Recently, several triangular-lattice magnets with delafossite structure have been found to display spin-liquid behavior down to the lowest temperatures. Remarkably, applying a magnetic field destroys the spin liquid, which then gives way to symmetry-breaking states, identified as semiclassical coplanar states including a magnetization plateau at 1/3 total magnetization. Here we provide a theoretical approach rationalizing this dichotomy, utilizing a Schwinger-boson theory that captures both ordered and disordered magnetic phases. We show that a zero-field spin liquid, driven by strong frustration, is naturally destabilized in a magnetic field via spinon condensation. Symmetry-breaking order akin to the standard triangular-lattice Heisenberg model then arises via an order-by-disorder mechanism. We discuss implications for pertinent experiments.

We study electron states, bound to topological textures such as skyrmions and domain walls in a Néel antiferromagnet. In certain limits, we find the dependence of bound states on the geometry of the texture, and estimate the bound-state contribution to its energy. This contribution proves significant compared with the purely magnetic energy, and thus substantially influences the shape of the texture. It also considerably shifts the transition line between the modulated and the uniform phase in favor of the latter.

The precompression effect of foreign elements affords a promising route to metallizing hydrogen atoms under moderate pressure and achieving superconductivity with high transition temperature $\left(T_{\mathrm{C}}\right)$ in polyhydrides. Recently, lutetium (Lu) polyhydrides are receiving considerable attention due to their fully filled $f$ orbitals, which are favorable for realizing high $T_{\mathrm{C}}$. The highest $T_{\mathrm{C}}$ observed in the as-synthesized lutetium polyhydrides up to now has reached 65–71 K under pressures of 181–218 GPa, which was attributed to the $Pm\overline{3}n$ ${\mathrm{Lu}}_4{\mathrm{H}}_{23}$ phase but without computational analysis. Here we perform first-principles calculations on the bonding feature, the stability, the electronic property, and the superconductivity of ${\mathrm{Lu}}_4{\mathrm{H}}_{23}$. ${\mathrm{Lu}}_4{\mathrm{H}}_{23}$ presents robust metallicity due to the orbital hybridizations associated with the H-H covalent and Lu-H ionic bonds, as well as the charge transfer from Lu to H atoms. Our calculations reveal that ${\mathrm{Lu}}_4{\mathrm{H}}_{23}$ can be stabilized at pressures above ∼200 GPa, whose $T_{\mathrm{C}}$ is estimated to be 69–225 (103–210) K at 200 (218) GPa and maintains at the average values around 150–200 K under the pressures exceeding 250 GPa. We propose that the $Fm\overline{3}m$ LuH plays a possible role in stabilizing ${\mathrm{Lu}}_4{\mathrm{H}}_{23}$ at pressures lower than 200 GPa, and, simultaneously, decreases the $T_{\mathrm{C}}$ of superconductivity in the as-synthesized lutetium polyhydrides. Our work provides a computational analysis on ${\mathrm{Lu}}_4{\mathrm{H}}_{23}$, and the predicted high $T_{\mathrm{C}}$ is expected to be experimentally realized by increasing pressure and optimizing synthesis process to eliminate the LuH phase.

Unconventional superconductivity occurs often in materials with low dimensionality. Here, we report superconductivity observed in layered ${\mathrm{Li}}_x{\mathrm{SnSe}}_2$ with the superconducting transition temperature $T_c\sim 6$ K. Through $\mathrm{L}{\mathrm{i}}^{+}$ intercalation in semiconducting ${\mathrm{SnSe}}_2$ via electrochemical process, ${\mathrm{Li}}_x{\mathrm{SnSe}}_2$ is formed with an extremely small $x$ value as estimated from the $c$-axis lattice parameter, carrier concentration, and first-principles calculations. Electrical resistivity and magnetic susceptibility measurements allow the construction of both lower $\left(H_{c1}^{ab}\right)$ and upper critical field ($H_{c2}^{ab}\mathrm{and}{H}_{c2}^c$) phase diagrams. While $H_{c2}^c$ obtained from 10% and 50% resistivity drops exhibits linear temperature dependence, $H_{c2}^c\left(90\%\mathrm{resistivity}\mathrm{drop}\right)$, $H_{c1}^{ab}$, and $H_{c2}^{ab}$ can be described by the empirical formula $H_{ci}\left(T\right)=H_{ci}\left(0\right)\left[1-{\left(\frac{T}{T_{c0}}\right)}^2\right]$ ($i=1$, 2), giving $H_{c2}^c\left(0\right)=754$ Oe, $H_{c1}^{ab}$(0) = 27 Oe, and $H_{\mathrm{c}2}^{ab}$(0) = 1652 Oe. Using the Ginzburg-Landau formula, we further estimate that the $c$-axis penetration depth ${\lambda }_c$(0) = 396 nm and the coherence length anisotropy ${\xi }_{ab}$(0) = 66.1 nm and ${\xi }_c$(0) = 30.3 nm. The fact that ${\xi }_c$(0) is much longer than the interlayer separation implies three-dimensional superconductivity with superconducting anisotropy ${\xi }_{ab}\left(0\right)/{\xi }_c$(0) ∼ 2.2.

We present a theoretical investigation of the $t_{2g}$ electronic structure reconstruction of body-centered-tetragonal (bct) ${\mathrm{NbO}}_2$ employing a combination of density functional theory and dimer-embedded dynamical mean-field theory calculations. We describe the electronic structure of the insulating phase of bct ${\mathrm{NbO}}_2$ and propose an orbital-selective mechanism for the gap opening. The resulting insulating state of the dimerized $x^2\text{−}{y}^2$ orbital along the rutile $c$ axis is shown to be adiabatically connected to an entangled Mott-Peierls insulator, but the true nature of insulating bct ${\mathrm{NbO}}_2$ compared with experiment is better characterized as a Mott localized electronic state due to strong dynamical intrasite multiorbital correlations. Our orbital-selective, strong-coupling scenario could be tested in future studies of ${\mathrm{NbO}}_2$ Mott memristors for neuromorphic computing.

We systematically derive the quantum kinetic equation in full phase space for any quadratic Hamiltonian of bosonic fields, including in the absence of translational invariance. This enables the treatment of boundaries, inhomogeneous systems, and states with nontrivial textures, such as skyrmions in the context of magnetic bosons. We relate the evolution of the distribution of bosons in phase space to single-electron, band-diagonal physical quantities such as Berry curvature and energy magnetization by providing a procedure to “diagonalize” the Hamiltonian in phase space, using the formalism of the Moyal product. We obtain exact equations, which can be expanded order by order, for example, in the “smallness” of the spatial gradients, providing a “semiclassical” approximation. In turn, at first order, we recover the usual full Boltzmann equation and give a self-contained and exact derivation of the intrinsic thermal Hall effect of bosons. The formulation clarifies the contribution from “energy magnetization” in a natural manner, and does not require the inclusion of Luttinger's pseudogravitational field to obtain thermal transport quantities.

We report hard x-ray photoelectron spectroscopy on ${\mathrm{SrFeO}}_3$ which is one of the classical conducting transition-metal oxides with a noncollinear magnetic structure. The obtained spectra show a detailed charge-transfer (CT) satellite structure, the Fe $2p_{3/2}$ main peak exhibits multiplet splitting, and the deterioration signs present in previous reports are absent here, allowing for a better determination of its intrinsic electronic structure. The results are well described by a ${\mathrm{FeO}}_6$ cluster model with a charge-transfer energy of about $-1.0$ eV, confirming the values obtained in the previous works. The negative CT energy indicates that the electronic configuration of the tetravalent Fe is $d^{5}L$ rather than $d^4$ where $L$ represents an O $2p$ hole. The small spectral weight observed at the Fermi level indicates the correlated metallic state with localized Fe $3d$ electrons and mobile O $2p$ holes which are governed by a large $d\text{−}d$ Coulomb interaction and negative CT energy.

We introduce a topological theory to study quasiparticles in interacting and/or disordered many-body systems, which have a finite lifetime due to inelastic and/or elastic scattering. The one-body quasiparticle Hamiltonian includes both the Bloch Hamiltonian of band theory and the self-energy due to interactions, which is non-Hermitian when the quasiparticle lifetime is finite. We study the topology of non-Hermitian quasiparticle Hamiltonians in momentum space, whose energy spectrum is complex. The interplay of band structure and quasiparticle lifetime is found to have remarkable consequences in zero- and small-gap systems. In particular, we predict the existence of topological exceptional points and bulk Fermi arcs in Dirac materials with two distinct quasiparticle lifetimes.

Determinant quantum Monte Carlo (DQMC) is a widely used unbiased numerical method for simulating strongly correlated electron systems. However, the update process in DQMC is often a bottleneck for its efficiency. To address this issue, we propose a generalized delay update scheme that can handle both on-site and extended interactions. Our delay update scheme can be implemented in both zero-temperature and finite-temperature versions of DQMC. We apply the delay update scheme to various strongly correlated electron models and evaluate its efficiency under different conditions. Our results demonstrate that the proposed delay update scheme significantly improves the efficiency of DQMC simulations, enabling it to simulate larger system sizes.

We perform a computational study of confined photonic states that appear in a three-dimensional (3D) superlattice of coupled cavities, resulting from a superstructure of intentional defects. The states are isolated from the vacuum by a 3D photonic band gap, using a diamondlike inverse woodpile crystal structure, and they exhibit “Cartesian” hopping of photons in high-symmetry directions. We investigate the confinement dimensionality to verify which states are fully 3D-confined, using a recently developed scaling theory to analyze the influence of the structural parameters of the 3D crystal. We create confinement maps that trace the frequencies of 3D-confined bands for select combinations of key structural parameters, namely the pore radii of the underlying regular crystal and of the defect pores. We find that a certain minimum difference between the regular and defect pore radii is necessary for 3D-confined bands to appear, and that an increasing difference between the defect pore radii from the regular radii supports more 3D-confined bands. In our analysis, we find that their symmetries and spatial distributions are more varied than electronic orbitals known from solid-state physics. We surmise that this difference occurs since the confined photonic orbitals derive from global Bloch states governed by the underlying superlattice structure, whereas single-atom orbitals are localized. Based on this realization, we suggest that the extent symmetries of “photonic orbitals” could possibly translate to novel macroscopic behaviors of “photonic solid-state matter,” never before seen in the standard electronic solid-state systems. We also discover pairs of degenerate 3D-confined bands with $p$-like orbital shapes and mirror symmetries matching the symmetry of the superlattice. Finally, we investigate the enhancement of the local density of optical states for cavity quantum electrodynamics applications. We find that donorlike superlattices, i.e., where the defect pores are smaller than the regular pores, provide greater enhancement in the air region than acceptorlike structures with larger defect pores, and thus offer better prospects for doping with quantum dots and ultimately for 3D networks of single photons steered across strongly coupled cavities.

Using bosonization, which maps fermions coupled to a ${\mathbb{Z}}_2$ gauge field to a qubit system, we give a simple form for the three-fermion nontrivial quantum cellular automaton (QCA) as realizing a phase depending on the framing of flux loops. We relate this framing-dependent phase to a pump of eight copies of a $p+ip$ state through the system. We give a resolution of an apparent paradox, namely that the pump is a shallow depth circuit (albeit with tails), while the QCA is nontrivial. We discuss also the pump of fewer copies of a $p+ip$ state and describe its action on topologically degenerate ground states. One consequence of our results is that a pump of $n p+ip$ states generated by a free Fermi evolution is a free fermion unitary characterized by a nontrivial winding number $n$ as a map from the third homotopy group of the Brilliouin Zone 3-torus to that of $SU\left(N_b\right)$, where $N_b$ is the number of bands. Using our simplified form of the QCA, we give higher-dimensional generalizations that we conjecture are also nontrivial QCAs, and we discuss the relation to Chern-Simons theory.

Understanding the origin of hot carrier relaxation in nanowires (NWs) with one-dimensional (1D) geometry is significant for designing efficient hot carrier solar cells with such nanostructures. Here, we study the influence of Auger heating and the Shockley-Read-Hall recombination on hot carrier dynamics of catalyst-free InGaAs-InAlAs core-shell NWs. Using steady-state and time-resolved photoluminescence (PL) spectroscopy the dependences of hot carrier effects on the degree of confinement of photogenerated carriers induced by the nanowire diameter are determined at different lattice temperatures. Analysis of excitation-power dependent data and temperature-dependent PL linewidth broadening reveal that at low temperatures, strong Auger recombination and phonon-bottleneck are responsible for hot carrier effects. Our analysis gives also insights into electron-phonon and ionized impurity scattering, showing opposing trends with NW diameter, and it allows one to estimate the Fröhlich coupling constant for the InGaAs NWs. Conversely, with increasing lattice temperature, hot carrier relaxation rates increase due to enhanced Shockley-Read-Hall and surface recombination. Time-resolved spectroscopy reveals a fourfold increase in the rate of Shockley-Read-Hall recombination from 6 ns at 10 K to 1.5 ns at 150 K. The findings suggest that minimizing defect densities in the bulk and surfaces of these NWs will be key to enhance hot carrier effects towards higher temperatures.

In the calculations of lattice thermal conductivity (${\kappa }_{\text{L}}$), vital contributions stemming from four-phonon scattering is often neglected. The significance of four-phonon scattering in the thermal transport properties of monolayer (ML) ${\mathrm{MoS}}_2$ has been unraveled using first-principles calculations combined with the Boltzmann transport equation. If only three-phonon scattering processes are considered then the ${\kappa }_{\text{L}}$ is found to be significantly overestimated ($\sim 115.8{\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K). With the incorporation of the four-phonon scattering processes, the ${\kappa }_{\text{L}}$ reduces to 24.6 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, which is found to be closer to the experimentally measured ${\kappa }_{\text{L}}$ of 34.5 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. Four-phonon scattering significantly impacts the carrier lifetime ($\tau$) of the low-energy out-of-plane acoustic mode (ZA) phonons and thereby, suppresses its contribution in ${\kappa }_{\text{L}}$ from 64% (for three-phonon scattering) to 16% (for both three- and four-phonon scatterings). The unusually high four-phonon scattering rate (${\tau }_4^{-1}$) of the ZA phonons is attributed to the distinctive quadratic dispersion, along with the combined effects of the acoustic-optical frequency gap, strong anharmonicity, and the selection rule imposed by reflection symmetry. The strong coupling between the dispersion characteristics of the ZA mode and the ${\tau }_4^{-1}$ is discovered through the application of mechanical strain. The strain induced increase in the linearity of the ZA mode dispersion dramatically reduces the significance of the four-phonon scattering in ML-${\mathrm{MoS}}_2$, both qualitatively and quantitatively. These findings offer valuable insights into the thermal transport phenomena of ML-${\mathrm{MoS}}_2$, as well as other 2D materials.

There are many ways of engineering the band gap of nanoribbons, including application of stress, electric field, and functionalization of the edges. In this paper, we investigate separately the effects of these methods on armchair graphene and boron nitride nanoribbons. By means of density functional theory calculations, we show that, despite their similar structures, the two materials respond in opposite ways to these stimuli. By treating them as perturbations of a heteroatomic ladder model based on the tight-binding formalism, we connect the two behaviors to the different symmetries of the top valence and bottom conduction wave functions. These results indicate that opposite and complementary strategies are preferable to engineer the gap width of armchair graphene and boron nitride nanoribbons.

Sliding ferroelectric materials present a promising avenue for achieving out-of-plane ferroelectricity in two-dimensional systems, yet further research is imperative to elucidate their intricate microscopic mechanisms. Here, we investigated the charge doping effect on sliding ferroelectricity by first-principles calculations. We found that the ferroelectric polarization and conductivity can coexist at low doping concentrations. The ferroelectric polarization decreases with increasing doping concentration when introducing electrons/holes into the bilayer sliding ferroelectricity with semiconducting character. The rate of decrease is determined by the occupancy ratio of each layer in the conduction and valence bands.

The magnetic dilution of vanadyl phthalocyanine (VOPc) within the isostructural diamagnetic titanyl phthalocyanine (TiOPc) affords promising molecular spin qubit platforms for solid-state quantum computing. The development of quantitative methods for determining how the interactions with a supporting substrate impact the electronic structure of the system are fundamental to determine their potential integration in physical devices. In this work we propose a combined approach based on x-ray absorption spectroscopy (XAS), atomic multiplet calculations, and density functional theory (DFT) to investigate the $3d$ orbital level structure of VOPc on TiOPc/Ag(100). We characterize VOPc in different molecular environments realized by changing the thickness of the TiOPc interlayer and adsorption configuration on Ag(100). Depending on the molecular film structure, we find characteristic XAS features that we analyze using atomic multiplet calculations. We use a Bayesian optimization algorithm to accelerate the parameter search process in the multiplet calculations and identify the ground state properties, such as the $3d$ orbital occupancy and splitting, as well as intra-atomic interactions. Our analysis indicates that VOPc retains its spin $S=\frac{1}{2}$ character in all configurations. Conversely, the energy separation and sequence of the unoccupied V $3d$ orbitals sensitively depend on the interaction with the surface and TiOPc interlayer. We validate the atomic orbital picture obtained from the multiplet model by comparison with DFT, which further allows us to understand the VOPc electronic properties using a molecular orbital description.

Here, we present an in-depth theoretical analysis of the linear electro-optic effect in low-symmetry three-dimensional (3D) conductive materials with large Berry curvature dipoles. Our study identifies two distinct kinetic contributions to the linear electro-optic effect: a gyrotropic Hermitian (conservative) piece and a non-Hermitian term that can originate optical gain. We concentrate on the study of 3D materials belonging to the 32 ($D_3$) point group subject to a static electric bias along the trigonal axis. Our investigation shows that doped trigonal tellurium has promising properties, with its gyrotropic electro-optic response offering the potential for realizing electrically biased electromagnetic isolators and inducing significant optical dichroism. Most notably, it is demonstrated that under sufficiently large static electric bias, tellurium's non-Hermitian electro-optic response may lead to optical gain. Using first-principles calculations, it is shown that $n$-doped tellurium is particularly promising, as it can host significantly larger Berry curvature dipoles than the more common $p$-doped tellurium.