Layered oxide heterostructures are the new route to tailor desired electronic and magnetic phases emerging from competing interactions involving strong correlation, orbital hopping, tunneling, and lattice coupling phenomena. Here, we propose a half-metal/insulator superlattice that intrinsically forms a spin-polarized two-dimensional electron gas (2DEG) following a mechanism very different from the widely reported 2DEG at the single perovskite polar interfaces. From a density functional theory plus $U$ study on a ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6/{\mathrm{La}}_2{\mathrm{CoMnO}}_6$ (001) superlattice, we find that a periodic quantum well is created along [001] which breaks the threefold $t_{2g}$ degeneracy to separate the doubly degenerate $xz$ and $yz$ states from the planar $xy$ state. In the spin-down channel, the dual effect of quantum confinement and strong correlation localizes the degenerate states, whereas the dispersive $xy$ state forms the 2DEG, which is robust against perturbations to the superlattice symmetry. The spin-up channel retains the bulk insulating. Both spin polarization and orbital polarization make the superlattice ideal for spintronic and orbitronic applications. The suggested 2DEG mechanism widens the scope of fabricating the next generation of oxide heterostructures.

Bilayer graphene, in the presence of a one-sided spin-orbit interaction (SOI) induced by a suitably chosen substrate, is predicted to exhibit unconventional quantum Hall states. The new states arise due to strong SOI-induced splittings of the eight zeroth Landau levels, which are strongly layer polarized, residing fully or partially on one of the two graphene layers. In particular, an Ising SOI on the meV scale is sufficient to invert the Landau level order between the $n=0$ and $n=1$ orbital levels under moderately weak magnetic fields $B\lesssim 10$ T. Furthermore, when the Ising field opposes the $B$ field, the order of the spin-polarized levels can also be inverted. We show that, under these conditions, three different compensated electron-hole phases, with equal concentrations of electrons and holes, can occur at $\nu =0$ filling. The three phases have distinct edge conductivity values. One of the phases is especially interesting, since its edge conduction can be turned on and off by switching the sign of the interlayer bias.

We explore the thermoelectric properties of gold atomic chains bridging gold electrodes by means of ab initio and semiempirical calculations, and heuristic reasoning. We predict the thermoelectric voltage induced by a temperature difference across such junctions to oscillate, repeatedly changing sign, as a function of the number of atoms $N$ making up the atomic chain. We also predict the amplitude of the oscillations to be proportional to $N$ for long atomic chains. Further, we predict the thermoelectric voltage to change sign, in some cases, if the junction is stretched without changing $N$. Our predictions apply regardless of whether regular or irregular electrode grain boundaries are present and whether the electrodes are symmetric or asymmetric. Our findings may pave the way to the realization of new mechanically controllable voltage switches, enabling direct conversion of heat into electricity in energy harvesting applications.

We consider a single-layer graphene with high ripples, so that the pseudomagnetic fields due to these ripples are strong. If the magnetic length corresponding to a typical pseudomagnetic field is smaller than the ripple size, the resulting Landau levels are local. Then the effective properties of the macroscopic sample can be calculated by averaging the local properties over the distribution of ripples. We find that this averaging does not wash out the Landau quantization completely. Average density of states (DOS) contains a feature (inflection point) at energy corresponding to the first Landau level in a typical field. Moreover, the frequency dependence of the ac conductivity contains a maximum at a frequency corresponding to the first Landau level in a typical field. This nontrivial behavior of the effective characteristics of randomly strained graphene is a consequence of nonequidistance of the Landau levels in the Dirac spectrum.

Recently, realizing new fermions, such as type-I and type-II Dirac/Weyl fermions in condensed matter systems, has attracted considerable attention. Here, we show that the transition state from type-I to type-II Dirac fermions can be viewed as a “type-III” Dirac fermion, which exhibits unique characteristics, including a Dirac-line Fermi surface with a nontrivial topological invariant and critical chiral anomaly effect, distinct from previously known Dirac semimetals. Most importantly, we discover ${\mathrm{Zn}}_2{\mathrm{In}}_2{\mathrm{S}}_5$ is a type-III Dirac semimetal, characterized by a pair of Dirac points in the bulk and Fermi arcs on the surface. We further propose a solid-state realization of the black-hole-horizon analog in inhomogeneous ${\mathrm{Zn}}_2{\mathrm{In}}_2{\mathrm{In}}_5$ to simulate black-hole evaporation with a high Hawking temperature. We envision that our findings will stimulate researchers to study the physics of type-III Dirac fermions, as well as astronomical problems in a condensed matter analog.

We have performed angle-resolved photoemission spectroscopy of the layered ternary telluride ${\mathrm{Ta}}_3{\mathrm{SiTe}}_6$ which is predicted to host nodal lines associated with nonsymmorphic crystal symmetry. We found that the energy bands in the valence-band region show Dirac-like dispersions which present a band degeneracy at the $R$ point of the bulk orthorhombic Brillouin zone. This band degeneracy extends one dimensionally along the whole $SR$ high-symmetry line, forming nodal lines protected by the glide mirror symmetry of the crystal. We also observed a small band splitting near $E_{\mathrm{F}}$ which supports the existence of hourglass-type dispersions predicted by the calculation. The present results provide an excellent opportunity to investigate the interplay between exotic nodal fermions and nonsymmorphic crystal symmetry.

We investigate the impact of Coulomb correlations on the low-energy collective excitations in two-dimensional electron systems in the quantum Hall state $\nu =2$. The collective excitations are studied using inelastic light scattering in the set of MgZnO/ZnO heterostructures with high-mobility two-dimensional electrons. The study focuses on the lowest-energy collective excitations in the paramagnetic state $\nu =2$, the cyclotron spin-flip magnetoexcitons (CSFMs), which are associated with inter-Landau-level transitions with a simultaneous spin flip. The Coulomb contribution to the CSFM energy is measured independently of the Zeeman term as a function of electron density. It is established that in the range of electron concentrations corresponding to the Wigner-Seitz parameter $r_s\sim 5–7$, the combination of the Coulomb and Zeeman contributions leads to a softening of the CSFM. This softening accompanies spontaneous switching of the spin configuration in the $\nu =2$ quantum Hall system from paramagnetic to ferromagnetic. Theoretical estimates are given for the correlation energy terms of the CSFM responsible for its softening and a concomitant ferromagnetic transition.

Based on first-principles calculations, we show that stoichiometric ${\mathrm{TaSe}}_3$, synthesized in space group $P{2}_1/m$, belongs to a three-dimensional strong topological insulator (TI) phase with $Z_2$ invariants ($1;100$). The calculated surface spectrum shows clearly a single Dirac cone on surfaces, with helical spin texture at a constant-energy contour. To check the stability of the topological phase, strain effects have been systematically investigated, showing that many topological phases survive in a wide range of the strains along both the $a$ and $c$ axes, such as strong TI, weak TI, and Dirac semimetal phases. ${\mathrm{TaSe}}_3$ provides us an ideal platform for experimental study of topological phase transitions. More interestingly, since superconductivity in ${\mathrm{TaSe}}_3$ has been reported for a long time, the coexistence of topological phases and a superconducting phase suggests that ${\mathrm{TaSe}}_3$ is a realistic system to study the interplay between topological and superconducting phases in the future.

We study the quantum phases driven by interaction in a semimetal with a quadratic band touching at the Fermi level. By combining the density matrix renormalization group (DMRG), analytical power expanded Gibbs potential method, and the weak coupling renormalization group, we study a spinless fermion system on a checkerboard lattice at half-filling, which has a quadratic band touching in the absence of interaction. In the presence of strong nearest-neighbor ($V_1$) and next-nearest-neighbor ($V_2$) interactions, we identify a site nematic insulator phase, a stripe insulator phase, and a phase separation region, in agreement with the phase diagram obtained analytically in the strong coupling limit (i.e., in the absence of fermion hopping). In the intermediate interaction regime we establish a quantum anomalous Hall phase in the DMRG as evidenced by the spontaneous time-reversal symmetry breaking and the appearance of a quantized Chern number $C=1$. For weak interaction we utilize the power expanded Gibbs potential method that treats $V_1$ and $V_2$ on equal footing, as well as the weak coupling renormalization group. Our analytical results reveal that not only the repulsive $V_1$ interaction, but also the $V_2$ interaction (both repulsive and attractive), can drive the quantum anomalous Hall phase. We also determine the phase boundary in the $V_1\text{−}{V}_2$ plane that separates the semimetal from the quantum anomalous Hall state. Finally, we show that the nematic semimetal, which was proposed for $|V_2|\gg V_1$ at weak coupling in a previous study, is absent, and the quantum anomalous Hall state is the only weak coupling instability of the spinless quadratic band touching semimetal.

Motivated by work on the bulk topological proximity effect and the topological bootstrap, we consider two coupled layers of quantum anomalous Hall (QAH) insulators with opposite signs of time-reversal breaking, which leads to a trivial band insulator at half-filling. We study the impact of interactions in this model within slave rotor theory, which leads to a layer-selective Mott transition, resulting in a fractionalized quantum anomalous Hall insulator ${\mathrm{QAH}}^{*}$ where a Chern band insulator coexists with a chiral spin liquid. We also compute the edge electron spectral function in the vicinity of the ${\mathrm{QAH}}^{*}$ phase.

We experimentally investigate second harmonic generation from strongly coupled localized and propagative phonon-polariton modes in arrays of silicon carbide nanopillars. Our results clearly demonstrate the hybrid nature of the system's eigenmodes and distinct manifestation of strong coupling in the linear and nonlinear responses. While in linear reflectivity the intensity of the two strongly coupled branches is essentially symmetric and well explained by their respective localized or propagative components, the second harmonic signal presents a strong asymmetry. Analyzing it in detail, we reveal the importance of interference effects between the nonlinear polarization terms originating in the bulk and in the phonon-polariton modes.

The iron-sulfur system is important for planetary interiors and is intensely studied, particularly for better understanding of the cores of the Earth and of terrestrial planets. Yet, there is a paradox about high-pressure stability of FeS: ab initio global optimization (at DFT level) predicts a $Pmmn$ phase (with a distorted rocksalt structure) to be stable at pressures above $\sim 120$ GPa, which has not yet been observed in the experiments, which instead revealed a CsCl-type phase which, according to density functional calculations, should not be stable. Using quasiharmonic free energy calculations and dynamical mean-field theory, we show that this apparent discrepancy is removed by proper account of electron correlations and entropic effects.

Although spin injection at room temperature in an IrMn metallic antiferromagnet strongly depends on the transport regime, and is more efficient in the case of magnonic transport, in this article, we present experimental data demonstrating that the enhanced efficiency of spin injection caused by spin fluctuations near the ordering temperature can be as efficient for the electronic and magnonic transport regimes. By selecting representative interacting environments, we also demonstrated that the amplification of spin injection near the ordering temperature of the IrMn antiferromagnet is independent of exchange coupling with an adjacent NiFe ferromagnet. In addition, our findings confirm that the spin current carried by magnons penetrates deeper than that transported by conduction electrons in IrMn. Finally, our data indicates that the value of the ordering temperature for the IrMn antiferromagnet is not significantly affected by either the electronic or magnonic nature of the spin-current probe, or by exchange coupling.

For the paradigmatic frustrated spin-half Heisenberg antiferromagnet on the kagome lattice we performed large-scale numerical investigations of thermodynamic functions by means of the finite-temperature Lanczos method for system sizes of up to $N=42$. We present the dependence of magnetization as well as specific heat on temperature and external field and show in particular that a finite-size scaling of specific heat supports the appearance of a low-temperature shoulder below the major maximum. This seems to be the result of a counterintuitive motion of the density of singlet states towards higher energies. Other interesting features that we discuss are the asymmetric melting of the 1/3 magnetization plateau as well the field dependence of the specific heat that exhibits characteristic features caused by the existence of a flat one-magnon band. By comparison with the unfrustrated square-lattice antiferromagnet the tremendous role of frustration in a wide temperature range is illustrated.

Artificial spin ices (ASIs) consisting of arrays of magnetic bars are key systems in the study of geometric frustration in magnetic systems. Of particular interest are quasicrystal (QC) ASIs, in which the lack of translational symmetry and the varying coordination number of interacting bars allow for topologically enhanced frustrated magnetization to occur. We have directly observed the formation of magnetic vortexes within the vertices of a QC-ASI as a metastable transient state, during the magnetization reversal process. We observed that the vortexes primarily form in a specific subset of the vertex motif types. Micromagnetic simulations show that although these magnetic vortexes result in an increase in the local energy of the vertex before the magnetization of the bars reverses to align with the applied magnetic field, the overall energy increase is lower than the higher energy motif configurations that would result from reversal of the magnetization in the connecting bar.

${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ undergoes a bulk insulator-to-metal transition at $x\approx 0.04$. Through careful analysis of previously published data ($x=0.053,0.061,0.076$), we find an extended region below the Debye temperature in which the resistivity appears to scale linearly with temperature. Meanwhile, resonant (in)elastic x-ray scattering data ($x=0.065$) suggest a possible crossover from quantum paramagnetic to quantum critical phenomenology between 100 and 200 K. We put this into context with other results, and propose a possible phase diagram as a function of doping.

We explore the interplay of a charge density wave (CDW) order and $s$-wave superconductivity in a disordered system. Recent experiments on $1T-{\mathrm{TiSe}}_2$, where the pristine sample has a commensurate CDW order and the superconductivity appears upon copper intercalation, motivates our study. Starting with an extended Hubbard model, with parameters which yield a CDW ground state within Hartree-Fock-Bogoliubov formalism in pure systems, we show that the addition of disorder quickly wipes out the global charge order by disrupting periodic modulation of density at some (low) strength of disorder. Along with this, the subdominant superconducting order emerges in regions that spatially anticorrelate with islands of strong local CDW order. The short-range density modulations, however, continue to persist and show discernible effects up to a larger disorder strength. The local CDW puddles reduce in size with increasing disorder and they finally lose their relevance in effecting the properties of the system. Our results have strong implications for the experimental phase diagram of transition-metal dichalcogenides.

Multiferroics, where at least two primary ferroic orders are present and coupled in a single system, constitute an important class of materials. They have attracted special consideration as they present both intriguing fundamental physics problems and technological importance for potential multifunctional devices. Here, we present the evidence of multiferroicity and magnetoelectric (ME) coupling in $\alpha -\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_3$; a unique binary perovskite. Corresponding to the antiferromagnetic (AFM) ordering around 80 K, a clear frequency-independent transition is observed in the dielectric permittivity. We showed that electric polarization emerges near the AFM regime that can be modulated with a magnetic field. The detailed structural analysis using synchrotron radiation x-ray diffraction demonstrates the increase in structural distortion with decreasing temperature, as well as changes in the unit cell parameters and bond lengths across the ferroelectric and magnetic ordering temperatures. This observation of multiferroicity and magnetoelastic coupling in $\alpha -\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_3$ provides insights for the exploration of ME coupling in related materials.

We consider the separate spin channel contributions to the charge conductance in superconducting/ferromagnetic spin valve $F_1/N/F_2/S$ structures. We find that the up- and down-spin conductance contributions may have a very different behavior in the subgap bias region (i.e., there is a spin-split conductance). This leads to a subgap peak in the total conductance. This peak behavior, which can be prominent also in $N/F/S$ systems, is strongly dependent, in a periodic way, on the thickness of the intermediate ferromagnetic layer. We study this phenomenon for the ballistic scattering regime using a numerical self consistent method, with additional insights gained from an approximate analytic calculation for an infinite $N/F/S$ structure. We study also the angular dependence on the relative magnetization angle between $F_1$ and $F_2$ of both the spin-split and the total conductance. We do so for realistic material parameters, layer thicknesses, and interface quality values relevant to previous [Jara et al., Phys. Rev. B 89, 184502 (2014)] experimental studies on such devices. We also find that the spin-split conductance is highly dependent on the interfacial scattering in these devices, and we carefully include these effects for realistic systems. A strong valve effect is found for the angularly dependent subgap peak conductance that is largely independent on the scattering and may prove useful in actual realizations of a superconducting spin valve device.

Within the nonequilibrium Green's function (NEGF) formalism, the generalized Kadanoff-Baym ansatz (GKBA) has stood out as a computationally cheap method to investigate the dynamics of interacting quantum systems driven out of equilibrium. Current implementations of the NEGF-GKBA, however, suffer from a drawback: real-time simulations require noncorrelated states as initial states. Consequently, initial correlations must be built up through an adiabatic switching of the interaction before turning on any external field, a procedure that can be numerically highly expensive. In this work, we extend the NEGF-GKBA to allow for correlated states as initial states. Our scheme makes it possible to efficiently separate the calculation of the initial state from the real-time simulation, thus paving the way for enlarging the class of systems and external drivings accessible by the already successful NEGF-GKBA. We demonstrate the accuracy of the method and its improved performance in a model donor-acceptor dyad driven out of equilibrium by an external laser pulse.