Typical Wannier-function downfolding starts with a mean-field or density functional set of bands to construct the Wannier functions. Here, we carry out a controlled approach, using density matrix renormalization group computed natural orbital bands, to downfold the three-band Hubbard model to an effective single-band model. A sharp drop-off in the natural orbital occupancy at the edge of the first band provides a clear justification for a single-band model. Constructing Wannier functions from the first band, we compute all possible two-particle terms and retain those with significant magnitude. The resulting single-band model includes two-site density-assisted hopping terms with $t_n\sim 0.6t$. These terms lead to a reduction of the ratio $U/t_{\mathrm{eff}}$, and are important in capturing the doping-asymmetric carrier mobility, as well as in enhancing the pairing in a single-band model for the hole-doped cuprates.

Polarization switching in ferroelectrics by applying external fields is a typical thermodynamic process. Heat dissipation plays an inevitable role, especially in ultrafast and nonequilibrium switching processes. Its role has long been neglected in existing theoretical models focusing on conservative actions. In this paper, a dynamic phase diagram is derived to consider the heat dissipation on equal footing with the work done by external field. Their competition leads to four switching mechanisms. The dynamic and thermodynamic analyses are then performed to verify the inevitable mechanisms dominated by heat dissipation. Our finding refreshes the understanding of polarization dynamics.

With rapid progress, the current study of topological properties in condensed matter systems has been further extended from the electronic scope to the phononic perspective. Based on first-principles calculations, we present a systematic investigation on topological phononic states in a series of biatomic alkali-metal sulfides AB $(\mathrm{A}=\mathrm{Na},\mathrm{K}; \mathrm{B}=\mathrm{S},\mathrm{Se},\mathrm{Te})$. Taking NaSe as an example, clear topological features of the nodal line and nodal surface states are revealed, and these two states, originating from the same two bands, concatenate together into a cladded nodal frame structure. Detailed symmetry arguments and effective Hamiltonian analysis show that the occurrence of the cladded nodal frame structure is determined by the combination of both symmorphic and nonsymmorphic symmetries. The phonon surface spectrum is calculated on the (010) surface and clean surface states are observed, which not only confirms their nontrivial topological feature, but also benefits their experimental verification. This cladded nodal frame structure can provide an ideal platform to examine the exotic topological phononic states and, with the present material candidates, especially those that have been synthesized, the corresponding experimental studies can be immediately advanced.

The coexistence of a left-handed polarized spin wave (LPSW) and a right-handed polarized spin wave (RPSW) in antiferromagnets provides a scheme for designing novel magnonic devices. To do so, it is necessary to manipulate the attenuation and the polarization of the LPSW and RPSW separately. We address this issue by studying theoretically the effect of spin-transfer torques (STTs) and an external magnetic field on spin waves (SWs) propagating in antiferromagnets. We find that Slonczewski-type STT can increase the attenuation length of one of the SW modes and decrease that of the other, while Zhang-Li type STT enhances (or weakens) the attenuation of both the LPSW and the RPSW simultaneously. Using the combined effect of the two kinds of STTs or the STT and an external magnetic field, we demonstrate control of the LPSW and RPSW attenuations separately, thus we can manipulate the SW polarization. We further suggest a magnon spin filter that can filter out one of the SW modes and preserve the other, which generates a pure magnonic spin current in antiferromagnets.

Recent years have seen a number of instances where magnetism and superconductivity intrinsically coexist. Our focus is on the case where spin-triplet superconductivity arises out of ferromagnetism, and we make a hydrodynamic analysis of the effect of a charge supercurrent on magnetic topological defects like domain walls and merons. We find that the emergent electromagnetic field that arises out of the superconducting order parameter provides a description for not only the physical quantities such as the local energy flux density and the interaction between current and defects but also the energy dissipation through magnetic dynamics of the Gilbert damping, which becomes more prominent compared to the normal state as superconductivity attenuates the energy dissipation through the charge sector. In particular, we reveal that the current-induced dynamics of domain walls and merons in the presence of the Gilbert damping give rise to the nonsingular $4\pi$ and $2\pi$ phase slips, respectively, revealing the intertwined dynamics of spin and charge degrees of freedom in ferromagnetic superconductors.

One of the strongest justifications for the continued search for superconductivity within the single-band Hubbard Hamiltonian originates from the apparent success of single-band ladder-based theories in predicting the occurrence of superconductivity in the cuprate coupled-ladder compound ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$. Recent theoretical works have, however, shown the complete absence of quasi-long-range superconducting correlations within the hole-doped multiband ladder Hamiltonian including realistic Coulomb repulsion between holes on oxygen sites and oxygen-oxygen hole hopping. Experimentally, superconductivity in ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ occurs only under pressure and is preceded by dramatic transition from one to two dimensions that remains not understood. We show that understanding the dimensional crossover requires adopting a valence transition model within which there occurs transition in Cu-ion ionicity from $+2$ to $+1$, with transfer of holes from Cu to O ions [S. Mazumdar, Phys. Rev. B 98, 205153 (2018)]. The driving force behind the valence transition is the closed-shell electron configuration of ${\mathrm{Cu}}^{1+}$, a feature shared by cations of all oxides with a negative charge-transfer gap. We make a falsifiable experimental prediction for ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ and discuss the implications of our results for layered two-dimensional cuprates.

Magnetic chains on superconductors hosting Majorana zero modes (MZMs) have attracted a great deal of interest due to their possible applications in fault-tolerant quantum computing. However, this is hindered by the lack of a detailed, quantitative understanding of these systems. As a significant step forward, we present a first-principles computational approach based on a microscopic relativistic theory of inhomogeneous superconductors applied to an iron chain on the top of Au-covered Nb(110) to study the Shiba band structure and the topological nature of the edge states. Contrary to contemporary considerations, our method enables the introduction of quantities indicating band inversion without fitting parameters in realistic experimental settings, holding thus the power to determine the topological nature of zero-energy edge states in an accurate ab initio based description of the experimental systems. We confirm that ferromagnetic Fe chains on an Au/Nb(110) surface do not support any separated MZM; however, a broad range of spin-spirals can be identified with robust zero-energy edge states displaying signatures of MZMs. For these spirals, we explore the structure of the superconducting order parameter, shedding light on the internally antisymmetric triplet pairing hosted by MZMs. We also reveal a twofold effect of spin-orbit coupling: although it tends to enlarge the topological phase regarding spin spiraling angles, it also extends the localization of MZMs. Due to the presented predictive power, our work fills a big gap between experimental efforts and theoretical models while paving the way for engineering platforms for topological quantum computation.

Recent advances in electron spin resonance techniques have allowed the manipulation of the spin of individual atoms, making magnetic atomic chains on superconducting hosts one of the most promising platform where topological superconductivity can be engineered. Motivated by this progress, we provide a detailed, quantitative description of the effects of manipulating spins in realistic nanowires by applying a first-principles-based computational approach to a recent experiment: an iron chain deposited on top of an Au/Nb heterostructure. As a continuation of the preceding paper, experimentally relevant computational experiments are performed in spin spiral chains that shed light on several concerns about practical applications and add new aspects to the interpretation of recent experiments. We explore the stability of topological zero-energy states, the formation and distinction of topologically trivial and nontrivial zero energy edge states, the effect of local changes in the exchange fields, the emergence of topological fragmentation, and the shift of Majorana zero modes along the superconducting nanowires, opening avenues toward the implementation of a braiding operation.

A nonperturbative dynamical coupling approach based on tight-binding molecular dynamics is used to evaluate the electron-ion (electron-phonon) coupling parameter in irradiated semiconductors as a function of the electronic temperature up to $\sim 25000\mathrm{K}$. The method accounts for arbitrary electronic distribution function via the Boltzmann equation, enabling a comparative analysis of various models: fully equilibrium electronic distribution, band-resolved local equilibria (distinct temperatures and chemical potentials of electrons in the valence and the conduction band), and a full nonequilibrium distribution. It is demonstrated that the nonequilibrium produces the electron-phonon coupling parameter different by at most $\sim 35\%$ from its equilibrium counterpart for identical deposited energy density, allowing us to use the coupling parameter as a function of the single electronic equivalent (or kinetic) temperature. The following 14 semiconductors are studied here: group IV: Si, Ge, SiC; group III–V: AlAs, AlP, GaP, GaAs, GaSb; oxides: ZnO, $\mathrm{Ti}{\mathrm{O}}_2, {\mathrm{Cu}}_2\mathrm{O}$; layered $\mathrm{Pb}{\mathrm{I}}_2$; ZnS and ${\mathrm{B}}_4\mathrm{C}$.

The spin-chain compound ${\mathrm{Sr}}_3{\mathrm{NiPtO}}_6$ is known to have a nonmagnetic ground state. We investigate the nature of the ground state of ${\mathrm{Sr}}_3{\mathrm{NiPtO}}_6$ using the magnetic susceptibility $\chi \left(T\right)$, heat capacity $C_{\mathrm{p}}\left(T\right)$, muon spin relaxation ($\mu \mathrm{SR}$), and inelastic neutron scattering (INS) measurements. $\chi \left(T\right)$ and $C_{\mathrm{p}}\left(T\right)$ do not exhibit any pronounced anomaly that can be associated with a phase transition to a magnetically ordered state. Our $\mu \mathrm{SR}$ data confirm the absence of long-range magnetic ordering down to 0.04 K. Furthermore, the muon spin relaxation rate increases below 20 K and exhibits temperature-independent behavior at low temperature, very similar to that observed in a quantum spin-liquid system. The INS data show a large excitation near 8 meV, and the analysis of the INS data reveals a singlet crystal-electric-field (CEF) ground state with a first excited CEF doublet state at ${\mathrm{\Delta }}_{\mathrm{CEF}}=7.7$ meV. The estimated CEF parameters reveal strong planar anisotropy in the calculated $\chi \left(T\right)$, consistent with the reported behavior of $\chi \left(T\right)$ of single-crystal ${\mathrm{Sr}}_3{\mathrm{NiPtO}}_6$. We propose that the nonmagnetic singlet ground state and a large ${\mathrm{\Delta }}_{\mathrm{CEF}}$ (much larger than the exchange interaction ${\mathcal{J}}_{\mathrm{ex}}$) are responsible for the absence of long-range magnetic ordering and can mimic a classical spin-liquid behavior in this quasi-one-dimensional spin-chain system ${\mathrm{Sr}}_3{\mathrm{NiPtO}}_6$. The classical spin-liquid ground state observed in ${\mathrm{Sr}}_3{\mathrm{NiPtO}}_6$ is due to the single-ion property, which is different from the quantum spin-liquid ground state observed in geometrically frustrated systems, in which two-ion exchanges play an important role.

We present a detailed investigation of the stoichiometric ${\mathrm{CeRh}}_4{\mathrm{Al}}_{15}$ single-crystal compound using the temperature dependence of the heat capacity $C_{\text{P}}(T$), electrical resistivity $\rho (T$), magnetic susceptibility $\chi (T$), and magnetization $M(H$) measurements for a magnetic field $H$ applied in the basal plane and along the $c$ axis. The low-temperature power-law behavior of $C/T\propto \chi \propto T^{-1+\alpha }$, the scaling behaviors of field-dependent magnetization at different temperatures up to 50 K and temperature-dependent heat capacity at various fields up to 9 T, and the $T$-linear resistivity are found to be consistent with the formation of quantum Griffiths singularities in the non-Fermi-liquid (NFL) regime. We further investigate the spin dynamics of a polycrystalline sample of ${\mathrm{CeRh}}_4{\mathrm{Al}}_{15}$, using zero-field (ZF) and longitudinal-field (LF) muon spin relaxation ($\mu \mathrm{SR}$) measurements. ZF-$\mu \mathrm{SR}$ measurements do not reveal any sign of long-range magnetic ordering down to 70 mK. The electronic relaxation rate $\lambda$ below 0.5 K increases rapidly and shows a thermal-activation-like characteristic $\left[T\log \right(\lambda )\sim T]$ over the entire measured temperature range between 70 mK and 4 K, indicating the presence of low-energy spin fluctuations in ${\mathrm{CeRh}}_4{\mathrm{Al}}_{15}$. LF-$\mu \mathrm{SR}$ measurements show a time-field ($t/H^{\eta }$) scaling of the $\mu \mathrm{SR}$ asymmetry, indicating the quantum critical behavior of this compound. Furthermore, an inelastic neutron scattering study of the polycrystalline sample reveals two crystal field excitations near 19 and 33 meV. These features collectively provide strong evidence of NFL behavior in ${\mathrm{CeRh}}_4{\mathrm{Al}}_{15}$ due to the formation of the Griffiths phase close to a $T\to$ 0 K quantum critical point.

Near-infrared phosphors based on ${\mathrm{Cr}}^{3+}$ have a wide range of important applications. The design and optimization of these phosphors require accurate determination of site occupancy and prediction of the luminescence mechanism of ${\mathrm{Cr}}^{3+}$ in solids. The former can typically be achieved through first-principles calculations of formation energies, while the latter can be accomplished through first-principles calculations aided by ligand field (LF) analyses. This study shows that it is possible to obtain all the information needed to construct adiabatic potential energy surfaces, including LF parameters and orbitals, the energies and equilibrium structures of certain states, and electron-phonon parameters, through calculation. The reliability of these results is confirmed by comparing them with experimental data. This improves the accuracy of the predictions for excitation, emission, and Stokes shift energies in the photoluminescence of ${\mathrm{Cr}}^{3+}$ ions. These calculations are then used to interpret the multipeak luminescence observed in Cr-doped La-gallogermanate systems. The results show that the ${\mathrm{Cr}}^{3+}$ ions located at a typical distorted-octahedral site in the host and another octahedral site similar to that in $\beta$–${\mathrm{Ga}}_2{\mathrm{O}}_3$ are responsible for producing the low and high bands in the luminescence spectra of various La-gallogermanates, respectively. This study demonstrates that combining first-principles calculations with LF analysis is an effective approach for obtaining the low excited states of $3d$ transition metal activators, which has important implications for the design and optimization of luminescent materials.

We propose a procedure that characterizes free-fermion or interacting classes of higher-order topological phases via their bulk entanglement structure. To this end, we construct nested entanglement Hamiltonians by first applying an entanglement cut to the ordinary many-body ground state, and then iterating the procedure by applying further entanglement cuts to the (assumed unique) ground state of the entanglement Hamiltonian. We argue that some classes of $n\mathrm{th}$-order topological phases can be characterized by the features of their $n\mathrm{th}$-order nested entanglement Hamiltonian, e.g., degeneracy in the entanglement spectrum. We explicitly compute nested entanglement spectra for a set of higher-order, free and interacting, fermionic and bosonic multipole and boundary obstructed phases and show that our method successfully identifies such phases.

Immersed in external magnetic fields $\left(B\right)$, buckled graphene constitutes an ideal tabletop setup, manifesting a confluence of time-reversal symmetry $\left(\mathcal{T}\right)$ breaking Abelian $\left(B\right)$ and $\mathcal{T}$-preserving strain-induced internal axial $\left(b\right)$ magnetic fields. In such a system, here we numerically compute two-terminal conductance $\left(G\right)$, and four- as well as six-terminal Hall conductivity $\left({\sigma }_{xy}\right)$ for spinless fermions. On a flat graphene $(b=0)$, the $B$ field produces quantized plateaus at $G=\pm |{\sigma }_{xy}|=(2n+1)e^2/h$, where $n=0,1,2,\cdots$. The strain-induced $b$ field lifts the twofold valley degeneracy of higher Landau levels and leads to the formation of additional even-integer plateaus at $G=\pm |{\sigma }_{xy}|=(2,4,\cdots )e^2/h$, when $B>b$. While the same sequence of plateaus is observed for $G$ when $b>B$, the numerical computation of ${\sigma }_{xy}$ in Hall bar geometries in this regime becomes unstable. A plateau at $G={\sigma }_{xy}=0$ always appears with the onset of a charge-density-wave order, causing a staggered pattern of fermionic density between two sublattices of the honeycomb lattice.

We investigate the spontaneous emission lifetime of a quantum emitter near a substrate coated with phosphorene under the influence of uniaxial strain. We consider both electric dipole and magnetic dipole-mediated spontaneous transitions from the excited to the ground state. The modeling of phosphorene is achieved by employing a tight-binding model that goes beyond the usual low-energy description. We demonstrate that both electric and magnetic decay rates can be widely tuned by the application of uniform strain, ranging from a near-total suppression of the Purcell effect to a remarkable enhancement of more than $1300\%$, all due to the high flexibility associated with the puckered lattice structure of phosphorene. We also unveil the use of strain as a mechanism to tailor the most probable decay pathways of the emitted quanta. Our results show that uniaxially strained phosphorene is an efficient and versatile material platform for the active control of light-matter interactions thanks to its extraordinary optomechanical properties.

We numerically study the Seebeck and Nernst effects of pseudospin-1 fermions in the $\alpha -T_3$ model under magnetic fields by combining the nonequilibrium Green's function and the Landauer-Büttiker formalism with the Stréda formula. In the $\alpha =0$ limit under strong magnetic fields, our results are in good agreement with theoretical results of graphene. Distinguishing from the case of $\alpha =0$, we find three characteristic features and discuss them in detail analytically for $\alpha >0$: (i) The inverse of the peak height of the Seebeck coefficient is $\frac{-e}{ln2k_B}[n_p+sign\left(n_p\right)\delta ]$, with $\delta =1/2$ for $\alpha >0$ while $\delta =0$ for $\alpha =0$, where $n_p$ denotes the $n_p\mathrm{th}$ peak near the Dirac point. We show that $\delta$ is determined by the Hall plateau series. (ii) The highest peak of the Seebeck coefficient is much higher than other peaks. This highest peak is ascribed to the vanishing of the Hall conductivity due to the energy gap between the flat band and lowest Landau level. (iii) There exists a negative peak near the Dirac point. We relate this unique behavior to the vanishing of the Hall conductivity near the Dirac point and the isolated flat band. Meanwhile, the peak of the Nernst coefficient for $\alpha =0$ splits into double peaks by increasing $\alpha$ and the height is enhanced. Additionally, the effects of temperatures, disorders, the strength of magnetic field, and edge patterns are also investigated. These findings provide theoretical foundation for future experimental studies on the thermoelectric properties based on the $\alpha -T_3$ materials.

A main source of capacity fading in lithium-ion batteries is the degradation of the active cathode materials caused by the series of volume changes during charge and discharge cycles. The quaternary colquiriite-type fluorides ${\mathrm{Li}}_x{\mathrm{CaFeF}}_6$ and ${\mathrm{Li}}_x{\mathrm{CaCoF}}_6$ were reported to have negligible volume changes in specific Li concentration ranges, making the underlying colquiriite structure a promising candidate for so-called zero-strain behavior. Using first-principles electronic structure calculations based on density functional theory with a Hubbard-$U$ correlation correction on the transition-metal ions, we systematically investigate the equilibrium volumes of the colquiriite-type fluorides ${\mathrm{Li}}_x{\mathrm{Ca}\mathit{M}\mathrm{F}}_6$ with $M=\mathrm{Ti}$, V, Cr, Mn, Fe, Co, and Ni at the Li concentrations $x=0$, 1, and 2. We elucidate the connection between the total volume of the structures and the local volumes of fluorine coordinated octahedra around the cations, and we find trends along the series of the 3d transition-metal elements. In the lithiation step from $x=1$ to $x=2$ we find volume changes of about 10%, and we discuss the discrepancy to the experimentally reported smaller value for ${\mathrm{Li}}_x{\mathrm{CaFeF}}_6$. The suitability as cathode material was furhter investigated by calculating the theoretical voltages and capacities. From $x=0$ to $x=1$ we describe the compensating structural mechanisms that lead to an exceptionally small volume change of ${\mathrm{Li}}_x{\mathrm{CaMnF}}_6$, which posseses a high theoretical voltage and moderate capacity. This compound is therefore a particularly promising zero-strain cathode material.

The recent discovery of pressure-induced superconductivity in the bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ (LNO) has opened a new platform for the study of unconventional superconductors. In this publication, we investigate theoretically the whole family of bilayer 327-type nickelates $R_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ ($R$ = rare-earth elements) under pressure. From La to Lu, the lattice constants and volume decrease, leading to enhanced in-plane and out-of-plane hoppings, resulting in an effectively reduced electronic correlation $U/W$. Furthermore, the Ni's $t_{2g}$ states shift away from the $e_g$ states, while the crystal-field splitting between $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ is almost unchanged. In addition, six candidates were found to become stable in the $Fmmm$ phase, with increasing values of critical pressure as the atomic number increases. Similar to the case of LNO, the $s_{\pm }$-wave pairing tendency dominates in all candidates, due to the nesting between the $\mathbf{\text{M}}=(\pi ,\pi )$ and the $\mathbf{\text{X}}=(\pi ,0)$ and $\mathbf{\text{Y}}=(0,\pi )$ points in the Brillouin zone. Then, $T_c$ is expected to decrease as the radius of rare-earth (RE) ions decreases. Our results suggest that LNO is already the “optimal” candidate, with Ce a close competitor, among the whole of the RE bilayer nickelates, and to increase $T_c$ we suggest growing on special substrates with larger in-plane lattice spacings.