Graphene nanoribbon quantum dot qubits have been proposed as promising candidates for quantum computing applications to overcome the spin-decoherence problems associated with typical semiconductor (e.g., GaAs) quantum dot qubits. We perform theoretical studies of the electronic structures of graphene nanoribbon quantum dots by solving the Dirac equation with appropriate boundary conditions. We then evaluate the exchange splitting based on an unrestricted Hartree-Fock method for the Dirac particles. The electronic wave function and long-range exchange coupling due to the Klein tunneling and the Coulomb interaction are calculated for various gate configurations. It is found that the exchange coupling between qubits can be significantly enhanced by the Klein tunneling effect. The implications of our results for practical qubit construction and operation are discussed.

A combination of scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, ab initio random structure searching, and density functional theory electronic structure calculations was applied to elucidate the atomic arrangement and electron band structure of the $(\mathrm{Au},\mathrm{Al})/\mathrm{Si}\left(111\right)2\times 2$ two-dimensional compound formed upon Al deposition onto the mixed $5\times 2/\sqrt{3}\times \sqrt{3}$ Au/Si(111) surface. It was found that the most stable $2\times 2$-(Au, Al) compound incorporates four Au atoms, three Al atoms, and two Si atoms per $2\times 2$ unit cell. Its atomic arrangement can be visualized as an array of meandering Au atomic chains with two-thirds of the Al atoms incorporated into the chains and one-third of the Al atoms interconnecting the chains. The compound is metallic and its electronic properties can be controlled by appropriate Al dosing since energetic location of the bands varies by $\sim 0.5\mathrm{eV}$ during increasing of Al contents. The $2\times 2$-(Au, Al) structure appears to be lacking the $C_{3v}$ symmetry typical for the hexagonal lattices. The consequence of the peculiar atomic structure of the two-dimensional alloy is spin splitting of the metallic states, which should lead to anisotropy of the current-induced in-plane spin polarization.

We present a molecular dynamics study providing evidence that it is possible to conceive efficient thermal diodes by a suitable hydrogen decoration of graphene nanoribbons. We estimate thermal rectifications at graphane/graphene interfaces with vertical, triangular, and T-shaped morphologies, and we report a significant thermal rectification up to $\sim 54\%$ for the triangular one. The dependence of the thermal rectification on the nanoribbon dimensions, vertex angle, and temperature gradient is also explored. The physical origin of the observed rectification is analyzed in terms of the different temperature dependence of the thermal conductivity in the pristine materials and the overlap of phonon densities in the different morphologies. Finally, we propose an effective continuum model to describe thermal rectification, which is only based on the steady state temperature profile rather than the actual heat flux. The model quantitatively predicts thermal rectification in very good agreement with the standard analysis based on the heat flux estimate.

Thouless's quantum adiabatic pumping is of fundamental interest to condensed-matter physics. It originally considered a zero-temperature equilibrium state uniformly occupying all the bands below a Fermi surface. In light of recent direct simulations of Thouless's concept in cold-atom systems, this paper investigates the dynamics of quantum adiabatic pumping subject to dephasing for rather general initial states with nonuniform populations and possibly interband coherence. Using a theory based on pure-dephasing Lindblad evolution, we find that the pumping is contributed by two parts of different nature: a dephasing-modified geometric part weighted by initial Bloch state populations and an interband-coherence-induced part compromised by dephasing, both of them being independent of the pumping time scale. The overall pumping reflects an interplay of the band topology, initial state populations, initial state coherence, and dephasing. Theoretical results are carefully checked in a Chern insulator model coupled to a pure-dephasing environment, providing a useful starting point to understand and coherently control quantum adiabatic pumping in general situations.

The $\alpha \text{−}{T}_3$ model is characterized by a variable Berry phase that changes continuously from $\pi$ to 0. We take advantage of this property to highlight the effects of this underlying geometrical phase on a number of physical quantities. The Hall quantization of the two limiting cases is dramatically different: a relativistic series is associated with a Berry phase of $\pi$, and a nonrelativistic series is associated with the other limit. We study the quantization of the Hall plateaus as they continuously evolve from a relativistic to a nonrelativistic regime. Additionally, we describe two physical quantities that retain knowledge of the Berry phase in the absence of a motion-inducing magnetic field. The variable Berry phase of the $\alpha \text{−}{T}_3$ model allows us to explicitly describe the Berry phase dependence of the dynamical longitudinal optical conductivity and of the angular scattering probability.

Through $s-d$ coupling, a superconducting thin film interfaced to a noncollinear magnetic insulator inherits its magnetic order, which may induce unconventional superconductivity that hosts Majorana edge states. We present a unified formalism that covers the cycloidal, helical, and tilted conical order discovered in multiferroics, as well as Bloch and Neel domain walls of ferromagnetic insulators, and show that they induce $\left(p_x+p_y\right)$-wave pairing that supports Majorana edge modes. The advantages over one-dimensional proposals are that the Majorana states can exist without fine tuning of the chemical potential, can be stabilized in a much larger parameter space, and can be separated over the distance of long-range noncollinear order that is known to reach a macroscopic scale. A skyrmion spin texture, on the other hand, induces a nonuniform $\left(p_r+i{p}_{\phi }\right)$-wave-like pairing under the influence of an emergent electromagnetic field, yielding a vortex state that displays both a bulk persistent current and a topological edge current.

We provide a route to use elastic and quasielastic neutron spin echo spectroscopy (Larmor labeling) for a unified approach to study both the static and dynamic properties of incommensurate magnetic structures with small propagation vectors. The archetypal noncentrosymmetric spiral magnet ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$ serves as a model system for our study. We show how Larmor labeling can efficiently decouple instrumental wavelength resolution and effective-$\vec{Q}$ resolution and give high-precision access to the structural properties of the incommensurate domains of ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$.

The electronic band structure of the (Ga,Mn)As system has been one of the most intriguing problems in solid state physics over the past two decades. Determination of the band structure evolution with increasing Mn concentration is a key issue to understand the origin of ferromagnetism. Here, we present room-temperature photoluminescence and ellipsometry measurements of $\mathrm{G}{\mathrm{a}}_{100\%-x}\mathrm{M}{\mathrm{n}}_x\mathrm{As}$ alloy. The upshift of the valence band is proven by the redshift of the room temperature near band-gap emission from the $\mathrm{G}{\mathrm{a}}_{100\%-x}\mathrm{M}{\mathrm{n}}_x\mathrm{As}$ alloy with increasing Mn content. It is shown that even a doping by 0.02% of Mn affects the valence-band edge, and it merges with the impurity band for a Mn concentration as low as 0.6%. Both x-ray diffraction pattern and high-resolution cross-sectional transmission electron microscopy images confirmed full recrystallization of the implanted layer and GaMnAs alloy formation.

We formulate a quantum waveguide theory of the Josephson effect in multiband superconductors, with special emphasis on iron-based materials. By generalizing the boundary conditions of the scattering problem, we first determine the Andreev levels spectrum and then derive an explicit expression for the Josephson current which generalizes the formula of the single-band case. In deriving the results, we provide a second quantization field theory, allowing us to evaluate the current-phase relation and the Josephson current fluctuations in multiband systems. We present results for two different order parameter symmetries, namely $s_{\pm }$ and $s_{++}$, which are relevant in multiband systems. The obtained results show that the $s_{\pm }$ symmetry can support $\pi$ states which are absent in the $s_{++}$ case. We also argue that there is a certain fragility of the Josephson current against phase fluctuations in the $s_{++}$ case. The temperature dependence of the Josephson critical current is also analyzed and we find, for both the order parameter symmetries, remarkable violations of the Ambegaokar-Baratoff relation. The results are relevant in view of possible experiments aimed at investigating the order parameter symmetry of multiband superconductors using mesoscopic Josephson junctions.

Recent experiments show that charge-density-wave correlations are prevalent in underdoped cuprate superconductors. The correlations are short ranged at weak magnetic fields but their intensity and spatial extent increase rapidly at low temperatures beyond a crossover field. Here we consider the possibility of long-range charge-density-wave order in a model of a layered system where such order competes with superconductivity. We show that in the clean limit, low-temperature long-range order is stabilized by arbitrarily weak magnetic fields. This apparent discrepancy with the experiments is resolved by the presence of disorder. Like the field, disorder nucleates halos of charge-density wave, but unlike the former it also disrupts interhalo coherence, leading to a correlation length that is always finite. Our results are compatible with various experimental trends, including the onset of longer range correlations induced by interlayer coupling above a characteristic field scale.

We study the dynamics of the half-filled zeroth Landau level of Dirac fermions using mirror symmetry, a supersymmetric duality between certain pairs of $2+1$-dimensional theories. We show that the half-filled zeroth Landau level of a pair of Dirac fermions is dual to a pair of Fermi surfaces of electrically neutral composite fermions, coupled to an emergent gauge field. Thus, we use supersymmetry to provide a derivation of flux attachment and the emergent Fermi-liquid-like state for the lowest Landau level of Dirac fermions. We find that in the dual theory the Coulomb interaction induces a dynamical exponent $z=2$ for the emergent gauge field, making the interactions classically marginal. This enables us to map the problem of $2+1$-dimensional Dirac fermions in a finite transverse magnetic field, interacting via a strong Coulomb interaction, into a perturbatively controlled model. We analyze the resulting low-energy theory using the renormalization group and determine the nature of the BCS interaction in the emergent composite Fermi liquid.

In this work we propose a simple model that supports Chern semimetals. These gapless topological phases share several properties with the Chern insulators like a well-defined Chern number associated with each band, topologically protected edge states and topological phase transitions that occur when the bands touch each, with linear dispersion around the contact points. The tight-binding model, defined on the Lieb lattice with intra-unit-cell and suitable nearest-neighbor hopping terms between three different species of spinless fermions, supports a single Dirac-like point. The dispersion relation around this point is fully relativistic and the $3\times 3$ matrices in the corresponding effective Hamiltonian satisfy the Duffin-Kemmer-Petiau algebra. We show the robustness of the topologically protected edge states by employing the entanglement spectrum. Moreover, we prove that the Chern number of the lowest band is robust with respect to weak disorder. For its simplicity, our model can be naturally implemented in real physical systems like cold atoms in optical lattices.

We study the electronic and magnetic structures of the newly discovered compound ${\mathrm{KCr}}_3{\mathrm{As}}_3$. The nonmagnetic state has five Fermi surface sheets involving three quasi-one-dimensional and two three-dimensional energy bands. However, the ground state is magnetic, exhibiting an interlayer antiferromagnetic order where the basic block-spin state of a unit Cr triangle retains a high spin magnitude. Moreover, the magnetic Fermi surface involves three one-dimensional sheets only, manifesting the reduced dimensionality. By fitting a twisted spin tube model, the magnetic frustrations are found to be relaxed, leading to gapless spin excitations. A frustration-induced transition to the disordered low block-spin state is expected upon increasing the intralayer exchange interaction.

The Hikami-Larkin-Nagaoka (HLN) formula [S. Hikami, A. I. Larkin, and Y. Nagaoka, Prog. Theor. Phys. 63, 707 (1980)] describes the quantum corrections to the magnetoconductivity of a quasi-2D electron gas (quasi-2DEG) with parabolic dispersion. It predicts a crossover from weak localization to antilocalization as a function of the strength of scattering off spin-orbit impurities. Here, we derive the conductivity correction for massless Dirac fermions in 3D topological insulators (TIs) in the presence of spin-orbit impurities. We show that this correction is always positive and therefore we predict weak antilocalization for every value of the spin-orbit disorder. Furthermore, the correction to the diffusion constant is surprisingly linear in the strength of the impurity spin-orbit. Our results call for a reinterpretation of experimental fits for the magnetoconductivity of 3D TIs, which have so far used the standard HLN formula.

We investigate the early stages of Pt nanocluster formation on $\mathrm{SrTi}{\mathrm{O}}_3$ (001) and the emergence of the Schottky barrier at the interface as a function of submonolayer Pt coverage using density functional theory. We find that there is a critical coverage of a 3/8 monolayer (ML), below which the Pt-oxygen interaction is dominant, leading to an ordered reconstructed surface with local Pt(II)-oxide structures. Beyond the critical coverage of 3/8 ML, the system undergoes a crossover at which the Pt-Pt interaction becomes dominant, leading to the formation of closed-packed hexagonal Pt nanoclusters. We show that cluster-induced gap states that pin the Fermi level at 1.3 eV above the valence band edge emerge at the same time as this crossover, thus showing the critical role it plays in determining the Schottky barrier height of the $\mathrm{Pt}/\mathrm{SrTi}{\mathrm{O}}_3$ (001) interface.

We demonstrate that an antiferromagnet can be employed for a highly efficient electrical manipulation of a ferromagnet. In our study, we use an electrical detection technique of the ferromagnetic resonance driven by an in-plane ac current in a NiFe/IrMn bilayer. At room temperature, we observe antidampinglike spin torque acting on the NiFe ferromagnet, generated by an in-plane current driven through the IrMn antiferromagnet. A large enhancement of the torque, characterized by an effective spin-Hall angle exceeding most heavy transition metals, correlates with the presence of the exchange-bias field at the NiFe/IrMn interface. It highlights that, in addition to the strong spin-orbit coupling, the antiferromagnetic order in IrMn governs the observed phenomenon.

We present an ab initio theory of the Gilbert damping in substitutionally disordered ferromagnetic alloys. The theory rests on introduced nonlocal torques which replace traditional local torque operators in the well-known torque-correlation formula and which can be formulated within the atomic-sphere approximation. The formalism is sketched in a simple tight-binding model and worked out in detail in the relativistic tight-binding linear muffin-tin orbital method and the coherent potential approximation (CPA). The resulting nonlocal torques are represented by nonrandom, non-site-diagonal, and spin-independent matrices, which simplifies the configuration averaging. The CPA-vertex corrections play a crucial role for the internal consistency of the theory and for its exact equivalence to other first-principles approaches based on the random local torques. This equivalence is also illustrated by the calculated Gilbert damping parameters for binary NiFe and FeCo random alloys, for pure iron with a model atomic-level disorder, and for stoichiometric FePt alloys with a varying degree of $L{1}_0$ atomic long-range order.

We investigate the statistical orthogonality catastrophe (STOC) in single-particle and many-body localized systems by studying the response of the many-body ground state to a local quench. Using scaling arguments and exact numerical calculations, we establish that the STOC gives rise to a wave function overlap between the pre- and postquench ground states that has an exponential decay with the system size, in sharp contrast to the well-known power law Anderson orthogonality catastrophe in metallic systems. This exponential decay arises from a statistical charge transfer process where a particle can be effectively “transported” to an arbitrary lattice site. In a many-body localized phase, this nonlocal transport and the associated exponential STOC phenomenon persist in the presence of interactions. We study the possible experimental consequences of the exponential STOC on the Loschmidt echo and spectral function, establishing that this phenomenon might be observable in cold atomic experiments through Ramsey interference and radio-frequency spectroscopy.

We have studied the influence of electron-electron interaction on the fractal butterfly spectrum of Dirac fermions in biased bilayer graphene in the fractional quantum Hall effect (FQHE) regime. We demonstrate that the butterfly spectrum exhibits remarkable phase transitions between the FQHE gap and the butterfly gap for chiral electrons in bilayer graphene, when the periodic potential strength or the bias voltage is varied. We also find that, in addition to those phase transitions, by varying the bias voltage one can effectively control the periodic potential strength experienced by the electrons. The electron-electron interaction causes the butterfly spectrum to exhibit new gaps inside the Bloch sub-bands not found in the single-particle case. We expect that both the observed phase transition and other new features in the butterfly spectrum of interacting Dirac fermions will be of great interest to researchers from diverse fields.

Large exciton binding energies are a distinguishing feature of two-dimensional semiconductors because of reduced screening, potentially leading to unique optoelectronic applications. Here we use electronic structure methods to calculate the properties of a two-dimensional material class: group-IV monosulfides including SiS, GeS, and SnS. Bulk SiS is predicted to be a metastable layered material. Quasiparticle excitations are calculated with the $G_0{W}_0$ method and the Bethe-Salpeter equation is are used to include electron-hole interactions. For monolayers, strongly bound excitons are found below the quasiparticle absorption edge. The predicted excitonic binding energies are as high as 0.7 eV. Due to large excitonic effects, these group-IV sulfide monolayers have great potential for nanoscale optoelectronic applications.