Recently many people discuss unconventional density waves (i.e.unconventional charge density waves (UCDW) and unconventional spin densitywaves (USDW)). Unlike in conventional density waves, the quasiparticle spectrumin these systems is gapless. Also these systems remain metallic. Indeed itappears that there are many candidates for UDW. The low temperature phase ofalpha-(BEDT-TTF)_2KHg(SCN)_4, the antiferromagnetic phase in URu_2Si_2, the CDWin transition metal dichalcogenite NbSe_2, the pseudogap phase in high T_ccuprate superconductors, the glassy phase in organic superconductorkappa-(BEDT-TTF)_2Cu[N(CN)_2]Br. After a brief introduction on UCDW and USDW,we shall discuss some of the above systems, where we believe we have evidencefor unconventional density waves.
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