We analyze the experimentally obtained spin current related magnetoresistance (MR) in epitaxial Pt/Co bilayers by using a drift-diffusion model that incorporates both bulk spin Hall effect and interfacial Rashba-Edelstein effect (REE). The MR analysis yields, for the Pt/Co interface, a temperature-independent Rashba parameter in the order of ${10}^{-11}\mathrm{eV}\mathrm{m}$ that agrees with theoretical calculations, along with an effective interfacial REE thickness of several angstroms, which is in overall consistency with our previous spin-orbit torque analysis. Our results suggest that both bulk and interface charge-spin current interconversions need to be taken into account for the spin current related MR analysis in highly conductive magnetic heterostructures such as the epitaxial Pt/Co bilayers.

A diagrammatic derivation of the Ginzburg-Landau equations for impure bulk superconductors, whose symmetry group includes the polar axis $\mathbf{c}$, is presented. The results obtained are valid at an arbitrary relationship between the critical temperature $k_B{T}_C$, the inverse impurity scattering time $\hslash /\tau$, and the spin-orbit energy $(\alpha /\hslash )p_F$, where $\alpha$ is the value of the spin-orbit coupling of current carriers, which is inherent in such compounds. The magnetoelectric effect (the spin polarization induced by the supercurrent) is also evaluated under the same conditions. Analogous results for two-dimensional asymmetric superconductors are presented too.

We study an intrinsic relation between the topology of bulk electronic states and magnetic responses of Majorana Kramers pairs, Kramers pairs of Majorana fermions, on a surface of time-reversal-invariant topological superconductors. Majorana Kramers pairs respond to an applied magnetic field anisotropically due to the interplay between time-reversal and crystalline symmetries. In this paper, we propose a systematic procedure to determine such surface magnetic responses in systems with an order-two symmetry. From the analysis of topological invariants associated with an order-two symmetry, it is found that magnetic responses are classified into four types, which are attributed to different topological invariants and exhibit distinguishable, characteristic magnetic responses. For a Kramers pair of Majorana fermions protected by ${\mathbb{Z}}_2$ topological invariants, we clarify that types of magnetic responses are determined only from Fermi-surface topology and symmetry of Cooper pairs. Finally, we apply our theory to the topological nonsymmorphic crystalline superconducting state in UCoGe, which exhibits a biaxially anisotropic magnetic response.

A recent development in magnetic materials research on a non-Joulian magnetostriction phenomenon [H. D. Chopra and M. Wuttig, Nature 521, 340 (2015)] highlights peculiar cellular magnetic domains with zigzag boundaries in Fe-Ga alloys. The cause of zigzag boundaries of cellular domains is attributed to hypothetical charge density waves beyond classical magnetic domain theory. In this paper, we report observations in Fe-Ga alloys of zigzag boundaries that form conventional stripe domains. The responses of stripe domains to external magnetic fields are observed, and the behaviors of zigzag boundaries are examined, which are further compared with those in cellular domains. It shows that both cellular and stripe domains and the constituent zigzag boundaries in magnetostrictive Fe-Ga alloys can be explained well by classical magnetic domain theory, without resorting to theory based on hypothetical charge density waves. In particular, our findings provide convincing evidence that zigzag boundaries in Fe-Ga alloys are conventional V lines commonly observed in cubic magnetic crystals like Fe-Si alloys. The intricate cellular domain structure is shown to correlate with the simple stripe domain structure in terms of zigzag V lines and flux-closure surface domains.

The magnetic nature of a quasi-one-dimensional compound, ${\mathrm{BaVSe}}_3$, has been investigated with positive muon spin rotation and relaxation (${\mu }^{+}\mathrm{SR}$) measurements at ambient and high pressures. At ambient pressure, the ${\mu }^{+}\mathrm{SR}$ spectrum recorded under zero external magnetic field exhibited a clear oscillation below the Curie temperature ($T_C\sim 41\mathrm{K}$) due to the formation of quasistatic ferromagnetic order. The oscillation consisted of two different muon spin precession signals, indicating the presence of two magnetically different muon sites in the lattice. However, the two precession frequencies, which correspond to the internal magnetic fields at the two muon sites, could not be adequately explained with relatively simple ferromagnetic structures using the muon sites predicted by density functional theory calculations. The detailed analysis of the internal magnetic field suggested that the V moments align ferromagnetically along the $c$ axis but slightly canted toward the $a$ axis by ${28}^{\circ }$ that is coupled antiferromagnetically. The ordered V moment (${\mathit{M}}_{\mathrm{V}}$) is estimated as (0.59, 0, 1.11) ${\mu }_{\mathrm{B}}$. As pressure increased from ambient pressure, $T_C$ was found to decrease slightly up to about 1.5 GPa, at which point $T_C$ started to increase rapidly with the further increase of the pressure. Based on a strong ferromagnetic interaction along the $c$ axis, the high-pressure ${\mu }^{+}\mathrm{SR}$ result revealed that there are two magnetic interactions in the $ab$ plane; one is an antiferromagnetic interaction that is enhanced with pressure, mainly at pressures below 1.5 GPa, while the other is a ferromagnetic interaction that becomes predominant at pressures above 1.5 GPa.

Antiferromagnetic materials have the possibility to offer ultrafast, high-data-density spintronic devices. A significant challenge is the reliable detection of the state of the antiferromagnet, which can be achieved using exchange bias. Here, we develop an atomistic spin model of the athermal training effect, a well-known phenomenon in exchange-biased systems where the bias is significantly reduced after the first hysteresis cycle. We find that the setting process in granular thin films relies on the presence of interfacial mixing between the ferromagnetic and antiferromagnetic layers. We systematically investigate the effect of the intermixing and find that the exchange bias, switching field, and coercivity all increase with increased intermixing. The interfacial spin state is highly frustrated leading to a systematic decrease in interfacial ordering of the ferromagnet. This metastable spin structure of initially irreversible spins leads to a large effective exchange coupling and thus large increase in the switching field. After the first hysteresis cycle these metastable spins drop into a reversible ground state that is repeatable for all subsequent hysteresis cycles, demonstrating that the effect is truly athermal. Our simulations provide insights into the role of interface mixing and the importance of metastable spin structures in exchange-biased systems which could help with the design and optimization of antiferromagnetic spintronic devices.

We formulate a strong-coupling theory for perpendicular standing spin waves (PSSWs) in ferromagnetic bilayers with the interlayer exchange coupling (IEC). Employing the Hoffmann boundary condition and the energy-flow continuity across the interface, we show that the PSSWs are still quantized but with nonintegral quantum numbers, similar to the case with a surface pinning but in sharp contrast to that with a free surface. The magnon-magnon coupling is characterized by the spectrum splitting which is linear with the IEC in the weak-coupling region but getting saturated in the strong-coupling limit. Analytical predictions are verified by full micromagnetic simulations with good agreement.

Motivated by a recent inelastic neutron scattering experiment on ${\mathrm{YbMgGaO}}_4$, I study the homogeneous spin model on the triangular lattice using the cluster mean-field theory and exact diagonalization. Phase diagrams with and without magnetic field are computed. The results show that the magnetic field can induce the phase transition from the spin liquid state (or the stripe state) to the ${120}^{\circ }$ antiferromagnetic state. These phase transitions are suppressed by the next-nearest-neighbor exchange interaction $J_2/J_1$ and vanish as $J_2/J_1>0.11$. I analyze a parameter space at $J_2/J_1=0.1$, where a field induced phase transition can occur, and find that the deviation of the theoretical spin excitation energy from the experimental data is only about 0.054. These results imply that an effective homogeneous spin model still applies to ${\mathrm{YbMgGaO}}_4$.

Hybrid superconductor-semiconductor heterostructures are promising platforms for realizing topological superconductors and exploring Majorana bound state physics. Motivated by recent experimental progress, we theoretically study how magnetic insulators offer an alternative to the use of external magnetic fields for reaching the topological regime. We consider different setups, where (1) the magnetic insulator induces an exchange field in the superconductor, which leads to a splitting in the semiconductor by proximity effect, and (2) the magnetic insulator acts as a spin-filter tunnel barrier between the superconductor and the semiconductor. We show that the spin splitting in the superconductor alone cannot induce a topological transition in the semiconductor. To overcome this limitation, we propose to use a spin-filter barrier that enhances the magnetic exchange and provides a mechanism for a topological phase transition. Moreover, the spin-dependent tunneling introduces a strong dependence on the band alignment, which can be crucial in quantum-confined systems. This mechanism opens up a route towards networks of topological wires with fewer constraints on device geometry compared to previous devices that require external magnetic fields.

The prediction of topological states in rare-earth monopnictide compounds has attracted renewed interest. Extreme magnetoresistance (XMR) has also been observed in several nonmagnetic rare-earth monopnictide compounds. The origin of XMR in these compounds could be attributed to several mechanisms, such as topologically nontrivial electronic structures and electron-hole carrier balance. YBi is a typical rare-earth monopnictide exhibiting XMR and is expected to have a nontrivial electronic structure. In this work, we perform a direct investigation of the electronic structure of YBi by combining angle-resolved photoemission spectroscopy and theoretical calculations. Our results show that YBi is topologically trivial without the expected band inversion, and they rule out the topological effect as the cause of XMR in YBi. Furthermore, we directly observed nearly perfect electron-hole compensation in the electronic structure of YBi, which could be the primary mechanism accounting for the XMR.

In this paper, we present a quantum information framework for the entanglement behavior of the low-energy quasiparticle (QP) excitations in various quantum phases in one-dimensional (1D) systems. We first establish an exact correspondence between the correlation matrix and the QP entanglement Hamiltonian for free fermions and find an extended in-gap state in the QP entanglement Hamiltonian as a consequence of the position uncertainty of the QP. A more general understanding of such an in-gap state can be extended to a Kramers theorem for the QP entanglement Hamiltonian, which also applies to strongly interacting systems. Further, we present a set of ubiquitous entanglement spectrum features, dubbed entanglement fragmentation, conditional mutual information, and measurement-induced nonlocal entanglement for QPs in 1D symmetry protected topological phases. Our result thus provides another framework to identify different phases of matter in terms of their QP entanglement.

We theoretically investigate the features of Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between two magnetic impurities, mediated by the interfacial bound states inside a domain wall (DW). The latter separates the two regions with oppositely signed inversion symmetry broken terms in graphene and Weyl semimetal. The DW is modeled by a smooth quantum well which hosts a number of discrete bound states including a pair of gapless, metallic modes with opposite chiralities. We find clear signatures of these interfacial chiral bound states in spin response (RKKY exchange interaction) which is robust to the deformation of the quantum well.

The relative twist angle in heterostructures of two-dimensional materials with similar lattice constants results in a dramatic alteration of the electronic properties. Here, we investigate the electrical and magnetotransport properties in bilayer graphene encapsulated between two hexagonal boron nitride (hBN) crystals, where the top and bottom hBN are rotationally aligned with the bilayer graphene with a twist angle ${\theta }_t\sim 0^{\circ }\text{and}{\theta }_b<1^{\circ }$, respectively. This results in the formation of two moiré superlattices, with the appearance of satellite resistivity peaks at carrier densities $n_{s1}$ and $n_{s2}$, in both hole- and electron-doped regions, together with the resistivity peak at zero carrier density. Furthermore, we measure the temperature ($T$) dependence of the resistivity ($\rho$). The resistivity shows a linear increment with temperature within the range 10 to 50 K for the density regime $n_{s1}<n<n_{s2}$ with a large slope $d\rho /dT\sim 8.5\mathrm{\Omega }/\mathrm{K}$. The large slope of $d\rho /dT$ is attributed to the enhanced electron-phonon coupling arising due to the suppression of Fermi velocity in the reconstructed minibands, which was theoretically predicted recently in doubly aligned graphene with top and bottom hBN. Our result establishes the ability of the doubly aligned moiré system to tune the strength of electron-phonon coupling and to modify the electronic properties of multilayered heterostructures.

The interplay between structural phase and electronic correlations has been an intriguing topic of research. A prominent example is the pressure-induced uncollapsed to collapsed tetragonal phase transition observed in ${\mathrm{CaFe}}_2{\mathrm{As}}_2$, which is accompanied with the emergence of superconductivity in the collapsed phase. Recently, a very similar structural phase transition was discovered in ${\mathrm{LaFe}}_2{\mathrm{As}}_2$, but in contrast to ${\mathrm{CaFe}}_2{\mathrm{As}}_2$, superconductivity was only observed in the uncollapsed phase, not the collapsed phase. Previous studies have attributed this puzzling observation to the differences in the two materials' band coherence, orbital occupation, and Fermi-surface topology. Here, we present a comparative study of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ using the density functional $\text{theory}+\text{dynamical}$ mean-field theory method. Surprisingly, we find that although La appears to have a valence higher than Ca, the doped one electron actually primarily resides on the La site. This leads to almost the same total $\mathrm{Fe}\text{−}3d$ occupancy and electronic correlation strength as well as a similar Lifshiftz transition in the Fermi-surface topology for the two materials. In addition, we show that the two materials in both structural phases belong to the category of Hund's metals. Our results indicate that the electronic structures of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ are not too different, which further suggests that superconductivity might also be induced in the collapsed phase of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ under similar nonhydrostatic conditions as for ${\mathrm{CaFe}}_2{\mathrm{As}}_2$.

We investigate the adiabatic topological charge pumping in a topological superconductor utilizing a quantum anomalous Hall insulator proximity coupled to an $s$-wave superconductor. We show that topological pumping is characterized by the appearance of protected Majorana edge states during the course of a pumping cycle. In a topological superconductor with a single Majorana edge state, the Majorana state will be pushed into the electrode in a cycle. This leads to a half-integer quantized pumped charge, because a Majorana fermion can be viewed as half of a fermion. The half-integer quantized charge pumping would serve as a fingerprint of a Majorana fermion.

Experiments on graphene bilayers, where the top layer is rotated with respect to the one below, have displayed insulating behavior when the moiré bands are partially filled. We calculate the charge distributions in these phases, and estimate the excitation gaps.

We establish the universal torus low-energy spectra at the free Dirac fixed point and at the strongly coupled chiral Ising fixed point and their subtle crossover behavior in the Gross-Neuveu-Yukawa field theory with $n_{\text{D}}=4$ component Dirac spinors in $D=(2+1)$ dimensions. These fixed points and the field theories are directly relevant for the long-wavelength physics of certain interacting Dirac systems, such as repulsive spinless fermions on the honeycomb lattice or $\pi$-flux square lattice. The torus energy spectrum has been shown previously to serve as a characteristic fingerprint of relativistic fixed points and is a powerful tool to discriminate quantum critical behavior in numerical simulations. Here, we use a combination of exact diagonalization and quantum Monte Carlo simulations of strongly interacting fermionic lattice models, to compute the critical torus energy spectrum on finite-size clusters with periodic boundaries and extrapolate them to the thermodynamic limit. Additionally, we compute the torus energy spectrum analytically using the perturbative expansion in $\epsilon =4-D$, which is in good agreement with the numerical results, thereby validating the presence of the chiral Ising fixed point in the lattice models at hand. We show that the strong interaction between the spinor field and the scalar order-parameter field strongly influences the critical torus energy spectrum and we observe prominent multiplicity features related to an emergent symmetry predicted from the quantum field theory. Building on these results we are able to address the subtle crossover physics of the low-energy spectrum flowing from the chiral Ising fixed point to the Dirac fixed point, and analyze earlier flawed attempts to extract Fermi velocity renormalizations from the low-energy spectrum.

Thermomagnetic and inelastic neutron-scattering measurements on ${\mathrm{Na}}_2\mathrm{Pr}{\mathrm{O}}_3$ are reported. This material is an antiferromagnetic honeycomb magnet based on the tetravalent lanthanide ${\mathrm{Pr}}^{4+}$ and has been proposed to host dominant antiferromagnetic Kitaev interactions. These measurements reveal magnetic fluctuations in ${\mathrm{Na}}_2\mathrm{Pr}{\mathrm{O}}_3$ below an energy of 2 meV as well as crystal-field excitations around 230 meV. The latter energy is comparable to the scale of the spin-orbit interaction and explains both the very small effective moment of around $1.0{\mu }_{\mathrm{B}}$ per ${\mathrm{Pr}}^{4+}$ and the difficulty to uncover any static magnetic scattering below the ordering transition at $T_{\mathrm{N}}=$ 4.6 K. By comparing the low-energy magnetic excitations in ${\mathrm{Na}}_2\mathrm{Pr}{\mathrm{O}}_3$ to those of the isostructural spin-only compound, ${\mathrm{Na}}_2\mathrm{Tb}{\mathrm{O}}_3$, a microscopic model of exchange interactions is developed that implicates dominant and surprisingly large Heisenberg exchange interactions $J\approx 1.1\left(1\right)$ meV. Although antiferromagnetic Kitaev interactions with $\mathit{\text{K}}\le 0.2J$ cannot be excluded, the inelastic neutron-scattering data of ${\mathrm{Na}}_2\mathrm{Pr}{\mathrm{O}}_3$ is best explained with a $\mathrm{\Delta }=1.22$ easy-axis XXZ exchange anisotropy.

Hugoniot data for $\mathrm{Pb}{\mathrm{F}}_2$ single crystals were measured for pressures up to 1 TPa through gas gun and laser shock experiments. Experimental results show that $\mathrm{Pb}{\mathrm{F}}_2$ transforms to a less compressible phase at 29 GPa and possibly melts at 57 GPa. Quantum molecular-dynamics simulations were also performed to calculated the liquid Hugoniot for $\mathrm{Pb}{\mathrm{F}}_2$. The simulated results are in good agreement with the experimental results, supporting the high-pressure phase is liquid. The current study, combined with previous studies on $\mathrm{Ti}{\mathrm{O}}_2, \mathrm{Si}{\mathrm{O}}_2$, and $\mathrm{Ca}{\mathrm{F}}_2$, suggests that for $A{X}_2$ compounds, decreasing of Hugoniot slope can be taken as an indication of melting, and the Hugoniot slope approaches 1.2 at ultrahigh pressure.