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We report a study of the cyclotron resonance (CR) transitions to and from the unusual $n=0$ Landau level (LL) in monolayer graphene. Unexpectedly, we find the CR transition energy exhibits large (up to 10%) and non-monotonic shifts as a function of the LL filling factor, with the energy being largest at half-filling of the $n=0$ level. The magnitude of these shifts, and their magnetic field dependence, suggests that an interaction-enhanced energy gap opens in the $n=0$ level at high magnetic fields. Such interaction effects normally have limited impact on the CR due to Kohn's theorem [W. Kohn, Phys. Rev. {\bf 123}, 1242 (1961)], which does not apply in graphene as a consequence of the underlying linear band structure., Comment: 4 pages, 4 figures. Version 2, edited for publication. Includes a number of edits for clarity; also added a paragraph contrasting our work w/ previous CR expts. in 2D Si and GaAs

A remarkable manifestation of the quantum character of electrons in matter is offered by graphene, a single atomic layer of graphite. Unlike conventional solids where electrons are described with the Schrodinger equation, electronic excitations in graphene are governed by the Dirac Hamiltonian. Some of the intriguing electronic properties of graphene, such as massless Dirac quasiparticles with linear energy-momentum dispersion, have been confirmed by recent observations. Here we report an infrared (IR) spectromicroscopy study of charge dynamics in graphene integrated in gated devices. Our measurements verify the expected characteristics of graphene and, owing to the previously unattainable accuracy of IR experiments, also uncover significant departures of the quasiparticle dynamics from predictions made for Dirac fermions in idealized, free standing graphene. Several observations, including a marked deviation of the energy bands from the simple linear dispersion, point to the relevance of many body interactions to the rather exotic electromagnetic response in graphene., Comment: New references are added. This is the submitted version. The published version is modified

We report on infrared spectroscopy of bilayer graphene integrated in gated structures. We observed a significant asymmetry in the optical conductivity upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer. From the conductivity data, the energy difference of the two sublattices is determined., Comment: 4 pages, 3 figures

The resistivity of ultra-clean suspended graphene is strongly temperature dependent for 5K0.5*10^11 cm^-2, the resistivity increases with increasing T and is linear above 50K, suggesting carrier scattering from acoustic phonons. At T=240K the mobility is ~120,000 cm^2/Vs, higher than in any known semiconductor. At the charge neutral point we observe a non-universal conductivity that decreases with decreasing T, consistent with a density inhomogeneity <10^8 cm^-2.

We have achieved mobilities in excess of 200,000 cm^2/Vs at electron densities of ~2*10^11 cm^-2 by suspending single layer graphene. Suspension ~150 nm above a Si/SiO_2 gate electrode and electrical contacts to the graphene was achieved by a combination of electron beam lithography and etching. The specimens were cleaned in situ by employing current-induced heating, directly resulting in a significant improvement of electrical transport. Concomitant with large mobility enhancement, the widths of the characteristic Dirac peaks are reduced by a factor of 10 compared to traditional, non-suspended devices. This advance should allow for accessing the intrinsic transport properties of graphene., Comment: 4 pages, 3 figures, references updated

We present the first measurements of cyclotron resonance of electrons and holes in bilayer graphene. In magnetic fields up to B = 18 T we observe four distinct intraband transitions in both the conduction and valence bands. The transition energies are roughly linear in B between the lowest Landau levels, whereas they follow \sqrt{B} for the higher transitions. This highly unusual behavior represents a change from a parabolic to a linear energy dispersion. The density of states derived from our data generally agrees with the existing lowest order tight binding calculation for bilayer graphene. However in comparing data to theory, a single set of fitting parameters fails to describe the experimental results., Comment: to appear in Phys. Rev. Lett. Updated version with two added references and minor text editing

We present a spectrum of experimental data on the fractional quantum Hall effect (FQHE) states in the first excited Landau level, obtained in an ultrahigh mobility two-dimensional electron system (2DES) and at very low temperatures and report the following results: For the even-denominator FQHE states, the sample dependence of the nu=5/2 state clearly shows that disorder plays an important role in determining the energy gap at nu=5/2. For the developing nu=19/8 FQHE state the temperature dependence of the Rxx minimum implies an energy gap of ~5mK.The energy gaps of the odd-denominator FQHE states at nu=7/3 and 8/3 also increase with decreasing disorder, similar to the gap at 5/2 state. Unexpectedly and contrary to earlier data on lower mobility samples, in this ultra-high quality specimen, the nu=13/5 state is missing, while its particle-hole conjugate state, the nu=12/5 state, is a fully developed FQHE state. We speculate that this disappearance might indicate a spin polarization of the nu=13/5 state. Finally, the temperature dependence is studied for the two-reentrant integer quantum Hall states around nu=5/2 and is found to show a very narrow temperature range for the transition from quantized to classical value., Comment: to be published

The conductivity of graphene samples with various levels of disorder is investigated for a set of specimens with mobility in the range of $1-20\times10^3$ cm$^2$/V sec. Comparing the experimental data with the theoretical transport calculations based on charged impurity scattering, we estimate that the impurity concentration in the samples varies from $2-15\times 10^{11}$ cm$^{-2}$. In the low carrier density limit, the conductivity exhibits values in the range of $2-12e^2/h$, which can be related to the residual density induced by the inhomogeneous charge distribution in the samples. The shape of the conductivity curves indicates that high mobility samples contain some short range disorder whereas low mobility samples are dominated by long range scatterers., Comment: 4 pages 4 figures

We investigate the quantum Hall (QH) states near the charge neutral Dirac point of a high mobility graphene sample in high magnetic fields. We find that the QH states at filling factors $\nu=\pm1$ depend only on the perpendicular component of the field with respect to the graphene plane, indicating them to be not spin-related. A non-linear magnetic field dependence of the activation energy gap at filling factor $\nu=1$ suggests a many-body origin. We therefore propose that the $\nu=0$ and $\pm1$ states arise from the lifting of the spin and sub-lattice degeneracy of the $n=0$ LL, respectively., Comment: 4 pages, 4 figures, to appear in Phys. Rev. Lett

We report infrared studies of the Landau level (LL) transitions in single layer graphene. Our specimens are density tunable and show \textit{in situ} half-integer quantum Hall plateaus. Infrared transmission is measured in magnetic fields up to B=18 T at selected LL fillings. Resonances between hole LLs and electron LLs, as well as resonances between hole and electron LLs are resolved. Their transition energies are proportional to $\sqrt{B}$ and the deduced band velocity is $\tilde{c}\approx1.1\times10^6$ m/s. The lack of precise scaling between different LL transitions indicates considerable contributions of many-particle effects to the infrared transition energies., Comment: 4 pages, 3 figures, to appear in Phys. Rev. Lett

The quantum Hall effect (QHE), one example of a quantum phenomenon that occur on a truly macroscopic scale, has been attracting intense interest since its discovery in 1980 and has helped elucidate many important aspects of quantum physics. It has also led to the establishment of a new metrological standard, the resistance quantum. Disappointingly, however, the QHE could only have been observed at liquid-helium temperatures. Here, we show that in graphene - a single atomic layer of carbon - the QHE can reliably be measured even at room temperature, which is not only surprising and inspirational but also promises QHE resistance standards becoming available to a broader community, outside a few national institutions., Comment: Published in Science online 15 February 2007

We perform a direct study of the magnitude of the anomalous splitting in the cyclotron resonance (CR) of a two-dimensional electron system (2DES) as a function of sample disorder. In a series of AlGaAs/GaAs quantum wells, identical except for a range of carbon doping in the well, we find the CR splitting to vanish at high sample mobilities but to increase dramatically with increasing impurity density and electron scattering rates. This observation lends strong support to the conjecture that the non-zero wavevector, roton-like minimum in the dispersion of 2D magnetoplasmons comes into resonance with the CR, with the two modes being coupled via disorder., Comment: accepted to PRB Rapid Comm

We describe a technique for fabricating nanometer-scale gaps in Pt wires on insulating substrates, using individual single-walled carbon nanotubes as shadow masks during metal deposition. More than 80% of the devices display current-voltage dependencies characteristic of direct electron tunneling. Fits to the current-voltage data yield gap widths in the 0.8-2.3 nm range for these devices, dimensions that are well suited for single-molecule transport measurements.

The quantum Hall (QH) effect in two-dimensional (2D) electrons and holes in high quality graphene samples is studied in strong magnetic fields up to 45 T. QH plateaus at filling factors $\nu=0,\pm 1,\pm 4$ are discovered at magnetic fields $B>$20 T, indicating the lifting of the four-fold degeneracy of the previously observed QH states at $\nu=\pm(|n|+1/2)$, where $n$ is the Landau level index. In particular, the presence of the $\nu=0, \pm 1$ QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting sublattice and spin degeneracy. The QH effect at $\nu=\pm 4$ is investigated in tilted magnetic field and can be attributed to lifting of the spin-degeneracy of the $n=1$ Landau level., Comment: 11 pages including 4 figures, to appear in PRL

The magnetoresistance, Rxx, at even-denominator fractional fillings, of an ultra high quality two-dimensional electron system at T ~ 35 mK is observed to be strictly linear in magnetic field, B. While at 35mK Rxx is dominated by the integer and fractional quantum Hall states, at T~1.2K an almost perfect linear relationship between Rxx vs B emerges over the whole magnetic field range except for spikes at the integer quantum Hall states. This linear Rxx cannot be understood within the Composite Fermion model, but can be explained through the existence of a density gradient in our sample.

We determine the spin susceptibility $\chi$ in the weak interaction regime of a tunable, high quality, two-dimensional electron system in a GaAs/AlGaAs heterostructure. The band structure effects, modifying mass and g-factor, are carefully taken into accounts since they become appreciable for the large electron densities of the weak interaction regime. When properly normalized, $\chi$ decreases monotonically from 3 to 1.1 with increasing density over our experimental range from 0.1 to $4\times10^{11} cm^{-2}$. In the high density limit, $\chi$ tends correctly towards $\chi\to 1$ and compare well with recent theory., Comment: Submitted to Physical Review B

We have observed quantization of the diagonal resistance, R_xx, at the edges of several quantum Hall states. Each quantized R_xx value is close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, R_xy. Peaks in R_xx occur at different positions in positive and negative magnetic fields. Practically all R_xx features can be explained quantitatively by a ~1%/cm electron density gradient. Therefore, R_xx is determined by R_xy and unrelated to the diagonal resistivity rho_xx. Our findings throw an unexpected light on the empirical resistivity rule for 2D systems.

We report on the temperature dependence of the mobility, $\mu$, of the two-dimensional electron gas in a variable density AlGaN/GaN field effect transistor, with carrier densities ranging from 0.4$\times10^{12}$ cm$^{-2}$ to 3.0$\times10^{12}$ cm$^{-2}$ and a peak mobility of 80,000 cm$^{2}$/Vs. Between 20 K and 50 K we observe a linear dependence $\mu_{ac}^{-1} = \alpha$T indicating that acoustic phonon scattering dominates the temperature dependence of the mobility, with $\alpha$ being a monotonically increasing function of decreasing 2D electron density. This behavior is contrary to predictions of scattering in a degenerate electron gas, but consistent with calculations which account for thermal broadening and the temperature dependence of the electron screening. Our data imply a deformation potential D = 12-15 eV., Comment: 3 pages, 2 figures, RevTeX. Submitted to Appl Phys Lett

We have investigated the behavior of electronic phases of the second Landau level under tilted magnetic fields. The fractional quantum Hall liquids at $\nu=$2+1/5 and 2+4/5 and the solid phases at $\nu=$2.30, 2.44, 2.57, and 2.70 are quickly destroyed with tilt. This behavior can be interpreted as a tilt driven localization of the 2+1/5 and 2+4/5 fractional quantum Hall liquids and a delocalization through melting of solid phases in the top Landau level, respectively. The evolution towards the classical Hall gas of the solid phases is suggestive of antiferromagnetic ordering.

We determine the density-dependent electron mass, m*, in two-dimensional (2D) electron systems of GaAs/AlGaAs heterostructures by performing detailed low-temperature Shubnikov deHaas measurements. Using very high quality transistors with tunable electron densities we measure m* in single, high mobility specimens over a wide range of r_s (6 to 0.8). Toward low-densities we observe a rapid increase of m* by as much as 40%. For 2>r_s>0.8 the mass values fall ~10% below the band mass of GaAs. Numerical calculations are in qualitative agreement with our data but differ considerably in detail., Comment: to be published in Physical Review Letters

A resistively detected NMR technique was used to probe the two-dimensional electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate $(1/T_{1})$ was extracted at near complete filling of the first Landau level by electrons. The nuclear spin of $^{75}$As is found to relax much more efficiently with $T\to 0$ and when a well developed quantum Hall state with $R_{xx}\simeq 0$ occurs. The data show a remarkable correlation between the nuclear spin relaxation and localization. This suggests that the magnetic ground state near complete filling of the first Landau level may contain a lattice of topological spin texture, i.e. a Skyrmion crystal.

At a very low temperature of 9mK, electrons in the 2nd Landau level of an extremely high mobility two-dimensional electron system exhibit a very complex electronic behavior. With varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed $\nu=2+2/5$ state separated from the even-denominator $\nu=2+1/2$ state by an insulating phase and a $\nu=2+2/7$ and $\nu=2+1/5$ state surrounded by such phases. A developing plateau at $\nu=2+3/8$ points to the existence of other even-denominator states.

Magnetotransport measurements were performed in a ultra-high mobility GaAs/AlGaAs quantum well of density $\sim 3.0 \times 10^{11}$ $cm^{-2}$. The temperature dependence of the magnetoresistance $R_{xx}$ was studied in detail in the vicinity of $\nu={9/2}$. In particular, we discovered new minima in $R_{xx}$ at filling factor $\nu\simeq 4{1/5}$ and $4{4/5}$, but only at intermediate temperatures $80\lesssim T\lesssim 120$ mK. We interpret these as evidence for a fractional quantum Hall liquid forming in the N=2 Landau level and competing with bubble and Wigner crystal phases favored at lower temperatures. Our data suggest that a magnetically driven insulator-insulator quantum phase transition occurs between the bubble and Wigner crystal phases at T=0., Comment: Phys. Rev. Lett.93 266804 (2004)

We present data on mobility lifetime, $\tau_t$, quantum lifetime, $\tau_q$, and cyclotron resonance lifetime, $\tau_{CR}$, of a sequence of high-mobility two-dimensional electron gases in the AlGaN/GaN system, covering a density range of $\sim1-4.5\times10^{12}$cm$^{-2}$. We observe a large discrepancy between $\tau_q$ and $\tau_{CR}$ ($\tau_q\sim\tau_{CR}$/6) and explain it as the result of density fluctuations of only a few percent. Therefore, only $\tau_{CR}$ --and not $\tau_q$ -- is a reliable measure of the time between electron scattering events in these specimens. The ratio $\tau_t / \tau_{CR}$ increases with increasing density in this series of samples, but scattering over this density range remains predominantly in the large-angle scattering regime.

Using cyclotron resonance, we measure the effective mass, $m$*, of electrons in AlGaN/GaN heterostructures with densities, $n_{2D}\sim 1-6\times10^{12}$cm$^{-2}$. From our extensive data, we extrapolate a band edge mass of $(0.208\pm0.002) m_e$. By comparing our $m$* data with the results of a multi-band \textbf{k.p} calculation we infer that the effect of remote bands is essential in explaining the observed conduction band non-parabolicity (NP). Our calculation of polaron mass corrections -- including finite width and screening - suggests those to be negligible. It implies that the behavior of $m*(n_{2D})$ can be understood solely in terms of NP. Finally, using our NP and polaron corrections, we are able to reduce the large scatter in the published band edge mass values.

Cyclotron-resonance (CR) measurements on two-dimensional (2D) electrons in AlGaN/GaN heterojunctions reveal large splittings (up to 2 meV) of the CR line for all investigated densities, $n_{2D}$, from 1 to $4\times 10^{12}cm^{-2}$ over wide ranges of magnetic field. The features resemble a level anti-crossing and imply a strong interaction with an unknown excitation of the solid. The critical energy of the splitting varies from 5 to 12 meV and as $\sqrt{n_{2D}}$. The phenomenon resembles data from AlGaAs/GaAs whose origin remains unresolved. It highlights a lack of basic understanding of a very elementary resonance in solids., Comment: 4 pages, 2 figures

At low Landau level filling of a two-dimensional electron system, typically associated with the formation of an electron crystal, we observe local minima in Rxx at filling factors nu=2/11, 3/17, 3/19, 2/13, 1/7, 2/15, 2/17, and 1/9. Each of these developing fractional quantum Hall (FQHE) states appears only above a filling factor-specific temperature. This can be interpreted as the melting of an electron crystal and subsequent FQHE liquid formation. The observed sequence of FQHE states follow the series of composite fermion states emanating from nu=1/6 and nu=1/8.

In a GaAs/AlGaAs quantum well of electron density 1x10^{11} cm^{-2} we observe a fractional quantum Hall effect (FQHE) at filling factors nu=4/11, and 5/13, and weaker states at nu=6/17, 4/13, 5/17 and 7/11. These sequences of fractions do not fit into the standard series of integral quantum Hall effects (IQHE) of composite fermions (CF) at nu=p/(2mp +/- 1). They rather can be regarded as the FQHE of CFs attesting to residual interactions between these composite particles. In tilted magnetic fields the nu=4/11 state remains unchanged, strongly suggesting it to be spin-polarized. The weak nu=7/11 state vanishes quickly with tilt.

We determine the spin susceptibility in a two dimensional electron system in GaAs/AlGaAs over a wide range of low densities from 2$\times10^{9}$cm$^{-2}$ to 4$\times10^{10}$cm$^{-2}$. Our data can be fitted to an equation that describes the density dependence as well as the polarization dependence of the spin susceptibility. It can account for the anomalous g-factors reported recently in GaAs electron and hole systems. The paramagnetic spin susceptibility increases with decreasing density as expected from theoretical calculations., Comment: 5 pages, 2 eps figures, to appear in PRL

We observe density induced 90$^{\circ}$ rotations of the anisotropy axes in transport measurements at half-filled high Landau levels in the two dimensional electron system, where stripe states are proposed ($\nu$=9/2, 11/2, etc). Using a field effect transistor, we find the transition density to be $2.9\times10^{11}$cm$^{-2}$ at $\nu$=9/2. Hysteresis is observed in the vicinity of the transition. We construct a phase boundary in the filling factor-magnetic field plane in the regime $4.4<\nu<4.6$. An in-plane magnetic field applied along either anisotropy axis always stabilizes the low density orientation of the stripes., Comment: 1 revtex file, 3 eps figures

We study the \nu=5/2 even-denominator fractional quantum Hall effect (FQHE) over a wide range of magnetic (B) field in a heterojunction insulated gate field-effect transistor (HIGFET). The electron density can be tuned from n=0 to 7.6 \times 10^{11} cm^{-2} with a peak mobility \mu = 5.5 \times 10^6 cm^2/Vs. The \nu=5/2 state shows a strong minimum in diagonal resistance and a developing Hall plateau at magnetic fields as high as 12.6T. The strength of the energy gap varies smoothly with B-field. We interpret these observations as strong evidence for a spin-polarized ground state at \nu=5/2., Comment: new references added

At very large tilt of the magnetic (B) field with respect to the plane of a two-dimensional electron system the transport in the integer quantum Hall regime at $\nu$ = 4, 6, and 8 becomes strongly anisotropic. At these filling factors the usual {\em deep minima} in the magneto-resistance occur for the current flowing {\em perpendicular} to the in-plane B field direction but develop into {\em strong maxima} for the current flowing {\em parallel} to the in-plane B field. The origin of this anisotropy is unknown but resembles the recently observed anisotropy at half-filled Landau levels., Comment: 4 pages, 4 figures

We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are {\em isotropic} in perpendicular magnetic field but become strongly {\em anisotropic} at $\nu$ = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, $B_{ip} \sim$ 2.5T, with the easy-current direction {\em parallel} to $B_{ip}$ but rotates by 90$^{\circ}$ at $B_{ip} \sim$ 10T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model., Comment: 4 pages, 4 figures

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, high-quality 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., Comment: 10 pages, 3 figures, to appear in Nature

We have measured the effective mass ($m^*$) of the four flux composite fermion at Landau level filling factor $\nu = 1/4$ ($^4$CF), using the activation energy gaps at the fractional quantum Hall effect (FQHE) states $\nu$ = 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de Haas (SdH) oscillations around $\nu = 1/4$. We find that the energy gaps show a linear dependence on the effective magnetic field $B_{eff}$ ($\equiv B-B_{\nu=1/4}$), and from this linear dependence we obtain $m^* = 1.0 m_e$ and a disorder broadening $\Gamma \sim$ 1 K for a sample of density $n = 0.87 \times 10^{11}$ /cm$^2$. The $m^*$ deduced from the temperature dependence of the SdH effect shows large differences for $\nu > 1/4$ and $\nu < 1/4$. For $\nu > 1/4$, $m^* \sim 1.0 m_e$. It scales as $\sqrt{B_{\nu}}$ with the mass derived from the data around $\nu =1/2$ and shows an increase in $m^*$ as $\nu \to 1/4$, resembling the findings around $\nu =1/2$. For $\nu < 1/4$, $m^*$ increases rapidly with increasing $B_{eff}$ and can be described by $m^*/m_e = -3.3 + 5.7 \times B_{eff}$. This anomalous dependence on $B_{eff}$ is precursory to the formation of the insulating phase at still lower filling., Comment: 5 pages, 3 figures

We observe sharp peaks and strong hysteresis in the electronic transport of a two-dimensional electron gas (2DEG) in the region of the integral quantum Hall effect. The peaks decay on time scales ranging from several minutes to more than an hour. Momentary grounding of some of the contacts can vastly modify the strength of the peaks. All these features disappear under application of a negative bias voltage to the backside of the specimen. We conclude, that a conduction channel parallel to the high mobility 2DEG is the origin for the peaks and their hysteretic behavior., Comment: 7 pages, 3 eps figures. Submitted to Phys. Rev. Lett, minor typos corrected

We report ultra-low temperature experiments on the obscure fractional quantum Hall effect (FQHE) at Landau level filling factor $\nu$=5/2 in a very high mobility specimen of $\mu=1.7 \times 10^7$ cm$^2$/Vs. We achieve an electron temperature as low as $\sim$ 4~mK, where we observe vanishing $R_{xx}$ and, for the first time, a quantized Hall resistance, $R_{xy}=h/(5/2e^2)$ to within 2 ppm. $R_{xy}$ at the neighboring odd-denominator states $\nu$=7/3 and 8/3 is also quantized. The temperature dependences of the $R_{xx}$-minima at these fractional fillings yield activation energy gaps $\Delta_{5/2}$=0.11K, $\Delta_{7/3}$=0.10K, and $\Delta_{8/3}$=0.055K., Comment: 5 pages, 3 figures

We have investigated the influence of an increasing in-plane magnetic field on the states at half-filling of Landau levels ($\nu$ = 11/2, 9/2, 7/2, and 5/2) of a two-dimensional electron system. In the electrically anisotropic phase at $\nu$ = 9/2 and 11/2 an in-plane magnetic field of $\sim$ 1-2 T overcomes its initial pinning to the crystal lattice and {\it reorient} this phase. In the initially isotropic phases at $\nu$ = 5/2 and 7/2 an in-plane magnetic field {\it induces} a strong electrical anisotropy. In all cases, for high in-plane fields, the high resistance axis is parallel to the direction of the in-plane field., Comment: 6 page, 3 figues

Low-temperature, electronic transport in Landau levels N>1 of a two-dimensional electron system is strongly anisotropic. At half-filling of either spin level of each such Landau level the magnetoresistance either collapses to form a deep minimum or is peaked in a sharp maximum, depending on the in-plane current direction. Such anisotropies are absent in the N=0 and N=1 Landau level, which are dominated by the states of the fractional quantum Hall effect. The transport anisotropies may be indicative of a new many particle state, which forms exclusively in higher Landau levels., Comment: 12 pages, 3 Postscript figures

We investigate the properties of composite fermions with four attached flux quanta through tilted field experiments near Landau level filling factor nu = 3/4. The observed collapse of fractional quantum Hall gaps in the vicinity of this quarter-filling state can be comprehensively understood in terms of composite fermions with mass and spin. Remarkably, the effective mass and g-factor of these four flux quanta composite fermions around nu = 3/4 are very similar to those of two flux quanta composite fermions around nu = 3/2., Comment: 4 pages, 1 table, 3 postscript figures