Coherence Transition Research Articles

Resonant inverse photoemission study of CeCoGe 3− xSi x

We have measured the resonant inverse photoemission spectra (RIPES) of CeCoGe 3−x Si x (0⩽x⩽3) as functions of composition and temperature. The strong composition dependence of 4f-state just above the Fermi level ( E F) shows that the localized 4f electrons in the compound become itinerant due to the enhanced Kondo effect. Temperature dependence of the f 1-peak indicates a transition from the paramagnetic state at room temperature to a coherent Fermi-liquid regime at low temperatures. The most remarkable variation of the spectra has been observed above 200 K with increasing temperature in CeCoSi 3 which has a very high Kondo temperature T K of about 900 K. This suggests that the change in the 4f-electronic state of CeCoSi 3 may be scaled by the coherence transition temperature rather than T K.

Observation of the coherence transition into a collective dense Kondo state by resonant inverse photoemission spectroscopy

By the resonant inverse photoemission spectroscopy (RIPES), we study the crossover from mixed-valent to heavy-fermion and Kondo regime in ${\mathrm{CeCoGe}}_{3\ensuremath{-}x}{\mathrm{Si}}_{x}$ $(0<~x<~3).$ We have measured the RIPE spectra as functions of composition and temperature. The strong composition dependence of $4f$ state (``${f}^{1}$ peak'') just above the Fermi level shows that the localized $4f$ electrons in the compound become itinerant due to the enhanced Kondo effect within the range $1.0<~x<~1.5.$ Temperature dependence of the ${f}^{1}$ peak indicates a transition from the paramagnetic state at room temperature to a coherent Fermi-liquid regime at low temperatures where the Kondo effect dominates. The most remarkable variation of the spectra has been observed above 200 K with increasing temperature in ${\mathrm{CeCoSi}}_{3}$ which has a very high Kondo temperature ${T}_{K}$ of about 900 K. This suggests that the change in the $4f$-electronic state of ${\mathrm{CeCoSi}}_{3}$ may be scaled by the coherence transition temperature rather than ${T}_{K}.$

Phase Coherence and Microphase Separation Transitions in Diblock Copolymer Thin Films

In thin films of diblock copolymers it was found that microphase separation and the establishment of phase coherence can occur as two distinct events in diblock copolymer thin films, at widely separated temperatures. At temperatures far removed from the bulk microphase separation transition temperature increasing overlap of the decaying concentration profiles from the substrate and air interfaces leads to a sharp onset of phase coherence throughout a thin film and the discretization of the film thickness, despite only weak microphase separation in the center of the film. A further transition to a fully microphase separated state occurs at temperatures close to the microphase separation transition temperature, but this transition becomes broader and less distinct with decreasing film thickness. In thicker films, on the other hand, phase coherence is not established until the ordering transition temperature is reached, due to the first-order nature of the transition and the finite coherence length in the disordered state. In thick films, phase coherence throughout the film is established either simultaneously or after complete microphase separation has occurred.

Coherence transition in granular high temperature superconductors

We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa 2Cu 3O 7− δ , YBa 2(Cu 2.98Zn 0.02)O 7− δ and GdBa 2Cu 3O 7− δ . The results show that the transition proceeds in two stages. Careful analysis of the conductivity in the regime of approach to the zero resistance state reveals the occurrence of a coherence transition, which is related to the connective nature of the granular samples. This transition occurs when the fluctuating phases of the order parameter in individual grains become long-range ordered. We obtain the exponent for fluctuation conductivity and the relevant critical temperature, T co, which is close to the point where resistivity vanishes. The specific heat results, when analyzed as d C/d T, show a weak but reproducible cusp-like anomaly at T co. This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.

Excess conductivity studies in the paracoherence region of Bi-Sr-Ca-Cu-O superconductors

The temperature variation of resistivity of several Samples of 85 K and 110 K superconductors of BiSrCaCuO system prepaed under different conditions was analysed in terms of excess conductivity due to phase fluctuations in the paracoherence region. The average values of estimated excess conductivity critical exponent (γ) for the 85 K and 110 K superconductors are respectively 2.55 and 2.75. The paracoherence to coherence transition in a granular superconductor is compared to the paramagnetic to ferromagnetic transition of 3D XY ferromagnets. The γ values obtained for these superconducting samples differ from the predicted susceptibility critical exponent value for the pure 3D XY ferromagnets. However, the obtained γ values are comparable to the susceptibility critical exponent of the disordered 3D Heisenberg ferromagnet system.

Disorder and non-linear magnetic response of high T c superconductors

We measure the low frequency magnetic response of YBa 2Cu 3O 6.7F x (0≤×<0.2) ceramics in a wide range of a.c. fields (10 −7 T ≤ μ 0H 0 ≤ 10 −4 T). When changing the amount of disorder (varying x) on the microscopic level we find the same non linear response with field amplitude H 0 as in granular conventional superconductors. The real part of the susceptibility appears as a universal function of H 1(T)/H 0 where H 1(T) is the field of first flux penetration. The power law dependence found for H 1(T) can be understood in the framework of the coherence transition of granular superconductors with random couplings.

Phase coherence and nonadiabatic transition at a level crossing in a periodically driven two-level system.

Some aspects of the Landau-Zener-type transition of a periodically driven two-level system with dephasing are clarified. Under the condition that the transition is well localized around the level-crossing region, the transition dynamics for the typical cases is described by simple formulas; namely, by the transfer-matrix method for the long-range coherence, by the application of the incoherent Landau-Zener formula for the short-range coherence, and by the strong-dephasing formula in the limit of complete decoherence.

Scaling behaviour and crossover of resistive transition in YBa 2Cu 3O 7- x

Our resistivity measurements on YBa 2Cu 3O 7- x ceramics, in the limit I=0, confirm a second-order phase transition at a critical temperature T c(0)< T cs where T cs is the critical temperature of the superconducting phase transition. We assume a percolation phase transition as it has been proposed in usual disordered granular superconductors for the onset of a superconducting order. Moreover, we show the existence of a crossover temperature T ∗( I). For the temperatures T > T ∗( I) > T c(0), σ∼ t -γ with γ⋍0.9 and t = (T-T c(I))/T c(0) where T c( I) corresponds to the critical temperature shift due to I. This conductivity critical behaviour is identical to the one observed as T→ T c(0) + and I=0. For the temperature range T c( I)< T⪌ T ∗ ( I), σ∼ A( I) t −γ̃ where the critical exponent value γ̃⋍1.3 appears to be in good agreement with the one computed elsewhere for the pure 3D XY system. We suggest that phase fluctuations account for the excess conductivity above the percolation threshold and we propose an intergranular coherence transition at T c( I).

Critical and gaussian fluctuations in superconducting oxides

Abstract We discuss the transport properties of high-Tc cuprates as resulting from a two-step transition: a superconducting intragranular transition at T cs and an intergrandular coherence transition at Tc < T cs. The first one can be described by mean-field theory, while the second one displays scaling properties characteristic of critical phenomena. A variety of behaviours are predicted and found for the maximum supercurrent at T < Tc . Critical phase fluctuations typical of granular systems account for the excess conductivity at T ≳ Tc . The rounding of the resistive transition at T ≳ Tcs can be explained by small (gaussian) fluctuations of the order parameter amplitude, once the role of granularity is taken into account. We also propose an explanation for the apparently intrinsic granularity of superconducting oxides.

Générateur de machines séquentielles autotestables pour circuits intégrés spécifiques

This paper presents an original method using a ROM memory, of systematically synthesizing on-line and offline self-testing synchronous sequential machines (automata, sequencers,), for the design of compiled ASIC’S. Choices about state transition graph coding and circuit architecture are related to simplicity, compactness, operating rate and especially to testability. The on-line and off-line testing is based on an error detecting code of a recurrent type: in addition to useful data, the ROM memory contains redundant information which is distributed between two consecutive states. This method guarantees both present state coherence and past transition conformity. On-line testing is based in this type of coding, called distributed redundancy coding. Its fault coverage is given for different classes of hardware failures. Off-line testing, which is exhaustive, uses the distributed redundancy coding technique and consists in scanning all the possible graph transitions in memory address order.

Generalized Bose-gas condensation model for T c in multi-dimensional superconductivity

Strong quantum interaction in a Bose-Einstein gas is sufficient to derive reasonable estimates of Tc, the energy gap, and the coherence length for all classes of superconductors such as the ceramic oxides, the metallics, heavy-Fermion metals, metallic hydrogen, and neutron stars for 3-dimensional, quasi-2 and quasi- 1 dimensional states. For the ceramic oxides this yields 10K, 40K, and 300K in 3, Q2, and Q1 dimensions. Interpreting the ceramic oxide case as one in which the interchain interactions are equal to the intrachain interactions, leads to Tc = (Tc1 Tc2)12 = (300K × 40K)12 = 110K as the approximate transition temperature for these materials when there is clearly a combined linear and planar structure. It is noteworthy that without specifying a coupling mechanism or coupling strength, this general model does well in calculating coherence lengths, transition temperatures, and coupling strengths over nine orders of magnitude (1K to 109K and meV to MeV) from the heavy Fermion metals to neutron stars.

Spin fluctuations and superconductivity in heavy fermions

We study within a scaling theory, recently proposed to describe the coherence transition in heavy fermions, the importance of spin fluctuations for the superconductivity of these systems.

Quantum coherence and phase transitions in granular superconductors with dissipation. II. Effect of disorder

We study the effect of disorder on a superconducting phase transition in granular arrays with dissipation. It is shown that, for a quite weak Gaussian disorder, this effect does not lead to a qualitative change of the phase diagram. Local quantum fluctuations suppress Cooper pair tunneling for a part of Josephson junctions and thus effectively increase disorder. For strongly disordered systems, the critical resistance which corresponds to a dissipative phase transition is found to be universal for a wide range of parameters.

Quantum coherence and phase transitions in granular superconductors with dissipation. I. Ordered arrays

We investigate the effect of dissipative currents on a superconducting phase transition in two- and three-dimensional granular arrays. We find the corresponding phase diagrams and show that, depending on dissipation, both long-range and local quantum fluctuations may be effective in destroying the global phase coherence. For large values of charging-to-Josephson coupling energy ratio and atT=0, we find the universal critical resistanceRc only for three dimensional arrays while for two dimensional onesR c −1 diverges logarithmically with increasing of the grain charging energy. For large values of a Josephson coupling energy, quantum phase slip effects play the main role and lead to the universal critical resistance in both cases. We also investigate the effect of dissipation on the critical temperature of theXY model phase transition and show that strong dissipation is extremely effective in suppressing quantum effects.

Phase transitions in granular superconducting matter and the magnetic properties of high T c ceramics

The paracoherence-coherence transition of weakly coupled superconducting grains is analogous to the paramagnetic-ferromagnetic transition of X-Y ferromagnets. The critical exponents β, γ and ν measured in granular superconductors differ from the X-Y model calculations for translationally invariant systems. We ascribe this discrepancy to disorder. We discuss the magnetic properties of YBaCuO sintered and powdered samples in the framework of the coherence transition. This approach allows us to explain the “incomplete Meissner effect” of superconducting ceramics and to show the existence of several levels of granularity; the latter however do not affect the critical behaviour.

Critical approach to the coherence transition in Kondo lattices.

We study the Kondo lattice from the point of view of the scaling theory of critical phenomena. The coherence temperature ${\mathrm{scrT}}_{c}$ is associated with a crossover from a paramagnetic state with local moments to a Fermi-liquid regime with strong magnetic correlations. We obtain an expression for ${\mathrm{scrT}}_{c}$ in terms of the Kondo-lattice parameters and a crossover exponent ${\ensuremath{\varphi}}_{t}$. The effect of a magnetic field is investigated and characterized by another critical exponent. These exponents obey scaling relations with a shifted dimensionality.

Coherent surface fluorescence versus thermally activated energy transfer to the bulk in the anthracene crystal: Model calculations and some experimental results

The spontaneous emission from a thermally disordered 2D exciton should show a sharp transition between fast coherent emission at low temperatures and incoherent domain emission at higher temperatures. Looking for this transition, we investigated the anthracene surface emission (intensity and lineshape) as a function of temperature. The observed steep decrease of the surface emission intensity with temperature was attributed to an activated quenching by the underlying bulk states. We devised a crude model which accounts qualitatively for the observed temperature dependence, showing that the coherence transition was masked in our experiments by the bulk quenching and should be sought in other systems. Using our model, we analyzed the different surface decay channels, determined the surface-bulk coupling constant and predicted that the surface emission lifetime cannot be longer than 1 ps at any temperature.

Granular superconductivity in R1Ba

Present methods of fabricating thin films and bulk samples of the high-${T}_{c}$ oxide superconductor ${R}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ frequently produce a granular morphology with stoichiometric grains separated by weakly superconducting material, Josephson, or proximity effect junctions. In these films, the bulk superconducting properties (transition width, critical current density) are controlled primarily by the strength of coupling between the grains and not by the superconductivity of the grains themselves. We have examined the $I\ensuremath{-}V$ characteristics of thin-film oxide superconductors as a function of temperature and find evidence for a coherence transition in which isolated grains couple to form a bulk superconductor. The transition manifests itself in the scaling behavior $V\ensuremath{\sim}{(I\ensuremath{-}{I}_{c})}^{x}$ where $x$ changes abruptly at coherence. The magnetotransport is in support of the model.

Phase coherence and quantized circulation transitions in superfluid helium.

Phase coherence and quantized circulation transitions in superfluid helium.