Theoretical methods that are accurate for both short-distance observables and long-wavelength collective modes are still being developed for the Hubbard model. Here, we benchmark an approach that combines dynamical mean-field theory (DMFT) observables with the two-particle self-consistent theory (TPSC). This offers a way to include non-local correlations in DMFT while also improving TPSC. The benchmarks are published diagrammatic quantum Monte Carlo results for the two-dimensional square lattice Hubbard model with nearest-neighbor hopping. This method ($\mathrm{TPSC}+\mathrm{DMFT}$) is relevant for weak to intermediate interaction, satisfies the local Pauli principle, and allows us to compute a spin susceptibility that satisfies the Mermin-Wagner theorem. The DMFT double occupancy determines the spin and charge vertices through local spin and charge sum rules. The TPSC self-energy is also improved by replacing its local part with the local DMFT self-energy. With this method, we find improvements for both spin and charge fluctuations and for the self-energy. We also find that the accuracy check developed for TPSC is a good predictor of deviations from benchmarks for this model. $\mathrm{TPSC}+\mathrm{DMFT}$ can be used in regimes where quantum Monte Carlo is inaccessible. In addition, this method paves the way to multiband generalizations of TPSC that could be used in advanced electronic structure codes that include DMFT.

Recently, charge density waves (CDWs) have been observed in $\mathrm{CaCu}{\mathrm{O}}_2\text{−}\mathrm{analogous}$ infinite-layer nickelates $R\mathrm{Ni}{\mathrm{O}}_2$ ($R=\mathrm{La}$, Nd) but exhibit very different hole doping dependent behaviors compared to that in cuprates, raising a challenging question on its origin. In this paper, employing density functional theory, many-body dynamic mean field theory, and determinant quantum Monte Carlo calculations, we propose a synergetic contribution from both electronic instability (EI) and moment-dependent electron-phonon coupling (MEPC). Unexpectedly, the EI and MEPC are mainly contributed by Ni $3d_{x2-y2}$ and $R 5d_{z2}$, highlighting the unique multiorbital feature. Interestingly, a strong Fermi surface nesting (FSN) induced by the unique feature of van Hove singularity (VHS) across the Fermi level exists in $R\mathrm{Ni}{\mathrm{O}}_2$, which is sensitive to hole doping. The hole doping can rapidly reduce FSN of Ni $3d_{x2-y2}$ by shifting VHS and decrease the occupation of $R 5d_{z2}$, which can largely weaken EI and MEPC in $R\mathrm{Ni}{\mathrm{O}}_2$. Remarkably, the temperature-insensitive feature of EI and MEPC could be a hint for rather high-temperature CDWs observed in undoped $R\mathrm{Ni}{\mathrm{O}}_2$. Our theory may offer one possible explanation to the experimentally observed CDW formation and its hole-doping dependence in nickelates, and also establishes a unified understanding of the hole doping dependent EI and MEPC in nickelates and cuprates.

The fabrication of ultrathin (Cd,Mn)Te nanowires with diameters reaching values below 10 nm is reported. The thinning procedure relies on a thermally induced reverse reaction process applied to (Cd,Mn)Te nanowires grown using the vapor-liquid-solid method. The impact of quantum confinement is manifested by a distinct blueshift of the excitonic emission from individual nanowires. Cathodoluminescence mapping reveals that the optical emission originates from relatively short nanowire segments, which is explained in terms of exciton localization on irregularities of nanowire core diameter. Magneto-optical study performed on several individual nanowires reveals a change from light hole to heavy hole character of the excitonic emission depending on the emission energy, i.e., on the size of the emitting object. The results are interpreted in terms of the influence of axial and radial quantum confinement on excitons in ultrathin nanowires.

In this paper, we analyze the impact of physically large baths used in terminals of thermal multitransistor systems formed using two-level systems (TLSs). In particular, we simulate the effects of a two-transistor system introduced as a thermal Darlington pair (DP). The size and proximity of the baths can cause multiple interactions with the transistor terminals represented by the TLS, not just the TLS directly connected to them. Such interactions can ultimately suppress the heat flows or impair the transistor action. However, we demonstrate that the DP model can achieve more than a $50\%$ increase in heat flows. Using the engineered interactions leading to the correlated TLS-thermal bath interactions, we establish an incoherent (no quantum coherence in the density matrix at the steady state) yet correlated (joint excitation of two TLSs due to bath interaction) heat transfer model to a two-transistor arrangement in a substrate. This model helps us to interpret the environmental effects on the device by treating the common environment as separate thermal baths and all the transitions in the system as independent. We also show that this model can be mapped to contain dark-states. These dark-states can provide an external channel for transistor switching. We use this knowledge to design thermal counterparts of electronic AND and OR gates, and to study their switching time and operation, paving the way to realizing thermal logic gates.

We present nonlinear optical four-wave mixing in a silicon nanodisk dimer metasurface. Under the oblique incident plane waves, the designed metasurface exhibits a multiresonant feature with simultaneous excitations of three quasi-bound states in the continuum (BIC). Through employing these quasi-BICs with maximizing electric field energy at the input bump wavelengths, significant enhancement of third-order nonlinear processes including third-harmonic generation, degenerate and nondegenerate four-wave mixing are demonstrated, giving rise to ten frequencies in the visible wavelengths. This work may lead to a new frontier of ultracompact optical mixer for applications in optical circuitry, ultrasensitive sensing, and quantum nanophotonics.

A well-known feature of the classical monoatomic gas is that its bulk viscosity is strongly suppressed because the single-particle dispersion is quadratic. On the other hand, in condensed matter systems the effective single-particle dispersion is altered by lattice effects and interactions. In this work, we study the bulk viscosity of one-dimensional Fermi gases with generic energy-momentum dispersion relations. As an application, viscous dissipation arising from lattice effects is analyzed for the tight-binding model. In addition, we investigate how weak interactions affect the bulk viscosity. Finally, we discuss viscous dissipation in the regime in which the Fermi gas is not fully equilibrated, as can occur when the system is driven at frequencies that exceed the rate of fermion backscattering. In this case, the Fermi gas is described by three bulk viscosities, which we obtain for a generic single-particle dispersion.

Two-dimensional (2D) Janus materials due to their asymmetric structures show fascinating spintronic and piezoelectric properties making them a research hot spot in recent years. In this work, inspired by Janus group-III monochalcogenides, we propose $\mathrm{Zn}AXY$ ($A=\mathrm{Si}$, Ge, Sn, and $X/Y=\mathrm{S}$, Se, Te, $X<Y$) monolayers as a novel structure with broken inversion and mirror symmetry. By calculating cohesive energy and phonon dispersion, eight of nine possible $\mathrm{Zn}AXY$ monolayers are proved to be dynamically stable. In addition, thermal stability of these eight structures is confirmed by ab initio molecular dynamics simulations. The electronic band structures of $\mathrm{Zn}AXY$ monolayers indicate that all of them are indirect semiconductors with strong spin-orbit coupling effects. Lack of inversion symmetry gives rise to Zeeman-type spin splitting at the $K$ point of the conduction band with the highest value of 136 meV for ZnSiSeTe. Furthermore, out-of-plane asymmetry results in Rashba spin splitting (RSS) at the Γ point of the valence and conduction bands in the most compositions of Janus $\mathrm{Zn}AXY$ monolayers. Among them, ZnGeSTe with ${\alpha }_R^{{\mathrm{\Gamma }}_V}$ of $1.79\mathrm{eV}\AA$ and ${\alpha }_R^{{\mathrm{\Gamma }}_c}$ of $0.862\mathrm{eV}\AA$ and ZnSiSTe with ${\alpha }_R^{{\mathrm{\Gamma }}_V}$ of $1.537\mathrm{eV}\AA$ and ${\alpha }_R^{{\mathrm{\Gamma }}_c}$ of $0.756\mathrm{eV}\AA$ are found to be great materials for future spintronic applications. Interestingly, in addition to large RSS, Mexican hat dispersion is observed at the Γ point of the topmost valence band in these materials. Moreover, the calculated elastic coefficients for the hexagonal $\mathrm{Zn}AXY$ monolayers confirm the mechanical stability of the predicted structures. Finally, Janus $\mathrm{Zn}AXY$ monolayers possess high in-plane (up to 7.46 pm/V) and out-of-plane (up to 0.67 pm/V) piezoelectric coefficients making them appealing alternatives for prevalent piezoelectric materials.

${\mathrm{NiS}}_2$ has been widely regarded as a model system to study the bandwidth-controlled Mott transition, as enabled by isovalent Se chemical substitution on the S sites. Motivated by advances in electrostatic gating, we theoretically investigate the filling-controlled Mott transition induced by gating, which has the advantage of avoiding disorder introduced by dopants and stoichiometric changes. We use combined density-functional theory (DFT) and dynamical mean-field theory (DMFT) to study such a filling-controlled transition and compare it with the case of bandwidth control. We draw a temperature-filling phase diagram and find that the Mott-insulator to metal transition occurs with modest added electron concentrations, well within the capabilities of existing electrostatic gating experiments. We find that there is significant incoherent weight at the Fermi level in the metallic phase when the transition is induced by gating. In contrast, the spectral weight remains rather coherent in the case of the bandwidth-controlled transition.

Floquet engineering is an important way to manipulate the electronic states of condensed matter physics. Recently, the discovery of the magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and its family provided a valuable platform to study magnetic topological phenomena, such as the quantum anomalous Hall effect, the axion insulator state, and the topological magnetoelectric effect. In this work, based on the effective model and first-principles calculations in combination with the Floquet theory, we reveal that the circularly polarized light (CPL) induces the sign reversal of the Chern number of odd-septuple-layer (SL) ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ thin films. In contrast, the CPL drives the axion insulator state into the quantum anomalous Hall state in even-SL ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ thin films. More interestingly, if the topmost van der Waals gap between the surface layer and the below bulk in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ films is slightly expanded, a high Chern number $\left|C\right|=2$ can be realized under the CPL. Our work demonstrates that the light can induce rich magnetic topological phases in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ films, which might have potential applications in optoelectronic devices.

The coupling of the spin and the motion of charge carriers is an important ingredient for the manipulation of the spin degree of freedom and for the emergence of topological matter. Creating domain walls in the spin-orbit interaction at the nanoscale may turn out to be a crucial resource for engineering topological excitations suitable for universal topological quantum computing or for new schemes for spin quantum bits. Realizing this in natural platforms remains a challenge. Using circuit quantum electrodynamics magnetospectroscopy, we investigate the spinful states of a double quantum dot made in a single wall carbon nanotube with lithographically patterned magnetically textured gates. While a full understanding of the behavior of our magnetic textures would be helpful, the experimental signals are consistent with a change of the spin-orbital structure of the states above each gate. The coherence of the data, backed up by extensive theoretical modeling and a control device, points towards the existence of a synthetic staggered spin-orbit interaction in our device.

In spin-based architectures of quantum devices, the hyperfine interaction between the electron spin qubit and the nuclear spin environment remains one of the main sources of decoherence. This paper provides a short review of the current advances in the theoretical description of the qubit decoherence dynamics. Next, we study the qubit-environment entanglement using negativity as its measure. For an initial maximally mixed state of the environment, we study negativity dynamics as a function of environment size, changing the numbers of environmental nuclei and the total spin of the nuclei. Furthermore, we study the effect of the magnetic field on qubit-environment disentangling timescales.

Titanium cobalt oxide $\mathrm{CoTi}{\mathrm{O}}_3$ (CTO) with ilmenite structure has attracted considerable attention in functional materials owing to their multiferroic and chemical stability characteristics similar to $\mathrm{FeTi}{\mathrm{O}}_3$ and perovskites. However, information about structural stability and lattice deformation of CTO under high pressure remains unclear. Here, we present a unique pressure gauge through a quantitative investigation on the vibrational properties of compressed CTO by in situ high-pressure Raman experiments up to 40 GPa. Neither phase transitions nor amorphization has been observed in such pressure range, demonstrating good lattice stability of CTO. These results have been validated by both the Jahn-Teller effect and Fermi resonance. More importantly, we have attempted to elucidate the evolution of Raman signals for a pressure gauge based on the synergistic effect of plasmon-resonance field enhancement. Our findings not only provide direct experimental evidence that CTO stays phase stable up to 40 GPa, but also offer insight to understanding the high-pressure Raman behaviors for other ilmenite based functional materials.

We propose a method for controlling the high-harmonic generation (HHG) with a high dynamic range in single-layer graphene. We find that, by utilizing shear strain, a significant enhancement or quenching of HHG is possible over a range of several orders of magnitude. This feature is made possible by the resonance mechanism at a van Hove singularity. Therein, the shear strain controls the configurations of the two Dirac cones, resulting in changes in the energy and dipole moment at the saddle point of the band dispersion. Our findings provide a way for modulating or switching light by using a nano-optomechanical device composed of single-layer graphene.

Microdiamonds with color centers have unique photophysical properties. We discover extremely narrow spectral lines with full width at half maximum down to 0.5 nm (≈390 GHz at wavelength 630.15 nm) at room temperature in photoluminescence of numerous high-pressure–high-temperature microdiamonds. Correlation spectroscopy proves that these lines originate from single photon emitters. Comprehensive experimental characterization of the emitters is performed by means of complementary scanning electron and fluorescence microscopy that allows morphological characterization and in time monitoring of the same labeled emitters. Study of photoluminescence and photoluminescence excitation spectra of single emitters showed negligible phonon sideband and narrowband excitation. Temperature sensitivity, stable photoluminescence, brightness, linear polarization, and fluorescence lifetime ≈1.5–2.5 ns are demonstrated. The results show great potential of the discovered “enigmatic” emitters for applications in quantum optics, fluorescence labeling, and high-resolution thermometry.

Topological insulators are attracting researchers to reveal related fundamental physics, whose fractal versions are expanding topological physics into the fractal regime. The fractal topological systems, however, show a challenge to the Kitaev formula, because it leads to a noninteger Chern number. The Chern number within the band gap oscillates more and more actively as the system size increases. We uncover that such a challenge should be attributed to the coupling, brought about by the lattice fractalization, between the internal and external edge states. To confirm this, examples of periodic, quasicrystal, and fractal Floquet topological insulators are compared with each other by analyzing the topological edge states and local Chern markers. The sum of the local Chern markers on all the edges of the fractal lattice presents a fractional power relation with the system size, reflecting the fractalization of the topological edge states. We reveal that once such a coupling is shut down, the Kitaev formula would once again result in integers.

We consider critical eigenstates in a two-dimensional quasicrystal and their evolution as a function of disorder. By exact diagonalization of finite-size systems we show that the evolution of properties of a typical wave function is nonmonotonic. That is, disorder leads to states delocalizing, until a certain crossover disorder strength is attained, after which they start to localize. Although this nonmonotonic behavior is only present in finite-size systems and vanishes in the thermodynamic limit, the crossover disorder strength decreases logarithmically slowly with system size, and is quite large even for very large approximants. The nonmonotonic evolution of spatial properties of eigenstates can be observed in the anomalous dimensions of the wave-function amplitudes, in their multifractal spectra, and in their dynamical properties. We compute the two-point correlation functions of wave-function amplitudes and show that these follow power laws in distance and energy, consistent with the idea that wave functions retain their multifractal structure on a scale which depends on disorder strength. Dynamical properties are studied as a function of disorder. We find that the diffusion exponents do not reflect the nonmonotonic wave-function evolution. Instead, they are essentially independent of disorder until disorder increases beyond the crossover value, after which they decrease rapidly, until the strong localization regime is reached. The differences between our results and earlier studies on geometrically disordered “phason-flip” models lead us to propose that the two models are in different universality classes. We conclude by discussing some implications of our results for transport and a proposal for a Mott hopping mechanism between power-law localized wave functions, in moderately disordered quasicrystals.

Phonon densities of states (DOSs) of body-centered cubic chromium were measured by time-of-flight inelastic neutron scattering at temperatures up to 1493 K. Density functional theory calculations with both quasiharmonic (QH) and anharmonic (AH) methods were performed at temperatures above the Néel temperature. Features in the phonon DOSs decrease in energy (soften) substantially with temperature. A Born–von Kármán analysis using fits to the experimental DOSs reveals a softening of almost 17% of the high-transverse phonon branch between 330 and 1493 K. The low-transverse branch changes by approximately half this amount. The AH calculations capture the observed behavior of the two transverse phonon branches, but the QH calculations give some inverted trends. Vibrational entropies from phonons and electrons are obtained, and their sum is in excellent agreement with the entropy of chromium obtained by calorimetry, indicating that above 330 K, no explicit temperature-dependent magnetic contributions are necessary.

Programmable logic devices with multiple functions are key ingredients in information technology. Here we propose a multifunction logic device based on the current-driven skyrmion motion in a single nanotrack, where the spatially modulated Dzyaloshinskii-Moriya interaction introduces a potential barrier to the skyrmion. Three stable stages of the skyrmion location under a driving current of different magnitudes are demonstrated at room temperature through micromagnetic simulations, according to which we implement all necessary logic gates for computing applications. Our proposal provides a feasible scheme to design skyrmion-based programmable logic devices.