Signature of Superconductivity in Orthorhombic CoSb Monolayer

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Signature of Superconductivity in Orthorhombic CoSb Monolayer Films on SrTiO(001) 3

Cui Ding, Guanming Gong, Yanzhao Liu, Fawei Zheng, Zhiyu Zhang, Haohao Yang, Zhe Li, Ying Xing, Jun Ge, Ke He, Wei Li, Ping Zhang, Jian Wang, Lili Wang, and Qi-Kun Xue ACS Nano, Just Accepted Manuscript • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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Signature of Superconductivity in Orthorhombic CoSb Monolayer Films on SrTiO3(001) Cui Ding1‡, Guanming Gong1‡, Yanzhao Liu2‡, Fawei Zheng3, Zhiyu Zhang1, Haohao Yang1, Zhe Li1, Ying Xing2,4, Jun Ge2, Ke He1,5, Wei Li1,5, Ping Zhang3, Jian Wang2,5,6,7*, Lili Wang1,5*, Qi-Kun Xue1,5,7*

1State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084

2International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China

3Institute of Applied Physics and Computational Mathematics, Beijing 100088, China

4Department of Materials Science and Engineering, School of New Energy and Materials, China University of Petroleum, Beijing 102249, China

ACS Paragon Plus Environment

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5Collaborative Innovation Center of Quantum Matter, Beijing 100084

6CAS Center for Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, China

7Beijing Academy of Quantum Information Sciences, Beijing 100193, China

Corresponding Authors *E-mail (Jian Wang): [email protected];

*E-mail (Lili Wang): [email protected];

*E-mail (Qi-Kun Xue): [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ABSTRACT: We prepare orthorhombic CoSb monolayer films on SrTiO3(001) substrate by molecular beam epitaxy, and observe symmetric gap around the Fermi level with


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coherence peaks at ± (6-7) meV by in-situ scanning tunneling spectroscopy. Ex-situ magnetization measurements of the films protected by Te and Si films consistently reveal a diamagnetic transition at 14 K. These results suggest the occurrence of superconductivity in orthorhombic CoSb monolayers on SrTiO3(001).

KEYWORDS: CoSe/SrTiO3, interface superconductor, molecular beam epitaxy, scanning tunneling microscopy/spectroscopy, magnetization

Cuprate, iron-pnictide and iron-chalcogenide high temperature superconductors consist of two-dimensional (2D) superconducting layers like CuO2, FeAs and FeSe layers, respectively, sandwiched between various charge reservoir layers.1-3 Thus, quasi-2D atomic and electronic structures are on empirical grounds known as a route to high Tc superconductivity. This quasi-2D feature has stimulated tremendous research interests in fabricating a superconducting layer on oxide substrates by state-of-the-art molecular beam epitaxy (MBE) or pulsed laser deposition.4 A recent notable example is the single unit cell FeSe film on TiO2–terminated SrTiO35-7 that exhibits a superconducting gap of ~ 20 meV, one order of magnitude larger compared with bulk FeSe.3 The discovery


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suggests a way for exploring high temperature superconductivity by interface engineering. The CuO2, FeSe and FeAs superconducting layers share similar features: they are late transition metal compounds with square sub-lattices of d-cations under 3d9 (Cu2+) or 3d6 (Fe2+) configurations, and the d orbitals with the strongest in-plane d-p hybridization near the Fermi energy dominate the quasi-2D electronic environment that hosts high Tc superconductivity.8,9 Between Cu 3d9 and Fe 3d6 are two other 3d late transition metals,

i.e. Co 3d7 and Ni 3d8. Bulk Co and Ni pnictides/chalcogenides have NiAs-type three dimensional

hexagonal

structures.10

Whether

2D

Co

and

Ni

analogs

host

superconductivity remains an interesting but challenging topic. Despite great efforts, only Nax(H3O)zCoO2·yH2O,11 LaCo2B2,12 ANi2Se2 (A= alkali metal) and BaNi2As213,14 have been reported to superconduct with Tc < 5 K, where superconductivity occurs in the CoO2 square lattice, anti-PbO-type CoB, NiSe, and NiAs layers, respectively. Bulk CoSb has NiAs-type hexagonal structure, which is reported as a nonmagnetic metal.15 A very recent first-principle calculation proposed that CoSb monolayer on SrTiO3 is isovalent to and shares the similar planar crystal structure with the single unit cell FeSe,


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and further predicted that the layered CoSb hosts superconductivity as well (Zhenyu zhang, personal communication, October, 2018). Inspired by this work, we carried out MBE growth of CoSb on SrTiO3(001) substrate and obtained an orthorhombic CoSb monolayer. Our in-situ scanning tunneling microscopy/spectroscopy (STM/STS) study reveals a symmetric gap around the Fermi level with coherence peaks at ± (6-7) meV.

Ex-situ magnetization measurements reveal a vibrating sample magnetometer (VSM) magnetization drop at around 14 K and magnetic hysteresis at low temperatures. The results suggest the occurrence of superconductivity in monolayer CoSb on SrTiO3. RESULTS AND DISCUSSION CoSb grows via a quasi-layer-by-layer mode and forms monolayer islands with in-plane rectangular lattices on SrTiO3(001) substrates. Figs. 1(a)-(d) depict a series of typical STM topographic images of CoSb films at various coverages taken at room temperature. At 0.3 monolayer (ML), flat CoSb islands with uniform height (8.0 Å at sample biases of 2-5 V) distribute randomly on the surface, as shown in Fig. 1(a). With increasing coverage, the CoSb islands become larger (Fig. 1(b)) and coalesce into nearly complete film at the coverage of 1.0 ML (Fig. 1(c)). Some CoSb patches with thickness of 8.0 Å but rough


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surface form on the first layer CoSb islands. With the coverage is increased to 2.3 ML, the surface is covered by randomly distributed islands of similar heights (Fig. 1(d)), indicating a quasi-layer-by-layer growth mode. To detect the electronic structure of CoSb, we conduct low temperature STM/STS measurements. The apparent contrast of the CoSb monolayer becomes bias-dependent, increasing from 4.0 Å to 9.0 Å with sample bias from 5 V to 1 V (not shown). Displayed in Figs. 1(e) and (f) are atomically resolved images of CoSb monolayer films taken at 4.8 K. Periodic stripes are present (left corner of Fig. 1(e)) with an inter-stripe separation of 5.9 Å. When the STM tip moves closer to the surface, rectangular lattice is clearly resolved (Fig. 1(f)). The corresponding fast Fourier transform (FFT) image indicates in-plane lattice constants of a = 3.2 Å and b = 5.9 Å. In order to determine the atomic structure, we employ the swarm-intelligence based CALYPSO structure prediction method16,17 to find the stable structure based on the experimental lattice parameters. The result is shown in Fig. 1(g). It consists of Sb-Co-Sb triple-layer stacking along the out-of-plane (001) direction with inplane orthorhombic lattice of Sb layer and tetragonal lattice of Co layer. The red frame in Fig. 1(g) indicates a single unit cell with in-plane lattice parameters of 5.92 Å × 3.24 Å


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and out-of-plane triple-layer thickness of 3.11 Å. The latter is close to the observed minima of 4.0 Å by STM. Different from anti-PbO-type structure wherein the middle cation atoms vertically overlap with anion atoms in either top or bottom layers, the CALYPSO structure prediction disclosed that the Co atoms occupy the middle positions of Sb atoms in adjacent layers (bottom panel in Fig. 1(g)). Intriguingly, the monolayer films exhibit exclusively symmetric tunneling gap around the Fermi level. Displayed in Figs. 2(b) and 2(c) are dI/dV spectra of monolayer CoSb islands with lateral size of 30 nm × 50 nm in a large bias range (-500 mV – 500 mV) and small bias range (-20 mV – 20 mV), respectively, taken at the marked points on the morphology image shown in Fig. 2(a). As displayed in Fig. 2(b), the periodic density of state (DOS) right below the Fermi level shows spatial variation. Typically, the relatively dark areas exhibit sharp DOS change below the Fermi level with a peak at approximately - 100 meV, in contrast to wide DOS occupation in the bright areas. The most important observation is the symmetric gap around the Fermi level, characterized by coherence peaks at ± (6-7) meV (Fig. 2(c)), suggesting the occurrence of superconductivity. In the second layer, this superconducting-like gap disappears. Instead, a featureless insulating


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state occurs, as shown in Fig. 2(d). This thickness dependence is similar to our previous observation in FeSe/SrTiO3.5 The superconducting-like gaps become suppressed with reduced lateral size. Fig. 2(e) displays the dI/dV spectra taken on two monolayer CoSb islands of 8 nm × 9 nm and 13 nm × 15 nm, revealing discrete peaks with respective energy separations of 16-20 meV and 8-10 meV at negative biases, marked by red and black bars, respectively. The energy separation decreases with increasing lateral size, consistent with previous observations in Coulomb blockade regime.18 Around EF, the island with lateral size of 8 nm × 9 nm exhibits a large energy space of 27 meV, while the 13 nm × 15 nm one exhibits a symmetric gap with peaks at ± 6 meV. Clearly, the spectrum of monolayer CoSb island with lateral size smaller than 10 nm show features of Coulomb blockade effects, while that between 10-20 nm comes from combined superconductivity and Coulomb blockade effects, as previously observed on Pb islands.19 The superconductivity in CoSb monolayer is further demonstrated by ex-situ magnetization measurement. Figs. 3(a) and (b) show the temperature dependence of VSM magnetization of Te/CoSb/SrTiO3 and Te/SrTiO3 samples, respectively, measured


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in both zero-field cooling (ZFC) and field cooling (FC) modes. The magnetic field is 1000 Oe, applied parallel to the SrTiO3 (001) plane. Clearly, for Te/CoSb/SrTiO3, the ZFC and FC magnetization versus temperature (M-T) curves exhibit a deviation at 14 K; while those for Te/SrTiO3 coincide with each other in the whole temperature range of 2-50 K. Subtracting the diamagnetic FC M-T curve of Te/CoSb/SrTiO3 as the background mainly from SrTiO3 substrate and Te protection layer from the ZFC M-T curve shown in Fig. 3(a), we obtain the ZFC M-T curve of the CoSb islands, as shown in Fig. 3(c). A magnetization drop can be observed at Tc ~ 14 K, reminiscent of superconducting Meissner effect. We further measure the magnetization as a function of magnetic field (M-H) of the CoSb islands, which are shown in Figs. 3(e) and 3(f). Distinct magnetic hysteresis loops can be observed

at

low

temperatures,

which

is

a

typical

behavior

of

type-II

superconductors.16,20,21 The hysteresis loops disappear above 13 K, in good agreement with the M-T results. On the other hand, the M-H curve of Te/SrTiO3 measured at 1.8 K (Fig. 3(d)) shows no hysteresis loop, confirming that the hysteresis loops are from the CoSb/SrTiO3. One may speculate that the Te atoms in capping layer may react with Co/Sb to form CoTex, SbTex, etc. Previous studies point out that CoTex is ferromagnetic


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or ferrimagnetic at 1 < x < 1.2, and the magnetic behavior changes to paramagnetic when

x > 1.2,22-25 which is different from the diamagnetic signal in our measurements. Sb2Te3 crystals are diamagnetic with a temperature independent magnetic susceptibility26,27 and the Co-doped Sb2Te3 nanoplates are paramagnetic from 5 K to 300 K.28 These facts suggest that the magnetization transition at 14 K is intrinsic to the CoSb/SrTiO3. This is further supported by ex-situ magnetization measurements under Si capping layer protection that consistently reveal magnetization drop at Tc ~ 13-14 K, as shown in Fig. 4. Here, the magnetization drops measured in surface-normal magnetic field (Figs. 4(b) and 4(d)) are relatively weak compared with surface-parallel magnetic field (Figs. 4(a) and 4(c)), similar to our previous results of one-unit-cell FeSe films on SrTiO3.6,

29

Furthermore, the M-T curve in FC mode shows no sign of downturn (Figs. 3(a), 4(a) and 4(b)), which is inconsistent with the feature of magnetization in the presence of the charge density wave (CDW) gaps.30,31 This probably excludes the CDW origin of the gap in Fig. 2(c).


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It is worth noting that monolayer CoSb films are stable only at relative low growth temperature. At higher growth temperature, nanowires networks form. Fig. 5 displays the morphologies and dI/dV tunneling spectra taken on samples prepared at a significantly increased substrate temperature of 380 °C. Clearly, nanowires form along two mutually perpendicular directions. The nanowire coverages in Figs. 5(a) and (b) are 0.26 ML and 0.45 ML, respectively. Notably, clusters occur in Fig. 5(b), indicating a nanowires coverage limit of ~ 0.45 ML, probably due to inter-wire repulsion. The nanowires range from 2 nm to 5 nm in width and exhibit stripes with nearly constant period of ~1 nm on the surface (inset of Fig. 5(c)). On isolated nanowires, DOS oscillation is resolved, as shown in Fig. 5(f). The energy separations decrease with increasing wire length (Fig. 5(g)), which we have attributed to Coulomb blockade effect. On connected nanowires, the DOS oscillation disappears (Fig. 5(d)) at expense of opening of gaps (~ 6-7 meV) around Fermi level (Fig. 5(e)). The results are consistent with the size effect in CoSb monolayer islands discussed above. CONCLUSIONS


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In summary, we successfully prepared orthorhombic CoSb monolayer films on SrTiO3 and observed superconductor-like gaps with prominent coherence peaks. The superconductivity transition at 14 K has been confirmed by ex-situ magnetization measurements. Since the monolayer CoSb shows completely different crystalline structure from its bulk counterpart, the system represents a ferromagnetic-element-based interface superconductor. METHODS We use Nb-doped (0.5 wt %) SrTiO3(001) and intrinsic SrTiO3(001) single crystals as substrates for STM/STS and SQUID measurements, respectively. MBE growth of CoSb films is conducted in an Omicron ultrahigh vacuum MBE system equipped with a room temperature STM by co-evaporating Co (99.995 %) and Sb (99.999 %) from Knudsen cells. The stoichiometry of CoSb compound is quite sensitive to the flux ratio between Co and Sb. To achieve orthorhombic CoSb monolayer films, the flux ration has to be controlled precisely. We set Co and Sb cell temperatures at 1090 °C (1100 °C) and 345 °C (355 °C), respectively, while keeping the substrates at 200 °C (380 °C) for islands (nanowires) growth. The corresponding growth rate is 0.02 ML per minute. The low


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temperature STM/STS characterization is conducted in a Createc STM system at 4.8 K. A vacuum suitcase with base pressure better than 1.0×10−8 Torr is used for sample transfer from MBE growth system to the low temperature STM system. PtIr tips are used throughout the experiments. STM topographic images are acquired in a constant current mode, with the bias voltage (Vs) applied to the sample. Tunneling spectra are measured by disabling the feedback circuit, sweeping the sample voltage Vs, and extracting the differential conductance dI/dV using a standard lock-in technique with a small bias modulation (~1 % of the sweeping range) at 937 Hz. For ex-situ magnetization experiments in a magnetic property measurement system (Quantum Design MPMS3), the samples are capped with either ~10 nm Te films or Si cluster films to protect the CoSb films from abient oxidation and contamination. ACKNOWLEDGMENT

The work is supported by the National Natural Science Foundation of China (Grant Nos 11888101, 11574174, 11774193, 11790311, and 11774008), the National Basic Research Program of China (Grant Nos 2015CB921000, 2018YFA0305604 and


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2017YFA0303302), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB28000000), and the Beijing Natural Science Foundation (Grant No. Z180010).

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Signature of Superconductivity in Orthorhombic CoSb Monolayer

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