@article { WOS:000655871200003,
title = {Classical model for sub-Planckian thermal diffusivity in complex crystals},
journal = {Phys. Rev. B},
volume = {103},
number = {18},
year = {2021},
month = {MAY 10},
publisher = {AMER PHYSICAL SOC},
type = {Article},
abstract = {Measurements of thermal diffusivity in several insulators have been shown to reach a Planckian bound on thermal transport that can be thought of as the limit of validity of semiclassical phonon scattering. Beyond this regime, the heat transport must be understood in terms of incoherent motion of the atoms under strongly anharmonic interactions. In this work, we propose a model for heat transport in a strongly anharmonic system where the thermal diffusivity can be lower than the Planckian thermal diffusivity bound. Similar to the materials that exhibit thermal diffusivity close to this bound, our scenario involves complex unit cells with incoherent intra-cell dynamics. We derive a general formalism to compute thermal conductivity in such cases with anharmonic intra-cell dynamics coupled to nearly harmonic inter-cell coupling. Through direct numerical simulation of the nonlinear unit-cell motion, we explicitly show that our model allows sub-Planckian thermal diffusivity. We find that the propagator of the acoustic phonons becomes incoherent throughout most of the Brillouin zone in this limit. We expect these features to apply to more realistic models of complex insulators showing sub-Planckian thermal diffusivity, suggesting a multispecies generalization of the thermal diffusivity bound that is similar to the viscosity bound in fluids.},
issn = {2469-9950},
doi = {10.1103/PhysRevB.103.184305},
author = {Wu, Huan-Kuang and Sau, Jay D.}
}
@article { WOS:000655905200002,
title = {Generating function for tensor network diagrammatic summation},
journal = {Phys. Rev. B},
volume = {103},
number = {20},
year = {2021},
month = {MAY 28},
publisher = {AMER PHYSICAL SOC},
type = {Article},
abstract = {The understanding of complex quantum many-body systems has been vastly boosted by tensor network (TN) methods. Among others, excitation spectrum and long-range interacting systems can be studied using TNs, where one however confronts the intricate summation over an extensive number of tensor diagrams. Here, we introduce a set of generating functions, which encode the diagrammatic summations as leading-order series expansion coefficients. Combined with automatic differentiation, the generating function allows us to solve the problem of TN diagrammatic summation. We illustrate this scheme by computing variational excited states and the dynamical structure factor of a quantum spin chain, and further investigating entanglement properties of excited states. Extensions to infinite-size systems and higher dimension are outlined.},
issn = {2469-9950},
doi = {10.1103/PhysRevB.103.205155},
author = {Tu, Wei-Lin and Wu, Huan-Kuang and Schuch, Norbert and Kawashima, Naoki and Chen, Ji-Yao}
}
@article {wu_competing_2020,
title = {Competing quantum phases of hard-core bosons with tilted dipole-dipole interaction},
journal = {Phys. Rev. A},
volume = {102},
number = {5},
year = {2020},
note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article},
month = {nov},
abstract = {Different quantum phases of a hard-core boson induced by dipole-dipole interaction with varying angles of polarization are discussed in this work. We consider the two most influential leading terms with anisotropy due to the tilted polarization of the on-site boson in the square lattice. To ensure the concreteness of this truncation, we compare our phase diagrams, obtained numerically from the cluster mean-field theory (CMFT) and infinite projected entangled-pair state (iPEPS), with that of the long-range interacting model from quantum Monte Carlo. Next, we focus on the case where the azimuthal angle is fixed to phi = pi/4. Using the mean-field analysis where the quantum spin operators are replaced by c numbers, we aim to search for the underlying phases, especially the supersolid. Our results show a competing scenario mainly between two ordered phases with different sizes of unit cell, where a first-order transition takes place in between them. With the help of the CMFT and variational iPEPS, the phase boundaries predicted by the mean-field theory are determined more precisely. Our discoveries elucidate the possible underlying supersolid phases which might be seen in the ultracold experiments with strongly dipolar atoms. Moreover, our results indicate that an effective triangular optical lattice can be realized by fine tuning the polarization of dipoles in a square lattice.},
issn = {2469-9926},
doi = {10.1103/PhysRevA.102.053306},
author = {Wu, Huan-Kuang and Tu, Wei-Lin}
}
@article { ISI:000562277400001,
title = {Frustration-induced supersolid phases of extended Bose-Hubbard model in the hard-core limit},
journal = {J. Phys.-Condes. Matter},
volume = {32},
number = {45},
year = {2020},
month = {OCT 28},
pages = {455401},
publisher = {IOP PUBLISHING LTD},
type = {Article},
abstract = {We investigate exotic supersolid phases in the extended Bose-Hubbard model with infinite projected entangled-pair state, numerical exact diagonalization, and mean-field theory. We demonstrate that many different supersolid phases can be generated by changing signs of hopping terms, and the interactions along with the frustration of hopping terms are important to stabilize those supersolid states. We argue the effect of frustration introduced by the competition of hopping terms in the supersolid phases from the mean-field point of view. This helps to give a clearer picture of the background mechanism for underlying superfluid/supersolid states to be formed. With this knowledge, we predict and realize thed-wave superfluid, which shares the same pairing symmetry with high-T(c)materials, and its extended phases. We believe that our results contribute to preliminary understanding for desired target phases in the real-world experimental systems.},
keywords = {Bose-Hubbard model, exact diagonalization, phase diagrams, projected entangled pair states, superfluids, supersolids},
issn = {0953-8984},
doi = {10.1088/1361-648X/aba383},
author = {Tu, Wei-Lin and Wu, Huan-Kuang and Suzuki, Takafumi}
}
@article {ISI:000473009200001,
title = {Theory of coherent phase modes in insulating Josephson junction chains},
journal = {Phys. Rev. B},
volume = {99},
number = {21},
year = {2019},
month = {JUN 21},
pages = {214509},
publisher = {AMER PHYSICAL SOC},
type = {Article},
abstract = {Recent microwave reflection measurements of Josephson junction chains have suggested the presence of nearly coherent collective charge oscillations deep in the insulating phase. Here we develop a qualitative understanding of such coherent charge modes by studying the local dynamical polarizability of the insulating phase of a finite length sine-Gordon model. By considering parameters near the noninteracting fermion limit where the charge operator dominantly couples to soliton-antisoliton pairs of the sine-Gordon model, we find that the local dynamical polarizability shows an array of sharp peaks in frequency representing coherent phase oscillations on top of an incoherent background. The strength of the coherent peaks relative to the incoherent background increases as a power law in frequency as well as exponentially as the Luttinger parameter approaches a critical value. The dynamical polarizability also clearly shows the insulating gap. We then compare the results in the high frequency limit to a perturbative estimate of phase-slip-induced decay of plasmons in the Josephson junction chain.},
issn = {2469-9950},
doi = {10.1103/PhysRevB.99.214509},
author = {Wu, Huan-Kuang and Sau, Jay D.}
}