npj Quantum Materials最新文献

Following the successful prediction of the superconducting phase diagram for infinite-layer nickelates, here we calculate the superconducting T_c vs. the number of layers n for finite-layer nickelates using the dynamical vertex approximation. To this end, we start with density functional theory, and include local correlations non-perturbatively by dynamical mean-field theory for n = 2–7. For all n, the Ni ({d}_{{x}^{2}-{y}^{2}}) orbital crosses the Fermi level, but for n > 4 there are additional (π, π) pockets or tubes that slightly enhance the layer-averaged hole doping of the ({d}_{{x}^{2}-{y}^{2}}) orbitals beyond the leading 1/n contribution stemming from the valence electron count. We finally calculate T_c for the single-orbital ({d}_{{x}^{2}-{y}^{2}}) Hubbard model by dynamical vertex approximation.

Ferroelectric materials exhibit a wealth of topological polar structures that hold promise for high-density, energy-efficient information technologies. Ferroelectric polarization configurations can be flipped by non-uniform mechanical stresses and associated lattice deformations and can be understood in the quasi-static regime based on flexoelectricity, but little is known about the dynamic mechanical excitations that generate topological ferroelectric structures. Here, we discover stable centre-type skyrmion-like polar nanodomains in super-tetragonal BiFeO₃ thin films generated by vibrational tapping using scanning probe microscope tips. Vibrational tapping can bidirectionally switch out-of-plane polarization by exerting strong dynamic force onto the elastically soft state emerging from strain-driven morphotropic phase transitions, which may be attributed to unconventional non-linear flexoelectric effects in the large strain-gradient regime. Our study provides a novel pathway into not only dynamic mechanoelectric coupling and topological polar structures, but also dynamic mechanical excitation technologies applicable to various fields.

α-MnTe, an A-type collinear antiferromagnet, has recently attracted significant attention due to its pronounced spin splitting despite having net zero magnetization, a phenomenon unique for a new class of magnetism dubbed altermagnetism. In this work, we develop a minimal effective Hamiltonian for MnTe based on realistic orbitals near the Fermi level at both the Γ and A points based on group representation theory, first-principles calculations, and tight-binding modeling. The Hamiltonian exhibits qualitatively distinct electron transport characteristics between these high-symmetry points and for different in-plane Néel vector orientations along the ([11bar{2}0]) and ([1bar{1}00]) directions. Although the spin–orbit coupling (SOC) is believed to be not important in altermagnets, we show the dominant role of SOC in the spin splitting and valence electrons of MnTe. These findings provide critical insights into altermagnetic electron transport in MnTe and establish a model playground for future theoretical and experimental studies.

Topological magnets exhibit fascinating physics like topologically protected surface states and anomalous transport. Although these states and phenomena are expected to strongly depend on the magnetic order, their experimental manipulation has been scarcely studied. Here, we demonstrate the magnetic field control of the topological band structure in Co₃Sn₂S₂ by magneto-optical spectroscopy. We resolve a magnetic field-induced redshift of the nodal loop resonance as the magnetization is rotated into the kagome plane. Our material-specific theory, capturing the observed field-induced spectral reconstruction, reveals the emergence of a gapless nodal loop for one of the in-plane magnetization directions. The calculations show that the additionally created Weyl points for in-plane fields marginally contribute to the optical response. These findings demonstrate that breaking underlying crystal symmetries with external fields provides an efficient way to manipulate topological band features. Moreover, our results highlight the potential of low-energy magneto-optical spectroscopy in probing variations of quantum geometry.

Interlayer interactions in few-layer NiPS₃ were investigated by analyzing low-frequency interlayer vibration modes and Davydov splitting of an intralayer, A_1g vibration mode at ~255 cm^–1 by Raman spectroscopy as a function of temperature. The interlayer force constants were estimated from the low-frequency Raman spectra by using the linear chain model. The out-of-plane direction interlayer force constant could also be estimated separately from the Davydov splitting, which agrees well with the linear chain model analysis. The dependence of the low-frequency shear and breathing modes and the Davydov splitting on the number of layers provide a unique, reliable tool for determining the number of layers.

Recently, the unconventional charge density wave (CDW) order with loop currents has attracted considerable attention in the Kagome material family AV₃Sb₅ (A = K, Rb, Cs). However, experimental signatures of loop current order remain elusive. In this work, based on the mean-field free energy, we analyze the collective modes of unconventional CDW order in a Kagome lattice model. Furthermore, we point out that phase modes in the imaginary CDW (iCDW) order with loop current orders result in time-dependent stray fields. We thus propose using nitrogen-vacancy (NV) centers to detect these time-dependent stray fields, providing a potential experimental approach to identifying loop current order.

In the long-wavelength limit, Bloch-band Berry curvature has no effect on the bulk plasmons of a two-dimensional electron system. In this Letter, we show instead that bulk plasmons are a probe of real-space topology. In particular, we focus on orbital Skyrme textures in twisted transition metal dichalcogenides, presenting detailed semiclassical and quantum mechanical calculations of the optical conductivity and plasmon spectrum of twisted MoTe₂.

Elucidating the factors limiting quantum coherence in real materials is essential to the development of quantum technologies. Here we report a strategic approach to determine the effect of lattice dynamics on spin coherence lifetimes using oxygen deficient double perovskites as host materials. In addition to obtaining millisecond T₁ spin-lattice lifetimes at T ~ 10 K, measurable quantum superpositions were observed up to room temperature. We determine that T₂ enhancement in Sr₂CaWO_6-δ over previously studied Ba₂CaWO_6-δ is caused by a dynamically-driven increase in effective site symmetry around the dominant paramagnetic site, assigned as W⁵⁺ via electron paramagnetic resonance spectroscopy. Further, a combination of experimental and computational techniques enabled quantification of the relative strength of spin-phonon coupling of each phonon mode. This analysis demonstrates the effect of thermodynamics and site symmetry on the spin lifetimes of W⁵⁺ paramagnetic defects, an important step in the process of reducing decoherence to produce longer-lived qubits.

Van der Waals (vdW) materials are featuring intertwined electronic order and collective phenomena. Elucidating the dynamics of the elementary excitations within the fundamental electronic degrees of freedom is of paramount importance. Here we performed resonant inelastic X-ray scattering (RIXS) to elaborate the spin-orbital excitations of the vdW antiferromagnet FePS₃ and their role for magnetism. We observed the spectral enhancement of spin-orbital multiplet excitations at about ~100 and ~220 meV, as well as the quasielastic response, when entering the antiferromagnetic phase with an order-parameter-like evolution in temperature. By comparing with model calculations, we discovered the trigonal lattice distortion, spin-orbit interaction and metal-ligand charge-transfer to be essential for these emergent excitations. We further reveal their spectral robustness down to the few atomic-layer limit by mechanical exfoliation, in accordance with the persistent antiferromagnetism reported previously. Our study highlights the crucial role of lattice and orbital anisotropy for stabilizing the quasi-two-dimensional magnetism and tailoring vdW magnets.

The layered cobaltate CaCoO₂ exhibits a unique herringbone-like structure. Serving as a potential prototype for a new class of complex lattice patterns, we study the properties of CaCoO₂ using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). Our results reveal a significant inter-plane hybridization between the Ca 4s- and Co 3d- orbitals, leading to an inversion of the textbook orbital occupation of a square planar geometry. Further, our RIXS data reveal a strong low energy mode, with anomalous intensity modulations as a function of momentum transfer close to a quasi-static response. These findings indicate that the newly discovered herringbone structure exhibited in CaCoO₂ may serve as a promising laboratory for the design of materials having strong electronic, orbital and lattice correlations.