The low-energy properties of transition metal oxides (TMOs) are governed by the electrons occupying strongly correlated d-orbitals that are hybridized with surrounding ligand oxygen p orbitals to varying degrees. Their physics is thus established by a complex interplay between the transition-metal (TM)-ligand hopping t, charge transfer energy Delta(CT), and on-site TM Hubbard repulsion U. Here, we study the spectral properties of a one dimensional analog of such a pd system, with alternating TM d and ligand anion p orbitals situated along a chain. Using the density matrix renormalization group method, we study the model's single-particle spectral function, x-ray absorption spectrum, and dynamical spin structure factor as a function of Delta(CT) and U. In particular, we present results spanning from the Mott insulating (Delta(CT) > U) to negative charge transfer regime Delta(CT) < 0 to better understand the ground-state and momentum-resolved excited-state properties of these different regimes. Our results can guide new studies on TMOs that seek to situate them within the Mott-Hubbard/charge transfer insulator classification scheme.

We numerically investigate the telltale signs of pair-density-wave order (PDW) in the Kondo-Heisenberg chain by focusing on the momentum resolved spectrum in different parameter regimes. Density matrix renormalization group calculations reveal that this phase is characterized by a dispersion with two minima and four Fermi points, indicating the emergence of an effective next-nearest-neighbor hopping that arises as a second-order effect to avoid magnetic frustration. The pairs appear in the spectrum as in-gap bound states with weight concentrated in the hole pockets. The low-energy physics can be understood by means of a generalized t-J model with next-nearest-neighbor hopping. Our results offer a guide for searching for experimental signatures, and for other models that can realize PDW physics.

We present a class of one-dimensional generic spinless fermion lattice Hamiltonians that express quasi-Fermi liquid physics, manifesting both Luttinger and Fermi liquid features due to solely irrelevant interactions. Using infinite matrix product state techniques, we unveil its universal structure by calculating static and dynamic responses. Key features include a finite discontinuity in the momentum distribution at the Fermi level, despite power-law singularities in the spectral function protected by particle-hole symmetry. Away from half-filling Landau quasiparticles emerge. Charge dynamics show either high-energy bound states or concentration of spectral weight within the continuum for attractive or repulsive interactions, respectively. These universal features are realized across multiple models and energy scales thus reifying the quasi-Fermi liquid as a unique paradigm for one-dimensional fermions.

Ultrafast-light-driven strongly correlated antiferromagnetic insulators, such as prototypical NiO with large energy gap 4 eV, have recently attracted experimental attention using either above-gap [K. Gillmeister et al., Nat. Commun. 11, 4095 (2020)] or subgap [H. Qiu et al., Nat. Phys. 17, 388 (2021)] energy photons that are of fundamental interest in far-from-equilibrium quantum matter or spintronic applications, respectively. In the latter context, emission of THz radiation is also observed from NiO/Pt bilayers, where heavy metal (HM) Pt introduces strong spin-orbit coupling (SOC). However, microscopic mechanisms of such emission remain obscure because spintronic THz emitters have been amply studied using FM/HM (FM-ferromagnetic metal of conventional type) bilayers, where ultrafast demagnetization takes place and is directly related to THz emission. Conversely, in NiO total magnetization is zero prior to the fs laser pulse (fsLP) application. Here we employ the two-orbital Hubbard-Hund-Heisenberg model and study, via numerically exact nonequilibrium quantum many-body methods, the dynamics of its Neel vector and nonequilibrium magnetization. Additionally, we compute electromagnetic radiation by both time-dependent magnetization and local charge currents arising in either plain NiO or NiO with proximity SOC introduced by HM layer. Our analysis reveals nonclassical dynamics of Neel vector and nonequilibrium magnetization, changing only in length while not rotating, where the former is substantially reduced only in the case above-gap fsLP. In the plain NiO case, THz radiation of interest to applications is insignificant, but adding SOC enhances both current and magnetic dipole contributions to it. Above THz range, we find integer high-harmonic generation, as well as unusual noninteger harmonics for above-gap fsLP pump.

Ultrafast light-driven strongly correlated antiferromagnetic insulators, such as prototypical NiO with a large Mott energy gap ≃4  eV, have recently attracted experimental attention using photons of both subgap [H. Qiu et al., Nat. Phys. 17, 388 (2021)NPAHAX1745-247310.1038/s41567-020-01061-7] and above-gap energy [K. Gillmeister et al., Nat. Commun. 11, 4095 (2020)NCAOBW2041-172310.1038/s41467-020-17925-8]. In the former context, which is also of great interest to applications, emission of terahertz (THz) radiation is observed from NiO/Pt bilayers, where heavy metal (HM) Pt introduces strong spin-orbit coupling (SOC) effects. However, in contrast to amply studied spintronic THz emitters using femtosecond laser pulse (fsLP)-driven FM/HM (where FM represents a ferromagnetic metal of the conventional type, such as Fe, Ni, or Co) bilayers, where ultrafast demagnetization takes place and is directly related to THz emission, microscopic mechanisms of electromagnetic (EM) radiation from NiO/HM bilayers remain obscure, as the total magnetization of NiO is zero before fsLP application. We employ the two-orbital Hubbard-Hund-Heisenberg model and study, via numerically exact quantum many-body methods, the dynamics of its Néel vector and nonequilibrium magnetization. This reveals nonclassical (i.e., not describable by the Landau-Lifshitz equation) dynamics of Néel vector and nonequilibrium magnetization, changing only in length while not rotating, where the former is substantially reduced in the case of above-gap fsLPs. Additionally, we compute EM radiation by time dependence of magnetization or of local charge currents, finding that both contributions are significant in the THz frequency range only in NiO with proximity SOC introduced by the HM layer. Outside the THz range, we find an integer high-harmonic generation, as well as unusual noninteger harmonics for the above-gap fsLP pump.

In atomistic spin dynamics simulations, the time cost of constructing the space- and time-displaced pair correlation function in real space increases quadratically as the number of spins $N$, leading to significant computational effort. The GEMM subroutine can be adopted to accelerate the calculation of the dynamical spin-spin correlation function, but the computational cost of simulating large spin systems ($>40000$ spins) on CPUs remains expensive. In this work, we perform the simulation on the graphics processing unit (GPU), a hardware solution widely used as an accelerator for scientific computing and deep learning. We show that GPUs can accelerate the simulation up to 25-fold compared to multi-core CPUs when using the GEMM subroutine on both. To hide memory latency, we fuse the element-wise operation into the GEMM kernel using $\mathtt{CUTLASS}$ that can improve the performance by 26% $\sim$ 33% compared to implementation based on $\mathtt{cuBLAS}$. Furthermore, we perform the on-the-fly calculation in the epilogue of the GEMM subroutine to avoid saving intermediate results on global memory, which makes the large-scale atomistic spin dynamics simulation feasible and affordable.

We develop a theory that explains the low-energy optical excitations near 1.5 eV observed by optical experiments in NiPS_{3}. Using ab initio methods, we construct a two-band Hubbard model for two effective Ni orbitals. The dominant effective hopping corresponds to third-nearest neighbors. This model exhibits triplet-singlet excitations of energy near 2 times the Hund exchange. We derive an effective model for the movement of two singlets in an antiferromagnetic background, that we solve using a generalized self-consistent Born approximation, disentangling the nature of these novel excitations, which move coherently as "singlet polarons".

With the introduction of x-ray free electron laser sources around the world, new scientific approaches for visualizing matter at fundamental length and time-scales have become possible. As it relates to magnetism and "magnetic-type" systems, advanced methods are being developed for studying ultrafast magnetic responses on the time-scales at which they occur. We describe three capabilities which have the potential to seed new directions in this area and present original results from each: pump-probe x-ray scattering with low energy excitation, x-ray photon fluctuation spectroscopy, and ultrafast diffuse x-ray scattering. By combining these experimental techniques with advanced modeling together with machine learning, we describe how the combination of these domains allows for a new understanding in the field of magnetism. Finally, we give an outlook for future areas of investigation and the newly developed instruments which will take us there.

Using the nascent concept of quantum spin -transfer torque [A. Zholud et al., Phys. Rev. Lett. 119, 257201 (2017); M. D. Petrovic ' et al., Phys. Rev. X 11, 021062 (2021)], we demonstrate that a current pulse can be harnessed to entangle quantum localized spins of two spatially separated ferromagnets (FMs) which are initially unentangled. The envisaged setup is composed of a spin-polarizer (FMp) and a spin -analyzer (FMa) FM layers separated by a normal metal (NM) spacer. The injection of a current pulse into the device leads to a time -dependent superposition of many -body states characterized by a high degree of entanglement between the spin degrees of freedom of the two distant FM layers. The nonequilibrium dynamics are due to the transfer of spin angular momentum from itinerant electrons to the localized spins via a quantum spin -torque mechanism that remains active even for collinear but antiparallel arrangements of the FMp and FMa magnetizations (a situation in which the conventional spin torque is absent). We quantify the mixed -state entanglement generated between the FM layers by tracking the time evolution of the full density matrix and analyzing the build-up of the mutual logarithmic negativity over time. The effect of decoherence and dissipation in the FM layers due to coupling to bosonic baths at finite temperature, the use of multielectron current pulses, and the dependence on the number of spins are also considered in an effort to ascertain the robustness of our predictions under realistic conditions. Finally, we propose a "current-pump-x-ray-probe" scheme, utilizing ultrafast x-ray spectroscopy, that can witness nonequilibrium and transient entanglement of the FM layers by extracting its time -dependent quantum Fisher information.

Van der Waals (vdW) magnetic materials are comprised of layers of atomically thin sheets, making them ideal platforms for studying magnetism at the two-dimensional (2D) limit. These materials are at the center of a host of novel types of experiments, however, there are notably few pathways to directly probe their magnetic structure. We report the magnetic order within a single crystal of NiPS$_3$ and show it can be accessed with resonant elastic X-ray diffraction along the edge of the vdW planes in a carefully grown crystal by detecting structurally forbidden resonant magnetic X-ray scattering. We find the magnetic order parameter has a critical exponent of $\beta\sim0.36$, indicating that the magnetism of these vdW crystals is more adequately characterized by the three-dimensional (3D) Heisenberg universality class. We verify these findings with first-principle density functional theory, Monte-Carlo simulations, and density matrix renormalization group calculations.

Recent years have witnessed an enormous rise in research interest in magnetic skyrmions owing to their capability to improve over contemporary spintronic devices. An overview of the various magnetic interactions responsible for the formation of skyrmion together with distinct noncentrosymmetric and centrosymmetric skyrmion candidates is given in this review article. The magnetic interactions known as Dzyaloshinskii-Moriya interactions (DMI) have been extensively studied over the years to better understand the mechanism of skyrmions in chiral magnets that have larger skyrmion sizes. Because of their low skyrmion size, the centrosymmetric frustrated magnets are dwelling to skyrmions controlled by long-range interactions such as the Ruderman-Kittel-Kasuya-Yosida interaction (RKKY), which may be useful in the development of high density memory devices. To lay a solid foundation for the magnetic interactions involved in skyrmion formations and many other special physical properties, more research in the field of centrosymmetric skyrmions is required. Apart from studying candidates with low skyrmion sizes, one of the main goals for the future is to better understand the dynamics of skyrmion using polarized magnons, which has the potential to be extremely beneficial for spintronic applications.

We present a density matrix renormalization group (DMRG) study of an extended $t-J$ model with hopping to the first and second neighbors -- the one dimensional $t_1-t_2-J$ model. The full phase diagram as a function of the density $n$ and exchange strength $J$, for both positive and negative values of $t_2$, is obtained. For $t_2=-0.5$ we observe that, in the strongly interacting region, Nagaoka Ferromagnetism (FM) is accompanied by a triplet pair density wave (PDW) upon doping. As the spin exchange $J$ increases, a charge density wave (CDW) phase emerges and then gives way to singlet superconductivity (SC). This phase behaves as a singlet PDW with vanishing spacial average of the order parameter and a spin gap. When $t_2=0.5$, the physics is basically reminiscent of the conventional $t-J$ model, undergoing a transition from a metallic to a SC phase as a function of $J$.

We consider a multiband Hubbard model H_m for Cu and O orbitals in Ba_2-xSr_xCuO_3+δ similar to the tree-band model for two-dimensional (2D) cuprates. The hopping parameters are obtained from maximally localized Wannier functions derived from ab initio calculations. Using the cell perturbation method, we derive both a generalized t-J model H_tJ and a one-band Hubbard model H_H to describe the low-energy physics of the system. H_tJ has the advantage of having a smaller relevant Hilbert space, facilitating numerical calculations, while additional terms should be included in H_H to accurately describe the multi-band physics of H_m. Using H_tJ and DMRG, we calculate the wave-vector resolved photoemission and discuss the relevant features in comparison with recent experiments. In agreement with previous calculations, we find that the addition of an attractive nearest-neighbor interaction of the order of the nearest-neighbor hopping shifts weight from the 3 k_F to the holon-folding branch. Kinetic effects also contribute to this process.

We present a time-dependent computational approach to study Auger electron spectroscopy (AES) beyond the ultra-short core-hole relaxation time approximation and, as a test case, we apply it to the paradigmatic example of a one-dimensional Mott insulator represented by a half-filled Hubbard chain. The Auger spectrum is usually calculated by assuming that, after the creation of a core-hole, the system thermalizes almost instantaneously. This leads to a relatively simple analytical expression that uses the ground-state with a core-hole as a reference state and ignores all the transient dynamics related to the screening of the core-hole. In this picture, the response of the system can be associated to the pair spectral function. On the other hand, in our numerical calculations, the core hole is created by a light pulse, allowing one to study the transient dynamics of the system in terms of the pulse duration and in the non-perturbative regime. Time-dependent density matrix renormalization group calculations reveal that the relaxation process involves the creation of a polarization cloud of doublon excitations that have an effect similar to photo-doping. As a consequence, there is a leak of spectral weight to higher energies into what otherwise would be the Mott gap. For longer pulses, these excited states, mostly comprised of doublons, can dominate the spectrum. By changing the duration of the light-pulse, the entire screening process can be resolved in time.

Understanding the nature and origin of collective excitations in materials is of fundamental importance for unraveling the underlying physics of a many-body system. Excitation spectra are usually obtained by measuring the dynamical structure factor, S ( Q , ω ), using inelastic neutron or x-ray scattering techniques and are analyzed by comparing the experimental results against calculated predictions. We introduce a data-driven analysis tool which leverages ‘neural implicit representations’ that are specifically tailored for handling spectrographic measurements and are able to efficiently obtain unknown parameters from experimental data via automatic differentiation. In this work, we employ linear spin wave theory simulations to train a machine learning platform, enabling precise exchange parameter extraction from inelastic neutron scattering data on the square-lattice spin-1 antiferromagnet La 2 NiO 4 , showcasing a viable pathway towards automatic refinement of advanced models for ordered magnetic systems.

Atomistic spin dynamics simulations provide valuable information about the energy spectrum of magnetic materials in different phases, allowing one to identify instabilities and the nature of their excitations. However, the time cost of evaluating the dynamical correlation function $S(\mathbf{q}, t)$ increases quadratically as the number of spins $N$, leading to significant computational effort, making the simulation of large spin systems very challenging. In this work, we propose to use a highly optimized general matrix multiply (GEMM) subroutine to calculate the dynamical spin-spin correlation function that can achieve near-optimal hardware utilization. Furthermore, we fuse the element-wise operations in the calculation of $S(\mathbf{q}, t)$ into the in-house GEMM kernel, which results in further performance improvements of 44\% - 71\% on several relatively large lattice sizes when compared to the implementation that uses the GEMM subroutine in OpenBLAS, which is the state-of-the-art open source library for Basic Linear Algebra Subroutine (BLAS).

We consider a multiband Hubbard model Hm for Cu and O orbitals in Ba2-xSrxCuO3+s similar to the three-band model for two-dimensional cuprates. The hopping parameters are obtained from maximally localized Wannier functions derived from ab initio calculations. Using the cell perturbation method, we derive both a generalized t -J model HtJ and a one-band Hubbard model HH to describe the low-energy physics of the system. HtJ has the advantage of having a smaller relevant Hilbert space, facilitating numerical calculations, while additional terms should be included in HH to accurately describe the multiband physics of Hm. Using HtJ and the density matrix renormalization group method, we calculate the wave-vector-resolved photoemission and discuss the relevant features in comparison with recent experiments. In agreement with previous calculations, we find that the addition of an attractive nearest-neighbor interaction of the order of the nearest-neighbor hopping shifts the weight from the 3kF to the holon-folding branch. Kinetic effects also contribute to this process.

We present numerical evidence for a paradigm in one-dimensional interacting fermion systems, whose phenomenology has traits of both Luttinger liquids and Fermi liquids. This state, dubbed a quasi-Fermi liquid, possesses a discontinuity in its fermion occupation number at the Fermi momentum. The excitation spectrum presents particlelike quasiparticles and absence of holelike quasiparticles, giving rise instead to edge singularities. Such a state is realized in a one-dimensional spinless fermion lattice Hamiltonian by fine-tuning the interactions to a regime where they become irrelevant in the renormalization group sense. We show, using uniform infinite matrix products states and finite-entanglement scaling analysis, that the system ground state is characterized by a Luttinger parameter $K = 1$ and a discontinuous jump in the fermion occupation number. We support the characterization with calculations of the spectral function that show a particle-hole asymmetry reflected in the existence of well-defined Landau quasiparticles above the Fermi level and edge singularities without the associated quasiparticles below. These results indicate that the quasi-Fermi liquid paradigm can be realized beyond the low-energy perturbative realm.