Atomically thin materials, exemplified by graphene, have shown remarkable properties due to interlayer decoupling. Monolayer NbSe₂, for instance, exhibits enhanced charge-density wave (CDW) order and Ising superconductivity. However, bulk materials generally outperform monolayers in terms of scalability, stability, and practicality. Therefore, achieving interlayer decoupling in bulk crystals is highly desirable. While methods exist for interlayer decoupling in monolayers and few-layer materials (twisting, lattice mismatch), these are not applicable to conventional bulk crystals. Computational methods often simulate decoupling using thick vacuum layers, but a practical alternative is needed. Previous attempts with insulating block layers in superlattices like Ba₃Nb₁₁S₂₈ have shown partial success but lacked complete electron decoupling. This study explores the use of aerogel-like materials, known for their exceptionally low thermal conductivity, as block layers to achieve both electrical and vibrational decoupling in a bulk superlattice.

Extensive research on 2D materials has highlighted the significant impact of interlayer decoupling on their properties. Graphene’s exceptional electronic and mechanical characteristics, monolayer MoS₂'s direct band gap transition, and the enhanced CDW order and superconductivity in monolayer NbSe₂ are prominent examples. The challenges of working with monolayer materials such as fabrication, stability, and quantity, however, push the need to develop strategies to achieve similar properties in bulk materials. While techniques like mechanical exfoliation, chemical vapor deposition, and introducing twist angles or lattice mismatch have been used to modify interlayer interactions, these methods are not scalable or suitable for all materials. Recent studies explored the use of insulating block layers in bulk superlattices, but these did not achieve complete decoupling. The use of aerogel-like structures, inspired by their extremely low thermal conductivity, is a novel approach to achieving both electronic and vibrational decoupling in bulk crystals.

Single crystals of (AOH)_xMX₂ (A = Na, K; M = Ta, Nb; X = S, Se) were synthesized using a hydrothermal method, employing MX₂ single crystals as precursors. (NaOH)_0.5NbSe₂ was selected as a model system. X-ray diffraction (XRD), both powder and single crystal, was used to characterize the crystal structure and confirm the incorporation of NaOH layers. Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were employed to analyze the elemental composition and oxidation states. Density functional theory (DFT) calculations were performed to understand the electronic structure and interlayer coupling. Electrical transport properties (resistivity) were measured along both in-plane (a-b) and out-of-plane (c-axis) directions to determine electrical anisotropy. Raman spectroscopy was used to investigate vibrational modes and confirm the absence of interlayer coupling. Heat capacity measurements helped identify localized phonon modes. Time-domain thermoreflectance (TDTR) and steady-state heat flow methods were used to determine out-of-plane and in-plane thermal conductivity, respectively. Finally, magnetic torque measurements were performed to study the CDW transition and superconductivity.

XRD data revealed that the introduction of NaOH layers significantly increased the interlayer spacing in (NaOH)_0.5NbSe₂. The out-of-plane resistivity showed insulating behavior, while in-plane resistivity remained metallic, indicating electrical decoupling. DFT calculations confirmed this by showing that the electronic bands and Fermi surfaces were similar to monolayer NbSe₂, with little to no interlayer interaction. Raman spectroscopy showed absence of interlayer vibrational modes, confirming vibrational decoupling. Heat capacity measurements showed that a significant portion (91%) of the phonon vibrations were localized. The out-of-plane thermal conductivity was dramatically reduced (0.28 W m⁻¹ K⁻¹ at room temperature), which is only 7% of the value for bulk NbSe₂. This indicates extremely low interlayer coupling. The in-plane thermal conductivity, however, remained high, leading to a very high thermal conductivity anisotropy (~350 at 300 K). DFT calculations showed that the interlayer coupling energy was significantly reduced (2.5 meV/Ų, about 9% of bulk NbSe₂). Raman spectroscopy revealed a much higher CDW transition temperature (>110 K) than bulk NbSe₂ (33 K), consistent with monolayer NbSe₂. Magnetic torque measurements further confirmed the enhanced CDW transition temperature and demonstrated that the CDW phase retained D_3h symmetry. Superconductivity was observed with a critical temperature of 1.2 K and a Pauli violation ratio up to 4.0, characteristic of Pauli-breaking Ising superconductivity.

The results demonstrate that the incorporation of aerogel-like NaOH layers effectively decouples the NbSe₂ layers in the bulk superlattice, both electrically and vibrationally. The observed properties, including the enhanced CDW transition temperature, the extremely low out-of-plane thermal conductivity and the Pauli-breaking 2D superconductivity are consistent with the behavior observed in monolayer NbSe₂. This strongly suggests that the bulk (NaOH)_0.5NbSe₂ behaves effectively as a collection of decoupled NbSe₂ monolayers. The significant anisotropy in both electrical and thermal conductivity underscores the success of the interlayer decoupling strategy. This approach opens up possibilities for creating bulk materials with tailored 2D properties.

This research presents a novel method for achieving interlayer decoupling in bulk materials by introducing aerogel-like layers. The (NaOH)_0.5NbSe₂ superlattice exhibits drastically enhanced CDW transition temperature and Pauli-breaking 2D superconductivity, mirroring monolayer NbSe₂. This technique allows for the large-scale fabrication of materials with intrinsic 2D characteristics, paving the way for exploring new functionalities in bulk materials. Future studies could investigate other 2D materials and different aerogel-like layers to expand the applicability of this approach and further understand the relationship between structure and properties.

The study focuses primarily on (NaOH)_0.5NbSe₂, limiting the generalizability of the findings to other material systems. The hydrothermal synthesis method may present challenges in scaling up production. Further research is needed to fully explore the impact of different aerogel-like materials and their influence on the decoupling process. The study assumes a perfectly ordered structure for the superlattice, while in reality, some structural imperfections might exist.