We report the precessional rotation of magnetically isotropic ferromagnetic nanoparticles in a viscous liquid that are subjected to a rotating magnetic field. In contrast to magnetically anisotropic nanoparticles, the rotation of which occurs due to coupling between the magnetic and lattice subsystems through magnetocrystalline anisotropy, the rotation of isotropic nanoparticles is induced only by magnetic dissipation processes. We propose a theory of this phenomenon based on a set of equations describing the deterministic magnetic and rotational dynamics of such particles. Neglecting inertial effects, we solve these equations analytically, find the magnetization and particle precessions in the steady state, determine the components of the particle angular velocity, and analyze their dependence on the model parameters. The possibility of experimental observation of this phenomenon is also discussed.

Recently it was shown [I. A. Gruzberg, A. Klümper, W. Nuding and A. Sedrakyan, Phys. Rev. B 95, 125414 (2017)] that taking into account random positions of scattering nodes in the network model with $U\left(1\right)$ phase disorder yields a localization length exponent $2.37\pm 0.011$ for plateau transitions in the integer quantum Hall effect. This is in striking agreement with the experimental value of $2.38\pm 0.06$. Randomness of the network was modeled by replacing standard scattering nodes of a regular network by pure tunneling (respectively, reflection) with probability $p$ where the particular value $p=1/3$ was chosen. Here we investigate the role played by the strength of the geometric disorder, i.e., the value of $p$. We consider random networks with arbitrary probability $0<p<1/2$ for extreme cases and show the presence of a line of critical points with varying localization length indices having a minimum located at $p=1/3$.

High-quality magnets such as yttrium iron garnet (YIG) are electrically insulating and very complex. By implementing a quantum thermostat into atomistic spin dynamics we compute YIG's key thermodynamic properties, viz., the magnon power spectrum and specific heat, for a large temperature range. The results differ (sometimes spectacularly) from simple models and classical statistics, but agree with available experimental data.

We propose an application of a parity-time symmetric non-Hermitian Su-Schrieffer-Heeger (SSH) model by embedding it in a two-dimensional square lattice tube. The coalescence state at the exceptional point of the non-Hermitian SSH model is chiral and selectively controls helical transport and amplification. Two typical helicity-dependent scattering dynamics are observed. If the incidence has an identical helicity with the embedded non-Hermitian SSH model, we observe a perfect transmission without reflection. However, if the incidence has an opposite helicity with the embedded non-Hermitian SSH model, except for a full transmission, we observe an amplified transmission with different helicity from the incidence; but the amplified reflection has identical helicity with the incidence. These intriguing features are completely unexpected in a Hermitian system. Moreover, the helical amplification at high efficiency can be triggered by an arbitrary excitation. The different dynamics between incidences with opposite helicities are the results of unidirectional tunneling, which is revealed to be capable of realizing without introducing magnetic field. We give a discussion about the helical dynamics under system imperfections. Our findings open a direction in an all-optical device and provide perspectives in non-Hermitian transport.

We study the effect of a magnetic field on the low-energy description of Mott insulators with strong spin-orbit (SO) coupling. In contrast to the standard case of the Hubbard model without SO coupling, we show that Peierls phases can modulate the magnetic exchange at leading order in the interaction. Our mechanism crucially depends on the existence of distinct exchange paths between neighboring magnetic ions enclosing a well-defined area. Thus it will generically be present in any solid state realization of the Kitaev model and its extensions. We explicitly calculate the variation of the exchange constants of the so-called $JK\mathrm{\Gamma }$ model as a function of the magnetic flux. We discuss experimental implications of our findings for various settings of candidate Kitaev spin liquids.

Magnetic frustration in atomically ordered crystals gives rise to a broad range of quantum phenomena. The mineral bixbyite, $\mathrm{F}{\mathrm{e}}_{2-x}\mathrm{M}{\mathrm{n}}_x{\mathrm{O}}_3$, has magnetic frustration induced by atomic disorder. The end-members $\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_3$ and $\beta -\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$ both have magnetically ordered ground states, but the bixbyite crystal studied here, with a composition $\mathrm{F}{\mathrm{e}}_{1.12}\mathrm{M}{\mathrm{n}}_{0.88}{\mathrm{O}}_3$, instead undergoes a spin-glass-like transition near 32 K. In this study, the magnetic correlations and atomic structure of bixbyite are followed across the spin-glass transition using powder and single-crystal neutron scattering. Refinement of Bragg intensities from neutron diffraction data allows precise determination of the average Fe and Mn concentration on the different crystallographic sites of the structure. No structural transitions occur in bixbyite upon cooling. Using the three-dimensional magnetic difference pair distribution function (3D-mΔPDF), the magnetic correlations are resolved spatially and their temperature dependence determined. The results demonstrate that the spin-spin correlations are strong well above the glass transition temperature, consistent with the spin-glass nature of the magnetism in bixbyite.

We theoretically study the ground-state phase diagram of strongly interacting bosons on a generalized zigzag ladder model, the railroad-trestle (RRT) model. By means of analytical arguments in the limits of decoupled chains and the case of vanishing fillings as well as extensive DMRG calculations, we examine the rich interplay between frustration and interaction for various parameter regimes. We distinguish three different cases, the fully frustrated RRT model where the dispersion relation becomes doubly degenerate and an extensive chiral superfluid regime is found, the antisymmetric RRT with alternating $\pi$ and 0 fluxes through the ladder plaquettes and the sawtooth limit, which is closely related to the latter case. We study detailed phase diagrams which include besides different single-component superfluids, the chiral superfluid phases, the two component superfluids, and different gaped phases, with dimer and a charge-density wave order.

The $S=1/2$ spin ladder compound ${\left({\mathrm{C}}_5{\mathrm{H}}_{12}\mathrm{N}\right)}_2{\mathrm{CuBr}}_4$ (BPCB) is studied by means of high-resolution inelastic neutron scattering. In agreement with previous studies we find a band of triplet excitations with a spin gap of $\sim 0.8$ meV and a bandwidth of $\sim 0.6$ meV. In addition, we observe a distinct splitting of the triplet band of $50\left(1\right)\mu \mathrm{eV}$ or $40\left(2\right)\mu \mathrm{eV}$ at the band minimum or maximum, respectively. By comparison to a strong-coupling expansion calculation of the triplet dispersion for a spin ladder with anisotropic exchange, weakly anisotropic leg interactions are identified as the dominant source of magnetic anisotropy in BPCB. Based on these results, we discuss the nature of magnetic exchange anisotropy in BPCB and in related transition-metal insulators.

We consider phase-separated states in magnetic oxide (MO) thin films. We show that these states have a nonzero electric polarization. Moreover, the polarization is intimately related to a spatial distribution of magnetization in the film. Polarized states with opposite polarization and opposite magnetic configuration are degenerate. An external electric field removes the degeneracy and allows one to switch between the two states. So, one can control electric polarization and magnetic configuration of the phase-separated MO thin film with the external electric field.

We reassess the phase diagram of high-pressure solid hydrogen using mean-field and many-body wave function-based approaches to determine the nature of phase III of solid hydrogen. To discover the best candidates for phase III, density functional theory calculations within the metageneralized gradient approximation by means of the strongly constrained and appropriately normed (SCAN) semilocal density functional are employed. We study 11 molecular structures with different symmetries, which are the most competitive phases, within the pressure range of 100 to 500 GPa. The SCAN phase diagram predicts that the $C2/c-24$ and $P{6}_{1}22-36$ structures are the best candidates for phase III with an energy difference of less than 1 meV/atom. To verify the stability of the competitive insulator structures of $C2/c-24$ and $P{6}_{1}22-36$, we apply the diffusion Monte Carlo (DMC) method to optimize the percentage $\alpha$ of exact exchange in the trial many-body wave function. We found that the optimized $\alpha$ equals $40\%$ and denote the corresponding exchange and correlation functional as PBE1. The energy gain with respect to the well-known hybrid functional PBE0, where $\alpha =25\%$, varies with density and structure. The PBE1-DMC enthalpy-pressure phase diagram predicts that the $P{6}_{1}22-36$ structure is stable up to 210 GPa, where it transforms to the $C2/c-24$. Hence, we predict that phase III of high-pressure solid hydrogen is polymorphic.

The Hall conductivity ${\sigma }_{xy}$ of many condensed matter systems presents a step structure when a uniform perpendicular magnetic field is applied. We report the quantum Hall effect in buckled AB-bottom-top bilayer silicene and its robust dependence on the electronic valley and spin-orbit coupling. With the unique multivalley electronic structure and the lack of spin degeneracy, the quantization of the Hall conductivity in this system is unlike the conventional sequence as reported for graphene. Furthermore, the conductivity plateaus take different step values for conduction ($2e^2/h$) and valence ($6e^2/h$) bands since their originating valleys present inequivalent degeneracy. We also report the emergence of fractions under significant effect of a uniform external electric field on the quantum Hall step structure by the separation of orbital distributions and the mixing of Landau levels from distinct valleys. The valley- and spin-dependent quantum Hall conductivity arises from the interplay of lattice geometry, atomic interaction, spin-orbit coupling, and external magnetic and electric fields.

Understanding microscopic properties of ${\mathrm{LiNiO}}_2$, a Li-ion battery cathode material with extraordinarily high reversible capacity, has remained a challenge for decades. Based on extensive electronic structure calculations, which reveal a large number of nearly degenerate phases involving the local Jahn-Teller effect as well as bond and oxygen-based charge disproportionation, we propose that ${\mathrm{LiNiO}}_2$ exists in a high-entropy charge-glass-like state at and below ambient temperatures. Recognizing the glassy nature of ${\mathrm{LiNiO}}_2$ not only explains its key experimental features, but also opens a path in designing entropy-stabilized battery cathodes with superb capacities.

Recently, perovskite-related $\mathrm{BaPb}{\mathrm{O}}_3$ has attracted attention due to its hidden topological properties and, moreover, has been used as a thin layer in heterostructures to induce two-dimensional superconductivity. Here we investigate the normal-state electronic transport properties of thin films of $\mathrm{BaPb}{\mathrm{O}}_3$. Temperature- and magnetic-field-dependent sheet resistances are strongly affected by two-dimensional quantum effects. Our analysis decodes the interplay of spin-orbit coupling, disorder, and electron-electron interaction in this compound. Like for recently discussed topological materials, we find that weak antilocalization is the dominant protagonist in magnetotransport, whereas electron-electron interactions play a pronounced role in the temperature dependence. A systematic understanding of these quantum effects is essential to allow for accurate control of properties not only of thin films of $\mathrm{BaPb}{\mathrm{O}}_3$ but also of topological heterostructures.

The quantum anomalous Hall effect in magnetic topological insulators provides an ideal platform for building topological phases and devices. Here we construct a bilayer structure consisting of two magnetic topological insulator films with drastically different coercive field. In the intermediate field regime, the two quantum anomalous insulators have opposite spin orientation and counter-propagating edge states, thus realizing a synthetic quantum spin Hall phase. Multi-terminal transport measurements show that a moderate magnetic field can tune the system between chiral and helical edge state transport regimes. The interlayer transport realizes quantized spin-biased resistance, and the coupling between the two edges can be tuned by an epitaxially grown spacer layer in a sandwich structure. The tunable chiral and helical edge states in the quantum anomalous Hall bilayer may find unique applications in spintronics and topological quantum computation.

Synchronization is a ubiquitous phenomenon in nature and we propose its new perspective in ultrafast dynamics in interacting electron systems. In particular, using graphene irradiated by an intense bicircular pulse laser as a prototypical and experimental viable example, we theoretically investigate how to selectively generate a coherent oscillation of electronic order such as charge density orders (CDOs). The key is to use tailored fields that match the crystalline symmetry broken by the target order. After the pump, a macroscopic number of electrons start oscillating and coherence is built up through a transition. The resulting physics is detectable as a coherent light emission at the synchronizion frequency and may be used as a purely electronic way of realizing Floquet states respecting exotic space-time crystalline symmetries. In the process, we also explore possible flipping of existing static CDOs and generation of higher harmonics. The general framework for the coherent electronic order is found to be analogous with the celebrated Kuramoto model, describing the classical synchronization of coupled pendulums.

By a semianalytical Bethe ansatz method and a $T$-matrix approach we study the scattering of a spinon, the elementary quantum many-body topological excitation in the one-dimensional (1D) Heisenberg model, by local and phonon potentials. In particular, we contrast the scattering of a spinon to that of a free spinless fermion in the XY model to highlight the effect of strong correlations. For the one-spinon scattering in an odd-site chain, we find regular behavior of the scattering coefficients. In contrast, in an even-site chain there is a transfer of transmission probability between the two spinon branches that grows exponentially with system size. We link the exponent of the exponential behavior to the dressed charge that characterizes the critical properties of the 1D Heisenberg model, an interplay of topological and critical properties. The aim of this study is a microscopic understanding of spinon scattering by impurities, barriers, or phonons, modeled as prototype potentials, an input in the analysis of quantum spin transport experiments.

Numerical simulations of domain-wall propagation in thin nanotubes when an external magnetic field is applied along the nanotube axis have shown an unexpected behavior, described as a transition from a linear to a magnonic regime. As the applied magnetic field increases, the initial regime of linear growth of speed with the field is followed by a sudden change in slope accompanied by the emission of spin waves. In this work an analytical formula for the speed of the domain wall that explains this behavior is derived by means of an asymptotic study of the Landau-Lifshitz-Gilbert equation for thin nanotubes. We show that the dynamics can be reduced to a one-dimensional hyperbolic reaction diffusion equation, namely, the damped double sine-Gordon equation, which shows the transition to the magnonic regime as the domain-wall speed approaches the speed of spin waves. This equation has been previously found to describe domain-wall propagation in weak ferromagnets with mobility proportional to the Dzyaloshinskii-Moriya interaction constant; for Permalloy nanotubes the mobility is proportional to the nanotube radius.

The issue of whether the quantum critical point (QCP) is hidden inside unconventional superconductors is a matter of hot debate. Although a prominent experiment on London penetration depth has demonstrated the existence of the QCP in the isovalent-doped iron-based superconductor ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$, with the observation of a sharp peak in the penetration depth in the vicinity of the disappearance of magnetic order at zero temperature, the nature of such an emerging QCP remains unclear. Here, we provide a unique picture to understand well the phenomena of the QCP based on the framework of linear response theory. Evidence from the density of states and superfluid density calculations suggests the nodeless-to-nodal pairing transition accompanied the appearance of a sharp peak in the penetration depth in ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$. Such a pairing transition originates from the three-dimensional electronic properties with a strong interlayer superconducting pairing. This finding provides a significant insight into the understanding of the QCP observed in experiment in ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$.

Two-dimensional semi-Dirac fermions are quasiparticles that disperse linearly in one direction and quadratically in the other. We investigate instabilities of semi-Dirac fermions toward charge and spin density wave and superconducting orders, driven by short-range interactions. We analyze the critical behavior of the Yukawa theories for the different order parameters using Wilson momentum shell renormalization group. We generalize to a large number $N_f$ of fermion flavors to achieve analytic control in $2+1$ dimensions and calculate critical exponents at one-loop order, systematically including $1/N_f$ corrections. The latter depend on the specific form of the bosonic infrared propagator in $2+1$ dimensions, which needs to be included to regularize divergencies. The $1/N_f$ corrections are surprisingly small, suggesting that the expansion is well controlled in the physical dimension. The order parameter correlations inherit the electronic anisotropy of the semi-Dirac fermions, leading to correlation lengths that diverge along the spatial directions with distinct exponents, even at the mean-field level. We conjecture that the proximity to the critical point may stabilize novel modulated order phases.