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Transverse Correlation Effects and Plastic Properties in the CDW Conductor NbSe3 Studied by X-ray Microdiffraction. A.F. Isakovic, J. Kmetko, K. Cicak, R. E. Thorne Physics Dept. LASSP, Cornell University P.G. Evans, Mat. Sci. and Engr. Dept., University of Wisconsin-Madison

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Transverse Correlation Effects and Plastic Properties in the

CDW Conductor NbSe3 Studied by X-ray Microdiffraction

A.F. Isakovic, J. Kmetko, K. Cicak, R. E. Thorne

Physics Dept. LASSP, Cornell University

P.G. Evans, Mat. Sci. and Engr. Dept., University of Wisconsin-Madison

B.Lai, Z. Cai, Argonne National Lab

  • X-ray microdiffraction as a tool in studying a collective dynamics

  • in electronic crystals

  • CDW q-vector rotation and shear strain for inhomogeneous pinning

  • Estimate of shear modulus 1.8 x 107 N/m2

Work supported in part by NSF/DMR04-05500


CDW depinning and structural change

Most crystals have thickness steps

10mm

CDW pinning depends on thickness

How do thickness steps/nonuniform pinning affect CDW

structure and transport?


Geometry

a*

c*

Previous work:

X-ray topography

b*

t

w

CDW shear and wavefront deformation

due to thickness-dependent pinning


j (mdeg)

y (mm)

Microdiffraction setup

Coherent

X-ray beam

sample

CCD

detector

zone

plate

2j

micro-

diffraction

OSA

vertical

slits

j

a*

incident

beam

c*

b*

diffracted

beam

Spatial Resolution: 300 nm

Sensitivity to Q rotations: 5 mdeg


Sample B, 120 K

E = 0

ET

step

thick

2ET

2.9ET

Intensity color scale

3.8ET

c*

5 mm

30 m

0

b*

Scans in b*-c* plane

  • Depinning of the thick, weakly pinned

  • side is accompanied by a loss of

  • intensity on that side (consistent

  • with topography experiment)

  • Spatial resolution is 300 nm

  • (~ 4 microns in topography)

2 10-3 counts/ monitorcount


thick

j

y

50 mdeg

2 mm

  • Most of the effects seen are due to electric field

  • (zero-field subtracted)

  • Depinning on the thick side + two types of

  • rotations (in b*-c* plane and in a-b* plane)

  • What do we really see in these images?

a – b*plane rocking images


C

B

A

6 mm

Individual rocking curves: El. field as parameter

  • CDW wavevector rotation visible in

  • curves’ centre-of-mass shift of

  • 25–40 mdeg

  • Peak width broadens from the

  • depinned to pinned side

thin

thick


0 mm

15 mm

Individual rocking curves: lateral position as a parameter


Rotation of CDW q-vector in a - b* plane

  • Significant rotation of the q-vector in the vicinity of the step

  • Rotates by 25-40 mdeg, depending on bias and position

  • Near the step edge region (dj/dy)MAX is ~30 mdeg/mm


FWHM across the step

as a function of bias

Fitting of the rocking curves

  • Rotation is pronounced AND

  • bias dependent across the step

  • and in the thick region

  • Rotation in step region occurs in

  • opposite direction for negative bias


0 mm

15 mm

0 mm

8 mm

FWHM: Comparison of samples

  • Samples with different:

  • height/thickness ratio

  • width of the step


Review of some other el./pl. NbSe 3 parameters

  • Consistent picture of longitudinal and transverse properties

  • Combined X-ray and transport measurements - powerful tool set

  • Significant differences between the bulk material parameter and

  • CDW parameter


Shear modulus

from both direct readout

and fit, we get ~ 30 mdeg/mm

Max. shear strain

Shear strength

Shear modulus

Conclusions

  • X-ray microdiffraction very useful tool in

  • structural studies of electronic crystals

  • CDW shear imaged with enhancement

  • over previous techniques (topograhy)

  • Rocking curves used to elucidate the

  • details of the CDW q-vector rotation

  • Analysis determines the shear strain

  • modulus G~ 1.8 x 107N/m2

  • We also find interesting behavior of the

  • FWHM in the vicinity of the step edge.


v

A hint from raw data:

The worst case integrated intensity

variations: +/- 10%

(between different fields)


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