1. Giorgi Ghambashidze Institute of Condensed Matter Physics, Tbilisi State University, GE-0128 Tbilisi, Georgia Muon Spin Rotation Studies of the Pressure Effect on the Static Stripe Magnetic Order in the La 2-x Ba x CuO 4 (x=1/8) System

2. Collaboration University of ZürichTbilisi State University Prof. Alexander Shengelaya Zurab Guguchia

3. Introduction Suppression of Superconductivity in La 2-x Ba x CuO 4 (x=1/8) System A. R. Moodenbaugh et. al., PRB 38, 4596 (1987)

4. In 1995, using neutron scattering, it was found, that in some cuprate superconductors, including La 2-x Ba x CuO 4, doped holes form an array of charge and spin ordered stripes. J.M. Tranquada, et. al., Nature, 375, 561 (1995)

5. Problem! What is the reason of stripes formation at low temperatures for x=1/8 concentration? One of the explanation is related to low-temperature structural phase transition But the exact reason not yet known.

6. At Ambient pressure and room temperature LBCO (x=1/8) compound has the high-temperature tetragonal (HTT) structure. On Cooling below T HT, the structure transforms to the low-temperature orthorhombic (LTO) phase, and below T LT one reaches the low- -temperature tetragonal (LTT) phase – which can be one of the possible reason of stripes formation.

7. Pressure effect on structural phase transition Recently it was shown that, it’s possible to suppress low-temperature tetragonal (LTT) structural phase transition in LBCO (x=1/8) compound by applying pressure, but the charge order (CO) continues to exist beyond p c (critical pressure). It was not shown how the magnetic order related with static stripes is affected by pressure. It was our main purpose of research! M. Hucker at. al., PRB 104, 057004 (2010)

8. Muon Spin Rotation Technique What are muons?

9. Why Muons? Use their spin (magnetic moment) as a magnetic probe Sensitive and accurate magnetometer Measures internal field and its variations (distribution in space, variation in time) Application in superconductivity, magnetism, conductive polymers…

10. Muons Production

11. Muon Precession 3 cm in length is pure phase of the single crystal

12. Muons Production

13. Pressure Cell 001 We used cylindrical high pressure cell for applying high pressure. It is made of half hard beryllium-copper and has one close end. This type of pressure cell can be applied at maximum 25 kbar pressure.

14. X-ray diffraction pattern for La 2-x Ba x CuO 4 sample

15. ZF- μSR spectra at 4 K temperature and under 15 kbar pressure

16. ZF- μSR spectra at 32 K temperature and under 15 kbar pressure

17. Obtained Results Internal Magnetic Field vs. Temperature for different pressures It is known, that structural phase transition is suppressed at 18.5 kbar!

18. Conclusions LTT structural-phase transition is not the reason of spin magnetic ordered stripes formation, since we suppressed structural transition under pressure in La 2-x Ba x CuO 4 (x=1/8) system. Magnetic order continues to exist. Magnetic transition temperature only slightly decreased by applied pressure. Therefore other mechanisms should be found which could lead to static stripe order in LBCO compound at 1/8 concentration.

19. Thanks For Attention!