Interplay of Nuclear Magnetism and Superconductivity

The ratio of nuclear saturation magnetization and superconducting critical field, ₀ M _sat / B _S0*, classifies the strength of mutual influence of nuclear magnetism and superconductivity. In order to investigate the interplay of both phenomena for the three distinct cases 1, 1, and 1 we have measured the ac susceptibility of Al, of the intermetallic compound AuIn ₂ , and of the metal hydride TiH _2.07 at ultralow temperatures, 17 K T 1 K, as function of static field 0 B 15 mT. For Al, the interplay enables an absolute measurement of the nuclear magnetization. For AuIn ₂ , we get a steep decrease of B _S (T) and a broadening of the superconducting transition in its nuclear ferromagnetic phase. Surprisingly, the nuclear ferromagnetic state coexists with type-I superconductivity in AuIn ₂. The metal hydride TiH _2.07 , which is under present investigation, is a good candidate to show reentrant superconductivity.     

Magnetic Impurities in Glass and Silver Powder at Milli- and Microkelvin Temperatures

In search of an ideal paramagnet for thermometry at very low temperatures, we have studied the magnetic behaviour of impurity moments (localized Fe³⁺, impurity concentration x = (180±10) ppm) dissolved in a structural borosilicate glass, in the temperature range 0.07 T 300 mK by means of dynamic ac susceptibility, and at 1.6T 300 K by static dc magnetization. In order to improve the thermal coupling of the insulating glass at the lowest temperatures, it was pulverized and mixed with silver powder of submicron grain size; the composite was subsequently compacted to a cylindrical sample by applying a pressure of a few kbar. This contact method which is applicable to other materials with bad thermal conductivity as well improved the accessible minimum temperature for the glass down to 0.1 mK. At low temperatures, we observe a Curie Weiss law for the dynamic susceptibility of the magnetic Fe³⁺ impurities in the glass down to 0.6 mK, a broad maximum of at T 0.38 mK and a decrease towards even lower temperatures. Compared to the frequently used, highly diluted metallic spin glasses, the magnetic behaviour of the glass makes inductance thermometry applicable in a much larger temperature range. In addition, we have investigated the low temperature magnetic properties of a sample of compacted silver powder. The small amount of paramagnetic impurities in the Ag particles (x = (4 ± 1) ppm) exhibits a low temperature susceptibility which can be described by the Kondo effect with an unexpectedly small Kondo temperature of T _K 1 mK.     

Optical Properties, Electronic Structure and Magnetism of α′-Na x V2O5

The optical properties of sodium-deficient α′-Na _x V₂O₅ (0.85 ≤ x ≤ 1.00) single crystals are analyzed using ellipsometry, and infrared reflectivity techniques. In sodium deficient samples, the optical absorption peak associated to the fundamental electronic gap develops in the middle of the pure α′-NaV₂O₅ gap at ～0.44 eV, and the material remains insulating up to the maximal achieved hole concentration of about 15%. Nonmetallic behavior under hole doping provoked reinterpretation of the α′-NaV₂O₅ optical spectra. We argue that the absorption peak at about 0.9 eV corresponds to the photoionization energy of a large polaron.     

Electronic conduction in bulk Se1−x Te x glasses at high pressures and at low temperatures

The electrical resistivity of bulk Se_1−x Te _x glasses is reported as a function of pressure (up to 8 GPa) and temperature (down to 77K). The activation energy for electronic conduction has been calculated at different pressures. The samples with 0⩽x⩽0·06 show a single activation energy throughout the temperature range of investigations. On the other hand samples with 0·08⩽x⩽0·3 show two activation energies in the different regions of temperature. The observed behaviour has been explained on the basis of band picture of amorphous semiconductors.     

Pressure Effect on Superconductivity and Magnetism in α-FeSe x

In this paper, the pressure effect on superconductivity and magnetism has been investigated in FeSe _x (x=0.80,0.88). The magnetization curves display anomaly at T _s1∼106 K and T _s2∼78 K except for the superconducting diamagnetic transition around T _c ∼8 K. The magnetic anomaly at T _s1 and T _s2 can be related to a ferromagnetic and an antiferromagnetic phase transition, respectively, as revealed by specific heat measurements. The application of pressure not only raises T _c , but also increases both T _s1 and T _s2.      

The Electronic Structures and Magnetism in Al Doped 4H-SiC: The First-Principles Calculation

In the field of a diluted magnetic semiconductor, a nontransition metal doped semiconductor or oxide is a way to avoid the ferromagnetic clusters of secondary phase. Since there is no d electron in such system, the origin of magnetism is a surprise. In this article, first-principles calculations were used to investigate the origin of magnetism in Al doped 4H-SiC. It is found that a hole was introduced into the SiC:Al system, which is located on one C atom adjacent to the Si vacancy. A neutral Si vacancy with a substituting Al lead to the formation of a net moment less than 1.0μ _B, while the Al itself has no contribution to the magnetic moment, but plays a key role to form them. The coupling between such local moments is paramagnetic.     

Electronic Structures,Stabilities, and Magnetism of SrTiO3 Monolayer and Ultrathin Nanotubes

Journal of Superconductivity and Novel Magnetism - Based on density functional theory, we predict the electronic structures, stabilities, and magnetism of ultrathin SrTiO3 nanotubes (u-STONTs) with...     

The Impact of Nuclear Magnetism on Superconductivity in a Metal with Nuclear Electric Quadrupole Splitting: Indium

We report the first investigation of the impact of nuclear magnetism on superconductivity in the tetragonal metal indium. We have measured the superconducting critical field B_c(T) and in its vicinity the nuclear magnetic heat capacity at ultralow temperatures, 170 KT200 mK. We compare the measured quantities with calculations which consider the nuclear magnetic Zeeman and the dominating nuclear electric quadrupole interaction in indium. The heat capacity data support the occurence of a positive sign of the electrical field gradient at nuclear sites and in consequence the existence of a nuclear low spin ground state. Surprisingly, at lowest investigated temperatures, 170 KT1 mK, the reduction of the critical field B_c(T) clearly exceeds the size of the calculated magnetization ₀ M(B_c, T) which is limited by the nuclear low spin ground state. In all other materials the interplay of nuclear magnetism and superconductivity has been studied so far (Al, AuAl ₂ , AuIn ₂ , Rh, and Sn), the bare nuclear magnetization appeared as an upper limit of the reduction of the critical field.     

Thermal Conductivity of Glasses Induced by Nuclear Quadrupole Interaction at Ultra Low Temperatures

It is investigated how nuclear degrees of freedom of tunneling system (TS) inherent in amorphous solids influence its acoustic properties. It was shown in our previous papers that below 10 mK nuclear quadrupole interaction breaks down the coherent tunneling. This phenomenon results in appearance of the quasi-gap in the distribution function for the tunneling amplitude splitting. The quasi-gap is responsible for the plateau in the temperature dependence of the real part of a dielectric permittivity or speed of sound. In this paper we are interested in ultrasonic absorption and thermal conductivity which are intimately connected. We demonstrate that there exists a temperature interval in a millikelvin region where the sound absorption behavior changes drastically from the behavior predicted by the standard tunneling model (STM). In particular, the sound absorption increases approximately by an order of magnitude. Since in the millikelvin region the heat transport is due to acoustic phonons, the thermal conductivity also should demonstrate a strong increase as compared to standard tunneling model. The application of a strong magnetic field is known to restore the coherent tunneling and the standard distribution for the tunneling splitting amplitude. Thus, one can expect that in a strong magnetic field the thermal conductivity should drop in the temperature interval where the coherent tunneling was initially destroyed.     

μSR Study on Slowing-Down Behavior of the Cu-Spin Fluctuations at High Temperatures in La2−x Sr x CuO4

Zero-field muon-spin-relaxation measurements have been carried out in order to investigate changes of the Cu-spin fluctuations in a wide hole-concentration range of La_2–x Sr _x CuO₄. An increase of the muon-spin depolarization rate with decreasing temperature has been observed between about 70 K and 140 K in the underdoped samples. The characteristic temperature, where the muon-spin depolarization rate starts to increase with decreasing temperature, has been found to decrease with increasing x and seems to vanish around x=0.15 after showing a local maximum around x=0.115. The present results are discussed in terms of the dynamical stripe and the pseudogap.     

金刚石在不同温度和压力下的强度及断裂

本文利用软压头技术,测定了人造金刚石和天然金刚石在不同温度下的强度．在室温,利用软压头技术测得的金刚石强度与常规方法所得的结果一致．高温条件下,金刚石的断裂行为取决于试验温度、平均接触压力及摩擦系数.观察到两种类型的裂纹,｛111｝解理裂纹与低温、高压力有关.而应变诱发裂纹产生于｛110｝面上,与高温、低应力有关.根据试验结果,提出了金刚石在不同温度和应力状态下的强度图．     

Pressure Effect on the Coexistence of Magnetism and Superconductivity in Ho1−x Dy x Ni2B2C

We have measured the transport properties of Ho_(1–x)Dy _x Ni₂B₂C (x=0.1, 0.4) single crystals to study the effects of high pressure conditions on the superconductivity and magnetism. The pressure affects the pair-breaking interaction by changing the magnetic exchange integral J _sf. In the case of x=0.1, the superconducting transition temperature T _c is higher than the Néel temperature T _N and decreases as the pressure increases. On the other hand, for x=0.4, when have lower T _N than T _c, T _c does not change with increasing pressure. These results are due to the effect of the pressure on J _sf.     

Nuclear Magnetism of Normal 3He and 3He-4He Mixtures in Aerogel

We report NMR experiments at 8 T on ³ He and ³ He- ⁴ He mixtures filling the pores of 95% porous aerogel, for temperatures T 6 mK. Magnetization measurements of pure ³ He reveal a localized layer approximately one monolayer thick. The longitudinal relaxation includes a component logarithmic in time, which is apparently associated with a fraction of the localized ³ He atoms. When the localized ³ He is displaced by adding ⁴ He the logarithmic relaxation disappears and T ₁ for the dominant exponential relaxation increases. Measurements of the spin diffusion coefficient with the aerogel filled with dilute solution in equilibrium with bulk phase-separated mixture provide an unambiguous determination of the spin mean free path,_s = 58 nm     

低延性陶瓷材料高温KR-曲线行为

在高温下采用加载－卸载技术与柔度标定相结合的方法测量了SiC陶瓷材料的KR－曲线行为。结果表明：在高温下陶瓷材料KR随裂纹长度的增加而提高。随温度提高，陶瓷材料韧化指数m减小，裂纹阻力降低，其KR曲线越平坦。陶瓷材料在高温下增大的KR曲线行为，与裂纹的转向及裂纹尾区裂纹表面之间的相互作用密切相关，加载－卸载技术与柔度标定相结合的试验方法，对测量陶瓷材料高温KR－曲线是一种有效简便的方法。     

不同温度烧结的赝两元合金(PbTe)1-x(snTe)x电学性能

首次采用粉末冶金法在大成分范围内制备了赝两元合金(PbTe)_1-x(SnTe)_x(x=0～1),并对其电学性能进行了系统地研究.实验发现:经550℃烧结后,随合金中SnTe摩尔分数x增大,合金的最大Seebeck系数值减小.当摩尔分数x≤0.6时,最大Seebeck系数所对应的温度逐渐升高;摩尔分数x>0.6时基本不变.当摩尔分数x=0.6时电导率达最大值.经高温烧结后两参数值也有类似的变化规律,但摩尔分数>0.2的各合金Seebeck系数值高于550℃烧结后的合金,电导率降低.从而得出:烧结温度对赝两元合金(PbTe)_1-x(SnTe)_x的综合电学性能影响不大.     

Magnetism and magnetotransport in sonochemically-prepared amorphous Co(100-x)Pt(x) nanoalloys

Amorphous nanoalloys of Co(100-x)Pt(x) (0 < or = x < or = 50) prepared sonochemically exhibit a negative magnetotransport effect, showing a maximum for platinum volume fraction of 10%. The oscillatory peak maxima observed in x = 30 composition for residual resistivity, rho0 and Tmin corresponds to the peak minima for the negative MR% in the range of compositions studied. We observe that in weakly exchange coupled amorphous alloys, the local magnetic order at Tmin and the disorder effects observed from rho0 become crucial.     

The Long Range Interaction and the Relaxation in Glasses at Low Temperatures

We describe the interaction stimulated relaxation in the ensemble of two-level systems, responsible for low temperature kinetics and thermodynamics properties of amorphous solids. This relaxation gets significant at sufficiently low temperature when phonons are substantially frozen out. We show that in the realistic experimental situation the measuring field strongly accelerates the interaction stimulated relaxation. The characteristic temperature and field dependencies of the relaxation rate are found when the rate is affected both by the interaction between two level systems and by the external field. PACS numbers: 61.43.Fs, 75.50.Lk, 77.22.Ch