Enhanced Polarization from Hollow Cube-like ZnSnO3 Wrapped by

Surface Review and Letters, 1841001 (18 pages) c World Scienti¯c Publishing Company ° DOI: 10.1142/S0218625X18410019

Surf. Rev. Lett. Downloaded from www.worldscientific.com by IOWA STATE UNIVERSITY on 01/11/19. Re-use and distribution is strictly not permitted, except for Open Access articles.

SCANNING TUNNELING MICROSCOPIC STUDY OF THE INTERFACE SUPERCONDUCTIVITY YANG WANG, CAN-LI SONG, LILI WANG, XU-CUN MA* and QI-KUN XUE State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, P. R. China *[email protected] Received 25 June 2018 Revised 4 October 2018 Accepted 11 November 2018 Published 4 January 2019 It has been 30 years since the Nobel Prize was awarded for scanning tunneling microscope (STM) in the year 1986, and there have been many instrumental developments and experimental achievements based on STM. In consideration of the strong capability and the extreme versatility in imaging, manipulating, and spectroscopy at the atomic level, STM has witnessed remarkable breakthroughs in many disciplines of condensed matter physics. In this paper, we will focus on recent STM studies on the interface superconductivity, which demonstrate a novel platform for exploring two dimensional superconductors and even high temperature superconductors by means of interface engineering. Keywords: Scanning tunneling microscopy; scanning tunneling spectroscopy; thin ¯lms; interface superconductivity.

1. Introduction The invention of STM by Binnig and Rohrer came in the year 1982, after which a new era of atomic-scale exploration was ushered in Refs. 1 and 2. Manifested by the ongoing advances, STM has revealed itself as the ideal tool in the surface and interface causes.3,4 Its sub-atomic spatial resolution nicely matches the atomic level requirements in recording the surface morphology and structure property.5,6 Together with the sub-millivolt energy resolution at low temperature, detailed depictions of the electronic structure landscapes are thus available by means of the scanning tunneling spectroscopy (STS).7,8 Given its *Corresponding

capability, STM has been applied to the superconductivity researches as well. Characterized by the appearance of zero resistance and perfect diamagnetism, superconductivity refers to the state where Fermionic electrons, with the assistance of arbitrary attractive force, condense into bosonic Cooper pairs.9 Attempts to explore the superconducting properties via tunneling trace back to the 1960s, when Giaever studied the superconductor/insulator/normal metal (SIN) planner junctions.10 Later on, this was further developed by Levinstein and Kunzler into the point contact technique, the very rudiment of the STM.11 The appearance and developments of STM enabled us to

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investigate the superconducting properties of a material in a more comprehensive and microscopic way. In addition to the information on surface morphology, one could directly probe the quasiparticle density of states (DOS) and measure the superconducting gap at the Fermi energy.12 When a magnetic ¯eld is applied normal to the sample surface, the superconducting vortices may appear to give further insight into the pairing symmetry.13 Another route to the pairing symmetry information lies in the impurity effect,14 that the magnetic ones impact both the conventional and unconventional superconductors,15,16 while for the non-magnetic ones, only the unconventional superconductors may be in°uenced.17,18 By low temperature STM/S, the impurity responses can be located with single atom accuracy. Therefore, STM has attracted much attention even in the ¯eld of high temperature superconductivity (HTS).12,19 On the other hand, interfaces, being one of the most fertile lands in condensed matter physics, exhibit a broad spectrum of marvelous phenomena in the vanguard of current researches.20,21 However, experimental researches in this direction just got into stride in the last two decades with the development of advanced thin-¯lm deposition techniques that allow single-atomic-layer precision. Boosted by the progresses in molecular beam epitaxy (MBE),22 arti¯cial heterostructures sprang up in these years, which, guaranteed by the elaborate selection of the constituent materials, enable the creation of exotic phenomena at will. The interface superconductivity, as can be traced back to \surface superconductivity" proposed by Ginzburg in 1964,23 has exempli¯ed itself as an excellent illustration of this situation.24,25 Generally speaking, dimension reduction is often accompanied by the enhancement in thermal °uctuation, and disorders increase as well. This could lead to localization, and the concomitant quantum phase °uctuation is by no means helpful in the development of superconductivity. From this superconductivity had long been thought missing in low dimensional systems, that the ground state of two dimensional (2D) systems should be insulating. Despite these speci¯cations of the dimensional restrictions, however, superconductivity does occur in reduced dimensionality.26–30 Even at the 2D limit, robust superconductivity has been identi¯ed in one-monolayer (1ML) Pb and In ¯lms on Si(111) substrates.30 Accounting for the observed superconductivity, the electron–phonon coupling

(e–ph coupling) constant in the metal/semiconductor heterostructure was disclosed to be greatly enhanced compared with the of the much thicker ¯lms,30,31 apparently owing to the presence of the metal/Si interface. These works not only suggest a new route toward 2D superconductivity, but that the interface may play a role in mediating the dilemma, helping the fragile superconductivity survive the ingravescent dimension con¯nements. Interface-enhanced high temperature superconductivity in one-unit-cell FeSe ¯lms on SrTiO3(001) (1UC FeSe/STO) was discovered in 2012.32 Compared with its bulk counterpart, which is characterized by a Tc around 8 K,33 the monolayer ¯lm exhibits an unexpected high transition temperature of about 65 K or even 109 K.32,34 The importance is selfevident, since at present only the copper oxides are known to have a Tc above 77 K in the ambient conditions. These ¯ndings not only have attracted much attention to the interface superconductivity, but also have inspired a re-examination of the unconventional superconductors, with both the cuprate and the ironbased families included. Like the FeSe (or the FeAs) layers to the iron based superconductors,35,36 the cuprates all share the same CuO2 layer in common. The other constituents are generally sorted as the charge reservoirs supplying carriers into the superconducting CuO2 layers.37,38 In such sense the high-Tc complex is nothing more than an in¯nite repetition of the superconducting/non-superconducting interfaces, belonging to the broad interface superconductivity category as well. Provided with e±cient charge transfer, superconductivity should emerge even for a single isolated interface regardless of modi¯cations outside.39 The happy surprise is that this circumvents any possible interference inevitable in the bulk case, thus serving as the ideal platform investigating the pristine superconducting properties. For this purpose, lots of e®orts have been devoted to the interface issue between the CuO2 and charge reservoir layers.39,40 Commencing with the above introduction, in this paper we will focus on several examples to discuss the interface superconductivity. We ¯rst summarize the robust superconductivity in superconducting ¯lms at the two-dimensional limit (e.g. Pb/Si and Ga/GaN). Afterwards, we apply such interface thinking to the more complex unconventional superconductors with special emphasis on the FeSe/STO system. We also include in this review the more recent progresses in

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the direct probing of the CuO2 layers of the cuprates, which yielded results helpful in unraveling the high-Tc conundrum.


2. Robust Superconductivity in Ultrathin Films Being the earliest studied low-dimensional systems, it has long puzzled the community as how thin the metals can be before losing their superconductivity. As early as 1938, Shalnikov observed superconductivity in Pb and Sn ¯lms tens of nanometers thick.41 Afterwards, Buckel and Hilsch unveiled an Tc enhancement in thin metal ¯lms, like Al, and even in amorphous Bi ¯lms which were regarded non-superconducting.42 Albeit these ¯ndings stimulated many valuable investigations, the ¯lms under research had lots of defects and were far from the 2D limit. With the developments in the sample preparation technique, the pursuit toward the 2D ultimate resurged again in the 1990s. Among them, the Pb/Si(111) heterostructure served as a perfect paradigm elucidating the 2D superconducting state. Stimulated by the observation

of the oscillating Tc in ultrathin Pb ¯lms,27–29 1ML Pb ¯lms on Si(111) substrate were soon achieved using MBE, namely the striped incommensurate pffiffiffi phase of Pb on Si(111) (SIC-Pb/Si) and the 7 pffiffiffi 3-Pb/Si(111).30 Summarized in Fig. 1 are the structure and superconducting properties of the SICPb/Si phase. Detailed accounts of its structures can be found in Fig. 1(a). In-situ temperature dependent STS evolution indicated a particle-hole symmetric gap around the Fermi level (EF ) characterized by two sharp coherence peaks (0.42 K), which are gradually suppressed with increasing temperature (Fig. 1(b)). Fitting the data using the BCS gap function yields ð0Þ ¼ 0:35 meV and Tc ¼ 1:83 K (Fig. 1(c)). Furthermore, superconductivity in the SIC-Pb/Si phase is con¯rmed by the presence of superconducting vortices under magnetic ¯eld (Figs. 1(d)–1(f)). Similar behaviors took place for the 1ML In ¯lms on Si(111) substrates as well. ARPES measurements revealed that the e–ph coupling parameter for the monolayer ¯lms is greatly enhanced compared with that for the much thicker ones,30,31 suggesting a signi¯cant contribution to superconductivity of the phonon modes from interface bonding. In addition,

Fig. 1. (Color online) (a) Schematic structure model (upper panel) and atomic resolution image (bottom panel) of the SICPb phase. Image conditions are: V ¼ 0:1 V, I ¼ 0:05 nA. (b) Temperature dependent STS evolution measured with a Nb tip on (a). The tunneling junctions were set as V ¼ 0:01 mV, I ¼ 0:2 nA. Data have been vertically shifted for clarity. (c) The BCS ¯t (blue curve) of the order parameters. (d)–(f) Vortices at di®erent magnetic ¯elds (0.03T, 0.06T and 0.09T, respectively) measured at 0.42K using a PtIr tip. In each vortex image, the superconducting regions of low conductance are shown in maroon color and the gapless normal state inside the vortex cores are yellow colored. The tunneling junction was set at V ¼ 0:01 mV, I ¼ 0:1 nA. (From Ref. 30) 1841001-3


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. (b) Atomic Fig. 2. (a) Topographic image (V ¼ 3:0 V, I ¼ 0:05 nA, 1 1 m2) of 2 ML Ga ¯lms, with a step height of 2.5 A resolution STM image (V ¼ 0:22 V, I ¼ 0:05 nA, 8 8 nm2) of a Ga ¯lm, with the inset being its corresponding FFT pattern. The bright spots correspond to Ga atoms at the top layer. (c) A series of di®erential tunneling conductance spectra (V ¼ 10 mV, I ¼ 0:1 nA) at various temperatures, normalized to the normal conductance spectrum at 10 K. (d) Temperature-dependent superconducting gap magnitude (dark squares) and their best ¯t to BCS gap function (red curve) for 2 ML Ga ¯lms. (e) Three-dimensional plots of tunneling conductance measured at various magnetic ¯elds at 2.7 K. Spectra measured at 0, 1.2, and 5.0T are labeled by black dashes. (From Ref. 44)

this interface e–ph coupling scenario can ¯nd its accord in ¯rst principle calculations too,43 lending further support to this interpretation. More recent progresses introduced new member into the superconducting thin metal ¯lms family. Presented in Figs. 2(a) and 2(b) are the surface topography and the atom-resolved images of 2ML Ga on GaN(0001) substrates, respectively.44 Since no (meta)stable Ga phases ever exhibit a hexagonal lattice, the observed 2ML Ga ¯lms are most likely stabilized by the wurtzite structure of the underlying GaN(0001) substrate, linking to the pseudo 1 1 phase at room temperature.45,46 Veri¯ed by the temperature dependent spectra evolution (Fig. 2(c)), the Ga ¯lms possess a Tc of 5.2 K, much higher than the transition temperature of the bulk -Ga, which is 1.08 K.47 The BCS ¯tting, as enclosed in Fig. 2(d), gives ð0Þ ¼ 1:01 meV, Tc ¼ 5:2 K and a BCS ratio of 2=kB Tc ¼ 4:5, indicative of a strong coupling superconductor. Figure 2(e) illustrates the dI/dV spectra as a function of the applied magnetic ¯eld normal to the sample surface (B? ). With increasing B? , the zero bias conductance progressively increases while the two superconducting coherence peaks gradually

smear out, providing solid evidences for the occurrence of superconductivity in the 2ML Ga ¯lms. Likewise, the high Tc in Ga/GaN(0001) was con¯rmed by ex-situ systematic electrical magnetotransport and magnetization measurements.44,48 In addition, an anisotropy in critical magnetic ¯eld and the Berezinski–Kosterlitz–Thouless-like transition were observed, which spoke for the 2D superconductivity as well. This ¯vefold increase of Tc in 2ML Ga/GaN(0001) hybrid heterostructure may ¯nd its origin from two aspects. At ¯rst glance, one may expect the dimensionality e®ect to play the role. However, extensive experiments have revealed a strong suppression in Tc as a superconductor gets thinner, e.g. Pb49 and FeSe.50 It is therefore inappropriate to ascribe the observed high Tc of 5.4 K to the dimensionality e®ects, e.g. by reducing the Ga ¯lms to only 2ML. In contrast, the high Tc here is most likely linked to the interface e®ect between Ga and GaN.44 The nearly perfect lattice matching (0.319 nm for GaN and 0.318 nm for Ga), as revealed by the STM images, hints at a strong interface interaction, while on the other hand, the non-centrosymmetric crystal

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structure of GaN may result in polarization induced interface carrier accumulation, further boosting Tc .44,51,52 Such 2D electron gas (2DEG), prerequisite to the occurrence of superconductivity, is indeed observed in other wurtzite heterostructures such as AlGaN/GaN,53 and has been explained based on similar polarization e®ects as well.

3. Enhanced Superconductivity in 1UC FeSe/STO To further demonstrate the feasibility of controlling 2D superconductivity via interface engineering, additional heterostructures with enhanced superconductivity are particularly in need. We then move to the oxide interfaces, which have been the hot topics these days with several systems of interest including the LaAlO3/SrTiO3 interface.54,55 We found that the high Tc can also be achieved in 1UC FeSe ¯lms prepared on SrTiO3 substrates, perhaps the most celebrated one with drastically enhanced Tc .32,56,57 The reason for its long lasting fame lies not only in the

extraordinarily high Tc , but also its reference values for the cuprates. In a word, it combines the merits of both the iron pnictides and the cuprates. Depicted in Fig. 3(a) is the schematic structure of FeSe, which is the simplest among all the Fe-based superconductors.36,50 Note that the -phase FeSe is a bulk superconductor with Tc 8 K at ambient pressure.33 More importantly, when reduced to the 1UC limit, its electronic structure evolves into a very simple one with only electron pockets at the four M points.58–60 This resembles the one band cuprates, which have a simple Fermi surface structure as well.61 Enhanced HTC in 1UC FeSe/STO serves as the ideal platform for the investigation of the high-Tc superconductivity. Story began with the pursuit for higher quality samples. Despite of its rather simple structure, FeSe was proved di±cult to synthesize at ¯rst. In 2011, Song et al. resorted to MBE to grow high quality FeSe ¯lms on graphitized SiC(0001) substrates (Fig. 3(b)), and they determined a V-shaped superconducting gap ( 2:2 meV, Fig. 3(c)) with diminishing Tc down to 2 K for 2UC.50,62 Referring to the ultimate

Fig. 3. (a) The schematic structure of FeSe. (b) Atomic resolution image (V ¼ 10 mV, I ¼ 0:1 nA, 5 nm 5 nm) of FeSe on graphene. (c) STS (V ¼ 10 mV, I ¼ 0:1 nA) on FeSe/graphene at various temperatures. (d) STM topography (V ¼ 3:1 V, I ¼ 29 pA, 67 nm 67 nm) of the 1-UC-thick FeSe ¯lm on STO(001). Grain boundaries appear as trenches along the h100i or h010i direction. (e) Atomic resolution STM topography (V ¼ 0:6 V, I ¼ 51 pA, 12:8 nm 12:8 nm) showing the Se terminated FeSe(001) lattice. (f) STS of 1ML FeSe/STO (001) at 4.2 K. ((a) from Ref. 50, (b)–(c) from Ref. 62, and (d)–(f) from Ref. 32) 1841001-5


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2D case, however, the 1UC FeSe ¯lms seem nonsuperconductive, in line with the dimension predictions.50 As regards the interface, the graphitized SiC is an inert one that interacts weakly with the FeSe ¯lms, and therefore the FeSe ¯lms are close to the freestanding limit. Radical changes take place when substituting STO(001) for the graphitized SiC. Here it should be emphasized that the STO substrate has a speci¯c TiO2-terminated surface before the FeSe epitaxy. Exhibited in Figs. 3(d) and 3(e) are the typical topography and the atomic resolution image of the 1UC FeSe/STO, respectively, with its characteristic STS enclosed in Fig. 3(f). Characterized by an overall U-shaped two-gap structure, the larger superconducting gap ¼ 20:1 meV is almost an order of magnitude larger than ¼ 2:2 meV for bulk cases.32,33,62Ex-situ transport and magnetic measurements validated the high Tc of 1UC FeSe/STO.63 More recent insitu studies even revealed a Tc as high as 109 K,34 far beyond the record Tc 55 K of the iron pnictide family.64 Intriguingly, such interface enhancement abruptly ceases at the second layer that a semiconducting spectrum turns out.32,65 Along with the comparison to the FeSe/graphene case, the signi¯cance of the interface e®ects provided by the STO is self-evident. Then what happens at the interface favoring the superconductivity? With the extensive investigations in the past ¯ve years three di®erent viewpoints have been put forward in response to this question. In the following we will go through these candidates based on the experimental e®orts, and try to ¯gure out the predominant ones indispensable for the drastic enhancements. (1) Charge transfer. Compared with bulk FeSe single crystals, no hole pockets show up at the point of the Brillouin zone for the monolayer FeSe on STO.59,60 From this the charge transfer may make a di®erence. Actually, the in°uential role of interface charge transfer has been noticed in the very initial stage of the researches.32 Compared with its 2UC and bulk cousins, the hole pocket is absent only for the 1UC FeSe/STO,58–60 suggesting the possible connections between high Tc and doping. More recent e®orts managed to con¯rm this by tuning the nonsuperconductive multilayer FeSe ¯lms to a Tc around 40 K by doping.66–70 Except for these excellent

illustrations by ARPES and transport measurements, STM/STS also contributed a lot to this subject. Zhang et al. studied the dependence of the superconducting properties of 1UC FeSe/STO on annealing by in-situ STM/STS (Fig. 4).71 It is clear that the superconducting gap enlarges with increasing annealing temperature, hinting the modi¯ed charge transfer from STO to FeSe (Figs. 4(a) and 4(b)). Combined with the ARPES results, the electron concentration estimated from the Fermi-surface volume can reach a value as high as 0.12 (electrons per Fe) for 1UC FeSe ¯lms.60 Temperature dependent investigations yielded a Tc 68 K for the 1UC FeSe/ STO ¯lms (Figs. 4(c) and 4(d)), in line with the ARPES60,72,73 and transport reports74 as well as the recent quasiparticle ultrafast dynamic results.75 Equivalent to the changes from the underlying STO side, carrier concentration of the FeSe ¯lms can also be tuned in a well-controlled manner by surface K adsorption.66 Tang et al. studied such K coated FeSe mono/multilayer ¯lms on STO.76 The resulting STS reads that the second UC FeSe ¯lms transformed into the superconducting state after surface K doping, regardless of whether the ¯rst UC ¯lms superconduct or not. This holds for thicker ¯lms as well, which turned into superconductivity with an optimal K coverage around 0.2–0.25 ML. Being capable of tuning the superconducting properties e®ectively,70 the above results undisputedly point to the importance of interface charge transfer. Judging from these pronounced e®ects, it turns fascinating as whether carrier density modi¯cations can bridge the Tc gap between the bulk and monolayer FeSe. To address this, Song et al. resorted to the multilayer FeSe ¯lms atop the graphitized SiC (0001) substrates to trace the doping evolution of the superconductivity by surface K adsorption.77 Outlined in Figs. 5(a)–5(e) is the topographic evolution with increasing surface K dose, while the corresponding STS transition is enclosed right below (Figs. 5(f)– 5(j)). In line with the above statements, pristine FeSe multilayer ¯lms prior to K doping exhibit a superconducting gap 2.2 meV.50,62,77 This indeed represents the bulk case, because these FeSe ¯lms are close to the free standing limit with diminishing interface interactions. What surprises us is the evolution of the gap with doping, that it ¯rst undergoes a suppression ((f)–(h)), after which gradually resurges ((h)–(i)) and ¯nally evolves into the high Tc case (j) with the

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Fig. 4. (a) The dI/dV spectra of 1UC FeSe/STO taken after annealing at 450 C, 480 C, 500 C, 510 C and 530 C. The corresponding gap is 9.1, 11.8, 12.5, 13.2, 15.4 meV, respectively. (b) Dependence of superconducting gap with annealing temperature. (c) A series of normalized dI/dV spectra taken at various temperatures after annealing the 1-UC FeSe ¯lms at 530 C. (d) The temperature dependence of ZBC extracted from the dI/dV spectra in (c). The spectra in (a) and (c) are shifted along y-axis at a ¯xed value of 0.5 for clarity. (From Ref. 71)

characteristic two gap structure similar to the 1UC FeSe/STO case. Taken together, a two domed phase diagram as the one in Fig. 5(k) best describes the experimental observations. The two domed picture helps reconcile the inconsistency between multilayer FeSe ¯lms deposited on STO and graphene, since only the latter one turns out

to be superconductive. Provided by carrier injection from the STO substrates, it may be the case that only the ¯rst layer of FeSe ¯lms on STO is doped with su±cient electrons to enter the second high Tc dome while the thicker ones may have a carrier concentration at the saddle region between the two domes, thus non-superconducting at all. With surface alkali atoms

Fig. 5. (Color online) (a)–(e) Topographies [(a) 5 nm 5 nm. (b)–(e) 30 nm 30 nm] and (f)–(j) dI/dV spectra of multilayer FeSe ¯lms on graphitized SiC with varying doses of K, as indicated. Blue arrows denote the superconducting gap edges or coherence peaks. The absence of an apparent EF -symmetric gap in (h) shows the full suppression of superconductivity in FeSe by an intermediate dose of K. Tunneling conditions: (a) V ¼ 10 mV, I ¼ 100 pA; (b) V ¼ 4:0 V, I ¼ 20 pA; (c) V ¼ 4:0 V, I ¼ 10 pA; (d) V ¼ 1:0 V, I ¼ 10 pA; (e) V ¼ 3:0 V, I ¼ 20 pA; (f) V ¼ 10 mV, I ¼ 100 pA; (g)–(j) V ¼ 20 V, I ¼ 100 pA. (k) Electron doping level x dependent two-dome-shaped phase diagram of the electron-doped FeSe. (From Ref. 77) 1841001-7


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providing excess electrons, these multilayer FeSe ¯lms on STO can then travel into the second dome and achieve the high Tc therein. Therefore, this experiment clearly reveals two separate superconducting domes, in which the parent phase ceases with doping for the other high-Tc phase at higher doping levels. Meanwhile, this analysis excludes nematicity and spin °uctuations (present in the parent phase) as precursors for high Tc . Last but not least, we argue that the superconductivity in multilayer FeSe ¯lms on SrTiO3 is killed by weak electron doping from the substrate, contrary to superconductivity in single layer FeSe being enhanced by interfacial coupling to phonons of SrTiO3, as will be discussed as below. It is worthy pointing out that even though thick FeSe ¯lms do superconduct with K coating, they never reach the Tc record of their monolayer counterpart. As a matter of fact, the enhancements attenuate in moving away from the interface, that the maximum superconducting gap sizes by K adsorption monotonously decrease with increasing ¯lm thickness.78 Together with the fact that only the ¯rst layer FeSe ¯lms superconduct for the undoped FeSe/STO interface,32 the above results undisputedly point to the occurrence of other interface e®ects beside the charge transfer. (2) Electron phonon (e-ph) coupling. This mechanism was proposed in the very ¯rst report.32 The Ushaped superconducting gap spontaneously calls for a more conventional pairing mechanism, so the e–ph coupling, serving as the predominant pairing glue in conventional superconductors,9 turns to be the promising candidate. In this way, one expects an enhanced e–ph coupling with the additional phonon modes provided by the STO substrates to contribute to the strengthening of superconductivity in 1UC FeSe/STO. Indeed, interface-enhanced e–ph coupling has been documented by both extensive experiments (e.g. ARPES,73 STS,78 ultrafast optical spectroscopy,75 and high-resolution electron energy loss spectroscopy (HREELS)79 and theoretical calculations.80–84 To be speci¯c, Lee et al. performed an ARPES study and discovered replica bands o®set by 100 meV for each of the primary bands of 1UC FeSe/STO, and this has been ascribed to bosonic shake-o®.73 Subsequent calculations pointed to the optical O phonon band and the surface Ti–O phonon modes of STO as the

probable candidate.81,85 Utilizing the ultrafast optical spectroscopy technique, the superconducting characteristics for the 1UC FeSe/STO have been determined by Tian et al. as Tc ¼ 68ð5= þ 2)K, ¼ 20:2 1:5 meV and the e–ph coupling constant ¼ 0:48.75 It should be emphasized that this experimentally determined e–ph coupling constant of 0.5 is about three times larger than the value of bulk FeSe,86 strongly sustaining the interfacial e–ph coupling perspective. Taken into account the experimentally identi¯ed phonon energy of 100 meV,73,79 e–ph coupling constant of 0.5,73,75 chemical potential of 60 meV59,72,73 and neglects the Coulomb repulsion due to the huge dielectric constant of STO, an e–ph coupling driven high Tc up to 77 K for the 1UC FeSe/ STO can be obtained by theoretical calculations. 82 These results demonstrate the crucial role of e–ph coupling in the high temperature superconductivity of the 1 UC FeSe/STO system. Referring to this, tunneling experiment best exempli¯es its capability in the direct probe of the e–ph coupling. In binding the electrons into the Cooper pairs, the bosonic mediator leaves its traces in the concomitant DOS reconstructions, which is in turn re°ected in the dI/dV (or the d 2 I=dV 2 ) spectra as the characteristic features at energies ( þ ), where is the superconducting gap energy and stands for the energy of the bosonic mode. This applies to both the conventional87 and unconventional superconductors, with the dip-hump feature in the latter family been controversially interpreted as ¯ngerprints of phonon or spin °uctuations.88,89 To be speci¯c, Song et al. reported the STM observation of a bosonic mode outside the superconducting gap of the FeSe ¯lms grown on graphene substrates, with its coupling to the FeSe electrons con¯rmed as well.90 Tang et al. extended this scheme to the FeSe ¯lms on the STO substrates. Displayed in Figs. 6(a) and 6(b) are the typical spectra for the 1UC FeSe ¯lms and K coated 3UC FeSe ¯lms on STO, respectively. After taking the second derivative of the normalized STS, clear dip-hump features representing the bosonic modes (10.7 meV and 19.8 meV for 1UC FeSe ¯lms, and 10.0 meV and 22.3 meV for the K coated 3UC FeSe ¯lms) are readily identi¯ed in the bottom panels.78 By comparison with the previous neutron scattering and Raman scattering measurements, these bosonic modes ¯nd their correspondence to the

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Fig. 6. (Color online) (a)–(b) Black curves show raw dI/dV (top panel, V ¼ 50 mV, I ¼ 100 pA), normalized dI/dV (middle panel), and d 2 I/dV 2 (bottom panel) spectra on bare 1-UC ¯lms and 3-UC ¯lms at the K coverage of 0.20ML, respectively. The normalization was performed by dividing the raw dI/dV spectrum by its background, which was extracted from a cubic ¯t to the conductance for jV j > 20 mV (the dashed line in the top panel). The pink, blue, and red dashes show the approximate energy positions of , ð þ 1 ) and ð þ 2 ), respectively. (c) Normalized dI/dV spectra (V ¼ 50 mV, I ¼ 100 pA) at temperatures ranging from 4.6 to 30.2 K taken on 2-UC FeSe ¯lms at the K coverage of 0.20ML. The horizontal bars indicate zero conductance position of each curve. The pink dashes and the red dashes are parallel and show the synchronous change in coherence peak and feature of phonon 2 . The spectra in (a)–(c) are smoothed by Gaussian ¯ltering. (From Ref. 78)

FeSe Eg (Se) phonons at frequency of 12–13.1 meV, A1g (Se) phonons at frequency 19.8 meV, and the STO TiO2 phonon at frequency of 21.7 meV.85,91,92 Moreover, theses bosonic features and the superconducting coherent peaks degrade synchronously toward elevating temperatures (Fig. 6(c)), suggesting an intimate underlying relation. This can be generalized to basically the whole FeSe family, that the presence of the same phonon modes is established in FeSe ¯lms with varying thicknesses and K coverages. According to these experiments, in spite of the signi¯cant variations in the superconducting gap sizes ranging from 6.5 meV to 19 meV, the energy distribution of the phonons uniformly collapses into two distinct groups centered at 11.2 meV and 21.5 meV. Judging from the omnipresence of the same phonon modes, the e–ph coupling should play a vital role in the superconducting properties of the FeSe related materials.

In view of the above analysis, the interface high-Tc of the 1UC FeSe ¯lms should be the result of combined charge transfer and the e–ph coupling.57,93 Roughly speaking, the carrier injection may take the responsibility for the ¯rst stage enhancement characterized by a preliminary enlargement of the gap size from about 2.2 meV to 10–14 meV. For the sake of additional boost, however, e–ph coupling is indispensable. Since the e–ph coupling is strongest right at the interface and attenuates in moving away, this explains the relatively lower Tc for the K induced superconductivity in thick FeSe ¯lms, and also accounts for the decreasing Tc maximum with increasing ¯lm thickness. In addition, suppression of superconductivity in monolayer FeSe ¯lms by surface K adsorbates is interpreted likewise, that the doped electrons from the opposite side may result in a counteraction of the interface electric dipoles, which in turn weakens the screening e®ect and thus the Cooper pairing.

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(3) Strain e®ects. Having established the predominant roles of the charge transfer and e–ph coupling, it remains inquisitive if other mechanisms contribute as well. Here the one regarding to the lattice mismatch mentioned above seems most promising.72,94 Judging from the lattice mismatch between the FeSe (0.3765 nm) and the STO (0.3905 nm), the FeSe should experience a tensile strain at the interface. This may enhance the antiferromagnetic interactions. On the other hand, however, carrier injections will destroy such correlations and turn it into the spin °uctuation high-Tc . So the net e®ect is a strengthened spin °uctuation. Where it responsible for the Cooper pairing at the interface, the 1UC FeSe ¯lms may enjoy a higher Tc with increasing in-plane lattice constant.72,94 Based on this image, various substrates have been adopted including STO(110),95,96 anatase TiO2 (001),97 rutile TiO2(001),98 MgO69 and even BaTiO3.99 Here it should be pointed out that choosing anatase TiO2(001) for the FeSe growth helps clarify the essential role of the substrates in realizing the high-Tc .97 As

sketched in Fig. 7(a), anatase TiO2 is characterized by a distinct O–Ti–O triple-layered structure in sharp contrast to the single-TiO2-terminated SrTiO3. This leads to a divergence between their physical properties, such as lattice constants, phonon modes, and dielectric constants. Speci¯cally, the in-plane lattice constant (0.3782 nm) of anatase TiO2(001), as compared with STO(0.3905 nm), is much closer to that of FeSe (0.3765 nm). As thus, the anatase TiO2(001) serves as an advisory system to distinguish whether or not the above-mentioned °uctuation mechanism bears a primary responsibility for the enhanced high-Tc superconductivity in FeSe-related heterostructures. Figures 7(b)–7(e) describe the surface morphology of the FeSe/TiO2(001) heterostructure, with the atomic resolution image of the 4 1 reconstructed anatase TiO2 (001) surface and the epitaxied FeSe monolayer ¯lms presented in Figs. 7(c) and 7(e), respectively. We can see that the FeSe lattice constant shrinks from 0.3905 nm to 0.38 nm in changing the substrate from STO (001) to TiO2(001), indicating a much relaxed interface for FeSe/TiO2. Nevertheless,

Fig. 7. (a) Schematic of the FeSe/TiO2(001) heterostructure. The surface reconstructions of SrTiO3(001) and anatase TiO2(001) are not shown for clarity. For the typical TiO2(001) thick ¯lms studied, its in-plane lattice constant has been well relaxed to 0.380 from 0.3905 nm. (b) STM topography (500 nm 500 nm, V ¼ 3:0 V, I ¼ 0:03 nA) showing an anatase TiO2(001) island with a thickness of 15 nm supported by a SrTiO3(001) substrate. (c) Enlarged STM topography (70 nm 70 nm, V ¼ 1:5 V, I ¼ 0:03 nA) acquired on a TiO2 island, showing clear 4 1 reconstruction plus oxygen vacancies (bright spots). (d) STM topography (100 nm 100 nm, V ¼ 3:0 V, I ¼ 0:03 nA) showing coexisting single UC (SUC) and double UC (DUC) FeSe ¯lms on anatase TiO2(001). (e) Atomic resolution STM topography (20 nm 20 nm, V ¼ 50 mV, I ¼ 0:1 nA) of 1UC FeSe ¯lm. (f) Low energy dI/dV spectra (V ¼ 50 mV, I ¼ 0:4 nA) taken on various 1UC FeSe/TiO2(001) ¯lms revealing the superconducting gap. (From Ref. 97)

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the STS shows no big di®erence. From the various STS on the FeSe ¯lms in Fig. 7(f), a two gap structure shows up. Compared with the case of 1UC FeSe/ STO, the two gap sizes show only slight di®erence with 1 ¼ 17 meV and 2 ¼ 8:5 meV. STS with larger gap sizes (1 ¼ 21 meV, 2 ¼ 11 meV) can sometimes be identi¯ed as well. Given the signi¯cant relaxation of the interface strain, and thus the concomitant variation in the °uctuation amplitude, the superconducting gap sizes should not be so close. By this token the spin °uctuation scenario is subordinate to the above two in enhancing Tc with only minor, if any, e®ects. Indeed, Tc of the FeSe monolayer ¯lms do not vary much for all the above-mentioned substrates, further con¯rming this statement. By the way, the charge transfer at the FeSe/TiO2 interface has been clari¯ed as well. It has been long believed that the oxygen vacancies in the STO substrates may act as the electron donors for the top FeSe ¯lms.95,96,100 Yet this has not been veri¯ed. By controlling the oxygen vacancies on the anatase TiO2, Ding et al. concluded that the oxygen vacancies have only very limited e®ects on the superconductivity, thus excludes them as the primary source of the

charge transfer from the substrates to the FeSe ¯lms.97 More recently, the work function of FeSe was determined to be larger than that of SrTiO3.101 Upon contact, interface band alignment would lead to an electron transfer from TiO2 to FeSe, consistent with our observation. In light of the two major enhancing factors, pairing in the ¯lms should be conventional. This is what has been observed by means of quasiparticle interference (QPI).102 By the fast Fourier transform of the dI/dV mappings at speci¯c energies, Fan et al. found that the QPI images are capable of deconvolving the band dispersions through the Bogoliubov quasiparticle scatterings within/between di®erent electron pockets. Illustrated in Figs. 8(a) to 8(f) are the QPI images of the 1UC FeSe ¯lms on STO. Ring like scattering structures and Bragg spots of the top Se lattice and underlying Fe lattice are readily observable. Through comparison with the schematic Fermi surface exhibited in Fig. 8(g), we can identify the di®erent rings as resulting from the intra pocket scattering (q1 ), nearest neighbor inter pocket scattering (q2 ) and the second nearest neighbor scattering (q3 ). Figure 8(h) demonstrates the numerical

Fig. 8. (Color online) (a)–(f) Fast Fourier transformation (FFT) of real-space dI/dV maps taken at various biases as labelled. The FFTs are four-fold symmetrized and shown on a logarithmic scale. The positions of scattering vectors (q1 , q2 , q3 ) are marked in (a) Note that q2 and q3 coincide with the Bragg spots of the top Se lattice and underlying Fe lattice, respectively. The yellow arrow in (a) indicates the spots of the 2 1 reconstruction. (g) Schematic of the Brillouin zone (BZ) and Fermi surface of 1UC FeSe/SrTiO3(001). The black solid square is the unfolded BZ with a full width of 2/ aFeFe . The dashed square is the folded BZ. The blue solid (dashed) ellipse is the electron (folded) pocket at the M points. Possible scattering vectors (q1 , q2 , q3 ) are marked by red arrows. (h) Simulated FFT image according to the electronic structure in the unfolded zone, at energies outside of the superconducting gap. A band ellipticity of 0.9 is used. (i) Integrated intensity (normalized at 30 meV) of the three sets of scattering rings, as functions of the energy. (From Ref. 102) 1841001-11


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simulation of the QPI process based on the Fermi topography in Fig. 8(g), which is in excellent accordance with the experimental results. To gather information on the pairing symmetry, detailed intensity analysis of the QPI has been conducted. Due to the coherence factors in the scattering matrix element, nonmagnetic scattering potentials will enhance the sign changing scatterings while suppress those which preserve the sign of k .103–105 In the context of d-wave paring, to be speci¯c, only q2 represents such sign changing scattering and thus should be enhanced. This is not what happened, as Fig. 8(i) shows that the intensity of the three rings share the same energy dependence. It thus follows that q2 has nothing special, and should be the sign preserving scatterings just as q1 and q3 . In this way, the pairing in the monolayer FeSe ¯lms should not be d-wave. Succeeding experiments on the surface impurity e®ects con¯rm that the superconductivity is immune to non-magnetic Zn, Ag and K impurities but is strongly suppressed by magnetic Cr and Mn impurities. This further rule out the sþ scenario and again speaks for the plain s-wave perspective.102

4. Interface Superconductivity in Cuprates The excellent illustration of the interface aspects of the FeSe/STO system necessitates a retrospect of the other high-Tc family. As mentioned, the cuprates are composed of essentially the same stacking of the superconducting/non-superconducting blocks. From this perspective, the interfaces may take charge of the exceptionally high Tc of cuprates as well. Ideally, the measurements should be conducted directly on the superconducting side of the interface, otherwise it is like resorting to the STO substrates for the superconducting FeSe monolayer ¯lms. However, only few works managed to do so, and the results are rather counterintuitive. In a representative example, Misra et al. found that the accidentally acquired CuO2 layer has a U-shaped STS devoid of near EF excitations, inconsistent with a d-wave pairing.40 Albeit the inclusion of a ¯ltering factor may partially explain the observations, it implies that CuO2 cannot be fully represented by the charge reservoirs. In order to achieve a reliable preparation of the CuO2 layer, the argon-ion bombardment and

annealing (IBA) was carried out onto optimally doped Bi2Sr2CaCu2O8+ (Bi2212) single crystals.106,107 The results in Fig. 9(a) demonstrate that almost all the constituent layers can be acquired including the two CuO2 layers buried far beneath the exfoliated BiO surface. Figure 9(c) indicates that the STS on the two CuO2 layers exhibits a two-gap structure, di®erent from the pseudogap appearance of the BiO layers. Temperature-dependent results read that only the inner gap is related with superconductivity, with the outer one merely the pseudogap. What's more, the SrO (I) layer, which belongs to the charge reservoirs as the BiO layers, is not insulating but metallic with a van Hove singularity at the Fermi energy.107 These ¯ndings touch on a fundamental question regarding to the cuprate, namely, the relationship between the pseudogap and the superconductivity. With respect to the two main interpretations, that one regards the pseudogap as the precursor of superconductivity while the other views them as competing orders,12,105 our results strongly disavow the former. Keeping in mind that the one band superconductivity is characterized by a single gap, the di®erent temperature dependences of the two gaps107,108 immediately rule out possible connections between them. In view of the metallic SrO(I) layer, the well-de¯ned pseudogap observed on the exposed BiO layers should not stem from the underlying CuO2 layer. This is further testi¯ed by succeeding experiments, that the pseudogap is still observable even on epitaxial BiOx islands up to 4 nm thick. Given the extreme sensitivity of STM on the tip-sample separation, this, and together with the previous observations of similar pseudogap dispersions on other strongly correlated non-superconducting materials (like La1:2 Sr1:8 Mn2O7109 and Sr2IrO4110–112), lead to the conclusion that the pseudogap may be inherent to some oxides including the BiOx here. The above ¯ndings shake the d-wave pairing recognition either. Previous ARPES results consented to a d-wave Fermi surface gap below the pseudogap onset temperature T ,61,113 and this has been corroborated by the QPI results from STM.114–116 However, all of them were conducted on the charge reservoir layers, where the pseudogap happens.106,107 It remains elusive whether the observed d-wave formalism re°exes the superconducting gap or the pseudogap. Albeit direct experiments on CuO2 were realized via IBA, we still su®er from the pseudogap

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Fig. 9. (Color online) STM topographic image of the as-sputtered and annealed Bi-2212 cuprate, showing various terminating planes (V ¼ 1:5 V, I ¼ 30 pA). (b) Crystallographic structure of Bi-2212, with repeated inverted oxide layers along the c-axis direction. (c) Topographies (upper panel) and electronic spectra (bottom panel) on various planes of BiO(I), SrO, CuO2(I), CuO2(II), and BiO(II) in Bi-2212, respectively. White squares mark the respective in-plane unit cells of every exposed planes, with a periodicity of 3.8 A. Black and blue arrows indicate the PG and superconducting gap in di®erent energy scales, respectively. The VHS on the SrO plane lies near EF. (From Ref. 107)

interference manifested as the larger outer gap. In addition, the CuO2 surface obtained via IBA is too small to systematically investigate its superconducting property. For it, the CuO2 ¯lms have been constructed from the bottom up. Figures 10(a) and 10(b) depict the typical topography and the atomic resolution image

of 1ML CuO2 ¯lms on optimal doped Bi2212 single crystals, respectively. The high quality of the ¯lm is self-evident. By means of STS, we found the surface is characterized by two kinds of low energy excitations: the U-shaped one (upper one in Fig. 10(c)) and the V-shaped one (bottom one in Fig. 10(c)). In the intermediate regions, a two-gapped structure turns

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Fig. 10. (Color online) (a)–(b) STM topography (V ¼ 1:5 V, I ¼ 20 pA, 40 nm 40 nm) and atomic resolution image (V ¼ 0:5 V, I ¼ 15 pA, 10 nm 10 nm) of monolayer CuO2 ¯lms on the Bi2212 substrate. (c) dI/dV spectra of the CuO2 ¯lms, revealing two distinct energy scale gaps (s and p ) in the low-energy QP excitations. The U-shaped spectra exhibit vanishing and particle-hole-symmetric DOS over a ¯nite energy range near EF. (d) Dependence of s (blue squares) and p (black circles) on p;BiO . The red diagonal dashes show the curve for p ¼ p;BiO . The error bars refer to the standard deviation of the statistical gap values. (e)–(f) Temperature dependence of the superconducting gap s and PG p . Distinct from a gradual DOS ¯lling for p , the DOS near EF for s shows a more dramatic increase with temperature. (V ¼ 0:1 V, I ¼ 100 pA). (From Ref. 117)

out with the two sets of peaks referring to the two energy scales, indicative of a smooth electronic transition.117 So what is the physics behind these two gaps? To address this, we have collected more than three thousands spectra from the ¯lms, and have summarized into Fig. 10(d) their statistics together with those of the pseudogap from the exposed Bi2212 substrates. Clearly, the V-shaped gap sizes (black circles) are linear with the pseudogap sizes of Bi2212. Moreover, the dispersions of the gap sizes are also similar, indicated by the two error bars superimposed on the data points. This means the V-shaped gap is actually related to the pseudogap from the substrates. The U-shaped ones, on the other hand, are more uniform and seem saturated toward under doping, in line with previous reports on the superconducting gap.118 Temperature-dependent experiments con¯rmed the superconductivity conjectures. Figure 10(e) shows that the U gap completely vanishes around Tc . The V gap, on the other hand, survives the highest available temperature, further validating its pseudogap identity.117 At this point, the violation of the d-wave symmetry is evident. From the one-band model of the strongly correlated cuprates, the nodal lines should intercept the Fermi surface at the four equivalent points of (=2, =2), leading to the occurrence of gap

nodes in ARPES spectra and the V-shaped line-shape of the STS.105 This contradicts the U-shaped appearance of the superconducting gap which favors a nodeless pairing superconductivity. More evidences are available in support of a nodeless s-wave superconductivity. In actual, the superconductivity is immune to non-magnetic K, Cs and Ag surface adsorbates, and neither the pure d-wave nor a ¯ltered d-wave gap function yield a satisfying ¯t to the U-gapped STS.117 The modulation-doping scenario is proposed on account of these ¯ndings. Famous in semiconducting physics,119 charge transfer may take place in response to the band bending at the interface of two materials, such as the Alx Ga1x As/GaAs interface. We believe the same happens at the CuO2/Bi2212 interface only that holes rather than electrons are transferred. This simple rigid band model also shed some new lights on the complex phase diagram of the cuprates. The emergence of superconductivity at the critical doping corresponds to the situation where su±cient holes are transferred into the CuO2 layers to form 2D hole liquid (2DHL). With further doping, EF of the charge reservoirs progressively decreases, so more holes are °ooded into the CuO2 layers to enjoy the Tc increase. Things continue until optimal doping is arrived, that the quantum well at the interface is saturated with holes. This is why Tc reaches its maximum, and from

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now on further doping the charge reservoirs will give rise to the formation of parallel conductive channels that the quantum well supporting the 2DHL starts to collapse. Owing to this, the e®ective doping level of the CuO2 layer and thus Tc decrease along the overdoped side and ¯nally, superconductivity disappears as EF touches the charge transfer band (CTB). Within this modulation doping model, the above results and the complicated phase diagram of cuprate high-Tc superconductors can be understood. Further con¯rmation of this model might open new avenue for building up new high-Tc materials.

understanding of their properties di®erent from the bulk. Furthermore, STM has been enjoying the great success in the ¯eld of condensed matter physics, becoming one of the most versatile and powerful tools in the interface superconductivity and the high temperature superconductivity causes as of today.

Acknowledgments We acknowledge all the collaborators. This work was ¯nancially supported by Natural Science Foundation, Ministry of Science and Technology and Ministry of Education of China.

5. Summary and Perspective The foregoing sections o®er a brief review over the STM perspective of the interface superconductivity. The interface could provide additional support to the fragile Cooper pairing in the 2D ultimate, and meanwhile opens up new avenue elucidating the convoluted unconventional phenomena in the high Tc iron pnictide and cuprates. Moreover, the successful identi¯cation of superconductivity in the 2D limit, as for the Pb/Si, In/Si and Ga/GaN cases, along with the drastic Tc promotion in the FeSe/STO system, guarantee practical applications in the future. Provided with incoming advances, even higher Tc may be achieved. In addition, the interface contributes to the clari¯cation of the long standing high Tc puzzle as well. Realization of the high Tc in monolayer FeSe ¯lms not only demonstrates the applicability of the BCS paradigm toward the complex iron pnictides and chalcogenides, but shatters the Macmillan limit imposed upon phonon coupled electron pairs. In light of the same build-in multi-interface structure of cuprates, we postulate that the same interface charge transfer plus e–ph coupling processes would take place as such. Especially, the discovery of the nodeless pairing at the CuO2/Bi-2212 interface is unprecedented and implies that the pairing might be mediated by an attractive interaction in k space, probably associated with e–ph interaction as in conventional superconductors. From this perspective, exploring the isotope e®ect on Tc will help unravel the pairing mechanism in the future. In a word, STM has been proved ideal to study the nanoscale materials and have in turn deepened our

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