Hackenbroich, Anna Sophie (2021): Entanglement in chiral topological systems. Dissertation, LMU München: Faculty of Physics 

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Abstract
Topological phases of matter describe systems beyond the paradigm of spontaneous symmetry breaking, giving rise to unconventional phenomena with possible applications in topological quantum computing. Almost 40 years after their discovery, topological phases remain among the most active research fields of condensed matter physics. As evidenced by the recent discovery of higher order topological insulators (HOTIs), novel physics can emerge even in noninteracting systems. Theoretical investigations of strongly correlated systems are extremely difficult due to the exponential growth of the quantummechanical Hilbert space and frequently require the application of approximative methods such as model wave functions. Tensor network states (TNS) are a class of variational wave functions which allow an efficient encoding of relevant quantum many body states and which form the basis for very successful numerical algorithms. Projected entangled pair states (PEPS), a class of TNS in two and higher dimensions, permit the exact representation of many interacting topological phases with time reversal symmetry. However, the description of experimentally relevant chiral topological phases like the integer and fractional quantum Hall effects is much more subtle using the TNS framework. The first part of this thesis focuses on the description of chiral topological phases using PEPS. Firstly, we study a previously proposed chiral PEPS that was conjectured to possess anyonic excitations. By a careful analysis of its symmetries we are able to align important entanglement observables more closely with the expected universal behavior. This highlights that efficient PEPS can possess characteristic properties of chiral topological phases. Secondly, we focus on the issue that all known chiral PEPS have algebraic bulk correlation functions and therefore cannot be the ground states of gapped local Hamiltonians. Since this problem arises already for noninteracting chiral topological phases, we focus on two examples of such systems and show that their ground states can be represented exactly by efficient PEPS in a hybrid lattice with one momentumspace direction. After an inverse Fourier transform, the PEPS with only realspace coordinates requires an exponentially growing number of parameters for an exact representation. This provides a concrete illustration of the impossibility to encode gapped chiral phases exactly with efficient PEPS. Finally, we analyze a model wave function for a threedimensional (3D) HOTI with strong intrinsic correlations using largescale variational Monte Carlo simulations. This wave function is obtained by projection of two copies of a noninteracting HOTI with chiral hinge states. We characterize the gapless hinge states of the interacting system and show that they are of the same nature as the edge states of the $1/2$ Laughlin state. Surprisingly, the gapped surfaces host a twodimensional (2D) phase whose topological entanglement entropy is half of that of the $1/2$ Laughlin state. Such a value cannot be obtained by any of the known 2D topological orders, showing a clear departure not only from the Laughlin $1/2$ physics but from conventional 2D topological order. This demonstrates that 3D topological phases can host rich phenomena that are not yet fully understood.
Item Type:  Theses (Dissertation, LMU Munich) 

Subjects:  500 Natural sciences and mathematics 500 Natural sciences and mathematics > 530 Physics 
Faculties:  Faculty of Physics 
Language:  English 
Date of oral examination:  23. February 2021 
1. Referee:  Delft, Jan von 
MD5 Checksum of the PDFfile:  3e366acddcf9712017ccf1441ac7c243 
Signature of the printed copy:  0001/UMC 27770 
ID Code:  27564 
Deposited On:  15. Mar 2021 10:42 
Last Modified:  15. Mar 2021 10:43 