Department of complex matter

Dynamics of complex systems

We investigate ultrafast relaxation processes in strongly correlated electron systems using femtosecond time-resolved spectroscopy. The dynamics of photoexcited electrons is studied in several systems, including superconductors, CDW systems, manganites, heavy fermions, MgB2 and systems with electronically driven phase transitions.

The availability of reliable ultrafast laser systems in the last couple decades or so has made possible the use of femtosecond real-time techniques to study electron-spin-lattice dynamics in various technologically interesting functionalized materials. The stroboscopic pump-probe technique, having the time resolution exceeding 20 femtoseconds (fs = 10-15 s), enabled investigations of Cooper-pair breaking and superconducting state recovery dynamics in superconductors, real-time studies of collective mode dynamics in charge-density wave systems, and study of electron-phonon coupling.

Another important topic of research are highly nonequlibrium states in correlated materials leading to a possibility of novel electronic states that cannot be observed in thermal equilibrium. Ultrafast relaxation from such states can lead to metastable hidden states of matter that show electronic properties different from equilibrium states. Due to ultrafast transition to hidden states such states could lead to development of novel ultrafast computer memory elements. Such an example is the  hidden metallic state in 1T–TaS2, which enables rapid switching from an insulator to a metal.

Schematic description of the switching into a hidden state in 1T-TaS2. The equilibrium resistance is shown on cooling (blue data points ) and on heating (red data points) . The green curve shows the resistance in the hidden state after switching by a single 35-fs optical pulse.  The inset shows a schematic description of the experimental setup.

Some milestone in superconductivity research

1987  The first Raman spectra on superconducting LaSrCuO and LaCuO

1987  Report of a “pseudogap” with “pre-formes pairs” up to 180 K

1988-1992  Reports of anharmonicity of apical O vibrations in YBCO and other cuprates

1990  Discovery of pyroelectric and piezoelectric effect in YBCO

1990 Application of polaron theory to describe the infrared response of cuprates

1994  First report of localised states in superconducting YBaCuO

1996  Review of 2-component superconductivity

1997  Report of 2-component ultrafast ultrafast response in YBCO

1999 Theory and systematic study of the temperature dependance of the pairing dynamics in HTS

2000 – Formulation of the finitive wavevector Jahn-Teller interaction

2002 Proposal for percolative superconductivity between pairs and stripes in the cuprates

2004  Description of strongly doped cuprates with the JT+Coulomb interaction using Monte Carlo method

Dynamics of complex system