Correlations between different regions of a quantum many-body system can be quantified through measures based on entropies of (reduced) subsystem states. For closed systems, several analytical and numerical tools, e.g., hydrodynamic theories or tensor networks, can accurately capture the time evolution of subsystem entropies, thus allowing for a profound understanding of the unitary dynamics of quantum correlations. However, these methods either cannot be applied to open quantum systems or do not permit an efficient computation of quantum entropies for mixed states. Here we make progress in solving this issue by developing a dissipative quasiparticle picture—describing quantum entropies and the mutual information in the limit of large space-time coordinates with their ratio being fixed—and showing its validity for quadratic open quantum systems. Our results demonstrate that the open quantum many-body dynamics of correlations can be understood in terms of propagating (dissipative) quasiparticles.

Over the last decade, the interest in the spin-$1/2$ Heisenberg antiferromagnet (HAF) on the square kagome (also called shuriken) lattice has been growing as a model system of quantum magnetism with a quantum paramagnetic ground state, flat-band physics near the saturation field, and quantum scars. A further motivation to study this model comes from the recent discovery of a gapless spin liquid in the square kagome magnet $\mathrm{K}{\mathrm{Cu}}_6\mathrm{Al}\mathrm{Bi}{\mathrm{O}}_4{\left(\mathrm{S}{\mathrm{O}}_4\right)}_5\mathrm{Cl}$ [M. Fujihala et al., Nat. Commun. 11, 3429 (2020)]. Here, we present large-scale numerical investigations of the specific heat $C\left(T\right)$, the entropy $S\left(T\right),$ as well as the susceptibility $\chi \left(T\right)$ by means of the finite-temperature Lanczos method for system sizes of $N=18,24,30,36,42,48$, and $N=54$. We find that the specific heat exhibits a low-temperature shoulder below the major maximum which can be attributed to low-lying singlet excitations filling the singlet-triplet gap, which is significantly larger than the singlet-singlet gap. This observation is further supported by the behavior of the entropy $S\left(T\right)$, where a change in curvature is present just at about $T/J=0.2$, the same temperature where the shoulder in $C$ sets in. For the susceptibility the low-lying singlet excitations are irrelevant, and the singlet-triplet gap leads to an exponentially activated low-temperature behavior. The maximum in $\chi \left(T\right)$ is found at a pretty low temperature $T_{\max }/J=0.146$ (for $N=42$) compared to $T_{\max }/J=0.935$ for the unfrustrated square-lattice HAF signaling the crucial role of frustration also for the susceptibility. We find a striking similarity of our square kagome data with the corresponding ones for the kagome HAF down to very low $T$. The magnetization process featuring plateaus and jumps and the field dependence of the specific heat that exhibits characteristic peculiarities attributed to the existence of a flat one-magnon band are discussed as well.

Elastic constants, thermal expansion, magnetostriction, and heat capacity measurements were performed with and without applied magnetic field on a single crystal of ${\mathrm{UIrSi}}_3$. The elastic properties were interpreted within the theory of the strain-exchange effect. The exchange-striction model together with the Ising model for the behavior of magnetic localized $5f$ electrons and itinerant electrons of uranium correctly reproduces the main features of our magneto-acoustic experiments in ${\mathrm{UIrSi}}_3$. Data on thermal expansion and magnetostriction confirm the conclusion that the dominant contribution of the measured temperature and field change in the speed of sound comes from the change in the elastic modulus itself. Based on the analysis of heat capacity measurements in the magnetic field, we explain the significant anomalies at the second-order branch of the phase transition boundary as a manifestation of the tricritical fluctuations, dominating the region below the tricritical point. The outcome confirms the three-dimensional Ising model, at zero and low magnetic fields, with a crossover to the mean-field tricritical behavior at fields close to the tricritical point, where tricritical fluctuations dominate the temperature evolution of the given property.

FeSe undergoes a transition from a tetragonal to a slightly orthorhombic phase at 90 K and becomes a superconductor below 8 K. The orthorhombic phase is sometimes called a nematic phase because quantum oscillation, neutron, and other measurements detect a significant asymmetry in $x$ and $y$. How nematicity affects superconductivity has recently become a matter of intense speculation. Here, we employ an advanced ab initio Green's function description of superconductivity and show that bulk tetragonal FeSe would, in principle, superconduct with almost the same critical temperature $T_c$ as the nematic phase. The mechanism driving the observed nematicity is not yet understood. Since the present theory underestimates it, we simulate the full nematic asymmetry by artificially enhancing the orthorhombic distortion. For benchmarking, we compare theoretical spin susceptibilities against experimentally observed data over all energies and relevant momenta. When the orthorhombic distortion is adjusted to correlate with observed nematicity in spin susceptibility, the enhanced nematicity causes spectral weight redistribution in the $\mathrm{Fe}\text{−}3d_{xz}$ and $\mathrm{Fe}\text{−}{d}_{yz}$ orbitals, but it leads to at most a $10–15\%$ increment in $T_c$. This is because the $d_{xy}$ orbital always remains the most strongly correlated and provides most of the source of the superconducting glue. Nematicity suppresses the density of states at the Fermi level; nevertheless, $T_c$ increases, in contradiction to both BCS theory and the theory of Bose-Einstein condensation. We show how the increase is connected to the structure of the particle-particle vertex. Our results suggest that while nematicity may be an intrinsic property of bulk FeSe, it is not the primary force driving the superconducting pairing.

We discuss the interplay between many-body localization and spin symmetry. To this end, we study the time evolution of several observables in the anisotropic $t\text{−}J$ model. Like the Hubbard chain, the studied model contains charge and spin degrees of freedom, yet it has smaller Hilbert space and thus allows for numerical studies of larger systems. We compare the field disorder that breaks the ${\mathbb{Z}}_2$ spin symmetry and a potential disorder that preserves the latter symmetry. In the former case, sufficiently strong disorder leads to localization of all studied observables, at least for the studied system sizes. However, in the case of symmetry-preserving disorder, we observe that odd operators under the ${\mathbb{Z}}_2$ spin transformation relax towards the equilibrium value at relatively short timescales that grow only polynomially with the disorder strength. On the other hand, the dynamics of even operators and the level statistics within each symmetry sector are consistent with localization. Our results indicate that localization exists within each symmetry sector for symmetry-preserving disorder. Odd operators' apparent relaxation is due to their time evolution between distinct symmetry sectors.

${\mathrm{NaOsO}}_3$ has recently attracted significant attention for the strong coupling between its electronic band structure and magnetic ordering. Here, we used time-resolved magnetic x-ray diffraction to determine the timescale of the photoinduced antiferromagnetic dynamics in ${\mathrm{NaOsO}}_3$. Our measurements are consistent with a sub-100 fs melting of the antiferromagnetic long-range order that occurs significantly faster than the lattice dynamics as monitored by the transient change in intensity of selected Bragg structural reflections, which instead show a decrease of intensity on a timescale of several ps.

We propose a nonperturbative ab initio approach to calculate the electrical conductivity of a liquid metal. Our approach is based on the Kubo formula and the theory of electron-phonon coupling (EPC) and, unlike the conventional empirical approach based on the Kubo-Greenwood formula, fully takes into account the effect of coupling between electrons and moving ions. We show that the electrical conductivity at high temperature is determined by an EPC parameter ${\lambda }_{\mathrm{tr}}$, which can be inferred, nonperturbatively, from the correlation of electron scattering matrices induced by ions. The latter can be evaluated in a molecular dynamics simulation. Based on the density-functional theory and pseudopotential methods, we implement the approach in an ab initio manner. We apply it to liquid sodium and obtain results in good agreement with experiments. This approach is efficient and based on a rigorous theory suitable for applying to general metallic liquid systems.

Bulk hybride halide perovskites are governed by significant Rashba and Dresselhaus splitting. This indicates that such effects will not only affect their optoelectronic properties, but also those of their two-dimensional layered relatives. This work aims at understanding how different ways of symmetry breaking influence these effects in those materials. For this purpose, model structures are adopted where the organic compounds are replaced by Cs atoms. Disregarding possible distortions in the inorganic layers, results in structures with composition ${\mathrm{Cs}}_{n+1}{\mathrm{Pb}}_n{\mathrm{I}}_{3n+1}$. Using the all-electron full-potential density-functional-theory code exciting, the impact of atomic displacement on the band structure is systematically studied for $n=1$, 2, 3, and $\infty$. The displacement patterns that yield Rashba or Dresselhaus splitting are identified, and the amount of the splitting is determined as a function of displacement. Furthermore, the spin textures in the electronic states around the band gap are analyzed to differentiate between Rashba and Dresselhaus effects. This study reveals in-plane Pb displacements as the origin of the strongest effects.

As a tunable spintronic device, the bipolar magnetic semiconductor (BMS) has been intensively studied in recent years, but the candidate materials are limited in the magnetic system. In this work, we report an alternative strategy to realize it in a nonmagnetic system called bipolar semiconductor (BS). Utilizing doping-induced ferromagnetism, the enantiomorphic flat bands in BS are used to switch the spin carriers, achieving the same function as the BMS. Based on the frontier orbital of the molecular building blocks, a universal approach is proposed to search the BS in two-dimensional (2D) covalent organic frameworks (COFs), which is confirmed by first-principles calculations in the experimentally synthesized fluoride-graphdiyne (F-GDY). Moreover, the spin-polarized channels and giant anomalous Hall effect can also be created in F-GDY through the light irradiation. Our results demonstrate a new spintronic application for 2D COFs without the intrinsic magnetism.

Conditional-probability density functional theory (CP-DFT) is a formally exact method for finding correlation energies from Kohn-Sham DFT without evaluating an explicit energy functional. We present details on how to generate accurate exchange-correlation energies for the ground-state uniform gas. We also use the exchange hole in a CP antiparallel spin calculation to extract the high-density limit. We give a highly accurate analytic solution to the Thomas-Fermi model for this problem, showing its performance relative to Kohn-Sham and it may be useful at high temperatures. We explore several approximations to the CP potential. Results are compared to accurate parametrizations for both exchange-correlation energies and holes.

In this paper we present the exact representation of a fully correlated electronic wavefunction as the single-particle basis approaches completeness. It consists of a half-infinite chain of matrices of exponentially increasing size. The complete basis limit is illustrated numerically using the density-matrix renormalization-group method by computing the core-valence entanglement in the ${\mathrm{C}}_2$ ground state in increasing subsets of cc-pVTZ and pVQZ bases until convergence is reached.

We investigate the ground-state properties of the fractional quantum Hall effect at the filling factor $\nu =2/3$ and $2+2/3$, with a special focus on their typical edge physics. Via a topological characterization scheme in the framework of the density matrix renormalization group, the nature of the $\nu =2/3$ and $2+2/3$ states is identified as an Abelian hole-type Laughlin state, as evidenced by the fingerprint of entanglement spectra, central charge, and topological spin. Crucially, by constructing an interface between the $2/3$ $(2+2/3)$ state and different integer quantum Hall states, we study the structures of the interfaces from many aspects, including charge density and dipole moment. In particular, we demonstrate the edge reconstruction by visualizing edge channels comprised of two groups: the outermost $1/3$ channel and the inner composite channel made of a charged mode and neutral modes.

We consider the Aharonov-Bohm (AB) effect on a confined electron ground state in a quantum ring defined electrostatically within the phosphorene monolayer. The strong anisotropy of effective masses in phosphorene quenches ground-state oscillations for a circular ring because of interrupted persistent current circulation around the ring. An elliptic deformation of the confinement potential can compensate for the anisotropy of the effective masses and produce ground-state parity transformations with the AB periodicity. Moreover, a specific ratio of the semiaxes is determined for which the spectrum becomes identical to that of a circular quantum ring and an isotropic effective mass. We identify a generalized angular momentum operator which commutes with the continuum Hamiltonian for the chosen ratio of the semiaxes that closes the avoided crossings of energy levels for states of the same parity and spin. Ground-state oscillations for the two-electron ground state are also discussed.

Strain engineering of epitaxial transition metal oxide heterostructures offers an intriguing opportunity to control electronic structures by modifying the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, we demonstrate that the electronic structure, magnetic and transport properties of ${\mathrm{La}}_{0.9}{\mathrm{Ba}}_{0.1}{\mathrm{MnO}}_3$ thin films can be effectively controlled by epitaxial strain. Spectroscopic studies and first-principles calculations reveal that the orbital occupancy in Mn $e_g$ orbitals can be switched from the $d_{3z^2-r^2}$ orbital to the $d_{x^2-y^2}$ orbital by varying the strain from compressive to tensile. The change of orbital occupancy associated with Mn $3d$-O $2p$ hybridization leads to dramatic modulation of the magnetic and electronic properties of strained ${\mathrm{La}}_{0.9}{\mathrm{Ba}}_{0.1}{\mathrm{MnO}}_3$ thin films. Under moderate tensile strain, an emergent ferromagnetic insulating state with an enhanced ferromagnetic Curie temperature of 215 K is achieved. These findings not only deepen our understanding of electronic structures, magnetic and transport properties in the ${\mathrm{La}}_{0.9}{\mathrm{Ba}}_{0.1}{\mathrm{MnO}}_3$ system, but also demonstrate the use of epitaxial strain as an effective knob to tune the electronic structures and related physical properties for potential spintronic device applications.

We study the energy spectra of Dirac fermions confined in the quantum dot formed in the graphene/transition metal dichalcogenides (TMD) system under a perpendicular magnetic field. The obtained spin-valley polarized spectra can be classified into three types according to the values of the energy gap, which strongly depends on the pseudospin potential and the valley-dependent spin-orbit coupling peculiar to the electronic structure of the graphene/TMD system. For the first case, the electrons for both valleys and both spins possess linear dispersion. The energy spectra of the quantum dot exhibit sudden jumps of levels due to the Berry phase jumps. For the second case, the electrons for the $K\left(K^{\prime }\right)$ valley with up (down) spin have linear dispersion while the electrons for the $K\left(K^{\prime }\right)$ valley with down (up) spin have parabolic dispersion. The sudden jumps and the degeneracy-splitting-recombination behaviors of levels coexist in the energy spectra. The latter behavior is a result of the continuous change of the Berry phase. For the third case, the electrons for both valleys and both spins have parabolic dispersion. The degeneracy-splitting-recombination behaviors make the energy spectra both spin and valley distinguished. In addition, the evolutions of the levels from the sudden jumps to the continuous changes by adjusting the valley-dependent spin-orbit coupling and the pseudospin potential are also investigated. These results provide the possibility to control the specific spin-valley freedom of electrons in the graphene/TMD system using the quantum dot setup.

We study the pressure-induced structural transformation of ${\mathrm{Ge}}_{136}$ clathrate by ab initio molecular dynamics and metadynamics. The system under pressure first undergoes amorphization followed by an ultrafast recrystallization to the $\beta$-tin structure on the time scale of 30 ps. The initial pressure-induced amorphization of clathrate is triggered by high pressure while the subsequent fast recrystallization to $\beta$-tin is driven by low temperature. Interestingly, the amorphous intermediate is still diffusive even at room temperature in spite of very strong undercooling, making the ultrafast recrystallization possible. The system provides an explicit example of structural transformation between two crystalline phases proceeding via noncrystalline intermediate. On fast decompression of the amorphous structure with incipient crystalline order, the recrystallization is blocked and the system instead proceeds to the tetrahedral low-density amorphous (LDA) phase.

We analyze the properties of magnons in metallic electron systems with spiral magnetic order. Our analysis is based on the random phase approximation for the susceptibilities of tight-binding electrons with a local Hubbard interaction in two or three dimensions. We identify three magnon branches from poles in the susceptibilities, one associated with in-plane, the other two associated with out-of-plane fluctuations of the spiral order parameter. We derive general expressions for the spin stiffnesses and the spectral weights of the magnon modes, from which the magnon velocities can also be obtained. Moreover, we determine the size of the decay rates of the magnons due to Landau damping. While the decay rate of the in-plane mode is of the order of its excitation energy, the decay rate of the out-of-plane mode is smaller so that these modes are asymptotically stable excitations even in the presence of Landau damping.

The high computational cost of first-principles electronic structure methods together with the successful applications of machine learning (ML) techniques in atomistic simulations resulted in a surge of interest in ML-based interatomic potentials. Despite great progress in the field, there remain some challenges to be solved such as the best way of incorporating long-range interactions, as well as nonlocal charge transfer. The first generation of the charge equilibration via neural network technique (CENT) was a major step forward in concurrently taking into account both aforementioned points. Within structure prediction methods, it turned out to be a powerful tool in discovering novel polymorphs of ionic systems. On the other hand, the method is not expected to be appropriate for multicomponent systems with reference data sets in which some or all elements are subject to varying oxidation states. Here, we present the second generation of CENT, with multiple improvements to the original variant that lead to a more accurate treatment of electrostatic interactions. To do this, it aims at reproducing the electric potential function, which is directly related to the charge distribution, rather than only considering total energies. In addition, a charge-free term is added to correct for the difference between the reference energies and those obtained with the energy functional of CENT. Moreover, the Green's function within the Hartree energy is modified to substantially shield interactions from charges in the neighborhood of each point. Also, the charge density is split into ionic and electronic parts, which allows for a better approximation of the electron density. The utility of this method is examined for magnesium oxide clusters, and multiple comparisons with the first generation are made, demonstrating that much more physical electrostatic interactions can be expected from the second generation of CENT.

Polarized inelastic neutron-scattering experiments have been performed to elucidate the anisotropic behavior of the low-energy spin-wave excitations in a multiferroic ${\mathrm{BiFeO}}_3$, which shows a cycloidal spin structure below 640 K. Using neutron polarization analysis for single magnetic domain crystals, magnetic excitation modes in and out of the cycloidal plane below 6 meV were separated successfully. The magnetic excitation spectra were analyzed using linear spin-wave theory. The low-energy magnon density of states consist of several magnon modes, including the two anisotropic modes, $\mathrm{\Phi }$ and $\mathrm{\Psi }$ modes, distributed in and out of the cycloidal plane, respectively, which were previously observed using optical spectroscopies. Furthermore, there are other magnon modes that are not active in optical measurements. A model spin Hamiltonian, which reproduces the spin-wave frequencies observed using optical spectroscopies, explains the overall spectra reasonably well.

Optical pump-probe experiments carried out in the time domain reveal both the intrinsic low-energy dynamics and its connections to higher-energy excitations in correlated electron systems. In this work, we propose two microscopic mechanisms for the optical generation of coherent magnetic modes in van der Waals magnets, and derive the corresponding effective light-spin interactions: either through pumping atomic orbital excitations resonantly or via a light-induced Floquet spin Hamiltonian, the ground state of the system is driven out of equilibrium. The subsequent long-time relaxational dynamics can then be probed using, e.g., the magneto-optical Kerr effect or transient grating spectroscopy. As an example, we apply our framework to ${\mathrm{NiPS}}_3$, which is magnetically ordered in the bulk, and is conjectured to realize the XY model in the monolayer limit. Our theory makes explicit how the material's low-energy response depends sensitively on the microscopic details of the light-spin coupling as well as pump fluence, frequency, and polarization. For the case of bulk ${\mathrm{NiPS}}_3$, we find quantitative agreement with recent experiments [D. Afanasiev et al., Sci. Adv. 7, eabf3096 (2021)]. We further propose pump-probe experiments for monolayer ${\mathrm{NiPS}}_3$ and detail how anomalous relaxational behavior may reveal excitations of a (proximate) Berezinskii-Kosterlitz-Thouless phase in a proposed effective XY model.