X-ray Microdiffraction Study of CDW Conductor NbSe3

1. 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

2. 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?

3. Geometry a* c* Previous work: X-ray topography b* t w CDW shear and wavefront deformation due to thickness-dependent pinning

4. 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

5. 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

6. 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

7. 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

8. 0 mm 15 mm Individual rocking curves: lateral position as a parameter

9. 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

10. 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

11. 0 mm 15 mm 0 mm 8 mm FWHM: Comparison of samples • Samples with different: • height/thickness ratio • width of the step

12. Review of some other el./pl. NbSe3 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

13. 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.

14. v A hint from raw data: The worst case integrated intensity variations: +/- 10% (between different fields)