Charge- and spin-density waves in existing superconductors: competition between Cooper pairing and Peierls or excitonic instabilities

Characteristic features of the exotic superconductors

The non-stationary Josephson (-), and quasiparticle (J-) currents through symmetrical and non-symmetrical tunnel junctions involving superconductors with charge-density waves were calculated. In both cases the voltage dependences of the currents have logarithmic singularities and jumps at positions determined by the non-trivial combinations of the superconducting, , and dielectric, , order parameters. The current J in non-symmetrical junctions is an asymmetrical function of the voltage, and depends on the sign of . For symmetrical junctions this current may be either symmetrical or not, depending on the relationship between the signs of the s on either side of the junction.

Josephson tunnelling involving superconductors with charge-density waves

Some of the unusual properties observed experimen-tally in the high temperature superconductors (HTSC)can be explained in terms of the eﬀective two componentmodels, which involve boson and fermion degrees of free-dom. There are numerous examples of such theoreticalscenarios, for instance: electron ﬁelds coupled to gaugeﬂuctuations, the RVB spinon-holon theory, the coupledelectron-phonon systems, etc. In this paper we considerone of these models, where the conduction band parti-cles (fermions) coexist and interact with the localizedelectron pairs (hard-core bosons). This, so called BosonFermion (BF) model, had initially been introduced [13] acouple of years before the discovery of HTSC materials.The authors have conjectured that it might describe anemerging physics of the electron-phonon systems for theintermediate coupling strength.From a formal point of view, the Hamiltonian of thisBF model can be considered as an eﬀective one for severalmodels, which are often used in the solid state physics. Ithas been derived so far: (1) from the generalized periodicAnderson model with the largeon-site attraction by elim-inating the hybridization between the wide and narrowband electrons via canonical transformation [2], (2) froma purely fermionic single band system described by theextended Hubbard model using the concept of bosoniza-tion for lattice fermions [3], (3) from the Hubbard modelon the two dimensional plaquettized lattice in the stronginteraction limit using the contractor method [4], and (4)from the resonating valence bond state of the t−J modelin the path integral technique [5].The same BF model has been considered also by manyother authors who postulated it in a more ad hoc way.For example in the Ref. [6] author has proposed the BFtype charge transfer Hamiltonian basing his argumentson interpretation of the optical experiments. Other ideascould be found in the Ref. [7]. Authors have representedthe patches of the 2D Brillouin zone near the so calledhot spots via dispersionless bosons, which are coupledto fermions of a remaining part of the Brillouin zone.The BF model is not only considered in a context ofHTSC. There are recent attempts to apply the same typeof picture for a description of the magnetically trappedatoms of alkali metals [8].Unconventional mechanism of superconductivitywithin the BF model has been studied in a number ofpapers [1, 2, 7, 10, 11, 12, 13, 14]. Fermions acquiresuperconducting coherence via exchange of the hard-corebosons. The same processes lead also to the eﬀectivemobility of bosons. Fermions/bosons can undergo aphase transition into the superconducting/superﬂuidphase at identically the same critical temperatureT

Upward curvature of the upper critical field in the boson-fermion model

A superconducting transition in the temperature dependence of the ab-plane resistivity of underdoped YBa2Cu3O6+x crystals in the range Tc≲30 K has been investigated. Unlike the case of samples with the optimal level of doping, the transition width increased insignificantly with magnetic field, and in the range T≲13 K it decreased with increasing magnetic field. The transition point Tc(B) was determined by analyzing the fluctuation conductivity. The curves of Bc2(T) measured in the region T/Tc≳0.1 did not show a tendency to saturation and had a positive second derivative everywhere, including the immediate neighborhood of Tc. The only difference among the curves of Bc2(T) for different crystal states is the scales of Tand B, so they can be described in terms of a universal function, which fairly closely follows Alexandrov’s model of boson superconductivity.

Resistive transition and upper critical field in underdoped YBa2Cu3O6+x single crystals

The theory by Werthamer-Helfand-Hohenberg 1 ~WHH! predicts the universal behavior of the upper critical field Hc2(T) in superconductors with weak electron-phonon coupling. The behavior can be described by a universal function hc2(T) expressed in the reduced variables for magnetic field h5H@Tc(2dHc2 /dT)T5T c # 21 and temperature t5T/Tc . The function hc2(t) has a negative second derivative and saturates to the value hc2’0.7 at t50. The WHH theory has described successfully properties of conventional superconductors but many new superconductors discovered during the last decade exhibit behavior inconsistent with the theory. Amorphous alloys based on transition metals 2 and K 3C 60 ~Ref. 3! shows the linear dependence Hc2(T) down to low temperatures. Some organic superconductors 4 and electrondoped high-temperature superconductors @L 22x Ce x CuO 42y where L5Pr,Sm,Nd ~Refs. 5 and 6!# even have a positive second derivative. Furthermore, in stark contrast to WHH a diverging Hc2(T) was observed in overdoped Tl 2Ba 2CuO 6 films, 7 where Hc2 increased rapidly with decreasing temperature in the whole temperature range 1.t.0.001, and in

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G. E. Tsydynzhapov

Papers

Positive curvature in the temperature dependence of H-c2 in KxBa1-xBiO3