We develop the Gutzwiller approximation method to obtain the renormalized Hamiltonian of the SU(4) $t\text{−}J$ model with the corresponding renormalization factors. Subsequently, a mean-field theory is employed on the renormalized Hamiltonian of the model on the honeycomb lattice under the scenario of a cooperative condensation of carriers moving in the resonating valence bond state of flavors. In particular, we find that the extended $s$-wave superconducting state is more favorable than the $d\pm id$-wave superconducting state in the doping range close to quarter filling. The pairing states of the SU(4) case reveal the property that the spin-singlet pairing and the spin-triplet pairing can coexist simultaneously. Our results might provide new insights into the twisted bilayer graphene system.

We analyze the paramagnetic state of a two-band Hubbard model with finite Hund's coupling close to integer filling at $n=2$ in two spacial dimensions. Previously, a Mott metal-insulator transition was established at $n=2$ with a coexistence region of a metallic and a bad metal state in the vicinity of that integer filling. The coexistence region ends at a critical point beyond which a charge instability persists. Here we investigate the transition into negative electronic compressibility states for an extended filling range close to $n=2$ within a slave boson setup. We analyze the separate contributions from the (fermionic) quasiparticles and the (bosonic) multiparticle incoherent background and find that the total compressibility depends on a subtle interplay between the quasiparticle excitations and collective fields. Implementing a Blume-Emery-Griffiths model approach for the slave bosons, which mimics the bosonic fields by Ising-like pseudospins, we suggest a feedback mechanism between these fields and the fermionic degrees of freedom. We argue that the negative compressibility can be sustained for heterostructures of such strongly correlated planes and results in a large capacitance of these structures. The strong density dependence of these capacitances allows to tune them through small electronic density variations. Moreover, by resistive switching from a Mott insulating state to a metallic state through short electric pulses, transitions between fairly different capacitances are within reach.

It is by now well established that periodically driven quantum many-body systems can realize topological nonequilibrium phases without any equilibrium counterpart. Here we show that, even in the absence of time translation symmetry, nonequilibrium topological phases of matter can exist in aperiodically driven systems for tunably parametrically long prethermal lifetimes. As a prerequisite, we first demonstrate the existence of long-lived prethermal Anderson localization in two dimensions under random multipolar driving. We then show that the localization may be topologically nontrivial with a quantized bulk orbital magnetization even though there are no well-defined Floquet operators. We further confirm the existence of this anomalous random multipolar driven insulator by detecting quantized charge pumping at the boundaries, which renders it experimentally observable.

In solid state physics, the electron-phonon interaction (EPI) is central to many phenomena. The theory of the renormalization of electronic properties due to EPIs became well established with the theory of Allen-Heine-Cardona (AHC), which is usually applied to second order in perturbation theory. However, this is only valid in the weak coupling regime, while strong EPIs have been reported in many materials. As a result, and with AHC becoming more established through density-functional perturbation theory, some nonperturbative (NP) methods have started to arise in the last years. However, they are usually not well justified and it is not clear to what degree they reproduce the exact theory. To address this issue, we present a stochastic approach for the evaluation of the nonperturbative interacting Green's function in the adiabatic limit, and show it is equivalent to the Feynman expansion to all orders in the perturbation. Also, by defining a self-energy, we can reduce the effect of broadening needed in numerical calculations, improving convergence in the supercell size. In addition, we clarify whether it is better to average the Green's function or self-energy. Then we apply the method to a graphene tight-binding model, and we obtain several interesting results: (i) The Debye-Waller term, which is normally neglected, does affect the change of the Fermi velocity $v_F$, and should be included to obtain accurate results. (ii) Although at room temperature second-order perturbation theory (P2) agrees well with the NP change of $v_F$ and of the self-energy close to the Dirac point, at high temperatures there are significant differences. For other $\mathbf{k}$ points, the disagreement between the P2 and NP self-energies is visible even at low temperatures, raising the question of how well P2 works in other materials. (iii) Close enough to the Dirac point, positive- and negative-energy peaks merge, giving rise to a single peak. (iv) At strong coupling and high temperatures, a peak appears at $\omega =0$ for several states, which is consistent with previous works on disorder and localization in graphene. (v) The spectral function becomes more asymmetric at stronger coupling and higher temperatures. Finally, we show that the method has better convergence properties when the coupling is strong relative to when it is weak, and discuss other technical aspects.

Halide perovskites show great potential for a variety of optoelectronic applications. The presence of robust exciton resonance in these materials makes them promising for use in information photonics. In this paper, we demonstrate the possibility of writing and delayed reading of the optical coherence in halide perovskite by the spontaneous (two-pulse) photon echo in ${\mathrm{MAPbI}}_3$ (${\mathrm{MA}}^{+}$ = ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$) lead-halide perovskite single crystal on picosecond time scales. The spectral and polarimetric measurements of the photon echo signal confirm the free excitonic origin of the excitation under study. Observed relatively long dephasing time, high exciton oscillator strength, and weakly pronounced excitation-induced dephasing make halide perovskites a promising media for applications.

Materials with inversion asymmetries can exhibit strong spin Hall effect (SHE) in the presence of Dresselhaus and Rashba spin-orbit coupling (D/R SOC) interactions. Ideally, in a two-dimensional crystal, inversion asymmetry could be modulated by stacking order and external perturbations. Here, using first-principles calculations, we systematically investigate the interplay between DSOC and RSOC and their influences on SHE in mono- and bilayer InSe. We show that in the presence of DSOC, the introduction of Rashba interaction through gate voltages in monolayer InSe increases Zeeman-like spin splitting around the Brillouin-zone center and contributes to the enhancement of spin Hall conductivity (SHC), which reaches a saturation point due to the RSOC-enforced spiral-spin texture. The SHC in the unperturbed centrosymmetric $\mathit{AB}$ stacked bilayer shows a peak associated with the Mexican-hat-like valence-band edge; however, in a wide energy range the SHC stays insignificant. In the $\mathit{AB}$′ stacked bilayer with the intrinsic RSOC present, the value of SHC can be comparable to that of $\mathit{AB}$ stacked bilayer with an external electric field. Moreover, we show that the spin-momentum locking in the $\mathit{AB}$′ stacked bilayer is switchable by a gate voltage. These findings provide a promising route for spintronic and magneto-optical applications by exploiting the rich physics of spin-orbit effects.

The Debye temperature ${\mathrm{\Theta }}_{\mathrm{D}}$ characterizes the vibrations of a solid and marks the transition between quantized and classical behaviors of nuclear motion. While thermodynamical theories suggest that for nanoparticles ${\mathrm{\Theta }}_{\mathrm{D}}$ should be lower than the bulk limit and increase with increasing nanoparticle size, various experimental measurements have reported intriguing variations with size, including values above the bulk limit and possibly decreasing with size. In this paper we have theoretically determined the Debye temperature of iron nanoparticles of up to about 7 nm diameter at the atomistic level of detail, using complementary approaches based on the equilibrium heat capacity or the mean-square atomic displacement. Both methods consistently indicate steady finite-size effects in the scaling regime, with no evidence for values higher than the bulk or varying in opposite ways, but they also produce marked quantitative differences. Further comparison with the melting temperature $T_m$ indicates that the two quantities correlate with each other through $T_m\propto {\mathrm{\Theta }}_{\mathrm{D}}^{\alpha }$, but with an exponent $\alpha$ close to 3 that deviates from the expected value of 2 that classical thermodynamical theories predict based on pure bulk ingredients.

We study the one-dimensional nonreciprocal lattices with real nearest-neighboring hopping and find that the energy spectra under open boundary conditions can be entirely real or imaginary. We further investigate the spectral properties and the non-Hermitian skin effect in the one-dimensional mosaic lattices with real nonreciprocal hopping introduced at equally spaced sites. The eigenenergies of such lattices undergo a real-complex-imaginary or real-complex transition as the nonreciprocity varies. Moreover, the skin effect exhibits phase transitions depending on the period of the mosaic nonreciprocity. The bulk states are abruptly shifted from one end of the lattice to the opposite one by crossing the critical points, accompanied by the closing and reopening of point gaps in the spectra under periodic boundary conditions. The phase diagrams of the transition are presented and the critical boundaries are analytically determined. Our work unveils the intriguing properties of the energy spectrum and skin effect in non-Hermitian systems.

Recent experimental discoveries of two-dimensional (2D) magnets have triggered intense research activities to search for atomically thin magnetic systems. Using first-principles calculations, we predict the emergence of 2D magnetism in the monolayers (MLs), few layers, and surfaces of nonmagnetic layered electrides ${\mathrm{Hf}}_{2}X$ $(X=\text{S}, \mathrm{Se}, \mathrm{Te})$ consisting of three-atom-thick $\mathrm{Hf}\text{−}X\text{−}\mathrm{Hf}$ stacks. It is revealed that each bulk ${\mathrm{Hf}}_{2}X$ hosts a quantum state of Dirac nodal lines with a high density of states arising from $\mathrm{Hf}\text{−}5d$ cationic and interlayer anionic electrons around $-0.9 \mathrm{eV}$ below the Fermi level $E_F$. However, for the MLs, few layers, and surfaces of ${\mathrm{Hf}}_{2}X$, such hybridized states are shifted toward $E_F$ to generate van Hove singularities, leading to a Stoner instability. The resulting surface ferromagnetism gives rise to strongly spin-polarized topological surface states at ${\mathrm{Hf}}_{2}X\left(001\right)$, demonstrating that anionic electrons, 2D magnetism, and band topology are entangled with each other. Our findings will open different perspectives for the discovery of 2D magnets via exploiting surface effects in nonmagnetic layered electrides.

Twisted Van der Waals systems offer the unprecedented possibility to tune different states of correlated quantum matter with external noninvasive electrostatic doping. The nature of the superconducting order presents a recurring open question in this context. In this work, we assess quantitatively the case of spin-fluctuation-mediated pairing for $\mathrm{\Gamma }$-valley twisted transition metal dichalcogenide homobilayers. We calculate self-consistently and dynamically the doping-dependent superconducting transition temperature $T_{\mathrm{c}}$ revealing a superconducting dome with a maximal $T_{\mathrm{c}}\approx 0.1–1\mathrm{K}$ depending on twist angle. We compare our results with conventional phonon-mediated superconductivity, and we identify clear fingerprints in the doping dependence of $T_{\mathrm{c}}$, which enable experiments to distinguish between different pairing mechanisms.

We report the emergence of a significant degree of intrinsic circular polarization of exciton photoluminescence in a twisted ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterostructure upon nonresonant driving with a linearly polarized laser. The effect is not related to the polarization of the incident light. Moreover, it is present at zero magnetic field, and reacts perceptibly to a perpendicularly applied magnetic field that, unexpectedly, can strongly diminish this effect. The giant magnitude of the polarization, which cannot be explained by natural optical activity or circular dichroism of the twisted lattice, suggests a kinematic origin arising from an emergent pyromagnetic symmetry in our structure, which we exploit to gain insight into the microscopic optical processes of our device.

Multi-principal-component alloys have attracted great interest as a novel paradigm in alloy design, with often unique properties and a vast compositional space auspicious for materials discovery. High entropy alloys (HEAs) belong to this class and are being investigated for prospective nuclear applications with reported superior mechanical properties including high-temperature strength and stability compared to conventional alloys. Computational materials design has the potential to play a key role in screening such alloys, yet for high-temperature properties, challenges remain in finding an appropriate balance between accuracy and computational cost. Here we develop an approach based on density-functional theory (DFT) and thermodynamic integration aided by machine learning based interatomic potential models to address this challenge. We systematically evaluate and compare the efficiency of computing the full free energy surface and thermodynamic properties up to the melting point at different stages of the thermodynamic integration scheme. Our new approach provides a $\times 4$ speed-up with respect to comparable free energy approaches at the level of DFT, with errors on high-temperature free energy predictions less than 1 meV/atom. Calculations are performed on an equiatomic HEA, TaVCrW—a low-activation composition and therefore of potential interest for next generation fission and fusion reactors.

Magnetization dynamics and spin waves in ferromagnets are investigated using the inertial Landau-Lifshitz-Gilbert equation. Taking inertial magnetization dynamics into account, dispersion relations describing the propagation of nutation spin waves in an arbitrary direction relative to the applied magnetic field are derived via Maxwell's equations. It is found that the inertia of magnetization causes the hybridization of electromagnetic waves and nutation spin waves in ferromagnets, hybrid nutation spin waves emerge, and the redshift of frequencies of precession spin waves is initiated, which transforms to precession-nutation spin waves. These effects depend sharply on the direction of wave propagation relative to the applied magnetic field. Moreover, the waves propagating parallel to the applied field are circularly polarized, while the waves propagating perpendicular to that field are elliptically polarized. The characteristics of these spin nutation waves are also analyzed.

We study the recently discovered even-odd effects in the normal state of single-electron devices manufactured at strontium titanium oxide/lanthanum aluminum oxide interfaces (STO/LAO). Within the framework of the number parity-projected formalism and a phenomenological fermion-boson model, we find that, in sharp contrast to conventional superconductors, the crossover temperature $T^{*}$ for the onset of number parity effect is considerably larger than the superconducting transition temperature $T_c$ due to the existence of a pairing gap above $T_c$. Furthermore, the finite lifetime of the preformed pairs reduces by several orders of magnitude the effective number of states $N_{\mathrm{eff}}$ available for the unpaired quasiparticle in the odd-parity state of the Coulomb blockaded STO/LAO island. Our findings are in qualitative agreement with the experimental results reported by Levy and coworkers for STO/LAO-based single electron devices.

Erbium-doped materials can serve as spin-photon interfaces with optical transitions in the telecom $C$ band, making them an exciting class of materials for long-distance quantum communication. However, the spin and optical coherence times of ${\mathrm{Er}}^{3+}$ ions are limited by currently available host materials, motivating the development of new ${\mathrm{Er}}^{3+}$-containing materials. Here we demonstrate the use of ion implantation to efficiently screen prospective host candidates, and show that disorder introduced by ion implantation can be mitigated through post-implantation thermal processing to achieve inhomogeneous linewidths comparable to bulk linewidths in as-grown samples. We present optical spectroscopy data for each host material, which allows us to determine the level structure of each site, allowing us to compare the environments of ${\mathrm{Er}}^{3+}$ introduced via implantation and via doping during growth. We demonstrate that implantation can generate a range of local environments for ${\mathrm{Er}}^{3+}$, including those observed in bulk-doped materials, and that the populations of these sites can be controlled with thermal processing.

Recently, the carbonaceous sulfur hydride was synthesized [E. Snider et al., Nature (London) 586, 373 (2020)], and the measured superconducting transition temperature ($T_c$) is about 278 K under 267 GPa, placing it as a promising candidate of the room temperature superconductor. However, the ultrahigh synthesis pressure is indeed a challenge for experimental technique, stimulating us to search other high temperature superconductors under relatively low pressure. Here we perform comprehensively structural searches of ternary alkaline (Am) boron hydrides. Our calculations indicate that the ternary ${\mathrm{AmB}}_2{\mathrm{H}}_8$ hydrides are energetically stable at about 10 GPa. Interestingly, the metallic $\mathrm{Rb}/{\mathrm{CsB}}_2{\mathrm{H}}_8$ polyhydrides are superior superconductors with high $T_c$ of $\sim 100$ K at 25 GPa. The electron-phonon coupling calculations suggest that the Am atoms are promising electron contributors, which effectively improve the $T_c$ and decrease the synthesized pressure in complex hydrides. These findings elucidate the superconductivity mechanism of ${\mathrm{AmB}}_2{\mathrm{H}}_8$ hydrides and offer crucial insights for design and synthesis of novel superconductors.

Whereas at low-temperatures ferroelectrics have a well understood ordered spatial dipole arrangement, the fate of these dipole configurations in the higher temperature paraelectric (PE) phase remains poorly understood. Using density functional theory (DFT), we find that, unlike the case in nonpolar $AB{\mathrm{O}}_3$, perovskites such as cubic $\mathrm{BaZr}{\mathrm{O}}_3$ that do not lower their energy by any form of positional symmetry breaking, the origin of distribution of the $B$-site off-centering in cubic PE such as $\mathrm{BaTi}{\mathrm{O}}_3$ is an intrinsic, energy lowering due to symmetry breaking. Minimizing the internal energy $E_0$ of a constrained cubic phase represented by a large enough supercell to accommodate symmetry breaking already reveals the presence of a distribution of local displacements (i.e., a polymorphous network) that (i) locally mimics the symmetries of the low temperature phases, while (ii) being the precursors of what finite temperature density functional theory (DFT) molecular dynamics (MD) finds as thermal motifs when equilibrating the free energy $E_0-TS$. Analyzing the DFT-derived configurations of the PE cubic supercell by projecting its displacements onto irreducible representations reveals that it is best described as a temperature-dependent superposition of numerous modes, including ferroelectric $\left({{\mathrm{\Gamma }}_4}^{–}\right)$ and antiferroelectric (${M_2}^{–}$ and ${X_5}^{+}$), rather than a single mode, (e.g., ${X_5}^{+}$) representing a well-defined long-range ordered configuration. This suggests that the electronic and dielectric properties of such PE phases are best calculated from a polymorphous distribution of interrelated local motifs in large supercells rather than from either purely disordered or long-range ordered models. In this respect, PE phases share a common feature with paramagnetic (PM) and paraelastic (PEL) perovskite phases whose central feature is a polymorphous distribution of local motifs—dipole moments in PE, magnetic moments in PM, and ordinary octahedral distortion modes in PEL—all computable by DFT supercells and useable in calculating electronic and magnetic properties of paraphases.

The $\mathrm{spin}-1/2$ chain with ferromagnetic exchange $J_1<0$ between first neighbors and antiferromagnetic $J_2>0$ between second neighbors supports two spin-Peierls (SP) instabilities depending on the frustration $\alpha =J_2/\left|J_1\right|$. Instead of chain dimerization with two spins per unit cell, $J_1-J_2$ models with $\alpha >0.65$ and linear spin-phonon coupling are unconditionally unstable to sublattice dimerization with four spins per unit cell. Unequal $J_1$ to neighbors to the right and left extends the model to gapped ($\gamma >0$) chains with conditional SP transitions at $T_{\mathrm{SP}}$ to dimerized sublattices and a weaker specific heat $C\left(T\right)$ anomaly. The spin susceptibility $\chi \left(T\right)$ and $C\left(T\right)$ are obtained in the thermodynamic limit by a combination of exact diagonalization of small systems with $\alpha >0.65$ and density matrix renormalization group (DMRG) calculations of systems up to $N\sim 100$ spins. Both $J_1-J_2$ and $\gamma >0$ models account quantitatively for $\chi \left(T\right)$ and $C\left(T\right)$ in the paramagnetic phase of $\beta \text{−}{\mathrm{TeVO}}_4$ for $T>8$ K, but lower $T$ indicates a gapped chain instead of a $J_1-J_2$ model as previously thought. The same parameters and $T_{\mathrm{SP}}=4.6$ K generate a $C\left(T\right)/T$ anomaly that reproduces the anomaly at the 4.6 K transition of $\beta \text{−}{\mathrm{TeVO}}_4$, but not the weak $\chi \left(T\right)$ signature.

We report the first nonjellium, systematic, density functional theory (DFT) study of intrinsic and extrinsic defects and defect levels in zinc-blende (cubic) gallium nitride. We use the local moment counter charge (LMCC) method, the standard Perdew-Becke-Ernzerhoff (PBE) exchange-correlation potential, and two pseudopotentials, where the Ga $3d$ orbitals are either in the core $\left(d^0\right)$ or explicitly in the valence set $\left(d^{10}\right)$. We studied 64, 216, 512, and 1000 atom supercells, and demonstrated convergence to the infinite limit, crucial for delineating deep from shallow states near band edges, and for demonstrating the elimination of finite cell-size errors. Contrary to common claims, we find that exact exchange is not required to obtain defect levels across the experimental band gap. As was true in silicon, silicon carbide, and gallium arsenide, the extremal LMCC defect levels of the aggregate of defects yield an effective LMCC defect band gap that is within 10% of the experimental gap (3.3 eV) for both pseudopotentials. We demonstrate that the gallium vacancy is more complicated than previously reported. There is dramatic metastability–a nearest-neighbor nitrogen atom shifts into the gallium site, forming an antisite, nitrogen vacancy pair, which is more stable than the simple vacancy for positive charge states. Our assessment of the $d^0$ and $d^{10}$ pseudopotentials yields minimal differences in defect structures and defect levels. The better agreement of the $d^0$ lattice constant with experiment suggests that the more computationally economical $d^0$ pseudopotentials are sufficient to achieve the fidelity possible within the physical accuracy of DFT, and thereby enable calculations in larger supercells necessary to demonstrate convergence with respect to finite size supercell errors.