arXiv subjects

Qing-Dong Jiang

Publications and source records attributed to Qing-Dong Jiang.

At least 37 records · Page 2Linked to original sources

arXiv2024

Braids and Higher-order Exceptional Points from the Interplay Between Lossy Defects and Topological Boundary States

We show that the perturbation of the Su-Schrieffer-Heeger chain by a localized lossy defect leads to higher-order exceptional points (HOEP). Depending on the location of the defect, third- and fourth- order exceptional points (EP3 \& EP4) appear in the space of Hamiltonian parameters. On the one hand, they arise due to the non-Abelian braiding properties of exceptional lines (EL) in parameter space. Namely, the HOEPs lie at intersections of mutually non-commuting ELs. On the other hand, we show that such special intersections happen due to the fact that the delocalization of edge states, induced by the non-Hermitian defect, hybridizes them with defect states. These can then coalesce together into an EP3. When the defect lies at the midpoint of the chain, a special symmetry of the full spectrum can lead to an EP4. In this way, our model illustrates the emergence of interesting non-Abelian topological properties in the multiband structure of non-Hermitian perturbations of topological phases.

cond-mat.mes-hall↗

arXiv2024

Geometry fluctuations in chiral superfluids

The coupling of chiral superfluids and superconductors to the background geometry leads to surprising geometric induction phenomena. We show that this coupling bears important consequences even in a nearly flat background, through its signature in thermal fluctuations. Starting from the Ginzburg-Landau free energy of a chiral superfluid minimally coupled to the background geometry, we show that the interaction strength between vortices gets renormalized by geometry fluctuations. In our setup, these arise from the shape fluctuations of the underlying two-dimensional substrate, and are controlled by its bending rigidity and tension. In the tensionless limit, the fluctuations lower the vortex interaction strength at large distances, which leads to a lowering of the BKT transition temperature. We study this effect in terms of the renormalization group flow of a dual sine-Gordon theory of the superfluid transition coupled to the substrate shape. It shows that, in turn, the chiral superfluid order can suppress the amplitude of shape fluctuations, resulting in an extended phase diagram which links the superfluid transition to the crumpling transition of the substrate. These reveal thermodynamic signatures of chirality in the superfluid transition.

cond-mat.supr-con↗

arXiv2024

Shaping the topology of twisted bilayer graphene via time-reversal symmetry breaking

Symmetry breaking is an effective tool for tuning the transport and topological properties of 2D layered materials. Among these materials, twisted bilayer graphene (TBG) has emerged as a promising platform for new physics, characterized by a rich interplay between topological features and strongly correlated electronic behavior. In this study, we utilize time-reversal symmetry breaking (TRSB) to manipulate the topological properties of TBG. By varying the strength of TRSB, we discover a topological phase transition between a topological insulating phase, which exhibits a pair of flat bands with opposite Chern numbers, and a novel insulating state where the Chern number, but not the Berry curvature, of the flat bands vanishes. We demonstrate that this topological transition is mediated by a gap closing at the $Γ$ point, and we construct a three-dimensional phase diagram as a function of the twisting angle, the symmetry-breaking parameter, and the mismatch coupling between AA and AB stacking regions. Finally, we show that this novel electronic phase can be identified in the lab by measuring, as a function of the Fermi energy, its non-quantized anomalous Hall conductivity that is induced by the Berry dipole density of the lowest flat bands.

cond-mat.mes-hall↗

arXiv2024

Engineering flat bands in twisted-bilayer graphene away from the magic angle with chiral optical cavities

Twisted bilayer graphene (TBG) is a recently discovered two-dimensional superlattice structure which exhibits strongly-correlated quantum many-body physics, including strange metallic behavior and unconventional superconductivity. Most of TBG exotic properties are connected to the emergence of a pair of isolated and topological flat electronic bands at the so-called magic angle, $θ\approx 1.05^{\circ}$, which are nevertheless very fragile. In this work, we show that, by employing chiral optical cavities, the topological flat bands can be stabilized away from the magic angle in an interval of approximately $0.8^{\circ}<θ<1.3^{\circ}$. As highlighted by a simplified theoretical model, time reversal symmetry breaking (TRSB), induced by the chiral nature of the cavity, plays a fundamental role in flattening the isolated bands and gapping out the rest of the spectrum. Additionally, TRSB suppresses the Berry curvature and induces a topological phase transition, with a gap closing at the $Γ$ point, towards a band structure with two isolated flat bands with Chern number equal to $0$. The efficiency of the cavity is discussed as a function of the twisting angle, the light-matter coupling and the optical cavity characteristic frequency. Our results demonstrate the possibility of engineering flat bands in TBG using optical devices, extending the onset of strongly-correlated topological electronic phases in moiré superlattices to a wider range in the twisting angle.

cond-mat.mes-hall↗

arXiv2024

Emergent Haldane Model and Photon-Valley Locking in Chiral Cavities

The realization of Haldane's topological graphene model in practical materials has presented significant challenges. Here, we propose achieving this model by embedding graphene in chiral cavities, using the asymptotically decoupled framework detailed in Ref. [Phys. Rev. Lett. 126, 153603 (2021)]. Additionally, we introduce an equilibrium strategy for achieving valley polarization in this system with C2-symmetry breaking. Through numerical methods, we quantify the locking of photon numbers with Bloch electrons and calculate the topology-induced imbalance of valley photons. Furthermore, we elucidate that topological phase transition is characterized by the sign change of photon numbers during interband excitation. These findings underscore the remarkable potential of utilizing cavity quantum fluctuations to engineer electronic and photonic properties specific to valleys and topologies, particularly within the realm of strong light-matter coupling.

cond-mat.mes-hall↗

arXiv2024

Geometric phase driven Josephson junction: Possible experimental scheme for the search of spin superfluidity

We use the Gross-Pitaevskii equation to study Josephson tunneling between two weakly coupled Bose-Einstein condensates, which compose spin-1 bosons. We show that a rotating magnetic field on one side can produce a phase difference across the junction, resulting in an oscillatory tunneling spin current. Besides numerical calculation, we derive analytical results in two extreme cases, namely the low- and high-frequency limits: in the low-frequency limit (magnetic field rotates adiabatically), a non-Abelian geometric phase arises and leads to the oscillatory spin current. By sharp contrast, the physics is intrinsically different in the high-frequency limit, where an average Zeeman energy difference leads to an oscillatory spin current. This proposed apparatus should be promising for the future experimental search of spin superfluidity.

cond-mat.quant-gas↗

arXiv2024

Geometric inhibition of superflow in single-layer graphene suggests a staggered-flux superconductivity in bilayer and trilayer graphene

In great contrast to the numerous discoveries of superconductivity in layer-stacked graphene systems, the absence of superconductivity in the simplest and cleanest monolayer graphene remains a big puzzle. Here, through realistic computation of electronic structure, we identify a systematic trend that superconductivity appears to emerge only upon alteration of the low-energy electronic lattice from the underlying honeycomb atomic structure. We then demonstrate that this inhibition can result from from geometric frustration of the bond lattice that disables quantum phase coherence of the order parameter residing on it. In comparison, upon deviating from the honeycomb lattice, relief of geometric frustration allows robust superfluidity with non-trivial spatial structure. For the specific examples of bilayer and trilayer graphene under an external electric field, such bond centered order parameter would develop superfluidity with staggered flux that breaks the time-reversal symmetry. Our study also suggests the possible realization of the long-sought superconductivity in single-layer graphene via the application of uni-directional strain.

cond-mat.supr-con↗

arXiv2023

Vacuum-Induced Symmetry Breaking of Chiral Enantiomer Formation in Chemical Reactions

A material with symmetry breaking inside can transmit the symmetry breaking to its vicinity by vacuum electromagnetic fluctuations. Here, we show that vacuum quantum fluctuations proximate to a parity-symmetry-broken material can induce a chirality-dependent spectral shift of chiral molecules, resulting in a chemical reaction process that favors producing one chirality over the other. We calculate concrete examples and evaluate the chirality production rate with experimentally realizable parameters, showing the promise of selecting chirality with symmetry-broken vacuum quantum fluctuations.

quant-ph↗

arXiv2022

Geometric Induction in Chiral Superfluids

We explore the properties of chiral superfluid thin films coating a curved surface. Due to the vector nature of the order parameter, a geometric gauge field emerges and leads to a number of observable effects such as anomalous vortex-geometric interaction and curvature-induced mass/spin supercurrents. We apply our theory to several well-known phases of chiral superfluid $\rm ^3 He$ and derive experimentally observable signatures. We further discuss the cases of flexible geometries where a soft surface can adapt itself to compensate for the strain from the chiral superfluid. The proposed interplay between geometry and chiral superfluid order provides a fascinating avenue to control and manipulate quantum states with strain.

cond-mat.supr-con↗

arXiv2022

Liouvillian Skin Effect in an Exactly Solvable Model

The interplay between dissipation, topology and sensitivity to boundary conditions has recently attracted tremendous amounts of attention at the level of effective non-Hermitian descriptions. Here we exactly solve a quantum mechanical Lindblad master equation describing a dissipative topological Su-Schrieffer-Heeger (SSH) chain of fermions for both open boundary condition (OBC) and periodic boundary condition (PBC). We find that the extreme sensitivity on the boundary conditions associated with the non-Hermitian skin effect is directly reflected in the rapidities governing the time evolution of the density matrix giving rise to a Liouvillian skin effect. This leads to several intriguing phenomena including boundary sensitive damping behavior, steady state currents in finite periodic systems, and diverging relaxation times in the limit of large systems. We illuminate how the role of topology in these systems differs in the effective non-Hermitian Hamiltonian limit and the full master equation framework.

quant-ph↗

arXiv2021

On the Sign of Fermion-Mediated Interactions

We develop a unified framework for understanding the sign of fermion-mediated interactions by exploiting the symmetry classification of Green's functions. In particular, we establish a theorem regarding the sign of fermion-mediated interactions in systems with chiral symmetry. The strength of the theorem is demonstrated within multiple examples with an emphasis on electron-mediated interactions in materials.

cond-mat.mes-hall↗

arXiv2020

Geometric Induction in Chiral Superconductors

We consider a number of effects due to the interplay of superconductivity, electromagnetism and elasticity, which are unique for thin membranes of layered chiral superconductors. Some of them should be within the reach of present technology, and could be useful for characterizing materials. More speculatively, the enriched control of Josephson junctions they afford might find useful applications.

cond-mat.supr-con↗

arXiv2020

Experimental sensing quantum atmosphere of a single spin

Understanding symmetry-breaking states of materials is a major challenge in the modern physical sciences. Quantum atmosphere proposed recently sheds light on the hidden world of these symmetry broken patterns. But the requirements for exquisite sensitivity to the small shift and tremendous spatial resolution to local information pose huge obstacles to its experimental manifestation. In our experiment, we prepare time-reversal-symmetry conserved and broken quantum atmosphere of a single nuclear spin and successfully observe their symmetry properties. Our work proves in principle that finding symmetry patterns from quantum atmosphere is conceptually viable. It also opens up entirely new possibilities in the potential application of quantum sensing in material diagnosis.

quant-ph↗

arXiv2019

Quantum Atmospherics for Materials Diagnosis

Symmetry breaking states of matter can transmit symmetry breaking to nearby atoms or molecular complexes, perturbing their spectra. We calculate one such effect, involving the `axion electrodynamics' relevant to topological insulators, quantitatively, and identify a signature for T violating superconductivity. We provide an operator framework whereby effects of this kind can be analyzed systematically.

cond-mat.mes-hall↗

arXiv2019

Axial Casimir Force

Quantum fluctuations in vacuum can exert a dissipative force on moving objects, which is known as Casimir friction. Especially, a rotating particle in the vacuum will eventually slow down due to the dissipative Casimir friction. Here, we identify a dissipationless force by examining a rotating particle near a bi-isotropic media that generally breaks parity symmetry or/and time-reversal symmetry. The direction of the dissipationless vacuum force is always parallel with the rotating axis of the particle. We therefore call this dissipationless vacuum force the axial Casimir force.

cond-mat.other↗

arXiv2018

Chiral Casimir Forces: Repulsive, Enhanced, Tunable

Both theoretical interest and practical significance attach to the sign and strength of Casimir forces. A famous, discouraging no-go theorem states that "The Casimir force between two bodies with reflection symmetry is always attractive." Here we identify a loophole in the reasoning, and propose a universal way to realize repulsive Casimir forces. We show that the sign and strength of Casimir forces can be adjusted by inserting optically active or gyrotropic media between bodies, and modulated by external fields.

cond-mat.mes-hall↗

arXiv2016

Chiral Wave-packet Scattering in Weyl Semimetals

In quantum mechanics, a particle is best described by the wave packet instead of the plane wave. Here, we study the wave-packet scattering problem in Weyl semimetals with the low-energy Weyl fermions of different chiralities. Our results show that the wave packet acquires a chirality-protected shift in the single impurity scattering process. More importantly, the chirality-protected shift can lead to an anomalous scattering probability, thus, affects the transport properties in Weyl semimetals. We find that the ratio between the transport lifetime and the quantum lifetime increases sharply when the Fermi energy approaches to the Weyl nodes, providing an explanation on the experimentally observed the ultrahigh mobility in topological (Weyl or Dirac) semimetals.

cond-mat.mes-hall↗

arXiv2015

Tantalum Monoarsenide: an Exotic Compensated Semimetal

Compared with the semiconductors such as silicon and gallium arsenide which have been used widely for decades, semimetals have not received much attention in the field of condensed matter physics until very recently. The realization of electronic topological properties has motivated interest of investigations on Dirac semimetals and Weyl semimetals, which are predicted to show unprecedented features beyond the classical electronic theories of metals. In this letter for the first time we report the electric transport properties of a robust Weyl semimetal candidate proposed by recent theoretical calculations, TaAs. Our study shows that this bulk material manifests ultrahigh carrier mobility ($\mathrm{5\times10^5 cm^2/V\cdot{s}}$) accompanied by an extremely large, unsaturated linear magnetoresistance ($\mathrm{MR}$), which reaches 5400 at 10 Kelvins in a magnetic field of 9 Teslas and 2.47$\times$10$^4$ at 1.5 Kelvins in a magnetic field of 56 Teslas. We also observed strong Shubnikov-de Haas (SdH) oscillations associated with an extremely low quantum limit ($\sim$8 Teslas). Further studies on TaAs, especially in the ultraquantum limit regime, will help to extend the realization of the topological properties of these exotic electrons.

cond-mat.mtrl-sci↗