Model Systems and Correlated Matter

This research area addresses model systems that exhibit a reduced complexity. Therefore, model systems provide an ideal basis for understanding fundamental mechanisms that result in correlations in complex materials. Examples of systems established within MAINZ include ultracold quantum gases and magnon gases, which also allow for a precise understanding and tailoring of interaction strengths. Rational design of correlated systems, including molecular magnets, superconductors and charge transfer salts, makes use of this understanding. Future research will focus on tailoring structure and interactions to optimize materials properties by varying the chemical or physical environment. Manipulating properties by ultra-fast excitations constitutes a further strategy for studying the dynamics of processes in highly correlated materials.

The research area Model Systems and Correlated Matter contains prime examples of systems with varying complexity. It comprises on the one hand correlated materials (e.g. complex superconductors, magnetic materials, etc.), on the other hand simpler model systems such as ultracold quantum gases and magnon gases.

Many of the peculiar properties of complex correlated matter directly emerge from the fundamental quantum-mechanical characteristics of its constituents. Model systems are particularly suited to study these fundamental effects in correlated systems since their reduced complexity allows for more precise analysis and engineering of correlations and interaction strengths. Fundamental problems, which are encountered in real 3D materials but are difficult to understand, are tackled under idealized conditions by first studying simpler lower dimensional model systems. At the same time real materials help to identify the most relevant questions for applications, which afterwards can be investigated in model systems. Here strong theoretical input and experimental expertise as well as application-oriented material engineering are brought together. With this, the research area can provide an intrinsic research supply chain to enhance the understanding and the development of building blocks for the next generation of complex materials. Spectacular phenomena in these materials include high-Tc superconductivity, novel states of matter such as topological insulators, or the giant magnetoresistance (GMR) effect. The potential impact of the resulting applications can be tremendous, which becomes obvious for the GMR effect, which is at the heart of the exponential capacity increase in hard disk data storage.

We now describe our research efforts in more detail starting with systems that exhibit weak interactions. In the regime of coherent phenomena, correlations are usually weak. Coherence effects such as Bose-Einstein condensation (BEC), superfluidity, superconductivity or decoherence phenomena of low-dimensional systems are studied within the scope of CRC/TRR49. Coherent systems can often be described by a single wave function, where all particles share the samequantum-mechanical properties (Ott, Widera, Fleischhauer, Sirker). An example are ultracold quantum gases where interaction strengths and correlations can be engineered. Coherence is also at the root of magnonic model systems, where the magnons – the bosonic quasi-particle excitations of the magnetization – generate spin currents and spin injection resulting in possible applications in low power spintronic devices for Green IT applications (Hillebrands).

The transfer from model systems to real materials is often accompanied by an increase in the interaction strength. The onset of correlations with increasing interactions imposes a strong challenge to their theoretical description and translates into new properties of materials, such as unconventional quantum phases, new inter-particle coupling mechanisms, and complex ordering. This challenging theory has been identified as a key prerequisite for the rational design of new materials. Within MAINZ, this area has been strengthened by a new Junior Research Group, which complements the theoretical efforts on correlated effects in molecules (Gauss).

Examples of interdisciplinary research on correlated matter guided by our scientific objectives are:

  • Molecular magnets: These complex structures are rationally designed from single molecule constituents including exchange-coupled transition metal ions (Rentschler). They possess magnetic properties of a quantum magnet but also many properties normally associated with organic polymeric compounds, establishing a link to the research area of Functional Polymers. Molecular magnetism is therefore a prime example where the physics of single spin systems is combined with the chemical ligand design, leading to amolecular assembly that is promising for non-volatile spintronic devices.
  • Nanographene: An example for engineering Function-through-Structure is the synthesis of nanographene structures from low-dimensional molecules. These elements can act as modelsystems, since they exhibit a wide range of electronic structures depending on the atomic arrangement. The edge structure at the atomic level determines magnetic correlations and spin transport properties as investigated within the National Priority Programm Graphene (Müllen, Kläui, Schönhense, Felser). High-resolution investigation of these molecules is particularly interesting on non-conducting surfaces to provide electronic decoupling from the underlying substrate (Kühnle).
  • Charge Transfer Salts: Advanced synthesis also allows for the Rational Design of new acceptors and donors as attempted within CRC/TRR49. These can be combined into new charge transfer salts thus building “artificial bulk metal” from single low dimensional molecules. This research is one of many examples for the close collaboration between physicists and chemists at MAINZ to bridge research areas to design new matter.
  • Heusler compounds: These and related low-dimensional materials, such ZrCuSiAs, ZrBe-Si, ThCr2Si2 and PbFCl-structure types are among the most promising candidates to rationally design and develop new superconductors, which could provide lossless energy transport. Their spin-dependent band structure makes them candidates for non-volatile low power spintronic applications. Thermo-electric Heusler compounds are promising to convert waste heat into useable energy. Iron pnictides belong to related structure types and to a class of superconductors, which attracts significant interest due to their high critical temperature and tunability of correlations at the border between magnetism and superconductivity. These materials are extensively studied in MAINZ using various characterization techniques (Felser, Aeschlimann, Hillebrands, Kläui).