A Mott insulator continuously connected to iron pnictide superconductors

Transcription

1 Reeived 1 Jul 216 Aepted 8 Nov 216 Pulished 19 De 216 DOI: 1.138/nomms13879 OPEN A Mott insultor ontinuously onneted to iron pnitide superondutors Yu Song 1, Zhr Ymni 2, Chongde Co 1,3,YuLi 1, Chenglin Zhng 1, Justin S. Chen 1, Qingzhen Hung 4, Hui Wu 4,5, Jing To 6, Yimei Zhu 6, Wei Tin 7, Songxue Chi 7, Huio Co 7, Yo-Bo Hung 8,9, Mrus Dntz 8, Thorsten Shmitt 8,RongYu 1,11,12, Andriy H. Nevidomskyy 1, Emili Morosn 1, Qimio Si 1 & Pengheng Di 1 Iron-sed superondutivity develops ner n ntiferromgneti order nd out of d-metl norml stte, whih hs een interpreted s originting from proximte Mott trnsition. Whether n tul Mott insultor n e relized in the phse digrm of the iron pnitides remins n open question. Here we use trnsport, trnsmission eletron mirosopy, X-ry sorption spetrosopy, resonnt inelsti X-ry sttering nd neutron sttering to demonstrte tht NFe 1 x Cu x As ner xe.5 exhiits rel spe Fe nd Cu ordering, nd re ntiferromgneti insultors with the insulting ehviour persisting ove the Néel temperture, inditive of Mott insultor. On deresing x from.5, the ntiferromgneti-ordered moment ontinuously dereses, yielding to superondutivity Bx ¼ 5. Our disovery of Mott-insulting stte in NFe 1 x Cu x As thus mkes it the only known Fe-sed mteril, in whih superondutivity n e smoothly onneted to the Mott-insulting stte, highlighting the importnt role of eletron orreltions in the high-t superondutivity. 1 Deprtment of Physis nd Astronomy nd Rie Center for Quntum Mterils, Rie University, Houston, Texs 775, USA. 2 Cndin Neutron Bem Centre, Chlk River Lortories, Chlk River, Ontrio KJ 1J, Cnd. 3 Deprtment of Applied Physis, Northwestern Polytehnil University, Xin 7172, Chin. 4 NIST Center for Neutron Reserh, Ntionl Institute of Stndrds nd Tehnology, Githersurg, Mrylnd 2899, USA. 5 Deprtment of Mterils Siene nd Engineering, University of Mrylnd, College Prk, Mrylnd , USA. 6 Deprtment of Condensed Mtter Physis nd Mterils Siene, Brookhven Ntionl Ntionl Lortory, Upton, New York 11973, USA. 7 Quntum Condensed Mtter Division, Ok Ridge Ntionl Lortory, Ok Ridge, Tennessee 37831, USA. 8 Pul Sherrer Institute, Swiss Light Soure, CH-5232 Villigen PSI, Switzerlnd. 9 Beijing Ntionl Lortory for Condensed Mtter Physis, nd Institute of Physis, Chinese Ademy of Sienes, Beijing 119, Chin. 1 Deprtment of Physis nd Beijing Key Lortory of Opto-eletroni Funtionl Mterils & Miro-nno Devies, Renmin University of Chin, Beijing 1872, Chin. 11 Deprtment of Physis nd Astronomy, Shnghi Jio Tong University, Shnghi 224, Chin. 12 Collortive Innovtion Center of Advned Mirostrutures, Nnjing 2193, Chin. Correspondene nd requests for mterils should e ddressed to Q.S. (emil: qmsi@rie.edu) or to P.D. (emil: pdi@rie.edu). NATURE COMMUNICATIONS 7:13879 DOI: 1.138/nomms

2 NATURE COMMUNICATIONS DOI: 1.138/nomms13879 At the hert of understnding the physis of the iron-sed superondutors is the interply of superondutivity, mgnetism nd d-metl ehviour 1 6. A key question is whether superondutivity emerges due to strong eletroni orreltions 6 11 or nested Fermi surfes Superondutivity ours in the viinity of ntiferromgneti (AF) order, oth in the iron pnitides nd iron hlogenides 2 5. In ddition, the norml stte hs very lrge room temperture resistivity, whih rehes the Ioffe Mott Regel limit 2,4,5. This d-metl ehviour hs een ttriuted to the proximity to Mott trnsition 7 11, with the Coulom repulsion of the multi-oritl 3d eletrons of the Fe ions eing lose to the threshold for eletroni loliztion. In the iron hlogenide fmily, severl ompounds hve een found to e AF nd insulting with hrteristi fetures of Mott insultor There is lso evidene for n oritl-seletive Mott phse in A x Fe 2 y Se 2 (A ¼ K, R) 19. However, in these iron hlogenide mterils, one nnot ontinuously tune the AF Mott-insulting stte into superondutor. On the other hnd, the iron pnitide NFe 1 x Cu x As is plusile ndidte for Mott insultor sed on trnsport nd snning tunnelling mirosopy (STM) mesurements ner x ¼.3 (refs 2,21); however, the insulting ehviour my lso e indued y Anderson loliztion vi Cudisorder. The prent ompounds of iron pnitide superondutors suh s AFe 2 As 2 (A ¼ B, Sr, C) nd NFeAs hve rystl strutures shown in Fig. 1,, respetively 2. They exhiit tetrgonl-to-orthorhomi struturl phse trnsition t temperture T s, followed y prmgneti to AF phse trnsition t T N (T s ZT N ) with olliner mgneti struture, where the spins re ligned ntiferromgnetilly long the xis of the orthorhomi lttie (Fig. 1) 22,23. When Fe in AFe 2 As 2 is repled y Cu to form ACu 2 As 2 (ref. 24), the Cu toms hve nonmgneti 3d 1 eletroni onfigurtion with Cu 1 þ nd filled d shell due to the presene of ovlent ond etween the As toms within the sme unit ell [As] 3 [As As] 4 /2 shown s the shded As As ond in Fig , s predited y nd struture lultions 29. Sine the rystl struture of NFe 1 x Cu x As does not llow similr ovlent ond (Fig. 1) 2, it would e interesting to explore the mgneti stte of NFe 1 x Cu x As in the hevily Cu-doped regime. From trnsport mesurements on single rystls of NFe 1 x Cu x As with xr.3, it ws found tht the in-plne resistivity of the system eomes insulting-like for xz.11 (ref. 2). STM showed tht lol density of sttes for NFe 1 x Cu x As with x ¼.3 resemles eletron-doped Mott insultors 21, lthough it is unler wht the undoped Mott-insulting stte is or whether it exhiits AF order. Using trnsport, neutron sttering, trnsmission eletron mirosopy (TEM), X-ry sorption spetrosopy (XAS) nd resonnt inelsti X-ry sttering (RIXS), we demonstrte tht hevily Cu-doped NFe 1 x Cu x As exhiits Fe nd Cu ordering, nd eomes n AF insultor when x pprohes.5. The insulting ehviour persists into the prmgneti stte, providing strong evidene for Mott-insulting stte. This onlusion is orroorted y our theoretil lultions sed on the U(1) slve-spin pproh 3. Our lultions demonstrte enhned orreltions nd Mott loliztion from the omined effet of ndwidth redution, whih results from Cu-site lokge of the kineti motion of the Fe 3d eletrons, nd hole doping from 3d 6 to 3d 5, oth of whih re mde possile y the fully oupied Cu 3d shell 7,9 11,3. On deresing x from xe.5, the orreltion lengths of Fe nd Cu ordering, nd mgneti ordering, s well s the ordered mgneti moment ontinuously derese, onneting smoothly with the superonduting phse in NFe 1 x Cu x As ppering ner x ¼ 5, highlighting the role of eletroni orreltions in iron pnitides. Results Resistivity mesurements. Single rystls of NFe 1 x Cu x As were prepred using the self-flux method. Trnsport mesurements were rried out using ommeril physil property mesurement system with the stndrd four-proe method (the Methods setion). Figure 1e shows temperture dependene of the in-plne resistivity for the x ¼ 16 smple. In ddition to superondutivity t T ¼ 11 K, the norml stte resistivity r is B.4 mo m t room temperture, onsistent with previous work 2. Figure 1f plots the temperture dependene of the resistivity on log sle for smples with x ¼.18,.38,.44 nd.48. Resistivity in the x ¼.18 smple exhiits insulting-like ehviour onsistent with erlier work 2. For the xz.39 smples, resistivity further inreses y n order of mgnitude ompred to the x ¼.18 smple, signifying tht eletrons eome muh more lolized in NFe 1 x Cu x As, when x pprohes.5. Neutron sttering results on NFe.56 Cu.44 As. Unpolrized nd polrized elsti neutron sttering were rried out on NFe 1 x Cu x As single-rystl smples (the Methods setion). For these mesurements, we use the orthorhomi unit ell nottion suitle for the AF-ordered stte of NFeAs (refs 22,23), nd define momentum trnsfer Q in threedimensionl reiprol spe in Å 1 s Q ¼ H* þ K* þ L*, where H, K nd L re Miller indies nd * ¼ ^2p/, * ¼ ^2p/ nd * ¼^2p/ (Fig. 1). Single rystls re ligned in the (H,,L) nd (H, H, L) sttering plnes in these mesurements. In the (H,, L) sttering geometry, the olliner mgneti struture in NFeAs (Fig. 1) gives mgneti Brgg peks elow T N t Q AF ¼ (H,,L), where H ¼ 1, 3,? nd L ¼.5, 1.5,?, positions 22,23. Figure 2 shows typil elsti sns for NFe.56 Cu.44 As long the [H,,.5] diretion t T ¼ 3.6 nd 3 K. Beuse of twinning in the orthorhomi stte, this is equivlent to elsti sns long the [, K,.5] diretion. While the sttering hs ler pek entred round Q ¼ (1,,.5)/(, 1,.5) t room temperture, the sttering is enhned mrkedly on ooling to 3.6 K, suggesting the presene of stti AF order. The temperture differene plot etween 3.6 nd 3 K in Fig. 2d indites tht the low-temperture intensity gin is essentilly instrumentl resolution limited (horizontl r). Figure 2e shows the temperture differene long the [1,, L]/ [, 1, L] diretion tht pprently is not resolution limited. The temperture dependene of the sttering t (1,,.5)/ (, 1,.5) plotted in Fig. 2f revels ler evidene of mgneti ordering elow T N E2 K, suggesting tht the smll pek oserved t room temperture in Fig. 2 ours in the prmgneti phse nd is thus of struturl (super-lttie) origin indued y Cu sustitution, sine suh pek is foridden for NFeAs. Fe nd Cu ordering in NFe 1 x Cu x with xe.5. To onlusively determine the origin of super-lttie peks nd the rystl struture of NFe.56 Cu.44 As, we hve rried out highresolution TEM mesurements. The inset in Fig. 2 shows n imge of the smple, nd the eletron diffrtion ptterns were olleted from res B1 nm in dimeter within single domin of the smple (see the rrow in the inset of Fig. 2). A typil diffrtion pttern long the [1] zone xis t room temperture is shown in Fig. 2. While we see ler super-lttie refletions t H ¼ 1, 3,? positions long the [H,, ] diretion, 2 NATURE COMMUNICATIONS 7:13879 DOI: 1.138/nomms

3 NATURE COMMUNICATIONS DOI: 1.138/nomms13879 ARTICLE BCu 2 As 2 NFe 1 x Cu x As NFeAs d NFe.5 Cu.5 As N B Fe/Cu Cu As ~2.5 Å ~6.9 Å T T ~5. Å ρ (mω m).5 e x = Temperture (K) f T N = 2 K Temperture (K) x =.18 x =.39 x =.48 1, 1 1 ρ (mω m) Ordered moment (μ B ) Fe 3d 5 Mott insultor 1..5 g Single rystl Powder NFeAs moment 5 T Fe 3d 6 d metl NFe 1 x Cu x As SC Figure 1 Summry of trnsport nd neutron sttering results. () The rystl struture of ACu 2 As 2 in the tetrgonl stte, where T is the in-plne lttie prmeter. The As As ovlent onding distne is B2.5 Å. () The rystl struture of NFe 1 x Cu x As, where similr As As ovlent onding is not possile. () The olliner mgneti struture of NFeAs, where the AF order nd moment diretion re long the orthorhomi xis 22,23. Only Fe toms re plotted in the figure for lrity. (d) Rel spe struture nd spin rrngements of NFe.5 Cu.5 As in the AF orthorhomi unit ell similr to NFeAs. In NFe.5 Cu.5 As, sine Fe nd Cu form stripes, N nd As lso shift slightly from their high-symmetry positions. (e) In-plne resistivity for x ¼ 16 smple, where ulk superondutivity ours elow T ¼ 11 K (ref. 2). (f) Temperture dependene of the in-plne resistivity for x ¼.18,.39,.44 nd.48 smples. The vertil rrow mrks the position of T N for the x ¼.44 smple determined from neutron sttering. (g) Evolution of ordered moment with doping in NFe 1 x Cu x As. The two ordered phses re seprted y the superonduting dome mrked s SC, with no mgneti order for x ¼ 16 ner optiml superondutivity. The right nd left insets show the in-plne mgneti strutures of NFeAs nd the new AF insulting phse, respetively. For xr5, the phse digrm from ref. 2 is plotted. Vertil error rs re from lest-squre fits (1 s.d.). they re sent t the K ¼ 1, 3,? positions long the [, K, ] diretion (Fig. 2). This mens tht the rystl struture of NFe.56 Cu.44 As is orthorhomi nd oeys two-fold rottionl symmetry. From this informtion nd from intensities of the super-lttie peks in (H,, L) sttering plne, we onlude tht Fe nd Cu toms in NFe.5 Cu.5 As (whih is pproximted y NFe.56 Cu.44 As) form rel spe stripe-like-ordered struture in Fig. 1d, in the spe group Im (Supplementry Note 1; Supplementry Tle 1). This onlusion is further orroorted y single-rystl neutron diffrtion mesurements (Supplementry Tle 2). The Fe Cu ordering is struturl nlogue of the mgneti order in NFeAs nd struturl superlttie peks our t the sme positions s mgneti peks in NFeAs. Fe nd Cu ordering in NFe 1 x Cu x As ffets the intensities of Brgg peks lredy present in NFeAs very little (Supplementry Note 1; Supplementry Fig. 1), nd the indued NATURE COMMUNICATIONS 7:13879 DOI: 1.138/nomms

4 NATURE COMMUNICATIONS DOI: 1.138/nomms13879 K (r.l.u.) Intensity profile (.u.) [H,,] [,K,] H (r.l.u.) [H,,] / [K,,] (r.l.u.) Intensity (ounts per 1 s) K 3 K d K 1 5 Intensity (ounts per 1 s) [H,,.5] / [,K,.5] (r.l.u.) [H,,.5] / [,K,.5] (r.l.u.) Intensity (ounts per 1 s) e K f Q = (1,,.5) / (,1,.5) T N Intensity (ounts per 1 s) [1,,L] / [,1,L] (r.l.u.) Temperture (K) Intensity (ounts/15min) 3, 2, 1, g L h (1,,.5) H/K (1,1,) K/H 2 K SF 24 K SF 2 K NSF 24 K NSF [H,,.5] / [,K,.5] (r.l.u.) [H,H,] (r.l.u.) Figure 2 TEM nd neutron sttering results on the struture nd mgneti order for NFe.56 Cu.44 As. () Using 2 kev inident eletrons, n eletron diffrtion pttern in the [H, K] plne ws otined from NFe.56 Cu.44 As prtile with its TEM rel spe imge shown in the inset. Sle r, is 1 mm. () Cuts of long the [H, ] nd [, K] diretions. () Unpolrized neutron diffrtion sns long the [H,,.5]/[, K,.5] diretion for the x ¼.44 smple t 3 nd 3.6 K. The pek t 3 K is super-lttie pek rising from Fe Cu ordering. Temperture differene plots etween 3.6 nd 3 K long the (d) [H,,.5]/[, K,.5] nd (e) [1,, L]/[, 1, L] diretions. Solid lines re Gussin fits nd the horizontl rs indite instrumentl resolution. (f) Temperture dependene of the sttering t Q ¼ (1,,.5)/(, 1,.5) shows T N E2 K. (g) Neutron polriztion nlysis of the mgneti Brgg pek t (1,,.5)/(, 1,.5). Neutron SF nd NSF ross-setions re mesured t 2 nd 24 K. The pek t 24 K is nonmgneti nuler sttering giving rise to NSF sttering. (h) Similr sns for the (1, 1, ) pek. The inset shows positions of these two peks in reiprol spe. All vertil error rs represent sttistil error (1 s.d.). super lttie re reltively wek. Aside from ordering of Fe nd Cu, other spets of the struture of NFe 1 x Cu x As remin identil to tht of NFeAs, orroorted y the similr neutron powder diffrtion ptterns (Supplementry Note 1; Supplementry Fig. 2; Supplementry Tle 3). Mgneti struture of NFe 1 x Cu x As with xe.5. Hving estlished the rel spe Cu nd Fe ordering nd rystl struture of NFe.56 Cu.44 As, it is importnt to determine the mgneti sttes of Cu nd Fe. Assuming tht the ovlent As As onding is not possile euse of the lrge distne etween the 4 NATURE COMMUNICATIONS 7:13879 DOI: 1.138/nomms

5 NATURE COMMUNICATIONS DOI: 1.138/nomms13879 ARTICLE neighouring As ions (Fig. 1), As ions in NFe 1 x Cu x As n only e in the As 3 stte. From XAS nd RIXS experiments on NFe.56 Cu.44 As (Supplementry Note 2; Supplementry Fig. 3), we onlude tht Cu in NFe 1 x Cu x As is in the nonmgneti Cu 1 þ onfigurtion. As onsequene, Fe should e in the Fe 3 þ 3d 5 stte, sine N n only e in the N 1 þ stte. Assuming Fe in NFe.56 Cu.44 As is indeed in the Fe 3 þ 3d 5 stte, we n determine the mgneti struture of the system y systemtilly mesuring mgneti peks t different wve vetors (Supplementry Note 3; Supplementry Figs 4 nd 5) nd using neutron polriztion nlysis (Supplementry Note 3; Supplementry Fig. 6). By polrizing neutrons long the diretion of the momentum trnsfer Q, neutron spin-flip (SF) sttering is sensitive to the mgneti omponents perpendiulr to Q, wheres the non-sf (NSF) sttering proes pure nuler sttering 31,32. Figure 2g shows SF nd NSF sns long the [H,,.5]/[, K,.5] diretion t T ¼ 2 nd 24 K. Inspetion of the dt revels ler mgneti sttering t 2 K tht disppers t 24 K, in ddition to the temperture-independent NSF nuler super-lttie refletion. Figure 2h shows the SF nd NSF sns long the [H, H, ] diretion t 2 nd 24 K. While the SF sttering shows ler pek t 2 K tht disppers on wrming to 24 K, the NSF sttering shows no evidene of the super-lttie peks. From these results nd from systemti determintion of mgneti nd super-lttie sttering intensity t different wve vetors, we onlude tht NFe.56 Cu.44 As forms olliner mgneti struture with moments ligned long the xis s shown in the right inset in Fig. 1g. The ordered mgneti moment is 1.12±9 m B /Fe t 4 K determined from neutron powder diffrtion (Supplementry Fig. 2e). For this mgneti struture, mgneti peks re expeted nd oserved t (, 1,.5) nd (1, 1, ) s shown in Fig. 2g,h (Supplementry Fig. 7). The mgneti peks t (, 1,.5) overlp with super-lttie peks t (1,,.5) due to twinning. Doping dependene of mgneti order in NFe 1 x Cu x As. To determine the Cu-doping dependene of the NFe 1 x Cu x Asphse digrm, we rried out dditionl mesurements on single rystls of NFe 1 x Cu x As with x ¼.18 nd.39. Figure 3, ompre the wve vetor sns long the [H,,.5]/[, K,.5] nd [1,, L]/[, 1, L] diretions for x ¼.18,.39 nd.44. In eh se, the sttering intensity is normlized to the (2,, ) nuler Brgg pek. With inresing Cu doping, the sttering profile eomes nrrower nd stronger, hnging from rod pek inditive of the short-rnge mgneti order in x ¼.18 nd.39 to n essentilly instrument resolution limited pek with long-rnge mgneti order t x ¼.44. Figure 3 shows the doping evolution of the spin spin orreltion length x in NFe 1 x Cu x As, suggesting tht the inresing spin orreltions in NFe 1 x Cu x As re relted to the inresing Cu doping nd the onomitnt inrese in orreltion length of Fe nd Cu ordering (Supplementry Note 3; Supplementry Fig. 5f). The evolution of the temperture dependent mgneti order prmeter with Cu doping is shown in Fig. 3d. For x well o.5, the mgneti trnsition is grdul nd spin-glss-like. However, with x pprohing.5, the trnsition t T N eomes more well defined. In the undoped stte, NFeAs hs smll ordered moment of.17±3 m B /Fe (ref. 23). On smll Cu doping, superondutivity is indued t x ¼ 2 nd the stti AF order is suppressed 2. With further Cu doping, the system eomes n AF insultor, where the ordered moment rehes B1.1 m B /Fe t x ¼.44 (the ordered moment per Fe site ws determined in the struture for NFe.5 Cu.5 As (Fig. 1d)). Sine the iron moment in NFe 1 x Cu x As inreses with inresing Cu doping, our determined moment is therefore lower ound for the ordered moment for Fe in the idel NFe.5 Cu.5 As, whih for Fe 3d 5 n e in either S ¼ 3/2 or S ¼ 5/2 spin stte. Disussion The ehviour in NFe 1 x Cu x As is entirely different from the iprtite mgneti prent phses seen in the iron oxypnitide superondutor LFeAsO 1 x H x, where mgneti prent phses on oth sides of the superonduting dome re metlli ntiferromgnets 33. In Cu-doped Fe 1 x Cu x Se, metl insultor trnsition hs een oserved B4% Cu doping, nd lolized moment with spin glss ehviour is found ner xe.12 (refs 34,35). Although the density funtionl theory (DFT) lultions suggest tht Cu ours in the 3d 1 onfigurtion nd the metl insultor trnsition is y disorder indued Anderson loliztion in Fe 1 x Cu x Se (ref. 36), the 2 3% soluility limit of the system 34 mens it is unler whether Fe 1 x Cu x Se is lso n AF insultor t xe.5. It is useful to ompre our experimentl findings with the results of the nd struture lultions sed on DFT (the Methods setion; Supplementry Note 4; Supplementry Figs 8 1). Both the DFT nd DFT þ U lultions lerly predit the prmgneti phse of NFe.5 Cu.5 As to e metl (Supplementry Fig. 9). By ontrst, our mesurements hve shown tht the insulting ehviour of the resistivity persists ove the Néel temperture (Fig. 1f), implying tht NFe.5 Cu.5 As is Mott insultor. This is lso onsistent with STM mesurements on lower Cu-doping NFe 1 x Cu x As, where the overll line shpe of the eletroni spetrum t x ¼.3 is similr to those of lightly eletron-doped opper oxides lose to the prent Mott insultor 21. To understnd the origin of the Mott-insulting ehviour, we ddress how Cu doping ffets the strength of eletron orreltions (the Methods setion; Supplementry Note 5; Supplementry Figs 11 nd 12). Our strting point is tht the lol (ioniztion) potentil differene etween the Fe nd Cu ions, s illustrted in Fig. 4 nd desried y the energy shift D in Fig. 4, will suppress the hopping integrl etween Fe nd Cu sites. This uses redution in the kineti energy or, equivlently, the eletron ndwidth. The rtio of the on-site Coulom repulsion U (inluding the Hund s oupling J H ) reltive to the ndwidth will effetively inrese, even if the rw vlues of U nd J H remin the sme s in pure NFeAs. This would enhne the tendeny of eletron loliztion even without onsidering the disorder effets with inresing x (refs 37,38). The se of x ¼.5 llows detiled theoretil nlysis. Here, the Cu nd Fe ions in NFe 1 x Cu x As hve rel spe ordering, s disussed ove. We study the metl insultor trnsition in multi-oritl Hurd models for NFe.5 Cu.5 As vi the U(1) slve-spin men-field theory 3. We use the tight-inding prmeters for NFeAs, nd onsider the limit of lrge lol potentil differene etween Cu nd Fe, whih trnsfers hrge from Fe to Cu to Cu 1 þ (n ¼ 1). The fully oupied Cu 3d shell mkes the Cu ions essentilly s vnies. Redution of the Fe 3d eletron ndwidth is used y this kineti loking effet (Supplementry Fig. 11). The resulting ground-stte phse digrm is shown in Fig. 4. For relisti prmeters, illustrted y the shded region, Mott loliztion tkes ple. Our understnding is in line with the generl theoretil identifition of Mott-insulting phse in n overll phse digrm 3, whih tkes into ount kineti energy redution indued inrese of the effetive intertions nd derese of the 3d eletron-filling from 6 per Fe 2 þ to 5 per Fe 3 þ (refs 7,9 11,3). Our work unovers Mott insultor, NFe.5 Cu.5 As, nd provides its understnding in n overll phse digrm of oth ndwidth nd eletron-filling ontrols 3. Our results suggest tht NATURE COMMUNICATIONS 7:13879 DOI: 1.138/nomms

6 NATURE COMMUNICATIONS DOI: 1.138/nomms13879 Intensity rtio (1,,.5) / (2,,) Correltion length ξ (Å) x = x = [H,,.5] / [,K,.5] Along (1,,) Along (,,1) Doping x x =.18 2 x = [1,,L] / [,1,L] d x =.18 x = Temperture (K) Intensity rtio (1,,L) / (2,,) Normlized intensity Figure 3 Cu-doping evolution of the mgneti order in NFe 1 x Cu x As. () Comprison of wve vetor sns long the [H,,.5]/[, K,.5] diretion for NFe 1 x Cu x As single rystls with x ¼.18,.39 nd.44. The dt re normlized to the (2,, ) nuler Brgg pek. Note tht intensity in the x ¼.18 nd.39 smples re multiplied y 2 nd 5 times, respetively. () Similr sns long the [1,, L]/[, 1, L] diretion. () Cu-doping evolution of the spin spin orreltion length. The orreltion length long (1,, ) for the x ¼.44 smple is resolution limited. (d) Cu-doping evolution of the mgneti order prmeter for NFe 1 x Cu x As. All error rs represent sttistil error (1 s.d.). the eletron orreltions of the iron pnitides, while weker thn those of the iron hlogenides, re suffiiently strong to ple these mterils in proximity to the Mott loliztion. This finding mkes it nturl tht the eletron orreltions nd the ssoited d-metl ehviour nd mgnetism indue similrly high superonduting trnsition temperture in the iron pnitides s in the iron hlogenide fmily. Suh ommonlity, in spite of the very different hemil omposition nd eletroni struture of these two rod lsses of mterils, introdues onsiderle simpliity in the quest for unified frmework of the iron-sed superondutivity. More generlly, the proximity to the Mott trnsition links the superondutivity of the iron pnitides to tht rising in the opper oxides, orgni hrge-trnsfer slts 39 nd lkli-doped fullerides 4, nd suggests tht the sme frmework my pply to ll these strongly orrelted eletroni systems. Note dded fter sumission: very reently, ngle-resolved photoemission spetrosopy mesurements on NFe.56 Cu.44 As smples lso onfirmed its Mott-insulting nture 41. Methods Smple preprtion nd experimentl detils. NFe 1 x Cu x As single rystls were grown y the self-flux method using the sme growth proedure s for NFe 1 x Co x As desried in erlier work 42. The Cu-doping levels used in this pper were determined y indutively oupled plsm tomi-emission spetrosopy. Smples with nominl Cu onentrtions of x ¼ 2, 2, 5, 75 nd 9% were prepred, resulting in tul Cu onentrtions of x ¼ 1.6, 18.4(.4), 38.9(2.8), 44.2(1.7) nd 48.4(3.4)%. For eh nominl doping exept x ¼ 2%, five to six smples were mesured nd the s.d. in these mesurements re tken s the unertinty of the tul onentrtions. For simpliity, the tul onentrtions re noted s x ¼ 1.6, 18, 39, 44 nd 48% in the rest of the pper. This suggests tht the soluility limit of Cu in NFe 1 x Cu x As single rystls y our growth method is E5%. For resistivity mesurements, smples were mounted onto resistivity puk inside n Ar filled glove ox y the four-proe method nd overed in Apiezon N grese. The prepred puk is then trnsferred in n Ar-seled ottle nd is only riefly exposed in ir, while eing loded into physil property mesurement system for mesurements. After mesurements, no visul deteriortion of the smples were seen under mirosope. For single-rystl elsti neutron sttering mesurements, smples were overed with hydrogen-free glue nd then stored in vuum ottle t ll times exept during smple loding efore the neutron sttering experiment. While our lrgest rystls grown re up to 2 g, we typilly used thin plte-like smples B5 5mm 2 in size with mss E.2 g for elsti neutron sttering mesurements. Mesurement of the smple with x ¼.18 ws rried out on the N5 triple-xis spetrometer t the Cndin Neutron Bem Center, Chlk River Lortories. Pyroliti grphite (PG) monohromtor nd nlyser (E i ¼ mev) were used with none-36 -smple ollimtion set-up. A PG filter ws pled fter the smple to eliminte ontmintion from higher-order neutron wvelengths. The experiment on x ¼.39 nd.44 smples were done on the HB-3 triple-xis spetrometer nd the HB-1A fixed-inident-energy triple-xis spetrometer, respetively, t the High Flux Isotope Retor, Ok Ridge Ntionl Lortory. On HB-3, PG monohromtor (E i ¼ 14.7 mev), nlyser nd two PG filters (one efore nd the other fter the smple) were used with ollimtion set-up smple HB-1A uses two PG monohromtors (E i ¼ 14.6 mev) nd two PG filters (mounted efore nd fter seond monohromtor), resulting in negligile higher-order wvelengths neutrons in the inident em. A PG nlyser is pled fter the smple. The ollimtion used ws smple In ll ses ove, the smples were ligned in [H,, L] sttering plne. The polrized single-rystl neutron sttering mesurements were rried out on the C5 polrized em triple-xis spetrometer t Cndin Neutron Bem Center, Chlk River Lortories. The neutron ems were polrized with Heusler (1, 1, 1) rystls with vertilly fousing monohromtor nd flt nlyser (E f ¼ 13.7 mev). A PG filter ws pled fter the smple nd none ollimtion ws used. Neutron polriztion ws mintined y using permnent mgnet guide fields. Mezei flippers were pled efore the smple to llow the mesurement of neutron SF nd NSF sttering ross-setions. A 5-oil Helmholtz ssemly ws used to ontrol the neutron spin orienttion t the smple position y produing mgneti field of the order of 1 G. The orienttion of the mgneti field t the smple position ws utomtilly djusted to llow the mesurements to e performed for the neutron spin to e prllel or perpendiulr to the momentum trnsfer. The flipping rtio, defined s the rtio of nuler Brgg pek intensities in NSF nd SF hnnels, ws mesured to e B1:1 for vrious field onfigurtions. The smple ws studied in oth [H,, L] nd [H, H, L] sttering plnes. The XAS nd RIXS mesurements on the x ¼.44 smples were rried out t the Advned Resonnt Spetrosopy emline of the Swiss Light Soure, Pul Sherrer Institut, Switzerlnd 43. Smples were leved in situ nd mesured in 6 NATURE COMMUNICATIONS 7:13879 DOI: 1.138/nomms

7 NATURE COMMUNICATIONS DOI: 1.138/nomms13879 ARTICLE J/U NFe.5 Cu.5 As, n Fe = 5, n Cu = μ Fe Metl U MT μ Cu O S M P Fe 3+ 3d 5 U OSM U (ev) vuum etter thn mr. X-ry sorption ws mesured in totl eletron yield mode y reording the drin urrent from the smples nd in totl fluoresene yield with photodiode. Linerly polrized X-rys with E ¼ ev resonnt t the Cu L 3 edge were used for the RIXS mesurements. The totl momentum trnsfer ws kept onstnt, ut the omponent of momentum trnsfer in the plne hs een vried. For neutron powder diffrtion mesurements, the smples were ground from 2g(x ¼ 16,.18,.39 nd.44) of single rystls nd seled in vndium smple ns inside He-filled glove ox. Neutron powder diffrtion mesurements were rried out t room temperture (3 K) on the BT1 high-resolution powder diffrtometer t NIST Center for Neutron Reserh. The Ge(3, 1, 1) monohromtor ws used to yield the highest neutron intensity nd est resolution t low sttering ngles. For the x ¼.44 smple, mesurement t 4 K using Cu(3, 1, 1) monohromtor ws lso rried out. For the single-rystl neutron diffrtion mesurement, smple with x ¼.44 (25 mg) ws used. The experiment ws rried out t the four-irle diffrtometer HB-3A t the High Flux Isotope Retor, Ok Ridge Ntionl Lortory. The dt ws mesured t 25 K with neutron wvelength of 1.3 Å from ent Si(3, 3, 1) monohromtor using n Anger mer detetor. Brgg peks ssoited with the NFeAs struture were mesured with 1 s per point y rrying out roking sns. Super-lttie peks were mesured with 1 min per point t eh position. DFT-sed eletroni struture lultions. To gin insight into the insulting nture of Cu-doped NFeAs, we hve performed series of DFT-sed eletroni struture lultions. The eletroni nd struture lultions hve een performed using full-potentil liner ugmented plne-wve method s implemented in the WIEN2K pkge 44. The exhnge-orreltions funtion ws tken within the generlized grdient pproximtion (GGA) in the prmeteriztion of Perdew, Burke nd Ernzerhof 45. For the tomi spheres, the muffin-tin rdii (R MT ) were hosen to e 2.5 Bohr for N sites, 2.38 Bohr for Fe nd Cu, nd 2.26 Bohr for As sites, respetively. The numer of plne wves ws limited y utoff prmeter (R MT K mx ) ¼ 7.5. To ount for the on-site orreltions in the form of the Hurd prmeter U, the GGA þ U pproh in Hrtree Fok-like sheme ws E F Fe Mott insultor Δ Cu Cu 1+ 3d 1 Figure 4 Shemti 3d eletroni sttes nd prmeter regimes of Mott insultor in the theoretil phse digrm for NFe.5 Cu.5 As. Shemti of the eletroni sttes of Cu ion longside those of n Fe ion, shown in terms of the tomi levels () nd the eletroni density of sttes (). The reltive potentil differene ssoited with the two ions is speified in terms of n energy shift, D, s illustrted in. () The groundstte phse digrm of multi-oritl Hurd model for NFe.5 Cu.5 As. U OSM nd U MT refer to the ritil U vlues for n oritl-seletive Mott (OSM) trnsition nd Mott trnsition (MT), respetively into n oritlseletive Mott phse (OSMP) nd Mott insultor. The shded region shows the physil prmeter regime s determined y ompring the theoretilly lulted ndwidth renormliztion ftors of the three t 2g oritls with those determined y the ngle-resolved photoemission spetrosopy (ARPES) results (Supplementry Note 5). used 46,47, with the vlue U ¼ 3.15 ev s lulted in LiFeAs using the onstrined rndom-phse pproximtion (RPA) pproh 48. While the neutron diffrtion studies were performed on the x ¼.44 smple, it is muh esier theoretilly to study the 5% Cu-doped NFe.5 Cu.5 As. To model the rystl struture, the experimentlly determined lttie prmeters nd tomi oordintes of NFe.56 Cu.44 As t 4 K were used (Supplementry Tle 3). The differene etween the struture t x ¼.44 nd.5 ws deemed inessentil in view of the wek dependene of the lttie prmeters on Cu onentrtion x (Supplementry Tle 3). While the experimentl struture n e fitted with symmetry group P4/nmm with disordered Cu/Fe, the high-resolution TEM mesurements found tht the super-lttie peks indite stripe-like-ordered pttern (Fig. 1d) onsistent with the spe group Im. We hve therefore used the Im spe group, with four formul units per unit ell, to perform the initio lultions. Interestingly, the results of the lultions turn out to e insensitive to the detils of the tomi rrngement: for instne, rrnging Cu/Fe toms in the hekerord pttern results in very similr density of sttes s tht otined for the stripe-like tomi onfigurtion. We lso note tht the dopted ordered rrngement of Fe/Cu sites neglets the effets of disorder nd Anderson loliztion, whih my e importnt t low doping levels xo.5. Those effets re however eyond the grsp of the first priniples DFT lultions. U(1) slve-spin men-field theory. To tke into ount the orreltion effets eyond the Htree Fok level, we further study the metl insultor trnsition in multi-oritl Hurd models for oth NFeAs nd NFe.5 Cu.5 As vi the U(1) slve-spin men-field theory 49. The model Hmiltonin ontins tightinding prt nd lol intertion prt. Detiled desription of the model is given in ref. 49. The tight-inding prmeters for NFeAs re tken from ref. 5, nd the eletron density is fixed to n ¼ 6 per Fe. As for NFe.5 Cu.5 As, the very lrge lol potentil differene etween Cu nd Fe, s estimted from our nd previous 37 DFT lultions, leds to hrge trnsferring from Fe to Cu. As result, Fe is effetively hole doped when the Cu-doping onentrtion is lose to.5. This piture is supported y our experimentl results whih suggest tht the Cu is in 3d 1 onfigurtion with n ¼ 1, nd Fe onfigurtion is 3d 5 with n ¼ 5. As zerothorder pproximtion to the fully oupied Cu 3d shell, we tret Cu ions s vnies. They orgnize themselves in olumnr-order pttern in NFe.5 Cu.5 As, s shown in Fig. 1d. Dt vilility. The dt tht support the findings of this study re ville from the orresponding uthors on request. Referenes 1. Kmihr, Y., Wtne, T., Hirno, M. & Hosono, H. Iron-sed lyered superondutor L[O 1 x F x ]FeAs (x ¼ 5.12) with T ¼ 26 K. J. Am. Chem. So. 13, (28). 2. Stewrt, G. R. Superondutivity in iron ompounds. Rev. Mod. Phys. 83, (211). 3. Slpino, D. J. A ommon thred: the piring intertion for the unonventionl superondutors. Rev. Mod. Phys. 84, 1383 (212). 4. Di, P. C. Antiferromgneti order nd spin dynmis in iron sed superondutors. Rev. Mod. Phys. 87, 855 (215). 5. Si, Q., Yu, R. & Arhms, E. High temperture superondutivity in iron pnitides nd hlogenides. Nt. Rev. Mter. 1, 1617 (216). 6. Yin, Z. P., Hule, K. & Kotlir, G. Kineti frustrtion nd the nture of the mgneti nd prmgneti sttes in iron pnitides nd iron hlogenides. Nt. Mter. 1, (211). 7. Si, Q. & Arhms, E. 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8 NATURE COMMUNICATIONS DOI: 1.138/nomms Chen, T.-K. et l. Fe-vny order nd superondutivity in tetrgonl -Fe 1 x Se. Pro. Ntl Ad. Si. USA 111, (214). 19. Yi, M. et l. Oservtion of temperture-indued rossover to n oritlseletive Mott phse in A x Fe 2 y Se 2 (A ¼ K, R) superondutors. Phys. Rev. Lett. 11, 673 (213). 2. Wng, A. F. et l. Phse digrm nd physil properties of NFe 1 x Cu x As single rystls. Phys. Rev. B 88, (213). 21. Ye, C. et l. Strong similrities etween the lol eletroni struture of insulting iron pnitide nd lightly doped uprte. Phys. Rev. X 5, 2113 (215). 22. Li, S. et l. Struturl nd mgneti phse trnsitions in N 1 d FeAs. Phys. Rev. B 8, 254 (R) (29). 23. Tn, G. T. et l. Eletron doping evolution of struturl nd ntiferromgneti phse trnsitions in NFe 1 x Co x As iron pnitides. Phys. Rev. B 94, 1459 (216). 24. Pfisterer, M. & Ngorsen, G. On the struture of ternry rsenides. Z. Nturforsh. B 35B, (198). 25. Annd, V. K. et l. 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Doping Mott insultor: physis of hightemperture superondutivity. Rev. Mod. Phys. 78, (26). 4. Tkyshi, Y. et l. The disorder-free non-bcs superondutor Cs 3 C 6 emerges from n ntiferromgneti insultor prent stte. Siene 323, (29). 41. Mtt, C. E. et l. NFe.56 Cu.44 As: pnitide insulting phse indued y on-site oulom intertion. Phys. Rev. Lett. 117, 971 (216). 42. Tntr, M. A. et l. Evolution of norml nd superonduting properties of single rystls of N 1 d FeAs upon intertion with environment. Phys. Rev. B 85, 1451 (212). 43. Stroov, V. N. et l. High-resolution soft X-ry emline ADRESS t the Swiss Light Soure for resonnt inelsti X-ry sttering nd ngleresolved photoeletron spetrosopies. J. Synhrotron Rdit. 17, (21). 44. Blh, P., Shwrz, K., Mdsen, G. K. H., Kvsnik, D. & Luitz, J. in An Augmented Plne Wve nd Lol Oritls Progrm for Clulting Crystl Properties (ed. Shwrz K.) (Tehnil University of Wien, 21). 45. Perdew, J. P., Burke, K. & Ernzerhof, M. Generlized grdient pproximtion mde simple. Phys. Rev. Lett. 77, (1996). 46. Anisimov, V. I., Znen, J. & Andersen, O. K. Bnd theory nd Mott insultors: Hurd U insted of Stoner I. Phys. Rev. B 44, (1991). 47. Liehtenstein, A. I., Anisimov, V. I. & Znen, J. Density-funtionl theory nd strong intertions: oritl ordering in Mott-Hurd insultors. Phys. Rev. B 52, R5467 (1995). 48. Miyke, T., Nkmur, K., Arit, R. & Imd, M. Comprison of initio low-energy models for LFePO, LFeAsO, BFe 2 As 2, LiFeAs, FeSe, nd FeTe: eletron orreltion nd ovleny. J. Phys. So. Jpn 79, 4475 (21). 49. Yu, R. & Si, Q. U(1) slve-spin theory nd its pplition to Mott trnsition in multioritl model for iron pnitides. Phys. Rev. B 86, 8514 (212). 5. Yu, R., Zhu, J.-X. & Si, Q. Oritl-seletive superondutivity, gp nisotropy, nd spin resonne exittions in multioritl t-j 1 -J 2 model for iron pnitides. Phys. Rev. B 89, 2459 (214). Aknowledgements We thnk X.H. Chen, B.J. Cmpell nd Lijun Wu for helpful disussions, Lelnd Hrriger, Sott Crr, Weiyi Wng nd Binod K. Ri for ssisting with some experiments. The singlerystl growth nd neutron sttering work t Rie is supported y the U.S. DOE, BES under ontrt no. DE-SC12311 (P.D.). A prt of the mteril synthesis work t Rie is supported y the Roert A. Welh Foundtion Grnt No. C-1839 (P.D.). The theoretil work t Rie ws in prt supported y the Roert A. Welh Foundtion Grnt No. C-1818 (A.H.N.), C-1411 (Q.S), y U.S. NSF grnts DMR (A.H.N.) nd DMR (Q.S.), nd y the Alexnder von Humoldt Foundtion (Q.S.). E.M. nd J.C. knowledge support from the DOD PECASE. The eletron mirosopy study t Brookhven Ntionl Lortory ws supported y the U.S. DOE, BES, y the Mterils Sienes nd Engineering Division under Contrt No. DE-AC2-98CH1886. The use of ORNL s High Flux Isotope Retor ws sponsored y the Sientifi User Filities Division, Offie of BES, U.S. DOE. XAS nd RIXS experiments hve een performed t the Advned Resonnt Spetrosopy emline of the Swiss Light Soure t the Pul Sherrer Institute. T.S nd M.D. knowledge funding through the Swiss Ntionl Siene Foundtion within the D-A-CH progrmme (SNSF Reserh Grnt 221L ). R.Y. knowledges the support from the Ntionl Siene Foundtion of Chin Grnt numer , nd the Fundmentl Reserh Funds for the Centrl Universities nd the Reserh Funds of Remnin University of Chin Grnt numer 14XNLF8. C.C. knowledges the support from the Ntionl Nturl Siene Foundtion of Chin Grnt No , the Ntionl Key Reserh nd Development Progrm of Chin Grnt No. 216YFB1111 nd Shnxi Interntionl Coopertion Progrm. Author ontriutions Most of the single-rystl growth nd neutron sttering experiments were rried out y Y.S. with ssistne from C.C., Y.L., C.Z., Q.H., H.W., W.T., S.C. nd H.C. The polrized neutron sttering experiments were performed y Z.Y. TEM mesurements were rried out y J.T. nd Y.Z. Trnsport nd indutively oupled plsm mesurements were rried out y C.C. nd J.S.C., under the supervision of E.M. The XAS nd RIXS experiments were performed y Y.-B.H., M.D. nd T.S. The theoretil work ws done y R.Y., A.H.N nd Q.S. P.D. provided the overll led of the projet. The pper ws written y P.D., Y.S., A.H.N., R.Y. nd Q.S. All uthors provided omments. Additionl informtion Supplementry Informtion ompnies this pper t ntureommunitions Competing finnil interests: The uthors delre no ompeting finnil interests. Reprints nd permission informtion is ville online t reprintsndpermissions/ How to ite this rtile: Song, Y. et l. A Mott insultor ontinuously onneted to iron pnitide superondutors. Nt. Commun. 7, doi: 1.138/nomms13879 (216). Pulisher s note: Springer Nture remins neutrl with regrd to jurisditionl lims in pulished mps nd institutionl ffilitions. This work is liensed under Cretive Commons Attriution 4. Interntionl Liense. The imges or other third prty mteril in this rtile re inluded in the rtile s Cretive Commons liense, unless indited otherwise in the redit line; if the mteril is not inluded under the Cretive Commons liense, users will need to otin permission from the liense holder to reprodue the mteril. To view opy of this liense, visit r The Author(s) NATURE COMMUNICATIONS 7:13879 DOI: 1.138/nomms