Fluctuations In Superconductors Research Articles

Case of thermodynamic failure in the Ginzburg-Landau approach to fluctuation superconductivity

Case of thermodynamic failure in the Ginzburg-Landau approach to fluctuation superconductivity

No superconductivity in Pb$_9$Cu$_1$(PO$_4$)$_6$O found in orbital and spin fluctuation exchange calculations

Finding a material that turns superconducting under ambient conditions has been the goal of over a century of research, and recently Pb_{10-x}10−xCu_xx(PO_44)_66O aka LK-99 has been put forward as a possible contestant. In this work, we study the possibility of electronically driven superconductivity in LK-99 also allowing for electron or hole doping. We use an ab initio derived two-band model of the Cu e_geg orbitals for which we determine interaction values from the constrained random phase approximation (cRPA). For this two-band model we perform calculations in the fluctuation exchange (FLEX) approach to assess the strength of orbital and spin fluctuations. We scan over a broad range of parameters and enforce no magnetic or orbital symmetry breaking. Even under optimized conditions for superconductivity, spin and orbital fluctuations turn out to be too weak for superconductivity anywhere near to room-temperature. We contrast this finding to non-self-consistent RPA, where it is possible to induce spin singlet d-wave superconductivity at T_cTc ≥300K if the system is put close enough to a magnetic instability.

Probing electron-electron interaction along with superconducting fluctuations in disordered TiN thin films

In two-dimensional (2D) disordered superconductors prior to superconducting (SC) transition, the appearance of a resistance peak in the temperature dependent resistance $R(T)$ measurements indicates the presence of weak localization (WL) and electron-electron interaction (EEI) in the diffusion channel and SC fluctuations in the Cooper channel. Here, we demonstrate an interplay between SC fluctuations and EEI by low-temperature magnetotransport measurements for a set of 2D disordered TiN thin films. While cooling the sample, a characteristic temperature ${T}^{*}$ is obtained from the $R(T)$ at which SC fluctuations start to appear. The upturn in $R(T)$ above ${T}^{*}$ corresponds to WL and/or EEI. By the temperature and field dependences of the observed resistance, we show that the upturn in $R(T)$ originates mainly from EEI with a negligible contribution from WL. Further, we have used the modified Larkin's electron-electron attraction strength $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$, containing a field-induced pair-breaking parameter, in the Maki-Thompson SC fluctuation term. Here, the temperature dependence of $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$ obtained from the magnetoresistance (MR) analysis shows a diverging behavior close to ${T}_{\mathrm{c}}$, and it remains almost constant at higher temperature within the limit of $\mathrm{ln}(T/{T}_{\mathrm{c}})<1$. Interestingly, the variation of $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$ on the reduced temperature $(T/{T}_{\mathrm{c}})$ offers a common trend which has been closely followed by all the concerned samples presented in this paper. Finally, the temperature dependence of inverse phase scattering time (${\ensuremath{\tau}}_{\ensuremath{\phi}}^{\ensuremath{-}1}$), as obtained from the MR analysis, clearly shows two different regimes; the first one close to ${T}_{\mathrm{c}}$ follows the Ginzburg-Landau relaxation rate $({\ensuremath{\tau}}_{\mathrm{GL}}^{\ensuremath{-}1})$, whereas the second one at high temperature varies almost linearly with temperature, indicating the dominance of inelastic electron-electron scattering for the dephasing mechanism. These two regimes are followed in a generic way by all the samples despite being grown under different growth conditions.

Superconductivity and density-wave fluctuations in an extended triangular Hubbard model: an application to SnSe2

We employ the fluctuation-exchange approximation to study the relation of superconducting pairing symmetries and density-wave fluctuations based on the extended triangular Hubbard model upon electron doping and interactions, with an possible application to the layered metal dichalcogenide SnSe2. For the case where the interactions between electrons contain only the on-site Hubbard term, the superconducting pairings are mainly mediated by spin fluctuations, and the spin-singlet pairing with the d-wave symmetry robustly dominates in the low and moderate doping levels, and a d-wave to extended s-wave transition is observed as the electron doping reaches n = 1. When the near-neighbor site Coulomb interactions are also included, the charge fluctuations are enhanced, and the spin-triplet pairings with the p-wave and f-wave symmetries can be realized in the high and low doping levels, respectively.

Incommensurate magnetic states induced by ordering competition in

Quantum criticality near a certain magnetic phase transition beneath the superconducting (SC) dome of is attentively studied by virtue of a phenomenological theory in conjunction with the renormalization group approach. We report that ordering competition between magnetic and SC fluctuations is capable of coaxing incommensurate (IC) magnetic states to experience distinct fates depending upon their spin configurations. The C 2-symmetry IC magnetic stripe with a perpendicular magnetic helix dominates over other C 2-symmetry magnetic competitors and hints at a potential candidate for the unknown C 2-symmetry magnetic state. Of the C 4-symmetry IC magnetic phases, the IC charge spin density wave is substantiated to be superior, shedding light on the significant intertwining of charge and spin degrees of freedom. Meanwhile, ferocious fluctuations render a sharp fall in superfluid density alongside a dip in critical temperature as well as intriguing behavior of the London penetration depth.

A Ginzburg-Landau approach to field theories for single, isolated zero-dimensional superconductors

The letter presents a Ginzburg-Landau model of fluctuations in superconductors in the zero-dimensional (0D) limit. The central descriptive tool is a complex order parameter interpreted as a self-consistent field describing the centre-of-mass motion of electrons pairs and highlighting a resistance between Cooper pairs. We describe the temperature and size dependences of superconductivity in single, isolated 0D superconductors taking into account an intrinsic energy parameter related to confined geometries. It is shown that all the superconducting effects arise only from the existence of fluctuation-induced superconductivity. The results obtained show that the electronic structure of 0D superconductors goes through several phases with qualitatively different properties. The dimensional dependence of thermodynamic quantities is particularly highlighted and thermodynamic properties of 0D fields are discussed and compared in relation to those of bulk fields. Individual grains of Al127 and Pb207 as well as granular superconductors are provided in support of the analysis.

Vortex lattice melting line in superconductors with paramagnetic pair breaking

Recent experiments on the iron-based superconductor FeSe in a high magnetic field have suggested the presence of both the fluctuation-induced vortex liquid regime and a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice. To get a general picture of the magnetic phase diagram in type II superconductors with strong superconducting (SC) fluctuation and strong paramagnetic pair breaking (PPB) such as FeSe, the vortex lattice melting curve ${H}_{m}(T)$ is theoretically investigated in situations where a FFLO state is expected to occur. In general, PPB tends to narrow the vortex liquid regime intervening between ${H}_{c2}(T)$ and ${H}_{m}(T)$. In particular, the vortex liquid regime is found to rapidly shrink upon entering; by cooling, the temperature range in which the FFLO state with a periodic modulation parallel to the magnetic field is stable in mean-field theory. Based on the present results, the high-field SC phase diagrams of FeSe in the parallel and perpendicular field configurations are discussed.

Suppression of fluctuations in a two-band superconductor with a quasi-one-dimensional band

Chain-like structured superconductive materials (such as A_2Cr_3As_3, with A = K, Rb, Cs) exhibit the multiband electronic structure of single-particle states, where coexisting quasi-one-dimensional (Q1D) and conventional higher-dimensional energy bands take part in the creation of the aggregate superconducting condensate. When the chemical potential approaches the edge of a Q1D band in a single-band superconductor, the corresponding mean-field critical temperature increases significantly but the superconductivity is quenched by fluctuations. However, recent investigation has revealed that when a Q1D band is coupled to a higher dimensional one by the interband Cooper-pair transfer, the thermal superconductive fluctuations can be suppressed so that the resulting critical temperature can be close to its mean-field value. In the present work, we calculate the mean-field T_c0 and fluctuation-shifted T_c critical temperatures for a two-band superconductor where a Q1D band coexists with a higher-dimensional band, and investigate how the thermal fluctuations are sensitive to the system parameters. We find that T_c is close to T_c0 in a wide range of microscopic parameters, and even the dimensionality of the higher-dimensional band does not play an essential role. Thus, the screening mechanism for suppressing fluctuations via the pair-exchange coupling between the bands is indeed relevant for a large class of Q1D multiband superconducting materials, encouraging further experiments aimed at reaching larger critical temperatures in such multiband superconductors.

Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor

To elucidate the origin of re-enhanced high-$T_c$ phase in the heavily electron-doped Fe-pnictides, systematic $^{75}$As NMR studies are performed on heavily electron-doped LaFe$Pn$O$_{0.75}$H$_{0.25}$ by controlling the pnictogen height ($h_{Pn}$) from the Fe plane through the substitution at $Pn$(=As) site with Sb or P. The measurements of nuclear spin relaxation rate (1/$T_1$) and Knight shift ($K$) reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gap like feature at low energies are more enlarged in higher $T_c$ compounds with higher $h_{Pn}$, while those are totally suppressed in non-superconducting compounds with lower $h_{Pn}$. This implies that the contribution of the finite energy part in the spin fluctuation spectrum is crucial for enhancing $T_c$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from $d_{xy}$ orbital that rises when $h_{Pn}$ is high, together with the enhanced electron correlation effects.

Quantum Magnetic Properties and Metal‐to‐Insulator Transition in Chemically Doped Calcium Ruthenate Perovskite

Ruthenates provide a comprehensive platform to study a plethora of novel properties, such as quantum magnetism, superconductivity, and magnetic fluctuation mediated metal–insulator transition (MIT). Herein, an overview of quantum mechanical phenomenology in calcium ruthenium oxide with varying compositions is provided. While the stoichiometric composition of exhibits non‐fermi liquid (FL) behavior with quasi‐criticality, chemically doped compounds depict prominent signatures of quantum magnetic fluctuations at low temperature that in some cases are argued to mediate in metal–insulator transition. In the case of cobalt‐doped , an unusual continuum fluctuation is found to persist deep inside the glassy phase of the material. These observations reflect the richness of the ruthenate research platform in the study of quantum magnetic phenomena of fundamental importance.

Direct visualization of a static incommensurate antiferromagnetic order in Fe-doped Bi2Sr2CaCu2O8+δ

Significance The pairing mechanism of high-temperature superconductivity, especially in cuprates, remains as the most challenging issue in condensed matter physics. The antiferromagnetic (AF) order/correlation is supposed to be important for inducing pairing and superconductivity. In this paper, we report the observation of a static incommensurate AF order by spin-polarized scanning tunneling microscope in Fe-doped Bi2212 with a nearly optimally doped state of superconductivity. The local incommensurate AF order can be directly visualized in regions with some Fe impurities. Our measurements provide a basis to study the interplay between superconductivity and AF spin fluctuations in cuprates.

Study of fluctuation conductivity in YBa2Cu3O7−δ films in strong magnetic fields

We report the effect of the ab-plane magnetic field B up to 8 T on the resistivity ρ(T) and fluctuation conductivity σ′'(T) in YBa2Cu3O7-δ thin films. As expected, up to ∼ 2.5 T the magnetic field monotonously increases ρ, the width of the resistive transition, ΔTc, and coherence length along the c axis, ξC(0), but decreases both Tc and the range of superconducting (SC) fluctuations ΔTfl. The fluctuation conductivity exhibits a crossover at characteristic temperature T0 from the 3D Aslamasov-Larkin (AL) theory near Tc to the 2D fluctuation theory of Maki-Thompson (MT). However, at B = 3 T, the MT term is completely suppressed, and above T0 σ′'(T) is unexpectedly described by the fluctuation contribution of 2D AL, suggesting the formation of a 2D vortex lattice in the film under the action of a magnetic field. At the same time, ΔTf sharply increases by a factor of about 7, and ξC(0) demonstrates a very unusual dependence on Tc when B increases above 3 T. Our results demonstrate the possibility of the formation of a vortex state in YBCO and its evolution with increasing B.

Superconductivity and thermal fluctuation of Y1-x Mx: 123 systems with (M = Pr,Ca) & (0.00 < x < 0.50): Comparative study

We report here a comparative study between the effects of Pr and Ca in place of Y on superconductivity and thermal fluctuation of the Y1-x Mx: 123 system with M = Pr and Ca and (0.00 < x < 0.50). Furthermore, some remarks on the hole carrier per Cu ions in high Tc superconductors are revaluated. It is found that numerical values of hole carrier per Cu ion based on M.R. Presland et al., Physica C 176, 95 (1991) disagree with those deduced from thermoelectric power (TEP) and some modifications to the M.R. Presland formula are necessary. On the other hand, the orthorhombic distortion is decreased, while the c-parameter is increased as x increases in Y1-x Mx: 123, but the rate of decrease/increase is dependent on the chosen M. The oxygen content has gradually decreased for the Ca series, whereas it is nearly unchanged (y = 6.94) for the Pr series. The doping density and the doping distance L are almost constant with increasing Pr up to 0.30 and then sharply decrease to Pr = 0.40. In contrast, a gradual sharp decrease in them is obtained for the Ca series. The spacing between Cu ions has gradually increased for the Pr series, but it has decreased for the Ca series. The critical temperature Tc is decreased as x increases for both series, but the rate of decrease is higher for Ca than for Pr as well as for hole carriers per Cu ion. The order parameter exponent (OPD) is two dimensional (2D) in and above the mean field region (MFR) as x increases up to 0.20 for both series. But with increasing x above 0.20, the OPD remains (2D) for the Ca series and becomes zero dimensional (0D) for the Pr series. While in the critical field region (CFR), the OPD is three dimensional (3D) for both series, but it becomes (2D) above x = 0.20 for the Pr series. The upper critical fields and critical currents are sharply increased for the Ca series up to x = 0.50, while they are slightly decreased by Pr up to 0.20 followed by an increase at 0.40. Our results are discussed in terms of the balance between the whole carriers generated/lost by substitution with those lost/introduced by oxygen vacancies in the Cu–O2 planes. As compared to Pr, Ca makes Y: 123 more compact and has higher interlayer coupling, higher Ginzburg-Landau parameter and lower anisotropy, which as possible highlights the present work.

Spectroscopic evidence of superconductivity pairing at 83\u2009K in single-layer FeSe/SrTiO3 films

Single-layer FeSe films grown on the SrTiO3 substrate (FeSe/STO) have attracted much attention because of their possible record-high superconducting critical temperature (Tc) and distinct electronic structures. However, it has been under debate on how high its Tc can really reach due to the inconsistency of the results from different measurements. Here we report spectroscopic evidence of superconductivity pairing at 83 K in single-layer FeSe/STO films. By preparing high-quality single-layer FeSe/STO films, we observe strong superconductivity-induced Bogoliubov back-bending bands that extend to rather high binding energy ~ 100 meV by high-resolution angle-resolved photoemission measurements. They provide a new definitive benchmark of superconductivity pairing that is directly observed up to 83 K. Moreover, we find that the pairing state can be further divided into two temperature regions. These results indicate that either Tc as high as 83 K is achievable, or there is a pseudogap formation from superconductivity fluctuation in single-layer FeSe/STO films.

Hole-like Fermi surface in the overdoped non-superconducting Bi1.8 Pb0.4 Sr2CuO6+δ

In high-temperature cuprate superconductors, the anti-ferromagnetic spin fluctuations are thought to have a very important role in naturally producing an attractive interaction between the electrons in the d-wave channel. The connection between superconductivity and spin fluctuations is expected to be especially consequential at the overdoped end point of the superconducting dome. In some materials, that point seems to coincide with a Lifshitz transition, where the Fermi surface changes from the hole-like centered at to the electron-like, centered at the Γ point causing a loss of large momentum anti-ferromagnetic fluctuations. Here, we study the doping dependence of the electronic structure of Bi1.8Pb0.4Sr2CuO in angle-resolved photoemission and find that the superconductivity vanishes at lower doping than that at which the Lifshitz transition occurs. This requires a more detailed re-examination of a spin fluctuation scenario.

Watt W. Webb: His measurements of the seemingly inaccessible broadened the horizons of biophysics

Watt W. Webb’s scientific career was a series of triumphs over challenges to make difficult measurements of important physical and biological phenomena: “impossible problems of experimental physiology,” as he described them (1). A common theme was his use of light microscopy in new ways to reveal equilibrium and dynamic properties in biomolecular systems and in organisms. Webb’s deep understanding of physics directly contributed to new instruments and methods for detecting and interpreting the hitherto undetectable and undecipherable. Webb and his collaborators opened approaches adopted far and wide to investigate important questions in areas of physics, biology, biochemistry, and biophysics. Webb was born on August 27, 1927 in Kansas City, Missouri. He entered the Massachusetts Institute of Technology at 16, where he majored in business and engineering administration. He then worked as an industrial engineer at Union Carbide. During that period, he resumed studies at the Massachusetts Institute of Technology and by 1955 had completed a Doctorate of Science in materials science physics and mathematics. Watt W. Webb. Image credit: Cornell University, licensed under CC BY-NC-ND. Beginning in the late 1950s and continuing through the early 1970s, Webb contributed to a wide variety of physics topics, including crystal growth and dislocations, magnetization, continuous transitions at critical liquid–vapor interfaces, and fluctuations in superconductors. Even at this early stage in his career, Webb showed an interest in statistical fluctuations, albeit in quantum systems, statistical noise, and phase transformations that prefigured his later work on more biologically oriented areas. Webb’s entry into biophysics in the early 1970s was driven by a tantalizing challenge: How to measure the kinetics of chemical reactions without displacing them from equilibrium. Even in equilibrium the concentrations of chemical reactants fluctuate spontaneously about their equilibrium values. The reaction kinetics could be determined from the time courses of these tiny … [↵][1]1Email: elson{at}wustl.edu. [1]: #xref-corresp-1-1

Nature of the magnetic stripes in fully oxygenated La2CuO4+y

We present triple-axis neutron scattering studies of static and dynamic magnetic stripes in an optimally oxygen-doped cuprate superconductor, La$_{2}$CuO$_{4+y}$, which exhibits a clean superconducting transition at $T_{\rm c}=42$ K. Polarization analysis reveals that the magnetic stripe structure is equally represented along both of the tetragonal crystal axes and that the fluctuating stripes display significant weight for in-plane as well as out-of-plane spin components. Both static magnetic order as well as low-energy fluctuations are fully developed in zero applied magnetic field and the low-energy spin fluctuations at $\hbar \omega = 0.3-10$ meV intensify upon cooling. We interpret this as an indication that superconductivity and low-energy spin fluctuations co-exist microscopically in spatial regions which are separated from domains with static magnetic order.

Erratum to: Superconductivity and spin fluctuations

In the original publication of this article the DOI number is incorrect. It should be doi:10.1007/s11467-011-0221-0.

Upper critical field and superconductor-metal transition in ultrathin niobium films

Recent studies suggest that in disordered ultrathin films superconducting (SC) state may be intrinsically inhomogeneous. Here we investigate the nature of SC state in ultrathin Nb films, of thickness d ranging from 1.2 to 20 nm, which undergo a transition from amorphous to polycrystalline structure at the thickness d simeq 3.3 nm. We show that the properties of SC state are very different in polycrystalline and amorphous films. The upper critical field (H_{c2}) is orbitally limited in the first case, and paramagnetically limited in the latter. The magnetic field induced superconductor-metal transition is observed, with the critical field approximately constant or decreasing as a power-law with the film conductance in polycrystalline or amorphous films, respectively. The scaling analysis indicates distinct scaling exponents in these two types of films. Negative contribution of the SC fluctuations to conductivity exists above H_{c2}, particularly pronounced in amorphous films, signaling the presence of fluctuating Cooper pairs. These observations suggest the development of local inhomogeneities in the amorphous films, in the form of proximity-coupled SC islands. An usual evolution of SC correlations on cooling is observed in amorphous films, likely related to the effect of quantum fluctuations on the proximity-induced phase coherence.

Analysis of fluctuation conductivity in Y1−хCdxBa2Cu3O7−δ (x = 0−0.4)

The effect that the partial substitution of Cd for Y has on the mechanism of excess conductivity formation in polycrystalline Y1−хCdxBa2Cu3O7−δ with x = 0 (Y1), 0.1 (Y2), 0.3 (Y3), and 0.4 (Y4) is investigated. The resistivity ρ of the samples increases markedly with increasing x, and the critical temperature of the superconducting (SC) state transition, Tc, decreases. The mechanism responsible for the formation of fluctuation conductivity, σ'(T), is considered within the framework of the Aslamazov–Larkin theory near Tc. The Ginzburg temperature (TG), the critical temperature in the mean-field approximation (Tcmf), the temperature of the 3D–2D crossover (T0), and T01, which limits the region of the SC fluctuations from above, are determined. It is shown that doping with Cd at x = 0−0.4 increases the coherence length along the c axis, ξc(0), by 2.7 times, and the distance between the CuO2 planes, d01, by 2.2 times. The temperature dependences of the pseudogap (PG), Δ*(T), are determined by analyzing the excess conductivity within the framework of the local pair model. It is found that with an increase in substitution, the maximum value of the PG Δ*(Tpair) decreases from 250.2 to 215.7 K, while the real value of the PG, measured at TG,Δ*(TG), increases from 217.4 to 224.2 K.