We develop the theory of quantum friction in two-dimensional topological materials. The quantum drag force on a metallic nanoparticle moving above such systems is sensitive to the nontrivial topology of their electronic phases, shows a novel distance scaling law, and can be manipulated through doping or via the application of external fields. We use the developed framework to investigate quantum friction due to the quantum Hall effect in magnetic field biased graphene, and to topological phase transitions in the graphene family materials. It is shown that topologically nontrivial states in two-dimensional materials enable an increase of two orders of magnitude in the quantum drag force with respect to conventional neutral graphene systems.

Spin-lattice relaxation of the nuclear spin system in $p$-type GaAs is studied using a three-stage experimental protocol including optical pumping and measuring the difference of the nuclear spin polarization before and after a dark interval of variable length. This method allows us to measure the spin-lattice relaxation time $T_1$ of optically pumped nuclei “in the dark,” that is, in the absence of illumination. The measured $T_1$ values fall into the subsecond time range, being three orders of magnitude shorter than in earlier studied $n$-type GaAs. The drastic difference is further emphasized by magnetic-field and temperature dependencies of $T_1$ in $p$-GaAs, showing no similarity to those in $n$-GaAs. This unexpected behavior finds its explanation in the spatial selectivity of the optical pumping in $p$-GaAs, that is only efficient in the vicinity of shallow donors, together with the quadrupole relaxation of nuclear spins, which is induced by electric fields within closely spaced donor-acceptor pairs. The developed theoretical model explains the whole set of experimental results.

In this paper we critically reconsider the Casimir repulsion between surfaces that carry the Chern-Simons interaction (corresponding to the Hall-type conductivity). We present a derivation of the Lifshitz formula valid for arbitrary planar geometries and discuss its properties. This analysis allows us to resolve some contradictions in the previous literature. We compute the Casimir energy for two surfaces that have constant longitudinal and Hall conductivities. The repulsion is possible only if both surfaces have Hall conductivities of the same sign. However, there is a critical value of the longitudinal conductivity above which the repulsion disappears. We also consider a model where both parity odd and parity even terms in the conductivity are produced by the polarization tensor of surface modes. In contrast to the previous publications [L. Chen and S.-L. Wan, Phys. Rev. B 84, 075149 (2011); Phys. Rev. B 85, 115102 (2012)], we include the parity anomaly term. This term ensures that the conductivities vanish for infinitely massive surface modes. We find that at least for a single mode, regardless of the sign and value of its mass, there is no Casimir repulsion.

Simulations [e.g., X. W. Zhou et al., Phys. Rev. B 79, 115201 (2009)] show nonlocal effects of the ballistic/diffusive crossover. The local temperature has nonlinear spatial variation not contained in the local Fourier law $\vec{j}\left(\vec{r}\right)=-\kappa \vec{\nabla }T\left(\vec{r}\right)$. The heat current $\vec{j}\left(\vec{r}\right)$ depends not just on the local temperature gradient $\vec{\nabla }T\left(\vec{r}\right)$ but also on temperatures at points ${\vec{r}}^{\prime }$ within phonon mean free paths, which can be micrometers long. This paper uses the Peierls-Boltzmann transport theory in nonlocal form to analyze the spatial variation $\mathrm{\Delta }T\left(\vec{r}\right)$. The relaxation-time approximation (RTA) is used because the full solution is very challenging. Improved methods of extrapolation to obtain the bulk thermal conductivity $\kappa$ are proposed. Callaway invented an approximate method of correcting RTA for the $\vec{q}$ (phonon wave vector or crystal momentum) conservation of N (Normal as opposed to Umklapp) anharmonic collisions. This method is generalized to the nonlocal case where $\kappa \left(\vec{k}\right)$ depends on the wave vector of the current $\vec{j}\left(\vec{k}\right)$ and temperature gradient $i\vec{k}\mathrm{\Delta }T\left(\vec{k}\right)$.

In the present work $\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}$ (CFA) thin films were deposited by ion beam sputtering on Si (100) substrates at the optimized deposition temperature of 300 °C. A series of CFA films with different thicknesses ($t_{\mathrm{CFA}}$), 8, 10, 12, 14, 16, 18, and 20 nm, were prepared and all samples were capped with a 5-nm-thick β-Ta layer. The thickness-dependent static and dynamic properties of the films were studied by SQUID magnetometry, in-plane as well as out-of-plane broadband vector network analyzer–ferromagnetic resonance (FMR) measurements, and angle-dependent cavity FMR measurements. The saturation magnetization and the coercive field were found to be weakly thickness dependent and lie in the range 900–950 kA/m and 0.53–0.87 kA/m, respectively. The effective damping parameter (${\alpha }_{\mathrm{eff}}$) extracted from in-plane and out-of-plane FMR results reveals a $\frac{1}{t_{\mathrm{CFA}}}$ dependence, the values for the in-plane ${\alpha }_{\mathrm{eff}}$ being larger due to two-magnon scattering (TMS). The origin of the ${\alpha }_{\mathrm{eff}}$ thickness dependence is spin pumping into the nonmagnetic β-Ta layer and in the case of the in-plane ${\alpha }_{\mathrm{eff}}$, also a thickness-dependent TMS contribution. From the out-of-plane FMR results, it was possible to disentangle the different contributions to ${\alpha }_{\mathrm{eff}}$ and to the extract values for the intrinsic Gilbert damping (${\alpha }_G$) and the effective spin-mixing conductance ($g_{\mathrm{eff}}^{\uparrow \downarrow }$) of the CFA/ β-Ta interface, yielding ${\alpha }_G=\left(1.1\pm 0.2\right)\times {10}^{-3}$ and $g_{\mathrm{eff}}^{\uparrow \downarrow }=\left(2.90\pm 0.10\right)\times {10}^{19}{\mathrm{m}}^{-2}$.

The reversal of uniform axial magnetization in a ferromagnetic nanotube (FNT) has been predicted to occur through the nucleation and propagation of vortex domains forming at the ends. We provide experimental evidence for this behavior through dynamic cantilever magnetometry measurements of individual FNTs. In particular, we identify the nucleation of the vortex end domains as a function of applied magnetic field and show that they mark the onset of magnetization reversal. We find that the nucleation field depends sensitively on the angle between the end surface of the FNT and the applied field. Micromagnetic simulations substantiate the experimental results and highlight the importance of the ends in determining the reversal process. The control over end-vortex nucleation enabled by our findings is promising for the production of FNTs with tailored reversal properties.

Using the semiclassical theory of electron dynamics, we derive a gauge-invariant expression for the spin toroidization in a periodical crystal. We show that the spin toroidization is comprised of two contributions: One is due to the configuration of a classical spin array, while the other comes from the coordinate shift of the electron as spin carrier in response to the inhomogeneous magnetic field. We then establish a direct and elegant relation between our spin toroidization and the antisymmetric magnetoelectric polarizability in insulators. Finally, we demonstrate our spin toroidization in a tight-binding model and show that it is a genuine bulk quantity.

We analyze the coupling of two qubits via an epitaxial semiconducting junction. In particular, we consider three configurations that include pairs of transmons or gatemons as well as gatemonlike two qubits formed by an epitaxial four-terminal junction. These three configurations provide electrical control of the interaction between the qubits by applying voltage to a metallic gate near the semiconductor junction and can be utilized to naturally realize a controlled-Z gate (CZ). We calculate the fidelity and timing for such a CZ gate. We demonstrate that in the absence of decoherence, the CZ gate can be performed under 50 ns with gate error below ${10}^{-4}$.

It has been predicted that emergent chiral magnetic fields can be generated by crystal deformation in Weyl/Dirac metals and superconductors. The emergent fields give rise to chiral anomaly phenomena as in the case of Weyl semimetals with usual electromagnetic fields. Here, we clarify effects of the chiral magnetic field on Cooper pairs in Weyl/Dirac superconductors on the basis of the Ginzburg-Landau equation microscopically derived from the quasiclassical Eilenberger formalism. It is found that Cooper pairs are affected by the emergent chiral magnetic field in a dramatic way, and the pseudo-Lorentz force due to the chiral magnetic field stabilizes the Fulde-Ferrell state and causes a charge/spin supercurrent, which flows parallel to the chiral magnetic field in the case of Weyl/Dirac superconductors. This effect is in analogy with the chiral magnetic effect of Weyl semimetals. In addition, we elucidate that neither Meissner effect nor vortex state due to chiral magnetic fields occurs.

Thermal conductivity of three high purity synthetic single crystalline diamonds has been measured with high accuracy at temperatures from 6 to 410 K. The crystals grown by chemical vapor deposition and by high-pressure high-temperature technique demonstrate almost identical temperature dependencies $\kappa \left(T\right)$ and high values of thermal conductivity, up to 24 $\mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{K}}^{-1}$ at room temperature. At conductivity maximum near 63 K, the magnitude of thermal conductivity reaches 285 $\mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{K}}^{-1}$, the highest value ever measured for diamonds with the natural carbon isotope composition. Experimental data were fitted with the classical Callaway model for the lattice thermal conductivity. A set of expressions for the anharmonic phonon scattering processes (normal and umklapp) has been proposed which gives an excellent fit to the experimental $\kappa \left(T\right)$ data over almost the whole temperature range explored. The model provides the strong isotope effect, nearly 45%, and the high thermal conductivity ($>24$ $\mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{K}}^{-1}$) for the defect-free diamond with the natural isotopic abundance at room temperature.

The spin-momentum locking of the surface states in a three-dimensional topological insulator (TI) allows a charge current on the surface of the TI induced by an applied spin current onto the surface, which is known as the inverse Edelstein effect (IEE), that could be achieved either by injecting pure spin current by spin-pumping from a ferromagnetic metal (FM) layer or by injecting spin-polarized charge current by direct tunneling of electrons from the FM to the TI. Here, we present a theory of the observed IEE effect in a TI-FM heterostructure for the spin-polarized tunneling experiments. If an electrical current is passed from the FM to the surface of the TI, because of density-of-states polarization of the FM, an effective imbalance of spin-polarized electrons occurs on the surface of the TI. Due to the spin-momentum helical locking of the surface states in the TI, a difference of transverse charge accumulation appears on the TI surface in a direction orthogonal to the direction of the magnetization of the FM, which is measured as a voltage difference. Here, we derive the two-dimensional transport equations of electrons on the surface of a diffusive TI, coupled to a FM, starting from the quantum kinetic equation, and analytically solve the equations for a rectangular geometry to calculate the voltage difference.

We present a structural and spectroscopic study of the compound $\mathrm{N}{\mathrm{i}}_2\mathrm{MnTe}{\mathrm{O}}_6$, closely related to the polar antiferromagnet $\mathrm{N}{\mathrm{i}}_3\mathrm{Te}{\mathrm{O}}_6$ known to show a colossal magnetoelectric effect and pronounced elementary magnetoelectric excitations. We prepared single crystals and polycrystalline samples of $\mathrm{N}{\mathrm{i}}_2\mathrm{MnTe}{\mathrm{O}}_6$ showing the same polar structure as $\mathrm{N}{\mathrm{i}}_3\mathrm{Te}{\mathrm{O}}_6$ from room temperature down to 4 K with the $R3$ space-group symmetry. Magnetic and dielectric measurements have indicated an antiferromagnetic phase transition at $T_{\mathrm{N}}\approx 70\mathrm{K}$, almost 20 K higher than that of $\mathrm{N}{\mathrm{i}}_3\mathrm{Te}{\mathrm{O}}_6$. Extensive infrared, Raman, and terahertz spectroscopy experiments were employed for investigating lattice and spin excitations, revealing all phonons predicted by the factor group analysis. Terahertz spectra below $T_{\mathrm{N}}$ reveal one new excitation, which is strongly influenced by external magnetic field, thus assigned to a magnon.

Topological superconductor (TSC) hosting Majorana fermions has been established as a milestone that may shift our scientific trajectory from research to applications in topological quantum computing. Recently, superconducting Pd-Bi binaries have attracted great attention as a possible medium for the TSC phase as a result of their large spin-orbit coupling strength. Here, we report a systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) study on the normal state electronic structure of superconducting $\alpha \text{−}{\mathrm{PdBi}}_2(T_c=1.7$ K). Our results show the presence of Dirac states at higher-binding energy with the location of the Dirac point at 1.26 eV below the chemical potential at the zone center. Furthermore, the ARPES data indicate multiple band crossings at the chemical potential, consistent with the metallic behavior of $\alpha \text{−}{\mathrm{PdBi}}_2$. Our detailed experimental studies are complemented by first-principles calculations, which reveal the presence of surface Rashba states residing in the vicinity of the chemical potential. The obtained results provide an opportunity to investigate the relationship between superconductivity, topology, and the Majorana fermion, as well as explore pathways to possible future platforms for topological quantum computing.

Superconductivity in iron pnictides is unconventional and pairing may be mediated by magnetic fluctuations in the Fe sublattice. Pressure is a clean method to explore superconductivity in iron based superconductors by tuning the ground state continuously without introducing disorder. Here we present a systematic high pressure transport study in $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals with $x=0.057$, which is near the antiferromagnetic instability. Resistivity $\rho ={\rho }_0+A{T}^n$ was studied under applied pressure up to 7.90 GPa. The parameter $n$ approaches a minimum value of $n\approx 1$ at a critical pressure $P_c=3.65$ GPa. Near $P_c$, the superconducting transition temperature $T_c$ reaches a maximum value of 25.8 K. In addition, the superconducting diamagnetism at 2 K shows a sudden change around the same critical pressure. These results may be associated with a possible quantum critical point hidden inside the superconducting dome, near optimum $T_c$.

It is shown that, in $d$-dimensional systems, the vertex corrections beyond the random phase approximation (RPA) or $GW$ approximation scales with the power $d-\beta -\alpha$ of the Fermi momentum if the relation between Fermi energy and Fermi momentum is ${\epsilon }_{\mathrm{f}}\sim p_{\mathrm{f}}^{\beta }$ and the interacting potential possesses a momentum power law of $\sim p^{-\alpha }$. The condition $d-\beta -\alpha <0$ specifies systems where RPA is exact in the high-density limit. The one-dimensional structure factor is found to be the interaction-free one in the high-density limit for contact interaction. A cancellation of RPA and vertex corrections render this result valid up to second order in contact interaction. For finite-range potentials of cylindrical wires a large-scale cancellation appears and is found to be independent of the width parameter of the wire. The proposed high-density expansion agrees with the quantum Monte Carlo simulations.

Gapped quantum spin chains with symmetry $\text{PSU}\left(N\right)=\text{SU}\left(N\right)/{\mathbb{Z}}_N$ are known to possess $N$ distinct symmetry protected topological phases. Besides the trivial phase, there are $N-1$ Haldane phases which are distinguished by the occurrence of massless boundary spins. We give an expression for a universal AKLT Hamiltonian for an arbitrary symmetry group, including the case where inversion symmetry is broken. Using a diagrammatic method, we construct Hamiltonians for two of the topological classes in $\text{SU}\left(N\right)$ that arise when the physical spin is in the adjoint representation. Since our results are valid for all $N$, we also discuss the large-$N$ limit.

We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) $\mathrm{G}{\mathrm{a}}_{0.98}\mathrm{M}{\mathrm{n}}_{0.02}\mathrm{P}$ and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimental data, as well as theoretical calculations, to understand the role of the Mn dopant in the emergence of ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between $\mathrm{G}{\mathrm{a}}_{0.98}\mathrm{M}{\mathrm{n}}_{0.02}\mathrm{P}$ and GaP in both angle-resolved and angle-integrated valence spectra. The $\mathrm{G}{\mathrm{a}}_{0.98}\mathrm{M}{\mathrm{n}}_{0.02}\mathrm{P}$ bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of $\mathrm{G}{\mathrm{a}}_{0.97}\mathrm{M}{\mathrm{n}}_{0.03}\mathrm{As}$ and GaAs [Gray et al., Nat. Mater. 11, 957 (2012)], demonstrating the strong similarity between these two materials. The Mn $2p$ and $3s$ core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.

Spin-polarized two-dimensional electron gas (2DEG) at the interface of two insulating perovskite oxides has been a focus of intensive studies in recent years. So far all attempts to construct magnetic 2DEG are based on the selection of an appropriate buffer layer or cap layer in $\mathrm{SrTi}{\mathrm{O}}_3$-based heterostructures, and the magnetic effect thus produced on 2DEG is indirect and weak. Here, we fabricated the 2DEG based on Fe-doped $\mathrm{SrTi}{\mathrm{O}}_3$ that is superparamagnetic rather than diamagnetic like $\mathrm{SrTi}{\mathrm{O}}_3$. In addition to good metallicity, considerable Kondo effect, and negative magnetoresistance, the most striking observation of the present work is the occurrence of the anomalous Hall effect up to room temperature. This is transport evidence for the existence of spin-polarized 2DEG at high temperatures. As suggested by the monotonic increase of Curie temperature with carrier density, the magnetic exchange between magnetic ions could be mediated by the itinerant electrons of the 2DEG. The present work opens an avenue for the exploration of spin-polarized 2DEG.

A defining feature of a symmetry protected topological phase (SPT) in one dimension is the degeneracy of the Schmidt values for any given bipartition. For the system to go through a topological phase transition separating two SPTs, the Schmidt values must either split or cross at the critical point in order to change their degeneracies. A renormalization group (RG) approach based on this splitting or crossing is proposed, through which we obtain an RG flow that identifies the topological phase transitions in the parameter space. Our approach can be implemented numerically in an efficient manner, for example, using the matrix product state formalism, since only the largest first few Schmidt values need to be calculated with sufficient accuracy. Using several concrete models, we demonstrate that the critical points and fixed points of the RG flow coincide with the maxima and minima of the entanglement entropy, respectively, and the method can serve as a numerically efficient tool to analyze interacting SPTs in the parameter space.

Doping of silicon via phosphine exposures alternating with molecular beam epitaxy overgrowth is a path to Si:P substrates for conventional microelectronics and quantum information technologies. The technique also provides a well-controlled material for systematic studies of two-dimensional lattices with a half-filled band. We show here that for a dense ($n_s=2.8\times {10}^{14} {\mathrm{cm}}^{-2}$) disordered two-dimensional array of P atoms, the full field magnitude and angle-dependent magnetotransport is remarkably well described by classic weak localization theory with no corrections due to interaction. The two- to three-dimensional crossover seen upon warming can also be interpreted using scaling concepts developed for anistropic three-dimensional materials, which work remarkably except when the applied fields are nearly parallel to the conducting planes.