A.M. Gabovich

Temperature-dependent pseudogap-like features in tunnel spectra of high-Tc cuprates as a manifestation of charge-density waves

Temperature, T, variations of the tunnel conductance G(V) were calculated for junctions between a normal metal and a spatially inhomogeneous superconductor with a dielectric gap on the nested sections of the Fermi surface or between two such superconductors. The dielectric gapping was considered to be a consequence of the charge density wave (CDW) appearance due to the electron–phonon (for a Peierls insulator) or a Coulomb (for an excitonic insulator) interactions. Spatial averaging was carried out over random domains with varying parameters of the CDW superconductor (CDWS). The calculated tunnel spectra demonstrate a smooth transformation from asymmetric patterns with a pronounced dip–hump structure at low T into those with a pseudogap depletion of the electron densities of states at higher T in the vicinity or above the actual critical temperatures of the superconducting transition for any of the CDWS domains. Thus, it is demonstrated that both the dip-hump structure and pseudogapping are manifestations of the same phenomenon. A possible CDW-induced asymmetry of the background contribution to G(V) is also touched upon. The results explain the peculiar features of G(V) for Bi2Sr2CaCu2O8+δ and other related high-Tc cuprates.

Competition of Superconductivity and Charge Density Waves in Cuprates: Recent Evidence and Interpretation

Explicit and implicit experimental evidence for charge density wave (CDW) presence in high- superconducting oxides is analyzed. The theory of CDW superconductors is presented. It is shown that the observed pseudogaps and dip-hump structures in tunnel and photoemission spectra are manifestations of the same CDW gapping of the quasiparticle density of states. Huge pseudogaps are transformed into modest dip-hump structures at low temperatures, , when the electron spectrum superconducting gapping dominates. Heat capacity jumps at the superconducting critical temperature and the paramagnetic limit are calculated for CDW superconductors. For a certain range of parameters, the CDW state in a -wave superconductor becomes reentrant with , the main control quantity being a portion of dielectrcally gapped Fermi surface. It is shown that in the weak-coupling approximation, the ratio between the superconducting gap at zero temperature and has the Bardeen-Cooper-Schrieffer value for -wave Cooper pairing and exceeds the corresponding value for -wave pairing of CDW superconductors. Thus, large experimentally found values are easily reproduced with reasonable input parameter values of the model. The conclusion is made that CDWs play a significant role in cuprate superconductivity.

How to explain the non-zero mass of electromagnetic radiation consisting of zero-mass photons

The mass of electromagnetic radiation in a cavity is considered using the correct relativistic approach based on the concept of a scalar mass not dependent on the particle (system) velocity. It is shown that due to the non-additivity of mass in the special theory of relativity the ensemble of chaotically propagating mass-less photons in the cavity has a finite overall mass.

What do we mean when using the acronym ‘BCS’? The Bardeen–Cooper–Schrieffer theory of superconductivity

The history and use of the acronym ‘BCS’ (named after Bardeen, Cooper and Schrieffer) in the science of superconductivity is traced and analysed. It is shown that a number of different theories are labelled ‘BCS’. The confusion in the application of the term ‘BCS’ is shown to be common because the term ‘theory’ itself is not precisely defined in physics. Recommendations are given to physics readers and students on how to distinguish between various theories referred to as ‘BCS’. Contributions from scientists in different laboratories to the creation and formation of the superconductivity theory are indicated.

Transport study of newly synthesized polycrystalline Sr1−xKxPbO3−δ solid solutions

Abstract New ceramics of Sr 1− x K x PbO 3− δ with x =0, 0.05, 0.1, 0.15 and 0.2 were synthesized and analyzed. Temperature, T , variations of resistivity, ρ , and thermoelectric power, S , were measured for 20 T T , the thermal resistivity coefficients d ρ /d T for all compositions were found to be negative, which suggests localization of current carriers or the appearance of a dielectric gap in the electron structure. The analysis of the ρ ( T ) curve shows that the electrical conduction has at least a substantial contribution from the tunneling processes at the grain boundaries. The thermoelectric power S is negative for all compositions and for all T , thus indicating electron-like conduction. The collection of data demonstrates that the parent substance SrPbO 3 is on the verge of the metal–insulator transition and different kinds of doping may substantially change the transport properties and the degree of current carrier degeneracy. The system seems to be promising as a building block for new superconducting families.

Influence of the spatially inhomogeneous gap distribution on the quasiparticle current in c-axis junctions involving d-wave superconductors with charge density waves

The quasiparticle tunnel current J(V) between the superconducting ab-planes along the c-axis and the corresponding conductance [Formula: see text] were calculated for symmetric junctions composed of disordered d-wave layered superconductors partially gapped by charge density waves (CDWs). Here, V is the voltage. Both the checkerboard and unidirectional CDWs were considered. It was shown that the spatial spread of the CDW-pairing strength substantially smears the peculiarities of G(V) appropriate to uniform superconductors. The resulting curves G(V) become very similar to those observed for a number of cuprates in intrinsic junctions, e.g. mesas. In particular, the influence of CDWs may explain the peak-dip-hump structures frequently found for high-T c oxides.

Resistive noise and heat effects in the granular oxide superconductor BaPb0.75Bi0.25O3

For weak applied magnetic fields the frequency spectrum of the voltage fluctuations was measured for Josephson ceramic samples of BaPb0.75Bi0.25O3 with Tc approximately=10 K. The measurements were carried out in the resistive state. It is shown that the spectral density Sv(f) in the frequency region 20 Hz or approximately=10 Oe. For currents I>or approximately=0.8 mA through the sample, oscillations with a fundamental frequency f1 or approximately=2 and the emergence of the oscillations apparently occur owing to heating effects.

Flicker-noise in superconducting glass - ceramics BaPb0.75Bi0.25O3

In ceramic superconductor BaPb0.75Bi0.25O3 with current-voltage characteristics of the tunnel-type the spectrum of voltage fluctuations was measured for T = 4.2 K. The noise spectral density SV(f) in the frequency range 20 Hz ≤ f ≤ 500 Hz is described by the formula f-α (1 ≤ a ≤ 2.2), where α is substantially dependent on the external magnetic field H ≤ 16.6 Oe. The observed noise is apparently associated with the emergence of the superconducting glass state. The spectra contain also voltage oscillations of thermal origin with the fundamental frequency fo≈ 100 Hz.

Quasiparticle conductance-voltage characteristics for break junctions involving d-wave superconductors: charge-density-wave effects

Quasiparticle tunnel conductance-voltage characteristics (CVCs), [Formula: see text], were calculated for break junctions (BJs) made up of layered d-wave superconductors partially gapped by charge-density waves (CDWs). The current is assumed to flow in the ab-plane of electrodes. The influence of CDWs is analyzed by comparing the resulting CVCs with CVCs calculated for BJs made up of pure d-wave superconductors with relevant parameters. The main CDW-effects were found to be the appearance of new CVC peculiarities and the loss of CVC symmetry with respect to the V-sign. Tunnel directionality was shown to be one of the key factors in the formation of [Formula: see text] dependences. In particular, the orientation of electrodes with respect to the current channel becomes very important. As a result, [Formula: see text] can acquire a large variety of forms similar to those for tunnel junctions between superconductors with s-wave, d-wave, and mixed symmetry of their order parameters. The diversity of peculiarities is especially striking at finite temperatures. In the case of BJs made up of pure d-wave superconductors, the resulting CVC can include a two-peak gap-driven structure. The results were compared with the experimental BJ data for a number of high-T c oxides. It was shown that the large variety of the observed current-voltage characteristics can be interpreted in the framework of our approach. Thus, quasiparticle tunnel currents in the ab-plane can be used as an additional mean to detect CDWs competing with superconductivity in cuprates or other layered superconductors.

Charge-density-wave features in tunnel spectra of high-Tc superconductors

Tunnel conductances G(V) = dJ/dV(V), where J(V) is the quasiparticle current and V is the voltage, have been calculated for nonsymmetric (CDWS-I-N) and symmetric (CDWS-I-CDWS) tunnel junctions (N, I, and CDWS stand for a normal metal, insulator, and electronically inhomogeneous CDW superconductor, respectively). The CDWS inhomogeneity was shown to be responsible for the appearance of smooth but conspicuous dip-hump structures (DHSs) in G(V) at T » Tc. At higher T, the DHS transforms into a broad pseudogap depletion in the density of states. Good qualitative agreement was obtained between theoretically calculated and experimental G(V) dependences.