Semiconducting Ground State Research Articles

Semiconducting Heusler Compounds beyond the Slater-Pauling Rule

Heusler compounds with semiconducting properties represent an important class of functional materials. Usually, research on these systems is guided by simple electron-counting rules, such as the Slater-Pauling principle. Here, we report on the discovery of Heusler-type semiconductors, significantly deviating from the Slater-Pauling rule. We theoretically predict the occurrence of nonmagnetic semiconducting ground states in various highly off-stoichiometric full-Heusler alloys, where self-substitution leads to a band-gap opening. This unexpected trend is confirmed experimentally by thermoelectric transport measurements on a multitude of Fe2−2xV1−xAl1+3x samples with up to 20% substitution of Fe and V atoms. The band-gap opening leads to an exceptionally large Seebeck coefficient in p-type Fe2VAl thermoelectrics, previously limited by bipolar conduction and low-density-of-states effective mass. Consequently, our work presents a paradigm to tune the band gap of Heusler compounds by self-substitution and introduces a hitherto unexplored class of semiconductors with exceptional thermoelectric properties, offering significant potential for advancements in energy science and sustainable-energy technologies. Published by the American Physical Society 2024

Structural and magnetic properties of Gd4Ga2O9 oxide with a large cryogenic magnetocaloric effect

The magnetocaloric effect has been extensively investigated in various types of magnetic solids to not only develop practical magnetic cooling applications but also deepen the understanding of these materials’ underlying inherent properties. Through experimental determination and theoretical calculation, we conducted a systematic investigation of Gd4Ga2O9 oxide in terms of its structural, magnetic, and magnetocaloric properties and found it to crystallize in a monoclinic structure belonging to the space group P21/c and to possess an antiferromagnetic semiconducting ground state. The consistent elements were uniformly distributed up to the nanoscale and presented as Gd3+, Ga3+, and O2 states, respectively. The magnetocaloric performances of Gd4Ga2O9 oxide were checked according to the parameters of maximum magnetic entropy change, refrigerant capacity, and temperature-averaged entropy changes (lift of 5 K), which were determined to be 25.77 J/kgK, 218.27 J/kg, and 23.43 J/kgK under 0–7 T magnetic field change. These parameters of Gd4Ga2O9 oxide are comparable to those of some recently reported materials with prominent performances, which means that it can be considered for cryogenic magnetic cooling applications.

Dimerization Effects and Negative Strain Energy in Silicon Monosulfide Nanotubes.

We report on the construction and characterization of silicon monosulfide nanotubes that were obtained by rolling up two-dimensional materials isoelectronic to phosphorene in the recently discovered layered Pmma and β phases. We relaxed and studied the nanotube structures using computational methods within density functional theory (DFT). We found that the nanotubes with a thick Pmma layer remain stable at room temperature, and their electronic properties depend on their diameters. Small-diameter nanotubes display metallic character, while nanotubes with increasing diameter show semiconducting ground states due to the dimerization in the silicon-silicon distances that opens a gap, leading to interesting optical properties in the near-infrared region. Furthermore, we discovered β SiS monolayer nanotubes having negative strain energies, similar to the well-known imogolite inorganic nanotubes. The combined thermal stability, compelling optical properties, and diverse applications of these silicon monosulfide nanotubes underscore the demand for novel synthesis methods to fully explore their potential in various fields.

Crystal and electronic structure of the ternary Zintl bismuthide BaLiBi

AbstractReported is the accurate refinement of the structure of the ternary bismuthide BaLiBi, based on single‐crystal X‐ray diffraction data. This compound crystallizes with the ZrBeSi structure type with the space group P63/mmc (no. 194), a=4.9917(6) Å, c=9.079(2) Å, V=195.92(7) Å3 with two formula units per unit cell. In addition to being a colored ternary variant of the AlB2 type, the crystal structure of BaLiBi can be also viewed as a “stuffed” variant of the NiAs structure, where the Bi atoms form a hexagonal close packing, the Ba atoms occupy the octahedral voids in this packing, and the Li atoms are located between adjacent tetrahedral voids on their common triangular faces. In the absence of direct Bi–Bi interactions, the BaLiBi crystal structure rationalized according to the notation (Ba2+)(Li+)(Bi3−), suggesting an electron‐balanced composition, i. e., a Zintl phase. In line with this notation, scalar‐relativistic first‐principle calculations with the LMTO code reveal a semiconducting ground state, with a bandgap of about 0.6 eV. Fully relativistic electronic structure calculations predict a semimetallic ground state.

Low-temperature infrared spectroscopy of the strongly correlated semiconductor Tm0.19Yb0.81B12 with dynamic charge stripes

Tm1–x Yb x B12 dodecaborides represent model objects for the studies of quantum critical behavior, metal–insulator transitions (MITs) and complex charge-spin–orbital–phonon coupling phenomena. In spite of intensive investigations, the mechanism of semiconducting ground state formation both in YbB12 and in the Yb-based strongly correlated electron systems remains a subject of active debates. We have performed first systematic measurements of temperature-dependent spectra of infrared conductivity of Tm0.19Yb81B12 at frequencies 40–35 000 cm−1 and in the temperature range 10–300 K. Analysis of the temperature evolution of the observed absorption resonances is performed allowing to associate these with the cooperative dynamic Jahn–Teller instability of the boron sub-lattice. This ferrodistortive effect of B12-complexes induces the rattling modes of the rare earth ions leading to emergence of both the intra-gap mixed-type collective excitations and the dynamic charge stripes. We estimate the temperature-dependent effective mass of charge carriers and propose the scenario of transformation of the many-body states in the multiple relaxation channels. We attribute the MIT to the localization of electrons at the vibrationally coupled Yb–Yb pairs, which is accompanied by the electronic phase separation and formation of the nanoscale filamentary structure of electron density (stripes) in Tm1–x Yb x B12 compounds.

Ferromagnetic correlations in the layered van der Waals sulfide FeAl2S4

Bulk cleavable crystals of the FeAl2S4 van der Waals sulfide were obtained by chemical vapor transport. The observed ferromagnetic instability indicates strong electronic correlations within the semiconducting ground state.

Optical phonon mode assisted thermal conductivity in p-type ZrIrSb half-Heusler alloy: a combined experimental and computational study

Half-Heusler (HH) alloys with an 18 valence electron count have attracted significant interest in the area of research related to thermoelectrics (TEs). Understanding the novel transport properties exhibited by these systems with semiconducting ground state is an important focus area in this field. The large thermal conductivity shown by most of the HH alloys poses a major hurdle to improving the figure of merit (ZT). In addition, understanding the mechanism of thermal conduction in heavy-constituent HH alloys is an interesting aspect. Here, we have investigated the high-temperature TE properties of ZrIrSb through experimental studies, phonon dispersion and electronic band structure calculations. ZrIrSb is found to exhibit a substantially lower magnitude of resistivity and Seebeck coefficient near room temperature due to the existence of anti-site disorder between Ir/Sb and vacant sites. Interestingly, in ZrIrSb, lattice thermal conductivity is governed by a coupling between the acoustic and low-frequency optical phonon modes, which originates due to heavier Ir/Sb atoms. This coupling leads to an enhancement in the Umklapp processes due to the optical phonon excitations near the zone boundary, resulting in a lower magnitude of κ L. Our studies point to the fact that the simultaneous existence of two heavy mass elements within a simple unit cell can substantially decrease the lattice degrees of freedom.

Narrow-gap semiconducting behavior in antiferromagnetic Eu11InSb9

Here we investigate the thermodynamic and electronic properties of ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_{9}$ single crystals. Electrical transport data show that ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_{9}$ has a semiconducting ground state with a relatively narrow band gap of 320 meV. Magnetic susceptibility data reveal antiferromagnetic order at low temperatures, whereas ferromagnetic interactions dominate at high temperature. Specific heat, magnetic susceptibility, and electrical resistivity measurements reveal three phase transitions at ${T}_{N1}=9.3$ K, ${T}_{N2}=8.3$ K, and ${T}_{N3}=4.3$ K. Unlike ${\mathrm{Eu}}_{5}{\mathrm{In}}_{2}{\mathrm{Sb}}_{6}$, a related europium-containing Zintl compound, no colossal magnetoresistance (CMR) is observed in ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_{9}$. We attribute the absence of CMR to the smaller carrier density and the larger distance between Eu ions and In-Sb polyhedra in ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_{9}$. Our results indicate that ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_{9}$ has potential applications as a thermoelectric material through doping or as a long-wavelength detector due to its narrow gap.

Robust Narrow-Gap Semiconducting Behavior in Square-Net La3Cd2As6

ABSTRACT: Narrow-gap semiconductors are sought-after materials due to their potential for long-wavelength detectors, thermoelectrics, and more recently non-trivial topology. Here we report the synthesis and characterization of a new family of narrow-gap semiconductors, $R$$_{3}$Cd$_{2}$As$_{6}$ ($R=$ La, Ce). Single crystal x-ray diffraction at room temperature reveals that the As square nets distort and Cd vacancies order in a monoclinic superstructure. A putative charge-density ordered state sets in at 279~K in La$_{3}$Cd$_{2}$As$_{6}$ and at 136~K in Ce$_{3}$Cd$_{2}$As$_{6}$ and is accompanied by a substantial increase in the electrical resistivity in both compounds. The resistivity of the La member increases by thirteen orders of magnitude on cooling, which points to a remarkably clean semiconducting ground state. Our results suggest that light square net materials within a $I4/mmm$ parent structure are promising clean narrow-gap semiconductors.

Electro-optical and mechanical properties of Zinc antimonide (ZnSb) monolayer and bilayer: A first-principles study

Latest synthesis of ZnSb monolayer, encouraged us to conduct density functional theory (DFT) simulations in order to study the structural, magnetic, electronic/optical and mechanical features of the sp2-hybridized honeycomb ZnSb monolayer (ML-ZnSb) and bilayer (BL-ZnSb). Our structural optimizations reveal that ML-ZnSb is an anisotropic hexagonal structure while BL-ZnSb is composed of shifted ZnSb layers which are covalently binded. ML-ZnSb is found to be a ferromagnetic metal, in contrast BL-ZnSb has a non-magnetic indirect band gap semiconducting ground state. For the in-plane polarization, first absorption peak of ML-ZnSb and BL-ZnSb confirm the absorbance of the light within the infrared domain wand visible range, respectively. Moreover, our results reveal that the layer-layer chemical bonding in BL-ZnSb significantly enhances the mechanical response of ML-ZnSb whose in-plane sti ness is the smallest among all 2D materials (2DM). Notably, the strong in-plane anisotropy of ML-ZnSb in its sti ness reduces in BL-ZnSb.

Two good metals make a semiconductor: A potassium-nickel compound under pressure

We predict a potassium-nickel intermetallic compound ${\mathrm{K}}_{2}\mathrm{Ni}$ at high pressure and identify it as the long-sought structure of the only known K-Ni compound to date [Parker et al., Science 273, 95 (1996)]. Although both constituent elements are metallic, ${\mathrm{K}}_{2}\mathrm{Ni}$ exhibits a semiconducting ground state with an indirect band gap of 0.65 eV. Electron instability due to the degeneracy at the Fermi level arises from the particular motif of the structure, which in turn induces symmetry-breaking Peierls distortion and a nonmetallic ground state. The results indicate that the chemical properties of elements can change dramatically under extreme conditions and have significant implications for the postulation that potassium is incorporated in Earth's core.

Electronic and Magnetic Properties of Transition‐Metal‐Doped ScN for Spintronics Applications

Motivated by the ongoing interest in nitrides as materials for spintronics applications, the effects of doping with magnetic transition‐metal elements (T = Cr, Mn, Fe, Co, and Ni) on the electronic properties of semiconducting scandium nitride (ScN) are studied. Using density functional theory together with the generalized gradient approximation (GGA) and PBE0r hybrid functional (with different mixing of the exact exchange), two different doping amounts of 25% (Sc0.75T0.25N) and 10% (Sc0.9T0.1N) are investigated. This is done in comparison to the reference compound ScN with a strong focus on identifying candidates for half‐metallic or semiconducting ferromagnetic ground states. Within GGA, only Sc0.75T0.25N and Sc0.75Mn0.25N are found to be semiconducting and half‐metallic, respectively. The use of hybrid functional changes these findings drastically, where Sc0.75Fe(Co,Ni)0.25N becomes the half‐metal and Sc0.75Cr(Mn)0.25N the semiconductor. However, additional calculations assuming antiferromagnetic ordering reveal that Sc0.75T0.25N is the only compound of this series, which prefers an antiferromagnetic (and semiconducting) ground state. For the lower concentration, Sc0.9T0.1N, similar results are predicted, and all the doped nitrides are found to prefer ferromagnetic ground state over an antiferromagnetic one.

Electronic structure of pristine and Ni-substituted LaFeO3 from near edge x-ray absorption fine structure experiments and first-principles simulations

We present a joint theoretical and experimental study of the oxygen $K$-edge spectra for LaFeO$_3$ and homovalent Ni-substituted LaFeO$_3$ (LaFe$_{0.75}$Ni$_{0.25}$O$_3$), using first-principles simulations based on density-functional theory with extended Hubbard functionals and x-ray absorption near edge structure (XANES) measurements. Ground-state and excited-state XANES calculations employ Hubbard on-site $U$ and inter-site $V$ parameters determined from first principles and the Lanczos recursive method to obtain absorption cross sections, which allows for a reliable description of XANES spectra in transition-metal compounds in a very broad energy range, with an accuracy comparable to that of hybrid functionals but at a substantially lower cost. We show that standard gradient-corrected exchange-correlation functionals fail in capturing accurately the electronic properties of both materials. In particular, for LaFe$_{0.75}$Ni$_{0.25}$O$_3$ they do not reproduce its semiconducting behaviour and provide a poor description of the pre-edge features at the O $K$ edge. The inclusion of Hubbard interactions leads to a drastic improvement, accounting for the semiconducting ground state of LaFe$_{0.75}$Ni$_{0.25}$O$_3$ and for a good agreement between calculated and measured XANES spectra. We show that the partial substitution of Fe for Ni affects the conduction-band bottom by generating a strongly hybridized O($2p$)-Ni($3d$) minority-spin empty electronic state. The present work, based on a consistent correction of self-interaction errors, outlines the crucial role of extended Hubbard functionals to describe the electronic structure of complex transition-metal oxides such as LaFeO$_3$ and LaFe$_{0.75}$Ni$_{0.25}$O$_3$ and paves the way to future studies on similar systems.

O-C240: a new sp3-dominated allotrope of carbon

We report a new allotrope of carbon predicted from first principles simulations. This allotrope is formed in a simulated conversion of two-dimensional polymeric C60 precursor subjected to uniaxial compression at high temperature. The structure is made up of 240 carbon atoms in an orthorhombic unit cell (termed as o-C240) having a mixed sp2/sp3 hybridization with the ratio of about 1:5. o-C240 is stable at ambient condition and exhibits superior mechanical performance including optimum Vickers hardness (45 GPa) and fracture toughness (4.10 MPa m1/3), outperforming most of widely used hard ceramics. The electronic structure reveals semiconducting ground state with an indirect band gap of 1.72 eV. The simple reaction pathway could accelerate discovery of this allotrope in laboratory, and the simultaneous occurrence of high fracture toughness, superhardness and semiconductivity is expected to find applications for this material.

Electronic Structure and Magnetic Anisotropy of Single-Layer Rare-Earth Oxybromide.

The discovery of intrinsic magnetism in two-dimensional (2D) limit has triggered increasing investigations in layered magnetic materials. However, most of the available candidates involves 3d transition metals, while the layered rare-earth magnetic materials are largely unexplored at present. Here, we proposed a series of 2D rare-earth magnetic semiconductors REOBr (RE = Tb, Dy, Ho, Er and Tm) with large magnetic moments and magnetic anisotropy energies using the PBE + U method. Our calculations indicate a half-metallic meta-stable state and a low-energy semi-conducting ground state in these 4f single-layers, which can be characterized by the location of the two-fold degenerate x(x2 – 3y2) orbital. The dynamical stability of single-layer REOBr is further confirmed using phonon dispersions. The predicted energy gaps ranging from 2.47 to 4.26 eV decrease with the atomic number of the rare-earth element. Meanwhile, very large spin moments and orbital moments up to 6.018 and 2.872 μB are found, which seem to be insensitive to the magnetic state. Furthermore, the magnetic anisotropy energies are evaluated and understood by a fourth-order non-uniaxial anisotropy mode. Diverse anisotropy energy landscapes including easy cone, easy plane, and easy axis are found, and an extremely high magnetic anisotropy energy of about 8 meV per RE atom is found in the single-layer DyOBr. Our investigations provide a unique insight into layered rare-earth magnetic materials and suggest the single-layer REOBr as competing candidates for low-dimensional data storage applications.

Ferromagnetic, antiferromagnetic, and Peierls distortion states in IVA-VA nanoribbons

Based on first-principles density-functional theory calculations, we have systematically investigated the electronic and magnetic properties of zigzag IVA-VA nanoribbons (ZIV-VNRs). We find that the ZIV-VNRs can exhibit ferromagnetic semiconducting ground states for CP and CAs while simultaneously exhibiting antiferromagnetic states for SiP and SiAs in their edges. What is more, their spin-polarized energy and magnetic moment are quite large in such special systems. Further, we find that a Peierls-like distortion semiconducting state could also take place when the bonding of the edge state is weaker enough as the atom number increases. The physical origin is the competition between charge, spin, and lattice degrees of freedom in different localization characteristics of ZIV-VNR edge states' p orbitals. Our work provides a route for potential functionalities of spintronics devices at the nanoscale.

Optical spectroscopy of SmN: Locating the 4f conduction band

The rare-earth nitride ferromagnetic semiconductors owe their varying magnetic properties to the progressive filling of 4f shell across the series. Recent electrical transport measurements on samarium nitride, including the observation of superconductivity, have been understood in terms of a contribution from a 4f transport channel. Band structure calculations generally locate an empty majority 4f-band within the conduction band although over a wide range of possible energies. Here we report optical reflection and transmission measurements on samarium nitride between 0.01 eV to 4 eV, that demonstrate clearly that the 4f band forms the bottom of the conduction band. Results at the lowest energies show no free carrier absorption, indicating a semiconducting ground state, and supporting earlier conclusions based on transport measurements.

Zero-energy-state-oriented tunability of spin polarization in zigzag-edged bowtie-shaped graphene nanoflakes under an electric field

A comprehensive first-principles study of the correlation between zero-energy states and the tunability of the spin-selective semiconducting properties of zigzag-edged bowtie-shaped graphene nanoflakes under an electric field is presented for the first time. We demonstrate that the spin degenerate semiconducting ground state can be lifted by the electric field. In particular, we find that the number of zero-energy states (’the nullity’) defined by the structural configuration determines the complexity and efficiency of the tunability of spin polarization. The fine-tuning of spin-dependent properties by the electric field originates from the manipulation of spin-polarized molecular orbital energies. We expect this study to aid the design of more effective and controllable low-dimensional molecular spintronics.

Transport and Thermodynamic Properties of CeRu<sub>2</sub>Al<sub>10</sub> Controlled by Pressure at around Critical Pressure

We present transport and thermodynamic properties of CeRu2Al10 controlled by pressure in a vicinity of a critical pressure PC ~ 4GPa, where antiferromagnetic ordering disappears. The resistivity under pressure was measured with DC four terminal method and the AC specific heat under pressure was measured by Joule heating type technique. The pressure was applied by cubic-anvil-apparatus and palm-cubic-anvil-apparatus. The results of AC specific heat indicate TN holds at high temperature up to 3.9 GPa but suddenly disappears above this pressure. We confirmed TN from thermodynamic properties. Although CeRu2Al10 is in a Kondo semiconducting ground state at 4 GPa, temperature dependences of electrical resistivity at 4.6 GPa and 5.9 GPa indicate metallic ground state in these pressures. CeRu2Al10 does not show superconductivity down to 0.7 K at 4.6 GPa and 5.9 GPa.

Crystal structure of the layered arsenide Rb3Cu3As2.

The crystal structure of a new arsenide, Rb3Cu3As2 (trirubidium tricopper diarsenide), has been established from single-crystal X-ray diffraction data. This compound crystallizes in the K3Cu3P2 type, with layers of interlinked CuAs2 units. The partitioning of the available valence electrons yields the charge-balanced composition (Rb+)3(Cu+)3(As3-)2, placing this phase in a broad field of transition-metal-containing Zintl phases. First-principles calculations confirm a semiconducting ground state, in accordance with electron-counting considerations. Chemical bonding analysis reveals strong covalent Cu-As bonds and ionic Rb...As interactions. In addition, a weak attraction is found between the Cu atoms, possibly pointing toward cuprophilic interactions.