Emergence of exceptional points in two dimensions is one of the remarkable phenomena in non-Hermitian systems. We here elucidate the impacts of symmetry on the non-Hermitian physics. Specifically, we analyze chiral symmetric correlated systems in equilibrium where the non-Hermitian phenomena are induced by the finite lifetime of quasi-particles. Intriguingly, our analysis reveals that the combination of symmetry and non-Hermiticity results in novel topological degeneracies of energy bands which we call symmetry-protected exceptional rings (SPERs). We observe the emergence of SPERs by analyzing a non-Hermitian Dirac Hamiltonian. Furthermore, by employing the dynamical mean-field theory, we demonstrate the emergence of SPERs in a correlated honeycomb lattice model whose single-particle spectrum is described by a non-Hermitian Dirac Hamiltonian. We uncover that the SPERs survive even beyond the non-Hermitian Dirac Hamiltonian, which is related to a zero-th Chern number.

We analyze a two-dimensional Kondo lattice model with special emphasis on non-Hermitian properties of the single-particle spectrum, following a recent proposal by Kozii and Fu. Our analysis based on the dynamical mean-field theory elucidates that the single-particle spectral weight shows the exceptional points (EPs). Correspondingly, the spectral weight exhibits the band touching, resulting in a structure similar to the "Fermi arc". Furthermore, we find an intriguing phenomenon arising from the periodicity of the lattice. The EPs generated by two distinct Dirac points merge and change into a hybrid point which vanishes as the exchange interaction is increased. Accordingly, the paramagnetic phase in the low temperature region shows a significant difference from non-interacting fermions: the imaginary part of the self-energy yields the "Fermi loop" without any defective points.

We analyze the effects of the local Coulomb interaction on a topological band insulator (TBI) by applying the dynamical mean field theory to a generalized Bernevig-Hughes-Zhang model having electron correlations. It is elucidated how the correlation effects modify electronic properties in the TBI phase at finite temperatures. In particular, the band inversion character of the TBI inevitably leads to the large reduction of the spectral gap via the renormalization effect, which results in the strong temperature-dependence of the spin Hall conductivity. We clarify that a quantum phase transition from the TBI to a trivial Mott insulator, if it is nonmagnetic, is of first order with a hysteresis. This is confirmed via the interaction dependence of the double occupancy and the spectral function. A magnetic instability is also addressed. All these results imply that the spectral gap does not close at the transition.Comment: 6 pages, 6 figure

We investigate properties of a topological Mott insulator in one dimension by examining the bulk topological invariant and the entanglement spectrum of a correlated electron model. We clarify how gapless edge states in a non-interacting topological band insulator evolve into spinon edge states in a topological Mott insulator. Furthermore, we propose a topological Mott transition, which is a new type of topological phase transition and never observed in free fermion systems. This unconventional transition occurs in spin liquid phases in the Mott insulator and is accompanied by zeros of the single-electron Green's function and a gap closing in the spin excitation spectrum.PACS numbers: 71.27.+a, 71.10.Fd Introduction-For many years, characterization of quantum phases has been done in terms of spontaneous broken symmetry and the corresponding order parameter, which is well known as the Ginzburg-Landau paradigm. Recently, however, it has become clear that topological phases of matter are out of this framework. These topological states are gapped in the bulk and differ from trivial ones in the topology of their electronic states [1, 2]. The big difference between them is the existence of gapless edge modes which are a source of a variety of intriguing physics. For instance, gapless edge modes play an essential role for the quantization of the Hall conductivity [3], topological magnetoelectric effects in three-dimensional topological insulators in bismuth based compounds [4][5][6], realization of Majorana fermions [7] which are useful for quantum computation, etc. In many cases, such topological phases have been regarded as free fermion systems.Recently, realizations of topological phases in d-and f -electron systems have been proposed [8][9][10][11], highlighting the importance of topological aspects of strongly correlated systems. Indeed, this issue has been extensively studied so far [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], and an interaction-induced topological phase is proposed within a Hartree-Fock approximation [24,31]. Furthermore, it has been reported that correlation effects reduce the number of nontrivial phases; two distinct topological phases of free fermion systems can be adiabatically connected if the intermediate states are correlated [2, 3,32,[35][36][37].In spite of these extensive studies, there are still important issues to be solved concerning Mott physics, which leads to a topological spin liquid in the strong interaction region. (i) One of them is how the gapless edge modes, which are a source of exotic and rich physics, are affected by correlation effects. It is proposed that edge states composed only of spinons appear in topological Mott insulators [12]. However, this has been demonstrated only for a particular model, and it is desirable to establish gapless edge spinons in topological Mott insulators from a more general point of view. (ii) Even for bulk systems, there is an important issue to be elucidated, i.e., properties of a topological phase trans...

The synthesis and characterization of thermoresponsive hydrogels on the basis of N‐isopropylacrylamide (IPAAm) copolymers crosslinked with biodegradable poly(amino acids) are described. This hydrogel was prepared with two kinds of reactive IPAAm‐based copolymers containing poly(amino acids) as the side‐chain groups and activated ester groups. We introduced the graft chains by decarboxylation polymerization of amino acid N‐carboxyanhydrides initiated from lateral amino groups in the PIPAAm copolymer. The hydrogels easily crosslinked with degradable poly(amino acid) chains by only mixing the copolymer aqueous solutions. The gelling method in this study would provide some of the following innovative features: (1) no necessary removal of unreacted monomers and so forth, (2) simpler loading of drugs into the hydrogels (only mixing when gelling), and (3) easier insertion into the body. On the basis of the swelling ratio measurement of the hydrogel, large volume changes dependent on temperature changes were observed. Moreover, the enzymatic temperature‐dependent degradation was confirmed. The results suggested that these hydrogels could be used for an injectable or implantable matrix of temperature‐modulated drug release. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 779–787, 2003