The magnetic state of a ferromagnet can affect the electrical transport properties of the material; for example, the relative orientation of the magnetic moments in magnetic multilayers underlies the phenomenon of giant magnetoresistance. The inverse effect--in which a large electrical current density can perturb the magnetic state of a multilayer--has been predicted and observed experimentally with point contacts and lithographically patterned samples. Some of these observations were taken as indirect evidence for current-induced excitation of spin waves, or 'magnons'. Here we probe directly the high-frequency behaviour and partial phase coherence of such current-induced excitations, by externally irradiating a point contact with microwaves. We determine the magnon spectrum and investigate how the magnon frequency and amplitude vary with the exciting current. Our observations support the feasibility of a spin-wave maser' or 'SWASER' (spin-wave amplification by stimulated emission of radiation).

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Micro-contacts between metals at low temperatures reveal non-linear structures in the current-voltage characteristics. These deviations from Ohm's law allow an energy-resolved spectroscopy of the interaction of the conduction electrons with elementary excitations (e.g. phonons) in a metal. To explain the method, the important parameters (electron mean free path versus contact dimension) in point-contact spectroscopy will be discussed together with examples of spectroscopic information obtained in various systems. Local temperature gradients in the contact region offer the possibility to study thermo-electric phenomena in small constrictions, such as thermal voltages in non-homogeneous contacts and quenching of the phonon-drag term in the thermo-power in homogeneous contacts. Besides these aspects of the point-contact technique, recent experiments will be shown with applications of point contacts other than just spectroscopy: magneto-resistance of a point contact, high-frequency rectification with a point contact as the non-linear element, electron focusing using a double point-contact set-up, electrical noise in constrictions and generation of phonons by means of point contacts.

We present a study of Nernst effect in underdoped La(2-x)Sr(x)CuO4 in magnetic fields as high as 28 T. At high fields, a sizable Nernst signal was found to persist in the presence of a field-induced nonmetallic resistivity. By simultaneously measuring resistivity and the Nernst coefficient, we extract the entropy of vortex cores in the vicinity of this field-induced superconductor-insulator transition. Moreover, the temperature dependence of the thermoelectric Hall angle provides strong constraints on the possible origins of the finite Nernst signal above T(c), as recently discovered by Xu et al. [Nature (London) 406, 486 (2000)].

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InAs-AlSb quantum wells are investigated by transport experiments in magnetic fields tilted with respect to the sample normal. Using the coincidence method we find for magnetic fields up to 28 T that the spin splitting can be as large as 5 times the Landau splitting. We find a value of the g-factor of |g| ≈ 13. For small even-integer filling factors the corresponding minima in the Shubnikov-de Haas oscillations cannot be tuned into maxima for arbitrary tilt angles. This indicates the anti-crossing of neighboring Landau and spin levels. Furthermore we find for particular tilt angles a crossover from even-integer dominated Shubnikov-de Haas minima to odd-integer minima as a function of magnetic field.73. 50.-h, 72.20.-i, 72.90.+y

We observe pronounced transport anisotropies in magnetotransport experiments performed in the twodimensional electron system of a Si͞SiGe heterostructure. They occur when an in-plane field is used to tune two Landau levels with opposite spin to energetic coincidence. The observed anisotropies disappear drastically for temperatures above 1 K. We propose that our experimental findings may be caused by the formation of a unidirectional stripe phase oriented perpendicular to the in-plane field. DOI: 10.1103/PhysRevLett.86.866 PACS numbers: 73.21.-b, 71.70.Di, 73.40.Lq, 73.43.-f Two-dimensional electron systems (2DESs) in high magnetic fields show many interesting fundamental effects. Widely known is the quantization of the energy spectrum into discrete Landau levels (LLs). For high magnetic fields the electron-electron interaction becomes important and novel states emerge. One of the most prominent of such features is the fractional quantum Hall effect (FQHE) [1] where new collective quasiparticles for electrons in the lowest two LLs appear.For higher LLs it has been proposed that an ordered charge-density wave forms an energetically lower lying state compared to the FQHE states [2,3]. In the situation when a higher LL is half filled the electrons arrange in stripes where the LL considered is either totally full or totally empty. Experimental evidence for the existence of such a stripe phase at half LL filling was found recently by several groups in 2DESs of very high mobility GaAs͞AlGaAs heterostructures [4][5][6][7]. With an additional magnetic field B ip parallel to the 2DES the orientation of the stripes can be tuned to the direction perpendicular to B ip [6][7][8].In this Letter we report on magnetotransport experiments in the 2DES of a Si͞SiGe heterostructure. Adding an in-plane field B ip , with the normal field component left constant, two neighboring LLs with opposite spin can be tuned to half filling simultaneously. We will show that huge maxima in the Shubnikov-de Haas (SdH) oscillations appear if the current direction I is oriented along B ip . Such an enhancement of the SdH maxima is not observed when I is oriented perpendicular to B ip . We will propose the formation of a unidirectional "stripe phase" to explain the huge transport anisotropies and possible physical origins of the stripe formation will be discussed.Our sample is a Si͞SiGe heterostructure with a 25-nm-thick strained Si channel embedded between two Si 0.7 Ge 0.3 barriers [9]. The electrons are provided by doping the top barrier with Sb starting 12 nm away from the Si channel. The resulting band structure leads to a high mobility 2DES formed in a triangular potential at the heterojunction between the Si channel and the top SiGe barrier (electron concentration n 7.2 3 10 15 m 22 ; mobility m 20 m 2 ͞V s). In order to perform transport experiments a 100-mm-wide Hall bar was patterned on the sample along the ͓100͔ direction.In a magnetic field the energy level structure of the 2DES consists of discrete LLs at energies E N ͑N 1 1͞2͒ ͑heB n...