We present transport measurements of cleaved edge overgrowth GaAs quantum wires. The conductance of the first mode reaches 2 e 2 /h at high temperatures T > ∼ 10 K, as expected. As T is lowered, the conductance is gradually reduced to 1 e 2 /h, becoming T -independent at T < ∼ 0.1 K, while the device cools far below 0.1 K. This behavior is seen in several wires, is independent of density, and not altered by moderate magnetic fields B. The conductance reduction by a factor of two suggests lifting of the electron spin degeneracy in absence of B. Our results are consistent with theoretical predictions for helical nuclear magnetism in the Luttinger liquid regime.Conductance quantization is a hallmark effect of ballistic one-dimensional (1D) non-interacting electrons [1][2][3][4]. One mode of conductance e 2 /h opens for each spin, giving conductance steps of 2 e 2 /h for spin degenerate electrons. In presence of electron-electron (e-e) interactions, strongly correlated electron behavior arises, described by Luttinger liquid (LL) theory [5][6][7]. Salient LL signatures include ubiquitous power-law scaling [8][9][10][11][12], separation of spin and charge modes, and charge fractionalizationall recently observed [13][14][15][16] in cleaved edge overgrowth (CEO) GaAs quantum wires [17], thus establishing CEO wires as a leading realization of a LL. Interestingly, the conductance of a clean 1D channel is not affected by interactions, since it is given by the contact resistance in the Fermi liquid leads [18][19][20][21][22]. In presence of disorder, however, the conductance is reduced with LL powerlaws [23,24]. While short constrictions display universal quantization [2,3], the ballistic CEO wires exhibit steps reduced below 2 e 2 /h at temperatures T ≥ 0.3 K [25,26], presenting an unresolved mystery [11,13,25,27].In this Letter, we revisit the conductance quantization in CEO wires, investigating for the first time low temperatures down to T ∼ 10 mK. We find that the conductance of the first wire mode drops to 1 e 2 /h at T ∼ 100 mK and remains fixed at this value for lower T , while the electron temperature cools far below 100 mK. At high T > ∼ 10 K, the conductance approaches the expected universal value 2 e 2 /h [25]. This behaviour suggests a lifting of the electron spin degeneracy at low T , in absence of an external magnetic field B. The observed quantization values are quite robust, appearing in several devices, unaffected by moderate magnetic fields, and independent of the overall carrier density. A recent theory [28][29][30] predicts a drop of the conductance by a factor of two in presence of a nuclear spin helix -a novel quantum state of matter. Our data agree well with this model, while other available theories are inconsistent with the experiments, thus offering a resolution of the non-universal conductance quantization mystery.Ultra-clean GaAs CEO double wires (DWs) were measured (inset, Fig. 1), similar to Refs. [13][14][15][16], offering Arrows indicate VG above which modes start to contribute to g, as label...

We present Silver-epoxy filters combining excellent microwave attenuation with efficient wire thermalization, suitable for low temperature quantum transport experiments. Upon minimizing parasitic capacitances, the attenuation reaches ≥ 100 dB above ≈ 150 MHz and -when capacitors are added -already above ≈ 30 MHz. We measure the device electron temperature with a GaAs quantum dot and demonstrate excellent filter performance. Upon improving the sample holder and adding a second filtering stage, we obtain electron temperatures as low as 7.5 ± 0.2 mK in metallic Coulomb blockade thermometers.

We present an improved nuclear refrigerator reaching 0.3 mK, aimed at microkelvin nanoelectronic experiments, and use it to investigate metallic Coulomb blockade thermometers (CBTs) with various resistances R. The high-R devices cool to slightly lower T , consistent with better isolation from the noise environment, and exhibit electron-phonon cooling ∝ T 5 and a residual heat-leak of 40 aW. In contrast, the low-R CBTs display cooling with a clearly weaker T -dependence, deviating from the electron-phonon mechanism. The CBTs agree excellently with the refrigerator temperature above 20 mK and reach a minimum-T of 7.5 ± 0.2 mK.

We present measurements of the electron temperature using gate-defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1 mK, the device electron temperature saturates, remaining close to 10 mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature.

Cooling nanoelectronic devices below 10 mK is a great challenge since thermal conductivities become very small, thus creating a pronounced sensitivity to heat leaks. Here, we overcome these difficulties by using adiabatic demagnetization of both the electronic leads and the large metallic islands of a Coulomb blockade thermometer. This reduces the external heat leak through the leads and also provides on-chip refrigeration, together cooling the thermometer down to 2.8 ± 0.1 mK. We present a thermal model which gives a good qualitative account and suggests that the main limitation is heating due to pulse tube vibrations. With better decoupling, temperatures below 1 mK should be within reach, thus opening the door for μK nanoelectronics.