Charge-Density Waves Survive the Pauli Paramagnetic Limit

Abstract

Measurements of the resistance of single crystals of (Per)2Au(mnt)2 have been made at magnetic fields B of up to 45 T, exceeding the Pauli paramagnetic limit of BP ≈ 37 T. The continued presence of non-linear charge-density wave electrodynamics at B ≥ 37 T unambiguously establishes the survival of the charge-density wave state above the Pauli paramagnetic limit, and the likely emergence of an inhomogeneous phase analogous to that anticipated to occur in superconductors.PACS numbers: 71.45. Lr, 71.20.Ps, 71.18.+y Fundamental changes can occur within paired-electron condensates subjected to intense magnetic fields [1, 2,3]. If the state is a spin-singlet (with electron spins opposed), as in charge-density waves (CDWs) [4] and s-and d-wave superconductors [5], the energy of the partially spinpolarized electrons of the uncondensed metal eventually becomes lower than condensate energy above a characteristic field known as the Pauli paramagnetic limit [6,7,8]. Continued survival of the condensate requires the formation of a lower energy spatially-inhomogeneous phase, in which pairing is between spin-polarized quasiparticle states [8,10,11,12,13]. The existence of such a phase in superconductors becomes questionable owing to the field-induced kinetic energy of orbital currents, which often suppresses superconductivity more strongly [14,15]. By contrast, pure CDW systems are free from orbital currents, yet their high condensation energies increase the demand on magnetic field strength required to reach the Pauli paramagnetic limit [4]. (Per) 2 Au(mnt) 2 is a rare example where this limit (≈ 37 T) falls within reach of the highest available quasi-static magnetic fields of 45 T [16,17]. In this paper we use temperatures down to 25 mK (roughly one-thousandth of the energy gap) to show that the CDW state surpasses the Pauli paramagnetic limit, signalling the likely appearance of an inhomogeneous phase.Both superconductivity [5] and CDWs [4] form as a consequence of electron-phonon interactions. Superconductors are ground states in which gauge symmetry is broken (where the variation of the magnetic field is dependent on topology) [5], while CDWs exhibit a periodic charge modulation that breaks translational symmetry [4]. The BCS (Bardeen-Cooper-Schrieffer) formalism that applies to superconductivity [18], also conveniently describes the electronic structure of CDWs, with the gap in the electronic energy spectrum in the zero temperature limit being given by 2∆ 0 = ζk B T c , where T c is the transition temperature. Whereas the ratio ζ = 3.52 in weak-coupling BCS theory, 5 < ζ < 10 in CDWs owing to their strong coupling to the ionic lattice [4]. Upon lowering the temperature through T c , a metal-insulator transition occurs, below which normal carriers must be thermally excited across the gap to conduct. The presence of a magnetic field B lowers T c [8]; simple theory predicts T c → 0 at the Pauli paramagnetic limit defined as, where g is the electron g-factor, s is the electron spin and µ B is the electron Bohr magneton.Ap...

Temperature-driven transition from the Wigner crystal to the bond-charge-density wave in the quasi-one-dimensional quarter-filled band

“…18 The effects of magnetic field on 1 4 -filled band CDWs is of strong recent interest. 74,75 We are currently investigating the consequences of the Zeeman interaction on the mixed spin-charge excitations of the BCDW.…”

1H and 195Pt NMR Study of the Parallel Two-Chain Compound Per2[Pt(mnt)2]

“…If it is assumed that the spectrum of d-electron spin ratio of the relaxation rates proximity R to the nearest Pt (13) between an NMR nucleus and the Pt from a Pt electronic spin to a Pt nucleus at a from a neighboring Pt electronic spin, the ratio of relaxation rates is only a factor of Pt) would need to be about 3 Å, less mplies that the Pt relaxation and spectra are Pt nucleus and its own on-site like in symmetry, more than half of its spin density is distributed over the [Pt(mnt) 2 ] − anion [24]. We may roughly estimate a lower bound on the hyperfine coupling by considering the shift in the spectral position from 1.74 K to 4.2 K in Figure 12a, using the value for χ at 4.2 K. Here we find A hf ~ 3700 Oe/µ B , again a value significantly higher than that for the proton sites.…”

“…In recent years there have been extensive studies of electrical transport in Per 2 [Pt(mnt) 2 ] in magnetic fields where the resistance of the sample yields information about how the CDW ground state changes with field and temperature [7], and B-T phase diagrams have been produced that show the CDW is suppressed at high magnetic fields (~20 T at 1 K), but a new type of insulating state appears at higher fields [4,13]. Since similar studies of the spin-zero isostructural compound Per 2 [Au(mnt) 2 ] show that significantly larger fields (~30 T at 1 K) are necessary to suppress the conventional CDW ground state [14], one can reasonably ask what role the spin −½ chain plays in altering the B-T phase diagram, and if and how the spin and conducting chain order parameters couple.…”

Sliding charge-density wave in manganites

“…The manganite CDW is a fully gapped system with no screening electrons, which has previously only been observed in extremely clean organic materials 27 . However, the manganite CDW exists with a high level of impurities, leading to dramatic hysteresis effects in the resistance.…”