Resonantly Enhanced Tunneling in a Double Layer Quantum Hall Ferromagnet

Abstract

The tunneling conductance between two parallel 2D electron systems has been measured in a regime of strong interlayer Coulomb correlations. At total Landau level filling n T 1 the tunnel spectrum changes qualitatively when the boundary separating the compressible phase from the ferromagnetic quantized Hall state is crossed. A huge resonant enhancement replaces the strongly suppressed equilibrium tunneling characteristic of weakly coupled layers. The possible relationship of this enhancement to the Goldstone mode of the broken symmetry ground state is discussed. PACS numbers: 71.10.Pm, 73.40.Hm, 73.40.Gk When two parallel two-dimensional electron systems (2DES) are sufficiently close together, interlayer Coulomb interactions can produce collective states which have no counterpart in the individual 2D systems [1][2][3]. One of the simplest, yet most interesting, examples occurs when the total electron density, N T , equals the degeneracy, eB͞h, of a single spin-resolved Landau level produced by a magnetic field B. In the balanced case (i.e., with layer densities N 1 N 2 N T ͞2), the Landau level filling factor of each layer viewed separately is n hN T ͞2eB 1͞2. If the separation d between the layers is large, they behave independently and are well described as gapless composite fermion liquids. No quantized Hall effect (QHE) is seen. On the other hand, as d is reduced, the system undergoes a quantum phase transition [4-6] to an incompressible state best described by the total filling factor n T 1͞2 1 1͞2 1. A quantized Hall plateau now appears at r xy h͞e 2 . Both Coulomb interactions and interlayer tunneling contribute to the strength of this QHE but there is strong evidence from experiment [3,7] and theory [1,8] that the incompressibility survives in the limit of zero tunneling. This remarkable collective state exhibits a broken symmetry [9][10][11][12], spontaneous interlayer phase coherence, and may be viewed as a kind of easy-plane ferromagnet. The magnetization of this ferromagnet exists in a pseudospin space; electrons in one layer are pseudospin up, while those in the other layer are pseudospin down. Numerous interesting properties are anticipated, including linearly dispersing Goldstone collective modes (i.e., pseudospin waves), a finite temperature Kosterlitz-Thouless transition, dissipationless transport for currents directed oppositely in the two layers, and bizarre topological defects in the pseudospin field [9][10][11][12][13]. To date, most experimental results on this system have derived from electrical transport measurements [3,4,7,14,15] although recently an optical study has been reported [16].In this paper we report on a new study of the double layer n T 1 ferromagnetic quantum Hall state, and its transition at large layer separation to a compressible phase, using the method of tunneling spectroscopy. Earlier experiments have shown that there is a strong suppression of the equilibrium tunneling between two widely separated parallel 2DESs at high magnetic field [17,18]. This suppression is a ...

Spontaneous coherence in a cold exciton gas

“…The onset of spontaneous coherence was evidenced by a strong enhancement of the recombination [25] and tunneling [26] rate, respectively. The results of other transport and optical experiments were also consistent with spontaneous coherence of indirect excitons [24,25,[27][28][29][30][31][32][33].…”

“…This way, one can engineer indirect excitons with radiative lifetimes and spin relaxation times orders of magnitude longer than those of regular excitons [21][22][23]. Due to their long lifetimes, indirect excitons can cool down well below the temperature of quantum degeneracy [24].In earlier studies, evidence for spontaneous coherence was obtained for indirect excitons in coupled quantum wells (CQW) [25] and for indirect excitons in quantum Hall bilayers [26,27]. The onset of spontaneous coherence was evidenced by a strong enhancement of the recombination [25] and tunneling [26] rate, respectively.…”

Competing Abelian and non-Abelian topological orders inν=1/3+1/3quantum Hall bilayers

“…The incompressible state is represented by the Halperin (111) state, which can also be viewed as a pseudo-spin ferromagnet [76]. This wavefunction encodes the physics of exciton superfluidity, with an associated Goldstone mode [77] and vanishing of Hall resistivity in the "counterflow" measurement setup [78,79]. The existence of an incompressible state (consistent with an exciton superfluid) has been established in numerics [80][81][82][83][84] The case of total filling ν = 2/3, which is the subject of this paper, has been less studied compared to previous examples.…”

Mutual Composite Fermion and Composite Boson approaches to balanced and imbalanced bi-layer quantum Hall system: An electronic analogy of the Helium 4 system

“…However, when d is smaller than a critical distance d c , even in the absence of interlayer tunneling, the system undergoes a quantum phase transition into a novel spontaneous interlayer coherent incompressible phase 2 . At low temperature, with extremely small interlayer tunneling amplitude, Spielman et al discovered a very pronounced narrow zero bias peak in this interlayer coherent incompressible state 4 . M. Kellogg et al also observed quantized Hall drag resistance at h/e 25 .…”