npj Quantum Materials最新文献

Altermagnetism has been detected in several materials using spin-sensitive probes. These measurements require rather complex setups that make it challenging to track variations in altermagnetic order, e.g., to identify a temperature-tuned altermagnetic phase transition. We propose a simple transport measurement that can probe the order parameter for d-wave altermagnetism. We suggest magnetoconductivity anisotropy—the difference between the two principal values of the magnetoconductivity tensor. This quantity can be easily measured as a function of temperature, without any spin-selective apparatus. It acquires a nonzero value in a C₄K phase, where C₄ rotations and time reversal K are not symmetries but their combination is. This effect can be traced to the modification of phase space density due to Berry curvature, which we demonstrate using semiclassical equations of motion for band electrons. As an illustration, we build a minimal tight-binding model with altermagnetic order that breaks C₄ and K symmetries while preserving C₄K.

The recent discovery of two-dimensional (2D) moiré vortex patterns reveals new possibilities for nanoscale polar topology engineering and unexplored physical phenomena. However, the physical origin and detailed topological characteristics of these moiré vortex patterns have still not been understood. In this study, based on the lattice polarization coupling of ferroelectrics, we analytically determined the discovered strain state in twisted bilayer systems by elastic theory. Furthermore, the resulting moiré vortex patterns are investigated via phase-field simulations. Our findings demonstrate that the in-plane moiré vortex patterns arise from periodic displacement vorticity induced by moiré stacking. The complex interplay among elastic, flexoelectric, and gradient energy is identified as the energetic driving force behind the formation of these vortex patterns. Through three-dimensional simulation, we reveal that each polar vortex exhibits significant in-plane divergence and out-of-plane chirality, with the latter being tunable via external electric fields. These findings offer new avenues for manipulating nanoscale ferroelectric topologies.

Orbital magnetoelectric effect is closely related to the band topology of bulk crystalline insulators. While the half quantized Chern-Simons coupling is well defined only for insulators with zero Chern number, such as three dimensional (3D) axion insulators, the orbital magnetoelectric effects in 3D Chern insulators with nonzero (layer) Chern numbers remain unexplored. In this work, we propose a never-mentioned quantization rule for the layer-resolved orbital magnetoelectric response in quasi-3D and genuine 3D Chern insulators, showing that its spatial gradient is exactly quantized in unit of e²/h. We theoretically demonstrate that such quantized orbital magnetoelectric response, protected by Chern number, is exact for various types of interlayer hoppings and stackings, remaining robust even against disorder and lack of crystalline symmetries. Two promising material platforms for observing this effect are also proposed thanks to recent developments in detecting spatial magnetic-field distributions in device systems.

We combine ultra-high-resolution inelastic neutron scattering and quantum Monte Carlo simulations to study thermodynamics and spin excitations in the spin-supersolid phase of the triangular lattice XXZ antiferromagnet K₂Co(SeO₃)₂ under zero and non-zero magnetic field. BKT transitions signaling the onset of Ising and supersolid order are clearly identified, and the Wannier entropy is experimentally recovered just above the supersolid phase. At low temperatures, with an experimental resolution of about 23 μeV, no discrete coherent magnon modes are resolved within a broad scattering continuum. Alongside gapless excitations, a pseudo-Goldstone mode with a 0.06 meV gap is observed. A second, higher-energy continuum replaces single-spin-flip excitations of the Ising model. Under applied fields, the continuum evolves into coherent spin waves, with Goldstone and pseudo-Goldstone sectors responding differently. The experiments and simulations show excellent quantitative agreement.

Topological quantum materials with kagome lattice have become the emerging frontier in the context of condensed matter physics. Kagome lattice harbors strong magnetic frustration and topological electronic states generated by the unique geometric configuration. Kagome lattice has the peculiar advantages in the aspects of magnetism, topology as well as strong correlation when the spin, charge, or orbit degrees of free is introduced, and providing a promising platform for investigating the entangled interactions among them. In this paper, we will systematically introduce the research progress on the kagome topological materials and give a perspective in the framework of the potential future development directions in this field.

While there are many different mechanisms which have been proposed to understand the physics behind light induced “superconductivity”, what seems to be common to the class of materials in which this is observed are strong pairing correlations, which are present in the normal state. Here we argue, that the original ideas of Eliashberg are applicable to such a pseudogap phase and that with exposure to radiation the fermions are redistributed to higher energies where they are less deleterious to pairing. What results then is a photo-induced state with dramatically enhanced number of nearly condensed fermion pairs. In this phase, because the a.c. conductivity, σ(ω) = σ₁(ω) + iσ₂(ω), is dominated by the bosonic contribution, it can be computed using conventional (Aslamazov Larkin) fluctuation theory. We, thereby, observe the expected fingerprint of this photoinduced “superconducting” state which is a 1/ω dependence in σ₂ with fits to the data of the same quality as found for the so-called photo-enhanced (Drude) conductivity scenario. Here, however, we have a microscopic understanding of the characteristic low energy scale which appears in transport and which is necessarily temperature dependent. This approach also provides insight into recent observations of concomitant diamagnetic fluctuations. Our calculations suggest that the observed light-induced phase in these strongly paired superconductors has only short range phase coherence without long range superconducting order.

A Kekulé lattice is an exotic, distorted lattice structure exhibiting alternating bond lengths, distinguished from naturally formed atomic crystals. Despite its evident applicability, the formation of a Kekulé lattice from topological solitons in magnetic systems has remained elusive. Here, we propose twisted bilayer easy-plane Néel antiferromagnets as a promising platform for achieving a “Meron Kekulé lattice”—a distorted topological soliton lattice comprised of antiferromagnetic merons as its lattice elements. We demonstrate that the cores of these merons are stabilized into the Kekulé-O pattern with different intracell and intercell bond lengths across moiré supercells, thereby forming a Meron Kekulé lattice. Moreover, the two bond lengths of the Meron Kekulé lattice can be fine-tuned by adjusting the twist angle and specifics of the interlayer exchange coupling, suggesting extensive control over the meron lattice configuration in contrast to conventional magnetic systems. These discoveries pave the way for exploring topological solitons with distinctive Kekulé attributes.

Materials in the family RV₆Sn₆ (R = rare earth) provide a unique platform to investigate the interplay between local moments from R layers and nonmagnetic vanadium kagome layers. Yet, the investigation of actinide members remains scarce. Here we report the synthesis of UV₆Sn₆ single crystals through the self-flux technique. Physical property measurements reveal two uranium-driven antiferromagnetic transitions at T_N1 = 29 K and T_N2 = 24 K, a complex field-temperature phase diagram, and unusual negative domain-wall magnetoresistance. Specific heat and angle-resolved photoemission spectroscopy measurements show a moderate f-electron enhancement to the density of states at the Fermi level (E_F), whereas our band structure calculations place the vanadium flat bands 0.25 eV above E_F. These findings point to a materials opportunity to expand the uranium 166 family with the goal of enhancing correlations by tuning 5f and 3d flat bands to E_F.

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.