Strong electronic correlations in transition-metal-oxides give rise to many exciting phenomena such as unconventional superconductivity, multiferroic states or competing interactions. In this section we report how the electronic and magnetic properties can be tailored by the crystal growth and how the new properties can be observed as well as theoretically understood. In particular, we investigate the complex iridium oxide Na3Ir3O8 with a chiral hyper-kagome lattice, the inverse-perovskites M3TrO (Tr = tetrel element: Si, Ge, Sn, Pb), and also iron-based superconductors such as Fe2Se2. From the spectroscopic side, we employ inelastic neutron scattering, spin-polarized scanning tunneling microscopy, and magnetization measurements. Finally, we analyze the resulting electronic correlations theoretically, in particular, their non-Fermi-liquid behavior, the origin of quantum interference and provide insights to a Mott-insulating state by a many-electron wavefunction.
Looking for new functionality in heterostructures, with greater flexibility and tunability compared to conventional semiconducting devices, may be achieved while using complex oxide and organic interfaces. One of the focuses of the Institute’s research is in combining different transition metal oxides with competing many-body interactions where a variety of ground states not present in the parent materials can be stabilized when heterostructured. Vivid examples given in this section include: the superconducting vortex state in the insulating regime of the LaAlO3–SrTiO3 interfaces, the ferromagnetic metallic state in atomically thin SrRuO3, spin and charge ordered states in nickelates and manganites as precisely controled via epitaxial strain and oxygen deficiency. Coming to organic electronics, one of the highlights reports on the successful fabrication of fast organic complementary circuits on flexible plastic substrates.
Quantum mechanical effects are primarily manifested in experiments probing low-dimensional entities. In this section, two examples of studying spin states in individual nanomagnets are presented: a magnetic STM tip and shallow implanted nitrogen-vacancy centers were used to probe spin dynamics of atomic Fe chains and single ferritin molecules. Quantum interfaces bridging nanoelectronics and nanophotonics are exemplified by a single molecule embedded in a tunnel junction gating the electrical plasmon generation. Several reports in this section present the most recent developments based on advanced nanofabrication technologies in the traditional field of two-dimensional electron systems (2DES). In particular, charge density modulated phases in the quantum Hall regime are studied by patterning 2DES with different combinations of quantum dots and constrictions in parallel paths from source to drain or by using comb-shaped gate structures to control the propagation of surface acoustic waves. One of the highlights demonstrates a non-destructive rote to covalently functionalize graphene via hyperthermal reaction.
Harvesting light in a way that its energy is stored as a fuel with a high energy density may combine a lot of advantages for the future energy sector. Hydrogen is of particular interest as a fuel (e.g. it can immediately be used in fuel cells producing electrical energy with high efficiency), and light driven water electrolysis could be the process used for its production. Here, we report two interesting materials (carbon nitride nano sheets and a covalent organic framework) which both show remarkable photocatalytic activity for water splitting. When it comes to the storage of electrical energy, storing large quantities of e.g. lithium at a relatively constant chemical potential is key to a high effective energy density. Along which pathways lithium is incorporated into adequate materials (e.g. LiFePO4) and how it is stored (e.g. at abrupt junctions) is investigated in detail. Since materials with a multitude of different kinds of charge carriers are quite common in materials for electrochemical energy conversion, the transport behavior of a model system (with protons, oxide ion vacancies and electronic holes being the charge carriers) is described in a quantitative way.
The paradigm of low-dimensional quantum systems is a two-dimensional electron system (2DES) embedded in semiconductor nanostructures. The emergence of the integer and fractional quantum hall effects in 2DES exposed to a magnetic field is one of the most fascinating phenomena in condensed matter physics. Several reports of this section present the most recent developments in this field. In particular, recent results provide evidence for Wigner crystallization and for modulation of the electron density in the plane of 2DES in the quantum Hall regime. Low-dimensional entities such as individual atoms, molecules and nanowires are also included in the subject of this section. Some of the recent developments involve the use of individual molecules as gates for plasmonic light emission, exploring the ultrafast dynamics of carriers in a single nanowire, and mapping the full three-dimensional magnetic anisotropy field of individual atoms.
One of the important focuses of the Institute's research is the design and exploration of heterostructures that have greater functionality compared to conventional semiconductor devices. Several highlights in this section are devoted to the unconventional electronic and magnetic properties that can be generated by oxide heterostructures. Current research efforts not only comprise further search for new combinations of different transition metal oxides and systematic incorporation of multiple interfaces with atomic precision, but also the development of integrated logic circuits that utilize two-dimensional electron gases at oxide interfaces. Another way to complement Si-based technology is making the electronics flexible and wearable by integrating inorganic and organic materials. One of the highlights demonstrates the feasibility of the fabrication of organic transistors on flexible plastic substrates.
Unconventional superconductivity and electronic correlations
Superconductivity can become unconventional when complex materials or exotic pairing mechanisms are involved. In modern solid state physics, unconventional superconductivity often occurs in the presence of competing interactions which are the consequence of electronic correlations involving charge, spin and orbital degrees of freedom. In these cases the description of superconductivity then requires both new experimental techniques and theoretical approaches. In this connection, several highlights of this section are devoted to complex materials revealing competing interactions: First, an important detection of the competition between superconductivity and a Charge-Density-Wave is found both in the cuprate high-temperature superconductor YBa2Cu3O6.6 using inelastic X-ray scattering as well as in the Cu-intercalated superconductor PdTe2. Next, we analyze the role of orbital degrees of freedom if a competition between triplet superconductivity and ferromagnetism takes place followed by a general theoretical approach employing functional Renormalization Group combined with Mean-Field Theory to study the competition between singlet superconductivity and antiferromagnetism in the ground state of the 2D Hubbard model. Further electronic correlations are demonstrated by measuring magnon lifetimes in the 2D and 3D antiferromagnets Rb2MnF4 and MnF2, respectively, by a new high-resolution neutron technique as well as in the spin-1/2 linear chain ferromagnet CuAs2O4. This section will be closed by the discovery of the Kondo effect in the weak-coupling regime in a purely organic molecule and by a new extension of the Density Functional Theory applied to the dilute magnetic semiconductor GaMnAs.
The functioning of electrochemical and thermionic energy conversion and storage devices such as fuel cells, batteries and thermionic devices rely on the availability of materials with very specific properties. These may be efficient oxygen reduction catalysts (relevant for fuel cells), ion conducting separator materials and stable but electrochemically active electrode materials (for batteries). In this section, a few fundamental approaches for analyzing existing and developing new functional materials are presented.
Solids may be unusual, in that they exhibit an unusual structure or show a rare effect. Here, we present a natural mineral which is a topological insulator, we demonstrate how unusual structures may be assembled from exfoliated nano-sheets, and we describe a diffraction experiment following the phase transition of a "jumping crystal".
The most traditional area of our research is an understanding of solids through their comprehensive characterization, rationalizing the synthesis, and prediction of new materials. In the classical field of semiconductor physics, by analyzing the properties of II-IV-V2 semiconductors, we gained insight into why the conventional increase in the band gap with decreasing temperature turns into an anomalous non-monotonic behavior when the divalent cation is replaced by monovalent d-electron copper or silver, e.g. in AgGaSe2. Even less expected was the recent discovery of superconductivity controlled by copper intercalation between the Se layers in the prototypical topological insulator Bi2Se3. The growth of single crystals of CuxBi2Se3 with various Cu contents has been optimized. Interest to structures with intercalated metal atoms makes it very topical to develop methods for accurate reconstruction of their electron density distribution from X-ray diffraction data. A general method using maximum entropy has been developed for localization of missing intercalated metal atoms in apatites. Titanium oxides continue to surprise with their unusual electron transport and structural properties which can be tuned by introducing dislocations or by applying a magnetic field.
Strong electron correlations in transition-metal compounds give rise to a multitude of unusual and interesting phenomena, such as, metal-insulator transitions, superconductivity at high temperature, and spin-orbital or multiferroic ordered states. These strong correlation effects cannot be understood within simple non-interacting single-electron theories. However, it is possible to describe certain aspects of these strong correlation phenomena in terms of extensions of single-particle approaches (e.g., combined local-density-functional and Gutzwiller theory), or by employing simplified model Hamiltonians (e.g., Kugel-Khomskii or Hubbard-type models). Besides these theoretical studies, this section also includes experimental investigations of two strongly correlated materials, namely the multiferroic chain compound CuBr2 and the iron-selenide superconductor Rb2Fe4Se5.
The quantum Hall effect is one of the most fascinating phenomena in condensed matter physics. This quantum mechanical state of matter is observed in two-dimensional metals in strong magnetic fields and is characterized by a Hall resistance, which is nearly constant over certain ranges of electron density and magnetic field strength. The reports of this section present some of the most recent experimental developments in this area of research. In particular, transport and scanning probe measurements of fractional and integer quantum Hall states in GaAs/AlGaAs-heterostructures as well as in graphene sheets are discussed.
The design and exploration of heterostructures grown from complex materials with atomically precise interfaces are at the focus of the Institute. We are particularly interested in combining different complex oxide materials with competing quantum many-body states where a variety of exotic two-dimensional electronic systems could be stabilized. Recent investigations have shown that both magnetic and superconducting phases can be induced at the interface between the relatively simple insulating materials SrTiO3 and LaAlO3. The electronic compressibility of this electronic system has been studied with Kelvin probe microscopy. The long-range transfer of electron-phonon coupling through the interfaces between oxide materials has been observed and demonstrates that epitaxial oxide superlattices offer novel opportunities to generate vibrational modes that do not exist in the bulk. The complexity of artificial heterostructures requires the development of new methods of rational calculation of their electronic structure. A new methodology has been developed to derived atomic effective pseudopotentials for semiconductor superlattices.
Nano-sized materials exhibit unusual magnetic, electrical, and chemical properties which are very different from those present in larger sized systems. This is in part due to quantum mechanical effects, which are much stronger at the nanoscale, and part due to the increased surface-to-volume ratio, which leads to an enhanced chemical reactivity. In this section, two examples of nanosized magnets are presented: a manganese-12-acetate molecular magnet and small clusters of iron atoms on monoatomic Cu2N surfaces. Furthermore, different fabrication methods of nanomaterials are discussed, such as stereoselective self-assembly and van der Waals epitaxial growth procedures.
A considerable part of our research contributes to the bases of further technological progress, some project even being motivated by potential applications in diverse technologies. A few examples are given in this section comprising a project on two-dimensional photonic crystals which may find interesting applications in sensing (e.g. pH) devices and several activities relying on the bulk and surface properties of titania, an oxide with very high relevance in electrochemical energy storage technologies (batteries). When it comes to storing energy chemically, visible-light driven hydrogen evolution is one of the most important reactions which is shown to be effectively catalysed by triazine-based carbon nitrides. Coming to electronic and opto-electronic devices, air-stable organic n-channel transistors may help to reduce the power consumption of mobile devices, while the fabrication of plasmonic nano-structures aligned to stable solid-state quantum emitters may pave the way to advanced plasmonic nanocircuits.
Superconductivity with its various unconventional properties remains one of the classical problems of physics. Since the quality of known compounds gets largely improved and also new classes of superconductors are discovered, one is now able to obtain deep insights into the formation of Cooper-pairs and the resulting characteristic spectroscopic properties. In this section, several research highlights are presented: First, the presence of a magnetic resonance mode below Tc in Fe-based superconductors can be demonstrated. These measurements were only possible, because high-quality crystals had been grown. The resonant mode is a strong feedback effect of superconductivity on the magnetic spectrum. It is intimately related with the response at higher energies in Resonant Inelastic X-ray Scattering (RIXS) measurements, allowing a quantitative analysis of the strength of the magnetic pairing glue in the normal state. Analyzing a new class of non-centrosymmetric superconductors in which both singlet- and triplet-pairing simultaneously occur, one can find topologically-protected surface states that may be observed by tunneling spectroscopy. Further it is demonstrated that this technique can also be employed to determine the gap structure of the two-dimensional electron liquid formed at the interface of LaAIO3 and SrTiO3. These results may be contrasted with the behavior of conventional superconductors such as doped picene. Finally, the role of the important Coulomb pseudopotential can be clarified by a modern Dynamical Mean Field Theory (DMFT) approach.
Another important focus of the Institute's research is the exotic physical properties of low-dimensional carbon structures, such as fullerenes, carbon nanotubes, or graphene. These materials are at the crossroad of fundamental research and the search for intriguing industrial applications, such as the possibility to use carbon nanotubes in nucleic acid biosensors or as minute field-effect-transistors in future high-performance integrated circuits. Several highlights in this section are devoted to the unconventional properties of different forms of graphene, where the mechanical, optical, and quantum-mechanical properties and the possibility to control its electronic structure by chemical doping are investigated.
Sometimes, it is the structural or chemical complexity of materials that creates their particular properties: A pair of two-dimensional electron gases may be formed so that coupling of holes and electrons leads to the formation of excitonic BCS states; the optical properties of multilayer structures can be tuned by external parameters such as temperature or gas composition. Two-dimensional electron liquids are not only formed at the interface of well-established semiconductors, but also at the interface of well-tailored perovskite-type oxides, which can reversibly undergo insulator/metal transitions, and the width of quantum wells can be controlled in order to observe different fractional quantum Hall states. For tailoring complex materials it is important to consider metastable states, which are represented by so-called "extended phase diagrams".
Albeit oriented toward a fundamental understanding of materials and device properties, some research fields are of particular technological relevance. This not only comprises the deeper understanding of technologically well-established materials, but also the exploration of new approaches in the development and implementation of new materials. Examples are electrodes and electrolytes for electrochemical energy conversion devices, such as batteries and fuel cells, unsaturated metal sites for the catalytic dissociation of oxygen, organic semiconductors for "plastic electronics" and nano-optical antennas for building single photon detectors.
Synthesis, characterization and physical understanding of crystalline solids form one of the cornerstones of the Institute's research. It is a particular challenge to determine or predict the structures of complex materials with structural frustration, mesostructured frameworks and crystal lattices arranged of similar building blocks, as in the case of Na1+xCuO2 Wigner crystals, or those involving large stable molecules, such as fullerenes. New methods of structural refinement are applied to characterize compounds with structural instabilities, which can be tuned by chemical substitution or electron doping. To reach a microscopic physical understanding of the diverse phenomena in crystalline compounds, first-principles calculations become indispensable, as they possess predictive power in describing electronic structures, vibrational properties of complex clusters, or explain the lattice distortions in compounds with heavy elements, in which relativistic effects start playing a major role.
The complex interactions between electrons involving charge, spin, and orbital degrees of freedom result in strong correlations. Their description often requires a combination of ab initio schemes and many-body methods; the results may be tested by various experimental techniques. Several highlights in this section are devoted to unconventional properties of low-dimensional materials with strong electronic correlations: First, the success of novel ab initio cluster calculations is investigated which include the full Coulomb vertex for NiO. Next, the interesting Spin-Peierls transition in TiPO4 both with magnetic measurements as well as with Density-Functional Theory (DFT) calculations is discussed. The measured anisotropic optical response of NaCu2O2 shows fair agreement with the results of the LSDA+U method. This section will be closed by an experimental realization of the two-impurity Kondo problem by using Scanning Tunneling Microscopy (STM) and by theoretical investigations of a nematic quantum critical point.