We show that an unpolarized electric current incident perpendicular to the plane of a thin ferromagnet can excite a spin-wave instability transverse to the current direction if source and drain contacts are not symmetric. The instability, which is driven by the current-induced "spin-transfer torque", exists for one current direction only.PACS numbers: 75.75.+a, 75.40.Gb, Ferromagnets serve as spin filters for an electrical current passing through the magnet: the spin of the electrons that are transmitted through a ferromagnet becomes partially polarized parallel or antiparallel to the direction of the magnetization whereas spin current perpendicular to the magnetization direction is absorbed. Spin filtering is the root cause for the "spintransfer torque", the phenomenon that a polarized current impinging on a ferromagnet affects its magnetization direction [1,2,3]. The source of the spin polarized current can either be a different ferromagnet, or, for a thick magnet, a region of the same ferromagnet upstream or downstream in the current flow. The "spin-transfer torque" gives rise to magnetization reversal in ferromagnet-normal-metal-ferromagnet trilayers [1, 2], which has been observed experimentally by several groups [4,5,6,7,8,9,10]. Dynamic manifestations of the spin-transfer torque include domain wall motion in bulk ferromagnets [11,12,13,14] and the excitation of spin waves by polarized currents in ferromagnetic multilayers or wires [2,3,4,5,6,7,8,15,16,17]. In all these manifestations, the current-induced spin torque can be distinguished from effects arising from the current induced magnetic field, the main difference being that spintransfer torque effects depend on the current direction, whereas magnetic field induced effects do not.In this letter, we show that an unpolarized current can also exert a spin-transfer torque on a ferromagnet, even if the magnet is so thin that its magnetization direction does not change along the current flow: Although an unpolarized current cannot exert a spin-transfer torque that changes the over-all magnetization direction, it can create a transverse spin wave instability for sufficiently high current densities if the source and drain contacts to the ferromagnet are not symmetric. This spin wave instability can be identified unambiguously as a spin-torque effect because of its dependence on current direction: the spin-wave instability is present for one current direction and absent for the other. The spin-wave instability should lead to a non-hysteretic feature in the currentvoltage characteristic of the ferromagnetic film that exists for one current direction only. In thick ferromagnets, such features have been observed in recent experiments [4,17] instability, asymmetric contacts to source and drain, is generically fulfilled in experiments on nanoscale magnets [5]. The issue of current-induced spin-wave excitation has significant practical relevance for devices based on the spin-torque effect in ferromagnetic multilayers. Whereas, experimentally, the presence of dynamica...

Motivated by a recent experiment by L.-H. Reydellet et al., Phys. Rev. Lett. 90, 176803 (2003), we discuss an interpretation of photon-assisted shot noise in mesoscopic multiprobe conductors in terms of electron-hole pair excitations. AC-voltages are applied to the contacts of the sample. Of interest are correlations resulting from the fact that electrons and holes are generated in pairs. We show that with two out-of-phase ac-potentials of equal magnitude and frequency, applied to different contacts, it is possible to trace out the Hanbury Brown Twiss exchange interference correlations in a four probe conductor. We calculate the distribution of Hanbury Brown Twiss phases for a four-probe single channel chaotic dot.

Random matrix theory can be used to describe the transport properties of a chaotic quantum dot coupled to leads. In such a description, two approaches have been taken in the literature, considering either the Hamiltonian of the dot or its scattering matrix as the fundamental random quantity of the theory. In this paper, we calculate the first four moments of the distribution of the scattering matrix of a chaotic quantum dot with a time-dependent potential, thus establishing the foundations of a "random scattering matrix approach" for time-dependent scattering. We consider the limit that the number of channels N coupling the quantum dot the reservoirs is large. In that limit, the scattering matrix distribution is almost Gaussian, with small non-Gaussian corrections. Our results reproduce and unify results for conductance and pumped current previously obtained in the Hamiltonian approach. We also discuss an application to current noise.

We study the current noise through an unbiased quantum electron pump and its mesoscopic fluctuations for arbitrary temperatures and beyond the bilinear response. In the bilinear regime, we find the full distributions of the noise power and the current-to-noise ratio for a chaotic quantum dots with single-channel and many-channel ballistic point contacts. For a dot with many-channel point contacts we also calculate the ensemble-averaged noise at arbitrary temperature and pumping strength. In the limit of strong pumping, a new temperature scale appears that corresponds to the broadening of the electron distribution function in the dot as a result of the time-dependent perturbations.

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