7,947 research outputs found
Probing Spin Accumulation induced Magnetocapacitance in a Single Electron Transistor
The interplay between spin and charge in solids is currently among the most
discussed topics in condensed matter physics. Such interplay gives rise to
magneto-electric coupling, which in the case of solids was named
magneto-electric effect, as predicted by Curie on the basis of symmetry
considerations. This effect enables the manipulation of magnetization using
electrical field or, conversely, the manipulation of electrical polarization by
magnetic field. The latter is known as the magnetocapacitance effect. Here, we
show that non-equilibrium spin accumulation can induce tunnel
magnetocapacitance through the formation of a tiny charge dipole. This dipole
can effectively give rise to an additional serial capacitance, which represents
an extra charging energy that the tunneling electrons would encounter. In the
sequential tunneling regime, this extra energy can be understood as the energy
required for a single spin to flip. A ferromagnetic single-electron-transistor
with tunable magnetic configuration is utilized to demonstrate the proposed
mechanism. It is found that the extra threshold energy is experienced only by
electrons entering the islands, bringing about asymmetry in the measured
Coulomb diamond. This asymmetry is an unambiguous evidence of spin accumulation
induced tunnel magnetocapacitance, and the measured magnetocapacitance value is
as high as 40%.Comment: 19 pages, 6 figure
Floquet topological insulator phase in a Weyl semimetal thin film with disorder
We investigate the effects of periodic fields and disorder on topological
properties of a Weyl-semimetal thin film. The two periodic fields, i.e., a
periodic magnetic field and elliptically polarized light, are discussed
respectively. By use of the Floquet theory, we find that both the two periodic
drives can resonantly induce the topological transitions from normal insulator
(NI) phases to Floquet topological insulator (FTI) phases. The Floquet
topological transitions are characterized by variation of Chern number.
Moreover, we show that the Floquet topological transitions can be explained by
a combination of the quantum well approximation and the rotating wave
approximation. In the disordered Weyl-semimetal thin film model under periodic
fields, we calculate the Bott index to characterize topological phase. It is
found that the FTI phase is robust against weak disorder, and collapses for
strong disorder strength. Interestingly, we find that disorder can also induce
a topological transition from a topological trivial phase to an FTI phase,
establishing the Floquet topological Anderson insulator (FTAI) phase. Finally,
an effective-medium theory based on the Born approximation further confirms the
numerical conclusions
Time-varying Bang-bang Property of Minimal Controls for Approximately Null-controllable Heat Equations
In this paper, optimal time control problems and optimal target control
problems are studied for the approximately null-controllable heat equations.
Compared with the existed results on these problems, the boundary of control
variables are not constants but time varying functions. The time-varying
bang-bang property for optimal time control problem, and an equivalence theorem
for optimal control problem and optimal target problem are obtained.Comment: 13 page
Topological Anderson insulator phase in a Dirac-semimetal thin film
The recently discovered topological Dirac semimetal represents a new exotic
quantum state of matter. Topological Dirac semimetals can be viewed as three
dimensional analogues of graphene, in which the Dirac nodes are protected by
crystalline symmetry. It has been found that quantum confinement effect can gap
out Dirac nodes and convert Dirac semimetal to a band insulator. The band
insulator is either normal insulator or quantum spin Hall insulator depending
on the thin film thickness. We present the study of disorder effects in thin
film of Dirac semimetals. It is found that moderate Anderson disorder strength
can drive a topological phase transition from normal band insulator to
topological Anderson insulator in Dirac semimetal thin film. The numerical
calculation based on the model parameters of Dirac semimetal NaBi shows
that in the topological Anderson insulator phase a quantized conductance
plateau occurs in the bulk gap of band insulator, and the distributions of
local currents further confirm that the quantized conductance plateau arises
from the helical edge states induced by disorder. Finally, an effective medium
theory based on Born approximation fits the numerical data
Photon echo using imperfect X-ray pulse with phase fluctuation
We study the impact of inter-pulse phase fluctuation in free-electron X-ray
laser on the signal in the photon echo spectroscopy, which is one of the
simplest non-linear spectroscopic methods. A two-pulse echo model is considered
with two-level atoms as the sample. The effect of both fluctuation amplitude
and correlation strength of the random phase fluctuation is studied both
numerically and analytically. We show that the random phase effect only affects
the amplitude of the photon echo, yet not change the recovering time. Such
random phase induces the fluctuation of recovering amplitude in the photon echo
signals among different measurements. We show the normal method of measuring
coherence time retains by averaging across the signals in different repeats in
current paper.Comment: 7 pages, 4 figure
Disorder-induced topological phase transitions on Lieb lattices
Motivated by the very recent experimental realization of electronic Lieb
lattices and research interest on topological states of matter, we study the
topological phase transitions driven by Anderson disorder on spin-orbit coupled
Lieb lattices in the presence of spin-independent and dependent potentials. By
combining the numerical transport and self-consistent Born approximation
methods, we found that both time-reversal invariant and broken Lieb lattices
can host disorder-induced gapful topological phases, including the quantum spin
Hall insulator (QSHI) and quantum anomalous Hall insulator (QAHI) phases. For
the time-reversal invariant case, this disorder can induce a topological phase
transition directly from normal insulator (NI) to the QSHI. While for the
time-reversal broken case, the disorder can induce either a QAHI-QSHI phase
transition or a NI-QAHI-QSHI phase transition. Remarkably, the time-reversal
broken QSHI phase can be induced by Anderson disorder on the spin-orbit coupled
Lieb lattices without time-reversal symmetry.Comment: accepted for publication in Phys. Rev.
Achieve Higher Efficiency at Maximum Power with Finite-time Quantum Otto Cycle
The optimization of finite-time thermodynamic heat engines was intensively
explored recently, yet limited to few cycles, e.g. finite-time Carnot-like
cycle. In this paper, we supplement a new type of finite-time engine with
quantum Otto cycle and show the better performance. The current model can be
widely utilized benefited from the general \mathcal{C}/\tau^{2} scaling of
extra work for finite-time adiabatic process with long control time \tau. Such
scaling allows analytical optimization of the generic finite-time quantum Otto
cycle to surpass the efficiency at maximum power for the Carnot-like engine. We
apply the current perturbation method to the quantum piston model and calculate
the efficiency at maximum power, which is validated with exact solution.Comment: 14 pages, 10 figure
Finite-size effects in non-Hermitian topological systems
We systematically investigate the finite-size effects in non-Hermitian
one-dimensional (1D) Su-Schrieffer-Heeger (SSH) and two-dimensional (2D) Chern
insulator models. Using a combination of analytical and numerical calculations,
we show that the non-Hermitian intra-cell hoppings in the SSH model can modify
the localization lengths of bulk and end states, giving rise to a complex
finite-size energy gap that exhibits an oscillating exponential decay as the
chain length grows. However, the imaginary staggered on-site potentials in the
SSH model only change the end-state energy, leaving the localization lengths of
the system unchanged. In this case, the finite-size energy gap can undergo a
transition from real values to imaginary values. We observed similar phenomena
for the finite-size effect in 2D Chern insulator systems.Comment: 12 pages, 12 figures. Accepted by Physical Review
The A-Cycle Problem for Transverse Ising Ring
Traditionally, the transverse Ising model is mapped to the fermionic c-cycle
problem, which neglects the boundary effect due to thermodynamic limit. If
persisting on a perfect periodic boundary condition, we can get a so-called
a-cycle problem that has not been treated seriously so far (Lieb et al., 1961
\textit{Ann. of Phys.} \textbf{16} 407). In this work, we show a little
surprising but exact result in this respect. We find the odevity of the number
of lattice sites, , in the a-cycle problem plays an unexpected role even in
the thermodynamic limit, , due to the boundary constraint.
We pay a special attention to the system with ,
which is in contrast to the one with , because
the former suffers a ring frustration. As a new effect, we find the ring
frustration induces a low-energy gapless spectrum above the ground state. By
proving a theorem for a new type of Toeplitz determinant, we demonstrate that
the ground state in the gapless region exhibits a peculiar longitudinal
spin-spin correlation. The entangled nature of the ground state is also
disclosed by the evaluation of its entanglement entropy. At low temperatures,
new behavior of specific heat is predicted. We also propose an experimental
protocol for observing the new phenomenon due to the ring frustration.Comment: 24 pages, 9 figure
Chiral Symmetry of Double-Walled Carbon Nanotubes detected in First-principles Optical Absorption Spectra
The linear polarizability absorption spectra of the double-walled carbon
nanotubes (DWNTs) have been calculated by using the tight-binding (TB) model
and sum-over-state (SOS) method, supplemented by the first principles CASTEP
calculations. It is found that the chiral symmetries of both outer and inner
tubes in the DWNTs can always be identified distinctly by the characteristic
peaks in the absorption spectra of the DWNTs, no matter what kind of the outer
tube is, offering a powerful experimental tool to measure precisely the chiral
angle of the inner tube of a DWNT.Comment: 10 pages, 5 figure
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