M.F. Hundley

On the 17.5K transition in URu2Si2: thermopower and elasticity

Abstract We report thermoelectric power (TEP) and elasticity measurements on the heavy Fermion compound URu2Si2. The TEP is positive and roughly independent of temperature between 300K and 120K, becomes increasingly negative below 70K, and displays a sharp change in slope at Tp=17.5K. At 15K there is a dramatic reversal in the TEP. The Youngs modulus shows a strong dip near 18K, accompanied by an increase in the internal friction. The TEP and elastic anomalies are consistent with a Fermi surface instability at Tp, and we interpret our results in terms of charge or spin density wave formation.

Magnetothermopower of NbSe3

Abstract We report longitudinal thermopower measurements on the charge density wave (CDW) conductor NbSe 3 in transversely applied magnetic (H) fields up to 75 kG. In the lower CDW state, increasing H results in a positive increase in the thermopower, indicating that the magnetic field acts to destroy electron-like states on the Fermi surface. We interpret our results in terms of H-induced “one-dimensionalization” of the electronic band structure, and an associated normal-to-CDW carrier conversion.

A phase-slip model of switching☆

We review recent experiments which suggest that charge-density wave (CDW) amplitude fluctuations play a critical role in switching, and present a model of switching based on amplitude collapse at phase-slip1 centers.

Anomalous magnetoresistance in charge density wave compounds: Is NbSe3 unique?

Abstract The partially gapped charge density wave (CDW) conductor NbSe 3 displays a dramatic magnetoresistance in the lower CDW state. We have made magnetoresistance measurements on the related partially gapped CDW compounds ZrTe 3 and K 3 Cu 8 S 6 . Despite structural and electronic similarities of these materials to NbSe 3 , we find non-existent or very small magnetoresistance effects. These results suggest that the anomalous magnetoresistance observed in NbSe 3 may be limited to partially gapped chain-like CDW conductors that contain a depinnable CDW.

Phase strain model for charge density wave sub-domain formation in an applied temperature gradient

Abstract The dynamics of a sliding charge density wave (CDW) condensate subjected to a uniform temperature gradient are examined within an elastic medium model. As the temperature gradient is increased, it becomes energetically favorable for the CDW to break up into a series of velocity sub-domains of equal size separated by phase slip centers. The number of sub-domains scales with the temperature gradient and with the length of the CDW crystal. Unusual dynamical asymmetries are also predicted depending on the relative direction of heat and current flow through the CDW crystal.

Electronic interference, elasticity, and scanning tunneling microscopy studies of the charge density wave conductor K0.3MoO3

Abstract Charge density wave (CDW) dynamics and statics are explored in the blue bronze K 0.3 MoO 3 . For very thin (optically transparent, d≈0.2 μ m) platelets highly coherent response obtains, and in the presence of dc electric fields the ac conductivity shows high frequency interference structure. This interference is discussed in terms of a resonant circuit analog. At low temperatures, the zero differential resistance state is associated with dramatic changes in the elastic constants of the crystal. However, the effects of sample core heating may be important. Both above and below the CDW transition temperature, the surface of K 0.3 MoO 3 has been examined by atomic resolution scanning tunneling microscopy. Although the underlying lattice is clearly resolved, no CDW superstructure has been imaged on the crystal surface.

A RF-induced dynamic coherence length in NbSe3☆

Abstract We have investigated Shapiro step mode-locking in NbSe 3 as a function of sample dimension, using both a series of samples with different total length, and individual samples with a variable spacing between voltage sensing probes. The rf-induced dynamic coherence length is orders of magnitude larger than the Fukuyama-Lee-Rice length.

Noise and Shapiro step interference in the charge-density-wave conductor K0.3MoO3☆

Abstract Extremely thin (transverse dimension ∼0.2 μm) samples of the charge density wave (CDW) conductor K 0.3 MoO 3 are found to display unusually coherent narrow-band noise spectra and well defined Shapiro step interference structure. The results suggest an intrinsic pinning potential with periodicity equal to the CDW wavelength. The Shapiro step interference appears to arise from a coupling of the external electric field to the low frequency dielectric relaxation mode rather than to the high frequency pinned phason mode.

Model of dynamic charge density wave break-up in an applied temperature gradient

Abstract We examine theoretically the high-velocity dc dynamics of a charge density wave (CDW) condensate subjected to a uniform longitudinal temperature gradient. The condensate is treated within an elastic medium model where internal strain and carrier conversion are treated explicitly. For sufficiently large temperature gradients, the CDW breaks up into a series of N coexisting subdomains with independent phase velocities. N scales directly with the magnitude of the temperature gradient and with the length of the crystal. Unusual dynamical asymetries are also predicted depending on the relative directions of heat and electrical current flow through the CDW crystal.

Temperature-gradient-induced subdomain scaling in NbSe3

Abstract In a temperature gradient, the charge density were (CDW) in NbSe 3 breaks up into a series of N coexisting “subdomains” with independent CDW phase velocities. With ΔT the temperature difference applied across the ends of the sample, we find N ∼(Δ T 2 3 . This scaling relation has been predicted by a phase-strain model of CDW dynamics.