Interlayer Sliding in Bulk 3R-NbSe2

• The paper introduces a growth-engineered stacking reconstruction where excess Nb drives an in-plane shift that creates a noncentrosymmetric, weakly coupled layered system.
• The modified structure yields quasi-2D superconductivity with pronounced Ising spin splitting and competing Rashba effects, as evidenced by a two-fold suppression in the upper critical field.
• Interlayer sliding also stabilizes an unconventional, monolayer-like charge-density-wave state, demonstrating a unified route to tune intertwined electronic phases in bulk NbSe2.

Searching arXiv for the specified paper and closely related NbSe2 work to ground the article. {"^^^^^^^^2query2 OR title:\2^^^^^^^^"Sliding two-dimensional superconductivity and charge-density-wave state in a bulk crystal\"","max_results":5,"sort_by":"submittedDate","sort_order":"descending"} {"^^^^^^^^2query2 Ising superconductivity monolayer charge density wave 3R noncentrosymmetric","max_results":^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2query2^^^^,"sort_by":"relevance","sort_order":"descending"} Interlayer sliding in bulk PRESERVED_PLACEHOLDER_^^^^2query2^^^^-NbSePRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^ denotes a growth-engineered modification of the stacking sequence in which NbSe$_2$ layers undergo an in-plane shift relative to the pristine $3R$ ABC registry, producing a bulk crystal that behaves electronically as a weakly coupled stack of two-dimensional superconducting sheets. In the reported sliding phase, self-intercalated Nb atoms occupy interlayer octahedral sites, reconstruct local bonding, and drive the system into an incommensurate rhombohedral arrangement with $0^\circ$-aligned layers and a unit cell consistent with the noncentrosymmetric space group $R3m$. The resulting structure deliberately disrupts $[001]$ mirror symmetry, strongly suppresses interlayer coupling, stabilizes Ising-type superconductivity in the bulk, and coexists with an unconventional charge-density-wave state akin to that of monolayer $2H$-NbSe$_2$ (&&&^^^^2query2^^^^&&&).

^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^. Structural definition and growth-induced sliding

The central structural operation is interlayer sliding: adjacent NbSe$_2$ layers are no longer arranged in the pristine PRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2query2^^^^ stacking sequence but are shifted in plane. In the reported system, this is engineered during growth by modulating the Nb content in PRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^-NbPRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\22^^^^SePRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^3. The excess Nb occupies interlayer octahedral sites, reconstructs the local bonding environment, and drives the layers into a new rhombohedral arrangement rather than merely changing carrier concentration. STEM and XRD directly reveal interlayer Nb atoms, satellite peaks from the incommensurate superstructure, and a sliding vector consistent with layer displacement (&&&^^^^2query2^^^^&&&).

This structural description matters because the chemistry is presented not as conventional doping but as a way to reprogram stacking. The distinction is central to the interpretation of the superconducting and charge-ordered phases. The paper explicitly frames the growth strategy as a route that uses self-intercalation to alter registry and symmetry, thereby changing the effective dimensionality of the electronic ground state. A plausible implication is that the relevant control parameter is the reconstructed interlayer geometry rather than the nominal stoichiometric deviation alone.

2. Symmetry breaking and suppression of interlayer coupling

The sliding structure has two stated symmetry consequences. First, it breaks the PRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^4 mirror symmetry. In an H-NbSePRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^5 layer, the two Se sublayers are arranged symmetrically, whereas in the globally noncentrosymmetric sliding PRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^6 lattice that out-of-plane mirror symmetry is no longer restored throughout the bulk. Second, the shifted and intercalated configuration strongly reduces interlayer coupling, effectively decoupling neighboring NbSePRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^7 sheets (&&&^^^^2query2^^^^&&&).

The experimental signature of this decoupling is the angular dependence of the upper critical field PRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^8. Near in-plane field orientation, PRESERVED_PLACEHOLDER_^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^9 follows a 2D Tinkham-like form rather than a 3D Ginzburg–Landau form. The paper further extracts a superconducting thickness of about $_2$^^^^2query2^^^^ nm from the in-plane critical-field fit, which is far smaller than the full crystal thickness but still much larger than a single layer spacing. This places the material in an intermediate regime: it is not a monolayer, yet its superconducting response is quasi-2D because the layers are electronically decoupled. Thickness-dependent measurements from bulk down to $_2$^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^ nm are reported to evolve consistently with reduced interlayer coupling and increasing SOC effects.

The significance of this regime is not simply dimensional crossover in a geometric sense. The authors argue that weakened interlayer coupling constrains the orbital trajectories available for pair breaking under in-plane magnetic field, so the condensate becomes effectively confined to each layer or weakly coupled layer stack. This suggests that the essential novelty is bulk realization of a two-dimensional superconducting electrodynamics through internal structural reconstruction rather than exfoliation.

3. Ising and Rashba spin-orbit coupling in the sliding phase

A major consequence of the noncentrosymmetric sliding structure is the persistence of Ising-like spin splitting in a bulk crystal. The paper writes the effective normal-state Hamiltonian as

$_2$2

where $_2$3 is the kinetic energy, $_2$4 labels the valley, $_2$5 is the Ising SOC term, $_2$6 is the Rashba term, and $_2$7 is the Zeeman field (&&&^^^^2query2^^^^&&&).

The physical contrast is made with bulk $_2$8-NbSe$_2$9, where the Ising fields from adjacent layers cancel because of centrosymmetric stacking. In sliding $3R$^^^^2query2^^^^-NbSe$3R$^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^, they do not cancel globally. The bulk crystal therefore retains a net Ising-like spin splitting at the $3R$2 valleys, which is why the superconductivity is described as Ising-type even though the sample is not atomically thin. At the same time, the globally noncentrosymmetric lattice permits Rashba SOC, especially because $3R$3 mirror symmetry is broken.

The paper emphasizes competition rather than coexistence in a purely additive sense. Ising SOC tends to protect singlet Cooper pairs against in-plane magnetic fields by pinning spins out of plane and suppressing Pauli paramagnetic depairing, whereas Rashba SOC introduces in-plane spin canting and weakens that protection. DFT is reported to show strong Ising splitting at the $3R$4 point together with smaller Rashba splitting near other high-symmetry regions such as $3R$5/$3R$6-related points. This mixed SOC landscape is presented as the microscopic basis for the anomalous critical-field behavior.

4. Upper critical field, Pauli limitation, and quasi-2D superconductivity

The reported superconductivity is characterized by a sharp transition near $3R$7 K and zero resistance around $3R$8 K. Magnetization shows a strong Meissner response and large superconducting volume fraction, and specific heat shows a superconducting anomaly together with an anisotropic gap fit (&&&^^^^2query2^^^^&&&). These observations are used to argue that the phase is bulk rather than filamentary.

The upper critical field phenomenology is central. In a pure Ising superconductor, the in-plane critical field is expected to exceed the conventional Pauli limit

$3R$9

because out-of-plane spin locking protects singlet pairs. In the sliding phase, however, the measured $0^\circ$^^^^2query2^^^^ is reported to show a pronounced low-temperature suppression relative to the ideal Ising-enhanced expectation (&&&^^^^2query2^^^^&&&). The paper interprets this as evidence that Cooper-pair protection is weakened by competing Rashba SOC rather than governed by a single SOC mechanism.

The authors analyze the data with a WHH-like picture and state that a conventional spin-orbit scattering interpretation yields unphysical parameters in some samples. On that basis, they favor intrinsic SOC competition over extrinsic disorder as the origin of the low-$0^\circ$^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^ behavior. The resulting picture is that the suppressed $0^\circ$2 at low temperature is a fingerprint of nontrivial pair breaking in a noncentrosymmetric, weakly coupled bulk layered superconductor. This also clarifies a common misconception: the quasi-2D nature of the superconductivity does not imply monolayer physics in a literal structural sense; rather, it reflects electronic decoupling within a bulk crystal.

5. Charge-density-wave phenomenology and monolayer-like character

The charge-density-wave state in the sliding phase is described as unconventional relative to pristine bulk $0^\circ$3-NbSe$0^\circ$4. Raman spectroscopy shows a low-frequency amplitude-mode-like feature and a temperature-dependent softening, and these signatures are reported to resemble the CDW phenomenology of monolayer $0^\circ$5-NbSe$0^\circ$6 rather than that of ordinary bulk $0^\circ$7 or pristine bulk $0^\circ$8 material (&&&^^^^2query2^^^^&&&).

The interpretation offered is that interlayer sliding stabilizes a 2D-like CDW state in the bulk. The accompanying discussion and calculations suggest a $0^\circ$9 CDW instability, with the energetically favored distortion similar to the hollow-site configuration known from monolayer NbSe$R3m$^^^^2query2^^^^. In that sense, the same structural manipulation that weakens interlayer coupling and enables bulk Ising-type superconductivity is also said to preserve or resurrect a monolayer-like charge order.

This juxtaposition of superconductivity and CDW order is structurally rather than chemically framed. The paper’s claim is not that a foreign intercalant or conventional doping produces an unrelated CDW instability, but that self-intercalation reconstructs stacking and symmetry in a way that simultaneously promotes two-dimensional superconducting behavior and monolayer-like CDW phenomenology. A plausible implication is that stacking reconstruction can act as a unified tuning parameter for intertwined ordered states in layered compounds.

6. Two-fold superconducting symmetry and open microscopic interpretations

One of the most striking reported observations is a robust two-fold symmetry in the superconducting state. During in-plane field rotation, the in-plane angular magnetoresistance shows a $R3m$^^^^2id:(Liu et al., 2 Aug 2025) OR title:\2^^^^ modulation even though the crystal has nominal three-fold rotational symmetry. The paper states that this anisotropy is intrinsic rather than an artifact of current direction, vortex motion, or demagnetization (&&&^^^^2query2^^^^&&&).

Two microscopic interpretations are discussed without a unique assignment. One is asymmetric SOC: in a globally noncentrosymmetric lattice with an unbalanced local environment, the Bogoliubov quasiparticle spectrum can become anisotropic enough to yield a two-fold superconducting response. The other is a multicomponent pairing order parameter, meaning that the condensate may mix symmetry channels or develop nematic order, analogous to scenarios discussed in few-layer NbSe$R3m$2. The authors explicitly refrain from overclaiming a single mechanism.

This restraint is important for interpretation. The observation rules out a simple isotropic $R3m$3-wave bulk superconductor as a complete description of the superconducting response, but it does not by itself uniquely identify the pairing symmetry. The broader significance is therefore methodological as well as physical: interlayer sliding is presented as a symmetry-breaking tool that can expose hidden anisotropies and competing spin textures in bulk layered matter without resorting to extrinsic intercalation by foreign species or chemical doping. In the specific case of $R3m$4-NbSe$R3m$5, the resulting phase combines broken $R3m$6 mirror symmetry, suppressed interlayer coupling, persistent Ising spin-valley locking, allowed Rashba SOC, quasi-2D superconductivity, and a monolayer-like CDW state within a single bulk crystal.