Dependence Of Upper Critical Field Research Articles

A semi-empirical formula for the dependence of upper critical field with component and strain in Nb3Sn

We include the effects of Sn content in the scaling law to generalize the upper critical field function of temperature and strain in Nb3Sn. The basis for this generalization comes from a theory developed by the same author [Scientific Reports 7, 1133 (2017)]. The important assumption in the current work is that, in determining the upper critical field the contributions of strain, temperature and Sn content are independent of each other. It is followed that we write down a semi-empirical expression where the strain function and are affected only by strain and temperature, respectively, and the variation of tin content enters in the function through This separable form of allows one to incorporate Sn content in the scaling law in a natural way, while this variable is not considered by traditional scaling laws. In addition to this advance, one can see a more flexible function with triple variables which could be an effective tool to fit or predict experiments. As the last but most important outcome, this semi-empirical function instructs one how to understand the role of component content played in scaling behaviors of Nb3Sn, so as to reduce the off-stoichiometric composition inducing discrepancy between the results by scaling laws and experiments.

Multi-band analysis on physical properties of superconducting FeSe films

The origins of superconductivity and pairing symmetry of order parameters are still controversial problems for FeSe thin films up to date. Under the Neumann boundary conditions, the electromagnetic properties of this system are investigated using the two-band Ginzburg–Landau theory. We calculate the temperature dependence of upper critical field in arbitrary direction and critical supercurrent density through the FeSe film. It is revealed that the normalized upper critical field is independent of the film thickness and all of our theoretical results are in accordance with the experimental data. These thus strongly indicate the existence of two-gap s-wave superconductivity in this material.

Ginzburg–Landau Analysis on the Physical Properties of the Kagome Superconductor CsV3Sb5

The kagome lattice consisting of corner-sharing triangles has been studied in the context of quantum physics for more than seventy years. For the novel discovered kagome superconductor CsV3Sb5, identifying the pairing symmetry of order parameter remained an elusive problem until now. Based on the two-band Ginzburg–Landau theory, we study the temperature dependence of upper critical field and magnetic penetration depth for this compound. All theoretical results are consistent with the experimental data, which strongly indicates the existence of two-gap s-wave superconductivity in this system. In addition, it is worth noting that the anisotropy of effective masses in the band with large (or small) gap is about 70 (or 2.4). With the calculation of the Kadowaki–Woods ratio as 0.58×10−5μΩ cm mol2 K2 mJ−2, the semi-heavy-fermion feature is suggested in the compound CsV3Sb5.

Planckian dissipation and non-Ginzburg-Landau type upper critical field in Bi2201

Resistivity and Hall effect measurements have been carried out on a micro-fabricated bridge of Bi2201 single crystal at low temperatures down to 0.4 K under high magnetic fields. When superconductivity is crashed by a high magnetic field, the recovered "normal state" resistivity still shows a linear temperature dependence in low temperature region. Combining with the effective mass and the charge carrier density, we get a linear scattering rate $1/\tau = \alpha k_{B} T/\hbar$ with $0.77<\alpha<1.16$, which gives a strong evidence of the Planckian dissipation. Furthermore, our results reveal a new type of temperature dependence of upper critical field, $H_{c2}(T)=H^*\sqrt{(1-t)/(t+0.154)}$, which is totally different from the expectation of the Ginzburg-Landau theory, and suggests uncondensed Cooper pairs above $H_{c2}(T)$ line.

Discovery of superconductivity in AlB2-type hexagonal YGa2

A crystalline sample YGa2 with the AlB2-type hexagonal structure has been synthesized using the self-flux method. We measured electrical resistivity and specific heat down to 0.4 K and in fields up to 200 mT. The obtained data reveal type-II superconductivity. Unusual behaviour of the temperature dependence of upper critical field H c2(T c) was attributed to the crystal twinning of two phases possessing close lattice parameters. We determined thermodynamic parameters in the superconducting and normal states of YGa2. DFT calculations indicated a large change in the Fermi velocity of carriers nearby cylinder-like structured Fermi surface along the Γ − A line.

Theoretical study on the magnetic properties of the superconductor FeSe

Identifying the form of superconducting order parameter is still a controversial problem for the iron-based superconductor FeSe. Based on anisotropic two-component Ginzburg–Landau theory, we study the temperature dependence of upper critical field and London penetration depth for FeSe. Without including the spin paramagnetic effect, all of our theoretical calculations fit the experimental data well in a broad temperature range. Our results thus show that FeSe is a two-gap s-wave superconductor. And the anisotropy of effective masses in the band with larger (or smaller) gap can be estimated as about 10 (or 2) respectively.

Investigation of dimensionality in superconducting NbN thin film samples with different thicknesses and NbTiN meander nanowire samples by measuring the upper critical field**Project supported by the Chinese Academy of Sciences (Grant No. XDB25000000).

We study superconducting properties of NbN thin film samples with different thicknesses and an ultra-thin NbTiN meander nanowire sample. For the ultra-thin samples, we found that the temperature dependence of upper critical field (Hc2) in parallel to surface orientation shows bending curvature close to critical temperature Tc, suggesting a two-dimensional (2D) nature of the samples. The 2D behavior is further supported by the angular dependence measurements of Hc2 for the thinnest samples. The temperature dependence of parallel upper critical field for the thick films could be described by a model based on the anisotropic Ginzburg–Landau theory. Interestingly, the results measured in the field perpendicular to the film surface orientation show a similar bending curvature but in a much narrow temperature region close to Tc for the ultra-thin samples. We suggest that this feature could be due to suppression of pair-breaking caused by local in-homogeneity. We further propose the temperature dependence of perpendicular Hc2 as a measure of uniformity of superconducting ultra-thin films. For the thick samples, we find that Hc2 shows maxima for both parallel and perpendicular orientations. The Hc2 peak for the perpendicular orientation is believed to be due to the columnar structure formed during the growth of the thick films. The presence of columnar structure is confirmed by transmission electron microscopy (TEM). In addition, we have measured the angular dependence of magneto-resistance, and the results are consistent with the Hc2 data.

Dimensional reduction and ionic gating induced enhancement of superconductivity in atomically thin crystals of 2H-TaSe2

The effects of dimensional reduction and ion intercalation on superconductivity (SC) in the presence of charge density waves (CDWs) in two-dimensional crystals of 2H-TaSe2 were characterized. We prepared atomically thin crystals by mechanical exfoliation and performed electrical transport measurements on devices made by photolithography. The superconducting transition temperature (TcSC) was found to increase monotonically as the thickness decreased, changing from 0.14 K in the bulk to higher than 1.4 K for a 3-nm-thick crystal. The temperature dependence of upper critical field was found to be anomalous. The CDW transition temperature (TcCDW) was found to decrease, but to a less extent than TcSC, from 120 K in the bulk to around 113 K for the 3-nm-thick crystal. In addition, ion intercalation was found to increase TcSC and suppress TcCDW in an atomically thin crystal of 2H-TaSe2. The implications of these findings are discussed. We suggest that dimensional reduction and ion intercalation are potentially effective ways to engineer material properties for layered transition metal chalcogenides.

Mean-field analysis on the magnetic properties of non-centrosymmetric superconductor LaNiC2

Based on two-band isotropic Ginzburg–Landau theory, we study the temperature dependence of upper critical field and London penetration depth for non-centrosymmetric superconductor LaNiC2. All the theoretical calculations fit the experimental data very well, especially the upward curvature of upper critical field near the critical temperature. Our results thus indicate that the two-gap scenario is better to account for the superconductivity of LaNiC2, and the Cooper pairs of this superconductor are in the conventional s-wave state.

Emergent exotic superconductivity in artificially engineered tricolor Kondo superlattices

In the quest for exotic superconducting pairing states, the Rashba effect, which lifts the electron-spin degeneracy as a consequence of strong spin-orbit interaction (SOI) under broken inversion symmetry, has attracted considerable interest. Here, to introduce the Rashba effect into two-dimensional (2D) strongly correlated electron systems, we fabricate non-centrosymmetric (tricolor) superlattices composed of three kinds of $f$-electron compounds with atomic thickness; $d$-wave heavy fermion superconductor CeCoIn$_5$ sandwiched by two different nonmagnetic metals, YbCoIn$_5$ and YbRhIn$_5$. We find that the Rashba SOI induced global inversion symmetry breaking in these tricolor Kondo superlattices leads to profound changes in the superconducting properties of CeCoIn$_5$, which are revealed by unusual temperature and angular dependences of upper critical fields that are in marked contrast with the bulk CeCoIn$_5$ single crystals. We demonstrate that the Rashba effect incorporated into 2D CeCoIn$_5$ block layers is largely tunable by changing the layer thickness. Moreover, the temperature dependence of in-plane upper critical field exhibits an anomalous upturn at low temperatures, which is attributed to a possible emergence of a helical or stripe superconducting phase. Our results demonstrate that the tricolor Kondo superlattices provide a new playground for exploring exotic superconducting states in the strongly correlated 2D electron systems with the Rashba effect.

Anisotropic upper critical field of theBiS2-based superconductorLaO0.5F0.5BiS2

Superconducting properties under high magnetic fields for the novel BiS2-based layered superconductor LaO0.5F0.5BiS2 (polycrystalline sample prepared using the high-pressure synthesis method) have been investigated. The anisotropy of the upper critical field was discussed by analyzing the temperature dependence of the temperature derivative of resistivity. The temperature dependence of upper critical field showed an anomalous behavior with a characteristic magnetic field of 8 T. We revealed that the anomalous behavior was caused by the existence of three kinds of upper critical field, HC2max, HC2mid and HC2min in LaO0.5F0.5BiS2. The lowest upper critical field HC2min is regarded as the Hc2 where the superconducting states of the grains with an orientation of H // c are suppressed. The HC2max and HC2mid are regarded as the Hc2 where the superconducting states of the grains with an orientation of H // ab are suppressed. The difference between the HC2max and HC2mid could be explained by the anisotropy of superconducting states within the Bi-S plane. The estimated anisotropy parameter of the upper critical fields was about 7.4 for the high-pressure-synthesized LaO0.5F0.5BiS2 polycrystalline sample. Since the value was clearly lower than the anisotropy parameter of over 30 observed in the previous report on the LaO0.5F0.5BiS2 single crystals grown under ambient pressure, we concluded that a larger anisotropy of superconductivity was not essential for the enhancement of Tc, and lower anisotropy might be important for a higher Tc in LaO1-xFxBiS2.

Angular dependence of the resistive upper critical field of the iron-based superconductor Fe1+δ (Te,Se) in high magnetic fields

We have investigated the angular dependence of the upper critical fields (µ0Hc2) of a single crystal of Fe1+δ(Te,Se) in pulsed high magnetic fields up to 52 T. At T = 10 K, the µ0Hc2 increases with increasing θ, which is the angle between the c-axis and the direction of the applied magnetic field. Using the effective-mass model based on the three-dimensional anisotropic Ginzburg-Landau (G-L) theory, we estimate the effective-mass anisotropy ratio γ = (mc* /mab*)1/2 to be 1.6 at 10 K. This result is consistent with that of our previous study on the anisotropy coefficient of the resistive upper critical field, Γ ≡ Hc2ab/Hc2c.

Role of crystal anisotropy on the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase

We demonstrate that the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase may be very different depending on the field orientation relative to the crystalline axes. We calculate numerically the upper critical field near the tricritical point taking into account the modulation of the order parameter along the magnetic field as well as the higher Landau levels. For s-wave superconductors with the anisotropy described by an elliptical Fermi surface we propose a general scheme of the analysis of the angular dependence of upper critical field at all temperatures on the basis of the exact solution for the order parameter. Our results show that the transitions (with tilting magnetic field) between different types of mixed states may be a salient feature of the FFLO phase. Moreover we discuss the reasons for the first-order phase transition into the FFLO state in the case of CeCoIn5 compound.

Superconductivity and magnetism of complex rhodium borides

A number of complex rhodium borides with an LuRu4B4-type structure is synthesized; these are DyRh4B4 (samples HP) with T c ≈ 4.5 K, DyRh3.8Ru0.2B4 (samples AM) with T c ≈ 4.5 K, Dy0.8Er0.2Rh3.8Ru0.2B4 (samples AM) with T c ≈ 6.3 K, and HoRh3.8Ru0.2B4 (samples AM) with T c ≈ 6.0 K. The temperature dependence of upper critical field B c2(T) for all the samples under study exhibits an anomalous behavior. In all cases, the curve B c2(T) demonstrates a point of inflection, after which the curve deviates from the classical parabolic law abruptly upward for DyRh4B4 and DyRh3.8Ru0.2B4 (the 1st group of compounds) and downward for the Dy0.8Er0.2Rh3.8Ru0.2B4 and HoRh3.8Ru0.2B4 compounds (the 2nd group). These compounds are found to be characterized by of the following phase transitions: paramagnet → ferrimagnet → superconductor (retained ferrimagnetism) → antiferromagnet (retained superconductivity). The latter transition to the antiferromagnetic state occurs only in the compounds of the 1st group. It is found that, for the DyRh3.8Ru0.2B4 compound, no traditional Meissner effect is observed but the so-called Volleben effect (paramagnetic Meissner effect) takes place.

Theoretical analysis on the upper critical field of superconductor NbS2

From the two-band Ginzburg-Landau theory, we study the temperature dependence of upper critical field on the layered superconductor NbS2. The temperature dependence of the anisotropic parameter for upper critical filed is also obtained. All the results fit the experimental data well in a broad temperature range. Thus our results show strong evidence that two-gap scenario is better to account for the superconductivity of NbS2. The anisotropic parameter of the upper critical field for NbS2 starts to decrease from 5.0 K, and this behavior is similar to those of MgB2 and NbSe2. However for NbS2 this number is about 7.3, which is much greater than the ones in MgB2 and NbSe2. The results also show that the band with the larger gap exhibits that the effective mass ratio between the in-plane and out-of-plane direction is about 54, and the other band indicates that the effective mass ratio is almost isotropic.

Two-band calculations on the upper critical field of superconductor NbSe 2

Based on two-band Ginzburg–Landau theory, we study the temperature dependence of upper critical field for superconducting crystal NbSe 2. The results reproduce the experimental data in a broad temperature range, especially the upward curvature near the transition temperature. Our calculations also indicate that in NbSe 2 the band with the smaller gap is almost isotropic, and most probably located on the bonding Nb Fermi sheets.

Pressure Dependence of Upper Critical Field in β-Pyrochlore Oxides

We report the results of the low-temperature specific heat measurements on the β-pyrochlore oxide superconductor RbOs 2 O 6 ( T c =6.3 K) in magnetic field under high pressure. We investigated the upper critical field H c2 at several pressures and found that the slope of H c2 at T c monotonically increases with increasing pressure in low-pressure superconducting phase SC1, indicating that the effective mass m * is enhanced by the pressure. Moreover, the jump of the specific heat at T c is also enhanced by the application of the pressure in SC1, suggesting that the pressure gives rise to the enhancement of the electron–phonon coupling and consequently enhances T c .

Mean-field calculations on the magnetic properties of the two-gap superconductor Lu2Fe3Si5

Based on Ginzburg–Landau theory, we study the temperature dependence of upper critical field and London penetration depth for the superconducting crystal Lu2Fe3Si5. All the results fit the experimental data well. We also indicate that the larger gap opens in the quasi-one-dimensional part of Fermi surfaces.

Effect of two length scales on the properties ofMgB2 for arbitrary applied magnetic field

Experiments carried out on the intermetallic superconducting materialMgB2 have shown anomalous magnetic field dependence of upper critical field, small angleneutron scattering form factor, specific heat, critical current etc. Similarly, scanningtunnelling microscopy (STM) experiments on vortex structures have shown unusuallylarge vortex core size and two different magnetic and spatial field scales. Also,whereas the specific heat measurements and isotope shift experiments have shownBardeen–Cooper–Schrieffer-like (BCS-like) behaviour, the temperature dependences ofthe penetration depth experiments have shown non-BCS-like behaviour. Theseanomalous behaviours have been attributed to the multiband superconductivityof this material and the nature of the local spatial behaviour of the magneticinduction and the order parameter components having two length scales. Wereport an analytical investigation of the effect of two length scales on thetemperature and the applied magnetic field dependence of several properties ofMgB2, such as, the penetration depth, single vortex and vortex lattice structure, vortex coreradius, reversible magnetization, critical current, small angle neutron scattering form factorand the shear modulus of the vortex lattice within the framework of two-order parameterGinzburg–Landau theory. We solve the corresponding nonlinear Ginzburg–Landauequations numerically exactly using an iterative method for arbitrary applied fieldHc1 < H < Hc2, the Ginzburg–Landau parameter and vortex lattice symmetry. This enablesus to compute the local spatial behaviour of the magnetic induction and theorder parameters accurately for arbitrary applied field and a wide rangeof temperature. Comparison of the analytical results with experiments onMgB2 gives very good agreement.

(Ba, K)Fe2As2 超伝導体の高密度多結晶試料の合成とその臨界特性

A bulk iron arsenide superconductor of (Ba, K)Fe2As2 was synthesized by a combined process of high temperature heat treatment and deformation using a Ta sheath. Observations by an optical microscope and a scanning electron microscope and the result of x-ray diffraction indicate that the sample has a polycrystalline dense structure with much less impurity phases compared to the samples prepared by conventional sintering process. Magneto-resistance and magnetization measurements have been carried out on the obtained (Ba, K)Fe2As2 sample in order to evaluate superconducting critical properties. The temperature dependence of upper critical field, -dHc2(T)/dT is~6.3 T/K, which corresponds to the value of single crystals. The temperature dependence of irreversibility fields, -dHirr(T)/dT, has also a steep slope at about 10 K below Hc2(T), suggesting that (Ba, K)Fe2As2 superconductors has good potential for magnetic applications at medium temperatures like MgB2. However, the global Jc at 5 K estimated from the magnetization hysteresis results of bulk and powder samples and the transport Jc at 20 K measured by a standard four probe method are as low as ~2×107 A/m2 and ~105 A/m2, respectively, in applied magnetic fields. We also estimated the local Jc from the magnetization hysteresis of powdered sample, which are the order of 109 A/m2 at 5 K and 108 A/m2 at 20 K. Those results indicate that current path is still largely limited in the bulk sample despite its high density and weak link at the grain boundary is most plausible origin of current limitation in the bulk sample.