Influence of structural defects on magnetic properties of the submicrometer ceramic Mn3O4

A high-density single-phase submicrometer ceramic Mn₃O₄ with a grain size d ≥ 0.06 μm has been obtained by using spherically converging shock waves. The action of shock waves reduces the unit cell volume and increases the Curie temperature T _C . The shape of the temperature dependences of reciprocal susceptibility x ^?1 of a polycrystal and the submicrometer ceramic at T > T _C is hyperbolic, which is typical of ferrimagnets. It is shown that the susceptibility of the ceramic in the region of short-range order at 78 K ≤ T ≤ 300 K is larger than that of the polycrystal, while the field dependences of the magnetization are nonlinear. The bombardment by electrons with a small dose of Φ = 0.8 × 10¹⁸ cm^?2 increases the susceptibility of the submicrometer ceramic as well as that of the polycrystal. When the fluence increases to Φ = 5 × 10¹⁸ cm^?2, the susceptibility decreases, which can be explained by radiation-induced disordering and a change in the “local” exchange interactions.     

Low temperature Mössbauer spectroscopy and magnetism of synthetic annite mica

Annite mica has a complicated magnetically ordered ground state with T_C?58K. An Fe²⁺ paramagnetic contribution persists far below T_C (down to 4.2K) and presumably corresponds to those Fe²⁺ moments that are most weakly coupled to the two-dimensional ferromagnetic backbone. The Fe³⁺ exists in at least three families of “sites” (i. e. positions on the backbone?) that give distinct spectral contributions. The low T Mössbauer spectroscopy of biotites in general is elucidated.     

The classification of superconductors using muon spin rotation

In this paper we explore the role of muon spin rotation (μSR) techniques in the characterization and classification of superconducting materials. In particular we focus upon the Uemura classification scheme which considers the correlation between the superconducting transition temperature,T _c, and the effective Fermi temperature,T _F, determined from μSR measurements of the penetration depth. Within this scheme strongly correlated “exotic” superconductors, i.e, hightT _C cuprates, heavy fermions, Chevrel phases and the organic superconductors, form a common but distinct group, characterized by a universal scaling ofT _C withT _F such that 1/100 <T _C/T _F<1/10. For conventional BCS superconductorsT _C/T _F<1/1000. The results of new μSR measurements of the penetration depth in superconducting Y(Ni_1?x Co _x )₂B₂C and YB₆ are also presented. In Y(Ni_1?x Co _x )₂B₂C the decrease ofT _C with increasing Co concentration is linked to a marked decrease in the carrier density from 2.9·10²⁸ m^?3 atx=0 to 0.6·10²⁸ m^?3, atx=0.1, while the carrier mass enhancement remains almost constant at approximately 10. For YB₆ we find evidence of a modest enhancement of the carrier mass (m ^*/m=3), and a relatively low carrier density of 0.24·10²⁸ m^?3. These results are discussed within the Uemura classification scheme. It is found that neither Y(Ni_1?x Co _x )₂B₂C withT _c/T _F>>1/250 nor YB₆ withT _C/T _F>>1/340 can be definitively classified as either “exttic” or “conventional”, but instead the compounds display behavior which interpolates between the two regimes.     

Phenomenological trends for heavy fermi liquids and narrow-band metals III: Characteristic temperatures

A correlation between the ratio of the Fermi temperature T_F¹ and the Debye temperature θ_D and the occurrence of superconductivity or magnetic order is described. Values of T_F¹/θ_D⪢ 1 are found to favor “conventional” superconductivity, whereas T_F¹/θ_D≲ 1 favors magnetic order. “Heavy fermion” superconductivity is found to be restricted to the case T_F¹/θ_D < 1.     

Is the glass transition in some super-cooled polyphenyls preceded by molecular cluster formation?

A recently reported anomalous behaviour of the positronium annihilation rate with temperature in certain super-cooled organic liquids is explained in terms of a simple free-volume model modified in the presence of molecular cluster formation. This model apart from showing the entropic origin of the phenomenon accounts for the existence of the transition temperature “T_r” much above the glass-transition temperature T_g. It also predicts for the threshold temperature “T_r” fo clusters to commence formation a value of 310_-7.0^+8.0 K as against the experimental value of 304 K for ortho-terphenyl.     

Comment on the low temperature specific heat of ferroelectrics,antiferroelectrics, and related materials

Lawless has reported accurate, low temperature (2–30°K) specific heat measurements (C_Exp) of a large number of ferroelectrics, antiferroelectrics, and related materials. A plot of C_Exp T^?3 vs. T (T is temperature) reveals an “excess’ specific heat in the 10–30K range. Lawless has interpreted and fit this excess heat with an Einstein term determining the number of oscillators and the Einstein frequence for each material. He has related these large number of extra oscillators to modes observed in some Raman experiments. Using a more realistic density of states, we suggest that the experimental data can be understood in terms of straightforward harmonic lattice dynamics and that there are no extra modes. The “extra” heat capacity merely arises from the differences between the more realistic density of states and that of a Debye solid.     

Atomic tunneling states with dissipation in glasses: temperature-dependent two-level systems

By taking account of the influence of dissipation on atomic tunneling states in glasses, we introduce a density of states of two-level systems which is non-linearly temperature dependent. Based on this model, we can explain the “excess T³” anomaly in the specific heat and the “plateau” in the thermal conductivity of glasses.     

Work function measurements on germanium by thermionic emission

In the present paper, we report the results of experiments carried out to determine the effect of surface states on the work function of intrinsic, germanium crystals. This investigation is realised by using the Thermionic Emission Theory at the intrinsic temperature range T (from 900 to 1050 K). The “apparent work function” φ¹, obtained from the experimental ln(i_sT²) versus 1/T curves, is found different from the Richardson-Dushman equation one: “the true work fucntion” φ. This difference is mainly due to the work function and electron reflection coefficient dependence on temperature. φ is deduced from φ¹ and from RD modified equation taking into account the semiconductor case. The value of φ varies with T: from 3.97 to 4.16 eV. These results are compared with those obtained, by several authors, using other techniques. We present an analysis of these results taking into consideration the effect of surface states shifts on the surface barrier height, and on the work function consequently.     

钙钛矿Mn基氧化物的电子顺磁共振行为的实验研究

借助电子顺磁共振技术,对Mn³⁺/Mn⁴⁺=2∶1和1∶1的两个典型体系 在居里温度T_C以上温区的顺磁行为进行了实验研究.实验表明,存在一特征温度 T_min,通过该温度将T_C以上的磁行为分成两个明显可区分的温区： 在T_min以上,共振谱由一对称的单线构成,Lande 因子g值与温度无关且接近 自由电子的值(～20023),线宽随温度呈线性变化等;而在T_{min关键词： 稀土锰氧化物 电子顺磁共振 交换变窄 自旋-自旋相互作用 自旋-晶格相互作用}     

EPR evidence for local pseudo freeze out of impurity dynamics in KH2PO4 type ferroelectrics and antiferroelectrics

The temperature dependence of the reorientation rates of the AsO₄^4? centers in KH₂AsO₄ and NH₄H₂AsO₄ as well as of the CrO₄^3? centers in KH₂PO₄ shows a strong deviation from an Arrhenius behavior which can be quantitatively described as a result of the local “pseudo freeze-out” of the dynamics of these centers and the associated polarization cloud far above the transition temperature T_C of the host crystal.     

On the physics of λφ4 in aℝ2×S 1 space

The physics of λφ⁴ quantum field theory in a space with a closed dimension (?²×S ¹) is studied on the basis of a varitional approach, which supports the existence of two interacting phases of the minkowskian λφ⁴ on nonperturbative grounds. As the lengthL of the closed dimension decreases (1/L becomes the relevant scale), triviality restoration is encountered in the “precarious” phase, as well as symmetry restoration in the “autonomous” phase. The close relation to the finite temperature formalism allows to uncover a temperature symmetry restoration of theT=0 spontaneously broken phase.     

NMR studies of singlet-ground-state rare-earth ions in high-T c superconductors

Many of presently known high-T _c superconductors contain rare-earth (RE) ions with an even number of electrons in an unfilled 4f-shell (Pr³⁺, Tb³⁺, Ho³⁺, Tm³⁺). If the ground state of 4f-electrons is non-degenerate and separated from excited states by high enough energy intervals, one can observe the so-called “enhanced NMR” of RE nuclei at low temperatures. In the present paper some aspects of the enhanced NMR are analyzed in applications to the crystal and electron structure of high-T _c superconductors.     

What can we learn from the finite temperature renormalization group?

The renormalization group for finite temperature quantum field theories is studied, in particular for λ?⁴. It is shown that the “high” temperature limit can only be discussed perturbatively ifT dependent renormalization schemes are implemented. Zero temperature renormalization schemes or renormalization at some fixed reference temperatureT _o are both inadequate as they imply perturbative expansions about fixed points of the renormalization group which are associated with a zero temperature system and a system at temperatureT _o respectively.T dependent schemes give rise to an expansion about the true fixed point of the system, the resulting renormalization group allows the entire crossover between high and low temperature behaviour to be investigated.     

Pseudogap behavior of nuclear spin relaxation in high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> superconductors in terms of phase separation

We analyze anew experiments on the NMR in cuprates and find an important information on their phase separation and its strip character hidden in the dependence of 1/⁶³T₁ on the degree of doping. In a broad class of materials, 1/⁶³T₁ is the sum of two terms: the temperature-independent one attributed to “ incommensurate” strips that occur at external doping and a “universal” temperature-dependent term ascribed to moving metallic and antiferromagnetic subphases. We argue that the frustrated first-order phase transition in a broad temperature interval bears a dynamical character.     

Some peculiarities of spin-lattice relaxation of impurity rare-earth ions in crystals caused by the structure defects

The spin-lattice relaxation times for Nd³⁺ ions in yttrium-aluminum garnets (YAG) and for Yb³⁺ ions in CaF₂ in the low-temperature range have been measured. For the first system the temperature dependence of the relaxation rate is determined to a great extent by the method of sample preparation. For samples grow by the method of the horizontally oriented crystallization the dependence is described asT ₁ ^?1 =AT ⁿ ,n ? 4.7, which is an evidence of an influence of local structure disordering on the relaxation. The temperature dependence of the relaxation rate in CaF₂:Yb is also “anomalous”:T ₁ ^?1 =AT ^3.3. The results are compared with the previous data on the relaxation in similar systems, and with other cases of observation of “anomalous” temperature dependences. Different manifestations of the local crystal defects in spin-lattice relaxation are discussed.     

A study of Fe-doped Bi-Sr-Ca-Cu superconductors

We have substituted 1.5% of Fe for Cu in several “2212” and “2223” Bi-Sr-Ca-Cu superconductors. All of the samples show a reduction ofT _c about 13 K due to the Fe impurities. Mössbauer measurements at room temperature reveal the structural characteristics such as stacking faults and intergrowth of different phases in these Bi-based compounds on the microscopic scale. The susceptibility ofT _c to Fe-doping in the Bi-“2212” or “2223” system is comparable to that of the “123” system but much smaller than that of the “214” system. The interplanar correlation existing in the “123” and the Bi-“2212” and “2223” systems seems to play an important role in sustaining the high temperature superconductivity and weakening the detrimental effect of impurity elements on superconductivity in these two systems.     

The effect of temperature on libron frequencies in molecular crystals: Implications for TTF-TCNQ

Results for the effect of pressure and oemperature on lattice-phonon frequencies in six crystals (As₄S₄, S₈, S₄N₄, C₁₀H₈, C₁₄H₁₀, C₁₆H₁₀) have been analyzed to separate the phonon-excitation-driven (“explicit”) and the thermal-expansion-driven (“implicit”) contributions to dv/dT. The data establish that the volume-driven implicit effect dominates the temperature dependence of external-mode frequencies in molecular crystals. This finding has a bearing upon recent proposals for the role of librons in the electron transport of TTF-TCNQ.     

Kondo scattering and properties of TmSe in the infrared region

The conductivity and permittivity spectra of the intermediate-valence semiconductor TmSe have been measured by terahertz and infrared spectroscopy in a frequency range of 10–10⁴ cm^?1 and a temperature range of 5–300 K. At low temperatures (5 K < T < 100 K), the spectra contain a gap Δ ≈ 2.5 meV, whose appearance is considered to be related to conduction-electron localization at local magnetic moments. At high temperatures (100 K < T < 300 K), the dielectric response is specified by two electronic components: “ light” conduction electrons and “heavy” hybridized f-d states. The microscopic parameters of both components, such as the concentration, mobility, effective mass, relaxation frequency, and the plasma frequency, are determined.     

Temperature dependence of fluorescence quantum yields of some side-substituted anthracene sulphonates

Of the four anthracene sulphonates (Na-salt), anthracene-1-sulphonate (1-AS), anthracene-2-sulphonate (2-AS) and anthracene-1, 5-disulphonate (1, 5-AS) show temperature dependence of φ_f, whereas for anthracene-1, 8-disulphonate (1, 8-AS) φ_f is independent of temperature. E_a for 1-AS, 2-AS and 1, 5-AS are 939 cm^?1, 1144 cm^?1 and 445 cm^?1, respectively. A low energy T_nπ¹ level supplied by ?SO^?₃ group could be responsible for efficient ISC in 1, 8-AS of symmetry C_2v. The possibility of tunnelling due to molecular distortion i.e. “geometry factor” cannot be ruled out also.