npj Quantum Materials最新文献

Hybrid lattice-light modes, known as phonon polaritons, represent the backbone of advanced protocols based on THz pumping of infrared modes. Here we provide a theoretical framework able to capture the different roles played by phonon polaritons in experimental protocols based either on Raman-like pump and probe schemes, typical of four-wave mixing processes, or on THz pump-visible probe three-wave mixing protocols. By using a many-body description of the nonlinear optical kernel, along with a perturbative solution of nonlinear Maxwell’s equations, we discuss the limitations of all-optical four-wave mixing protocols and we highlight the advantages of exploiting broadband THz pumps to enlarge the phase space of the phonon polariton dispersion at low momenta accessible in a single experiment. Besides providing a quantitative description of existing and future experiments, our results offer a general framework for the theoretical modeling of the hybridization between light and lattice degrees of freedom in time-resolved experiments.

The charge density wave in (TaSe₄)₂I has drawn much attention recently as a controversial candidate for an axion insulator where the CDW breaks the chiral symmetry of the Weyl semimetal. Here we use ultrafast x-ray scattering to study the collective modes of this CDW. By measuring several diffraction peaks we find that the order parameter involves coupled optical and acoustic modes. For strong near-infrared excitation, the dynamics of the x-ray diffraction show evidence of photoinduced inversion of both components of the CDW order parameter, and associated domain walls. These results demonstrate the potential of ultrafast methods to induce topological defects through highly nonequilibrium dynamics. In (TaSe₄)₂I these defects should lead to exotic electronic states due to the nontrivial topology of the band structure.

Transition-metal ions with 5d² electronic configuration in a cubic crystal field are prone to have a vanishing dipolar magnetic moment but finite higher-order multipolar moments, and they are expected to exhibit exotic physical properties. Through an investigation using resonant inelastic X-ray scattering (RIXS), Raman spectroscopy, and theoretical ligand-field (LF) multiplet and ab initio calculations, we fully characterized the local electronic structure of Ba₂CaOsO₆, particularly, the crystal-field symmetry of the 5d² electrons in this anomalous material. The low-energy multiplet excitations from RIXS at the oxygen K edge and Raman-active phonons both show no splitting. These findings are consistent with the ground state of Os ions dominated by magnetic octupoles. Obtained parameters pave the way for further realistic microscopic studies of this highly unusual class of materials, advancing our understanding of spin-orbit physics in systems with higher-order multipoles.

We show that certain crystalline topological defects in the gapless Kitaev honeycomb spin liquid model generate a chirality and Majorana fermion orbital magnetization that depends in a universal manner on their emergent flux. Focusing on 5–7 dislocations as building blocks, consisting of pentagon and heptagon disclinations, we identify the Kitaev bond label configurations that preserve solvability. By computing two formulations of local markers M(r) we find that the 5 and 7 lattice defects generate a real-space contribution to Chern number and an associated Majorana fermion orbital magnetization proportional to M(r). The sign of the M(r) contribution from each 5/7 defect, i.e. its q_M = ± 1 chirality, is determined by the defect Frank angle sign F and emergent gauge field flux W = ± i through the expression q_M = − iFW. Remarkably, though lattice curvature and torsion can interplay with the surrounding gapless background to modify the profile of M(r), its sign q_M is determined locally, implying that crystalline defects in the Kitaev spin liquid can generate a robust and observable chirality.

We present our study of (0001) oriented Mn₃Sn (c-Mn₃Sn) thin films synthesized directly on an MgO (111) substrate via molecular beam epitaxy. We identify a growth window where Mn₃Sn growth can be controlled through slight adjustments of the Mn flux, achieving either μm²-sized high crystalline-quality islands or an almost completely continuous film. High-resolution X-ray diffraction results indicate that both films are highly (0001) oriented. The atomic resolution images show clear film-substrate interfaces displaying an epitaxial relationship. Scanning precession electron diffraction measurements reveal that the island featured sample has highly crystallized Mn₃Sn. The sample featuring a high continuity exhibits defects in some areas but retains the dominant Mn₃Sn structure. This work demonstrates a potential method for synthesizing high crystalline-quality Mn₃Sn films with substantial coverage, facilitating the study of Mn₃Sn films without the influence of an additional buffer layer and promoting their application in integrated spintronics.

Materials featuring hypervalent bismuth motifs have generated immense interest due to their extraordinary electronic structure and exotic quantum transport. In this study, we synthesized high-quality single crystals of La₃ScBi₅ characterized by one-dimensional hypervalent bismuth chains and performed a systematic investigation of the magnetoresistive behavior and quantum oscillations. The metallic La₃ScBi₅ exhibits a low-temperature plateau of electrical resistivity and quasi-linear positive magnetoresistance, with anisotropic magnetoresistive behaviors suggesting the presence of anisotropic Fermi surfaces. This distinctive transport phenomenon is perfectly elucidated by first-principles calculations utilizing the semiclassical Boltzmann transport theory. Furthermore, the nonlinear Hall resistivity pointed towards a multiband electronic structure, characterized by the coexistence of electron and hole carriers, which is further supported by our first-principles calculations. Angle-dependent de Haas-van Alphen oscillations are crucial for further elucidating its Fermiology and topological characteristics. Intriguingly, magnetization measurements unveiled a notable paramagnetic singularity at low fields, which might suggest the nontrivial nature of the surface states. Our findings underscore the interplay between transport phenomena and the unique electronic structure of hypervalent bismuthide La₃ScBi₅, opening avenues for exploring novel electronic applications.

Predetermining the as-grown polarization of ferroelectric thin films is essential to integrate their reliable properties into electronic devices. However, studies have so far focused mainly on the control of the polarization state of a single ferroelectric layer. Here we report a strategy for the artificial modulation of pristine polarization in BiFeO₃ bilayer films. We have fabricated multilayers of BiFeO₃/SrTiO₃/BiFeO₃ on single-crystalline SrTiO₃ (001) substrates. It is found that the out-of-plane polarization components of the BiFeO₃ bilayer can be controlled by modifying the surface terminations of SrTiO₃ interlayer and SrTiO₃ substrate. Using aberration-corrected scanning transmission electron microscopy, we directly visualize the head-to-head and tail-to-tail polarization configurations formed by the BiFeO₃ bilayers. Polar discontinuity at the ferroelectric/non-ferroelectric interface is the reason for tuning the orientation of electrical polarization. Our work provides an effective route to design fascinating ferroelectric multilayers with well-defined polarization direction.

Understanding the formation of skyrmions in centrosymmetric materials is a problem of fundamental and technological interest. GdRu₂Si₂ is a candidate material that hosts a variety of multi-Q magnetic phases, including in zero-field. Here, inelastic neutron scattering is used to measure the spin excitations in the field-polarized phase of GdRu₂Si₂. Linear spin wave theory and a method of interaction invariant path analysis are used to derive a Hamiltonian accounting for the spectra. The Hamiltonian, dominated by bilinear (Ruderman-Kittel-Kasuya-Yosida) Heisenberg exchange, compares favorably to ab initio calculations. Dipolar interactions are a secondary energy scale to consider, with J_D.D~ 0.05J_RKKY. However, it is shown that in the field-polarized phase the dipolar interactions ‘self screen’ so that their effect is largely suppressed. No specific evidence for higher-order exchange is found. These aspects are discussed in the context of the lower field multi-Q states and the anisotropy of the system.

Altermagnets are magnetic states with fully compensated spins and broken PT (PT: parity times time reversal) symmetry (i.e., spin-split bands). We classify three kinds of altermagnets in terms of broken P and T. Furthermore, strong altermagnets have spin-split bands without spin-orbit coupling (SOC), and weak altermagnets has spin-split bands only with non-zero SOC. These strong vs. weak altermagnets can be identified from the total number of symmetric spin rotation operations.

In standard quantum electrodynamics (QED), the so-called non-minimal (Pauli) coupling is suppressed for elementary particles and has no physical implications. Here, we show that the Pauli term naturally appears in a known family of Dirac materials—the lead-halide perovskites, suggesting a novel playground for the study of analog QED effects. We outline measurable manifestations of the Pauli term in the phenomena pertaining to (i) relativistic corrections to bound states (ii) the Klein paradox, and (iii) spin effects in scattering. In particular, we demonstrate that (a) the binding energy of an electron in the vicinity of a positively charged defect is noticeably decreased due to the polarizability of lead ions and the appearance of a Darwin-like term, (b) strong spin-orbit coupling due to the Pauli term affects the exciton states, and (c) scattering of an electron off an energy barrier with broken mirror symmetry produces spin polarization in the outgoing current. Our study adds to the understanding of quantum phenomena in lead-halide perovskites and paves the way for tabletop simulations of analog Dirac-Pauli equations.