Ultrafast dynamics in multiferroics HoMnO 3 revealed by fs spectroscopy. Chih Wei Luo ( 羅志偉 )

Transcription

1 Ultrafast dynamics in multiferroics HoMnO 3 revealed by fs spectroscopy Chih Wei Luo ( 羅志偉 )

2 Outline Introduction of femtosecond (fs) laser pulses Ultrafast dynamics in multiferroics HoMnO 3 Summary I High-T c superconductor YBa 2 Cu 3 O 7 nanodots Summary II

3 Introduction of fs laser pulses What is the ultrashort pulse? ~10-6 s ~10-9 s ~10-12 s ~10-15 s

4 Introduction of fs laser pulses Timescales 10 fs light pulse Computer clock cycle Camera flash 1 minute One month Age of pyramids Human existence Age of universe femtosecond 1 picosecond Time (seconds) a pulse : 1 minute ~ 1 minute : age of universe

5 Introduction of fs laser pulses Which one is true? 1 / 1 min 1 / 0.5 min 1 / 1 sec Idea from 石訓全

6 Introduction of fs laser pulses Ultrafast camera!!

7 Introduction of fs laser pulses The possibility for nuclear fusion! Short pulse = intense peak power 100 mj, 100 fs = 1 TW W/cm φ = 10 μm (10 10 V/cm) Institute of Laser Engineering Osaka University Seed Pump Verdi Mira Long pulse, high energy Pump Evolution Short pulse, low energy Amplifier Legend Short pulse, high energy

8 Introduction of fs laser pulses The evolution of pulse width The shorter pulse duration, the more papers! Femtosecond in Web of Science Pulse duration (sec.) First laser (Ruby) Active mode-locking Passive mode-locking Colliding pulse mode-locking intra-cavity pulse compression XUV excitation pulse No. of Publications Year Year Prof. Ahmed Zewail Prof. Theodor W. Hänsch The 1999 Nobel Prize in Chemistry The 2005 Nobel Prize in Physics

9 Ultrafast dynamics in HoMnO 3 Multiferroic Ferromagnets (ferroelectrics) form a subset of magnetically (electrically) polarizable materials such as paramagnets and antiferromagnets (paraelectrics and antiferroelectrics) W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442, 759 (2006).

10 Ultrafast dynamics in HoMnO 3 Multiferroic ReMnO 3 Hexagonal structure v.s. Orthorhombic structure Seongsu Lee, et al Nature 451,805 (2008) S. Satpathy, et al PRL 76,960 (1996) W. Prellier, et al, JPCM 17, 803 (2005)

11 Ultrafast dynamics in HoMnO 3 Hexagonal HoMnO 3 MnO 5 bipyramids form a layered structure on a b plane. T C = 875 K P z = 5.6 μc cm 2 T N = 76 K T SR = 33 K T Ho = 5 K Coexistence between FE and AFM B. Lorenz, et al PRB 71, (2005)

12 Ultrafast dynamics in HoMnO 3 Magnetoelectric coupling effect on hexagonal HoMnO 3 Dielectric constant Heat capacity Lattice constant B. Lorenz, et al PRL 92, (2004) B. Lorenz, et al PRB 71, (2005) C. Dela Cruz, et al PRB 71,060407R (2005)

13 Ultrafast dynamics in HoMnO 3 Optical properties of hexagonal HoMnO 3 A. B. Souchkov, et al PRL 91, (2003) Transmittance and reflectance measurements were performed using a Fourier transform spectrometer a 1g in a frequency range from 10 to cm -1 (1.2 mev to 5.6 ev) e 2g 1.7 ev absorption peak comes from d d e 1g transitions. ~0.15 ev blueshift as decreasing temperature. Associate with the magnetic phase transition.

14 Ultrafast dynamics in HoMnO 3 Optical properties of hexagonal HoMnO 3 Rare earth : Gd Tb Dy Ho T N Woo Seok Choi, et al PRB 78, (2008)

15 Ultrafast dynamics in HoMnO 3 Crystal structure and magnetic property Out of plane : c axis In plane : ab axis T N = 76 K T SR = 33 K T Ho = 5 K 1.2x10-5 Intensity (arb. units) HMO(002) HMO(004) HMO(006) χ (emu/oe) 1.0x x x x x10-6 T Ho ZFC 100 Oe H//c-axis T SR 1/χ (Oe/emu) Curie-Weiss Law Temperature (K) θ (degree) Temperature (K)

16 Ultrafast dynamics in HoMnO 3 Pump-probe and optical spectroscopy Tunable photon energy from 1.52 to 1.69 ev Normalize Intentsity (arb. units) 740nm 755nm 770nm 785nm 800nm 815nm Wavelength (nm)

17 Ultrafast dynamics in HoMnO 3 Temperature-dependent transient reflectivity change (ΔR/R) Wavelength : 800 nm Wavelength : 770 nm Wavelength : 740 nm T=290K T=290K T=290K ΔR/R (arb. units) x3 x3 T=250K T=210K T=170K T=150K T=140K T=120K T=100K T=60K ΔR/R (arb. units) T=250K T=210K T=190K T=170K T=130K T=110K T=95K T=85K ΔR/R (arb. units) T=220K T=180K T=140K T=100K T=80K T=71K T=67K T=60K Delay Time (ps) Delay Time (ps) Delay Time (ps)

18 Ultrafast dynamics in HoMnO 3 Oscillation component 2 2 Strain Pulse Model τ osc ( λprobe / 2υ sound n sin θ ) 800nm 740nm ΔR/R (arb. units) Delay time (ps) T=290K LuMnO 3 D. Lim, et al APL 83,4800 (2003)

19 Ultrafast dynamics in HoMnO 3 Charge transfer from e 2g to a 1g by pump pulses Mn 3 + 3d levels Pump energy :1.52 ev Normalized amplitude of ΔR/R T 0 =140 K 815nm Room temperature Low temperature Pump energy T=290K T=140K Pump energy d 2 2 3z r d 2 ( x y 2 ),( xy) E Temperature (K) Observed the blueshift of energy gap! Woo Seok Choi, et al PRB 78, (2008)

20 Ultrafast dynamics in HoMnO 3 Charge transfer from e 2g to a 1g by pump pulses Normalized amplitude of ΔR/R Temperature (K) 815nm 800nm Mn 3 + 3d levels Room temperature Pump energy T=290K T=140K Pump energy :1.55 ev Low temperature Pump energy d 2 2 3z r d 2 ( x y 2 ),( xy) E Observed the blueshift of energy gap!

21 Ultrafast dynamics in HoMnO 3 Charge transfer from e 2g to a 1g by pump pulses Mn 3 + 3d levels Pump energy :1.55 ev Normalized amplitude of ΔR/R T 0 =117 K 815nm 800nm Room temperature Low temperature Pump energy T=290K T=140K T=117K Pump energy d 2 2 3z r d 2 ( x y 2 ),( xy) E Temperature (K) Observed the blueshift of energy gap!

22 Ultrafast dynamics in HoMnO 3 Charge transfer from e 2g to a 1g by pump pulses Energy gap E dd (ev) Normalized amplitude of ΔR/R AFM T 0 Temperature (K) Temperature (K) Slope nm 800nm 785nm 770nm 755nm 740nm Mn 3 + 3d levels Room temperature Pump energy Temperature (K) χ (emu/oe) T=290K T=63K 1.2x x x x x x10-6 Pump energy :1.68 ev Low temperature T Ho ZFC 100 Oe H//c-axis Pump energy T SR d 2 2 3z r d 2 ( x y ),( xy) /χ (Oe/emu) Temperature (K) Curie-Weiss Law Temperature (K) E

23 Ultrafast dynamics in HoMnO 3 Charge transfer from e 2g to a 1g by pump pulses Energy gap E dd (ev) Normalized amplitude of ΔR/R AFM T 0 Temperature (K) Temperature (K) Slope nm 800nm 785nm 770nm 755nm 740nm Mn 3 + 3d Temperature (K) levels Room temperature Pump energy T=290K T=63K T=63K Pump energy :1.68 ev Low temperature Pump energy Pump energy d 2 2 3z r d 2 ( x y d 2 ( x y 2 2 ),( xy) ),( xy) Extra blueshift comes from longrange AFM ordering!! E

24 Ultrafast dynamics in HoMnO 3 Demagnetization dynamics τ m T=290K T=180K ΔR/R (arb. units) 75K τ c 290K 180K 75 K nm 785nm 770nm 755nm 740nm τ m Delay time (ps) 3 T=75K T e T l 2 τ m T s Delay time (ps) Temperature (K)

25 Summary ΔR/R (arb. units) 800nm 740nm T=290K Delay time (ps) The oscillation due to the strain pulse was clearly observed in ΔR/R by fs spectroscopy. Normalized amplitude of ΔR/R nm nm 785nm nm 755nm 740nm -0.2 T Temperature (K) A distinct blueshift of the Mn 3+ d-d optical transition comes from the appearance of AFM long-range ordering. T=290K T=180K τ m 75 K T=75K τ m Delay time (ps) T e T l T s The demagnetization time (τ m ) in a few ps scale and its recovering time (τ c ) in a few 100 ps scale were shown in the ΔR/R.

26 YBCO nanodots Sample reparation: 400 (001) YBa 2 Cu 3 O 7 (YBCO) / (100) LaAlO 3 Vacuum Pumps Excimer Laser Lens intensity (a. u.) YBCO(001) YBCO(002) YBCO(003) LAO(100) YBCO(004) YBCO(005) YBCO(006) LAO(200) XRD YBCO(007) Heater Vacuum Gauges P Y θ (degrees) O 2 Vacuum Chamber 15 SEM Resistance (Ω) 10 5 Tc = 90.1 K Temperature (K)

27 YBCO nanodots Experimental setup: (spot size~110 μm) fs Laser 19.6 cm YBCO 光路徑 LaAlO 3

28 Results surface morphology YBCO nanodots Fluence = 0 J/cm 2 Fluence = 0.26 J/cm 2 Fluence = 0.53 J/cm 2 Fluence = 0.21 J/cm 2 Fluence = 0.32 J/cm 2 C. W. Luo, C. C. Lee, et al., Optics Express 16, (2008)

29 YBCO nanodots Results structure Fluence = 0 J/cm 2 Fluence = 0.21 J/cm 2 Fluence = 0.26 J/cm 2 Fluence = 0.32 J/cm 2 Fluence = 0.53 J/cm 2 XRD signals of YBCO thin films at various laser fluences.

30 YBCO nanodots Results superconductivity Fluence = 0 J/cm 2 Fluence = 0.21 J/cm 2 Fluence = 0.26 J/cm 2 Fluence = 0.32 J/cm 2 Fluence = 0.53 J/cm 2

31 YBCO nanodots Results composition Fluence = 0 J/cm 2 Fluence = 0.21 J/cm K > 1897 K (Ba) Fluence = 0.26 J/cm 2 Fluence = 0.32 J/cm K > 3345 K (Y) EDS spectra show the composition of area 1 and area 2. Fluence = 0.53 J/cm 2 ΔT = W/ CV W 0.1mJ C = V = J m m 3-3 K -1

32 Summary The surface microstructure of YBCO thin films can be manipulated by properly controlling the fluence of the irradiating femtosecond laser. A ripple pattern was clearly observed on the surface of one YBCO thin film. The (001)-YBCO film turns into nanodot array with the superconductivity remains almost intact. Serve as a new way of engineering the material surfaces into nanometer scale structures. Formation of nano-textured conical microstructures in titanium metal surface B. K. Nayak, et al., Appl. Phys. A 90, 399 (2008)

33 Acknowledgements Students: H. C. Shih, C. C. Lee, H. I. Wang, W. T. Tang Solid State Lab: K. H. Wu, J. Y. Juang, J.-Y. Lin, T. M. Uen NSRRC: J. M. Chen, J. M. Lee Thank you for your attention!!