Abstract

Ferroelectricity at Ultimately Small Length Scales

Umesh V Waghmare

Theoretical Sciences Unit, J Nehru Centre for Advanced Scientific Research, Jakkur PO, Bangalore 560 064 INDIA

Introducing fundamental aspects of ferroelectricity in perovskite oxide crystals, we highlight how its sensitivity to surrounding makes its evolution to nano-scale films of materials quite interesting and relevant to miniaturization of devices. Using first-principles theoretical analysis, we predict ferroelectricity in 2-D materials like 1T-MoS₂ and h-NbN, and demonstrate how it originates from strongly coupled electrons and phonons. While these allow dipolectronic devices based on ultimately thin active channels, experimental realization of such devices and their scalable fabrication are quite challenging. To this end, we report scale-free ferroelectricity in which sub-nano meter thin ferroelectric layers are naturally embedded in 2-dimensional crystalline architecture of orthorhombic structure of HfO₂, a material that is already a part of current semiconductor technologies, opening up ultimately high density ferroelectric memory devices.

Work done in collaboration with Anuja Chanana, S Anand, K Mohan, Anjali Singh and Sharmila N Shirodkar and Jun Hee Lee group at UNIST.

Fractionalized polar skyrmions in BiFeO₃

Chan-Ho Yang

Department of Physics, KAIST, Daejeon, Korea

We report the electrical and mechanical creations of stable fractionalized polar skyrmions via flexoelectric effect in a prototypical multiferroic BiFeO3 thin film [1,2], which are directly imaged by angle-resolved piezoresponse force microscopy technique [3]. We also demonstrate an approach to enhance piezoresponse at the nanoscale. Phase-field simulations reveal that a tip-driven flexoelectric effect allows the formation of an ordered polar skyrmionlike configuration that is topologically protected by the converse flexoelectric effect. Our findings open up avenues for designing and controlling spatially localized topological textures in rhombohedral ferroelectrics, and also could contribute to future developments for ferroelectric-based nanoelectronic devices.

[1] F. Zhuo and C.-H. Yang, Phys. Rev. B 102, 214112 (2020).

[2] J. Kim, M. You, K.-E. Kim, K. Chu, and C.-H. Yang, npj Quantum Mater. 4, 29 (2019).

[3] K. Chu and C.-H. Yang, Rev. Sci. Instrum. 89, 123704 (2018).

Orbital-Selective Superconductivity in infinite-layer Nickelates:

Role of nonzero f-ness

Tanusri Saha-Dasgupta

S.N.Bose National Centre for Basic Sciences, JD Block, Salt Lake, Kolkata 700106, India

We propose a first-principles derived low-energy model Hamiltonian in infinite-layer nickelate compounds, consisting of two orbitals: Ni x₂-y₂, and an axial orbital. The axial orbital is constructed out of A-site d, Ni 3z₂-r₂, and Ni-s characters. Calculation of the superconducting pairing symmetry and pairing eigenvalue of the spin fluctuation mediated pairing interaction underlines the crucial role of the interorbital Hubbard interaction in superconductivity, which turns out to be orbital selective.[1] The axial orbital brings in material dependence to the problem, making LaNiO₂ different from NdNiO₂ or PrNiO₂, thereby controlling the interorbital Hubbard interaction-assisted superconductivity.[2]

[1] Priyo Adhikary, Subhadeep Bandyopadhyay, Tanmoy Das, Indra Dasgupta, and Tanusri Saha-Dasgupta, Phys. Rev. B 102, 100501(R) (2020). [2] Subhadeep Bandyopadhyay, Priyo Adhikary, Tanmoy Das, Indra Dasgupta, and Tanusri Saha- Dasgupta, Phys. Rev. B 102, 220502(R) (2020)

Concurrent topological transitions of zone-center van Hove singularity and critical type-II Dirac fermions in a new 2D carbon crystal

Young-Woo Son

Korea Institute for Advanced Study, Seoul, Korea

We study electronic properties of a new planar carbon crystal formed through networking biphenylene molecules. Unprecedented electronic features among carbon materials such as zone-center saddle point and peculiar type-II Dirac fermionic states are shown to exist in the low energy electronic spectrum. The type-II state here has a nearly flat branch and is close to a transition to type-I. With a moderate uniaxial strain, a pair of Dirac points merge with the zone center saddle point, realizing concurrent Lifshitz transitions of van Hove singularity as well as pair annihilation of the Dirac fermions. A new effective Hamiltonian encompassing all distinctive low energy states is constructed, revealing a finite winding number of the pseudo-spin texture around the Dirac point, quantized Zak phases, and topological grain boundary states. Possible magnetic instabilities are also discussed.

"Order-by-disorder" in the Hilbert space and anomalous thermalization in Rokhsar-Kivelson models

Arnab Sen

School of Physical Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032

Nonequibrium quantum matter is a rich field with several open questions. In this talk, I discuss how paradigmatic frustrated models like the quantum link and the quantum dimer models with Rokhshar-Kivelson Hamiltonians harbor anomalous “quantum scars” in their many-body spectrum which lead to anomalous thermalization starting from certain simple initial states. The mechanism for the formation of these quantum scars is akin to “order-by-disorder”, but in the Hilbert space, where highly delocalized and degenerate zero modes in the Hilbert space superpose to form localized quantum scars. For certain lattice geometries, such scars can be shown to exist even when one of the linear dimensions of the system diverges. If time permits, some other minimal spin models that are analytically tractable and yet show emergent physics like fractons will be discussed.

References

1. Paul A. McClarty, Masudul Haque, Arnab Sen, Johannes Richter, Phys. Rev. B 102, 224303 (2020)

2. Debasish Banerjee and Arnab Sen, Phys. Rev. Lett. 126, 220601 (2021)

3. Bhaskar Mukherjee, Debasish Banerjee, K. Sengupta, Arnab Sen, Phys. Rev. B 104, 155117 (2021)

4. Saptarshi Biswas, Debasish Banerjee, Arnab Sen (to be submitted)

High-pressure tuning of anomalous Hall effects, density waves, and superconductivity in 2 D materials

Kee Hoon Kim

Center for Novel States of Complex Materials Research, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea

Opportunities for investigating correlated electronic systems using extreme physical conditions such as high fields and high-pressures are greatly developing in recent years. In particular, high pressure is as an effective route for tuning electronic states of solids, of which applications to solids can lead to findings of e.g., unexplored exotic phases of various quantum matters or their putative quantum mechanical ground states. Herein, I’ll update exciting science progresses in several systems; (1) Firstly, pressure induced tuning of band topologoloy and temperature induced gap reduction in stacked 2D honeycomb lattice materials CrSiTe₃ and CrGeTe₃ with simultaneous development ferrimagnetic order. With discussions on the development of ferromagnetic moment at high pressure, we also discuss the anomalous Hall effects mainly originating from the nontrivial topology change in the band structure of CrSiTe₃ with pressure. (2) Secondly, we present doping and pressure induced optimization of superconductivitiy, particularly focusing on tuning of electronic states in Pd-intercalated TaSe₂, i.e., Pd_xTaSe₂, in which superconducting transition is optimized from 0.14 (TaSe₂) to 3.1 K (x=~0.08) with simultaneous suppression of a commensurate charge density wave (CDW) state. We found that the Pd intercalation can involve a Lifshitz transition in the underlying electronic states at normal states without CDW ground states, which seems to be useful for increasing density of states (DOS) and electron-phonon coupling and in turn two band BCS superconductivity at low temperatures. Although it was not clear with intercalation, with tuning of pressure, we found that a second order quantum phase transition of a CDW transition can occur around 22 GPa and fluctuating CDW ground states can instigate the superconductivity up to 8.3 K. With systematic Raman and transport results, we discuss how a CDW quantum criticality and related fluctuation can be involved in the process of boosting up superconductivity with pressure.

Dulmage-Mendelsohn percolation: Geometry of maximally-packed dimer models and topologically-protected zero modes on diluted bipartite lattices

Kedar Damle

Tata Institute of Fundamental Research, Mumbai 400005, India

The classic combinatorial construct of {\em maximum matchings} probes the random geometry of regions with local sublattice imbalance in a site-diluted bipartite lattice. We demonstrate that these regions, which host the monomers of any maximum matching of the lattice, control the localization properties of a zero-energy quantum particle hopping on this lattice. The structure theory of Dulmage and Mendelsohn provides us a way of identifying a complete and non-overlapping set of such regions. This motivates our large-scale computational study of the Dulmage-Mendelsohn decomposition of site-diluted bipartite lattices in two and three dimensions. Our computations uncover an interesting universality class of percolation associated with the end-to-end connectivity of such monomer-carrying regions with local sublattice imbalance, which we dub {\em Dulmage-Mendelsohn percolation}. Our results imply the existence of a monomer percolation transition in the classical statistical mechanics of the associated maximally-packed dimer model and the existence of a phase with area-law entanglement entropy of arbitrary many-body eigenstates of the corresponding quantum dimer model. They also have striking implications for the nature of collective zero-energy Majorana fermion excitations of bipartite networks of Majorana modes localized on sites of diluted lattices, for the character of topologically-protected zero-energy wavefunctions of the bipartite random hopping problem on such lattices, and thence for the corresponding quantum percolation problem, and for the nature of low-energy magnetic excitations in bipartite quantum antiferromagnets diluted by a small density of nonmagnetic impurities

Octupolar order and Ising quantum criticality in osmate double perovskites

Arun Paramekanti,

University of Toronto, Toronto, Canada M5S1A7

Traditionally, magnetism in solids deals with ordering patterns of the electron magnetic dipole moment, as probed, for instance, via neutron diffraction. f-electron heavy fermion systems are well-known candidates for more complex forms of symmetry breaking, involving higher-order magnetic or electric multipoles. In this talk, I will discuss our recent theoretical proposal for Ising octupolar order in d-orbital systems, which explains a wide range of experiments in 5d^2 osmate double perovskites with spin-orbit coupling. The proposed Ising ferro-octupolar order is shown to be linked to a type of orbital loop-current order. Deviations from cubic symmetry, via strain or surfaces, induces a transverse field on the octupolar order which can lead to surface quantum phase transitions, or transitions in thin films or in strained 3D crystals. We propose further experimental tests of our proposal.

1. S. Voleti, A. Haldar, A. Paramekanti, arXiv:2109.13266

2. S. Voleti, D. Maharaj, B. D. Gaulin, G. M. Luke, A. Paramekanti, PRB 101, 155118 (2020)

3. D. D. Maharaj, et al, Phys. Rev. Lett. 124, 087206 (2020)

4. A. Paramekanti, D. D. Maharaj, B. D. Gaulin, PRB 101, 054439 (2020)

Colossal angular magnetoresistance in a nodal-line ferrimagnetic semiconductor Mn₃Si₂Te₆

Jun Sung Kim

Department of Physics, POSTECH, Pohang, Korea

Efficient magnetic control of electronic conduction is at the heart of spintronic functionality for memory and logic applications. Magnets with topological bandcrossings serve as a good material platform for such control, because their topological band degeneracy can be readily tuned by spin congurations, dramatically modulating electronic conduction. Here we propose that the topological nodal-line degeneracy of spin-polarized bands in magnetic semiconductors induces an extremely large angular response of magnetotransport. Taking a layered ferrimagnet Mn₃Si₂Te₆ and its derived compounds as a model system, we show that the topological band degeneracy, driven by chiral molecular orbital states, is lifted depending on spin orientation, which leads to a metal-insulator transition in the same ferrimagnetic phase. The resulting variation of angular magnetoresistance with rotating magnetization exceeds a trillion percent per radian, which we call colossal angular magnetoresistnace. Our findings demonstrate that magnetic nodal-line semiconductors are a promising platform for realizing extremely sensitive spin- and orbital-dependent functionalities.

Spin-phonon Coupling and Magnons in Two-dimensional Antiferromagnets

Subhadeep Datta

School of Physical Sciences, Indian Association for the Cultivation of Science,

Jadavpur, Kolkata – 700032, India

Mermin-Wagner theorem restricts long range magnetic order at finite temperature in a two-dimensional (2D) isotropic ferromagnet [1, 2]. However, magnetic anisotropy can counter the thermal fluctuations and lift this restriction, thus leading to magnetism in two-dimensions (2D) [1,3]. Recently, magneto-optical Kerr measurement (MOKE) revealed that a 2D Ising ferromagnet (e.g., monolayer Chromium Iodide (CrI₃ )) possesses intrinsic long-range ferromagnetic order (T_c ~ 45 K)). On the other hand, bulk van der Waals magnetic materials such as Cr₂Ge₂Te₆ and transition metal phosphorus trisulfides (MPS₃, M = transition metal) well reveal magnetic anisotropy and have been studied via superconducting quantum interference device (SQUID) and neutron scattering [4]. But realization of magnetic order down to mono/few atomic layer magnets with complex in-plane and out-of-plane spin-arrangement and the nature of elementary excitations is technically challenging via above mentioned techniques. Raman spectroscopy, unlike conventional SQUID or neutron scattering techniques, is an indirect tool to study layer dependent coupling mechanism in 2D. Besides, evolution of characteristic Raman modes with temperature and pressure appearing at different energy scales imparts specific information such as spin-lattice coupling, spin-wave excitations together with structural/magnetic phase transition.

In this work, we aim to explore pristine atomic layers of 2D antiferromagnet and their heterostructures with other 2D van der Waals materials (vdW) having different electronic and magnetic orders [5,6]. Mechanical stacking of 2D atomic layers in preferred location has been done using a micromanipulator-based assembly in the laboratory set up. Spin-phonon coupling, probed by low temperature micro-Raman spectroscopy, in few layers of FePS₃ (Antiferromagnet in bulk with Néel temperature 120 K) confirms the retention of magnetic ordering even in the quasi-2D limit. In addition, evolution of Raman peaks at lower wavenumbers (around 100 cm^-1 ) below T_N can be identified as a result of zone folding effect in spin lattice at low temperature. Interestingly, emergence of a distinct peak at 120 cm^-1 at a temperature much lower than T_N has been confirmed as a signature of antiferromagnetic magnon. Quantum fluctuation in few layer systems at lower temperature may cause strongly correlated electronic effects which may result in the suppression of magnon temperature from AFM ordering temperature. Raman spectroscopy can thus predict the magnetic transition temperature (here, T_N ) of a few atomic layers via spin-phonon coupling. Moreover, identification of the characteristic Raman modes and their evolution would facilitate the study of stacked magnetic vdW heterostructure involving different quantum materials, like topological insulators. We envision 2D magnets and related vdW heterostructures as promising candidates for ultra-fast magnonic device applications.

[1] D. Mermin and H. Wagner, Phys. Rev. Lett. 17, 11331136 (1966).

[2] L. Onsager, Phys. Rev. 65, 117 (1944).

[3] J. L. Lado and J. Fernndez-Rossier, 2D Mater. 4, 35002 (2017)

[4] A. Banerjee et al., Science 356, 1055 (2017).

[5] A. Ghosh, M. Palit, S. Maity, V. Dwij, S. Rana and S. Datta, Phys. Rev. B 103, 064431 (2021).

[6] D. Vaclavkova, M. Palit, J. Wyzula, S. Ghosh, A. Delhomme, S. Maity, P. Kapuscinski, A. Ghosh, M.

Veis, M. Grzeszczyk, C. Faugeras, M. Orlita, S. Datta, and M. Potemski, Phys. Rev. B 104, 134437

Electric Quantum Oscillation in Weyl Semimetals: A New Form of Quantum Oscillation

Kwon Park

Korea Institute for Advanced Study, Seoul, Korea

One of the most important features of quantum mechanics is the formation of standing waves, or energy eigenstates via the constructive interference of wave functions. In fact, this is the underlying principle for the existence of atoms and, for that matter, all matters in the universe. In condensed matter systems, the constructive interference can occur in macroscopic scales, giving rise to various different forms of oscillatory behaviors collectively known as quantum oscillation. In this talk, I would like to talk about a new form of quantum oscillation that can occur in Weyl semimetals under electric fields. Specifically, it is shown that there is a rich structure in the chiral anomaly transport in Weyl semimetals, including the negative magnetoresistance, the non-Ohmic behavior, the Esaki-Tsu peak, and finally the resonant oscillation of the dc electric current as a function of electric fields, called the electric quantum oscillation.

Weak-coupling to strong-coupling quantum criticality crossover in a Kitaev quantum spin liquid α-RuCl₃

Jaehoon Park

POSTECH

TBA

Superconductivity with re-writable magnetic memory at LaVO₃/SrTiO₃ interfaces

Goutam Sheet

IISER Mohali

The conducting interfaces of perovskite oxide insulators are potential playgrounds of novel quantum phenomena. I will present our discovery of superconductivity at the interface of two insulators – a Mott insulator LaVO₃ and a band insulator SrTiO₃. The superconducting phase coexists and interplays with an unusual in-situ magnetic state. The magnetic state is found to be unconventional. The magnetic state displays spontaneous magnetization (Ms) that gets oriented in a direction opposite to a magnetic field, leading to a negative remanence (NR). I will explain how the NR might be the consequence of an exotic spin-canting in the LaVO₃ layer. The unusual NR causes counter-intuitive effects in the superconducting state, such as surprising enhancement of superconductivity under applied magnetic fields. Exploiting these unusual properties, we devised well-characterized magnetic field cycling protocols to controllably write, read, erase and re-write magnetic information at the interfaces. Therefore, apart from the intriguing physics of low-dimensional superconductivity interplaying with NR at a Mott-insulator/band-insulator interface, our discovery opens up an avenue towards an all-oxide superconducting information read-write platform.

Fe₃O₄ nanoparticles and Verwey transition: gateway to nanochemistry

Je-Geun Park^1,2

¹Center for Quantum Materials, Seoul National Univ., Korea

²Department of Physics & Astronomy, Seoul National University, Korea

Oxidation is arguably one of the most important chemical processes in nature and affects everyday life all around us, if not the whole human civilization. It is so common that we cannot think of our life without it. A case in point is the oxygen delivery through our blood vessels that keep us alive day and night. Therefore, it is natural for us to learn every element's oxidation status at the beginning of our training as condensed matter physicists, experimentalists, and theoreticians. This education gives us the impression that we understand the basic oxidation process.

Well, only up to a point. When it comes to the oxidation process on a nanometer scale, it may not be too exaggerated to say that nothing is known very much. And the problem of understanding oxidation on such tiny scales remains simply unchartered territory for chemists and all material scientists. In this talk, I would like to demonstrate that the Verwey transition of Fe₃O₄ can aid us in this enormous challenge. Using the size and time dependence of the Verwey transition in Fe₃O₄ nanoparticles, we can shed important light on this fundamentally important problem of science.

[1] Jisoo Lee, Soonku Kwon, Je-Geun Park, Taeghwan Hyeon, Nano Letters 15, 4337 (2015).

[2] Taehun Kim, Sangwoo Sim, Sumin Lim, Midori Amano Patino, Jaeyoung Hong, Jisoo Lee,

Taeghwan Hyeon, Yuichi Shimakawa, Soonchil Lee, J. Paul Attfield, and Je-Geun Park, Nature Communications 12, 6356 (2021).