## News

# 2016.11.03 Quantum Domain Walls Induce Incommensurate Supersolid Phase on the Anisotropic Triangular Lattice

### Xue-Feng Zhang*, Shijie Hu*, Axel Pelster, and Sebastian Eggert

We investigate the extended hard-core Bose-Hubbard model on the triangular lattice as a function of spatial anisotropy with respect to both hopping and nearest-neighbor interaction strength. At half-filling the system can be tuned from decoupled one-dimensional chains to a two-dimensional solid phase with alternating density order by adjusting the anisotropic coupling. At intermediate anisotropy, however, frustration effects dominate and an incommensurate supersolid phase emerges, which is characterized by incommensurate density order as well as an anisotropic superfluid density. We demonstrate that this intermediate phase results from the proliferation of topological defects in the form of quantum bosonic domain walls. Accordingly, the structure factor has peaks at wave vectors, which are linearly related to the number of domain walls in a finite system in agreement with extensive quantum Monte Carlo simulations. We discuss possible connections with the supersolid behavior in the high-temperature superconducting striped phase.

Phys. Rev. Lett. **117**, 193201 (2016)

# 2015.02.09 The phase diagram of the antiferromagnetic XXZ model on the triangular lattice (Editor's Suggestion)

### Daniel Sellmann, Xue-Feng Zhang*, and Sebastian Eggert

We determine the quantum phase diagram of the antiferromagnetic spin-1/2 XXZ model on the triangular lattice as a function of magnetic field and anisotropic coupling Jz. Using the density matrix renormalization group algorithm in two dimensions, we establish the locations of the phase boundaries between a plateau phase with 1/3 Neel order and two distinct coplanar phases. The two coplanar phases are characterized by a simultaneous breaking of both translational and U(1) symmetries, which is reminiscent of supersolidity. A translationally invariant umbrella phase is entered via a first-order phase transition at relatively small values of Jz compared to the corresponding case of ferromagnetic hopping and the classical model. The phase transition lines meet at two tricritical points on the tip of the lobe of the plateau state, so that the two coplanar states are completely disconnected. Interestingly, the phase transition between the plateau state and the upper coplanar state changes from second order to first order for large values of Jz larger than 2.5J.

Phys. Rev. B **91**, 081104(R) (2015)

# 2014.11.14 Phase diagram of the triangular extended Hubbard model

### Luca F. Tocchio, Claudius Gros, Xue-Feng Zhang, and Sebastian Eggert

We study the extended Hubbard model on the triangular lattice as a function of filling and interaction strength. The complex interplay of kinetic frustration and strong interactions on the triangular lattice leads to exotic phases where long-range charge order, antiferromagnetic order, and metallic conductivity can coexist. Variational Monte Carlo simulations show that three kinds of ordered metallic states are stable as a function of nearest neighbor interaction and filling. The coexistence of conductivity and order is explained by a separation into two functional classes of particles: part of them contributes to the stable order, while the other part forms a partially filled band on the remaining substructure. The relation to charge ordering in charge transfer salts is discussed.

Phys. Rev. Lett. ** 113**, 246405 (2014)

# 2013.09.30 Chiral Edge States and Fractional Charge Separation in a System of Interacting Bosons on a Kagome Lattice

### Xue-Feng Zhang and Sebastian Eggert

We consider the extended hard-core Bose-Hubbard model on a kagome lattice with boundary conditions on two edges. We find that the sharp edges lift the degeneracy and freeze the system into a striped order at 1/3 and 2/3 filling for zero hopping. At small hopping strengths, holes spontaneously appear and separate into fractional charges which move to the edges of the system. This leads to a novel 1 edge liquid phase, which is characterized by fractional charges near the edges and a finite edge compressibility but no superfluid density. The compressibility is due to excitations on the edge which display a chiral symmetry breaking that is reminiscent of the quantum Hall effect and topological insulators. Large scale Monte Carlo simulations confirm the analytical considerations.

Phys. Rev. Lett. **111**, 147201 (2013)

# 2013.02.28 Rydberg Polaritons in a Cavity: A Superradiant Solid (Editor's suggestion)

### Xue-Feng Zhang, Qing Sun, Yu-Chuan Wen, Wu-Ming Liu, Sebastian Eggert, and An-Chun Ji

We study an optical cavity coupled to a lattice of Rydberg atoms, which can be represented by a generalized Dicke model. We show that the competition between the atom-atom interaction and atomlight coupling induces a rich phase diagram. A novel superradiant solid (SRS) phase is found, where both the superradiance and crystalline orders coexist. Different from the normal second order superradiance transition, here both the solid-1=2 and SRS to SR phase transitions are first order. These results are confirmed by large scale quantum Monte Carlo simulations.

Phys. Rev. Lett. **110**, 090402 (2013)

# 2011.11.20 Supersolid phase transitions for hard-core bosons on a triangular lattice

### Xue-Feng Zhang*, Raoul Dillenschneider, Yue Yu, and Sebastian Eggert

Hard-core bosons on a triangular lattice with nearest-neighbor repulsion are a prototypical example of a system with supersolid behavior on a lattice. We show that in this model the physical origin of the supersolid phase can be understood quantitatively and analytically by constructing quasiparticle excitations of defects that are moving on an ordered background. The location of the solid to supersolid phase transition line is predicted from the effective model for both positive and negative (frustrated) hopping parameters. For positive hopping parameters the calculations agree very accurately with numerical quantum Monte Carlo simulations. The numerical results indicate that the supersolid to superfluid transition is first order.

Phys. Rev. B **84**, 174515 (2011)