Electric-field distribution near current contacts of anisotropic materials

Abstract

We have measured the nonuniformity of the electric field near lateral current contacts of the charge-densitywave materials NbSe 3 and o-TaS 3 . In this contact geometry, the electric field increases considerably near a current contact. Fitting our data to an existing model yields values for the conduction anisotropy and a characteristic longitudinal length scale. This length scale is on the same order as the mesoscopic phenomena in charge-density-wave devices. DOI: 10.1103/PhysRevB.65.033403 PACS number͑s͒: 71.45.Lr, 72.15.Nj Anisotropic conductors with a chainlike structure can undergo a phase transition to a charge-density-wave ͑CDW͒ state.1 Electrons condense into a collective state in which the charge density is periodically modulated. The CDW can slide through the crystal under the application of an electric field. However, defects in the crystal lattice pin the CDW at low fields and a threshold has to be overcome. The experimental study of CDW dynamics usually involves transport measurement along the chain direction. Electrical contact is most often made by connecting metal electrodes to the crystal at various positions on the top or bottom side ͑face͒ of the crystal 2 or, more recently, by etching side contacts in the crystal itself.3 Most transport studies have been performed on samples with contact spacings of 10 m and larger.Recently, interest has grown to study the mesoscopic CDW regime.3,4 As the length scale for mesoscopic phenomena is on the order of microns in the longitudinal direction, it is essential to reduce contact spacings to this scale. Since both types of lateral contacts mentioned above apply current in the transverse direction, a region of nonuniform electric field exists in the vicinity of current contacts. This nonuniformity is known under the name ''fringing effects.'' In previous experiments so far, measurements of fringing effects were limited by perturbing contacts and large contact separation.In case of anisotropic materials, fringing effects are particularly pronounced. In such materials, the length scale over which fringing effects are important is ͱA times larger than in the isotropic case. Here, A is the anisotropy, which is the ratio of the conductivity along and perpendicular to the crystal length.We have measured the electric field distribution on submicron length scales in the longitudinal direction. We find good agreement with existing models which indicate that fringing effects are important up to distances of tͱA from the current contact, where t is the crystal thickness. From our data, we can deduce the corresponding A. We find it to be ϳ100 for NbSe 3 and ϳ1000 for o-TaS 3 at Tϭ120 K. This is the first measurement of A in the crystallographic a axis of NbSe 3 . Our measurements are performed in the pinned state, so that our data concerns geometrical effects only, and does not explore the complicated current dependent field profiles that develop when the CDW depins. 5,6 Experiments are performed on crystals of NbSe 3 and o-TaS 3 . Both materials have an anisotropi...

Nonlocal transport in the charge density waves ofo-TaS3

“…We found that nonlocal voltages appeared in the CDW of o-TaS 3 , both below and above the threshold field for CDW sliding. The sign of the observed nonlocal voltage was opposite to that of the spreading resistance [14,15]. In addition to this, the temperature dependence of the nonlocal voltage suggests that the observed nonlocal voltage originates from the CDW.…”

“…We first compared our observation with a trivial source, known as spreading resistance, which could affect the nonlocal voltage probes [14,15]. When the current is injected from a tiny electrode into a bulk sample, it cannot flow homogeneously until it spreads over a cross section.…”

“…where E denotes an applied field, l is the electrode length, and t ′ = t √ A is a reduced thickness with A being the anisotropic ratio of conductivity [14,15]. In addition, the effective cross section for the current is limited to the specific distance ∼ t ′ , and an additional contribution to resistance emerges, known as spreading resistance.…”

End-current injection contacts for anisotropic materials: Fabrication and application to the quasi-one-dimensional conductorNbSe3

“…2,3 Contacts applied to the side of these elongated and often whiskerlike crystals inject current along the low-conductance direction, leading to large contact resistances, large local Ohmic heating, and inhomogeneous current and electric field profiles in the vicinity of the contact. 4 In charge-density-wave ͑CDW͒ conductors such as K 0.3 MoO 3 and o-TaS 3 where the Peierls transition fully gaps the Fermi surface, CDW depinning, and motion, which dramatically increase the longitudinal conductance, can increase the conductance anisotropy to 10 6 or more at helium temperatures. 2 The inhomogeneities in field and current densities associated with side contacts ͓Fig.…”

“…1͑a͔͒ occur on a longitudinal scale of roughly the crystal thickness times the square root of the conductance anisotropy, or 5 -50 m for typical samples. 4 As a result, side contacts are poorly suited to probing CDW physics at length scales of 10 m and below. They can produce shear and phase slip in the CDW condensate, smearing out the depinning transition and the spectral width of the coherent oscillations that accompany CDW motion, and generating large-amplitude low-frequency electrical noise.…”