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Our main research interests are states of matter which arise as a result of strong interactions between electrons related magnetic interactions in solids. Such ``exotic'' ground states as superconductivity and spin liquids (quantum disordered magnetic states) originate from these interactions. Interestingly, the same interactions between electrons, which lead to unconventional superconductivity, can produce an insulator when tuned to be somewhat stronger. This kind of ``fine tuning'' of certain control parameters also allow physicists to tune materials from magnetically ordered to quantum disordered spin liquid state. A better understanding of what parameters of materials to control can not only reveal the basic physical phenomena, but allow us to tune material properties.
One of the ways to study fundamental properties of materials is to measure their excitation spectra. We can obtain information about masses of atoms and strength of chemical bonds in a system build of atoms, such as a molecule or a crystal, by exciting vibrations. Similarly, we can learn about interactions between electrons and spins in materials by exciting the electronic and magnetic degrees of freedom. One of the ways to study all of these excitations, vibrational, electronic, and magnetic ones, is inelastic light scattering, so-called Raman scattering. Below you can find examples of research projects done in our lab.
New research in organic quantum materials as presented at ISCOM 2024 conference:
Recordings of the tutorials on molecular quantum materials
Recordings of the session on metal-organic-frameworks as quantum materials
"Everything material" Nd2Ir2O7: Raman scattering evidence for Weyl nodes and quantum spin ice fluctuations of Nd magnetic moments. Video of Natalia's talk at a conference at KITP, August 2023
"Everything material" Nd2Ir2O7: Raman scattering evidence for Weyl nodes and quantum spin ice fluctuations of Nd magnetic moments. Video of Natalia's talk at a conference at KITP, August 2023
Observation of a quantum dipole liquid state in an organic quasi-two-dimensional material
Observation of a quantum dipole liquid state in an organic quasi-two-dimensional material
A metal in which the repulsion between conduction electrons dominates over their kinetic energy becomes an insulator at low temperatures. Such Mott insulators are commonly pictured with electrons localized on lattice sites. Their low-energy physics involves spins only. We experimentally demonstrate that new charge degrees of freedom emerge in a molecule-based Mott insulator k-(BEDT-TTF)2Hg(SCN)2Br resulting in quantum dipole liquid state. When electrons localize on molecular dimer lattice sites on triangular lattice, they form electric dipoles which do not order at low temperatures and fluctuate with a frequency detected experimentally in our Raman spectroscopy experiments. The heat capacity and Raman scattering response support a scenario where the composite spin and electric dipole degrees of freedom remain fluctuating down to the lowest temperatures.
Raman spectroscopy of SmB6: break down of bulk hybridization gap and Kondo topological insulator state due to Sm vacancies.
Raman spectroscopy of SmB6: break down of bulk hybridization gap and Kondo topological insulator state due to Sm vacancies.
SmB6 is a proposed topological Kondo insulator where the presence of topological nontriviality can be tuned by variations in the Sm valence. Sm valence can be changed by tuning stoichiometry of SmB6. We investigated a range of samples of SmB6 with different number of Sm vacancies. We showed that Raman scattering can detect a presence of vacancies down to 1 % of Sm sites in SmB6 crystal by probing the intensity of defect-induced scattering of the acoustic phonon branch at 10 meV. In the electronic Raman spectra of SmB6 at temperatures below 130 K, we observe features developing in A1g and Eg symmetries at 100 and 41 meV which we assign to excitations between hybridized bands, and depressed spectral weight below 20 meV associated with the hybridization gap. With the increased number of Sm vacancies up to 1 % we observe an increase of spectral weight below 20 meV showing that the gap is filling in with electronic states. For the sample with the lowest number of vacancies the in-gap exciton excitations with long lifetimes protected by hybridization gap are observed at 16-18 meV in Eg and T2g symmetries. The excitation features broaden with a decrease in the lifetime on increasing number of vacancies and are quenched by the presence of in-gap states at concentration of Sm vacancies of about 1 %. Based on this study we suggest that only the most stoichiometric SmB6 samples have a bulk gap necessary for topological Kondo insulators.
Magnetic excitations and magnetoelastic coupling in α-SrCr2O4
Magnetic excitations and magnetoelastic coupling in α-SrCr2O4
Using Raman scattering technique we follow energies and temperature dependence of two-magnon excitations in the helical antiferromagnet α-SrCr2O4. We also detect local distortions of the crystal lattice resulting from magnetoelastic coupling.
Competing magnetic interactions lead to a suppression of a simple magnetic order and an enhanced importance of small parameters of a system which define its ground state. Comparing experiment and DFT calculations on a triangular lattice antiferromagnet α-SrCr2O4, we show that even small variations from a perfect triangular lattice lead to large differences in the exchange interactions, which define the geometry of the resulting magnetic order.
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