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 $U(t,0)\equiv P(t)e^{\frac{-i}{\hbar}H_F t}$ with $P(t+T)=P(t)$, and $H_F$ 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 $\frac 1 \omega$ is derived, where $\omega$ 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$\acute{\textrm{e}}$-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/Y$_3$Fe$_5$O$_{12}$ (YIG) system are tuned significantly by spin orbital coupling strength $\xi$ 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