Surface-induced orbital-selective band reconstruction in kagome superconductor CsV3Sb5

Linwei Huai(淮琳崴) Yang Luo(羅洋) Samuel M.L.Teicher Brenden R.Ortiz Kaize Wang(王鎧澤)Shuting Peng(彭舒婷) Zhiyuan Wei(魏志遠(yuǎn)) Jianchang Shen(沈建昌) Bingqian Wang(王冰倩) Yu Miao(繆宇)Xiupeng Sun(孫秀鵬) Zhipeng Ou(歐志鵬) Stephen D.Wilson and Junfeng He(何俊峰)

1CAS Key Laboratory of Strongly-coupled Quantum Matter Physics and Department of Physics,University of Science and Technology of China,Hefei 230026,China

2Materials Department and California Nanosystems Institute,University of California Santa Barbara,Santa Barbara,California 93106,USA

Keywords: photoemission,kagome superconductor,band structure

1. Introduction

Due to the exotic lattice geometry, kagome materials provide a novel platform to explore various intriguing quantum phenomena.[1–7]Recently, a new family of kagome metalsAV3Sb5(A= K, Rb and Cs) has been discovered.[7–10]Superconductivity,[7–9,11–15]charge density wave,[7,8,16–21]pair density wave[21]and non-trivial topology[7,22,23]have all been reported in this family of materials. Distinct electronic structures have also been revealed in both bulk and surface states of the materials,[23–33]which successfully accommodate various novel phenomena in this material system. An exciting next step is to manipulate the associated electronic states,which would not only help understand the origin of mysterious physical interactions but also open a new window for exploring emergent quantum phenomena.

In this paper, we unveil the electronic band structure of CsV3Sb5via systematic ARPES measurements and report the discovery of a distinct momentum-dependent band evolution as a function of time. A significant energy shift of the electron-like band aroundΓis observed while only a moderate energy shift of the hole-like band aroundMis revealed. This phenomenon is reproduced in a much shorter time scale byin-situannealing the CsV3Sb5sample. Orbitalresolved density functional theory (DFT) calculations show different orbital characters for the bands aroundΓandMrespectively, which indicates an orbital-selective band reconstruction.Through careful analysis,we attribute this time-and momentum-dependent band reconstruction to the formation of Cs vacancies on the sample surface. Our observation of the surface-induced orbital-selective band reconstruction uncovers a promising method to manipulate the properties of the surface and realize possible orbital-selective control of the electronic structure.

2. Experimental details and results

Single crystals of CsV3Sb5were grown by the self-flux method.[7,10]The samples were cleavedin-situwith a base pressure of better than 8×10-11torr.The photoemission measurements were carried out at our lab-based ARPES system with 21.2 eV photons. The energy resolution was～5 meV for the measurements. The Fermi level was referenced to that of a polycrystalline Au in electrical contact with the samples.DFT calculations were performed without spin–orbit-coupling in VASP v5.4.4 using identical parameters to a recent study.[7]

CsV3Sb5is a new kagome superconductor characterized by a layered crystal structure withP6/mmm(No. 191) space group (Fig. 1(a)). The V atoms form a perfect kagome sublattice in this material. This kagome net is interwoven with Sb atoms, which form a hexagonal net in the same plane(Figs.1(b)and 1(c)). This V3Sb layer is then bounded above and below by Sb honeycomb lattices and Cs hexagonal lattices (Fig. 1(a)). The corresponding three-dimensional (3D)Brillouin zone (BZ) is presented in Fig. 1(d). As shown in Figs.1(e)and 1(f),the measured Fermi surface map and band structure alongΓ–Mdirection are consistent with theoretical calculations.[7]For simplicity,we label the electron-like band aroundΓpoint asαband, and the hole-like band aroundMpoint asβband,hereafter(Fig.1(f)).

Systematic measurements have been performed as a function of time at a fixed temperature (115 K). We note that the measurement temperature is much higher than the transition temperature of any electronic order in this system. By tracking the energy position of the band bottom(top)via the EDC peak atΓ(M)(Fig.2),the systematic time evolution of theα(β) band is shown in Fig. 2(c) (Fig. 2(f)). It is clear that theαband moves significantly towards a lower binding energy,while theβband shows a moderate energy change as a function of time,giving rise to an apparent momentum-dependent band evolution with time.

Fig. 1. Crystal structure and band structure of CsV3Sb5. (a) The 3D lattice structure of CsV3Sb5 with space group P6/mmm. (b) Top view of the crystal structure. (c)The kagome sublattice formed by V atoms. (d)Schematic of the 3D BZ.(e)Fermi surface measured at 200 K.(f)Band structure along Γ–M direction measured at 120 K.The measurements were performed on fresh sample surfaces.

Fig.2. The evolution of band structure as a function of time. (a)Photoelectron intensity plot of the α band near Γ. (b)EDC at Γ with its energy range marked by the black dashed line in (a). (c) Time evolution of the EDC in (b). The intensity of the EDC peak is shown by the color scale.(d)–(f)Same as(a)–(c),but for the β band near M.

To better understand the band evolution,we have further performed measurements before and after anin-situvacuum annealing process(Fig.3). Figure 3(a)shows the band structure alongΓ–Mdirection measured before and after a twohour-annealing at 200°C respectively. These two measurements have been carried out with the same condition and at the same temperature(200 K).By comparing the energy position ofα(β)band bottom(top)via the EDC peak before and after the annealing process,a momentum-dependent energy shift of the bands is observed,mimicking that observed in the previous time-dependent experiment. We note that the same amount of energy shift is now realized in a much shorter time scale with the annealing process(Fig.3(b)). The significant energy shift of theαband can also be resolved from the constant energy map at various binding energies. As the binding energy becomes larger, the constant energy contour gradually shrinks from a circular shape to a point. The location of this vanishing point is at a smaller binding energy in the measurement after annealing, comparing to that before the annealing process (Fig. 3(c)). The vacuum annealing not only reproduces the momentum-dependent energy shift of the bands, but also accelerates this band evolution.

3. Discussion

We next consider the origins of the time-and momentumdependent band reconstruction. First, a time-dependent band evolution is presumably associated with changes on the sample surface.[15,33]This is confirmed by the experimental fact that the original band structure (without any energy shift of the bands) appears again when a fresh surface is produced by re-cleaving the measured samples. However,the observed momentum-dependent band evolution is not due to a trivial aging effect. Sample aging would typically broaden the measured spectrum but should not give rise to an energy shift of the bands. Carrier doping due to a potential absorption of atoms or molecules on the sample surface may induce a rigid band shift, but it cannot explain the different evolution betweenαband andβband observed in our experiment. To deepen the understanding of the momentum-dependent band evolution,orbital-resolved DFT calculations have been carried out. The calculated band structure(Fig.4(a))matches well with our experiment results. As shown in Figs.4(c)and 4(d),theαband is dominated by Sb pz-orbital,whereas theβband mainly contains V d-orbital. The pz-orbital is presumably more sensitive to the out-of-plane interaction. As such, theαband shows a more evident response to the surface changes, which is consistent with our experiment results. Combining the experiments and theoretical calculations, a surface-induced orbitalselective band reconstruction is presented in this compound.

The last thing to discuss is the origin of the sample surface change. We attribute it to the formation of Cs vacancies on the surface. First,due to the chemical activity of Cs atoms and the weak interlayer interaction, the Cs atoms can slowly desorb from the topmost layers of the cleaved surface, producing Cs vacancies on the surface. The formation of alkalimetal-atom vacancies on the cleaved sample surface has been observed by STM measurements.[33]Theoretically, the carriers from the Cs atoms hybridize with both Sb pz-orbital electrons and V d-orbital electrons,but the hybridization strength is different.In the calculations,the appearance of Cs vacancies induces a significant energy shift of theαband and a moderate energy shift of theβband.[15,34]The same calculations[15]also reveal significant changes of the bands nearEFaround theMpoint, which are also consistent with the experimental observations in Fig. 3(a)). If the above scenario is at work,and the number of desorbed Cs atoms on the sample surface is increasing with time,then the observed time-and momentumdependent energy shift of the bands can be well explained.This is also aligned with the fact that high temperature annealing can accelerate the desorption process of Cs atoms on the sample surface,which in turn produces the same momentumdependent band evolution in a much shorter time. Nevertheless, the high temperature annealing process might be more complicated,since the Cs vacancies may also exist in the bulk sample,at least in a few layers below the surface.In this sense,a direct hole doping effect may appear on top of the surfaceinduced orbital-selective band reconstruction.It would also be interesting to explore how the physical properties are affected by the Cs vacancies in the material. It has been reported that the loss of Cs atoms in CsV3Sb5thin films can enhance the superconducting transition temperature and suppress the charge density wave.[15]

Fig.4. Orbital-resolved DFT band structure and schematic diagram of the Cs desorption on the sample surface. (a)Calculated band structure along the Γ–M direction. (b)Schematic diagram of the Cs desorption on the sample surface. (c)and(d)Orbital-resolved calculations for α band and β band,respectively.

4. Conclusions

In summary, utilizing high-resolution ARPES measurements, we have identified a surface induced orbital-selective energy shift of the bands in CsV3Sb5. A momentumdependent band evolution is observed as a function of time in the normal state of CsV3Sb5. This process is reproduced in a much shorter time scale by annealing the sample in vacuum.Combining the experimental results with orbital-resolved DFT calculations, we illustrate that the observed band evolution represents an orbital-selective energy shift of the bands,which is likely due to the desorption of Cs atoms on the sample surface. These results may pave a special way to manipulate the electronic structure of kagome superconductors via the sample surface. Further studies are stimulated to explore various ways beyond the vacuum annealing to modify the surface of kagome superconductors.

Acknowledgments

We thank useful discussions with X. H. Chen, T. Wu,Z. Y. Wang, J. J. Ying, Z. J. Xiang and K. Jiang. The work at University of Science and Technology of China(USTC) was supported by the Fundamental Research Funds for the Central Universities(Grant Nos.WK3510000008 and WK3510000012) and USTC start-up fund. Work at UC Santa Barbara was supported by the UC Santa Barbara NSF Quantum Foundry funded via the Q-AMASE-i program under award DMR-1906325. This research made use of the shared facilities of the NSF Materials Research Science and Engineering Center at UC Santa Barbara (DMR-1720256). B. R.O. acknowledges support from the California NanoSystems Institute through the Elings Fellowship program. S.M.L.T has been supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1650114.