a r X i v :c o n d -m a t /0403231v 1 [c o n d -m a t .s u p r -c o n ] 9 M a r 2004
Electronic Transport in Underdoped YBCO Nanowires:Possible Observation of
J.A.Bonetti,∗D.S.Caplan,D.J.Van Harlingen,and M.B.Weissman
Department of Physics and Materials Research Laboratory,University of Illinois at Urbana-Champaign,Urbana,IL 61801
We have measured the transport properties of a series of underdoped YBa 2Cu 3O 7−δnanowires fab-ricated with widths of 100-250nm.We observe large telegraph-like ﬂuctuations in the resistance between the pseudogap temperature T ∗and the superconducting transition temperature T c ,consis-tent with the formation and dynamics of a domain structure such as that created by charge stripes.We also ﬁnd anomalous hysteretic steps in the current-voltage characteristics well below T c .
Despite years of intense study of the high-T c cuprates,many key questions remain unresolved,particularly re-garding the microscopic details of the pairing mechanism,the nature of the pseudogap state,and the relationship between the superconducting and pseudogap states.One prominent category of high-T c models involves the pres-ence of a symmetry-breaking,non-superconducting order parameter that competes or coexists with superconduc-tivity.Examples of such ordering are various orbital cur-rent models [1,2,3]and charge stripe ordering [4,5,6,7].A signature of underlying order would be the formation of domains separating various phases or orientations of the order parameter.To date,experimental evidence for such domains is suggestive but inconclusive.Neutron scattering data [8,9,10]and transport measurements have been interpreted as support for charge ordering and stripe formation.STM spectroscopy imaging has re-vealed electronic phase separation into mesoscopic scale superconducting and pseudogap domains ,and stripe features that have been attributed to charge stripes ,quasiparticle interference fringes ,and incommensu-rate spin-density waves .
In this Letter,we report transport measurements on nanoscale YBCO samples that show large amplitude switching noise in the resistance at temperatures above the superconducting transition temperature T c but be-low the pseudogap temperature T ∗,and hysteretic volt-age steps in the current-voltage characteristics well below T c .We argue that the resistance ﬂuctuations may be a signature of the formation and dynamics of a symmetry-breaking domain structure arising from charge ordering into stripes.We consider whether the voltage steps could arise from a similar mechanism or are caused by heating or phase slips in the nanowire.
The key to the experiments is the fabrication of nanowires without signiﬁcant degradation of their super-conducting properties.We start with YBCO thin ﬁlms (thickness 50-100nm),grown by pulsed laser deposition on LaAlO 3(LAO)substrates,with T c ’s from ∼92K (optimal doping)to ∼60K on the underdoped side,as determined by a two-coil inductive measurement.We then dc sputter a carbon layer (∼200nm)and a thin
gold layer (∼20nm)to serve as an etching template and to protect the ﬁlm during processing.Patterning is done by electron beam lithography and ion beam etching on a liquid nitrogen cooled stage to avoid ﬁlm degradation.Finally,the protective layers are removed and standard photolithography is used to attach leads to the nanowire.Four-point resistance measurements on short (∼500nm)segments of the nanowires revealed surprising re-sults.The overall shape of the resistance vs.temperature curves is as expected for underdoped ﬁlms:as the tem-perature is lowered,the typical linear decrease in resistiv-ity becomes sublinear below a temperature T ∗normally associated with the pseudogap temperature,followed by the transition into the superconducting state.This is shown in Figure 1(a)for both a 3µm line and a 250nm wide nanowire.The transition width is typically 3-5K with an onset at most a few degrees below the transi-tion temperature of the starting uniform ﬁlm.However,
FIG.segment of a 250nm wide nanowire compared to that of a 3µm line.(b)Expanded section showing telegraph-like switching ﬂuc-tuations at temperatures below approximately 150K .