123 research outputs found

### Short time dynamics of molecular junctions after projective measurement

In this work, we study the short time dynamics of a molecular junction
described by Anderson-Holstein model using full-counting statistics after
projective measurement. The coupling between the central quantum dot (QD) and
two leads was turned on at remote past and the system is evolved to steady
state at time $t=0$, when we perform the projective measurement in one of the
lead. Generating function for the charge transfer is expressed as a Fredholm
determinant in terms of Keldysh nonequilibrium Green's function in the time
domain. It is found that the current is not constant at short times indicating
that the measurement does perturb the system. We numerically compare the
current behaviors after the projective measurement with those in the transient
regime where the subsystems are connected at $t=0$. The universal scaling for
high-order cumulants is observed for the case with zero QD occupation due to
the unidirectional transport at short times. The influences of electron-phonon
interaction on short time dynamics of electric current, shot noise and
differential conductance are analyzed

### The focusing of electron flow in a bipolar Graphene ribbon with different chiralities

The focusing of electron flow in a symmetric p-n junction (PNJ) of graphene
ribbon with different chiralities is studied. Considering the PNJ with the
sharp interface, in a armchair ribbon, the electron flow emitting from $(-L,0)$
in n-region can always be focused perfectly at $(L,0)$ in p-region in the whole
Dirac fermion regime, i.e. in whole regime $E_0<t$ where $E_0$ is the distance
between Dirac-point energy and Fermi energy and $t$ is the nearest hopping
energy. For the bipolar ribbon with zigzag edge, however, the incoming electron
flow in n-region is perfectly converged in p-region only in a very low energy
regime with $E_0<0.05t$. Moreover, for a smooth PNJ, electrons are
backscattered near PNJ, which weakens the focusing effect. But the focusing
pattern still remains the same as that of the sharp PNJ. In addition, quantum
oscillation in charge density occurs due to the interference between forward
and backward scattering. Finally, in the presence of weak perpendicular
magnetic field, charge carriers are deflected in opposite directions in the
p-region and n-region. As a result, the focusing effect is smeared. The lower
energy $E_0$, the easier the focusing effect is destroyed. For the high energy
$E_0$ (e.g. $E_0=0.9t$), however, the focusing effect can still survive in a
moderate magnetic field on order of one Tesla.Comment: 29 pages, 16 figure

### Symmetry and transport property of spin current induced spin-Hall effect

We study the spin current induced spin-Hall effect that a longitudinal spin
dependent chemical potential $qV_{s=x,y,z}$ induces a transverse spin
conductances $G^{ss'}$. A four terminal system with Rashba and Dresselhaus
spin-orbit interaction (SOI) in the scattering region is considered. By using
Landauer-B$\ddot u$ttiker formula with the aid of the Green function, various
spin current induced spin-Hall conductances $G^{ss'}$ are calculated. With the
charge chemical potential $qV_c$ or spin chemical potential $qV_{s=x,y,z}$,
there are 16 elements for the transverse conductances $G^{\mu
\nu}_p=J_{p,\mu}/V_{\nu}$ where $\mu,\nu=x,y,z,c$. Due to the symmetry of our
system these elements are not independent. For the system with $C_2$ symmetry
half of elements are zero, when the center region only exists the Rashba SOI or
Dresselhaus SOI. The numerical results show that of all the conductance
elements, the spin current induced spin-Hall conductances $G^{ss'}$ are usually
much greater (about one or two orders of magnitude) than the spin Hall
conductances $G^{sc}$ and the reciprocal spin Hall conductances $G^{cs}$. So
the spin current induced spin-Hall effect is dominating in the present device.Comment: 7 pages, 6 figure

### Transient dynamics of molecular devices under step-like pulse bias

We report first principles investigation of time-dependent current of
molecular devices under a step-like pulse.Our results show that although the
switch-on time of the molecular device is comparable to the transit time, much
longer time is needed to reach the steady state. In reaching the steady state
the current is dominated by resonant states below Fermi level. The contribution
of each resonant state to the current shows the damped oscillatory behavior
with frequency equal to the bias of the step-like pulse and decay rate
determined by the life time of the corresponding resonant state. We found that
all the resonant states below Fermi level have to be included for accurate
results. This indicates that going beyond wideband limit is essential for a
quantitative analysis of transient dynamics of molecular devices

### First-principles investigation of dynamical properties of molecular devices under a steplike pulse

We report a computationally tractable approach to first principles
investigation of time-dependent current of molecular devices under a step-like
pulse. For molecular devices, all the resonant states below Fermi level
contribute to the time-dependent current. Hence calculation beyond wideband
limit must be carried out for a quantitative analysis of transient dynamics of
molecules devices. Based on the exact non-equilibrium Green's function (NEGF)
formalism of calculating the transient current in Ref.\onlinecite{Maciejko}, we
develop two approximate schemes going beyond the wideband limit, they are all
suitable for first principles calculation using the NEGF combined with density
functional theory. Benchmark test has been done by comparing with the exact
solution of a single level quantum dot system. Good agreement has been reached
for two approximate schemes. As an application, we calculate the transient
current using the first approximated formula with opposite voltage
$V_L(t)=-V_R(t)$ in two molecular structures: Al-${\rm C}_{5}$-Al and Al-${\rm
C}_{60}$-Al. As illustrated in these examples, our formalism can be easily
implemented for real molecular devices. Importantly, our new formula has
captured the essential physics of dynamical properties of molecular devices and
gives the correct steady state current at $t=0$ and $t\rightarrow \infty$.Comment: 15 pages, 8 figure

### Crossed Andreev effects in two-dimensional quantum Hall systems

We study the crossed Andreev effects in two-dimensional
conductor/superconductor hybrid systems under a perpendicular magnetic field.
Both a graphene/superconductor hybrid system and an electron gas/superconductor
one are considered. It is shown that an exclusive crossed Andreev reflection,
with other Andreev reflections being completely suppressed, is obtained in a
high magnetic field because of the chiral edge states in the quantum Hall
regime. Importantly, the exclusive crossed Andreev reflection not only holds
for a wide range of system parameters, e.g., the size of system, the width of
central superconductor, and the quality of coupling between the graphene and
the superconductor, but also is very robust against disorder. When the applied
bias is within the superconductor gap, a robust Cooper-pair splitting process
with high-efficiency can be realized in this system.Comment: 10 pages, 10 figure

### The parity of specular Andreev reflection under mirror operation in zigzag graphene ribbon

It is known that the parity of reflection amplitude can either be even or odd
under the mirror operation. Up to now, all the parities of reflection amplitude
in the one-mode energy region are even under the mirror operation. In this
paper, we give an example of odd parity for Andreev reflection (AR) in a
three-terminal graphene-supercondutor hybrid systems. We found that the parity
is even for the Andreev retroreflection (ARR) and odd for specular Andreev
reflection (SAR). We attribute this remarkable phenomenon to the distinct
topology of the band structure of graphene and the specular Andreev reflection
involving two energy bands with different parity symmetry. As a result of odd
parity of SAR, the SAR probability of a four-terminal system with two
superconducting leads (two reflection interfaces) can be zero even when the
system is asymmetric due to the quantum interference of two ARs.Comment: 11 pages, 3 figure

- â€¦