title: Thermal noise of mechanical oscillators in steady states with a heat flux.
abstract: We present an experimental investigation of the statistical properties of the position fluctuations of low-loss oscillators in nonequilibrium steady states. The oscillators are coupled to a heat bath, and a nonequilibrium steady state is produced by flowing a constant heat flux, setting a temperature difference across the oscillators. We investigated the distribution of the measurements of the square of the oscillator position and searched for signs of changes with respect to the equilibrium case. We found that, after normalization by the mean value, the second, third, and fourth standardized statistical moments are not modified by the underlying thermodynamic state. This differs from the behavior of the absolute, i.e., not normalized, second moment, which is strongly affected by temperature gradients and heat fluxes. We illustrate this with a numerical experiment in which we study via molecular dynamics the fluctuations of the length of a one-dimensional chain of identical particles interacting via anharmonic interparticle potentials, with the extremes thermostated at different temperatures: we use the variance of the length in correspondence to its first elastic mode of resonance to define an effective temperature which we observe to depart from the thermodynamic one in the nonequilibrium states. We investigate the effect of changing the interparticle potential and show that the qualitative behavior of the nonequilibrium excess is unchanged. Our numerical results are consistent with the chain length being Gaussian distributed in the nonequilibrium states. Our experimental investigation reveals that the position variance is the only, and crucially easily accessible, observable for distinguishing equilibrium from nonequilibrium steady states. The consequences of this fact for the design of interferometric gravitational wave detectors are discussed.
authors.affiliation: Istituto Nazionale di Fisica Nucleare, Via Marzolo 8, I-35131 Padova, Italy.
authors.forename: Livia
authors.initials: L
authors.lastname: Conti
authors.affiliation: Istituto Nazionale di Fisica Nucleare, Via Marzolo 8, I-35131 Padova, Italy.
authors.forename: Claudia
authors.initials: C
authors.lastname: Lazzaro
authors.affiliation: Istituto Nazionale di Fisica Nucleare, Via Marzolo 8, I-35131 Padova, Italy.
authors.forename: Gagik
authors.initials: G
authors.lastname: Karapetyan
authors.affiliation: Institute of Materials for Electronics and Magnetism, Nanoscience-Trento-FBK Division, 38123 Povo (Trento), Italy and Istituto Nazionale di Fisica Nucleare, Trento Institute for Fundamental Physics and Applications, 38123 Povo (Trento), Italy.
authors.forename: Michele
authors.initials: M
authors.lastname: Bonaldi
authors.affiliation: Istituto Nazionale di Fisica Nucleare, Via Marzolo 8, I-35131 Padova, Italy.
authors.forename: Matteo
authors.initials: M
authors.lastname: Pegoraro
authors.affiliation: Istituto Nazionale di Fisica Nucleare, Via Marzolo 8, I-35131 Padova, Italy.
authors.forename: Ram-Krishna
authors.initials: RK
authors.lastname: Thakur
authors.affiliation: Istituto Nazionale di Fisica Nucleare, Via Marzolo 8, I-35131 Padova, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: De Gregorio
authors.affiliation: Dipartimento di Scienze Matematiche and Graphene@PoliTO Lab, Politecnico di Torino Corso Duca degli Abruzzi 24, 10129 Torino, Italy and Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via P. Giura 1, 10125 Torino, Italy.
authors.forename: Lamberto
authors.initials: L
authors.lastname: Rondoni
mesh_terms: D006358:Hot Temperature; D055595:Mechanical Phenomena; D008670:Metals; D008962:Models, Theoretical
chemical_list: D008670:Metals