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Register a new userHall detection of a ppm spin polarization
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Hall effects have been employed as sensitive detectors of magnetic fields and magnetizations. In spintronics, exotic phenomena often emerge from a non-equilibrium spin polarization or magnetization, that is very difficult to measure directly. The challenge is due to the tiny total moment, which is out of reach of superconducting quantum interference devices and vibrating sample magnetometers or spectroscopic methods such as X-ray magnetic circular dichroism. The Kerr effect is sufficiently sensitive only in direct gap semiconductors, in which the Kerr angle can be resonantly enhanced. Here we demonstrate that even one excess spin in a million can be detected by a Hall effect at room temperature. The novel Hall effect is not governed by the spin Hall conductivity but by its energy derivative thereby related to the spin Nernst effect.
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The spin Hall effect creates a spin current in response to a charge current in a material that has strong spin-orbit coupling. The size of the spin Hall effect in many materials is disputed, requiring independent measurements of the effect. We develop a novel mechanical method to measure the size of the spin Hall effect, relying on the equivalence between spin and angular momentum. The spin current carries angular momentum, so the flow of angular momentum will result in a mechanical torque on the material. We determine the size and geometry of this torque and demonstrate that it can be measured using a nanomechanical device. Our results show that measurement of the spin Hall effect in this manner is possible and also opens possibilities for actuating nanomechanical systems with spin currents.
We report on results of numerical studies of the spin polarization of the half filled second Landau level, which corresponds to the fractional quantum Hall state at filling factor $ u=5/2$. Our studies are performed using both exact diagonalization and Density Matrix Renormalization Group (DMRG) on the sphere. We find that for the Coulomb interaction the exact finite-system ground state is fully polarized, for shifts corresponding to both the Moore-Read Pfaffian state and its particle-hole conjugate (anti-Pfaffian). This result is found to be robust against small variations of the interaction. The low-energy excitation spectrum is consistent with spin-wave excitations of a fully-magnetized ferromagnet.
We have carried out tilt magnetic field (B) studies of the u=12/5 fractional quantum Hall state in an ultra-high quality GaAs quantum well specimen. Its diagonal magneto-resistance Rxx shows a non-monotonic dependence on tilt angle (theta). It first increases sharply with increasing theta, reaches a maximal value of ~ 70 ohms at theta ~ 14^o, and then decreases at higher tilt angles. Correlated with this dependence of Rxx on theta, the 12/5 activation energy (Delta_{12/5}) also shows a non-monotonic tilt dependence. Delta_{12/5} first decreases with increasing theta. Around theta = 14^{o}, Delta_{12/5} disappears as Rxx becomes non-activated. With further increasing tilt angles, Delta_{12/5} reemerges and increases with theta. This tilt B dependence at u=12/5 is strikingly different from that of the well-documented 5/2 state and calls for more investigations on the nature of its ground state.
We extend the semiclassical Boltzmann formalism for the anomalous Hall effect (AHE) in nondegenerate multiband electron systems to the spin Hall effect (SHE) and unconventional Edelstein effect (UEE, cannot be accounted for by the conventional Boltzmann equation, unlike the conventional Edelstein effect). This extension is confirmed by extending the Kohn-Luttinger density-matrix transport theory in the weak disorder-potential regime. By performing Kubo linear response calculations in a prototypical multiband model, the Boltzmann scaling for the AHE/SHE and UEE is found to be valid only if the disorder-broadening of bands is quite smaller than the minimal intrinsic energy-scale around the Fermi level. Discussions on this criterion in various multiband systems are also presented. A qualitative phase diagram is proposed to show the influences of changing independently the impurity density and strength of disorder potential on the AHE/SHE and UEE.
We propose two experimental setups that allow for the implementation and the detection of fractional solitons of the Goldstone-Wilczek type. The first setup is based on two magnetic barriers at the edge of a quantum spin Hall system for generating the fractional soliton. If then a quantum point contact is created with the other edge, the linear conductance shows evidence of the fractional soliton. The second setup consists of a single magnetic barrier covering both edges and implementing a long quantum point contact. In this case, the fractional soliton can unambiguously be detected as a dip in the conductance without the need to control the magnetization of the barrier.
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