arXiv subjects

S. Lyon

Publications and source records attributed to S. Lyon.

5 recordsLinked to original sources

arXiv2012

Indirect Exchange Interaction in Fully Metal-Semiconductor Separated SWCNTs Revealed by ESR

The ESR response from highly metal-semiconductor(M-SC) separated SWCNTs for temperatures T between 0.39 and 200 K is characteristically different for the two systems. The signal originates from defect spins but interaction with free electrons leads to a larger line width for M tubes. The latter decreases with increasing T whereas it increases with T for SC tubes. The spins undergo a ferromagnetic phase transition below around 10 K. Indirect exchange is suggested to be responsible for the spin-spin interaction, supported by RKKY interaction in the case of M tubes. For SC tubes spin-lattice relaxation via an Orbach process is suggested to determine the line width.

cond-mat.mtrl-sci↗

arXiv2006

Linear temperature dependence of conductivity in Si two-dimensional electrons near the apparent metal-to-insulator transition

In a high mobility two-dimensional electron system in Si, near the critical density, $n_c=0.32\times10^{11}$cm$^{-2}$, of the apparent metal-to-insulator transition, the conductivity displays a linear temperature ($T$) dependence around the Fermi temperature. When $σ_0$, the extrapolated T=0 conductivity from the linear T-dependence, is plotted as a function of density, two regimes with different $σ_0(n)$ relations are seen, suggestive of two different phases. Interestingly, a sharp transition between these two regimes coincides with $n_c$, and $σ_0$ of the transition is $\sim$ $e^2/h$, the quantum conductance, per square. Toward T=0, the data deviate from linear $σ(T)$ relation and we discuss the possible percolation type of transition in our Si sample.

cond-mat.mes-hall↗

arXiv2006

Valley splitting of Si/SiGe heterostructures in tilted magnetic fields

We have investigated the valley splitting of two-dimensional electrons in high quality Si/Si$_{1-x}$Ge$_x$ heterostructures under tilted magnetic fields. For all the samples in our study, the valley splitting at filling factor $ν=3$ ($Δ_3$) is significantly different before and after the coincidence angle, at which energy levels cross at the Fermi level. On both sides of the coincidence, a linear density dependence of $Δ_3$ on the electron density was observed, while the slope of these two configurations differs by more than a factor of two. We argue that screening of the Coulomb interaction from the low-lying filled levels, which also explains the observed spin-dependent resistivity, is responsible for the large difference of $Δ_3$ before and after the coincidence.

cond-mat.mes-hall↗

arXiv2005

Observation of Inter-Valley Gap Anomaly of Two Dimensional electrons in Silicon

We report here a systematic study of the energy gaps at the odd-integer quantum Hall states $ν=3$ and 5 under tilted magnetic (B) fields in a high quality Si two-dimensional electron system. Out of the coincidence region, the valley splitting is independent of the in-plane B fields. However, the $ν=3$ valley gap differs by about a factor of 3 ($Δ_v\sim$ 0.4K vs. 1.2K) at either side of the coincidence. More surprisingly, instead of reducing to zero, the energy gaps at $ν=3$ and 5 rise rapidly when approaching the coincidence angles. We believe that such anomaly is related to strong coupling of the nearly degenerate Landau levels.

cond-mat.mes-hall↗

arXiv2004

The Two-flux Composite Fermion Series of Fractional Quantum Hall States in Strained Si

Magnetotransport properties are investigated in a high-mobility two-dimensional electron system in the strained Si quantum well of a (100) Si_0.75Ge_0.25/Si/Si_0.75Ge_0.25 heterostructure, at temperatures down to 30mK and in magnetic fields up to 45T. We observe around ν=1/2 the two-flux composite fermion (CF) series of the fractional quantum Hall effect (FQHE) at ν=2/3, 3/5, 4/7, and at ν=4/9, 2/5, 1/3. Among these FQHE states, the ν=1/3, 4/7 and 4/9 states are seen for the first time in the Si/SiGe system. Interestingly, of the CF series, the 3/5 state is weaker than the nearby 4/7 state and the 3/7 state is conspicuously missing, resembling the observation in the integer quantum Hall effect regime that the ν=3 is weaker than the nearby ν=4 state. Our data indicate that the two-fold degeneracy of the CFs is lifted and an estimated valley splitting of ~1K.

cond-mat.mes-hall↗