Charge-and spin-density-wave superconductors (original) (raw)
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TOPICAL REVIEW: Charge and spin-density-wave superconductors
Superconductor Science & Technology, 2001
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.
Charge- and spin-density-wave superconductors. Topical review:
Superconding Scence and Technology, 2001
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, NbSe 3 , and compounds with the A15 and C15 structures among others. There is much evidence to show that high-T c oxides may also belong to this group of materials. The SDW superconductors include URu 2 Si 2 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. Corresponding authors. become a so-called 'ideal conductor' with a zero resistance, rather than a true superconductor exhibiting the Meissner and Josephson effects . It is remarkable that the concept of the electron spectrum energy gap in the superconducting state had also been proposed by Bardeen [12] almost simultaneously with Fröhlich and before the full microscopic Bardeen-Cooper-Schrieffer (BCS) theory was developed .
Physics Reports-review Section of Physics Letters, 2002
This review is an up-to-date snapshot of the current situation in studies of materials with competing electron spectrum instabilities, namely Cooper pairing, on the one hand, and charge-density waves (CDWs) or spin-density waves (SDWs), on the other. The CDW- or SDW-driven instabilities in electron spectra with reduced dimensionalities may result from either the Fermi surface (FS) nesting or the existence of the Van Hove saddle points and lead to the appearance of the Peierls (excitonic) gap on parts of the FS (partial gapping). CDW superconductors include layered dichalcogenides, NbSe3, some organic substances and compounds with A15 and C15 structures among others. A large body of data is presented which may be considered as an evidence that high-Tc oxides also belong to this class of materials. In particular, an interpretation is given for the pseudogap phenomena in cuprates as having the CDW origin. The SDW superconductors include heavy-fermion compounds and UPd2Al3, Cr–Re alloys and a number of organic superconductors. We discuss the experimental information on the existence of CDW and SDW instabilities in a wide range of different superconductors, and assess their contention with the Cooper pairing on the Fermi surface. The main emphasis is placed on the properties of the existing substances with the interplay between superconductivity and CDW- or SDW-waves rather than on theoretically analyzing the possibility of such mixed phases in the framework of idealized microscopic theoretical models. The problem of symmetry for relevant order parameters is also touched upon. The theoretical description of the partially gapped CDW and SDW superconductors is presented based mostly on the phenomenological Bilbro–McMillan model. Various thermodynamical and electrodynamical properties are analyzed in this framework. Much emphasis is given to the nonstationary Josephson effect in tunnel junctions involving CDW or SDW superconductors. A comparison is carried out with the experiment, with special attention paid to high-Tc oxides.
Superconductors with charge- and spin-density waves: theory and experiment (Review)
Low Temperature Physics, 2000
The properties of existing superconductors with ejectron spectrum instabilities, namely chargedensity waves ͑CDWs͒ and spin-density waves ͑SDWs͒, are reviewed. In such substances the superconducting gap exists over the whole Fermi surface, whereas the dielectric gap emerges only on its nested sections. In particular, CDW superconductors include layered dichalcogenides, NbSe 3 , compounds with the A15 and C15 structures, etc. There is a lot of evidence that high-T c oxides also belong to this group of materials. SDW superconductors include, e.g., URu 2 Si 2 and related heavy-fermion compounds, Cr-Re alloys and organic superconductors. The theoretical description given in this review is based mostly on the Bilbro-McMillan model of the partially dielectrized metal. Various thermodynamic and electrodynamic properties are calculated in the framework of this model. The main subject of the review is the nonstationary Josephson effect in tunnel junctions involving CDW or SDW superconductors. A new effect of symmetry breaking in symmetrical tunnel junctions is predicted by the authors. A comparison with experiment is given.
Superconductors with charge- and spin-density waves: theory and experiment (Review article)
Low Temp. Phys., 2000
The properties of existing superconductors with ejectron spectrum instabilities, namely chargedensity waves ͑CDWs͒ and spin-density waves ͑SDWs͒, are reviewed. In such substances the superconducting gap exists over the whole Fermi surface, whereas the dielectric gap emerges only on its nested sections. In particular, CDW superconductors include layered dichalcogenides, NbSe 3 , compounds with the A15 and C15 structures, etc. There is a lot of evidence that high-T c oxides also belong to this group of materials. SDW superconductors include, e.g., URu 2 Si 2 and related heavy-fermion compounds, Cr-Re alloys and organic superconductors. The theoretical description given in this review is based mostly on the Bilbro-McMillan model of the partially dielectrized metal. Various thermodynamic and electrodynamic properties are calculated in the framework of this model. The main subject of the review is the nonstationary Josephson effect in tunnel junctions involving CDW or SDW superconductors. A new effect of symmetry breaking in symmetrical tunnel junctions is predicted by the authors. A comparison with experiment is given.
Coexistence of the Upper Charge-Density-Wave and the Superconductivity in NbSe3
Journal of the Physical Society of Japan, 2005
We have extensively measured the resistance of NbSe 3 near critical pressure (P 1 ¼ 3:2 GPa) for the upper charge-density-wave (T 1-CDW) phase. We reveal that the T 1-CDW coexists with the superconducting (SC) phase between 2.0 GPa and 2.9 GPa. Near P 1 on the T 1-CDW side, the T 1-CDW transition temperature appears to be singularly, T 1 $ ðP 1 À PÞ 0:49AE0:03 and the SC transition is very broad and becomes much sharpened with increasing pressure. We discuss mechanism of the coexistence of the T 1-CDW and the SC phase.
Physical Review Materials
Despite intense efforts on all known quasi-two-dimensional superconductors, the origin and exact boundary of the electronic orderings, particularly charge density waves and superconductivity, are still attractive problems with several open questions. Here, in order to reveal how the superconducting gap evolves, we report on high quality complementary measurements of magneto-optical imaging, specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and London penetration depth λ ab (T) measurements supplemented with theoretical calculations for 2H-NbSe 2 and 2H-NbS 2 single crystals. The temperature dependence of λ ab (T) calculated from the lower critical field and Andreev spectroscopy can be well described by using a two-band model with s-wave-like gaps. The effect of pressure on the superconducting gap of both systems illustrates that both bands are practically affected. Upon compression, the Fermi surfaces do not change significantly, and the nesting remains almost unaffected compared to that at ambient condition. However, a strong bending in the upper critical fields (H c2) curves is obtained under pressure and support the presence of a strong Pauli paramagnetic effect. In NbSe 2 , using a two-band model with s-wave-like gaps, the temperature dependence H c2 (T) can be properly described. In contrast to that, the behavior of H c2 for NbS 2 is ruled by the spin paramagnetic effect. The estimated values of the penetration depth at T = 0 K confirm that NbSe 2 and NbS 2 superconductors depart from a Uemura-style relationship between T c with λ −2 ab (T), the in-plane superconducting penetration depth.
Scientific Reports, 2021
By means of first-principles cluster expansion, anisotropic superconductivity in the transition metal dichalcogenide Nb(Se$$_{x}$$ x S$$_{1-x}$$ 1 - x )$$_{2}$$ 2 forming a van der Waals (vdW) layered structure is observed theoretically. We show that the Nb(Se$$_{0.5}$$ 0.5 S$$_{0.5}$$ 0.5 )$$_{2}$$ 2 vdW-layered structure exhibits minimum ground-state energy. The Pnnm structure is more thermodynamically stable when compared to the 2H–NbSe$$_{2}$$ 2 and 2H–NbS$$_{2}$$ 2 structures. The characteristics of its phonon dispersions confirm its dynamical stability. According to electronic properties, i.e., electronic band structure, density of states, and Fermi surface indicate metallicity of Nb(Se$$_{0.5}$$ 0.5 S$$_{0.5}$$ 0.5 )$$_{2}$$ 2 . The corresponding superconductivity is then investigated through the Eliashberg spectral function, which gives rise to a superconducting transition temperature of 14.5 K. This proposes a remarkable improvement of superconductivity in this transition m...
d-wave superconductivity near charge instabilities
Physical Review B, 1996
We investigate the symmetry of the superconducting order parameter in the proximity of a phase-separation or of an incommensurate charge-density-wave instability. The attractive effective interaction at small or intermediate transferred momenta is singular near the instability. This strongly q-dependent interaction, together with a residual local repulsion between the quasiparticles and an enhanced density of states for band structures appropriate for the high temperature superconducting oxides, strongly favors the formation of d-wave superconductivity. The relative stability with respect to superconductivity in the s-wave channel is discussed in detail, finding this latter hardly realized in the above conditions. The superconducting temperature is mostly determined by the closeness to the quantum critical point associated to the charge instability and displays a stronger dependence on doping with respect to the simple proximity to a Van Hove singularity. The relevance of this scenario and the generic agreement of the resulting phase diagram with the properties displayed by high temperature superconducting oxides is discussed.
Pristine and intercalated transition metal dichalcogenide superconductors
Physica C: Superconductivity and its Applications, 2015
Transition metal dichalcogenides (TMDs) are quasi-two-dimensional layered compounds that exhibit strongly competing effects of charge-density wave (CDW) formation and superconductivity (SC). The weak van der Waals interlayer bonding between hexagonal layers of octahedral or trigonal prismatic TMD building blocks allows many polytypes to form. In the single layer 1T polytype materials, one or more CDW states can form, but the pristine TMDs are not superconducting. The 2H polytypes have two or more Fermi surfaces and saddle bands, allowing for dual orderings, which can be coexisting CDW and SC orderings, two SC gaps as in MgB2, two CDW gaps, and possibly even pseudogaps above the onset TCDWs of CDW orderings. Higher order polytypes allow for multiple CDW gaps and at least one superconducting gap. The CDW transitions TCDWs usually greatly exceed the superconducting transitions at their low Tc values, their orbital order parameters (OPs) are generally highly anisotropic and can even contain nodes, and the SC OPs can be greatly affected by their simultaneous presence. The properties of the CDWs ubiquitously seen in TMDs are remarkably similar to those of the pseudogaps seen in the high-Tc cuprates. In 2H-NbSe2, for example, the CDW renders its general s-wave SC OP orbital symmetry to be highly anisotropic and strongly reduces its Josephson coupling strength (IcRn) with the conventional SC, Pb. Hence, the pristine TMDs are highly "unconventional" in comparison with Pb, but are much more "conventional" than are the ferromagnetic superconductors such as URhGe. Applied pressure and intercalation generally suppress the TMD CDWs, allowing for enhanced SC formation, even in the 1T polytype materials. The misfit intercalation compound (LaSe)1.14(NbSe2) and many 2H-TMDs intercalated with organic Lewis base molecules, such as TaS2(pyridine) 1/2 , have completely incoherent c-axis transport, dimensional-crossover effects, and behave as stacks of intrinsic Josephson junctions. Except for the anomalously large apparent violation of the Pauli limit of the upper critical field of (LaSe)1.14(NbSe2), these normal state and superconducting properties of these intercalation compounds are very similar to those seen in the high-Tc superconductor, Bi2Sr2CaCu2O 8+δ and in the organic layered superconductor, κ-(ET)2Cu[N(CN)2]Br, where ET is bis(ethylenedithio)tetrathiafulvalene. Electrolytic intercalation of TMDs with water and metallic ions leads to compounds with very similar properties to cobaltates such as NaxCoO2 • yH2O.