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Zhao, X.

Publications and source records attributed to Zhao, X..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Computational design of highly signalling-active membrane receptors through solvent-mediated allosteric networks

Abstract Protein catalysis and allostery require the atomic-level orchestration and motion of residues and ligand, solvent and protein effector molecules. However, the ability to design protein activity through precise protein–solvent cooperative interactions has not yet been demonstrated. Here we report the design of 14 membrane receptors that catalyse G protein nucleotide exchange through diverse engineered allosteric pathways mediated by cooperative networks of intraprotein, protein–ligand and –solvent molecule interactions. Consistent with predictions, the designed protein activities correlated well with the level of plasticity of the networks at flexible transmembrane helical interfaces. Several designs displayed considerably enhanced thermostability and activity compared with related natural receptors. The most stable and active variant crystallized in an unforeseen signalling-active conformation, in excellent agreement with the design models. The allosteric network topologies of the best designs bear limited similarity to those of natural receptors and reveal an allosteric interaction space larger than previously inferred from natural proteins. The approach should prove useful for engineering proteins with novel complex protein binding, catalytic and signalling activities.

Chemistry↗
DOE OSTI2022

The level-1 trigger for the SuperCDMS experiment at SNOLAB

The SuperCDMS SNOLAB dark matter search experiment aims to be sensitive to energy depositions down to Script O(1 eV). This imposes requirements on the resolution, signal efficiency, and noise rejection of the trigger system. To accomplish this, the SuperCDMS level-1 trigger system is implemented in an FPGA on a custom PCB. A time-domain optimal filter algorithm realized as a finite impulse response filter provides a baseline resolution of 0.38 times the standard deviation of the noise, σn, and a 99.9% trigger efficiency for signal amplitudes of 1.1 σ n in typical noise conditions. Embedded in a modular architecture, flexible trigger logic enables reliable triggering and vetoing in a dead-time-free manner for a variety of purposes and run conditions. The trigger architecture and performance are detailed in this article.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2021

Quantum spin state transitions in the spin-1 equilateral triangular lattice antiferromagnet Na 2 BaNi(PO 4 ) 2

We report we have grown single crystals of Na 2 BaNi(PO 4 ) 2 , a spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat, and thermal conductivity measurements at ultralow temperatures. The main results are as follows: (i) At zero magnetic field, Na 2 BaNi(PO 4 ) 2 exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $\textit{c}$ axis. This suggests a canted 120° spin structure, which is in a plane including the crystallographic $\textit{c}$ axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the $\textit{c}$ axis. (ii) With increasing field along the $\textit{c}$ axis, a 1/3 magnetization plateau is observed, which means that the canted 120° spin structure is transformed into an up-up-down (UUD) spin structure. With even higher fields, the UUD phase further evolves into possible V and V' phases. (iii) With increasing field along the $\textit{a}$ axis, the canted 120° spin structure is possibly transformed into an umbrella phase and a V phase. Therefore Na 2 BaNi(PO 4 ) 2 is a rare example of a spin-1 ETLAF with single-crystalline form that exhibits easy-axis spin anisotropy and a series of quantum spin state transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Survival of itinerant excitations and quantum spin state transitions in YbMgGaO 4 with chemical disorder

A recent focus of quantum spin liquid (QSL) studies is how disorder/randomness in a QSL candidate affects its true magnetic ground state. The ultimate question is whether the QSL survives disorder or the disorder leads to a “spin-liquid-like” state, such as the proposed random-singlet (RS) state. Since disorder is a standard feature of most QSL candidates, this question represents a major challenge for QSL candidates. YbMgGaO 4 , a triangular lattice antiferromagnet with effective spin-1/2 Yb 3+ ions, is an ideal system to address this question, since it shows no long-range magnetic ordering with Mg/Ga site disorder. Despite the intensive study, it remains unresolved as to whether YbMgGaO 4 is a QSL or in the RS state. Here, through ultralow-temperature thermal conductivity and magnetic torque measurements, plus specific heat and DC magnetization data, we observed a residual κ 0 /T term and series of quantum spin state transitions in the zero temperature limit for YbMgGaO 4 . These observations strongly suggest that a QSL state with itinerant excitations and quantum spin fluctuations survives disorder in YbMgGaO 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Thermodynamic non-ideality and disorder heterogeneity in actinide silicate solid solutions

Non-ideal thermodynamics of solid solutions can greatly impact materials degradation behavior. We have investigated an actinide silicate solid solution system (USiO 4 –ThSiO 4 ), demonstrating that thermodynamic non-ideality follows a distinctive, atomic-scale disordering process, which is usually considered as a random distribution. Neutron total scattering implemented by pair distribution function analysis confirmed a random distribution model for U and Th in first three coordination shells; however, a machine-learning algorithm suggested heterogeneous U and Th clusters at nanoscale (~2 nm). The local disorder and nanosized heterogeneous is an example of the non-ideality of mixing that has an electronic origin. Partial covalency from the U/Th 5f–O 2p hybridization promotes electron transfer during mixing and leads to local polyhedral distortions. The electronic origin accounts for the strong non-ideality in thermodynamic parameters that extends the stability field of the actinide silicates in nature and under typical nuclear waste repository conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Unconventional Hall response in the quantum limit of HfTe 5

Interacting electrons confined to their lowest Landau level in a high magnetic field can form a variety of correlated states, some of which manifest themselves in a Hall effect. Although such states have been predicted to occur in three-dimensional semimetals, a corresponding Hall response has not yet been experimentally observed. Here, we report the observation of an unconventional Hall response in the quantum limit of the bulk semimetal HfTe 5 , adjacent to the three-dimensional quantum Hall effect of a single electron band at low magnetic fields. The additional plateau-like feature in the Hall conductivity of the lowest Landau level is accompanied by a Shubnikov-de Haas minimum in the longitudinal electrical resistivity and its magnitude relates as 3/5 to the height of the last plateau of the three-dimensional quantum Hall effect. Our findings are consistent with strong electron-electron interactions, stabilizing an unconventional variant of the Hall effect in a three-dimensional material in the quantum limit.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2012

Global Network of Slow Solar Wind

The streamer belt region surrounding the heliospheric current sheet (HCS) is generally treated as the primary or sole source of the slow solar wind. Synoptic maps of solar wind speed predicted by the Wang-Sheeley-Arge model during selected periods of solar cycle 23, however, show many areas of slow wind displaced from the streamer belt. These areas commonly have the form of an arc that is connected to the streamer belt at both ends. The arcs mark the boundaries between fields emanating from different coronal holes of the same polarity and thus trace the paths of belts of pseudostreamers, i.e., unipolar streamers that form over double arcades and lack current sheets. The arc pattern is consistent with the predicted topological mapping of the narrow open corridor or singular separator line that must connect the holes and, thus, consistent with the separatrix-web model of the slow solar wind. Near solar maximum, pseudostreamer belts stray far from the HCS-associated streamer belt and, together with it, form a global-wide web of slow wind. Recognition of pseudostreamer belts as prominent sources of slow wind provides a new template for understanding solar wind stream structure, especially near solar maximum.

solar wind↗
NASA NTRS1996

Computerized Design and Generation of Gear Drives With a Localized Bearing Contact and a Low Level of Transmission Errors

A general approach developed for the computerized simulation of loaded gear drives is presented. In this paper the methodology used to localize the bearing contact, provide a parabolic function of transmission errors, and simulate meshing and contact of unloaded gear drives is developed. The approach developed is applied to spur and helical gears, spiral bevel gears, face-gear drives, and worm-gear drives with cylindrical worms.

Litvin, F.↗
NASA NTRS1996

Computerized Design and Analysis of Face-Milled, Uniform Tooth Height, Low-Noise Spiral Bevel Gear Drives

A new method for design and generation of spiral bevel gears of uniform tooth depth with localized bearing contact and low level of transmission errors is considered. The main features of the proposed approach are as follows: (1) the localization of the bearing contact is achieved by the mismatch of the generating surfaces. The bearing contact may be provided in the longitudinal direction, or in the direction across the surface; and (2) the low level of transmission errors is achieved due to application of nonlinear relations between the motions of the gear and the gear head-cutter. Such relations may be provided by application of a CNC machine. The generation of the pinion is based on application of linear relations between the motions of the tool and the pinion being generated. The relations described above permit a parabolic function of transmission errors to be obtained that is able to absorb almost linear functions caused by errors of gear alignment. A computer code has been written for the meshing and contact of the spiral bevel gears with the proposed geometry. The effect of misalignment on the proposed geometry has also been determined. Numerical examples for illustration of the proposed theory have been provided.

Litvin, F. L.↗
NASA NTRS1996

Computerized Design and Analysis of Face-Milled, Uniform Tooth Height, Low-Noise Spiral Bevel Gear Drives

A new method for design and generation of spiral bevel gears of uniform tooth depth with localized bearing contact and low level of transmission errors is considered. The main features of the proposed approach are as follows: (1) The localization of the bearing contact is achieved by the mismatch of the generating surfaces. The bearing contact may be provided in the longitudinal direction, or in the direction across the surface; and (2) The low level of transmission errors is achieved due to application of nonlinear relations between the motions of the gear and the gear head-cutter. Such relations may be provided by application of a CNC machine. The generation of the pinion is based on application of linear relations between the motions of the tool and the pinion being generated. The relations described above permit a parabolic function of transmission errors to be obtained that is able to absorb almost linear functions caused by errors of gear alignment. A computer code has been written for the meshing and contact of the spiral bevel gears with the proposed geometry. The effect of misalignment on the proposed geometry has also been determined. Numerical examples for illustration of the proposed theory have been provided.

Litvin, F. L.↗
NASA NTRS1995

The solar/interplanetary event of 14 April 1994 observed by Yohkoh/SXT

The polar crown event of April 14 1994 is one of the largest scale eruptive events observed by the Yohkoh/SXT. Associated with the formation of an arcade of soft X-ray loops at the Sun was the detection of an interplanetary forward/reverse shock event by the Ulysses spacecraft some 4-7 days later. The relationship between the coronal and interplanetary signatures of these events is important if we are to address fully the initialization and consequent acceleration of interplanetary phenomena, such as CMEs and counter-streaming electrons, originating at the Sun. From detailed analysis of the energetics of the arcade formed during the eruption of April 14 1994, we find peak temperatures and emission measures of approximately 5MK and approximately 10(exp 48)cm(exp -3) respectively. The total thermal content of the arcade loop structure observed in soft X-rays is calculated to be some 5 x 10(exp 29) ergs. The development of these parameters as the event proceeds and their relationship to the dynamics of the eruption are investigated. Although spanning a longitudinal range of some 150 degrees the April 14 event displayed the typical helmet streamer structure normally associated with coronal mass ejections These helmet streamers are thought to be related to the global solar magnetic field through the heliospheric current sheet (HCS). The arcade formation, together with the eruption of material into interplanetary space, signifies a large-scale reconfiguration of the coronal magnetic field. We examine the effects of the formation of such a coronal arcade structure on the HCS and discuss the dynamics involved with the passage of a large scale disturbance through the interplanetary magnetic field.

Alexander, D.↗