arXiv subjects

Shekhar Sharma

Publications and source records attributed to Shekhar Sharma.

4 recordsLinked to original sources

arXiv2026

Structural stability, electronic structure, and magnetic properties of the single-layer trilayer La3Ni2O7 polymorph

A polymorph of the bilayer nickelate La3Ni2O7 that displays an alternating single-layer (SL) and trilayer (TL; 1313) stacking pattern has recently been discovered. Signatures of superconductivity under pressure have been found in this phase. At ambient pressure, La3Ni2O7-1313 has been reported to crystallize in three different space-group symmetries Cmmm, Imma, and Fmmm. Unlike the commonly observed tilted NiO6 octahedra in perovskite nickelates, the Cmmm phase exhibits no NiO6 tilts, implying that this structural feature alone may be insufficient to give rise to superconductivity in Ruddlesden-Popper nickelates. Here, we employ first-principles calculations and group theory analysis to study the pressure dependence of the structural instabilities in this SL-TL La3Ni2O7 polymorph. At ambient pressure, we identify multiple unstable phonon branches in the highest symmetry (Cmmm) structure at various high-symmetry points of the Brillouin zone. Distortions associated with these instabilities lead to one of the other experimentally reported space groups (Imma) that does display octahedral tilts. The magnetic tendencies indicate that the electronic structure of La3Ni2O7-1313 at ambient pressure is dominated by the TL block, as the SL is in a Mott-insulating regime. Under pressure, a tetragonal P4/mmm structure becomes stable, in agreement with experiments.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional polaronic ground state in superconducting LiTi$_2$O$_4$

Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel Li$B_2$O$_4$ materials. On the $B$-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi$_2$O$_4$ and heavy fermion behavior in LiV$_2$O$_4$. To date, however, LiTi$_2$O$_4$ has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic groundstates. Here, we present a multimodal investigation of LiTi$_2$O$_4$, combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi$_2$O$_4$ from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi$_2$O$_4$. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

cond-mat.supr-con↗

arXiv2024

Pressure effects on the electronic structure and magnetic properties of infinite-layer nickelates

Motivated by the discovery of superconductivity in infinite-layer nickelates RNiO$_2$ (R= rare-earth), and the subsequent enhancement of their T$_c$ with pressure, we investigate the evolution of the electronic structure and magnetic properties of this family of materials via first-principles calculations employing hydrostatic and chemical pressure as tuning knobs. Overall, our analysis shows that pressure tends to increase the R-$5d$ self-doping effect, as well as the Ni-$d _{x^{2}-y^{2}}$ bandwidth, the $e_g$ energy splitting, the charge transfer energy, and the superexchange ($J$). Using the energy scale of $J$ as a predictor of superconducting tendencies, we anticipate that pressure can indeed be a feasible means to further increase the T$_c$ in this family of materials.

cond-mat.supr-con↗

arXiv2021

Counterdiabatic route for preparation of state with long-range topological order

We propose here a counterdiabatic (CD) strategy for fast preparation of a state with long-range topological order by magnetic field tuning of an initial separable state. For concreteness, we consider the ground state of the honeycomb Kitaev model whose long-range topological order together with the anyonic excitations make it an interesting candidate for fault-tolerant universal quantum computation and storage. The required CD perturbation is found to be local, having the form of the off-diagonal exchange interactions reminiscent of trigonal deformations in Kitaev Hamiltonians. We show that the counterdiabatically produced state can have high fidelity and retain numerous desired entanglement properties.

quant-ph↗