We construct a general theory of adiabatic propagation of spinor exciton polaritons in waveguides of arbitrary shape, accounting for the effects of TE-TM splitting in linear polarizations and Zeeman splitting in circular polarizations. The developed theory is applied for the description of waveguides of periodically curved shape. We show that in this geometry the periodic rotation of the effective in-plane magnetic field produced by TE-TM interaction results in a nontrivial band-gap structure, which can be additionally tuned by application of an external magnetic field. It is also demonstrated that spin-dependent interactions between polaritons lead to the formation of stable gap solitons.

${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ is a centrosymmetric magnet in which a skyrmion lattice has recently been discovered. Here, we investigate the magnetic structure of the zero-field ground state using neutron diffraction on single crystal and polycrystalline ${{}^{160}\mathrm{GdRu}}_2{\mathrm{Si}}_2$. In addition to observing the principal propagation vectors ${\mathbf{q}}_1$ and ${\mathbf{q}}_2$, we discover higher-order magnetic satellites, notably ${\mathbf{q}}_1+2{\mathbf{q}}_2$. The appearance of these satellites is explained within the framework of a double-$Q$ constant-moment solution. Using powder diffraction, we implement a quantitative refinement of this model. This structure, which contains vortexlike motifs, is shown to have a one-dimensional topological charge density.

We demonstrate the potential of van der Waals magnets for spintronic applications by reporting long-distance magnon spin transport in the electrically insulating antiferromagnet chromium thiophosphate ($\mathrm{Cr}\mathrm{P}{\mathrm{S}}_4$) with perpendicular magnetic anisotropy. We inject and detect magnon spins nonlocally by Pt contacts and monitor the nonlocal resistance as a function of an in-plane magnetic field up to 7 T. We observe a nonlocal resistance over distances up to at least a micron below the Néel temperature ($T_{\mathrm{N}}=38$ K) close to magnetic field strengths that saturate the sublattice magnetizations.

Atomic simulations using machine learning interatomic potential (MLIP) have gained a lot of popularity owing to their accuracy in comparison to conventional empirical potentials. However, the transferability of MLIP to systems outside the training set poses a significant challenge. Here, we compare the transferability of three MLIP approaches: (i) neural network potentials (NNP), (ii) physical LassoLars interactions potential (PLIP) and (iii) linear potentials with Belher-Parrinello descriptors, trained over a small but diverse configuration of zinc oxide polymorphs. We compared the obtained models with density functional theory reference results for physical properties including bulk lattice parameters, surface energies, and vibrational density of states and showed the superiority of both NNP and PLIP models. However, the NNP model performed poorly when compared to the other two linear models for the structural optimization of nanoparticles and molecular dynamics simulation of liquid phases, which are systems outside the training set. While providing less accurate prediction for solid Zinc Oxides phases, both linear models appear more transferable than NNP when testing for nanoscale systems and liquid phases. Our results are finally rationalized by a combination of different statistical analysis including spread in force evaluation, information imbalance, convex hull calculation, and density in descriptor space.

We formulate a semiclassical circuit model to clarify the role of quantum entanglement in the recently discovered encoding phase transitions in quantum circuits with measurements. As a starting point, we define a random circuit model with nearest neighbor classical gates interrupted by erasure errors. In analogy with the quantum setting, this system undergoes a purification transition at a critical error rate above which the classical information entropy in the output state vanishes. We show that this phase transition is in the directed percolation universality class, consistent with the fact that having zero entropy is an absorbing state of the dynamics; this classical circuit cannot generate entropy. Adding an arbitrarily small density of quantum gates in the presence of errors eliminates the transition by destroying the absorbing state: the quantum gates generate internal entanglement, which can be effectively converted to classical entropy by the errors. We describe the universal properties of this instability in an effective model of the semiclassical circuit. Our model highlights the crucial differences between information dynamics in classical and quantum circuits.

We report the investigation of spin dynamics in the antiferromagnetic metal ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ with the Zintl phase by using time-resolved magneto-optical Kerr effect (TR-MOKE) and transient reflectivity spectroscopy. In TR-MOKE measurement, we observe a spin precession mode with a frequency of about 18 GHz below 1.4 T and a new emerging coherent acoustic phonon mode near 35 GHz at a higher field where the latter shows a field-independent behavior. With temperature increasing, the spin precession disappears above Néel temperature $T_N$, whereas the acoustic phonon persists up to 25 K due to existence of magnetic-field-induced polarized paramagnetism. In contrast, the quasiparticle dynamics of ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ does not show dramatic change in the vicinity of $T_N$. By comparing the spin and quasiparticle dynamics, we attribute the appearance of an acoustic phonon in TR-MOKE to photoinduced transient perturbation of magneto-optical coefficient. Our paper provides an in-depth investigation on the dynamical magnetic properties and broadens the understanding of the ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ system.

Topological solitons are crucial to many branches of physics, such as models of fundamental particles in quantum field theory, information carriers in nonlinear optics, and elementary entities in quantum and classical computations. Chiral magnetic materials are a fertile ground for studying solitons. In the past few years, a huge number of all kinds of topologically protected localized magnetic solitons have been found. The number is so large, and a proper organization and classification are necessary for their future development. Here we show that many topological magnetic solitons can be understood from the duality of particle and elastic continuum-medium nature of skyrmions. In contrast to the common belief that a skyrmion is an elementary particle that is indivisible, skyrmions behave like both particle and continuum media that can be torn apart to bury other objects, reminiscing particle-wave duality in quantum mechanics. Skyrmions, like indivisible particles, can be building blocks for cascade skyrmion bags and target skyrmions. They can also act as bags and glues to hold one or more skyrmions together. The principles and rules for stable composite skyrmions are explained and presented, revealing their rich and interesting physics.

We report electrical and thermal transport properties of single-crystalline kagome metals $A{\text{Ti}}_3{\text{Bi}}_5(A=\text{Rb},\mathrm{Cs})$. Different from the structrually similar kagome superconductors $A{\mathrm{V}}_3{\mathrm{Sb}}_5$, no charge density wave instabilities are found in $A{\mathrm{Ti}}_3{\mathrm{Bi}}_5$. At low temperatures below 5 K, signatures of superconductivity appear in $A{\mathrm{Ti}}_3{\mathrm{Bi}}_5$ as seen in magnetization measurements. However, bulk superconductivity is not evidenced by specific heat results. Similar to $A{\mathrm{V}}_3{\mathrm{Sb}}_5, A{\mathrm{Ti}}_3{\mathrm{Bi}}_5$ show a nonlinear magnetic field dependence of the Hall effect below about 70 K, pointing to a multiband nature. Unlike $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ in which phonons and electron-phonon coupling play important roles in thermal transport, the thermal conductivity in $A{\mathrm{Ti}}_3{\mathrm{Bi}}_5$ is dominated by electronic contributions. Moreover, our calculated electronic structures of ${\mathrm{ATi}}_3{\mathrm{Bi}}_5$ suggest that van Hove singularities are sitting well above the Fermi energy. Compared with $A{\mathrm{V}}_3{\mathrm{Sb}}_5$, the absence of charge orders in $A{\mathrm{Ti}}_3{\mathrm{Bi}}_5$ is closely associated with minor contributions from electron-phonon coupling and/or van Hove singularities.

We describe temporally correlated noise processes that influence the idle evolution of a superconducting transmon qubit. To model the composite qubit-environment system we use quantum circuit theory, and we show how a circuit Hamiltonian can be derived for transverse noise affecting the qubit. Based on the time-convolutionless projection operator method, we construct a time-local master equation which, when transformed to its canonical Lindblad form, exhibits a decay rate that is negative at all times, corresponding to eternally non-Markovian dynamics. By expressing the solution of the master equation in the Kraus representation, we identify two crucial non-Markovian phenomena: periodic revivals of coherence, and the appearance of additional frequencies far from the qubit frequency in the precession of the qubit state. When a single qubit gate acts on the qubit state, these extra frequency terms rotate undesirably and they effectively act as the memory of the state prior to the rotation around the Bloch sphere.

Single crystals of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ were studied by high-resolution capacitance dilatometry to obtain in-plane ($B\parallel ab$) and out-of-plane ($B\parallel c$) thermal expansion and magnetostriction at temperatures between 2 and 300 K and in magnetic fields up to 15 T. The anomalies in both response functions lead to the “magnetoelastic” phase diagrams and separate the paramagnetic (PM), ferromagnetic low-temperature/low-field (LTF) and aligned ferromagnetic (FM) phases. Different signs of magnetostriction anomalies as well as the evolution of thermal expansion anomalies at small fields $B\parallel ab$ of different magnetic-field dependence clearly supports the scenario of an intermediate region separating PM and LTF phases in finite external in-plane magnetic fields and implies a triple point in the magnetic phase diagram. Simulations of the magnetostriction using the Stoner-Wohlfarth model for uniaxial anisotropy demonstrate that the observed quadratic-in-field behavior in the LTF phase is in line with a rotation of the spins from the preferred $c$ direction into the $ab$ plane. Both the LTF and the PM phase close to $T_{\mathrm{C}}$ exhibit very strong pressure dependencies of the magnetization, $\partial ln{M}_{\mathrm{ab}}/\partial p_{\mathrm{ab}}$, of several hundred %/GPa and the transition from the LTF to the FM phase strongly depends on $p_{\mathrm{ab}}$ ($\sim -280$%/GPa), indicating a strong decrease in the uniaxial anisotropy under applied in-plane pressure. Our data clearly demonstrate the relevance of critical fluctuations and magnetoelastic coupling in ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$.

High coercivity is an important property of permanent magnets for application in energy conversion devices. The Nd magnet, ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$, is a typical material. Because coercivity is a long-time relaxation phenomenon, which originates from a strong metastable magnetic state, it is difficult to estimate coercive field (coercive force) studying the time evolution dynamics simulation of a model with atomistic parameters under the limitation of the simulation time. In our recent study [M. Nishino et al., Phys. Rev. B 102, 020413(R) (2020)], we presented a method to estimate coercivity using a statistical method to extend the limitation of simulation time and evaluated appropriately the coercive field of a single grain of the Nd magnet. In the present study, we propose an alternative method to estimate coercivity more conveniently using the field-dependent survival (nonreversal) probability generated by a time evolution simulation under a field sweep. We demonstrate that the coercive field of the single grain can be estimated. In this method, not only coercive field but also the zero-field energy barrier and field for the zero-energy barrier can be estimated. We discuss detailed features of the estimation of these quantities.

We report an in-plane thermal transport study of the honeycomb Kitaev material ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ at sub-Kelvin temperatures. In zero magnetic field, the $\kappa \left(T\right)$ displays a rather weak $T$ dependence but has a nonzero residual term ${\kappa }_0/T$, indicating strong phonon scattering by magnetic excitation and the possibility of itinerant spinonlike excitations coexisting with an antiferromagnetic order below 27 K. We propose the zero-field ground state is a fractionalized antiferromagnetic (AF*) state with both magnetic order and fractionalized excitations. With both the heat current and magnetic field along the $a*$ (Co-Co bond) direction, the ${\kappa }_{a*}\left(B\right)$ exhibits two sharp minima at 7.5 and 10 T, which confirms the phase boundaries of the reported field-induced intermediate state. No such intermediate phase was found in the ${\kappa }_a\left(B\right)$ for the current and field along the $a$ (zigzag chain) direction. Finally, ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ displays a strongly anisotropic magneto-thermal conductivity since the in-plane (out-of-plane) field strongly enhances (suppresses) ${\kappa }_{a*}$ and ${\kappa }_a$.

Motivated by the experimental identification of a higher-order Van Hove singularity (VHS) in $A{\text{V}}_3{\mathrm{Sb}}_5$ kagome metals, we study electronic instabilities of two-dimensional lattice models with higher-order VHS and flavor degeneracy. In contrast to conventional VHSs, the larger power-law density of states and the weaker nesting propensity of higher-order VHSs conspire together to generate distinct competing instabilities. After discussing the occurrence of higher-order VHSs on square and kagome lattice models, we perform unbiased renormalization group calculations to study competing instabilities and find a rich phase diagram containing ferromagnetism, antiferromagnetism, superconductivity, and Pomeranchuk orders. Remarkably, there is a generic transition from superconductivity to a $d$-wave Pomeranchuk order with increasing flavor number. Implications for the intriguing quantum states of $A{\text{V}}_3{\mathrm{Sb}}_5$ kagome metals are also discussed.

We investigate the effect of spin-orbit coupling on proximity-induced superconductivity in a normal metal attached to a superconductor. Specifically, we consider a heterostructure where the presence of interfaces gives rise to a Rashba spin-orbit coupling. The properties of the induced superconductivity in these systems are addressed within the tunneling Hamiltonian formalism. We find that the spin-orbit coupling induces a mixture of singlet and triplet pairing and, under specific circumstances, odd-frequency, even-parity, spin-triplet pairs can arise. We also address the effect of impurity scattering on the induced pairs and discuss our results in the context of heterostructures consisting of materials with spin-momentum locking.

The multiterminal generalization of the steady-state density functional theory for the description of electronic and thermal transport (referred to as iq-DFT) is presented. The linear-response regime of the framework is developed leading to exact expressions for the many-body transport coefficients and thermoelectric efficiency purely in terms of quantities accessible to the framework. The theory is applied to a multiterminal interacting quantum dot in the Coulomb blockade regime for which accurate parametrizations of the exchange-correlation kernel matrix are given. The thermoelectric efficiency and output power of the multiterminal system are studied. Surprisingly, the strong-interaction limit of these quantities can be understood in terms of the noninteracting one.

Excitons in two dimensions and other low-dimensional semiconductors are known to deviate from simple Wannier model descriptions due to nonlocal dielectric screening. As a consequence, energy levels do not follow a simple hydrogenic Rydberg series. Recently, a Kratzer model including a repulsive core potential has been suggested as an analytically solvable model of nonhydrogenic excitons. We adopt this model to describe both static and dynamic Stark effects in low-dimensional semiconductors. An exact formula for the exciton polarizability valid for arbitrary dimension and core potential is obtained. Moreover, analytical oscillator strengths allow for analyses of the dynamic Stark effect in various dimensions and materials.

The enhancement of two-photon spontaneous emission (2PSE) from trivalent and divalent rare earth ions in proximity to graphene and graphene nanoribbons is calculated for achievable experimental conditions using a combination of finite difference time domain simulations and direct computation of transition rates between energy levels in rare earths. For ${\mathrm{Er}}^{3+}$, we find that the 2PSE rate is initially 8 orders lower than the single-photon spontaneous emission rate but that, with enhancement, 2PSE can reach 2.5% of the overall decay. When graphene nanoribbons are used, we also show that the emission of free-space photon pairs from ${\mathrm{Er}}^{3+}$ at 3–3.2 $\mu \mathrm{m}$ via 2PSE can be increased by $\sim 400$. Our calculations show significantly less relative graphene-enhanced 2PSE than previous works, and we attribute this variation to differences in emitter size and assumed graphene mobility. We also show that the internal energy structure of the ion can have an impact on the degree of 2PSE enhancement achievable and find that divalent rare earths are more favorable.