The discovery of two-dimensional (2D) van der Waals magnetic materials and their heterostructures provided an exciting platform for emerging phenomena with intriguing implications in information technology. Here, based on a multiscale modeling approach that combines first-principles calculations and a Heisenberg model, we demonstrate that interfacing a ${\mathrm{CrTe}}_2$ layer with various Te-based layers enables the control of the magnetic exchange and Dzyaloshinskii-Moriya interactions as well as the magnetic anisotropy energy of the whole heterobilayer, and thereby the emergence of topological magnetic phases such as skyrmions and antiferromagnetic Néel merons. The latter are novel particles in the world of topological magnetism since they arise in a frustrated Néel magnetic environment and manifest as multiples of intertwined hexamer textures. Our findings pave a promising road for proximity-induced engineering of both ferromagnetic and long-sought antiferromagnetic chiral objects in the very same 2D material, which is appealing for information technology devices employing quantum materials.

Cerium is a fascinating element due to its diverse physical properties, which include forming various crystal structures ($\gamma , \alpha , {\alpha }^{\prime }, {\alpha }^{\prime \prime }$, and $\epsilon$), mixed valence behavior, and superconductivity, making it an ideal platform for investigating the interplay between different electronic states. Here, we present a comprehensive transport study of cerium under quasihydrostatic pressures up to 54 GPa. Upon applying pressure, cerium undergoes the $\alpha \to {\alpha }^{\prime \prime }$ transition at around 4.9 GPa, which is accompanied by the appearance of superconductivity with $T_{\mathrm{c}}$ of 0.4 K, and $T_{\mathrm{c}}$ slightly increases to 0.5 K at 11.4 GPa. At 14.3 GPa, $T_{\mathrm{c}}$ suddenly increases when the ${\alpha }^{\prime \prime }$ phase transforms into the $\epsilon$ phase, reaching a maximum value of 1.25 K at around 17.2 GPa. Upon further increasing the pressure, $T_{\mathrm{c}}$ monotonically decreases. Together with the results of previous studies, our findings suggest that the evolution of superconductivity in cerium is closely correlated with the multiple pressure-induced structural transitions and corresponding unusual electronic structures.

In noncentrosymmetric superconductors the pair potential has both even-parity singlet and odd-parity triplet components. If time-reversal symmetry is broken, the superconducting phase of these components is not the same, for example in anapole superconductors. In this paper it is shown that breaking time-reversal symmetry by a phase difference between the two components significantly alters both the density of states and the conductance in $s+\mathrm{helical} p$-wave superconductors. The density of states and conductance in $s+\mathrm{chiral} p$-wave superconductors are less influenced by adding a phase difference because time-reversal symmetry is already broken in the $s+p$-wave superconductor. The Tanaka-Nazarov boundary conditions are extended to 3D superconductors, allowing us to investigate a greater variety of superconductors, such as Balian-Werthamer superconductors, in which the direction of the $d$ vector is parallel to the direction of momentum. The results are important for the determination of pair potentials in potentially time-reversal symmetry broken noncentrosymmetric superconductors.

Spin squeezing protocols successfully generate entangled many-body quantum states, the key pillars of the second quantum revolution. In our recent work [Phys. Rev. Lett. 129, 090403 (2022)] we showed that spin squeezing described by the one-axis twisting model can be generated in the Heisenberg spin-$1/2$ chain with periodic boundary conditions when accompanied by a position-dependent spin-flip coupling induced by a single laser field. In this work, we show analytically that the change in boundary conditions from the periodic to the open ones significantly modifies spin squeezing dynamics. A broad family of twisting models can be simulated by the system in the weak-coupling regime, including one- and two-axis twisting under specific conditions, providing the Heisenberg level of squeezing and acceleration of the dynamics. Our analytical findings are confirmed by full numerical simulations.

We introduce a quantum optics platform featuring the minimal ingredients for the description of a spintronically pumped magnon condensate, which we use to promote driven-dissipative phase transitions in the context of spintronics. We consider a Dicke model weakly coupled to an out-of-equilibrium bath with a tunable spin accumulation. The latter is pumped incoherently in a fashion reminiscent of experiments with magnet-metal heterostructures. The core of our analysis is the emergence of a hybrid lasing-superradiant regime that does not take place in an ordinary pumped Dicke spin ensemble, and which can be traced back to the spintronics pumping scheme. We interpret the resulting nonequilibrium phase diagram from both a quantum optics and a spintronics standpoint, supplying a conceptual bridge between the two fields. The outreach of our results concern dynamical control in magnon condensates and frequency-dependent gain media in quantum optics.

The effective medium theory is a key concept in metamaterials, with a growing interest in the temporal effective medium when the constitutive parameters are modulated with a period much shorter than the signal period. However, the effective medium with both frequency dispersion and time-varying resonating parameters is not clearly understood. In this work, we establish the temporal effective medium theory for acoustic metamaterials with time-varying frequency dispersion for compressibility. The formulas for the effective medium vary with the modulation of different resonance parameters and the transition from frequency-dispersive to -nondispersive regimes. Through experiments and theoretical models, we determine that time-varying resonance strength corresponds to the temporal average of compressibility β, time-varying resonating frequency corresponds to the temporal average of $1/\left(\beta \text{–}{\beta }_0\right)$ in the frequency-dispersive regime, and time-varying modulation amplitude in the nondispersive regime corresponds to the temporal average of 1/β. These results clarify the effective medium for frequency-dispersive time-varying metamaterials and have potential applications in different classical waves such as acoustics, elastodynamics, and electromagnetism for designing temporal functional devices.

In this paper, we study the non-Hermitian physics emerging from a predator-prey ecological model described by a generalized Lotka-Volterra equation. In the phase space, this nonlinear equation exhibits both chaotic and localized dynamics, which are separated by a critical point. These distinct dynamics originate from the interplay between the periodicity and non-Hermiticity of the effective Hamiltonian in the linearized equation of motion. Moreover, the dynamics at the critical point, such as algebraic divergence, can be understood as an exceptional point in the context of non-Hermitian physics.

Inelastic neutron scattering experiments combining time-of-flight and polarized techniques yield a comprehensive picture of the magnon dispersion in multiferroic ${\mathrm{TbMnO}}_3$ including the dynamic chirality. Taking into account only ${\mathrm{Mn}}^{3+}$ moments, spin-wave calculations including nearest-neighbor interactions, frustrating next-nearest neighbor exchange, as well as single-ion anisotropy and antisymmetric terms describe the energy dispersion and the distribution of neutron scattering intensity in the multiferroic state very well. Polarized neutron scattering reveals strong dynamic chirality of both signs that may be controlled by external electric fields in the multiferroic phase. Also above the onset of long-range multiferroic order in zero electric field, a small inelastic chiral component can be inverted by an electric field. The microscopic spin-wave calculations fully explain also the dynamic chirality of magnetic excitations, which is imprinted by the static chirality of the multiferroic phase. The ordering of ${\mathrm{Tb}}^{3+}$ moments at lower temperature reduces the broadening of magnons but also renders the magnon dispersion more complex.

Magnetotoroidal order, also called ferrotoroidicity, is the most recently established type of ferroic state. It is based on a spontaneous and uniform alignment of unit-cell-sized magnetic whirls, called magnetotoroidal moments, associated with a macroscopic toroidization. Because of its intrinsic linear magnetoelectric coupling, this new ferroic state could be useful in the development of spintronic devices. We exploit two-dimensional periodic arrays of magnetostatically coupled nanomagnets as model systems for the investigation of long-range magnetotoroidal order. We present two pathways promoting this order, namely, (i) structures comprising a ring of uniformly magnetized sub-micrometer-sized bar magnets and (ii) structures in which each magnetic building block itself hosts a magnetic vortex. For both cases, calculations of the magnetic-dipole interaction and micromagnetic simulations reveal the conditions for the formation of spontaneous magnetotoroidal order. We confirm this order and the formation of magnetotoroidal domains in our arrays with magnetic force microscopy. We identify the presence of two types of domain-wall states emerging from the competition of two intrinsic microscopic couplings. Our work not only identifies the microscopic conditions promoting spontaneous magnetotoroidal order but also highlights the possibility to tailor mesoscale magnetic arrays toward elusive types of ferroic order.

Antiferromagnetic (AF) spintronics offers the advantages of ultrahigh operating speed and stability to a device in the presence of a magnetic field. To fully exploit these advantages, the device should be comprised of all-AF materials. Although achieving a noticeable magnetoresistance (MR) effect in an all-AF device is difficult, this effect is essential for AF spintronic applications. Herein, we investigated the tunneling MR (TMR) effect in all-AF $\mathrm{Nb}\text{/}{\mathrm{Mn}}_2\mathrm{Au}\text{/}\mathrm{CdO}\text{/}{\mathrm{Mn}}_2\mathrm{Au}\text{/}\mathrm{Nb}$ magnetic tunnel junctions (MTJs) using first-principles scattering theory. Through calculations, we predicted the presence of high TMR, which was found to be of the order of $1000\%$ in the case of some symmetric junctions. This could be attributed to the interfacial resonance-tunneling effect related to the $k_{\left|\right|}$-dependent complex band structures of CdO and ${\mathrm{Mn}}_2\mathrm{Au}$ and to the high spin polarization of the interfacial magnetic atoms. Moreover, we studied the effects of voltage bias and interfacial disorder, such as oxygen vacancies, manganese vacancies, and manganese–cadmium exchanges, at the ${\mathrm{Mn}}_2\mathrm{Au}\text{/}\mathrm{CdO}$ interface. Our findings indicate that all-AF $\mathrm{Nb}\text{/}{\mathrm{Mn}}_2\mathrm{Au}\text{/}\mathrm{CdO}\text{/}{\mathrm{Mn}}_2\mathrm{Au}\text{/}\mathrm{Nb}$ MTJs are highly promising materials for spintronic applications and that rocksalt CdO is a potential symmetry-filtering material for such applications.

Three-dimensional massless topological semimetals exhibit linear energy band crossing points that act as monopoles of Berry curvature. Here, an alternative class of massless semimetals is introduced, featuring linear $N$-fold crossing points each of which acts as a source of a Berry dipole. We construct continuum and lattice models for such massless multifold Hopf semimetals (MMHSs) with $N=3,4,5$ bands and study nontrivial effects of a Berry dipole crossing: (i) A Landau level spectrum that is strongly tunable by the orientation of the magnetic field relative to the dipole axis. (ii) An anomalous Hall conductivity that is an odd function of the Fermi level. (iii) Weak-field dissipative magnetoconductivities that resemble the chiral anomaly, chiral magnetic and magnetochiral effects familiar from a pair of coupled Weyl nodes, but that are even functions of the Fermi level. By gapping out MMHSs, multiband Hopf insulators with Hopf numbers as high as ${\mathcal{N}}_{\text{Hopf}}=10$ are obtained, providing a fertile playground to explore delicate topology.

Motivated by the realization of coupled Luttinger liquids in a split condensate for quantum sine-Gordon (SG) model [Haller et al., Nature 466, 597 (2010); Schweiglet et al., Nature 545, 323 (2017); Nat. Phys. 17, 559 (2021)], here we study the phase transition in coupled condensates using the bosonization method. We show that when these condensates have different velocities $u_i$ and Luttinger parameters $K_i$, the physics is described by a generalized SG model, which is also exactly solvable. We use various approaches, including the Wilson renormalization group, correlation function, variational wave function, and the equivalent Coulomb gas methods, demonstrating the phase transition at $2=1/\left(4K_1\right)+1/\left(4K_2\right)$. The physical meaning of this boundary is explained by the density-phase uncertainty principle, showing how the many-body interactions drive the coupled condensates from two gapless phases to one gapless phase. The relative phase between the condensates may fundamentally affect the band structure, yet it will not affect this boundary. Furthermore, we also discuss the relevance of our result in the two-dimensional models for the Berezinskii-Kosterlitz-Thouless transition, from which the physical meaning of this boundary is self-evident. Our results can be used to study the phase transitions in one-dimensional coupled models with Fermions and Bosons, and even the Kane-Fisher model with impurities, and study their phase transitions, non-Gaussian dynamics, and thermalization, which may be of great interest in quantum simulation.

We present a detailed high pressure investigation on the magnetoelectric material ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ by means of Raman spectroscopy, synchrotron x-ray diffraction, and dielectric constant measurements combined with the density functional theory (DFT) based simulations. Our experimental results demonstrate three isostructural phase transitions at about 5.2 (P1), 8.5 (P2), and 11.5 GPa (P3) as evidenced by the distinct slope change in lattice parameters along with the splitting of certain Raman modes. Profound anomalies have been observed in linewidth and mode shift of several Raman modes as a consequence of isostructural transitions in the vicinity of P1, P2, and P3. Low temperature Raman spectroscopic investigation shows that strong spin-phonon coupling is present in the system up to 225 K due to short-range magnetic interactions. The onset of two crystal symmetry transitions at around 17 (P4) and 21.5 (P5) is manifested by the appearance of new Raman modes and the emergence/splitting of several Bragg peaks, which are corroborated by DFT calculations through enthalpy crossover. The system exhibits a mixed phase of P-3c1 and monoclinic C2/c in the pressure range of 17-20 GPa. At P5, the P-3c1 phase is replaced by a more distorted monoclinic $\mathrm{P}{2}_1/\mathrm{c}$ phase, which coexists with the C2/c phase up to 24 GPa. As per the DFT insight, the huge charge loss by the O atom in the buckled honeycomb layer around the pressure range 9–13 GPa may attributes to the electronic origin of structural transition.

We present ${}^{127}\mathrm{I}$ nuclear quadrupole resonance spectra and nuclear relaxation of $\alpha \text{−}{\left(\text{BEDT-TTF}\right)}_2{\mathrm{I}}_3$ that undergoes a charge-ordering transition. Only one of the two ${\mathrm{I}}_3$ anion sites shows a significant differentiation in the electric field gradients across the first-order transition. The charge modulation only in the BEDT-TTF layers cannot reproduce; instead, an anion-donor interaction accompanied by hydrogen bonding is necessary. The dominating source for the nuclear relaxation is the local libration of the ${\mathrm{I}}_3$ anions, but an anomalous peak is detected just below the transition, as observed by ${}^{13}\mathrm{C}$ NMR.

The single electron spectrum and wave functions in quasicrystals continue to be a fascinating problem, with few known exact solutions. We investigate the energy spectra and gap structures for a quasiperiodic tiling in two dimensions. Varying a continuous parameter, we follow the evolution of the band structure from discrete molecular or atomic states, to the multifractal states well known from previous studies. We propose a gap labeling scheme for finite approximants that is different from, but converges to, one introduced by Kellendonk and Putnam [Directions in Mathematical Quasicrystals, edited by M. Baake and R. Moody, CRM Monograph Series] for infinite tilings.

Quantum anomalous Hall effect (QAHE) is the real topological state without magnetic field that is robust against any perturbations, and is related to a bulk topological number $\mathcal{C}$, counting the number of chiral edge modes. Such chiral edge modes also exist at the magnetic domain walls between regions with different Chern numbers. Here, we systematically investigate the electronic properties of topological junctions formed at boundaries of QAHEs with different Chern numbers. We find that the number of chiral edge modes along the junction is determined by the difference of Chern numbers of adjacent regions, which can be understood from the coupling between counterpropagating channels along the junction. Finally, we show that the current partition of topological junctions can be flexibly manipulated by tuning the number of quantum anomalous Hall regions, Chern numbers, and the magnetization directions. Our work provides an ideal platform to design multichannel low-power devices for electronic circuits and switching applications.

Electron states with the spin-momentum-locked Dirac dispersion at the surface of a three-dimensional (3D) topological insulator are known to lead to weak antilocalization (WAL), i.e., low temperature and low-magnetic-field quantum interference-induced positive magnetoresistance (MR). In this work, we report on the MR measurements in ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\left(\mathrm{Te},\mathrm{Se}\right)}_3$ 3D topological insulator thin films epitaxially grown on Si(111), demonstrating an anomalous WAL amplitude. This anomalously high amplitude of WAL cannot be explained by parabolic or linear MR and indicates the existence of an additional MR mechanism. Another supporting observation is not linear in the classically weak magnetic field Hall effect in the same films. The increase of the low-field Hall coefficient, with respect to the higher-field value, reaches $10\%$. We consistently explain both transport features within a two-liquid model, where the mobility of one of the components strongly drops in a weak magnetic field. We argue that this dependence may arise from the Zeeman-field-induced gap opening mechanism.