1. Phase diagram of FeSe by nematic ultrafast dynamics
J.-Y. Lin,1 C. W. Luo2, P. C. Cheng2, S.-H. Wang1, J.-C. Chiang1, K. H. Wu2, J. Y. Juang2, D. A. Chareev3, O. S. Volkova4, A. N. Vasiliev4
1Institute of Physics, National Chiao Tung University, Hsinchu 30010, Taiwan
2Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
3Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow District, Russia
4Department of Low Temperature Physics and Superconductivity, Faculty of Physics, M V Lomonosov Moscow State University, Moscow , Russia

2. Contents Fe-based superconductors Nematicity Issues on FeSe
Nematic ultrafast dynamics in FeSe
Phase diagram of FeSe

3. Fe-Based superconductors: One of the most intriguing chapters in superconductivity and modern condensed matter physics.
1111
122
111 11
Material
LaFeAsO
BaFe2As2
LiFeAs
FeSe
Layer distance (Å)
8.3
6.5
6.3
5.5
Tc (K)
~56
~38
~18
~10(100)
FeSe has the simplest structure, namely the tetragonal PbO-type structure, among the iron-based superconductors.
† G. R. Stewart, Rev. Mod. Phys. 83, 1589 (2011)

4. Nematicity breaks the rotational symmetry by making the x and y directions
in the plane non-equivalent, while preserving the time-reversal symmetry.
One of the most common liquid-crystal phases is the nematic. The word nematic comes from the Greek νήμα (Greek: nema), which means "thread".
The local nematic director, which is also the local optical axis, is given by the spatial and temporal average of the long molecular axes.

5. One example of nematicity and it fluctuations in pnictides
Temperature dependence of the in-plane resistivity ρa (green) and ρb (red) of Ba(Fe1−xCox)2As2 for Co concentrations from x = 0 to

6. Nematicity is generally considered driven by magnetism in pnictides.

7. When it comes to FeSe, the magnetic picture seems to collapse!
Orthorhombic
Tetragonal
No magnetic order found in FeSe!

8. Who is the driver of nematicity in FeSe?
150
Spin?
200
250
Charge/orbital?
Temperature

9. Issues on FeSe
Fermi surfaces and the electronic structure: their temperature evolution
The origin of T*
The origin of Nematic order
Why is there no static magnetic order?
The orbital or spin physics?

10. Pump-probe spectroscopy
Probe the dynamics of electron, phonon, spin in time domain.

11. FeSe single crystals

12. As for the study of FeSe, this was long long ago……

13. Experimental system Pump (400 nm): 39.7 J/cm2
Probe (800 nm): 2.3 J/cm2 Pulse width : ~100 fs
Repetition Rate : 5.2 MHz

16. Experimental data fitting
In order to have a more quantitative analysis, the relaxation processes (t > 0) of ΔR/R transients in FeSe single crystals is phenomenologically described by
The first term: the decay of the photoexcited electrons (or quasiparticles, QPs) with an initial population number A1, through phonon-coupling with a relaxation time of τ1.
The second term: the decay of QPs with an initial population number A2, through spin-coupling with a corresponding decay time of τ2. *
The third term: the energy loss from the hot spot to the ambient environment within the timescale of microsecond, which is much longer than the period of the measurement (~50 ps) and hence is taken as a constant.
*Ogasawara, T. et al. Phys. Rev. Lett. 94, (2005).

19. Phase diagram of FeSe

20. Who is the drive?

23. Johnston, 2010

25. Orbital limit
Pauli limit

26. orbital kx ky  M
spin Fe

28. Summary
T* has a magnetic origin.
Mechanism: double exchange??

29. Suzuki et al.