npj Quantum Materials最新文献

Heterostructures made from 2D transition-metal dichalcogenides are known as ideal platforms to explore excitonic phenomena ranging from correlated moiré excitons to degenerate interlayer exciton ensembles. So far, it is assumed that the atomic reconstruction appearing in some of the heterostructures gives rise to a dominating localization of the exciton states. We demonstrate that the center-of-mass wavefunction of the excitonic states in reconstructed MoSe₂/WSe₂ heterostructures can extend well beyond the moiré periodicity of the investigated heterostructures. The results are based on real-space calculations yielding a lateral potential map for interlayer excitons within the strain-relaxed heterostructures with weak random disorder, as expected for realistic samples, and the corresponding real-space center-of-mass excitonic wavefunctions. We combine the theoretical results with cryogenic photoluminescence experiments, which support the computed level structure and relaxation characteristics of the interlayer excitons.

Generalized Wigner crystals (GWC) on triangular moiré superlattices, formed from stacking two layers of transition metal chalcogenides, have been observed at multiple fractional fillings [Nature 587, 214–218 (2020), Nat. Phys. 17, 715–719 (2021), Nature 597, 650–654 (2021)]. Motivated by these experiments, tied with the need for accurate microscopic descriptions of these materials, we explore the origins of GWC at n = 1/3 and 2/3 filling. We demonstrate the general limitations of theoretical descriptions relying on finite-range, versus long-range interactions, however, we clarify why some properties are captured by an effective nearest-neighbor model. We study both classical and quantum effects at zero and finite temperatures, discussing the role of charge frustration, identifying a “pinball” phase, a partially quantum melted GWC, with no classical analog. Our work addresses several experimental observations and makes predictions for how many of the theoretical findings can be potentially realized in future experiments.

Magneto-optical measurements in La_0.8Sr_0.2NiO₂ and Nd_0.825Sr_0.175NiO₂ reveal an intriguing new facet of infinite-layer nickelate superconductors: the onset of spin-glass behavior at a temperature far exceeding the superconducting critical temperature T_c. This discovery sharply contrasts with copper oxide superconductors, where magnetism and superconductivity remain largely exclusive. Moreover, the magnitude and onset temperature of the polar Kerr effect in Nd_0.825Sr_0.175NiO₂ fabricated on SrTiO₃ and (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 substrates differ dramatically, while T_c does not.

It is a distinct possibility that spin fluctuations are the pairing interactions in numerous unconventional superconductors. In the high-transition-temperature (high-T_c) cuprates, superconductivity emerges upon doping antiferromagnetic Mott insulators, and spin fluctuations might furthermore drive unusual pseudogap phenomena. Here we use magnetic neutron scattering to study the highly underdoped cuprate HgBa₂CuO_4+δ (hole concentration p ≈ 0.064). In contrast to prior results for other underdoped cuprates, we find no evidence of incommensurate magnetic order associated with spin-density-wave or stripe correlations. Instead, the antiferromagnetic response in both the superconducting and pseudogap states is gapped below Δ_AF ≈ 6 meV, commensurate over a wide energy range, and disperses above about 55 meV. Given the pristine nature of HgBa₂CuO_4+δ, which exhibits high structural symmetry and minimal point disorder effects, this behavior likely signifies the unmasked response of the underlying CuO₂ planes near the Mott-insulating state. These results serve as a benchmark for a refined theoretical understanding of the cuprates.

Stacking and twisting van der Waals materials provides a powerful tool to engineer quantum matter. For instance, 1T-TaS₂ monolayers are Mott insulators, whereas layered 1H-TaS₂ is metallic and superconducting; thus, the T/H bilayer, where heavy fermions and unconventional superconducting phases are expected from localized spins (1T) coexisting with itinerant electrons (1H), has been intensively studied. However, recent studies revealed significant charge transfer that questions this scenario. Here, we propose a T/T/H trilayer heterostructure where the T/T bilayer is a flat-dispersion band insulator with localized electrons, whereas the 1H layer remains metallic with a weak spin polarization. Varying the T/T stacking configuration tunes the flat-band filling, enabling a crossover from a doped-Mott regime to a Kondo-like state. Such a trilayer heterostructure provides, therefore, a rich novel platform to study strong correlation phenomena and unconventional superconductivity.

Nonlinear light-matter interaction at low energy, particularly in the terahertz (THz) frequency range, hosts unique phenomena distinct from the optical excitation with photon energy of a few eV. In cuprate superconductors Bi₂Sr₂CaCu₂O_8+x, the THz nonlinear response is identified via the optical reflectivity change and interpreted as the amplitude mode of the superconducting condensate, namely the Higgs mode [K. Katsumi et al., Phys. Rev. Lett. 120, 117001 (2018)]. However, the origin of the THz nonlinearity has been questioned because the pair-breaking process, identified in Raman spectroscopy, can also contribute to it. Here, we reexamined the THz-driven nonequilibrium dynamics in cuprates Bi₂Sr₂CaCu₂O_8+x by comparing it with the Raman susceptibility. In the optical reflectivity change, we found an oscillatory behavior following the squared THz waveform (THz Kerr signal), as well as the relaxation of the quasiparticle excitation. Careful insight into the data revealed that the oscillatory and decaying contributions exhibit different doping dependence. Remarkably, the doping and temperature evolutions of the THz Kerr signal are distinct from those of the Raman susceptibility, which is described by the pair-breaking due to diamagnetic light–matter interaction. These results indicate the importance of the paramagnetic light–matter coupling in the THz Kerr signal in the cuprate superconductors, likely arising from the Higgs mode.