Speaker Abstracts

Ultrafast excitation by a femtosecond laser pulse followed by a weak probe pulse allows us to control and probe transient processes in quantum materials. Photo-excited carrier dynamics involves many important physical processes such as quasi-thermalization by electron-electron interaction followed by intraband and interband relaxation processes. My talk will discuss our recent results on optical pump-terahertz probe of ultrathin nanowires [1] as well as nanocrystals [2] of topological insulator Bi₂Te₃. These experiments give quantitative insights into the contributions of Dirac electrons at the surface and bulk carriers to the photoconductivity in the terahertz range. In nanowires, we establish the absorption of THz radiation by the Dirac surface states plasmon oscillations of the photoexcited charge carriers in topological surface states.

References:

[1] K.P. Mithun, S. Kar, Abinash Kumar, D V S Muthu,N. Ravishankar and A.K. Sood, Nanoscale (2021) [2] K.P. Mithun, Abinash Kumar, Subhajit Kundu, N. Ravishankar and A.K. Sood (2021)

Hybrid perovskites like (C₄H₉NH₃)₂PbI₄ have fascinating layered crystal structure with periodic nanoscale interfaces between the inorganic {PbI₄}^2- and organic C₄H₉NH₃⁺ layers. Because of these interfaces, electron and hole are confined in atomically thin {PbI₄}^2- inorganic well layers. Therefore, these layered perovskites are considered as electronically 2D systems, irrespective of their crystallite sizes.[1] Importantly, the crystal structure is flexible, allowing a number of combinations of different organic cations and inorganic anions. So a rational design of the nanoscale interfaces, and hence, tunable optoelectronic properties are feasible. For example, excitonic binding energy can be controlled over an order of magnitude from a few tens of meV to a few hundreds of meV, with simple variation of composition of organic cations. Furthermore, we introduce new cation-pi interactions between organic cations, resulting into completely water stable low dimensional hybrid perovskite.[2] In this talk, I will discuss about how controlling nanoscale interface between organic and inorganic layers can yield interesting optical, optoelectronic and chemical properties.[3] But note that the nanoscale properties will be discussed using millimeter sized single crystals.

References:

1. Sheikh et al, ACS Energy Lett. 2018, 3, 2940.

2. Sheikh et al, Angew. Chem. Int. Ed. 2021, DOI: 10.1002/anie.202105883

3. Sheikh et al, Angew. Chem. Int. Ed. 2020, 59, 11653.

Multi-band/multi-orbital materials such as transition-metal oxides, iron superconductors or twisted bilayer systems offer a fertile platform for exploring the physics of strong electronic correlations. In this context, the interplay of the `Hubbard U’ with Hund’s rule and spin-orbit coupling, as well as orbital differentiation, lead to rich physics beyond the paradigmatic ‘Mottness’. In recent years, the concept of `Hund’s metals has emerged and has successfully explained the properties of iron superconductors and ruthenates. In this talk, I will consider mostly Sr₂RuO₄ – a remarkable material which also serves as a precision laboratory for many-body physics. I will report on very recent high-resolution ARPES experiments which allow to put the Dynamical Mean-Field Theory framework to a direct test, review how Hund’s coupling is responsible for strong correlations in this material and emphasize the importance of spin-orbit coupling. Time permitting, I will also briefly discuss recent advances on the superconducting state.

Most of the magnetic thin films hosting nontrivial topology are characterized by the ferromagnet/heavy-metal interfaces in order to generate a strong interfacial Dzyaloshinskii-Moriya interaction and usually an external magnetic field is applied to stabilize the topological spin textures from the chiral magnetic ground state. However, in particular from an application point of view, both the use of heavy metals and the need of an external magnetic field are limitations. Therefore, it is highly desirable to have light metal-based non-collinear magnetic systems which is capable of hosting skyrmions at zero field and room temperature. Here, we will demonstrate how such light atom multilayers can be employed for the stabilization of isolated magnetic skyrmions at room temperature and zero magnetic field via interlayer exchange coupling.

Combined effects of double-exchange, p-d-exchange, and super-exchange interactions lead to complex ferromagnetism in transition-metal compounds such as diluted ferromagnetic semiconductors [1] and perovskite oxides [2]. In 2D versions of these materials, the realization of perpendicular magnetic anisotropy (PMA) has been pursued from the application point of view.

We have studied the single-ion anisotropies (SIA) of magnetism in 2D ferromagnets using angle-dependent XMCD measurements and subsequent cluster-model calculation. In (Ba,K)(Zn,Mn)₂As₂, a 2D version of GaMnAs, doped holes entering the spin-orbit-split yz/zx level of Mn explain the observed PMA [3]. In the Fe-intercalated TiS₂, the trigonal crystal-field splitting of Fe 3d is found to be tiny, but the resulting SIA is sufficient to explain the strong PMA owing to the large orbital magnetic moment of the Fe²⁺ ion [4]. In the van der Waals ferromagnet Cr₂Ge₂Te₆, the calculated SIA is smaller than the observed PMA by an order of magnitude [5]. This is consistent with the scenario that super-exchange interaction between the Cr ions through ligand p orbitals having strong spin-orbit coupling is responsible for the observed PMA.

References:

[1] P. Mahadevan et al., Phys. Rev. Lett. 93, 177201 (2004).

[2] D. D. Sarma et al., Phys. Rev. Lett. 85, 2549 (2000).

[3] S. Sakamoto et al., ACS Appl. Electron. Mater. 3 (2021).

[4] G. Shibata et al., J. Phys. Chem. C; DOI: 10.1021/acs.jpcc.1c02345.

[5] M. Suzuki et al., in preparation.

Recently discovered 2M phase of bulk WS₂ was observed to exhibit superconductivity with a critical temperature of 8.8 K, the highest reported among superconducting transition metal dichalcogenides [1,2]. Also predicted to support protected surface states, it could be a potential topological superconductor. Based on the insights gained from an ab-initio analysis of the bulk phase, we predict bilayer 2M WS₂ as a new two-dimensional topological material [3]. The broken inversion symmetry in this newly proposed bilayer leads to the presence of Berry curvature dipole and resulting non-linear responses. We propose that such non-linear signals, which are absent in the centrosymmetric bulk phase, can be signatures of the bilayer. We hope our predictions lead to the experimental realization of this as-yet-undiscovered two-dimensional topological material.

References:

[1] Y. Fang et al., Adv. Materials 1901942 (2019)

[2] Y. Yuan et al., Nat. Phys. 15, 1046 (2019)

[3] N. B. Joseph and A. Narayan, arXiv:2009.00849

The entanglement between electron-correlation, spin-orbit coupling (SOC) and lattice vibration has emerged as a rich playground for the disclosure of novel quantum states of matter where SOC plays a central role. In this talk we shall present and discuss a few examples of SOC-driven phenomena in transition metal oxides, including: non-collinear (multipolar) magnetic orderings, metal-to-insulator transitions and topological behaviors.

Semiconductors have played an important role in the development of information and communications technology, solar cells, solid state lighting. Nanowires are considered as building blocks for the next generation electronics and optoelectronics. In this talk, I will present the results on optoelectronic devices such as lasers/LEDs, THz detectors, energy devices such as solar cells, photoelectrochemical (PEC) water splitting and Neuro-electrodes. Future prospects of the semiconductor nanowires will be discussed.

The promise of inducing a Mott-insulating to a conducting state transition (Mott-breakdown) through application of an electric current or an electric field constitute a very active field of research.

Here, we shall present experimental evidences of current-induced Mott-breakdown phenomenon in a doped titanate spinel oxide system at record-low threshold electric fields. The system under investigation exhibits a sharp drop of resistance by several orders-of-magnitude under a modest electric current flow (with a corresponding electric voltage which is orders of magnitude smaller than typical breakdown voltages). We shall also discuss experimental evidences to rule out a Joule-heating driven resistivity transition and the necessity to invoke an electronic mechanism for understanding the resistivity-state transition. We shall further discuss the underlying microscopic mechanism which drives such a unique phenomenon using a combination of various experimental probes and first-principles calculation. We, thus, introduce a novel and general mechanism to induce Mott-breakdown effect in presence of ultra-low threshold fields in similar kind of systems.

References:

(1) A. Rahaman et al. Phys. Rev. B 100, 115162 (2019)

(2) A. Rahaman et al. Phys. Rev. B – Accepted (2021)

Halide perovskites (HaPs) have, for most of the past decade, been intensively studied for their excellent optoelectronic properties with emphasis on PV cells and light emission. The most common members of this family have the general formula ABX₃, where A is typically a monovalent cation (often organic as in alkyl amine but also inorganic), B is a divalent inorganic cation (with Pb²⁺ being the most common) and X a halide.

In 2002, metal-free HaPs were first reported where A is a large divalent amine cation and B is the ammonium cation [1]. The octahedral perovskite framework in these materials is believed to be held together by hydrogen bonding as well as ionic bonding.

In this talk, I give a brief overview of these metal-free HaPs and our own work on single crystals of this interesting family followed by our studies which emphasize optoelectronic properties and theoretical/XPS studies of their band structures.

Gold is precious, attractively shiny and almost entirely chemically inert. Unlike other metals, it does not rust when exposed to air, and retains its luster indefinitely, justifying its usage in jewelry. We have synthesized a new type of gold that exhibits properties distinctly more novel than the conventional gold. This new form is obtained in the form of microcrystals measuring between 2 and 10 micrometers in length and diameters, around 500 nm. The crystals are obtained by heating a gold-organic precursor mixed with small quantities of structure guiding agents, in air! The appearance is bipyramidal with penta-twinned tips, covered with nanofacets of unusually high miller indices. The crystallites can withstand mercury treatment and exhibit low dissolution rates in aqua-regia unlike the cubic gold, which readily dissolves. The emergence of such extraordinary properties is attributed to the occurrence of non-cubic phases borne out of the lattice stain.

The presentation will provide an overview of the results obtained with specific reference to the distribution of non-cubic lattices within the crystallite volume.

Quantum anomalous Hall (QAH) insulator can be realized in a topological magnetic insulator with non-trivial band topology combined with magnetic order. In principle, QAH insulators could be stable at ambient conditions, but their experimental realizations have been demonstrated only at extremely low temperatures. In this talk, I will review recent research developments searching for topological magnetic insulators among two-dimensional (2D) materials. Some transition metal chalcogenides and halides have interesting features in their electronic band structure, which lead to novel magnetic interactions and topological characteristics. From density-functional-theory calculations, we demonstrate that a class of 2D transition-metal compounds becomes a ferromagnetic insulator with a non-trivial Chern number. While transition metal atoms are responsible for the ferromagnetic ground state, the band topology depends on the hopping matrix elements through chalcogen atoms. The non-trivial band topology is confirmed to have a nonzero Chern number, quantized Hall conductivity, and chiral edge states by using the Wannier function analysis. We also predict that a two-dimensional metal-organic framework of a single layer of the transition-metal bis-dithiolene complex can become a ferromagnetic insulator with a non-trivial Chern number.

The recent progress in synthesizing and discovering states and phenomena beyond the Landau symmetry-breaking paradigm in quantum materials has been quite extraordinary. These new modalities confront our views of fermions and bosons’ possible behavior in solids, yet in bulk remain frequently concealed from the modern experimental probes. To exacerbate, though by now topological phases are well-known for non-correlated compounds, they are scarcely found in correlated electron systems. In my talk, I will discuss a way of addressing these challenges by creating and exploring new synthetic templates of thin films with rich many-body behavior derived from the rare-earth pyrochlore iridates.

Specifically, I will focus on quantum states of (111)-oriented rare-earth iridium pyrochlore thin-films La₂Ir₂O₇ (La = Pr ... Lu) which is (1) a model system for an exotic nodal non-Fermi liquid metal known as the Luttinger-Abrikosov-Beneslavskii state, and (2) time-reversal broken Weyl semimetal. During the talk, I will discuss the challenges of growing complex materials of platinum group metal oxides and finding direct signatures for the presence of Weyl fermions.

The study of graphene has drawn much attention due to its rich physics arising from linear band structure giving rise to massless excitations and potential use for next-generation electronic devices. Experiments show significant anomalies at the Dirac point even in pristine graphene. Doping of charge carriers leads to additional complexity in the electronic structure. We study the properties of the Dirac states in pristine and hole-doped graphene employing angle-resolved photoemission spectroscopy and density functional theory. Experimental data exhibit anomalous intensities in a large energy window across the Dirac point indicative of an energy gap along with in-gap states and/or presence of plasmaron excitations. Employing symmetry-selective measurements for each band, we discover dispersive linear energy bands crossing at a distinct Dirac point within the anomalous region. No gap is observed even after 5% boron substitution that reduced the carrier concentration significantly. The anomalies at the Dirac point arises from the lifetime and momentum broadening. The substitution of boron at the graphitic sites essentially leads to a band renormalization and a shift of the Dirac point towards the Fermi level. These results suggest that SiC-graphene is a good platform to realize interesting science as well as advanced technology where the carrier concentration, mobility can be tuned keeping the Dirac Fermionic properties protected.

Here we discuss the nature of the magnetic coupling between different antiferromagnetic layers, taking NiO and CoO as prototypical antiferromagnetic systems. We show that the nature of the interaction between the antiferromagnetic layers depends on the thickness of the films as well as the substrate on which these films are grown. We use the temperature-dependent exchange-scattered electron intensities from the antiferromagnetic lattice to measure the surface Neel temperatures. Our experiments show that the surface Neel temperatures are significantly influenced around the interface due to the magnetic proximity effect, similar to ferromagnets. Our work also suggests that the magnetic proximity effect is a combination of a short-range magnetic exchange coupling at the interface and a weaker and longer-range coupling mediated by magnetic correlations, explaining the long magnetic order propagation length in antiferromagnetic bilayers and superlattices.

Conventional gas refrigeration technology is having drawbacks in terms of power consumption and greenhouse gas emissions. Taking into account these issues, alternate solutions for cooling applications are being explored. Magnetic refrigeration (MR) technology which works on the principle of magnetocaloric effect is one such alternative. Rare-earth based oxides are predicted to be suitable candidates for low-temperature magnetic cooling due to their low magnetic ordering temperature in the rare-earth sub-lattices, large magnetization, weak magnetic exchange interactions, negligible thermal and field hysteresis. The phenomena of magnetocaloric effect in various rare-earth oxides of type AA’BO₃ where A, A’ = Gd, Dy and B = Cr, Mn, Fe and Al, their structural, electronic, magnetic and magnetocaloric investigations will be discussed. GdAlO₃ shows a giant isothermal magnetic entropy change (ΔSM) as compared to GdMnO₃, but with similar relative cooling power (RCP). However, the absence of magnetic and thermal hysteresis in GdAlO₃ makes it a more efficient magnetic refrigerant than GdMnO₃. DyMnO₃ shows moderate magnetocaloric effect, but it could be enhanced with Gd- doping suggesting that rare earth mixing plays a vital role in modifying the magnetocaloric properties. GdCrO₃ shows exceptionally high values for ΔSM, ΔT_ad and RCP (41 Jkg^-1K^-1, 20.8 K and 560 Jkg^-1, respectively) for a field change of 7 T, making it one of the best candidates for MR at low temperatures.

Spin electronics is largely concerned with spin polarized electron transport in thin films of ferromagnets and metals with strong spin-orbit coupling. Antiferromagnets have attracted attention in recent times on account of their high frequency spin dynamics, the absence of any stray field and the possibility of switching the antiferromagnetic axis through 90° in crystal structures of appropriate symmetry. Metallic ferrimagnets offer the best of both worlds, plus some unique properties of their own. When half-metallic, they combine high spin polarization with little or no net magnetization or stray field near compensation. Domains can be imaged directly. The magnetization dynamics can be excited electrically by spin-orbit torque, resonance frequencies are high, coercivity and anisotropy field can be huge, there are prospects of switching a single layer by spin-orbit torque and ultra-fast all-optical toggle switching can be observed, with re-switching on a 10 ps timescale. These features will be illustrated with reference to the original zero moment half metal, Mn₂Ru_xGa with the XA Heusler structure. Future prospects and challenges will be outlined.

We present a general method of constructing in-situ pseodopotentials from first principles, all-electron, full-potential electronic structure calculations of a solid. The method is applied to bcc Na, at equilibrium volume. The essential steps of the method involve (i) calculating an all-electron Kohn-Sham eigenstate. (ii) Replacing the oscilllating part of the wave function (inside the mun-tin spheres) of this state, with a smooth function. (iii) Representing the smooth wave function in a Fourier series, and (iv) inverting the Kohn-Sham equation, to extract the pseudopotential that produces the state generated in steps (i)-(iii). It is shown that an in-situ pseudopotential can reproduce an all-electron full-potential eigenvalue up to the sixth significant digit. A comparison of the all-electron theory, in-situ pseudopotential theory and the standard nonlocal pseudopotential theory demonstrates good agreement, e.g., in the energy dispersion of the 3s band state of bcc Na.

Double double perovskites AA'BB'O₆ with ordering of cations at both A and B sites are rare, but a new family exemplified by NdMnMnSbO₆ was discovered in 2016. These have tetragonal P42/n symmetry with columnar order of A cations and rocksalt B site order. A large number of such materials have since been discovered using high pressure synthesis and the family will be reviewed in this talk. A remarkable variety of magnetic properties has been discovered, with ferro-, ferri-, and antiferro- magnetic orders all observed, and also cluster spin glass behaviour. Recent discovery of double double to double perovskite transitions in some materials enables magnetic properties of A-site ordered and disordered forms of the same composition to be compared.

When a droplet containing nanoparticle (NP) dispersion evaporates under ambient or controlled environment, attractive assemblies/patterns are formed that are interesting both from fundamental and application point of view. At the fundamental level, the increased curiosity stems from the reason that such NP dispersion is a perfect system to study (in both length and time scale) the assembly/pattern formation under equilibrium/non-equilibrium conditions. Historically speaking, investigation on such microscopic systems started once the two phase synthesis (as proposed by Brus et al.) of ligand protected, nearly monodispersed metal NPs was mastered. In this context, we have been working on “digestive ripening” process in which a colloidal suspension in a solvent when refluxed at or above the solvent boiling temperature in the presence of the surface active agent leads to the conversion of a highly polydisperse colloid into a nearly monodisperse one (s < 5%).[1] It is hypothesized that the surface active groups of such digestive ripening agents bind and remove reactive surface atoms/clusters from big nanoparticles and redeposit them on smaller nanoparticles. In this way, large particles become smaller, while small particles become larger and eventually, an equilibrium size is obtained. One of the gratifying features of this digestive ripening process is the self-assembling of these nearly monodispersed NPs into 2D and 3D crystal structures (superlattices). One of the noteworthy facts of these large-area 2D superlattices is that, in all the cases where formation of a good quality assemblies/patterns have been seen, excess free ligand was invariably used during the experiments. Though there were considerable efforts in elucidating the underlying mechanism for the formation of such non-equilibrium geometrical structures both by theory and modeling combined with experimental studies, the exact role of excess ligand (that was invariably being used in the experiments) in the final pattern formation was either missing or inadequately explained. In this talk we will briefly present a brief overview of the digestive ripening process, first. We will then describe our recent results where we have shown that addition of excess ligand is necessary for a 2D NP pattern formation over large areas under non-equilibrium conditions.[2] Our investigations revealed that the attractive solvent-ligand interactions increases the evaporation time and the repulsive NP-ligand interactions result in depletion induced spinodal phase separation with conserved surface fractal parameters. The details of our studies will be briefed during the talk.

References:

1. J. R. Shimpi, D. S. Sidhaye and B. L. V. Prasad, Langmuir, 2017, 33, 9491−9507.

2. K. Bhattacharjee, K. Biswas and B. L V. Prasad, J. Phys. Chem. C 2020, 124, 23446−23453

The twisted bilayers of two-dimensional materials have recently become the playground of strong correlation physics, with the possibilities of charge and spin density wave instabilities. In this talk I will present our recent work on trying to understand the unusual semiconductor-metal-semiconductor transitions that occur in hole doped transition metal dichalcogenides for small twist angles.

This is work done in collaboration with Sumanti Patra and Poonam Kumari. A part of this work has appeared in Phys. Rev B 102, 205415 (2020).

Transition metal dichalcogenide (TMD) nanosheets with defect-rich and vertically aligned edges are highly advantageous for various catalytic applications. Synthesis of TMDs using the colloidal techniques opens various possibilities to tune the electronic and optical properties of these 2D materials. As an example, we choose MoSe₂ nanosheets that have plenty of defects. The defect sites are responsible for adsorption on the surface thereby yielding excellent electrocatalytic hydrogen evolution and other catalytic activities on the surface.

Further, these defects can be employed as seeding points to grow other materials on them. Cu₂S in these defect sites leads to a Type-II semiconductor heterojunction that allows for charge separation and therefore the MoSe₂-Cu₂S forms a superior material for generation of photocurrent.

Now even heterojunctions of MoSe₂, a hexagonal crystal with CsPbBr₃ – a perovskite have been enabled by use of a linker molecule 4 – aminothiophenol. Enhanced photocurrents are obtained with such a nanoheterostructure. This methodology further opens up avenues for forming heterostructures with large lattice mismatches and can therefore be of great potential use.

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Predictive control over defect minimization is the most important aspect during growth of any material. While it is undesirable to have defects in the system, it very often leads to unexpected modulation of physical properties which could have marked technological applications. Such is the case with complex oxides where oxygen vacancies (OVs) are the most common point defects. In the first part of the talk, the effect of electrostatic gating on transport properties of the conducting interface between two band insulators 𝛾-Al₂O₃ and SrTiO₃ (STO) will be presented. The conductivity in this heterostructure primarily originates from the presence of excess OVs. Our transport measurements reveal time-dependent charge trapping phenomena which can continue for several hours. Most importantly, we detect an additional source of charge trapping (detrapping) at (from) ferroelastic twin walls of STO. The amount of trapped/detrapped charges at the twin wall is electric field tunable and is controlled by the net polarity of the wall.

In the second part of the talk, OV induced emergent phenomena in a non-magnetic band insulator KTaO₃ (KTO) will be discussed. Creation of OV in KTO makes it a metal. Unexpectedly, oxygen deficient KTO shows signature of topological Hall effect (THE). Ab initio calculations reveal simultaneous formation of the magnetic moment on Ta atoms around an isolated OV and Rashba-type spin texturing of conduction electrons, which are responsible for THE.

We study the spectral properties of and spectral-crossovers between different random matrix ensembles (Poissonian, GOE, GUE) in correlated spin-chain systems, in the presence of random magnetic fields, and the scalar spin-chirality term, competing with the usual isotropic and time-reversal invariant Heisenberg term. We have investigated these crossovers in the context of the level-spacing distribution and the level-spacing ratio distribution. We use Random Matrix Theory (RMT) analytical results to fit the observed Poissonian-to-GOE and GOE-to-GUE crossovers, and examine the relationship between the RMT crossover parameter λ, and scaled physical parameters of the spin-chain systems. We find that the crossover behavior exhibits universality, in the sense that it becomes independent of lattice size in the large Hamiltonian matrix dimension limit.

Quasicrystals have fascinated scientists from the time of their discovery. Quasicrystals have forbidden symmetry and exhibit remarkable physical properties such as high resistivity and low thermal conductivity. However, it is still unclear why nature prefers the quasicrystalline symmetry at some parts of the phase diagram. An elemental quasicrystal in the bulk form has not been discovered until date. In this talk, we report formation of quasiperiodic Sn film with clathrate structure grown on a 5-fold Al-Pd-Mn substrate. The Sn film is of 3-4 nm thickness, that is largest reported so far for any element [V. K. Singh et al., Phys. Rev. Res. 2, 013023 (2020)].

It is known that the green phase compounds, R₂BaCuO₅ (R = Sm and Gd) crystallize in the centrosymmetric orthorhombic (Pnma) structure. Magnetization and specific heat measurements reveal the long-range antiferromagnetic ordering of Cu²⁺ and Sm³⁺-ions in Sm₂BaCuO₅ at T_N1 = 23 K and T_N2 = 5 K, respectively. Applied magnetic field induces dielectric anomaly at T_N1 whose magnitude increases with field. Interestingly, an electric polarization below T_N1 under applied magnetic fields and the polarization varies linearly up to the maximum field of 9 T with the magnetoelectric coefficient α ~ 4.4 ps/m, demonstrating high magnetoelectric coupling [1]. On the other hand, powder neutron diffraction study reveals that Gd and Cu-moments in Gd₂BaCuO₅ order at the same temperature (T_N=11.8 K) in an elliptical cycloidal configuration with an incommensurate modulation vector (0, 0, g), which is accompanied by the emergence of ferroelectric polarization. With decreasing temperature, it undergoes a lock-in transition at T_loc ~ 6 K, below which the magnetic structure becomes commensurate with kc = (0, 0, ½) and strongly noncollinear, which causes an additional contribution to the electric polarization resulting from the polar magnetic space group (Paca21) [2].

Reference:

[1] Premakumar Yanda, N. V. Ter-Oganessian and A. Sundaresan, Phys. Rev. B, 100, 104417 - 104420 (2019).

[2] Premakumar Yanda, et. al., Phys. Rev. Research 2. 023271 (2020).

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]

References:

[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)

Since BCS theory, phonons have been understood as a prototype pairing glue for the low-T_c superconductivity. However, recent development of the room temperature superconductivity of the pressurized hydrides, revive the interesting role of phonon for the high-T_c superconductor. First, I will discuss the important issues of vertex correction of the adiabatic as well as non-adiabatic phonons and the necessary modification of the T_c-formula including the vertex correction. In addition, I will discuss the cases of high-T_c superconductors (cuprates and FeSe-monolayer) where phonons (forward scattering or isotropic scattering) can play an important role to boost T_c together with other non-phononic pairing bosons.