arXiv subjects

N. Freytag

Publications and source records attributed to N. Freytag.

3 recordsLinked to original sources

arXiv2002

How free are Composite Fermions at $ν= 1/2$? A NMR investigation

NMR measurements of the electron spin polarization have been performed on a 2D electron system at and around half-filled lowest Landau level. Comparing the magnetic field and the temperature dependence of the spin polarization to models of free and interacting composite fermions (CF), we confirm the existence of a Fermi sea and show that residual interactions are important. Independent measurements of the CF effective mass, g-factor and Fermi energy are obtained from the thermal activation of the spin polarization in tilted fields. The filling factor dependence of the spin polarization for $2/5 < ν< 2/3$ reveals a broken particle-hole symmetry for the partially polarized CF Fermi sea.

cond-mat.mes-hall↗

arXiv2001

Observation of a new phase transition between fully and partially polarized quantum Hall states with charge and spin gaps at $ν= 2/3$

The average electron spin-polarization $\cal P$ of two-dimensional electron gas confined in $\rm GaAs/GaAlAs$ multiple quantum-wells was measured by nuclear magnetic resonance (NMR) near the fractional quantum Hall state with filling factor $ν={2/3}$. Above this filling factor (${2/3} \leq ν< 0.85$), a strong depolarization is observed corresponding to two spin flips per additional flux quantum. The most remarkable behavior of the polarization is observed at $ν={2/3}$, where a quantum phase transition from a partially polarized (${\cal P} \approx {3/4}$) to a fully polarized (${\cal P} = 1$) state can be driven by increasing the ratio between the Zeeman and the Coulomb energy above a critical value $η_{c} = \frac{Δ_{Z}}{Δ_{C}} = 0.0185$.

cond-mat.mes-hall↗

arXiv1999

NMR Determination of 2D Electron Spin Polarization at $ν=1/2$

Using a `standard' NMR spin-echo technique we determined the spin polarization of two-dimensional electrons, confined to GaAs quantum wells, from the hyperfine shift of Ga nuclei in the wells. Concentrating on the temperature and magnetic field dependencies of spin polarization at Landau level filling factor $ν=1/2$, we find that the results are described well by a simple model of non-interacting composite fermions, although some inconsistencies remain when the two-dimensional electron system is tilted in the magnetic field.

cond-mat.mes-hall↗