Browsing by Author "Wang, L. Y."

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A side-by-side comparison of Daya Bay antineutrino detectors
(1970-01-01)
An, F. P.
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An, Q.
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Bai, J. Z.
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Balantekin, A. B.
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Band, H. R.
"The Daya Bay Reactor Neutrino Experiment is designed to determine precisely the neutrino mixing angle theta(13) with a sensitivity better than 0.01 in the parameter sin(2) 2 theta(13) at the 90% confidence level. To achieve this goal, the collaboration will build eight functionally identical antineutrino detectors. The first two detectors have been constructed, installed and commissioned in Experimental Hall 1, with steady data-taking beginning September 23, 2011. A comparison of the data collected over the subsequent three months indicates that the detectors are functionally identical, and that detector-related systematic uncertainties are smaller than requirements. (C) 2012 Elsevier B.V. All rights reserved."
All-electric spin pumping in quantum channels with a single finger-gate capacitor
(2017-02-07)
Wang, L. Y.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
In this paper, we show that a single finger-gate capacitor (FGC) can generate pure spin pumping in a quantum channel (QC). Two dynamic fields, ac spin-orbit interaction and ac potential energy, both induced by the FGC onto the QC, are the agents driving the spin pumping. Smooth spatial profiles of the two ac fields are taken into account both perturbatively and full numerically for the nonadiabatic spin pumping. Our perturbative approach reveals that the spin-pumping mechanism is resonant sideband processes associated with simultaneous coupling of the two ac fields with traversing carriers. Full sideband-process treatment is carried out numerically by a time-dependent scattering matrix method. The same spin-pumping mechanism holds also for the case of a single finger-gated QC, albeit with smaller pumping amplitudes.
Asymmetries in intrinsic spin-Hall effect in low in-plane magnetic field
(2008-10-01)
Wang, L. Y.
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Chu, C. S.
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Mal'shukov, A. G.
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電子物理學系
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Department of Electrophysics
Effects of low in-plane magnetic field on bulk spin densities and edge spin accumulations of a diffusive two-dimensional semiconductor stripe are studied. Focusing upon the Dresselhaus-type intrinsic spin-orbit interaction (SOI), we look for the symmetry, or asymmetry, characteristics in two magnetic-field orientations: along and transverse to the stripe. For longitudinal field, the out-of-plane spin density S(z) exhibits odd parity across the stripe and even parity in the magnetic field and is an edge accumulation. For transverse field, the out-of-plane S(z) becomes asymmetric in both spatial and field dependencies and has finite bulk values for finite magnetic fields. Our results support utilizing low in-plane magnetic fields for the probing of the underlying SOI.
Gate-induced localized states in graphene: Topological nature in their formation
(2018-12-26)
Wang, L. Y.
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Chang, Che-Yuan
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
In this work, we show that an N-C-chain gate potential along the armchair chain (direction (y) over cap) of a graphene sheet gives rise to topological localized states (LSs): one branch for N-C = 1 and two branches for N-C >= 2. These LSs are shown to form whenever the gate-induced potential V-0 is nonzero. The topological nature behind the formation of these LSs is revealed (for N-C = 1, 2) by showing, for V-0 not equal 0, that the LS-secular equation can be cast into a pseudospin-rotation form on which rotation upon a valley-associated pseudospin is to equate with another valley-associated pseudospin. Both pseudospins are on the same side of the gate potential. That the rotation angle of the pseudospin-rotation operator falls within the range of variation of the relative angle Delta theta(p) between the two pseudospins, as the energy E varies across the entire energy gap for a given k(y), demonstrates the topological nature and the inevitability of the LS branch formation. These topological LS branches exhibit Dirac-point characteristics, with dispersion relations leading out from the Dirac point (at k(y) = 0). For general multiple (N-C > 1) carbon chain gate-potential cases, the number N-LS of LS branches are found to increase with V-0, up to a maximum of N-LS,(max) = N-C. Yet LS branches carrying the Dirac-point characteristics are found to be fixed at two.
Improved measurement of electron antineutrino disappearance at Daya Bay
(2013-01-01)
An, F. P.
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An, Q.
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Bai, J. Z.
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Balantekin, A. B.
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Band, H. R.
We report an improved measurement of the neutrino mixing angle theta(13) from the Daya Bay Reactor Neutrino Experiment. We exclude a zero value for sin(2)2 theta(13) with a significance of 7.7 standard deviations. Electron antineutrinos from six reactors of 2.9 GW(th) were detected in six antineutrino detectors deployed in two near (flux-weighted baselines of 470 m and 576 m) and one far (1648 m) underground experimental halls. Using 139 days of data, 28909 (205308) electron antineutrino candidates were detected at the far hall (near halls). The ratio of the observed to the expected number of antineutrinos assuming no oscillations at the far hall is 0.944 +/- 0.007(stat.)+/- 0.003(syst.). An analysis of the relative rates in six detectors finds sin(2)2 theta(13)= 0.089 +/- 0.010(stat.)+/- 0.005(syst.) in a three-neutrino framework.
Independent measurement of the neutrino mixing angle theta(13) via neutron capture on hydrogen at Daya Bay
(2014-10-03)
An, F. P.
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Balantekin, A. B.
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Band, H. R.
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Beriguete, W.
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Bishai, M.
A new measurement of the theta(13) mixing angle has been obtained at the Daya Bay Reactor Neutrino Experiment via the detection of inverse beta decays tagged by neutron capture on hydrogen. The antineutrino events for hydrogen capture are distinct from those for gadolinium capture with largely different systematic uncertainties, allowing a determination independent of the gadolinium-capture result and an improvement on the precision of the theta(13) measurement. With a 217-day antineutrino data set obtained with six antineutrino detectors and from six 2.9 GWth reactors, the rate deficit observed at the far hall is interpreted as sin(2)2 theta(13) = 0.083 +/- 0.018 in the three-flavor oscillation model. When combined with the gadolinium-capture result from Daya Bay, we obtain sin(2)2 theta(13) = 0.089 +/- 0.008 as the final result for the six-antineutrino-detector configuration of the Daya Bay experiment.
Nonuniversality of the intrinsic inverse spin-Hall effect in diffusive systems
(2012-04-02)
Wang, L. Y.
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Mal'shukov, A. G.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We studied the electric current induced in a two-dimensional electron gas by the spin current, in the presence of Rashba and cubic Dresselhaus spin-orbit interactions. We found out that the factor relating the electric and spin currents is not universal, but rather depends on the origin of the spin current. Drastic distinction has been found between two cases: the spin current created by diffusion of an inhomogeneous spin density, and the pure homogeneous spin current. We found that in the former case the inverse spin-Hall effect electric current is finite, while it turns to zero in the latter case, if the spin-orbit coupling is represented by Rashba interaction.
Observation of Electron-Antineutrino Disappearance at Daya Bay
(2012-04-23)
An, F. P.
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Bai, J. Z.
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Balantekin, A. B.
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Band, H. R.
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Beavis, D.
The Daya Bay Reactor Neutrino Experiment has measured a nonzero value for the neutrino mixing angle 0(13) with a significance of 5.2 standard deviations. Antineutrinos from six 2.9 GW(th) reactors were detected in six antineutrino detectors deployed in two near (flux-weighted baseline 470 m and 576 m) and one far (1648 m) underground experimental halls. With a 43 000 ton-GW(th)-day live-time exposure in 55 days, 10 416 (80 376) electron-antineutrino candidates were detected at the far hall (near halls). The ratio of the observed to expected number of antineutrinos at the far hall is R = 0.940 +/- 0.011(stat.) +/- 0.004(syst.). A rate-only analysis finds sin(2)2 theta(13) = 0.092 +/- 0.016(stat.) +/- 0.005(syst.) in a three-neutrino framework.
Rashba-type spin accumulation near a void at a system edge
(2011-09-30)
Wang, L. Y.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We consider the spatial distribution of spin accumulation S(z) near a void (edge void) located at a system edge. The edge void is semicircular in shape with a radius R(0) approximate to l(so), the spin-relaxation length, and it is formed out of a Rashba-type two-dimensional electron system in the diffusive regime. The nonuniform driving field provides the essential condition, and diffusive contributions to the spin currents from spin polarizations provide the primary impetus for the spin accumulation. The edge void reveals an underlying asymmetry of the bulk-void counterpart, namely, a finite bulklike spin flow at the system edge. This bulklike spin flow gives rise to spin accumulation when open boundary occurs. The bulklike spin flow is found to exhibit a similar spatial profile as that of Sz at the system edge. This bulklike spin flow is expected to be a primary impetus for local spin injection at the sample edge near the edge void when a local protrusion occurs. Spin orientation of the bulklike spin flow varies with the locations on the edge-void boundary.
Results from the Daya Bay Reactor Neutrino Experiment
(2014-01-01)
Tsang, K. V.
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An, F. P.
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An, Q.
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Bai, J. Z.
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Balantekin, A. B.
The Daya Bay Reactor Neutrino Experiment was designed to achieve a sensitivity on the value of sin(2) 2 theta(13). to better than 0.01 at 90% CL. The experiment consists of eight antineutrino detectors installed underground at different baselines from six nuclear reactors. With data collected with six antineutrino detectors for 55 days, Daya Bay announced the discovery of a non-zero value for sin(2) 2 theta(13) with a significance of 5.2 standard deviations in March 2012. The most recent analysis with 139 days of data acquired in a six-detector configuration yields sin(2) 2 theta(13) = 0.089 +/- 0.010(stat.) +/- 0.005(syst.), which is the most precise measurement of sin(2) 2 theta(13) to date.
Robust level coincidences in the subband structure of quasi-2D systems
(2012-12-01)
Winkler, R.
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Wang, L. Y.
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Lin, Y. H.
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Chu, C. S.
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電子物理學系
Recently, level crossings in the energy bands of crystals have been identified as a key signature for topological phase transitions. Using realistic models we show that the parameter space controlling the occurrence of level coincidences in energy bands has a much richer structure than anticipated previously. In particular, we identify robust level coincidences that cannot be removed by a small perturbation of the Hamiltonian compatible with the crystal symmetry. Different topological phases that are insulating in the bulk are then separated by a gapless (metallic) phase. We consider HgTe/CdTe quantum wells as a specific example. (c) 2012 Elsevier Ltd. All rights reserved.
Search for a Light Sterile Neutrino at Daya Bay
(2014-10-01)
An, F. P.
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Balantekin, A. B.
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Band, H. R.
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Beriguete, W.
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Bishai, M.
A search for light sterile neutrino mixing was performed with the first 217 days of data from the Daya Bay Reactor Antineutrino Experiment. The experiment's unique configuration of multiple baselines from six 2.9 GW(th) nuclear reactors to six antineutrino detectors deployed in two near (effective baselines 512 m and 561 m) and one far (1579 m) underground experimental halls makes it possible to test for oscillations to a fourth (sterile) neutrino in the 10(-3) eV(2) < vertical bar Delta m(41)(2)vertical bar < 0.3 eV(2) range. The relative spectral distortion due to the disappearance of electron antineutrinos was found to be consistent with that of the three-flavor oscillation model. The derived limits on sin(2) 2 theta(14) cover the 10(-3) eV(2) less than or similar to vertical bar Delta m(41)(2)vertical bar less than or similar to 0.1 eV(2) region, which was largely unexplored.
Spectral Measurement of Electron Antineutrino Oscillation Amplitude and Frequency at Daya Bay
(2014-02-10)
An, F. P.
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Balantekin, A. B.
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Band, H. R.
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Beriguete, W.
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Bishai, M.
A measurement of the energy dependence of antineutrino disappearance at the Daya Bay reactor neutrino experiment is reported. Electron antineutrinos ((v) over bar (e)) from six 2.9 GW(th) reactors were detected with six detectors deployed in two near (effective baselines 512 and 561 m) and one far (1579 m) underground experimental halls. Using 217 days of data, 41 589 (203 809 and 92 912) antineutrino candidates were detected in the far hall (near halls). An improved measurement of the oscillation amplitude sin(2) 2 theta(13) = 0.090(-0.009)(+0.008) and the first direct measurement of the (v) over bar (e) mass-squared difference vertical bar Delta m(ee)(2)vertical bar = (2.59(-0.20)(+0.19) x 10(-3) eV(2)) is obtained using the observed (v) over bar (e) rates and energy spectra in a three-neutrino framework. This value of vertical bar Delta m(ee)(2)vertical bar eejis consistent with vertical bar Delta m(mu mu)(2)vertical bar measured by muon neutrino disappearance, supporting the three-flavor oscillation model.
Spin generation in a Rashba-type diffusive electron system by nonuniform driving field
(2010-03-01)
Wang, L. Y.
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Chu, C. S.
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Mal'shukov, A. G.
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電子物理學系
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Department of Electrophysics
We show that the Rashba spin-orbit interaction contributes to edge spin accumulation S(z) in a diffusive regime when the driving field is nonuniform. Specifically, we solve the case of nonuniform driving field in the vicinity of a circular void locating in a two-dimensional electron system and we identify the key physical process leading to the edge spin accumulation. The void has radius R(0) in the range of spin-relaxation length l(so) and is far from both source and drain electrodes. The key physical process we find is originated from the nonuniform in-plane spin polarizations. Their subsequent diffusive contribution to spin current provides the impetus for the edge spin accumulation S(z) at the void boundary. The edge spin accumulation is proportional to the Rashba coupling constant alpha and is in a spin-dipole form oriented transversely to the driving field. We expect similar spin accumulation to occur if the void is at the sample edge.
Spin-Hall interface resistance in terms of Landauer-type spin dipoles
(2007-02-01)
Mal'shukov, A. G.
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Wang, L. Y.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We considered the nonequilibrium spin dipoles induced around spin-independent elastic scatterers by the intrinsic spin-Hall effect associated with the Rashba spin-orbit coupling. The spin polarization normal to the two-dimensional electron gas (2DEG) has been calculated in the diffusion range around the scatterer. Although around each impurity this polarization is finite, we found that the corresponding macroscopic spin density obtained via averaging of individual spin dipole distributions over impurity positions is zero in the bulk. At the same time, the spin density is finite near the boundary of 2DEG, except for a special case of a hard wall boundary, that is, when it turns to 0. The boundary value of the spin polarization can be associated with the interface spin-Hall resistance determining the additional energy dissipation due to spin accumulation.
Temporal universal conductance fluctuations in RuO(2) nanowires due to mobile defects
(2011-10-17)
Lien, An-Shao
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Wang, L. Y.
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Chu, C. S.
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Lin, Juhn-Jong
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電子物理學系
Temporal universal conductance fluctuations (TUCF's) are observed in RuO(2) nanowires at cryogenic temperatures. The fluctuations persist up to very high T similar to 10 K. Their root-mean-square magnitudes increase with decreasing T, reaching similar to 0.2e(2)/h at T less than or similar to 2 K. These fluctuations are shown to originate from scattering of conduction electrons with rich amounts of mobile defects in artificially synthesized metal oxide nanowires. TUCF characteristics in both one-dimensional saturated and unsaturated regimes are identified and explained in terms of current theories. Furthermore, the TUCF's as a probe for the characteristic time scales of the mobile defects (two-level systems) are discussed.
Temporal universal conductance fluctuations in RuO2 nanowires due to mobile defects
(2011-10-17)
Lien, An-Shao
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Wang, L. Y.
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Chu, C. S.
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Lin, Juhn-Jong
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電子物理學系
Temporal universal conductance fluctuations (TUCF's) are observed in RuO2 nanowires at cryogenic temperatures. The fluctuations persist up to very high T similar to 10 K. Their root-mean-square magnitudes increase with decreasing T, reaching similar to 0.2e(2)/h at T less than or similar to 2 K. These fluctuations are shown to originate from scattering of conduction electrons with rich amounts of mobile defects in artificially synthesized metal oxide nanowires. TUCF characteristics in both one-dimensional saturated and unsaturated regimes are identified and explained in terms of current theories. Furthermore, the TUCF's as a probe for the characteristic time scales of the mobile defects (two-level systems) are discussed.
The detector system of the Daya Bay reactor neutrino experiment
(2016-03-01)
An, F. P.
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Bai, J. Z.
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Balantekin, A. B.
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Band, H. R.
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Beavis, D.
The Daya Bay experiment was the first to report simultaneous measurements of reactor antineutrinos at multiple baselines leading to the discovery of (nu) over bar (e) oscillations over km-baselines. Subsequent data has provided the world\'s most precise measurement of sin 2 2013 and the effective mass splitting Delta m(ee)(2). The experiment is located in Daya Bay, China where the cluster of six nuclear reactors is among the world\'s most prolific sources of electron antineutrinos. Multiple antineutrino detectors are deployed in three underground water pools at different distances from the reactor cores to search for deviations in the antineutrino rate and energy spectrum due to neutrino mixing. Instrumented with photomultiplier tubes, the water pools serve as shielding against natural radioactivity from the surrounding rock and provide efficient muon tagging. Arrays of resistive plate chambers over the top of each pool provide additional muon detection. The antineutrino detectors were specifically designed for measurements of the antineutrino flux with minimal systematic uncertainty. Relative detector efficiencies between the near and far detectors are known to better than 0.2%. With the unblinding of the final two detectors\' baselines and target masses, a complete description and comparison of the eight antineutrino detectors can now be presented. This paper describes the Daya Bay detector systems, consisting of eight antineutrino detectors in three instrumented water pools in three underground halls, and their operation through the first year of eight detector data-taking. (C) 2015 Elsevier B.V. All rights reserved.
The muon system of the Daya Bay Reactor antineutrino experiment
(2015-02-11)
An, F. P.
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Balantekin, A. B.
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Band, H. R.
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Beriguete, W.
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Bishai, M.
The Daya Bay experiment consists of functionally identical antineutrino detectors immersed in pools of ultrapure water in three well-separated underground experimental halls near two nuclear reactor complexes. These pools serve both as shields against natural, low-energy radiation, and as water Cherenkov detectors that efficiently detect cosmic muons using arrays of photomultiplier tubes. Each pool is covered by a plane of resistive plate chambers as an additional means of detecting muons. Design, construction, operation, and performance of these muon detectors are described. (C) 2014 Elsevier By. All rights reserved.
Universal conductance fluctuations in indium tin oxide nanowires
(2012-02-14)
Yang, Ping-Yu
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Wang, L. Y.
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Hsu, Yao-Wen
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Lin, Juhn-Jong
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電子物理學系
Magnetic-field-dependent universal conductance fluctuations (UCFs) are observed in weakly disordered indium tin oxide nanowires from 0.26 K up to similar to 25 K. The fluctuation magnitudes increase with decreasing temperature, reaching a fraction of e(2)/h at T less than or similar to 1 K. The shape of the UCF patterns is found to be very sensitive to thermal cycling of the sample to room temperature, which induces irreversible impurity reconfigurations. On the other hand, the UCF magnitudes are insensitive to thermal cycling. Our measured temperature dependence of the root-mean-square UCF magnitudes is compared with the existing theory [C. W. J. Beenakker and H. van Houten, Phys. Rev. B 37, 6544 (1988)]. A notable discrepancy is found, which seems to imply that the experimental UCFs are not cut off by the thermal diffusion length L-T, as would be expected by the theoretical prediction when L-T < L-phi, where L-phi is the electron dephasing length. The approximate electron dephasing length is inferred from the UCF magnitudes and compared with that extracted from the weak-localization magnetoresistance studies. A reasonable semiquantitative agreement is observed.