We employ a quantum Liouville equation with relaxation to model the recently observed anomalous Hall effect in graphene irradiated by an ultrafast pulse of circularly polarized light. In the weak-field regime, we demonstrate that the Hall effect originates from an asymmetric population of photocarriers in the Dirac bands. By contrast, in the strong-field regime, the system is driven into a nonequilibrium steady state that is well described by topologically nontrivial Floquet-Bloch bands. Here, the anomalous Hall current originates from the combination of a population imbalance in these dressed bands together with a smaller anomalous velocity contribution arising from their Berry curvature. This robust and general finding enables the simulation of electrical transport from light-induced Floquet-Bloch bands in an experimentally relevant parameter regime and creates a pathway to designing ultrafast quantum devices with Floquet-engineered transport properties.

When subjected to parallel electric field $\mathit{E}$ and magnetic field $\mathit{B}$, Weyl semimetals exhibit the exotic transport property known as the chiral anomaly due to the pumping of electrons between Weyl cones of opposite chiralities. When one or both electromagnetic (EM) fields are replaced by strain-induced chiral pseudoelectromagnetic (pseudo-EM) fields, other types of quantum anomalies occur. In the present paper, we will show that such quantum anomalies can be reproduced in a completely different system—a Weyl ferromagnet whose magnonic structure remarkably encodes Weyl physics. By analytical and numerical calculations, we will show that the magnon bands can be Landau-quantized by either an inhomogeneous electric field $\mathit{E}$ or a chiral pseudoelectric field $\mathit{e}$ induced by a torsional strain, and magnons can be pumped along bands by either an inhomogeneous magnetic field $\mathit{B}$ or a chiral pseudomagnetic field $\mathit{b}$ due to a dynamic uniaxial strain. We list the magnon quantum anomalies and the associated anomalous spin and heat currents in the Weyl ferromagnet, and show they are distinct from their Weyl semimetal counterparts.

We have investigated the critical phenomenon associated with the magnetic phase transition in the half-metallic full-Heusler ${\mathrm{Co}}_2\mathrm{TiGe}$. The compound undergoes a continuous paramagnetic-to-ferromagnetic phase transition below the Curie temperature $T_C=371.5$ K. The analysis of magnetization isotherms in the vicinity of $T_C$, following a modified Arrott plot method, the Kouvel-Fisher technique, and a critical isotherm plot, yields the asymptotic critical exponents $\beta =0.495, \gamma =1.324$, and $\delta =3.67$. The self-consistency and reliability of the obtained exponents are further verified by the Widom scaling relation and scaling equations of state. The mean-field-like value of the critical exponent $\beta$ suggests a long-range nature of the exchange interactions, whereas the values of the critical exponents $\gamma$ and $\delta$ imply sizable critical spin fluctuations. The half-metallic itinerant character of ${\mathrm{Co}}_2\mathrm{TiGe}$ in the presence of magnetic inhomogeneity may result in such a strong deviation from the three-dimensional Heisenberg values ($\beta =0.369, \gamma =1.38$, and $\delta =4.8$) of the critical exponents toward the mean-field values ($\beta =0.5, \gamma =1$, and $\delta =3)$. The results suggest the complex nature of exchange couplings that stabilize the long-range ferromagnetic ordering in the system and are consistent with the earlier theoretical studies on the exchange mechanism in ${\mathrm{Co}}_2\mathrm{TiGe}$.

Achieving magnetometers with ultrahigh sensitivity at room temperature is an outstanding problem in physical sciences and engineering. Recently developed non-Hermitian cavity spintronics offers new possibilities. In this work we predict an exceptional magnetic sensitivity of cavity magnon polaritons with the peculiar parity-time ($\mathcal{P}\mathcal{T}$) symmetry. Based on the input-output formalism, we demonstrate a “Z”-shape spectrum including two side-band modes and a dark-state branch with an ultranarrow linewidth in the exact $\mathcal{P}\mathcal{T}$ phase. The spectrum evolves to a step function when the polariton touches the third-order exceptional point, accompanied by an ultrahigh sensitivity with respect to the detuning. The estimated magnetic sensitivity can approach ${10}^{-15}\mathrm{T}{\mathrm{Hz}}^{-1/2}$ in the strong coupling region, which is two orders of magnitude higher than that of the state-of-the-art magnetoelectric sensor. We derive the condition for the noiseless sensing performance. Purcell-like effect is observed when the $\mathcal{P}\mathcal{T}$ symmetry is broken. A possible experimental scheme to realize our proposal is also discussed.

The origin of the effect of “colossal magnetoresistance” (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudocubic ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ along the Mn-O-Mn bond direction at four $x$ doping values ($x\le 0.5$) and various temperatures and report a lattice dynamics study at $x_0=0.2$, representative of optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum $E_n^{\mathrm{mag}}\left(q\right)$ with $n$ levels, characteristic of the internal excitations of “orbital polarons” defined by ${\mathrm{Mn}}^{3+}$ neighbors surrounding a central ${\mathrm{Mn}}^{4+}$ hole. Because of its hopping, the hole mixes dynamically all the possible orbital configurations of its surrounding ${\mathrm{Mn}}^{3+}$ with degenerate energies. The $E_n^{\mathrm{mag}}$ values indicate a lifting of orbital degeneracy by phonon excitations. The $n$ value and the $q$ range are used to characterize these orbital polarons in direct space. At $x=0.125$ and $x=0.3$ the spectrum reveals two-dimensional polarons coupled by exchange and three-dimensional “free” polarons, respectively, with sizes $\ell =1.67a\le 2a$ in all bond directions. At $x_0=0.2$, the spin and the lattice dynamics provide evidence for chains of orbital polarons of size $\ell =2a$ with a periodic distribution over $\approx 3a$ and an interaction energy $\approx 3$ meV. At $T\le T_c$ the charges propagate together with the longitudinal acoustic phonons along the chains enhancing their ferromagnetic character. The phase separation between metallic and ferromagnetic chains in a nonmetallic matrix may be crucial for CMR.

${\mathrm{LaCrGe}}_3$ is an itinerant ferromagnet with a Curie temperature of $T_{\mathrm{C}}=85$ K and exhibits an avoided ferromagnetic quantum critical point under pressure through a modulated antiferromagnetic phase as well as tricritical wing structure in its temperature-pressure-magnetic field ($T\text{−}p\text{−}H$) phase diagram. In order to understand the static and dynamical magnetic properties of ${\mathrm{LaCrGe}}_3$, we carried out ${}^{139}\mathrm{La}$ nuclear magnetic resonance (NMR) measurements. Based on the analysis of NMR data, using the self-consistent-renormalization (SCR) theory, the spin fluctuations in the paramagnetic state are revealed to be isotropic ferromagnetic and three dimensional (3D) in nature. Moreover, the system is found to follow the generalized Rhodes-Wohlfarth relation which is expected in 3D itinerant ferromagnetic systems. As compared to other similar itinerant ferromagnets, the Cr $3d$ electrons and their spin fluctuations are characterized to have a relatively high degree of localization in real space.

Using torque magnetometry, we reveal remarkably simple universal behavior of the superconducting (SC) precursor in the cuprates by tracking the nonlinear diamagnetism above $T_c$ in four distinct compounds: single-${\mathrm{CuO}}_2$-layer ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }, {\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ and ${\mathrm{Bi}}_2$(Sr,${\mathrm{La})}_2{\mathrm{CuO}}_{6+\delta }$, and double-layer ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_{0.95}{\mathrm{Y}}_{0.05}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$. We find that SC diamagnetism vanishes in an exponential manner, characterized by a universal temperature scale that is approximately independent of compound and $T_c$. We discuss the possibility that this unusual behavior signifies the proliferation of SC clusters as a result of intrinsic inhomogeneity inherent to the cuprates.

Through a series of x-ray diffraction experiments, combined with first-principles calculations, we show that the nobility of platinum can be overcome, reacting with lithium under compression at room temperature. Pressures as low as 2.3 GPa lead to the synthesis of ${\mathrm{Li}}_{11}{\mathrm{Pt}}_2$, exhibiting the highest lithium content of any known intermetallic compound. Above 16 GPa, ${\mathrm{Li}}_{11}{\mathrm{Pt}}_2$ expels Li and transforms to ${\mathrm{Li}}_2\mathrm{Pt}$. This change is accompanied by a transformation in bonding characterized by the formation of one-dimensional Pt-Pt bonds with increasingly covalent character at higher densities.

Motivated by the discovery of near-room-temperature superconductivity in the sodalite-like clathrate hydride ${\mathrm{LaH}}_{10}$, we report ab initio calculations of the superconducting properties of two closely related hydrides ${\mathrm{YH}}_6$ and ${\mathrm{YH}}_{10}$ for which an even higher $T_{\mathrm{c}}$ has been predicted. Using fully anisotropic Migdal-Eliashberg theory with Coulomb corrections, we find almost isotropic superconducting gaps, resulting from a uniform distribution of the coupling over states of both Y and H sublattices. The Coulomb screening is rather weak, resulting in a Morel-Anderson pseudopotential ${\mu }^{*}=0.11$, at odds with claims of unusually large ${\mu }^{*}$ in lanthanum hydrides. The corresponding critical temperatures at 300 GPa exceed room temperature ($T_{\mathrm{c}}=290$ and 310 K for ${\mathrm{YH}}_6$ and ${\mathrm{YH}}_{10}$), in agreement with a previous isotropic-gap calculation. We estimate anharmonic effects to be weak. The different response of these two compounds to external pressure along with a comparison to low-$T_{\mathrm{c}}$ superconducting ${\mathrm{YH}}_3$ may inspire strategies to improve the superconducting properties of this class of hydrides.

van der Waals (vdW) layered compounds provided a fruitful research platform for the realization of superconductivity. However, a vdW layered superconductor with a high transition temperature ($T_c$) at ambient conditions is still rare. Here, using variable-composition evolutionary structure predictions, we systematically explored the stable compounds in the P-S system up to 20 GPa. Opposed to the complex stoichiometries at ambient conditions, only one compound, ${\mathrm{PS}}_2$, is predicted to be thermodynamically stable above 8 GPa. Strikingly, ${\mathrm{PS}}_2$ is a vdW layered material isostructural to $3R-{\mathrm{MoS}}_2$ exhibiting a predicted $T_c$ of around 11 K at ambient pressure, both in the bulk and the monolayer form. ${\mathrm{PS}}_2$ has been successfully synthesized via high-pressure experiments following the theoretical predictions. This enables replacing transition metals with group V elements in transition-metal dichalcogenides and paves the way in the search for vdW layered materials with superior properties.

The Cooper-pair transistor (CPT), a small superconducting island enclosed between two Josephson weak links, is the atomic building block of various superconducting quantum circuits. Utilizing gate-tunable semiconductor channels as weak links, the energy scale associated with the Josephson tunneling can be changed with respect to the charging energy of the island, tuning the extent of its charge fluctuations. Here, we directly demonstrate this control by mapping the energy level structure of a CPT made of an indium arsenide nanowire with a superconducting aluminum shell. We extract the device parameters based on the exhaustive modeling of the quantum dynamics of the phase-biased nanowire CPT and directly measure the even-odd parity occupation ratio as a function of the device temperature, relevant for superconducting and prospective topological qubits.

The mixed quantum-classical nonadiabatic molecular dynamics (NAMD) is a powerful tool to study many phenomena, especially ultrafast carrier transport and cooling. Carrier decoherence and detailed balance are two major issues in NAMD. So far, there is no computationally inexpensive approach to incorporate both effects. While the decoherence effect can be easily included in the state density matrix formalism and the detailed balance can be included in surface hopping or the wave function collapse approach, it is difficult to include both of them in a unified formalism. In this work we introduce a state density matrix formalism (referred to as P-matrix) including both the decoherence and detailed balance effects for NAMD. This method is able to explicitly treat the decoherence between different pairs of adiabatic states. Moreover, the off-diagonal density matrix elements are divided into two parts, corresponding to energy-increasing and energy-decreasing transitions. The detailed balance is then enforced by a Boltzmann factor applied to the energy-increasing transition part. The P-matrix formalism is applied to study hot-hole cooling and transfer processes in Si quantum dot (QD) systems. The calculated hot-carrier relaxation time is consistent with experiments. In a QD-pair system, the hot-hole cooling time shows weak dependence on the QD spacing. However, the hot-carrier transfer rate from one QD to another is found to decrease exponentially with the QD-QD distance. When the QD spacing is small ($\sim 1$ nm), the hot-carrier transfer can be very efficient. It is also shown that the explicit treatment of decoherence time is important in order to treat this hot-carrier transfer correctly.

We have performed ab initio lattice dynamics and molecular-dynamics studies of $\mathrm{L}{\mathrm{i}}_{2}X$ ($X=\mathrm{O}$, S and Se) to understand the ionic conduction in these compounds. The inelastic neutron scattering measurements on $\mathrm{L}{\mathrm{i}}_2\mathrm{O}$ have been performed across its superionic transition temperature of about 1200 K. The experimental spectra show significant changes around the superionic transition temperature, which is attributed to large diffusion of lithium as well as its large vibrational amplitude. We have identified a correlation between the chemical pressure (ionic radius of $X$ atom) and the superionic transition temperature. The simulations are able to provide the ionic diffusion pathways in $\mathrm{L}{\mathrm{i}}_{2}X$.

We present a computational method to determine the exchange constants in isotropic spin models. The method uses the Hamiltonian and overlap matrices computed from density functional schemes that are based on nonorthogonal basis sets. We demonstrate that the proposed method as implemented on top of the siesta code reproduces the Heisenberg interactions of simple metallic bulk ferromagnets as obtained from former well-established computational approaches. Then we address $sp$ magnetism in graphene nanostructures. For fluorinated graphene we obtain exchange interactions in fairly good agreement with previous calculations using maximally localized Wannier functions and we confirm the theoretical prediction of a ${120}^{\circ }$ Néel state. Associated with the magnetic edge states of a zigzag graphene nanoribbon we find rapidly decaying exchange interactions, however, with an unconventional distance dependence of $exp(-\sqrt{r/\delta })$. We show that the stiffness constant derived from the exchange interactions is consistent with a previous estimate based on total energy differences of twisted spin configurations. We highlight that our method is an efficient tool for the analysis of novel hybrid nanostructures where metallic and organic components are integrated to form exotic magnetic patterns.

The magnetoplasmonic properties of Ag-Co composite nanohole arrays are investigated. It is observed that both plasmonic and magnetic properties of the Ag-Co composite nanohole arrays strongly depend on the composition ratio of Ag and Co. The enhanced optical transmission due to plasmonic resonance decreases with the increase of the Co component while the Faraday rotation effect increases monotonically. The magnetization dynamics of a composite thin film is also modified by the composition as well as the introduction of hole arrays. The Ag-Co composite nanohole arrays with the Co content of 30% show high plasmonic–magneto-optics performances compared with that of Ti-Co composite nanohole array, and annealing of the Ag-Co composites in vacuum can further improve this property. All experimental results are confirmed by the finite-difference time-domain calculations. Such a magnetoplasmonic composite material can act as a class of materials for magnetoplasmonic devices or metamaterial applications.

The Spirit framework is designed for atomic-scale spin simulations of magnetic systems with arbitrary geometry and magnetic structure, providing a graphical user interface with powerful visualizations and an easy-to-use scripting interface. An extended Heisenberg-type spin-lattice Hamiltonian including competing exchange interactions between neighbors at arbitrary distances, higher-order exchange, Dzyaloshinskii-Moriya and dipole-dipole interactions is used to describe the energetics of a system of classical spins localized at atom positions. A variety of common simulation methods are implemented including Monte Carlo and various time evolution algorithms based on the Landau-Lifshitz-Gilbert (LLG) equation of motion. These methods can be used to determine static ground-state and metastable spin configurations, sample equilibrium and finite-temperature thermodynamical properties of magnetic materials and nanostructures, or calculate dynamical trajectories including spin torques induced by stochastic temperature or electric current. Methods for finding the mechanism and rate of thermally assisted transitions include the geodesic nudged elastic band method, which can be applied when both initial and final states are specified, and the minimum mode-following method when only the initial state is given. The lifetimes of magnetic states and rates of transitions can be evaluated within the harmonic approximation of transition-state theory. The framework offers performant central processing unit (CPU) and graphics processing unit (GPU) parallelizations. All methods are verified and applications to several systems, such as vortices, domain walls, skyrmions, and bobbers are described.

We study the spectral function of two-leg Hubbard ladders with the time-dependent density matrix renormalization group method (tDMRG). The high-resolution spectrum displays features of spin-charge separation and a scattering continuum of excitations with coherent bands of bound states “leaking” from it. As the interleg hopping is increased, the continuum in the bonding channel moves to higher energies and spinon and holon branches merge into a single coherent quasiparticle band. Simultaneously, the spectrum undergoes a crossover from a regime with two minima at incommensurate values of $k_x$ (a Mott insulator), to one with a single minimum at $k_x=\pi$ (a band insulator). We identify the presence of a continuum of scattering states consisting of a triplon and a polaron. We analyze the processes leading to quasiparticle formation by studying the time evolution of charge and spin degrees of freedom in real space after the hole is created. At short times, incoherent holons and spinons are emitted but after a characteristic time $\tau$ charge and spin form polarons that propagate coherently.

In typical flat-band models, defined as nearest-neighbor tight-binding models, flat bands are usually pinned to the special energies, such as top or bottom of dispersive bands, or band-crossing points. In this paper, we propose a simple method to tune the energy of flat bands without losing the exact flatness of the bands. The main idea is to add farther-neighbor hoppings to the original nearest-neighbor models, in such a way that the transfer integrals depend only on the Manhattan distance. We apply this method to several lattice models including the two-dimensional kagome lattice and the three-dimensional pyrochlore lattice, as well as their breathing lattices and non-line graphs. The proposed method will be useful for engineering flat bands to generate desirable properties, such as enhancement of $T_c$ of superconductors and nontrivial topological orders.

While the search for quantum anomalous Hall (QAH) insulators remains active, attention is also being paid to the controllable topological phase transitions, as they could enable the realization of both exotic quantum phenomena and novel spintronic devices. However, stable QAH insulators are innately rare to report since ferromagnetism often accompanies metallicity. Here we identify theoretically the realization of QAH effect in a stable two-dimensional ferromagnetic insulator, ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ quintuple layers, as evidenced by the analysis of Chern number and chiral edge states. Remarkably, under electric field, a topological phase transition between QAH insulator and quantum spin Hall (QSH) insulator is revealed with the calculated nontrivial spin Chern number and Wannier charge centers for the QSH insulator. Unlike the conventional QSH insulator, here gapless edge states survive even though the time-reversal symmetry is broken by the ferromagnetic ordering. Our findings not only extend the advanced concepts but also afford exciting opportunities for experimental engineering and implementations of magnetic topological states.