Fluctuations In Superconductors Research Articles

Comment on “Critical Fluctuations in Superconductors and the Magnetic Field Penetration Depth”

A Comment on the Letter by Igor E. Herbut and Zlatko Te\ifmmode \check{s}\else \v{s}\fi{}anovi\ifmmode \acute{c}\else \'{c}\fi{}, Phys. Rev. Lett. 76, 4588 (1996). The authors of the Letter offer a Reply.

Superconductivity fluctuation effects on the thermal conductivity of Bi2Sr2CaCu2O8.

The superconductivity fluctuation contribution ${\ensuremath{\kappa}}_{\mathrm{fl}}$ to the thermal conductivity of a ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{1.8}{\mathrm{Ca}}_{1.2}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ polycrystal is extracted from precise experimental data. The crossover from two-dimensional (2D) to 3D behavior theoretically predicted by Varlamov and Livanov is well marked. The crossover temperature ${T}_{\mathrm{VL}}$ and the amplitude of the fluctuation contribution lead to realistic values for the interlayer coupling energy ${J}_{C}$ and the transport relaxation time $\ensuremath{\tau}$ near ${T}_{c}$.

Anomalous microwave absorption in quasi-one-dimensional (Sr,Ca) 14Cu 24O 41

Anomalously large and higly reproducible cusps in the temperature-dependent surface resistance data have been observed using contactless microwave loss measurements at 9.2 GHz in (Sr,Ca) 14Cu 24O 41 — a novel cuprate with quasi-one-dimensional two-legged ladder Cu 2O 3 planse and edge-sharing CuO 3 chains. The onset temperatures of the microwave cusps, which are as high as 270 K, decrease with increasing calcium content and nitrogen annealing. No indications of bulk superconductivity are evident in dc magnetic susceptibility and ac resistance measurements in the 80 to 300 K and 15 to 300 K temperature ranges respectively. However, a rapid increase in intensity of the Cu 2+ electron paramagnetic resonance signal, which is likely to be associated with persistent currents in filamentary fractions of the samples, occurs at temperatures near the minima of the cusps. The observed cusps in the microwave data are tentatively interpreted as superconductive pairing fluctuations close to a charge density wave instability in the quasi-one-dimensionally structured ladder planes of optimally doped intergranular interfaces of the ceramic specimens.

On the critical fluctuations in superconductors

The field-theoretical gauge model for a superconductor, generalized to an n/2-component complex order parameter is considered. The question of the order of the phase transition occurring in this model is discussed. Previous renormalization-group calculations suggest that in such a model a fluctuation-induced first-order phase transition occurs. We re-examine previously obtained expressions for the renormalization-group functions in the two-loop approximation in three dimensions. Special attention is being paid to the fact that the corresponding series might be the asymptotic ones and therefore have zero radius of convergence. We discuss the possible ways of analytical continuation of the series obtained. Comparing results obtained by `direct' calculations with those obtained by Padé analysis and the Padé - Borel resummation technique, the conjecture is made that in the model under consideration there still exists a possibility for a second-order phase transition.

Critical fluctuations in superconductors and the magnetic field penetration depth.

The superconducting transition is studied within the one-loop renormalization group in fixed dimension $D\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$ and at the critical point. A tricritical behavior is found and, for $\ensuremath{\kappa}>{\ensuremath{\kappa}}_{c}$, an attractive charged fixed point, distinct from that of a neutral superfluid. The critical exponents of the continuous transition are evaluated, and it is shown that the anomalous dimension of the gauge field equals unity. This implies the proportionality of the magnetic field penetration depth and the superconducting correlation length below the transition. The penetration depth exponent is nonclassical. We argue that it cannot be extracted from the dual theory in a straightforward manner since it is not renormalized by fluctuations of the dual field.

Theory for the interdependence of high- Tc superconductivity and dynamical spin fluctuations

The doping dependence of the superconducting state for the 2D one-band Hubbard Hamiltonian is determined. By using an Eliashberg theory within the FLEX approximation, we find that the gap function Δ k has a d x 2− y 2 symmetry and T c becomes maximal for 13% doping. Furthermore, we find a strong interplay between d-wave superconductivity and short range anitferromagnetic order leading to the appearance of shadow states below T c . Since we determine the dynamical excitations directly from real frequency axis calculations, we obtain new results for the spectral density of states that explain the anomalous behavior of photoemission and tunneling experiments.

ChemInform Abstract: Superconductivity and Spin Fluctuations in Doped CeIr5.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Superconductivity and spin fluctuations in the electron-doped infinitely-layered high T c superconductor Sr 0.9La 0.1CuO 2 ( Tc = 42 K)

We report 63Cu NMR studies on uniaxially aligned and unaligned powder samples of an electron-doped infinitely-layered high T c cuprate superconductor Sr 0.9 La 0.1 CuO 2 ( T c = 42 K). We measured the nuclear spin-lattice relaxation rate 1 T 1 up to 800 K. We found from the temperature dependence of 1 T 1 T that the wave-vector-averaged spin susceptibility is highly enhanced by spin-fluctuations in the normal state, resulting in a Curie-Weiss behavior. Below T c, we did not observe a Hebel-Slichter coherence peak of 1 T 1 , suggesting an unconventional nature of the symmetry of the superconducting order parameter. These results are quite similar to those observed for some hole-doped high T c cuprates.

Superconductivity and spin fluctuations in doped CeIr 5

We report a detailed investigation of the binary compound CeIr 5. Owing to difficulty in being able to prepare single-phase CeIr 5, we studied the low-temperature properties of CeIr 5, by investigating the doping-stabilized CeIr 5, systems (Th 1− x Ce x )Ir 5, x ⩽ 0.7, and Ce(Pt 1− x Ir x ) 5, x ⩽ 0.85. Via measurements of the specific heat and magnetic susceptibility, we observed spin fluctuations in the Ce(Pt 1− x Ir x ) 5 system, arising from a Pauli paramagnetic state, and superconductivity in (Th 1− x Ce x )Ir 5.

Ward's identity and the suppression of spin fluctuation superconductivity

Of crucial importance in superconductors is the vanishing of certain coherence factors, such as pkq = ukuk+q - vkvk+q for bare fermion energies {epsilon}k = {epsilon}k+q. This result leads to the abrupt drop of the acoustic attenuation rate below T{sub c} in conventional superconductors. Similarly the interference of {phi}k and {phi}k+Q for the statically distorted normal crystal leads to gkQ {yields} O, corresponding to a deformation potential theorem for the zone boundary phonon.

Importance of phase fluctuations in superconductors with small superfluid density

THE superconducting state of a metal is characterized by a complex order parameter with an amplitude and a phase. In the BCS-Eliashberg mean-field theory1, which is a very good approximation for conventional metals, the phase of the order parameter is un-important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition. Here we argue that superconductors with low super-conducting carrier density (such as the organic and high-Tc oxide superconductors) are characterized by a relatively small phase 'stiffness9 and poor screening, both of which imply a significantly larger role for phase fluctuations. As a consequence, in these mat-erials the transition to the superconducting state may not display typical mean-field behaviour, and phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties. For some quasi-two-dimensional materials, notably underdoped high-temperature superconductors, the onset of long-range phase order controls the gross value of Tc as well as its systematic variation from one material to another.

Self consistent calculation of D-wave superconductivity in Hubbard model

Self consistent calculations of super-conductivity, including spin and charge fluctuations in a 2-dimensional Hubbard model, give a finite gap with d x 2-y 2 symmetry and a T c =0.016 times the in plane hopping (t) for Hubbard U very near the antiferromagnetic transition. The Stoner enhancement factors in the incommensurate peak of the imaginary part of the spin susceptibility in the 2-D Brillouin zone is found to be of order 150 to 200 and its absolute value at T c is close to the recently measured value for LaSrCuO but it is found to decrease faster with temperature than indicated in experiments.

Superconducting fluctuation conductivity in granular Al-Ge films above the metal-insulator transition

Superconducting fluctuation conductivities and magnetoconductances have been measured in thin granular Al-Ge films above the superconducting transition temperature. For very metallic films located well above the metal-insulator transition, the two-dimensional Aslamazov-Larkin and Maki-Thompson theories describe the conductivity and magnetoconductance data well. The Maki-Thompson expressions involve the temperature-dependent pair-breaking parameter delta . For films located near the metal-insulator transition, we have observed a dimensional crossover from three dimensions to two dimensions to a fractal dimension with decreasing temperatures. Values for the diffusion constant, ranging from 0.4 to 1.4 cm2 s-1, were obtained from the magnetoconductance fits, and these magnitudes compared favourably with values obtained from critical-field measurements taken below Tc.

Superconducting fluctuation effects in grain-aligned HgBa 2Ca 2Cu 3O 8+ δ

We have measured the normal state magnetization for grain-aligned HgBa 2Ca 2Cu 3O 8+ δ with high and low magnetic fields applied perpendicular to the CuO 2 planes. The high-field magnetization curves follow the three-dimensional scaling based on the Ginzburg-Landau theory. In addition, we observed significant superconducting fluctuation diamagnetism above T c using a low magnetic field, and analyzed it by utilizing the modified Lawrence-Doniach model. Our result demonstrates the dimensional crossover from 3D to 2D as the temperature increases, which is consistent with the scaling result of a high-field limit. From the analysis, we obtained the upper critical field slope, zero-temperature GL coherence length, and mass anisotropy.

Thermal conductivity of pure or iron-doped YBa2Cu3O7- delta with or without an excess of CuO

The thermal conductivities of YBa2(Cu1-xFex)3O7- delta +y% CuO ceramics for 0<x<0.03 and 0<y<5 have been measured. The thermal conductivities of all samples exhibit a minimum in the vicinity of the critical temperature Tc and a maximum near Tc/2. These results are interpreted with the help of an electronic model, i.e. supposing that the main contribution to the thermal conductivity below Tc is due to electron scattering in the CuO2 planes. Within a simple two-fluid model derived from kinetic theory, taking into account the temperature dependence of the electronic relaxation time and the normal charge carrier concentration, we obtain theoretical curves which reproduce the experimental results quite well. We also take into account the porosity of the samples (in term of an intergrain contribution) to derive the electronic thermal conductivity. The parameter values of the model and the observed minimum are explained in terms of physical properties. In particular, the contribution of superconductivity fluctuations to the thermal conductivity is shown to be negligible with respect to other mechanisms.

Resistive superconducting transition of ?-type BEDT-TTF organic superconductors in a magnetic field

The superconducting transition of the organic compounds [kappa]-(BEDT-TTF)[sub 2]X is studied by resistive measurement in a magnetic field up to 10 T applied normal to the conducting plane. For the salts with X = Cu[N(CN)[sub 2]]Br and X = CuCN[N(CN)[sub 2]] the transition shows fan-shaped broadening caused by superconductivity fluctuation. For the X = Cu(NCS)[sub 2] salt the resistivity shows a peak in the transition region in a magnetic field below 4 T. This phenomenon is suppressed in defect-reduced samples for intralayer conduction. We discuss this peak in relation to the thermal fluctuation on the Josephson junction structures in this salt. 16 refs., 3 figs.

Effect of superconductive fluctuations on nuclear magnetic relaxation in high-Tc superconductors.

Effect of superconductive fluctuations on nuclear magnetic relaxation in high-Tc superconductors.

Phase diagram of a high-temperature superconductor in the Bi 2Sr 2CuO y system

The resistive transition of the superconductivity was measured for Bi 2Sr 2CuO y (Bi-2201) single crystals with maximal T c in the configuration of H// c-axis. We observed the breaking of bulk superconductivity due to the external magnetic field at 1.6 K. The superconducting phase boundary was obtained from the irreversibility line defined by the zero-resistance temperature, and the full diagram of the superconductivity was determined in the H-T plane. We estimated the fluctuation superconductivity region whose onset boundary was located far from the bulk superconductivity.

Theory of Nested d-p Model with Strong Repulsion

The nested d-P model with a strong repulsion on d-sites is studied. Starting with Green functions of a normal paramagnetic state in an isolated atomic limit (transfer->0), expansions with respect to residual interactions are performed and scaling equations are solved. The attraction between d-electrons mediated by doped holes plays a dominant role. The possible orders in one­ dimension are of charge-density wave and singlet superconductivity for small doping, and of charge and spin-density wave for large doping, respectively. In quasi-one-dimension only charge-density wave establishes three-dimensional order and the fluctuation of singlet superconductivity develops in short range.

Coupling of fluctuation modes in the disordered strongly attractive Hubbard model

The coupling of two different types of soft modes related to interaction-generated superconducting and disorder-generated (pseudo)diffusive fluctuations in the large negative-U Hubbard model with random local energies is studied in the framework of a Grassmann field theoretic approach with modified semionic Matsubara frequencies. In leading loop order it is found that the sound-like order parameter phase fluctuation mode retains its massless nature with only its velocity being modified, as expected from the generalized Ward identity. The disorder-induced fluctuations which are originally completely local due to the locality of disorder correlations acquire true diffusive behaviour by interaction effects in the superconducting phase.