Charge Density Wave and Narrow Energy Gap at Room Temperature in 2D Pb_3–<i>x</i>Sb_1+<i>x</i>S₄Te_2−δ with Square Te Sheets

We report a new two-dimensional compound, Pb_3–<i>x</i>­Sb_1+<i>x</i>­S₄­Te_2−δ, that has a charge density wave (CDW) at room temperature. The CDW is incommensurate with <i>q</i>-vector of 0.248(6)<i>a</i>* + 0.246(8)<i>b</i>* + 0.387(9)<i>c</i>* for <i>x</i> = 0.29(2) and δ = 0.37(3) due to positional and occupational long-range ordering of Te atoms in the sheets. The modulated structure was refined from the single-crystal X-ray diffraction data with a superspace group <i>P</i>1̅(αβγ)­0 using (3 + 1)-dimensional crystallography. The resistivity increases with decreasing temperature, suggesting semiconducting behavior. The transition temperature (<i>T</i>_CDW) of the CDW is ∼345 K, above which the Te square sheets become disordered with no <i>q</i>-vector. First-principles density functional theory calculations on the undistorted structure and an approximate commensurate supercell reveal that the gap is due to the structure modulation

Semiconducting Ba₃Sn₃Sb₄ and Metallic Ba_7–<i>x</i>Sn₁₁Sb_{15–<i>y</i>} (<i>x</i> = 0.4, <i>y</i> = 0.6) Zintl Phases

We report the discovery of two ternary Zintl phases Ba₃Sn₃Sb₄ and Ba_7–<i>x</i>Sn₁₁­Sb_{15–<i>y</i>} (<i>x</i> = 0.4, <i>y</i> = 0.6). Ba₃Sn₃Sb₄ adopts the monoclinic space group <i>P</i>2₁/<i>c</i> with <i>a</i> = 14.669(3) Å, <i>b</i> = 6.9649(14) Å, <i>c</i> = 13.629(3) Å, and β = 104.98(3)°. It features a unique corrugated two-dimensional (2D) structure consisting of [Sn₃Sb₄]^6– layers extending along the <i>ab</i>-plane with Ba²⁺ atoms sandwiched between them. The nonstoichiometric Ba_6.6Sn₁₁Sb_14.4 has a complex one-dimensional (1D) structure adopting the orthorhombic space group <i>Pnma</i>, with unit cell parameters <i>a</i> = 37.964(8) Å, <i>b</i> = 4.4090(9) Å, and <i>c</i> = 24.682(5) Å. It consists of large double Sn–Sb ribbons separated by Ba²⁺ atoms. Ba₃Sn₃Sb₄ is an n-type semiconductor which has a narrow energy gap of ∼0.18 eV and a room temperature carrier concentration of ∼4.2 × 10¹⁸ cm^–3. Ba_6.6Sn₁₁­Sb_14.4 is determined to be a metal with electrons being the dominant carriers

Spin-Valve Effect in NiFe/MoS₂/NiFe Junctions

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have been recently proposed as appealing candidate materials for spintronic applications owing to their distinctive atomic crystal structure and exotic physical properties arising from the large bonding anisotropy. Here we introduce the first MoS₂-based spin-valves that employ monolayer MoS₂ as the nonmagnetic spacer. In contrast with what is expected from the semiconducting band-structure of MoS₂, the vertically sandwiched-MoS₂ layers exhibit metallic behavior. This originates from their strong hybridization with the Ni and Fe atoms of the Permalloy (Py) electrode. The spin-valve effect is observed up to 240 K, with the highest magnetoresistance (MR) up to 0.73% at low temperatures. The experimental work is accompanied by the first principle electron transport calculations, which reveal an MR of ∼9% for an ideal Py/MoS₂/Py junction. Our results clearly identify TMDs as a promising spacer compound in magnetic tunnel junctions and may open a new avenue for the TMDs-based spintronic applications

Thickness-Dependent Magnetic Breakdown in ZrSiSe Nanoplates

We report a study of thickness-dependent interband and intraband magnetic breakdown by thermoelectric quantum oscillations in ZrSiSe nanoplates. Under high magnetic fields of up to 30 T, quantum oscillations arising from degenerated hole pockets were observed in thick ZrSiSe nanoplates. However, when decreasing the thickness, plentiful multifrequency quantum oscillations originating from hole and electron pockets are captured. These multiple frequencies can be explained by the emergent interband magnetic breakdown enclosing individual hole and electron pockets and intraband magnetic breakdown within spin–orbit coupling (SOC) induced saddle-shaped electron pockets, resulting in the enhanced contribution to thermal transport in thin ZrSiSe nanoplates. These experimental frequencies agree well with theoretical calculations of the intriguing tunneling processes. Our results introduce a new member of magnetic breakdown to the field and open up a dimension for modulating magnetic breakdown, which holds fundamental significance for both low-dimensional topological materials and the physics of magnetic breakdown

Spin-Polarized Tunneling through Chemical Vapor Deposited Multilayer Molybdenum Disulfide

The two-dimensional (2D) semiconductor molybdenum disulfide (MoS₂) has attracted widespread attention for its extraordinary electrical-, optical-, spin-, and valley-related properties. Here, we report on spin-polarized tunneling through chemical vapor deposited multilayer MoS₂ (∼7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5–2% has been observed, corresponding to spin polarization of 5–10% in the measured temperature range of 300–75 K. First-principles calculations for ideal junctions result in a TMR up to 8% and a spin polarization of 26%. The detailed measurements at different temperature, bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS₂ spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomena that control their performance

Nondegenerate Integer Quantum Hall Effect from Topological Surface States in Ag₂Te Nanoplates

The quantum Hall effect is one of the exclusive properties displayed by Dirac Fermions in topological insulators, which propagates along the chiral edge state and gives rise to quantized electron transport. However, the quantum Hall effect formed by the nondegenerate Dirac surface states has been elusive so far. Here, we demonstrate the nondegenerate integer quantum Hall effect from the topological surface states in three-dimensional (3D) topological insulator β-Ag2Te nanostructures. Surface-state dominant conductance renders quantum Hall conductance plateaus with a step of e2/h, along with typical thermopower behaviors of two-dimensional (2D) massless Dirac electrons. The 2D nature of the topological surface states is proven by the electrical and thermal transport responses under tilted magnetic fields. Moreover, the degeneracy of the surface states is removed by structure inversion asymmetry (SIA). The evidenced SIA-induced nondegenerate integer quantum Hall effect in low-symmetry β-Ag2Te has implications for both fundamental study and the realization of topological magneto-electric effects

Controlling the Spin Texture of Topological Insulators by Rational Design of Organic Molecules

We present a rational design approach to customize the spin texture of surface states of a topological insulator. This approach relies on the extreme multifunctionality of organic molecules that are used to functionalize the surface of the prototypical topological insulator (TI) Bi₂Se₃. For the rational design we use theoretical calculations to guide the choice and chemical synthesis of appropriate molecules that customize the spin texture of Bi₂Se₃. The theoretical predictions are then verified in angular-resolved photoemission experiments. We show that, by tuning the strength of molecule–TI interaction, the surface of the TI can be passivated, the Dirac point can energetically be shifted at will, and Rashba-split quantum-well interface states can be created. These tailored interface propertiespassivation, spin-texture tuning, and creation of hybrid interface stateslay a solid foundation for interface-assisted molecular spintronics in spin-textured materials