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Theoretical physicists working at a blackboard collaboration pod in the Beecroft building.
Credit: Jack Hobhouse

Siddharth Parameswaran

Professor of Physics

Research theme

  • Fields, strings, and quantum dynamics
  • Quantum materials
  • Quantum optics & ultra-cold matter

Sub department

  • Rudolf Peierls Centre for Theoretical Physics

Research groups

  • Condensed Matter Theory
sid.parameswaran@physics.ox.ac.uk
Telephone: 01865 273968
Rudolf Peierls Centre for Theoretical Physics, room 70.29
  • About
  • Research
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  • Publications

Gate-tunable double-dome superconductivity in twisted trilayer graphene

Nature Physics Springer Nature (2025)

Authors:

Zekang Zhou, Jin Jiang, Paritosh Karnatak, Ziwei Wang, Glenn Wagner, Kenji Watanabe, Takashi Taniguch, Christian Schönenberger, Siddharth Ashok Parameswaran, Steven H Simon, Mitali Banerjee

Abstract:

Graphene moiré systems are ideal environments for investigating complex phase diagrams and gaining fundamental insights into the mechanisms that underlie them, as they permit controlled manipulation of electronic properties. Magic-angle twisted trilayer graphene has emerged as a key platform for exploring moiré superconductivity due to the robustness of its superconducting order and the ability to tune its energy bands with an electric field. Here we report the direct observation of two domes of superconductivity in the phase diagram of magic-angle twisted trilayer graphene. The dependence of the superconductivity of doped holes on the temperature, magnetic field and bias current shows that it is suppressed near a specific filling of the moiré flat band, leading to a double dome in the phase diagram within a finite range of the displacement field. The transport properties are also indicative of a phase transition and the potentially distinct nature of superconductivity in the two domes. Hartree–Fock calculations incorporating mild strain yield an incommensurate Kekulé spiral state whose effective spin polarization peaks in the regime where superconductivity is suppressed in the experiments.
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Classification of spin-12 fermionic quantum spin liquids on the trillium lattice

Physical Review B American Physical Society (APS) 112:10 (2025) 104429

Authors:

Ming-Hao Li, Sounak Biswas, SA Parameswaran

Abstract:

We study fermionic quantum spin liquids (QSLs) on the three-dimensional trillium lattice of corner-sharing triangles. We are motivated by recent experimental and theoretical investigations that have explored various classical and quantum spin liquid states on similar networks of triangular motifs with strong geometric frustration. Using the framework of projective symmetry groups (PSG), we obtain a classification of all symmetric Z 2 and U ( 1 ) QSLs on the trillium lattice. We find two Z 2 spin-liquids, and a single U ( 1 ) spin-liquid that is proximate to one of the Z 2 states. The small number of solutions reflects the constraints imposed by the nonsymmorphic symmetries in the space group of the trillium lattice. Using self-consistency conditions of the mean-field equations, we obtain the spinon band-structure and spin structure factors corresponding to these states. All three of our spin liquids are gapless at their saddle points: one of the two Z 2 QSLs is nodal, while the U ( 1 ) case hosts a spinon Fermi surface. One of our Z 2 spin liquids hosts a stable gapless nodal star that is protected by projective symmetries against additions of further neighbor terms in the mean-field ansatz. We comment on directions for further work.
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Emergent Interacting Phases in the Strong Coupling Limit of Twisted M-Valley Moiré Systems: Application to SnSe${}_2$

(2025)

Authors:

Ming-Rui Li, Dumitru Calugaru, Yi Jiang, Hanqi Pi, Ammon Fischer, Henning Schlömer, Lennart Klebl, Maia G Vergniory, Dante M Kennes, Siddharth A Parameswaran, Hong Yao, B Andrei Bernevig, Haoyu Hu

Chern-textured exciton insulators with valley spiral order in moiré materials

Physical Review B American Physical Society (APS) 112:3 (2025) 35130

Authors:

Ziwei Wang, Yves H Kwan, Glenn Wagner, Steven H Simon, Nick Bultinck, Sa Parameswaran

Abstract:

We explore the phase diagrams of moiré materials in search of a class of intervalley-coherent correlated insulating state: the Chern texture insulator (CTI). This phase of matter, proposed in a companion paper [Kwan , .], breaks valley <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow> <mi>U</mi> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> </math> symmetry in a nontrivial fashion wherein the valley order parameter is forced to texture in momentum space as a consequence of band topology. Using detailed Hartree-Fock studies, we establish that the CTI emerges as an energetically competitive intermediate-coupling ground state in several moiré systems that lack a twofold rotation symmetry that forbids the single-particle topology essential to the formation of the CTI valley texture. Published by the American Physical Society 2025
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Textured exciton insulators

Physical Review B (condensed matter and materials physics) American Physical Society 112:3 (2025) 035129

Authors:

Yves H Kwan, Ziwei Wang, Glenn Wagner, Steven SIMON, Siddharth ASHOK PARAMESWARAN, Nick Bultinck

Abstract:

We introduce and study interacting topological states that arise in time-reversal symmetric bands with an underlying obstruction to forming localized states. If the U(1) valley symmetry linked to independent charge conservation in each time-reversal sector is spontaneously broken, the corresponding “excitonic” order parameter is forced to form a topologically nontrivial texture across the Brillouin zone. We show that the resulting phase, which we dub a textured exciton insulator, cannot be given a local-moment description because of a form of delicate topology. Using toy models of bands with Chern or Euler obstructions to localization, we construct explicit examples of the Chern or Euler texture insulators (CTIs or ETIs) they support, and demonstrate that these are generically competitive ground states at intermediate coupling. We construct field theories that capture the response properties of these new states. Finally, we identify the incommensurate Kekulé spiral phase observed in magic-angle bi- and trilayer graphene as a concrete realization of an ETI.
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