arXiv subjects

P. -H. Chang

Publications and source records attributed to P. -H. Chang.

5 recordsLinked to original sources

arXiv2021

Voltage-controlled magnetic anisotropy in antiferromagnetic MgO-capped MnPt films

The magnetic anisotropy in MgO-capped MnPt films and its voltage control are studied using first-principles calculations. Sharp variation of the magnetic anisotropy with film thickness, especially in the Pt-terminated film, suggests that it may be widely tuned by adjusting the film thickness. In thick films the linear voltage control coefficient is as large as 1.5 and $-0.6$ pJ/Vm for Pt-terminated and Mn-terminated interfaces, respectively. The combination of a widely tunable magnetic anisotropy energy and a large voltage-control coefficient suggest that MgO-capped MnPt films can serve as a versatile platform for magnetic memory and antiferromagnonic applications.

cond-mat.mtrl-sci↗

arXiv2019

Pressure-induced high temperature superconductivity in H3X (X=As, Se, Br, Sb, Te and I)

The discovery of high critical temperature T_{c} superconductivity in highly compressed H_{3}S has opened up the question of searching for strong electron-phonon coupling in the hydrides outside the transition metal series. The specific objective of this work is to explore the possibility of discovering a material that exceeds the superconducting transition temperature of H_{3}S. Our study includes the materials H_{3}X (X=As, Se, Br, Sb, Te, and I), is limited to the Im\overline{3}m crystal structure. The procedure we adopt involves performing linearized augmented plane wave calculations for many different volumes to compute the electronic densities of states and their pressure variation. This is combined with Quantum-ESPRESSO calculations from which we obtain the phonon frequencies and the electron-phonon coupling constant λ, and followed by applying the multiple scattering-based theory of Gaspari and Gyorffy to obtain the Hopfield parameters and the McMillan-Allen-Dynes theory. It should be stressed that the GG approach decouples the electronic contribution to λfrom the corresponding phonon contribution, and provides additional insights for the understanding of superconductivity in these materials. Based on our analysis, the hydrogen is the main contributor to the T_{c} in these materials as it makes up 75\sim80 % of the total λ. Our calculations for H_{3}Se and H_{3}Br give a T_{c}{}{}\sim100 K. For the other materials in our study we find that H_{3}As is unstable and H_{3}Sb, H_{3}Te and H_{3}I have small values of the McMillan-Hopfield paramters which makes it unlikely to give high T_{c}. However, according to both of our rigid band model and virtual crystal calculations, we predict a T_{c}\sim150 K for H_{3}Br with a small amount of hydrogen doping.

cond-mat.supr-con↗

arXiv2018

Origin of spin reorientation transitions in antiferromagnetic MnPt-based alloys

Antiferromagnetic MnPt exhibits a spin reorientation transition (SRT) as a function of temperature, and off-stoichiometric Mn-Pt alloys also display SRTs as a function of concentration. The magnetocrystalline anisotropy in these alloys is studied using first-principles calculations based on the coherent potential approximation and the disordered local moment method. The anisotropy is fairly small and sensitive to the variations in composition and temperature due to the cancellation of large contributions from different parts of the Brillouin zone. Concentration and temperature-driven SRTs are found in reasonable agreement with experimental data. Contributions from specific band-structure features are identified and used to explain the origin of the SRTs.

cond-mat.mtrl-sci↗

arXiv2018

Origin of nonlocal resistance in multiterminal graphene on hexagonal-boron-nitride: Fermi surface edge currents rather than Fermi sea topological valley currents

The recent observation [R. V. Gorbachev et al., Science {\bf 346}, 448 (2014)] of nonlocal resistance $R_\mathrm{NL}$ near the Dirac point (DP) of multiterminal graphene on aligned hexagonal boron nitride (G/hBN) has been interpreted as the consequence of topological valley Hall currents carried by the Fermi sea states just beneath the bulk gap $E_g$ induced by the inversion symmetry breaking. However, the valley Hall conductivity $σ^v_{xy}$, quantized inside $E_g$, is not directly measurable. Conversely, the Landauer-Büttiker formula, as numerically exact approach to observable nonlocal transport quantities, yields $R_\mathrm{NL} \equiv 0$ for the same simplistic Hamiltonian of gapped graphene that generates $σ^v_{xy} \neq 0$. We combine ab initio with quantum transport calculations to demonstrate that G/hBN wires with zigzag edges host dispersive edge states near the DP that are absent in theories based on the simplistic Hamiltonian. Although such edge states exist also in isolated zigzag graphene wires, aligned hBN is required to modify their energy-momentum dispersion and generate $R_\mathrm{NL} \neq 0$ near the DP persisting in the presence of edge disorder. Concurrently, the edge states resolve the long-standing puzzle of why the highly insulating state of G/hBN is rarely observed. We conclude that the observed $R_\mathrm{NL}$ is unrelated to Fermi sea topological valley currents conjectured for gapped Dirac spectra.

cond-mat.mes-hall↗

arXiv2017

Proximity band structure and spin textures on both sides of topological-insulator/ferromagnetic-metal interface and their transport probes

The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin texture around their interfaces. Here we combine density functional theory with Green's function techniques to obtain the spectral function at any plane passing through atoms of Bi$_2$Se$_3$ and Co or Cu layers comprising the interface. In contrast to widely assumed but thinly tested Dirac cone gapped by the proximity exchange field, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi$_2$Se$_3$ in contact with Co near the Fermi level $E_F^0$, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions injected by metallic layers and pushed, due to charge transfer from Co or Cu layers, few tenths of eV below $E_F^0$ for both Bi$_2$Se$_3$/Co and Bi$_2$Se$_3$/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation [K. Kondou {\em et al.}, Nat. Phys. {\bf 12}, 1027 (2016)] of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of $E_F^0$. Interestingly, three monolayers of Co adjacent to Bi$_2$Se$_3$ host spectral functions very different from the bulk metal, as well as in-plane spin textures signifying the spin-orbit proximity effect. We predict that out-of-plane tunneling anisotropic magnetoresistance in vertical heterostructure Cu/Bi$_2$Se$_3$/Co, where current flowing perpendicular to its interfaces is modulated by rotating magnetization from parallel to orthogonal to current flow, can serve as a sensitive probe of spin texture residing at $E_F^0$.

cond-mat.mes-hall↗