We demonstrate that the electronic band structure extracted from quasiparticle interference spectroscopy [Nat. Phys. 9, 468 (2013)] and the theoretically computed form of the superconducting gaps [Proc. Natl. Acad. Sci. USA 111, 11663 (2014)] can be used to understand the $dI/dV$ line shape measured in the normal and superconducting state of ${\mathrm{CeCoIn}}_5$ [Nat. Phys. 9, 474 (2013)]. In particular, the $dI/dV$ line shape, and the spatial structure of defect-induced impurity states, reflects the existence of multiple superconducting gaps of $d_{x^2-y^2}$ symmetry. These results strongly support a recently proposed microscopic origin of the unconventional superconducting state.

Constrained density functional theory (CDFT) is used to evaluate the energy level alignment of a benzene molecule as it approaches a graphene sheet. Within CDFT the problem is conveniently mapped onto evaluating total energy differences between different charge-separated states, and it does not consist in determining a quasiparticle spectrum. We demonstrate that the simple local density approximation provides a good description of the level alignment along the entire binding curve, with excellent agreement to experiments at an infinite separation and to $GW$ calculations close to the bonding distance. The method also allows us to explore the effects due to the presence of graphene structural defects and of multiple molecules. In general, all our results can be reproduced by a classical image charge model taking into account the finite dielectric constant of graphene.

We report the magnetic-field-dependent shift of the electron chemical potential in bulk, $n$-type GaAs at room temperature. A transient voltage of $\sim 100\mu \mathrm{V}$ was measured across a Au-${\mathrm{Al}}_2{\mathrm{O}}_3$-GaAs metal-oxide-semiconductor capacitor in a pulsed magnetic field of $\sim 6\mathrm{T}$. Several spurious voltages larger than the signal that had plagued earlier researchers performing similar experiments were carefully eliminated. The itinerant magnetic susceptibility of GaAs is extracted from the experimentally measured data for four different doping densities, including one as low as $5\times {10}^{15}{\mathrm{cm}}^{-3}$. Though the susceptibility in GaAs is dominated by Landau-Peierls diamagnetism, the experimental technique demonstrated can be a powerful tool for extracting the total free carrier magnetization of any electron system. The method is also virtually independent of the carrier concentration and is expected to work better in the nondegenerate limit. Such experiments had been successfully performed in two-dimensional electron gases at cryogenic temperatures. However, an unambiguous report on having observed this effect in any three-dimensional electron gas has been lacking. We highlight the 50 year old literature of various trials and discuss the key details of our experiment that were essential for its success. The technique can be used to unambiguously yield only the itinerant part of the magnetic susceptibility of complex materials such as magnetic semiconductors and hexaborides, and thus shed light on the origin of ferromagnetism in such systems.

GeTe, a self-doping $p$-type semiconductor where the high free hole concentration is determined by Ge vacancies is a well-known base for high-efficiency $\mathrm{A}{\mathrm{g}}_x\mathrm{S}{\mathrm{b}}_x\mathrm{G}{\mathrm{e}}_{50-2x}\mathrm{T}{\mathrm{e}}_{50}$ (a tellurium-antimony-germanium-silver series) thermoelectric materials. Here it is shown that the replacement of Ge by Ag in GeTe (a $\mathrm{A}{\mathrm{g}}_x\mathrm{G}{\mathrm{e}}_{50-x}\mathrm{T}{\mathrm{e}}_{50}$ system) significantly decreases the Seebeck coefficient, whereas the replacement by Sb $\left(\mathrm{S}{\mathrm{b}}_x\mathrm{G}{\mathrm{e}}_{50-x}\mathrm{T}{\mathrm{e}}_{50}\right)$ increases it. These effects can be attributed to a change in carrier concentration and consistent with ${}^{125}\mathrm{Te}$ NMR spin-lattice relaxation measurements and NMR signal position, which is mostly dependent on the Knight shift. Opposite changes in carrier concentration in $\mathrm{A}{\mathrm{g}}_x\mathrm{G}{\mathrm{e}}_{50-x}\mathrm{T}{\mathrm{e}}_{50}$ and $\mathrm{S}{\mathrm{b}}_x\mathrm{G}{\mathrm{e}}_{50-x}\mathrm{T}{\mathrm{e}}_{50}$ can be explained by different valence electron configurations of Ag and Sb compared to that of Ge, which results in a different local electron imbalance and/or in a change in Ge vacancy formation energy and affects the total carrier concentration. Comparison of our data for GeTe, $\mathrm{A}{\mathrm{g}}_2\mathrm{G}{\mathrm{e}}_{48}\mathrm{T}{\mathrm{e}}_{50}$, and $\mathrm{S}{\mathrm{b}}_2\mathrm{G}{\mathrm{e}}_{48}\mathrm{T}{\mathrm{e}}_{50}$ with those for $\mathrm{A}{\mathrm{g}}_2\mathrm{S}{\mathrm{b}}_2\mathrm{G}{\mathrm{e}}_{46}\mathrm{T}{\mathrm{e}}_{50}$ shows that the effects from Ag and Sb compensate for each other and supports the formation of $[\mathrm{Ag}+\mathrm{Sb}]$ atomic pairs suggested earlier based on theoretical calculations.

The light-matter coupling between cyclotron transition and photon is theoretically investigated in a monolayer $\mathrm{Mo}{\mathrm{S}}_2$ system with consideration of the influence of electron-hole asymmetry. The results show that ultrastrong light-matter coupling can be achieved at a high filling factor of Landau levels. Furthermore, we show that, in contrast to the case for conventional semiconductor resonators, the $\mathrm{Mo}{\mathrm{S}}_2$ system shows a vacuum instability. In a monolayer $\mathrm{Mo}{\mathrm{S}}_2$ resonator, the diamagnetic term can still play an important role in determining magnetopolariton dispersion, which is different from a monolayer graphene system. The diamagnetic term arises from electron-hole asymmetry, which indicates that electron-hole asymmetry can influence the quantum phase transition. Our study provides new insights in cavity-controlled magnetotransport in the $\mathrm{Mo}{\mathrm{S}}_2$ system, which could lead to the development of polariton-based devices.

We present a fast and stable numerical technique to obtain the self-energy terms of electrodes for first-principles electron transport calculations. Although first-principles calculations based on the real-space finite-difference method are advantageous for execution on massively parallel computers, large-scale transport calculations are hampered by the computational cost and numerical instability of the computation of the self-energy terms. Using the orthogonal complement vectors of the space spanned by the generalized Bloch waves that actually contribute to transport phenomena, the computational accuracy of transport properties is significantly improved with a moderate computational cost. To demonstrate the efficiency of the present technique, the electron transport properties of a Stone-Wales (SW) defect in graphene and silicene are examined. The resonance scattering of the SW defect is observed in the conductance spectrum of silicene since the ${\sigma }^{*}$ state of silicene lies near the Fermi energy. In addition, we found that one conduction channel is sensitive to a defect near the Fermi energy, while the other channel is hardly affected. This characteristic behavior of the conduction channels is interpreted in terms of the bonding network between the bilattices of the honeycomb structure in the formation of the SW defect. The present technique enables us to distinguish the different behaviors of the two conduction channels in graphene and silicene owing to its excellent accuracy.

We show that the interaction of surface plasmons with quantum emitters can be described by an effective model that has the same structure as a lossy multimode cavity quantum electromagnetic interaction. This allows the coherent manipulation of quantum emitters dressed by surface plasmons at the nanoscale. We show that strong coupling in quantum plasmonics can be used to mediate efficiently the interaction between emitters via a decoherence-free channel, immune to the strong plasmon dissipation. Efficient and robust population transfer, as well as the deterministic generation of entanglement between emitters are numerically shown. These results pave the way for an efficient use of the quantum plasmonic platform beyond its inherent losses.

We present first-principles calculations of the electronic band structure and spin-orbit effects in graphene functionalized with methyl molecules in dense and dilute limits. The dense limit is represented by a $2\times 2$ graphene supercell functionalized with one methyl admolecule. The calculated spin-orbit splittings are up to $0.6$ meV. The dilute limit is deduced by investigating a large, $7\times 7$, supercell with one methyl admolecule. The electronic band structure of this supercell is fitted to a symmetry-derived effective Hamiltonian, allowing us to extract specific hopping parameters including intrinsic, Rashba, and pseudospin inversion asymmetry spin-orbit terms. These proximity-induced spin-orbit parameters have magnitudes of about 1 meV, giant compared to pristine graphene whose intrinsic spin-orbit coupling is about $10\mu \mathrm{eV}$. We find that the origin of this giant local enhancement is the $s{p}^3$ corrugation and the breaking of local pseudospin inversion symmetry, as in the case of hydrogen adatoms. Similarly to hydrogen, also methyl acts as a resonant scatterer, with a narrow resonance peak near the charge neutrality point. We also calculate STM-like images showing the local charge densities at different energies around methyl on graphene.

The Wang-Landau Monte Carlo algorithm is implemented within an effective Hamiltonian approach and applied to ${\mathrm{BaTiO}}_3$ bulk. The density of states obtained by this approach allows a highly accurate and straightforward calculation of various thermodynamic properties, including phase transition temperatures, as well as polarization, dielectric susceptibility, specific heat, and electrocaloric coefficient at any temperature. This approach yields rather smooth data even near phase transitions and provides direct access to entropy and free energy, which allow us to compute properties that are typically unaccessible by atomistic simulations. Examples of such latter properties are the nature (i.e., first order versus second order) of the phase transitions for different supercell sizes and the thermodynamic limit of the Curie temperature and latent heat.

The spin-orbit interaction is an inherent part of magnetism, which links up the independent world of spins to the atomic lattice, thus controlling many functional properties of magnetic materials. In the widely used $3d$ transition metal ferromagnetic films, the spin-orbit interaction is relatively weak, due to low atomic number. Here we show that it is possible to enhance and tune the spin-orbit interaction by adding $5d$ platinum dopants into permalloy (${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$) thin films by a cosputtering technique. This is achieved without significant changes of the magnetic properties, due to the vicinity of Pt to meeting the Stoner criterion for the ferromagnetic state. The spin-orbit interaction is investigated by means of transport measurements (the anisotropic magnetoresistance and anomalous Hall effect), ferromagnetic resonance measurements to determine the Gilbert damping, as well as by measuring the x-ray magnetic circular dichroism at the $L_3$ and $L_2$ x-ray absorption edges to reveal the ratio of orbital to spin magnetic moments. It is shown that the effective spin-orbit interaction increases with Pt concentration within the 0%–10% Pt concentration range in a way that is consistent with theoretical expectations for all four measurements.

We investigated the composition dependence of the magnetic susceptibility of the quasicrystal approximant Au-Al-Gd. A composition-driven ferromagnetic transition is observed in a quasicrystal approximant, which is attributed to the Ruderman-Kittel-Kasuya-Yosida (RKKY) oscillation via a variation in the Fermi wave vector. The ferromagnetic transition is most simply understood as a result of the close matching of the nearest and second-nearest spin distances with the maximum positions of the RKKY potential. The present work provides an idea that allows us to tailor the magnetic order via the electron concentration in quasicrystal approximants as well as in quasicrystals.

We determined the magnetic $B-T$ phase diagram of PdFe bilayer on $\mathrm{Ir}\left(111\right)$ surface by performing Monte Carlo and spin dynamics simulations based on an effective classical spin model. The parameters of the spin model were determined by ab initio methods. At low temperatures we found three types of ordered phases, while at higher temperatures, below the completely disordered paramagnetic phase, a large region of the phase diagram is associated with a fluctuation-disordered phase. Within the applied model, this state is characterized by the presence of skyrmions with finite lifetime. According to the simulations, this lifetime follows the Arrhenius law as a function of temperature.

Temperature dependent measurements of ${}^{57}\mathrm{Fe}$ Mössbauer spectra on ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ single crystals in the tetragonal and collapsed tetragonal phases are reported. Clear features in the temperature dependencies of the isomer shift, relative spectra area, and quadrupole splitting are observed at the transition from the tetragonal to the collapsed tetragonal phase. From the temperature dependent isomer shift and spectral area data, an average stiffening of the phonon modes in the collapsed tetragonal phase is inferred. The quadrupole splitting increases by $\sim 25\%$ on cooling from room temperature to $\sim 100$ K in the tetragonal phase and is only weakly temperature dependent at low temperatures in the collapsed tetragonal phase, in agreement with the anisotropic thermal expansion in this material. In order to gain microscopic insight about these measurements, we perform ab initio density functional theory calculations of the electric field gradient and the electron density of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ in both phases. By comparing the experimental data with the calculations we are able to fully characterize the crystal structure of the samples in the collapsed-tetragonal phase through determination of the As $z$ coordinate. Based on the obtained temperature dependent structural data we are able to propose charge saturation of the Fe-As bond region as the mechanism behind the stabilization of the collapsed-tetragonal phase at ambient pressure.

Using polarized neutron diffraction and x-ray resonant magnetic scattering (XRMS) techniques, multiple phase transitions were revealed in an underdoped, nonsuperconducting $\mathrm{Eu}({\mathrm{Fe}}_{1-x}{\mathrm{Ir}}_x){}_2{\mathrm{As}}_2$ $\left(x=0.06\right)$ single crystal. Compared with the parent compound ${\mathrm{EuFe}}_2{\mathrm{As}}_2$, the tetragonal-to-orthorhombic structural phase transition and the antiferromagnetic order of the $\mathrm{Fe}^{2+}$ moments are significantly suppressed to $T_S=111\left(2\right)$ K and $T_{\text{N,Fe}}=85\left(2\right)$ K by 6% Ir doping, respectively. In addition, the $\mathrm{Eu}^{2+}$ spins order within the $ab$ plane in the A-type antiferromagnetic structure similar to the parent compound. However, the order temperature is evidently suppressed to $T_{\text{N,Eu}}=16.0\left(5\right)$ K by Ir doping. Most strikingly, the XRMS measurements at the Ir $L_3$ edge demonstrates that the Ir $5d$ states are also magnetically polarized, with the same propagation vector as the magnetic order of Fe. With $T_{\text{N,Ir}}=12.0\left(5\right)$ K, they feature a much lower onset temperature compared with $T_{\text{N,Fe}}$. Our observation suggests that the magnetism of the Eu sublattice has a considerable effect on the magnetic nature of the $5d$ Ir dopant atoms and there exists a possible interplay between the localized $\mathrm{Eu}^{2+}$ moments and the conduction $d$ electrons on the FeAs layers.

We investigate the effects of magnetic and nonmagnetic impurities on the two-dimensional surface states of three-dimensional topological insulators (TIs). Modeling weak and strong TIs using a generic four-band Hamiltonian, which allows for a breaking of inversion and time-reversal symmetries and takes into account random local potentials as well as the Zeeman and orbital effects of external magnetic fields, we compute the local density of states, the single-particle spectral function, and the conductance for a (contacted) slab geometry by numerically exact techniques based on kernel polynomial expansion and Green's function approaches. We show that bulk disorder refills the surface-state Dirac gap induced by a homogeneous magnetic field with states, whereas orbital (Peierls-phase) disorder preserves the gap feature. The former effect is more pronounced in weak TIs than in strong TIs. At moderate randomness, disorder-induced conducting channels appear in the surface layer, promoting diffusive metallicity. Random Zeeman fields rapidly destroy any conducting surface states. Imprinting quantum dots on a TI's surface, we demonstrate that carrier transport can be easily tuned by varying the gate voltage, even to the point where quasibound dot states may appear.

Quantum spin models have been studied extensively in one and higher dimensions. Furthermore, these systems have been doped with holes to study $t\text{−}J$ models of $\mathrm{SU}\left(2\right)$ spin-1/2. Their anyonic counterparts can be built from non-Abelian anyons, such as Fibonacci anyons described by $\mathrm{SU}{\left(2\right)}_3$ theories, which are quantum deformations of the $\mathrm{SU}\left(2\right)$ algebra. Inspired by the physics of $\mathrm{SU}\left(2\right)$ spins, several works have explored ladders of Fibonacci anyons and also one-dimensional (1D) $t\text{−}J$ models. Here, we aim to explore the combined effects of extended dimensionality and doping by studying ladders composed of coupled chains of interacting itinerant Fibonacci anyons. We show analytically that in the limit of strong rung couplings these models can be mapped onto effective 1D models. These effective models can either be gapped models of hole pairs, or gapless models described by $t\text{−}J$ (or modified $t\text{−}J\text{−}V$) chains of Fibonacci anyons, whose spectrum exhibits a fractionalization into charge and anyon degrees of freedom. The charge degrees of freedom are described by the hardcore boson spectra while the anyon sector is given by a chain of localized interacting anyons. By using exact diagonalizations for two-leg and three-leg ladders, we show that indeed the doped ladders show exactly the same behavior as that of $t\text{−}J$ chains. In the strong ferromagnetic rung limit, we can obtain a new model that hosts two different kinds of Fibonacci particles, which we denote as the heavy $\tau$'s and light $\tau$'s. These two particle types carry the same (non-Abelian) topological charge but different (Abelian) electric charges. Once again, we map the two-dimensional ladder onto an effective chain carrying these heavy and light $\tau$'s. We perform a finite size scaling analysis to show the appearance of gapless modes for certain anyon densities, whereas a topological gapped phase is suggested for another density regime.

We study the extended Bose-Hubbard model on the square lattice at half-filling as a function of next-nearest neighbor hopping amplitude and interaction strength. To variationally map out the phase diagram of this model, we develop a two-parameter family of wave functions based on the parton construction which can describe both topological and broken symmetry phases on equal footing. In addition, our wave functions resolve long standing issues with more conventional short-range Jastrow wave functions. Using this variational ansatz, we show that a spontaneous time-reversal symmetry-breaking fractional Chern insulator is energetically favored over a critical region between two superfluid phases. In verifying the properties of these parton wave functions we exemplify a more robust way to identify topology through the Hall conductance.

We compute the ground-state phase diagram of the Hubbard and frustrated Hubbard models on the square lattice with density matrix embedding theory using clusters of up to 16 sites. We provide an error model to estimate the reliability of the computations and complexity of the physics at different points in the diagram. We find superconductivity in the ground state as well as competition between inhomogeneous charge, spin, and pairing states at low doping. The estimated errors in the study are below $T_c$ in the cuprates and on the scale of contributions in real materials that are neglected in the Hubbard model.