1. SDW Induced Charge Stripe Structure in FeTe
Department of Applied Physics, Hokkaido University
Y. Kawashima, K.Ichimura, J. Ishioka, K. Yamaya, S. Tanda
Department of Physics, Hokkaido University
T. Kurosawa, M. Oda
Thank 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.

2. Stripe Structure Usually occur with anisotropic interaction Long range
attractive interaction
Stripe 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.
Short range repulsive interaction
Ex)Dipole-Dipole interaction

3. Iron-based Superconductors
New Type Strong correlated system
Iron-based Superconductors
LaFePO(1111)
Fe-P
La-O
Iron-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 .
W. F. Wang et. al. New J. Phys (2009)
Coexistence of magnetism and superconductivity

4. Magnet New Phase? Magnets and Iron-based Superconductors Iron-based
ex)Fe, Ni, Co
Spin and Charge
ex)LaFePO,BaFe2As2
Spin Density Wave(SDW)
Main Character
Spin
rspin
Main Character
Charge
I 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.
Strong
Weak
Spin-Spin Interaction
Investigate the new electronic state
Our Purpose

5. FeTe SDW transition was suggested Methodb Fe Te
Cleaved a b c
To 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.
SDW transition was suggested
A. Subedi et. al, PRB, 78, (2008)
Investigate the new electronic state by using FeTe
Method

6. e- V I LT-UHV-STM/STS Sample was cleaved in ultra high vacuum
Scanning Tunneling Microscopy/Spectroscopy(STM/STS)
tip
sample
Feedback Circuit
Controller
DoS εF
εF+Vbias
Vbias E e-
tip V I
sample
This 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.
D(r,eV):Density of State of sample
LT-UHV-STM/STS
Temperature 6.8K　　Pressure 10-8Pa
Sample was cleaved in ultra high vacuum 6

7. Experiments Prepared sample Sample preparation
Sample was prepared by chemical vapor transport method using I2
nominal ratio Fe:Te=1:0.9
Put In evacuated quartz tube and keep 700℃ for one week.
The sample was evaluated by energy dispersive X-ray spectroscopy.
Prepared sample
Firstly, 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.
1mm
Good single crystalline sample was obtained

8. Electrical property measurement
3He cryostat system
T = 0.5K～300K DC four probes method
Resistivity measurement
Magnetization measurement
SQUID Magnetometer T = 2 K～300 K
We 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.

9. SDW transition DC four probes Heating anomaly at 58K SQUID H = 0.5T
Result of Resistivity and magetization measurement
58K
58K
DC four probes Heating
anomaly at 58K
SQUID　H = 0.5T
This 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.
AFM transition at 58K
SDW transition

10. STM Experimental Result (T=7.8K)
Vbias: 0.9V Itunnel: 0.7nA Current image
1nm
Cleaved c
3.8Å
Te atom
3.8Å
STM 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 7.8K
Discovery of
charge stripe structure

11. We can see the iron layer under the tellurium layer.
Analyzing Stripe Structure B A B A
To 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. Fe Te A 1 2 3
[nm] B 1 2 3
[nm]
We can see the iron layer under the tellurium layer.

12. C Analyzing Stripe Structure2 C C Two types of iron atoms
We 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. C 1 2
[nm]
Two types of iron atoms
form charge stripe structure

13. STS Experimental Result (T=7.8K)
TSDW~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.
SDW gap structure

14. The model of SDW induced charge stripe structure
When SDW was formed on iron layer.
Ferromagnetic structure
Antiferromagnetic structure
nesting vector[110]
Antiferromagnetic and ferromagnetic direction by SDW.
Crystal Structure: 4-fold rotation
Spin Structure: 2-fold rotation
We 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.

15. Other Electrons have strong coulomb interaction
+Reducing rotation symmetry caused by SDW
There 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.
Reduce off-site coulomb interaction
SDW induced charge stripe structure

16. The image of our model SDW Charge Stripe
This picture show result of our model.
SDW occur on this direction and charge stripe occur this direction.
Charge Stripe

17. Charge Order and Charge Stripe at Strong correlated system
J. M. Tranquada et. al, Nature,375, 561 (1995)
Charge stripe in HTSC
Charge order in organic conductors
There 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.
H. Seo, JPSJ, 69,No. 3,805(2000)
Anisotropic structure & Charge localize
Mott-insulator Base Stripe
Strong coulomb interaction relate to charge stripe structure.

18. Strong Correlative Systems
Organic Conductors
・Low transfer energy
・Anisotropic crystal structure
Cuprate Superconductors
・High on-site coulomb energy
・Isotropic crystal structure
・Mott-insulator base(half-filled)
We 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.
New E-Crystal MX1
Iron-based Superconductors
・High off-site coulomb energy
・Isotropic crystal structure
・(Semi)Metal base

19. How 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)
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.
Can Charges form stripe structure only themselves?

20. Thank you very much for your attention.
Summary
Results and Disscusion
Make single crystalline sample of FeTe
Discover the charge stripe structure on FeTe by STM
Propose the model of SDW induced charge stripe structure
MX1 series is new materials of two dimensional crystal.
Thank you very much for your attention.