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DSpace at MIT (77,621 recursos)
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MIT Open Access Articles

Mostrando recursos 41 - 60 de 13,927

41. Trion-Induced Negative Photoconductivity in Monolayer MoS[subscript 2] - Lee, Y.-H.; Ling, X.; Akselrod, G. M.; Lui, Chun Hung; Frenzel, Alex James; Pilon, Daniel Victor; Kong, Jing; Gedik, Nuh; Akselrod, G. M.
Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We, however, observe a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS[subscript 2]), a two-dimensional (2D) semiconductor, using ultrafast optical-pump terahertz-probe spectroscopy. In particular, the conductivity is reduced to only 30% of its equilibrium value at high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the 2D system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the conductivity.

42. Prolonged dopamine signalling in striatum signals proximity and value of distant rewards - Howe, Mark W.; Sandberg, Stefan G.; Phillips, Paul E. M.; Graybiel, Ann M.; Tierney, Patrick
Predictions about future rewarding events have a powerful influence on behaviour. The phasic spike activity of dopamine-containing neurons, and corresponding dopamine transients in the striatum, are thought to underlie these predictions, encoding positive and negative reward prediction errors. However, many behaviours are directed towards distant goals, for which transient signals may fail to provide sustained drive. Here we report an extended mode of reward-predictive dopamine signalling in the striatum that emerged as rats moved towards distant goals. These dopamine signals, which were detected with fast-scan cyclic voltammetry (FSCV), gradually increased or—in rare instances—decreased as the animals navigated mazes to reach...

43. Optogenetic Stimulation of Lateral Orbitofronto-Striatal Pathway Suppresses Compulsive Behaviors - Burguiere, Eric; Monteiro, Patricia; Feng, Guoping; Graybiel, Ann M.
Dysfunctions in frontostriatal brain circuits have been implicated in neuropsychiatric disorders, including those characterized by the presence of repetitive behaviors. We developed an optogenetic approach to block repetitive, compulsive behavior in a mouse model in which deletion of the synaptic scaffolding gene, Sapap3, results in excessive grooming. With a delay-conditioning task, we identified in the mutants a selective deficit in behavioral response inhibition and found this to be associated with defective down-regulation of striatal projection neuron activity. Focused optogenetic stimulation of the lateral orbitofrontal cortex and its terminals in the striatum restored the behavioral response inhibition, restored the defective down-regulation,...

44. Localized microstimulation of primate pregenual cingulate cortex induces negative decision-making - Amemori, Ken-ichi; Graybiel, Ann M.
The pregenual anterior cingulate cortex (pACC) has been implicated in human anxiety disorders and depression, but the circuit-level mechanisms underlying these disorders are unclear. In healthy individuals, the pACC is involved in cost-benefit evaluation. We developed a macaque version of an approach-avoidance decision task used to evaluate anxiety and depression in humans and, with multi-electrode recording and cortical microstimulation, we probed pACC function as monkeys performed this task. We found that the macaque pACC has an opponent process-like organization of neurons representing motivationally positive and negative subjective value. Spatial distribution of these two neuronal populations overlapped in the pACC, except...

45. Recursive FMP for distributed inference in Gaussian graphical models - Liu, Ying; Willsky, Alan S.
For inference in Gaussian graphical models with cycles, loopy belief propagation (LBP) performs well for some graphs, but often diverges or has slow convergence. When LBP does converge, the variance estimates are incorrect in general. The feedback message passing (FMP) algorithm has been proposed to enhance the convergence and accuracy of inference. In FMP, standard LBP is run twice on the subgraph excluding the pseudo-FVS (a set of nodes that breaks most crucial cycles) while nodes in the pseudo-FVS use a different protocol. In this paper, we propose recursive FMP, a purely distributed extension of FMP, where all nodes use...

46. Sampling from Gaussian graphical models using subgraph perturbations - Liu, Ying; Kosut, Oliver; Willsky, Alan S.
The problem of efficiently drawing samples from a Gaussian graphical model or Gaussian Markov random field is studied. We introduce the subgraph perturbation sampling algorithm, which makes use of any pre-existing tractable inference algorithm for a subgraph by perturbing this algorithm so as to yield asymptotically exact samples for the intended distribution. The subgraph can have any structure for which efficient inference algorithms exist: for example, tree-structured, low tree-width, or having a small feedback vertex set. The experimental results demonstrate that this subgraph perturbation algorithm efficiently yields accurate samples for many graph topologies.

47. Realistic litz wire characterization using fast numerical simulations - Zhang, Richard Y.; White, Jacob K.; Kassakian, John G.; Sullivan, Charles R.
The losses of realistic litz wires are characterized while explicitly accounting for their construction, using a procedure that computes the current-driven and magnetic-field-driven copper losses using fast numerical simulations. We present a case study that examines loss variation in one- and two-level litz wires as a function of twisting pitch, over a wide range of values and in small increments. Experimental confirmation is presented for predictions made by numerical simulations. Results confirm the capability and efficiency of numerical methods to provide valuable insights into the realistic construction of litz wire.

48. Optimizing Electrode Configuration for Electrical Impedance Measurements of Muscle via the Finite Element Method - Jafarpoor, Mina; Jia Li, Mina; White, Jacob K.; Rutkove, Seward B.
Electrical impedance myography (EIM) is a technique for the evaluation of neuromuscular diseases, including amyotrophic lateral sclerosis and muscular dystrophy. In this study, we evaluated how alterations in the size and conductivity of muscle and thickness of subcutaneous fat impact the EIM data, with the aim of identifying an optimized electrode configuration for EIM measurements. Finite element models were developed for the human upper arm based on anatomic data; material properties of the tissues were obtained from rat and published sources. The developed model matched the frequency-dependent character of the data. Of the three major EIM parameters, resistance, reactance, and...

49. Systematic parameter estimation in data-rich environments for cell signalling dynamics - White, Jacob K.; Nim, Tri Hieu; Luo, Le; Clement, Marie-Veronique; Tucker-Kellogg, Lisa
Motivation: Computational models of biological signalling networks, based on ordinary differential equations (ODEs), have generated many insights into cellular dynamics, but the model-building process typically requires estimating rate parameters based on experimentally observed concentrations. New proteomic methods can measure concentrations for all molecular species in a pathway; this creates a new opportunity to decompose the optimization of rate parameters. Results: In contrast with conventional parameter estimation methods that minimize the disagreement between simulated and observed concentrations, the SPEDRE method fits spline curves through observed concentration points, estimates derivatives and then matches the derivatives to the production and consumption of each species....

50. 2DEG electrodes for piezoelectric transduction of AlGaN/GaN MEMS resonators - Weinstein, Dana; Popa, Laura Cornelia
A 2D electron gas (2DEG) interdigitated transducer (IDT) in Gallium Nitride (GaN) resonators is introduced and demonstrated. This metal-free transduction does not suffer from the loss mechanisms associated with more commonly used metal electrodes. As a result, this transducer can be used for both the direct interrogation of GaN electromechanical properties and the realization of high Q resonators. A 1.2 GHz bulk acoustic resonator with mechanical Q of 1885 is demonstrated, with frequency quality factor product (f·Q) of 2.3×10[superscript 12], the highest measured in GaN to date.

51. Switchable piezoelectric transduction in AlGaN/GaN MEMS resonators - Weinstein, Dana; Popa, Laura Cornelia
This work presents a new switching mechanism in piezoelectric transduction of AlGaN/GaN bulk acoustic resonators. A piezoelectric transducer is formed in the AlGaN, between a top Schottky electrode and a 2D electron gas (2DEG) as a second electrode. In the off state, this 2DEG can be depleted by applying a negative bias to the top electrode, suppressing transduction and reducing capacitive loading. Switchable AlGaN/GaN resonators are demonstrated from 240MHz to 3.5GHz with frequency-quality factor products up to 1.7×10[superscript 12] in air. Switching is shown in both devices with passive piezoelectric and HEMT sensing, with >19dB suppression in the off state.

52. An ultralow power athermal silicon modulator - Timurdogan, Erman; Sorace-Agaskar, Cheryl M.; Sun, Jie; Shah Hosseini, Ehsan; Biberman, Aleksandr; Watts, Michael
Silicon photonics has emerged as the leading candidate for implementing ultralow power wavelength–division–multiplexed communication networks in high-performance computers, yet current components (lasers, modulators, filters and detectors) consume too much power for the high-speed femtojoule-class links that ultimately will be required. Here we demonstrate and characterize the first modulator to achieve simultaneous high-speed (25 Gb s[superscript −1]), low-voltage (0.5 V[subscript PP]) and efficient 0.9 fJ per bit error-free operation. This low-energy high-speed operation is enabled by a record electro-optic response, obtained in a vertical p–n junction device that at 250 pm V[superscript −1] (30 GHz V[superscript −1]) is up to 10 times larger than prior demonstrations. In addition, this...

53. Matrix Remodeling Maintains ESC Self-Renewal by Activating Stat3 - Przybyla, Laralynne M.; Theunissen, Thorold W.; Jaenisch, Rudolf; Voldman, Joel
While a variety of natural and synthetic matrices have been used to influence embryonic stem cell (ESC) self-renewal or differentiation, and ESCs also deposit a rich matrix of their own, the mechanisms behind how extracellular matrix affects cell fate are largely unexplored. The ESC matrix is continuously remodeled by matrix metalloproteinases (MMPs), a process that we find is enhanced by the presence of mouse embryonic fibroblast feeders in a paracrine manner. Matrix remodeling by MMPs aids in the self-renewal of ESCs, as inhibition of MMPs inhibits the ability of ESCs to self-renew. We also find that addition of the interstitial...

54. Electrokinetic confinement of axonal growth for dynamically configurable neural networks - Honegger, Thibault; Scott, Mark A.; Voldman, Joel; Yanik, Mehmet Fatih
Axons in the developing nervous system are directed via guidance cues, whose expression varies both spatially and temporally, to create functional neural circuits. Existing methods to create patterns of neural connectivity in vitro use only static geometries, and are unable to dynamically alter the guidance cues imparted on the cells. We introduce the use of AC electrokinetics to dynamically control axonal growth in cultured rat hippocampal neurons. We find that the application of modest voltages at frequencies on the order of 10[superscript 5] Hz can cause developing axons to be stopped adjacent to the electrodes while axons away from the...

55. Image-Predicated Sorting of Adherent Cells Using Photopatterned Hydrogels - Kovac, Joseph; Gerardin, Ylaine; Voldman, Joel
Using photopatterned hydrogels and selective cell encapsulation, populations of adherent cells are examined using microscopy and sorted into viable sub-populations predicated on their imaged phenotypes. The inexpensive method utilizes commercial reagents and equipment available in many labs, making image-predicated cell sorting an accessible technique for a large number of individual labs.

56. Probing Embryonic Stem Cell Autocrine and Paracrine Signaling Using Microfluidics - Przybyla, Laralynne; Voldman, Joel
Although stem cell fate is traditionally manipulated by exogenously altering the cells' extracellular signaling environment, the endogenous autocrine and paracrine signals produced by the cells also contribute to their two essential processes: self-renewal and differentiation. Autocrine and/or paracrine signals are fundamental to both embryonic stem cell self-renewal and early embryonic development, but the nature and contributions of these signals are often difficult to fully define using conventional methods. Microfluidic techniques have been used to explore the effects of cell-secreted signals by controlling cell organization or by providing precise control over the spatial and temporal cellular microenvironment. Here we review how...

57. Design and Fabrication of Ultralight Weight, Adjustable Multi-electrode Probes for Electrophysiological Recordings in Mice - Brunetti, Philip M.; Wimmer, Ralf D.; Liang, Li; Siegle, Joshua H.; Voigts, Jakob; Halassa, Michael M.; Wilson, Matthew A.
The number of physiological investigations in the mouse, mus musculus, has experienced a recent surge, paralleling the growth in methods of genetic targeting for microcircuit dissection and disease modeling. The introduction of optogenetics, for example, has allowed for bidirectional manipulation of genetically-identified neurons, at an unprecedented temporal resolution. To capitalize on these tools and gain insight into dynamic interactions among brain microcircuits, it is essential that one has the ability to record from ensembles of neurons deep within the brain of this small rodent, in both head-fixed and freely behaving preparations. To record from deep structures and distinct cell layers...

58. Emergent Majorana mass and axion couplings in superfluids - Wilczek, Frank
Axions (in the general sense) may acquire qualitatively new couplings inside superfluids. Their conventional couplings to fermions, in empty space, involve purely imaginary masses; the new couplings involve emergent Majorana masses. A generalized concept of Majorana mass is proposed, which allows continuous interpolation between Majorana and Dirac fermions, and also extension to bosons. The possibility of weak links for axions, recently put forward, is analyzed and replaced with a non-local analogue.

59. Coherent feedback that beats all measurement-based feedback protocols - Jacobs, Kurt; Wang, Xiaoting; Wiseman, Howard M.
We show that when the speed of control is bounded, there is a widely applicable minimal-time control problem for which a coherent feedback protocol is optimal, and is faster than all measurement-based feedback protocols, where the latter are defined in a strict sense. The superiority of the coherent protocol is due to the fact that it can exploit a geodesic path in Hilbert space, a path that measurement-based protocols cannot follow.

60. Single particle tracking reveals spatial and dynamic organization of the Escherichia coli biofilm matrix - Birjiniuk, Alona; Nance, Elizabeth; Hanes, Justin; Ribbeck, Katharina; Billings, Amanda Nicole; Doyle, Patrick S.
Biofilms are communities of surface-adherent bacteria surrounded by secreted polymers known as the extracellular polymeric substance. Biofilms are harmful in many industries, and thus it is of great interest to understand their mechanical properties and structure to determine ways to destabilize them. By performing single particle tracking with beads of varying surface functionalization it was found that charge interactions play a key role in mediating mobility within biofilms. With a combination of single particle tracking and microrheological concepts, it was found that Escherichia coli biofilms display height dependent charge density that evolves over time. Statistical analyses of bead trajectories and...

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