The experiment that the high spin selectivity and the length-dependent spin polarization are observed in double-stranded DNA [Science 331, 894 (2011)], is elucidated by considering the combination of the spin-orbit coupling, the environment-induced dephasing, and the helical symmetry. We show that the spin polarization in double-stranded DNA is significant even in the case of weak spin-orbit coupling, while no spin polarization appears in single-stranded DNA. Furthermore, the underlying physical mechanism and the parameters-dependence of the spin polarization are studied.PACS numbers: 87.14.gk, 87.15.Pc, Molecular spintronics, by combining molecular electronics with spintronics to manipulate the transport of electron spins in organic molecular systems, is regarded as one of the most promising research fields and is now attracting extensive interest [1][2][3][4], owing to the long spin relaxation time and the flexibility of organic materials. Unconventional magnetic properties of molecular systems reported in organic spin valves and magnetic tunnel junctions, are attributed to the hybrid states in the organicmagnetic interfaces [5][6][7][8][9] and to single-molecule magnet [4]. Organic molecules would not be suitable candidates for spin-selective transport because of their nonmagnetic properties and weak spin-orbit coupling (SOC) [10].However, very recently, Göhler et al. reported the spin selectivity of photoelectron transmission through selfassembled monolayers of double-stranded DNA (dsDNA) deposited on gold substrate [11]. They found that wellorganized monolayers of the dsDNA act as very efficient spin filters with high spin polarization at room temperature for long dsDNA, irrespective of the polarization of the incident light. The spin filtration efficiency increases with increasing length of the dsDNA and contrarily no spin polarization could be observed for single-stranded DNA (ssDNA). These results were further substantiated by direct charge transport measurements of single ds-DNA connected between two leads [12]. Although several theoretical models were put forward to investigate the spin-selective properties of DNA molecule based on single helical chain-induced Rashba SOC [13,14], the models neglect the double helix feature of the dsDNA and are somewhat inconsistent with the experimental results that the ssDNA could not be a spin filter. Until now the underlying physical mechanism remains unclear for high spin selectivity observed in the dsDNA [15,16].In this Letter, a model Hamiltonian, including the small environment-induced dephasing, the weak SOC, and the helical symmetry, is proposed to investigate the quantum spin transport through the ssDNA and dsDNA connected to nonmagnetic leads. We interpret the experimental results that the electrons transmitted through the dsDNA exhibit high spin polarization, the spin filtration efficiency will be enhanced by increasing the DNA length, and no spin polarization appears for the ssDNA. The physical mechanism arises from the combination of the dephasing, the S...

We study the disorder effect on the transport properties in the HgTe/CdTe semiconductor quantum wells. We confirm that at a moderate disorder strength, the initially un-quantized two terminal conductance becomes quantized, and the system makes a transition to the novel topological Anderson insulator (TAI). Conductances calculated for the stripe and cylinder samples reveal the topological feature of TAI and supports the idea that the helical edge states may cause the anomalous quantized plateaus. The influence of disorder is studied by calculating the distributions of local currents. Base on the above-mentioned picture, the phenomena induced by disorder in the quantum spin Hall region and TAI region are directly explained. Our study of the local current configurations shed further light on the mechanism of the anomalous plateau.

We report on a theoretical study of spin-dependent electron transport through single-helical molecules connected by two nonmagnetic electrodes, and explain the experiment of significant spin-selective phenomenon observed in α-helical protein and the contradictory results between the protein and single-stranded DNA. Our results reveal that the α-helical protein is an efficient spin filter and the spin polarization is robust against the disorder. These results are in excellent agreement with recent experiments [Mishra D, et al. (2013) S pintronics is a multidisciplinary field that manipulates the electron spin transport in solid-state systems and has been receiving much attention among the physics, chemistry, and biology communities (1-4). Recent experiments have made significant progress in this research field, finding that doublestranded DNA (dsDNA) molecules are highly efficient spin filters (5-7). This chiral-induced spin selectivity (CISS) is surprising because the DNA molecules are nonmagnetic and their spin-orbit couplings (SOCs) are small. Additionally, the CISS effect opens new opportunities for using chiral molecules in spintronic applications and could provide a deeper understanding of the spin effects in biological processes. For the above reasons, there has been considerable interest in the spin transport along various chiral systems including dsDNA (8-11), single-stranded DNA (ssDNA) (12-15), and carbon nanotubes (16). However, no spin selectivity was measured in the ssDNA above the experimental noise (5).Very recently, spin-dependent electron transmission and electrochemical experiments were performed on bacteriorhodopsinan α-helical protein of which the structure is single helicalembedded in purple membrane which was physisorbed on a variety of substrates (17). It was reported by means of two distinct techniques that the electrons transmitted through the membrane are spin polarized, independent of the experimental environments, implying that this α-helical protein can exhibit the ability of spin filtering. Meanwhile, a chiral-based magnetic memory device was fabricated by using self-assembled monolayer of another α-helical protein called polyalanine (18). All of these results seem to be inconsistent with previous experiments' conclusions that the single-stranded helical molecules, such as ssDNA, may not polarize the electrons (5). We note that the electron transport/transfer has been widely investigated in many proteins (19)(20)(21)(22)(23)(24)(25)(26). However, to our knowledge, the underlying physics is still unclear for spin-selective phenomenon observed in the α-helical protein and for the contradictory behaviors between the protein and the ssDNA.In this paper, we propose a model Hamiltonian to explore the spin transport through single-helical molecules connected by two nonmagnetic electrodes, and provide an unambiguous physical mechanism for efficient spin selectivity observed in the protein and for the contrary experimental results between the protein and the ssDNA. Our results reveal that t...

We report on a general theory for analyzing quantum transport through devices in the Metal-QD-Metal configuration where QD is a quantum dot or the device scattering region which contains Rashba spin-orbital and electron-electron interactions. The metal leads may or may not be ferromagnetic, they are assumed to weakly couple to the QD region. Our theory is formulated by second quantizing the Rashba spin-orbital interaction in spectral space (instead of real space), and quantum transport is then analyzed within the Keldysh nonequilibrium Green's function formalism. The Rashba interaction causes two main effects to the Hamiltonian: (i) it gives rise to an extra spin-dependent phase factor in the coupling matrix elements between the leads and the QD; (ii) it gives rise to an inter-level spin-flip term but forbids any intra-level spin-flips. Our formalism provides a starting point for analyzing many quantum transport issues where spin-orbital effects are important. As an example, we investigate transport properties of a Aharnov-Bohm ring in which a QD having Rashba spin-orbital and e-e interactions is located in one arm of the ring. A substantial spin-polarized conductance or current emerges in this device due to a combined effect of a magnetic flux and the Rashba interaction. The direction and strength of the spin-polarization are shown to be controllable by both the magnetic flux and a gate voltage.

The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has Weyl nodes in bulk excitations and Fermi arcs surface states. We study the effects of disorder and localization in WSMs and find three exotic phase transitions. (I) Two Weyl nodes near the Brillouin zone boundary can be annihilated pairwise by disorder scattering, resulting in the opening of a topologically nontrivial gap and a transition from a WSM to a three-dimensional (3D) quantum anomalous Hall state. (II) When the two Weyl nodes are well separated in momentum space, the emergent bulk extended states can give rise to a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (III) Two Weyl nodes can emerge near the zone center when an insulating gap closes with increasing disorder, enabling a direct transition from a normal band insulator to a WSM. We determine the phase diagram by numerically computing the localization length and the Hall conductivity, and propose that the exotic phase transitions can be realized on a photonic lattice.PACS numbers: 72.15. Rn, 73.20.Fz, Topological quantum states of matter have emerged as an important and growing field in condensed matter and materials physics recently [1,2]. The Weyl semimetal (WSM) is a newly proposed quantum state of the kind that breaks time-reversal symmetry or inversion symmetry [3][4][5][6][7][8][9][10]. A WSM exhibits a set of paired zero-energy Weyl nodes (linearly touching points of conduction and valence bands) in its bulk spectrum and Fermi arcs excitations localized on the surface. A number of candidate materials have been predicted to be WSMs, including pyrochlore iridates and magnetic topological insulator multilayers [3][4][5][6]. Recently, following the theoretical prediction [7], angle resolved photoemission experiments confirmed that TaAs is a WSM by the observation of the Fermi arcs surface states [8]. Both the Weyl nodes and the Fermi arcs have been observed in NbAs using a combination of soft X-ray and ultraviolet photoemission experiments [9]. Furthermore, the Weyl points have been predicted and subsequently observed remarkably in gyroid photonic crystals [10].In this Letter, we study both numerically and analytically the stability of the gapless Weyl nodes and Fermi arcs against random potential scattering and the novel disorder-induced metal-insulator transitions in WSM systems. Previous studies have concentrated on the properties of a single Weyl node, assuming that the disorder potential is smooth enough to avoid scattering between different nodes [11][12][13][14]. Indeed, a system with a single Weyl node is not subject to Anderson localization even for strong disorder [15]. However, the theorem of Nielsen and Ninomiya states that gapless Weyl nodes with opposite chirality must appear in pairs [16]. Thus, it is essential to study the localization properties of a pair of Weyl nodes since they can be annihilated pairwise when approaching each other in momentum space or by strong intervalley scattering [17,18]. To this end, we study a model syste...

We investigate the electron tunneling through a normal-metal-quantum-dot-superconductor ͑N-QD-S͒ system where multiple discrete levels of the QD are considered. By using the nonequilibrium-Green's-function method, the current I and the probability of the Andreev reflection T A () are derived and studied in detail. In addition to the resonant behavior of the Andreev tunneling as obtained in previous works, we find that the current I versus the gate voltage v g exhibits different kinds of peaks, depending on the bias voltage, the level spacing of the QD, and the energy gap of the superconducting electrode. Besides, in I-V characteristics extra peaks superimposed on the conventional current plateaus emerge, which stem from the resonant Andreev reflections. In the case with strongly asymmetric barriers, the BCS spectral density can be obtained directly from the I-V characteristics.

We investigate the figure of merit of a quantum dot (QD) in the Coulomb blockade regime. It is found that the figure of merit ZT may be quite high if only single energy level in the QD is considered. On the other hand, with two or multi energy levels in the QD and without the Coulomb interaction, the ZT is strongly suppressed by the bipolar effect due to small level spacing. However, in the presence of the Coulomb interaction, the effective level spacing is enlarged and the bipolar effect is weakened, resulting in ZT to be considerably high. Thus, it is more likely to find a high efficient thermoelectric QDs with large Coulomb interaction. By using the parameters for a typical QD, the ZT can reach over 5.

We find that in order to completely describe the spin transport, apart from spin current (or linear spin current), one has to introduce the angular spin current. The two spin currents respectively describe the translational and rotational motion (precession)of a spin. The definitions of these spin current densities are given and their physical properties are discussed. Both spin current densities appear naturally in the spin continuity equation. Moreover we predict that the angular spin current can also induce an electric field E, and in particular E scales as 1/r 2 at large distance r, whereas the E field generated from the linear spin current goes as 1/r 3 . Recently, a new sub-discipline of condensed matter physics, spintronics, is emerging rapidly and generating great interests. 1,2 The spin current, the most important physical quantity in spintronics, has been extensively studied. Many interesting and fundamental phenomena, such as the spin Hall effect 3,4,5,6 and the spin precession 7,8 in systems with spin-orbit coupling, have been discovered and are under further study.As for the charge current, the definition of the local charge current density j e (r, t) = Re[Ψ † (r, t)e vΨ(r, t)] and its continuity equation d dt ρ e (r, t) + ∇ • j e (r, t) = 0 is well-known in physics. Here Ψ(r, t) is the electronic wave function, v =ṙ is the velocity operator, and ρ e (r, t) = eΨ † Ψ is the charge density. This continuity equation is the consequence of charge invariance, i.e. when an electron moves from one place to another, its charge remains the same. However, in the spin transport, there are still a lot of debates over what is the correct definition for spin current. 9 The problem stems from that the spin s is no invariant quantity in the spin transport, so that the conventional defining of the spin current < v s > is no conservative. Recently, some studies have begun investigation in this direction 10,11,12 , e.g. a semi-classical description of the spin continuity equation has been proposed 10,11 , as well as studies introducing a conserved spin current under special circumstances. 9In this paper, we study the definition of local spin current density. We find that due to the spin is vector and it has the translational and rotational motion, one has to use two quantities, the linear spin current and the angular spin current, to completely describe the spin transport. Here the linear spin current describe the translational motion of a spin, and the angular spin current is for the rotational motion. The conventional linear spin current has been extensively studied. However, the physical meaning of the angular spin current is given for the first time. The definition of two spin current densities are given and they appear naturally in the quantum spin continuity equation. Moreover, we predict that the angular spin current can generate an electric field similar as with the linear spin current, and thus contains physical consequences.The paper is organized as follows. In Section II, we first discuss the flow of a class...