Magnetotransport experiments on high mobility two-dimensional electron gases in GaAs͞AlGaAs heterostructures have revealed striking anomalies near half filling of several spin-resolved, yet highly excited, Landau levels. These anomalies include strong anisotropies and nonlinearities of the longitudinal resistivity r xx which commence only below about 150 mK. These phenomena are not seen in the ground state or first excited Landau level but begin abruptly in the third level. Although their origin remains unclear, we speculate that they reflect the spontaneous development of a generic anisotropic many-electron state. [S0031-9007(98) A magnetic field applied perpendicular to the plane of a two-dimensional electron gas (2DEG) resolves the energy spectrum into discrete Landau levels (LLs). As the field increases, the Fermi level drops down through the Landau ladder in a series of steps until, at high field, it resides in the lowest (N 0) level. In this situation the kinetic energy of the electrons is quenched and electron-electron interactions dominate the physics with the fractional quantized Hall effect (FQHE) as the most spectacular consequence [1]. After more than 15 years of study, much is known about electron correlations in this lowest LL case. The same cannot be said when the Fermi level is in a higher Landau level. In the second LL (N 1), the FQHE is virtually absent; only fragile and poorly understood states at Landau filling fractions n 7͞3, 5͞2, and 8͞3 are seen in the best samples. In the third and higher LLs (N $ 2) still less is known, although there have been interesting suggestions of charge density waves in the clean limit [2,3]. At very high N, and therefore very low magnetic field, the Landau level splitting becomes insignificant and the 2DEG assumes the character of a weakly disordered Fermi liquid.In this paper we report the observation of several dramatic anomalies in the low temperature magnetotransport of clean 2DEGs when the Fermi level lies near the middle of a spin-resolved highly excited Landau level. These effects, which commence only below about 150 mK, abruptly begin and are strongest in the third (N 2) LL, but persist up to about N 6. Including strong anisotropies and intriguing nonlinearities of the resistivity, these effects suggest a considerably more interesting tableau at high N than independent electrons moving in a disordered Landau band.The samples used in this study are GaAs͞AlGaAs heterojunctions grown by molecular beam epitaxy (MBE). Data from six samples (A through F) will be discussed. Samples A, B, and C were taken from one MBE wafer, D and E from a second, and F from a third. Each wafer was rotated during growth to ensure high homogeneity of the electron density n s . These densities (in units of 10 11 cm 22 ) are close to n s 2.67 for samples A, B, and C; n s 2.27 for samples D and E; and n s 1.52 for sample F. The low temperature mobility of each is m $ 9 3 10 6 cm 2 ͞V s. Each sample was cleaved (along ͗110͘ directions) into a 5 3 5 mm square from its parent ͗001͘ wa...

We report the observation of an acute sensitivity of the anisotropic longitudinal resistivity of twodimensional electron systems in half-filled high Landau levels to the magnitude and orientation of an in-plane magnetic field. In the third and higher Landau levels, at filling fractions n 9͞2, 11͞2, etc., the in-plane field can lead to a striking interchange of the "hard" and "easy" transport directions. In the second Landau level the normally isotropic resistivity and the weak n 5͞2 quantized Hall state are destroyed by a large in-plane field and the transport becomes highly anisotropic. PACS numbers: 73.20.Dx, 73.40.Kp, 73.50.Jt In a recent paper, Lilly et al. [1] reported observations of several anomalies in the low temperature magnetotransport of high quality two-dimensional electron systems (2DES) when several Landau levels (LLs) are occupied. These anomalies include the development of large anisotropies and nonlinearities of the longitudinal resistivities (r xx and r yy ) near half filling of several spin-resolved high LLs. These effects are observed only at very low temperatures (T , 150 mK) and when at least three or more LLs are occupied. Taken together, the observations offer strong evidence for new correlated many-electron states in high LLs which are different than the familiar fractional quantized Hall states found in the lowest (N 0) LL. The fact that qualitatively identical phenomena are found in several adjacent LLs (having N $ 2) points to a generic mechanism. In this paper we report on an investigation of the behavior of these unusual phenomena when a magnetic field component B k in the plane of the 2D system is added to the perpendicular field B Ќ generating the Landau quantization. Our results show that the transport anisotropy can be quite sensitive to the in-plane magnetic field: in some cases B k appears to rotate the principal axes of the anisotropy by 90 ± . Figure 1 illustrates the anisotropy of the longitudinal resistance seen in high LLs at T 50 mK. The sample used to obtain these results is a modulation-doped GaAs͞ AlGaAs heterojunction containing a 2DES with sheet density of n s 2.7 3 10 11 cm 22 and a mobility of 11 3 10 6 cm 2 ͞V s. This structure was grown by molecular beam epitaxy (MBE) on a ͗001͘ GaAs substrate. The sample geometry (see insets) consists of a square mesa, 2.5 mm on a side, etched onto a larger square chip. The sides of the mesa are rotated 45 ± relative to the natural cleavage directions (͗110͘ and ͗110͘) of GaAs. Eight diffused indium Ohmic contacts are placed at the corners and midpoints of the sides of the square. For the data in Fig. 1a, the solid curve corresponds to current flowing between corner contacts along the diagonal of the square which is parallel to the ͗110͘ crystallographic direction, while for the dotted curve the current flow is between corner contacts along the diagonal parallel to ͗110͘. The inset to the figures identifies these diagonals. For each trace, the measured voltage is that between the two midpoint contacts on one side of the cur...

We report electrical transport measurements on GaAs/AlGaAs based electron-hole bilayers. These systems are expected to make a transition from a pair of weakly coupled two-dimensional systems to a strongly coupled exciton system as the barrier between the layers is reduced. Once excitons form, phenomena such as Bose-Einstein condensation of excitons could be observed. In our devices, electrons and holes are confined in double quantum wells, and carriers in the devices are induced with top and bottom gates leading to variable density in each layer. Separate contact to each layer allows Coulomb drag transport measurements where current is driven in one layer while voltage is measured in the other. Coulomb drag is sensitive to interlayer coupling and has been predicted to provide a strong signature of exciton condensation. Drag measurement on EHBLs with a 30 nm barrier are consistent with drag between two weakly coupled 2D Fermi systems where the drag decreases as the temperature is reduced. When the barrier is reduced to 20 nm, we observe a consistent increase in the drag resistance as the temperature is reduced. These results indicate the onset of a much stronger coupling between the electrons and holes which leads to exciton formation and possibly phenomena related to exciton condensation.

The intriguing re-entrant integer quantized Hall states recently discovered in high Landau levels of high-mobility 2D electron systems are found to exhibit extremely non-linear transport. At small currents these states reflect insulating behavior of the electrons in the uppermost Landau level. At larger currents, however, a discontinuous and hysteretic transition to a conducting state is observed. These phenomena, found only in very narrow magnetic field ranges, are suggestive of the depinning of a charge density wave state, but other explanations can also be constructed.Two-dimensional electron systems (2DES) in strong magnetic fields have proven to be a remarkably rich laboratory for many-body physics [1]. The continuing improvements in the techniques for creating 2DES in semiconductor heterostructures have been paralleled by a steady stream of discoveries of novel electron correlation phenomena. While the fractional quantum Hall effect (FQHE) in the lowest (N = 0) Landau level (LL) is the best known of these, there has been a recent realization that interactions among electrons in the excited LLs can give rise to whole new classes of many-body phenomena. For example, recent transport measurements [2,3] have revealed huge and unexpected anisotropies of the resistivity of the 2DES when the third and higher (N ≥ 2) LLs are half filled. These anisotropies are not seen in the lowest two LLs and appear only at very low temperatures and in the highest quality samples. The observations are in qualitative agreement with earlier theoretical suggestions of unidirectional charge density wave (CDW) ground states ("stripe phases") in the half-filled N ≥ 2 LLs [4,5]. More recent theoretical work [6-9], going beyond the Hartree-Fock approximation, has lent support to the stripe phase picture, albeit with possibly important modifications due to quantum fluctuations.The experiments in high Landau levels also reveal remarkable phenomena away from half filling, in the flanks of the LLs. In this regime both Lilly, et al.[2] and Du, et al. [3] reported the resistivity to be essentially isotropic and to fall to zero in narrow regions of magnetic field near 1 4 and 3 4 filling of the LLs. In these regions the Hall resistance is found to be accurately quantized but, quite surprisingly, at the value of the adjacent integer quantum Hall plateaus. These re-entrant integer quantum Hall effect (RIQHE) states, which have only been found in the N ≥ 2 LLs, suggest the existence of insulating phases of the electrons in the uppermost LL. In this paper we report the observation of a discontinuous transition from the insulating state to a conducting one when large electric fields are applied. This transition is found to be hysteretic and extremely temperature and magnetic field dependent. The results are suggestive of the depinning of CDWs [10], but they are also reminiscent of quantum Hall breakdown phenomena [11].The samples used in this investigation are modulationdoped GaAs/AlGaAs heterostructures grown by molecular beam epitaxy. Data from tw...

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Coulomb drag resulting from interlayer electron-electron scattering in double layer 2D electron systems in a high magnetic field has been measured. Within the lowest Landau level the observed drag resistance exceeds its zero magnetic value by factors of typically 1000. At half-filling of the lowest Landau level in each layer (n 1͞2) the data suggest that our bilayer systems are much more strongly correlated than recent theoretical models based on perturbatively coupled composite fermion metals.[ S0031-9007(98) PACS numbers: 73.40. Hm, 72.15.Qm, 73.20.Dx Double layer two-dimensional electron systems (2DES) have been the subject of intense recent interest, especially at high magnetic fields, owing to the diversity of manybody phenomena they exhibit which are not found in ordinary single layer systems. These new phenomena arise from the interplay of the intralayer and interlayer Coulomb interactions and the tunneling amplitude in the system. A particularly interesting case arises when the individual 2D layers are at Landau level filling fraction n 1͞2. If the separation between the layers is large, the system behaves as two independent 2DES's, each of which is widely believed to be a Fermi liquidlike state of Chern-Simons composite particles. On the other hand, when the layers are close together, they behave as a single system and exhibit a ferromagnetic quantized Hall state at total Landau filling factor n tot 1͞2 1 1͞2 1. The nature of this remarkable transition from two gapless Fermi liquids to a single gapped quantum Hall phase is not well understood and remains a frontier topic in the field [1].The strength of the Coulomb interaction between electrons in opposite layers is obviously a key ingredient of the physics. Recently a technique has been developed which provides a simple way to directly obtain the interlayer electronic momentum relaxation rate and thereby assess the strength of these critical interactions. In this technique the frictional drag between the two 2DES's is measured by observing the voltage which develops in one layer when a current is driven through the other. This voltage, which exists even though the two layers are electrically isolated, is directly proportional to the interlayer momentum relaxation rate arising from the scattering of electrons in one layer off those in the other. At zero magnetic field drag studies have yielded a quantitative measure of the Coulomb scattering rate between electrons in the two layers [2], provided evidence for momentum relaxation due to the exchange of phonons [3], and revealed the predicted plasmon enhancement of the drag [4][5][6][7]. Recent drag experiments performed in magnetic fields large enough to induce the integer quantized Hall effect have given evidence for the oscillatory screening effects expected from Landau quantization [8,9].In this paper we report the first Coulomb drag results from the extreme quantum limit, focusing especially on the situation where in each 2D layer the lowest Landau level is half-filled. Our measurements show that ...

We report resistivity measurements from 0.03 to 10 K in a dilute high mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a gated field-effect transistor geometry, a wide range of densities, 0.16 x 10(10) to 7.5 x 10(10) cm(-2), are explored. For high densities, the results are quantitatively shown to be due to scattering by acoustic phonons and impurities. In an intermediate range of densities, a peak in the resistivity is observed for temperatures below 1 K. This nonmonotonic resistivity can be understood by considering the known scattering mechanisms of phonons, bulk, and interface ionized impurities. Still lower densities appear insulating to the lowest temperature measured.

Deficiencies of total collagen, type III collagen, and elastin have been proposed to explain aneurysm formation. Infrarenal aortas were collected from 19 patients (age 70 +/- 7 years) undergoing operative repair of abdominal aortic aneurysms (diameter 7 +/- 2 cm) and from 13 autopsies (age 63 +/- 17 years) of patients without aneurysm disease (controls). Wall thickness and collagen and elastin concentration were determined in full-thickness aorta. Collagen types I and III were measured after digestion with cyanogen bromide, which solubilized nearly 90% of total collagen for typing. Cyanogen bromide peptides were separated by sequential carboxymethylcellulose and agarose chromatography and quantified by peak area measurement with computerized image analysis. Histologic examination revealed prominent inflammatory cell infiltration and deficient, fragmented elastin in the aneurysms. Aortic wall thickness was similar in aneurysms and in control specimens. In the aneurysms, collagen was increased (37% +/- 16% vs 24% +/- 5%; p less than 0.05) and elastin was decreased (1% +/- 1% vs 12% +/- 7%; p less than 0.001), expressed as a percentage of delipidized, decalcified dry weight. Collagen type I accounted for 74% +/- 4% of aneurysm and 73% +/- 4% of control collagen solubilized for typing, and collagen type III accounted for 26% +/- 4% of aneurysm and 27% +/- 4% of control collagen solubilized for typing. Neither patients with a family history of aneurysms nor those without a history of aneurysms had collagen type III deficiency. Atherosclerotic abdominal aortic aneurysms are associated with an inflammatory process and may result from elastin degradation and not a deficiency of type III collagen.