Sergej Moroz

Quantum Fluids

Technical University Munich

Department of Physics

James-Franck-Str. 1

85747 Garching

Tel. +49 89 289 12884


Group Webpage

As a rule, quantum collective behavior of a macroscopic ensemble of particles is very different from the physics of its elementary constituents. The richness and universality of the emergent physical theories inspires and fuels my research of quantum phases of matter.


Research focus: topological phases in condensed matter, quantum fluids, universal few-body quantum physics

The main areas of research conducted in our group are concentrated around two broad topics: effective field theories of quantum phases of matter and discrete lattice gauge theories interacting with fermionic matter.

Effective theories in condensed matter physics

Although microscopically condensed matter physics is about interaction between electrons, protons, neutrons and light, often the many-body nature of the problem gives rise to emergence of new degrees of freedom with intriguing collective behavior at low energies. These degrees of freedom constitute the building blocks of effective field theories that in addition are constrained by symmetries of the problem. This set-up provides a reliable micro-independent framework for non-perturbative understanding of strongly interacting quantum systems. In our group we are especially interested in the interplay of topology and geometry in quantum phases of matter. We develop effective theories of various low-dimensional many-body topological quantum fluids and crystals such as topological superfluids and superconductors, vortex crystals and quantum Hall fluids.

Selected Publications

  • Bosonic superfluid on lowest Landau level. Sergej Moroz, Dam Thanh Son, Phys. Rev. Lett. 122, 235301 (2019), [arXiv:1901.06088]
  • Effective field theory of a vortex lattice in a bosonic superfluid. Sergej Moroz, Carlos Hoyos, Claudio Benzoni, Dam Thanh Son, SciPost Phys. 5, 039 (2018), [arXiv:1803.10934]
  • Topological order, symmetry, and Hall response of two-dimensional spin-singlet superconductors. Sergej Moroz, Abhinav Prem, Victor Gurarie, Leo Radzihovsky, Phys. Rev. B 95, 014508 (2017), [arXiv:1606.03462]
  • Effective theory of chiral two-dimensional superfluids. Carlos Hoyos, Sergej Moroz, Dam Thanh Son, Phys. Rev. B 89, 174507 (2014), [arXiv:1305.3925]

Discrete lattice gauge theories interacting with fermionic matter

Gauge theories play a central role in our current description of Nature. During the last century gauge invariance proved to be a valuable guiding principle in physics, to the point that all the known fundamental interactions in particle physics beyond electromagnetism are now described by some non-Abelian Yang-Mills gauge theories. Lattice regularization is widely used in high energy physics to study strongly coupled quantum gauge theories. Lattice gauge theories also arise naturally in the context of nowadays condensed matter physics in lattice problems where low-energy excitations fractionalize.

Theoretical discovery of the Ising Z2 gauge theory led to a drastic shift of paradigm of our understanding of phase transitions and was the first example of a system that exhibits topological order. Since the Ising gauge field mediates attraction, when coupled to fermionic matter it leads to formation of an exotic superfluid state. In our group we use analytical methods and numerical density matrix renormalization group (DMRG) approach to understand quantum phases of fermions coupled to the Ising gauge theory in one and two spatial dimensions. Our research is partially motivated by recent advances in cold atom experiments, where prototypes of the Ising gauge theory coupled to matter are actively studied.

Selected Publications

  • Confined phases of one-dimensional spinless fermions coupled to Z2 gauge theory. Umberto Borla, Ruben Verresen, Fabian Grusdt, Sergej Moroz, [arXiv:1909.07399]


Bosonic superfluid on lowest Landau level

S. Moroz, D. T. Son.

Physic Review Letters 122, 235301 (2019).

Show Abstract

We develop a low-energy effective field theory of a two-dimensional bosonic superfluid on the lowest Landau level at zero temperature and identify a Berry term that governs the dynamics of coarse-grained superfluid degrees of freedom. For an infinite vortex crystal we compute how the Berry term affects the low-energy spectrum of soft collective Tkachenko oscillations and non-dissipative Hall responses of the particle number current and stress tensor. This term gives rise to a quadratic in momentum term in the Hall conductivity, but does not generate a non-dissipative Hall viscosity.

DOI: 10.1103/PhysRevLett.122.235301

Confined phases of one-dimensional spinless fermions coupled to Z2 gauge theory

U. Borla, R. Verresen, F. Grusdt, S. Moroz.

Physics Review Letters 124, 120503 (2020).

Show Abstract

We investigate a quantum many-body lattice system of one-dimensional spinless fermions interacting with a dynamical Z2 gauge field. The gauge field mediates long-range attraction between fermions resulting in their confinement into bosonic dimers. At strong coupling we develop an exactly solvable effective theory of such dimers with emergent constraints. Even at generic coupling and fermion density, the model can be rewritten as a local spin chain. Using the Density Matrix Renormalization Group the system is shown to form a Luttinger liquid, indicating the emergence of fermionic fractionalized excitations despite the confinement of lattice fermions. In a finite chain we observe the doubling of the period of Friedel oscillations which paves the way towards experimental detection of confinement in this system. We discuss the possibility of a Mott phase at the commensurate filling 2/3.

DOI: 10.1103/PhysRevLett.124.120503

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