The work of this group focuses on the physics of complex solids with strongly correlated electrons, one of the frontiers of modern condensed matter physics. In these solids, familiar concepts of band theory and classical magnetism, which have guided us through the electronic revolution in the past decades, fail to describe the macroscopic properties. This is because electronic correlations are not a perturbation on the kinetic energy and on the electron phonon coupling, but rather are a dominant energy scale. Thus, in analogy to the solid, liquid and gaseous states of aggregation of matter, many new phases emerge from strong electronic interactions, often preserving quantum properties in the macroscopic phenomena and up to high temperatures. This provides a rich playground and a seemingly endless variety of phases, which can serve as new paradigms for a number of new applications. High-Tc superconductivity, Colossal Magnetoresistance, and Mott physics are only a few of the best-known examples in such complex solids.
The highly nonlinear physics of strongly interacting many body systems makes these quantum phases extremely sensitive to changes of external conditions, with the system rearranging among many competing ground states for even subtle changes in doping, pressure or magnetic fields.
Irradiation with light, which acts as to redistribute charge among various sites or to distort the lattice, can be used to redirect the macroscopic properties of the system on ultrafast timescales. This was realized little more than a decade ago, and the number of applications in this field has grown enormously in the past few years.
The scientific focus of the Quantum Condensed Matter Dynamics Group is twofold.
First and foremost, we aim at controlling quantum condensed matter with light, seeking to generate phases that do not occur spontaneously, and to understand the properties of condensed matter away from equilibrium. Traditionally, quantum-phase discovery has been achieved by materials synthesis, or, alternatively, by using pressure or strong magnetic fields. The ability to generate sculpted fields of light that are both longitudinally and transversely coherent opens a wealth of new possibilities, directing various excitation along predetermined paths and acting on the macroscopic in ways that are only starting to be explored.
Can we manipulate charge, spin and orbital order and turn these effects to applications in high frequency data processing devices? Can we control charge density wave ground states and their competition with other quantum many-body ground states? Can we induce superconductivity at high temperatures? Can we use coherent excitations in complex solids for sensing? Can we reproduce, complement and extend the remarkable applications of control achieved in quantum gases over the past decade? Can we contribute to the quest by theoretical condensed matter physicists to find universality in non-equilibrium phase transitions? Does such Universality exist?
In our work, we have for example investigated the use of light to manipulate excitations coherently, driving charge and orbital excitations, phonons and superconducting Plasmon (Josephson excitations) in various complex solids. The recent discovery of the light-induced superconductivity effect, the bond selective control of conductivity and magnetism in complex manganites, as well as THz-field gating of interlayer transport in Cuprate superconductors are just three of the most recent achievements, which open the way to a new field of discovery in coherently driven quantum matter.
A second key theme is the application of ultrafast x-ray and electron sources to this class of problems.
As mentioned above, quantum condensed matter is controlled by electronic and magnetic interactions, which are coupled to the atomic motions through electron-lattice and spin-lattice interactions.
We use optical probes, from the THz to the hard x-rays to measure the femtosecond dynamics of electrons, atoms, spins and orbitals. In all these areas, the group has been making significant progress. At lower frequencies, we have measured time dependent optical conductivities from the sub THz range (millimeter wavelengths) to the visible. In this way, the conductivity of photo-stimulated Charge density wave compounds could be studied, and coupled to measurements of electronic structure with time resolved photo-emission spectroscopy. In the XUV, band structures have been mapped at all k vectors using 20-eV radiation from High order Harmonic beams with temporal resolutions of only few tens of femtoseconds.
Short pulse x-rays in the soft x-rays are used for ultrafast electronic spectroscopy, for example in time resolved NEXAFS, or for resonant soft x-ray diffraction, used to measure magnetic order in the time domain. These are used both at synchrotrons and at Free Electron Lasers to measure nanometer scale ordering of charges spins and orbital’s, and to detect their rearrangement in the time domain.
Finally, hard x-rays have been used to measure atomic positions with femtosecond resolution, an area in which we have lead the field, starting with measurements of atomic motions in solids using laser produced plasmas (1999-2003), sliced electron beams at synchrotrons (2003-2006) and most recently with Free Electron Lasers.