4,794 research outputs found
Justifying the Special Theory of Relativity with Unconceived Methods
Many realists argue that present scientific theories will not follow the fate of past scientific theories because the former are more successful than the latter. Critics object that realists need to show that present theories have reached the level of success that warrants their truth. I reply that the special theory of relativity has been repeatedly reinforced by unconceived scientific methods, so it will be reinforced by infinitely many unconceived scientific methods. This argument for the special theory of relativity overcomes the critics’ objection, and has advantages over the no-miracle argument and the selective induction for it
Investigation of inter-slice magnetization transfer effects as a new method for MTR imaging of the human brain
We present a new method for magnetization transfer (MT) ratio imaging in the brain that requires no separate saturation pulse. Interslice MT effects that are inherent to multi-slice balanced steady-state free precession (bSSFP) imaging were controlled via an interslice delay time to generate MT-weighted (0 s delay) and reference images (5-8 s delay) for MT ratio (MTR) imaging of the brain. The effects of varying flip angle and phase encoding (PE) order were investigated experimentally in normal, healthy subjects. Values of up to ∼ 50% and ∼ 40% were observed for white and gray matter MTR. Centric PE showed larger MTR, higher SNR, and better contrast between white and gray matter than linear PE. Simulations of a two-pool model of MT agreed well with in vivo MTR values. Simulations were also used to investigate the effects of varying acquisition parameters, and the effects of varying flip angle, PE steps, and interslice delay are discussed. Lastly, we demonstrated reduced banding with a non-balanced SSFP-FID sequence and showed preliminary results of interslice MTR imaging of meningioma
Spin correlations in the electron-doped high-transition-temperature superconductor Nd{2-x}Ce{x}CuO{4+/-delta}
High-transition-temperature (high-Tc) superconductivity develops near
antiferromagnetic phases, and it is possible that magnetic excitations
contribute to the superconducting pairing mechanism. To assess the role of
antiferromagnetism, it is essential to understand the doping and temperature
dependence of the two-dimensional antiferromagnetic spin correlations. The
phase diagram is asymmetric with respect to electron and hole doping, and for
the comparatively less-studied electron-doped materials, the antiferromagnetic
phase extends much further with doping [1, 2] and appears to overlap with the
superconducting phase. The archetypical electron-doped compound
Nd{2-x}Ce{x}CuO{4\pm\delta} (NCCO) shows bulk superconductivity above x \approx
0.13 [3, 4], while evidence for antiferromagnetic order has been found up to x
\approx 0.17 [2, 5, 6]. Here we report inelastic magnetic neutron-scattering
measurements that point to the distinct possibility that genuine long-range
antiferromagnetism and superconductivity do not coexist. The data reveal a
magnetic quantum critical point where superconductivity first appears,
consistent with an exotic quantum phase transition between the two phases [7].
We also demonstrate that the pseudogap phenomenon in the electron-doped
materials, which is associated with pronounced charge anomalies [8-11], arises
from a build-up of spin correlations, in agreement with recent theoretical
proposals [12, 13].Comment: 5 pages, 4 figure
Towards End-to-end Video-based Eye-Tracking
Estimating eye-gaze from images alone is a challenging task, in large parts
due to un-observable person-specific factors. Achieving high accuracy typically
requires labeled data from test users which may not be attainable in real
applications. We observe that there exists a strong relationship between what
users are looking at and the appearance of the user's eyes. In response to this
understanding, we propose a novel dataset and accompanying method which aims to
explicitly learn these semantic and temporal relationships. Our video dataset
consists of time-synchronized screen recordings, user-facing camera views, and
eye gaze data, which allows for new benchmarks in temporal gaze tracking as
well as label-free refinement of gaze. Importantly, we demonstrate that the
fusion of information from visual stimuli as well as eye images can lead
towards achieving performance similar to literature-reported figures acquired
through supervised personalization. Our final method yields significant
performance improvements on our proposed EVE dataset, with up to a 28 percent
improvement in Point-of-Gaze estimates (resulting in 2.49 degrees in angular
error), paving the path towards high-accuracy screen-based eye tracking purely
from webcam sensors. The dataset and reference source code are available at
https://ait.ethz.ch/projects/2020/EVEComment: Accepted at ECCV 202
Cardiac output in idiopathic normal pressure hydrocephalus: association with arterial blood pressure and intracranial pressure wave amplitudes and outcome of shunt surgery
<p>Abstract</p> <p>Background</p> <p>In patients with idiopathic normal pressure hydrocephalus (iNPH) responding to shunt surgery, we have consistently found elevated intracranial pressure (ICP) wave amplitudes during diagnostic ICP monitoring prior to surgery. It remains unknown why ICP wave amplitudes are increased in these patients. Since iNPH is accompanied by a high incidence of vascular co-morbidity, a possible explanation is that there is reduced vascular compliance accompanied by elevated arterial blood pressure (ABP) wave amplitudes and even altered cardiac output (CO). To investigate this possibility, the present study was undertaken to continuously monitor CO to determine if it is correlated to ABP and ICP wave amplitudes and the outcome of shunting in iNPH patients. It was specifically addressed whether the increased ICP wave amplitudes seen in iNPH shunt responders were accompanied by elevated CO and/or ABP wave amplitude levels.</p> <p>Methods</p> <p>Prospective iNPH patients (29) were clinically graded using an NPH grading scale. Continuous overnight minimally-invasive monitoring of CO and ABP was done simultaneously with ICP monitoring; the CO, ABP, and ICP parameters were parsed into 6-second time windows. Patients were assessed for shunt surgery on clinical grade, Evan's index, and ICP wave amplitude. Follow-up clinical grading was performed 12 months after surgery.</p> <p>Results</p> <p>ICP wave amplitudes but not CO or ABP wave amplitude, showed good correlation with the response to shunt treatment. The patients with high ICP wave amplitude did not have accompanying high levels of CO or ABP wave amplitude. Correlation analysis between CO and ICP wave amplitudes in individual patients showed different profiles [significantly positive in 10 (35%) and significantly negative in 16 (55%) of 29 recordings]. This depended on whether there was also a correlation between ABP and ICP wave amplitudes and on the average level of ICP wave amplitude.</p> <p>Conclusions</p> <p>These results gave no evidence that the increased levels of ICP wave amplitudes seen in iNPH shunt responders prior to surgery were accompanied by elevated levels of ABP wave amplitudes or elevated CO. In the individual patients the correlation between CO and ICP wave amplitude was partly related to an association between ABP and ICP wave amplitudes which can be indicative of the state of cerebrovascular pressure regulation, and partly related to the ICP wave amplitude which can be indicative of the intracranial compliance.</p
Superconformal Yang-Mills quantum mechanics and Calogero model with OSp(N|2,R) symmetry
In spacetime dimension two, pure Yang-Mills possesses no physical degrees of
freedom, and consequently it admits a supersymmetric extension to couple to an
arbitrary number, N say, of Majorana-Weyl gauginos. This results in (N,0) super
Yang-Mills. Further, its dimensional reduction to mechanics doubles the number
of supersymmetries, from N to N+N, to include conformal supercharges, and leads
to a superconformal Yang-Mills quantum mechanics with symmetry group
OSp(N|2,R). We comment on its connection to AdS_2 \times S^{N-1} and reduction
to a supersymmetric Calogero model.Comment: 1+28 pages, no figure; Refs added. To appear in JHE
Inter-slice blood flow and magnetization transfer effects as a new simultaneous imaging strategy
The recent blood flow and magnetization transfer (MT) technique termed alternate ascending/ descending directional navigation (ALADDIN) achieves the contrast using interslice blood flow and MT effects with no separate preparation RF pulse, thereby potentially overcoming limitations of conventional methods. In this study, we examined the signal characteristics of ALADDIN as a simultaneous blood flow and MT imaging strategy, by comparing it with pseudo-continuous ASL (pCASL) and conventional MT asymmetry (MTA) methods, all of which had the same bSSFP readout. Bloch-equation simulations and experiments showed ALADDIN perfusion signals increased with flip angle, whereas MTA signals peaked at flip angle around 45°-60°. ALADDIN provided signals comparable to those of pCASL and conventional MTA methods emulating the first, second, and third prior slices of ALADDIN under the same scan conditions, suggesting ALADDIN signals to be superposition of signals from multiple labeling planes. The quantitative cerebral blood flow signals from a modified continuous ASL model overestimated the perfusion signals compared to those measured with a pulsed ASL method. Simultaneous mapping of blood flow, MTA, and MT ratio in the whole brain is feasible with ALADDIN within a clinically reasonable time, which can potentially help diagnosis of various diseases
Coherent quantum state storage and transfer between two phase qubits via a resonant cavity
A network of quantum-mechanical systems showing long lived phase coherence of
its quantum states could be used for processing quantum information. As with
classical information processing, a quantum processor requires information bits
(qubits) that can be independently addressed and read out, long-term memory
elements to store arbitrary quantum states, and the ability to transfer quantum
information through a coherent communication bus accessible to a large number
of qubits. Superconducting qubits made with scalable microfabrication
techniques are a promising candidate for the realization of a large scale
quantum information processor. Although these systems have successfully passed
tests of coherent coupling for up to four qubits, communication of individual
quantum states between qubits via a quantum bus has not yet been demonstrated.
Here, we perform an experiment demonstrating the ability to coherently transfer
quantum states between two superconducting Josephson phase qubits through a
rudimentary quantum bus formed by a single, on chip, superconducting
transmission line resonant cavity of length 7 mm. After preparing an initial
quantum state with the first qubit, this quantum information is transferred and
stored as a nonclassical photon state of the resonant cavity, then retrieved at
a later time by the second qubit connected to the opposite end of the cavity.
Beyond simple communication, these results suggest that a high quality factor
superconducting cavity could also function as a long term memory element. The
basic architecture presented here is scalable, offering the possibility for the
coherent communication between a large number of superconducting qubits.Comment: 17 pages, 4 figures (to appear in Nature
Ripple Texturing of Suspended Graphene Atomic Membranes
Graphene is the nature's thinnest elastic membrane, with exceptional
mechanical and electrical properties. We report the direct observation and
creation of one-dimensional (1D) and 2D periodic ripples in suspended graphene
sheets, using spontaneously and thermally induced longitudinal strains on
patterned substrates, with control over their orientations and wavelengths. We
also provide the first measurement of graphene's thermal expansion coefficient,
which is anomalously large and negative, ~ -7x10^-6 K^-1 at 300K. Our work
enables novel strain-based engineering of graphene devices.Comment: 15 pages, 4 figure
Electrical generation and absorption of phonons in carbon nanotubes
The interplay between discrete vibrational and electronic degrees of freedom
directly influences the chemical and physical properties of molecular systems.
This coupling is typically studied through optical methods such as
fluorescence, absorption, and Raman spectroscopy. Molecular electronic devices
provide new opportunities for exploring vibration-electronic interactions at
the single molecule level. For example, electrons injected from a scanning
tunneling microscope tip into a metal can excite vibrational excitations of a
molecule in the gap between tip and metal. Here we show how current directly
injected into a freely suspended individual single-wall carbon nanotube can be
used to excite, detect, and control a specific vibrational mode of the
molecule. Electrons inelastically tunneling into the nanotube cause a
non-equilibrium occupation of the radial breathing mode, leading to both
stimulated emission and absorption of phonons by successive electron tunneling
events. We exploit this effect to measure a phonon lifetime on the order of 10
nanoseconds, corresponding to a quality factor well over 10000 for this
nanomechanical oscillator.Comment: 17 pages, 4 figure
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