Superconducting Fluctuation Effects Research Articles

Superconducting fluctuation effect on epitaxially layered films of superconductor NbN and half-metallic Heusler alloy Co2MnSi

Superconducting properties were investigated in epitaxially layered films consisting of superconductor NbN and half-metallic Heusler alloy Co2MnSi (CMS). Temperature dependence of the electrical resistivity ρ(T) was measured by applying perpendicular magnetic fields to the surface of NbN/CMS films. With the increase of the CMS thickness dCMS, the upper critical field μ0Hc2 decreased monotonically, but the superconducting transition temperature Tc had the minimum of 10.1 K at dCMS ≈ 5 nm. The Tc behavior was in qualitative agreement with the theory of the π-coupling. The pair-breaking parameter δ determined by the superconducting fluctuation theory took the maximum at dCMS = 3 ∼ 5 nm, which would be related to the minimum of Tc. The experimental results reveal that the superconductivity of the NbN layer in NbN/CMS films is affected by the interplay between the superconducting NbN layer and the half-metallic CMS layer.

Estimations of superconducting fluctuation effects in amorphous MoRu and MoRe alloy thin films

Herein, we investigated the superconducting properties of amorphous Mo alloy thin films: molybdenum ruthenium (MoRu) onto c-plane sapphire and molybdenum rhenium (MoRe) glass substrates. To extract the two-dimensional (2D) transport properties from the experimental data, we analyzed the magnetoconductance Δσ = 1/Rsq(H, T) − 1/Rsq(0, T) by the sum of the mechanisms due to the 2D superconducting fluctuations, weak localization, and electron-electron interaction theories. The result is consistent with the recent simulation result that considers disorder in thin films. In addition, we compared the device parameters of superconducting nitride films, such as niobium nitride and molybdenum nitride using the presents results.

Superconducting Fluctuations above Tc and pair breaking parameters of two dimensional Niobium Nitride Films

Transport properties have been investigated for the epitaxial superconducting NbN thin films. We analysed the excess conductance σ’ ≡ σ(T) - σN by the sum of the Aslamazov-Larkin (AL) and Maki-Thompson (MT) terms for thermal fluctuations above Tc, where the σN ≡1/RsqN is the normal state sheet conductance. We have found that the theoretical expression σ’theo (T) = σ’AL (T) + σ’MT (T,δ) can be well fitted to σ’exp (T) with use of the suitable value of the pair breaking parameter δ in the MT term relating to the inelastic scattering rate 1/τin(T) as δ = πħ/8kBTτin. The rate 1/τin(T) given by the sum of 1/τfluc(T), 1/τe-e(T) and 1/τe-ph (T) is determined from the analysis of the magneto-conductance Δσ = σ(H) – σ(0) by the sum of AL, MT and the localization terms, where the first, second and third terms correspond to the rate due to the superconducting fluctuation effect, electron-electron and electron-phonon interactions, respectively. The RsqN dependence of δ is expressed by δ = δ0 + αRsqN, where the first term δ0 due to 1/τe-ph (T) and the second term due to the sum of 1/τfluc(T) and 1/τe-e(T). Although we obtained a reasonable value of Debye temperature ΘD ≈630 K from the δ0, the magnitude of the α is about 5 times larger than the theoretical value.

Enhanced superconducting fluctuation effects on thermodynamic properties in the BCS-BEC crossover regime

Effects of superconducting fluctuation (SCF) on thermodynamic properties of electron systems in the so-called BCS-BEC-crossover regime are studied. As the attractive interaction between electrons becomes stronger upon approaching the BCS-BEC-crossover regime, importance of the mode coupling between SCF drastically increases. The enhanced mode coupling leads to lowering of both the zero-field superconducting critical temperature and the depairing field $B_\text{c2} (T)$. Consequently, SCF-induced contributions to the specific heat and the diamagnetic susceptibility can seemingly exceed the corresponding values in the Gaussian approximation. We discuss relevance of the present results to the anomalous SCF-induced diamagnetic response observed in the iron selenide (FeSe).

BCS-like critical fluctuations with limited overlap of Cooper pairs in FeSe

In conventional superconductors, very narrow superconducting fluctuation regions are observed above $T_c$ because of the strong overlapping of Cooper pairs in a coherence volume. In the bulk form of iron chalcogenide superconductor FeSe, it is argued that the system may locate in the crossover region of BCS to BEC, indicating a strong superconducting fluctuation. In this respect, we have carried out measurements of magnetization, specific heat and Nernst effect on FeSe single crystals in order to investigate the superconducting fluctuation effect near $T_c$. The region of diamagnetization induced by superconducting fluctuation seems very narrow above $T_c$. The crossing point of temperature dependent magnetization curves measured at different magnetic fields, which appears in many systems of cuprate superconductors and is regarded as indication of strong critical fluctuation, is however absent. The magnetization data can be scaled based on the Ginzburg-Landau fluctuation theory for a quasi-two-dimensional system, but the scaling result cannot be described by the theoretical function of the fluctuation theory because of the limited fluctuation regions. The specific heat jump near $T_c$ is rather sharp without the trace of strong superconducting fluctuation. This is also supported by the Nernst effect measurements which indicate a very limited region for vortex motion above $T_c$. Associated with very small value of Ginzburg number and further analyses, we conclude that the superconducting fluctuation is very weak above $T_c$ in this material. Our results are strongly against the picture of significant phase fluctuation in FeSe single crystals, although the system has a very limited overlapping of Cooper pairs in the coherence volume. This dichotomy provides new insights into the superconducting mechanism when the system is with a dilute superfluid density.

Different spin relaxation mechanisms between epitaxial and polycrystalline Ta thin films

We demonstrate that spin relaxation mechanisms are different between epitaxial Ta and disordered polycrystalline Ta thin films by determining the relationship between spin relaxation time and diffusion constant. To control the diffusion constant, epitaxial Ta and polycrystalline Ta thin films are prepared by sputtering on different substrates and at different growth temperatures. The spin relaxation time is extracted from the results of weak antilocalization analysis including the superconducting fluctuation effect. The dominant spin relaxation mechanism for polycrystalline Ta thin films is the Elliot–Yafet mechanism, as is expected for centrosymmetric metal films. In contrast, the D’yakonov–Perel’ mechanism plays a role in epitaxial Ta thin films.

Superconducting fluctuation effect in CaFe0.88Co0.12AsF

Out-of-plane angular dependent torque measurements were performed on CaFe0.88Co0.12AsF single crystals. Superconducting fluctuations, featured by magnetic field enhanced and exponential temperature dependent diamagnetism, are observed above the superconducting transition temperature Tc, which is similar to that of cuprate superconductors, but less pronounced. In addition, the ratio of Tc versus superfluid density follows well the Uemura line of high-Tc cuprates, which suggests the exotic nature of the superconductivity in CaFe0.88Co0.12AsF.

Superconducting fluctuations in isovalently substitutedBaFe2(As1−xPx)2: Possible observation of multiband effects

The nature of superconducting fluctuation effects in the isovalently-substituted iron pnictide BaFe$_2$(As$_{1-x}$P$_x$)$_2$ (x=0.35) is probed through measurements of the magnetization and magnetoconductivity around the superconducting transition. The results, obtained with magnetic fields up to 9 T applied in the two main crystal directions, are consistent with anisotropic Ginzburg-Landau (GL) approaches for finite applied magnetic fields. The analysis allowed to determine with accuracy the out-of-plane, $\xi_c(0)$, and in-plane, $\xi_{ab}(0)$, GL coherence lengths. Significant differences are found between the $\xi_c(0)$ values resulting from electrical transport and magnetization data. According to recent theoretical approaches, these differences could be interpreted in terms of the multiband nature of this material. The analysis of data in the low field region around the transition temperature also suggests that phase fluctuations, although possibly relevant in other Fe-based superconductors, may play a negligible role in this compound.

Colossal thermomagnetic response in the exotic superconductor URu2Si2

The Nernst coefficient is a measure of the transverse thermoelectric effect in a conductor. Superconducting fluctuations magnify this effect but in URu2Si2, the million-fold enhancement suggests that the fluctuations have an exotic origin. The superconducting fluctuation effect, due to preformed Cooper pairs above the critical temperature Tc, has been generally understood by the standard Gaussian fluctuation theories in most superconductors1. The transverse thermoelectric (Nernst) effect is particularly sensitive to the fluctuations, and the large Nernst signal found in the pseudogap regime of the underdoped cuprates2,3 has raised much debate. Here we report on the observation of a colossal Nernst signal due to the superconducting fluctuations in the heavy-fermion superconductor URu2Si2. The Nernst coefficient is anomalously enhanced (by a factor of ∼106) as compared with the theoretically expected value of the Gaussian fluctuations. Moreover, contrary to the conventional wisdom, the enhancement is more significant with a reduction of the impurity scattering rate. This unconventional Nernst effect intimately reflects the highly unusual superconducting state of URu2Si2. The results invoke possible chiral or Berry-phase fluctuations associated with the broken time-reversal symmetry4,5,6,7 of the superconducting order parameter.

Resistive phase transition of the superconducting Si(111)-( 7 × 3 )-In surface

Recently, superconductivity was found on semiconductor surface reconstructions induced by metal adatoms, promising a new field of research where superconductors can be studied from the atomic level. Here we measure the electron transport properties of the Si(111)-()-In surface near the resistive phase transition and analyze the data in terms of theories of two-dimensional (2D) superconductors. In the normal state, the sheet resistances (2D resistivities) R□ of the samples decrease significantly between 20 and 5 K, suggesting the importance of the electron-electron scattering in electron transport phenomena. The decrease in R□ is progressively accelerated just above the transition temperature (Tc) due to the direct (Aslamazov-Larkin term) and the indirect (Maki-Thompson term) superconducting fluctuation effects. A minute but finite resistance tail is found below Tc down to the lowest temperature of 1.8 K, which may be ascribed to a dissipation due to free vortex flow. The present study lays the ground for a future research aiming to find new superconductors in this class of materials.

Mode–mode coupling theory of itinerant electron antiferromagnetism in superconducting state

It has been considered since the first discovery of a high-Tc cuprate that an antiferromagnetic (AF) state and a superconducting (SC) state are separated in it. However, it is very intriguing that the coexistence of the AF and SC states has recently been observed in HgBa2Ca4Cu5O12+ (Hg-1245). Moreover, it is very novel that this coexistence of these two states appears if the SC-transition temperature Tc is higher than the AF-transition temperature TN. The mode–mode coupling theory can provide a clear elucidation of this novel phenomenon. A key point of this theory is that the AF susceptibility consists of the random-phase-approximation (RPA) term and the mode–mode coupling one. The RPA term works to make a positive contribution to the emergence of the antiferromagnetic critical point (AF-CP). In contrast, the mode–mode coupling term works to make a negative contribution to the emergence of the AF-CP. However, the growth of the SC-gap function in the dx2−y2-wave SC state works to suppress the negative contribution of the mode–mode coupling term to the emergence of the AF-CP. Moreover, the effect of SC fluctuations near the SC-transition temperature Tc suppresses the mode–mode coupling term of the AF susceptibility that works to hinder the AF ordering. For these two reasons, there is a possibility that the dx2−y2-wave SC state is likely to promote the emergence of the AF-CP. Namely, the appearance of the above-mentioned novel coexistence of the AF and SC states observed in Hg-1245 can be explained qualitatively on the basis of this idea.

In-plane and transverse superconducting fluctuation diamagnetism in the presence of charge-density waves in2H−NbSe2single crystals

The fluctuation-diamagnetism (FD) above the superconducting transition was measured in 2H-NbSe2 single crystals. The moderate uniaxial anisotropy of this compound, and some experimental improvements, allowed to measure the superconducting fluctuation effects in the two main crystallographic directions. These results reveal that the nonlocal electrodynamic effects on the FD are highly anisotropic, and they also discard a possible contribution to the FD coming from the charge-density waves (CDW) appearing below $Tcdw > Tc in 2H-NbSe2, in agreement with a phenomenological estimate.

Carrier concentration dependence of critical fluctuation in La 2− xSr xCuO 4

We investigated the superconducting fluctuation effects of overdoped La 2− x Sr x CuO 4 (LSCO) film with x = 0.19 by the microwave conductivity measurement. Temperature and frequency dependences of the fluctuation-induced excess conductivity in the vicinity of T c were analyzed using a dynamic scaling theory. We observed a two dimensional (2D) critical behavior of the fluctuation. Together with our previous work, the spatial dimension of the fluctuation does change from 2D (for underdoped LSCO) to 3D (for nearly optimally doped LSCO), and then, to 2D (for overdoped LSCO) as the carrier concentration increases.

Breakdown of the superconducting fluctuations by a magnetic field

We have measured the diamagnetism induced in the normal state by the superconducting fluctuations in Pb 1 - x In x alloys. The experiments were performed with external magnetic fields up to well above H C 2 ( 0 ) , the upper critical magnetic field extrapolated to T = 0 K . The results show that in dirty alloys the superconducting fluctuation effects are independent of the amount of impurities in all the measured H– T phase diagram above H C 2 ( T ) . Moreover, for all the studied alloys these fluctuation effects vanish when the applied field approaches the corresponding 1.1 H C 2 ( 0 ) . At these high fields the superconducting coherence length becomes of the order of its minimum value, the one at T = 0 K . So, we propose that these striking results could be due to the limits imposed by the uncertainty principle to the shrinkage, when H increases, of the superconducting wave function.

Experimental observation of the breakdown by a magnetic field of the superconducting fluctuations in the normal state

The effects induced on the normal state magnetization by fluctuating Cooper pairs have been measured in ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}$ alloys up to magnetic fields above ${H}_{C2}(0)$, the upper critical field extrapolated to $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Our results show that in dirty alloys these superconducting fluctuation effects are, in the entire $H\ensuremath{-}T$ phase diagram above ${H}_{C2}(T)$, independent of the amount of impurities and that they vanish when $H\ensuremath{\sim}1.1{H}_{C2}(0)$. These striking results seem to be consistent with the limits imposed by the uncertainty principle to the shrinkage of the superconducting wave function.

Quantum Resistive Behaviors in Vortex Liquid Regimes at Finite Temperatures

Motivated by a {\it mean field-like} resistive behavior in magnetic fields commonly seen in various superconducting (SC) cuprates and organics with strong fluctuation, {\it quantum} SC fluctuation effects on resistive behaviors are reexamined by putting emphasis on their roles in the so-called {\it thermal} vortex liquid regime. By incorporating the quantum fluctuation and a vortex pinning effect in the GL fluctuation theory, it is found that the resistivity curve shows not a fan-shaped broadening but a sharp drop at a 3d vortex-glass transition point far below an apparent upper critical field H_{c2}^*(T) as a result of a quantm fluctuation enhanced by an adequately small condensation energy or by a strong field. Fittings to data of cuprate and organic superconductors are performed by including effects of SC pseudogap region. It is argued on the cuprate materials that, irrespective of the presence of fluctuations of competing non-SC orders, the in-plane coherence length of hole-doped cuprates decreases on approaching the underdoped limit and that the condensation energy density to be measured from the heat capacity data is maximum near the optimal doping. The case of disordered quasi 2d s-wave films showing the field-tuned superconductor-insulator transition (FSIT) behavior is also studied for comparison, and an agreement with available data is found.

Superconducting Fluctuations Above TC and the Uncertainty Principle: How Small may the Coherence Length be?

We summarize here some of our recent results on the superconducting fluctuation effects above TC in different low- and high-TC superconductors, at high reduced-temperatures and magnetic fields. These results confirm our proposal that the collective behaviour of the fluctuating Cooper pairs in the short-wavelength fluctuation regime is dominated by the uncertainty principle, which imposes a limit to the shrinkage of the superconducting wave function when T increases well above TC or H becomes of the order of Hc2(0), the upper critical magnetic field amplitude extrapolated to T = 0 K .

Superconducting fluctuation effect on the de Haas–van Alphen oscillations in the vortex liquid state of quasi-2D superconductors

The damping of the de Haas–van Alphen (dHvA) oscillations in the mixed state of a 2D superconductor is studied within a new analytical approach to the problem of vortex-lattice fluctuations at high magnetic fields. It is shown that in the ‘liquid’ state around mean field H c2( T), averaging over phase fluctuations leads to damping of the dHvA osillations, which can be well described by the random vortex lattice model. The calculated damping rate is in a good quantitative agreement with recent experiments carried out on a quasi-2D organic superconductor. It is predicted that deep in the mixed state, the freezing into an ordered vortex lattice is accompanied by a sharp upward turn in the slope of the ‘Dingle plot’.

Spin-gap and the superconducting fluctuations in high- Tc cuprates probed by 63Cu NMR

From the 63Cu Knight shift and the nuclear spin–lattice relaxation rate under magnetic fields from zero up to 28 T in the underdoped (U) YBa 2Cu 4O 8 and the slightly overdoped (O) TlSr 2CaCu 2O 6.8, we find that the spin-gap (pseudogap) behaviors show contrasting response to magnetic fields in the U and O compounds. In the O compound, the spin-gap is strongly field dependent and can be accounted for by the superconducting fluctuation effects, but that in the U compound shows no field dependence. The implication is discussed.

Superconducting fluctuation effects on quasi-two dimensional d–p model

We study superconducting fluctuation effects on the quasi-two-dimensional d–p model. We adopt the fluctuation exchange approximation in order to derive both normal and anomalous vertices of renormalized d-electron propagators, and then solve the Bethe–Salpeter equation and derive t-matrix as a superconducting fluctuation propagator. We calculate the normal d-electron self-energy self-consistently, and get self-consistent solutions in which the system is close to an antiferromagnetic instability.