Metal-to-insulator crossover in YBa2Cu3Oy probed by low-temperature quasiparticle heat transport

It was recently demonstrated that in La2-xSrxCuO4 the magnetic-field (H) dependence of the low-temperature thermal conductivity kappa up to 16 T reflects whether the normal state under high magnetic field is a metal or an insulator. We measure the H dependence of kappa in YBa(2)Cu(3)O(y) (YBCO) at subkelvin temperatures for a wide doping range, and find that at low doping the kappa(H) behavior signifies the change in the ground state in this system as well. Surprisingly, the critical doping is found to be located deeply inside the underdoped region, about the hole doping of 0.07 hole/Cu; this critical doping is apparently related to the stripe correlations as revealed by the in-plane resistivity anisotropy.     

Evolution of the Hall coefficient and the peculiar electronic structure of the cuprate superconductors

Although the Hall coefficient R(H) is an informative transport property of metals and semiconductors, its meaning in the cuprate superconductors has been ambiguous because of its unusual characteristics. Here we show that a systematic study of R(H) in La2-xSrxCuO4 single crystals over a wide doping range establishes a qualitative understanding of its peculiar evolution, which turns out to reflect a two-component nature of the electronic structure caused by an unusual development of the Fermi surface recently uncovered by photoemission experiments.     

Low-temperature electronic heat transport in La2-xSrxCuO4 single crystals: unusual low-energy physics in the normal and superconducting states

The thermal conductivity kappa is measured in a series of La2-xSrxCuO4 (0 < or = x < or = 0.22) single crystals down to 90 mK to elucidate the evolution of the residual electronic thermal conductivity kappa(res), which probes the extended quasiparticle states in the d-wave gap. We found that kappa(res)/T grows smoothly, except for a 1/8 anomaly, above x approximately 0.05, and shows no discontinuity at optimum doping, indicating that the behavior of kappa(res)/T is not governed by the metal-insulator crossover in the normal state; as a result, kappa(res)/T is much larger than what the normal-state resistivity would suggest in the underdoped region, which highlights the peculiarities in the low-energy physics in the cuprates.     

Quantum phase transitions in the cuprate superconductor Bi2Sr2-xLaxCuO6+delta

To elucidate a quantum phase transition (QPT) in Bi(2)Sr(2-x)La(x)CuO(6+delta), we measure charge and heat transport properties at very low temperatures and examine the following characteristics for a wide range of doping: normal-state resistivity anisotropy under 58 T, temperature dependence of the in-plane thermal conductivity kappa(ab), and the magnetic-field dependence of kappa(ab). It turns out that all of them show signatures of a QPT at the 1/8 hole doping. Together with the recent normal-state Hall measurements under 58 T that signified the existence of a QPT at optimum doping, the present results indicate that there are two QPTs in the superconducting doping regime of this material.     

Magnon-hole scattering and charge order in Sr14-xCaxCu24O41

The magnon thermal conductivity kappa(mag) of the hole-doped spin ladders in Sr14-xCaxCu24O41 has been investigated at low doping levels x. The analysis of kappa(mag) reveals a strong doping and temperature dependence of the magnon mean free path l(mag), which is a local probe for the interaction of magnons with the doped holes in the ladders. In particular, this novel approach to studying charge degrees of freedom via spin excitations shows that charge ordering of the holes in the ladders leads to a freezing out of magnon-hole scattering processes.     

Electronic structure,physico-chemical,linear and non linear optical properties analysis of coronene, 6B-, 6N-, 3B3N- substituted C24H12 using RHF,B3LYP and wB97XD methods

In this work we have investigated the effects of substituting carbon atoms with B, N and BN on the electronic structure, physico-chemical, linear and non linear optical properties of Coronene (C24H12) using HF and DFT methods. We have calculated total electronic energy E₀, zero point vibrational energy ZPVE, the enthalpy H, entropy S, molar heat capacity at constant volume C_v, ionization potential IP, electron affinity EA, hardness \(\kappa\), softness \(\vartheta\), electronegativity EN, dipole moment µ, average polarizability \(< \alpha >\), anisotropy \(\Delta \alpha\), the first molecular hyperpolarizability β_mol, second order hyperpolarizability \(\gamma_{av}\), HOMO–LUMO Energy gap E_gap, work function E_F, refractive index n, susceptibility χ, dielectric constant ε and molar refractivity MR of coronene (C24H12), the 6B-, 6N- and 3B3N- substitute-doped C24H12 C18B6H12 C18N6H12 and C18B3N3H12. The E_gap values of the molecules are between 0.91 and 2.36 eV. We observed that β_mol changes slightly when C24H12 is doped with either 6B or 6N even though their β_mol values are too small. However, by doping C24H12 with both 3B and 3N, creating a strong donor–acceptor system, a very large increase in µ and β_mol was found for C18B3N3H12. This study was done using RHF, B3LYP and wB97XD methods with the cc-pVDZ basis set. The studies have shown that doping decreases some of the above properties significantly while some increases significantly compared to pure coronene, suggesting that 6B-, 6N-, and 3B3N-doped Coronene as serious candidates for electronics, optoelectronics and photonic devices.     

基于ICA与SVM算法的高光谱遥感影像分类

提出了一种利用独立分量分析(ICA)与支撑向量机(SVM)算法进行高光谱遥感影像分类的新方法。采用ICA算法对高光谱遥感影像(PHI传感器获取,80波段)进行了特征提取,并以提取出的影像数据(光谱维数为20)构建SVM分类器。对SVM算法进行核函数删选与参数寻优后,发现采用RBF核的SVM算法(C=103,γ=0.05)分类结果最佳,分类精度与Kappa系数分别达94.5127%与0.935 1,优于BP-神经网络(分类精度39.4758%,Kappa系数0.315 5)、波谱角分类(分类精度80.282 6,Kappa系数0.770 9)、最小距离分类(分类精度85.462 7%,Kappa系数0.827 7)以及最大似然分类(分类精度86.015 6%,Kappa系数0.835 1)4种方法。针对分类结果常出现的"椒盐"现象,利用形态学算子对SVM(RBF核)分类结果进行了类别集群处理,将分类精度与Kappa系数分别提高至94.758 4%与0.938 0,获得了更接近实况的分类图像。结果表明:ICA结合SVM算法准确率高,是高光谱遥感影像分类的优选方法,且类别集群是优化影像分类的有效方法之一。     

Thermal conductivity of the hole-doped spin ladder system Sr14-xCaxCu24O41

The thermal conductivity of the spin-1/2 ladder system Sr14-xCaxCu24O41 ( x = 0, 2, and 12) has been measured both along ( kappa(c)) and perpendicular to ( kappa(a)) the ladder direction at temperatures between 5 and 300 K. While the temperature dependence of kappa(a) is typical for phonon heat transport, an unusual double-peak structure is observed for kappa(c)(T). We interpret this unexpected feature as a manifestation of quasi-one-dimensional magnon thermal transport mediated by spin excitations along the ladders.     

In-plane thermal conductivity of Nd2CuO4: evidence for magnon heat transport

We report the temperature and magnetic field dependence of the in-plane thermal conductivity (kappa(ab)) of high-quality monocrystalline Nd2CuO4. Isothermal measurements of the field dependence of kappa(ab) at low temperatures (2 K相似文献   

Effects of absorbed hydrogen on the electronic properties of (Zr2Fe)(1-x)H(x) metallic glasses

The electrical conductivity (σ) of hydrogen doped (Zr(2)Fe)(1-x)H(x) metallic glasses has been measured in the temperature range from 290 down to 5 K. The decrease of the room temperature conductivity and the increase of its temperature coefficient are explained as consequences of increased disorder due to hydrogen doping. σ(T) for (Zr(2)Fe)(1-x)H(x) metallic glasses at low temperatures decreases with the increase of temperature, forming a minimum at T(min), before it starts a monotonic increase with increasing temperature. Both the functional forms and the magnitudes of the observed σ(T) are interpreted in terms of weak localization, electron-electron interaction and spin-fluctuation effects. Our results reveal that the electron-phonon scattering rate varies with the square of temperature from low temperatures up to 100 K and changes behaviour to a linear form at higher temperatures. At low temperatures, the minimum in σ(T) is shifted to higher temperatures, which is ascribed to the increase of the screening parameter of the Coulomb interaction F* associated with the enhancement of the spin fluctuations arising from the increase of the hydrogen doping. The spin-orbit scattering rate and the electron diffusion constant are reduced by hydrogen doping.     

Specific heat and thermal conductivity in the mixed state of MgB2

The specific heat C and the electronic and phononic thermal conductivities kappa(e) and kappa(ph) are calculated in the mixed state for magnetic fields H near H(c2), including the effects of supercurrent flow and Andreev scattering. The resulting function C(H) is nearly linear while kappa(e)(H) exhibits an upward curvature near H(c2). The slopes decrease with impurity scattering which improves the agreement with the data on MgB2. The ratio of phonon relaxation times tau(n)/tau(s)=g(omega(0),H) for phonon energy omega(0) is smeared out around omega(0)=2Delta and tends to one for increasing H. This leads to a rapid reduction of kappa(ph)(H) in MgB2 for relatively small fields due to the rapid suppression of the smaller energy gap.     

Weak-field thermal hall conductivity in the mixed state of d-wave superconductors

Thermal transport in the mixed state of a d-wave superconductor is considered within the weak-field regime. We express the thermal conductivity, kappa(xx), and the thermal Hall conductivity, kappa(xy), in terms of the cross section for quasiparticle scattering from a single vortex. Solving for the cross section (neglecting the Berry phase contribution and the anisotropy of the gap nodes), we obtain kappa(xx)(H,T) and kappa(xy)(H,T) in surprisingly good agreement with the qualitative features of the experimental results for YBa2Cu3O6.99. In particular, we show that the simple, yet previously unexpected, weak-field behavior, kappa(xy)(H,T) approximately T squareroot [H], is that of thermally excited nodal quasiparticles, scattering primarily from impurities, with a small skew component provided by vortex scattering.     

Understanding the friction behavior of sulfur-terminated diamond-like carbon films under high vacuum by first-principles calculations

Generally, the repulsive force was a key factor account for superlow friction of H or F doped diamond-like carbon (DLC) films under high vacuum. As we known, H or F doped DLC usually exhibited superlow friction under high vacuum. However, the superlow friction of S doped DLC under high vacuum was not found so far. This phenomenon was desirable to be well investigated. In this work, S-terminated diamond interfaces also exhibited strong repulsive force, however, the estimated friction coefficient was variable for S-terminated diamond interfaces. The lowest and largest friction coefficient was about 0.003 and 0.4 respectively, which indicated that the superlow friction of S doped DLC could achieve in theory. In order to well probe the unusual friction behavior of S doped DLC under high vacuum, using first-principles method, the repulsive interaction between sliding surfaces was well investigated in order to understand the unusual friction behavior of S doped DLC films.     

Low-energy theory of disordered graphene

At low values of external doping, graphene displays a wealth of unconventional transport properties. Perhaps most strikingly, it supports a robust "metallic" regime, with universal conductance of the order of the conductance quantum. We here apply a combination of mean-field and bosonization methods to explore the large scale transport properties of the system. We find that, irrespective of the doping level, disordered graphene is subject to the common mechanisms of Anderson localization. However, at low doping a number of renormalization mechanisms conspire to protect the conductivity of the system, to an extend that strong localization may not be seen even at temperatures much smaller than those underlying present experimental work.     

Doping dependence of infrared conductivity of camphor-sulphonic-acid-doped polyaniline

The reflectivity spectrum of a polyaniline CSA-doped in presence of m-cresol has been measured over the wide wavenumber range 25-15,000 cm^-1 (0.003-1.9 eV) for three different doping levels. Since spectra cannot be fitted correctly with the conventional Drude model, several extensions are tested. A model derived from the factorized form of the dielectric response and including the effect of Anderson localization in disordered metals, is proposed and found to yield good fit to data with a satisfactory physical meaning. Data are reduced to a small number of parameters potentially useful for further comparison with other conducting polymers or even other non-Drude conducting media like oxides. Received 6 February 2002 / Received in final form 12 August 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: gervais@delphi.phys.univ-tours.fr RID="b" ID="b"UMR 6157 CNRS/CEA     

掺杂物对MgB2超导电性能影响的研究进展

总结了不同掺杂物对MgB2超导电性能影响的研究现状,具体介绍了不同粒度、不同类型的掺杂物对MgB2超导电性的影响。目前的研究结果表明:所有掺杂都降低了MgB2的临界转变温度,而除Cu外的大部分掺杂物都可改善MgB2的不可逆磁场Hirr,提高其临界电流密度Jc;在所有掺杂物中,纳米级S iC掺杂对MgB2的超导电性能改进最大。基于目前的研究现状和结果,最后文中对MgB2超导材料的应用前景进行了展望。     

Absence of large nanoscale electronic inhomogeneities in the Ba(Fe1-xCox)2As2 pnictide

⁷⁵As NMR and susceptiblity were measured in a Ba(Fe_1-xCo_x)₂As₂ single crystal for x = 6% for various field H values and orientations. The sharpness of the superconducting and magnetic transitions demonstrates a homogeneity of the Co doping x better than ±0.25%. On the nanometer scale, the paramagnetic part of the NMR spectra is found very anisotropic and very narrow for H∥ab which allows to rule out the interpretation of reference [J. Phys. Soc. Jpn 78, 013711 (2009); Phys. Rev. B 79, R 140506 (2009)] in terms of strong Co induced electronic inhomogeneities. We propose that a distribution of hyperfine couplings and chemical shifts due to the Co effect on its nearest As explains the observed linewidths and relaxations. All these measurements show that Co substitution induces a very homogeneous electronic doping in BaFe₂As₂, from nano to micrometer lengthscales, on the contrary to the K doping.     

Competing orders and superconductivity in the doped mott insulator on the Shastry-Sutherland lattice

Quantum antiferromagnets on geometrically frustrated lattices often allow a number of unusual paramagnetic ground states. The fate of these Mott insulators upon doping is an important issue that may shed some light on the high T(c) cuprate problem. We consider the doped Mott insulator on the Shastry-Sutherland lattice via the t-J model. The U(1) slave-boson mean-field theory reveals the strong competition between different broken symmetry states. It is found that, in some ranges of doping, there exist superconducting phases with or without coexisting translational-symmetry-breaking orders such as the staggered flux or dimerization. Our results will be directly relevant to SrCu2(BO3)(2) when this material is doped in future.     

Weak localization of Dirac fermions in graphene

In the presence of the charged impurities, we study the weak localization effect by evaluating the quantum interference correction to the conductivity of Dirac fermions in graphene. With the inelastic scattering rate due to electron-electron interactions obtained from our previous work, we investigate the dependence of the quantum interference correction on the carrier concentration, the temperature, the magnetic field, and the size of the sample. It is found that weak localization is present in large size samples at finite carrier doping. Its strength becomes weakened or quenched when the sample size is less than a few microns at low temperatures as studied in the experiments. In the region close to zero doping, the system may become delocalized. The minimum conductivity at low temperature for experimental sample sizes is found to be close to the data.     

Unusual magnetic-field-induced phase transition in the mixed state of superconducting NbSe2

The thermal conductivity kappa in the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c axis, at several temperatures below T(c). With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H(c2), an unexpected hysteretic behavior of kappa(H) with all the generic features of a first order phase transition. The transition is not manifest in the kappa(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.