkable dependence of plasmonic features on the presence of adsorbates paves the way for plasmonic sensors based on Weyl semimetals operating in the mid-infrared.

By using the local-density approximation + dynamical mean-field theory approach, we study the low-energy electronic properties of ${\mathrm{Sr}}_2{\mathrm{RhO}}_4$ in a realistic setting, and compare to ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. We investigate the interplay of spin-orbit coupling, crystal field, and Coulomb interaction, including the tetragonal terms of the Coulomb tensor. We find that (i) differently than in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, the zero-frequency effective crystal-field “enhancement” due to Coulomb repulsion, $\mathrm{\Delta }{\varepsilon }_{\mathrm{CF}}(\omega =0)$, is small and, depending on the parameters, even negative. (ii) In addition, the effects of (realistic) anisotropic Coulomb terms are weak. (iii) Instead, the effective zero-frequency enhancement of the spin-orbit interaction doubles the value of the corresponding local-density approximation couplings. This explains the experimental Fermi surface and supports a previous proposal based on static mean-field calculations. We find that the sign of the Coulomb-induced spin-orbit anisotropy is influenced by the octahedral rotation. Based on these conclusions, we examine recent optical conductivity experiments. (iv) We show that the spin-orbit interaction is key for understanding them; differently than in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, the $t_{2g}$ intraorbital contributions are small; thus, the single-band picture does not apply.

We classify topological phases of non-Hermitian systems in the Altland-Zirnbauer classes with an additional reflection symmetry in all dimensions. By mapping the non-Hermitian system into an enlarged Hermitian Hamiltonian with an enforced chiral symmetry, our topological classification is thus equivalent to classifying Hermitian systems with both chiral and reflection symmetries, which effectively change the classifying space and shift the periodical table of topological phases. According to our classification tables, we provide concrete examples for all topologically nontrivial non-Hermitian classes in one dimension and also give explicitly the topological invariant for each nontrivial example. Our results show that there exist two kinds of topological invariants composed of either winding numbers or ${\mathbb{Z}}_2$ numbers. By studying the corresponding lattice models under the open boundary condition, we unveil the existence of bulk-edge correspondence for the one-dimensional topological non-Hermitian systems characterized by winding numbers, however we did not observe the bulk-edge correspondence for the ${\mathbb{Z}}_2$ topological number in our studied ${\mathbb{Z}}_2$-type model.

The semiconductor Bloch equations for a two-band model including inter- and intraband excitation are used to study the nonlinear absorption of single and multiple light pulses by direct-gap semiconductors. For a consistent analysis the contributions to the absorption originating from both the interband polarization and the intraband current need to be included. In the Bloch equation approach these contributions as well as different excitation pathways in terms of sequences of inter- and intraband excitations can be evaluated separately which allows for a transparent analysis, the identification of the dominant terms, and analyzing their dependence on the excitation conditions. In the perturbative regime we obtain analytical expressions for the multiphoton absorption coefficients for continuous-wave excitation. These results are shown to agree well with numerical results for short pulses and/or finite dephasing and relaxation times. We confirm the previously predicted strong enhancement of two-photon absorption for nondegenerate conditions for pulsed excitation. We discuss the dependencies of the multiphoton absorption on the light frequencies, initial band populations, and the time delay between the pulses. The frequency dependence of the two-photon absorption coefficient for nondegenerate excitation is evaluated perturbatively in third order. The higher-order contributions to the optical absorption include three- and four-photon absorption and show a rich frequency dependence including negative regions and dispersive line shapes. Nonperturbative solutions of the Bloch equations demonstrate a strongly nonmonotonous behavior of the intensity-dependent optical absorption for a single incident pulse and in a pump-probe setup.

We study the influence of spin-orbit coupling on the hole states in InAs/GaAs quantum dots grown on [001]- and [111]-oriented substrates belonging to symmetry point groups: $C_{2\mathrm{v}}$, $C_{3\mathrm{v}}$, and $D_{2\mathrm{d}}$. We investigate the impact of various spin-orbit mechanisms on the strength of coupling between $s$- and $p$-shell states, which is a significant spin-flip channel in quantum dots. We calculate spin relaxation rates between the states of lowest Zeeman doublet and show that the [111]-oriented structure offers one order of magnitude slower relaxation compared to the usual [001]-oriented self-assembled QD. The magnetic-field dependence of the hole states is calculated using multiband (up to 14 bands) $\mathit{k}·\mathit{p}$ model. We identify the irreducible representations linked to the states and discuss the selection rules, which govern the avoided-crossing pattern in magnetic-field dependence of the energy levels. We show that dominant contribution to the coupling between some of these states comes from the shear strain. On the other hand, we demonstrate no coupling between $s$- and $p$-shell states in the [111]-oriented structure.

Using scanning tunneling potentiometry, we have obtained spatial distribution of electrochemical potential on the $\mathrm{Si}\left(111\right)\text{−}7\times 7$ surface and estimated a ratio of conductance across surface steps and domain boundaries to that of the reconstructed terrace. Our measurements reveal significant resistance across the steps, indicating that the conductance on the $7\times 7$ surface measured in macroscopic methods is basically determined by the step density, which is uncontrolled in most of the macroscopic measurements. The domain boundaries also exhibit measurable resistance, implying underestimation (overestimation) of the $7\times 7$ terrace (step) conductance when these conductance values are estimated from the anisotropy in the conductance along and across the step directions without considering the resistance at the domain boundaries.

Time-resolved magneto-optic Kerr microscopy measures the effect of in-plane electric fields on the dynamics of a photoexcited spin distribution in a modulation-doped GaAs quantum well. The structure features nearly equal Dresselhaus and Rashba coefficients, such that there is negligible impact of spin-orbit coupling for electrons moving along the $\left[1\overline{1}0\right]$ or $\left[\overline{1}10\right]$ directions. Meanwhile, spin texture emerges for electrons moving in the $\left[110\right]$ or $\left[\overline{1}\overline{1}0\right]$ directions. The overall spin pattern resembles a persistent spin helix. An in-plane electric field, applied transverse to the spin texture (along the $\left[1\overline{1}0\right]$ or $\left[\overline{1}10\right]$ directions), introduces a drift of the spin packet and additional Larmor precessions, i.e., a marked decrease of the spatial periodicity of the spin pattern. The in-plane electric field also increases the temporal frequency of the evolving spin distribution, which is directly linked to the cubic Dresselhaus spin-orbit coupling term. Moreover, the in-plane field increases the diffusion coefficient by more than an order of magnitude. We attribute this effect to carrier heating and the separation of the photogenerated electron-hole dipole.

The absence of net magnetization, which forbids any stray magnetic fields, is one of the greatest advantages of antiferromagnets in device applications. In conventional antiferromagnets, however, spin current cannot be extracted without the aid of a static magnetic field. Here, we develop a theory of antiferromagnetic optospintronics to resolve this fundamental dilemma. By coupling a linearly polarized photon and nonreciprocal magnon bands, we construct a superposition state of left- and right-handed magnon states with opposite group velocities. We numerically demonstrate that by using this superposition state, an antiferromagnetic spin current can be efficiently generated without a net magnetic field including net magnetization. We also find that the breakdown of the superposition state induces the stripe superfluid phase of a two-component Bose-Einstein condensate. Our results lay the foundation for manipulating the superposition states of emergent particles in devices.

The critical current across grain boundaries is severely suppressed in high-temperature superconductors, such as the cuprate and iron-based compounds, if the grain boundary angle exceeds a few degrees. This is known from the low critical currents in untextured conductors and measurements on bicrystalline films. Textured conductors were developed to overcome this limitation; however, a quantitative understanding between the degree of texture and the macroscopic critical current is still missing. A model for the prediction of the self-field critical current as a function of grain alignment on the basis of experimental data obtained from bicrystals is presented. It is a mean-field approach based on percolation theory. Without any fit parameter, good agreement with recent studies on cuprates and iron-based superconductors is obtained, where the critical current and the texture were analyzed quantitatively. The simplified grain boundary physics hence describes the macroscopic properties of imperfectly textured materials.

The DFT–1/2 method in density functional theory [L. G. Ferreira et al., Phys. Rev. B 78, 125116 (2008)] aims to provide accurate band gaps at the computational cost of semilocal calculations. The method has shown promise in a large number of cases, however some of its limitations or ambiguities on how to apply it to covalent semiconductors have been pointed out recently [K.-H. Xue et al., Comput. Mater. Science 153, 493 (2018)]. In this work, we investigate in detail some of the problems of the DFT–1/2 method with a focus on two classes of materials: covalently bonded semiconductors and transition-metal oxides. We argue for caution in the application of DFT–1/2 to these materials, and the condition to get an improved band gap is a spatial separation of the orbitals at the valence band maximum and conduction band minimum.

With their optical wavelength in the near infrared (790–800 nm) and their unique spectroscopic properties at cryogenic temperatures, thulium-doped crystals are at the center of many architectures linked to classical signal processing and quantum information. In this work, we focus on Tm-doped YSO, a compound that was left aside in the mid-1990s due to its rather short optical coherence lifetime. By means of time-resolved hole-burning spectroscopy, we investigate the anisotropic enhanced nuclear Zeeman effect and demonstrate deep, sub-MHz, persistent spectral hole burning under specific magnetic field orientation, and magnitude. By estimating the experimental parameters corresponding to a real-scale ultrasound optical tomography setup using Tm:YSO as a spectral filter, we validate Tm:YSO as a promising compound for medical imaging in the human body.

We present the design, fabrication, and experimental investigation of a printed circuit board metagrating (MG) for perfect anomalous reflection. The design follows our previously developed analytical formalism, resulting in a single-element MG capable of unitary coupling of the incident wave to the specified (first-order) Floquet-Bloch (FB) mode while suppressing the specular reflection. We characterize the MG performance experimentally using a bistatic scattering pattern measurement, relying on an original beam-power integration approach for accurate evaluation of the coupling to the various modes across a wide frequency range. The results show that highly efficient wide-angle anomalous reflection is achieved, as predicted by the theory, with a relatively broadband response. In addition, the MG is found to perform well when illuminated from different angles, acting as a multichannel reflector, and to scatter efficiently also to higher-order FB modes at other frequencies, exhibiting multifunctional capabilities. Importantly, the merits of the utilized beam-integration approach, namely, its improved resilience to measurement inaccuracies or noise effects, and the implicit accommodation of different effective aperture sizes are emphasized, highly relevant in view of the numerous recent experimental reports on anomalous reflection metasurfaces. Finally, we discuss the source for losses associated with the MG; interestingly, we show that these correlate well with edge diffraction effects rather than (commonly assumed) power dissipation. These experimental results verify our theoretical synthesis scheme, showing that highly efficient anomalous reflection is achievable with a realistic fabricated MG, demonstrating the practical applicability and potential multifunctionality of analytically designed MGs for future wave-front manipulating devices.