Giant Low Temperature Heat Capacity of GaAs Quantum Wells near Landau Level Fillingν=1

Abstract

We report low temperature (T ) heat capacity (C) data on a multiplequantum-well GaAs/AlGaAs sample in the quantum Hall regime. Relative to its low field magnitude, C exhibits up to ∼ 10 5 -fold enhancement near ν=1where Skyrmions are the ground state of the confined two-dimensional electrons. We attribute the large C to a Skyrmion-induced, strong coupling of the nuclear spin system to the lattice. The data are consistent with the Schottky nuclear heat capacity of Ga and As atoms in the quantum wells, except at very low T where C vs T exhibits a remarkably sharp peak suggestive of a phase transition in the electronic system. PACS numbers: 65.40.Hq, 73.20.Dx, 73.40.Hm Typeset using REVT E X 1 Heat capacity is one of the most fundamental physical properties as it directly probes thermodynamic quantities such as entropy [1]. In the case of two-dimensional electron systems (2DESs) heat capacity can be a powerful probe of single-and many-body properties such as the Landau quantized density of states and the quantum Hall effect (QHE), both integral and fractional [2,3]. Measurements of 2DES heat capacity, however, are among the most challenging experiments because of the very small electron contribution. As a result, in contrast to the overwhelming number of magnetotransport and magnetooptical experiments reported for 2DESs in the QHE regime [4], very few heat capacity measurements have been reported so far [5,6]. In this Letter we report heat capacity measurements of a modulation-doped GaAs/Al x Ga 1−x As multiple-quantum-well heterostructure down to very low temperature (T ≥ 27 mK) and Landau level filling factor (ν > ∼ 0.5). As a function of increasing magnetic field, B, in addition to oscillations associated with the 2DESs' oscillating density of states at the Fermi level, we observe a dramatic increase of the low-T heat capacity (C) in the range 0.5 < ∼ ν < ∼ 1.5. For 0.7 < ∼ ν < ∼ 0.85, C exhibits a striking T -dependence, including a remarkably sharp peak suggestive of a phase transition at very low T . We interpret these unexpected observations in terms of the Schottky model [7] for the nuclear-spin heat capacity of Ga and As atoms which couple to the lattice via the 2DESs' low energy excitation spin textures (Skyrmions). The origin of the peak at very low T is discussed in relation with a phase transition in the electronic system. The experiments were performed on a multiple-quantum-well heterostructure grown by molecular-beam epitaxy and consisting of 100 GaAs quantum wells separated by Al 0.3 Ga 0.7 As barriers. The wells and barriers are 250 and 1850Å thick respectively, and the barriers are δ-doped with donors (Si) near their centers. Electrical resistivity data on samples from the same wafer exhibit well-developed fractional QHE and attest to the high quality of the sample [8]. A 7 × 7 mm 2 piece of the wafer was thinned to 65 µm and two carbon paint resistors were deposited on the substrate side and connected to the heat sink withNbTi wires. One carbon resistor was used as a thermometer while the other se...

Spin and Charge Ordering in the Quantum Hall Regime

“…3.8a is a consequence of these Goldstone modes [191]. A strong coupling between the electronic and nuclear spin system around = 1 was also inferred from heat capacity measurements [192]. The existence of skyrmions was further corroborated by the observation of a strongly decreasing electron spin polarization when going away from exact = 1 [183,193].…”

Magneto-optical probe of quantum Hall states in a wide parabolic well modulated by random potential

“…12 The data consistent with the presence of the skyrmions were obtained by means of heat capacity measured as a function of the temperature in Ref. 13. Magnetotransport measurements were also employed to study the skyrmions, and somewhat controversial results were obtained in Refs.…”

Soft Spin Wave nearν=1: Evidence for a Magnetic Instability in Skyrmion Systems

“…As a consequence of the additional symmetry breaking, the 2D electron system supports spin waves which, contrary to the ferromagnetic spin wave, remain gapless in the presence of the magnetic field [13,14,15]. Both a jump in the specific heat and the very short T 1 observed away from ν=1 were interpreted as indirect consequences of the enhanced coupling between the nuclear spins and the electron system due to the gapless spin waves (Goldstone mode) of the Skyrme crystal [16,17,18].While most studies of Skyrmions have focused solely on their impact on electron spin polarization, no experiment has explored up to now the possibility of exploring Skyrmion interactions and the magnetic ground state of the 2DES near ν=1 by probing long-wavelength spin wave excitations. Measurements of spin wave excitation are especially valuable since they are expected to reflect the presence of a broken symmetry state arising from XY spin ordering [14].…”