| Since Onnes H.K.found that Hg displays superconductivity at 4.2 K,people have been working hard on superconductivity for more than 100 years.It has strongly promoted the development of the correlated electron materials and practical application,which has given five chances for Nobel prizes.Although there have been many achievemnets in superconductors,such as,establishing the BCS superconducting microscopic theory,searching for higher critical temperature superconductor,application of superconductor in the field of energy,medicine,communication,traffic and defence-related science and technology,the more universal superconducting theory and the room temperature superconductor are still the final pursuits of all the researchers.As the most typical macroscopic phenomenon,superconductivity has been one of the research hotspots for more than one hundred years,because of the unique properties of electricity and magnetism,which has a high scientific value and application prospect.For now,if pursuing a higher critical temperature,we must work on polycompound.However,there are two big challenges in the superconducting research.First,a random permutation and combination of six chemical elements generates a huge number of possibilities.Second,even if the elements are the same,the varying stoichiometry will also change the property dramatically,which makes the superconducting phase diagrams display the characteristic of variability and multiformity.It will spend too much time to cover the whole database by trying out all the cases one by one.Therefore,these challenges can be concluded that,due to the lack of the big database for the polycompounds and the variability and multiformity of superconducting phase diagram,the cognition of superconductor still stays on the qualitative level,indicating the traditional superconducting research model is in dire need of adjustment.The rise of Materials Genome Initiative gives a new opportunity for the research of key materials,of which core is High-throughput technology.High-throughput technology is aimed at accelerating the process of the sample fabrications,property characterizations and establishing material phase-diagram database,which can expose the key rule.As the combination of Materials Genome technology and superconductor,a new high-throughput superconducting researching model will be born.The iron-base and the cuprate superconductors have played an important role on the field of superconductivity,which keep the second highest and the highest critical temperature record respectively.They have not only a huge scientific value on the mechanism investigation,but also the application prospect.In iron-based superconductors,FeSe possesses the simplest chemical composition and crystal structure but with the most complex properties,which can be regarded as the ideal platform for mechanism investigation.The critical temperature of FeSe can be adjusted from<2 K to 50 K.Investigating what happen during this process will give some key hints for the understanding of iron-base superconducting mechanism.For the high temperature cuprate superconductor,electron-doped cuprates have a more concise phase diagram and lower upper critical field than the hole-doped,which is beneficial for mechanism study.Among the electron-doped cuprates,only the La2-xCexCuO4±?system have the enough chemical stability to allow it covering the whole range from the optimal doping to heavy over doping regions,which is the ideal object for high-throughput research,above all the quantum critical phenomenon.Therefore,we choose the FeSe and La2-xCexCuO4±?system as the primary researching object to perform the high-throughput study,in order to establish material phase-diagram database and expose the key rule fast.Through combining the pulsed laser deposition,laser molecular beam epitaxy and high-throughput thin film technology,we have strived to develop the high-throughput superconducting researching paradigm and have made some important achievements on the FeSe and La2-xCexCuO4±?system respectively.1)FeSe thin film fabricationsUniform FeSe film fabrications:Based on the experience of more than 1500sample depositions,the high quality of FeSe thin films can be controlled precisely.The zero resistivity superconducting transition critical temperature can be manipulated from<2 K to 14 K through adjusting the thin films preparation technology,including:1)substrates,2)thickness,3)stoichiometry of the FeSe target,4)laser fluence and 5)off-stoichiometry of target surface induced from laser ablation.According to the data of more than 1500 FeSe thin films,Tc displays a positive correlation with lattice constant c and RRR respectively,which have a big error bar and only give a qualitative result.In general,the differences of lattice constant and RRR may be resulted from the off-stoichiometry of Fe:Se ratio,but the off-stoichiometry is too tiny to be distinguished by the existing analytical measurement of chemical composition,which indicating that the off-stoichiometry should be less than 1%.Those high quality FeSe thin films can be an important support of the in-depth physical property research and applications.Tc gradient FeSe high-throughput film fabrications:By inventing a combined laser technology,we successfully fabricate FeSe high-throughput films with a Tc gradient,i.e.a combinatorial material library with Tc gradient.On the same one combinatorial FeSe thin film,the samples from different regions are fabricated with the same condition and environment background,which can suppress the experimental error maximally.Benefited by the high veracity of high-throughput method,the FeSe high-throughput films with Tc gradient expose the accurate correlation among c axis,RRR and Tc,which is clearer than the result of uniform films.Previously,by uniform films,this work must bases on more than 1500 samples and takes 3 years,but still do not give an accurate experimental result.For now,thanks to Tc gradient FeSe high-throughput films,this work only needs one time of sample parallel synthesis and high-throughput measurement,and the whole process just spends one week.Moreover,it can give a more accurate result.It gives a full expression that how powerful is high-throughput superconducting researching method.Substrate temperature gradient FeSe high-throughput film fabrications:By inducing a gradient distribution of temperature to the substrate,we successfully fabricate FeSe high-throughput films with a deposition temperatures gradient.It not only increases the efficiency,but also gives a more accurate result,which can provide a new idea for film fabricating.Thickness gradient FeSe high-throughput film fabrications:Using a combined mask technology,we successfully fabricate FeSe high-throughput films with a thickness gradient.High-throughput superconducting research can give a strict control of single variable,of which data are much higher comparable.2)FeSe thin film property studyNormal resistivity study:We have performed temperature dependence of resistance measurement for different FeSe thin films,and have obtained the resistivity and Tc.The dR/dT data tell us that the resistances,from which films have various Tc values,display different temperature dependence.These variant behaviors of temperature dependence may link to some electronic states or orders.Clarifying the correlation between these orders with superconductivity should give some important information for mechanism study.In addition,the temperature dependence of resistance under various fields can give the anisotropic upper critical fields,which is benefitial for practical application.Magnetoresistance and Hall resistance:We have performed magneto resistance and Hall resistance measurements for different FeSe thin films,and have obtained the almost perfect data.MR is in proportion to B2,and the Hall constants are independent to B but vary at different temperature,which indicating that FeSe is a multi-band system.In-plane magnetoresistance:First,the in-plane angular dependent magneto resistances display different two-fold symmetries in superconducting state and normal state respectively,which may correlate to the nematicity or structure transition and deserve for in-depth study.Second,the in-plane magneto resistances possess a positive to negative transition with the increasing of temperature,and this behavior is much distinct to the out-plane magneto resistance,which may correlate to the nature of spin and deserve for in-depth study too.The relation among electronic structure,crystal structure and critical temperature:Based on the uniform FeSe films,we obtain the correct correlation of the electronic structure property and critical temperature.Above 120 K,the Hall coefficients of various samples almost overlap with each other,which indicating that their doping effects are very tiny.The Hall coefficients for all the samples change their sign from negative to positive at 120 K.Under 120 K,the Hall coefficients for various samples are pushed down with the increasing of critical temperature.Thus,we conclude that all the samples here exhibit electronic structure that is qualitatively similar above 120 K,but differs considerably below 120 K,which results from the band splitting and indicating all the samples have a same band splitting temperature.The angle resolved photoemission spectroscopy measurement gives a preliminary result of the electronic structure of FeSe thin films.Combining with the simultaneous fitting of magneto resistance and Hall resistance,we obtain the concrete values of hole and electron carriers for various samples and different temperatures.With the increasing of critical temperature,the Fermi level has no obvious filling effect,and the hole carrier density still remain the same,but the electron carrier density gradually rise and surpass the hole,which variation is as high as 7 times.This result cannot be completely explained by the simple electron filling picture,and all samples have no obvious doping effect.In order to understand this nature,we change the focus to the crystal structure.Although the uniform FeSe films only give the qualitative correlations among lattice constants a,c,c/a and critical temperature,but the high-throughput films can provide a more accurate and quantitative result.Following the increasing of critical temperature,the c-axis parameter gradually expands from5.51 to 5.57?,whereas the a-axis parameter gradually shrinks from 3.78 to 3.73?.Combining the data of uniform films and high-throughput films,we have successfully established the explicit database of electronic structure,crystal structure and critical temperature for FeSe thin films.The band structures of FeSe by first-principle calculation give the reasonable findings,which obtained from three sets of lattice parameters.With decreasing a-axis parameter and increasing c-axis parameter,the most noticeable change in the electronic structures takes place in the dxy band,the dxyy band shifts up in energy around?and shift down around M,while dxz/dyz bands exhibit little change.However,dxy band is only across the Fermi surface at the M point and sink down below the Fermi surface at?,so the change of dxy band just influence the electron carriers.Such orbital selectivity can reasonably explain the variation of electronic structure as changing the critical temperature.On the other hand,the increase of the dxy pocket at the Fermi level from the downshift of dxy band was found to decrease the nematicity.We further investigated the microstructure of the FeSe films by transmission electron microscopy,which told us that local lattice distortions by the impurity domains bring stress over the whole sample and modify the crystal lattice,which will change the band structure.Reducing the number of these domains will suppress the nematicity and enhance the superconductivity.3)La2-xCexCuO4±?thin films fabricationsThe cuprate high temperature superconductor has kept the highest critical temperature record for several decades.It always is a researching hotspot at the superconducting researching field,which includes hole-doping and electron-doping as two types.Compared to hole-doping cuprate,the electron-doping superconductor has a more concise phase diagram and lower upper critical field,which makes it become the ideal platform for mechanism investigation.In electron-doped cuprates,La2-xCexCuO4±?system can provide various thin films,which can cover the whole range from the optimal dope to heavy over dope.Therefore,La2-xCexCuO4±?is the ideal object for high-throughput research,above all the quantum critical phenomenon.Single-component La2-xCexCuO4±?films fabrications:By using laser molecular beam epitaxy technology,a series of La2-xCexCuO4±?single-component films,which possess a more high quality,are successfully obtained.During the film depositions,several fabricating factors are carefully optimized,such as,the perfect ratio of rare earth elements to Cu,the most appropriate oxygen content and the most sufficient atomic position relaxation,which makes sure that the La2-xCex CuO4±?thin films have a dense lattice structure and their superconductivity are independent to the following vacuum annealing time.Such high-precision fabricating control will be a powerful support to the next high-throughput films fabrications.Ce doping gradient La2-xCexCuO4±?high-throughput films fabrications:By using combinatorial mask technology,a series of high quality La2-x-x CexCuO4±?high-throughput films,which have a Ce doping gradient distribution,are successfully fabricated.Previously,by single-component films,establishing a superconducting phase diagram must base on 10 different Ce doping components,which needs more than hundreds of sample fabrications and spends 2 years,but still do not gives an accurate experimental result.For now,thanks to these Ce doping gradient La2-xCexCuO4±?high-throughput films,this work only need one time of sample parallel synthesis and high-throughput measurement,and the whole process just spend one or two month.Furthermore,as long as the probe sizes less than 2?m,the distinguishability of the Ce doping can precede 0.02%,which can be a powerful support to quantum critical phenomenon research.It gives a full expression that how powerful is high-throughput superconducting researching method. |
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