Parallel Critical Field Research Articles

Unusual superconductivity in the topological nodal-line semimetal candidate Sn x NbSe2-δ

We report the superconductivity of the topological nodal-line semimetal candidate Sn x NbSe2-δ with a noncentrosymmetric crystal structure. The superconducting transition temperature T c of Sn x NbSe2-δ drastically varies with the Sn concentration x and the Se deficiency δ, and reaches 12 K, relatively higher than those of known topological superconductors. The upper critical field of this compound shows unusual temperature dependence, inconsistent with the WHH theory for conventional type-II superconductors. In a low-Tc sample, the zero-temperature limit of the upper critical field parallel to the ab plane exceeds the Pauli paramagnetic limit estimated from the simple BCS weak coupling model by a factor of ∼ 2, suggestive of unusual superconductivity stabilized in Sn x NbSe2-δ . Together with the robust superconductivity against disorder, these observations indicate that Sn x NbSe2-δ is a promising candidate to explore topological superconductivity.

Numerical study of PbTe-Pb hybrid nanowires for engineering Majorana zero modes

Epitaxial semiconductor-superconductor (SM-SC) hybrid nanowires are potential candidates for implementing Majorana qubits. Recent experimental and theoretical works show that charged impurities in SM remain a major problem in all existing hybrid nanowires, in which the SM is either InAs or InSb while the SC is mainly Al. Here, we theoretically validate the recently proposed PbTe-Pb hybrid nanowire as a potential candidate for Majorana devices. By studying the electrostatic and electronic properties of PbTe nanowires, we demonstrate that the huge dielectric constant of PbTe endows itself a high tolerance of charged impurity, which is a potential advantage over InAs and InSb nanowires. Moreover, we find that the effective axial Land\'e $g$ factor and Rashba spin-orbit coupling strength of PbTe nanowires are comparable to those of InAs nanowires. The conceivable merits of using Pb as the SC are (i) Pb has a larger superconducting gap, higher critical temperature, and higher parallel critical magnetic field than those of Al; (ii) a superconducting gap comparable with those of InAs-Al and InSb-Al can be induced in PbTe-Pb even by a weak coupling between Pb and PbTe, which simultaneously relieves the adverse renormalization and induced disorder effects on SM from SC; and (iii) Pb film can be grown on PbTe with a thin buffer CdTe layer in between, solving the lattice mismatch problem as an important source of disorder. In the presence of a parallel magnetic field, we show that the typical BdG energy spectrum and tunneling spectroscopy of PbTe-Pb resemble those of InAs and InSb based hybrid nanowires exposed to a tilting magnetic field, as a result of the highly anisotropic Land\'e $g$ factors of PbTe nanowires. The calculated topological phase diagrams of PbTe-Pb indicate that the multivalley character of PbTe makes it easier than InAs and InSb to access topological superconducting phases. Our results could facilitate the experimental realization of PbTe-Pb hybrid nanowires and inspire further theoretical works.

Transition from three- to two-dimensional Ising superconductivity in few-layer NbSe2 by proximity effect from van der Waals heterostacking

We report the experimental observation of Ising superconductivity in 3-dimensional NbSe2 stacked with single-layer MoS2. The angular dependence of the upper critical magnetic field and the temperature dependence of the upper parallel critical field confirm the appearance of two-dimensional Ising superconductivity in the 3-dimensional NbSe2 with single-layer MoS2 overlay. We show that the superconducting phase has strong Ising spin-orbit correlations which make the holes spin non-degenerate. Our observation of Ising superconductivity in heterostructures of few-layer NbSe2 of thickness ~ 15 nm with single-layer MoS2 raises the interesting prospect of observing topological chiral superconductors with nontrivial Chern numbers in a momentum-space spin-split fermionic system.

Recent Advances in 2D Superconductors.

The emergence of superconductivity in 2D materials has attracted much attention and there has been rapid development in recent years because of their fruitful physical properties, such as high transition temperature (Tc ), continuous phase transition, and enhanced parallel critical magnetic field (Bc ). Tremendous efforts have been devoted to exploring different physical parameters to figure out the mechanisms behind the unexpected superconductivity phenomena, including adjusting the thickness of samples, fabricating various heterostructures, tuning the carrier density by electric field and chemical doping, and so on. Here, different types of 2D superconductivity with their unique characteristics are introduced, including the conventional Bardeen-Cooper-Schrieffer superconductivity in ultrathin films, high-Tc superconductivity in Fe-based and Cu-based 2D superconductors, unconventional superconductivity in newly discovered twist-angle bilayer graphene, superconductivity with enhanced Bc , and topological superconductivity. A perspective toward this field is then proposed based on academic knowledge from the recently reported literature. The aim is to provide researchers with a clear and comprehensive understanding about the newly developed 2D superconductivity and promote the development of this field much further.

Time-Dependent Ginzburg–Landau Simulations of the Critical Current in Superconducting Films and Junctions in Magnetic Fields

Understanding the magnetic field dependence of the critical current density (Jc) of superconductors is of considerable interest for optimizing their use in high field applications. Using time-dependent Ginzburg-Landau theory, we have completed simulations of the average electric field generated in thin film systems subject to transport currents in applied magnetic fields, and compared them to thin film systems containing narrow junctions of reduced critical temperature (Tc). For thin films in contact with insulating surfaces, Jc approaches the depairing current density at applied magnetic fields below the initial vortex penetration field and remains nonzero until close to Tinkham's parallel critical field. For thin films in contact with highly metallic surfaces, Jc was found to decrease to zero with decreasing film width. Adding a junction region to the film was found to broaden the transition to the normal state at all applied magnetic fields and reduce Jc of the film at zero field.

Parallel Critical Field in Thin Niobium Films: Comparison to Theory

For the first time, a comparison to the predicted behavior for parallel critical field is carried out for the model of Kogan and the model of Hara and Nagai. In this study, thin niobium films in the moderately dirty regime were considered. Experimental values of the $-C_{2}$ term are seen to be lower than those from the model of Hara and Nagai. A possible reason for this could be not including the non spherical Fermi surface of niobium into the model. There is clearly disagreement with the model of Kogan as the films get cleaner and thinner, and two films which should be below his critical thickness still show positive values of $-C_{2}$, in disagreement with his theory.

Zeeman-limited superconductivity in crystalline Al films

We report the evolution of the Zeeman-mediated superconducting phase diagram (PD) in ultrathin crystalline Al films. Parallel critical field measurements, down to 50 mK, were made across the superconducting tricritical point of films ranging in thickness from 7 to 30 ML. The resulting phase boundaries were compared with the quasiclassical theory of a Zeeman-mediated transition between a homogeneous BCS condensate and a spin-polarized Fermi liquid. Films thicker than $\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ showed good agreement with theory, but thinner films exhibited an anomalous PD that cannot be reconciled within a homogeneous BCS framework.

Ultrathin two-dimensional superconductivity with strong spin–orbit coupling

We report on a study of epitaxially grown ultrathin Pb films that are only a few atoms thick and have parallel critical magnetic fields much higher than the expected limit set by the interaction of electron spins with a magnetic field, that is, the Clogston-Chandrasekhar limit. The epitaxial thin films are classified as dirty-limit superconductors because their mean-free paths, which are limited by surface scattering, are smaller than their superconducting coherence lengths. The uniformity of superconductivity in these thin films is established by comparing scanning tunneling spectroscopy, scanning superconducting quantum interference device (SQUID) magnetometry, double-coil mutual inductance, and magneto-transport, data that provide average superfluid rigidity on length scales covering the range from microscopic to macroscopic. We argue that the survival of superconductivity at Zeeman energies much larger than the superconducting gap can be understood only as the consequence of strong spin-orbit coupling that, together with substrate-induced inversion-symmetry breaking, produces spin splitting in the normal-state energy bands that is much larger than the superconductor's energy gap.

Anisotropy reversal of the upper critical field at low temperatures and spin-locked superconductivity inK2Cr3As3

We report measurements of the anisotropic upper critical field ${H}_{c2}(T)$ for ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ single crystals up to 60 T and $Tg0.6\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Our results show that the upper critical field parallel to the Cr chains, ${H}_{c2}^{\ensuremath{\parallel}}(T)$, exhibits a paramagnetically limited behavior, whereas the shape of the ${H}_{c2}^{\ensuremath{\perp}}(T)$ curve (perpendicular to the Cr chains) has no evidence of paramagnetic effects. As a result, the curves ${H}_{c2}^{\ensuremath{\perp}}(T)$ and ${H}_{c2}^{\ensuremath{\parallel}}(T)$ cross at $T\ensuremath{\approx}4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, so that the anisotropy parameter ${\ensuremath{\gamma}}_{H}(T)={H}_{c2}^{\ensuremath{\perp}}/{H}_{c2}^{\ensuremath{\parallel}}(T)$ increases from ${\ensuremath{\gamma}}_{H}({T}_{c})\ensuremath{\approx}0.35$ near ${T}_{c}$ to ${\ensuremath{\gamma}}_{H}(0)\ensuremath{\approx}1.7$ at 0.6 K. The paramagnetically limited behavior of ${H}_{c2}^{\ensuremath{\parallel}}(T)$ is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains.

Asymmetric Avalanche Behavior in a Zeeman-Limited Superconductor

We have performed transport and tunneling density of states measurements of ultra-thin Al films through the first-order parallel critical field transition. The transition is intrinsically hysteretic and exhibits avalanche-like jumps in both resistivity and tunneling density states. Tunneling measurements on films with sheet resistances of a few hundred ohms show large avalanche-like collapses of the condensate on the superheating branch of the critical field hysteresis loop. In contrast, the transition back into the superconducting phase (i.e., along the supercooling branch) is always continuous.

Fabrication and properties of the meander nanowires based on ultra-thin Nb films

We report the fabrication and the study of superconducting properties of ultra-thin Nb superconducting meander nanowires, which can be used as superconducting nanowire single-photon detectors (SNSPDs). The ultra-thin (about 7-nm thick) Nb films are patterned into micro-bridges, and 100-nm wide meander nanowires by using e-beam lithography (EBL). The average transition temperature (Tc) of the nanowires is about 4.8 K and the critical current density jc is about 2.8 × 106 A/cm2. Superconducting characteristics of the specimens at different applied magnetic fields up to 8 T (parallel or perpendicular to the specimen) are systematically investigated. The normalized temperature t (= T/Tc) dependences of the parallel critical field (Hc‖) for both the micro-bridge and the meander nanowire are almost the same, following the Ginzburg and Landau (GL) formalism for ultra-thin films. However, in perpendicular field and in the vicinity of Tc (> 0.95Tc), the critical field Hc‖ of the nanowire exhibits a down-turn curvature nonlinear temperature dependence while the micro-bridge displays a linear temperature dependence. The nonlinear behavior of Hc⊥ in the nanowire is believed to be due to the fact that in the vicinity of Tc the coherence length becomes larger than the line width. Additionally, the localization of carriers in the nanowire could also contribute to the nonlinear behavior. The resistive transitions could be described by the phase-slip model for quasi-one-dimensional system. Moreover, the hysteresis in I–V curve of the meander nanowires can be illustrated by a simple model of localized normal hotspot maintained by Joule heating.

Two-dimensional superconductivity in the layered organic superconductorκH-(DMEDO-TSeF)2[Au(CN)4](THF) with thick dielectric insulating layers

We have systematically investigated the resistive superconducting transition in the layered organic superconductor ${\ensuremath{\kappa}}_{\mathrm{H}}$-(DMEDO-TSeF)${}_{2}$[Au(CN)${}_{4}$](THF) [where DMEDO-TSeF is dimethyl(ethylenedioxy)tetraselenafulvalene and THF is tetrahydrofuran], which consists of two crystallographically independent conducting layers and one independent thick dielectric insulating layer. Applying a slight pressure of up to 0.35 GPa suppresses the superconducting phase. The angular dependence of the upper critical field and the short interlayer coherence length indicate that the present compound is a highly two-dimensional superconductor. The upper critical field parallel to the conducting layers exceeds the Pauli paramagnetic limit. These superconducting properties are consistent with the crystal structure in which the superconducting layers are well separated by a thick anion insulating layer.

Superconductivity of 80NbN–20SiO2 granular films

The feasibility of creating granular superconducting films of NbN–SiO2 with a controlled grain size by a sol-gel method is demonstrated. A comprehensive study is made of the structural and transport properties of granular films of 80%NbN–20%SiO2 with different thicknesses. It is found that production of a complete superconducting transition requires that the samples have thicknesses greater than 750nm. The critical temperatures for a superconducting transition and the upper critical magnetic fields are roughly constant for films with different thicknesses and equal 4.5K and 4.4T, respectively. A crossover from 2D to 3D behavior is discovered in the temperature variation of the upper parallel critical field. It is shown that at low magnetic fields the resistive transitions obey an Arrhenius law. The mechanism for broadening of resistive transitions in magnetic fields is most likely magnetic flux creep. The magnetic field dependence of the activation energy is obtained. Typical initial signs of a magnetical...

Upper critical fields and critical current density of single crystal

The transport properties, upper critical fields, superconducting anisotropy, and critical current density in an iso-valent phosphorus-doped BaFe2As2 single crystal close to optimum doping are investigated in this report. Temperature dependent resistivity and susceptibility show a superconducting transition temperature, Tc, just below 31 K both with sharp transitions. The upper critical field parallel to the ab-plane, Hc2^ab, is above 77 Tesla while that along the c-axis direction, Hc2^c, is just above 36 Tesla, yielding a low superconducting anisotropy ratio ~ 2. The estimated inter-plane coherence length based on the Ginzburg-Landau (G-L) theory indicates BaFe2(As0.68P0.32)2 is still above the point for a dimensional crossover inferring the superconducting layers are not weakly-coupled in this system. The critical current density at 5 K obtained from magnetization loops measurement show a modest Jc as high as 10^5 A/cm2.

Fermi-liquid effects in the gapless state of marginally thin superconducting films

We present low-temperature tunneling density-of-states measurements in Al films in high parallel magnetic fields. The thickness range of the films, $t=6--9\text{ }\text{nm}$, was chosen so that the orbital and Zeeman contributions to their parallel critical fields were comparable. In this quasispin paramagnetically limited configuration, the field produces a significant suppression of the gap, and at high fields the gapless state is reached. By comparing measured and calculated tunneling spectra we are able to extract the value of the antisymmetric Fermi-liquid parameter ${G}^{0}$ and thereby deduce the quasiparticle density dependence of the effective parameter ${G}_{\text{eff}}^{0}$ across the gapless state.

Upper critical field of electron-dopedPr2−xCexCuO4−δin parallel magnetic fields

We report a systematic study of the resistive superconducting transition in the electron-doped cuprates Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta}$ down to 1.5 K for magnetic field up to 58 T applied parallel to the conducting ab-planes. We find that the zero temperature parallel critical field (H$_{c2\parallel ab}$(0)) exceeds 58 T for the underdoped and optimally-doped films. For the overdoped films, 58 T is sufficient to suppress the superconductivity. We also find that the Zeeman energy $\mu_B$H$_{c2\parallel ab}$(0) reaches the superconducting gap ($\triangle_0$), i.e. $\mu_B$H$_{c2\parallel ab}(0)\simeq \triangle_0$, for all the dopings, strongly suggesting that the parallel critical field is determined by the Pauli paramagnetic limit in electron-doped cuprates.

Coupling phenomena and vortex transitions in superconducting Ni/Nb multilayers

We have studied some structural and superconducting properties of Ni/Nb multilayered films prepared by magnetron sputtering. Magnetization measurements allowed us to establish a rich H × T diagram that indicates a 3D-2D dimensional crossover of the vortex system, as revealed by the parallel upper critical field behavior. Consistently, it was identified a possible decoupling line associated with the transition of Abrikosov vortex lines into vortex pancakes, starting around the same crossover region. An irreversibility line was also determined and it suggests different regimes of the vortex matter, where the role played by the Ni ferromagnetic layers might be relevant.

Density of states, entropy, and the superconducting Pomeranchuk effect in Pauli-limited Al films

We present low temperature tunneling density of states measurements of Pauli-limited Al films in which the Zeeman and orbital contributions to the critical field are comparable. We show that films in the thickness range of 6-7 nm exhibit a reentrant parallel critical field transition which is associated with a high entropy superconducting phase, similar to the high entropy solid phase of 3He responsible for the Pomeranchuk effect. This phase is characterized by an excess of states near the Fermi energy so long as the parallel critical field transition remains second order. Theoretical fits to the zero bias tunneling conductance are in good agreement with the data well below the transition but theory deviates significantly near the transition. The discrepancy is a consequence of the emergence of e-e interaction correlations as one enters the normal state.

Electrostatic and parallel-magnetic-field tuned two-dimensional superconductor-insulator transitions

The two-dimensional superconductor-insulator transition in disordered ultrathin amorphous bismuth films has been tuned both by electrostatic electron doping using the electric field effect and by the application of parallel magnetic fields. Electrostatic doping was carried out in zero and nonzero magnetic fields, and magnetic tuning was conducted at multiple strengths of electrostatically induced superconductivity. The various transitions were analyzed using finite size scaling to determine their critical exponent products. For the electrostatically tuned transition, the exponent product $\ensuremath{\nu}z=0.7\ifmmode\pm\else\textpm\fi{}0.1$, using data from intermediate temperatures down to $60\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. Here $\ensuremath{\nu}$ is the correlation length exponent and $z$ is the dynamical critical exponent. In the case of electrostatically tuned transitions in field, and the field-tuned transitions at various values of electrostatically induced superconductivity, scaling was successful with $\ensuremath{\nu}z=0.65\ifmmode\pm\else\textpm\fi{}0.1$ from intermediate temperatures down to about 100 or $150\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. The parallel critical magnetic field, ${B}_{c}$, increased with electron transfer as ${(\ensuremath{\Delta}n\ensuremath{-}\ensuremath{\Delta}{n}_{c})}^{0.33}$, and the critical resistance decreased linearly with $\ensuremath{\Delta}n$. However, at lower temperatures, in the insulating regime, the resistance became larger than expected from extrapolation of its temperature dependence at higher temperatures, and scaling failed. These observations imply that although the electrostatic and parallel magnetic-field-tuned superconductor-insulator transitions would appear to belong to the same universality class and to be delineated by a robust phase boundary that can be crossed either by tuning $\ensuremath{\Delta}n$ or $B$, in the case of the field-tuned transition at the lowest temperatures, some different type of physical behavior turns on in the insulating regime.

Spin Proximity Effect in Ultrathin Superconducting Be-Au Bilayers

We present a detailed study of the effects of interface spin-orbit coupling on the critical field behavior of ultrathin superconducting Be/Au bilayers. Parallel field measurements were made in bilayers with Be thicknesses in the range of d=2-30 nm and Au coverages of 0.5 nm. Though the Au had little effect on the superconducting gap, it produced profound changes in the spin states of the system. In particular, the parallel critical field exceeded the Clogston limit by an order of magnitude in the thinnest films studied. In addition, the parallel critical field unexpectedly scaled as [FORMULA: SEE TEXT], suggesting that the spin-orbit coupling energy was proportional to Delta0/d2. Tilted field measurements showed that, contrary to recent theory, the interface spin-orbit coupling induces a large in-plane superconducting susceptibility but only a very small transverse susceptibility.