570 research outputs found
Random Strain Induced Correlations in Materials with Intertwined Nematic and Magnetic Orders
Electronic nematicity is rarely observed as an isolated instability of a correlated electron system. Instead, in iron pnictides and in certain cuprates and heavy-fermion materials, nematicity is intertwined with an underlying spin-stripe or charge-stripe state. As a result, random strain, ubiquitous in any real crystal, creates both random-field disorder for the nematic degrees of freedom and random-bond disorder for the spin or charge ones. Here, we put forward an Ashkin-Teller model with random Baxter fields to capture the dual role of random strain in nematic systems for which nematicity is a composite order arising from a stripe state. Using Monte Carlo to simulate this random Baxter-field model, we find not only the expected breakup of the system into nematic domains, but also the emergence of nontrivial disorder-promoted magnetic correlations. Such correlations enhance and tie up the fluctuations associated with the two degenerate magnetic stripe states from which nematicity arises, leaving characteristic signatures in the spatial profile of the magnetic domains, in the configurational space of the spin variables, and in the magnetic noise spectrum. We discuss possible experimental manifestations of these effects in iron-pnictide superconductors. Our work establishes the random Baxter-field model as a more complete alternative to the random-field Ising model to describe complex electronic nematic phenomena in the presence of disorder
The Vice President-More than an Afterthought?
A round-table discussion among former U.S. Vice President Richard B. Cheney, Caruso Family Professor of Law and retired U.S. Ambassador Douglas Kmiec, and former U.S. Attorney General Edwin Meese III considered the practical implications of conceiving the Vice President as a legislative officer, an executive officer, or both. It was noted that until the second half of the twentieth century, the Office of the Vice President was conceived as legislative. Funding for the Office appeared in budget lines relating to Congress and physically, the Vice President’s office was in the Capitol. Beginning with Walter Mondale’s service as Vice President, presidents have been delegating increasing executive authority, seeing the Vice President as a “deputy president.” Perhaps the most aggressive and influential of the modern “deputy presidents” was Vice President Cheney himself. Attorney General Meese concurred and saw this as positive. Ambassador Kmiec was less approving, encouraging Vice President Cheney and Attorney General Meese to contemplate the benefits that a dual-natured legislative–executive Vice President supplies to maintaining a workable government. The capacity of the Vice President to assert independence, as late Justice Scalia explained in an Office of Legal Counsel opinion, is unique. Unlike members of the Cabinet, the Vice President is not removable by the President, and thus, the Vice President can use his dual nature to advance executive–legislative compromise. Vice President Cheney’s reliance upon his significant, but personal, legislative experience prior to his vice presidency to facilitate executive–legislative bargaining suggests qualities that presidential nominees might consider more directly in vice presidential selection, and not just geographic complementarity and ideological compatibility. While it has been commonplace to think of the vice presidential office as “an afterthought” borrowed from state charters at the time of the founding, this dialogue suggests how a vice president with a foot in each of the Legislative and Executive Branches can assist in overcoming dysfunctional periods when partisan division is great
The Dominion Range Ice Core, Queen Maud Mountains, Antarctica - General Site and Core Characteristics with Implications
The Transantarctic Mountains of East Antarctica provide a new milieu for retrieval of ice-core records. We report here on the initial findings from the first of these records, the Dominion Range ice-core record. Sites such as the Dominion Range are valuable for the recovery of records detailing climate change, volcanic activity, and changes in the chemistry of the atmosphere. The unique geographic location of this site and a relatively low accumulation rate combine to provide a relatively long record of change for this potentially sensitive climatic region. As such, information concerning the site and general core characteristics are presented, including ice surface, ice thickness, bore-hole temperature, mean annual net accumulation, crystal size, crystal fabric, oxygen-isotope composition, and examples of ice chemistry and isotopic composition of trapped gases
Pressure studies of impurity levels in AlxGa1-xAs
doi: 10.1088/0268-1242/4/4/033The authors present a study of the deep and shallow donor levels under hydrostatic pressure. The shallow levels follow the conduction bands, while the deep levels are strongly sublinear with pressure. The temperature dependence of the intensities and energies is used to obtain an energy level diagram of the deep levels at high pressures.This work was supported by theU S Army under grant number DAAL03-86K-0083, the US Department of Energy under grant number DE-AC02 84ER45048, and Amoco Corporation. M Chandrasekhar is a n A P Sloan Foundation Fellow
On the Nature of Memory and Rejuvenation in Glassy Systems
The memory effect in a single crystal spin glass
() has been measured using ac susceptibility techniques over a reduced temperature range of
and a model of the memory effect has been developed. A
double-waiting-time protocol is carried out where the spin glass is first
allowed to age at a temperature below , followed by a second aging at a
lower temperature after it has fully rejuvenated. The model is based on
calculating typical coincidences between the growth of correlated regions at
the two temperatures. It accounts for the absolute magnitude of the memory
effect as a function of both waiting times and temperatures. The data can be
explained by the memory loss being a function of the relative change in the
correlated volume at the first waiting temperature because of the growth in the
correlations at the second waiting temperature.Comment: 11 pages, 6 figure
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Development of a High Level Waste Tank Inspection System
The Westinghouse Savannah River Technology Center was requested by it`s sister site, West Valley Nuclear Service (WVNS), to develop a remote inspection system to gather wall thickness readings of their High Level Waste Tanks. WVNS management chose to take a proactive approach to gain current information on two tanks t hat had been in service since the early 70`s. The tanks contain high level waste, are buried underground, and have only two access ports to an annular space between the tank and the secondary concrete vault. A specialized remote system was proposed to provide both a visual surveillance and ultrasonic thickness measurements of the tank walls. A magnetic wheeled crawler was the basis for the remote delivery system integrated with an off-the-shelf Ultrasonic Data Acquisition System. A development program was initiated for Savannah River Technology Center (SRTC) to design, fabricate, and test a remote system based on the Crawler. The system was completed and involved three crawlers to perform the needed tasks, an Ultrasonic Crawler, a Camera Crawler, and a Surface Prep Crawler. The crawlers were computer controlled so that their operation could be done remotely and their position on the wall could be tracked. The Ultrasonic Crawler controls were interfaced with ABB Amdata`s I-PC, Ultrasonic Data Acquisition System so that thickness mapping of the wall could be obtained. A second system was requested by Westinghouse Savannah River Company (WSRC), to perform just ultrasonic mapping on their similar Waste Storage Tanks; however, the system needed to be interfaced with the P-scan Ultrasonic Data Acquisition System. Both remote inspection systems were completed 9/94. Qualifications tests were conducted by WVNS prior to implementation on the actual tank and tank development was achieved 10/94. The second inspection system was deployed at WSRC 11/94 with success, and the system is now in continuous service inspecting the remaining high level waste tanks at WSRC
Disorder-induced local strain distribution in Y-substituted TmVO4
We report an investigation of the effect of substitution of Y for Tm in
via low-temperature heat capacity measurements, with the
yttrium content varying from to . Because the Tm ions support a
local quadrupolar (nematic) moment, they act as reporters of the local strain
state in the material, with the splitting of the ion's non-Kramers crystal
field groundstate proportional to the quadrature sum of the in-plane tetragonal
symmetry-breaking transverse and longitudinal strains experienced by each ion
individually. Analysis of the heat capacity therefore provides detailed
insights into the distribution of local strains that arise as a consequence of
the chemical substitution. These local strains suppress long-range quadrupole
order for , and result in a broad Schottky-like feature for higher
concentrations. Heat capacity data are compared to expectations for a
distribution of uncorrelated (random) strains. For dilute Tm concentrations,
the heat capacity cannot be accounted for by randomly distributed strains,
demonstrating the presence of significant strain correlations between sites.
For intermediate Tm concentrations, these correlations must still exist, but
the data cannot be distinguished from that which would be obtained from a 2D
Gaussian distribution. The cross-over between these limits is discussed in
terms of the interplay of key length scales in the substituted material. The
central result of this work, that local strains arising from chemical
substitution are not uncorrelated, has implications for the range of validity
of theoretical models based on random effective fields that are used to
describe such chemically substituted materials, particularly when electronic
nematic correlations are present
Signatures of Z Vestigial Potts-nematic order in van der Waals antiferromagnets
Layered van der Waals magnets have attracted much recent attention as a
promising and versatile platform for exploring intrinsic two-dimensional
magnetism. Within this broader class, the transition metal phosphorous
trichalcogenides P stand out as particularly interesting, as they
provide a realization of honeycomb lattice magnetism and are known to display a
variety of magnetic ordering phenomena as well as superconductivity under
pressure. One example, found in a number of different materials, is
commensurate single- zigzag antiferromagnetic order, which spontaneously
breaks the spatial threefold rotation symmetry of the honeycomb
lattice. The breaking of multiple distinct symmetries in the magnetic phase
suggests the possibility of a sequence of distinct transitions as a function of
temperature, and a resulting intermediate -nematic phase which
exists as a paramagnetic vestige of zigzag magnetic order -- a scenario known
as vestigial ordering. Here, we report the observation of key signatures of
vestigial Potts-nematic order in rhombohedral FePSe. By performing linear
dichroism imaging measurements -- an ideal probe of rotational symmetry
breaking -- we find that the symmetry is already broken above the N\'eel
temperature. We show that these observations are explained by a general
Ginzburg-Landau model of vestigial nematic order driven by magnetic
fluctuations and coupled to residual strain. An analysis of the domain
structure as temperature is lowered and a comparison with zigzag-ordered
monoclinic FePS reveals a broader applicability of the Ginzburg-Landau
model in the presence of external strain, and firmly establishes the P
magnets as a new experimental venue for studying the interplay between
Potts-nematicity, magnetism and superconductivity.Comment: 6 pages, 4 figures + supplementary materia
Sublocalization of the gene encoding manganese superoxide dismutase (MnSOD/SOD2) to 6q25 by fluorescence in Situ hybridization and somatic cell hybrid mapping
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29772/1/0000110.pd
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