1. Kitaoka lab Itohara Keita
Antiferromagnetic order induced by an applied magnetic field in a high-temperature superconductor
Kitaoka lab
Itohara Keita
My name is Keita Itohara.
I am a member of Kitaoka laboratory.
Today,I will talk about “Antiferromagnetic order induced by an applied magnetic field in a high-temperature superconductor”

2. Contents ・Introduction Meissner effect TypeⅠand TypeⅡsuperconductor
La2-xSrxCuO4
Experimental method
Magnetic neutron diffraction
・Experimental data and discussions
・Summary
This is the contents of my presentation.
Firstly,I’ll introduce you the brief of Meissner effect.
Next,what the difference between type1 and type2 superconductor is.
And I’ll show you the crystal structure of LaSrCuO,which is the sample of this study and explain its features.
Then, I’ll talk about how authors did their jobs using magnetic neutron diffraction.
This is the main subject in my presentation, experimental data and discussions.
Finally I’ll summarize my presentation.

3. Meissner effect B=M+H=0 Magnetic field H
As you know, superconductor has a characteristic feature, the electric resistivity suddenly dropped to zero below transition temperature Tc.
And this is another important feature, it called “Meissner effect”
The Meissner effect is the expulsion of a magnetic field from a superconductor.
It is not surprising that this effect occurs below Tc.
This phenomenon is applied to a linear motor car floating.
Right figure shows the brief model of “Meissner effect”
If we give the external magnetic field H, superconductors in the Meissner state exhibit perfect diamagnetism, such that their magnetization M=-H
Diamagnetism is defined as the generation of a spontaneous magnetization of a material which directly opposes the direction of an applied field.
Magnetic field H

4. TypeⅠand typeⅡsuperconductor
Perfect
diamagnetism
Perfect
diamagnetism
Magnetic field H
Vortex state
There are two type of superconductors.
The one is called Type1 superconductor and the other called Type2.
If there is no magnetic field , both of them show perfect diamagnetism.
And no flux can penetrate inside it.
That’s because I showed you on the previous page.
In stronger field, in type2 a little flux can penetrate though type1 is still perfect diamagnetism.
In this state, partially, superconductivity has broken by field penetrating
On this picture, a partial colored with black means normal state, and with gray means supreconductivity.
And this state, coexisting normal state and superconductivity,is called “Vortex state”
Then in much stronger field, type 1 suddenly changes normal state.
And in type2, much more flux can penetrate than lower field, but it still shows vortex state.
As you can see in a last row,finally, type2 also has broken their superconductivity completely.
Normal state
Normal state

5. underdoped La2-xSrxCuO4
0.1
0.2
0.3 50
100
150
200
250
300
350
Temperature[K]
Sr doping　(x)
Antiferromagnetism
Spin glass
superconductivity
Here shows the crystal structure of LaSrCuO.
There exists Cu atom on the yellow ball, O on the gray, La on the blue one.
This material has one CuO2 plane.
And we can put carriers into it by chemical substitution,La3+ ions to Sr2+ ions.
The right side of this slide,this is the phase diagram.
The non-doped compound is an insulating antiferromagnet.
Introduction of charge carriers via Sr doping reduces the ordered moment until it vanishes at x<0.13
LaSrCuO becomes a superconductor for Sr doping of 0.06<x<0.25.
In this study, authors used single crystals of underdoped(x=0.1) compound.
Use single crystals of
　underdoped La2-xSrxCuO4

6. Neutron diffraction Bragg’s law 2dsinθ= nλ
Neutron diffraction is closely
related to X-ray powder diffraction
Merit
・Neutrons have stronger penetration
than X-ray and interact directly with
the nucleus of the atom
→Samples sizes are relatively
　 large compared to those used
　 in X-ray diffraction

7. Magnetic neutron diffraction
Neutrons are uncharged,
they carry a spin
interact with magnetic moments
reveal the microscopic magnetic
structure of a material

8. Experimental data (1) Superconductivity characterized
T(K)
Superconductivity characterized
by zero-resistivity at the much
lower temperature
→Tc=29K

9. Experimental data (2) The applied field that imposes
antiferromagnetism
The applied field that imposes
the vortex lattice also induces
antiferromagnetic order
superconductivity and antiferro coexist throughout the bulk of the material

10. Experimental data (3) The order increases rapidly
with small fields, reflecting the
linear dependence of vortex
density on field
Vortex core

11. 130Å av 500Å H=1T H=14.5T Magnetic correlation length →ζ>400Å
Superconducting coherent length
→ξ〜20Å
130Å av
500Å
H=1T
H=14.5T

12. Summary ・In the high-temperature superconductivity,
La2-xSrxCuO4(x=0.10),there is the field-induced
antiferromagnetism
・The field-induced antiferromagnetism is intrinsic
・And superconductivity and antiferromagnetism coexist
throughout the bulk of the material
0.1
0.2
0.3 50
100
150
200
250
300
350
Temperature[K]
Sr doping　(x) he
Antiferromagnetism
Spin glass
superconductivity