We theoretically investigate the optically induced demagnetization of ferromagnetic FePt using the time-dependent density functional theory (TDDFT). We compare the demagnetization mechanism in the perturbative and nonperturbative limits of light-matter interaction and show how the underlying mechanism of the ultrafast demagnetization depends on the driving laser intensity. Our calculations show that the femtosecond demagnetization in TDDFT is a longitudinal magnetization reduction and results from a nonlinear optomagnetic effect, akin to the inverse Faraday effect. The demagnetization scales quadratically with the electric field $E$ in the perturbative limit, i.e., $\mathrm{\Delta }{M}_z\propto E^2$. Moreover, the magnetization dynamics happens dominantly at even multiples $n{\omega }_0$, ($n=0,2,\cdots$) of the pump-laser frequency ${\omega }_0$, whereas odd multiples of ${\omega }_0$ do not contribute. We further investigate the demagnetization in conjunction to the optically induced change of electron occupations and electron correlations. Dynamical correlations within the Kohn-Sham local-density framework are shown to have an appreciable yet distinct effect on the amount of demagnetization depending on the laser intensity. Comparing the ab initio computed demagnetizations with those calculated from spin occupations, we show that electronic coherence plays a dominant role in the demagnetization process, whereas interpretations based on the time-dependent occupation numbers poorly describe the ultrafast demagnetization.

We report our systematic investigation of temperature-dependent Barkhausen volume behavior for CoFeB/Pd with perpendicular magnetic anisotropy by means of magneto-optical Kerr microscopy. In the temperature range where two-dimensional (2D) Ising-like and single-domain features are sustained, hysteresis parameters such as coercivity, hysteresis area, and saturation magnetization are quantitatively analyzed with respect to the temperature. Interestingly it is demonstrated that the Barkhausen length is directly proportional to the domain-wall width via the magnetic anisotropy in this single-domain and 2D Ising-like model system.

Co-doped ${\mathrm{MnNiGe}}_{1.05}$ alloys of nominal compositions ${\mathrm{MnNi}}_{1-x}{\mathrm{Co}}_x{\mathrm{Ge}}_{1.05}$ (for $x=0.075$ and 0.125), with a significant amount of vacancies at the transition-metal sites, have been explored through temperature-dependent structural and magnetic investigations. Such studies confirm the presence of martensitic phase transition (MPT) in both the alloys. However, the observed MPT is found to be incomplete in nature. The presence of a significant amount of high-temperature austenite phase, well below the MPT, results in several interesting features, including field-induced metamagnetic transition and nonmonotonic change in the inverse magnetocaloric behavior, etc. In addition, a possible spin-reorientation transition has also been noticed in the magnetically ordered austenite phase for both the alloys.

We demonstrate that a plethora of higher order topological phases emerge in magnet-superconductor hybrid (MSH) systems through the interplay of a stacked magnetic structure and an underlying triangular surface lattice; the latter being of great current experimental interest. Such lattices offer the ability to create three main types of edge terminations — called $x$-, $y$- and $y^{\prime }$-edges — of MSH islands that, in turn, give rise to a complex phase diagrams exhibiting various regions of higher order topological superconducting (HOTSC) phases. We identify the single adatom chain, as well as a pair of adjacent adatom chains (called a double-chain), as the basic building blocks for the emergence of HOTSC phases. Of particular interest are those HOTSC phases which arise from a competition between the topology of single- and double-chain blocks, which are absent for square lattices.

Dynamical mean-field theory describes the impact of strong local correlation effects in many-electron systems. While the single-particle spectral function is directly obtained within the formalism, two-particle susceptibilities can also be obtained by solving the Bethe-Salpeter equation. The solution requires handling infinite matrices in Matsubara frequency space. This is commonly treated using a finite frequency cutoff, resulting in slow linear convergence. A decomposition of the two-particle response in local and nonlocal contributions enables a reformulation of the Bethe-Salpeter equation inspired by the dual boson formalism. The reformulation has a drastically improved cubic convergence with respect to the frequency cutoff, considerably facilitating the calculation of susceptibilities in multi-orbital systems. This improved convergence arises from the fact that local contributions can be measured in the impurity solver. The dual Bethe-Salpeter equation uses the fully reducible vertex which is free from vertex divergences. We benchmark the approach on several systems including the spin susceptibility of strontium ruthenate ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, a strongly correlated Hund's metal with three active orbitals.

A model of the non-Abelian fractional quantum Hall effect is obtained from the diagonalization of the matrix model proposed by Dorey, Tong, and Turner (DTT). The Hamiltonian is reminiscent of a spin Calogero-Moser model but involves higher-order symmetric representations of the non-Abelian symmetry. We derive the energy spectrum and show that the Hamiltonian has a triangular action on a certain class of wave functions with a free-fermion expression. We deduce the expression of the ground-state eigenfunctions and show that they solve a Knizhnik-Zamolodchikov equation. Finally, we discuss the emergence of Kac-Moody symmetries in the large-$N$ limit using the level-rank duality, and we confirm the results obtained previously by DTT.

We propose an effective lattice model for the moiré structure of the twisted bilayer dice lattice. In the chiral limit, we find that there are flat bands at the zero-energy level at any twist angle besides the magic ones and these flat bands are broadened by small perturbation away from the chiral limit. The flat bands contain both bands with zero Chern number which originate from the destructive interference of the states on the dice lattice and the topological nontrivial bands at the magic angle. The existence of the flat bands can be detected from the peak-splitting structure of the optical conductance at all angles, while the transition peaks do not split and only occur at magic angles in twisted bilayer graphene.

We present low-temperature magnetoconductance measurements on ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.8}{\mathrm{Se}}_{1.2}$ kinks with ribbon-shaped legs. The conductance displays a clear dependence on the in-plane magnetic field orientation. The conductance modulation is consistent with orbital effect-driven trapping of the topological surface states on different side facets of the legs of the kink, which affects their transmission across the kink. This magnetic field driven trapping and conductance pattern can be explained with a semiclassical picture and is supported by quantum transport simulations. The interpretation is corroborated by varying the angle of the kink and analyzing the temperature dependence of the observed magnetoconductance pattern, indicating the importance of phase coherence along the cross-section perimeter of the kink legs.

We examine the impact of nonmagnetic disorder on the electronic states of a multilayer structure comprising layers of both topological and conventional band insulators. Employing the Burkov-Balents model with renormalized tunneling parameters, we generate phase diagrams correlating with disorder, demonstrating that nonmagnetic disorder can induce transitions between distinct topological phases. The subsequent section of our investigation focuses on the scenario where disorder is unevenly distributed across layers, resulting in fluctuations of the interlayer tunneling parameter, termed off-diagonal disorder. Furthermore, we determine the density of states employing the self-consistent single-site diagram technique, expanding the Green function in relation to the interlayer tunneling parameter (locator method). Our findings reveal that off-diagonal disorder engenders delocalized bulk states within the band gap. The emergence of these states may lead to the breakdown of the anomalous quantum Hall effect (AQHE) phase, a phenomenon that has garnered significant attention from researchers in the realm of topological heterostructures. Nonetheless, our results affirm the stability of the Weyl semimetal phase even under substantial off-diagonal disorder.

We report the results of a temperature-dependent reflection spectroscopy study, across a wide energy range ($\sim 0.01$ to 3 eV), of unsubstituted and 40% Zn substituted for Co, for the transition-metal phosphorus trichalcogenide, $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. We observe a transition from a paramagnetic to antiferromagnetic state with a Néel temperature at $T_{\mathrm{N}}\sim 120$ K that is completely suppressed in Zn-substituted samples. At 300 K we identify four narrow ($\sim 1$ meV) infrared active phonon modes while at 70 K (below $T_{\mathrm{N}}$) we observe that the low-energy phonons, dominated by Co motion, resolve into two modes. These low-energy modes are asymmetric, indicating coupling to a broad electronic continuum. We also report a broad ($\sim 30$ meV) low-temperature infrared absorption band that appears near $T_{\mathrm{N}}$ that we suggest is determined by a multiphonon-assisted 2-magnon absorption process. At 300 K in higher-energy spectra, we observe considerable absorption starting from $0.20\pm 0.02$ eV which we associate with inter-$\mathrm{C}{\mathrm{o}}^{2+}$ ion $3d$ transitions. At temperatures below $T_{\mathrm{N}}$ the number of electronic absorption bands increases from 4 to 6, indicating a lowering of the symmetry around the $\mathrm{C}{\mathrm{o}}^{2+}$ ions. On substituting 40% Zn for Co, the antiferromagnetic transition is suppressed along with temperature-dependent changes in the phonon and electronic spectra. The temperature-dependent spectral changes indicate strongly correlated behavior between the infrared active lattice vibrations, the electronic excitations, and magnetism in unsubstituted $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$.

Quantum spin liquids (QSLs) represent a novel state where spins are highly entangled but do not order even at zero temperature due to strong quantum fluctuations. Such a state is mostly studied in Heisenberg models defined on geometrically frustrated lattices. Here, we turn to a new triangular-lattice antiferromagnet ${\mathrm{PrMgAl}}_{11}{\mathrm{O}}_{19}$, in which the interactions are believed to be of Ising type. Magnetic susceptibility measured with an external field along the $c$ axis is two orders of magnitude larger than that with a field in the $ab$ plane, displaying an ideal easy-axis behavior. Meanwhile, there is no magnetic phase transition or spin freezing observed down to 1.8 K. Ultralow-temperature specific heat measured down to 50 mK does not capture any phase transition either, but a hump at 4.5 K, below which the magnetic specific heat exhibits a quasiquadratic temperature dependence that is consistent with a Dirac QSL state. The inelastic neutron scattering technique is also employed to elucidate the nature of its ground state. In the magnetic excitation spectra, there is a gapless broad continuum at the base temperature 55 mK, in favor of the realization of a gapless QSL. Our results provide a scarce example for the QSL behaviors observed in an Ising-type magnet, which can serve as a promising platform for future research on QSL physics based on an Ising model.

The implementation of an efficient self-consistent field (SCF) method including both scalar-relativistic effects and spin-orbit interaction in density functional theory (DFT) is presented. We make use of Gaussian-type orbitals and all integrals are evaluated in real space. Our implementation supports density functional approximations up to the level of meta-generalized gradient approximations for SCF energies and gradients. The latter can be used to compute the stress tensor and consequently allow us to optimize the cell structure. Considering spin-orbit interaction requires the extension of the standard procedures to a two-component formalism and a noncollinear approach for open-shell systems. Here, we implemented both the canonical and the Scalmani-Frisch noncollinear DFT formalisms, with hybrid and range-separated hybrid functionals being presently restricted to SCF energies. We demonstrate both efficiency and relevance of spin-orbit effects for the electronic structure of discrete systems and systems periodic in one to three dimensions.

Despite being successfully synthesized [Zhang et al., Nat. Mater. 20, 1073 (2021)], the monolayer structure of stable hexagonal ${\mathrm{TiO}}_2$ is unknown, and it is not even clear whether it can exist in a freestanding form. Through first-principles calculations, we have identified two previously uncharted stable structures, namely, distorted $1\times \sqrt{3} 1T-{\mathrm{TiO}}_2$ and $\sqrt{3}\times \sqrt{3} 1T-{\mathrm{TiO}}_2$, both of which are energetically more favorable than commonly adopted $1H$ and $1T$ phases. Here, structural distortions are characterized by the out-of-plane shifts of Ti atoms due to the pseudo-Jahn-Teller interactions, which break one and all two inversion symmetries of $1T$ configuration. As a consequence, the $1\times \sqrt{3} 1T$ remains centrosymmetric, while the $\sqrt{3}\times \sqrt{3} 1T$ exhibits out-of-plane ferroelectricity. Electronic structure calculations show that both are wide-band-gap semiconductors with band gaps larger than their bulk counterparts. In this paper, we not only deepen the understanding of structural instability in wide-gap semiconductors but also add a member to the rare family of two-dimensional out-of-plane ferroelectrics.

One-dimensional Mott insulators can be described using the sine-Gordon model, an integrable quantum field theory that provides the low-energy effective description of several one-dimensional gapped condensed matter systems, including recent realizations with trapped ultracold atoms. Employing the theory of generalized hydrodynamics, we demonstrate that this model exhibits separation of the transport of topological charge vs energy. Analysis of the quasiparticle dynamics reveals that the mechanism behind the separation is the reflective scattering between topologically charged kinks/antikinks. The effect of these scattering events is most pronounced at strong coupling and low temperatures, where the distribution of quasiparticles is narrow compared to the reflective scattering amplitude. This effect results in a distinctively shaped “arrowhead” light cone for the topological charge.

Groups IVB and VB transition-metal carbides (TMCs) have the same lattice structure but different preferred slip systems, of which group IVB TMCs favor ⟨110⟩{110} while group VB TMCs favor ⟨110⟩{111}. This distinction suggests that the mechanical properties of high-entropy carbide ceramics ${\left(\mathrm{HfTiZr}\right)}_{1-x}{\left(\mathrm{NbTa}\right)}_x\mathrm{C}$ can be designed by adjusting the concentration of different group metal elements to activate specific slip systems. In this work, we investigated the influence of the proportion and distribution of various metal atoms on mechanical behavior and slip systems using first-principles calculations. The results show that the mechanical properties of ${\left(\mathrm{HfTiZr}\right)}_{1-x}{\left(\mathrm{NbTa}\right)}_x\mathrm{C}$ near the equilibrium point follow the rule of mixture. However, their yield shear strength is influenced by the strength of the metal-C bond located on the slip plane, while the position of the slip plane is primarily determined by the unstable stacking fault energy. Moreover, dissociation on {111} slip plane occurs when $x\ge 0.5$ (i.e., valence electron concentration $\ge 8.5$ electrons per formula unit), suggesting the potential to shift the dominant ⟨110⟩{110} slip system of group IVB TMCs to the ⟨110⟩{111} slip system. By adjusting the proportion and distribution of different group metal atoms in ${\left(\mathrm{HfTiZr}\right)}_{1-x}{\left(\mathrm{NbTa}\right)}_x\mathrm{C}$, it is possible to customize the slip behavior and yield shear strength and plasticity.

In the quest to find quantum spin liquids, layered cobalt oxides ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ and ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ have been proposed as promising candidates for approximating the Kitaev honeycomb-lattice model. Yet, their suitability has been thrown into question due to observed long-range magnetic order at low temperatures and indications of easy-plane, rather than Kitaev-type, spin anisotropy. Here, we use muon-spin relaxation to reveal an unexpected picture: Contrary to the anticipated static nature of the long-range order, the systems show prevalent spin dynamics with a spatially uneven distribution and varied correlation times. This underlines that the magnetic ground states cannot be solely described by the long-range order, suggesting a significant role of quantum fluctuations. Our findings not only shed light on the complex physics of these systems but also underscore the need for a refined approach in the search for realizable quantum spin liquids.

An effective field theory (EFT) for dynamical axions in Weyl semimetals (WSMs) is presented. A pseudoscalar axion excitation is predicted in WSMs at sufficiently low temperatures, independently of the strength of the Weyl fermion self-coupling. For strong fermion self-coupling the axion is the gapless Goldstone boson of chiral $\mathrm{U}{\left(1\right)}^{\text{ch}}$ spontaneous symmetry breaking. For weak fermion self-coupling an axion is also generated at nonzero chiral density for Weyl nodes displaced in energy, as a gapless collective mode of correlated fermion pair excitations of the Fermi surface. This is an explicit example of the extension of Goldstone's theorem to symmetry breaking by the axial anomaly itself. In both cases, the axion is a chiral density wave or phason mode of the superfluid state of the WSM, and the Weyl fermions form a chiral condensate $\langle \overline{\psi }\psi \rangle$ at low temperatures. In the presence of an applied magnetic field, the axion mode becomes gapped, in analogy to the Anderson–Higgs mechanism in a superconductor. 't Hooft anomaly matching from ultraviolet to infrared scales is directly verified in the EFT approach. WSMs thus provide an interesting quantum system in which superfluid, non-Fermi liquid behavior, and a dynamical axion are predicted to follow directly from the axial anomaly in a consistent EFT that may be tested experimentally.

Shock compression experiments on platinum (Pt) were carried out by combining a two-stage gas gun with the unique high-Z three-stage gas gun launcher technique. The shock Hugoniot of Pt was determined in a wide pressure range of 0.2–1.1 TPa. These data fill the gap between the two-stage gas gun and magnetically driven shock experiments, and thereby we observe a clear softening behavior of the Pt Hugoniot at the beginning of ∼1.1 TPa. Ab initio molecular dynamics simulations were also performed to reveal the response of Pt under ultrahigh pressure compressions. The simulation results show that the melting of Pt under shock compression occurs at ∼570 GPa, and a sudden increase in conductivity and free electron number density emerges at the beginning of the Hugoniot softening. The further electronic structure calculations indicate that the Hugoniot softening may arise from the thermal excitation (ionization) of the $5d$ electrons and the localized $6s$ electrons in Pt under extreme shock compression. This work has gained further insight into the ionization behaviors of high-Z metal elements with complex electronic structures under ultrahigh pressure compressions, which is of significance for understanding the partial ionization effect in the transition region from condensed matter to warm dense matter.

The normal state in iron chalcogenides is metallic but highly unusual, with orbital and spin degrees of freedom partially itinerant or localized depending on temperature, leading to many unusual features. In this work, we report on the observations of two of such features, the orbital-selective Mott phase (OSMP) and spin nematicity, evidenced in magnetization and magnetotransport [resistivity, Hall effect, anisotropic magnetoresistance (AMR)] of ${\mathrm{Fe}}_{1-y}{\mathrm{Ni}}_y{\mathrm{Te}}_{0.65}{\mathrm{Se}}_{0.35}$ single crystals, with $0<y<0.21$. Substitution of Ni dopes crystals with electrons, what eliminates some of the hole pockets from Fermi level, leaving only one, originating from the $d_{xy}$ orbital. This leads to electron-dominated conduction at low $T$ for $y\gtrsim 0.06$. However, at high temperatures, $T\gtrsim 125–178$ K, the conduction reverses to hole dominated. Anomalies in magnetization and resistivity are observed at temperatures which approach the high-$T$ boundary of the electron-dominated region. Analysis of these effects suggests a link with the appearance of the $d_{z^2}$ hole pockets at $X$ points of the Brillouin zone in the OSMP phase, facilitated by the localization of the $d_{xy}$ orbital, as recently reported by angular-resolved photoemission experiments [J. Huang et al., Commun. Phys. 5, 29 (2022)]. The low-$T$ AMR shows mixed fourfold and twofold rotational symmetry of in-plane magnetocrystalline anisotropy, with the fourfold term the largest at small $y$, and suppressed at intermediate $y$. These results are consistent with the mixed stripe and bicollinear magnetic correlations at small $y$, and suppression of stripe correlations at intermediate $y$, indicating development of spin nematicity with increasing Ni doping, which possibly contributes to the suppression of superconductivity.