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1.
The quantum conductance of the quantum dots (QDs) made of two kinds of primary carbon nanotubes (CNTs), i.e., armchair and zigzag CNTs, threaded by an axial magnetic field, has been studied by using the tight binding approximation and constant interaction model. It is found that under increasing axial magnetic field, each conductance shell of the zigzag CNT-QDs could split into two groups with each group of two peaks moving up or down, respectively. And the up- and down-moving two peaks would re-group with other two peaks, down- and up-moving, in the neighboring shell, forming a new four-peak shell, and then re-splitting, re-grouping again due to the Aharonov-Bohm effect, which is in agreement with those of experiments. But, in contrast, the conductance shells of the armchair CNT-QDs do not split by the magnetic field. Our subsequent theoretical studies show further that the above phenomena, i.e., the conductance shell-splitting, re-grouping, and re-splitting again with increasing the magnetic field exist in all the CNT-QDs except for the armchair one. 相似文献
2.
X. W. Zhang Y. H. Zhu J. B. Xia 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(1):133-142
The Hamiltonian in the framework of eight-band effective-mass
approximation of the zinc-blende nanowires and nanorods in the
presence of external homogeneous magnetic field is given in the
cylindrical coordinate. The electronic structure, optical
properties, magnetic energy levels, and g factors of the nanowires
and nanorods are calculated. It is found that the electron states
consist of many hole-state components, due to the coupling of the
conduction band and valence band. For the normal bands which are
monotone functions of |kz|, long nanorods can be modeled by the
nanowires, the energy levels of the nanorods approximately equal
the values of the energy band E(kz) of the nanowires with the
same radius at a special kz, where kz is the wave vector
in the wire direction. Due to the coupling of the states, some of
the hole energy bands of the nanowires have their highest points
at kz≠0. Especially, the highest hole state of the InSb
nanowires is not at the kz=0 point. It is an indirect band
gap. For these abnormal bands, nanorods can not be modeled by the
nanowires. The energy levels of the nanorods show an interesting
plait-like pattern. The linear polarization factor is zero, when
the aspect ratio L/2R is smaller than 1, and increases as the
length increases. The gz and gx factors as functions of
the kz, radius R and length L are calculated for the
wires and rods, respectively. For the wires, the gz of the
electron ground state increases, and the gz of the hole
ground state decreases first, then increases with the kz
increasing. For the rods, the gz and gx of the electron
ground state decrease as the R or the L increases. The gx
of the hole ground state decreases, the gz of the hole ground
state increases with the L increasing. The variation of the
gz of the wires with the kz is in agreement with the
variation of the gz of the rods with the L. 相似文献
3.
F. Heidrich-Meisner G. B. Martins C. A. Büsser K. A. Al-Hassanieh A. E. Feiguin G. Chiappe E. V. Anda E. Dagotto 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,67(4):527-542
The numerical analysis of strongly interacting nanostructures
requires powerful techniques.
Recently developed methods, such as the time-dependent density matrix renormalization group (tDMRG) approach or the embedded-cluster
approximation (ECA), rely on the numerical solution of clusters of finite size. For the interpretation of numerical results,
it is therefore crucial to understand finite-size effects
in detail. In this work, we present a careful finite-size analysis for the examples of one quantum dot, as well as three serially
connected quantum
dots. Depending on “odd-even” effects, physically quite different
results may emerge from clusters that do not differ much in their size. We provide
a solution to a recent controversy over results obtained with ECA for three quantum dots.
In particular, using the optimum clusters discussed in this paper, the parameter range in which
ECA can reliably be applied is increased, as we show for the case of three quantum dots.
As a practical procedure, we propose that a comparison of results for static quantities against those of quasi-exact methods,
such as the ground-state density matrix renormalization group (DMRG) method or exact diagonalization,
serves to identify the optimum cluster type.
In the examples studied here, we find that to observe signatures of the Kondo effect in finite systems, the best clusters
involving dots and leads
must have a total z-component of
the spin equal to zero. 相似文献
4.
R.M. Stevenson R.J. Young P. See D.G. Gevaux K. Cooper P. Atkinson I. Farrer D.A. Ritchie A.J. Shields 《Physica E: Low-dimensional Systems and Nanostructures》2006,32(1-2):135
We demonstrate control of the fine-structure splitting of the exciton emission lines in single InAs quantum dots by the application of an in-plane magnetic field. The composition of the barrier material and the size and symmetry of the quantum dot are found to determine decrease or increase in the linear polarization splitting of the dominant exciton emission lines with increasing magnetic field. This enables the selection of dots for which the splitting can to be tuned to zero, within the resolution of our experiments. General differences in the g-factors and exchange splittings are found for different types of dot. 相似文献
5.
A. John Peter K. Lily Mary Eucharista 《Journal of magnetism and magnetic materials》2009,321(5):402-407
We report the effect of intense laser field on donor impurities in a semimagnetic Cd1-xinMnxinTe/Cd1-xoutMnxoutTe quantum dot. The spin polaronic energy of different Mn2+ is evaluated for different dot radii using a mean field theory in the presence of laser field. Magnetization is calculated for various concentrations of Mn2+ ions with different dot sizes. Significant magnetization of Mn spins can be obtained through the formation of polarized exciton magnetic polarons (EMPs). A rapid decrease of the laser dressed donor ionization energy for different values of dot sizes with increasing field intensity is predicted. Also, it is found that the polarization of EMPs increases rapidly at higher excitation energies. 相似文献
6.
A. Ramšak J. Mravlje 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(4):419-425
We investigate entanglement between electrons in
serially coupled double quantum dots attached to noninteracting
leads. In addition to local repulsion we consider the influence of
capacitive inter-dot interaction. We show how the competition
between extended Kondo and local singlet phases determines the
ground state and thereby the entanglement. The results are
additionally discussed in connection with the linear conductance
through the system. 相似文献
7.
R. Tigrine A. Khater B. Bourahla M. Abou Ghantous O. Rafil 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(1):59-64
A theoretical model is presented for the study of the scattering of magnons
at an extended symmetric atomic well in very thin magnetic films. The thin
film consists of three cubic atomic planes with ordered spins coupled by
Heisenberg exchange, and the system is supported on a non-magnetic
substrate, and considered otherwise free from magnetic interactions. The
coherent transmission and reflection scattering coefficients are derived as
elements of a Landauer type scattering matrix. Transmission and reflection
scattering cross sections are hence calculated specifically, as a function
of the varying local magnetic exchange on the inhomogeneous boundary.
Detailed numerical results for the individual incident film magnons, and for
the calculated overall magnon conductance, show characteristic transmission
properties, with associated Fano resonances, depending on the magnetic
boundary conditions and on the magnon incidence. 相似文献
8.
We consider the dynamics of a single electron in a chain of tunnel coupled quantum dots, exploring the formal analogies of this system with some of the laser-driven multilevel atomic or molecular systems studied by Bruce W. Shore and collaborators over the last 30 years. In particular, we describe two regimes for achieving complete coherent transfer of population in such a multistate system. In the first regime, by carefully arranging the coupling strengths, the flow of population between the states of the system can be made periodic in time. In the second regime, by employing a “counterintuitive” sequence of couplings, the coherent population trapping eigenstate of the system can be rotated from the initial to the final desired state, which is an equivalent of the STIRAP technique for atoms or molecules. Our results may be useful in future quantum computation schemes. 相似文献
9.
K. Kärkkäinen M. Borgh M. Manninen S. M. Reimann 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):225-228
We compare magnetism in two artificial lattice structures, a
quantum dot array formed in a two-dimensional electron gas and an optical lattice loaded with repulsive, contact-interacting
fermionic atoms. When the tunneling between the
lattice sites is strong, both lattices are non-magnetic. With reduced tunneling in the tight-binding limit, the shell-filling
of the single-site quantum wells combined with Hund's rule determines the
magnetism. This leads to a systematic magnetic phase diagram with non-magnetic, ferromagnetic and antiferromagnetic phases. 相似文献
10.
The ground- and first-excited states of magnetopolarons in two-dimensional quantum dots for all coupling strengths 总被引:2,自引:0,他引:2
B.S. Kandemir T. Altanhan 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(2):227-232
The ground- and first-excited state energies of a magnetopolaron in a two dimensional parabolic quantum dot are studied within
a variational calculation for all coupling strength. The Lee-Low-Pines-Huybrecht variational technique that is developed previously
for all coupling strength has been extented for polarons in a magnetic field. The dependence of the polaronic correction on
the magnetic field and the confinement length is investigated. The polarization potential and the renormalized cyclotron masses
as a function of electron-phonon coupling strength and the strength of both confinement potential and magnetic field are also
studied within the same approach.
Received 16 December 2002 / Received in final form 14 April 2003 Published online 4 June 2003
RID="a"
ID="a"e-mail: kandemir@science.ankara.edu.tr 相似文献
11.
A. Martín-Rodero 《物理学进展》2013,62(6):899-958
In this article, we review the state of the art on the transport properties of quantum dot systems connected to superconducting and normal electrodes. The review is mainly focused on the theoretical achievements, although a summary of the most relevant experimental results is also given. A large part of the discussion is devoted to the single-level Anderson-type models generalized to include superconductivity in the leads, which already contains most of the interesting physical phenomena. Particular attention is paid to the competition between pairing and Kondo correlations, the emergence of π-junction behavior, the interplay of Andreev and resonant tunneling, and the important role of Andreev bound states that characterized the spectral properties of most of these systems. We give technical details on the several different analytical and numerical methods which have been developed for describing these properties. We further discuss the recent theoretical efforts devoted to extend this analysis to more complex situations like multidot, multilevel or multiterminal configurations in which novel phenomena is expected to emerge. These include control of the localized spin states by a Josephson current and also the possibility of creating entangled electron pairs by means of non-local Andreev processes. 相似文献
12.
The third-harmonic generation (THG) coefficient for cylindrical quantum dots in a static magnetic field is investigated theoretically. By using the compact density-matrix approach and the iterative method, we obtain an analytical expression for the THG coefficient, and numerical calculations for typical GaAs/AlAs cylindrical quantum dots are presented. The results show that the THG coefficient can reach a magnitude of 10−10 m2/V 2. In addition to the radius R of the cylindrical quantum dots, both the parabolic confining potential and the static magnetic field have an influence on the THG coefficient. 相似文献
13.
The Coulomb blockade (CB) in quantum dots (QDs) is by now well documented. It has been used to guide the fabrication of single
electron transistors. Even the most sophisticated techniques for synthesizing QDs (e.g. MOCVD/MBE) result in an assembly in
which a certain amount of disorder is inevitable. On the other hand, theoretical approaches to CB limit themselves to an analysis
of a single QD. In the present work we consider two types of disorders: (i) size disorder; e.g. QDs have a distribution of
sizes which could be unimodal or bimodal in nature. (ii) Potential disorder with the confining potential assuming a variety
of shapes depending on growth condition and external fields. We assume a Gaussian distribution in disorder in both size and
potential and employ a simplified mean field theory. To do this we rely on the scaling laws for the CB (also termed as Hubbard
U) obtained for an isolated QD [1]. We analyze the distribution in the Hubbard U as a consequence of disorder and observe that Coulomb blockade is partially suppressed by the disorder. Further, the distribution
in U is a skewed Gaussian with enhanced broadening.
相似文献
14.
M. C. Desjonquères C. Barreteau G. Autès D. Spanjaard 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(1):23-27
We show that considerable orbital magnetic moments and magneto-crystalline anisotropy energies are obtained for a Fe monatomic
wire described in a tight-binding method with intra-atomic electronic interactions treated in a full Hartree Fock (HF) decoupling
scheme.
Even though the use of the orbital polarization ansatz with simplified Hamiltonians leads to fairly good results when the
spin magnetization is saturated this is not the case of unsaturated systems. We conclude that the full HF scheme is necessary
to investigate low dimensional systems. 相似文献
15.
X. W. Zhang J. B. Xia 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,58(3):263-268
The electronic structure, spin splitting energies, and g factors of
paramagnetic In1-xMnxAs nanowires under magnetic and electric
fields are investigated theoretically including the sp-d exchange
interaction between the carriers and the magnetic ion. We find that the
effective g factor changes dramatically with the magnetic field. The spin
splitting due to the sp-d exchange interaction counteracts the
Zeeman spin splitting. The effective g factor can be tuned to zero by the
external magnetic field. There is also spin splitting under an electric
field due to the Rashba spin-orbit coupling which is a relativistic effect.
The spin-degenerated bands split at nonzero kz (kz is the wave
vector in the wire direction), and the spin-splitting bands cross at
kz = 0, whose kz-positive part and negative part are symmetrical. A
proper magnetic field makes the kz-positive part and negative part of
the bands asymmetrical, and the bands cross at nonzero kz. In the
absence of magnetic field, the electron Rashba coefficient increases almost
linearly with the electric field, while the hole Rashba coefficient
increases at first and then decreases as the electric field increases. The
hole Rashba coefficient can be tuned to zero by the electric field. 相似文献
16.
H. Pan T. H. Lin 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(3):299-303
The supercurrent through an Aharonov-Bohm interferometer containing
two parallel quantum dots connected with two superconductor leads is
investigated theoretically. The possibility of controlling the
supercurrent is explored by tuning the quantum dot energy levels and
the total magnetic flux. By tuning the energy levels, both quantum
dots can be in the on-resonance or off-resonance states, and thus
the optimal modulation of the supercurrent can be achieved. The
supercurrent sign does not change by simply varying the quantum dot
energy levels. However, by tuning the magnetic flux, the
supercurrent can oscillate from positive to negative, which results
in the π-junction transition. 相似文献
17.
S. Lee M. Dobrowolska J.K. Furdyna 《Physica E: Low-dimensional Systems and Nanostructures》2005,26(1-4):271
We have investigated a series of double-layer structures consisting of a layer of self-assembled non-magnetic CdSe quantum dots (QDs) separated by a thin ZnSe barrier from a ZnCdMnSe diluted magnetic semiconductor (DMSs) quantum well (QW). In the series, the thickness of the ZnSe barrier ranged between 12 and 40 nm. We observe two clearly defined photoluminescence (PL) peaks in all samples, corresponding to the CdSe QDs and the ZnCdMnSe QW, respectively. The PL intensity of the QW peak is observed to decrease systematically relative to the QD peak as the thickness of the ZnSe barrier decreases, indicating a corresponding increase in carrier tunneling from the QW to the QDs. Furthermore, polarization-selective PL measurements reveal that the degree of polarization of the PL emitted by the CdSe QDs increases with decreasing thickness of the ZnSe barriers. The observed behavior is discussed in terms of anti-parallel spin interaction between carriers localized in the non-magnetic QDs and in the magnetic QWs. 相似文献
18.
G. Michałek B. R. Bułka 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(3):411-419
Conductance and other physical quantities are calculated in double quantum dots (DQD) connected in series in the limit of
coherent tunnelling using a Green's function technique. The inter-dot Coulomb repulsion and the exchange interaction are studied
by means of the Kotliar and Ruckenstein slave-boson mean-field approach. The crossover from the atomic to the molecular limit
is analyzed in order to show how the conductance in the model depends on the competition between the level broadening (dot-lead
coupling) and the dot-dot transmission. The double Kondo effect
was found in the gate voltage characteristics of the conductance in the atomic limit. In the case, when each dot accommodates
one electron, the Kondo resonant states are formed between dots and their adjacent leads and transport is dominated by hopping
between these two resonances. In the molecular limit the conductance vanishes for sufficiently low gate voltages, which means
the Kondo effect disappeared. For small dot-lead coupling the transport characteristics are very sensitive on the influence
of the inter-dot Coulomb repulsion and the position of the local energy level. The
resonance region is widened with increase of the inter-dot Coulomb interactions while the exchange interaction has opposite
influence. 相似文献
19.
H.-K. Zhao Q. Chen 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):419-427
We have investigated the spectral density of shot noise for the
system of a quantum dot (QD) coupled to two single-wall carbon
nanotube terminals irradiated with a microwave field on the QD. The
terminal features are involved in the shot noise through modifying
the self-energy of QD. The contributions of carbon nanotube
terminals to the shot noise exhibit obvious behaviors. The novel
side peaks are associated with the photon absorption and emission
procedure accompanying the suppression of shot noise. The shot noise
in balanced absorption belongs to sub-Poissonian, and it is
symmetric with respect to the gate voltage. The differential shot
noise displays intimate relation with the nature of carbon nanotubes
and the applied microwave field. It exhibits asymmetric behavior
for the unbalanced absorption case versus gate voltage. The Fano
factor of the system exhibits the deviation of shot noise from the
Schottky formula, and the structures of terminals obviously
contribute to it. The super-Poissonian and sub-Poissonian shot noise
can be achieved in the unbalanced absorption in different regime of
source-drain bias. 相似文献
20.
M. Pierre M. Hofheinz X. Jehl M. Sanquer G. Molas M. Vinet S. Deleonibus 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,70(4):475-481
We extend a simple model of a charge trap coupled to a single-electron box to energy ranges and parameters such that it gives
new insights and predictions readily observable in many experimental systems. We show that a single background charge is enough
to give lines of differential conductance in the stability diagram of the quantum dot, even within undistorted Coulomb diamonds.
It also suppresses the current near degeneracy of the impurity charge, and yields negative differential lines far from this
degeneracy. We compare this picture to two other accepted explanations for lines in diamonds, based respectively on the excitation
spectrum of a quantum dot and on fluctuations of the density-of-states in the contacts. In order to discriminate between these
models, we emphasize the specific features related to environmental charge traps. Finally we show that our model accounts
very well for all the anomalous features observed in silicon nanowire quantum dots. 相似文献