arXiv subjects

Wenshan Hong

Publications and source records attributed to Wenshan Hong.

At least 19 recordsLinked to original sources

arXiv2026

Enhancement of $J$$_c$ by Proton Irradiation in HgBa$_2$Ca$_2$Cu$_3$O$_8$$_+$$_\delta$ Single Crystals

Critical current density is the key parameter for the practical application of superconductivity. In this study, 3 MeV proton irradiation experiments were conducted on HgBa$_2$Ca$_2$Cu$_3$O$_8$$_+$$_\delta$ single crystals to introduce pinning centers. The critical current density is found to be strongly enhanced after the irradiation with its maximum at a dose of 1$\times$10$^{16}$/cm$^2$, where the self-field critical current density at 2 K is enhanced from 5.5 MA/cm$^2$ to 26 MA/cm$^2$. At 77 K, the self-field critical current density for all irradiated crystals is over 0.1 MA/cm$^2$. The power-law dependence of the critical current density on the magnetic field is observed after irradiation, with a large power-law exponent $\alpha$ close to 1. A monotonic magnetic field dependence of the normalized magnetic relaxation rate is observed, which could be attributed to the low irreversibility field caused by the large anisotropy in Hg1223 single crystals. Through the analysis of the pinning force density of the crystal before and after irradiation, a clear mechanism change has been observed.

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arXiv2026

T-linear specific heat in pressurized and magnetized Shastry-Sutherland Mott insulator SrCu2(BO3)2

The pressurized Shastry-Sutherland Mott insulator SrCu2(BO3)2 has been found to host a plaquette-singlet phase and an antiferromagnetic phase that break different symmetries spontaneously.The recent experiment showed that their transition is of a first order nature, which seems against the pursuit of exotic and deconfined degrees of freedom in this famous frustrated quantum magnet. We found a new direction in this study. By applying a magnetic field to the material, we discover that SrCu2(BO3)2 exhibits a universal and metallic T-linear specific heat behavior in a large magnetitic field range close to the pressure of zero-field first order transition between plaquette-singlet and antiferromagnetic phases. Such an unexpected gapless response from an electronically gapped Mott insulator could be attributed to magnetized Dirac spinons liberated by the combined effect of magnetic field and pressure, consistently seen from our quantum many-body thermal tensor network computation of the Shastry-Sutherland model under magnetic field. Such a robust and universal T-linear specific heat phase points out the richness of the phase diagram of the material expanded by the axes of pressure and magnetic field and is calling for new theoretical frameworks to its full explanation.

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arXiv2026

Electronic Structure and Superconducting Gap of HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

The spatially-resolved laser-based high resolution angle resolved photoemission spectroscopy (ARPES) measurements have been performed on the optimally-doped HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg1223) superconductor with a $T_c$ at 133 K. Two distinct regions are identified on the cleaved surface: the single Fermi surface region where only one Fermi surface is observed, and the double Fermi surface region where two Fermi surface sheets are resolved coming from both the inner (IP) and outer (OP) CuO$_2$ planes. The electronic structure and superconducting gap are measured on both of these two regions. In both cases, the observed electronic states are mainly concentrated near the nodal region. The momentum dependence of superconducting gap deviates from the standard d-wave form. These results indicate that the surface electronic structure of Hg1223 behaves more like that of underdoped cuprates.

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arXiv2026

Decoupled interband pairing in a bilayer iron-based superconductor evidenced by ultrahigh-resolution ARPES

We present direct experimental evidence of a weakly coupled multiband superconducting state in the bilayer iron-based superconductor ACa$_2$Fe$_4$As$_4$F$_2$ (A = K, Cs) via ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES). Remarkably, the K-containing compound exhibits two distinct transition temperatures, corresponding to two separate sets of bilayer-split bands, as evidenced by temperature-dependent superconducting gap and spectral weight near the Fermi energy, while its Cs counterpart displays conventional single transition behavior. These experimental observations are well described by the weakly coupled two-band model of Eilenberger theory, which identifies suppressed interband pairing interactions between the bilayer-split bands as the key mechanism. By exploring quantum phenomena in the weak-coupling limit within a multiband system, our findings pave the way for engineering exotic superconductivity via band-selective pairing control.

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arXiv2025

Strain-induced structural change and nearly-commensurate diffuse scattering in the model high-temperature superconductor HgBa$_2$CuO$_{4+\delta}$

We investigate the strain response of underdoped HgBa$_2$CuO$_{4+\delta}$ (Hg1201), by synchrotron X-ray diffraction and corresponding simulations of thermal diffuse scattering. The compression in the crystallographic $a$ direction leads to relatively small expansion in the $b$ and $c$ directions, with Poisson ratios $\nu_{ba}$=0.16 and $\nu_{ca}$=0.11, respectively. However, the Cu-O distance in the $c$ direction exhibits a notable 0.9% increase at 1.1% $a$-axis compression. We further find strain-induced diffuse scattering which corresponds to a new type of two-dimensional charge correlation. Interestingly, this signal is insensitive to the onset of superconductivity and instead corresponds to a short-range, nearly commensurate modulation with a wave vector close to (0.5, 0, 0) and a correlation length of approximately four unit cells. It closely resembles the charge order theoretically predicted in the phase diagram of the spin-liquid model with resonating valence bonds on a square lattice.

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arXiv2025

ac strain based thermodynamic criterion for vortex lattice in type-II superconductors

In type-I superconductors, zero electrical resistivity and perfect diamagnetism define two fundamental criteria for superconducting behavior. In contrast, type-II superconductors exhibit more complex mixed state physics, where magnetic flux penetrates the material above the lower critical field Hc1 in the form of quantized vortices, each carrying a single flux quantum. These vortices form a two dimensional lattice which persists up to another irreversible field (Hirr) and then melts into a dissipative liquid phase. The vortex lattice is fundamental to the magnetic and electrical properties of type II superconductors, ac strain susceptibility-a thermodynamic criterion-for identifying this phase has remained elusive. Here, we report the discovery of a dynamic magnetostrictive effect, wherein the geometry of the superconductor oscillates only under an applied alternating magnetic field due to the disturbance of the vortex lattice. This effect is detected by a thin piezoelectric transducer, which converts the excited geometric deformation into an in-phase ac voltage. Notably, we find a direct and nearly linear relationship between the signal amplitude and the vortex density in lattice across several representative type-II superconductors. In the vortex liquid phase above Hirr, the signal amplitude rapidly decays to zero near the upper critical field (Hc2), accompanied by a pronounced out-of-phase component due to enhanced dissipation. This dynamic magnetostrictive effect not only reveals an unexplored magnetoelastic property of the vortex lattice but also establishes a fundamental criterion for identifying the type-II superconductors.

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arXiv2025

Neutron Spin Resonance Near a Lifshitz Transition in Overdoped Ba$_{0.4}$K$_{0.6}$Fe$_2$As$_2$

Elucidating the relationship between spin excitations and fermiology is essential for clarifying the pairing mechanism in iron-based superconductors (FeSCs). Here, we report inelastic neutron scattering results on the hole overdoped Ba$_{0.4}$K$_{0.6}$Fe$_2$As$_2$ near a Lifshitz transition, where the electron pocket at $M$ point is nearly replace by four hole pockets. In the normal state, the spin excitations are observed at incommensurate wave vectors with chimney-like dispersions. By cooling down to the superconducting state, a neutron spin resonance mode emerges with a peak energy of $E_r=$ 14-15 meV weakly modulated along $L$-direction. The incommensurability notably increases at low energies, giving rise to downward dispersions of the resonance mode. This behavior contrasts sharply with the upward dispersions of resonance observed in optimally doped Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$ contributed by the hole to electron scattering, but resembles with the cases in KFe$_2$As$_2$ and KCa$_2$Fe$_4$As$_4$F$_2$ where the fermiology are dominated by hole pockets. These results highlight the critical role of electronic structure modifications near the Fermi level, especially in governing interband scattering under imperfect nesting conditions, which fundamentally shape the spin dynamics of FeSCs.

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arXiv2025

Dominant apical-oxygen electron-phonon coupling in HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$

How electron-phonon interactions influence high-temperature superconductivity in cuprates remains contested, and their role outside the CuO$_2$ planes has been largely overlooked. The most conspicuous evidence for such coupling is the ubiquitous 70-meV dispersion kink seen by photoemission, yet its microscopic origin is still debated. Here we use oxygen-$K$-edge resonant inelastic X-ray scattering (RIXS) to probe the trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg1223). When both incident photon energy and polarization are tuned to the apical-oxygen $1s\!\rightarrow\!2p_z$ transition, the RIXS spectra exhibit a ladder of at least ten phonon overtones, evenly spaced by 70 meV, whose intensities follow a Franck-Condon envelope, signalling exceptionally strong electron-phonon coupling. Quantitative modelling that incorporates core-hole lifetime evaluation yields an apical-phonon coupling energy of 0.25(1) eV, significantly larger than that of the planar stretching mode. Such a coupling strength offers a strong contender for explaining the universal 70-meV kink and suggests that the dominant electron-phonon channel resides outside the CuO$_2$ planes. By elevating inter-layer lattice dynamics from a peripheral factor to a central actor, our results provide a fresh starting point for theories seeking to reconcile strong correlations, lattice dynamics and high-temperature superconductivity.

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arXiv2025

Three Magnetization Peaks in HgBa$_2$Ca$_2$Cu$_3$O$_8$ Single Crystals

By measuring magnetization hysteresis loops of the superconducting HgBa$_2$Ca$_2$Cu$_3$O$_8$ single crystals ($T_{\rm c}$ = 133 K), we observed three magnetization peaks in a wide temperature region. This is in contrast to the previous observation that there are only two magnetization peaks in many superconductors. Detailed analysis finds that the second peak here evolves from a kinky structure at low temperatures and gets enhanced at high temperatures; the third peak evolves from a general broad peak at low temperatures and evolves into a sharp peak and even a step-like one at high temperatures. We propose a general phase diagram to interpret these peaks, the second peak is corresponding to the order-disorder transition, while the third peak is associated with the elastic-plastic crossover. Our work unifies the understanding of different "second peak" structures in different systems and thus sheds new light in understanding the vortex dynamics in type-II superconductors.

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arXiv2025

Preformed Cooper Pairs in a Triclinic Iron Pnictide Superconductor

Electron pairing along with phase coherence generates superconductivity below the critical temperature ($T_c$). In underdoped high-$T_c$ cuprates, these two quantum phenomena may occur at separate temperatures, which was lately confirmed in the quasi-two-dimensional (quasi-2D) iron chalcogenide superconductors. Here, we report a systematic investigation on the pre-pairing behavior in a triclinic iron pnictide superconductor (Ca$_{0.85}$La$_{0.15}$)$_{10}$(Pt$_3$As$_8$)(Fe$_2$As$_2$)$_5$ with $T_c \approx $ 30 K, where the superconductivity is quasi-2D manifested by the Berezinskii-Kosterlitz-Thouless behaviors. Inelastic neutron scattering experiments unambiguously reveal a spin resonance peak around $E_R =$ 13 meV in the superconducting state, but its intensity continuously decreases when warming up across $T_c$, accompanied with an anomaly around $T^{*}\approx$ 45 K in spin correlations, and a suppression by an in-plane magnetic field persisting to the same temperature. Below $T^{*}$, a significant Nernst signal and a reduction of density of states at the Fermi level are also observed. These results suggest that the precursor of spin resonance is highly related to the preformed Cooper pairs driven by phase fluctuations, much like the pseudogap case in cuprates.

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arXiv2024

Two Plaquette-Singlet Phases and Emergent SO(5) Deconfined Quantum Criticality in SrCu2(BO3)2

The deconfined quantum critical point (DQCP) has become a central open concept in the physics of quantum matter, and its proposed presence in the Shastry-Sutherland model was followed by the experimental observation of at least a minimal DQC scenario induced by an applied magnetic field in SrCu$_2$(BO$_3$)$_2$. However, the nature of the plaquette-singlet phase in SrCu$_2$(BO$_3$)$_2$ remains unresolved, and with it the identification of the DQCP symmetry from among several theoretical scenarios. Here we perform detailed high-pressure $^{11}$B NMR studies to reveal the presence of both the full-plaquette (FP) and empty-plaquette (EP) phases in SrCu$_2$(BO$_3$)$_2$, phase-separated at a first-order, pressure-driven transition with a volume-fraction effect. The field-driven transition from the EP to the antiferromagnetic (AFM) phase complements our previous observations of the FP--AFM transition, with both showing deconfined quantum criticality, while the scaling of the spin-lattice relaxation rate near the EP--AFM transition, $1/T_1 \propto T^{0.6}$, suggests a DQCP governed by a different universality class. We discuss possible extensions to the Shastry-Sutherland model that account for these pressure and field effects. The expanded phase space we discover mandates an SO(5) DQCP symmetry, and hence our results take an important step towards a complete understanding of deconfined quantum criticality in SrCu$_2$(BO$_3$)$_2$.

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arXiv2024

Necklace-like pattern of vortex bound states

Vortex is a topological defect in the superconducting condensate when a magnetic field is applied to a type-II superconductor, as elucidated by the Ginzburg-Landau theory. Due to the confinement of the quasiparticles by a vortex, it exhibits a circular shaped pattern of bound states with discrete energy levels, as predicted by the Caroli-de Gennes-Matricon theory in 1964. Here, however, we report a completely new type of vortex pattern which is necklace-like in an iron-based superconductor KCa2Fe4As4F2. Our theoretical analysis shows that this necklace-like vortex pattern arises from selective off-shell interference between vortex bound states of opposite angular momenta in the presence of rotational symmetry breaking due to disorders. This fascinating effect can be observed in a system with a small Fermi energy and wave vector, conditions fortuitously met in our samples. Our results not only disclose a novel vortex structure but also provide insights into comprehending the physics of the superconducting condensate.

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arXiv2024

Unprecedentedly large gap in HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ with the highest $T_c$ at ambient pressure

In cuprate superconductors, the highest $T_c$ is possessed by the HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg-1223) system at ambient pressure, but the reason remains elusive. Here we report the scanning tunneling measurements on the Hg-1223 single crystals with $T_c$ = 134 K. The observed gaps determined from the tunneling spectra (STS) can be categorized into two groups: the smaller gap $\Delta_1$ ranges from about 45 to 70 meV, while the larger gap $\Delta_2$ from about 65 to 98 meV. The STS was measured up to 200 K and the larger gap can persist well above $T_c$, indicating a pseudogap feature which may reflect the strong pairing energy in the inner layer. Interestingly, an extremely strong particle-hole asymmetry is observed in associating with a very robust coherence-like peak at the bias of the larger gap in the hole branch of the Bogoliubov dispersion. We argue that the observed asymmetry results may be from the interplay of a flat band (van Hove singularity) in the electronic spectrum and the large gap in the underdoped (inner) layer. A theoretical approach based on a trilayer model with an interlayer coupling can give a reasonable explanation. Our results provide deep insight into understanding the mechanism of superconductivity in cuprate superconductors.

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arXiv2024

Multiband superconductivity and a deep gap minimum from the specific heat in KCa$_2$(Fe$_{1-x}$Ni$_x$)$_4$As$_4$F$_2$ ($x$ = 0, 0.05, 0.13)

Specific heat can explore low-energy quasiparticle excitations of superconductors, so it is a powerful tool for bulk measurement on the superconducting gap structure and pairing symmetry. Here, we report an in-depth investigation on the specific heat of the multiband superconductors KCa$_2$(Fe$_{1-x}$Ni$_x$)$_4$As$_4$F$_2$ ($x$ = 0, 0.05, 0.13) single crystals and the overdoped non-superconducting one with $x$ = 0.17. Clear specific heat anomalies can be observed at the superconducting transition temperature of 33.6 K and 28.8 K for the samples with $x$ = 0 and $x$ = 0.05, respectively. For the two samples, the magnetic field induced specific heat coefficient $Δγ(H)$ in the low-temperature limit increases rapidly below 2 T, then it rises slowly above 2 T. Using the non-superconducting sample with $x$ = 0.17 as a reference, the specific heat of phonon background for various superconducting samples can be obtained and subtracted, which allows us to extract the electronic specific heat of the superconducting samples. Through comparative analyses, it is found that the energy gap structure including two $s$-wave gaps and an extended $s$-wave gap with large anisotropy can reasonably describe the electronic specific heat data. According to these results, we suggest that at least one anisotropic superconducting gap with a deep gap minimum should exist in this multiband system. With the doping of Ni, the superconducting transition temperature of the sample decreases along with the decrease of the large $s$-wave gap, but the extended $s$-wave gap increases due to the enlarged electron pockets via adding more electrons. Despite these changes, the general properties of the gap structure remain unchanged versus doping Ni.

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arXiv2023

Electronic Phase Propagation Speed in BaFe$_2$As$_2$ Revealed by Dilatometry

Thermal expansion offers deep insights into phase transitions in condensed matter physics. Utilizing an advanced AC-temperature dilatometer with picometer resolution, this study clearly resolves the antiferromagnetic and structural transition in BaFe$_2$As$_2$. The implementation of temperature oscillation reveals a hysteresis near the transition temperature $T_\mathrm{N}$ with unprecedented resolution. Unexpectedly, we find that the hysteretic width exhibits a universal dependence on the parameters of temperature oscillation and the sample's longidutinal dimension, which in turn reveals a finite transition speed. Our quantitative analysis shows that this phase boundary propagates at a mere 188 $\mu$m/s - a speed seven orders of magnitude slower than acoustic waves. It suggests a hidden thermodynamic constraint imposed by the electronic degrees of freedom. Our research not only sheds light on the dynamics of phase transitions between different correlated phases, but also establishes high precision dilatometry as a powerful tool for material studies. This measurement technique, when properly modified, can be extended to studies of other material properties such as piezoelectric, magneto-restriction, elastic modulus, etc.

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arXiv2023

Deconfined quantum critical point lost in pressurized SrCu2(BO3)2

In the field of correlated electron materials, the relation between the resonating spin singlet and antiferromagnetic states has long been an attractive topic for understanding of the interesting macroscopic quantum phenomena, such as the ones emerging from magnetic frustrated materials, antiferromagnets and high-temperature superconductors. SrCu2(BO3)2 is a well-known quantum magnet, and it is theoretically expected to be the candidate of correlated electron material for clarifying the existence of a pressure-induced deconfined quantum critical point (DQCP), featured by a continuous quantum phase transition, between the plaquette-singlet (PS) valence bond solid phase and the antiferromagnetic (AF) phase. However, the real nature of the transition is yet to be identified experimentally due to the technical challenge. Here we show the experimental results for the first time, through the state-of-the-art high-pressure heat capacity measurement, that the PS-AF phase transition of the pressurized SrCu2(BO3)2 at zero field is clearly a first-order one. Our result clarifies the more than two-decade long debates about this key issue, and resonates nicely with the recent quantum entanglement understanding that the theoretically predicted DQCPs in representative lattice models are actually a first-order transition. Intriguingly, we also find that the transition temperatures of the PS and AF phase meet at the same pressure-temperature point, which signifies a bi-critical point as those observed in Fe-based superconductor and heavy-fermion compound, and constitutes the first experimental discovery of the pressure-induced bi-critical point in frustrated magnets. Our results provide fresh information for understanding the evolution among different spin states of correlated electron materials under pressure.

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arXiv2022

Nodal s$_\pm$ Pairing Symmetry in an Iron-Based Superconductor with only Hole Pockets

The origin of the high temperature superconductivity in the iron-based superconductors remains elusive after being extensively studied for more than a decade. Determination of the pairing symmetry is essential in understanding the superconductivity mechanism. In the iron-based superconductors that have hole pockets around the Brillouin zone center and electron pockets around the zone corners, the pairing symmetry is generally considered to be s$_\pm$, endowing a sign change in the superconducting gap between the hole and electron pockets. For the iron-based superconductors with only hole pockets, however, a couple of pairing scenarios have been proposed but the exact symmetry is still highly controversial. Here we report our determination of the pairing symmetry in KFe$_2$As$_2$ which is a prototypical iron-based superconductor with hole pockets both around the zone center and around the zone corners. By taking laser-based angle resolved photoemission measurements with super-high resolution and at ultra-low temperature, we have precisely determined the superconducting gap distribution and identified the locations of the gap nodes on all the Fermi surface around the zone center and the zone corners. The complete superconducting gap structure, in combination with the observation of the spin resonance in neutron scattering, provides strong evidence on the s$_\pm$ pairing symmetry in KFe$_2$As$_2$ with a gap sign reversal between the hole pockets around the zone center and the hole pockets around the zone corners. These results unify the pairing symmetry in the hole-doped iron-based superconductors and point to the spin fluctuation as the pairing glue in generating superconductivity.

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arXiv2022

Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa$_2$Fe$_4$As$_4$F$_2$ bilayer pnictide superconductor

We performed magnetization measurements in a single crystal of the anisotropic bilayer pnictide superconductor KCa$_2$Fe$_4$As$_4$F$_2$, with $T_c$ $\simeq$ 34 K, for $H$$\parallel$$c$-axis and $H$$\parallel$$ab$-planes. A second magnetization peak (SMP) was observed in the isothermal $M(H)$ curves measured below 16 K for $H$$\parallel$$ab$-planes. A peak in the temperature variation of the critical current density, $J_{c}$($T$), at 16 K, strongly suggests the emergence of Josephson vortices at lower temperatures, which leads to the SMP in the sample. In addition, it is noticed that the appearance of Josephson vortices below 16 K renders easy magnetic flux penetration. A detailed vortex dynamics study suggests that the SMP can be explained in terms of elastic pinning to plastic pinning crossover. Furthermore, contrary to the common understanding, the temperature variation of the first peak field, $H_1$, below and above 16 K, behaves non-monotonically. A highly disordered vortex phase, governed by plastic pinning, has been observed between 17 K and 23 K, within a field region around an extremely large first peak field. Pinning force scaling suggests that the point defects are the dominant source of pinning for $H$ $\parallel$$ab$-planes, whereas, for H $\parallel$$c$-axis, point defects in addition to surface defects are at play. Such disorder contributes to the pinning due to the variation in charge carrier mean free path, $δ$$l$-pinning. Moreover, the large $J_c$ observed in our study is consistent with the literature, which advocates this material for high magnetic field applications.

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