Observation of “Photon-Assisted” Shot Noise in a Phase-Coherent Conductor

Abstract

We observe a novel signature of coherent transport in the nonequilibrium current fluctuations of a diffusive metallic conductor shorter than the electron phase-breaking length. Under illumination with monochromatic microwaves, the shot noise develops features at voltages corresponding to the photon energies, V nhn͞e, which are oscillatory functions of the microwave power, while the conductance and the I-V curve are unaffected. The observed effect bears a strong resemblance to photon-assisted tunneling, although this marks the first demonstration in a linear system, and in a system without an explicit tunnel barrier. [S0031-9007(98)05538-0] PACS numbers: 73.50. Td, 73.23.Ps, 73.50.Mx, 73.50.Pz Studies of the nonequilibrium current fluctuations have recently emerged as a probe of various mesoscopic systems [1]. The nature of the shot noise can reveal information about the transport which is not evident merely by examining the conductance or the time-averaged transport. For example, correlations due to the Fermi statistics of the electrons can lead to a partial suppression of the shot noise [2,3]. It has been demonstrated [4,5] that even a metallic wire will produce shot noise, provided that its length is short compared to the electron-electron inelastic length, L ee . Time-dependent phenomena in nanostructured systems have also become an active area of research lately [6], and it is interesting to ask what the effect of a time-dependent potential on the current fluctuations might be. Indeed, Lesovik and Levitov [7] recently proposed a new two-particle interference phenomenon, the "nonstationary Aharonov-Bohm effect," which should be observable in the shot noise of a phase-coherent conductor under a high-frequency excitation. As we describe below, this phenomenon is formally identical to the process of "photon-assisted tunneling" (PAT) [8], but applied to shot noise of the system rather than its current-voltage characteristic.In this Letter, we present measurements of the nonequilibrium noise of a diffusive, phase-coherent metallic conductor in the presence of both a high-frequency (2-40 GHz) ac excitation and a dc bias. The noise displays features generated by the time-dependent excitation that bear all the hallmarks of photon-assisted tunneling. Because of the linear nature of the conductance in this mesoscopic system, however, the photon-assisted signatures appear only in the noise, not in the conductance.The theory of photon-assisted tunneling [8] was developed some time ago to explain the features observed [9] in superconducting tunnel junctions when they were illuminated with microwave radiation. It was extended to derive the quantum-limited performance [10] of heterodyne detectors based on these devices. More recently, PAT has been demonstrated in a variety of systems, including semiconductor quantum dots [11], metallic singleelectron transistors [12], semiconductor superlattices [13], and other dual-barrier semiconductor devices [14]. In all of these systems, the conductance displays a strong nonlinearity...

Shot noise of photon-excited electron-hole pairs in open quantum dots

“…This explains why the expression for the noise in the ac-driven system of Refs. [5,6] fits the experimental data [2,3] so well: the dispersion of the noise is significant on a much larger frequency scale than applied in these experiments.…”

“…We find that dephasing leaves the noise invariant, but inelastic scattering decreases correlations eventually down to zero. Investigation of noise [1][2][3] generally provides information not available from current measurements, such as effective charge and quantum statistical properties of the carriers. Most of the work has focused on noise in the presence of stationary (dc) applied voltages.…”

Integer and fractional charge Lorentzian voltage pulses analyzed in the framework of photon-assisted shot noise

“…Applying a monochromatic sine wave to a contact, the noise singularities at dc voltage multiple of the frequency has been observed in a diffusive metallic wire by Schoelkopf et al [45] via the second derivative of the noise, giving the P l spectroscopy. Later the controlled suppression by a dc voltage bias of hole or electron excitation contribution to PASN has been discussed and observed in a Quantum Point Contact by Reydellet et al [48].…”

“…Using the separation of the voltage pulse into its mean value, or d.c. part < V (t) >= V dc and its ac component V ac (t) = V (t) − V dc as in ref. [28], the physics of voltage pulses can be conveniently discussed in the framework of Photo-Assisted Shot Noise (PASN) where both theoretical [41][42][43] and experimental results [45,48] are already available. At zero temperature, the excess noise ∆S I characterizing the extra excitations is then the difference between the PASN noise S P ASN I due to V (t) = V ac + V dc and the (transport) shot noise (TSN) S T SN I that we would have with only the dc voltage V (t) = V dc [35-40, 49, 50].…”

Scattering approach to counting statistics in quantum pumps

“…The experimental measurements of the shot noise 15,16,17,18 are now well established. The measurement of the higher cumulants, on the other hand, is still an unresolved problem actively discussed in the literature 19 .…”