Possible Evidence for Helical Nuclear Spin Order in GaAs Quantum Wires

Abstract

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...

Dynamic response functions and helical gaps in interacting Rashba nanowires with and without magnetic fields

“…Both mechanisms lead to very similar experimental signatures in conductance measurements, where the opening of a helical gap results in a halving of the conductance as the chemical potential is tuned close to the Dirac point. 21, 22 However, we will show that dynamic response functions, in particular the spectral function, the structure factor, and the tunnelling density of states, allow one to uniquely assign one of these origins to an observed helical gap. These functions are thus a much better probe of the helical gap than conductance measurements alone.…”

Magnetic cooling for microkelvin nanoelectronics on a cryofree platform

“…As thermal excitations represent an ubiquitous energy scale in solid state systems, advancing to lower temperatures might open up the way to the discovery of new physical phenomena such as fragile fractional quantum Hall states 1 and electron-mediated nuclear phase transitions, both in 2D and 1D systems [2][3][4] . To investigate such phenomena, one needs to access lower temperatures beyond what a dilution refrigerator could achieve.…”

On-and-off chip cooling of a Coulomb blockade thermometer down to 2.8 mK

“…Reaching ultralow temperatures in electronic transport experiments can be key to novel quantum states of matter such as helical nuclear spin phases [1][2][3] , full nuclear spin polarization 4 , quantum Hall ferromagnets 4 or fragile fractional quantum Hall states 5,6 . In addition, the coherence of semiconductor and superconducting qubits 7-9 as well as hybrid Majorana devices [10][11][12][13] could benefit from lower temperatures.…”