Enhanced superconductivity in atomically thin TaS2

Abstract

The ability to exfoliate layered materials down to the single layer limit has presented the opportunity to understand how a gradual reduction in dimensionality affects the properties of bulk materials. Here we use this top-down approach to address the problem of superconductivity in the two-dimensional limit. The transport properties of electronic devices based on 2H tantalum disulfide flakes of different thicknesses are presented. We observe that superconductivity persists down to the thinnest layer investigated (3.5 nm), and interestingly, we find a pronounced enhancement in the critical temperature from 0.5 to 2.2 K as the layers are thinned down. In addition, we propose a tight-binding model, which allows us to attribute this phenomenon to an enhancement of the effective electron-phonon coupling constant. This work provides evidence that reducing the dimensionality can strengthen superconductivity as opposed to the weakening effect that has been reported in other 2D materials so far.

ARTICLE

Received 27 Jun 2015 | Accepted 15 Feb 2016 | Published 17 Mar 2016

Efrn Navarro-Moratalla1,w, , Joshua O. Island2, , Samuel Manas-Valero1, Elena Pinilla-Cienfuegos1,w, Andres Castellanos-Gomez2,w, Jorge Quereda3, Gabino Rubio-Bollinger3,4, Luca Chirolli5,

Jose Angel Silva-Guilln5, Nicols Agrat3,4,5, Gary A. Steele2, Francisco Guinea5, Herre S.J. van der Zant2 & Eugenio Coronado1,

The ability to exfoliate layered materials down to the single layer limit has presented the opportunity to understand how a gradual reduction in dimensionality affects the properties of bulk materials. Here we use this topdown approach to address the problem of superconductivity in the two-dimensional limit. The transport properties of electronic devices based on 2H tantalum disulde akes of different thicknesses are presented. We observe that superconductivity persists down to the thinnest layer investigated (3.5 nm), and interestingly, we nd a pronounced enhancement in the critical temperature from 0.5 to 2.2 K as the layers are thinned down. In addition, we propose a tight-binding model, which allows us to attribute this phenomenon to an enhancement of the effective electronphonon coupling constant. This work provides evidence that reducing the dimensionality can strengthen super-conductivity as opposed to the weakening effect that has been reported in other 2D materials so far.

1 Universidad de Valencia (ICMol), Catedrtico Jos Beltrn Martnez n 2, Paterna 46980, Spain. 2 Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, Delft 2628 CJ, The Netherlands. 3 Departamento de Fsica de la Materia Condensada, Universidad Autnoma de Madrid, Campus de Cantoblanco, Madrid 28049, Spain. 4 Condensed Matter Physics Center (IFIMAC), Universidad Autnoma de Madrid, Madrid 28049, Spain. 5 Instituto Madrileno de Estudios Avanzados en Nanociencia (IMDEA- Nanociencia), Calle Farady 9, Cantoblanco, Madrid 28049, Spain. w Present addresses:

Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA (E.N.-M.); Valencia Nanophotonics Technology Center, Building 8F|1st Floor, Universidad Politcnica de Valencia, Cam de Vera, s/n, 46022 Valencia, Spain (E.P.-C.); Instituto Madrileno de Estudios Avanzados en Nanociencia (IMDEA- Nanociencia), Calle Farady 9, Cantoblanco 28049 Madrid, Spain (A.C.-G.). These authors contributed equally to this work. Correspondence and requests for materials should be addressed to E.N.-M. (email: mailto:[email protected]

Web End [email protected] ) or to J.O.I.

(email: mailto:[email protected]

Web End [email protected] ) or to E. C. (email: mailto:[email protected]

Web End [email protected] ).

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DOI: 10.1038/ncomms11043 OPEN

Enhanced superconductivity in atomically thin TaS2

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11043

The behaviour of superconductors in the two-dimensional (2D) limit is a long-standing problem in physics that has been the focus of extensive research in the eld16. The

bottomup approach has provided signs of the existence of superconductivity at the 2D limit in experiments performed on in situ-grown, ultrathin lead lms fabricated by evaporation7,8. However, for lms grown in this way, it is difcult to avoid the strong inuence from the substrate lattice, yielding typically highly disordered lms. A different approach takes advantage of the ability of certain van der Waals materials to be separated into individual layers, which may later be isolated as defect-free 2D crystals on a substrate of choice9. This topdown approach permits overcoming the lattice and chemical restrictions imposed by the substrate in the bottomup strategy in such a way that the coupling may be minimized by an appropriate choice of surface1012.

Although graphene is not an intrinsic superconductor, recent studies have brought forward the possibility of inducing superconductivity in this 2D material by garnishing its surface with the right species of dopant atoms or, alternatively, by using ionic liquid gating13,14. However, reported experiments have failed to show direct evidence of superconducting behaviour in exfoliated graphene, leaving out the archetypal material from studies of 2D superconductivity15.

An even more attractive family of 2D materials is provided by the transition metal dichalcogenides (TMDCs) since some of its members exhibit superconductivity in the bulk state16,17. Just as in graphene, TMDCs present a strong in-plane covalency and weak interlayer van der Waals interactions, which allow exfoliation of the bulk18. This has given rise to a very rich chemistry of hybrid multifunctional materials based on the restacking of TMDC nano-layer sols with functional counterparts19,20. In addition, the all-dry exfoliation methodologies have allowed for the deposition of TMDC akes on a variety of surfaces21,22. These micromechanical exfoliation techniques allow access to nearly defect-free, large surface area akes of virtually any TMDC, opening the door to the study of how a dimensionality reduction affects the properties of these materials2326. Surprisingly, despite the works reported in the literature searching for intrinsic superconductivity in atomically thin 2D crystals2729, for a long time the sole examples came from FeSe thin lms grown in situ on a substrate3033. Only very recently, several studies of niobium diselenide (NbSe2) akes have yielded the rst clear evidence of the existence of superconductivity in freshly cleaved specimens of less than three layers in thickness3437.

Tantalum disulphide (TaS2) is another member of the TMDC family. In its bulk state, TaS2 is composed of robust covalently bonded STaS planes that stack upon each other. A variety of polytypic phases originate from the distinct in-plane Ta coordination spheres described by the S2 ligands and by the stacking periodicity of the individual planes. For instance, the 2H and 1T polytypes present unit cells with two trigonal bipyramidal and one octahedral Ta-coordinated layers, respectively. Although extensively explored in the 1960s38, 1T and 2H polytypes are once again attracting major attention as they constitute ideal case studies for the investigation of competing orders, namely, superconductivity, charge density waves (CDW)39,40 and hidden phases41. In this scenario, the study of decoupled or isolated TaS2 layers may provide new insights into these exotic phenomena42.

Transport measurements of few-layer TaS2 akes have been reported in akes as thin as 2 nm, but superconductivity in TaS2 layers thinner than 8 nm has not been observed, probably due to the environmental degradation of the samples43.

Here, we explore 2D superconductivity in few-molecular-layer tantalum disulphide akes of different thicknesses, which have been mechanically exfoliated onto Si/SiO2 substrates. Interestingly, we observe that superconductivity persists down to the thinnest layer investigated (3.5 nm, approximately 5 covalent

planes), with a pronounced increase in the critical temperature (Tc) from 0.5 K (bulk crystal) to B2.2 K when the thickness of the layer is decreased. In search of the origin of these observations, we perform density functional theory (DFT) calculations and construct a simple tight binding model to study the change in the electronic band structure and density of states (DOSs) at the Fermi level as a function of reduced thickness. We ascribe the enhancement to an increase in the effective coupling constant (leff) for reduced thicknesses, which ultimately determines Tc.

ResultsFabrication of transport devices. Although the exfoliation of other TMDC members has been extensively studied, little has been reported on the controlled isolation of atomically thin 2H-TaS2 akes. This layered material appears to be difcult to exfoliate and is also particularly susceptible to oxidation in atmospheric conditions44, hindering the manipulation of very thin akes in open moist air. Although complex encapsulation techniques help preserving samples from oxidation36, we nd that a rapid integration of freshly exfoliated akes into nal devices and their immediate transfer to vacuum conditions for measurement also permits retaining the pristine properties of most TaS2 samples (vide infra).

The experimental process begins with the chemical vapour transport growth of bulk TaS2 crystals (vide infra in Methods), which are subsequently exfoliated onto Si/SiO2 substrates. To ensure a high-quality material, optical, Raman and atomic force microscopy characterization were performed on exfoliated akes of varying thicknesses (see Supplementary Figs 15 and Supplementary Note 1 and 2 for details). As already established for graphene and other TMDCs, inspection of the substrate surface by optical microscopy permits identifying the presence of nanometre thin TaS2 akes. In an attempt to access akes with a reduced number of atomic layers, we developed a modication of the micromechanical exfoliation method and optimized it for the controlled isolation of few-layer 2H-TaS2 akes45,46. The method relies on precisely controlling a uniaxial pressure applied directly with a single crystal over the accepting substrate and in combination with a shearing cleavage movement. This allows for the cleavage of very thin akes, down to 1.2 nm thick (see Supplementary Fig. 1 for details), corresponding to a single 2H-TaS2 unit-cell (see Fig. 1a) formed by two individual layers.

Unfortunately, all attempts to contact these akes and measure transport properties were unsuccessful, likely due to their instability in ambient conditions.

To avoid the oxidation of few layer akes, freshly exfoliated samples designated for device fabrication and transport measurements are immediately covered with an methyl methacrylate/poly (methyl methacrylate) double layer resist in preparation for subsequent device nanofabrication steps. Figure 1b shows an example of a fabricated device incorporating the thinnest ake measured with a thickness of 3.5 nm (corresponding to approximately 5 layers) and lateral dimensions of the order of a few micrometres as imaged by atomic force microscopy. The chromium/gold (Cr/Au, 5 nm/70 nm) electrodes were evaporated onto selected akes by employing standard e-beam lithography techniques (see Methods for details). All transport measurements were made using a four terminal current bias conguration in a temperature range of 20 mK to 4 K in a dilution fridge.

Transport properties and superconductivity. We present measurements on 12 akes of varying thicknesses in the E330 nm range, integrated in the described four-terminal devices, with the aim of studying the effect of dimensionality

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Figure 1 | Atomically thin TaS2 devices. (a) Ball and stick model of the crystal structure of the 2H polytype of TaS2. The dashed prism encloses the content of a single unit cell and the metal coordination geometry is highlighted by the red polyhedron. (b) Atomic force microscopy image of two devices fabricated on a 3.5-nm 2H-TaS2 ake. The scale bar is 4 mm in length.

The full colour scale of the topograph corresponds to a height of 100 nm. (c) Line prole of the ake taken at the location of the white dotted line in b.

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reduction on the superconducting properties of TaS2. All devices show a superconducting transition observed by four terminal current bias measurements as a function of temperature. Figure 2 shows the currentvoltage (IV) and resistancetemperature (RT) characteristics for three representative devices having thicknesses of 14.9 nm (Fig. 2a,b), 5.8 nm (Fig. 2c,d) and 4.2 nm (Fig. 2e,f). The transport data for the thinnest 3.5 nm ake can be found in the Supplementary Fig. 6. The zero bias, numerical derivatives (dV/dI) as a function of temperature show a clear superconducting transition for each device (Fig. 2b,d,f). From these (interpolated) curves, we estimate Tc, taken at 50% of the normal-state resistance. For the 14.9 nm ake, and despite the fact that the sample does not attain a zero resistive state, one may still appreciate that there is a phase transition centred at 540230 mK. This is in rough agreement with previously reported Tc values of 600 mK for bulk 2H-TaS2 material47.

Interestingly, and in contrast with studies on other 2D superconductors, the Tc values show a marked increase for the thinner akes of 5.8 nm (1.450.13 K) and 4.2 nm(1.790.20 K). This peculiar result is discussed in detail below. In addition, critical current densities increase by orders of magnitude as the devices become thinner (14.9 nm, JcE700 A cm 2, 5.8 nm, JcE7 104 A cm 2 and 4.2 nm,

JcE5 105 A cm 2). In thin lm superconductors with high

critical current densities, as those measured in our thinnest akes, Joule self-heating starts to play a role48. This explains the pronounced asymmetry in the IV characteristics for thinner akes (Fig. 2a versus Fig. 2e). As the current bias is swept from high negative values through zero, non-equilibrium Joule heating pushes the superconducting transition to a lower current value. This asymmetry decreases as the temperature approaches Tc,

where Joule heating effects become less signicant (Fig. 2e).

Effect of an external magnetic eld on superconductivity. To further characterize the devices at 50 mK, the upper critical eld (Bc2) of these type II superconductors is determined by applying an external magnetic eld, perpendicular to the surface of the ake. Figure 3 shows colour scale plots of dI/dVI curves as a function of external eld for the same three devices as in Fig. 2. Figure 3b shows the zero-bias differential resistance as a function of external eld. From these curves, we estimate the Bc2 as the external eld at which the device returns to the normal-state resistance. Once

again, in accordance with the upper critical eld reported for the

bulk material (110 mT), we measure a Bc2 of E130 mT for the

bulk-like 14.9 nm ake49. The thinner akes present higher upper critical elds of E0.9 T (5.8 nm) and E1.7 T (4.2 nm) following the interesting trend for Tc. The critical elds at 50 mK allow estimation of the superconducting GinzburgLandau coherence lengths given by: Bc2(50 mK) j0/2px(50 mK) (ref. 2). The

coherence lengths for the 4.2 and 5.8 nm akes are 13.9 and19.1 nm, respectively, suggesting that these akes are in the 2D limit. To further qualify the 2D nature of the thinnest akes, we analyse the IV and RT curves (such as those in Fig. 2) of selected devices at zero external eld in order to infer the typical signature of 2D superconductivity: the BerezinskiiKosterlitzThouless transition (see Supplementary Fig. 7 and Supplementary Note 3 for details). Note that this study can only be carried out for selected thinner samples for which sufcient data are available. We nd that the transport data are consistent with a BerezinskiiKosterlitz Thouless superconducting transition, further supporting the 2D nature of the thinnest TaS2 akes.

Effect of dimensionality on the superconducting state. We now turn our attention to the collective behaviour of our 12 devices and

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Figure 2 | Superconductivity in atomically thin crystals. Temperature dependence of three selected devices spanning the range of thicknesses studied. (a) Currentvoltage (IV) characteristics as a function of temperature for a bulk-like 14.9 nm device. (b) Resistance (zero bias numerical derivative) versus temperature for the 14.9 nm device. (c) IV characteristics as a function of temperature for a 5.8-nm device. (d) Resistance versus temperature for the 5.8-nm device. (e) IV characteristics as a function of temperature for a 4.2-nm device. (f) Resistance versus temperature for the 4.2-nm device.

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Figure 3 | Enhanced critical magnetic eld in thin akes. Perpendicular external magnetic eld dependence at 30 mK for three selected devices spanning the range of thicknesses studied. (a) Resistance (zero bias numerical derivative) versus applied eld for a bulk-like, 14.9 nm device. (b) Zero bias resistance versus applied eld for the 14.9-nm device. (c) Resistance (zero bias numerical derivative) versus applied eld for the 5.8-nm device. (d) Zero bias resistance versus applied eld for the 5.8-nm device. (e) Resistance versus applied eld for the 4.2-nm device. (f) Zero bias resistance versus applied eld for the 4.2-nm device.

Figure 4 | 2D superconductivity and enhanced Tc in atomically thin TaS2. (a) Variation of Tc as a function of the thickness of the TaS2 akes.

Devices exhibiting a non-zero residual resistance below Tc areplotted in red. The error bars are given by the temperatures at 10 and 90% of the normal state resistance. The solid black line marks the bulk Tc of 600 mK. The black dotted line is an exponential trend line, t to the data starting at the bulk limit. (b) Variation of Bc2 as a function

of ake thickness. The red circles mark the same devices in a having residual resistance. The black solid line indicates the bulk limit upper critical eld of 110 mT. The grey solid line plots the GL coherence lengths, calculated from the y axis Bc2 values, and marks the edge of the 2D limit.

the effect of reduced dimensionality on the superconducting properties of TaS2. Figure 4 illustrates the measured Tc and Bc2 for the devices reported. A bulk limit was found for samples over 10 nm in thickness, such as the one in Figs 2a,b and 3a,b, for which the superconducting properties were consistent with bulk crystals and did not depend on the number of layers. It is interesting to note that these types of akes exhibit a non-zero residual resistance (red data points) at base temperature, indicating a certain degree of crystalline inhomogeneity and providing a plausible explanation to the slight variation of Tc, similar to the variation in reported bulk values (0.6 and 0.8 K) (refs 25,50). The bulk-limit devices approach the edge of the 2D limit set by the Ginzburg-Landau (GL) coherence length (x 55 nm) estimated from the bulk Bc2 (see Fig. 4b).

DiscussionIn addition to thicker akes that behave in a way consistent with bulk properties, we also observe the superconducting transition in devices made out of thinner TaS2 akes, down to 3.5 nm (B5 layers). Interestingly, we observe a strong enhancement of Tc and Bc2 for thinner akes, up to more than a factor of four larger than in the bulk material. The Tc enhancement with

decreasing number of layers exhibited by the TaS2 samples is in strict contrast to the Tc suppression previously reported in elemental materials7, binary systems51 and even the closely related dichalcogenide family member, NbSe2 (ref. 29). A common theme in these studies is that as the material is thinned down, substrate interactions, either from induced strain or increased Coulomb interactions, suppress the formation of Cooper pairs. In NbSe2 devices, a clear correspondence can be made with a decrease in the residual resistance ratio (RRR) giving an indication of increased substrate interactions or more probable that ake degradation is more prevalent in thinner akes30. This agrees with our attempts to contact akes thinner than 3.5 nm showing a complete insulating state at room temperature. Correspondingly, the RRR values (see Supplementary Fig. 8 for details) for our TaS2 sample set show a signicant reduction for the two thinnest akes. However, devices as thin as 4.5 nm still maintain an RRR of 10, indicating pristine thin samples even below our bulk limit of 10 nm.

An initial point that needs to be addressed once trying to interpret the Tc enhancement is the possibility of electrochemical doping coming from either the original crystals, or through fabrication processes (lithography resists). Although it is well understood that the Tc of TaS2 crystals is particularly sensitive to

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intrinsic non-stoichiometric doping52, we may rule out this effect coming from the original crystals by having measured a bulk Tc of

B0.5 K. Now considering the potential doping coming from environmental or intercalation interactions, Raman spectroscopy provides us with strong evidence of the absence of such processes. In contrast with the remarkable peak shifts displayed by intercalated crystals of 2H-MX2 (ref. 53), the Raman spectra of exfoliated akes presented in the supplement (review Supplementary Figs 35) show no signicant change in crystal structure for akes of only four layers. Given that the akes are not undergoing intercalation through exfoliation or fabrication, there could indeed be some doping coming from surface contamination or from the oxide substrate. However, previous studies show that gate-induced or surface-induced electrostatic doping allows for a carrier density modulation of maximum ca 1012 cm 2 (ref. 54), which is at least three orders of magnitude lower than the estimated single-layer carrier concentration in these metallic TMDCs (ca 1015 cm 2) (refs 30,55). In this line, these doping effects have shown to modulate Tc in NbSe2 by 8% at most30. Finally, although substrate interactions have led to the interesting Tc enhancements found in epitaxial grown FeSe on

STO, we rule out such effects as the TaS2 akes presented here are weakly coupled to the substrate. This suggests a deeper mechanism as opposed to simple substrate interaction, intercalation or degradation reported in previous studies.

A possible mechanism at work could be the enhancement of the superconducting properties associated with a suppression of the commensurate CDW order, which is in direct competition with superconducting pairing19. This is consistent with the interpretation presented of the enhanced Tc and Bc2 observed in the studies of intercalation of TMDC, where it is argued that the in-plane chemical doping leads to the suppression of the charge density order, and in certain TMDCs under pressure where the same claim is made49,56,57. To explore the effect of the CDW on the DOS at the Fermi level as a function of reduced thickness, we calculate the DOS from an effective one-orbital tight-binding model and simulate the CDW at a mean eld level as a periodic potential that locally shifts the onsite energy (see Supplementary Figs 9 and 10 and Supplementary Note 4 for details). We nd that the DOS at the Fermi level is not appreciably affected by the CDW for reduced thicknesses. Ultimately, to determine if such a competition with CDWs could be playing a role, one could search for direct evidence of such suppression in STM studies of thin akes below the 10-nm bulk limit observed here.

An alternative explanation of the enhanced Tc could be a change of the band structure of the material in atomically thin akes. To explore this possibility, we perform DFT calculations and construct a simplied tight-binding model to study the electronic band structure and DOS nN(0) as a function of the sample thickness. The results of the calculation can be observed in the Supplementary Figs 11 and 12 and Supplementary Notes 5 and 6. The resulting 2D bands contain hole pockets and show saddle points below the Fermi level. These saddle points give rise to van Hove peaks, whose height increases as the number of layers is decreased, and ultimately diverge in the 2D limit. However, the DOS per layer at the Fermi level nN(0) decreases as the number of layer is reduced (see Supplementary Fig. 13 for details). For a simplied model with a constant attractive interaction V, the coupling constant, that ultimately determines the Tc, takes the usual BCS value l VnN(0). This behaviour of

the DOS would suggest at rst an analogous trend of Tc, which does not sufce to explain the experiments. The value of the superconducting gap and Tc can be inuenced by the interactions properties of the material. The effective coupling constant58 determining Tc is given by leff lm , where l is the electron

phonon coupling constant, and m , known as AndersonMorel

pseudo-potential, is a term that represents the renormalized repulsive Coulomb interaction. In usual 3D superconductors characterized by a featurelesshence constantDOS, the projection on the Fermi level of the high-energy degrees of freedom gives rise to a pseudo-potential of the form m m/(1 m

ln(W/o0)), with o0 the characteristic phonon frequency, W the system bandwidth and m the bare Coulomb repulsion. In a 2D system, with a DOS characterized by a van Hove singularity near the Fermi level, the renormalization of the bare m can be signicantly larger than in a 3D material. This effect is therefore strongly dependent on the number of layers. For a generic DOS nN(e), the pseudo-potential takes the form

m

m

1 m R

Wo0 deNe=e

withNe the total DOS normalized with its value at the Fermi

energy. Assuming a constant repulsive interaction U, one can estimate m UnN(0). For a weak repulsion, the renormalization is

negligible and the effective coupling constant follows the DOS at the Fermi level nN(0). For a relatively strong Coulomb repulsion, the value of the pseudo-potential at the Fermi level can be strongly affected by features of the DOS at higher energies, such as van Hove singularities. As the number of layers is decreased, the renormalization of a relatively strong repulsion for the band structure in the model is sufcient to reverse the dependence of Tc on the number of layers obtained from a simple electronphonon attractive interaction (see Supplementary Fig. 14 for details). This analysis points to a non-negligible role of the Coulomb repulsive interaction in superconducting 2H-TaS2, characterized by a predominant Ta 5d orbital character at the Fermi level. The Coulomb repulsion has also been proposed to be at the origin of superconductivity in MoS2 (refs 5961).

In conclusion, we have reported 2D superconductivity in 2H-TaS2 in atomically thin layers. In contrast to other van der

Waals superconductors such as NbSe2, we nd that the Tc of this material is strongly enhanced from the bulk value as the thickness is decreased. In addition to a possible charge-density wave origin, we propose a model in which this enhancement arises from an enhancement of the effective coupling constant, which determines the Tc. Our results provide evidence of an unusual effect of the reduction of dimensionality on the properties of a superconducting 2D crystal and unveil another aspect of the exotic manifestation of superconductivity in atomically thin transition metal dichalcogenides.

Methods

Crystal growth. Polycrystalline 2H-TaS2 was synthesized by heating stoichiometric quantities of Ta and S in an evacuated quartz ampoule at 900 C for 9 days. The growth of large single crystals from the polycrystalline sample was achieved by employing a three-zone furnace. The powder sample was placed in the leftmost zone of the furnace and the other two zones were initially brought to 875 C and kept at that temperature for 1 day. Following, the temperature of the source zone was risen to 800 C during the course of 3 h. The temperature of the centre zone was then gradually cooled down at a speed of 1 C min 1 until a

gradient of 125 C was nally established between the leftmost (875 C) and centre (750 C) zones. A gradient of 50 C was also set between the rightmost and growth zones. This temperature gradient was maintained for 120 h and the furnace was then switched off and left to cool down naturally. The crystals were then thoroughly rinsed with diethyl ether and stored under an N2 atmosphere.

Device fabrication. Contact pads and optical markers are rst created on the surface of the Si/SiO2 substrates to locate and design contacts to the transferred akes. The contacts (chromium 5 nm/ gold 70 nm) are then patterned with

standard e-beam lithography (Vistec, EBPG5000PLUS HR 100), metal deposition (AJA International) and subsequent lift-off in warm acetone. To preserve the sample integrity, it is crucial to exfoliate, pattern the electrodes and load into the dilution fridge within a few hours. In that respect and even after minimizing the fabrication time, all attempts to contact akes with thicknesses below 3.5 nm were unsuccessful because of sample degradation.

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Band structure calculations. The DFT simulation of the band structure of 2H-TaS2 has been performed using the Siesta code on systems with different number of layers62. We use the generalized gradient approximation, in particular, the functional of Perdew, Burke and Ernzerhoff63. In addition, we use a split-valence double-z basis set including polarization functions64. The energy cutoff of the real space integration mesh was set to 300 Ry and the Brillouin zone k sampling was set, within the MonkhorstPack scheme65, to 30 30 1 in the case

of multi-layer samples and 30 30 30 in the case of the bulk calculation. We use

the experimental crystal structure of 2H-TaS2 for all the calculations66.

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Acknowledgements

Financial support from the EU (ELFOS project and ERC Advanced Grant SPINMOL), the Spanish MINECO (Excellence Unit Mara de Maeztu MDM-2015-0538, Project Consolider-Ingenio in Molecular Nanoscience and projects MAT201125046 and MAT201457915-R, co-nanced by FEDER), Dutch organization for Fundamental Research on Matter (FOM), NWO/OCW and the Comunidad Autonoma de Madrid (MAD2D-CM -S2013/MIT-3007- and NANOFRONTMAG-CM -S2013/MIT-2850) and the Generalitat Valenciana (Prometeo Program) are gratefully acknowledged. AC-G acknowledges nancial support from the BBVA Foundation through the fellowshipI Convocatoria de Ayudas Fundacion BBVA a Investigadores, Innovadores y Creadores Culturales (Semiconductores ultradelgados: hacia la optoelectronica exible), from the MINECO (Ramn y Cajal 2014 program, RYC-2014-01406) and from the MICINN (MAT2014-58399-JIN). We are grateful to the Electronic Microscopy team at Central Support Service in Experimental Research (SCSIE, University of Valencia, Spain) for their kind and constant support.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the nal version of the manuscript. E.N.-M. jointly conceived the study with

J.O.I., designed and performed the measurements and prepared the manuscript; S.M.-V. and E.P.-C. helped with the preparation and characterization of samples and contributed to the edition of the manuscript. L.C. created the theoretical model with contributions from J.A.S.-G. and F.G. supervised the theoretical analysis and edited the manuscript. J.Q. and G.R.-B. carried out optical microcopy measurements model and interpret. A.C.-G., N. A., G.A.S., H.S.J. van der Z. and E.C. supervised the study and edited the manuscript.

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How to cite this article: Navarro-Moratalla, E. et al. Enhanced superconductivity in atomically thin TaS2. Nat. Commun. 7:11043 doi: 10.1038/ncomms11043 (2016).

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