Low temperature resistivity minimum and its correlation with magnetoresistance in La0.67Ba0.33MnO3 nanomanganites

A systematic investigation of structural, magnetic and electrical properties of nanocrystalline La_0.67Ba_0.33MnO₃ materials, prepared by citrate gel method has been undertaken. The temperature-dependant low-temperature resistivity in ferromagnetic metallic (∼50 K) phase shows upturn behavior and is suppressed with applied magnetic field. The experimental data (<75 K) can be best fitted in the frame work of Kondo-like spin-dependant scattering, electron-electron and electron-phonon interactions. It has been found that upturn behavior may be attributed to weak spin disorder scattering including both spin polarization and grain boundary tunneling effects, which are the characteristic features of extrinsic magnetoresistance behavior, generally found in nanocrystalline manganites. The variation of electrical resistivity with temperature in the high temperature ferromagnetic metallic part of electrical resistivity (75K<T<T_P) has been fitted with grain/domain boundary, electron-electron and magnon scattering mechanisms, while the insulating region (T>T_P) of resistivity data has been explained based on adiabatic small polaron hopping mechanism.     

Evidence for charge Kondo effect in superconducting Tl-doped PbTe

We report results of low-temperature thermodynamic and transport measurements of Pb1-xTlxTe single crystals for Tl concentrations up to the solubility limit of approximately x=1.5%. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature T(K) approximately 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high T(c) values.     

Influence of the low Mn intercalation on magnetic and electronic properties of 2H-TaS2 single crystals

The low-Mn-intercalated compounds 2H-Mn_xTaS₂ (x≤0.1) single crystals were successfully grown via the iodine chemical vapor transport technique; the electrical and magnetic properties have been investigated systematically. No signs of charge density wave and superconductivity transitions appear down to 2 K even when x=0.02, while a small resistivity upturn has been observed in the low-temperature region for Mn_xTaS₂ samples. A substantial magnetocrystalline anisotropy is observed and the Mn magnetic moments lie mainly parallel to the ab-plane. The glass behavior was observed in the low-temperature region firstly from the ac susceptibility measurement, the freezing of the glass may contribute to the upturn of the resistivity.     

Metallic low-temperature resistivity in a 2D electron system over an extended temperature range

We report measurements of the zero-field resistivity in a dilute 2D electron system in silicon at temperatures down to 35 mK. This extends the previously explored range of temperatures in this system by almost an order of magnitude. On the metallic side, the resistivity near the metal-insulator transition continues to decrease with decreasing temperature and shows no low-temperature upturn. At the critical electron density, the resistivity is found to be temperature independent in the entire temperature range from 35 mK to 1 K.     

Synthesis and electrical transport of SrHfO3 thin films grown on a SrTiO3 (001) substrate using a pulsed laser deposition

We report the deposition of epitaxial SrHfO₃ thin films on a SrTiO₃ (001) substrate in different substrate temperatures by using a pulsed laser deposition (PLD) method. We carried out X-ray diffraction (XRD), X-ray reflectivity (XRR), reciprocal space mapping (RSM), atomic force microscopy (AFM), resistivity, and Hall measurements to examine the crystallinity, morphology and electrical properties of these films. All films showed smooth and uniform morphology with small root mean square (RMS) roughness. While the SrHfO₃ sample grown at 750 °C is metallic, the films deposited at 600 °C, 650 °C, and 700 °C show an upturn at low temperatures. The temperature dependence of the metallic parts was analyzed based on the parallel resistor model that includes resistivity saturation. On the other hand, the low-temperature upturn was found to be well described by a weak localization mechanism. We also observed the possible emergence of non-Fermi liquid behavior when the upturn disappeared. All SrHfO₃ films have p-type charge carriers.     

Origin of the anomalous low temperature upturn in the resistivity of the electron-doped cuprate superconductors

The temperature, doping, and field dependences of the magnetoresistance (MR) in Pr2-xCexCuO4-delta films are reported. We distinguish between orbital MR, found when the magnetic field is applied perpendicular to the ab planes, and the nearly isotropic spin MR. The latter, the major MR effect in the superconducting samples, appears in the region of the doping-temperature phase diagram where drho/dT<0, or an upturn in the resistivity appears. We conclude that the upturn originates from spin scattering processes.     

Size-dependent electronic-transport mechanism and sign reversal of magnetoresistance in Nd0.5Sr0.5CoO3

A detailed investigation of electronic-transport properties of Nd(0.5)Sr(0.5)CoO(3) has been carried out as a function of grain size ranging from micrometre order down to an average size of 28 nm. Interestingly, we observe a size induced metal-insulator transition in the lowest grain-size sample while the bulk-like sample is metallic in the whole measured temperature regime. An analysis of the temperature dependent resistivity in the metallic regime reveals that the electron-electron interaction is the dominating mechanism while other processes like electron-magnon and electron-phonon scatterings are also likely to be present. The fascinating observation of enhanced low temperature upturn and minimum in resistivity on reduction of grain size is found due to electron-electron interaction (quantum interference effect). This effect is attributed to enhanced disorder on reduction of grain size. Interestingly, we observe a cross over from positive to negative magnetoresistance in the low temperature regime as the grain size is reduced. This observed sign reversal is attributed to enhanced phase separation on decreasing the grain size of the cobaltite.     

Finite temperature physics of 1D topological Kondo insulator: Stable Haldane phase,emergent energy scale and beyond

In recent years, interacting topological insulators have emerged as new frontiers in condensed matter physics, and the hotly studied topological Kondo insulator (TKI) is one of such prototypes. Although its zero-temperature ground-state has been widely investigated, the finite temperature physics on TKI is largely unknown. Here, we explore the finite temperature properties in a simplified model for TKI, namely the one-dimensional p-wave periodic Anderson model, with numerically exact determinant quantum Monte Carlo simulation. It is found that the topological Haldane phase established for groundstate is still stable against small thermal fluctuation and its characteristic edge magnetization develops at low temperature. Such facts emphasize the robustness of (symmetry-protected) topological order against temperature effect, which always exists at real physical world. Moreover, we use the saturated low-T spin structure factor and the \frac{1}{T}-law of susceptibility to detect the free edge spin moment, interestingly the low-temperature upturn behavior of the latter one is similar to experimental finding in SmB₆ at T<50 K. It implies that similar physical mechanism may work both for idealized models and realistic correlated electron materials. We have also identified an emergent energy scale T_cr, which signals a crossover into interesting low-T regime and seems to be the expected Ruderman–Kittel–Kasuya–Yosida coupling. Finally, the collective Kondo screening effect has been examined and it is heavily reduced at boundary, which may give a fruitful playground for novel physics beyond the wellestablished Haldane phase and topological band insulators.     

The effect of segregated transition metal ions on the grain boundary resistivity of gadolinium doped ceria: Alteration of the space charge potential

The effect of segregated transition metals on the grain boundary resistivity of 1 mol% Gd-doped ceria has been investigated. The main focus of interest is whether the space charge potential that causes the blocking effect of the grain boundaries of the ceria can be extrinsically modified. The introduction of a small amount (< 0.5 mol%) of transition metals (Fe, Co, Mn and Cu) to 1 mol% Gd-doped ceria results in significant reductions in only the grain boundary resistivities of the samples attributed to exclusive segregation of the transition metals into the grain boundary core. In the case of Co- and Fe-doped samples, the grain boundary resistivity is lowered by an order of magnitude. EELS line scans across the grain boundaries of the Fe-doped sample have indicated that the grain boundaries are free of a secondary phase of transition metal oxide and that the Fe in the grain boundary likely exists as point defects. These results strongly suggest that it is indeed possible to reduce the excess positive charge in the grain boundary core, and thus the grain boundary resistivity in a ceria electrolyte, extrinsically as initially postulated. A defect chemistry model which explains partial counterbalance to the positive grain boundary core charge has been suggested. The resistivity minimum shown for the samples with different Fe concentration indicates that there is an optimum concentration of transition metal in the grain boundary core of the ceria necessary for such a countbalance.     

Observation of bulk superconductivity in NaxCoO2.yH(2)O and NaxCoO2.yD(2)O powder and single crystals

Poly- and single-crystalline NaxCoO2 has been successfully intercalated with H2O and D2O as confirmed by x-ray diffraction and thermogravimetric analysis. Resistivity, magnetic susceptibility, and specific heat measurements show bulk superconductivity with T(c) close to 5 K in both cases. The substitution of deuterium for hydrogen has an effect on T(c) of less than 0.2 K. Investigation of the resistivity anisotropy of NaxCoO2.yH(2)O single crystals shows (a). almost zero resistivity below T(c), and (b). an abrupt upturn at T(*) approximately 52 K in both the ab plane and the c direction. The implications of our results on the possible superconducting mechanism will be discussed.     

Effect of oxygen on superconductivity and structure of Bi-system superconductor

A systematic study of the effects of oxygen content and distribution in 2223 single-phase Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_10+δ s superconductor has has been carried out using a combination of quenching in air and low-temperature annealing in nitrogen. With decreasing oxygen content a maximum was observed in T_c, accompanying the increase in room-temperature resistivity. Also the effect of oxygen redistribution has been studied by low-temperature annealing in nitrogen, which allows no oxygen content change. After this annealing, in T_c, the room temperature resistivity and the c-axis parameter an apparent change occurred as compared to the original quenching states. From tge measurement of in situ resistivity which increased gradually during low-temperature annealing, it seems that a metastable phase was formed by oxygen diffusion. The interchange of oxygen atoms between Bi₂O₂ and CuO₂ planes and the possible ordering of oxygen vacancies were proposed to explain the relevant experimental phenomena.     

Spin-glass and spin-fluctuation in Mo-doped Ca3Co4O9 system

Structural, magnetic, resistivity and thermal transport measurements have been performed to study the Mo-doping effect on a layered cobaltite Ca₃Co_4−xMo_xO₉(0≤x≤0.4) system. The results indicate that the low-temperature magnetic behavior of the system changes from a ferrimagnetic state to a spin-glass-like state upon Mo doping, which is due to the decrease in the average valence of Co ions. Moreover, all the Mo-doped samples have a higher resistivity and larger thermopower S compared with the Mo-free sample. The variation in the resistivity and thermopower between the Mo-doped and the Mo-free samples is dominated by the change in the carrier concentration of the samples. In the Mo-doped samples with x≥0.1, both the resistivity and thermopower decrease gradually with increasing Mo-doping level, which is suggested to mainly originate from the variation in the carrier mobility of the samples. In addition, an obvious thermopower upturn is observed in the S(T) curve of all the Mo-doped samples, which can be explained by the enhancement of spin-fluctuation induced by Mo-doping.     

Observation of novel coexistence of Kondo effect and room temperature magnetism in AlN/Al/AlN trilayer thin film

In this work for the first time, we are reporting the unusual observation of the Kondo effect with the coexistence of room temperature ferromagnetism in AlN/Al/AlN trilayer thin film. The grown film shows resistivity minimum at a temperature of ∼48K, which shifts to the lower temperature on the application of magnetic fields. After considering various possibilities for an upturn in resistivity, we found that the Kondo scattering is responsible for upturn at low temperature. The simultaneous presence of ferromagnetism and Kondo scattering is explained by spatial variation of nitrogen vacancy defects from the film surface to the Al sandwich layer. Furthermore, magneto-transport properties of the film measured at different temperature exhibits both negative and positive components described by localized magnetic moment model for the spin scattering of carriers and two-band model, respectively. This work provides insight into the novel co-existence of ferromagnetism and Kondo effect in crystalline AlN.     

Effect of Mn substitution on the delicate balance between structure and properties of Gd1.4Ce0.6Sr2RuCu2O10

Some new members of a ruthenocuprate(2212) series have been synthesized by Mn substitution for Ru in Gd_1.4Ce_0.6Sr₂RuCu₂O₁₀. Characterization by x-ray diffraction (XRD) phase analysis has been carried out. Changes in structural features on substitution, including a significant change in lattice parameter for a very low substitution level, have been observed. Four-probe resistivity studies indicate the coexistence of superconductivity and magnetism for the pristine compound and a semiconductor-like upturn in resistivity and the absence of superconductivity even for very low levels of Mn substitution. AC susceptibility measurements show a progressive suppression of the magnetic transition temperature as well as a smearing of the magnetic transition as a function of Mn substitution. Possible reasons for the absence of superconductivity have been discussed.     

Effect of grain boundary on electrical properties of polycrystalline lanthanum nickel oxide thin films

In the present work, lanthanum nickel oxide (LNO) thin films were prepared by the sol-gel method. Microstructures of the films were tailored by changing sol concentration so as to investigate the effect of grain boundary on the transport properties of electrons in the polycrystalline LNO films. Based on the temperature dependence of the resistivity and the magnetic field dependence of the magnetoresistance (MR) at various temperatures, the factors that dominate the transport behavior in the polycrystalline LNO films were explored in terms of weak localization and strong localization. The results show that the grain boundary has a significant influence on the transport behavior of the electrons in LNO films at a low-temperature region, which can be captured by a variable-range hopping (VRH) model. The increase of metal–insulator (M–I) transition temperature is ascribed to Anderson localization in grain boundary. At a high-temperature region, electron–electron scattering and electron–phonon scattering predominates in the films. In this case, the existence of more grain boundary shows a minor effect on the transport behavior of the electrons but elevates the residual resistivity of the films.     

Anomalous Hall Effect in Layered Ferrimagnet MnSb_2Te_4

We report on low-temperature electron transport properties of MnSb_2Te_4,a candidate of ferrimagnetic Weyl semimetal.Long-range magnetic order is manifested as a nearly square-shaped hysteresis loop in the anomalous Hall resistance,as well as sharp jumps in the magnetoresistance.At temperatures below 4K,a lnT-type upturn appears in the temperature dependence of longitudinal resistance,which can be attributed to the electron-electron interaction(EEI),since the weak localization can be excluded by the temperature dependence of magnetoresistance.Although the anomalous Hall resistance exhibits a similar lnT-type upturn in the same temperature range,such correction is absent in the anomalous Hall conductivity.Our work demonstrates that MnSb_2Te_4 microflakes provide an ideal system to test the theory of EEI correction to the anomalous Hall effect.     

H+ irradiation effect in Co-doped BaFe2As2 single crystals

We report the suppression of the critical temperature T_c in single crystalline Ba(Fe_1?xCo_x)₂As₂ at under-, optimal-, and over-doping levels by 3 MeV proton irradiation. T_c decreases and residual resistivity increases monotonically with increasing the dose. The low-temperature resistivity does not show the upturn in contrast with the α-particle irradiated NdFeAs(O,F), which suggests that proton irradiation introduces nonmagnetic scattering centers. Critical scattering rates for all samples obtained by three different ways are much higher than that expected in s±-pairing scenario based on inter-band scattering due to antiferro-magnetic spin fluctuations.     

Effect of the chiral chain length on structural and phase properties of ferroelectric liquid crystals

Studies of structural and phase properties obtained on several ferroelectric liquid crystalline materials with 2-alkoxypropionate group used as a chiral centre and without any lateral substitution are presented. In dependence on the chiral chain length these compounds exhibit the cholesteric N* phase, the ferroelectric smectic C* and a low-temperature SmX phase. Values of the spontaneous polarization and spontaneous tilt angle have been determined within the whole range of the SmC* phase. A low-temperature SmX phase has been identified as the orthogonal hexatic SmB* phase. The molecular parameters, namely the layer spacing in the SmC* and SmB* phases and the average intermolecular distances (D) between neighbouring parallel molecules in all investigated phases have been determined using the results of the X-ray diffraction obtained on non-oriented samples. The effect of the chiral chain length on mesomorphic, structural and physical properties of the studied ferroelectric liquid crystalline materials is discussed.     

Probing the topological exciton condensate via Coulomb drag

The onset of exciton condensation in a topological insulator thin film was recently predicted. We calculate the critical temperature for this transition, taking into account screening effects. Furthermore, we show that the proximity to this transition can be probed by measuring the Coulomb drag resistivity between the surfaces of the thin film as a function of temperature. This resistivity shows an upturn upon approaching the exciton-condensed state.     

铁磁金属中自旋波散射引起的电阻率

铁磁金属中自旋向上电子与自旋向下电子的能量由于磁化而造成的分裂,在低温下对自旋波散射引起的电阻率(磁电阻率)有重要的影响。由于这个分裂,单带情况下低温磁电阻率总是衰减的;但两带情况下则有可能增强。考虑到自旋波谱中的能隙用单带模型所得的磁电阻率表达式与Tb和Dy的实验资料比较表明:不但低温电阻率的温度依赖性符合,数值上也与高温磁电阻测量结果一致。 关键词：