For all collinear altermagnets, we sort out piezomagnetic free-energy invariants allowed in the nonrelativistic limit and relativistic piezomagnetic invariants bilinear in the N & eacute;el vector L and magnetization M, which include strain-induced Dzyaloshinskii-Moriya interaction. The symmetry-allowed responses are fully determined by the nonrelativistic spin Laue group. In the nonrelativistic limit, two distinct mechanisms are discussed: the band-filling mechanism, which exists in metals and is illustrated using the simple two-dimensional Lieb lattice model, and the temperature-dependent exchange-driven mechanism, which is illustrated using first-principles calculations for transition-metal fluorides. The leading second-order nonrelativistic term in the strain-induced magnetization is also obtained for CrSb. Piezomagnetism due to the strain-induced Dzyaloshinskii-Moriya interaction is calculated from first principles for transition-metal fluorides, MnTe, and CrSb. Finally, we discuss triplet superconducting correlations supported by altermagnets and protected by inversion rather than time-reversal symmetry. We apply the nonrelativistic classification of Cooper pairs to describe the interplay between strain and superconductivity in the two-dimensional Lieb lattice and in bulk rutile structures. We show that triplet superconductivity is, on average, unitary in an unstrained altermagnet, but becomes nonunitary under piezomagnetically active strain.

Donor impurities in wide direct band gap semiconductors provide a promising platform for spin–photon quantum technologies by combining a donor spin qubit with optically addressable transitions. In ZnO, the shallow donor with the largest reported binding energy has long been associated with the I_10 bound exciton line, but its microscopic origin has remained unresolved. Here we demonstrate the controlled formation and identification of this donor as a Sn–Li complex through a combination of ion implantation, annealing, optical spectroscopy, and first-principles calculations. Resonant two-laser coherent population trapping measurements reveal an electron–^119Sn hyperfine interaction of 392 ± 15 MHz, establishing a coupled electron–spin–1/2, nuclear–spin–1/2 system with one of the largest hyperfine couplings reported for shallow donors in semiconductors. Density functional theory calculations show that a nearest-neighbor Sn_Zn–Li_Zn complex has favorable formation energetics, donor character with the electron localized on Sn, and an extrapolated hyperfine interaction consistent with experiment. The large donor binding energy and excited-state structure indicate enhanced thermal robustness of the optical transition relative to conventional group–III donors, while the strong hyperfine interaction enables fast electron–nuclear spin control and prospects for direct nuclear–spin–photon interfaces. We further observe efficient optically induced nuclear spin polarization, highlighting a path toward nuclear spin initialization. More broadly, our results reveal how a donor–acceptor complex can access previously unexplored regimes of shallow donor physics, extending the design space of quantum defects beyond isolated substitutional dopants.

Quantum materials with layered structures offer an excellent platform for exploring emerging electronic, mechanical, and optical properties with potential for technological applications. Trigonally structured t-PtBi2 is such a material. When a femtosecond laser pulse is directed onto t-PtBi2, the resulting transient reflectivity reveals resonances of coherent acoustic phonons, arising from the interference between the diffracted light and light reflected at the t-PtBi2/SiO2 interface. The resonance frequency is found to decrease with increasing the sample thickness from 71.48 GHz for the 20-nm-thick sample to 2.62 GHz for the 276-nm-thick sample. By combining the resonance frequency and mass density, we determine the out-of-plane elastic constant C33, which decreases from 84 GPa for the 20-nm-thick sample to 27 GPa for the 276-nm-thick sample, indicating high sensitivity to the sample thickness. However, the theoretical C33 value of 60 GPa obtained from first-principles calculations is higher than the experimentally determined saturated value for bulk. The discrepancy may be explained by considering the temperature difference and possible point defects in t-PtBi2, which can cause softening through wave sliding. Our combined experimental and theoretical investigation demonstrates that the elastic properties of layered materials can be obtained via light-matter interaction, which is particularly powerful for t-PtBi2 due to its low thermal conductivity and high optical absorption.

We present a comprehensive first-principles investigation of defects in 4H_b-TaS_2. In this layered transition metal dichalcogenide, charge transfer between alternating Mott-insulating 1T and metallic 1H layers gives rise to exotic quantum phases such as the Kondo effect and topological superconductivity. Motivated by recent defect manipulation in 4H_b-TaS_2 via STM, we address their microscopic nature and impact on interlayer charge transfer. To this end, we systematically analyze over 90 defects using large-scale density functional theory (DFT) calculations. Our extensive dataset, compiled from STM simulations, defect formation energies, work functions, and charge transfer, establishes a foundational resource for future theoretical and experimental studies on defect engineering in 4H_b-TaS_2.

We present a systematic first-principles investigation of halide impurities (F and Cl) in Ga_2O_3 and Al_2O_3, considering both monoclinic and corundum phases. Our study of the structural properties, formation energies, and charge-state transition levels establishes the relative stability of different atomic configurations and charge states. We find that F and Cl on oxygen sites act as shallow donors in Ga_2O_3 in both the monoclinic and corundum phases. However, their behavior differs substantially as the band gap increases with greater Al compositions. Fluorine is prone to DX-center formation with increased Al composition, leading to self-compensation at 38

Complex magnetic materials are extremely attractive for revealing unconventional spin states and novel magnetic excitations. Here, we report the structural, thermodynamic, and magnetic properties of a novel magnetic material Li2Co3Se4O12 based on x-ray and neutron diffraction, specific heat, magnetization, and x-ray photoelectron spectroscopy measurements. X-ray and neutron diffraction refinements reveal two Co sites Co (1) and Co (2) even though both are in the octahedral environment. While they are not connected along the b and c directions, these octahedra are edge-shared forming the Co (2) - Co (1) - Co (2) trimer chain along the a direction. The magnetic susceptibility exhibits the Curie-Weiss (CW) temperature dependence at high temperatures (above similar to 50 K) with the negative CW temperature, a dip centered at T-* similar to 8.0 K, and an antiferromagnetic transition at T-N = 3.3 K. The specific heat confirms that there is a phase transition at T-N and a hump at T-*. The long-range magnetic transition at T-N implies that, in addition to the intra-chain interaction, there is strong inter-chain interaction, which is likely due to polarized SeO3 bridging between chains. Single crystal neutron diffraction refinement reveals a complex magnetic structure with the angle between Co (1) and Co (2) moments similar to 105 degrees. Within the Co (2) - Co (1) - Co (2) trimer, two Co (2) moments are parallelly aligned. Surprisingly, the Co (1) moment (1.92 mu(B)) is only half of the Co (2) moment (3.96 mu(B)). There is likely the spin-state change for Co (1) from the high-spin state at T > T-* to the low-spin state at T < T-*, causing a dip in the magnetic susceptibility and a hump in the specific heat. When the magnetic field is applied, multiple metamagnetic transitions are found in all directions, implying field-driven magnetic excitations. Our results demonstrate rich magnetic properties of Li2Co3Se4O12 that are sensitive to the external stimuli such as the magnetic field.

Chemical short-range order (CSRO) is prevalent across many metals and alloys and has recently gained particular attention in concentrated alloys. The advent of complex concentrated alloys has spurred renewed interest in understanding and controlling CSRO. Here, we review recent experimental and theoretical progress on CSRO, highlighting both advancements and ongoing controversies, particularly regarding its impact on the physical properties of concentrated alloys. For example, a highly debated issue is the effect of CSRO on mechanical strength, which remains unresolved due to limited experimental measurements confined to a narrow annealing-temperature range, even for widely studied alloys like CoCrNi Evaluation of the CSRO effects on various physical properties is critical to answer a central question: Can CSRO be transformed into a practical alloy-engineering tool? We also identify critical gaps in the experimental and theoretical frameworks to achieve this goal. Despite the extensive study of CSRO, there remains a need for methodologies that enable its practical application in alloy design. We explore potential solutions, emphasizing the promising roles of machine-learning potentials and additive manufacturing in creating novel avenues for CSRO control.

Point defects have been successfully utilized in various quantum technologies, serving as quantum qubits for quantum computation, single-photon emitters for quantum communication, and nanoscale sensors for quantum metrology. However, their further development faces key challenges, particularly in discovering and exploring suitable defect-host systems that meet the necessary criteria for quantum applications. Here, using polarization-resolved Raman spectroscopy and terahertz absorption spectroscopy, we discover three distinct chromium-vacancy-induced excitations in the uniaxial antiferromagnetic insulator, Cr_2O_3. These vacancy-induced excitations have an energy scale of a few tens of millielectronvolts and are twofold degenerate, and the lowest one at 64 cm^-1 is sharp and sensitive to the external magnetic field along the easy-axis direction, particularly close to the spin-flop regime around 6T, where the mode softens from 64 to 27cm^-1. Based on the defect supercell first-principles calculations, we interpret the mode at 64 cm^-1 as a local magnetic excitation of the local moment within the chromium vacancy state. Our results establish that the magnetic defect states in Cr_2O_3 have potential for quantum applications.

Interface engineering with an inherent symmetry in magnetic oxides is important both for fundamental science and applications of spintronic devices. However, previous efforts in manipulating inversion symmetry are mainly focused on heterostructures with ideal interfaces which precludes a large group of practically important materials. Here we demonstrate systematically tunable inversion symmetry through dynamically controllable interfacial disorders in the nominal (SrRuO3)(2)/(SrTiO3)(2) superlattice. By controlling the dynamic growth parameter - the pulsed laser ablation frequency, we realized controllable asymmetric Ru/Ti intermixing at the top and bottom interfaces of each supercell. Thus the inversion symmetry is absent at the two interfaces between SrRuO3 and SrTiO3, with the degree of the asymmetry tunable. Moreover, the manipulation of the inversion symmetry induces possible variation to the Berry curvature, with a maximal change of the anomalous Hall resistivity by 1530%. First-principle density functional theory calculations illustrate the strong tendency of Ti/Ru intermixing and enhanced Ti ferromagnetism which both coincide to experimental observations. Our study opens up a new avenue in controlling the inversion symmetry with a broad spectrum of material candidates.

Kagome metals have emerged as a frontier in condensed matter physics due to their potential to host exotic quantum states. Among these, CsV3Sb5 has attracted significant attention for the unusual coexistence of charge density wave (CDW) order and unconventional superconductivity, presenting an ideal system for exploring the emergent phenomena from the interplay of phonons, electronic fluctuations, and topological effects. The nature of CDW formation in CsV3Sb5 is unconventional and has sparked considerable debate. In this study, we examine the origin of the CDW state via ab initio finite-temperature simulations of the lattice dynamics. Through a comparative study of CsV3Sb5 and 2H-NbSe2, we demonstrate that the experimental absence of phonon softening-a hallmark of conventional CDW transition-in CsV3Sb5 along with the presence of a weakly first-order transition, can be attributed to quantum zero-point atomic motion. This zero-point motion smears the free energy landscape of CDW, effectively stabilizing the pristine structure even below the CDW transition temperature. We argue that this surprising behavior could cause coexistence of pristine and CDW structures across the transition and lead to a weak first-order transition. Our predicted lattice dynamical behavior is supported by coherent phonon spectroscopy in single-crystalline CsV3Sb5. Our results provide crucial insights into the formation mechanism of CDW materials that exhibit little to no phonon softening, including cuprates, and highlight the surprising role of quantum effects in emergent properties of relatively heavy-element materials like CsV3Sb5.

EuM2As2 (M = Zn, Cd, In, Sn etc.) is an excellent material system for studying magnetism-tuned topological properties. However, discrepancies exist between experimental data and theoretical calculations regarding the bulk and surface bandgaps. In this work, cleaved EuZn2As2 crystals are studied using scanning tunneling microscopy/spectroscopy and density functional theory calculations. Triangular-shaped defect-induced modifications in the local density of states help distinguish between Eu-terminated and AsZn-terminated surfaces. While large bandgaps (similar to 1.5 eV at 77 K) are observed on both pristine surfaces, the bandgap size is found to be highly sensitive to local heterogeneity, tending to decrease. By combining experimental observations with theoretical simulations, we conclude that the reduced bandgap in heterogeneous regions arises from Zn vacancies and/or substitution by As atoms, both impacting greater in the Eu surface electronic properties than those in the AsZn surface. This demonstrates the intimate relationship between the electronic structure and magnetism in EuZn2As2.

First-principles calculations are employed to explore avenues to increase the Néel temperature (T_N) of the magnetoelectric antiferromagnet Cr_2O_3 through doping. Employing the hybrid functional method, we calculate the formation energy of intrinsic defects and transition metal dopants (Mo, W, Nb, Ta, Zr, and Hf) to assess their likelihood of formation. Intrinsic defect calculations indicate that Cr interstitials and oxygen vacancies dominate under Cr-rich conditions, whereas Cr vacancies prevail under O-rich conditions. Notably, under Cr-rich conditions, the Fermi level can be pinned slightly above mid-gap due to the formation of Cr interstitials and oxygen vacancies. To assess the influence of dopant on T_N of Cr_2O_3, we calculate the enhancement of the exchange energy for the spin on the dopant site or on adjacent Cr site using the supercell method. Our study identifies isovalent Mo and W substitution on Cr site as the most promising candidates to increase Néel temperature due to the impurity-mediated enhanced exchange interaction for half-filled bands. Formation energy calculations indicate that Mo and W substitution on Cr are easier to form under Cr-rich conditions and a Fermi level near or slightly above the midgap renders a desirable neutral Mo and W defect. This is assisted by the formation of intrinsic Cr interstitial and O vacancy under Cr-rich conditions. These findings offer a route to utilize defects for higher T_N and enhanced performance of Cr_2O_3 in magnetoelectric devices and furnish invaluable insights for directing subsequent experimental endeavors.

Novel materials with large electro-optic (EO) coefficients are essential for developing ultra-compact broadband modulators and enabling effective quantum transduction. Compared to lithium niobate, the most widely used nonlinear optical material, wurtzite AlScN offers advantages in nano-photonic devices due to its compatibility with integrated circuits. We perform detailed first-principles calculations to investigate the electro-optic effect in $\mathrm{Al}_{1-x}\mathrm{Sc}_{x}\mathrm{N}$ alloys and superlattices. At elevated Sc concentrations in alloys, the EO coefficients increase; importantly, we find that cation ordering along the $c$ axis leads to enhanced EO response. Strain engineering can be used to further manipulate the EO coefficients of AlScN films. With applied in-plane strains, the piezoelectric contributions to the EO coefficients increase dramatically, even exceeding 251 pm/V. We also explore the possibility of EO enhancement through superlattice engineering, finding that nonpolar $a$-plane $\mathrm{(AlN)}_m/\mathrm{(ScN)}_n$ superlattices increase EO coefficients beyond 40 pm/V. Our findings provide design principles to enhance the electro-optic effect through alloy engineering and heterostructure architecture.

Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al2O3, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behavior of Cr in Ga2O3 through theoretical and experimental techniques. β-Ga2O3 samples are grown with the floating zone method and show evidence of a sharp photoluminescence signal, reminiscent of ruby. We calculate the energetics of formation of Cr from first principles, demonstrating that Cr preferentially incorporates as a neutral impurity on the octahedral site. Cr possesses a quartet ground-state spin and has an internal transition with a zero-phonon line near 1.8 eV. By comparing the calculated and experimentally measured luminescence lineshape function, we elucidate the role of coupling to phonons and uncover features beyond the Franck–Condon approximation. The combination of strong emission with a small Huang–Rhys factor of 0.05 and a technologically relevant host material renders Cr in Ga2O3 attractive as a quantum defect.

The band structure of ultrathin Pd(111) thin films grown on the Cr2O3(0001) surface was studied by angule-resolved photoemission spectroscopy (ARPES) combined with first-principles calculations. The Cr2O3(0001) interface and the expanded Pd lattice constant appears to significantly affect the occupied band structure of an ultrathin palladium film. A characteristic band splitting is seen in the experimental occupied electronic structure, forming a hexagonal pattern approximately half-way from theΓ¯point to the surface Brillouin zone boundary. The ARPES spectrum near the Fermi level is reproduced by the first-principles simulation for a Pd monolayer (ML) placed on the Cr-terminated Cr2O3surface with the energetically favorable lateral stacking. In this configuration, the Pd bands inside the Cr2O3band gap have a significant, momentum-dependent exchange splitting, which matches with the experimentally observed band splitting. The experimental band structure shows a band with an electron effective mass close to the free electron mass, neglecting mass enhancement effects close to the Fermi level. From the hybridization of the Pd ML with the Cr2O3(0001) substrate, a hole band with a mass of -0.48 ± 0.02 is identified.

Electron correlation is a main driver of exotic quantum phases and their interplay. The 4Hb-TaS2 system, possessing an intrinsic heterostructure of 1T-and 1H-TaS2 monolayers, offers a unique opportunity to control electron correlation by distorting the atomic lattice or tuning interlayer coupling. Here, we investigated intrinsically deformed charge-density waves (CDWs) in the 1T layer of 4Hb-TaS2 to elucidate and control their effects on flat bands using scanning tunneling microscopy and spectroscopy (STM/S) combined with first-principles calculations. We identified two types of CDW defects: Type 1 has structural distortion and locally suppressed flat bands, while Type 2 features an increased flat band filling factor of intact CDW structure. Density functional theory calculations indicate that a sulfur vacancy in the 1T layer distorts the CDW structure and gives rise to a Type 1, whereas a sulfur vacancy in the 1H layer reduces the interlayer charge transfer and leads to a Type 2. Furthermore, we demonstrated creating and erasing individual CDW defects via STM manipulation. Our findings provide a pathway to not only tune flat bands but also selectively manipulate the interaction between CDW, the atomic lattice, and interlayer coupling in strongly correlated systems with atomic precision.

An in-depth investigation of transition metal impurities (Hf, Zr, Nb, and W) is presented as shallow donors in monoclinic Ga2O3 using first-principles calculations within the framework of density functional theory. A combination of semilocal and hybrid functionals is used to predict their binding energies and hyperfine parameters. The generalized gradient approximation (GGA) allows performing calculations for supercells of up to 2500 atoms, enabling an extrapolation to the dilute limit. The shortcoming of GGA in correctly describing the electron localization is then overcome by the use of the hybrid functional. Results are presented and discussed in light of the application of these transition-metal elements as shallow donors in Ga2O3 and their identification in the experiment. The methodology applied here can be used in calculations for shallow donors in other systems.

Scandium nitride (ScN) is an attractive material for electronic applications due to its high n-type conductivity. Native defects and unintentional impurities may limit its electron concentration and reduce its mobility; therefore, it is important to control their formation and incorporation. Hydrogen and oxygen are unintentional impurities that are commonly present during growth and processing. They act as shallow donors in ScN and hence may be regarded as harmless or even favorable to achieving n-type conductivity. Here we show, using state-of-the-art first-principles calculations, that these impurities can be detrimental because they readily form complexes with scandium vacancies (VSc). Isolated VSc have relatively high formation energies and thus have low concentrations and little impact on electronic properties. However, complexes between VSc and either hydrogen or oxygen form more readily than the pristine vacancy and will act as both compensating and scattering centers. Our results point to the importance of controlling the incorporation of hydrogen and oxygen in ScN (and AlScN alloys) to avoid degradation of the electronic properties.

Nitride ferroelectrics have recently emerged as promising alternatives to oxide ferroelectrics due to their compatibility with mainstream semiconductor processing. ScAlN, in particular, has exhibited remarkable piezoelectric coupling strength (K2) comparable to that of lithium niobate, making it a valuable choice for RF filters in wireless communications. Recently, ScAlN has sparked interest in its use for nanophotonic devices, chiefly due to its large bandgap facilitating operation in blue wavelengths coupled with promises of enhanced nonlinear optical properties such as a large second-order susceptibility (χ(2)). It is still an open question whether ScAlN can outperform oxide ferroelectrics concerning the Pockels effect-an electro-optic coupling extensively utilized in optical communications devices. In this paper, we present a comprehensive theoretical analysis and experimental demonstration of ScAlN's Pockels effect. Our findings reveal that the electro-optic coupling of ScAlN, despite being weak at low Sc concentration, may be significantly enhanced and exceed LiNbO3 at high levels of Sc doping, which points the direction of continued research efforts to unlock the full potential of ScAlN.