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Topological quantum materials include:
Two-dimensional materials and their moire heterostructures
Quantum spin liquid
Topological materials
Unconventional superconductivity
Topological spin texture, like skyrmions
Heavy fermion material
Fig. 1, schematic view of terahertz electromagnetic waves generation using high-Tc cuprate superconductors
Fig. 2, spontaneous formation of π phase kink under current drive.
Mechanism of terahertz generation from high-Tc cuprate superconductors under current drive
Mechanism of terahertz generation from high-Tc cuprate superconductors under current drive
Based on computer simulations and theoretical analysis, a new dynamic state is found in inductively coupled intrinsic Josephson junctions in the absence of an external magnetic field. In this state, the plasma oscillation is uniform along the c axis with the fundamental frequency satisfying the ac Josephson relation. There are (2m+1)π phase kinks around the junction center, with m being an integer, periodic and thus nonuniform in the c direction. In the IV characteristics, the state manifests itself as current steps occurring at all cavity modes. Inside the current steps, the plasma oscillation becomes strong, which generates several harmonics in frequency spectra at a given voltage. The recent experiments on terahertz radiations from the mesa of a Bi2Sr2CaCu2O8+δ single crystal can be explained in terms of this state.
Massless Leggett Mode in Three-Band Superconductors with Time-Reversal-Symmetry Breaking
Massless Leggett Mode in Three-Band Superconductors with Time-Reversal-Symmetry Breaking
The Leggett mode associated with out-of-phase oscillations of the superconducting phase in multiband superconductors usually is heavy due to interband coupling, which makes its excitation and detection difficult. We report on the existence of a massless Leggett mode in three-band superconductors with time- reversal-symmetry breaking. The mass of this Leggett mode is small close to the time-reversal-symmetry- breaking transition and vanishes at the transition point, and thus locates within the smallest super- conducting energy gap, which makes it stable and detectable, e.g., by means of the Raman spectroscopy. The thermodynamic consequences of this massless mode and possible realization in iron-based super- conductors are also discussed.
Frustrated interband scatterings force Cooper pairs in different bands to carry different phases, which results in interband Josephson currents. Two dynamical modes associated with superconducting phases in three-band superconductors: the Leggett mode, where two phases oscillate out-of-phase while the third one stays unchanged, becomes massless at the time-reversal symmetry breaking transition (left), and the Bogoliubov-Anderson-Goldstone mode, where all the three phases rotate in the same direction during the propagation of plasma wave in space (right).
Face Centered Cubic and Hexagonal Close Packed Skyrmion Crystals in Centrosymmetric Magnets
Face Centered Cubic and Hexagonal Close Packed Skyrmion Crystals in Centrosymmetric Magnets
Skyrmions are disklike objects that typically form triangular crystals in two-dimensional systems. This situation is analogous to the so-called pancake vortices of quasi-two-dimensional superconductors. The way in which Skyrmion disks or “pancake Skyrmions” pile up in layered centrosymmetric materials is dictated by the interlayer exchange. Unbiased Monte Carlo simulations and simple stabilization arguments reveal face centered cubic and hexagonal close packed Skyrmion crystals for different choices of the interlayer exchange, in addition to the conventional triangular crystal of Skyrmion lines. Moreover, an inhomogeneous current induces a sliding motion of pancake Skyrmions, indicating that they behave as effective mesoscale particles.
S. -Z. Lin and C. D. Batista, Phys. Rev. Lett. 120, 077202 (2018)
Topological defects as relics of emergent continuous symmetry and Higgs condensation of disorder in ferroelectrics
Topological defects as relics of emergent continuous symmetry and Higgs condensation of disorder in ferroelectrics
Lars Onsager and Richard Feynman envisaged that the three-dimensional (3D) superfluid-to-normal λ transition in 4He occurs through the proliferation of vortices. This process should hold for every phase transition in the same universality class. The role of topological defects in symmetry-breaking phase transitions has become a prime topic in cosmology and high-temperature superconductivity, even though direct imaging of these defects is challenging. Here we show that the U(1) continuous symmetry that emerges at the ferroelectric critical point of multiferroic hexagonal manganites leads to a similar proliferation of vortices. Moreover, the disorder field (vortices) is coupled to an emergent U(1) gauge field, which becomes massive by means of the Higgs mechanism when vortices condense (span the whole system) on heating above the ferroelectric transition temperature. Direct imaging of the vortex network in hexagonal manganites o ers unique experimental access to this dual description of the ferroelectric transition, while enabling tests of the Kibble–Zurek mechanism.
Dual description of a phase transition with Z2 × Z3 symmetry. The phase transition can be described in terms of the order field φ (a) or the disorder field ψ (b). The local order parameter φ takes six values, represented by the even hours in the clock dials in a. They correspond to the six multiferroic states or domains α+ through γ − distinguished by the polarization direction (+ or −) and the trimerization phase (α, β, γ ), as described in the text. The multiferroic Z2 × Z3 vortices are line defects where the six domains meet with each other, as shown in c. Continuous U(1) symmetry emerges from Z2 × Z3 order parameter at the critical temperature. The disordered phase above Tc can be described as a condensation of the disorder field ψ signalled by the proliferation of vortex lines spanning the whole system (yellow lines). Only quickly fluctuating closed vortex loops (red lines) are present for T < Tc .
Topology and Localization in the Kondo Lattice Model
Topology and Localization in the Kondo Lattice Model
The Kondo lattice model describing the coupling between conduction electrons and localized magnetic moments is relevant for a large family of physical systems. Here we reveal that the one dimensional Kondo lattice model with an incommensurate magnetic elliptical spiral is a topological insulator with a Chern number 2ℤ in the two dimensional space with one physical dimension and one ancillary dimension spanned by the Goldstone mode of the spiral. Moreover, the electronic states can be localized for a strong local exchange coupling. The topological protected edge states are responsible for the pumping of electron charge, and give rise to multiferroic response. Our work uncovers a hitherto undiscovered nontrivial topology and Anderson localization in the Kondo lattice model.
S. Ying and S. -Z. Lin, arXiv:1809.06295
Spectrum of the 1D Kondo lattice model with extended bulk states (left) and localized bulk states (right). There are topological protected edges states inside the gaps.
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