The Bonn Journal Club on Condendensed Matter Theory \ Physikalisches Institut, Uni Bonn
We are now in the second year of the Journal Club. Details about talks from the previous season can be found here.
Season 1: WiSe22-SoSe23
Season 2: WiSe23-SoSe24
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Talks This Season:
| Speaker | Date | Title |
|---|---|---|
| 1. Dr. Franco Lisandrini (AG Kollath) | October 18, 2023 (Wednesday) | Partially disordered Hiesenberg antiferromagnet with short range stripe correlations |
| [Slides],[Abstract] | ||
| 2.Kriti Baweja (AG Luitz) | November 08, 2023 (Wednesday) | Weak measurements in the ground state of 2-D Antiferromagnetic Heisenberg model |
| [Slides],[Abstract] | ||
| 3.Julien Breiher (AG Luitz) | November 15, 2023 (Wednesday) | The simulation of Clifford Circuits : The Gottesman-Knill theorem |
| [Slides],[Abstract] | ||
| 4.Michael Hübner, Oliver Freyermuth (IT Dept.) | November 29, 2023 (Wednesday) | News from the IT department and Cluster Computing |
| [Slides],[Abstract] | ||
| 5.Dr. Sayak Ray (AG Kroha) | December 06, 2023 (Wednesday) | Bistability dynamics in the dissipative Dicke-Bose-Hubbard system |
| [Slides],[Abstract] |
Details of Talks:
1. October 18, 2023 (Wednesday)
Title: Partially disordered Hiesenberg antiferromagnet with short range stripe correlations
Speaker: Dr. Franco Lisandrini (AG Kollath)
Zero-point quantum fluctuations of a Néel order can produce effective interactions between quasi-orphan spins weakly coupled to the lattice. On the 3–√×3–√−distorted triangular lattice, this phenomenon leads to a correlated partially disordered phase. In this article, we use matrix product state methods to study a similar model: the S=1/2 stuffed square lattice. Tunning the exchange amplitudes we go from a square lattice plus orphan central spins at J′/J=0 , to the union jack lattice at J′/J=1, and a square lattice including all spins at J/J′=0. We calculate the complete antiferromagnetic phase diagram, dominated by ferrimagnetic and Néel orders, and compare with existing results. Most importantly, we find a partially disordered phase in the weakly frustrated regime. In this phase, the Néel order from the square lattice is unaffected, while the central spins form a collective state with exponentially decaying double striped correlations. We also study the role of quantum fluctuations by introducing an ordering staggered magnetic field on the square sublattice, and find that the central spins order ferromagnetically when fluctuations from the Néel order are suppressed.
2. November 08, 2023 (Wednesday)
Title: Weak measurements in the ground state of 2-D Antiferromagnetic Heisenberg model
Speaker: Kriti Baweja (AG Luitz)
In this thesis we developed a numerical technique which allows us to simulate local measurements on the ground state of the two-dimensional Antiferromagnetic Heisenberg Model. The system under consideration is highly entangled and has long-range order which makes it a particularly interesting system to study. To be able to do this we extended the framework of a pre-existing method in Quantum Monte Carlo known as Stochastic Series Expansion (SSE). The main advantage of using SSE is that we can go to large system sizes. We first start with implementing SSE for a simple system without measurement and benchmark it against Exact Diagonalization (ED). This work deals with two projection sectors namely the singlet and triplet and we derive the form of measurement operator in each of these sectors respectively. We then implemented SSE with measurement for some example systems and benchmark it against ED. We saw that measurements in some configurations lead to a sign problem (in computational basis) even for a bi-partite lattice. The only sign-free measurement found for the case of singlet measurement was when the local-measurements were performed on the neighbouring sites. It was found that measuring a singlet on a bond disentangles it from the system and kills the long-range order. Correspondingly, the sign-free triplet measurement was found to be on the diagonal bonds. In this case we saw that the long-range order enhanced in the post-measurement state. Despite the fact that results in both the sectors are qualitatively and quantitatively distinct, they share a common characteristic that the effects observed in the post-measurement state in both the sectors are non-local.
3. November 15, 2023 (Wednesday)
Title: The simulation of Clifford Circuits : The Gottesman-Knill theorem
Speaker: Julien Breiher (AG Luitz)
It is a well known fact that qubit systems have a computational and storage cost growing exponentially with the size of the system. In this instance of the Bonn condensed matter journal club, we will look at an alternative to this computational problem through the lens of stabilizer circuits. We will give an intuition towards the Gottesman-Knill theorem as well as an improvement on it and several ways it can be used for simulation of highly entangled states.
4. November 29, 2023 (Wednesday)
Title: News from the IT department and Cluster Computing
Speaker: Michael Hübner, Oliver Freyermuth (IT Dept.)
Zero-point quantum fluctuations of a Néel order can produce effective interactions between quasi-orphan spins weakly coupled to the lattice. On the 3–√×3–√−distorted triangular lattice, this phenomenon leads to a correlated partially disordered phase. In this article, we use matrix product state methods to study a similar model: the S=1/2 stuffed square lattice. Tunning the exchange amplitudes we go from a square lattice plus orphan central spins at J′/J=0 , to the union jack lattice at J′/J=1, and a square lattice including all spins at J/J′=0. We calculate the complete antiferromagnetic phase diagram, dominated by ferrimagnetic and Néel orders, and compare with existing results. Most importantly, we find a partially disordered phase in the weakly frustrated regime. In this phase, the Néel order from the square lattice is unaffected, while the central spins form a collective state with exponentially decaying double striped correlations. We also study the role of quantum fluctuations by introducing an ordering staggered magnetic field on the square sublattice, and find that the central spins order ferromagnetically when fluctuations from the Néel order are suppressed.
5. December 06, 2023 (Wednesday)
Title: Bistability dynamics in the dissipative Dicke-Bose-Hubbard system
Speaker: Dr. Sayak Ray (AG Kroha)
We consider a driven-dissipative system consisting of an atomic Bose-Einstein condensates in a two-dimensional Hubbard lattice and is coupled to a single mode of an optical cavity. Due to the interplay between strong, repulsive atomic interaction and the atom-cavity coupling, the system exhibits several phases of atoms and photons including the atomic superfluid (SF) and supersolid (SS). We investigate the dynamical behaviour of the system, where we include dissipation using the Lindblad master equation formalism. Due to the discontinuous nature of the Dicke transition for strong atomic repulsion, we find extended co-existence regions of different phases. We investigate the resulting switching dynamics, particularly between the coexisting SF and SS phases, which eventually become damped by dissipation.