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Frontiers of Physics

  • Cover Illustration

    2017, Vol.12  No.4

    Graphene based all-carbon nano-junction is structurally stable and mechanically flexible. It has been extensively recognized as a good candidate of nanoscale electronic and spin-electronic device. Jian-Wei Li et al. investigated the electronic and thermal electronic transport properties of kinds of graphene based all-carbon nano-junctions using first principles method. Huge TMR roughly equal to 105 was found for these structures. In addition, thermally ind [Detail] ...

    Table of Contents

, Volume 12 Issue 4

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REVIEW ARTICLE
Quantum dot behavior in transition metal dichalcogenides nanostructures Collection
Gang Luo, Zhuo-Zhi Zhang, Hai-Ou Li, Xiang-Xiang Song, Guang-Wei Deng, Gang Cao, Ming Xiao, Guo-Ping Guo
Front. Phys. . 2017, 12 (4): 128502 .   DOI: 10.1007/s11467-017-0652-3
Abstract   PDF (19682KB)

Recently, transition metal dichalcogenides (TMDCs) semiconductors have been utilized for investigating quantum phenomena because of their unique band structures and novel electronic properties. In a quantum dot (QD), electrons are confined in all lateral dimensions, offering the possibility for detailed investigation and controlled manipulation of individual quantum systems. Beyond the definition of graphene QDs by opening an energy gap in nanoconstrictions, with the presence of a bandgap, gate-defined QDs can be achieved on TMDCs semiconductors. In this paper, we review the confinement and transport of QDs in TMDCs nanostructures. The fabrication techniques for demonstrating two-dimensional (2D) materials nanostructures such as field-effect transistors and QDs, mainly based on e-beam lithography and transfer assembly techniques are discussed. Subsequently, we focus on electron transport through TMDCs nanostructures and QDs. With steady improvement in nanoscale materials characterization and using graphene as a springboard, 2D materials offer a platform that allows creation of heterostructure QDs integrated with a variety of crystals, each of which has entirely unique physical properties.

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RESEARCH ARTICLE
Phase diagram and transport properties of Sb-doped Ca0.88La0.12Fe2As2 single crystals
Xiang-Zhuo Xing,Wei Zhou,Chun-Qiang Xu,Nan Zhou,Fei-Fei Yuan,Yu-Feng Zhang,Xiao-Feng Xu,Zhi-Xiang Shi
Front. Phys. . 2017, 12 (4): 127401 .   DOI: 10.1007/s11467-016-0621-2
Abstract   PDF (2731KB)

The effects of isovalent Sb substitution on the superconducting properties of the Ca0.88La0.12Fe2(As1−ySby)2 system have been studied through electrical resistivity measurements. It is seen that the antiferromagnetic or structural transition is suppressed with Sb content, and a high-Tc superconducting phase, accompanied by a low-Tc phase, emerges at 0.02≤y≤0.06. In this intermediate-doping regime, normal-state transport shows non-Fermi-liquid-like behaviors with nearly T-linear resistivity above the high-Tc phase. With further Sb doping, this high-Tc phase abruptly vanishes for y>0.06 and the conventional Fermi liquid is restored, while the low-Tc phase remains robust against Sb impurities. The coincidence of the high-Tc phase and non-Fermi liquid transport behaviors in the intermediate Sb-doping regime suggests that AFM fluctuations play an important role in the observed non-Fermi liquid behaviors, which may be intimately related to the unusual nonbulk high-Tc phase in this system.

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First-principles study on the electronic and transport properties of periodically nitrogen-doped graphene and carbon nanotube superlattices Collection
Fuming Xu,Zhizhou Yu,Zhirui Gong,Hao Jin
Front. Phys. . 2017, 12 (4): 127306 .   DOI: 10.1007/s11467-017-0650-5
Abstract   PDF (1297KB)

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 nitrogendoped graphene and carbon nanotube nanostructures. 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 transmission spectrum through the pristine and 3×3 armchair carbon nanotube heterostructure shows quantized behavior under certain circumstances.

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Low conductance of nickel atomic junctions in hydrogen atmosphere Collection
Shuaishuai Li,Yi-Qun Xie,Yibin Hu
Front. Phys. . 2017, 12 (4): 127305 .   DOI: 10.1007/s11467-016-0647-5
Abstract   PDF (2278KB)

The low conductance of nickel atomic junctions in the hydrogen environment is studied using the nonequilibrium Green’s function theory combined with first-principles calculations. The Ni junction bridged by a H2 molecule has a conductance of approximately 0.7 G0. This conductance is contributed by the anti-bonding state of the H2 molecule, which forms a bonding state with the 3d orbitals of the nearby Ni atoms. In contrast, the Ni junction bridged by the two single H atoms has a conductance of approximately 1 G0, which is weakly spin-polarized. The spin-up channels were found to contribute mostly to the conductance at a small junction gap, while the spin-down channels play a dominant role at a larger junction gap.

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Impurity-limited quantum transport variability in magnetic tunnel junctions Collection
Jianing Zhuang,Yin Wang,Yan Zhou,Jian Wang,Hong Guo
Front. Phys. . 2017, 12 (4): 127304 .   DOI: 10.1007/s11467-016-0644-8
Abstract   PDF (875KB)

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 firstprinciples calculations of the fluctuations of the on/off currents and STT provide vital information for future predictions of the long-term reliability of spintronic devices, which is imperative for high-volume production.

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Conductance switching of a phthalocyanine molecule on an insulating surface Collection
Kun Peng Dou (豆坤鵬),Chao-Cheng Kaun (關肇正)
Front. Phys. . 2017, 12 (4): 127303 .   DOI: 10.1007/s11467-016-0623-0
Abstract   PDF (4770KB)

We study the electron transport through the double-barrier junction consisted of the phthalocyanine molecule adsorbed on a NaCl bilayer on a metal substrate and the STM tip from first principles. The hydrogen tautomerization reaction happened in the molecule changes the spatial extensions of the molecular π orbitals under the tip, leading to junction conductance switching. Shifting the molecule to locate on different ions also varies the conductance. The transport channels of the tautomers on different adsorbed sites are identified.

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Transport through a quantum dot coupled to two Majorana bound states
Qi-Bo Zeng,Shu Chen,L. You,Rong Lü
Front. Phys. . 2017, 12 (4): 127302 .   DOI: 10.1007/s11467-016-0620-3
Abstract   PDF (2826KB)

We investigate electron transport inside a ring system composed of a quantum dot (QD) coupled to two Majorana bound states confined at the ends of a one-dimensional topological superconductor nanowire. By tuning the magnetic flux threading through the ring, the model system we consider can be switched into states with or without zero-energy modes when the nanowire is in its topological phase. We find that the Fano profile in the conductance spectrum due to the interference between bound and continuum states exhibits markedly different features for these two different situations, which consequently can be used to detect the Majorana zero-energy mode. Most interestingly, as a periodic function of magnetic flux, the conductance shows 2π periodicity when the two Majorana bound states are nonoverlapping (as in an infinitely long nanowire) but displays 4π periodicity when the overlapping becomes nonzero (as in a finite length nanowire). We map the model system into a QD–Kitaev ring in the Majorana fermion representation and affirm these different characteristics by checking the energy spectrum.

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Weak localization of bismuth cluster-decorated graphene and its spin–orbit interaction Collection
Jian-Lei Ge,Tian-Ru Wu,Ming Gao,Zhan-Bin Bai,Lu Cao,Xue-Feng Wang,Yu-Yuan Qin,Feng-Qi Song
Front. Phys. . 2017, 12 (4): 127210 .   DOI: 10.1007/s11467-017-0677-7
Abstract   PDF (2114KB)

Weak-localization (WL) measurements were performed in a Bi cluster-decorated graphene sheet. The charge concentration was kept constant, and the amplitude of the conductance correction was suppressed after the Bi-cluster deposition. Detailed WL data were obtained while the gate and temperature were changed. Using E. McCann’s formula, the spin-relaxation time was extracted, which was found to increase with the elastic scattering time. This is attributed to the Elliott–Yafet spin relaxation and Kane–Mele type spin–orbit coupling (SOC). The SOC strength was enhanced to 2.64 meV as a result of the first deposition. The coverage effect is discussed according to the measurement after the second deposition.

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REVIEW ARTICLE
Landau quantization of Dirac fermions in graphene and its multilayers Collection
Long-Jing Yin (殷隆晶),Ke-Ke Bai (白珂珂),Wen-Xiao Wang (王文晓),Si-Yu Li (李思宇),Yu Zhang (张钰),Lin He (何林)
Front. Phys. . 2017, 12 (4): 127208 .   DOI: 10.1007/s11467-017-0655-0
Abstract   PDF (66703KB)

When electrons are confined in a two-dimensional (2D) system, typical quantum–mechanical phenomena such as Landau quantization can be detected. Graphene systems, including the single atomic layer and few-layer stacked crystals, are ideal 2D materials for studying a variety of quantum–mechanical problems. In this article, we review the experimental progress in the unusual Landau quantized behaviors of Dirac fermions in monolayer and multilayer graphene by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Through STS measurement of the strong magnetic fields, distinct Landau-level spectra and rich level-splitting phenomena are observed in different graphene layers. These unique properties provide an effective method for identifying the number of layers, as well as the stacking orders, and investigating the fundamentally physical phenomena of graphene. Moreover, in the presence of a strain and charged defects, the Landau quantization of graphene can be significantly modified, leading to unusual spectroscopic and electronic properties.

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RESEARCH ARTICLE
Spin filtering in transition-metal phthalocyanine molecules from first principles Collection
Li Niu,Huan Wang,Lina Bai,Ximing Rong,Xiaojie Liu,Hua Li,Haitao Yin
Front. Phys. . 2017, 12 (4): 127207 .   DOI: 10.1007/s11467-017-0671-0
Abstract   PDF (977KB)

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.

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REVIEW ARTICLE
Master equation approach to transient quantum transport in nanostructures Collection
Pei-Yun Yang,Wei-Min Zhang
Front. Phys. . 2017, 12 (4): 127204 .   DOI: 10.1007/s11467-016-0640-z
Abstract   PDF (4235KB)

In this review article, we present a non-equilibrium quantum transport theory for transient electron dynamics in nanodevices based on exact Master equation derived with the path integral method in the fermion coherent-state representation. Applying the exact Master equation to nanodevices, we also establish the connection of the reduced density matrix and the transient quantum transport current with the Keldysh nonequilibrium Green functions. The theory enables us to study transient quantum transport in nanostructures with back-reaction effects from the contacts, with non-Markovian dissipation and decoherence being fully taken into account. In applications, we utilize the theory to specific quantum transport systems, a variety of quantum decoherence and quantum transport phenomena involving the non-Markovian memory effect are investigated in both transient and stationary scenarios at arbitrary initial temperatures of the contacts.

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RESEARCH ARTICLE
Spin-resolved quantum transport in graphene-based nanojunctions Collection
Jian-Wei Li,Bin Wang,Yun-Jin Yu,Ya-Dong Wei,Zhi-Zhou Yu,Yin Wang
Front. Phys. . 2017, 12 (4): 126501 .   DOI: 10.1007/s11467-016-0614-1
Abstract   PDF (2023KB)

First-principles calculations were performed to explore the spin-resolved electronic and thermoelectric transport properties of a series of graphene-nanoribbon-based nanojunctions. By flipping the magnetic moments in graphene leads from parallel to antiparallel, very large tunneling magnetoresistance can be obtained under different gate voltages for all the structures. Spin-resolved alternating-current conductance increases versus frequency for the short nanojunctions but decreases for the long nanojunctions. With increasing junction length, the behavior of the junctions changes from capacitive-like to inductive-like. Because of the opposite signs of spin-up thermopower and spin-down thermopower near the Fermi level, pure spin currents can be obtained and large figures of merit can be achieved by adjusting the gate voltage and chemical potential for all the nanojunctions.

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Ratchet motion and current reversal of coupled Brownian motors in pulsating symmetric potentials
Chen-Pu Li,Hong-Bin Chen,Zhi-Gang Zheng
Front. Phys. . 2017, 12 (4): 120502 .   DOI: 10.1007/s11467-016-0622-1
Abstract   PDF (1034KB)

In this study, we investigate the collective directed transport of coupled Brownian particles in spatially symmetric periodic potentials under time-periodic pulsating modulations. We find that the coupling between two particles can induce symmetry breaking and consequently collective directed motion. Moreover, the direction of motion can be reversed under certain conditions. The dependence of directed current on various parameters is systematically studied. reverse motion can be achieved by modulating the coupling free length and the phase shift of the pulsating potential. The dynamical mechanism of these transport properties is understood in terms of the effective-potential theory and the space-time transformation invariance. The directed transport of coupled Brownian motors can be manipulated and optimized by adjusting the coupling strength, pulsating frequency, or noise intensity.

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REVIEW ARTICLE
Wave-function approach to Master equations for quantum transport and measurement Collection
Shmuel Gurvitz
Front. Phys. . 2017, 12 (4): 120303 .   DOI: 10.1007/s11467-016-0638-6
Abstract   PDF (728KB)

This paper presents a comprehensive review of the wave-function approach for derivation of the numberresolved Master equations, used for description of transport and measurement in mesoscopic systems. The review contains important amendments, clarifying subtle points in derivation of the Master equations and their validity. This completes the earlier works on the subject. It is demonstrated that the derivation does not assume weak coupling with the environment and reservoirs, but needs only high bias condition. This condition is very essential for validity of the Markovian Master equations, widely used for a phenomenological description of different physical processes.

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RESEARCH ARTICLE
Routing protocol for wireless quantum multi-hop mesh backbone network based on partially entangled GHZ state
Pei-Ying Xiong,Xu-Tao Yu,Zai-Chen Zhang,Hai-Tao Zhan,Jing-Yu Hua
Front. Phys. . 2017, 12 (4): 120302 .   DOI: 10.1007/s11467-016-0617-y
Abstract   PDF (2413KB)

Quantum multi-hop teleportation is important in the field of quantum communication. In this study, we propose a quantum multi-hop communication model and a quantum routing protocol with multihop teleportation for wireless mesh backbone networks. Based on an analysis of quantum multi-hop protocols, a partially entangled Greenberger–Horne–Zeilinger (GHZ) state is selected as the quantum channel for the proposed protocol. Both quantum and classical wireless channels exist between two neighboring nodes along the route. With the proposed routing protocol, quantum information can be transmitted hop by hop from the source node to the destination node. Based on multi-hop teleportation based on the partially entangled GHZ state, a quantum route established with the minimum number of hops. The difference between our routing protocol and the classical one is that in the former, the processes used to find a quantum route and establish quantum channel entanglement occur simultaneously. The Bell state measurement results of each hop are piggybacked to quantum route finding information. This method reduces the total number of packets and the magnitude of air interface delay. The deduction of the establishment of a quantum channel between source and destination is also presented here. The final success probability of quantum multi-hop teleportation in wireless mesh backbone networks was simulated and analyzed. Our research shows that quantum multi-hop teleportation in wireless mesh backbone networks through a partially entangled GHZ state is feasible.

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