Universal conductance fluctuations in three dimensional metallic single crystals of Si

In this paper we report the measurement of conductance fluctuations in 3D crystals of Si made metallic by heavy doping. ( L/L(straight phi) approximately 10(3), where L(straight phi) is the phase coherence length.) Temperature and magnetic field dependence of noise strongly indicate the universal conductance fluctuations as a predominant source of the observed magnitude of noise. Conductance fluctuations within a single phase coherent region of L(3)(straight phi) were found to be saturated at <(deltaG(straight phi))(2)> approximately (e(2)/h)(2). An accurate knowledge of the level of disorder enables us to calculate the change in conductance deltaG1 due to movement of a single scatterer as <(deltaG1)(2)> approximately (e(2)/h)(2), which is approximately 2 orders of magnitude higher than its theoretically expected value in 3D systems.     

Quantum criticality in a Mott pn junction in an armchair carbon nanotube

In an armchair carbon nanotube pn junction the p and n regions are separated by a region of a Mott insulator, which can backscatter electrons only in pairs. We predict a quantum-critical behavior in such a pn junction. Depending on the junction's built-in electric field E, its conductance G scales either to zero or to 4e(2)/h as the temperature T is lowered. The two types of the G(T) dependence indicate the existence, at some special value of E, of an intermediate quantum-critical point with a finite conductance G<4e(2)/h. This makes the pn junction drastically different from a simple potential barrier in a Luttinger liquid.     

Quantum dots with even number of electrons: kondo effect in a finite magnetic field

We show that the Kondo effect can be induced by an external magnetic field in quantum dots with an even number of electrons. If the Zeeman energy B is close to the single-particle level spacing Delta in the dot, the scattering of the conduction electrons from the dot is dominated by an anisotropic exchange interaction. A Kondo resonance then occurs despite the fact that B exceeds by far the Kondo temperature T(K). As a result, at low temperatures T相似文献   

Crossover from weak localization to Shubnikov-de Haas oscillations in a high-mobility 2D electron gas

We study the magnetoresistance deltarho(xx)(B)/rho(0) of a high-mobility 2D electron gas in the domain of magnetic fields B, intermediate between the weak localization and the Shubnikov-de Haas oscillations, where deltarho(xx)(B)/rho(0) is governed by the interaction effects. Assuming short-range impurity scattering, we demonstrate that in the second order in the interaction parameter lambda a linear B dependence, deltarho(xx)(B)/rho(0) approximately lambda(2)omega(c)/E(F) with a temperature-independent slope, emerges in this domain of B (here omega(c) and E(F) are the cyclotron frequency and the Fermi energy, respectively). Unlike previous mechanisms, the linear magnetoresistance is unrelated to the electron executing the full Larmour circle, but rather originates from the impurity scattering via the B dependence of the phase of the impurity-induced Friedel oscillations.     

Neutrino cosmology after WMAP 7-year data and LHC first Z' bounds

The gauge-extended U(1)(C)×SU(2)(L)×U(1)(I(R))×U(1)(L) model elevates the global symmetries of the standard model (baryon number B and lepton number L) to local gauge symmetries. The U(1)(L) symmetry leads to three superweakly interacting right-handed neutrinos. This also renders a B-L symmetry nonanomalous. The superweak interactions of these Dirac states permit ν(R) decoupling just above the QCD phase transition: 175 is < or approximately equal to T(ν(R))(dec)/MeV is < or approximately equal to 250. In this transitional region, the residual temperature ratio between ν(L) and ν(R) generates extra relativistic degrees of freedom at BBN and at the CMB epochs. Consistency with both WMAP 7-year data and recent estimates of the primordial 4He mass fraction is achieved for 3相似文献   

Local density of states around a magnetic impurity in high- T(c) superconductors based on the t-J model

The local density of states (LDOS) around a magnetic impurity in high- T(c) superconductors is studied using the two-dimensional t-J model with a realistic band structure. The order parameters are determined in a self-consistent way within the Gutzwiller approximation and the Bogoliubov-de Gennes theory. In sharp contrast with the nonmagnetic impurity case, the LDOS near the magnetic impurity shows two resonance peaks reflecting the presence of spin-dependent resonance states. It is also shown that these resonance states are approximately localized around the impurity. The present results have an immediate connection with the scanning tunneling spectroscopy observation of Bi2Sr2Ca(Cu1-xNi[Zn](x))(2)O(8+delta).     

Length-dependent conductance of a spin-incoherent Hubbard chain: Monte Carlo calculations

The dc conductance of a short spin-incoherent Hubbard chain coupled to leads is investigated using quantum Monte Carlo calculations. In contrast with the Luttinger liquid regime, where the conductance is equal to the noninteracting value, the spin-incoherent regime displays a conductance that decreases rapidly with chain length down to a value of roughly 1.5 e2/h for a four site chain followed by a slower decrease for longer chains. We also discuss the resistance contribution from scattering in the contacts.     

Tunneling between two luttinger liquids induced by a driving field

Tunneling between two Luttinger liquids driven by a time-dependent field with a frequency f is investigated using the zero-mode bosonization. We show that inclusion of the zero modes is essential in order to obtain correct results in the limit L(T)/L>1 ( L is the channel length and L(T) is the thermal length). We find that the tunneling current is quantized in units of "ef" and takes the form I approximately ef SUM(n = 1)(infinity)delta[mu(F)-2pi / LvPlanck's over 2pi(n-1 / 2)].     

Transport in a dissipative Luttinger liquid

We study theoretically the transport through a single impurity in a one-channel Luttinger liquid coupled to a dissipative (Ohmic) bath. For nonzero dissipation, the single impurity is always a relevant perturbation which suppresses transport strongly. At zero temperature, the current voltage relation of the link is I approximately exp(-E0/eV), where E0 approximately eta/kappa and kappa denotes the compressibility. At nonzero temperature T, the linear conductance is proportional to exp(-sqrt(CE0/kBT)). The decay of Friedel oscillation saturates for a distance larger than L(eta) approximately 1/eta from the impurity.     

Tenfold magnetoconductance in a nonmagnetic metal film

We present magnetoconductance (MC) measurements of homogeneously disordered Be films whose zero field sheet conductance ( G) is described by the Efros-Shklovskii hopping law G(T) = (2e(2)/h)exp-(T0/T)(1/2). The low field MC of the films is negative with G decreasing a factor of 2 below 1 T. In contrast the MC above 1 T is strongly positive. At 8 T, G increases tenfold in perpendicular field and fivefold in parallel field. In the simpler parallel case, we observe field enhanced variable range hopping characterized by an attenuation of T0 via the Zeeman interaction.     

Interaction effects at crossings of spin-polarized one-dimensional subbands

We report conductance measurements of ballistic one-dimensional (1D) wires defined in GaAs/AlGaAs heterostructures in an in-plane magnetic field, B. When the Zeeman energy is equal to the 1D subband energy spacing, the spin-split subband N upward arrow intersects (N+1) downward arrow, where N is the index of the spin-degenerate 1D subband. At the crossing of N=1 upward arrow and N=2 downward arrow subbands, there is a spontaneous splitting giving rise to an additional conductance structure evolving from the 1.5(2e(2)/h) plateau. With further increase in B, the structure develops into a plateau and lowers to 2e(2)/h. With increasing temperature and magnetic field the structure shows characteristics of the 0.7 structure. Our results suggest that at low densities a spontaneous spin splitting occurs whenever two 1D subbands of opposite spins cross.     

A nearly universal critical conductivity for semiconductor-metal alloys

Classical ionized impurity scattering is employed to calculate the conductivity at and in the vicinity of the critical point. The result sigma(iis)(x = x(c),T) = Asqrt[T], closely given by e(2)/Planck's over 2pilambda(dB) with the de Broglie wavelength lambda(dB) = h/(2m(*)kT)(1/2) in the nondegenerate regime epsilon(F)x(c), T) might also explain the linear scaling behavior sigma(x, T)-Asqrt[T] = sigma(0)(x/x(0)-1).     

Numerical renormalization group at marginal spectral density: application to tunneling in Luttinger liquids

Many quantum mechanical problems (such as dissipative phase fluctuations in metallic and superconducting nanocircuits or impurity scattering in Luttinger liquids) involve a continuum of bosonic modes with a marginal spectral density diverging as the inverse of energy. We construct a numerical renormalization group in this singular case, with a manageable violation of scale separation at high energy, capturing reliably the low energy physics. The method is demonstrated by a nonperturbative solution over several energy decades for the dynamical conductance of a Luttinger liquid with a single static defect.     

Kondo model for the "0.7 anomaly" in transport through a quantum point contact

Experiments on quantum point contacts have highlighted an anomalous conductance plateau around 0.7(2e(2)/h), with features suggestive of the Kondo effect. Here, an Anderson model for transport through a point contact analyzed in the Kondo limit. Hybridization to the band increases abruptly with energy but decreases with valence, so that the background conductance and the Kondo temperature T(K) are dominated by different valence transitions. This accounts for the high residual conductance above T(K). The model explains the observed gate-voltage, temperature, magnetic field, and bias-voltage dependences. A spin-polarized current is predicted even for low magnetic fields.     

One-dimensional transport in polymer nanofibers

We report our transport studies in quasi-one-dimensional (1D) conductors-helical polyacetylene fibers doped with iodine-and the data analysis for other polymer single fibers and tubes. We found that at 30 K相似文献   

Magnetization transport and quantized spin conductance

We analyze transport of magnetization in insulating systems described by a spin Hamiltonian. The magnetization current through a quasi-one-dimensional magnetic wire of finite length suspended between two bulk magnets is determined by the spin conductance which remains finite in the ballistic limit due to contact resistance. For ferromagnetic systems, magnetization transport can be viewed as transmission of magnons, and the spin conductance depends on the temperature T. For antiferromagnetic isotropic spin-1/2 chains, the spin conductance is quantized in units of order (gmu(B))(2)/h at T=0. Magnetization currents produce an electric field and, hence, can be measured directly. For magnetization transport in electric fields, phenomena analogous to the Hall effect emerge.     

Electron-positron outflow from black holes

Cosmological gamma-ray bursts (GRBs) appear as the brightest transient phenomena in the Universe. The nature of their central engine is a missing link in the theory of fireballs to stellar mass progenitors, and may be associated with low mass black holes. In contact with an external magnetic field B, black hole spin produces a gravitational potential on the wave function of charged particles. We show that a rapidly rotating black hole of mass M produces outflow from initially electrostatic equilibrium with normalized isotropic emission approximately 10(48)(B/B(c))(2)(M/7M)(2)sin (2) theta erg/s, where B(c) = 4.4x10(13) G. The half-opening angle satisfies theta >or = square root[B(c)/3B]. The outflow proposed as input to GRB fireball models.     

The nature of the magnetism in quasi-2D layered α-Ni(OH)2

The two layered hexagonal hydroxides of Ni are β-Ni(OH)(2) and α-Ni(OH)(2); β-Ni(OH)(2) is now known to be an antiferromagnet whereas the nature of the magnetism in α-Ni(OH)(2) is not yet well established. Here, the magnetic properties of α-Ni(OH)(2) with lattice parameters a = 3.02 ? and c = 8.6 ?, and flower-like morphology with petal thickness of approximately equal to 50 ? are reported. Temperature (2-300 K) and magnetic field (up to 65 kOe) dependence of the magnetization and ac susceptibility at f = 0.1-1000 Hz were measured. Analysis of the data yields ferromagnetic ordering in the system with T(C) is approximately equal to 16 K. In addition, a nanosize related blocking temperature T(B) = 8 K and spin-glass-like ordering of the surface spins near 3.5 K are inferred from the ac frequency and dc magnetic field dependence of these transitions. Fitting to the high temperature series and quasi-2D nature of the system is used to determine J(1)/k(B) = 4.38 K (J(2)/k(B) = 0.14 K) for the intraplane (interplane) exchange coupling between the Ni(2+) ions.     

Magnetic field dependence of ultracold inelastic collisions near a feshbach resonance

Inelastic collision rates for ultracold 85Rb atoms in the F = 2, m(f) = -2 state have been measured as a function of magnetic field. At 250 gauss (G), the two- and three-body loss rates were measured to be K2 = (1.87+/-0.95+/-0.19)x10(-14) cm(3)/s and K3 = (4.24(+0. 70)(-0.29)+/-0.85)x10(-25) cm(6)/s, respectively. As the magnetic field is decreased from 250 G towards a Feshbach resonance at 155 G, the inelastic rates decrease to a minimum and then increase dramatically, peaking at the Feshbach resonance. Both two- and three-body losses are important, and individual contributions have been compared with theory.     

Quasioptical study of antiferromagnetic resonance in YFeO3 at submillimeter wavelength under high pulsed magnetic fields

Transmission spectra, T(H), of linearly polarized electromagnetic waves through YFeO(3), weak ferromagnet, measured at frequencies nu=96-1000 GHz in long-pulsed magnetic fields (H||k||c-axis, Faraday geometry) exhibit strong rotation of the polarization plane near the quasiferromagnetic AFMR as well as low frequency impurity modes. New ascending impurity branch including five lines was observed at high magnetic field (10-30 T) at 96 GHz and 140 GHz in addition to the known low-field descending impurity branch. Behavior of all the impurity modes assigned to transitions in (6)S(5/2) multiplet of Fe(3+) "impurity" ions in c-sites was described self-consistently by one spin-Hamiltonian. A theoretical calculation of dynamical magnetic susceptibility at AFMR and impurity modes and further simulation of transmission spectra made it possible to describe the main features of the observed spectra T(H). It was found that the T(H) behavior is determined at resonances not only by non-diagonal components of the magnetic susceptibility but also by the anisotropy of the dielectric permittivity (epsilon(xx)(') not equal epsilon(yy)(')), i.e. birefringence.