arXiv subjects

Gaofeng Xu

Publications and source records attributed to Gaofeng Xu.

16 recordsLinked to original sources

arXiv2026

Magnetically induced Circular Photogalvanic Effect in Symmetric Two-dimensional Materials

Photocurrents that depend on the helicity of the incident light can be generated in both bulk and low-dimensional materials lacking inversion symmetry, known as the circular photogalvanic effect (CPGE). We propose that by employing a magnetic effect, the limitation on the inversion symmetry broken materials can be overcome, such that helicity-dependent photocurrent can be generated in a symmetric material, i.e., a magneto-circular photogalvanic effect (MCPGE). As a proof of principle, we elucidate the mechanism of such an MCPGE through an effective Hamiltonian of a monolayer SbH on a magnetic substrate with an adjustable magnetization. Moreover, the associated response in optical absorption is analyzed, both single-particle and excitonic, through a Bethe-Salpeter equation to describe the Coulomb interaction in excitons. Our result broadens the mechanism of CPGE and opens new opportunities for optoelectronic devices.

cond-mat.mes-hall↗

arXiv2026

Profound impacts of interlayer interactions in bilayer altermagnetic V2S2O

Two-dimensional altermagnets exhibit exceptional potential for low-power spintronics via nonrelativistic spin splitting and zero net magnetization. Here, we systematically investigate the influence of interlayer interactions on the electronic, magnetic and quantum transport properties of bilayer vanadium oxysulfide (V2S2O), a prototypical layered altermagnet, using DFT and NEGF calculations. Our results reveal that interlayer interactions predominantly modulate the p-orbital derived top valence bands, inducing a profound competitive valence band maximum position between Gamma-point pz and X/Y-point pxy orbitals, with an energy difference as small as 9 meV. Furthermore, interlayer interactions suppress the piezomagnetic effect and impose additional requirements on the type of strain for the bilayer system, compared to its monolayer counterpart. Out-of-plane external electric fields effectively weaken interlayer coupling by enlarging the energy difference of Gamma/X-Y top valence bands to 170 meV. Quantum transport simulations on a bilayer Au/V2S2O/Au two-probe device demonstrate the presence of pronounced spin current. Interlayer interactions reduce the transmission spin polarization from nearly 100% (monolayer) to 60% (bilayer) for energies above the Fermi level. Notably, gate-voltage modulation exhibits significant asymmetry in controlling charge-to-spin current conversion efficiency, originating from the out-of-plane symmetry breaking induced by the electrode geometry. Specifically, a positive gate voltage markedly enhances the contribution of the bottom layer to the overall spin polarization, while a negative gate voltage induces a marginal reduction of transmission spin polarization, attributed to the inherently weak polarization contribution of the bottom layer. These findings provide essential insights for the design and optimization of multilayer altermagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2025

The birefringent spin-laser as a system of coupled harmonic oscillators

Adding spin-polarized carriers to semiconductor lasers strongly changes their properties and, through the transfer of angular momentum, leads to the emission of circularly polarized light. In such spin-lasers, the polarization of the emitted light can be modulated an order of magnitude faster than its intensity in the best conventional lasers. This ultrafast operation in spin-lasers relies on large linear birefringence, usually viewed as detrimental in spin and conventional lasers, which couples the two linearly polarized emission modes. We show that the dynamical properties of birefringent spin-lasers under intensity and polarization modulation are accurately described as coupled harmonic oscillators. Our model agrees with the intensity-equation description which, unlike the common complex field components describing the role of birefringence in laser dynamics, uses simpler real quantities and allows analytical solutions. We further predict unexplored operation regimes and elucidate the difference between the weak and strong coupling in spin-lasers.

cond-mat.mes-hall↗

arXiv2024

Manipulating the Optical Response of TaIrTe4 Heterostructures through Band Alignment Strategy

Weyl semimetals, such as $TaIrTe_{4}$, characterized by their unique band structures and exotic transport phenomena, have become a central focus in modern electronics. Despite extensive research, a systematic understanding of the impact of heterogeneous integration on the electronic and optical properties of TaIrTe4 device remains elusive. We have carried out density functional theory combined with nonequilibrium Green's function formalism calculations for $TaIrTe_{4}/WTe_{2}$, $TaIrTe_{4}/MoTe_{2}$ and $TaIrTe_{4}/h-BN$ heterostructures, aiming to understand the manipulation of photoresponse through various band alignment strategies. The underlying impacts of interlayer interactions, charge transfer and build-in electric field on the electronic properties are carefully investigated. We design a dual-probe photodetector device to understand the overall photoresponse enhancement of the heterogeneous integration by decomposing into the specific strain, interlayer transition, band overlap and symmetry lowering mechanics. These van der Waals integrations provide an ideal platform for studying band alignment physics in self-powered optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Emergent bright excitons with Rashba spin-orbit coupling in atomic monolayers

Optical properties in van der Waals heterostructures based on monolayer transition-metal dichalcogenides (TMDs), are often dominated by excitonic transitions. While intrinsic spin-orbit coupling (SOC) and an isotropic band structure are typically studied in TMDs, in their heterostructures Rashba SOC and trigonal warping (TW), resulting in bands with threefold anisotropy, are also present. By considering a low-energy effective Hamiltonian and Bethe-Salpeter equation, we study the effect of Rashba SOC and TW on the band structure and absorption spectra. Rashba SOC is predicted to lead to emergent excitons, which are identified as an admixture between 1s and 2p symmetries. In contrast, for experimentally relevant values, TW has only a negligible effect on the absorption spectrum. These findings could guide experimental demonstrations of emergent bright excitons and further studies of the proximity effects in van der Waals heterostructure.

cond-mat.mes-hall↗

arXiv2023

Polarization response of spin-lasers under amplitude modulation

Lasers with injected spin-polarized carriers show an outstanding performance in both static and dynamic operation. In addition to the intensity response of conventional lasers, without spin-polarized carriers, both intensity and polarization of light can be exploited for optical communication in spin-lasers. However, the polarization dynamics of spin-lasers under amplitude modulation has been largely overlooked. Here we reveal, analytically and numerically, a nontrivial polarization response that accompanies the well-known intensity dynamics of a spin-laser under amplitude modulation. We evaluate the polarization and intensity response under the same amplitude modulation, and further assess the capability of such a polarization response in digital data transfer with eye diagram simulations. Our results provide a more complete understanding of the modulation response in spin-lasers and open up unexplored opportunities in optical communication and spintronics.

cond-mat.mes-hall↗

arXiv2022

Switching modulation of spin transport in ferromagnetic tetragonal silicene

We study the band structure and transport properties of ferromagnetic tetragonal silicene nanoribbons by using the non-equilibrium Green's function method. The band structure and spin-dependent conductance are discussed under the combined effect of the external electric field, potential energy, exchange field and the spin-orbit coupling. One can easily realize a phase transition from a semimetallic to a semiconducting state by changing the transverse width of the nanoribbon. Separation of spin-dependent conductances arises from the effect of exchange field and the spin-orbit coupling, while zero-conductance behaviors exhibit spin-dependent band gaps induced by the electric field. We propose a device configuration of four-terminal tetragonal silicene nanoribbon with two central channels. It is found that spin current can be controlled by utilizing two switches. The switch with a high potential barrier can block electrons flowing from the central scattering region into other terminals. Interestingly, applying only one switch can realize spin-dependent zero conductance and large spin polarization. Two switches can provide multiple operations for controlling spin-dependent transport properties. The two-channel ferromagnetic tetragonal silicene nanoribbon can realize an effective separation of spin current, which may be a potential candidate for spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Variational Hierarchical Directed Bounding Box Construction for Solid Mesh Models

Object oriented bounding box tree (OBB-Tree for short) has many applications in collision detection, real-time rendering, etc. It has a wide range of applications. The construction of the hierarchical directed bounding box of the solid mesh model is studied, and a new optimization solution method is proposed. But this part of the external space volume that does not belong to the solid mesh model is used as the error, and an error calculation method based on hardware acceleration is given. Secondly, the hierarchical bounding box construction problem is transformed into a variational approximation problem, and the optimal hierarchical directed bounding box is obtained by solving the global error minimum. In the optimization calculation, we propose that combining Lloyd clustering iteration in the same layer and MultiGrid-like reciprocating iteration between layers. Compared with previous results, this method can generate aired original solid mesh models are more tightly packed with hierarchical directed bounding box approximation. In the practical application of collision detection, the results constructed using this method can reduce the computational time of collision detection and improve detection efficiency.

cs.GR↗

arXiv2021

Threshold behavior in spin lasers: Spontaneous emission and nonlinear gain

A hallmark of spin-lasers, injected with spin-polarized carriers, is their threshold behavior with the onset of stimulated emission. Unlike the single threshold in conventional lasers with unpolarized carriers, two thresholds are expected in spin lasers. With the progress in scaled-down lasers and the use of novel two-dimensional materials it is unclear if the common description of spin lasers assuming a negligible spontaneous emission and linear optical gain remains relevant or even how to identify the lasing thresholds. Our rate-equation description addresses these questions by considering a large spontaneous emission and a nonlinear optical gain. We provide a transparent approach and analytical results to explore the resulting threshold behavior, its deviation from the prior studies, as well as guide future spin-lasers as a versatile platform for spintronics beyond magnetoresistance.

cond-mat.mes-hall↗

arXiv2020

Intensity equations for birefringent spin lasers

Semiconductor spin lasers are distinguished from their conventional counterparts by the presence of spin-polarized carriers. The transfer of angular momentum of the spin-polarized carriers to photons provides important opportunities for the operation of lasers. With the injection of spin-polarized carriers, which lead to the circularly polarized light, the polarization of the emitted light can be changed an order of magnitude faster than its intensity. This ultrafast operation of spin lasers relies on a large birefringence, usually viewed as detrimental in spin and conventional lasers. We introduce a transparent description of spin lasers using intensity equations, which elucidate the influence of birefringence on the intensity and polarization modulation of lasers. While intensity modulation is independent of birefringence, for polarization modulation an increase in birefringence directly increases the resonant frequency. Our results for dynamical operation of lasers provide a guide for their spin-dependent response and spintronic applications beyond magnetoresistance.

cond-mat.mes-hall↗

arXiv2020

Optically Probing Tunable Band Topology in Atomic Monolayers

In many atomically thin materials their optical absorption is dominated by excitonic transitions. It was recently found that optical selection rules in these materials are influenced by the band topology near the valleys. We propose that gate-controlled band ordering in a single atomic monolayer, through changes in the valley winding number and excitonic transitions, can be probed in helicity-resolved absorption and photoluminescence. This predicted tunable band topology is confirmed by combining an effective Hamiltonian and a Bethe-Salpeter equation for an accurate description of excitons, with first-principles calculations suggesting its realization in Sb-based monolayers.

cond-mat.mes-hall↗

arXiv2020

Spin-Lasers: Spintronics Beyond Magnetoresistance

Introducing spin-polarized carriers in semiconductor lasers reveals an alternative path to realize room-temperature spintronic applications, beyond the usual magnetoresistive effects. Through carrier recombination, the angular momentum of the spin-polarized carriers is transferred to photons, thus leading to the circularly polarized emitted light. The intuition for the operation of such spin-lasers can be obtained from simple bucket and harmonic oscillator models, elucidating their steady-state and dynamic response, respectively. These lasers extend the functionalities of spintronic devices and exceed the performance of conventional (spin-unpolarized) lasers, including an order of magnitude faster modulation frequency. Surprisingly, this ultrafast operation relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both viewed as detrimental in spintronics and conventional lasers. Spin-lasers provide a platform to test novel concepts in spin devices and offer progress connected to the advances in more traditional areas of spintronics.

cond-mat.mes-hall↗

arXiv2018

Ultrafast spin-lasers

The appeal of lasers can be attributed to both their ubiquitous applications and their role as model systems for elucidating nonequilibrium and cooperative phenomena. Introducing novel concepts in lasers thus has a potential for both applied and fundamental implications. Here we experimentally demonstrate that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 GHz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers. Surprisingly, this ultrafast operation relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both commonly viewed as detrimental in spintronics and conventional lasers. Our results overcome the key speed limitations of conventional directly modulated lasers and offer a prospect for the next generation of low-energy ultrafast optical communication.

cond-mat.mes-hall↗

arXiv2017

Magnetic Proximity Effects in Transition-Metal Dichalcogenides: Converting Excitons

The two-dimensional character and reduced screening in monolayer transition-metal dichalcogenides (TMDs) lead to the ubiquitous formation of robust excitons with binding energies orders of magnitude larger than in bulk semiconductors. Focusing on neutral excitons, bound electron-hole pairs, that dominate the optical response in TMDs, it is shown that they can provide fingerprints for magnetic proximity effects in magnetic heterostructures. These proximity effects cannot be described by the widely used single-particle description, but instead reveal the possibility of a conversion between optically inactive and active excitons by rotating the magnetization of the magnetic substrate. With recent breakthroughs in fabricating Mo- and W-based magnetic TMD-heterostructures, this emergent optical response can be directly tested experimentally.

cond-mat.mtrl-sci↗

arXiv2017

Wurtzite spin lasers

Semiconductor lasers are strongly altered by adding spin-polarized carriers. Such spin lasers could overcome many limitations of their conventional (spin-unpolarized) counterparts. While the vast majority of experiments in spin lasers employed zinc-blende semiconductors, the room temperature electrical manipulation was first demonstrated in wurtzite GaN-based lasers. However, the underlying theoretical description of wurtzite spin lasers is still missing. To address this situation, focusing on (In,Ga)N-based wurtzite quantum wells, we develop a theoretical framework in which the calculated microscopic spin-dependent gain is combined with a simple rate equation model. A small spin-orbit coupling in these wurtzites supports simultaneous spin polarizations of electrons and holes, providing unexplored opportunities to control spin lasers. For example, the gain asymmetry, as one of the key figures of merit related to spin amplification, can change the sign by simply increasing the carrier density. The lasing threshold reduction has a nonmonotonic depenedence on electron spin polarization, even for a nonvanishing hole spin polarization.

cond-mat.mes-hall↗

arXiv2015

Toward high-frequency operation of spin lasers

Injecting spin-polarized carriers into semiconductor lasers provides important opportunities to extend what is known about spintronic devices, as well as to overcome many limitations of conventional (spin-unpolarized) lasers. By developing a microscopic model of spin-dependent optical gain derived from an accurate electronic structure in a quantum well-based laser, we study how its operation properties can be modified by spin-polarized carriers, carrier density, and resonant cavity design. We reveal that by applying a uniaxial strain, it is possible to attain a large birefringence. While such birefringence is viewed as detrimental in conventional lasers, it could enable fast polarization oscillations of the emitted light in spin lasers which can be exploited for optical communication and high-performance interconnects. The resulting oscillation frequency ($>200$ GHz) would significantly exceed the frequency range possible in conventional lasers.

cond-mat.mes-hall↗