In-Chain Tunneling through Charge-Density-Wave Nanoconstrictions and Break Junctions

Abstract

We have fabricated longitudinal nanoconstrictions in the charge-density wave conductor (CDW) NbSe 3 using a focused ion beam and using a mechanically controlled break-junction technique. Conductance peaks are observed below the T P1 145 K and T P2 59 K CDW transitions, which correspond closely with previous values of the full CDW gaps 2 1 and 2 2 obtained from photoemission. These results can be explained by assuming CDW-CDW tunneling in the presence of an energy gap corrugation 2 comparable to 2 , which eliminates expected peaks at j 1 2 j. The nanometer length scales our experiments imply indicate that an alternative explanation based on tunneling through back-to-back CDW-normal-conductor junctions is unlikely. DOI: 10.1103/PhysRevLett.96.096402 PACS numbers: 71.45.Lr, 73.23.ÿb, 73.40.Gk, 74.50.+r Charge-density wave (CDW) conduction remains of major interest despite its experimental discovery nearly 30 years ago. Much of the existing work has focused on transport properties of as-grown single crystals [1]. More recently, micro-and nanofabrication methods for CDW materials has allowed the study of mesoscopic CDW physics [2 -4]. Structures for tunneling spectroscopy are of particular interest because of the unusual gap structure with large one-dimensional fluctuation effects expected in these highly anisotropic materials, and because of predictions of unusual midgap excitations of the collective mode [5]. Tunneling studies in fully gapped CDW conductors like the ''blue bronze'' K 0:3 MoO 3 suffer from band-bending effects at the interface akin to semiconductor-insulatormetal junctions. These effects are absent in the partially gapped CDW conductor NbSe 3 , which remains metallic down to 4.2 K.Tunneling perpendicular to the direction b of the quasione-dimensional chains, along which the CDW wave vector lies, has been studied in ribbonlike whiskers of NbSe 3 by scanning tunneling microscopy (STM) [6], by tunneling through Pb contacts evaporated over the native oxide on the b-c plane [7,8], and by tunneling through a gold wire or a NbSe 3 crystal that is laid across another NbSe 3 crystal, forming junctions in the a-b or b-c planes [9,10]. Peaks in the T 4:2 K differential conductance at 35 and 101 mV [6], 35 mV [7], 36 mV and 90 mV [9], and 37 mV and 100 mV [10] from metal-NbSe 3 junctions correspond well with the CDW gaps 1 110 mV and 2 45 mV for NbSe 3 's T P1 145 K and T P2 59 K CDWs as determined by angle-resolved photo emission (ARPES) [11]. Crossed NbSe 3 -NbSe 3 crystals [9] yield peak voltages of 60 mV and 142 mV, and interlayer tunneling in microfabricated NbSe 3 mesas yields peaks at 50 mV and 120 mV [3]. A single in-chain tunneling study [2] using a gold ribbon mechanically positioned near the end of a NbSe 3 crystal gave a peak at 100 mV for the T P1 CDW.Here we demonstrate that a small constriction in a NbSe 3 single crystal, produced by dry etching with a Ga focused ion beam (FIB), shows conductance peaks at 105 mV and 190 mV corresponding to 2 1 and 2 2 , as illustrated in Fig. 1. We reproduce ...

Charge density waves in NbSe3: The models and the experimental evidence

“…This generally accepted perception of CDW formation and ordering appeared to be in good accord with a variety of experiments performed on NbSe 3 : in addition to the nonlinear transport properties attributed to CDW sliding and pinning [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], there were also electron [28] and x-ray diffraction studies [29][30][31][32], nuclear magnetic resonance (NMR) [33][34][35] and angular resolved photoemission spectroscopy [36,37] performed. However, the model seems to be in disaccord with some transmission electron microscopy (TEM) results [38] and particularly with the latest LT STM observations [39][40][41][42].…”

Coherent-noncoherent tunneling crossover inNbSe3−NbSe3point contacts

“…Such a behavior correlates well with the results obtained for stacked mesa junctions 19 and for longitudinal nanoconstrictions. 23 The amplitude of the ZBCP increases when the temperature Temperature dependences of the zero bias differential resistance R d0 normalized to the resistance at low CDW transition temperature T p2 = 59 K for NbSe 3 -NbSe 3 contacts revealing either a zero bias resistance peak ͑ZBRP, closed circles͒ or a zero bias conductance peak ͑ZBCP, open circles͒. is decreased and saturates at T Ͻ 10-12 K; this agrees with the results obtained in Refs.…”

“…The same results were obtained in the case of interlayer tunneling [18][19][20] and for longitudinal nanoconstrictions. 23 Formally, that means that the carriers remaining in pockets cannot be involved as quasiparticle excitations for the low-temperature CDW. Such a situation is possible, for example, if the CDW order parameter has an unusual symmetry.…”

“…There are, however, some uncertainty in the absolute values of both energy gaps. For measurements based on tunneling ͑through Pb contacts, point contacts, or mesas͒, the different low temperature ⌬ p2 / e ranges from 52 mV, 23 24 mV ͑present work͒, 25 mV, 18 35 mV, 26,34 and 37 mV, 24,25 while ⌬ p1 / e ranges from 120 mV, 18 101 mV, 34 100 mV, 24,25 95 mV, 23 and 89 mV ͑present work͒. In spite of their dispersion, these values are in good agreement with those obtained from photoemission: 35,36 45 mV for ⌬ p2 / e and 110 mV for ⌬ p1 / e. Anisotropy of the CDW gap magnitude has also to be considered in Ref.…”

“…23 In contrast to stacked junctions, only in-plane electrical transport occurs in such a geometry. It was shown that such a constriction behaves like a CDW-I-CDW tunnel junction, and the observed energy gap singularities correspond well with the full CDW gaps 2⌬ p1 and 2⌬ p2 .…”