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Disorder and electron-electron interaction play essential roles in the physics of electron systems in condensed matter. In two-dimensional, quantum Hall systems, extensive studies of disorder-induced localization have led to the emergence of a scaling picture with a single extended state, characterized by a power-law divergence of the localization length in the zero-temperature limit. Experimentally, scaling has been investigated via measuring the temperature dependence of plateau-to-plateau transitions between the integer quantum Hall states (IQHSs), yielding a critical exponent κ≃0.42. Here we report scaling measurements in the fractional quantum Hall state (FQHS) regime where interaction plays a dominant role. Our Letter is partly motivated by recent calculations, based on the composite fermion theory, that suggest identical critical exponents in both IQHS and FQHS cases to the extent that the interaction between composite fermions is negligible. The samples used in our experiments are two-dimensional electron systems confined to GaAs quantum wells of exceptionally high quality. We find that κ varies for transitions between different FQHSs observed on the flanks of Landau level filling factor ν=1/2 and has a value close to that reported for the IQHS transitions only for a limited number of transitions between high-order FQHSs with intermediate strength. We discuss possible origins of the nonuniversal κ observed in our experiments.

An optical pumping technique was used to enhance and localize nuclear magnetic resonance (NMR) signals from an n-doped GaAs/Al0.1Ga0.9As multiple quantum well structure, permitting direct radio-frequency measurements of gallium-71 NMR spectra and nuclear spin-lattice relaxation rates (1/T1) as functions of temperature (1.6 K < T < 4.2 K) and the Landau level filling factor (0.66 < v < 1.76). The measurements reveal effects of electron-electron interactions on the energy levels and spin states of the two-dimensional electron system confined in the GaAs wells. Minima in 1/T1 at v approximately 1 and v approximately 2/3 indicate energy gaps for electronic excitations in both integer and fractional quantum Hall states. Rapid, temperature-independent relaxation at intermediate v values indicates a manifold of low-lying electronic states with mixed spin polarizations.

What are the ground states of an interacting, low-density electron system? In the absence of disorder, it has long been expected that as the electron density is lowered, the exchange energy gained by aligning the electron spins should exceed the enhancement in the kinetic (Fermi) energy, leading to a (Bloch) ferromagnetic transition. At even lower densities, another transition to a (Wigner) solid, an ordered array of electrons, should occur. Experimental access to these regimes, however, has been limited because of the absence of a material platform that supports an electron system with very high quality (low disorder) and low density simultaneously. Here we explore the ground states of interacting electrons in an exceptionally clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well. The large electron effective mass in this system allows us to reach very large values of the interaction parameter rs, defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases, qualitatively consistent with the above scenario: a paramagnetic phase at large densities, a spontaneous transition to a ferromagnetic state when rs surpasses 35, and then a phase with strongly nonlinear current-voltage characteristics, suggestive of a pinned Wigner solid, when rs exceeds ≃38. However, our sample makes a transition to an insulating state at rs ≃27, preceding the onset of the spontaneous ferromagnetism, implying that besides interaction, the role of disorder must also be taken into account in understanding the different phases of a realistic dilute electron system. [ABSTRACT FROM AUTHOR]

Hydrostatic pressure is a useful tool that can tune several key parameters in solid state materials. For example, the Landé g-factor in GaAs two-dimensional electron systems (2DESs) is expected to change from its bulk value g ≃ −0.44 to zero and even to positive values under sufficiently large hydrostatic pressure. Although this presents an intriguing platform to investigate electron-electron interaction in a system with g = 0, studies are quite limited because the GaAs 2DES density decreases significantly with increasing hydrostatic pressure. Here, we show that a simple model, based on pressure-dependent changes in the conduction band alignment, quantitatively explains this commonly observed trend. Furthermore, we demonstrate that the decrease in the 2DES density can be suppressed by more than a factor of 3 through an innovative heterostructure design. [ABSTRACT FROM AUTHOR]

Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, double quantum well systems can support exciton condensates, which consist of electrons in one well tightly bound to holes in the other. 'Perfect' drag is therefore expected; a steady transport current of electrons driven through one quantum well should be accompanied by an equal current of holes in the other. Here we demonstrate this effect, taking care to ensure that the electron-hole pairs dominate the transport and that tunnelling of charge between the quantum wells, which can readily compromise drag measurements, is negligible. We note that, from an electrical engineering perspective, perfect Coulomb drag is analogous to an electrical transformer that functions at zero frequency. [ABSTRACT FROM AUTHOR]

At high magnetic fields, where the Fermi level lies in the N=0 lowest Landau level (LL), a clean two-dimensional electron system (2DES) shows numerous incompressible liquid phases which exhibit the fractional quantum Hall effect (FQHE; ref. ). These liquid phases do not break rotational symmetry, exhibiting resistivities which are isotropic in the plane. In contrast, at lower fields, when the Fermi level lies in the N?2 third and several higher LLs, the 2DES exhibits a distinctly different class of collective states. In particular, near half-filling of these high LLs the 2DES exhibits a strongly anisotropic longitudinal resistance at low temperatures. These 'stripe' phases, which do not exhibit the quantized Hall effect, resemble nematic liquid crystals, possessing broken rotational symmetry and orientational order. Here we report a surprising new observation: an electronic configuration in the N=1 LL, the resistivity tensor of which simultaneously exhibits a robust fractionally quantized Hall plateau and a strongly anisotropic longitudinal resistance resembling that of the stripe phases. [ABSTRACT FROM AUTHOR]

Systems with coupled mechanical and optical or electrical degrees of freedom have fascinating dynamics that, through macroscopic manifestations of quantum behaviour, provide new insights into the transition between the classical and quantum worlds. Of particular interest is the back-action of electrons and photons on mechanical oscillators, which can lead to cooling and amplification of mechanical motion. Furthermore, feedback, which is naturally associated with back-action, has been predicted to have significant consequences for the noise of a detector coupled to a mechanical oscillator. Recently it has also been demonstrated that such feedback effects lead to strong coupling between single-electron transport and mechanical motion in carbon nanotube nanomechanical resonators. Here we present noise measurements which show that the mesoscopic back-action of electrons tunnelling through a radio-frequency quantum point contact causes driven vibrations of the host crystal. This effect is a remarkable macroscopic manifestation of microscopic quantum behaviour, where the motion of a mechanical oscillator—the host crystal, which consists of on the order of 1020 atoms—is determined by statistical fluctuations of tunnelling electrons. [ABSTRACT FROM AUTHOR]

When a strong magnetic field is applied perpendicularly (along z) to a sheet confining electrons to two dimensions (x–y), highly correlated states emerge as a result of the interplay between electron–electron interactions, confinement and disorder. These so-called fractional quantum Hall liquids form a series of states that ultimately give way to a periodic electron solid that crystallizes at high magnetic fields. This quantum phase of electrons has been identified previously as a disorder-pinned two-dimensional Wigner crystal with broken translational symmetry in the x–y plane. Here, we report our discovery of a new insulating quantum phase of electrons when, in addition to a perpendicular field, a very high magnetic field is applied in a geometry parallel (y direction) to the two-dimensional electron sheet. Our data point towards this new quantum phase being an electron solid in a ‘quasi-three-dimensional’ configuration induced by orbital coupling with the parallel field. [ABSTRACT FROM AUTHOR]

Semiconductor devices continue to press into the nanoscale regime, and new applications have been proposed for which a single dopant atom acts as the functional part of the device. Moreover, because shallow donors and acceptors are analogous to hydrogen atoms, experiments on small numbers of dopants have the potential to be a testing ground for fundamental questions of atomic and molecular physics. Although dopant properties are well understood with respect to the bulk, the study of configurations of dopants in small numbers is an emerging field. Here we present local capacitance measurements of electrons entering silicon donors in a gallium arsenide heterostructure. To the best of our knowledge, this study is the first example of single-electron capacitance spectroscopy carried out directly with a scanning probe tip. The precise position with respect to tip voltage of the observed single-electron peaks varies with the location of the probe, reflecting a random distribution of silicon within the donor plane. In addition, three broad capacitance peaks are observed independent of the probe location, indicating clusters of electrons entering the system at approximately the same voltages. These broad peaks are consistent with the addition energy spectrum of donor molecules, effectively formed by nearest-neighbour pairs of silicon donors. [ABSTRACT FROM AUTHOR]

Wigner solids in two-dimensional electron systems in high magnetic field B exhibit a striking, microwave or rf resonance, that is understood as a pinning mode. The temperature, Tm, above which the resonance is absent, is interpreted as the melting temperature of the solid. Studies of Tm for many B and many sample densities n show that Tm is a function of the Landau level filling ν alone for a given sample. This indicates that quantum mechanics figures importantly in the melting. Tm also appears to be increased by larger sample disorder. [ABSTRACT FROM AUTHOR]

The melting temperature Tm of a solid is generally determined by its solid–liquid transition on being heated at a fixed pressure, usually ambient pressure. It is also determined indirectly by the density n by means of the equation of state. This remains true even for solid helium, in which quantum effects often lead to unusual properties. Here, we present experimental evidence to show that for a two-dimensional (2D) solid formed by electrons in a semiconductor sample under a strong perpendicular magnetic field (B), Tm is not controlled by n, but effectively by the quantum correlation between the electrons through the Landau level filling factor ν=nh/eB (where h is the Planck constant and e is the electronic charge). Such melting behaviour, different from that of all other known solids (including a classical 2D electron solid at zero magnetic field), suggests the quantum nature of the magnetic-field-induced electron solid. Moreover, Tm increases with the strength of the sample-dependent disorder that tends to pin the electron solid in place. [ABSTRACT FROM AUTHOR]

Large cyclotron-resonance (CR) line splittings have been observed previously in two-dimensional (2D) electrons in AlGaN/GaN and in GaAs/AlGaAs heterojunctions. The features resemble a level anti-crossing and imply a strong interaction with an unknown excitation of the solid. The origin of this phenomenon remains unexplained. This paper reviews the existing data and presents some recently detected correlations of the splitting with other sample parameters. [ABSTRACT FROM AUTHOR]

We have measured high frequency magnetotransport of a high quality two-dimensional electron system (2DES) near the reentrant insulating phase (RIP) at Landau fillings (ν) between &rac15; and &frac29;. The magnetoconductivity in the RIP has resonant behavior around 150 MHz, showing a peak at &nu∼0.21. Our data support the interpretation of the RIP as due to some pinned electron solid. We have also investigated a narrowly confined 2DES recently found to have a RIP at ⅓<ν<½ and we have revealed features, not seen in DC transport, that suggest some intriguing interplay between the ⅓ FQHE and RIP. [ABSTRACT FROM AUTHOR]

We report the magnetotransport properties of a two-dimensional electron gas in a modulation-doped AlGaAs/GaAs heterostructure subjected to a lateral potential with honeycomb geometry. Periodic oscillations of the magnetoresistance and a delocalized-localized transition are shown by applying a gate voltage. We argue that electrons in such artificial-graphene lattices offer a promising approach for the simulation of quantum phases dictated by Coulomb interactions. [ABSTRACT FROM AUTHOR]

The surface charge generated on an Al0.24Ga0.76As/GaAs quantum well sample by electron bombardment was monitored by measuring the change in the conductivity of the channel. Upon turning off the electron bombardment the surface charge on adsorbed layers of xenon and water at 8 K decays in room temperature darkness with a lifetime τ=0.30±0.02 s. The average charging efficiency, μ0, defined as the ratio of the charge collected by the surface to the beam current times the charging time, is μ0≃0.001. Surface charging proves to be an effective method for contactless gating of field effect devices. [ABSTRACT FROM AUTHOR]

We experimentally determine the density dependence of the transport lifetime (τt) obtained from low-field Hall measurements and the quantum lifetime (τq) derived from analysis of the amplitude of Shubnikov–de Haas oscillations in a tunable high mobility two-dimensional electron gas (2DEG) in a Al0.06Ga0.94N/GaN heterostructure. Using an insulated gate structure, we are able to tune the 2DEG density from 2×1011 to 2×1012 cm-2, and thus, monitor the evolution of the scattering times in a single sample at T=0.3 K in a previously unexplored density regime. The transport lifetime τt is a strong function of electron density, increasing from ∼2.7 ps at ne=2×1011 cm-2 to ∼11 ps at ne=1.75×1012cm-2. Conversely, we find that the quantum scattering time τq is relatively insensitive to changes in electron density over this range. The data suggest that dislocation scattering accounts for the density dependence of τq as well as τt in our low-density sample. [ABSTRACT FROM AUTHOR]

A new technique for creating independent ohmic contacts to closely spaced two-dimensional electron systems in double quantum well (DQW) structures is described. Without use of shallow diffusion or precisely controlled etching methods, the present technique results in low-resistance contacts which can be electrostatically switched between the two-conducting layers. The method is demonstrated with a DQW consisting of two 200 Å GaAs quantum wells separated by a 175 Å AlGaAs barrier. A wide variety of experiments on Coulomb and tunnel-coupled 2D electron systems is now accessible. [ABSTRACT FROM AUTHOR]

We introduce an electrostatic lens for ballistic electrons in two-dimensional (2D) systems and demonstrate its focusing action in very high mobility GaAs-(AlGa)As heterostructures. This is the first refractive element for the control of 2D electrons. It exemplifies the close analogy between ballistic propagation in 2D electron systems and traditional geometrical optics. [ABSTRACT FROM AUTHOR]

We introduce three basic components for the manipulation of quasi-classical ballistic electrons in very high mobility two-dimensional electron systems and evaluate their effectiveness using the transverse electron focusing technique. We present an electron absorber or electron randomizer, demonstrate electron focusing in an area of tunable electron density, and present a simplified version of an injector/collector for ballistic carriers in two dimensions. [ABSTRACT FROM AUTHOR]

We report the observation of low-lying collective charge and spin excitations in the second Landau level at v=2+1/3 and also for the very fragile states at v=2+2/5 and 2+3/8 in inelastic light scattering experiments. These modes exhibit a clear dependence on filling factor and temperature substantiating the unique access to the characteristic neutral excitation spectra of the incompressible fractional quantum Hall effect (FQHE) states. A detailed mode analysis reveals low-energy modes at around 70 μeV and a sharp mode slightly below the Zeeman energy interpreted as gap and spin-wave excitation, respectively. The lowest-energy collective charge excitation spectrum at v=2+1/3 exhibits significant qualitative similarities with its cousin state in the lowest Landau level at v=1/3 suggesting similar magnetoroton minima in the neutral excitations. The mode energies differ by a scaling of 0.15 indicating different interaction physics in the N=0 and N=1 Landau levels. The striking polarization dependence in elastic and inelastic light scattering is discussed in the framework of anisotropic electron phases that allow for the stabilization of nematic FQHE states. The observed excitation spectra provide new insights by accessing quantum phases in the bulk of electron systems and facilitate comparison with different theoretical descriptions of those enigmatic FQHE states. [ABSTRACT FROM AUTHOR]

We have measured magnetic field dependences of the attenuation and velocity of surface acoustic waves in a high-mobility n-GaAs/AlGaAs structure with a wide quantum well. The results allowed us to find the complex conductance σ(ω) of the heterostructure for different frequencies, temperatures, and magnetic fields near filling factors v = 1,2. Observed behavior of σ(ω) versus magnetic field outside close vicinities of integer fillings reveals an oscillation pattern indicative of the rich fractional quantum Hall effect. Our result is that in very close vicinities of integer filling factors the AC response of a high-mobility two-dimensional structures behaves as that of a two-dimensional system of localized electrons. Namely, both real and imaginary parts of the complex AC conductance at low temperatures agree with the predictions for the two-site model for a two-dimensional hopping system. Another result is the specific temperature dependences of σ(ω), which are extremely sensitive to the filling factor value. These dependences indicate a sharp crossover between the localized modes and a pinned Wigner crystal. [ABSTRACT FROM AUTHOR]

We observe geometric resonance features of composite fermions on the flanks of the even-denominator ν = 1/2 fractional quantum Hall state in high-mobility two-dimensional electron and hole systems confined to wide GaAs quantum wells and subjected to a weak, strain-induced, unidirectional periodic potential modulation. The features provide a measure of how close to ν = 1/2 the system stays singlecomponent and supports a composite fermion Fermi sea before transitioning into a ν = 1/2 fractional quantum Hall state, presumably, the two-component Ψ331 state. [ABSTRACT FROM AUTHOR]

In a quasi-two-dimensional electron system with nonzero layer thickness, a parallel magnetic field can couple to the out-of-plane electron motion and lead to a severe distortion and eventual splitting of the Fermi contour. Here we directly and quantitatively probe this evolution through commensurability and Shubnikov-de Haas measurements on electrons confined to a 40-nm-wide GaAs (001) quantum well. We are able to observe the Fermi contour splitting phenomenon, in good agreement with the results of semiclassical calculations. Experimentally, we also observe intriguing features, suggesting magneticbreakdown- type behavior when the Fermi contour splits. [ABSTRACT FROM AUTHOR]

In a two-dimensional electron system subject to microwaves and a magnetic field, photovoltages emerge. They can be separated into two components originating from built-in electric fields and electric field domains arising from spontaneous symmetry breaking. The latter occurs in the zero resistance regime only and manifests itself in pulsed behavior, synchronous across the sample. The pulses show sign reversal. This implies a flip of the field in each domain, consistent with the existence of two equally probable electric field domain configurations due to the spontaneous symmetry breaking. [ABSTRACT FROM AUTHOR]

In high-quality two-dimensional electrons confined to GaAs quantum wells, near Landau level filling factors v = 1/2 and 1/4, we observe signatures of the commensurability of the electron-flux composite fermion cyclotron orbits with a unidirectional periodic density modulation. Focusing on the data near v = 1/2, we directly and quantitatively probe the shape of the composite fermions' cyclotron orbit, and therefore their Fermi contour, as a function of magnetic field (B∥) applied parallel to the sample plane. The composite fermion Fermi contour becomes severely distorted with increasing B∥ and appears to be elliptical, in sharp contrast to the electron Fermi contour which splits as the system becomes bilayerlike at high B∥. We present a simple, qualitative model to interpret our findings. [ABSTRACT FROM AUTHOR]

For a series of samples with two-dimensional electron systems in dilute AlxGa1-xAs, with varying x from 0% to 0.85%, we survey the pinning mode resonances of Wigner solids at the low Landau filling termination of the fractional quantum Hall effect series. For all x studied, the pinning modes are present with frequencies fpk that are consistent with collective weak pinning. For x⩾0.22% we find fpk vs B exhibits a rapid increase that is not present for x=0. We find the observed fpk is much smaller than values calculated with a simple Wigner solid model which neglects the effects of the disorder on the charge distribution of a carrier. [ABSTRACT FROM AUTHOR]

We demonstrate a severe Fermi contour anisotropy induced by the application of a parallel magnetic field to high-mobility electrons confined to a 30-nm-wide (001) GaAs quantum well. We study commensurability oscillations, namely, geometrical resonances of the electron orbits with a unidirectional, surface-strain-induced, periodic potential modulation, to directly probe the size of the Fermi contours along and perpendicular to the parallel field. Their areas are obtained from the Shubnikov-de Haas oscillations. Our experimental data agree semiquantitatively with the results of parameter-free calculations of the Fermi contours, but there are significant discrepancies. [ABSTRACT FROM AUTHOR]

Detailed measurements of the longitudinal thermopower of two-dimensional electrons in the first excited Landau level are reported. Clear signatures of numerous fractional quantized Hall states, including those at v = 5/2 and 7/3, are observed in the magnetic field and temperature dependence of the thermopower. An abrupt collapse of the thermopower is observed below about T = 40 mK at those filling factors where reentrant insulating electronic states have been observed in conventional resistive transport studies. The thermopower observed at v = 5/2 is discussed in the context of recent theories which incorporate non-Abelian quasiparticle exchange statistics. [ABSTRACT FROM AUTHOR]

We report the evolution of the fractional quantum Hall state (FQHS) at a total Landau level (LL) filling factor of v = 7/2 in wide GaAs quantum wells in which electrons occupy two electric subbands. The data reveal subtle and distinct evolutions as a function of density, magnetic field tilt angle, or symmetry of the charge distribution. At intermediate tilt angles, for example, we observe a strengthening of the v = 7/2 FQHS. Moreover, in a well with asymmetric change distribution, there is a developing FQHS when the LL filling factor of the symmetric subband vS equals 5/2 while the antisymmetric subband has a filling factor of 1 < vA < 2. [ABSTRACT FROM AUTHOR]

Microwave-induced resistance oscillations (MIROs) have been commonly observed in high-mobility GaAs/AlGaAs two-dimensional electron systems (2DESs) under microwave irradiation. In ultraclean GaAs/AlGaAs quantum wells, we have recently observed a very large resistance spike at the second harmonic of cyclotron resonance. In order to elucidate its origin, we have studied the response of microwave photoresistances in a two-axis magnetic field configuration, where the perpendicular (Bz) and the in-plane (Bx) components can be independently applied to the sample. The experiments reveal a distinctive response of the spike to the Bx compared with that of the MIROs. While the major MIRO peaks show an increasing phase shift toward a quarter period in increasing Bx, the spike position shows an essentially zero shift. This finding lends additional support for the notion that the spike is a unique effect in the microwave-driven 2DES. [ABSTRACT FROM AUTHOR]

We report the observation of a remarkably strong microwave photoresistivity effect in a high-mobility two-dimensional electron system subject to a weak magnetic field and low temperature. The effect manifests itself as a giant microwave-induced resistivity peak which, in contrast to microwave-induced resistance oscillations, appears only near the second harmonic of the cyclotron resonance and only at sufficiently high microwave frequencies. Appearing in the regime linear in microwave intensity, the peak can be more than an order of magnitude stronger than the microwave-induced resistance oscillations and can not be explained by existing theories. [ABSTRACT FROM AUTHOR]

Quasi-particles with fractional charge and statistics, as well as modified Coulomb interactions, exist in a two-dimensional electron system in the fractional quantum Hall (FQH) regime. Theoretical models of the FQH state at filling fraction v = 5/2 make the further prediction that the wave function can encode the interchange of two quasi-particles, making this state relevant for topological quantum computing. We show that bias-dependent tunneling across a narrow constriction at v = 5/2 exhibits temperature scaling and, from fits to the theoretical scaling form, extract values for the effective charge and the interaction parameter of the quasi-particles. Ranges of values obtained are consistent with those predicted by certain models of the 5/2 state. [ABSTRACT FROM AUTHOR]

The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors (ν≲1/6) has long been a topic of interest and controversy in condensed matter. Following the recent breakthrough in the quality of ultrahigh-mobility GaAs 2DESs, we revisit this problem experimentally and investigate the impact of reduced disorder. In a GaAs 2DES sample with density n=6.1×1010/cm² and mobility μ=25×106 cm²/V s, we find a deep minimum in the longitudinal magnetoresistance (Rxx) at ν=1/7 when T≃104 mK. There is also a clear sign of a developing minimum in Rxx at ν=2/13. While insulating phases are still predominant when ν≲1/6, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence ν=p/(2mp±1) even in the very low Landau level filling limit. The magnetic-field-dependent activation energies deduced from the relation Rxx∝eEA/2kT corroborate this view and imply the presence of pinned Wigner solid states when ν≠p/(2mp±1). Similar results are seen in another sample with a lower density, further generalizing our observations. [ABSTRACT FROM AUTHOR]

The fractional quantum Hall effect stands as a quintessential manifestation of an interacting two-dimensional electron system. One of the fractional quantum Hall effect's most fundamental characteristics is the energy gap separating the incompressible ground state from its excitations. Yet, despite nearly four decades of investigations, a quantitative agreement between the theoretically calculated and experimentally measured energy gaps is lacking. Here we report a systematic experimental study that incorporates very high-quality two-dimensional electron systems confined to GaAs quantum wells with fixed density and varying well widths. The results demonstrate a clear decrease of the energy gap as the electron layer is made thicker and the short-range component of the Coulomb interaction is weakened. We also provide a quantitative comparison between the measured energy gaps and the available theoretical calculations that takes into account the role of finite layer thickness and Landau level mixing. All the measured energy gaps fall below the calculations, but as the electron layer thickness increases, the results of experiments and calculations come closer. Accounting for the role of disorder in a phenomenological manner, we find better overall agreement between the measured and calculated energy gaps, although some puzzling discrepancies remain. [ABSTRACT FROM AUTHOR]

Interacting two-dimensional electrons confined in a GaAs quantum well exhibit isotropic transport when the Fermi level resides in the first excited (N=1) Landau level. Adding an in-plane magnetic field (B‖) typically leads to an anisotropic, stripelike (nematic) phase of electrons with the stripes oriented perpendicular to the B‖ direction. Our experimental data reveal how a periodic density modulation, induced by a surface strain grating from strips of negative electron-beam resist, competes against the B‖-induced orientational order of the stripe phase. Even a minute (<0.25%) density modulation is sufficient to reorient the stripes along the direction of the surface grating. [ABSTRACT FROM AUTHOR]