Effects of a Collective Spin Resonance Mode on the STM Spectra of D-Wave Superconductors

Ferroelectric and piezoelectric properties of tungsten substituted SrBi 2Ta 2O 9ferroelectric ceramicsIndrani Coondoo *,S.K.Agarwal a ,A.K.Jha ba Superconductivity and Cryogenics Division,National Physical Laboratory,Dr K.S.Krishnan Road,New Delhi 110012,India bDepartment of Applied Physics,Delhi College of Engineering,Bawana Road,Delhi 110042,India1.IntroductionDefects in crystals significantly influence physical and various other properties of materials [1].For instance,as it is well known,doping by other elements leads to significant changes in the electrical properties of silicon.Historically,‘‘defect engineering’’has been developed in the field of semiconducting materials such as compound semiconductors as well as in diamond,Si and Ge [2–4].Subsequently,the concept of defect engineering has been applied to other functional materials,and the significant improve-ment in material properties have been achieved in high transition-temperature superconductors [5],amorphous SiO 2[6],photonic crystals [7]and also in the field of ferroelectrics,such as BaTiO 3,Pb(Ti,Zr)O 3(PZT),etc.[8,9].Various structural and electrical properties of bismuth layer-structured ferroelectrics (BLSF)are also strongly affected on deviation from stoichiometric composi-tions and defects have been recognized as a crucially important factor [10–13].It has been found that in BLSF small changes in chemical composition result in significantly altered dielectric and ferroelectric properties including dielectric constant and remanent polarization.In SrBi 2Ta 2O 9(SBT)and SrBi 2Nb 2O 9(SBN),orthor-hombic structural distortions with non-centrosymmetric spacegroup A 21am cause spontaneous ferroelectric polarization (P s )along a axis [14,15].SBT,a member of the BLSF family,has occupied an important position among the Pb-free ferroelectric memory materials [16–18].Tungsten (W 6+)has recently been investigated as a dopant for bismuth titanates and lanthanum doped bismuth titanates,in which the remanent polarization was observed to enhance when a small amount of Ti 4+was substituted by W 6+[19,20].With the objective to improve structural,dielectric and ferroelectric proper-ties,the hexavalent tungsten (W 6+)was chosen as a donor cation for partial replacement of the pentavalent tantalum (Ta 5+)SBT.In this report,the effect of tungsten substitution in SBT (SBTW),on the microstructural,ferroelectric and piezoelectric properties is reported.The results including the improvement in polarization properties have been discussed.2.ExperimentalSamples of compositions SrBi 2(W x Ta 1Àx )2O 9(SBWT),with x =0.0,0.025,0.050,0.075,0.10and 0.20were synthesized by solid-state reaction method taking SrCO 3,Bi 2O 3,Ta 2O 5and WO 3(all from Aldrich)in their stoichiometric proportions.The powder mixtures were thoroughly ground and passed through sieve of appropriate size and then calcined at 9008C in air for 2h.The calcined mixtures were ground and admixed with about 1–1.5wt%polyvinyl alcohol (Aldrich)as a binder and then pressed at $300MPa into disk shaped pellets.The pellets were sintered at 12008C for 2h in air.Materials Research Bulletin 44(2009)1288–1292A R T I C L E I N F O Article history:Received 3October 2008Received in revised form 5December 2008Accepted 6January 2009Available online 15January 2009Keywords:A.CeramicsC.X-ray diffractionD.FerroelectricityA B S T R A C TTungsten substituted samples of compositions SrBi 2(W x Ta 1Àx )2O 9(x =0.0,0.025,0.050,0.075,0.10and 0.20)were synthesized by solid-state reaction method and studied for their microstructural,electrical conductivity,ferroelectric and piezoelectric properties.The X-ray diffractograms confirm the formation of single phase layered perovskite structure in the samples with x up to 0.05.The temperaturedependence of dc conductivity vis-a`-vis tungsten content shows a decrease in conductivity,which is attributed to the suppression of oxygen vacancies.The ferroelectric and piezoelectric studies of the W-substituted SBT ceramics show that the remanent polarization and d 33values increases with increasing concentration of tungsten up to x 0.05.Such compositions with low conductivity and high P r values should be excellent materials for highly stable ferroelectric memory devices.ß2009Elsevier Ltd.All rights reserved.*Corresponding author.Present address:Liquid Crystal Group,National Physical Laboratory,Dr K.S.Krishnan Road,New Delhi 110012,India.Tel.:+919810361727;fax:+911125170387.E-mail address:indrani_coondoo@ (I.Coondoo).Contents lists available at ScienceDirectMaterials Research Bulletinj o ur n a l h o m e p a g e :w w w.e l se v i e r.c om /l oc a t e /m a t r e sb u0025-5408/$–see front matter ß2009Elsevier Ltd.All rights reserved.doi:10.1016/j.materresbull.2009.01.001X-ray diffractograms of the sintered samples were recorded using a Bruker diffractometer in the range 108 2u 708with CuK a radiation.The sintered pellets were polished to a thickness of 1mm and coated with silver paste on both sides for use as electrodes and cured at 5508C for half an hour.Electrical conductivity was performed using Keithley’s 6517A Electrometer.The polarization–electric field (P –E )hysteresis measurements were done at room temperature using an automatic P –E loop tracer based on Sawyer–Tower circuit.Piezoelectric charge co-efficient d 33was measured using a Berlincourt d 33meter after poling the samples in silicone–oil bath at 2008C for half an hour under a dc electric field of 60–70kV/cm.3.Results and discussion3.1.Structural and micro-structural studiesThe phase formation and crystal structure of the ceramics were examined by X-ray diffraction (XRD),which is shown in Fig.1.The XRD patterns of the samples show the characteristic peaks of SBT.The peaks have been indexed with the help of a computer program–POWDIN [21]and the refined lattice parameters are given in Table 1.It is observed that a single phase layered perovskite structure is maintained in the range 0.0 x 0.05.Owing to the same co-ordination number i.e.6and the smallerionic radius of W (0.60A˚)in comparison to Ta (0.64A ˚),there is a high possibility of tungsten occupying the tantalum site.The observance of unidentified peak of very low intensity in the compositions with x >0.05indicates the solubility limit of W concentration in SBT.The unidentified peak is possibly due to tungsten not occupying the Ta sites in the structure as the intensity of this peak is observed to increase with tungsten content.Composition and sintering temperature influences the micro-structure such as grain growth and densification of the specimen,which in turn control other properties of the material [11,13].The effects of W substitution on the microstructure have been examined by SEM and the obtained micrographs are shown in Fig.2.It shows the microstructure of the fractured surface of the studied samples.It is clearly observed that W substitution has pronounced effect on the average grain size and homogeneity of the grains.Randomly oriented and anisotropic plate-like grains are observed in all the samples.It is also observed that the average grain size increases gradually with increasing W content.The average grain size in the sample with x =0.0is $2–3m m while that in the sample with x =0.20the size increases to $5–7m m.3.2.Electrical studiesThe electrical conductivity of ceramic materials encompasses a wide range of values.In insulators,the defects w.r.t.the perfect crystalline structure act as charge carriers and the consideration of charge transport leads necessarily to the consideration of point defects and their migration [22].Many mechanisms were put forward to explain the conductivity mechanism in ceramics.Most of them are approximately divided into three groups:electronic conduction,oxygen vacancies ionic conduction,and ionic and p-type mixed conduction [22].Intrinsic conductivity results from the movement of the component ions,whereas conduction resulting from the impurity ions present in the lattice is known as extrinsic conductivity.At low temperature region (ferroelectric phase),the conduction is dominated by the extrinsic conduction,whereas the conduction at the high-temperature paraelectric phase ($300–7008C)is dominated by the intrinsic ionic conduction [23,25].Fig.3shows the temperature dependence of dc conductivity (s dc )for the undoped and doped SBT samples.The curves show that the conductivity increases with temperature.This is indicative of negative temperature coefficient of resistance (NTCR)behavior,a characteristic of dielectrics [22].It is observed in Fig.3that throughout the temperature range,the dc conductivity of the doped samples are nearly two to three orders lower than that of the undoped sample.Two predominant conduction mechanisms indicated by slope changes in the two different temperature regions are observed in Fig.3.Such changes in the slope in the vicinity of the ferro-paraelectric transition region have been observed in other ferroelectric materials as well [23,24].In addition,it is also observed (Table 2)that the activation energy calculated using the Arrhenius equation [22]in the paraelectric phase increase from $0.80eV for the undoped sample to $2eV for the doped samples.The X-ray photoemission spectroscopic study has confirmed that when Bi 2O 3evaporates during high-temperature processing,vacancy complexes are formed in the (Bi 2O 2)2+layers [26].As a result,defective (Bi 2O 2)2+layers are inherently present in SBT.The undoped SBT shows n-type conductivity,since when oxygen vacancies are created,it leaves behind two trapped electrons [27]:O o !12O 2"þV o þ2e 0(1)where O o is an oxygen ion on an oxygen site,V o is a oxygen vacant site and e 0represents electron.The conductivity in the perovskites can be described as an ordered diffusion of oxygen vacancies [28].Their motion is manifested by enhanced ionic conductivity associated with an activation energy value of $1eV [26].These oxygen vacancies can be suppressed by addition of donors,since the donor oxide contains more oxygen per cation than the host oxide it replaces [29].It has been reported that conductivity in Bi 4Ti 3O 12(BIT)can be significantly decreased,up to three orders of magnitude with the addition of donors,such as Nb 5+and Ta 5+at the Ti 4+sites [23,30].A few other studies on layered perovskites have also reported a decrease inconductivityFig.1.XRD patterns of SrBi 2(W x Ta 1Àx )2O 9samples sintered at 12008C.Table 1Lattice parameters of SrBi 2(W x Ta 1Àx )2O 9samples.Concentration of W a (A ˚)b (A ˚)c (A ˚)0.0 5.5212 5.513924.92230.025 5.5214 5.520225.10790.05 5.5217 5.519925.05850.075 5.5191 5.504525.05670.10 5.5142 5.506125.0850.205.51335.493925.0861I.Coondoo et al./Materials Research Bulletin 44(2009)1288–12921289with addition of donors [23,24,31].In the present study,the Ta 5+-site substitution by W 6+in SBT can be formulated using a defect chemistry expression as WO 3þV o!Ta 2O 512W Ta þ3O o (2)It shows that the oxygen vacancies are reduced upon the substitution of donor W 6+ions for Ta 5+ions.Hence,it is reasonable to believe that the conductivity in SBT is suppressed by donor addition.As per the above discussion,the high s dc observed in the undoped SBT (Fig.3)can be attributed to the motion of oxygen vacancies.As already discussed,the doped samples show reduced conductivity because the transport phenomena involving oxygen vacancies are greatly reduced.The high E a value of $1.75–2eVcorresponding to the high-temperature region in the doped ceramics is consistent with the fact that in the donor-doped materials,the ionic conduction reduces [32].The activation energy E a in the low temperature ferroelectric region (Table 2)corre-sponds to extrinsic conduction.At lower temperatures the extrinsic conductivity results from the migration of impurity ions in the lattice.Some of these impurities may also be associated with lattice defects.Pure SBT has large number of Schottky defects (oxygen vacancies)in addition to impurity ions whereas in the doped samples,due to charge neutrality,there is relatively less content of oxygen vacancies.Thus,in the doped samples the conductivity in the low temperature region is largely due to the impurity ions only.This explains the high activation energy in pure SBT in the low temperature region compared to doped samples (Table 2).In the high-temperature region,the value of E a in the doped samples is observed to increase with W concentration up to x =0.05but beyond that,it decreases (Table 2).The decrease in the activation energy for samples with x >0.05suggests an increase in the concentration of mobile charge carriers [33].This observation can be ascribed to the existence of multiple valence states of tungsten.Since tungsten is a transitional metal element,the valence state of W ions in a solid solution most likely varies from W 6+to W 4+depending on the surrounding chemical environment [34].When W 4+are substituted for the Ta 5+sites,oxygen vacancies would be created,i.e.one oxygen vacancy would be created for every two tetravalent W ions entering the crystal structure,whichFig.3.Variation of dc conductivity with temperature in SrBi 2(W x Ta 1Àx )2O 9samples.Fig.2.SEM micrographs of fractured surfaces of SrBi 2(W x Ta 1Àx )2O 9samples with (a)x =0.0,(b)x =0.025,(c)x =0.050,(d)x =0.075,(e)x =0.10and (f)x =0.20Table 2Activation energy (E a )in the high-temperature paraelectric region and low temperature ferroelectric region;Curie temperature (T c )in SrBi 2(W x Ta 1Àx )2O 9samples.Concentration of W E a (high temp.)(eV)E a (low temp.)(eV)T c (8C)0.00.790.893110.025 1.920.593080.05 1.960.543250.075 1.940.543380.10 1.860.573680.201.740.54390I.Coondoo et al./Materials Research Bulletin 44(2009)1288–12921290explains the increase in the concentration of mobile charge carriers which ultimately results in an decrease in the E a beyond x>0.05. Hence it is reasonable to conclude that W ions in the SBWT exists as a varying valency state,i.e.at lower doping concentration they exist in hexavalent state(W6+)and at a higher doping concentra-tion,they tend to exist in lower valency states[8].The P–E loops of SrBi2(Ta1Àx W x)2O9are shown in Fig.4.It is observed that W-doping results in formation of well-defined hysteresis loops.Fig.5shows the compositional dependence of remanent polarization(2P r)and the coercivefield(2E c)of SrBi2(Ta1Àx W x)2O9samples.Both the parameters depend on W content of the samples.It is observed that2P rfirst increases with x and then decreases while2E cfirst decreases with x and then increases(Fig.5).The optimum tungsten content for maximum2P r ($25m C/cm2)is observed to be x=0.075.It is known that ferroelectric properties are affected by compositional modification,microstructural variation and lattice defects like oxygen vacancies[10,35,36].In hard ferroelectrics, with lower valent substituents,the associated oxide vacancies are likely to assemble in the vicinity of domain walls[37,38].These domains are locked by the defects and their polarization switching is difficult,leading to an increase in E c and decrease in P r[38]. On the other hand,in soft ferroelectrics,with higher valent substituents,the defects are cation vacancies whose generation in the structure generally increases P r.Similar observations have been made in many reports[38–41].Watanabe et al.[42]reported a remarkable improvement in ferroelectric properties in the Bi4Ti3O12ceramic by adding higher valent cation,V5+at the Ti4+ site.It has also been reported that cation vacancies generated by donor doping make domain motion easier and enhance the ferroelectric properties[43].Further,it is known that domain walls are relatively free in large grains and are inhibited in their movement as the grain size decreases[44].In the larger grains, domain motion is easier which results in larger P r.Also for the SBT-based system,it is known that with increase in the grain size the remanent polarization also increases[45,46].Based on the obtained results and above discussion,it can be understood that in the undoped SBT,the oxygen vacancies assemble at sites near domain boundaries leading to a strong domain pinning.Hence,as observed,well-saturated P–E loop for pure SBT is not obtained.But in the doped samples,the suppression of the oxygen vacancies reduces the pinning effect on the domain walls,leading to enhanced remanent polarization and lower coercivefield.Also,the increase in grain size in tungsten added SBT,as observed in SEM micrographs(Fig.2)contribute to the increase in polarization values.In the present study,the grain size is observed to increase with increasing W concentration.However, the2P r values do not monotonously increase and neither the E c decreases continuously with increasing W concentration(Fig.5). The variation of P r and E c beyond x>0.05,seems possibly affected by the presence of secondary phases(observed in XRD diffracto-grams),which hampers the switching process of polarization [47–50].Also,beyond x>0.05the increase in the number of charge carriers in the form of oxygen vacancies leads to pinning of domain walls and thus a reduction in the values of P r and increase in E c is observed.Fig.6shows the variation of piezoelectric charge coefficient d33 with x in the SrBi2(Ta1Àx W x)2O9.The d33values increases with increase in W content up to x=0.05.A decrease in d33values is observed in the samples with x!0.075.The piezoelectric coefficient,d33,increases from13pC/N in the sample with x=0.0to23pC/N in the sample with x=0.05.It is known that the major drawback of SBT is its relatively higher conductivity,which hinders proper poling[51].High resistivity is therefore important for maintenance of poling efficiency at high-temperature[52,53].The W-doped SBT samples show an electrical conductivity value up to three orders of magnitude lower than that of undoped sample(Fig.3).The positional variation of2P r and2E c in SrBi2(W x Ta1Àx)2O9samples.Fig.6.Variation of d33in SrBi2(W x Ta1Àx)2O9samples.Fig. 4.P–E hysteresis loops in SrBi2(W x Ta1Àx)2O9samples recorded at roomtemperature.I.Coondoo et al./Materials Research Bulletin44(2009)1288–12921291decrease in conductivity upon donor doping improve the poling efficiency resulting in the observed higher d33values.Moreover, since the grain size increases with W content in SBT,it is reasonable to believe that the increase in grain size will also contribute to the increase in d33values[54].The decrease in the value of d33for samples with x!0.075is possibly due to the presence of secondary phases as observed in diffractograms[1,51,55]and the increase in oxygen vacancies for samples with x>0.05.4.ConclusionsX-ray diffractograms of the samples reveal that the single phase layered perovskite structure is maintained in the samples with tungsten content x0.05.SEM micrographs reveal that the average grain size increases with increase in W concentration. The temperature dependence of the electrical conductivity shows that tungsten doping results in the decrease of conductivity by up to three order of magnitude compared to W free SBT.All the tungsten-doped ceramics have higher2P r than that of the undoped sample.The maximum2P r($25m C/cm2)is obtained in the composition with x=0.075.The reduced conductivity allows high-temperature poling of the doped samples.Such compositions with low loss and high P r values should be excellent materials for highly stable ferroelectric memory devices.The d33value is observed to increase with increasing W content up to x0.05.The value of d33 in the composition with x=0.05is$23pC/N as compared to$13 pC/N in the undoped sample.AcknowledgmentsThe authors sincerely thank Prof.P.B.Sharma,Dean,Delhi College of Engineering,India for his generous support and providing ample research infrastructure to carry out the research work.The authors are thankful to Dr.S.K.Singhal,Scientist, National Physical 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Femtosecond laser fabrication of long period fiber gratings and applications in refractive index sensingBenye Li a ,Lan Jiang a ,n ,Sumei Wang a ,Hai-Lung Tsai b ,Hai Xiao caLaser Micro-/Nano-Fabrication Laboratory,School of Mechanical Engineering,Beijing Institute of Technology,100081,PR Chinab Laser-Based Manufacturing Laboratory,Department of Mechanical and Aerospace Engineering,Missouri University of Science and Technology (Formerly University of Missouri-Rolla),Rolla,MO 65409,USA cDepartment of Electrical and Computer Engineering,Missouri University of Science and Technology (Formerly University of Missouri-Rolla),Rolla,MO 65409,USAa r t i c l e i n f oArticle history:Received 1December 2010Received in revised form 11April 2011Accepted 21April 2011Available online 5May 2011Keywords:Femtosecond laserLong period fiber gratings Refractive index sensora b s t r a c tAn improved point-by-point inscription method is proposed to fabricate long period fiber gratings (LPFGs)by using a laser operating at 800nm with 35fs duration pulses.The sensitivity to misalignment between the core and the focus is reduced by scanning a rectangular part on the fiber.LPFGs with an attenuation depth of 20dB are achieved within the wavelength range of 1465–1575nm.Characteriza-tion of the temperature sensitivity and thermal stability of the LPFGs is presented.A 5.6nm wavelength shift and a 1.2dB decrease in the attenuation peak are observed following heat treatment at 6001C for 4h.The fabricated LPFGs are used as refractive index sensors.The effect of heat treatment on the response of the LPFGs to refractive index changes is also studied.&2011Elsevier Ltd.All rights reserved.1.IntroductionLong period fiber gratings (LPFGs)are widely used in optical communications and sensing fields.In sensing fields,LPFGs present obvious advantages such as high sensitivity to tempera-ture and external refractive index changes [1].Various methods of LPFGs fabrication have been developed,including irradiation by the output from ultraviolet lasers [2,3],and CO 2lasers [4],exposure to electric arcs [5]and mechanical pressure [6].Recently,femtosecond lasers have been widely used to fabricate optical devices in transparent materials [7,8].LPFGs fabricated by an IR (800nm)femtosecond laser have superior aging characteristics and high resistance to thermal decay [9],but the attenuation depth is only about 8dB with a relatively large background loss in the irregular spectrum.By controlling the alignment of the focus within the fiber core along the length of the fiber,a more regular transmission spectrum is achieved [10,11].For the aforementioned IR femtosecond laser inscription method [9–11],the transmission spectrum is very sensitive to the misalignment between the fiber core and the focus of the inscribing beam,which is difficult to control accurately.The thermal stability and characteristics of LPFGs should be considered when they are used as sensors.The refractive indexmodulation of the LPFGs will be reduced [12]and the sensitivity to external refractive index changes will be different after heat treatment.A recent paper reports the thermal characteristics of LPFGs with the changes of external refractive indexes [13].However,the effect of heat treatment on the refractive index (RI)sensing has not been studied.In this paper,an improved point-by-point inscription method is adopted to reduce the sensitivity to misalignment between the core and the focus by scanning a rectangular part of the fiing this method,LPFGs with the attenuation depths of 20dB are obtained,which is the highest reported for LPFGs fabricated by IR femtosecond lasers.Characterization of the thermal response and thermal stability of the fabricated LPFGs are presented.The effect of heat treatment on the LPFG refractive index sensitivity is also studied.It is demonstrated that LPFGs fabricated by exposure to the output from femtosecond lasers can be used as refractive index sensors at high temperatures.2.Experimental setupA schematic diagram of the experimental setup is shown in Fig.1.An fs laser (Spectra-Physics)with a pulse width of 35fs is used to fabricate the LPFG sensors.The center wavelength and repetition rate of the laser are 800nm and 1kHz,respectively.A combination of a half-wave plate and a polarizer is used to reduce the laser power,and then several neutral density (ND)filters are used to further reduce the laser power to the desired values.Contents lists available at ScienceDirectjournal homepage:/locate/optlastecOptics &Laser Technology0030-3992/$-see front matter &2011Elsevier Ltd.All rights reserved.doi:10.1016/j.optlastec.2011.04.011nCorresponding author.Tel.:þ861068914517.E-mail addresses:jianglan@,jianglan@ (L.Jiang).Optics &Laser Technology 43(2011)1420–1423The laser beam is focused on the fiber core by a 20Âmicroscopic objective (NA ¼0.45)and the beam diameter at the focal plan is about 2m m.The pulse energy is within the range of 0.1–0.5m J.The optical fiber used in this experiment is an enhanced telecom fiber SMF-28e (Corning,Inc.)with a core diameter of 8.2m m,a cladding diameter of 125m m and a numerical aperture of 0.14.The fiber samples are translated by a 6-axis positioning system with a resolution of 1m m.The transmission spectrum of the fiber grating is monitored by the combination of a tunable laser (Agilent81980A,wavelength range 1465–1575nm)and an optical power meter (Agilent 81636B)during the inscription process.The tunable laser scans through its wavelength range (1465–1575nm)at the rate of 0.5nm per step and the power meter detects the power trans-mitted through the grating.The polymer coating of the fiber sample is removed and cleaned before the LPFG fabrication.The back-reflected light from the fiber is monitored by a CCD camera and the laser beam is considered to be focused on the center of the fiber core when the image of the back-reflected light becomes a linear shape along thefiber axis [14].The linear shape would be separated into two parts if there is a deviation between the focal point and the center of the fiber core.The fiber is then moved vertically to the fiber axis away from the center of the fiber core by 8m m in the focal plane.This position is the actual scanning starting point.The distance of 8m m is intended to ensure that the scanning area covers the whole fiber core during the fabrication process.Fig.2shows the beam path during the laser inscribing process with the step of 2m m and the microscope image of the fabricated fiber.A rectangular area of 16ÂL m m is formed after the laser scanning,where L is half of the grating period (duty ratio is 0.5).The scanning length perpendicular to the axis of the fiber is 16m m,as compared with the fiber core diameter of 8.2m m.It ensures that the focused beam scans over the whole fiber core in the y-direction as shown in Fig.2(a),and reduces the misalignment sensitivity as reported in Ref.[9].3.Results and discussion3.1.Effects of pulse energy,grating length and heat treatment on LPFG transmission spectraDuring the inscription process,the translation speed,grating period and duty ratio are 50m m/s,436m m and 0.5,respectively.Fig.3shows the transmission spectra of LPFGs fabricated with irradiation pulse energies of 0.16,0.24,0.32,0.4and 0.48m J.All gratings have the same length of 25.2mm.The transmission loss reaches a maximum with the increase in pulse energy (0.16–0.32m J),and then declines with further increase in pulse energy (0.32–0.48m J).The attenuation band has a red-shift with increas-ing irradiation fluence,i.e.,with the increase in refractive index modulation,which is in agreement with the coupled mode theory [15].For irradiation pulse energy of 0.32m J,the maximum transmission attenuation of 20dB is obtained at 1525.8nm and the background loss is only 1dB.The translation speed is known and the route of scanning path is certain,hence,the time required for each LPFG fabrication can be estimated.It takes about 35min to process one LPFG.After the laser beam propagated through a series of optics,the pulse duration is spread to about 50fs due to the dispersion effects,which is measured before the objective lens.The corresponding ablation threshold fluence of silica is measured to be about 3.6J/cm 2.However,there isn’t a grating signal when the irradiation fluence is 3.6J/cm 2.Fig.4shows the transmission spectra of LPFGs withFiberShutterTunable laserHalogen lightPolarizerFs laserHalf-wave plateND filter CCDDichroic mirrorIPCPower meterObjective lensHexapod 6-axis positioning systemxyzFig.1.Schematic diagram of the experimental setup for LPFG fabrication anddetection.Fig.2.(a)Beam path during the laser inscribing process and (b)microscope image of an inscribed LPFG.B.Li et al./Optics &Laser Technology 43(2011)1420–14231421different lengths for the same irradiation pulse energy of 0.12m J.The corresponding irradiation fluence is 3.8J/cm 2,a little higher than the ablation threshold fluence.The transmission loss increases with the increase in grating length and reaches maximum with the length of 39.24mm,while the maximum transmission attenuation is only 10dB.When the grating length is further increased to 40.11mm,the attenuation peak begins to decrease for the over-coupling occurs based on the coupled mode theory.Thus,for a given grating period and duty ratio,both the pulse energy and the grating length should be considered during the fabrication process for a high-quality LPFG.The inscription time is about 55min for the LPFG with a length of 40.11mm.During the experiments we also tried to fabricate the LPFGs with various periods in order to achieve significant grating signal in the limited wavelength range of our tunable laser (1465–1575nm).After many trials,it is found that the maximum attenuation peak is around the center of the wavelength range of the tunable laser with proper pulse energy and grating length when the grating period is 436m m.Some regular damage occurred in the cladding area near the incident surface because the used laser fluence was greater than the ablation threshold,as shown in Fig.2(b).The refractive indices of both the fiber core and cladding are changed after the irradiation,since the energy is absorbed by both the fiber core andcladding [2,3,16].The details of the refractive index modification are still being investigated by using the plasma model proposed by the authors [17].An LPFG with a length of 25.2mm is put into an electric Muffle furnace to test the temperature sensitivity.The LPFG has a maximum transmission attenuation of 20dB around 1525.8nm.The temperature changed from 30to 4001C at a step of 201C every 5min.The peak of the attenuation band has a red-shift when the temperature increases,as shown in Fig.5.The mea-sured temperature sensitivity is 0.091nm/1C.LPFG with background loss of 0.3dB and attenuation peak around 20dB is achieved by selecting the appropriate laser fluence and alignment between the inscription focal and the fiber core,as shown in Fig.6(a).However,background loss always varies from one LPFG to another,because of the motion-stage error,the fiber fabrication error and the laser fluence fluctuations.An LPFG with background loss around 2dB is put into the Muffle furnace for 4h at 6001C and the background loss is reduced to 0.4dB after the heat treatment.Furthermore,the heat treatment leads to a 5.6nm wavelength shift and a 1.2dB decrease in the attenuation peak,as shown in Fig.6(b).The highest working temperature of the LPFG inscribed by the fs laser is much higher than that of LPFGs fabricated by157nm F 2-lasers [3].3.2.Response to RI changes of the LPFGThe response of a 25-mm-long LPFG to refractive index changes is first investigated without heat treatment.All RI changes tests are at room temperature.The LPFG is fixed on a glass plate to avoid bending-induced signal change.Then,the glass plate is put into different liquids and the transmission spectra are recorded.The liquids are sucrose solution with different concentrations (0%,30%,35%,40%,45%,50%,55%and 60%),and their corresponding RI are 1.3330,1.3902,1.3997,1.4096,1.4200,1.4307and 1.4418,respec-tively.The device is cleaned by deionized water and air dried after each test of one concentration.The LPFG is put into the Muffle furnace,whose temperature is set at 4001C for 4h.After the heat treatment,the LPFG is left at room temperature for 24h and then the response of the LPFG to RI changes is investigated.When the surrounding refractive index is changed from n ex to n ex ,the wavelength shift dl 0can be expressed as [18]:dl 0ffiu 21l 30L 8p 3n cl r 31n 2clÀn 2exÀÁ1=2À1n 2clÀn 2ex 1=2264375ð1Þ-12-10-8-6-4-20T r a n s m i s s i o n (d B )wavelength (nm)Fig.4.Transmission spectra of LPFGs with different grating lengths.-505101520253035W a v e l e n g t h s h i f t (n m )Temperature (°C)Fig.5.Resonance wavelength shift of the LPFG versus temperature change.1460-25-20-15-10-500.48µJ0.4µJ0.32µJ0.24µJT r a n s m i s s i o n (d B )wavelength (nm)0.16µJ148015001520154015601580Fig.3.Transmission spectra of LPFGs fabricated at different laser pulse energies.B.Li et al./Optics &Laser Technology 43(2011)1420–14231422where l 0is the resonance wavelength in air,n cl is the cladding refractive index,r is the cladding radius,and u N is the m th root of the Bessel function J 0.Fig.7shows the wavelength shift of the LPFG with respect to RI changes.The numerical calculation is based on Eq.(1).A blue-shift is observed in the transmission spectra,which is in good agreement with the numerical calcula-tion.The wavelength shift is more accordant with the numerical calculation curve after the heat treatment.Heat treatment causes a change in refractive index modulation of the grating and more stable grating can be achieved by heat treatment [13].As a result,the LPFG sensor is more reliable after a proper heat treatment for RI detection.4.ConclusionsIn this study,an improved point-by-point inscription method is developed to make LPFGs with attenuation peak up to 20dB by using a 800nm,35fs laser.The temperature sensitivity of the LPFG is 0.091nm/1C and it shows high temperature stability at 6001C.Theoretically,the LPFG sensors fabricated by our proposed method can stand 12001C,which still needs further experimental verification.As an RI sensor,the LPFG becomes more stable after the heat treatment.The results present guidelines to make the LPFGs as temperature sensors and stable refractive index sensors.AcknowledgmentsThis research is supported by the National Natural Science Foundation of China (Grant nos.90923039and 50705009)and 863of Ministry of Science and Technology of China (Grant no.2008AA03Z301).References[1]James SW,Tatam RP.Optical fibre long-period grating sensors:character-istics and application.Meas Sci Technol 2003;14:R49–61.[2]Vengsarkar AM,Lemaire PJ,Judkins JB,Bhatia V,Erdogan T,Sipe JE.Long-periodfiber gratings as band-rejection filters.J Lightwave Technol 1996;14(1):58–65.[3]Chen KP,Herman PR,Zhang J,Tam R.Fabrication of strong long-periodgratings in hydrogen-free fibers with 157-nm F 2-laser radiation.Opt Lett 2001;26(11):771–3.[4]Rao Y-J,Wang Y-P,Ran Z-L,Zhu T.Novel fiber-optic sensors based on long-period fiber gratings written by high-frequency CO 2laser pulses.J Lightwave Technol 2003;21(5):1320–7.[5]Hwang IK,Yun SH,Kim BY.Long-period fiber gratings based on periodicmicrobends.Opt Lett 1999;24(18):1263–5.[6]Savin S,Digonnet MJF,Kino GS,Shaw HJ.Tunable mechanically induced long-period fiber gratings.Opt Lett 2000;25(10):710–2.[7]Smelser CW,Mihailov SJ,Grobnic D.Formation of type I-IR and type II-IR gratingswith an ultrafast IR laser and a phase mask.Opt Express 2005;13(14):5377–86.[8]Lin C-H,Jiang L,Xiao H,Chai Y-H,Chen S-J,Tsai.H-L.Fabry–Perot inter-ferometer embedded in a glass chip fabricated by femtosecond laser.Opt Lett 2009;34(16):2408–10.[9]Kondo Y,Nouchi K,Mitsuyu T,Watanabe M,Kazansky PG,Hirao K.Fabrica-tion of long-period fiber gratings by focused irradiation of infrared femtose-cond laser pulses.Opt Lett 1999;24(10):646–8.[10]Hindle F,Fertein E,Przygodzki C,Durr F,Paccou L,Bocquet R,et al.Inscription of long-period gratings in pure silica and germano–silicate fiber cores by femtosecond laser irradiation.IEEE Photon Technol Lett 2004;16(8):1861–3.[11]Zhang N,Yang JJ,Wang MW,Zhu XN.Fabrication of long-period fiber gratingsusing 800nm femtosecond laser pulses.Chin Phys Lett 2006;23(12):3281–4.[12]Kalachev AI,Pureur V,Nikogosyan DN.Investigation of long-period fibergratings induced by high-intensity femtosecond UV laser pulses.Opt Com-mun 2005;246:107–15.[13]Costa RZV,Kamikawachi RC,Muller M,Fabris JL.Thermal characteristics oflong-period gratings 266nm UV-point-by-point induced.Opt Commun 2009;282:816–23.[14]Fujii T,Fukuda T,Ishikawa S,Ishii Y,Sakuma K,Hosoya H.Characteristicsimprovement of long-period fiber gratings fabricated by femtosecond laser pulses using novel positioning technique.In:Proceedings of the Opt Fiber Commun Conference Los Angeles,CA,USA;2004,ThC6.[15]Erdogan T.Cladding-mode resonances in short-and long period fiber gratingfilters.J Opt Soc Am A 1997;14(8):1760–73.[16]Slattery SA,Nikogosyan DN.Long-period fiber grating inscription under high-intensity 352nm femtosecond irradiation:three-photon absorption and energy deposition in cladding.Opt Commun 2005;255:81–90.[17]Jiang L,Tsai.HL.Plasma modeling for ultrashort pulse laser ablation ofdielectrics.J Appl Phys 2006;100:023116.[18]Chiang KS,Liu Y,Ng MN,Dong X.Analysis of etched long-period fibre gratingand its response to external refractive index.Electron Lett 2000;36(17):966–7.-20-15-10-50-20-15-10-50T r a n s m i s s i o n (d B )Wavelength (nm)Wavelength (nm)1460148015001520154015601580Fig.6.Transmission spectra of the LPFGs with various background loss:(a)an LPFG with background loss around 0.3dB and (b)an LPFG with background loss around 2dB before heat treatment and 0.4dB after heat treatment.-30-25-20-15-10-50W a v e l e n g t h s h i f t (n m )Refractive indexFig.7.Wavelength shift as a function of refractive index.B.Li et al./Optics &Laser Technology 43(2011)1420–14231423。

Final state effect: : 1~10 eV
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15
XPS spectra of oxidized Al
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17
轨道电子结合能的近似计算方法
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3
能量分辨和化学成分分辨
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14
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PHYSICAL REVIEW B94,241110(R)(2016)Hubbard band versus oxygen vacancy states in the correlated electron metal SrVO3S.Backes,1T.C.R¨o del,2,3F.Fortuna,2E.Frantzeskakis,2P.Le F`e vre,3F.Bertran,3M.Kobayashi,4R.Yukawa,4 T.Mitsuhashi,4M.Kitamura,4K.Horiba,4H.Kumigashira,4R.Saint-Martin,5A.Fouchet,6B.Berini,6Y.Dumont,6A.J.Kim,1F.Lechermann,7,8H.O.Jeschke,1M.J.Rozenberg,9R.Valent´ı,1,*and A.F.Santander-Syro2,†1Institut f¨u r Theoretische Physik,Goethe-Universit¨a t Frankfurt,Max-von-Laue-Strasse1,60438Frankfurt am Main,Germany 2CSNSM,Univ.Paris-Sud,CNRS/IN2P3,Universit´e Paris-Saclay,91405Orsay Cedex,France3Synchrotron SOLEIL,L’Orme des Merisiers,Saint-Aubin-BP48,91192Gif-sur-Yvette,France 4Photon Factory,Institute of Materials Structure Science,High Energy Accelerator Research Organization(KEK),1-1Oho,Tsukuba305-0801,Japan5SP2M-ICMMO-UMR-CNRS8182Universit´e Paris-Sud,Universit´e Paris-Saclay,91405Orsay Cedex,France 6GEMaC,Universit´e de Versailles St.Quentin en Y.-CNRS,Universit´e Paris-Saclay,Versailles,France7Institut f¨u r Theoretische Physik,Universit¨a t Hamburg,Jungiusstrasse9,20355Hamburg,Germany8Institut f¨u r Keramische Hochleistungswerkstoffe,TU Hamburg-Harburg,D-21073Hamburg,Germany 9Laboratoire de Physique des Solides,CNRS,Univ.Paris-Sud,Universit´e Paris-Saclay,91405Orsay Cedex,France(Received22February2016;published19December2016)We study the effect of oxygen vacancies on the electronic structure of the model strongly correlatedmetal SrVO3.By means of angle-resolved photoemission spectroscopy(ARPES)synchrotron experiments,we investigate the systematic effect of the UV dose on the measured spectra.We observe the onset of a spuriousdose-dependent prominent peak at an energy range where the lower Hubbard band has been previously reportedin this compound,raising questions on its previous interpretation.By a careful analysis of the dose-dependenteffects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lowerHubbard band.We obtain the ARPES spectrum in the limit of a negligible concentration of vacancies,wherea clear signal of a lower Hubbard band remains.We support our study by means of state-of-the-art ab initiocalculations that include correlation effects and the presence of oxygen vacancies.Our results underscore therelevance of potential spurious states affecting ARPES experiments in correlated metals,which are associatedwith the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas atthe surface of insulating d0transition metal oxides.DOI:10.1103/PhysRevB.94.241110Introduction.A major challenge of modern physics is to understand the fascinating phenomena in strongly correlated transition metal oxides(TMOs),which emerge in the neighbor-hood of the Mott insulator state.Some preeminent examples that have gathered interest for almost30years are high temperature superconductivity,colossal magnetoresistance, heavy fermion physics,and,of course,the Mott metal-insulator transition itself[1].Significant theoretical progress was made with the introduction of dynamical meanfield theory (DMFT)and its combination with ab initio density functional methods[local density approximation(LDA)+DMFT],which allows treatment of the interactions promoting itinerancy and localization of electrons on equal footing[2–4].Among the most emblematic achievements of DMFT is the prediction of a Hubbard satellite,which separates from the conduction band of a metal.This satellite results from the partial localization of conduction electrons due to their mutual Coulomb repulsion. Early DMFT studies also showed that it is the precursor of the localized electronic states of a Mott insulator[5].Since then,these predictions promoted a large number of studies using photoemission spectroscopy,which is a technique to directly probe the presence of Hubbard bands.In this context, the TMO system SrVO3has emerged as the drosophila model system to test the predictions of strongly correlated electron *valenti@itp.uni-frankfurt.de†andres.santander@csnsm.in2p3.fr theories.In fact,SrVO3is arguably the simplest correlated metal.It is a simple cubic perovskite,with nominally one electron per V site,which occupies a threefold degenerate t2g conduction band.While the presence of a satellite in the photoemission spectra of Ni metal was already well known,in the context of correlated TMOs,the Hubbard band was originally reported in a systematic investigation of Ca1−x Sr x VO3[6],which was followed by many subsequent studies,including angle-resolved photoemission spectroscopy (ARPES)[7–9]and comparisons with theoretical predictions (see,for instance,Refs.[10–20],among others).One of the most salient features in SrVO3is the observation of a broad peak at an energy of about−1.5eV in angle integrated photoemission spectra(upper black curve in Fig.1), which is interpreted as a Hubbard satellite linked to the V t2g electrons.This feature is also seen in a large range of 3d1materials[21,22].The ratio of spectral strength between the quasiparticle(QP)state and the incoherent satellite in SrVO3is an important indicator of the magnitude of electron correlations[1,2].However,photoemission experiments using different photon energies or light brilliance have reported very dissimilar values for such a ratio[11],making the quantitative benchmarking of realistic ab initio theories for correlated electron systems difficult[6,11,18,23,24].Moreover,as shown in Fig.1,a broad peak at about the same energy is also observed in several d0TMO cubic perovskites,such as SrTiO3, KTaO3,or anatase TiO2.Nevertheless,in all these cases the feature has been clearly linked to the presence of oxygenS.BACKES et al.PHYSICAL REVIEW B94,241110(R)(2016)FIG.1.Integrated UV photoemission spectra for various per-ovskite oxides,showing a quasiparticle peak at E F and an in-gap state at energies between1and1.5eV.For SrVO3(upper black curve),a correlated-electron metal,the QP peak corresponds to the bulk conduction band,and as will be shown further,the in-gap state is a superposition of the lower Hubbard band and localized electronic states associated with oxygen vacancies.For the other d0 oxides,such as KTaO3(blue curve),anatase TiO2(green curve), or SrTiO3(red curve),the QP peak and in-gap state correspond respectively to a confined quasi-2D electron gas at the sample surface and to localized states,all formed by oxygen vacancies.The crystal orientation(normal to the samples’surface)is indicated in all cases. defects[25–32].Interestingly,recent ab initio calculations show that the spectral weight at−1.3eV in SrTiO3most likely is not of Ti t2g orbital character,but should be understood as an in-gap defect state with Ti e g character[33–36].Thus,we are confronted with the fact that at about1.5eV below the Fermi level(E F),wefind the lower Hubbard bands of d1systems as well as the in-gap states of oxygen-deficient d0systems.In view of these observations one may unavoidably wonder(and worry),despite the great success of DMFT methods,whether the putative Hubbard satellite of SrVO3might also originate from oxygen vacancy states.Moreover,one should also worry about the possibility of these extrinsic states affecting the features of the conduction band dispersion.In the present Rapid Communication we resolve these issues in a thorough manner.We present a systematic photoemission study of SrVO3,to demonstrate dramatic consequences in the spectra due to the creation of oxygen ing ARPES,we directly show that the UV or x-rays used for measurements can produce a large enhancement, of almost an order of magnitude,of the peak at−1.5eV, similar to the effect observed in d0oxide insulators[25–28,37].Despite these significant effects on the energy states around the Mott-Hubbard band,we are able to determine the bulk SrVO3photoemission spectrum in the limit of a negligible concentration of vacancies,where a clear signal of the dispersive correlated Hubbard band remains.We support the interpretation of the experimental data by means of state-of-the-art LDA+DMFT calculations on SrVO3with oxygen vacancies.Consistent with our experimental data,the calculations show that oxygen vacancies produce states(of e g symmetry)at energies near the Hubbard satellite.While our study provides definite evidence of a correlated Hubbard band in SrVO3as predicted by DMFT,it also underlines the significant effects due to oxygen vacancies,which may also affect photoemission data in other TMOs.Methods.The bulk-like relaxed,crystalline(001)oriented SrVO3thinfilms were grown by pulsed laser deposition (PLD)either at the GEMaC laboratory,then measured at the CASSIOPEE beamline of Synchrotron SOLEIL,or in a PLD chamber directly connected to the ARPES setup at beamline2A of KEK-Photon Factory(KEK-PF)[9,38,39]. To clean the surfaces in UHV prior to ARPES experiments at SOLEIL,the SrVO3thinfilms were annealed at550◦C for t=5–20min at pressures lower than2×10−8Torr.At KEK-PF,the PLD growth was performed under a pressure below10−7Torr,to obtain UHV-clean surfaces,using a Sr2V2O7target,which has excess oxygen with respect to SrVO3,thus minimizing the formation of vacancies during the growth.In all cases,the surface quality was confirmed right before ARPES measurements by low-energy electron diffraction(LEED).The thinfilms measured at KEK-PF showed a c(4×4)surface reconstruction,which does not affect the analysis and conclusions of this work.For the ARPES measurements we used linearly polarized photons in the energy range30–110eV and hemispherical electron analyzers with vertical slits at SOLEIL and horizontal slits at KEK-PF.The angular and energy resolutions were0.25◦and 15meV.The mean diameter of the incident photon beam was smaller than100μm.The UV light brilliance,measured using calibrated photodiodes,was≈5×109photons s−1μm−2at SOLEIL,and about100times smaller at KEK-PF.The samples were cooled down to T=20K before measuring.Unless specified otherwise,all data were taken at that temperature. The results were reproduced on more thanfive samples.All through this Rapid Communication,directions and planes are defined in the cubic unit cell of SrVO3.We denote [hkl]the crystallographic directions in real space, hkl the corresponding directions in reciprocal space,and(hkl)the planes orthogonal to those directions.The indices h,k,and l of hkl correspond to the reciprocal lattice vectors of the cubic unit cell of SrVO3.The Supplemental Material[40]presents further details about the sample growth and measurements.Experimental results.Figure2(a)shows the integrated photoemission spectra of SrVO3as a function of the UV dose, measured at CASSIOPEE SOLEIL under the same conditions of light brilliance of any standard ARPES experiment at a third-generation synchrotron.The measurements were done by continuously irradiating the sample with hν=33eV photons while recording the spectra as a function of irradiation time, with an accumulation time of about2min per spectrum.The blue and black curves show spectra for the lowest and highest measured doses,obtained respectively after∼2min and∼2h of irradiation.These data clearly demonstrate that the very UV or x-rays used for photoemission experiments can produce radical changes in the measured spectra of SrVO3.Note in fact that a similar effect has been observed for VO2[41].In particular,from Fig.2(a)we observe that the amplitude ofHUBBARD BAND VERSUS OXYGEN V ACANCY STATES IN THE...PHYSICAL REVIEW B94,241110(R)(2016)FIG.2.(a)Photoemission spectra of SrVO3as a function of UV dose,measured at Synchrotron SOLEIL(hν=33eV).The energy distribution curves(EDCs)were extracted from raw ARPES data around the 002point integrated along the k= 010 direction.(b)Corresponding momentum distribution curves(MDCs)integrated over50meV below E F.Peaks in the MDCs indicate the Fermi momenta.(c),(d)Same as(a),(b)for SrTiO3(hν=47eV).Thefilling of a2DEG upon UV irradiation is evidenced by the formation of QP peaks in the EDCs and MDCs at E F[inset of(c)and(d),respectively].All data were taken at20K.the in-gap state at−1.5eV,and,more significantly,the ratio of in-gap to quasiparticle(QP)amplitudes,strongly increase with increasing UV dose,going from about1:3in a pristine sample to more than2:1in a heavily irradiated sample. Importantly,note that the QP peak position remains basically dose independent,implying that the carriers created by the UV or x irradiation do not significantly dope the conduction band,and form dominantly localized states.This is confirmed in Fig.2(b),which shows that the Fermi momenta of the QP band,given by the peak positions in the momentum distribution curves(MDCs)at E F,are also dose independent. Additional data presented in the Supplemental Material further demonstrate that our measurements yield the expected3D bulk Fermi surface of SrVO3.Thus,the observed increase in intensity of the in-gap state upon UV irradiation cannot be ascribed to a change infilling of the conduction band,which could have affected the electron correlations.Instead,this unambiguously shows the light-assisted formation of localized defect states at essentially the same energy as that of the expected intrinsic lower Hubbard band—which should then resemble the in-gap peak observed at the lowest UV doses.In fact,as mentioned previously,it is well established that strong doses of UV or x-rays create a large concentration of oxygen vacancies in several d0perovskites[25–32,42]. As illustrated in Figs.2(c)and2(d)for the case of SrTiO3, the progressive doping of the surface region with oxygen vacancies,due to synchrotron UV irradiation,has two effects: the formation of a very intense in-gap state at about−1.3eV, and,in contrast to SrVO3,the simultaneous creation of a sharp QP peak at E F corresponding to a confined quasi-2D electron gas(2DEG)at the samples’surface.The effective mass of such2DEG,precisely determined by ARPES,matches the mass expected from density functional theory calcula-tions[25,26,43,44].Thus,as in SrVO3,the increase in intensityof the in-gap state observed in SrTiO3upon UV or x irradiationcannot be due to an onset or increase of electron correlations,and should be ascribed to an extrinsic effect.We therefore conclude that,in SrVO3,exposure to syn-chrotron UV or x-rays creates oxygen vacancies,which are inturn responsible for the extrinsic increase in intensity of thein-gap state evidenced by our measurements.This effect canseriously obscure the determination of the spectral function ofthis model system,thus hampering the advancement of validtheories for correlated electron systems.Thefindings described above imply that the correct ex-perimental determination of the vacancy-free photoemissionspectrum of SrVO3should(i)use samples that from thebeginning have the lowest possible concentration of oxygenvacancies,and(ii)use doses of UV or x-ray light low enoughto avoid light-induced changes in the measured spectra.Tothis end,we measured SrVO3thinfilms grown directly insitu at beamline2A of KEK-PF.As mentioned before,the growth protocol of such thinfilms minimizes the formationof vacancies,while the UV light brilliance at KEK-PF is ∼100times smaller than the one in Figs.2(a)and2(b)from measurements at SOLEIL.We checked(see the SupplementalMaterial)that under these conditions the spectra did notchange with time,even after several hours of measurements.The resulting energy-momentum ARPES map,and its secondderivative,are presented in Figs.3(a)and3(b).One clearlyobserves the dispersing QP band along with an also dispersivein-gap state of weaker intensity,corresponding to the intrinsiclower Hubbard band,as reported in previous works[7].Theintrinsic spectral function of SrVO3will then be given by such aphotoemission spectrum,which approaches the vacancy-freelimit,modulo dipole-transition matrix elements,inherent toS.BACKES et al.PHYSICAL REVIEW B94,241110(R)(2016)FIG.3.(a)Energy-momentum ARPES intensity map measured at KEK-PF with a low UV dose on a SrVO3sample prepared in situ,using a well-established protocol to minimize the formation of oxygen vacancies(see the main text and Supplemental Material).Note that due to the choice of light polarization,the heavy bands along(100)are not observed and only the contribution of the light d xy band is detected.The data were acquired at hν=88eV around ¯103.(b)Second derivative(negative values)of the map in(a).The use of second derivatives allows a better visualization of the dispersion of both the quasiparticle and Mott-Hubbard bands on the same color plot.The dispersionless feature at E F is a spurious effect of such a second derivative on the Fermi-Dirac cutoff.(c),(d)Same as(a),(b)after a strong UV irradiation dose,measured at SOLEIL(hν=33eV),typical of modern third-generation synchrotrons.The measurements were done at hν=33eV close to 002.All data were taken at20K.Note that at constant photon energy,ARPES maps out the electronic structure at a spherical surface of three-dimensional (3D)k space,which can be locally approximated to a plane for all our measurements(details in the Supplemental Material).The different choice of photon energies and k-space positions for measurements at KEK-PF[(a)and(b)]and SOLEIL[(c)and(d)]was dictated by the different geometrical configurations and constraints of the beamlines in both synchrotrons.the photoemission process,which can still modulate the intensity of the QP peak relative to the Hubbard peak.A calculation of such matrix elements requires a full one-step calculation of the photoemission process,which is beyond the scope of this work.By contrast,Figs.3(c)and3(d) show the momentum-resolved electronic structure of a sample, measured at SOLEIL,that was intensively irradiated.There, the peak at−1.5eV becomes broader,more intense,and nondispersive—all characteristic signatures of a high random concentration of oxygen vacancies.Numerical calculations.To rationalize from a microscopic point of view the influence of oxygen vacancies on the electronic structure of SrVO3,we performed charge self-consistent LDA+DMFT calculations for bulk SrVO3and var-ious relaxed oxygen-deficient SrVO3supercells.The latter are computationally demanding calculations.We shall focus here on the case of a2×2×3supercell with two oxygen vacancies located at opposite apical sites of one vanadium atom,as shown in the inset of Fig.4(b).We use such a vacancy arrangement as it is the prototypical one for d0compounds[43].For our LDA+DMFT calculations we chose values of U= 2.5eV and J=0.6eV for vanadium and included the effects of bandwidth renormalization due to dynamically screened Coulomb interactions by following the prescription suggested in Ref.[45](the LDA+DMFT unrenormalized data are shown in the Supplemental Material).In Figs.4(a)and4(c)we show, respectively,the results of the k-integrated and k-resolved spectral functions for bulk SrVO3without oxygen vacancies. Wefind the expected features of a t2g quasiparticle peak at the Fermi level and a lower Hubbard band at negative energies of the same t2g nature,in agreement with the photoemission spectra in Fig.2(a)and Figs.3(a)and3(b).The light band at E F along k 100 [Fig.4(c)]consists of two degenerate bands of d xy and d xz characters,while the heavy band along the same direction has d yz character.While comparing with the measured k-resolved spectral function[Figs.3(a)and3(b)],HUBBARD BAND VERSUS OXYGEN V ACANCY STATES IN THE...PHYSICAL REVIEW B94,241110(R)(2016)FIG.4.LDA+DMFT results for SrVO3including bandwidth renormalization effects[45].(a)k-integrated spectral function for bulk SrVO3.The V t2g orbitals show a quasiparticle peak at E F and a lower Hubbard band at−1.6eV.(b)Spectral function for the2×2×3supercell of SrVO3with two oxygen vacancies.An additional nondispersive V e g vacancy state originating from the V atom neighboring the oxygen vacancies leads to a sharp peak below the Fermi level at∼−1.0eV.The V t2g orbitals show a quasiparticle peak at E F and a lower Hubbard band at−1.8eV.(c)and(d)show the corresponding spectral functions(multiplied by a Fermi-Dirac function at20K)along the X- -X path.one should bear in mind that along -X(or -Y)the heavy d yz(or d xz)bands are silenced by dipole-transition selection rules in the experiment[25].Inclusion of bandwidth renormalization[45]renders the lower Hubbard band at an energy(−1.6eV)in reasonable agreement with experiment (−1.5eV).We adopted typical values for U and J from the literature.We did not attempt to further optimize the values to get a better quantitative agreement with the experimental data, for two reasons:First is the heavy numerical cost,and second, as we show next in the calculations with oxygen vacancies, the adopted values facilitate the distinct visualization of the contributions from the Hubbard and localized states to the incoherent peak at∼−1.5eV.The removal of oxygen atoms in the system leads to the donation of two electrons per oxygen to its surrounding. Already at the level of density functional theory(DFT)in the local density approximation(LDA)(see the Supplemental Material),wefind that most of the charge coming from the additional electrons is transferred to the3d z2orbitals of the neighboring V atom,developing into a sharp peak of e g symmetry located around−1.0eV,i.e.,at an energy close to the position of the experimentally observed oxygen vacancy states.In analogy to the experimental average over many lattice sites,note that averaging among various supercells with differentoxygen vacancy locations and concentrations(which is beyondthe scope of the present work)would result in a wider in-gape g band,as demonstrated for the case of SrTiO3(see Fig.3ofRef.[34])and for some cases in SrVO3(see the SupplementalMaterial,Fig.S7).By including electronic correlations within(bandwidth renormalized)LDA+DMFT we then see that allthe experimental observations qualitatively emerge.In fact,the conducting t2g orbitals develop a lower Hubbard bandpeaked at energies about−1.8eV[Figs.4(b)and4(d)],similarto the bulk case without oxygen vacancies.Most notably,this lower Hubbard satellite does not increase in amplitudewith the introduction of vacancies,but rather broadens.Inaddition,the oxygen vacancy defect states situated at about −1eV remain qualitatively unchanged by the correlation effects,but experience a broadening with respect to the pureLDA case.This is in agreement with the photoemission data,evidencing that the increase in intensity of the in-gap statein the oxygen-deficient SrVO3is not to be attributed to anincrease in population of the lower Hubbard satellite,butinstead to the manifestation of vacancy states of e g character.Conclusions.In summary,we performed a detailed study of the effects of oxygen vacancies in the spectroscopy of the archetypal strongly correlated electron system SrVO3.We found that oxygen vacancy states,which are created by UV or x-ray irradiation,occur at energies close to the Hubbard satellite.This dramatically affects the measured line shape of the Mott-Hubbard band and the ratio of intensities between the quasiparticle and the Mott-Hubbard peaks.By means of a systematic study under a controlled irradiation dose, using samples directly grown in situ,we were able to obtain the photoemission spectrum of the bulk SrVO3system in the limit of a negligible concentration of oxygen vacancies. 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