Collaborative Research Center 360
Constrained Quantum Matter
Less is more!
We use carefully chosen constraints to design and
manipulate quantum states in solids, seeking to create new quantum
materials and explore conceptual challenges from quantum information
theory to non-equilibrium physics.
Next events
Recent Articles
Kumar, H.; Köpf, M.; Telang, P.; Bura, N.; Jesche, A.; Gegenwart, P.; Kuntscher, C. A. Phys. Rev. B 110, 035140, 2024. @article{kumar_optical_2024, The synergy of strong spin-orbit coupling and electron-electron interactions gives rise to unconventional topological states, such as topological Mott insulator, Weyl semimetal, and quantum spin liquid. In this study, we have grown single crystals of the pyrochlore iridate Pr2Ir2O7 and explored its magnetic, lattice dynamical, and electronic properties. While Raman spectroscopy data reveal six phonon modes confirming the cubic Fd‾3m crystal symmetry, dc magnetic susceptibility data show no anomalies and hence indicate the absence of magnetic phase transitions down to 2 K. Both temperature-dependent electric transport and optical conductivity data reveal the metallic character of Pr2Ir2O7. The optical conductivity spectrum contains a midinfrared absorption band, which becomes more pronounced with decreasing temperature due to spectral weight transfer from high to low energies. The presence of the midinfrared band hints at the importance of correlation physics. The optical response furthermore suggests that Pr2Ir2O7 is close to the Weyl semimetal phase. |
Pichler, F.; Kadow, W.; Kuhlenkamp, C.; Knap, M. Probing magnetism in moiré heterostructures with quantum twisting microscopes Journal Article Phys. Rev. B 110, 045116, 2024. @article{pichler_probing_2024, Spin-ordered states close to metal-insulator transitions are poorly understood theoretically and challenging to probe in experiments. Here, we propose that the quantum twisting microscope, which provides direct access to the energy-momentum resolved spectrum of single-particle and collective excitations, can be used as a novel tool to distinguish between different types of magnetic order. To this end, we calculate the single-particle spectral function and the dynamical spin-structure factor for both a ferromagnetic and antiferromagnetic generalized Wigner crystal formed in fractionally filled moiré superlattices of transition metal dichalcogenide heterostructures. We demonstrate that magnetic order can be clearly identified in these response functions. Furthermore, we explore signatures of quantum phase transitions in the quantum twisting microscope response. We focus on the specific case of triangular moiré lattices at half filling that have been proposed to host a topological phase transition between a chiral spin liquid and a 120° ordered state. Our work demonstrates the potential for quantum twisting microscopes to characterize quantum magnetism in moiré heterostructures. |
Kunze, J.; Köpf, M.; Cao, Weizheng; Qi, Yanpeng; Kuntscher, C. A. Optical signatures of type-II Weyl fermions in the noncentrosymmetric semimetals RAlSi (R=La, Ce, Pr, Nd, Sm) Journal Article Phys. Rev. B 109, 195130, 2024. @article{kunze_optical_2024, Weyl semimetals with magnetic ordering provide a promising platform for the investigation of rare topological effects such as the anomalous Hall effect, resulting from the interplay of nontrivial bands with various spin configurations. The materials RAlSi, where R represents a rare-earth element, are prominent representatives of Weyl semimetals, where the Weyl states are induced by space inversion symmetry breaking, and in addition, for several rare-earth elements R, enhanced by time-reversal symmetry breaking through the formation of a magnetic order at low temperature. We report optical signatures of Weyl fermions in the magnetic compounds CeAlSi, PrAlSi, NdAlSi, and SmAlSi as well as the nonmagnetic family member LaAlSi by broad-frequency infrared spectroscopy at room temperature, i.e., in the paramagnetic phase. A similar profile of the optical conductivity spectrum and a metallic character are observed for all compounds, with LaAlSi showing the strongest free charge-carrier contribution. Furthermore, the linear-in-frequency behavior of the optical conductivity of all investigated compounds indicates the presence of Weyl nodes in close proximity to the Fermi energy, resulting from inversion symmetry breaking in noncentrosymmetric structures. According to the characteristics of these linear slopes, the RAlSi compounds are expected to host mainly type-II Weyl states with overtilted Weyl cones. The results are compared to the optical response of the closely related RAlGe materials, which are considered as potential hybridization-driven Weyl-Kondo systems. |
Impressions from past events
The research programme
We seek to design and utilize new quantum states by taking advantage of the recently developed capability to tailor electron systems in complex materials through a variety of complementary constraints, focusing on spin-momentum locking, gauge structures of interacting spin systems, and kinetic constraints.
News
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