Spin Effects in the Phase Transition of the ν_T = 1 Bilayer Electron System

K. MURAKI; P. GIUDICI; N. KUMADA; Y. HIRAYAMA; T FUJISAWA

São Pedro

Conferencia; High Magnetic Fields in Semiconductor Physics, HMF-18; 2008

Bilayer electron systems realized in semiconductor double quantum wells offer a uniqueopportunity to explore a many-electron system while tuning its interaction parameter in situthrough electrostatic gating. Particularly interesting is the case when the tunneling is absentand the degeneracy of the spin-split Landau levels produced by a perpendicular magneticfield equals the total electron density (i.e., total filling factor νT = 1), where a transition between a purely many-body quantum Hall (QH) state with strong interlayer correlation anda compressible state with weak interlayer correlation takes place upon changing density.Previous experiments have established that for sufficiently small tunneling this phase transition occurs when the ratio between the intralayer and interlayer Coulomb interactions,parameterized by the interlayer distance d and the magnetic length `B, crosses a criticalvalue, d/`B ≈ 2. Although this transition is generally believed to be driven by the collapseof the QH gap due to the softening of a collective excitation mode of pseudospin representing the layer degree of freedom, its exact nature remains an open issue. Furthermore, recent experiments have suggested that the spin degree of freedom is not completely frozen in this system for the relevant magnetic field range [1, 2]. Here we report a tilted-field experiment on a double quantum well with negligible tunneling and demonstrate that spin plays a decisive role in determining the phase boundary of this system. We establish a new phase diagram as a function of the Zeeman energy and d/`B and discuss its implications as to the nature of the phase transition [3].The sample consists of two 18-nm-wide GaAs quantum wells separated by a 10-nmthickAlAs/GaAs superlattice barrier (d = 28 nm), designed to have a very small tunnelinggap of ΔSAS ∼ 150 μK. For a magnetic field applied perpendicular to the sample plane,the QH state weakens with increasing density and collapses at a perpendicular field ofB⊥ = 3.1 T, corresponding to d/`B = 1.9, in accordance with previous reports. Whenan in-plane field component Bk is introduced by tilting the sample in a magnetic field, weobserve that the phase boundary shifts to higher densities (i.e., higher B⊥). The phaseboundary steadily shifts with tilting until it saturates at an unexpectedly high field of B⊥ =4.6 T (d/`B = 2.3). Using a simple model considering the Coulomb and Zeeman energiesof the two phases, we show that the observed shift of the phase boundary is a manifestationof the energetic competition between the fully polarized QH state and a partially polarizedcompressible state, with the saturation at high fields indicating the full polarization of thecompressible state. Our data thus present the first experimental observation of the intrinsictransition that has been discussed in theory for a spinless system, which is now shown tobe preempted by a transition to a partially polarized compressible state under the standardexperimental conditions. On the basis of these observations, we argue that the nature of thephase transition is contingent on the spin polarization of the compressible state.[1] N. Kumada et al., Phys. Rev. Lett. 94, 096802 (2005).[2] I. B. Spielman et al., Phys. Rev. Lett. 94, 076803 (2005).[3] P. Giudici et al., Phy. Rev. Lett. 100, 106803 (2008).