90 research outputs found
Amplified erosion above waterfalls and oversteepened bedrock reaches
None of the conventional bedrock erosion laws can predict incision immediately upslope of a waterfall lip where the flow is accelerating toward a freefall. Considering the expected increase in flow velocity and shear stress at the lip of a waterfall, we determine erosion amplification at a waterfall lip as [equation], where [equation] is the erosion rate at the upstream end of the flow acceleration zone above a waterfall, Fr is the Froude number at this setting, and n ranges between 0.5â1.7. This amplification expression suggests that erosion at the lip could be as much as 2â5 times higher relative to erosion at a normal setting with identical hydraulic geometry. Utilizing this erosion amplification expression in numerical simulations, we demonstrate its impact on reach-scale morphology above waterfalls. Amplified erosion at the lip of a waterfall can trigger the formation of an oversteepened reach whose length is longer than the flow acceleration zone, provided incision wave velocity (Vi) at the upstream edge of the flow acceleration zone is higher than the retreat velocity of the waterfall face. Such an oversteepened reach is expected to be more pronounced when Vi increases with increasing slope. The simulations also suggest that oversteepening can eventually lead to steady state gradients adjacent to a waterfall lip provided Vi decreases with increasing slope. Flow acceleration above waterfalls can thus account, at least partially, for prevalent oversteepened bedrock reaches above waterfalls. Using the cosmogenic isotope Cl-36, we demonstrate that incision wave velocity upstream of a waterfall at the Dead Sea western escarpment is probably high enough for freefall-induced oversteepening to be feasible
Si/SiGe bound-to-continuum quantum cascade emitters
Si/SiGe bound-to-continuum quantum cascade emitters designed
by self-consistent 6-band k.p modeling and grown by low energy
plasma enhanced chemical vapour deposition are presented
demonstrating electroluminescence between 1.5 and 3 THz. The
electroluminescence is Stark shifted by an electric field and
demonstrates polarized emission consistent with the design.
Transmission electron microscopy and x-ray diffraction are also
presented to characterize the thick heterolayer structure
Optical response from terahertz to visible light of electronuclear transitions in LiYF_4:Ho^(3+)
Because of its role as a model system with tunable quantum fluctuations and quenched disorder, and the desire for optical control and readout of its states, we have used high-resolution optical absorption spectroscopy to measure the crystal-field excitations for Ho^(3+) ions in LiHo_xY_(1âx)F_4 from the terahertz to visible regimes. We show that many of the excitations yield very narrow lines visibly split even by the nuclear hyperfine interaction, making Ho^(3+) in LiHo_xY_(1âx)F_4 a candidate host for optically addressable electronuclear qubits with quality factors as high as Q = 4.7 Ă 10^5, where the higher-lying levels are electronic singlets. Optical transitions in the easily accessible near- and mid-infrared are narrow enough to allow readout of the ground-state electronuclear qubits responsible for the interesting magnetism of LiHo_xY_(1âx)F_4. While many of the higher-lying states have been observed previously, we also report here detailed spectra of terahertz excitations. The strengths of the electric and magnetic dipole crystal-field transition lines of five of the lowest excited spin-orbit manifolds of dilute LiYF_4:Ho^(3+) were calculated and compared with measurement. The magnitude of the nuclear hyperfine coupling was used to assign the correct upper and lower states to transition lines
Optical response from terahertz to visible light of electronuclear transitions in LiYF4:Ho3+
Because of its role as a model system with tunable quantum fluctuations and quenched disorder, and the desire for optical control and readout of its states, we have used high-resolution optical absorption spectroscopy to measure the crystal-field excitations for Ho3+ ions in LiHoxY1âxF4 from the terahertz to visible regimes. We show that many of the excitations yield very narrow lines visibly split even by the nuclear hyperfine interaction, making Ho3+ in LiHoxY1âxF4 a candidate host for optically addressable electronuclear qubits with quality factors as high as Q = 4.7 Ă 105, where the higher-lying levels are electronic singlets. Optical transitions in the easily accessible near- and mid-infrared are narrow enough to allow readout of the ground-state electronuclear qubits responsible for the interesting magnetism of LiHoxY1âxF4. While many of the higher-lying states have been observed previously, we also report here detailed spectra of terahertz excitations. The strengths of the electric and magnetic dipole crystal-field transition lines of five of the lowest excited spin-orbit manifolds of dilute LiYF4:Ho3+ were calculated and compared with measurement. The magnitude of the nuclear hyperfine coupling was used to assign the correct upper and lower states to transition lines
Emergence of highly coherent quantum subsystems of a noisy and dense spin system
Quantum sensors and qubits are usually two-level systems (TLS), the quantum
analogs of classical bits which assume binary values '0' or '1'. They are
useful to the extent to which they can persist in quantum superpositions of '0'
and '1' in real environments. However, such TLS are never alone in real
materials and devices, and couplings to other degrees of freedom limit the
lifetimes - called decoherence times - of the superposition states. Decoherence
occurs via two major routes - excitation hopping and fluctuating
electromagnetic fields. Common mitigation strategies are based on material
improvements, exploitation of clock states which couple only to second rather
than first order to external perturbations, and reduction of interactions via
extreme dilution of pure materials made from isotopes selected to minimize
noise from nuclear spins. We demonstrate that for a dense TLS network in a
noisy nuclear spin bath, we can take advantage of interactions to pass from
hopping to fluctuation dominance, increasing decoherence times by almost three
orders of magnitude. In the dilute rare-earth insulator LiY1-xTbxF4, Tb ions
realize TLS characterized by a 30GHz splitting and readily implemented clock
states. Dipolar interactions lead to coherent, localized pairs of Tb ions, that
decohere due to fluctuating quantum mechanical ring-exchange interaction,
sensing the slow dynamics of the surrounding, nearly localized Tb spins. The
hopping and fluctuation regimes are sharply distinguished by their Rabi
oscillations and the invisible vs. strong effect of classic 'error correcting'
microwave pulse sequences. Laying open the decoherence mechanisms at play in a
dense, disordered and noisy network of interacting TLS, our work expands the
search space for quantum sensors and qubits to include clusters in dense,
disordered materials, that can be explored for localization effects.Comment: 39 page
Sediment source and mixing and the cycle of sediment transport: an example from NE Negev Desert, Israel.
Alluvial terraces represent the end product of sedimentary cycles; each includes sediment generation, transport, accumulation, and the ultimate incision that forms abandoned alluvial surfaces. We examine the middle Pleistocene to recent drainage system evolution in Makhtesh Hazera, Negev Desert, southern Israel and compare the characteristics of erosion and sediment transport in the present system with those expressed by the alluvial
terraces. The Hazera drainage basin lies at the margins of the arid to hyper-arid Dead Sea rift (DSR). Makhtesh (crater) Hazera is a deeply incised erosional structure (5X7 km) that has been excavated since the early Pliocene into the crest of the Hazera asymmetric anticline. The Makhtesh floor is surrounded by cliffs rising more than
400 meters high. The cliffs are built of Upper Cretaceous hard carbonates caprocks overlying Lower Cretaceous friable quartz sandstones. Bedrock knickpoints isolate the drainage basin in the Makhtesh and above it from a direct influence of the terminal base level of the DSR. Thus, the accumulation of sediment and abandonment of terraces are controlled by climate and bedrock barriers located at the Makhtesh outlet. We use cosmogenic isotope concentrations to determine bedrock denudation rates, ages of alluvial terraces, and basin wide erosion rates in different channels throughout the basin. The use of cosmogenic isotopes enables us to determine sediment sources
and reconstruct sedimentary cycles. OSL dating was used to determine the accumulation ages of alluvial sediment
in alluvial terraces. These two methods enable quantitative evaluation of fluvial processes. Bedrock erosion rates suggest a strong dependence of erosion on lithology. While the Lower Cretaceous sandstone
erodes at >100 mm ky-1, the overlying hard carbonate caprock yielded cosmogenic isotope concentrations that correspond to erosion rates of 1-3 mm ky-1. This significant difference in erosion rates maintains the dramatic relief of the Hazera drainage basin. We find that the quartz sediment in the present fluvial system of Makhtesh
Hazera originates from two predominant sources. One is the Lower Cretaceous sandstone that crops out along the base of the Makhtesh cliffs. The second source are un consolidated Miocene sands that fill the syncline which is located north west of the Makhtesh and is drained into it. 10Be concentrations in successive samples indicate
that the Miocene sand is gradually diluted by Lower Cretaceous sand as it flows down stream and the mixing of sediment from both sources is good. Alluvial terraces and bedrock units exposed inside the Makhtesh do not contribute a significant amount of sediment to the present drainage system.
Three major alluvial terrace levels were identified. The highest terrace level (MKT0) was abandoned at 279±19 ky. This level probably covered most of the Makhtesh surface. The deposition of the two lower levels, MKT1 and MKT2 (which were abandoned at 160±6 and 47±9 ky, respectively), was confined to the present drainage system. Analysis of cosmogenic depth profiles from the terraces suggests significant recycling of sediment within the Mekhtesh. This is in contrast to the present system that lacks recycled sediment. We explain this difference by the fact that the terraces are the final product of a sedimentary cycle while the present drainage system presents
a âsnapshotâ in time which does not represent the entire cycle only the present state of the system which is expressed by rapid incision and very little lateral migration.European Geosciences Unio
Optical response from terahertz to visible light of electronuclear transitions in LiYF_4:Ho^(3+)
Because of its role as a model system with tunable quantum fluctuations and quenched disorder, and the desire for optical control and readout of its states, we have used high-resolution optical absorption spectroscopy to measure the crystal-field excitations for Ho^(3+) ions in LiHo_xY_(1âx)F_4 from the terahertz to visible regimes. We show that many of the excitations yield very narrow lines visibly split even by the nuclear hyperfine interaction, making Ho^(3+) in LiHo_xY_(1âx)F_4 a candidate host for optically addressable electronuclear qubits with quality factors as high as Q = 4.7 Ă 10^5, where the higher-lying levels are electronic singlets. Optical transitions in the easily accessible near- and mid-infrared are narrow enough to allow readout of the ground-state electronuclear qubits responsible for the interesting magnetism of LiHo_xY_(1âx)F_4. While many of the higher-lying states have been observed previously, we also report here detailed spectra of terahertz excitations. The strengths of the electric and magnetic dipole crystal-field transition lines of five of the lowest excited spin-orbit manifolds of dilute LiYF_4:Ho^(3+) were calculated and compared with measurement. The magnitude of the nuclear hyperfine coupling was used to assign the correct upper and lower states to transition lines
Precise determination of low energy electronuclear Hamiltonian for LiYHoF
We use complementary optical spectroscopy methods to directly measure the
lowest crystal-field energies of the rare-earth quantum magnet
LiYHoF, including their hyperfine splittings, with more
than 10 times higher resolution than previous work. We are able to observe
energy level splittings due to the and
isotopes, as well as non-equidistantly spaced hyperfine transitions originating
from dipolar and quadrupolar hyperfine interactions. We provide refined crystal
field parameters and extract the dipolar and quadrupolar hyperfine constants
and
, respectively. Thereupon we determine all crystal-field
energy levels and magnetic moments of the ground state manifold,
including the (non-linear) hyperfine corrections. The latter match the
measurement-based estimates. The scale of the non-linear hyperfine corrections
sets an upper bound for the inhomogeneous line widths that would still allow
for unique addressing of a selected hyperfine transition. e.g. for quantum
information applications. Additionally, we establish the far-infrared,
low-temperature refractive index of LiYHoF.Comment: 9 pages, 6 Figures, 3 Table
Si/SiGe quantum cascade superlattice designs for terahertz emission
Quantum cascade lasers are compact sources that have demonstrated high output powers at THz frequencies.
To date all THz quantum cascade lasers have been realized in III-V materials. Results are presented from
Si1âxGex quantum cascade superlattice designs emitting at around 3 THz which have been grown in two
different chemical vapor deposition systems. The key to achieving successful electroluminescence at THz
frequencies in a p-type system has been to strain the light-hole states to energies well above the radiative
subband states. To accurately model the emission wavelengths, a 6-band k.p tool which includes the effects
of non-abrupt heterointerfaces has been used to predict the characteristics of the emitters. X-ray diffraction
and transmission electron microscopy have been used along with Fourier transform infrared spectroscopy to
fully characterise the samples. A number of methods to improve the gain from the designs are suggested
Precise determination of the low-energy electronuclear Hamiltonian of LiY1âx HoxF4
The insulating rare-earth magnet LiY1âxHoxF4 has received great attention because a laboratory field applied perpendicular to its crystallographic c axis converts the low-energy electronic spin Hamiltonian into the (dilute) transverse field Ising model. The mapping between the real magnet and the transverse field Ising model is strongly dependent on the exact nature of the low-energy Hamiltonian for the material, which can be determined by spectroscopy in the dilute limit. The energies of the eigenstates are in the difficult terahertz (THz) regime, and here we use THz time domain and Fourier transform spectroscopy to directly measure the lowest crystal-field levels of LiY1âxHoxF4 in the dilute limit, including nuclear hyperfine substructure. The high resolution of our measurements allows us to observe the nonequidistantly spaced Ho (I = 72 ) hyperfine transitions originating from dipolar and quadrupolar hyperfine interactions. We provide refined crystal-field parameters and extract the dipolar and quadrupolar hyperfine constants AJ = 0.027 03 ± 0.000 03 cmâ1 (810.3 ± 0.9 MHz) and B = 0.04 ± 0.01 cmâ1(1.2 ± 0.3 GHz), respectively. Thereupon we determine all crystal-field energy levels and magnetic moments of the 5/8 ground-state manifold, including the (nonlinear) hyperfine corrections. The latter improve the prediction precision by a factor of 60 compared to previous crystal-field parameters. Additionally,we establish the far-infrared, low-temperature refractive index of LiY1âxHoxF4
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