1,720,982 research outputs found
Low-temperature dielectric response of beta-alumina at 16 GHz
No full textMicrowave techniques are used to measure the dielectric response of M\sp+ beta alumina at 16 GHz for temperatures between 0.3 and 20K. (M\sp+ refers to the monovalent cations Na, K, Ag, or Li.) These measurements include cw observations of the absorption and the dispersion, and the recovery of the absorption signal following a high power pulse of microwaves. The cw measurements at 16 GHz are shown to exhibit qualitative features similar to those observed at audio frequencies (and at 10 GHz for Na) in beta alumina, which have been shown to be characteristic of highly disordered systems.In this temperature range the dielectric response in glasses is typically explained by modeling the structural disorder in terms of a broad energy independent distribution of localized atomic tunneling centers. In the case of beta alumina's unique structure this tunneling occurs within disordered planes separated by crystalline blocks of alumina. The tunneling theory has been used to explain both the frequency and temperature dependence of the observed dielectric response.We demonstrate that although the theory can be used to fit either the loss or dispersion at 16 GHz, they cannot be fit simultaneously with the same set of parameters. Since the theory predicts a linear and causal response, this indicates that the theoretical relationship between the absorption and the loss (based on the Kramers-Kronig relationships) is inconsistent with the observed results. Our measurements of the pulsed saturation recoveries, which probe the mechanism that mediates the dielectric response and therefore should clarify the problem, are shown to be inconclusive. They are, however, consistent with results obtained at radio frequencies which have been explained in terms of spectral diffusion. Finally, we demonstrate that the extension of the theory to the dielectric data above 0.5K is not supported by other low temperature properties for which the theory was originally created. This indicates either the theory needs to be modified or additional physical processes need to be included.Made available in DSpace on 2011-05-07T13:09:03Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9011031.pdf: 4791218 bytes, checksum: 2d61717d015a19f5d6a389b60ef013c2 (MD5)
Previous issue date: 1989Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:50:53Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:23:17-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Electron spin-lattice relaxation in ytterbium ion-doped silicate glass
No full text"Relaxation rates of Yb\sp{3+} ions incorporated in low concentrations into a host silicate glass have been measured using a pulse saturation/recovery technique at 9.5 GHz over the temperature range 1.5-7.0 K. Compared with similar measurements made on crystalline material, the temperature dependence of the recovery rates for the two-phonon Raman process is anomalously weak (T\sp6 instead of T\sp9). This anomaly suggests the need to modify the Debye density of states. Fractal models have been suggested for the thermal properties of glasses and for similarly anomalous spin relaxation behavior in proteins. An alternate model is proposed here that is supported by other measurements of low temperature thermal transport properties in glasses. The relaxation rates are interpreted in terms of a ""thermally effective"" density of states consisting of only low frequency extended vibrational modes. This is smaller than the total density of vibrational modes (extended and localized) measured, for example, by neutron scattering. An estimate for the localization frequency or crossover frequency between these two regimes can be extracted from fits to the lowest concentration data."Made available in DSpace on 2011-05-07T12:18:07Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9026326.pdf: 3627216 bytes, checksum: 1c91293924eee83199bb525bb60cfe82 (MD5)
Previous issue date: 1990Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:39:21Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:16:32-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Electron spin-lattice relaxation in two heme iron and two blue-copper proteins at liquid helium temperatures
No full textThe relaxation rates in frozen aqueous solutions of whale ferri-myoglobin azide, bovine ferri-hemoglobin azide, cupric azurin (P. aeruginosa) and cupric spinach plastocyanin were measured at 9.5 GHz using the pulse-saturation recovery method. Measurements covered a temperature range of 1.4 K to as high as 22 K, with corresponding relaxation rates up to 10\sp5/sec. Improvements in the equipment and the methods of analysis have enabled more stringent tests of the temperature dependence of the rates. In particular, several models proposed in the literature to explain the anomalous temperature dependence of the Raman rates in proteins are shown to be insufficient, including two fractal models. In addition, it is shown that any model based exclusively on the protein structure fails due to the diversity of the data under various solvent conditions. A general functional form consistent with a crossover in the vibrational properties is proposed instead, similar to the localization crossover in amorphous materials.The effect on the relaxation rate of several cosolvents and solutes is also examined. The effect on the direct process is much more pronounced than on the Raman region. The differences are shown to be consistent with changes in the velocity of sound at room temperature caused by the addition of cosolvents and solutes.Finally, the EPR recovery form is analyzed. We propose that the deviations in the recovery from an exponential form are due to a distribution of relaxation rates. The source of the distribution is most likely sample heating in the lower temperatures and a distribution of conformations frozen in near the paramagnetic site in the higher temperatures. It is not likely that it is caused by spin-spin interactions. The exact form of the distribution is unclear, but the most successful functional form for the recoveries is a stretched exponential with an exponent ranging from 0.5 to 1.0. However, a simple exponential fit to a limited portion of the recovery was the experimentally most viable and consistent method of extracting a rate.Made available in DSpace on 2011-05-07T12:35:05Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9114436.pdf: 10032254 bytes, checksum: 62d39169cc654a63a0d5040c03ea72d4 (MD5)
Previous issue date: 1990Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:42:55Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:18:43-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Electron spin-lattice relaxation in proteins, model heme complexes and ferricyanide solutions at helium temperatures
No full textElectron Spin-Lattice relaxation rates are reported for frozen solutions of the blue-copper proteins azurin and plastocyanin, the low-spin iron heme protein cytochrome-c, two (bis)imidazole ferric heme complexes in three different organic solvents and two ferricyanide solutions. Measurements were performed at X-band frequencies and temperatures between 1.4 and 22 K. Relaxation rates show a one-phonon direct process at low temperatures (T 4 K) with a temperature dependence that is approximately characterized by a simple power-law (T\sp{\rm n}), with fitted values of n between 4.9 and 7.45. The expected temperature dependence of a two-phonon (Raman) process in simple crystalline systems has been demonstrated to follow a power-law (T\sp{\rm n}), with n = 9.0. The anomalously weak temperature dependence for protein systems has been tentatively explained in terms of a fractal model of protein dynamics, where the temperature exponent is n = 4q + 2d - 1 and q = dd\sb{\varphi}/D. The localized vibrations (fractons) are characterized by a localization exponent d\sb{\varphi} on an underlying fractal lattice with Hausdorf dimension D and the fracton density of states with a spectral dimension d. A wide variation in the fitted n-values for different solvent conditions of the protein solutions suggests that it may not be possible to conclusively verify the fractal model as it is formulated. Studies of heme complexes and ferricyanide solutions show that the protein backbone is not essential for the observation of an anomalous temperature dependence. Two phenomenological models of a phonon density of states in amorphous systems are discussed and compared to relevant length scales.Made available in DSpace on 2011-05-07T13:15:26Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9114226.pdf: 6489651 bytes, checksum: 746bc57c9764b115b677ee565e4050d4 (MD5)
Previous issue date: 1990Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:52:22Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:24:06-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Anisotropic spin-lattice relaxation times of trivalent rare earth ions in anhydrous lanthanum trichloride
No full text"Measurements of the temperature and angular dependence
oft,he spin-lattice relaxation time Tl have been made for
cerium, praseodymium, neodymium, samarium, terbium, and erbium
,
in single crystals of anhydrous lanthanum tric~loride. The
experiments were performed on samples cqntaining ~ 2 at.%
rpre earth doping using pulse-saturation techniques at
X-ba~d frequencies. Angular m~asurements at constant
temperature and microwave ~requency were made for.,magnetic
field orientations between 0° and 90° ~ith respect to the
crystal c-axis in""the temperature range 1065 to 402°K. The
""results are interpreted as involving an angular d~pendent
direct process and isotropic Orbach and Raman mechanisms.
They are compared with theoretically predicted rates obtainedfrom
our computer calculations which are ba~ed upon Orbach's
approach and require a knowledge of the d9t.atic c~stal field
param,aters""., Since the rare earth trichlorides have been the
oboject of much optical investigation, these parameters 'are
available for all the ioris we trea,\ except cerium. '. Except·
for the ions cerium and samarium~ nearly all the predicted
relaxation rate temperature dependencies agree with experimental
results within a factor of five or less~ Theoretical
treatments which involve the mixing of excited state wave
functions into the ground magnetic doublet through the
anisotropic Zeeman interaction are required to expla~n the
observed angular dependence.. The theoretical·anisotropies
are followed almost exactly in every case.. One of the ions,
praseodymium, exhibits a phonon limited direct process relaxation
rate which is interesting because the apparent hot
phonon equilibration time is about 13 nsec, corresponding to
an equilibration path length of only 26 microns. Measurements
made before and after a crystal was cleaved to half its
original thickness gave an unaltered relaxation rate. After
neutron bombardment the bottlenecked r~laxation rate changed
with the phonon equilibration time app~rently reduced to 8 nsec.
Prior irradiation of this same sample with 100 KeV x-rays
produced no observable change in the relaxation time."Submitted by Carolyn Mead ([email protected]) on 2011-05-19T18:19:11Z
No. of bitstreams: 1
1965_mikkelson.pdf: 6344226 bytes, checksum: 15863cbff51950ee5825e3573fa431f8 (MD5)Made available in DSpace on 2011-05-19T18:19:11Z (GMT). No. of bitstreams: 1
1965_mikkelson.pdf: 6344226 bytes, checksum: 15863cbff51950ee5825e3573fa431f8 (MD5)
Previous issue date: 1965Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead ([email protected]) on 2011-05-19T18:19:11Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:16:10-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ThesisThesisU of I Onl
Microwave studies of amorphous silicon
No full textElectron paramagnetic resonance and electron spin relaxation rates of the intrinsic paramagnetic center (g=2.0055) in amorphous silicon have been studied in the 0.3 -1.2 K temperature range. Various sample preparation techniques were used, including ion implantation, sputtering, arid vacuum evaporation. The temperature dependence of the spin lattice relaxation rates depends somewhat on sample preparation but is always very
close to simple T power laws. The n values observed in this study fall into two ranges: 2.09 -2.36 and 3.26 -3.47. Comparison of measurements at 9.3 GHz and 16.5 GHz indicates that the observed rates are independent
or very nearly independent of microwave frequency and applied magnetic field.
Conventional one and two phonon spin lattice relaxation mechanisms cannot account for the observed temperature dependences. A theory involving spin lattice relaxation by coupling to a distribution of two level systems (TLS) is presented. The theory is adjusted so that it can be applied in the relevant temperature range and its predictions are compared to the experimental results. An attempt is made to identify the TLS and the TLS-spin coupling using electron-nuclear double rescnance (ENDOR), microwave frequency dielectric absorption, and magnSubmitted by Carolyn Mead ([email protected]) on 2011-06-07T14:10:40Z
No. of bitstreams: 1
1984_askew.pdf: 4877314 bytes, checksum: e1c4b813ceef1b7595ef79be0628d1ed (MD5)Made available in DSpace on 2011-06-07T14:10:40Z (GMT). No. of bitstreams: 1
1984_askew.pdf: 4877314 bytes, checksum: e1c4b813ceef1b7595ef79be0628d1ed (MD5)
Previous issue date: 1984Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead ([email protected]) on 2011-06-07T14:10:40Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:09:01-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ThesisThesisU of I Onl
Spin-lattice relaxation of trivalent cerium, neodymium and erbium in yttrium ethyl sulfate
No full textSubmitted by William Weathers ([email protected]) on 2012-03-02T20:23:00Z
No. of bitstreams: 1
1974_marchand.pdf: 2647975 bytes, checksum: 3b589de777f887aa54c67605a02ee305 (MD5)Made available in DSpace on 2012-03-02T20:23:00Z (GMT). No. of bitstreams: 1
1974_marchand.pdf: 2647975 bytes, checksum: 3b589de777f887aa54c67605a02ee305 (MD5)
Previous issue date: 1974-02Restriction data tranferred 2014-07-01T11:33:13-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: thesisItem marked as restricted to the 'UIUC Users [automated]' Group (id=2) by William Weathers ([email protected]) on 2012-03-02T20:23:00Z
Item is restricted indefinitely.thesisU of I Onl
Spin-lattice relaxation of trivalent praseodymium, terbium, and erbium in anhydrous lanthanum trichloride: Temperature and magnetic field dependence
No full textWe present the results of our measurements of the spinlattice
relaxation rates of several trivalent rare earth ions, in the
temperature range 0.25 - 4.2o
K. We analyze these results within the
framework of the Kronig-Van Vleck-Orbach theory, the main features of
which we derive in a semi-rigorous manner. We used the pulse saturation
method, operating at a microwave frequency of 15.5 GHz. Through
additional measurements of our own and of others on the same samples
at 9 GHz, we were able to check the frequency (or field) dependence
of the relaxation rates. The non-Kramers Pr:LaC13
system was bottlenecked
in the direct region and hence its relaxation rate was essentially
frequency independent. The Kramers system Er:LaC13
showed a direct
process frequency dependence which was very close to the fourth power
law of Van Vleck; the non-Kramers system Tb:LaC13 had a direct
relaxation process which varied with the square of the frequency, a
variation close to that predicted. Actually, the Tb:LaC13.
system is
somewhat unusual in that a crystal field splitting of its ground
doublet results in the composition of its two lowest states being a
function of the magnitude of the applied field. This situation should
result in a fairly strong field dependence for the Raman and Orbach
coefficients, a dependence which is observed experimentally. In
contrast, no frequency dependence was expected or observed for the
Raman and Orbach coefficients for Er:LaC13 ,
The low temperatures used in these experiments were attained
through the use of a helium-three cryostat. The construction of this
cryostat and the associated ESR pulse spectrometer are described in
some detail.Submitted by Carolyn Mead ([email protected]) on 2011-07-08T18:40:42Z
No. of bitstreams: 1
1968_bohan.pdf: 5058133 bytes, checksum: b47e7faecf6a64a92d258390ae725552 (MD5)Made available in DSpace on 2011-07-08T18:40:42Z (GMT). No. of bitstreams: 1
1968_bohan.pdf: 5058133 bytes, checksum: b47e7faecf6a64a92d258390ae725552 (MD5)
Previous issue date: 1968Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead ([email protected]) on 2011-07-08T18:40:42Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:33:06-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ThesisThesisU of I Onl
- …
