We report electron-hole conduction asymmetry in monolayer graphene. Previously, it has been claimed that electron-hole conduction asymmetry is due to imbalanced carrier injection from metallic electrodes. Here, we show that metallic contacts have negligible impact on asymmetric conduction and may be either sample or device-dependent phenomena. Electrical measurements show that monolayer graphene based devices exhibit suppressed electron conduction compared to hole conduction due to the presence of donor impurities which scatter electrons more efficiently. This can be explained by the relativistic nature of charge carriers in a graphene monolayer and can be reconciled with the fact that in a relativistic quantum system transport cross section does depend on the sign of scattering potential in contrast to a nonrelativistic quantum system.

We experimentally investigate Andreev transport through the interface between an indium superconductor and the edge of the InAs/GaSb bilayer. To cover all possible regimes of the InAs/GaSb spectrum, we study samples with 10-nm-, 12-nm-, and 14-nm-thick InAs quantum wells. For the trivial case of a direct band insulator in 10 nm samples, differential resistance demonstrates standard Andreev reflection. For InAs/GaSb structures with band inversion (12 and 14 nm samples), we observe distinct low-energy structures, which we regard as direct evidence for the proximity-induced superconductivity within the current-carrying edge state. For 14 nm InAs well samples, we additionally observe mesoscopiclike resistance fluctuations, which are subjected to threshold suppression in low magnetic fields.

Spin-orbit (SO) splitting in the conduction-band minimum (CBM) of monolayer $\mathrm{W}{\mathrm{S}}_2$ plays a pivotal role in spintronics for spin-valley coupled electron. Using first-principles calculation, a large SO splitting at the K point in the CBM is achieved in halogen doped monolayer $\mathrm{W}{\mathrm{S}}_2$ (83.55 meV for F doped 4 × 4 × 1 supercell) because of the strong spin-orbit coupling induced by the asymmetric electric field. We further clarify that the asymmetric electric field originates from the stronger trigonal prismatic ligand field and asymmetric surface charge distribution incurred by halogen doping. More importantly, halogen doping could be used to lift the degeneracy of K and K' valleys in the CBM of monolayer $\mathrm{W}{\mathrm{S}}_2$ by breaking the time-reversal symmetry. This research proposes a feasible method to enlarge the SO splitting in the CBM of transition-metal dichalcogenides, advancing their application in valley spintronic devices.

It is known that the interplay between strong spin-orbit coupling and broken inversion symmetry can lead to chiral configurations in low-dimensional magnetic systems deposited on heavy-element surfaces. This phenomenon is due to the Dzyaloshinskii-Moriya interaction, which favours noncollinear magnetic structures. Here we show by ab initio simulations that this interaction leads to a twisting of the two antiferromagnetically coupled edge-state spins of a zigzag graphene nanoribbon deposited on the topological insulator ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. To our knowledge, this is the first report of a chiral magnetic structure formed by a pair of one-dimensional states. This effect results in a finite net magnetization in the nanoribbons, which could lead to applications such as spin filters in graphene-based spintronics devices.

A method is presented for describing vibrational effects in x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) using a combination of the classical Franck-Condon (FC) approximation and classical trajectories run on the core-excited state. The formulation of RIXS is an extension of the semiclassical Kramers-Heisenberg formalism of Ljungberg et al. [Phys. Rev. B 82, 245115 (2010)] to the resonant case, retaining approximately the same computational cost. To overcome difficulties with connecting the absorption and emission processes in RIXS, the classical FC approximation is used for the absorption, which is seen to work well provided that a zero-point-energy correction is included. In the case of core-excited states with dissociative character, the method is capable of closely reproducing the main features for one-dimensional test systems, compared to the quantum-mechanical formulation. Due to the good accuracy combined with the relatively low computational cost, the method has great potential of being used for complex systems with many degrees of freedom, such as liquids and surface adsorbates.

Fe $L_{2,3}$-edge x-ray absorption spectra (XAS) and magnetic circular dichroism (MCD) in resonant inelastic x-ray scattering (RIXS) of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ were measured to identify the electronic structure responsible for its weak ferromagnetism caused by the Dzyaloshinskii-Moriya interaction (DMI) at room temperature. In contrast to negligible MCD in XAS, MCD in RIXS (RIXS-MCD) was clearly observed in the $dd$ excitation at 1.8 eV via excitation to charge-transfer states. Furthermore, RIXS-MCD showed a crystal orientation dependence, indicating that the observed RIXS-MCD originated from DMI. The observed RIXS-MCD is well described by ab initio charge-transfer multiplet calculations, revealing that the RIXS-MCD derives from spin flip excitations at delocalized $e_{\mathrm{g}}$ orbitals. By the combination of the experiments and calculations, RIXS-MCD has unraveled that the origin of DMI in $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ is the $e_{\mathrm{g}}$ orbitals, which are strongly hybridized with the $2p$ orbitals of oxygen atoms. The results demonstrate the importance of RIXS-MCD for identifying the electronic structure related to DMI.

Measurements of the Raman spectra in ${\mathrm{MgB}}_2$ up to 750 K are presented. The width of the $E_{2g}$ phonon line shows a pronounced maximum in the range of 350–400 K. In the same temperature range the frequency exhibits a sharp softening. The $E_{2g}$ phonon line is superimposed on the electronic background, whose frequency also strongly depends on temperature. The phonon self-energies and spectra of electronic Raman scattering were calculated, taking into account a renormalization of ab initio electronic structure due to an electron-phonon scattering. We obtained a satisfactory description of the temperature dependence of the phonon linewidth and proved that it is mainly determined by nonadiabatic effects. Since the wave vector of the $E_{2g}$ phonon is in the nonadiabatic range, the contribution of electrons to phonon damping dominates and determines its nonmonotonic behavior as a function of temperature. When the anharmonic contributions to the phonon frequency are taken into account, the total shift of the $E_{2g}$ phonon energy agrees with the experiment at 300 K, but the estimates of the anharmonic terms in the high-temperature range need further clarification.

Using a multiple scattering theory algorithm, we investigate numerically the transmission of ultrasonic waves through a disordered locally resonant metamaterial containing only monopolar resonators. By comparing the cases of a perfectly random medium with its pair correlated counterpart, we show that the introduction of short range correlation can substantially impact the effective parameters of the sample. We report, notably, the opening of an acoustic transparency window in the region of the hybridization band gap. Interestingly, the transparency window is found to be associated with negative values of both effective compressibility and density. Despite this feature being unexpected for a disordered medium of monopolar resonators, we show that it can be fully described analytically and that it gives rise to negative refraction of waves.

A linear unsaturating magnetoresistance at high perpendicular magnetic fields, together with a quadratic positive magnetoresistance at low fields, has been seen in many different experimental materials, ranging from silver chalcogenides and thin films of InSb to topological materials like graphene and Dirac semimetals. In the literature, two very different theoretical approaches have been used to explain this classical magnetoresistance as a consequence of sample disorder. The phenomenological random resistor network model constructs a grid of four terminal resistors, each with a varying random resistance. The effective medium theory model imagines a smoothly varying disorder potential that causes a continuous variation of the local conductivity. Here, we demonstrate numerically that both models belong to the same universality class and that a restricted class of the random resistor network is actually equivalent to the effective medium theory. Both models are also in good agreement with experiments on a diverse range of materials. Moreover, we show that in both cases, a single parameter, i.e., the ratio of the fluctuations in the carrier density to the average carrier density, completely determines the magnetoresistance profile.

Magnetic droplets are dynamical solitons that can be generated by locally suppressing the dynamical damping in magnetic films with perpendicular anisotropy. To date, droplets have been observed only in nanocontact spin-torque oscillators operated by spin-polarized electrical currents. Here, we experimentally demonstrate that magnetic droplets can be nucleated and sustained by pure spin currents in nanoconstriction-based spin Hall devices. Micromagnetic simulations support our interpretation of the data, and indicate that in addition to the stationary droplets, propagating solitons can be also generated in the studied system, which can be utilized for the information transmission in spintronic applications.

We present a study of the lower critical field, $H_{c1}$, of ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ as a function of carrier concentration with the aim of quantifying the superfluid density. At low carrier concentration (i.e., the underdoped side), superfluid density and the carrier concentration in the normal state are equal within experimental margin. A significant deviation between the two numbers starts at optimal doping and gradually increases with doping. The inverse of the penetration depth and the critical temperature follow parallel evolutions as in the case of cuprate superconductors. In the overdoped regime, the zero-temperature superfluid density becomes much lower than the normal-state carrier density before vanishing all together. We show that the density mismatch and the clean-to-dirty crossover are concomitant. Our results imply that the discrepancy between normal and superconducting densities is expected whenever the superconducting gap becomes small enough to put the system in the dirty limit. A quantitative test of the dirty BCS theory is not straightforward, due to the multiplicity of the bands in superconducting strontium titanate.

We devise a functional renormalization group treatment for a chain of interacting spinless fermions which is correct up to second order in interaction strength. We treat both inhomogeneous systems in real space as well as the translationally invariant case in a $k$-space formalism. The strengths and shortcomings of the different schemes as well as technical details of their implementation are discussed. We use the method to study two proof-of-principle problems in the realm of Luttinger liquid physics, namely, reflection at interfaces and power laws in the occupation number as a function of crystal momentum.

We use linked-cluster expansions to analyze the quantum phase transitions between symmetry-unbroken trivial and topological Haldane phases in two different spin-one chains. The first model is the spin-one Heisenberg chain in the presence of a single-ion anisotropy, while the second one is the dimerized spin-one Heisenberg chain. For both models, we determine the ground-state energy and the one-particle gap inside the nontopological phase as a high-order series using perturbative continuous unitary transformations. Extrapolations of the gap series are applied to locate the quantum critical point and to extract the associated critical exponent. We find that this approach works unsatisfactorily for the anisotropic chain, since the quality of the extrapolation appears insufficient due to the large correlation length exponent. In contrast, extrapolation schemes display very good convergence for the gap closing in the case of the dimerized spin-one Heisenberg chain.

Detailed comparative theoretical and experimental study of electronic properties and spin structure was carried out for a series of Pb-based quaternary compounds ${\mathrm{PbBi}}_2{\mathrm{Te}}_{4-x}{\mathrm{Se}}_x$. For all values of $x$, these compounds are theoretically predicted to be topological insulators, possessing at high Se content a remarkably large band gap and a Dirac point isolated from bulk states. Using spin- and angle-resolved photoemission spectroscopy, it was shown that the ${\mathrm{PbBi}}_2{\mathrm{Te}}_2{\mathrm{Se}}_2$ and ${\mathrm{PbBi}}_2{\mathrm{Te}}_{1.4}{\mathrm{Se}}_{2.6}$ compounds are characterized by well-defined spin-polarized topological surface state in the bulk gap. To define the probable distribution of atoms over the atomic sites for these samples, we performed ab initio calculations in ordered and disordered configurations of the unit cell. We found that theoretical calculations better reproduce photoemission data when Te atoms are placed in the outermost layers of the septuple layer block.

We report on the hydrogenation of $(3\times 3)/(4\times 4)$ silicene epitaxially grown on Ag(111) studied by in situ Raman spectroscopy and state-of-the-art ab initio calculations. Our results demonstrate that hydrogenation of $(3\times 3)/(4\times 4)$ silicene leads to the formation of two different atomic structures which exhibit distinct spectral vibrational modes. Raman selection rules clearly show that the Si atoms undergo a rehybridization in both cases from a mixed ${sp}^2\text{−}{sp}^3$ to a dominating ${sp}^3$ state increasing the distance between the two silicene sublattices. This results in a softening of the in-plane and a stiffening of the out-of-plane phonon modes. Nevertheless, hydrogenated epitaxial silicene retains a two-dimensional nature and hence can be considered as epitaxial silicane. The level of hydrogenation can be determined by the intensity ratio of the Raman modes with different symmetries.

A three-dimensional (3D) nodal-loop semimetal phase is exploited to engineer a number of intriguing phases featuring different peculiar topological surface states. In particular, by introducing various two-dimensional gap terms to a 3D tight-binding model of a nodal-loop semimetal, we obtain a rich variety of topological phases of great interest to ongoing theoretical and experimental studies, including a chiral insulator, degenerate-surface-loop insulator, and second-order topological insulator, as well as a Weyl semimetal with tunable Fermi arc profiles. The unique concept underlying our approach is to engineer topological surface states that inherit their dispersion relations from a gap term. The results provide one rather unified principle for the creation of novel topological phases and can guide the search for new topological materials. Two-terminal transport studies are also carried out to distinguish the engineered topological phases.

Spin-orbit coupling is a single-particle phenomenon known to generate topological order, and electron-electron interactions cause ordered many-body phases to exist. The rich interplay of these two mechanisms is present in a broad range of materials and has been the subject of considerable ongoing research and controversy. Here we demonstrate that interacting two-dimensional electron systems with strong spin-orbit coupling exhibit a variety of time reversal symmetry breaking phases with unconventional spin alignment. We first prove that a Stoner-type criterion can be formulated for the spin polarization response to an electric field, which predicts that the spin polarization susceptibility diverges at a certain value of the electron-electron interaction strength. The divergence indicates the possibility of unconventional ferromagnetic phases even in the absence of any applied electric or magnetic field. This leads us, in the second part of this work, to study interacting Rashba spin-orbit coupled semiconductors in equilibrium in the Hartree-Fock approximation as a generic minimal model. Using classical Monte Carlo simulations, we construct the complete phase diagram of the system as a function of density and spin-orbit coupling strength. It includes both an out-of-plane spin-polarized phase and in-plane spin-polarized phases with shifted Fermi surfaces and rich spin textures, reminiscent of the Pomeranchuk instability, as well as two different Fermi-liquid phases having one and two Fermi surfaces, respectively, which are separated by a Lifshitz transition. We discuss possibilities for experimental observation and useful application of these novel phases, especially in the context of electric-field-controlled macroscopic spin polarizations.

We theoretically demonstrate that the in-plane magnetization-induced quantum anomalous Hall effect (QAHE) can be realized in the atomic crystal layers of group-V elements with a buckled honeycomb lattice. Based on $s{p}^3$ tight-binding models with parameters being extracted from first-principles results, we show that for weak and strong spin-orbit couplings, the systems harbor QAHEs with Chern numbers of $\mathcal{C}=\pm 1$ and $\pm 2$, respectively. For $\mathcal{C}=\pm 1$ phases, we find the critical magnetization to realize QAHE can be extremely small by tuning the spin-orbit coupling strength. For $\mathcal{C}=\pm 2$ phases, we find that although the critical magnetization is larger, it can be decreased effectively by strain. Moreover, the band gap is large enough for a room-temperature observation. These features suggest that it is experimentally feasible to realize high-temperature QAHEs from in-plane magnetization in the atomic crystal layers of group-V elements.

We study the realization of anyon-permuting symmetries of topological phases on the lattice using tensor networks. Working on the virtual level of a projected entangled pair state, we find matrix product operators (MPOs) that realize all unitary topological symmetries for the toric and color codes. These operators act as domain walls that enact the symmetry transformation on anyons as they cross. By considering open boundary conditions for these domain wall MPOs, we show how to introduce symmetry twists and defect lines into the state.

Layered electrides, as typified by ${\mathrm{Ca}}_2\mathrm{N}$, are unconventional quasi-two-dimensional metals with low work functions, in which the conduction electrons are localized between the cation layers as well as outside the surface. We have investigated the electronic structure of the interface between a layered electride and another material, using graphene on ${\mathrm{Ca}}_2\mathrm{N}$ as an example. Our first-principles calculation shows that a graphene layer on ${\mathrm{Ca}}_2\mathrm{N}$ remains flat and is doped to an extremely high density of $n=5\times {10}^{14}{\mathrm{cm}}^{-2}$ with its Fermi level ($E_F$) aligned with the logarithmically divergent van Hove singularity (VHS) at a saddle point of graphene's ${\pi }^{*}$ band. This finding shows that graphene/${\mathrm{Ca}}_2\mathrm{N}$ is an ideal testing ground for the exploration of the many-body ground states, most notably superconducting states ($p+ip$ wave, $d$ wave, and $f$ wave), predicted to appear when $E_F$ is close to a VHS. The work function decreases abruptly upon monolayer attachment but reverts to that of ${\mathrm{Ca}}_2\mathrm{N}$ upon bilayer attachment. This peculiar behavior is explained in terms of the distinctive electronic structures of the constituent materials and their bonding.