22 Search Results

Electric currents have the intriguing ability to induce magnetization in nonmagnetic crystals with sufficiently low crystallographic symmetry. Some associated phenomena include the non-linear anomalous Hall effect in polar crystals and the nonreciprocal directional dichroism in chiral crystals when magnetic fields are applied. In this work, we demonstrate that the same underlying physics is also manifested in the electronic tunneling process between the surface of a nonmagnetic chiral material and a magnetized scanning probe. In the paramagnetic but chiral metallic compound Co _1/3 NbS ₂ , the magnetization induced by the tunneling current is shown to become detectable by its couplingmore » to the magnetization of the tip itself. This results in a contrast across different chiral domains, achieving atomic-scale spatial resolution of structural chirality. To support the proposed mechanism, we used first-principles theory to compute the chirality-dependent current-induced magnetization and Berry curvature in the bulk of the material. Our demonstration of this magnetochiral tunneling effect opens up an avenue for investigating atomic-scale variations in the local crystallographic symmetry and electronic structure across the structural domain boundaries of low-symmetry nonmagnetic crystals.« less

We comore » mbine synchrotron-based infrared absorption and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of hafnia under compression. We find that pressure drives HfO ${}_2$ :7%Y from the mixed monoclinic ( $P{2}_1/c$ ) $+$ antipolar orthorhombic ( $\mathit{Pbca}$ ) phase to pure antipolar orthorhombic ( $\mathit{Pbca}$ ) phase at approximately 6.3 GPa. This transformation is irreversible, meaning that upon release, the material is kinetically trapped in the $\mathit{Pbca}$ metastable state at 300 K. Compression also drives polar orthorhombic ( $Pca{2}_1$ ) hafnia into the tetragonal ( $P{4}_2/nmc$ ) phase, although the latter is not metastable upon release. These results are unified by an analysis of the energy landscape. The fact that pressure allows us to stabilize targeted metastable structures with less Y stabilizer is important to preserving the flat phonon band physics of pure HfO ${}_2$ .« less

In order to explore the consequences of spin–orbit coupling on spin–phonon interactions in a set of chemically similar mixed metal oxides, we measured the infrared vibrational properties of Co ₄B₂ O₉ (B = Nb, Ta) as a function of temperature and compared our findings with lattice dynamics calculations and several different models of spin–phonon coupling. Frequency vs temperature trends for the Co²⁺ shearing mode near 150 cm^–1 reveal significant shifts across the magnetic ordering temperature that are especially large in relative terms. Bringing these results together and accounting for noncollinearity, we obtain spin–phonon coupling constants of –3.4 and –4.3 cm^–1more » for Co₄Nb₂O₉ and the Ta analog, respectively. Analysis reveals that these coupling constants are derived from interlayer (rather than intralayer) exchange interactions and that the interlayer interactions contain competing antiferromagnetic and ferromagnetic contributions. At the same time, beyond-Heisenberg terms are minimized due to fortuitous symmetry considerations, different from most other 4d- and 5d-containing oxides. Comparison with other contemporary oxides shows that spin–phonon coupling in this family of materials is among the strongest ever reported, suggesting an origin for magnetoelectric coupling.« less

Here, we report direct visualization of spin-flip transition of the surface layer in antiferromagnet MnBi₄Te₇, a natural superlattice of alternating MnBi₂Te₄ and Bi₂Te₃ layers, using cryogenic magnetic force microscopy (MFM). The observation of magnetic contrast across domain walls and step edges confirms that the antiferromagnetic order persists to the surface layers. The magnetic field dependence of the MFM images reveals that the surface magnetic layer undergoes a first-order spin-flip transition at a magnetic field that is lower than the bulk transition, in excellent agreement with a revised Mills model. Our analysis suggests no reduction of the order parameter in themore » surface magnetic layer, implying robust ferromagnetism in the single-layer limit. The direct visualization of surface spin-flip transition not only opens up exploration of surface metamagnetic transitions in layered antiferromagnets, but also provides experimental support for realizing quantized transport in ultrathin films of MnBi₄Te₇ and other natural superlattice topological magnets.« less

Magnetic topological insulators provide a prominent material platform for quantum anomalous Hall physics and axion electrodynamics. However, the lack of material realizations with cleanly gapped surfaces hinders technological utilization of these exotic quantum phenomena. Here, using the Zintl concept and the properties of nonsymmorphic space groups, we computationally engineer magnetic topological insulators. Specifically, we explore Eu₅M₂X₆ (M=metal, X=pnictide) Zintl compounds and find that Eu₅Ga₂Sb₆, Eu₅Tl₂Sb₆, and Eu₅In₂Bi₆ form stable structures with nontrivial Z₂ indices. We also show that epitaxial and uniaxial strain can be used to control the Z₂ index and the bulk energy gap. Lastly, we discuss experimental progressmore » towards the synthesis of the proposed candidates and provide insights that can be used in the search for robust magnetic topological insulators in Zintl compounds.« less

We report a {001} planar defect found in SiGe nanopillars. The defect structure, determined by atomic-resolution electron microscopy, matches the Humble defect model proposed for diamond. We also investigate several possible variants of the Humble structure using first-principles calculations and find that the one lowest in energy is in agreement with the scanning transmission electron microscope images. The pillar composition has been analyzed with electron energy loss spectroscopy, which hints at how the defect is formed. Our results show that the structure and formation process of defects in nanostructured group IV semiconductors can be different from their bulk counterparts.

Hafnia (HfO₂) is a promising material for emerging chip applications due to its high-κ dielectric behavior, suitability for negative capacitance heterostructures, scalable ferroelectricity, and silicon compatibility. The lattice dynamics along with phononic properties such as thermal conductivity, contraction, and heat capacity are under-explored, primarily due to the absence of high quality single crystals. Herein, we report the vibrational properties of a series of HfO₂ crystals stabilized with yttrium (chemical formula HfO2: xY, where x = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. We untangle the effects of Y bymore » testing our calculations against the measured Raman and infrared spectra of the cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal the signature modes of polar orthorhombic hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO₂ and paves the way for an analysis of mode contributions to high-κ dielectric and ferroelectric properties for chip technologies.« less

Orientation-dependent electronic properties of wurtzite zinc oxide nanorods are characterized by aloof-beam electron energy-loss spectroscopy (EELS) carried out in a scanning transmission electron microscope (STEM). The two key crystal orientation differentiating transitions specific to the in-plane (13.0 eV) and out-of-plane (11.2 eV) directions with respect to the wurtzite structure are examined by first-principles density-functional theory calculations. We note some degree of orientation dependence at the onset of direct band gap transition near 3.4 eV. We demonstrate that good polarization selectivity can be achieved by placing the electron probe at different locations around the specimen with increasing impact parameter while keepingmore » the beam-specimen orientation fixed. The observed results are qualitatively elucidated in terms of the perpendicular electric fields generated by the fast electron (60 kV) used in the microscope. The fact that good polarization selectivity can be achieved by aloof-beam EELS without the requirement of sample reorientation is an attractive aspect from the characterization method point of view in the STEM-EELS community.« less

A two-dimensional material – Mg₂B₄C₂, belonging to the family of the conventional superconductor MgB₂, is theoretically predicted to exhibit superconductivity with critical temperature T_c estimated in the 47–48 K range (predicted using the McMillian-Allen-Dynes formula) without any tuning of external parameters such as doping, strain, or substrate-induced effects. The origin of such a high intrinsic T_c is ascribed to the presence of strong electron-phonon coupling and large density of states at the Fermi level. This system is obtained after replacing the chemically active boron-boron surface layers in a MgB₂ slab by chemically inactive boron-carbon layers. Hence, the surfaces of thismore » material are inert. Our calculations confirm the stability of 2D Mg₂B₄C₂. We also find that the key features of this material remain essentially unchanged when its thickness is increased by modestly increasing the number of inner MgB₂ layers.« less

In order to explore the properties of a two-sublattice ferroelectric, we measured the infrared and Raman scattering response of CuInP₂S₆ across the ferroelectric and glassy transitions and compared our findings to a symmetry analysis, calculations of phase stability, and lattice dynamics. In addition to uncovering displacive character and a large hysteresis region surrounding the ferroelectric transition temperature T_C, we identify the vibrational modes that stabilize the polar phase and confirm the presence of two ferroelectric variants with opposite polarizations. Below T_C, a poorly understood relaxational or glassy transition at T_g is characterized by local structure changes in the form ofmore » subtle peak shifting and activation of low frequency out-of-plane Cu- and In-containing modes. Here, the latter are due to changes in the Cu/In coordination environments and associated order-disorder processes. Moreover, T_g takes place in two steps with another large hysteresis region and significant underlying scattering. Combined with imaging of the room temperature phase separation, this effort lays the groundwork for studying CuInP₂S₆ under external stimuli and in the ultrathin limit.« less