Observation of pseudogap in SnSe₂ atomic layers grown on graphite

Abstract

Superconducting metal dichalcogenides (MDCs) present several similarities to the other layered superconductors like cuprates. The superconductivity in atomically thin MDCs has been demonstrated by recent experiments, however, the investigation of the superconductivity intertwined with other orders are scarce. Investigating the pseudogap in atomic layers of MDCs may help to understand the superconducting mechanism for these true two-dimensional (2D) superconducting systems. Herein we report a pseudogap opening in the tunneling spectra of thin layers of SnSe₂ epitaxially grown on highly oriented pyrolytic graphite (HOPG) with scanning tunneling microscopy/spectroscopy (STM/STS). A significant V-shaped pseudogap was observed to open near the Fermi level (E_F) in the STS. And at elevated temperatures, the gap gradually evolves to a shallow dip. Our experimental observations provide direct evidence of a pseudogap state in the electron-doped SnSe₂ atomic layers on the HOPG surface, which may stimulate further exploration of the mechanism of superconductivity at 2D limit in MDCs.

Graphical abstract

Keywords

scanning tunneling microscopy / pseudogap / metal dichalcogenides / SnSe₂ / van der Waals epitaxy

1 Introduction

Atomic layers of metal dichalcogenides (MDCs) are receiving intensive attention in recent years due to the intriguing properties dramatically different from their bulk counterparts [1]. Besides single-particle electronic properties, many-body collective phenomena, e.g., charge density waves (CDW) [2-4] and superconductivity [4-10] have been investigated in thin layers of transition metal dichalcogenides (TMDCs) approaching the exact 2D limit. It is worth noting that for high-temperature superconductors, such as superconducting cuprates, in addition to their CDW and superconducting behavior, there are also pseudogaps closely related to superconductivity [11-14]. This phenomenon is similar to some three-dimensional (3D) TMDCs that the existence of CDW, superconductivity and pseudogap can also be observed [15-18]. However, for TMDCs with the thickness of atomic layers, the existence of a pseudogap has only been reported in monolayer VSe₂ [19] and single-layer TiTe₂ [3]. Comparatively, much less effort has been made in the study of post-transition metal dichalcogenides (PMDCs). It would be highly interesting to explore the possible many-body collective phenomena of PMDCs without the transition metal d-electron contributions to bands near the Fermi surface. Among the PMDCs, SnSe₂ is particularly an interesting system because it bears some similarities to the TMDCs indicated from preliminary studies. For bulk SnSe₂, with doping or pressurization, the appearance of CDW and superconductivity can be achieved [20-22]. On the other hand, for single-layer SnSe₂, the superconductivity with Tc of several Kelvins has also been reported [23, 24]. But whether and how the pseudogap exists in the atomic layers SnSe₂ are unknown, which demands immediate experimental investigations.

In this work, we address the above questions by performing the low-temperature STM study on atomically thin SnSe₂ layers synthesized on the HOPG surface with van der Waals epitaxy. A striking asymmetric V-shaped gap was observed to open at the E_F for both monolayer (ML) and bilayer (BL) SnSe₂, which cannot be accounted for by a single-particle origin. The gap has a magnitude (Δ) of 16–22 meV with up to 90% loss of conductance at 5 K and it evolves to a shallow dip at 77 K for the BL SnSe₂. The temperature dependence of the nominal gap depth suggests that the gap is not fully closed even at temperature much higher than 77 K. By comparing the tunneling spectra of 2D SnSe₂ atomic layers and a bulk SnSe₂ sample, we found a considerable amount of charge transferring from the HOPG substrate to SnSe₂ layers. It may efficiently lower the conduction band minimum (CBM) of the latter and make the SnSe₂ atomic layers conducting. Our experimental study clearly shows that a pseudogap exists in the atomically thin SnSe₂ layers approaching the 2D limit, which might be closely related to the superconductivity discovered recently in similar systems.

2 Materials and methods

The substrate used for this study was prepared from a piece of freshly cleaved HOPG (Grade SPI-1, SPI Supplies). After the cleavage, the HOPG substrate was immediately transferred into the ultra-high vacuum (UHV) chamber (base pressure 1.5 × 10^-10 mbar) and degassed at 350 °C for 30 minutes. A clean graphite surface with atomically flat terraces was obtained. SnSe₂ films were grown by co-evaporating Sn (99.9999%) and Se (99.999%) with a flux ratio of 1:50 from an effusion cell (Quantech) and a homemade Knudson cell, respectively. The HOPG substrate was held at room temperature during the deposition. The as-grown SnSe₂ film was annealed at 210 °C for 30 minutes in the UHV chamber. The bulk SnSe₂ sample was prepared via chemical vapor transport method with iodine as a transport agent. A mixture of Sn, Se and I₂ powder was encapsulated in a vacuum quartz tube and placed into a two-zone temperature gradient furnace. The high-temperature zone and low-temperature zone were kept respectively at 800 °C and 700 °C for 1 day, then, both were cooled to room temperature in 2 days. The clean surface of bulk SnSe₂ was obtained by cleaving a SnSe₂ single crystal with Scotch tape in vacuum (base pressure 1 × 10^-6 mbar). All samples of SnSe₂ atomic layers on HOPG and bulk SnSe₂ were transferred in vacuo into the STM chamber for the low-temperature STM and STS measurements. The STM experiments were carried out with a low-temperature STM (Scienta Omicron GmbH) with a base pressure of 5 × 10^-11 mbar. A chemically etched tungsten tip was cleaned by Ar⁺ sputtering prior to all measurements. The dI/dV spectra were recorded with a lock-in amplifier using a sinusoidal modulation of 4 mV and 732 Hz.

3 Results and discussion

The schematic structure of the SnSe₂ thin film grown on HOPG is shown in Fig. 1(a). The SnSe₂ layers and underlying HOPG form a typical van der Waals heterostructure. The atomic structure of SnSe₂ shares the same arrangement as that of the 1T phase of TiSe₂. And each Sn atom is surrounded by two triangles of Se atoms [Fig. 1(b)], forming a trilayer sandwich structure. Figure 1(c) is a typical STM topographic image of a SnSe₂ film grown on HOPG showing both BL and ML SnSe₂. The atomically-resolved STM image taken on the BL SnSe₂ [Fig. 1(d)] shows a hexagonal lattice structure, corresponding to the atomic structure of the top Se layer. It is worth noting that the surface possesses a simple hexagonal lattice and no CDW order can be observed at all sample biases. The lattice constant a₀ is experimentally determined to be 3.81 Å for both ML and BL. More structural details of the sample are shown in Supplementary Fig. S1.

The wide-energy-range tunneling spectra taken on the BL SnSe₂ show a typical semiconducting behavior [Fig. 1(e)]. The differential conductance is quite low in the bias range of -1.0 V to 0 V, consistent with the calculated bandgap of the BL SnSe₂ [25]. However, when we check carefully the narrow-energy-range spectrum, finite conductance around the E_F and an unexpected gap opening are found. The high-resolution tunneling spectrum near the E_F [Fig. 1(f)] shows a sharp asymmetric V-shaped gap with two pronounced conductance peaks located at±16 mV. This energy gap is much larger than the superconducting gaps of the SnSe₂ thin layers reported recently [23, 24]. The possible reason for that we didn’t observe such a superconducting gap is that the working temperature of our STM is 5 K, which is not low enough for observing such a low-temperature superconductivity. This gap feature was robustly observed in all ML and BL terraces with different STM tips, despite the gap behavior being not very uniform across the whole BL surface. The dI/dV spectra taken on the ML SnSe₂ also show a prominent gap opening at E_F but with much stronger spatial inhomogeneity, so here we focus on the gap for BL SnSe₂. By analyzing over 300 spectra (taken on BL far away from defects and edges) we found that the magnitude 2Δ varies in a range of 32–44 meV and the nominal depth ranges from 65% to 90%, depending on the spatial locations. Figure 2(a) shows a set of dI/dV spectra taken along a 6-nm distance on the BL SnSe₂ in a defect-free area. The relative height of the conductance peaks and the symmetry of the gap shape vary from point to point, showing a considerable spatial inhomogeneity and suggesting a short coherence length on the nanometer scale. The short coherence length was also evidenced by the evolution of the gap near the terrace edges. As shown in Fig. 3, two sets of such measurements show that the gap could be locally suppressed near terrace edges, which are strong indications of correlated electronic behavior.

To further verify the origin of the V-shaped gap observed in the BL SnSe₂, we studied the temperature dependence of the low-energy tunneling spectra as shown in Fig. 2(b). The gap has the largest amplitude at the lowest measured temperature of 5 K and gradually evolves into a slight dip at 77 K. We are not able to derive a temperature dependence of Δ since the spatial deviation of Δ is comparable to the temperature-induced variation. However, we found that the depth of the gap shows a monotonic decreasing behavior with increasing temperature. The temperature dependence of the nominal gap depth is plotted in Fig. 2(c). The gap depth decreases rapidly from 5 K to 25 K then slowly to 77 K, implying a very high pseudogap temperature T^* (> 77 K), which is much higher than the T_c of superconductivity in SnSe₂ bulk or thin layers [20-24].

Such a gap opening in the tunneling spectra of BL SnSe₂ strongly suggests a pseudogap nature. First, it has a typical V-shaped behavior which differs from the phonon-induced steplike behavior [26]. Second, the largely dropped conductance in the gap (up to 90% in some areas) suggests that it is unlikely to be a CDW gap which usually accompanies large residual DOS at E_F [18, 27, 28]. This is also consistent with our STM measurements that no CDW order was observed at any bias on the surface. Third, the gap is accompanied with two pronounced side peaks located symmetrically with respect to E_F, the characteristic of superconducting or pseudogap coherence peaks [29-32], which rule out the possibilities of the Kondo effect. Moreover, the possibility of a superconducting gap is also excluded because the exceptional large gap size will give a huge T_c over 200 K, which cannot be true in this case with residual DOS at E_F at 5 K. All these observations point pseudogap as the most possible origin for the gap.

Many superconductors have a doping phase diagram containing CDW, superconductivity, pseudogap and metal phases, in which the pseudogap region can be reached by tuning the charge-carrier concentration [32-35]. Bulk SnSe₂ is an n-type semiconductor with low charge-carrier concentration typically from 10¹⁷ to 10¹⁸ cm^-3. So there arises a question whether the SnSe₂ atomic layers experience a considerable carrier doping on HOPG? We resolve this question by comparing the local work functions between the films and the HOPG substrate first. Supplementary Fig. S2 shows the dZ/dV spectra taken on HOPG, ML, BL and bulk SnSe₂ surfaces. A typical STM image of the bulk sample is shown in Supplementary Fig. S3. The work function differences of the samples can be directly deduced from the differences of the peaks. It is known that sharp resonances, known as the Gundlach oscillations, will appear in the tunneling junction if the applied sample bias voltage is close to or larger than the surface work function. The Gundlach oscillation is a phenomenon of field-emission resonance through standing-wave states in the tip-sample gap, which has been successfully employed to determine the work function differences of thin films. The occurrence of the first resonant peak of Gundlach oscillation corresponds to the onset of the field emission, which reflected the relative amplitude of the work function of the sample. From the dZ/dV spectra, it is clearly shown that the work function of HOPG (W_HOPG) is much lower than that of BL (W_BL) and ML (W_ML) SnSe₂, while bulk SnSe₂ has a highest work function (W_bulk) among all samples. Our results are consistent with earlier reports that bulk SnSe₂ has an exceptionally large work function (5.3 eV) [36] among MDC semiconductors, while HOPG has a much lower work function of 4.4 eV [37].

Furthermore, we analyzed the CBM of the SnSe₂ layers. In Fig. 4(a), we show the typical dI/dV spectra near E_F taken on the surfaces of ML, BL and bulk SnSe₂ to compare the thickness dependence of the CBM. All the spectra show pronounced conductance at positive sample biases. The spectrum of the bulk SnSe₂ shows nearly zero conductance at negative biases and rise of conductance just above E_F, exhibiting the characteristic behavior of heavily doped semiconductors. The spectrum of the BL SnSe₂ shows a rise of conductance located at around -60 mV. For ML SnSe₂, although a much higher spatial inhomogeneity is found in the tunneling spectra, most spectra show a rise of conductance around -100 mV as demonstrated in the representative spectrum. The CBM of each sample can thus be identified as the onset of the conductance rise in each spectrum, which is largely lowered to below E_F for BL and ML SnSe₂, resulting in significantly enhanced DOS at E_F. Similarly lowered CBM has been previously observed in superconducting SnSe₂–Co(Cp)₂ superlattice in which it is 150 meV below E_F [20]. The shift of E_F and CBM can be readily understood with an interfacial charge transfer scenario. Figure 4(b) shows the schematic band diagrams of a SnSe₂ sample before contacting and in contact with a HOPG substrate. As a result of W_HOPG < W_ML W_BL < W_bulk, the energy of the CBM in SnSe₂ is lower than the Fermi energy of HOPG. When SnSe₂ layers are in contact with a HOPG, a considerable amount of electrons migrate from the HOPG to the conduction band of SnSe₂. This electron transfer lifts the E_F of the interfacial layers of SnSe₂ from below CBM to above CBM, turning the layers from semiconducting to metallic. A Schematic diagram of band structures near E_F for bulk, ML and BL SnSe₂ is illustrated in Fig. 4(c). The Fermi surface consists of six electron pockets at the M points of the hexagonal Brillouin zone. Such a largely-enhanced DOS at E_F as well as the greatly-increased electron carrier concentration are believed to take responsibility for the emergence of the pseudogap state. To verify the effect of electron doping on the SnSe₂ thin layers, we theoretically calculated the density of states (DOS) of a freestanding SnSe₂ monolayer without and with electron doping. The calculations do show that the CBM of SnSe₂ monolayer can be greatly lowered below E_F even at a small doping level of 0.1 electron per cell, which is fully consistent with the experimental results.

4 Conclusion

In summary, we discovered an asymmetric V-shaped pseudogap in the tunneling spectra for SnSe₂ atomic layers grown on HOPG. This gap shows similarities to that of pseudogap for some superconductors in shape, energy, spatial inhomogeneity and temperature dependence. We expect more experimental investigations like transport measurements with electrostatic doping [38] and angle-resolved photoemission spectroscopy studies to uncover the nature and physical mechanism behind the pseudogap. The observation and investigation in this work present the first report of pseudogap state in superconducting SnSe₂ thin layers, which will greatly help to the future understanding of competing or intertwined orders closely related to superconductivity in MDCs.

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