Layer-charge instability in unbalanced bilayer systems in the quantum Hall regime

Abstract

Measurements in GaAs hole bilayers with unequal layer densities reveal a pronounced magnetoresistance hysteresis at the magnetic field positions where either the majority or minority layer is at Landau level filling factor one. At a fixed field in the hysteretic regions, the resistance exhibits an unusual time dependence, consisting of random, bidirectional jumps followed by slow relaxations. These anomalies are apparently caused by instabilities in the charge distribution of the two layers.PACS numbers: 71.70.Ej, 73.43.Qt Hysteretic phenomena are widespread in nature. They are common magnetic materials, and often indicate a non-equilibrium situation associated with a phase transition and the presence of domains [1]. Recently, hysteresis has also been reported in various two-dimensional (2D) carrier systems in semiconductor structures at low temperatures and high magnetic fields [2,3,4,5,6,7]. In these cases, magneto-resistance (ρ xx ) hysteresis appears in the quantum Hall (QH) regime when two Landau levels (LLs) with opposite spin are brought into coincidence. While the 2D systems studied have been notably different, the common thread in these experiments is that there is a magnetic transition involving the carrier spin [8].Here we present hysteretic ρ xx data in 2D bilayer systems in the QH regime. The hysteresis in these systems has a different origin and is caused by a non-equilibrium charge distribution in the two layers. We studied the magneto-transport coefficients of GaAs bilayer hole systems with unequal layer densities. When the interlayer tunneling is sufficiently small, ρ xx of the bilayer system exhibits a pronounced hysteresis at perpendicular magnetic field (B) positions close to where either the majority or minority layer is at LL filling factor one. Most remarkable is the time dependence of ρ xx at a fixed field in the hysteretic regime, when the two layers are closely spaced. As a function of time, ρ xx exhibits large, random, sudden jumps toward higher and lower values, followed by a slow decay in the opposite direction. The data may signal an instability in the charge distribution of the two layers, i.e., an instability associated with the pseudospin (layer), rather than spin, degree of freedom.We studied nine GaAs bilayer hole samples from six different wafers, all grown on GaAs (311)A substrates and modulation doped with Si. In all samples, the holes are confined to two 15nm-wide GaAs quantum wells which are separated by AlAs or AlAs/AlGaAs barriers with thickness 7.5 ≤ W ≤ 200nm. The rather thick barrier combined with the large effective mass of GaAs 2D holes [9] reduces considerably the tunneling between the two layers [10]. As grown, the samples have layer densities of ≤ 7 × 10 10 cm −2 , and low temperature (T ) mobilities of ∼ 35 m 2 /Vs. Metallic top and bottom gates were added to control the densities in the layers. We studied several types of devices, including 2.5×2.5mm square samples and ones with patterned Hall bars; in these samples the ohmic contacts contact both layers. ...

Charge metastability and hysteresis in the quantum Hall regime

“…These hysteretic effects have been associated with a number of physical phenomena, including non-equilibrium charge distributions [3][4][5] , long-lived eddy currents [6][7][8][9] within the interior of the 2DES, first order phase transitions involving the electron spin [10][11][12][13][14][15] or pseudospin [16][17][18][19] degrees of freedom, and metastable orientations of the electron nematic phases at high Landau level occupancy 20 . In the majority of these experiments the behavior of the collective electron state was inferred from measurements of magnetoresistance, which was found to depend strongly on the sweep rate and direction of the magnetic field.…”

Hysteresis in the quantum Hall regimes in electron double quantum well structures

“…This hysteretic electronic transport was first observed in a single, high electron mobility quantum well with a low mobility parallel conducting channel 12 , and later in hole DQW structures 13 . So far, no studies have been conducted in the most common DQW structures, the electron DQW's.…”

“…Recently, a new phenomenon has been discovered in the DQW structures: electronic transport hysteresis 12,13 . It was observed that, when the densities of two wells are different and tunneling is negligible, the magnetotransport coefficients show hysteretic behavior when the magnetic (B) field is swept up and down.…”

Hysteresis in the Quantum Hall Regimes in Electron Double-Quantum-Well Structures

“…In addition, since the distance (or the coupling) between the two quantum wells can be controllably tuned from a few tenths of nanometer to several microns, DQW structures have shown promise as possible future electronic devices for next generation information processing 11 . Recently, a new phenomenon has been discovered in the DQW structures: electronic transport hysteresis 12,13 . It was observed that, when the densities of two wells are different and tunneling is negligible, the magnetotransport coefficients show hysteretic behavior when the magnetic (B) field is swept up and down.…”