P6 Thermodynamics and dynamics of highly frustrated magnets

The competing interactions in highly frustrated magnets suppress classically ordered states and thus favour exotic ground states. Indeed, the huge (near-)degeneracy of these magnets shows up in a range of highly unusual features, e.g., the large entropy accumulation at a field-induced quantum critical point is predicted to give rise to an enhanced magnetocaloric effect. At the same time, the huge density of low-energy excitations allows their localisation in real space, i.e., they form flat bands in reciprocal space. In contrast to the well-studied static properties, dynamic properties of highly frustrated magnets are little explored. For example, in a single particle picture one expects only a small contribution to thermal conductivity from excitations in flat bands. However, many-body effects remain to be included, which will change this situation. The aim of this project is to study such dynamical properties of highly frustrated magnets using numerical methods. In particular, one-dimensional quantum magnets are ideal model systems since they can be tackled by different variants of the density-matrix-renormalization group (DMRG) method. This has been successfully demonstrated for the thermodynamic properties and the magnetic excitation spectrum of the highly frustrated quasi-1D magnet azurite. Still, variants of the DMRG method for studying finite-temperature dynamics are only just becoming sufficiently powerful to enable a reliable computation of dynamic properties. The first main goal of this project is to implement such methods and compute the dynamic spin structure factor at finite temperature as well as the thermal conductivity for one-dimensional model systems.

PhD student in charge: Alexander Tiegel

International Partner: A. Honecker

Publications:

Jonas Becker, Thomas Köhler, Alexander C. Tiegel, Salvatore R. Manmana, Stefan Wessel, Andreas Honecker, Finite-temperature dynamics and thermal intra-band magnon scattering in Haldane spin-one chains, open access version: arXiv:1703.04652 (2017)

Alexander C. Tiegel, Salvatore R. Manmana, Thomas Pruschke, and Andreas Honecker, Matrix product state formulation of frequency-space dynamics at finite temperatures, Phys. Rev. B 90, 060406(R) (2014), open access version: arXiv:1312.6044

Alexander C. Tiegel, Thomas Veness, Piet E. Dargel, Andreas Honecker, Thomas Pruschke, Ian P. McCulloch, and Fabian H. L. Essler, Optical conductivity of the Hubbard chain away from half filling, Phys. Rev. B 93, 125108 (2016), open access version:arXiv 1601.00648

Alexander C. Tiegel, Andreas Honecker, Thomas Pruschke, Alexey Ponomaryov, Sergei A. Zvyagin, Ralf Feyerherm, Salvatore R. Manmana, Dynamical properties of the sine-Gordon quantum spin magnet Cu-PM at zero and finite temperature, Phys. Rev. B 93, 104411 (2016), open access version: arXiv 1511.07880

E.S. Klyushina, A.C. Tiegel, B. Fauseweh, A.T.M.N. Islam, J.T. Park, B. Klemke, A. Honecker, G.S. Uhrig, S.R. Manmana, B. Lake, Magnetic excitations in the S = 1/2 antiferromagnetic-ferromagnetic chain compound BaCu2V2O8 at zero and finite temperature, Phys. Rev. B 93, 241109(R) (2016), open access version: arXiv:1602.06184

Alexander C. Tiegel, Finite-temperature dynamics of low-dimensional quantum systems with DMRG methods, Ph.D. thesis, Georg-August-Universität Göttingen (2016). Persisten Address: http://hdl.handle.net/11858/00-1735-0000-0028-8801-A

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