We investigate the spin-$\frac{1}{2}$ antiferromagnetic Heisenberg model with a Dzyaloshinskii-Moriya interaction on kagome lattice, making use of the variational Monte Carlo technique. An exotic quantum spin state is found to arise from a melting of the $\mathit{Q}=0$ long-range magnetic order by a topological transition, when a small anisotropic third-nearest-neighbor antiferromagnetic Heisenberg interaction is turned on. This state is a gapped quantum spin liquid, characterized by a topological order with ground-state degeneracy $n_g=4$ and topological entanglement entropy $\gamma =ln2$, suggesting it is an Abelian topological phase. Furthermore, the Chern numbers of the spin-up (-down) spinon occupied bands of this state are $C_{\uparrow \downarrow }=\pm 1$, respectively. From this perspective, this state is also a time-reversal symmetric (total Chern number $C_{\text{total}}=0$) topological insulator with spinons as the chiral edge states, which carry opposite spin and move in the opposite direction. It is analogous to the quantum spin Hall state but the spin current is carried by deconfined spinons in a quantum spin liquid, and so is dubbed as the spinon quantum spin Hall state.

Lifshitz transitions are topological transitions of a Fermi surface, whose signatures typically appear in the conduction properties of a host metal. Here, we demonstrate, using an extended Falicov-Kimball model of a two-flavor fermion system, that a Lifshitz transition which occurs in the noninteracting limit impacts interaction-induced insulating phases, even though they do not host Fermi surfaces. For strong interactions we find a first-order transition between states of different polarization. This transition line ends in a very unusual quantum critical endpoint, whose presence is stabilized by the onset of interflavor coherence. We demonstrate that the surfaces of maximum coherence in these states reflect the distinct Fermi-surface topologies of the states separated by the noninteracting Lifshitz transition. Experimental realizations of our results are discussed for both electronic and optical lattice systems.

We study viscoelastic response in Weyl semimetals with broken time-reversal symmetry. The principal finding is that topology and anisotropy of the Fermi surface are manifested in the viscoelasticity tensor of the electron fluid. In the dynamic (interband) part of this tensor, the anisotropy leads to a qualitatively different, compared with isotropic models, scaling with frequency and the Fermi energy. The components of the viscosity tensor determined by the Fermi-surface properties agree in the Kubo and kinetic formalisms; the latter, however, misses the anomalous Hall viscosity originating from filled states below the Fermi surface. The anisotropy of the dispersion relation is also manifested in the acceleration and relaxation terms of the hydrodynamic equations providing means to probe the anisotropy in transport experiments.

Topological band theory predicts a $\mathbb{Z}$ classification of three-dimensional (3D) Dirac semimetals (DSMs) at the single-particle level. Namely, an arbitrary number of identical bulk Dirac nodes will always remain locally stable and gapless in the single-particle band spectrum, as long as the protecting symmetry is preserved. In this work we find that this single-particle classification for $C_n$-symmetric DSMs will break down to ${\mathbb{Z}}_{n/\text{gcd}(2,n)}$ in the presence of symmetry-preserving electron interactions. Our theory is based on a dimensional reduction strategy which reduces a 3D Dirac fermions to one-dimensional building blocks, i.e., vortex-line modes, while respecting all the key symmetries. Using bosonization technique, we find that there exists a minimal number $N=n/\text{gcd}(2,n)$ such that the collection of vortex-line modes in $N$ copies of DSMs can be symmetrically eliminated via four-fermion interactions. While this gapping mechanism does not have any free-fermion counterpart, it yields an intuitive “electron-trion coupling” picture. By developing a topological field theory for DSMs and further checking the anomaly-free condition, we independently arrive at the same classification results. Our theory paves the way for understanding topological crystalline semimetallic phases in the strongly correlated regime.

Efficient $p$-type doping is essential and challenging for applications of ultra-wide-bandgap AlGaN with a high aluminum content. Although the band-edge-assisted ionization enabled by the superlattice of [0001] origination can reduce the ionization level due to the raised valence band maximum (VBM), the indispensable electric field induced by the spontaneous and piezoelectric polarization is in the entire range of alloy and quite negative to the photoelectric applications. In this work, we propose that band-edge-assisted ionization can be activated even without the universal electric field, which forms an ultralow ionization level of $-0.1$ eV for the ${\mathrm{Al}}_{0.8}{\mathrm{Ga}}_{0.2}\mathrm{N}\left[11\overline{2}0\right]$ digital alloy. The energy reduction is triggered by two factors: the intensive VBM states dominated by the coupling between GaN layers and the intriguing decrease in the unoccupied $p_z$ level, which is induced by a symmetry reduction from $C_{3v}$-like to $C_s$ at the dopant site. The transition of symmetry is triggered by a perturbation from the GaN layers, which is caused by a chainlike coupling of $p_x$ states between nitrogen at the VBM and dopant sites and can be partially canceled depending on the dopant sites. In addition to the conventional band bending in band-edge-assisted ionization, the reduction of ionization level activated by long-ranged orbital coupling offers a new view for efficient $p$-type doping in AlGaN systems.

Employing density functional theory, we explore the energetics, geometries, and electronic properties of Janus WSSe in terms of curvature effects. Total energy calculations on tubular forms of Janus WSSe reveal that Janus WSSe intrinsically possesses a scrolled conformation with a radius of curvature of 2.3 nm or larger. Scrolling releases compressive and tensile strains on the Se and S atoms, respectively, in the flat conformation. Curvature associated with scrolling does not affect the band edge alignment of the Janus WSSe. The internal electric field across a WSSe layer decreases monotonically with increasing curvature.

Fano resonance features asymmetric spectral line shape and holds numerous potential applications such as filters, sensors, lasers, nonlinear optics, and slow-light devices. However, general methods to achieve Fano resonance face substantial challenges because of their limited dynamic parametric tunability. Here, we present a theoretical demonstration of an alternate method to realize dynamically reversing the magnitude and line shape of the Fano profile in anisotropic bilayer metallic gratings by simply manipulating the polarization angle of incident light. By introducing an in-plane shift in one grating layer, the resonant phase of the hybridized plasmon modes formed within the bilayer system transforms between in phase and out of phase with the polarization direction, leading to the asymmetry reversal of the Fano line shape. This concept can be demonstrated and explained by applying a deduced temporal coupled-mode theory, which reveals that the asymmetry parameter $q$ undergoes a sign reversal along with the reversal of Fano line shape. This theory is further examined with the results of numerical simulations, finding excellent agreement between them. Our proposed approach enables completely and dynamically reversing the spectral asymmetry of the Fano resonance in an anisotropic bilayer grating, showing potential for applications involving active control and manipulation of Fano resonance.

In this work we investigated the effects of nanoscale roughness on Derjaguin-Landau-Verwey-Overbeek (DLVO) forces using the proximity force approximation. To that end, the force between a gold-coated sphere and a plate, both immersed in pure and salt-added $\left[\mathrm{NaCl}\right]=0.01M$ ethanol, was measured as a function of surface roughness with atomic force microscopy. The force data were compared with the predictions of DLVO theory with and without incorporating the contribution of surface roughness. It was observed that the surface roughness does not modify the qualitative behavior of the force-displacement curve but rather has a secondary influence by altering the values of the free parameters of the DLVO theory, namely, the Debye length and the electrostatic potential of the interacting bodies. In fact, it was found that the repulsive component of the force in pure ethanol increased with increasing roughness. These observations coincide with a positive correlation of the absolute value of the surface potentials with increasing surface roughness, suggesting that roughness effectively enhances the electric field near the surfaces, resulting in stronger electrostatic forces. For the salt-added solution, the contributions of the electrostatic forces are heavily suppressed, and the Casimir contribution plays the dominant role. Finally, we have shown that failure to incorporate the complete optical properties of the interacting metal bodies yields unphysical results for the free parameters of the electrostatic double-layer force.

The impact of the growth orientation on spin dynamics in GaAs/AlGaAs quantum wells (QWs) is explored through magnetophotoluminescence measurements. Samples grown on both (100) and (311)A GaAs surfaces exhibit intensity oscillations at high magnetic fields, characteristic of magnetoabsorption arising from interband transitions between Landau levels when excited near resonance. Notably, the (311)A surface reveals optical responses rarely observed on the conventional (100) surface, including an inversion in circular polarization degree influenced by both excitation power and temperature. To elucidate these findings, we develop an eight-level model encompassing electron and hole excited states. Our analysis demonstrates that thermalization is the primary and sufficient factor responsible for reproducing spin inversion with external parameters.

Pump-probe experiments with femtosecond time resolution can be used to disentangle the electronic dynamics from the lattice response and thus provide valuable insights into the nonequilibrium behavior of correlated materials. In Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$, a multiorbital Mott-Hubbard material which has been intensively investigated for decades, time-resolved experiments reported a photoinduced insulator-metal transition leading to a transient metal state with nonthermal properties. Here, we combine nonequilibrium dynamical mean-field theory with realistic first-principles modeling to simulate the ultrafast response of this material to a laser excitation. Our calculations reproduce the insulating initial state, with orbital occupations in agreement with experiment, and reveal an ultrafast pump-induced gap filling associated with a charge reshuffling between the $e_g^{\pi }$ and $a_{1g}$ orbitals. However, in contrast to the related compound ${\mathrm{VO}}_2$, the electronic system thermalizes within a few tens of femtoseconds and we find no evidence for the existence of a metastable nonthermal metal. This suggests that the reported nonthermal behavior in the experiments may be associated with the mismatch between the electronic and lattice temperatures.

We study a relationship between conformally invariant boundary conditions and anomalies of conformal field theories (CFTs) in $1+1$ dimensions. For a given CFT with a global symmetry, we consider symmetric “gapping potentials”, which are relevant perturbations to the CFT. If a gapping potential is introduced only in a subregion of the system, it provides a certain boundary condition to the CFT. From this equivalence, if there exists a Cardy boundary state, which is invariant under a symmetry, then the CFT can be gapped with a unique ground state by adding the corresponding gapping potential. This means that the symmetry of the CFT is anomaly free. Using this approach, we systematically deduce the anomaly-free conditions for various types of CFTs with several different symmetries. They include the free compact boson theory, Wess-Zumino-Witten models, and unitary minimal models. When the symmetry of the CFT is anomalous, it implies a Lieb-Schultz-Mattis type ingappability of the system. Our results are consistent with, where available, known results in the literature. Moreover, we extend the discussion to other symmetries including spin groups and generalized time-reversal symmetries.

By constructing an exactly solvable spin model, we investigate the critical behaviors of transverse-field Ising chains interpolated with cluster interactions, which exhibit various types of topologically distinct Ising critical points. Using fidelity susceptibility as an indicator, we establish the global phase diagram, including ferromagnetic, trivial paramagnetic, and symmetry-protected topological phases. Different types of critical points exist between these phases, encompassing both topologically trivial and nontrivial Ising critical points, as well as Gaussian critical points. Importantly, we demonstrate the existence of a Lifshitz transition between these topologically distinct Ising critical points, with central charge and critical exponents determined through finite-size scaling. This work serves as a valuable reference for further research on phase transitions within the gapless quantum phase of matter.

We report a comprehensive study of the synthesis of epitaxial SmN thin films on ${\mathrm{LaAlO}}_3$ (100) by molecular beam epitaxy under slow-growth conditions. By carefully tuning the substrate temperature and the molecular nitrogen partial pressure, we are able to control the nitrogen content of our samples and produce near-stoichiometric, well-ordered SmN thin films. The influence of nitrogen defects on the samarium ion valence and the electronic structure is investigated by x-ray absorption and photoelectron spectroscopy, suggesting that the empty Sm $5d$ states may act as a charge reservoir stabilizing the $4f^5$ configuration in the nitrogen-deficient compound. The spectra also reveal a significant hybridization of the Sm $4f$ and N $2p$ states.

We investigate the band structure of the three-dimensional Hofstadter model on cubic lattices, with an isotropic magnetic field oriented along the diagonal of the cube with flux $\mathrm{\Phi }=2\pi m/n$, where $m,n$ are coprime integers. Using reduced exact diagonalization in momentum space, we show that, at fixed $m$, there exists an integer $n\left(m\right)$ associated with a specific value of the magnetic flux, that we denote by ${\mathrm{\Phi }}_c\left(m\right)\equiv 2\pi m/n\left(m\right)$, separating two different regimes. The first one, for fluxes $\mathrm{\Phi }<{\mathrm{\Phi }}_c\left(m\right)$, is characterized by complete band overlaps, while the second one, for $\mathrm{\Phi }>{\mathrm{\Phi }}_c\left(m\right)$, features isolated band-touching points in the density of states and Weyl points between the $m\mathrm{th}$ and the $(m+1)$-th bands. In the Hasegawa gauge, the minimum of the $(m+1)$-th band abruptly moves at the critical flux ${\mathrm{\Phi }}_c\left(m\right)$ from $k_z=0$ to $k_z=\pi$. We then argue that the limit for large $m$ of ${\mathrm{\Phi }}_c\left(m\right)$ exists and it is finite: ${lim}_{m\to \infty }{\mathrm{\Phi }}_c\left(m\right)\equiv {\mathrm{\Phi }}_c$. Our estimate is ${\mathrm{\Phi }}_c/2\pi =0.1296\left(1\right)$. Based on the values of $n\left(m\right)$ determined for integers $m\le 60$, we propose a mathematical conjecture for the form of ${\mathrm{\Phi }}_c\left(m\right)$ to be used in the large-$m$ limit. The asymptotic critical flux obtained using this conjecture is ${\mathrm{\Phi }}_c^{\left(\mathrm{conj}\right)}/2\pi =7/54$.

Strain represents an ubiquitous feature in semiconductor heterostructures, and can be engineered by different means in order to improve the properties of various devices, including advanced metal-oxide-semiconductor field-effect transistors and spin-based qubits. However, its treatment within the envelope function framework is well established only for the homogeneous case, thanks to the theory of Bir and Pikus. Here, we generalize this theory to the case of inhomogeneous strain. By fully accounting for the relativistic effects and metric aspects of the problem, we derive a complete envelope-function Hamiltonian, including the terms that depend on first and second spatial derivatives of the strain tensor.

MXene-like MOenes have shown intriguing properties, such as superconductivity in ${\mathrm{Ti}}_2\mathrm{O}$ monolayers, direct band gaps, exotic quantum phase transitions, and strong light harvesting in ${\mathrm{Ti}}_2\mathrm{O}{X}_2$ ($X$ = F, Cl) monolayers. However, stoichiometry engineering as a commonly tunable factor should be extended to study the thickness-dependent properties of MOenes. In this study, ${\mathrm{Ti}}_{n+1}{\mathrm{O}}_n$ ($n$ = 2, 3) MOenes and their functionalized counterparts are systematically investigated using ab initio calculations. Similar to the $2H\text{−}{\mathrm{Ti}}_2\mathrm{O}$ monolayer with a superconducting transition temperature $T_c$ of 4.7 K, $2H\text{−}{\mathrm{Ti}}_3{\mathrm{O}}_2$ and ${\mathrm{Ti}}_4{\mathrm{O}}_3$ monolayers are superconductors with a $T_c$ of 4.4 and 3.7 K, respectively. More interestingly, $2H\text{−}{\mathrm{Ti}}_3{\mathrm{O}}_2{\mathrm{F}}_2$ and ${\mathrm{Ti}}_4{\mathrm{O}}_3{\mathrm{F}}_2$ monolayers exhibit quantum spin Hall effects at room temperature with nontrivial gaps of 0.22 and 0.20 eV, respectively. The $d\text{−}d$ band inversion mechanism is crucial for their topological nature. Therefore, the thickness-dependent features in ${\mathrm{Ti}}_{n+1}{\mathrm{O}}_n$ MOenes will draw more attention to this emerging family of MOenes.

Non-charge-based logic in single-hole spin of semiconductor quantum dots (QDs) can be controlled by anisotropic gate potentials providing a notion for making next-generation solid-state quantum devices. In this study, we investigate the isotropic and anisotropic behavior of phonon-mediated spin relaxation of heavy-hole spin hot spots in QDs. For the electron spin in isotropic QDs, hot spots are known to be always present due to the Rashba spin-orbit coupling. But for heavy holes in isotropic dots, we show that the occurrences of spin hot spots are sensitive to the bulk $g$ factor. The hot spot for Rashba coupling in InAs and GaSb dots arises because these materials possess negative bulk $g$ factor, while that for the Dresselhaus coupling in GaAs and InSb dots is found due to their positive bulk $g$ factor. For anisotropic QDs, on the other hand, the spin hot spot is universally present due to their broken in-plane rotational symmetry. Further, the increasing electric field, that strengthens the Rashba coupling, is shown to cover a wide range of magnetic field by the hot spots. Results demonstrate that the magnetic field, choice of dot materials, and size anisotropy can act as effective control parameters, which can be experimentally used to design the device for detecting the phonon-mediated heavy-hole spin-relaxation behavior of III-V semiconductor QDs.

Competing spin-orbit coupling, electron correlation, and structural distortion are crucial factors influencing the physics of oxides, including iridates. We investigate ultrathin chemically bonded layered ${\mathrm{Na}}_2{\mathrm{IrO}}_3$, which exhibits characteristics of a proximate quantum spin liquid in its bulk form. Employing first-principles calculations, we explore the interplay between Heisenberg and Kitaev interactions within the two-dimensional limit. Contrary to the conventional understanding of van der Waals materials, magnetism in ultrathin ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is reinforced in the two-dimensional limit. As the zigzag antiferromagnetic state stabilizes, it diverges further from the Kitaev spin-liquid state due to enhanced Heisenberg and off-diagonal exchange interactions. Furthermore, carrier doping can tune the electronic and magnetic states, resulting in combined Mott insulator-to-metal and antiferromagnetic-to-ferromagnetic transitions. These findings provide compelling insights into the magnetism of a two-dimensional realm in non-van der Waals correlated oxide flakes.

Evidence is growing that a second dome of high-$T_{\mathrm{c}}$ superconductivity can be accessed in the cuprates by increasing the doping beyond the first dome. Here, we use ab initio methods without invoking any free parameters, such as the Hubbard $U$, to reveal that pressure could turn ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ into an ideal candidate for second dome superconductivity, displaying the predicted signature of strongly hybridized $d_{x^2-y^2}$ and $d_{z^2}$ orbitals. Notably, pressure is found to induce a phase transition replacing the antiferromagnetic phases with an orbitally degenerate $d-d$ phase. Our study suggests that the origin of the second dome is correlated with the oxygen-hole fraction in the ${\mathrm{CuO}}_2$ planes and the collapse of the pseudogap phase.