arXiv subjects

Zhi Wang

Publications and source records attributed to Zhi Wang.

At least 379 records · Page 21Linked to original sources

arXiv2014

Viewing Majorana Bound States by Rabi Oscillations

Rabi oscillation is a pure quantum phenomenon where the system jumps forth and back between two quantum levels under stimulation of a microwave, and a resonance occurs when the energy difference is matched by the photon energy. Rabi oscillations have been observed in various quantum systems so far, and especially are used to demonstrate the quantum coherence of quantum bits. In the present work, we explore Rabi oscillation in a system accommodating the elusive Majorana bound states (MBSs) under intensive search recently. The proposed setup is constructed by a quantum dot (QD) and a superconducting quantum interference device (SQUID), where a semiconductor nanowire with spin-orbital coupling in the topological state is introduced to form Josephson junction. When the coupling between QD and the Josephson junction is tuned by an ac gate voltage, Rabi oscillation takes place among quantum states formed by QD and MBSs, which makes it possible to reveal bizarre properties of MBSs by sensing the electron occupation on QD. Especially, one should be able to observe the fractional Josephson relation $\cos(πΦ/Φ_0)$ unique to MBSs with $Φ$ the magnetic flux applied in the SQUID and $Φ_0=hc/2e$ the flux quantum. The system has been investigated in terms of the analytic Floquet theorem and numerical simulations with fine agreement.

cond-mat.mes-hall↗

arXiv2013

Exploring Scientists' Working Timetable: A Global Survey

In our previous study (Wang et al., 2012), we analyzed scientists' working timetable of 3 countries, using realtime downloading data of scientific literatures. In this paper, we make a through analysis about global scientists' working habits. Top 30 countries/territories from Europe, Asia, Australia, North America, Latin America and Africa are selected as representatives and analyzed in detail. Regional differences for scientists' working habits exists in different countries. Besides different working cultures, social factors could affect scientists' research activities and working patterns. Nevertheless, a common conclusion is that scientists today are often working overtime. Although scientists may feel engaged and fulfilled about their hard working, working too much still warns us to reconsider the work - life balance.

cs.DL↗

arXiv2013

Terahertz Wave Guiding by Femtosecond Laser Filament in Air

Femtosecond laser filament generates strong terahertz (THz) pulse in air. In this paper, THz pulse waveform generated by femtosecond laser filament has been experimentally investigated as a function of the length of the filament. Superluminal propagation of THz pulse has been uncovered, indicating that the filament creates a THz waveguide in air. Numerical simulation has confirmed that the waveguide is formed because of the radially non-uniform refractive index distribution inside the filament. The underlying physical mechanisms and the control techniques of this type THz pulse generation method might be revisited based on our findings. It might also potentially open a new approach for long-distance propagation of THz wave in air.

physics.optics↗

arXiv2013

Terahertz imaging with sub-wavelength resolution by femtosecond laser filament in air

Terahertz (THz) imaging provides cutting edge technique in biology, medical sciences and non-destructive evaluation. However, due to the long wavelength of the THz wave, the obtained resolution of THz imaging is normally a few hundred microns and is much lower than that of the traditional optical imaging. We introduce a sub-wavelength resolution THz imaging technique which uses the THz radiation generated by a femtosecond laser filament in air as the probe. This method is based on the fact that the femtosecond laser filament forms a waveguide for the THz wave in air. The diameter of the THz beam, which propagates inside the filament, varies from 20 μm to 50 μm, which is significantly smaller than the wavelength of the THz wave. Using this highly spatially confined THz beam as the probe, THz imaging with resolution as high as 20 μm (~λ/38) can be realized.

physics.optics↗

arXiv2013

Fidelity in topological superconductors with end Majorana fermions

Fidelity and fidelity susceptibility are introduced to investigate the topological superconductors with end Majorana fermions. A general formalism is established to calculate the fidelity and fidelity susceptibility by solving Bogoliubov-de Gennes equations. It is shown that the fidelity susceptibility manifest itself as a peak at the topological quantum phase transition point for homogeneous Kitaev wire, thus serves as a valid indicator for the topological quantum phase transition which signals the appearance of Majorana fermions. The effect of disorders is investigated within this formalism. We consider three disordered systems and observe fidelity susceptibility peak in all of them. By analyzing the susceptibility peak, we notice that the local potential disorders and the hopping disorders can shift the phase transition point, while off-diagonal disorders have no obvious influence. Our results confirm that the existence of topological quantum phase transition is robust to these disorders, while the behavior of the phase transition might be influenced by disorders.

cond-mat.supr-con↗

arXiv2013

Proposal for Manipulation of Majorana Fermions in Nano-Patterned Semiconductor-Superconductor Heterostructure

We investigate a heterostructure system with a spin-orbit coupled semiconductor sandwiched by an s-wave superconductor and a ferromagnetic insulator, which supports Majorana fermions (MFs) at the superconducting vortex cores. We propose a scheme of transporting and braiding the MFs, which only requires application of point-like gate voltages in a system with nano-meter patterns. By solving the time-dependent Bogoliubov-de Gennes equation numerically, we monitor the time evolutions of MF wave-functions and show that the braiding of MFs with non-Abelian statistics can be achieved by adiabatic switching within several nano seconds.

cond-mat.mes-hall↗

arXiv2013

Topological Single Electron Pumping Assisted by Majorana Fermions

Single electron pumping based on the topological property of Majorana fermions (MFs) is proposed. The setup consists of a quantum dot and four nano topological superconductors (TSs) connected by constriction junctions, with an additional vortex located in the loop of TSs. Operation is performed by gate voltages at constriction junctions. Simulations with Bogloliubov-de Gennes equation demonstrate successfully quantum protection during switching operation.

cond-mat.mes-hall↗

arXiv2012

Tracing scientist's research trends realtimely

In this research, we propose a method to trace scientists' research trends realtimely. By monitoring the downloads of scientific articles in the journal of Scientometrics for 744 hours, namely one month, we investigate the download statistics. Then we aggregate the keywords in these downloaded research papers, and analyze the trends of article downloading and keyword downloading. Furthermore, taking both the download of keywords and articles into consideration, we design a method to detect the emerging research trends. We find that in scientometrics field, social media, new indices to quantify scientific productivity (g-index), webometrics, semantic, text mining, open access are emerging fields that scientometrics researchers are focusing on.

cs.DL↗

arXiv2012

Exploring scientists' working timetable: Do scientists often work overtime?

A novel method is proposed to monitor and record scientists' working timetable. We record the downloads information of scientific papers real-timely from Springer round the clock, and try to explore scientists' working habits. As our observation demonstrates, many scientists are still engaged in their research after working hours every day. Many of them work far into the night, even till next morning. In addition, research work also intrudes into their weekends. Different working time patterns are revealed. In the US, overnight work is more prevalent among scientists, while Chinese scientists mostly have busy weekends with their scientific research.

cs.DL↗

arXiv2012

Ratchet potential and rectification effect in Majorana fermion SQUID

Motivated by a recent experimental progress in realizing Majorana fermions (MFs) in a heterostructure of a spin-orbit coupling nanowire and superconductor (V. Mourik et al., Science.1222360), we investigate a SQUID formed by the novel superconductor-nanowire-superconductor Josephson junction which contains MFs and a conventional superconductor-insulator-superconductor junction. It is shown that the critical current of the SQUID is different for the two current directions. Since the asymmetric Josepshon current forms a ratchet potential for the dynamics of superconducting phase, a rectification effect is expected when the SQUID is driven by an ac current. These novel properties are expected to be useful for probing the elusive MFs as well as for their dynamics.

cond-mat.supr-con↗

arXiv2011

Manipulating Edge Majorana Fermions in Vortex State

A vortex in a model spinless px+ipy superconductor induces two Majorana fermions (MFs), one in the core and the other at the sample edge. In the present work, we show that edge MF can be generated, fused, transported, and braided easily by tuning gate voltages at point-like constriction junctions. Solving the time-dependent Bogoliubov-de Gennes equation, we demonstrate that the braiding of edge MFs obeys the non-Abelian statistics. The present setup is therefore a promising implementation for topological quantum computation, and has the advantage of easy manipulation and simple device structure.

cond-mat.supr-con↗

arXiv2011

Detecting Majorana fermions by nonlocal entanglement between quantum dots

Nonlocal entanglement between two quantum dots can be generated through Majorana fermions. The two Majorana fermions at the ends of an one-dimensional topological superconductor form a nonlocal fermion level, coupling to the occupation states of two quantum dots put close to the two ends, and the entire system will come into an entangled state. After introducing a charging energy by a capacitor, entanglement of the entire system can manifest itself through the nonlocal entanglement between the two quantum dots. That is, when measuring the electron occupations of the quantum dots, the measurement result of one quantum dot will influence the measurement result of the other quantum dot. This nonlocal entanglement between the two quantum dots is a strong evidence of the nonlocal nature of the fermion level constructed by two Majorana fermions.

cond-mat.mes-hall↗

arXiv2011

Time-Reversal-Symmetry-Broken Superconductivity Induced by Frustrated Inter-Component Couplings

Superconductivity is associated with spontaneously broken gauge symmetry. In some exotic superconductors the time-reversal symmetry is broken as well, accompanied with internal magnetic field. A time-reversal symmetry broken (TRSB) superconductivity without internal magnetic field involved can be induced by frustrated inter-component couplings, which becomes a realistic issue recently due to the discovery of iron-pnictide superconductors. Here we derive stability condition for this novel TRSB state using the Ginzburg-Landau (GL) theory. We find that there are multiple divergent coherence lengths, and that this novel superconductivity cannot be categorized by the GL number into type I or type II. We reveal that the critical Josephson current of a constriction junction between two bulk superconductors of different chiralities is suppressed significantly from that for same chirality. This effect provides a unique way to verify experimentally this brand new superconductivity.

cond-mat.supr-con↗

arXiv2010

Interference and Switching of Josephson current carried by nonlocal spin-entangled electrons in a SQUID-like system with quantum dots

Josephson current of spin-entangled electrons through the two branches of a SQUID-like structure with two quantum dots exhibits a magnetic-flux response different from the conventional Josephson current. Due to their interference, the period of maximum Josephson current changes from $h/2e$ to $h/e$, which can be used for detecting the Cooper-pair splitting efficiency. The nonlocal spin entanglement provides a quantum mechanical functionale for switching on and off this novel Josephson current, and explicitly a switch is formulated by including a pilot junction. It is shown that the device can be used to measure the magnitude of split-tunneling Josephson current.

cond-mat.supr-con↗

arXiv2010

Beam optimization of RFQ and SFRFQ combined accelerator at Peking University

The Peking University Integral Split Ring Radio Frequency Quadrupole(ISR RFQ) accelerator was constructed in 1999 with a high duty factor 16.7% and repetition frequency 166Hz, and it was able to accelerate N+, O+,O-, C+ and He+ from 1.6kev/u to 65keV/u. It was later upgraded as an injector of the Separated Function RFQ (SFRFQ). The experiments indicated that the maximum accelerated O+ beam current could exceed 3.2mA with energy 1.03MeV and an energy spread (FWHM) 3.1%. Then the beam transports through a 1m-long magnetic triplet to the entrance of SFRFQ and is finally accelerated to 1.64MeV. The beam conditioning of RFQ were carefully optimized to satisfy the requirements of the SFRFQ. The combined accelerator eventually can deliver 0.53mA O+ beam with energy 1.65MeV, which has sufficiently demonstrated the feasibility of the SFRFQ structure.

physics.acc-ph↗

arXiv2010

Reduced fidelity in Kitaev honeycomb model

We study the reduced fidelity and reduced fidelity susceptibility in the Kitaev honeycomb model. It is shown that the reduced fidelity susceptibility of two nearest site manifest itself a peak at the quantum phase transition point, although the one-site reduced fidelity susceptibility vanishes. Our results directly reveal that the reduced fidelity susceptibility can be used to characterize the quantum phase transition in the Kitaev honeycomb model, and thus suggest that the reduced fidelity susceptibility is an accurate marker of the topological phase transition when it is properly chosen, despite of its local nature.

cond-mat.str-el↗

arXiv2010

Extinction of quasiparticle scattering interference in cuprate superconductors

The quasiparticle scattering interference phenomenon characterized by the peaks in the local density of states is studied within the kinetic energy driven superconducting mechanism in the presence of a single impurity. By calculation of the Fourier transformed ratio of the local density of states at opposite energy, it is shown that the quasiparticle scattering interference phenomenon can be described qualitatively by a single impurity in the kinetic energy driven homogeneous d-wave superconducting state. The amplitude of the peak increases with increasing energy at the low energy, and reaches a maximum at the intermediate energy, then diminishes to zero at the high energy. The theory also predicts that with increasing doping, the position of the peak along the nodal direction moves towards to the center of the Brillouin zone, while the position of the peak along the antinodal direction is shifted to large momentum region.

cond-mat.supr-con↗

arXiv2009

Momentum dependence of quasiparticle spectrum and Bogoliubov angle in cuprate superconductors

The momentum dependence of the low energy quasiparticle spectrum and the related Bogoliubov angle in cuprate superconductors are studied within the kinetic energy driven superconducting mechanism. By calculation of the ratio of the low energy quasiparticle spectra at positive and negative energies, it is shown that the Bogoliubov angle increase monotonically across the Fermi crossing point. The results also show that the superconducting coherence of the low energy quasiparticle peak is well described by a simple d-wave Bardeen-Cooper-Schrieffer formalism, although the pairing mechanism is driven by the kinetic energy by exchanging spin excitations.

cond-mat.supr-con↗