Motivated by recent experiments realizing correlated phenomena and superconductivity in two-dimensional (2D) van der Waals devices, we consider the general problem of whether correlation effects may be enhanced by modifying band structure while keeping a fixed weak interaction strength. Using determinantal quantum Monte Carlo, we study the 2D Hubbard model for two different band structures: a regular nearest-neighbor tight-binding model and a partially flat-band structure containing a nondispersing region, with identical total noninteracting bandwidth $W_{\mathrm{tot}}$. For both repulsive and attractive weak interactions ($\left|U\right|\ll W_{\mathrm{tot}}$), correlated phenomena are significantly stronger in the partially flat model. In the repulsive case, even with $U$ being an order of magnitude smaller than $W_{\mathrm{tot}}$, we find the presence of a Mott insulating state near half filling of the flat region in momentum space. In the attractive case, where generically the ground state is superconducting, the partially flat model exhibits significantly enhanced superconducting transition temperatures. These results suggest the possibility of engineering correlation effects in materials by tuning the noninteracting electronic dispersion.

We report on magnetic and electronic properties of the $B$-site-ordered double-perovskite $\mathrm{L}{\mathrm{a}}_2\mathrm{CrMn}{\mathrm{O}}_6$ films grown by pulsed-laser deposition. The magnetic-field dependence of magnetization curves showed hysteresis at low temperatures regardless of the degree of Cr/Mn order and the saturation magnetization became smaller for the higher-Cr/Mn-ordered film. The x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements suggested antiferromagnetic coupling between $\mathrm{C}{\mathrm{r}}^{3+}$ and $\mathrm{M}{\mathrm{n}}^{3+}$ ions, resulting in ferrimagnetism of the $B$-site-ordered double-perovskite $\mathrm{L}{\mathrm{a}}_2\mathrm{CrMn}{\mathrm{O}}_6$. A band structure was established by combining these results together with the synchrotron-radiation photoemission and optical spectra. We discuss the magnetic states among the $B$-site-ordered $\mathrm{L}{\mathrm{a}}_2{B}^{\prime }\mathrm{Mn}{\mathrm{O}}_6$ ($B^{\prime }:3d$ transition-metal elements from V to Ni) being basically consistent with the Kanamori-Goodenough rule.

We study the symmetric periodic Anderson model of the conduction electrons hybridized with the localized correlated electrons on a square lattice. Using the canonical representation of electrons by B. Kumar [Phys. Rev. B 77, 205115 (2008)], we do a self-consistent theory of its effective charge and spin dynamics, which produces an insulating ground state that undergoes continuous transition from the Kondo singlet to the Néel antiferromagnetic phase with decreasing hybridization and uncovers two inversion transitions for the charge quasiparticles. With suitably inverted quasiparticle bands for moderate to weaker effective Kondo couplings, this effective charge dynamics in the magnetic field coupled to electronic motion produces magnetic quantum oscillations with frequency corresponding to the half Brillouin zone.

We present systematic ab initio calculations of the charge carrier correlations between adjacent layers of two-dimensional materials in the presence of both charged impurity and strain disorder potentials using the examples of monolayer and bilayer graphene. Our analysis yields unambiguous first-principles quantum corrections to the Thomas-Fermi densities for interacting two-dimensional systems described by orbital-free density functional theory. Specifically, using density-potential functional theory, we find that quantum corrections to the quasiclassical Thomas-Fermi approximation have to be taken into account even for heterostructures of mesoscopic size. In order for the disorder-induced puddles of electrons and holes to be anticorrelated at zero average carrier density for both layers, the strength of the strain potential has to exceed that of the impurity potential by at least a factor of ten, with this number increasing for smaller impurity densities. Furthermore, our results show that quantum corrections have a larger impact on puddle correlations than exchange does, and they are necessary for properly predicting the experimentally observed Gaussian energy distribution at charge neutrality.

We consider the interaction-driven Mott transition at zero temperature from the viewpoint of microscopic Fermi liquid theory. To this end, we derive an exact expression for the Landau parameters within the dynamical mean-field theory (DMFT) approximation to the single-band Hubbard model. At the Mott transition, the symmetric and the antisymmetric Landau parameters diverge. The vanishing compressibility at the Mott transition directly implies the divergence of the forward-scattering amplitude in the charge sector, which connects the proximity of the Mott phase to a tendency toward phase separation. We verify the expected behavior of the Landau parameters in a DMFT application to the Hubbard model on the triangular lattice at finite temperature. Exact conservation laws and the Ward identity are crucial to capture vertex divergences related to the Mott transition. We furthermore generalize Leggett's formula for the static susceptibility of the Fermi liquid to the static fermion-boson response function. In the charge sector, the limits of small transferred momentum and frequency of this response function commute at the Mott transition.

Implementing topological insulators as elementary units in quantum technologies requires a comprehensive understanding of the dephasing mechanisms governing the surface carriers in these materials, which impose a practical limit to the applicability of these materials in such technologies requiring phase coherent transport. To investigate this, we have performed magnetoresistance (MR) and conductance fluctuations (CF) measurements in both exfoliated and molecular beam epitaxy grown samples. The phase breaking length ($l_{\varphi }$) obtained from MR shows a saturation below sample dependent characteristic temperatures, consistent with that obtained from CF measurements. We have systematically eliminated several factors that may lead to such behavior of $l_{\varphi }$ in the context of TIs, such as finite size effect, thermalization, spin-orbit coupling length, spin-flip scattering, and surface-bulk coupling. Our work indicates the need to identify an alternative source of dephasing that dominates at low $T$ in topological insulators, causing saturation in the phase breaking length and time.

We develop a unified numerical approach for modeling semiconductor-superconductor heterostructures. All the key physical ingredients of these systems—orbital effect of magnetic field, superconducting proximity effect, and electrostatic environment—are taken into account on equal footing in a realistic device geometry. As a model system, we consider indium arsenide (InAs) nanowires with an epitaxial aluminum (Al) shell, which is one of the most promising platforms for Majorana zero modes. We demonstrate qualitative and quantitative agreement of the obtained results with the existing experimental data. Finally, we characterize the topological superconducting phase emerging in a finite magnetic field and calculate the corresponding topological phase diagram.

Ruthenium oxide ($\mathrm{S}{\mathrm{r}}_4\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{10}$) exhibits a second magnetic transition at a temperature $T_M$ below its ferromagnetic (FM) transition ($T_C\sim 105$ K), which has attracted considerable attention in the past two decades. Here, we successfully grew $\mathrm{S}{\mathrm{r}}_4{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{F}{\mathrm{e}}_x\right)}_3{\mathrm{O}}_{10}$ ($x=0.01$) crystal and systematically investigated the magnetotransport property of its nanosheets. We found a feature that there is an in-plane FM order aligned along ⟨110⟩ directions below $T_C$ without the occurrence of the second magnetic transition at $T_M$, which is in contrast to that expected in pure $\mathrm{S}{\mathrm{r}}_4\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{10}$ nanosheets, where the in-plane FM order will rotate to the $c$ direction below $T_M$. Furthermore, a metamagnetic-like transition is also observed below ∼50 K at about 2 T when the magnetic field is applied along the $c$ axis. These results unambiguously demonstrate that the second magnetic transition at $T_M$ is actually a consequence of spin reorientation induced by the inherent coupling between the spin and lattice, which can be effectively tuned by element doping.

We report thickness dependent inhomogeneous antiferromagnetic (AFM) to ferromagnetic (FM) phase transitions in ultrathin $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) films grown on the $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. When the films are 4–7 unit cells (UCs) thick, FM domains appear as isolated nanodisks in the AFM matrix that together floats on top of the three AFM base UCs, leading a superparamagnetic blocking behavior. Our first principles calculations unravel the rather counterintuitive physical origin of this mixed phase state; the formation of FM/AFM domain boundaries is energetically favorable. At 8 UCs, an abrupt shear strain relief occurs in the LSMO thin film and twinning patterns with two unit cell periodicity form along the [010] and [100] directions. Our studies reveal the complexity of the magnetic phase transition at the nanometer scale and open a door for the development of quantum devices and statistical theories.

Recently, topological superconductors based on Josephson junctions in two-dimensional electron gases with strong Rashba spin-orbit coupling have been proposed as attractive alternatives to wire-based setups. Here, we elucidate how phase-controlled Josephson junctions based on quantum wells with [001] growth direction and an arbitrary combination of Rashba and Dresselhaus spin-orbit coupling can also host Majorana bound states for a wide range of parameters as long as the magnetic field is oriented appropriately. Hence, Majorana bound states based on Josephson junctions can appear in a wide class of two-dimensional electron gases. We study the effect of spin-orbit coupling, the Zeeman energies, and the superconducting phase difference to create a full topological phase diagram and find the optimal stability region to observe Majorana bound states in narrow junctions. Surprisingly, for equal Rashba and Dresselhaus spin-orbit coupling, well localized Majorana bound states can appear only for phase differences $\varphi \ne \pi$ as the topological gap protecting the Majorana bound states vanishes at $\varphi =\pi$. Our results show that the ratio between Rashba and Dresselhaus spin-orbit coupling or the choice of the in-plane crystallographic axis along which the superconducting phase bias is applied offer additional tunable knobs to test Majorana bound states in these systems. Finally, we discuss signatures of Majorana bound states that could be probed experimentally by tunneling conductance measurements at the edge of the junction.

The electronic, lattice dynamical, electron-phonon coupling, and superconducting properties of alkali-metal hydrides LiH, NaH, and KH, metalized through doping with alkaline-earth metals Be, Mg, and Ca, respectively, are investigated within the framework of density functional perturbation theory. The alloys were modeled by the self-consistent virtual crystal approximation, and the effect of zero-point energy contribution is consistently taken into account. For all three alloys, a steady increase of the electron-phonon coupling constant $\lambda$ is found with progressive alkaline-earth metal doping, reaching values as high as 0.47 for (Li/Be)H, 1.26 for (Na/Mg)H, and 1.69 for (K/Ca)H. The growth of $\lambda$ with doping is the result of two effects: the softening of the phonon spectrum, mainly of the H-optical modes, and the increase of the density of states at the Fermi level. Estimates of the superconducting critical temperature reach values of 2.1 K for ${\mathrm{Li}}_{0.95}{\mathrm{Be}}_{0.05}\mathrm{H}$, 28 K for ${\mathrm{Na}}_{0.8}{\mathrm{Mg}}_{0.2}\mathrm{H}$, and even 49 K for ${\mathrm{K}}_{0.55}{\mathrm{Ca}}_{0.45}\mathrm{H}$, demonstrating that doping is an alternative route to high transition temperatures in this material class without the need to apply high external pressure.

Ferromagnetic resonance (FMR) in yttrium iron garnet (YIG)/Pt with a suitable YIG thickness in the submillimeter range show pure spin current contributions from the uniform FMR (UFMR) mode and various spin-wave resonance (SWR) modes, each with a different absorption intensity, inverse spin Hall voltage, and temperature dependence. The higher order mode of SWR with nonzero moment transferring along the thickness direction induces a maximum dc voltage among all modes. The spin current driven by UFMR and SWR has a linear $T$ and power-law $T$ dependence, respectively. In addition, the mixing conductance is derived for both modes, indicative of power-law $T$ dependences. There is an absence of evidence for coherent spin pumping in YIG/Pt.

We examine a modified drift-diffusion formalism to describe spin transport near an ultrathin magnet whose thickness is similar to or less than the spin dephasing length. Most of the previous theories on spin torque assume the transverse component of a injected spin current dephases perfectly thus are fully absorbed into the ferromagnet. However, in the state-of-art multilayer systems under consideration of recent studies, the thicknesses of ferromagnets are on the order of or less than a nanometer, thus one cannot safely assume the spin dephasing to be perfect. To describe the effects of a finite dephasing rate, we adopt the concept of transmitted mixing conductance, whose application to the drift-diffusion formalism has been limited. For a concise description of physical consequences, we introduce an effective spin transparency. Interestingly, for an ultrathin magnet with a finite dephasing rate, the spin transparency can be even enhanced and there arises a non-negligible fieldlike spin-orbit torque even in the absence of the imaginary part of the spin mixing conductance. The effective spin transparency provides a simple extension of the drift-diffusion formalism, which is accessible to experimentalists analyzing their results.

Motivated by the rich physics of electric-field control of magnetic properties observed in recent experiments, we use the first-principles calculations to investigate charge-transfer-induced magnetism of the full $d$-band nonmagnetic metals. A ferromagnetic phase transition initiated by an electric-field-induced charge transfer was found for all full $d$-band nonmagnetic metals. Beside the ordinary Hund's rules in Cu and Ag, a more complex correlation between the charge and the induced magnetization has been found, especially in Pd. The origin of the abnormal charge-induced magnetization is determined by the Coulomb interaction-induced instability of spin degeneracy at specific charges. In addition, a large nontrivial anomalous Hall effect has been discovered, and the sign of the anomalous Hall conductivity can change as charge number varies, a feature potentially useful for future spintronic applications.

Materials with optical activity can rotate the polarization plane of transmitted light. The most typical example is the natural optical activity, which has the symmetry property of changing sign after space inversion but being invariant to time inversion. Faraday rotation exhibits the opposite: It is invariant to space inversion but changes sign after time reversal. Here, we demonstrate that in a magnetoelectric material, another type of polarization rotation is possible. This effect is investigated in magnetoelectric ${\mathrm{YbAl}}_3{\left({\mathrm{BO}}_3\right)}_4$ under the viewpoint of time and space inversion symmetry arguments. We observe the sign change of the rotation sense under either time or space reversal. This investigation proves that the polarization rotation in ${\mathrm{YbAl}}_3{\left({\mathrm{BO}}_3\right)}_4$ must be classified as gyrotropic birefringence, which has been discussed within the idea of time-reversal breaking in underdoped cuprates. The diagonal terms in the magnetoelectric susceptibility are responsible for the observed signal of gyrotropic birefringence. Further analysis of the experimental spectra reveals a substantial contribution of the natural optical activity to the polarization rotation. We also demonstrate that the observed activity originates from the magnetoelectric susceptibility.

Superconductors with the A15 structure are prototypical type-II $s$-wave superconductors which have generated considerable interest in early superconducting material history. However, the topological nature of the electronic structure remains unnoticed so far. Here, using first-principles band structure calculations based on density-functional theory, we show that the A15 superconductors (${\mathrm{Ta}}_3\mathrm{Sb}$, ${\mathrm{Ta}}_3\mathrm{Sn}$, and ${\mathrm{Ta}}_3\mathrm{Pb}$) have nontrivial band topology in the bulk electronic band structures, leading to the formation of topological surface states near the Fermi energy. Due to the bulk superconductivity, the proximity effect in the topological surface states would induce topological superconductivity even without heterostructure of a topological insulator and an $s$-wave superconductor. Our results indicate that the A15 superconductors are promising candidates for the realization of the topological superconductivity and the Majorana fermion.

We report the evolution of charge density wave states under pressure for two $\mathrm{Nb}{\mathrm{S}}_3$ phases: triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and x-ray diffraction (XRD) techniques are applied. The x-ray studies on the monoclinic phase under pressure show a compression of the lattice at different rates below and above ∼7 GPa but without a change in space group symmetry. The Raman spectra of the two phases evolve similarly with pressure; all peaks almost disappear in the ∼6–8 GPa range, indicating a transition from an insulating to a metallic state, and peaks at new positions appear above 8 GPa. The results suggest suppression of the ambient charge-density waves and their subsequent recovery with new orderings above 8 GPa.

The negative electronic compressibility refers to the lowering of the chemical potential of a metallic system when the carrier density increases. This effect has often been invoked in the past to explain the enhancement of the capacitance beyond the classical limits in capacitors with two-dimensional electron gases as electrodes. Based on experiments on strongly confined semiconductor quantum wells (QWs), it has been traditionally ascribed to the electron exchange energy as the main driving force. Recent research, however, has revealed that analogous effects can occur in other classes of materials systems, such as polar oxide interfaces, whose characteristics drastically depart from those of the previously considered cases. To rationalize these new results, it is necessary to revisit the established theory of confined electron gases, and test whether its conclusions are valid beyond the specifics of semiconductor-based QWs. Here we find, based on first-principles calculations of jellium slabs, that one must indeed be very careful when extrapolating existing results to other realistic physical systems. In particular, we identify a number of additional, previously overlooked mechanisms (e.g., related to the displacement of the electronic cloud and to the multiband structure of the delocalized gas), that enter into play and become sources of negative capacitance in the weak-confinement regime. Our detailed analysis of these emerging contributions, supported by analytic models and multiple test cases, will provide a useful guidance in the ongoing quest for nanometric capacitors with enhanced performance.

As far-field thermal emission is limited by blackbody radiation, the significant enhancement of thermal radiation in the near-field plays a vital role in a variety of applications such as infrared sensing, radiation cooling, and thermophotovoltaics. Yet the techniques of exporting the near-field signal to the far field are still not mature, typically relying on complex instrumentation capable of being applied only to the surfaces of model systems and structures. Here we develop an efficient method of extracting near-field thermal radiation to the far field by integrating a nanoscale thermal emitter with a high-index optical waveguide. By directly depositing the emitter onto the optical waveguide, it requires no vacuum gap and enables efficient coupling of near-field thermal radiation into propagating wave-guided modes. A single-mode planar waveguide incorporated with an indium-tin oxide (ITO) film as a thermal emitter is theoretically investigated to prove the feasibility of thermal extraction. The Wiener-chaos expansion method is applied to directly calculate the thermal radiation from the emitter-waveguide system. We experimentally demonstrate the fidelity of the optical-waveguide-assisted near-field thermal extraction by measuring the emission spectrum of an ITO-coated optical fiber and comparing with theoretical predictions. Our experimental demonstration in conjunction with the direct simulation paves the way for effectively extracting near-field thermal radiation with applications in chemical sensing, infrared imaging, and near-field thermal energy management.

We propose a systematic approach to constructing microscopic models with fractional excitations in three-dimensional (3D) space. Building blocks are quantum wires described by the ($1+1$)-dimensional conformal field theory (CFT) associated with a current algebra $\mathfrak{g}$. The wires are coupled with each other to form a 3D network through the current-current interactions of ${\mathfrak{g}}_1$ and ${\mathfrak{g}}_2$ CFTs that are related to the $\mathfrak{g}$ CFT by a nontrivial conformal embedding $\mathfrak{g}\supset {\mathfrak{g}}_1\times {\mathfrak{g}}_2$. The resulting model can be viewed as a layer construction of a 3D topologically ordered state, in which the conformal embedding in each wire implements the anyon condensation between adjacent layers. Local operators acting on the ground state create pointlike or looplike deconfined excitations depending on the branching rule. We demonstrate our construction for a simple solvable model based on the conformal embedding $SU{\left(2\right)}_1\times SU{\left(2\right)}_1\supset U{\left(1\right)}_4\times U{\left(1\right)}_4$. We show that the model possesses extensively degenerate ground states on a torus with deconfined quasiparticles, and that appropriate local perturbations lift the degeneracy and yield a 3D $Z_2$ gauge theory with a fermionic $Z_2$ charge.