arXiv subjects

Tiantao Qu

Publications and source records attributed to Tiantao Qu.

6 recordsLinked to original sources

arXiv2026

Fragility of Unidirectional Transport in Weakly Disordered Photonic Chern Insulators

Photonic Chern insulators enable unidirectional light transport protected by nontrivial band topology -- essential for robust photonic integrated circuits and error-free communication. However, disorder from impurities or defects inevitably exists in practical applications, yet how weak disorder affects topological chiral edge states remains insufficiently understood. Here, we reveal a previously unrecognized mechanism by which weak disorder can disrupt robust propagation of chiral edge states in photonic Chern insulators, despite the preservation of global topological invariants. By randomly replacing a small number of magnetized rods with nonmagnetized impurities in a magnetic photonic crystal, we find that when the excitation frequency approaches the single impurity defect state frequency, weak coupling between spatially extended defect states forms a topologically trivial impurity band inside the topological gap. This enables coexistence and coupling of defect states and chiral edge states. The reciprocal "necklace state" transport channels formed by coupled defect states break the expected unidirectional propagation in topological Chern insulators with weak disorder. Our work reveals that topological chiral edge state and disorder interactions are more intricate than previously understood and provides new insights into stability and control of topological transport in realistic applications.

physics.optics↗

arXiv2026

Magnetic doping-induced second-order and first-order topological phase transition inthe photonic alloy

The bulk-edge correspondence principle, a cornerstone of topological physics, ensures that first-order topological systems host robust chiral edge states in two dimension. This was later extended to higher-order phases, where second-order topological insulators exhibit localized, topologically protected corner states. While the transition between these distinct phases has been demonstrated in periodic systems, its existence in disordered platforms remains an open question. Here, we demonstrate a controllable topological phase transition between a second-order topological phase and a first-order topological phase in a two-dimensional photonic alloy. By tuning the magnetic doping concentration - implemented by attaching permanent magnets randomly to nonmagnetized yttrium iron garnet rods in an alternately magnetized honeycomb lattice with C3 rotational symmetry - we flexibly control the system's topology. At zero doping, we observe higher-order corner states, confirmed by a trivial Chern number and non-zero bulk polarizations of 1/3. As doping concentration increases, these corner states progressively merge with the bulk states, culminating in the closure of the bulk transmission gap. After the bulk transmission gap reopens with further increased doping, the system transitions to a first-order topological phase, characterized by a nontrivial Chern number of -1 and the emergence of a chiral edge state. This transition is reversible, providing a highly tunable and experimentally simple platform for flexibly switching between localized corner states and delocalized chiral edge states within a single photonic system.

cond-mat.dis-nn↗

arXiv2025

Disorder-driven exceptional points and concurrent topological phase transitions in non-Hermitian systems

Exceptional points (EPs) are spectral degeneracies unique to non-Hermitian systems which underpin phenomena from enhanced sensing to unconventional topology. While disorder is usually viewed as detrimental, it can also drive topological phase transitions (TPTs). Here, we show that random disorder alone can generate EPs and concurrent TPTs in a multiorbital non-Hermitian lattice with nonreciprocal hopping. Increasing disorder induces successive real-complex-real spectral transitions accompanied by band inversion and quantized changes in the spin Bott index. Using effective medium theory and large-scale simulations, we trace these transitions to a competition between disorder-induced energy-level renormalization and nonreciprocity-driven hybridization. The resulting phase diagram reveals extended EP lines that emerge from the Hermitian TPT point and persist over a broad parameter range. Our results establish disorder as an active mechanism for engineering exceptional point mediated topology in non-Hermitian matter.

cond-mat.dis-nn↗

arXiv2024

Band alignment effect in the topological photonic alloy

Recently, a photonic alloy with non-trivial topological properties has been proposed, based on the random mixing of Yttrium Iron Garnet (YIG) and magnetized YIG rods. When the doping concentration of magnetized YIG rods is less than one, a chiral edge state (CES) of the topological photonic alloy appears in the frequency range of the non-trivial topological gap of the magnetized YIG crystal. In this work, we surprisingly find that by randomly mixing the Perfect Electric Conductor (PEC) and magnetized YIG rods in a square lattice, the photonic alloy system with appropriate doping concentrations can present CES in special frequency intervals even when both components support the propagation of bulk states. Analyzing the band structure of two components, we noticed a shift between the first trivial bandgap for PEC and the first topological bandgap for magnetized YIG. When calculating the transmission spectrum of the photonic alloy, we discovered that the frequency range for the topological gap gradually opens from the lower limit frequency of the bandgap for PEC to the bandgap for the magnetized YIG rods. The topological gap opening occurs as the doping concentration of magnetized YIG rods increases, creating an effective band alignment effect. Moreover, the topological gap for the photonic alloy is confirmed by calculating the reflection phase winding with the scattering method. Lastly, the gradual appearance of the CES is identified by applying Fourier transformation to real-space electromagnetic fields. Our work broadens the possibilities for flexible topological gap engineering in the photonic alloy system.

physics.optics↗

arXiv2024

Topological photonic alloy

We present the new concept of photonic alloy as a non-periodic topological material. By mixing non-magnetized and magnetized rods in a non-periodic 2D photonic crystal configuration, we realized photonic alloys in the microwave regime. Our experimental findings reveal that the photonic alloy sustains non-reciprocal chiral edge states (CESs) even at very low concentration of magnetized rods. The non-trivial topology and the associated edge states of these non-periodic systems can be characterized by the winding of the reflection phase. Our results indicate that the threshold concentrations for the investigated system within the first non-trivial band gap to exhibit topological behavior approach zero in the thermodynamic limit for substitutional alloys, while the threshold remains non-zero for interstitial alloys. At low concentration, the system exhibits an inhomogeneous structure characterized by isolated patches of non-percolating magnetic domains that are spaced far apart within a topologically trivial photonic crystal. Surprisingly, the system manifests CESs despite a local breakdown of time-reversal symmetry rather than a global one. Photonic alloys represent a new category of disordered topological materials, offering exciting opportunities for exploring topological materials with adjustable gaps.

cond-mat.mes-hall↗

arXiv2023

Topological Anderson amorphous insulator

The topological phase in amorphous systems adds a new dimension to the topological states of matter. Here, we present an interesting phenomenon dubbed the topological Anderson amorphous insulator (TAAI). Anderson disorder can drive topologically trivial amorphous systems with structural disorders into noncrystalline topological insulators. The gap closing and reopening, spin Bott index, robust edge states, and quantized conductance characterize the Anderson disorder-induced nontrivial topology in amorphous systems. More importantly, phase diagrams are given for the topological phase transition (TPT). It is found that amorphous structural disorder and Anderson disorder are synergistic to drive the s-p band inversion of the system and hence the TPT, which is further confirmed by the effective medium theory. Our findings report a disorder-induced topological phenomenon in noncrystalline systems and shed light on the physical understanding of the interplay between the coexistence of two types of disorder effects and topology.

cond-mat.mes-hall↗