| 1. |
- Artyukhin, Sergey, et al.
(författare)
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Solitonic lattice and Yukawa forces in the rare-earth orthoferrite TbFeO3
- 2012
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Ingår i: Nature Materials. - 1476-4660. ; 11:8, s. 694-699
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Tidskriftsartikel (refereegranskat)abstract
- The random fluctuations of spins give rise to many interesting physical phenomena, such as the 'order-from-disorder' arising in frustrated magnets and unconventional Cooper pairing in magnetic superconductors. Here we show that the exchange of spin waves between extended topological defects, such as domain walls, can result in novel magnetic states. We report the discovery of an unusual incommensurate phase in the orthoferrite TbFeO3 using neutron diffraction under an applied magnetic field. The magnetic modulation has a very long period of 340 angstrom at 3 K and exhibits an anomalously large number of higher-order harmonics. These domain walls are formed by Ising-like Tb spins. They interact by exchanging magnons propagating through the Fe magnetic sublattice. The resulting force between the domain walls has a rather long range that determines the period of the incommensurate state and is analogous to the pion-mediated Yukawa interaction between protons and neutrons in nuclei.
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| 2. |
- Bramberger, Max, et al.
(författare)
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Formation of CuO2 sublattices by suppression of interlattice correlations in tetragonal CuO
- 2023
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Ingår i: SciPost Physics. - 2542-4653. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the tetragonal phase of the binary transition metal oxide CuO (t-CuO) within the context of cellular dynamical mean-field theory. Due to its strong antiferromagnetic correlations and simple structure, analysing the physics of t-CuO is of high interest as it may pave the way towards a more complete understanding of high-temperature superconductivity in hole-doped antiferromagnets. In this work we give a formal justification for the weak-coupling assumption that has previously been made for the interconnected sublattices within a single layer of t-CuO by studying the non-local self-energies of the system. We compute momentum-resolved spectral functions using a Matrix Product State (MPS)-based impurity solver directly on the real axis, which does not require any numerically ill-conditioned analytic continuation. The agreement with photoemission spectroscopy indicates that a single-band Hubbard model is sufficient to capture the material’s low energy physics. We perform calculations on a range of different temperatures, finding two magnetic regimes, for which we identify the driving mechanism behind their respective insulating state. Finally, we show that in the hole-doped regime the sublattice structure of t-CuO has interesting consequences on the symmetry of the superconducting state.
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| 3. |
- Köhler, Thomas, et al.
(författare)
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Efficient and flexible approach to simulate low-dimensional quantum lattice models with large local Hilbert spaces
- 2021
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Ingår i: SciPost Physics. - : SCIPOST FOUNDATION. - 2542-4653. ; 10:3
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Tidskriftsartikel (refereegranskat)abstract
- Quantum lattice models with large local Hilbert spaces emerge across various fields in quantum many-body physics. Problems such as the interplay between fermions and phonons, the BCS-BEC crossover of interacting bosons, or decoherence in quantum simulators have been extensively studied both theoretically and experimentally. In recent years, tensor network methods have become one of the most successful tools to treat such lattice systems numerically. Nevertheless, systems with large local Hilbert spaces remain challenging. Here, we introduce a mapping that allows to construct artificial U(1) symmetries for any type of lattice model. Exploiting the generated symmetries, numerical expenses that are related to the local degrees of freedom decrease significantly. This allows for an efficient treatment of systems with large local dimensions. Further exploring this mapping, we reveal an intimate connection between the Schmidt values of the corresponding matrix-product-state representation and the single-site reduced density matrix. Our findings motivate an intuitive physical picture of the truncations occurring in typical algorithms and we give bounds on the numerical complexity in comparison to standard methods that do not exploit such artificial symmetries. We demonstrate this new mapping, provide an implementation recipe for an existing code, and perform example calculations for the Holstein model at half filling. We studied systems with a very large number of lattice sites up to L = 501 while accounting for N-ph = 63 phonons per site with high precision in the CDW phase.
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| 4. |
- Paeckel, Sebastian, et al.
(författare)
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Detecting superconductivity out of equilibrium
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:18
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Tidskriftsartikel (refereegranskat)abstract
- Recent pump-probe experiments on underdoped cuprates and similar systems suggest the existence of a transient superconducting state above T-c. This poses the question of how to reliably identify the emergence of long-range order, in particular superconductivity, out of equilibrium. We investigate this point by studying a quantum quench in an extended Hubbard model and by computing various observables, which are used to identify (quasi-)long-range order in equilibrium. Our findings imply that, in contrast to current experimental studies, it does not suffice to study the time evolution of the optical conductivity to identify superconductivity. In turn, we suggest to utilize time-resolved angle-resolved photoemission spectroscopy experiments to probe for the formation of a condensate in the two-particle channel.
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| 5. |
- Paeckel, Sebastian, et al.
(författare)
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Time-evolution methods for matrix-product states
- 2019
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Ingår i: Annals of Physics. - : ACADEMIC PRESS INC ELSEVIER SCIENCE. - 0003-4916 .- 1096-035X. ; 411
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Tidskriftsartikel (refereegranskat)abstract
- Matrix-product states have become the de facto standard for the representation of one-dimensional quantum many body states. During the last few years, numerous new methods have been introduced to evaluate the time evolution of a matrix-product state. Here, we will review and summarize the recent work on this topic as applied to finite quantum systems. We will explain and compare the different methods available to construct a time-evolved matrix-product state, namely the time-evolving block decimation, the MPO W-I,W-II method, the global Krylov method, the local Krylov method and the one- and two-site time-dependent variational principle. We will also apply these methods to four different representative examples of current problem settings in condensed matter physics. (C) 2019 The Author(s). Published by Elsevier Inc.
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| 6. |
- Stolpp, Jan, et al.
(författare)
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Comparative study of state-of-the-art matrix-product-state methods for lattice models with large local Hilbert spaces without U(1) symmetry
- 2021
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Ingår i: Computer Physics Communications. - : Elsevier. - 0010-4655 .- 1879-2944. ; 269
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Tidskriftsartikel (refereegranskat)abstract
- Lattice models consisting of high-dimensional local degrees of freedom without global particle-number conservation constitute an important problem class in the field of strongly correlated quantum many body systems. For instance, they are realized in electron-phonon models, cavities, atom-molecule resonance models, or superconductors. In general, these systems elude a complete analytical treatment and need to be studied using numerical methods where matrix-product states (MPSs) provide a flexible and generic ansatz class. Typically, MPS algorithms scale at least quadratic in the dimension of the local Hilbert spaces. Hence, tailored methods, which truncate this dimension, are required to allow for efficient simulations. Here, we describe and compare three state-of-the-art MPS methods each of which exploits a different approach to tackle the computational complexity. We analyze the properties of these methods for the example of the Holstein model, performing high-precision calculations as well as a finite-size scaling analysis of relevant ground-state observables. The calculations are performed at different points in the phase diagram yielding a comprehensive picture of the different approaches.
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| 7. |
- Wilke, Reja H., et al.
(författare)
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Symmetry-protected Bose-Einstein condensation of interacting hardcore bosons
- 2023
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Ingår i: Communications Physics. - : Springer Nature. - 2399-3650. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- The large practical potential of exotic quantum states is often precluded by their notorious fragility against external perturbations or temperature. Here, we introduce a mechanism stabilizing a one-dimensional quantum many-body phase exploiting an emergent Z(2) -symmetry based on a simple geometrical modification, i.e. a site that couples to all lattice sites. We illustrate this mechanism by constructing the solution of the full quantum many-body problem of hardcore bosons on a wheel geometry, which are known to form Bose-Einstein condensates. The robustness of the condensate against interactions is shown numerically by adding nearest-neighbor interactions, which typically destroy Bose-Einstein condensates. We discuss further applications such as geometrically inducing finite-momentum condensates. Since our solution strategy is based on a generic mapping, our findings are applicable in a broader context, in which a particular state should be protected, by introducing an additional center site.
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