SDW Induced Charge Stripe Structure in FeTeDepartment of Applied Physics, Hokkaido UniversityY. Kawashima, K.Ichimura, J. Ishioka, K. Yamaya, S. TandaDepartment of Physics, Hokkaido UniversityT. Kurosawa, M. OdaThank you very much for giving me an opportunity to talk about our recent work.The title is SDW induced Charge Stripe Structure in FeTe.I am Yuuki Kawashima from Hokkaido University, Japan.This work is studied by this members.1Stripe StructureUsually occur with anisotropic interactionLong rangeattractive interaction Short range repulsive interactionEx)Dipole-Dipole interactionStripe structure usually occurs with anisotropic interaction such as dipole-dipole interaction.When the anisotropic interaction consists of a long range attractive interaction and a short range repulsive interaction, the Stripe structure occurs like this image.Connect along attractive interaction direction and separate along repulsive interaction direction.Dipole-Dipole interaction is example of this type of anisotropic interaction.2

W. F. Wang et. al. New J. Phys. 11 045003(2009)Iron-based Superconductors

LaFePO(1111)Fe-PLa-OCoexistence of magnetism and superconductivityNew Type Strong correlated systemIron-based superconductors is new type of strong correlated systems.The main characteristic of the iron-based superconductor is coexistence of magnetism and superconductivity.In iron-based superconductors, SDW state and SC state coexist.The crystal structure is two dimensional. So Fermi surface is two dimensional.Two dimensional Fermi surface cause SDW by Fermi surface nesting .3Magnets and Iron-based SuperconductorsMagnetIron-based Superconductorex)Fe, Ni, Coex)LaFePO,BaFe2As2Main CharacterSpinMain CharacterChargeSpin-Spin InteractionStrongWeakNew Phase?Spin and ChargeSpin Density Wave(SDW)

Investigate the new electronic stateOur PurposerspinI sort the magnets and iron-based superconductors by view point of spin-spin interaction.By this view point, I can consider the new phase between magnets and iron-based superconductors.The ground state of the magnets is ferromagnetic or antiferromagnetic.The main characteristic of magnet consist of a spin element of a electron.The spin-spin interaction in the magnet is strong.By contrast, if the strong spin-spin interaction on the magnet disapper, that material can show superconductivity.In this point, Iron-based superconductor is weak spin-spin interaction magnet.Then the attractive interaction of cooper pairs are formed by a charge element of electron.From the point of view of spin-spin interaction, we think that there is new phase.The main characteristic of new phase is both of a spin element and a charge element.We look on this new phase.

It is said that SDW occur at the non-superconducting phase.The spin structure is formed by SDW.The ground state related with charge is not known.4

FeTeInvestigate the new electronic state by using FeTeMethodFeTeabc

abSDW transition was suggestedA. Subedi et. al, PRB, 78, 134514 (2008)CleavedTo study the new phase, FeTe is good material.Because FeTe has no blocking layer, so FeTe is simplest crystal structure among iron-based superconductors.FeTe cleave easily at here. This is important point at surface sensibility measurement like STM.FeTe show high pressure effect. It is good for control electron property without doping.FeTe have many advantages on experiment.We use FeTe to investigate the electron state of the new phase.5

D(r,eV):Density of State of sampleSample was cleaved in ultra high vacuumTemperature 6.8KPressure 10-8PaScanning Tunneling Microscopy/Spectroscopy(STM/STS)LT-UHV-STM/STS

Feedback CircuitController

DoSDoSFF+VbiasVbiasEe-VItipsampletipsample6This is a picture of scanning tunneling microscopy.STM use electron tunneling. So STM have atomic resolution.Tunneling current between tip and sample is proportional to integral of density of state of sample.Differential conductance is proportional to density of state of sample.Then STM has atomic resolution and high energy resolution.We use low temperature ultra high vacuum STM.The instrument spec is this.Sample was cleaved in ultra high vacuum and measured at clean surface.6ExperimentsSample preparationSample was prepared by chemical vapor transport method using I2nominal ratio Fe:Te=1:0.9Put In evacuated quartz tube and keep 700 for one week.The sample was evaluated by energy dispersive X-ray spectroscopy.

Prepared sample1mmGood single crystalline sample was obtainedFirstly, We made FeTe single crystalline sample by chemical vapor transport method using iodine.The Fe, Te and iodine put in evacuated quartz tube and keep 700 degree of Celsius for one week.The sample was evaluated by energy dispersive X-ray spectroscopy.We obtained good single crystalline samples.7Electrical property measurement3He cryostat systemT = 0.5K300K DC four probes methodResistivity measurementMagnetization measurementSQUID Magnetometer T = 2 K300 KWe measure resistivity and magnetization of sample for measuring electrical property.He3 cryostat system was used on resistivity measurement.The resistivity was measured by DC 4 probe method.The magnetization was measured by SQUID magnetometer.8

58KSQUIDH = 0.5TAFM transition at 58K

58KDC four probes Heatinganomaly at 58KSDW transitionResult of Resistivity and magetization measurementThis is experimental result of resistivity and magnetization measurements.Left figure show result of magnetic susceptibility dependence on temperature.Horizontal axis shows temperature. Vertical axis shows magnetic susceptibility.The magnetic susceptibility rapid decrease under 58K.By magnetization measurement, FeTe have antiferromagnetic transition.Right figure show result of resistivity dependence on temperature.The resistivity slightly increase from room temperature to 58K.The resistivity decrease at 58K and increase again.By resistivity measurement, FeTe show resistivity anomaly at 58K.These measurement show SDW transition at 58K.

Next, We show STM result under transition temperature.9STM Experimental Result (T=7.8K)3.8Te atom

Vbias: 0.9V Itunnel: 0.7nA Current image1nmDiscovery of charge stripe structure3.8

CleavedcSTM experimental result on 7.8K.This figure is STM current image at T=7.8K.Red circle form lattice on the image.The lattice length correspond to length between tellurium atoms.The side of lattice is 0.38nm.This image see at tellurium layer and from c-axis.There are line structures indicated by blue line.The lines form a stripe structure on the image.We discover charge stripe structure on FeTe at 7.8K10

AB

A0123[nm]

B0123[nm]

ABWe can see the iron layer under the tellurium layer.TeFeAnalyzing Stripe StructureTo analyze the stripe structure detail.We take line profiles.Left below line profile is taken along A line indicated by blue line.Red arrows peaks correspond to tellurium atoms.There are other peaks indicated by blue arrows between red arrows.These peaks correspond to Fe atoms.Right figure shows view from c-axis.The line profile is taken along this A lines.Right below line profile is taken along B line indicated by red line on the figure.There are only iron atoms peaks.We can see the iron layer under the tellurium layer.11

C012[nm]

CCTwo types of iron atoms form charge stripe structureAnalyzing Stripe Structure2We take the line profile along the line C indicated by yellow line.This line profile is taken along iron sites.The line profile show that there are two type of iron sites.The stripe structure occurred by two different iron sites.12STS Experimental Result (T=7.8K)SDW gap structureTSDW~58K

DSDW=9meV

We also measure tunnel differential conductance.Horizontal axis is bias voltage.Vertical axis is tunnel differential conductance.The differential conductance is proportional to DOS.There are kink structure at +-10meV indicated by blue arrows.The tunnel differential conductance around 0mV is almost zero. Then the kink structure is gap structure.The gap structure is same as expectation from 58K at mean field approximation.Then the gap structure is caused by SDW transition.We observed SDW gap on STS measurement.13The model of SDW induced charge stripe structureWhen SDW was formed on iron layer.nesting vector[110]

Ferromagnetic structureAntiferromagnetic structureAntiferromagnetic and ferromagnetic direction by SDW.

Crystal Structure: 4-fold rotationSpin Structure: 2-fold rotationWe propose the model of SDW induced charge stripe structure.This picture shows the spin structure when the SDW occur on the iron layer.SDW nesting vector is [110].When the SDW occur on the iron layer, the spin structure form 2-fold rotational symmetry.2-fold rotational symmetry is lower symmetry than the crystal symmetry which is 4-fold rotation symmetry.The SDW cause symmetry breaking on spin structure.14Other Electrons have strong coulomb interaction+Reducing rotation symmetry caused by SDW

SDW induced charge stripe structureReduce off-site coulomb interactionThere are other Fermi surface which does not join on the SDW.Because of the two dimensional Fermi surface, nesting on the FeTe is incomplete.Other electrons have strong coulomb interaction.If the other electrons form a stripe structure with spin rotational symmetry, these electrons reduce the off-site coulomb interaction.In this model, the stripe structure can reduce the off-site coulomb interaction and it is caused by spin rotational symmetry breaking by SDW.15

SDWCharge StripeThe image of our modelThis picture show result of our model.SDW occur on this direction and charge stripe occur this direction.

16Charge Order and Charge Stripe at Strong correlated systemCharge order in organic conductorsH. Seo, JPSJ, 69,No. 3,805(2000)Strong coulomb interaction relate to charge stripe structure.J. M. Tranquada et. al, Nature,375, 561 (1995)

Charge stripe in HTSC

Anisotropic structure & Charge localizeMott-insulator Base StripeThere are charge orders and charge stripes structure on organic conductors and cuprate superconductors.Left picture is image of charge order in organic conductors.Charges are rich at the large circle position and poor at other positions.The charges form stripe structure in the picture.Right picture is image of charge stripe in cuprate superconductors.There are hole at the black circle positions and electron at white circle position. ( point the position of charges)The holes form stripe structure in the picture.

The charge stripe structure is occurred at strong correlative materials.Recently, the new type of strong correlative materials discovered named iron-based superconductors.17Strong Correlative SystemsOrganic ConductorsLow transfer energyAnisotropic crystal structureCuprate SuperconductorsHigh on-site coulomb energyIsotropic crystal structure Mott-insulator base(half-filled)Iron-based SuperconductorsHigh off-site coulomb energyIsotropic crystal structure(Semi)Metal base

New E-Crystal MX1We sort the strong correlative system with there characteristics.Characteristics of Organic conductor are low transfer energy and anisotropic crystal structure.The charge order in organic conductor is occurred by anisotropic crystal structure and charge localization.Characteristics of cuprate superconductors are high on-site coulomb energy and isotropic crystal structure.The filling of cuprate superconductors is half filled, then, cuprate superconductors are mott-insulator base materials.The charge stripe occur on the mott-insulator back ground and strong relate with back ground.Characteristics of Iron-based superconductors are high off-site coulomb energy and isotropic crystal structure.Iron-based superconductors base on metal.The charge stripe occur by coulomb interaction on the metal.We discover new type charge stripe based on the metallic material.These three materials have different characteristics.

18How do we form the stripe structure by isotropic interaction without structural anisotropy?

Isotropic repulsive interaction particle form stripe structure on simulation.G. Malescio and G. Pellicane, Nature Materials, 2, 97 (2003)

Can Charges form stripe structure only themselves?In previous slide, I talked a stripe structure is usually formed by an anisotropic interaction.How about an isotropic interaction such as coulomb interaction between charges?If there are structural anisotropy like organic conductors, charges can form the stripe structure.Then, if there are not structural anisotropy, it is not understood yet.Our motivation is Charges can form stripe structure without structural anisotropy.Theoretically, it is said that an isotropic repulsive interaction form a stripe structure by simulation.Lets consider about a charge stripe structure on experiments.19SummaryMake single crystalline sample of FeTeDiscover the charge stripe structure on FeTe by STMPropose the model of SDW induced charge stripe structureResults and DisscusionThank you very much for your attention.MX1 series is new materials of two dimensional crystal.20