Temperature dependence of nuclear magnetization and relaxation

The temperature dependences of nuclear magnetization and relaxation rates are reviewed theoretically and experimentally in order to quantify the effects of temperature on NMR signals acquired by common imaging techniques. Using common sequences, the temperature dependences of the equilibrium nuclear magnetization and relaxation times must each be considered to fully understand the effects of temperature on NMR images. The temperature dependence of the equilibrium nuclear magnetization is negative because of Boltzmann's distribution for all substances at all temperatures, but the combined temperature dependences of the equilibrium magnetization and relaxation can be negative, weak or positive depending on the temperature (T), echo time (T(E)), repetition time (T(R)), and the temperature dependences of the relaxation times T(1)(T) and T(2)(T) in a pulse sequence. As a result, the magnitude of the NMR signal from a given substance can decrease, increase or stay somewhat constant with increasing temperature. Nuclear thermal coefficients are defined and predictions for spin echo and other simple sequences are verified experimentally using a number of substances representing various thermal and NMR properties.     

Proton longitudinal NMR relaxation of poly(p-phenylene sulfide) in the laboratory and the rotating frames reference

Spin-lattice NMR relaxation times T1 in the laboratory frame and T1rho(off) as well as T1rho(off) in the rotating frame off-resonance were employed to the study of molecular dynamics of both pristine PPS and thermally treated poly(p-phenylene sulfide) (PPS). The temperature dependence of T1 was exponential in the whole temperature range studied, whereas T1rho only in low temperatures. In the high temperature range the distribution of relaxation times T1rho and correlation times tau(c) as well as activation energy Ea was observed. The distribution of activation energy determined from T1 minima at 15 and 30 MHz and from low temperature slopes of T1rho dependence as well as from spectral density functions (estimated from proton off-resonance technique) was attributed to the reorientation of phenylene groups around the sulfur-phenyl-sulfur axis in amorphous and crystalline phases of PPS. Furthermore, it is suggested that an additional relaxation mechanism related to interactions of protons with paramagnetic centers is operative in a low temperature range. After thermal treatment of PPS the low temperature minima disappeared and the relaxation times shortened in the low temperature regime. Both these facts were attributed to an increased contribution of spin diffusion in the relaxation process.     

Controlling electron-phonon interactions in graphene at ultrahigh carrier densities

We report on the temperature dependent electron transport in graphene at different carrier densities n. Employing an electrolytic gate, we demonstrate that n can be adjusted up to 4 × 10(14) cm(-2) for both electrons and holes. The measured sample resistivity ρ increases linearly with temperature T in the high temperature limit, indicating that a quasiclassical phonon distribution is responsible for the electron scattering. As T decreases, the resistivity decreases more rapidly following ρ(T) ～ T(4). This low temperature behavior can be described by a Bloch-Grüneisen model taking into account the quantum distribution of the two-dimensional acoustic phonons in graphene. We map out the density dependence of the characteristic temperature Θ(BG) defining the crossover between the two distinct regimes, and show that, for all n, ρ(T) scales as a universal function of the normalized temperature T/Θ(BG).     

Temperature dependence of the P-hit single event transient pulse width in a three-transistor inverter chain

A comparison of the temperature dependence of the P-hit single event transient (SET) in a two-transistor (2T) inverter with that in a three-transistor (3T) inverter is carried out based on a three-dimensional numerical simulation. Due to the significantly distinct mechanisms of the single event change collection in the 2T and the 3T inverters, the temperature plays different roles in the SET production and propagation. The SET pulse will be significantly broadened in the 2T inverter chain while will be compressed in the 3T inverter chain as temperature increases. The investigation provides a new insight into the SET mitigation under the extreme environment, where both the high temperature and the single event effects should be considered. The 3T inverter layout structure (or similar layout structures) will be a better solution for spaceborne integrated circuit design for extreme environments.     

An interstitialcy theory of structural relaxation and related viscous flow of glasses

A theory of isothermal structural relaxation and creep of glasses below the glass transition temperature is given. According to the interstitialcy theory, the supercooled liquid state does not exist below a Kauzmann "pseudocritical" temperature T(k), which lies above the temperature T(K), commonly called the "Kauzmann temperature." Structural relaxation is simply a reduction with time of the interstitialcy concentration to the crystalline state for TT(k). The predicted viscosity eta is universal, given by eta=eta(0) + eta(T)t, in agreement with experiment. eta is continuous in T, with eta discontinuous at T(k) but linear in 1/T above and below T(k). The dependence of eta on the shear modulus directly connects kinetic and thermodynamic properties of glasses and liquids.     

Thermodynamics of heat-shock response

Production of heat-shock proteins is induced when a living cell is exposed to a rise in temperature. The heat-shock response of protein DnaK synthesis in E.coli for temperature shifts T-->T+DeltaT and T-->T-DeltaT is measured as a function of the initial temperature T. We observe a reversed heat shock at low T. The magnitude of the shock increases when one increases the distance to the temperature T0 approximately 23 degrees C, thereby mimicking the nonmonotonous stability of proteins at low temperature. This suggests that stability related to hot as well as cold unfolding of proteins is directly implemented in the biological control of protein folding.     

Dielectric investigation of the temperature dependence of the nonexponentiality of the dynamics of polymer melts

Using broad band dielectric spectroscopy (10(-5)-10(9) Hz), combining time domain and frequency domain techniques, we study the temperature dependence of the non-Debye character of the alpha relaxation of polymer melts in the glass transition temperature T(g) range. The alpha relaxation process is described in terms of the Kohlrausch-Williams-Watts relaxation function which has a single parameter beta to characterize the nonexponentiality of the relaxation. At high temperatures, beta remains nearly insensitive to temperature changes, whereas in the vicinity of T(g) a nearly linear increasing of beta with temperature is found. The temperature range where the change of the beta(T) behavior occurs is located for all the polymers investigated around 1.2T(g). Moreover, our results indicate a common value of beta approximately equal to 1/3 at the temperature where the relaxation time diverges. The beta(T) behavior near T(g) is discussed in terms of a "rugged landscape" phase space which allows us to rationalize both the beta(T) behavior observed as well as the similarities of our findings near T(g) with the results reported in simulations on Ising spin glasses and other model systems.     

Activated dynamics and effective temperature in a steady state sheared glass

We conduct nonequilibrium molecular dynamics simulations to measure the shear stress sigma, the average inherent structure energy E{IS}, and the effective temperature T{eff} of a sheared model glass as a function of bath temperature T and shear strain rate gamma. For T above the glass transition temperature T0, the rheology approaches a Newtonian limit and T{eff}-->T as gamma-->0, while for T相似文献   

Relaxation times in paramagnetically doped agarose gels as a function of temperature and ion concentration

Tissue equivalent gels of NMR phantoms have been investigated at 3.4 MHz. The proton T1 and T2 relaxation times have been measured in Ni++ and Cu++ doped agarose gels as a function of temperature and ion concentration. Ni-agarose gels have the lower T1 temperature dependence, but gels containing both Cu++ and Ni++ can be produced for which T1 has virtually no temperature dependence.     

Specific heat of a fermionic atomic cloud in the unitary regime

In the unitary regime, when the scattering amplitude greatly exceeds in magnitude the average interparticle separation, and below the critical temperature thermal properties of an atomic fermionic cloud are governed by the collective modes, specifically the Bogoliubov-Anderson sound modes. The specific heat of an atomic cloud in an elongated trap, in particular, has a rather complex temperature dependence, which changes from an exponential behavior at very low temperatures (T < h omega(parallel)), to proportional T for h omega(parallel) < T < h omega(perpendicular) and then continuously to proportional T4 at temperatures just below the critical temperature, when the surface modes play a dominant role. Only the low (h omega(parallel) < T < h omega(perpendicular)) and high (h omega(perpendicular) < T < T(c)) temperature power laws are well defined. For the intermediate temperatures one can introduce at most a gradually increasing with temperature exponent.     

无序CuZr和CuY合金的电阻和磁电阻

以熔化-旋转法制备了Cu70Zr30和Cu100-xYx( x = 28, 67)非晶带试样并在1～300 K温度范围内测量了电阻和磁电阻随温度变化的规律.非晶Cu70Zr30电阻率ρ(T)的温度系数(TCR)在整个测量温区内都是负值,并且在两个不同的温区表现出-T1/2行为.对于类似的Cu100-xYx合金系统,在1～200 K温区内也做了同类测量.在低温1～4 K, 两个不同的无序系统CuZr和CuY的 TCR都准确地表现出-T1/2行为,这表明无序系统在极低温条件下的量子相干效应.这主要应归因于在粒子-空穴通道的电子-电子相互作用.而无序Cu70Zr30在宽广的中低温区60～300 K以更大斜率表现出的-T1/2行为,可以用初始定域化理论解释.无序CuZr和CuY的低温磁电阻ρ(B,T)测量结果与定域化理论进行了拟合和讨论.     

Temperature dependence of proton relaxation times in vitro

Accurate measurement of tissue relaxation characteristics is dependent on many factors, including field strength and temperature. The purpose of this study was to evaluate the relationship between sample temperature, viscosity and proton spin-lattice relaxation time (T1) and spin-spin relaxation time (T2). A review of two basic models of relaxation the simple molecular motion model and the fast exchange two state model is given with reference to their thermal dependencies. The temperature dependence for both T1 and T2 was studied on a 0.15 Tesla whole body magnetic resonance imager. Thirteen samples comprising both simple and complex materials were investigated by using a standard spin-echo (SE) technique and a modified Carr-Purcell-Meiboom-Gill (CPMG) multi-echo sequence. A simple linear relationship between T1 and temperature was observed for all samples over the range of 20 degrees C to 50 degrees C. There is an inverse relationship between viscosity and T1 and T2. A quantity called the temperature dependence coefficient (TDC) is introduced and defined as the percent rate of change of the proton relaxation time referenced to a specific temperature. The large TDC found for T1 values, e.g. 2.37%/degrees C for CuSO4 solutions and 3.59%/degrees C for light vegetable oils at 22 degrees C, indicates that a temperature correction should be made when comparing in-vivo and in-vitro T1 times. The T2 temperature dependence is relatively small.     

Hyperthermia induces T1 relaxation and blood flow changes in tumors. A MRI thermometry study in vivo

Regional hyperthermia in combination with chemotherapy and/or radiotherapy has proven to be an effective treatment concept for locally advanced deep-seated tumors. Simultaneous MR-imaging could be a promising approach for therapy optimization. Purpose of this study was the in vivo investigation of temperature induced longitudinal relaxation time (T(1)) and blood flow changes in a tumor model. Using a 1.5 Tesla MR system, the T(1) sensitivity on temperature and the onset of tissue changes at temperatures >44 degrees C were investigated in muscle samples. Also, fourteen Syrian Golden Hamsters with amelanotic melanoma A-MEL-3 were examined during heating of the tumors. Temperature induced blood flow and T(1) changes were determined continuously during hyperthermia. Changes of T(1) correlated linearly with temperature over a wide range (27-44 degrees C) in the tissue sample. Tissue changes became notable above 44 degrees C. In the tumor model, relative changes of T(1) (unlike blood flow) showed linear correlation with temperature over the entire range of hyperthermia relevant temperatures (32-44 degrees C). For a low thermal dose, T(1) allows the assessment of temperature changes in tumors in vivo. At higher thermal doses, in addition to temperature changes, T(1) also shows tissue changes. Both temperature and tissue changes supply important information for hyperthermia.     

在二带京兹堡朗道理论下MgB2的超导参量的研究

文中通过G-L理论二带模型来研究相干长度ξ(T)、伦敦穿透深度λ(T)以及京兹堡朗道参量κ(T)=λ(T)/ξ(T)在Tc附近对温度T的依赖关系。理论计算结果与MgB2样品的实验数据符合的很好。同时与单带模型算出的结果做了对比,发现G-L理论二带模型能更精确描述MgB2的超导特性。     

Spin-reorientation, ferroelectricity, and magnetodielectric effect in YFe(1-x)Mn(x)O3(0.1 ≤ x ≤ 0.40)

We report the observation of magnetoelectric and magnetodielectric effects at different temperatures in Mn-substituted yttrium orthoferrite, YFe(1-x)Mn(x)O(3)(0.1 ≤ x ≤ 0.40). Substitution of Mn in antiferromagnetic YFeO(3)(T(N) = 640 K) induces a first-order spin-reorientation transition at a temperature, T(SR), which increases with x whereas the Néel temperature (T(N)) decreases. While the magnetodielectric effect occurs at T(SR) and T(N), the ferroelectricity appears rather at low temperatures. The origin of magnetodielectric effect is attributed to spin-phonon coupling as evidenced from the temperature dependence of Raman phonon modes. The large magnetocapacitance (18% at 50 kOe) near T(SR) = 320 K and high ferroelectric transition temperature (～115 K) observed for x = 0.4 suggest routes to enhance magnetoelectric effect near room temperature for practical applications.     

Thermodynamics of a Fermi liquid beyond the low-energy limit

We consider the nonanalytic temperature dependences of the specific heat coefficient, C(T)/T, and spin susceptibility, chi(s)(T), of 2D interacting fermions beyond the weak-coupling limit. We demonstrate within the Luttinger-Ward formalism that the leading temperature dependences of C(T)/T and chi(s)(T) are linear in T, and are described by the Fermi liquid theory. We show that these temperature dependences are universally determined by the states near the Fermi level and, for a generic interaction, are expressed via the spin and charge components of the exact backscattering amplitude of quasiparticles. We compare our theory to recent experiments on monolayers of He3.     

Metamagnetic quantum criticality revealed by 17O-NMR in the itinerant metamagnet Sr3Ru2O7

We have investigated the spin dynamics using 17O-NMR in the bilayered perovskite Sr3Ru2O7, which sits close to a metamagnetic quantum critical point. The nuclear spin-lattice relaxation rate divided by temperature 1/T1T is enhanced on approaching the metamagnetic critical field of approximately 7.9 T, and at the critical field 1/T1T continues to increase and does not show Fermi-liquid behavior down to 0.3 K. The temperature dependence of T1T in this region suggests the critical temperature Theta to be approximately 0 K, which is strong evidence that the spin dynamics possesses a quantum critical character. Comparison between uniform susceptibility and 1/T1T reveals that antiferromagnetic fluctuations instead of two-dimensional ferromagnetic fluctuations dominate the spin fluctuation spectrum at the critical field, which is unexpected for itinerant metamagnetism.     

Glass formation criterion for various glass-forming systems

A conceptual approach to evaluate glass-forming ability for various glass-forming systems has been proposed from a physical metallurgy point of view. It was found that the glass-forming ability for noncrystalline materials was related mainly to two factors, i.e., 1/(T(g)+T(l)) and Tx (wherein Tx is the onset crystallization temperature, T ( g) the glass transition temperature, and T(l) the liquidus temperature), and could be predicated by a unified parameter gamma defined as T(x)/(T(g)+T(l)). This approach was confirmed and validated by experimental data in various glass-forming systems including oxide glasses, cryoprotectants, and metallic glasses.     

Existence of temperature on the nanoscale

We consider a regular chain of quantum particles with nearest neighbor interactions in a canonical state with temperature T. We analyze the conditions under which the state factors into a product of canonical density matrices with respect to groups of n particles each and under which these groups have the same temperature T. In quantum mechanics the minimum group size n(min) depends on the temperature T, contrary to the classical case. We apply our analysis to a harmonic chain and find that n(min)=const for temperatures above the Debye temperature and n(min) proportional to T(-3) below.     

Resonant production of topological defects

We describe a novel phenomenon in which vortices are produced due to resonant oscillations of a scalar field which is driven by a periodically varying temperature T, with T remaining much below the critical temperature T(c). Also, in a rapid heating of a localized region to a temperature below T(c), far separated vortex and antivortex can form. We compare our results with recent models of defect production during reheating after inflation. We also discuss possible experimental tests of our predictions of topological defect production without ever going through a phase transition.