103 research outputs found
Electron momentum distribution of a single mobile hole in the t-J model
We investigate the electron momentum distribution function (EMDF) for the
two-dimensional t-J model. The results are based on the self-consistent Born
approximation (SCBA) for the self-energy and the wave function. In the Ising
limit of the model we give the results in a closed form, in the Heisenberg
limit the results are obtained numerically. An anomalous momentum dependence of
EMDF is found and the anomaly is in the lowest order in number of magnons
expressed analitycally. We interpret the anomaly as a fingerprint of an
emerging large Fermi surface coexisting with hole pockets.Comment: M2S - submitted to Physica
Geometrical view of quantum entanglement
Although a precise description of microscopic physical problems requires a
full quantum mechanical treatment, physical quantities are generally discussed
in terms of classical variables. One exception is quantum entanglement which
apparently has no classical counterpart. We demonstrate here how quantum
entanglement may be within the de Broglie-Bohm interpretation of quantum
mechanics visualized in geometrical terms, giving new insight into this
mysterious phenomenon and a language to describe it. On the basis of our
analysis of the dynamics of a pair of qubits, quantum entanglement is linked to
concurrent motion of angular momenta in the Bohmian space of hidden variables
and to the average angle between these momenta
Conductance of a molecule with a center of mass motion
We calculate the zero temperature conductance and characteristic correlation
functions of a molecule with a center of mass (CM) motion which modulates
couplings to the leads. In the first model studied, the CM vibrational mode is
simultaneously coupled to the electron density on the molecule. The conductance
is suppressed in regimes corresponding to non-integer occupancy of the
molecule. In the second model, where the CM mode is not directly coupled to the
electron density, the suppression of conductance is related to the dynamic
breaking of the inversion symmetry.Comment: to appear in Phys. Rev.
Spin qubits in double quantum dots - entanglement versus the Kondo effect
We investigate the competition between pair entanglement of two spin qubits
in double quantum dots attached to leads with various topologies and the
separate entanglement of each spin with nearby electrodes. Universal behavior
of entanglement is demonstrated in dependence on the mutual interactions
between the spin qubits, the coupling to their environment, temperature and
magnetic field. As a consequence of quantum phase transition an abrupt switch
between fully entangled and unentangled states takes place when the dots are
coupled in parallel.Comment: 3 figure
Kondo effect in triple quantum dots
Numerical analysis of the simplest odd-numbered system of coupled quantum
dots reveals an interplay between magnetic ordering, charge fluctuations and
the tendency of itinerant electrons in the leads to screen magnetic moments.
The transition from local-moment to molecular-orbital behavior is visible in
the evolution of correlation functions as the inter-dot coupling is increased.
Resulting novel Kondo phases are presented in a phase diagram which can be
sampled by measuring the zero-bias conductance. We discuss the origin of the
even-odd effects by comparing with the double quantum dot.Comment: 4 pages, 4 figure
Spin-fluctuation mechanism of superconductivity in cuprates
The theory of superconductivity within the t-J model, as relevant for
cuprates, is developed. It is based on the equations of motion for projected
fermionic operators and the mode-coupling approximation for the self-energy
matrix. The dynamical spin susceptibility at various doping is considered as an
input, extracted from experiments. The analysis shows that the
superconductivity onset is dominated by the spin-fluctuation contribution. We
show that T_c is limited by the spin-fluctuation scale and shows a
pronounced dependence on the next-nearest-neighbor hopping t'. The latter can
offer an explanation for the variation of T_c among different families of
cuprates.Comment: Color figure
Conductance of deformable molecules with interaction
Zero temperature linear response conductance of molecules with Coulomb
interaction and with various types of phonon modes is analysed together with
local occupation, local moment, charge fluctuations and fluctuations of
molecular deformation. Deformation fluctuations are quantitatively related to
charge fluctuations which exhibit similarity also to static charge
susceptibility.Comment: 4 pages, color figure
Kondo effect and channel mixing in oscillating molecules
We investigate the electronic transport through a molecule in the Kondo
regime. The tunneling between the electrode and the molecule is asymmetrically
modulated by the oscillations of the molecule, i.e., if the molecule gets
closer to one of the electrodes the tunneling to that electrode will increase
while for the other electrode it will decrease. The system is described by a
two-channel Anderson model with phonon-assisted hybridization, which is solved
with the Wilson numerical renormalization group method. The results for several
functional forms of tunneling modulation are presented. For a linearized
modulation the Kondo screening of the molecular spin is caused by the even or
odd conduction channel. At the critical value of the electron-phonon coupling
an unstable two-channel Kondo fixed point is found. For a realistic modulation
the spin at the molecular orbital is Kondo screened by the even conduction
channel even in the regime of strong coupling. A universal consequence of the
electron-phonon coupling is the softening of the phonon mode and the related
instability to perturbations that break the left-right symmetry. When the
frequency of oscillations decreases below the magnitude of such perturbation,
the molecule is abruptly attracted to one of the electrodes. In this regime,
the Kondo temperature is enhanced and, simultaneously, the conductance through
the molecule is suppressed.Comment: published versio
Arbitrary qubit transformations on tuneable Rashba rings
An exact solution is presented for the time-dependent wave function of an initial ground-state Kramers-doublet qubit that is driven around a quantum ring. We show that the initial qubit may be transformed to an arbitrary point on the Bloch sphere for an integral number of revolutions around the ring. Full coverage of the Bloch sphere is achieved by dividing the total rotation into segments, changing the rotation axis after each segment by an adiabatic shift in the Rashba spin-orbit interaction. Prospects and challenges for possible realizations are discussed for which rings based on InAs quantum wires are promising candidates
Entanglement between static and flying qubits in a semiconducting carbon nanotube
Entanglement can be generated by two electrons in a spin-zero state on a
semiconducting single-walled carbon nanotube. The two electrons, one weakly
bound in a shallow well in the conduction band, and the other injected into the
conduction band, are coupled by the Coulomb interaction. Both transmission and
entanglement are dependent on the well characteristics, which can be controlled
by a local gate, and on the kinetic energy of the injected electron. Regimes
with different degrees of electron correlation exhibit full or partial
entanglement. In the latter case, the maximum entanglement can be estimated as
a function of width and separation of a pair of singlet-triplet resonances.Comment: 17 pages and 12 figures, accepted to J. Phys. Cond. Ma
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