arXiv subjects

Jikun Chen

Publications and source records attributed to Jikun Chen.

16 recordsLinked to original sources

arXiv2026

Counterintuitive inverse superconducting transition beyond 4He-cooling limit

Thermally driven quantum-orders observed in exceptional instances may redefine the role of thermal-fluctuation from a source of decoherence to a resource for coherent-state engineering. While preliminary signs of counterintuitive temperature-rise-triggered superconductivity manifested in CeCu2Si2, ErRh4B4, Ho1.2Mo6S8 and (La,Ce)Al2, their critical-temperatures (Tc-inv) remain below Kelvin-range, precluding substantial applications. Here, we report field-modulated inverse-superconducting-transitions above 4He-cooling-limit in Eu-based infinite-layer nickelates (EuxNd1-xNiO2 and EuxPr1-xNiO2) grown on a substrate under both overdoped and underdoped regimes. Paradigmatically, superconductivity with zero-resistance is confined between Tc-inv (2.6-5.4 K) and another higher normal-Tc, rising and decreasing with applied magnetic-field, respectively. Starting from the resistive-state below Tc-inv, the inverse-superconducting-transition is driven by not only temperature-rising, but also current-density, while superconductivity further vanishes at higher temperature and current thresholds. The Kelvin-range inverse superconducting transition is plausibly explained by temperature-induced alternating dominance of effective magnetic-fields arising from Eu2+4f7 related compensations relative to the upper-critical-field. Furthermore, an extended-phenomenological-framework is also supported by reemerged superconductivity below 300 mK under magnetic-field, giving rise to an unprecedented temperature-induced reentrant superconductivity. Our findings establish magnetic-interaction-reconfigured high-Tc systems as fertile platforms for exploring quantum phenomena that reverse thermal-decoherence paradigm, also enabling antithetical-designs to unlock untapped application-scenarios for quantum-phase-transition devices.

cond-mat.supr-con↗

arXiv2025

A chemical avenue to manipulate field-reentrant superconducting rivalries in infinite layer nickelates

Recently, preliminary magnetic field-reentrant superconductivity manifested in high-temperature (Tc) Eu-doped infinite-layer (IL) nickelates, beyond analogous discoveries exclusively in low-Tc systems. This evokes intriguing fundamental issues about potential quantum-phase boundary and criticality between unconventional superconductivity and field-reentrant-one, which are inexplicable owing to formidable challenges in growing IL-nickelates towards later-series rare-earths. Herein, we open up chemical avenues to enable effective growth of (RE1-yRE'y)1-xEuxNiO2 (RE/RE': Pr, Nd, Sm, Gd, Dy), giving rise to discoveries of RE-4f-related quantum competition between high-Tc and reentrant superconductivity. Robust magnetic-field-reentrant superconductivity with uniaxial anisotropy is observed at superconducting-dome boundaries, stemming from Eu2+-4f7 associated competition between magnetic-fluctuation promoted pairing and exchange-field interactions. Their quantum-criticality is further modulable via RE(RE')-magnetism, which either reinforces reentrancy or elevates Tc (40.1 K) with more robust critical-current-density (~266 kA/cm2 at 2 K) beyond Sr-/Ca-doped counterparts. Our synthetic route enables the establishment of an ideal platform via IL-nickelates for studying 4f-related unconventional superconductivity and quantum-criticality.

cond-mat.supr-con↗

arXiv2024

SPDiffusion: Semantic Protection Diffusion Models for Multi-concept Text-to-image Generation

Recent text-to-image models have achieved impressive results in generating high-quality images. However, when tasked with multi-concept generation creating images that contain multiple characters or objects, existing methods often suffer from semantic entanglement, including concept entanglement and improper attribute binding, leading to significant text-image inconsistency. We identify that semantic entanglement arises when certain regions of the latent features attend to incorrect concept and attribute tokens. In this work, we propose the Semantic Protection Diffusion Model (SPDiffusion) to address both concept entanglement and improper attribute binding using only a text prompt as input. The SPDiffusion framework introduces a novel concept region extraction method SP-Extraction to resolve region entanglement in cross-attention, along with SP-Attn, which protects concept regions from the influence of irrelevant attributes and concepts. To evaluate our method, we test it on existing benchmarks, where SPDiffusion achieves state-of-the-art results, demonstrating its effectiveness.

cs.CV↗

arXiv2024

Irrelevance of 1H composition to the superconductivity in the infinite-layer nickelates: judging from the MeV energy scale

The discovery of the superconductivity in the infinite-layer nickelates, as topotactically reduced from their respective perovskite percussors via co-annealing with CaH2, extends the understanding in superconductivity. Nevertheless, whether the incorporated 1H composition is critical to the infinite-layer superconductivity recently arouses considerable debates, while the central challenge lies in the quantification of 1H that is easily interfered by the conventional electron or orbital associated processes. Herein, we demonstrate the irrelevance between the superconductivity in the infinite-layer nickelates and their incorporated 1H composition, assisted by nuclear reaction analysis (NRA) and heavy ion energy recoil detection analysis (HIERDA) based on the nuclear interactions at MeV energy scale. These approaches completely overwhelm the conventional interferes, such as ionization, activation and chemical bonds, and achieves the 1H quantification within superconducting La0.8Sr0.2NiO2 (or Nd0.8Sr0.2NiO2). A large diversity of 1H composition far beyond the previously expected critical dome was observed, while their TC were not changed significantly. Furthermore, the superconductivity was demonstrated to be achievable for La0.8Sr0.2NiO2 reduced by Al without any hydrogen associated process, while the superconducting properties for the CaH2 reduced La0.8Sr0.2NiO2 is rather stable after long term exposure in air, despite the high volatility of 1H within oxides. All these results indicate that the 1H incorporation composition is not critical to the superconductivity of the infinite-layer nickelates.

cond-mat.supr-con↗

arXiv2023

Non-Equilibrium Spark Plasma Reactive Doping Enables Highly Adjustable Metal to Insulator Transitions and Improved Mechanical Stability for VO2

Although vanadium dioxide (VO2) exhibits the most abrupt metal to insulator transition (MIT) properties near room-temperature, the present regulation of their MIT functionalities is insufficient owing to the high complexity and susception associated with V4+. Herein, we demonstrate a spark plasma assisted reactive sintering (SPARS) approach to simultaneously achieve in situ doping and sintering of VO2 within largely short period (~10 minutes). This enables high convenience and flexibility in regulating the electronic structure of VO2 via dopant elements covering Ti, W, Nb, Mo, Cr and Fe, leading to a wide adjustment in their metal to insulator transition temperature (TMIT) and basic resistivity. Furthermore, the mechanical strengths of the doped-VO2 were meanwhile largely improved via the compositing effect of high melting-point dopant oxide. The high adjustability in MIT properties and improved mechanical properties further paves the way towards practical applications of VO2 in power electronics, thermochromism and infrared camouflage.

cond-mat.mtrl-sci↗

arXiv2022

Adversarial Representation Sharing: A Quantitative and Secure Collaborative Learning Framework

The performance of deep learning models highly depends on the amount of training data. It is common practice for today's data holders to merge their datasets and train models collaboratively, which yet poses a threat to data privacy. Different from existing methods such as secure multi-party computation (MPC) and federated learning (FL), we find representation learning has unique advantages in collaborative learning due to the lower communication overhead and task-independency. However, data representations face the threat of model inversion attacks. In this article, we formally define the collaborative learning scenario, and quantify data utility and privacy. Then we present ARS, a collaborative learning framework wherein users share representations of data to train models, and add imperceptible adversarial noise to data representations against reconstruction or attribute extraction attacks. By evaluating ARS in different contexts, we demonstrate that our mechanism is effective against model inversion attacks, and achieves a balance between privacy and utility. The ARS framework has wide applicability. First, ARS is valid for various data types, not limited to images. Second, data representations shared by users can be utilized in different tasks. Third, the framework can be easily extended to the vertical data partitioning scenario.

cs.CR↗

arXiv2021

Hydrogen induced electronic transition within correlated perovskite nickelates with heavy rare-earth composition

Although discovery in hydrogen induced electronic transition within perovskite family of rare-earth nickelate (ReNiO3) opens up a new paradigm in exploring both the new materials functionality and device applications, the existing research stays at ReNiO3 with light rare-earth compositions. To further extend the cognition towards heavier rare-earth, herein we demonstrate the hydrogen induced electronic transitions for quasi-single crystalline ReNiO3/LaAlO3 (001) heterostructures, covering a large variety of the rare-earth composition from Nd to Er. The hydrogen induced elevations in the resistivity of ReNiO3 (RH/R0) show an unexpected non-monotonic tendency with the atomic number of the rare-earth composition, e.g., firstly increase from Nd to Dy and afterwards decreases from Dy to Er. Although ReNiO3 with heavy rare-earth composition (e.g. DyNiO3) exhibits large RH/R0 up to 107, their hydrogen induced electronic transition is not reversible. Further probing the electronic structures via near edge X-ray absorption fine structure analysis clearly demonstrates the respective transition in electronic structures of ReNiO3 from Ni3+ based electron itinerant orbital configurations towards the Ni2+ based electron localized state. Balancing the hydrogen induced transition reversibility with the abruption in the variations of material resistivity, we emphasize that the ReNiO3 with middle rare-earth compositions (e.g. Sm) to be most suitable that caters for the potential applications in correlated electronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Revealing the role of interfacial heterogeneous nucleation in metastable thin film growth of rare earth nickelates electronic transition materials

Although rare earth nickelates (ReNiO3) exhibit abundant electronic phases and widely adjustable metal to insulator electronic transition properties, their practical electronic applications are largely impeded by their intrinsic meta stability. Apart from elevating oxygen reaction pressures, heterogeneous nucleation is expected as an alternative strategy that enables the crystallization of ReNiO3 at low meta stability. In this work, the respective roles of high oxygen pressure and heterogeneous interface in triggering ReNiO3 thin films growth at metastable state are revealed. The ReNiO3 (Re:Nd, Sm, Eu, Gd, and Dy) heterogeneous thin films growth on LaAlO3 single crystal substrate have an effective crystallization at atmosphere without the necessity to apply high oxygen pressures, suggesting the interfacial bonding between the ReNiO3 and substrates can sufficiently reduce the positive Gibbs formation energy of ReNiO3, which is further verified by the first principles calculations. Nevertheless, the abrupt electronic transitions only appear in ReNiO3 thin films grown at high oxygen pressures, in which cases the oxygen vacancies are effectively eliminated via high oxygen pressure reactions as indicated by near edge X ray absorption fine structure (NEXAFS). This work unveils the synergistic effects of heterogeneous nucleation and high oxygen pressure on the growth of high quality ReNiO3 thin films.

cond-mat.mtrl-sci↗

arXiv2020

AC-frequency switchable correlated transports in rare-earth perovskite nickelates

Whilst electron correlations were previously recognized to trigger beyond conventional direct current (DC) electronic transportations (e.g. metal-to-insulator transitions, bad metal, thermistors), their respective influences to the alternation current (AC) transport are largely overlooked. Herein, we demonstrate active regulations in the electronic functionalities of d-band correlated rare-earth nickelate (ReNiO3) thin films, by simply utilizing their electronic responses to AC-frequencies (fAC). Assisted by temperature dependent near edge X-ray absorption fine structure analysis, we discovered positive temperature dependences in Coulomb viscosity of ReNiO3 that moderates their AC impedance. Distinguished crosslinking among R(Real)-fAC measured in nearby temperatures is observed that differs to conventional oxides. It enables active adjustability in correlated transports of ReNiO3, among NTCR-, TDelta- and PTCR- thermistors, via fAC from the electronic perspective without varying materials or device structures. The TDelta-fAC relationship can be further widely adjusted via Re composition and interfacial strains. The AC-frequency sensitivity discovered in ReNiO3 brings in a new freedom to regulating and switching the device working states beyond the present semiconductor technologies. It opens a new paradigm for enriching novel electronic applications catering automatic transmission or artificial intelligence in sensing temperatures and frequencies.

cond-mat.mtrl-sci↗

arXiv2019

Overlooked transportation anisotropies in d-band correlated rare-earth perovskite nickelates

Anisotropies in electronic transportations conventionally originate from the nature of low symmetries in crystal structures, and were not anticipated for perovskite oxides, the crystal asymmetricity of which is far below, e.g. van der Waals or topological crystal. Beyond conventional expectations, herein we demonstrate pronounced anisotropies in the inter-band coulomb repulsion dominated electronic transportation behaviors under low-dimensional confinement for the perovskite family of rare-earth nickelates (ReNiO3). From one aspect, imparting bi-axial interfacial strains upon various lattice planes results in extrinsic anisotropies in the abrupt orbital transitions of ReNiO3, and their metal to insulator transition behaviors that elevates the transition temperature beyond the existing merit. From the other aspect, the in-plane orbital entropy associated to the in-plane symmetry of the NiO6 octahedron within ReNiO3 causes intrinsic anisotropies for the gradually orbital transition with temperature to further improve their thermistor transportation properties. The present work unveils the overlooked role of the electronic orbital directionality within low dimensional correlated perovskites that can trigger anisotropic transportation behaviors, in spite of their relatively symmetric crystal structures. Establishing anisotropic transportations integrating the electron correlation and quantum confinement effects will bring in a new freedom for achieving further improvement in transportation properties of multi-functional perovskite oxides.

cond-mat.mtrl-sci↗

arXiv2019

Entropy driven reverse-metal-to-insulator transition and delta-temperatural transports in metastable perovskites of correlated rare-earth nickelate

The metal to insulator transition (MIT) in Mott-Hubbard systems is one of the most important discoveries in condensed matter physics, and results in abrupt orbital transitions from the insulating to metallic phases by elevating temperature across a critical point (TMIT). Although the MIT was previously expected to be mainly driven by the orbital Coulomb repulsion energy, the entropy contribution to the orbital free energy that also determines the relative stability of the metallic and insulating phases was largely overlooked. Herein, we demonstrate an orbital-entropy dominated reversible electronic phase transition in the metastable perovskite family of correlated rare-earth nicklates (ReNiO3), in addition to their previously known MIT driven by orbital Coulomb energies. In reverse to MIT, the resistivity of ReNiO3 abruptly increases by 2-3 orders by elevating T across another critical point (TR-MIT) below TMIT, and such transition is named as reverse-metal to insulator transition (R-MIT). Combining the afterwards exponentially decreasing resistivity in the insulating phase of ReNiO3 at further temperature elevation, a distinguished delta-temperatural transport character is established, which is potentially applicable for locking the working temperatures range for electric devices. The TR-MIT is shown to be enhanced via reducing the compositional complexity and size of Re or imparting bi-axial compressive strains, and meanwhile the transition sharpness of delta-temperatural transport is reduced. Our discovery indicates that temperature range for a thermodynamically stable insulating phase of ReNiO3 is in between of TR-MIT and TMIT, while a new conductive phase with high orbital entropy is formed by further descending temperature below TR-MIT.

physics.app-ph↗

arXiv2018

Instantaneous effects of photons on electrons in semiconductors

The photoelectric effect established by Einstein is well known, which indicates that electrons on lower energy levels can jump up to higher levels by absorbing photons, or jump down from higher levels to lower levels and give out photons1-3. However, how do photons act on electrons and further on atoms have kept unknown up to now. Here we show the results that photons collide on electrons with energy-transmission in semiconductors and pass their momenta to electrons, which make the electrons jump up from lower energy levels to higher levels. We found that (i) photons have rest mass of 7.287exp(-38) kg and 2.886exp(-35) kg, in vacuum and silicon respectively; (ii) excited by photons with energy of 1.12eV, electrons in silicon may jump up from the top of valance band to the bottom of conduction band with initial speed of 2.543exp(3) m/s and taking time of 4.977exp(-17) s; (iii) acted by photons with energy of 4.6eV, the atoms who lose electrons may be catapulted out of the semiconductors by the extruded neighbor atoms, and taking time of 2.224exp(-15) s. These results make reasonable explanation to rapid thermal annealing, laser ablation and laser cutting.

physics.atom-ph↗

arXiv2018

Robust formation of topological Hall effect in MnGa/heavy metal bilayers

We have investigated the topological Hall effect (THE) in MnGa/Pt and MnGa/Ta bilayers induced by interfacial Dzyaloshinskii-Moriya interaction (DMI). The most evident THE signals have been found based on the MnGa films with small critical DMI energy constant Dc. The large topological portion of the Hall signal from the total Hall signal has been extracted in the whole temperature range from 5 to 300 K. These results open up the exploration of the DMI induced magnetic behavior based on the bulk perpendicular magnetic anisotropy materials for fundamental physics and magnetic storage technologies.

cond-mat.mtrl-sci↗

arXiv2017

A micrometer-thick oxide film with high thermoelectric performance at temperature ranging from 20-400 K

Thermoelectric (TE) materials achieve localised conversion between thermal and electric energies, and the conversion efficiency is determined by a figure of merit zT. Up to date, two-dimensional electron gas (2DEG) related TE materials hold the records for zT near room-temperature. A sharp increase in zT up to ~2.0 was observed previously for superlattice materials such as PbSeTe, Bi2Te3/Sb2Te3 and SrNb0.2Ti0.8O3/SrTiO3, when the thicknesses of these TE materials were spatially confine within sub-nanometre scale. The two-dimensional confinement of carriers enlarges the density of states near the Fermi energy3-6 and triggers electron phonon coupling. This overcomes the conventional σ-S trade-off to more independently improve S, and thereby further increases thermoelectric power factors (PF=S2σ). Nevertheless, practical applications of the present 2DEG materials for high power energy conversions are impeded by the prerequisite of spatial confinement, as the amount of TE material is insufficient. Here, we report similar TE properties to 2DEGs but achieved in SrNb0.2Ti0.8O3 films with thickness within sub-micrometer scale by regulating interfacial and lattice polarizations. High power factor (up to 103 μWcm-1K-2) and zT value (up to 1.6) were observed for the film materials near room-temperature and below. Even reckon in the thickness of the substrate, an integrated power factor of both film and substrate approaching to be 102 μWcm-1K-2 was achieved in a 2 μm-thick SrNb0.2Ti0.8O3 film grown on a 100 μm-thick SrTiO3 substrate. The dependence of high TE performances on size-confinement is reduced by ~103 compared to the conventional 2DEG-related TE materials. As-grown oxide films are less toxic and not dependent on large amounts of heavy elements, potentially paving the way towards applications in localised refrigeration and electric power generations.

cond-mat.mtrl-sci↗

arXiv2017

Effect of photons on atoms in crystallization of amorphous silicon films

The preferentially (220) orientated polycrystalline silicon films have been fabricated by performing photon-involved rapid thermal annealing upon amorphous silicon films. In contrast, conventional thermal annealing of the silicon amorphous films results in the preference of (111) orientated crystallization. This difference reveals the overlooked impaction of photons upon silicon atoms, which results in the preferential orientation of lattice plans with the smallest density of atoms on their projection plane rather than the lowest energies.

cond-mat.mtrl-sci↗

arXiv2016

The product of dependent random variables with applications to a discrete-time risk model

Let $X$ be a real valued random variable with an unbounded distribution $F$ and let $Y$ be a nonnegative valued random variable with a unbounded distribution $G$, which satisfy that \begin{eqnarray*} P(X>x|Y=y)\sim h(y)P(X>x) \end{eqnarray*} holds uniformly for $y\geq0$ as $x\to \infty$. Under the condition that $\overline{G}(bx)=o(\overline H(x))$ holds for all constant $b>0$, we proved that $F\in\mathcal{L}(γ)$ for some $γ\geq 0$ implied $H\in \mathcal{L}(γ/β_G)$ and that $F\in\mathcal{S}(γ)$ for some $γ\geq 0$ implied $H\in \mathcal{S}(γ/β_G)$, where $H$ is the distribution of the product $XY$, and $β_G$ is the right endpoint of $G$, that is, $β_G=\sup\{y:~G(y)<1\}\in (0,\infty],$ and when $β_G=\infty$, $γ/β_G$ is understood as 0. Furthermore, in a discrete-time risk model in which the net insurance loss and the stochastic discount factor are equipped with a dependence structure, a general asymptotic formula for the finite-time ruin probability is obtained when the net insurance loss has a subexponential tail.

math.PR↗