# Possible spin excitation structure in monolayer FeSe grown on SrTiO

###### Abstract

Based on recent high-resolution angle-resolved photoemission spectroscopy measurement in monolayer FeSe grown on SrTiO, we constructed a tight-binding model and proposed a superconducting (SC) pairing function which can well fit the observed band structure and SC gap anisotropy. Then we investigated the spin excitation spectra in order to determine the possible sign structure of the SC order parameter. We found that a resonance-like spin excitation may occur if the SC order parameter changes sign along the Fermi surfaces. However, this resonance is located at different locations in momentum space compared to other FeSe-based superconductors, suggesting that the Fermi surface shape and pairing symmetry in monolayer FeSe grown on SrTiO may be different from other FeSe-based superconductors.

###### pacs:

74.70.Xa, 74.78.-w, 74.20.-z, 75.40.Gb## I introduction

In most iron pnictide superconductors, there exist hole and electron pockets in the Brillouin zone (BZ) center and corner, respectively and it is commonly accepted that the spin fluctuation induced by the nesting between the hole and electron pockets leads to the unconventional superconductivity and high transition temperature T. In this case, the superconducting (SC) pairing symmetry is -wave, where the sign of the SC order parameter on the hole pockets is opposite to that on the electron ones.mazin At early stages it was believed that, if either the hole or electron pocket disappears, then the nesting condition between the hole and electron pockets will be broken and superconductivity will be suppressed, as in the hole-overdoped BaKFeAs ding1 and electron-overdoped BaFeCoAs.Takahashi In this regard, the recent findings of high-T superconductivity in a family of FeSe-based iron pnictide superconductors, such as LiFeOHFeSe,chenxh ; Johrendt ; zhaozx Li(NH)(NH)FeSe clarke and AFeSe (A=Rb, Cs, K),chenxl ; chenxh2 ; conder as well as monolayer FeSe grown on SrTiO,xueqk1 are quite surprising and have attracted much attention among the community, since in these materials, there are no hole pockets, but only electron ones in the BZ.zhouxj1 ; zhouxj2 ; fengdl1 ; fengdl2 ; shenzx1 ; zhouxj3 ; fengdl4 ; ding ; zhouxj4 ; fengdl5 ; fengdl6

At present, the SC pairing symmetry in the FeSe-based superconductors is hotly debated. Theoretical investigations suggest that, in the absence of the hole pockets, the nesting is now between the electron ones and in this case, a nodeless -wave pairing symmetry is the leading candidate,aoki ; scalapino ; leedh which, when folded into the 2Fe/cell BZ, will lead to gap nodes or extreme minima in the vicinity of the BZ boundaries.mazin2 Experimentally, most angle-resolved photoemission spectroscopy (ARPES) measurements found that the SC gap magnitude is nearly isotropic along the Fermi surfaces, with no apparent nodes or extreme minima.zhouxj1 ; shenzx1 ; zhouxj3 ; fengdl4 ; zhouxj4 ; fengdl5 ; fengdl6 Inelastic neutron scattering (INS) experiments observed a spin resonance in LiFeODFeSe,Boothroyd ; zhaoj1 Li(ND)(ND)FeSe Boothroyd1 and AFeSe (A=Rb, Cs, K),Inosov1 ; Inosov2 ; Boothroyd2 ; Inosov3 ; zhaoj which was interpreted as the signature of the sign change of the SC order parameter. However, scanning tunneling microscopy (STM) experiments performed in LiFeOHFeSe fengdl7 and monolayer FeSe grown on SrTiO fengdl3 ruled out any sign change of the SC order parameter along the Fermi surfaces.

Among the above mentioned FeSe-based superconductors, monolayer FeSe grown on SrTiO is of particular interest. T in this material is above 50K,xueqk1 and can even reach up to 100K, jiajf the highest among all iron pnictide superconductors. It has the simplest structure where the Fe atoms form a strictly two-dimensional (2D) lattice, with the Se atoms sitting below and above the Fe plane. In contrast to its simple structure, the SC mechanism is complicated and the high T may be resulted from a combination of the electron-electron interaction within the monolayer FeSe and the electron-phonon interaction between FeSe and the SrTiO substrate,shenzx1 ; leedh2 where the interface effect may play a vital role in enhancing T. Recently its Fermi surfaces have been precisely mapped out by high-resolution ARPES and clear gap anisotropy has been observed, putting strong constraint on the possible pairing symmetry.shenzx2 Furthermore, since ARPES measures only the magnitude of the SC gap, but not its phase, a phase-sensitive measurement is thus needed to determine the exact pairing symmetry. Therefore in this work, we investigate the spin excitation spectra in monolayer FeSe grown on SrTiO, which can be measured by INS, to determine the possible sign structure of the SC order parameter.

## Ii method

We adopt a 2D tight-binding model proposed in Ref. gaoy_fese, , where each unit cell contains two inequivalent sublattices and . The coordinate of the sublattice in the unit cell is while that for the sublattice is , with being . The Hamiltonian can be written as , where

(1) |

Here creates a spin up electron with momentum and on the orbital of the sublattice . , , , and . In addition, and are the tight-binding and pairing parts of the system, respectively. Throughout this work, the momentum is defined in the 2Fe/cell BZ and the energies are in units of 0.1eV. In the following we set and to fit the band structure measured by ARPES. Under this set of parameters, the average electron number is (the system is about electron doped). Fig. 1 shows the calculated band structure and Fermi surfaces. The hole pockets sink below the Fermi level while two electron pockets and exist around with their sizes similar to the ARPES data (). A slight ellipticity in the electron pockets and is induced by setting . Both the electron number and the Fermi surface topology are consistent with ARPES.zhouxj1 ; shenzx2 The reason we fit our tight-binding model to ARPES instead of the LDA band structure is that the bottom of the electron band in the LDA-calculated band structure is 500meV below the Fermi level, almost an order of magnitude deeper than the ARPES results.sadovskii Furthermore, if the correlation effect can account for the shallow electron band, a hole pocket will show up around .sadovskii2 In contrast, in our model, the top of the hole band and the bottom of the electron band are both located 100meV below the Fermi level, agreeing with ARPES much better than the LDA-calculated band structure.

The band structure as well as the pairing function in the band basis can be obtained through a unitary transform as

(2) | |||||

(3) |

Here are the energies of the two hole bands which sink below the Fermi level and are those of the two electron bands (). The diagonal components in represent the pairing function on each band while the off-diagonal components signify the inter-band pairing. Since we are interested in the spin excitation spectra, we calculate the multiorbital dynamical spin susceptibility as kuroki

(4) |

with and the experimentally measured spin susceptibility is

Within the random-phase approximation (RPA) and writing the spin susceptibility in a matrix form, we have

with the bare spin susceptibility being

at . Here , , , and is the usual coherence factor of the spin susceptibility. is the th diagonal element of and . In deriving Eq. (II) we have neglected the off-diagonal components of and assumed that the orbital-band matrix element for the anomalous Green’s functions is the same as that for the normal Green’s functions, as done in Ref. scalapino, . We have verified that the above assumptions do not alter the spin susceptibility qualitatively, which is also found in Refs. scalapino, and siqm, . The nonzero elements of the interaction matrix are for , for , for and for . In addition we have taken and . From Eq. (II) we have, for and ,

(8) |

and is related to by the Kramers-Kronig (KK) relations. In fully gapped superconductors, for a fixed , due to the function in Eq. (8), is zero when , where is a threshold energy determined by the minimal value of . Meanwhile, if , then at , there will exist a step discontinuity in which will lead to a logarithmic divergence in . In this case, will diverge at (for arbitrary ), where and this signifies the formation of a spin resonance [at , and , leading to in Eq. (II)]. On the contrary, if , then the step discontinuity and the logarithmic divergence will disappear in and , respectively, leading to the disappearance of any spin resonance for .

For small but finite , the step discontinuity in will become step-like, but nonvanishing and continuous across , therefore the logarithmic divergence in will acquire a finite height. In this case, can only be met with a sufficiently large and a peak with finite height instead of a divergence will show up in , which also signifies a spin resonance.

In monolayer FeSe grown on SrTiO, there is no signature of static magnetic order when T is the highest, therefore for arbitrary in the normal state, we should have , which determines the upper bound of to be in our model [see Fig. 2(a)]. On the other hand, since it may be close to a magnetic instability,zhouxj2 ; fengdl1 ; gong therefore we set in the following. Here we need to point out that the value of in the present work (0.6eV) seems to be smaller than those adopted in Refs. aoki, and scalapino, (1eV). The reason is that, our band structure is a fit to the ARPES-measured one, which can be viewed as the LDA band structure renormalized by the correlation effect. We notice that the band width in Refs. aoki, and scalapino, is 4eV, while in our model it is 1.5eV, correspondingly a smaller in our model can lead to the magnetic instability.

## Iii results and discussion

Having elucidated the origin of the spin resonance in the spin excitation spectrum, we then apply the above argument to our model of monolayer FeSe. In our previous work gaoy_fese we have proposed a pairing function that can well reproduce the anisotropy of the SC gap observed by ARPES,shenzx2 which can be expressed as

(9) |

where and . However in Ref. gaoy_fese, we did not consider the ellipticity of the electron pockets. Here we show, even in the presence of a slight ellipticity, the SC gap derived from our pairing function still agrees quite well with the experiment. As we can see from Fig. 2(b), the gap magnitude is relatively larger on than that on , while on both and , the gap maxima (minima) are located along the () line. The gap anisotropy derived here is consistent with both the ARPES data shenzx2 and our previous result.gaoy_fese In the inset of Fig. 2(b) we show the density of states (DOS) in the SC state, where two pairs of the SC coherence peaks appear at (0.14, red arrows) and (0.17, blue arrows), respectively. This two gap structure is also consistent with STM experiments.xueqk1 ; fengdl3 ; xueqk4 ; xueqk5 Further inspection of the pairing function shows that, on the two electron bands, the SC order parameters are negative () while on the hole bands, there exist sign changes in and (see Fig. 3). The pairing symmetry is generally -wave and since the hole bands do not form Fermi surfaces, thus we denote this pairing symmetry as a hidden sign-changing -wave symmetry. In Ref. gaoy_fese, it was shown that this pairing symmetry can be distinguished from the conventional -wave one by investigating the impurity-induced in-gap bound states. In this work we intend to study the possible spin excitation structure that may be observed by INS.

In the following we consider three -wave pairing cases. The pairing function of the first one is given by Eq. (9) (case \@slowromancapi@). For the second case, we artificially set and to be zero, i.e., we assume that the SC pairing does not take place in the two hole bands (case \@slowromancapii@) and this is the conclusion drawn in Ref. bang2, . For the last case, in addition to setting and to be zero, we further set to be . In this way, there will exist a sign change of the SC order parameter between and (case \@slowromancapiii@), as proposed in Ref. chubukov, [see Fig. 4(g) of Ref. chubukov, ]. Considering the SC order parameters along the Fermi surfaces, in all the three cases, their magnitudes are consistent with ARPES and are the same as that shown in Fig. 2(b) while their signs can be seen in Fig. 4. Here we do not consider the nodeless -wave pairing as proposed in Ref. scalapino, , for two reasons. The first one is, as can be seen in Fig. 4 of Ref. scalapino, , along one pocket, the gap magnitude of the nodeless -wave pairing changes from the maxima to the minima as varies from to , which is not consistent with the ARPES data since the gap magnitude should be both the local maxima at and [see Fig. 4(c) in Ref. shenzx2, ]. The inconsistency of the nodeless -wave pairing can also be seen from Fig. 4(d) of Ref. shenzx2, . Secondly, the nodeless -wave pairing state requires that along and (in the 2Fe/cell BZ), should be zero. However in our proposed gap function, this cannot be satisfied as can be seen from Fig. 3. Therefore, we cannot assume any pairing function whose gap magnitude is consistent with ARPES while respects the nodeless -wave pairing symmetry.

In calculating , we set and . In order to see the superconductivity-induced intensity change, we subtract in the SC state by its normal state value. We find that the intensity change is most prominent along the red arrow in the inset of Fig. 4(a) and the results are shown in Fig. 5. As we can see, for cases \@slowromancapi@ and \@slowromancapii@, generally decreases once entering the SC state while for case \@slowromancapiii@, there exists a clear increase of intensity close to [the position of is shown in the inset of Fig. 4(a)] and around . To see it more clearly, we plot in Fig. 6 the spin susceptibility as a function of , at . We find that the spin susceptibilities in the normal state, as well as in cases \@slowromancapi@ and \@slowromancapii@, are mostly featureless. In contrast, for case \@slowromancapiii@, there exists a step-like jump in around , which leads to a peak in the maximal eigenvalue of at the same . The origin of these is because, at , the threshold energy due to the function in Eq. (8) is approximately determined by , where and are momenta on and , respectively, as denoted by the gray arrow in Fig. 4(b). In both cases \@slowromancapi@ and \@slowromancapii@, the coherence factor associated with this process, is zero since and . On the contrary, in case \@slowromancapiii@, since , the above mentioned coherence factor reaches its maximal value two. Therefore in cases \@slowromancapi@ and \@slowromancapii@, the step-like jump in , as well as the peak in the maximal eigenvalue of , are suppressed while they are enhanced in case \@slowromancapiii@. Careful inspection of shows that, in cases \@slowromancapi@ and \@slowromancapii@, decreases once entering the SC state and no resonance-like structure exists at . In comparison, for case \@slowromancapiii@, a broad hump centered around shows up, whose intensity exceeds its normal-state counterpart while at , is suppressed in the SC state compared to that in the normal state. Since the hump structure is located at , at a first glance, it seems that a spin resonance exists in case \@slowromancapiii@. However as we can see, the hump around in already exists in , whose maximum is located at while the profiles of and are similar, except for an increased intensity in . The hump structure in therefore does not signify a spin resonance since at , is not close to zero. Furthermore, since the largest maximal eigenvalue of is at , the condition cannot be met for any . Therefore the collective spin excitation at and is highly damped. In INS experiments, when entering the SC state, an intensity increase at is commonly attributed to the formation of spin resonances, where usually is the STM-measured SC gap. Here we show, since the SC order parameters are anisotropic on the Fermi surfaces, can be nonzero at while it will be zero if the SC gaps are isotropic. Consequently, at , there may exist an increase of when entering the SC state as long as is increased compared to its normal-state value and this intensity increase does not signify a spin resonance. As shown for case \@slowromancapiii@, the hump structure in is very robust since it already appears in . Even if the exact value of is unknown, for , the hump structure will always appear at . Therefore, cases \@slowromancapi@ and \@slowromancapii@ can be distinguished from case \@slowromancapiii@ by their different behaviors at .

In order to investigate whether cases \@slowromancapi@ and \@slowromancapii@ can be distinguished from each other by INS, we study the spin susceptibilities at [the position of can be seen in the inset of Fig. 4(a)]. As we can see from the inset of Fig. 7, this wave vector connects the momenta on the electron pockets with those on the hole bands which sink below the Fermi level. In the usual iron pnictide superconductors, INS experiments commonly found a spin resonance at this wave vector, which supports the sign-changing characteristics of the SC order parameter between the electron and hole pockets, therefore naively we expect the same phenomenon may occur in case \@slowromancapi@, but not case \@slowromancapii@. Numerical results in Fig. 7 shows that, already in the normal state, there exist two step-like jumps in and two peaks in the maximal eigenvalue of , which are related by the KK relations. Below the first step, is zero since the hole bands sink below the Fermi level and the threshold energy associated with the scattering process in the inset of Fig. 7 must overcome the energy gap between the top of the hole band (band 2) and the Fermi level. Similarly, the second step jump in is resulted from the scattering between the electron pockets and another hole band (band 1) which is even deeper below the Fermi level. Similar structures exist in cases \@slowromancapi@ and \@slowromancapii@ as well, which seems to be insensitive to the relative sign of the SC order parameters connected by . The reason is that, suppose is on the electron pockets and is on the hole bands, then in the coherence factor we have and , leading to . Meanwhile since the hole bands are located at meV below the Fermi level, we further have . Therefore in this case is in the vicinity of one, irrespective of the sign of and the step jumps in , as well as the peaks in the maximal eigenvalue of , will also exist in cases \@slowromancapi@ and \@slowromancapii@. Similar behaviors can also be found in , where a peak shows up in the normal state, as well as in cases \@slowromancapi@ and \@slowromancapii@. Here the peaks are truly the signatures of spin resonances since the conditions and can be simultaneously met. For as we considered, the crossings between the gray dotted line and the curves of the maximal eigenvalue of in the middle panel of Fig. 7 determine the location of the peaks in . As decreases, the gray dotted line will shift up and the peaks in will move to higher energy. Finally if is infinitesimally small, will be the same as . Therefore, at , in contrast to our naive expectation, the spin susceptibilities in cases \@slowromancapi@ and \@slowromancapii@ are similar and cannot be distinguished from each other.

Finally, we comment on the spin susceptibilities at , whose location is shown in the inset of Fig. 4. A spin resonance at this wave vector has been theoretically predicted scalapino and experimentally observed in LiFeODFeSe,Boothroyd ; zhaoj1 Li(ND)(ND)FeSe Boothroyd1 and AFeSe (A=Rb, Cs, K).Inosov1 ; Inosov2 ; Boothroyd2 ; Inosov3 ; zhaoj Beside the coherence factor, the existence of this spin resonance relies closely on the square or rectangular shape of the Fermi surfaces. As shown in Fig. 4(b) of Ref. Inosov2, and Fig. 7 of Ref. Boothroyd, , the scatterings between the flat parts of the Fermi surfaces lead to the nesting wave vector . However, as we can see from Fig. 7 of Ref. Boothroyd, , the Fermi surfaces are perpendicular to the line, i.e., if viewing in the 2Fe/cell BZ, and are both perpendicular to the line, as shown by the blue dotted lines in Fig. 1(b). Apparently this is not the case in monolayer FeSe grown on SrTiO since high-resolution ARPES found elliptical Fermi surfaces with no flat parts. shenzx2 Indeed, the calculated spin susceptibilities at are negligibly small compared to those at (not shown here), suggesting that there will not exist spin resonances at in monolayer FeSe grown on SrTiO. In contrast, the spin excitation should be peaked at , which connects the parts of the electron pockets close to the BZ boundaries, similar to that shown in Fig. 1(a) of Ref. leedh2, . In addition we infer, even if the pairing symmetry is a nodeless -wave, as schematically shown in Fig. 4(h) of Ref. chubukov, , should still be peaked around , similar to case \@slowromancapiii@.

## Iv summary

In summary, we have investigated possible spin excitation spectra in monolayer FeSe grown on SrTiO, based on a model fitting the ARPES-measured band structure and gap anisotropy. We found that, if the SC order parameter changes sign between the inner and outer pockets, a resonance-like spin excitation may occur, which is highly damped. On the contrary, if the SC order parameter maintains its sign along the Fermi surfaces, no such spin excitation can occur. Furthermore, since the hole bands sink 100meV below the Fermi level, INS experiments are unable to tell whether or not Cooper pairs form on the hole bands, let alone the sign of the SC order parameter. Finally we propose that the observed spin resonance at in other FeSe-based superconductors may not exist in monolayer FeSe grown on SrTiO, since the Fermi surfaces in this material are elliptical, rather than square or rectangular-like. Possibly the SC pairing symmetry in monolayer FeSe grown on SrTiO may also be different from other FeSe-based superconductors, since at least in LiFeOHFeSe, the maxima and minima of the gap are located at exactly the opposite locations compared to monolayer FeSe grown on SrTiO (see Fig. 7 of Ref. wenhh, ). Beside INS, the spin excitation spectra proposed in this work may also be measured by STM in real space as proposed in a recent work.Wahl

## Acknowlegements

This work is supported by the Natural Science Foundation from Jiangsu Province of China (Grants No. BK20160094 and No. BK20141441), the NSFC (Grant No. 11374005) and NSF of Shanghai (Grant No. 13ZR1415400). QHW is supported by NSFC (under grant No.11574134).

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