arXiv subjects

Zihan Cui

Publications and source records attributed to Zihan Cui.

6 recordsLinked to original sources

arXiv2026

Emergent quantum phenomena via phase-coherence engineering in infinite-layer nickelate superconductors

Dimensionality of a physical system, conventionally an invariant geometric characteristic, fundamentally governs the universality class of phase transitions and the landscape of emergent collective phenomena. In low-dimensional or layered high-temperature superconductors, the macroscopic phase coherence of superconducting orders is typically confined in two dimensions, underscoring the critical role of phase fluctuations in determining the overall phase diagrams. Here, we strategically enhance the phase fluctuations by fabricating periodically arranged nano-holes in the infinite-layer nickelate superconducting films, effectively constructing Josephson junction arrays. In the nano-patterned films, the weakening of macroscopic phase coherence drives a two-stage superconducting transition towards an anomalous metallic ground state with saturated resistance. The emergence of charge-2e quantum oscillations manifests the coherence across the array, while an anomalous zero-field magnetoresistance peak signifies the extreme quantum phase fluctuations persisting to ultralow temperatures. Remarkably, with quantum fluctuations enhanced synergistically by nano-patterning and magnetic fields, an anomalous reversal of superconducting anisotropy is observed in Nd-nickelates, where in-plane critical fields fall below out-of-plane values. The evolution of anisotropy may unmask an internal exchange-Zeeman field coupled to the collective electronic states. Our results unveil how superconductivity evolves in response to phase fluctuations, establishing nano-patterning as a powerful paradigm to uncover hidden intertwined orders in strongly correlated systems.

cond-mat.supr-con↗

arXiv2025

Exceedingly large in-plane critical field of finite-momentum pairing state in bulk superlattices

Magnetic flux profoundly influences the phase factor of charge particles, leading to exotic quantum phenomena. A recent example is that the orbital effect of magnetic field could induce finite-momentum pairing state in nanoflakes, which offers a new pathway to realize the spatially modulated superconductivity distinct from the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state induced by Zeeman effect. However, whether such intriguing state can exist in the bulk materials under extremely large magnetic field remains elusive. Here we report the orbital effect induced finite-momentum pairing state with exceedingly large in-plane critical field in a bulk superconducting superlattice. Remarkably, the in-plane critical field shows a pronounced upturn behavior, exceeding eight times the Pauli limit which is comparable to monolayer Ising superconductor. Under high in-plane magnetic fields, significant anisotropic transport behavior between the interlayer and intralayer directions is detected, highlighting the critical role of suppressed interlayer coherence in the orbital effect induced finite-momentum pairing state. Crucially, this finite-momentum pairing state remains robust against moderate disorder. Our findings suggest that van der Waals superlattices, with strong Ising spin-orbit coupling and tunable interlayer coherence, offer new avenues for constructing and modulating unconventional superconducting states.

cond-mat.supr-con↗

arXiv2025

Early Stopping in Contextual Bandits and Inferences

Bandit algorithms sequentially accumulate data using adaptive sampling policies, offering flexibility for real-world applications. However, excessive sampling can be costly, motivating the devolopment of early stopping methods and reliable post-experiment conditional inferences. This paper studies early stopping methods in linear contextual bandits, including both pre-determined and online stopping rules, to minimize in-experiment regrets while accounting for sampling costs. We propose stopping rules based on the Opportunity Cost and Threshold Method, utilizing the variances of unbiased or consistent online estimators to quantify the upper regret bounds of learned optimal policy. The study focuses on batched settings for stability, selecting a weighed combination of batched estimators as the online estimator and deriving its asymptotic distribution. Online statistical inferences are performed based on the selected estimator, conditional on the realized stopping time. Our proposed method provides a systematic approach to minimize in-experiment regret and conduct robust post-experiment inferences, facilitating decision-making in future applications.

math.ST↗

arXiv2025

High-temperature field-free superconducting diode effect in high-Tc cuprates

The superconducting diode effect (SDE) is defined by the difference in the magnitude of critical currents applied in opposite directions. It has been observed in various superconducting systems and attracted high research interests. However, the operating temperature of the SDE is typically low and/or the sample structure is rather complex. For the potential applications in non-dissipative electronics, efficient superconducting diodes working in zero magnetic field with high operating temperatures and a simple configuration are highly desired. Here, we report the observation of a SDE under zero magnetic field with operating temperatures up to 72 K and efficiency as high as 22% at 53 K in high-transition-temperature (high-Tc) cuprate superconductor Bi2Sr2CaCu2O8+{\delta} (BSCCO) flake devices. The rectification effect persists beyond two hundred sweeping cycles, confirming the stability of the superconducting diode. Our results offer promising developments for potential applications in non-dissipative electronics, and provide insights into the mechanism of field-free SDE and symmetry breakings in high-Tc superconductors.

cond-mat.supr-con↗

arXiv2022

The activated scaling behavior of quantum Griffiths singularity in two-dimensional superconductors

Quantum Griffiths singularity is characterized by the divergence of the dynamical critical exponent with the activated scaling law and has been widely observed in various two-dimensional superconductors. Recently, the direct activated scaling analysis with the irrelevant correction has been proposed and successfully used to analyze the experimental data of crystalline PdTe2 and polycrystalline \b{eta}-W films, which provides new evidence of quantum Griffiths singularity. Here we show that the direct activated scaling analysis is applicable to the experimental data in different superconducting films, including tri-layer Ga films and LaAlO3/SrTiO3 interface superconductor. When taking the irrelevant correction into account, we calculate the corrected sheet resistance at ultralow temperatures. The scaling behavior of the corrected resistance in a comparably large temperature regime and the theoretical fitting of the phase boundary give unambiguous evidence of quantum Griffiths singularity. Compared to the previous method based on the finite size scaling, the direct activated scaling analysis represents a more direct and precise way to analyze the experimental data of quantum Griffiths singularity in diverse two-dimensional superconductors.

cond-mat.supr-con↗

arXiv2021

High-Temperature Anomalous Metal States in Iron-Based Interface Superconductors

The nature of the anomalous metal state has been a major puzzle in condensed matter physics for more than three decades. Here, we report systematic investigation and modulation of the anomalous metal states in high-temperature interface superconductor FeSe films on SrTiO3 substrate. Remarkably, under zero magnetic field, the anomalous metal state persists up to 20 K in pristine FeSe films, an exceptionally high temperature standing out from previous observations. In stark contrast, for the FeSe films with nano-hole arrays, the characteristic temperature of the anomalous metal state is considerably reduced. We demonstrate that the observed anomalous metal states originate from the quantum tunneling of vortices adjusted by the Ohmic dissipation. Our work offers a perspective for understanding the origin and modulation of the anomalous metal states in two-dimensional bosonic systems.

cond-mat.supr-con↗