Electron-counting rules were applied to understand the stability, structural preference, and physical properties of metal disilicides. Following predictions made by 14 electron counting rules, the ordered semiconductor ReGaSi, the first ternary phase in this system, is proposed and successfully synthesized. It crystallizes with a primitive tetragonal structure (space group P4/nmm) closely related to that of MoSi2-type ReSi2, but with Ga and Si orderly distributed in the unit cell. The band structure, density of states, and crystal orbital calculations confirm the electron count hypothesis to predict new stable compounds. Calculations, based on 14 electrons per ReGaSi units, show a small indirect band gap of ~0.2 eV around Fermi level between full and empty electronic states. Additionally, first-principles calculations confirm the site preference of Ga and Si which is observed through the structural refinement. Experimental magnetic measurements verified the predicted non-magnetic properties of ReGaSi.
Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels a…
Parameters of the energy gap and, consequently, electronic, magnetic and galvanomagnetic properties in different X$_2$YZ Heusler alloys can vary quite strongly. In particular, half-metallic ferromagnets (HMFs) and spin gapless semiconductors (SGSs) with almost 100% spin polarization of charge carriers are promising materials for spintronics. The changes in the electrical, magnetic and galvanomagnetic properties of the Co$_2$YSi (Y = Ti, V, Cr, Mn, Fe) and Co$_2$MnZ Heusler alloys (Z = Al, Si, Ga, Ge) in possible HMF and/or SGS states were followed and their interconnection was established. Sig…
Intermetallic semiconductors with the cubic Half-Heusler structure (XYZ) have excellent thermoelectric properties. This has been attributed to the high degeneracy of the carrier pockets in the band structure, but large differences are found between different material compositions. Half-Heuslers are often interpreted within Zintl chemistry, making a clear distinction between an electropositive cation ($X^{n+}$) and an extended polyanion ($YZ^{n-}$). Based on quantitative real space chemical bonding analysis, we unravel large degrees of covalent bonding between the formal cation and anion, makin…
Based on high throughput density functional theory calculations, we performed systematic screening for spin-gapless semiconductors (SGSs) in quaternary Heusler alloys XX 0 YZ (X, X 0 , and Y are transition metal elements without Tc, and Z is one of B, Al, Ga, In, Si, Ge, Sn, Pb, P, As, Sb, and Bi). Following the empirical rule, we focused on compounds with 21, 26, or 28 valence electrons, resulting in 12, 000 possible chemical compositions. After systematically evaluating the thermodynamic, mechanical, and dynamical stabilities, we successfully identified 70 stable SGSs, confirmed by explicit …
Photoexcitation in solids brings about transitions of electrons/holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous photocurrent due to the topological character of the constituting electronic bands; the Berry connection. This photocurrent, termed shift current, is expected to emerge on the time-scale of primary photoexcitation process. We observed ultrafast time evolution of the shift current in a prototypical ferroelectric semiconductor by detecting emitted terahertz electromagnetic waves. By sweeping the excita…
Single crystals of CrSbSe$_3$, a structurally pseudo-one-dimensional ferromagnetic semiconductor, were grown using a high-temperature solution growth technique and were characterized by x-ray diffraction, anisotropic, temperature- and field-dependent magnetization, temperature-dependent resistivity and optical absorption measurements. A band gap of 0.7 eV was determined from both resistivity and optical measurements. At high temperatures, CrSbSe$_3$ is paramagnetic and isotropic with a Curie-Weiss temperature of $\sim$145 K and an effective moment of $\sim$4.1 $μ_B$/Cr. A ferromagnetic transit…
A viable and accurate route to GW simulations of metals and doped semiconductors
Istituto Nanoscienze- CNR, S3（意大利国家研究委员会纳米科学研究所）；Università degli Studi di Modena e Reggio Emilia（摩德纳和雷焦艾米利亚大学）；Dipartimento di Fisica, Università di Roma La Sapienza（罗马第一大学物理系）
Two-dimensional (2D) materials family with its many members and different properties has recently drawn great attention. Thanks to their atomic thickness and smooth surface, 2D materials can be constructed into heterostructures or homostructures in the fashion of out-of-plane perpendicular stacking or in-plane lateral stitching, resulting in unexpected physical and chemical properties and applications in many areas. In particular, 2D metal-semiconductor heterostructures or homostructures (MSHSs) which integrate 2D metals and 2D semiconductors, have shown great promise in future integrated elec…
In semiconductor physics, many essential optoelectronic material parameters can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For monolayer transition-metal dichalcogenide semiconductors, this field scale is substantial --tens of teslas or more-- due to heavy carrier masses and huge exciton binding energies. Here we report absorption spectroscopy of monolayer MoS$_2$, MoSe$_2$, MoTe$_2$, and WS$_2$ in very high magnetic fields to 91~T. We follow the diamagnetic shifts and valley Zeeman splittings of not only the exciton's $1s$ ground state but also …
We present photoluminescence measurements in monolayer WSe$_2$, which point to the importance of the interaction between charged particles and excitonic complexes. The theoretical analysis highlights the key role played by exchange scattering, referring to cases wherein the particle composition of the complex changes after the interaction. For example, exchange scattering renders bright excitonic complexes dark in monolayer WSe$_2$ on accounts of the unique valley-spin configuration in this material. In addition to the ultrafast energy relaxation of hot excitonic complexes following their inte…
Semiconductor coupled quantum dots provide a unique opportunity of tuning bandgaps by tailoring band offsets, making them ideal for photovoltaic and other applications. Here, we have studied stability, trends in the band gap, band offsets, and optical properties for a series of coupled quantum dots comprised of II-VI semiconductor using a hybrid functional method. We have shown how the quantum confinement and interfacial strain considerably affect the band gap and band offsets for these heterostructures at the nanoscale. We show that the trend in band offsets obtained from our first-principles…
Time-resolved Kerr rotation measurements were performed in InGaAs/GaAs quantum wells nearby a doped Mn delta layer. Our magneto-optical results show a typical time evolution of the optically-oriented electron spin in the quantum well. Surprisingly, this is strongly affected by the Mn spins, resulting in an increase of the spin precession frequency in time. This increase is attributed to the variation in the effective magnetic field induced by the dynamical relaxation of the Mn spins. Two processes are observed during electron spin precession: a quasi-instantaneous alignment of the Mn spins wit…
