npj Quantum Materials最新文献

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.

Inelastic neutron scattering (INS) measurements of powder D₃(⁷Li)(¹⁹³Ir)₂O₆ reveal low energy magnetic excitations with a scattering cross-section that is broad in ∣Q∣ and energy transfer. The magnetic nature of the excitation spectrum is demonstrated by longitudinally polarized neutron scattering. The total magnetic moment of 1.8(4)μ_B/Ir inferred from the observed magnetic scattering cross-section is consistent with the effective moment inferred from magnetic susceptibility data and expectations for the J_eff = 1/2 single ion state. The rise in the dynamic correlation function ({mathcal{S}}(Q,omega )) for ℏω < 5 meV can be described by a simple model assuming nearest-neighbor anisotropic spin exchange, such as that found in the Kitaev model. Exchange disorder associated with the D site likely plays an important role in stabilizing the low T quantum fluctuating state^1,2.

The layered honeycomb magnet α-RuCl₃ has emerged as a promising candidate for realizing a Kitaev quantum spin liquid. Previous studies have reported oscillation-like anomalies in the longitudinal thermal conductivity and half-integer quantized thermal Hall conductivity above the antiferromagnetic critical field H_c, generating significant interest. However, the origins of these phenomena remain contentious due to strong sample dependence. Here we re-examine the magnetothermal transport properties using recently available ultra-pure α-RuCl₃ single crystals to further elucidate potential signatures of the spin liquid state. Our findings reveal that while anomalies in thermal conductivity above H_c persist even in ultraclean crystals, their magnitude is significantly attenuated, contrary to the quantum oscillations hypothesis. This suggests that the anomalies are likely attributable to localized stacking faults inadvertently introduced during magnetothermal transport measurements. The thermal Hall conductivity exhibits a half-quantized plateau, albeit with a narrower width than previously reported. This width reduction can be understood through two distinct mechanisms: sample-dependent magnetic critical fields that influence the lower boundary of the plateau region, and the decoupling between chiral Majorana edge currents and phononic thermal transport that determines the upper boundary. These results indicate that structural imperfections exert a substantial influence on both the oscillation-like anomalies and quantization effects observed in magnetothermal transport measurements of α-RuCl₃.

Inspired by the striking discovery of metastable superconductivity in K₃C₆₀ at 100K, far above T_c = 20 K, we discuss possible mechanisms for long-lived, photo-induced superconductivity. Starting from a model of optically-driven Raman phonons coupled to inter-band electronic transitions, we develop a microscopic mechanism for photo-controlling the pairing interaction. Leveraging this mechanism, we first investigate long-lived superconductivity arising from the thermodynamic metastable trapping of the driven phonon. We then propose an alternative route, where the superconducting gap created by an optical drive leads to a dynamical bottleneck in the equilibration of quasi-particles. We conclude by discussing the implications of both scenarios for experiments that can be used to discriminate between them. Our work provides falsifiable explanations for the nanosecond-scale photo-induced superconductivity found in K₃C₆₀, while simultaneously offering a theoretical basis for exploring metastable superconductivity in other quantum materials.

Vanadium dioxide (VO₂) is a prototypical material that undergoes a structural phase transition (SPT) from a monoclinic (M1) to rutile (R) structure and an insulator-to-metal transition (IMT) when heated above 340 K or excited by an ultrafast laser pulse. Due to the strong electron–electron and electron–lattice interactions, modeling the ultrafast IMT in VO₂ has proven challenging. Here, we develop an efficient theoretical approach to the light-induced phase transitions by combining a tensor network ansatz for the electrons with a semiclassical description of the nuclei. Our method is based on a quasi-one-dimensional model for the material with the important multiorbital character, electron–lattice coupling, and electron–electron correlations being included. We benchmark our method by showing that it qualitatively captures the ground state phase diagram and finite-temperature phase transitions of VO₂. Then, we use the hybrid quantum-classical tensor network approach to simulate the dynamics following photoexcitation. We find that the structure can transform faster than the harmonic phonon modes of the M1 phase, suggesting lattice nonlinearity is key in the SPT. We also find separate timescales in the evolution of dimerization and tilt lattice distortions, as well as the loss and subsequent partial restoration behavior of the displacements, explaining the complex dynamics observed in recent experiments. Moreover, decoupled SPT and IMT dynamics are observed, with the IMT occurs quasi-instantaneously. Our model and approach, which can be extended to a wide range of materials, reveal the unexpected non-monotonic transformation pathways in VO₂ and pave the way for future studies of non-thermal phase transformations in quantum materials.

1T-TaS₂ is a non-magnetic Mott insulating transition-metal dichalcogenide with an odd number of electrons per unit cell, making it a potential spin-liquid candidate. This behavior arises from miniband reconstructions in the charge density wave state, producing a nearly flat band at half-filling. We revisit its electronic band structure using a nearest-neighbor tight-binding model, emphasizing the importance of often-neglected “spin-flip” terms in the spin-orbit coupling. By comparing with density functional theory calculations, we estimate the strength of these couplings. We also apply our theory to 1T-TaSe₂, which is found to be a promising candidate for a topologically non-trivial flat band. Our findings have significant implications for correlated physics in the flat band, including the emergent spin-spin Hamiltonian at half-filling, identified as a J-K-Γ-(Gamma ^{prime}) model on a triangular lattice, and for tuning electronic properties away from half-filling.