arXiv subjects

K. Lai

Publications and source records attributed to K. Lai.

24 records · Page 2Linked to original sources

arXiv2005

Planets in Stellar Clusters Extensive Search. IV. A detection of a possible transiting planet candidate in the open cluster NGC 2158

We have undertaken a long-term project, Planets in Stellar Clusters Extensive Search (PISCES), to search for transiting planets in open clusters. In this paper we present the results for NGC 2158, an intermediate age, populous cluster. We have monitored the cluster for over 260 hours, spread over 59 nights. We have detected one candidate transiting low luminosity object, with eclipse depth of 3.7% in the R-band. If the host star is a member of the cluster, the eclipse depth is consistent with a 1.7 R_J object. Cluster membership of the host is supported by its location on the cluster main sequence (MS) and its close proximity to the cluster center (2'). We have discovered two other stars exhibiting low-amplitude (4-5%) transits, V64 and V70, but they are most likely blends or field stars. Given the photometric precision and temporal coverage of our observations, and current best estimates for the frequency and radii of short-period planets, the expected number of detectable transiting planets in our sample is 0.13. We have observed four outbursts for the candidate cataclysmic variable V57. We have discovered 40 new variable stars in the cluster, bringing the total number of identified variables to 97, and present for them high precision light curves, spanning 13 months.

astro-ph↗

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↗

arXiv2005

Modulation of the high mobility two-dimensional electrons in Si/SiGe using atomic-layer-deposited gate dielectric

Metal-oxide-semiconductor field-effect transistors (MOSFET's) using atomic-layer-deposited (ALD) Al$_2$O$_3$ as the gate dielectric are fabricated on the Si/Si$_{1-x}$Ge$_x$ heterostructures. The low-temperature carrier density of a two-dimensional electron system (2DES) in the strained Si quantum well can be controllably tuned from 2.5$\times10^{11}$cm$^{-2}$ to 4.5$\times10^{11}$cm$^{-2}$, virtually without any gate leakage current. Magnetotransport data show the homogeneous depletion of 2DES under gate biases. The characteristic of vertical modulation using ALD dielectric is shown to be better than that using Schottky barrier or the SiO$_2$ dielectric formed by plasma-enhanced chemical-vapor-deposition(PECVD).

cond-mat.mes-hall↗

arXiv2005

Two-dimensional metal-insulator transition and in-plane magnetoresistance in a high mobility strained Si quantum well

The apparent metal-insulator transition is observed in a high quality two-dimensional electron system (2DES) in the strained Si quantum well of a Si/Si_{1-x}Ge_x heterostructure with mobility μ=1.9 x 10^5 cm^2/Vs at density n=1.45 x 10^{11} cm^{-2}. The critical density, at which the thermal coefficient of low T resistivity changes sign, is ~ 0.32 x 10^{11} cm^{-2}, so far the lowest observed in the Si 2D systems. In-plane magnetoresistance study was carried out in the higher density range where the 2DES shows the metallic-like behavior. It is observed that the in-plane magnetoresistance first increases as ~ B_{ip}^2 and then saturates to a finite value ρ(B_c) for B_{ip} > B_c. The full spin-polarization field B_c decreases monotonically with n but appears to saturate to a finite value as n approaches zero. Furthermore, ρ(B_c)/ρ(0) ~ 1.8 for all the densities ranging from 0.35 x 10^{11} to 1.45 x 10^{11} cm^{-2} and, when plotted versus B_{ip}/B_c, collapses onto a single curve.

cond-mat.mes-hall↗

arXiv2004

Coulomb Drag near the metal-insulator transition in two-dimensions

We studied the drag resistivity between dilute two-dimensional hole systems, near the apparent metal-insulator transition. We find the deviations from the $T^{2}$ dependence of the drag to be independent of layer spacing and correlated with the metalliclike behavior in the single layer resistivity, suggesting they both arise from the same origin. In addition, layer spacing dependence measurements suggest that while the screening properties of the system remain relatively independent of temperature, they weaken significantly as the carrier density is reduced. Finally, we demonstrate that the drag itself significantly enhances the metallic $T$ dependence in the single layer resistivity.

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↗