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

This review deals with the properties of superconductors with competing electron spectrum instabilities, namely, charge-density waves (CDWs) and spin-density waves (SDWs). The underlying reasons of the electron spectrum instability may be either Fermi surface nesting or the existence of Van Hove saddle points for lower dimensionalities. CDW superconductors include layered dichalcogenides, NbSe3, and compounds with the A15 and C15 structures among others. There is much evidence to show that high-Tc oxides may also belong to this group of materials. The SDW superconductors include URu2Si2 and related heavy-fermion compounds, Cr-Re alloys and organic superconductors. We review the experimental evidence for CDW and SDW instabilities in a wide range of different superconductors, and assess the competition between these instabilities of the Fermi surface and the superconducting gap. Issues concerning the superconducting order parameter symmetry are also touched upon. The accent is put on establishing a universal framework for further theoretical discussions and experimental investigations based on an extensive list of available and up-to-date references.

http://iopscience.iop.org/article/10.1088/0953-2048/14/4/201/pdf

Charge- and spin-density-wave superconductors

BaMnF4 is a pyroelectric ferromagnet which displays a number of unusual physical characteristics, some of which are unique. It has the only known continuous antiferroelectric phase transition. It has two- and three-dimensional antiferromagnetic ordering temperatures. It is a weak ferromagnet with ferromagnetism caused by the linear magnetoelectric effect-the only case yet known. It exhibits dielectric anomalies at its Neel temperature. At high temperatures its dielectric constant diverges with increasing temperature; a ferroelectric phase transition would occur if the crystals did not melt first. At high temperatures BaMnF4 is also an anisotropic ionic conductor. The antiferroelectric phase is incommensurate. The incommensurate phase appears unusual in that its translation vector is temperature-independent. Sound velocity measurements made near the antiferroelectric phase transition temperature demonstrate the presence of and characteristics for a relaxational mode; this mode couples strongly to transverse acoustic phonons and is probably the 'phason' predicted theoretically for incommensurate lattices. This article reviews theory and experiment for this unusual material, including neutron, Raman and Brillouin scattering; X-ray, dielectric and conductivity measurements; magnetic resonance and susceptibility studies.

http://iopscience.iop.org/article/10.1088/0034-4885/42/6/003/pdf

Phase transitions in BaMnF4

Superconducting A15-type compounds were discovered 25 years ago. Several thousand original publications have appeared on this growing subject. The author reviews the essential structural aspects together with the characteristic superconducting properties and information on electron and phonon spectra.

https://iopscience.iop.org/article/10.1088/0034-4885/43/5/003/pdf

A15-type superconductors

The incommensurate (I) states of the triple charge density waves (CDW) in 1T-TaS 2 are studied in details by taking into account a sufficiently large number of harmonics. A domain-like structure of the I-CDW state is shown to be realized as the fundamental wave vector of the I state approaches to the commensurate (“3×1” superstructure) wave vector; the domain boundary is characterized generally by a change of the phase and amplitude of the CDW, and inside the domain an approximately commensurate CDW is stabilized. The nature of the commensurability energy and the coexistence of three CDW's are reflected in the pattern of the domain-like structure. The successive first-order transitions in 1T-TaS 2 are discussed in this connection. The presence of a domain-like structure, which our theory predicts for the nearly commensurate phase of 1T-TaS 2 , is consistent with the recent X-ray photoemission experiment by Hughes and Pollak.

Domain-like Incommensurate Charge-Density-Wave States and the First-Order Incommensurate-Commensurate Transitions in Layered Tantalum Dichalcogenides. I. 1T-Polytype

The martensitic phase transition in the A-15 compounds is treated by expanding the free energy in terms of the order parameters and its spatial gradients. The model used is a modified version of that recently proposed by Gorkov. The predictions for a wide range of physical properties are in good agreement with experimental results for V/sub 3/Si and Nb/sub 3/Sn lending support to the veracity of the model. 25 refs.

Landau theory of the martensitic transition in A − 15 compounds

A dynamical Landau theory is proposed to describe the coupled motion of phonons and charge-density waves (CDW) near the CDW instability in transition-metal dichalcogenides. The dynamic structure factor is shown to exhibit a Kohn anomaly and a diverging central mode.

Ravindra N. Bhatt

Papers

Landau theory of the martensitic transition in A − 15 compounds

Theory of phonon dynamics near a charge-density-wave instability

Theory of anomalous dispersion in liquid He 4