10 Publications from the project Correlations in antiferromagnets Sebastian Eggert (TU Kaiserslautern) Summary Achievements Motivation The role of this project within the Transregio is to analyze correlations in antiferromagnets which are tunable by the chemical composition or the magnetic field in order to identify interesting new phases and quantum critical points. Topics In the coming funding period three major topics are considered (i) Frustrated spin systems. (ii) Coupled Spin Clusters (iii) Dimensional transition of coupled chain systems. Methods The following method will be used: Bosonization Chain mean field theory Spin wave theory / field theory Quantum Monte Carlo Strong coupling expansion People We apply for funding of one graduate student: Annabelle Bordth starting in early 2016. Current Ph.D. student Dominik Straßel is expected to graduate in 2015. The project will be assisted by the principal investigator, Dr. Imke Schneider, and Dr. Xue-Feng Zhang, who are financed by state funds (Grundausstattung, GA). Magneto-caloric effects and criticality [P10] We found exact linear scaling of magneto-caloric effect of 2D coupled spin dimer system in a collaboration with project Cooling rate and susceptibility show crossover to BKT behavior well above BKT phase transition. For experiments, see project A8 B1 Fig. 4: schematic (left) and numerical (right) phase diagram of 2D coupled spin dimers. (from P[10]) Chiral edge states with fractional charges in 2D [P4] The XXZ model on the Kagome lattice with hard-wall boundary conditions shows edge states with fractional charge, that are connected by topological string excitations. Such fractional edge excitations were previously only known in 1D (e.g. the edge spin-1/2 in the Haldane chain). Fig. 5: fluctuating strings and edge excitations in the kagome lattice with hard walls. Supersolid phase transitions on the triangular lattice [P9] Mediated entanglement between spins [P3] Generalized effective potential Landau theory for the analytic determination of phase diagrams [P5] Spin and charge transport in 1D systems [P8,P9] in collaboration with project Theoretical spectral analysis of magnetic molecules [P6] A11E Achievements The frustrated J1-J2 Heisenberg Model [P2] The nature of the intermediate phase of the J1-J2 model has been controversial. We considered an anisotropic generalization of the model with renormalization group method and find an intermediate dimerized phase. 10 Publications from the project [P1] A. Vogler, R. Labouvie, S. Barontini, S. Eggert, V. Guarrera, and H. Ott, Dimensional phase transition from an array of 1D Luttinger liquids to a 3D Bose-Einstein condensate, Phys. Rev. Lett. 113, 215301 (2014). [P2] A. Metavitsiadis, D. Sellmann, and S. Eggert, Spin liquid versus dimer phases in an anisotropic J1 − J2 frustrated square antiferromagnet, Phys. Rev. B 89, 241104(R) (2014). [P3] A. Metavitsiadis, R. Dillenschneider, and S. Eggert, Impurity entanglement through electron scattering in a magnetic field, Phys. Rev. B 89, 155 406 (2014). [P4] X.-F. Zhang and S. Eggert, Chiral edge states and fractional charge separation in interacting bosons on a kagome lattice, Phys. Rev. Lett. 111, 147201 (2013). [P5] T. Wang, X.-F. Zhang, S. Eggert, and A. Pelster, Generalized Effective Potential Landau Theory for Bosonic Quadratic Superlattices, Phys. Rev. A 87, 063615 (2013). [P6] A. Machens, N. P. Konstantinidis, O. Waldmann, I. Schneider, and S. Eggert, The even-odd effect in short antiferromagnetic Heisenberg chains, Phys. Rev. B 87, 144409 (2013). [P7] N. Sedlmayr, D. Morath, I. Affleck, J. Sirker, and S. Eggert, Conducting fixed points for inhomogeneous quantum wires: a conformally invariant boundary theory, Phys. Rev. B 89, 045133 (2014). [P8] N. Sedlmayr, J. Ohst, I. Affleck, J. Sirker, and S. Eggert, Transport and scattering in inhomogeneous quantum wires, Phys. Rev. B 86, 121302(R) (2012). [P9] X.-F. Zhang, R. Dillenschneider, Y, Yu, and S. Eggert, Supersolid phase transitions for hard-core bosons on a triangular lattice, Phys. Rev. B 84, 174515 (2011). [P10] D. Straßel, P. Kopietz, and S. Eggert, Quantum critical points in 2D coupled spin dimer systems and the BKT transition, preprint arXiv:1412.0266 submitted (2015). Fig. 1: Anisotropic model Fig. 2: The phase diagram of the generalized anisotropic J1-J2 model (from [P2]) Dimensional transition of coupled chains [P1] In a collaboration with experimental project the phase transition of coupled 1D bosonic chains to a 3D condensate was mapped out as a function of coupling strength and filling. This collaboration continues in the next funding period. Coupled spin chains will also be studied. A9 Fig. 3: Analysis of local density (top) yields the phase diagram (left), from [P1]. Transregional Collaborative Research Centre SFB/TR 49 Frankfurt / Kaiserslautern / Mainz