Superconductivity at 41 K and its competition with spin-density-wave instability in layered CeO1-xFxFeAs

A series of layered CeO1-xFxFeAs compounds with x=0 to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high T_{c} strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce 4f electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.     

Growth and transport features of electron-doped superconductor Pr1-xLaCexCuO4-δ thin films

<正>T’-phase electron-doped superconductor Pr_1-xLaCe_xCuO_4-δ（PLCCO） thin films are successfully prepared on SrTiO₃（100） substrates by using the dc magnetron sputtering method.It is found that the films each have a highly oriented structure along the c-axis.For optimally doped films with x≈0.10,the superconducting transition temperature T_c is 23.5 K,which is similar to that of a single crystal.The quadratic temperature dependence of the resistivity is observed when T > T_c,which can be attributed to the two-dimensional Fermi liquid behaviour.Besides,the optimal conditions for preparing the T’-phase PLCCO thin films are also discussed in detail.     

Superconductivity in CuIr2-xAlxTe4 telluride chalcogenides

The relationship between charge-density-wave (CDW) and superconductivity (SC), two vital physical phases in condensed matter physics, has always been the focus of scientists' research over the past decades. Motivated by this research hotspot, we systematically studied the physical properties of the layered telluride chalcogenide superconductors CuIr$_{2-x}$Al$_{x}$Te$_{4}$ ($0 \leqslant x \leqslant 0.2$). Through the resistance and magnetization measurements, we found that the CDW order was destroyed by a small amount of Al doping. Meanwhile, the superconducting transition temperature ($T_{\rm c}$) kept changing with the change of doping amount and rose towards the maximum value of 2.75 K when $x=0.075$. The value of normalized specific heat jump ($\Delta C/\gamma T_{\rm c}$) for the highest $T_{\rm c}$ sample CuIr$_{1.925}$Al$_{0.075}$Te$_{4}$ was 1.53, which was larger than the BCS value of 1.43 and showed the bulk superconducting nature. In order to clearly show the relationship between SC and CDW states, we propose a phase diagram of $T_{\rm c}$ vs. doping content.     

Doping-induced change in the interlayer transport mechanism of Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} near the superconducting transition temperature

We perform a detailed study of temperature, bias, and doping dependence of interlayer transport in the layered high temperature superconductor Bi_{2}Sr_{2}CaCu_{2}O_{8+delta}. We observe that the shape of interlayer characteristics in underdoped crystals exhibits a remarkable crossover at the superconducting transition temperature: from thermal activation-type above T_{c} to almost T-independent quantum tunneling-type below T_{c}. Our data provide insight into the nature of interlayer transport and indicate that its mechanism changes with doping: from the conventional single quasiparticle tunneling in overdoped to a progressively increasing Cooper pair contribution in underdoped crystals.     

Transport and percolation in a low-density high-mobility two-dimensional hole system

We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime (p > or approximately4x10;{9} cm;{-2}), the nonmonotonic temperature dependence ( approximately 50-300 mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of T=50 mK, the conductivity versus density data indicate an inhomogeneity driven percolation-type transition to an insulating state at a critical density of 3.8x10;{9} cm;{-2}.     

Superfluid density in a highly underdoped YBa_{2}Cu_{3}O_{6+y} superconductor

The superfluid density rho_{s}(T) identical with1/lambda;{2}(T) has been measured at 2.64 GHz in highly underdoped YBa_{2}Cu_{3}O_{6+y}, at 37 dopings with T_{c} between 3 and 17 K. Within limits set by the transition width DeltaT_{c} approximately 0.4 K, rho_{s}(T) shows no evidence of critical fluctuations as T-->T_{c}, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that rho_{s} displays the behavior expected for a quantum phase transition in the (3+1)-dimensional XY universality class, with rho_{s0} proportional, variant(p-p_{c}), T_{c} proportional, variant(p-p_{c});{1/2}, and rho_{s}(T) proportional, variant(T_{c}-T);{1} as T-->T_{c}.     

Strong electronic correlations in LixZnPc organic metals

Nuclear magnetic resonance, electron paramagnetic resonance and magnetization measurements show that bulk LixZnPc are strongly correlated one-dimensional metals. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T_{1} and of the static uniform susceptibility chi_{S} on approaching room temperature are characteristic of a Fermi liquid. Moreover, while for x approximately 2 the electrons are delocalized down to low temperature, for x-->4 a tendency towards localization is noticed upon cooling, yielding an increase both in 1/T_{1} and chi_{s}. The x dependence of the effective density of states at the Fermi level D(E_{F}) displays a sharp enhancement for x approximately 2, at the half filling of the ZnPc lowest unoccupied molecular orbitals. This suggests that LixZnPc is on the edge of a metal-insulator transition where enhanced superconducting fluctuations could develop.     

Scaling of the low-temperature dephasing rate in Kondo systems

We present phase coherence time measurements in quasi-one-dimensional Ag wires doped with Fe Kondo impurities of different concentrations n_{s}. Because of the relatively high Kondo temperature T_{K} approximately 4.3 K of this system, we are able to explore a temperature range from above T_{K} down to below 0.01T_{K}. We show that the magnetic contribution to the dephasing rate gamma_{m} per impurity is described by a single, universal curve when plotted as a function of T/T_{K}. For T>0.1T_{K}, the dephasing rate is remarkably well described by recent numerical results for spin S=1/2 impurities. At lower temperature, we observe deviations from this theory. Based on a comparison with theoretical calculations for S>1/2, we discuss possible explanations for the observed deviations.     

Phase coherence of conduction electrons below the Kondo temperature

We have measured the phase decoherence rate tau_{varphi};{-1} of conduction electrons in disordered Ag wires implanted with 2 and 10 ppm Fe impurities, by means of the weak-localization magnetoresistance. The Kondo temperature of Fe in Ag, T_{K} approximately 4 K, is in the ideal temperature range to study the progressive screening of the Fe spins as the temperature T falls below T_{K}. The contribution to tau_{varphi};{-1} from the Fe impurities is clearly visible over the temperature range 40 mK-10 K. Below T_{K}, tau_{varphi};{-1} falls rapidly until T/T_{K} approximately 0.1, in agreement with recent theoretical calculations. At lower T tau_{varphi};{-1} deviates from theory with a flatter T-dependence. Understanding this anomalous dephasing for T/T_{K}<0.1 may require theoretical models with larger spin and number of channels.     

Exciton gas compression and metallic condensation in a single semiconductor quantum wire

We study the metal-insulator transition in individual self-assembled quantum wires and report optical evidence of metallic liquid condensation at low temperatures. First, we observe that the temperature and power dependence of the single nanowire photoluminescence follow the evolution expected for an electron-hole liquid in one dimension. Second, we find novel spectral features that suggest that in this situation the expanding liquid condensate compresses the exciton gas in real space. Finally, we estimate the critical density and critical temperature of the phase transition diagram at n{c} approximately 1 x 10;{5} cm;{-1} and T{c} approximately 35 K, respectively.     

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.     

Ac vortex-dependent torsional oscillation response and onset temperature T0 in solid 4He

Detailed studies of ac velocity V_{ac} and T dependence of torsional oscillator responses of solid 4He are reported. A characteristic onset temperature T_{0} approximately 0.5 K is found, below which a significant V_{ac}-dependent change occurs in the energy dissipation for the samples at approximately 32 bar and for one at 49 bar. A V_{ac} dependence of the so-called "nonclassical rotational inertia" fraction also appears below approximately T_{0}. The log(V_{ac}) linear dependence, which suggests involvement of quantized vorticies, was examined in the nonclassical rotational inertia fraction. We find a common 1/T;{2} dependence for this linear slope change in all of the samples for 30相似文献   

Spontaneous magnetization and resistivity jumps in bilayered manganite （La0.8Eu0.2）4/3Sr5/3Mn2O7 single crystals

Owing to the inhomogeneous state resulting from the doping of a small number of Eu ions into Laa/3Sr5/3Mn2O7, from the resulting single crystal （La0.8Eu0.2）a/3Sr5/3Mn2O7 we have observed the magnetization jump, the resistivity jump, as well as the relaxation phenomena. For （Lao.sEuo.2）a/3Sr5/3Mn2O7, it has a very delicate ground state due to the interplays among spin, charge, orbital, lattice degrees of freedom. Consequently, the magnetization state is sensitive to temperature, magnetic field, as well as time. Meanwhile, the evolution of the magnetization with time shows a spontaneous jump when both the temperature and the magnetic field are constant. Similar step-like behaviours are also observed in resistivity. All these results suggest that Eu doping can greatly modulate the physical properties of Laa/3Sr5/3Mn2O7 and cause such interesting behaviours.     

一种新型的红外成像材料

基于超大磁阻(CMR)材料在绝缘体—金属转变(I—M)点附近的巨大电阻变化，本研究了La位Gd掺杂对La0.7-xGdxSr0.3MnO3(x=0.20、0.30、0.40、0.50)电阻温度系数(TCR)的影响。实验结果表明：Gd掺杂将引起电阻率曲线的急剧变化，导致出现大的TCR；而且随Gd掺杂的增加，TCRR在x＝0．30出现峰值，然后随掺杂量增加逐步降低，大的TCR行为将成为新型的红外成像材料。     

Evidence for a quantum phase transition in Pr2-xCe(x)CuO4-delta from transport measurements

The doping and temperature dependences of the Hall coefficient, R(H), and ab-plane resistivity in the normal state down to 350 mK is reported for oriented films of the electron-doped high-T(c) superconductor Pr(2-x)Ce(x)CuO(4-delta). The doping dependences of beta (rho=rho(0)+ATbeta) and R(H) (at 350 mK) suggest a quantum phase transition at a critical doping near x=0.165.     

Strong correlations produce the Curie-Weiss phase of NaxCoO2

Within the t-J model we study several experimentally accessible properties of the 2D-triangular lattice system NaxCoO2, using a numerically exact canonical ensemble study of 12 to 18 site triangular toroidal clusters as well as the icosahedron. Focusing on the doping regime of x approximately 0.7, we study the temperature dependent specific heat, magnetic susceptibility, and the dynamic Hall coefficient R_{H}(T,omega) as well as the magnetic field dependent thermopower. We find a crossover between two phases near x approximately 0.75 in susceptibility and field suppression of the thermopower arising from strong correlations. An interesting connection is found between the temperature dependence of the diamagnetic susceptibility and the Hall coefficient. We predict a large thermopower enhancement, arising from transport corrections to the Heikes-Mott formula, in a model situation where the sign of hopping is reversed from that applicable to NaxCoO2.     

Cu site substitution effect on the Hall coefficient of La2−xSrxCuO4

The Cu site substitution effects on the Hall coefficient R_H and the electrical resistivity have been studied for La_2−xSr_xCuO₄. In a small x region, R_H decreases largely with Zn doping and increases with Ni doping. In the Ni doped samples, shows the characteristic temperature dependence similar to those of the unsubstituted samples where the decrease of the slope of vs. T curve is observed for x<0.1 above 600K but in the Zn doped samples, the change of the slope of becomes smaller,i.e. the temperature dependence of below 600K becomes smaller. These results indicate that the origin of the change of the slope of around 600K for x<0.1 is magnetic and the spin correlation or the electronic state is rather different below and above 600K. The unusual Zn doping dependences of R_H and are naturally explained by considering that the electronic state at high temperatures above 600K which has a small Hall coefficient expected for the large Fermi surface comes down to lower temperatures by the Zn doping.     

Temperature Dependence of In-plane Resistivity and Inverse Hall Angle in NLED Holographic Model

In the strange metal phase of the high-Tc cuprates, it is challenging to explain the linear temperature dependence of the in-plane resistivity and the quadratic temperature dependence of the inverse Hall angle. In this paper, we investigate the temperature dependence of the in-plane resistivity and inverse Hall angle in the nonlinear electrodynamics holographic model developed in our recent work. Maxwell electrodynamics and Born-Infeld electrodynamics are considered. Both cases support a wide spectrum of temperature scalings in parameter space. For Maxwell electrodynamics, the T-linear in-plane resistivity generally dominates at low temperatures and survives into higher temperatures in a narrow strip-like manner. Meanwhile, the T-quadratic inverse Hall angle dominates at high temperatures and extends down to lower temperatures. The overlap between the T-linear in-plane resistivity and the T-quadratic inverse Hall angle, if occurs, would generally present in the intermediate temperate regime. The Born-Infeld case with a 0 is quite similar to the Maxwell case. For the Born-Infeld case with a 0, there can be a constraint on the charge density and magnetic field. Moreover, the overlap can occur for strong charge density.     

Effect of Cu doping on the magnetic and electrical properties of n=2 Ruddlesden-Popper manganates

A series of polycrystalline Cu-doped n=2 Ruddlesden-Popper manganates La1.2Sr1.8CuzMn（2-x）O7 （x=0, 0.04, 0.13） were synthesized by the solid state reaction method. The effect of Cu doping on the magnetic and transport properties has been studied. It is found that Cu substitution for Mn greatly affects the magnetic and electrical properties of the parent phase La1.2Sr1.8Mn2O7. With the increase of Cu content, the system undergoes a transition from longrange ferromagnetic order to the spin glass state and further to an antiferromagnetic order. A little of Cu dopant can lead to the samples showing semiconductor or insulator behaviour in the whole observed temperature range while the parent phase has a metal-insulator transition. These samples show colossal magnetoresistance at low temperatures and the value of it decreases with increasing Cu content.     

Anomalous Hall effect of facing-target sputtered ferrimagnetic Mn4N epitaxial films with perpendicular magnetic anisotropy

Epitaxial Mn$_{4}$N films with different thicknesses were fabricated by facing-target reactive sputtering and their anomalous Hall effect (AHE) is investigated systematically. The Hall resistivity shows a reversed magnetic hysteresis loop with the magnetic field. The magnitude of the anomalous Hall resistivity sharply decreases with decreasing temperature from 300 K to 150 K. The AHE scaling law in Mn$_{4}$N films is influenced by the temperature-dependent magnetization, carrier concentration and interfacial scattering. Different scaling laws are used to distinguish the various contributions of AHE mechanisms. The scaling exponent $\gamma > 2$ for the conventional scaling in Mn$_{4}$N films could be attributed to the residual resistivity $\rho_{xx0}$. The longitudinal conductivity $\sigma_{xx}$ falls into the dirty regime. The scaling of $\rho_{\rm AH}=\alpha \rho_{xx0} +b\rho_{xx}^{n}$ is used to separate out the temperature-independent $\rho_{xx0}$ from extrinsic contribution. Moreover, the relationship between $\rho_{\rm AH}$ and $\rho_{xx}$ is fitted by the proper scaling to clarify the contributions from extrinsic and intrinsic mechanisms of AHE, which demonstrates that the dominant mechanism of AHE in the Mn$_{4}$N films can be ascribed to the competition between skew scattering, side jump and the intrinsic mechanisms.