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 ν = 2 + 2/5 state separated from the even-denominator ν = 2 + 1/2 state by an insulating phase and a ν = 2 + 2/7 and ν = 2 + 1/5 state surrounded by such phases. A developing plateau at ν = 2 + 3/8 points to the existence of other even-denominator states.Low-temperature electron correlation in the lowest Landau level (LL) of a two-dimensional electron system (2DES) separates largely into two regions. At very low filling factor, ν ≤ 1/5, an insulating phase exists, which has now quite convincingly been determined to be a pinned electron solid [1,2,3]. At higher filling factor 1 > ν >∼ 1/5 the multiple sequences of fractional quantum Hall effect (FQHE) liquids [4,5, 6,7] dominate, which show the characteristic vanishing magneto resistance, R xx , and quantized Hall resistance, R xy , at many odd-denominator rational fractional fillings ν = p/q [8].Altogether about fifty such FQHE states have been observed in this region. Their multiple sequences can largely be described within the composite fermion (CF) model [9,10, 11,12], with the exact origin of some higher order states still being argued. The electrical behavior between FQHE states carries no particularly strong transport signature, being thought of as arising largely from the conduction of excited quasiparticles of the neighboring FQHE states, with CF liquids occurring at some even-denominator fractions.At high LL's a very different pattern seems to emerge. There charge density wave (CDW) or liquid crystal like states dominate, often referred to as electronic stripe and bubble phases [13,14,15]. Characteristically these states are pinned to the lattice, immobilizing the electrons of this LL, which leads to transport properties identical to those of the neighboring integer quantum Hall effect (IQHE) states. FQHE states are absent in these high LL's, except for the recent observation of two FQHE features in the third LL, at elevated temperatures [16]. Of course, high LL fillings typically occur at lower magnetic fields and hence at poorer resolution of potential FQHE features. However, very general theoretical arguments [17,18] based on an increasing extent of the wavefunction with increasing LL index, hence the increasing importance of exchange and the diminishing applicability of point-like interactions, clearly support this trend.It is in the 2nd LL where electron liquids and electron solids collide. The larger extent of the wavefunction as compared to the lowest LL and its additional zero allows for a much broader range of electron correlations to be favorable, leading to an ever changing competition between multiple electronic phases as the filling factor is varied and as the temperature is low...

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 twodimensional electron system (2DES) and at very low temperatures and report the following results: For the even-denominator FQHE states, the sample dependence of the ν=5/2 state clearly shows that disorder plays an important role in determining the energy gap at ν=5/2. For the developing ν=19/8 FQHE state the temperature dependence of the R xx minimum implies an energy gap of ~5mK.The energy gaps of the odd-denominator FQHE states at ν=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 ν=13/5 state is missing, while its particle-hole conjugate state, the ν=12/5 state, is a fully developed FQHE state. We speculate that this disappearance might indicate a spin polarization of the ν=13/5 state.Finally, the temperature dependence is studied for the two-reentrant integer quantum Hall states around ν=5/2 and is found to show a very narrow temperature range for the transition from quantized to classical value.

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Two capacitive pressure gauges which have been used extensively in the study of liquid and solid helium are described. The first gauge, constructed of Be–Cu, has a sensitivity of Δp∼2×10−6 atm for pressures of 100 atm. It is highly stable, has essentially no hysteresis, and is affected very little by changes in temperature below 4.2 K. The second gauge is of nylon construction and is suitable for NMR or with paramagnetic salt thermometry. A detailed description of the construction and use of both gauges is given.

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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 the 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.

The solidification of 4 He in Vycor glass and in sintered Ag powder has been studied between 0.8 and 2.5 K. At 0.8 K, elevation of the melting pressure by 12 bars is seen in Vycor and by 0.3 bar in the Ag sinter. In each material a range of melting pressures, corresponding to different pore sizes, is observed. The location of the \ transition in the liquid in Vycor is determined up to solidification pressure.PACS numbers: 67.40. Kh, 67.80.Gb Several studies have been made of the solidification and superfluidity of 4 He in confined geometries. 1 " 3 Experiments on helium in Vycor glass have indicated that it remained a liquid at pressures of ~-15 bars above the bulk melting pressure. 2 Recently Beamish et al 3 (BHTE) have observed solidification and the superfluid transition of helium in Vycor from changes in sound velocity. However, an accurate phase diagram has not been available because of uncertainties in the presence of the helium in the Vycor.We report measurements of pressure versus temperature along isochores and of the cooling rate (related to the heat capacity) of helium in Vycor. We identify solidification, melting, and the X transition and obtain a phase diagram up to P = 60 bars and T = 2.5 K. There is a range of pressures where both solid and liquid may exist, which, at r = 0.8 K, extends from 36.3 to 38.0 bars. Similar behavior is seen in the Ag sinter, but at only ~ 0.3 bar above bulk melting. The X transition of helium in Vycor has been followed to above solidification pressure, made possible by supercooling of the liquid.The cell is depicted in Fig. 1 (inset). Measurement of the pressure of 4 He in Vycor above bulk melting is difficult because of the long time constant required for communication to a transducer. We obtained a short time constant by grinding the Vycor into a powder with grains ^ 74 pm (200 mesh sieve) thereby reducing the distance for pressure communication. 4 The powder loosely filled the cell to a filling factor of 0.5, allowing individual grains to move with the bulk helium surrounding them and to transmit pressure to the capacitive transducer. 5 Bulk helium existed in the spaces between grains which were ~ 10 4 times the dimension of the 60-A pores in the Vycor.A layer of 700-A sintered Ag powder, pressed to a filling factor of 0.5, provided thermal contact to the helium. The cell was mounted on the still of a dilution refrigerator with a weak thermal link to allow slow cooling of the cell with a time constant ~ 500 times that for equilibrium within the cell. Temperatures were measured with a carbon resistor calibrated against the 4 He melting pressure. 6 Cooling along the bulk melting curve at T = 2 K was at a rate \dT/dt\^4 mK/min, with the cell reaching 0.8 K in ~5 h. The cooling rate is related to the heat current from the cell Q by ti-ll dt = Ceff dT dt where C/ and c s are the specific heats of the liquid 20.0 20.2 20.4 20.6 20.8 21.0 21.2 V(cm 3 /mole) FIG. 1. Upper-right inset, the cell: (a) thermal link, (b) heater, (c) thermometer, (d) Ag sinter, (e) Vycor powder...

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