The ground state for the half-filled $t-t^{\prime }$ Hubbard model is treated within the Hartree-Fock approximation and the slave-boson approach including correlations. The criterium for the metal-insulator transition in the Slater scenario is formulated using an analytical free-energy expansion in the next-nearest-neighbor transfer integral $t^{\prime }$ and in direct antiferromagnetic gap $\mathrm{\Delta }$. The correlation effects are generally demonstrated to favor the first-order transition. For a square lattice with a strong van Hove singularity, accidental close degeneracy of AFM and paramagnetic phases is analytically found in a wide parameter region. As a result, there exists an interval of $t^{\prime }$ values for which the metal-insulator transition is of the first order due to the existence of the van Hove singularity. This interval is very sensitive to model parameters (direct exchange integral) or external parameters. For the simple and body-centered cubic lattices, the transition from the insulator AFM state with increasing $t^{\prime }$ occurs to the phase of an AFM metal and is a second-order transition, which is followed by a transition to a PM metal. These results are quantitatively modified when taking into account the intersite Heisenberg interaction, which can induce first-order transitions. A comparison with the Monte Carlo results is performed.

The theory for the vanishing of Néel order in the spin $S=1/2$ square lattice antiferromagnet has been the focus of attention for many decades. A consensus appears to have emerged in recent numerical studies on the antiferromagnet with first and second neighbor exchange interactions (the $J_1\text{−}{J}_2$ model): A gapless spin liquid is present for a narrow window of parameters between the vanishing of the Néel order and the onset of a gapped valence bond solid state. We propose a deconfined critical SU(2) gauge theory for a transition into a stable ${\mathbb{Z}}_2$ spin liquid with massless Dirac spinon excitations; on the other side of the critical point, the SU(2) spin liquid (the ‘$\pi$-flux’ phase) is presumed to be unstable to confinement to the Néel phase. We identify a dangerously irrelevant coupling in the critical SU(2) gauge theory, which contributes a logarithm-squared renormalization. This critical theory is also not Lorentz invariant and weakly breaks the SO(5) symmetry which rotates between the Néel and valence bond solid order parameters. We also propose a distinct deconfined critical U(1) gauge theory for a transition into the same gapless ${\mathbb{Z}}_2$ spin liquid; on the other side of the critical point, the U(1) spin liquid (the ‘staggered flux’ phase) is presumed to be unstable to confinement to the valence bond solid. This critical theory has no dangerously irrelevant coupling, dynamic critical exponent $z\ne 1$, and no SO(5) symmetry. All of these phases and critical points are unified in a SU(2) gauge theory with Higgs fields and fermionic spinons which can naturally realize the observed sequence of phases with increasing $J_2/J_1$: Néel, gapless ${\mathbb{Z}}_2$ spin liquid, and valence bond solid.

Experimental and theoretical studies of laser-induced breakdown in dielectrics provide conflicting conclusions about the possibility to trigger ionization avalanche on the subpicosecond time scale and the relative importance of carrier-impact ionization over field ionization. On the one hand, current models based on a single ionization-rate equation do not account for the gradual heating of the charge carriers, which, for short laser pulses, might not be sufficient to start an avalanche. On the other hand, kinetic models based on microscopic collision probabilities have led to variable outcomes that do not necessarily match experimental observations as a whole. In this paper, we present a rate-equation model that accounts for the avalanche process phenomenologically by using an auxiliary differential equation to track the gradual heating of the charge carriers and define the collisional impact rate dynamically. The computational simplicity of this dynamical rate-equation model offers the flexibility to extract effective values from experimental data. This is demonstrated by matching the experimental scaling trends for the laser-induced damage threshold of several dielectric materials for pulse durations ranging from a few fs to a few ps. Through numerical analysis, we show that the proposed model gives results comparable to those obtained with multiple rate equations and identify potential advantages for the development of large-scale, three-dimensional electromagnetic methods for the modeling of laser-induced breakdown in transparent media.

Colloidal quantum dots (QDs) of group II-VI are key ingredients of next-generation QD light-emitting diodes technology for display and lighting, yet the understanding of their luminescent characteristics are far from being mature. Using a hybrid time-dependent density functional theory, we have studied the electronic and excitonic properties of blue-emitting colloidal QDs within group II-VI containing a thousand atoms or more, including CdSe, CdS, ZnSe, and ZnS QDs, considering both quantum confinement and surface ligand effects. It is found that the calculated optical gaps are in excellent quantitative agreement with experiment, irrespective of the QD nature. Scaling laws of size-dependent energy gaps governed solely by quantum confinement effects have further been explored at both single-particle level and correlated excitonic level for all QDs. With concurrently stoichiometric control and enhancing quantum confinement effects, we have predicted an unusual switching of symmetry character of the highest occupied molecular orbital state from a ${\mathrm{\Gamma }}_3$ to a ${\mathrm{\Gamma }}_1$ symmetry at ultrasmall size ($\sim 1$ nm) for all QDs. After the switching, pronounced linearly polarized band-edge excitonic emission is activated. The radiative exciton decay lifetime is found to increase monotonically with increasing the QD size and tends to saturate at larger sizes. Finally, we have explored the surface passivation mechanism of inorganic chloride ligand, and identified various favorable Cd-Cl bonding configurations which enable an effective surface passivation resembling the commonly applied pseudohydrogen passivation scheme. We find that chloride ligand serves as a hole delocalization ligand and tends to redshift the absorption spectra, reduce the absorption intensity, and significantly enhance the exciton decay lifetime. Our results provide a guideline for spectroscopic studies of excitonic characteristics of colloidal QDs within group II-VI.

Recent progress in manipulating individual quantum systems has led to the development of quantum thermal transistors, which control the thermal conductivity between two of its terminals according to an input signal on its third terminal. With several models for individual thermal transistors already developed, the next natural step is to investigate how multiple thermal transistors can interconnect to build useful composite devices. In electronics literature, the Darlington pair is a two-transistor configuration commonly used to construct electronic amplifiers with higher gain than is possible with a single transistor. We create the electronic Darlington pair's thermal equivalent using two individual thermal transistors in a similar configuration. Unlike previously studied models, multitransistor configurations like this contain internal transistor interconnections whose temperatures cannot be biased externally but are determined by the individual transistors' internal dynamics. We refine previous models to incorporate these transistor-transistor interactions and introduce an intermediate thermal bath to facilitate the thermal energy exchange between the Darlington pair transistors. We investigate temperature-based and optical field-based control strategies of the Darlington pair in terms of both steady-state and transient thermal flow characteristics through numerical simulations. Under both control strategies, the thermal Darlington pair's steady-state performance exhibits superior thermal amplification, sensitivity, and thermodynamic efficiency than an equivalent single thermal transistor. Our results closely mirror those expected from the corresponding electronic Darlington pair. Hence, we envision that we may readily adapt the intermediate bath formalism we developed in this work to translate a wide variety of useful electronic multitransistor configurations to their thermal equivalents.

Electronic states in the gap of a superconductor inherit intriguing many-body properties from the superconductor. Here, we create these in-gap states by manipulating Cr atomic chains on the $\beta -\mathrm{Bi}{}_2\mathrm{Pd}$ superconductor. We find that the topological properties of the in-gap states can greatly vary depending on the crafted spin chain. These systems make an ideal platform for nontrivial topological phases because of the large atom-superconductor interactions and the existence of a large Rashba coupling at the Bi-terminated surface. We study two spin chains, one with atoms two lattice parameters apart and one with $\sqrt{2}$ lattice parameters. Of these, only the second one is in a topologically nontrivial phase, in agreement with the spin interactions for this geometry.

Despite the rapid progress in understanding the first intrinsic magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, its electronic structure remains a topic under debates. Here we perform a thorough spectroscopic investigation into the electronic structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ via laser-based angle-resolved photoemission spectroscopy. Through quantitative analysis, we estimate an upper bound of 3 meV for the gap size of the topological surface state. Furthermore, our circular dichroism measurements reveal band chiralities for both the topological surface state and quasi-2D bands, which can be well reproduced in a band hybridization model. A numerical simulation of energy-momentum dispersions based on a four-band model with an additional step potential near the surface provides a promising explanation for the origin of the quasi-2D bands. Our study represents a solid step forward in reconciling the existing controversies in the electronic structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, and provides an important framework to understand the electronic structures of other relevant topological materials ${\mathrm{MnBi}}_{2\mathrm{n}}{\mathrm{Te}}_{3\mathrm{n}+1}$.

We study the disorder-induced phase transition of higher-order Weyl semimetals (HOWSMs) and the fate of the topological features of disordered HOWSMs. We obtain a global phase diagram of HOWSMs utilizing the scaling theory of Anderson localization. Specifically, a phase transition from the Weyl semimetal (WSM) to the HOWSM is obtained, distinguishing the disordered HOWSMs from the traditional WSMs. Furthermore, we confirm the robustness of Weyl nodes for HOWSMs. Interestingly, the unique topological properties of HOWSMs show different behaviors: (i) the quantized quadrupole moment and the corresponding quantized charge of hinge states are fragile to weak disorder; (ii) the hinge states show moderate stability which enables the feasibility in experimental observation. Our study deepens the understanding of the topological nature of HOWSM and paves a possible way to its characterization in experiments.

Here, we report both ac and dc magnetization, thermodynamic, and electric properties of hexagonal ${\mathrm{Ba}}_3{\mathrm{NiIr}}_2{\mathrm{O}}_9$. ${\mathrm{Ni}}^{2+}$ (spin 1) forms a layered triangular lattice and interacts antiferromagnetically, while ${\mathrm{Ir}}^{5+}$ is believed to act as a magnetic link between the layers. This complex magnetic interaction results in magnetic frustration, leading to a spin-glass transition at $T_f\sim 8.5$ K. The observed magnetic relaxation and aging effect also confirm the nonequilibrium ground state. The system further shows large exchange bias which is tunable with cooling field. Below the Curie-Weiss temperature ${\theta }_{CW}$ ($\sim -29$ K), the magnetic specific heat $C_m$ displays a broad hump and at low temperature follows $C_m=\gamma T^{\alpha }$ dependence, where both $\gamma$ and $\alpha$ show dependence on temperature and magnetic field. A sign change in magnetoresistance is observed which is due to an interplay among the magnetic moment, field, and spin-orbit coupling.

Magnons (the quanta of spin waves), unlike electrons, do not suffer from Ohmic losses, which makes them an attractive information carrier in communication and data processing. The possibility of nonreciprocal magnon propagation is of great practical relevance to directional communications, particularly when the nonreciprocity is steerable through magnons themselves in a monologic magnonic device. This work demonstrates explicitly such nonreciprocity in the mode profiles of magnons in ${\mathrm{Ni}}_{79}{\mathrm{Fe}}_{21}$ thin films. Evidence of nonreciprocal emergence of hybridized dipole-exchange spin waves at two surfaces of ${\mathrm{Ni}}_{79}{\mathrm{Fe}}_{21}$ nanofilms deposited on a surface oxide silicon substrate is provided by studying magnon transmission and asymmetry via Brillouin light scattering measurements. The dipole-dominated spin wave and exchange-dominated spin wave are found to be localized near the top and bottom surfaces, respectively, and travel along opposite directions. The nonreciprocity and the localization can be tuned by an in-plane magnetic field. Our findings provide a simple and flexible approach to nonreciprocal all-magnon logic devices highly compatible with silicon-based integrated circuit technology.

Despite extensive experimental and theoretical work, consensus on the crystal structure and stability of the helium-hydrogen system is lacking. In this study, we investigate the possibility of helium forming a stable compound with hydrogen by using a first-principles structure search method. As a result of first-principles simulations, we predict a helium-hydrogen compound that is mechanically stable below 8 GPa with a $\mathrm{He}{\left({\mathrm{H}}_2\right)}_3$ stoichiometry. Topological analysis of electron density at the bond critical points shows that there exists a quantifiable level of van der Waals interaction between helium and hydrogen in this $\mathrm{He}{\left({\mathrm{H}}_2\right)}_3$ crystal. Our current results provide a case of weak interaction in a mixed hydrogen-helium system, offering insights for the evolution of interiors of giant planets such as Jupiter and Saturn.

We study theoretically the influence of temperature and disorder on the spin-wave spectrum of the magnonic crystal ${\mathrm{Fe}}_{1-c}{\mathrm{Co}}_c$. Our formalism is based on the analysis of a Heisenberg Hamiltonian by means of the wave vector and frequency-dependent transverse magnetic susceptibility. The exchange integrals entering the model are obtained from the ab initio magnetic force theorem. The coherent potential approximation is employed to treat the disorder and random phase approximation in order to account for the softening of the magnon spectrum at finite temperatures. The alloy turns out to exhibit many advantageous properties for spintronic applications. Apart from a high Curie temperature, its magnonic band gap remains stable at elevated temperatures and is largely unaffected by the disorder. We pay particular attention to the attenuation of magnons introduced by the alloying. The damping turns out to be a nonmonotonic function of the impurity concentration due to the nontrivial evolution of the value of exchange integrals with the Co concentration. The disorder-induced damping of magnons is estimated to be much smaller than their Landau damping.

The magnetization orientation dependence of Gilbert damping and ultrafast demagnetization were investigated in single-crystalline ${\mathrm{Co}}_{50}{\mathrm{Fe}}_{50}$ films grown on a MgO(100) substrate by the time-resolved magneto-optical Kerr effect technique. The intrinsic Gilbert damping coefficient extracted from the time evolution of magnetization precessions shows a remarkably large anisotropy with a ratio of more than 300% for magnetization orientations along the $\langle 100\rangle$ and $\langle 110\rangle$ axes. Such a large anisotropy of Gilbert damping persists to high frequencies up to $50\mathrm{GHz}$, where the effect of two-magnon scattering is suppressed. In contrast to the anisotropic Gilbert damping, the ultrafast demagnetization time and ratio for different in-plane magnetization orientations are nearly isotropic with the magnetization orientation. Although anisotropic Gilbert damping can be explained by the variation of spin-orbit coupling with the magnetization orientations, our results demonstrate that the role of spin-orbit coupling in the high nonequilibrium state after ultrafast laser excitation is isotropic in driving ultrafast spin-flip processes.

Motivated by experimental observations, Samajdar et al. [Nat. Phys. 15, 1290 (2019)] have proposed that the insulating Néel state in the parent compounds of the cuprates is proximate to a quantum phase transition to a state in which Néel order coexists with semion topological order. We study the manner in which proximity to this transition can make the phonons chiral, by inducing a significant phonon Hall viscosity. We describe the spinon-phonon coupling in a lattice spinon model coupled to a strain field, and also using a general continuum theory constrained only by symmetry. We find a nonanalytic Hall viscosity across the transition, with a divergent second derivative at zero temperature.

Motivated by the recently experimental reported signatures of the tunable Mott insulating state and superconductivity in an ABC graphene trilayer superlattice, we investigate the charge compressibility, the spin correlation, and the superconducting instability within the Hubbard model on a three-layer honeycomb lattice. It is found that an antiferromagnetically ordered Mott insulator emerges beyond a critical $U_c$ at half-filling, and the electronic correlation drives a $d+id$ superconducting pairing to be dominant over other pairing patterns in a wide doped region. The effective pairing interaction with $d+id$ pairing symmetry is strongly enhanced with the increasing of on-site interaction and suppressed as the interlayer coupling strength increases. Our intensive numerical results demonstrate that the insulating state and superconductivity in an ABC graphene trilayer are driven by strong electric correlation, and it may offer an attractive systems to explore rich correlated behaviors.

Microwave pulse sequences are the basis of coherent manipulation of the electronic spin ground state in nitrogen vacancy (NV) centers. In this work we demonstrate stimulated Raman transitions (SRTs) and stimulated Raman adiabatic passage (STIRAP), two ways to drive the dipole-forbidden transition between two spin sublevels in the electronic triplet ground state of the NV center. This driving is achieved by a two-photon Raman microwave pulse which simultaneously drives two detuned transitions via a virtual level for SRTs or via two adiabatic and partially overlapping resonant microwave pulses for STIRAP. We lay the theoretical framework of SRT and STIRAP dynamics and verify experimentally the theoretical predictions of population inversion by observing the dipole-forbidden transition in the ground state of a single NV center. A comparison of the two schemes showed better robustness and success of the spin swap for STIRAP compared to SRT.

Crystals of plasmonic metal nanoparticles have intriguing optical properties. They reach the regimes of ultrastrong and deep strong light-matter coupling, where the photonic states need to be included in the simulation of material properties. We propose a quantum description of the plasmon polaritons in supercrystals that starts from the dipole and quadrupole excitations of the nanoparticle building blocks and their coupling to photons. Our model excellently reproduces the results of finite difference time domain simulations. It provides detailed insight into the emergence of the polariton states. Using the example of a face centered cubic crystal we show that the dipole and quadrupole states mix in many high-symmetry directions of the Brillouin zone. A proper description of the plasmon and plasmon-polariton band structure is only possible when including the quadrupole-derived states. Our model leads to an expression of the reduced coupling strength in nanoparticle supercrystals that we show to enter the deep strong-coupling regime if the metal nanoparticles take up $>60\%$ of the total volume. In addition to the plasmon-polariton energies, we analyze the relative contributions of the dipole, quadrupole, and photonic states to their eigenfunctions and are able to demonstrate the decoupling of light in the deep strong-coupling regime. Our results pave the way for a better understanding of the quantum properties of metallic nanoparticle supercrystals in the ultrastrong- and deep strong-coupling regime.

We apply the mean-field Hartree Fock theory of gapped electronic states at charge neutrality in bilayer graphene to thin films of rhombohedral graphite with up to thirty layers. For the ground state, the order parameter (the separation of bands at the valley center) saturates to a constant nonzero value as the layer number increases, whereas the band gap decreases with layer number. We consider chiral symmetry breaking disorder in the form of random layer potentials and chiral preserving disorder in the form of random values of the interlayer coupling. The former reduces the magnitude of the mean band gap whereas the latter has a negligible effect, which is due to self-averaging within a film with a large number of layers. We determine the ground state in the presence of an individual stacking fault which results in two pairs of low-energy bands and we identify two separate order parameters. One of them determines the band gap at zero temperature, the other determines the critical temperature leading, overall, to a temperature dependence of the band gap that is distinct to that of pristine rhombohedral graphite. In the presence of stacking faults, each individual rhombohedral section with $m$ layers contributes a pair of low-energy flat bands producing a peak in the Berry curvature located at a characteristic $m$-dependent wave vector. The Chern number per spin-valley flavor for the filled valence bands in the ground state is equal in magnitude to the total number of layers divided by two, the same value as for pristine rhombohedral graphite.

The coupling between electronic and lattice degrees of freedom lies at the core of many important properties of solids. Nevertheless, surprisingly little is known about the entanglement between these degrees of freedom. Here, we calculate the entanglement entropy at zero temperature as well as the mutual information and the logarithmic entanglement negativity at finite temperatures between the electrons and the lattice for a one-dimensional chain. The electrons are described within Luttinger-liquid theory. Our results show that the entanglement entropy diverges when one approaches the limit of stability, the so-called Wentzel-Bardeen singularity. We find that the mutual information and the logarithmic entanglement negativity decrease with temperature. The mutual information reaches a finite value in the infinite-temperature limit, which is a consequence of the infinite linear electron spectrum of Luttinger theory. The logarithmic entanglement negativity becomes exactly zero above a certain temperature; that is, the lattice and the electrons become nonentangled above this temperature. If the electron-electron interaction is unscreened or weakly screened, this characteristic temperature diverges with the system size. However, if the interaction is strongly screened, the characteristic temperature is finite and independent of size, indicating a phase transition in the thermodynamic limit.