32,775 research outputs found
Hall conductance of two-band systems in a quantized field
Kubo formula gives a linear response of a quantum system to external fields,
which are classical and weak with respect to the energy of the system. In this
work, we take the quantum nature of the external field into account, and define
a Hall conductance to characterize the linear response of a two-band system to
the quantized field. The theory is then applied to topological insulators.
Comparisons with the traditional Hall conductance are presented and discussed.Comment: 6 pages, 7 figure
Population transfer driven by far-off-resonant fields
For a two-level system, it is believed that a far-off-resonant driving can
not help coherent population transfer between the states. In this work, we
propose a scheme to implement the coherent transfer with far-off-resonant
driving. The scheme works well with both constant driving and Gaussian driving.
The total time to finish population transfer is also minimized by optimizing
the detuning and coupling constants. We find that the scheme is sensitive to
spontaneous emission much more than dephasing. It might find potential
applications in X-ray quantum optics and population transfer in Rydberg atoms
as well.Comment: arXiv admin note: text overlap with arXiv:1011.4423 by other author
Floquet theorem for open systems and its applications
For a closed system with periodic driving, Floquet theorem tells that the
time evolution operator can be written as with , and is Hermitian and
time-independent called Floquet Hamiltonian. In this work, we extend the
Floquet theorem from closed systems to open systems described by a Lindblad
master equation that is periodic in time. Lindbladian expansion in powers of
is derived, where is the driving frequency. Two
examples are presented to illustrate the theory. We find that appropriate trace
preserving time-independent Lindbladian of such a periodically driven system
can be constructed by the application of open system Floquet theory, and it
agrees well with the exact dynamics in the high frequency limit.Comment: 5 pages, 4 figure
Engineering the coupling between Majorana bound states
We study the coupling between Majorana bound states (CMBS), which is mediated
by a topologically trivial chain in the presence of pairing coupling and
long-range coupling. The results show that CMBS can be enhanced by the pairing
coupling and long-range coupling of the trivial chain. When driving the trivial
chain by periodic driving field, we deduce the analytical expressions of CMBS
in the high-frequency limit, and demonstrate that CMBS can be modulated by the
frequency and amplitude of driving field. Finally we exhibit the application of
tunable CMBS in realizing quantum logic gates.Comment: 8 pages, 8 figure
Anomalous carrier density independent superconductivity in iron pnictides
Dome-shape superconductivity phase diagram can commonly be observed in
cuprate and iron-based systems via tuning parameters such as charge carrier
doping, pressure, bond angle, and etc. We report doping electrons from
transition-metal elements (TM = Co, Ni) substitution can induce high-Tc
superconductivity around 35 K in Ca0.94La0.06Fe2As2, which emerges abruptly
before the total suppression of the innate
spin-density-wave/anti-ferromagnetism (SDW/AFM) state. Unexpectedly, the onset
critical temperature for the high-Tc superconductivity stays constant for a
wide range of TM doping. Possible extrinsic factors like phase separation,
chemical inhomogeneity, and charge carrier cancelation effect are all excluded.
This anomalous charge carrier density independent SC is very similar to the
interface superconductivity in La2-xSrxCuO4-La2CuO4 bilayer system. The further
verified two-dimensional (2D) nature of superconductivity by the Tinkham's
angular-dependent critical field model as well as by the angle-resolved
magneto-resistance measurements jointly supports the idea of interfacial effect
induced high-Tc superconductivity
The influence of localization transition on dynamical properties for an extended Aubry-Andr\'e-Harper model
We show the localization transition and its effect on two dynamical processes
for an extended Aubry-Andr\'e-Harper model with incommensurate on-site and
hopping potentials. After specifying an extended Aubry-Andr\'e-Harper model, we
check the localization transition for all the eigenstates and eigenenergy band
splitting behavior versus a system parameter. To examine the effect of
localization transition on dynamical processes, firstly, the slowly pumping of
the edge states are examined. In the dynamical processes, the system acts as
conductor for the excitation in the nonlocal region and insulator in the
localized region. Then by quantum Lyapunov control method with different
control Hamiltonians, we prepare an edge localized state which exists in the
nonlocal region. Compared to that in the nonlocal region, the control effect is
suppressed in the localized region. Then we employ the entropy and occupation
imbalance between even and odd sites to indicate the localization transition
further. Finally, the experimental schemes based on cold atoms trapped
quasiperiodic optical lattice and coupled optical waveguide arrays are
suggested
Possible Phase Separation and Transport Properties in Large Superconducting Ca0.77La0.18Fe0.90As2 Crystals
We synthesized large superconducting single crystal Ca0.77La0.18Fe0.90As2
('112' type) of 2 millimeter size. Scanning electron microscopy (SEM)
measurements revealed that bright and dark stripes alternately spread on the
surface of crystals, indicating possible existence of intrinsic phase
separation. Temperature (T) -dependent resistivity, Hall effect and
magneto-resistance (MR) were measured with magnetic field (H) applied to
different directions of crystal. The upper critical field (Hc2) anisotropy was
estimated with a moderate value around 2.8. Positive Hall coefficient (RH) and
anisotropic MR were found and showed strong T-dependent feature. Below T about
100 K, abnormal behaviors appear simultaneously in resistivity derivative, Hall
coefficient and MR, which indicates that other scattering mechanisms more than
conventional multiband effect may exist.Comment: This paper has been withdrawn by the author due to a crucial sign
error of the Hall coefficien
Preparation of edge states by shaking boundaries
Preparing topological states of quantum matter, such as edge states, is one
of the most important directions in condensed matter physics. In this work, we
present a proposal to prepare edge states in
Aubry-Andr-Harper (AAH) model with open boundaries, which
takes advantage of Lyapunov control to design operations. We show that edge
states can be obtained with almost arbitrary initial states. A numerical
optimalization for the control is performed and the dependence of control
process on the system size is discussed. The merit of this proposal is that the
shaking exerts only on the boundaries of the model. As a by-product, a
topological entangled state is achieved by elaborately designing the shaking
scheme.Comment: 11 pages, 11 figure
Thermal noise limited higher-order mode locking of a reference cavity
Higher-order mode locking has been proposed to reduce the thermal noise limit
of reference cavities. By locking a laser to the HG02 mode of a 10-cm long all
ULE cavity, and measure its performance with the three-cornered-hat method
among three independently stabilized lasers, we demonstrate a thermal noise
limited performance of a fractional frequency instability of 4.9E-16. The
results match the theoretical models with higher-order optical modes. The
achieved laser instability improves the all ULE short cavity results to a new
low level.Comment: 12pages, 4figure
Tuning Magnetotransport in PdPt/Y3Fe5O12: Effects of magnetic proximity and spin orbital coupling
Anisotropic magnetoresistance (AMR) ratio and anomalous Hall conductivity
(AHC) in PdPt/YFeO (YIG) system are tuned significantly by spin
orbital coupling strength through varying the Pt concentration. For both
Pt/YIG and Pd/YIG, the maximal AMR ratio is located at temperatures for the
maximal susceptibility of paramagnetic Pt and Pd metals. The AHC and ordinary
Hall effect both change the sign with temperature for Pt-rich system and vice
versa for Pd-rich system. The present results ambiguously evidence the spin
polarization of Pt and Pd atoms in contact with YIG layers. The global
curvature near the Fermi surface is suggested to change with the Pt
concentration and temperature
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