In a coherent experimental effort that combines thermodynamic methods (magnetization, specific heat and thermal expansion), high field electron spin resonance and nuclear magnetic resonance spectroscopies we investigate magnetic and electronic properties of materials with strong electronic correlations. These are complex transition metal oxides, iron pnictides and other related compounds where quantum entanglement of spins, orbitals and charges gives rise to novel quantum ground states and exotic spin excitations. Our main goal is to obtain fundamental insights into the properties of emerging unconventional spin phases in low-dimensional and frustrated quantum magnets, to examine their low-energy spin dynamics as well as to explore the interplay between magnetism and superconductivity in unconventional high-temperature superconductors.
Quantum magnetic phenomena in complex transition oxides
In many insulating transition metal oxides (TMO) interacting localized electron spins form low-dimensional (low-D) networks, such as spin chains, spin ladders or spin planes of different symmetry. The spatial spin confinement boosts the role of fluctuations. Of strong importance for the fluctuations is also the smallness of the magnetic spin quantum number of the constituting spins, where the strongest quantum nature is present for S=1/2 systems. The characteristic of such low-dimensional quantum spin systems are novel ground states and spin excitations which can radically differ from those known for classical magnets. Besides (or in addition to) the reduction of the spin dimensionality, strong frustration of the exchange interactions in the spin lattice due to its specific topology or competing interactions inhibits long-range spin order and yields a huge degeneracy of the ground states. Under certain conditions novel states, such as quantum spin liquids, magnetic monopole excitations and other exotic phenomena may arise. A coupling of an electron spin to the orbital degrees of freedom opens a plethora of new phenomena in quantum spin systems. In particular, very recently a novel kind of spin-orbital Mott insulators on the basis of iridium oxides has been proposed, where an unusual insulating or conducting behavior is controlled by both Coulomb repulsion and the strength of the spin-orbit coupling. The primary objective of our work in this research topic is to search for new magnetic phenomena in complex TMO and to provide important insights for the verification and development of fundamental models of low-D and frustrated quantum magnets. (selected publications)
Interplay of magnetism and superconductivity in novel superconductors (cuprates, iron pnictides)
The iron pnictide family of superconductors, with transition temperatures as high as 55 K, became the second family of materials capable of achieving high Tc, ending the monopoly of the cuprates in this field. The pnictides are similar to the cuprates in that both families are quasi-2d layered compounds with having an antiferromagnetically ordered ground state in the undoped parent compounds. With doping, the magnetic ordering is suppressed and superconductivity emerges. However, the two families are quite different as well. The pnictides have a multiband electronic structure as opposed to the single band physics of the cuprates. Furthermore, the undoped parent compounds of pnictides are metallic instead of Mott insulators as in the case of the cuprates. For sure, a comparison and contrast between the pnictides and cuprates will help to better understand high Tc superconductivity by identifying the common necessary contributing ingredients. One important issue in the field is e.g. whether the magnetism in the pnictides is driven by local moment physics like it is the case for the cuprates or by itinerant physics as well as to better understand the interplay between the magnetic properties and superconductivity in both iron pnictide and cuprate compounds. (selected publications)
In the recent past the field of molecular magnetism has experienced an enormous growth both because of the discovery of new magnetic quantum phenomena of a fundamental physical interest, but also because of recognizing a potentially great impact of this field for the emerging technologies of information storage, spin electronics, and quantum computation. Synthesis and magnetic characterization of molecules with steadily increasing number of paramagnetic transition metal ions in the molecule’s core has yielded the discovery of a number of Single Molecule Magnets, i.e. molecules possessing stable and large magnetic moments with sizable magnetic anisotropy of a pure molecular origin regardless the state of aggregation. Here we focus on the determination of the electronic ground state, magnetic anisotropy, the low-energy spectrum of the spin states and spin dynamics which is crucial for the understanding of correlations between the chemical composition, bonding geometry and magnetic properties of polynuclear metal organic complexes and consequently for a targeted design of the molecules with specific magnetic functionalities. (selected publications)
Nuclear Magnetic Resonance (NMR) is a powerful local probe technique for investigating the properties of Li ion battery materials. In LiMnPO4, we have shown by comparing the NMR spectra of two different nuclei, 7Li and 31P, that disorder in the Mn sublattice leads to the relatively poor electrochemical properties of this material. Our results are perfectly consistent with a recent theoretical study which found a formation of a vacancy-polaron complex owing to lattice distortion. In SiCN, we could successfully determine the activation energy EA and the correlation time 0 of the Li ion hopping process from temperature dependent 7Li linewidth and spin lattice relaxation rate measurements. From the NMR spectra, we find evidence that the carbon phases are the major electrochemically active sites for Li storage. (selected publications)
Electron spin resonance (ESR), also commonly named electron paramagnetic resonance (EPR), is a powerful spectroscopic tool in experimental condensed matter physics. With this technique one can selectively tune different electron spin ensembles into resonance by exposing the substance to high frequency electromagnetic radiation in the presence of magnetic field. ESR yields valuable information about crystal fields, spin-orbit and spin-spin interactions, spin- and lattice dynamics, spin structures and low-energy excitations in magnetically ordered states. At the IFW Dresden we have developed a cutting edge high frequency high-field electron spin resonance (HF-ESR) instrumentation that enables high-resolution and sensitive spectroscopic measurements over a broad range of control parameters, such as frequency up to 1 THz, magnetic field up to 16 T and temperature down to 300 mK.
The applied thermodynamic methods at the IFF comprise high-resolution magnetometry, dilatometry and calorimetry at low temperatures (3He and 4He temperatures), high magnetic fields (up to 18 T) and high pressures (up to 6 GPa) in order to study the magnetic and thermodynamic properties of strongly correlated electron systems. In addition to the AC and DC magnetization as well as the specific heat, thermal expansion and magnetostriction measurements give us the opportunity to evaluate not only the temperature but also the pressure dependence of the entropy, which in turn gives information on the pressure dependence of the magnetic transition temperature Tc (TN). In this context, a unique low-background hydrostatic pressure-cell up to ~6 GPa serves as an additional tool to experimentally determine the pressure dependence of the magnetization. Particular focus is given to quantum magnets with reduced dimensionality, frustration and the interplay of spin, charge, structure and orbitals in complex transition metal oxide systems as well as on novel FeAs superconductors.
The method Nuclear Magnetic Resonance (NMR) makes use of the hyperfine coupling of the nuclei to their electronic environment to gain information about local magnetic and electronic properties of superconductors, magnetic materials, Li ion battery materials, etc. The nuclei are ideal probes for static as well as dynamic properties since their magnetic moments are tiny compared to the electronic moments, thereby minimizing the influence on the electronic system. In addition, the nuclear quadrupole moment allows for probing the local charge distribution via Nuclear Quadrupole Resonance (NQR). All experiments can be performed in magnetic fields up to 16 Tesla, temperatures from 1.5 to 500 K, and pressures up to 3 GPa.