Vortex Phase Transition Research Articles

Flux dynamics, anisotropy in Jc and vortex phase diagram of H+-intercalated FeSe single crystal

This paper investigates the superconductivity, pinning, and vortex dynamics of FeSe before and after protonation. The study shows that, after being protonated, FeSe's superconducting critical temperature Tc has been increased to more than 40 K and the critical current density Jc has also been increased from 2.1 × 104 A/cm2 to 1.2 × 106 A/cm2, with significantly larger magnetic relaxation rates (Q). Meanwhile, the critical current density anisotropy (γ) decreases substantially while the upper critical field Hc2 increases from 13 T to 91 T after protonation. In addition, the reasons for the absence of the second peak effect in Hx-FeSe were explored. This investigation also reveals the existence of the vortex phase transition associated with the pinning behavior in protonated FeSe, demonstrated by the foot-like characteristics of resistivity. Based on these results, the vortex phase diagrams of FeSe before and after protonation were established. These findings provide important insights for exploring new high-temperature superconductors and their magnetic flux dynamics through protonation.

Order–disorder phase transition and elastic-to-plastic vortex creep crossover in a triclinic iron pnictide superconductor (Ca_{0.85}La_{0.15})_{10}(Pt_3As_8)(Fe_2As_2)_5

Vortex matter in layered high-T_c superconductors, including iron-pnictides, undergo several thermodynamic phase transitions due to the complex interplay of pinning energy, thermal energy and elastic energy. Moreover, the presence of anisotropy makes their vortex physics even more intriguing. Here, we report a detailed vortex dynamics study, using dc magnetization measurements, in a triclinic iron-pnictide superconductor (Ca_{0.85}La_{0.15})_{10}(Pt_3As_8)(Fe_2As_2)_5, with a superconducting transition temperature, T_csim 31 K. A second magnetization peak (SMP) feature is observed for magnetic field perpendicular (Hparallelc) and parallel (Hparallelab) to the crystal plane. However, its fundamental origin is quite different in both directions. For Hparallelc, the SMP can be well explained using an elastic-to-plastic vortex creep crossover, using collective creep theory. In addition, a possible rhombic-to-square vortex lattice phase transition is also observed for fields in between the onset-field and peak-field related to the SMP. On the other hand, for Hparallelab, a clear signature of an order-disorder vortex phase transition is observed in the isothermal M(H) measurements at Tge 6 K. The disordered phase exhibits the characteristics of entangled pinned vortex-liquid. We construct a comprehensive vortex phase diagram by displaying characteristic temperatures and magnetic fields for both crystal geometries in this unique superconducting compound. Our study sheds light on the intricate vortex dynamics and pinning in an iron-pnictide superconductor with triclinic symmetry.

Vortex Phase Dynamics in Yttrium Superhydride YH6 at Megabar Pressures.

A comprehensive study of vortex phases and vortex dynamics is presented for a recently discovered high-temperature superconductor YH6 with Tc(onset) of 215 K under a pressure of 200 GPa. The thermal activation energy (U0) is derived within the framework of the thermally activated flux flow (TAFF) theory. The activation energy yields a power law dependence U0 ∝ Hα on magnetic field with a possible crossover at a field around 8-10 T. Furthermore, we have depicted the vortex phase transition from the vortex-glass to vortex-liquid state according to the vortex-glass theory. Finally, vortex phase diagram is constructed for the first time for superhydrides. Very high estimated values of flux flow barriers U0(H) = (1.5-7) × 104 K together with high crossover fields make YH6 a rather outstanding superconductor as compared to most cuprates and iron-based systems. The Ginzburg number for YH6 Gi = (3-7) × 10-3 indicates that thermal fluctuations are not so strong and cannot broaden superconducting transitions in weak magnetic fields.

Significant enhancement of critical current density in H+-intercalated FeSe single crystal

Superconducting transition temperature (T c) and critical current density (J c) are two key factors that are not only crucial for probing high temperature superconducting mechanism, but also for practical applications. The simple crystal structure of FeSe is very favorable for the fabrication of thin films and wires, but its application is limited by the relatively low T c and small J c. Previous studies have found that the T c of FeSe can be significantly enhanced over 40 K by using protonation method. Here, we present a systematic study of J c and vortex properties of H+-intercalated FeSe (H x -FeSe) single crystals. The value of J c for H x -FeSe single crystal is significantly enhanced, exceeding 1.3 × 106 A cm−2 at 4 K, which is more than two orders of magnitude larger than 1.1 × 104 A cm−2 of pristine FeSe. The vortex pinning mechanism of H x -FeSe is found to be surface pinning, which is different from the dominant strong point-like pinning in pristine FeSe. Moreover, the systematic study of the vortex phase transition and the underlying mechanism provides a wealth of information for the vortex phase diagram of H x -FeSe single crystal. Our results confirm that the introduction of H+ intercalations into FeSe not only enhance the T c but also significantly increases the value of J c, which is favorable for practical applications.

Chiral Majorana edge modes and vortex Majorana zero modes in superconducting antiferromagnetic topological insulator

The antiferromagnetic topological insulator (AFTI) is topologically protected by the combined time-reversal and translational symmetry . In this paper we investigate the effects of the s-wave superconducting pairings on the multilayers of AFTI, which breaks symmetry and can realize quantum anomalous Hall insulator with unit Chern number. For the weakly coupled pairings, the system corresponds to the topological superconductor (TSC) with the Chern number C = ±2. We answer the following questions whether the local Chern numbers and chiral Majorana edge modes of such a TSC distribute around the surface layers. By the numerical calculations based on a theoretic model of AFTI, we find that when the local Chern numbers are always dominated by the surface layers, the wavefunctions of chiral Majorana edge modes must not localize on the surface layers and show a smooth crossover from spatially occupying all layers to only distributing near the surface layers, similar to the hinge states in a three dimensional second-order topological phases. The latter phase, denoted by the hinged TSC, can be distinguished from the former phase by the measurements of the local density of state. In addition we also study the superconducting vortex phase transition in this system and find that the exchange field in the AFTI not only enlarges the phase space of topological vortex phase but also enhances its topological stability. These conclusions will stimulate the investigations on superconducting effects of AFTI and drive the studies on chiral Majorana edge modes and vortex Majorana zero modes into a new era.

Suppression of vortex-induced vibration of a cactus-inspired cylinder near a free surface

Bionic ribs inspired by those of a cactus are adopted in order to suppress the vortex-induced vibration (VIV) responses of cylinders located near free surfaces. The VIV responses of cactus-inspired cylinders located near free surface are investigated numerically. The range of the Reynolds number is 8.0 × 103 ≤ Re ≤ 5.6 × 104, and the range of the Froude number is 0.08 ≤ Fr ≤ 0.56. The distance between the cylinder and the free surface is 2D (D is diameter of the cylinder), and the cactus-inspired ribs with height ratios (Ks/D, where Ks is the height of the ribs) include 0.0 (M1), 0.025 (M2), 0.05 (M3), and 0.1 (M4). The VIV response of a smooth cylinder (M0) far from a free surface is used as a reference. Different ribs with height ratios have different effects on the VIV responses of cylinders located near free surfaces. The suppression effect of the bionic ribs on the VIV response is largest when Ks/D = 0.1 (M4). The maximum cross-flow amplitude ratio, the maximum in-line amplitude ratio, and the maximum mean drag coefficient of the cactus-inspired cylinder (M4) are 53.57%, 53.13%, and 49.21% lower than that of the smooth cylinder (M1). The cactus-inspired cylinders are easier to desynchronize from the lock-in region. The free surface causes the vortex phase transition to happen earlier. The free surface has a better suppression effect on VIV response in the high Froude number range. When the rib height ratio is small (M2, Ks/D = 0.025), the free surface enhances the VIV response. However, the VIV response is mainly affected by the ribs when the rib height ratio exceeds a certain value (Ks/D ≥ 0.05). When the cactus-inspired rib with height ratio increases, the critical Reynolds number of the vortex phase transition decreases. The free surface and the cactus-inspired ribs are conducive to dissipation of wakes and lead to decreases in the wake turbulence intensity. Because the cactus-inspired ribs can cause the separation line to develop away from the cylinder wall and effectively control the wake intensity, the VIV response of the cactus-inspired cylinder is suppressed effectively. For the cactus-inspired cylinder, galloping responses can be avoided in high-Reynolds-number range, and boundary-layer separation occurs at the top of the ribs. Unlike the boundary-layer separation process of the smooth cylinder (M1), the ribs of the cactus-inspired cylinder destroy the boundary layer, causing smaller vortices to be produced.

Double Majorana vortex zero modes in superconducting topological crystalline insulators with surface rotation anomaly

The interplay of time-reversal and $n$-fold rotation symmetries ($n=2,4,6$) is known to bring a new class of topological crystalline insulators (TCIs) having $n$ surface Dirac cones due to surface rotation anomaly. We show that the proximity-induced $s$-wave superconductivity on the surface of these TCIs yields a topological superconducting phase in which two Majorana zero modes are bound to a vortex, and that $n$-fold rotation symmetry ($n=2,4,6$) enriches the topological classification of a superconducting vortex from $\mathbb{Z}_2$ to $\mathbb{Z}_2\times\mathbb{Z}_2$. Using a model of a three-dimensional high-spin topological insulator with $s$-wave superconductivity and two-fold rotation symmetry, we show that, with increasing chemical potential, the number of Majorana zero modes at one end of a vortex changes as $2\to1\to0$ through two topological vortex phase transitions. In addition, we show that additional magnetic-mirror symmetry further enhances the topological classification to $\mathbb{Z} \times \mathbb{Z}$

Vortex phase diagram in 12442-type RbCa2Fe4As4F2 single crystal revealed by magneto-transport and magnetization measurements

The recently discovered 12442-type ACa2Fe4As4F2 (A = K, Rb, Cs) compounds are the only iron-based superconductors (IBSs) with double FeAs layers between neighboring insulating layers, analogous to the double CuO2 layers in some high-T c cuprates. Here, we report the study of vortex phase diagram of RbCa2Fe4As4F2 single crystal via magneto-transport and magnetization measurements. The resistive transition under magnetic fields shows a foot-like kink at a characteristic temperature, T s, followed by a resistive tail in nearly zero resistivity region. Such behavior is ascribed to a vortex slush transition at T s, below which the vortex state has short-range vortex lattice correlation, and then a second-order transition into the vortex glass phase occurs with further decreasing temperature. Above T s, the Arrhenius plot of resistivity shows two linear regions that are separated by a crossover line T cr(B), which is associated with a crossover from collective to plastic pinning or different flux pinning behaviors resulted from different types of defect. In addition, the magnetic hysteresis loops reveal a second magnetization peak (SMP), which is shifted to lower fields with increasing temperature for T< 12 K. However, the SMP unexpectedly moves back to a higher field at T= 12 K, and then gradually turns into a shoulder or kink that moves to higher fields at high temperatures, such anomalous behavior has never been observed in IBSs. According to the magneto-transport and magnetization data, the vortex phase diagram of RbCa2Fe4As4F2 is finally constructed. Details on the different vortex phase transitions and relevant physical scenarios are given and discussed.

Vortex and Surface Phase Transitions in Superconducting Higher-order Topological Insulators

Topological insulators, having intrinsic or proximity-coupled s-wave superconductivity, host Majorana zero modes (MZMs) at the ends of vortex lines. The MZMs survive up to a critical doping of the TI at which there is a vortex phase transition that eliminates the MZMs. In this work, we show that the phenomenology in higher-order topological insulators (HOTIs) can be qualitatively distinct. In particular, we find two distinct features. (i)We find that vortices placed on the gapped (side) surfaces of the HOTI, exhibit a pair of phase transitions as a function of doping. The first transition is a surface phase transition after which MZMs appear. The second transition is the well-known vortex phase transition. We find that the surface transition appears because of the competition between the superconducting gap and the local T-breaking gap on the surface. (ii)We present numerical evidence that shows strong variation of the critical doping for the vortex phase transition as the center of the vortex is moved toward or away from the hinges of the sample. We believe our work provides new phenomenology that can help identify HOTIs, as well as illustrating a promising platform for the realization of MZMs.

Effect of Dzyaloshinskii-Moriya interaction on magnetic vortex switching driven by radial spin waves

We theoretically investigate the radial-spin-wave induced magnetic vortex switching in the presence of Dzyaloshinskii-Moriya interaction (DMI). From micromagnetic simulations, we observe a circular-to-radial vortex phase transition by increasing the DMI strength. The radial spin-wave excitation spectrum for each magnetization configuration is analyzed, showing that the frequency of spin-wave mode with a given radial node number monotonically increases (decreases) with the DMI parameter of the radial (circular) vortex. Interestingly, we find that the DMI can significantly facilitate the polarity switching of the circular vortex driven by radial spin waves. Our work provides a new insight into the DMI effect on the vortex dynamics and is helpful for designing fast all-magnonic memory devices.

Hexagonal warping effect on Majorana zero modes at the ends of superconducting vortex lines in doped strong 3D topological insulators

In a superconducting topological insulator, superconducting vortex line may trap 1-dimensional topological band with Majorana zero modes (MZMs) localized at its ends. In this work, we study the effect of hexagonal warping to the vortex phase transition. We carry out both analytical calculations based on a semiclassical formula and numerical calculations based on full quantum mechanics of Bogliubov de Gennes equation. We find that the hexagonal warping term enlarges the topological region of the vortex line as the chemical potential changes, and leads to the MZMs even in the absence of topological surface states.

Fulde\u2013Ferrell\u2013Larkin\u2013Ovchinnikov and vortex phases in a layered organic superconductor

Superconductivity is one of the most intriguing topics in solid state physics. Generally, the superconducting Cooper pairs are broken by the Zeeman effect, which gives the so-called Pauli paramagnetic limit HPauli. However, when the superconductivity is in the clean limit and the orbital effect is strongly quenched, the Cooper pairs can survive even above HPauli, which is the so-called Fulde and Ferrell, and Larkin and Ovchinnikov (FFLO) phase. Extensive efforts have been devoted to the discovery of the FFLO phase. However, vortex phase transitions have given rise to considerable ambiguity in the interpretation of the experimental data. Here, we report comprehensive magnetocaloric and torque studies of the FFLO phase transition in a highly two-dimensional organic superconductor. We observe the FFLO phase transition clearly distinct from vortex melting transitions. The phase diagram provides crucial information on the stability of the FFLO phase in magnetic fields.

Entropy jump at the first-order vortex phase transition in Bi2Sr2CaCu2O8+δ with columnar defects

Abstract We study the entropy jump associated with the first-order vortex melting transition (FOT) in Bi2Sr2CaCu2O8+δ crystals by means of Hall probe magnetometry. The samples present a diluted distribution of columnar defects (CD) introduced by irradiation with Xe ions. The FOT is detected in ac transmittivity measurements as a paramagnetic peak, the height of which is proportional to the enthalpy difference entailed by the transition. By applying the Clausius-Clapeyron relation, we quantify the evolution of the entropy jump Δs as a function of the FOT temperature, TFOT, in both pristine crystals and crystals with CD. On increasing the density of CD, Δs decreases monotonically with respect to values found in pristine samples. The Δs versus TFOT dependence in the case of pristine samples follows reasonably well the theoretical prediction of dominant electromagnetic coupling for a model neglecting the effect of disorder. The data for samples with a diluted distribution of CD are not properly described by such a theoretical model.

Thermally activated flux flow, vortex-glass phase transition and the mixed-state Hall effect in 112-type iron pnictide superconductors

The transport properties in the mixed state of high-quality Ca0.8La0.2Fe0.98Co0.02As2 single crystal, a newly discovered 112-type iron pnictide superconductor, are comprehensively studied by magneto-resistivity measurement. The field-dependent activation energy, U0, is derived in the framework of thermally activated flux flow (TAFF) theory, yielding a power law dependence U0~Hα with a crossover at a magnetic field around 2 T in both H⊥ab and H//ab, which is ascribed to the different pinning mechanisms. Moreover, we have clearly observed the vortex phase transition from vortex-glass to vortex-liquid according to the vortex-glass model, and vortex phase diagrams are constructed for both H⊥ab and H//ab. Finally, the results of mixed-state Hall effect show that no sign reversal of transverse resistivity ρxy(H) is detected, indicating that the Hall component arising from the vortex flow is also negative based on the time dependent Ginzburg-Landau (TDGL) theory. Meanwhile, the transverse resistivity ρxy(H) and the longitudinal resistivity ρxx(H) follow the relation |ρxy(H)|=Aρxx(H)β well with an exponent β~2.0, which is in line with the results in theories or experiments previously reported on some high-Tc cuprates.

Vortex phase transition and superconducting properties in quasi-two-dimensional k-(BEDT-TTF)2Cu[N(CN)2]Br organic superconductor

We report investigations of the low temperature dc susceptibility and the magnetization on the layered organic superconductor k-(BEDT-TTF)2Cu[N(CN)2]Br near 80 K and the effect of disorder on the superconducting transition temperature Tc. The shielding effect (S) and the critical current density Jc were studied (with H parallel to the c axis of the crystal). Jc can be estimated by analysis of magnetic hysteresis measurement using the Bean model. For each temperature value, we observed two regimes in the critical current density Jc(H). Our results show that the magnetic properties of these compounds depend strongly on the cooling rate. The structural transformation which occurs at the vicinity of 80 K very strongly influences the physics of vortex lattice and the associated magnetic behavior. #organic_superconductor #critical_current #shielding_effect #magnetic_susceptibility #vortex_pinning

Vortex phase transition and anisotropy behavior of optimized (Li1−xFexOH)FeSe single crystals

(Li1−xFexOH)FeSe single crystals have been successfully synthesized by a hydrothermal ion-exchange technique. We have carried out systematic research on the influence of the concentration of selenourea (cm) on the superconducting properties of (Li1−xFexOH)FeSe single crystals. The optimized specimens possess an onset superconducting transition temperature Tc of up to 42 K obtained at cm = 0.8 mol l−1, and the corresponding residual resistivity ratio (RRR = ρ (300 K)/ρ (Tc)) is estimated to be 11. The vortex liquid-to-glass phase transition has been discussed based on the collective pinning model. The large distance between the Hg and Hc2 lines, as well as the much narrower region of the pinned vortex-liquid phase, indicate a weak vortex-pinning ability in (Li1−xFexOH)FeSe single crystals. The big out-plane anisotropy behavior versus temperature has been explored according to the anisotropic Ginzburg–Landau theory, and the anisotropy factor Γ displays linear behavior towards temperature similar to that of SmFeAsO1−xFx superconductors.

Magnification of signatures of a topological phase transition by quantum zero point motion

In this letter we show that the zero-point motion of the vortex in superconducting doped topological insulators leads to significant changes in the electronic spectrum at the topological phase transition in this system. This topological phase transition is tuned by the doping level and the corresponding effects manifest in the density of states at energies which are of the order of the fluctuations frequency. This frequency might be much larger than the electronic energy gap in the spectrum generated by a stationary vortex. As a result the quantum zero-point motion can move the spectral signature of the topological vortex phase transition to energies which are well within the resolution of scanning tunneling microscopy. Moreover, the phenomena studied in this letter present novel effects of Magnus force on the vortex spectrum which are not present in the ordinary s-wave superconductors. Our results show that quantum zero point fluctuations can bring the fingerprints of topological phase transitions to more experimentally accessible grounds and point to the importance of study of quantum fluctuations in different candidates for realizing topological phase transitions.

Angle-resolved vortex glass transition and pinning properties in BaFe1.8Co0.2As2 single crystals

The vortex phase transition in BaFe1.8Co0.2As2 single crystals was studied systematically via measuring the current–voltage (I-V) characteristics by rotating the crystals in magnetic fields from H // c to H // ab. The angle-resolved I-V curves clearly show a three-dimensional vortex glass (VG) scaling collapse. The angular dependent VG transition field Hg and critical current density Jc can be approximately described by the anisotropic Ginzburg-Landau model, and no peaks representing c-axis correlated pinning were recorded for H // c, which reveals that the vortex pinning is mainly caused by randomly distributed uncorrelated-defects. While in the vicinity of H // ab, Jc deviates from the model due to the in-plane intrinsic pinning. Based on the above results, the angle-resolved vortex phase diagram is constructed, indicating the anisotropic superconducting properties of the system.

Meissner to vortex phase transition in a two-leg ladder in artificial gauge field

We consider a two-leg boson ladder in artificial gauge field with hard-core intraleg and negligible interleg interactions. Using numerical simulations based on the Density Matrix Renormalization Group (DMRG) algorithm, combined with a bosonization approach, we study its commensurate-incommensurate transition to a vortex phase at a critical flux. We discuss the finite-size scaling behavior of the longitudinal current near the transition. For weak interchain boson hopping, the finite size scaling is in agreement with the predictions from bosonization.

Effect of injected spins with different polarized orientations on the vortex phase transition in La0.7Sr0.3MnO3/La1.85Sr0.15CuO4 heterostructure

The current-voltage (I-V) characteristics with spin injection were investigated for the epitaxial La0.7Sr0.3MnO3/La1.85Sr0.15CuO4 heterostructure rotated from H//c to H//ab in magnetic fields up to 14 T. It is found that all the I-V curves in various magnetic fields can be scaled with a three dimensional (3D) vortex glass model, and the spin injection can induce a better 3D scaling behavior, which is closely related to the decrease of the anisotropy parameter. A vortex phase diagram for the evolution of vortex glass transition field (Hg) and upper critical field (Hc2) indicates that both Hg and Hc2 are suppressed by spin injection, and this effect becomes more obvious in the case of H//ab, which probably originates from the different suppression on the superconducting pairing strength by different injected spins' orientations.