Daniel G. Chica

Bound states in the continuum (BICs) are quantum states that remain localized despite lying within a continuum of extended, delocalized states. Whereas optical BICs have been realized in photonic systems, achieving electronic bound states embedded in a metallic background remains an ongoing challenge. In this study, we used scanning tunneling microscopy to identify two defect-induced electronic states that remain localized within the metallic continuum of Pd ₅ AlI ₂ , a two-dimensional van der Waals metal. Our calculations suggest that electron-hopping interference on the decorated checkerboard lattice of Pd ₅ AlI ₂ gives rise to a (quasi) flat band built from spatially localized eigenstates that are orthogonal to the dispersive continuum. Atomic defects within this frustrated lattice shift the associated flat-band localized states into the metallic continuum, forming robust, symmetry-protected BICs.

Heteroanionic solids offer unique levers for tuning structures and properties, but accessing multianion phase space remains challenging due to mismatches in anion reactivity. Here, we describe a new family of A ₂ Hf ₃ Q ₆ O (A = K, Rb, Cs; Q = S, Se) oxychalcogenides, the first reported in the A─Hf─Q─O quaternary space, which are prepared by reacting HfQ ₂ with the corresponding alkali carbonate (A ₂ CO ₃ ). The resulting structures are dimensionally reduced variants of the 1T‐HfQ ₂ parent phase, where rows of O atoms insert into the transition metal dichalcogenide (TMD) layers. This yields nanometer‐wide 1D‐[Hf ₃ Q ₆ ] ribbons that link into step‐like sheets through distinctive Hf─O─Hf bridges, which connect heteroleptic HfQ ₅ O polyhedra at the ribbon edges. UV/Vis and photoluminescence spectra reveal tunable absorption and emission features spanning 1.8–2.6 eV. Calculated band structures show direct‐gap transitions and highly dispersive bands along the [Hf ₃ Q ₆ ] nanoribbons, yielding effective masses more than two orders of magnitude lower than in other crystallographic directions. Charge‐density plots further illustrate quasi‐1D electronic anisotropy, showing frontier orbitals confined to the nanoribbons due to covalent Hf─Q bonding. We anticipate heteroanion‐driven dimensional reduction can be used to engineer anisotropy in semiconductors and offers new routes to synthetically challenging multianion systems.

Heteroanionic solids offer unique levers for tuning structures and properties, but accessing multianion phase space remains challenging due to mismatches in anion reactivity. Here, we describe a new family of A ₂ Hf ₃ Q ₆ O (A = K, Rb, Cs; Q = S, Se) oxychalcogenides, the first reported in the A─Hf─Q─O quaternary space, which are prepared by reacting HfQ ₂ with the corresponding alkali carbonate (A ₂ CO ₃ ). The resulting structures are dimensionally reduced variants of the 1T‐HfQ ₂ parent phase, where rows of O atoms insert into the transition metal dichalcogenide (TMD) layers. This yields nanometer‐wide 1D‐[Hf ₃ Q ₆ ] ribbons that link into step‐like sheets through distinctive Hf─O─Hf bridges, which connect heteroleptic HfQ ₅ O polyhedra at the ribbon edges. UV/Vis and photoluminescence spectra reveal tunable absorption and emission features spanning 1.8–2.6 eV. Calculated band structures show direct‐gap transitions and highly dispersive bands along the [Hf ₃ Q ₆ ] nanoribbons, yielding effective masses more than two orders of magnitude lower than in other crystallographic directions. Charge‐density plots further illustrate quasi‐1D electronic anisotropy, showing frontier orbitals confined to the nanoribbons due to covalent Hf─Q bonding. We anticipate heteroanion‐driven dimensional reduction can be used to engineer anisotropy in semiconductors and offers new routes to synthetically challenging multianion systems.

Anderson localization is anticipated to play a pivotal role in the manifestation of superconducting phases, optimal thermoelectric properties, and the quantum anomalous Hall effect. Dimensionality plays a central role in the physics of Anderson localization. Previous studies of localization in 2D conductors have relied on thin metallic films produced by vapor deposition, but challenges such as domain size effects, grain boundaries, and strain continue to complicate interpretation. Here, we overcome these challenges by investigating localization physics in Pd_{5}AlI_{2}, a van der Waals metal that can be exfoliated to the 2D limit. Using scanning tunneling spectroscopy, we demonstrate Anderson localization of electrons in monolayer Pd_{5}AlI_{2} from the presence of a Coulomb gap at the Fermi level and a unique density of state distribution characteristic of wave function localization. Moreover, we show evidence of phonon localization in Pd_{5}AlI_{2} from the layer-dependent broadening and softening of phonon modes. These signatures of electron and phonon localization, absent in multilayer samples, provide spectroscopic evidence for the theoretical predictions that localization is enhanced at reduced dimensionality.

Antiferromagnetic spintronics offers the potential for higher-frequency operations and improved insensitivity to magnetic fields compared to ferromagnetic spintronics. However, previous electrical techniques to detect antiferromagnetic dynamics have utilized large, millimeter-scale bulk crystals. In this work, we demonstrate direct electrical detection of antiferromagnetic resonance in structures on the few-micrometer scale using spin-filter tunneling in platinum ditelluride (PtTe ₂ )/bilayer chromium sulfide bromide (CrSBr)/graphite junctions in which the tunnel barrier is the van der Waals antiferromagnet CrSBr. This sample geometry allows not only efficient detection but also electrical control of the antiferromagnetic resonance through spin-orbit torque from the PtTe ₂ electrode. The ability to efficiently detect and control antiferromagnetic resonance enables detailed studies of the physics governing these high-frequency dynamics.

In materials with 1D electronic bands, electron–electron interactions can produce intriguing quantum phenomena, including spin‐charge separation and charge density waves (CDW). Most of these systems, however, are non‐magnetic, motivating a search for anisotropic materials where the coupling of charge and spin may affect emergent quantum states. Here, chemical intercalation of the van der Waals magnetic semiconductor CrSBr yields Li_0.17(2)(tetrahydrofuran)_0.26(3)CrSBr, which possesses an electronically driven quasi‐1D CDW with an onset temperature above room temperature. Concurrently, electron doping increases the magnetic ordering temperature from 132 to 200 K and switches its interlayer magnetic coupling from antiferromagnetic to ferromagnetic. The spin‐polarized nature of the anisotropic bands that give rise to this CDW enforces an intrinsic coupling of charge and spin. The coexistence and interplay of ferromagnetism and charge modulation in this exfoliatable material provide a promising platform for studying tunable quantum phenomena across a range of temperatures and thicknesses.

Magnetism in two-dimensional (2D) van der Waals (vdW) crystals offers promising new directions for low-dimensional physics and devices. In this work, mega-electron volt (MeV) ultrafast electron diffraction was employed to investigate the ultrafast atomic dynamics of a novel, 2D vdW magnetic single-crystal CrSBr. Femtosecond (fs) optical pump pulses excited non-equilibrium atomic displacements shown to be coherent acoustic phonons (CAPs). Phonon frequencies were extracted by analyzing oscillations of different Bragg peak (BP) intensities and were determined to be GHz acoustic disturbances that propagated as strain waves. Phonon modes exhibit anisotropy with respect to the a and b crystal axes. Subharmonic phonon frequencies were also observed, and this provided a signature of nonlinear oscillatory coupling between the laser-induced pumping phonon frequency and secondary phonon frequencies. Thus, CrSBr was found to serve as a nonlinear phononic frequency converter. The ultrafast time dependence of the Bragg intensity was simulated by incorporating an oscillating deviation parameter ansatz into expressions for the dynamical scattering intensity yielded excellent modeling of the ultrafast structural dynamics of the photo-excited 2D crystal. Our work provides a foundation for exploring how fs light pulses can influence phonon dynamics in materials with strong spin-lattice coupling. These results suggest that CAPs can match the magnon frequencies and show the promise of CrSBr for use in optical-to-microwave transducers and phononic devices.

The growth of layered 2D compounds is a key ingredient in finding new phenomena in quantum materials, optoelectronics, and energy conversion. Here, we report SnP ₂ Se ₆ , a van der Waals chiral ( R 3 space group) semiconductor with an indirect bandgap of 1.36 to 1.41 electron volts. Exfoliated SnP ₂ Se ₆ flakes are integrated into high-performance field-effect transistors with electron mobilities >100 cm ² /Vs and on/off ratios >10 ⁶ at room temperature. Upon excitation at a wavelength of 515.6 nanometer, SnP ₂ Se ₆ phototransistors show high gain (>4 × 10 ⁴ ) at low intensity (≈10 ⁻⁶ W/cm ² ) and fast photoresponse (< 5 microsecond) with concurrent gain of ≈52.9 at high intensity (≈56.6 mW/cm ² ) at a gate voltage of 60 V across 300-nm-thick SiO ₂ dielectric layer. The combination of high carrier mobility and the non-centrosymmetric crystal structure results in a strong intrinsic bulk photovoltaic effect; under local excitation at normal incidence at 532 nm, short circuit currents exceed 8 mA/cm ² at 20.6 W/cm ² .

The transport of energy and information in semiconductors is limited by scattering between electronic carriers and lattice phonons, resulting in diffusive and lossy transport that curtails all semiconductor technologies. Using Re ₆ Se ₈ Cl ₂ , a van der Waals (vdW) superatomic semiconductor, we demonstrate the formation of acoustic exciton-polarons, an electronic quasiparticle shielded from phonon scattering. We directly imaged polaron transport in Re ₆ Se ₈ Cl ₂ at room temperature, revealing quasi-ballistic, wavelike propagation sustained for a nanosecond and several micrometers. Shielded polaron transport leads to electronic energy propagation lengths orders of magnitude greater than in other vdW semiconductors, exceeding even silicon over a nanosecond. We propose that, counterintuitively, quasi-flat electronic bands and strong exciton–acoustic phonon coupling are together responsible for the transport properties of Re ₆ Se ₈ Cl ₂ , establishing a path to ballistic room-temperature semiconductors.