In-Plane Magnetic Field-Induced Spin Polarization and Transition to Insulating Behavior in Two-Dimensional Hole Systems

Abstract

Using a novel technique, we make quantitative measurements of the spin polarization of dilute [ (3.4-6.8)x10(10) cm(-2)] GaAs (311)A two-dimensional holes as a function of an in-plane magnetic field. As the field is increased the system gradually becomes spin polarized, with the degree of spin polarization depending on the orientation of the field relative to the crystal axes. Moreover, the behavior of the system turns from metallic to insulating before it is fully spin polarized. The minority-spin population at the transition is approximately 8x10(9) cm(-2), close to the density below which the system makes a transition to an insulating state in the absence of a magnetic field.

Spin–valley phase diagram of the two-dimensional metal–insulator transition

“…This is because both the expected loss of screening upon valley polarization, and the smaller mobility (along the current direction, i.e., [100]) of the X valley to which the electrons are transferred, lead to a rise in ρ. , we observe that the positive magneto-resistivity nearly saturates at high fields, beyond a field B P , as marked in Figs. 1(e) and 2(b). The field B P represents the magnetic field beyond which the 2DES is fully spin-polarized [16,17,18,19,28,29,31]. The fields B C and B P are close to each other but, as pointed out by Tutuc et al [19], the fact that B C < B P implies that the transition to the insulating phase occurs not at the full spin polarization field but rather when the population of minority spin electrons reaches below a threshold value.…”

“…The field B P represents the magnetic field beyond which the 2DES is fully spin-polarized [16,17,18,19,28,29,31]. The fields B C and B P are close to each other but, as pointed out by Tutuc et al [19], the fact that B C < B P implies that the transition to the insulating phase occurs not at the full spin polarization field but rather when the population of minority spin electrons reaches below a threshold value. We find that the situation is similar for the onset of insulating behavior with valley polarization also: As seen in Fig.…”

“…The associated metal-toinsulator transition (MIT) has subsequently become the subject of intense interest and controversy [3]. While behavior similar to that of Ref.[2] has now been reported for a wide variety of 2D carrier systems such as n-AlAs, 10], and p-Si/SiGe [11,12], the origin of the metallic state and its transition into the insulating phase remain major puzzles in solid state physics.Several experiments have demonstrated the important role of the spin degree of freedom in the MIT problem, either in systems with a strong spin-orbit interaction [10,13,14], or via the application of an external magnetic field to spin polarize the carriers [15,16,17,18,19]. The latter experiments have shown that a magnetic field applied parallel to the 2DES plane suppresses the metallic temperature dependence, ultimately driving the 2DES into the insulating regime as the 2DES is spin polarized.…”

“…Several experiments have demonstrated the important role of the spin degree of freedom in the MIT problem, either in systems with a strong spin-orbit interaction [10,13,14], or via the application of an external magnetic field to spin polarize the carriers [15,16,17,18,19]. The latter experiments have shown that a magnetic field applied parallel to the 2DES plane suppresses the metallic temperature dependence, ultimately driving the 2DES into the insulating regime as the 2DES is spin polarized.…”

Reappearance of linear hole transport in an ambipolar undoped GaAs/AlGaAs quantum well

“…Because in-plane fields only affect the spin degree of freedom and not the in-plane orbital motion, this suggests that spin degeneracy of holes is responsible for nonmonotonic behaviour of ( ) ρ T in zero field. Over the entire p range explored, magnetoresistance measurements show that figure 5(b)) to fields up to 10 T. Notably, no tendency to crossover to linear behaviour, as is expected for a fully spin-polarised 2DHG [14], was observed. The Zeeman energy splitting for GaAs 2DHGs in in-plane fields depends on density, crystal orientation and the shape of the confining potential, due to spin-orbit coupling [15].…”