Hong-Chen Jiang

Numerical evidence of fluctuating stripes in the normal state of high-Tc cuprate superconductors

Numerics converging on stripes The Hubbard model (HM) describes the behavior of interacting particles on a lattice where the particles can hop from one lattice site to the next. Although it appears simple, solving the HM when the interactions are repulsive, the particles are fermions, and the temperature is low—all of which applies in the case of correlated electron systems—is computationally challenging. Two groups have tackled this important problem. Huang et al. studied a three-band version of the HM at finite temperature, whereas Zheng et al. used five complementary numerical methods that kept each other in check to discern the ground state of the HM. Both groups found evidence for stripes, or one-dimensional charge and/or spin density modulations. Science, this issue p. 1161, p. 1155 Multiple numerical methods are used to study the ground-state and finite-temperature solutions of the Hubbard model. Upon doping, Mott insulators often exhibit symmetry breaking where charge carriers and their spins organize into patterns known as stripes. For high–transition temperature cuprate superconductors, stripes are widely suspected to exist in a fluctuating form. We used numerically exact determinant quantum Monte Carlo calculations to demonstrate dynamical stripe correlations in the three-band Hubbard model, which represents the local electronic structure of the copper-oxygen plane. Our results, which are robust to varying parameters, cluster size, and boundary conditions, support the interpretation of experimental observations such as the hourglass magnetic dispersion and the Yamada plot of incommensurability versus doping in terms of the physics of fluctuating stripes. These findings provide a different perspective on the intertwined orders emerging from the cuprates’ normal state.

Dynamical time-reversal symmetry breaking and photo-induced chiral spin liquids in frustrated Mott insulators

The search for quantum spin liquids in frustrated quantum magnets recently has enjoyed a surge of interest, with various candidate materials under intense scrutiny. However, an experimental confirmation of a gapped topological spin liquid remains an open question. Here, we show that circularly polarized light can provide a knob to drive frustrated Mott insulators into a chiral spin liquid, realizing an elusive quantum spin liquid with topological order. We find that the dynamics of a driven Kagome Mott insulator is well-captured by an effective Floquet spin model, with heating strongly suppressed, inducing a scalar spin chirality Si · (Sj × Sk) term which dynamically breaks time-reversal while preserving SU(2) spin symmetry. We fingerprint the transient phase diagram and find a stable photo-induced chiral spin liquid near the equilibrium state. The results presented suggest employing dynamical symmetry breaking to engineer quantum spin liquids and access elusive phase transitions that are not readily accessible in equilibrium.Exotic quantum phases like spin liquids have long been investigated theoretically but it is difficult to find materials that realize these states in equilibrium. Here the authors propose that optical driving could be used to induce chiral spin liquid behaviour in frustrated Mott insulators.

Stripe order from the perspective of the Hubbard model

A microscopic understanding of the strongly correlated physics of the cuprates must account for the translational and rotational symmetry breaking that is present across all cuprate families, commonly in the form of stripes. Here we investigate emergence of stripes in the Hubbard model, a minimal model believed to be relevant to the cuprate superconductors, using determinant quantum Monte Carlo (DQMC) simulations at finite temperatures and density matrix renormalization group (DMRG) ground state calculations. By varying temperature, doping, and model parameters, we characterize the extent of stripes throughout the phase diagram of the Hubbard model. Our results show that including the often neglected next-nearest-neighbor hopping leads to the absence of spin incommensurability upon electron-doping and nearly half-filled stripes upon hole-doping. The similarities of these findings to experimental results on both electron and hole-doped cuprate families support a unified description across a large portion of the cuprate phase diagram.Strongly correlated electrons: spin stripes emerge in the Hubbard modelThe phase diagram of the Hubbard model is studied numerically by varying parameters and suggests that spin stripe order can be observable at accessible temperatures. A team led by Thomas P. Devereaux from Stanford University and colleagues from SLAC National Accelerator Laboratory and University of North Dakota investigate emergence of spin stripe orders in the Hubbard model by tuning various parameters in the determinant quantum Monte Carlo simulations and the density matrix renormalization group calculations. They show that including the next-nearest-neighbor hopping term, which was often neglected in previous studies, in the Hubbard model leads to nearly half-filled spin stripes upon hole-doping, while no stripes upon electron-doping. The consistence of these findings with experimental results on both electron and hole-doped cuprate superconductors supports a unified description across a large portion of the cuprate phase diagram.

Holon Wigner Crystal in a Lightly Doped Kagome Quantum Spin Liquid

We address the problem of a lightly doped spin liquid through a large-scale density-matrix renormalization group study of the t-J model on a kagome lattice with a small but nonzero concentration δ of doped holes. It is now widely accepted that the undoped (δ=0) spin-1/2 Heisenberg antiferromagnet has a spin-liquid ground state. Theoretical arguments have been presented that light doping of such a spin liquid could give rise to a high temperature superconductor or an exotic topological Fermi liquid metal. Instead, we infer that the doped holes form an insulating charge-density wave state with one doped hole per unit cell, i.e., a Wigner crystal. Spin correlations remain short ranged, as in the spin-liquid parent state, from which we infer that the state is a crystal of spinless holons, rather than of holes. Our results may be relevant to kagome lattice herbertsmithite upon doping.

Nature of a single doped hole in two-leg Hubbard and t-J ladders

In this paper, we have systematically studied the single hole problem in two-leg Hubbard and

Superconductivity in the Hubbard model and its interplay with charge stripes and next-nearest hopping t'

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Absence of Superconductivity in Doping Kagome Quantum Spin Liquid

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Dynamical Time-Reversal Symmetry Breaking and Photo-Induced Chiral Spin Liquid in Frustrated Mott Insulators

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Fluctuating spin stripes in the normal state of high-Tc cuprate superconductors

ladders by large-scale density-matrix renormalization group calculations. We found that the doped hole in both models behaves similarly with each other while the three-site correlated hopping term is not important in determining the ground state properties. For more insights, we have also calculated the elementary excitations, i.e., the energy gaps to the excited states of the system. In the strong rung limit, we found that the doped hole behaves as a Bloch quasiparticle in both systems where the spin and charge of the doped hole are tightly bound together. In the isotropic limit, while the hole still behaves like a quasiparticle in the long-wavelength limit, its spin and charge components are only loosely bound together with a nontrivial mutual statistics inside the quasiparticle. Our results show that this mutual statistics can lead to an important residual effect which dramatically changes the local structure of the ground state wavefunction.