Projects Currently Underway

1. Applications of duality in condensed matter systems

   Shantanu Mukherjee (SNB), Amitabha Lahiri (SNB), Flavio Nogueira (IFW), Jeroen van den Brink (IFW),

   This project deals with a model of magnetization, described by a non-linear sigma model (NLSM), interacting with a superconductor described by a gauged Landau- Ginzburg (LG) theory via an electromagnetic coupling term, more specifically a Zeeman term. The Euler-Lagrange equation shows that this leads to a topological structure in which the vortex is coupled to a Skyrmion-like configuration. In particular, Shantonu showed that the solution for the spin field describes a Skyrmion-like configuration within the region of extension of the magnetic field due to the vortex – the extension and radius of the Skyrmion depends on the magnetic length scale of the vortex.

2. ARPES studies on the Topological Systems

   Susmita Changdar (SNB), S. Thirupathaiah (SNB), Sergey Borisenko (IFW), Bernd Buechner (IFW)

   Our study on TaSb2 single crystals unveiled intriguing electronic properties. Notably, surface states near the Fermi level were revealed by comparing ARPES data with DFT slab calculations. Given the lack of experimental exploration into the band structure of TaSb2 to date, we are currently drafting a manuscript based on our findings from the collected ARPES data. Additionally, we plan further to investigate the topological nature of these surface bands.In our investigation on PtBi2, we successfully observed the pattern of the Fermi arc in the k x -k y plane. This observation was facilitated by the high-resolution data (energy resolution of 2-3 meV) collected with our Laser-ARPES set-up. Moving forward, our plans include thoroughly investigating Fermi arcs in additional samples to obtain a comprehensive understanding. Furthermore, we aim to explore the potential anisotropy in superconductivity to gain deeper insights into the material’s behavior.

3. Application of Machine learning in Search of new cuprate compounds

   Aishwaryo Ghosh (SNB), Saha-Dasgupta (SNB), Oleg Janson (IFW), van der Brink (IFW)

   Low-dimensional quantum spin systems hosting transition metal ions with small spin have attracted the attention of researchers for a long time. These systems provide a fascinating playground to study properties dictated by quantum fluctuations. These compounds are considered as low-dimensional as the magnetic or electronic dimensionality is reduced from 3D, as important electronic/magnetic interactions are found to be in one or two dimensions. Very often the true nature of the exchange networks cannot be foreseen from their crystal structure. In other words, electronic dimensionality is not always obvious from geometric dimensionality. This calls the need for time consuming detailed calculation for each compound. Is there a way to bypass these detailed calculations by employing a method with predictive power? In this study, based on a data driven approach, we explore this considering cuprate compounds containing oxides of Cu 2+ .

4. The nature of magnetic phases in 2D van der Waal magnet and their heterostructure

   Riju Pal (SNB), Atindra Nath Pal (SNB), Laura Corredor Bohórquez (IFW), Alexey Alfonsov (IFW), Louis Veyrat (IFW), Bernd Buechner (IFW)

   This project aims to understand the unique magnetic and electronic properties of the near-room-temperature (TC ~ 270 K) van der Waals (vdW) ferromagnet Fe4GeTe2. Fe4GeTe2 features a peculiar spin reorientation transition at TSR ∼ 110 K, suggesting a non-trivial temperature evolution of the magnetic anisotropy (MA). We further unveil the non-trivial physical properties utilizing electron spin resonance (ESR) spectroscopy. First, we provide quantitative insights into the unusual temperature evolution of MA of Fe4GeTe2, which additionally gives a detailed understanding of this peculiar spin reorientation. At high temperatures, the demagnetization effect mostly gives the total MA with a small contribution of the counteracting intrinsic MA of an easy-axis type, whose growth below a characteristic temperature Tshape ∼ 150 K renders the sample seemingly isotropic at TSR. Below one further temperature Td ∼ 50 K, the intrinsic MA becomes even more complex.

5. Exploration of Quantum Spin Liquid states in low-dimensional RuCl3

   Manodip Routh (SNB), M. Kumar (SNB), Satoshi Nishimoto (IFW), J. van der Brink (IFW)

   In this study, we examined the properties of the QP phase in this model on two geometries: the two-legged ladder and the two-dimensional honeycomb lattice, using exact diagonalization (ED) and density matrix renormalization group (DMRG) methods. Our analysis revealed a comprehensive ground-state phase diagram, showing the stability of QP order over a wide range of the phase diagram, particularly near the KSL phases. This enhancement is particularly intriguing as it occurs in the absence of long-range spin-spin correlations, suggesting a nuanced relationship between QP order and QSLs. In fact, a positive third-order susceptibility, which is indicative of a general signature of QP order, has been observed in the vicinity of the ferromagnetic-Kitaev QSL phase in α-RuCl 3 . This study shows that the QSLs in the phase co-exist higher spin order phase like QP or spin-nematic phase. We believe that it is a significant contribution in this fields. We have revised the quantum phase diagram of this model and show that there are two types of dimer phase in the system.