The interplay of hopping parameters that can give rise to flat bands in consequence of quantum interference in electronic, photonic, and other interesting materials has become an extensively studied topic. Most of the recognized structures having flat bands are lattices that can be understood by the mathematical theory of line graphs, such as the Lieb, kagome, and checkerboard lattices. Here, we demonstrate that the structures that can realize the same kinds of flat bands given by those well-known lattices hosting exotic quantum phases are more flexible. The flat bands belonging to the recognized structures can be ideally embedded into new structures that cannot be considered as the original ones in terms of a unitary transformation. The uncovered mechanism enriches the understanding of physics behind the localized quantum states and broadens the choice of materials that can be used for designing electronic and photonic devices from the zero band dispersion.

We investigate the structural and electronic properties of bulk ZnS in zinc blende as well as in wurtzite phase, and ${\mathrm{Zn}}_n{\mathrm{S}}_n$ nanoclusters by using the Hubbard model $\left(\mathrm{DFT}+U\right)$. It provides an on-site Coulomb correction to mitigate some of the commonly known limitations of traditional DFT-GGA method such as the underestimation of band gap and inaccurate description of electronic band structure. Especially for the nanoclusters, the traditional DFT method cannot reproduce all properties accurately that are observed in the experiments. Within the framework of $\mathrm{DFT}+U$ method, our model is first able to predict various properties of bulk ZnS (zinc blende and wurtzite) as well as in nanoclusters with high accuracy. We empirically determined the Hubbard correction parameters $U_d$ and $U_p$ for $\mathrm{Zn}\text{−}3d$ and $\mathrm{S}\text{−}3p$ states, respectively, that could reproduce the measured values of band gaps, $d$-band positions, $p$-states bandwidths, lattice parameters, etc. with a reasonable agreement. It was found that our model can be compared very well with more accurate hybrid functionals at only a fraction of the computational cost. Further, the selected pair of $U_d$ and $U_p$ values are used to investigate the structural and electronic properties of ZnS nanoclusters and results agree well with the higher levels of theory.

We report the observation of the pressure-induced, fully collapsed tetragonal phase of $\mathrm{CaRbF}{\mathrm{e}}_4\mathrm{A}{\mathrm{s}}_4$ for $P\sim 22\mathrm{GPa}$ via high-pressure x-ray diffraction and magnetotransport measurements. The x-ray measurements, along with resistivity measurements, show that there is an initial half-collapsed tetragonal phase for $6<P<22\mathrm{GPa}$, in which superconductivity is continuously suppressed from $T_{\mathrm{c}}=35\mathrm{K}$ at $P=3.1\mathrm{GPa}$ to $T_{\mathrm{c}}<2\mathrm{K}$ for $P\ge 17.2\mathrm{GPa}$, as well as signs of the fully collapsed tetragonal phase near $P=22\mathrm{GPa}$. Density functional calculations suggest that both of these transitions are driven by increased As-As bonding, first across the Ca layer, and then at the second transition, across the Rb layer. Although electrical resistivity measurements in the fully collapsed tetragonal phase do not show superconductivity, there is a change in the slope of both the Hall coefficient and the longitudinal resistance near 22 GPa, suggesting a strong correlation between the electronic and lattice degrees of freedom in this new iron-based superconductor.

An acceleration of continuous time quantum Monte Carlo (CTQMC) methods is a potentially interesting branch of work as they are matchless as impurity solvers of a dynamical mean field theory (DMFT) approach for the description of strongly correlated systems. The inversion of the $k\times k$ matrix with $k^2$ operations given by the diagram expansion order $k$ in the CTQMC fast update and the multiplication of the $k\times k$ matrix, and the noninteracting properties with $k\times {\omega }_{n_{\text{max}}}^{m-1}$ operations to measure the $m$-point correlators, are computationally time consuming. Here we propose the CTQMC method in combination with a machine learning technique, which eliminates the $k\times {\omega }_{n_{\text{max}}}$ and $k\times {\omega }_{n_{\text{max}}}^3$ operations for the two-point impurity Green's functions $G_{\sigma }\left(i{\omega }_n\right)$ and four-point vertices ${\chi }_{\sigma ,\overline{\sigma }}(i{\omega }_{n{}_1},i{\omega }_{n{}_2},i{\omega }_{n{}_3},i{\omega }_{n{}_4})$, respectively. This method not only predicts the accurate physical properties at low temperature, but also dramatically decreases the computational times of ${\chi }_{\sigma ,\overline{\sigma }}(i{\omega }_{n{}_1},i{\omega }_{n{}_2},i{\omega }_{n{}_3},i{\omega }_{n{}_4})$ for the nonlocal extension of DMFT approximation.

We report a dual resonance feature in the ballistic conductance through a quantum Hall graphene nanoribbon with a magnetic quantum dot. Such a magnetic quantum dot localizes Dirac fermions exhibiting anisotropic eigenenergy spectra with broken time-reversal symmetry. Interplay between the localized states and quantum Hall edge states is found to be twofold, showing Breit-Wigner and Fano resonances, which is reminiscent of a double quantum dot system. By fitting the numerical results with the Fano-Breit-Wigner line shape from the double quantum dot model, we demonstrate that the twofold resonance is due to the valley mixing that comes from the coupling of the magnetic quantum dot with quantum Hall edge channels; an effective double quantum dot system emerges from a single magnetic quantum dot in virtue of the valley degree of freedom. It is further confirmed that the coupling is weaker for the Fano resonance and stronger for the Breit-Wigner resonance.

Electrical and optoelectronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) can be tuned by exploiting their structural phase transitions. Here semiconducting (2H) to metallic (1T) phase transition is investigated in a strained ${\mathrm{MoWSe}}_2$ monolayer using molecular dynamics (MD) simulations. Novel intermediate structures called $\alpha$ and $\beta$ are found between the 2H and 1T phases. These intermediate structures are similar to those observed in a 2D $\mathrm{Mo}{\mathrm{S}}_2$ by scanning transmission electron microscopy. A deep generative model, namely the variational autoencoder (VAE) trained by MD data, is used to generate novel heterostructures with $\alpha$ and $\beta$ interfaces. Quantum simulations based on density functional theory show that these heterostructures are stable and suitable for novel nanoelectronics applications.

On the basis of a Berry-phase analysis, we study the ground state of the $J_1\text{−}{J}_2$ Heisenberg chain for $S=2,3,4$. We find that changes in the Berry phase occur $S$ times for spin-$S$ systems, indicating sequential phase transitions. The sequential phase transitions are associated with the change in valence-bond configurations of the ground states. To demonstrate this numerically, we connect the $J_1\text{−}{J}_2$ Heisenberg Hamiltonian with the generalized Affleck-Kennedy-Lieb-Tasaki model and show that the different phases are connected to the different valence-bond solid states.

In quantum critical heavy fermion systems, local moments are coupled to both collective spin fluctuations and conduction electrons. As such, the Bose-Fermi Anderson model, describing the coupling of a local moment to both a bosonic and a fermionic bath, has been of extensive interest. For the model in the presence of SU(2) spin rotational symmetry, questions have been raised about its phase diagram. Here we develop a version of continuous-time quantum Monte Carlo (CT-QMC) method suitable for addressing this issue; this procedure can reach sufficiently low temperatures while preserving the SU(2) symmetry. Using this method for the Bose-Fermi Anderson model, we clarify the renormalization-group fixed points and the phase diagram for the case with a constant fermionic-bath density of states and a power-law bosonic-bath spectral function ${\rho }_b\left(\omega \right)\propto {\omega }^s$ ($0<s<1$). Importantly, we find that two types of Kondo destruction quantum critical point (QCP) can arise in a single model. They are distinguished by the nature of the Kondo destroyed state: The local spin correlation either decays in imaginary time as a power law or remains a constant in the long-time limit. Specifically, for the model with $s^{*}<s<1$, both types of QCPs exist and, in the parameter regime accessible by an analytical $\epsilon$-expansion renormalization-group calculation (here $\epsilon =1-s$), the CT-QMC result is fully consistent with prior predictions by the latter method. For $s<s^{*}$, there is only one type of QCP. At both types of Kondo destruction QCPs, we find that the exponent of the local spin susceptibility $\eta$ obeys the relation $\eta =\epsilon$, which has important implications for Kondo destruction QCP in the Kondo lattice problem.

We calculate indirect exchange interaction between two magnetic impurities in pristine graphene in the presence of a circular potential. In bulk graphene structures indirect exchange interaction, also known as RKKY (Ruderman-Kittel-Kasuya-Yosida) interaction, shows a power-law decay with distance for both doped and undoped cases. Here we show that under a circular electric potential quasibound states lead to enhanced RKKY interactions between magnetic moments located in the vicinity of the potential well. It is shown that the strength of the potential well and Fermi energy can be tuned to create enhanced, nondecaying, long ranged RKKY interactions. We show that when the Fermi level lies at the quasibound state energy, the scattering processes between the states of the same chirality dominate over the other scattering channels and this leads to a predominantly ferromagnetic, nondecaying interaction between the impurities at long distances. The predicted effect can enable electrical control of RKKY interactions in graphene or other two-dimensional materials.

Transition-metal dichalcogenides own a variety of structures as well as electronic properties which can be modulated by structural variations, element substitutions, ion or molecule intercalations, etc. However, there is very limited knowledge on metastable phases of this family, especially the precise regulation of structural changes and accompanied evolution of electronic properties. Here, based on a new developed field-effect transistor with solid ion conductor as the gate dielectric, we report a controllable structural and electronic phase transitions in metastable ${\mathrm{MoS}}_2$ thin flakes driven by electric field. We found that the metastable structure of $1{\mathrm{T}}^{\prime \prime \prime }-{\mathrm{MoS}}_2$ thin flake can be transformed into another metastable structure of $1{\mathrm{T}}^{\prime }$-type upon intercalation of lithium regulated by electric field. Moreover, the metastable $1{\mathrm{T}}^{\prime }$ phase persists during the cycle of intercalation and de-intercalation of lithium controlled by electric field, and the electronic properties can be reversibly manipulated with a remarkable change of resistance by four orders of magnitude from the insulating $1{\mathrm{T}}^{\prime }-{\mathrm{LiMoS}}_2$ to superconducting $1{\mathrm{T}}^{\prime }-{\mathrm{MoS}}_2$. Such reversible and dramatic changes in electronic properties provide intriguing opportunities for development of novel nanodevices with highly tunable characteristics under electric field.

Compared to metamaterials based on regulation of real material parameters, artificial materials designed by controlling complex parameters exhibit richer physics and different functions. In this work, we present acoustic conjugate metamaterials, which possess complex mass densities and bulk compressibilities with opposite phases, to manipulate acoustic waves. By adjusting the phases of the complex material parameters, we can change the handedness and loss/gain of the material, and then, produce unique phenomena in acoustic fields. Theoretical analysis and numerical simulations demonstrate that placing a slab of an acoustic conjugate metamaterial in a traditional medium can create acoustic laser modes, perfect absorption or coherent perfect absorption. Moreover, even if total reflection arises at the interface, extraordinary transmission is observed, which differs from the phenomenon of traditional total reflection. Therefore, acoustic conjugate metamaterials exhibit unprecedented functions that have not been observed in the metamaterials on account of real parameter manipulation.

We explore different ways to simplify the evaluation of the smooth overlap of atomic positions (SOAP) many-body atomic descriptor [Bartók et al., Phys. Rev. B 87, 184115 (2013).]. Our aim is to improve the computational efficiency of SOAP-based similarity kernel construction. While these improved atomic descriptors can be used for general characterization and interpolation of atomic properties, their main target application is accelerated evaluation of machine-learning-based interatomic potentials within the Gaussian approximation potential (GAP) framework [Bartók et al., Phys. Rev. Lett. 104, 136403 (2010)]. We achieve this objective by expressing the atomic densities in an approximate separable form, which decouples the radial and angular channels. We then express the elements of the SOAP descriptor (i.e., the expansion coefficients for the atomic densities) in analytical form given a particular choice of radial basis set. Finally, we derive recursion formulas for the expansion coefficients. This new SOAP-based descriptor allows for tenfold speedups compared to previous implementations, while improving the stability of the radial expansion for distant atomic neighbors, without degradation of the interpolation power of GAP models.

Short-range polar order in the relaxor ferroelectric material ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3-28\%{\mathrm{PbTiO}}_3$ (PMN-28PT) have been studied using neutron diffuse scattering. An external electric field along the [110] direction can affect the diffuse scattering in the low temperature rhombohedral/monoclinic phase. Diffuse scattering intensities associated with [110] short-range polarizations are partially suppressed, while those arising from $\left[1\overline{1}0\right]$ polarizations are enhanced. On the other hand, short-range polar order along other equivalent $\langle 110\rangle$ directions, i.e., [101], $\left[10\overline{1}\right]$, [011], and $\left[01\overline{1}\right]$ directions, are virtually unaffected by the field. Our results, combined with previous work, strongly suggest that most parts of short-range polar order in PMN-$x\mathrm{PT}$ relaxor systems are robust in the low temperature phase, where they couple strongly to ferroelectric polarizations of the surrounding ferroelectric domains, and would only respond to an external field indirectly through ferroelectric domain rotation.

We calculate the fermionic spectral function $A_k\left(\omega \right)$ in the spiral spin density wave (SDW) state of the Hubbard model on a quasi-2D triangular lattice at small but finite temperature $T$. The spiral SDW order $\mathrm{\Delta }\left(T\right)$ develops below $T=T_N$ and has momentum $\mathbf{K}=(4\pi /3,0)$. We pay special attention to fermions with momenta $\mathbf{k}$, for which $\mathbf{k}$ and $\mathbf{k}+\mathbf{K}$ are close to Fermi surface in the absence of SDW. At the mean-field level, $A_k\left(\omega \right)$ for such fermions has peaks at $\omega =\pm \mathrm{\Delta }\left(T\right)$ at $T<T_N$ and displays a conventional Fermi liquid behavior at $T>T_N$. We show that this behavior changes qualitatively beyond mean field due to singular self-energy contributions from thermal fluctuations in a quasi-2D system. We use a nonperturbative eikonal approach and sum up infinite series of thermal self-energy terms. We show that $A_k\left(\omega \right)$ shows peak/dip/hump features at $T<T_N$, with the peak position at $\mathrm{\Delta }\left(T\right)$ and hump position at $\mathrm{\Delta }(T=0)$. Above $T_N$, the hump survives up to $T=T_p>T_N$, and in between $T_N$ and $T_p$ the spectral function displays the pseudogap behavior. We show that the difference between $T_p$ and $T_N$ is controlled by the ratio of in-plane and out-of-plane static spin susceptibilities, which determines the combinatoric factors in the diagrammatic series for the self-energy. For certain values of this ratio, $T_p=T_N$, i.e., the pseudogap region collapses. In this last case, thermal fluctuations are logarithmically singular, yet they do not give rise to pseudogap behavior. Our computational method can be used to study pseudogap physics due to thermal fluctuations in other systems.

Site-occupation embedding theory (SOET) [B. Senjean et al., Phys. Rev. B 97, 235105 (2018)] is an in-principle exact embedding method combining wave-function theory and density functional theory that gave promising results when applied to the one-dimensional Hubbard model. Despite its overall good performance, SOET faces a computational cost problem as its auxiliary impurity-interacting system remains the size of the full system (which is problematic as the computational cost increases exponentially with system size). In this work, this issue is circumvented by employing the Schmidt decomposition, thus leading to a drastic reduction of the computational cost while retaining the same accuracy. We show that this projected version of SOET (P-SOET) is competitive with other embedding techniques such as density matrix embedding theory (DMET) [G. Knizia and G. K.-L. Chan, Phys. Rev. Lett. 109, 186404 (2012)]. In contrast to the latter, density functional contributions come naturally into play in P-SOET's framework without any additional computational cost or double counting effect. As an important result, the density-driven Mott-Hubbard transition in one dimension (which is displayed by multiple impurity sites in DMET or in dynamical mean-field theory) is well described, for the first time, with a single impurity site.

The surface plasmon resonances of Au nanoparticles have attracted wide interest for the promising functionalities in nonlinear plasmonics and nanophotonics in recent decades. However, the third-order susceptibility ${\chi }^{\left(3\right)}$ of Au nanostructures, one of the crucial parameters to describe the nonlinear optical properties, had been experimentally described at only a few wavelengths. These fragmental results have precluded the understanding of the physical origins. Here, we have evaluated the real and imaginary components of ${\chi }^{\left(3\right)}$ in a broad range (1.50–3.10 eV) through a combined analysis of spectroscopic ellipsometry and pump-probe spectroscopy. The results show that the macroscopic and intrinsic ${\chi }^{\left(3\right)}$ are strongly wavelength dependent and consist of several successive peaks. Furthermore, by analyzing and comparing the Au thin film to nanoparticles, the contributions of the interband and intraband transitions in the Au nanoparticle are clarified.

We investigate nuclear spin hyperpolarization from optically polarized nitrogen vacancy centers in isotopically enriched diamonds with ${}^{13}\mathrm{C}$ concentrations up to 100%. ${}^{13}\mathrm{C}$ enrichment leads to a nitrogen vacancy electron spin resonance with a complex hyperfine structure and dynamic nuclear polarization enhancement profile. We show that strongly coupled ${}^{13}\mathrm{C}$ spins in the first shell surrounding a nitrogen vacancy center generate resolved hyperfine splittings, but do not act as an intermediary in the transfer of hyperpolarization of bulk nuclear spins. High levels of ${}^{13}\mathrm{C}$ enrichment are desirable to increase the efficiency of hyperpolarization for magnetic resonance signal enhancement, imaging contrast agents, and as a platform for quantum sensing and many-body physics.

In this paper we introduce an exactly solvable Kondo lattice model without any fine-tuning local gauge symmetry. This model describes itinerant electrons interplaying with a localized magnetic moment via only longitudinal Kondo exchange. Its solvability results from conservation of the localized moment at each site, and is valid for arbitrary lattice geometry and electron filling. A case study on a square lattice shows that the ground state is a Néel antiferromagnetic insulator at half-filling. At finite temperature, paramagnetic phases including a Mott insulator and correlated metal are found. The former is a melting antiferromagnetic insulator with a strong short-range magnetic fluctuation, while the latter corresponds to a Fermi liquidlike metal. Monte Carlo simulation and theoretical analysis demonstrate that the transition from paramagnetic phases into the antiferromagnetic insulator is a continuous two-dimensional Ising transition. Away from half-filling, patterns of spin stripes (inhomogeneous magnetic order) at weak coupling, and phase separation at strong coupling are predicted. With established Ising antiferromagnetism and spin stripe orders, our model may be relevant to a heavy fermion compound $\mathrm{CeCo}{\left({\mathrm{In}}_{1-x}{\mathrm{Hg}}_x\right)}_5$ and novel quantum liquid-crystal order in a hidden order compound ${\mathrm{URu}}_2{\mathrm{Si}}_2$.

Optical lattice experiments which probe the effect of disorder on superfluidity often use a speckle pattern for generating the disorder. Such speckle disorder is spatially correlated. While fermionic superfluidity in the presence of uncorrelated disorder is well studied, the impact of correlated disorder, particularly on thermal properties of the superfluid, is poorly understood. We provide a detailed study of the impact of speckle disorder, for varying speckle size and disorder magnitude, on the ground state and thermal properties of a Fermi superfluid. We work in the coupling regime of BCS-BEC crossover in a two-dimensional lattice. For a fixed disorder strength, an increase in speckle size leads to smoothening of the self-consistent background potential, increase in the critical disorder needed for a superfluid-insulator transition, and an increase in superfluid $T_c$. Along with these hints at decrease in effective disorder, speckle correlations also suppress the superfluid gap and the gap formation temperature—effects normally associated with increasing disorder. We correlate these effects with the effective potential and the single-particle localization effects in the ground state.