Metastable Superconductivity in Two-Dimensional IrTe2

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Metastable superconductivity in two-dimensional IrTe crystals Masaro Yoshida, Kazutaka Kudo, Minoru Nohara, and Yoshihiro Iwasa Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00673 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Metastable superconductivity in two-dimensional IrTe2 crystals Masaro Yoshida1,2, Kazutaka Kudo3, Minoru Nohara3 and Yoshihiro Iwasa1,2* 1

RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan. 2Department of

Applied Physics and Quantum-Phase Electronics Centre (QPEC), The University of Tokyo, Tokyo 113-8656, Japan. 3Research Institute for Interdisciplinary Science (RIIS), Okayama University, Okayama 700-8530, Japan. *E-mail: [email protected] KEYWORDS. Two-dimensional material, superconductivity, metastable state, IrTe2.

ABSTRACT. Two-dimensional (2D) materials exhibit unusual physical and chemical properties that are attributed to the thinning-induced modification of their electronic band structure. Recently, reduced thickness was found to dramatically impact not only the static electronic structure, but also the dynamic ordering kinetics. The ordering kinetics of first-order phase transitions becomes significantly slowed with decreasing thickness, and metastable supercooled states can be realized by thinning alone. We therefore focus on layered iridium ditelluride (IrTe2), a charge-ordering system that is transformed into a superconductor by suppressing its first-order transition. Here, we discovered a persistent superconducting zero-resistance state in

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mechanically-exfoliated IrTe2 thin flakes. The maximum superconducting critical temperature (Tc) was identical to that which is chemically optimized, and the emergent superconductivity was revealed to have a metastable nature. The discovered robust metastable superconductivity suggests that 2D material is a new platform to induce, control, and functionalize metastable electronic states that are inaccessible in bulk crystals.

TEXT. Various layered materials have recently been made into monolayers or ultrathin flakes,1,2 which possess exotic properties that are not found in their bulk counterparts. For instance, the distinctive electronic structures in monolayers host the quantum Hall effect in graphene,3,4 strong photoluminescence in semiconducting MoS2,5,6 and enhanced Pauli limits in superconducting NbSe2.7 However, the reduced thickness has recently been revealed to exert a considerable influence on the ordering kinetics of first-order phase transitions.8−11 These discontinuous phase transitions accompany temperature hystereses, and require finite time to be completed. In layered 1T-TaS2, a first-order phase transition system with charge density wave (CDW) ordering, the hysteresis increases with decreasing thickness.8−11 In ultrathin flakes, the CDW transition can be kinetically avoided even with slow cooling, so that the room-temperature state can be easily quenched and stabilized at low temperatures, as if the system was rapidly cooled.8−10 These thinning-induced slow ordering kinetics8,10,11 are probably important for retaining the light- or current-induced metastable states that were recently discovered in 1T-TaS2 thin flakes.10−14 Such slow ordering kinetics appears to be universal in 2D materials that exhibit first-order phase transitions, and it enables us to induce unprecedented metastable electronic states.


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Figure 1. IrTe2 as a 2D material. (a) Crystal structure of the layered IrTe2. The tellurium (Te) atoms are arranged around the iridium (Ir) atom on the corners of an octahedron, which polytype is called “1T.” (b) Temperature (T) dependence of resistivity (ρ) of a bulk single crystal. IrTe2 is a pure Ir3+ normal metal at room temperature, whereas it exhibits a striped Ir3+–Ir4+ chargeordered state below the transition temperature, in the vicinity of 280 K. (c) Schematic electronic phase diagram of IrTe2, where the suppression of the striped charge-ordering state induces the superconductivity. (d) Optical microscope imaging of a microdevice containing an IrTe2 thin flake.

In the study presented here, we focused on IrTe2 (see Fig. 1a), a layered charge-ordering material that exhibits a unique first-order phase transition.15 This material’s room-temperature phase is that of a normal metal, mainly with Ir3+ (5d6) and Te1.5- valences because of the strong interlayer Te−Te coupling. Upon cooling, Ir4+ (5d5) –Ir4+ (5d5) dimerization occurs, and IrTe2


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undergoes a first-order phase transition to an Ir3+–Ir4+ phase in which the charge order exhibits a striped pattern. The transition is peculiarly associated with the valence change of anions from Te1.5- to Te2-. We note that the Ir4+(5d5) forms the localized spin-orbit Mott state, and the Hubbard gap is enhanced by the dimer formation.15 As shown in Fig. 1b, the transition appears as a steep increase in resistivity (ρ) at the transition temperature (TCO) in the vicinity of 280 K. Relevant to this finding, the presence of iridium dimers has been suggested in the normal metallic phase despite the absence of a long-range order.16 One of the intriguing aspects of IrTe2 is the emergence of superconductivity. Figure 1c shows the conceptual electronic phase diagram of the behavior exhibited by doped IrTe2 bulk crystal. Here, the striped charge-ordered phase and the superconductive phase compete with each other,17−19 and superconductivity appears when the striped charge order is suppressed. This was confirmed by an x-ray diffraction study on the bulk superconducting phase.20 Also of note, at the surfaces of both doped and non-doped crystals, a hexagonal charge order appears that is characterized by quasi-ordered hexagons of Ir4+ atoms with a periodicity of 2 nm, where an opening of the superconducting gap is observed.21−23 However, the hexagonal charge-ordered superstructure is absent in the bulk superconducting phase.20 Therefore, suppressing the striped charge order is essential to induce superconductivity in IrTe2. As in the case of 1T-TaS2,10 the first-order phase transition to the striped charge order can be kinetically avoided by reducing thickness, and metastable superconductivity can appear in IrTe2 thin flakes. We fabricated four-terminal devices to investigate IrTe2 thin flakes. Their layered nature enables us to mechanically exfoliate the IrTe2 bulk crystals, and we obtained thin flakes on silicon dioxide (300 nm)/silicon substrates. The typical size of cleaved thin flakes was 10 × 10 µm2 and the thickness was several tens of nanometers. We established metal contacts through an


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electron beam lithography process, followed by the sequential deposition of chromium (5 nm) and gold (120 nm). Using the microdevices as shown in Fig. 1d, we conducted four-terminal resistivity measurements. We obtained temperature-dependent resistivity data using lock-in amplifiers with a constant current of 5 µA at the cooling/warming rate of 1 K/min. Figure 2a shows the normalized resistivity [ρ(T)/ρ(300 K)] versus temperature (T) curves for representative IrTe2 thin flakes with thicknesses ranging from 71 to 89 nm. Mechanical exfoliation rarely yielded ultrathin flakes, probably because of the strong interlayer bonding between Te atoms (see Fig. S1 in the supporting information). Some thin flakes (e.g., sample #1) exhibited clear jumps in resistivity, indicating the occurrence of the normal-to-striped charge order transition, whereas others exhibited no jump (e.g., sample #6). Significantly, all the thin flakes underwent superconducting transitions at low temperatures, as shown in Fig. 2b. Every sample exhibited a relatively sharp resistivity decrease, distinct from the vague superconductivity occasionally observed in bulk crystals with Ir vacancies or excess Te.24,25 The resistivity of sample #3 went to zero at 3.2 K, which is almost identical to the critical temperature that is chemically optimized in Ir0.96Pt0.04Te2.18 The flakes in our samples were not atomically thin, and superconductivity is not found in pristine IrTe2 bulk crystal.17−19 The superconductivity revealed in this work seems not to originate from the reduced dimensionality that is responsible for the enhanced superconductivity in 2H-TaS2.26


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Figure 2. Superconducting 2D IrTe2 single crystals. (a) Temperature (T) dependence of normalized resistivity [ρ(T)/ρ(300 K)] for the thin single crystals. (b) T dependence of ρ(T)/ρ(4 K). (c) ρ(T)/ρ(300 K) versus T curves for thin and bulk crystals. (d) T dependence of Hall coefficient (RH) of thin flakes.

To clarify the reason for the emergence of superconductivity in IrTe2 thin flakes, Fig. 2c compares ρ(T)/ρ(300 K) versus T curves for bulk (thickness, t = 100 µm) and thin (t = 75 nm) crystals. We chose sample #1 as a representative thin flake because it exhibited the clearest jumps in resistivity at TCO. Figure 2c shows that TCO values in the bulk crystal are 278 and 285 K during cooling and heating, respectively, whereas in the thin flake, the corresponding TCO values


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are 194 K and 271 K. Here, TCO during cooling is significantly decreased with reduced thickness, indicating that it takes more time for the thin flake than for the bulk crystal to undergo the normal-to-striped charge-ordered first-order phase transition. The temperature hysteresis of the thin flake (∆T = 77 K) is therefore much wider than that of the bulk crystal (∆T = 7 K). These results suggest that the ordering kinetics of the thin flake are much slower than that of the bulk crystal. The normal-tostriped charge order transition was probably kinetically suppressed in the thin flake, resulting in the metastable superconductivity. We cannot find clear thickness dependence of ordering kinetics in the examined thin flakes, probably because we focused on the narrow thickness range. By increasing the thickness, we can eventually reach the critical thickness for the emergence of superconductivity with the cooling rate of 1 K/min. To obtain more insights into the properties of thin flakes, we conducted Hall effect measurements in the superconducting thin flakes as shown in Fig. 2d. In CuxIrTe2 systems, an increase in resistivity indicates the emergence of the striped charge-ordered domains, and that an increase in RH upon cooling reflects the existence of non-striped charge-ordered domains.19 Here, irrespective of the occurrence of the jumps in resistivity, all the thin flakes showed a dramatic increase in RH with decreasing temperature. These results suggest that the thin flakes are composed mainly of non-striped charge-ordered domains. The increase in resistivity, the degree of which varied among the samples, implies the coexistence of striped and non-striped chargeordered domains. The large extent of the non-striped charge-ordered domains further demonstrates the slow ordering kinetics in thin flakes.


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Figure 3. Multiple ground states revealed by repeating thermal cycles. (a) Resistance (R) versus T curves generated by cooling and warming a 75-nm-thick crystal three times. (b) Magnified R– T curves at low temperatures.

Additional resistivity measurements in IrTe2 thin flakes revealed the metastable nature of their low-temperature state. We cooled and heated the sample three times in a 75-nm-thick crystal, and observed different values of residual resistance in each thermal cycle (see Fig. 3). This observation suggests the presence of a complex structure consisting of domains with striped charge-order and those without striped charge-order. Because the striped charge-ordered phase possesses anisotropic conducting planes,27 the resistivity of the striped charge-ordered domains can be higher than that of the isotropic non-striped ones. Therefore, the larger residual resistance values indicate the larger volume of the striped charge-ordered domains. The result shown in Fig. 3 indicates the existence of multiple metastable states, which originates from the varied volumes of the striped domains that randomly nucleated and grew in the IrTe2 thin flake during cooling.


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The metastability is a characteristic feature of pristine 2D materials, because in systems with such a small number of defects, nucleation can occur randomly. In order to access the multiple metastable states of the striped charge-order and superconductivity, we performed current switching experiments. Data were collected in a DC setup using a Keithley 2400 source meter and 2182 nanovoltmeter. We cooled down a sample from 300 K to a certain temperature, and fed current at the rate of 0.2 ~ 0.3 mA/s. Figure 4a shows the current (I) dependence of the resistance (R) measured at different temperatures. The employed sample was identical to the 75-nm-thick crystal shown in Fig. 3. We observed a hysteresis loop in the R–I curve at low temperatures, whereas the hysteresis was absent above 200 K. The maximum resistance value during the hysteresis was 1.4 Ω with increasing current, which is identical to that upon heating shown in Fig. 3a. The value of the current at which the hysteresis closes in the forward scan increased with a reduced base temperature. These features suggest that the temperature of the entire thin flake increased homogeneously with increasing current. When injecting a current of 50 mA at 10 K, resistance increased up to 1.8 Ω, which is significantly above the resistance value at 300 K of 1.4 Ω. Therefore, the temperature of the entire thin flake can vary within a wide range. We note that the local temperature of the thin flake is rapidly increased during injecting current. For example, during increasing current at 200 K, the local temperature is raised above 300 K within 3 min. Such time is too short for the first-order normal-to-striped charge-order transition to be completed, resulting in the absence of hysteresis in the R–I curve at 200 K.


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Figure 4. Metastable nature of the superconductivity revealed by injecting pulsed high current. (a) Current (I) dependence of resistance (R) for a 75-nm-thick crystal, where R is shifted upward by 0.7 Ω to distinguish the different measurement sets, and each colored arrow denotes the current at which the hysteresis loop closes in the forward scan. (b) Temperature (T) dependence of R for a 78-nm-thick crystal before (s0) and after (s1, s2 and s3) applying pulsed voltages. (c) Time dependence of R (left ordinate) and applied pulsed in-plane voltage (V, right ordinate) measured at 4 K for the 78-nm-thick crystal. The duration of the pulsed voltage was 0.5 s. (d) Schematics of the low-temperature states consisting of both metallic striped charge-ordered and superconducting non-striped domains.

The current injection enabled us to control the sample temperature effectively, and thus we could quickly access one of the multiple metastable states by applying a pulsed voltage with a


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concomitant high current at a low temperature. We prepared a 78-nm-thick crystal, which in the initial state (s0) went to the zero-resistance state at 2.3 K, as represented by the green R–T curve in Fig. 4b. We set the base temperature at 4 K, and sequentially applied a pulsed voltage over a duration of 0.5 s. Figure 4c shows the time evolution of the low-field resistance at 4 K with intermittently applied pulsed voltage. After the first application of the pulsed voltage of 2 V, the residual resistance decreased by 20 %. To characterize the resultant metastable state (s1), we then cooled the sample to 2 K. As represented by the yellow R–T curve in Fig. 4b, the resistance went to zero at 2.6 K, which temperature is higher than that of the initial state s0. The residual resistance always changed in response to the application of voltage, which corresponds to the realization of multiple metastable states. We sometimes observed a dramatic increase in residual resistance after applying a low voltage of 1.0 V, and consequently resistance was finite above 2 K in this metastable state (s2, see the dark blue R–T curve in Fig. 4b). The residual resistance then significantly decreased after applying a 2 V pulse, followed by the realization of a metastable state (s3) showed by the pink R–T curve in Fig. 4b. These results demonstrate the strong thermal cycle dependence for the occurrence of the zero-resistance state, which in turn indicates the microscopic coexistence of metallic striped charge-ordered domains and superconducting nonstriped domains, as schematized in Fig. 4d. Once a current pulse is applied, the coordination of striped and non-striped domain changes. The volume fraction of non-striped domains depends on thermal cycles, followed by the difference in the temperature where zero-resistance state is realized. The thermal cycled dependence of the TCO further supports the assertion that thinninginduced slow ordering kinetics is the key mechanism stimulating the emergence of


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superconductivity. In Fig. 2c, TCO during heating (TCO_H) of the thin flake is slightly lower than that of the bulk crystal. This observation may suggest unintentional carrier doping in the thin flake IrTe2 device. However, unintentional carrier doping is unlikely because TCO_H was revealed to depend significantly on the thermal cycle. As shown in Fig. S2 in the supporting information, current switching resulted either in increased TCO_H (see Fig. S2a), or in the suppression of the transition to the striped charge-ordered phase such that it became impossible to define TCO_H after applying a high current (see Fig. S2b). Such strong thermal cycle dependence further highlights the metastable character of the thin flakes, and thus the superconductivity is attributed to the slow ordering kinetics. The present results provide an insight into the origin of the superconductivity in IrTe2. We here recall that superconductivity was initially discovered in chemically doped systems, where the band filling is changed. In contrast, the reduction in thickness is not accompanied by the change of electronic structure. Chemical doping changes the band filling to reduce the Ir–Ir dimerization energy, whereas the decrease in thickness slows the ordering kinetics to suppress the Ir–Ir dimerization. Because striped charge-order is inhibited and superconductivity emerges in both doped and thinned IrTe2 crystals, it is highly likely that the suppression of the Ir–Ir dimerization is essential to induce superconductivity. We finally mention the underlying picture of the thinning effect on the first-order phase transition that consists of the two processes; nucleation and growth. In Fig. 2c, we find that the reduction in thickness decreases the transition temperature on cooling, indicating that the nucleation probability is decreased by thinning. This reminds us of a classical theory of homogeneous nucleation, where the nucleation rate is proportional to the system volume.28 On the other hand, the thin flakes in Fig. 2 show multiple jumps in resistivity with cooling, implying


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the slow growth of striped charge-ordered domains after their nucleation. We note that in a 1TTaS2 thin flake, a single sharp transition takes place,8−11 and it is likely that domains grow rapidly once they nucleate. The present results revealed that the reduction in thickness can have the influence not only on the rate of nucleation but also on the growth rate, dependent on materials. Further research on various kinds of first-order phase transition materials is required to elucidate the thickness dependent nature of the ordering kinetics. In conclusion, we discovered zero-resistance superconductivity in IrTe2 thin flakes. Comparing bulk and thin crystals suggests that thinning-induced slow ordering kinetics play a significant role in the emergence of superconductivity. Repeated thermal cycles demonstrated the metastable nature of the superconductivity, and the discovery of metastable superconductivity in IrTe2 thin flakes further indicates the novel opportunities available in 2D materials, especially for realizing nonequilibrium states that may be accessible only in reduced dimensions.


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ASSOCIATED CONTENT Supporting Information. 16-nm-thick crystal, and current switching effect on temperature dependence of resistance. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions M.Y. fabricated the thin flake devices and conducted the resistivity measurements. K.K. and M.N. grew the single crystal. M.Y. and Y.I. planned the study and wrote the manuscript. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank F. Kagawa, H. Oike, and T. Sato for engaging with us in productive discussions. This work was supported by Grants-in-Aid for Scientific Research (grant numbers 25000003, 15H05886, 16K05451) from the Japan Society for the Promotion of Science (JSPS). REFERENCES 1. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102,


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two-dimensional 1T-TaS2 crystals. Sci. Adv. 2015, 1, e1500606. 11. Tsen, A. W.; Hovden, R.; Wang, D.; Kim, Y. D.; Okamoto, J.; Spoth, K. A.; Liu, Y.; Lu, W. J.; Sun, Y. P.; Hone, J. C.; Kourkoutis, L. F.; Kim, P.; Pasupathy, A. N. Structure and control of charge density waves in two-dimensional 1T-TaS2. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 15054-15059. 12. Stojchevska, L.; Vaskivskyi, I.; Mertelj, T.; Kusar, P.; Svetin, D.; Brazovskii, S.; Mihailovic, D. Ultrafast switching to a stable hidden quantum state in an electronic crystal. Science 2014, 344, 177-180. 13. Vaskivskyi, I.; Mihailovic, I. A.; Brazovskii, S.; Gospodaric, J.; Mertelj, T.; Svetin, D.; Sutar, P.; Mihailovic, D. Fast electronic resistance switching involving hidden charge density wave states. Nat. Commun. 2016, 7, 11442. 14. Yoshida, M.; Gokuden, T.; Suzuki, R.; Nakano, M.; Iwasa, Y. Current switching of electronic structures in two-dimensional 1T-TaS2. Phys. Rev. B 2017, 95, 121405(R). 15. Ko, K. T.; Lee, H. H.; Kim, D. H.; Yang, J. J.; Cheong, S. W.; Eom, M. J.; Kim, J. S.; Gammag, R.; Kim, K. S.; Kim, H. S.; Kim, T. H.; Yeom, H. W.; Koo, T. Y.; Kim, H. D.; Park, J. H. Charge-ordering cascade with spin-orbit Mott dimer states in metallic iridium ditelluride. Nat. Commun. 2015, 6, 7342. 16. Joseph, B.; Bendele, M.; Simonelli, L.; Maugeri, L.; Pyon, S.; Kudo, K.; Nohara, M.; Mizokawa, T.; Saini, N. L. Local structural displacements across the structural phase transition in IrTe2: order-disorder of dimers and role of Ir-Te correlations. Phys. Rev. B 2013, 88, 224109. 17. Yang, J. J.; Choi, Y. J.; Oh, Y. S.; Hogan, A.; Horibe, Y.; Kim, K.; Min, B. I.; Cheong, S. W. Charge-orbital density wave and superconductivity in the strong spin-orbit coupled IrTe2:Pd.


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Phys. Rev. Lett. 2012, 108, 116402. 18. Pyon, S.; Kudo, K.; Nohara, M. Superconductivity induced by bond breaking in the triangular lattice of IrTe2. J. Phys. Soc. Jpn. 2012, 81, 053701. 19. Kamitani, M.; Bahramy, M. S.; Arita, R.; Seki, S.; Arima, T.; Tokura, Y.; Ishiwata, S. Superconductivity in CuxIrTe2 driven by interlayer hybridization. Phys. Rev. B 2013, 87, 180501(R). 20. Ivashko, O.; Yang, L.; Destraz, D.; Martino, E.; Chen, Y.; Guo, C. Y.; Yuan, H. Q.; Pisoni, A.; Matus, P.; Pyon, S.; Kudo, K.; Nohara, M.; Forró, L.; Rønnow, H. M.; Hücker, M. V.; Zimmermann, M.; Chang, J. Charge-stripe order and superconductivity in Ir1-xPtxTe2. Sci. Rep. 2017, 7, 17157. 21. Fujisawa, Y.; Machida, T.; Igarashi, K.; Kaneko, A.; Mochiku, T.; Ooi, S.; Tachiki, M.; Komori, K.; Hirata, K.; Sakata, H. Visualizing the Pt doping effect on surface and electronic structure in Ir1-xPtxTe2 by scanning tunnelling microscopy and spectroscopy. J. Phys. Soc. Jpn. 2015, 84, 043706. 22. Ruan, W.; Tang, P.; Fang, A.; Cai, P.; Ye, C.; Li, X.; Duan, W.; Wang, N.; Wang, Y. Structural phase transition and electronic structure evolution in Ir1-xPtxTe2 studied by scanning tunnelling microscopy. Sci. Bull. 2015, 60, 798-805. 23. Kim, H. S.; Kim, S.; Kim, K.; Min, B. I.; Cho, Y. H.; Wang, L.; Cheong, S. W.; Yeom, H. W. Nanoscale superconducting honeycomb charge order in IrTe2. Nano Lett. 2016, 16, 42604265. 24. Fang, A. F.; Xu, G.; Dong, T.; Zheng, P.; Wang, N. L. Structural phase transition in IrTe2: a combined study of optical spectroscopy and band structure calculations. Sci. Rep. 2013, 3, 1153.


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25. Pyon, S.; Kudo, K.; Nohara, M. Emergence of superconductivity near the structural phase boundary in Pt-doped IrTe2 single crystals. Physica C 2013, 494, 80-84. 26. Navarro-Moratalla, E.; Island, J. O.; Manas-Valero, S.; Pinilla-Cienfuegos, E.; CastellanosGomez, A.; Quereda, J.; Rubio-Bollinger, G.; Chirolli, L.; Silva-Guillen, J. A.; Agrait, N.; Steele, G. A.; Guinea, F.; van der Zant, H. S. J.; Coronado, E. Enhanced superconductivity in atomically thin TaS2. Nat. Commun. 2016, 7, 11043. 27. Toriyama, T.; Kobori, M.; Konishi, T.; Ohta, Y.; Sugimoto, K.; Kim, J.; Fujiwara, A.; Pyon,

S.; Kudo, K.; Nohara, M. Switching of conducting planes by partial dimer formation in IrTe2. J. Phys. Soc. Jpn. 2014, 83, 033701. 28. Turnbull, D. J. Chem. Phys. 1952, 20, 411-424.

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19

a

d Nano Letters

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ition trans

Temperature

r rde

ρ (µΩ cm)

to

1 2 2µm V 3 Ir Te 4 b5 40 c 6 7 IrTe2 bulk 8 1s Normal single crystal 9 30 metal 10 11 1220 Stripe 13 Charge Normal Order 14 (Ir3+) 1510 Stripe Supercond. 16 Charge Order 3+ 4+ (Ir ,Ir ) 17 ACS Paragon Plus Environment Doping 180 0 100 200 300 19 Thinning T (K) 20

A

aPage 21 of 24 1.0

Nano Letters

Thickness (t) = 75 nm (#1) 75 nm (#2) 77 nm (#3)

ρ/ρ(4K)

RH (x10-4cm3/C)

ρ/ρ(300K)

ρ/ρ(300K)

1 2 3 4 5 6 1.0 7 0.5 b #6 #5 8 #3#4 9 0.5 #2 #1 10 86 nm (#4) 11 89 nm (#5) 0.0 12 71 nm (#6) 2.0 2.5 3.0 3.5 13 T (K) 140.0 0 50 100 150 200 250 300 15 T (K) 16 17 c181.5 d 3.0 #1 Thin flake (#1) #3 19 t = 75 nm #6 201.0 2.0 21 220.5 1.0 Bulk crystal 23 t = 100 µm 240.0 0.0 ACS Paragon Plus Environment 25 0 100 200 300 0 100 200 300 26 T (K) T (K) 27

a 1.5

Nano Letters

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R (Ω )

R (Ω)

1 2 3 4 1.0 5 2nd scan 6 b 0.4 7 2nd 8 9 0.5 3rd 0.2 1st scan 10 1st 11 3rd scan 0.0 12 2.0 2.5 3.0 3.5 13 t = 75 nm T (K) 14 150.0 ACS Paragon Plus Environment 0 50 100 150 200 250 300 16 T (K) 17

b 0.6 Nano Letters

a 23 of 24 Page 1

200 K

t = 78 nm

V

R (Ω)

R (Ω)

RT = 10 K (Ω)

A

Ir4+ Ir3+

Heat/Cool V

3.5

A

Heat/Cool...

s2

t = 78 nm T=4K

s3 4 2 100 s

0

V (V)

s2 1 0.4 120 K 0 2 3 70 K 0 s0 2 4 0.2 5 0 30 K 1 6 s3 0 s1 7 10 K t = 75 nm 8 0 0 0.0 0 10 20 30 40 50 2.0 2.5 3.0 9 I (mA) T (K) 10 11 c s0 12 13 0.5 14 15 s1 16 17 0.4 18 19 ACS Paragon Plus Environment 20 21 0.3 Time (s) 22 R (Ω)

d

1 2 3 4 5

ρ/ρ(4K)

Nano Letters 1.0

0.5

Page 24 of 24 Bulk Thin


Ir

Te

0.0 2.0

2.5

3.0

T (K)

3.5

Metastable Superconductivity in Two-Dimensional IrTe2

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