Low-temperature Maximum Research Articles

Peculiar low-temperature properties of metallicNd2−xCexCuO4caused by interactions between Nd moments and conduction electrons

Low-energy magnetic excitations in ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ with $0<~x<~0.2$ have been investigated by measurements of the specific heat. For all Ce-doped samples $(x>~0.05),$ a linear term $\ensuremath{\gamma}T$ was found at very low temperatures $(T\ensuremath{\lesssim}0.5\mathrm{K}),$ with ${0.3\mathrm{}\mathrm{J}/\mathrm{K}}^{2}{\ensuremath{\lesssim}\mathrm{\ensuremath{\gamma}}\ensuremath{\lesssim}0.6\mathrm{}\mathrm{J}/\mathrm{K}}^{2}$ per mole Nd for $0.05<~x<0.14,$ and ${3\mathrm{}\mathrm{J}/\mathrm{K}}^{2}{\ensuremath{\lesssim}\mathrm{\ensuremath{\gamma}}\ensuremath{\lesssim}4\mathrm{}\mathrm{J}/\mathrm{K}}^{2}$ per mole Nd for higher Ce concentrations. In overdoped ${\mathrm{Nd}}_{1.8}{\mathrm{Ce}}_{0.2}{\mathrm{CuO}}_{4},$ \ensuremath{\gamma} is rapidly suppressed in magnetic fields exceeding 1.5 T. The temperature dependence of the electrical resistance parallel to the ${\mathrm{CuO}}_{2}$ planes corresponds to that of two-dimensional metals. The magnetoresistance of ${\mathrm{Nd}}_{1.8}{\mathrm{Ce}}_{0.2}{\mathrm{CuO}}_{4}$ single crystals and $c$-axis-oriented thin films in magnetic fields parallel to the ${\mathrm{CuO}}_{2}$ planes is monotonously negative at temperatures above 1 K. Below about 500 mK, the magnetoresistance is positive in small fields, it goes through a maximum near 2 T, and it is negative for $B>2\mathrm{T}.$ These features are similar to those observed in heavy-fermion systems. In contrast, in ${\mathrm{Pr}}_{1.8}{\mathrm{Ce}}_{0.2}{\mathrm{CuO}}_{4},$ with nonmagnetic Pr, the magnetoresistance is negative at all temperatures of our measurements down to 20 mK. This proves unambiguously that interactions between conduction electrons and Nd moments give rise to the low-temperature maximum of the magnetoresistance. Furthermore, a comparison of the magnetoresistance with the modification of the specific heat by magnetic fields supports the interpretation of the large linear term as resulting from the interaction between Nd moments and conduction electrons.

Phonon-phonon interaction and thermal conductivity in strongly anisotropic quasi-two-dimensional systems

The phonon-phonon interaction and phonon thermal conductivity in strongly anisotropic quasi-two-dimensional systems are considered. Much attention is being given to the low-frequency bending (out-of-plane) phonons which can give rise to a low-temperature local maximum in the $T$ dependence of the in-plane thermal conductivity \ensuremath{\lambda}. The effect of the system anisotropy on \ensuremath{\lambda} is consistently studied. The results lead to an interesting interpretation of the anomalous behavior of the in-plane thermal conductivity in strongly anisotropic systems.

Thermally stimulated conductivity in disordered semiconductors at low temperatures

Theoretical interpretation of the thermally stimulated conductivity (TSC) is suggested for the temperatures at which transport is via hopping of carriers through localized bandtail states. The description is not based on specific assumptions used in the most of the previous TSC theories, such as, e.g., neglect of the retrapping of mobile carriers, etc. Our approach is based on the general concept of the transport energy according to which the transport path of hopping electrons in the bandtail is temperature dependent. This theory allows for a natural extension to higher temperatures and accounts for all existing experimentally observed trends in the TSC including the puzzling so far pinning of the low-temperature maximum on the TSC curves at different initial temperatures. Experimental results for the low-temperature TSC are presented, which are consistent with the theoretical predictions.

Specific Heat of the Half-Filled One-Dimensional Hubbard Model: Feynman Path-Integral Simulations

The Feynman path-integral quantum Monte Carlo (PI QMC) approach has been employed to study the specific heat C of the one-dimensional (1D) half-filled Hubbard chain as a function of temperature. For not too strong many-body interactions C exhibits a single maximum only. With increasing correlation strength the C curve shows an intermediate minimum separating a sharp low-temperature maximum and a broad high-temperature profile. The first peak is caused by spin excitations and the high-temperature values of C are determined by charge degrees of freedom. The contribution of both quantities to the net specific heat curve has been quantified in the present theoretical setup. The specific heat data are interpreted with the aid of calculated charge fluctuations, probabilities Pi(n) to find n = 2, 1, 0 electrons at the respective lattice sites, a spin density wave (SDW) order parameter and the electronic chemical potential μ. Die Wärmekapazität C der halbgefüllten eindimensionalen (1D) Hubbard-Kette wurde mit Hilfe von Feynman-Pfadintegral-Quanten-Monte-Carlo (PI-QMC)Simulationen als Funktion der Temperatur untersucht. Für nicht zu starke Vielteilchen-Wechselwirkungen zeigt C nur ein Maximum. Mit zunehmender Korrelationsstärke bildet sich ein intermediäres Minimum in der C-Kurve aus, die ein Tieftemperatur-Maximum und eine Hochtemperatur-Flanke trennt. Der erste Peak wird durch Spin-Anregungen verursacht, während die Hochtemperatur-Flanke von C durch Ladungs-Freiheitsgrade hervorgerufen wird. Die Beiträge beider Größen zu den Netto-Wärmekapazitätskurven werden in der vorliegenden Arbeit diskutiert. Die Wärmekapazitäts-Daten werden auf der Basis von Besetzungswahrscheinlichkeiten Pi(n) interpretiert, die die Wahrscheinlichkeit angeben, an Gitterplatz i n = 2, 1 oder 0 Elektronen zu finden. Weitere Hilfsgrößen sind ein Ordnungsparameter für Spindichtewellen (SDW) sowie das elektronische chemische Potential μ.

Polynuclear copper(II) complexes with μ2-1,1-azide bridges. Structural and magnetic properties

Two novel, weakly antiferromagnetically coupled, tetranuclear copper(II) complexes [Cu 4(PAP) 2( μ 2-1,1-N 3) 2( μ 2-1,3-N 3) 2( μ 2-CH 3OH) 2(N 3) 4 ( 1) (PAP = 1,4-bis-(2′-pyridylamino)phthalazine) and [Cu 4(PAP3Me) 2 ( μ 2-1,1-N 3) 2( μ 2-1,3-N 3) 2(H 2O) 2(NO 2) 2]- (NO 3) 2 ( 2) (PAP3Me = 1,4-bis-(3′-methyl-2′-pyridyl)aminophthalazine) contain a unique structural with two μ 2-1,1-azide intramolecular bridges, and two μ 2-1,3-azide intermolecular bridges linking pairs of copper(II) centers. Four terminal azide groups complete the five-coordinate structures in 1, while two terminal waters and two nitrates complete the coordination spheres in 2. The dinuclear complexes [Cu 2(PPD)( μ 2-1,1-N 3)(N 3) 2(CF 3SO 3)]CH 3OH) ( 3) and [Cu 2(PPD)( μ 2-1,1-N 3)(N 3) 2(H 2O)(ClO 4)] ( 4) (PPD = 3,6-bis-(1′-pyrazolyl)pyridazine) contain pairs of copper centers with intramolecular μ 2-1,1-azid and pyridazine bridges, and exhibit strong antiferromagnetic coupling. A one-dimensional chain structure in 3 occurs through intermolecular μ 2-1,1-azide bridging interactions. Intramolecular Cu-N 3-Cu bridge angles in 1 and 2 are small (107.9 and 109.4°, respectively), but very large in 3 and 4 (122.5 and 123.2°, respectively), in keeping with the magnetic properties. 2 crystallizes in the monoclinic system, space group C2/ c with a = 26.71(1), b = 13.51(3), c = 16.84(1) A ̊ , β = 117.35(3)° and R = 0.070, R w = 0.050. 3 crystallizes in the monoclinic system, space group P2 1/ c with a = 8.42(1), b = 20.808(9), c = 12.615(4) A ̊ , β = 102.95(5)° and R = 0.045, R w = 0.039. 4 crystallizes in the triclinic system, space group P1, with a = 10.253(3), b = 12.338(5), c = 8.072(4) A ̊ , α = 100.65(4), β = 101.93(3), γ = 87.82(3)° and R = 0.038, R w = 0.036 . The magnetic properties of 1 and 2 indicate the presence of weak net antiferromagnetic exchange, as indicated by the presence of a low temperature maximum in χ m (80 K ( 1), 65 K ( 2)), but the data do not fit the Bleaney-Bowers equation unless the exchange integral is treated as a temperature dependent term. A similar situation has been observed for other related compounds, and various approaches to the problem will be discussed. Magnetically 3 and 4 are well described by the Bleaney-Bowers equation, exhibiting very strong antiferromagnetic exchange (− 2 J = 768(24) cm −1 ( 3); − 2 J = 829(11) cm −1 ( 4)).

Temperature dielectric anomalies of triglycine sulphate crystals admixtured with L-α-alanine at different frequencies of the measuring field

Abstract The permittivity ϵ′ and loss tangent tg δ were measured for TGS crystals admixtured with different amounts of L-α-alanine, in a measuring field with frequencies varying from 1 kHz to 10000 kHz and for temperatures from 100 to 330 K. With increasing amount of L-α-alanine admixture, the low temperature maximum decreases, and for the crystal with the maximum concentration of L-α-alanine it disappears.

Low temperature heat capacity measurements of U6FeH15 hydride

The heat capacity Cp(T) of U6FeH15 hydride was meausured in the temperature range 4.5–255 K. The complex anomaly with two critical temperatuers was observed, which arises from the magnetic phase transitions. The low-temperature maximum reveals thermal hysteresis. The higher temperature anomaly is close to the Tc observed for β-UH3. The change in entropy of the transitions was estimated to be 5.43 J mol−1 K−1. An electronic specific heat coefficient γ∗ = 33.8 mJ mol−1 K−2 was derived.

Low temperature mechanical energy dissipation phenomena in praseodymium-doped yttrium superconductors

Studies of the anelastic properties of Y 1− x Pr x Ba 2Cu 3O 7–8 ceramics (0 ⩽ × ⩽ 1) were performed. The internal friction and Young modulus measurements were carried out by the vibrating reed technique in the temperature range from 20 to 180 K. The measurement frequency was in the range 90–760 Hz. The aim of this work was to investigate the influence of praseodymium content on the anelastic properties of 1–2–3 material. The main phenomena which are caused by doping with praseodymium can be seen in the low temperature part of the internal friction spectra (20–40 K). The maximum at about 40 K, known as maximum 1 in the spectra of yttrium 1–2–3 material, gradually decreases with increasing praseodymium content. Relaxation processes located in the conducting planes, possibly electronic phenomena, have been considered as an origin of maximum 1. The samples which contained more than 50% Pr showed another low temperature maximum 25–30 K. This maximum also reflects thermally activated relaxation phenomena. Its estimated activation energy is very low (0.03 eV). The common feature of all the Y 1− x Pr x Ba 2Cu 3O 7–8 samples regardless of the dopant content is the presence of internal friction maxima observed in our frequency range at 80 and 100 K (maxima 3 and 4 in the spectra of 1–2–3 material). These processes were not significantly affected by the presence of praseodymium.

Susceptibility of Y2Cu2O5Hx.

Temperature dependence of the real and imaginary part of the ac susceptibility was measured for polycrystalline Y 2 Cu 2 O 5 H x for x=0, 0.34, 0.72, 1.52, and 1.88. It was found that the effective magnetic moment of the copper ions decreases with increasing hydrogen content. Also, the height of the maximum at 13 K associated with the antiferromagnetic transition of Cu ions decreases while its position shifts slightly toward higher temperatures. Another maximum at about 5.8 K disappears almost completely for hydrogen content of x=1.52. A steady magnetic field also reduces the height of the lower temperature maximum, shifting it negligibly. For an external magnetic field above 200 Oe, this maximum disappears completely

Thermogravimetric, infrared and mass-spectroscopic analysis of the desorption of methanol, propan-1-ol, propan-2-ol and 2-methylpropan-2-ol from montmorillonite

AbstractThe desorption of methanol (MeOH), propan-1-ol (n-PrOH), propan-2-ol (i-PrOH) and 2-methylpropan-2-ol (t-BuOH) from Na+-, Ca2+-, Al3+-, Cr3+- and Fe3+-exchanged montmorillonite has been studied using variable temperature infrared (IR) spectroscopy and thermogravimetric analysis (TGA). Alcohol-saturated trivalent cation (M3+) exchanged samples exhibit maxima in the derivative thermograms at 20 and 110°C (MeOH), 30 and 160°C (n-PrOH), 20 and 110°C (i-PrOH) and 20, 55 and 80°C (t-BuOH). Alcohol-saturated Na+ and Ca2+-exchanged montmorillonite samples exhibit maxima at higher temperatures in the i-PrOH (20 and 140°C) and t-BuOH (30, 90 and 110°C) desorption profiles but at the same temperatures for MeOH and n-PrOH. Mass spectroscopic analysis of the vapours desorbed from the alcohol-treated samples show that the low-temperature maxima in the alcohol desorption from the M3+-exchanged clays are due to unchanged alcohol, whilst those occurring at 80°C (t-BuOH), 110°C (i-PrOH) and 160°C (n-PrOH) are due, in the main, to alkene produced from the intramolecular dehydration of the respective alcohol. Changes in the IR spectra of the adsorbed alcohols occur at temperatures which are in accord with the mass spectral data. No mass spectral evidence was found for the formation of dialkylethers via the competing intermolecular process but dimerisation and oligomerisation of t-BuOH were observed.

Low-temperature mechanical energy dissipation phenomena in lanthanum superconductors

Abstract The anelastic properties of ceramics which belong to the lanthanum family of superconductors were studied. The internal friction and Young's modulus measurements were carried out by the vibrating reed technique in the temperature range from 20 to 300 K. The measurement frequency was in the range of 90–760 Hz. A large internal friction, corresponding to a maximum of the relaxation, was observed at a temperature about 50–60 K. An interesting correlation of both the peak height and the relaxation strength with the dopant content x has been found. An electronic relaxation phenomenon was considered as a possible source of the low-temperature internal friction maximum. The well known HTT→LTO phase transition was observed through both a pronounced change in Young's modulus and the internal friction maximum. Also the LTO→LTT phase transition was indirectly observed through an anomalous change in the Young's modulus.

Domain-wall formation in late-time phase transitions.

We examine domain wall formulation in late time phase transitions. We find that in the invisible axion domain wall phenomenon, thermal effects alone are insufficient to drive different parts of the disconnected vacuum manifold. This suggests that domain walls do not form unless either there is some supplemental (but perhaps not unreasonable) dynamics to localize the scalar field responsible for the phase transition to the low temperature maximum (to an extraordinary precision) before the onset of the phase transition, or there is some non-thermal mechanism to produce large fluctuations in the scalar field. The fact that domain wall production is not a robust prediction of late time transitions may suggest future directions in model building.

Fractionation of thylakoid membranes from Porphyridium purpureum using the detergent N-lauryl-?-iminodipropionate

Two green fractions, thought to represent the chlorophyll-antennae of photosystems I (PSI) and II (PSII), were isolated from the red alga Porphyridium purpureum by solubilisation of the thylakoid membranes using the detergent N-lauryl-ß-iminodipropionate and subsequent sucrose-density-gradient centrifugation. No release of pigments from the pigment-protein complexes was detected during isolation. The fractions were analyzed with respect to their chlorophyll-protein pattern, spectral properties and pigment composition. The supposed PSII antenna fraction contained both the major carotenoids of P. purpureum, β-carotene and zeaxanthin, and showed a long-wavelength absorption maximum at 672 nm and a low-temperature fluorescence maximum at 692-694 nm. Polypeptides of this fraction cross-reacted with antibodies raised against the PSII polypeptides D1, CP43 and CP47 from higher plants. The PSI fraction could perform P-700 photooxidation and showed a long-wavelength absorbance maximum at 679 nm and a low-temperature fluorescence maximum at 718 nm. It contained β-carotene as the only carotenoid. The fluorescence excitation spectrum of the fraction and measurements of the photochemical activity of a thylakoid preparation excited with light that is preferentially absorbed either by chlorophyll (433 nm) or by carotenoids (495 nm) indicate that β-carotene serves as a very efficient antenna-pigment in PSI. In contrast, only a small amount of energy transfer from the carotenoids to chlorophyll could be observed with the supposed PSII fraction.

AC susceptibility in the reentrant spin-glass system (Fe 0.65Ni 0.35) 1− xMn x

The temperature dependent Alternating Current Susceptibility (ACS) measured on the alloy system (Fe 0.65Ni 0.35) 1− x Mn x in the concentration range of 0.08 ≤ x ≤ 0.15 and in weak external magnetic fields shows several peaks related to the magnetic transitions. Beside the Curie temperature T C peak we observed two further maxima at lower temperatures T L and T H which appear near the reentrant ferromagnet ↔ spin-glass transition) where T L < T H. These low temperature maxima exhibit different frequency dependencies: T L is nearly frequency independent whereas T H is shifted to higher temperatures with increasing frequency. We observed that T L disappears in field cooling experiments.

Isolation, purification and partial characterization of a 30‐kDa chlorophyll‐a/b‐binding protein from spinach

A 30-kDa chlorophyll-a/b-binding protein was purified from photosystem II membrane fragments using Ca(2+)-chelating Sepharose 6B chromatography. The protein binds approximately four chlorophyll a molecules, one chlorophyll b molecule and carotenoids. Its 77-K fluorescence-emission spectrum exhibits a maximum at 680 +/- 1 nm. The protein has a high tendency to form a dimer in the presence of Ca2+.Ca2+ binding affects the low-temperature fluorescence-emission maximum, leading to a decrease in its intensity and a blue shift of 1 nm. Similar spectral changes were obtained in the presence of Mg2+, possibly indicating a common binding domain for both cations. We interpret these observations as cation-induced conformational changes of the protein, which were reversible upon subsequent incubation in EDTA. Evidence is presented for the involvement of carboxyl groups in the coordination sphere of the bivalent cations. The possible structural and functional role of the protein is discussed.

ELECTRICAL RESISTIVITY OF THE WEAK-DISPERSION HEAVY-FERMION ALLOY MODEL

The electrical resistivity of the heavy-fer mion alloy system is calculated through the Yoshimori-Kasai model with the application of the slave-boson technique. The residual resistivity of the system is found in the mean-field approximation by using the CPA method-The low-temperature resistivity maximum in the concentrated alloy is aslo obtained as the Gaussian fluctuation of the slave-boson field is included.

Specific heat of UPt3 in magnetic fields up to 24.5 T.

We report the first high-precision specific-heat measurements on the heavy-fermion compound ${\mathrm{UPt}}_{3}$, over a wide temperature (1.4 KT40 K) and field range (B24.5 T), thus covering the metamagnetic transition at 20 T. The data, taken on a single-crystalline sample, show a pronounced low-temperature maximum in the enhanced ${\ensuremath{\gamma}}^{\mathrm{*}}$ value at 20 T for a field direction in the hexagonal plane. We also investigate the presence of a ${\mathit{T}}^{3}$ln(T/${\mathit{T}}^{\mathrm{*}}$) contribution to the specific heat in field.

Transport properties of CeB 6: Comparison with specific heat data

Electrical resistivity (ϱ), thermal conductivity ( K), and thermoelectric power ( Q) measurements were performed on a high quality single crystal of CeB 6 ( ϱ residual = 0.33μΩ cm ). Comparison is made with specific heat data obtained on another sample. The low temperature maximum in Q(T) which is associated with a large contribution in K(T), a T 4 dependence of ϱ, and a T 3 dependence of C, may be due to magnon drag effects.

The origin of the low-temperature resistivity anomalies in antiferromagnetic Cr-Fe alloys

The effect of pressure and V doping on the resistivity behavior of Cr-Fe antiferromagnetic alloys has been investigated. In both cases the low-temperature resistivity behavior is found to be dependent upon the Fermi level position relative to the impurity resonance level. It is concluded that the low-temperature resistivity minimum, maximum and other anomalies in Cr-Fe alloys are caused by the impurity resonance scattering mechanism and not by the Kondo effect.

Temperature dependence of inorganic nitrogen uptake and assimilation in Antarctic sea-ice microalgae

The influence of temperature on NO3-and NH4+uptake, and the activity of the assimilatory enzyme NO3-reductase (NR) was compared to inorganic C uptake (photosynthesis) in natural assemblages of Antarctic sea-ice microalgae. NO3-and NH4+uptake reached a maximum between 0.5°–2.0°C and 2.0°–3.0°C, respectively, which was close to that for photosynthesis (2.5°–3.0°C). NR showed a distinctly higher temperature maximum (10.0°–12.0°C) and a lower Q10 value than inorganic N and C transport. Our data imply that, owing to differential temperature characteristics between N transport and N assimilation at in situ temperature (-1.9°C), the incorporation of extracellular NO3-into cellular macromolecules, may be limited by transport of NO3-into the cell rather than the intracellular reduction of NO3-to NH4+. Despite differences in temperature maxima between N transport and N assimilation, the overall low temperature maxima of inorganic N metabolism characterizes Antarctic sea-ice microalgae as psychrophilic. Our study is the first to examine the temperature dependence of inorganic N uptake and assimilation in sea-ice microbial communities.