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1.
We report the temperature and magnetic field dependence of the conductivity of multiwall carbon nanotube mat in the temperature range 1.4-150 K and in magnetic fields up to 10 T. It is observed that charge transport in this system is governed by Mott’s variable-range hopping of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Mott’s various parameters, such as localization length, hopping length, hopping energy and density of states at the Fermi level are deduced from the variable-range hopping fit. The resistance of the sample decreases with the magnetic field applied in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states with the well-known consequence of a decrease in Mott’s T0 parameter in variable-range hopping. The application of magnetic field lowers the crossover temperature at which three-dimensional variable-range hopping turns to two-dimensional variable-range hopping. The conductivity on the lower temperature side is governed by the weak localization giving rise to positive magnetoconductance. Finally, a magnetic field-temperature diagram is proposed showing different regions for different kinds of transport mechanism. 相似文献
2.
A.A. Zvyagin A. Klümper J. Zittartz 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(1):25-36
The exactly solvable model of supersymmetric t - J chains (STJC) of correlated electrons with next-nearest-neighbour (NNN) interactions is proposed and studied. The model with
interactions between nearest neighbours and NNN interactions in one chain can also be considered as a two-chain model with
zigzag-like coupling between the chains. The NNN interaction (coupling between chains) causes the onset of additional Dirac
seas for low-lying charge and/or spin excitations. These Dirac seas change the low-energy (conformal) behavior of the model.
The filling of those seas depends on the values of the NNN coupling (interactions between chains), external magnetic field
and applied voltage. We identify the new ground state phases which appear due to the NNN as incommensurate ones. The NNN coupling
in the incommensurate phases induces spontaneous magnetization and/or spontaneous filling of the Dirac sea for charge excitations
(“spontaneous charge ordering”). The onset of this order implies a first order quantum phase transition driven by the field
with hysteresis phenomena.
Received 13 September 2000 相似文献
3.
One-parameter general coherent states of the spl(2,1) superalgebra are constructed and their properties are discussed in detail.
One-parameter matrix elements of the spl(2,1) generators in the one-parameter general coherent-state space are calculated.
The parameter α may be related to the interaction parameter U in one exactly solvable model for correlated electrons. 相似文献
4.
We study the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction in doped armchair nanotube in the presence of gap parameter. The effects of both next nearest neighbor hopping parameter and electron-Holstein phonon on RKKY interaction have been addressed. RKKY interaction as a function of distance between localized moments have been analyzed. In order to calculate the exchange interaction along arbitrary direction between two magnetic moments, we should obtain the transverse static spin susceptibility of armchair graphene nanoribbon in the presence of electron-phonon coupling and gap parameter. The spin susceptibility components are calculated using Green’s function approach for Holstein model Hamiltonian. The effects of electron doping on dependence of exchange interaction on distance between moments are investigated via calculating correlation function of spin density operators. Our results show the influences of next nearest neighbor hopping parameter on the spatial behavior of RKKY interactions are different in the presence of electron phonon coupling. 相似文献
5.
R.S. Markiewicz J. Lorenzana A. Bansil 《Journal of Physics and Chemistry of Solids》2011,72(5):333-336
We have investigated magnetic and charge instabilities of the cuprates within the Gutzwiller approximation RPA (GA+RPA). Interestingly, in GA the dressed Hubbard U is not a single parameter, but has different forms in the spin and charge responses, with distinct doping dependencies. While there are a number of competing magnetic instabilities for hole-doped cuprates, we fail to find any purely electronic charge density waves. The dominant magnetic instabilities are associated with ‘double nesting’, and the phase diagrams are material dependent, with LSCO differing from other cuprates. 相似文献
6.
H. Yi K.-H. Ahn 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,76(2):283-287
We investigate dynamical transport properties of interacting electrons moving in a vibrating nanoelectromechanical wire in
a magnetic field. We have built an exactly solvable model in which the electron-electron interaction is considered nonperturbatively
and the electric current and mechanical vibration are treated fully quantum mechanically on an equal footing. We demonstrate
our theory by calculating the admittance of a finite-size wire, which is influenced by the magnetic field strength, the electron-electron
interaction, and the complex interplay between the mechanical and the electrical energy scales. Nontrivial features including
sharp resonance peaks appear in the admittance, which may be experimentally observable. 相似文献
7.
We have used a tight-binding Hamiltonian of an ABA-stacked trilayer zigzag graphene nanoribbon with β-alignment edges to study the edge magnetizations. Our model includes the effect of the intralayer next-nearest-neighbor hopping, the interlayer hopping responsible for the trigonal warping and the interaction between electrons, which is considered by a single band Hubbard model in the mean field approximation. Firstly, in the neutral system we analyzed the two magnetic states in which both edge magnetizations reach their maximum value; the first one is characterized by an intralayer ferromagnetic coupling between the magnetizations at opposite edges, whereas in the second state that coupling is antiferromagnetic. The band structure, the location of the edge-state bands and the local density of states resolved in spin are calculated in order to understand the origins of the edge magnetizations. We have also introduced an electron doping so that the number of electrons in the ribbon unit cell is higher than in neutral case. As a consequence, we have obtained magnetization steps and charge accumulation at the edges of the sample, which are caused by the edge-state flat bands. 相似文献
8.
P. Schlottmann A.A. Zvyagin 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):325-335
We consider the one-dimensional t
-
J model, which consists of electrons with spin S on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the lattice sites and spin-exchange J between neighboring sites. The model is integrable at the supersymmetric point, J
=
t. Without spoiling the integrability we introduce an Anderson-like impurity of spin S (degenerate Anderson model in the limit), which interacts with the correlated conduction states of the host. The lattice model is defined by the scattering
matrices via the Quantum Inverse Scattering Method. We discuss the general form of the interaction Hamiltonian between the impurity and
the itinerant electrons on the lattice and explicitly construct it in the continuum limit. The discrete Bethe ansatz equations
diagonalizing the host with impurity are derived, and the thermodynamic Bethe ansatz equations are obtained using the string
hypothesis for arbitrary band filling as a function of temperature and external magnetic field. The properties of the impurity
depend on one coupling parameter related to the Kondo exchange coupling. The impurity can localize up to one itinerant electron
and has in general mixed valent properties. Groundstate properties of the impurity, such as the energy, valence, magnetic
susceptibility and the specific heat coefficient, are discussed. In the integer valent limit the model reduces to a Coqblin-Schrieffer impurity.
Received: 31 December 1997 / Accepted: 17 March 1998 相似文献
9.
A solvable open quantum system: The strongly driven micromaser 总被引:1,自引:0,他引:1
F. Casagrande B. -G. Englert P. Lougovski A. Lulli E. Solano H. Walther 《Optics and Spectroscopy》2005,99(2):301-306
10.
Saeed Ghanbari P. Blair Blakie Peter Hannaford Tien D. Kieu 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,70(3):305-310
The accessibility of the critical parameters for the superfluid to Mott insulator quantum phase transition in a 2D permanent
magnetic lattice is investigated. We determine the hopping
matrix element J, the on-site interaction U, and hence the ratio J/U, in the harmonic oscillator wave function approximation.
We show that for a range of realistic parameters the critical values of J/U, predicted by different methods for the Bose-Hubbard
model in 2D, such as mean field theory and Monte Carlo simulations, are accessible in a 2D permanent magnetic lattice. The
calculations are performed for a 2D permanent magnetic lattice created by two crossed arrays of parallel rectangular magnets
plus a bias magnetic field. 相似文献
11.
M. A. Alexanyan K. S. Aramyan 《Journal of Contemporary Physics (Armenian Academy of Sciences)》2007,42(2):49-54
We study the equations of motion of a spherical oscillator model suggested by Bellucci and Nersessian, in the presence of a constant magnetic field. This model is shown to be exactly solvable classically in contrast to the Higgs oscillator which is not exactly solvable in magnetic fields. 相似文献
12.
If massive neutrinos possess magnetic moments, they can undergo spin flip in a magnetic field. The magnetic fields needed for a meaningful measurement of neutrino moments could be very high and may occur in astronomical objects such as some supernovae or active galactic nuclei: they are typically chaotic ones. We develop the general theory of the passage of neutrinos through such fields. We also develop a simple model which becomes solvable in the high energy limit. Both helicities occur with equal probability, independently of the initial distribution. Observational consequences are discussed. 相似文献
13.
15.
We calculate the tunnel current between two parallel two-dimensional electron systems in a strong perpendicular magnetic field. We model the strongly correlated electron systems by Wigner crystals, and describe their low-energy dynamics in terms of magnetophonons. The effects of the magnetophonons on the tunneling processes can be described by an exactly solvable independent-boson model. A tunneling electron shakes up magnetophonons, which results in a conductance peak that is displaced away from zero voltage and broadened compared with the case of no magnetic field. At low temperatures and low enough voltages the tunneling conductance is strongly suppressed, and the I–V characteristics exhibit a power-law behavior. The zero-voltage conductance is thermally activated with an activation temperature 10 K. The results are in very good agreement with experiment. 相似文献
16.
Yong-Qing Chen 《International Journal of Theoretical Physics》2007,46(11):2956-2960
One-parameter inhomogeneous differential realization of the SPL(2,1) superalgebra on the space of homogeneous polynomials
and the corresponding boson-fermion realization are studied. The parameter α may be related to the interaction parameter U in one exactly solvable model for correlated electrons. 相似文献
17.
Tie-Jun LiYa-Min Quan Da-Yong LiuLiang-Jian Zou 《Journal of magnetism and magnetic materials》2012,324(6):1046-1050
We study the magnetic phase diagram within an extended half-filled Hubbard model, focusing on the roles of the next-nearest-neighbor (NNN) and the next-next-nearest-neighbor (3rd NN) hoppings in the magnetic configurations. We find that due to the spin frustration from the long range hopping and the competition between long-range hopping and Coulomb correlation, the striped antiferromagnetic (AFM) phase is stable when the NNN hopping is dominant, while the bicollinear AFM phase is robust when the 3rd NN hopping is considerably large. The triple points are found in various magnetic phase diagrams. Possible applications of the present theory on intermediately correlated LaFeAsO and strongly correlated FeTe are discussed. 相似文献
18.
Universal spin-dependent variable range hopping in wide-band-gap oxide ferromagnetic semiconductors
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This paper proposes a universal spin-dependent variable
range hopping theoretical model to describe various experimental
transport phenomena observed in wide-band-gap oxide ferromagnetic
semiconductors with high transition metal concentration. The
contributions of the `hard gap' energy, Coulomb interaction,
correlation energy, and exchange interaction to the electrical
transport are considered in the universal variable range hopping
theoretical model. By fitting the temperature and magnetic field
dependence of the experimental sheet resistance to the theoretical
model, the spin polarization ratio of electrical carriers near the Fermi
level and interactions between electrical carriers can be
obtained. 相似文献
19.
《Nuclear Physics B》1998,516(3):501-528
The correlation functions for a strongly correlated exactly solvable one-dimensional boson system on a finite chain as well as in the thermodynamic limit are calculated explicitly. This system, which we call the phase model, is the strong coupling limit of the integrable q-boson hopping model. The results are presented as determinants. 相似文献
20.
We show that correlated hopping of triplets, which is often the dominant source of kinetic energy in dimer-based frustrated quantum magnets, produces a remarkably strong tendency to form supersolid phases in a magnetic field. These phases are characterized by simultaneous modulation and ordering of the longitudinal and transverse magnetization, respectively. Using quantum Monte Carlo and a semiclassical approach for an effective hard-core boson model with nearest-neighbor repulsion on a square lattice, we prove, in particular, that a supersolid phase can exist even if the repulsion is not strong enough to stabilize an insulating phase at half-filling. Experimental implications for frustrated quantum antiferromagnets in a magnetic field at zero and finite temperature are discussed. 相似文献