The Seminar, in its current format, started in November 2022 and is typically held one or two times per month on Thursdays. Its aims are:
- to foster German-Ukrainian collaboration and exchange of ideas among researchers working in the field of quantum materials, providing a forum for researchers to present their work, share their ideas, and get feedback from their peers;
- to introduce students and early career researchers to the field of quantum materials research, providing an opportunity for students and early career researchers to learn about the latest developments in the field and potentially identify new research opportunities.
Link to join: https://zoom.us/j/99281703595?pwd=QnVqN09BRGZKYlBhSk4wakpHd2trUT09
Meeting ID: 992 8170 3595 // Passcode: 963908
Upcoming and previous seminars:
Lecture #10: May 4, 2023 at 14:00 Kyiv time (13:00 Dresden time)
Speaker: Prof. R. Moessner, MPIPKS, Dresden
Title: Absence of diffusion in certain non-random magnets
Abstract: Thanks to their ubiquity and relative simplicity, magnets have played a central role in our understanding of the collective behaviour of matter. This talk presents very simple magnetic models which host highly unusual dynamical phenomena thanks to an emergent gauge field: one has fractionalised quasiparticles that move on a purely dynamical fractal invisible to thermodynamics; and the other exhibits localisation in the absence of disorder. While the former is based on an existing magnetic material called spin ice, various phenomena in these models -- known and unknown -- are in search of an experimental realisation.
Science 378, 1218 (2022)
Phys. Rev. Lett. 118, 266601 (2017)
Lecture #9: April 20, 2023 at 14:00 Kyiv time (13:00 Dresden time)
Speaker: Alexander Yaresko, Max Planck Institute for Solid State Research, Stuttgart
Title: How much anisotropy is enough? LDA+U study of anisotropic magnetic interactions in Iridium oxide
More info: https://kau.org.ua/news/sci/973-qs-yaresko-2023
Abstract: Because of strong spin-orbit coupling of Iridium 5d electrons magnetic interaction in many Ir4+ oxides cannot be described by an isotropic Heisenberg-like model and anisotropic exchange interactions become important. In this work LSDA+U band structure calculations with constrained magnetization direction are used to study magnetic properties of some Ir oxides with edge- and corner-sharing IrO octahedra. Effective magnetic interactions are estimated by mapping calculated total energy differences onto appropriate effective models which include isotropic Heisenberg as well as bond-dependent anisotropic interactions. In Na2IrO3, with edge sharing IrO octahedra forming a honeycomb lattice, magnetic interactions are dominated by the symmetric anisotropic Kitaev term. In R2Ir2O7 iridates, with a pyrochlore crystal structure, nearest neighbor Ir-Ir bonds do not have inversion symmetry and antisymmetric Dzyaloshinskii-Moriya (DM) interaction is allowed. It stabilizes noncollinear all-in-all-out magnetic order in pyrochlore iridates with a small radius of a rear-earth ion R.
Lecture #8: April 6, 2023 at 14:00 Kyiv time (13:00 Dresden time)
Speaker: Mikhail Belogolovskii, Kyiv Academic University
Title: Quantum interference patterns in new types of Josephson junctions
More info: https://kau.org.ua/news/sci/959-qs-belogolovskii-2023
Abstract: Josephson junction formed by two superconducting films connected with a weak link, e.g., a tunnel barrier or a normal interlayer, can support a non-dissipative supercurrent driving by a phase difference between the electrodes. The introduction of magnetic fields into such devices or more complex structures based on them leads to fundamentally new phenomena. The Fraunhofer-like pattern of the maximum Josephson current in a single Josephson junction and the periodic field dependence of the critical current in a two-junction quantum interferometer (SQUID) are analogues of corresponding classical interference effects in optics (one-slit and two-slit interference patterns, respectively). In this talk, I present two new types of stacked Josephson junctions developed and studied together with US colleagues (I.P. Nevirkovets and J.B. Ketterson, Northwestern University, Evanston, IL), namely, the first Josephson device with two amorphous superconducting electrodes demonstrating ideal high-uniform tunneling characteristics and the first Josephson junction formed by ordinary metallic films that exhibits SQUID-like supercurrent-magnetic field dependencies. We believe that the former samples will avoid non-homogeneous current distributions inside a weak link, which can lead, in particular, to supercurrent behavior mimicking a so-called 0 - Pi transition. The latter devices with edge-dominated currents across a weak link are expected to be promising candidates for a new generation of magnetic field nanosensors.
Lecture #7: March 16, 2023 at 14:00 Kyiv time (13:00 Dresden time)
Speaker: Hans-Henning Klauss, TU Dresden
Title: Degenerated superconductivity and fluctuation induced phases in multiband systems: Sr2RuO4 and Ba1-xKxFe2As2
More info: https://kau.org.ua/news/sci/947-qs-klauss-2023
Abstract: From the very beginning of quantum mechanics the concept of degeneracy of states has been an important topic throughout the quantum world. For example, the Landau-Zener dynamics of avoided level crossing describes quantum tunneling in single-molecule magnets as well as the deficiency in the flux of solar electron neutrinos at the earth. In solid state physics degeneracy can lead to multi-component order-parameters in symmetry broken phases such as superconductors and magnets. The topologically non-trivial skyrmion phase in the itinerant ferromagnet MnSi can be described as a triple-q antiferromagnet.
In particular in multi-band superconductors the degeneracy of superconducting order parameters with the same or different symmetry can lead to a coherent superposition. Unlike in magnets these states are experimentally difficult to detect. One indication is the appearance of time-reversal symmetry breaking (TRSB) superconductivity.
In my talk I will discuss two important systems in this context. In Sr2RuO4 we used uniaxial and hydrostatic pressure muon spin relaxation (µSR) experiments [1,2] to prove the two-component nature of the superconducting state using the concept of explicit symmetry breaking under uniaxial pressure. I will present uniaxial pressure studies along (100) and (110) directions in this system and discuss the implications on the superconducting state.
In Ba1-xKxFe2As2 we found TRSB superconductivity in µSR experiments at high doping x~0.8 due to the competition of single-component order parameters with different symmetry . This competition appears close to a Lifshitz transition of the multi-band fermi surface. Supported by detailed thermodynamic and transport experiments we found a new TRSB phase also above TC. This state can be understood as a fluctuation-induced Z2 symmetry breaking with four fermion correlations .
 V. Grinenko, S. Ghosh, et al., Nat. Phys. 17, 748 (2021),  V. Grinenko, et al., Nat. Comm. 12, 3920 (2021),  V. Grinenko, et al., Nat. Phys. 16, 789 (2020),  V. Grinenko, et al., Nat. Phys. 17, 1245 (2021).
Lecture #6: March 2nd, 2023 at 14:00 EET (13:00 CET) // IFW Dresden - D2E.27
Speaker: Pavel Sukhachov, Yale University
Title: Anomalous sound attenuation and electromagnetic field penetration in Weyl and Dirac materials
More info: https://kau.org.ua/news/sci/941-qs-sukhachov-2023
Abstract: A salient feature of Weyl and Dirac materials is the possibility to realize the chiral anomaly due to their relativistic-like electronic spectra and nontrivial topology. In this talk, I will present my recent results related to the manifestations of the chiral anomaly in sound attenuation and electromagnetic field penetration. Due to the interplay of intra- and inter-node scattering processes as well as screening, an external magnetic field generically reduces the sound absorption. A nontrivial dependence on the relative direction of the magnetic field and the sound wave vector, i.e., the magnetic sound dichroism, can occur in materials with nonsymmetric Weyl nodes. Also, I will demonstrate that the current response to an electromagnetic field in a Weyl or Dirac semimetal becomes nonlocal due to the chiral anomaly even under the conditions of the normal skin effect. Signatures of this nonlocality may be found in the transmission of electromagnetic waves.
Lecture #5: February 9, 2023 at 15:00 Kyiv time (14:00 Dresden time, IFW Dresden - D2E.27)
Speaker: Dmytro Inosov, TU Dresden
Title: Natural minerals as an inspiration for magnetic quantum materials
More info: https://kau.org.ua/news/sci/940-qs-inosov-2023
Abstract: The rapidly developing field of quantum magnetism relies heavily on the existence of real materials that serve as platforms for the experimental observation of various emergent phenomena predicted by theoretical models. These models usually assume a certain level of complexity and are very sensitive to a delicate balance of many tunable parameters. Therefore, the choice of an appropriate model system requires careful consideration of many candidate compounds. Using several examples from the forefront of research in magnetic frustration, I will show that natural copper minerals offer a rich pool of compounds for finding such model systems, and can inspire and guide both theorists in the development of interesting new spin models, and chemists in the synthesis of suitable materials to realize them. When it comes to crystal structures of high complexity, natural minerals are an inexhaustible source of new magnetic materials, and understanding their low-temperature properties represents a rapidly emerging interdisciplinary field, bridging mineralogy with low-temperature condensed-matter physics and quantum chemistry.
Lecture #4: January 26, 2023 at 15:00 Kyiv time (14:00 Dresden time, IFW Dresden - D2E.27)
Speaker: Alexander Kordyuk, KAU
Title: Multiband superconductors for quantum applications
More info: https://kau.org.ua/news/sci/928-qs-kordyuk-2023
Abstract: Multiband electronic structure is an inherent feature of high temperature superconductors and could be a key property responsible for both the mechanisms of high temperature superconductivity and novel applications of these materials. I will discuss a few examples in which we try to apply our experience in HTSC materials research to new quantum applications, namely: the Fermi surface filter for Josephson junctions, vortex dissociation effects, and an approach to mechanical quantum oscillator.
Lecture #3: January 12, 2023 at 15:00 Kyiv time (14:00 Dresden time, IFW Dresden - D2E.27)
Speaker: Sergey Borisenko, IFW Dresden
Title: Towards topological superconductivity using ARPES
More info: https://kau.org.ua/news/sci/918-qs-borisenko-2023
Abstract: We use angle-resolved photoemission spectroscopy (ARPES) to study the fine details of the fermiology of superconductors and topological materials in an attempt to detect topological superconductivity. I will present a novel ARPES technique that can more effectively study the dynamics of electrons at low energy, opening up the possibility for long-sought high-throughput measurements. I will also give an overview of our recent results on various candidates and related materials.
Lecture #2: December 8, 2022 at 12:00 Kyiv time (11:00 Dresden time, IFW Dresden - D2E.27)
Speaker: Jeroen van den Brink, IFW Dresden
Title: How surfaces Berry-curve Bloch-electrons
More info: https://kau.org.ua/news/sci/908-qs-jeroen-2022
Abstract: In recent years it has become clear that electronic Berry curvature (BC) is a key concept to understand and predict physical properties of crystalline materials. A wealth of interesting Hall-type responses in charge, spin and heat transport are caused by the BC associated to electronic bands inside a solid: anomalous Hall effects in magnetic materials, and various nonlinear Hall and Nernst effects in non-magnetic systems that lack inversion symmetry. However, for the largest class of known materials---non-magnetic ones with inversion symmetry---electronic BC is strictly zero. Here we show that precisely for these bulk BC-free materials, a finite BC can emerge at their surfaces and interfaces. This immediately activates certain surfaces in producing Hall-type transport responses. We demonstrate this by first principles calculations of the BC at bismuth, mercury-telluride (HgTe) and rhodium surfaces of various symmetries, revealing the presence of a surface Berry curvature dipole and associated quantum nonlinear Hall effects at a number of these. This opens up a plethora of materials to explore and harness the physical effects emerging from the electronic Berry curvature associated exclusively to their boundaries.
Lecture #1: November 10, 2022 at 12:00 Kyiv time (11:00 Dresden time, IFW Dresden - D2E.27)
Speaker: Bernd Büchner, IFW Dresden
Title: Iron pnictides and beyond: New insights on superconductivity
More info: https://kau.org.ua/news/sci/902-guqumat-seminar-01