The ${\mathrm{Bi}}^{3+}$ ion is an excellent activator and sensitizer for luminescent materials. However, the complexity and variety of the ${\mathrm{Bi}}^{3+}$-related transitions bring a great challenge to the study of luminescence processes of ${\mathrm{Bi}}^{3+}$ doped materials. Here, we presented first-principles calculations to determine the excitation, relaxation, and emission processes of ${\mathrm{Bi}}^{3+}$ activated materials by using $\mathrm{Ca}M{\mathrm{O}}_3:{\mathrm{Bi}}^{3+}\left(M=\mathrm{Zr},\mathrm{Sn},\mathrm{Ti}\right)$ as prototype systems, where ${\mathrm{Bi}}^{3+}$ substitutes ${\mathrm{Ca}}^{2+}$ in similar coordinate environments but presents tremendously different excitation and emission spectra. The equilibrium geometric structures of excited states were calculated based on density-functional theory (DFT), with appropriately constraining the electron occupation and including the spin-orbit couplings. Then the hybrid DFT calculations were carried out to obtain the electronic structures and defect levels. Different metastable excited states and Stokes shift were obtained for $M=\mathrm{Zr}$, Sn, and Ti, which explain the remarkable differences in the measured emission spectra. The energies of three types of transitions are obtained from the calculations, including intra-${\mathrm{Bi}}^{3+}$ bands transition and charge transfer between ${\mathrm{Bi}}^{3+}$ ions and the band edges. This leads to a clear and reliable interpretation of all the excitation spectra in the series. The method and its applications to $\mathrm{Ca}M{\mathrm{O}}_3:{\mathrm{Bi}}^{3+}$ show the potential of first-principles calculations in analyzing and predicting luminescent properties of ${\mathrm{Bi}}^{3+}$ activated materials.

Based on local-density approximation plus dynamical mean-field theory ($\mathrm{LDA}+\mathrm{DMFT}$) calculations, we perform a comprehensive analysis of electronic structure reconstruction of the ThFeAsN superconductor, showing how the normal and $s$ wave superconducting spectra are reshaped by many-particle electron-electron interactions. Here, the ThFeAsN parent compound is described as an orbital-selective marginal Fermi liquid metal, with coexisting Fermi liquid quasiparticles and pseudogapped electronic states. Upon electron doping, an additional Kondo insulating state is predicted to exist in this system. Furthermore, we show how dynamical correlations strongly renormalize the bare Bogoliubov quasiparticles in the $s$ wave superconducting state into totally correlated lineshapes. These findings contribute to the microscopic understanding of the role played by dynamical multiorbital electronic correlations in the low energy spectrum relevant to unconventional Fe-based superconductors.

The dynamical stability of three-dimensional (3D) Lennard-Jones (LJ) crystals has been studied for many years. The fcc and hcp structures are dynamically stable, while the bcc structure is stable only for long-range LJ potentials that are characterized by relatively small integer pairs $(m,n)$. Here, we study the dynamical stability of two-dimensional (2D) LJ crystals, where the planar hexagonal, the buckled honeycomb, and the buckled square structures are assumed. We demonstrate that the stability property of 2D and 3D LJ crystals can be classified into four groups depending on $(m,n)$. The instabilities of the planar hexagonal, the buckled square, and the bcc structures are investigated within analytical expressions. The structure-stability relationship between the LJ crystals and the elemental metals in the periodic table is also discussed.

We develop a non-Markovian approach to study quantum dissipation and thermal transport in the nonequilibrium two-level system based on a canonical transformation and full-counting statistics. Specifically, we analytically study dissipative dynamics of population difference, quantum coherence, and heat current in terms of the canonical transformed Redfield approach. It demonstrates that the dynamics of both the population difference and the energy flow show monotonic decaying behaviors with the identical decay rate, whereas the quantum coherence is dominated by the oscillation showing the non-Markovian effect. Moreover, through tuning the temperature bias of thermal baths, negative differential thermal conductance is clearly exhibited beyond the weak system-bath coupling limit and Markovian approximation. The results may provide guidance for the efficient control of energy and the information transport in nanodevices.

As first recognized in 2010, epitaxial graphene on SiC(0001) provides a platform for quantized Hall resistance (QHR) metrology unmatched by other two-dimensional structures and materials. Here we report graphene parallel QHR arrays, with metrologically precise quantization near $1000\mathrm{\Omega }$. These arrays have tunable carrier densities, due to uniform epitaxial growth and chemical functionalization, allowing quantization at the robust $\nu =2$ filling factor in array devices at relative precision better than ${10}^{-8}$. Broad tunability of the carrier density also enables investigation of the $\nu =6$ plateau. Optimized networks of QHR devices described in this work suppress Ohmic contact resistance error using branched contacts and avoid crossover leakage with interconnections that are superconducting for quantizing magnetic fields up to 13.5 T. Our work enables more direct scaling of resistance for quantized values in arrays of arbitrary network geometry.

Asymmetric Weyl semimetals, which possess an inherently chiral structure, have different energies and dispersion relations for left- and right-handed fermions. They exhibit certain effects not found in symmetric Weyl semimetals, such as the quantized circular photogalvanic effect and the helical magnetic effect. In this work, we derive the conditions required for breaking chiral symmetry by applying an external field in symmetric Weyl semimetals. We explicitly demonstrate that in certain materials with the $T_d$ point group, magnetic fields along low symmetry directions break the symmetry between left- and right-handed fermions; the symmetry breaking can be tuned by changing the direction and magnitude of the magnetic field. In some cases, we find an imbalance between the number of type I left- and right-handed Weyl cones (which is compensated by the number of type II cones of each chirality).

Graphene magnetoplasmons, collective excitations of massless Dirac fermions, can be actively tuned by an external magnetic field in addition to electrostatic gating. Their magneto-optic properties are highly determined by the edges of the structure and their interaction. Here, we focus on the fundamental dipolar magnetoplasmon, which has strong optical activity, and report its extinction spectra in graphene circular rings, eccentric rings, and split rings. We find that, as compared to traditional graphene disks, circular rings still sustain symmetrical mode splitting (${\omega }_{+}$ and ${\omega }_{-}$) but with less energy difference $\mathrm{\Delta }\omega ={\omega }_{+}-{\omega }_{-}$. Increasing the ratio of inner radius to outer radius, $\mathrm{\Delta }\omega$ will decrease further and eventually vanish at a critical ratio, which can be well described as a function of the only product of cyclotron frequency and relaxation time. In eccentric rings, a small translation of the inner hole will not change the mode splitting and thus the value of $\mathrm{\Delta }\omega$, but for a larger translation, $\mathrm{\Delta }\omega$ increases dramatically, with one magnetoplasmonic mode (polarization dependent) getting dark gradually. In split rings, due to symmetry breaking abruptly, even a very tiny split will cause the disappearance of mode splitting. Our findings contribute to a basic understanding of graphene magnetoplasmons which, in turn, can boost the application of graphene magneto-optic devices at terahertz and infrared frequencies.

Narrow-gap semiconductors with strong spin-orbit coupling such as bismuth tellurohalides have become popular candidates for spintronic applications. But driving spin-polarized photocurrents in these materials with circularly polarized light requires picosecond lifetimes of the photoexcited carriers and low spin-flip scattering rates. In search of these essential ingredients, we conducted an extensive study of the carrier dynamics on the Te-terminated surface of BiTeI, which exhibits a giant Rashba splitting of both surface and bulk states. We observe a complex interplay of surface and bulk dynamics after photoexcitation. Carriers are rapidly rearranged in momentum space by quasielastic phonon and defect scattering, while a phonon bottleneck leads to a slow equilibration between bulk electrons and lattice. The particular band dispersion opens an inelastic decay channel for hot carriers in the form of plasmon excitations, which are immanent to Rashba-split systems. These ultrafast scattering processes effectively redistribute excited carriers in momentum and energy space and thereby inhibit spin-polarized photocurrents.

Ferroic systems under considerable geometrical restrictions at nanoscale have successfully introduced novel phases such as multiferroic and topological phases. However, ferroic orders completely disappear below the critical size limit of several nanometers and the geometry cannot be relied upon to produce a variety of phases. Here, via first-principles calculations, we demonstrate that a rich variety of phases and their transitions can be realized by dislocations in paraelectric ${\mathrm{SrTiO}}_3$. We show that atomic-scale ferroelectricity and (anti)ferromagnetism are induced by the strain concentration and nonstoichiometry intrinsic to dislocations in ${\mathrm{SrTiO}}_3$, resulting in ferroelectric-(anti)ferromagnetic-multiferroic phase transitions depending on the core structure. Furthermore, we also show that electrical polarization configurations strongly depend on the strain distribution around a dislocation and topological phases can be realized without geometrical restrictions. The present result suggests that the utilization of defects in a material is a powerful strategy to design ferroic orders below the critical size, thereby expanding the application of ferroic nanostructures to the atomic scale.

The spin-orbit entangled (SOE) $J_{\mathrm{eff}}$ state has been a fertile ground to study quantum phenomena. Contrary to the conventional weakly correlated $J_{\mathrm{eff}}=1/2$ state of $4d$ and $5d$ transition metal compounds, the ground state of $\mathrm{Cu}{\mathrm{Al}}_2{\mathrm{O}}_4$ hosts a $J_{\mathrm{eff}}=1/2$ state with a strong correlation of Coulomb $U$. Here, we report that, surprisingly, ${\mathrm{Cu}}^{2+}$ ions of $\mathrm{Cu}{\mathrm{Al}}_2{\mathrm{O}}_4$ overcome the otherwise usually strong Jahn-Teller distortion and instead stabilize the SOE state, although the cuprate has relatively small spin-orbit coupling. From the x-ray absorption spectroscopy and high-pressure x-ray diffraction studies, we obtained definite evidence of the $J_{\mathrm{eff}}=1/2$ state with a cubic lattice at ambient pressure. We also found the pressure-induced structural transition to a compressed tetragonal lattice consisting of the spin-only $S=1/2$ state for pressure $P_c>8\mathrm{GPa}$. This phase transition from the Mott insulating $J_{\mathrm{eff}}=1/2$ to the $S=1/2$ states is a unique phenomenon. Our study offers an example of the SOE $J_{\mathrm{eff}}$ state under strong electron correlation and its pressure-induced transition to the $S=1/2$ state.

We report a characterization of the high-pressure behavior of zinc iodate, $\mathrm{Zn}{\left(\mathrm{I}{\mathrm{O}}_3\right)}_2$. By the combination of x-ray diffraction, Raman spectroscopy, and first-principles calculations we have found evidence of two subtle isosymmetric structural phase transitions. We present arguments relating these transitions to a nonlinear behavior of phonons and changes induced by pressure on the coordination sphere of the iodine atoms. This fact is explained as a consequence of the formation of metavalent bonding at high pressure which is favored by the lone-electron pairs of iodine. In addition, the pressure dependence of unit-cell parameters, volume, and bond distances is reported. An equation of state to describe the pressure dependence of the volume is presented, indicating that $\mathrm{Zn}{\left(\mathrm{I}{\mathrm{O}}_3\right)}_2$ is the most compressible iodate among those studied up to now. Finally, phonon frequencies are reported together with their symmetry assignment and pressure dependence.

We investigate laser-induced torques in magnetically noncollinear ferromagnets with a spin-spiral magnetic structure using ab initio calculations. Since spin spirals may be used to approximate the magnetization gradients locally in domain walls and skyrmions, our method may be used to obtain the laser-induced torques in such objects from a multiscale approach. Employing the generalized Bloch theorem, we obtain the electronic structure computationally efficiently. We employ our method to assess the laser-induced torques in bcc Fe, hcp Co, and $L_{1}0$ FePt when a spin-spiral magnetic structure is imposed. We find that the laser-induced torques in these magnetically noncollinear systems may be orders of magnitude larger than those in the corresponding magnetically collinear systems and that they exist for both linearly and circularly polarized light. This result suggests that laser-induced torques driven by noncollinear magnetic order or by magnetic fluctuations may contribute significantly to processes in ultrafast magnetism.

We study the phase diagram of the ${\text{SO}}_q\left(3\right)$ quantum group invariant spin-1 bilinear-biquadratic spin chain for real values of $q>1$. Numerical computations suggest that the chain has at least three clearly distinguished phases: A chiral analog of the Haldane phase, a dimerized phase, and a ferromagnetic phase. In contrast, the counterpart of the extended critical region that is known to exist for $q=1$ remains elusive. Our results show that the Haldane phase fails to exhibit a twofold degeneracy in the entanglement spectrum but that the degeneracy is restored upon a suitable $q$ deformation of the entanglement Hamiltonian, which can be interpreted as a Zeeman field. The structure of the phase diagram is confirmed through analytical calculations in the extreme anisotropic limit $q\to \infty$. Our results suggest that symmetries of the form ${\mathcal{U}}_q\left[su\left(2\right)\right]$ for distinct choices of $q$ should be interpreted as one single family instead of separate symmetries when defining SPT phases, leading naturally to the notion of a $q$-SPT phase.

We present a study of the thermal Hall effect in the extended Heisenberg model with $XXZ$ anisotropy in the kagome lattice. This model has the particularity that, in the classical case, and for a broad region in parameter space, an external magnetic field induces a chiral symmetry breaking: the ground state is a doubly degenerate $q=0$ order with either positive or negative net chirality. Here, we focus on the effect of this chiral phase transition in the thermal Hall conductivity using linear-spin-waves theory. We explore the topology and calculate the Chern numbers of the magnonic bands, obtaining a variety of topological phase transitions. We also compute the magnonic effect to the critical temperature associated with the chiral phase transition ($T_c^{SW}$). Our main result is that, the thermal Hall conductivity, which is null for $T>T_c^{SW}$, becomes nonzero as a consequence of the spontaneous chiral symmetry breaking at low temperatures. Therefore, we present a simple model where it is possible to “switch” on/off the thermal transport properties introducing a magnetic field and heating or cooling the system.

The electromagnetic helicity of the free electromagnetic field and the static magnetic helicity are shown to be two different embodiments of the same physical quantity, the total helicity. The total helicity is the sum of two terms: a term that measures the difference between the number of left-handed and right-handed photons of the free field, and another term that measures the screwiness of the static magnetization density in matter. Each term is the manifestation of the total helicity in different frequency regimes: $\omega >0$ and $\omega =0$, respectively. This unification establishes the theoretical basis for studying the conversion between the two embodiments of total helicity upon light-matter interaction.

We derive a complete classification of Floquet phases of interacting bosons and fermions with $U\left(1\right)$ symmetry in two spatial dimensions. According to our classification, there is a one-to-one correspondence between these Floquet phases and rational functions $\pi \left(z\right)=a\left(z\right)/b\left(z\right)$, where $a\left(z\right)$ and $b\left(z\right)$ are polynomials obeying certain conditions and $z$ is a formal parameter. The physical meaning of $\pi \left(z\right)$ involves the stroboscopic edge dynamics of the corresponding Floquet system: in the case of bosonic systems, $\pi \left(z\right)=\frac{p}{q}·\tilde{\pi }\left(z\right)$, where $\frac{p}{q}$ is a rational number that characterizes the flow of quantum information at the edge during each driving period and $\tilde{\pi }\left(z\right)$ is a rational function which characterizes the flow of $U\left(1\right)$ charge at the edge. A similar decomposition exists in the fermionic case. We also show that $\tilde{\pi }\left(z\right)$ is directly related to the time-averaged $U\left(1\right)$ current that flows in a particular geometry. This $U\left(1\right)$ current is a generalization of the quantized current and quantized magnetization density found in previous studies of noninteracting fermionic Floquet phases.

Magnonic crystals offer a wide playground to study the emergent properties of spin waves, and ferromagnetic antidot lattices are leading candidates for magnonic devices due to the faster propagation of spin waves combined with wide-frequency tunability. Despite having a broad range of studies on periodic and quasiperiodic systems, a combination of quasiperiodic lattice with a complex basis is absent in the literature. The quasiperiodicity of octagonal lattice, along with a complex triangular antidot basis lacking reflection symmetry provides newer and richer spin-wave dynamics. Such complex magnonic crystal exhibits a strong eightfold anisotropy superposed with a weak threefold anisotropy. This is in contrast to a strong fourfold anisotropy superposed with a weak threefold anisotropy observed in a square lattice with triangular antidot basis. The spatial profiles of spin waves revealed the presence of resonant modes with both even and odd-mode quantization number, besides a mode conversion from extended to quantized mode with the systematic variation of the in-plane bias magnetic field orientation. These are in consonance with the strong anisotropic behavior of the spin-wave modes. The strong modifications of the asymmetric potential energy landscape in these magnonic crystals lead to the stark modulation of the rich spin-wave dynamics, thus opening avenues to reprogrammable magnonics with complex geometry.

The three-dimensional Edwards-Anderson spin-glass model presents strong spatial heterogeneities well characterized by the so-called backbone, a magnetic structure that arises as a consequence of the properties of the ground state and the low-excitation levels of such a frustrated Ising system. Using extensive Monte Carlo simulations and finite size scaling, we study how these heterogeneities affect the phase transition of the model. Although we do not detect any significant difference between the critical behavior displayed by the whole system and that observed inside and outside the backbone, surprisingly, a selective bond dilution of the complement of this magnetic structure induces a change of the universality class, whereas no change is noted when the backbone is fully diluted. This finding suggests that the region surrounding the backbone plays a more relevant role in determining the physical properties of the Edwards-Anderson spin-glass model than previously thought. Furthermore, we show that when a selective bond dilution changes the universality class of the phase transition, the ground state of the model does not undergo any change. The opposite case is also valid, i. e., a dilution that does not change the critical behavior significantly affects the fundamental level.

Identifying quantum phases and phase transitions is key to understanding complex phenomena in statistical physics. In this work, we propose an unconventional strategy to access quantum phases and phase transitions by visualization based on the distribution of ground states in Hilbert space. By mapping the quantum states in Hilbert space onto a two-dimensional feature space using an unsupervised machine learning method, distinct phases can be directly specified and quantum phase transitions can be well identified. Our proposal is benchmarked on gapped, critical, and topological phases in several strongly correlated spin systems. As this proposal directly learns quantum phases and phase transitions from the distributions of the quantum states, it does not require priori knowledge of order parameters of physical systems, which thus indicates a perceptual route to identify quantum phases and phase transitions particularly in complex systems by visualization through learning.