An international team of researchers from the European XFEL together with colleagues from the Max-Born Institute in Berlin – partners of OPTOlogic, Universities of Berlin and Hamburg, The University of Tokyo, the Japanese National Institute of Advanced Industrial Science and Technology (AIST), the Dutch Radboud University, Imperial College London, and Hamburg Center for Ultrafast Imaging, have presented new ideas for ultrafast multi-dimensional spectroscopy of strongly correlated solids. This work has now been published in Nature Photonics.

Cartoon view of the key many-body states corresponding to the spectroscopic signal at energies of the LHB, QP and UHB.

“Strongly correlated solids are complex and fascinating quantum systems in which new electronic states often emerge, especially when they interact with light,” says Alexander Lichtenstein from Hamburg University and Eu-XFEL. Strongly correlated materials, which include high-temperature superconductors, certain types of magnetic materials, and twisted quantum materials among others, both challenge our fundamental understanding of the microcosm and offer opportunities for many exciting applications ranging from materials science to information processing to medicine: for example, superconductors are used by MRI scanners. This is why understanding the hierarchy and the interplay of the diverse electronic states arising in strongly correlated materials is very important.

At the same time, it challenges our experimental and theoretical tools, because transformations between these states are often associated with phase transitions. Phase transitions are transformations that do not develop smoothly from one stage to the next but may occur suddenly and quickly, in particular when the material is interacting with light. What are the pathways of charge and energy flow during such a transition? How quickly does it occur? Can light be used to control it and to sculpt the electron correlations? Can the light bring the material into a state that the material wouldn’t find itself in under the usual circumstances? These are the types of questions that can be addressed with powerful and sensitive devices like X-ray lasers such as the European XFEL in Schenefeld near Hamburg, and/or with the modern optical tools of attosecond science (1 attosecond = 10-18 sec).

In their work, the international team now presents a completely new approach that makes it possible to monitor and decipher the ultrafast charge motion triggered by short laser pulse illuminating a strongly correlated system. They have developed a variant of ultrafast multi-dimensional spectroscopy, taking advantage of the attosecond control of how multiple colors of light add to form an ultrashort laser pulse. The sub-cycle temporal resolution offered by this spectroscopy shows the complex interplay between the different electronic configurations and demonstrates that a phase transition from a metallic state to an insulating state can take place within less than a femtosecond – i.e. in less than one quadrillionth of a second.

“Our results open up a way of investigating and specifically influencing ultrafast processes in strongly correlated materials that goes beyond previous methods,” says Olga Smirnova from the Max-Born institute and Berlin TU, awardee of the Mildred Dresselhaus prize of the Hamburg Centre for Ultrafast Imaging, “we have thus developed a key tool for accessing new ultrafast phenomena in correlated solids.”

Reference: Sub-cycle multidimensional spectroscopy of strongly correlated materials, V. N. Valmispild, E. Gorelov, M. Eckstein, A. I. Lichtenstein,  H. Aoki,  M. I. Katsnelson,  M. Yu. Ivanov &  O. Smirnova,  Nature Photonics (2024)

ICREA Prof at ICFO Maciej Lewenstein is among members selected in this year’s class.

The Board of Directors of  Optica (formerly OSA) recently elected 129 members from 26 countries to the Society’s 2024 Fellow Class. Optica Fellows are selected based on several factors, including outstanding contributions to research, business, education, engineering and service to Optica and its community. ICREA Professor at ICFO Dr. Maciej Lewenstein was among the members selected to belong to this year’s Fellows Class “for outstanding theoretical contributions to atto-optics, atto-science, quantum optics, and quantum information.”

“Congratulations to the 2024 class of Optica Fellows,” said Michal Lipson 2023 Optica President. “It is a pleasure to honor these members who are advancing our field and society. We are grateful for their exceptional work and dedication.”

Fellows are Optica members who have served with distinction in the advancement of optics and photonics. The Fellow Members Committee, led by Chair Ofer Levi, University of Toronto, Canada, reviewed 216 nominations submitted by current Fellows. As Fellows can account for no more than 10 percent of the total membership, the election process is highly competitive. Candidates are recommended by the Fellow Members Committee and approved by the Awards Council and Board of Directors.

The new Fellows will be honored at Optica conferences and events throughout 2024.

About Optica

Optica (formerly OSA), Advancing Optics and Photonics Worldwide, is the society dedicated to promoting the generation, application, archiving and dissemination of knowledge in the field. Founded in 1916, it is the leading organization for scientists, engineers, business professionals, students and others interested in the science of light. Optica’s renowned publications, meetings, online resources and in-person activities fuel discoveries, shape real-life applications and accelerate scientific, technical and educational achievement.

In a recently study published in Reports on Progress in Physics, researchers Irénée Frérot, Matteo Fadel and ICREA Prof. at ICFO Maciej Lewenstein, review methods that allow one to detect and characterize quantum correlations in many-body systems, with a special focus on approaches which are scalable.

Maciej Lewenstein gives a brief overview about the study in the following video abstract:

Link to the paper in ROPP

Link to the paper in ArXiv

In a recent study in Nature, an international team of researchers demonstrates theoretically and experimentally the capabilities that solid-state simulators could have in reproducing peculiar states of matter by mimicking electronic behaviour and observing their outcomes.

To describe matter on a microscopic level, physicists rely on very simple quantum-mechanical models. Amongst the most famous of these is the Hubbard model in which quantum particles hop between neighbouring lattice sites, and repel each other when they are at the same site.

Intriguing macroscopic behaviour can emerge even from this very simple model: if the interaction is dominant, particles tend to isolate on individual sites, and if the number of particles equals the number of sites (or a multiple thereof), the interactions turn the system into an insulating phase known as the Mott state. On the other hand, if the hopping is dominant, particles tend to spread over many sites, giving rise to metallic or even superfluid behaviour (where all atoms form a coherent wavepacket spread over the lattice and forms a non-viscous state of matter).

More scenarios become possible if interactions act over extended ranges, i. e. particles to repel each other at longer distances. A striking effect of these longer-range interactions is the possibility of stabilization of an insulating phase distinct from the Mott phase, when only half of the sites are occupied. The particles then seek a so-called “checkerboard” configuration in which every second site remains empty on a bipartite periodic structure like a square lattice.

For the first time, experimental observations of such a phase of quasi-particles, known as excitons (a bound state of an electron and a hole), have now been witnessed by experimentalists Camille Lagoin and ICFO alumnus François Dubin, from CNRS and Sorbonne Université, in collaboration with theoretical researchers and OPTOlogic partners Utso Bhattacharya, Tobias Grass, Tymoteusz Salamon and ICREA Professor Maciej Lewenstein, from ICFO, Ravindra Chhajlany from Institute of Spintronics and Quantum Information of the Adam Mickiewicz University, and Markus Holzmann from CNRS and Université Grenoble. In their study, the team of researchers have been able to observe this phenomenon by optically exciting a semiconductor sample fabricated in PRISM by Kirk Baldwin and Loren Pfeiffer, from Princeton University. The results have been published in Nature.

In this study, a laser pulse injects electrons and holes in two coupled GaAs quantum wells. The spatially-separated and oppositely charged particles then attractively bind together to form a composite particle called an exciton, with an electric dipole moment pointing from the hole to the electron. It is this dipole moment which through the dipolar forces, makes individual excitons feel each other, even if they are far apart. Additionally, gate electrodes at the surface imprint a square lattice potential for these excitons. With these ingredients, the system turns out to be well represented by a generalized Hubbard model with extended range of interactions. Cooling down to a temperature of 300 milli-Kelvin (mK) reveals suppressed compressibility of the system, providing evidence of the sought-after insulating phase in the half-filled lattice, and advanced multi-orbital theoretical modelling reveals the presence of the checkerboard pattern. Entering this regime, constitutes a well identified research advancement, where quantum particles are shown to spontaneously break the lattice symmetry and arrange themselves into a crystalline structure distinct from the underlying lattice. On tuning the exciton density per site, the experiment also reveals the existence of the incompressible Mott phase at unitary filling. Thus, by varying the filling which serves as a tuning parameter, one can trace the competition between various insulating phases at finite temperatures.

An important difference between electrons and excitons could allow in the future to reach even more exotic phases: while electrons are so-called fermions (particles that cannot simultaneously occupy the same quantum state according to the Paul exclusion principle) excitons are bosons, and hence can form a condensate, like Cooper pairs of electrons in superconductivity, Helium-4 atoms forming superfluids, or some atomic gases forming Bose Einstein condensates. At very low temperatures, still not attained so far, the three key ingredients: 1) excitons’ ability to condense, 2) the surrounding lattice potential, and 3) the long-range interactions between excitons, may conspire to give rise to a striking “supersolid” phase, in which the constituents would simultaneously show crystalline order (like in a solid) and flow without viscosity (like in a superfluid).

Thus, the observations achieved by this study highlight the ability of dipolar excitons to enable a controlled environment for the quantum simulation of the “Extended Bose-Hubbard model” at 300 mK.  Lowering this temperature to around 10 mK is within experimental reach and an objective for the near future, and will allow the long-coveted lattice supersolids to be expected as a stable phase of matter. By further exploiting the excitons’ orbital and spin degrees of freedom in the lattice, more exotic phases like multi-component supersolids may also be feasible. The experimental achievement attained in this study sets a major step forward and establishes a milestone in research in atomic to condensed matter physics, opening the door to staggering possibilities for the future.