The 8-Pmmn borophene is one kind of new elemental monolayer, which hosts anisotropic and tilted massless Dirac fermions (MDF). The planar p-n junction (PNJ) structure as the basic component of various novel devices based on the monolayer material has attracted increasing attention. Here, we analytically study the transport properties of anisotropic and tilted MDF across 8-Pmmn borophene PNJ. Similar to the isotropic MDF across graphene junctions, perfect transmission exists but its direction departures the normal direction of borophene PNJ induced by the anisotropy and tilt, i.e., oblique Klein tunneling. The oblique Klein tunneling does not depend on the doping levels in N and P regions of PNJ as the normal Klein tunneling but depends on the junction direction. Furthermore, we analytically derive the special junction direction for the maximal difference between perfect transmission direction and the normal direction of PNJ and clearly distinguish the respective contribution of anisotropy and tilt underlying the oblique Klein tunneling. In light of the rapid advances of experimental technologies, we expect the oblique Klein tunneling to be observable in the near future.

Electric currents carrying a net spin polarization are widely used in spintronics, whereas globally spin-neutral currents are expected to play no role in spin-dependent phenomena. Here we show that, in contrast to this common expectation, spin-independent conductance in compensated antiferromagnets and normal metals can be efficiently exploited in spintronics, provided their magnetic space group symmetry supports a non-spin-degenerate Fermi surface. Due to their momentum-dependent spin polarization, such antiferromagnets can be used as active elements in antiferromagnetic tunnel junctions (AFMTJs) and produce a giant tunneling magnetoresistance (TMR) effect. Using RuO2 as a representative compensated antiferromagnet exhibiting spin-independent conductance along the [001] direction but a non-spin-degenerate Fermi surface, we design a RuO2/TiO2/RuO2 (001) AFMTJ, where a globally spin-neutral charge current is controlled by the relative orientation of the Néel vectors of the two RuO2 electrodes, resulting in the TMR effect as large as ~500%. These results are expanded to normal metals which can be used as a counter electrode in AFMTJs with a single antiferromagnetic layer or other elements in spintronic devices. Our work uncovers an unexplored potential of the materials with no global spin polarization for utilizing them in spintronics.

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which exploits the Néel vector to control the topological electronic states and the associated spin-dependent transport properties. A recently discovered Néel spin-orbit torque has been proposed to electrically manipulate Dirac band crossings in antiferromagnets; however, a reliable AFM material to realize these properties in practice is missing. In this letter, we predict that room temperature AFM metal MnPd2 allows the electrical control of the Dirac nodal line by the Néel spin-orbit torque. Based on firstprinciples density functional theory calculations, we show that reorientation of the Néel vector leads to switching between the symmetry-protected degenerate state and the gapped state associated with the dispersive Dirac nodal line at the Fermi energy. The calculated spin Hall conductivity strongly depends on the Néel vector orientation and can be used to experimentally detect the predicted effect using a proposed spin-orbit torque device. Our results indicate that AFM Dirac nodal line metal MnPd2 represents a promising material for topological AFM spintronics.

We study the mobility of Dirac fermions in monolayer graphene on a GaAs substrate, restricted by the combined action of the extrinsic potential of piezoelectric surface acoustical phonons of GaAs (PA) and of the intrinsic deformation potential of acoustical eigen-phonons in graphene (DA). In the high temperature (T ) regime the momentum relaxation rate exhibits the same linear dependence on T but different dependences on the carrier density n, corresponding to the mobility µ ∝ 1/ √ n and 1/n, respectively for the PA and DA scattering mechanisms. In the low T Bloch-Grüneisen regime, the mobility shows the same square-root density dependence, µ ∝ √ n, but different temperature dependences, µ ∝ T −3 and T −4 , respectively for PA and DA phonon scattering.Graphene [1] due to its unique linear chiral electronic dispersion [2] exhibits novel transport properties [3,4] and has great potential as a desirable material for future electronic and optical technologies [2,5]. Momentum relaxation is a key phenomenon that governs transport of Dirac fermions in graphene [6]. It is of practical interest for developing high-speed electronics and in recent years has been extensively studied both theoretically [7][8][9] and experimentally [10][11][12]. The scattering by defects [7,[13][14][15][16], impurities [8,12,[17][18][19][20][21], and phonons [9,11,16,[22][23][24][25][26] have been investigated to determine and control the dominant mechanism that limits the carrier mobility in graphene.In device structures used so far, graphene is often deposited on an oxidized silicon wafer (SiO 2 /Si), which due to various scattering mechanisms imposes constraints on its excellent transport properties observed in suspended graphene devices [10,27]. Recently, structures on other promising substrate materials such as h-BN [28,29] and GaAs [30,31] have been fabricated and studied with the intention for high-quality graphene electronics. Along with its superior surface quality and strong hydrophilicity preventing folding of large-scale graphene flakes, GaAs has a substantially larger dielectric constant in comparison with SiO 2 and h-BN and hence improved electrical screening. In such high purity GaAs structures, electronphonon scattering can be a decisive factor in limiting the mobility of Dirac fermions and the piezoelectric GaAs substrate can serve as a powerful tool for studying electronic properties of graphene by means of remote piezoelectric surface acoustical phonons.In the present work we study the temperature and density dependence of the carrier mobility in monolayer graphene at finite doping on a GaAs substrate. We calculate the mobility limited by scattering from the piezoelectric potential of remote surface acoustical phonons of the substrate (PA phonons) versus the deformation potential of acoustical eigen-phonons of the graphene lattice (DA phonons). In experiment the typical wavelength of phonons taking part in scattering events is much larger than the distance, d, of several angströms between the graphene sheet and the GaAs sub...

Antiferromagnetic (AFM) spintronics exploits the Néel vector as a state variable for novel spintronic devices. Recent studies have shown that the field-like and antidamping spin-orbit torques (SOT) can be used to switch the Néel vector in antiferromagnets with proper symmetries. However, the precise detection of the Néel vector remains a challenging problem. In this letter, we predict that the nonlinear anomalous Hall effect (AHE) can be used to detect the Néel vector in most compensated antiferromagnets supporting the antidamping SOT. We show that the magnetic crystal group symmetry of these antiferromagnets combined with spin-orbit coupling produce a sizable Berry curvature dipole and hence the nonlinear AHE. As a specific example, we consider half-Heusler alloy CuMnSb, which Néel vector can be switched by the antidamping SOT. Based on density functional theory calculations, we show that the nonlinear AHE in CuMnSb results in a measurable Hall voltage under conventional experimental conditions. The strong dependence of the Berry curvature dipole on the Néel vector orientation provides a new detection scheme of the Néel vector based on the nonlinear AHE. Our predictions enrich the material platform for studying non-trivial phenomena associated with the Berry curvature and broaden the range of materials useful for AFM spintronics.

We present a theoretical study on interactions of electrons in graphene with surface acoustic waves (SAWs). We find that owing to momentum and energy conservation laws, the electronic transition accompanied by the SAW absorption cannot be achieved via inter-band transition channels in graphene. For graphene, strong absorption of SAWs can be observed in a wide frequency range up to terahertz at room temperature. The intensity of SAW absorption by graphene depends strongly on temperature and can be adjusted by changing the carrier density. This study is relevant to the exploration of the acoustic properties of graphene and to the application of graphene as frequency-tunable SAW devices

We predict a novel quantum interference based on the negative refraction across a semiconductor P-N junction: with a local pump on one side of the junction, the response of a local probe on the other side behaves as if the disturbance emanates not from the pump but instead from its mirror image about the junction. This phenomenon is guaranteed by translational invariance of the system and matching of Fermi surfaces of the constituent materials, thus it is robust against other details of the junction (e.g., junction width, potential profile, and even disorder). The recently fabricated P-N junctions in 2D semiconductors provide ideal platforms to explore this phenomenon and its applications to dramatically enhance charge and spin transport as well as carrier-mediated long-range correlation.PACS numbers: 73.40. Lq, 75.30.Hx, 72.80.Vp Half a century ago, Veselago proposed the concept of negative refraction for electromagnetic waves [1][2][3][4]: upon transition from a medium with positive refractive index across a sharp interface into a negative index medium, a diverging pencil of rays is coherently refocused to form a sharp image or "quantum mirage" [5], similar to the bending of light to create mirages in the atmosphere. In the past decade, negative refraction and mirage have been observed for electromagnetic waves of various frequencies (see Ref.[6] for a review) and for cold atoms [7,8]. In 2007, Cheianov et al. [9] proposed the interesting idea that a sharp P-N junction of graphene can exhibit negative refraction and hence focus electrons out of a local pump into a sharp quantum mirage. This effect has been widely used in theoretical proposals to control charge and/or spin transport for massless Dirac fermions in semiconductors (see for a few examples). However, a sharp quantum mirage requires the junction to be sharp compared with the electron wavelength (∼ a few nanometers), otherwise it would disappear due to the path-dependent phase accumulation inside the junction. This makes the observation and application of this effect an experimentally challenging task [13].In this letter, we theoretically demonstrate that in many situations where the quantum mirage is no longer visible, its effect still exists, which could make the response across the P-N junction independent of distance. As a basic observation in physics, the response amplitude in a d-dimensional uniform system decays at least as fast as 1/R (d−1)/2 with distance R, irrespective of the energy dispersion, spin-orbit coupling, etc. This directly leads to rapid decay of many physical properties, such as the charge and spin conductivity [14] As an example, we demonstrate that the P-N junction could dramatically enhance the carriermediated long-range interaction between localized magnetic moments by several orders of magnitudes.For an intuitive physical picture about the hidden quantum mirage, we start from a sharp P-N junction as shown in Fig. 1(a). Here the plane waves excited by a local pump (filled red circle) in the N region is perfectly focuse...

Massless Dirac fermions (MDFs) emerge as quasiparticles in various novel materials such as graphene and topological insulators, and they exhibit several intriguing properties, of which Veselago focusing is an outstanding example with a lot of possible applications. However, up to now Veselago focusing merely occurred in p-n junction devices based on the isotropic MDF, which lacks the tunability needed for realistic applications. Here, motivated by the emergence of novel Dirac materials, we investigate the propagation behaviors of anisotropic MDFs in such a p-n junction structure. By projecting the Hamiltonian of the anisotropic MDF to that of the isotropic MDF and deriving an exact analytical expression for the propagator, precise Veselago focusing is demonstrated without the need for mirror symmetry of the electron source and its focusing image. We show a tunable focusing position that can be used in a device to probe masked atom-scale defects. This study provides an innovative concept to realize Veselago focusing relevant for potential applications, and it paves the way for the design of novel electron optics devices by exploiting the anisotropic MDF.