arXiv subjects

Shu-Hui Zhang

Publications and source records attributed to Shu-Hui Zhang.

30 records · Page 2Linked to original sources

arXiv2019

Anomalous caustics and Veselago focusing in 8-Pmmn borophene p-n junctions with arbitrary junction directions

Negative refraction usually demands complex structure engineering while it is very natural for massless Dirac fermions (MDFs) across the \textit{p-n} junction, this leads to Dirac electron optics. The emergent Dirac materials may exhibit hitherto unidentified phenomenon due to their nontrivial band structures in contrast to the isotropic MDFs in graphene. Here, as a specific example, we explore the negative refraction induced caustics and Veselago focusing of tilted MDFs across 8-\textit{Pmmn} borophene \textit{p-n} junctions. To this aim, we develop a technique to effectively construct the electronic Green's function in \textit{p-n} junctions with arbitrary junction directions. Based on analytical discussions and numerical calculations, we demonstrate the strong dependence of interference pattern on the junction direction. As the junction direction perpendicular to the tilt direction, Veselago focusing or normal caustics (similar to that in graphene) appears resting on the doping configuration of the \textit{p-n} junctions, otherwise anomalous caustics (different from that in graphene) occurs which is manipulated by the junction direction and the doping configuration. Finally, the developed Green's function technique is generally promising to uncover the unique transport of emergent MDFs, and the discovered anomalous caustics makes tilted MDFs potential applications in Dirac electron optics.

cond-mat.mes-hall↗

arXiv2019

Velocity-determined anisotropic behaviors of RKKY interaction in 8-\textit{Pmmn} borophene

As a new two-dimensional Dirac material, 8-\textit{Pmmn} borophene hosts novel anisotropic and tilted massless Dirac fermions (MDFs) and has attracted increasing interest. However, the potential application of 8-\textit{Pmmn} borophene in spin fields has not been explored. Here, we study the long-range RKKY interaction mediated by anisotropic and tilted MDFs in magnetically-doped 8-\textit{Pmmn} borophene. To this aim, we carefully analyze the unique real-space propagation of anisotropic and tilted MDFs with noncolinear momenta and group velocities. As a result, we analytically demonstrate the anisotropic behaviors of long-range RKKY interaction, which have no dependence on the Fermi level but are velocity-determined, i.e., the anisotropy degrees of oscillation period and envelop amplitude are determined by the anisotropic and tilted velocities. The velocity-determined RKKY interaction favors to fully determine the characteristic velocities of anisotropic and tilted MDFs through its measurement, and has high tunability by engineering velocities shedding light on the application of 8-\textit{Pmmn} borophene in spin fields.

cond-mat.mes-hall↗

arXiv2019

Tilted Klein tunneling across atomically sharp interfaces

Despite many similarities between electronics and optics, the hopping of the electron on a discrete atomic lattice gives rise to energy band nonparabolicity and anisotropy. The crucial influences of this effect on material properties and its incorporation into the continuum model have received widespread attention in the past half century. Here we predict the existence of a different effect due to the hopping of the electron across an atomically sharp interface. For a general lattice, its influence on transport could be equally important as the energy band nonparabolicity/anisotropy, but cannot be incorporated into the continuum model. On the honeycomb lattice of graphene, it leads to the breakdown of the conventional Klein tunneling -- one of the exotic phenomena of relativistic particles -- and the onset of tilted Klein tunneling. This works identifies a unique feature of the discrete atomic lattice for transport, which is relevant for ballistic electronic devices at high carrier densities.

cond-mat.mes-hall↗

arXiv2019

Dirac nodal line metal for topological antiferromagnetic 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. Here, we predict that room temperature AFM metal MnPd$_{2}$ allows the electrical control of the Dirac nodal line by the Néel spin-orbit torque. Based on first-principles 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 MnPd$_{2}$ represents a promising material for topological AFM spintronics.

cond-mat.mes-hall↗

arXiv2018

Selective generation and amplification of RKKY interactions by P-N interface

We propose a physical mechanism to generate and selectively amplify anisotropic Rudermann-Kittel-Kasuya-Yosida (RKKY) interactions between two local spins. The idea is to combine the deflection of the carrier velocity by a P-N interface and the locking of this velocity to the carrier spin orientation via spin-orbit coupling. We provide analytical and numerical results to demonstrate this mechanism on the surface of a topological insulator P-N junction. This work identifies the P-N interface as a second knob which, together with the carrier density, enables independent control of the strength and anisotropy of the RKKY interaction over a wide range. These findings may be relevant to scalable quantum computation and two-impurity quantum criticality.

cond-mat.mes-hall↗

arXiv2018

Tunable two-dimensional Dirac nodal nets

Nodal line semimetals are characterized by symmetry-protected band crossing lines and are expected to exhibit nontrivial electronic properties. Connections of the multiple nodal lines, resulting in nodal nets, chains, or links, are envisioned to produce even more exotic quantum states. In this work, we propose a feasible approach to realize tunable nodal line connections in real materials. We show that certain space group symmetries support the coexistence of the planar symmetry enforced and accidental nodal lines, which are robust to spin-orbit coupling and can be tailored into intricate patterns by chemical substitution, pressure, or strain. Based on first-principles calculations, we identify non-symmorphic centrosymmetric quasi-one-dimensional compounds, K$_{2}$SnBi and MX$_{3}$ (M = Ti, Zr, Hf and X = Cl, Br, I), as materials hosting such tunable 2D Dirac nodal nets. Unique Landau levels are predicted for the nodal line semimetals with the 2D Dirac nodal nets. Our results provide a viable approach for realize the novel physics of the nodal line connections in practice.

cond-mat.mes-hall↗

arXiv2018

Oblique Klein tunneling in 8-Pmmn borophene p-n junctions

The 8-\textit{Pmmn} borophene is one kind of new elemental monolayer, which hosts anisotropic and tilted massless Dirac fermions (MDF). The planar \textit{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-\textit{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 \textit{N} and \textit{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.

cond-mat.mes-hall↗

arXiv2018

Veselago focusing of anisotropic massless Dirac fermions

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.

cond-mat.mes-hall↗

arXiv2018

Perfect transmission at oblique incidence by trigonal warping in graphene P-N junctions

We develop an analytical mode-matching technique for the tight-binding model to describe electron transport across graphene P-N junctions. This method shares the simplicity of the conventional mode-matching technique for the low-energy continuum model and the accuracy of the tight-binding model over a wide range of energies. It further reveals an interesting phenomenon on a sharp P-N junction: the disappearance of the well-known Klein tunneling (i.e., perfect transmission) at normal incidence and the appearance of perfect transmission at oblique incidence due to trigonal warping at energies beyond the linear Dirac regime. We show that this phenomenon arises from the conservation of a generalized pseudospin in the tight-binding model. We expect this effect to be experimentally observable in graphene and other Dirac fermions systems, such as the surface of three-dimensional topological insulators.

cond-mat.mes-hall↗

arXiv2017

Focusing RKKY interaction by graphene P-N junction

The carrier-mediated RKKY interaction between local spins plays an important role for the application of magnetically doped graphene in spintronics and quantum computation. Previous studies largely concentrate on the influence of electronic states of uniform systems on the RKKY interaction. Here we reveal a very different way to manipulate the RKKY interaction by showing that the anomalous focusing - a well-known electron optics phenomenon in graphene P-N junctions - can be utilized to refocus the massless Dirac electrons emanating from one local spin to the other local spin. This gives rise to rich spatial interference patterns and symmetry-protected non-oscillatory RKKY interaction with a strongly enhanced magnitude. It may provide a new way to engineer the long-range spin-spin interaction in graphene.

cond-mat.mes-hall↗

arXiv2017

General Green's function formalism for layered systems: Wave function approach

The single-particle Green's function (GF) of mesoscopic structures plays a central role in mesoscopic quantum transport. The recursive GF technique is a standard tool to compute this quantity numerically, but it lacks physical transparency and is limited to relatively small systems. Here we present a numerically efficient and physically transparent GF formalism for a general layered structure. In contrast to the recursive GF that directly calculates the GF through the Dyson equations, our approach converts the calculation of the GF to the generation and subsequent propagation of a scattering wave function emanating from a local excitation. This viewpoint not only allows us to reproduce existing results in a concise and physically intuitive manner, but also provides analytical expressions of the GF in terms of a generalized scattering matrix. This identifies the contributions from each individual scattering channel to the GF and hence allows this information to be extracted quantitatively from dual-probe STM experiments. The simplicity and physical transparency of the formalism further allows us to treat the multiple reflection analytically and derive an analytical rule to construct the GF of a general layered system. This could significantly reduce the computational time and enable quantum transport calculations for large samples. We apply this formalism to perform both analytical analysis and numerical simulation for the two-dimensional conductance map of a realistic graphene p-n junction. The results demonstrate the possibility of observing the spatially-resolved interference pattern caused by negative refraction and further reveal a few interesting features, such as the distance-independent conductance and its quadratic dependence on the carrier concentration, as opposed to the linear dependence in uniform graphene.

cond-mat.mes-hall↗

arXiv2016

Hidden quantum mirage by negative refraction in semiconductor P-N junctions

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.

cond-mat.mtrl-sci↗