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Register a new userHexagonal Sr0.6Ba0.4MnO3: Spin and Dipole Coupling via Local Structure
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Hexagonal Sr0.6Ba0.4MnO3 (SBMO) follows P63/mmc symmetry where MnO6 octahedra are both face-shared (Mn2O9 bi-octahedra) and corner-shared via oxygen anion. It undergoes ferroelectric (FE) and antiferromagnetic (AFM) orderings close to the room temperature. Magnetic properties appear to be governed by intricate exchange interactions among Mn4+ ions within and in adjacent Mn2O9 bi-octahedra, contingent upon the local structural changes. Calculations based on our model spin-Hamiltonian reveal that the dominant linear AFM fluctuations between the Mn4+ ions of two oxygen-linked bi-octahedra result in short range correlations, manifest as a smooth drop in magnetization below 325 K. Competition between spin-exchange and local-strain is reckoned as responsible for the atypical magneto-electricity, obtained near the room temperature.
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The crystal structure of hexagonal-Sr0.6Ba0.4MnO3 allows various competing superexchange interactions, leading to intriguing magnetic properties. Local structural changes modify overlapping between Mn and oxygen ions with temperature. Calculations based on our model spin-Hamiltonian reveal that the dominant linear antiferromagnetic superexchange interaction between the oxygen-linked Mn4+ ions results in short range correlations (SRC), manifesting a smooth drop in magnetization below 325K. Dominance of superexchange interaction changes its allegiance towards the non-linear oxygen-linked Mn-O-Mn interactions, onsetting long-range correlations (LRC) below 225K. Below the SRC-LRC crossover temperature, electrical response arising from the interacting dipoles exhibits power-law divergent behaviour of relaxation time, upon cooling. Non-ergodic character of the dipole-cluster glass state is examined via the indispensable aging and rejuvenation effects, similar to the spin glasses. Competitive-frustration among spin-exchange and local-strain is reckoned as responsible for the electrical glass origin.
Despite the recognition of two-dimensional (2D) systems as emerging and scalable host materials of single photon emitters or spin qubits, uncontrolled and undetermined chemical nature of these quantum defects has been a roadblock to further development. Leveraging the design of extrinsic defects can circumvent these persistent issues and provide an ultimate solution. Here we established a complete theoretical framework to accurately and systematically design quantum defects in wide-bandgap 2D systems. With this approach, essential static and dynamical properties are equally considered for spin qubit discovery. In particular, many-body interactions such as defect-exciton couplings are vital for describing excited state properties of defects in ultrathin 2D systems. Meanwhile, nonradiative processes such as phonon-assisted decay and intersystem crossing rates require careful evaluation, which compete together with radiative processes. From a thorough screening of defects based on first-principles calculations, we identify promising single photon emitters such as SiVV and spin qubits such as TiVV and MoVV in hexagonal boron nitride. This work provided a complete first-principles theoretical framework for defect design in 2D materials.
Imaging and spectroscopy performed in a low-voltage scanning transmission electron microscope (LV-STEM) are used to characterize the structure and chemical properties of boron-terminated tetravacancies in hexagonal boron nitride (h-BN). We confirm earlier theoretical predictions about the structure of these defects and identify new features in the electron energy-loss spectra (EELS) of B atoms using high resolution chemical maps, highlighting differences between these areas and pristine sample regions. We correlate our experimental data with calculations which help explain our observations.
We present high energy x-ray (67 keV) and neutron scattering measurements on a single crystal of K$_{1-x}$Li$_x$TaO$_3$ for which the Li content ($x=0.02$) is less than $x_c = 0.022$, the critical value below which no structural phase transitions have been reported in zero field. While the crystal lattice does remain cubic down to T=10 K under both zero-field and field-cooled ($E le 4$ kV/cm) conditions, indications of crystal symmetry lowering are seen at $T_C=63$ K where the Bragg peak intensity changes significantly. A strong and frequency-dependent dielectric permittivity is observed at ambient pressure, a defining characteristic of relaxors. However an extensive search for static polar nanoregions, which is also widely associated with relaxor materials, detected no evidence of elastic neutron diffuse scattering between 300 K and 10 K. Neutron inelastic scattering methods were used to characterize the transverse acoustic and optic phonons (TA1 and TO1 modes) near the (200) and (002) Bragg peaks. The zone center TO1 mode softens monotonically with cooling but never reaches zero energy in either zero field or in external electric fields of up to 4 kV/cm. These results are consistent with the behavior expected for a dipolar glass in which the local polar moments are frozen and exhibit no long-range order at low temperatures.
Heavy metals are key to spintronics because of their high spin-orbit coupling (SOC) leading to efficient spin conversion and strong magnetic interactions. When C60 is deposited on Pt, the molecular interface is metallised and the spin Hall angle in YIG/Pt increased, leading to an enhancement of up to 600% in the spin Hall magnetoresistance and 700% for the anisotropic magnetoresistance. This correlates with Density Functional Theory simulations showing changes of 0.46 eV/C60 in the SOC of Pt. This effect opens the possibility of gating the molecular hybridisation and SOC of metals.
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