Low temperature transport and evidence for nuclear order in GaAs Quantum Wires

Abstract

In this thesis we focus on low temperature transport through cleaved edge overgrowth (CEO) quantum wires. This thesis is motivated by recent theoretical work on one hand that predicts the formation of a nuclear helimagnet in presence of a Luttinger liquid (LL) below a critical system temperature [1] and previous measurements on CEO wires on the other hand that might indicate the onset of such a phase transition [2]. The first task for this thesis was to create suitable measurement conditions to approach the theoretically predicted strongly correlated state of matter. More precisely,electron sample temperatures much lower than the predicted ordering temperature of 75mK for GaAs CEO wires have to be demonstrated. For this purpose, cryogenic microwave filters with very low cut-off frequency and good thermalization properties were developed and installed for all measurement cables. With that a minimum electron temperature of 7.5mK was reached in metallic coulomb blockade thermometers [3],and furthermore, for the first time in these devices, a deviation from pure electron-phonon cooling is observed [4]. At low refrigerator temperatures T_R, the CEO (double) wires show pronounced and completely reproducible conductance oscillations as a function of density. We show that these oscillations, also present in the zero magnetic field tunneling current between the parallel quantum wires, emerge as 1D Fabry Perot resonances in the ballistic CEO wires [5]. We analyze the maximum transmission (T=1) through these wires, i.e the oscillation maxima, in the single mode regime as a function of temperature. While the quantum wires approach universal conductance quantization of 2e^2/h for a single quantum wire only at quite large T_R>15K, we find that the conductance saturates below T_R=75mK at 1e^2/h [6]. Furthermore, we give strong evidence that the conductance saturation is not related to insuffcient thermalization, i.e.the CEO wires cool far beyond the saturation temperature of 75mK. This seems to indicate lifting of electron spin degeneracy at zero external magnetic field, consistent with the theo- retically predicted low temperature limit for a clean LL in the ordered helicalstate [1]. We can further exclude other potential mechanisms (temperature dependent contact resistance, freeze-out of weakly disordered LL, Wigner crystal formation/incoherent LL), leaving only nuclear spins as candidates for the source of the (possibly) observed lifted spin degeneracy as spin-orbit coupling is rather weak in GaAs, and the saturation at 1e^2/h is observed in absence of an external magnetic field. This might resolve the long-standing mystery of the temperature-dependent (non-universal) conductance quantization in GaAs cleaved edge overgrowth quantum wires, and furthermore might give first experimental evidence for a new, strongly correlated state of matter, namely (helical) nuclear order induced by the strongly interacting electrons via hyperfine coupling. We also measure real-time tunneling in a GaAs few electron double quantum dot (DQD) by means of an adjacent quantum dot as charge sensor. At low temperatures, in the limit of negligible interdot tunneling and low tunnel rates to source and drain, we observe metastable charge state switching. The metastability only occurs within diamond shaped regions that are centered between associated triple points of the charge stability diagram (CSD). We show that these charge fluctuations arise as an intrinsic property in DQDs, and take place via fast intermediate states that include an electron exchange with the leads [7]. Due to the geometrical shape of the diamond (in very good agreement with our model of thermally activated electron exchange with the leads), its large energy scale (>1.7K) and due to its visibility even at charge sensor bias voltages as small as 5µV, we exclude extrinsic effects such as phonon or photon assisted tunneling. Furthermore, the simultaneous observation of the diamond shaped region of metastable charge state switching at various points in the CSD and its pinning to associated triple points upon reshaping the DQD, make charge traps and other defects a very unlikely explanation.