Non‐Destructive Laser Nanopatterning of Superconducting Heterostructures in Topological Sn Thin Films

ABSTRACT

Heterostructures composed of superconductors and topological materials have emerged as compelling platforms for realizing topological superconductivity and fault‐tolerant quantum computation. A critical bottleneck, however, lies in achieving atomically clean and structurally coherent interfaces between dissimilar materials. Here, we report the fabrication of high‐quality planar heterostructures composed of the topological Dirac semimetal (TDS) α‐Sn and the superconducting β‐Sn phase, achieved by focused laser irradiation on α‐Sn thin films. The irradiated regions undergo a phase transition from α‐Sn to β‐Sn, exhibiting atomically smooth surfaces with a root mean square (RMS) roughness of just 0.75 nm. The laser‐induced β‐Sn demonstrates superconductivity with a critical temperature of 3.7 K and a Ginzburg–Landau coherence length (ξ _GL) of 68.2 nm. Notably, β‐Sn nanowires patterned through this method exhibit a pronounced superconducting diode effect, reaching a maximum rectification ratio (η) of 10.8%. These findings establish laser irradiation as a versatile, non‐destructive, and scalable approach for fabricating high‐quality α‐Sn/β‐Sn heterostructures, offering a promising route toward next‐generation superconducting quantum devices.

We demonstrate successful fabrication of high‐quality α‐Sn/β‐Sn planar nanostructures with arbitrary shapes by focused laser irradiation on topological Dirac semimetal α‐Sn thin films. The irradiated regions transform into atomically smooth, superconducting β‐Sn with a critical temperature of 3.7 K. Patterned β‐Sn nanowires exhibit a pronounced superconducting diode effect. This non‐destructive and scalable method enables precise nanofabrication of superconducting/topological heterostructures for quantum device applications.

1. Introduction

Topological superconductors garner significant attention as a crucial pathway for realizing Majorana quasiparticles, which are highly sought‐after yet enigmatic candidates for quantum bits in fault‐tolerant quantum computing [1, 2, 3, 4]. To date, extensive research on topological superconductivity has focused on heterostructures of topologically nontrivial materials, such as topological insulators [5, 6], TDS [7, 8], and topological Weyl semimetals [9, 10], in conjunction with conventional s‐wave superconductors, exploiting the superconducting proximity effect [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. To realize a material platform for large‐scale quantum computing, the critical challenges in such heterostructures are establishing high‐quality interfaces between topological materials and superconductors while utilizing cost‐effective and scalable processes for material growth and nanofabrication.

A previous work [16] successfully achieved these heterostructures based on Sn. Body‐centered tetragonal β‐Sn is a well‐known metal that exhibits conventional BCS‐type superconductivity below 4 K. On the other hand, α‐Sn, with a diamond‐type crystal structure, behaves as a Luttinger semimetal with no bandgap in bulk. However, under tensile or compressive strain, it can transform into a 3D topological insulator (3D‐TI) or a TDS, respectively. Notably, TDSs [17, 18, 19] feature 3D linear‐dispersion Dirac cones with strong spin‐orbit‐momentum locking, leading to exotic phenomena such as Fermi arcs on the surface and chiral anomaly. Both the 3D Luttinger semimetal and TDS have been theoretically predicted to become topological superconductors capable of hosting Majorana quasiparticles in their superconducting states [20, 21].

Among the rare examples of experimentally confirmed TDSs [17, 18, 19, 22, 23, 24, 25, 26, 27, 28], α‐Sn [17, 24, 25, 26, 27] stands out as the only elemental material that can be grown with high quality on semiconductor substrates like InSb or CdTe. Furthermore, α‐Sn undergoes a phase transition to β‐Sn upon heating, providing a new avenue to introduce superconductivity into the already‐rich topological phase diagram of α‐Sn. This transition can be achieved by irradiating a focused ion beam (FIB), resulting in superconducting β‐Sn regions within the TDS α‐Sn films with high‐quality interfaces [16]. Although it is a universal and cost‐effective method for fabricating nanoscale Sn‐based superconductor/TDS planar heterojunctions of arbitrary shapes, there is a drawback that irradiating the FIB causes significant damage in the bulk β‐Sn. In order to induce a phase transition from α‐Sn to β‐Sn, FIB transfers energy to α‐Sn both by the collisions of the ions and by thermal diffusion. Considering the fact that β‐Sn at the center bulk area was damaged while β‐Sn near the α‐Sn/β‐Sn interface region showed clean structures, direct ion collisions are thought to damage β‐Sn, and heating creates clean β‐Sn [16]. In this work, we demonstrate an alternative method to form nanostructures of superconducting β‐Sn in an α‐Sn thin film using laser beam irradiation, which is fabricated only by heating and thus can avoid damages by ion collisions.

2. Results and Discussion

We study an α‐Sn (40 nm)/InSb structure, where α‐Sn is expected to be a TDS, which was epitaxially grown onto an InSb (001) substrate using molecular beam epitaxy as described in previous work [17]. As shown in Figure 1a, a maskless laser lithography system was used for laser irradiation (See Methods). The laser source is a single‐mode semiconductor diode laser emitting light at a wavelength of 405 nm. The laser power is 120 mW, and the minimum feature size is 300 nm. This system facilitates maskless exposure through Computer‐Aided Design (CAD), offering the flexibility to irradiate lasers onto areas of varying shapes and sizes.

FIGURE 1.

(a) Schematic image of α‐Sn/β‐Sn planar nanostructures fabricated by laser irradiation (left panel). Phase transition from topological Dirac semimetal α‐Sn to superconducting metal β‐Sn induced by local heating by irradiating a laser beam (right panel). (b) Optical microscopy image of a 750 nm‐wide β‐Sn nanowire (white) embedded in an α‐Sn matrix (gray). (c) β‐Sn/α‐Sn/β‐Sn Josephson junction structure with a 500 nm‐wide central α‐Sn region (gray). (d) Arbitrarily designed β‐Sn patterns (white). All of these patterns were formed by laser irradiation, demonstrating high flexibility and compatibility with standard nanofabrication techniques.

We irradiated a laser beam onto the α‐Sn thin film, promoting the phase transition from α‐Sn to β‐Sn. Figure 1b–d show the irradiation results of several patterns, including a β‐Sn nanowire embedded in the α‐Sn plane (Figure 1b), a β‐Sn/α‐Sn/β‐Sn junction (Figure 1c), and a more complicated shape of the logo of the University of Tokyo (Figure 1d). Note that the area that was exposed to the laser beam turns white, indicating a phase transition from α‐Sn to β‐Sn. The minimum width of the nanowire and Josephson junction was 750 and 500 nm, respectively, indicating that nanofabrication is possible.

Then, the surface morphology of as‐grown, laser‐irradiated, and FIB‐irradiated Sn is analyzed using atomic force microscopy (AFM). Here, the FIB‐irradiated Sn area was formed using the same conditions described in the previous work [16]. The AFM measurement in Figure 2a reveals a root mean square (RMS) value of 0.29 nm for the as‐grown α‐Sn, indicating an atomically flat film. Meanwhile, the RMS value of the laser‐irradiated area is 0.75 nm, smaller than 1.96 nm of the FIB‐irradiated area. These results suggest that laser irradiation yields β‐Sn areas with reduced roughness compared with FIB irradiation. Furthermore, the FIB‐irradiated Sn shows grains with a diameter of 0.1 µm, while the laser‐irradiated Sn is much smoother. However, there is a striped pattern with an interval of 0.1 µm at the β‐Sn/α‐Sn interface in the laser‐irradiated structure, which is possibly caused by volume contraction of 27% [29, 30] due to the α‐to‐β phase transition in the laser‐irradiated Sn area. These differences in the surface morphology result from the different mechanisms of the allotropic transformation from α‐Sn into β‐Sn in the two methods. FIB induces phase transition not only by thermal diffusion but also by direct collision of Ga ions, whereas the laser provides only a heating effect when transforming Sn.

FIGURE 2.

(a) AFM images of the interfaces between as‐grown α‐Sn and β‐Sn induced by FIB (left) and laser (right) irradiation. The RMS roughness values of the FIB‐ and laser‐irradiated β‐Sn regions are 1.96 nm and 0.75 nm, respectively. (b) Inverse pole figure map of the FIB‐ (top) and laser‐irradiated (bottom) β‐Sn regions. The normal direction (ND), the reference direction (RD), and the transverse direction (TD) correspond to the perpendicular [001], the in‐plane [010], and the in‐plane [100] directions of the InSb (001) substrate, respectively. A color code, as defined in the unit triangle in the inset, is used to indicate the crystal plane in each domain that is facing the probing direction. For the laser‐induced β‐Sn regions, the dominant crystal axis in each probing direction is indicated at each unit triangle.

The crystallinity of the β‐Sn areas was investigated by electron backscattering diffraction (EBSD) measurements. EBSD measurements were conducted at 15 kV with a 70° tilt, over a 10 µm × 10 µm area and 30 nm step size. Figure 2b shows inverse pole figures for the FIB‐irradiated and laser‐irradiated Sn areas, revealing domain orientations of the induced β‐Sn relative to the InSb substrate: ND, RD and TD probing directions correspond to the [001] axis (perpendicular to the film plane), [010] and [100] axes (in the film plane) of the InSb (001) substrate, respectively, as illustrated in the inset. The figure maps crystallographic directions of the detected domains in each probing direction using a color code linked to crystal planes, as illustrated in the triangular inset. The result for the FIB‐irradiated Sn area (top panel) indicates multiple small domains with varied orientations, whose area‐weighted average grain size is ∼230 nm. Black regions suggest the presence of amorphous areas and small‐grain polycrystalline zones. In contrast, the result for the laser‐induced Sn area (bottom panel) shows single‐crystal domains in more than 80% of the β‐Sn area (the average size is ∼ 8 µm). From the color code and intensity mapping of the β‐Sn area in Figure 2b, one can see a tendency for β‐Sn [$\overline{4}52$] to align with the [001] direction of the InSb (001) substrate, β‐Sn [$22\overline{1}$] to align—slightly rotated— with the [100] direction of the InSb (001) substrate, and β‐Sn [102] to align with the [010] direction of the InSb (001) substrate, respectively. These EBSD data thus confirm much better crystal quality of the laser‐induced β‐Sn, in comparison with that formed by FIB.

Next, we characterize the superconducting properties of the laser‐irradiated Sn area, where a phase transition to β‐Sn is expected. Figure 3a displays the temperature dependence of resistance (R—T curves) for three regions: as‐grown α‐Sn (green), InSb substrate (black), and a laser‐irradiated Sn (blue) at 300 – 2 K (All the data are normalized using the resistance at 300 K for comparison). Here, the R—T curves were obtained in the as‐grown Sn sample using Hall bar geometry, in the InSb substrate and the laser‐irradiated Sn using Van der Pauw geometry. Only the region irradiated by the laser shows zero resistance at 3.7 K, which indicates the appearance of superconductivity. In Figure 3b, the R‐T measurements focus exclusively on the laser‐irradiated β‐Sn region, with the data collected under various in‐plane magnetic field strengths from 0 – 0.25 T. Here, the critical magnetic field was estimated to be 0.66 T at 2 K. Increasing the magnetic field strength monotonously lowers the critical temperature. The observed two‐step decrease in resistance during the superconducting transition is attributed to the inhomogeneity of the SC region resulting from the reverse phase transition of β‐Sn to α‐Sn, a phenomenon called Sn‐pest. Sn‐pest generally occurs in pure bulk Sn at 13°C, where it undergoes an allotropic transformation from β‐Sn to α‐Sn. Measurements conducted at low temperatures can induce unexpected Sn‐pest, as we observed in some of our laser‐induced β‐Sn nanostructures (see Supporting Information).

FIGURE 3.

(a) Temperature dependence of resistance for laser‐irradiated Sn (blue), as‐grown α‐Sn (green), and an InSb subtrate (black). (b) Resistance vs. temperature for laser‐irradiated Sn under various in‐plane magnetic fields applied along the [110] direction of the InSb (001) substrate. (c) Magnetic moment of laser‐irradiated Sn as a function of temperature measured under field‐cooling (FC) and zero‐field‐cooling (ZFC) conditions. (d) Magnetic moment of laser‐irradiated Sn vs. magnetic field at different temperatures. (e) Experimental results and fitting curves of the critical magnetic field of laser‐irradiated Sn as a function of temperature for both perpendicular (red, H // [001]) and in‐plane (blue, H // [110]) field directions.

Second, diamagnetism in the laser‐irradiated Sn was characterized using a superconducting quantum interference device (SQUID). Figure 3c illustrates the temperature dependence of the magnetic moment measured in a 1.2 mm × 2.9 mm area of laser‐irradiated Sn, conducted under both field cooling (FC) and zero‐field cooling (ZFC) conditions. For FC, a magnetic field of 5 mT was continuously applied during cooling until the temperature reached 2 K, after which the magnetic moments were measured from 2 to 10 K at 5 mT. In contrast, for ZFC, no magnetic field was applied during cooling until the temperature reached 2 K, and then the magnetic moments were measured over the same temperature range from 2 to 10 K at 5 mT. As shown in Figure 3c, the laser‐irradiated sample displays characteristics consistent with those of a clean type‐I superconductor (SC), as there is diamagnetism with no discernible differences in the magnetic moment measured under FC and ZFC conditions. Then, the magnetic field dependence of the magnetic moment was measured at various temperatures as shown in Figure 3d. Magnetic fields were applied between −50 and 50 mT, ranging temperatures from 2 to 4.5 K. As the applied magnetic field is swept, the magnetization below 3.7 K exhibits negative hysteresis, a typical behaviour observed in SCs.

Subsequently, the temperature dependence of the critical magnetic fields for in‐plane and perpendicular fields was examined, as shown in Figure 3e. The laser‐induced β‐Sn SC film satisfies the following equations,

$$H_{\mathrm{C}\parallel }=\frac{\sqrt{3}{\varphi }_0}{\pi {\xi }_{\mathrm{GL}}d}\sqrt{1-\frac{T}{T_{\mathrm{C}}}}$$ (1)

$$H_{\mathrm{C}\perp }=\frac{{\varphi }_0}{2\pi {\xi }_{\mathrm{GL}}^2}\left(1-\frac{T}{T_{\mathrm{C}}}\right)$$ (2)

where Φ ₀ = h/(2e) = 2.07 × 10⁻¹⁵ Wb (Φ ₀ is the magnetic flux quantum, h is the Planck constant, e is the elementary charge), ξ _GL is Ginsburg–Landau superconducting coherent length, and d represents its thickness [31]. The fitted values are ξ _GL = 68.2 nm and d = 16.9 nm, respectively. Although the estimated ξ _GL is shorter than the coherence length observed in bulk β‐Sn (over 230 nm) [32, 33], a similar reduction also occurred in an α‐Sn/β‐Sn mixed film (40.3 nm) [34]. The discrepancy between the fitted thickness (16.9 nm) and the sample thickness (40 nm) is also attributed to Sn‐pest, particularly in regions away from the directly irradiated area.

Notably, we observed superconducting diode effect (SDE) in β‐Sn nanowires (NW) with a width W of 750 nm embedded in TDS α‐Sn, as shown in Figure 4a,b. In Figure 4a,b, we measured the critical current (I _C) while applying a magnetic field H nearly parallel to the nanowire with the field strength varied from 0 T to 0.35 T, above which the SC weakens and disappears. When the current is swept from zero, we define the critical current I _C as the value just before the resistance increases above 0.05 Ω. The critical currents were measured when the superconducting current was parallel (I _C+) and antiparallel (I _C‐) to H . Then, the superconducting rectification ratio η [= (I _C+—I _C‐)/(I _C+ + I _C‐)] was estimated for each magnetic field, which is 3.5 ∼ 5.5% on average, as shown in Figure 4c. The most striking feature is that η is non‐zero even without a magnetic field and exhibits no oscillation in its sign. On the other hand, in the FIB‐irradiated NWs, the SDE requires applying an external magnetic field, upon which η reaches a maximum of 35% with oscillations and sign reversals [16]. The origin of the SDE in the FIB‐irradiated NWs was attributed to the superconducting transport in the TDS α‐Sn induced by the proximity effect from the neighbouring β‐Sn. In contrast, the SDE in laser‐irradiated NW does not reverse the sign and does not require a magnetic field, implying that it might have a different origin.

FIGURE 4.

(a) Superconducting diode effect (SDE) in a laser‐induced β‐Sn nanowire aligned along the [110] direction of InSb, with a width W = 750 nm and a length L = 20 µm, embedded in a 40‐nm thick TDS α‐Sn film. Measurements were performed at 2 K with a magnetic field H applied nearly parallel to the current. When the current is swept from zero, the resistance increases from zero (black area) to above 0.05 Ω (red area) at critical currents. (b) Superconducting critical current I _C for each applied magnetic field when the current I is parallel (I _C+) and antiparallel (I _C‐) to H , extracted from the data in (a). (c) Rectification ratio η [= (I _C+ − I _C‐)/(I _C+ + I _C‐)] estimated from (b). (d) Angular dependence of the superconducting critical current measured at 2 K under a fixed in‐plane magnetic field H = 0.1 T, rotated with 5° increments. The angle θ is defined between H and the current I , which is applied in the NW along the [110] direction of InSb. (e) Even component of the critical current, I _EVEN = (I _C+ + I _C−)/2, and (f) odd component of the critical current, I _ODD = (I _C+ − I _C−)/2, both extracted from the angular measurements in (d). (g) Illustration of a possible scenario; an interface of β‐Sn/α‐Sn is formed at the bottom of the NW due to Sn pest. When H is applied perpendicular to I , there will be a Lorentz force (F _L) acting to vortices in the Sn NW. The direction of the Lorentz force is reversed when the direction of either I or H is reversed. Due to the asymmetry between the upper β‐Sn/vacuum surface and the bottom β‐Sn/α‐Sn/InSb interface, SDE is induced.

Subsequently, the in‐plane magnetic field angle dependence of SDE was measured at 2 K under a fixed magnetic field strength H of 0.1 T. The angle (θ) was defined as the angle between H and I (as depicted in Figure 4d). We focus on the even component (I _EVEN = (I _C+ + I _C‐)/2, Figure 4e) and the odd component (I _ODD = (I _C+—I _C‐)/2, Figure 4f), which were measured while rotating H with an interval of 5°. Notably, the odd component disappears when H is parallel to I . Peaks in the odd component, indicative of SDE, are observed when 45° ≦ θ ≦ 135°. The maximum of η reaches ∼10.8% when θ = 45°, 135°, 225°, 315°. These results are largely different from those in the FIB‐induced β‐Sn NWs¹⁶, where the largest SDE was observed at θ = 0° and 112°. In contrast, the even component I _EVEN displays a twofold symmetry and is notably suppressed when H ⊥ I (θ = 90°, 270°). This behaviour is again distinct from that of the FIB‐induced β‐Sn NWs, where the I _EVEN​ showed twofold symmetry with maxima at θ = 45° and 225°. These observations suggest that the SDE in laser‐irradiated β‐Sn NWs arises from a mechanism fundamentally different from that in their FIB‐fabricated counterparts. It is also worth noting that the in‐plane field dependence of the SDE indicates a possible misalignment of the magnetic field H from the nanowire axis by approximately 5–10° in our measurements shown in Figure 4a,b.

Achieving a large, magnetic‐field‐free SDE has been a central goal in the development of next‐generation superconducting circuit technologies. Field‐free SDE has recently been reported in systems with geometric asymmetry [35], strain‐induced effects [36], or inversion‐symmetry breaking [37], although its underlying mechanism remains under debate. To gain insight into the field‐free SDE observed in this work, we first compare laser‐ and FIB‐induced nanowires (NWs). Although their geometries are similar, the crystallinity and surface quality of the resulting β‐Sn differ substantially. In FIB‐irradiated NWs, ion bombardment severely damages the central β‐Sn region, leaving relatively intact β‐Sn only near the interfaces with α‐Sn [16]. Consequently, the superconducting critical current is predominantly governed by these β‐Sn/α‐Sn interfaces at the two edges of the NWs. In this scenario, the topological band structure of the α‐Sn regions, which acquire superconductivity via the proximity effect, has been proposed to play a key role in generating the SDE. In contrast, laser‐irradiated NWs exhibit clean, homogeneous β‐Sn across the entire structure. Here, conventional BCS‐type superconductivity of β‐Sn dominates the transport behavior. This fundamental difference in structural quality likely explains the markedly different SDE characteristics observed between the two types of Sn nanowires.

These observations naturally raise the question of the origin of the SDE. Nonreciprocal transport requires the breaking of inversion symmetry. In the planar direction of the 750‐nm‐wide laser‐irradiated Sn NWs, inversion symmetry is preserved, and the β‐Sn crystal structure itself is also inversion symmetric, except at the β‐Sn/α‐Sn boundaries near the nanowire edges. However, inversion symmetry can be broken along the growth direction, where the top and bottom interfaces are intrinsically different. During cooling, Sn pest can induce the formation of α‐Sn near the bottom Sn/InSb interface, effectively introducing an additional β‐Sn/α‐Sn boundary, as illustrated in Figure 4g. This scenario is supported by the estimated effective thickness of the superconducting Sn channel, d = 16.9 nm, which is significantly smaller than the nominal film thickness of 40 nm (see also Figure S3). Consequently, inversion symmetry would be broken along the vertical (growth) direction across the entire nanowire, potentially giving rise to the observed SDE.

Considering the abovementioned scenario, the suppression of superconductivity (I _EVEN) and enhancement of SDE (I _ODD) when H is rotated in the film plane from parallel to perpendicular to the current I , as seen in Figure 4d–f, can be reasonably explained by the vortex‐depinning scenario. Although bulk β‐Sn is a type I superconductor, a 40 nm‐thick β‐Sn thin film can behave effectively as a type II superconductor, especially under an in‐plane magnetic field, due to enhanced penetration depth λ, reduced cohenrence length ξ, and surface or disorder effects that push the ratio κ = λ/ξ beyond the type I—type II boundary (see Methods). When applying H ⊥ I (in plane), the influence of the Lorentz force on vortices in β‐Sn pushes the vortices to the upper β‐Sn/vacuum surface or the bottom β‐Sn/α‐Sn/InSb interface. This causes suppression of the critical superconducting current (I _EVEN) comparing to that in the H // I configuration (Figure 4e). Furthermore, due to the asymmetry between the upper β‐Sn/vacuum surface and the bottom β‐Sn/α‐Sn/InSb interface, the critical current differs depending on the current I or magnetic field H directions, leading to the appearance of SDE (Figure 4f).

The remaining question is why the SDE persists even at zero applied magnetic field. In certain BCS‐type systems, SDE has been observed under extremely small perpendicular magnetic fields (on the order of several Oe), often attributed to extrinsic factors such as geometric asymmetry [38]. Although we carefully demagnetized the superconducting magnet prior to measurement, the presence of a minute remanent field cannot be completely excluded. On the other hand, magnetic‐field‐free SDE has also been experimentally reported in superconducting systems with broken inversion symmetry [37] or strain [38], albeit without a comprehensive theoretical description. Considering the possible formation of α‐Sn near the bottom of the nanowires, both inversion‐symmetry breaking and residual strain are expected at the β‐Sn/α‐Sn interface, which could induce a similar zero‐field SDE. Additionally, theoretical work has predicted that in superconducting topological Dirac semimetals, crystal deformation can produce a spatially varying shift of the Dirac points, effectively acting as a fictitious vector potential and giving rise to an emergent chiral magnetic field [39]. This chiral magnetic field can exert a pseudo‐Lorentz force on Cooper pairs formed between electrons with opposite chirality and generate a spin supercurrent even when time‐reversal symmetry is preserved [39]. Along the rough β‐Sn/α‐Sn interface at the bottom of the nanowires, the α‐Sn regions may become superconducting via the proximity effect and contribute to the observed field‐free SDE. Nevertheless, further investigations, particularly detailed microstructural characterization of the Sn nanowires, will be essential to fully clarify the underlying mechanism.

3. Conclusions

In summary, we successfully fabricate TDS α‐Sn / superconducting β‐Sn planar heterostructures by irradiating a laser beam. Structural analysis indicates that this method is less damaging than FIB, and the superconducting β‐Sn fabricated by laser irradiation is a clean type‐I superconductor. Superconducting β‐Sn NWs show SDE with the maximum rectification ratio η reaching 10.8% even without a magnetic field, which is promising for realizing superconducting logic devices. These findings validate the laser‐irradiation method as a non‐destructive approach for achieving clean superconducting β‐Sn/α‐Sn nanostructures, which are potential avenues for topological quantum physics and devices.

4. Methods

4.1. Sample Growth

The single‐crystalline α‐Sn thin film was epitaxially grown on an InSb (001) substrate using low‐temperature molecular beam epitaxy. First, a 150 nm‐thick InSb buffer layer was grown on the InSb (001) substrate. Subsequently, the substrate holder was cooled to –70°C using liquid nitrogen, and a 40 nm‐thick α‐Sn film was grown at a growth rate of 10 Å/min.

4.2. Fabrication of Laser‐Irradiated Sn

We used a DWL 66+ from Heidelberg Inst., whose specifications indicate a minimum laser feature size of 300 nm, and the 3σ value of edge roughness is 50 nm. To ensure stability during laser irradiation, the sample was securely affixed to a Si substrate using correction fluid, with the Si substrate further secured using tape. An optical microscope image of the sample captured immediately after laser irradiation shows the laser‐irradiated regions appearing as white areas, indicating a transformation from α‐Sn to β‐Sn. Nanowires and rectangular structures of laser‐irradiated Sn were formed at different laser powers (See Figure S1). At 75 – 80 mW, sparse β‐Sn regions and non‐uniformities were noticeable, particularly in the nanowire structures and along the scanning direction. This is attributed to heating inhomogeneity when the laser power barely reaches the threshold for a phase transition to β‐Sn. When the laser power was increased to 100 mW, the nanowire structures were uniformly and cleanly processed, indicating an appropriate power level. However, at 300 mW, inhomogeneities persisted, particularly in the center of the nanowire structures and square regions, suggesting potential Sn melting due to excessive laser power (the melting point of Sn at 231.9°C). Therefore, we concluded that a laser power ranging from 100 mW to 200 mW would be suitable for fabricating α‐Sn/β‐Sn planar heterostructures using this laser lithography system.

Another factor that contributes to improved β‐Sn homogeneity is thermal diffusion during laser irradiation, which induces β‐Sn transformation not only within the laser spot but also in its surrounding area. As shown in Figure S2, when the laser power exceeds 100 mW, the nanowire width observed by SEM expands by approximately 300 nm on both sides. This expansion indicates that β‐Sn formation occurs up to about 300 nm beyond the laser‐irradiated region. Consequently, minor misalignments or small gaps between laser scan paths are effectively compensated by thermal diffusion, ensuring continuous β‐Sn coverage across the entire nanowire area. Furthermore, given that the Sn film thickness is only 40 nm, it is reasonable to conclude that the β‐Sn phase forms across the full film thickness immediately after the laser irradiation (afterwards, α‐Sn is formed near the bottom Sn/InSb interface by Sn pest, as illustrated in Figure 4g).

4.3. Type II Superconductivity in Sn Thin Films

Bulk Sn is a type‐I superconductor with a Ginzburg–Landau parameter $\kappa ={\lambda }_0/{\xi }_0\approx 34\mathrm{nm}/230\mathrm{nm}\approx 0.15<1/\sqrt{2}$, where λ₀ and ξ₀ are the penetration depth and the coherence length of Sn, respectively. However, a 40‐nm Sn film can crossover into effective type‐II behaviour because thin‐film disorder significantly renormalizes both the penetration depth and coherence length. In the dirty limit, the effective parameters follow ${\lambda }_{\mathrm{eff}}={\lambda }_0\sqrt{{\xi }_0/l}$ and ${\xi }_{\mathrm{eff}}=0.855\sqrt{{\xi }_{0}l}$, where l is the electronic mean free path. Consider the thickness of our β‐Sn channel is 40 nm or less because of the Sn pest, assuming a realistic value of l ∼ 40 nm in the thin Sn film yields λ _eff ≈ 82 nm and ξ _eff ≈ 82 nm, giving κ  = λ _eff /ξ _eff ≈ 1, above the type‐I/II boundary ${\kappa }_c=1/\sqrt{2}\approx 0.707$. In this regime, Abrikosov vortices become energetically stable. These quantitative estimates demonstrate that a 40‐nm Sn film naturally exhibits type‐II–like mixed‐state behaviour due to increased scattering and reduced dimensionality. We note that thin Sn films have previously been demonstrated to behave effectively as type‐II superconductors [40, 41, 42]. These works support our proposed scenario in which a 40‐nm Sn film can effectively show type‐II–like behaviour.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma72204‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.