arXiv subjects

Guodong Jiang

Publications and source records attributed to Guodong Jiang.

10 recordsLinked to original sources

arXiv2026

Magnetic Field Walls in Flat-band Superconductors

Superconductors of different types have distinct magnetic properties; for example, they can form Abrikosov vortices or alternating normal-superconducting domains. We predict that, in flat bands, a superconducting phase exhibiting walls of magnetic flux is stable in an applied magnetic field. This phase relies on the lack of a single particle energy penalty for forming condensates of any momentum in flat bands and, consequently, their superconducting free energy being a negative and periodic function of the vector potential at low temperatures. Using a minimal lattice-periodic model of free energy, we study two types of soliton modes of the wall phase: the kink and breather solitons. They determine the lower critical field and the high-field behavior of the wall phase, respectively. The competition between the walls and vortices in flat bands is also discussed. Our results suggest that flat bands help sustain superconductivity in the presence of large magnetic fields.

cond-mat.supr-con↗

arXiv2025

Ideal quantum geometry of the surface states of rhombohedral graphite and its effects on the surface superconductivity

The interplay of quantum geometry and interactions determines the correlated state properties of flat bands. Here, we investigate the ideal quantum geometry (IQG), i.e., the property that the trace of quantum metric equals the Berry curvature, in the surface flat bands of rhombohedrally stacked graphene (RG) multilayers. We show that RG represents a class of semimetals with IQG, among which only RG has a nonvanishing IQG at the center of the surface bands. In the presence of long-range hoppings, a displacement field polarizes the electron density to one of the two surfaces, stabilizing the IQG and boosting transitions to correlated phases like superconductivity. Analyzing the superfluid stiffness of the superconducting state in a many-layer RG, we reveal a heavy-fermion picture where the "heavy electrons" carry a nonzero supercurrent. This unusual behavior arises from the IQG of the surface states, which suggests a large fraction of supercurrent running on the surface of RG.

cond-mat.mes-hall↗

arXiv2024

Superfluid weight cross-over and critical temperature enhancement in singular flat bands

Non-analytic Bloch eigenstates at isolated band degeneracy points exhibit singular behavior in the quantum metric. Here, a description of superfluid weight for zero-energy flat bands in proximity to other high-energy bands is presented, where they together form a singular band gap system. When the singular band gap closes, the geometric and conventional contributions to the superfluid weight as a function of the superconducting gap exhibit different crossover behaviors. The scaling behavior of superfluid weight with the band gap is studied in detail, and the effect on the Berezinskii-KosterlitzThouless (BKT) transition temperature is explored. It is discovered that tuning the singular band gap provides a unique mechanism for enhancing the supercurrent and critical temperature of two-dimensional (2D) superconductors.

cond-mat.supr-con↗

arXiv2024

Geometric superfluid weight of composite bands in multiorbital superconductors

The superfluid weight of an isolated flat band in multi-orbital superconductors contains contributions from the band's quantum metric and a lattice geometric term that depends on the orbital positions in the lattice. Since the superfluid weight is a measure of the superconductor's energy fluctuation, it is independent of the lattice geometry, leading to the minimal quantum metric of a band [Phys. Rev. B 106, 014518 (2022)]. Here, a perturbation approach is developed to study the superfluid weight and its lattice geometric dependence for composite bands. When all orbitals exhibit uniform pairing, the quantum geometric term contains each band's contribution and an inter-band contribution between every pair of bands in the composite. Based on a band representation analysis, they provide a topological lower bound for the superfluid weight of an isolated composite of flat bands. Using this perturbation approach, an analytical expression of the lattice geometric contribution is obtained. It is expressed in terms of Bloch functions, providing a convenient formula to calculate the superfluid weight for multi-orbital superconductors.

cond-mat.supr-con↗

arXiv2023

Pair Density Waves from Local Band Geometry

A band-projection formalism is developed for calculating the superfluid weight in two-dimensional multi-orbital superconductors with an orbital-dependent pairing. It is discovered that, in this case, the band geometric superfluid stiffness tensor can be locally non-positive-definite in some regions of the Brillouin zone. When these regions are large enough or include nodal singularities, the total superfluid weight becomes non-positive-definite due to pairing fluctuations, resulting in the transition of a BCS state to a pair-density wave (PDW). This geometric BCS-PDW transition is studied in the context of two-orbital superconductors, and proof of the existence of a geometric BCS-PDW transition in a generic topological flat band is established.

cond-mat.supr-con↗

arXiv2023

Gravitational Response of Topological Quantum States of Matter

Identifying novel topological properties of topological quantum states of matter, such as exemplified by the quantized Hall conductance, is a valuable step towards realizing materials with attractive topological attributes that guarantee their imperviousness to realistic imperfections, disorder and environmental disturbances. Is the gravitational coupling coefficient of topological quantum states of matter a promising candidate? Substantially building on well established results for quantum Hall states, using disclinations as tools for controlled creation of pristine spatial curvature free of undesirable artifacts such as would interfere with the electronic motion of interest, herein we report that a large class of lattice topological states of matter exhibit gravitational response, i.e., charge response to intrinsic spatial curvature. This phenomenon is characterized by a topologically quantized coupling constant. Remarkably, the charge-gravity relationship remains linear in the curvature, up to the maximum curvature achievable on the lattice, demonstrating absence of higher order nonlinear response. Our findings facilitate articulating the physical principles underlying the topological quantization of the gravitational coupling constant, in analogy with the insights offered by the Chern number description of the quantized Hall conductance.

cond-mat.mes-hall↗

arXiv2021

Dyn-Backdoor: Backdoor Attack on Dynamic Link Prediction

Dynamic link prediction (DLP) makes graph prediction based on historical information. Since most DLP methods are highly dependent on the training data to achieve satisfying prediction performance, the quality of the training data is crucial. Backdoor attacks induce the DLP methods to make wrong prediction by the malicious training data, i.e., generating a subgraph sequence as the trigger and embedding it to the training data. However, the vulnerability of DLP toward backdoor attacks has not been studied yet. To address the issue, we propose a novel backdoor attack framework on DLP, denoted as Dyn-Backdoor. Specifically, Dyn-Backdoor generates diverse initial-triggers by a generative adversarial network (GAN). Then partial links of the initial-triggers are selected to form a trigger set, according to the gradient information of the attack discriminator in the GAN, so as to reduce the size of triggers and improve the concealment of the attack. Experimental results show that Dyn-Backdoor launches successful backdoor attacks on the state-of-the-art DLP models with success rate more than 90%. Additionally, we conduct a possible defense against Dyn-Backdoor to testify its resistance in defensive settings, highlighting the needs of defenses for backdoor attacks on DLP.

cs.AI↗

arXiv2019

Tuning insulator-semimetal transitions in 3D topological insulator thin films by inter-surface hybridization and in-plane magnetic fields

A pair of Dirac points (analogous to a vortex-antivortex pair) associated with opposite topological numbers (with $\pmπ$ Berry phases) can be merged together through parameter tuning and annihilated to gap the Dirac spectrum, offering a canonical example of a topological phase transition. Here, we report transport studies on thin films of BiSbTeSe$_2$ (BSTS), which is a 3D TI that hosts spin-helical gapless (semi-metallic) Dirac fermion surface states (SS) for sufficiently thick samples, with an observed resistivity close to $h/4e^2$ at the charge neutral point. When the sample thickness is reduced to $\sim$10 nm thick, the Dirac cones from the top and bottom surfaces can hybridize (analogous to a "merging" in the real space) and become gapped to give a trivial insulator. Furthermore, we observe that an in-plane magnetic field can drive the system again towards a metallic behavior, with a prominent negative magnetoresistance (MR, up to $\sim$$-$95\%) and a temperature-insensitive resistivity close to $h/2e^2$ at the charge neutral point. The observation is interpreted in terms of a predicted effect of an in-plane magnetic field to reduce the hybridization gap (which, if small enough, may be smeared by disorder and a metallic behavior). A sufficiently strong magnetic field is predicted to restore and split again the Dirac points in the momentum space, inducing a distinct 2D topological semimetal (TSM) phase with 2 single-fold Dirac cones of opposite spin-momentum windings.

cond-mat.mes-hall↗

arXiv2018

Geometric response of quantum Hall states to electric fields

Exploiting novel aspects of the quantum geometry of charged particles in a magnetic field via gauge-invariant variables, we provide tangible connections between the response of quantum Hall fluids to non-uniform electric fields and the characteristic geometry of electronic motion in the presence of magnetic and electric fields. The geometric picture we provide motivates the following conjecture: non-uniform electric fields mimic the presence of spatial curvature. Consequently, the gravitational coupling constant also appears in the charge response to non-uniform electric fields.

cond-mat.mes-hall↗

arXiv2018

Connection topology of step edge state bands at the surface of a three dimensional topological insulator

Topological insulators in the Bi$_2$Se$_3$ family manifest helical Dirac surface states that span the topologically ordered bulk band gap. Recent scanning tunneling microscopy measurements have discovered additional states in the bulk band gap of Bi$_2$Se$_3$ and Bi$_2$Te$_3$, localized at one dimensional step edges. Here numerical simulations of a topological insulator surface are used to explore the phenomenology of edge state formation at the single-quintuple-layer step defects found ubiquitously on these materials. The modeled one dimensional edge states are found to exhibit a stable topological connection to the two dimensional surface state Dirac point.

cond-mat.mtrl-sci↗