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Register a new userMagnetotransport in low-density $p$-Si/SiGe heterostructures: From metal through hopping insulator to Wigner glass
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We study DC and AC transport in low-density $p-$Si/SiGe heterostructures at low temperatures and in a broad domain of magnetic fields up to 18 T. Complex AC conductance is determined from simultaneous measurement of velocity and attenuation of a surface acoustic wave propagating in close vicinity of the 2D hole layer. The observed behaviors of DC and AC conductance are interpreted as an evolution from metallic conductance at B=0 through hopping between localized states in intermediate magnetic fields (close to the plateau of the integer quantum Hall effect corresponding to the Landau-level filling factor $ u$=1) to formation of the Wigner glass in the extreme quantum limit ($Bgtrsim 14$, $T lesssim 0.8$ K).
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The magnetoresistance components $rho_{xx}$ and $rho_{xy}$ were measured in two p-Si/SiGe/Si quantum wells that have an anisotropic g-factor in a tilted magnetic field as a function of temperature, field and tilt angle. Activation energy measurements demonstrate the existence of a ferromagnetic-paramagnetic (F-P) transition for a sample with a hole density of $p$=2$times10^{11}$,cm$^{-2}$. This transition is due to crossing of the 0$uparrow$ and 1$downarrow$ Landau levels. However, in another sample, with $p$=7.2$times10^{10}$,cm$^{-2}$, the 0$uparrow$ and 1$downarrow$ Landau levels coincide for angles $Theta$=0-70$^{text{o}}$. Only for $Theta$ > 70$^{text{o}}$ do the levels start to diverge which, in turn, results in the energy gap opening.
A metal-insulator transition was induced by in-plane magnetic fields up to 27 T in homogeneously Sb-doped Si/SiGe superlattice structures. The localisation is not observed for perpendicular magnetic fields. A comparison with magnetoconductivity investigations in the weakly localised regime shows that the delocalising effect originates from the interaction-induced spin-triplet term in the particle-hole diffusion channel. It is expected that this term, possibly together with the singlet particle-particle contribution, is of general importance in disordered n-type Si bulk and heterostructures.
We perform detailed magnetotransport studies on two-dimensional electron gases (2DEGs) formed in undoped Si/SiGe heterostructures in order to identify the electron mobility limiting mechanisms in this increasingly important materials system. By analyzing data from 26 wafers with different heterostructure growth profiles we observe a strong correlation between the background oxygen concentration in the Si quantum well and the maximum mobility. The highest quality wafer supports a 2DEG with a mobility of 160,000 cm^2/Vs at a density 2.17 x 10^11/cm^2 and exhibits a metal-to-insulator transition at a critical density 0.46 x 10^11/cm^2. We extract a valley splitting of approximately 150 microeV at a magnetic field of 1.8 T. These results provide evidence that undoped Si/SiGe heterostructures are suitable for the fabrication of few-electron quantum dots.
We use a van-der-Waals pickup technique to fabricate different heterostructures containing WSe$_2$(WS$_2$) and graphene. The heterostructures were structured by plasma etching, contacted by one-dimensional edge contacts and a topgate was deposited. For graphene/WSe$_2$/SiO$_2$ samples we observe mobilities of $sim$12 000 cm$^2$/Vs. Magnetic field dependent resistance measurements on these samples show a peak in the conductivity at low magnetic field. This dip is attributed to the weak antilocalization (WAL) effect, stemming from spin-orbit coupling. Samples where graphene is encapsulated between WSe$_2$(WS$_2$) and hBN show a much higher mobility of up to $sim$120 000 cm$^2$/Vs. However, in these samples no WAL peak can be observed. We attribute this to a transition from the diffusive to the quasiballistic regime. At low magnetic field a resistance peak appears, which we ascribe to a size effect, due to boundary scattering. Shubnikov-de Haas oscillations in fully encapsulated samples show all integer filling factors, due to complete lifting of the spin and valley degeneracy.
In the present work we have investigated the transport properties in a number of Si/SiGe samples with square antidot lattices of different periods. In samples with lattice periods equal to 700 nm and 850 nm we have observed the conventional low-field commensurability magnetoresistance peaks consistent with the previous observations in GaAs/AlGaAs and Si/SiGe samples with antidot lattices. In samples with a 600 nm lattice period a new series of well-developed magnetoresistance oscillations has been found beyond the last commensurability peak which are supposed to originate from periodic skipping orbits encircling an antidot with a particular number of bounds.
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