The Generalized Joint Density of States and Its Application to Exploring the Pairing Symmetry of High-Tc Superconductors

We introduce a generalized joint density of states(GJDOS),which incorporates the coherent factors into the JDOS,to study quasiparticle interference(QPI)in superconductors.The intimate relation between the Fourier-transformed local density of states and GJDOS is revealed:they correspond respectively to the real and imaginary parts of a generalized impurity-response function,and particularly share the same angular factors and singular boundaries,as seen from our approximate analytic results for d-wave superconductors.Remarkably,our numerical GJDOS analysis agrees well with the QPI patten of d-wave cuprates and s_(±)-wave iron-based superconductors.Moreover,we illustrate that the present GJDOS scenario can uncover the sign features of the superconducting gap and thus can be used to explore the pairing symmetry of the A_(1-x)Fe_(2-y)Se_(2)(A=K,Cs,etc)superconductors.

Ferromagnetic barrier-induced negative differential conductance on the surface of a topological insulator

The effect of the negative differential conductance of a ferromagnetic barrier on the surface of a topological insulat（ is theoretically investigated. Due to the changes of the shape and position of the Fermi surfaces in the ferromagnetic barrie the transport processes can be divided into three kinds： the total, partial, and blockade transmission mechanisms. The bias voltage can give rise to the transition of the transport processes from partial to blockade transmission mechanisms, which results in a considerable effect of negative differential conductance. With appropriate structural parameters, the currenl voltage characteristics show that the minimum value of the current can reach to zero in a wide range of the bias voltag and then a large peak-to-valley current ratio can be obtained.

All-electrically reading out and initializing topological qubits with quantum dots

We analyze the reading and initialization of a topological qubit encoded by Majorana fermions in one-dimensional semiconducting nanowires, weakly coupled to a single level quantum dot （QD）. It is shown that when the Majorana fermions are fused by tuning gate voltage, the topological qubit can be read out directly through the occupation of the QD in an energy window. The initialization of the qubit can also be realized via adjusting the gate voltage on the QD, with the total fermion parity conserved. As a result, both reading and initialization processes can be achieved in an all-electrical way.

Dynamic Control of Tunneling Conductance in Ferroelectric Tunnel Junctions

We investigate the dynamic characteristics of electric polarization P(t)in a ferroelectric junction under ac applied voltage and stress,and calculate the frequency response and the cut-off frequency f0,which provides a reference for the upper limit of the working frequency.Our study might be significant for sensor and memory applications of nanodevices based on ferroelectric junctions.

Controlling Fusion of Majorana Fermions in One-Dimensional Systems by Zeeman Field

We propose the realization of Majorana fermions （MFs） on the edges of a two-dimensional topological insulator in the proximity with s-wave superconductors and in the presence of transverse exchange field h. It is shown that there appear a pair of MFs localized at two junctions and that a reverse in the direction of h can lead to permutation of two MFs. With decreasing h, the MF states can either be fused or form one Dirac fermion on the π-junctions, exhibiting a topological phase transition. This characteristic can be used to detect physical states of MFs when they are transformed into Dirac fermions MFs is also given. localized on the π-junction. A condition of decoupling two

Spin-Filter Effect Induced by Magnetic Edge States of Zigzag Carbon Nanotube

Spin-filter effect is predicted in a weak coupled junction composed of a nonmagnetic metal electrode and a zigzag carbon nanotube. This effect is induced by the magnetic edge states of the nanotube, and can produce spin- polarized current in the absence of an external magnetic field. We find that the spin polarization of the current changes its sign at the half-filling point of the nanotube, thus electric field control of spin transport can be realized. Furthermore, we find the coupling strength of the junction may cause a magnetic transition on the edge of the nanotube.

An Exotic Type of Fulde-Ferrel-Larkin-Ovchinnikov States in Spin-Orbit Coupled Condensates

We find for the first time that a model Hamiltonian or s-wave superconductors in tne presence or spm-orbit interactions and a Zeeman field is exactly solvable. Most intriguingly, based on the exact solutions, a novel type of Fulde Ferrel-Larkin-Ovchinnikov ground state is rigorously revealed, in which the center-of-mass momentum of the fermion pair is proportional to the Zeeman field. We also generalize our exact analysis to the spin-orbit- coupled Bose-Einstein condensate.

First-principles study on the electronic and transport properties of periodically nitrogen-doped graphene and carbon nanotube superlattices

Prompted by recent reports on √3×√3 graphene superlattices with intrinsic inter-valley interactions, we perform first-principles calculations to investigate the electronic properties of periodically nitrogen- doped graphene and carbon nanotube nanostruetures. In these structures, nitrogen atoms substitute one-sixth of the carbon atoms in the pristine hexagonal lattices with exact periodicity to form perfect √3×√3 superlattices of graphene and carbon nanotubes. Multiple nanostructures of √3×√3 graphene ribbons and carbon nanotubes are explored, and all configurations show nonmagnetic and metallic behaviors. The transport properties of √3×√3 graphene and carbon nanotube superlattices are calculated utilizing the non-equilibrium Green＇s function formalism combined with density functional theory. The translnission spectrum through the pristine and √3×√3 armchair carbon nanotube heterostructure shows quantized behavior under certain circumstances.

Anderson Localization Enhanced Spin Selective Transport of Electrons in DNA

Recent experiments revealed the unusual strong spin effects with high spin selective transmission of electrons in double-stranded DNA. We propose a new mechanism that the strong spin effects could be understood in terms of the combination of the ehiral structure, spin-orbit coupling, and especially spin-dependent Anderson localization. The presence of chiral structure and spin-orbit coupling of DNA induce weak Fermi energy splitting between two spin polarization states. The intrinsic Anderson localization in generic DNA molecules may result in remarkable enhancement of the spin selective transport. In particular, these two spin states with energy splitting have different localization lengths. Spin up/down channel may have shorter/longer localization length so that relatively less/more spin up/down electrons may tunnel through the system. In addition, the strong length dependence of spin selectivity observed in experiments can be naturally understood. Anderson localization enhanced spin selectivity effect may provide a deeper understanding of spin-selective processes in molecular spintronics and biological systems.

Simulation of the spin-boson model with superconducting phase qubit coupled to a transmission line

Based on the rapid experimental developments of circuit QED,we propose a feasible scheme to simulate the spin-boson model with superconducting circuits,which can be used to detect quantum Kosterlitz-Thouless(KT) phase transition.We design the spinboson model by using a superconducting phase qubit coupled to a semi-infinite transmission line,which is regarded as a bosonic reservoir with a continuum spectrum.By tuning the bias current or the coupling capacitance,the quantum KT transition can be directly detected through tomography measurement on the states of the phase qubit.We also estimate the experimental parameters using the numerical renormalization group method.

Relativistic quantum effects of Dirac particles simulated by ultracold atoms

Quantum simulation is a powerful tool to study a variety of problems in physics, ranging from high-energy physics to condensed-matter physics. In this article, we review the recent theoretical and experimental progress in quantum simulation of Dirac equation with tunable parameters by using ultracold neutral atoms trapped in optical lattices or subject to light-induced synthetic gauge fields. The effective theories for the quasiparticles become relativistic under certain conditions in these systems, making them ideal platforms for studying the exotic relativistic effects. We focus on the realization of one, two, and three dimensional Dirac equations as well as the detection of some relativistic effects, including particularly the well-known Zitterbewegung effect and Klein tunneling. The realization of quantum anomalous Hall effects is also briefly discussed.

Nonlinear optics in the electron-hole continuum in 2D semiconductors:two-photon transition, second harmonic generation and valley current injection

We investigate two-photon transitions to the electron-hole scattering continuum in monolayer transition-metal dichalcogenides, and identify two contributions to this nonlinear optical process with opposite circularly polarized valley selection rules. In the non-interacting limit, the competition between the two contributions leads to a crossover of the selection rule with the increase of the two-photon energy. With the strong Coulomb interaction between the electron and hole, the two contributions excite electron-hole scattering states in orthogonal angular momentum channels, while the strength of the transition can be substantially enhanced by the interaction. Based on this picture of the two-photon transition, the second harmonic generation(SHG) in the electron-hole continuum is analyzed, where the Coulomb interaction is shown to greatly alter the relative strength of different cross-circular polarized SHG processes. Valley current injection by the quantum interference of one-photon and two-photon transition is also investigated in the presence of the strong Coulomb interaction, which significantly enhances the injection rate.

Entanglement charge of thermal states

Entanglement charge is an operational measure to quantify nonlocalities in ensembles consisting of bipartite quantum states.Here we generalize this nonlocality measure to single bipartite quantum states.As an example,we analyze the entanglement charges of some thermal states of two-qubit systems and show how they depend on the temperature and the system parameters in an analytical way.

Topological quantum memory interfacing atomic and superconducting qubits

We propose a scheme to manipulate a topological spin qubit which is realized with cold atoms in a one-dimensional optical lattice.In particular, by introducing a quantum opto-electro-mechanical interface, we are able to first transfer a superconducting qubit state to an atomic qubit state and then to store it into the topological spin qubit. In this way, an efficient topological quantum memory could be constructed for the superconducting qubit. Therefore, we can consolidate the advantages of both the noise resistance of the topological qubits and the scalability of the superconducting qubits in this hybrid architecture.

Asymmetric ferromagnetic criticality in pyrochlore ferromagnet Lu2V2O7

We study the ferromagnetic criticality of the pyrochlore magnet Lu2 V2 O7 at the ferromagnetic transition TC% 70 K from the isotherms of magnetization MeHT via an iteration process and the Kouvel-Fisher method. The critical exponents associated with the transition are determined: b = 0.32(1), c = 1.41(1),and d ? 5:38. The validity of these critical exponents is further verified by scaling all the MeHT data in the vicinity of TConto two universal curves in the plot of M=jejbversus H=jejbtc, where e ? T=TCà 1.The obtained b and c values show asymmetric behaviors on the T < TCand the T > TCsides, and are consistent with the predicted values of 3 D Ising and cubic universality classes, respectively. This makes Lu2 V2 O7 a rare example in which the critical behaviors associated with a ferromagnetic transition belong to different universality classes. We describe the observed criticality from the Ginzburg-Landau theory with the quartic cubic anisotropy that microscopically originates from the anti-symmetric Dzyaloshinskii-Moriya interaction as revealed by recent magnon thermal Hall effect and theoretical investigations.

Impurity-limited quantum transport variability in magnetic tunnel junctions

We report an extensive first-principles investigation of impurity-induced device-to-device variability of spin-polarized quantum tunneling through Fe/MgO/Fe magnetic tunnel junctions （MTJ）. In particular, we calculated the tunnel magnetoresistance ratio （TMR） and the average values and variances of the currents and spin transfer torque （STT） of an interfacially doped Fe/MgO/Fe MTJ. Further, we predicted that N-doped MgO can improve the performance of a doped Fe/MgO/Fe MTJ. Our first- principles calculations of the fluctuations of the on/off currents and STT provide vital information for future predictions of the long-term reliability of production. spintronic devices, which is imperative for high-volume

Spin filtering in transition-metal phthalocyanine molecules from first principles

Using first-principles calculations based on density functional theory and the nonequilibrium Green＇s function formalism, we studied the spin transport through metal-phthalocyanine （MPc, M=Ni, Fe, Co, Mn, Cr） molecules connected to aurum nanowire electrodes. We found that the MnPc, FePc, and CrPc molecular devices exhibit a perfect spin filtering effect compared to CoPc and NiPc. Moreover, negative differential resistance appears in FePc molecular devices. The transmission coefficients at different bias voltages were further presented to understand this phenomenon. These results would be useful in designing devices for future nanotechnology.

Constraining the relativistic mean-field models from PREX-2 data:effective forces revisited

Based on the current measurement of the neutron distribution radius(R_(n))of ^(208)Pb from the PREX-2 data,we revisited the recently developed G3 and IOPB-I force parameters by fine-tuning some specific couplings within the relativistic mean-field(RMF)model.Theω-ρ-mesons coupling and theρ-meson coupling are constrained to the experimental neutron radius of^(208)Pb without compromising the bulk properties of finite nuclei and infinite nuclear matter observables.The modified parameter sets are applied to calculate the gross properties of finite nuclei such as binding energies,charge distributions,nuclear radii,pairing gaps,and single-particle energies.The root-mean-square deviations in binding energy and charge radius are estimated with respect to the available experimental data for 195 even-even nuclei,and the results compare favourably with the well-calibrated effective interactions of Skyrme,Gogny and other relativistic mean-field parametrizations.The pairing gap estimations for modified G3 and IOPB-I for Sn isotopes are also compared with the Hartree-Fock-Bogoliubov calculation with the Gogny(D1S)interaction.The isotopic shift and single-particle energy spacing are also calculated and compared with the experimental data for both original and modified versions of the G3 and IOPB-I parameter sets.Subsequently,both the modified parameter sets are used to obtain the various infinite nuclear matter observables at saturation.In addition to these,the force parameters are adopted to calculate the properties of a high isospin asymmetry dense system such as neutron star matter and tested for validation using the constraint from GW170817 binary neutron star merger events.The tuned forces predict relatively good results for finite and infinite nuclear matter systems and the current limitation on the neutron radius from PREX-2.A systematic analysis using these two refitted parameter sets over the nuclear chart will be communicated shortly.