arXiv subjects

Runnan Zhang

Publications and source records attributed to Runnan Zhang.

5 recordsLinked to original sources

arXiv2026

Universal 3:1 Scaling of Quantum-Confined Stark Spectra Revealed by a Three-Dimensional Profile

We report that the quantum-confined Stark effect spectrum exhibits a nearly rigid redshift while preserving its characteristic peak spacing patterns when increasing the electric field strength F. Using InGaN as a model system, we uncover two electric-field-independent scaling laws for the spectral peaks in both the sub-bandgap and above-bandgap regions and the coefficient ratio is near 3:1. With a novel three-dimensional (3D) visualization, we reveal that the sub-bandgap peak spacings scale as $\frac{12\pi\hbar^2}{L^2\sqrt{m_em_h}}$ while the above-bandgap peak spacings scale as $\frac{4\pi\hbar^2}{L^2\sqrt{m_em_h}}$, explaining the origin of the 3:1 ratio. This scaling behavior, validated in both InGaN and GaAs systems and at electroluminescence working conditions, shows that increasing F only expands the energy range and increases the number of peaks without altering the spacing. Beyond these laws, the 3D profile offers new insights into the Tauc background, Franz-Keldysh oscillations and coherence length, providing a powerful tool for the design and diagnostics of electro-optic devices.

cond-mat.mtrl-sci↗

arXiv2024

Atomic observation on diamond (001) surfaces with non-contact atomic force microscopy

To achieve atomic-level characterization of the diamond (001) surface, persistent efforts have been made over the past few decades. The motivation behind the pursuit extends beyond investigating surface defects and adsorbates; it also involves unraveling the mystery of the smooth growth of diamond. However, the inherently low conductivity and the short C-C bonds render atomic resolution imaging exceptionally challenging. Here, we successfully overcame these challenges by employing non-contact atomic force microscopy with reactive Si tips. Atomic resolution imaging was achieved even at room temperature. With density-functional-theory calculations, we clarified that the critical factors for atomic resolution are in the formation of tilted C-Si bonds between scanning probes and surfaces, along with reordering of the surface C-C dimers. Implications of the findings extend beyond the realm of surface characterization. The present atomic-resolution microscopies drive future advancements in diamond technologies by providing avenues for identifying dopants and constructing artificial nanostructures.

cond-mat.mtrl-sci↗

arXiv2023

Laser induced surface magnetization in Floquet-Weyl semimetals

We investigate optically induced magnetization in Floquet-Weyl semimetals generated by irradiation of a circularly-polarized continuous-wave laser from the group II-V narrow gap semiconductor Zn$_3$As$_2$ in a theoretical manner. Here, this trivial and nonmagnetic crystal is driven by the laser with a nearly resonant frequency with a band gap to generate two types of Floquet-Weyl semimetal phases composed of different spin states. These two phases host nontrivial two-dimensional surface states pinned to the respective pairs of the Weyl points. By numerically evaluating the laser-induced transient carrier-dynamics, it is found that both spins are distributed in an uneven manner on the corresponding surface states due to significantly different excitation probabilities caused by the circularly-polarized laser with the nearly resonant frequency. It is likely that such spin-polarized surface states produce surface magnetization, and furthermore the inverse Faraday effect also contributes almost as much as the spin magnetization. To be more specific, excited carries with high density of the order of $10^{21}\: {\rm cm}^{-3}$ are generated by the laser with electric field strength of a few MV/cm to result in the surface magnetization that becomes asymptotically constant with respect to time, around 1 mT. The magnitude and the direction of it depend sharply on both of the intensity and frequency of the driving laser, which would be detected by virtue of the magneto-optic Kerr effect.

cond-mat.mtrl-sci↗

arXiv2022

Floquet-Weyl semimetals generated by an optically resonant interband-transition

Floquet-Weyl semimetals (FWSMs) generated by irradiation of a continuous-wave laser with left-hand circular polarization (rotating in counterclockwise sense with time) on the group II-V narrow gap semiconductor Zn$_3$As$_2$ are theoretically investigated, where the frequency of the laser is set nearly {\it resonant} with a band gap of the crystal. It is found that the excitation of the crystal by such a laser induce two types of FWSM phases that differ absolutely in characters. To be specific, the associated two pairs of Weyl points are stably formed by band touching between Floquet sidebands ascribable to a valence band labeled as $J_z=\pm3/2$ and a conduction band labeled as $J_z=\pm 1/2$, where $J_z$ represents the $z$-component of total angular momentum quantum number of $\Gamma$-point and a double sign corresponds. Here, one FWSM state composed of the up-spin Floquet sidebands relevant to $J_z=3/2$ and $1/2$ shows almost quadratic band-touching in the vicinity of the associated pair of Weyl points, while the other FWSM state composed of the down-spin Floquet sidebands relevant to $J_z=-3/2$ and $-1/2$ shows linear band-touching. Further, it is revealed that both up-spin and down-spin sidebands host nontrivial two-dimensional surface states that are pinned to the respective pairs of the Weyl points. Both surface states also show different energy dispersions and physical properties. More detailed discussion is made in the text on the origin of the above findings, chirality of the FWSM phases, alteration of topological order, laser-induced magnetic properties, and so on.

cond-mat.mtrl-sci↗

arXiv2019

Topological Hall effect in bulk ferromagnet Cr$_2$Te$_3$ embedded with black-phosphorus-like bismuth nanosheets

We implement the molecular beam epitaxy method to embed the black-phosphorus-like bismuth nanosheets into the bulk ferromagnet Cr$_2$Te$_3$. As a typical surfactant, bismuth lowers the surface tensions and mediates the layer-by-layer growth of Cr$_2$Te$_3$. Meanwhile, the bismuth atoms precipitate into black-phosphorus-like nanosheets with the lateral size of several tens of nanometers. In Cr$_2$Te$_3$ embedded with Bi-nanosheets, we observe simultaneously a large topological Hall effect together with the magnetic susceptibility plateau and magnetoresistivity anomaly. As a control experiment, none of these signals is observed in the pristine Cr$_2$Te$_3$ samples. Therefore, the Bi-nanosheets serve as seeds of topological Hall effect induced by non-coplanar magnetic textures planted into Cr$_2$Te$_3$. Our experiments demonstrate a new method to generates a large topological Hall effect by planting strong spin-orbit couplings into the traditional ferromagnet, which may have potential applications in spintronics.

cond-mat.mtrl-sci↗