As part of the Light for Graphene (L4G) seminars on December 12th, 2022, ICFO welcomed Mikhail Ivanov, who delved into processes associated with electron dynamics in correlated materials. Professors Maciej Lewenstein and Jens Biegert graciously hosted the session. Below, you’ll find details encompassing the abstract of the discussion alongside Dr. Ivanov’s professional trajectory.

Abstract:

Attosecond spectroscopy and strong light fields boldly embarked into solid-state physics about a decade ago.

Throughout this decade, we have come to appreciate that strong light fields drive electrons in solids not solely on the time-scale of the pulse envelope but also on the sub-cycle time scale. This control over light oscillations and polarization at the sub-cycle level presents intriguing opportunities to manipulate electronic responses on a sub-femtosecond time scale. Furthermore, I will exemplify these opportunities with a series of cases, ranging from PHz valleytronics in graphene to attosecond spectroscopy of the 2D Hubbard model.

Should time permit for the preparation of new slides, I intend to shift the focus entirely towards discussing our latest findings. These findings delve into resolving the mystery of insanely fast 1 fsec dephasing times in strongly driven solids and probing the enigmatic behavior of not-so-massless, massless Fermions in Weyl semimetals.

Biography:

Education and Early Career

Mikhail Ivanov heads the Theory Department at Max Born Institute and also holds a Professor position in the Department of Physics at Humboldt University.

Born in the USSR and educated at Moscow State University, Misha left Moscow within a year after the USSR ceased to exist.

International Work Experience

Since then, he has worked extensively in Canada, Poland, the UK, and Germany. Currently, he leads the Department of Theory at the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy in Berlin, alongside his professorship at Humboldt University Berlin.

Current Positions and Affiliations

Despite his global journey, Misha maintains his professorial role at Imperial College London due to his fondness for the city. Additionally, he holds a visiting professor appointment at the Technion in Haifa, Israel, reflecting his deep affection for the country.

Researchers from ICFO and Aarhus University report in Nature Communications a novel technique that uses orbital angular momentum to detect entanglement in the attosecond regime.

The Significance of Entanglement in Quantum Physics and Attosecond Physics

Entanglement is one of the sweet words we listen to lately when referring to quantum physics. It has provided staggering results in quantum simulations and computing, proving to optimize time in calculations, or even enhance imaging processes. In addition, attosecond physics is the field of physics that studies processes that occur in matter on the scale of attoseconds, i.e. 10^-18s the time it takes for electrons to move through atoms and molecules. The combination of entanglement techniques and attoscience has led to a new field where attosecond imaging methods exploit quantum phenomena like interference, however, the role of entanglement remains unclear/unexplored.

Recent Advances in Studying Entanglement in Attoscience

Now, the potential for entanglement to optimize or improve attosecond imaging has been unexplored so far because, among other things, scientists never assumed that it would have such a leading role in the process. However, in recent years, there has been a growing interest in entanglement in attoscience, with the main focus on the entanglement between electrons and ions, revealing a connection between them that allows better understanding of coherence. Most of the focus has been on the connection between electrons and ions, but studies on the entanglement between two ionized electrons have received less attention. And those few studies that have tackled the issue have based their work on the entanglement of continuous quantities, which are challenging to compute and interpret, and often impossible to measure.

Demonstrating Entanglement through Electron Vortex States

In a recent study published in Nature Communications, ICFO researchers Andrew S. Maxwell, who then transferred to Aarhus carrying this study, and ICREA Prof. at ICFO Maciej Lewenstein, in collaboration with Lars Madsen from Aarhus University, have shown that the production and measurement of electron vortex states, which are free electrons with helical wavefront that may carry orbital angular momentum (OAM), offer a solution to the above problems.

Quantifying Entanglement and Developing Detection Methods

In their study, they exploit the discrete degree of freedom, the OAM—inherent to all free particles, to clearly demonstrate the manifestation of entanglement in non-sequential double ionization (NSDI), a highly correlated two-electron ionization process. Through known conservation laws and the superposition of intermediate excited states, they demonstrate that the OAM of the two ionized electrons in NSDI is in fact entangled.

They then use the logarithmic negativity as a way to quantify the entanglement for a wide range of targets and parameters. They construct an entanglement witness, which provides a novel way for detecting the entanglement in an experiment in a much simpler way, avoiding full tomographic measurements. The interplay of intermediate excited states allows the photoelectrons to approach maximally entangled states. Furthermore, the entanglement is robust as it survives incoherent averaging over the focal volume of the laser.

Implications of OAM Entanglement for Attosecond Imaging

The use of this new technique involving OAM in attosecond processes provides a new pathway for improving imaging techniques and learning how to control matter on ultrafast times scales. Furthermore, the OAM entanglement of the photoelectrons demonstrates the fundamental non-classical nature of NSDI.

 

Links

Link to paper

Link to the research group led by ICREA Prof. at ICFO Maciej Lewenstein

A team of researchers has uncovered hidden gems lurking behind the Abelian-Higgs model in one spatial dimension and time.

The Promise of Quantum Simulators

While universal fault-tolerant quantum computing with error correction remains elusive, special-purpose quantum computers, i.e. quantum simulators, hold the future. Although the physics community strongly believes that quantum simulators have already led to quantum advantage, especially in studies of quantum dynamics and quantum disordered systems, there are no rigorous proofs yet. Moreover, most of the applications of quantum simulators have concerned models of condensed matter physics. However, this paradigm is beginning to change. Increasingly, impressive applications to quantum chemistry and optimization problems are emerging. In physics, the current focus is on quantum simulation of fundamental models of high energy physics: Lattice Gauge Theories and Quantum Field Theories.

The Higgs mechanism is an essential ingredient of the Standard Model of particle physics that explains the ‘mass generation’ of gauge bosons. While its seemingly simple one-dimensional lattice version may serve as an interesting novel quantum simulator, until now, researchers had not explored it.

New Insights from Recent Research

In a recent study published in Physical Review Letters, Titas Chanda with Jakub Zakrzewski from Jagiellonian University in Cracow, Luca Tagliacozzo from the Institute of Fundamental Physics IFF-CSIC and ICREA professor Maciej Lewenstein from ICFO took up the challenge to fill this gap. Unlike the system in the continuum, researchers identified two distinct regions in the lattice version. Specifically, they found the confined and Higgs regions. Moreover, these two regions are separated by a line of first-order phase transitions that ends in a second-order critical point. Furthermore, above this critical point, the regions are smoothly connected by a crossover. Additionally, the presence of a second-order critical point allows researchers to construct an unorthodox continuum limit of the theory described by a conformal field theory (CFT). This work strongly motivates current prospects of quantum simulations of quantum gauge theories and opens a path toward observing the Higgs mechanism in experiments with cold atomic setups.

Cited article: Titas Chanda, Maciej Lewenstein, Jakub Zakrzewski, and Luca Tagliacozzo. Phase Diagram of $1+1D$ Abelian-Higgs Model and Its Critical Point. Phys. Rev. Lett. 128, 090601

A team of OPTOlogic researchers describes in Nature Quantum Materials a technique to probe the orbital texture of a metal using photoemission spectroscopy.

In the article, published in the journal Nature Quantum Materials, researchers from the Fritz Haber Institute of the Max Planck Society, together with researchers from the University of Bordeaux, the SLAC National Accelerator Laboratory, the University of Würzburg, the New Technologies Research Center of the University of West Bohemia and the Institute for Optics and Atomic Physics of the Technical University of Berlin, describe how to access the orbital texture – the momentum-dependent orbital character of crystalline solids – using novel a measurement method in photoemission spectroscopy.

Researchers also went through some of their latest publications. So far, the consortium members have published more than ten publications in peer-reviewed journals and have several in preprint. This is a remarkable achievement, considering that the project has been ongoing for about six months.

Samuel Beaulieu, the first author of the study, explains, “During my postdoc with Prof. Ralph Ernstorfer, I started to investigate the electronic structure of 2D materials using angle-resolved photoemission spectroscopy. I had the intuition that measuring the modulation of the photoemission intensity upon a specific crystal rotation could give us insight beyond the band structure”.

Using multidimensional photoemission spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) is an experimental technique that allows measuring the momentum-resolved energy bands in crystalline solids. In this method, the electrons inside a solid absorb a photon with larger energy than the material’s work function and escape into the vacuum. A key aspect of this technique is that the energy- and momentum-dependence of the photoemission signal is proportional to the single-particle spectral function, giving key information about the many-body interactions inside the material.

In addition to being sensitive to the spectral function, the ARPES signal is modulated by the so-called matrix element effects, which encode information about the orbitals forming the energy bands. However, being a complex quantity, extracting information about orbitals from matrix element effects is not a trivial task.

Novel Techniques and Findings in Orbital Texture

First, the team used extreme ultraviolet angle-resolved photoemission spectroscopy, allowing us to access 3D photoemission intensities, resolved in energy and both momentum components of the electrons. In addition, they measured the photoemission intensity modulation upon sample rotation – a procedure that they recently developed- which allowed them to disentangle signatures of the orbital texture of a layered 2D metal (1T-TiTe₂).

Then, to strengthen the robustness of the results, they compared the experimental measurements with the theoretical calculations performed by two theory research groups, which used two complementary approaches. The synergy between the novel experimental approach and these state-of-the-art theoretical calculations allowed linking the orbital texture of the material and the measured angle-resolved photoemission intensity modulation upon sample rotation.

The results represent a significant step towards moving from the typical band structure mapping to accessing the electronic wave function and the orbital texture of solids. In the future, this methodology could be used to investigate the ultrafast non-equilibrium modification of the orbital texture upon photoexciting materials using ultrashort light flashes.

“Our discovery could be used to generate very precise knowledge about the non-equilibrium behavior of nature, specifically quantum materials, upon absorption of light. Because light-matter interaction governs a great number of devices in our everyday life, who knows, maybe our discovery could one day have an impact on everyday life”, comments Beaulieu.

Cited article: Beaulieu, S., Schüler, M., Schusser, J. et al. Unveiling the orbital texture of 1T-TiTe₂ using intrinsic linear dichroism in multidimensional photoemission spectroscopy. npj Quantum Mater. 6, 93 (2021). https://doi.org/10.1038/s41535-021-00398-3

In a study recently published in Communications Chemistry, researchers from ICFO and partners of OPTOlogic report on a newly-developed machine learning algorithm for LIED to extract the three-dimensional structure of large and complex molecules.

Revolutionary Discovery: Visualizing Molecular Transformation

Until very recently, the very idea of witnessing how molecules break or transform during chemical reactions was unfathomable. However, in 2016, researchers from ICFO achieved a breakthrough by developing mid-IR-driven laser-induced electron diffraction (LIED) with kinematic coincidence detection. This groundbreaking technique enabled them to image the position of each atom within a single molecule using one of its own electrons. The remarkable picometer spatial and attosecond temporal resolution achieved through LIED allowed them to actually image and track the molecular bond breakup in acetylene (C₂H₂) a mere nine femtoseconds after its ionization, a method they aptly coined as the “molecular selfie”.

Application of LIED to Small Molecules: A Glimpse into the Molecular World

Initially, applying the LIED technique to capture snapshots of small gas-phase molecules proved to be an immensely powerful tool for understanding the intricate interactions of molecules, revealing how they react, change, break, and bend. Nevertheless, extending this technique to more complex molecular structures posed a significant challenge. As the size of the molecule increases, so does the difficulty of structural retrieval. Consequently, it becomes necessary to calculate thousands of molecular configurations for all possible orientations, a task that could take an impractical amount of time.

In a recent study published in Chemistry Communications, ICFO researcher Xinyao Liu, Kasra Amini, Aurelien Sanchez, Blanca Belsa, Tobias Steinle, led by led by ICREA Professor at ICFO Jens Biegert, report on a solution to this problem with a newly-developed machine learning algorithm for LIED to extract the three-dimensional structure of large and complex molecules in their experiment, the team of researchers developed a machine learning model and combined it with a Convolutional Neural Network (CNN) algorithm which, according to the researchers, is “well suited for problems in image recognitions to identified subtle features from an image at a different level of complexity similar to a human brain”. Using the CNN-ML framework, the pre-calculated database of configurations could be drastically reduced to unambiguously identify a complex chiral molecular structure such as the Fenchone molecule.

Importancia del Avance: Determinación Eficiente de Estructuras Moleculares 3D

This result is of major importance because being able to calculate the 3D molecular structure of complex molecules with the sufficient structural resolution has been, so far, a very difficult challenge to overcome. This study is a major step forward in this field, where the combination of LIED, machine learning and CNN network, have not only shown that they can predict and determine the structure of these large molecules but also do it within a completely reasonable computing processing time.

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Cited article:  A. Sanchez, K. Amini, S.-J. Wang, T. Steinle, B. Belsa, J. Danek, A. T. Le, X. Liu, R. Moshammer, T. Pfeifer, M. Richter, J. Ullrich, S. Gräfe, C. D. Lin & J. Biegert. Molecular structure retrieval directly from laboratory-frame photoelectron spectra in laser-induced electron diffraction Communications Chemistry volume 12, Article number: 1520 (2021).