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We propose a simple semiclassical method for calculating higher-order cumulants of current in multichannel mesoscopic conductors. To demonstrate its efficiency, we calculate the third and fourth cumulants of current for a chaotic cavity with multichannel leads of arbitrary transparency and compare the results with ensemble-averaged quantum-mechanical quantities. We also explain the discrepancy between the quantum-mechanical results and previous semiclassical calculations.Comment: 7 pages, 4 figure

We present a semiclassical theory of shot noise in diffusive superconductor-normal-metal ͑SN͒ contacts. At subgap voltages, we reproduce the doubling of shot noise with respect to conventional normal-metal contacts, which is interpreted in terms of an energy balance of electrons. Above the gap, the voltage dependence of the noise crosses over to the standard one with a voltage-independent excess noise. The semiclassical description of noise leads to correlations between currents at different electrodes of multiterminal SN contacts which are always of the fermionic type, i.e., negative. Using a quantum extension of the Boltzmann-Langevin method, we reproduce the peculiarity of noise at the Josephson frequency and obtain an analytical frequency dependence of noise at above-gap voltages.

We calculate the third cumulant of current in a chaotic cavity with contacts of arbitrary transparency as a function of frequency. Its frequency dependence drastically differs from that of the conventional noise. In addition to a dispersion at the inverse RC time characteristic of charge relaxation, it has a low-frequency dispersion at the inverse dwell time of electrons in the cavity. This effect is suppressed if both contacts have either large or small transparencies.

We calculate the corrections to the Sharvin conductance of ballistic multimode microcontacts that result from electron-electron scattering in the leads. Using a semiclassical Boltzmann equation, we obtain that these corrections are positive and scale with temperature as T(2)ln(EF/T) for three-dimensional contacts and as T for two-dimensional ones. These results are relevant to recent experiments on two-dimensional electron gas contacts.

The shot noise in long diffusive superconductor-normal-metal-superconductor contacts is calculated using the semiclassical approach. At low frequencies and for purely elastic scattering, the voltage dependence of the noise is of the form S(I) = (4Delta+2eV)/3R. The electron-electron scattering suppresses the noise at small voltages resulting in vanishing noise yet infinite dS(I)/dV at V = 0. The distribution function of electrons consists of a series of steps, and the frequency dependence of noise exhibits peculiarities at omega = neV, omega = neV-2Delta, and omega = 2Delta-neV for integer n.

It is often argued that a small non-degenerate quantum system coupled to a bath has a fixed energy in its ground state since a fluctuation in energy would require an energy supply from the bath. We consider a simple model of a harmonic oscillator (the system) coupled to a linear string and determine the mean squared energy fluctuations. We also analyze the two time correlator of the energy and discuss its behavior for a finite string.The coupling of two subsystems is a central problem in many areas of physics. The interaction of atoms with the radiation field has been central in the development of quantum mechanics 1 . The division into subsystems often depends on the questions asked: For instance dephasing in mesoscopic systems investigates the quantum motion of a single electron in a conductor viewing all other conduction electrons as the bath 2 . In this work we are interested in the energy fluctuations in a test system coupled to another system. Of particular interest is the zero-temperature limit. It is often argued that in the zero-temperature limit two subsystems can not exchange energy 3,4 . The argument is based on the assumption that both subsystems are in their separate ground state (which they would assume in the absence of any coupling). Since both the test system and the bath are in their ground state neither of the two can supply an energy to the other 3,4 . Below we consider a simple system of an oscillator (the test system) coupled to a linear string (the bath) and investigate the energy fluctuations of the test system. This is an exactly solvable model which demonstrates the existence of energy fluctuations in the zero-temperature limit. These fluctuations are a consequence of the finite coupling energy between the test system and the bath.Often the interaction of two subsystems is treated in terms of the Einstein coefficients for the absorption and the spontaneous and stimulated emission of the test system. In such an approach rates of transitions of the test system to change energy form one eigenstate E n to another eigenstate E m are investigated. These are inelastic transition rates which vanish when both systems are in the ground state. The resulting probabilities are used in a classical stochastic equation for the probabilities of occupation of the test system. Only probabilities enter in the description of the reservoir system interaction and this approach again yields no energy fluctuations in the ground state. Such a description is challenged both in Laser physics 1 and in the more recent discussions of macroscopic quantum coherence 5,6 .The role of zero-point fluctuations in mesoscopic conductors is a hotly debated issue and we cite here only Ref. 7 as an entry to the literature. The point of view taken here has been applied to investigate the persistent current of a small mesoscopic loop with a quantum dot capacitively coupled to a transmission line 8 . The persistent current is a measure of the quantum coherence of the ground state. It was found that the ground state undergoes a cro...