Two-body Dirac equation versus KDP equation

A brief review of two-body Dirac and Kemmer-Duffin-Petiau approaches for the bound state problem of two fermions is presented from an algebraic point of view in a comparative manner. Reduction of the direct product of two Dirac spaces is discussed.Work supported by the T.B.T.A.K. under TBAG/CG-1.     

La0.67Pb0.33MnO3的磁性及输运特性

利用固相反应烧结法制备了La067Pb033MnO3单相多晶样品.研究了其结构、磁性及输运特性.结果表明，样品呈菱面相晶体结构，空间群为R3C，居里温度TC(=353K)非常接近TMI(=360K).在居里温度附近，发生铁磁-顺磁转变，导电特性由金属特征向半导体特征过渡.磁电阻在居里点达到极大值.当H=16T时，磁电阻的极大值为145%；当H=08T时，磁电阻的极大值为9%.输运性质表明，TTMI时 关键词： 磁电阻 输运特性 磁极化子 钙钛石     

Delocalized quasiparticles in the vortex state of an overdoped high-T(c) superconductor probed by 63Cu NMR

We report the spin Knight shift (K(s)) and the nuclear spin-lattice relaxation rate (1/T1) in the vortex state as a function of magnetic field (H) up to 28 T in the high-Tc superconductor TlSr2CaCu2O6.8 (Tc = 68 K). At low temperatures well below Tc, both K(s) and 1/T1 measured around the middle point between the two nearest vortices (saddle point) increase substantially with increasing field, which indicate that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of states (DOS) around the saddle point is found to be kappaN(0)square root[H/H(c2)], with kappa = 0.5-0.7 and N0 being the normal-state DOS.     

Thermal conductivity in the vicinity of the quantum critical end point in Sr3Ru2O7

Thermal conductivity of Sr3Ru2O7 was measured down to 40 mK and at magnetic fields through the quantum critical end point at Hc=7.85 T. A peak in the electrical resistivity as a function of the field was mimicked by the thermal resistivity. In the limit as T-->0 K, we find that the Wiedemann-Franz law is satisfied to within 5% at all fields, implying that there is no breakdown of the electron despite the destruction of the Fermi liquid state at quantum criticality. A significant change in disorder [from rho0(H=0 T)=2.1 to 0.5 microOmega cm] does not influence our conclusions. At finite temperatures, the temperature dependence of the Lorenz number is consistent with ferromagnetic fluctuations causing the non-Fermi liquid behavior as one would expect at a metamagnetic quantum critical end point.     

Gate Tunable Supercurrent in Josephson Junctions Based on Bi_2Te_3 Topological Insulator Thin Films

We report transport measurements on Josephson junctions consisting of Bi_2Te_3 topological insulator(TI) thin films contacted by superconducting Nb electrodes.For a device with junction length L=134 nm,the critical supercurrent I_c can be modulated by an electrical gate which tunes the carrier type and density of the TI film.I_c can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state.In the p-type regime the Josephson current can be well described by a short ballistic junction model.In the n-type regime the junction is ballistic at 0.7 K T 3.8 K while for T 0.7 K the diffusive bulk modes emerge and contribute a larger I_c than the ballistic model.We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p-n junctions.Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices.     

Field-induced quantum critical point in CeCoIn5

The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a function of temperature, down to 25 mK and in magnetic fields of up to 16 T applied perpendicular to the basal plane. With increasing field, we observe a suppression of the non-Fermi liquid behavior, rho approximately T, and the development of a Fermi liquid state, with its characteristic rho=rho(0)+AT2 dependence. The field dependence of the T2 coefficient shows critical behavior with an exponent of 1.37. This is evidence for a field-induced quantum critical point (QCP), occurring at a critical field which coincides, within experimental accuracy, with the superconducting critical field H(c2). We discuss the relation of this field-tuned QCP to a change in the magnetic state, seen as a change in magnetoresistance from positive to negative, at a crossover line that has a common border with the superconducting region below approximately 1 K.     

抗艾滋病药物司他夫定与血液蛋白相互作用的研究

研究药物和血液中载体蛋白的相互作用对阐明药物在体内的转运、分布、代谢和药效等具有重要意义。运用稳态荧光、紫外-可见吸收光谱、动力学瞬态发射光谱和循环伏安法研究了抗艾滋病(HIV)药物司他夫定(stavudine,D4T)对人血清白蛋白(HSA)、牛血清白蛋白(BSA)和血红蛋白(Hb)三种血液蛋白的荧光猝灭机制,均为静态猝灭;得出不同温度(300 K,310 K,320 K)下D4T和载体的结合常数K_a(K_a的大小顺序为Hb＞HSA＞BSA)和结合位点数n(n均为1);分析二者结合过程的热力参数ΔH,ΔS和ΔG,三种血液蛋白均为ΔG＞0,ΔH＞0,说明D4T和载体的结合是一种自发的放热过程,同时由ΔH＜0,ΔS＜0,推测出D4T与HSA,BSA和Hb之间的结合力都为氢键和范德华力;根据Frster非辐射能量转移理论(FRET)分析了供体(蛋白)和受体(D4T)之间发生能量转移的可能性并计算了结合距离R₀和r,其中r＜7 nm且0.5R₀＜r＜1.5R₀,表明从HSA,BSA和Hb到D4T之间发生能量转移的可能性很大。同时利用同步荧光,三维荧光和圆二色谱法得出,D4T与载体结合时对载体(HSA,BSA和Hb)的二级结构无影响,且三级构象变化不大。通过本文实验可知,HSA,BSA和Hb三种血液蛋白均可作为运输D4T到靶位置的良好载体蛋白,这些结果为更深入研究D4T药物分子设计和抗HIV作用的应用提供有利的实验依据。     

Tight-binding description of Landau levels of graphite in tilted magnetic fields

The electronic structure of Bernal-stacked graphite subject to tilted magnetic fields is studied theoretically. The minimal nearest-neighbor tight-binding model with the Peierls substitution is employed to describe the structure of Landau levels. We show that, while the orbital effect of the in-plane component of the magnetic field is negligible for massive Dirac fermions in the vicinity of the K point of the graphite Brillouin zone, at the H point it leads to the experimentally observable splitting of Landau levels, which grows approximately linearly with the in-plane field intensity.     

Long-range ferromagnetism of Mn12 acetate single-molecule magnets under a transverse magnetic field

We use neutron diffraction to probe the magnetization components of a crystal of Mn12 single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the crystallographic c axis. The application of a magnetic field H(perpendicular) perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. For T approximately < 0.9(1) K, this equilibrium state shows spontaneous magnetization, indicating the onset of ferromagnetism. These long-range magnetic correlations nearly disappear for mu0H(perpendicular) approximately > 5.5 T, possibly suggesting the existence of a quantum critical point.     

Observation of modulated quadrupolar structures in PrPb3

Neutron diffraction measurements have been performed on the cubic compound PrPb(3) in a [001] magnetic field to examine the quadrupolar ordering. Antiferromagnetic components with q = (1/2 +/- delta 1/2 0), (1/2 1/2 +/- delta 0) (delta approximately 1/8) are observed below the transition temperature T(Q) (0.4 K at H = 0) whose amplitudes vary linear with H and vanish at zero field, providing the first evidence for a modulated quadrupolar phase. For H < 1 T, a nonsquare modulated state persists even below 100 mK suggesting quadrupole moments associated with a Tau(3) doublet ground state to be partially quenched by hybridization with conduction electrons.     

Shot noise in ballistic graphene

We have investigated shot noise in graphene field effect devices in the temperature range of 4.2-30 K at low frequency (f=600-850 MHz). We find that for our graphene samples with a large width over length ratio W/L, the Fano factor F reaches a maximum F ~ 1/3 at the Dirac point and that it decreases strongly with increasing charge density. For smaller W/L, the Fano factor at Dirac point is significantly lower. Our results are in good agreement with the theory describing that transport at the Dirac point in clean graphene arises from evanescent electronic states.     

Heavy fermion superconductivity and magnetic order in noncentrosymmetric CePt3Si

CePt3Si is a novel heavy fermion superconductor, crystallizing in the CePt3B structure as a tetragonally distorted low symmetry variant of the AuCu3 structure type. CePt3Si exhibits antiferromagnetic order at T(N) approximately 2.2 K and enters into a heavy fermion superconducting state at T(c) approximately 0.75 K. Large values of H(')(c2) approximately -8.5 T/K and H(c2)(0) approximately 5 T refer to heavy quasiparticles forming Cooper pairs. Hitherto, CePt3Si is the first heavy fermion superconductor without a center of symmetry.     

k=0处的类狄拉克锥

狄拉克锥在电子和经典波体系中分别被发现, 由于其线性能带关系, 伴随着很多独特的现象. 除了一般存在于布里渊区边界处的狄拉克锥, k=0处也存在包含线性能带关系的类狄拉克锥. 这个类狄拉克锥可以由单极子和偶极子的偶然简并而形成. k=0处的类狄拉克锥可以通过两维电介质光子晶体来实现, 利用等效媒质理论, 此时的光子晶体在类狄拉克点频率可以等效为介电常数和磁导率都为零的材料. 电介质双零折射率材料既可以避免阻抗的不匹配, 也可以避免体系推广到高频所引起的强烈损耗. 此外, k=0处的类狄拉克锥与双零折射率的概念可以从两维体系拓展到三维体系, 而且还可以从电磁波体系推广到声波和弹性波体系. 利用具有类狄拉克点的两维光子晶体, 在材料参数都偏离类狄拉克点条件的两个半无限大光子晶体所构成的界面中, 一定存在界面态. 这些界面态的存在可以通过层状多重散射理论得到的表面阻抗以及体能带的几何相位来彻底解释.     

Metal to insulator transition in films of molecularly linked gold nanoparticles

We report a metal to insulator transition (MIT) in disordered films of molecularly linked gold nanoparticles (NPs). As the number of carbons (n) of alkanedithiol linker molecules (C(n)S2) is varied, resistance (R) at low temperature (T = 2 K) and at 200 K, as well as trends in R vs T data at intermediate temperatures, all point to an MIT occurring at n = 5. We describe these results in a context of a Mott-Hubbard MIT. We find that all insulating samples (n > or = 5) exhibit a universal scaling behavior R approximately exp[(T0/T)nu] with nu = 0.65, and all metallic samples (n < or = 5) exhibit weaker R-T dependencies than bulk gold. We discuss these observations in terms of competitive thermally activated processes and strong, T-independent elastic scattering, respectively.     

YbRh2Si2: spin fluctuations in the vicinity of a quantum critical point at low magnetic field

We report a 29Si NMR study on aligned single crystals of YbRh2Si2 which shows behavior characteristic of a quantum critical point (QCP: T(N)-->0). The Knight shift K and the nuclear spin-lattice relaxation rate 1/T(1) of Si show a strong dependence on the external field H, especially below 5 kOe. At the lowest H used in this measurement (H approximately 1.5 kOe), it was found that 1/T(1)T continues to increase down to 50 mK, whereas K stays constant with a large magnitude below 200 mK. This result strongly suggests the development of antiferromagnetic fluctuations with finite q vectors that compete with q=0 spin fluctuations in the vicinity of the QCP near H=0.5 kOe.     

Critical point and the nature of the pseudogap of single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta superconductors

We apply strong magnetic fields of H=28.5 to 43 T to suppress superconductivity (SC) in the cuprates Bi2Sr2-xLaxCuO6+delta (x=0.65, 0.40, 0.25, 0.15, and 0), and investigate the low temperature (T) normal state by 63Cu nuclear spin-lattice relaxation rate (1/T1) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at x approximately 0.05, which corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid that persists as the ground state when superconductivity is removed by the magnetic field. A comparison of the superconducting state with the H-induced normal state in the x=0.40 (Tc=32 K) sample indicates that there remains substantial part of the Fermi surface even in the fully developed PG state, which suggests that the PG and SC are coexisting matters.     

Anisotropic effect of Cd and Hg doping on the Pauli limited superconductor CeCoIn5

We studied the effect of impurity on the first order superconducting (SC) transition and the high field-low temperature (HFLT) SC state of CeCoIn5 by measuring the specific heat of CeCo(In1-xCdx)_{5} with x=0.0011, 0.0022, and 0.0033 and CeCo(In1-xHgx)_{5} with x=0.000 16, 0.000 32, and 0.000 48 at temperatures down to 0.1 K and fields up to 14 T. Cd substitution rapidly suppresses the crossover temperature T0, where the SC transition changes from second to first order, to T=0 K with x=0.0022 for H parallel[100], while it remains roughly constant up to x=0.0033 for H parallel[001]. The associated anomaly of the proposed FFLO state in Hg-doped samples is washed out by x=0.000 48, while remaining at the same temperature, indicating high sensitivity of that state to impurities. We interpret these results as supporting the nonmagnetic, possibly FFLO, origin of the HFLT state in CeCoIn5.     

Magnetic behavior of Eu(3)Ni(4)Ga(4): antiferromagnetic order and large magnetoresistance

The results of the magnetic susceptibility, isothermal magnetization, heat capacity, electrical resistivity and magnetoresistance measurements on polycrystalline Eu(3)Ni(4)Ga(4) are presented. Eu(3)Ni(4)Ga(4) forms in Na(3)Pt(4)Ge(4)-type cubic crystal structure (space group [Formula: see text]). The temperature dependence of the magnetic susceptibility of Eu(3)Ni(4)Ga(4) confirms the divalent state (Eu(2+)) of Eu ions with an effective magnetic moment μ(eff)?=?7.98?μ(B). At low fields, e.g.?at 0.01?T, a magnetic phase transition to an antiferromagnetically ordered state occurs at T(N)?=?10.9?K, which is further confirmed by the temperature dependence of the heat capacity and electrical resistivity. The field dependence of isothermal magnetization at 2?K reveals the presence of two field induced metamagnetic transitions at H(c1) and H(c2)?=?0.55 and 1.2?T, respectively and a polarized phase above H(PO)?=?1.7?T. The reduced jump in the heat capacity at the transition temperature, ΔC|(T(N))?=?13.48?J/mol-Eu?K would indicate an amplitude modulated (AM) antiferromagnetic structure. An interesting feature is that a large negative magnetoresistance, MR?=?[ρ(H)?-?ρ(0)]/ρ(0), is observed in the vicinity of magnetic transition even up to 2T(N). Similar large magnetoresistance has been observed in the paramagnetic state in some Gd and Eu based alloys and has been attributed to the magneto-polaronic effect.     

Superconductivity in the Einstein solid V Al(10.1)

We used magnetic susceptibility, resistivity and heat capacity measurements to characterize the superconducting state in the Einstein solid V Al(10.1). We find that V Al(10.1) is a weak-coupling, type-II superconductor with T(c)?=?1.53?K and an upper critical field of H(c2)(0)?=?800?Oe. The heat capacity data in the range 0.07?K?相似文献   

Lifetime measurements and particle-rotor calculations for 42 105 Mo63

The half-lives of seven levels in¹⁰⁵Mo have been determined with high precision at the fission-product separator Josef. The-- triple coincidence method with plastic and BaF₂ scintillators was used for a determination of the time between the _q feeding of a level of interest and its depopulation. A value of _q =0.38 (1) for the deformation parameter has been deduced from the half-lives of the first and second excited states. The results of Particle-Rotor model-calculations with the Nilsson parameters=0.084 and=0.28 support the assignment of the Nilsson orbital [532] 5/2^– to the ground state of¹⁰⁵Mo.The authors acknowledge with pleasure fruitful discussions with Profs. W.R. Phillips, I. Ragnarsson and O.W.B. Schult.