Condensed Matter Seminars This Term

Since the discovery of cuprate superconductors, layered compounds have been one of important target systems for developing new exotic superconductors and studying unconventional mechanisms. In this seminar, I will talk about material development of layered BiCh₂-based (Ch: S, Se) superconductors, which were first discovered in 2012 [1,2]. In the BiCh₂-based compounds, intrinsic local structural disorder (local distortion) has been observed by various experimental probes, and the suppression of local disorder is essential for the emergence of bulk superconductivity in the system [3]. After obtaining bulk superconductors with less local disorder, we could see some interesting superconducting properties in BiCh₂-based superconductors: absence of isotope effect [4], nematic superconductivity [5], and huge upper critical fields [6] in La(O,F)Bi(S,Se)₂ will be briefly explained.

[1] Y. Mizuguchi et al., PRB 86, 220510 (2012)

[2] Y. Mizuguchi et al., JPSJ 81, 114725 (2012)

[3] (review article) Y. Mizuguchi, JPSJ 88, 041001 (2019)

[4] K. Hoshi, Y. Goto, Y. Mizuguchi, PRB 97, 094509 (2018)

[5] K. Hoshi et al., JPSJ 88, 033704 (2019)

[6] K. Hoshi et al., Sci. Rep. 12, 288 (2022)

Height models and random tiling are well-studied objects in classical statistical mechanics and combinatorics that lead to many interesting phenomena, such as arctic curve, limit shape and Kadar-Parisi-Zhang scaling. We introduce quantum dynamics to the classical hexagonal dimer, and six-vertex model to construct frustration-free Hamiltonians with unique ground state being a superposition of tiling configurations subject to a particular boundary configuration. An internal degree of freedom of color is further introduced to generate long range entanglement that makes area law violation of entanglement entropy possible. The scaling of entanglement entropy between half systems is analysed with the surface tension theory of random surfaces and under a q-deformation that weighs random surfaces in the ground state superposition by the volume below, it undergoes a phase transition from area law to volume scaling. At the critical point, the scaling is L logL due to the so-called "entropic repulsion” of Gaussian free fields conditioned to be positive. An exact holographic tensor network description of the ground state is give with one extra dimension perpendicular to the lattice. We also discuss an alternative realisation with six-vertex model, inhomogeneous deformation to obtain sub-volume intermediate scaling, and possible generalisations to higher dimension.

The Heisenberg antiferromagnet on the kagome lattice is an archetypal example of how large ground state degeneracies arise, and how they may get resolved by thermal and quantum fluctuations. Augmenting the Heisenberg model by chiral spin interactions has proved to be of particular interest in the discovery of chiral quantum spin liquids. I will focus on the classical variant of this chiral kagome model, which exhibits, similar to the classical Heisenberg antiferromagnet, a remarkably large and structured ground-state manifold combining continuous and discrete degrees of freedom. This allows for a rich set of order-by-disorder phenomena. Degeneracy lifting by thermal and quantum fluctuations occurs in a highly selective way that, among other interesting effects, provides a semiclassical route to an emergent Z_2 spin liquid.