We study the conditions to realize an excitonic condensed phase in an electron-hole bilayer system with local Hubbard-like interactions at half-filling, where we can address the interplay with Mott localization. Using dynamical mean-field theory, we find that an excitonic state is stable in a sizable region of a phase diagram spanned by the intralayer ($U$) and interlayer ($V$) interactions. The latter term is expected to favor the excitonic phase which is indeed found in a slice of the phase diagram with $V>U$. Remarkably, we find that, when $U$ is large enough, the excitonic region extends also for $U>V$, in contrast with naïve expectations. The extended stability of the excitonic phase can be linked to in-layer Mott localization and interlayer spin correlations. Using a mapping to a model with attractive interlayer coupling, we fully characterize the condensate phase in terms of its superconducting counterpart, thereby addressing its coherence and correlation length.

Intense “blue-green” phosphorescence is commonly observed in near-colorless laboratory-grown high-pressure high-temperature diamonds following optical excitation at or above the indirect band gap. We have employed a holistic combination of optically excited time-resolved techniques (in addition to standard spectroscopic characterization techniques) to study the physics of this long-lived phosphorescence and understand luminescence-related charge-transfer processes. It is shown that the properties of the broad “blue-green” luminescence and phosphorescence band can be fully explained by emission from neutral substitutional nitrogen-boron donor-acceptor pairs (${\text{N}}_{\text{S}}^0\cdots {\text{B}}_{\text{S}}^0$), once the configurational change between charge states is considered, and both tunneling between defects and thermal ionization of donors and acceptors is considered. Significant concentrations of metastable ${\text{N}}_{\text{S}}^{-}$ are identified after optical excitation at or above the indirect band gap. ${\text{N}}_{\text{S}}^{-}$ is much shallower ($\sim 0.2$ eV) than previously thought and plays a key role in resetting the ${\text{N}}_{\text{S}}^0\cdots {\text{B}}_{\text{S}}^0$ donor-acceptor pairs.

Enhanced room-temperature electromechanical coupling in the equimolar lead-free ferroelectric solid solution of barium zirconate titanate and barium calcium titanate is attributed to the existence of an intermediate morphotropic phase region. Initial studies suggested that the existence of a polar orthorhombic phase in the morphotropic region between terminal solid solutions of rhombohedral barium zirconate titanate and tetragonal barium calcium titanate causes this enhancement. However, recent experiments suggest the coexistence of two or more of these phases in the morphotropic region. Such coexistence can often arise due to changes in anisotropy in electrostriction. To understand the effect of anisotropy in electrostriction on the stability of these phases and the consequent polarization switching characteristics in the equimolar solid solution, we performed systematic phase-field simulations of domain evolution to the steady state in this system under stress-free conditions as a function of anisotropy in electrostriction defined by $Q_z=2Q_{44}/(Q_{11}-Q_{12})$, where $Q_{11}, Q_{12}$, and $Q_{44}$ are the independent coefficients of the electrostriction tensor (expressed using Voigt notation). Our results show a systematic reduction in polarization anisotropy with increasing $Q_z$. For example, a single-phase configuration consisting of orthorhombic variants corresponding to isotropic electrostrictive moduli ($Q_z=1$) transforms to a three-phase configuration containing tetragonal, orthorhombic, and rhombohedral variants when the moduli show strong anisotropy ($Q_z=2.5$). We use the effective in-plane and out-of-plane piezoelectric coefficients and their ratio as a measure of the electromechanical switching characteristics. Unlike polarization anisotropy behavior, these characteristics do not exhibit such monotonic behavior. The effective piezo coefficients depend not only on the number of coexisting polar phases but also on their spatial configuration with respect to the applied field. Our predictions pertaining to the coexistence of tetragonal and orthorhombic variants and the corresponding effective out-of-plane piezoelectric coefficient show good agreement with experimental findings. We also discuss how clamped or constrained conditions can modify the phase stability observed under stress-free conditions.

Barium titanate ($\mathrm{BaTi}{\mathrm{O}}_3$, BTO) is a typical ferroelectric material with a series of complex phase transitions. Understanding its phase transitions and related properties from an atomic perspective is of scientific significance and practical importance. In this paper, based on first-principles calculation, a second-principles model is constructed to investigate the phase transition and related properties of BTO at the atomic level. By using the constructed second-principles model, the full phase-transition sequence from high-temperature paraelectric cubic phase to the low-temperature rhombohedral phase through tetragonal and orthorhombic intermediate phases have been successfully predicted at the atomic level. To reveal the underlying mechanism of phase transition, the influences of different anharmonic terms on phase transition are investigated in detail. We found that the interaction between strain and Ti-O bond plays an essential role in phase transition. Furthermore, dielectric properties and the $P\text{−}E$ loop of BTO are successfully obtained with the model. In this paper, we not only provide a model to predict the phase transitions and related properties of BTO from the atomic level but also extend the application of the second-principles method.

The nearly free-electron metal next to a localized Mott-insulating state has been recently proposed as a way to probe Kondo lattice physics and to gain insight into how the two extremes of localized and delocalized electron states interact [V. Sunko et al., Sci. Adv. 6, eaaz0611 (2020)]. Although high harmonic generation has been used extensively to investigate the gas phase, its extension to solids is relatively recent, and has not yet been applied to interfaces. Here, we investigate the field-induced dielectric breakdown at the metal/Mott-insulator interface using high harmonic generation, emitted when the interface is subjected to an ultrafast laser pulse. We show that the intensity of high harmonic emission correlates closely with doublon production and the corresponding loss of short-range antiferromagnetic order. For strong interlayer coupling, the harmonic intensity is defined by a phase transition between states that do not exist in a pure Mott-insulator case. For weak interlayer coupling, the threshold for dielectric breakdown is considerably lowered due to the presence of a metallic layer. This suggests that interlayer coupling can be used as an additional knob to control magnetic-insulator breakdown, with implications for using Mott insulators as memristors in neuromorphic circuits.

Antiferromagnetic Weyl semimetals with spin chirality offer excellent platforms to address the Berry phase physics, which manifests prominently in several of their electro-optical and electromagnetic responses including as a large anomalous Hall effect (AHE) and spin Hall conductivity. The ${\mathrm{Mn}}_3\mathrm{Sn}$ and ${\mathrm{Mn}}_3\mathrm{Ge}$ compounds, where the Mn spins arrange in a kagome lattice, are examples of this class of materials. Here, we report on measurements of magnetotransport in $c$-axis textured ${\mathrm{Mn}}_3\mathrm{Sn}$ thin films grown on the [111] plane of single-crystal MgO by dc magnetron sputtering. At room temperature, these films display a weak uncompensated magnetic moment of $\approx 0.12{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ in the basal plane and a longitudinal resistivity (${\rho }_{xx}$) $\approx 3.8\mu \mathrm{\Omega }\mathrm{m}$, which matches well with the bulk value. A residual resistivity ratio $[{\rho }_{xx}\left(300K\right)/{\rho }_{xx}\left(2K\right)]$ of $\approx 3.92$ further indicates the high quality of the films. While at 300 K a weak AHE together with field-linear Hall resistivity (${\rho }_{xy}$) is observed in magnetic fields ($H$) applied perpendicular to the kagome planes, the temperature ($T$) dependence of ${\rho }_{xy}$ shows prominent signatures of three magnetic phases in the temperature regime of 2–300 K. The ${\rho }_{xy}$ also derives a nontrivial topological contribution (${\rho }_{\mathrm{THE}}\sim 1\mathrm{n}\mathrm{\Omega }\mathrm{m}$) in the spin-glass phase which appears at $T\le 100$ K. The origin of the ${\rho }_{\mathrm{THE}}$ is attributed to spin textures which may appear in a frustrated chiral spin order. Our measurements of anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) over a wide $H-T$ phase space reveal the hitherto unseen effects in the three magnetic phases of ${\mathrm{Mn}}_3\mathrm{Sn}$. While the AMR and PHE are negative in the inverse triangular spin phase ($250\mathrm{K}\le T\le T_{\mathrm{N}}$), the helical phase ($100\le T\le 250\mathrm{K}$) is devoid of anisotropic in-plane resistivity, and the spin-glass phase shows a sign reversal of AMR with the increasing magnetic field. The origin of this sign change in AMR/PHE is attributed to the emergence of topologically protected spin textures like skyrmions where the fictitious effective magnetic field is estimated to be $\approx 4.4$ Tesla.

We have studied the film thickness dependence of the current-induced magneto-optical Kerr effect in Pt and V thin films. The Kerr signal for Pt shows little dependence on the thickness in the range studied (20–80 nm). In contrast, the signal for V increases with increasing thickness and saturates at a thickness near 100 nm to a value significantly larger than that of Pt. These experimental results are accounted for assuming that spin and orbital Hall effects are responsible for the Kerr signal. We show that the Kerr signal is proportional to the product of the dc spin (orbital) Hall conductivity and the energy derivative of the ac spin (orbital) Hall conductivity. Contributions from the spin and orbital Hall effects mostly add up for V whereas they cancel out for Pt. Assuming that the orbital Hall conductivity matches that predicted from first-principles calculations, the thickness dependence of the Kerr signal suggests that the orbital diffusion length of V is considerably smaller compared to its spin diffusion length.

A series of ${\mathrm{Fe}}_x{\mathrm{Rh}}_{100-x}$ ($30\le x\le 57$) films were epitaxially grown using magnetron sputtering, and were systematically studied by magnetization, electrical resistivity, and Hall resistivity measurements. After optimizing the growth conditions, phase-pure ${\mathrm{Fe}}_x{\mathrm{Rh}}_{100-x}$ films were obtained, and their magnetic phase diagram was revisited. The ferromagnetic (FM) to antiferromagnetic (AFM) transition is limited at narrow Fe contents with $48\le x\le 54$ in the bulk ${\mathrm{Fe}}_x{\mathrm{Rh}}_{100-x}$ alloys. By contrast, the FM-AFM transition in the ${\mathrm{Fe}}_x{\mathrm{Rh}}_{100-x}$ films is extended to cover a much wider $x$ range between 33% and 53%, whose critical temperature slightly decreases with increasing Fe content. The resistivity jump and magnetization drop at the FM-AFM transition are much more significant in the ${\mathrm{Fe}}_x{\mathrm{Rh}}_{100-x}$ films with $\sim 50\%$ Fe content than in the Fe-deficient films; the latter have a large amount of paramagnetic phase. The magnetoresistivity (MR) is rather weak and positive in the AFM state, while it becomes negative when the FM phase shows up, and a giant MR appears in the mixed FM and AFM state. The Hall resistivity is dominated by the ordinary Hall effect in the AFM state, while in the mixed state or high-temperature FM state, the anomalous Hall effect takes over. The absence of topological Hall resistivity in ${\mathrm{Fe}}_x{\mathrm{Rh}}_{100-x}$ films with various Fe contents implies that the previously observed topological Hall effect is most likely extrinsic. We propose that the anomalous Hall effect caused by the FM iron moments at the interfaces nicely explains the hump-like anomaly in the Hall resistivity data. Our systematic investigations may offer valuable insights into the spintronics based on iron-rhodium alloys.

We present a scattering formalism to analyze the spin-momentum locking in structured holey plasmonic metasurfaces. It is valid for any unit cell for arbitrary position and orientation of the holes. The spin-momentum locking emergence is found to originate from the unit-cell configuration. Additionally, we find that there are several breakdown terms spoiling the perfect spin-momentum locking polarization. We prove that this breakdown also appears in systems with global symmetries of translation and rotation of the whole lattice, like the kagome lattice. Finally, we present the excitation of surface-plasmon polaritons as the paramount example of the spin-momentum locking breakdown.

This article reports a comparative study of bulk and surface properties in the transition metal dichalcogenide $1T\text{−}{\mathrm{TaS}}_2$. When heating the sample, the surface displays an intermediate insulating phase that persists for $\sim 10$ K on top of a metallic bulk. The weaker screening of Coulomb repulsion and a stiffer charge density wave (CDW) explain such resilience of a correlated insulator in the topmost layers. Both time-resolved angle-resolved photoelectron spectroscopy and transient reflectivity are employed to investigate the dynamics of electrons and CDW collective motion. It follows that the amplitude mode is always stiffer at the surface and displays variable coupling to the Mott-Peierls band, stronger in the low-temperature phase and weaker in the intermediate one.

We investigate vortex matter in a proximity-coupled $\mathrm{Mg}/\mathrm{MgO}/\mathrm{Mg}{\mathrm{B}}_2$ nanocomposite with $\sim 30$ vol. % of $\mathrm{Mg}{\mathrm{B}}_2$ using magneto-optical imaging, scanning superconducting quantum interface device microscopy, and pinning-force analysis. This nanocomposite was prepared by spark plasma sintering (SPS) of the MgO/$\mathrm{MgO}/\mathrm{Mg}{\mathrm{B}}_2$ mixture powders obtained from a solid-phase reaction between Mg and ${\mathrm{B}}_2{\mathrm{O}}_3$ [Uchino et al., Phys. Rev. B 101, 035146 (2020)]. The resulting nanocomposite is characterized by scale-free (or fractal) distributions of $\mathrm{Mg}{\mathrm{B}}_2$ components and the atomically clean $\mathrm{MgO}/\mathrm{Mg}{\mathrm{B}}_2$ interfaces. It is also found that high SPS temperature ($1200{}^{\circ }\mathrm{C}$) is the key to improve the proximity-induced superconducting properties of the $\mathrm{Mg}/\mathrm{MgO}/\mathrm{Mg}{\mathrm{B}}_2$ nanocomposite. This proximity-coupled system acts as a fully phase-coherent superconductor with isotropic pinning and strong superfluid phase stiffness irrespective of the low volume fraction of $\mathrm{Mg}{\mathrm{B}}_2$. Our results demonstrate that in contrast to the case of conventional granular superconductors, the grain boundaries in the present system carry high critical currents and have high vortex-pinning efficiency, implying an excellent phase-coherent capability of the proximity-coupled fractal network.

Infinite-layer nickelate superconductors have recently been discovered to share both similarities and differences with cuprate superconductors. Notably, the incorporation of hydrogen (H) through topotactic reduction has been found to play a critical role in their electronic structure and, consequently, their superconductivity. In this study, we utilized a theoretical approach combining density-functional theory and impurity approximation to design three characteristic multi-orbital Hubbard models representing low, moderate, and high concentrations of topotactic-hydrogen. Consistent with experimental findings, our simulations revealed that both low and high concentrations of topotactic-hydrogen induce high-spin states ($S=1)$ that are composed by holes at $d_{x^2-y^2}$ and $d_{z^2}$ orbitals and consequently the emergent intersite hopping between $d_{z^2}$ to $d_{x^2-y^2}$ is unfavorable for superconductivity. Conversely, an optimal concentration of 25% H aligns with the single Ni-$d_{x^2-y^2}$ band picture of superconductivity in infinite-layer nickelates, demonstrating its beneficial effect on promoting superconducting behavior.

Three-dimensional topological insulators support gapless Dirac fermion surface states whose rich topological properties result from the interplay of symmetries and dimensionality. Their topological properties have been extensively studied in systems of integer spatial dimension but the prospect of these surface electrons arranging into structures of noninteger dimension like fractals remains unexplored. In this work, we investigate a class of states arising from the coupling of surface Dirac fermions to a time-reversal symmetric fractal potential, which breaks translation symmetry while retaining self-similarity. Employing large-scale exact diagonalization, scaling analysis of the inverse participation ratio, and the box-counting method, we establish the onset of self-similar Dirac fermions with fractal dimension for a symmetry-preserving surface potential with the geometry of a Sierpiński carpet fractal with fractal dimension $D\approx 1.89$. Dirac fractal surface states open a fruitful avenue to explore exotic regimes of transport and quantum information storage in topological systems with fractal dimensionality.

Controlling the anomalous valley Hall (AVH) effect by external means is crucial for valleytronic devices in practical applications; however, most of the previously proposed control approaches are either irreversible or volatile. Here, we present a general scheme for achieving nonvolatile electrical control of the AVH effect based on multiferroic van der Waals heterostructure. Using density functional theory calculations and $\mathit{k}·\mathit{p}$ model analysis, we further demonstrate the feasibility of this design principle by stacking ferromagnetic monolayer $\mathrm{V}{\mathrm{Se}}_2$ on ferroelectric monolayer ${\mathrm{Al}}_2{\mathrm{S}}_3$. The reversible switching of AVH can be finely manipulated by reversing the ferroelectric polarization of ${\mathrm{Al}}_2{\mathrm{S}}_3$ via electric field. The regulated AVH state of $\mathrm{V}{\mathrm{Se}}_2$ can be stably preserved due to the ferroelectric nonvolatility of ${\mathrm{Al}}_2{\mathrm{S}}_3$. In addition, the sign of valley polarization can be simultaneously inverted by reversing ferroelectric polarization. Our results not only provide the basis for an intrinsic ferroelectricity controlled ferrovalley, but also uncover an outstanding candidate for realizing bidirectional and nonvolatile switching valleytronic devices.

Electronic correlation and reconstruction are two important factors that play a critical role in shaping the magnetic and electronic properties of correlated low-dimensional systems. Here, we report a competition between the electronic correlation and structural reconstruction in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ti}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures by modulating material polarity and interfacial strain, respectively. The heterostructures exhibit a critical thickness ($t_c$) at which a metal to insulator transition abruptly occurs at certain thickness, accompanied by the coexistence of two- and three-dimensional (2D and 3D) carriers. Intriguingly, the $t_c$ exhibits a $\mathsf{V}\text{−}\mathrm{shaped}$ dependence on the doping concentration of Sr, with the smallest $t_c$ value at $x=0.5$. We attribute this $\mathsf{V}\text{−}\mathrm{shaped}$ dependence to the competition between the electronic reconstruction (modulated by the polarity) and the electronic correlation (modulated by strain), which are borne out by the experimental results, including strain-dependent electronic properties and the evolution of 2D and 3D carriers. Our findings underscore the significance of the interplay between electronic reconstruction and correlation in the realization and utilization of emergent electronic functionalities in low-dimensional correlated systems.

Low-dimensional germanium hole devices are promising systems with many potential applications such as hole spin qubits, Andreev spin qubits, and Josephson junctions, and can serve as a basis for the realization of topological superconductivity. This vast array of potential uses for Ge largely stems from the exceptionally strong and controllable spin-orbit interaction (SOI), ultralong mean free paths, long coherence times, and compatibility with complementary metal-oxide-semiconductor (CMOS) technology. However, when brought into proximity with a superconductor (SC), metallization normally diminishes many useful properties of a semiconductor, for instance, typically reducing the $g$ factor and SOI energy, as well as renormalizing the effective mass. In this paper, we consider metallization of a Ge nanowire (NW) in proximity to a SC, explicitly taking into account the three-dimensional (3D) geometry of the NW. We find that proximitized Ge exhibits a unique phenomenology of metallization effects, where the 3D cross section plays a crucial role. For instance, in contrast to expectations, we find that SOI can be enhanced by strong coupling to the superconductor. We also show that the thickness of the NW plays a critical role in determining both the size of the proximity-induced pairing potential and metallization effects, since the coupling between the NW and SC strongly depends on the distance of the NW wave function from the interface with the SC. In the absence of electrostatic effects, we find that a sizable gap opens only in thin NWs ($d\lesssim 3\mathrm{n}\mathrm{m}$). In thicker NWs, the wave function must be pushed closer to the SC by electrostatic effects in order to achieve a sizable proximity gap such that the required electrostatic field strength can simultaneously induce a strong SOI. The unique and sometimes beneficial nature of metallization effects in SC-Ge NW devices evinces them as ideal platforms for future applications in quantum information processing.

We investigated the properties of quantum electron solids of different symmetries in two-dimensional double-layer systems in transition-metal dichalcogenides stacked on opposite sides of thin layers of boron nitride. For the parameters of our experimental structure with a screened interaction, we discovered that both the hexagonal and the square lattices are stable, whereas for a pure Coulomb interaction with no screening, only the hexagonal lattice is stable. The hexagonal lattice possesses an energy lower than that of the square lattice and thus is expected to form at low temperatures. We found that the square lattice has a higher melting temperature and thus will replace the hexagonal lattice of the electron solid state as the temperature is increased. This result agrees with our Coulomb drag resistance measurements at different temperatures which exhibit peaks at different gate voltages, corresponding to phases with different chemical potentials. Our system provides for a system to study quantum structural transformations.

The Casimir-Lifshitz interaction, a long-range force that arises between solids and molecules due to quantum fluctuations in electromagnetic fields, has been widely studied in solid-state physics. The degree of polarization in this interaction is influenced by the dielectric properties of the materials involved, which in turn are determined by factors such as band-to-band transitions, free carrier contributions, phonon contributions, and exciton contributions. Gapped metals, a new class of materials with unique electronic structures, offer the potential to manipulate dielectric properties and, consequently, the Casimir-Lifshitz interaction. In this study, we theoretically investigate the finite temperature Casimir-Lifshitz interaction in ${\mathrm{La}}_3{\mathrm{Te}}_4$-based gapped metal systems with varying off-stoichiometry levels. We demonstrate that off-stoichiometric effects in gapped metals can be used to control the magnitude and, in some cases, even the sign of Casimir-Lifshitz interactions. We predict measurable corrections due to stoichiometry on the predicted Casimir force between a ${\mathrm{La}}_3{\mathrm{Te}}_4$ surface and a gold sphere, attached to an atomic force microscopy tip.