# Quantum Matter Theory

The research within the programme Quantum Matter Theory focuses on condensed matter where quantum mechanical effects in a very prominent way determine the properties.

Current research interests span several contemporary areas in modern physics, such as unconventional superconductivity, topological matter, non-equilibrium physics, strongly correlated systems, ultracold atoms, and quantum information. Beyond a natural connection between several of these areas, a strongly unified focus of the research is the methodology, where low-energy models for complex and quantum mechanical many-body systems play a central role. While the research is naturally linked to more material specific research, the overarching focus here is on understanding the physics of matter on a conceptual and unified level.

## Topological and Dirac materials

Topological and Dirac materials are large classes of recently discovered materials that have received immense amount of attention, including the Nobel Prizes in Physics in 2010 and 2016.

We undertake research on a number of topological and Dirac materials and phenomena, spanning from general effective low-energy models to full ab-initio studies of both advanced materials and engineered structures.

Topological materials have an intrinsic property, topological order, which gives rise to protected surfaces states with exotic properties. These lead to everything from dissipation-free surface transport in topological insulators to exotic Majorana fermions in topological superconductors.

In Dirac materials the low-energy excitations form a linear Dirac dispersion, usually only found for high-energy electrons in vacuum. Graphene, surface states in topological insulators, and high-temperature cuprate superconductors are all Dirac materials, which despite their diversity share unifying physical properties.

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## Superconductivity

Superconductivity is the astonishing physical phenomenon of a material conducting electrical current without any resistance, i.e. without any ohmic losses. Ever since its discovery more than 100 years ago, superconductivity has fascinated scientists, who have attempted to develop suitable theories. To increase the understanding of novel forms of superconductivity, we develop theoretical methods and selfconsistent calculations.

The theory of Bardeen, Cooper and Schrieffer provides a basic description of conventional superconductivity mediated by quantized lattice vibrations. However, new and unusual forms of superconductivity have been discovered, which require new conceptual understanding. Among these are high-temperature superconductivity in copper-oxides and other unconventional forms that may appear in two-dimensional materials, heterostructures, and topological materials, as well as quasi-1D materials (Bechgaard and Fabre salts, chromium pnictide). Coupling of 1D systems with strong intrinsic unconventional pairing to clean metallic substrates may further result in micron-length superconducting devices working at high temperatures. To investigate these unconventional superconductors we use low-energy effective models, develop the anisotropic multiband Eliashberg framework for computing high-temperature superconductivity, and employ and extend the numerical density matrix renormalization group to parallel supercomputers (both stand-alone and in conjunction with mean-field methods). Other areas of interest are odd-frequency pairing and strong-coupling superconductivity.

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## Non-equilibrium Nano Physics

Physical phenomena occur in general under non-equilibrium conditions, because of time-dependence, influence from external force fields, and local variations in the environment. Measurements intrinsically invoke disturbances which give rise to fluctuations that may or may not be desired. Our task is to develop new theoretical framework for studies of dynamical aspects of correlated materials and apply it to address properties of concrete systems.

Developments of new theoretical framework for studies of dynamical aspects of correlated materials under non-equilibrium conditions is one of our central tasks. The primary focus is on fundamental mechanisms for couplings between electronic and internal degrees of freedom, which may be of, e.g., magnetic, electric, and mechanical nature, and they may be coupled effectively through the electronic structure. It is crucial to deeply analyse the importance of those interactions in the bigger picture, since they may dramatically influence the physical dynamics. Of great importance is testing our developments to applications to address properties of concrete systems. For instance, we study dynamical magnetic exchange interactions between magnetic molecules, electric and thermal field control of magnetic exchange interactions.

The theme and focus is devoted to physical phenomena that happen under general non-equilibrium conditions, including time-dependence and strong perturbations, or distortions, under external force fields, e.g., voltage bias and thermal gradient. We develop new theoretical framework for studies of dynamical aspects of correlated materials and apply it to address properties of concrete systems.

The research can be very much partitioned into three branches.

- Chiral induced spin selectivity – fundamental theory for the mechanisms behind the phenomenom.
- Developmenst of new theoretical framework for studies of dynamical aspects of correlated materials under non-equilibrium conditions.
- Applications of new developments to address properties of concrete systems, such as dynamical magnetic exchange interactions between magnetic molecules.

### Funding

The work is supported by external grant from Stiftelsen Olle Engqvist Byggmästare.

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## Quantum Information

Quantum information is a cross-disciplinary field with great potential impact on future information technology. Key applications in the field are information processing, secure communication, simulations of quantum systems, and metrology.

We use geometric and topological techniques to develop robust logical gates for quantum computation and to seek a deeper understanding of non-classical correlations, in particular quantum entanglement and Bell-type non-locality.

We work on various other topics in quantum mechanics, such as Berry phases, weak measurements, photon and neutron optics, cold atom dynamics, as well as quantum properties of magnetic systems.

### Publications

- J. Zhang, T. H. Kwyaw, S. Filipp, L.-C. Kwek, E. Sjöqvist, and D. M. Tong,
*Geometric and holonomic quantum computation*, Physics Reports**1027**, 1 (2023). - J. Larson, E. Sjöqvist, and P. Öhberg,
*Conical intersections in physics: an introduction to synthetic gauge theories*, Springer Lecture Notes in Physics (2020). - E. Sjöqvist,
*Geometric phases in quantum information*, International Journal of Quantum Chemistry**115**, 1311 (2015). - E. Sjöqvist,
*A new phase in quantum computation*, Physics (APS)**1**, 35 (2008).

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## Staff

## Publications

Part of Physica. B, Condensed matter, 2024

Part of Experimental dermatology, 2024

- DOI for A novel professional-use synergistic peel technology to reduce visible hyperpigmentation on face: Clinical evidence and mechanistic understanding by computational biology and optical biopsy
- Download full text (pdf) of A novel professional-use synergistic peel technology to reduce visible hyperpigmentation on face: Clinical evidence and mechanistic understanding by computational biology and optical biopsy

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## Contact

- Quantum Matter Theory
- Annica Black-Schaffer