We report the observation of a highly unusual Hall current in the helical magnet MnSi in an applied pressure P = 6-12 kbars. The Hall conductivity displays a distinctive step-wise field profile quite unlike any other Hall response observed in solids. We identify the origin of this Hall current with the effective real-space magnetic field due to chiral spin textures, which may be a precursor of the partial-order state at P >14.6 kbar. We discuss evidence favouring the chiral spin mechanism for the origin of the observed Hall anomaly.

According to recent theories, a system of electrons at the half-filled Landau level can be transformed to an equivalent system of composite fermions at zero effective magnetic field. In order to test for these new particles, we have studied transport in antidot superlattices in a two-dimensional electron gas. At low magnetic fields electron transport exhibits well-known resonances at fields where the classical cyclotron orbit becomes commensurate with the antidot lattice. At v=y we observe the same dimensional resonances. This establishes the semiclassical behavior of composite fermions.PACS numbers: 73.40.HmDuring the past decade two-dimensional electron systems at low temperature and high magnetic field have repeatedly surprised us with exotic electron correlation phenomena. The quantum liquids of the fractional quantum Hall effect (FQHE) [1-3], the still enigmatic electron crystal [4] at very low filling factors, and the vanishing and reappearance of certain quantum Hall states in double layer electron systems [5][6][7] all reflect the dominance of electron-electron interaction at very high magnetic fields. Recently the nature of the electronic interaction at the half-filled Landau level has received much attention. There is now mounting evidence for a novel particle called a "composite fermion" [8] that plays a crucial role in the physics of two-dimensional (2D) electron systems in the lowest Landau level. In this paper we present results of an experiment that demonstrates the semiclassical motion of such a particle.The significance of the physics at the half-filled Landau level was foreshadowed in exceptional electrical transport and surface acoustic wave anomalies exactly at v = y. At this filling fraction Jiang et al.[9] observed a deep minimum in the magnetoresistivity p xx that persisted to unusually high temperatures, exhibiting a temperature dependence distinctly different from the neighboring FQHE states. Surface acoustic wave (SAW) experiments by Willett et al [10] at v= y revealed attenuation and velocity changes that were opposite to those observed in the regime of the FQHE liquids.Independently, the hierarchical model of the FQHE that orders the various odd-denominator states at v=p/q (p=integer, g=odd integer) had come under increasing criticism. Starting from the Laughlin liquids [11] condensed from electrons at v = l/m and v = l-1/m, the higher order FQHE states are derived from lower order states as Laughlin states of fractionally charged quasiparticles [12,13]. In particular, the two prominent series of liquids at v=p/(2p ± 1) represent a succession of parental and daughter states starting from v=y and v=y and converging towards v = y. However, questions were raised [14] regarding the density of quasiparticles and their apparent noninteracting nature.Jain proposed an innovative model for these series of FQHE liquids based on hypothetical particles which he termed composite fermions [14]. The liquids of the FQHE are then derived as the integral quantum Hall effect of such composite fermions. T...

The edge of a two-dimensional electron system (2DES) in a magnetic field consists of one-dimensional (1D) edge-channels that arise from the confining electric field at the edge of the specimen 1−3 . The crossed electric and magnetic fields, E x B, cause electrons to drift parallel to the sample boundary creating a chiral current that travels along the edge in only one direction. Remarkably, in an ideal 2DES in the quantum Hall regime all current flows along the edge 4−6 .Quantization of the Hall resistance, R xy = h/Ne 2 , arises from occupation of N 1D edge channels, each contributing a conductance of e 2 /h 7−11 . To explore this unusual one-dimensional property of an otherwise two-dimensional system, we have studied tunneling between the edges of 2DESs in the regime of integer quantum Hall effect (QHE). In the presence of an atomically precise, highquality tunnel barrier, the resultant interaction between the edge states leads to the formation of new energy gaps and an intriguing dispersion relation for electrons traveling along the barrier. The absence of tunneling features due to the electron spin and the persistence of a conductance peak at zero bias are not consistent with a model of weakly interacting edge states.

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Experiments on a nearly spin degenerate two-dimensional electron system reveals unusual hysteretic and relaxational transport in the fractional quantum Hall effect regime. The transition between the spin-polarized (with fill fraction nu = 1/3) and spin-unpolarized (nu = 2/5) states is accompanied by a complicated series of hysteresis loops reminiscent of a classical ferromagnet. In correlation with the hysteresis, magnetoresistance can either grow or decay logarithmically in time with remarkable persistence and does not saturate. In contrast to the established models of relaxation, the relaxation rate exhibits an anomalous divergence as temperature is reduced. These results indicate the presence of novel two-dimensional ferromagnetism with a complicated magnetic domain dynamic.